Vikas S. Padalkar*a,
Daisuke Sakamakia,
Kenji Kuwadaa,
Norimitsu Tohnaib,
Tomoyuki Akutagawac,
Ken-ichi Sakaid and
Shu Seki*a
aDepartment of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto, 615-8510, Japan. E-mail: vikaspadalkar@gmail.com; seki@moleng.kyoto-u.ac.jp
bDepartment of Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
cInstitute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, 980-8577, Japan
dDepartment of Bio- & Material Photonics, Chitose Institute of Science and Technology, Chitose 066-8655, Japan
First published on 2nd March 2016
Two novel donor–π bridge–acceptor compounds containing excited state intramolecular proton transfer (ESIPT) and non-ESIPT units based on triphenylamine–benzothiazole were synthesized via Suzuki coupling reaction. Their photophysical properties were studied in the solid state as well as in solutions with solvents of different polarities. The fluorophores showed absorption in the UV region and emission in the visible region in both solutions and solid state. The optical properties of these compounds are highly dependent on solvent polarity. Significant positive solvatochromism (∼30 nm absorption and ∼80 nm emission red shift in polar solvents) were observed for both the compounds. Large Stokes shift (∼15000 cm−1), polarity sensitive optical properties and very high quantum efficiencies (∼90%) in solvents and the solid state are the striking features of the synthesized compounds. Intramolecular charge transfer (ICT) characteristics of the compounds were supported experimentally and computationally. Halochromism and the intermolecular charge transfer phenomenon were used for investigation of ESIPT emission for compound 9 over ICT emission.
Designing of donor–acceptor (D–A)/donor–π–acceptor (D–π–A)/acceptor–donor–acceptor (A–D–A) type of chromophore is more desirable approach for tuning the optical properties of the chromophores by controlling the strength of donor or/and acceptor and length of π-conjugated system.28,29 Plenty of chromophores have been reported in last two decade for potential applications based on these frameworks because of efficient intramolecular charge transfer properties of these types of molecules.30 Various donors (carbazole, aromatic amines, thiophenes, thienothiophenes, etc.) and acceptors (cyanoacrylic acids, azoles, diketopyrrolopyrrole, bithiazole, iso-indigo, benzothiadiazole, benzotriazole, and quinoxalines, etc.) have been explored for designing such chromophores.28–34 With appropriate choices of the donor and acceptors, the levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) as well as the emission color of the D–A, D–π–A, D–A–D, and A–D–A molecule can be tuned by chemical alteration.35,36 Among them, most functional chromophores typically feature a hydrophobic electron-donating triphenylamine (TPA) moiety connected to different electron accepting units through π-system.30
TPA is well known as an electron donating unit providing simultaneously hole-transporting pathways in luminescent material, however it suffers from the effect of ACQ in condensed state.37 Recently, significant efforts have been put on transformation of ACQ active TPA chromophores to AIE active chromophores having excellent luminescent properties in the aggregate and solid state10,38–40 by introducing AIEgens units. Benzothiazole (BT) is a heterocyclic unit with electron-rich sulfur and nitrogen atom having electron acceptor property. 2-Hydroxy analogue of BT is known for excited state intramolecular proton transfer (ESIPT) process.41 Large Stokes shift due to ESIPT emission (keto tautomer) without self-absorption is a unique property of ESIPT systems.42 This characteristic can be used as a new design strategy for producing fluorescent organic materials to reduce self-quenching using ESIPT properties.43 Considering the advantages of the combination of TPA and BT respectively as electron-donating and accepting units, in the present study we have synthesized two push-full chromophores and studied their photophysical properties systematically.
Yield after column chromatography: 52%, white solid.
1H-NMR (400 MHz, CDCl3, 20 °C): δ ppm 12.70 (s, 1H, –OH), 7.97–7.99 (d, J = 8.0 Hz, 1H), 7.89–7.91 (d, J = 8.0 Hz, 1H), 7.49–7.54 (m, 2H), 7.40–7.44 (m, 1H), 7.29 (d, J = 2.0 Hz, 1H), 7.10–7.07 (dd, J1 = 2.0 Hz, J2 = 8.0 Hz, 1H).
MALDI-TOF (m/z): calculated: 304.95, found: 305.92.
Yield after column chromatography: 58%, white solid.
1H-NMR (400 MHz, CDCl3, 20 °C): δ ppm 8.19–8.21 (d, J = 8.0 Hz, 1H), 8.06–8.08 (d, J = 8.0 Hz, 1H), 7.91–7.93 (d, J = 8.0 Hz, 1H), 7.49–7.54 (m, 2H), 7.39–7.44 (m, 2H), 2.48 (s, 3H).
MALDI-TOF (m/z): calculated: 348.21, found: 349.97.
Yield after column chromatography: 45%, buff white solid.
1H-NMR (400 MHz, CDCl3, 20 °C): δ ppm 7.60–7.65 (m, 2H), 7.21–7.23 (m, 2H), 7.02–7.14 (m, 7H), 4.09–4.15 (t, 8H), 2.00–2.06 (m, 4H).
MALDI-TOF (m/z): calculated: 413.08, found: 413.22.
Yield after column chromatography: 48%, yellow solid.
1H-NMR (400 MHz, CDCl3, 20 °C): δ ppm 8.35–8.37 (d, J = 8.0 Hz, 2H), 8.08–8.10 (d, J = 8.0 Hz, 2H), 7.92–7.94 (d, J = 8.0 Hz, 2H), 7.61–7.63 (d, J = 8.0 Hz, 2H), 7.53–7.58 (m, 4H), 7.49–7.51 (t, 2H), 7.45–7.46 (d, J = 2.4 Hz, 2H), 7.39–7.43 (t, 2H), 7.31–7.35 (t, 2H), 7.19–7.21 (m, 6H), 7.10–7.13 (t, 1H), 2.53 (s, 6H).
13C-NMR (100 MHz, CDCl3, 20 °C): δ ppm 169.24, 162.36, 153.05, 148.61, 147.72, 147.01, 143.87, 135.25, 133.12, 130.60, 129.53, 127.97, 126.34, 125.29, 125.19, 124.38, 124.30, 124.00, 123.86, 123.27, 121.52, 21.76.
MALDI-TOF (m/z): calculated: 779.19, found: 779.30.
Elemental analysis; mol. formula: C48H33N3O4S2 (actual: C: 73.92, H: 4.26, S: 8.22, N: 5.39; found: C: 73.86, H: 4.27, S: 7.94, N: 5.22).
Yield after column chromatography: 76%, yellow solid.
1H-NMR (400 MHz, CDCl3, 20 °C): δ ppm 12.56 (s, 1H, –OH), 8.35–8.37 (d, J = 8.0 Hz, 1H), 8.08–8.10 (d, J = 8.0 Hz, 1H), 7.99–8.01 (d, J = 8.0 Hz, 1H), 7.90–7.94 (t, 2H), 7.73–7.75 (d, J = 8.0 Hz, 1H), 7.49–7.63 (m, 8H), 7.46 (d, J = 12.0 Hz, 1H), 7.39–7.43 (t, 2H), 7.29–7.34 (m, 3H), 7.20–7.22 (m, 6H), 7.10–7.13 (t, 1H), 2.53 (s, 3H).
13C-NMR (100 MHz, CDCl3, 20 °C): δ ppm 169.25, 169.05, 162.38, 162.34, 158.21, 153.05, 151.90, 148.60, 147.77, 147.57, 147.05, 144.85, 143.90, 135.25, 133.90, 133.01, 132.55, 130.61, 129.52, 128.85, 127.95, 127.89, 126.70, 126.32, 125.45, 125.30, 125.19, 124.38, 124.28, 123.99, 123.95, 123.83, 123.27, 122.08, 121.52, 121.35, 117.98, 115.43, 115.29, 21.81.
MALDI-TOF (m/z): calculated: 737.89, found: 737.31.
Elemental analysis; mol. formula: C46H31N3O3S2 (actual: C: 74.87, H: 4.23, S: 8.69, N: 5.69; found: C: 74.67, H: 4.27, S: 8.22, N: 5.33).
Comps | Medium | λabsmax (nm) | ε (mol−1 dm3 cm−1) | λemmax (nm) | Stoke shift (nm) | Stoke shift (cm−1) | Quantum efficiency Φ (%) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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a Measured on thin film, spin-cast from (1 wt%) dichloromethane solution.b Measured from 10−5 M solution.c Absolute quantum yields in solid state.d Quantum yields measured by relative method using quinine sulphate standard (10−5 M).e Not calculated. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
8 | Solid filma | 405, 311 | e | 493 | 88, 182 | 4407, 11![]() |
21c | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Tolueneb | 389, 302 | 65![]() |
448 | 59, 146 | 3385, 10![]() |
86d | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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THFb | 389, 300 | 69![]() |
478 | 89, 178 | 4786, 12![]() |
85d | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Chloroformb | 390, 299 | 64![]() |
475 | 85, 176 | 4588, 12![]() |
88d | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Acetonitrileb | 385, 300 | 68![]() |
526 | 141, 226 | 6962, 14![]() |
75d | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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DMSOb | 395, 305 | 63![]() |
528 | 133, 223 | 6377, 13![]() |
78d | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
43![]() |
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9 | Solid filma | 409, 305 | e | 490 | 81, 185 | 4041, 12![]() |
21c | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Tolueneb | 394, 297 | 62![]() |
450 | 56, 153 | 3158, 11![]() |
75d | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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THFb | 393, 295 | 63![]() |
483 | 90, 188 | 4741, 13![]() |
78d | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Chloroformb | 394, 297 | 63![]() |
474 | 80, 177 | 4283, 12![]() |
80d | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
36![]() |
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Acetonitrileb | 388, 295 | 64![]() |
525 | 137, 230 | 6725, 14![]() |
67d | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
35![]() |
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DMSOb | 394, 302 | 56![]() |
533 | 139, 231 | 6618, 14![]() |
69d | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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UV-visible absorption spectra of compounds 8 and 9 are presented in Fig. 1 and S1.† Both compounds have the same donor and acceptor and differ only by the substitution at 2-position of the benzothiazole unit. Both the compounds show two absorption bands in near UV and visible region in various solvents between 270 to 450 nm. The absorption band located ∼300 nm can be assigned to π–π* electronic transition of the respective TPA and BT units, and ∼390 nm due to the transition of the conjugated TPA–BT backbone. Spectral peak positions of the absorption bands were almost unchanged for both the compounds in polar (acetonitrile and DMSO), non-polar (toluene and chloroform), and moderately polar (THF) solvents with small contribution from tailing of the lower transition energies. This indicates that there is little influence of solvent polarity on the ground state and a negligible charge transfer process between the TPA donor and BT acceptor in the ground state. In acetonitrile absorption maxima was 385 and 388 nm for compound 8 and 9 respectively, while slightly red shifted absorption was observed in other studied solvents. In solid state, compounds 8 and 9 showed absorption maxima at 405 and 409 nm respectively which were ∼20 nm red shifted absorptions as compared to absorption maxima in acetonitrile (Fig. S2†).
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Fig. 1 Steady state absorption spectra of compounds: 8 (left) and 9 (right) in different solvents (10−5 M concentration) at room temperature. |
Interestingly, the emission properties of the compounds are dependent on solvent polarity (Fig. 2). In all the studied solvents both the compounds showed single emission maxima with very large Stokes shift. Both compounds show positive solvatochromism effect, indicating the significant dipole moment change in the excited state. In toluene compounds emitted ∼450 nm, while in DMSO it was ∼530 nm which is ∼80 nm red shifted (Fig. 3), suggesting intramolecular charge transfer emission for these compounds in solution.45
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Fig. 2 Steady state emission spectra of compounds 8 (left) and 9 (right) in different solvents (10−5 M concentration) at room temperature (λex: 380 nm). |
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Fig. 3 Normalised emission spectra of compounds 8 (left) and 9 (right) in different solvents (10−5 M concentration) at room temperature (λex: 380 nm). |
As compound 9 contains ESIPT unit, it was expected to have different emission pattern than compound 8. However, the emission patterns of the compounds 9 and 8 are almost identical. In non-polar solvents (toluene and chloroform), 9 showed emission maxima at 450 and 474 nm, red-shifting gradually at 483 nm in moderately polar solvent (THF) and at 525 and 533 nm in polar solvents (acetonitrile and DMSO). These emissions are due to cis-enol emission or cis-keto emission (ESIPT) or intramolecular charge transfer emissions? The identical emission pattern of the ESIPT compound 9 and non-ESIPT compound 8 suggests that the emissions of the compound 9 would be due to ICT emission but not due to ESIPT process. Emission spectra of compound 9 in different solvents indicate that intramolecular charge transfer is dominating over ESIPT process. To understand the emission properties of the compound 9, systematic studies have been performed to control ICT process over ESIPT. Initially, considering difference in basicity of amine nitrogen and thiazole nitrogen, effect of acidity on absorption and emission properties were studied by assuming stepwise protonation mechanism of nitrogen atoms (i.e. protonation of TPA nitrogen first which would block the intramolecular charge transfer (ICT) and ESIPT process would be expected). However, the absorption and emission spectra of compound 9 in presence of different equivalent of TFA indicates that protonation occur simultaneously (Fig. 4) instead of stepwise. In absence of TFA, 9 showed absorption bands at ∼290 and ∼390 nm, while upon increase in concentration of TFA the intensity of 290 nm band slightly decrease with red shift and intensity of 394 nm significantly decreases with blue shift in absorption. Appearance of new band with gradual increase of TFA was observed at 490 nm with isosbestic point at 353 and 425 nm with 394 nm absorption band. New absorption peak at 490 nm can be assigned to protonated species of compound 9 (Scheme S1†).
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Fig. 4 (Left) Absorption spectra, (right) emission spectra of compound 9 in presences of various eq. of TFA; (10−5 M concentration, room temperature); λex: 380 nm. |
In case of emission spectra, the fluorescence intensity decreases gradually (Fig. 4) upon addition of TFA and compound 9 became non fluorescent at high concentration of TFA without piece of information about presence of ESIPT process (Fig. S3†). To have more understanding of ICT versus ESIPT emission in compound 9 another control experiment was performed by addition of electron acceptor (DDQ) compound which would participate in intermolecular charge transfer between TPA and DDQ and will control the intramolecular charge process. TPA is electron rich unit and DDQ is more electron deficient unit. Charge complexes can be formed by combinations of particular donor with acceptor.46 UV-visible and fluorescence spectra were recorded for compound 9 in presence of various amount of DDQ (Fig. S4†). The absorption, emission intensity and wavelength of compound 9 remain same even after addition of 10 eq. of DDQ. These results conclude that intermolecular charge transfer process is not occurring between TPA and DDQ and observed emission is due to intermolecular charge transfer from TPA to BT unit. ICT or ESIPT emission for 9 is still under investigation.‡
Both the compounds are highly emissive in solvents of different polarity with very high fluorescence quantum efficiencies. In non-polar (Φf: 86–88% and Φf: 75–80% for 8 and 9 resp.) and moderately polar (Φf: 85% for 8 and Φf: 75% for 9) solvents compounds show high fluorescence quantum efficiencies in comparison to polar solvents (Φf: 75–78% for 8 and Φf: 67–69% for 9) indicating a better stabilization of the emitting species in non-polar or THF solvents. The slight quenching of quantum efficiencies in polar solvents may be due to slight twisting of the donor acceptor units in the excited state due to high solvent polarity which is a common characteristic of the molecules exhibiting high dipole moment in the excited state.47
Similar to emission properties in the solvents, compounds 8 and 9 are also emissive in the solid state. The emission spectra of the compounds 8 and 9 in the solid state are shown in Fig. 5 and S5.† In solid state compounds 8 and 9 emit at 493 and 490 nm respectively, which is red shifted emission in comparison to non-polar solvents and blue shifted in comparison to polar solvents. The fluorescence images of the compounds 8 and 9 in solution, solid state and in film state are shown in Fig. 6.
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Fig. 7 Normalised absorption spectra of compounds 8 (left) and 9 (right) in THF and THF–water mixture (10−5 M concentration, room temperature, water fraction (vol%)). |
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Fig. 8 Emission spectra of compounds 8 (left) and 9 (right) in THF and THF–water mixture (10−5 M concentration, room temperature, λex: 380 nm, water fraction (vol%)). |
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Fig. 9 Normalised emission spectra of compounds 8 (left) and 9 (right) in THF and THF–water mixture (10−5 M concentration, room temperature, λex: 380 nm, water fraction (vol%)). |
This blue shifted emission with increase in the fluorescence intensity is assigned to formation of aggregates by suppression of ICT effect.51 Compounds 8 and 9 exist in aggregate state when the water percentage is more than 60% (fw ≥ 60%). In this state, ICT process cannot be stabilized due to hydrophobic environment generated in the aggregate state which suppresses the ICT process.50 The fluorescence intensity is high at 80% water fraction, but it again decreases in 90 and 95% water (Fig. 10). This can be assigned to formation of more nanocrystalline particles at 80% water fraction and in random agglomerate at 90 and 95% water fraction. This change in fluorescence behavior is due to the change in the packing mode of molecules in the aggregates.52 At 80% water fraction, the molecules may steadily assemble in an ordered fashion to form more emissive crystalline aggregates. In the mixture with 90–95% water fraction, however, the molecules may quickly agglomerate in a random way to form less emissive smaller (amorphous) particles. Amorphous particles cause lowering of fluorescence intensity while crystalline particles leads to enhancement.52 This observation concludes that both the compounds are AIE active. In aggregate state, ∼20 nm red shift in absorption was observed due to formation of J-type of aggregates which is a typical characteristic of J-aggregation10–12 (Fig. 7). In order to confirm type of molecular packing for compounds 8 and 9 many attempts were performed to develop single crystal in different solvents/mixture of solvents but we were unable to obtain single crystals suitable for X-ray analysis. However, optimized structures by DFT computations to support this observation are discussed in next section. The phase transition from crystalline to amorphous in aggregate state may be the probable reason for quenching of fluorescence quantum efficiencies in the solid state. The amorphous nature of the compounds in solid state is confirmed by powder-XRD measurement Fig. 11 and S7.† The powder XRD result concludes that compound 9 is comparatively more amorphous in nature than compound 8.
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Fig. 12 DSC heating and cooling cycles for compounds 8 and 9 (left) first cycle; (right) second cycle. |
Optimized geometries of both the compounds are shown in Fig. S9.† Both the compounds adopt twisted conformations. The torsion between any two phenyl rings in TPA core is ∼40°. Such twisted and non-planar conformations would hamper the strong π–π stacking which enable to emit the molecules in the condensed state efficiently. The TPA core and benzothiazole core of both the molecules are slightly twisted (θ ∼ 30°), which makes the chromophores moderately conjugated, which results in efficient intramolecular charge transfer process within the system. In short, twisted co-planar conformations make these molecules emissive in solid and aggregated states. Compounds 8 and 9 contain two benzothiazole units attached to TPA core. The orbital distribution of HOMOs and LUMOs conclude that transfer of charge density from TPA to BT units takes place simultaneously. Compound 8 is symmetrical V-shaped framework contain same acceptor groups on TPA core, hence charge transfer probability is equal for both the side as confirmed by Mulliken charge distribution (Fig. S10†). However, in case of compound 9, acceptors are different. As confirmed by HOMO and LUMOs, charge transfer takes place for both the side but may not be in equal probability. Mulliken charge distribution also suggest that BT unit with –OH group at 2 position is comparatively strong acceptor (N: −0.617|e|) than BT unit with –OCOCH3 group (N: −0.505|e|) (Fig. S10†).
Footnotes |
† Electronic supplementary information (ESI) available: Optical properties, powder-XRD data, DSC data and DFT optimised structures of compounds 8 and 9. See DOI: 10.1039/c6ra02417c |
‡ Investigation of intramolecular charge transfer emission over ESIPT emission is still under study for compound 9. |
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