Lalit N. Patnaik

Organic photoconductors : dark and photoconduction studies in two p-dimethylamino styryl dyes derived from pyridine-2 and pyridine-4

Results of our experiments on the dark and photoconduction studies in two p-dimethylamino styryl dyes derived from pyridine-2 (PDMS-P2) and pyridine-4 (PDMS-P4) in their pure form without any dopant or additive are reported. Measurements on surface-type (raster pattern) cells show that the dark and photocurrents are dependent on the applied potential, temperature, and the photocurrent on the intensity of the incident radiation and the wavelength. Action spectra of the samples could not be recorded as the intensity of the monochromatic radiation from the monochromator reaching the sample was too low to induce any noticeable photocurrent. The compounds show a low dark conductivity. The dark and photocurrents show a perfect ohmic behaviour in the temperature range studied (288–328 K). Since photoconduction could only be observed in a vacuum, this clearly indicates that the compounds are n-type semiconductors. The observed rise and decay kinetics of the photocurrents indicate the presence of traps in the forbidden zone. The dyes show an enhanced photoconduction on illumination with visible radiation only. The marked open circuit voltage and the short-circuit current observed in these compounds indicate a possible application in solar photovoltaics.

The kinetics of retardation by phenol of the polymerization of acrylonitrile

Abstract The present communication discusses the kinetics of the retarding effect of phenol on the polymerization of acrylonitrile initiated by azobisisobutyronitrile (AIBN). The results are treated on the basis of the scheme proposed by Tudos et al.

Rate parameters for transfer reactions of some polymer radicals: in terms of Swain and Lupton's f and r and the unique positional weighting factors of Williams and Norrington

Abstract Rate parameters of the reaction of phenols, deuterated phenols and nitrobenzenes with polyvinylacetate radicals and of aromatic thiols with polymethylmethacrylate radicals have been correlated with Swain and Luptons substituent constants (Fk and Rk) and Williams and Norringtons unique positional weighting factors (fj and rj) by the following equation: P i = α i ƒ j F k + β i r j R k + e i + P 0 i where Pis are the rate parameters, P0i being that for a standard reference state. The correlations were found to be quite satisfactory. The sign and magnitude, and the ratio of the reaction dependent parameters αi and βi throw light on the nature of the transition state and the relative contributions of the mesomeric and inductive effects. The present studies also show that the mesomeric effect of meta-substituents is significantly greater than reported by earlier workers.

Conformational studies on somep-dimethylaminostyryl dyes

Abstract PCILO computations have been carried out on the conformations of five p -dimethylaminostyryl dyes derived from quinoline-4 (I), quinoline-2 (II), pyridine-4 (III), pyridine-2 (IV) and benzothiazole (V). The stable conformations of I–IV are found to be nonplanar, while V is almost planar. The results have been explained in terms of various possible steric interactions.

Structural dependence on the photovoltaic and the photoconduction behaviour of some P-dimethylamino styryl dyes derived from different heterocyclic nuclei

Abstract The results on the structural dependence of the photovoltaic and the photoconduction behaviour of seven P-dimethylamino styryl dyes derived from different heterocyclic nuclei is reported. It has been confirmed that the plane-to-plane stacking of the molecules in the solid state enhances the π-electron overlap and hence to the higher photovoltaic and the photoconduction behaviour observed in these compounds.

Thin-layer chromatographic behaviour of some styryl cyanine dyes derived from pyridine

Abstract The chromatographic R M values for some styryl cyanine iodides derived from pyridine were obtained by thin-layer chromatography using propanol—water—acetic acid solvent systems in various proportions. Linear free energy relationship studies were carried out with some electronic, steric and lipophilic parameters, either singly or in combination with each other. The results indicate that the steric factor has the greatest influence on the chromatographic behaviour. An attempt was made to establish the nature of dye-substrate adsorption by correlation with the theoretical charge densities (CNDO/2 and INDO).

Structural dependence on the photoconduction and photovoltaic behaviour of merocyanine dyes-III

Abstract The present results are concerned with the photoconduction and photovoltaic behaviour of two new merocyanine dyes (derived from two different heterocyclic nuclei) in thin film form on a surface type multi-electrode cell structure. It is found that the dominant trapping level in photoconduction is different from that in dark conduction. A band model has been used to evaluate the charge carrier transport parameters and the generation of charge carriers is explained on the basis of an exciton model which is confirmed by the intensity variation of the photocurrent in these dyes. The dyes were found to behave as p-type semiconductors. The distribution of traps which is exponential in nature is suggested from the intensity dependence of the short-circuit current for photovoltaic properties. The results were sought to rationalise in terms of the structure of the dyes with their conduction properties.

Rise and decay time kinetics of photocurrent in merocyanine dyes

Abstract The dependence of rise and decay time of photocurrent on the applied voltage, sample temperature and incident light intensity has been studied for a new series of merocyanine dyes (six in numbers). The samples were prepared in thin film form on a multi-electrode raster type surface cell by vacuum deposition. Two orders of magnitude difference was found in the photocurrent when the samples were studied in air and vaccum of the order of 10−5 Torr in the conductivity cell. Rise and decay times were found to be less in vacuum. At higher temperatures, the photocurrent more rapidly approaches its stationary value because the ratios of concentrations of filled traps and free carriers (nt/n and pt/p) decrease as the temperature is increased. The dyes were found to behave as p-type semiconductors.

Ortho Effect and Additivity of Substituent Parameters in the Reaction between Phenols and Poly(vinyl Acetate) Radicals

Abstract The ortho effect and the additivity of substituent parameters in the reaction between phenols and poly(vinyl acetate) radicals have been investigated in the light of theories postulated by Swain and Lupton on substituent constants Fk and Rk and that of Williams and Norrington on the unique positional weighting factors fj and rj. The present correlation studies show that while Chartons theory that the ortho-substituent effect is primarily electrical is true for most of the substituents, some steric considerations have to be invoked while dealing with groups like o-tert-butyl. The additivity of fjFk and rjRk is much less satisfactory than that of Hammetts [sgrave] or Brown-Okamotos electrophilic [sgrave]+ substituent constants.

Substituent effects in the reaction between acetophenones and polyacrylonitrile radicals

The reactivity of substituted acetophenones towards polyacrylonitrile radical has been correlated with Swain and Luptons field and resonance parameters inclusive of the unique positional weighting factors suggested by Williams and Norrington. From the sign and magnitude of the reaction dependent regression constants, the transition state has been found to be essentially ‘non-polar’ in character.