Exciton Adventures

Conspectus

Excitons are mobile excited states in materials with properties and dynamics that underpin many applications of conjugated organic polymers. Understanding these quasiparticles in organic materials requires knowledge of classical molecular photophysics and solid-state physics-based descriptions of semiconducting polymers. The molecular interpretation of an exciton is an excited state with physical size determined by the region over which bond lengths/angles are relaxed from their equilibrium ground state geometries. The physics representation is that excitons are quasiparticles comprising a bound electron–hole pair. Specifically, in the excited state the hole is a vacancy in the HOMO (valence band) and the electron is an unpaired electron in the LUMO (conduction band). Excitons move through multiple mechanisms. These include Förster energy transfer wherein the dipole from a virtual electronic emission is captured by a coupled chromophore to create a new excited state. The Förster mechanism is facilitated by the natural tendency of the high aspect ratio of conjugated polymer chromophores to align, the relatively low dielectric constant of the materials, and the high spectral overlap of the absorptions and emissions. In a second mechanism, the exciton moves through band transport, wherein thermal phonons (traveling compression/expansion nuclear motion excitations) nudge the excitons and cause movement. This mechanism requires direct orbital interactions (strong electronic coupling) and is related to Dexter energy transfer in the molecular systems, wherein collisional interactions between molecules allow for energy transfer. Both molecular and solid-state physics perspectives are valuable in understanding exciton dynamics/mobility. Exciton transport is central to photovoltaics and photodetectors excitons wherein these electron–hole pairs diffuse to interfaces and separate to create electrical potentials and/or photocurrents. Ultrasensitive chemical sensors and biosensors based on conjugated organic polymers exhibit amplified sensor responses enabled by exciton diffusion. This natural amplification has been widely exploited for trace chemical and biological sensing, and explosive sensors have been commercial now for more than 20 years based on this principle. Moreover, the polychromophore character of conjugated organic polymers creates very large absorption cross sections and when combined with high emission quantum yields makes them exceptionally bright fluorescent labels, which has found use in biology. Exciton diffusion enhances photoredox catalysis wherein fast exciton diffusion increases the probably of encounter of the reactive excited state within a nanosecond timeline. My studies of excitons have been guided by classical physical organic chemistry wherein hypotheses are tested through materials design and synthesis. I will detail how receptor integration, control of interpolymer organization, precision assembly of aligned polymers, and modulation of excited state lifetimes have provided understanding and empowered key applications.