Low molecular weight 4,4’-quinocyanines for in vivo NIR-II fluorescence imaging

Fluorescence-guided surgery (FGS) is limited by the shallow penetration and high autofluorescence of currently available NIR-I fluorophores. To address these constraints, we developed a class of low molecular weight organic fluorophores, 4,4′-quinocyanines (QuCy), that extend emission into the NIR-II (950–1700 nm) region. Rational design through substitution of indolenine head groups with N-substituted quinolinium moieties enhanced π-conjugation, reduced HOMO–LUMO gaps, and induced bathochromic shifts of up to 225 nm, confirmed by density functional theory and spectroscopic analyses. Five derivatives (JAM317–319, JAP331, JAP334) were synthesized via a modular route and exhibited emission maxima of 976–1004 nm. Compared with NIR-I dyes, QuCy dyes demonstrated improved depth penetration and contrast due to reduced tissue autofluorescence. In vivo evaluation of JAM317 revealed efficient tumor cell uptake, low cytotoxicity, and high-resolution vascular imaging. These findings establish QuCy dyes as a promising platform for clinically translatable NIR-II contrast agents.