Resolution of <i>trans</i>-1-(2-Naphthyl)-2-phenylethene Fluorescence in the Presence of Tri-<i>n</i>-butylamine into Pairs of Monomer and Exciplex Spectra. Selectivity in Conformer Quenching

Fluorescence spectra of trans-1-(2-naphthyl)-2-phenylethene (NPE) obtained at different excitation wavelengths and tri-n-butylamine (TBA) concentrations in methylcyclohexane are resolved into four distinct components by application of principal component analysis with self-modeling. Two of the components are the NPE conformer fluorescence spectra, and the other two are the corresponding NPE·TBA exciplexes. Pure component combination coefficients are based on optimization of global (λexc-independent) Stern−Volmer plots for the monomers and exciplex fluorescence plots for the exciplexes. Conformer-specific fluorescence quenching rate constants by the amine are obtained for the first time. These are indistinguishable from the corresponding exciplex formation rate constants. Fortuitously, the Stern−Volmer constants for the two singlet excited conformer/TBA interactions differ by only ∼25%, revealing high conformer selectivity by TBA. This selectivity is consistent with the large shift between the λmax of the two exciplex fluorescence spectra which reveals the more favorable (2.9 kcal/mol) reduction potential of NPEA, the less extended conformer.