Charge transfer processes in conjugated triarylamine-oligothiophene-perylenemonoimide dendrimers

The synthesis and charge transfer properties of triarylamine-oligothiophene-perylenemonoimide dendrimers, TPA(T2-PMI)3 and TPA(T4-PMI)3, are described. The fluorescence quantum yields indicate strong emission quenching by electron transfer [¼THF ) 0.004 for TPA(T2-PMI)3, ¼THF ) 0.003 for TPA(T4-PMI)3, and ¼THF ) 0.8 for PMI]. Moreover, with the increase of the solvent polarity, the quantum yields decrease indicating that the A+¥D-¥ (acceptor/donor) couple is more stabilized. The femtosecond transient absorption spectra show a very fast charge separation process (2 ps; kcs 5 1011 s-1) and a charge recombination of more than 1 order of magnitude slower (50 ps; kcr 2 1010 s-1), as observed from the rise time and decay of the radical anion and radical cation absorption bands. The analysis of the transient absorption spectroscopy and of the energetics of the process using Marcus theory indicates that in the electron transfer process the thiophene unit is the first electron donor. The triarylamine is not functioning as a second electron donor, as also substantiated by the absence of an effect of the addition of acid on the emission intensity of the dendrimers. The presence of the triarylamine and/or the proximity of the oligothiophenes does improve the donor capabilities of the oligothiophene unit slightly and enhances its conjugation as seen in the absorption spectra and the transients of the radial cations. These results can be used for a better design of molecular materials for, e.g., photovoltaic applications.