Tuning the Photophysical Properties of Symmetric Squarylium Dyes: Investigation on the Halogen Modulation Effects

Article abstract of DOI:10.1002/chem.201804393

A series of symmetric squarylium dyes (SQDPA-X) with different halogen (X=F, Cl, Br, I) substituents have been developed. The photophysical properties could be facilely tuned by the halogen modulation effects. The strategy of incorporating different halogen substitutions into AIE active luminogens enables a facile approach for exploring new intriguing organic fluorescent dyes.

Full text of DOI:10.1002/chem.201804393

A Journal of
Accepted Article
Title: Tuning the photophysical properties of symmetric squarylium
dyes: investigation on the halogen modulation effects
Authors: Weiben Chen, Simeng Zhang, Gaole Dai, Ying Chen, Miao Li,
Xiaoyu Zhao, Yulan Chen, and Long Chen
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To be cited as: Chem. Eur. J. 10.1002/chem.201804393
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Tuning the photophysical properties of symmetric squarylium
dyes: investigation on the halogen modulation effects
Weiben Chen,[a]† Simeng Zhang,[a]† Gaole Dai,[b] Ying Chen,[a] Miao Li,[a] Xiaoyu Zhao,[a] Yulan Chen*[a]
and Long Chen*[a]
Abstract: A series of symmetric squarylium dyes (SQDPA-X) with
different halogen (X = F, Cl, Br, I) substituents have been developed.
The photophysical properties could be facilely tuned by the halogen
modulation effects. The strategy of incorporating different halogen
substitutions into AIE active luminogens enables a facile approach for
exploring new intriguing organic fluorescent dyes.
been explored. In this context, we herein reported a new series of
symmetric SQDs derivatives (i.e. SQDPA-F, SQDPA-Cl, SQDPA-
Br, SQDPA-I) with typical AIE, mechanochromic and vapochromic
properties by different halogen (X= F, Cl, Br, I) modulation effects
(Scheme 1). These SQDs were ambiguously characterized by
UV-vis absorption spectroscopy, powder X-ray diffraction (PXRD),
differential scanning calorimetry (DSC), single crystal diffraction
analysis and theoretical modelling.
Organic fluorescent dyes have gained increasing interest on
account of their substantial applications in organic light-emitting
diodes,[1] bioimaging,[2] dye-sensitized solar cells,[3] nonlinear
optical materials,[4] etc. It is well known that most of the
luminophores usually tolerate a pronounced decreasing of their
fluorescence in aggregated states, known as aggregation caused
quenching (ACQ) effect.[5] In contrast, some particular
chromophores are emissive in the aggregated states. This
unconventional photophysical phenomenon is coined as
aggregation-induced emission (AIE) by Tang and co-workers.[6]
The comprehensive mechanism for the unique phenomenon was
clarified to be the restriction of intramolecular rotations (RIR).[7]
Most of AIE active luminogens (AIEgens) possess propeller-like
or flexible moieties such as diphenylamine,[8] triarylamine,[9]
tetraphenylethylene[10] etc. Currently, except for the above-
mentioned applications, AIEgens based stimuli-responsive
fluorochromic materials[11] have also been developed.[12]
Fluorochromic materials with reversible colour change
triggered by external stimuli are an important category of smart
materials.[13] Up to now, several types of fluorochromic materials
have been developed such as photochromic,[14] electrochromic,
[15] thermochromic,[16] mechanochromic,[17] solvatochromic[18] and
vapochromic[19] materials. Among them, mechanochromic
materials with reversible fluorescence change by mechanical
stress have made rapid progress.[20]
The SQDs were facilely synthesized by the condensation of
squaric acid with diphenylamine derivatives in toluene and n-butyl
alcohol at 110 oC (Scheme 1).[23] The photophysical properties of
all SQDs was first investigated by UV-vis spectra performed in
DMF solutions. As shown in Figure 1, the intense absorption
peaks at 406 to 425 nm can be correlated to the HOMO-LUMO
transitions (385.18 nm to 396.05 nm, oscillator strength f = 0.6071
to 0.7601) based on the time dependent density functional theory
(TD DFT) calculations (CAM-B3LYP/6-31G**) (Table S4, Figure
S32). The red shift sequence of simulated absorption maxima
peaks also match with the experimental spectra, i.e. SQDPA-F,
SQDPA-H, SQDPA-Cl, SQDPA-Br, SQDPA-I. Obvious red-shifts
in absorption and emission maximum of these SQDs were
observed as the halogen atomic number increased.[24] To further
compare the electronic structures of these SQDs, DFT
calculations at the B3LYP/6-31G** level were carried out.[25] The
HOMOs for all SQDs are π orbitals mainly localized on the central
cyclobutene cores, while π* orbitals of the LUMO are primarily
distributed over the peripheral groups (Figure S31 and Table S3).
The observed optical bandgaps (Eobg) were 2.29 eV, 2.27 eV, 2.24
eV, 2.23 eV, 2.20 eV for SQDPA-F, SQDPA-H, SQDPA-Cl,
SQDPA-Br, SQDPA-I, respectively, which are in accordance with
the calculated energy band gaps (Eg), following the same trend of
SQDPA-F (3.334 eV) > SQDPA-H (3.323 eV) > SQDPA-Cl (3.283
eV) > SQDPA-Br (3.278 eV) > SQDPA-I (3.261 eV) (Table S3,
Figure S31). These results illustrate that the introduction of
halogen atom can modulate the photophysical properties of
squarylium dyes.
Squaric acid is an electron deficient motif with a four
membered ring, which can react with electron donating groups to
form a donor–acceptor–donor (D–A–D) system.[21] Theoretical
simulation suggests that the ground state and excited singlet state
of squarylium dyes (SQDs) exhibit distinctive intramolecular
charge transfer (ICT) behavior.[22] To the best of our knowledge,
SQDs with both AIE and fluorochromic properties have not yet
[a]
W. Chen, S. Zhang, Dr. Y. Chen, M. Li, X. Zhao, Prof. Y. Chen,
Prof. L. Chen
Department of Chemistry, Institute of Molecular Plus, and Tianjin
Key Laboratory of Molecular Optoelectronic Science, Tianjin
University, Tianjin 300072, China.
[b]
Dr. G. Dai
Institute of Functional Nano & Soft Materials (FUNSOM), Soochow
University, Jiangsu 215123, China.
E-mail: yulan.chen@tju.edu.cn; long.chen@tju.edu.cn
These authors contributed equally to this work.
[†]
Scheme 1. Synthesis of symmetric squarylium dyes containing different
halogen substituted diphenylamine derivatives
Supporting information for this article is given via a link at the end of
the document.
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Figure 1. (a) UV-vis absorption spectra of SQDs (1.0 × 10-5 mol/L in DMF); (b) solid-state fluorescence spectra and (c) solid-state absorption spectra of SQDPA-H
(black line), SQDPA-F (red line), SQDPA-Cl (blue line), and SQDPA-Br (pink line) SQDPA-I (green line); (d) the fluorescence images of all SQDs
The AIE characteristics of these new luminogens (i.e.
SQDPA-F, SQDPA-Cl, SQDPA-Br, SQDPA-I) were elucidated in
detail (Figure 2 and Figure S15-S16). All these newly halogen
substituted SQDs were nonluminous in DMF or THF solutions but
emitted bright green light when proper amount of water was
added. The fluorescence significantly enhanced when the water
fraction (fw) exceeded a critical point (fw = 30%, 20%, and 30% for
SQDPA-Cl, SQDPA-Br, SQDPA-I, respectively). The emission
further enhanced with adding more water. However, the
fluorescence intensity descended when the fraction of water fw
exceeded 60%, 40%, and 70% for SQDPA-Cl, SQDPA-Br,
SQDPA-I, respectively. It was demonstrated that large
nanoaggregates were formed and precipitated in these cases by
the dynamic light scattering analysis (Figure S24-S26).[26]
Luminophores with bright emission in solid state usually
exhibit tunable optical properties upon external stimuli, which
reminds us to explore the mechanofluorochromism (MFC)
behaviour of these SQDs. For example, SQDPA-F displays
distinct fluorescence enhancement upon grinding. The
fluorescence quantum yield (φF) slightly increased from 4.80% to
5.45% (Table S2) and the solid-state fluorescence lifetime also
increased from 0.37 ns to 0.55 ns after grinding (Table S5). In
Powder X-ray diffraction (PXRD) measurements were
conducted to unveil the origin of the mechanochromism (Figure
S17-S18). For the ground SQDPA-F, SQDPA-Cl and SQDPA-Br
samples, most of the diffraction peaks decreased compared with
the pristine ones, which demonstrated that the crystalline
structures have been substantially demolished to the partial
amorphous states. The remained weak diffraction peaks might be
ascribed to the insufficient grinding. The diffraction peaks of the
ground SQDPA-Cl and SQDPA-Br samples were greatly
enhanced after fuming with THF, which confirmed the recovery of
an ordered crystalline state. Differential scanning calorimetry
measurement was carried out for all SQDs (Figure S28). Only
SQDPA-Cl and SQDPA-Br exhibited apparent endothermic peaks
at 145 °C and 160°C, respectively, which refered to the phase
conversion from metastable amorphous state to the ordered
contrast,
SQDPA-Cl
and
SQDPA-Br
exhibit
typical
mechanochromic behaviours upon external mechanical
stimulation (Figure 3). The emission bands of SQDPA-Cl (Figure
3a) and SQDPA-Br (Figure 3b) were broadened upon grinding.
And the fluorescence quantum yields of SQDPA-Cl and SQDPA-
Br in the pristine state (49% and 34%, respectively) were higher
than those in the ground state (15% and 14%, respectively). The
solid-state fluorescence lifetime of SQDPA-Cl and SQDPA-Br
also decreased (Table S5). The largest fluorescence red-shift was
56 nm for SQDPA-Cl and 65 nm for SQDPA-Br after grinding. The
emission maxima of the ground SQDs was blue-shifted to the
original state after fuming with THF vapor and the intensity was
enhanced as well. The reversible fluorescence switching has
been demonstrated with SQDPA-Cl and SQDPA-Br for at least
five cycles without significant changes (Figure S20). Such good
fluorescence fatigue resistance is of great significance for
application in sensors or security inking.[27]
Figure 2. Fluorescence spectra of the dilute solutions of (a) SQDPA-Cl and (c)
SQDPA-Br in DMF/H2O with different fw; plot of fluorescence intensity maximum
of (b) SQDPA-Cl and (d) SQDPA-Br; Inset: fluorescence images of (b) SQDPA-
Cl and (d) SQDPA-Br in DMF/H2O mixtures
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show apparent endothermic peaks at 105 °C, which was due to
the crystal state conversion (Figure S29). We further measured
the UV-vis absorption spectra of SQDPA-I at different color states
(Figure S22b). It revealed that the absorbance maximum was
blue-shifted from 478 nm to 423 nm, which was similar to the
absorbance maximum in CHCl3 (425 nm) (Figure S22a). The
above results indicate that the molecular packing could be loosen
when SQDPA-I is fuming with CHCl3, while it could be re-
condensed upon fuming with EA. The different photophysical
properties should originate from the different packing modes.
Figure 3. Normalized solid-state fluorescence spectra of (a) SQDPA-Cl and (b)
SQDPA-Br before (black line) and after (red line) grinding and upon THF (pink
line) fuming. Inset: Photographs of (a) SQDPA-Cl and (b) SQDPA-Br before and
after grinding and organic solvent fuming.
Figure 4. (a) fluorescence spectra of SQDPA-I before (black line) and after (red
line) grinding and CHCl3 (purple line) and ethyl acetate (EA) (blue line) fuming.
(b) photographs of SQDPA-I upon CHCl3 and EA fuming, respectively
microcrystalline state.[28]
The mechanochromism of SQDPA-I was not obvious, but it
showed distinctive vapochromic feature toward different organic
solvents (Figure 4). The red orange colour of the pristine SQDPA-
I changed to bright yellow when fuming with CHCl3, CH2Cl2 or THF
(good solvents). The emission maximum was blue-shifted from
the original 577 nm to 537 nm. Meanwhile, the fluorescence
quantum yield of pristine SQDPA-I was dramatically increased to
29.4% upon CH2Cl2 fuming (Table S2). The solid-state
fluorescence lifetime was also increased from 0.22 ns to 1.44 ns
(Table S5). It is interesting that the color of SQDPA-I could be
recovered to the initial red orange upon fuming with ethyl acetate
(EA, poor solvent). Moreover, this reversible color change could
be repeated for at least five cycles upon alternative fuming with
CHCl3 and EA vapor (Figure S19b). The PXRD pattern of
SQDPA-I slightly exhibited attenuation after grinding. However,
the PXRD pattern of SQDPA-I greatly changed when exposed to
CHCl3 vapor (Figure S19a). In addition, PXRD patterns of
SQDPA-I after CHCl3 fuming (red line) was similar to the
simulation SQDPA-I XRD (black line) based on the single crystal
data (Figure S23). Meanwhile, the DSC of SQDPA-I after CHCl3
In order to understand the different AIE and fluorochromic
behavior of these SQDs, single crystals of the four new SQDs
were obtained through slow vapor diffusion (Figure S11-S14). All
SQDs adopt propeller-like conformations in the single crystal
structure, which suggests that the AIE properties might be
attributed to the restriction of intramolecular rotations of
diphenylamine units.[29] The dihedral angle SQDPA-F between
two phenyl groups of the side diphenylamine is 116.41°. The
hydrogen bond C-F···H-C length (2.434 Å) of SQDPA-F is shorter
than the interaction between hydrogen and other halogen atoms
(Figure S11). The drastic friction force might destruct the stronger
C-F···H-C interaction of SQDPA-F to cause enhanced emission.
The distance of two layers in SQDPA-Cl is 7.389 Å and no
halogen bond and C-Cl···H-C interaction was observed between
adjacent molecules (Figure S12). Molecular packing of SQDPA-
Br molecules was driven by the halogen bonds C-Br···O-C (3.258
Å) and hydrogen bond C-Br···H-C (3.021 Å) in single crystal
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Keywords: symmetric squarylium dyes • halogen • aggregation-
induced emission • mechanochromism • vapochromism
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Entry for the Table of Contents (Please choose one layout)
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Weiben Chen, Simeng Zhang, Gaole
Dai, Ying Chen, Miao Li, Xiaoyu Zhao,
Yulan Chen* and Long Chen*
Page No. – Page No.
Title
Four symmetric squarylium dyes with different halogen substituents have been
facilely synthesized. The photophysical properties exhibit distinctive dependence on
the halogen substitutions.
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