Influences of molecular structures on the spectral properties and photostability of rhodamine dyes

Although the syntheses and applications of rhodamine dyes have been extensively researched, their photophysical properties and photostability have rarely been studied. In this study, we systematically investigated the photophysical properties and photostability of six rhodamine dyes by the spectrum measurements, photodegradation experiments and theoretical calculations. It was found that the following structural modifications can lead to the redshift of absorption and fluorescence emission spectra: increasing the alkyls of amino on xanthene; forming rigid ring between the N-linked alkyl and xanthene; and esterification of –COOH on benzene ring. The degradation experiments under 525-nm light irradiation indicated that increasing the alkyls of amino on xanthene improves the photostability, fixing the N-linked alkyls on the xanthene ring to form rigid rings significantly reduces the photostability of rhodamine dyes, and the esterification of –COOH on benzene ring can increase their photostability to some extent. The results of DFT calculations showed that the esterification of –COOH decreases the dihedral angle D1 and increases D2, the variation trends of calculated absorption and emission wavelengths are consistent with the measured results, and the rhodamine dyes with higher electrophilic index usually have higher photostability.