Structural Integration of Carbazole and Tetraphenylethylene: Ultrafast Excited‐State Relaxation Dynamics and Efficient Electroluminescence

Abstract

We report the synthesis, characterisation, and spectroscopic investigations of a new solution‐processable N‐styrylcarbazole‐linked tetraphenylethylene (TPE) derivative (TPE‐BCzS), exhibiting green‐orange emission. While the material displayed weak photoluminescence in solution with a low photoluminescence quantum yield (PLQY of 4±0.2%), significant photoluminescence enhancement in neat‐film PLQY (55±8%) was observed. Studies using steady‐state spectroscopy, and femtosecond and nanosecond transient absorption spectroscopy revealed details of the excited‐state dynamics, consisting of the Franck‐Condon (FC) state, non‐radiative conformational relaxation, and formation and decay of the triplet excited‐state generated via intersystem crossing upon ultrafast photoexcitation. Solution processed OLEDs based on TPE‐BCzS displayed maximum external quantum efficiencies of 1.8% and 1.7% for neat and blend films [20wt% in a 4,4′‐bis(N‐carbazolyl)‐1,1′‐biphenyl host], approaching the theoretical efficiency limit for the determined PLQYs of the films. While TPE materials are typically associated with aggregation‐induced emission, we report that the enhanced PLQYs in the solid state are due to restriction of structural relaxations in the solid state—and not to the commonly misunderstood aggregation effect.