Photochemical cis-trans Isomerization in the Triplet State

The variation of fluorescence yield as a function of exciting wavelength for the title compounds has been examined. 1,3,5-Trifluorobenzene shows a maximum in fluorescence yield at an exciting wavelength λex = 2680 Å; for 1,2,3-trifluorobenzene, this maximum occurs near 2740 Å. Triplet yields for both molecules, based on the biacetyl sensitized emission technique, have been determined at various exciting wavelengths. The maximum triplet yield of 1,3,5-trifluorobenzene = 0.40 and occurs at λex = 2640 Å, while that of 1,2,4-trifluorobenzene = 0.64 at λex = 2780 Å. The triplet of the 1,2,4 isomer appears to lie below the triplet of cis-butene-2 and thus the olefin cis–trans isomerization technique cannot be used as a diagnostic for the triplet state of this molecule. The effect of two inert gases, cyclohexane and cis-butene-2, on both fluorescence and triplet yield has been studied.

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Energieübertragung von angeregtem Chlorophyll auf C40-Polyene mit verschiedenen chromophoren Gruppen

Zeitschrift für Naturforschung B ◽

10.1515/znb-1961-0708 ◽

1961 ◽

Vol 16 (7) ◽

pp. 445-454 ◽

Cited By ~ 39

Author(s):

Hedwig Claes

Keyword(s):

Energy Transfer ◽

Protective Effect ◽

Triplet State ◽

Photochemical Reactions ◽

Minimum Length ◽

Double Bonds ◽

Anaerobic Conditions ◽

Trans Isomerization ◽

Quenching Process ◽

Direct Energy

Three chlorophyll sensitized light reactions (irreversible photoxydation and reversible photoreduction of chlorophyll, and cis-trans isomerization of carotenes) were studied in presence of acyclic C40-polyenes with 11, 9, 7, 5, and 3 conjugated double bonds.The protective effect of carotenes containing 11 and 9 conjugated double bonds against photoxydation of chlorophyll saturates with increasing carotene concentrations and remains incomplete at carotene concentrations which fully prevent the anaerobic photoreduction.The inefficiency of polyenes with shorter chromophores to protect chlorophyll against photoxydation can be slowly compensated by increasing the concentration of ζ-carotene but not of phytofluene.Apparently the interaction between excited chlorophyll and carotenes becomes more sensitive to inhibition by O2 with increasing saturation of the polyenes: The slight interference of phytofluene with photochemical reactions of chlorophyll, observed under anaerobic conditions, disappears in presence of air. ζ-carotene behaves like the less saturated polyenes in the absence of O2 but not in solutions saturated with air.Inhibition of energy transfer from excited chlorophyll to carotenes can also be observed with p-benzoquinone in place of O2. The normally rapid cis-trans isomerization of polyenes sensitized by chlorophyll in light is slowed down in presence of O2 or p-benzoquinone.These results indicate a direct energy transfer from excited chlorophyll to carotenoids. But this happens only with the polyenes having a minimum length of the chromophore. O2 is not necessary for this process. Apparently O2, p-benzoquinone and carotenoids act indepedently as efficient quenchers of the triplet state of excited chlorophyll in the manner shown by FUJIMORI and LIVINGSTON 24. When present together, they compete for the quenching process.

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