9 M"1 s'1],20 which has a much lower quenching rate than I2.The molecular and electronic structure of the quencher must be important in its quenching mechanism. The lowest values of the rate constants by CHI3 occur in ethanol, suggesting that the quenching efficiency is not increased by the polarity of the solvent and confirming the results obtained with iodomethane. The rate constants are very close to the diffusion-controlled limit, and a discussion of the mechanism is here more complex. By use of the same procedure adopted for I2, values of A around 1013 s"1 were obtained from the general equation ( 22). This equation does not take into account solvent cage effects,34 which increase the expected A values.It can be concluded that heavy-atom quenching is a complex problem still not completely understood and deserving further study.Acknowledgment. This workjvas supported by INIC (Instituto Nacional de Investigado Científica). Financial support from the Fundado Calouste Gulbenkian for the purchase of a Spex Fluorolog 112 spectrofluorometer is gratefully acknowledged. I acknowledge V. R. Pereira for his help in the experimental part of the work.
Photon-gated persistent spectral hole burning was observed for metal-free tetraphenylporphine with halogenated anthracene derivatives in poly(methylmethacrylate) or poly(ethylene) at liquid-helium temperatures. The hole formation yield was markedly dependent on the polymer matrices and the sample composition as well as on the gating wavelength. The irreversible broadening of holes in the systems measured by temperature cycling experiment was smaller than that by proton tautomerization of tetraphenylporphine. We concluded that the hole formation mechanism is donor-acceptor electron transfer on the basis of combined analysis of the matix and acceptor concentration dependence of hole formation yield, the action spectrum of the gating photon and the photoproduct spectrum.
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