Photophysics of Anionic Bis(4 H ‐imidazolato)Cu I Complexes

In this paper, the photophysical behavior of four panchromatically absorbing, homoleptic bis(4 H ‐imidazolato)Cu I complexes, with a systematic variation in the electron‐withdrawing properties of the imidazolate ligand, were studied by wavelength‐dependent time‐resolved femtosecond transient absorption spectroscopy. Excitation at 400, 480, and 630 nm populates metal‐to‐ligand charge transfer, intraligand charge transfer, and mixed‐character singlet states. The pump wavelength‐dependent transient absorption data were analyzed by a recently established 2D correlation approach. Data analysis revealed that all excitation conditions yield similar excited‐state dynamics. Key to the excited‐state relaxation is fast, sub‐picosecond pseudo‐Jahn‐Teller distortion, which is accompanied by the relocalization of electron density onto a single ligand from the initially delocalized state at Franck‐Condon geometry. Subsequent intersystem crossing to the triplet manifold is followed by a sub‐100 ps decay to the ground state. The fast, nonradiative decay is rationalized by the low triplet‐state energy as found by DFT calculations, which suggest perspective treatment at the strong coupling limit of the energy gap law.