Flexible Designing Strategy to Construct Activatable NIR-II Fluorescent Probes with Emission Maxima beyond 1200 nm.

Activatable second near-infrared (NIR-II) fluorescent probes that can be lighted up by specific targets have attracted great attention because of their high specificity and resolution, which hold great promise in deep-tissue imaging. However, such probes were relatively rarely reported so far, and the emission maximum is still limited (mainly located at 900-1000 nm). To solve the problem, herein, we proposed a flexible strategy to modulate the emission wavelength of NIR-II fluorescent probes, and four proof-of-concept probes (, , , and ) based on D-π-A molecular skeleton were obtained. These probes can be activated by HS and the emission maximum located from 925 to 1205 nm, which was attributed to the cooperation of elongating the π-conjugated system and enhancing the electron-donating ability of the donor region. In these probes, exhibited the combination of long excitation/emission (925/1140 nm) and moderate quantum yield as well as high sensitivity toward HS, enabling us to track and image HS with high contrast. We expected that such a molecular design strategy will become an important approach to developing activatable NIR-II fluorescent probes with long emission.