Tunable lifetimes and efficiencies of room temperature phosphorescent liquids by modulating the length and number of alkyl chains

Abstract

Organic room temperature phosphorescence (RTP) liquid composites exhibit the potential to make innovative changes in large area flexible lighting applications, and it is extremely challenging to achieve high-efficiency RTP in pure organic solvent-free liquid systems. The excited state properties and inner lighting mechanisms of these composites are unclear; therefore, a theoretical perspective to design high efficiency RTP liquids with tunable lifetime is highly desired. Herein, we systematically investigate the photophysical properties of a series of long swallow-tailed bromonaphthalimide (BT unit) molecules by the newly proposed optimally tuned range-separated (RS) functional method, and a state-of-the-art RTP molecule with an absolute quantum yield (Φ_RTP) of 57.1% and a lifetime (τ) of 160 ms in solvent-free liquid is obtained. Moreover, theoretical results show that the energy gap between the lowest singlet excited state (S₁) and triplet excited state (T₁) can be reduced and the non-radiative energy consumption process can be restricted by modulating the length and number of alkyl chains in organic RTP molecules. Thus, a wise molecular design strategy is proposed and five additional efficient RTP molecules with tunable lifetimes (43, 19, 136, 0.11 and 0.005 ms) and efficiencies (11.3%, 6.8%, 5.9%, 0.2% and 0.05%) are theoretically proposed. This study sheds light on the relationship among molecular structure, lifetime and efficiency, and can provide an important prototype to explore high-efficiency RTP by pure organic solvent-free liquid systems.