One- and two-photon-induced isomerization of styryl compounds possessing A-π-A′ structure

Article abstract of DOI:10.1016/j.dyepig.2016.05.001

Two styryl compounds with A-π-A′ structure that feature a fixed pyridine ring as an electron acceptor (A) and pyridine or 5,6,7,8-tetrahydroisoquinoline cation as the other acceptor (A′) have been synthesized. Their structures were elucidated by means of

Full text of DOI:10.1016/j.dyepig.2016.05.001

Dyes and Pigments 132 (2016) 237e247
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One- and two-photon-induced isomerization of styryl compounds
0
possessing A-
p
-A structure
a
,
*
b
b
a
Beata J e˛ drzejewska , Marta Gordel , Janusz Szeremeta , Małgorzata A. Kaczorowska ,
c
b
Marek J oꢀ zefowicz , Marek Samo ꢀc
Faculty of Chemical Technology and Engineering, UTP University of Science and Technology, Seminaryjna 3, 85-326 Bydgoszcz, Poland
a
b
Advanced Materials Engineering and Modelling Group, Faculty of Chemistry, Wrocław University of Science and Technology, Wybrze z_ e Wyspia nꢀ skiego 27,
5
0-370 Wroclaw, Poland
c
Institute of Experimental Physics, Faculty of Mathematics, Physics and Informatics, University of Gda nꢀ sk, ul. Wita Stwosza 57, 80-952 Gda nꢀ sk, Poland
a r t i c l e i n f o
a b s t r a c t
0
Article history:
Received 10 March 2016
Received in revised form
Two styryl compounds with A-
p
-A structure that feature a fixed pyridine ring as an electron acceptor (A)
0
and pyridine or 5,6,7,8-tetrahydroisoquinoline cation as the other acceptor (A ) have been synthesized.
Their structures were elucidated by means of NMR and IR spectroscopy. One-photon fluorescence,
fluorescence quantum yields and lifetimes were investigated. It was found that both visible (408 nm) and
near-infrared (800 nm) light promotes the conversion of the E-isomer of the studied compounds to the
Z-isomer. Under the UV irradiation (310 nm) the obtained Z-isomer reverts to the initial form in tens of
minutes. The observed inter-conversion was found to be fast and efficient. The trans-to-cis isomerization
was followed by decomposition of the compound upon longer than 100 s exposure to light as indicated
from mass spectra.
30 April 2016
Accepted 3 May 2016
Available online 7 May 2016
Keywords:
Styryl dyes
Synthesis
One- and two-photon induced
isomerization
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
Their spectral properties depend on the chain length and the ter-
minating moieties. It is known that, at first, elongation of the pol-
Processes of photoisomerization of olefins and polyenes have
been a subject of intense research for a long time since they serve as
examples of unimolecular reactions in condensed phase [1].
Polymethine dyes are a large class of compounds, which have
been extensively used in a wide range of applications from lasers
ymethine chain increases the fluorescence quantum yields, but, as
it grows further, it causes a decrease of the efficiency [10].
0
One example of the class of A-
p
-A compounds are styryl dyes.
6 5
These dyes incorporate the univalent group C H eCH]CHethat is
derived from styrene. Thus, classical styryl dyes consist of aromatic
and heterocyclic rings on opposite sides of the vinylene bond. There
are many nitrogen-heterocyclic rings, which may be present in
their structure, e.g. quinoline, pyridine, indole, benzothiazole,
benzoxazole, benzoselenazole and others. In most cases styryl dyes
bearing azaaromatic moieties are salts because they contain a
quaternized heterocyclic nitrogen, although there are examples of
neutral dyes in which the heterocyclic nitrogen is not quaternized.
Moreover, a variety of substituents can be added to the two ter-
minal moieties, allowing further diversification of the dye
structure.
[2] and photography [3] to diagnostics by fluorescent detection of
analytes [4,5] and sensitizers for photodynamic therapy [6,7]. The
basic structure of these dyes is characterized by a conjugated
(
polymethine) chain terminated at each end by a heterocyclic
moiety. In general, polymethine dyes have a poor resistance to light
compared with other chromophores such as azo and quinone dyes,
but many structural modifications improving light-fastness and
resistance can be made [8]. When the two end groups are different,
the molecules possess a permanent electric dipole moment and
show second-order nonlinear optical behavior [9].
Polymethine dyes are usually highly fluorescent and they absorb
and emit light mostly in the visible region of the optical spectrum.
Because of the flexibility of the styryl dyes, several distinct
isomers can be formed by rotation around the bonds of the styryl
unit. The “all-trans” isomer conformation is generally more stable
at room temperature and can be partially converted to the cis-form
by applying light. This leads to an equilibrium state between the
populations of trans and cis isomers. The cis-form can be reverted to
*
Corresponding author.
E-mail address: beata@utp.edu.pl (B. J e˛ drzejewska).
http://dx.doi.org/10.1016/j.dyepig.2016.05.001
143-7208/© 2016 Elsevier Ltd. All rights reserved.
0

238
B. J e˛ drzejewska et al. / Dyes and Pigments 132 (2016) 237e247
the trans isomer when the light of a corresponding wavelength is
used to promote this reaction. Isomers formed by photoexcitation
may remain stable at low temperatures [11e13]. These isomers
present, in general, slightly shifted absorption spectra and weaker
intensity bands [14,15].
Good photostability is main pre-requisite in laser-induced
fluorescence, dye laser technology and all application involving
fluorescence when either high sensitivity or a high signal rate is
required [5]. A few hemicyanines present sufficient chemical sta-
bility in solution for practical use. For longer polymethine chains,
the stability is improved with “rigidified” structures in which the
possible rotation around the chain bonds is blocked by appropriate
substituents [16,17]. On the other hand, photodegradation plays an
important role in environmental applications. The rate constants
Analysesysteme GmbH, Germany) operating with the VARIOEL
software (version 5.14.4.22).
Melting points (uncorrected) were determined on the Bo e€ thius
apparatus (type PHMK 05, Germany).
ꢁ
5
Absorption (the concentration of the dye 1.0 ꢀ 10 M) and
ꢁ
6
emission spectra (the concentration of the dye 3.0 ꢀ 10 M) in
dimethylsulfoxide (DMSO), acetonitrile (MeCN), tetrahydrofuran
(THF), dichloromethane (CH
recorded at room temperature using
2
Cl
2
) and 1,4-dioxane (1,4-Dx) were
Shimadzu UVevis
a
Multispec-1501 spectrophotometer (Japan) and a Hitachi F-4500
spectrofluorimeter (Japan), respectively.
The fluorescence quantum yields of the studied dyes (
4
dye) were
calculated using the Eq. (1):
2
for photodegradation may be influenced by
a
fast photo-
n
IdyeAref
dye
4
¼
4
ref
$
;
(1)
isomerization that, depending on molecular parameters and irra-
diation conditions, determines the rate of light absorption by the
isomer mixture [18]. For this reason, it is important to investigate
whether the tested compounds can be photoisomerized (or pho-
todegraded) and under which conditions.
dye
2
Iref Adye
n
ref
where:
4
ref is the fluorescence quantum yield of reference (Coumarine
In this respect, two compounds of styryl derivatives (Chart 1)
were designed and their photochromic properties evaluated. The
kinetics of the photoisomerization process was investigated by
1
) in ethanol (
4
ref ¼ 0.64 [19]), Adye and Aref are the absorbances
of the dye and reference samples at the excitation wavelengths
1
(Az0.1 at 404 nm), Idye and Iref are the integrated intensities
measuring H NMR and UVeVis spectra. The interconversion was
(areas) of dye and standard spectra, respectively, ndye and nref are
induced by absorption of either one or two photons by a molecule.
the refractive indices of the solvents used for the dyes and the
reference, respectively.
2
. Experimental
For fluorescence lifetime measurements a time-correlated sin-
gle-photon counting (TCSPC) system was used. It contains a cooled
fast microchannel plate photodetector and is accessorized with a
monochromator at the observation. As the excitation source a
2
.1. Measurements
All reagents and solvents were purchased from Aldrich Chemical
Co. and used without further purification. For thin layer chroma-
tography, aluminum oxide IB-F flexible sheets (thickness 0.2 mm)
were purchased from J.T. Baker Chemical Co., Germany.
HPLC analyses were done by an HPLC system equipped with
UVeVis detector (detection wavelength was 360 nm or 400 nm),
Binary HPLC Pump and a Symmetry C18 column (3.5
4
with 1.0 mL/min flow rate, r.t., 20 mL injection volume and HPLC
375 nm laser diode was used. This laser has a pulse width of 58 ps
and maximal average power of 5 mW. The fluorescence decays
were fitted using double-exponential deconvolution model, where
IRF(t) is the instrument response function at time t,
amplitude of the decay of the i-th component at time t and
lifetime of the i-th component. Based on the data the average
lifetime, av was calculated as )/(S ), The radiative (k
a
i
is the
t
i
is the
mm,
.6 ꢀ 75 mm). Separation was conducted under isocratic conditions
t
t
av ¼ (S
i
a
i
t
i
i
a
i
r
)
and the total non-radiative (knr) rate constants for the singlet-
grade THF or acetonitrile containing trifluoroacetic acid (0.05%) as a
mobile phase.
excited state of the studied compounds were estimated from the
av
fluorescence quantum yield (4dye) and the average lifetime (t )
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
a Bruker Ascend™ 400 NMR spectrometer, respectively. Perdeu-
values using the following equations
4
dye
k
r
¼
;
(2)
(3)
6
terated dimethylsulfoxide (DMSO-d ) was used as the solvent and
tetramethylsilane (TMS) as internal standard.
The IR spectra of the synthesized salts were recorded using a
Bruker Vector 22 FT-IR spectrophotometer (Germany) in the range
4
t
av
ꢀ
ꢁ
1
ꢁ
t
4
dye
ꢁ1
k
nr
¼
:
00e4500 cm , by KBr pellet technique.
av
The elemental analysis was made with a Vario EL III (Elementar
Nonlinear optical absorption properties of the compounds 2 and
3
were characterized at 800 nm using the recently developed f-scan
technique [20], which is a modification of the Z-scan technique
used in our previous contributions (e.g. [21e27]). The difference
between the techniques is that while in the Z-scan the sample is
moved along the laser beam (in z direction), in f-scan its position is
fixed but instead the focal length (f) of the lens is changed. We have
used here an electrically focus-tunable lens (Optotune EL-10e30
series) capable of rapid changes of f. The advantage of f-scan over Z-
scan is thus a much shorter time of a single scan, which minimizes
the exposure time of organic compounds that are potentially un-
stable to the high-power pulsed laser radiation, e.g. show photo-
chromic changes as in the present case. The investigated samples
were placed in 1 mm path length, stoppered glasses cuvettes. The
800 nm laser pulses (~130 fs) at the repetition rate of 1 kHz were
+
CH CH
CH CH
N CH
3
N
-
I
2
+
N
CH
3
CH CH
-
N
I
3
Chart 1.

B. J e˛ drzejewska et al. / Dyes and Pigments 132 (2016) 237e247
239
supplied by a Quantronix Integra-C regenerative amplifier followed
by a Quantronix-Palitra-FS optical parametric amplifier. The
required wavelength was separated from the Palitra output using
polarizing wavelength separators and color glass filters. The mea-
surement was repeated three times for each dye in order to provide
an error bar.
Mass spectrometry analysis was performed on a Q-Exactive
mass spectrometer (Thermo Scientific). Samples of all examined
compounds were dissolved in methanol to give a final concentra-
aldehyde) was collected and subsequently washed repeatedly with
copious water. The filtrate was neutralized with 3 M NaOH. The
crude product was collected by filtration and air-dried. Then it was
dissolved in hot ethyl acetate, and the insoluble materials were
removed by filtration. The formyl compound (1) obtained after
evaporation of the solvent (5.4 g) was recrystallized from CHCl
acetone to yield 4.1 g (57%) of pale-yellow powder.
3
/
ꢂ
ꢂ
C
C
14
H11NO; 209.24 g/mol; mp. 85.5e86.7 C (lit. mp. 84e86
[28]).
tion of 10
m
M and were introduced into the mass spectrometer by
1H NMR (DMSO-d
) d (ppm): 7.316 (m, 1H, Ar), 7.519e7.559 (d,
6
3JH,H ¼ 16.0 Hz, 1H, eCH¼), 7.599e7.619 (d, JH,H ¼ 8.0 Hz, 1H, Ar),
3
use of electrospray source. Data acquisition and analysis were
conducted using the Xcalibur (Thermo Scientific) software.
3
7.754e7.794 (d, JH,H ¼ 16.0 Hz, 1H, eCH¼), 7.840 (m, 1H, Ar), 7.932
(
m, 4H, Ar), 8.621 (m, 1H, Ar), 10.013 (s, 1H, CHO).
13
2.1.1. One-photon-induced isomerization
6
C NMR (DMSO-d ) d (ppm): 123.48, 123.60, 128.08, 130.48,
The photochemical process was studied in a quartz cuvette with
131.19, 131.86, 137.45, 150.14, 192.93 (CH), 136.02, 142.79, 154.82 (C).
IR (KBr): 2925, 1694, 1600, 1470, 1431, 1214, 1169, 975, 820, 533.
dimensions 4 ꢀ 1 ꢀ 1 cm. In order to ensure complete absorption of
light, the cuvette was placed in a horizontal position and irradiated
with diode pumped solid state (DPSS) laser light (
xenon lamp equipped with monochromator,
l
l
EM ¼ 408 nm) (or
2.2.2. General procedure for synthesis of styryl dyes [29e31]
Quaternary salt (10 mmol), 2-[2-(4-formylphenyl)ethenyl]pyri-
dine (10 mmol) and 25 mL ethanol were placed in a 100 mL one-
necked flack with a stirrer and a condenser. Then three drops of
piperidine were added as a catalyst and the resultant mixture was
refluxed for 12 h. Upon cooling, microcrystals of the iodide salt
were collected by filtration under reduced pressure and recrystal-
lised from ethanol.
EM ¼ 310 nm)
through the bottom wall (the optical path length ¼ 4 cm). The laser
beam power was 20 mW. The solution was stirred during irradia-
tion. After each period of irradiation the electronic absorption
spectra were recorded on a Shimadzu UVevis Multispec-1501
spectrophotometer (Japan).
The photobleaching quantum yield (
the following formula:
4bl) was determined using
2.2.2.1. trans,trans-4-{4-[(2-(pyridine-2-yl)ethenyl]-styryl}-N-meth-
N $c$h$V$
DA
ylpyridine iodide (2). The compound 2 was prepared according to
the general procedure using 1,4-dimethylpyridinium iodide (2.35 g,
A
4
¼
(4)
bl
l$ε$t$P$l
1
0 mmol) and 2-[2-(4-formylphenyl)ethenyl]pyridine (2.09 g,
where N
A
is the Avogadro’s number, c is velocity of light in vacuum,
A is the difference
between absorbances at the peak wavelength ( ) prior and post
10 mmol). The dark yellow solid was crystallized from ethanol:
h is Planck’s constant, V is the reaction volume,
D
ꢂ
C
H
21 19
N
2
I; 426.29 g/mol; 3.15 g (73.8% yield); mp 284.4 C. The
purity of the dye was checked by TLC (methanol, aluminum oxide
IB-F, R
l
irradiation, t is the irradiation time, ε is the molar absorption co-
efficient of the dye, P is the power of absorbed light and l is the
length of solution that the light passes through (the optical path
length).
f
¼ 0.29).
1
þ
H NMR (DMSO-d
6
)
J
d
(ppm): 4.246 (s, 3H, N CH
3
), 7.267 (m, 1H,
3
Ar), 7.439e7.399 (d,
H,H ¼ 16.0 Hz, 1H, eCH¼), 7.559e7.519 (d,
3
3
J
H,H ¼ 16.0 Hz, 1H, eCH¼), 7.577e7.557 (d, JH,H ¼ 8.0 Hz, 1H, Ar),
3
7.706e7.666 (d,
J
H,H ¼ 16.0 Hz, 1H, eCH¼), 7.756 (m, 5H, Ar),
3
2.1.2. Two-photon-induced isomerization
8.028e7.988 (d, JH,H ¼ 16.0 Hz, 1H, eCH¼), 8.231e8.214 (d,
3
The process was studied in either a 50
mL or 4 mL quartz cuvette.
JH,H ¼ 6.8 Hz, 2H, Ar), 8.579 (m, 1H, Ar), 8.865e8.848 (d,
3J
¼ 6.8 Hz, 2H, Ar).
The cuvette was placed in a vertical position (the optical path
length ¼ 1 cm) and irradiated with 800 nm pulses from the
Quantronix Integra amplifier (1.0 mJ/pulse, 130 fs pulse length,
H,H
1
3
þ
C NMR (DMSO-d )
6
d
(ppm): 46.905 (N CH ), 122.565, 122.707,
3
123.144, 123.395, 127.612, 128.530, 129.404, 131.010, 136.790,
139.882, 144.897, 149.464 (CH), 134.889, 138.243, 152.239, 154.544
(C).
IR (KBr): 3004, 1644, 1621, 1520, 1431, 1336, 1190, 1050, 972, 839,
810, 767, 743, 620, 545.
1
kHz repetition rate). After each period of irradiation the electronic
absorption spectra were recorded on
spectrophotometer.
In the case of monitoring the isomerization process by H NMR
measurements, the sample was irradiated in NMR tubes for
a
JASCO V-670
1
Elemental analysis: calc. 59.2%C, 4.5%H, 6.6%N, 29.8%I, measured
59.13%C, 4.54%H, 6.54%N.
2
e60 min. The samples were diluted to 7.5 mM with DMSO-d
6
1
before the H NMR spectra were recorded on a Bruker Ascend™
00 NMR spectrometer. The initial concentration was 14.5 mM and
the volume 150 L. For studies of the dependence of the initial
4
2.2.2.2. trans,trans-4-{4-[(2-(pyridine-2-yl)ethenyl]-styryl}-N-
methyl-5,6,7,8-tetrahydroisoquinolinium iodide (3). The compound
3 was prepared according to the general procedure using N-
methyl-5,6,7,8-tetrahydroisoquinolinium iodide (2.75 g, 10 mmol)
and 2-[2-(4-formylphenyl)ethenyl]pyridine (2.09 g, 10 mmol). The
m
reaction rate on laser power, samples were irradiated in NMR tubes
for 6 min to give a low conversion for which the initial rate con-
ditions are applicable.
yellow solid was crystallized from ethanol: C24
mol; 3.99 g (85.5% yield); mp 257.8e259.6 C. The purity of the dye
H
23
N
2
I; 466.36 g/
ꢂ
2
2
.2. Synthetic procedure
was checked by TLC (methanol, aluminum oxide IB-F, R
f
¼ 0.18).
1
.2.1. Synthesis of 2-[2-(4-formylphenyl)ethenyl]pyridine (1) [28]
2
H NMR (DMSO-d
4H, eCH
6
)
d
(ppm): 1.811 (m, 2H, eCH
2
e), 2.890 (m,
þ
-[2-(4-formylphenyl)ethenyl]pyridine (1) was synthesized based
on the condensation of 3.4 g (25.3 mmol) of terephthalaldehyde
and 4.8 g (53.2 mmol) of -picoline in 3 mL of acetic acid and 5 mL
2
e), 4.239 (s, 3H, N CH
3
), 7.276 (m, 1H, Ar), 7.388e7.428 (d,
3
J
H,H ¼ 16.0 Hz, 1H, eCH¼), 7.569 (s, 1H, eCH¼), 7.602e7.622 (d,
3
3
a
JH,H ¼ 8.0 Hz, 2H, Ar), 7.687e7.727 (d, JH,H ¼ 16.0 Hz, 1H, eCH¼),
3
of acetic anhydride. The reaction mixture was refluxed for 12 h at
7.751e7.771 (d,
J
H,H ¼ 8.0 Hz, 2H, Ar), 7.819 (m, 2H, Ar),
ꢂ
3
135 C, cooled down to room temperature and placed in 100 mL of
8.500e8.517 (d, J ¼ 6.8 Hz, 1H, Ar), 8.584 (m, 1H, Ar), 8.698e8.715
3
aqueous 6 M HCl. The resulting precipitate (the unreacted
(d, JH,H ¼ 6.8 Hz, 1H, Ar), 8.807 (s, 1H, Ar).

240
B. J e˛ drzejewska et al. / Dyes and Pigments 132 (2016) 237e247
1
3
C NMR (DMSO-d
6
)
d
(ppm): 21.023, 26.496, 26.955 (eCH
2
e),
þ
4
6.610 (N CH
3
), 121.352, 122.499, 122.576, 127.044, 129.077,
1
1
30.672, 131.098, 134.070, 136.757, 141.488, 144.394, 149.431 (CH),
31.611, 135.468, 136.003, 136.604, 150.873, 154.599 (C).
IR (KBr): 3030, 2931, 1640, 1603, 1587, 1472, 1322, 1233, 1162,
1055, 1014, 985, 960, 845, 822, 766, 746, 549.
Elemental analysis: calc. 61.8%C, 5.0%H, 6.0%N, 27.2%I, measured
61.61%C, 5.15%H, 5.94%N.
3
. Results and discussion
3.1. Synthesis
The terephthalaldehyde was found to be reasonably reactive in
cyanine condensations. Thus, the compound reacts with a-picoline
in acetic anhydride to provide 2-[2-(4-formylphenyl)ethenyl]pyri-
dine in a good yield. The reaction of the obtained 2-[2-(4-
formylphenyl)ethenyl]pyridine with quaternary salt using piperi-
dine as catalyst leads to long conjugated organic compounds 2e3
Fig. 1. Normalized and scaled [32,33] electronic absorption (solid line) and fluores-
cence (dash line) spectra of and in DMSO and THF at room temperature;
EX ¼ 404 nm.
2
3
(Scheme 1). Synthesis of the compounds is based on the method
l
reported by us and other authors [28e31]. We characterized the
intermediate and objective compounds using standard spectro-
scopic methods and obtained satisfactory analysis of data corre-
sponding to their molecular structure.
(TCSPC). Analyzing the data collected in Table 1, it follows that, for
compound 2, the fluorescence quantum yield clearly increases with
increasing solvent polarity from 13% in THF to 20% in DMSO,
whereas for 3, the fluorescence quantum yield is found to be very
low (below 1%) and almost independent of the microenvironment.
The high value of fluorescence quantum yield in DMSO results from
the increased viscosity of the solvent.
The low fluorescence quantum yield of compound 3 might be
connected with some interactions between the excited solute
molecule and solvent molecules, which modifies the excited-state
configuration and the fluorescence spectrum intensity, e.g. prox-
imity effect and conformational changes [35e38]. In a partially
stiffened molecule, twisting of the alkyl bridge probably disturbs
planarity of the compound. Thus, especially in non-polar solvents,
3.2. Spectral properties
Normalized and scaled between wavelength and wavenumber
representation [32,33] UVevisible absorption and fluorescence
spectra (excited at 404 nm) of compounds 2 and 3 in organic sol-
vents of different polarity are shown in Fig. 1.
The styryl compounds 2 and 3 show intense long-wavelength
(
LW) absorption band ascribed to
p / p* transition with
maximum located between 383 and 397 nm. The LW band of 2 is
ꢁ1
shifted to longer wavelength by about 200 cm in comparison to 3.
The molar absorption coefficient of the LW band of the studied
4
4
ꢁ1
ꢁ1
compounds ranges from 4.4 ꢀ 10 (3) to 5.6 ꢀ 10
M
cm (2).
the emitting polar
perturbation by the proximity effect and vibronic coupling to an
energetically close lying n- * state. This opens an effective non-
p-p* state might be destabilized to experience
The results suggest that the bonding of the methine group to the
pyridine ring leads to a decrease in
p
/
p* transitions. The fluo-
p
rescence maxima are located at ca. 536 nm and 530 nm for com-
pounds 2 and 3, respectively. Rigidity of the methine group causes
the blue shift of the fluorescence band probably because of poorer
radiative deactivation pathway for the excited singlet state, which
may explain the observed reduction in the fluorescence intensity.
The fluorescence decay profiles were double-exponential.
Analyzing the fluorescence decay data obtained for both studied
p
electron conjugation caused by twisting of the alkyl bridge
(
disturbed planarity of the compound in the ground state). The
molecules, it is found that the temporal profile exhibits a fast (t1)
ꢁ1
large Stokes shifts (~6500e7300 cm ) suggest that the geometry
of the compounds changes after excitation. The spectral properties
of compounds 2 and 3 are listed in Table 1.
and a slower decay (t2) components. The presence of the broad
long-wavelength steady-state fluorescence band and two decay
components in the studied molecules dissolved in polar solvents,
suggest that two emitting states exist for the investigated fluo-
rophores in polar solvents: a non-relaxed intramolecular charge
transfer state (ICT)NR and a relaxed ICT state (ICT)R.
In order to obtain more insight into the photophysics of 2 and 3,
we determined their quantum yields in different solvents with
Coumarine 1 in ethanol as the reference and their fluorescence
lifetimes using time correlated single photon-counting technique
Scheme 1. General route for the synthesis of the styryl compounds.

B. J e˛ drzejewska et al. / Dyes and Pigments 132 (2016) 237e247
241
Table 1
Photophysical dataa for compounds 2 and 3.
Compound 2
Compound 3
DMSO
396
MeCN
392
THF
390
CH
2
Cl
2
1,4-Dx
392
DMSO
389
MeCN
386
THF
383
CH
2
Cl
2
1,4-Dx
abs
max
416
405
381
l
ε
50800
538
56000
536
e
48400
538
e
44400
527
46200
530
e
44300
537
e
fl
max
536
511
532
474
l
Dn
6665
19.74
0.393
21.18
0.958
78.82
0.838
1.06
2.36
0.96
4.1
0.952
1.9
6853
18.39
0.317
22.87
0.695
77.13
0.609
1.01
3.02
1.34
4.4
6984
13.40
0.374
37.52
0.952
62.48
0.735
1.14
1.82
1.18
6.5
5451
11.52
0.327
26.09
0.700
73.91
0.603
1.11
1.91
1.47
7.68
1.01
5941
4.20
0.216
93.61
1.352
6.39
0.289
1.53
1.46
3.32
22.8
e
6732
0.69
7039
0.66
7313
0.59
0.299
40.06
1.02
59.94
0.731
1.45
0.08
1.36
169.9
e
6069
0.27
5150
0.57
0.246
33.19
1.155
66.81
0.853
1.34
0.07
1.17
174.4
e
4
fl
t
1
0.033
75.46
1.067
24.54
0.287
1.48
0.24
3.46
144.1
0.931
1.7
0.076
71.15
1.302
28.85
0.430
1.33
0.15
2.31
150.1
0.914
1.8
0.062
27,53
0.631
72,47
0.474
1.46
0.06
2.10
369.4
0.96
1.7
a
1
t
2
a
2
t
av
2
c
av
k
r
avknr
nr/k
os
k
r
f
1.02
2.1
e
s
e
1.8
e
e
e
m
12
8.91
9.16
e
9.39
e
8.72
8.59
e
9.06
e
a
ab
fl
ꢁ1
ꢁ1
ꢁ1
Absorption ð
l
; nmÞ, fluorescence ð
l
; nmÞ maxima, Stokes shift (Dn; cm ), molar absorption coefficient (εmax; M cm ), fluorescence quantum yield (
4
fl
; %),
max
max
; %) and correlation coefficients (c2), radiative (
av
k
r
; 108
s
ꢁ
1
) and nonradiative ( k s
av
nr; 109
ꢁ
1
) rate constants, fos oscillator
fluorescence lifetime (
strength, one-photon absorption cross-section (
t
; ns), their amplitudes (
a
2
s
; Å ) and
m
12 in D calculated based on the oscillator strength and the energy of the ICT maximum according to B.J. Coe et al.
[34].
Because both studied dyes show the multimode emission
initial state. The observed changes in the electronic absorption
spectra of compounds 2 and 3 under 408 nm irradiation are
depicted in SI (Fig. S1).
Based on UVeVis data, the kinetics of the trans-cis photo-
chemical isomerization was studied by measuring the absorbance
at a fixed wavelength as a function of time. The relative absorbance
A(t) was calculated based on Eq. (5) [39]:
character, the average values of the fluorescence decay times have
been calculated. The calculated average lifetime was in the sub-
nanosecond range and changed from 838 ps (2 in DMSO) to
2
87 ps (3 in DMSO). Radiative and non-radiative rate constants
calculated from quantum yields and fluorescence lifetimes show
that the non-radiative rate constants are two orders of magnitude
larger than the radiative decay for compound 3 and only four or six
times larger for compound 2.
Aðtrans; tÞ
AðtÞ ¼
;
(5)
Using the f-scan technique, the effective two-photon absorption
Aðtrans; t Þ
0
eff.
2
cross-section (s ) in DMSO was measured to be 156 ± 12 GM and
119 ± 9 GM at 800 nm for compounds 2 and 3, respectively.
where A(trans, t) and A(trans, t
0
) are absorbances at
lmax before and
These interesting electronic properties of the newly synthesized
after different times of irradiation, respectively.
dyes have prompted an intensive investigation of the trans-cis
photoisomerization processes in them.
The kinetic data were found to fit well to a mono-exponential
function defined by Eq. (6).
À
Á
AðtÞ ¼ Aðtrans;PSSÞ þ Atrans;t0 ꢁ Atrans;PSS expð ꢁ k$tÞ;
(6)
3
.3. Photochemical observations
where k is the observed rate constant and t is the time of irradiation
[
One- and two-photon induced photochemistry of compounds 2
1
39e41].
The apparent rate constant, k, for isomerization is expressed by
the sum of the forward, ktrans/cis, and backward rate constant,
and 3 was studied using a combination of UVeVis and H NMR
spectroscopy upon laser irradiation.
ktrans)cis [39].
3.3.1. One-photon induced isomerization
As shown in Fig. S1, the visible light irradiation (CW, 408 nm) of
k
trans/csðhvÞ
the investigated compounds in acetonitrile solutions leads to a
decrease of the long-wavelength (LW) absorption band at max (ca.
8e55%). This behavior is accompanied by increase of the short-
trans
cis
(7)
(8)
l
ktrans)cisðhvÞ
4
wavelength (SW) absorption band with the maximum localized
at ~310 nm. After a few minutes of irradiation the photostationary
state (PSS) is reached and no further change in the intensity of the
absorption occurs up to ca. 100 s of irradiation. We suppose that the
observed changes are due to the E-Z photoisomerization process.
Thus, the photostationary-state mixture generated after irradiation
with visible light (408 nm) contains the formed cis-isomer and the
residual trans-isomer with no change in the trans:cis ratio, or an
appearance of any new products. The well-defined isosbestic points
k ¼ ktrans/cisðh
n
Þ þ ktrans)cisðh Þ
n
Fig. 2 shows the plots of relative absorbance vs. time of irradi-
ation (Eq. 6) during the trans/cis isomerization of 2 and 3 in MeCN
and DMSO, respectively. The observed rate constants (k) for cis-2
and cis-3 in DMSO and MeCN at room temperature are collected in
Table 2.
For both compounds an exponential relationship is observed. As
shown in Table 2, the k values in DMSO are lower than those in
MeCN. This indicates that the trans-cis isomerization is dependent
on the polarity and viscosity of the medium [39].
(
340 and 250 nm) support the conclusion that both trans-2 and
trans-3 are converted into respective single photoproducts. The
obtained Z-isomer is thermally stable under given conditions but
the UV irradiation (310 nm) causes the return of the molecule to its
The obtained results allow also to calculate the photobleaching
quantum yield (Table 2). The values observed for compound 2 are

242
B. J e˛ drzejewska et al. / Dyes and Pigments 132 (2016) 237e247
1
[
42]. It should be noted that the H NMR spectrum of compound 3
1.0
0.8
0.6
0.4
compound 2
in MeCN
in DMSO
compound 3
after 408 nm irradiation, also displays the same changes i.e. the
disappearance of the trans isomer and the increase of the
photoproduct.
As can be seen in Figs. S2 and S3, when the trans solution is
irradiated for ca. 2 h, the signals specific to trans-compound,
disappear while the peaks characteristic for the cis photoproduct
become more intense. The analysis of the NMR spectra does not
reveal any new peaks which might be attributed to other photo-
products for the given conditions. By integrating the cis isomer
signals corresponding to the vinylic doublets and/or the methyl
groups relative to the trans peaks, it was found that the cis integral
gradually increased before reaching a plateau of 56% or 53% relative
to the sum of the trans and cis isomers of 2 and 3, respectively after
in MeCN
in DMSO
0
20
40
60
6
ca. 2 h of 408 nm irradiation in DMSO-d . Fig. 4 compares the ki-
netic behavior for the tested styryl derivatives.
Time (s)
Fig. 2. The kinetics of absorbance changes of 2 (red) and 3 (blue) obtained in MeCN
circles) and DMSO (triangles); the trans/cis isomerization was induced with 408 nm
irradiation (DPSS laser, 20 mW). (For interpretation of the references to color in this
(
3.3.2. Two-photon induced isomerization
Similarly to one-photon irradiation, upon 800 nm light ab-
sorption the intensity of the long-wavelength band corresponding
figure legend, the reader is referred to the web version of this article.)
to the
wavelength band attributed to the cis form (centered at around
10 nm) increases for the two compounds (Fig. 5). The results
indicate the transformation of the more stable E-isomer to the Z-
isomer that absorbs at shorter wavelengths. An isosbestic point can
be clearly seen between these two bands, which indicates the
presence of a precursor-successor relationship between the two
absorbing states. After 20 min of irradiation the photostationary
state was achieved.
In order to confirm the two-photon induced process, Fig. 6 il-
lustrates the relationship between the initial rate of photo-
isomerization and the incident laser power. The slope of the
logelog plot should give the number of photons absorbed per ab-
sorption event [43]. The experimental data points are fitted with
lines having slopes of 1.94 ± 0.1 and 1.91 ± 0.1 for compounds 2 and
p / p* transition of trans form decreases and the short-
higher than for compound 3, however the differences are not very
significant. This behavior is understandable because the trans form
of 3 possesses one partly stiffened vinylic group and has better
photostability, whereas the compound 2 with freely rotating vinylic
groups is less stable. Moreover, the photobleaching quantum yield
3
ꢁ
3
N
is higher in MeCN (
h
¼ 0.341$10 Pa s; E ¼ 0.46) than in DMSO
T
ꢁ
3
N
(h
¼ 1.996$10 Pa s; E ¼ 0.444). Lower polarity and higher vis-
T
cosity of the solvent increases photostability of the studied
compounds.
To confirm the trans-cis photoisomerization and to exclude
other photoinduced processes an NMR analysis was performed.
Fig. 3 shows the NMR spectra of parent sample (trans-2), sample
ꢂ
stored in the dark, heated in the dark at 70 C and irradiated at
4
08 nm.
The trans and cis isomers of styryl derivatives can be distin-
3, respectively, suggesting a two-photon excitation mechanism.
3
The slight deviation of the slopes from the theoretical value of two
is believed to be due to the saturation effects at higher photon flux
[41,44].
guished by their distinctive
analysis of H NMR spectra of 2 in DMSO-d
JHH coupling constants [38]. The
1
6
ꢂ
indicates that storing
the solution in the dark and heating at 70 C for 5 h did not cause
any changes in signals positions. In the region 7e9 ppm, there are
well-separated pairs of doublets for the vinylic protons. The
The corresponding 1H NMR spectra for the sample before and
after infra-red irradiation to the photostationary state (PSS) are
shown in Fig. 7. Each of the resonances is assigned to either the
trans or the cis form of 3. Prior to irradiation, the sample is exclu-
sively in the trans form. The pure trans-2 displays a large coupling
coupling constant 3
J
H,H ¼ 16.0 Hz is characteristic for the trans
configuration (see experimental part and SI). After irradiation, the
formation of the cis isomer is accompanied by a change of the peak
positions. Indeed, the intensity of the peaks corresponding to the
vinylic protons, located at 8.03, 7.71, 7.56 and 7.44 ppm for trans-2,
decreased, while new pairs of doublets with the coupling constant
equal ca. 12.0 Hz appeared at 7.71, 7.39, 7.25 and 6.84 ppm for cis-2,
respectively. The observed changes in chemical shifts and the
coupling constants are in perfect agreement with cis arrangement
3
constant ( JH,H) between vinylic protons equal to 16.0 Hz, typical of
1
the trans-stilbene isomer. As expected, the H NMR spectrum for
the photogenerated cis isomer contains a signal pattern similar to
that of the trans form, but with different chemical shifts (the
3
coupling constants between vinylic protons JH,H ¼ 12 Hz).
As the signals for the trans and cis forms are well separated, the
trans:cis ratio was readily determined. Analysis of the data collected
Table 2
a
Results of probing the sample of 2 and 3 after one- and two-photon irradiation.
Ratio cis/trans formb
From UVeVis
kb
4
b
Ratio cis/trans formc
kc
4
bl
c
Ratio cis/trans formd
kd
No
bl
From 1H NMR
1.32 ꢀ 10ꢁ
4
2
3
1PI
PI
1PI
PI
0.55/0.45
0.55/0.45
0.49/0.51
0.50/0.50
0.442
0.298
0.221
0.54/0.46
0.55/0.45
0.48/0.52
0.53/0.47
0.159
0.135
0.133
0.56/0.44
0.56/0.44
0.53/0.47
0.53/0.47
ꢁ
3
3
ꢁ3
ꢁ3
ꢁ3
2
8.06 ꢀ 10
3.65 ꢀ 10
3.93 ꢀ 10
4.17 ꢀ 10
3.43 ꢀ 10
ꢁ4
0.308
0.272
ꢁ3
2
4.51 ꢀ 10ꢁ
3.24 ꢀ 10
a
ꢁ1
Observed rate constants (k, s ); quantum yields (
In MeCN.
In DMSO.
Ratio based on integration of the vinylic doublets ([trans]/[trans]þ[cis]) in DMSO-d
4
bl) of photobleaching process.
b
c
d
6
.

B. J e˛ drzejewska et al. / Dyes and Pigments 132 (2016) 237e247
243
Fig. 3. 1H NMR spectra of compound 2 in DMSO-d
(from bottom): a) parent sample (trans form), b) sample stored in a dark, c) sample heated at 70 C, d) and after one-photon
ꢂ
6
irradiation at 408 nm leading to the photostationary state.
Dye þ h
n
/Dye*
(9)
(10)
(11)
Dye þ O /Dye þ O $2 ꢁ
*
$þ
2
$
ꢁ
Dye þ O /degradation
2
As can be seen from the above equations the photodegradation
might follow either zero or first order kinetics. In the first case the
interaction of the excited dye with oxygen (Eq. (10)) becomes the
rate determining process. For the first-order reaction the limiting
step of the process is dye excitation (Eq. (9)) [45,46]. The photo-
degradation process was monitored in UVeVis and MS spectra.
Analysis of the electronic absorption spectra (Fig. 9) recorded after
different irradiation time revealed that short exposure of the
sample to light (up to 100 s) resulted in trans-cis isomerization and
then the destruction of the compounds occurred. Although the
difference between dye 2 and dye 3 is not very large, dye 3 with one
partly stiffened vinylic group exhibits less photostability than dye 2
with two ethyl groups. After irradiation for 7 h, the dye 3 showed
Fig. 4. Kinetics of one-photon isomerization of compounds 2 (red circles) and 3 (blue
triangles) upon irradiation at 408 nm (70 mW), as monitored by 1H NMR spectroscopy.
(
For interpretation of the references to color in this figure legend, the reader is referred
80e82% photobleaching, but dye 2 showed only 62e64% decrease
to the web version of this article.)
in maximal absorbance. This may suggest that the photo-oxidative
fading of the dyes 2 and 3 occurs as a result of an oxygen attack onto
the vinylene group upon light exposure. However, these com-
pounds have different photostabilities. Compound 2 has much
better photostability than 3. According to literature data [47], when
the methine chain length is shorter, the photostability of cyanine
dyes greatly increases. Thus, we expected that the compound 3,
having one vinylene group stiffened, should reveal worse photo-
bleaching behavior. The effect illustrated in Fig. 9 is inconsistent
with this expectation but it might be related to the fluorescence
quantum yields of compound 2 (18.39%) and compound 3 (0.66%)
(see Table 1). We suppose that the excited molecule 2 is quenched
quicker than 3 by intra-molecular electron transfer and goes back to
the ground state, and thus the compound is protected from pho-
tobleaching. A similar effect was observed by Peng et al. for a series
of heptamethine 5-sulfo-3H-indocyanine dyes [48]. Additionally,
the increase of the solvent viscosity (from MeCN to DMSO)
in Table 2 revealed that UVeVis data and NMR spectroscopy led to
similar conclusion about the trans-cis conversion (Fig. 8).
3.3.3. Photodegradation
The presented above results indicate that the tested styryl de-
rivatives undergo photoisomerization upon both 408 nm and
00 nm light absorption. However, this is not the only process.
8
Additional processes like photobleaching or photodegradation
should be also considered. From the literature [45,46] it is well
known that the photodegradation of the styryl dyes is promoted by
oxygen. Since oxygen is a good electron acceptor the sequence of
reactions leading to photodegradation of the styryl dyes is likely to
be that illustrated below (Eqs. 9e11) [45,46]:

244
B. J e˛ drzejewska et al. / Dyes and Pigments 132 (2016) 237e247
Fig. 5. The dependence of UVeVis absorption spectra of 2 (left) and 3 (right) in MeCN upon irradiation with Quantronix Integra femtosecond laser (800 nm, 1.0 mJ/pulse, 130 fs
2
pulse length, 1 kHz, 64 GW/cm ). Arrows indicate the direction of changes with time of irradiation.
increases the photostability of the compounds which is also
consistent with the increase of the fluorescence quantum yield in
DMSO.
The photodegradation of dyes 2 and 3 was followed at absorp-
tion maximum of the pure trans form. In both cases, there was a
linear relationship between absorbance and irradiation time in the
time range from 100 s to 7 h, as shown in Fig. 9, which means that
the photodegradation followed zero-order kinetics.
The photodegradation rate constants for the tested compounds
were affected by the experimental conditions such as the presence
or absence of oxygen. The kinetics curves shown in Fig.10 represent
the relationship between the relative absorbance of 2 and 3 and
time in the acetonitrile solution saturated with oxygen, air or
helium.
For saturated solutions of 2, the photodegradation rate con-
ꢁ
5
ꢁ5
and
stants were calculated to be 1.6
ꢀ
10 , 1.5
ꢀ
10
.4 ꢀ 10ꢁ
5 ꢁ1
s
in O , air and He, respectively. Saturated solutions of
1
2
ꢁ5
ꢁ5
ꢁ5 ꢁ1
s
3
gave rate constants of 6.3 ꢀ 10 , 5.7 ꢀ 10 , and 5.4 ꢀ 10
Fig. 6. The dependence of the initial rate of cis isomer formation on the average laser
power at 800 nm, as monitored by UVeVis spectroscopy. The solid line shows the best
straight line fit to the data.
2
for O , air, and He, respectively. The data indicate that photo-
bleaching rate constants are enhanced in the presence of oxygen in
Fig. 7. 1H NMR spectra of compound 3 in DMSO-d
: before and after 10 min of two-photon irradiation at 800 nm (64 GW/cm ) with Quantronix Integra Ti:sapphire femtosecond
2
6
laser (stacked spectra from bottom).

B. J e˛ drzejewska et al. / Dyes and Pigments 132 (2016) 237e247
245
Table 3
The major ions (m/z) produced from photodegradation of 2.
m/z (measured) m/z (calculated) Assignment
Mass error (ppm)
þ
299.15428
285.13824
226.08591
210.09109
152.07047
138.05486
124.03938
299.1548
285.1392
226.0868
210.0919
152.0711
138.0555
124.0398
Product 2, [C21
H
19
N
2
]
1.73
3.36
3.93
3.85
4.14
4.63
3.38
þ
[C20
[C14
[C14
H
H
H
17 2
N ]
þ
þ
þ
12 NO
2
]
12 NO]
[C
8
[C
7
[C
6
H
10 NO
2
]
þ
þ
H
H
8
NO
NO
2
]
]
6
2
Table 4
The major ions (m/z) produced from photodegradation of 3.
m/z (measured) m/z (calculated) Assignment
Mass error (ppm)
þ
3
3
2
2
39.18518
25.16949
94.14844
85.13837
339.1861
325.1705
294.1494
285.1392
277.1341
262.1232
224.1075
197.1204
Product 3, [C24
H
23
N
2
]
2.71
3.10
3.26
2.91
2.70
3.39
3.61
3.44
þ
[C23
[C19
[C20
[C18
[C18
[C15
[C14
H
H
H
H
H
H
H
21
20 NO
2
N ]
þ
2
]
þ
17
17
N ]
N O]
2
2
Fig. 8. Kinetics of two-photon isomerization of compound 2 (red circles) and 3 (blue
þ
þ
þ
2
1
277.13335
triangles, inset) upon irradiation at 800 nm (64 GW/cm ), as monitored by H NMR
spectroscopy. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
2
2
1
62.12231
24.10669
97.11972
16 NO]
14 NO]
þ
15 N]
long-time irradiation the mass spectra were recorded. Due to dif-
ficulty in isolation of the principal products the measurements
were performed for the parent sample, the sample irradiated to the
photostationary state and fully bleached dye. Given the high mass
accuracy of the HRMS mass spectrometry there can be no question
as to the elemental composition or charge of the ions formed. The
structures of the formed ions have been confirmed by the appli-
cation of the collision induced dissociation (CID) tandem mass
spectrometry method. The major fragmentation ions observed in
the mass spectrum of the product mixture are listed in Tables 3 and
4
. Figs. S9eS14 show the ESI(þ) HRMS mass spectra of the dyes
under mentioned above conditions.
We suspect that the degradation of the dyes begins with the
rupture of the vinylene bond. Then the second eCH]CHe bond is
attacked to produce the remaining products shown in Fig.11 for dye
2. However, it is necessary to emphasize that the products detected
by MS might arise from both the photodegradation and thermal
decomposition of the dyes as previously detected for cyanine and
merocyanine dyes [45,46]. So, it is difficult to evaluate the mech-
anism leading to the detected products. However, the detected
photofragmentation products are consistent with the involvement
of oxygen [49,50].
Fig. 9. Kinetics of photobleaching upon irradiation at 408 nm (20 mW), as monitored
by UVeVis spectroscopy.
the sample but in a small extent.
In order to confirm the degradation of the compounds upon
Fig. 10. Comparison of the 2 (left) and 3 (right) photofading profiles. Irradiation source: DPSS laser, 408 nm, 70 mW.

246
B. J e˛ drzejewska et al. / Dyes and Pigments 132 (2016) 237e247
Fig. 11. Proposed main products for the photodegradation of compound 2. (The ortho pyridine derivatives are also possible).
4
. Conclusion
Two styryl dyes were prepared and characterized by UVeVis
dx.doi.org/10.1016/j.dyepig.2016.05.001.
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