We explored three common CAs—DA, Epi, and NE—along with
Cat as a control species. Excitation spectra of the Tb(ligand) (CA)
complexes exhibit one band near 255 nm and another centered
around 290–295 nm (Fig. 10). The former is assigned as the singlet
p!p* transition La(in the notation of Platt ( 139 )) of the Cat
dianion (256 nm) ( 140 ), while the latter is consistent with the Lb
transition of the Cat dianion, which is blue shifted from 308 to
290 nm when bound to a metal ion ( 141 ). As expected, the Tb^3 þ
complex with DOTA disfavored coordination by the analyte (all
four CAs were investigated), as evidenced by the absence of a band
at 290 nm and negligible emission intensity. Interestingly, the
Tb^3 þcomplex with heptadentate DO3A, which should have suffi-
cient space in its inner sphere for a bidentate chelate such as a
CA, behaved similarly to [Tb(DOTA)]. Notably, the two
hexadentate ligands, DO2A and EDTA, formed luminescent Tb^3 þ
ternary complexes with all three CAs and Cat. The emission spec-
tral profiles for the Tb(DO2A)(CA) complexes are all very similar,
FIG. 10 Excitation spectra (lem¼544 nm) of various Tb(ligand)(CA)
complexes in 50 mM CAPS buffer, pH 13.5. 10mM CA, 1.0 mM Tb
(ligand) complex. Clockwise from upper left: catechol (Cat), dopamine
(DA), norepinephrine (NE), and epinephrine (Epi).
LUMINESCENT LANTHANIDE SENSORS 25