Issue 0, 1978

Structural imperfections and the delayed fluorescence of anthracene crystals

Abstract

Steady-state and temporal dependences of delayed fluorescence as a function of temperature have been measured for a range of high purity anthracene crystals with the aim of investigating the triplet exciton trapping characteristics. Direct excitation of triplets with weakly absorbed red light of low and high intensity has been employed and the response of the crystals to (a) low frequency (≈5–20 Hz) chopped-light and (b) a δ-function excitation has been recorded.

For high intensity excitation there is an inflection in the decay for both modes of excitation, revealing the existence of trap saturation over the entire temperature range, 350–4 K. Our study indicates that even at high temperatures the triplet exciton is localised in deep structural traps of low concentration (≈10–11 M/M). Even in the purest samples available the triplet exciton lifetimes reflect the fact that the routes controlling triplet exciton decay are extrinsic via trapping sites.

The highest technical lifetime measured from delayed fluorescence is ≈27 ms for sealed melt-grow crystals; this value shows an appreciable reduction upon cleavage. All observations may be interpreted in terms of the perturbed anthracene molecules, both as separate entities and in pairs (“incipient dimers”), associated with dislocations and a polymorphic modification, that yield a quasi-continuum of triplet trapping levels extending downwards from ≈4,700 cm–1. The deepest traps are removed upon prolonged u.v. irradiation of the crystals under conditions that yield the photodimer, whereupon other shallower traps appear.

Article information

Article type
Paper

J. Chem. Soc., Faraday Trans. 2, 1978,74, 618-629

Structural imperfections and the delayed fluorescence of anthracene crystals

J. O. Williams and Z. Zboiński, J. Chem. Soc., Faraday Trans. 2, 1978, 74, 618 DOI: 10.1039/F29787400618

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