Synthesis, electronic structure, linear and nonlinear photophysical properties of novel asymmetric branched compounds

Article abstract of DOI:10.1016/j.dyepig.2019.108115

A series of novel asymmetric branched compounds that utilize a 1,3,5-triazine core and feature D-π-A-(π-D′-π-A′)_0-2 configurations (D = donor, A = acceptor, π = conjugated bridge) were designed, successfully synthesized, and fully characterized by ¹H NMR, ¹³C NMR, FT-IR, and HRMS. Their photophysical properties including linear absorption, one-photon excited fluorescence, two-photon absorption, and frequency up-converted fluorescence, were systematically investigated in different solvents. With a rise in the polarity of solvents, the peak positions of the one-photon excited fluorescence are red-shifted and the Stokes shifts increase, while the linear absorption wavelengths change slightly. In addition, the target compounds except CZ show the positive solvatokinetic effect. With a rise in the number of branches, the red shifts of the absorption and emission maxima, the hyperchromicity of the molar absorption coefficients, and the decrease of the Stokes shifts are observed. The peripheral electron donors (carbazole, phenothiazine) and acceptors (pyridine, benzimidazole) also exert an important influence on the photophysical properties. Under excitation of 690–930 nm fs laser pulses, all the target compounds emit frequency up-converted fluorescence with the maximal peaks at 471–575 nm, and the two-photon absorption cross-sections in THF are 132 (PTZ), 182 (CZ), 453 (CZ-Py1), 844 (CZ-Py2), 1244 (CZ-BI1), and 2072 (CZ-BI2) GM, respectively. Their two-photon response is found to be nearly additive with respect to the number of branches. The time-dependent density functional theory calculations were conducted to gain an insight into their electronic structures and to better understand the structure-photophysical property relationships. The results clearly indicate the importance of appropriate structural units on the enhancement of two-photon absorption properties.

Full text of DOI:10.1016/j.dyepig.2019.108115

Journal Pre-proof
Synthesis, electronic structure, linear and nonlinear photophysical properties of novel
asymmetric branched compounds
Zhi-Bin Cai, Li-Jun Chen, Sheng-Li Li, Qing Ye, Yu-Peng Tian
PII:
S0143-7208(19)31941-2
DOI:
https://doi.org/10.1016/j.dyepig.2019.108115
DYPI 108115
Reference:
To appear in: Dyes and Pigments
Received Date: 16 August 2019
Revised Date: 6 December 2019
Accepted Date: 6 December 2019
Please cite this article as: Cai Z-B, Chen L-J, Li S-L, Ye Q, Tian Y-P, Synthesis, electronic structure,
linear and nonlinear photophysical properties of novel asymmetric branched compounds, Dyes and
Pigments (2020), doi: https://doi.org/10.1016/j.dyepig.2019.108115.
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition
of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of
record. This version will undergo additional copyediting, typesetting and review before it is published
in its final form, but we are providing this version to give early visibility of the article. Please note that,
during the production process, errors may be discovered which could affect the content, and all legal
disclaimers that apply to the journal pertain.
©
2019 Published by Elsevier Ltd.

Synthesis, electronic structure, linear and nonlinear
photophysical properties of novel asymmetric branched
compounds
a,
a
b
a
b
Zhi-Bin Cai *, Li-Jun Chen , Sheng-Li Li , Qing Ye , Yu-Peng Tian
a
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou
10014, P. R. China
3
b
Department of Chemistry, Anhui Province Key Laboratory of Functional Inorganic
Materials, Anhui University, Hefei 230039, P. R. China
*
Corresponding author.
E-mail address: caizbmail@126.com (Z.-B. Cai).
Abstract: A series of novel asymmetric branched compounds that utilize a
1
,3,5-triazine core and feature D-π-A-(π-D'-π-A')0-2 configurations (D = donor, A =
acceptor, π = conjugated bridge) were designed, successfully synthesized, and fully
1
13
characterized by H NMR, C NMR, FT-IR, and HRMS. Their photophysical
properties including linear absorption, one-photon excited fluorescence, two-photon
absorption, and frequency up-converted fluorescence, were systematically
investigated in different solvents. With a rise in the polarity of solvents, the peak
positions of the one-photon excited fluorescence are red-shifted and the Stokes shifts
increase, while the linear absorption wavelengths change slightly. In addition, the
target compounds except CZ show the positive solvatokinetic effect. With a rise in the
number of branches, the red shifts of the absorption and emission maxima, the
hyperchromicity of the molar absorption coefficients, and the decrease of the Stokes
shifts are observed. The peripheral electron donors (carbazole, phenothiazine) and
1

acceptors (pyridine, benzimidazole) also exert an important influence on the
photophysical properties. Under excitation of 690-930 nm femtosecond laser pulses,
all the target compounds emit frequency up-converted fluorescence with the maximal
peaks at 471-575 nm, and the two-photon absorption cross-sections in THF are 132
(PTZ), 182 (CZ), 453 (CZ-Py1), 844 (CZ-Py2), 1244 (CZ-BI1), and 2072 (CZ-BI2)
GM, respectively. Their two-photon response is found to be nearly additive with
respect to the number of branches. The time-dependent density functional theory
calculations were conducted to gain an insight into their electronic structures and to
better understand the structure-photophysical property relationships. The results
clearly indicate the importance of appropriate structural units on the enhancement of
two-photon absorption properties.
Keywords: two-photon absorption, fluorescence, asymmetric branched compound,
structure-property relationship
1
. Introduction
Two-photon absorption (TPA) is a nonlinear optical phenomenon that involves the
excitation of a molecule by simultaneous absorption of two photons [1]. TPA
materials are of great interest in current research due to their many potential
applications such as optical power limiting [2], two-photon pumped up-converted
lasing [3], two-photon fluorescence imaging [4], three-dimensional optical data
storage [5], three-dimensional microfabrication [6], and two-photon dynamic therapy
[
7].
The TPA cross-section (σ) is the most important parameter used to evaluate TPA
properties of materials. Therefore, to realize the above-mentioned applications, great
efforts have been focused on the design and synthesis of novel materials with large σ.
Many studies reveal that σ can substantially increase by incorporating several dipolar
2

or quadrupolar chromophores into one molecular structure through a common core, in
which multi-branched architectures produce the cooperative enhancement effect. For
example, Zhao et al. [8] synthesized novel multi-branched TPA dyes containing
ketocoumarin, the largest σ was obtained to be 1117 GM. He et al. [9] synthesized a
new series of linear and multi-branched bithiazole-based compounds possessing
terminal triphenylamine groups, the σ values of the multi-branched compounds (1132
and 1665 GM) increase obviously compared with those of the linear counterparts (173
and 429 GM). Thus, multi-branched architectures have become an effective strategy
in the molecular design of TPA materials.
Multi-branched architectures greatly extend the molecular π-conjugated systems. A
high degree of electron delocalization in the whole π-system is considered to be the
key factor to improve σ. So in the design of multi-branched TPA materials, it is
important to select appropriate cores, peripheral moieties, and branch configurations
which can facilitate intramolecular charge transfer (ICT). Recently, many
heterocycle-based TPA materials with large σ have been reported [10]. It was found
that π-deficient and π-excessive heterocycles may act as efficient electron acceptors
(A) and donors (D), respectively. Additionally, molecules based on heterocycle(s)
often emit strong fluorescence, which is an important prerequisite for some
TPA-based applications such as fluorescence microscopy. 1,3,5-Triazine is a
heterocycle, analogous to the six-membered benzene ring but with three alternate
carbon atoms replaced by nitrogen atoms. It is more π-electron-deficient than other
heterocyclic rings and thus can be used as an effective electron acceptor. In addition,
the structural symmetry and the spatial coplanarity make it easier to become a core
which can be functionalized with a variety of desired molecules. For example, Wang
et al. [11] investigated several modified triple-branched compounds based on a
1
,3,5-triazine core, the results indicate that these compounds exhibit good TPA and
two-photon fluorescence properties. Yang et al. [12] synthesized
a
diketopyrrolopyrrole-based multi-branched derivative with a 1,3,5-triazine core,
which exhibits good TPA and aggregation-induced emission properties. The aromatic
heterocycle carbazole has perfect coplanarity and its electron density is much richer
3

compared with benzene and fluorene, for a lone pair of electrons on the nitrogen atom
participates in forming its large π-bond [13]. Phenothiazine is a recognized
pharmaceutical molecule bearing electron-rich nitrogen and sulfur heteroatoms, and
its ring is nonplanar with a butterfly conformation in the ground state, which can
impede the molecular aggregation and the formation of intermolecular excimer [14].
Both carbazole and phenothiazine can be used as efficient electron donors. Likewise,
as nitrogen-containing heterocyclic molecules, pyridine and imidazole have strong
electron-withdrawing ability and good coplanarity, rendering them to be ideal electron
acceptors for nonlinear optical materials [15]. Up till now, the reported
multi-branched TPA materials generally adopt exactly the same branches [16], while it
is rarely reported that different structural units are introduced into branches or
different branch configurations are combined to construct asymmetric multi-branched
TPA materials [17]. Therefore, the relationship between structures and properties of
multi-branched TPA materials modulated by asymmetry remains largely unclear. In
addition, the multi-branched TPA materials reported so far usually adopt two branch
configurations: a D-π-A dipolar structure [18] and a D-π-D quadrupolar structure [19].
Compared with D-π-D quadrupolar structures, A-π-D-π-A quadrupolar structures
introduce electron acceptors [20], large σ may be achieved by stronger ICT from the
central donor to the peripheral acceptors through various conjugated linkages upon
optical excitation. For example, Liu et al. [21] synthesized a D-π-D molecule based
on two triphenylamine donors and its σ is 75 GM. Katan et al. [22] synthesized a
A-π-D-π-A molecule based on a triphenylamine center and two octylsulfonyl end
groups, whose σ (420 GM) increases obviously.
In this work, we designed a series of novel asymmetric single-branched (PTZ, CZ),
double-branched (CZ-Py1, CZ-BI1), and triple-branched (CZ-Py2, CZ-BI2)
compounds built from a 1,3,5-triazine core, in which carbazole (or phenothiazine) and
pyridine (or benzimidazole) were selected as the peripheral electron donor and
acceptor, respectively, and the electron-donating methoxy groups were introduced in
the middle of the branch as the auxiliary donors (Fig. 1). Their configurations are in
the form of D-π-A (PTZ, CZ), D-π-A-π-D'-π-A' (CZ-Py1, CZ-BI1), and
4

D-π-A-(π-D'-π-A')2 (CZ-Py2, CZ-BI2), respectively. To our knowledge, the
asymmetric multi-branched TPA materials formed by incorporating dipolar D-π-A and
quasi-quadrupolar A-π-D-π-A' branches into a 1,3,5-triazine core have not been
reported. All the target compounds were successfully synthesized and fully
1
13
characterized by H NMR, C NMR, FT-IR, and HRMS. Their linear and nonlinear
photophysical properties were systematically investigated in different solvents. The
time-dependent density functional theory (TDDFT) calculations were used to
complement and to rationalize experimental findings.
CH3
N
CH3
N
N
N
S
N
CH3
N
CH3
N
N
H3CH2C
CH2CH3
CZ
PTZ
CH3
N
CH3
N
N
N
OCH3
OCH3
H3C
N
N
N
3
H CO
N
N
H3CO
H3CH2C
N
H3CH2C
N
CZ-Py1
CZ-BI1
N
N
N
CH3
H3CO
H3CO
OCH3
OCH3
N
N
N
N
OCH3
OCH3
H3C
N
N
N
N
3
H CO
N
3
H CO
H3CH2C
H3CH2C
N
N
CZ-Py2
CZ-BI2
Fig. 1. Molecular structures of the target compounds.
5

2
. Results and discussion
2
.1. Synthesis and characterization
The synthetic routes of the target compounds are shown in Scheme 1.
,4,6-Trimethyl-1,3,5-triazine (1a) [23], 2,5-dimethoxy-1,4-benzenedicarboxaldehyde
2
(
2a)
[24],
1,2-dimethyl-1H-benzimidazole
(3a)
[25],
and
1
9
0-ethyl-10H-phenothiazine-3-carboxaldehyde
[26],
-ethyl-9H-carbazole-3-carboxaldehyde (4a) [27] were synthesized according to the
literature procedures. 2,5-Dimethoxy-4-[(1E)-2-(4-pyridinyl)ethenyl]benzaldehyde
2b) [24] and
,5-dimethoxy-4-[(1E)-2-(1-methyl-1H-benzimidazol-2-yl)ethenyl]benzaldehyde (2c)
were prepared by the condensation of 4-methylpyridine and
,2-dimethyl-1H-benzimidazole with 2a in a solution of acetic anhydride and acetic
acid, respectively. 2c was reported here for the first time. The alkaline condensation of
a with the corresponding aldehydes (3a, 4a) in methanol gave two new target
(
2
1
1
compounds (PTZ, CZ). Then CZ continued to undergo alkaline condensation with 2b
or 2c to give the other four new target compounds (CZ-Py1, CZ-Py2, CZ-BI1, and
CZ-BI2) in moderate yields.
Column chromatographic purification of the target compounds was performed on
1
13
silica gel (200-300 mesh). All of them were fully characterized by H NMR, C NMR,
1
FT-IR, and HRMS. The H NMR spectra show the doublets for olefinic protons at
8
.49-8.57, 8.02-8.47, 7.71-8.20, 7.45-7.62, 7.37-7.43, and 7.03-7.32 ppm with the
coupling constants of 15.8-16.6 Hz, which suggests the E-configurations. The protons
in the carbazole ring exhibit seven groups of peaks: four doublets at 8.68-8.74,
8
.25-8.31, 7.68-7.71, and 7.66-7.67 ppm, one double doublet at 7.93-7.98 ppm, and
two triplets at 7.50-7.52 and 7.27-7.29 ppm. The protons in the pyridine ring exhibit
two groups of peaks: two doublets at 8.57 and 7.56-7.57 ppm. The protons in the
benzimidazole ring exhibit three groups of peaks: two double doublets at 7.64-7.65
and 7.56 ppm and one multiplet at 7.21-7.28 ppm. For the single- and
double-branched compounds (PTZ, CZ, CZ-Py1, and CZ-BI1), the singlets at
6

2
.53-2.63 ppm are attributed to the protons in the methyl group(s) linked to the
triazine ring, which disappear completely in the triple-branched compounds (CZ-Py2,
CZ-BI2). Moreover, it can be seen that the chemical shifts of the protons in the
olefinic group linked to the benzimidazole ring (about 7.62 and 8.20 ppm) move to
lower fields compared with those linked to the pyridine ring (about 7.45 and 7.71
ppm). This indicates that benzimidazole has stronger electron-withdrawing ability
than pyridine. In the FT-IR spectra, the characteristic absorption peaks appearing at
-1
-1
1
232-1246 cm and 1328-1346 cm are attributed to the stretching vibration of -C-N
-
1
and =C-N, respectively. The peaks at 1211 cm belong to the C-O-C asymmetric
-1
stretching vibration, and those at 1040-1042 cm belong to the C-O-C symmetric
stretching vibration. The skeleton vibration absorption peaks of the triazine ring
-
1
-1
-1
appear at 1504-1530 cm , whose values decrease from 1524 cm to 1519 (1519) cm
-1
and 1504 (1505) cm when going from CZ to CZ-Py1 (CZ-BI1) and CZ-Py2
-1
(
CZ-BI2). The peaks at 973-980 cm correspond to the C-H out-of-plane bending
vibration of the trans olefinic groups, which further supports the E-configurations.
7

CH3
N
NH
NH
CH C OC H
C H OH
N
N
2
5
K CO
AcOH
2
3
CH CN
CH C OC H · HCl
H C
3
3
3
2
5
3
2
5
HCl
CH3
1a
OCH3
OCH3
CHO
OCH3
CH Br
(
HCHO)n
(1) N (CH )
4 2 6
BrH C
OHC
H CO
2
2
HBr / AcOH
(2) AcOH / H O
2
H CO
3
H CO
3
3
2
a
OCH3
CHO
N
CH3
N
Ac O, AcOH
2
H CO
3
2
b (71.3%)
N
CH3
N
N
N
OCH3
CH3
CHO
Ac O, AcOH
2
CH3
H CO
3
2c (70.2%)
S
S
S
CHO
CH CH Br
3
2
DMF
1a
PTZ (63.8%)
POCl3
KOH
N
H
N
N
CH CH
CH2CH3
2
3
3
a
CZ-Py1 (45.7%)
CZ-Py2 (67.4%)
CZ-BI1 (44.4%)
2
b
KOH
CHO
DMF
1a
CZ
69.1%)
POCl3
KOH
N
N
(
H CH C
H3CH2C
3
2
2
c
4
a
KOH
CZ-BI2 (61.2%)
Scheme 1. Synthetic routes of the target compounds.
2
.2. Liner absorption and TDDFT calculations
The linear photophysical properties of the target compounds in different solvents
8

are listed in Table 1. Their linear absorption spectra in THF are shown in Fig. 2. As
shown in Fig. 2, PTZ exhibits two major absorption bands, while the other target
compounds (CZ, CZ-Py1, CZ-Py2, CZ-BI1, and CZ-BI2) show three major
absorption bands. For the double- and triple-branched compounds, the bands observed
at 293-298 and 325-363 nm can be attributed to the π-π* transition of the carbazole
ring and styrene, respectively, the low-energy bands located at 410-433 nm originate
from an ICT process. For the single-branched compounds, the absorption maxima
abs
max
(
λ
) of PTZ with a phenothiazine donor appear at 310-312 nm, which are attributed
abs
max
to the π-π* transition of the phenothiazine ring, while the
λ
of CZ with a
carbazole donor around 380 nm results from an ICT process. It can be seen from the
abs
data in Table 1 that the
λ
values change slightly (≤ 6 nm) with an increase in the
max
solvent polarity from toluene to DMF, which indicates that the surrounding solvent
molecules have little influence on the transition energy of the target compounds. As
abs
the number of branches increases, the red shifts (
λ
) accompanied by the
max
hyperchromicity (the maximum molar absorption coefficient, εmax ) are observed.
abs
4
-1
For example, the
λ
(
εmax ) values in THF increase from 381 nm (3.64×10 mol
max
-1
4
-1
-1
4
-1
-1
L cm ) to 410 nm (4.63×10 mol L cm ) and 418 nm (16.22×10 mol L cm )
when going from CZ to CZ-Py1 and CZ-Py2. These can be due to the extended
π-conjugated systems formed by incorporating more branches into the central triazine
ring. Compared CZ-Py1 with CZ-BI1 and CZ-Py2 with CZ-BI2, respectively,
CZ-BI1 and CZ-BI2 have stronger light-absorbing ability and their transition energy
between the ground and excited electronic states is lower than that of CZ-Py1 and
CZ-Py2.
9

Table 1
Linear photophysical properties of the target compounds.
abs a
max
−4
b
OPEF c
max
d
e
λ
10
ε
λ
∆ν
Φ
Compound
Solvent
-1
-1
-1
(nm)
(mol L cm )
(nm)
550
564
583
594
435
458
469
481
472
485
491
500
479
491
499
509
482
499
503
512
485
504
509
(cm )
14076
14424
14899
15216
3397
4413
4925
5320
3204
3772
3905
4038
2990
3557
3713
3939
3006
3545
3704
3888
2801
3469
3555
3827
PTZ
Toluene
THF
310
311
312
312
379
381
381
383
410
410
412
416
419
418
421
424
421
424
424
427
427
429
431
433
2.08
0.31
0.21
0.05
0.02
0.04
0.06
0.13
0.27
0.49
0.35
0.24
0.01
0.37
0.23
0.08
0.005
0.58
0.47
0.39
0.02
0.45
0.30
0.14
0.006
2.49
CH Cl2
2.70
2
DMF
2.51
CZ
Toluene
THF
2.65
3.64
CH Cl2
4.05
2
DMF
3.13
CZ-Py1
CZ-Py2
CZ-BI1
Toluene
THF
4.94
4.63
CH Cl2
4.88
2
DMF
4.14
Toluene
THF
13.43
16.22
15.88
14.34
6.92
CH Cl2
2
DMF
Toluene
THF
5.12
CH Cl2
5.09
2
DMF
6.64
CZ-BI2
Toluene
THF
14.34
17.24
19.28
20.45
CH Cl2
2
DMF
519
a
-5
-1
Maximum linear absorption wavelength, c = 1×10 mol L .
1
0

b
c
d
e
Maximum molar absorption coefficient.
-6
-1
Maximum one-photon excited fluorescence wavelength, c = 1×10 mol L .
Stokes shift.
Fluorescence quantum yield.
-5
Fig. 2. Linear absorption spectra of the target compounds in THF (c = 1×10 mol
-
1
L ).
To further rationalize the absorption spectra, the TDDFT calculations were
performed using the Gaussian 09 program for the target compounds. The non-local
density function of B3LYP with 6-31G (d) basis sets was used for the calculations.
The ground state geometries of the target compounds were fully optimized. Table 2
abs
suggests that the calculated
λ
values are in consistent with the experimental ones.
max
The absorption peaks with the strongest oscillator strength are at 325 (PTZ), 373
CZ), 440 (CZ-Py1), 452 (CZ-Py2), 456 (CZ-BI1), and 477 (CZ-BI2) nm,
(
11

respectively, which are primarily due to the transition from the HOMO to the LUMO
percent contributions: 81.4%-96.5%) except PTZ (HOMO-1→LUMO, 66.7%). The
(
molecular orbital energy levels mainly involved are plotted, as shown in Fig. 3. The
calculated HOMO-LUMO energy gaps (∆E) are 3.616 (CZ), 3.148 (CZ-Py1), 3.067
(CZ-Py2), 2.978 (CZ-BI1), and 2.934 (CZ-BI2) eV, respectively, corresponding with
abs
max
the sequence of
λ
(CZ < CZ-Py1 < CZ-Py2 < CZ-BI1 < CZ-BI2). The
molecular orbits mainly involved are illustrated in Fig. 4. It can be seen clearly that
the electronic transition of all the target compounds is accompanied by significant
charge transfer. For PTZ, CZ, CZ-Py1, and CZ-Py2, the electron clouds of the
HOMOs are distributed on the electron donors (phenothiazine, carbazole, and
methoxy) and π-conjugated bridges (1,2-vinylene and 1,4-phenylenedivinylene),
whereas in the LUMOs, most or all of the electron clouds on the above-mentioned
donors have transferred to the electron acceptors (triazine and pyridine). For CZ-BI1
and CZ-BI2, it is notable that the electron clouds of the HOMOs are not spread over
the D-π-A branch, but are essentially concentrated on the methoxy donor, the
benzimidazole acceptor, and the 1,4-phenylenedivinylene π-bridge of the A-π-D'-π-A'
branch. In the LUMOs, the electron cloud density localized on methoxy and
benzimidazole decreases, while that on the strong electron-withdrawing triazine core
increases obviously.
Table 2
Calculated and experimental absorption maxima, oscillator strength, and major orbital
contributions for the target compounds.
Calculated Experimental
Oscillator
Compound Transition
Major orbital contribution
abs
abs
λ
(nm)
λ
(nm)
strength, f
max
max
a
PTZ
CZ
S
S
0
0
→S
3
1
325
373
310-312
379-383
0.6386
HOMO-1→LUMO (66.7%),
HOMO→LUMO+2 (20.4%)
HOMO→LUMO (96.5%)
a
→S
0.7346
1
2

a
a
CZ-Py1
CZ-Py2
S
S
0
0
→S
1
2
440
452
410-416
418-424
0.9124
1.0468
HOMO→LUMO (95.4%)
HOMO→LUMO (83.2%),
HOMO-1→LUMO (9.7%)
HOMO→LUMO (90.5%)
HOMO→LUMO (81.4%),
HOMO-1→LUMO (14.8%)
→S
a
a
CZ-BI1
CZ-BI2
S
S
0
0
→S
1
1
456
477
421-427
427-433
1.7891
1.8904
→S
a
Data from the linear absorption spectra in different solvents.
Fig. 3. Molecular orbital energy levels of the target compounds mainly involved in the
TDDFT calculations.
1
3

PTZ
CZ
CZ-Py1
CZ-Py2
CZ-BI1
CZ-BI2
LUMO+2
LUMO
HOMO
HOMO-1
Fig. 4. Molecular orbits of the target compounds mainly involved in the TDDFT
calculations.
1
4

2
.3. One-photon excited fluorescence
The one-photon excited fluorescence (OPEF) spectra of the target compounds are
shown in Figs. 5 and 6. The corresponding data are listed in Table 1. Upon
photoexcitation, all the target compounds exhibit a fluorescence peak located within
the range from 435 to 594 nm, which can be assigned to the ICT emission. It is
noticeable that the effect of solvent polarity on the emission behavior is much more
pronounced than that on the absorption behavior. The empirical parameter E (30) has
T
been used fairly effectively to describe solvent polarity, whose values of toluene, THF,
CH Cl , and DMF are 33.9, 37.4, 40.7, and 43.2, respectively [28]. As the solvent
2
2
OPEF
) show positive solvatochromic behavior.
max
polarity increases, the OPEF maxima (
λ
OPEF
max
From toluene to DMF, the
λ
values range from 550 to 594 nm for PTZ, from
4
35 to 481 nm for CZ, from 472 to 500 nm for CZ-Py1, from 479 to 509 nm for
CZ-Py2, from 482 to 512 nm for CZ-BI1, and from 485 to 519 nm for CZ-BI2. The
OPEF
max
Stokes shifts (∆v) exhibit the same solvent polarity dependencies as the
λ
. These
can be explained by the fact that the excited state may possess higher polarity than the
ground state, as a result, the solute-solvent interaction in the excited state is enhanced
as compared with that in the ground state, which leads to a lower energy lever for the
excited state and finally decreases the energy gap between the ground and excited
abs
max
OPEF
max
states. Similar to the
λ
, the
λ
is also red-shifted with the increase of
OPEF
max
branches owing to the large extension of π-systems. For example, the
λ
values
in THF increase from 458 nm of CZ to 485 (499) nm of CZ-Py1 (CZ-BI1) and 491
504) nm of CZ-Py2 (CZ-BI2). However, the ∆v values display a corresponding
(
decline, which implies that the polarity of these branched compounds in the excited
state can be sequenced as CZ > CZ-Py1 > CZ-Py2 and CZ > CZ-BI1 > CZ-BI2. In
OPEF
max
abs
addition, inspection of the
λ
as well as the
λ
reveals that the electronic
max
structures of the double-branched compounds CZ-Py1 and CZ-BI1 may be more
similar to those of the corresponding triple-branched compounds CZ-Py2 and
1
5

OPEF
max
abs
max
CZ-BI2 (3-9 nm shifts for
λ
and 5-9 nm shifts for
λ
), rather than that of the
OPEF
max
single-branched compound CZ (19-47 nm shifts for
λ
and 29-44 nm shifts for
abs
max
OPEF
max
λ
). The influence of the peripheral electron donor on the
λ
(PTZ cf. CZ) is
OPEF
max
more remarkable than that of the peripheral electron acceptor on the
λ
(CZ-Py1
OPEF
max
cf. CZ-BI1 and CZ-Py2 cf. CZ-BI2). For example, in THF, the
λ
of PTZ at
5
64 nm is red-shifted by 106 nm relative to that of CZ at 458 nm (phenothiazine
donor cf. carbazole donor), while for CZ-Py1, CZ-Py2, CZ-BI1, and CZ-BI2
OPEF
max
(pyridine acceptor cf. benzimidazole acceptor), the differences among
λ
do not
exceed 14 nm. From the fluorescence photographs in THF (Fig. 6), it is obvious that
PTZ and CZ emit fluorescence of different colors varied from orange-yellow to blue,
while the colors of fluorescence are all green for CZ-Py1, CZ-Py2, CZ-BI1, and
CZ-BI2. These observations indicate that the radiative transition energy of PTZ is far
lower than that of CZ probably due to its significant geometric reorganization and
strong solvation effect in the excited state, while the radiative transition energy of
CZ-Py1, CZ-Py2, CZ-BI1, and CZ-BI2 is close to each other.
PTZ
1
6

PTZ
CZ
CZ-Py1
1
7

CZ-Py2
CZ-BI1
1
8

CZ-BI2
Fig. 5. One-photon excited fluorescence spectra of the target compounds in different
-6
-1
solvents (c = 1×10 mol L ).
Fig. 6. One-photon excited fluorescence spectra and the fluorescence photographs of
the target compounds in THF.
1
9

It is well accepted that the dipole moment changes are related to the optical
properties of organic compounds. The Lippert-Mataga equation is widely used to
estimate the dipole moment changes between the excited and ground states (
∆µeg ),
which is as follows [29]:
2
eg
2∆µ
∆
ν
=
∆f + constant
(1)
3
hca
where ꢀν
absorption (emission) maximum. ꢀµeg corresponds to (µe
moments of the excited and ground states, respectively. h is Planck’s constant, c is the
= νabs – vem represents the Stokes shift, vabs (v ) is the wavenumber of the
em
–
µg), µe and µg are the dipole
speed of light,
1) / (2
a
is the radius of the Onsager spherical cavity.
2
2
∆
f
=
(ε
−
ε
+
1)
−
(n
−
1) / (2n
+
1) represents the orientation polarizability,
ε
is
the dielectric constant and n is the refractive index of the solvent.
The Lippert-Mataga plots for CZ-Py1 CZ-Py2 CZ-BI1, and CZ-BI2 are
displayed in Fig. 7. The ꢀν values are found to depend linearly on the orientation
polarizability of the solvents ( f). The slopes (k) derived from the fitted lines are 3172
,
,
ꢀ
-1
(
CZ-Py1), 3475 (CZ-Py2), 3265 (CZ-BI1), and 3774 (CZ-BI2) cm , respectively.
1
2
eg
3
According to the Lippert-Mataga relationship (
∆µ
= khca ), the
∆µeg is
2
proportional to the k and the Onsager radius (a). Considering the k and the a together,
the order of the CZ-Py1 CZ-BI2 CZ-BI1 CZ-BI1
µeg should be CZ-Py2
CZ-Py1, and CZ-BI2 CZ-Py2. The increase of the
µeg is expected to result in
the enhancement of TPA.
∆
>
,
>
,
>
>
∆
2
0

Fig. 7. Lippert-Mataga plots for CZ-Py1, CZ-Py2, CZ-BI1, and CZ-BI2.
The fluorescence quantum yields (
Equation (2) [30]. Fluorescein in 0.1 mol L sodium hydroxide was used as the
reference ( = 0.9 [30]).
Φ) in different solvents were calculated by
-1
Φ
2
A n F
s
r
s
2
Φs
=
Φr
(2)
A n F
r
s
r
where the subscripts s and r denote the sample and the reference, respectively. A is the
absorbance at the excitation wavelength, n is the refractive index of the relevant
solution, and F is the integrated area under the corrected emission spectrum.
OPEF
max
As can be seen from Fig. 5 and Table 1, both the maximum OPEF intensity (
I
)
and the
solvatokinetic effect. For example, the
times that in slightly polar toluene. This should be due to the proximity effect [31].
The vibronic coupling of the n- * state with the lowest * state increases
non-radiative decay (internal conversion and / or intersystem crossing), which can be
weakened in polar solvents. In contrast to CZ, the other target compounds (PTZ
Φ
of CZ increase upon increasing the solvent polarity, showing the negative
Φ
of CZ in highly polar DMF is more than six
π
π-π
,
2
1

CZ-Py1
,
CZ-Py2
,
CZ-BI1, and CZ-BI2) show the positive solvatokinetic effect.
OPEF
max
With a rise in the solvent polarity, both the
and significantly. Especially, the
I
and the Φ decrease consistently
Φ
values dramatically drop to 0.02 for PTZ, 0.01 for
CZ-Py1, 0.005 for CZ-Py2, 0.02 for CZ-BI1, and 0.006 for CZ-BI2 in highly polar
DMF. This fluorescent behavior may be attributed to the formation of the twisted
intramolecular charge transfer (TICT) state which is usually characterized by
enhanced charge transfer accompanied by a twist of the bond joining the donor and
the acceptor [32]. The polarity-dependent TICT state is non-emissive because of rapid
intersystem crossing to non-radiative triplets.
2
.4. Two-photon properties
As a consequence of their relatively strong fluorescence, the TPA response of all
the target compounds in the near infrared (NIR) region was experimentally
determined by the well-known two-photon induced fluorescence method [33]. Their
corresponding nonlinear photophysical properties in toluene and THF are listed in
Table 3. The representative two-photon excited fluorescence (TPEF) spectra of
CZ-BI2 in THF are shown in Fig. S1. Under excitation of 690-930 nm femtosecond
laser pulses, they all emit frequency up-converted fluorescence with the maximum
TPEF
TPEF wavelengths (
λ
) at 471-561 nm (toluene) and 496-575 nm (THF).
max
OPEF
max
TPEF
max
Compared with the
λ
, the
λ
values are obviously red-shifted owing to the
reabsorption effect. Since the linear absorption spectra in the long wavelength sides
and the OPEF spectra in the short wavelength sides overlap. Furthermore,
-3
-1
-6
-1
concentrated solution (1×10 mol L ) instead of dilute solution (1×10 mol L ) was
used in the TPEF measurement. The reabsorption of the short wavelength
fluorescence can no longer be neglected. As shown in Fig. S1, it is obvious that the
TPEF intensity is significantly associated with the excitation wavelengths, while the
excitation wavelengths slightly affect the TPEF peak shapes and positions. All the
target compounds exhibit a single TPEF peak. Considering the introduction of
electron donors and acceptors which may produce the strong push-pull electronic
2
2

effect, this phenomenon indicates that the emitting state here should not be the locally
excited (LE) state but the ICT state.
Table 3
Noninear photophysical properties of the target compounds.
TPEF a
λmax
b
c
σ max
σ max / MW
Compound
Solvent
-
1
(
nm)
(GM)
104
(GM g mol)
0.289 (0.8)
0.367 (0.7)
0.351 (1.0)
0.555 (1.0)
0.566 (1.6)
0.782 (1.4)
0.641 (1.8)
1.017 (1.8)
1.245 (3.5)
1.968 (3.5)
1.603 (4.6)
PTZ
Toluene
THF
561
575
471
496
511
520
518
524
521
529
525
536
132
CZ
Toluene
THF
115
182
CZ-Py1
CZ-Py2
CZ-BI1
Toluene
THF
328
453
Toluene
THF
532
844
Toluene
THF
787
1244
1500
2072
CZ-BI2
Toluene
THF
2.214 (4.0)
a
-3
-1
Maximum two-photon excited fluorescence wavelength, c = 1×10 mol L .
b
c
-50
4
-1
Maximum two-photon absorption cross-section, 1 GM = 1×10 cm s photon .
Maximum two-photon absorption cross-section divided by molecular weight. The
numbers in the parentheses are the relative values.
It is observed from Fig. 2 that the target compounds are highly transparent above
00 nm, indicating that the laser wavelengths (690-930 nm) used in the TPEF
5
measurement are out of the linear absorption region. The representative TPEF spectra
of CZ-BI2 in THF at 830 nm under different pump powers are shown in Fig. 8. The
2
3

inset displays the logarithmic plot of its fluorescence integral (I ) versus different
out
pump powers (I ). The slope derived from the fitted line is 2.05. For the other target
in
compounds, the relationship between fluorescence intensity and pump powers also
follows the square law. These validate that the detected up-converted fluorescence
emission really comes from the TPA process.
-3
Fig. 8. Two-photon excited fluorescence spectra of CZ-BI2 in THF (c = 1×10 mol
-1
L ) at 830 nm under different pump powers. The inset is the logarithmic plot of the
fluorescence integral (I ) of CZ-BI2 versus different pump powers (I in).
out
The σ values are obtained by comparing the TPEF intensity of the samples with that
of a reference compound using the following Equation (3) [33]:
Fs Φr nr cr
σs
=
σ r
(2)
Fr Φs ns cs
where the subscripts s and r denote the sample and the reference, respectively. F and
represent the TPEF integral intensity and the fluorescence quantum yield. n and c
are the refractive index and the concentration of the solution. In this work, we selected
Φ
2
4

-
1
-3
-1
fluorescein in 0.1 mol L sodium hydroxide (c = 1
values at different excitation wavelengths had been determined by Webb et al.
33b].
×10 mol L ) as the reference. Its
σ
[
Fig. 9. Two-photon absorption cross-sections of the target compounds in THF under
different excitation wavelengths.
Figs. 9 and S2 show the
One can see that the optimal excitation wavelengths in both solvents (THF and
toluene) are 730 (CZ), 790 (PTZ), 810 (CZ-Py1 CZ-Py2), and 830 (CZ-BI1
σ values of the target compounds in the 690-930 nm region.
,
,
CZ-BI2) nm, respectively, which appear at about twice the wavelengths of the
low-energy one-photon absorption bands. This phenomenon indicates that the
absorption of the ICT state plays a dominant role. The
polarity dependencies. As can be seen by comparing Fig. 9 with Fig. S2, the
obtained in moderately polar THF are larger than those obtained in slightly polar
toluene, which may be due to the stronger solute-solvent interaction in THF. The
maxima (σmax) of the single-branched compounds CZ and PTZ in THF (toluene) are
σ
values also display solvent
σ
values
σ
2
5

1
82 (115) and 132 (104) GM, respectively. The small decrease from CZ to PTZ is
ascribed to the nonplanar structure of the phenothiazine ring, which may impede
electron transport. One can observe moderate or significant enhancement of the σmax
for the double-branched compounds by factors of 2.5-2.9 (CZ-Py1) and 6.8 (CZ-BI1),
relative to the single-branched compound (CZ). This can be attributed to the
introduction of a quasi-quadrupolar A-
dipolar D- -A branch, which not only elongates the conjugation pathway, but also
modulates the push-pull electronic mode. As the number of branches is changed
systematically from two to three by introducing another identical A- -D- -A' branch,
the σmax values are found to increase almost additively. For example, the σmax of
CZ-BI2 rises by 713 (828) GM in toluene (THF) with respect to that of CZ-BI1
π-D-π-A' branch different from the original
π
π
π
,
which is approximately the same as the increase in the σmax observed from CZ to
CZ-BI1 (672 GM in toluene and 1062 GM in THF). This fact reveals that the
coupling between the branches is weak. For the series (CZ, CZ-BI1, and CZ-BI2),
the σmax values rise in a proportion of 1.0 : 6.8 : 11.4-13.0, and the ratio of σmax / MW,
which is defined as the σmax divided by the molecular weight, is 1.0 : 3.5 : 4.0-4.6. It is
obvious that both the σmax (1.0 : 2.5-2.9 : 4.6) and the σmax / MW (1.0 : 1.4-1.6 : 1.8)
of the series (CZ
different TPA magnitude of the two series is closely related to the fact that the
peripheral pyridine acceptor (CZ-Py1 CZ-Py2) is replaced with the benzimidazole
, CZ-Py1, and CZ-Py2) are greatly smaller. The significantly
,
acceptor (CZ-BI1
,
CZ-BI2). Compared with pyridine, benzimidazole has stronger
1
electron-withdrawing ability, which can be proved from the H NMR spectra. The
protons in the olefinic group linked directly to benzimidazole exhibit higher chemical
shifts. Therefore, a reasonable explanation is that the benzimidazole acceptor is able
to promote electron delocalization to a greater extent. At the microscopic level, the
is related to the imaginary part of the molecular third-order nonlinear susceptibility
second-order hyperpolarizability, ). A simplified model expression for the is
σ
(
γ
γ
obtained from the Sum-Over-States (SOS) expression, which can be written as
Equation (4) [34]:
2
6

2
ge
2
ge
4
ge
2
ge
2
ee'
M ∆
µ
M
E
M M
γ xxxx[model] = 24
− 24
+ 24
(4)
3
3
ge
∑
2
E
E E
ge
e'
ge ge'
Where Mge ,
∆µge , and Ege are the transition dipole moment, the difference in the
dipole moments, and the transition energy between the ground state and the first
excited state, respectively. Mee' is the transition dipole moment between the first
excited state and the two-photon allowed higher excited states. Ege' is the transition
energy between the ground state and the two-photon allowed higher excited states.
According to Equation (4), the
σ
can be enhanced by (1) decreasing the transition
µge ), and (3)
energy ( Ege Ege' ), (2) increasing the difference in the dipole moments (
,
∆
increasing the transition dipole moment ( Mge , M ). It can be seen from the linear
ee'
absorption spectra and the TDDFT calculation results that the Ege and the Ege'
decrease monotonically following the order: CZ-BI2
Based on the Lippert-Mataga equation, the
µge deduced from the relationship
between the ꢀν and the f can be sequenced as CZ-BI1
CZ-Py2. The square of the Mge shows positive correlation with the
seen in Table 1, an increase in the
CZ-Py2) to CZ-BI1
CZ-BI2), indicating a change in the Mge
< CZ-BI1 < CZ-Py2 < CZ-Py1.
∆
ꢀ
>
CZ-Py1 and CZ-BI2
>
ε
[35]. As
ε
values is evident when going from CZ-Py1
CZ-BI1 CZ-Py1
(
(
(
>
and CZ-BI2
experimental results, which is that the σmax values of CZ-BI1 and CZ-BI2 are much
larger than those of CZ-Py1 and CZ-Py2
> CZ-Py2). All the above evidence further explains the observed
.
3
. Conclusion
A series of novel asymmetric branched compounds that utilize a 1,3,5-triazine core
and feature D- -A-( -D'- -A')0-2 configurations were synthesized using the
dehydration condensation reaction and fully characterized. Their linear and nonlinear
π
π
π
2
7

photophysical properties were studied in detail through the experiments and the
TDDFT calculations. Some interesting structure-property relationships were also
revealed. Our investigation shows that the target compounds do not exhibit the strong
cooperative effect associated with the non-additive enhancement of
σ. But based on
the values of CZ-BI1 and CZ-BI2 (787-2072 GM), which are much larger than
σ
those of the analogous compounds CZ-Py1 and CZ-Py2 (328-844 GM), it is possible
to infer that the benzimidazole acceptor can be a very useful TPA structural unit. We
will further functionalize CZ-BI1 and CZ-BI2 by forming the benzimidazolium salts
with the expectation that they will possess good water solubility combined with
desirable one- and two-photon absorption and luminescence properties for the
application in bioimaging.
4
. Experimental section
4
.1. Materials and instruments
The solvent DMF was pre-dried before use. The other materials were commercially
available and were used without further purification.
Melting points were measured on an X-4 micromelting point apparatus without
1
13
correction. H and C NMR spectra were collected on a Bruker AVANCE
Ⅲ 500
apparatus, with TMS as an internal standard and DMSO-d as a solvent. FT-IR spectra
6
were recorded on a Thermo Nicolet 6700 spectrometer using KBr pellets. Mass
spectra were taken on a Thermo Scientific LTQ Orbitrap XL mass spectrometry
instrument.
The linear absorption spectra were measured on a Shimadzu UV-2550 UV-visible
spectrophotometer. The OPEF spectra measurements were performed using a
RF-5301PC fluorescence spectrophotometer. The TPEF spectra were measured using
a femtosecond Ti : Sapphire laser (Chameleon Ultra
Ⅱ) as the pump source with a
pulse width of 140 fs and a repetition rate of 80 MHz, and tuned by step of 20 nm in
the wavelength range of 690-930 nm.
2
8

4
.2. Synthesis
4
.2.1.
2,5-dimethoxy-4-[(1E)-2-(1-methyl-1H-benzimidazol-2-yl)ethenyl]
benzaldehyde (2c
)
A mixture of 1,2-dimethyl-1H-benzimidazole (1.46 g, 10 mmol), 2a (1.94 g, 10
mmol), acetic anhydride (6 mL), and acetic acid (3 mL) was heated under reflux for 6
h, then cooled to room temperature. After concentrated hydrochloric acid (15 mL)
was added, the mixture was filtered. The filtrate was neutralized with 30% aqueous
sodium hydroxide solution (30 mL) and gave a precipitate. The precipitate was
purified by column chromatography on silica gel using petroleum ether / ethyl acetate
(8 : 1) as eluent to give yellow crystalline powder (2.26 g, 70.2%). m.p. 244-246 ºC;
1
H NMR (DMSO-d , 500 MHz)
δ: 10.36 (s, 1H), 8.17 (d, J = 16.0 Hz, 1H), 7.75 (s,
6
1
6
3
1
3
4
H), 7.73 (d, J = 16.0 Hz, 1H), 7.64 (dd, J = 6.9 Hz, J = 1.3 Hz, 1H), 7.58 (dd, J =
1
2
1
.9 Hz, J = 1.3 Hz, 1H), 7.31 (s, 1H), 7.22-7.28 (m, 2H), 4.03 (s, 3H), 3.99 (s, 3H),
2
.92 (s, 3H); FT-IR (KBr)
ν: 3046, 2944, 2868, 1669, 1602, 1465, 1402, 1334, 1267,
+
210, 1039, 973, 745, 602; HRMS (ESI): m/z calcd for C H N O [M+H] :
1
9
19
2
3
23.1396; found: 323.1392.
.2.2. 3-[(1E)-2-(4,6-dimethyl-1,3,5-triazin-2-yl)ethenyl]-9-ethyl-9H-carbazole (CZ
To a mixture of 1a (0.55 g, 4.5 mmol), potassium hydroxide (0.3 g), and methonal
30 mL) was added a solution of 4a (0.67 g, 3 mmol) in methanol (30 mL) dropwise.
)
(
The reaction mixture was refluxed for 20 h and then the solvent was removed. The
residue was purified by column chromatography on silica gel using petroleum ether /
ethyl acetate (10 : 1) as eluent to give bright yellow needle crystals (0.68 g, 69.1%).
1
m.p. 156-157 ºC; H NMR (DMSO-d , 500 MHz)
δ: 8.68 (d, J = 1.1 Hz, 1H), 8.35 (d,
6
J = 15.9 Hz, 1H), 8.25 (d, J = 7.6 Hz, 1H), 7.93 (dd, J = 8.6 Hz, J = 1.1 Hz, 1H),
1
2
7
7
.68 (d, J = 8.6 Hz, 1H), 7.66 (d, J = 8.2 Hz, 1H), 7.50 (t, J = 7.6 Hz, 1H), 7.27 (t, J =
.6 Hz, 1H), 7.21 (d, J = 15.9 Hz, 1H), 4.49 (q, J = 7.1 Hz, 2H), 2.56 (s, 6H), 1.34 (t,
1
3
J = 7.1 Hz, 3H); C NMR (DMSO-d , 125 MHz)
δ: 175.40, 170.76, 142.90, 140.72,
6
1
1
1
40.06, 126.26, 126.23, 125.88, 122.73, 122.27, 122.17, 121.38, 120.72, 119.42,
09.62, 109.50, 37.15, 25.21, 13.72; FT-IR (KBr)
ν: 3051, 2967, 2874, 1622, 1524,
+
475, 1388, 1337, 1234, 976, 742; HRMS (ESI): m/z calcd for C H N [M+H] :
2
1
21
4
2
9