MODELING THE OPTICAL SPECTROSCOPY OF AGGREGATES OF DIPOLAR AND QUADRUPOLAR DYE MOLECULES

Abstract

The field of organic electronics is currently receiving a great deal of attention from theoretical and experimental chemists, physicists and material scientists, especially regarding energy transport and optical behavior. The conventional understanding of photophysical and transport processes in organic aggregates, films and crystals is based on the Frenkel exciton model. However, despite its success, the Frenkel model cannot properly account for several important properties of aggregates of highly polarizable chromophores, such as the charge-transfer character of the ground and excited states (and their associated dipole moments) which can change in response to intermolecular interactions. The essential state model (ESM) is investigated as an alternative to the Frenkel exciton model for understanding absorption and emission in aggregates of polar and high-polarizable chromophores. Polar and high-polarizable chromophores consist of donor and acceptor components linked by a c-conjugated bridge. In thiswork, the ESM is expanded to include local vibronic coupling via an Holsteinstyle Hamiltonian. Emphasis is placed on the divergences from the conventional Kasha model which is based on the Frenkel exciton Hamiltonian and is commonly used to understand the relationship between molecular packing and photophysical properties. Divergences between the current approach and the Kasha model are thoroughly explored for the DA dimers in the weak and strong intermolecular coupling limits. Vibronic signatures which reveal information about molecular packing are compared to those derived from Frenkel exciton theory Specific applications include Davydov splitting in covalently bonded squaraine (DAD) complexes and the general photophysical response of dimers of DA chromophores such as merocyanines dyes.