Harvesting infrared photons with croconate dyes

Excited-state properties of the croconate dyes were investigated with an aim to utilize them as light harvesting assemblies in the infrared (IR) region (λmax ∼ 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.