title: Twisting the phenyls in aryl diphosphenes (Ar-P=P-Ar). Significant impact upon lowest energy excited states.
abstract: 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-tBu(3)C(6)H(2))-diphosphene (Mes*-P=P-Mes*) and bis-(2,6-Mes(2)C(6)H(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 orbitals (KS-MOs) exhibit stabilization and mixing of the two highest energy frontier orbitals: the n(+) and pi 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(1) <-- S(0)) does not always correspond to the LUMO <-- HOMO configuration. Coupling between the phenyl rings and central -P=P- destabilizes the pi-pi* dominated state. Hence, the S(1) is always n(+)-pi* in nature, even with a pi-type HOMO. This coupling of the ring and -P=P- pi systems engenders complexity in the UV/vis absorption region, and may be the origin of the variety of photobehaviors observed in diphosphenes.
authors.affiliation: Department of Chemistry and the Center for Chemical Dynamics, Case Western Reserve University, Cleveland, Ohio, USA.
authors.forename: Huo-Lei
authors.initials: HL
authors.lastname: Peng
authors.forename: John L
authors.initials: JL
authors.lastname: Payton
authors.forename: John D
authors.initials: JD
authors.lastname: Protasiewicz
authors.forename: M C
authors.initials: MC
authors.lastname: Simpson
mesh_terms: D001555:Benzene Derivatives / Q000737:chemistry*; D003198:Computer Simulation; D008956:Models, Chemical; D010720:Phosphines / Q000737:chemistry*; D010777:Photochemistry; D011789:Quantum Theory
chemical_list: D001555:Benzene Derivatives; D010720:Phosphines