Excitons, polarons and polaritons play a critical role in the electronic and optical properties of aggregates, thin films and crystals of π-conjugated molecules and polymers in free space and in microcavities. In such systems, local electronic excitations couple strongly to each other as well as to nuclear degrees of freedom involving the symmetric vinyl-stretching mode (the aromatic-quinoidal vibration when conjugated rings are present) resulting in delocalized neutral and charged quasiparticles, respectively known as excitons and polarons. In microcavities excitons and polarons are further coupled to a resonant cavity mode to form polaritons. In this talk the photophysical properties of excitons and polarons in organic materials are compared and contrasted within the framework of Holstein-style Hamiltonians, which treat electronic coupling and local electron-vibrational coupling on equal footing. Applications are made to excitons and positively-charged polarons (holes) in poly(3-hexylthiophene) films. For exciton-polaritons, the Holstein-Tavis -Cummings (HTC) Hamiltonian is employed to demonstrate vibronic decoupling as well as the existence of “dark” polaritons, exotic quasi-particles which do not absorb radiation but nevertheless undergo side-band luminescence.

Understanding excitons, polarons and polaritons in organic materials using a Holstein multiparticle approach

ACS Spring 2022

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