Real-Time Tracking of Photoinduced Metal–Metal Bond Formation in a d⁸d⁸ Di-Iridium Complex by Vibrational Coherence and Femtosecond Stimulated Raman Spectroscopy

We report real-time dynamics of photoinduced metal–metal bond formation acquired from ultrafast time-resolved stimulated emission and femtosecond stimulated Raman spectra (FSRS) of [Ir₂(2,5-dimethyl-2,5-diisocyanohexane)₄]²⁺ (Ir(TMB)) in the region of low-frequency vibrations. Interpretation was supported by impulsive stimulated Raman experiments and time-dependent density-functional theory (TDDFT) calculations. The Ir–Ir stretching frequency doubled on going from ground to the lowest singlet excited state ¹dσ*pσ, from 53 to 126 cm^–1, demonstrating Ir–Ir bond formation. Spectral evolution during the first 4 ps after excitation showed extremely large-amplitude coherent oscillations of stimulated emission as well as FSRS signal intensities, which occurred with the excited-state Ir–Ir stretching frequency combined with frequencies of several deformation vibrations and the first Ir–Ir overtone. Corresponding vibrations were observed in FSRS directly but most of them vanished in the first 3 ps, indicating that they belonged to transiently populated hot vibrational states. Fourier transforms of intensity oscillations plotted against FSRS frequencies produced two-dimensional (2D-FSRS) maps with diagonal and off-diagonal features due to Franck–Condon-excited and anharmonically coupled vibrations, some of which acquired Raman intensity through coupling with the Ir–Ir stretch. We concluded that optical excitation impulsively shortens the Ir–Ir distance and increases its stretching force constant, assisted by a simultaneously excited network of coupled deformation modes. The electronically/vibrationally excited system then relaxes through periodic strengthening and weakening of the Ir–Ir interaction and changing conformations of the TMB ligand framework, forming a metal–metal bonded ¹dσ*pσ state after 4–5 ps.