Broad-Band NIR Transient Absorption Spectroscopy of an “All-Carbon”-Bridged Bimetallic Radical Cation Complex

Broad-band
near-infrared (NIR) transient absorption (TA) spectroscopy
has been used for the first time to probe an all-carbon-bridged organometallic
radical cation complex. The compound [{Ru­(PPh3)2Cp}2(μ-CCCC)]+ ([1]+) was investigated in dichloromethane and acetonitrile
solutions, using laser excitation at 700, 800, 900, and 1000 nm; these
wavelengths span the NIR absorption band envelope. The resulting TA
spectra were found to be independent of excitation wavelength and
consist of an excited state absorption feature with a peak at ca.
1150 nm and the corresponding bleach signal of the ground-state NIR
absorption band, which both decay to 0 over the 50 ps time window
investigated. Data were analyzed globally and fit collectively for
each of the four different excitation wavelengths, with the resulting
best fit to a biexponential decay function indicating two processes
with slightly different time scales of ca. 1.5 and ca. 9.0 ps involved
in the relaxation to the ground state.