Novel dimeric leuco-TAM dyes, 1,4-bis{(1E,3Z)-1,3-bis(1,3,3-trimethylindolin-2-ylidene)propan-2-yl}benzene derivatives: Structure and spectroscopic characterization

Novel dimeric leuco-triarylmethane (LTAM) dyes, 1,4-bis{(1E,3Z)-1,3-bis(1,3,3-trimethylindolin-2-ylidene)propan-2-yl}benzene derivatives, as precursors of dimeric TAM+ and TAM++ dyes, were synthesized and characterized by 1D and 2D NMR experiments including DEPT, COSY, HSQC, HMBC and NOESY. Judging from the 1H NMR analysis, the dimeric LTAM molecules were suggested to have a dual-propeller shaped structure. For the prepared dimeric leuco-TAM dyes, the ZE/EZ isomers were formed as the sole products from the reaction of 2–3 M excess Fischer base and terephthalaldehyde in absolute ethanol. The ZE/EZ isomers were equilibrated with other diastereomers (EE/EE and ZZ/ZZ) in organic solvents. UV–Vis spectroscopy of dimeric TAM++ dyes in organic solvents show an absorption band at >700 nm in the near-infrared (NIR) region. Formation of the dimeric TAM++ molecules was further confirmed by comparison of CV diagrams for monomeric TAM+ and dimeric TAM++ molecules.