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 (lambda(max) similar to 865 nm and M-1 c(-1) = (1.4-4.2) x 10(5) 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-triple energy transfer method show relatively long life (lifetime 7.2 mu s). 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.