Light Atom Influenced Room Temperature Phosphorescence in Functionalized Benzophenones: Experimental and Theoretical Correlations

Abstract

Organic room temperature phosphorescence (RTP) compounds have attracted considerable attention owing to their promising emissive properties and broad potential applications. Despite this interest, the understanding of the RTP mechanism in the solid-state remains limited, highlighting the need for a judicious structural design to develop efficient organic RTP materials. For this purpose, the current investigation was performed in derivatives of benzophenone, specifically in the presence of organic fluorine and/or methoxy groups, i.e., a heavy atom-free benzophenone framework. In this study, the origin of the observed experimental RTP behaviour was assessed via computational support from theoretical electronic structure calculations, with a particular emphasis on the quantitative atomic contributions to the HOMO and LUMO orbitals that account for the observed luminescent behaviour. Further, crystal packing analyses were conducted to explore the influence of multiple weak non-covalent intermolecular interactions such as C-H⋯O/F/π, C-H⋯H-C and C-F⋯F-C among molecules within the crystalline lattice. Importantly, the energetic contributions in these benzophenone derivatives were influenced by dispersion forces rather than electrostatic interactions. Additionally, theoretical analysis confirmed that the lone pairs on the carbonyl oxygen atom are available for the observed electronic transitions of (n, π*) character. Moreover, the time-dependent density functional theory (TDDFT) calculations revealed the existence of multiple low-lying triplet states below the first excited singlet state that contribute to the prolonged lifetimes observed in all the benzophenone derivatives.