Development of benzo[cd]indolenyl cyanine dyes for NIR-absorbing films and elucidation of molecular structure–spectroscopic relationship

Abstract

Benzo[cd]indolenyl-substituted heptamethine cyanine dyes exhibit unique optical properties, notably pronounced near-infrared (NIR) absorption beyond 1000 nm. Still, their application in films remains challenging due to severe aggregation behaviors arising from increased π–π interactions of the extended heteroaryl rings. In this study, four benzo[cd]indolenyl-substituted heptamethine cyanine dyes were synthesized by modifying the carbocyclic ring structures at the center of the polymethine chain. Additionally, the perchlorate (ClO₄⁻) counter anion was substituted with the tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (BARF) anion, leading to a substantial improvement in the compatibility and thermal stability of dyes within the film. Interestingly, unlike Cy-5 with a cyclopentene ring, dyes with a cyclohexene ring showed significant subpeak absorption in the visible light range. The influence of the central ring structure on the differences in optical properties was comprehensively elucidated using density functional theory calculations. NIR-absorbing films using Cy-5B with a cyclopentene central ring demonstrated outstanding visible light transmittance, but high-concentration films still exhibited dye aggregation due to increased molecular planarity. Conversely, for cyanine dyes containing a cyclohexene moiety, despite the lower visible light transmittance caused by subpeak absorption, the introduction of steric hindrance groups to the central ring effectively inhibited dye aggregation in Cy-t6B and Cy-p6B films. Introducing a steric hindrance group into the central 5-membered ring would be a promising strategy to inhibit aggregation in benzo[cd]indolenyl cyanine dye films while maintaining excellent visible light transmittance.