Study of non-conjugated bichromophoric systems, the so-called ‘jaw photochromic materials’. Part 1. Photocyclomerization and fluorescence of bis-9-anthrylmethyl ethers

Abstract

Bis-9-anthrylmethyl ether and eight derivatives of the type XACH₂OCH₂AY (Ia–i) where A is 9,10-anthrylidene and X and Y are substituents have been prepared and investigated. The three-member ether linkage was chosen in order to obtain more easily than with a polymethylene chain the relevant conformations for efficient photocyclomerization. Indeed, the majority of these compounds photocyclomerized in fluid solution at 366 nm with a quantum yield of 0.16–0.50. Photodissociation at 254 nm has also a high efficiency (0.27–0.64). Cyclomerization occurs even for compounds where both X and Y are bulky (CH₃, Br). An increase in the photocyclization quantum yield was noted in acetonitrile relative to methylcyclohexane (MCH). These systems are thus photochromic. A study of the u.v. absorption spectra of (Ia–h) reveals considerable hypochromism relative to those of model compounds and this is consistent with the presence of ground state conformations having a high degree of interaction between the aromatic groups; in contrast (Ii) exhibits hyperchromism with respect to the reference compound and this is ascribable to different conformations arising from steric crowding of the substituents. Excimer fluorescence was recorded in fluid solution at room temperature for the diphenyl derivative (Ii) which does not react and in MCH matrices at 77 K for the cyclomers cleaved at 254 nm by the method of Chandross and Ferguson. Their maximum emission (550–575 nm) points to the presence of strong overlapping conformations.