Excitation-Wavelength-Dependent Photophysics of d(8)d(8) Di-isocyanide Complexes

Binuclear Rh(I) and Ir(I) TMB (2,5-dimethyl-2,5-diisocyanohexane) and dimen (1,8-diisocyanomenthane) complexes possess d sigma*p sigma and d pi p sigma singlet and triplet excited states that can be selectively excited in the visible and UV spectral regions. Using perturbational spin-orbit TDDFT, we unraveled the detailed character and spin mixing of these electronic transitions and found that delocalization of p sigma and d pi orbitals over C N-groups makes C N stretching vibrations sensitive reporters of electron density and structural changes upon electronic excitation. Picosecond time-resolved infrared spectra measured after visible light, 375 nm, and 316 nm excitation revealed excitation-wavelength-dependent deactivation cascades. Visible light irradiation prepares the (1)d sigma*p sigma state that, after one or two (sub)picosecond relaxation steps, undergoes 70-1300 ps intersystem crossing to (3)d sigma*p sigma, which is faster for the more flexible dimen complexes. UV-excited (1,3)d pi p sigma states decay with (sub)picosecond kinetics through a manifold of high-lying triplet and mixed-spin states to (3)d sigma*p sigma with lifetimes in the range of 6-19 ps (316 nm) and 19-43 ps (375 nm, Ir only), bypassing (1)d sigma*p sigma. Most excited-state conversion and some relaxation steps are accompanied by direct decay to the ground state that is especially pronounced for the most flexible long/eclipsed Rh(dimen) conformer.