Improved performance of a photosynthetic antenna–reaction
center mimicking supramolecular solar cell is demonstrated. Toward
this, porphyrin–phthalocyanine conjugates connected by amide
linkers, as wide-band capturing solar energy harvesting materials,
have been newly synthesized and characterized. Efficient singlet–singlet
energy transfer from the zinc or free-base porphyrin to phthalocyanine
is evidenced from steady-state emission and transient absorption studies
in nonpolar and polar solvents. Further, the dyad was immobilized
via axial coordination of zinc porphyrin of the dyad on semiconducting
TiO2 surface modified with axial coordinating ligand functionality,
phenylimidazole. Photoelectrochemical studies revealed improved performance
of this cell compared to either zinc porphyrin or zinc phthalocyanine
only modified electrodes under similar experimental conditions. Transient
absorption studies performed on the dyad immobilized on glass/TiO2 surface suggested that upon excitation of the axially coordinated
zinc porphyrin of the dyad singlet–singlet energy transfer
to zinc phthalocyanine occurs within 0.2 ps instead of a competing
charge injection reaction from the singlet excited zinc porphyrin
to TiO2. Further, sequential photoinduced electron transfer
from the newly formed singlet excited zinc phthalocyanine to zinc
porphyrin producing ZnP•––ZnPc•+ with a 2 ps time constant and followed by electron
injection from the ZnP•– to TiO2 within 30 ps has been proposed as a mechanism of photocurrent generation
in the biomimetic supramolecular photocell.
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