Crystal phosphorescence excitation spectrum

In pure crystals, singlet excitons can be created by mutual annihilation of triplet excitons. The intensity of the singlet exciton fluorescence depends quadratically on the triplet exciton concentration and is therefore proportional to the square of the singlet-triplet extinction coefficient. It is interesting to compare such a delayed fluorescence excitation spectrum, observed by Avakian et cd. 52) on naphthalene, with a corresponding phosphorescence excitation spectrum (Fig. 22). 

Let us note that there is a small gap of about 30 cm-i between the O —0" band observed in phosphorescence excitation and phosphorescence emission (Fig. 20). Such an effect is typical for crystals where emission occurs generally from traps that lie 20—50 cm i below the host level. The phosphorescence spectrum is dominated by the strong 0 - 0" band, an indication that there is little geometry change between So and T. ... 

Figure 15.11 Absorption and emission spectra of anthracene. These are at room temperature, except for the phosphorescence that was measured at 77 K. The singlet-triplet absorption is the triplet excitation spectrum in single-crystal anthracene. The dotted lines are the simulated absorption ( s=26,400 cm, <7 =350 cm , 5a = 1-18, v =1450 cm ) and fluorescence ( s=26200 cm", ...

Hutchison reported the first ESR spectrum of a metastable phosphorescent state by study of naphthalene oriented in durene crystals.4 Since then, similar spectra have been recorded for several other polynuclear aromatics both oriented in host crystals and randomly suspended in glassy matrices. D values for all these ir,n excited states are quite low, indicating little interaction between the unpaired electrons. Interestingly, D for the quinolines equals 0.10cm"1 just as in naphthalene,197 indicating that the presence of a heteroatom does not necessarily change the ir,w nature of the lowest triplet state very much. A similar conclusion has been reached from a comparison of the ESR spectra of fluorene, carbazole, dibenzofuran, and dibenzothiophene.198... 

We now consider hydrogen transfer reactions between the excited impurity molecules and the neighboring host molecules in crystals. Prass et al. [1988, 1989] and Steidl et al. [1988] studied the abstraction of an hydrogen atom from fluorene by an impurity acridine molecule in its lowest triplet state. The fluorene molecule is oriented in a favorable position for the transfer (Figure 6.18). The radical pair thus formed is deactivated by the reverse transition. H atom abstraction by acridine molecules competes with the radiative deactivation (phosphorescence) of the 3T state, and the temperature dependence of transfer rate constant is inferred from the kinetic measurements in the range 33-143 K. Below 72 K, k(T) is described by Eq. (2.30) with n = 1, while at T>70K the Arrhenius law holds with the apparent activation energy of 0.33 kcal/mol (120 cm-1). The value of a corresponds to the thermal excitation of the symmetric vibration that is observed in the Raman spectrum of the host crystal. The shift in its frequency after deuteration shows that this is a libration i.e., the tunneling is enhanced by hindered molecular rotation in crystal. 

This description of the relative spectral linewidths of the lowest excited toi states applies to the whole family of aromatic hydrocarbons. It also applies to the manifold of triplet jui states. In the case of benzene, Burland, Castro and Robinson 24> and Burland and Castro 25> have used phosphorescence and delayed fluorescence excitation techniques, respectively, to measure the absorption spectrum of the lowest triplet state, 3Biu of ultrapure crystals at 4 K. The origin is located at 29647 cm-1. Unlike all the earlier studies on the lowest singlet triplet absorption spectrum, this was not an 02 perturbation experiment. Here widths of less than 3 cm-1 were obtained. This result should be compared with the much broader bands 150-1 observed for the suspected second triplet ZE i in 5 cm crystals of highly purified benzene 26>. The two triplet states are separated by 7300 cm"1. 

ODMR has been applied to the study of sub-levels of trans-stilbene it is found that the phosphorescent state is not rigorously planar in single crystals B. in crystals of stilbene cyanobenzene complexes the triplet excitation is almost completely on the stilbene. The phosphorescence spectrum of 1,4-diazatriphenylene has also been published °. 

The lowest excited triplet state Ti of this crystal belongs, in contrast, only to the anthracene molecule and corresponds to triplet Frenkel excitons with no polar character, as we described in Sect. 6.5. This follows from the energetic position (not shown here) and the vibronic structure of the phosphorescence spectrum [36]. 

Chapter 7 introduces the reader to solutions of many selected problems in molecular physics. In particular, the following important problems are studied in detail the fluorescence spectrum ofp-terphenyl crystal, the vibrational fine structure of the spin-allowed absorption band of rans-[Co(CN)2(f )2]Cl3H20, and transport phenomena of electronic excitation in pentacene-doped molecular crystals. It is followed by an analysis of phosphorescence and radiationless transition in aromatic molecules with nonbonding electrons as well as predissociation of the 82 state of H2O+ by nonadiabatic interaction via conical intersection. 

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