Facile Access to Benz[c,d]indolium-containing Dyes: C-N and C-C Bond Formation and Their Photophysical Properties

Abstract

This work explores an efficient synthesis method for 1,8-Np derivatives, focusing specifically on the relatively inactive site (site II) of 1,8-Np, as well as the low-activity NH site present in secondary amine compounds. Derivatives Np-A-1∼9 are synthesized by forming C-N bonds with POCl3 acting as the activating reagent, achieving good yields (30∼62%). Next, substrate expansion on 1,8-Np proceed with other secondary amine compounds. Np-B and C are synthesized by forming C-N bonds, however, Np-D-1, 2 are synthesized by forming C-C bonds. This is a mild approach for forming C-N and C-C bonds, which is beneficial for the preparation of near-infrared (NIR) fluorescent dyes. Optical properties in solvents are discussed for Np-A∼D. This system shows large Stokes shifts (4513∼9105cm-1). The AIE phenomenon and solid-state emission properties are observed in Np-A-1∼9, which exhibit strong AIE phenomena, featuring 1.3 to 7.1-fold fluorescence intensity enhancement and solid fluorescence quantum yield ranging from 6.08 to 15.27%. The substitution on the indole unit is beneficial for enhancing the AIE effect and increasing the solid fluorescence quantum yield. Single crystal analysis and DFT calculation further elucidate the structures and the optical properties.