Alkylene-bridged N,N,N′N′-Tetrasubstituted Bis(2-amino-5-thiazolyl)methinium Salts-A New Class of Strongly Fluorescent Dyes

The fluorescence-enhancing effect of the oxygen bridge in pyronine and rhodamine di- or triphenylmethane dyes, is connected to a significant hypsochrome shift of the absorption bands. This shift is absent when the bridge is an alkylene unit. While the preparation of such compounds is difficult, that of the heterocyclic analogues 1 is relatively simple and gives rise to intensely fluorescent dyes with absorptions at long wavelength.