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
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.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.