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

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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.