THE ISOMERIZATION OF MESO-AMINE SUBSTITUTED HEPTAMETHINE DYES

From investigations into the absorption and fluorescence characteristi cs of several new meso-amine substituted heptamethine cyanine dyes and the commercially available laser dyes IR140, IR132 and IR144, it appe ars that these dyes exist in at least two interconvertible ground stat e isomeric conformations in organic solvents. From the position and na ture of absorption and fluorescence characteristics, it is concluded t hat these conformers differ in the contribution to the chromophore of the meso-amine substituent, which either acts as an electron-donor by resonance or else withdraws by induction. Fluorescence emission below 900 nm can only be attributed to the conformer in which the meso-amine groups acts as an electron-donor by resonance. This gives rise to a s ystem that is analogous to the allopolar isomerism exhibited by holome ropolar cyanine dyes.