Time-Dependent Afterglow Color in a Single-Component Organic Molecular Crystal

An organic crystal of 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (pCBP) exhibits time-dependent afterglow color from blue to orange over 1 s. Both experimental and computational data confirm that the color evolution results from well-separated, long-persistent thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) with different but comparable decay rates. TADF is enabled by a small S₁ -T₁ energy gap of 0.7 kcal mol^-1 . The good separation of TADF and RTP is due to a 11.8 kcal mol^-1 difference in the S₀ energies of the S₁ and T₁ structures, indicating that apart from the excited-state properties, tuning the ground state is also important for luminescence properties. This afterglow color evolution of pCBP allows its applications in anticounterfeiting and data encryption with high security levels.