Optical and Structural Properties of ESIPT Inspired HBTâFluorene

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Optical and Structural Properties of ESIPT Inspired HBTFluorene Molecular Aggregates and Liquid Crystals Vikas S Padalkar, Yusuke Tsutsui, Tsuneaki Sakurai, Daisuke Sakamaki, Norimitsu Tohnai, Kenichi Kato, Masaki Takata, Tomoyuki Akutagawa, Ken-ichi Sakai, and Shu Seki J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b08073 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 27, 2017

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The Journal of Physical Chemistry

Optical and Structural Properties of ESIPT Inspired HBT− HBT−Fluorene Molecular Aggregates and Liquid Crystals a

a

a

a

b

c

Vikas S. Padalkar, Yusuke Tsutsui, Tsuneaki Sakurai,* Daisuke Sakamaki, Norimitsu Tohnai, Kenichi Kato, Masaki Takac d e a ta, Tomoyuki Akutagawa,* Ken-ichi Sakai, and Shu Seki* a

Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan. b

Department of Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan.

c

RIKEN SPring-8 Center, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan.

d

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan.

e

Department of Bio- & Material Photonics, Chitose Institute of Science and Technology, Chitose 066-8655, Japan.

ABSTRACT: In bulk materials, positional isomers not only help in understanding how slight difference in molecular structure alters the crystal packing and optical properties, but also play a key role in developing new type of materials for functional applications. A detailed study on the photophysical properties of fluorene–HBT positional isomers in solution and in the solid state providing a molecular level understanding of the factors which influence fluorescence behavior is reported. Two molecules Ia and IIa were synthesized by Suzuki coupling reaction and their photophysical properties were compared to positional isomers Ib and IIb. Crystal structure analyses and density functional theory (DFT) computation studies were performed to understand structure– properties relation and the results reveal that changing substitution pattern has a marked influence on their packing modes and luminescence properties. Strong noncovalent interactions (π–π) in the solid state hamper the excited state intramolecular proton transfer (ESIPT) process which causes fluorescence quenching in the solid state (Ia and IIa = Φf: 28–40%; Ib and IIb = Φf: 55–67%). Compounds show solvent–responsive and aggregation induced emission (AIE) fluorescence properties. Bent structure of Ia with double and symmetric substitution of ESIPT motifs exhibit particularly unique condensed phase upon heating, confirmed as a nematic liquid crystalline phase, and this is the first report on the ESIPT and AIE active liquid crystalline materials with a bananashaped molecule.

In π-conjugated chromophores, solid state emission properties are switchable/tunable by controlling the mode of molecular packing7. Different factors such as molecular framework/conformation13, molecular configuration14, hydrogen bonding15, external stimuli16, noncovalent intermolecular interactions17, and others determine the molecular packing. Understanding the molecular packing based on these factors and relation with optical properties is still challenging and meaningful. Novel solid-state emissive materials would be obtained by (i) alteration of molecular packing of known chromophores by physical/chemical change (ii) development of new fluorophores. Due to known synthesis process, optical and structural properties, the former has been recognized as a more promising way to tune the solid-state emission properties. In recent years, tuning of solid state emission by polymorphism18 and by positional isomers14,19–21 have been attracting more attention. However, in viewpoint of molecular packing and reproducibility, polymorphism dependent emission is still difficult and challenging22. In positional isomer, the positions of functional group or substituents are different

INTRODUCTION Solid state emissive materials have been an interesting research topic in recent years due to their potential applications in organic laser1, data storage2, WOLEDs3, photoswitch4, organic electronics5, large-area flexible display6 etc. Commonly these materials are used as solid state for above applications. Unfortunately, most of the organic chromophores show fluorescence quenching in solid state against the solution7 due to the excitonic coupling and formation of excimer and exciplexes8. In 2001 Tang et al. reported a new strategy for solid state emission by aggregation method9. Various approaches such as restriction of intramolecular rotation (RIR), restriction of twisted intramolecular charge transfer (RTICT), restriction of intramolecular vibration (RIV), restriction of intramolecular motion (RIM), formation of J-aggregates over H-aggregates, intramolecular planarization, via excited state intramolecular proton transfer (ESIPT), restriction of photo-isomerization and photo–cyclization have been proposed for AIE phenomena10–12. However, none of them can be used universally. 1

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with same atomic composition. This small change in molecular structure makes a big difference in the crystal stacking, electronic structure, and optical properties14,19–21 and, thus would be a great help to researchers for tuning/switching the solid state emission properties. ESIPT (excited state intramolecular proton transfer) is an extremely fast photo-tautomerization process13,23–26. It is a four level photo−cyclic process (Enol→ Enol*→ Keto*→ Keto) mediated by intramolecular hydrogen bonding occurring upon photoexcitation13,23–26. Multiple emissions and large Stokes shift without self absorption27 are the striking features of the ESIPT chromophores. These unique photophysical properties make this process more interesting for practical applications such as photo-stabilizers, laser dyes, and luminescent materials for molecular probes28,13,23–26. Various types of chromophores have been reported for above maintained applications13,23–26. However, solid-state emissive ESIPT chromophores with very high quantum efficiencies are not explored extensively. Recently, ESIPT chromophores with AIE29 and AIEE30 have been reported by researchers31,32. AIE with ESIPT would definitely be a desirable approach to obtain desirable emissive properties in solid state, which was confirmed by reported AIE-ESIPT systems31,32. However, optical and structural properties of AIE-ESIPT active positional isomers are not reported yet. Fluorene derivatives are widely used as electroluminescent materials33. The emission properties of fluorene are easily tunable by altering the substitution pattern at 7,7′ or 9,9 position34. Considering these advantages of fluorene unit, we have chosen this fluorene unit for study. ESIPT unit helps to achieve red shifted solid state emission without selfabsorption35. Fluorene–HBT aromatic units coupled each other through single bond to avoid intramolecular rotation32. Two HBT units were introduced at 7,7′-position to obtain appropriate molecular stacking for solid state emission (Jaggregation)28 and alkyl groups at 9,9-position for solubility in solvents and for steric hindrance to avoid perfect π–π stacking. The photophysical and structural properties of synthesized chromophores Ia and IIa were studied in solution, aggregate and solid states. The results of optical and structural properties were compared with previously published positional isomers36 Ib and IIb from our group to understand the molecular packing–optical properties relationship. The structures of the positional isomers are summarized in Figure 1. Optical properties of analogues of IIa with one or two methyl group (IIa0 and IIa1) are also studied (Figure 2) to confirm the role of hydroxy groups on photophysical properties and molecular packing in the condensed phases.

Figure 1. Structures of Ia and IIa and their positional isomers Ib and IIb.

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Figure 2. Structures of non-ESIPT and ESIPT compounds.

EXPERIMENTAL METHODS The details about materials, methods and characterizations techniques used are summarized in the Supporting Information. RESULTS AND DISCUSSION Design and Synthesis of Compounds Compounds Ia and IIa were synthesized by a similar procedure as followed for compounds Ib and IIb36 (Scheme S1) and compounds IIa0 and IIa1 were prepared by Suzuki coupling between boronic acid and methylated HBT followed by demethylation reaction (Scheme S2). The intermediate boronic esters (2a and 2b) of fluorene were prepared from corresponding dibromo compounds (1a and 1b) by lithiation reaction (n-BuLi, –78°C) followed by substitution of trimethylborate and 1,3-propanediol sequentially. Phosphorus trichloride catalyzed condensation of 4-bromo-2-hydroxybenzoic acid (4) and o-aminothiophenol (3) followed by air oxidation reactions gave white color 2(2′−hydroxy) benzothiazole (HBT) (5) in good yield37. NaH mediated methylation reaction of intermediate 5 gave intermediate 6. Furthermore, boronic esters (2a and 2b) or boronic acid 7 were coupled with HBT (5) or methylated HBT 6 via Suzuki coupling using Pd(PPh3)4 catalyst under basic medium to obtain Ia, IIa and IIa0 in 42, 40 and 74 % yields, respectively. Compound IIa0 on controlled demethylation reaction using BBr3 yields mixture of monohydroxy (IIa1) and dihydroxy compounds (IIa). All the compounds have good solubility in nonpolar solvents as a result they could be easily well purified by chromatography techniques for spectral and optical studies. All the intermediates and target compounds were characterized by NMR and MS spectral analyses and results are in well accordance as expected (Synthesis Schemes S1 & S2, & spectral details: see the Supporting Information).

Figure 3. Normalized absorption spectra of (a) Ia and (b) IIa in –5 different solvents at room temp (10 M conc).

36

Steady State Measurements Absorption, emission and fluorescence quantum efficiencies of the compounds Ia and IIa were investigated in solvents of 2


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different polarities and in solid state. Both the compounds are soluble in organic solvents due to the presence of alkyl chains at 9,9-position of fluorene unit. Compounds (Ia and IIa) have same core structure and differ only by alkyl chain attached to fluorene unit. UV−vis absorption spectra of the compounds in solvents are shown in Figure 3 and Figure S2 and the corresponding data are summarized in Table 1. The absorption spectra of the compounds Ia and IIa in different solvents is between 374–382 nm and 370–382 nm, respectively which can be assigned to π−π* transition of fluorene– HBT backbone similar to HBT unit. In polar and nonpolar solvents absorption spectra of these compounds show slight change38. In nonpolar solvents (toluene and chloroform) absorption maxima is slightly red shifted (~382 nm) in comparison to absorption maxima (~375 nm) in polar solvents (acetonitrile and DMF). The absorption bands of Ia and IIa are significantly red shifted (~50 nm) in comparison to their positional isomers Ib and IIb (Table 1). This larger spectral red shifts can be ascribed to better conjugation in Ia and IIa 39. The appearance of shoulder peaks only in DMF suggests that Ia and IIa would be partially deprotonated, which has been confirmed by the deprotonation reaction of the compounds in DMF by NaH (Figure S3)38. In comparison with absorption spectra, the emission spectra of both the compounds showed significant response with solvent polarity (Figure 4 and Figure S4). The compounds showed dual (or triple) emissions in studied solvents. The detailed mechanism of ESIPT giving dual emission from HBTbased molecules has been well discussed in terms of kinetic traces of fluorescence by Chou, et al.40,41, proposing the precursor–successor type reaction patterns. Solvent polarity in the system mainly controls the barrier between the states, leading to the relative changes in the intensities of the dual emissions. In nonpolar solvents with the lower barrier between the two states, the compounds showed intense emission band ~510 and ~515 nm (cis-keto*) in chloroform and toluene, respectively along with shoulder bands ~420 and ~440 nm (cis-enol*), shifting more into cis-keto* forms in their equilibrium38,42. Isomeric species of cis-enol/cis-keto* form might be responsible to the third emission in nonpolar solvents along with cis-enol* and cis-keto* emission38. For the former two emission bands, In acetonitrile and DMF, emission band ~440 nm can be assigned to protonated cisenol/trans-enol/trans-keto*38. In our previous report, single emission band ~552 nm was reported for positional isomers Ib and IIb in non-polar solvents which is ~30 nm red shifted in comparison to Ia and IIa. However, short wavelength emission bands do not have significant differences in emission maxima for all compounds. The Stokes shifts of the Ia and IIa are comparatively less (≤ 7,500 cm–1) in different solvents in comparison to their positional isomers Ib and IIb (≤ 12,000 cm–1). The fluorescence quantum efficiencies of the compounds Ia and IIa are 3–4 fold higher in nonpolar solvents (Φf = 13%) in comparison to DMF solvent (Φf = 2−4 %) (Table 1). This indicates better stabilization of excited state in non-polar solvents. Quenching of fluorescence in DMF may be due to partial twisting between fluorene and HBT units in the excited state that is common characteristic of the molecules having large dipole moment in the excited state.

Figure 4. Steady state emission spectra of (a) Ia and (b) IIa in –5 different solvents at room temperature (10 M concentration), λex = 375 nm.

Similar to solid state emission properties of positional isomers Ib (Φf = 67%) and IIb (Φf = 55%)36, compounds Ia and IIa are also strongly emissive in the solid state. The photographs of solid-state fluorescence behaviors of the compounds Ia and IIa and their positional isomers36 as well as the emission spectra of Ia and IIa are shown in Figure 5. In solid state, both the compounds show dual emissions, short wavelength emission ~455 nm (cis-enol*) and long wavelength (cis-keto*) ESIPT emission ~520 nm. However, single emission was reported for positional isomers Ib and IIb. This clearly indicates that in case of positional isomer the strong intramolecular hydrogen bonding stabilizes cis-enol form with maximum populations at ground state, upon photoexcitation cis-enol tautomarises to cis-keto* form resulting in single long wavelength emission. In case of Ia and IIa along long wavelength emission, short wavelength emission was observed because cis-enol* species does not tautomarizes completely to cis-keto* form. This may be due to strong noncovalent intermolecular interactions. The absorption and emission spectra of the compounds in solid state and in solution are almost identical, whereas traditional donor–acceptor systems show aggregation caused quenching (ACQ) problem and red shift in the solid state43. The fluorescence quantum efficiencies of the compounds Ia (Φf = 40%) and IIa (Φf = 27 %) are 4−10 fold higher in solid state in comparison to studied solvents (Φf = 2−13 %). The quenching of the fluorescence efficiencies in solution can be assigned to intramolecular rotation, intermolecular hydrogen bonding or conformational changes occurring in solvents which hamper the ESIPT process13,28. However, in rigid media these factors are not dominant over ESIPT process due to physical constraint13,28. The lifetime of the emission has been also estimated as 2.01 and 1.40 ns respectively for Ia and IIa (Figure S5), suggesting short enough ones typically observed for ESIPT systems, and thus in studied molecules fluorescence emission is assigned to ESIPT process. In positional isomers the fluorescence efficiencies are much higher (Ib: Φf = 67% and IIb: Φf = 54%) than Ia (Φf = 40%) and IIa (Φf = 27 %). This behavior is may be due difference in molecular packing. In bulk material, molecular packing determines the optical properties. To have better understanding of variation of quantum efficiencies in solid state with positional isomers, aggregation induced emission studies were performed for Ia as an representative example and results were compared with reported AIE properties of positional isomer Ib36. 3



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concentration, room temperature, water fraction (vol%), λex = 375 nm).

Addition of water in THF solution of compounds induced not protonation but a molecular aggregation which is further confirmed by NMR study in THF-d8 and THF-TFA solvents (Figure S7). In aggregate state ~54 nm red shift with significant drop in absorption intensity was observed (Mie light scattering)44. This indicates the formation of J-type aggregates which is a typical characteristic of AIE active chromophore10–12. The formation of J-type aggregate was further supported by single crystal X-ray analyses. In case of emission spectra, short wavelength emission band showed ~20 nm red shift (λabs: from 417 nm to 430 nm), and emission band at 435 nm is shifted to 450 nm, but long wavelength emission band remains the same as emission band in pure THF. Gradual decrease of short wavelength emission intensity and gradual increase in long wavelength emission intensity was observed as the percentage of water increases from 10–60%, while significant enhancement of emission intensity of long wavelength was observed in aggregate state (≥ 70 %)45. In aggregate state, ~10 fold enhancements in quantum efficiency (Φf = 33%) were observed in comparison to quantum efficiency in pure THF (Φf = 3.4%). The fluorescence quantum efficiency in aggregate state is higher at 70% water fraction, but it again quenched in 80–95% water fraction (Φf = ~20 %) (Figure 7). This may be assigned to structural transition from more ordered nanoaggregates to random agglomerates46. In 70% water fraction, molecules might be assembled in well-ordered fashion to form crystalline species which are more emissive with characteristic optical absorption over 500 nm range. Corresponding planar aggregates are observed on graphite surfaces by AFM as seen in Figure S8, suggesting well packed nano-aggregate precursors presented specifically in these solvent mixtures. The further increase in water fraction up to 80–95% water fraction molecules might be quickly agglomerate in random way to form amorphous species which are less emissive as observed by AFM on the identical substrates with randomly-dispersed random nanoparticles47. Thus, AIE study clearly indicates that compound Ia is AIE active similar to reported positional isomer Ib. To have further depth about molecular packing and solid-state emission properties structural parameters were studied experimentally and theoretically.

Figure 5. (a) Day light and UV light images (λex = 365 nm) of Ia and IIa and their positional isomer Ib and IIb. (b) Normalized solid-state emission spectra of compounds Ia and IIa, λex = 375 nm.

Aggregation Induced Emission (AIE) Study AIE study of Ia was performed in various ratios of water/THF. The AIE data is summarized in Figure 6 and Table S1. In absence of water (100 % THF) Ia shows absorption at 382 nm and emission at 423, 442 and 518 nm. As percentage of water fraction increases from 10–60% the absorption maxima remain identical to pure THF (λabs = 382 nm). A dramatic red shift in the absorption spectra was observed (λabs: from 382 nm to 436 nm) when the water fraction is 70%, which would be related to the aggregate formation (i.e. transition from homogenous solution to the nanoaggregates phase (Figure S6).

Figure 6. (a) Steady state absorption spectra of Ia in THF and –5 THF/water mixture. (b) steady state emission spectra (10 M

Table 1: Optical properties of Ia and IIa and positional isomers Ib and IIb in solution and solid state Comps

Medium a

Solid Film Chloroform b Toluene b DMF b Acetonitrile a Solid Film b Chloroform b Toluene b DMF b

Ia

IIa

λ (nm) 380 383 382 381 377 379 382 381 370

ε (mol–1 dm3 cm– 1 ) e 122720 117240 93800 33650 e 116340 112740 65650

λ (nm)

Stokes shift (nm)

Stokes shift (cm–1)

442, 457, 520 418, 441, 510 420, 440, 517 417, 440, 510 417, 440, 515 455, 523 420, 440, 509 421, 445, 515 415, 438, 495

62, 77,140 35, 58, 127 38, 58,135 36, 59, 129 40, 63, 138 76, 144 38, 58, 127 40, 64, 134 55, 68, 125

3690, 4430, 7080 2180, 3430, 6500 2370, 3450, 6840 2260, 3520, 6640 2540, 3780, 7100 4410, 7260 2370, 3450, 6530 2490,3770, 6830 3500, 4120, 6820

4


Quantum efficiency Φ (%) c

40 12 d 13 d 4 f c 28 d 13 d 12 d 2

Emission Lifetime (ns) 2.01

d

0.95

1.40 0.93

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The Journal of Physical Chemistry b

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Acetonitrile Solid Film b Chloroform b Toluene b DMF

3050, 3690, 6990 f c 11436 67.2 d 11824 1.8 d 11701 1.5 g Ib d 5489 7.5 5460, 6774, b Acetonitrile 333 2300 407, 430, 543 74, 97, 210 f 11613 a c Solid Film 336 e 552 216 11645 54.6 b d Chloroform 334 81900 552 218 11824 1.7 b d Toluene 336 72200 551 215 11613 1.4 g IIb b d DMF 338 47200 405 67 4894 7.2 5641, 6955, b Acetonitrile 331 6230 407,430, 552 76, 99, 221 f 12095 a b –5 c Measured on thin film, spin-cast from (1 wt %) dichloromethane solution. Measured from 10 M solution. Absolute quantum yields d –5 e f in solid state. Quantum yields measured by relative methods using quinine sulphate standard (10 M concentration). Not calculated. g 36 Not measured, Literature data . a

377 338 334 335 338

59430 e 80400 69400 21900

426, 438, 512 551 552 551 415

49, 61, 135 213 218 216 77

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band at 440 nm in solution and solid states with a small Stokes shift (~4900 cm–1). However, in case of compound IIa1, observed emissions are long wavelength emission with large Stokes shift (8000 cm–1). The large stokes shift, dual, single broad or multiple emission are unique characteristics of ESIPT system. The result summarized in Table S2 clearly indicates that the emission properties of compound IIa1 is due to ESIPT. The emission spectra of compound IIa1 are similar to emission spectra of positional isomers Ia, IIa, Ib and IIb. The ESIPT phenomenon of compound IIa1 was further supported by aggregation induced emission study. It was expected that, compound IIa1 show AIE emission due to restriction of intramolecular rotation by forming strong intramolecular hydrogen bonding in the aggregated or solid states. i.e. formation of lock structure (Figure S10). Compound IIa0 shows small spectral shift at high fraction of water (Figure S9), which might be assigned to polarity effect and not due to AIE. In the absence of -OH (Intramolecular hydrogen bonding) this molecule has little chance to form lock structure like compounds IIa1 and IIa that could result in intramolecular rotation between thiazole and methoxy phenyl ring, which resulted in the quenching of the fluorescence. The AIE results of the compound IIa0 also shows lowering in the fluorescence quantum efficiency at higher percentage of water (Figures S11). In case of compounds IIa1 and IIa significant change in the emission spectra were observed in mixture of THF/water. In pure THF compounds IIa0 and IIa show dual emission ~420 nm (enol) and ~520 nm (keto) (Figure S9). Upon addition of water to the THF solution of IIa0 and IIa, the polarity of media was enhanced and was expected to favor normal emission due to stabilization of enol* species in polar medium, but the observations are contradictory to expected results. At high fraction of water keto emissions were significant for both the compounds. This uncommon observation could be assigned to planar nature of compounds IIa0 and IIa due to RIR in aggregated state. Aggregation effect caused restriction of intramolecular torsional motion and would help for planar conformation resulted in proton transfer from -OH to imine nitrogen and enhancement of the fluorescence quantum efficiency.

Figure 7. Plot of fluorescence quantum efficiency against fraction of water (vol%) in THF/water system.

Figure 8. Absorption spectra of (a) IIa0 and (b) IIa1 in different –5 solvents at room temperature (10 M conc.).

Figure 9. Steady state emission spectra of (a) IIa0 and (b) IIa1 in –5 different solvents and solid state at room temperature (10 M conc. for solution), λex = 370 nm.

Effects of Hydroxy Groups on AIE and ESIPT Behaviors To have clear understanding of the emission properties of fluorene–HBT based positional isomers, the optical properties of the non-ESIPT (IIa0) and ESIPT (IIa1) compounds were studied in solution and solid state in detail. The absorption and emission spectra of the compounds are presented in Figure 8 and Figure 9 respectively and data is summarized in Table S2 & Figure S9. Their absorption spectra are almost identical. The identical absorption spectra of the compounds in different solvents indicate that stable ground state of the compounds and absence of intramolecular charge process between fluorene and HBT unit. The absorption bands ~370 nm can be assigned to π–π* transition of fluorene–HBT backbone. The emission spectra of the compounds are completely different in studied solvents and solid state. Compound IIa0 shows an intense emission band at 420 nm with a shoulder

Liquid Crystalline States To understand phase transition at different temperatures, differential scanning colorimetric (DSC) measurement was performed under nitrogen flow at 10 °C min–1 scanning rate. The DSC curves for two successive cycles are shown in Figure 10 and Figure S12. Compound Ia displays a couple of endothermic transition peaks at 250°C and 192°C upon cooling, as well as the peaks observed for IIa at 196°C and 180°C, respectively. The peaks at the higher temperatures are corresponding to melting point, and there is no significant phase transition below the lower peaks. The crystalline nature of compounds was confined by powder-XRD experiment (Figure S13) around room temperature. To characterize the condensed phases of Ia and IIa at the higher temperature, the crossed polarized optical microscope images (POM) of Ia and IIa were recorded during the cooling processes, where the separation 6


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of the set of thermal transition peaks are larger, suggesting the clear texture of liquid crystalline (LC) phases observed for both compounds. The textures below the lower peaks show typical ones in crystalline phases, thus the upper peaks are assigned to clearing points, and the lower to crystalline-LC mesophase transitions (melting point). It is noteworthy that heating/cooling cycles of IIa0 and IIa1 gave no LC phase transition until their melting points as seen in the corresponding DSC traces with a single endothermic peak (Figure S14). Hampering the intramolecular hydrogen bondings, at least, of one end causes dramatic change of disappearing the LC phases, suggesting the strong demand for the planar structure of the molecules, and hence locking into the banana-shape of the molecule,48–50 to present their LC mesophases. Isotropic liquid phase was observed above the peak. In the LC mesophase temperature regions, characteristic optical textures, different from those in the Cr phases, were observed (Figure 10). In particular, compound Ia with a wide temperature-range mesophase showed a Schlieren texture that is typical of nematic phase51–53. In fact, variable-temperature XRD measurements revealed that the mesophase of Ia did not show any sharp diffraction peaks (Figure S15). Together with the characteristic textures as well as the relatively small transition enthalpy of mesophase-isotropic (∆Hm-iso, ∆Hm-iso= 3.3 kJ mol–1 in Ia and = 1.7 kJ mol–1 in IIa) to those of Cr-meso (∆Hcr-m, ∆Hcr-m = 103 kJ mol–1 in Ia and = 80 kJ mol–1 in IIa), the mesophases were assigned as nematic LC phases. The isotropic phase transition enthalpy (∆Hcr-iso) in IIa0 and IIa1 are also observed as 21 kJ mol–1 and 12 kJ mol–1 (on 2nd heating), respectively, which are apparently smaller than those of IIa exhibiting nematic LC phases. Rotational motion in these intra-molecular hydrogen bonding hampered the reserve of ∆Hcr-iso in part, which is consistent with the strong demand for the planar structure preserved by the ESIPT motifs. Although the presence of LC phases has been reported for planar molecules with ESIPT motifs giving discotic LC phase54, this is the first example of nematic LC phases of banana-shaped ESIPT molecules that are feasible to control molecular alignment by external stimuli such as electric field.

Figure 10. DSC curves of Ia and IIa (left) on second heating and –1 cooling cycle at 10 °C min . Cr and N denote crystalline and nematic liquid crystalline phases, respectively. Right Panels: Crossed polarized optical microscopic images (POM) and optical ones (OM) of Ia at 249°C (N) and 90°C (Cr), and of IIa at 190°C (N) and 180°C (Cr).

The emissive LC phase of the compounds was also characterized by the photoemission spectroscopy under the variable temperature ranging from 80°C to over 250°C as shown in Figure 11. The emission from both keto and enol forms are suppressed above the respective melting points. The suppression is rather remarkable in the emission from enol form, suggesting the accelerated ESIPT processes under high temperature shifting the equilibrium into keto forms of the compounds at the excited states. However, the overall quantum efficiency declined over one order of magnitude lower than those in crystalline state, which is suggestive of significant contribution from indirect relaxation of excited energy via the enhanced thermal motion of the molecules. It is noteworthy that the emission intensity from LC phases were reduced down to 30–50% in comparison with those in crystalline phases but kept almost constant under elevating temperature up to the melting points particularly in Ia. This suggests the feasibility of Ia as emissive LC materials55,56, and emission polarization may be also under control by the molecular alignment in LC phases.

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Figure 11. Emission spectra of (a) Ia and (b) IIa upon cooling from 300°C (Ia) or 250°C (IIa) with the cooling rate of 5 °C upon excitation at 280 nm. Bluish, yellow-greenish, and reddish colors are indicative of crystalline, LC, and isotropic phases of respective compounds.

Crystal Structure Single crystal is most accurate and important tool for understanding the molecular packing and optical properties correlation. In order to have better understanding of fluorescence enhancement/quenching with change in structure skeleton, single crystal of Ia was developed in dichloromethane/hexane mixture for X-ray single crystallographic analysis and results were compared to crystal data of positional isomer Ib. Single crystal data of the compounds Ia and positional isomer Ib are summarized in Table S3. The single crystal structures of Ia and Ib are shown in Figure 12. As summarized in Table S3 compounds Ia and Ib crystallize in the triclinic system and have nearly identical cell densities. Both the compounds have coplanar conformation with slight twisting between HBT and fluorene unit (Figure 12). The dihedral angles between the terminal thiazole ring and hydroxyphenyl ring are ~2 and 5° which confirm the suitable geometry for ESIPT (Figure S16). The distance between acidic center and basic nitrogen O1−N1 and O2−N2 were 2.68 Å and 2.53 Å for compound Ia and 2.59 Å and 2.61 Å for compound Ib respectively (Figure S15), which indicates a favorable bond distance for intramolecular hydrogen bonding which results in ESIPT process. The torsion between HBT–fluorene units indicates that compound Ia is having more planar conformation (θ = 29° and 34°) in comparison to compound Ib (θ = 40°).

Figure 12. (a) X−ray single crystal of Ia. (b) X−ray single crystal of Ib (Hydrogen atoms are omitted for clarity. Thermal ellipsoids are set at 50% probability). (c) Interplanar distance between stacked molecules of Ia. (d) Interplanar distance between stacked molecules of Ib. (e) Stacking image of Ia, (f) stacking image of Ib.

This conformational change could be the key factor for molecular packing and fluorescence quantum efficiency. Both the crystals have antiparallel slip-stacking (Figure 12). In case of Ia, the fused thiazole part overlapped with the phenyl part of the stacked molecule with inter-planar distance of 3.31 Å suggesting strong π–π intermolecular interaction (Figure 12b). However, in case of Ib, only thiazole ring is overlapped with phenyl part of stacked molecule with inter-planar distance of 3.55 Å, phenyl ring fused with thiazole core is overlaid (Figure 12b). This result indicates that the molecular packing modes are significantly different in the positional isomers, but the conformation is nearly identical. Strong π–π intermolecular interactions and more stacked framework factors are assigned to fluorescence quenching for compound Ia 40% in comparison to positional isomer Ib in solid state 8


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The Journal of Physical Chemistry bond and it is directly proportional59,60. Torsion between fluorene and HBT unit were (C10−C11−C15−C14 and C3−C2−C29−C30) 35° and (C15−C14−C11−C12 and C33−C28−C2−C1) 37° at ground state for compounds Ia and Ib respectively which are in good line with single crystal data. In the S1 state, the computed torsions were 21° (both side) for Ia and 23° and 29° for Ib, implying that excited state of compounds are more planar than ground state and have suitable conformation for efficient conjugation between fluorene and HBT unit.

67%. Strong intermolecular stacking (π–π) and different noncovalent interactions (C…H–N, C…H–O) affect the intramolecular hydrogen bonding resulting in fluorescence quenching7,57. Crystal packing also suggests that stacking is less slipped in compound Ia in comparison to the compound Ib (Figure 12). Compounds Ia and Ib show substantial displacement along the long axis with significant π-overlap with head to tail arrangement of the stacked molecules that is characteristic of J-aggregation. This result also supports the conclusion of AIE study28, i.e. formation of J-type of aggregates. Electronic structure To understand the relation between the optical properties, electronic structures and structural parameters, density functional theory (DFT) computation studies were performed on Gaussian 0958. The ground and excited states (S1) of the compound were optimized using B3LYP functional and 6-31G(d,p) basis set. Small alkyl (methyl) chains at 9,9-position were considered for simplicity. The optimized structures of the ground state (cis-enol) and excited state (cis-enol*) are shown in Figure S17 and the frontier MO diagram is shown in Figure 13. The results summarized in Table S4 clearly indicate that hydrogen transfer occurs at the excited state for both the compounds. In ground and excited states, the calculated torsion between thiazole and hydroxyphenyl rings is ~0° which indicates the planar nature of HBT unit suitable for ESIPT process. This observation also supports the experimental results about planarity of HBT units obtained by single crystal data. In the ground state, bond length between thiazole nitrogen and phenolic hydrogen (N72−H56, N73−H63 and N74−H57, N75−H65) was found to be 1.753Å and 1.733Å for compound Ia and Ib respectively (Figure S17). At the same time the bond length between acidic hydrogen and oxygen (O27−H56, O41−H63 and O27−H57, O41−H65) was 0.992Å for both the compounds. At the excited state, significant change in bond lengths were observed for both the compounds. The N−H bond lengths (N72−H56, N73−H63 and N74−H57, N75−H65) decreased to 1.721 Å and 1.720 Å for Ia and 1.611 Å and 1.706 Å for compound Ib, while O−H bond lengths (O27−H56, O41−H63 and O27−H57, O41−H65) extended to 0.994 for Ia and 1.022 Å and 0.998 Å for Ib. In addition, the bond lengths between thiazole and hydroxyphenyl ring and N=C− also show significant change in the excited state. Lengthening of O−H bonds and shortening of N−H bonds in the excited state clearly suggest that increase in intramolecular hydrogen bond strength in S1 state resulted in ESIPT process. Results of geometrical parameters conclude that ESIPT process is favorable for both the compounds. However, change in bond lengths, bond angles in the excited state are not in equal proportion for both the compounds. Significant change was observed for Ib in comparison to Ia. The N−H bond lengths of compounds in the excited state (S1) clearly indicate that intramolecular hydrogen bonding is strong in compound Ib (1.611 Å and 1.706 Å) over Ia (1.721 Å and 1.720 Å). This observation supports the experimental results of fluorescence quantum efficiencies of both the compounds in solid state (Ia: Φf = 40%; Ib: Φf = 67%). In ESIPT chromophores, solid state fluorescence quantum efficiency depends on strength of hydrogen

Figure 13. Frontier molecular orbitals of Ia of enol form.

CONCLUSION The molecular packing, electronic structure, and optical property of fluorene–HBT based systems are sensitive to the substitution pattern. Molecules Ia and IIa pack into a slipped stack (anti-parallel, head-to-tail) with strong π–π interactions (3.314 Å) and the π-electrons are averagely distributed on the whole backbone (HOMO and LUMO). Similar to Ia and IIa, isomers Ib and IIb packed into anti-parallel head-to-tail slipped stacked with weak π–π interactions (3.554 Å) and πelectrons are distributed on fluorene (HOMO) or HBT (LUMO) units. Ia and IIa exhibit thermos-tropic liquid crystalline (LC) phases, which is the first report on a nematic LC phase based on “banana-shaped” molecules with ESIPT motifs. In contrast, non-rod-shaped Ib and IIb did not show any mesophase. Also Ia0 and Ia1 do not have locking structures with intramolecular hydrogen bondings and thus were found not to serve as a mesogen. Compounds Ia and IIa are comparatively less fluorescent in solid state in comparison to positional isomers Ib and IIb. Crystal structure analyses and theoretical calculations conclude that intramolecular hydrogen bonding (ESIPT process) is strong in Ib over Ia that causes enhancement of fluorescence quantum efficiency in the solid state. Compounds (Ia, IIa and Ib, IIb) display AIE behavior (J-aggregation) that can be ascribed to ESIPT or restriction of intramolecular rotation (RIR) process. The red shift in absorption spectra of compounds Ia and IIa is assigned to better conjugation between fluorene–HBT units. In short, molecular packing influences the optical and electronic properties of fluorene–HBT based ESIPT chromophores, and particularly the LC phases of the molecules allow us to dynamic modulation and switching of emission properties including intensity, wavelength, as well as the polarization.

ASSOCIATED ASSOCIATED CONTENT 9



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This material is available free of charge via the Internet at http://pubs.acs.org., Steady state measurements, DSC, powder XRD and single crystal data of Ia and Ib. DFT calculation using B3LYP/6–31G(d,p) of Ia and Ib (optimized structure of cis–enol form: S0 and S1 state).

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AUTHOR INFORMATION Corresponding Author Email: [email protected] (T.S.), [email protected] (T.A.), [email protected] (S.S.).

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT V. S. P. and D. S. thank the JSPS Research Fellowship. This work was partially supported by a Grant-in-Aid for Scientific Research on Innovative Areas "π-System Figuration: Control of Electron and Structural Dynamism for Innovative Functions" from Japan Society for the Promotion of Science (JSPS) (No. JP2604063, JP26102011, JP26810023, JP26102001, 15K21721) and for Young Scientists (A) (No. 17H04880), and a research grant from Research Institute for Production Development. The synchrotron 61 radiation XRD experiments were performed at BL44B2 in SPring-8 with the approval of RIKEN (Proposal No. 20160014). We also acknowledge JACSO Co. for variable-temperature fluorescence spectroscopy measurements.

ABBREVIATIONS ESIPT, excited-state intramolecular proton transfer; ICT, intramolecular charge transfer; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital; DSC, differential scanning calorimetry; NMR, nuclear magnetic resonance; MALDI-TOF, matrix-assisted laser desorption ionization time-offlight; XRD, X-ray powder diffraction; HBT, hydroxyl benzothiazole; AIE, aggregation induced emission, ACQ, aggregation caused quenching, RIR, restriction of intramolecular rotation, TLC, thin layer chromatography; HPLC, high performance liquid chromatography.

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Optical and Structural Properties of ESIPT Inspired HBTâFluorene

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Optical and Structural Properties of ESIPT Inspired HBTâFluorene