Photophysics of the Phenylnaphthalenes and Binaphthyls: Evidence for Conformers

Abstract

<p>The simple biaryls 1- and 2- phenylnaphthalene 1, 1' - ,1,2'- and 2,2'-binaphthyl may, in principle, exist as distinct molecular conformers differing chiefly in their rotational configuration about the interannular bond. The steady-state fluorescence and fluorescence decay spectroscopy of these molecules in dilute solution has been examined for evidence of such single bond rotational conformers.</p> <p>While the fluorescence of 1-phenylnaphthalene originates from essentially a unique molecular conformation in fluid solution or in rigid organic glasses at 77K, singlet decay measurements of 2-phenylnaphthalene at 295K and 77K indicate the presence of at least two emissive components. This is confirmed by the emission spectroscopy at low temperature and by the effect of selective oxygen quenching on the room tmeperature fluorescence. In a polycrytalline matrix of methylcyclohexane at 77K the 2-phenylnaphthalene molecule appears constrained to a single, more planar conformation.</p> <p>Of the binaphthyl only the 2,2'-isomer exhibits complex fluorescence behaviour in the fluid hydrocarbon solvents, the spectrum showing a distinct excitation wavelength dependence while the fluorescence decay can be best analyzed as the sum of long and short lived exponential decays. Molecular Oxygen was used to enhance these spectroscopic differences, differentially quenching the longer lived conformer of 2,2'-binaphthyl. The Shpol'skii (polycrystalline) emission spectrum of this compound at 77k indicated the presence of spectroscopically distinct conformers different from those seen in fluid media and separated by ~525cm⁻¹ in their fluorescence origins. 1,1'- and 1,2'- binaphthyl, while giving no evidence of confromeric fluorescence at 295K did give a biexponential fluorescence decay in organic glasses at 77K.</p> <p>These studies suggest that relatively stable single bond conformers may be more ubiquitous than is generally supposed. Where these conformers differ spectroscopically the technique of fluorescence emission and decay spectroscopy may be successfully applied in conjunction with the selective quenching techniques.</p>