Supramolecular Donor–Acceptor
Assembly Derived
from Tetracarbazole–Zinc Phthalocyanine Coordinated to Fullerene:
Design, Synthesis, Photochemical, and Photoelectrochemical Studies
- Publication date
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Abstract
A functional photosynthetic antenna–reaction center
mimicking
donor–acceptor polyad has been newly designed, synthesized,
and characterized. The polyad was comprised of four entities of carbazole
covalently linked to the macrocycle periphery of a zinc phthalocyanine
(ZnPc). Efficient singlet excitation transfer from the carbazole to
zinc phthalocyanine has been witnessed from the emission studies.
Axial coordination of phenylimidazole-functionalized fulleropyrrolidine
to ZnPc served as an electron acceptor in the polyad. Optical absorption
and emission along with computational studies revealed stable complex
formation wherein the evaluated binding constant <i>K</i> was 7.7 ± 0.2 × 10<sup>5</sup> M<sup>–1</sup>,
an order of magnitude higher than that observed earlier for similar
complexes due to the electronic effect induced by the carbazole entities.
From the free-energy calculations, photoinduced electron transfer
from the <sup>1</sup>ZnPc* to fullerene within the polyad was established
to be an exothermic process. Kinetics of charge separation, <i>k</i><sub>CS</sub>, monitored by time-resolved emission was
found to be 2.8 × 10<sup>9</sup> s<sup>–1</sup>, indicating
a relatively fast charge-separation process. The electron-transfer
products were characterized by nanosecond transient absorption spectroscopic
technique; the presence of ZnPc<sup>+•</sup> radical cation
at 890 nm and fulleropyrrolidine anion radical at 1000 nm was clear
from this study. The kinetics of charge recombination, <i>k</i><sub>CR</sub>, evaluated from the decay of either of the radical
ions, was found to be 6.25 ± 0.2 × 10<sup>7</sup> s<sup>–1</sup>, revealing the persistence of the radical ion-pair
species to some extent. Further, photoelectrochemical studies, performed
by constructing photocells by electrophoretic deposition of the studied
polyad on nanocrystalline SnO<sub>2</sub> modified surface, revealed
a higher value of incident photon-to-current conversion efficiency
covering the wide visible–near IR spectral region and good
on–off switchability. Better charge injection from the excited
polyad to the conduction band of the semiconductor was evident from
the electrochemical impedance spectral measurements of electron recombination
resistance calculations