Real-time observation of molecular flattening and intersystem crossing in [(DPEPhos)Cu(i)(PyrTet)] via ultrafast UV/Vis- and mid-IR spectroscopy on solution and solid samples

Abstract

The primary photo-induced processes in the mononuclear, heteroleptic Cu(I) complex [(DPEPhos)Cu(PyrTet)] (1), relevant for OLED applications, were investigated in various solvents and in solid state samples via femtosecond (fs) time resolved UV/Vis and fs time resolved mid-IR transient absorption spectroscopy (TA) with MLCT excitation around 340 nm. UV/Vis fs-TA on 1 in solution reveals (i) a severe blue-shift of excited state absorption on the time scale of a few picoseconds (τ₂) that is not observed in solids, and (ii), on the time scale of several tens of picoseconds (τ₃), a process with very similar dynamics in all samples. Mid-IR fs-TA in solution indicates structural changes with τ₂. Transient absorption anisotropy experiments temporally resolve a viscosity-dependent change of the excited state transition dipole moment orientation with τ₂, as quantitatively predicted for the relaxed S₁-state via TD-DFT. The results strongly suggest flattening distortion (FD) and structural rearrangement of the PyrTet-moiety to occur on the time scale of τ₂ in liquid phase, and to be suppressed in solid phase. Moreover, intersystem crossing (ISC) is assigned to the process described by τ₃, in line with its direct observation via time-resolved luminescence spectroscopy on 1 by Bergmann et al. (Sci. Adv., 2016, 2, e1500889). Overall, the use of structure-sensitive methods and the direct comparison of different preparations of 1 (i.e. solution vs. solid state), are a unique basis for a clear assignment of spectro-temporal characteristics to fundamental deactivation processes such as FD and ISC.