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 s1 (1.2 ps) and 5.5 1011 s1 (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 s1 (119 ps) and 5.4 109 s1 (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.
