Decisive role of heavy-atom orientation for efficient enhancement of spin–orbit coupling in organic thermally activated delayed fluorescence emitters

Abstract

In view of the rapidly growing interest in the hybrid materials for heavy-metal-free optoelectronics, the research described here aimed to confirm the potential ability of abundant heavy atoms (HAs) in improving key parameters of organic emitters with thermally activated delayed fluorescence. Namely, the enhancement of reverse intersystem crossing (rISC) while keeping a reasonable value of fluorescence rate was investigated in red emitters with bromine atom(s) introduced into the ortho positions of the N,N-ditolylaniline donor fragment. The results of photophysical investigations and quantum chemical calculations indicate that selective acceleration of rISC by HAs without a substantial decrease of the fluorescence rate is possible. Molecular design principles of such hybrid materials, however, do not seem simple. In the investigated emitters, the oscillator strength of the S₁–S₀ transition which defines the fluorescence rate is not directly influenced by the bromine atoms, and remains similar or decreases weakly for the brominated emitters. The maximal enhancement of spin–orbit coupling (SOC) does not depend directly on the number of HAs either, but on their relative position and orientation in the emitter. The analysis of numerous rotational isomers of emitters revealed that SOC enhancement cannot be explained by either the internal or the external HA effect. Taking into account the lack of significant contribution of bromines in orbital and spin momentum of the T₁–S₁ transition, but yet significant SOC enhancement, we explain the observed phenomenon as a heavy-atom field effect (HAFE) which increases the total angular momentum. The most impressive SOC enhancement by the HAFE up to 60 times is observed when HAs align asymmetrically and are oriented towards the chromophore fragment with the largest orbital momentum change. Another important observation reveals that, in the case of symmetrical structures, the field of a heavy atom can be compensated by another one leading to almost zero SOC and 650-fold rISC inhibition. Such species should be avoided at the stage of molecular design planning.