Anthracene photoconductivity

Photoconductivity has been observed on polymers to which the active polynuclear aromatic groups had been added by chemical attachment. For example, poly(L-lysine) with anthracene groups is claimed to be useful in electrophotography65. Similarly, poly(p-alkyl glutamates) with the attached 9-anthryl, 9-anthryl-methyl and other groups were claimed to be photoconductive66. ... 

A number of photoconductuve resins made by condensation of anthracene, pyrene etc. with formaldehyde, benzaldehyde etc. have been disclosed in the patent literature. Their structure is however ai defined and no exact photoconductivity measurements have been reported in the literature. 

Surface-enhanced exciton dissociation models are based on the assumption that the absorption of a photon creates an exciton that diffuses to the surface where it dissociates into a free electron-hole pair, or a free and deeply trapped carrier of opposite sign, through an interaction with a donor or acceptor center associated with the surface. Such a process was first proposed by Lyons (1955). Evidence for this was largely based on early studies of anthracene, where the photogeneration efficiency increases with the absorption coefficient. Further evidence was that gases, particularly O2, adsorbed on the surface of anthracene crystals significantly change the photoconductivity, even for weakly absorbed radiation. 

Photoconductivity was first observed by Smith (1873, 1873a) and Adams and Day (1876) in Se layers. Photoconductivity in organic solids was first reported for anthracene in the early part of this century (Pochettino, 1906 ... 

Single-boundary three-dimensional aromatic hydrocarbons do have high photoconductivities (41), but the relationship between molecular structure and photoconductivity remains unclear. In this regard, the red shift of structure 4 in comparison with the presumably planar parent compound, namely tetrabenz[a,c,/i,j]anthracene (24) may be significant. In this book, Fetzer (15) gives UV-visible spectra for these kinds of polycyclic aromatic com-... 

Pochettino [1] discovered in 1906 that anthracene is a photoconductor. The anthracene crystal was investigated later, in particular after 1950, as the prototype for photoconductivity and many other electrical, optical and optoelectronic properties... 

In 1959/1960, Le Blanc [9] and Kepler [10] investigated the transient photoconductivity of high-purity anthracene crystals. The charge carriers were activated by a UV pulse and their mobihties were measured (see Sect. 8.4.1). Both for holes and for electrons, they found values between 0.3cm /Vs and 3cm /Vs at room temperature. These were dependent on the crystal orientation relative to the electric field and increased on cooling. Their results are still vahd today (see Table 8.2 in Sect. 8.5.2). 

In the treatment of photoconductivity in Sect. 8.4.1, we simply mentioned in passing the primary process the generation of the excess charge carriers using the internal photoeffect. In this section, we will treat the details of this process and choose again as an example the anthracene crystal. 

The conductivities of these crystals lie typically in the range of 1(T to 10 Q cm clearly higher than those of pure anthracene crystals, for example. Their ground states, as shown by photoelectron spectra, are similar to anthracene, with 5-10% CT character. The triplet states Ti are likewise similar to anthracene, as seen in their optical spectra. The Si <- So absorption is, however, very different it is a typical CT absorption. As its photoconductivity shows, the charge separation in anthracene-PMDA takes place in the Si state with an activation energy of 0.14 eV. 

Suitable materials for the use in such systems include anthracene, sulfur, and selenium. In addition to anthracene, other organic photoconductive materials most attractive to PVK, have been the focus of interest in electrophotography. Poly(vinylcarbazole)s, when sensitized with TNF exhibit a good photo-response and discharge characteristics and a low dark decay. However, the dark decay depends upon the polarity of the surface charge. 

Alkyl substituted anthracene has found application in plasticization of poly(N-vinyl carba-zole) residing in photoconductive layer of imaging member. 

Purification. The purity of the polymer has a decisive influence on various electrophotographic characteristics, such as photoconductivity and photosensitivity. A content of less than 100 ppm is required with respect to the monomer. Regardless of the method of preparation, the polymer is contaminated with up to 6% of NVK, up to 500 ppm carbazole, anthracene, and with sulfur compounds in the ppm range [25]. 

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