Two nitro-substituted 1,4-distyrylbenzene-like compounds have been investigated using stationary and time-resolved (ns/fs) spectrometric techniques as a function of solvent polarity. In the two ...compounds the central benzene ring is substituted with a p-nitrostyryl group at one side while, at the other side, compound 1 (asymmetric) bears a pyrid-4-ylethenyl group and compound 2 (symmetric) another p-nitrostyryl group. The solvent dependent intramolecular charge transfer (ICT) in the singlet manifold was found to strongly affect the competition among fluorescence, intersystem crossing and trans-cis photoisomerization. The presence of nitro-groups in the 1,4-distyrylbenzene skeleton causes the usual strong decrease of fluorescence in favour of intersystem crossing to a reactive triplet state. However, the favoured formation of the ICT state in polar solvents induces an unexpected important increase of the fluorescence quantum yield (three/two order of magnitude for the nitro and dinitro derivatives, respectively). The ultrafast spectral transients helped to understand the solvent effects measured by stationary techniques and gave information on the dynamics of the locally excited singlet state ((1)LE*) and the (1)ICT* state, fast produced in polar solvents. Evidence of dual fluorescence in a limited range of solvent polarity, particularly for compound 1, is also reported. The role of an upper triplet state in a non-polar solvent is discussed also based on quantum-mechanical calculations (TD-DFT method) and temperature effects on the photophysical parameters.
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► The solvatochromism of six ethenyl and four ethynyl push–pull compounds was investigated. ► The ground (μg) and excited state (μe) dipole moments were calculated. ► The μe(ICT) of ...the solvent-stabilized state formed by the ICT process were obtained. ► The quadratic hyperpolarizability, βCT, was then derived. ► These photostable compounds can offer potential interest for their NLO responses.
Emission or both absorption/emission solvatochromic effects induced by intramolecular charge transfer (ICT) have been used to determine the excited-state dipole moments of the locally excited μe(LE) and the polar-solvent induced μe(ICT) states and the first hyperpolarizability parameter βCT of six asymmetric distyrylbenzene analogues, two 9,10-(diarylethynyl)anthracene derivatives and two 2,7-(diarylethynyl)fluorene analogues. The change in dipole moment under excitation (Δμeg) was also evaluated by theoretical calculations and compared with experimental data. All compounds display more or less remarkable positive solvatochromism which leads to relatively high hyperpolarizabilities and makes them interesting candidates for applications associated with photoinduced ICT. The structural effect on the transfer is also briefly discussed and related to the relaxation properties (previously investigated) of these compounds.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The isomerization and cyclization photoreactions of the cis (Z) isomers of n-styrylnaphthalenes (n = 1 and 2), n-styrylphenanthrenes (n = 1, 2, 3, 4, and 9), and two related compounds, ...3-styrylchrysene and 3-styrylbenzocphenanthrene, were investigated by spectrophotometric and chromatographic techniques. The quantum yields of the two photoreactions were measured in aerated and deaerated nonpolar solvent at room temperature and compared with those reported in the literature for some of the molecules investigated and other related compounds. The combined use of high-performance liquid chromatographic and spectrophotometric techniques made easier the separation of the components of the photoreaction mixtures thus simplifying the study of the isomerization/cyclization competition and the measurement of the UV−vis absorption spectra and the thermal decay kinetics of the dihydrophenanthrene-type intermediates. The conformational equilibria in the ground state and the positional isomerism (n values) notably affect the prevalence of one or the other competitive photoreaction. Oxygen also plays an important role: practically it does not affect the quantum yield of the Z → E process but has a drastic effect on the formation of the final oxidation product which can proceed even in the presence of traces of air. In three cases, non-negligible formation of a side colorless product was detected. It was assigned to an isomer of the normal photocyclization intermediate (4a,4b-dihydrophenanthrene-type), formed by shift of one or both hydrogen atom(s) to other positions of the rings. Parallel ab initio calculations of the potential energy of the possible conformers helped to explain the structure effects on the competitive photoreactive relaxation pathways. Some correlations between the computed reagent/product energy difference in the ground state and the photocyclization yield and thermal stability of the intermediates were also verified.
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IJS, KILJ, NUK, PNG, UL, UM
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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The photobehavior of two 1,4-distyrylbenzene analogues where the central benzene ring is asymmetrically substituted with a pyrid-4-ylethenyl group at one side and thien-2-ylethenyl or a p-nitrostyryl ...group at the other side, has been studied in two solvents at room temperature. The four geometrical isomers (EE, ZE, EZ, and ZZ) of each compound were separated by the combined use of HPLC and spectrophotometric techniques. The radiative/reactive competition in their excited state relaxation was particularly examined: the diabatic/adiabatic contributions were estimated and a reasonable interpretation of the photoisomerization mechanism was proposed. The role of the conformational isomers was also investigated by measured and computed spectral data. Since the different electron donor/acceptor character of the side groups of these molecules can induce charge transfer phenomena that can affect the relaxation pathways of their excited states, the photobehavior was compared in inert and polar solvents to clarify the role of the intramolecular charge transfer. The latter was found to affect markedly the relaxation properties and to induce interesting fluorosolvatochromic effects, particularly in the p-nitro derivative. The participation of the triplet state in the reaction mechanism of the latter was also investigated by flash photolysis and sensitized experiments.
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IJS, KILJ, NUK, PNG, UL, UM
•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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
► 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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The emission properties of some aza-stilbenes (2-, 3- and 4-styrylpyridine) and thio-stilbenes 2- and 3-styrylthiophene and 1,2-di-(3-thienyl)ethene have been investigated after inclusion in ...commercial (NaY) and cation-exchanged (TlY) faujasite zeolites to get information on the triplet properties through population of the T
1 state induced by the heavy atom effect. The fluorescence properties in NaY and TlY were compared with those reported in solution. The phosphorescence spectra, observed in TlY at liquid nitrogen temperature, allowed the energy levels of the T
1 states to be obtained. Phosphorescence lifetimes were also measured. Their comparison with the lifetime known for stilbene showed that the radiative decay is little affected by the heteroatoms.
► The exchange of Na
+ with heavy Tl
+ cations in faujasite zeolites allowed the triplet properties of some hetero-stilbenes to be obtained. ► The absorption and fluorescence spectra in NaY and TlY were measured and compared with those in fluid solutions. ► The triplet energy levels and lifetimes of three aza-stilbenes and three thio-stilbenes in TlY were determined by measuring their phosphorescence emission at liquid nitrogen temperature.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
