Computational Approach to Investigate Structure-Property Relationship of a series of Carbazole Containing Thermally Activated Delayed Fluorescent Molecules

TL;DR

This study uses density functional theory to analyze how molecular structure influences the electronic properties and TADF efficiency of carbazole-based emitters, aiding the design of better organic light-emitting devices.

Contribution

It provides a detailed structure-property relationship for carbazole TADF emitters, highlighting the importance of triplet state energy, spin orbit coupling, and reorganization energy in TADF performance.

Findings

Triplet excited state energy depends on molecular structure.

Close $_{ST}$ and high spin orbit coupling enhance RISC rates.

Small optical absorption peak shifts with SOC are observed.

Abstract

Donor-acceptor type compounds are an important category of organic materials that show properties suitable for light emission applications. To achieve a full understanding of the mechanism of thermally activated delayed fluorescence (TADF) process, we studied the structure-property relationship for a series of carbazole based TADF emitters, 2CzPN, 4CzPN, 4CzIPN, 4CzBN and 5CzBN. We applied density functional theory to investigate kinetic and electronic properties. We find that the energetic position of triplet excited state of these emitters depends on their molecular structure. Our findings emphasize that to enable reverse intersystem crossing and eventually TADF, strong spin orbit coupling and minimal energy difference between singlet and triplet states $\Delta E_{ST}$ must be obtained simultaneously. We also find that the reverse intersystem crossing rates $k_{RISC}$ values are higher where $\Delta E_{ST}$ values are closer to reorganization energy. Furthermore, a small change in the absorption peak of optical absorption spectra with and without spin orbit coupling (SOC) is observed for each emitter. This result is extremely beneficial for the design of new TADF molecules, and we believe that our work contributes to the progress of future development of high-performance organic molecular light-emitting devices.