Studies on the radiolytically produced transients of neutral red: triplet and reduced radicals

The spectral and kinetic properties of reduced radicals and the triplet state of neutral red (NR), a phenazine-based dye, have been investigated using pulse radiolysis technique. A mixed water-isopropanol-acetone solvent has been used to study the reduced radicals of NR for a wide pH range of about 1-13, due to limitation of solubility of the dye in aqueous solutions particularly above pH 8. From pH-dependent absorption studies it has been established that the reduced radicals of NR can exist in four different prototropic forms in solution. Three pKa values for the corresponding prototropic equilibria have been estimated. The formation and decay rate constants of reduced radicals have also been measured. The triplet state characteristics of the dye have been investigated in neat benzene solutions, both in the presence and in the absence of triplet sensitizers. The T1-->Tn absorption spectrum and decay kinetics of the triplet state have been measured. The triplet state energy (ET) of NR in benzene have been estimated to be within 36-42 kcal mol-1, using an energy transfer method.