Ultrafast Wiggling and Jiggling: Ir₂(1,8-diisocyanomenthane)₄²⁺

Binuclear complexes of d⁸ metals (Pt^II, Ir^I, Rh^I,) exhibit diverse photonic behavior, including dual emission from relatively long-lived singlet and triplet excited states, as well as photochemical energy, electron, and atom transfer. Time-resolved optical spectroscopic and X-ray studies have revealed the behavior of the dimetallic core, confirming that M-M bonding is strengthened upon dσ∗ â' pσ excitation. We report the bridging ligand dynamics of Ir₂(1,8-diisocyanomenthane)₄²⁺ (Ir(dimen)), investigated by fs-ns time-resolved IR spectroscopy (TRIR) in the region of C=N stretching vibrations, 2000-2300 cm^-1. The v(C=N) IR band of the singlet and triplet dσpσ excited states is shifted by-22 and-16 cm^-1 relative to the ground state due to delocalization of the pσ LUMO over the bridging ligands. Ultrafast relaxation dynamics of the ¹dσpσ state depend on the initially excited Franck-Condon molecular geometry, whereby the same relaxed singlet excited state is populated by two different pathways depending on the starting point at the excited-state potential energy surface. Exciting the long/eclipsed isomer triggers two-stage structural relaxation: 0.5 ps large-scale Ir-Ir contraction and 5 ps Ir-Ir contraction/intramolecular rotation. Exciting the short/twisted isomer induces a â5 ps bond shortening combined with vibrational cooling. Intersystem crossing (70 ps) follows, populating a ³dσpσ state that lives for hundreds of nanoseconds. During the first 2 ps, the Î(C=N) IR bandwidth oscillates with the frequency of the Î(Ir-Ir) wave packet, ca. 80 cm^-1, indicating that the dephasing time of the high-frequency (16 fs)^-1 C=N stretch responds to much slower (â400 fs)^-1 Ir-Ir coherent oscillations. We conclude that the bonding and dynamics of bridging di-isocyanide ligands are coupled to the dynamics of the metal-metal unit and that the coherent Ir-Ir motion induced by ultrafast excitation drives vibrational dephasing processes over the entire binuclear cation.