Lifetime Shortening and Fast Energy-Tansfer Processes upon Dimerization of a A-π-D-π-A Molecule

Authors

  • Dr. Elisabetta Collini,

    1. Dipartimento di Scienze Chimiche and UdR INSTM, Università di Padova, Via Marzolo 1, 35131 Padova (Italy)
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  • Dr. Luca Bolzonello,

    1. Dipartimento di Scienze Chimiche and UdR INSTM, Università di Padova, Via Marzolo 1, 35131 Padova (Italy)
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  • Dr. Mirco Zerbetto,

    1. Dipartimento di Scienze Chimiche and UdR INSTM, Università di Padova, Via Marzolo 1, 35131 Padova (Italy)
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  • Prof. Camilla Ferrante,

    Corresponding author
    1. Dipartimento di Scienze Chimiche and UdR INSTM, Università di Padova, Via Marzolo 1, 35131 Padova (Italy)
    • Dipartimento di Scienze Chimiche and UdR INSTM, Università di Padova, Via Marzolo 1, 35131 Padova (Italy)

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  • Dr. Norberto Manfredi,

    1. Department of Materials Science and Milano-Bicocca Solar Energy Research Center - MIB-Solar, University of Milano-Bicocca, UdR INSTM, Via Cozzi 53, I-20125, Milano (Italy)
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  • Prof. Alessandro Abbotto

    1. Department of Materials Science and Milano-Bicocca Solar Energy Research Center - MIB-Solar, University of Milano-Bicocca, UdR INSTM, Via Cozzi 53, I-20125, Milano (Italy)
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Abstract

Time-resolved fluorescence and transient absorption experiments uncover a distinct change in the relaxation dynamics of the homo-dimer formed by two 2,5-bis[1-(4-N-methylpyridinium)ethen-2-yl)]-N-methylpyrrole ditriflate (M) units linked by a short alkyl chain when compared to that of the monomer M. Fluorescence decay traces reveal characteristic decay times of 1.1 ns and 210 ps for M and the dimer, respectively. Transient absorption spectra in the spectral range of 425–1050 nm display similar spectral features for both systems, but strongly differ in the characteristic relaxation times gathered from a global fit of the experimental data. To rationalize the data we propose that after excitation of the dimer the energy localizes on one M branch and then decays to a dark state, peculiar only of the dimer. This dark state relaxes to the ground state within 210 ps through non-radiative relaxation. The nature of the dark state is discussed in relation to different possible photophysical processes such as excimer formation and charge transfer between the two M units. Anisotropy decay traces of the probe-beam differential transmittance of M and the dimer fall on complete different time scales as well. The anisotropy decay for M is satisfactorily ascribed to rotational diffusion in DMSO, whereas for the dimer it occurs on a faster time scale and is likely caused by energy-transfer processes between the two monomer M units.

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