title: Lifetime shortening and fast energy-tansfer processes upon dimerization of a A-π-D-π-A molecule.
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.
authors.affiliation: Dipartimento di Scienze Chimiche and UdR INSTM, Università di Padova, Via Marzolo 1, 35131 Padova (Italy).
authors.forename: Elisabetta
authors.initials: E
authors.lastname: Collini
authors.forename: Luca
authors.initials: L
authors.lastname: Bolzonello
authors.forename: Mirco
authors.initials: M
authors.lastname: Zerbetto
authors.forename: Camilla
authors.initials: C
authors.lastname: Ferrante
authors.forename: Norberto
authors.initials: N
authors.lastname: Manfredi
authors.forename: Alessandro
authors.initials: A
authors.lastname: Abbotto
keywords: charge transfer; fluorescence; molecular dimers; relaxation dynamics; supramolecular chemistry