Photochemistry and DNA-affinity of some stilbene and distyrylbenzene analogues containing pyridinium and imidazolium iodides

Article abstract of DOI:10.1016/j.jphotochem.2010.09.007

The relaxation properties of the excited states of some trans-1,2-diarylethene analogues (where one aryl group is a methylpyridinium or dimethylimidazolinium group and the other one is a π-excessive furyl or pyrrolyl group) and two all-trans-distyrylbenzene analogues (where the central ring is a methylpyridinium group and the side rings are furyl or methylpyrrolyl groups) have been investigated in buffered (pH 7) aqueous solutions. The compounds of the diarylethene series generally undergo efficient trans → cis photoisomerization while the yield of the radiative deactivation is very small at room temperature. The corresponding distyrylbenzenes display small yields of radiative/reactive pathways and mainly deactivate by internal conversion. The solvent effect on the spectral behaviour indicates the occurring of intramolecular charge transfer ("push-pull" compounds) which can induce interesting non-linear optical properties. Some experiments on the interactions with DNA, which might affect the cell metabolism, showed a modest binding affinity for the compounds with one ethenic bond and two aromatic rings. The complexation constant increases substantially in compounds with two ethenic bonds (three aromatic rings) and in the halogen-substituted compounds. The formation of ligand-DNA complexes affects only slightly the competition of the radiative/reactive relaxation from the lowest excited singlet state.

Full text of DOI:10.1016/j.jphotochem.2010.09.007

Journal of Photochemistry and Photobiology A: Chemistry 216 (2010) 66–72
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photochemistry and DNA-affinity of some stilbene and distyrylbenzene
analogues containing pyridinium and imidazolium iodides
C.G. Fortunaa, U. Mazzucatob, G. Musumarraa, D. Pannaccib, A. Spallettib,∗
a Dipartimento di Scienze Chimiche, Università di Catania, 95125 Catania, Italy
b Dipartimento di Chimica, Università di Perugia, 06123 Perugia, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 August 2010
Received in revised form 2 September 2010
Accepted 6 September 2010
Available online 16 September 2010
The relaxation properties of the excited states of some trans-1,2-diarylethene analogues (where one aryl
group is a methylpyridinium or dimethylimidazolinium group and the other one is a ␲-excessive furyl
or pyrrolyl group) and two all-trans-distyrylbenzene analogues (where the central ring is a methylpyri-
dinium group and the side rings are furyl or methylpyrrolyl groups) have been investigated in buffered (pH
7) aqueous solutions. The compounds of the diarylethene series generally undergo efficient trans → cis
photoisomerization while the yield of the radiative deactivation is very small at room temperature. The
corresponding distyrylbenzenes display small yields of radiative/reactive pathways and mainly deac-
tivate by internal conversion. The solvent effect on the spectral behaviour indicates the occurring of
intramolecular charge transfer (“push–pull” compounds) which can induce interesting non-linear optical
properties.
Keywords:
Photoisomerization (cis/trans)
Conformers
Quaternized azastilbenes
Binding with DNA
Some experiments on the interactions with DNA, which might affect the cell metabolism, showed a
modest binding affinity for the compounds with one ethenic bond and two aromatic rings. The complex-
ation constant increases substantially in compounds with two ethenic bonds (three aromatic rings) and
in the halogen-substituted compounds. The formation of ligand–DNA complexes affects only slightly the
competition of the radiative/reactive relaxation from the lowest excited singlet state.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
ation of these molecules is generally very scarce (with yields
<1%), slightly increasing in a polar solvent and reaching high val-
As well known for stilbene-like molecules, trans isomers of
styrylpyridinium salts are generally very sensitive to UV irradiation
rapidly isomerizing to a cis/trans (Z/E) equilibrium mixture. 1,2-
Diarylethenes where one aryl group is a ␲-excessive pentatomic
group acting as a donor (D) and the other one is a ␲-deficient pyri-
dinium group acting as an acceptor (A), separated by an ethene
bridge (E), have been described to behave as push–pull (D–E–A)
systems displaying interesting intramolecular charge transfer (ICT)
properties, useful for applications in optoelectronics [1]. They can
also form complexes with DNA [2], possibly affecting the cell
metabolism, thus being of interest for potential antitumor activity
[2–4].
In principle, the peculiar properties of the quaternized aza-
stilbenes and azadistyrylbenzenes are expected to affect the
photobehaviour of the corresponding neutral molecules. Previous
studies (often carried out in surfactant assemblies for microen-
vironment characterization [5]) were mainly concerned with the
4-styryl-substituted pyridinium cations [6]. The radiative relax-
ues, close to 1, in rigid matrices at low temperatures only. The
photoreactivity (trans-cis isomerization, dimerization, association)
strongly depends on the experimental conditions (substituents
at the phenyl group, nature of the anion, solvent and temper-
ature) [6]. Conductivity measurements on some iodides have
indicated that the compounds are fully dissociated in polar solvents
at low concentration [7]. In these conditions the photoisomer-
ization quantum yields of 1-methyl-4-styrylpyridinium iodide is
∼50% (little affected by substituents and nature of the anion),
a maximum value expected for the diabatic mechanism. In fact,
similarly to the neutral molecules, the cations are assumed to iso-
merize by this common mechanism implying a twisting around
the ethenic bond towards an energy minimum at the perpen-
dicular configuration (perp, at about 90◦), followed by a S1 → S0
internal conversion (IC) or T1 → S0 intersystem crossing (ISC) and
relaxation to the ground-state cis and trans isomers in roughly
a 1:1 branching ratio. The photoreaction can be thus schema-
1
tized: 1,3trans* → 1,3perp* → perp → ˛1cis + (1 − ˛)1trans, where
the partitioning factor ˛ is assumed to be ∼0.5 in most cases [8]
(it has to be noted that the recent literature prefers to describe
the radiationless processes at the perp region as conical intersec-
tions [9]). In dilute solutions the trans isomers of the cations were
∗
Corresponding author. Tel.: +39 075 5855575; fax: +39 075 5855598.
E-mail address: faby@unipg.it (A. Spalletti).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.09.007

C.G. Fortuna et al. / Journal of Photochemistry and Photobiology A: Chemistry 216 (2010) 66–72
67
by quaternization of E-2-styrylpyridine, synthesized for previous
works by standard procedures [12]. The compounds 2 and 4,
reported in a previous paper [10], are included in Scheme 1 for
comparison. The cis isomers were obtained by UV irradiation and
separated by semi-preparative HPLC using a Phenomenex Jupiter
C18, 250 × 21.2 mm, 10 ␮m, column. The photophysical and photo-
chemical behaviour was studied in buffered (ETN) water at pH 7.0.
Some measurements were also carried out in acetonitrile (MeCN)
from Fluka and chloroform (CH) from J.T. Baker.
Ar =
2
2
2
2
N
Ar
S
O
+
N
CH3
CH3
Ph (1)
2T (2)
2F (3)
2Py (4)
1-Ar,2-(1-methylpyridinium)ethenes
Ar =
2
6
2
2
Perkin-Elmer Lambda 800 and Varian Cary 4E spectrophotome-
ters were used for the absorption measurements. The fluorescence
spectra were measured in dilute solutions (absorbance <0.1 at the
excitation wavelength, ꢀexc) by a Spex Fluorolog-2 F112AI spec-
trofluorimeter. The emission quantum yields were determined
at ꢀexc corresponding to the maximum of the first absorption
band (ꢀmax) using 9,10-diphenylanthracene in cyclohexane as
fluorimetric standard (ꢁF = 0.90 in de-aerated solvent [13]). For
photochemical measurements (potassium ferrioxalate in water as
actinometer), a 150 W high pressure xenon lamp coupled with a
monochromator was used. The trans → cis photoreaction (solute
concentrations ∼10−4 M) was monitored by HPLC using a Waters
apparatus equipped with an analytical (Phenomenex Jupiter C18;
250 × 4.6 mm, 5 ␮m) column and a UV detector. Water/MeCN
mixtures were used as eluents. The monitoring wavelength was
at the isosbestic point. The conversion percentage was held at
below 8% to avoid the competition from the back photoreaction.
The photostationary state (pss) composition, shifted towards
the cis isomer for the compounds with one double bond, was
found to be reversed for 5 and 6 with two isomerizable double
bonds. The [cis]/[trans] ratio at pss was very low and made rather
difficult the measurements of the quantum yields of the direct
(ꢁt → c) photoprocess of these two compounds. In the case of 6,
it prevented to isolate the cis isomer; its yield was then evalu-
ated from the pss composition at the isosbestic point through
the relationship [cis,trans]/[trans,trans] = (εtrans,trans/εcis,trans) ×
(ꢁtrans,trans → cis,trans/ꢁcis,trans → trans,trans). The quantum yields
reported in Table 2 for 6 have to be taken with caution.
Fluorescence lifetimes (obtained for compound 5 only, owing to
the very low emission yields of the others) were measured by a Spex
Fluorolog-␶2 apparatus, based on the phase modulation technique.
All measurements were carried out in de-aerated solutions by
purging with nitrogen. The parameters reported in the Tables are
averages of at least three independent experiments with mean
deviations of ca. 15% and 10% for fluorescence and photoisomer-
ization quantum yields, respectively. The uncertainty of the latter
is higher for 6 (see above). Other details on the experimental mea-
surements are reported elsewhere [14].
The tests on the binding of the ethenic compounds to salmon
DNA (from Sigma–Aldrich) were carried out by measuring the
changes of the absorption spectra under successive additions of
an aqueous buffered (ETN, 0.01 M NaCl, pH 7.0) solution of DNA
to an initial ∼2 × 10−5 M concentration of ligand in the same
buffer. The complexation constants (K) were obtained by absorp-
tion/concentration plots according to the equation of McGhee and
von Hippel [15]. The experiments, repeated at least two times,
also varying in some cases the monitoring wavelength, had a
reproducibility within 30% owing to the rather small changes in
absorbance and additional errors associated to corrections for dilu-
tion.
Ar
N
Ar
O
N
+
CH3
CH3
2F (5)
2Py (6)
2,6-di-(Ar-ethenyl)-1-methylpyridinium
H3C
+
N
Br
2
Ar =
N
2
4
O
CH3
Cl
2-ClPh (7)
Ar
4-BrPh (8)
2-[5-Ar-furan-2-yl-ethenyl-1,3-dimethyl-imidazolium
Scheme 1. Trans cations of the iodide salts investigated (cations 2 and 4 were
studied in a previous work [10] and here reported for comparison).
found to generally isomerize in the singlet manifold, unless the
substituent induces a significant triplet population [6].
In a previous paper we investigated some trans-1-heteroaryl-
2-(1-methylpyridinium-2-yl)ethene iodides where the heteroaryl
group was a thienyl, Br-thienyl or pyrrolyl group [10]. In the present
work we extend the photophysical and photochemical investiga-
tionto some1,2-diarylethenes whereonearyl groupis apyridinium
or imidazolinium iodide and the other one is a phenyl, a furan-
2-yl or a 5-Ar-furan-2-yl (with Ar = o-Cl- or p-Br-phenyl) group.
The behaviour of two distyrylbenzene analogues with a central
2,6-substituted pyridinium group and side furan-2-yl and pyrrol-
2-yl groups was also studied. Aim of this work was to study the
competitive relaxation processes of the lowest excited states of
these compounds, examining also the occurring of ICT processes.
For some of them, the reactive trans/cis process was found to pre-
vail whereas internal conversion is the dominant deactivation for
others and the radiative pathway is generally very scarce.
It was also interesting to measure their binding affinity towards
DNA. When these salts bearing one or more isomerizable double
bonds are associated to DNA, their photobehaviour (isomerization
quantum yield and photostationary state, pss, composition) may
change significantly because of the different binding affinity of DNA
towards the trans or cis geometrical isomer [2]. The results obtained
on the interaction between DNA and the six salts here investigated
showed a modest binding affinity for the pyridinium derivatives
with one ethenic bond and two aromatic rings. The complexation
constant increased substantially in compounds with two ethenic
bonds (three aromatic rings) and in the halogen-substituted com-
pounds. The changes in the competition of the radiative/reactive
relaxation from the lowest excited singlet state induced by the
formation of ligand–DNA complexes were also investigated.
2. Experimental
Theoretical calculations of the electronic spectra, conforma-
tional equilibria [16] and heats of formation of the cations (see
Appendix A) were carried out using the HyperChem computa-
tional package (version 7.5). The computed transition energies and
oscillator strengths were obtained by ZINDO/S using optimized
geometries (according to PM3 method) with 9 × 9 configuration
interaction.
The trans isomers investigated (compounds 3 and 5–8 in
Scheme 1) have been synthesized at the Catania laboratory. The
synthesis, spectroscopic characterization as well as preliminary
in vitro antitumor tests have been reported elsewhere [3,4,11].
The styryl compound (1) was prepared at the Perugia laboratory

68
C.G. Fortuna et al. / Journal of Photochemistry and Photobiology A: Chemistry 216 (2010) 66–72
Table 1
Spectral properties of the iodide salts 1–8 in buffered (ETN) water and solvatochromic properties [red-shift from water (W) to chloroform (Ch)].
max
ab
max
ab
Compound
ꢀ
in W (nm)
ꢀmF ax (nm)
Stokes shift (cm−1
)
ꢀ
in Ch (nm)
−ꢂꢀ(Ch–W) (cm−1
)
1
2
3
4
5
6
7
8
333
365a
367
412a
402
447
359
368
440
480a
460
510a
530
585
458
470
7300
6560
5510
4660
6010
5280
6020
5900
347
382
384
442
421
483
372
380
1210
1220
1210
1650
1120
1670
970
860
a
From Ref. [10].
3. Results and discussion
CH3
+
N
3.1. Spectral properties
N
+
CH3
The spectral data of the six compounds investigated are col-
lected in Table 1 while Fig. 1 shows their absorption spectra in
max
1-A
1-B
buffered (ETN) water. After a careful check, some ꢀ
values
ab
in Table 1 were now corrected with respect to those previously
reported [4a,11]. Replacement of the phenyl group of 1 by pen-
tatomic groups leads to a net red-shift of the absorption/emission
spectra (see Section 3.4). An even higher bathochromic shift was
observed for compounds with two ethenic bonds, two side donor
groups and a central acceptor group (D–E–A–E–D, 5 and 6).
Scheme 2. Conformational equilibrium of compound 1.
3.2. Conformational equilibria
The compounds investigated are expected to exist in solution
as mixtures of different conformations [16]. The conformer equi-
librium for the reference compound 1 is shown in Scheme 2. The
calculations (Appendix A) indicated that the conformer A has the
smallest formation enthalpy difference (ꢂHf◦) and is the prevalent
species in solution.
Also in compounds 2–4 (four possible conformers, see the case
of compound 3, as an example, in Scheme 3), the most stable species
are those of the A type, with a prevalence of the s-cis species
(produced by rotation around the quasi-single bond with the pen-
tatomic ring), as previously reported for the thienyl (2) and pyrrolyl
(4) derivatives [10].
In the case of compounds 5 and 6 with two ethenic bonds (10
possible conformers), the computed ꢂHf◦ values indicated a preva-
lence of the three elongated species with a largest abundance for
the s-cis, s-cis species, particularly for 6 (86%). Rotation around the
quasi-single bond adjacent to the pyridinium group leads to species
of much higher energy, strongly deviating from planarity (see semi-
compressed and compressed species in Scheme 4 for compound 5,
as an example). Finally, for compounds 7 (four conformations) and
8 (two possible species), the s-cis conformer is largely prevalent.
No sign of dimerization or association was observed in the
dilute solutions examined. The computed spectral properties (see
Appendix A Supplementary Data) were in reasonable agreement
with the experimental positions and intensities of the absorption
bands. In all compounds, the first band corresponds to a ␲,␲*
HOMO → LUMO electronic transition with partial CT character.
As to the very weak fluorescence spectra, a strong Stokes shift
with respect to the corresponding absorption bands was observed,
indicating that the emitting state has a relaxed geometry rather
different from that of the Franck–Condon state reached by absorp-
tion. Contrary to the absorption spectrum, which shifts to the blue,
the emission spectrum does not shift or shifts slightly to the red
in polar solvents. This behaviour points to an excited state of very
small dipole moment in agreement with the result of a deep inves-
tigation [17] on the fluorescence behaviour of the 4-pyrydinium
analogue of our compound 4 in 20 solvents of varying solvent polar-
N
ity parameter, ET [18].
1
3
5
CH3
N
5
6
4
2
0
6
7
8
7
8
N
+
X
O
CH3
O
39%
A (s-trans)
B (s-trans)
1
3
CH3
+
N
O
O
N
X
+
CH3
200
300
400
500
61%
λ
(nm)
A (s-cis)
B (s-cis)
Fig. 1. Absorption spectra of compounds 1, 3, 5 and 6–8 in buffered (ETN) water at
room temperature.
Scheme 3. Possible conformers of compound 3.

C.G. Fortuna et al. / Journal of Photochemistry and Photobiology A: Chemistry 216 (2010) 66–72
69
O
O
O
N
N
N
+
+
+
CH3
CH3
CH3
O
O
O
elongated
semicompressed
compressed
Scheme 4. Conformers of compound 5 produced by rotation around the bond with the central pyridinium group.
Table 2
ation pathway is very weak, its yield being <0.001 (except for
compound 5 with ꢁF = 0.006) with a consequent scarce help of
the usually informative fluorescence properties for the interpreta-
tion of the relaxation mechanism. The photoreactivity (rotation of
one double bond, trans,trans → cis,trans) of the two compounds of
the di-(arylethenyl)pyridinium series, 5 and 6, is very small, again
similarly to the corresponding neutral molecules [20], but with a
modest compensation of the usually coupled radiative deactivation
whose yield remains small, even if, for 5 (bearing two side furan
rings), it increases by more than one order of magnitude. In any
case, internal conversion is the main deactivation channel of these
quadrupolar molecules.
Quantum yields of the radiative and reactive relaxation of the iodide salts of trans-2-
styryl-(1-methylpyridinium) and some hetero-analogues in buffered (ETN) water.
Compound
ꢁF (×104)
ꢁt → c
ꢁc → t
1
2a
3
3
1.2
4
4.0
60
4
7
8
0.40
0.60
0.52
0.37
0.04
∼0.01
0.44
0.49
0.20
0.22
0.13
0.46
0.26
∼0.5
0.16
0.25
4a
5
6
7
8
a
From Ref. [10].
As to the back photoisomerization quantum yield from cis to
trans (ꢁc → t values reported in Table 2), it is of the same order
than the direct one (ꢁt → c) for compound 4 only; for the others
it is smaller but still substantial. An efficient radiationless pathway
contributes to the deactivation of the S1 state of the cis isomers.
Interestingly, irradiation of the trans, cis isomer of 5 led prevalently
to trans, trans but a small yield (∼0.7%) of the cis,cis isomer was also
detected. The photochemical formation of cis,cis is rather unusual
in the series of distyrylbenzene analogues [21].
The fluorescence excitation spectra did well overlap the cor-
responding absorption spectra (see two examples in Fig. 2) as
expected from the calculations that support the prevalence of one
conformer in solution. In our experimental conditions, the fluo-
rescence lifetime (ꢃF = 0.12 ns) was measurable for 5 only, which
has the highest ꢁF value. The monoexponential decay indicates
that the prevalent elongated conformers (quasi-isoenergetic) have
undistinguishable emission properties.
The quantum yield of the reactive deactivation was measured
also in MeCN. Generally, it tends to increase in the non-protic sol-
vent, particularly for the pyridinium salts bearing the furyl group, 3
and 5. The ꢁt → c value goes from 0.52 to 0.75 for 3 and from 0.05 to
0.25 for 5. This behaviour, reported also for the 4-pyridinium posi-
tional isomer of our 2-pyridinium derivative (4) mentioned above
[17], could be tentatively explained by the interaction of water with
the oxygen of the furan ring which opposes the twisting around the
double bond leading to isomerization.
3.3. Radiative and reactive relaxation quantum yields
Table 2 shows the quantum yields of the radiative and reactive
relaxation of the compounds investigated. The main deactivation
pathway of the compounds of the diarylethene series is trans → cis
photoisomerization, similarly to the related neutral molecules
[19]. The photobehaviour of our reference compound, 1-methyl-
2-styrylpyridinium iodide (1) in buffered (ETN) water is similar to
that previously reported for the protonated compound (ꢁF = 0.003,
ꢁt → c = 0.55 and ꢁc → t = 0.28) in 50/50 water/ethanol at pH 2 [19b].
The presence of a pentatomic group (2T, 2F, 2Py) in compounds
2–4, 7 and 8 affects the fluorescence/isomerization competition at a
negligible extent. Their common characteristic is a high trans → cis
photoisomerization quantum yield, which, according to the dia-
batic mechanism, which implies a trans → perp quantum yield
almost the double of the trans → cis yield, approximately accounts
for the absorbed quanta. In fact, the competitive radiative relax-
3.4. Intramolecular charge transfer effects
Asymmetrical 1,2-diarylethenes where the two side aryl groups
have different D/A properties are generally characterized by high
polarizability and hyperpolarizability and can behave as NLO sys-
tems. An exhaustive investigation carried out on stilbene-like
molecules bearing quaternized azine groups (mainly of the tri-
flate salts of 4-methylpyridinium analogues) demonstrated that
excitation
emission
excitation
absorption
emission
1
5
absorption
300
400
(nm)
500
300
400
500
(nm)
600
700
λ
λ
Fig. 2. Fluorescence emission and excitation spectra of 1 and 5 in buffered (ETN) water at room temperature (absorption spectra shown for comparison).

70
C.G. Fortuna et al. / Journal of Photochemistry and Photobiology A: Chemistry 216 (2010) 66–72
0.6
0.4
0.2
0.0
5 + DNA
7 + DNA
421 nm
387 nm
300
350
400
450
350
400
450
500
λ (nm)
λ (nm)
Fig. 3. Absorption spectra of compounds 5 and 7 (corrected for dilution) recorded after successive addition of a DNA solution up to 8 × 10−4 M.
Table 3
the spectral shifts induced by an increase in the donating proper-
ties of D, the quaternization and the solvent polarity can be useful
parameters indicating them as suitable candidates for components
of materials having interesting two-photon absorption properties
and frequency up-converted lasing activity [1,22].
Complexation constants of compounds 1, 3 and 5–8 with DNA in buffered (ETN)
water at room temperature.
Compound
K (M−1
)
n
1
3
5
6
7
8
2700
5700
13,600
58,500
58,000
119,000
6.5
7.5
3.0
1.1
3.4
1.6
A comparison of the spectral behaviour of diarylethenes 2–4, 7
and 8 with that of the corresponding phenyl derivative 1 (Table 1)
shows that the red shift induced by the heterocyclic group (see
Section 3.1) is substantial for all compounds and greatest for the
pyrrolyl-derivative 4, which has the best donor properties. The shift
becomes even larger in the quadrupolar compounds 5 and 6 with a
2,6-disubstituted pyridinium group. This indicates that these com-
pounds can display interesting NLO properties [1,22]. An evaluation
of the interaction between the two D/A heterocycles and conse-
quent ICT effects in the present compounds was roughly obtained
by measuring the dependence of the absorption spectrum on the
solvent polarity. Solubility problems drastically limits the range of
the solvent to be investigated. In any case, measurements in Ch,
MeCN and water showed that the absorption maximum shifts to
shorter wavelengths and the absorption coefficient εmax decreases
on increasing the solvent polarity. It has to be noted that differ-
ent solvents may also affect the equilibrium between conformers
discussed in Section 3.1. The negative solvatochromism shown in
Table 1 for all of our compounds indicates a lower dipole moment
in the excited state, due to the CT character of the electronic tran-
sition. This behaviour is in agreement with the dipole moment of
the excited state roughly estimated by calculations (see Appendix
A). The hypsochromic shift in water with respect to Ch confirms
the push–pull character of the compounds investigated. The shift
increases in compounds bearing the best accepting pyrrole ring (4
and 6).
A treatment of the absorbance changes according to the equation
of McGhee and von Hippel [15]:
ꢀ
ꢁ
n−1
ꢄ
c
1 − nꢄ
= K(1 − nꢄ)
1 − (n − 1)ꢄ
where c is the concentration of the free ligand and ꢄ is the binding
density (i.e. the ratio between the bound ligand and DNA) allowed
the complexation constant (K) and the number of the binding sites
excluded when the ligand is bound to DNA (n) to be estimated
(Table 3). An example of the treatment for compound 8 is shown in
Fig. 4.
The K values reported [10] for compounds 2 and 4 were of
the order of 3500 M−1; the little reproducible n values were not
reported there.
The ligand–DNA complexation constants are in the range
3–6 × 103 in the case of the pyridinium derivatives with one ethenic
bond (1–4). These values are probably too low for therapeutic appli-
cation (K values reported for analogous compounds are higher by
one order of magnitude or more [2]). The K value increases to the
3.5. Interactions with DNA
K = 119000 3000
n = 1.55 0.04
R2 = 0.974
100000
80000
60000
40000
Spectrophotometric measurements showed that all the trans
isomers investigated interact with DNA, as evidenced by the red-
shift and the intensity decrease of their absorption maximum under
successive additions of DNA to a constant olefin concentration
(∼2 × 10−5 M). For compounds 1 and 3 the spectral shift was very
small indicating that the complex formation involves relatively
weak interactions. Much more evident spectral changes were found
for 5 and 6. Two examples of the spectral evolvement due to the for-
mation of trans-diarylethene-DNA complexes are shown in Fig. 3
for compounds 5 and 7.
λan =368 nm
0.1
0.2
0.3
0.4
By increasing substantially the DNA concentration, a final spec-
trum was obtained which was assumed to correspond to an
equilibrium completely shifted towards the complexed form. Its
maximum is shifted towards the red by about 20 nm in both cases.
ν
Fig. 4. Plot to obtain the association constant of the system 8-DNA according to the
equation of McGhee and von Hippel [15].

C.G. Fortuna et al. / Journal of Photochemistry and Photobiology A: Chemistry 216 (2010) 66–72
71
Table 4
Spectral properties and fluorescence quantum yields of the complexes with DNA in buffered (ETN) water.
max
ab
Compound
ꢀ
(nm)
εmax (M−1 cm−1
)
ꢀmF ax (nm)
ꢀisosb (nm)
ꢁF (×104)
1
3
5
6
7
8
340
372
425
476
376
381
20,800
21,400
34,700
16,600
9100
440
455
515
569
464
474
349
386
420
474
387
400
4.0
3.0
200
130
46
30
13,000
range 1–6 (×104) for the di-substituted compounds, 5 and 6, and
is even higher for the imidazole derivatives, 7 and 8. The high K
values found for the compounds with two ethenic bonds (5 and 6)
and those halogen-substituted (7 and 8) are interesting and can be
related to the observation that the presence of three aromatic rings
and of halogen substituents might increase the anti-proliferative
activity [4b]. Indeed, the latter, tested against two tumor cell lines
(breast and prostate carcinomas) following the method described
elsewhere [3], has been reported to be particularly high for these
compounds. The n value in Table 3 is near to 1 (typical of inter-
calated complexes [15]) for two compounds only, indicating that
intercalation of the ligand is probable for 6 and 8, whereas in the
other cases groove binding is favoured in our experimental condi-
tions.
1.0x105
excitation
emission
compound
free
5
+DNA
5.0x104
0.0
400
500
600
700
3.6. Photobehaviour of the complexes with DNA
λ
(nm)
When styrylpyridinium ligands are organized on the DNA, the
photophysical and photochemical relaxations of the excited state
can change substantially with respect to that of the free molecule,
depending on the aryl moiety properties of the ligand and its DNA-
binding mode [2].
Fig. 5. Fluorescence intensity of the free and DNA-complexed compound 5 in
buffered (ETN) water at room temperature.
In our previous work on the thienyl (2) and pyrrolyl (4) deriva-
tives, characterized by rather small K values (of the order of few
thousands M−1), it was found that the emission yield of the com-
plexes remained almost unchanged for 2 but increased markedly
for 4 [10]. Table 4, which collects the spectral properties and
emission yields of the new compounds investigated (1, 3 and
5–8), shows that the radiative relaxation is always (more or less)
favoured by complexation (compared with data in Table 2). The
complexes of compounds 5 and 6 with two quadrupolar sys-
tems (three heterocyclic groups) display the largest bathocromic
absorption/emission shift and, interestingly, the highest fluores-
cence yields, probably due to an increase of planarity of the ligand
in the complex. The observed fluorescence enhancement, particu-
larly efficient for 5–8, reflects a reduced flexibility of the complexed
ligands which could affect both the reactive and non-reactive radia-
tionless deactivations. However, it should be noted that the quanta
involved in the emission process remain a too small fraction to
involve sizable effects on the competitive processes (see below).
An example of the changes in the fluorescence yield is shown
for compound 5 in Fig. 5.
incertitude and no clear coupling between the radiative/reactive
relaxation was found in the case of these two compounds [10].
We have here investigated the effect of the interaction with DNA
on photoisomerization for two compounds having a small K value
and high n value (compound 3) and viceversa (compound 8). It was
not possible to investigate compound 6 (with n = 1.1) since it prac-
tically did not isomerize even in the absence of DNA (Table 2). The
results obtained were ꢁE → Z = 0.47 and 0.55 for 3 and 8, respec-
tively. The changes found with respect to the free ligands were of
the order of magnitude of the experimental uncertainty thus indi-
cating that the interaction (both intercalation and groove binding
modes) leaves the reaction center enough space to undergo twist-
ing of the double bond. In the case of intercalation, prevalent for
8, this behaviour should imply an incomplete insertion of the lig-
and into the intercalation pocket so that the protruding part of the
ligand has enough space to isomerize.
A detailed investigation on the photoreactivity of these com-
plexes will be the subject of a next paper.
As to the reactive relaxation, it has been reported that DNA
affects the photostationary state (pss) composition through its
preference in binding affinity towards trans or cis isomers [2]. In
the diluted ligand solutions used in this work (∼10−5 mol dm−3),
irradiation gave only a mixture of the two geometrical isomers
without formation of cyclobutane photodimers, which were often
detected in styrylpyridinium salt photochemistry [2]. In our previ-
ous paper on compounds 2 and 4, it was found that complexation
did not alter the photobehaviour substantially. In the case of com-
pound 2, both the pss ([Z/][E]) composition and E → Z quantum yield
decreased with respect to the free ligand while a quite larger red-
shift of the absorption spectrum but a negligible decrease of ꢁE → Z
was reported for compound 4. It has to be noted that both the K
and ꢁF values of these compounds were characterized by a large
4. Conclusions
The information obtained gives a general description of the S1
relaxation of this series of salts. The stilbene analogues display a
photobehaviour similar to that of stilbene, that is a largely preva-
lent relaxation by essentially diabatic isomerization and an even
smaller competition by fluorescence. On the contrary, the pho-
tobehaviour of the two distyrylbenzene analogues (5 and 6) is
quite different from that of the corresponding neutral compounds
(2,6-distyrylpyridine and 1,3-distyrylbenzene) where the preva-
lent deactivation path is the radiative one and the photoreactivity
is very low or quite negligible. In the present case, both main relax-
ation processes are little efficient and internal conversion becomes
the prevalent deactivation process.

72
C.G. Fortuna et al. / Journal of Photochemistry and Photobiology A: Chemistry 216 (2010) 66–72
The complexation with DNA did not change much the photobe-
[4] (a) F.P. Ballistreri, V. Barresi, G. Consiglio, C.G. Fortuna, M.L. Longo, G.
Musumarra, ARKIVOC, Part (i) (2003) 105–117;
haviour of this series of free ligands. In any case, a general tendency
to favour the very small yield of the radiative decay and to decrease
the yield of the reactive pathway was observed but the coupling of
the radiative/reactive relaxation, typical of stilbenoid compounds,
was not clearly verified in these poorly emitting complexes. How-
ever, it is probable that the decay is complicated by a different
contribution of the internal conversion in the relaxation of the
complexed ligands.
The results obtained indicate that the structure/property rela-
tionship (spectral and DNA-complexation properties) are in some
way related to other properties useful for material applications. Sol-
vatochromism indicators can be used as probes of the ICT and its
potential applications in the field of two photon pumping and fre-
quency up-converted lasing activity [1,22]. On the other hand, the
complexation constants with DNA can provide a probe for poten-
tial antitumor activity [2,4b]. The promising results obtained so far
need gaining deeper insight into the ground state properties and
the photobehaviour of this series of compounds.
(b) F.P. Ballistreri, V. Barresi, P. Benedetti, G. Caltabiano, C.G. Fortuna, M.L.
Longo, G. Musumarra, Bioorg. Med. Chem. 12 (2004) 1689–1695.
[5] (a) D.G. Whitten, J.C. Russell, R.H. Schmehl, Tetrahedron 38 (1982) 2455–2487;
(b) D.G. Whitten, Acc. Chem. Res. 26 (1993) 502–509.
[6] H. Görner, A. Fojtik, J. Wróblewski, L.J. Currell, Z. Naturforsch 40a (1985)
525–537;
H. Görner, H. Gruen, J. Photochem. 28 (1985) 329–350.
[7] H. Güsten, D. Schulte-Frohlinde, Z. Naturforsch 34b (1979) 1556–1558.
[8] J. Saltiel, Y.-P. Sun, in: H. Dürr, H. Bouas-Laurent (Eds.), Photochromism:
Molecules and Systems, Elsevier, Amsterdam, 1990, pp. 64–162 (and references
therein).
[9] (a) M.A. Robb, F. Bernardi, M. Olivucci, Pure Appl. Chem. 67 (1995) 783–789;
(b) M.J. Bearpark, F. Bernardi, S. Cliford, M. Olivucci, M.A. Robb, T. Vreven, J.
Phys. Chem. A 101 (1997) 3841–3847.
[10] E. Marri, U. Mazzucato, C.G. Fortuna, G. Musumarra, A. Spalletti, J. Photochem.
Photobiol. A: Chem. 179 (2006) 314–319.
[11] M. Fichera, C.G. Fortuna, G. Impallomeni, G. Musumarra, Eur. J. Org. Chem.
(2002) 145–150.
[12] G. Galiazzo, Gazz. Chim. It. 95 (1965) 1322–1334.
[13] G. Bartocci, F. Masetti, U. Mazzucato, A. Spalletti, I. Baraldi, F. Momicchioli, J.
Phys. Chem. 91 (1987) 4733–4743.
[14] P. Gajdek, R.S. Becker, F. Elisei, U. Mazzucato, A. Spalletti, J. Photochem. Photo-
biol. A: Chem. 100 (1996) 57–64.
[15] J.D. McGhee, P.H. von Hippel, J. Mol. Biol. 86 (1974) 469–489.
[16] (a) U. Mazzucato, F. Momicchioli, Chem. Rev. 91 (1991) 1679–1719;
(b) G. Bartocci, A. Spalletti, U. Mazzucato, in: J. Waluk (Ed.), Conformational
Analysis of Molecules in Excited States, Wiley-VCH, New York, 2000 (Chapter
5 and references therein).
[17] C.-L. Zhan, D.-Y. Wang, J. Photochem. Photobiol. A: Chem. 147 (2002) 93–101.
[18] C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, 2nd ed., VCH,
Weinheim, 1988.
Acknowledgements
The authors thank Dr. A. Mazzoli for her technical assistance in
the final part of this work.
Appendix A. Supplementary data
[19] (a) U. Mazzucato, Pure Appl. Chem. 54 (1982) 1705–1721 (and references
therein);
(b) G. Bartocci, P. Bortolus, U. Mazzucato, J. Phys. Chem. 77 (1973) 605–610;
(c) G. Marconi, G. Bartocci, U. Mazzucato, A. Spalletti, F. Abbate, L. Angeloni,
E. Castellucci, Am. Inst. Phys. Conf. Proc 364 (1996) 175–182 (and references
therein).
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2010.09.007
[20] L. Giglio, U. Mazzucato, G. Musumarra, A. Spalletti, Phys. Chem. Chem. Phys. 2
(2000) 4005–4012.
References
[21] E. Marri, D. Pannacci, G. Galiazzo, U. Mazzucato, A. Spalletti, J. Phys. Chem. A
107 (2003) 11231–11238 (and references therein).
[22] (a) G.S. He, L. Yuan, P.N. Prasad, A. Abbotto, A. Facchetti, G.A. Pagani, Opt. Com-
mun. 140 (1997) 49–52;
[1] S. Bradamante, A. Facchetti, G.A. Pagani, J. Phys. Org. Chem. 10 (1997) 514–524
(and references cited therein).
[2] B. Juskowiak, M. Chudak, Photochem. Photobiol. 79 (2004) 137–144 (and ref-
erences cited therein).
[3] V. Barresi, D.F. Condorelli, C.G. Fortuna, G. Musumarra, S. Scirè, Bioorg. Med.
Chem. 10 (2002) 2899–2904.
(b) A. Abbotto, L. Beverina, R. Bozio, A. Facchetti, C. Ferrante, G.A. Pagani, D.
Pedron, R. Signorini, Org. Lett. 4 (2002) 1495–1498 (and references therein).