Synthesis and photophysical properties of two-photon chromophores containing 1H-benzimidazole residue

Article abstract of DOI:10.1016/j.dyepig.2014.06.007

The non-centrosymmetric, trans,trans-2-{4-[2-(1H-benzimidazol-2-yl)ethenyl] -styryl}-1,3-dimethylbenzimidazole iodide and centrosymmetric, trans,trans-1,4-bis[4-(2-(1H-benzimidazol-2-yl)ethenyl)]benzene polymethine compounds based on 1H-benzimidazole residue were synthesized and characterized. Their photophysical properties in solvents of different polarity were thoroughly studied by means of absorption, emission and fluorescence quantum yield measurements. Analysis of the results clearly indicates that the compounds have strong one-photon absorption bands in the near ultraviolet region, relatively high fluorescence quantum yields and large Stokes' shifts due to the charge transfer effect. The one- and two-photon absorption properties of a selected non-centrosymmetric dye were compared to those of a centrosymmetric polymethine dye. Their two-photon absorption cross-sections in DMF were determined to be ～330 GM and ～380 GM at ca. 650 nm for the centrosymmetric and non-centrosymmetric polymethine compounds, respectively. Density functional theory calculations of one- and two-photon absorption electronic spectra of the tested compounds using CAM-B3LYP, LC-BLYP and LC-ωPBE functionals were carried out to support the experimental data.

Full text of DOI:10.1016/j.dyepig.2014.06.007

Dyes and Pigments 111 (2014) 162e175
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and photophysical properties of two-photon chromophores
containing 1H-benzimidazole residue
,
*
b
c
ꢀ c
Beata Jędrzejewska a , Przemysław Krawczyk , Marta Gordel , Marek Samoc
a University of Technology and Life Sciences, Faculty of Chemical Technology and Engineering, Seminaryjna 3, 85-326 Bydgoszcz, Poland
Nicolaus Copernicus University, Collegium Medicum, Department of Physical Chemistry, Kurpinskiego 5, 85-950 Bydgoszcz, Poland
Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wyb. Wyspianskiego 27, 50-370 Wroclaw, Poland
b
ꢀ
c
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
The non-centrosymmetric, trans,trans-2-{4-[2-(1H-benzimidazol-2-yl)ethenyl]-styryl}-1,3-dimethylben
zimidazole iodide and centrosymmetric, trans,trans-1,4-bis[4-(2-(1H-benzimidazol-2-yl)ethenyl)]ben-
zene polymethine compounds based on 1H-benzimidazole residue were synthesized and characterized.
Their photophysical properties in solvents of different polarity were thoroughly studied by means of ab-
sorption, emission and fluorescence quantum yield measurements. Analysis of the results clearly indicates
that the compounds have strong one-photon absorption bands in the near ultraviolet region, relatively high
fluorescence quantum yields and large Stokes' shifts due to the charge transfer effect.
The one- and two-photon absorption properties of a selected non-centrosymmetric dye were
compared to those of a centrosymmetric polymethine dye. Their two-photon absorption cross-sections
in DMF were determined to be ~330 GM and ~380 GM at ca. 650 nm for the centrosymmetric and non-
centrosymmetric polymethine compounds, respectively.
Received 27 March 2014
Received in revised form
5 June 2014
Accepted 6 June 2014
Available online 15 June 2014
Keywords:
Polymethine dyes
Synthesis
Absorption and emission spectra
Fluorescence lifetime
DFT calculations
Density functional theory calculations of one- and two-photon absorption electronic spectra of the
Two-photon absorption
tested compounds using CAM-B3LYP, LC-BLYP and LC-
experimental data.
u
PBE functionals were carried out to support the
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
g
(ꢀ
three distinct structural elements: electron-donating (D) groups,
electron-accepting (A) groups and conjugated -bridges con-
u
;
u
,ꢀ
u,u). This quantity can be maximized by combining
The development of organic compounds with large two-
photon absorption (TPA) cross-sections has recently been the
focus of research interest due to their various applications in fields
such as two-photon laser scanning microscopy [1,2], 3D micro-
fabrication [3,4], optical data storage [5], optical limiting [6], and
photodynamic therapy [7]. For all of these applications, molecules
with large TPA cross-sections in the visible and near-infrared
spectral region are desired [8e11]. Many strategies have been
identified to maximize the TPA cross-section (s2) of organic ma-
terials. s2 is a measure of probability of simultaneous absorption
of two photons by a molecule, usually expressed in Goeppert-
Mayer units (1 GM ¼ 10ꢀ50 cm4 s photonꢀ1 moleculeꢀ1). From a
fundamental point of view, the TPA cross-section is proportional
to the imaginary part of the second hyperpolarizability
p
necting donor and acceptor moieties [10e12]. Thus, based on the
intra-molecular interactions, typical classes of non-linear optical
molecules are non-centrosymmetric dipolar compounds (A-
centrosymmetric quadrupolar compounds (D- -D, A- -A, D-
-D, Ae -D- -A) and non-centrosymmetric octupolar com-
pounds (D-( -A)3, A-( -D)3) and there are also many more com-
p
-D),
p
p
p-A-
p
p
p
p
p
plex ones. Among the simplest nonlinear dyes with small
molecule size and moderately large s2 one can mention stilbene
derivatives which are based on a “pushepull” system. The non-
linear optical properties of these chromophores depend on the
ability of the donor to provide electrons and the acceptor to
withdraw electrons. An example of the donating group is a ter-
tiary amine group whereas the methyl pyridinium group can
constitute an electron acceptor [13e15].
The TPA cross-sections are most often determined either from
two-photon excited fluorescence [16] or directly by transmission
* Corresponding author. Tel.: þ48 52 374 9034; fax: þ48 52 3749009.
E-mail address: beata@utp.edu.pl (B. Jędrzejewska).
http://dx.doi.org/10.1016/j.dyepig.2014.06.007
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

B. J
ędrzejewska et al. / Dyes and Pigments 111 (2014) 162e175
163
measurements in the Z-scan technique [17]. The two-photon fluo-
rescence technique requires a suitable reference probe [18] and
exclusion of one-photon excited fluorescence from the collected
signal. On the other hand, the Z-scan technique can determine s2
also for non-fluorescent molecules and even in the range of
wavelengths of excited state absorption [2,19].
The non-linear optical responses of new chromophores can be
also predicted by theoretical calculations [20]. There are three main
types of computational methods: a classical method, molecular
mechanics, and two quantum chemical methods: ab initio and
semiempirical. In turn, ab initio methods exist as two large families
of methods: HartreeeFock and density functional.
The purity of the dye was checked by TLC technique. The sample
was dissolved in methanol-DMF 10:1 v/v mixture, applied on the
plate (aluminium oxide IB-F) and eluted with methanol-chloroform
1:10 v/v mixture.
HPLC analyses were done by Waters HPLC systems equipped
with Waters 2489 UVeVis detector (detection wavelength was
370 nm), Waters 1525 Binary HPLC Pump and a Symmetry C18
column (3.5
m
m, 4.6 ꢁ 75 mm). Separation was conducted under
isocratic conditions with 0.8 ml/min flow rate, 25 ꢂC, 10
ml injection
volume and HPLC grade methanol as a mobile phase.
Melting points were determined on the Buchi M-565 Melting
Point apparatus.
Semi-empirical methods are usually parametrized for the
ground state (e.g. AM1) and the excited states active in the linear
spectroscopy (e.g. INDO/S model, fitted to reproduce UVevis ab-
sorption spectra at the CI singles (CIS) level). These models are
often in error when applied to the two-photon transitions where
higher excited-state energy levels are involved and double excita-
tions are important [21e24]. Methods including higher-order cor-
relations are computationally expensive and often result in the over
correlated ground-state wave function [21,25]. In addition, size
consistency is not guaranteed and special care needs to be taken
when choosing the right configurations [21]. Alternatively, the TPA
cross-section may be obtained as implemented for ab initio calcu-
lations e that is, by taking the residue of the second-order response
function or the first hyperpolarizability [25].
The current method of choice for calculating the excited-state
structure of large molecular systems is based on adiabatic time-
dependent density functional theory (TD-DFT) [26] in the Kohne-
Sham (KS) formalism [27e29]. TD-DFT was shown to give a better
agreement with experiment than both semi-empirical and low
level ab initio calculations for two-photon absorption (TPA) calcu-
lations in large quadrupolar conjugated organic chromophores [21]
and small molecules [25,30].
Absorption and emission spectra were recorded at room tem-
perature on a Shimadzu UVevis Multispec-1501 spectrophotom-
eter (Japan) and a Hitachi F-4500 spectrofluorimeter (Japan),
respectively. The spectra were recorded in the following solvents:
tetrahydrofuran (THF), acetone (AcMe); N,N-dimethylformamide
(DMF), dimethylsulfoxide (DMSO), methanol (MeOH), acetonitrile
(MeCN) and water (H2O). The concentration of the dye in the so-
lution was 1.0 ꢁ 10ꢀ5 M and 1.0 ꢁ 10ꢀ6 M for absorption and
fluorescence measurements, respectively. All solvents were spec-
troscopic grade and were used without any additional purification.
They were characterized by their static dielectric constant (ε) and
refractive index (n) at 20 ꢂC. The solvent polarity function, f(ε,n), is
given by Eq. (1) [31].
"
#
ꢀ
ꢁ
ꢀ
ꢁ
2n2 þ 1
n2 ꢀ 1
n2 þ 2
ðε ꢀ 1Þ
ðε þ 2Þ
ꢀ
ꢁ
ꢀ
ꢁ
f ðε; nÞ ¼
$
ꢀ
(1)
n2 þ 2
The fluorescence quantum yields (
F
) were measured by using a
standard method under the same experimental conditions for all
compounds. Dilute Coumarin 1 in ethanol (Fref ¼ 0.64 [32]) at the
same optical density as the other samples (A z 0.1 at 404 nm) was
used as the reference. The quantum yield of the tested dye (
calculated using equation:
F
) was
We describe here investigations of new chromophores
involving vinylidene-linked benzimidazole moieties. The BMe
compound is based on pushepull system, which consists of 1H-
benzimidazole an electron-donating group (D) and 1,3-
dimethylbenzimidazolium iodide an electron-withdrawing group
n2
n2ref
IAref
F ¼ Fref
$
(2)
Iref
A
(A) coupled through a
p-conjugated spacer. The BH is a centro-
symmetric chromophore bearing on both sides 1H-benzimid-
azoles. Herein, we report the preparation and spectroscopic
properties of these compounds. Their one- and two-photon ab-
sorption properties have been characterized by both experimental
and theoretical methodologies.
where:
Fref is the fluorescence quantum yield of the reference
(Coumarin 1) sample in ethanol, A and Aref are the absorbances
of the compound under the study and reference sample at the
excitation wavelength (404 nm), I and Iref are the integrated
emission intensity for the tested compounds and reference
sample, n and nref are the refractive indexes of the solvents used
for the compounds and the reference, respectively.
2. Experimental
2.1. Materials and measurements
All reagents and solvents (spectroscopic grade) were purchased
from Aldrich Chemical Co. or Alfa Aesar Co, and used without
further purification.
The fluorescence lifetimes were measured using an Edinburgh
Instruments single-photon counting system (FLS920P Spectrome-
ters). The excitation was provided by a picosecond diode laser
generating pulses of about 55 ps at 375 nm. Short laser pulses in
combination with a fast microchannel plate photodetector and ul-
trafast electronics make it possible to analyse the fluorescence
decay signals with a resolution of few picoseconds. The compounds
were studied at concentration needed to provide absorbance of
0.2e0.3 in a 10 mm cell at 375 nm. The fluorescence decays were
fitted as sums of two exponentials. The average lifetime, tav was
calculated as tav ¼ (Siaiti)/(Siai), where ai and ti are the amplitudes
and lifetimes.
The 1H (200 MHz or 400 MHz) and 13C (50 MHz or 100 MHz)
NMR spectra were recorded on a Varian Gemini 200 or Bruker
Ascend™ 400 NMR spectrometers, respectively. Dimethylsulfoxide
(DMSO-d6) was used as the solvent and tetramethylsilane (TMS) as
internal standard. Chemical shifts are reported in ppm (d). Coupling
constants, J, are reported in Hz.
The IR spectra of the synthesized salts were recorded using a
Bruker Vector 22 FT-IR spectrophotometer (Germany) in the range
400e4500 cmꢀ1, by KBr pellet technique.
The elemental analysis was performed with an Elementar
Analyser Vario EL III instrument (Germany) operating with the
VARIOEL software (version 5.14.4.22).
The two-photon absorption spectrum was determined using the
open-aperture Z-scan technique [17] using a setup and procedures
described elsewhere [33,34]. Briefly,
a
tunable amplified

164
B. Jędrzejewska et al. / Dyes and Pigments 111 (2014) 162e175
femtosecond laser system consisting of a Quantronix Integra
regenerative amplifier (800 nm, 1.2 mJ/pulse, 130 fs pulse length)
and a Quantronix Palitra optical parametric amplifier was used and
the spectral range of the measurements was 600e1600 nm. The Z-
scan data obtained on a solution of the dye were calibrated against
closed-aperture Z-scan measurements on a 4.66 mm thick silica
plate [33,34].
were carried out employing the CAM-B3LYP functionals and the 6-
311þþG(d,p) basis set.
2.3. Synthetic procedure
The procedure for the preparation of the compounds under
study is shown in Scheme 1.
2.3.1. Synthesis of trans-2-[2-(4-formylphenyl)ethenyl]
2.2. Computational details
The molecular geometry for considered molecules was carried
out using the Gaussian 09 program package [35] and the hybrid
exchange-correlation B3LYP [36] functional in conjunction with the
6e311þþG(d,p) basis set for the ground and first excited state. The
optimization of geometry was performed both in vacuo and with
the inclusion of solvent effects by means of the Integral equation
formalism for the polarizable continuum model (IEFePCM) [37]. It
was confirmed in all cases by means of harmonic vibrational
analysis that the stationary points correspond to minima on the
potential energy surface.
benzimidazole (BCHO)
and
trans,trans-1,4-bis[4-(2-benzimidazolyl)ethenyl]benzene (BH)
These compounds were obtained in parallel synthesis by the
reaction of terephthaldialdehyde with 2-methyl-[1H]-benzimid-
azole [48e50]. The detailed synthetic procedure together with
spectroscopic and HPLC analysis is described in our previous paper
[50].
The electronic absorption and fluorescence spectra were ob-
tained using time-dependent density functional theory (TD-DFT)
using the Gaussian 09 suite of programs. The long-range corrected
LC-BLYP [38,39], LC-uPBE [40e42] and CAM-B3LYP [43] func-
tionals, and the 6-311þþG(p,d) basis set were employed for these
calculations, carried out both in vacuo and with the inclusion of the
solvents effect using the PCM model.
2.3.2. Synthesis of trans,trans-2-{4-[2-(1H-benzimidazol-2-yl)
ethenyl]-styryl}-N,N-dimethylbenzimidazolium iodide (BMe)
In the case of a molecule absorbing two photons of the same
energy in isotropic media, the degenerate two-photon transition
probability in an isotropic medium using a linearly polarized laser
beam is given by [44]:
[51e53]
ꢂ
ꢅ
ꢃ
ꢄ
ꢃ
ꢄ
X
*
*
1
15
OF
ii
OF
jj
OF
ij
OF
ij
〈dOF〉 ¼
S
S
þ 2S
S
(3)
1,2,3-Trimethylbenzimidazolium iodide (0.84 g, 3.4 mmol), 2-
[2-(4-formylphenyl)ethenyl]benzimidazole (0.97 g, 3.4 mmol), and
anhydrous methanol (25 mL) were placed in a 100 mL one-necked
flask with a stirrer and a condenser. Then three drops of piperidine
were added as a catalyst and the resultant mixture was heated
under reflux for 12 h. Upon cooling in a refrigerator, orange mi-
crocrystals of the iodide salt precipitated and were collected by
filtration under reduced pressure; 518.39 g/mol; 1.17 g, 66.7% yield,
mp 283e285 ꢂC, Rf ¼ 0.17 and Rf ¼ 0.76 for methanol-chloroform
1:10 v/v and 1:5 v/v, respectively. Elemental Anal. Calcd for
ij
In this equation, SOijF, is the second-order transition moment
given by:
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
"
〈0 z $_m K〉〈K z $_m F〉
ꢆ
ꢆ
ꢆ
ꢆ
X
1
i
2
j
1
Z
OF
S
ðz1; z2Þ ¼
ij
ua ꢀ u1
K
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
#
ꢆ
ꢆ
ꢆ
〈0 z $_m K〉〈K z $_m F〉
ꢆ
ꢆ
ꢆ
ꢆ
2
1
i
j
þ
(4)
C26H23N4I: C, 60.2; H, 4.5; N, 10.8; I, 24.5%. Found: C, 60.19; H, 4.76;
ua ꢀ u2
N, 10.83%.
where Zu þ Zu should satisfy the resonance condition and
ꢆ
ꢆ
1
2
1H NMR (DMSO-d6)
d (ppm): 3.54 (1H, NH), 4.17 (s, 6H, CH3), 7.14
ꢆ
ꢆ
〈0 z $_m K〉 stands for the transition moment between electronic
ꢆ
ꢆ
1
i
3
(m, 2H, BIm), 7.40e7.44 (d, JH,H ¼ 16.0 Hz, 1H, eCHdbnd), 7.54
states j0> and jK>, respectively.
z is the vector defining polarization
3
(m, 2H, BIm), 7.63e7.59 (d, JH,H ¼ 16.0 Hz, 1H, eCHdbnd), 7.70
of photons.
3
(m, 2H, BIm), 7.75e7.71 (d, JH,H ¼ 16.0 Hz, 1H, eCHdbnd),
It should be considered that determined experimental spectra
7.84e7.82 (d, 3JH,H ¼ 8.0 Hz, 2H, Ar), 7.88e7.84 (d, 3JH,H ¼ 16.0 Hz,
1H, eCHdbnd), 7.99e8.01 (d, 3JH,H ¼ 8.0 Hz, 2H, Ar), 8.07 (m, 2H,
BIm)
(
s(2)) with line broadening are connected to 〈dOF〉 through [45]:
8p3a2Z3 u2gðuÞ
ð2Þ
〈dOF
〉
(5)
13C NMR (DMSO-d6)
d
(ppm): 33.0 (NþCH3), 108.1, 113.0, 118.1,
s
¼
$
OF
e4
GF=2
119.4, 126.5, 127.6, 129.3, 133.3, 133.7, 145.7 (CH), 132.0, 134.6,
136.2, 138.4, 148.1, 150.7, 150.9 (C)
where
a
is a fine structure constant,
u
is the frequency of absor-
IR (KBr) (cmꢀ1): 3054, 1631, 1596, 1562, 1528, 1485, 1415, 1311,
1278, 1211, 1164, 1143, 1024, 968, 812, 741, 518.
bed photons (assuming one source of photons), GF is the broad-
ening of the final state (F) due to its finite lifetime and g(
provides the spectral line profile, which often is assumed to be a
u
)
d-function.
3. Results and discussion
In order to characterize the allowed TPA states, the quadratic
response functions formalism [25,45] combined with the Kohne-
Sham method was used as implemented in the DALTON 2011
program [46,47]. Solvent effects were taken into account with the
self-consistent reaction field (SCRF) model. All the TPA calculations
3.1. Preparation
The synthesis of BH was undertaken in acetic anhydride in the
presence of acetic acid whilst the BMe compound was synthesized

B. J
ędrzejewska et al. / Dyes and Pigments 111 (2014) 162e175
165
Scheme 1. General route for the synthesis of 1H-benzimidazole derivatives.
in moderate yield (67%) from trans-2-[2-(4-formylphenyl)ethenyl]
benzimidazole (BCHO) and 1,2,3-trimethylbenzimidazolium iodide
on heating by using piperidine as a catalyst. Their chemical struc-
tures and purity were characterized by 1H, 13C NMR and IR spectra
and by thin layer and HPLC chromatography. The data were found
to be in good agreement with the proposed structure. As expected,
eCHdbndCHe bonds in both the centrosymetric and non-
centrosymetric chromophores have a trans configuration, as evi-
denced by the value of coupling constants (J ¼ 16 Hz) for olefinic
protons (see Ref. [50] and Experimental section). All of the aromatic
vibration for CeN bonds were observed in the 1250e1020 cmꢀ1
range, as medium bands.
The NMR and IR spectra together with HPLC analysis are pre-
sented in Supplementary Materials.
The studied molecules consist of a typical donore
pedonor
structure, BH, and donor- -acceptor structure, BMe, where the
p
protons are observed around
NMR spectrum of the BMe dye exhibited a signal near
which corresponds to the NeH proton of the benzimidazole ring
and ca.
4.2 ppm assigned to the methyl groups. The 13C NMR and
d
7.0e8.2 ppm. Additionally, the 1H
3.5 ppm
d
d
IR spectral data of the dyes are summarized in the Experimental
section and [50], and are also in good agreement with structures of
the compounds.
The IR spectra showed typical vibrations for aromatic rings
(CdbndC, 1600e1630, 1500e1530, 1400e1420 cmꢀ1
; dCeH,
700e900 cmꢀ1), resonance-conjugated unsaturated stretching
modes in the chromophore (CdbndC, CdbndN, 1370e1480 cmꢀ1),
chromophoric CH out-of-plane bending (CH, 940e1030 cmꢀ1) and
bands for the appropriate functional group like the methyl group
which gives low intensity absorptions around 1380 cmꢀ1 and
1480 cmꢀ1. They were identified as the deformation vibrations
Fig. 1. Normalized electronic absorption and fluorescence spectra of the heterocycle-
dCH3 sym. and
dCH3 asym., respectively. The amine stretching
based molecules recorded in DMF.

166
B. Jędrzejewska et al. / Dyes and Pigments 111 (2014) 162e175
Table 1
indicate involvement of charge transfer, most likely involving par-
tial donation of the nitrogen lone pair of electrons in the benz-
imidazole residue to the charged benzimidazolium moiety through
Spectral characteristic of BH and BMe compounds in solvents of different polarity
lex ¼ 404 nm).
(
f(ε,n)
THF
Acetone DMF
DMSO
MeOH
MeCN
Water
the conjugated
p-system. The broadness of the absorption spec-
0.5491 0.7904 0.8356 0.8400 0.8546 0.8593 0.9137
trum may arise due to a contribution from more than one electronic
state to the absorption spectrum. Another reason may be that it
reflects a broad distribution of conformers (solvent-solute or
intramolecular) in the ground state [54].
BH
ab
l
ðnmÞ 389
395
390
392
4.72
384
4.37
382
5.28
e
e
max
ε (104 M1 5.29
e
5.16
cmꢀ1
)
fl
As shown, the structural modification (BH vs. BMe) leads to
l
ðnmÞ 428; 454 427; 450 433; 458 439; 461 422; 446 427; 449 e
max
Dn (cmꢀ1
)
3681
0.855
0.386
12.49
1.112
87.51
1.021
8.37
3618
0.609
0.44
13.57
1.168
86.43
1.069
5.70
3807
0.775
0.432
22.54
1.162
77.46
0.997
7.77
3818
0.630
0.4
30.81
1.15
69.19
0.919
6.86
3620
0.738
0.482
19.09
1.196
80.95
1.060
6.96
3906
0.679
0.477
15.18
1.234
84.82
1.119
6.07
e
e
e
e
e
e
e
e
negligible changes in the
p-conjugated length and so results in
Ffl
linear absorption spectra with similar maximum position, lmax
.
tf1l (ns)
a1 (%)
tf2l (ns)
However, the emission maxima and the Stokes' shifts of methylated
dye (BMe) were red-shifted by approximately 125 nm compared to
those of non-methylated compound (BH). It is worth mentioning
that the Stokes' shift is ca. three-times larger in BMe than in BH
(Table 1, Dn). The observed large Stokes' shift suggests that the
emitting state is not the FranckeCondon S1 state reached in the
absorptive transition and indicates that significant charge redis-
tribution occurs upon excitation prior to emission. This suggests a
quite substantial dipole moment change caused by the excitation
[54].
a2 (%)
fl
av
t
(ns)
kr
(108 sꢀ1
)
)
knr
fos
1.42
0.94
3.66
2.26
0.95
4.02
0.87
2.47
0.84
2.87
0.99
e
e
(108 sꢀ1
e
BMe
ab
l
ðnmÞ 363
386
389
392
5.73
381
5.30
377
5.24
376
5.18
max
ε (104 M1
e
5.49
5.15
Similar to the absorption spectrum, the fluorescence spectrum
of BH shows vibrational structure for all used solvents at room
temperature, while for BMe the fluorescence intensity distribution
is described by a Gaussian shape. Such shape is characteristic for
fluorescence spectra when the motions in the fluorophore envi-
ronment occur simultaneously or faster than the emission. For
these cases a very large number of different solute-environment
space configurations are possible. They possess a large variation
of interaction energies, forming an ensemble of sub-states. Each
sub-state can possess a sharp maximum, but when added their
contributions give a broadband emission spectrum of a Gaussian
shape [55]. The range of the inhomogeneous broadening depends
on the values of dipole moments of the dye molecule in the ground
and excited states and on the dielectric properties and structure of
the solvent [55]. The fluorescence emission spectra bands are
somewhat narrower than the absorption spectra (see Table 1). Its
fluorescence full-width at half-maximum FWHMfl values are
smaller than that determined for the long-wave absorption band,
FWHMab. The difference between FWHMfl and FWHMab amounts
to 46% for low-polar THF, 15% for polar nonprotic DMSO and 10% for
polar protic MeOH. If the broadening of the spectra is understood as
deriving from the population of the thermally available conformers,
then according to Rettig [56], the observed results can be taken as
the evidence that some of these conformers are non-emissive
(reduction of fluorescence band width). This type of conformer is
obtained by twisting of the olefinic double bonds. The deactivation
of this state should occur mainly by radiationless processes because
the energy gap between its excited state and the ground state is
very small [56,57].
The steady-state absorption and emission spectra of BH and
BMe in the fairly limited range of solvent polarity show small var-
iations as a function of polarity. Generally, the BH and BMe dyes,
which include a strongly electron donating benzimidazole nucleus,
should rather possess negative solvatochromism. However, no
reasonable correlation has been found between either the ab-
sorption or fluorescence peak maxima and the solvent polarity
parameter, f(ε,n). The same is true for the dependence of the Stokes'
shift parameter on f(ε,n). The possible reason behind the lack of
correlation between the fluorescence maxima or the Stokes' shift
and the solvent polarity parameter (Table 1), is the emission from
more than one conformer formed following optical excitation.
Since the solvent effect on the fluorescence maxima of the unde-
composed spectra is not a simple reflection of the solvent depen-
dence of a particular excited state conformation, the spectral shape
cmꢀ1
)
fl
l
ðnmÞ 576
580
582
584
4181
567
4102
576
4243
574
4136
max
FWHMab 5694
4279
4281
(cmꢀ1
FWHMfl
(cmꢀ1
)
3082
3499
3527
3535
3677
3554
3918
)
Dn (cmꢀ1
Ffl
)
10,187 8665
8525
0.120
0.449
21.61
1.288
78.39
1.107
1.02
8387
0.114
0.463
18.13
1.389
81.87
1.221
0.96
8610
0.067
0.334
17.34
0.865
82.66
0.773
1.01
9164
0.109
0.452
11.45
1.255
88.55
1.163
0.94
9174
0.044
0.384
96.9
1.552
3.1
0.157
0.442
12
1.561
88
0.103
0.489
19.08
1.301
80.92
1.146
0.94
tf1l(ns)
a1 (%)
tf2l (ns)
a2 (%)
fl
av
t
(ns)
1.427
0.98
0.420
1.05
kr
(108 sꢀ1
)
)
knr
fos
6.03
7.78
1.25
8.01
1.18
7.23
1.29
11.9
1.14
7.66
1.17
22.8
1.11
(108 sꢀ1
e
heterocyclic ring and styryl group are employed as acceptor and
conjugated moieties, respectively, while 1,3-
dimethylbenzimidazolium moiety constitutes an acceptor group.
The two heterocycle-based molecules have been used in this workto
investigate the effect of nitrogen alkylation on the one- and two-
photon spectroscopic properties.
p-
3.2. Optical properties
The normalized electronic absorption and fluorescence spectra
of the heterocycle-based organic molecules under investigation at a
concentration of 1.5e2 ꢁ 10ꢀ5 M are presented in Fig. 1. Table 1
collects the photophysical properties of the studied compounds.
There are: absorption (lmax), emission (lmax), Stokes' shift (Dn),
relative fluorescence quantum yield (Ffl) fluorescence lifetimes (tfl
)
)
with their amplitudes (
rate constants and oscillator strength (fos).
a), the radiative (kr) and non-radiative (knr
As can be seen in Fig. 1, the absorption spectrum of BH in DMF,
with a maximum at 390 nm, shows a vibrational structure. Analysis
of this complex vibronic spectrum with the use of the Gaussian
function showed that the absorption bands occur at ca. 368 nm,
389 nm and 411 nm. The molar extinction coefficients of BH range
from 4.37 to 5.29 ꢁ 10ꢀ4 Mꢀ1 cmꢀ1 in solvents of different polarity.
On the other hand, the absorption spectrum of BMe is structureless
and broad, having half-widths of 4281 cmꢀ1. The large bandwidth
and the large extinction coefficient value (in the range of
5.15e5.73 ꢁ 104 Mꢀ1 cmꢀ1) for the BMe absorption probably

B. J
ędrzejewska et al. / Dyes and Pigments 111 (2014) 162e175
167
as well as the solvent shift can also depend on the relative fluo-
rescence quantum yields of the conformers [58].
summarized in Table 1, show the contributions of the long (ca.
1.2 ns) and the short (ca. 0.4 ns) lifetime components. The ampli-
tude of the short lifetime component is the highest in the sample
displaying the fastest decay and the amplitude of the long lifetime
component is the highest in the sample displaying the slowest
decay.
The observed hypsochromic shift occurring in methanol relative
to THF or acetone can be a result of the solute-solvent interaction
through intermolecular hydrogen bond formation between meth-
anol and the lone pair electrons of benzimidazole nitrogen atom.
This slightly decreases the electron density on the benzimidazole
nitrogen atom, and consequently decreases to some extent the
Fig. 2 shows the fluorescence decays of BH and BMe in solvents
of different polarity. From this figure, it is clear that the fluores-
cence lifetimes of BMe significantly decrease with increasing sol-
vent polarity (from THF to water). However, there is only a little
change in lifetimes of BH with applied solvents. Using the experi-
mental Ffl and tav values, one can calculate values for the radiative
(kr) and nonradiative (knr) rate constants. The values of kr are in the
range from 8.37 ꢁ 108 sꢀ1 to 0.64 ꢁ 108 sꢀ1 for BH and from
0.94 ꢁ 108 sꢀ1 to 1.05 ꢁ 108 sꢀ1 for BMe (Table 1). For the BMe dye,
the kr value does not vary much as a function of solvent properties.
The largest kr value is found in lower polarity solvents (Table 1).
This is due to the fact that, for a constant transition dipole, kr is
proportional to the square of the refractive index of the medium
[59]. The fluorescence quantum yield and lifetime for the studied
compounds decrease when the solvent changes from THF to water,
due to an increase of the nonradiative rate constant knr in more
polar solvents. The large knr values for both BMe and BH in polar
solvent indicate that the nonradiative channel is dominant in the
excited-state deactivation.
To get a deeper insight into fluorescence behaviour of BH and
BMe, the fluorescence decay traces in DMF were recorded at
different emission wavelengths, lEM (Fig. 3). The decays were found
to be dependent on the monitoring wavelengths. Thus, a fast decay,
as compared to that at 458 nm and 582 nm for BH and BMe,
respectively, was observed at the blue edge, and a fast growth
followed by a relatively slow decay was observed at the red edge of
the emission spectra.
mobility of the attached
p electrons over the conjugated pathway.
The same is realized when the spectra recorded in aqueous solution
are compared to those recorded in methanol.
Likewise as the fluorescence maxima the fluorescence quantum
yield (Ffl) shows changes with increasing solvent polarity. Fluo-
rescence quantum yields are higher for the non alkylated com-
pound BH compared to the ionic dye BMe. The modification of the
symmetrical compound with N-methyl groups leads to ca. seven-
fold decrease in fluorescence quantum yield. Solvent choice also
has a considerable influence on the fluorescence quantum yield of
the compounds. BH and BMe in water are the samples with the
lowest fluorescence quantum yields, whereas BH and BMe in THF
are the samples with the highest Ffl values. In general, the quantum
yields of the synthesized compounds decreased in protic solvents
like methanol and water and in solvents of high polarity as
compared to THF.
Apart from fluorescence quantum yields, the fluorescence life-
times of the two chromophores were determined as described in
the Experimental section. The fluorescence decays were fitted with
the minimum number of exponential terms that resulted in
random residuals. Consistent with a previous paper [50], the fluo-
rescence decays measured with the compounds under the study
were two-exponential (Fig. 2). The results of the individual fits,
To reveal the reason behind the wavelength-dependent changes
in the emission characteristics, fluorescence transient decays were
converted to time-resolved emission spectra (TRES) (Fig. 4).
The analysis of the TRES spectra indicates that initially an in-
crease in the fluorescence intensity is observed, followed by
gradual decreases. There is a small discrepancy in the maximum
wavelengths between the TRES and the steady state spectra, but it
does not influence the overview of the dynamic properties of the
system.
Normalized fluorescence spectra of BH and BMe in DMF are
shown in Fig. 5 for various time delays after electronic excitation.
The data are fitted by log-normal functions. The spectra are slightly
shifted to the red with an invariant shape and show only one iso-
emissive point which is observed at 465 nm and 565 nm for BH
and BMe, respectively. Observation of an iso-emissive point in
time-resolved area-normalized emission spectra (TRANES) is an
unambiguous confirmation for the presence of two emissive spe-
cies in the sample, irrespective of their origin [60].
3.3. Photostability
To characterize photophysical properties of the studied com-
pounds it is very important to check their photostability in solution.
The photostability arises primarily as a consequence of molecular
flexibility about the central carbonecarbon double bonds in the
singlet excited state and accounts for the characteristic photo-
chemical reaction shown by trans-cis photoisomerization. It was
found that the optical properties of the compounds in the dark at
room temperature remained constant for over 3 days but photo-
degradation was observed upon irradiation of the dye solutions
with a 20 mW DPSS laser beam (lEM ¼ 408 nm). Fig. 6 shows
photofading behaviour of BH and BMe in acetonitrile.
Fig. 2. Fluorescence decay curves of BH and BMe in solvents of different polarity
(
lEx ¼ 373 nm, lEM ¼ 450 or 550 nm for BH and BMe, respectively).

168
B. Jędrzejewska et al. / Dyes and Pigments 111 (2014) 162e175
The results indicate that visible light irradiation of BMe and BH
in MeCN causes distinct changes in the electronic absorption
spectra, i.e. after 200 s of irradiation, BMe showed a 30% decrease in
maximal absorbance accompanied by hypsochromic shift of about
16 nm whereas the BH showed 25% photofading with blue shift of
ca. 12 nm.
3.4. Theoretical approach
To gain further insight into the correlation between structure
and the physical properties, quantum chemistry computations
were conducted. The optimized parameters (bond lengths, bond
angles and dihedral angles) of the investigated molecules in their
ground and first excited state (cf. Fig. 7) in selected solvents are
listed in Table S1 (See Supporting Information).
When the geometries of the S0 and S1 states are compared, the
following observations are made: (a) the molecular structure of the
BMe is not planar, in both the ground and excited state. In the first
case, the rotation is bigger; (b) during the excitation of the mole-
cule, the dihedral angle between C6eC7eC8eN9 decreases and in
the same time C6eC7eC8eN11 increases. Simultaneously there is
an increase of C7eC8eN9 and C7eC8eN11 bond angles while
C8eN9eC10 and C8eN11eC10 are reduced. These differences
make the benzimidazole fragment with methyl groups more
twisted in the ground state and flatter in the excited state BMe; (c)
in the case of carbonecarbon p-bridges, transition of the molecule
from the ground to the excited state causes the single bonds to
become shorter, and the double bonds longer in BMe. These ob-
servations do not apply to BH; (d) in both cases, the molecule is
characterized by a planar structure and its excitation does not in-
fluence its geometry significantly.
Fig. 3. Fluorescence decay profile for BH (top) and BMe (bottom) in aqueous DMF at
different emission wavelengths.
Table 2 contains theoretically determined spectroscopic pa-
rameters characterizing the lowest-lying singlet excited states. It
Fig. 4. Time-resolved emission spectra (TRES) of BH (top panel) and BMe (bottom panel) in DMF at room temperature at various times after excitation (lEx ¼ 373 nm). Direction of
the arrow indicates from 0 to 10 ns every 0.1 ns.

B. J
ędrzejewska et al. / Dyes and Pigments 111 (2014) 162e175
169
Fig. 6. Changes in absorbance intensity for BH and BMe, respectively, after irradiation
with a blue DPSS laser (line at 408 nm) with the power of 20 mW.
while CAM-B3LYP overestimates them, about 45 nm for BMe and
38 nm for BH. On the other hand, the latter gives the values nearest
to the experimental ones. Using CAM-B3LYP gives values at the
level of the average relative error of 3.5% for non-centrosymmetric
compound, which decreases to 0.5% for centrosymmetric. Similarly
Fig. 5. Time-resolved area-normalized emission spectra (TRANES) with time for BH
(top) and BMe (bottom) in DMF.
was found that for both molecules the first excited state is the
good performance of this functional in computing spectra of p-
charge-transfer (CT) state of
pe
p* character and characterized by
conjugated organic molecules has also been reported by other au-
thors [61e65]. More importantly, the values contained in the pre-
sented tables indicate that, while in the case of BMe passing from
the gas phase to the solvent is accompanied by a decline of the OPA
value (hipsochromic shift), for BH these values increase (bato-
chromic shift). The performed theoretical studies indicate that for
the first molecule there is a gradual but slight increase in the value
of the excitation energy, which allows for searching after negative
solvatochromism. However, the presence of MeCN and DMF sol-
vents caused a slight shift of the absorption band maximum in the
direction of longer wavelength relative to the less polar solvents
the highest value of the oscillator strength. This transition is
dominated by HOMO/LUMO excitation (see Fig. 8). The figure
shows that the HOMO is localized on the fragment without sub-
stituents, while the LUMO is focused on a part with methyl groups.
Thus, during the excitation to the S1 state there is a substantial shift
of the electron density distribution in direction of the benzimid-
azole with substituents and this constitutes an intramolecular
charge transfer. Despite the fact that the methyl groups are
electron-donating, it accumulates on the opposite side of the pos-
itive charge of the molecule which results from the fact that hy-
drolysis of iodide salt is so strong that it produces the charge shift
(
D
EMeCNeMeOH ¼ ꢀ0.0004 eV and
further shift to a more polar DMSO and water results in the re-
growth of the excitation energy (
D
EDMFeMeCN ¼ ꢀ0.0146 eV), so a
from the benzimidazole part, yielding the form of D-p-A. In the
case of centrosymmetric BH compound, the HOMO is stretched
across the entire molecule but LUMO is focused on the central
phenyl ring, with the result that there is little intramolecular charge
transfer.
The wavelengths of the absorption maxima (S0 / S1), oscillator
strengths, fluorescence maxima (S0/S1) and Stokes' shifts calcu-
lated in vacuum and solvents are presented in Table 2. Comparison
of functionals used in the evaluation of one-photon absorption
bands (OPA) indicates that the LC functionals give similar values,
DEDMSOeDMF ¼ þ0.0055 eV). This
behaviour of BMe might be discerned in the nature of sol-
uteesolvent interaction.
Fig. 9 shows a comparison of the absorption spectra deter-
mined experimentally and theoretically (TD/CAM-B3LYP) in
methanol and DMF. Firstly, if the calculated maximum absorption
band is shifted slightly in the direction of longer wavelengths, in
the case of the BMe compound in both the media the obtained
spectra have similar shapes. Furthermore, a rather intense band at

170
B. Jędrzejewska et al. / Dyes and Pigments 111 (2014) 162e175
Fig. 7. Optimized geometries of BMe (top) and BH (bottom).
211 nm was recorded in methanol, which is much less intense in
theoretical spectrum and occurs at the wavelength of 283 nm. As
mentioned earlier, this band is probably associated with the for-
mation of H-bonds in more polar DMF and for this reason it dis-
appears in this solvent. For BH, an almost identical position of the
absorption band peaks was obtained, determined both theoreti-
cally and experimentally. However, in this case, the calculation
does not provide additional bands that appear during the mea-
surements. This proves the existence of specific interactions in the
solute-solvent system, for which the TD-DFT method is not
sensitive.
Table 3 collects the computed emission wavelengths lEM in gas
phase and the solvent effect on the position the emission peaks.
Similarly to the OPA, when changing from gas phase to the solvent,
the position of the emission shifted. In the case of BMe it is a blue-
and for BH a red-shift. The change in peak position in different
solvents is an indication that the emission wavelengths are sensi-
tive to the environmental factors of the solvents. For this reason, for
both molecules with increasing solvent polarity a hypsochromic
effect is observed. On the other hand, it can also be seen that lEM
exhibits the red-shift when going from less polar MeOH to more
polar MeCN or DMF and then a blue-shift when the most polar
solvents are taken into account. Comparing the determined lEM for
both compounds it is easy to see that the presence of the two
methyl groups causes a red shift on the position of the emission
peaks. In addition, BMe is characterized by higher values of the
Stokes' shift.
account the value of the Stokes' shift, its higher value was obtained
in the case of LC-BLYP and LC- PBE. Generally, it can be concluded
that in the presence of solvents, the CAM-B3LYP functional pro-
vides the value on average 0.63/0.61, LC-BLYP 0.72/0.70 and LC-
u
u
PBE 0.70/0.68 for BMe and BH, respectively. Only in the case of
BMe, the Stokes' shift value determined in the least polar THF is
slightly undervalued by an average of about 12%.
The dipole moments for the ground (m1) and the first excited
state (m2) for the investigated compounds calculated within the TD-
DFT formalism are listed in Table 4. Firstly, in each of the media the
dipole moments obtained for the BH molecule are zero, for both the
S0 and S1 state (symmetry). In the case of the BMe structure, for the
S0 and S1, an increase in the value of the dipole moment during
transition from the gas phase to the solvent is observed. The values
collected in Table 4 indicate that with an increase of the solvent
polarity the ground state dipole moment increases. However,
analysis of the CT shows that, while the transition from vacuum to
THF is characterized by an increase in m2, the transition to the more
polar environment is accompanied by a decrease of the dipole
moment, and is more than 3D. This observation seems to be a little
surprising because it is not associated with any observation result-
ing from the absorption and fluorescence analysis, where growth
trends were observed for MeCN and DMF. Further enhancing po-
larity of the medium results in an increase of m2, although initially
increases are becoming smaller
(
Dm2(MeOHeAcetone)
¼
0.47 D;
Dm(2MeCNeMeOH) ¼ 0.07 D; Dm2(DMFeMeCN) ¼ 0.04 D) and from the DMF
was getting bigger (Dm2(DMSOeDMF) ¼ 0.15 D; Dm2(WatereDMSO) ¼ 0.22 D).
It also clear that the excitation of the charge-transfer (CT) state is
accompanied by a decrease of the dipole moment values. For the
CAM-B3LYP functional this value is more than 16D and 9D less
relative to the m1, for the gas phase and solvents, respectively.
However, for THF Dm1e2 is three times smaller compared tothe other
polar media. It is also worth mentioning that polarity of the low-
The data presented in Table 3 indicate that LC-BLYP and LC-
u
PBO describe the position of the emission peak at a similar level,
and the differences between them are lower than 0.9 nm (1.4 in the
gas phase) and 0.5 nm for BMe and BH respectively. On the other
hand, CAM-B3LYP overestimates the value of lEM relative to LC-
functionals by about 52 nm (BMe) and 46 nm (BH). Taking into

B. J
ędrzejewska et al. / Dyes and Pigments 111 (2014) 162e175
171
Table 2
3.5. NLO characterization
Calculated values of excitation energies (in nm, lmax) and oscillator strengths (fOS
)
for the BMe and BH molecules.
In addition to the linear spectral calculations presented and
discussed above, two-photon absorption spectra are also of inter-
est. As mentioned earlier, for the compounds with positive sol-
vatochromism, the two-photon absorption spectrum shifts toward
longer wavelengths with increasing polarity of the solvent [67]. All
presented conclusions indicate that the studied molecule, similarly
as BTP1 described previously [50], is characterized by a negative
solvatochromism. Because there is not enough detailed reports in
the subject literature on the impact of the environment with
different polarity on the two-photon absorption cross-section value
for this class of compounds [43,68,69], the results presented below
should be treated as demonstrative and a continuation of the ones
provided for the BTP1 molecule.
CAM-B3LYP
LC-BLYP
LC-uPBE
lmax
fOS
lmax
fOS
lmax
fOS
BMe
Gas chase
p
ne
pep*
ne
pep*
ne
pep*
ne
pep*
ne
pep*
ne
pep*
ne
pep*
ne
e
p
*
*
473
357
405
306
394
300
392
299
392
299
394
301
393
300
391
299
1.79
0.047
2.29
0.038
2.31
0.028
2.31
0.026
2.32
0.026
2.34
0.026
2.34
0.025
2.32
397
289
359
280
352
276
350
276
350
276
352
277
351
277
349
275
2.12
0.028
2.37
0.0066
2.36
0.0067
2.35
0.0067
2.36
0.0068
2.38
0.0071
2.37
0.0071
2.36
402
292
361
282
354
279
352
278
352
278
354
279
353
279
351
278
2.11
0.037
2.40
0.0064
2.39
0.0065
2.38
0.0065
2.39
0.0066
2.41
0.0069
2.41
0.0069
2.39
p
THF
p
*
Acetone
MeOH
MeCN
DMF
p
*
p
*
p
*
The TPA cross-sections in a.u. (
d
OF) and in GM ðsOð2FÞÞ for the
p
*
compounds considered in this study are given in Table 5 for two
lowest singlet excited states. The values indicate that in the case of
BMe the most dominating TPA state is S1, which is also the most
dominating in OPA. This is accordance with observation about other
asymmetric compounds presented by Agren and co-workers [70].
However, for BH a different relation is observed. This molecule
exhibits efficient TPA under excitation into the second excited state
which is not unexpected for a centrosymmetric structure. Indeed,
because of the presence of the centre of symmetry the mutual
exclusion selection rule is operative in the case of BH: since tran-
sition S0eS1 is one-photon allowed, it is two-photon disallowed, on
the other hand, two-photon excitation into S2 is possible but this
transition is not one-photon allowed. However, it needs to be noted
that while the selection rules lead to differences between OPA and
TPA for BH and BMe, the chromophores reach different excited
states upon absorption but, due to fast radiationless relaxation they
emit from the same, lowest excited state.
DMSO
Water
p
*
p
*
0.025
0.0069
0.0066
BH
Gas chase
ne
p
p
*
*
379
299
389
304
387
303
386
302
386
303
388
304
388
304
386
302
2.26
0.000
2.44
0.000
2.43
0.000
2.43
0.000
2.43
0.000
2.45
0.000
2.45
0.000
2.43
342
274
350
280
348
279
348
278
348
279
350
280
349
280
348
278
2.25
0.000
2.40
0.000
2.40
0.000
2.39
0.000
2.40
0.000
2.41
0.000
2.41
0.000
2.39
343
276
351
282
350
281
349
280
349
281
351
282
351
282
349
280
2.29
0.000
2.45
0.000
2.44
0.000
2.44
0.000
2.44
0.000
2.46
0.000
2.46
0.000
2.44
pe
THF
nep*
pe
p
*
Acetone
MeOH
MeCN
DMF
nep*
pe
p
*
nep*
pe
p
*
nep*
pe
p
*
nep*
pe
p
*
DMSO
Water
nep*
pe
p
*
nep*
pe
p
*
0.000
0.000
0.000
Organic solvatochromic compounds may exhibit significant
changes under the influence of the solvent in terms of not only
structural and linear optical properties (OPA) but also of the
nonlinear optical properties, such as TPA. Considering the values
expressed in atomic units, in both cases the transition from vacuum
to solvent is accompanied by an increase of the dOF by more than
60,000 a.u. In general, increasing the dielectric constant of the
lying excited state is decreasing with the polarity of the solvent. This
observation indicates that the ground state is better stabilized by the
polar solvent than the CT excited state, which leads to the
enhancement of the excitation energy of the solute. Similar results
were obtained for trans,trans-2-{4-[2-(1H-benzimidazol-2-yl)
ethenyl]-styryl}-N-methylpyridine iodide (BTP1) [50]. The pre-
sented conclusions, as well as studies carried out by the group of
Bartkowiak [66] and our research conducted for the BTP1 molecule
allow to classify the investigated BMe compound as one exhibiting
negative solvatochromism.
environment causes a gradual increase in the value of d
OF. For the
BMe molecule a deviation from this dependence is observed,
because least polar THF leads to the TPA almost identical as in the
case of more polar water. The transition from THF to acetone is
accompanied by a fall of two-photon cross-section by about 2500,
but further increase of the solvent polarity contributes to the
Fig. 8. The plots of orbital contour surfaces for BMe (left panel) and BH (right panel). The molecular orbitals were calculated at the CAM-B3LYP/6-311þþG(d,p) level of theory. The
lower plot presents contour surface of HOMO and the upper presents contour surface of LUMO. Shown are the contour surfaces of orbital amplitude 0.02 (red) and ꢀ0.02 (blue). (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

172
B. Jędrzejewska et al. / Dyes and Pigments 111 (2014) 162e175
Fig. 9. The comparison of the absorption spectra determined experimentally (the solid line) and theoretically (the dotted line) for BH (right panel) and BMe (left panel).
growth of
d
OF. In the case of a centrosymmetrical compound (BH)
acetone e 0.477 eV, methanol e 0.464 eV, MeCN e 0.467 eV, DMF e
0.479 eV, DMSO e 0.472 eV and water e 0.464 eV. Looking closely at
the two-photon cross-section values are almost twice lower as
compared to the asymmetric chromophore (BMe). Electronic
excitation of BH molecule induces a transfer of a significant electron
density from the benzimidazole part in the direction of the centre
this table one finds that, as in the case d
OF, the change of the
environment is accompanied by change in the value of sðO2FÞ. It
should be noted, that in both cases the cross-section tends to in-
crease with the polarity of the solvent, but only up to MeCN. Then,
the transition from DMF to the more polar solvents results in a
gradual increase of this parameter. In the case of BMe, in order to
clarify these discrepancies, one should refer to the two-state
approximation [67,71,72]. According to this, the TPA depends on
three parameters: oscillator strength, the transition energy and the
polarity of the excited state. As presented earlier, in any environ-
ment fOS value is very high and almost equal in all solvents. On the
of the
p-bridge. Such excitation results in a change of the quad-
rupole moment and thus leads to the enhancement of the
d
OF value.
Table 5 shows the values of TPA cross-section also in GM units
that are more useful for comparisons to the measured values. In all
simulations, the following values of GF (see Eq. (5)), determined on
the basis of our experimental data, were used: THF e 0.554 eV,
Table 3
Calculated emission values (in nm, lEM) and oscillator strengths (fOS) for the BMe and
BH molecules.
Table 4
Values of dipole moments for the ground and first lowest-lying singlet electronic
states for BMe calculated at the TDDFT/6-311þþG(d,p) level of theory. All values are
given in [D].
CAM-B3LYP
LC-BLYP
LC-uPBE
lEM
fOS
lEM
fOS
lEM
fOS
BMe
Gas phase
THF
Acetone
MeOH
MeCN
DMF
DMSO
Water
BH
Gas phase
THF
Acetone
MeOH
MeCN
DMF
CAM-B3LYP
LC-BLYP
LC-uPBE
507
495
492
491
491
495
495
491
1.88
2.40
2.44
2.44
2.44
2.46
2.46
2.44
455
439
441
440
441
444
443
441
1.84
2.44
2.44
2.44
2.44
2.46
2.46
2.44
457
441
442
441
441
445
444
441
1.85
2.46
2.46
2.46
2.46
2.48
2.48
2.46
Gas phase
THF
m1
m2
m1
m2
m1
m2
m1
m2
m1
m2
m1
m2
m1
m2
m1
m2
23.30
6.59
24.60
12.06
28.82
26.50
29.35
23.56
29.46
23.88
29.47
23.93
29.48
23.94
29.51
24.04
29.56
24.20
24.57
11.36
28.85
26.34
29.39
23.15
29.50
23.49
29.51
23.54
29.52
23.55
29.55
23.65
29.60
23.82
28.34
25.34
28.98
19.24
29.11
19.71
29.13
19.78
29.14
19.82
29.17
19.97
29.23
20.19
Acetone
MeOH
MeCN
DMF
446
479
478
477
478
482
481
478
2.29
2.49
2.48
2.48
2.48
2.50
2.50
2.48
404
433
433
433
433
436
436
433
2.25
2.41
2.41
2.40
2.41
2.42
2.42
2.41
404
433
433
432
433
436
435
433
2.30
2.46
2.45
2.45
2.45
2.47
2.47
2.45
DMSO
Water
DMSO
Water

B. J
ędrzejewska et al. / Dyes and Pigments 111 (2014) 162e175
173
Table 5
Values of two-photon absorption (TPA) cross-section 〈dOF 〉 in atomic units and sOð2FÞ in
GM obtained using Eqs. (3) and (5) respectively for the BMe and BH calculated at the
CAM-B3LYP/6-311þþG(d,p) level of theory.
BMe
BH
ð2Þ
ð2Þ
dOF ꢁ 10ꢀ3
s
dOF ꢁ 10ꢀ3
s
OF
OF
S1
S2
S1
S2
2.4
S1
S2
S1
S2
Gas phase
THF
Acetone
MeOH
MeCN
DMF
301.8
368.9
365.4
367.1
367.2
367.1
367.8
368.7
1.3
7.3
7.6
8.1
8.1
8.1
8.3
8.6
314.3
355.3
418.4
431.4
428.7
417.8
428.6
436.4
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
113.7
176.9
193.7
197.1
197.9
197.9
198.9
200.7
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
296.7
398.8
500.2
519.9
518.6
505.4
516.6
529.4
12.8
15.7
17.2
17.2
16.8
17.5
18.3
DMSO
Water
other hand, the dipole moment differences increase with the
dielectric constants. Thus in this case, the excitation energy value
Fig. 11. Two-photon absorption spectrum of BMe derived from Z-scan measurements
in DMF. The data are compared with one-photon spectrum replotted against the
doubled wavelength.
has a decisive influence on the two-photon absorption. Therefore,
ð2Þ
the characteristic s
in terms of environmental impact on the
OF
obtained value analogous resulting conclusions as in the case of
OPA. It is also worth mentioning that higher values of two-photon
the OPA spectra replotted against the doubled wavelength, except
for their intensity. The reason for this similarities, despite the dif-
ferences in the selection rules due to different symmetries of the
molecules are not clear at this stage. Nevertheless, it can be
concluded that both BMe and BH are found to be moderate strength
two-photon absorbers.
Summing up, the non-linear optical properties of the non-
centrosymmetrical dipolar compound (BMe) and centrosym-
metric quadrupolar compound (BH) revealed very similar two-
photon properties, thus, the experimental spectrum appears to be
split into two bands, the actual peak value being found in the
shorter wavelength component of the composite band and there is
no nonlinear absorption at longer wavelengths (1000e1600 nm).
ð2Þ
absorption (TPA) cross-section s in solutions are obtained for BH
OF
than for BMe.
It is known that the value of TPA expressed in GM is affected by
the GF. When the only theoretical work are presented, the value of
the broadening of the final state is unknown and is assumed to be
constant for all solvents. For this reason, we also calculate the value
of the two-photon absorption cross-section for constant value of GF.
The obtained results are illustrated in Fig. 10. It is clear that the
ð2Þ
higher values of s are obtained for BH compound.
OF
The results of theoretical calculations were compared to the
experimental data. As shown in Fig.1, the experimental one-photon
absorption (OPA) spectra of BMe and BH in DMF are very similar to
each other in peak positions and intensities (see data in Table 1).
This is also true for the two-photon absorption (TPA) spectra
(Figs. 11 and 12). The largest OPA peaks are observed at 3.18 eV and
3.19 eV for BH and BMe, respectively with the second small peak
located at 4.13 eV. In the TPA spectra, both compounds have a main
peak at 1.91 eV, with almost the same magnitude of the cross-
section (330 and 380 GM for BH and BMe, respectively). More-
over, at a photon energy lower than 1.8 eV, a smaller peak is also
found in both spectra. Both the TPA bands are well reproduced in
4. Conclusion
Optical properties of two compounds based on the benzimid-
azole skeleton have been investigated. The dyes were synthesized,
in reasonably good yields, by the Knovenagel condensation reac-
tion. They show the typical absorption and emission features of
styryl fluorophores, i.e., absorption bands that are blue-shifted with
increasing solvent polarity, a highly Stokes-shifted emission band
Fig. 10. Comparison of the values the two-photon absorption expressed in GM at a
constant value of the broadening of the final state. The solid line refers to BMe and the
dotted line to BH.
Fig. 12. Two-photon absorption spectrum of BH derived from Z-scan measurements in
DMF. The data are compared with one-photon spectrum replotted against the doubled
wavelength.

174
B. Jędrzejewska et al. / Dyes and Pigments 111 (2014) 162e175
[16] Xu C, Webb WW. Measurement of two-photon excitation cross-sections of
molecular fluorophores with data from 690 to 1050 nm. J Opt Soc Am B
1996;13:481e91.
[17] Sheik-Bahae M, Said AA, Wei TH, Hagan DJ, Van Stryland EW. Sensitive
measurement of optical nonlinearities using a single beam. IEEE J Quantum
Electron 1990;26:760e9.
[18] Makarov NS, Drobizhev M, Rebane A. Two-photon absorption standards in the
550e1600 nm excitation wavelength range. Opt Express 2008;16:4029e47.
[19] Padilha LA, Webster S, Hu HH, Przhonska OV, Hagan DJ, Van Stryland EW,
et al. Excited state absorption and decay kinetics of near IR polymethine dyes.
Chem Phys 2008;352:97e105.
[20] Terenziani F, Katan C, Badaeva E, Tretiak S, Blanchard-Desce M. Enhanced
two-photon absorption of organic chromophores: theoretical and experi-
mental assessments. Adv Mater 2008;20:4641e78.
of mirror image shape and a relatively high fluorescence quantum
yield which decreases with increasing solvent polarity.
The two-photon absorption measurements show no significant
change in cross-section as a result of substituting alkyl (dye BMe)
groups onto the nitrogen of the benzimidazole ring. The non-
centrosymmetrical dipolar compound (A-
centrosymmetric quadrupolar compound (D-
p
p
-D, BMe) and the
-D, BH) have mod-
erate TPA cross-section of ca. 350 GM at around 625 nm.
The one- and two-photon experimental data were supported by
the results of quantum chemical calculations based on the density
functional theory with CAM-B3LYP, LC-BLYP and LC-uPBE func-
[21] Masunov A, Tretiak S. Prediction of two-photon absorption properties for
organic chromophores using time-dependent density-functional theory.
J Phys Chem B 2004;108:899e907.
tionals. The theoretical calculations show differences in the ab-
sorption and fluorescence peak position between BMe and BH
chromophores. Furthermore, the quantum-chemical calculations
allowed the estimation of the TPA cross-section for studied
compounds.
[22] Dewar MJS, Zoebisch EG, Healy EF, Stewart JJP. Development and use of
quantum mechanical molecular models. 76. AM1: a new general purpose
quantum mechanical molecular model. J Am Chem Soc 1985;107:3902e9.
[23] Janesko BG, Yaron D. Using molecular similarity to construct accurate semi-
empirical electronic structure theories. J Chem Phys 2004;121:5635e45.
[24] Pond SJK, Rumi M, Levin MD, Parker TC, Beljonne D, Day MW, et al. One- and
two-photon spectroscopy of donor-acceptor-donor distyrylbenzene de-
rivatives: effect of cyano substitution and distortion from planarity. J Phys
Chem A 2002;106:11470e80.
[25] Sałek P, Vahtras O, Guo J, Luo Y, Helgaker T, Ågren H. Calculation of two-
photon absorption cross-sections by means of density-functional theory.
Chem Phys Lett 2003;374:446e52.
[26] Casida ME. Recent advances in density-functional methods, part 1, vol. 3.
Singapore: World Scientific; 1995.
[27] Dreuw A, Head-Gordon M. Single-reference ab initio methods for the calcu-
lation of excited states of large molecules. Chem Rev 2005;105:4009e37.
[28] Magyar RJ, Tretiak S. Dependence of spurious charge-transfer excited states
on orbital exchange in TDDFT: large molecules and clusters. J Chem Theory
Comput 2007;3:976e87.
[29] Miura M, Aoki Y, Champagne B. Assessment of time-dependent density
functional schemes for computing the oscillator strengths of benzene, phenol,
aniline, and fluorobenzene. J Chem Phys 2007;127:084103.
Acknowledgements
This research was supported in part by the Ministry of Science
and Higher Education (MNiSW) (BS-9/2014), PL-Grid Infrastruc-
ture, the Foundation for Polish Science under the “Welcome” Pro-
gram, and by a statutory activity subsidy from the Polish Ministry of
Science and Higher Education for the Faculty of Chemistry of WUT.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2014.06.007.
[30] Kobko N, Masunov A, Tretiak S. Calculations of the third-order nonlinear
optical responses in push-pull chromophores with a time-dependent density
functional theory. Chem Phys Lett 2004;392:444e51.
References
[31] Kawski A, Bojarski P. Comments on the determination of excited state dipole
moment of molecules using the method of solvatochromism. Spectrochim
Acta Part A 2011;82:527e8.
[1] He GS, Markowicz PP, Line P-C, Prasad PN. Observation of stimulated emission
by direct three-photon excitation. Nature 1999;415:767e70.
[2] Larson DR, Zipfel WR, Williams RM, Clark SW, Bruchez MP, Wise FW, et al.
Water-soluble quantum dots for multiphoton fluorescence imaging in vivo.
Science 2003;300:1434e6.
[3] Kawata S, Sun H-B, Tanaka T, Takada K. Finer features for functional micro-
devices. Nature 2001;412:697e8.
[4] Zhou W, Kuebler SM, Braun KL, Yu T, Cammack JK, Ober CK, et al. An efficient
two-photon-generated photoacid applied to positive-tone 3D micro-
fabrication. Science 2002;296:1106e9.
[32] Olmsted III J. Calorimetric determinations of absolute fluorescence quantum
yields. J Phys Chem 1979;83(20):2557e682.
ꢀ
ꢀ
[33] Olesiak-Banska J, Gordel M, Kołkowski R, Matczyszyn K, Samoc M. Third-order
nonlinear optical properties of colloidal gold nanorods.
J Phys Chem C
2012;116:13731e7.
ꢀ
ꢀ
[34] Szeremeta J, Nyk M, Wawrzynczyk D, Samoc M. Wavelength dependence of
nonlinear optical properties of colloidal CdS quantum dots. Nanoscale
2013;5(6):2388e93.
[5] Belfield KD, Schafer KJ. A new photosensitive polymeric material for WORM
optical data storage using multichannel two-photon fluorescence readout.
Chem Mater 2002;14:3656e62.
[35] Frisch MJ, Trucks GW, Schlegel HB, et al. Gaussian 09. Revision A.1. Wall-
ingford CT: Gaussian. Inc.; 2009.
[36] Becke AD. Density-functional thermochemistry. III. The role of exact ex-
change. J Chem Phys 1993;98:5648e52.
€
[6] Ehrlich JE, Wu XL, Lee I-YS, Hu Z-Y, Rockel H, Marder SR, et al. Two-photon
absorption and broadband optical limiting with bis-donor stilbenes. Opt Lett
1997;22:1843e5.
[7] Bhawalkar JD, Kumar ND, Zhao CF, Prasad PN. Two-photon photodynamic
therapy. J Clin Laser Med Surg 1997;15:201e4.
[8] Ramakrishna G, Goodson III T. Excited-state deactivation of branched two-
photon absorbing chromophores: a femtosecond transient absorption inves-
tigation. J Phys Chem A 2007;111(6):993e1000.
ꢀ
[37] Cances MT, Mennucci B, Tomasi J. A new integral equation formalism for the
polarizable continuum model: theoretical background and applications to
isotropic and anisotropic dielectrics. J Chem Phys 1997;107:3032e41.
[38] Iikura H, Tsuneda T, Yanai T, Hirao K. Long-range correction scheme for
generalized-gradient-approximation exchange functionals.
J Chem Phys
2001;115:3540e4.
[39] Tawada Y, Tsuneda T, Yanagisawa S, Yanai T, Hirao K. A long-range-cor-
rected time-dependent density functional theory. J Chem Phys 2004;120:
8425e33.
[40] Vydrov OA, Scuseria GE. Assessment of a long range corrected hybrid func-
tional. J Chem Phys 2006;125:234109e18.
ꢀ
ꢀ
ꢀ
[9] Olesiak-Banska J, Hanczyc P, Matczyszyn K, Norden B, Samoc M. Nonlinear
absorption spectra of ethidium and ethidium homodimer. Chem Phys
2012;404:33e5.
[10] Karton-Lifshin N, Albertazzi L, Bendikov M, Baran PS, Shabat D. “Donor-two-
acceptor” dye design: a distinct gateway to NIR fluorescence. J Am Chem Soc
2012;134:20412e20.
[41] Vydrov OA, Scuseria GE, Perdew JP. Tests of functionals for systems with
fractional electron number. J Chem Phys 2007;126:154109e18.
[42] Yanai T, Tew D, Handy N. A new hybrid exchange-correlation functional using
the Coulomb-attenuating method (CAM-B3LYP). Chem Phys Lett 2004;393:
51e7.
[11] Karton-Lifshin N, Segal E, Omer L, Portnoy M, Satchi-Fainaro R, Shabat D.
A
unique paradigm for a turn-ON near-infrared cyanine-based probe:
noninvasive intravital optical imaging of hydrogen peroxide. J Am Chem Soc
2011;133:10960e5.
ꢀ
[43] Zalesny R, Bartkowiak W, Styrcz S, Leszczynski J. Solvent effects on con-
[12] Sumalekshmy S, Fahrni CJ. Metal-ion-responsive fluorescent probes for two-
photon excitation microscopy. Chem Mater 2011;23:483e500.
[13] Zhou H, Wang J, Zhou F, Xu D, Cao Y, Liu G, et al. New five-coordinated
mercury (II) dyes based on a novel 2,20:60,200-terpyridineligand: struc-
tures, photophysical properties and DFT calculations to evaluate the halogen
effect on the two-photon absorption. Dyes Pigm 2012;95:723e31.
formationally induced enhancement of the two-photon absorption cross-
section of
a pyridinium-N-phenolate betaine dye. A quantum chemical
study. J Phys Chem A 2002;106:4032e7.
[44] Norman P, Cronstrand P, Ericsson J. Theoretical study of linear and nonlinear
absorption in platinum-organic compounds. Chem Phys 2002;285:207e20.
[45] Olsen J, Jorgensen P. Linear and nonlinear response functions for an exact
state and for an MCSCF state. J Chem Phys 1985;82:3235e64.
ꢀ
[14] Dumur F, Mayer CR, Dumas E, Miomandre F, Frigoli M, Secheresse F. New
chelating stilbazonium-like dyes from Michler's Ketone. Org Lett 2008;10:
321e4.
[15] Tan Y, Zhang Q, Yu J, Zhao X, Tian Y, Cui Y, et al. Solvent effect on two-photon
absorption (TPA) of three novel dyes with large TPA cross-section and red
emission. Dyes Pigm 2013;97:58e64.
[46] DALTON.
A molecular electronic structure program. Release Dalton2011;
2011. see: http://daltonprogram.org/.
[47] LSDALTON. A linear scaling molecular electronic structure program. Release
Dalton2011; 2011. see: http://daltonprogram.org/.

B. J
ędrzejewska et al. / Dyes and Pigments 111 (2014) 162e175
175
ꢀ
[48] Xue P, Lu R, Zhao L, Xu D, Zhang X, Li K, et al. Hybrid self-assembly of a
p
[61] Jacquemin D, Bouhy M, Perpete EA. Excitation spectra of nitro-
diphenylaniline: accurate time-dependent density functional theory pre-
dictions for charge-transfer dyes. J Chem Phys 2006;124:204321.
gelator and fullerene derivative with photoinduced electron transfer for
photocurrent generation. Langmuir 2010;26(9):6669e75.
ꢀ
[49] Huang Z-L, Lei H, Li N, Qiu Z-R, Wang H-Z, Guo J-D, et al. Novel heterocycle-
based organic molecules with two-photon induced blue fluorescent emission.
J Mater Chem 2003;13(4):708e11.
[62] Jacquemin D, Preat J, Wahtelet V, Fontaine M, Perpete EA. Thioindigo dyes:
highly accurate visible spectra with TD-DFT.
J Am Chem Soc 2006;128:
2072e83.
ꢀ
[50] Jędrzejewska B, Krawczyk P, Pietrzak M, Gordel M, Matczyszyn K, Samoc M,
[63] Peach MJG, Helgaker T, Sałek P, Keal TW, Lutnaes OB, Tozer DJ, et al. Assess-
ment of a Coulomb-attenuated exchange-correlation energy functional. Phys
Chem Chem Phys 2006;8:558e62.
et al. Styryl dye possessing donor-p-acceptor structure e synthesis, spectro-
scopic and computational studies. Dyes Pigm 2013;99:673e85.
[51] Yan Y-X, Fan H-H, Lam C-K, Huang H, Wang J, Hu S, et al. Synthesis, structures,
and two-photon absorption properties of two new heterocycle-based organic
chromophores. Bull Chem Soc Jpn 2006;79:1614e9.
[52] Cao D-X, Liu Z-Q, Zhang G-H, Cao F-X, Chen H-Y, Li G-Z. Synthesis, structure
and photophysical properties of three new hemicyanine dyes. Dyes Pigm
2008;76:118e24.
ꢀ
[64] Jacquemin D, Perpete EA, Scalmani G, Frisch MJ, Kobayashi R, Adamo C.
Assessment of the efficiency of long-range corrected functionals for some
properties of large compounds. J Chem Phys 2007;126:144105.
[65] Jacquemin, Perpete EA, Scuseria GE, Ciofini I, Adamo C. Extensive TD-DFT
ꢀ
investigation of the first electronic transition in substituted azobenzenes.
Chem Phys Lett 2008;465:226e9.
[53] Jędrzejewska B, Rudnicki A. The synthesis and spectroscopic investigation of
dichromophoric hemicyanine dyes. Dyes Pigm 2009;80:297e306.
[54] Palit DK, Singh AK, Bhasikuttan AC, Mittal JP. Relaxation dynamics in the
excited states of LDS-821 in solution. J Phys Chem A 2001;105:6294e304.
[66] Bartkowiak W. In: Papadopoulos MG, et al., editors. Non-linear optical prop-
erties of matter. Springer; 2006. pp. 299e318.
[67] Krawczyk P. DFT study of linear and nonlinear optical properties of donor-
acceptor substituted stilbenes, azobenzenes and benzilideneanilines. J Mol
Model 2010;16:659e68.
ꢀ
[55] Jozefowicz M, Kozyra KA, Heldt JR, Heldt J. Effect of hydrogen bonding on the
ꢀ
intramolecular
charge
transfer
fluorescence
of
6-dodecanoyl-2-
[68] Wielgus M, Bartkowiak W, Samoc M. Two-photon solvatochromism. I. Solvent
dimethylaminonaphtalene. Chem Phys 2005;320:45e53.
effects on two-photon absorption cross-section of 4-dimethylamino-40-
[56] Strehmel B, Seifert H, Rettig W. Photophysical properties of fluorescence probes.
2. A model of multiple fluorescence for stilbazolium dyes studied by global
analysis and quantum chemical calculations. J Phys Chem 1997;101:2232e43.
nitrostilbene (DANS). Chem Phys Lett 2012;554:113e6.
ꢀ
ꢀ
[69] Wielgus M, Zalesny R, Murugan N, Kongsted J, Ågren H, Samoc M, et al. Two
photon solvatochromism. 2. Experimental and theoretical study of solvent
effects on the two-photon absorption spectrum of Reichardt's dye. Chem-
PhysChem 2013;14:3731e9.
ꢀ
[57] Wroblewski S, Trzebiatowska K, Jędrzejewska B, Pietrzak M, Gawinecki R,
Pączkowski J. Development of fluorescence probes based on stilbazolium salts
for monitoring free radical polymerization processes. J Chem Soc Perkin Trans
2 1999:1909e17.
[70] Wang CK, Macak P, Luo Y, Ågren H. Effects of
p centers and symmetry on two-
photon absorption cross-sections of organic chromophores. J Chem Phys
2001;114(22):9813e20.
[71] Oudar JL, Chemla DS. Hyperpolarizabilities of the nitroanilines and their
relations to the excited state dipole moment. J Chem Phys 1977;66(6):
2664e8.
[72] Murugan NA, Kongsted J, Rinkevicius Z, Aidas K, Mikkelsen KV, Ågren H.
Hybrid density functional theory/molecular mechanics calculations of two-
photon absorption of dimethylamino nitro stilbene in solution. Phys Chem
Chem Phys 2011;13(27):12506e16.
[58] Noukakis D, Van der Auweraer M, Toppet S, De Schryver FC. Photophysics of a
thiacarbocyanine dye in organic solvents. J Phys Chem 1995;99:11860e6.
[59] Qin W, Rohand T, Baruah M, Stefan A, Van der Auweraer M, Dehaen W, et al.
Solvent-dependent photophysical properties of borondipyrromethene dyes in
solution. Chem Phys Lett 2006;420:562e8.
[60] Ira, Koti ASR, Krishnamoorthy G, Periasamy N. TRANES spectra of fluorescence
probes in lipid bilayer membranes: an assessment of population heteroge-
neity and dynamics. J Fluoresc 2003;13(1):95e103.