The photophysical properties of a mononuclear Cu(dppb)(pz2Bph2) complex have been investigated by employing the thermal vibration correlation function (TVCF) approach. The harmonic oscillator model with origin displacement, distortion, and Duschinsky rotation effects for the potential energy surfaces are considered. Absorption spectrum obtained by the scalar relativistic density functional theory combined with restricted open-shell configuration interaction including spin-orbit coupling effects is in excellent agreement with the experimental data. We found that the intersystem crossing (ISC) from the first excited singlet state (S1) to the triplet state (T1) is forbidden by direct spin-orbit coupling at the first-order perturbation, but becomes allowed through combined with vibronic coupling. The reverse intersystem crossing (RISC) proceeds at a rate of KRISC = 3.98 × 108 s−1 at room temperature 300 K, which is about 6 order of magnitude larger than the mean phosphorescence rate, KP, av = 7.3 × 102 s−1. At the same time, the ISC rate KISC = 3.06 × 109 s−1 is again about 3 order of magnitude larger than the fluorescence rate KF = 6.47 × 106 s−1. This implies that the S1 state can be populated from the T1 state, TADF should be observed and TADF decay time is τ(300 K) = 2.32 μs by fitting calculation. But at 30 K, the situation will change. The RISC rate becomes very small, about KRISC = 1.19 × 101 s−1, while the ISC rate only decreases slightly from KISC = 3.06 × 109 s−1 to KISC = 1.93 × 109 s−1. As a consequence, the Cu(dppb)(pz2Bph2) complex is highly attractive candidates for applications of TADF.