Substitution‐Dependent Modulation of Photophysical Properties of 2,7‐ and 3,6‐Dinaphthyl‐9‐phenylcarbazoles in Solution and Solid State

The photophysical properties of two positional isomers of dinaphthyl-9-phenylcarbazoles (2,7-DNpPCz and 3,6-DNpPCz) were systematically investigated in solution and solid states. Experimental measurements revealed a pronounced positional dependence, although both compounds exhibit fluorescence in solution and the solid state. On average, the 2,7-isomers exhibit larger fluorescence rate values than the 3,6-isomers, although the magnitude of this difference varies among individual compounds. To gain mechanistic insight, density function theory (DFT) and time-dependence DFT calculations were performed at the (TD)-B3LYP/6-31+G(d) level. The results show that 2,7-DNpPCz derivatives possess delocalized frontier orbitals and large oscillator strengths for the S1 ← S0 transition, whereas 3,6-DNpPCz derivatives exhibit smaller oscillator strengths with mixed orbital contributions, leading to weaker transition probabilities. These combined findings demonstrate that substitution geometry critically governs radiative decay dynamics in carbazole-based luminophores and provide valuable guidelines for the molecular design of efficient solid-state organic emitters.