Investigation of Ligand-to-Ligand Charge Transfer in Heteroleptic Zinc(II) 4'-Substituted 2,2':6',2''-Terpyridine Complexes

Molecular photosensitizers can use a donor-chromophore-acceptor architecture to achieve long-lived charge separated states that may more efficiently initiate the single electron transfer processes involved in photocatalysis or photovoltaic devices. These photosensitizers are typically metal complexes wherein the dominant lowest energy photon-absorbing state is a metal-to-ligand charge transfer, having molar absorptivities on the order of 10^4 cm^-1 M^-1, and charge separation is achieved through careful redox leveling of ancillary electron donating and accepting groups. Taking advantage of direct ligand-to-ligand charge transfer states introduces new excited state pathways that can also lead to charge separated states. In this work, we present a series of heteroleptic zinc(II) 2,2':6',2''-terpyridine photosensitizers containing a pendant pyridyl group and carbazole or diphenylamine electron donating groups capable of undergoing direct ligand-to-ligand charge transfer with up to 42% photoluminescence quantum yields in solution or solid state, and up to 86% when arrested in a polymer matrix due to overcoming aggregation induced quenching effects.