The details of the photophysics of a diphosphene DmpPPDmp (Dmp: 2,6-Mes2C6H3) have been examined experimentally and computationally. Femtosecond transient absorption spectroscopy has been used to probe the dynamics of the S1 and S2 excited states of DmpPPDmp, through excitation at 480 and 400 nm, respectively. The molecule returns to S0 on sub-nanosecond timescales; no irreversible photochemistry is observed. The S2 state is observed in the transient spectra with an absorption feature at the red end of the visible spectrum. Its lifetime was measured to be 275 fs. The S1 state does not absorb appreciably in the probe wavelength range. Excitation into either of these states leads to transient absorption signals in the 400–600 nm region that exhibit a rise time longer than the measured instrument response function, indicating that they do not arise from the initially excited state. These bands decay biexponentially, with lifetimes of 20 ps and of a few hundred ps. Calculations at the CASSCF(8,6)/6-31G** and CASPT2(8,6)/6-31G**//CASSCF(8,6)/6-31G** levels support these assignments, and underpin an initial working model that involves participation of phenyl torsional twisting motions and the possibility of rapid intersystem crossing to the low-lying triplet manifold.
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