Experimental evidence of the excited-state mixing in the blue emitter for organic light-emitting diodes

High hopes have been placed on organic emitters, which are supposed to solve the problem of low stability of blue OLEDs. A peculiar phenomenon of thermally activated delayed fluorescence (TADF), which brought such emitters to the range of the top-studied materials for organic optoelectronics within the last decade, remains poorly understood. Here, we report the results of comprehensive photophysical studies of one of the most successful candidates for blue TADF OLEDs, the TMCz-BO emitter (9-(5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-1,3,6,8-tetramethyl-9H-carbazole) characterised by outstanding triplet-harvesting properties. One of the main aims of this work is to understand the reason for these unique properties. Steady-state and time-resolved spectroscopic investigations in media of various polarity, viscosity, and temperature reveal that at least five excited states of different characters and multiplicity are responsible for the emissive and spin-flip transitions in the TMCz-BO molecular systems. First of all, in contrast to typical donor-acceptor TADF emitters, the S1 state of TMCz-BO does not have a pure charge-transfer character but shows a considerable contribution of the locally-excited state of the acceptor fragment, which provides a fast radiative rate. The T1 state is a superposition of two locally excited and one charge-transfer states, providing reasonable spin-orbit coupling. Regarding the TADF mechanism in various media, reverse intersystem crossing follows the T1 -> S1 model, considering the excited-state mixing, a notion introduced here to explain the triple and dual nature of the respective states. Such a mixing is dynamic in low-viscosity solutions due to low barriers for molecular vibrations. In films with a host matrix, a static excited-state mixing occurs, assisted by the low-amplitude vibrations within the local energetic minimum of the emitting species. The high efficiency of the excited-state mixing in TMCz-BO is explained by the rigid structure of its donor and acceptor fragments and their limited but still active mutual rotations. This provides negligible structural differences between various electronic states, enabling low reorganisation energies favourable for radiative and spin-flip processes while maintaining vibrational activation of spin-orbit coupling. Despite a lower reverse intersystem crossing rate in media of high viscosity, TMCz-BO shows rare near-UV TADF in films with the non-polar host. Our results thus highlight the unique and intriguing properties of TMCz-BO, opening up new perspectives for further research and potential improvements in OLED applications.