Direct measurements of energy transfer between triplet states of molecules in liquids using picosecond pulses

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Abstract
We have measured the buildup and decay rates of triplet‐triplet absorption of triplet donor solute molecules dissolved in acceptor solvents following picosecond excitation of the first excited singlet state of the solute at room temperature. We have used harmonic pulses from a mode‐locked Nd+3 /glass laser for exciting (3545 Å) and probing (5300 Å) the samples. We report buildup (k) and decay rates (kQ ) for the following solutions: benzophenone in cis ‐piperylene (k −1 =7 psec , k Q −1 =9 psec ) and trans ‐piperylene (k −1 =7 psec , k Q −1 =11 psec ) , benzophenone in 1‐methylnaphthalene (k −1 <10 psec , k Q −1 =20 psec ) , and 1‐nitronaphthalene in cis‐piperylene (k −1 =8 psec , k Q −1 =678 psec ) . We discuss our results in terms of a model of triplet‐triplet energy transfer which depends on good overlap of donor and acceptor energy levels with large Franck‐Condon factors. This model indicates that energy may compete with vibrational relaxation particularly for the benzophenone in piperylene systems, but that thermally assisted energy transfer will be required for 1‐nitronapthalene in cis ‐piperylene. These predictions are in good agreement with our experimental results. We also report a novel technique of measuring the position of t =0 and the laser pulse duration for calibrating the type of experimental apparatus we use.

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