Harvesting infrared photons with croconate dyes

Abstract

Excited-state properties of the croconate dyes were investigated with an aim to utilize them as light harvesting assemblies in the infrared (IR) region (λ_max 865 nm and M^-1 c^-1 = (1.4-4.2) × 10⁵ M^-1 s^-1). The excited singlet of the monomeric dye quickly deactivates (lifetime 4-7 ps) without undergoing intersystem crossing to generate triplet. The triplet excited-state produced via triplet-triplet energy transfer method show relatively long life (lifetime 7.2 µs). The dye molecules when deposited as thin film on optically transparent electrodes or on nanostructured TiO2 film form H-aggregates with a blue-shifted absorption maximum around 660 nm. The excitons formed upon excitation of the dye aggregates undergo charge separation at the TiO₂ and SnO₂ interface. The H-aggregates in the film are photoactive and produce anodic current (IPCE of 1.2% at 650 nm) when employed in a photoelectrochemical cell. Spectroscopic and photoelectrochemical experiments that highlight the usefulness of croconate dyes in IR light harvesting applications are described.