Intersystem crossing mechanism of thermally activated delayed fluorescence copper(I) thiolate complex: The roles of exchange coupling and magnetic spin interactions

The exchange coupling (J) and magnetic dipole (D) interactions of thermally activated delayed fluorescence copper(I) thiolate complex have been investigated by employing computational chemistry methods. The situation of strict orthogonality of the "hole"-"electron" orbitals with the overlap integral <phi 175 alpha vertical bar phi 175 beta > = 0 leads to small J and excessive triplet T-1 population, and is unfavorable to the formation of singlet 1 state via the exchange coupling induced reverse intersystem crossing in the triplet state 1. In magnetic dipole interactions, the mutually perpendicular orbitals provides a significant one-center heavy atomic contribution to the SOC, <(1)phi vertical bar h(y)(so)vertical bar(3)phi > = -41.76 and <(1)phi vertical bar h(z)(so)vertical bar(3)phi > = -63.14 cm(-1), which will enhance the probability of intersystem crossing from the T-1 to S-1, but system needs to overcome a large intersystem crossing barrier height of 11.05 kcal/mol relative to that of the T-1 state at the CAM-B3LYP/def2-TZVP(-f) level. Under magnetic field limit, S-1-T-0, S-1-T+ 1, and S-1-T-1 mixing are all inefficient because the large energy gap between T-1 and S-1, vertical bar 2J vertical bar >> g beta B prohibits significant mixing in the triplet state T-1, but for the S-1/T-1 crossing point, the S-1-T-1 energy gap is comparable to that of the S-1-T-0 energy gap, allowing for mixing of S-1 with T-1 and T-0, vertical bar T-z > = 56%vertical bar 1, 0> + 20%vertical bar 1 1, -1> + 16%vertical bar 0, 0>.