PP bond photophysics in an Ar-PP-Ar diphosphene

The details of the photophysics of a diphosphene DmpPPDmp (Dmp: 2,6-Mes ₂C ₆H 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 DmpPPDmp, 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.