Absolute rate constants for reaction of Sod.-with substituted benzenes and benzoates have been determined by pulse radiolysis. The values are found to range from about 5 X lo9 M-' s-I for anisole to < I O 6 M-' s-I for nitrobenzene. A correlation of the rate constants with the Hammett substituent constant u gave p = -2.4 for both series of compounds. It is concluded that the reaction takes place by an electron transfer from the ring to so4*-.The reaction of the sulfate radical anion, SO4.-, with several aromatic compounds has been shown by ESR to result in the production of hydroxycyclohexadienyl radicak2 In the case of methoxy derivatives radical cations have been observed3 and in the case of several carboxy derivatives decarboxylation has been found to take place, resulting in the production of phenyl radicah4q5 These observations can be explained by various mechanisms. The hydroxycyclohexadienyl radicals can be produced by addition to the ring followed by hydrolysis as suggested by Norman et al.? or by electron transfer from the ring to the SOY-, as suggested by several authors,6-8 followed by hydroxylation with water or OH-. The radical cations from anisoles can also be produced by direct electron transfer3 or via addition followed by elimination. Decarboxylation can occur through direct oxidation of the carboxyl group, as is the case with aliphatic carboxylic acids,2s8 or again following oxidation of the ring.5 It appeared that examination of the effect of substituents on the rate constant for reaction of s04.-with aromatic compounds might shed some light on the mechanism.Absolute rate constants for reactions of SO4--radical can be determined by kinetic spectrophotometric pulse radiolysis as described p r e v i o u~l y .~-~~ The pseudo-first-order decay rate of the SO4.-absorption is measured in the presence of increasing concentrations of the aromatic compound and the second-order rate constant calculated from a linear plot of the rate vs. the concentration. The rate constants were found to vary from 5 X lo9 M-I s-l for anisole down to

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Two new water soluble squaraine dyes, bis[3-(p-carboxybenzyl)benzothiazol-2-ylidene]squaraine (Sq1) and bis [3-(carboxymethyl)benzothiazol-2-ylidene]squaraine (Sq2), have been synthesized and their photophysical properties have been characterized. Sq1 and Sq2 form dimer aggregates in water that have absorption bands blue-shifted to those of the monomeric forms. Aggregate formation is more preferred in D 2 O than in H 2 O. In the presence of low concentrations (<3 × 10 -4 M) of poly(vinylpyrrolidone) (PVP), enhancement in aggregate formation of Sq1 is observed, whereas that of Sq2 remains unaffected. Higher concentration (>3 × 10 -4 M) of PVP leads to the disruption of the aggregate and formation of a new species with absorption bands red-shifted to those of the corresponding monomers. The nature of these interactions has been investigated. It is proposed that hydrophobic interaction between the chromophoric units is the major driving force for the formation of the H-type (sandwich) aggregates in water. The red-shifted species are attributed to the monomeric forms, microencapsulated in a hydrophobic environment provided by PVP. Picosecond laser flash photolysis studies of the aggregates show clear evidence for the breakup of the aggregate from the excited state to yield an excited state-ground state monomer pair that rapidly recombines to reform the ground state dimer.

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FT-IR spectra of 5-methyl-2-(p-methylaminophenyl)benzoxazole was recorded and analysed. The vibrational frequencies of the compound have been computed using the Hartree-Fock/6-31G* basis and compared with the experimental values.

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