Mechanism for the Sensitized Luminescence of a Lanthanide Ion Macrocycle Appended to a Cyclodextrin

Aromatic hydrocarbons trigger a green luminescence on their association to a β-cyclodextrin (CD) modified with a Tb 3+ diethylenetriaminepentaacetic acid (DTPA) macrocycle strapped across the bottom of the CD cup, [β-CD∪ 2 (Tb⊂DTPA)]. The mechanism of this enhanced luminescence response is investigated with biphenyl as the aromatic hydrocarbon. Time-resolved emission spectroscopy reveals that excitation energy at the Tb 3+ ion appears 12 μs after light is absorbed by biphenyl included within the CD cup. Excitation spectra are consistent with a photophysical mechanism comprising absorption-energy transfer-emission (AETE) from the aromatic hydrocarbon bound to the cavity of CD to the DTPA-encapsulated Tb 3+ ion. A comparison of the luminescence decay kinetics for biphenyl associated to [β-CD∪ 2 (Tb⊂DTPA)] and [β-CD∪ 2 (Gd⊂DTPA)] complexes indicates that the pathway for the AETE process is absorption to the singlet excited state followed by intersystem crossing to the triplet, from which energy is transferred to the lanthanide ion.