Laser flash photolysis of aromatic azo-hydrazone systems

Abstract

Laser flash photolysis using 347 nm excitation and pulse radiolysis have been used to study the transient species produced from solutions of a series of eight hydrazone compounds synthesised from substituted (R¹,R²) 1,3-diones coupled to substituted (Ar) aromatic amines. Three classes of transient behaviour were found in hexane solution. Compound (8) gave a very short-lived transient (t_½ca. 45 ns, λ_max. 510 nm) which was identified as a triplet species. The other three compounds from unsymmetrical dione derivatives gave short-lived transients (t_½ca. 50–100 µs) in the spectral region 440–460 nm and long-lived transients (t_½ca. 0.5–2.0 s) occurring at ca. 425 nm, whereas the four symmetrical derivatives (R¹= R²) gave only the short-lived transients in the region 450–470 nm. The lifetimes of the longer-lived transients were shown to be dependent on solvent polarity and added acid. It is proposed that the longer-lived transients are associated with isomerism about the CN bond, induced by photoexcitation into the singlet excited state. Rate constants for quenching of various triplet species by three of the hydrazones were determined and shown to correlate with the triplet energy of the compounds being quenched. Assuming quenching occurred by electronic energy transfer the triplet energies of the hydrazones were estimated to lie in the range 150–190 kJ mol^–1. The photochemical breakdown products arising from extensive u.v. irradiation have been examined by chromatographic and spectroscopic techniques. It was shown that the compounds are relatively stable to irradiation above 330 nm.