Photoconductive Polymers Based on Carbazole

As stated above, the primary mechanism for charge-carrier generation in poly (vinyl carbazole) appears to be due to the excitation of the carbazole rings to their excited singlet states [112]. While the singlet excited state is not a conductive species, the conductivity is believed to be the result of an associated electron-positive hole pair migrating through the solid polymeric material. Dissociation of the electron pair produces a separate electron and a positive hole in such a way that the electron ends up in the conducting state [94]. This requires acquisition of more energy. One way that can be accomplished is by exciton–surface interaction [243]. Regensburger published an absorption spectrum, fluorescence spectrum, and photocurrent spectrum for a 7.6 mm films of poly (N-vinyl carbazole) [244]. The shape of the response of the photoconductor to the wavelength of the light flash is very close to the shape of the absorption spectra. Bauser and Klopffer explain this as a result of interaction of singlet excitons with trapped holes [245]. Lyoo used a low-temperature initiator, 2,20-azobis(2,4-dimethyl-valeronitrile), to polymerize N vinyl carbazole in a heterogeneous solution in a mixture of methyl and t-butyl alcohols [247]. The polymer that formed has the Mn molecular weights >3x10⁶. The author emphasized that this method provides ultrahigh-molecular-weight polymer and conversions greater than 80%.

The optical transparency of poly (vinyl carbazole) films produced by this room temperature process appears to be quite high, although transparency decreases at high conversions. In film form, this material is useful for photoconductors, charge-transfer complexes, and electroluminescent devices. The higher polymer molecular weight typically enhances film mechanical properties [247].

Horhold and Rathe [248] reported that they have prepared poly (9-methylcarbazole-3,6-diyl 1,2dipenylvinylene). The polymer (Mn ¼ 10,000) was formed by dehalogenating polycondensation of 3,6-bis (a, a-dichlorobenzyl)-10,9-methylcarbazole with chromium (II) acetate. This polymer was found to be also highly photoconductive. Its dark conductivity increases by doping it with arsenic pentafluoride [248]. Photoconductive polymers and copolymers were also synthesized by Haque et al. [249] from 2-(9 carbazolyl)-1-propenyl vinyl ether:

The polymers and copolymers form by a cationic polymerization mechanism, using boron trifluoride or ethylalumionum dichloride as the catalysts [249]. Charge transfer complexes also form from poly (vinyl carbazole) that acts as the donor, with poly [2-(methacroyloxy) ethyl-4,5,7-trinitro-9-oxo-2-fluorene-carboxylate] that acts as the acceptor [250]:

The required mole ratio of components in the complex is 1:1 [250]. Charge transfer complexation occurs in a similar manner in poly(2-carbazolylethyl acrylate) molecularly doped with 2,4,7-trinitrofluorene. Quantum efficiency of the hole propagation of the copolymer with the 0.05–1.0 molar ratio of trinitro fluorene to carbazole chromophores is higher than in the corresponding trinitro fluorene and ethyltrinitrofluorene doped homopolymer of poly (2 carbazolylethyl acrylate) [251]. Kim and Webber studied delayed emission spectra of poly (vinyl carbazole) that was doped with dimethyl terephthalate and pyrene [252]. On the basis of their results, they concluded that at room temperature dimethyl terephthalate does not completely quench the triplet excitation state of poly (vinyl carbazole). They also concluded that phosphorescent states of poly (vinyl carbazole) dimethyl terephthalate are similar, implying a significant charge-transfer character in the former. In 1985, polymeric triphenylmethane dyes based on condensation polymers such as polyesters and polyurethanes were prepared [252]:

These dyes were then shown to sensitize photo-conductivity of poly (N-vinyl carbazole). Polyacetylene derivatives exhibit unique characteristics such as semi conductivity, high gas per meability, helix inversion, and nonlinear optical properties [253]. Attempts were made, therefore, to incorporate carbazole into polyacetylene in hope of attaining enhance properties [253].

It was found [122] that the current conducted by this polymer during irradiation is 40–50 times higher than it is in the dark. On the other hand, the electron mobility of the di-t-butyl carbazole polyacetylene (shown above) is lower than in poly (vinyl carbazole). This was attributed to the bulkiness of the butyl groups [254]. Siloxanes with pendant carbazole groups were synthesized by Strohriegl [254] by the following technique:

The material, however, did not turn out to be photoconductive. The photo-conductivity of copolymers was also investigated. Copolyacrylates with pendant donor and acceptor chromophores, such as 2-N-4,5,7-trinitrofluorenone-2-ethyl acrylate, do exhibit photo-generation properties [254]. These copolymers

can be prepared by free-radical copolymerization of the appropriate monomers. Photo-conductivity in the visible is obtained by charge transfer complexation. Similar work was done earlier by Natansohn [255], who copolymerized N-methyl, 1,3 hydroxymethyl carbazolyl acrylate with acryloyl-30-hyroxypropyl-3,5-dinitrobenzoate:

Illumination of the copolymer samples induces a certain degree of ionization accompanied by proton transfer. There is a permanent increase in the quantity of radicals generated by light. Another number of radicals apparently forms during illumination, but disappears in the dark [254]. It was reported [256] that a polymer formed by condensation of N-(3-isopentyl)-3,6-diformyl carbazole and 4-14-bis[(-aminoethyl) aminophenyl-azo] nitrobenzene yields a new polyazomethine, carbazole-azo polymer. The product is soluble in organic solvents. The polymer possesses carbazole moieties and azo type nonlinear optical chromophores in the backbone. It shows high photo conductivity and nonlinear optical properties.

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مواضيع ذات صلة

Polymer-Based Solar Cells

Photo-Conducting Polymers That Are Not Based on Carbazole

Photo-Conducting Polymers

Liquid Crystalline Alignment

Application to Optical Data Storage

Changes in Viscosity and Solubility of Polymeric Solutions

Photo-Isomerization of the Azo Group

Photo-Isomerization of the Olefinic Group

Copolymers of Methacrylates with Cholesteric Monomers

Polymers with Norbornadiene Moieties

Photo-Responsive Polymers

Polymers Designed to Cross-link Upon Irradiation with Laser Beams