Intrinsic Properties of Single Graphene Nanoribbons in Solution: Synthetic and Spectroscopic Studies

We report a novel type of structurally defined graphene nanoribbons (GNRs) with uniform width of 1.7 nm and average length up to 58 nm. These GNRs are decorated with pending Diels–Alder cycloadducts of anthracenyl units and N-n-hexadecyl maleimide. The resultant bulky side groups on GNRs afford excellent dispersibility with concentrations of up to 5 mg mL–1 in many organic solvents such as tetrahydrofuran (THF), two orders of magnitude higher than the previously reported GNRs. Multiple spectroscopic studies confirm that dilute dispersions in THF (<0.1 mg mL–1) consist mainly of nonaggregated ribbons, exhibiting near-infrared emission with high quantum yield (9.1%) and long lifetime (8.7 ns). This unprecedented dispersibility allows resolving in real-time ultrafast excited-state dynamics of the GNRs, which displays features of small isolated molecules in solution. This study achieves a breakthrough in the dispersion of GNRs, which opens the door for unveiling obstructed GNR-based physical properties and potential applications.


Materials.
The chemicals and organic solvents used in this work were purchased from Sigma-Aldrich, Admas-beta, Alfa Aesar and J＆K suppliers and used as received, unless otherwise mentioned.

Instrumentation and Methods
Analytical thin layer chromatography (TLC) was conducted on silica gel coated substrates (60 F254) from Merck. Liquid-phase nuclear magnetic resonance ( 1 H NMR and 13 C NMR) spectra were performed on Mercury Plus 400 (400 MHz for S2 proton, 100 MHz for carbon) spectrometer using tetramethylsilane (TMS) as the internal reference. Data are reported via chemical shift (ppm, the solvent resonance as the internal standard), integration and multiplicity (Abbreviations: s = singlet, d = doublet, t = triplet, m = multiplet). Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra were obtained using a Bruker Reflex II utilizing a 337 nm nitrogen laser, calibrated with poly(ethylene glycol) (3000 g/mol).
Molecular weights of the polyphenylene precursors were recorded by gel permeation chromatography (GPC) with a Wyatt Optilab DSP differential refractometer and a Wyatt miniDAWN multi-angle laser light scattering detector, using tetrahydrofuran (THF) as the eluent and standard polystyrene as the reference. Recycling GPC was conducted on Shodex K-2003 GPC columns with chloroform as the eluent. Fourier transform infrared (FTIR) spectra were performed on a Spectrum 100 (Perkin Elmer, Inc., USA) spectrometer. Raman spectroscopy was conducted on a Horiba XploRA PLUS Raman spectrometer, equipped with 2400 lines/mm and 1800 lines/mm gratings, at 532 nm and 638 nm laser wavelengths, respectively. A 100X objective was used and the laser power was kept low (<0.5 mW) in order to avoid damage and heat induced effects. Dynamic Light Scattering (DLS) was conducted on a Malvern Zetasizer Nano S apparatus with a 4.0 mW laser (λ = 633 nm). The measurements were performed at a scattering angle of 90° under room temperature.
The CONTIN program was used for the processing of the data. Atomic Force Microscope (AFM) measurements were carried out using a Multimode AFM with a S3 Nanoscope VIII controller (Veeco/Digital Instruments) and a Cypher AFM (Asylum Research) in intermittent contact mode. AFM images were obtained via Scanning Probe Image Processor (SPIP, Image Metrology ApS) and WSxM (Nanotec Electronica, Spain) softwares. Ultraviolet-Visible (UV-vis) absorption spectra were obtained on HITACHI U-4100 and JASCO V570 spectrophotometers.
Photoluminescence spectra were collected via a Princeton Instruments Acton 2500i grating spectrometer.

Synthetic procedures of GNR-AHM
Scheme S1. The synthetic route of structurally defined GNRs functionalized with AHM bulky side groups (6, GNR-AHM). MgSO4. Subsequently, the mixture was filtered and the solvent was evaporated.

Synthesis of GNR-AHMs
The method described in 3.3 was followed to convert the PPP precusors into GNR-AHMs. Firstly, 50 mg PPP-AHM was dissolved in 50 mL DCM. Then the solution of ferric chloride (562 mg, 7.5 eqv./H) in 2.0 mL nitromethane was added.
Subsequently, a stream of nitrogen saturated with dichloromethane was passed through the mixture for 2 h. The mixture was stirred at room temperature for 24 h.
After the reaction, an excess amount of methanol was added to produce precipitate.
The precipitate was collected by extraction filtration, washed with water and methanol for at least 5 cycles. After vacuum drying, 46 mg black powders (GNR-AHM) were obtained, with a yield of 91%.
The comparison of the FTIR spectra of the PPP precursors and GNR samples are shown below (Figures S14-S16).

Solid-State NMR measurements of PPP-AHM and GNR-AHM samples
All solid-state 1 H magic-angle-spinning (MAS) and 13  kHz. To achieve sufficiently good signal-to-noise ratios, 32768 transients were recorded and co-added using a recycling delay of 2.5 s.  Figure S18b, which includes several separate 1 H signals with significantly narrower line width. Again, these characteristic spectral differences can be attributed to the increased rigidity of the resulting GNRs.      Figure S24). The domains S24 usually extend over tens of thousands square nanometers. AFM topographic images clearly show that monolayer thick films (3.9 ± 0.9 Å) are formed (supporting information Figure S25).   Table S1. Quantum yields and PL lifetime of the GNR-AHM samples ). The excitation wavelength was 600 nm. b Fluorescence quantum yield was obtained using quinine sulfate as the standard. 21 Figure S26. Relationship of the PL maximum intensity (at 740 nm) and the GNR concentration in THF.

Transient absorption experimental setup
The transient absorption (TA) setup is fed by a 100-fs, 1-kHz repetition rate Ti:sapphire laser system (Libra, Coherent) with a central wavelength of 800 nm. The nm, or on a 3 mm thick YAG plate for the NIR WLC extending from 815 to 1100 nm.
For the TA spectra in Figure 3 of the main text, the output of the second OPA at 1225 nm was focused on a 2 mm thick sapphire plate, generating a WLC, displayed in Figure

2DES experimental setup
Two-dimensional Electronic Spectroscopy (2DES) is an optical frequency analogue of multidimensional NMR techniques, and can also be seen as an extension of TA spectroscopy in which sub-10 fs temporal resolution is made compatible with high excitation frequency resolution. Measurements were performed in the partially collinear pump-probe geometry. Both pump and probe pulses are generated by a non-collinear optical parametric amplifier (NOPA) in the NIR range, fed by an amplified Ti:sapphire laser (Coherent Libra) beam at 800 nm with 100 fs pulse S27 duration and 1 kHz repetition rate. The details of the design of the NOPA are given in reference [22]. Briefly, the second harmonic of the Ti:sapphire is used to amplify the WLC (shown in Figure S27) in a quasi-collinear geometry. The NOPA pulses are compressed through chirped mirrors and the pulse duration was measured by the PG-FROG technique 23 Figure S28a, TA spectra of GNR-AHM-1 are characterized by the same shape as the spectra of GNR-AHM-3 ( Figure S28b). In order to obtain the time constants of the dynamics of the entire dataset, we performed global analysis with the Glotaran software 26 . We observed that a three exponential decay model (plus one that is much longer than the longest delay measured) fits well the data. The evolution associated spectra (EAS) give the wavelength dependent amplitudes associated to each time constant. In Figure S30, we plot the EAS spectra of GNR-AHM-3 sample pumped with λ PUMP =730 nm. As already seen, the fastest component is related to the decay around 740 nm. Figure S30. EAS spectra obtained by fitting GNR-AHM-3 TA maps with Glotaran. In black the spectra related to the fastest decay constant τ 1 (250 fs), in red the one related to the second decay constant τ 2 (9 ps) and in green, spectrum related to the longest component τ 3 (290 ps).

Concentration dependent linear absorption
Linear absorption spectra were acquired at room temperature using a V-570 Jasco spectrophotometer. The background has been corrected accounting for solvent absorption.   Figure S31 shows concentration dependence of GNR-AHM-1 linear absorption spectra. On highly concentrated samples, we observe additional weak absorption bands between 850 and 1000 nm, which gradually disappears as the concentration is lowered, being thus assigned to aggregated species in the dispersions.
The strong inhomogeneous broadening observed with 2DES (Figure 3c) was discussed to originate from intermolecular interactions through the bulky AHM side chains. This is corroborated by the fact that the individual peaks in the absorption get narrower as the concentration is lowered ( Figure S31b, right panel).

Spectral diffusion
Knowing from 2DES that the lowest electronic transition is inhomogeneously broadened from time zero, we can investigate in more detail the spectral diffusion up to 900 ps with the narrowband TA, which corresponds to a horizontal cut of the 2DES maps. Here, the pump spectrum is about 10 nm wide around 730 nm, which is sufficiently narrow to see that the spectral diffusion manifests itself as a broadening of the transition. Figure S32. Series of normalized TA spectra of GNR-AHM-3 in toluene from 0.06 to 900 ps. A slight broadening is observed in the first 500 fs, after which the lineshape retains a constant width up to 900 ps. Because the width of this band is much narrower compared to the diagonal peak widths in 2DES, we conclude that much of the microscopic structures causing inhomogeneous broadening is very rigid and stable. Because of ample evidence to discard the presence of large aggregates, the inhomogeneity cannot be assigned to the distributions of GNR aggregate structures but likely to the configurations of AHM side chains.