Altering molecular photophysics by merging organic and inorganic chromophores

Photofunctional molecules and assemblies lie at the heart of many important fundamental processes in nature, and researchers have generated multitudes of artificial chromophores intended to mimic these naturally occurring systems. As dynamic spectroscopic techniques are becoming more widely available, ultrafast techniques in particular, substantial insight continues to be gleaned from the initial photon stimulation event through internal conversion, structural rearrangements, intersystem crossing, energy migration, electron transfer events, and ultimately regeneration of the ground state chromophores in both naturally occurring and inspired chromophores. This Account details research endeavors motivated by the concept that merging organic and inorganic chromophores can lead to new molecules exhibiting novel excited state properties. Moreover, these excited state properties can be fundamentally understood using combinations of static and dynamic spectroscopic tools, yielding systematic improvements to molecules poised for application in diverse research areas including light-harvesting, lifetime engineering, photocatalysis, and photochemical upconversion. Initial explorations focused on utilizing Förster energy transfer processes in Ru(II)-based metal-organic chromophores for solar light-harvesting while maintaining long excited state lifetimes. This eventually led to molecules exhibiting triplet-triplet energy transfer between energetically proximate triplet states featuring thermally activated photoluminescence from the upper charge transfer excited state with markedly extended lifetimes. Interest in systematically producing long-lived excited states with concomitant large Stokes shifts inspired the development of numerous Pt(II) bipyridyl and terpyridyl acetylide charge transfer chromophores featuring ultrafast intramolecular energy migration, high quantum yield ligand-localized phosphorescence at room temperature, and synthetically tunable excited state absorption properties. This structural motif also made it possible to access the triplet excited states of perylenediimide chromophores, permitting quantitative examination of internal conversion and intersystem crossing processes in these complex molecules. The generation of new metal-organic structures featuring unique photophysics appears limitless and simply requires the continued ingenuity of researchers.