Insight into the relationship between the photostability and molecular structure of rhodamine dyes

It is still challenging to clarify the intrinsic relationship between the photostability and molecular structures of rhodamine dyes. Herein, we investigated the photostability of three rhodamine dyes RhB, Rh101, and Rh101-ME in water and 20%EtOH–H₂O solutions under 525 nm LED light irradiation. The photostability of RhB is much higher than that of Rh 101 and Rh101-ME, indicating that forming rigid ring between the N-linked alkyl and xanthene can significantly decrease the photostability of rhodamine dyes, while the esterification of carboxyl on benzene ring can improve their photostability. The EPR analyses and radical trapping experiments proved that ¹O₂ is the most important reactive oxygen species responsible for dye degradation, and the Rh101 and Rh101-ME have higher ¹O₂ and •OH production rates than RhB. The cyclic voltammetry tests showed that forming rigid ring between the N-linked alkyls and xanthene can decrease the oxidation potential of RhB and consequently lower down its photostability. The theoretical calculations disclosed that the higher HOMO energy levels in ground states and lower singlet–triplet energy gaps of Rh101 and Rh101-ME are the main factor that results in their lower photostability when compared with RhB.