MECHANISM FOR THE SENSITIZED LUMINESCENCE OF A LANTHANIDE ION MACROCYCLE APPENDED TO A CYCLODEXTRIN

Aromatic hydrocarbons trigger a green luminescence on their associatio n to a beta-cyclodextrin (CD) modified with a Tb3+ diethylenetriaminep entaacetic acid (DTPA) macrocycle strapped across the bottom of the CD cup, [beta-CD boolean OR2(Tb subset of DTPA)]. The mechanism of this enhanced luminescence response is investigated with biphenyl as the ar omatic hydrocarbon. Time-resolved emission spectroscopy reveals that e xcitation energy at the Tb3+ ion appears 12 mu s after light is absorb ed by biphenyl included within the CD cup. Excitation spectra are cons istent with a photophysical mechanism comprising absorption-energy tra nsfer-emission (AETE) from the aromatic hydrocarbon bound to the cavit y of CD to the DTPA-encapsulated Tb3+ ion. A comparison of the lumines cence decay kinetics for biphenyl associated to [beta-CD boolean OR2(T b subset of DTPA)] and [beta-CD boolean OR2(Gd subset of DTPA)] comple xes indicates that the pathway for the AETE process is absorption to t he singlet excited state followed by intersystem crossing to the tripl et, from which energy is transferred to the lanthanide ion.