New cationic spiropyrans with photoswitchable NIR fluorescence

Visible-light-mediated photochromic compounds with NIR absorption and fluorescence are of great interest for use in different biomedical applications. In this work, new representatives of spiropyrans with conjugated cationic 3H-indolium substituents in different positions of 2H-chromene moiety were synthesized. The electron -donating methoxy groups were introduced in the uncharged indoline and charged indolium cycles to form the effective conjugation chain between the hetarene moiety and the cationic fragment for reaching NIR absorption and fluorescence. The molecular structure and the effects of cationic fragment position on the mutual stability of the spirocyclic and merocyanine forms of compounds were carefully studied in the solutions and solid state by NMR, IR, HRMS, single-crystal XRD, and quantum chemical calculations. It was found that the obtained spi-ropyrans demonstrate positive or negative photochromism depending on the cationic fragment's position. One of spiropyrans has shown bidirectional photochromic properties induced exclusively by visible light of different wavelengths in both directions. The photoinduced merocyanine forms of compounds possessed far-red shifted absorption maxima and NIR fluorescence, which makes them prospective fluorescent probes for bioimaging.

Automated summary

In this study, a series of new photochromic compounds with near-infrared (NIR) absorption and fluorescence were synthesized. These compounds have conjugated cationic 3H-indolium substituents in different positions of the 2H-chromene moiety. The electron-donating methoxy groups were introduced to form an effective conjugation chain, enabling NIR absorption and fluorescence. The compounds demonstrated positive or negative photochromism depending on the position of the cationic fragment, and one of the compounds showed bidirectional photochromic properties induced by visible light of different wavelengths. The photoinduced merocyanine forms of the compounds exhibited far-red shifted absorption maxima and NIR fluorescence, making them potential fluorescent probes for bioimaging.