New Photoswitchable Cationic Spiropyrans Unveil Promising NIR Fluorescence for Biomedical Imaging

Researchers from the Southern Federal University in the Russian Federation have developed a series of novel cationic spiropyrans with photoswitchable near-infrared (NIR) fluorescence. The findings, published in Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, offer promising opportunities for applications in various biomedical fields (1).

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The study focused on the synthesis of spiropyran derivatives featuring conjugated cationic 3H-indolium substituents positioned at different locations of the 2H-chromene framework. To achieve NIR absorption and fluorescence, electron-donating methoxy groups were introduced into the uncharged indoline and charged indolium rings, forming an effective conjugation chain between the hetarene moiety and the cationic fragment.

The conjugation of indolium substituents with the spiropyran framework enhances energy transfer, resulting in improved fluorescence intensity and sensitivity. The cationic groups facilitate cellular uptake and localization, enabling precise imaging of specific cell types or subcellular compartments. The photoswitchable nature of these derivatives allows for reversible modulation of fluorescence properties, offering dynamic control over imaging signals. Finally, their NIR fluorescence is advantageous for biomedical imaging, as it allows deeper tissue penetration and improved imaging depth and resolution.

The team extensively investigated the molecular structure and the influence of the cationic fragment's position on the stability of the spiropyran and merocyanine forms of the compounds. They employed various analytical techniques, including nuclear magnetic resonance (NMR), infrared spectroscopy (IR), high-resolution mass spectrometry (HRMS), single-crystal X-ray diffraction (XRD), and quantum chemical calculations. These analyses provided valuable insights into the behavior of the compounds in both solution and solid states.

The researchers discovered that the synthesized spiropyran derivatives exhibited positive or negative photochromism, depending on the position of the cationic fragment. Notably, one particular spiropyran demonstrated bidirectional photochromic properties, enabling reversible switching between its two forms using visible light of different wavelengths. The photoinduced merocyanine forms of the compounds exhibited absorption maxima shifted towards the far-red region and displayed NIR fluorescence, making them promising candidates for bioimaging applications.

The development of these cationic spiropyrans with photoswitchable NIR fluorescence opens up new avenues for optical imaging and sensing techniques in biomedicine. The ability to control the fluorescence properties using visible light offers advantages in terms of reduced phototoxicity and improved imaging depths. Furthermore, the compounds' structural versatility allows for potential modifications to fine-tune their properties and tailor them for specific applications.

The study represents an important step forward in the field of photochromic compounds with NIR fluorescence. The findings contribute to the growing body of knowledge on novel materials for biomedical research and hold promise for advancements in bioimaging technology.

Reference

(1) Kozlenko, A. S.; Ozhogin, I. V.; Pugachev, A. D.; Rostovtseva, I. A.; Makarova, N. I.; Demidova, N. V.; Tkachev, V. V.; Borodkin, G. S.; Metelitsa, A. V.; El-Sewify, I. M.; Lukyanov, B. S. New cationic spiropyrans with photoswitchable NIR fluorescence. Spectrochimica Acta Part A: Mol. Biomol. Spectrosc. 2023, 297, 122712. DOI: 10.1016/j.saa.2023.122712