Solid-State Photophysical Properties of Chiral Perylene Diimide Derivatives: AIEnh-Circularly Polarized Luminescence from Vacuum-Deposited Thin Films

Abstract

Abstract Studies on the optical properties of chiral perylene diimide derivatives, (S,S)- and (R,R)-BPP (BPP = N,N′-bis(1-phenylethyl)perylene-3,4,9,10-tetracarboxylic diimide), through absorption, steady-state and time-resolved fluorescence, and chiroptical spectroscopies revealed that the vacuum-deposited thin films of (S,S)- and (R,R)-BPP exhibit aggregation-induced enhanced (AIEnh) circularly polarized luminescence (CPL), as observed when they are dispersed in KBr and polymer matrices. Fluorescence lifetimes revealed that two emissive species exist in the deposited thin film: the dimer state with a fast decay and the weakly coupled excimer state with a slow decay. Unlike the dilute solutions of chiral BPPs, the deposited thin films displayed circular dichroism (CD) and CPL, indicating solid-state aggregation-induced chiroptical properties of chiral BPPs. Time-dependent density functional theory calculations on isolated monomer and π-stacked dimer suggested that the formation of a chirally twisted molecular arrangement through intermolecular π–π interactions is essential for the chiroptical properties of (S,S)- and (R,R)-BPP in the solid states. Moreover, we fabricated organic light-emitting diodes (OLEDs) using (S,S)- and (R,R)-BPP isomers as CPL emitters. The devices exhibited circularly polarized electroluminescence (CPEL) although the EL dissymmetry factor (gEL) was significantly low, as has been frequently noted in earlier studies.