Electronic Structure and Excited State Dynamics of the NIR-II Emissive Molybdenum(III) Analog to the Molecular Ruby

Photoactive chromium(III) complexes saw a conceptual breakthrough with the discovery of the prototypical molecular ruby, mer-[Cr(ddpd)2]3+ (ddpd = N,N’-dimethyl-N,N’-dipyridin-2-ylpyridine-2,6-diamine), that shows intense long-lived near-infrared (NIR) phosphorescence from metal-centered spin-flip states. In contrast to the numerous studies on chromium(III) photophysics, only ten luminescent molybdenum(III) complexes have been reported so far. Here, we present the synthesis and characterization of mer-MoX3(ddpd) (1: X = Cl, 2: X = Br) and cisfac-[Mo(ddpd)2]3+ (cisfac-[3]3+), an isomeric heavy homolog of the prototypical molecular ruby. For cisfac-[3]3+, we found strong zero-field splitting using magnetic susceptibility measurements and electron paramagnetic resonance (EPR) spectroscopy. Electronic spectra covering the spin-forbidden transitions show that the spin-flip states in mer-1, mer-2 and cisfac-[3]3+ are much lower in energy than in comparable chromium(III) compounds. While all three complexes show weak spin-flip phosphorescence in the NIR-II, the emission of cisfac-[3]3+ peaking at 1550 nm is particularly low in energy. Femtosecond-transient absorption spectroscopy reveals a short excited state lifetime of 1.4 ns, six orders of magnitude shorter than that of mer-[Cr(ddpd)2]3+. Using density functional theory and ab initio multi-reference calculations, we break down the reasons for this disparity, and derive principles for the design of future stable photoactive molybdenum(III) complexes.