Macroscopic Wires from Fluorophore-Quencher Dyads with Long-Lived Blue Emission

Article abstract of DOI:10.1021/acs.jpca.7b08268

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 cyanobenzoa

Full text of DOI:10.1021/acs.jpca.7b08268

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Article
Macroscopic Wires from Fluorophore-
Quencher 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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Macroscopic Wires from FluorophoreꢀQuencher
Dyads with LongꢀLived Blue Emission
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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
1School 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 smallꢀmolecular fluorophoreꢀquencher dyads
(FQDs, average MW = 425 Da), comprised of carbazole, melatonin and cyanobenzoate moieties.
These FQDs are nonꢀemissive 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 chargeꢀtransfer,
localized triplet state, and possibly delayed fluorescence emissions with intrinsic luminescence
lifetimes ranging from 0.228 to 21333 ꢀs, based on luminescence, transient absorption
measurements, Xꢀray diffraction and calculations.
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KEYWORDS macroscopic assembly, blue luminescence, roomꢀtemperature phosphorescence,
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transient absorption, organic dyad
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INTRODUCTION
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Roomꢀtemperature phosphorescence (RTP) from purely organic molecules has gained
renewed interests in the past few years due to its potential application in environmentꢀfriendly,
lowꢀcost and highꢀefficient organic lightꢀemitting diodes (OLEDs).1ꢀ5 A key breakthrough is the
successful design and synthesis of organic molecules with nearly 100% thermallyꢀactivated
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 aggregationꢀcaused 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 aggregationꢀ
induced emission (AIE).8ꢀ11 Apart from application in OLEDs, organic RTP molecules also find
practical use as backgroundꢀfree imaging agents in biology,12ꢀ13 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 microsecondꢀlong RTP as nanoscopic assemblies.14 These
solidꢀstate emission from the fluorophoreꢀquencher dyads (FQDs) is evidenced to stem from an
almost entirely intermolecular chargeꢀtransfer (CT) state, i.e., excitation comprised of nonꢀ
overlapping 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
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a result, luminescence lifetimes may substantially increase from the
ꢀs range to ms. In the
current study, we present a class of cyanobenzoate FQDs (Scheme 1, left), where the donors are
alkoxylꢀsubstituted 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
40ꢀfold increase compared to that of the nitrobenzoate counterpart, and with lifetimes in the ꢀsꢀ
ms 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 nonꢀlinear optics,16 chemical sensing17 and displaying
technology.2
Scheme 1. Left: proposed energy diagram to produce photoluminescence (PL) involving
localized triplet state for prolonged excitedꢀstate lifetime; right: a lower 3CT state leads to redꢀ
shifted PL emission and reduced lifetime.
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EXPERIMENTAL
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Materials. Carbazole (CP, Aladdin Reagent Co.), 4ꢀhydroxycarbazole (98%, Energy Chemical
Reagent Co.), Iodoethane (99%, Energy Chemical Reagent Co.), 1ꢀBromohexane (99%, Energy
Chemical Reagent Co.), 1ꢀBromobutane (98%, Energy Chemical Reagent Co.), Benzyl bromide
(98%, Aladdin Reagent Co.), 1ꢀBromododecane (98%, Aladdin Reagent Co.), 2ꢀBromoethanol
(95%, Aladdin Reagent Co.), 3,4ꢀdihydroꢀ2Hꢀpyran (95%, Aldrich Reagent Co.), Sodium
hydride (60% dispersion in mineral oil, Energy Chemical Reagent Co.), 4ꢀCyanobenzoic acid
(98%, Aladdin Reagent Co.), pꢀtoluenesulfonic acid monohydrate (98.5%, Aladdin Reagent Co.),
Nꢀ(3ꢀDimethylaminopropyl)ꢀN’ꢀethylcarbodiimide hydrochloride (98.5%, Aladdin Reagent Co.),
4ꢀdimethylaminopyridine (99%, Aladdin Reagent Co.) were used as received. All other reagents
and solvents were purchased from Sinopharm Chemical Reagent Co. and used as received.
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Method. H 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. MALDIꢀTOF mass spectra (ESI) were collected
from Atouflex Speed mass spectrometer. UV/Vis absorption spectra were recorded on a Beijing
Persee TUꢀ1901 UVꢀVis spectrometer. UVꢀVisꢀNIR absorption spectra were recorded on a
SOLID3700 UVꢀVisꢀNIR spectrometer. Excitation and steadyꢀstate fluorescence emission
spectra and absolute quantum yield were conducted on a FluoroMaxꢀ4 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. 9Hꢀcarbazolꢀ4ꢀol (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 roundꢀbottom 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
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(petroleum/ethyl acetate = 9:1), yielding a white solid (3.25 g, 94%). H NMR (300MHz, d6ꢀ
DMSO), 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. 4ꢀethoxyꢀ9Hꢀcarbazole (2.5 g, 11.85
mmol), sodium hydride (60%, 0.95 g, 23.75 mmol) and 50ml DMF were added to a roundꢀ
bottom flask equipped with a magnetic stir bar. Then the solution was conducted at room
temperature for an hour. After Sodiumiodide and 2ꢀ(2ꢀbromoethoxy)tetrahydroꢀ2Hꢀpyran (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 pꢀtoluenesulfonic 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
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white solid (2.46 g, 81%). H NMR (300 MHz,d6ꢀDMSO) , δ(TMS, ppm): 8.19 (d, J = 7.7 Hz,
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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).
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Synthesis of 2-(4-ethoxy-9H-carbazol-9-yl)ethyl-4-cyanobenzoate. 2ꢀ(4ꢀethoxyꢀ9Hꢀcarbazolꢀ
9ꢀyl)ethanꢀ1ꢀol (1 g, 5.88 mmol), and 4ꢀcyanobenzoic acid (2.59 g, 17.64mmol) were added to a
roundꢀbottom 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
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solid (1.8 g, 80 %) . H NMR (300 MHz, d6ꢀDMSO) , 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 4ꢀethoxyꢀ
9Hꢀcarbazole. 1H NMR (300 MHz, d6ꢀDMSO) , 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), 1.70ꢀ1.51(m, 2H), 1.01 (t, J = 7.4 Hz,
3H).
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Synthesis of 4-hexyloxy-9H-carbazole. H NMR (300 MHz, d6ꢀDMSO) , δ(TMS, ppm): 11.24
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(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).
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Synthesis of 4-dodecyloxy-9H-carbazole. H NMR (300 MHz, d6ꢀDMSO), 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).
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Synthesis of 4-benzyloxy-9H-carbazole. H NMR (300 MHz, d6ꢀDMSO), 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 2ꢀ
(4ꢀethoxyꢀ9Hꢀcarbazolꢀ9ꢀyl)ethanꢀ1ꢀol. 1H NMR (300 MHz, d6ꢀDMSO) , 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).
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Synthesis of 2-(4-butoxy-9H-carbazol-9-yl)ethan-1-ol. H NMR (300 MHz, d6ꢀDMSO) ,
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, d6ꢀDMSO) ,
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).
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Synthesis of 2-(4-(dodecyloxy)-9H-carbazol-9-yl)ethan-1-ol. 1H NMR (300 MHz, d6ꢀDMSO) ,
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).
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Synthesis of 2-(4-(benzyloxy)-9H-carbazol-9-yl)ethan-1-ol. H NMR (300MHz, d6ꢀDMSO) ,
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).
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N-(2-(1-(2-hydroxyethyl)-5-methoxy-1H-indol-3-yl)ethyl)acetamide. H NMR (300 MHz, d6ꢀ
DMSO) , 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
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synthesis of 2ꢀ(4ꢀethoxyꢀ9Hꢀcarbazolꢀ9ꢀyl)ethyl 4ꢀcyanobenzoate. H NMR (300MHz, d6ꢀ
DMSO) , 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
3
4
5
6
363.1104, found 363.1113.
7
8
9
1
Synthesis of 2-(4-butoxy-9H-carbazol-9-yl)ethyl-4-cyanobenzoate. H NMR (300 MHz, d6ꢀ
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DMSO) , 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), 7.46ꢀ7.25 (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.
1
Synthesis of 2-(4-(hexyloxy)-9H-carbazol-9-yl)ethyl 4-cyanobenzoate. H NMR (300 MHz,
d6ꢀDMSO) , 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.
1
Synthesis of 2-(4-(benzyloxy)-9H-carbazol-9-yl)ethyl-4-cyanobenzoate. H NMR (300 MHz,
d6ꢀDMSO) , 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.
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2-(3-(2-acetamidoethyl)-5-methoxy-1H-indol-1-yl)ethyl-4-cyanobenzoate. 1H NMR (300
MHz, d6ꢀDMSO) , 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 1ꢀ7 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 1HꢀNMR 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 broadꢀ
band 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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UVꢀVis 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.
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Figure 1. a) Chemical structures of fluorophoreꢀquencher dyads (FQDs) 1ꢀ7; b) photo showing
the concentrationꢀdependent behavior of FQD 4 in 2ꢀmethyꢀtetrahydrofuran at 77 K,
concentrations from left to right: 1×10ꢀ5, 1×10ꢀ4 , 5×10ꢀ4, 1×10ꢀ3, 5×10ꢀ3, and 1×10ꢀ2 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 1ꢀ3 are
distinct wireꢀlike structures while the rest are foamꢀlike. The fluorescence microscopic images
are shown in Figure 2a, where the morphologies of these FQDs are wireꢀlike in general (Figure
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S4) except for the benzylꢀsubstituted FQD (6), which are needleꢀlike crystals. For FQDs with
increasingly longer alkoxyl substitution, the trend is manifested by the gradual decrease in the
diameter of the wires. The hydrogenꢀsubstituted FQD (1) has the largest diameter (~10ꢀ30 ꢀm),
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followed by the ethyloxy (2, ~5ꢀ20 ꢀm, Figure 1c), nꢀbutyloxyl (3, ~1ꢀ10 ꢀm with mixed belt
structures up to 20ꢀꢀm wide), nꢀhexyloxy (4, <1~3 ꢀm) and dodecyloxy (5, ~1ꢀ2 ꢀm)
counterparts. It has to be noted that the large aspect ratios of 4 and 5 have rendered these FQDs
prone to severe entangling, compared to 2 and 3, which are moderately intertwined.
The absorption and steadyꢀstate emission spectra of the collected FQD assemblies are
shown in Figure 2b with the individual chemical structure shown. At room temperature in air, all
PL spectra look similar and do not exhibit any vibronic progression; the emission maxima are
slightly different and vary from 446 to 466 nm. From the PL spectra, FQD 3 (nꢀbutoxy) exhibits
the most blueꢀshifted emission maximum (λem = 446 nm), followed by 6, 3, 2, 1, and finally 5
exhibits the most redꢀshifted PL. The shapes of the PL spectra are almost identical for all the
FQDs under vacuum. At 77 K vibronic features appear for FQDs 3 and 4. The PL lifetimes of the
FQD wires under different conditions are presented in Table 1. It has to be noted that the
measured PL lifetimes contain two different components with a short one in the nanosecond
range (τF = 35.14ꢀ72.50 ns) and a long one in the microsecond range (τDF = 0.932ꢀ134.80 ꢀs)
except for 1 and 2. At 77 K, however, substantially longer “afterꢀglows” with subꢀsecond long
lifetimes (0.2054ꢀ0.6586 s) could be recorded for all assemblies (Table 1 and Table S1).
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Figure 2. a) Fluorescence micrograph images of FQD (1ꢀ6) assemblies by adding water (70%, v)
into THF (30%, v); scale bar = 100 ꢀm. b) absorption (black dotted lines) and steadyꢀstate
emission spectra in air (black lines), vacuum (red lines) and at 77 K (blue lines) of the above
assemblies excited at 365 nm.
Table 1. Photoluminescence properties of fluorophoreꢀquencher dyads 1ꢀ6.
h
λair
λvacuum λ77k
λ77k
τF
τDF
τ77k
ΦPL
τF0
τDF0
(nm)a (nm)b (nm)c (nm)d (ns)e
(s)g
(ꢀs)f
(ꢀs)i
(ꢀs)j
462
461
460
461
469
458
512 84.64
504 57.70
ꢀ
ꢀ
0.2054 0.061 1.39
0.4035 0.253 0.228
ꢀ
ꢀ
1
2
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449
446
466
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454
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455
447
448
446
500 93.25 59.57 0.3108 0.390 0.239
504 35.14 18.23 0.6586 0.101 0.348
509 72.50 134.80 0.5253 0.037 1.96
152
180
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3643
507 46.02 0.931 0.4297 0.236 0.195 3.94
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a. steadyꢀstate emission maxima in air at room temperature (λex = 365 nm)
b. steadyꢀstate emission maxima under vacuum at room temperature (λex = 365 nm)
c. steadyꢀstate emission maxima at 77 K (λex = 365 nm)
d. delayed emission maxima at 77 K (λex = 365 nm)
e. preꢀexponent weightꢀaveraged lifetimes in air at room temperature (nanoLED at λex =
365 nm); nonꢀaveraged lifetimes are provided in Table S1 due to space constraint
f. preꢀexponent weightꢀaveraged lifetimes in air at room temperature (SpectraLED at λex =
365 nm) ; nonꢀaveraged lifetimes are provided in Table S1 due to space constraint
g. preꢀexponent weightꢀaveraged lifetimes at 77 K (excited with SpectraLED at λex = 365
nm) ; nonꢀaveraged lifetimes are provided in Table S1 due to space constraint
h. absolute luminescence quantum yields at room temperature in air
i. calculated intrinsic photoluminescence lifetimes for fluorescence in air at room
temperature
j. calculated intrinsic photoluminescence lifetimes for delayed fluorescence in air at room
temperature
Based on our design strategy (Scheme 1) and the spectroscopic data, we assign the shortꢀ
lived PL emission (subꢀꢀs to ꢀs) to the singlet intermolecular CT state (exciplexꢀlike) from
carbazole to cyanobenzoate (1CT) based on Xꢀray diffraction and calculations (Figure S5). We
ascribe the longerꢀlived residual as thermallyꢀactivated delayed fluorescence (TADF)6,7,18 from
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the reversible intersystem crossing (RISC) of localized carbazole triplet state (3πꢀπ*) to CT
5
6
1
3
(Scheme 1,). The energy difference between the CT and πꢀπ* states can be estimated by
measuring the steadyꢀstate and delayed PL spectra at 77 K (Figure S6), where the RISC may be
sufficiently blocked. The calculated singletꢀtriplet energy splittings for solidꢀstate FQDs 1ꢀ6 are
5.12, 5.70, 5.65, 7.23, 7.64 and 7.71 kcal/mol, respectively. Such small energy differences will
allow for efficient forward intersystem crossing to achieve triplet excitons; while the energy gaps
may still appear big for the RISC at room temperature, we suspect that many modes of
vibrational states, which further reduce the energy gap, can assist the backꢀpopulation process.
For 1, the delayed PL, or lowꢀtemperature phosphorescence (LTP), showed distinct vibrational
progression similar to localized carbazole triplet excited state; for 2ꢀ6, however, the vibrational
features of the LTP are absent likely due to the heteroatom (ꢀOR) participation of the π
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3
conjugation and increased density of states. It is noted that the emission maximum of the πꢀπ*
state (LTP) for each molecule varies slightly, possibly due to the influence of excitonic coupling,
19
which is common in the solid state. The situation is in contrast to a previous report from us,
where the quencher moiety is nitrobenzoate (NB) and a stronger electron acceptor (Scheme 1).
As a consequence, the CT emission has much lower energy (~560ꢀ620 nm) vs. the 3πꢀπ* state of
carbazole correspondingly, the lifetimes of NBꢀbased FQDs are on the order of microseconds
even at 77 K.
To further assist our claim, we also performed transient absorption (TA) measurements on
these dyads. Figure 3 shows the TA spectra and corresponding kinetics for 2 in in both solutions
(THF and DMSO) and the solid state. Starting at 350 fs (Figure 3a), only one peak at ~370 nm is
observed (the negative peak at 387 nm is likely due to the stimulated emission of carbazole). At
470 fs, the peak is shifted to a longer wavelength (~400 nm) and is further redꢀshifted to 425 nm
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at 910 fs and stops at 488 nm after 2 ps and starts to diminish after 8 ns, which is assigned as the
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CT band (S1). It is difficult to infer whether CT (T1) was involved given its close proximity in
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energy. The TA is similar in DMSO (Figure 3c) but decays much more rapidly when monitored
at 488 nm with a fitted lifetime of 194.7 ps (vs. 3.69 ns in THF, Figure 3b and 3d). Compared to
THF, DMSO has a larger dielectric constant value and thus stronger ability to stabilize charges.
Since the exciton is delocalized throughout the fluorophore and quencher, it is possible that the
faster kinetics is caused by better solvation of DMSO which generates ion pairs via electron
transfer and quenches the excited state.20 In the solid state (Figure 3e), however, a broad peak at
~400 nm is present in additional to the broad CT peak at ~500 nm, which is most likely to be the
carbazole triplet state.21 The intensities at the two peaks almost cease to change after the
measurement range of 8 ns with a fixed ratio (Figure 3f), indicating a state of equilibrium
between 3CT and 3πꢀπ*.
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a)
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0.91
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3030
5020
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τ = 3690 ps
at 488 nm
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THF solution
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τ
(ns)
Wavelength (nm)
c)
e)
d)
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4.85
35.4
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96.4
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τ = 194.7 ps
at 488 nm
DMSO solution
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400 450 500 550 600 650
τ
(ns)
Wavelength (nm)
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0.0
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0.32
0.44
0.58
709
2350
6790
solid state
τ > 8000 ps
at 498 nm
0
2
4
6
8
350 400 450 500 550 600 650
τ
(ns)
Wavelength (nm)
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
1ꢀ6, the emission maximum of 7 in the assembled state (Figure 4c) is redꢀshifted by ~20ꢀ40 nm,
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to 483 nm (
Φ
= 0.165), which is perhaps due to the stronger electronꢀdonating ability of
5
melatonin. Similarly, the measured lifetime of
7 at roomꢀtemperature in air also contains a fast
6
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component (114.50 ns) and a slow one (3.52 ms), which are assigned to CT fluorescence and
TADF, respectively. At 77 K, the singletꢀtriplet energy difference estimated from Figure 4c is
~4.32 kcal/mol. It is also interesting to notice that the otherwise photoꢀlabile melatonin is rather
resistant to persistent UV irradiation in the dyad assembly, which indicates that the
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cyanobenzoate acceptor can suppress its photochemistry by stabilizing the nꢀ
melatonin.22
π* state of
Figure 4. a) Chemical structure of melatoninꢀcyanobenzoate dyad
7 and fluorescence
micrograph image produced from adding water to the THF solution ; b) solidꢀstate absorption
and steadyꢀstate PL emission; c) steadyꢀstate 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 fluorophoreꢀquencher 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 chargeꢀtransfer (CT)
and phosphorescence, respectively, based on both PL and transient absorption measurements.
The choice of a quencher moiety with lesser electronꢀaccepting strength renders the excitedꢀstate
energy levels of CT and localized triplet states very close and significantly prolongs the apparent
emissive lifetimes.
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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: zxym@ustc.edu.cn
*Dr. Chuanyao Zhou: chuanyaozhou@dicp.ac.cn
*Prof. Guoqing Zhang: gzhang@ustc.edu.cn
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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of China (2015AA033903 to X. Z.) and the Youth Innovation Promotion Association of CAS
(2017224 to C. Z.).
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REFERENCES
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