Intramolecular Triplet‐Triplet Energy Transfer in Polyadenylic Acids: The Influence of Triplet Energy Trapping on the Decay Kinetics of Delayed Fluorescence Produced by a Triplet‐Triplet Annihilation

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Abstract
A delayed fluorescence is observed from both polyriboadenylate and polydeoxyriboadenylate helices, but not from a random conformer of polyrA. The delayed fluorescence depends approximately on the square of the triplet concentration and is believed to originate from a triplet‐triplet annihilation occurring within the polymer helices. The decay of the delayed fluorescence is nonexponential, with a short‐lived component of 70 msec decay, and a longer‐lived component of 670 msec. Increasing temperature from 77°K decreases the fractional contribution of the short component. The failure of phosphorescence intensity to increase linearly with exciting light intensity and its having a shorter rise time to steady state levels than decay indicate that the emission occurs from an exciting‐light‐saturable site. As there was no evidence for a depletion of the ground state by optical pumping to the triplet level at light intensities which gave appreciable reduction in rate of approach to steady state, all the phosphorescence emissions must originate from an intrinsic, lower energy trap site. Previous studies of the Stokes' shift of phosphorescence from the polymers have found an unexplained addition to the Stokes' shift of 15 mm−1 which is not present in oligomer helices. These observations were consistent with a simple trapping model for the production of delayed fluorescence. The triplet excitons became rapidly trapped by these lower energy sites, and delayed fluorescence is formed by thermal activation to the polymer host level, followed by a diffusive migration and anihilation with a trapped triplet exciton. This model is fully consistent with the transition metal quenching of phosphorescence, where, although extensive reduction in phosphorescence yield occurred, no alteration in lifetime was found.