Donald F. Sears

Dynamics of cis-stilbene photoisomerization: the adiabatic pathway to excited trans-stilbene

Abstract Fluorescence excitation spectra of cis-stilbene in n-hexane monitored at 350 nm and 404 nm, are resolved into cis-stilbene and trans-stilbene contributions using principal component analysis with self-modeling. The results confirm that both trans and cis fluorescence originating from excitation of pure cis-stilbene solutions are due to single photon excitation of 1c to 1c*. The quantum yield for adiabatic 1c*→1t* conversion is φct*=0.0020, independent of excitation wavelength. The corrected fluorescence excitation spectrum of cis-stilbene faithfully tracks its UV absorption spectrum. The fluorescence spectrum of cis-stilbene is nearly temperature independent in n-hexane, 0–58 °C. Assuming a 1c* → 1p* → 1t* pathway for adiabatic 1t* formation where 1p* is the perpendicular phantom singlet state, leads to the conclusion that 1p* and 1t* are nearly isoenergetic.

Fluorescence, fluorescence-excitation and absorption spectra of trans-1-(2-anthryl)-2-phenylethene conformers

Fluorescence spectra of trans-1-(2-anthryl)-2-phenylethene (APE) obtained under varying conditions of excitation wavelength and oxygen concentration in toluene are resolved into two distinct components by application of principal component analysis with self-modeling (PCA-SM). Self-modeling is guided by the constraint that Stem-Volmer quenching plots for the individual conformers be independent of excitation wavelength. The same process applied on a matrix of fluorescence-excitation spectra leads to resolved conformer-specific fluorescence-excitation spectra. Consistency between the fluorescence and the fluorescence-excitation spectrum of each conformer is established. The fluorescence-excitation spectr and literature fluorescence quantum yields are used to resolve the UV absorption spectrum of APE. The pure conformer spectra obtained in this work are compared with those from earlier PCA-SM treatments in which self-modeling procedures were based on the Lawton and Sylvestre nonnegativity constraint and on a maximal spectral dissimilarity constraint.

The effect of solvent polarizability on the fluorescence of trans-1-(2-naphthyl)-2-phenylethene conformers. Conformer-specific fluorescence from the cis isomer.

A matrix of trans-1-(2-naphthyl)-1-phenylethene (t-NPE) fluorescence spectra obtained from benzene solutions at 20 degrees C by varying the excitation wavelength (lambdaexc) and the oxygen concentration is resolved into pure conformer fluorescence spectra by use of principal component analysis with self-modeling based on optimum global Stern Volmer constant criteria. The resulting fractional contributions of the two components to the fluorescence spectra are combined with observed fluorescence quantum yields as a function of lambdaexc to obtain the conformer-specific quantum yields. These quantum yields and fluorescence lifetimes are used to determine conformer-specific radiative and radiationless rate constants. Comparison with results from an analogous study in methylcyclohexane reveals pronounced, diferential enhancements of all these rate constants in benzene (Bz). Preliminary measurements of emission from a cis-1-(2-naphthyl)-1-phenylethene (c-NPE) solution in Bz at 20 degrees C, under static cell conditions, are dominated by sequential two-photon-induced t-NPE fluorescence due to excitation of t-NPE formed photochemically from the cis isomer. The spectra also reveal a weak structureless broad emission, which is assigned to 1c-NPE* by comparison with the published low temperature fluorescence spectrum of c-NPE in a hydrocarbon glass. The conformational origin of this emission is addressed.