Photophysics of Untethered ZnTPP-Fullerene Complexes in Solution

The spectroscopy and dynamic behavior of the self-assembled, Soret-excited zinc tetraphenylporphyrin (ZnTPP) plus fullerene (C-60) model system in solution has been examined using steady state fluorescence quenching, nanosecond time-correlated single photon counting, picosecond fluorescence upconversion, and picosecond transient absorption methods. Evidence of ground state complexation is presented. Steady-state quenching of the S-2 and S-1 fluorescence of ZnTPP by C-60 reveals that the quenching processes only occur in the excited complexes, are ultrafast, and proceed at different rates in the two states. Only uncomplexed ZnTPP is observed by fluorescence lifetime methods; the locally excited complexes are either dark or, more likely, rapidly relax to products that do not radiate strongly. Both short-range (Dexter) energy transfer and electron transfer relaxation mechanisms are evaluated. Picosecond transient absorption data obtained from the subtle differences between the spectra of Soret-excited ZnTPP with and without a large excess of added C-60 reveal the formation, on a subpicosecond time scale, of relatively long-lived charge-separated species. Soret excitation of ZnTPP center dot center dot center dot C-60 does not produce a quantitative yield of species in the lower S-1 excited state.