Styryl dye possessing donor-π-acceptor structure - Synthesis, spectroscopic and computational studies

Article abstract of DOI:10.1016/j.dyepig.2013.06.008

Abstract A novel hemicyanine dye possessing donor-π-acceptor structure that features a fixed benzimidazole ring as an electron donor and pyridinium moiety as an electron acceptor was synthesized. The structure of the hemicyanine was elucidated by means of NMR and IR spectroscopy. The dye was studied using steady-state absorption and emission spectroscopy. The analysis of experimentally recorded spectra was supported by quantum chemical calculations using density functional theory with CAM-B3LYP, LC-BLYP, LC-ωPBE and PBE0 functionals. The measurements and calculations were performed in solvents of different polarity. In particular, we found that the CAM-B3LYP and LC-ωPBE excitation energies are in very good agreement with the experimental data. Two-photon absorption of the dye was studied by Z-scan measurements.

Full text of DOI:10.1016/j.dyepig.2013.06.008

Dyes and Pigments 99 (2013) 673e685
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Styryl dye possessing donor-p-acceptor structure e Synthesis,
spectroscopic and computational studies
,
c
a
b
Beata Je˛drzejewska a , Przemys1aw Krawczyk , Marek Pietrzak , Marta Gordel ,
*
b
c
ꢀ b
Katarzyna Matczyszyn , Marek Samoc , Piotr Cysewski
a University of Technology and Life Sciences, Faculty of Chemical Technology and Engineering, Seminaryjna 3, 85-326 Bydgoszcz, Poland
b Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wyb. Wyspianskiego 27, 50-370 Wroclaw, Poland
c
ꢀ
Nicolaus Copernicus University, Collegium Medicum, Department of Physical Chemistry, Kurpinskiego 5, 85-950 Bydgoszcz, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel hemicyanine dye possessing donor-p-acceptor structure that features a fixed benzimidazole ring
Received 26 February 2013
Received in revised form
3 June 2013
Accepted 4 June 2013
Available online 21 June 2013
as an electron donor and pyridinium moiety as an electron acceptor was synthesized. The structure of the
hemicyanine was elucidated by means of NMR and IR spectroscopy. The dye was studied using steady-
state absorption and emission spectroscopy. The analysis of experimentally recorded spectra was sup-
ported by quantum chemical calculations using density functional theory with CAM-B3LYP, LC-BLYP, LC-
u
PBE and PBE0 functionals. The measurements and calculations were performed in solvents of different
polarity. In particular, we found that the CAM-B3LYP and LC- PBE excitation energies are in very good
agreement with the experimental data. Two-photon absorption of the dye was studied by Z-scan
measurements.
u
Keywords:
Styryl dyes
Synthesis
Absorption and emission spectra
Fluorescence lifetime
Computational calculations
Two-photon absorption
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
optical power limiting [7], and two-photon three-dimensional
microfabrication [8e10]. All of the applications are primarily based
Two-photon absorption (TPA) is a nonlinear absorption process
wherein two photons are absorbed simultaneously. Characteristic
features are adherence to even-parity selection rules and quadratic
intensity dependence, while one-photon absorption processes
typically conform to odd-parity selection rules and linear intensity
dependence. An important consequence of two-photon absorption
is the possibility of the presence of upconverted fluorescence
emission [1,2].
Although the two-photon absorption process was predicted
theoretically by Göppert-Mayer as early as in 1931 [3] and experi-
mentally observed [4] in the 1960s, organic molecules exhibiting
significant nonlinear optical effects are still an intensely investi-
gated subject of various scientific studies.
on accessing higher-energy excited states using relatively low-
energy laser sources.
To realize these applications, intense worldwide effort has been
focused on the design of organic materials with a large TPA cross-
section (d) at desirable wavelengths. Up to now, several efficient
design strategies have been put forward to enhance dTPA [11]. One
of the most successful types of two-photon organic dyes is
constituted by the pushepull architecture, based on a dipolar D-
A structure: the donor group (D) is an electron-rich unit, linked
through a bridge spacer to the electron-acceptor group (A). The
p-
p
vast majority of pushepull chromophores are neutral organic
molecules, with fluorene, biphenyl, or naphthyl groups used as the
electron bridge [12]. At the same time, organic salts with an NLO-
active cation have also received considerable attention. They
usually contain the N-methylpyridinium cationic moiety as an
acceptor and are known as “stilbazolium” derivatives. This group
of NLO chromophores brings several interesting features: (1)
cationic quaternary nitrogen is a very strong acceptor; (2) varia-
tion of counter ions is an easy way to tune the crystalline packing,
(3) they are promising as amphiphilic materials for NLO-active
LangmuireBlodgett (LB) films [13e15].
Research into novel frequency upconversion materials has been
of considerable interest due to their potential use in emerging
applications. These new technologies include three-dimensional
optical data storage [5], photodynamic therapy [6], two-photon
* Corresponding author. Tel.: þ48 52 374 9034; fax: þ48 52 3749009.
E-mail address: beata@utp.edu.pl (B. Je˛drzejewska).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.06.008

674
B. Je˛drzejewska et al. / Dyes and Pigments 99 (2013) 673e685
ꢀ
ꢁ
ꢀ
ꢀ
ꢁ
ꢁ
An extremely powerful and comparably low cost tool in the
ꢂ
ꢃ
2n2 þ 1
n2 ꢀ 1
n2 þ 2
ð
ð
3
ꢀ 1Þ
design of new organic molecules with large two-photon absorption
cross-sections are ab initio or quantum mechanical calculations
which are able to accurately foresee the electronic and spectro-
scopic properties of the chromophores. The computational meth-
odologies based on density functional theory (DFT) and time-
dependent density functional theory (TD-DFT) provide accurate
results and accurately reproduce the optical properties of com-
pounds allowing us to predict and screen, in some cases, novel
synthetic approaches [16].
In this paper we describe the synthesis and spectral properties
of a novel compound belonging to the class of styryl dyes. Our
intention was to support experimental data with an analysis of the
nature of the photoexcitation, with a view towards assessment of
the reliability of predictions of electronic spectra for this compound
calculated using the frequently applied conventional global hybrid
PBE0 functional, as well as the hybrid exchangeecorrelation
ꢀ
ꢁ
f ð
3
; nÞ ¼
$
ꢀ
(1)
n2 þ 2
3 þ 2Þ
The fluorescence quantum yields for the dyes in solvents of
different polarity were determined as follows. The fluorescence
spectrum of a dilute dye solution (A z 0.1 at 404 nm) was recorded
by excitation at the absorption band maximum of the standard.
Dilute Coumarin 1 in ethanol (
F
¼ 0.64 [19]) was used as reference.
The fluorescence spectrum of Coumarin 1 was obtained by excita-
tion at its absorption maximum at 404 nm. The quantum yield of
the tested dye (Fdye) was calculated using equation:
n2dye
IdyeAref
Fdye
¼
Fref
$
(2)
n2ref
Iref Adye
where:Fref is the fluorescence quantum yield of the reference
(Coumarin 1) sample in ethanol, Adye and Aref are the absorbances of
the dye and reference samples at the excitation wavelength
(404 nm), Idye and Iref are the integrated emission intensity for the
dyes and reference samples, ndye and nref are the refractive indexes
of the solvents used for the dyes and the reference, respectively.
The fluorescence lifetimes were measured using an Edinburgh
Instruments, single-photon counting system (FLS920P Spectrome-
ters). The apparatus utilizes for the excitation 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
ultrafast electronics make a successful analysis of fluorescence
decay signals with a resolution of few picoseconds possible. The
dyes were studied at concentration able to provide equivalent
absorbance at 375 nm (0.2e0.3 in the 10 mm cell) to be obtained.
The fluorescence decays were fitted to two exponentials. The
average lifetime, sav is calculated as sav ¼ (Siaisi)/(Siai), where ai
CAM-B3LYP and long-range corrected LC-uPBE and LC-BLYP func-
tionals. In addition to the linear optical properties, nonlinear optical
properties are also considered.
2. Experimental
2.1. Measurements
All reagents and solvents (spectroscopic grade) were purchased
from Aldrich Chemical Co. and used without further purification.
For thin layer chromatography, aluminium oxide IB-F flexible
sheets (thickness 0.2 mm) were purchased from J.T. Baker Chemical
Co., Germany.
The 1H (200 MHz or 400 MHz) and 13C (50 MHz or 100 MHz)
NMR spectra were recorded with the use of a Varian Gemini 200 or
Bruker AscendÔ 400 NMR spectrometers, respectively. Dime-
thylsulfoxide (DMSO-d6) was used as the solvent and tetrame-
thylsilane (TMS) as internal standard. Chemical shifts are reported
and si are the amplitudes and lifetimes.
The two-photon absorption spectrum was determined using the
open-aperture Z-scan technique using a setup and procedures
described elsewhere [20e22]. Briefly, a tunable amplified femto-
second laser system consisting of a Quantronix Integra regenerative
amplifier (800 nm, 1.2 mJ/pulse, 130 fs pulse length) and a Quan-
tronix Palitra optical parametric amplifier was used and the spec-
tral range of the measurements was 600e1600 nm. The Z-scan data
obtained on a solution of the dye were calibrated against mea-
surements on a 4.66 mm thick silica plate [20,21].
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.
HPLC analyses were done by Shimadzu HPLC systems equipped
with UVeVis detector (detection wavelength was 370 nm) and a
Supelcosil LC-NH2 column (250 mm ꢁ 4.6 mm ꢁ 5
Separation was conducted under isocratic conditions with 0.8 ml/
min flow rate, 30 ꢂC, 20
l injection volume and HPLC grade
methanol as a mobile phase.
mm) Supelco.
m
According to Coe et al., a two-state analysis of ICT transitions
gives [23]:
Melting points (uncorrected) were determined on the Boëthius
apparatus (type PHMK 05, Germany).
Dm2ab
¼
Dm212 þ 4m212
(3)
Absorption and emission spectra were recorded at room tem-
perature using a Shimadzu UVevis Multispec-1501 spectropho-
tometer (Japan) and a Hitachi F-4500 spectrofluorimeter (Japan),
respectively. The spectra were recorded in the following solvents:
tetrahydrofuran (THF), acetone; 1-methyl-2-pyrrolidone (MP), N,N-
dimethylformamide (DMF), dimethylsulfoxide (DMSO), methanol
(MeOH), acetonitrile (MeCN), formamide (FA) and water (H2O). The
final concentration of the dye in the solution was 1.0 ꢁ 10ꢀ5 M. The
spectroscopic measurements were also performed in the studied
solvents containing either 10% or 20% of methanol. In that case the
samples were prepared as follows: the appropriate amount of the
chromophore was dissolved in methanol, then 0.1 mL (or 0.2 mL) of
the concentrated (ca. 1 mM) stock solution was added to a 10 mL
volumetric flask containing spectroscopic grade solvents under the
study. All solvents were the spectroscopic grade and were used
without any additional purification. They were characterized by
where Dmab is the dipole-moment change between the diabatic
states and Dm12 is the observed (adiabatic) dipole-moment change.
The value of m12 can be determined from the oscillator strength fos
of the transition by:
ꢄ
ꢅ
1=2
fos
j
m12j ¼
(4)
ꢀ5
1:08 ꢁ 10
E
max
where Emax is the energy of the ICT maximum (in wavenumbers)
ꢁ
and m12 is in e A. The latter is converted into Debye units upon
multiplying by 4.803. The degree of delocalization c2b and electronic
coupling matrix element Hab for the diabatic states are given by
"
!
#
1=2
Dm212
Dm212 þ 4m212
1
2
cb2
¼
1 ꢀ
(5)
their static dielectric constant (
3
) and refractive index (n) at 20 ꢂC.
,n), is given by Eq. (1) [17,18].
The solvent polarity function, f(
3

B. Je˛drzejewska et al. / Dyes and Pigments 99 (2013) 673e685
675
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
the final state (F) due to its finite lifetime and g(
spectral line profile, which often is assumed to be a d-function.
u
) provides the
Emax
ð
m12
Dmab
Þ
jHabj ¼
(6)
In this study we present calculations of two-photon absorption
probability (computed as single residue of quadratic response
functions) which were done using the DALTON 2011 program
[35,36]. Solvent effects were taken into account with the self-
consistent reaction field (SCRF) model.
If the hyperpolarizability tensor b0 has only nonzero elements
along the ICT direction, then this quantity is given by
3ðDm12
Þ
m212
b0
¼
(7)
ðEmaxÞ2
2.3. Synthetic procedure
2.2. Computational details
2.3.1. Synthesis of trans-2-[2-(4-formylphenyl)ethenyl]
benzimidazole (BT)
The geometry optimization of all molecules in their ground state
was carried out both in vacuo and with the inclusion of solvent
effects by means of the polarizable continuum model (PCM) [24] as
implemented in the Gaussian 09 package [25]. The geometry of the
solute was optimized using the hybrid exchange-correlation B3LYP
[26] functional with the 6-311þþG(d,p) basis set, whereas in the
case of a complex containing the solute molecule with two meth-
anol molecules, the optimization was performed using the B97D
[27] functional which includes the correction for dispersion inter-
action as suggested by Grimme. The latter calculations employed 6-
31 þ g(d) basis set. In all cases, the Hessian was computed in order
to confirm that the stationary points correspond to minima on the
potential energy surface.
H
N
O
CH CH
C
N
H
The compound was obtained based on the synthesis described in
literature [37,38]. A mixture of terephthalaldehyde (2.68 g, 0.02 mol)
and 2-methylbenzimidazole (2.66 g 0.02 mol) was stirred at 120 ꢂC
for 6 h in a mixture of acetic acid (3 mL) and acetic anhydride (6 mL)
solvents. The reaction mixture was cooled down to room tempera-
ture. After adding concentrated hydrochloric acid (30 mL), the
mixture was left for an hour. The resulting precipitate was collected
and subsequently washed repeatedly with copious water. The filtrate
was neutralized with 30% NaOH (60 mL) and gave yellowish pre-
cipitate that was filtered and dried. The crystals were dissolved in hot
ethyl acetate, and the insoluble materials were removed by filtration
to yield trans,trans-1,4-bis[4-(2-benzimidazolyl)ethenyl]benzene
(BTI) (see supporting information). The filtrate was evaporated to
dryness. The crude product was recrystallized from CHCl3/acetone to
yield 2.7 g (54%) of the trans-2-[2-(4-formylphenyl)ethenyl]benz-
imidazole; C16H12N2O; 248.28 g/mol.
Spectroscopic parameters characterizing one-photon excitation
spectra, i.e. oscillator strengths and excited state dipole moments
were determined using time-dependent density functional theory
(TD-DFT) using the Gaussian 09 suite of programs. The standard
hybrid PBE0 [28], long-range corrected LC-BLYP [29], LC-uPBE [29e
31] and CAM-B3LYP [32] functionals, and the 6-311þþG(d,p) basis
set were employed for these calculations, carried out both in vacuo
and with the inclusion of the solvents effect using the PCM model.
In the case of 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 Ref. [33]:
1H NMR (DMSO-d6)
d (ppm): 7.1990 (m, 2H, Ar), 7.3912-7.4324 (d,
J ¼ 16.48 Hz, 1H, eCH¼), 7.4974e7.5162 (d, J ¼ 7.52 Hz, 1H, Ar),
7.6103e7.6291 (d, J ¼ 7.52 Hz,1H, Ar), 7.7129e7.7542 (d, J ¼ 16.52 Hz,
1H, eCH¼), 7.8797e7.9001 (d, J ¼ 8.16 Hz, 2H, Ar), 7.9460e7.9666 (d,
J ¼ 8.24 Hz, 2H, Ar), 10.0211 (s, 1H, CHO), 12.7248 (s, 1H, NH).
D
E
h
ꢇ
OF
ij
ꢈ
ꢇ
ꢈ i
X
*
*
1
15
dOF
¼
S
S
þ 2S
S
(8)
OF
ii
OF
OF
ij
OF
ij
jj
ij
2.3.1.1. 13C NMR (DMSO-d6)
d (ppm): 192.387 (CHO), 113.104,
In this equation, S is the second-order transition moment
113.804, 125.893, 128.487, 130.116, 140.775 (CH), 131.627, 136.989,
139.337, 141.6205 (C). IR (KBr): 3052, 2890, 2837, 1693, 1600, 1568,
1505, 1419, 1388, 1310, 1279, 1212, 1169, 1144, 1032, 1001, 969, 919,
810, 735, 506, 441.
given by:
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
D
ED
E
"
_
m
_
m
0
z
$
K
K
z
$
F
X
1
2
i
j
1
Z
OF
ij
S
ð
z1
;
z2Þ ¼
ua
ꢀ
u1
K
2.3.2. Synthesis of trans,trans-2-{4-[(2-(1H-benzimidazol-2-yl)
ethenyl]-styryl}-N-methylpyridine iodide (BTP1)
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
ꢆ
D
ED
E
#
_
i
_
j
ꢆ
ꢆ
0
z
2
$
m
K
K
z
ꢆ
$
m
F
ꢆ
1
þ
(9)
ua
ꢀ
u2
H
N
CH CH
CH CH
where Zu1 þ Zu2 should satisfy the resonance condition and
N
CH
_
N
I
h0j:z1
mij:Ki stands for the transition moment between electronic
,
3
states j0> and jK>, respectively.
z is the vector defining polarization
of photons.
1,2-Dimethylpyridinium iodide (0.59 g, 2.5 mmol), 2-[2-(4-
formylphenyl)ethenyl]benzimidazole (0.62 g, 2.5 mmol), and
methanol (20 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 microcrystals of the
iodide salt were collected by filtration under reduced pressure
[39,40]; C23H20N3I; 465.33 g/mol; 0.71 g, 65.2% yield, mp 267e
270 ꢂC. The purity of the dye was checked by TLC (methanol-DMF
5:0.5 v/v, aluminum oxide IB-F, Rf ¼ 0.25).
In order to compare the calculated values of the two-photon
absorption probability with the two-photon absorption cross-
section which was determined experimentallysðO2FÞ, the following
relation might be used [34]:
D
E
8
p3a2Z3
e4
u
2gð
u
Þ
ð2Þ
s
¼
$
dOF
(10)
OF
G
F=2
where
a
is a fine structure constant,
u
is the frequency of absorbed
photons (assuming one source of photons), GF is the broadening of

676
B. Je˛drzejewska et al. / Dyes and Pigments 99 (2013) 673e685
1H NMR (DMSO-d6)
d
(ppm): 4.4065 (s, 3H, NþCH3), 7.1781 (m,
C]N and C]C bonds were observed in the frequency range of
1600e1430 cmꢀ1, as a strong or medium band and for CeH in the
3090e3000 cmꢀ1 region as a weak band. The peaks corresponding
to aromatic bending vibrations out-of-plane for CeH appeared at
about 540 cmꢀ1, 740 cmꢀ1 and 820 cmꢀ1 and were of medium or
strong intensity. Absorption for the ethylenic group was located
within the same range. Stretching vibrations of the aromatic CeC
bond gave a strong band between 1597 cmꢀ1 and 1616 cmꢀ1. The
aromatic bonds were also to be found between 1500 and 1540 cmꢀ1
2H, Ar), 7.3633e7.4045 (d, J ¼ 16.48 Hz, 1H, eCH¼), 7.5003e7.5186
(d, J ¼ 7.32 Hz, 1H, Ar), 7.6056e7.6245 (d, J ¼ 7.56 Hz, 1H, Ar),
7.6621e7.7021 (d, J ¼ 16.00 Hz, 1H, eCH¼), 7.7295e7.7707 (d,
J ¼ 16.48 Hz, 1H, eCH¼), 7.8122e7.8332 (d, J ¼ 8.4 Hz, 2H, Ar),
7.9070 (1H, Ar), 7.9374e7.9582 (d, J ¼ 8.32 Hz, 2H, Ar), 7.9789e
8.0189 (d, J ¼ 16.0 Hz, 1H, eCH¼), 8.5207 (1H, Ar), 8.5524e8.5693
(d, J ¼ 6.76 Hz, 1H, Ar), 8.9347e8.9501 (d, J ¼ 6.16 Hz, 1H, Ar),
12.8129 (s, 1H, NH).
and around 1450 cmꢀ1
.
2.3.2.1. 13C NMR (DMSO-d6)
d
(ppm): 46.055 (NþCH3), 111.157,
The methyl group gives low intensity absorptions around
117.745, 118.718, 119.343, 121.657, 122.724, 124.942, 125.158, 127.581,
129.220, 133.325, 142.233, 144.268, 146.154 (CH), 134.581, 135.139,
138.140, 143.998, 150.723, 152.306 (C). IR (KBr): 3081, 1633, 1616,
1569, 1522, 1459, 1415, 1326, 1278, 1214, 1175, 1145, 1007, 962, 817,
771, 744, 538.
1380 cmꢀ1 and 1480 cmꢀ1. They were identified as the deformation
vibrations
dCH3 sym. and dCH3 asym., respectively. The amine
stretching vibration for CeN bonds were observed in the 1250e
1020 cmꢀ1 range, as strong or medium bands.
The characteristic stretching vibration of O]C in the BT was
located at 1695 cmꢀ1 whereas the O]CeH stretching vibration
gave absorption band at 2743 cmꢀ1
.
3. Results and discussion
The NMR and IR spectra as well as HPLC chromatograms of the
compounds are presented in Supplementary materials.
The chromophore was poorly soluble in many organic solvents
of low or medium polarity such as dichloromethane, chloroform,
THF, ethyl acetate, n-hexane, benzene and toluene at room
temperature.
3.1. Synthesis
Synthesis of benzimidazolium chromophore BTP1 is depicted in
Fig. 1.
Starting 2-methyl-[1H]-benzimidazole and terephthaldialdehyde
are available commercially, while 1,2-dimetylpyridinium iodide was
prepared from 2-methypyridine according to the typical procedure
[40]. Styrylbenzimidazole with formyl group, the key compound in
the preparation of the BTP1 chromophore, was obtained by the re-
action of terephthaldialdehyde with 2-methyl-[1H]-benzimidazole.
The condensation was carried out in acetic anhydride e acetic acid
mixture to yield trans-2-[2-(4-formylphenyl)ethenyl]benzimidazole
as a major product, accompanied by the formation of byproduct,
trans,trans-1,4-bis[4-(2-benzimidazolyl)ethenyl]benzene.
BTP1 chromophore was prepared via Knoevenagel condensation
of trans-2-[2-(4-formylphenyl)ethenyl]benzimidazole with 1,2-
dimethylpyridinium iodide in methanol in the presence of a few
drops of piperidine used as catalyst according to the procedures
described in literature [39e41].
3.2. Spectral properties
Fig. 2 displays a few typical sets of steady-state absorption and
fluorescence spectra of BTP1 in formamide, DMF and AcCN at room
temperature. The absorption spectra are structureless and broad,
having half-widths of about 4000 cmꢀ1. The large bandwidth and
the large extinction coefficient values (about 5.0 ꢁ 104 Mꢀ1 cmꢀ1
)
for the absorption probably indicate involvement of charge transfer,
most likely involving partial donation of the nitrogen lone pair of
electrons in the benzimidazole residue to the charged pyridinium
moiety through the conjugated
around 300 nm is due to localized
p
p
-system. The absorption band
/ p
* transition of the stilbene
moiety. The broadness of the absorption spectrum may arise due to
a contribution from more than one electronic state to the absorp-
tion spectrum. The other reason is that it probably reflects a broad
distribution of conformers (solvent-solute or intramolecular) in the
ground state [42]. The photophysical characteristics of BTP1 are
listed in Tables 1 and 2.
As expected for this highly polar chromophore, its solubility is
limited to the more polar solvents. For that reason, only medium
and high polarity solvents have been used in this study. The steady-
state absorption spectra of BTP1 in the fairly limited range of sol-
vent polarity show small variations as a function of polarity
Structures and purity of BTP1 was confirmed by IR, 1H and 13C
NMR spectra, and by thin layer and HPLC chromatography. The data
were found to be in good agreement with the proposed structure.
According to the 1H NMR data, BTP1 crystallizes as a solvent free
molecule. As expected, CH ¼ CH bonds in the chromophore have a
trans configuration, as evidenced by the value of coupling constants
for olefinic protons (see experimental part and supplementary
materials).
The IR spectra indicate some characteristic bands for the syn-
thesized compounds. Thus, the aromatic stretching vibrations for
Fig. 1. Synthesis of the chromophore.

B. Je˛drzejewska et al. / Dyes and Pigments 99 (2013) 673e685
677
Table 2
The electronic absorption and one photon emission spectra parameters for BTP1
compound in solvents of different polarity. Each sample contained either 10 or 20%
of methanol (lex ¼ 404 nm).
ab
fl
max
Solvent
l
ðnmÞ;
l
ðnmÞ
Dn (cmꢀ1
)
max
3
(104 Mꢀ1 cmꢀ1
)
10% MeOH 20% MeOH 10% MeOH 20% MeOH 10% MeOH 20% MeOH
THF
403.5;
4.52
404;
4.54
398;
4.80
402;
5.35
402;
4.91
394;
4.91
614.6
621.2
622.6
623.2
625.2
602.4
608.6
621.4
624.0
624.8
624.4
8512
9059
8938
8768
9418
9273
8321
9033
8850
8871
9365
Acetone 397.5;
4.85
DMF
400;
5.13
DMSO 403;
4.72
MeCN 393.5;
4.82
Water 386.5;
4.78
Fig. 2. Normalized electronic absorption and one-photon fluorescence spectra BTP1 in
formamide, DMF and AcCN at 293 K.
in this part produces the electron distribution from the benzimid-
azole fragment, producing the form of D- -A.
p
(Table 1). Generally, with increasing solvent polarity the maxima of
the absorption spectra shift to the blue.
The experimentally observed negative solvatochromism of BTP1
constitutes convincing evidence for a ground state that is more
polar than the charge-transfer excited state, and hence lower dipole
moment value of the latter state as compared to the former is
evident. These results are in perfect agreement with previous ob-
servations on many other chromophores [12,42e44].
The fluorescence spectra of BTP1 were recorded in the same
solvents as the steady-state absorption spectra. Generally, a broad
and structureless band was observed whose position depends on
the excitation wavelength and the solvent (see Figs. 1 and 4). The
solvent dependent shape of the emission spectra indicates the
possibility of the presence of more than one conformer, with either
or both in the ground and excited electronic states [42].
As shown in Fig. 2, the relationship between the absorption and
the emission spectra displays, to some extent, a mirror image
suggesting a weak geometrical relaxation of Franck-Condon singlet
excited state. It should be underlined, however, that the maximum
of the emission spectrum recorded in each of the solvents at room
temperature shows a large red shift, more than ca. 8000 cmꢀ1 with
respect to the absorption maximum. The observed large Stokes’
Solvatochromism is generally explained through differential
solvation of the ground state and Franck-Condon excited state.
Negative solvatochromism will result from better stabilization by
polar solvents of the ground state in comparison to the excited state.
The molecular orbitals involved in intramolecular charge transfer
(ICT) in BTP1 are depicted in Fig. 3. The HOMO, as expected, is
dominated by benzimidazolium group, and the LUMO by the qua-
ternary pyridinium moiety. On excitation, electron density is trans-
ferred from the donor end of the molecule to the positively charged
quaternary heterocyclic moiety, providing charge compensation,
and thus indeed resulting in an excited state that is less polar than
the ground state, and hence less stabilized by polar solvents.
Therefore, negative solvatochromism should be expected [12,42e
44]. On the other hand, the methyl substituent (electron-donating)
is also attached to the pyridine fragment which may cause the
opposite effect. However, the tested compound in its ground state is
a cation derived from an iodide salt, and the iodide anion is asso-
ciated with the pyridine ring. Consequently, positive charge formed
Table 1
Spectral characteristic of BTP1 compound in solvents of different polarity (lex ¼ 404 nm).
THF
Acetone
MP
DMF
DMSO
MeOH
MeCN
FA
Water
f(
3
ab
,n)
ðnmÞ
0.5491
402.5
0.7904
399.5
0.8125
403.5
4.72
4275
619.8
3388
8649
0.199
0.484
3.62
1.408
96.38
1.37
1.45
5.83
0.8356
402.5
5.09
4033
623.4
3468
8804
0.124
0.536
3.41
1.197
96.59
1.17
1.05
7.46
0.8400
402.5
5.35
3952
625.2
3468
8850
0.119
0.400
2.16
1.361
97.84
1.34
0.89
6.57
0.8546
394.5
5.68
0.8593
395.0
4.54
3952
621.4
3468
9224
0.186
0.382
2.21
1.686
97.79
1.66
1.12
4.91
0.8948
396.5
4.32
4113
617.2
3468
9018
0.080
0.617
48.1
0.878
51.90
0.75
1.06
12.2
0.9137
387.5
5.78
l
max
3
(104 M1 cmꢀ1
)
FWHMab (cmꢀ1
)
5001
618.4
2904
8674
0.164
0.339
4.11
1.797
95.89
1.74
4194
619.4
3388
8887
0.294
0.433
3.65
1.542
96.35
1.50
3710
610.6
3629
8971
0.135
0.492
10.72
0.936
89.28
0.89
3871
603.4
3871
9234
0.019
0.178
98.23
0.802
1.77
0.19
1.01
51.9
1.22
fl
l
ðnmÞ
max
FWHM (cmꢀ1
)
fl
Dn (cmꢀ1
Ffl
)
sf1l(ns)
a1 (%)
sf2l(ns)
a2 (%)
sfalv(ns)
kr (108 sꢀ1
)
0.94
4.81
1.96
4.70
1.52
9.74
1.24
knr (108 sꢀ1
fos
)
1.15
1.15
1.19
1.01
1.00
m12 (D)
9.88
9.87
10.08
ꢀ240
0.15
10.14
ꢀ234
0.15
9.16
9.16
9.98
b0 (10ꢀ30 esu)
cb2
ꢀ235
ꢀ233
ꢀ194
ꢀ195
ꢀ221
0.15
0.15
0.14
0.13
0.15
jHabj(103 cmꢀ1
)
8.8
8.9
9.0
9.2
8.7
8.6
9.2
lex ¼ 404 nm; FWHM e full with at half maximum; Dm12 dipole moment change between excited and ground states was taken from quantum chemical calculation for gas faze
and is equal to ꢀ19.44 D.

678
B. Je˛drzejewska et al. / Dyes and Pigments 99 (2013) 673e685
shift indicates that significant charge redistribution occurs upon
excitation, prior to emission, and suggests a quite substantial dipole
moment change caused by the excitation [42].
The compound has a moderate fluorescence quantum yield (F,
Table 1), which also depends on the solvent polarity. It was found
that the fluorescence intensity of the chromophore gradually de-
creases with increasing solvent polarity from THF to DMF. These
results indicate that the chromophore shows typical asymmetric
chromophore behaviour [45].
In order to clarify the role of solvent polarity in modifying both
the ground and excited states of molecule, we tested the fluores-
cence excitation and emission characteristics of BTP1 in selective
solvents. As illustrated in Fig. 4, right panel, for molecule under
study dissolved in polar and highly polar solvents, only a single
fluorescence band is observed and the location of the fluorescence
peak remains invariant on lexc. Fluorescence measurements carried
out in medium polar solvents (THF) yield two partially overlapping
emission bands.
Fig. 4 shows the excitation wavelength dependence of fluores-
cence spectra (fluorescence maxima positions and shape of the
spectra) of BTP1 in THF at room temperature. Upon increasing the
excitation wavelength, the intensity of the short-wavelength band
at about 444 nm decreases. This behaviour is accompanied by an
increase of the long-wavelength emission band intensity at 618 nm.
Fig. 3. The plots of orbital contour surfaces for model A. 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 interpre-
tation of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
Fig. 4. Electronic absorption, excitation (left panel) and fluorescence (right panel) spectra of BTP1 in selected solvents.

B. Je˛drzejewska et al. / Dyes and Pigments 99 (2013) 673e685
679
The observation of the dual fluorescence indicates the existence of
two emitting species characterized by the molecules possessing
different spatial conformations.
As can be seen (Fig. 4), the steady-state fluorescence excita-
tion spectrum of the title compound is not only solvent depen-
dent but also depends on the observation wavelength. The
fluorescence excitation spectra were recorded for two emission
wavelengths (detection at the maximum of the short-wavelength
and long-wavelength bands of the emission spectrum). As can be
seen in Fig. 4, fluorescence excitation and absorption spectra are
superimposable when recorded for long-wavelength emission
band [46].
The differences between absorption and excitation spectra of
BTP1 show the existence of the excited molecules that differ in
spatial conformations and/or differ in solvent relaxation [46].
The steady-state fluorescence maximum slightly shifts to the
red, due to an increase in solvent polarity, which is evident from
Table 1. However, no reasonable correlation has been found be-
tween either the absorption or fluorescence peak maxima and the
Fig. 5. Fluorescence decay curves of BTP1 in solvents of different polarity
(
lex ¼ 375 nm, lem ¼ 615 nm).
solvent polarity parameter, f(
3 ,n). The same is true for the depen-
dence of the Stokes’ shift parameter on f(
3
,n). The possible reason
ꢇ
ꢈ
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 undecomposed spectra is not a simple reflection of the solvent
dependence of a particular excited state conformation, the spectral
shape as well as the solvent shift can also depend on the relative
quantum yields of the conformers [47].
To further understand the excited state dynamics, fluorescence
decay lifetimes of the BTP1 were measured in the studied solvents.
These measurements were performed on an Edinburgh FLS920
spectrofluorimeter with a picosecond diode laser as the excitation
1 ꢀ Ffl
sav
knr
¼
(12)
In aprotic solvents, kr and knr is not so well correlated with
viscosity of the solvents used. The radiative rate constant is rather
independent of the medium which, indicates that the electronic
structure of the fluorescent state is insensitive to the environ-
mental perturbation induced by the solvents. The non-radiative
rate constant can be represented as a sum of the rates associated
with internal conversion, photoisomerization, intersystem crossing
and specific solvent effects. A closer inspection of Fig. 6 suggests
that the nonradiative rate constant increases with increasing sol-
vent viscosity. These facts indicate that in the deactivation process
of the S1 state, the nonradiative relaxation process dominates,
however, in addition to this process, the intramolecular rear-
rangements, e.g., solvation phenomena as well as other specific
kinds of soluteesolvent interactions might also play a role [42,45].
Fig. 7 shows the absorbance spectra of BTP1 in MeCN, before and
after different irradiation times. The photoreactions were carried
out in quartz cells (1 cm in width) where the sample solution was
irradiated with a blue DPSS laser with an intensity of light of
20 mW at room temperature.
source. The fluorescence lifetimes (sf) have been determined by
single photon counting technique. The fluorescence decay function
is seen to be double exponential when monitoring the fluorescence
decay at 615 nm (Fig. 5). For all the decays in this experiment, c
2 is
bigger than 1.0 and less than 1.3, which indicates a good fit. The
lifetime of the longer component may be considered as the lifetime
of the S1 state of the most stable conformer of BTP1 molecule.
Fig. 5 shows the fluorescence decays of BTP1 in various solvents.
From this figure and data collected in Table 1, it is clear that the
fluorescence lifetimes decrease with increasing solvent polarity
(from THF to water).
We noted that the fluorescence average lifetimes do not corre-
late well with the fluorescence quantum yields for this chromo-
phore in different solvents. However, the results from the quantum
yield measurement and lifetime calculation of the compound can
clearly demonstrate the sensitivity of its fluorescence behaviour to
the solvents. The fluorescence quantum yield and lifetime for BTP1
decreases with increase of solvent polarity due to the strong
interaction between the strongly polarized molecule and solvents
in the excited state. Moreover, the much lower fluorescence
quantum yield in a proton solvent, water, can be attributed to the
hydrogen bond interaction between the molecule and surrounding
solvent, which results in an additional non-radiative decay as
observed in other molecules [48,49].
Based on the average lifetimes and quantum yields of fluores-
cence data, radiative (kr) and nonradiative (knr) rate constants have
been calculated according to Eqs. (11) and (12). The values are
collected in Table 1.
Ffl
sav
Fig. 6. Radiative (solid sign) and non-radiative (open sign) rate constants as a function
of viscosity of the aprotic solvents. The lines are used to guide the eye.
kr
¼
(11)

680
B. Je˛drzejewska et al. / Dyes and Pigments 99 (2013) 673e685
Visible light irradiation of BTP1 in MeCN do not cause distinct
changes in the electronic absorption spectra, i.e. after 20 s of irradi-
ation, BTP1 showed 5% decrease in maximal absorbance accompa-
nied by hypsochromic shift of about 4 nm. After irradiation times
higherthan20s,nofurtherchangeswereobserved. Thissuggeststhat
obtained from synthesis in the trans configuration, BTP1 is photo-
chemically stable and does not undergo isomerization to cis form.
The BTP1 compound shows a blue shift in the absorption band
and very low emission in the protic solvent, water, with its excited
state decaying non-radiatively, involving the hydrogen bond in-
teractions. The strong hydrogen bonding solvent forms hydrogen
bonds with the nitrogen lone pair of the dye molecule, which re-
duces the magnitude of the ICT interaction within the dye mole-
cule. However, when the investigated compound BTP1 was
dissolved in solvents containing either 10% or 20% of methanol no
significant changes in the spectral properties were observed as
shown in Table 2.
The intensity and maximum of the absorption and fluorescence
band depend on the solvent but do not depend on methanol con-
tent. Fig. 8 presents the examples of the absorption spectra of the
BTP1 compound in pure DMF and after addition of 10% or 20% of
methanol to each sample.
Fig. 8. Normalized electronic absorption spectra of BTP1 in pure DMF and with
addition of MeOH at 293 K.
It has long been recognized that the longest wavelength ab-
sorption band of “pushepull” compounds is responsible for the
second-order NLO response, according to the well-known and
widely used “two-level model” [50]. In this model, the hyper-
polarizability (b0) can be described in terms of a ground and first
excited-state having charge-transfer character and is related to the
energy of the optical transition (E), its oscillator strength (fos), and
the difference between ground and excited-state dipole moment
type, the electronic spectrum of BTP1 exhibits very intense band
associated with so-called intramolecular charge-transfer. It was
mentioned in the experimental section, that BTP1 was dissolved in
solution containing 10% of methanol in order to improve its solu-
bility. Therefore, it was very important to verify how this addition
affects the computed electronic structure parameters. In our
theoretical calculations, the presence of admixture of methanol
was taken into account by placing explicitly two methanol mole-
cules near the nitrogen atoms from benzimidazole groups. Here-
after, the solute molecule surrounded by dielectric continuum will
be denoted as model A, while the complex of a solute and two
methanol molecules surrounded by dielectric continuum will be
denoted as model B.
(Dm12) [51]. It should be noted that this analysis constitutes a major
oversimplification because the compounds that reveal multiple
low-energy excited states, cannot be accurately described as two-
state systems [23]. Nevertheless, it is notable that the chromo-
phore does display one dominant visible maximum, and the
derived b0 values are rather large. As we can conclude from Table 1,
the indirectly derived b0 values are slightly dependent on the po-
larity of the solvent.
Tables 3 and 4 contain the values of the theoretically deter-
mined spectroscopic parameters characterizing the lowest-lying
singlet excited states, obtained using model A and B, respectively.
In the case of both models, we found that the first electronic excited
state is the charge transfer-state of
pep* character and the corre-
3.3. Theoretical calculations
sponding transition is dominated by the HOMO/LUMO excitation
as was mentioned earlier (cf. Figs. 3 and 10). Fig. 10 presents the
plots of orbital contour surfaces for model B. The results show that
for BTP1 compound the lowest one-photon transition in vacuo,
which is an allowed transition, does not change for the calculations
including the presence of two methanol molecules.
The compound studied in this work (BTP1, see Fig. 9) belongs to
the D-p-A class of compounds. Similarly to other molecules of this
Spectroscopic parameters assembled in Tables 3 and 4 show that
transition to the first excited state is accompanied by the largest
value of oscillator strength. Experimental studies have shown the
inability to classify it as a chemical exhibiting a positive or negative
solvatochromism. The same observation is confirmed by the
theoretical calculations using four different functionals. Although
the comparison of the data computed for BTP1 in vacuo and in
solvent show the increase in the excitation energy (hypsochromic
shift), changing the solvent from less to more polar does not result
in significant changes in the position of the band maximum. The
presence of two methanol molecules slightly increases the values of
oscillator strength but shifts the absorption band towards longer
wavelengths with an average of 60 nm in the gas phase and 30 nm
in all solvents. On the other hand, this batochromic shift results in
the values obtained for the complex with two methanol molecules
being slightly closer to the experimental values.
In both cases, the functional PBE0 gives values of the transition
energy (
DE) significantly underestimated relative to the experi-
Fig. 7. Changes in absorbance intensity for BTP1 dye after irradiation of a Blue DPSS
laser (line at 408 nm) with intensity of light of 20 mW.
mental value. The average difference of DDE between them is

B. Je˛drzejewska et al. / Dyes and Pigments 99 (2013) 673e685
681
Fig. 9. Chemical structure of the investigated molecule a) monomer and b) with two-methanol compounds obtained after optimization in G09.
3710.1 cmꢀ1 for the monomer and 5162 cmꢀ1 for the complex. The
asymptotically corrected CAM-B3LYP, LC-BLYP and LC- PBE func-
addition, we can see that for the ground state these differences are
smaller (Dm < 0.3 D), and slightly larger for the CT excited state
(Dm > 0.3 D). In both cases, for S0 and S1, an increase in the value of
u
tionals give the results more comparable with those obtained
experimentally. Here the average deviation is 403.2 cmꢀ1 and
-1532.5 cmꢀ1 for CAM-B3LYP, 3548.8 cmꢀ1 and 1371.1 cmꢀ1 for LC-
the dipole moment during transition from the gas phase to the
solvent is observed. In general, it can be stated that the increase of
the solvent polarity is followed by the increase of the value of m1
and m2, and these differences are minimal with an average of 0.03e
0.1 D. It is also clear, that in both cases the excitation to the CT
excited state is accompanied by a decrease of the dipole moment
values, and its value is more than 6 D less relative to the m1. It is
worth mentioning, that the polarity of the low-lying excited state is
decreasing with the polarity of the solvent. The values contained in
Tables 5 and 6 indicate, that for the gas phase Dm12 is more than 2
times smaller than for water. The presence of two molecules of
methanol does not change significantly the value of Dm12 if solvent
effects are taken into account; however, one should not overlook
that reduction of Dm12 roughly by a factor of two is found for the
molecule in vacuo. 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. In this connection, according to research carried out by
Bartkowiak and others [52], we classify the investigated system as
the one exhibiting negative solvatochromism.
BLYP, 3387.5 cmꢀ1 and 1290.5 cmꢀ1 for LC-
uPBE, in the case of the
monomer and the complex, respectively. Among these three, the LC
functionals give similar values, while the CAM-B3LYP on average
predicts the absorption band maximum to be 40 nm larger than
that observed experimentally. On the other hand, the values ob-
tained in the case of model A using CAM-B3LYP, and in the case of
model B using LC-uPBE are closer to the experimental ones. In
general, the values for the complex are closer to the experimental
ones and the presence of two molecules of methanol causes an
overestimation of the
with experiment, therefore LC-
l
value using the CAM-B3LYP in comparison
PBE more correctly describes the
u
spectroscopic parameters of this compound.
The dipole moments for the ground (m1) and first excited state
(m2) for the investigated monomer and complex calculated within
TDDFT formalism are presented in Tables 5 and 6, respectively.
Firstly, the presence of two molecules of methanol does not affect
significantly the obtained values of the dipole moment. In the gas
phase differences in the values of this parameters between the
investigated systems are in the region of 2e5 D, but the presence of
the solvent minimizes to less than 0.7 D, except for the PBE0. In
In the case of compounds with a positive solvatochromic
behaviour, the two-photon cross-section (TPA) increases with
Table 3
Calculated and experimental values of excitation energies (in nm, lmax) and oscillator strengths (fos) for the molecule without two-methanol compounds (model A). The bold
type was distinguished values of DFT functionals giving more reliable results in relation to experimental values.
Solvent
Gas phase
THF
CAM-B3LYP
LC-BLYP
LC-
u
PBE
PBE0
lmax
fos
lmax
fos
lmax
fos
lmax
fos
p
nep*
e
p
*
494.05
368.67
367.24
398.64
281.07
314.45
390.73
310.27
277.25
388.70
309.08
276.88
388.86
309.23
276.86
390.59
310.50
276.95
389.92
273.57
310.09
387.52
308.46
276.59
1.0804
0.0443
0.3590
1.9197
0.1707
0.0328
1.9664
0.0260
0.0263
1.9694
0.0248
0.0205
1.9758
0.0244
0.0203
2.0006
0.0234
0.0217
2.0005
0.0670
0.0231
1.9823
0.0232
0.0180
385.33
298.52
265.55
351.71
287.04
256.61
347.85
284.74
255.94
346.71
283.96
255.78
346.90
284.14
255.77
348.26
285.29
255.86
347.87
285.01
255.81
346.21
283.69
255.66
1.7229
0.0278
0.1273
2.0800
0.0153
0.0152
2.0833
0.0120
0.0147
2.0782
0.0113
0.0143
2.0828
0.0112
0.0145
2.1047
0.0111
0.0155
2.1027
0.0109
0.0153
2.0823
0.0106
0.0144
391.59
302.38
272.23
353.83
289.30
259.49
349.65
286.84
258.75
348.45
286.03
258.56
348.63
286.20
258.57
350.00
287.36
258.65
349.60
287.08
258.59
347.89
285.73
258.44
1.6831
0.0311
0.1596
2.0914
0.0178
0.0143
2.0986
0.0139
0.0138
2.0940
0.0130
0.0134
2.0989
0.0130
0.0136
2.1215
0.0128
0.0146
2.1196
0.0125
0.0144
2.0990
0.0123
0.0134
747.61
608.78
534.05
488.94
376.21
375.81
466.81
362.71
358.36
461.91
359.94
354.79
461.72
359.71
354.30
463.53
360.26
354.18
462.00
359.41
353.11
458.07
357.56
351.39
0.4923
0.0091
0.3147
1.1870
0.1061
0.3912
1.3213
0.4584
0.0313
1.3442
0.4529
0.0347
1.3560
0.4493
0.0356
1.3920
0.4371
0.0379
1.3981
0.4353
0.0390
1.3822
0.4416
0.0389
pe
p
p
*
*
pe
nep*
pe
p
p
*
*
Acetone
MeOH
MeCN
DMF
pe
nep*
pe
p
p
*
*
pe
nep*
pe
p
p
*
*
pe
nep*
pe
p
p
*
*
pe
nep*
pe
p
p
*
*
DMSO
Water
pe
nep*
pe
p
p
*
*
pe
nep*
pe
p*

682
B. Je˛drzejewska et al. / Dyes and Pigments 99 (2013) 673e685
Table 4
Calculated and experimental values of excitation energies (in nm, lmax) and oscillator strengths (fos) for the molecule with two-methanol compounds (model B). The bold type
was distinguished values of DFT functionals giving more reliable results in relation to experimental values.
Solvent
Gas phase
THF
CAM-B3LYP
LC-BLYP
LC-
u
PBE
PBE0
lmax
fos
lmax
fos
lmax
fos
lmax
fos
p
nep*
e
p
*
540.18
387.89
356.03
434.67
321.48
293.74
422.25
316.62
287.11
419.17
315.24
286.35
419.31
315.41
286.29
421.53
316.90
286.38
420.53
316.43
286.18
417.21
314.53
285.79
1.6078
0.0320
0.2530
2.0462
0.0114
0.1072
2.0723
0.0135
0.0810
2.0712
0.0134
0.0496
2.0774
0.0135
0.0472
2.1033
0.0144
0.0482
2.1022
0.0143
0.0432
2.0813
0.0135
0.0359
449.97
314.24
278.02
383.91
296.04
264.06
376.04
292.84
262.72
374.01
291.84
262.42
374.16
292.03
262.40
375.87
293.33
262.50
375.20
292.98
262.41
372.82
291.43
262.18
1.9404
0.0061
0.0715
2.1462
0.0348
0.0184
2.1442
0.0287
0.0170
2.1379
0.0269
0.0165
2.1426
0.0269
0.0167
2.1653
0.0283
0.0179
2.1627
0.0277
0.0177
2.1415
0.0256
0.0164
455.50
317.62
286.85
385.94
298.14
267.40
377.73
294.79
265.93
375.62
293.76
265.60
375.77
293.94
265.58
377.50
295.26
265.68
376.81
294.90
265.58
374.37
293.32
265.34
1.9163
0.0032
0.0504
2.1568
0.0363
0.0186
2.1578
0.0302
0.0171
2.1528
0.0281
0.0165
2.1578
0.0282
0.0167
2.1809
0.0297
0.0179
2.1785
0.0290
0.0177
2.1572
0.0269
0.0164
680.05
588.63
516.92
523.61
404.74
371.34
499.13
384.95
361.31
493.41
380.87
359.16
493.29
380.31
358.96
495.90
380.15
359.29
494.08
378.93
358.63
489.07
376.94
357.27
1.0715
0.0476
0.0004
1.5012
0.0355
0.6709
1.5784
0.0418
0.6256
1.5890
0.0434
0.6100
1.5996
0.0439
0.6050
1.6367
0.0452
0.5937
1.6391
0.0457
0.5885
1.6163
0.0457
0.5874
pe
p
p
*
*
pe
nep*
pe
p
p
*
*
Acetone
MeOH
MeCN
DMF
pe
nep*
pe
p
p
*
*
pe
nep*
pe
p
p
*
*
pe
nep*
pe
p
p
*
*
pe
nep*
pe
p
p
*
*
DMSO
Water
pe
nep*
pe
p
p
*
*
pe
nep*
pe
p*
increasing polar environment [53]. Conclusions presented in this
work indicate that the molecule tested by us should exhibit nega-
fact that there are difficulties in the dissolution of the compound,
the TPA spectra discussed below should be treated as demonstra-
tive and indicating the impact of the solvent polarity on the non-
linear response.
tive solvatochromism. For this group of compounds, there are no
ð2Þ
detailed reports in the subject literature on the value of
s
in
OF
various media. To our best knowledge, there is only one theoretical
work regarding TPA in the gas phase and in the polar solvent and in
most cases the works concern the impact of polar environment on
the first- (b) and second- (g) order hyperpolarizabilities for the
non-centrosymmetric organic molecules [33,54,55]. Despite the
Fig. 11 presents the comparison of the values of the TPA cross-
section for monomer and the complex with two molecules of
methanol in selected environments. Similarly to the results of one-
photon absorption calculations, in the case of functional CAM-
B3LYP almost identical values of this parameters were obtained
for both systems. For this reason, further discussions will be
focused solely on the complex. The CAM-B3LYP was designed to
predict charge-transfer excitations more accurately by improving
the long range behaviour of the exchange potential, so for this
reason, further consideration will be based on values obtained us-
ing this functional [56].
Two-photon absorption cross-sections in a.u. (d
OF) and in GM
ð
sðO2FÞÞ for the molecule in this study are given in Table 7. In all
simulations, the following values of GF (see eq. (10)), determined on
the basis of our spectroscopic data for BTP1 solution containing 10%
of methanol, were used: THF e 4470.7 cmꢀ1, acetone e 3844 cmꢀ1
,
methanol e 3744.8 cmꢀ1, acetonitrile e 3769.8 cmꢀ1, DMF e
3867.4 cmꢀ1, DMSO e 3805.3 cmꢀ1 and water e 3745.6 cmꢀ1
.
Looking closely at this table one finds that the most dominating TPA
state is also the most dominating one-photon absorption state
(OPA). These observation are in accordance with the results pre-
sented by Agren and co-workers, i.e. for asymmetric compounds
both the dominant OPA and the dominant TPA state are the same
[57]. Considering the CT excited state, it is difficult to clearly
determine the impact of the environment on the obtained value of
the TPA cross-section. First, quite a surprising observation is that
the obtained values of both parameters do not differ significantly
from each other in the following media. These small differences can
be related to change of position of absorption band maxima. The
presence of the solvent, as it has been shown previously, does not
affect substantially the value of excitation energy. In accordance
with the two-state approximation [31,50,58], the obtained value of
dOF depends on three factors: oscillator strength, dipole moment
difference and transition energy. Because of the fact that Dm12 and
Fig. 10. The plots of orbital contour surfaces for molecule with presence of two-
methanol compounds. 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.)

B. Je˛drzejewska et al. / Dyes and Pigments 99 (2013) 673e685
683
Table 5
Values of dipole moments for the ground and first lowest-ling singlet electronic states for the model A calculated at the TDDFT/6-311þþG(d,p) level of theory. All values are
given in D.
Solvent
Ground state m1
First excited state m2
CAM-B3LYP
LC-BLYP
LC-
u
PBE
PBE0
CAM-B3LYP
LC-BLYP
LC-
u
PBE
PBE0
Gas phase
THF
Acetone
MeOH
MeCN
DMF
24.77
30.59
31.31
31.47
31.49
31.50
31.55
31.63
26.33
31.24
31.86
31.99
32.01
32.02
32.06
32.13
26.10
31.15
31.77
31.91
31.93
31.94
31.98
32.05
22.66
29.70
30.56
30.75
30.76
30.79
30.84
30.93
5.33
17.46
19.95
20.49
20.58
20.66
20.83
21.05
11.77
24.1
10.71
23.38
25.11
25.49
25.55
25.58
25.7
5.72
6.70
7.86
8.26
8.35
8.51
8.65
8.77
25.76
26.13
26.18
26.21
26.32
26.49
DMSO
Water
25.88
fos are almost equal in case of all solvents, the energy transition will
be a factor responsible for saturation of the two-photon cross-
section.
that, as in the case of spectroscopic parameters, the LC-
u
PBE would
be more appropriate, but it is not available in the applied
computing program. However, these differences are insignificant
and overall agreement is quite satisfactory. Therefore, it is believed
that the values of determined for the other solvents are not
burdened with a bigger mistake, and the calculation scheme used
provides a correct value of this parameters and thus supports
experimental research.
The results discussed above are obtained using the values of GF
from the one-photon absorption calculations. As a consequence of
eq. (10), this coefficient has influence on the values of TPA obtained
in GM. It is easy to assume that if this factor changes, the sðO2FÞwill
change significantly. For this reason, the TPA was also calculated
using GF ¼ 3892.4 cmꢀ1, which is an average value of the broad-
ening of the final state for all solvents. The results are presented in
parenthesis in Table 5. When different values of GF are used, the TPA
in water is about 50 GM higher than in THF. In the case of constant
absorption line width, the TPA in water is about 16 GM smaller than
in THF so the relation is reversed. Moreover, for a medium of
relative permittivity there are observed almost the same values of
TPA cross-sections with increasing solvent polarity when
GF ¼ const is used. However, first calculations indicated more non-
monotonic behaviour of the TPA with respect to solvent polarity
In the case of the values expressed in atomic units the transition
from gas phase to solvent is accompanied by a slight decline of the
d
OF. However, in the case of sðO2FÞ, it can be attributed to the upward
trend. These observations, which may be due to the presence of the
negative solvatochromism, are significantly different from the
molecules with positive solvatochromism, where there is a clear
increase in both parameters [53]. Referring to the azobenzene,
stilbene and benzilideneanilines derivatives, ð2inÞ the case of the
studied molecule the values of both d
OF and sOF are several times
higher. This indicates that the tested molecule is characterized by a
nonlinear response at a much higher level and it should be ex-
pected for the higher values of the molecular hyperpolarizabilities.
Moreover, the component of the first hyperpolarizability is asso-
ciated with TPA by equation (13), therefore similar trends in the
obtained values are expected.
ꢇ
ꢈ
zz
d
w
bzzzDm12;z
(13)
gꢀCT
The two-photon absorption cross-section values in GM deter-
mined in both theoretical and experimental are in good agreement.
In the case of CAM-B3LYP functional the obtained value is about
54 GM lower and for the B3LYP about 70 GM higher relative to the
value measured in DMSO. Considering the computational methods
used in the function of the average relative deviation of the method,
which is characteristic for the solvatochromic reversal [58]. In the
ð2Þ
second case, the values of
s
show a monotonic decrease with
OF
increasing solvent polarity which is characteristic for either nega-
tive or positive solvatochromic shifts. This observation indicates
that for BTP1 the direct solvent effect has significant influence on
behaviour of the TPA cross-section, but it is effectively reduced by
the other factors like the broadening of the final state. On the other
hand, these discrepancies clearly illustrated the importance of GF in
correct description of the TPA cross-section.
the best value of the TPA cross-section appears to be provided by
ð2Þ
CAM-B3LYP functional. The use of this method leads to a
s
OF
affected by an error of 13%, while for B3LYP this value increases to
16%. The type of model solvent used may be partly responsible for
these discrepancies and in particular, the parameters that define a
cavity solvation in SCRF method. In the context of computational
validation protocol, it should be mentioned that the Z-scan method,
by which the experimentally determined TPA cross-sections has
been flawed, gives error values often exceeding 20%. Moreover, this
discrepancy has its roots in the applied functional. It is expected
3.4. Comparison with experimentally derived two-photon spectra
Fig. 12 shows the comparison of the experimentally obtained
two-photon absorption spectrum with the one-photon absorption
Table 6
Values of dipole moments for the ground and first lowest-ling singlet electronic states for the model B calculated at the TDDFT/6-311þþG(d.p) level of theory. All values are
given in D.
Solvent
Ground state m1
First excited state m2
CAM-B3LYP
LC-BLYP
LC-
u
PBE
PBE0
CAM-B3LYP
LC-BLYP
LC-
u
PBE
PBE0
Gas Phase
THF
Acetone
MeOH
MeCN
DMF
22.19
30.19
31.24
31.47
31.51
31.52
31.59
31.71
24.06
31.02
31.94
32.15
32.18
32.19
32.26
32.36
23.9
19.82
29.06
30.28
30.55
30.59
30.61
30.69
30.82
10.99
20.46
20.93
20.96
21.04
21.09
23.57
21.50
16.07
24.85
25.32
25.77
25.83
25.84
25.97
26.20
15.63
24.44
24.73
25.18
25.24
25.26
25.45
25.63
15.69
13.17
13.02
13.33
13.39
13.48
13.59
13.69
30.93
31.85
32.06
32.09
32.10
32.17
32.26
DMSO
Water

684
B. Je˛drzejewska et al. / Dyes and Pigments 99 (2013) 673e685
is found to be a strong two-photon absorber in the expected
wavelength range, i.e. at twice the wavelength of the main one-
photon absorption band. It is comforting to find that the peak
value of the two-photon absorption in this range is found to be
w430 GM, quite close to the predicted theoretically range of values.
However, 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. In addition, at the long wave-
length tail of the two-photon absorption band we detected the
presence of a higher order absorption process, tentatively ascribed
to three-photon absorption (within w850e950 nm, the s3 value at
950 nm appeared to be about 1.2 10ꢀ77 cm6 s2). There was no
nonlinear absorption at longer wavelengths (1000e1600 nm).
4. Conclusions
Synthesis, spectral and photophysical properties of BTP1 were
studied in medium and high polarity solvents. It has been found
that negative solvatochromizm in the absorption spectra and
bathochromic shifts of the emission bands occurs with increased
solvent polarity which is a well-known manifestation of the charge
transfer character of the solvent-relaxed emissive state. However,
the plot of the emission maxima vs. polarity of the solvents is not
linear but this plot of the absorbance maxima is much more linear,
indicating that the specific interactions between solvent and solute
occur rather in the excited state than in the ground state. Performed
theoretical calculations showed that TD-DFT properly describes the
one-photon absorption of the investigated compound. Obtained
transition energies are in good agreement with experimental data,
and in the same way indicate the trends in the behaviour of the
molecule in the presence of solvent.
Additionally, Stark spectroscopy of the ICT bands affords esti-
mated static first hyperpolarizabilities b0 of the compound in sol-
vent of different polarity. The indirectly derived b0 values are large
and slightly depend on the polarity of the solvent. The two-photon
absorption cross-section for the investigated compound were
determined using the quadratic response functions formalism
combined with density functional theory. For the DMSO, the
theoretical value in terms of the average relative deviation does not
exceed of 16%, relative to the experimental value. This coefficient is
quite satisfactory and indicates the possibility of applying the
presented computational scheme in prediction of two-photon ab-
sorption cross-section for this class of compounds.
Fig. 11. Comparison of the values of the two-photon absorption cross-section
d in a.u.
(dotted line) and
s in GM (straight line) designated for the monomer (triangle-shaped
marker) and a complex with two molecules of methanol (wheel-shaped marker).
Table 7
Values of two-photon absorption (TPA) cross-section (d
OF) in atomic units and s02F in
GM calculated using Eqs. (9) and (10) respectively for the complex with two
methanol molecules.
Solvent
CAM-B3LYP
B3LYP
d
ꢁ 10ꢀ50
s
d
ꢁ 10ꢀ50
s
Gas Phase
THF
Acetone
MeOH
MeCN
DMF
4.16
3.71
3.48
3.50
3.49
3.49
3.48
3.46
364.4
4.49
8.42
8.82
8.90
8.92
8.92
8.94
8.98
190.5
330.95 (381.41)
373.81 (370.41)
381.86 (368.62)
381.17 (370.41)
371.60 (370.47)
376.23 (369.05)
379.95 (366.86)
380.42 (438.42)
484.20 479.80)
506.65 (489.08)
504.38 (490.15)
491.60 (490.10)
500.86 (491.31)
516.31 (498.52)
DMSO
Water
replotted at twice the wavelength. The results can be seen to
confirm some of the theoretical predictions although there are also
some striking features of the experimental spectrum that may be
considered not fully understood at the present stage. Indeed, BTP1
The experimental and theoretical results indicate that this un-
symmetrical cyanine dye is thus a promising candidate for appli-
cations as a nonlinear absorber.
Acknowledgements
This research was supported in part by the Ministry of Science
and Higher Education (MNiSW) (BS-12/2010), 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.2013.06.008.
References
Fig. 12. Two-photon absorption spectrum of BTP1 derived from Z-scan measurements
in DMSO solvent. The data are compared with the one-photon spectrum re-plotted
against the doubled wavelength. The data for the wavelength greater than w850 nm
contained contributions from a three-photon process.
[1] Denk W, Strickler JH, Webb WW. Two-photon laser scanning fluorescence
microscopy. Science 1990;248:73e6.

B. Je˛drzejewska et al. / Dyes and Pigments 99 (2013) 673e685
685
[2] Andrade CD, Yanez CO, Rodriguez L, Belfield KD. A series of fluorene-based
two-photon absorbing molecules: synthesis, linear and nonlinear character-
ization, and bioimaging. J Org Chem 2010;75:3975e82.
[32] 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.
ꢀ
[33] Zalesny R, Bartkowiak W, Styrcz S, Leszczynski J. Solvent effects on con-
[3] Göppert-Mayer M. Über. Elementarakte mit zwei Quantensprüngen. Ann Phys
1931;9:273e94.
formationally induced enhancement of the two-photon absorption cross-
section of
a pyridinium-N-phenolate betaine dye. A quantum chemical
[4] Peticolas WL. Multiphoton spectroscopy. Annu Rev Phys Chem 1967;18:233e
60.
[5] Cumpston BH, Ananthavel SP, Barlow S, Dyer DL, Ehrlich JE, Erskine LL, et al.
Two-photon polymerization initiators for three-dimensional optical data
storage and microfabrication. Nature 1999;398:51e4.
study. J Phys Chem A 2002;106:4032e7.
[34] Norman P, Cronstrand P, Ericsson J. Theoretical study of linear and nonlinear
absorption in platinum-organic compounds. Chem Phys 2002;285:207e20.
[35] Release Dalton2011DALTON. a molecular electronic structure program. see,
http://daltonprogram.org/; 2011.
[6] Drobizhev M, Stepanenko Y, Dzenis Y, Karotki A, Rebane A, Taylor PN, et al.
Extremely strong near-IR two-photon absorption in conjugated porphyrin
dimers: quantitative description with three-essential-states model. J Phys
Chem B 2005;109:7223e36.
[7] He GS, Lin T-C, Prasad PN, Cho C-C, Yu L-J. Optical power limiting and stabi-
lization using a two-photon absorbing neat liquid crystal in isotropic phase.
Appl Phys Lett 2003;82:4717e9.
[8] Zhou WH, Kuebler SM, Braun KL, Yu TY, Cammack JK, Ober CK, et al. An
efficient two-photon-generated photoacid applied to positive-tone 3D
microfabrication. Science 2002;296:1106e9.
[9] Kawata S, Sun HB, Tanaka T, Takada K. Micromachines can be created with
higher resolution using two-photon absorption. Nature 2001;412:697e8.
[10] Miwa M, Juodkazis S, Kawakami T, Matsuo S, Misawa H. Femtosecond two-
photon stereo lithography. Appl Phys A 2001;73:561e6.
[36] Release Dalton2011LSDALTON. a linear scaling molecular electronic structure
program. see, http://daltonprogram.org/; 2011.
[37] Xue P, Lu R, Zhao L, Xu D, Zhang X, Li K, et al. Hybrid self-assembly of a
p
gelator and fullerene derivative with photoinduced electron transfer for
photocurrent generation. Langmuir 2010;26(9):6669e75.
[38] 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.
[39] 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.
[40] 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.
[11] Zhou H, Zhou F, Tang S, Wu P, Chen Y, Tu Y, et al. Two-photon absorption dyes
[41] Je˛drzejewska B, Rudnicki A. The synthesis and spectroscopic investigation of
dichromophoric hemicyanine dyes. Dyes Pigm 2009;80:297e306.
[42] 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.
[43] Coe BJ, Harris JA, Hall JJ, Brunschwig BS, Hung S-T, Libaers W, et al. Syntheses
and quadratic nonlinear optical properties of salts containing benzothiazo-
lium electron-acceptor groups. Chem Mater 2006;18(25):5907e18.
with thiophene as
p electron bridge: synthesis, photophysical properties and
optical data storage. Dyes Pigm 2011;92:633e41.
[12] Quist F, Vande Velde CML, Didier D, Teshome A, Asselberghs I, Clays K, et al.
Pushepull chromophores comprising benzothiazolium acceptor and thio-
phene auxiliary donor moieties: Synthesis, structure, linear and quadratic
non-linear optical properties. Dyes Pigm 2009;81:203e10.
[13] Belfield KD, Hagan DJ, Van Stryland EW, Schafer KJ, Negres RA. New two-
photon absorbing fluorene derivatives: synthesis and nonlinear optical
characterization. Org Lett 1999;1:1575e8.
[14] Coe BJ, Harris JA, Asselberghs I, Wostyn K, Clays K, Persoons A, et al. Quadratic
optical nonlinearities of N-methyl and N-aryl pyridinium salts. Adv Funct
Mater 2003;13(5):347e57.
ꢀ
[44] Je˛drzejewska B, Kabatc J, Osmia1owski B, Pa˛czkowski J. Bischromophoric
hemicyanine dyes. Spectroscopic properties of 1,3-bis-(p-N, N-dia-
lkylaminostyryl)pyridiniumpropane dibromides. Spectrochimica Acta Part A
2007;67:306e15.
[45] Xiao H, Mei C, Wang Y, Li H, Qian S, Yin H, et al. Novel triphenylamine-cored
two-photon absorbing dyes for labeling of biomolecules. Mater Chem Phys
2011;130:897e902.
[15] Coe BJ, Harris JA, Brunschwig BS, Garin J, Orduna J. Molecular salts with
diquat-based electron acceptors for nonlinear optics.
2005;127(10):3284e5.
J
Am Chem Soc
[46] Józefowicz M, Heldt JR, Bajorek A, Pa˛czkowski J. Spectroscopic properties of
ethyl 5-(4-dimethylaminophenyl)-3-amino-2.4-dicyanobenzoate. Chem Phys
2009;363:88e99.
[47] 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.
[48] Chew S, Wang P, Hong Z, Kwong HL, Tang J, Sun S, et al. Photoluminescence
and electroluminescence of a novel green-yellow emitting materiale5.6-Bis-
[16] Pastore M, Mosconi E, De Angelis F, Grätzel M. A computational investigation
of organic dyes for dye-sensitized solar cells: benchmark, strategies, and open
issues. J Phys Chem C 2010;114:7205e12.
[17] Je˛drzejewska B, Pietrzak M. Dipole moment determination of 4-[(N-(5,6,7,8-
tetrahydroisoquinolinium-5-ylidene)methyl]-N, N-dialkylaniline iodides in
solution. Spectrochimica Acta Part A 2011;79:985e92.
[4-(naphthalene-1-yl-phenyl-amino)-phenyl]pyrazine-2.3
dicarbonitrile.
[18] Je˛drzejewska B, Pietrzak M, Pa˛czkowski J. Solvent effects on the spectroscopic
properties of styrylquinolinium dyes series. J Fluoresc 2010;20:73e86.
[19] Olmsted III J. Calorimetric determinations of absolute fluorescence quantum
yields. J Phys Chem 1979;83(20):2557e682.
J Luminesc 2007;124:221e7.
[49] Shao J, Guan Z, Yan Y, Jiao C, Xu Q-H, Chi C. Synthesis and characterizations of
star-shaped octupolar triazatruxenes-based two-photon absorption chromo-
phores. J Org Chem 2011;76:780e90.
[50] Oudar JL, Chemla J. Hyperpolarizabilities of the nitroanilines and their re-
lations to the excited state dipole moment. J Chem Phys 1977;66:2664e9.
[51] Lacroix PG, Malfant I, Payrastre C, Wolf J-G, Bonvoisin J, Nakatani K. Synthesis,
crystal structure, and second-order nonlinear optical properties of a new
phase-matchable cyanine dye. Chem Mater 1998;10:1135e40.
ꢀ
ꢀ
[20] Olesiak-Banska J, Gordel M, Ko1kowski R, Matczyszyn K, Samoc M. Third-order
nonlinear optical properties of colloidal gold nanorods.
J Phys Chem C
2012;116:13731e7.
ꢀ
ꢀ
[21] Szeremeta J, Nyk M, Wawrzynczyk D, Samoc M. Wavelength dependence of
nonlinear optical properties of colloidal CdS quantum dots. Nanoscale
2013;5(6):2388e93.
[52] Bartkowiak W. Solvatochromism and nonlinear optical properties of donor-
[22] 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.
acceptor
p-conjugated molecules. In: Papadopoulos MG, Sadlej AJ,
ꢀ
Leszczynski J, editors. Non-linear optical properties of matter. Berlin/Heidel-
berg: Springer; 2006. p. 299e318.
[23] Coe BJ, Fielden J, Foxon SP, Harris JA, Helliwell M, Brunschwig BS, et al. Diquat
derivatives: highly active, two-dimensional nonlinear optical chromophores
with potential redox switchability. J Am Chem Soc 2010;132:10498e512.
[24] Cancés 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.
[25] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al.
Gaussian 09. Revision A.1. Wallingford CT: Gaussian. Inc.; 2009.
[26] Becke AD. Density-functional thermochemistry. III. The role of exact ex-
change. J Chem Phys 1993;98:5648e52.
[53] Krawczyk P. DFT study of linear and nonlinear optical properties of donor-
acceptor substituted stilbenes, azobenzenes and benzilideneanilines. J Mol
Model 2010;16:659e68.
[54] Wielgus M, Bartkowiak W, Samoc M. Two-photon solvatochromism. I. Solvent
effects on two-photon absorption cross section of 4-dimethylamino-40-
nitrostilbene (DANS). Chem Phys Lett 2012;554:113e6.
ꢀ
ꢀ
[55] 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, [submitted
for publication]
[27] Grimme S. Semiempirical GGA-type density functional constructed with a
long-range dispersion correction. J Comp Chem 2006;27:1787e99.
[28] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made
simple. Phys Rev Lett 1996;77:3865e8.
[56] Rudberg E, Sa1ek P, Helgaker T, Ågren H. Calculations of two-photon charge-
transfer excitations using Coulomb-attenuated density-functional theory.
J Chem Phys 2005;123:184108e15.
[57] Wang CK, Macak P, Luo Y, Ågren H. Effects of
p centers and symmetry on two-
[29] Tawada Y, Tsuneda T, Yanagisawa S, Yanai T, Hirao K. A long-range-corrected
time-dependent density functional theory. J Chem Phys 2004;120:8425e33.
[30] Vydrov OA, Scuseria GE. Assessment of a long range corrected hybrid func-
tional. J Chem Phys 2006;125:234109e18.
[31] Vydrov OA, Scuseria GE, Perdew JP. Tests of functionals for systems with
fractional electron number. J Chem Phys 2007;126:154109e18.
photon absorption cross-sections of organic chromophores. J Chem Phys
2001;114:9813e21.
[58] 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;13:12506e16.