A study of the photochemical behaviour and relaxation mechanisms of anti-syn isomerisation around quinazolinone -N-N= bonds

High-resolution NMR experiments revealed that differently substituted quinazolinone-based Schiff bases undergo anti to syn isomerisation on exposure to ultraviolet light in DMSO solution. The degree anti to syn conversion varied significantly upon substitution (between 5% and 100%) and also showed two noteworthy features: that relaxation back to the anti-form goes far faster (by at least 3 orders of magnitude) when the C-6 rings B and C have ortho-OH substituents, and that relaxation can also be significantly sped up by addition of acid. Two possible mechanisms explaining the differences in relaxation process have been proposed: (I) the interaction of the azomethine hydrogen with the carbonyl oxygen results in slower reversion to the anti-form and/or (II) suppression of conjugation of the N3 lone pair with the N=CH double bond by protonation and/or internal H-bonding. Both of these mechanisms have been analysed theoretically.

Automated summary

High-resolution NMR experiments showed that differently substituted quinazolinone-based Schiff bases undergo anti to syn isomerisation under exposure to ultraviolet light in DMSO solution. The degree of anti to syn conversion varies significantly with different substitutions, and in some cases, the relaxation rate can be accelerated by addition of acid.