Rotamerism in 2,2′-binaphthyl. A study based on fluorescence analysis and CS-INDO/CI calculations

Abstract

The torsional isomerism of 2,2′-binaphthyl in the ground state and lowest excited singlet states has been investigated theoretically and experimentally. The combined results of a fluorescence steady-state and time-resolved analysis and of CS-INDO/CI calculations showed that this molecule exists in the ground state in two rotameric forms in which the naphthyls are rotated around the interconnecting quasi-single bond by different angles (ϕ≈ 35 and 145°). The absorption spectrum of 2,2′-binaphtyl was assigned and resolved into the individual spectra of the two rotamers. The fluorescence spectra, lifetimes and quantum yields of the two isomers at room temperature were also derived. The effects of solvent viscosity increase and of temperature decrease on the measured fluorescence lifetimes were found to be small. Lastly, a discussion of the rotational behaviour of this molecule as opposed to that of other biaryls is offered on the basis of the calculated torsional energy curves.