Anthracene crystals

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

Emission spectra at these points are shown in Figure 8.2d. The band shapes were independent of the excitation intensity from 0.1 to 2.0 nJ pulse . The spectrum of the anthracene crystal with vibronic structures is ascribed to the fluorescence originating from the free exdton in the crystalline phase [1, 2], while the broad emission spectra of the pyrene microcrystal centered at 470 nm and that of the perylene microcrystal centered at 605 nm are, respectively, ascribed to the self-trapped exciton in the crystalline phase of pyrene and that of the a-type perylene crystal. These spectra clearly show that the femtosecond NIR pulse can produce excited singlet states in these microcrystals. 

Figure 8.2e shows the dependence of the fluorescence intensity on the excitation power of the NIR light for the microcrystals measured with a 20x objective. In this plot, both axes are given in logarithmic scales. The slope of the dependence for the perylene crystal is 2.8, indicating that three-photon absorption is responsible for the florescence. On the other hand, slopes for the perylene and anthracene crystals are 3.9 for anthracene and 4.3 for pyrene, respectively. In these cases, four-photon absorption resulted in the formation of emissive excited states in the crystals. These orders of the multiphoton absorption are consistent with the absorption-band edges for each crystal. The four-photon absorption cross section for the anthracene crystal was estimated to be 4.0 x 10 cm s photons by comparing the four-photon induced fluorescence intensity of the crystal with the two-photon induced fluorescence intensity of the reference system (see ref. [3] for more detailed information). 

Nishimura, H., Yamaoka, T., Hattori, K., Matsui, A. and Mizuno, K. (1985) Wavelength-dependent decay times and time-dependent spectra of the singlet-exciton luminescence in anthracene crystals./. Phys. Soc. Jpn., 54, 4370-4381. Matsui, A. and Nishimura, H. (1980) Luminescence of free and self trapped excitons in pyrene. J. Phys. Soc. Jpn., 49, 657-663. 

K. Kojima, Photoplastic Effect in Anthracene Crystals, Appl. Phys. Lett., 38, 530 (1981) see also, ibid., 56, 927 (1984). 

W Helfrich and WG Schneider, Recombination radiation in anthracene crystals, Phys. Rev. Lett., 14 229-231, 1965. 

Aaviksoo, J., and Reinot, T. 1992. Ballistic propagation of luminescence pulse in anthracene crystal flakes. MoZ. Cryst. Liq. Cryst. 217 147. 

Hnppert, D., and Rojansky, D. 1985. Picosecond stndy of electronic energy transfer in tet-racene-doped anthracene crystal. Chem. Phys. Lett. 114 149. 

Figure 5.6 Electron micrograph of a network of dislocations in anthracene crystal. (Courtesy of G.M. Parkinson J.M. Thomas, University of Cambridge.)...

Anthracene crystals are highly fluorescent (4>f = 1-0) but in dissolved state emission is much reduced ( / = 0.25). A recent explanation of this large difference is that the second triplet state T% of anthracene lies above the first singlet in anthracene crystal but below it in the dissolved state. thereby enhancing the nonradiative dissipative processes. Molecular adsorption on a substrate also enhances the fluorescence. Hydrogen... 

Occasionally, long-range disorder and/or different phases may coexist within a crystalline material. Arrangement of molecules in the different regions will necessarily be different in at least some respects. One of the earliest reports of invocation of this phenomenon involves the photodimerization of anthracene in the crystalline state [219]. In the crystal structure of anthracene, the faces of no molecules are separated by <4 A. Yet upon irradiation, a dimer is readily formed. Thomas, Jones, and co-workers used electron microscopy to reveal the coexistence inside normal anthracene crystals of regions of a metastable phase. In the minor phase (space group PI), the C9- -C9. distance is 4.2 A, whereas in the stable crystal it is 4.5 A. The dimerization is proposed to originate in the minor phase of the crystal. 

Hydrogen atom transfer from anthracene, excited into its lowest excited singlet state, to anthraquinone impurity molecules creates a radical pair that strongly quenches the fluorescence from anthracene crystals. The reverse transfer rate constant, found from measurements of fluorescence intensity and its characteristic lifetime at different moments after the creation of the radical pair, varies from 106 to 10s s 1 in the range 110-65 K, kc = 4 x 104 s 1, TC = 60K. The kc values drops to 102 s 1 in the deuteroanthracene crystal [Lavrushko and Benderskii, 1978]. 

Fig. 8 Typical current transients produced by motion of (i)(a) electrons, (b) holes through an undeformed anthracene crystal (ii)(a) electrons, (b) holes through a deformed anthracene crystal. All measurements at room temperature following electron beam irradiation with in (i) F = 5.8 x 105 V m-1 and crystal thickness 1.72 mm and in (ii) F = 5 x 105 V m with crystal thickness 1.04 mm. (After Aris et al., 1973)...

Fig. 9 Current transients (regions DE and de) characteristic of rapid recombination of the charge carriers near the illuminated electrode in an anthracene crystal with damaged surface (a)...

Fig. 14 Photocurrent excitation spectrum of an undoped anthracene crystal whose traps were filled at 83 K current, 3 x 10-I3A full scale. (After Rohrbacher and Karl, 1975)...

Fig. 20 Plots of H/kTc against 1 kTQ for electron and hole injection into anthracene crystals grown under different atmospheres open symbols refer to electrons and closed symbols to positive holes. (After Owen et al, 1974)...

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