Investigating electron-transfer processes of supramolecular donor-acceptor complexes using femtosecond transient absorption spectroscopy

Abstract

A femtosecond transient absorption spectroscopy system was developed and used to characterize the charge-transfer processes which take place between photo-excited porphyrin and C60-fullerene supramolecular donor-acceptor complexes, specifically the free-base (TPP) and zinc metal porphyrins (ZnTPP). A comprehensive photophysical characterization of the TPP-C60 and ZnTPPC60 complexes, along with the individual moieties, was performed which included ground-state absorption, steady-state fluorescence, and femtosecond transient absorption spectroscopic data, from which Jablonski diagrams and kinetic models could be constructed. The electron transfer processes were monitored along spectral traces at 1000 nm in the difference absorption spectra, where the C􀀀 60 anion appears. Upon photo-excitation at 387.5 nm with pulses of 150 fs time-duration and pump beam fluency of 1.02 mJ/cm2, the kinetic rates of charge-separation (kCS) from donor to acceptor were determined to be 8.3 1011 s􀀀1 (1.2 ps) and 5.5 1011 s􀀀1 (1.8 ps) for the TPP-C60 and ZnTPP-C60 complexes, respectively. Charge-recombination (kCR) rates from the acceptor to donor were 8.4 109 s􀀀1 (119 ps) and 5.4 109 s􀀀1 (183 ps) for the TPP-C60 and ZnTPP-C60 complexes, respectively. The ratio of chargeseparation and charge-recombination kinetic rates (kCS/kCR) exceeded 100 for both TPP-C60 and ZnTPP-C60 complexes, showing that these simple, selfassembled, supramolecular complexes have the ability to extend the process of charge-recombination after fast separation, a key characteristic necessary for an effective solar cell.