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Two-Photon-Induced [2 + 2] Cycloaddition of Bis-thymines: A Biocompatible and Reversible Approach
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Two-Photon-Induced [2 + 2] Cycloaddition of Bis-thymines: A Biocompatible and Reversible Approach
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Cite this: ACS Omega 2020, 5, 20, 11547–11552
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https://doi.org/10.1021/acsomega.0c00770
Published May 12, 2020

Copyright © 2020 American Chemical Society. This publication is licensed under CC-BY-NC-ND.

Abstract

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Despite having great value across a wide variety of scientific fields, two-photon polymerizations currently suffer from two significant problems: the need for photoinitiators, which generate toxic side products, and the irreversibility of the process. Hence, the design of a versatile approach that circumvents these issues represents a major scientific challenge. Herein, we report a two-photon absorption strategy where reversible [2 + 2] cycloaddition of bis-thymines was achieved without the need for any photoinitiator. The cycloaddition and cycloreversion reactions could be induced by simply changing the irradiation wavelength, and repeated writing and erasing cycles were performed. The simplicity, reversibility, and biocompatibility of this strategy open up a whole new toolbox for applications across a wide variety of scientific fields.

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Introduction

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Two-photon (TP) excitation was theoretically predicted in 1930, (1) empirically demonstrated 30 years later, (2) and transformed the future of material science thereafter. (3) It is defined as the simultaneous absorption of two photons, each carrying nearly half of the energy required to excite a molecule to higher energy states. (4,5) Its success relies on two key ingredients. First, the two-photon absorption depends quadratically on the incident light intensity, thus sharply decreasing outside the focal volume. As a consequence, any TP-induced polymerization can be confined in a small region of the sample by tightly focusing the excitation light. This way, different patterns can be drawn, (6) and potentially erased, and the material refractive index can be changed only in the irradiated zones. (7) Optical storage devices and optical gratings, based on direct laser writing, represent a few examples of possible applications. (8) Second, TP-induced reactions use long-wavelength light such as near-infrared, which confers a significant benefit in biological applications, where the phototoxicity is significantly reduced and extended penetration depths can be achieved. (9,10) These features, combined with the high spatial confinement, result in an exceptional cocktail, which enables the use of TP-based polymer particles for the controlled release of biomolecules in vivo. (11,12) As a result, a considerable volume of research has been carried out in the field of TP-induced polymerization, and significant achievements have been reached hitherto. (13) However, existing TP-based polymerization schemes still suffer from a few significant pitfalls. The chemical bonds formed during the reactions are typically stable, rendering the process irreversible. (14) In addition, they normally require the use of photoinitiators to trigger the reactions, which generate radicals that create unwanted and highly (cyto)toxic side products, such as reactive oxygen species. (15,16) To achieve a universal platform for TP-induced reactions, it is crucial to develop a versatile approach that overcomes these issues.
[2 + 2] Cycloaddition reactions represent promising candidates to be studied by TP absorption, thanks to the significant control over the stereochemistry of the polymer chains and the eco-friendly hallmarks. (17) So far, few TP-induced reactions based on [2 + 2] cycloaddition have been designed, which involve polymers containing coumarin and maleimide side chains. Here, however, photoinitiators are still required and complete reversibility remains a challenge. (18,19)
To knock over these barriers, we report a two-photon polymerization strategy, where different chemical structures can be easily obtained by only changing the irradiation wavelength and without the need of any photoinitiator.
We previously designed a bis-thymine monomer, n-butyl-linked-bis(thymine propanoate), which can undergo one-photon-induced [2 + 2] cycloaddition and cycloreversion upon irradiation with UV light in the solid crystalline state. (20−22) Here, we demonstrate that the same reaction could be achieved upon TP absorption (Figure 1).

Figure 1

Figure 1. [2 + 2] Cycloaddition and cycloreversion reactions of n-butyl-linked-bis(thymine propanoate). The reacting moieties are highlighted in green and blue, while the red dashed lines indicate the bond formation pathway.

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Results and Discussion

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Cycloaddition

In the first stage, we investigated whether the bis-thymine monomer could undergo cycloaddition upon TP absorption. In a typical experiment, a monomer crystal was irradiated with 600 nm focused light (1.6 mW, 1 min, 200 fs, 80 MHz). As depicted in Figure 2a, changes in the crystal occurred after irradiation (round spots) and are provoked either by a modification in the material refractive index or by a crack generated in the crystal. Such alteration is, in turn, caused by the strain imparted on the crystal lattice upon reorientation of the molecules, as reported by previous studies. (20)

Figure 2

Figure 2. Cycloaddition and cycloreversion reactions of bis-thymines induced via two-photon excitation. (a) Transmission image of the bis-thymine monomer crystal after inducing cycloaddition in three spots. (b) Raman mapping of a sample area, featuring the monomer (green) and the cyclobutane (blue). (c) Raman spectrum of the cyclobutane structure compared with the reference spectrum. (d) Transmission image recorded after erasing one spot of previously photoinduced cyclobutane. (e) Raman mapping of the same sample area, confirming the complete cycloreversion. (f) Raman spectrum of the obtained monomer compared with the reference spectrum. Scale bar: 5 μm.

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To investigate which product resulted from the irradiation, we initially conducted NMR measurements, which unfortunately resulted in no signal due to the low amount of sample. Consequently, we used Raman spectroscopy, as its sensitivity allows to analyze reactants at sub-micrometer scale. Not to mention, Raman spectroscopy represents an ideal candidate in chemical reaction studies, thanks to its remarkable fingerprinting capabilities, which provide chemical and structural information unique for each species. Particularly, [2 + 2] cycloaddition reactions represent straightforward subjects of study, as the bis-thymine and cyclobutane structures give distinct Raman fingerprints. (23−25) Investigations through Raman spectroscopy require reference spectra of the compounds under study, which are compared with the spectra obtained during the reactions. In the light of this finding, the Raman spectra, corresponding to the bis-thymine molecules and photoinduced cyclobutane samples synthesized via one-photon reaction, were acquired and kept as a reference throughout all this work. An extensive assignment of the vibrational bands is available in the Supporting Information (SI) (Figure S2). The Raman spectra collected from the TP-induced spots display bands associated with the skeletal stretching vibrations of cyclobutane ring at 498, 707, and 922 cm–1 (Figure 2c, vertical dotted lines) and are in good agreement with the reference spectrum. The Pearson correlation coefficient (PCC), which can be regarded as a measure of similarity, was calculated between the reference and extracted spectra (more information in SI). The obtained value of PCC is 0.84, which indicates a high correlation, thus confirming the successful TP-induced cycloaddition.
Small variations in the vibrational bands between the one-photon (reference) and TP-generated samples (photoinduced) were, however, noticeable, and were due to the difference in crystallinity (Figure S3).
Notably, we discovered that the cycloaddition yield strongly depends on the crystal orientation with respect to the light polarization (Figure S4). We hypothesize that such orientation dependence arises from the anisotropic nature of the crystals, in which molecules are packed unidirectionally. (20,26)
Next, we aimed at evaluating the confinement volume of the reaction, and Raman mapping represents an excellent approach to asses it. Hence, a designated area in the sample (Figure 2a, red rectangle) was scanned with a laser and Raman spectra were collected point-by-point. A false-color image was then generated from the peak intensities corresponding to the cyclobutane stretching vibration (blue 922 cm–1) and the thymine deformations (green 1638, 1658 cm–1) (Figure 2b). The image obtained reveals that the cycloaddition is limited to the irradiated spots.
We then confirmed the TP nature of this process. The UV–vis spectra recorded from the reference samples show no absorption in the spectral region above 300 nm, excluding that the reactions take place via one-photon absorption (Figure S5). The typical evidence for TP process is given by power dependence investigations as, in a TP process, the absorption exhibits quadratic dependence on the irradiation power. Consequently, different spots in the monomer crystal were irradiated with various laser powers, and after each irradiation step, a Raman spectrum was collected (Figure 3a). The peak intensity corresponding to the cyclobutane ring vibration at 922 cm–1 was plotted as a function of the laser power (Figure 3b). The scattered data were fitted with a power function, I = Pa, where I is the peak intensity, P is the laser power, and a is the power of the function. From this fitting, the estimated value was 1.75, confirming the occurrence of TP absorption. The deviation from the predicted value 2 can be attributed to the saturation of polymer product within the 600 nm irradiation spot. (27)

Figure 3

Figure 3. Power dependence measurements. (a) Raman spectra collected after irradiation with different femtosecond laser powers. (b) Intensity of the peak associated with the cyclobutane structure (vertical dotted line) against the laser power. Both ordinate and abscissa are given as logarithmic scales.

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Cycloreversion

Cycloreversion reactions were investigated in an entirely analogous fashion. Accordingly, one spot of the previously photoinduced cyclobutane was irradiated with 450 nm pulsed light (0.3 mW, 1 min, 200 fs, 80 MHz). The irradiated spot could be erased, and the monomer crystal completely regenerated (Figure 2d). This result suggests that the spots originating after 600 nm irradiation (Figure 2a) were caused by a change in the refractive index and not by cracks formed in the crystal. Following the same approach as outlined for the cycloaddition, we collected the Raman spectra corresponding to the irradiated area and compared them with the Raman fingerprint of the original monomer (Figure 2f, PCC value: 0.98). In the spectrum collected from the photoinduced spot, bands related to the cyclobutane ring vanished entirely, while peaks attributed to the thymine moieties at 447, 459, 507, 521, 1638, and 1656 cm–1 re-emerged (Figure 2f, red dotted lines). These spectral changes suggest the successful cleavage of the cyclobutane rings, as well as the formation of the initial bis-thymine units. Once again, Raman mapping in the same area was carried out (Figure 2e). The image obtained further confirms the loss of the cyclobutane structures.
In addition, we investigated the wavelength dependence of the process, as the irradiation wavelength appears to have a strong influence on the reaction products. Surprisingly, we discovered that when 500 nm light was employed instead of 450 nm light, a different compound was formed (Figure S6). We hypothesize that a partial cycloreversion occurred via charge separation, a mechanism previously shown by Tsurkan et al. in [2 + 2] cycloaddition reactions. (19) A more in-depth investigation of such a new structure is beyond the purpose of this work, although it deserves future investigations.

Cycloaddition and Cycloreversion Cycles

The last set of experiments was aimed at exploring whether our compounds could undergo repeated cycles of cycloaddition and cycloreversion. The same spot in the monomer crystal was irradiated repeatedly by alternating 600 and 450 nm light. As revealed by the resulting Raman spectra, the monomer and cyclobutane structures could be obtained after each irradiation step (Figure 4a). To better visualize this achievement, peak intensities related to the bis-thymine (1638 cm–1) and the cyclobutane (922 cm–1) were plotted against the irradiation step (Figure 4b).

Figure 4

Figure 4. Repeated cycles of cycloaddition and cycloreversion of bis-thymines. (a) Raman spectra collected from the same spot after alternate irradiation with 600 and 450 nm light. (b) Peak intensities associated with the thymine (green bands) and cyclobutane (blue bands) against the irradiation cycles.

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Conclusions

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Although two-photon polymerizations are in the spotlight of an ever-growing research endeavor, limitations related to the need for photoinitiators and the irreversibility of the process have not been overcome, yet. In this work, two-photon-induced [2 + 2] cycloaddition of bis-thymine monomers was achieved without any photoinitiator and in a fully reversible fashion. The key to the success of this strategy lies in its versatility. The reactions can be induced by only varying the irradiation wavelength, rendering this approach easy to perform. The complete reversibility of the process enables repeated writing/erasing/rewriting cycles, along with repeated modifications of the material refractive index. Moreover, the absence of photoinitiators renders this material biocompatible, suggesting potential for its implementation in biomedical applications.

Experimental Section

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Sample Preparation

Thymine-based compounds were synthesized and characterized by the group of Prof. K. Saito following a previously described protocol. (22) The monomer powder was dissolved in a 50/50 (v/v) mixture of acetonitrile/H2O (10 mg·mL–1, 21 μM). The solution was vortexed to ensure that the whole monomer was dissolved and 70 μL was drop-casted on a clean glass coverslip. The solvent was allowed to evaporate overnight to yield white crystals, which were attached to the glass coverslip (Figure S1). A single-photon synthesized polymer was either dissolved in a 50/50 (v/v) mixture of chloroform/H2O (10 mg·mL–1) to yield a filmlike morphology, or in a 50/50 (v/v) mixture of acetonitrile/H2O (10 mg·mL–1) to yield a crystal-like morphology. Both solutions were then drop-casted onto a glass coverslip following the same procedure as described for the monomer. After the evaporation of the solvent, film and crystals structures appeared (Figure S3).

Two-Photon Reaction

The two-photon experiments were performed on the same setup as that used for Raman spectroscopy. A glass coverslip containing either a monomer or a polymer was placed on the piezoelectric stage of the microscope. A Ti:sapphire femtosecond laser (MaiTai, Spectra-Physics, 820 nm, 120 fs, 80 MHz) coupled with an optical parametric oscillator (OPO) (Inspire, Spectra-Physics) was used to produce femtosecond pulses (80 MHz, 120 fs, linearly polarized) to induce two-photon reactions. The light was deflected by a dichroic mirror (Z633RDC, Chroma) before being focused on the sample by the same objective as that used for Raman spectroscopy. Three different wavelengths were used, i.e., 600 nm for cycloaddition (1.6 mW), 450 nm for cycloreversion (0.3 mW), and 500 nm for obtaining the unknown compound (1 mW). For a typical experiment, an irradiation time of 1 min was used.

Raman Spectroscopy

Raman measurements were conducted using an inverted optical microscope (Ti–U, Nikon) equipped with a piezoelectric stage (P517.3CL, Physik Instrument). Continuous-wave He–Ne laser light (632.8 nm) was reflected by a dichroic mirror (Z633RDC, Chroma) and then focused on the sample by an objective lens (Plan Apo 60x, N.A. 0.95 or Plan Fluor 40x, N.A. 0.60, Nikon). Circular polarization at the sample was achieved by tuning half-wavelength (λ/2) and quarter-wavelength (λ/4) waveplates. The Raman signals were collected by a spectrometer (iHR 320, Horiba) equipped with a cooled electron-multiplying charge-coupled device (CCD) camera (Newton 920, Andor).

Pearson Correlation Coefficient (PCC)

The Pearson correlation coefficient (PCC) between the two spectra was estimated according to the following equation
where N is the total number of considered Raman shifts; xn and yn are the Raman intensities registered for the first and second spectra, respectively, at the nth Raman shift; and x̅ and y̅ are the mean intensities recorded for the first and second spectra, respectively. rx,y can be regarded as a measure of the overall similarity between the two profiles being studied. Even though there exists no widespread consensus on the interpretation of the PCC, the coefficients calculated in this work reflect relatively high correlations given the noise level of the measurements and the high amount of experimental factors influencing them. To obtain an unbiased estimation of the PCC, each pair of spectra was preliminarily baseline-corrected by asymmetric least squares (AsLS). (28)

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c00770.

  • Experimental procedures; Raman characterization and peak assignment; morphology dependence; orientation dependence; UV–vis spectra; and reaction with 500 nm light (PDF)

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Author Information

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  • Corresponding Authors
  • Authors
    • Monica Ricci - Department of Chemistry, KU Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium
    • Martin G.T.A. Rutten - Department of Chemistry, KU Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium;  Present Address: Institute for Complex Molecular Systems, Eindhoven University of Technology, 5612 AZ Eindhoven, The Netherlands
    • Shuichi Toyouchi - Department of Chemistry, KU Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium;  Orcidhttp://orcid.org/0000-0002-4771-8015
    • Sepa Nanayakkara - School of Chemistry, Monash University, Clayton, VIC 3800, Australia
    • Beatrice Fortuni - Department of Chemistry, KU Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium
    • Raffaele Vitale - Department of Chemistry, KU Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium;  Laboratoire de Spectrochimie Infrarouge et Raman, Université de Lille, Villeneuve d’Ascq Cedex C5, 59655 Lille, France;  Orcidhttp://orcid.org/0000-0002-7497-1673
    • Susana Rocha - Department of Chemistry, KU Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium;  Orcidhttp://orcid.org/0000-0003-1258-9396
    • Daniela A. Wilson - Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands;  Orcidhttp://orcid.org/0000-0002-8796-2274
    • Johan Hofkens - Department of Chemistry, KU Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium;  Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany;  Orcidhttp://orcid.org/0000-0002-9101-0567
  • Author Contributions

    M.R. and M.G.T.A.R. contributed equally to this work.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was funded by the JST PRESTO (JPMJPR1515) to K.S., the Research Foundation, Flanders (Grant Nos. G.0B39.15, G.0B49.15, G098319N, G0D4519N, G081916N, and ZW15_09-GOH6316), and the KU Leuven Research Fund (C14/15/053) to H.U. and J.H., and JSPS Kakenhi (JP17H03003) to H.U. The Erasmus + grant to M.G.T.A.R. and FWO scholarship (1S33117N and 12X1419N) to M.R. and B.F., respectively, are greatly acknowledged. D.A.W. acknowledges NWO Chemische Wetenschappen VIDI grant (723.015.001) and the Ministry of Education, Culture and Science (024.001.035). J.H. acknowledges the support of the Flemish government through long-term structural funding Methusalem (CASAS2, Meth/15/04) and the Hercules Foundation (HER/11/14). This collaborative work was supported by the JSPS Core-to-Core Program, A. Advanced Research Networks.

Abbreviations

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TP

two-photon

NMR

nuclear magnetic resonance

PCC

Pearson correlation coefficient

References

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    Chen, Z.; He, S.; Butt, H. J.; Wu, S. Photon Upconversion Lithography: Patterning of Biomaterials Using Near-Infrared Light. Adv. Mater. 2015, 27, 2203– 2206,  DOI: 10.1002/adma.201405933
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    Photon Upconversion Lithography: Patterning of Biomaterials Using Near-Infrared Light
    Chen, Zhijun; He, Shuqing; Butt, Hans-Juergen; Wu, Si
    Advanced Materials (Weinheim, Germany) (2015), 27 (13), 2203-2206CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    We developed a new protein patterning method using near IR light.
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    Croissant, J.; Maynadier, M.; Gallud, A.; Peindy N’Dongo, H.; Nyalosaso, J. L.; Derrien, G.; Charnay, C.; Durand, J. O.; Raehm, L.; Serein-Spirau, F. Two-Photon-Triggered Drug Delivery in Cancer Cells Using Nanoimpellers. Angew. Chem., Int. Ed. 2013, 52, 13813– 13817,  DOI: 10.1002/anie.201308647
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    Angewandte Chemie, International Edition (2013), 52 (51), 13813-13817CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Mesoporous silica nanoparticles with azobenzene moieties (nanoimpellers) were synthesized that contained a two-photon fluorophore that possessed a high two-photon absorption cross-section, leading to nanoimpellers configured for two-photon excitation (TPE) with fluorescence resonance energy transfer from the fluorphore to the azobenzene. The residual fluorescence of the nanoimpellers allowed TPE fluorescence imaging in cancer cells. A camptothecin-loaded nanoimpeller with a high energy transfer quantum yield from the fluorophore to the azobenzene moiety could induce human breast cancer cell death under these TPE conditions and was not toxic to cells without irradn. at low concns.
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    Caged compounds: photorelease technology for control of cellular chemistry and physiology
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    Nature Methods (2007), 4 (8), 619-628CODEN: NMAEA3; ISSN:1548-7091. (Nature Publishing Group)
    A review. Caged compds. are light-sensitive probes that functionally encapsulate biomols. in an inactive form. Irradn. liberates the trapped mol., permitting targeted perturbation of a biol. process. Uncaging technol. and fluorescence microscopy are 'optically orthogonal': the former allows control, and the latter, observation of cellular function. Used in conjunction with other technologies (for example, patch clamp and/or genetics), the light beam becomes a uniquely powerful tool to stimulate a selected biol. target in space or time. Here the author describes important examples of widely used caged compds., their design features and synthesis, as well as practical details of how to use them with living cells.
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    Acta Biomaterialia (2015), 18 (), 186-195CODEN: ABCICB; ISSN:1742-7061. (Elsevier Ltd.)
    Fabrication of three-dimensional (3D) hydrogel microenvironments with predefined geometry and porosity can facilitate important requirements in tissue engineering and regenerative medicine. Chitosan (CH) is well known as a biocompatible hydrogel with prospective biol. properties for biomedical aims. So far, microstructuring of this soft material presents a great limitation for its application as functional supporting material for guided tissue formation. Enabling photopolymn., chem. modified CH can be applied for the biofabrication of reproducible 3D scaffolds using rapid prototyping techniques like two-photon polymn. (2PP) or others. The application of this technique allows precise serial fabrication of computer-designed microstructure geometries by scanning a femtosecond laser beam within a photosensitive material. This work explores a new synthesis of water-sol. photosensitive chitosan and the fabrication of well-defined microstructures from the generated materials. To modulate the mech. and biochem. properties of the material, CH was combined and cross-linked with synthetic poly(ethylene glycol) diacrylate. For a biol. adaptation to the in vivo situation, CH was covalently crosslinked with a photosensitive modified vascular endothelial growth factor (VEGF). Performed in vitro studies reveal that modified CH is biocompatible. VEGF enhances CH bioactivity. Furthermore, a 3D CH scaffold can be successfully seeded with cells. Therefore, the established CH holds great promise for future applications in tissue engineering.
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    Rodriguez-Emmenegger, C.; Preuss, C. M.; Yameen, B.; Pop-Georgievski, O.; Bachmann, M.; Mueller, J. O.; Bruns, M.; Goldmann, A. S.; Bastmeyer, M.; Barner-Kowollik, C. Controlled Cell Adhesion on Poly(Dopamine) Interfaces Photopatterned with Non-Fouling Brushes. Adv. Mater. 2013, 25, 6123– 6127,  DOI: 10.1002/adma.201302492
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    Controlled Cell Adhesion on Poly(dopamine) Interfaces Photopatterned with Non-Fouling Brushes
    Rodriguez-Emmenegger, Cesar; Preuss, Corinna M.; Yameen, Basit; Pop-Georgievski, Ognen; Bachmann, Michael; Mueller, Jan O.; Bruns, Michael; Goldmann, Anja S.; Bastmeyer, Martin; Barner-Kowollik, Christopher
    Advanced Materials (Weinheim, Germany) (2013), 25 (42), 6123-6127CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    We present the successful prepn. of photopatterned interfaces to precisely control cell adhesion. The strategy consists of merging bioinspired polydopamine (PDA) films with antifouling poly(oligoethylene glycol Me ether methacrylate) (poly(MeOEGMA)) brushes utilizing the nitrile imine-mediated tetrazole-ene cycloaddn. photoclick protocol. PDA functionalized with a novel tetrazole was photopatterned with an atom transfer radical polymn. initiator, subsequently used for growing cell-repellent polymer brushes. The photopatterned surfaces supported the formation of confluent layers of rat embryonic fibroblasts, while poly(MeOEGMA) brushes withstand cell adhesion resulting in a high fidelity patterns of cells.
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    Tischer, T.; Rodriguez-Emmenegger, C.; Trouillet, V.; Welle, A.; Schueler, V.; Mueller, J. O.; Goldmann, A. S.; Brynda, E.; Barner-Kowollik, C. Photo-Patterning of Non-Fouling Polymers and Biomolecules on Paper. Adv. Mater. 2014, 26, 4087– 4092,  DOI: 10.1002/adma.201401006
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    Photo-Patterning of Non-Fouling Polymers and Biomolecules on Paper
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    Advanced Materials (Weinheim, Germany) (2014), 26 (24), 4087-4092CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    We resent the use of the photo-triggered NITEC (nitrile imine-mediated tetrazole-ene cycloaddns.) reaction to pattern cellulose with ultra-low fouling poly(CBAA-3) (poly(carboxybetaine acrylamide)) and to immobilize streptavidin. The strategy consists of the facile arylation of cellulose with a tetrazole acid chloride which was further exploited to pattern a maleimide-functional initiator to graft poly(CBAA-3) brushes. The use of SET-LRP (single electron transfer living radical polymn.) accounted for fast polymn. without the need of any purifn. step to remove catalysts and enabled access to cellulose surface fully resistant to fetal calf serum. In a similar fashion streptavidin functionalized with maleimide was immobilized onto Cel-Tet with remarkable efficiency. We envision that the reported strategy can be translated to the development of novel bioactive papers and μPADs (microfluidic paper-based anal. devices) necessary for the forthcoming developments in medicine.
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    Forbidden Chemistry: Two-Photon Pathway in [2+2] Cycloaddition of Maleimides
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    Two-photon excitation provides high spatial resoln. in three dimensions of the corresponding chem. or phys. processes, allowing submicrometer structuring in stereolithog. and three-dimensional (3D) microfabrication. While studying two-photon structuring applications, we obsd. an undescribed phenomenon in photochem. that dictates reactivity of maleimide groups in two-photon mode. A low-absorbance transition formerly ignored in classical photochem. has been found for maleimides. This transition was assigned to symmetry-breaking donor-acceptor complex formation, which revealed a formally forbidden pathway in [2+2] cycloaddn. reactions of maleimide moieties. This synthetic pathway allowed for the creation of hydrogel materials under physiol. conditions at low laser excitation energy (0.1 J/cm2 at 800 nm) without the use of photoinitiators, which makes it truly two-photon click chem.
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    Udagawa, Akihiro; Johnston, Priscilla; Sakon, Aya; Toyoshima, Ryosuke; Uekusa, Hidehiro; Koshima, Hideko; Saito, Kei; Asahi, Toru
    RSC Advances (2016), 6 (109), 107317-107322CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    Solid-state photo-polymn. in crystals can produce stereoregular polymer mols. in environmentally friendly solvent-free systems. The polymn. mechanism of bis-thymine derivs., such as dimethyl-3,3'-(3,3'-(butane-1,4-diyl)bis(5-methyl-2,4-dioxo-3,4-dihydropyrimidine-3,1(2H)-diyl))dipropanoate (1), known as unique mols. that can topochem. and reversibly polymerize in the cryst. state via [2 + 2]-cycloaddn. reactions upon UV irradn., remained to be solved. In this manuscript, the crystal structure of the polymeric photoproduct (1P) from a bis-thymine deriv. 1 was detd. using ab initio powder X-ray diffraction data and applied to investigate the polymn. mechanism of bis-thymine derivs. The topochem. polymn. was found to be achieved via [2 + 2]-cycloaddn. with the flexible Bu chain between thyminyl rings relieving the distortion of whole structure derived from cyclobutane formation. The crystal structure of 1P also showed that it polymd. stereoregularly with trans-anti cyclobutane conformations.
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  • Abstract

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    Figure 1

    Figure 1. [2 + 2] Cycloaddition and cycloreversion reactions of n-butyl-linked-bis(thymine propanoate). The reacting moieties are highlighted in green and blue, while the red dashed lines indicate the bond formation pathway.

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    Figure 2

    Figure 2. Cycloaddition and cycloreversion reactions of bis-thymines induced via two-photon excitation. (a) Transmission image of the bis-thymine monomer crystal after inducing cycloaddition in three spots. (b) Raman mapping of a sample area, featuring the monomer (green) and the cyclobutane (blue). (c) Raman spectrum of the cyclobutane structure compared with the reference spectrum. (d) Transmission image recorded after erasing one spot of previously photoinduced cyclobutane. (e) Raman mapping of the same sample area, confirming the complete cycloreversion. (f) Raman spectrum of the obtained monomer compared with the reference spectrum. Scale bar: 5 μm.

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    Figure 3

    Figure 3. Power dependence measurements. (a) Raman spectra collected after irradiation with different femtosecond laser powers. (b) Intensity of the peak associated with the cyclobutane structure (vertical dotted line) against the laser power. Both ordinate and abscissa are given as logarithmic scales.

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    Figure 4

    Figure 4. Repeated cycles of cycloaddition and cycloreversion of bis-thymines. (a) Raman spectra collected from the same spot after alternate irradiation with 600 and 450 nm light. (b) Peak intensities associated with the thymine (green bands) and cyclobutane (blue bands) against the irradiation cycles.

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      Kaiser, W.; Garrett, C. G. B.
      Physical Review Letters (1961), 7 (), 229-31CODEN: PRLTAO; ISSN:0031-9007.
      The generation of blue fluorescent light (λ = 4250 A.) by exciting CaF2:Eu++ crystals with red light (λ = 6943 A.) from a ruby maser is studied as an example of 2-photon excitation. CaF2.0.1% EuF2 crystals exhibit strong absorption between 30,000 and 25,000 cm.-1 and are caused to emit a b brilliant blue fluorescence about 300 A. wide. A crystal of 1 mm. thickness is mounted in front of a quartz-spectrometer, and light from a 0.1-joule output ruby maser is focused on it through 2 red filters of transmission less than 10-4 for λ <6100 A. to prevent stray light from the flash lamp from reaching the crystal. Between the crystal and the spectrometer prism a blue filter with about 10-3 transmission at 7000 A. is interposed. A photograph of the spectrum is shown. Pure CaF2 gives no effect. In another expt., photomultipliers are used to compare the blue and the red part of the spectrum and to measure the no. of about 108 photons per flash. It is assumed that a real state of Eu++ is excited by a 2-photon process. The theory of the probability of a process by which an atom is excited by 2 photons hν1 and hν2, the sum of which corresponds to an excited state of the atom, had been published earlier (Goeppert-Mayer, CA 25, 3911). The results are in reasonably good agreement with an unpublished theory by D. Kleinman.
    3. 3
      Pawlicki, M.; Collins, H. A.; Denning, R. G.; Anderson, H. L. Two-Photon Absorption and the Design of Two-Photon Dyes. Angew. Chem., Int. Ed. 2009, 48, 3244– 3266,  DOI: 10.1002/anie.200805257
      3
      Two-photon absorption and the design of two-photon dyes
      Pawlicki, Milosz; Collins, Hazel A.; Denning, Robert G.; Anderson, Harry L.
      Angewandte Chemie, International Edition (2009), 48 (18), 3244-3266CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      This review introduces the theory of two-photon absorption, surveys the wide range of potential applications, and highlights emerging structure-property correlations that can serve as guidelines for the development of efficient two-photon dyes.
    4. 4
      Rumi, M.; Perry, J. W. Two-Photon Absorption: An Overview of Measurements and Principles. Adv. Opt. Photonics 2010, 2, 451,  DOI: 10.1364/AOP.2.000451
      4
      Two-photon absorption: an overview of measurements and principles
      Rumi, Mariacristina; Perry, Joseph W.
      Advances in Optics and Photonics (2010), 2 (4), 451-518CODEN: AOPAC7; ISSN:1943-8206. (Optical Society of America)
      A review. The range of org. compds. whose degenerate two-photon absorption (2PA) spectrum has been reported has increased rapidly in recent years, in parallel with the growing interest in applications based on the 2PA process. The comparison of results from different techniques is not always straightforward, and exptl. conditions employed may vary significantly. We overview the concepts underlying 2PA measurements and the common assumptions and approxns. used in the data anal. for various techniques. The importance of selecting appropriate excitation regimes under which measurements should be performed and of avoiding contributions from absorption mechanisms in addn. to 2PA will be emphasized.
    5. 5
      Kobayashi, Y.; Mutoh, K.; Abe, J. Stepwise Two-Photon Absorption Processes Utilizing Photochromic Reactions. J. Photochem. Photobiol., C 2018, 34, 2– 28,  DOI: 10.1016/j.jphotochemrev.2017.12.006
      5
      Stepwise two-photon absorption processes utilizing photochromic reactions
      Kobayashi, Yoichi; Mutoh, Katsuya; Abe, Jiro
      Journal of Photochemistry and Photobiology, C: Photochemistry Reviews (2018), 34 (), 2-28CODEN: JPPCAF; ISSN:1389-5567. (Elsevier B.V.)
      A review. Stepwise two-photon absorption (2PA) is one of the nonlinear photoresponses where two photons are sequentially absorbed to proceed a photophys. or photochem. reaction. The stepwise 2PA can selectively produce a higher excited state with relatively high spatial selectivity, and therefore, it can be an efficient tool to explore and realize nonlinear photoresponsive materials beyond Kasha's rule. Moreover, if a photogenerated chem. species is used as an intermediate state of the stepwise 2PA, the power threshold to promote the stepwise 2PA can be greatly reduced compared with that of a simultaneous 2PA, and conventional LEDs and even sunlight could be used to induce the stepwise 2PA. Because of these advantages, the stepwise 2PA has been widely studied more than a half century ago not only for fundamental physics and chem. but also for applications to optical memory, holog., and artificial photosynthesis. In this review, we overview various types of stepwise 2PA processes from the viewpoints of history, fundamentals, and applications. Esp. we focus on the stepwise 2PA processes related to photochromic reactions because the combination of photochromic reactions and stepwise 2PA processes has realized various potentially-applicable nonlinear photoresponsive materials and found several anomalous phenomena beyond the one-photon photophys. and photochem. reactions. The stepwise 2PA processes are mainly divided into two categories involving an electronic excited state or a photogenerated transient species as the intermediates states of the stepwise 2PA. These two types of the stepwise 2PAs are explained resp., and finally, the summary and outlook are presented.
    6. 6
      Sabatyan, A.; Rafighdoost, J. Grating- and Checkerboard-Based Zone Plates as an Optical Array Generator with a Favorable Beam Shape. Appl. Opt. 2017, 56, 5355,  DOI: 10.1364/AO.56.005355
      6
      Grating- and checkerboard-based zone plates as an optical array generator with a favorable beam shape
      Sabatyan, Arash; Rafighdoost, Jila
      Applied Optics (2017), 56 (19), 5355-5359CODEN: APOPAI; ISSN:2155-3165. (Optical Society of America)
      The combination of a grating as a periodic object and a Fresnel zone plate with a quasi-periodic structure leads us to propose some novel array generator elements. It is shown that diffraction performance of the proposed element depends on the grating structure, so that the diffraction performance may efficiently be increased if the grating is replaced with a checkerboard structure. Depending on the kind of zone plate, we have demonstrated that the corresponding array generator is also created. For instance, a focused array of vortices is generated when the zone plate is replaced with a spiral zone plate. We have also clarified that the method allows us to generalize it to the other class of the zone plate-based elements, e.g., a cross zone plate. Namely, when the cross zone plate is used instead of the Fresnel zone plate, a spot array generator is again created whose diffractive performance is improved by modifying its phase structure. Also, it is clearly shown that the focusing feature of the element depends on the grating period. Finally, there is a good agreement between the simulation results and the corresponding exptl. works.
    7. 7
      Li, X.; Zhang, Q.; Chen, X.; Gu, M. Giant Refractive-Index Modulation by Two-Photon Reduction of Fluorescent Graphene Oxides for Multimode Optical Recording. Sci. Rep. 2013, 3, 1339  DOI: 10.1038/srep02819
      7
      Ratio of Aβ42/P-tau181p in CSF is associated with aberrant default mode network in AD
      Li, Xiaozhen; Li, Tie-Qiang; Andreasen, Niels; Kristoffersen Wiberg, Maria; Westman, Eric; Wahlund, Lars-Olof
      Scientific Reports (2013), 3 (), 1339, 5 pp.CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      The default mode network (DMN) is particularly relevant to Alzheimer's disease (AD) since its structures are vulnerable to deposition of amyloid. Decreased levels of β-amyloid1-42 (Aβ42) and increased total tau protein (T-tau) and tau phosphorylated at position threonine 181 (P-tau181p) in cerebrospinal fluid (CSF) have been established as valid biomarkers for the diagnosis and prognosis of AD. However, the relationship between CSF biomarkers and change in the DMN is still unknown. In this study we investigated the correlation between the functional connectivity within the DMN and the ratio of Aβ42/P-tau181p in the CSF. We found that the ratio of Aβ42/P-tau181p was moderately pos. correlated with the functional connectivity within the DMN in the left precuneus/cuneus. This finding implicates that the brain functional connectivity within DMN is affected by pathol. changes at early stage in AD. This may provide a better understanding of AD pathol. progression and improve AD diagnosis.
    8. 8
      Sun, H. B.; Kawata, S. Two-Photon Photopolymerization and 3D Lithographic Microfabrication. Adv. Polym. Sci. 2004, 170, 169– 273,  DOI: 10.1007/b94405
      8
      Two-photon photopolymerization and 3D lithographic microfabrication
      Sun, Hong-Bo; Kawata, Satoshi
      Advances in Polymer Science (2004), 170 (NMR, 3D Analysis, Photopolymerization), 169-273CODEN: APSIDK; ISSN:0065-3195. (Springer GmbH)
      This review discusses the development and current progress of femtosecond laser micro-nanofabrication based on multiphoton absorption, and particular emphasis is placed on two-photon photopolymn. Femtosecond laser interaction with matter differs essentially from those with longer pulses or CW lasers in its significant nonlinearity, ultrafast characteristics, and the possibility of high localization of reaction vol., with these features enabling three-dimensional (3D) micro-nanofabrication in solid and liq. media. In two-photon photopolymn., when a near-IR femtosecond laser is tightly focused into a photopolymerizable resin, 3D polymer micro-nanostructures are produced by pinpoint photopolymn. of liq. precursory resins. Using this direct laser writing scheme, various photonic, micro-optical components and micromech. devices have been readily produced.
    9. 9
      Helmchen, F.; Denk, W. Deep Tissue Two-Photon Microscopy. Nat. Methods 2005, 2, 932– 940,  DOI: 10.1038/nmeth818
      9
      Deep tissue two-photon microscopy
      Helmchen, Fritjof; Denk, Winfried
      Nature Methods (2005), 2 (12), 932-940CODEN: NMAEA3; ISSN:1548-7091. (Nature Publishing Group)
      A review. With few exceptions biol. tissues strongly scatter light, making high-resoln. deep imaging impossible for traditional - including confocal - fluorescence microscopy. Nonlinear optical microscopy, in particular two photon-excited fluorescence microscopy, has overcome this limitation, providing large depth penetration mainly because even multiply scattered signal photons can be assigned to their origin as the result of localized nonlinear signal generation. Two-photon microscopy thus allows cellular imaging several hundred microns deep in various organs of living animals. Here the authors review fundamental concepts of nonlinear microscopy and discuss conditions relevant for achieving large imaging depths in intact tissue.
    10. 10
      Chen, Z.; He, S.; Butt, H. J.; Wu, S. Photon Upconversion Lithography: Patterning of Biomaterials Using Near-Infrared Light. Adv. Mater. 2015, 27, 2203– 2206,  DOI: 10.1002/adma.201405933
      10
      Photon Upconversion Lithography: Patterning of Biomaterials Using Near-Infrared Light
      Chen, Zhijun; He, Shuqing; Butt, Hans-Juergen; Wu, Si
      Advanced Materials (Weinheim, Germany) (2015), 27 (13), 2203-2206CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We developed a new protein patterning method using near IR light.
    11. 11
      Croissant, J.; Maynadier, M.; Gallud, A.; Peindy N’Dongo, H.; Nyalosaso, J. L.; Derrien, G.; Charnay, C.; Durand, J. O.; Raehm, L.; Serein-Spirau, F. Two-Photon-Triggered Drug Delivery in Cancer Cells Using Nanoimpellers. Angew. Chem., Int. Ed. 2013, 52, 13813– 13817,  DOI: 10.1002/anie.201308647
      11
      Two-Photon-Triggered Drug Delivery in Cancer Cells Using Nanoimpellers
      Croissant, Jonas; Maynadier, Marie; Gallud, Audrey; Peindy N'Dongo, Harmel; Nyalosaso, Jeff L.; Derrien, Gaelle; Charnay, Clarence; Durand, Jean-Olivier; Raehm, Laurence; Serein-Spirau, Francoise; Cheminet, Nathalie; Jarrosson, Thibaut; Mongin, Olivier; Blanchard-Desce, Mireille; Gary-Bobo, Magali; Garcia, Marcel; Lu, Jie; Tamanoi, Fuyuhiko; Tarn, Derrick; Guardado-Alvarez, Tania M.; Zink, Jeffrey I.
      Angewandte Chemie, International Edition (2013), 52 (51), 13813-13817CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Mesoporous silica nanoparticles with azobenzene moieties (nanoimpellers) were synthesized that contained a two-photon fluorophore that possessed a high two-photon absorption cross-section, leading to nanoimpellers configured for two-photon excitation (TPE) with fluorescence resonance energy transfer from the fluorphore to the azobenzene. The residual fluorescence of the nanoimpellers allowed TPE fluorescence imaging in cancer cells. A camptothecin-loaded nanoimpeller with a high energy transfer quantum yield from the fluorophore to the azobenzene moiety could induce human breast cancer cell death under these TPE conditions and was not toxic to cells without irradn. at low concns.
    12. 12
      Ellis-Davies, G. C. R. Caged Compounds: Photorelease Technology for Control of Cellular Chemistry and Physiology. Nat. Methods 2007, 4, 619– 628,  DOI: 10.1038/nmeth1072
      12
      Caged compounds: photorelease technology for control of cellular chemistry and physiology
      Ellis-Davies, Graham C. R.
      Nature Methods (2007), 4 (8), 619-628CODEN: NMAEA3; ISSN:1548-7091. (Nature Publishing Group)
      A review. Caged compds. are light-sensitive probes that functionally encapsulate biomols. in an inactive form. Irradn. liberates the trapped mol., permitting targeted perturbation of a biol. process. Uncaging technol. and fluorescence microscopy are 'optically orthogonal': the former allows control, and the latter, observation of cellular function. Used in conjunction with other technologies (for example, patch clamp and/or genetics), the light beam becomes a uniquely powerful tool to stimulate a selected biol. target in space or time. Here the author describes important examples of widely used caged compds., their design features and synthesis, as well as practical details of how to use them with living cells.
    13. 13
      Selimis, A.; Mironov, V.; Farsari, M. Direct Laser Writing: Principles and Materials for Scaffold 3D Printing. Microelectron. Eng. 2015, 132, 83– 89,  DOI: 10.1016/j.mee.2014.10.001
      13
      Direct laser writing: Principles and materials for scaffold 3D printing
      Selimis, Alexandros; Mironov, Vladimir; Farsari, Maria
      Microelectronic Engineering (2015), 132 (), 83-89CODEN: MIENEF; ISSN:0167-9317. (Elsevier B.V.)
      A review. For a great variety of research fields extending from photonics to tissue engineering applications, the requests for the construction of three-dimensional structures with high resoln. grow more and more imperative. Towards this aim, the direct laser writing technique by multi-photon polymn., due to its unique properties and characteristics, has proven to be an indispensable tool to high accuracy structuring and has been put on the map as an emerging technol. for scaffold 3D printing. In the present review, the basic principles of multi-photon polymn. are presented, the exptl. set-up requirements are described and the employed materials demands are thoroughly mentioned as well as the most representative examples of the recent developments in the field.
    14. 14
      Kufelt, O.; El-Tamer, A.; Sehring, C.; Meißner, M.; Schlie-Wolter, S.; Chichkov, B. N. Water-Soluble Photopolymerizable Chitosan Hydrogels for Biofabrication via Two-Photon Polymerization. Acta Biomater. 2015, 18, 186– 195,  DOI: 10.1016/j.actbio.2015.02.025
      14
      Water-soluble photopolymerizable chitosan hydrogels for biofabrication via two-photon polymerization
      Kufelt, Olga; El-Tamer, Ayman; Sehring, Camilla; Meissner, Marita; Schlie-Wolter, Sabrina; Chichkov, Boris N.
      Acta Biomaterialia (2015), 18 (), 186-195CODEN: ABCICB; ISSN:1742-7061. (Elsevier Ltd.)
      Fabrication of three-dimensional (3D) hydrogel microenvironments with predefined geometry and porosity can facilitate important requirements in tissue engineering and regenerative medicine. Chitosan (CH) is well known as a biocompatible hydrogel with prospective biol. properties for biomedical aims. So far, microstructuring of this soft material presents a great limitation for its application as functional supporting material for guided tissue formation. Enabling photopolymn., chem. modified CH can be applied for the biofabrication of reproducible 3D scaffolds using rapid prototyping techniques like two-photon polymn. (2PP) or others. The application of this technique allows precise serial fabrication of computer-designed microstructure geometries by scanning a femtosecond laser beam within a photosensitive material. This work explores a new synthesis of water-sol. photosensitive chitosan and the fabrication of well-defined microstructures from the generated materials. To modulate the mech. and biochem. properties of the material, CH was combined and cross-linked with synthetic poly(ethylene glycol) diacrylate. For a biol. adaptation to the in vivo situation, CH was covalently crosslinked with a photosensitive modified vascular endothelial growth factor (VEGF). Performed in vitro studies reveal that modified CH is biocompatible. VEGF enhances CH bioactivity. Furthermore, a 3D CH scaffold can be successfully seeded with cells. Therefore, the established CH holds great promise for future applications in tissue engineering.
    15. 15
      Rodriguez-Emmenegger, C.; Preuss, C. M.; Yameen, B.; Pop-Georgievski, O.; Bachmann, M.; Mueller, J. O.; Bruns, M.; Goldmann, A. S.; Bastmeyer, M.; Barner-Kowollik, C. Controlled Cell Adhesion on Poly(Dopamine) Interfaces Photopatterned with Non-Fouling Brushes. Adv. Mater. 2013, 25, 6123– 6127,  DOI: 10.1002/adma.201302492
      15
      Controlled Cell Adhesion on Poly(dopamine) Interfaces Photopatterned with Non-Fouling Brushes
      Rodriguez-Emmenegger, Cesar; Preuss, Corinna M.; Yameen, Basit; Pop-Georgievski, Ognen; Bachmann, Michael; Mueller, Jan O.; Bruns, Michael; Goldmann, Anja S.; Bastmeyer, Martin; Barner-Kowollik, Christopher
      Advanced Materials (Weinheim, Germany) (2013), 25 (42), 6123-6127CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We present the successful prepn. of photopatterned interfaces to precisely control cell adhesion. The strategy consists of merging bioinspired polydopamine (PDA) films with antifouling poly(oligoethylene glycol Me ether methacrylate) (poly(MeOEGMA)) brushes utilizing the nitrile imine-mediated tetrazole-ene cycloaddn. photoclick protocol. PDA functionalized with a novel tetrazole was photopatterned with an atom transfer radical polymn. initiator, subsequently used for growing cell-repellent polymer brushes. The photopatterned surfaces supported the formation of confluent layers of rat embryonic fibroblasts, while poly(MeOEGMA) brushes withstand cell adhesion resulting in a high fidelity patterns of cells.
    16. 16
      Tischer, T.; Rodriguez-Emmenegger, C.; Trouillet, V.; Welle, A.; Schueler, V.; Mueller, J. O.; Goldmann, A. S.; Brynda, E.; Barner-Kowollik, C. Photo-Patterning of Non-Fouling Polymers and Biomolecules on Paper. Adv. Mater. 2014, 26, 4087– 4092,  DOI: 10.1002/adma.201401006
      16
      Photo-Patterning of Non-Fouling Polymers and Biomolecules on Paper
      Tischer, Thomas; Rodriguez-Emmenegger, Cesar; Trouillet, Vanessa; Welle, Alexander; Schueler, Vincent; Mueller, Jan O.; Goldmann, Anja S.; Brynda, Eduard; Barner-Kowollik, Christopher
      Advanced Materials (Weinheim, Germany) (2014), 26 (24), 4087-4092CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We resent the use of the photo-triggered NITEC (nitrile imine-mediated tetrazole-ene cycloaddns.) reaction to pattern cellulose with ultra-low fouling poly(CBAA-3) (poly(carboxybetaine acrylamide)) and to immobilize streptavidin. The strategy consists of the facile arylation of cellulose with a tetrazole acid chloride which was further exploited to pattern a maleimide-functional initiator to graft poly(CBAA-3) brushes. The use of SET-LRP (single electron transfer living radical polymn.) accounted for fast polymn. without the need of any purifn. step to remove catalysts and enabled access to cellulose surface fully resistant to fetal calf serum. In a similar fashion streptavidin functionalized with maleimide was immobilized onto Cel-Tet with remarkable efficiency. We envision that the reported strategy can be translated to the development of novel bioactive papers and μPADs (microfluidic paper-based anal. devices) necessary for the forthcoming developments in medicine.
    17. 17
      Junkers, T. [2 + 2] Photo-Cycloadditions for Polymer Modification and Surface Decoration. Eur. Polym. J. 2015, 62, 273– 280,  DOI: 10.1016/j.eurpolymj.2014.08.005
      17
      [2+2] Photo-cycloadditions for polymer modification and surface decoration
      Junkers, Thomas
      European Polymer Journal (2015), 62 (), 273-280CODEN: EUPJAG; ISSN:0014-3057. (Elsevier Ltd.)
      A review. Photo-induced reactions play an important role in precision polymer design. Photoreactions are often cost-effective reactions that proceed under relatively mild reaction conditions. Addnl., photo reactions add spatial and temporal control over reactions, which gives rise to a large no. of applications that would otherwise be inaccessible. Among the various photochemistries available, [2+2] cycloaddns. are a prominent example for effective conversions. In the field of polymer chem., such reactions are often used for crosslinking. If reaction conditions are chosen carefully, they can, however, also be employed for advanced polymer transformations, such as polymer end-group modification or specific ligation to substrates. Esp. when applying flow chem. approaches, very high reaction efficiencies are gained, bringing [2+2] cycloaddns. close in use to other well known click-like techniques such as thiol-ene chem.
    18. 18
      Iliopoulos, K.; Krupka, O.; Gindre, D.; Salle, M. Reversible Two-Photon Optical Data Storage in Coumarin-Based Copolymers. J. Am. Chem. Soc. 2010, 132, 14343– 14345,  DOI: 10.1021/ja1047285
      18
      Reversible Two-Photon Optical Data Storage in Coumarin-Based Copolymers
      Iliopoulos, Konstantinos; Krupka, Oksana; Gindre, Denis; Salle, Marc
      Journal of the American Chemical Society (2010), 132 (41), 14343-14345CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A functionalized polymer film allowing for a complete and straightforward second-harmonic generation (SHG)-assisted high-contrast writing-reading-erasing-writing sequence is proposed. The whole process is supported by the reversible photoinduced dimerization of a coumarin chromophore and enables efficient optical data storage that can be detected only by SHG imaging.
    19. 19
      Tsurkan, M. V.; Jungnickel, C.; Schlierf, M.; Werner, C. Forbidden Chemistry: Two-Photon Pathway in [2 + 2] Cycloaddition of Maleimides. J. Am. Chem. Soc. 2017, 139, 10184– 10187,  DOI: 10.1021/jacs.7b04484
      19
      Forbidden Chemistry: Two-Photon Pathway in [2+2] Cycloaddition of Maleimides
      Tsurkan, Mikhail V.; Jungnickel, Christiane; Schlierf, Michael; Werner, Carsten
      Journal of the American Chemical Society (2017), 139 (30), 10184-10187CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Two-photon excitation provides high spatial resoln. in three dimensions of the corresponding chem. or phys. processes, allowing submicrometer structuring in stereolithog. and three-dimensional (3D) microfabrication. While studying two-photon structuring applications, we obsd. an undescribed phenomenon in photochem. that dictates reactivity of maleimide groups in two-photon mode. A low-absorbance transition formerly ignored in classical photochem. has been found for maleimides. This transition was assigned to symmetry-breaking donor-acceptor complex formation, which revealed a formally forbidden pathway in [2+2] cycloaddn. reactions of maleimide moieties. This synthetic pathway allowed for the creation of hydrogel materials under physiol. conditions at low laser excitation energy (0.1 J/cm2 at 800 nm) without the use of photoinitiators, which makes it truly two-photon click chem.
    20. 20
      Udagawa, A.; Johnston, P.; Sakon, A.; Toyoshima, R.; Uekusa, H.; Koshima, H.; Saito, K.; Asahi, T. Crystal-to-Crystal Photo-Reversible Polymerization Mechanism of Bis-Thymine Derivative. RSC Adv. 2016, 6, 107317– 107322,  DOI: 10.1039/C6RA24229D
      20
      Crystal-to-crystal photo-reversible polymerization mechanism of bis-thymine derivative
      Udagawa, Akihiro; Johnston, Priscilla; Sakon, Aya; Toyoshima, Ryosuke; Uekusa, Hidehiro; Koshima, Hideko; Saito, Kei; Asahi, Toru
      RSC Advances (2016), 6 (109), 107317-107322CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      Solid-state photo-polymn. in crystals can produce stereoregular polymer mols. in environmentally friendly solvent-free systems. The polymn. mechanism of bis-thymine derivs., such as dimethyl-3,3'-(3,3'-(butane-1,4-diyl)bis(5-methyl-2,4-dioxo-3,4-dihydropyrimidine-3,1(2H)-diyl))dipropanoate (1), known as unique mols. that can topochem. and reversibly polymerize in the cryst. state via [2 + 2]-cycloaddn. reactions upon UV irradn., remained to be solved. In this manuscript, the crystal structure of the polymeric photoproduct (1P) from a bis-thymine deriv. 1 was detd. using ab initio powder X-ray diffraction data and applied to investigate the polymn. mechanism of bis-thymine derivs. The topochem. polymn. was found to be achieved via [2 + 2]-cycloaddn. with the flexible Bu chain between thyminyl rings relieving the distortion of whole structure derived from cyclobutane formation. The crystal structure of 1P also showed that it polymd. stereoregularly with trans-anti cyclobutane conformations.
    21. 21
      Johnston, P.; Wheldale, D.; Braybrook, C.; Saito, K. Topochemical Polymerization Using Bis-Thyminyl Monomers. Polym. Chem. 2014, 5, 4375– 4384,  DOI: 10.1039/C4PY00265B
      21
      Topochemical polymerization using bis-thyminyl monomers
      Johnston, Priscilla; Wheldale, Dylan; Braybrook, Carl; Saito, Kei
      Polymer Chemistry (2014), 5 (14), 4375-4384CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
      Topochem. polymn. is one way to synthesize novel macromol. architectures with stereoregular chain structures. The [2π + 2π]-cycloaddn. of certain conjugated diolefin monomers is one such form of topochem. polymn. Unfortunately, the design of new monomers that can undergo topochem. [2π + 2π]-polymn. to form linear polymers can be a formidable challenge due to the strict geometrical criteria governing the reactivity of monomer crystals. In this research ten bis-thymine monomers were synthesized, that varied with respect to the N1-functionality, the size of the N3-N3-polymethylene spacer, or the type of rigid N3-N3-aryl spacer employed. On irradn., five of the cryst. monomers were found to undergo topochem. photo-reactions. The crystal structures of the bis-thyminyl monomers were used to investigate the monomer structure-reactivity relationships. The mol. wts. of the photo-products were detd. and the thermal properties of some of the new materials were also evaluated using thermo anal. techniques.
    22. 22
      Johnston, P.; Braybrook, C.; Saito, K. Topochemical Photo-Reversible Polymerization of a Bioinspired Monomer and Its Recovery and Repolymerization after Photo-Depolymerization. Chem. Sci. 2012, 3, 2301– 2306,  DOI: 10.1039/c2sc20380d
      22
      Topochemical photo-reversible polymerization of a bioinspired monomer and its recovery and repolymerization after photo-depolymerization
      Johnston, Priscilla; Braybrook, Carl; Saito, Kei
      Chemical Science (2012), 3 (7), 2301-2306CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      Solid-state topochem. polymn. is one way to synthesize novel macromol. architectures with stereoregular chain structures. Topochem. reactions are attractive, "green" synthetic pathways in material design, since they occur in solvent-free conditions and in response to external stimuli, such as heat and light. For the first time, we have used the reversible [2π + 2π]-cycloaddn. of a bioinspired bis-thymine monomer to topochem. synthesize a polymer. The polymer can be fully photo-depolymd. to the monomer and then reversibly and repeatedly photo-polymd. and photo-depolymd. This is the first demonstration of complete photo-depolymn. and subsequent monomer recycling using this mechanism.
    23. 23
      Chen, D.; Han, X.; Jin, W.; Du, Y.; Xu, P. In Situ Raman Monitoring of [2 + 2] Cycloaddition of Pyridine Substituted Olefins Induced by Visible Laser. Chem. Commun. 2014, 50, 15631– 15633,  DOI: 10.1039/C4CC06808D
      23
      In situ Raman monitoring of [2+2] cycloaddition of pyridine substituted olefins induced by visible laser
      Chen, Dengtai; Han, Xijiang; Jin, Wen; Du, Yunchen; Xu, Ping
      Chemical Communications (Cambridge, United Kingdom) (2014), 50 (98), 15631-15633CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Visible laser induced [2+2] cycloaddn. of solid-state pyridine substituted olefins into cyclobutane has been monitored by an in situ Raman technique. The laser power and wavelength can dramatically alter the reaction kinetics, as a prior melting process (heating from laser irradn.) is required for this [2+2] photoreaction.
    24. 24
      Miller, F. A.; Capwell, R. J.; Lord, R. C.; Rea, D. G. Infrared and Raman Spectra of Cyclobutane and Cyclobutane-D8. Spectrochim. Acta, Part A 1972, 28, 603– 618,  DOI: 10.1016/0584-8539(72)80029-1
      24
      Infrared and Raman spectra of cyclobutane and cyclobutane-d8
      Miller, Foil A.; Capwell, Robert J.; Lord, R. C.; Rea, Donald G.
      Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy (1972), 28 (4), 603-18CODEN: SAMCAS; ISSN:1386-1425.
      New ir and Raman measurements are reported for cyclobutane and cyclobutane-d8. They include ir spectra on the gas at room temp. and on the solid at ∼ 100°K. Raman spectra were measured for the gas and liq. at room temp. and for the solid at 113°K. These data allow the assignments (for D2d) to be put on a much firmer basis. Many of the tentative ones of Lord and Nakagawa were confirmed and made more exact. Several of the fundamentals were obsd. for the 1st time. All of the 21 spectroscopically active fundamentals for C4H8 are well assigned except for 2 b1 and 1 b2 modes (ν9, ν10, and ν15).
    25. 25
      Socrates, G. Infrared and Raman Characteristic Group Frequencies Tables and Charts; John Wiley & Sons, 1970; Vol. 74.
      There is no corresponding record for this reference.
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      Rekštytė, S.; Jonavičius, T.; Gailevičius, D.; Malinauskas, M.; Mizeikis, V.; Gamaly, E. G.; Juodkazis, S. Nanoscale Precision of 3D Polymerization via Polarization Control. Adv. Opt. Mater. 2016, 4, 1209– 1214,  DOI: 10.1002/adom.201600155
      26
      Nanoscale Precision of 3D Polymerization via Polarization Control
      Rekstyte, Sima; Jonavicius, Tomas; Gailevicius, Darius; Malinauskas, Mangirdas; Mizeikis, Vygantas; Gamaly, Eugene G.; Juodkazis, Saulius
      Advanced Optical Materials (2016), 4 (8), 1209-1214CODEN: AOMDAX; ISSN:2195-1071. (Wiley-VCH Verlag GmbH & Co. KGaA)
      In this paper, a systematic anal. through modeling and expts. is presented in order to reveal polarization effects, their influence on the feature size (resoln.), and the coupling between thermal gradient and polarization in direct laser writing.
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      Xu, C.; Webb, W. W. Measurement of Two-Photon Excitation Cross Sections of Molecular Fluorophores with Data from 690 to 1050 Nm. J. Opt. Soc. Am. B 1996, 13, 481,  DOI: 10.1364/JOSAB.13.000481
      27
      Measurement of two-photon excitation cross sections of molecular fluorophores with data from 690 to 1050 nm
      Xu, Chris; Webb, Watt W.
      Journal of the Optical Society of America B: Optical Physics (1996), 13 (3), 481-491CODEN: JOBPDE; ISSN:0740-3224. (Optical Society of America)
      Measurements of two-photon fluorescence excitation (TPE) spectra are presented for 11 common mol. fluorophores in the excitation wavelength range 690 nm < λ < 1050 nm. Results of excitation by ∼100-fs pulses of a mode-locked Ti:sapphire laser are corroborated by single-mode continuous-wave Ti:sapphire excitation data in the range 710 nm < λ < 840 nm. Abs. values of the TPE cross section for Rhodamine B and Fluorescein were obtained by comparison with 1-photon-excited fluorescence, assuming equal emission quantum efficiencies. TPE action cross sections for the other nine fluorophores are also detd. No differences between 1-photon- and two-photon-excited fluorescence emission spectra are found. TPE emission spectra are independent of excitation wavelength. With both pulsed and continuous-wave excitation the fluorescence emission intensities are strictly proportional to the square of the excitation intensity to within ±4% for excitation intensities sufficiently below excited-state satn.
    28. 28
      Eilers, P. H. C. Anal. Chem. 2003, 75, 3631– 3636,  DOI: 10.1021/ac034173t
      28
      A Perfect Smoother
      Eilers, Paul H. C.
      Analytical Chemistry (2003), 75 (14), 3631-3636CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      The A. Savitzky and M. Golay (1964) filter has several disadvantages. A very attractive alternative is a smoother based on penalized least squares, extending ideas presented by E. T. Whittaker (1923). This smoother is extremely fast, gives continuous control over smoothness, interpolates automatically, and allows fast leave-one-out cross-validation. It can be programmed in a few lines of Matlab code. Theory, implementation, and applications are presented.

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    • Experimental procedures; Raman characterization and peak assignment; morphology dependence; orientation dependence; UV–vis spectra; and reaction with 500 nm light (PDF)


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