Different substituents at the sydnone ring can drive the photoreactivity towards different products. In particular, phenyl-substitution favours the formation of benzodiazepine derivatives
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•Different substituents at the sydnone ring can drive the photoreactivity towards different products.•Phenyl-substitution at the sydnone ring favours the formation of benzodiazepine derivatives interesting for biomedical applications.•Ns flash photolysis gave insight in the transient precursors responsible for the different reactivity of the substituted sydnones.•Oxygen effect and energy transfer from triplet donors measurements added information about the mechanism of the photoreactions.•QM calculations evidenced low-lying forbidden states with CT character or completely localized on the sydnone, the most photoreactive moiety.
This work follows an earlier one where some sydnone derivatives, interesting for their biological and biomedical applications, were synthesized and the effect of substituents on their photoreactivity was studied in irradiation conditions of high intensity and high substrate concentrations. The aim of the present work is to investigate the deactivation pathways of the excited states of three of these compounds, bearing a stilbene moiety, and to determine the quantum yield of the competitive fluorescence/photoreactions in soft irradiation conditions. Flash photolysis measurements with nanosecond resolution were also carried out to obtain information on the transient intermediates responsible for the photoproducts found in the previous work. The oxygen effect on the photoproducts and the energy transfer from triplet donors were also investigated. The experimental results, supported by quantum chemical calculations, showed that different substituents at the sydnone ring can drive the photoreactivity towards different products, thus confirming the results of the previous work. In particular, phenyl-substitution favours the formation of benzodiazepine derivatives.
Strong increase of basicity under excitation of n-pyridyl,5-oxazolylethenes. The pKa* of S1 derived by the Förster cycle and directly measured by fluorimetric titration for the longer lived isomer ...with n=3.
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•The acid-base properties of the positional isomers of E-n-pyridyl,5-oxazolyl-ethene (n=2, 3 and 4) in the ground and excited states were investigated.•The pKa* of the S1 state, estimated by the Förster cycle and measured by fluorimetric titration for the longer-lived isomer with n=3, showed that they become much stronger bases under excitation.•The deactivating effect of the n,π* states of the pyridyl group, observed in non-polar solvent, disappears in the protonating medium for n=2 and 4 which photoisomerize with high yield.•The isomerization yield of the protonated isomer with n=3 is instead quite low due to changes in the excited state character from ethenic to charge transfer.•Quantum-mechanical calculations helped to describe the conformational equilibria and the charge transfer process from the oxazole group towards the pyridinium moiety.
The effect of protonation on the excited state properties of the three positional isomers of E-5-2-(pyrid-n-yl)ethenyloxazole’s (n-PyEt-5Ox with n=2, 3 and 4) was studied in buffered water by spectrophotometric, fluorimetric and chromatographic measurements. The clear shift to the red of the absorption and emission spectra by protonation indicates that the basicity of these compounds increases strongly (by ∼7pH units) under excitation. Ground state pKa values (around 5) were measured by absorption (for n=2 and 4) and fluorescence (for n=3) titration and excited state pKa* values were estimated by application of the Förster cycle. For the compound with n=3, which displays a rather higher fluorescence lifetime, it was possible to perform a direct fluorimetric titration which showed two inflection points corresponding to pKa (4.77) and pKa* (12.27). Previous works on these compounds had shown a marked deactivating effect of the pyridine n,π* states on the emitting and reactive π,π* states (proximity effect) favouring relaxation by internal conversion with a marked reduction of the competitive fluorescence and trans-cis (E–Z) isomerisation yields, particularly for n=2 and 4 in non-polar cyclohexane. The new results indicate that the aqueous solvent, and mainly protonation, affect sensibly the spectral, photochemical and photophysical behaviour increasing again the radiative and reactive parameters of compounds with n=2 and 4. The isomer with n=3, whose n,π* states play a much smaller deactivating effect, shows a peculiar behaviour with a very low isomerisation yield at acid pH, reasonably due to intramolecular charge transfer from the Ox ring to the PyH+ moiety, which hinders the twisting process. All these results are compared with those reported in the literature for the n-styrylpyridine analogues to point out the role of the oxazole group.
•Competitive photoreactions of n-styryloxazole (n=2, 4 and 5) were investigated.•Positional isomerism affects photocyclization more than photoisomerization.•Effects of conformers and intramolecular ...Hbonds evidenced by QM calculations.•Analog compound bearing two heteroaromatic rings (oxazole and furan) was also studied.
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 Hbonds 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.
► Six new push–pull distyrylbenzene analogues have been synthesized. ► Spectral and relaxation properties have been measured. ► The effect of furan, thiophene or pyridine central groups has been ...investigated. ► Pyridine derivatives have the main effect on the emission/reaction competition.
Six novel asymmetric 2,5-distyryl-furan, 2,5-distyryl-thiophene and 2,6-distyrylpyridine derivatives, bearing an electron acceptor (p-nitro) group at one side and an electron donor (p-methoxy or p-dimethylamino) group at the other side, have been prepared. The experimental absorption properties have been measured and compared with the computed parameters. Theoretical and experimental results indicate that one conformational isomer (the compressed one) is largely prevalent in all compounds. The measured radiative and reactive relaxation properties of these donor/acceptor disubstituted compounds were also compared with those of the unsubstituted analogues previously investigated. The presence of the donor/acceptor groups leads to a significant increase of the charge transfer character of both the ground and the excited states and to strong red shifts of the absorption spectra, an effect that can be useful for potential applications in material science. The fluorescence/photoisomerization competition of the furan and thiophene derivatives was found to be rather similar to that of the unsubstituted analogues whereas significant changes were found for the pyridine derivatives where a drastic decrease of fluorescence, a sizable increase of photoisomerization and a predominance of radiationless deactivation to the ground state becomes operative.
•The chemical and photochemical yields of isomerization and cyclization of 5-2-(pyrid-n-yl)ethenyloxazoles were determined.•The n,π* states play an important deactivating role dependent of the ...position (n) of the pyridine moiety.•QM calculations helped to describe the role of conformers on photoreactivity and the isomerization/cyclization competition.•A very high regioselectivity was found for the cyclization of one of the conformers with n=3.•Solvent photoaddition and formation of a second dihydrophenanthrene-type intermediate were evidenced for compound with n=4.
This paper describes the results obtained in the study of the photobehaviour of heteroanalogs of stilbene bearing oxazole and pyridine rings at the opposite sides of the ethene moiety. The effect of the positional isomerism of the n-pyridyl group (n=2, 3 and 4) on the competitive relaxation processes of the excited states (fluorescence, isomerization and cyclization) was investigated and compared with the behaviour previously reported for the 5-styryloxazole analogue.
The group in Zagreb prepared the compounds and followed their photochemistry in preparative conditions while the group in Perugia studied the spectral properties of the trans/cis isomers and the cyclization products and measured the quantum yields of the competitive processes in mild conditions with the main aim to clarify the mechanism of the primary stages after excitation. The photobehaviour revealed an important deactivation effect of the n,π* states introduced by the nitrogen atom. This effect reflects the one reported for styrylpyridines, the analogous compounds with a phenyl instead of an oxazolyl ring. Quantum-mechanical Hyperchem calculations proved to be useful to describe the conformational equilibria and the role of conformers on photoreactivity while more refined DFT calculations on the Z isomers allowed the structure dependent competition between their isomerization/cyclization processes and the possible role of intramolecular H-bonds on the deactivation pathways to be explained. For the compound with n=4, side processes of hydrogen shift in the primary dihydrophenanthrene-like intermediate and of solvent addition accompanying the photocyclization process were evidenced.
The heteroatom effect on the radiative and reactive relaxations of
E,
E-1,2-distyrylbenzene (DSB) has been studied by stationary and time-resolved techniques on three compounds where the side phenyl ...(Ph) groups were replaced by 4′-pyridyl (4P), 2′-thienyl (2T) and 2′-furyl (2F) groups. Contrary to the case of the hydrocarbon, whose photoisomerization is accompanied by substantial degradation, the
E,
E
→
Z,
E photoisomerization prevails in the hetero-analogues, particularly in the thienyl derivative. The lack of conjugation in these 1,2-substituted benzenes leads to a decrease in the fluorescence yield with respect to their 1,4 and 1,3 analogues (particularly in the presence of the heteroatoms), which is accompanied by an increase of the photoreaction quantum yield for the pyridyl- (significant) and thienyl- (very small) derivatives, whilst internal conversion is the prevalent deactivation pathway of the furyl derivative.
Sintetska organska fotokemija, kemija pobuđenih stanja molekula, razlikuje se od kemije u osnovnom stanju i omogućuje jednostavan pristup kompliciranim strukturama do kojih se često vrlo teško dolazi ...klasičnim sintetskim putem. U svome radu koristimo fotokemijsku metodologiju u sintezi hetero-policikličkih spojeva, potencijalno biološki aktivnih spojeva. Tako je priređeno niz novih konjugiranih polienskih sustava s heterocikličkom jezgrom (furan, tiofen, pirol, oksazol, sidnon i sl.) koji se osvjetljavanjem u određenim uvjetima prevode u hetero-policikličke derivate.
U ovom pregledu dan je naglasak na fotokemijsku sintezu policikličkih struktura s kisikom polazeći iz o-supstituiranih furanskih heterostilbena
New heteroaryl‐substituted o‐divinylbenzenes, 2,2′‐(1,2‐phenylenedivinylene)difuran (9), 2,2′‐(1,2‐phenylenedivinylene)bisbenzobfuran (10), and 2,2′‐(1,2‐phenylenedivinylene)bisnaphtho2,1‐bfuran ...(11), were prepared and irradiated at various concentrations; intramolecular photocycloaddition and intermolecular 2+2 twofold photoaddition reactions took place to give bicyclo3.2.1octadiene derivatives 12–14 and cyclophane derivatives 15–17, respectively. Compound 11 was the most selective of these o‐divinylbenzenes, which, owing to π–π intra‐ or intermolecular complexation, gave only the exo‐bicyclo3.2.1octadiene derivative 14 at low concentrations, and only the cyclophane derivative 17 at high concentrations.
A matter of concentration: The β,β′‐diheteroaryl‐substituted o‐divinylbenzenes 1–3 were irradiated and the photoproducts examined (see scheme): the bisnaphthofuran derivative 3 is the most selective, which, owing to π–π intra‐ or intermolecular complexation, gives only the exo‐bicyclo3.2.1octadiene derivative 4 upon irradiation at low concentrations and only the cyclophane derivative 9 at high concentrations.
A new 2-{2-2-(4-methylphenyl)ethenylphenyl}pyrrole (
11
) was prepared and transformed by a photochemical reaction furnishing two cyclised products 4,5-dihydro-4-(
p-methylphenyl)benzo
gindole (
12
) ...and 4
H-4-(
p-methylbenzyl)pyrrolo2,1-
aisoindole (
13
), and a reduction product, 2-{2-2-(4-methylphenyl)ethylphenyl}pyrrole (
14
).
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