Benzophenone, photoreduction phosphorescence

Quantitative photophysical, photochemical and photopolymerisation data are presented on five novel water soluble benzophenone photoinitiators. Phosphorescence quantum yields, triplet lifetimes and transient formation on conventional flash photolysis correlate with the ability of the initiators to photoinduce the polymerisation of 2-hydroxyethylmethacrylate and a commercial monoacrylate resin in aqueous media. The results indicate that the lowest excited triplet state of the initiator is abstracting an electron from the tertiary amine cosynergist probably via a triplet exciplex followed by hydrogen atom abstraction. This is confirmed by a detailed analysis on the effect of oxygen, pH and the ionisation potential of the amine on transient formation and photopolymerisation. Using photocalorimetry a linear correlation is found between the photopolymerisation quantum yields of the initiators and their photoreduction quantum yields in aqueous media. 

Data obtained from the photoreduction of benzophenone in isopropanol indicates that the quantum yield (for the formation of acetone) is nearly constant using irradiation at several different wavelengths between 366 and 254 mp..16 The quantum yield for phosphorescence emission from the carbonyl n,ir triplet state is independent of exciting wavelength.22 Furthermore, the singlet excitation spectrum for the phosphorescence emission has been found to be very similar (if not identical) to the absorption spectrum.23 There are other examples where irradiation in the region of a given transition has ultimately led to a triplet of a different type.24... 

Direct evidence for triplet-triplet transfer has been provided by sensitizing both the isomerization of cis- or ira/w-olefins,28 38 79,80 and the dimerization of some cyclic olefins81-83 with carbonyl compounds. Furthermore, the phosphorescence of some carbonyl compounds can be quenched by olefins (for example, acetone with 2-pentene30 and phenyl-cyclopropyl ketone with 2-methyl-2-butene37). On the other hand, the phosphorescence of benzophenone is not quenched by 2-methyl-2-butene37 nor is the photoreduction of benzophenone quenched efficiently by cyclohexene (Table II). 

---