From Excitons to Excimers: Understanding the Steady-State Absorption and Photoluminescence Features of Perylene Diimide Dyes

Abstract

There is currently a great interest to develop and market organic electronic devices, and theoretical models are needed to provide physical insight and quality predictions when designing these materials. Many organic molecules absorb in the UV-vis region of light, and therefore, UV-vis spectroscopy is a relatively simple tool that can help experimentalists "see" the packing arrangements of the molecules within each material, as long as there is a solid theoretical understanding of the photophysics that links the interactions between molecules to changes in optical features. For example, the Kasha spectral shifts have been used for decades to identify J-aggregate and H-aggregate packing arrangements from red- and blue- spectral shifts, respectively. The innate presence of vibronic coupling in organic molecules gives rise to a unique set of additional spectral signatures that are far more reliable than the Kasha spectral shifts for inferring packing arrangements. Moreover, the Kasha shifts are based entirely on Coulomb coupling between molecules, which leads to the creation of delocalized Frenkel excitons. For many π-conjugated organic molecules, however, dispersion forces in π-conjugated chromophores encourage close packing distances of about 3.5-4 Å between organic monomers, which further introduces intermolecular couplings beyond the Coulomb coupling, due to intermolecular charge transfer (CT). Therefore, much theoretical research has focused on incorporating all these effects through a Frenkel-CT-Holstein Hamiltonian, in order to better understand how different packing arrangements within a given material can be identified through specific changes in steady-state absorption and photoluminescence features. In this thesis, the Frenkel-CT-Holstein model is specifically applied to study the absorption and photoluminescent spectra of various derivatives of perylene diimide (PDI), which are of great interest as non-fullerene acceptors in organic photovoltaic design. PDIs display a plethora of packing arrangements and corresponding spectral signatures just by varying the substituents within the PDI core. This thesis first aims to understand the exciton band structure of two different PDI micro-crystals that both experience similar Frenkel-CT interference, but with one system displaying dominant Coulomb interactions while the other undergoes dominant Frenkel-CT coupling. Both are close to what is called a “null”-point, and the work in this thesis explores the photoluminescent signature as a reliable means to track which side of the “null”-point the Frenkel-CT interference lies. While the Frenkel-CT-Holstein model is successful in modeling mostly absorption spectra of aggregates composed of PDI monomers, one challenge has been that aggregates of PDIs often exhibit so-called excimer features in their photoluminescence spectra, which the model cannot account for. Systems that emit broad, structureless and red-shifted excimer peaks typically display inefficient exciton transport in organic semiconductors. The bulk of this thesis has been to expand the model to account for excimer emission, which is made possible by utilizing a Holstein-Peierls (HP) Hamiltonian that incorporates the effects of both local vibronic coupling and nonlocal Frenkel-CT coupling to intermolecular motions within a dimer. The experimental spectra for two different PDI dimer systems that display different excimer features is successfully reproduced with the new theory. This thesis concludes by analyzing how nonlocal coupling, which account for changes in the Frenkel-CT mixing along an intermolecular vibrational mode, can lead to various types of excimers. Different phase relations within the electron and hole nonlocal coupling parameters can combine with different phase relations within the electron and hole Frenkel-CT coupling parameters, leading to a rich array of excimer properties, especially when combined with the additional effects of Coulomb coupling, as well as local intermolecular vibronic coupling, which can either enhance or diminish the excimer photoluminescence. Overall, the Holstein-Peierls approach offers insight into the roles of Frenkel and CT excitons in excimer formation, and highlights the importance of the magnitude and phase of the intermolecular electron and hole transfer integrals in the ground and excited state geometries in producing distinct excimer features. The model provides further insight into the origin of excimers, which lays a foundation for future theoretical and experimental studies in designing organic materials.