Effect of the positional isomerism on the photoreactivity of styryloxazoles

Article abstract of DOI:10.1016/j.jphotochem.2015.10.022

This paper describes the results obtained in the study of the photobehaviour of heteroanalogs of stilbene bearing an oxazole ring. The competitive relaxation processes (fluorescence, isomerization and cyclization) of the excited states of n-styryloxazoles (n = 2 and 4) were investigated and compared with the behaviour previously reported for n = 5. After preparation of the unknown compound with n = 4 and of its positional isomer with n = 2 (the latter with a new method of synthesis), their photobehaviour was firstly investigated in preparative conditions by NMR analysis to measure the chemical yields of their photoproducts. The study was then continued in mild irradiation conditions to measure the quantum yields of the competitive photoreactions in the primary irradiation steps. The effects of the position of the styryl group at the oxazole ring, the relative abundance of the various conformers and the possible formation of intramolecular H-bonds on the deactivation pathways are described. Quantum-mechanical Hyperchem calculations proved to be very useful to describe the conformational equilibria and the role of conformers on photoreactivity while more refined DFT calculations on the Z isomers allowed to explain the structure dependent competition between their isomerization/cyclization processes. The effect of the replacement of the phenyl ring with a second heteroaromatic group of electron donor character was investigated for the 5-(2-(furan-2-yl) ethenyl) oxazole.

Full text of DOI:10.1016/j.jphotochem.2015.10.022

Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 95–103
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Effect of the positional isomerism on the photoreactivity of
styryloxazoles
b
b,
a
V. Bottia, F. Eliseia, U. Mazzucatoa, , I. Šagud , M. Šindler-Kulyk , A. Spalletti
*
*
a
Department of Chemistry, Biology and Biotechnology and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN), University of Perugia,
via Elce di Sotto 8, Perugia 06123, Italy
b
ꢀ
Department of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, Marulicev trg 19, Zagreb 10000, Croatia
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 16 July 2015
Received in revised form 20 October 2015
Accepted 25 October 2015
Available online 29 October 2015
This paper describes the results obtained in the study of the photobehaviour of heteroanalogs of stilbene
bearing an oxazole ring. The competitive relaxation processes (fluorescence, isomerization and
cyclization) of the excited states of n-styryloxazoles (n = 2 and 4) were investigated and compared with
the behaviour previously reported for n = 5. After preparation of the unknown compound with n = 4 and
of its positional isomer with n = 2 (the latter with a new method of synthesis), their photobehaviour was
firstly investigated in preparative conditions by NMR analysis to measure the chemical yields of their
photoproducts. The study was then continued in mild irradiation conditions to measure the quantum
yields of the competitive photoreactions in the primary irradiation steps.
The effects of the position of the styryl group at the oxazole ring, the relative abundance of the various
conformers and the possible formation of intramolecular Hꢀꢀbonds on the deactivation pathways are
described. Quantum-mechanical Hyperchem calculations proved to be very useful to describe the
conformational equilibria and the role of conformers on photoreactivity while more refined DFT
calculations on the Z isomers allowed to explain the structure dependent competition between their
isomerization/cyclization processes. The effect of the replacement of the phenyl ring with a second
heteroaromatic group of electron donor character was investigated for the 5-(2-(furan-2-yl) ethenyl)
oxazole.
ã 2015 Elsevier B.V. All rights reserved.
1. Introduction
No contribution of other pathways, such as dimerization and
intersystem crossing (ISC), were observed. Internal conversion (IC)
The two main photoreactions (isomerization and cyclization) of
stilbene have been under continuous investigation over many
decades [1–13]. However, some mechanistic insights are still deeply
debated in the modern literature [14–18]. In comparison, stilbene-
like compounds bearing one or more heteroaryl groups have been
less deeply investigated. The study of structural effects on their
photobehaviouristheninterestingsincethepresenceandpositionof
the heteroatom(s) can affect sensibly their relaxation pathways.
In a previous paper [19] the photobehaviour of the trans (E) and
cis (Z) isomers of 5-styryloxazole (5-StOx) and their p-OMe
derivatives was investigated in cyclohexane in mild conditions
(room temperature, short and moderate-intensity irradiation
times and low concentration) to measure reliable quantum yields
of the competitive relaxation pathways of the excited states
(fluorescence, E ! Z and Z ! E isomerization and cyclization) [19].
was likely operative in the case of the E-OMe derivative and in both
Z isomers because the sum of emission and isomerization yields
did not account for all the absorbed quanta [19].
The reversible photoisomerization was reported to occur in the
singlet manifold by the well known diabatic mechanism [1,2].
Starting from the trans isomer, it involves twisting around the
ethenic bond towards an energy minimum at the perpendicular
configuration (at about 90ꢁ, 1perp*) followed by a S1 ! S0 IC and
relaxation to the ground-state E and Z isomers in roughly a
1
1
1:1branchingratio[1E* ! perp* ! perp ! a1Z + (1 ꢀ
a
)1E], where
the partitioning factor
a
is generally assumed to be ꢂ0.5 [1,2].
The cyclization photoreaction, which starts from the Z isomer
and is generally accepted to occur in the singlet manifold [3–
13,20], led, as usual, to the unstable coloured intermediate (4a,4b-
dihydrophenanthrene-type, DHP) which, in the presence of an
oxidant (generally oxygen), lost the two hydrogen atoms in trans
stereochemistry to give the stable polynuclear phenanthrene-type
(P) arene [19]. It should be recalled that two different cyclization
intermediates (DHP and DHP’) have been sometimes reported in
the literature since the initial DHP, generally in anaerobic
* Corresponding author.
E-mail addresses: ugo.mazzucato@unipg.it (U. Mazzucato), msindler@fkit.hr
(M. Šindler-Kulyk).
http://dx.doi.org/10.1016/j.jphotochem.2015.10.022
1010-6030/ã 2015 Elsevier B.V. All rights reserved.

96
V. Botti et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 95–103
conditions, can produce an isomer DHP’ (through an intramolecu-
lar 1,n-hydrogen shift) which does not revert to Z and loses the
hydrogen atoms to give the final P-type product [3,4]. Moreover,
relatively good reactivity predictions about the cyclization path
were reported in the literature on the basis of the sum of the free
valence numbers of the excited state for the reacting C atom pair
N
(SFr*) [3]. Even better results were obtained with an alternative
method where differences in the electronic overlap population
(EOP) in the ground and excited states were used as general
reactivity indices for photocyclization [3].
As to the quantum yields of the competing relaxation pathways
of the parent 5-StOx compound, a modest fluorescence yield (fF of
ꢂ7%) was measured for the E isomer whereas, as usual, no
emission was detected for the Z isomer in these experimental
conditions. Photoisomerization was the largely prevailing deacti-
vation pathway, particularly for the E isomer (fE!Z = 0.57). For the
reverse reaction of the Z isomer, a smaller yield, smaller than that
of stilbene (0.35) [1,2,18], was reported (fZ!E = 0.15). A small
Scheme 2. Synthesis of E/Z-2-styryloxazole (2-StOx), E/Z-4-styryloxazole (4-StOx)
and E-5-(2-(furan-2-yl) ethenyl) oxazole (2-FEt-5Ox).
described [23], while the novel 4-StOx was prepared by Wittig
reaction from oxazole-4-carbaldehyde [27] and benzyltriphenyl-
phosphonium salt. To prepare 2-StOx [28] we developed a new
“one pot” procedure which includes formylation of oxazole and the
Wittig reaction.
Preparative irradiation experiments were performed under
aerobic conditions and addition of iodine in benzene solution in a
Rayonet reactor equipped with 300 nm lamps as described for 5-
StOx [23]. Irradiation of n-StOx (n = 2, 4 and 5) was also performed
in deuterated benzene in NMR tubes in aerobic conditions not
purged with oxygen (oxygen from the air present in the tube) and
the process followed by 1H NMR (Bruker, AV300 and AV600)
spectrometry. Further details are given in the Supplementary
Information.
formation of DHP, with lmax = 378 nm and t = 210 min in deaerated
solution, was obtained. In the presence of oxygen DHP led to the
stable cyclization product (naphtho[1,2-d]oxazole, NPh[1,2-d]Ox)
with a yield fZ!DHP of 0.013 [19].
Similar results were reported for the p-OMe derivative which
displayed slightly smaller isomerization and higher cyclization
yields. The ratio fZ!E/fZ!DHP decreased from the value of 11.54 for
5-StOx to 8.9 for the p-OMe derivative, in favour of the cyclization
process [19].
In conclusion, the photobehaviour of the two E-styryloxazoles
previously investigated was found to be rather similar to that of
stilbene [1–13] while that of the Z isomers showed a reduced
reactivity, particularly for cyclization. It should be noted that even
the few quantitative data of relaxation quantum yields available in
the literature for E-styryl-substituted five-membered heteroaryl
units (such as thiophene [4,21], pyrrole [22] and indole [22]) show
a similar photobehaviour, namely efficient isomerization and weak
fluorescence. On the other hand, the cyclization was reported to
generally display lower yield than stilbene and in some cases not to
be operative at all.
Since the study of stilbene-like compounds with heterocyclic
groups replacing one or both phenyl rings is important from
various points of view (synthesis of polycyclic compounds [23],
fluorescence sensors [24] and preparation of candidates for
potential applications in non-linear optics [25] and for the therapy
of human pathologies [26]), the present work aims to investigate
the effects of the position of the styryl group at the oxazole ring on
the photobehaviour of n-styryloxazoles (n = 2, 4 and 5) and the
effect of the replacement of the phenyl ring with a second
heteroaromatic group on the photobehaviour of 5-(2-(furan-2-yl)
ethenyl) oxazole (2-FEt-5Ox) (Scheme 1).
2.1.1. Synthesis of 2-StOx
To a solution of oxazole (0.9 g, 13.05 mmol) in dried ether under
nitrogen at ꢀ70 ꢁC n-BuLi (1.6 M, 8 mL, 15.6 mmol) was added
dropwise and the mixture was stirred for 30 min. To this mixture
1 mL of dried DMF (0.96 g, 13.05 mmol) was added, stirred for
30 min and then left to warm to room temperature. Benzyltriphe-
nylphosphonium bromide (4 g, 9.2 mmol) was added in portions of
0.5 g and the mixture stirred overnight. The reaction mixture was
poured on ice water and extracted with benzene. Organic layers
were combined and dried over anhydrous MgSO4, filtered and
concentrated under vacuum. By multiple extraction with petro-
leum ether 0.434 g (16%) of product [28] is gained as a mixture of Z-
and E-2-StOx (Z:E = 9:1). The crude material was purified by
column chromatography (petroleum ether/ether variable ratio).
Z-2-StOx: Oil, Rf (PE/E = 10:2) = 0.60; IR nmax/cmꢀ1: 3128, 3068,
1632, 1507, 1499, 1164, 1108, 918; UV (EtOH) lmax/nm (e
/dm3
molꢀ1 cmꢀ1): 220 (11248), 294 (12824); 1H NMR (CDCl3, 600 MHz):
l /ppm 7.66–7.63 (m, 2H, H-ar), 7.52/d, J4,5 = 0.59 Hz,1H, H-5), 7.39–
7.31 (m, 3H, H-ar), 7.17 (d, J4,5 = 0.59 Hz, 1H, H-4), 6.86 (d,
Jet = 12.85 Hz, 1H, H-et), 6.44 (d, Jet = 12.85 Hz, 1H, H-et); 13C
NMR (CDCl3,150 MHz): l /ppm 160.51 (s),137.88 (d, C-5),136.39 (d,
C-et), 135.67 (s), 129.42 (d, C-ar), 128.51 (d, C-ar), 128.20 (d, C-4),
128.02 (d, C-ar), 114.65 (d, C-et). HRMS (MALDI–TOF/TOF) za
2. Experimental
2.1. Synthesis and irradiation of 2-StOx, 4-StOx and 2-FEt-5Ox
C
19H17NO: (M + K)+calcd = 210.0316 (M + Kꢁ)+exp = 210.0315.
Two approaches were applied to synthesize the investigated
compounds (Scheme 2). The furyl derivative 2-FEt-5Ox was
prepared by the Van Leusen reaction from the corresponding
carbaldehyde and p-tolylsulfonyl)methyl isocyanide (TosMIC) as
E-2-StOx: Yellow powder, mp 52-54 C; Rf (PE/E = 10:2) = 0.40;
UV (EtOH) lmax/nm (
234 (Sh 5526), 302 (25,193), 313 (Sh 23516); IR nmax/cmꢀ1: 3201,
1643,1539,1448, 1106, 966; 1H NMR (CDCl3, 600 MHz):
/ppm 7.63
e
/dm3 molꢀ1 cmꢀ1): 221 (8741), 227 (8569),
d
(d, J4,5 = 0.60 Hz, 1H, H-5), 7.47 (d, Jet = 16.40 Hz,1H, H-et), 7.55–7.52
(m, 2H, H-ar), 7.40–7.32 (m, 2H, H-ar), 7.35–7.32 (m, 1H, H-ar), 7.19
(d, J4,5 = 0.60 Hz, 1H, H-4), 6.97 (d, Jet = 16.40 Hz, 1H, H-et); 13C NMR
(CDCl3, 150 MHz): d/ppm 161.80 (s), 138.15 (d, C-5), 136.16 (d, C-et),
135.55 (s), 129.16 (d, C-ar), 128.86 (d, C-ar), 128.55 (d, C-4), 127.18
(d, C-ar), 113.99 (d, C-et); MS m/z (EI): 171 (100%), 109 (10%).
Scheme 1. Compounds investigated: E-2- and E-4-styryloxazole (E-n-StOx with
n = 2 and 4) and E-5-(2-(furan-2-yl) ethenyl) oxazole (2-FEt-5Ox).

V. Botti et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 95–103
97
2.1.2. Synthesis of 4-StOx
The procedure used to measure the E ! Z photoreaction
quantum yields was as follows: diluted solutions of the E isomers
in de-aerated (by bubbling nitrogen) CH were irradiated up to a
maximum conversion percentage of ꢂ10% to minimize competi-
tion of the back photoreaction. To follow the photoreactions of the
Z isomers (not available as pure separated compounds) irradiation
was initially performed at 313 nm to reach a photostationary 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 Z isomer 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 compo-
nents: E, Z, DHP and small amounts of the P-type dehydrogenated
compound. The air-equilibrated solutions were then left overnight
in the dark to allow the decay of the DHP intermediates to go to
completion. The residual components were separated by HPLC
(using MeCN/water mixtures as eluent) and identified spectro-
photometrically. Only in the case of 4-StOx the irradiation was
carried out on a pure Z sample, previously separated from the E
isomer by HPLC.
Synthesis of 4-StOx: To an absolute ethanol solution of
benzyltriphenylphosphonium bromide (3.08 g, 7.1 mmol, 50 mL)
NaOEt in ethanol was added dropwise and simultaneously with a
solution of oxazole-4-carbaldehyde (0.69 g, 7.1 mmol) [27] and the
reaction mixture was stirred for 24 h. After removal of the solvent
the residue was dissolved in water and extracted with benzene.
Organic layers were dried over anhydrous MgSO4, filtered and the
solvent was removed under reduced pressure. By multiple
extractions with petroleum ether 0.673 g (56%) of product was
obtained as a mixture of Z- and E-4-StOx (Z:E = 3:2). By multiple
column and thin layer chromatography, 0.411 g of Z-4-StOx was
isolated (contaminated with 11% of E-4-StOx).
Z-4-StOx: Oil, Rf (PE/E = 3:1) = 0.41; IR nmax/cmꢀ1: 3024, 1517,
1097, 1065, 910; UV (EtOH) lmax/nm (
(8719); 1H NMR (CDCl3, 600 MHz):
/ppm 7.78 (s, 1H, H-2), 7.38–
7.33 (m, 5H, H-ar), 7.28 (s, 1H, H-5), 6.72 (d, 1H, Jet = 12.0 Hz, H-et),
6.44 (d, Jet = 12.0 Hz, 1H, H-et);13C NMR (CDCl3, 150 MHz)
/ppm:
e
/dm3 molꢀ1 cmꢀ1): 275
d
d
150.1 (d, C-2), 137.6 (s), 136,71 (s), 136.2 (d, C-5), 132.2 (d, C-et),
128.5 (d, C-Ar), 128.3 (d, C-Ar), 127.6 (d, C-Ar), 120.0 (d, C-Ar);
HRMS (MALDI–TOF/TOF) for C11H9NO: (M + K)+calcd = 210.0316,
(M + H)+exp = 210.0311.
The thermal stability of DHPs at room temperature was
measured by following spectrophotometrically the disappearance
of the visible absorption band in both oxygenated and de-aerated
CH (by bubbling oxygen and nitrogen, respectively). The quantum
yields reported in the Tables are generally averages of two/three
independent experiments with mean deviations of ca. 10% for
E-4-StOx: Oil, Rf (PE/E = 3:1) = 0.40; 1H NMR (CDCl3, 600 MHz)
d
/ppm (values are derived from the spectrum of mixture of
isomers): 7.88 (s, 1H, H-2), 7.68 (s, 1H, H-5), 7.50 (d, Jar = 7.3 Hz, 1H,
H-ar), 7.38–7.33 (m, 2H, H-ar), 7.36 (d, Jet = 16.0 Hz, H-et), 7.29–7.28
(m, 1H, H-ar), 6.94 (d, Jet = 12.0 Hz, 1H, H-et).
f . Higher uncertainty (15–20%) was found in the more delicate
E!Z
2.1.3. Synthesis of NPh[2,1-d]Ox
measurements of the photoreaction yields starting from the Z
isomer (fZ!E and fZ!P), particularly in the presence of very small
cyclization yield (<2%).
Quantum-mechanical calculations were carried out using the
HyperChem computational package (version 7.5). 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 ꢃ 20) single excited configurations.
Irradiation of 4-StOx gave naphtho[2,1-d]oxazole (NPh[2,1-d]
Ox) [29]: 92% yield, colorless crystals, mp 50–51 ꢁC; Rf (PE/
E = 10:2) = 0.43; UV (EtOH) lmax/nm (e/dm3 molꢀ1 cmꢀ1): 222
(29,823), 233 (31,946), 311 (1545), 326 (2098); IR nmax/cmꢀ1
:
3099, 2921, 1498, 1261, 1087, 1066; 1H NMR (CDCl3, 600 MHz): d/
ppm 8.26 (d, Jar = 8.6 Hz, 1H, H-ar), 8.25 (s, 1H, H-2), 8.00 (d,
Jar = 8.2 Hz, 1H, H-ar), 7.86 (d, Jet = 8.7 Hz, 1H, H-et), 7.82 (d,
Jet = 8.7 Hz, 1H, H-et), 7.67–7.64 (m, 1H, H-ar), 7.59–7.56 (m, 1H, H-
ar); 13C NMR (CDCl3, 150 MHz): d/ppm 151.2 (d), 145.6 (s), 136.1 (s),
131.4 (s), 128.1 (d), 126.5 (d), 125.5 (d), 125.0 (d), 120.0 (s), 119.8 (d),
118.4 (d).
More refined quantum-mechanical calculations on Z isomers
were carried out in CH at the B3LYP/6-311G + (2d,p) level of theory
[31] using the Gaussian 09 package [32]. The geometry optimi-
zations were undertaken using tight convergence limits and with
no symmetry constraints. The final energies for each optimized
structure, the electron density changes and the frontier molecular
orbitals were calculated using the CPCM model [33] to simulate the
dielectric medium effect.
2.2. Spectral and photoreactivity measurements
For the spectroscopic and photoreaction measurements,
cyclohexane (CH) from Fluka, spectrophotometric grade, was used
as solvent. In some experiments also acetonitrile (MeCN), benzene
and a mixture of 9/1 (v/v) methylcyclohexane/3-methylpentane
(MCH/3MP) from Fluka were used after further purification by
standard procedures.
3. Results and discussion
PerkinElmerLambda800andCary4EVarianspectrophotometers
were used for the absorption measurements. The fluorescence
spectra were measured with a Spex Fluorolog-2 F112AI spectrofluo-
rimeter. Dilute solutions (absorbance <0.1 at the excitation
wavelength, lexc) were used for fluorimetric measurements. The
3.1. n-Styryloxazoles
The initial part of the photochemical work, carried out at the
Zagreb laboratory, was particularly aimed to optimize the
fluorescence quantum yields
corresponding to the maximum of the first absorption band (lmax).
2-(1-naphthyl)-5-phenyl-3,4-oxadiazole ( -NPD) in CH was used
(fF) were determined at lexc
a
as fluorometric standard (fF = 0.70 in deaerated solvent) [30].
For photochemical measurements (potassium ferrioxalate in
water as actinometer) a 150 W high pressure xenon lamp coupled
with a monochromator and a 500 W mercury lamp and an
interference filter at 313 nm were used. The photoreaction (solute
concentrations ꢂ10ꢀ4 M) was monitored by HPLC using a Waters
apparatus equipped with analytical Gemini C18 (4.6 ꢃ 250 mm; 5
m
m) and Prontosil 200-3-C30 (4.6 ꢃ 250 mm; 3
mm) columns and
an UV diode-array detector.
Fig. 1. Graphical presentation of irradiation of (E/Z)-4-StOx in NMR tubes in C6D6.

98
V. Botti et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 95–103
O
N
O
N
hv
E-(B)
E-(A)
4
N
N
O
O
hv
Z-(A) (2%)
Z-(B) (98%)
H
O2, I2
- H2
N
N
H
O
O
DHP
naphtho[2,1-d]oxazole
Scheme 3. Photoreactions of Z-4-StOx.
cyclization process. It started from irradiation of the E isomer or
mixtures of E/Z isomers under suitable conditions for a preparative
work (high light intensity, long irradiation times of tenths of hours,
concentrated solutions and addition of iodine as efficient oxidant).
The evolution of the photoreactive pathways in aerobic conditions
(see experimental), measured in the present work also for the
previously investigated 5-StOx, was followed by NMR analysis, as
shown in Fig. 1 for n = 4, as an example, and in Figs SI.1 and SI.2 in
the Supplementary Information for the other two isomers.
The results thus obtained gave interesting information on the
photobehaviour. Irradiation of E-2-StOx showed only the E–Z
isomerization which led to photostationary state (E:Z = 3:1) after
40 min. Irradiation of 4-StOx (E:Z = 1:1) showed the E–Z isomeri-
zation and the photostationary state (E:Z = 3:1) was reached after
ꢄ2.5 h followed by the electrocyclization after ꢄ8 h. The 5-StOx,
measured for the sake of comparison, displayed a photobehaviour
similar to 4-StOx with the more effective process of electro-
cyclization. The long induction period for the formation of the
cyclised P product recorded by NMR can be explained by slow
formation of the DHP intermediate due to the competition of its
return to the starting Z isomer by thermal and mainly photochem-
ical processes. It should also be noted that the final P product
should accumulate enough before the sensitivity of the NMR
method allows its detection.
to characterize the absorption/emission spectral properties and to
measure the quantum yields of the competing relaxation
processes.
The different position of the styryl group with respect to the
oxygen atom in the five-membered ring is expected to change the
electronic distribution and affect in some way the competition of
the different relaxation pathways. Scheme 3 shows the photo-
reactions of these compounds for 4-StOx, as an example. The
spectral properties of the E isomers are shown in Fig. 2 and
collected in Table 1.
The E isomers of 2- and 5-StOx absorb in the same spectral
region with the tail of 2-StOx broadened towards the red and a
more structured shape for n = 5; the spectrum of 4-StOx is shifted
towards the blue as also predicted by calculations (see below,
Table 3). The absorption coefficients of the E isomers were
measured directly, those of the polycyclic final products were
assumed equal to those in ethanol (see spectra of Fig. SI.3) and
those of the Z isomers and DHP intermediates were obtained by
normalization at the E/Z and DHP/P isosbestic points, respectively.
For n = 2, no sign of formation of the DHP intermediate and its
dehydrogenation product P was observed. It should be noted that
the spectral shape of Z-2-StOx is unusually structured (see Fig. SI.4)
indicating a planar and more conjugated geometry favoured by an
interaction of Hꢀꢀbond type between the N atom of the oxazole
ring and a H atom of the phenyl group, as also suggested by the
computed results (see below). The naphtho[2,1-d]oxazole (NPh
[2,1-d]Ox), polycyclic product formed under irradiation of Z-4-
StOx, shows the structured spectrum typical of these compounds
while its DHP precursor has a bell-shaped band shifted towards
400 nm because of the extended conjugation in the non-aromatic
The experimental work was then continued at the Perugia
laboratory under mild experimental conditions to follow the
photobehaviour in the first stages of irradiation with the main aim
p
-system typical of these intermediates (Fig. 3).
The almost specular absorption/emission spectra are structured
and the excitation spectra are practically superimposed to the
absorption spectra (Figs SI.5 and SI.6). Deviations are within the
experimental uncertainty due to the weak signal and the use of low
transmittance filters to avoid photodegradation. In the case of the
compound with n = 4 (as previously found for 5-StOx) [19], no lexc
effects on the emission spectra were observed (Fig. SI.7) whereas
changes in the spectral shape were found for n = 2, consistent with
the presence (confirmed by calculations) of conformational
equilibria among fluorophores with slightly different spectral
properties (Fig. SI.8).
Since these flexible molecules exist in solutions as mixtures of
conformers [34,35] due to rotation of the aryl groups around the
quasi-single bond with the ethenic bridge (see Scheme 4 for Z-4-
StOx as an example), calculations of total energies were performed
to evaluate their relative abundances in the ground state and
spectral properties. Tables 2 and 3 show the results obtained for
the two compounds. The last column shows the experimental
Fig. 2. Absorption and normalized emission spectra of the E isomers of 2- and 4-
StOx in CH compared with n = 5 from Ref. [19].

V. Botti et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 95–103
99
Table 1
~
Spectral properties and stokes shifts (DvS) of two positional isomers of 2- and
O
N
4-StOx in two solvents. Data for n = 5 from ref. 19 are also reported for
comparison purposes.a
O
N
emax
max
A rotamer (2%)
B rotamer (98%)
n
solvent
D
l
(nm)
lmF ax(nm)
abs
(Mꢀ1 cmꢀ1
28800
)
ꢀ1
~
vS(cm
)
Scheme 4. Conformational equilibrium of Z-4-StOx.
2
CH
325
309
299
328
312
302
334
352
369
5040
5390
3420
significant contribution of IC is found for the Z isomers, particularly
for n = 4 and n = 5. The isomer with n = 4 displays a cyclization yield
almost one order of magnitude higher than that of n = 5. It should
be noted that the metastable DHP intermediate is characterized by
three deactivation pathways: under irradiation it undergoes ring
opening and reverts to Z while in the dark it is thermally oxidized
to P or undergoes H-shift [4] followed by dehydrogenation to P
even in the absence of oxidant. Therefore, the cyclization yield
depends obviously on three rate constants, kZ!DHP,kDHP!Z and
kDHP!P, the latter representing the oxidation rate, kox. From the
information of the chromatographic analysis we assume kDHP!P >>
kDHP!Z in the presence of the oxidant and then fZ!DHP
approximates fZ!P, the formation photoefficiency of the final
product P (Fig. 4).
The thermal evolution of the absorption spectrum of DHP for
n = 4 (Fig. 5) was followed in the dark in deaerated (nitrogen
atmosphere) and aerated (oxygen atmosphere) CH and MeCN
solutions. In deaerated solution, tDHP = 210 and 160 min in CH and
MeCN, respectively (Table 4) were derived from the initial rates of
the DHP disappearance (Fig. 5B for n = 4). These lifetimes, reduced
by 50% by bubbling oxygen, are very similar to those reported for 5-
StOx and slightly longer than the value of stilbene in aerated
solution (140 min) [3]. A rough estimation of kox in CH (5.6 ꢃ10ꢀ3
and 8.9 ꢃ10ꢀ3 Mꢀ1 sꢀ1 for n = 4 and 5, respectively) was derived by
the lifetimes in Table 4, showing the importance of the presence of
an oxidant to accelerate the DHP ! P process. In deaerated
solutions the disappearance kinetics of DHP is probably due to
different contributions, namely the thermal DHP ! Z cyclorever-
sion and formation of P by oxygen traces not completely removed
by bubbling nitrogen. However, production of P in anaerobic
conditions cannot be excluded [20].
EtOH
25300
4
CH
306
286
278
304
285
277
25650
25500
312
327
341
MeCN
5
CH
322
307
296
318
304
293
25200
28600
327
343
360
MeCN
a
The main maxima are underlined.
maxima for comparison: there is good agreement for the
conformers of the E isomer, less for those of the Z isomers. In
the case of E-2-StOx, both conformers are abundant while one
largely prevails for the Z isomer, namely that one where C and N
atoms are faced at the reaction center (see below).
For 4-StOx, one conformer largely prevails for both E and Z
isomers. Similar results were reported for n = 5 [19].
The two n-StOx isomers with n = 2 and 4 display a rather similar
behaviour, the main difference being that n = 4 cyclizes with a good
yield, similar to that of stilbene (fZ!DHP = 0.1) [6,18], whereas n = 2
does not cyclize at all. The comparison with n = 5 indicates that the
latter is characterized by a larger E ! Z isomerization yield while
the Z isomer is less reactive, particularly in the cyclization pathway
(almost one order of magnitude smaller). Perusal of data in Table 4
indicates that, contrary to the case of the E isomers with n = 5 and
n = 4, where isomerization, with a small additional contribution of
Some preliminary experiments on the effect of temperature and
solvent on the relaxationproperties were carried out for 4-StOx. The
experiments at different T on the E isomer evidenced, as expected,
the coupling between the reactive and emissive deactivations since
in the rigid matrix, when the isomerization is inhibited, the
fluorescence yield approximates a value of 100%. An investigation
on the solvent effect carried out on Z-4-StOx showed that polar
solvents favour the isomerization to the detriment of cyclization
fluorescence, accounts for all excitation quanta (assuming
a
ffi 0.5,
the partitioning factor towards E and Z for the decay from the
1,3perp* configuration in the diabatic mechanism) [1], internal
conversion (IC) contributes to the relaxation of n = 2. A more
(fZ!DPH decreases from 0.063 in CH to 0.014 in MeCN). Similar
results were described also for Z-stilbene where the cyclization is
particularlyaffected (two-three times smaller inpolar solvents) [8].
We tried to explain the different cyclization yields for the
conformers of Z-n-StOx able to cyclize and for those indicated by
the Hyperchem calculations as the most abundant species (see
above) by performing DFT calculations with the CAM-B3LYP/6-
311G + (2d,p) method, simulating the solvent effect by the CPCM
model. Theresultsreproduced satisfactorilythe experimentaltrend
of the absorption properties and showed that the spectral band is
assignable to the first S0 ! S1 transition, described ꢂ97% by the
E
25000
Z
DHP
20000
P
15000
10000
5000
0
HOMO ! LUMO,
p
!
p*-type, configuration.
The computed changes in electron density (EDC) on going from
S0 to S1 under excitation (Table 5) are interesting. Indeed, they
showed, in addition to the well-known decrease of the ethenic
bond order, responsible for the twisting towards the E isomer, a
density increase between the two carbon atoms implied in the ring
closing process (reaction center) for the isomers with n = 5 and 4,
particularly for the latter. This indicates that the Franck–Condon
state reached by irradiation is already disposed towards DHP
250
300
350
400
450
λ (nm)
Fig. 3. Absorption spectra of E-4-StOx and its photoproducts in CH.

100
V. Botti et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 95–103
Table 2
Computed dipole moment (m) and total energy (ETOT), derived relative abundance (%) at room temperature, transition energy (l) and oscillator strength (f) of the conformers
of the E and Z isomers of 2-StOx.a
Conformer
m
(D)
ETOT (kcal mol-1)
ꢀ343868.531
% (293K)
36
l
(nm)
f
lexp
(nm)
E(A)
0.72
1.67
0.81
312
279
260
241
217
0.94
0.0003 (TC)
0.009 (n,
0.09
0.37
S1
S2
S3
S4
S6
299
p*)
227
299
E(B)
Z(A)
ꢀ343868.875
ꢀ343865.125
64
99
311
278
257
241
217
0.91
0.0003 (TC)
0.01 (n,
0.13
S1
S2
S3
S4
S6
p*)
0.37
227
294
318
280
279
245
221
0.71
0.002 (TC)
0.01 (n,
0.13
S1
S2
S3
S4
S6
p*)
0.3
224
Z(B)
1.75
-343862.625
1
a
The last column shows the experimental transition energies for comparison.
formation and reflects the experimental results (highest fZ!DHP
for n = 4). As said above, for n = 2, where the N atom of the oxazole
ring is faced to the C atom of the phenyl ring at the reaction center,
no cyclization was observed. CN bond formation has been
described in a limited number of cases (see, for example, the
earliest case of the formation of benzimidazo[1,2-a]quinoline from
2-styrylimidazoles [36] and the recent deep investigation on 2-
styrylquinolines [37]. However, no sign of CN cyclization was found
in the present case.
A planar structure was computed for the latter isomer (see
Table SI.1), probably stabilized by a Hꢀꢀbond between N and the
ortho H atom of the phenyl ring, as suggested by the computed
Table 3
Computed dipole moment (m) and total energy (ETOT), derived relative abundance (%) at room temperature, transition energy (l) and oscillator strength (f) of the conformers
of the E and Z isomers of 4-StOx.a
Conformer
m
(D)
ETOT (kcal molꢀ1
ꢀ343865.750
)
% (293 K)
99
l
(nm)
f
lexp (nm)
E (A)
1.05
285
280
275
251
215
1.01
0.11(TC)
0.08
0.01 (n,p*)
0.41
S1
S2
S3
S4
S7
286
278
E (B)
1.40
ꢀ343863.188
1
Z (A)
1.32
1.68
ꢀ343859.656
ꢀ343861.875
2
Z (B)
98
268
266
249
244
196
195
0.006 (Ph)
0.06 (H–L)
S1
S2
S3
S4
S11
S12
272
220
0.01 (n,
0.45
p*)
0.76
0.75
a
The last column shows the experimental transition energies for comparison.

V. Botti et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 95–103
101
30000
1.0
0.8
0.6
0.4
0.2
0.0
not irradiated
E
Z
irr. time = 208 min
25000
20000
15000
10000
5000
0
250
300
350
400
250
300
350
λ
(nm)
λ (nm)
Fig. 6. Absorption spectra of E- and E-2-FEt-5Ox in CH.
Fig. 4. Spectral evolution of E-2-StOx in CH at increasing irradiation times
(lexc = 299 nm) up to the photostationary state.
O
N
O
O
ꢁ
^
N
distance between N and H (2.1 Å) and NꢀꢀHꢀꢀC angle (147 ), both
values typical of a “real” H-bond formation [38]. It should be noted
that Z-2-StOx displays the highest reactivity towards the E isomer
(Table 4).
O
A rotamer (0%)
B rotamer (15%)
It is difficult to compare the photoreaction quantum yields
measured at the first stages of irradiation with the results obtained
by prolonged irradiation in preparative conditions (where the
E ! Z ! DHP consecutive process is followed), as described above.
Anyway the comparison shows that Z-2-StOx does not cyclize in
both conditions.
O
N
O
O
N
O
C rotamer (85%)
D rotamer (0%)
Scheme 5. Conformational equilibrium of Z-2-FEt-5Ox.
Table 4
Quantum yields of the radiative and reactive processes and lifetime of the DHP
intermediate for the positional isomers of n-styryloxazole in deaerated CH.
3.2. E-5-(2-(furan-2-yl) ethenyl) oxazole (2-FEt-5Ox)
Replacement of the phenyl group of 5-StOx with another
heteroaromatic group of different electron donor/acceptor (D/A)
properties can modify the photobehaviour. We examined the effect
of an electron rich furyl group (the E isomer is shown in Scheme 1).
It should be noted that the D/A character of the oxazole group is
rather ambiguous and depends on the attachment position. Its
second heteroatom (the tertiary nitrogen) contributes one electron
to the aromaticity of the ring and the lone pair of electrons in the
n-StOx
n = 2
n = 4
n = 5a
fF (E isomer)
fE ! Z
fZ ! E
fZ ! DPH
tDHP (min)
0.010
0.38
0.40
Not observed
0.036
0.48
0.18
0.072
0.57
0.14
0.06
0.014
210 (116)b
200 (90)b
a
Data for 5-StOx from Ref. [19] reported for comparison.
Data in parenthesis refer to oxygenated solutions.
b
molecular plane lowers the
p
-orbital energy level. The oxazole is
0.10
A
B
0.08
0.08
0.06
0.04
0.02
0.00
0.07
0.06
0.05
+ nitrogen
+ oxygen
350
400
450
0
50
100
150
t (min)
200
250
300
350
λ
(nm)
Fig. 5. Spectral evolution of the DHP formed from 4-StOx in deaerated CH kept 1040 min in the dark at room temperature (A) and corresponding kinetics in deaerated and
oxygenated solutions (B) (at short wavelengths the formation of the first peak of the final P product at 325 nm can be also observed).

102
V. Botti et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 95–103
Table 5
Frontier molecular orbitals and changes in electron density (EDC, isodensity = 0.0004) for the S0 ! S1 transition (violet and light-blue colours refer to an increase and a
decrease in the ED, respectively) obtained by TDDFT calculations.
Z
pH
pLÃ
EDC
St
2-StOx(A)
4-StOx(B)
5-StOx(A)
2-FEt-5Ox(A)
2-FEt-5Ox(C)
then characterized by higher ionization potentials (9.83 eV) and
displays weaker electron donor properties with respect to furan
to what described above for 2-StOx, the formation of a strong
interaction of Hbond character. The only conformer (A) suitable to
a ring closure (see the sizable electron density in Table 5) has a
distorted structure owing to severe steric interactions (Table SI.1).
The consequent rather higher energy level of this isomer implies in
practice its absence in the conformational equilibrium at room
temperature and could then explain why no trace of DHP was
found in our experiments (see below).
The fE!Z value (0.38) was found slightly smaller than those
previously reported for 5-StOx and its p-OMe derivative [19].
Considering also the very low emission yield, a non-negligible part
of the excited molecules probably decays by IC. However, a
contribution of ISC to the triplet manifold cannot be excluded. The
fZ!E value (0.24) is similar to that measured for the analogous 5-
StOx bearing the donor p-OMe substituent [19]. Despite the
absence of DHP and P (P = furobenzoxazole, FBOx) formation in the
mild conditions used for our irradiations (short irradiation time,
low conversion, low light dose and low concentration), the
experiments on preparative scale, using iodine as oxidant and
performed in benzene at ꢂ40 ꢁC until full conversion of the
starting reagents (ꢂ24 h of irradiation), led to FBOx with a
notable chemical yield of 63% [23]. Evidently the presence of
iodine, able to accelerate the oxidation rate (kox) to P to detriment
of the ring opening of DHP, was crucial for the formation of the
final product.
(8.88 eV) and other
density map of the oxazole ring indicates that it still remains a
weak donor [40,41]. In any case, the nature of the two side
p-rich analogues [39]. In any case the electron
p
heteroaryl groups of 2-FEt-5Ox suggests a very weak charge
transfer (CT) character of the S0 ! S1 transition and the probable
absence of further intramolecular (ICT) processes in the excited
state.
The weak CT character is in agreement with the absence of
important changes of the absorption spectrum (Fig. 6) with solvent
polarity, the lmax in MeCN being shifted by only 2 nm towards the
blue (321 nm) with respect to CH (323 nm). The fluorescence yield
was found to be very low (fF = 0.007 in CH) and the fluorescence
spectrum did not shift on going from CH to anisole and MeCN
pointing to the absence of any fluorosolvatochromism. In principle,
four conformers (Scheme 5) are expected for Z-2-FEt-5Ox.
The calculations of Table SI.2 indicate that the most stable ones
are those where OH intramolecular H-bonds can favour planar
structures (B and C rotamer in Scheme 5). However, they cannot
lead to a ring closure which would imply formation of a new CC
bond, as confirmed by the DFT calculations on the C conformer
(Table 5) showing the absence of electron density at the reaction
^
center. The computed distance between O and H (2.2 Å) and OHC
angle (125ꢁ) of the C conformer indicate also in this case, similarly

V. Botti et al. / Journal of Photochemistry and Photobiology A: Chemistry 316 (2016) 95–103
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Acknowledgements
The authors thank the Ministero per l’Università e la Ricerca
Scientifica e Tecnologica, MIUR (Rome, Italy) [PRIN “Programmi di
Ricerca di Interesse Nazionale” 2010–2011 (2010FM738P) as well
as the Ministry of Science, Education and Sports of the Republic of
Croatia (grant nos. 125-0982933-2926) for fundings.
The authors thank also Mr. Danilo Pannacci for his technical
assistance in HPLC measurements.
Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version, at http://dx.doi.org/10.1016/j.
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