P=P bond photophysics in an Ar-P=P-Ar diphosphene.

The details of the photophysics of a diphosphene DmpP=PDmp (Dmp: 2,6-Mes(2)C(6)H(3)) have been examined experimentally and computationally. Femtosecond transient absorption spectroscopy has been used to probe the dynamics of the S(1) and S(2) excited states of DmpP=PDmp, through excitation at 480 and 400 nm, respectively. The molecule returns to S(0) on sub-nanosecond timescales; no irreversible photochemistry is observed. The S(2) 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 S(1) 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.