Highly solvatochromic fluorescence of anthraquinone dyes based on triphenylamines

Article abstract of DOI:10.1016/j.dyepig.2017.05.024

Five donor-acceptor anthraquinone dyes based on phenylamine/triphenylamines with different substituted groups were synthesized by Suzuki reaction in good yields, and the photophysical properties were studied in organic solvents with different polarity. The title dyes exhibited remarkable solvatochromic fluorescence (>190 nm emission shift in polar media), which was derived from intramolecular charge transfer (ICT) character that revealed by DFT/TD-DFT calculation. And large Stokes shifts (210–306 nm) were observed in different organic solvents, along with rich color changes from blue to green, yellow, orange and even purple-red. The Stokes shifts were linearly dependent on the solvent polarity function E_T(30). Interestingly, it was found from the single-crystal X-ray diffraction analysis that dye 3 displayed two different interleaved channel structures, which was seldom seen in organic compounds and could be used as host for complexation potentially. Electrochemical characterization suggested that the different substituted phenylamine groups attached to anthraquinone unit could lead to tunable potentials and energy levels.

Full text of DOI:10.1016/j.dyepig.2017.05.024

Accepted Manuscript
Highly solvatochromic fluorescence of anthraquinone dyes based on triphenylamines
Yanxia Li, Tingfeng Tan, Shirong Wang, Yin Xiao, Xianggao Li
PII:
S0143-7208(17)30713-1
10.1016/j.dyepig.2017.05.024
DYPI 5986
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 4 April 2017
Revised Date: 8 May 2017
Accepted Date: 9 May 2017
Please cite this article as: Li Y, Tan T, Wang S, Xiao Y, Li X, Highly solvatochromic fluorescence
of anthraquinone dyes based on triphenylamines, Dyes and Pigments (2017), doi: 10.1016/
j.dyepig.2017.05.024.
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ACCEPTED MANUSCRIPT
Highly solvatochromic fluorescence of anthraquinone dyes based on
triphenylamines
Yanxia Lia, b, c, Tingfeng Tanc,*, Shirong Wanga, b, Yin Xiaoa, b, Xianggao Lia, b,
*
a School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072,
China
b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin
300072, China
c School of Science, Tianjin Chengjian University, Tianjin 300384, China
*Corresponding
tantingfeng@126.com (Tingfeng Tan), Tel: +86 22 2740 4208
ABSTRACT: Five donor-acceptor anthraquinone
author:
E-mail:
lixianggao@tju.edu.cn
(Xianggao
Li),
dyes
based
on
phenylamine/triphenylamines with different substituted groups were synthesized by
Suzuki reaction in good yields, and the photophysical properties were studied in detail in
organic solvents with different polarity. The title dyes exhibited remarkable
solvatochromic fluorescence (>190 nm emission shift in polar media) derived from
intramolecular charge transfer (ICT) character which was revealed by DFT/TD-DFT
calculation. And large Stokes shifts (210-306 nm) were observed in different organic
solvents, along with rich color changes from blue to green, yellow, orange and even
purple-red. The Stokes shifts were linearly dependent on the solvent polarity function
ET(30). Interestingly, by single-crystal X-ray diffraction analysis, dye 3 displayed two
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different interleaved channel structures, which was seldom seen in organic compounds
and could be used as host for complexation potentially. Electrochemical characterization
suggested that the different substituted phenylamine groups attached to AQ unit could
lead to tunable potentials and energy levels.
Key words: Anthraquinone, Triphenylamines, ICT, Solvatochromic, Channel structures.
1. Introduction
Organic molecules with an electron donor (D) and an acceptor (A) typically possess
intramolecular charge-transfer (ICT) characters and are widely used as various
optoelectronic devices [1-5]. In addition, such types of structural compounds are also
known to exhibit solvatochromic character resulting in the changes of position, shape,
and intensity of absorption/emission bands upon various environment polarity, along with
the changes of color and brightness [5-11]. Usually, emission spectra are more sensitive
than absorption one to solvent polarity. Based on highly environmental sensitivity and
good selectivity, the solvatochromic fluorescent dyes have attracted growing interest for
promising
potential
application
as
sensitive
probes
to
monitor
microenvironment/biomolecules [8, 12-13]. These dyes are generally highly polar and
undergo significant changes of their dipole moment upon excitation, which could be
reflected by the spectral shifts and intensity changes (enhancement or quenching)
[14-15]. Additionally, a bathochromic shift is usually called positive solvatochromism
(more common) for ICT compounds and the corresponding hypsochromic shift is
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denoted as negative one with an increase in solvent polarity [16]. However, many
elements should be considered in practical application such as emission wavelength,
quantum yield and Stokes shift, which mainly depend on the fluorescence molecule,
polarity of the solvent and solvation [15, 17]. Also, solvatochromic effects in the visible
spectrum region would be beneficial to the application of polar probes. Therefore, a large
number of solvatochromic compounds through tuning the groups electron donor and/or
acceptor strengths have been designed and developed to meet desired properties and
functions.
It is well known that anthraquinones (AQs) as electron acceptors have attracted much
attention due to potential applications in biology and pharmaceutics [18-21], anion
sensing [21-24], organic photovoltaics [25-27]. Besides being as acceptor,
anthraquinones are also the parent substance of many dyes, and lots of solvatochromic
fluorescence molecules using anthraquinones as electron-accepter have been investigated
[10, 28-31]. On the other hand, as a unique and widely accepted donating group,
triphenylamines (TPAs) show very good electron donating ability and charge transfer
feature [9, 32-33]. Therefore, it would bring great performance by combining two
desirable traits into one molecule.
With this background and aim, five ICT fluorescent dyes were synthesized using
anthraquinone as electron acceptor and phenylamine/triphenylamines as electron donor.
Solvent effects on fluorescence spectra derived from ICT and emission colors in solvents
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of different polarity were investigated. Interestingly, Dye 3 exhibited nanometer cavity
structure in crystal, which was confirmed by TEM.
2. Experimental section
2.1. Materials and methods
All starting materials were obtained commercially without further purification.
Solvents were dried and distilled before used. NMR spectra were collected on a Bruker
AVANCE III 400 (400MHz) spectrometer with CDCl3 as the solvent and
tetramethylsilane (TMS) as an internal standard. HRMS (high resolution mass
spectrometry) data were performed on a Bruker micrOTOF-QII mass spectrometer.
UV−vis spectra, Fluorescence spectra and Infrared spectra were recorded with PE
LAMBDA35 spectrometer, F-380 spectrometer and Thermo Nicolet iS5 spectrometer,
respectively. Fluorescence lifetime measurements were recorded on Edinburg FLS-980
fluorescence spectrometer using time-correlated single photon counting (TCSPC). The
single crystal X-ray diffraction data were obtained by a Rigaku XtaLAB P200
diffractometer. The morphology investigation was conducted using a JEM-2100F
transmission electron microscope (TEM) at 200 kV. Cyclic voltammetry experiments
were performed on a CHI660E electrochemistry workstation.
2.2 Quantum chemical calculation
All molecular geometries were fully optimized at hybrid Density Functional Theory
(DFT) levels by B3LYP functions with 6–31G(d) basis set [34]. Time-dependent density
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functional theory (TD-DFT) was employed to calculate 10 excitation states and obtain
absorption spectra using B3LYP/6-31G (d) basis set. All the calculations were performed
on a Gaussian 03 program.
The electronic structures and the frontier molecular orbital (MO) energies of 1-5 are
illustrated in Fig.2, and the corresponding TD-DFT results are shown in Fig.1b and
Table 2.
2.3 Synthesis
The synthetic route of anthraquinones bearing phenylamine/triphenylamine moieties
1-5 are given in Scheme 1. Triphenylamine derivatives (6a-6c) were synthesized by our
group. Compounds 7 [35], 10 [36], 12 [37], 13a-13c [38-40] were prepared according to
established procedures. The target compounds were synthesized by Suzuki reaction. All
1
as-synthesized compounds were confirmed by H NMR, 13C NMR spectroscopy, high
resolution mass spectroscopy and FTIR spectroscopy. The structure of dye 3 was also
verified by X-ray crystallography.
2.3.1
Synthesis
of
4,4'-((4-(9,10-dioxo-9,10-dihydroanthracen-2-yl)phenyl)azanediyl)dibenzaldehyde
(dye 1)
Compound 7 (0.38 g, 1.0 mmol), 10 (0.44 g, 1.3 mmol), tetrabutylammonium (0.16 g,
0.5 mmol) were dissolved in toluene (40 mL) under argon, and K2CO3 (2M, 2.5 mL) was
added. The mixture was stirred at room temperature for 15 min, and then Pd(PPh3)4
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catalyst (0.05 g, 0.04 mmol) was added. The reaction mixture was heated to reflux for 12
h. After cooling, the product was extracted with CH2Cl2, washed with water, dried over
anhydrous MgSO4. The organic layer was evaporated and the compound was purified by
flash chromatography on silica gel column (petroleum ether:ethyl acetate=10:1 as eluent)
to give orange solid 0.28 g, yield: 54.90%, m.p. 182-183 °C IR (KBr, ν
, cm−1): 1694
.
1
(C=O), 1673(C=O); H NMR (400 MHz, CDCl3) δ (ppm): 9.93 (s, 2H, CHO), 8.54 (s,
1H, Ar-H), 8.43 – 8.29 (m, 3H, Ar-H), 8.03 (d, J = 7.9 Hz, 1H, Ar-H), 7.79 (dd, J = 29.8,
8.0 Hz, 8H, Ar-H), 7.29 (dd, J = 17.4, 8.2 Hz, 6H, Ar-H); 13C NMR (100 MHz, CDCl3) δ
(ppm): 190.47 (s), 183.15 (s), 182.73 (s), 151.72 (s), 146.33 (s), 145.55 (s), 136.22 (s),
134.30 (s), 134.14 (s), 134.03 (s), 133.67 (s), 133.59 (s), 132.27(s), 131.98 (s) , 131.83 (s)
, 131.43 (s), 128.96 (s), 128.22 (s), 127.30 (d, J = 3.9 Hz), 126.84 (s), 125.29 (s), 123.39
(s); HRMS(ACPI+): m/z calcd for C34H22NO4 [M+H]+: 508.1549, found: 508.1543.
2.3.2 Synthesis of 2-(4-aminophenyl)anthracene-9,10-dione (dye 2)
Dye 2 was synthesized by a similar procedure to dye 1, which is different from the
literature [41]. After purification by flash chromatography on silica gel column
(petroleum ether:ethyl acetate=4:1 as eluent), red solid (0.21 g) was obtained, yield:
70.23%. m.p. 222-223 °C. IR (KBr, ν
, cm−1): 1672 (C=O); 1H NMR (400 MHz, CDCl3) δ
(ppm): 8.47 (d, J = 1.4 Hz, 1H, Ar-H), 8.36 – 8.27 (m, 3H, Ar-H), 7.95 (dd, J = 8.1, 1.5
Hz, 1H, Ar-H), 7.79 (dd, J = 5.5, 3.3 Hz, 2H, Ar-H), 7.58 (d, J = 8.4 Hz, 2H, Ar-H), 6.80
(d, J = 8.4 Hz, 2H, Ar-H), 3.90 (s, 2H, NH2); 13C NMR (100 MHz, CDCl3) δ (ppm):
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183.49 (s), 182.86 (s), 147.48 (s), 146.79 (s), 134.08 (s), 133.87 (t, J = 5.4 Hz), 133.71 (s),
131.17 (d, J = 3.3 Hz), 128.76 (s), 128.41 (s), 128.05 (s), 127.19 (d, J = 6.6 Hz), 124.28
(s), 115.41 (s); HRMS(ACPI+): m/z calcd for C20H14NO2 [M+H]+: 300.1025, found:
300.1017.
2.3.3 Synthesis of 2-(4-(diphenylamino)phenyl)anthracene-9,10-dione (dye 3)
Dye 3 was synthesized by a similar procedure to dye 1. After purification by flash
chromatography on silica gel column (petroleum ether:ethyl acetate=50:1 as eluent), red
solid (0.38 g) was obtained, yield: 84.26%. m.p. 204-205 °C IR (KBr, ν
, cm−1): 1672
.
1
(C=O); H NMR (400 MHz, CDCl3) δ (ppm): 8.50 (s, 1H, Ar-H), 8.32 (dd, J = 7.9, 3.9
Hz, 3H, Ar-H), 7.97 (d, J = 8.1 Hz, 1H, Ar-H), 7.84 – 7.74 (m, 2H, Ar-H), 7.61 (d, J =
8.6 Hz, 2H, Ar-H), 7.30 (t, J = 7.8 Hz, 4H, Ar-H), 7.21 – 7.12 (m, 6H, Ar-H), 7.08 (t, J =
7.3 Hz, 2H, Ar-H); 13C NMR (100 MHz, CDCl3) δ (ppm): 183.30 (s), 182.75 (s), 148.82
(s), 147.27 (s), 146.28 (s), 134.09 (s), 133.92 (d, J = 4.7 Hz), 133.78 (s), 133.68 (s),
131.91 (s), 131.57 (s), 131.45 (s), 129.45 (s), 128.04 (d, J = 4.6 Hz), 127.19 (d, J = 5.9
Hz), 125.04 (s), 124.69 (s), 123.64 (s), 122.93 (s); HRMS(ACPI+): m/z calcd for
C32H22NO2 [M+H]+: 452.1651, found: 452.1646.
2.3.4 Synthesis of 2-(4-(di-p-tolylamino)phenyl)anthracene-9,10-dione (dye 4)
Dye 4 was synthesized by a similar procedure to dye 1. After purification by flash
chromatography on silica gel column (petroleum ether:ethyl acetate=50:1 as eluent), red
solid (0.41 g) was obtained, yield: 85.59%. m.p. 180-181°C IR (KBr, ν
, cm−1): 1672
.
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(C=O); 1H NMR (400 MHz, CDCl3) δ (ppm): 8.49 (d, J = 1.8 Hz, 1H, Ar-H), 8.36 – 8.28
(m, 3H, Ar-H), 7.96 (dd, J = 8.2, 1.9 Hz, 1H, Ar-H), 7.84 – 7.75 (m, 2H, Ar-H), 7.58 (d,
J = 8.6 Hz, 2H, Ar-H), 7.17 – 6.99 (m, 10H, Ar-H), 2.34 (s, 6H, CH3); 13C NMR (100
MHz, CDCl3) δ (ppm): 183.33 (s), 182.72 (s), 149.21 (s), 146.38 (s), 144.73 (s), 134.05
(s), 134.07 – 133.52 (m), 133.69 (s), 133.42 (s), 131.34 (d, J = 8.6 Hz), 130.85 (s),
130.08 (s), 128.04 (s), 127.86 (s), 127.17 (d, J = 5.8 Hz), 125.29 (s), 124.52 (s), 121.64
(s), 20.87 (s); HRMS(ACPI+): m/z calcd for C34H26NO2 [M+H]+: 480.1964, found:
480.1957.
2.3.5 Synthesis of 2-(4-(bis(4-methoxyphenyl)amino)phenyl)anthracene-9,10-dione
(dye 5)
Dye 5 was synthesized by a similar procedure to dye 1. After purification by flash
chromatography on silica gel column (petroleum ether:ethyl acetate=50:1 as eluent), red
solid (0.46 g) was obtained, yield: 90.02%. m.p. 188-189 °C IR (KBr, ν
, cm−1): 1672
.
(C=O); 1H NMR (400 MHz, CDCl3) δ (ppm): 8.48 (d, J = 1.7 Hz, 1H, Ar-H), 8.36 – 8.28
(m, 3H, Ar-H), 7.95 (dd, J = 8.2, 1.9 Hz, 1H, Ar-H), 7.83 – 7.75 (m, 2H, Ar-H), 7.56 (d,
J = 8.6 Hz, 2H, Ar-H), 7.11 (d, J = 7.8 Hz, 4H, Ar-H), 7.00 (d, J = 7.9 Hz, 2H, Ar-H),
6.87 (d, J = 8.9 Hz, 4H, Ar-H), 3.82 (s, 6H, OCH3); 13C NMR (100 MHz, CDCl3) δ
(ppm): 183.21 (s), 182.56 (s), 156.43 (s), 149.64 (s), 146.28 (s), 140.22 (s), 133.98 (s),
133.98 – 133.55 (m), 133.62 (s), 131.17 (s), 131.03 (s), 129.73 (s), 127.99 (s), 127.81(s),
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127.10 (d, J = 6.7 Hz), 124.25 (s), 119.76 (s), 114.90 (s), 55.49 (s); HRMS(ACPI+): m/z
calcd for C34H26NO4 [M+H]+: 512.1862, found: 512.1857.
3. Results and discussion
3.1 Photophysical properties
The UV–vis absorption spectra of dyes 1-5 in toluene are shown in Fig. 1a. Dyes 2-5
with electron-donating substituents in phenylamine ring show multiple absorption bands.
The longest wavelengths in 400-550 nm of dyes 3-5 and 370-500 nm of dye 2, which are
characterized by broad bands, arise from the HOMO→LUMO transitions, where the
HOMOs (highest occupied molecular orbitals) of dyes 1-5 are dominantly localized on
phenylamine/TPA units and the LUMOs (lowest unoccupied molecular orbitals) are
mainly localized on AQ moieties (Table 1), indicating typical ICT structural features
from donor (TPAs for 3-5 and phenylamine for 2) to acceptor (AQ) [42-43]. The
absorption regions in 300-400 nm of dyes 3-5 and 300-350 nm of dye 2 can be assigned
to the π−π* transitions, which results from HOMO→LUMO +1 transitions. It is worth
noting that the obvious bathochromic shifts of ICT bands of dyes 2-5 are observed with
increasing the electron-donating ability or the expansion of conjugation system of donor.
As for dye 2, the absorption band emerges at 417 nm, while those of dyes 3-5 are
redshifted to 454, 470 and 481 nm, respectively.
Nevertheless, the LUMO+1 orbitals of dye 1 concentrates on aldehyde groups and the
adjacent benzene rings due to the electron-withdrawing ability of aldehyde groups, which
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is different from those of dyes 2-5 (Fig. 2). The oscillator strength of the transition from
HOMO to LUMO is 0.1397, which is much lower than that of the transitions from
HOMO to LUMO+1 (0.4468) and HOMO to LUMO+2 (0.5137) (Table 1). Therefore, a
broad and strong absorption band of dye 1 concentrates at 376 nm, and two weak
shoulder peaks around 332 and 422 nm are observed, indicating weak ICT (Fig. 1b). In
other words, a possible explanation is the combined effect of the π→π* transition and
ICT band. In addition, dyes 1-5 in solvents with different polarity exhibit similar maxima
absorption bands in the range of 368-485 nm (Table 2 and Fig. S1), and small redshifts
are observed with increasing solvent polarity, indicating that the ground states of dyes 1-5
must be polar. This theoretical analysis is essentially in agreement with the experimental
results (Fig. 1b).
Unlike the case of absorption maxima, the fluorescence derived from the ICT band of
dyes 1-5 depend on solvent polarity dramatically. As shown in Fig. 3, the influence of the
solvent polarity on the emission maxima is more pronounced than that of absorption
maxima (Table 2), demonstrating that the dipole moments of excited states are higher
compared with those of ground states [5, 44]. Dyes 1-5 exhibit pronounced positive
polarity dependent solvatochromic shifts of 192, 183, 184, 138 and 149 nm, respectively,
along with spectral broadening and large Stokes shifts (210-306 nm). And all these are
ascribed to the energetic stabilization of excited-states and low energy gaps derived from
the solvent dipole relaxation in polar solvents [6, 32, 45]. From the fluorescence image of
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dyes 1-5 (Fig. 4), dye 1 has outstanding solvatochromism from non-polar to polar
solvents, showing good solvent selectivity. Also, dyes 2-5 have more diverse colors from
non-polar to weak solvents. In addition, the emission maxima of dyes 1-5 increase with
the order of 1 < 2 < 3 < 4 < 5 in the same solvent (Table 2), which is consistent with
absorption spectra.
The corresponding fluorescence quantum yields (ΦF) and fluorescence lifetimes (τ)
of dyes 1-5 are also illustrated in Table 2. Fig. 5 shows fluorescence decay curves of
dyes 1-5 in toluene. As the solvent polarity is increased, the ΦF of dye 1 shows an
increase from 0.0024 in hexane to 0.3007 in toluene and then a dramatic decrease,
indicating two different non-radiative deactivation channels of the excited states [47].
Similarly, theΦF of dye 2 has the same change tendency, and the highest ΦF is found in
CCl4. Unlikely, the ΦF of dyes 3-5 are found in non-polar hexane with values of 0.392,
0.403 and 0.265, respectively, and decrease further to about 0.004 in weak polar solvents.
The analogous changing trends are observed in their fluorescence lifetimes. However, the
fluorescence lifetimes of dyes 3-4 in CCl4 are longer than those in hexane, which is
agreement with the report of literature [48]. Surprisingly, the fluorescence is quenched
dramatically and even disappears completely when solvent polarity increases to DCM
through THF for dyes 2-4, and to DCM through ether for dye 5. This may be attributed to
the fact that strong de-excitation of excited states in polar solvents, resulting from ICT
character, favors the quenching [48].
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To deeply understand the solvatochromic behaviors of dyes 1-5, the Stokes shifts
(∆
ν
) are plotted as a function of solvent polarity parameter ET(30) [49-50], known as an
empirical scale and commonly used scale of solvent polarity to analyze solvatochromism.
As shown in Fig. 6, dyes 1-5 exhibit overall increase of Stokes shifts from non-polar to
polar solvents. And there are relatively good linear correlations between Stokes shifts and
empirical polarity parameter ET(30) with R2 equal to 0.88, 0.97, 0.93, 0.89 and 0.94,
respectively, implying strong dependence of ICT fluorescence emission on the
solvents of different polarity[51].
3.2 Single-crystal X-ray analysis
The single crystal of dye 3 is obtained via slow evaporation of cosolvent system
(ethyl acetate and petroleum ether). The data have been deposited at the Cambridge
Crystallographic Data Centre (deposition number is CCDC 1407658) and the pertinent
crystallographic data are summarized in Table S1. The crystal structure of dye 3 (Fig.
7a) shows that dihedral angles between three benzene rings (C15-C20, C21-C26,
C27-C32) of TPA and the benzene ring (C1 C2 C3 C12 C13 C14) of AQ are 28.3°,
136.7° and 47.6°, respectively. From the crystal cell figure viewed in b-axis of molecule
3 as shown in Fig. 7b, two hydrogen bonds in C22−H···O1 and C9−H···O2 are the
predominant intermolecular interactions and their bond lengths are 2.494 Å and 2.388 Å,
respectively. Interestingly, viewing down the c-axis (Fig. 8), two kinds of channels are
observed clearly, which are attributed to two hydrogen bonds stated above (the formation
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of 3D molecular packed structure can be seen in the video of SI). The dimensions for the
rhombic cavity and another polygonal one are around 4.2×10.1 Å and 4.0×6.9 Å,
respectively, Surprisingly, the apertures of the cavity structure are up to nanometer level
by TEM (Fig. 9), which is rarely seen in small organic molecules and as host for
complexation potentially [52].
3.3 Electrochemical properties
Cyclic voltammograms (CV) were employed to estimate redox potentials as well as
EHOMO and ELUMO levels. The redox properties of dyes 1-5 were measured using three
electrode systems with glassy carbon as working electrode, Pt wire as counter electrode
and Ag+/Ag as reference electrode [53]. The pseudo-reference was calibrated externally
using a 1.0 mM solution of ferrocene (Fc/Fc+). As shown in Fig. 10, there are three
similar quasi-reversible processes for dyes 1 and dyes 3-5, while two quasi-reversible and
an irreversible process for dye 2. In the cathodic side, two quasi-reversible reduction
peaks of dyes 1-5, almost unaffected by phenylamine/TPA groups on AQ moiety, are
observed corresponding to AQ units. And in the anodic region, the oxidation processes
are attributed to the oxidation of the phenylamine/TPA groups, which are strongly
influenced by N- substituted phenylamines. Compared with dye 1, the oxidation
processes of dyes 2-5 occur at lower potentials. The corresponding energy levels based
on potentials are exhibited in Table 3. Furthermore, the HOMO-LUMO optical energy
gaps (Eg) of dyes 2-5 are reduced by 0.05, 0.27, 0.38, 0.42 eV in comparison with that of
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dye 1, indicating that the introduction of electron-donating groups in the N-substituents
could enhance the ICT process [54]. Additionally, the HOMO levels of dyes 2-5 are in
the range of −5.34 to −5.04 eV and the Eg are 2.09 to 2.46 eV, which is efficient for hole
transport and beneficial for photovoltaic applications [55].
4. Conclusion
A series of intramolecular charge transfer (ICT) anthraquinone dyes with TPAs/PA
units have been synthesized and exhibit outstanding solvatochromic emission. Their
emissions are highly sensitive to solvents and tunable with large red-shifts up to 138-192
nm from non-polar to less polar solvents, accompanied by rich color changes of emission
image. Notably, the large Stokes shifts of dyes 1-5 are observed with the values of
210-306 nm and have relatively good linear correlations with solvent parameter ET(30).
The synthesized dyes have ideal candidates as probes for the detection of less polar
solvents. The cyclic voltammetry and DFT calculations suggest that the as-synthesized
dyes could be potentially applied to the wide field of optoelectronics. In addition, dye 3
would be as host for complexation potentially due to forming cavity structures in crystal.
Acknowledgements
The authors acknowledge National Natural Science Foundation of China (21676188,
21102098) and Tianjin Science and technology plan projects (16JCZDJC37100) for
financial supports. The calculation in this work was supported by high performance
computing center of Tianjin University, China.
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Scheme 1 Synthetic routes of dyes 1-5
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Fig. 1 Experimental UV-Vis absorption spectra of dyes 1-5 in toluene (2.0×10−5 mol L−1) (a)
and calculated by TD-DFT at the B3LYP/6-31G(d) level (b)
Fig. 2 HOMO and LUMO electron distributions and energy levels of dyes 1-5
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Fig. 3 Normalized fluorescence spectra of dyes 1-5 in different solvents (2.0×10−5 mol L−1) using their
longest absorption wavelength as excitation wavelength. (a) 1; (b) 2; (c) 3; (d) 4; (e) 5
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Fig. 4 Fluorescence image in different solvents after irradiation of dyes 1-5 with 365 nm illumination
under a UV-transilluminator
Fig. 5 Fluorescence decay curves of dyes 1-5 in toluene
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Fig. 6 Plots of Stokes shifts (∆ν) of dyes 1-5 versus the solvent polarity parameter ET(30)
Fig. 7 Crystal ORTEP figure (30% probability) (a) and crystal cell along the b axis (b) of dye 3
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Fig. 8 Crystal packing diagram of dye 3 along c-axis
Fig. 9 TEM images of dye 3
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Fig. 10 Cyclic voltammograms of dyes 1-5 (1.0 mM) in DCM solutions containing 0.1 M Bu4NPF6 at
a scan rate of 0.10 V·s−1
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