Photoisomerization and Photocyclization of 5-Styryloxazole

Article abstract of DOI:10.5562/cca2443

This paper describes the competition of the relaxation processes (fluorescence, isomerization and cyclization) of 5-styryloxazole in the first excited singlet state. The study is carried out under mild conditions and in the first stage of irradiation to compare the photobehaviour with that reported in a previous work on preparative scale. The quantum yields of the radiative and reactive deactivation are compared with those of the analogous compounds containing pentatomic heteroaryl groups.

Full text of DOI:10.5562/cca2443

CROATICA CHEMICA ACTA
CCACAA, ISSN 0011-1643, e-ISSN 1334-417X
| hkd ^ROATICA
1926^1 h EMICA
Croat. Chem. Acta 87 (4) (2014) 327-333.
T
http://dx.doi.org/10.5562/cca2443
Original Scientific Article
Photoisomerization and Photocyclization of S-Styryloxazole1^
Ivana Šagud,3Marija Šindler-Kulyk,a* Anna Spalletti,b* and Ugo Mazzucatob
aDepartment ofOrganic Chemistry, Faculty ofChemical Engineering and Technology, University ofZagreb,
Marulićev trg 19, HR-10000 Zagreb, Croatia
bDepartment ofChemistry, Biology andBiotechnologies and Centro di Eccellenza Materiali lnnovativi Nanostrutturati
(CEMIN), University ofPerugia, Piazza Universita 1, 06123 Perugia, Italy
RECEIVED MARCH 19,2014; REVISED MAY 6,2014; ACCEPTED MAY 9,2014
Abstract. This paper describes the competition of the relaxation processes (fluorescence, isomerization
and cyclization) of 5-styryloxazole in the first excited singlet state. The study is carried out under mild
conditions and in the first stage of irradiation to compare the photobehaviour with that reported in a previ-
ous work on preparative scale. The quantum yields ofthe radiative and reactive deactivation are compared
with those of the analogous compounds containing pentatomic heteroaryl groups.
Keywords: electrocyclization, fluorescence, oxazoles, photochemistry
INTRODUCTION
toisomerization of the more stable E isomer, is generally
accepted to occur in the singlet manifold. In the case of
the model molecule stilbene, photocyclization leads to
an unstable coloured 4a,4b-dihydrophenanthrene (DHP)
which, in the presence of an oxidant, loses the two hy-
drogen atoms in trans stereochemistry to give the stable
polynuclear phenanthrene-type (P) arene.4-6 At least
(see later) four components (E, Z, DHP and P) are thus
generally found in an irradiated solution of the E or Z
isomers.
The competition between the radiative (fluorescence)
and reactive (geometrical isomerization and cyclization)
relaxation of stilbene and its analogues has been a sub-
ject widely investigated in the last decades as model
photoprocesses to understand the dynamics of photoin-
duced molecular rearrangements.1
The E -y Z isomerization occurs generally by the
well known diabatic mechanism implying twist around
the ethylenic bond towards an energy minimum at the
perpendicular configuration (perp), at about 90°, fol-
lowed by a Si -» So internal conversion (IC) or Ti -> S0
intersystem crossing (ISC) and relaxation to the ground-
state trans (E) and cis (Z) isomers in roughly a 1:1
branching ratio. The photoreaction can be thus schema-
tised: UE* —» 1,3perp* —» 'perp -> a'Z + (l-a)'E, where
the partitioning factor a is assumed to be ~0.5 in most
cases.2 In some less common cases, specific structures
may stabilize the Si state of the E geometry and increase
the energy barrier to twisting thus leading to a practical-
ly photostable 1,3E* state and a 1,3Z* isomer able to
reach the fluorescent E* state on the same potential
energy curve (adiabatic mechanism, I>3Z* -> 1>3E*),
followed by the relaxation pathways of l,3E* seen
above.3
The ratio of the isomerization/cyclization quantum
yields ( ^ z ->e / ^ z -> d p ) depends on the Z structure. In the
h
case of stilbene, the ratio in non-polar solvents is about
70:30 %. However, only one third of the cyclization
pathway in the excited state leads to the metastable
ground state DHP (half life, tm, of the order of 100
minutes) while the rest reverts to Z with the result of a
Z -» DHP quantum yield of about 10 % in non-polar
solvents.7-9 In the dark and in the absence of oxidants,
or under irradiation, DHP undergoes ring opening and
return to Z, whereas in the presence of oxidants (e.g.,
oxygen or iodine) it converts thermally and irreversibly
to the cyclic product P by hydrogen elimination,
sometimes in competition with ring opening to Z.
In some particular cases, more than one DHP can
be present in the mixture since the initial DHP, general-
ly in anaerobic conditions, can produce an isomeric
intermediate, DHP’, which does not revert to Z and
loses (sometimes slowly in deaerated media) the hydro-
While the cis-trans (Z-E) photoisomerization can
occur in both the singlet and triplet states, photocycliza-
tion, which starts from the Z isomer produced by pho-
t Dedicated to Dr. Mirjana Eckert-Maksić on the occasion of her 70thbirthday.
* Authors to whom correspondence should be addressed. (E-mail: anna.spalletti@unipg.it; msindler@fkit.hr)

I. Šagud et al., Photoisomerization andPhotocyclization of5-Styryloxazole
328
obtained for the first three cyclized products (naphthox-
azole, furobenzoxazole and thienobenzoxazole).22
The present paper deals with the quantum yields
of the competitive photoreactions (isomerization/cy-
clization) of 5-StOx and its /j-methoxy derivative,
5-(4-methoxystyryl)oxazole (p-OMe-5-StOx). Our main
aim is the study of the photoisomerization and photode-
hydrocyclization under mild conditions, starting from
the E isomers, to get information on the quantum yields
and photoreaction mechanism in the primary steps of
irradiation. The results obtained will be briefly com-
pared with those reported for stilbene and with the
scarce quantitative information available for five-
membered heteroaryl analogues.
gen atoms to give the final stable P-type photoproduct.
The process implies an intramolecular l,«-hydrogen
shift5 which can be base-catalysed, e.g. by a primary
amine, the latter operating as the transferring agent.10
The photobehaviour of stilbene-analogues contain-
ing one or two heteroatoms, comparatively less studied
with respect to stilbene, is interesting because the pres-
ence of heteroatoms can significantly affect the photo-
reaction mechanism. Pioneering work on compounds
bearing pyridine or pentatomic groups [fiiran (F), thio-
phene (T) and pyrrole (Py)] has been carried out in the
‘60s by various groups.11-13 More recent papers deal
particularly with the solvent and oxygen dependent
photocyclization pathways and with the competitive
photoreactions induced by l,«-hydrogen shifts in stil-
bene-like compounds containing the pyridyl group14 or
five-membered heterocycles.15"18 In fact, the photocy-
clization of styryl derivatives of F, T and Py bearing
various substituents at the phenyl ring and/or in specific
media was described to be accompanied by high chemi-
cal yields of competitive photoproducts implying a
novel skeletal rearrangement. In a study of the para-
substituted Z-styrylfuran in dichloromethane15 a good
chemical yield of 5-(3-oxo-l-butenylbenzo[b]furan was
found in the absence of oxygen and explained by the
fact that the furan oxygen might cause an higher acidity
of the hydrogen atom that initiates a [l,9]-hydrogen
shift, followed by ring opening and rearrangement to
this new final product. Similar results were obtained for
styrylthiophene and styrylpyrrole analogues.16,17 A de-
tailed flash photolysis study of 4’-benzyl-5-styrylfuran
in DCM allowed to detect the spectra of all the interme-
diates and again reported the abundant competitive
formation of the butadienyl derivative in the absence of
oxygen.18 Another group studied the dye-sensitized
photooxidation of 3-styrylthiophene and described the
prevalent formation of the P-type product accompanied
by formation of various oxidation products via a super-
oxide radical anion pathway.19
The different n,n* excited state distribution and
different sensitivity to oxidants relative to stilbene
makes pyrrole and other Ti-excessive five-membered-
ring derivatives easily decomposable unless a protective
electron-acceptor substituent is present.20
The photobehaviour of stilbene analogues where a
phenyl group is replaced by a pentatomic group with
two heteroatoms, such as oxazole (Ox), thiazole (Th)
and imidazole (Im) derivatives has been little investi-
gated. A paper on styrylbenzothiazole describes the ring
size effects on its photochemistry and photophysics.21
An extensive study of 5-styryloxazole (5-StOx) and
analogous compounds where the other heteroaryl group
was furan or thiophene, was recently carried out by
some of us in the presence of iodine until full conver-
sion. Similar good chemical yields (50-60 %) were
EXPERIMENTAL
The compounds investigated (Scheme 1) were prepared
by the Van Leusen reaction from corresponding cinnam-
aldehydes and TosMIC reagent in the Zagreb laboratory
as described previously.22 The solvents used were cy-
clohexane (CH) and acetonitrile (MeCN) from Sigma-
Aldrich. The use of combined HPLC and UV spectro-
metric techniques allowed the separate spectra of the
components, produced under irradiation of solutions of
the starting E isomers, to be obtained.
X = H (5-StOx)
X
X = OCH3 (p-OMe-5-StOx)
Scheme 1. Investigated compounds.
A Perkin-Elmer Lambda 800 spectrophotometer
was used for the absorption measurements. The fluores-
cence spectra were measured with a Spex Fluorolog-2
F112AI spectrofluorimeter using dilute solutions (ab-
sorbance <0.1 at the excitation wavelength, Z^c)- The
fluorescence quantum yield (</>f, uncertainty ± 15 %)
was determined at /Uxc corresponding to the maximum
of the first absorption band (/Wx) using a-NPD (</>f =
0.70)23in de-aerated CH as fluorimetric standard.
For photochemical measurements (potassium fer-
rioxalate in water was used as actinometer) a 150 W
high pressure xenon lamp coupled with a monochroma-
tor was used. The photoreaction (solute concentrations
~10-4 mol dm"3) was monitored by HPLC using a Wa-
ters apparatus equipped with analytical Symmetry C l8
(4.6 x 250 mm; 5 pm) and Prontosil 200-3-C30 (4.6 x
250 mm; 3 pm) columns and UV diode-array detector.
The procedure used to measure the photoreaction
quantum yields was as follows. Diluted solutions of the
Croat. Chem. Acta 87 (2014) 327.

I. Šagud et al., Photoisomerization andPhotocyclization o f5-Styryloxazole
329
E isomers in de-aerated (by bubbling nitrogen or argon)
cyclohexane were irradiated up to a maximum conver-
sion percentage of 10 % to minimize competition of the
back photoreactions. To follow the photoreactions of the
Z isomers (not available as pure separated compounds)
irradiation was initially performed at 313 nm (5-StOx)
and 333 nm (OCTE-derivative) to reach a photostation-
ary state enriched in the Z isomer (in the presence of a
side source of visible light to destroy DHP and avoid P
formation at this stage) and then at 254 nm where the
absorption of the latter largely prevails. Samples of the
irradiated solutions, analyzed immediately after the
removal of the irradiating source by combined HPLC
and spectrophotometric techniques, generally contain
four components: E, Z, DHP and small amounts of
P-type dehydrogenated arene. The air-equilibrated
solutions were then left overnight in the dark to allow
the decay of the DHP intermediates to go to comple-
tion. The residual components were separated by
HPLC (using acetonitrile as eluent, a small addition of
water being necessary in some cases to better separate
the chromatographic peaks) and identified spectropho-
tometrically.
In the case of the p-OMe derivative, where the
possible conformers of the Z isomer have comparable
abundances (see Results and discussion), the quantum
yields of E and P formation (tpz->e and <fe->p) measured
at Aexc = 254 nm, could be affected by different absorp-
tion spectra of the two conformers. In this case, the
reported quantum yields are to be considered mean
values for the conformer mixture.
The thermal stability of DHPs, usually expressed
by the half-lives (ty2) at room temperature, was meas-
ured by following spectrophotometrically the disappear-
ance of the visible absorption band in both aerated and
de-aerated (by bubbling nitrogen or argon) CH. The
quantum yields reported in the Tables are generally the
averages of three independent experiments with mean
deviations of ca. 5 % for <p^z. Higher uncertainties
(10-15 %) were found in the more delicate measure-
ments of the photoreactions starting from the Z isomer
(<pz-*Eand <pz->p), particularly considering the very small
cyclization yield.
Figure 1. Absorption spectra of the two geometrical isomers
(E and Z) of 5-StOx in CH together with the DHP intermedi-
ate and its oxidation P-type product.
Total energies and dipole moments were obtained for
geometries optimized by HF ab initio method (3-21-G
level). The computed transition energies and oscillator
strengths were obtained by ZINDO/S at the optimized
geometries, the configuration interaction including 400
(20 x 20) single excited configurations.
RESULTS AND DISCUSSION
5-styryloxazole (5-StOx)
Figure 1 shows the absorption spectra of E-5-StOx and its
photoproducts in CH while Table 1 collects the main
spectral properties of the E and Z isomer, respectively,
together with those of the p-OMe derivative, in CH and
MeCN. The spectra of the Z isomer and the DHP-type
intermediate were obtained by normalization at the Z/E
and DHP/P isosbestic points, respectively. The spectrum
of the final P-type product in CH was considered to have
the same absorption coefficient as that previously report-
ed in EtOH.22 While the P spectrum displays a slightly
structured shape, the DHP spectrum shows a typical bell-
shaped band shifted to the red at 380 nm, characteristic of
an increased conjugation in the closed form.4,5
Quantum-mechanical calculations were carried out us-
ing the HyperChem computational package (version 7.5).
The presence of the N heteroatom (not implied in
the aromatic sextet) as a new site of basicity decreases
Table 1. Absorption and fluorescence maxima of the E isomer of the compounds investigated in two solvents
Compound
5-StOx
Solvent
CH
2ab / nm ta)
£abmax/ mol 1dm3cm-1
Afi / nm (a)
25190
28580
296, 308, 322
293.304.318
328, 343, 359
MeCN
CH
306, 319, 333sh
304, 316, 330
30830
32270
344, 361.378
p-OMe-5-StOx
MeCN
(a)The main maximum is underlined; sh means shoulder.
Croat. Chem. Acta 87 (2014) 327.

330
I. Šagud et al., Photoisomerization and Photocyclization o f5-Styryloxazole
the Tt-rich character of the oxazole ring with respect to
the furan and pyrrole analogues, as also indicated by its
higher ionization potential (9.83 eV vs. 8.88 and 8.90
eV for F and Py, respectively).24 This effect bestows on
this azole moiety a weaker electron donor character and
leads to an electron deficiency on the C atoms of the
ring. It has to be noted that the decrease of the electron
density at the 4-C (the reactive position of the cycliza-
tion reaction) can affect the yield of the latter photoreac-
tion, see below).
The donor properties of the Ox ring are expected
to give some charge transfer (CT) character to the
So—»-Si transition even if the spectral behaviour detected
in MeCN (higher polarity but lower polarization) is very
similar to that in CH with maxima only slightly shifted
towards the blue. The computed dipole moment of the
ground state (pg = 2.42 and 1.82 D for the E and Z iso-
mer, respectively, see below, Table 2) reveals a slightly
unsymmetrical charge distribution.
Figure 2. Normalized absorption, fluorescence (emission and
excitation) spectra ofF-5-StOx in CH.
depend on the excitation wavelength (AeXC) while the
excitation spectrum shows a non-negligible change in the
relative intensities of the two side peaks that does not
exclude a small effect of conformational equilibria.25,26
Theoretical calculations were then carried out
on the formation enthalpy to evaluate the presence of
conformational equilibria in our solutions (Scheme 2).
Figure 2 shows the normalized absorption and flu-
orescence emission/excitation spectra of T-5-StOx in
CH. The spectra are almost specular and slightly struc-
tured with an absorption band displaying a main peak at
308 nm in CH and two side peaks with vibrational pro-
gression typical of stilbenoid compounds. The emission
spectrum, showing a Stokes shift of 3420 cm-1, does not
Table 2. Computed dipole moment (// / D) and total energy (£ t o t / kcal mol '), derived relative abundance (%) at room tempera-
ture, transition energy (2 / nm) and oscillator strength ( f ) of the conformers of the E and Z isomers of 5-StOx
u ! D
rotamer
% (at 293 K)
Z / nm
Et o t / kcal m ol-1
/
301
254
217
198
190
1.06
0.07
0.31
0.07
0.39
Si
S3
St
S9
F-5-StOx (A)
2.42
-343864.719
1
S
11
312
259
218
199
0.98
0.18
0.40
0.11
Si
S
3
F-5-StOx (B)
Z-5-StOx (A)
Z-5-StOx (B)
2.42
1.82
1.84
-343867.188
-343862.469
-343860.500
99
97
3
St
S9
282
237
213
196
0.50
0.09
0.09
0.80
Si
Ss
S
11
S
12
286
232
192
189
0.34
0.52
0.77
0.91
s ,
S5
S
12
S 13
Table 3. Absorption properties of the Z and DHP photoproducts and DHP lifetime for the two compounds investigated in CH
Compound
5-StOx
Zz / nm (a)
s f ' m / mol 1dm3cm-1
Zd h p / nm (a)
378
rr>HP / min
200
288. 306sh, 320sh
297. 318sh, 330sh
11100
15520
p-OMe-5-StOx
378
100
<a)The main maximum is underlined; sh means shoulder.
Croat. Chem. Acta 87 (2014) 327.

I. Šagud et ah, Photoisomerization and Photocyclization of5-Styryloxazole
331
Table 2 shows the computed spectral properties and
Et o t values for the E and Z isomers of 5-StOx.
s-cis rotamer (A)
s-trans rotamer (B)
Scheme 2. Conformational isomers of£-5-StOx.
The results for 5-StOx showed a good agreement
between the experimental and computed absorption
maxima while the Et o t values indicated that one con-
former is expected to be largely prevalent for both geo-
metrical isomers, the second one being practically neg-
ligible (1 % for E and 3 % for Z).
A/nm
Figure 3. Spectral evolvement under irradiation of 5-StOx in
CH: this solution was previously irradiated for 30 min atX= 313
nm to produce DHP and then further irradiated at X= 380 nm.
The quantum yields of the radiative and reactive
processes of 5-StOx are shown in Table 4 together with
those of its substituted analogue. The fluorescence yield
of 5-StOx was found to be <j>v= 0.072 in deaerated CH
while the corresponding lifetime was found to be shorter
than the resolution time of our apparatus (0.5 ns). An
As a matter of fact, accurate analysis of the
reaction mixture evidenced the formation of a very
small amount of a fifth component X. The possibility
that such intermediate could be produced through a
1,«-hydrogen shift was explored in details but with
negative result. In fact, the HPLC analysis showed that X
was induced by the polar eluent mixture (MeCN contain-
ing small amounts of water) while mass-chromatographic
analysis indicated that it contained oxygen and thus was
assignable to a negligible side product of oxidation fa-
voured by the eluent in aerated conditions.
increase in the
value which seemed to be at the limits
of the experimental uncertainty was observed using /Uxc
longer than the absorption maximum.
As tested in a previous work,27 the combined use
of HPLC and spectrophotometric techniques was very
useful for the separation of the components of the pho-
toreaction mixture thus allowing the evaluation of the
isomerization/cyclization competition and the measure-
ment of the spectra and decay kinetics of the DHP-type
intermediate. Our experiments in CH were carried out in
different steps. Initial irradiation at 313 nm was used to
measure the E —> Z photoisomerization quantum yield.
Prolonged irradiation produced a E/Z photostationary
state. In order to enrich the mixture in the Z isomer the
solution was subjected to a parallel irradiation with light
of Aexc > 350 nm to avoid the ring closure. The mixture
thus obtained (—80 % Z) underwent a further irradiation
step at 254 nm to follow the competitive Z —> E and
Z —> DHP photoreactions. Left overnight in aerated
conditions, the mixture showed a decrease of DHP and
an increase in P. Otherwise, when irradiated at 380 nm
with light absorbed by DHP only, the solution showed an
increase (from 20 to 80 %) in the Z component confirm-
ing the photochemical DHP —*• Z back ring opening, as
well known for the corresponding hydrocarbon.4,5
Going to the photoreaction quantum yields sum-
marized in Table 4 for the two compounds investigated,
the E —» Z photoisomerization of 5-StOx, was found to
have a quantum yield ^ e-»z = 0.57, a value close to that
of stilbene (~0.50).1,2 The high value of the reaction
quantum yield (even higher than the value of 0.5 ex-
pected by the diabatic mechanism2), accompanied by
the small emission yield, indicates that decay by internal
conversion (IC) can be excluded for this compound and
probably the photoreaction occurs in the singlet mani-
fold through a diabatic mechanism. It should be noted
that the few quantitative data of relaxation quantum
yields available in the literature for £-styryl-substituted
five-membered heteroaryl units (T,28,29 Py,30 indole30)
indicate that these structures are characterized by a
similar photobehaviour, namely efficient isomerization
and weak fluorescence. Only for 2-styrylbenzothiazole a
much less efficient reactive pathway was reported with
E -> Z quantum yields of the order of 10 %, probably
Table 4. Fluorescence and photoreaction quantum yields of the compounds investigated in CH
Compound
5-StOx
(/>?
<f>e ->z
<j>z ->e
<j>z->p
0.072 + 0.009
0.026 + 0.003
0.57 + 0.01
0.41 +0.03
0.15 + 0.01
0.24 + 0.02
0.013 ± 0.001s
0.027 + 0.003
p-OMe-5-StOx
Croat. Chem. Acta 87 (2014) 327.

332
I. Šagud et al., Photoisomerization andPhotocyclization of5-Styryloxazole
due to a sizable contribution of ICT processes competi-
tive with isomerization.21
It is known that methoxy-substitution at the ortho
and para positions leads to a decrease in the ionization
potential (favouring the donor properties) and in the
energy gap (increase of the HOMO energy and red shift
of the spectrum) of the transition. The higher computed
dipole moment of the E isomer (p = 3.38 D) reflects the
combined effect of the donor properties of the two side
groups. However, the expected increased contribution of
ICT was not verified, the effect of the solvent polarity
on the absorption spectrum (small hypsochromic and
auxochromic shifts) being similar to that measured for
the unsubstituted compound. A most important effect
was found on the emission properties. A modest but
significant reduction in both <f>p and </>e-+z values (0.026
and 0.41, respectively) was found for the p-OMe
derivative in CH with respect to the unsubstituted
compound. Even in this case, the fa value showed a
slight increase using longer 2eXCafter the absorption
maximum. The possible contribution of ISC to the tri-
plet state was not investigated but a moderate triplet
production cannot be excluded. It was interesting to
observe that the fluorescence emission, weak in CH,
becomes undetectable in MeCN.
Experiments carried out with increasing oxygen
concentrations showed an expected decrease in the DHP
lifetime: from 200 to 150 and 90 min on going from de-
aerated (by bubbling nitrogen) to aerated and oxygen-
saturated CH solution, respectively. A similar value (96
min) was reported for stilbene in a non-polar 2:1 mix-
ture of methylcyclohehane/isohexane.4
The quantum yield of the competitive Z —>E and
Z —>P in Table 4 was preferably investigated using 2eXC
of 254 nm where Z is the main absorber. The values
thus obtained were (fe->E = 0.15 and </>z->p= 0.013. The
isomerization yield is rather smaller than that reported
for stilbene (0.35) while the cyclization yield (to give
naphtho[l,2-d]oxazole, NOx) is almost one order of
magnitude smaller than the value of 0.10 reported for
stilbene.7-9 As said above, the small cyclization yield
based on the DHP interconversion to P in the presence
of oxidant (left overnight in the dark) has a higher
uncertainty with respect to the yields of the other pho-
toreactions.
5-(4-methoxystyryl)oxazole (p-OMe-5-StOx)
The photobehaviour of Z-p-OMe-5-StOx was stud-
ied with the procedure used for the parent compound and
gave similar results. The spectrum of the DHP intermedi-
ate in CH was clearly observable with Amax = 380 nm in
CH. The lifetime measured in aerated solution in the first
two hours of its bleaching was about 100 min with a
tendency to increase after further decay, probably be-
cause of the consumption of the oxidant. The absorption
spectrum of the final cyclized product (8-methoxy-
naphtho[l,2-d]oxazole, 8-OMe-NOx) in MeCN was in
good agreement with that previously reported in EtOH.22
The spectral properties of this compound bearing a
donor para-substituent are rather similar to those of
5-StOx. They showed a bathochromic shift of 12 nm
and an increase in the absorption coefficient of 22 and
13 % in non-polar and polar solvent, respectively, with
respect to the parent compound.
Table 5 shows the computed spectral properties
and Et o t values for the E and Z isomers of p-OMe-5-
StOx. Even for this compound the Et ot values indicat-
ed that one conformer (A) largely prevails for the E
isomer while a mixture of two conformers is foreseen
for the Z isomer.
The (fc->e (0.24) and
p (0.027) were found almost
doubled with respect to the unsubstituted compound.
Table 5. Computed dipole moment (ju / D) and total energy (Et o t / kcal m of'), derived relative abundance (%) and electronic
spectra [transition energy (2 / nm) and oscillator strength (f) ] of the conformers ofthe E and Z isomers ofp-OMe-5-StOx
rotamer
p / D
% (at T = 293 K)
Et o t / kcal m o f1
2 / nm
/
E-p-MeO-5-StOx (A)
3.38
-414934.625
99
315
260
216
1.08
0.14
0.40
Si
S3
S7
E-p-MeO-5-StOx (B)
Z-p-MeO-5-StOx (A)
3.63
1.73
-414932.156
^414929.062
1
26
310
257
225
217
216
0.63
0.38
0.08
0.26
0.08
s,
S3
S6
S7
S8
Z-p-MeO-5-StOx (B)
2.19
^114929.660
74
285
228
220
203
194
0.50
0.19
0.11
0.21
0.40
Si
S6
S8
S10
S13
Croat. Chem. Acta 87 (2014) 327.

I. Šagud et al, Photoisomerization and Photocyclization o f 5-Styryloxazole
333
2. J. Saitiel and Y.-P. Sun, in H. Durr, H. Bouas-Laurent (Eds.)
Photochromism: Molecules and Systems, Elsevier, Amsterdam,
1990, pp. 64—162, and references therein.
3. G. Bartocci, U. Mazzucato, and A. Spalletti, Trends Phys. Chem.
12 (2007) 1-36.
CONCLUSIONS
The study of 5-StOx and p-OMe-5-StOx under mild
conditions showed a photobehaviour similar to that of
stilbene, at least regarding the deactivation of the E
isomer. Taken together, the yields of the abundant
isomerisation and relatively weak fluorescence account
for all the quanta absorbed. Both yields are slightly
lower for the p-OMe-derivative where a weak contribu-
tion of internal conversion cannot be excluded. The
situation is different for the Z isomer since the two
competitive photoreactions (back isomerization to E and
cyclization) are characterized by lower yields compared
to stilbene, particularly for the unsubstituted compound
where the cyclization yield decreases by almost an order
of magnitude with respect to stilbene. This effect could
be due to the reduced electron density (with respect to
the analogous five-membered rings with only one het-
eroatom, F and Py), caused by the presence of the basic
N heteroatom, at the reaction center, namely on the
carbon atom implied in the ring closing reaction. A
general quantitative study of the photobehaviour of
these stilbene analogues bearing different five-
membered heteroaryl groups would be necessary to
better understand the structure effect on the relaxation
properties and the possible interest for applications. We
plan to extend this research line on structurally-
modified analogous compounds where the presence of
pentatomic heteroaryl group will probably induce larger
effects. The continuation of this work in different direc-
tions has the aim of investigating the effects of: i) posi-
tional isomers bearing the styryl group in different posi-
tions (2- and 4-StOx); ii) fused-ring bicyclic benzologs
(styrylbenzoxazoles), and iii) diarylethenes containing
two heteroaryl groups with the same (symmetric) or
different (asymmetric) electron donor/acceptor proper-
ties. The latter are expected to be of specific interest
since the occurring of intramolecular CT processes in
the excited state should strongly affect the radiative and
reactive relaxation leading to properties of potential
application.
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Acknowledgements. The authors recognize the financial sup-
port by MIUR (Ministero dell’Universita e della Ricerca,
Rome, Italy) and the University of Perugia (PRIN 2010-2011
n. 2010FM738P) as well as the Ministry of Science, Education
and Sports of the Republic of Croatia (grant nos. 125-
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Croat. Chem. Acta 87 (2014) 327.

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