Abstract. Pulsed laser photolysis at 347nm has been used to study the transient spectroscopy of alloxazine, lumichrome, lumiflavin, and riboflavin in acidic (pH 2.2) aqueous solution and in ethanol. Intersystem crossing quantum yields (φISC) were determined by a modification of the comparative laser excitation method which utilizes the variation of the triplet yield with intensity in conjunction with a kinetic model for the various photophysical and photochemical processes occurring during the pulse. Fluorescence quantum yields and lifetimes are also reported. Correction for quenching of the excited singlet state by H+ ions shows that, in neutral aqueous solution, intersystem crossing for flavins is an efficient process (φISC˜ 0.7) which, in conjunction with fluorescence, accounts for the fate of all absorbed photons. For alloxazine (φISC˜ 0.45) and lumichrome (φISC˜ 0.7) the results are more difficult to interpret owing to interconversion between alloxazine and isoalloxazine structures in the singlet excited state. For all four compounds, the quantum yield of products derived from the singlet excited state is estimated as ˜0.04. There is evidence of biphotonic product formation at high laser energies. In ethanol, where φISC for lumichrome is about twice that of lumiflavin, internal conversion between the excited singlet and ground states appears to be a significant process. Complete triplet‐triplet absorption spectra in the region 260–750nm are reported. For lumichrome at pH 2.2 there is spectral evidence for isomeric triplet states which appear to be in equilibrium.

The pulsed ruby laser (347 nm) flash photolysis technique has been used to measure the triplet-triplet absorption spectra and triplet lifetimes of trans -retinal, N-trans -retinylidene-nbutylamine (NRBA), and protonated NRBA (NRBAH') at room temperature. In methylcyclohexane solution, the triplet lifetimes are in the range 10-2Ops and decrease in the order NRBAH' > NRBA > trans-retinal. Intersystem-crossing efficiencies ($,sc) were determined by a comparison technique using anthracene and 1,Zbenzanthracene as reference compounds. For trans -retinal, +, , , is 0.50 k 0.05 in methylcyclohexane and 0.08 in methanol, which confirms that earlier values of 0.11 and 0.017 in these solvents are in error. For NRBA and NRBAH' in methylcyclohexane, QlSc values are 0.008 and < 0.001, respectively. Evidence is presented for a significant solvent effect in the isomerization of retinal via the triplet state, and that cis +trans isomerization occurs from the triplet state of NRBAH'. The relation between the intersystem-crossing properties of model compounds and the photochemistry of rhodopsin is discussed.423

The gas-phase electronic absorption spectra of triethylenediamine and quinuclidine have been measured. Each compound shows two strong bands in the regions 1650–2500 Å and 1650–2300 Å, respectively, which have well-resolved vibrational structure, as well as very weak, structured absorption at longer wavelengths (2560–2700 Å for triethylenediamine and 2300–2500 Å for quinuclidine). An essentially complete vibrational analysis of all but one of these bands has been accomplished. The results indicate a substantial change in the equilibrium positions of the nuclei in the excited states relative to the positions in the ground state. For quinuclidine, the most prominent excited state vibrational mode can be correlated with the ground-state mode which corresponds to an approach to planarity of the nitrogen-bearing end of the molecule. For triethylenediamine, ambiguities in the ground-state assignments preclude conclusions about the upper state geometry. By comparisons with the spectra of other amines, the strong electronic bands of triethylenediamine and quinuclidine are assigned as n → p Rydberg transitions. Exciton model calculations of the shift of the first strong band of triethylenediamine relative to the corresponding band in quinculidine predict changes of about 3000 cm−1 to lower energy for an n → 3pz transition and about 1500 cm−1 to higher energy for an n → px y transition. The observed shift of the 0–0 band is 3920 cm−1 to lower energy, which is clearly in accord with z polarization. The weak, longer-wavelength absorption of the cage amines is tentatively considered to arise from sub-Rydberg transitions.

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