Studies of fluorescent dyes: part 2. An investigation of the synthesis and electronic spectral properties of substituted 3-(2-benzimidazolyl)coumarins

A series of 3-(2ʹ-benzimidazolyl) coumarins containing a variety of substituents in the benzene ring of the coumarin system have been synthesised and their spectral properties investigated. The results of PPP molecular orbital calculations, using previously optimised parameters, were found to provide a reasonable account of the electronic absorption spectra for this series of dyes, except for some compounds which are likely to be non-planar as a result of steric congestion. The absorption and emission properties of the dyes are discussed in terms of their electronic structures.