Tris(1‐naphthyl)borane Studied by Laser Flash Photolysis, Matrix Isolation, and Low‐Temperature NMR Spectroscopy: Two Isomers with Significantly Different Triplet Excited State Properties

Abstract

AbstractLaser flash photolysis (308 nm) of tris(1‐naphthyl)borane (TNB, 1) in cyclohexane solution yields a transient spectrum that is best interpreted as the superposition of the spectra of two transient species with λmax = 460 nm (transient 1, τ = 10 μs) and λmax = 640 nm (transient 2, τ = 20 μs). Transient 2 also shows some absorption at λ > 800 nm. Both transient species exhibit behaviour typical of triplet excited states. The seemingly paradoxical situation of two long‐lived triplet excited states being generated by excitation of a single compound can be resolved in terms of two isomeric structures of TNB. The activation energy for isomerization is very low and has been calculated (AM1/RHF) to be of the order of 2.7 kcal/mol. Further evidence comes from low‐temperature 1H‐NMR spectroscopy. At T = 183 K, some NMR bands of 1 are significantly broadened, while the shape of other bands remains nearly constant. This indicates that the broadening effect observed is due to the slowing‐down of an intramolecular rearrangement, and not to a general increase in solvent viscosity. The phosphorescence spectrum of 1, matrix‐isolated in Ar at 10 K, consists of three bands, which can be attributed to two different emitting isomers. The corresponding triplet energies differ significantly and are 57.8 and ca. 48 kcal/mol, respectively. The LFP experiments performed with 1 gave no evidence for the formation of the hypovalent boron species 1‐naphthylborene or di‐1‐naphthylboryl radical.