Picosecond diffuse reflectance and transmission laser flash photolysis study of various triaryl-2-pyrazolines

Abstract

In this study the first ever reported application of diffuse reflectance laser flash photolysis for the observation of sub-nanosecond transient absorption decays is presented. The compounds studied are various triaryl-2-pyrazolines, both as microcrytals and contained within polycarbonate films. The microcrystalline samples were studied using pump—probe laser flash photolysis in diffuse reflectance mode and the observed transient absorption decay could be fitted using a biexponential model with, in the case of 1, 5-diphenyl-3-styryl-2-pyrazoline, lifetimes of 1.6 × 10 −10 and 1.3 × 10 −9s for the first and second decay components, respectively. This model could also be used to fit the decay kinetics obtained from transmission pump—probe laser flash photolysis experiments conducted upon polycarbonate films containing this same compound, the lifetimes in this instance being 5.5 × 10 −12 and 1.7 × 10 −10s for the first and second decay components, respectively. In addition, a study of the quenching of the pyrazoline excited states in a polycarbonate matrix by disulphone magenta was undertaken. In this case it was necessary to modify the second term of the biexponential model with a term to allow for Förster type long range energy transfer, the Förster critical transfer distance being determined as 25 Å. This biexponential model is rationalized as initial excitation being to the S 2 state, the first decay component being relaxation to the S 1 state and the second component decay of the S 1 state to the ground state, by radiative and non-radiative relaxation and, when DSM is present, long range energy transfer to this energy acceptor.