Optimization of the Structural Rigidity of a Deep‐NIR Scaffold for Bioimaging

Bright deep‐NIR dyes are actively sought after for their potential in fluorescence‐guided surgery and disease theranostics. The major bottleneck lies with the rigidification of the conjugative backbone to suppress non‐radiative deactivation. EC5 is a notable deep‐NIR absorbing/emitting scaffold, which we first reported in 2017. We recently discovered that its diphenyl ether moiety exhibited structural freedom, which was detrimental to its fluorescence brightness. We proposed to enhance the structural rigidity of EC5 via ring‐contraction, i.e., changing the diphenyl ether moiety of EC5E into a biphenyl of EC5B, its low‐frequency normal modes were largely suppressed as predicted by theoretical calculations, and a 55.0% increase of fluorescence brightness in CH2Cl2 was rendered experimentally. The bright EC5B was feasible for high‐contrast in vivo imaging. EC5B has broad potential in practical biomedical applications.