Unusual high fluorescence of two nitro-distyrylbenzene-like compounds induced by CT processes affecting the fluorescence/intersystem-crossing competition.

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.

Photobehavior of the geometrical isomers of two 1,4-distyrylbenzene analogues with side groups of different electron donor/acceptor character.

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.

Competition between photoisomerization and photocyclization of the cis isomers of n-styrylnaphthalenes and -phenanthrenes.

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-styrylbenzo[c]phenanthrene, 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.

Adiabatic pathways in the conformational and geometrical photoisomerizations of the 1,2-distyrylbenzene isomers.

The photophysical behavior of the geometrical isomers of 1,2-distyrylbenzene (1,2-DSB) has been investigated as a function of temperature by stationary and pulsed fluorimetric techniques in a nonpolar solvent. The temperature effect on the spectral properties, fluorescence quantum yields, and decay profiles allowed the role played by the conformational isomers of these molecules in the relaxation processes of the lowest excited states to be clarified. In the case of the EE geometrical isomer, a conformer (or rotamer) re-equilibration, implying an adiabatic interconversion during the lifetime in the S(1) state, was found to be operative. The deactivation channels of the excited cis isomers (ZE and ZZ) markedly depend on the conformational geometry, which is responsible for the adiabatic (rotamer-specific) cis --> trans photoisomerization. The radiative and radiationless relaxation rate parameters of the isomers and conformers of 1,2-DSB, derived from the experimental and calculation results, allowed a complete and reasonably quantitative description of their behavior in the S(1) state.

Fluorescence/photoisomerization competition in trans-aza-1,2-diarylethenes.

The effect of the presence and position of the nitrogen heteroatom on the photobehaviour of a series of symmetric and asymmetric 1,2-diarylethenes, where one or both the one-, two- or three-membered aryl groups, linked to the ethene bridge, contain(s) nitrogen heteroatoms, has been investigated by fluorimetric and photochemical techniques. The yield of the radiative pathway is generally reduced with respect to the corresponding hydrocarbons, due to a smaller energy barrier for trans-->cis (E-->Z) photoisomerization in the singlet state, which often can be slightly favoured in the aza-compounds. All compounds investigated exist in solution as mixtures of two or more conformers, which can have different spectral and kinetic properties. Attempts to separate the intrinsic properties by selective photoexcitation, at a wavelength where the light is mainly absorbed by one conformer, allowed in some favourable cases the spectra and decay times of the largely prevalent (quasi-pure) conformer to be obtained. In two cases, the formation of intramolecular (hydrogen-bond-type) interactions between the nitrogen atom and the nearest hydrogen atom of the ethene bridge (proved by NMR measurements and confirmed by theoretical calculations) were found to stabilize a specific conformer thus affecting a selective relaxation of the excited molecule.

Competitive radiative and reactive relaxation channels in the excited state decay of some thio-analogues of EE-distyrylbenzene.

The photophysical and photochemical properties of some thio-analogues of symmetrically substituted EE-1,4-distyrylbenzene (linear conjugation) and EE-1,3-distyrylbenzene (crossed conjugation), where thiophene rings replace the side benzene rings or the central benzene ring, have been investigated. The kinetic competition between the radiative and reactive relaxation channels of the lowest excited singlet state has been compared with that found for the corresponding hydrocarbons. The photobehaviour markedly depends on the type of conjugation and on the position (2' or 3') of the ethenic bridge with respect to the sulfur atom. The main effect of the heteroatom is an increase in the photoisomerization yield due to a decrease of the torsional barrier in S(1) and to an increase in the S(1)--> T(1) intersystem crossing, which opens the way to isomerization in the triplet manifold. Conformational equilibria, due to restricted rotation around the quasi-single bonds with the ethenic carbons, have also been investigated by selective photoexcitation for the compounds with side 3'-thienyl groups.