Energy Transfer Pathway in Luminescent Lanthanide Complexes Based on Dansyl-N-methylaminobenzoic Acid through Intramolecular Charge Transfer State for Near Infrared Emission

Abstract

We have investigated the photophysical properties of dansyl-N-methylaminobenzoic acid (DABAH) as a ligand and its lanthanide (Ln 3+ )-cored complexes (Ln 3+ -(DABA) 3 (terpy)) in order to determine the main energy transfer pathway for sensitized near infrared emission of Ln 3+ ions (Ln 3+ = Nd 3+ and Er3 + ) in Ln 3+ -(DABA) 3 (terpy). The fluorescence spectrum of DABAH shows a large Stokes shift with increasing solvent polarity. This large Stokes shift might be due to the formation of a twisted intramolecular charge transfer (TICT) state, as demonstrated by the large dipole moment in the excited state. It is in good agreement with the result that the phosphorescence even in the Gd 3+ -cored complex based on the DABAH ligand was not observed, maybe due to the highly forbidden character of the S 1 → T 1 transition in the DABAH ligand. A short decay component (ca. 1 ns) was observed in Er3+-(DABA) 3 (terpy) whereas the fluorescence lifetimes of DABAH and its Gd 3+ -(DABA) 3 (terpy) are observed about ∼10 ns. The short component could be originated from the energy transfer process between the ligand and the Ln 3+ ion. Based on the fluorescence of DABAH its Ln 3+ -(DABA) 3 (terpy), the sensitization of Ln 3+ luminescence in the Ln 3+ -(DABA) 3 (terpy) takes place by the energy transfer via the TICT state of DABAH in the excited singlet state rather than via the excited triplet state.