Observation of the low-energy 1Ag state of diphenylhexatriene by two-photon excitation spectroscopy

Resolution of Three Fluorescence Components in the Spectra of all-trans-1,6-Diphenyl-1,3,5-hexatriene under Isopolarizability Conditions

all-trans-1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence in solution consists of emissions from the S1 (2(1)A(g)) and S2 (1(1)B(u)) states of the s-trans,s-trans conformer (s-t-DPH) and emission from the S1 state of the s-cis,s-trans conformer (s-c-DPH). The contribution of s-c-DPH fluorescence increases upon excitation at longer wavelengths, and both minor emissions, s-c-DPH and 1(1)B(u) s-t-DPH fluorescence, contribute more at higher temperatures (Ts). Resolution of a spectrothermal matrix of DPH fluorescence spectra by principal component analysis with self-modeling (PCA-SM) is hampered by T-dependent changes in the spectra of the individual components. We avoided differential polarizability-dependent spectral shifts by measuring the spectra in n-alkanes (Cn, C8 to C16 with n even) at T values selected to keep the index of refraction constant, hence under isopolarizability conditions. Compensation of the spectra for T-induced broadening allowed resolution of the spectral matrix into its three components. The optimum van't Hoff plot gives Delta H = 2.83 kcal/mol for s-c-DPH/s-t-DPH equilibration, somewhat smaller than the 3.4 kcal/mol calculated value, and the optimum Boltzmann distribution law plot gives Delta E(ab) = 4.09 kcal/mol for 1(1)B(u)/2(1)A(g) equilibration. The 1(1)B(u) fluorescence spectrum bears mirror-image symmetry with the DPH absorption spectrum, and the energy gap, 1431 cm(-1), is consistent with the 1615 cm(-1) difference between the lowest energy bands in the 1(1)B(u) and 2(1)A(g) fluorescence spectra. The results give V(ab) = 198 +/- 12 cm(-1) for the vibronic matrix coupling element between the 2(1)A(g) and 1(1)B(u) states. Fluorescence quantum yields and lifetimes under isopolarizability conditions reveal an increase in the effective radiative rate constant of s-t-DPH with increasing T.

Light quenching of pyridine2 fluorescence with time-delayed pulses

We describe the effects of time-delayed long-wavelength pulses on the intensity and anisotropy decays of pyridine2. The sample was exposed to a continuous train of 360 nm excitation pulses and time-delayed 720 nm pulses. The long-wavelength pulses, which overlapped the emission spectrum of pyridine2, resulted in a spatially localized decrease in intensity at the point of beam overlap. The time-delayed quenching pulses caused a stepwise decrease in the intensity and anisotropy decays, as seen by oscillations in the frequency-domain data. The time-resolved anisotropy was shown to decrease below zero (-0.2) following the vertically polarized quenching pulse. The extent of light quenching depended on the time delay between the excitation and quenching pulses, and can be used to measure the decay time. Light quenching and/or multipulse methods may provide a new class of experiments for fluorescence spectroscopy and imaging.

Fluorescence of tyrosine and tryptophan in proteins using one- and two-photon excitation

We examined the emission spectra of tyrosine- and tryptophan-containing proteins using one-photon (270-310 nm) and two-photon (565-610 nm) excitation. Emission spectra for two-photon excitation of native and denatured human serum albumin and of three purine nucleoside phosphorylases indicated an absence of the tyrosine emission normally seen for one-photon excitation below 290 nm. We examined the one-photon and two-photon excitation spectra of tyrosine-tryptophan mixtures to determine the origin of selective excitation of the tryptophan residues. These results confirmed a short-wavelength shift of the tyrosine two-photon excitation spectrum relative to that of tryptophan, as recently reported by Rehms and Callis (1993) Chem. Phys. Lett. 208, 276-282.

Effects of two-photon absorption on degenerate four-wave mixing in solutions of diphenyl polyenes

We have observed double-peaked phase-conjugate pulses in the degenerate four-wave mixing of nanosecond laser pulses in solutions of diphenyl polyenes. We attribute this to the superposition of fast and slow gratings, where the slow grating is due to two-photon absorption. This is supported by the results of time delay and polarization experiments as well as evidence of a second spatial harmonic grating.