Photochemistry of 2-phenylbenzophenone. An unusual bichromophoric molecule

Abstract

2-Phenylbenzophenone is proposed to have two nearby triplets, a low lying π,π* state similar to that of biphenyl and a benzophenone-like triplet lying ~4 kcal/mol higher. The triplet lifetimes of this photostable ketone are extremely short, e.g. 0.56 and 2.5 ns at 300 K in toluene and isooctane, respectively, and are believed to be controlled by population of the benzophenone-like triplet and decay from this state via interaction of the carbonyl chromophore with the phenyl ring in the ortho position. The two triplets interconvert via ring rotation. The Arrhenius parameters for triplet decay in five solvents show some dependency with the temperature dependence of the solvent viscosity.

Evaluation of N-Aromatic Maleimides as Free Radical Photoinitiators: A Photophysical and Photopolymerization Characterization

“…However, the similarity in lifetimes may simply connote two species with similar or coupled molecular structure. (b) Two distinct populated excited triplet states may exist for planar ARMIs, as in the case of 2-phenylbenzophenone (2PBP) described by Scaiano, where he claimed that two energetically close (Δ ≈ 4 kcal/mol) excited triplets states exist for 2PBP, which may interconvert by rotation of one of the phenyl rings . In the case of planar ARMIs, there is little barrier to planarity or perpendicularity of the two rings, vide supra from the AM1 rotational conformer calculations previously reported …”