Anthracene and Naphthalene Crystals Two-dimensional Triplet Excitons

Anthracene and Naphthalene Crystals Two-dimensional Triplet Excitons 

In non-doped anthracene crystals, as in non-doped naphthalene crystals at T = 300 K, two sharp Lorentzian ESR lines from excitons are observed (Figs. 7.18 and 7.21). They have, in complete analogy to the M lines of mini-excitons, a large and anisotropic fine structure (Fig. 7.19). The fine-structure constants of the triplet excitons in the anthracene crystal are D /he = -0.00575 cm and B /he = -1-0.0330 cm . The choice of axes is here the same as for the mini-excitons (cf. Fig. 7.11) z = b. From the measurements, the angle Z(x, a) = 27.25° is determined. VBth the methods treated in Sect. 7.4, this yields the fine-structure constants of the isolated anthracene molecule at room temperature, D/he = -1-0.0694 cm and E/hc = -0.00836 cm . (The small deviations from the values obtained for isolated anthracene molecules in a single-crystal matrix (see 

Taking the lifetime of the triplet excitons to be r 10 ms, we find the mean squared displacement or diffusion length L for triplet excitons in anthracene crystals at room temperature to be 

It is thus about 1000 times longer than that of singlet excitons in anthracene crystals (see Sect. 6.9.1), and therefore makes energy transfer via the long-lived triplet states possible over long distances, i.e. even to traps or reaction centres whose concentrations are very low. 

The large mean squared displacement can by the way explain the small residual linewidth of the triplet ESR lines in a natural way (for small A - B spacing) this was also observed for the triplet excitons in naphthalene crystals (Eig. 7.21). It is due to the complete averaging-out of the hyperfine structure, because the number N of molecules over which the average is carried out is not 2, as for the mini-excitons, but rather is very large (N 1). The second moment is then not reduced by the factor V2, as for the mini-excitons, but instead by the factor 1/N. The exciton ESR linewidth is therefore narrowed in relation to the linewidths of the isolated 

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