Lanthanide complexes with benzothiazole derivatives (Btz-R, R = OCH3 and OH) and terpyridine (tpy) ligands were synthesized, and their photophysical properties were precisely investigated. The free Btz-OCH3 ligand in toluene, excited with UV light, produced the normal emission bands around 410 nm, whereas Btz-OH produced a strong excited-state intramolecular proton transfer (ESIPT) band at 510 nm. The Ln(III) complexes (Ln = Nd, Er, and Yb) exhibited sensitized near-IR luminescence when the Btz-R ligands were excited. The sensitized luminescence quantum yields (Phi(Ln)) of the lanthanide complexes were markedly enhanced by ESIPT: for [Nd(Btz-R)(tpy)] in toluene solution, Phi(Ln) = 0.04% for Btz-OCH3 and 0.39% for Btz-OH. The sensitized luminescence of the Er(III) complexes (Phi(Ln) = 0.002% for Btz-OCH3 and 0.009% for Btz-OH) was less efficient than that of the Nd(III) complexes. This difference is due to the smaller energy gap between the emitting and ground levels of the Er(III) ion. The rate constants for the energy transfer from Btz-R to Ln(III) were about similar to 10(9) s(-1), as evaluated by the Forster resonance energy transfer mechanism.