A perpendicular phenyl-induced exceedingly efficient solid-state excited state intramolecular proton transfer fluorophore based on 2-(2-hydroxyphenyl)benzothiazole

Article abstract of DOI:10.1016/j.dyepig.2016.11.055

Fluorescent solid-state organic materials have been intensively researched in recent years. In this study, two 2-(2-hydroxyphenyl)benzothiazole (HBT) derivatives: phenylethylene-modified HBT (HBT-s-Ph) and diphenylethylene-modified HBT (HBT-d-Ph), were sy

Full text of DOI:10.1016/j.dyepig.2016.11.055

Accepted Manuscript
A perpendicular phenyl-induced exceedingly efficient solid-state
excited state intramolecular proton transfer fluorophore based on 2-(2-
hydroxyphenyl)benzothiazole
Qin Wang, Longfei Xu, Yahui Niu, Yuxiu Wang, Mao-Sen Yuan, Yanrong Zhang
PII:
S0143-7208(16)30949-4
10.1016/j.dyepig.2016.11.055
DYPI 5624
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 13 October 2016
Revised Date: 29 November 2016
Accepted Date: 29 November 2016
Please cite this article as: Wang Q, Xu L, Niu Y, Wang Y, Yuan M-S, Zhang Y, A perpendicular phenyl-
induced exceedingly efficient solid-state excited state intramolecular proton transfer fluorophore based
on 2-(2-hydroxyphenyl)benzothiazole, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2016.11.055.
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ACCEPTED MANUSCRIPT

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A perpendicular phenyl-induced exceedingly efficient solid-state excited state
intramolecular proton transfer fluorophore based on
2-(2-hydroxyphenyl)benzothiazole
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Qin Wang, Longfei Xu, Yahui Niu, Yuxiu Wang, Mao-Sen Yuan*
and Yanrong Zhang*
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College of Science, Northwest A&F University, Yangling, Shaanxi 712100, P. R.
China
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Corresponding author:
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College of Science, Northwest A&F University, Yangling, Shaanxi 712100, P. R.
China
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Tel: +86-29-87092226; E-mail: zhangyr@nwsuaf.edu.cn (Y. R. Zhang);
yuanms@nwsuaf.edu.cn (M.-S. Yuan)
Abstract
Fluorescent solid-state organic materials have been intensively researched in recent
years. In this study, two 2-(2-hydroxyphenyl)benzothiazole (HBT) derivatives:
phenylethylene-modified HBT (HBT-s-Ph) and diphenylethylene-modified HBT
(HBT-d-Ph), were synthesized by facile Suzuki cross-coupling reactions. In toluene
solutions, the fluorescence quantum yield (Φf) of HBT-s-Ph is slightly larger than that
of HBT-d-Ph, because of the presence of a stronger intramolecular H-bond in
HBT-s-Ph than in HBT-d-Ph. In crystalline forms, both of their Φf has substantial
improvement, and the Φf of HBT-d-Ph reaches 78%, which is twice that of HBT-s-Ph.
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The crystal structure revealed that the striking emission enhancement of HBT-d-Ph
was attributable to its distinct molecular packing model and the resultant relatively
weak intermolecular interactions induced by the single phenyl group cis to the HBT
moiety.
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Keywords: Solid-state fluorescence; Crystal packing; Excited state intramolecular
proton transfer; Steric hindrance.
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1. Introduction
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Organic fluorescent materials have been intensively researched because of the huge
range of diverse molecular structures available via chemical modification as well as
their potential applications in various fields such as light emitting diodes, optical
sensors, and photoemitters.[1-4] However, organic fluorophores may suffer from
aggregation-caused quenching (ACQ) at high concentrations or in the aggregated state
because of the strong intermolecular vibronic interactions of the resulting excimers or
exciplexes.[5-6] Although the introduction of an extended π-conjugated system is
often an efficient strategy for the construction of red-shifted fluorophores,[7,8]
concentration quenching is especially problematic in these compounds, which show
enhanced intermolecular contact via π-π interactions in the solids.[9]
Aggregation-induced emission enhancement (AIEE), first reported by Tang et al. and
Park et al., is a unique photophysical phenomenon that acts oppositely to the ACQ
effect.[10,11] AIEE results from a reduction in the irradiative energy loss that occurs
when the assembly into aggregates restricts the intramolecular rotation of
fluorophores.[5,12] However, only a few organic fluorophores display AIEE.[13-16]
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It remains a challenge to enlarge the family of AIEE-active compounds by facile
synthesis to achieve high luminescence efficiencies. Organic dyes bearing
intramolecular hydrogen bonds (H-bonds) usually exhibit an abnormally large Stokes
shift in their fluorescence emissions, compared with that of conventional dyes.[17]
This change occurs because the excited state intramolecular proton transfer (ESIPT)
mechanism results in significant differences between both the structural and electronic
configurations of the absorbing and emitting species (enol and keto tautomers,
respectively). It should be noted that fluorophores that show large Stokes shifts are
more desirable since the shift reduces the interference of the exciting light, which is
essential for the fabrication of high-performance devices.[18-20]
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2-(2-Hydroxyphenyl)benzothiazole (HBT) has been extensively studied as a typical
ESIPT molecule because of its structural simplicity and good photophysical properties.
[21,22] Sakai et al. reported four alkoxy-substituted dibenzothiazolyphenol
compounds, among which the butoxy (OBt) derivative exhibited a maximum
fluorescence quantum yield (Φf) of 0.44 in the crystal state.[23] Yang et al. conjugated
5-tert-butylisophthalic acid with 2-(4-amino-2-hydroxyphenyl)benzothiazole, and
reported that the resulting compound demonstrated a higher Φf when dissolved in a
mixture of THF/H2O than in pure THF.[24] Herein, phenylethylene-modified HBT
(HBT-s-Ph) and diphenylethylene-modified HBT (HBT-d-Ph) were designed and
readily synthesized through Suzuki cross-coupling reactions (Scheme 1) to develop
HBT derivatives with an extended π-conjugation system and further explore their
structure-property relationships. The emissions of both HBT derivatives were
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remarkably enhanced in their crystalline forms compared to those in solution, but
different degrees of enhancement were observed for the various HBT derivatives. The
analysis of each compound by single-crystal X-ray diffraction (XRD) revealed that
the striking emission enhancement of HBT-d-Ph was attributable to its distinct
molecular packing model and the resultant relatively weak intermolecular interactions
induced by the single phenyl group cis to the HBT moiety. The weak intermolecular
interaction between two HBT-d-Ph molecules was confirmed by the lower melting
point measured by differential scanning calorimetry (DSC).
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Scheme 1. Synthetic routes of HBT-s-Ph and HBT-d-Ph.
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2. Experimental
2.1 Materials and Instruments
(Z)-Styrylboronic acid was from Alfa Aesar (Lancaster, England). 2-Aminothiophenol,
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5-bromosalicylaldehyde, and Pd(PPh3)4 were purchased from Energy Chemical
(Shanghai, China). Ethene-1,1-diyldibenzene was purchased from Heowns (Tianjin,
China). All other materials were purchased from local commercial suppliers and were
of analytical reagent grade, unless otherwise stated. Solvents were purified by
standard procedures. All reactions were monitored by thin-layer chromatography
(TLC) with detection by UV.
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1H and C NMR spectra of the products were recorded on a Bruker 500 MHz NMR
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spectrometer in CDCl3 solution using tetramethylsilane (TMS) as the internal standard,
and chemical shifts were given in ppm and coupling constants (J) in Hz. High
resolution mass spectra (HRMS) were recorded on a Bruker instrument using standard
conditions (electron spray ionization, ESI). Fourier-transform infrared spectroscopy
(FT-IR) was recorded on a Nicolet 5700 spectrophotometer at wavelengths of
400-4000 cm-1 and a resolution of 3 cm-1over 32 scans. UV absorption spectra were
obtained on a UV-vis spectrophotometer (U-3310). The fluorescence spectra were
recorded on a fluorospectrophotometer (F-7000). The fluorescence quantum yield (Фf)
in solution was determined by using quinine sulfate (Фf = 0.55 in 0.1 M H2SO4) as a
standard.[25] Quantum yields of the solid-state powder were determined with a PTI
C-701 calibrated integrating sphere system. [26]
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The single crystals of HBT-s-Ph and HBT-d-Ph were obtained by the slow
diffusion of their respective CH2Cl2 solution for several days at room temperature.
The data collection was done at room temperature on a Bruker SMART APEX-II
CCD area detector using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å).
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Data reduction and integration were done by the INTEGRATE program of the APEX2
software. Semi-empirical absorption correction was applied using the SCALE
program. The structure was solved by direct method and refined by the full matrix
least-squares method on F2 using SHELX.
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2.2 Synthesis and Characterization
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2.2.1 2-(Benzo[d]thiazol-2-yl)-4-bromophenol (1) was synthesized according our
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previously reported literature.[27] White solid, yield 84%. H NMR (500 MHz,
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CDCl3) δ 12.59 (1H, s), 8.03 (1H, d, J = 8.5 Hz), 7.95 (1H, d, J = 8.5 Hz), 7.58– 7.55
(1H, m), 7.51 – 7.46 (2H, m), 7.04 (1H, d, J = 9.0).
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2.2.2 (Z)-2-(Benzo[d]thiazol-2-yl)-4-styrylphenol (HBT-s-Ph): Pd(PPh3)4 (0.05
mmol, 57.8 mg) was added to a thoroughly degassed solution of 1 (0.5 mmol, 153.2
mg), (Z)-styrylboronic acid (0.6 mmol, 88.8 mg), K2CO3(1.5 mmol, 207.3 mg), in a
mixture of toluene (15.0 mL)/ ethanol (5.0 mL)/ H2O (1.25 mL). The resulting
mixture was stirred at 100 ºC for 24 h under argon atmosphere. After cooling, the
solvent was evaporated and the residue was subjected to column chromatography on
silica (PE/DCM= 4/1) to yield the desired product. Saffron yellow crystalline solid,
yield 63%. Melting point (m.p.) 194 -196 ºC. IR (KBr, cm-1): 3419.5, 3018.8, 1584.4,
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1491.2, 1214.3, 1171.3, 785.0, 753.3. H NMR (500 MHz, CDCl3) δ 12.65 (1H, s),
8.06 – 8.04 (1H, m), 7.99-7.60 (1H, m), 7.82 (1H, d, J = 2.0 Hz), 7.64 (1H, dd, J = 8.5,
2.0 Hz), 7.59 – 7.50 (3H, m), 7.50 – 7.46 (1H, m), 7.44 – 7.41(2H, m), 7.33 – 7.30
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(1H, m), 7.16 (1H, d, J = 8.5), 7.15 (1H, d, J = 16 Hz,), 7.08 (1H, d, J = 16 Hz). C
NMR (126 MHz, CDCl3) δ 169.18, 157.72, 151.87, 137.36, 132.66, 130.53, 129.28,
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128.75, 127.54, 127.42, 126.81, 126.65, 126.40, 125.68, 122.27, 121.60, 118.39,
116.84. HRMS (ESI) found: [M+H]+ 330.09470; ‘molecular formula C21H16NOS’
requires [M+H]+ 330.09471.
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2.2.3 2-Bromo-1,1-diphenylethene 2: To a solution of ethene-1,1-diyldibenzene (2.0 g,
11.1mmol) in chloroform (40 mL) was added a solution of bromine (1.6 g, 10 mmol)
in chloroform (20 mL) slowly. After decolouration of the solution, the solvent was
evaporated and the residue was subjected to column chromatography on silica
(PE/DCM =7/1) to yield the desired product. Yield 66%. 1H NMR (500 MHz, CDCl3)
δ 7.49 – 7.46 (2H, m), 7.45 – 7.43 (1H, m), 7.42 – 7.40 (1H, m), 7.39 (1H, d, J = 1.5
Hz), 7.37 – 7.35 (3H, m), 7.30 – 7.26 (2H, m), 6.84 (1H, s), which was consistent
with previously reported data.[28]
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2.2.4
2-(2,2-Diphenylvinyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
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(2-bromoethene-1,1-diyl)dibenzene 2 (1.87 g, 7.2 mmol), bis(pinacolato) diboron (2.1
g, 8.28 mmol), 1,10-bis(diphenylphosphino) ferrocene-palladium(II)dichloride
dichloromethane (177 mg, 0.22 mmol) and potassium acetate (2.12 g, 21.6 mmol)
were dissolved in dioxane (40 mL). The solution was degassed by bubbling nitrogen
for 30 min. The reaction mixture was refluxed for 20 h. After cooling down to room
temperature, the reaction mixture was extracted with ethyl acetate. The combined
organic layers were collected and concentrated in vacuo. The residue was subjected to
column chromatography on silica (PE/DCM= 3/1) to yield the desired product. Yield
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80 %. H NMR (500 MHz, CDCl3) δ 7.37 – 7.35 (5H, m), 7.34 – 7.30 (5H, m), 6.05
(1H, s), 1.20 (12H, s), which was consistent with previously reported data.[29]
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2.2.5 2-(Benzo[d]thiazol-2-yl)-4-(2,2-diphenylvinyl)phenol (HBT-d-Ph): Pd(PPh3)4
(0.2 mmol, 231 mg) was added to a thoroughly degassed solution of 1 (2 mmol, 612
mg), 3 (2 mmol, 612 mg), K2CO3(6mmol, 828 mg), in a mixture of toluene (50 mL)/
ethanol (2.0 mL)/ H2O (4.5 mL). The resulting mixture was stirred at 100 ºC for 24 h
under argon atmosphere. After cooling, the mixture was evaporated and the residue
was subjected to column chromatography on silica (PE/DCM= 3/1) to yield the
desired product. Crystalline solid, yield 64%. Melting point (m.p.) 165 -166 ºC. IR
(KBr, cm-1): 3423.5, 3022.8, 1584.4, 1489.9, 1430.8, 1261.0, 1200.1, 762.5, 690.3. 1H
NMR (500 MHz, CDCl3) δ 12.56 (1H, s), 7.99 (1H, d, J = 8.0 Hz), 7.91 (1H, d, J =
8.0 Hz), 7.55 – 7.51 (1H, m), 7.51 – 7.44 (3H, m), 7.44 – 7.37 (6H, m), 7.37 – 7.33
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(3H, m), 7.17 (1H, dd, J = 8.5, 2.0 Hz), 7.01 (1H, s), 6.96 (1H, d, J = 8.5 Hz). C
NMR (126 MHz, CDCl3) δ 169.42, 156.82, 151.79, 143.21, 141.57, 140.41, 134.52,
132.73, 130.30, 129.16, 129.08, 129.03, 128.30, 127.79, 127.50, 127.44, 126.64,
125.49, 122.13, 121.54, 117.52, 116.33. HRMS (ESI) found: [M+H]+ 406.12604;
‘molecular formula C27H20NOS’ requires [M+H]+ 406.12601.
3. Results and discussion
3.1 Synthesis
The synthetic procedure and molecular structures of compounds HBT-s-Ph and
HBT-d-Ph are outlined in Scheme 1. These two HBT derivatives were readily
synthesized by the palladium-catalyzed Suzuki reaction between organoboron
compounds and organic halides. 2-(Benzo[d]thiazol-2-yl)-4-bromophenol (1) was
synthesized according to our previously reported method.[27] HBT-s-Ph was
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synthesized from the commercially available (Z)-styrylboronic acid and compound 1
(yield 84%). HBT-d-Ph was synthesized in three steps: firstly, the bromination of
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ethene-1,1-diyldibenzene
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the
1’-position
under
bromine
yielded
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(2-bromoethene-1,1-diyl)dibenzene, 2. Following bromination, borylation of 2 with
bis(pinacolato)diboron gave boronic acid ester 3. Finally, compound HBT-d-Ph was
obtained by a facile Suzuki reaction (yield 80%). These new compounds were
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unambiguously characterized by H and 13C nuclear magnetic resonance (NMR)
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spectroscopy and high-resolution mass spectrometry (HRMS) (SI, Fig. S9 to S17).
Single-crystal XRD analysis was also performed for both HBT-s-Ph and HBT-d-Ph
(CCDC 1507695 and 1507694, respectively). [30]
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3.2 Absorption and fluorescence properties in solution.
As previously reported, the existence of a stable intramolecular H-bond between the
proton and nitrogen atom in the ground state is required to facilitate the
transformation from enol tautomer to keto tautomer in the excited state.[31] This
transformation involves the transfer of a proton from the oxygen atom of the phenolic
hydroxyl to the nitrogen atom in the benzothiazole (BT) ring. Toluene, as a
non-H-bonding solvent, cannot effectively compete with the H-bond acceptor for the
phenolic hydroxyl proton to form an intermolecular H-bond. Therefore, the enol-cis
closed form is the most stable conformer of ESIPT dyes in toluene. ESIPT dyes form
the keto tautomer by ESIPT and thereby undergo keto emission on excitation, as
shown in Scheme 2.[32]
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Scheme 2. Schematic representation of the photophysical process for HBT-based
ESIPT dyes.
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Table 1. Photophysical properties of HBT-s-Ph and HBT-d-Ph in toluene.
λabs / nm εmax
λem / nm △λ/ nm Φf /% τ/ ns
δ/ppm) kns/107s-1
HBT-s-Ph 370
HBT-d-Ph 371
0.93
0.97
566
573
196
203
1.02
0.91
0.40
0.63
12.65
12.56
2.47
1.58
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Figure. 1 (A) Absorption and (B) fluorescence spectra of HBT-s-Ph and HBT-d-Ph in
toluene (λem: 370 nm and 371 nm, respectively).
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We measured the steady-state absorption spectra and fluorescence spectra of
HBT-s-Ph and HBT-d-Ph in toluene, as shown in Fig. 1. The accompanying
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photophysical data for HBT-s-Ph and HBT-d-Ph are summarized in Table 1. The
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UV-Vis absorption spectra of HBT-s-Ph and HBT-d-Ph in toluene (Fig. 1a) exhibit
two main absorption bands. The high-energy absorption band (~280–340nm)
corresponds to the absorbance of the BT moiety,[33] while the lower energy
absorption band (~350–470nm) is assigned to the S0-S1 (π–π*) transition of the closed
enol tautomer. Although the structure of HBT-d-Ph contains one more conjugated
phenyl ring than is present in HBT-s-Ph, both HBT derivatives exhibit almost the
same maximum absorption wavelengths. Previous reports suggest that the absorption
band intensity can be correlated with the planarity of the dyes.[34] Based on the
results of this approach, we conclude that HBT-s-Ph was less planar than HBT-d-Ph in
toluene because the ratio of the high-energy and low-energy bands was slightly
smaller for HBT-d-Ph. The difference in the planarity of these dyes was also indicated
by considering the maximum molar extinction coefficient values (0.93 and 0.97 × 104
M-1cm-1 for HBT-s-Ph and HBT-d-Ph, respectively), where a less planar structure
presents a lower probability for the π–π* transition.[35] Given that the absorption
spectra are insensitive to the presence of the additional phenyl group in HBT-d-Ph, it
is plausible that, in the ground state, steric hindrance may result in the phenyl group in
the 2,2-diphenylvinyl unit that is cis to the HBT moiety aligning in a non-planar
conformation. Therefore, the orthogonally-aligned phenyl group does not participate
in the electronic delocalization of HBT-d-Ph. The fluorescence emission spectra of
HBT-s-Ph and HBT-d-Ph in toluene (Fig. 1b) show that HBT-d-Ph exhibits a red-shift
in its keto emission compared with HBT-s-Ph. The large Stokes shifts were found for
HBT-s-Ph and HBT-d-Ph (196 nm and 203 nm, respectively), which are characteristic
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of ESIPT dyes. Their ESIPT properties were further confirmed by the fluorescence
spectra in various solvents (Fig. S2 and Fig. S3). The Φf of HBT-s-Ph was slightly
larger than that of HBT-d-Ph, which we suggest is because of the slightly stronger
intramolecular H-bond in the HBT-s-Ph molecule. Previous reports have established
that the strength of intramolecular H-bonds often positively correlate with the
Φf.[18,23] The strength of these H-bonds is reflected by the chemical shift of the
phenolic hydroxyl proton, which was found to be 12.65 and 12.56 ppm for HBT-s-Ph
and HBT-d-Ph, respectively.[27] The slight differences in the values of Φf in toluene
solution further suggests that the incorporation of an extra phenyl group had only a
small effect on the electronic conjugation of the molecular structure of HBT-d-Ph,
compared to that of HBT-s-Ph.
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Fig. 2 (A) Emission spectra of compounds HBT-s-Ph and HBT-d-Ph crystal (λem: 370
nm and 371 nm, respectively); the inset shows digital images of photoluminescence
recorded for HBT-s-Ph and HBT-d-Ph crystal under UV light irradiation (from left to
right). (B) Fluorescence lifetime profiles of HBT-s-Ph and HBT-d-Ph crystal.
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Table 2. Fluorescence properties of HBT-s-Ph and HBT-d-Ph in the crystalline state.
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λem/ nm Φf / %
τ/ ns
3.45
4.46
kns/108s-1
1.73
HBT-s-Ph
HBT-d-ph
557
568
40.41
78.18
0.49
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3.3 Fluorescence properties of crystalline samples.
Single crystals of HBT-s-Ph and HBT-d-Ph emitted strong fluorescence under UV
light irradiation (Fig. 2), and their solid-state photophysical properties are summarized
in Table 2. Solid-state fluorescence spectroscopy (Fig. 2a) showed that, compared to
their solution-state photophysical properties, the maximum emission wavelengths
shifted hypsochromically by 9 nm and 5 nm for HBT-s-Ph and HBT-d-Ph,
respectively. Furthermore, the Φf of HBT-s-Ph and HBT-d-Ph showed 39.6 and
85.9-fold improvements, respectively (Fig. S5 and Fig. S6), and τ increased by 3.05
and 3.83 ns for HBT-s-Ph and HBT-d-Ph, respectively (Fig. 2b). Additionally, the
fluorescence spectra in THF/H2O systems were also measured (Fig. S4). The results
demonstrated that when the water content increased, the fluorescence intensity
gradually increased. Therefore, we conclude that both dyes were AIEE-active.
Restricted intramolecular motion has been proposed as one of the central mechanisms
by which the AIEE phenomenon occurs.[36-39] It should be noted that, in toluene
solution, the Φf of HBT-s-Ph is only slightly larger than that of HBT-d-Ph, while the
Φf of HBT-d-Ph is almost twice that of HBT-s-Ph in the solid state. To the best of our
knowledge, the Φf value observed for HBT-d-Ph is the highest reported for an
ESIPT-based solid-state dye, and is competitive with other reported fluorescent
emitters. [1, 40-44]
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3.4 Crystal structure analysis
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Fig. 3 View of the intramolecular H-bonding interaction of HBT-s-Ph and HBT-d-Ph
crystal.
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Table 3. Hydrogen bond parameters for HBT-s-Ph and HBT-d-Ph.
N···O
O···H
H···N
<OHN / °
length /
Å
length /
Å
length / Å
HBT-s-Ph
HBT-d-Ph
2.599
2.621
0.820
0.820
1.871
1.891
147.28
147.85
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The crystal structures of HBT-s-Ph and HBT-d-Ph were determined by single-crystal
XRD to elucidate the origin of the differences between the Φf values of HBT-based
dyes in solution and in the solid state. Their crystal structures are shown in Fig. 3, and
the H-bond parameters are presented in Table 3. The HBT-s-Ph and HBT-d-Ph
molecules show intramolecular H-bonding interactions with bond distances of 1.871
and 1.891 Å, respectively. These bonds stabilize the enol form in the ground state,
which facilitates the generation of the keto tautomer in the excited state. The
intramolecular H-bond of HBT-s-Ph is expected to be stronger than that of HBT-d-Ph,
which was confirmed by the decreased distances between N1···O1 and H1···N1 in
HBT-s-Ph, compared to HBT-d-Ph. This relative H-bonding strength order was also
verified by the lengths of the C–C bonds linking the phenol ring to the BT group,
which were 1.453 and 1.456 Å in HBT-s-Ph and HBT-d-Ph respectively. The strength
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of these H-bonds is often positively correlated with the Φf value. This trend holds for
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our reported HBT derivatives bearing various substituent groups with different
Hammett constants in toluene,[27] and is also supported by the difference of Φf
between HBT-s-Ph and HBT-d-Ph in toluene. However, it is evident that the same
correlation is not observed for HBT-s-Ph and HBT-d-Ph in the solid state.
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Fig. 4 Molecular distance in HBT-s-Ph and HBT-d-Ph crystal.
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Fig. 5 DSC curves for HBT-s-Ph and HBT-d-Ph crystal.
The molecular spatial configuration is well known to be vital in determining the
stacking mode of a dye molecule and thus influences the photophysical properties of
the resultant aggregation state.[1,4] HBT-s-Ph and HBT-d-Ph crystallize in the
monoclinic space group P2(1)/c and the triclinic space group P-1, respectively. The
crystallographic data are summarized in Table S1. For HBT-s-Ph, the three planar
π-units (i.e., the phenol, BT, and phenyl rings) are almost coplanar: the dihedral angle
between the phenol and the BT ring is 4.85°, and the dihedral angle between the
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phenol and the phenyl ring is 2.33°. The dihedral angle between the phenol and the
BT in HBT-d-Ph is 4.69°, and the similarity between this angle and the corresponding
angle in HBT-s-Ph is suggested to occur because of the intramolecular H-bonding
effect. However, the dihedral angle between the phenol and the benzene ring trans to
the BT ring in HTB-d-Ph is 8.07°, which is larger than the corresponding angle in
HBT-s-Ph. The dihedral angle between the phenol and the phenyl ring cis to the BT
ring in HTB-d-Ph is 88.8°, which indicates that the phenyl ring cis to the BT ring
makes almost no contribution to the extension of π conjugation. The distance between
two parallel HBT-s-Ph molecules was found to be 3.495 Å, which is characteristic of
the crystal stacking of planar π-conjugated systems.[45] However, the same distance
in HBT-d-Ph was 5.821 Å, as shown in Fig. 4. It should be noted that in two adjacent
HBT-d-Ph molecules, the phenyl rings perpendicular to the planar moiety, point
towards the adjacent molecule. This phenyl group impedes tight molecular packing
and reduces the intermolecular interactions in HBT-d-Ph. This relatively loose
molecular packing was consistent with DSC observations (Fig. 5),which showed that
the melting point of HBT-d-Ph was lower than that of HBT-s-Ph (165.89 °C and
196.5 °C for HBT-d-Ph and HBT-s-Ph, respectively).
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4. Conclusion
HBT-s-Ph and HBT-d-Ph were both found to be AIEE-active. The fluorescence
quantum yield in the crystalline dyes was higher than in solution by 39.6 and 86.6
times for HBT-s-Ph and HBT-d-Ph, respectively. Although the stronger H-bond in
HBT-s-Ph resulted in a slightly higher fluorescence quantum yield in solution for
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HBT-s-PH, this difference did not play a decisive role in the crystalline dye. Further
single-crystal XRD analysis revealed that the different stacking modes, solely induced
by the incorporation of the additional phenyl ring cis to the HBT moiety in the
HBT-d-Ph molecule, were responsible for the differing extents of emission
enhancement. As far as we know, this is the first report of a perpendicular
phenyl-induced highly efficient solid-state ESIPT fluorophore based on HBT.
Therefore, HBT-d-Ph is a potential candidate application in high-performance
emitting devices. Moreover, the introduction of the diphenylethylene group into other
molecular skeletons is a facile modification that could offer a general strategy for
improving the solid-state luminous efficiency of other systems. This strategy could be
applied for fabricating high-performance luminescent and sensory materials for
optical displays, biomedical imaging, and molecular electronics.
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Acknowledgements
This study was supported by the National Natural Science Foundation of China
(21405122), the Natural Science Foundation of Shaanxi (2016JM2009) and the
Fundamental Research Funds for the Central Universities (2452015428).
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ꢀ HBT derivatives bearing styryl and 2,2-diphenylvinyl, respectively, were
synthesized.
ꢀ In solution state Φf of HBT-s-Ph was larger due to stronger intramolecular
H-bond.
ꢀ In crystal state Φf of HBT-d-Ph 78% was larger due to weak intermolecular
interaction.
ꢀ The results were verified through crystal analysis and DSC.
ꢀ A perpendicular phenyl induced exceedingly efficient solid-state ESIPT
fluorophore.