The excited-state intramolecular proton transfer fluorescence of HBT derivative induced by solvent polarity

Article abstract of DOI:10.1080/15421406.2016.1200923

In this contribution, a dye (THBT) with 2,5-dimethylthiophene substituted to 2-(2-hydroxyphenyl)benzothiazole was synthesized and characterized by NMR and mass spectroscopic techniques. Its emission behavior in various solvents was investigated and obvious negative solvatochromism was observed. The emission maximum of THBT varied from deep blue (425?nm) in DMSO to orange (550?nm) in chloroform. It is also highly emissive in polycarbonate film and solid powder with the emission peaks 546 and 554?nm, respectively. The ESIPT effect, together with the relaxation of free rotation of thiophene in the excited states, shifted the solid emission 34?nm toward longer wavelength furtherly, with respect the 520?nm (green) emission of HBT. Quantum calculation of frontier molecular orbitals uncovers the underlying mechanism of negative solvatochromism.

Full text of DOI:10.1080/15421406.2016.1200923

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
The excited-state intramolecular proton transfer
fluorescence of HBT derivative induced by solvent
polarity
Xiaochuan Li, Dandan Shan, Changkyeom Kim & Young-A Son
To cite this article: Xiaochuan Li, Dandan Shan, Changkyeom Kim & Young-A Son
(2016) The excited-state intramolecular proton transfer fluorescence of HBT derivative
induced by solvent polarity, Molecular Crystals and Liquid Crystals, 635:1, 158-166, DOI:
10.1080/15421406.2016.1200923
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1200923
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Date: 08 November 2016, At: 06:38

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
The excited-state intramolecular proton transfer fluorescence of
HBT derivative induced by solvent polarity
Xiaochuan Lia, Dandan Shana, Changkyeom Kimb, and Young-A Sonb
aCollaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory
of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering,
Henan Normal University, Xinxiang, Henan, P. R. China; bBK, Department of Advanced Organic Materials
Engineering, Chungnam National University, South Korea
ABSTRACT
KEYWORDS
-(-hydroxyphenyl)
benzothiazole; negative
solvatochromism; highly
emissive; molecular orbitals;
ESIPT
In this contribution, a dye (THBT) with 2,5-dimethylthiophene sub-
stituted to 2-(2-hydroxyphenyl)benzothiazole was synthesized and
characterized by NMR and mass spectroscopic techniques. Its emission
behavior in various solvents was investigated and obvious negative
solvatochromism was observed. The emission maximum of THBT varied
from deep blue (425 nm) in DMSO to orange (550 nm) in chloroform. It
is also highly emissive in polycarbonate film and solid powder with the
emission peaks 546 and 554 nm, respectively. The ESIPT effect, together
with the relaxation of free rotation of thiophene in the excited states,
shifted the solid emission 34 nm toward longer wavelength furtherly,
with respect the 520 nm (green) emission of HBT. Quantum calculation
of frontier molecular orbitals uncovers the underlying mechanism of
negative solvatochromism.
Introduction
Investigation of small organic functional molecules fascinates chemists, physicists, and bio-
chemists by its application in materials sciences and structural biotechnology. Out of various
types of physical signal (electricity, light, magnetism, acoustic, etc) and variety of fascinat-
ing behavior, the signal processing of fluorescence is the most convenient way to realize the
complicated logic operation [1–3]. Therefore, a series of fluorescent organic molecules were
intensively investigated and developed. The typical fluorophores are the coumarins, BODIPY,
naphthalimide, bisindolylmalimide, rhodamine, and perylenediimide, in which the emission
color varies from deep blue to deep red with the emission maximum spanning from 440 to
750 nm [4–12].
Excited state intramolecular proto transfer (ESIPT) chromophores, highly emissive small
molecules, have been applied in molecular probes, logic gates, biomarkers, and various emit-
ting materials [13–18]. ESIPT is a photochemical tautomerization that yields an excited
keto form (K∗) from the original enol form (E) in the subpicosecond time region [19,
20]. The excited-state keto form relaxes radioactively to give a fluorescence spectrum with
CONTACT Young-A Son
University,  Gung-dong, Daejeon, -, South Korea; Xiaochuan Li
Chemical Engineering, Henan Normal University, East Jianshe Rd. , Xinxiang, Henan, , China.
yason@cnu.ac.kr
DepartmentofAdvancedOrganicMaterialsEngineering, ChungnamNational
lixiaochuan@htu.cn School of Chemistry and
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
159
a large Stokes shift or non-radioactively to the ground-state keto form, which undergoes
thermal re-enolization to give the stable enol form. The large Stokes-shifted fluorescence
induced by ESIPT has an attractive character of unusual emission at longer wavelength
from a small molecule and therefore is not reabsorbed even in a high concentration of
chromophores. Despite such a unique character, the efficiency of the large Stokes-shifted
fluorescence is generally low. The typical ESIPT chromophores are those containing 2-(2-
hydroxyphenyl)benzothiazole (HBT) or 2-(2-hydroxyphenyl)benzoxazole (HBO). In this
contribution, an electron-rich thiophene unit was introduced to HBT. With such an attach-
ment to HBO, the isomerization efficiency from the excited state may be diminished because
a much larger volume change is required for the deactivation process. The suppression of
the isomerization efficiency in ESIPT molecules may lead to an increase in the efficiency of
the large Stokes-shifted fluorescence. Our intention was to achieve and understand the cor-
responding properties through detailed study and structural analysis, which is of significant
for the application in biological science and technology [21, 22].
Experimental
General procedures and materials
The solvents used in the reaction were carefully dried according to the standard procedure
˚
and stored over 4 A molecular sieve. All the reagent-grade chemicals were purchased from
Sigma-Aldrich CO. LLC. (South Korea) and used without further purification. Melting points
were determined on a Mel-Temp IA9200 digital melting point apparatus in a glass capillary
®
and were uncorrected. All synthesized compounds were routinely characterized by TLC and
1H NMR. TLC was performed on aluminum-backed silica gel plates (Merck DC. Alufolien
Kieselgel 60 F254).
1H and 13C NMR spectroscopy
1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Brucker AM-400
spectrometer operating at frequencies of 400 MHz for proton 100 MHz for carbon in CDCl3.
Proton chemical shifts (δ) are relative to tetramethylsilane (TMS, δ = 0) as internal standard
and expressed in parts per million. Spin multiplicities are given as s (singlet), d (doublet), t
(triplet), and m (multiplet) as well as b (broad). Coupling constants (J) are given in Hertz.
High resolution mass spectra (HRMS)
The mass spectra measured on a LC-MS (Waters UPLC-TQD) mass spectrometer. High res-
olution mass spectra (HRMS) were measured on a Brucker micrOTOF II Focus instrument.
UV-Vis and emission spectra
The absorption spectra were measured with a PERSEE TU-1900 and an Agilent 8453 spec-
trophotometer. Emission spectra were measured with Shimadzu RF-5301PC fluorescence
spectrophotometer. The solvents used in photochemical measurement were spectroscopic
grade and were purified by distillation. The stock solution of compounds (2×10−3 M) was pre-
pared in THF, and a fixed amount of these concentrated solutions were added to each exper-
imental solution. All the experiments were done repeatedly, and reproducible results were

160
X. LI ET AL.
Scheme . Synthesis of -(,-dimethylthiophen--yl)--(-hydroxyphenyl)benzothiazole (THBT). ) NBS,
dichloromethane, r.t.; ) −°C, n-BuLi,·-isopropoxy-,,,-tetramethyl-,,-dioxaborolane, THF; )
Ethanol, reflux; ) N, Pd(PPh),  M NaCO, THF, reflux  h.
obtained. Prior to the spectroscopic measurements, solutions were deoxygenated by bubbling
nitrogen through them.
Theoretical calculations
For the theoretical study of the excited state photo-physics of the dye, the DMol3 program,
which is available as part of Material Studio (Accelrys Inc., San Diego, California, United
States), was used. Both the ground state geometries and the frontier molecular orbital of the
dye were calculated using the density function theory (DFT) with the B3LYP hybrid func-
tional and the double numerical plus d-functions (DND) atomic orbital basis set.
Synthesis
Synethetic routes of the ESIPT dye, 4-(2,5-dimethylthiophen-3-yl)-2-(2-hydroxyphenyl)
benzothiazole (THBT), are outlined in Scheme 1. The 2,5-dimethylthiophene was con-
veniently brominated with NBS in dichloromethane to give 1 [23, 24]. And it was
treated with n-BuLi and 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane and yielded 2
(2-(2,5-dimethylthiophen-3-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane) [25]. HBT-Br was
obtained by condensation of 5-bromo-2-hydroxybenzaldehyde and 2-aminobenzenethiol
[22, 26, 27]. Finally, HBT-Br and 2 was coupled under standard Suzuki coupling conditions
and yielded THBT.
4-(2,5-dimethylthiophen-3-yl)-2-(2-hydroxyphenyl)benzothiazole (THBT)
Under nitrogen atmosphere, HBT-Br (153 mg, 0.5 mmol), Na2CO3 (1.06 g, 10 mmol),
Pd(PPh3)4 (28.9 mg), and H2O (5 mL) were charged in a 100 mL flame dried Schlenk tube. 2
(143 mg, 0.6 mmol) in THF (50 mL) was added by syringe. Next, the mixture was heated to
reflux for 48 h and monitored by TLC. After HBT-Br was fully reacted, as confirmed by TLC,
the reaction mixture was cooled to room temperature and poured in to distilled water (60 mL).
The mixture was stirred for 30 m, then it was extracted with dichloromethane (25 mL×3). The
organic phase was combined and washed with brine (75 mL). Next, the solvent was dehydrated
over anhydrous MgSO4 and evaporated in a vacuum. The residue was purified by silica gel
(200–300 mesh) column chromatography with hexane and afforded THBT (168 mg, 60%).

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
161
Table . Optical data of the THBT in various solvents.
UV-Vis
Fluorescence
λem (nm)
Stokes shift (nm)
Stokes shift (nm)
Solvent
DMSO
DMF
λabs (nm)
log εmax
ꢀF
















.
.

.

.
.
.
.
.
.
.
.
.
.
.
.
.
.


.
.


THF
Acetonitrile

.

Dioxane


.
.


Dichloromethane
Chloroform


.
.


Hexane
Methanol
Polycarbonate film
a
a
a
—
—
—

.

a
The data was not shown due to the weak emission.
Mp: 225–228°C; 1H NMR (400 MHz, CDCl3) δ: 8.00 (d, 1 H, J = 8.0 Hz), 7.90 (d, 1 H, J =
8.0Hz), 7.68 (s, 1 H), 7.51 (t, 1 H, J = 8.0Hz), 7.40 (m, 3H), 7.14 (d, 1 H, J = 8.0Hz), 6.73
(s, 1 H), 2.48 (s, 6H); 13C NMR (100 MHz, CDCl3) δ 169.3, 156.7, 151.9, 137.1, 135.9, 133.1,
132.6, 131.5, 128.6, 128.2, 127.1, 126.8, 125.6, 122.2, 121.6, 117.8, 116.6, 15.2, 14.1. HRMS
(ESI): [M+H]+ C19H16NOS2 requires 338.0673; found [M+H]+ 338.0680.
Result and discussion
The spectra properties of THBT was investigated in various solvents, such as DMSO, DMF,
THF, acetonitrile, dioxane, dichloromethane, chloroform, hexane, and methanol. It was found
that the solubility of THBT in organic media was enhanced due to the attachment of thio-
phene unit to HBT. At the same time, the photostability of THBT in solvents is robust. Long
time exposure to the room light does not decompose the compound. However, the attach-
ment of thiophene to HBT makes it sensitive to the external environment. Negative solva-
tochromism was induced by the variation of solvent polarity. The basic absorption/emission
peaks, absorption coefficients, quantum yields, and Stokes shifts of THBT are presented in
Table 1. The absorption spectra of THBT in various solvents were recorded in Figure 1.
In dioxane, chloroform, and acetonitrile, the longest absorption maximum was positioned
around 400–500 nm. In other tested solvents, including polar and nonpolar media, the absorp-
tion was very weak in visible region. In all the solvents, the stronger absorptions were in the
UV region (∼350 nm).
The emission of THBT is significantly environment dependent, which is different from
that of HBT. In most of the organic solvents, THBT is highly emissive. However, the fluores-
cence was almost totally quenched in protonic solvent, such as methanol. In aprotic solvents,
THBT exhibits various emission characters irregularly. Detailed emission properties are listed
Table 1. Figure 2 shows the emission spectra in various aprotic solvents, by which highly
emission was induced. Interestingly, a negative solvatochromism was observed. In DMSO, the
emission maximum of THBT is shortest (425 nm, deep blue) among all the tested solvents.
It was shifted to 475 nm (light blue) with the solvent displaced by DMF. With the polarity

162
X. LI ET AL.
Figure . Absorption spectra of THBT in various solvents (.×− M).
decreasing, the emission maximum was shifted to 495/505 nm (green) in THF and acetoni-
trile, respectively. Further decreasing of solvent polarity, the emission maximum of THBT
was red-shift to about 550 nm (yellow). In medium and weak polar solvent, the emission
of THBT was sensitive enough to the polar environment. Except the difference of emission
intensities, the emission peaks are almost identical to each other. It can be seen that THBT
solvatochromic shift from chloroform to DMSO is 125 nm toward the shorter wavelength,
comparable to the largest solvatochromic effects observed in heavily studied merocyanine
derivatives. It is known to all that the solvatochromism depends on the difference in dipole
moment between the ground state and the excited states of the chromophore. Within the con-
ventional interpretation of solvatochromic interactions, the negative solvatochromism indi-
cates that the value of the dipole moment in the excited electronic state (μe) is smaller than in
the ground state (μg). The attaching of electron rich heterocycle thiophene to THBT enhances
the total electron density of molecules, which facilitates the enlargement of ground state dipole
moment.
Figure . Emission spectra of THBT in various solvents (.×− M).

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
163
Figure . Emission spectra of THBT/HBT in solid powder and THBT in film at with  wt.% blended in poly-
carbonate.
The Stokes shift of HBT derivatives is generally larger than conventional dyes due to the
ESIPT. The emission maximum in aggregate state shifted to green region due to the mech-
anism of ESIPT. For the unsubstituted HBT fluorophore, the molecular framework is rigid
due to the intramolecular hydrogen bonding, and thus leads to small geometry relaxation.
With thienyl substitution to HBT, the molecular flexibility was increased; that is, the dihe-
dral angle between the thienyl and the benzene ring may be changed upon excitation. This
enhanced geometry relaxation will radiationlessly consume absorption energy, then lower the
emission energy correspondingly, red shift the emission wavelength. This was fully verified
by the emission behavior of THBT in chloroform (200 nm Stokes shift). The quantum yields
of THBT in different solvents varied in the range of 0.18–0.58. There seems to be no regular
trend for the quantum yield variation in different polarity solvents. The emission efficiency
in chloroform and hexane is higher than other tested solvents. In contrary, it was significant
lowered in acetonitrile/dichloromethane, and totally quenched in protonic solvent, such as
methanol.
THBT is highly emissive not only in solution but also in the film blended with polycar-
bonate and in the solid powder state. The emission spectra are shown in Figure 3. Without
the attaching of thiophene, the solid powder emission of HBT is greenish with the emission
peak 522 nm. With the attaching of thiophene unit, the film emission of THBT blended in
polycarbonate (5 -wt.%) red-shifted to 546 nm due to the effect of ESIPT and aggregation
in the polymer matrix. Further red-shift of THBT was observed in solid powder (554 nm),
which is closer to that in film. Additionally, the emission behavior in the film/solid powder is
similar to that in chloroform/hexane. The full width at half maximum (FWHM) of THBT in
the solid powder was estimated to be 57 nm, which is narrower than that in chloroform. Gen-
erally, tightly packing in the solid powder will broaden the FWHM of emission. Therefore, the
ESIPT effect plays a major role in the solid powder emission of THBT. The variation of emis-
sion in solution/film/solid (blue to orange) was characterized by CIE coordinates (Figure 4).
Notable CIE coordinate change exhibits the fluorescence color variation induced by solvent
polarity (Figure 4a). With the polarity decreasing, the CIE coordinates crowded in orange
region. The CIE coordinates of film and solid emission is adjacent to each other, which is
facilitates to the future application in device (Figure 4b).

164
X. LI ET AL.
Figure . CIE  chromaticity diagram of the emissions of THBT in solution (a), and HBT (in polycarbonate
film), THBT (in polycarbonate film and solid powder).
To better comprehend the geometrical, electronic, and optical properties of HBM, we
undertook a comprehensive computational investigation using Material Studio. To reduce the
run times in the first instance, the ground-state energy-minimized structures were calculated
using DFT and LDA/DN basis set [28, 29]. Further refinement and optimization on structures
were undertaken using DND/B3LYP basis set.
The size and signs of frontier molecules orbitals are illustrated in Figure 5. Distinctive con-
tributions of each atomic orbital are discernible in the HOMO and LUMO diagrams, which

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
165
Fiugre . HOMO and LUMO diagrams of THBT obtained from Dmol calculations and the energy minimized
structure.
dominated the strong absorption band in UV-Visible region [8, 30, 31]. In HOMO, the elec-
tron density is well distributed over the thiophene and phenolic rings, reflecting the electron-
rich character. However, the LUMO distribution moved away from thiophene ring to ben-
zothiazole ring. While the phenolic rings contributes always to the HOMO and LUMO dis-
tribution. The enhanced electron density due to the attaching of thiophene ring was ease to
induce the ground state charge separation in polar environment. The higher electron density
location at nitrogen in LUMO indicates the interaction between hydrogen and nitrogen atoms.
˚
The distance also can be estimated to be 1.747 A by calculation, which limits the free rotation
of benzothiazole ring around the single covalent bond. The enhanced rigidity of molecular
framework contributes mainly to its solid powder emission. While, the rotation of thiophene
ring around the single covalent bond is freely. The energy dissipation of excited states shifts
the emission spectra to orange, which was supported by the calculated dipole moment of
ground/excited state (μe = 3.623D, μg = 3.196D). According to the optimized excited stated,
the absorption and emission energy can be estimated to be 2.53 eV and 2.21 eV, respectively.
The absorption and emission wavelength could be calculated to be 490 and 561 nm, which is
in agreement with the experimental value mentioned above.
Conclusions
In summary, a 2,5-dimethylthiophene attached dye (THBT) was synthesized under Suzuki
conditions and fully characterized. The electron rich nature of substituent leads to higher elec-
tron density in the molecular system. An obvious negative solvatochromism was observed for
the emission behavior of THBT in different polarity solvent. The emission color varied from
deep blue in DMSO (425 nm) to orange in hexane/chloroform (∼550 nm). Increase of sol-
vent polarity blue shifts the emission maximum. Additionally, THBT is also highly emissive
in polycarbonate film and solid powder with the emission peaks 546 and 554 nm respectively.
The emission color of THBT in solution/film/solid powder was characterized by CIE coordi-
nates in detail. The calculated HOMO distribution indicates that the higher electron density
in thiophene and phenolic rings. It is the higher dipole moment in ground state induced by
polar solvent result the negative solvatochromism.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (21272060), PCSIRT
(grant no. IRT1061), and the Program for Innovative Research Team in University of Henan Province
(15IRTSTHN003). This study was supported by the Basic Science Research Program through the

166
X. LI ET AL.
National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Plan-
ning (Grant No. 2015021972).
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