Anthracene, irradiated crystals

Although a signal was detected also in irradiated pyrimidine solutions under similar conditions, it was attributed to dissociation products of the pyrimidine molecule and not to photoionization. If the purine solutions contained ethanol, then the signal of the CH3CHOH radical could also be detected this did not happen in ethanolic pyrimidine solutions. The possible importance of photoionization as the primary process in the photochemistry of nucleic acid derivatives has been supported by Kearns, from a study of photoionization in anthracene single crystals.115 He concluded that anthracene photoionization occurred via the interaction of two singlet excitons. 

Energy transfer in molecular crystals seems to be a well-established phenomenon. Irradiated crystals of anthracene containing only a trace of naphthacene show the characteristic green fluorescence of naphthacene rather than the violet of the primary constituent. If the material is dissolved in benzene, the anthracene fluorescence predominates. 

Emission Centres in Anthracene Single Crystals. Effects of Irradiation. D.P. Craig and J. Rajikan. 

PARAMAGNETIC CENTERS IN GAMMA-IRRADIATED ANTHRACENE SINGLE CRYSTALS. 

For instance, head-head photodimers are predicted from the crystal structures of 9-cyanoanthracene and 9-anthraldehyde, but the head-tail isomer is produced. Craig and Sarti-Fantoni and later others found that photoreactions of 9-cyanoanthracene and 9-anthraldehyde take place at defect sites [96,215], Systematic photochemical and crystallographic studies by Schmidt and co-workers uncovered many cases of substituted anthracenes which behave in an unexpected fashion (Scheme 40) [216,217]. Examples shown in Scheme 40 clearly illustrate that, unlike cinnamic acid derivatives, the stereochemistry of the product dimer from anthracenes cannot be predicted on the basis of crystal packing. An example from the laboratories of Venkatesan is noteworthy in this context [218], Irradiation of crystals of 7-... 

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. 

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)...

A. L. Sweeting, L. M. unpublished study). Disorder is not common in the 9-substituted anthracenes, but is observed for anthryl esters 7 and 9. We conclude that disorder may provide local dissymmetry, deforming the lattice and allowing charge accumulation. Unfortunately crystallography cannot determine how the sites of different conformation are distributed in the crystal. We examined the local environment by testing some of the materials for the generation of a second harmonic on laser irradiation no correlation with triboluminescence was found. 

X-ray irradiation of single crystals of anthracene peroxide (33, Scheme 23) gives mixed crystals of anthraquinone (34) and anthrone (35), with retention of the shape of the original crystal [118]. This transformation has been monitored by... 

Interest in the monomerization of anthracene photodimers continues the head-to-tail dimer (290) (see p. 361) produced by irradiation of 9-cyanoanthracene has been investigated by two groups. " From results of differential scanning calorimetry and emission spectroscopy of cleaved surfaces of partially monomerized single crystals, it is suggested that the first exothermic peak (and the subsequent ones) arises from monomer crystallization in the photodimer host matrix. " Theocharis and Jones have also examined this retroprocess and using single crystal X-ray techniques they have shown that the first step in the solid-state thermal monomerization of this dimer is a... 

The whole truth, however, is not nearly as simple as this. The experimental evidence has not been collected systematically, and it is not known, in most cases, whether singlet or triplet states are involved. In the last ten years, at least sixty papers have been published in which reasonably reliable structures have been assigned to the cyclobutane dimers produced by irradiation of a great variety of unsymmetrical olefins in solution. (The stereo- and regioselectivity in solid-state photodimerization reactions have also received a lot of attention, but they are determined by the alignment of the monomers in the crystal lattice, not by frontier orbital effects.) For irradiation in solution, head-to-head dimers are described as major or sole products in forty of these papers, and head-to-tail dimers are described in twenty of them. In addition, almost all photodimerizations of 9-substituted anthracenes (479) give the head-to-head dimer (480),385 whereas the... 

It was first reported that the topochemical photopolymerization of diolefin crystals gave rise to cracks and deformation [7]. An atomic force microscopic (AFM) study made possible the observation that the photodimerizations of trans-cinnamic acids and anthracenes in the crystalline state induced surface morphological changes at the tens and hundreds of nanometers level by the transportation and rebuilding of the surface molecules [8]. The appearance of a surface relief grating on the single crystal of 4-(dimethylamino)azobenzene was demonstrated by repeated irradiation with two coherent laser beams [9]. 

Photoemission from excited single states produced by photoionization of anthracene crystals occurs after two step laser excitation Biphotonic excitation of phenanthrene under 208 nm irradiation is a complex process involving both ionization andT-T annihilation. Change transfer exciton band structures have been characterized with samples of crystalline tetracene . Measurement of the photoionization efficiency in trans-stilbene crystals as a function of excitation energy shows that ionization occurs after rapid vibronic relaxation o. 

Cyanoanthracene has a 3-type crystal structure but produces a head-to-tail photodimer. Unlike cinnamic acids and their derivatives, the stereochemistry of the photoproduct dimer from anthracenes cannot often be predicted on the basis of their crystal packing [78]. 9-Methoxyanthracene has a photostable y-type crystal structure. 9-CNA host crystals doped with 9-MeOA form a solid solution, which upon irradiation yield a heterodimer (3> = 0.12). A reaction mechanism via an exciplex is proposed. The exciplex fluorescence was observed [32] ... 

In the case of double salts of anthracene-9-carboxylic acid (9-AC), which usually cyrstallized with included water and/or solvent, the 9-AC" component in the crystal underwent decarboxylation and reduction along with dimerization (Scheme 45) [77]. Although the head-to-tail dimer and a mixed dimer were obtained, the head-to-head one was not produced. The reactivity was influenced by the amount of the included solvenL e.g., (9-AC )2(c-chxiiH2 )(EtbH)xi was photostable, while (9-AC )2(c-chxnH2 )(EtOH)o,5 gave six photoproducts. Incidentally, the elusive head-to-head dimer was selectively obtained by solid-state irradiation of the 9-AC homocrystal as a thermally unstable product [78]. 

Temperature Dependence. At room temperature the exponential phosphorescence decay is absent, presumably because of the removal of triplet states by the temperature sensitive quenching process found at low temperatures. The decay from 5 /xsec. to 5 msec, did not fit any simple decay scheme although the mean slope of the decay on a log-log plot was —1. In the first 200 psec. after irradiation the room temperature emission is more intense than at 93 °K. A similar temperature dependence of the luminescence of anthracene crystals has been observed following ultraviolet excitation (1, 23). This behavior was interpreted as being caused by the enhanced intersystem crossing to the triplet states at the higher temperatures. This model, however, would not explain why the luminescence intensity of hot adenine powder in Figure 7 was lower than... 

The role of crystal imperfections in the dimerization of substituted anthracenes has been described in the case of l,8-dichloro-9-methylanthracene.178 Similar studies have now been conducted for the 10-methyl isomer.179 In order to explain how the topochemically forbidden / -dimer (head to tail) is produced from irradiation in the solid phase, optical and electron microscopic examinations of the (010) faces of the orthorhombic crystals of the monomer have been carried out, together with differential-enthalpic and dielectric measurements. Again it is shown that the dimer nuclei appear at emergent dislocations. 

Similar experiments performed at higher CTAB concentrations near the phase transition from isotropic solution to lyotropic liquid crystals show that the phase transition temperature is affected by the presence of rheologically active compounds (155,161). Figure 13 demonstrates that the phase transition temperature increases when small amounts of 9-anthracene carboxylic acid are solubilized. Irradiation at X = 366 nm, i.e. photodimerization, removes the effect, and reirradiation at X = 254 nm (splitting of the dimers) causes a reincrease of the phase transition temperature. 

Figure 13. Phase transition temperature of the system of isotropic solution and hexagonal liquid crystal. Pure CTAB-HjO, CTAB-HjO + 0.62% 9-anthracene carboxylic acid before irradiation, x after irradiation at 366 nm, after reirradiation at 280 nm, O. From Ref. 155 with the permission of VCH Veriagsgesellschafl.

The fundamental observable of exciton mobility and energy conduction in molecular crystals is sensitised fluorescence (compare Fig. 6.20). One irradiates a host crystal H, for example anthracene, which has a very low doping or impurity concentration of a guest G, for example tetracene, with light that can be absorbed by the host... 

As a second example, Fig. 7.7 shows the optical detection of the zero-field resonance (ODMR) from X-traps (cf Sect. 4.1) in an anthracene crystal at T = 1.2 K [6]. Here, both the variation of the intensity of the phosphorescence. Alp, and also the changes in the intensity AIdf of the delayed fluorescence (cf Sect. 6.9.2) were measured as functions of the microwave frequency. With both methods, aU three zero-field resonances were detected. The optical detection of the resonance at 1850MHz requires simultaneous irradiation at one of the other two resonance frequencies. The method of (optical) detection of this resonance is therefore referred to as electron-electron double resonance (FEDOR). From the three zero-field resonances and their structures, the three fine-structure parameters of two different X-traps in the anthracene crystal were found to be... 

Fig. 7.7 ODMR signals from X-traps in an anthracene crystal at T = 1.2 K. The variations A Ip in the phosphorescence intensity and A Iqp of the intensity of the delayed fluorescence are plotted as functions of the microwave frequency vpp. The signal at 1850 MHz (EEDOR) was detected via simultaneous irradiation with one of the other resonance frequencies.

Fig. 8.14 The photoelectron quantum yield as a function of the applied field strength F in an anthracene crystal at room temperature with an excitation wavelength of 225 nm. a a pristine costal, left-hand ordinate, b a crystal which was irradiated before the experiment, right-hand ordinate. In the linear region, a and...

Fig. 8.18 The dependence of the number of photo charge carriers in an anthracene crystal on the intensity of the irradiation at high intensities. The wavelengths of the excitation light were in the range of the absorption of the singlet excitons in which single-photon photogeneration of charge carriers is negligible...

Deeper traps can in some cases be emptied by irradiation with light. Figure 8.42 shows as an example the excitation spectrum of the photocurrent in an anthracene crystal which was doped with 10 tetracene molecules and whose hole traps were previously filled at a lower temperature, so that the anthracene crystal contained tetracene radical-cations before the excitation. The excitation spectrum shows the 0,0 transition and the vibronic series of the energetically lowest doublet-doublet transition Di Dq in the tetracene radical-cation. The combined evaluation of the thermally and the optically-stimulated currents yielded in this special case a value = 0.42 eV for the depth of the hole traps represented by tetracene in an... 

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