Ilijana Kikaš

Photophysics of Push–Pull Distyrylfurans, Thiophenes and Pyridines by Fast and Ultrafast Techniques

Time-resolved transient absorption and fluorescence spectroscopy with nano- and femtosecond time resolution were used to investigate the deactivation pathways of the excited states of distyrylfuran, thiophene and pyridine derivatives in several organic solvents of different polarity in detail. The rate constant of the main decay processes (fluorescence, singlet-triplet intersystem crossing, isomerisation and internal conversion) are strongly affected by the nature [locally excited (LE) or charge transfer (CT)] and selective position of the lowest excited singlet states. In particular, the heteroaromatic central ring significantly enhances the intramolecular charge-transfer process, which is operative even in a non-polar solvent. Both the thiophene and pyridine moieties enhance the S1 →T1 rate with respect to the furan one. This is due to the heavy-atom effect (thiophene compounds) and to the (1) (π,π)*→(3) (n,π)* transition (pyridine compounds), which enhance the spin-orbit coupling. Moreover, the solvent polarity also plays a significant role in the photophysical properties of these push-pull compounds: in fact, a particularly fast (1) LE*→(1) CT* process was found for dimethylamino derivatives in the most polar solvents (time constant, τ≤400 fs), while it takes place in tens of picoseconds in non-polar solvents. It was also shown that the CT character of the lowest excited singlet state decreased by replacing the dimethylamino side group with a methoxy one. The latter causes a decrease in the emissive decay and an enhancement of triplet-state formation. The photoisomerisation mechanism (singlet/triplet) is also discussed.

Synthesis, Photochemistry, and Photophysics of Butadiene Derivatives: Influence of the Methyl Group on the Molecular Structure and Photoinduced Behavior

Novel butadiene derivatives display diverse photochemistry and photophysics. Excitation of 2-methyl-1-(o-vinylphenyl)-4-phenylbutadiene leads to the dihydronaphthalene derivative, whereas photolysis of the corresponding model o-methyl analogue results in the formation of the naphthalene-like derivative, deviating from the nonmethylated analogue of the prior starting compound and producing benzobi- and -tricyclic compounds. The effect of the methyl substituents is even more dramatic in the case of the dibutadienes. The parent unsubstituted compound undergoes photoinduced intramolecular cycloaddition giving benzobicyclo[3.2.1]octadiene, whereas the photochemical reaction of the corresponding dimethylated derivative shows only geometrical isomerization due to the steric effect of the substituents. Methyl groups on the butadiene backbones reduce the extent of conjugation, causing a blue-shift of the characteristic absorption band. The fluorescence efficiency is dramatically decreased, as a consequence of nonplanarity and reduced rigidity of the molecules due to the crowding by the methyl and phenyl groups together. Four molecules of very similar structures show dramatically different photoinduced behavior, revealing how changes of the nature and position of the substituents are valuable in understanding the photophysics and photochemistry of these types of compounds.