Generation of Triplet Excited States via Photoinduced Electron Transfer in meso-anthra-BODIPY: Fluorogenic Response toward Singlet Oxygen in Solution and in Vitro
- Mikhail A. Filatov
- ,
- Safakath Karuthedath
- ,
- Pavel M. Polestshuk
- ,
- Huguette Savoie
- ,
- Keith J. Flanagan
- ,
- Cindy Sy
- ,
- Elisabeth Sitte
- ,
- Maxime Telitchko
- ,
- Frédéric Laquai
- ,
- Ross W. Boyle
- , and
- Mathias O. Senge
Abstract

Heavy atom-free BODIPY-anthracene dyads (BADs) generate locally excited triplet states by way of photoinduced electron transfer (PeT), followed by recombination of the resulting charge-separated states (CSS). Subsequent quenching of the triplet states by molecular oxygen produces singlet oxygen (1O2), which reacts with the anthracene moiety yielding highly fluorescent species. The steric demand of the alkyl substituents in the BODIPY subunit defines the site of 1O2 addition. Novel bis- and tetraepoxides and bicyclic acetal products, arising from rearrangements of anthracene endoperoxides were isolated and characterized. 1O2 generation by BADs in living cells enables visualization of the dyads distribution, promising new imaging applications.
Optical probes based on photoinduced electron transfer (PeT) in donor–acceptor dyads have broad use in diagnostics, particularly for detection of biomolecules, metal ions, reactive oxygen species (ROS) and measurement of intracellular pH. (1) The PeT process leads to formation of non-emissive charge-separated states (CSS) that decay back to the ground state via different pathways. Among those is recombination of CSS, which may lead to locally excited triplet states of the molecule. (2) Recently this process has attracted attention as a method to increase intersystem crossing without relying on the heavy atom effect. (3)
The possibility of singlet oxygen (1O2) generation by donor–acceptor dyads has not been realized so far in a practical sense. It could be expected that 1O2 generation by PeT-based optical probes in biological environments would affect their optical response and simultaneously induce cytotoxicity. This is of concern especially in the case of ROS detection, where sensitization of 1O2 by the probe itself may lead to false positives and incorrect interpretations. (4) On the other hand, PeT-mediated 1O2 generation could provide a new tool for theranostic applications, because the process of charge separation can be turned on/off by various stimuli. Herein we report readily accessible heavy atom-free BODIPY-anthracene dyads (BADs) that can act as efficient triplet sensitizers, and become fluorescent in response to the generated 1O2.
Although a number of triplet sensitizers based on halogenated BODIPYs have been reported in the past decade, (5) observations of triplet excited states formation in heavy atom-free BODIPYs are rare. (6) In our search for efficient donor–acceptor photosensitizers, we focused on BADs 1 and 2 (Scheme 1). BODIPYs are known to be efficient energy and electron acceptors when combined with anthracene. (7) Although compound BAD1 has been reported to exhibit PeT, no triplet excited states formation has been noted. (8)
Scheme 1

Upon broad-band visible light irradiation of air-saturated solutions of BAD1 in a range of polar solvents we observed, to our surprise, completely selective formation of BAD1-BE, which could be isolated in 5% yield along with recovered unreacted starting material (Scheme 1). In contrast, irradiation of BAD2 under the same conditions resulted in complete conversion of the substrate and formation of two products, bicyclic acetal derivative (BAD2-BA) and tetraepoxide (BAD2-TE), which were isolated in 80% and 10% yields, respectively. The structures of the products were confirmed by NMR spectroscopy and X-ray crystallography (for details, see Supporting Information (SI)). Unlike BADs 1 and 2, isolated compounds exhibit bright fluorescence independent of the solvent polarity. For instance, the emission quantum yields of BAD1-BE in CH2Cl2 and hexane were determined to be 0.91 and 0.89, respectively.
The formation of these products appears to be due to the sensitization of oxygen and subsequent [4+2] cycloaddition of the resulting 1O2, which is typical for anthracene derivatives. (9) Singlet oxygen quantum yields were measured using 1,3-diphenylisobenzofuran as 1O2 trap, giving values of 0.67 and 0.38 in ethanol, for BAD1 and BAD2, respectively. To understand the mechanism of 1O2 formation, we studied the excited state dynamics of the dyad BAD1 by broad-band vis-NIR subpico- to microsecond transient absorption (TA) pump–probe spectroscopy.
Immediately after photoexcitation with fs pulses at 355 nm, a broad band around 360 nm due to the anthracene’s singlet excited state (S1) absorption, which partially overlaps with the ground state photo bleach (PB), was observed (Figure 1a). The concomitant decay of this band and simultaneous rise of the bleach at 505 nm indicate ultrafast energy transfer (EnT) from anthracene to BODIPY subunit, populating the singlet state of the latter, SBDP. Furthermore, another absorption band grows at 580 nm, and it was assigned to the BODIPY radical-anion (BDP–•), (10) forming due to the PeT process. This band rises during the first 100 ps, simultaneously shifting to 570 nm, indicating a transition of the radical-anion to another excited state (see inset of Figure 1a). Synchronously with the rise of the BDP–• absorption (580 nm), yet another absorption band, centered at 680 nm rises, presumably due to the anthracene radical-cation (Ant+•), in line with previous reports. (11) Fitting of the PB decays at 380 and 400 nm and the rise of BDP–• and Ant+• bands (Figure 1b) yields time constants of 1.15 and 0.54 ps for the EnT and PeT processes, respectively.
Figure 1

Figure 1. (a) ps–ns Transient absorption spectra of BAD1 in dimethylformamide upon excitation at 355 nm with 35 fs pulses at delay times of 600 fs (black line), 1 ps (red line), 100 ps (green line), and 5 ns (blue line). The inset shows the blue shift of the TA spectra to 570 nm. (b) Kinetics monitored at 380 nm (black line), 400 nm (red line), 425 nm (green line), 505 nm (violet line), 570 nm (cyan line), and 680 nm (magenta line) as indicated by vertical colored bars. (c) ns−μs Transient absorption spectra of degassed BAD1 solutions following excitation at 355 nm by 700 ps laser pulses. The spectra were integrated from 3 to 5 ns (black line), 10–100 ns (red line), 0.1–1 μs (green line), 1–5 μs (blue line), and 10–100 μs (cyan line). (d) Kinetics observed for the bands at 570 and 680 nm, assigned to the BODIPY triplet state and anthracene radical-cation, respectively, in the absence and presence of oxygen (solid and dotted lines, respectively).
In the ns−μs TA experiments, a rise of an absorption band at 570 nm over 1 μs was observed, indicating formation of long-lived states (Figure 1c). Previous reports on the TA spectra of BODIPY support the assignment of this band to the BODIPY triplet state (TBDP) absorption. (10b) The band at 570 nm was quenched and decayed faster in the presence of oxygen (Figure 1d). In contrast, the anthracene radical cation-absorption band at 680 nm was impacted by oxygen significantly less. The TBDP lifetime in the absence of O2 was determined to be 41 μs. The observed transition from the bands originating in CSS to the absorption by the triplet suggests that the formation of CSS is a prerequisite for populating of TBDP.
The frontier molecular orbitals diagram (Figure 2a) shows that the two highest occupied orbitals πant and πBDP located on the anthracene and BODIPY subunit, respectively, are nearly degenerate. Density functional theory (DFT) calculations (see SI for computational details) confirm that in BAD1 PeT could take place from πant to singly occupied πBDP, leading to singlet charge transfer state SCSS that is 0.4 eV more stable than the SBDP excited state. Unlike the valence excited states, CSS has very low ferromagnetic exchange coupling integral due to negligible overlap of singly occupied orbitals πant and π*BDP located in mutually orthogonal molecular moieties, thus leading to a very small singlet–triplet energy gap (S-T gap). Two pathways for triplet state generation from CSS may yield the lowest local TBDP state (Figure 2b): spin–orbit charge transfer intersystem crossing (SOCT-ISC) and radical pair intersystem crossing (RP-ISC)), followed by triplet charge recombination. (12) As shown in extensive works of Wasielewski and co-workers, (13) SOCT-ISC prevails for systems with strong electronic couplings, requiring short distances between the subunits (4.3 Å in BAD1). On the other hand, due to the small S-T gap in the RP state, mixing of SCSS and TCSS states is possible due to, e.g., electron–nuclear hyperfine coupling. More detailed studies are necessary to distinguish between mechanisms governing spin interconversion in BADs.
Figure 2

Figure 2. (a) Frontier molecular orbitals and their energies (in a.u.) for BADs 1 and 2. (b) Diagram demonstrating transitions between excited states in BAD1.
The observed PeT process is clearly manifested in the spectroscopic properties of BADs. The fluorescence of the BODIPY is quenched in polar solvents as evidenced by the negligible values of Φf, compared to the strong emission in nonpolar solvents (Table S3). A broad emission band at 610 nm was observed in polar solvents. Such red-shifted broad emission bands arising from charge transfer excited states were reported for various donor–acceptor systems. (14) DFT calculations in vacuo show that SCSS state is approximately 0.2 eV higher in energy than the valence SBDP state. The dipole moment for the SCSS state was computed to be μ = 19 D in vacuo, much higher than that for the valence SBDP state (5 D). Interactions of CSS with polar solvent result in a decrease of the SCSS state dipole moment to 1.1 D and change the relative energy ordering of the SBDP and SCSS states, making PeT process favorable.
When irradiated with monochromatic or broad-band visible light, air-saturated solutions of BADs in polar solvents showed a gradual increase in fluorescence (Figure 3b). For instance, irradiation of BAD2 solution results in up to 100-fold increase of fluorescence intensity due to formation of compounds BAD2-BA and BAD2-TE. No change in the emission was observed upon irradiation of the solutions in hexane even for longer periods of time (Figure S6), confirming that the dyads do not generate 1O2 in the absence of the PeT process.
Figure 3

Figure 3. (a) Absorption and emission spectra of BAD1. (b) Changes of the emission intensity upon irradiation of BADs and reference compound solutions in CH2Cl2 (5 × 10–5 M) with broad-band visible light. (c) Change of BAD2 emission upon irradiation with 532 nm laser (10 mW cm–2). (d) Photo of BAD2 solution before and after 5 min of irradiation, taken under excitation with 365 nm light.
The formation of bicyclic acetal and tetraepoxide products from BAD2 is likely to take place via a 9,10-endoperoxide intermediate (Scheme 2). The rearrangement of endoperoxides into bisepoxides can be induced either thermally or photochemically. (9) The process is caused by homolytic cleavage of the peroxide O–O bond, followed by rearrangement to more stable bisepoxides. Commonly, such bisepoxides containing a cyclohexadiene ring could not be isolated but only trapped with dienophiles. (15) Indeed, we found no traces of this intermediate in the reaction mixture. According to previous reports, the formation of a bicyclic acetal from bisepoxide may take place via heterolytic cleavage of the epoxide C–C bond, leading to an ylide-type bipolar intermediate. (15b) This is followed by C–O bond rupture of a second epoxide fragment, leading to rearomatization of the lateral ring and formation of the acetal bridge. The rearrangement competes with addition of 1O2 molecule to the diene moiety leading to BAD2-TE.
Scheme 2

In contrast, bisepoxide BAD1-BE is stable and showed no formation of the rearrangement products. Its formation likely proceeds via the mechanism discussed above, involving O–O homolytic cleavage and subsequent isomerization. The addition of 1O2 to the outer ring in this case is surprising, as the central 9,10-site is the most reactive, based on frontier molecular orbital analysis. The influence of steric factors on the regioselectivity of endoperoxide formation has been reported for acenes with bulky substituents at the ortho positions of the aryl groups. (16) Comparison with BAD2 shows that the unusual reactivity of BAD1 can be attributed to the effect of methyl substituents in position 4 of the BODIPY core. This can be seen in the XRD data, where C-4 methyl substituents in BAD1 are forming a steric-like shield of the C-9 position of the anthracene unit. Introduction of methyl groups into the BODIPY pyrrole rings shields the inner ring of the orthogonal anthracene residue, making the approach of 1O2 molecule difficult. Different reactivity of BADs toward 1O2 accounts for the variations in their fluorescence response (Figure 3b) due to the cycloaddition to the anthracene moiety, which takes place considerably faster for BAD2.
The rise of BAD2 fluorescence due to cycloaddition reaction is manifested even at 1 μM concentration, and it reaches intensities comparable to those of emission of a strongly fluorescent reference BODIPY compound (Figure S7). It was of special interest to investigate whether the sensitization process can be reproduced in live cells. For this purpose, we generated appropriate water-soluble derivatives (Scheme 3). Substitution of fluorine atoms with N,N-dimethylaminopropyne-1 residues gave corresponding BADs 3 and 4. Quaternization of the dimethylamino group with 1,3-propane sultone then gave BADs 5 and 6, bearing zwitterionic fragments (betaine) that imparted the desired aqueous solubility.
Scheme 3

To examine the fluorescence response of BADs 5 and 6 toward self-sensitized 1O2 in cells, human breast cancer (MDA-MB-468) cells were incubated with BADs 5 and 6 (1 μM) followed by irradiation with broad-band visible light (400–700 nm, 23.8 mW cm–2). Cells were irradiated for 0, 2.5, and 5 min and visualized by confocal fluorescence microscopy. Over the time course of irradiation, an increase in the fluorescence intensity was observed for BAD6 (Figure 4). This indicates first that the chromophore entered the cells, rather than simply associating with the external cell membrane; and secondly, that the fluorescence increased in a similar way to that observed for BAD2 in homogeneous solution. However, this behavior was not replicated in the case of BAD5, which showed no observable fluorescence on this time scale, even when irradiated with higher light doses. Lower fluorogenicity of BAD5 is in accord with the behavior of parent BAD1, which was shown to react with 1O2 considerably slower than BAD2.
Figure 4

Figure 4. Confocal microscopy images of cells incubated with 1 μM of BAD6 after the irradiation with broad-band visible light (400–700 nm, 23.8 mW cm–2) for (a) 2.5 min and (b) 5 min.
At higher concentrations of BADs, evidence of morphological changes to the cells upon irradiation, most noticeably “blebbing” of the cell membrane, was observed (Figure S12), indicating apoptotic behavior. Cell viabilities after incubation with a range of concentrations (1–50 μM) of BADs 5 and 6, followed by light treatment (23.8 mW cm–2), were assayed by MTT protocol. The results obtained indicate that both water-soluble BADs induce a significant cytotoxic effect on the cells, whereas negligible cytotoxic effects were observed in the control group under otherwise identical conditions, but without irradiation (Figure S13). Median lethal doses (LD50) of BADs were found to be 4 μM, thus the lower dose of 1 μM was selected for imaging experiments.
In conclusion, we have demonstrated that heavy atom-free donor–acceptor dyads can be used as 1O2 sensitizers, whereby the triplet excited states form by way of photoinduced electron transfer. Moreover, the described dyads are capable of forming strongly fluorescent species with self-sensitized 1O2 in biological media. The fluorescent response allows visualization of 1O2 formation within the cells and, consequently, fine-tuning of the photon doses required to cause oxidative stress. These sensitizers may give rise to a promising new class of materials for photonic applications that depend on triplet excited states generation. Studies to extend the scope such systems are underway.
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b00551.
Experimental details; data for BAD1, BAD1-BE, BAD2-BA, BAD2-TE
Terms & Conditions
Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.
Acknowledgment
This work was supported by grants from the European Commission (M.A.F., CONSORT, Grant No. 655142), Science Foundation Ireland (M.O.S. SFI IvP 13/IA/1894), and by funding from King Abdullah University of Science and Technology (KAUST). We thank Prof. Sergei Vinogradov for helpful discussions.
References
This article references 16 other publications.
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Now, in benzonitrile, the charge-transfer state (CTS) is positioned slightly below the triplet levels, such that charge recombination restores the ground state. However, in CH2Cl2 or methyltetrahydrofuran, the CTS is slightly higher in energy than the triplet levels, and decays, in part, to form the triplet state localized on the C60 residue. This step is highly specific and does not result in direct formation of the triplet excited state localized on the Bodipy unit. Subsequent equilibration of the two triplets takes place on a relatively slow timescale.(b) Amin, A. N.; El-Khouly, M. E.; Subbaiyan, N. K.; Zandler, M. E.; Fukuzumi, S.; D’Souza, F. Chem. Commun. 2012, 48, 206– 208 DOI: 10.1039/C1CC16071KGoogle ScholarThere is no corresponding record for this reference.(c) Huang, L.; Yu, X.; Wu, W.; Zhao, J. Org. 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Three thymine-contg. visible light-harvesting Bodipy derivs. (B-1, B-2 and B-3, which show absorption at 505 nm, 630 nm and 593 nm, resp.) were used as H-bonding modules, and 1,6-diaminopyridine-appended C60 was used as the complementary hydrogen bonding module (C-1), in which the C60 part acts as a spin converter for triplet formation. Visible light-harvesting antennae with methylated thymine were prepd. as refs. (B-1-Me, B-2-Me and B-3-Me), which are unable to form strong H-bonds with C-1. Triple H-bonds are formed between each Bodipy antenna (B-1, B-2 and B-3) and the C60 module (C-1). Singlet energy transfer from the Bodipy antenna to the C60 module was confirmed by fluorescence quenching studies. The intersystem crossing of the latter produced the triplet excited state. The nanosecond transient absorption spectroscopy showed that the triplet state is either localized on the C60 module (for assembly B-1·C-1), or on the styryl-Bodipy antenna (for assemblies B-2·C-1 and B-3·C-1). 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- 5Kamkaew, A.; Lim, S. H.; Lee, H. B.; Kiew, L. V.; Chung, L. Y.; Burgess, K. Chem. Soc. Rev. 2013, 42, 77– 88 DOI: 10.1039/C2CS35216HGoogle Scholar5https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhslKrurbJ&md5=69fddf735bafa0b2b33a70835af1430fBODIPY dyes in photodynamic therapyKamkaew, Anyanee; Lim, Siang Hui; Lee, Hong Boon; Kiew, Lik Voon; Chung, Lip Yong; Burgess, KevinChemical Society Reviews (2013), 42 (1), 77-88CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)A review. BODIPY dyes tend to be highly fluorescent, but their emissions can be attenuated by adding substituents with appropriate oxidn. potentials. Substituents like these have electrons to feed into photoexcited BODIPYs, quenching their fluorescence, thereby generating relatively long-lived triplet states. Singlet oxygen is formed when these triplet states interact with 3O2. In tissues, this causes cell damage in regions that are illuminated, and this is the basis of photodynamic therapy (PDT). The PDT agents that are currently approved for clin. use do not feature BODIPYs, but there are many reasons to believe that this situation will change. This review summarizes the attributes of BODIPY dyes for PDT, and in some related areas.
- 6Zhao, J.; Xu, K.; Yang, W.; Wang, Z.; Zhong, F. Chem. Soc. Rev. 2015, 44, 8904– 8939 DOI: 10.1039/C5CS00364DGoogle Scholar6https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhs1Gjsb3I&md5=26bdbc5fa8c7f5348f6a2e831126ab0cThe triplet excited state of Bodipy: formation, modulation and applicationZhao, Jianzhang; Xu, Kejing; Yang, Wenbo; Wang, Zhijia; Zhong, FangfangChemical Society Reviews (2015), 44 (24), 8904-8939CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)Boron dipyrromethene (Bodipy) is one of the most extensively investigated org. chromophores. Most of the investigations are focused on the singlet excited state of Bodipy, such as fluorescence. In stark contrast, the study of the triplet excited state of Bodipy is limited, but it is an emerging area, since the triplet state of Bodipy is tremendously important for several areas, such as the fundamental photochem. study, photodynamic therapy (PDT), photocatalysis and triplet-triplet annihilation (TTA) upconversion. The recent developments in the study of the prodn., modulation and application of the triplet excited state of Bodipy are discussed in this review article. The formation of the triplet state of Bodipy upon photoexcitation, via the well known approach such as the heavy atom effect (including I, Br, Ru, Ir, etc.), and the new methods, such as using a spin converter (e.g. C60), charge recombination, exciton coupling and the doubly substituted excited state, are summarized. All the Bodipy-based triplet photosensitizers show strong absorption of visible or near IR light and the long-lived triplet excited state, which are important for the application of the triplet excited state in PDT or photocatalysis. Moreover, the methods for switching (or modulation) of the triplet excited state of Bodipy were discussed, such as those based on the photo-induced electron transfer (PET), by controlling the competing Forster-resonance-energy-transfer (FRET), or the intermol. charge transfer (ICT). Controlling the triplet excited state will give functional mols. such as activatable PDT reagents or mol. devices. It is worth noting that switching of the singlet excited state and the triplet state of Bodipy may follow different principles. Application of the triplet excited state of Bodipy in PDT, hydrogen (H2) prodn., photoredox catalytic org. reactions and TTA upconversion were discussed. The challenges and the opportunities in these areas were briefly discussed.
- 7(a) Wan, C. W.; Burghart, A.; Chen, J.; Bergström, F.; Johansson, L. B.; Wolford, M. F.; Kim, T. G.; Topp, M. R.; Hochstrasser, R. M.; Burgess, K. Chem. - Eur. J. 2003, 9, 4430– 4441 DOI: 10.1002/chem.200304754Google Scholar7ahttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3sXnvVygtLg%253D&md5=2b1cca3bdab4d5cc998f337abcaeadefAnthracene-BODIPY cassettes: syntheses and energy transferWan, Chi-Wai; Burghart, Armin; Chen, Jiong; Bergstroem, Fredrik; Johansson, Lennart B.-A.; Wolford, Matthew F.; Kim, Taeg Gyum; Topp, Michael R.; Hochstrasser, Robin M.; Burgess, KevinChemistry - A European Journal (2003), 9 (18), 4430-4441CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)Compds. based on the 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (BODIPY) framework are excellent fluorescent markers. When BODIPY dyes of this type are conjugated to functionalities that absorb at relatively short wavelengths, those functionalities can, in some mols., transmit the absorbed energy to the BODIPY which then fluoresces. In such cases the BODIPY fragment acts as an acceptor while the other group serves as a donor. Energy transfer efficiencies in such donor-acceptor cassette systems must vary with the relative orientation of the two components, and with the structure of the linkers that attach them. This study was designed to probe these issues for the special case in which the linkers between the donor and acceptor fragments are conjugated. To do this, 8 donor-acceptor cassettes were prepd. Electrochem. studies were performed to provide insight into the degree of donor-acceptor conjugation in these systems. X-ray crystallog. studies on single crystals of 2 of the cassettes revealed the favored conformations of the donor and acceptor fragments in the solid state. Absorption, fluorescence, and time-resolved fluorescence spectra of the compds. were recorded, and quantum yields for the cassettes excited at the donor λmax were measured. Fluorescence steady-state anisotropy data were detd. 2 cassettes to provide information about the mutual direction of the transition dipole moments.(b) Benniston, A. C.; Harriman, A.; Whittle, V. L.; Zelzer, M.; Harrington, R. W.; Clegg, W. Photochem. Photobiol. Sci. 2010, 9, 1009– 1017 DOI: 10.1039/c0pp00039fGoogle Scholar7bhttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXotVOrsbg%253D&md5=f64bfd83c4b2a4b44d52c9477d984665Exciplex-like emission from a closely-spaced, orthogonally-sited anthracenyl-boron dipyrromethene (Bodipy) molecular dyadBenniston, Andrew C.; Harriman, Anthony; Whittle, Victoria L.; Zelzer, Mischa; Harrington, Ross W.; Clegg, WilliamPhotochemical & Photobiological Sciences (2010), 9 (7), 1009-1017CODEN: PPSHCB; ISSN:1474-905X. (Royal Society of Chemistry)A mol. dyad, ANTBD, has been prepd. that incorporates a boron dipyrromethene (Bodipy) group functionalized at the meso position with an anthracenyl unit. Emission from the dyad contains contributions from both localized fluorescence from the Bodipy unit and exciplex-like emission assocd. with an intramol. charge-transfer state. The peak position, intensity and lifetime of this exciplex emission are solvent dependent and the shift in the emission max. shows a linear relationship to the solvent polarity function (Δf). The calcd. dipole moment for the exciplex is 22.5 ± 2.2 D. The radiative rate const. (kRAD) for exciplex emission decreases progressively with increasing solvent polarity. In this latter case, kRAD shows an obvious dependence on the energy gap between the exciplex state and the first-excited singlet state resident on the Bodipy unit. The emission characteristics for ANTBD dissolved in perfluorooctane are used to characterize the refractive index and dielec. const. of the solvent.
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- 13Dance, Z. E. X.; Mi, Q.; McCamant, D. W.; Ahrens, M. J.; Ratner, M. A.; Wasielewski, M. R. J. Phys. Chem. B 2006, 110, 25163– 25173 DOI: 10.1021/jp063690nGoogle ScholarThere is no corresponding record for this reference.(b) Dance, Z. E. X.; Mickley, S. M.; Wilson, T. M.; Ricks, A. B.; Scott, A. M.; Ratner, M. A.; Wasielewski, M. R. J. Phys. Chem. A 2008, 112, 4194– 4201 DOI: 10.1021/jp800561gGoogle Scholar13bhttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXktFShu7o%253D&md5=80c3fc256cfc6bd04003e9c200d9334fIntersystem Crossing Mediated by Photoinduced Intramolecular Charge Transfer: Julolidine-Anthracene Molecules with Perpendicular π SystemsDance, Zachary E. X.; Mickley, Sarah M.; Wilson, Thea M.; Ricks, Annie Butler; Scott, Amy M.; Ratner, Mark A.; Wasielewski, Michael R.Journal of Physical Chemistry A (2008), 112 (18), 4194-4201CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)Time-resolved ESR studies show that the primary mechanism of triplet formation following photoexcitation of julolidine-anthracene mols. linked by a single bond and having perpendicular π systems is a spin-orbit, charge-transfer intersystem crossing mechanism (SOCT-ISC). This mechanism depends on the degree of charge transfer from julolidine to anthracene, the dihedral angle (θ1) between their π systems, and the magnitude of the electronic coupling between julolidine and anthracene. We compare 4-(9-anthracenyl)-julolidine with the more sterically encumbered 4-(9-anthracenyl)-3,5-dimethyljulolidine and find that fixing θ1 equiv. 90° serves to enhance SOCT-ISC by increasing the change in orbital angular momentum accompanying charge transfer. Given that the requirements for the SOCT-ISC mechanism are quite general, we expect it to occur in a variety of electron donor-acceptor systems.(c) Colvin, M. T.; Ricks, A. B.; Scott, A. M.; Co, D. T.; Wasielewski, M. R. J. Phys. Chem. A 2012, 116, 1923– 1930 DOI: 10.1021/jp212546wGoogle ScholarThere is no corresponding record for this reference.
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- 15(a) Rigaudy, J.; Baranne-Lafont, J.; Defoin, A.; Kim Cuong, N. Tetrahedron 1978, 34, 73– 82 DOI: 10.1016/0040-4020(78)88039-9Google ScholarThere is no corresponding record for this reference.(b) Rigaudy, J.; Scribe, P.; Breliere, C. Tetrahedron 1981, 37, 2585– 2593 DOI: 10.1016/S0040-4020(01)98961-6Google ScholarThere is no corresponding record for this reference.
- 16(a) Zehm, D.; Fudickar, W.; Hans, M.; Schilde, U.; Kelling, A.; Linker, T. Chem. - Eur. J. 2008, 14, 11429– 11441 DOI: 10.1002/chem.200801355Google Scholar16ahttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXkt1OmsQ%253D%253D&md5=574221bd3c146d1d98e1c3557a9c6aba9,10-diarylanthracenes as molecular switches: syntheses, properties, isomerizations and their reactions with singlet oxygenZehm, Daniel; Fudickar, Werner; Hans, Melanie; Schilde, Uwe; Kelling, Alexandra; Linker, TorstenChemistry - A European Journal (2008), 14 (36), 11429-11441CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)A series of 9,10-diarylanthracenes with various substituents at the ortho positions have been synthesized by palladium-catalyzed cross-coupling reactions. Such compds. exhibit interesting phys. properties and can be applied as mol. switches. Despite the high steric demand of the substituents, products were formed in moderate-to-good yields. In some cases, microwave conditions further improved yields. Bis-coupling afforded two isomers (syn and anti) that do not interconvert at room temp. These products were easily sepd. and their relative stereochemistries were unequivocally assigned by NMR spectroscopy and X-ray anal. The syn and anti isomers exhibit different phys. properties (e.g., m.ps. and solubilities) and interconversion by rotation around the aryl-aryl axis commences at <100°C for fluoro-substituted diarylanthracenes and at >300°C for alkyl- or alkoxy-substituted diaryl-anthracenes. The reactions with singlet oxygen were studied sep. and revealed different reactivities and reaction pathways. The yields and reactivities depend on the size and electronic nature of the substituents. The anti isomers form the same 9,10-endoperoxides as the syn species, occasionally accompanied by unexpected 1,4-endoperoxides as byproducts. Thermolysis of the endoperoxides exclusively yielded the syn isomers. The interesting rotation around the aryl-aryl axis allows the application of 9,10-diarylanthracenes as mol. switches, which are triggered by light and air under mild conditions. Finally, the oxygenation and thermolysis sequence provides a simple, synthetic access to a single stereoisomer (syn) from an unselective coupling step.(b) Baral, R. N.; Thomas, S. W. J. Org. Chem. 2015, 80, 11086– 11091 DOI: 10.1021/acs.joc.5b01692Google Scholar16bhttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhslCktrfP&md5=ed6e68d75a85489a15593180418216d6Steric and Electronic Substituent Effects Influencing Regioselectivity of Tetracene EndoperoxidationBaral, Rom Nath; Thomas, Samuel W., IIIJournal of Organic Chemistry (2015), 80 (21), 11086-11091CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)This paper describes the influence of steric and electronic factors in the regioselectivity of endoperoxide formation of tetracene derivs. using 1O2. A combination of kinetics expts. and product distributions resulting from these photosensitized oxidns. demonstrates that, while the steric effect of o-alkyl groups on aryl substituents is highly localized to the substituted ring, the resistance to oxidn. based on phenylethynyl substituents is more evenly distributed between the two reactive rings. These results are important for the rational design of highly persistent acenes.
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Abstract
Scheme 1
Scheme 1. Photoinduced Transformations of BADsFigure 1
Figure 1. (a) ps–ns Transient absorption spectra of BAD1 in dimethylformamide upon excitation at 355 nm with 35 fs pulses at delay times of 600 fs (black line), 1 ps (red line), 100 ps (green line), and 5 ns (blue line). The inset shows the blue shift of the TA spectra to 570 nm. (b) Kinetics monitored at 380 nm (black line), 400 nm (red line), 425 nm (green line), 505 nm (violet line), 570 nm (cyan line), and 680 nm (magenta line) as indicated by vertical colored bars. (c) ns−μs Transient absorption spectra of degassed BAD1 solutions following excitation at 355 nm by 700 ps laser pulses. The spectra were integrated from 3 to 5 ns (black line), 10–100 ns (red line), 0.1–1 μs (green line), 1–5 μs (blue line), and 10–100 μs (cyan line). (d) Kinetics observed for the bands at 570 and 680 nm, assigned to the BODIPY triplet state and anthracene radical-cation, respectively, in the absence and presence of oxygen (solid and dotted lines, respectively).
Figure 2
Figure 2. (a) Frontier molecular orbitals and their energies (in a.u.) for BADs 1 and 2. (b) Diagram demonstrating transitions between excited states in BAD1.
Figure 3
Figure 3. (a) Absorption and emission spectra of BAD1. (b) Changes of the emission intensity upon irradiation of BADs and reference compound solutions in CH2Cl2 (5 × 10–5 M) with broad-band visible light. (c) Change of BAD2 emission upon irradiation with 532 nm laser (10 mW cm–2). (d) Photo of BAD2 solution before and after 5 min of irradiation, taken under excitation with 365 nm light.
Scheme 2
Scheme 2. Tentative Mechanism of the Formation of Fluorescent ProductsScheme 3
Scheme 3. Synthesis of Water-Soluble BADs DerivativesFigure 4
Figure 4. Confocal microscopy images of cells incubated with 1 μM of BAD6 after the irradiation with broad-band visible light (400–700 nm, 23.8 mW cm–2) for (a) 2.5 min and (b) 5 min.
References
ARTICLE SECTIONSThis article references 16 other publications.
- 1Chan, J.; Dodani, S. C.; Chang, C. J. Nat. Chem. 2012, 4, 973– 984 DOI: 10.1038/nchem.1500Google Scholar1https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhslahurnM&md5=2ab2ef20873f40a189b64fb5e2888998Reaction-based small-molecule fluorescent probes for chemoselective bioimagingChan, Jefferson; Dodani, Sheel C.; Chang, Christopher J.Nature Chemistry (2012), 4 (12), 973-984CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)A review. The dynamic chem. diversity of elements, ions and mols. that form the basis of life offers both a challenge and an opportunity for study. Small-mol. fluorescent probes can make use of selective, bioorthogonal chemistries to report on specific analytes in cells and in more complex biol. specimens. These probes offer powerful reagents to interrogate the physiol. and pathol. of reactive chem. species in their native environments with minimal perturbation to living systems. This Review presents a survey of tools and tactics for using such probes to detect biol. important chem. analytes. The authors highlight design criteria for effective chem. tools for use in biol. applications as well as gaps for future exploration.
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- 3(a) Ziessel, R.; Allen, B. D.; Rewinska, D. B.; Harriman, A. Chem. - Eur. J. 2009, 15, 7382– 7393 DOI: 10.1002/chem.200900440Google Scholar3ahttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXovVChsLc%253D&md5=dd158aaadb86e171e2d4b071dcd44245Selective Triplet-State Formation during Charge Recombination in a Fullerene/Bodipy Molecular Dyad (Bodipy = Borondipyrromethene)Ziessel, Raymond; Allen, Ben D.; Rewinska, Dorota B.; Harriman, AnthonyChemistry - A European Journal (2009), 15 (30), 7382-7393, S7382/1-S7382/2CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)A conformationally restricted mol. dyad has been synthesized and subjected to detailed photophys. examn. The dyad comprises a borondipyrromethene (Bodipy) dye covalently linked to a buckminsterfullerene C60 residue, and is equipped with hexadecyne units at the boron center in order to assist soly. The linkage consists of a diphenyltolane, attached at the meso position of the Bodipy core and through an N-methylpyrrolidine ring at the C60 surface. Triplet states localized on the two terminals are essentially isoenergetic. Cyclic voltammetry indicates that light-induced electron transfer from Bodipy to C60 is thermodynamically favorable and could compete with intramol. energy transfer in the same direction. The driving force for light-induced electron abstraction from Bodipy by the singlet excited state of C60 depends critically on the solvent polarity. Thus, in non-polar solvents, light-induced electron transfer is thermodynamically uphill, but fast excitation energy transfer occurs from Bodipy to C60 and is followed by intersystem crossing and subsequent equilibration of the two triplet excited states. Moving to a polar solvent switches on light-induced electron transfer. Now, in benzonitrile, the charge-transfer state (CTS) is positioned slightly below the triplet levels, such that charge recombination restores the ground state. However, in CH2Cl2 or methyltetrahydrofuran, the CTS is slightly higher in energy than the triplet levels, and decays, in part, to form the triplet state localized on the C60 residue. This step is highly specific and does not result in direct formation of the triplet excited state localized on the Bodipy unit. Subsequent equilibration of the two triplets takes place on a relatively slow timescale.(b) Amin, A. N.; El-Khouly, M. E.; Subbaiyan, N. K.; Zandler, M. E.; Fukuzumi, S.; D’Souza, F. Chem. Commun. 2012, 48, 206– 208 DOI: 10.1039/C1CC16071KGoogle ScholarThere is no corresponding record for this reference.(c) Huang, L.; Yu, X.; Wu, W.; Zhao, J. Org. Lett. 2012, 14, 2594– 2597 DOI: 10.1021/ol3008843Google ScholarThere is no corresponding record for this reference.(d) Guo, S.; Xu, L.; Xu, K.; Zhao, J.; Kucukoz, B.; Karatay, A.; Yaglioglu, H. G.; Hayvali, M.; Elmali, A. Chem. Sci. 2015, 6, 3724– 3737 DOI: 10.1039/C4SC03865GGoogle Scholar3dhttps://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXmt1WqtLw%253D&md5=647c0401bf1214ff640f9211d1b73fabBodipy-C60 triple hydrogen bonding assemblies as heavy atom-free triplet photosensitizers: preparation and study of the singlet/triplet energy transferGuo, Song; Xu, Liang; Xu, Kejing; Zhao, Jianzhang; Kucukoz, Betul; Karatay, Ahmet; Yaglioglu, Halime Gul; Hayvali, Mustafa; Elmali, AyhanChemical Science (2015), 6 (7), 3724-3737CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)Supramol. triplet photosensitizers based on hydrogen bonding-mediated mol. assemblies were prepd. Three thymine-contg. visible light-harvesting Bodipy derivs. (B-1, B-2 and B-3, which show absorption at 505 nm, 630 nm and 593 nm, resp.) were used as H-bonding modules, and 1,6-diaminopyridine-appended C60 was used as the complementary hydrogen bonding module (C-1), in which the C60 part acts as a spin converter for triplet formation. Visible light-harvesting antennae with methylated thymine were prepd. as refs. (B-1-Me, B-2-Me and B-3-Me), which are unable to form strong H-bonds with C-1. Triple H-bonds are formed between each Bodipy antenna (B-1, B-2 and B-3) and the C60 module (C-1). Singlet energy transfer from the Bodipy antenna to the C60 module was confirmed by fluorescence quenching studies. The intersystem crossing of the latter produced the triplet excited state. The nanosecond transient absorption spectroscopy showed that the triplet state is either localized on the C60 module (for assembly B-1·C-1), or on the styryl-Bodipy antenna (for assemblies B-2·C-1 and B-3·C-1). Intra-assembly forward-backward (ping-pong) singlet/triplet energy transfer was proposed. In contrast to the H-bonding assemblies, slow triplet energy transfer was obsd. for the non-hydrogen bonding mixts. As a proof of concept, these supramol. assemblies were used as triplet photosensitizers for triplet-triplet annihilation upconversion.(e) Cheriya, R. T.; Joy, J.; Alex, A. P.; Shaji, A.; Hariharan, M. J. Phys. Chem. C 2012, 116, 12489– 12498 DOI: 10.1021/jp303406pGoogle ScholarThere is no corresponding record for this reference.(f) Mani, T.; Niedzwiedzki, D. M.; Vinogradov, S. A. J. Phys. Chem. A 2012, 116, 3598– 3610 DOI: 10.1021/jp301345hGoogle ScholarThere is no corresponding record for this reference.
- 4Ragàs, X.; Jiménez-Banzo, A.; Sánchez-García, D.; Batllori, X.; Nonell, S. Chem. Commun. 2009, 2920– 2922 DOI: 10.1039/b822776dGoogle ScholarThere is no corresponding record for this reference.
- 5Kamkaew, A.; Lim, S. H.; Lee, H. B.; Kiew, L. V.; Chung, L. Y.; Burgess, K. Chem. Soc. Rev. 2013, 42, 77– 88 DOI: 10.1039/C2CS35216HGoogle Scholar5https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhslKrurbJ&md5=69fddf735bafa0b2b33a70835af1430fBODIPY dyes in photodynamic therapyKamkaew, Anyanee; Lim, Siang Hui; Lee, Hong Boon; Kiew, Lik Voon; Chung, Lip Yong; Burgess, KevinChemical Society Reviews (2013), 42 (1), 77-88CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)A review. BODIPY dyes tend to be highly fluorescent, but their emissions can be attenuated by adding substituents with appropriate oxidn. potentials. Substituents like these have electrons to feed into photoexcited BODIPYs, quenching their fluorescence, thereby generating relatively long-lived triplet states. Singlet oxygen is formed when these triplet states interact with 3O2. In tissues, this causes cell damage in regions that are illuminated, and this is the basis of photodynamic therapy (PDT). The PDT agents that are currently approved for clin. use do not feature BODIPYs, but there are many reasons to believe that this situation will change. This review summarizes the attributes of BODIPY dyes for PDT, and in some related areas.
- 6Zhao, J.; Xu, K.; Yang, W.; Wang, Z.; Zhong, F. Chem. Soc. Rev. 2015, 44, 8904– 8939 DOI: 10.1039/C5CS00364DGoogle Scholar6https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhs1Gjsb3I&md5=26bdbc5fa8c7f5348f6a2e831126ab0cThe triplet excited state of Bodipy: formation, modulation and applicationZhao, Jianzhang; Xu, Kejing; Yang, Wenbo; Wang, Zhijia; Zhong, FangfangChemical Society Reviews (2015), 44 (24), 8904-8939CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)Boron dipyrromethene (Bodipy) is one of the most extensively investigated org. chromophores. Most of the investigations are focused on the singlet excited state of Bodipy, such as fluorescence. In stark contrast, the study of the triplet excited state of Bodipy is limited, but it is an emerging area, since the triplet state of Bodipy is tremendously important for several areas, such as the fundamental photochem. study, photodynamic therapy (PDT), photocatalysis and triplet-triplet annihilation (TTA) upconversion. The recent developments in the study of the prodn., modulation and application of the triplet excited state of Bodipy are discussed in this review article. The formation of the triplet state of Bodipy upon photoexcitation, via the well known approach such as the heavy atom effect (including I, Br, Ru, Ir, etc.), and the new methods, such as using a spin converter (e.g. C60), charge recombination, exciton coupling and the doubly substituted excited state, are summarized. All the Bodipy-based triplet photosensitizers show strong absorption of visible or near IR light and the long-lived triplet excited state, which are important for the application of the triplet excited state in PDT or photocatalysis. Moreover, the methods for switching (or modulation) of the triplet excited state of Bodipy were discussed, such as those based on the photo-induced electron transfer (PET), by controlling the competing Forster-resonance-energy-transfer (FRET), or the intermol. charge transfer (ICT). Controlling the triplet excited state will give functional mols. such as activatable PDT reagents or mol. devices. It is worth noting that switching of the singlet excited state and the triplet state of Bodipy may follow different principles. Application of the triplet excited state of Bodipy in PDT, hydrogen (H2) prodn., photoredox catalytic org. reactions and TTA upconversion were discussed. The challenges and the opportunities in these areas were briefly discussed.
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Supporting Information
Supporting Information
ARTICLE SECTIONSThe Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b00551.
Experimental details; data for BAD1, BAD1-BE, BAD2-BA, BAD2-TE
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