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

Authors

  • Dr. Benedetta Carlotti,

    Corresponding author
    1. Department of Chemistry and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN), University of Perugia, via Elce di Sotto 8, 06123 Perugia (Italy), Fax: (+39) 755855598
    • Department of Chemistry and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN), University of Perugia, via Elce di Sotto 8, 06123 Perugia (Italy), Fax: (+39) 755855598
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  • Dr. Ilijana Kikaš,

    1. Department of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, Marulicev trg 19, 10000 Zagreb (Croatia)
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  • Prof. Irena Škorić,

    1. Department of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, Marulicev trg 19, 10000 Zagreb (Croatia)
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  • Prof. Anna Spalletti,

    1. Department of Chemistry and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN), University of Perugia, via Elce di Sotto 8, 06123 Perugia (Italy), Fax: (+39) 755855598
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  • Prof. Fausto Elisei

    1. Department of Chemistry and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN), University of Perugia, via Elce di Sotto 8, 06123 Perugia (Italy), Fax: (+39) 755855598
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Abstract

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 1LE*→1CT* 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.

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