Twisting the phenyls in aryl diphosphenes (Ar-P=P-Ar). Significant impact upon lowest energy excited states

Aryl diphosphenes (Ar-P=P-Ar) possess features that may make them useful in photonic devices, including the possibility for photochemical E-Z isomerization. Development of good models guided by computations is hampered by poor correspondence between predicted and experimental UV/vis absorption spectra. A hypothesis that the phenyl twist angle (i.e., PPCC torsion) accounts for this discrepancy is explored, with positive findings. DFT and TDDFT (B3LYP) were applied to the phenyl-P=P-phenyl (Ph-P=P-Ph) model compound over a range of phenyl twist angles, and to the Ph-P=P-Ph cores of two crystallographically characterized diphosphenes: bis-(2,4,6-iBu ₃C ₆H ₂)-diphosphene (Mes*-P=P-MeS*) and bis-(2,6-Mes ₂C ₆H ₃)diphosphene (Dmp-P=P-Dmp). A shallow PES is observed for the model diphosphene: the full range of phenyl twist angles is accessible for under 5 kcal/mol. The Kohn-Sham orbitais (KS-MO _s) exhibit stabilization and mixing of the two highest energy frontier orbitais: the n- and π localized primarily on the -P=P- unit. A simple, single-configuration model based upon this symmetry-breaking is shown to be consistent with the major features of the measured UV/vis spectra of several diphosphenes. Detailed evaluation of singlet excitations, transition energies and oscillator strengths with TDDFT showed that the lowest energy transition (S; -S ₀) does not always correspond to the LUMO -HOMO configuration. Coupling between the phenyl rings and central -P=P- destabilizes the π-π dominated state. Hence, the Si is always n+-π* in nature, even with a π-type HOMO. This coupling of the ring and -P=P- π systems engenders complexity in the UV/vis absorption region, and may be the origin of the variety of photobehaviors observed in diphosphenes.