Formation of dimeric and cation-‘capped’ complexes by crown-ether styryl dyes: A DFT and X-ray study

DFT calculations for the benzothiazole styryl dyes containing a benzo-18-crown-6-ether fragment and an ammonioalkyl or ammonio-p-xylylenyl substituent at the quaternary nitrogen atom of the benzothiazolium residue were carried out. The effect of the length and rigidity of the N-substituent on the structure and the formation energy of the cation-‘capped’ Z isomers of the dyes arising on exposure to light as a result of intramolecular interaction involving the hydrogen bonds of the N-substituent ammonium group (spacer) with the oxygen atoms of the crown-ether fragment was studied. It was found that the dyes in solution are likely to exist predominantly as ion pairs with one perchlorate anion and the most pronounced stabilization of cation-‘capped’ complexes is attained for dyes having rather long ammoniohexyl or ammonio-p-xylylenyl spacer. The DFT results are in good agreement with X-ray diffraction data for the dimeric complex of the dye with an N-ammoniohexyl substituent.