Photoinduced Charge Transfer in a Conformational Switching Chlorin Dimer–Azafulleroid in Polar and Nonpolar Media

Abstract

In the present study, a biomimetic reaction center model, that is, a molecular triad consisting of a chlorin dimer and an azafulleroid, is synthesized and its photophysical properties are studied in comparison with the corresponding molecular dyad, which consists only of a chlorin monomer and an azafulleroid. As evidenced by (1) H NMR, UV/Vis, and fluorescence spectroscopy, the chlorin dimer-azafulleroid folds in nonpolar media into a C2 -symmetric geometry through hydrogen bonding, resulting in appreciable electronic interactions between the chlorins, whereas in polar media the two chlorins diverge from contact. Femtosecond transient absorption spectroscopy studies reveal longer charge-separated states for the chlorin dimer-azafulleroid; ≈1.6 ns in toluene, compared with the lifetime of ≈0.9 ns for the corresponding chlorin monomer-azafulleroid in toluene. In polar media, for example, benzonitrile, similar charge-separated states are observed, but the lifetimes are inevitably shorter: 65 and 73 ps for the dimeric and monomeric chlorin-azafulleroids, respectively. Nanosecond transient absorption and singlet oxygen phosphorescence studies corroborate that in toluene, the charge-separated state decays indirectly via the triplet excited state to the ground state, whereas in benzonitrile, direct recombination to the ground state is observed. Complementary DFT studies suggest two energy-minima conformations, that is, a folded chlorin dimer-azafulleroid, which is present in nonpolar media, and another conformation in polar media, in which the two hydrophobic chlorins wrap the azafulleroid. Inspection of the frontier molecular orbitals shows that in the folded conformation, the HOMO on each chlorin is equivalent and is shared owing to partial π-π overlap, resulting in delocalization of the conjugated π electrons, whereas the wrapped conformation lacks this stabilization. As such, the longer charge-separated lifetime for the dimer is rationalized by both the electron donor-acceptor separation distance and the stabilization of the radical cation through delocalization. The chlorin folding seems to change the photophysical properties in a manner similar to that observed in the chlorophyll dimer in natural photosynthetic reaction centers.

Dimeric Corrole Analogs of Chlorophyll Special Pairs

“…Comparison of through-space distances reveals that Mg 2+ ions are closer in P 700 than in PbRC, at 6.3 and 7 Å, respectively . It follows that there can be substantial differences in slipped-cofacial electronic interactions, and that such differences likely affect functional features, such as excited-state lifetimes and reduction potentials. − Notably, cofacial interactions within the PSII special pair (P 680 , wherein the number discloses the absorption maximum) − must be optimally tuned for ultrafast electron transfer to the primary acceptor. ,, …”

Synthesis of a Chlorin Fullerene Dyad for Artificial Photosynthesis

“…chlorophyll a for efficient light harvesting and energy conversion, most of the artificial photosynthetic devices investigated consist of completely unsaturated porphyrin donors. So far only a few chlorin‐based dyads with fullerene acceptors have been reported . In general, chlorins 2 show red‐shifted absorption wavelengths and substantial stronger absorptions relative to porphyrins.…”

Geometry‐Independent Ultrafast Energy Transfer in Bioinspired Arrays Containing Electronically Coupled BODIPY Dimers as Energy Donors

“…Chlorins, synthetic analogs of chlorophylls, are a promising class of biomimetic chromophores, with strong absorption bands at ~400 nm and > 600 nm as well as excited state and redox properties suitable for solar energy conversion applications, [37][38][39] and have previously been incorporated into synthetic energyand electron-transfer arrays. [23,34,35,[40][41][42][43][44][45][46][47][48][49] BODIPYs are likewise a broad class of chromophores with widely tunable optical properties that provide high potential for application for solar energy conversion. [50][51][52] Previously, we reported a series of arrays composed of chlorin as an energy acceptor and monomeric BODIPY as an energy donor.…”