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If the idea that the excitation energy is dissipated among the
stretching vibrational modes of the cluster of associated water
molecules is correct, then we can look upon these stretching
vibrations as the “promoting modes” for the nonradiative
transition in a manner similar to that adopted by Siebrand32 in
his treatment of nonradiative transitions in aromatic hydrocar-
bons. In these compounds,33 a distinct deuterium isotope effect
is observed. The Franck-Condon factors associated with the
transition are greatly reduced upon deuteration, which affects
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We are continuing our studies of these systems to expand
the range of structures under consideration, for instance to
determine whether the addition of substituents onto the 4-amino
group has an effect on the photophysics in these aqueous
alcohols.
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Conclusion
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The observed decrease in the fluorescence quantum yields
and lifetimes of two 4-aminonaphthalimides in aqueous ethanol
can be attributed to increased nonradiative deactivation as the
proportion of water in the solvent increases. The increase in
the nonradiative rate constant is not a linear function of the
water concentration, and the experimental data have therefore
been analyzed in terms of a model involving multiple quenching
species developed by Moore et al.27 In ethanol/water mixtures,
a cluster of some seven or eight water molecules is required to
effect the quenching, possibly corresponding to the excitation
of one quantum of a stretching mode in each water molecule.
Similar results are observed in deuterated solvents, although
the observed increase in the nonradiative decay is smaller and
the number of deuterated water molecules required to effect the
enhancement on the basis of the model is rather larger than
would be expected if one quantum of a stretching mode was
excited in each molecule. The behavior noted here is similar
to that observed for other systems such as the aminonaphtha-
lenes, aminonaphthalene sulfonates, and acridine, and it is
possible that the data in these systems could be treated in a
manner similar to that adopted here.
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Acknowledgment. We thank the University of Central
Lancashire for consumable support and for a research student-
ship (D.Y.) and the EPSRC for granting access to the Daresbury
Laboratory.
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References and Notes
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