Photostability of D-π-A nonlinear optical chromophores containing a benzothiazolium acceptor

Article abstract of DOI:10.1002/poc.1782

For the practical application of second-order NLO materials, not only a high molecular quadratic hyperpolarizability β but also good thermal, chemical, and photochemical stabilities are required. Most of the state-of-the-art chromophores with high NLO res

Full text of DOI:10.1002/poc.1782

Research Article
Received: 10 April 2010,
Revised: 14 July 2010,
Accepted: 20 July 2010,
Published online in Wiley Online Library: 28 September 2010
(wileyonlinelibrary.com) DOI 10.1002/poc.1782
Photostability of D–p–A nonlinear optical
chromophores containing a benzothiazolium
acceptor
a
b
b
´ ˇa
´
´
*
´
Marek Cigan , Anton Gaplovsky , Ivica Sigmundova , Pavol Zahradnı´k ,
c
d
´
´
˘
Roman Dedic and Magdalena Hromadova
For the practical application of second-order NLO materials, not only a high molecular quadratic hyperpolarizability b
but also good thermal, chemical, and photochemical stabilities are required. Most of the state-of-the-art chromo-
phores with high NLO response cannot be put to use because they are photochemically highly unstable. Good thermal
and photochemical stabilities with preserved high hyperpolarizabilities can be achieved by replacement of an
aromatic ring with easily delocalizable heteroaromatics, e.g., with benzothiazole. Furthermore, desirable modifi-
cations of the benzothiazole fragment lead to improvement in b values. Here we report results of a comprehensive
investigation of the photochemical stability of seven D–p–A push–pull molecules based on a N-methyl-
benzothiazolium acceptor and a N,N-dimethylaminophenyl donor with a different length of conjugated bridge
and different acceptor strength. The quantum yield (F) and the kinetic parameters of photoreactions were
determined for existing photodegradation pathways on irradiation at 300–850 nm in MeOH. Trans–cis photoisome-
rization is proposed as a fast but inefficient photobleaching mechanism for these irradiation wavelengths. Self-
sensitized photooxidation by 1O2 makes very slow parallel photodegradation pathway and, albeit to small value of F,
plays a dominant role in the photodegradation of the compounds investigated. Both structural modifications
(extension of conjugated bridge and an additional acceptor group bonded to heterocycle) resulting in an increase
of NLO response led to a decrease in photostability due to the self-sensitized 1O2 photooxidative attack. Thus a
compromise should be found between an increase in NLO response and a decrease in photostability to make a choice
of studied compounds for practical applications. Copyright ß 2010 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper.
Keywords: benzothiazolium D–p–A NLO chromophores; photooxidation; photostability; singlet oxygen; trans–cis
photoisomerization
benzothiazole-derived dyes with a D–p–A setup were found to be
INTRODUCTION
promising candidates for NLO applications.[10] Further improve-
ment in b values was achieved by proper introduction of donor
and acceptor substituents onto the benzothiazole core due to its
nonsymmetric character and quaternization of the benzothiazole
nitrogen.[11] Benzothiazolium styryl dye and its derivatives with
different donors and acceptors were widely investigated in last
In the past fifteen years, a considerable effort has been focused
on the development of organic molecules with enhanced second
order nonlinear optical (NLO) properties due to their potential
applications in various areas such as optical modulation, frequency
doubling and molecular switching.[1–5] The most widely studied
second-order (quadratic) NLO effects such as second harmonic
generation (SHG) arise from high first molecular hyperpolarizabil-
ities b. The most common design of molecules with large b values
comprises strong electron donors and acceptors connected by a
p-conjugated system (donor–p–acceptor or ‘push–pull’ chromo-
phores).[1,2] In such donor–p–acceptor (D–p–A) systems, the
donor and acceptor moieties provide the necessity of ground-state
charge asymmetry, whereas the p-conjugated bridge provides a
pathway for the redistribution of electron density under the
influence of external electric fields.[6]
For the practical application of second-order NLO materials,
not only a high molecular quadratic hyperpolarizability b, but
also good thermal, chemical and photochemical stabilities are
required.[7,8] Good thermal and photochemical stabilities of
NLO chromophores with preserved high hyperpolarizabilities can
be achieved by replacement of an aromatic ring with easily
delocalizable heteroaromatics.[9] In respect of these findings,
´ ˇ
*
Correspondence to: M. Cigan, Institute of Chemistry, Faculty of Natural
Sciences, Comenius University, SK-842 15 Bratislava, Slovakia.
E-mail: cigan@fns.uniba.sk
´ ˇ
´
´
a
b
c
d
M. Cigan, A. Gaplovsky
Institute of Chemistry, Faculty of Natural Sciences, Comenius University,
SK-842 15 Bratislava, Slovakia
´
I. Sigmundova, P. Zahradn´ık
Department of Organic Chemistry, Faculty of Natural Sciences, Comenius
University, SK-842 15 Bratislava, Slovakia
˘
R. Dedic
Department of Chemical Physics and Optics, Faculty of Mathematics and
Physics, Charles University Prague, CZ-12116 Prague, Czech Republic
´
M. Hromadova
Department of Molecular Electrochemistry, J. Heyrovsky Institute of Physical
´
Chemistry of ASCR, v.v.i, CZ-182 23 Prague, Czech Republic
J. Phys. Org. Chem. 2011, 24 450–459
Copyright ß 2010 John Wiley & Sons, Ltd.

PHOTOSTABILITY OF NONLINEAR OPTICAL CHROMOPHORES
century as good laser[12] and NLO dyes.[13] Over the last few years,
our research group synthesized a large number of push–pull 3-
alkyl-benzothiazolium salts with different number n of etheny-
lene units in a p-conjugated bridge and various donor and
acceptor substituents.[11,14–17] The benzothiazolium salts were
found to be much more effective NLO-phores in comparison with
the corresponding neutral benzothiazoles. It was shown that the
length of the p-conjugated bridge between the electron-donating
group and benzothiazolium moiety, as well as the donor and
acceptor ability, have a significant influence on b. Recently, Coe
et al.[18] reported the results of Hyper-Rayleigh scattering (HRS)
measurements for 3-methyl-benzothiazolium salts (n ¼ 1–4; D ¼
N,N-dimethylaminophenyl) in comparison with the analogous
1-methylpyridinium salts. Experimental measurements revealed
that the static molecular quadratic hyperpolarizability b0 increases
with the length of polyene chain, and the benzothiazolium salts
exhibit larger NLO responses than their pyridinium analogues.
In most cases, donors and acceptors in D–p–A systems are
Figure 1. Chemical structure of the studied D–p–A benzothiazolium
salts
Material) and has been published in Ref. [11]. The pyridine or
piperidine have been used as the base. All compounds have
all-trans configuration of double bonds in the conjugated bridge.
Experimental techniques
Electronic absorption spectra were obtained on a Hewlett
Packard HP 8452A diode array spectrophotometer and fluor-
escence measurements were performed on a Hitachi F-2000
fluorescence spectrophotometer. The solvent used in all mea-
surements was methanol (MeOH) because of the good solubility
of the compounds investigated and the good photostability
of 2,5-dimethylfuran (2,5-DMF) and 2,5-diphenylisobenzofurane
(DPIBF) as actinometric indicators of photooxidative attack of
singlet oxygen (1O2) in this solvent (Sections 2 and 3 in
Experimental). All photodegradation measurements were carried
out at 25 8C in the dark with only the 100-W xenon lamp as the
light source. Concentrations of 1–7 never exceeded 2 ꢀ
10ꢁ5 mol dmꢁ3, to avoid molecule aggregation and photoche-
mical dimerization processes.
—
connected via the conjugated bridge containing C C bonds.
—
However, this bond readily undergoes trans–cis photoisomeriza-
tion, which may hamper the material efficiency and lifetime. It was
previously reported that the primary causes of photochemical
—
instability of NLO chromophores containing C C bonds are
—
photooxidation and photoisomerization.[19–24] In either case, the
nonlinear activity is significantly diminished, the charge-transfer
(CT) system is reduced, and new absorption features appear
elsewhere in the spectrum, usually at a much shorter wavelength.
The identity and the number of specific reactions and their relative
rates are influenced in a complicated way by the exact identity of
the chromophore, the influence of atmospheric environment, the
irradiation wavelength, the temperature, etc. Photostability of
organic dyes is consequently often considered the Achilles Heel of
organic photonic materials[8,20,25,26] and limits their practical
applications. Most of the state-of-the-art chromophores which
were designed to have high electro-optic coefficients and hyper-
polarizabilities cannot be put to use because they are highly
unstable and degrade because of a photochemically generated
singlet oxygen.[8,27]
Photodegradation measurements
All photochemical measurements were performed using the
apparatus described elsewhere (Fig. 7 without ultrasonic horn
H).[28] The light source was a 100-W xenon lamp (300–850 nm;
I0 ¼ 2.05 ꢂ 0.05 ꢀ 10ꢁ5 mol sꢁ1 dmꢁ3). The actual concentration
of compounds studied in air-saturated solutions during irradia-
tion was measured spectrophotometrically (HP 8452A) at shiel-
ding the incident photon flux from the light source. Absorbed
photon flux Ia (by compound X) was calculated from emission
curves for the light source measured on an Ocean Optics SD 2000
spectrophotometer using the following equation:
Here we report results of a comprehensive investigation of the
photochemical stability, particularly the photooxidative stability
of seven benzothiazolium iodides with a different length of
conjugated bridge and a different acceptor strength of benzo-
thiazolium core caused by withdrawing substituents. The studied
compounds 1–7 are dipolar push–pull chromophores containing
an N-methylbenzothiazolium electron-acceptor group, an alke-
nylene conjugation spacer with 1–3 double bonds and an
N,N-dimethylamino group as a donor in the para position of the
substituted phenyl group. The acceptor strength of 4–7 is
enhanced by introducing an additional withdrawing substituent
into the benzothiazole fragment (NO2 or CN).The chemical
structure of the compounds studied is presented in Fig. 1.
Experiments were carried out to determine the role of
photoisomerization and photooxidation in the photodegradation
of 1–7 and to evaluate the overall stability and availability of such
molecules for practical applications.
!
P
1
Z
ꢁ
Ali
AlX
Il
i
P
Ia ¼ I0ꢁIT ¼ I0
1ꢁ10
dl
1
R
Ali
Ildl
0
i
0
R
R
S0dlꢁ ST dl
l
l
R
¼ I0
;
(1)
S0dl
l
R
R
where S0dl and STdl denote the total area under the emission
spectrum of the light source after passing through a cuvette
without and with X, respectively, I0 and IT the incident and the
transmitted photon flux, respectively, A means absorbance (the
value of absorbed photon flux during photodegradation depends
on time – for detailed calculation of F during photodegradation
of studied compounds using time-dependent values of Ia refer
Eqns (7) and (10)). The incident photon flux I0 in all measurements
was determined using the integrated form of the equation for
EXPERIMENTAL
Synthesis
The compounds under study have been prepared by the con-
densation reactions as is presented in Fig. S1 (Supplementary
J. Phys. Org. Chem. 2011, 24 450–459
Copyright ß 2010 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

´ ˇ
M. CIGAN ET AL.
Figure 2. Changes in the concentrations of 1–3 (A) and 4–7 (B) in MeOH on irradiation with a 100-W Xe lamp (300–850 nm; I0 ¼ 2.05 ꢂ
0.05 ꢀ 10ꢁ5 mol sꢁ1 dmꢁ3). Time-dependent concentrations are fitted by single exponential functions [dotted line]
F1O2 determination for a solution of a sensitizer with known
F1O2 and an acceptor with known b (Eqn (5)).
1–7) that absorbs a photon flux Ia (mol sꢁ1 dmꢁ3) (Fig. 3):[34,35]
dcA
dt
d½DMFꢃ
1O2ðXÞ
¼ IaF
dt
kDr MF½DMFꢃ
ꢁ
¼ ꢁ
kd þ kDr MF½DMFꢃ þ kQðXÞ½Xꢃ
kQðXÞ½Xꢃ
½DMFꢃ
Z þ ½DMFꢃ
¼
;
Z ¼ b þ
:
(5)
kDr MF
Determination of singlet oxygen quenching rate constants (kQ)
The rate constant kQ for 1O2 quenching (relative reactivity index
bQ ¼ kd/kQ; in this case bQ includes both physical and chemical
quenching) by 1–7 necessary for more accurate F1O2 determi-
nation were calculated based on inhibition of DPIBF oxidation
(b ¼ 1.1ꢀ 10ꢁ4 M in MeOH[29,30]) by competitive 1O2 quench-
ing[31,32] using the following equations (in the primary model we
replaced the concentrations with the corresponding absorbances):
In this approach, we involved quenching of 1O2 by a sensitizer
using parameter Z. Parameter Z is the sum of two components
1
and includes overall (physical and chemical) quenching of O2
by acceptor A (2,5-DMF; kr ¼ 3.9.108 dmꢁ3 molꢁ1 sꢁ1 [34]) and
sensitizer X, and should be used for every sensitizer with high
value of kQ (e.g., omitting the parameter Z for sensitizer 3 with
high value of kQ leads to a reduction in quantum yield of about
60%). Parameter b is the reactivity index of the acceptor (the ratio
between the rate constants for nonradiative unimolecular decay
of 1O2 for the appropriate solvent kd and acceptor oxidation
kr).[36] Its value represents the acceptor concentration at which
Q
0
Q
0
krð½ADPIBF="DPIBFꢃt ꢁ½ADPIBF="DPIBFꢃt Þþkdlnð½ADPIBFꢃt =½ADPIBFꢃt Þ
kQ¼
Q
½Qꢃlnð½ADPIBFꢃ0=½ADPIBFꢃt Þ
(2)
2
3
7
ꢀ
ꢁ
1
0
d½ADPIBF="DPIBF
dt
ꢃ
half the reactive O2 species will be trapped. Constant kQ is the
ꢁ
kd þ kr½DPIBFꢃ0
½Qꢃ
6
4
overall quenching constant for sensitizer X. Integrating the Eqn
(5) and deducing acceptor concentrations from absorbance
values gives:
ꢀ
ꢁ
or kQ
¼
ꢁ1 ;
(3)
5
d½ADPIBF="DPIBF
dt
ꢃ
ꢁ
and by monitoring the decrease of the DPIBF absorption band
at different quencher concentration using Young’s kinetic
technique:[33]
1O2ðXÞ
Aa þ "DMFZllnAa ¼ Aa0 þ "DMFZllnAa0ꢁIa"DMFlF
t; (6)
where Aa0 and Aa are the acceptor absorbance (e.g., at the
position of maximum absorption) before and at various irra-
diation times, l is the optical path length (cm) and e is the molar
extinction coefficient of the acceptor. The slope of a plot of this
equation against irradiation time gives F1O2 when the other
parameters are known.
ꢂ
S0
= ¼ 1 þ Q= ½Qꢃ ¼ 1 þ
kd
k
1
½Qꢃ:
(4)
S
bQ
Superscripts 0 and Q denote the absence and presence of
quencher Q, [DPIBF]0 is the initial concentration of DPIBF, [Q] is
the quencher concentration and S is the slope of the plot of a
first-order disappearance of DPIBF at a given [Q]. Both methods
were used to eliminate errors for time-dependent absorption
measurements. 1O2 was generated by MB sensitization
(F1O2(MB) ¼ 0.50 ꢂ 0.01 in MeOH[29,34]).
Superoxide anion (Oꢁ2 ) detection by EPR
Electron paramagnetic resonance (EPR) spectra were recorded on
an ERS 230 instrument (ZWG Berlin, Germany), which operates in
the X-band (ꢄ9.3 GHz) with a modulation amplitude of 0.1 mT
and microwave power of 5 mW. A sample containing 2.5 ꢀ
10ꢁ5 mol dmꢁ3 of one of the derivatives studied was continuously
irradiated at ꢄ60 W mꢁ2 by a 250-W halogen lamp through a 10-cm
water filter for 20 min. The superoxide anion radical generated was
Determination of the quantum yield of singlet oxygen production
(F1O2)
Values of F1O2 were determined using an equation that
describes the change in concentration cA (mol dmꢁ3) of a singlet
oxygen acceptor (with known b value) after irradiation for a given
time t (s) in the presence of a singlet oxygen generator (sensitizer
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 450–459

PHOTOSTABILITY OF NONLINEAR OPTICAL CHROMOPHORES
Figure 3. Kinetic changes in the absorption spectra of 2,5-DMF in MeOH during irradiation of
2 (A) and 7 (B) at 300–850 nm
(I0 ¼ 2.05 ꢂ 0.05 ꢀ 10ꢁ5 mol sꢁ1 dmꢁ3). Absorbed photon flux at t ¼ 0 s was Ia0 ¼ 1.38 ꢀ 10ꢁ5 mol sꢁ1 dmꢁ3 for 2 and Ia0 ¼ 1.05 ꢀ 10ꢁ5 mol sꢁ1 dmꢁ3
for 7
indirectly monitored using a spin trap with 5,5-dimethyl-4,5dihydro-
xypyrrole-N-oxide (DMPO, Sigma; cꢄ 3.5 ꢀ 10ꢁ4 mol dmꢁ3).[37,38]
carbon electrode with an area of 3.79 ꢀ 10ꢁ3 cm2. The auxiliary
electrode was a cylindrical platinum net. Ferrocene was used as
an internal reference and all data are referred to the formal redox
potential of the ferrocene/ferrocenium (Fc/Fcþ) couple equal to
þ0.52 V in this system. Oxygen was removed from the solution by
Phosphorescence measurements (determination of the energy of a
triplet state ET)
a stream of argon, which blanketed the solution throughout
0
Phosphorescence spectra were detected using a home-built
set-up for detection of weak near-IR luminescence described in
Ref. [39]. Excitation pulses of ꢄ10 mJ and 4.5 ns wide were
provided by the dye laser Lambda Physik FL1000 pumped by
the excimer laser ATL Lasertechnik ATLEX 500i. The excitation
wavelengths and laser dyes were chosen to match different
absorption spectra of the investigated molecules, namely 524 nm
(Coumarin 307), 540 nm (Coumarin 153), and 575 nm (Rhodamin
6G). The samples were excited through optically polished bottoms
of fluorescence cells. Luminescence was collected by lens
assembly through two long-pass filters Schott RG7 and high-
luminosity monochromator Jobin Yvon H20IR to the infrared-
sensitive photomultiplier Hamamatsu R5509 cooled to ꢁ50 8C by
liquid nitrogen. The pulses from the photomultiplier were fed
through the Becker–Hickl HF AC-26dB preamplifier to the
Becker–Hickl MSA 200 photon counter/multiscaler with time
resolution of 5 ns per channel. The spectra were obtained as
integrals of the counts in each of the kinetics at individual
wavelengths in the spectral region from 750 to 1350 nm and
corrected in respect to the spectral sensitivity of the detection
set-up as well as to the equivalent absorbed energy. The kinetics
were fitted by double exponential decays to obtain phosphor-
escence lifetimes.
the measurements. E0 values were obtained from the cyclic
voltammetric measurements of 0.5 mM solutions of 1–7 in
acetonitrile and 0.1 M tetrabutylammonium hexafluoropho-
0
sphate supporting electrolyte. Error in the E0 determination is
0
ꢂ0.005 V. Estimated values E0 for 1–3 agree well with the
literature data.[18]
RESULTS AND DISCUSSION
Spectral characteristics
The UV–visible absorption spectra of salts 1–7 in MeOH were
measured previously.[11] These spectra are dominated by intense
low-energy bands in the visible region (lA 520–630 nm)
*
attributable to p!p ICT transitions from the terminal electro-
n-donating N,N-dimethyl groups to the central benzothiazolium
skeleton. Less intense bands in the spectra are associated with
*
nondirectional p!p transitions at higher energy. Basic absorp-
tion characteristics of ICT transitions (absorption maxima lA,
extinction coefficient log e, oscillator strength f, ICT energies EA)
and calculated values of first hyperpolarizabilities b (at the
semiempirical PM3 level by the finite-field method)[11] of
benzothiazolium NLO chromophores 1–7 are presented in
Table 1. ICT bands move predictably to lower energy with
increasing conjugation length and with increasing acceptor
strength of the benzothiazolium skeleton. An additional acceptor
group bound to the benzothiazolium ring (NO2 or CN) has a
controversial effect on the NLO response of 4–7. Addition of this
group to chromophore 1 with single double bond enhances the
value of b and addition to chromophore 2 with two double bonds
leads to a modest decrease in NLO response. A rising number of
double bonds enhances the NLO response of benzothiazolium
chromophores. Different equilibrium between benzenoid form and
quinoid form(s) in the ground and the excited state together with
possible isomerization in longer linkers[40] could explain the lowered
values of log e; and f in absorption spectra of compound 6.
0
Determination of oxidation potential (E0 )
Iodides 1–7 were metathesized to their corresponding hexa-
fluorophosphate salts (due to interfering signal of Iꢁ anion) by
precipitation from the MeOH/MeCN/aqueous KPF6. Electroche-
mical measurements were done in a three-electrode arrange-
ment using a home-made fast rise-time potentiostat. The instru-
ment was interfaced to a personal computer via an IEEE-interface
card (PC-Lab, AdvanTech Model PCL-848) and a data acquisition
card (PCL-818) using 12-bit precision. Positive iR compensation
just short of oscillations was used in all the measurements. The
reference electrode AgjAgClj1M LiCl was separated from the test
solution by a salt bridge. The working electrode was a glassy
J. Phys. Org. Chem. 2011, 24 450–459
Copyright ß 2010 John Wiley & Sons, Ltd.
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´ ˇ
M. CIGAN ET AL.
Table 1. Photophysical properties of the molecules studied
Dye
lA (nm)
EA (kJ molꢁ1
)
log e
f
lP (nm)
ET (kJ molꢁ1
)
t1 (ns)
t2 (ms)
b (10ꢁ30 esu)
1
2
3
4
5
6
7
520
562
580
562
551
630
618
230
213
206
213
217
190
194
4.83
4.78
4.75
4.90
4.91
4.61
4.83
1.07
1.24
1.33
0.99
1.01
0.62
1.14
841
851
861
854
863
852
850
142
141
139
140
139
140
142
85 ꢂ 3
76 ꢂ 4
79 ꢂ 8
82 ꢂ 9
90 ꢂ 8
83 ꢂ 1
83 ꢂ 2
1.3 ꢂ 0.4
1.2 ꢂ 0.1
1.4 ꢂ 0.1
1.2 ꢂ 0.1
1.2 ꢂ 0.1
1.2 ꢂ 0.2
1.1 ꢂ 0.1
98.5
370.1
744.8
183.3
183.7
317.3
321.0
lA, the absorption maximum; log e, the extinction coefficient; f, the oscillator strength; EA, the ICT energies; lP, the phosphorescence
maximum; ET, the energy of triplet state; t1 and t2, fast and slow decay time of luminescence signals; b, the first hyperpolarizability.
Fluorescence spectra showed very low intensity in MeOH
(quantum yields of fluorescence FF never exceeded 0.005). This
can be attributed to enhanced ICT interaction, which often
results in fluorescence quenching due to enhanced probability of
nonluminiscent ICT and TICT (twisted intramolecular charge
transfer) state population in polar solvents.[41] The same effect
was observed in a few push–pull D–p–A salts and it has also
a practical application.[42] The evidence of the final relaxed
nonluminiscent dark TICT state in the commercial benzothiazo-
lium push–pull laser dye LDS 751 (which is structurally very close
to dye 2) was recently confirmed experimentally using the FS TR
SEP FD method (femtosecond time resolved stimulated emission
pumping fluorescence depletion).[43] Hydrogen bonding effect of
the solvent can also contribute to the nonradiative internal
conversion.[41]
phosphorescence. However, the second alternative is quite
unusual for most organic triplets. All the respective lifetimes are
quite similar, one can only say that longer component is always
(for all different excitation wavelengths) slower in sample 3 (refer
Table 1). The striking differences in phosphorescence intensities
of the different materials are therefore caused by different
triplet state quantum yields and not by different quenching of
the emitting states. Thus, the extension of the conjugated
bridge and an additional acceptor group bonded to the
heterocycle have a substantial effect on the triplet state
population although they do not significantly influence the
energy of the triplet states.
Photostability
Phosphorescence spectra were measured to estimate the
energy of triplet states (the position of the main phosphor-
escence maxima lP, Table 1) as one of the major factors that
As already mentioned, the NLO chromophores photostability is a
very important characteristic with regard to their practical usage.
There are two dominant processes leading to photodegradation
of NLO chromophores containing a carbon–carbon double bond
as part of a p-conjugated bridge: trans–cis isomerization and
photooxidation. To study the influence of the extension of the
conjugated bridge and the addition of an acceptor group to
the heterocycle on the photostability of 1–7, MeOH solutions of
the compounds were subjected to irradiation with a 100-W Xe lamp.
1
influence the production and quenching of singlet oxygen O2
(Sections 2.2 and 2.3 in Results and Discussion). All the studied
compounds exhibit similar luminescence spectra with lP around
852 ꢂ 11 nm and a shoulder around 910 nm. Bathochromic shift
of the main phosphorescence maximum with the length of the
conjugated bridge was observed for chromophores 1–3. On the
contrary, no such shift was found for compounds 4 and 6 and a
hypsochromic shift was obtained for compounds 5 and 7.
Luminescence intensity exhibits very strong dependence on the
length of the conjugated bridge and on addition of the acceptor
to N-methylbenzothiazolium moiety. It increases significantly
with increasing length of the conjugated bridge. This effect was
the most pronounced in chromophores 1–3, where compound 1
exhibited only approximately 3% of the total phosphorescence
intensity of compound 2, while the phosphorescence intensity of
compound 3 was ꢄ27 times higher than that of compound 2.
This effect was also observed in compounds 4 and 6 as well as in
5 and 7 where the total phosphorescence intensity increased 5
and 7 times, respectively, in the compounds with longer
conjugated bridges. Similarly an addition of the acceptor to
heterocycle significantly increases the luminescence intensity.
Time-resolved luminescence signals were accurately fitted by
biexponential functions. We distinguished fast (t1) and slow (t2)
decay times of approximately 80 ꢂ 10 ns and 1.3 ꢂ 0.1 ms,
respectively, with the short component fractional yield of
ꢄ70–75% from the biexponential fits. The short component of
the luminescence signal is assumed to result from a deactivation
of the dark TICT state[43] or it may be as well a component of
Trans–cis photoisomerization
Changes in the main absorption band (ICT transition) were
monitored to determine the photodegradation quantum yield in
air-saturated MeOH solutions under one-photon excitation at
room temperature. Kinetic changes in the absorption spectra of
1–7 in MeOH on irradiation at 300–850 nm are presented in Figs.
S2 (Supplementary Material) and 2. The main absorption during
this irradiation corresponds to ICT transitions for all compounds
studied. In all cases, the ICT absorption band initially decreased
with irradiation time and the absorption bands were shifted to a
shorter wavelength. After longer irradiation, a photostationary
state was attained and the spectra of the reaction mixtures
did not change with further irradiation (small concentration
changes due to self-sensitized 1O2 photooxidative attack were
not detectable in such short time intervals). These results can be
rationalized by the existence of trans–cis photoisomerization,
leading to a photostationary state. The data was fitted to a
monoexponential decay function and the decay rate constants
determined from the fitted curves were used to calculate the
quantum yield of photodegradation (i.e., photoisomerization)
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J. Phys. Org. Chem. 2011, 24 450–459

PHOTOSTABILITY OF NONLINEAR OPTICAL CHROMOPHORES
Table 2. Photodegradation characteristics of 1–7 on irradiation at 300–850 nm
1
Dye
k (10ꢁ4 sꢁ1
)
t0.5 (min)
t0.9 (min)
Fdeg ꢀ 10ꢁ4
% initial
% O2 contribution
1
2
3
4
5
6
7
1800
1150
76.0
7.8
10.0
9.1
0.07
0.10
1.52
15
12
13
0.22
0.33
5.05
49
38
42
4.33
5.60
0.57
0.15
0.13
0.66
0.18
99.6
99.2
97.9
96.7
97.9
92.8
97.2
0.002
0.060
2.2
3.2
1.4
0.6
1.5
10.0
12
38
k, the photodegradation rate constant; t0.5, the time for 50% degradation; t0.9, the time for 90% degradation; Fdeg, the quantum yield
for photodecomposition; % initial, the percent of initial isomer in the photostationary state; % 1O2 contribution, the contribution of
1
self-sensitized photooxidation by O2 to overall photodegradation during photoisomerization.
according to:
process), respectively, both of which can contribute to dye
photofading.[44,45] Therefore, we conducted experiments to
determine whether 1–7 generate these reactive oxygen species
and whether there is evidence of self-sensitized photooxidation.
ct
R
ꢁ
dc
c0ꢁct
c0ꢁct
2
ln10
c0
ꢃ
ꢄ
Fdeg
¼
¼
¼_
;
t0:9
¼
:
n
Ia0þIa0:9
t
P
R
Iai þIaiþ1
k
t0:9
t0:9
nþ1
Iadt
2
Production of the superoxide anion radical Oꢁ2 . To study the
generation of superoxide anion radical O:2ꢁ after photoexcitation
of 1–7, EPR spin-trapping was carried out with DMPO as the spin
trapper.[34,35] After 20 min of irradiation with a 250-W halogen
lamp only 4 showed a very low-intensity EPR signal (Fig. S3 in
Supplementary Material) in the region of free radicals (g ¼ 2.004).
This signal seemed to represent DMPO-Oꢁ2 adduct with three
coupling constants due to the nitrogen atom and two hydrogen
atoms in the b and g positions in this molecule (AN ¼ 13.3 G,
AaH ¼ 10.1 G and AbH¼1.5 G). Experimental coupling constants
agree well with literature data.[37,38] Electron transfer (formation
of a superoxide anion radical) and subsequent oxidation reactions
(followed by various radical recombinations) or nucleophilic
addition of O:2ꢁ thus may contribute to the overall degradation
of 4.
i¼0
0
(7)
Values for the photodegradation rate constant (k), the time for
50% degradation (t0.5), the time for 90% degradation (t0.9), the
quantum yield for photodecomposition (Fdeg), the percent of
initial isomer in the photostationary state and the contribution of
1
self-sensitized photooxidation by O2 to overall photodegrada-
tion during photoisomerization are summarized in Table 2.
Fdeg exhibited no concentration dependence, consistent with
a first-order photodecomposition process. The values of Fdeg
ranged from (0.11–0.14) ꢀ 10ꢁ4 (2% photodegradation) for 5
to (5.5–5.7) ꢀ 10ꢁ4 (0.8% photodegradation) for 2, with the
percentage photodegradation calculated from the asymptotes of
the fitted exponential curves. Small values of Fdeg are most likely
connected with the extent of CT character in the molecules
studied. Substantial competition of the ICT (and/or TICT) excited-
states population with the nonradiative phantom-state popu-
lation (from the second one trans–cis isomerization occurs) and
moreover the hydrogen bonding effect of MeOH may enhance
the nonradiative internal conversion.[41] Addition of the third
double bond to the conjugated bridge and an additional
acceptor bound to position 6- of N-methylbenzothiazolium
decreased the value of Fdeg, although the overall percentage
photodegradation slightly increased. Very rapid cis–trans photo-
isomerization of the initially formed cis isomer may be respon-
sible for a low contribution of this pathway to the overall
photodegradation. Trans–cis photoisomerization thus seems to
be a fast (mainly for 1–3) but inefficient (ꢄ1–7% decrease of
initial concentration) photobleaching mechanism for these
irradiation wavelengths. Contribution of self-sensitized photo-
oxidation by 1O2 (Section 2.4 in Results and Discussion) as a
parallel degradation pathway to the overall photodecomposition
during trans–cis photoisomerization of 1–7 is not significant
(0–3%).
Production of singlet oxygen 1O2. Values for the quantum yield of
singlet oxygen production (F1O2) are summarized in Table 4 and
outlined in Fig. 3. Relatively low values of F1O2 for all seven
derivatives can be attributed to the ICT character of these
molecules, which manifests its effect in sensitizer–oxygen contact
complexes.[46,47] CT character in the sensitizer can influence
the energy, yield, and lifetime of the triplet state, all of which
can, likewise, ultimately be reflected in the singlet oxygen yield.
Increasing CT interactions reduce the overall quantum yield of
singlet oxygen formation F1O2 and the decrease is further
enhanced in polar solvents due to the enhancement of CT
interactions (stabilization of exciplexes of triplet excited sensitizer
and O2 in moving from nonpolar to highly polar solvents). The
extent of CT, both within the sensitizer itself (i.e., intramolecular
CT) as well as in the sensitizer-oxygen complex (i.e., inter-
molecular CT), thus have a large adverse effect on the efficiency
with which singlet oxygen is generated.[48]
The increasing value of F1O2 after addition of the acceptor
groups to the N-methylbenzothiazolium moiety of 1 (simul-
0
Photooxidation
taneous increase of E0 ) demonstrates an effect of intermolecular
CT (molecules 4 and 5). This behavior does not take effect on
going from molecule 2 to structure 6 and 7 and even a slight
decrease of F1O2 in both cases was observed. These results
There are two possible mechanisms for interaction between an
excited dye molecule and dioxygen, which lead to the formation
of singlet oxygen (Type II process) and a superoxide anion (Type I
J. Phys. Org. Chem. 2011, 24 450–459
Copyright ß 2010 John Wiley & Sons, Ltd.
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´ ˇ
M. CIGAN ET AL.
1
Table 3. Parameters for characterization of physical and chemical quenching of O2 by 1–7
0
kQ
(109 dm3 molꢁ1 sꢁ1
bQ
(10ꢁ5 mol dmꢁ3
E0 vs. Fc/
Fcþ (V)
kr
br
Dye
)
)
m (D)
(106 dm3 molꢁ1 sꢁ1
)
(10ꢁ2 mol dmꢁ3
)
1
2
3
4
5
6
7
0.17
1.50
5.00
1.20
0.41
3.70
7.10
65.0
7.3
2.2
9.2
26.8
3.0
1.6
0.567
0.381
0.264
0.626
0.610
0.435
0.424
2.4
4.1
7.3
12.4
7.3
12.0
6.1
ꢅ 0.1
ꢅ 0.1
ꢅ 0.1
3.2
1.5
2.2
ꢆ10
ꢆ10
ꢆ10
3.5
7.5
5.0
7.3
1.5
kQ, the rate constant for overall 1O2 quenching; bQ, the relative reactivity index (includes both physical and chemical quenching) for
0
1O2 quenching; E0 , the formal redox potential; m, the ground-state dipole moment; kr, the rate constant for 1O2 photooxidative attack;
1
br, the reactivity index of dye with O2.
suggest a controversial role of a combination of both inter- and
intramolecular CT on the 1O2 production efficiency, possibly
affecting the quantum yield of the triplet state population and/or
a triplet state deactivation pattern. As was previously mentioned
by Nielsen et al.,[48] one cannot always rely on rule-of-thumb
guidelines when attempting to construct either efficient or ineffi-
cient 1O2 sensitizers and a full investigation of the photophysical
properties of the system studied is generally required.
The value of F1O2 for 1–3 represents again an unexpected
decrease of F1O2 of 1–3 with increasing value of E0. This
increasing tendency of a quantum yield with extension of
conjugated bridge copies the ability of phosphorescence
emission by molecules 1–3 although only qualitative conclusions
of the emission probability could be done. This behavior is
consistent with the fact that the S0–S1 excitation energy changes
more with the conjugate chain length than that of S0–T1.[49,50]
Enhancement in rate constants for intersystem crossing for
longer chains was observed and attributed to a lowering of
the S1–T1 energy gap for longer chains.[49,51] Thus, molecules with
longer conjugation lengths seem to have a higher triplet
quantum yield. The first excited singlet states of 1–7 should not
contribute markedly to overall 1O2 production owing to their
short lifetimes t0 ꢅ 10 ns (t0 ¼ 1.5/n2maxf where nmax is the
wavenumber corresponding to the absorption maximum). The
1O2 production efficiency of the studied molecules is clearly
the greatest for 3 and the least for 1.
Quenching of singlet oxygen. Estimated values of 1O2 quenching
rate constants kQ (bQ) (Table 3) exceeded the values expected
for an electronic-to-vibrational energy transfer (converting
electronic excitation energy of the O2 molecule into vibration
of O2 and quencher Q) by orders of magnitude. These high values
1
of kQ regarding relatively high triplet energies (ET >130 kJ molꢁ1
)
0
and low oxidation potentials (E0 < 0.65 V) can be attributed to a
CT-induced quenching of 1O2, where the deactivation of the
initially formed singlet encounter complex 1(Q1D)EC is enhanced
by the formation of a singlet exciplex 1(Q1D)CT, which is stabilized
by the transfer of electric charge from the quencher to the
oxygen molecule.[52] Iodides do not contribute significantly to
such high kQ values in protic solvents.[53] The formation of a
1
singlet exciplex (Q1D)CT is followed by isc to the ground-state
CT complex, by chemical reaction, or by separation of free
ions. Thus, the overall quenching rate constant kQ can be
additively composed of a physical and a chemical component,
i.e., kQ ¼ kp þ kr.
kp
1
1
1
3
3
Q þ D ! ðQ1DÞEC ! ðQ1DÞCT ꢁ! ðQ3SÞCT ꢁ! Q þ S
Rate constants kQ of 1–7 showed very strong dependence on
the length of the conjugated bridge and an addition of acceptor
group to the N-methylbenzothiazolium moiety. Values of kQ
increased significantly with both characteristics. Both quenching
1
Table 4. Parameters for characterization of O2 production by 1-7 and subsequent self-sensitized photooxidation
0
Dye
F1O2
E0 vs. Fc/Fcþ (V)
t1/2(1O2) (days)
t9/10(1O2) (days)
Fr(1O2)
1
2
3
4
5
6
7
0.0006
0.0014
0.0098
0.0012
0.0009
0.0011
0.0012
0.567
0.381
0.264
0.626
0.610
0.435
0.424
0
80.5
29.5
4.1
13.5
38.8
25.0
27.0
362
124
15
0.0063 ꢀ [C]
0.014 ꢀ [C]
0.098 ꢀ [C]
0.034 ꢀ [C]
0.012 ꢀ [C]
0.021 ꢀ [C]
0.017 ꢀ [C]
60
177
115
116
F1O2, the quantum yield of 1O2 production; E0 , the formal redox potential; t1/2(1O2) and t9/10(1O2), the half-life time of the
self-sensitized photooxygenation reaction and the time of degradation of 90% of the compound at these reaction; Fr(1O2), the
1
quantum yield of the self-sensitized photodegradation by O2.
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J. Phys. Org. Chem. 2011, 24 450–459

PHOTOSTABILITY OF NONLINEAR OPTICAL CHROMOPHORES
1
of O2 and O2 production depends on two main parameters:
1
from the slope of the plot
triplet-state energy ET and oxidation potential of the sensitizer.
The oxidation process is reversible for all studied compounds and
lnðA0=AtÞ ¼ Kt;
(9)
0
its formal redox potential E0 is reported in Table 3. Because of the
almost constant value of ET, increasing kQ with extension of the
conjugated bridge can be rationalized by the decreasing value of
where K ¼ [C][1O2].
Figure 4 shows the photostability of 4–7 in MeOH against 1O2
photooxigenative attack. The linear relationships between ln[C]0/
[C]t and t indicate that these are first-order kinetics reactions. The
slopes of the plots indicate that the photofading rate constant is
the greatest for 4 and the smallest for 7 (Fig. 4B).
If the rate of 1O2 formation kf is known, then we can calculate
an objective value of the second-order rate constant kr (or br)
for 1O2 photooxidative attack. The kf value was obtained by
multiplying the known value F1O2 for MB in MeOH by the value
of Ia absorbed by this sensitizer. Results are summarized in
Table 3.
0
E0 . This is an established phenomenon.[52] Albeit to the opposite
0
effect of NO2 (CN) group addition on E0 , kQ considerably
increased after incorporation of these groups to the benzothia-
zolium moiety. Considering again the relatively constant values
of ET, these results indicate a significant role of intramolecular
charge-transfer (ICT) in the CT-induced quenching of 1O2. ICT
most likely plays an important role not only in the production of
1
1O2, but also in the quenching of O2. The increasing tendency
of kQ copies the increasing tendency of ground-state dipole
moments m,[11] although absolute correlation does not exist.
Polarizability of the chromophore most likely contributes as
another factor to CT-induced quenching of 1O2. An opaque trend
of kQ behavior after the exchange of a CN group for an NO2 group
in pairs 4–5 and 6–7 could be explained by different equilibrium
between benzenoid and quinoid form of compound 6 in the
ground state (values of log e and f in Table 1).
Concentrations of chromophores 1–3 did not change after 2 h
on irradiation at the same experimental conditions. This does not
necessarily mean that molecules 1–3 do not react with O2, but
1
the second-order rate constant kr should not exceed
105 dm3 molꢁ1 sꢁ1 (br ꢆ 0.1 mol dmꢁ3).
To determine the kinetic parameters of self-sensitized
photooxidation due to 1O2 production and subsequent reaction
between 1O2 and the sensitizer, we first calculated the time-
dependent concentration values (ciþ1) for this photodegradation
pathway using the following equations:
Self-sensitized photooxidation by 1O2. In many cases, CT quench-
ing of 1O2 additionally competes with chemical reactions, which
are often far from negligible.[52] To determine the fraction of
F
I
ð1ꢁ10ꢁac
Þ
Þ
1
aiDt
i
O
2
ciþ1 ¼ ciexpꢁ
;
Iai ¼ Ia0
;
b
1
1
r
chemical reaction with O2 in the overall quenching of O2, we
carried out a series of experiments in which the decrease in
ð1ꢁ10ꢁac
0
(10)
i ¼ 0; 1; 2; . . .; n; Dt ¼ 50000s
1
absorbance of 1–7 due to MB-generated O2 in the absence of
and then subtracted the half-life times (t1/2(1O2)) of the self-
sensitized photooxygenation reaction and the times of degra-
dation of 90% of the compound (t9/10(1O2)) at these reaction from
the corresponding graphs ciþ1 ¼ f(ti) (Fig. 5) (coefficients a were
determined using OO SD 2000 spectrophotometer).
any other quencher was monitored. According to the mechanism
of photooxidation by 1O2, the reaction rate at low concentration
(br > >[C]) (assuming that [1O2] is constant) is given by the
following equations:[33,34]
d½Cꢃ
dt
½Cꢃ
½Cꢃ þ br
ꢆ
½Cꢃ
½Cꢃ
To compare the kinetic parameters of autooxidation by 1O2 for
molecules with (4–7) and without (1–3) an acceptor group bonded
to the heterocycle, the maximum values of kr ¼ 105 dm3 molꢁ1 sꢁ1
(br ꢆ 0.1 mol dmꢁ3, respectively) for all three chromophores 1–3
was taken. Results are summarized in Table 4. The obtained order
of self-sensitized photodegradation for 1–3 coincides with
1O2ðMBÞ
ꢁ
¼ FrIa ¼ F
Ia
¼ kf
¼ kf
¼
½Cꢃ þ br
br
ꢅ
kr
¼ kf ½Cꢃ ¼ kr½Cꢃ 1O2 ¼ K½Cꢃ:
(8)
kd
The photofading rate constant K for the dyes was obtained
Figure 4. (A) Photodegradation of 4 in MeOH due to photooxidation by MB-generated 1O2 (F1O2 ¼ 0.50 ꢂ 0.01, Ia ¼ 4.5 ꢂ 0.5 ꢀ 10ꢁ6 mol sꢁ1 dmꢁ3 for
MB; color filter with transmittance >550 nm was used to avoid direct photodegradation of 4). (B) Photostability of MeOH solutions of 4–7 against 1O2
photooxidative attack (dotted lines, linear regression of experimental data)
J. Phys. Org. Chem. 2011, 24 450–459
Copyright ß 2010 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

´ ˇ
M. CIGAN ET AL.
1
Figure 5. Calculated photodegradation curves of (A) 1–3 and (B) 4–7 in MeOH due to self-sensitized O2 photooxidative attack during irradiation at
300–850 nm (incident photon flux I0 ¼ 2.05 ꢀ 10ꢁ5 mol sꢁ1 dmꢁ3
)
the order of photodegradation after 3 weeks of the exposure of
solution of 1–3 (10ꢁ4 mol dmꢁ3) to daily sunlight. For a
more accurate estimation of kinetic parameters describing this
degradation pathway for 1–3, further experiments with higher
irradiation intensities and more effective sensitizer should be
carried out.
CONCLUSIONS
In this paper, the photochemical stability of seven D–p–A
benzothiazolium salts as candidates for NLO chromophores
were investigated. The studied compounds differed in length of
conjugated bridge and presence (or absence) of an additional
acceptor group (NO2 or CN) bound to position 6- of the
N-methylbenzothiazolium moiety. The photoreaction quantum
yield and the kinetic parameters were determined for existing
photodegradation pathways on irradiation at 300–850 nm in
MeOH. The most rapid reaction for any of the seven substrates is
trans–cis photoisomerization, which leads to a photostationary
state and contributes with a low percent (ꢄ1–7% decrease of
initial concentration) to the overall photodegradation. Photo-
The quantum yields (Fr(1O2)) of these self-sensitized photo-
degradations were calculated according to:
1O2
F
Frð102Þ
¼
½Cꢃ:
(11)
br
Values of Fr(1O2) are expressed as number ꢀ [C] to point out
their dependence on the concentration of the corresponding
chromophore (Table 4). Times t1/2(1O2) and t9/10(1O2) are relatively
high (except derivate 3 with t1/2(1O2) ꢄ4 days) and showed strong
dependence on addition of an acceptor group to the heterocycle
and the length of the conjugated bridge. Both structural changes
decrease the overall photostability although the change is not so
significant after the addition of an acceptor group to skeleton 2.
Self-sensitized photooxidation by 1O2 thus makes a slow parallel
degradation pathway for irradiation at 300–850 nm and, albeit
to the small F, contributes with large fraction to the overall
photodegradation of 4–7 (and probably also of 1–3).
Exchange of the NO2 group for CN causes a large increase in
the photostability of compound 5, but had only a very
small influence on the photostability of 7. This difference could
be most likely due to the equilibrium between resonance-forms
of the ground state of compound 6. The photostabilities of
the chromophores can be placed in the order:
1 > 5 > 2 > 7 > 6 > 4 > 3. It is apparent that a compromise
should be made between an increase in NLO response and a
decrease in photostability to enable a choice of NLO chromo-
phores for practical applications.
1
oxidation by O2 gives a second very slow parallel degradation
pathway and, albeit to small values of F, plays a dominant role in
the photodegradation of 1–7. Electron transfer (formation of a
superoxide anion radical) and subsequent oxidation reactions
(followed by various radical recombinations) or nucleophilic
addition of Oꢁ2 may contribute to the overall degradation of
compound 4. Both structural modifications increasing NLO
response (extension of the conjugated bridge and an additional
acceptor group bonded to the heterocycle) led to a decrease in
photostability due to manifestation of the main photodegrada-
tion pathway (self-sensitized 1O2 photooxidative attack). From
the values of the kinetic parameters of self-sensitized photo-
oxidation by 1O2, it can be concluded that a compromise should
be made between an increasing NLO response and a decreasing
photostability to enable a choice of NLO chromophores 1–7 for
practical applications. The existence of photostationary states at
ꢄ1–2% degradation by photoisomerization and low photoche-
mical quantum yield (F ¼ 0.014 ꢀ [C] and 0.012 ꢀ [C]) for the
main photodegradation pathway, together with high b values
mean that 2 and 5 are promising derivatives for NLO applications.
Finally, the minimal overall degradation (ꢄ1–2%) due to trans–
cis photoisomerization, the relatively low quantum yield for
self-sensitized photooxidation by 1O2 as the main photodegrada-
tion pathway (0.014 ꢀ [C] and 0.012 ꢀ [C]) and the high b values
(370 ꢀ 10ꢁ30 esu and 184 ꢀ 10ꢁ30 esu) mean that compounds 2
and 5 are promising for NLO applications. Compound 3, albeit to
very large value of b, is not a good candidate for practical NLO
applications due to its poor photostability.
Acknowledgements
This work was supported by the Slovak Research and Develop-
ment Agency through APVV Grant No. 0259-07, by the Slovak
Grant Agency for Science (VEGA No. 1/ 0639/08), UK Grant No.
UK/229/2007, Grant Agency of the Czech Republic (GACR 203/08/
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Copyright ß 2010 John Wiley & Sons, Ltd.
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PHOTOSTABILITY OF NONLINEAR OPTICAL CHROMOPHORES
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