Microsolvation-Driven\nHours-Long Spectral Dynamics\nin Phenoxazine Dyes

The phenoxazine class of dyes has found widespread applications\nin chemistry and biology for more than a century, particularly for\nlipid membrane studies. Here, we report a general phenomenon on the\nensemble spectral stability of traditional phenoxazine class of dyes\n(nile red, cresyl violet, and nile blue) that exhibit hours-long microstructural\ntransitions reflected through systematic changes of electronic spectra\nover an hour. Mechanistic investigations reveal that such spectral\ndynamics of the dyes can be mitigated by tuning microenvironments,\nwhere microsolvation plays an underlying role. These microsolvation-induced\nmicrostructural changes in a single dye species tend to follow zeroth-order\nkinetics. The half-life values of such processes systematically vary\nwith solvent hydrogen bonding strength and ionic radius of the dyes’\ncounteranions. In so doing, using a model lipid membrane, we demonstrate\nthat the spectral response of a phenoxazine dye must be utilized appropriately\nfor studying membrane properties. These findings of the phenoxazine\nclass of dyes are of high significance for their careful applications\nin chemistry and biology.