Energy transfer mechanism in luminescence Eu(III) and Tb(III) complexes of coumarin-3-carboxylic acid: A theoretical study.

Excited state energy level diagrams of coumarin-3-carboxylic acid (HCCA) chromophore, Eu(CCA)Cl(HO) (1), Eu(CCA)Cl(HO) (2) Eu(CCA)(HO) (3), Tb(CCA)Cl(HO) (4) and Tb(CCA)(NO)(HO) (5) in gas phase and polar solution have been calculated by means of DFT/TDDFT/ωB97XD methods. Based on these results, the ability of CCA to sensitize Eu(III) and Tb(III) luminescence has been examined. The competitive excited state processes in the complexes - fluorescence, intersystem crossing (ISC) and phosphorescence, were analyzed depending on the environment, number of the ligands, Ln(III) ion type (Eu and Tb) and counteranion (Cl and NO). It has been found that the environment altered the S state energy, oscillator strength, fluorescence lifetime as well as the S character - polar solution stabilized the S(ππ*) state, whereas non-polar solution (gas phase, solid state) stabilized the S(nπ*) state. The S(nπ*) state was decisive for the efficient energy transfer as it suppressed the S emission of CCA and favored ISC or direct transfer to the emitting levels of Eu(III). The HCCA triplet (T) state minimum energy (~2.7, ~2.6 eV) and (ππ*) character were retained in Eu/Tb-CCA complexes regardless of the environment. The energy gap between the higher energy T donor state and the acceptor levels D of Eu(III) (~0.5 eV) and D of Tb(III) (~0.1 eV) provided optimal resonance conditions for effective energy transfer for Eu(III), but less probability for Tb(III). The nonradiative energy (CCA → Eu(III)) transfer rates and quantum luminescence yield for 2 and 3 were calculated by a strategy combining DFT geometries, INDO/S excitation energies and calculated Judd-Ofelt parameters. The excitation channel T → D through an exchange mechanism was predicted as the most probable one to populate the main emissive Eu-centered state in complexes 2 and 3. The more efficient luminescence of 3 than that of 2 was discussed and explained.