Organic Photophysics Governed by Conformational Heterogeneity

Woojae Kim

Department of Chemistry, Yonsei University

E-mail: woojae@yonsei.ac.kr

The majority of optoelectronic processes in organic semiconducting materials are typically intermolecular in nature. This indicates that the kinetics or efficiencies of light-induced phenomena such as energy and electron transfer, excimer formation, singlet fission, and triplet-triplet annihilation are strongly influenced by intermolecular geometries. Therefore, understanding the structure-property relationships is crucial to rationally controlling the intermolecular interactions driving these processes in the solid state. However, overcoming the intrinsic structural disorder of the solid state remains a significant challenge. Consequently, tailored molecular dimers have been widely studied as model systems to understand fundamental solid-state photophysics. Molecular dimers possess more easily defined structures than chromophores in thin films, enabling control of through-bond and through-space couplings. This generally facilitates the development of design principles based on a comprehensive library of structure-property relationships underlying the excited-state photophysics of organic semiconductors. However, this approach assumes that molecular dimers have a well-defined static structure. In this talk, I will discuss in depth the caveats to this approach and the importance of dynamic and static structural heterogeneity in organic photophysics. First, I will introduce structurally heterogeneous singlet fission dynamics occurring in a pentacene dimer, highlighting the significant impact of dynamic and static effects on the formation of initial triplet pairs and the generation of final free triplets. Next, I will discuss the Janus-type photophysics observed in an anthracene dimer, where its excited state dynamics behave oppositely despite the only structural difference being syn- or anti-type connectivity.