Abstract

Biological applications of rhodium(III) complexes are relatively rare due to their weak luminescence and the requirement of high excitation energy.  In this work, a series of rhodium(III) complexes containing a rhodamine moiety was designed, synthesized, and characterized.  Upon photoexcitation, these complexes exhibited moderate rhodamine fluorescence in solutions under ambient conditions and generated a considerable amount of singlet oxygen (1O2).  Time-resolved transient absorption (TA) spectroscopic results suggested the generation of a dark rhodamine triplet state due to the incorporation of the rhodium(III) center, and such a triplet state should be the lowest-lying state contributing to the enhanced 1O2 generation ability.  Our data showed that instead of intersystem crossing from the singlet (S1) to the triplet state (T1) of rhodamine, a substantial part of the energy was cascaded from the S1 state to a rhodium-based triplet (T1’), and subsequently to the rhodamine T1, depending on the energy level of the T1’ state.  For this reason, the 1O2 generation ability can be readily controlled by a judicious choice of the cyclometalating ligands, while an excellent balance between 1O2 generation and fluorescence can be maintained.  This hybrid rhodamine–rhodium(III) system is anticipated to function as innovative theragnostic agents for both fluorescence imaging and photodynamic therapy (PDT).