AZACYANINE DYES WITH ACRIDINIUM TERMINAL GROUPS - A NEW SERIES OF NEAR-INFRARED ABSORBING DYES

Condensation of the 9,10-dimethylacridium cation with 4-nitroso-N,N-dialkylanilines gives highly bathochromic azacyanines in good yields. In dichloromethane these show intense absorption in the near infrared (c. 720-820 nm). However, in more polar solvents, particularly water, the cations are strongly aggregated and absorb at much shorter wavelengths (c. 550-707 nm). Aggregation appears to be reduced if the arylamine has a large steric requirement. The related xanthene and thioxanthene systems are much more hypsochromic. Annelation of the acridinium dyes to give the benz[a]acridinium analogues results in a further bathochromic shift of the absorption band.