Intrinsic Analysis of Thermally Activated Delayed Fluorescence (TADF) for Ag(I) Complex Based on the Path Integral Approach: Origin of the Effective Spin-Flipping Channel and Vibrational Spin-Orbit Coupling Effect

In order to design new Ag(I)-based materials for thermally activated delayed fluorescence (TADF), it is vital to develop a detailed understanding of the current best performing materials. The quantitative predictions of the photophysical processes of the Ag(dmp)(P-2-nCB) -to TADF complex are calculated using time-dependent density functional theory (TD-DFT) combined with the path integral approach for dynamics including the Herzberg-Teller effects. All calculated results are in good agreement with the experimentally available data, demonstrating the validity of our applied theoretical approach. Analysis of ETS-NOCV (extended transition state natural orbital for chemical valence) shows that there is a weak bond interaction dominated by electrostatic interactions and accompanied by some covalent components between Ag(I) and dmp ligands due to the introduction of the strongly electron-donating negatively charged P-2-nCB ligand, thus giving a small energy separation between the lowest singlet S-1 and triplet T-1 states of Delta E(S-1 - T-1) approximate to 532 cm(-1). The SOC strongly depends on the geometrical alteration caused by the molecular promotion vibrations. Our study has revealed that a few promotion vibrational modes, that is, omega(4)(6) and omega(227), effectively induce the strong SOC between S-1 and T-1 and speed up the reverse intersystem crossing (RISC) process dramatically. The computed k(RISC) value is 1.19 x 10(7) s(-1) for the solid phase at 300 K, which are about S orders of magnitude larger than the mean phosphorescence rate, k(p) = 9.56 X 10(2) s(-1), and it is also far larger than ISC k(ISC)(0) = 7.84 X 10(2) s(-1) rates from T-1 to S-0. The S-1 state thus can be an efficient thermal repopulation from the T-1 state by the RISC pathway. Finally, we also note that the diabatic vibration coupling triplet pair T-1/T-2 will also be important for efficient and practical RISC. Our investigation will be of great utility toward designing and improving the Ag(I)-based TADF complexes.

Automated summary

In order to design new thermally activated delayed fluorescence (TADF) materials based on Ag(I), it is important to understand the current best performing materials. The photophysical processes of the Ag(dmp)(P-2-nCB) to TADF complex were quantitatively predicted using time-dependent density functional theory (TD-DFT) combined with the path integral approach. The results were in good agreement with experimental data, validating the theoretical approach. The study revealed the importance of certain molecular vibrational modes in inducing strong spin-orbit coupling (SOC) and accelerating the reverse intersystem crossing (RISC) process.