An excited-state solute—solvent complex (exciplex) has been shown to be responsible for a large red shift and loss of vibrational structure in the fluorescence spectra of indole and indole derivatives in polar solvents. Solute—solvent stoichiometry of 1:2 and 1:1 is observed with associating and nonassociating solvents, respectively. Hydrogen bonding between the indole >N–H group and solvent is shown not to be responsible for the interaction. It is suggested that the exciplex state is a charge-transfer state and is an intermediate in the process of electron transfer from the solute to the solvent.

Rate constants for both radiative and non-radiative relaxation of the fluorescent state of indole and several methyl-indoles in solution have been calculated from fluorescence lifetime and quantum yield values.Radiative relaxation for indoles is solvent dependent while non-radiative relaxation is attributed to at least two processes, one of which is both solvent and temperature dependent. The latter process measured in aqueous solvents manifests a pattern of compensation of activation enthalpy by activation entropy with an isokinetic temperature for indole of 283°K.A quantitatively consistent description of the excited-state behavior of 1-Me-indole in both free and exciplex state is provided. Small deviations in fluorescence parameters for indole suggest a more complicated set of events.

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SUMMARYMost of the carbon fixed in photosynthesis by Xanthoria aureola (.Y. parietina var. aureola) initially accumulates in sugar alcohols and relatively little is incorporated into insoluble compounds at first. Two kinds of sugar alcohol are involved-mannitol and a pentitol. During the early stages of photosynthesis, much more carbon accumulates in the pentitol than in mannitol. Within 9-15 minutes of exposure to NaH'^COj in the hght, the level of •^' ' ' C in metabolic intermediates presumably common to both symbionts (PGA, hexose monophosphates, organic acids) became more or less steady and remained so for several hours. It was concluded from this that it is likely that the fiow of carbon from alga to fungus is relatively rapid and efficient, and achieved a steady rate within 15 minutes of the start of the experiment.