Twisting the Phenyls in Aryl Diphosphenes (Ar-P=P-Ar). Significant Impact upon Lowest Energy States

Abstract

Aryl diphosphenes (Ar-PdP-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-PdP-phenyl (Ph-PdP-Ph) model compound over a range of phenyl twist angles, and to the Ph-PdP-Ph cores of two crystallographically characterized diphosphenes: bis-(2,4,6-tBu3C6H2)-diphosphene (Mes*-PdP-Mes*) and bis-(2,6-Mes2C6H3)- diphosphene (Dmp-PdP-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 orbitals (KS-MOs) exhibit stabilization and mixing of the two highest energy frontier orbitals: the n+ and π localized primarily on the -PdP- 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 (S1 r S0) does not always correspond to the LUMO r HOMO configuration. Coupling between the phenyl rings and central -PdP- destabilizes the π-π* dominated state. Hence, the S1 is always n+-π* in nature, even with a π-type HOMO. This coupling of the ring and -PdP- π systems engenders complexity in the UV/vis absorption region, and may be the origin of the variety of photobehaviors observed in diphosphenes.