Formation of free radicals in photo‐irradiated cellulose. IV. Effect of ferric ions

Abstract

AbstractThe effects of wavelength and concentration of ferric‐ion photosensitizer on radical formation in cellulose irradiated with ultraviolet light were studied by means of electron spin resonance spectroscopy. Irradiations were carried out at 77°K. The absorption spectra of ferric complexes with cellulose model compounds, namely, glucose, lactose, and cellobiose, indicate that light absorption of iron–cellulose complex takes place at 365 nm, to initiate free‐radical formation. A three‐line electron spin resonance spectrum with a relative signal intensity 1:1:1 was observed when the sample was treated with 0.1‐mmole/l. ferric ion and irradiated with light λ > 3400 Å. Five‐line spectra with different signal intensities were observed when the sample was irradiated with light λ > 2800 Å and λ > 2537 Å, respectively. Further, the 1:1:1 three‐line spectrum was immediately changed to a five‐line spectrum when the sample was re‐irradiated with light λ > 2537 Å. The concentration of ferric ion strikingly affected the radical formation in cellulose and caused changes of the line‐shape and of the relative signal intensities of the spectra. The sample with 0.1‐mmole/l. ferric ion exhibited the 1:1:1 three‐line spectrum; however, when the concentration was increased to 20 mmoles/l., a prominent five‐line spectrum with relative signal intensity 1:2:0.8:2:1 was observed, when the sample was irradiated with light λ > 3400 Å. On the basis of these findings, it is apparent that several kinds of radical species can be formed by employing suitable wavelengths and varying concentrations of ferric ion.