Femtosecond fluorescence upconversion study of rigid dendrimers containingperyleneimide chromophores at the rim

The kinetics of a newly synthesized series of a first generation of polyphe nylene dendrimers in which one phenyl in a dendritic arm was para-substitut ed by a peryleneimide chromophore are reported. One such peryleneimide chro mophore is attached to 1, 3 or 4 arms. The results are compared to a series of polyphenylene dendritic compounds, which are identical except for the s ubstitution at a meta-position of a phenyl ring. The para-substitution yiel ds a better spatial definition of the peryleneimide units relative to one a nother and the influence of this aspect on the kinetics is studied. Four di fferent kinetic components were resolved for both groups of dendrimers. An ultra-short component varying from 500 fs to 2 ps and attributed to intramo lecular vibrational redistribution (IVR) is identical for both series. The decay time of a second component, which is comprized of the vibrational rel axation and a singlet-singlet annihilation process observed in both substit uted dendrimer series, is shorter in the para-substituted dendrimers compar ed to the meta-substituted ones. It is also shown that the annihilation pro cess, which is only present in the multi-chromophoric compounds of both the series and resolved with an excitation energy dependent study, has definit ely a larger contribution in the partial amplitudes for the para-substitute d compounds. This is related to the relative orientation of the transition dipoles of the chromophores in both the series. (C) 2001 Elsevier Science B .V. All lights reserved.