Laser photolysis of photographic image dyes

2017-06-02T15:41:27Z (GMT) by David R. Worrall
Studies have been carried out investigating the photochemical and photophysical properties of pyrazolotriazole azomethine dyes. Such compounds can be used as magenta images in the subtractive colour photographic process. This work has been carried out both in dilute fluid solution and in high concentration, high viscosity systems designed to mimic the photographic product. Picosecond pump-probe laser flash photolysis studies have provided the first observation of photoinduced transient absorption changes attributable to the excited states of this class of dyes, and have allowed assignment of the excited singlet state lifetime as being in the range 1 to 3ps at room temperature, with little dependence upon solvent properties or the pattern of substituents on the dye skeleton. Using picosecond laser flash photolysis it has also been possible to observe evolution of the population along the ground state potential surface to the two isomeric forms, the rate of this process showing some solvent dependence. Nanosecond and picosecond laser flash photolysis studies have been used to investigate the process of syn-anti and anti-syn isomerisation about the azomethine linkage. The syn-anti photoisomerisation occurs on picosecond timescales, the anti isomer so produced relaxing thermally back to the syn form on timescales ranging from microseconds to milliseconds. The rate constant for this process is a complex function of solvent properties as well as being dependent upon steric factors within the molecule, and the reasons for this are discussed. Triplet energy sensitisation studies have demonstrated that a pathway exists for this isomerisation process via the triplet manifold, and has allowed determination of minimum isomerisation quantum yields from the triplet state. Such studies have also allowed estimation of limits for the molar decadic absorption coefficients of the anti isomer. The photographic product dyes have been demonstrated to be efficient quenchers of singlet molecular oxygen, the quenching mechanism being predominantly physical in nature. The quantum yields of singlet oxygen production are too small to be measured using time resolved techniques. These factors are used to explain in part the resistance of such compounds to oxidative photodegradation. Steady state irradiation of azomethine dyes in systems designed to simulate the photographic product by a high intensity source of known spectral profile has allowed determination of the quantum yields of photodegradation, which in combination with the results obtained in fluid solution provide a basis for explaining the behaviour of image dyes in the photographic product environment.