Unsymmetric leuco-TAM dyes, (2E, 2′E)-2,2′-{(E)-4-phenylpent-2-ene-1,5-diylidene}bis(1,3,3-trimethylindoline) derivatives. Part II: X-ray crystal structure

The unequivocal solid-state structure and stereochemistry of the unsymmetric leuco-triarylmethane (Un-LTAM) dye 2,2′-{(E)-4-(phenyl)pent-2-ene-1,5-diylidene}bis(1,3,3-trimethylindoline) derivatives were established using X-ray single-crystal analysis. The X-ray analysis showed that the EEE isomers were formed stereoselectively with an unsymmetric three-bladed propeller conformation from the reaction of a Fischer's base and cinnamaldehyde derivatives. The 5,5′-dichloro-4″-nitro-triarylmethane dye displayed a triclinic crystal structure with the space group P-1. The molecules stack alternately to form a dimer, adopting intermolecular distances of 8.53 and 9.40 Å for rings B and C, respectively. Unsymmetric triarylmethane cations, the oxidized form of these dyes, have UV absorptions over 680 nm and selected examples absorb close to the near-IR region.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The solid-state structure of the Un-LTAM dyes were established using X-ray crystal analysis. ► The EEE isomers are formed stereoselectively with a three-bladed propeller conformation. ► The molecules stack alternately to form a dimer with the intermolecular distances of 8.0–9.0 Å. ► Un-TAM+ dyes have their UV absorptions in the range of 700 nm, which is close to the near-ir region.