engleski

Ultrafast photoinduced intramolecular charge transfer in push−pull distyrylfuran, thiophene and pyridine

Six novel asymmetric distyryl−furan, thiophene and pyridine derivatives, bearing an electron acceptor (nitro) and an electron donor (methoxy or dimethylamino) group, have been prepared. Steady −state and time−resolved transient absorption and fluorescence spectroscopies with ns− and fs− resolution were used to obtain a completedescription of the excited state dynamics in several organic solvents.1, 2 These compounds display remarkable positive fluorosolvatochromism in agreement with a strong enhancement of dipole moment under excitation and relatively high hyperpolarizabilities, which make them interesting for NLOapplications. The rate constants of fluorescence, intersystem crossing, isomerization and internal conversion are strongly affected by the nature (locally excited, LE, or charge transfer, CT) and relative position of the lowest excited singlet states. The main result found in this study is the effect of the central heteroaromaticring, which strongly favours the intramolecular charge transfer by acting as an auxiliary acceptor/donor unit. This role can explain why the 1CT* state here was surprisingly detected even in solvents of very low polarity, such as cyclohexane. Solvent polarity plays a significant role: a particularly fast 1LE* → 1CT* process was found in the more polar solvents (τ ≤ 400 fs) while it takes place in tenths of ps in non−polar solvents. It was also shown that the CT character of the lowest excited singlet decreased by replacing the dimethylaminegroup by a methoxy one. This latter causes an enhancement of the triplet state formation. Both the thienyl and pyridyl moieties enhance the S1 → T1 rate with respect to the furyl one. This is due to the heavy atom effect (thienyl−compounds) and to the 1(π, π)* → 3(n, π)* transition (pyridyl−compounds) which enhance the spin−orbit coupling. TD−DFT calculationsallowedinformation on the singlet and tripletstateorder and nature to be obtained. The photoisomerization mechanism (singlet/triplet) was also clarified. A low reactivity was found in the case of the furan derivatives in agreement with a rotation in the singlet manifold, a mixed mechanism was proposed for onethiophene derivativewhile the short−living triplet states reported for the pyridine−derivatives point to an isomerizationoccurring in the triplet manifold.

intramolecular charge transfer; isomerization mechanism; push−pull compounds

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