2 research outputs found
Photoinduced Electron and Energy Transfer in a Molecular Triad Featuring a Fullerene Redox Mediator
In
order to investigate the possibility of a fullerene acting as
an electron and/or singlet energy relay between a donor chromophore
and an acceptor, a triad consisting of a fullerene (C<sub>60</sub>) covalently linked to both a porphyrin energy and electron donor
(P) and a β-tetracyanoporphyrin energy and electron acceptor
(CyP) was synthesized. Steady state and time-resolved spectroscopic
investigations show that the porphyrin first excited singlet state
donates singlet excitation and an electron to the fullerene and also
donates singlet excitation to the CyP. All three processes differ
in rate constant by factors of ≤1.3, and all are much faster
than the decay of <sup>1</sup>P–C<sub>60</sub>–CyP by
unichromophoric processes. The fullerene excited state accepts an
electron from P and donates singlet excitation energy to CyP. The
P<sup>•+</sup>–C<sub>60</sub><sup>•–</sup>–CyP charge-separated state transfers an electron to CyP to
produce a final P<sup>•+</sup>–C<sub>60</sub>–CyP<sup>•–</sup> state. The same state is formed from P–C<sub>60</sub>–<sup>1</sup>CyP. Overall, the final charge-separated
state is formed with a quantum yield of 85% in benzonitrile, and has
a lifetime of 350 ps. Rate constants for formation and quantum yields
of all intermediate states were estimated from results for the triad
and several model compounds. Interestingly, the intermediate P<sup>•+</sup>–C<sub>60</sub><sup>•–</sup>–CyP
charge-separated state has a lifetime of 660 ps. It is longer lived
than the final state in spite of stronger coupling of the radical
ions. This is ascribed to the fact that recombination lies far into
the inverted region of the Marcus rate constant vs thermodynamic driving
force relationship
Spectroscopic Analysis of a Biomimetic Model of Tyr<sub>Z</sub> Function in PSII
Using
natural photosynthesis as a model, bio-inspired constructs
for fuel generation from sunlight are being developed. Here we report
the synthesis and time-resolved spectroscopic analysis of a molecular
triad in which a porphyrin electron donor is covalently linked to
both a cyanoporphyrin electron acceptor and a benzimidazole–phenol
model for the Tyr<sub>Z</sub>-D<sub>1</sub>His190 pair of PSII. A
dual-laser setup enabled us to record the ultrafast kinetics and long-living
species in a single experiment. From this data, the photophysical
relaxation pathways were elucidated for the triad and reference compounds.
For the triad, quenching of the cyanoporphyrin singlet excited state
lifetime was interpreted as photoinduced electron transfer from the
porphyrin to the excited cyanoporphyrin. In contrast to a previous
study of a related molecule, we were unable to observe subsequent
formation of a long-lived charge separated state involving the benzimidazole–phenol
moiety. The lack of detection of a long-lived charge separated state
is attributed to a change in energetic landscape for charge separation/recombination
due to small differences in structure and solvation of the new triad