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Abstract: The phenothiazine–phenylene–isoalloxazine dyad, 3-methyl-10-[4-(10-heptyl-10H-phenothiazin-3-yl)-phenyl]-10H-benzo[g]pteridine-2,4-dione, dissolved in either dichloromethane or acetonitrile is characterized by absorption and emission spectroscopy. Absorption cross-section spectra, stimulated emission cross-section spectra, fluorescence quantum distributions, fluorescence quantum yields, and degrees of fluorescence polarisation are determined. The fluorescence decay is determined by time-resolved measurements. The dye photo-stability is investigated by observation of absorption spectral changes due to prolonged blue-light excitation. The absorption spectrum of the dyad resembles the superposition of the absorption of the isoalloxazine (flavin) moiety and of the phenylphenothiazine moiety. Photo-excitation of the flavin moiety causes fluorescence quenching by ground-state reductive electron transfer from phenylphenothiazine to isoalloxazine followed by charge recombination. Photo-excitation of the phenothiazine moiety causes (i) efficient excited-state oxidative electron transfer from phenothiazine to isoalloxazine with successive recombination, and (ii) moderate energy transfer followed by ground-state phenothiazine electron transfer and recombination. [Copyright &y& Elsevier]

Abstract: The pyrene-flavin (isoalloxazine) dyad, PFD {C44H31N5O5; CA Index name: 1-pyrenepropanoic acid, α-[[4,10-dihydro-2,4-dioxo-10-phenylbenzo[g]pteridin-3(2H)-yl)acetyl]amino]-, phenylmethyl ester, (αR)–(9Cl); CA Registry number: 618907-57-6}, dissolved in either dichloromethane or acetonitrile is characterized by absorption and emission spectroscopy. Absorption cross-section spectra, stimulated emission cross-section spectra, fluorescence quantum distributions, quantum yields, and degrees of fluorescence polarisation are determined. The fluorescence decay after femtosecond pulse excitation is determined by fluorescence up-conversion. The ground-state absorption recovery is determined by picosecond pump and probe transmission measurements. The dye photo-stability is investigated by observation of absorption spectral changes due to prolonged blue-light excitation. The absorption spectrum of PFD dyad resembles the superposition of the absorption of isoalloxazine (flavin) and 1-methylpyrene. Long-wavelength photo-excitation of the flavin moiety causes fluorescence quenching by ground-state electron transfer from pyrene to isoalloxazine. Short-wavelength photo-excitation of the pyrene moiety causes (i) excited-state electron transfer from pyrene to isoalloxazine, and (ii) Förster-type energy transfer from pyrene to flavin followed by ground-state electron transfer from pyrene to flavin. [Copyright &y& Elsevier]

Abstract: The flavoquinone dyes 10-phenyl-isoalloxazine-3-acetic acid ethyl ester (1) and 10-(4-bromo-phenyl)-3-methyl-isoalloxazine (2) in dichloromethane, acetonitrile, and methanol are characterized by absorption and emission spectroscopy. Absorption cross-section spectra, stimulated emission cross-section spectra, fluorescence quantum distributions, quantum yields, lifetimes, and degrees of fluorescence polarization are determined. The blue-light photo-degradation of the dyes is studied. Mass spectroscopic measurements reveal the formation of phenyl-benzo-pteridine (isoalloxazine) derivatives, tetraaza-benzo-aceanthrylene derivatives, dihydro-quinooxaline derivatives, and pyrazino-carbazole derivatives. An enhancement of photo-degradation is observed by the formed photo-fragments. [Copyright &y& Elsevier]

Abstract: (αR)-α-[[[10-[4-(10-heptyl-10H-phenothiazin-3-yl)phenyl]-4,10-dihydro-2,4-dioxobenzo[g]pteridin-3(2H)-yl]acetyl]amino]pyrenepropionic acid phenylmethyl ester, a pyrene–isoalloxazine(flavin)–phenothiazine triad (abbreviated as PYFPT), was designed to mimic the dye-based functions of cryptochromes. PYFPT dissolved in dichloromethane and acetonitrile is characterized by absorption and fluorescence spectroscopy. Absorption cross-section spectra, fluorescence quantum distributions and quantum yields, degrees of fluorescence polarisation, and fluorescence lifetimes are determined. The triad is highly photo-stable as has been shown by monitoring absorption spectra under prolonged blue-light exposure. The absorption spectrum of PYFPT resembles the superposition of the absorption of the isoalloxazine (flavin), pyrene, and phenothiazine subchromophores. Blue light (λ exc =428nm) excites exclusively the isoalloxazine subunit causing reductive electron transfer from the phenothiazine subchromophore to the locally excited isoalloxazine with a time constant of approximately 600fs under formation of a charge transfer state which subsequently undergoes charge recombination. Near ultraviolet radiation (λ exc =311nm) causes simultaneous excitation of the isoalloxazine, pyrene, and phenothiazine chromophores. Both, the locally excited pyrene and phenothiazine moieties cause oxidative electron transfer to the flavin moiety with time constants of about 80ps and 50ps followed by subsequent charge recombination. Förster-type energy transfer processes have been found to be one to two orders of magnitude slower. [Copyright &y& Elsevier]