CPM Seminar

Excitons and Excimers in Molecular π-Stacks

Frank Spano

Department of Chemistry
Temple University

Organic π-conjugated molecules and polymers continue to receive widespread attention as semiconducting materials for field effect transistors, light emitting diodes and solar cells. In the aggregated and crystalline phases of such materials, the collective electronic excitations - known as “excitons” - are important for energy transport as well as light emission, processes which are often hindered by the formation of excimers. Excimers are stabilized excited states, characterized by broad, red-shifted emission with low quantum yield. In this talk, the photophysical properties of excitons and excimers in organic materials are described with a theoretical model based on a Holstein-Peierls Hamiltonian, which, in addition to electronic coupling between molecules, includes local and nonlocal coupling to inter- and intramolecular vibrations. Applications are made to π-stacks of perylene diimide (PDI) chromophores.[1, 2] In such stacks, the electronic interaction between molecules is determined by an interference between short-range super-exchange coupling due to wave function overlap, and long-range Coulomb coupling arising from transition dipole-dipole interactions.[3, 4] The nature of the interference can be deduced from vibronic signatures derived from a vinyl-stretching, progression-forming mode appearing in the absorption and photoluminescence spectra. A novel mechanism for excimer formation is presented based on nonlocal coupling to an intermolecular torsional mode and used to account for excimer emission in covalently-bound PDI dimers. [2]

[1] A. Oleson, T. Zhu, I.S. Dunn, D. Bialas, Y. Bai, W. Zhang, M. Dai, D.R. Reichman, R. Tempelaar, L.B. Huang, F.C. Spano, Perylene Diimide-Based Hj- and hJ-Aggregates: The Prospect of Exciton Band Shape Engineering in Organic Materials, Journal of Physical Chemistry C 123 (2019) 20567-20578.
[2] A.L. Bialas, F.C. Spano, A Holstein­Peierls Approach to Excimer Spectra: The Evolution from Vibronically Structured to Unstructured Emission, The Journal of Physical Chemistry C 126 (2022) 4067-4081.
[3] N.J. Hestand, F.C. Spano, Expanded Theory of H- and J- Molecular Aggregates: The Effects of Vibronic Coupling and Intermolecular Charge Transfer, Chem. Rev. 118 (2018) 7069­7163.
[4] N.J. Hestand, F.C. Spano, Molecular Aggregate Photophysics beyond the Kasha Model: Novel Design Principles for Organic Materials, Acc. Chem. Res. 50 (2017) 341-350.