Intersystem crossing, phosphorescence, and spin-orbit coupling. Two contrasting Cu(I)-TADF dimers investigated by milli- to micro-second phosphorescence, femto-second fluorescence, and theoretical calculations

Applying femto-second time-resolved spectroscopy, we study intersystem crossing times τ(ISC) of two Cu(I) dimers, Cu₂Cl₂(dppb)₂ 1 (dppb = 1,2-bis-diphenylphosphino)benzene) and Cu₂Cl₂(N^P)₂ 2 (N^P = 2-(diphenylphosphino)-6-methylpyridine) used as powders. τ(ISC) varies by more than one order of magnitude from 39 ps (1) to 3.7 ps (2). A similar trend is displayed in the radiative phosphorescence decay time τ^r(T₁) and the zero-field splitting (ZFS) amounting to τ^r(T₁) = 4.2 ms and ZFS < 1 cm⁻¹ (0.1 meV) for 1 and 45 µs and 15 cm⁻¹ (1.9 meV) for 2. Simple quantum mechanical considerations and TD-DFT calculations allow us to correlate these different photophysical mechanisms. This is related to the efficiency of spin–orbit coupling (SOC), in particular with respect to the triplet state T₁. Presumably, inter-metallic interactions, occurring in 2, are the source of the much more efficient SOC between T₁ and neighboring states, thus, providing a relation between structure and photophysical properties. Accordingly, easily accessible phosphorescence data or even just structure data might already help to predict trends in ISC, at least for Cu(I) dimers. As for TADF emitters applied in OLEDs, τ(ISC) should be as fast as possible, the presented discussions might help to better understand ISC processes, in particular, with respect of speeding them up.