A simple and effective "capping" approach to readily tune the fluorescence of near-infrared cyanines.

Heptamethine cyanines are favorable for fluorescence imaging applications in biological systems owing to their near-infrared (NIR) absorption and emission. However, it is very difficult to quench the fluorescence of NIR dyes by the classic photoinduced electron transfer mechanism due to their relatively high-lying occupied molecular orbital energy levels. Herein, we present a simple and effective "capping" approach to readily tune the fluorescence of NIR cyanines. The resulting new functional NIR ( = , , or ) dyes not only retain the intact tricarbocyanine scaffold, but also have a built-in switch to regulate the fluorescence by spiro-cyclization. When compared to traditional cyanines, novel dyes have a superior character in that their NIR optical properties can be readily tuned by the intrinsic spiro-cyclization mechanism. We expect that this "capping" strategy can be extended across not only the visual spectrum but also to structurally distinct fluorophores.