The Role of the Ligand-to-Metal Charge-Transfer State in the Dipivaloylmethanate-Lanthanide Intramolecular Energy Transfer Process

Two new bis(dipivaloylmethanate)nitrato-bis(triphenylphosphine oxide)lanthanide(III) complexes [Eu(dpm)(2)(NO3)(tppo)(2)] (1) and [Tb(dpm)(2)(NO3)(tppo)(2)] have been prepared in monomeric form. The structural behavior of these coordination compounds is significantly different from those of [Eu-2(dpm)(6)] and [Tb-2(dpm)(6)] dimeric complexes. The photoluminescent properties of the monomeric and dimeric complexes are dependent on the energy positions of the low-lying ligand-to-metal charge-transfer (LMCT) state, which may be correlated with the chelating and chelate bridging coordination modes of the dpm ligand, respectively. The monomeric complex presents higher luminescence intensity than the dimeric complex due to the higher energy of the LMCT state in this system. Although the excited energy levels of the Eu-III ion in complex 1 undergo efficient depopulation by LMCT, the same is not observed for the D-5(0) emitting level, which presents values of the emitting lifetime that are almost temperature independent. The triboluminescence phenomenon of the [Ln(dpm)(2)(NO3)(tppo)(2)] complex is also discussed.