Long-Range Excitation Energy Migration in a Cationic BODIPY/Clay Nanosheet Light-Harvesting Antenna

Abstract

Natural photosynthetic antennas arrange chromophores at high density without aggregation, concentrating excitation energy at the reaction center with near-unity quantum efficiency. Reproducing this function is a key challenge for delivering light energy to artificial photosynthetic reaction centers. Here we designed a trivalent cationic BODIPY derivative (BODIPY3+) whose molecular size matches the interanionic-site distance (ca. 1.2 nm) of saponite (SSA) clay nanosheets. Through this size-matching effect, BODIPY3+ adsorbed as isolated monomers, without aggregation, up to 81% of the cation exchange capacity (CEC). Fixation on the rigid clay surface suppressed nonradiative deactivation and lengthened the fluorescence lifetime from 0.8 ns in water to 2.0 ns. The Förster equation gave a homoenergy-migration rate constant of kmg = 6.2 × 1010 s–1, more than 2 orders of magnitude faster than the excited-state decay, so the excitation samples on the order of 102 donor molecules within its lifetime. With coadsorbed cationic porphyrin (p-TMPyP) as the acceptor, the energy-transfer efficiency remained ca. 60% even at a donor/acceptor ratio of 48:1─far exceeding a random-distribution model and evidencing multistep migration. This BODIPY3+/clay system functions as an artificial light-harvesting antenna and demonstrates the value of inorganic nanosheets as reaction fields for artificial photosynthesis.