Macroscopic Wires from Fluorophore-Quencher Dyads with Long

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Macroscopic Wires from FluorophoreQuencher Dyads with Long-Lived Blue Emission Tao Wang, Ziye Wu, Wei Sun, Shengye Jin, Xing Yuan Zhang, Chuanyao Zhou, Jun Jiang, Yi Luo, and Guoqing Zhang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b08268 • Publication Date (Web): 30 Aug 2017 Downloaded from http://pubs.acs.org on September 5, 2017

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The Journal of Physical Chemistry

Macroscopic Wires from Fluorophore-Quencher Dyads with Long-Lived Blue Emission Tao Wang,1,2 Ziye Wu,2 Wei Sun, 1,2 Shengye Jin,3 Xingyuan Zhang,*1 Chuanyao Zhou,*3 Jun Jiang, 1,2 Yi Luo,1,2 and Guoqing Zhang*2 1

School of Chemistry and Materials Science and 2Hefei National Laboratory for Physical

Sciences at the Micro Scale, University of Science and Technology of China, Hefei, China 230026 3

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics,

Chinese Academy of Sciences, Dalian, China, 116023

ABSTRACT. We report the formation of macroscopic wires up to centimeters in length from a series of structurally flexible, covalently tethered small-molecular fluorophore-quencher dyads (FQDs, average MW = 425 Da), comprised of carbazole, melatonin and cyanobenzoate moieties. These FQDs are non-emissive in organic solutions but become moderately to highly luminescent (ΦF = 0.037 – 0.39) upon formation of wires with emission maxima in the blue region (446 – 483 nm). The blue photoluminescence (PL) is ascribed to a combination of singlet charge-transfer, localized triplet state, and possibly delayed fluorescence emissions with intrinsic luminescence lifetimes ranging from 0.228 to 21333 µs, based on luminescence, transient absorption measurements, X-ray diffraction and calculations.

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KEYWORDS macroscopic assembly, blue luminescence, room-temperature phosphorescence, transient absorption, organic dyad INTRODUCTION Room-temperature phosphorescence (RTP) from purely organic molecules has gained renewed interests in the past few years due to its potential application in environment-friendly, low-cost and high-efficient organic light-emitting diodes (OLEDs).1-5 A key breakthrough is the successful design and synthesis of organic molecules with nearly 100% thermally-activated delay fluorescence (TADF),6,7 which allows for more efficient harvesting of electrically generated triplet excitons while maintaining a relatively fast radiative decay from the singlet state, in comparison to traditional OLED materials. Since OLED materials are to be used in the solid state, a common issue during application is aggregation-caused quenching despite their highly luminescent nature as discrete molecules in solution. The problem can be circumvented by introducing bulky substituents to the molecules or by designing molecules with aggregationinduced emission (AIE).8-11 Apart from application in OLEDs, organic RTP molecules also find practical use as background-free imaging agents in biology,12-13 provided that the lifetime is adequately long. Recently, we discovered that by covalently linking a carbazole fluorophore and a nitrobenzoate quencher through an alkyl spacer (Scheme 1, right), the resulting dyad is not luminescent in solution but exhibits microsecond-long RTP as nanoscopic assemblies.14 These solid-state emission from the fluorophore-quencher dyads (FQDs) is evidenced to stem from an almost entirely intermolecular charge-transfer (CT) state, i.e., excitation comprised of nonoverlapping orbitals from the carbazole donor and the nitrobenzoate acceptor, respectively. The very small singlet and triplet energy gap from the nanoassemblies thus leads to RTP from


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degenerate singlet and triplet excited states. From the energy diagram, it is plausible to conclude that should an acceptor moiety with lesser strength (i.e., cyanobenzoate vs. nitrobenzoate) be used, the localized triplet state of the fluorophore is thus able to participate the emissive state. As a result, luminescence lifetimes may substantially increase from the µs range to ms. In the current study, we present a class of cyanobenzoate FQDs (Scheme 1, left), where the donors are alkoxyl-substituted carbazoles and melatonin, respectively (Figure 1a). In both cases, intense blue photoluminescence (PL) can be observed in the solid state with a maximum QY of 0.39, a 40-fold increase compared to that of the nitrobenzoate counterpart, and with lifetimes in the µsms range. Furthermore, these FQDs are capable of forming microscopic assemblies such as wires up to centimeter long under appropriate conditions. To the best of our knowledge, microscopic assemblies constructed from small organic molecules are very rare and may find potential applications in optoelectronics,15 non-linear optics,16 chemical sensing17 and displaying technology.2

Scheme 1. Left: proposed energy diagram to produce photoluminescence (PL) involving localized triplet state for prolonged excited-state lifetime; right: a lower 3CT state leads to redshifted PL emission and reduced lifetime.


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EXPERIMENTAL Materials. Carbazole (CP, Aladdin Reagent Co.), 4-hydroxycarbazole (98%, Energy Chemical Reagent Co.), Iodoethane (99%, Energy Chemical Reagent Co.), 1-Bromohexane (99%, Energy Chemical Reagent Co.), 1-Bromobutane (98%, Energy Chemical Reagent Co.), Benzyl bromide (98%, Aladdin Reagent Co.), 1-Bromododecane (98%, Aladdin Reagent Co.), 2-Bromoethanol (95%, Aladdin Reagent Co.), 3,4-dihydro-2H-pyran (95%, Aldrich Reagent Co.), Sodium hydride (60% dispersion in mineral oil, Energy Chemical Reagent Co.), 4-Cyanobenzoic acid (98%, Aladdin Reagent Co.), p-toluenesulfonic acid monohydrate (98.5%, Aladdin Reagent Co.), N-(3-Dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride (98.5%, Aladdin Reagent Co.), 4-dimethylaminopyridine (99%, Aladdin Reagent Co.) were used as received. All other reagents and solvents were purchased from Sinopharm Chemical Reagent Co. and used as received. Method. 1H NMR (300 MHz) spectra were recorded on a Bruker AV300 NMR spectrometer operated in the Fourier transform mode. NMR chemical shifts were reported in standard format as values in ppm relative to deuterated solvents. MALDI-TOF mass spectra (ESI) were collected from Atouflex Speed mass spectrometer. UV/Vis absorption spectra were recorded on a Beijing Persee TU-1901 UV-Vis spectrometer. UV-Vis-NIR absorption spectra were recorded on a SOLID3700 UV-Vis-NIR spectrometer. Excitation and steady-state fluorescence emission spectra and absolute quantum yield were conducted on a FluoroMax-4 spectrofluorometer (Horiba Scientific) and analyzed with an Origin integrated software FluoroEssence (v2.2). Fluorescence and phosphorescence lifetime data were acquired with a 1MHz LED laser with the excitation peak at 370 nm. Lifetime data were analyzed with DataStation v6.6 (Horiba Scientific). Delayed photoluminescence spectra (77k) were recorded on SpectraSuite (Ocean Optics, v2008).


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Synthesis of 4-ethoxy-9H-carbazole. 9H-carbazol-4-ol (3 g, 16.4 mmol), Potassium carbonate (4.5 g, 32.8 mmol), Tetrabutyl ammonium bromide (0.528 g, 0.164mmol ) and 160ml ethyl acetate were added to a round-bottom flask equipped with a magnetic stir Bar. The reaction was first conducted at room temperature for an hour . Then iodoethane (7.67 g, 49.2mmol) was added to the solution, and heated under reflux for 24h. After the reaction finished, the solution was filtered, extracted with water (100mL×3). After drying over anhydrous Na2SO4, ethyl acetate was removed on a rotary evaporator. The crude product was purified by column chromatography (petroleum/ethyl acetate = 9:1), yielding a white solid (3.25 g, 94%). 1H NMR (300MHz, d6DMSO), δ(TMS, ppm): 11.24 (s, 1H, -NH), 8.15 (d, J = 7.7 Hz, 2H), 7.44 (d, J = 8.1 Hz, 1H,), 7.39 - 7.22 (m, 2H), 7.20 - 7.08 (m, 1H), 7.06 (d, J = 8.0 Hz, 1H), 6.68 (d, J = 7.9 Hz, 1H), 4.25 (q, J = 7.0 Hz, 2H), 1.52 (t, J = 6.9 Hz, 3H). Synthesis of 2-(4-ethoxy-9H-carbazol-9-yl)ethan-1-ol. 4-ethoxy-9H-carbazole (2.5 g, 11.85 mmol), sodium hydride (60%, 0.95 g, 23.75 mmol) and 50ml DMF were added to a roundbottom flask equipped with a magnetic stir bar. Then the solution was conducted at room temperature for an hour. After Sodiumiodide and 2-(2-bromoethoxy)tetrahydro-2H-pyran (3.7 g, 18.78 mmol) added, the reaction was conducted at 85 °C for another 24 h. The reaction was quenched with excess water, and the solvent was removed in vacuo. The solid was dissolved in CH2Cl2 (70 mL), extracted with water (70 mL×3). After drying over anhydrous Na2SO4, CH2Cl2 was removed on a rotary evaporator. Then p-toluenesulfonic acid monohydrate (2.25 g, 11.85 mmol) and 50mL methanol were added to the flask, and the reaction was conducted at 60ºC overnight. After the reaction was quenched with excess NaOH solution, the solvent was removed on a rotary evaporator. Then the solid was dissolved in CH2Cl2, extracted with water (70 mL×3). After drying over anhydrous Na2SO4, CH2Cl2 was removed on a rotary evaporator. The crude


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product was purified by column chromatography (petroleum/ethyl acetate = 3:1), yielding a white solid (2.46 g, 81%). 1H NMR (300 MHz,d6-DMSO) , δ(TMS, ppm): 8.19 (d, J = 7.7 Hz, 1H), 7.57 (d, J = 8.2 Hz, 1H), 7.45 - 7.28 (m, 2H), 7.18 (dt, J = 7.5, 3.4 Hz, 2H), 6.73 (d, J = 7.9 Hz, 1H), 4.86 (t, J = 5.5 Hz, 1H), 4.40 (t, J = 5.8 Hz, 2H), 4.27 (q, J = 7.0 Hz, 2H), 3.76 (q, J = 5.7 Hz, 2H), 1.52 (t, J = 6.9 Hz, 3H). Synthesis of 2-(4-ethoxy-9H-carbazol-9-yl)ethyl-4-cyanobenzoate. 2-(4-ethoxy-9H-carbazol9-yl)ethan-1-ol (1 g, 5.88 mmol), and 4-cyanobenzoic acid (2.59 g, 17.64mmol) were added to a round-bottom flask containing 90 mL CH2Cl2. After cooling to 0ºC, EDC•HCl (1.69 g, 8.82mmol) and DMAP (0.72 g, 5.89 mmol), dissolved in CH2Cl2 (30 mL), were added to the solution dropwise. Then the reaction was conducted at 25 ºC overnight. After the reaction finished, the solvent was extracted with 5% NaHCO3 solution (120 mL×2) and water (120 mL×2). After drying over anhydrous Na2SO4, CH2Cl2 was removed in vacuum. The crude product was purified by column chromatography (petroleum/ethyl acetate= 3:1), yielding a white solid (1.8 g, 80 %) . 1H NMR (300 MHz, d6-DMSO) , δ(TMS, ppm): 8.17 (d, J = 7.7 Hz, 1H), 7.88 (d, J = 8.2 Hz, 2H), 7.79 (d, J = 8.3 Hz, 2H,), 7.69 (d, J = 8.2 Hz, 1H), 7.46 - 7.24 (m, 3H), 7.19 (t, J = 7.5 Hz, 1H), 6.74 (d, J = 7.6 Hz, 1H), 4.82 (t, J = 5.0 Hz, 2H), 4.66 (t, J = 5.0 Hz, 2H), 4.25 (q, J = 7.0 Hz, 2H), 1.51 (t, J = 6.9 Hz, 3H). MS (ESI): m/z [M+H]+ Calcd for C24H21N2O3 385.15467, found 385.15384 Synthesis of 4-butoxy-9H-carbazole. This synthesis process is similar to synthesis of 4-ethoxy9H-carbazole. 1H NMR (300 MHz, d6-DMSO) , δ(TMS, ppm): 11.24 (s, 1H), 8.14 (d, J = 7.8 Hz, 1H), 7.45 (d, J = 8.1 Hz, 1H), 7.39 - 7.22 (m, 2H,), 7.06 (d, J = 8.0 Hz, 1H), 6.68 (d, J = 7.9 Hz, 1H), 4.20 (t, J = 6.3 Hz, 2H), 1.88 (dt, J = 8.2, 6.4 Hz, 2H), 1.70-1.51(m, 2H), 1.01 (t, J = 7.4 Hz, 3H).


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Synthesis of 4-hexyloxy-9H-carbazole. 1H NMR (300 MHz, d6-DMSO) , δ(TMS, ppm): 11.24 (s, 1H), 8.14 (d, J = 7.8 Hz, 1H), 7.50 - 7.40 (m, 1H), 7.39 - 7.22 (m, 2H), 7.14 (td, J = 7.5, 1.1 Hz, 1H), 7.06 (d, J = 8.0 Hz, 1H), 6.68 (d, J = 7.9 Hz, 1H), 4.20 (t, J = 6.3 Hz, 2H), 1.99 - 1.83 (m, 2H), 1.58 (m, J = 7.1 Hz, 2H), 1.37 (td, J = 8.4, 7.6, 4.2 Hz, 4H, -OCH2CH2CH2 CH2CH2CH3), 0.90 (t, J = 6.9 Hz, 3H). Synthesis of 4-dodecyloxy-9H-carbazole. 1H NMR (300 MHz, d6-DMSO), δ(TMS, ppm):: 11.24 (s, 1H, -NH), 8.14 (d, J = 7.7 Hz, 1H), 7.45 (d, J = 8.1 Hz, 1H), 7.39 - 7.22 (m, 2H), 7.19 7.01 (m, 2H), 6.67 (d, J = 7.9 Hz, 1H), 4.19 (t, J = 6.2 Hz, 2H), 1.90 (m, J = 6.5 Hz, 2H), 1.57 (m, J = 6.9 Hz, 2H), 1.46 - 1.34 (m, 2H), 1.26 (d, J = 17.0 Hz, 14H), 0.90 - 0.79 (m, 3H). Synthesis of 4-benzyloxy-9H-carbazole. 1H NMR (300 MHz, d6-DMSO), δ(TMS, ppm): 11.29 (s, 1H), 8.12 (d, J = 7.8 Hz, 1H), 7.60 (d, J = 7.0 Hz, 2H), 7.46 (dt, J = 7.5, 3.2 Hz, 3H), 7.41 7.26 (m, 3H), 7.17 - 7.05 (m, 2H), 6.81 (d, J = 7.9 Hz, 1H), 5.36 (s, 2H). Synthesis of 2-(9H-carbazol-9-yl)ethan-1-ol. This synthesis process is similar to synthesis of 2(4-ethoxy-9H-carbazol-9-yl)ethan-1-ol. 1H NMR (300 MHz, d6-DMSO) , δ(TMS, ppm): 8.14 (dd, J = 7.8, 1.2 Hz, 2H), 7.60 (d, J = 8.2 Hz, 2H), 7.43 (ddd, J = 8.3, 7.1, 1.3 Hz, 2H), 7.21 (d, J = 1.0 Hz, 1H), 4.88 (t, J = 5.5 Hz, 1H), 4.43 (t, J = 5.7 Hz, 2H), 3.78 (q, J = 5.7 Hz, 2H). Synthesis of 2-(4-butoxy-9H-carbazol-9-yl)ethan-1-ol. 1H NMR (300 MHz, d6-DMSO) , δ(TMS, ppm): 8.17 (d, J = 7.7 Hz, 1H), 7.57 (d, J = 8.2 Hz, 1H), 7.45 - 7.28 (m, 2H), 7.18 (dt, J = 7.5, 3.4 Hz, 2H), 6.73 (d, J = 7.9 Hz, 1H), 4.86 (t, J = 5.5 Hz, 1H), 4.40 (t, J = 5.8 Hz, 2H), 4.22 (t, J = 6.3 Hz, 2H), 3.76 (q, J = 5.7 Hz, 2H), 1.98 - 1.81 (m, 2H), 1.70 - 1.50 (m, 2H), 1.01 (t, J = 7.4 Hz, 3H).


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Synthesis of 2-(4-(hexyloxy)-9H-carbazol-9-yl)ethan-1-ol. 1H NMR (300 MHz, d6-DMSO) , δ(TMS, ppm): 8.18 (d, J = 7.7 Hz, 1H), 7.58 (d, J = 8.2 Hz, 1H), 7.45 - 7.28 (m, 2H), 7.24 - 7.11 (m, 2H), 6.73 (d, J = 7.9 Hz, 1H), 4.86 (t, J = 5.5 Hz, 1H), 4.40 (t, J = 5.8 Hz, 2H), 4.21 (t, J = 6.3 Hz, 2H), 3.76 (q, J = 5.7 Hz, 2H,), 1.91 (m, J = 6.6 Hz, 2H), 1.58 (m, J = 7.2 Hz, 2H), 1.37 (tq, J = 12.8, 7.5, 7.0 Hz, 4H), 0.90 (t, J = 6.9 Hz, 3H). Synthesis of 2-(4-(dodecyloxy)-9H-carbazol-9-yl)ethan-1-ol. 1H NMR (300 MHz, d6-DMSO) , δ(TMS, ppm): 8.17 (d, J = 7.7 Hz, 1H), 7.57 (d, J = 8.2 Hz, 1H), 7.45 - 7.28 (m, 2H), 7.16 (t, J = 7.7 Hz, 2H), 6.72 (d, J = 7.9 Hz, 1H), 4.86 (t, J = 5.5 Hz, 1H), 4.40 (t, J = 5.8 Hz, 2H), 4.20 (t, J = 6.2 Hz, 2H), 3.76 (q, J = 5.7 Hz, 2H), 1.90 (m, J = 6.6 Hz, 2H), 1.57 (m, J = 7.0 Hz, 2H), 1.26 (d, J = 15.7 Hz, 14H), 0.90 - 0.79 (m, 3H). Synthesis of 2-(4-(benzyloxy)-9H-carbazol-9-yl)ethan-1-ol. 1H NMR (300MHz, d6-DMSO) , δ(TMS, ppm): 8.16 (d, J = 7.7 Hz, 1H), 7.65 - 7.54 (m, 3H), 7.52 - 7.30 (m, 5H), 7.26 - 7.10 (m, 2H), 6.86 (d, J = 7.9 Hz, 1H), 5.38 (s, 2H), 4.87 (t, J = 5.5 Hz, 1H), 4.42 (t, J = 5.8 Hz, 2H), 3.77 (q, J = 5.7 Hz, 2H). N-(2-(1-(2-hydroxyethyl)-5-methoxy-1H-indol-3-yl)ethyl)acetamide. 1H NMR (300 MHz, d6DMSO) , δ(TMS, ppm): δ 7.94 (t, J = 5.7 Hz, 1H), 7.30 (d, J = 8.9 Hz, 1H), 7.12 (s, 1H), 7.02 (d, J = 2.4 Hz, 1H), 6.75 (dd, J = 8.9, 2.4 Hz, 1H), 4.83 (t, J = 5.3 Hz, 1H), 4.09 (t, J = 5.7 Hz, 2H), 3.76 (s, 3H), 3.67 (t, J = 5.5 Hz, 2H), 3.33 - 3.23 (m, 2H), 2.75 (t, J = 7.5 Hz, 2H), 1.80 (s, 3H). Synthesis of 2-(9H-carbazol-9-yl)ethyl-4-cyanobenzoate. This synthesis process is similar to synthesis of 2-(4-ethoxy-9H-carbazol-9-yl)ethyl 4-cyanobenzoate. 1H NMR (300MHz, d6DMSO) , δ(TMS, ppm): 8.14 (d, J = 7.7 Hz, 2H), 7.89 (d, J = 8.4 Hz, 2H), 7.80 (d, J = 8.4 Hz, 2H), 7.73 (d, J = 8.2 Hz, 2H), 7.45 (ddd, J = 8.3, 7.1, 1.2 Hz, 2H), 7.26 - 7.14 (m, 2H), 4.86 (t, J


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= 5.1 Hz, 2H), 4.68 (t, J = 5.0 Hz, 2H). MS (ESI): m/z [M+Na]+ Calcd for C22H16N2O2Na 363.1104, found 363.1113. Synthesis of 2-(4-butoxy-9H-carbazol-9-yl)ethyl-4-cyanobenzoate. 1H NMR (300 MHz, d6DMSO) , δ(TMS, ppm): 8.15 (d, J = 7.7 Hz, 1H), 7.89 (d, J = 8.4 Hz, 2H), 7.80 (d, J = 8.4 Hz, 2H), 7.70 (d, J = 8.2 Hz, 1H), 7.46-7.25 (m, 3H), 7.20 (t, J = 7.5 Hz, 1H), 6.75 (d, J = 7.6 Hz, 1H), 4.82 (d, J = 5.1 Hz, 2H), 4.66 (t, J = 4.9 Hz, 2H), 4.21 (t, J = 6.3 Hz, 2H), 1.89 (m, J = 6.5 Hz, 2H), 1.59 (m, J = 7.5 Hz, 2H), 1.00 (t, J = 7.4 Hz, 3H). MS (ESI): m/z [M+H]+ Calcd for C26H25O3N2 413.18597, found 413.18512. Synthesis of 2-(4-(hexyloxy)-9H-carbazol-9-yl)ethyl 4-cyanobenzoate. 1H NMR (300 MHz, d6-DMSO) , δ(TMS, ppm): 8.16 (d, J = 7.7 Hz, 1H), 7.88 (d, J = 8.4 Hz, 2H), 7.79 (d, J = 8.4 Hz, 2H), 7.69 (d, J = 8.2 Hz, 1H), 7.46 - 7.24 (m, 3H), 7.19 (t, J = 7.4 Hz, 1H), 6.74 (d, J = 7.7 Hz, 1H), 4.82 (t, J = 4.9 Hz, 2H), 4.66 (t, J = 5.0 Hz, 2H), 4.20 (t, J = 6.3 Hz, 2H), 1.89 (m, J = 6.5 Hz, 2H), 1.56 (m, J = 7.2 Hz, 2H), 1.36 (s, 4H), 0.89 (t, J = 6.9 Hz, 3H). MS (ESI): m/z [M+H]+ Calcd for C28H29O2N3 441.21727, found 441.21594. Synthesis of 2-(4-(dodecyloxy)-9H-carbazol-9-yl)ethyl-4-cyanobenzoate. 1H NMR (300 MHz, CDCl3) , δ(TMS, ppm): 8.35 (dt, J = 7.8, 1.0 Hz, 1H), 7.89 - 7.74 (m, 2H), 7.65 - 7.53 (m, 2H), 7.48 - 7.39 (m, 2H), 7.36 (t, J = 8.1 Hz, 1H), 7.30 - 7.26 (m, 1H), 7.05 (d, J = 8.1 Hz, 1H), 6.69 (d, J = 8.0 Hz, 1H), 4.71 (s, 4H), 4.23 (t, J = 6.4 Hz, 2H), 1.99 (m, J = 6.6 Hz, 2H), 1.63 (q, J = 7.8, 7.2 Hz, 2H), 1.43 (m, 16H), 0.95 - 0.80 (m, 3H). MS (ESI): m/z [M+H]+ Calcd for C34H41O2N3 525.31117, found 525.31116.

Synthesis of 2-(4-(benzyloxy)-9H-carbazol-9-yl)ethyl-4-cyanobenzoate. 1H NMR (300 MHz, d6-DMSO) , δ(TMS, ppm): 8.14 (d, J = 7.7 Hz, 1H), 7.87 (d, J = 8.4 Hz, 2H), 7.78 (d, J = 8.4 Hz,


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2H), 7.71 (d, J = 8.3 Hz, 1H), 7.63 - 7.55 (m, 2H), 7.51 - 7.27 (m, 6H), 7.17 (t, J = 7.5 Hz, 1H), 6.92 - 6.81 (m, 1H), 5.37 (s, 2H), 4.84 (t, J = 5.0 Hz, 2H), 4.67 (t, J = 5.0 Hz, 2H). MS (ESI): m/z [M+H]+ Calcd for C29H23O2N3 447.17032, found 447.17130. 2-(3-(2-acetamidoethyl)-5-methoxy-1H-indol-1-yl)ethyl-4-cyanobenzoate.

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H NMR (300

MHz, d6-DMSO) , δ(TMS, ppm): δ 7.99 (d, J = 1.1 Hz, 4H), 7.96 - 7.87 (m, 1H), 7.43 (d, J = 8.9 Hz, 1H), 7.23 (s, 1H), 7.02 (d, J = 2.4 Hz, 1H), 6.77 (dd, J = 8.8, 2.4 Hz, 1H), 4.52 (dd, J = 10.8, 4.4 Hz, 4H), 3.76 (d, J = 1.0 Hz, 3H), 3.26 (q, J = 6.8 Hz, 2H), 2.74 (t, J = 7.6 Hz, 2H), 1.78 (s, 3H). MS (ESI): m/z [M+H]+ Calcd for C23H23N3O4 406.17613, found 406.17551

RESULTS AND DISCUSSION From Figure 1a, dyads 1-7 are synthesized via routine coupling chemistry between the –NH and -CH2Br groups, respectively. The products were purified by silica column chromatography to give white powders and were characterized by 1H-NMR and HRMS (Supporting Information, SI). When dissolved in common organic solvents, such as acetone, tetrahydrofuran (THF), dichloromethane (DCM), none of the FQDs exhibits photoluminescence (PL) under a broadband UV lamp (λex = 254 and 365 nm). At 77 K, however, the PL of any of the dyads can be gradually “turned on” with increasing concentrations (Figure 1b, Figure S1). When inspected by


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UV-Vis spectroscopy in THF (Figure S2), the dyads absorption spectra appear to be a simple overlay of the carbazole (or melatonin) fluorophore and the cyanobenzoate quencher, indicating very weak interactions between the two components in solution. The absence of fluorescence emission for these FQD solutions is presumably due to the quenching by cyanzobenzoate moiety given that the fluorophores by themselves are highly photoluminescent.

Figure 1. a) Chemical structures of fluorophore-quencher dyads (FQDs) 1-7; b) photo showing the concentration-dependent behavior of FQD 4 in 2-methy-tetrahydrofuran at 77 K, concentrations from left to right: 1×10-5, 1×10-4 , 5×10-4, 1×10-3, 5×10-3, and 1×10-2 M; c) photo showing macroscopic photoluminescent wires of 2 formed by adding water (70%, v%) into THF (30%, v%) after six hours (scale bar: 1 cm) . When water is added to the THF solution of the dyads, turbidity could be observed beyond 70% water content in most cases. However, photoluminescence was not immediately observable; the suspension exhibited progressively stronger PL over time. The substituent on the carbazole fluorophore affects the time dependence of the PL (Figure S3): the bulkier the substituent group on the fluorophore moiety, the slower for the suspension to reach maximal PL intensity after the turbidity point. The photophysical properties of the dyad suspensions were investigated after 48 h, long after the solid morphologies and PL ceased to change. Under ambient light, FDQs 1-3 are distinct wire-like structures while the rest are foam-like. The fluorescence microscopic images are shown in Figure 2a, where the morphologies of these FQDs are wire-like in general (Figure


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S4) except for the benzyl-substituted FQD (6), which are needle-like crystals. For FQDs with increasingly longer alkoxyl substitution, the trend is manifested by the gradual decrease in the diameter of the wires. The hydrogen-substituted FQD (1) has the largest diameter (~10-30 µm), followed by the ethyloxy (2, ~5-20 µm, Figure 1c), n-butyloxyl (3, ~1-10 µm with mixed belt structures up to 20-µm wide), n-hexyloxy (4, 8000 ps at 498 nm

0.8

∆mOD

400

0.4

0.0

0.0 350

1

5

∆mOD

12

∆ mOD

3

1

350 400 450 500 550 600 650

e)

4

2

0

c)

τ = 3690 ps at 488 nm

5

∆mOD

∆ dOD

THF solution

∆ mOD

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400

450

500

550

600

650

Wavelength (nm)

0

2

4

6

8

τ (ns)

Figure 3. Transient absorption spectra (a, c and e) and kinetics (b, d, f) probed at the indicated wavelengths of 2 in THF (a, b), DMSO (c, d) and the solid state (e, f). The samples are pumped at 350 nm; the solid lines in b, d and f are the fits of the kinetics by exponential functions. To demonstrate the generality of the FQD design concept, we replaced the donor moiety from carbazole to melatonin (Figure 4a), a naturally occurring sleep hormone molecule that is rich in electrons. The absorption and PL spectra of FQD 7 are shown in Figure 4b: compared to 1-6, the emission maximum of 7 in the assembled state (Figure 4c) is red-shifted by ~20-40 nm,


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to 483 nm (Φ = 0.165), which is perhaps due to the stronger electron-donating ability of melatonin. Similarly, the measured lifetime of 7 at room-temperature in air also contains a fast component (114.50 ns) and a slow one (3.52 ms), which are assigned to CT fluorescence and TADF, respectively. At 77 K, the singlet-triplet energy difference estimated from Figure 4c is ~4.32 kcal/mol. It is also interesting to notice that the otherwise photo-labile melatonin is rather resistant to persistent UV irradiation in the dyad assembly, which indicates that the cyanobenzoate acceptor can suppress its photochemistry by stabilizing the n-π* state of melatonin.22

Figure 4. a) Chemical structure of melatonin-cyanobenzoate dyad 7 and fluorescence micrograph image produced from adding water to the THF solution ; b) solid-state absorption and steady-state PL emission; c) steady-state and delayed PL emission spectra at 77 K; d) Transient absorption decay at 498 nm excited at 350 nm.

CONCLUSION


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In conclusion, we have successfully constructed a series of fluorophore-quencher dyads (FQDs) which exhibit photoluminescence only when they formed ordered assemblies at the macroscopic scale. The photoluminescence (PL) properties of the FQD assemblies were investigated, where they show moderate to strong blue PL which contains a fast component and a slow one. The origins of the fast and slow emissions are ascribed to singlet charge-transfer (CT) and phosphorescence, respectively, based on both PL and transient absorption measurements. The choice of a quencher moiety with lesser electron-accepting strength renders the excited-state energy levels of CT and localized triplet states very close and significantly prolongs the apparent emissive lifetimes.

ASSOCIATED CONTENT Supporting Information. Materials and method, synthetic procedures, characterization data and

supporting tables and figures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author

*Prof. Xingyuan Zhang: [email protected] *Dr. Chuanyao Zhou: [email protected] *Prof. Guoqing Zhang: [email protected] ACKNOWLEDGMENT

We acknowledge support from the Fundamental Research Funds for the Central Universities (WK2340000068 to G. Z.), the National High Technology Research and Development Program


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