TRIS(1-NAPHTHYL)BORANE STUDIED BY LASER FLASH-PHOTOLYSIS, MATRIX-ISOLATION, AND LOW-TEMPERATURE NMR-SPECTROSCOPY - 2 ISOMERS WITH SIGNIFICANTLY DIFFERENT TRIPLET EXCITED-STATE PROPERTIES

Laser flash photolysis (308 nm) of tris(1-naphthyl)borane (TNB, 1) in cyclohexane solution yields a transient spectrum that is best interpre ted as the superposition of the spectra of two transient species with lambda(max) = 460 nm (transient 1, tau = 10 mu s) and lambda(max) = 64 0 nm (transient 2, tau = 20 mu s). Transient 2 also shows some absorpt ion at lambda > 800 nm. Both transient species exhibit behaviour typic al of tripler excited states. The seemingly paradoxical situation of t wo 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 be en calculated (AM1/RHF) to be of the order of 2.7 kcal/mol. Further ev idence comes from low-temperature H-1-NMR spectroscopy. At T = 183 K, some NMR bands of 1 are significantly broadened, while the shape of ot her bands remains nearly constant. This indicates that the broadening effect observed is due to the slowing-down of an intramolecular rearra ngement, and not to a general increase in solvent viscosity. The phosp horescence spectrum of 1, matrix-isolated in Ar at 10 K, consists of t hree bands, which can be attributed to two different emitting isomers. The corresponding tripler 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.