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 iota(ISC) of two Cu(I) dimers, Cu2Cl2(dppb)(2) 1 (dppb = 1,2-bis-diphenylphosphino)benzene) and Cu2Cl2((NP)-P-boolean AND)(2) 2 ((NP)-P-boolean AND = 2-(diphenylphosphino)-6-methylpyridine) used as powders. '(ISC) varies by more than one order of mag-nitude from 39 ps (1) to 3.7 ps (2). A similar trend is displayed in the radiative phosphorescence decay time iota(r)(T1) and the zero-field splitting (ZFS) amounting to iota(r)(T-1) = 4.2 ms and ZFS < 1 cm(-1) (0.1 meV) for 1 and 45 ls and 15 cm(-1) (1.9 meV) for 2. Simple quantum mechanical considerations and TD-DFT cal-culations 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 T-1. Presumably, inter-metallic interactions, occurring in 2, are the source of the much more efficient SOC between T-1 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, iota(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. (c) 2022 Elsevier B.V. All rights reserved.

Automated summary

Using femto-second time-resolved spectroscopy, we investigated the intersystem crossing times (ISC) of two Cu(I) dimers, Cu2Cl2(dppb)(2) 1 and Cu2Cl2((NP)-P-bool   AND)(2) 2. The ISC times ranged from 39 ps (1) to 3.7 ps (2). The radiative phosphorescence decay time (iota(r)(T1)) and zero-field splitting (ZFS) showed a similar trend, with values of 4.2 ms and ZFS < 1 cm(-1) for 1, and 45 µs and 15 cm(-1) for 2, respectively. Quantum mechanical considerations and TD-DFT calculations allowed us to establish a correlation between these photophysical properties and spin-orbit coupling (SOC), particularly in the triplet state T-1.