Modulation of lanthanide luminescence by carbamoylmethylphosphine oxide ligand: A theoretical study

The antenna capacity of Phenacyldiphenylphosphine oxide (Phenac), combining C=O and P=O donor groups was established by theoretical examination of its chromophore properties, binding ability to encapsulate and stabilize lanthanide complexes and potential to sensitize the Eu-III and Tb-III luminescence in Eu(Phenac)(2)(NO3)(3)(H2O) (1), Eu(Phenac)(2)(NO3)(3) (2), Eu(Phenac)(3)(NO3)(3) (3) and Tb(Phenac)(2)(NO3)(3)(H2O) (4) complexes in vacuum, solution and solid phase. The ligand-centered singlet and triplet excited state energies, the rates and lifetimes for the rival nonradiative (S-1 -> T-1,T-2 intersystem crossing (ISC)) and radiative and nonradiative S-1 -> S-0 and T-1 -> S-0 relaxation were predicted by means of DFT/TDDFT/omega B97XD calculations. Similar relaxation paths were found for all complexes. The absorbed energy by S-n > 1(pi-pi*) states relaxed to T-1min by major ISC mechanism S-1 -> T-2 -> T-1. The long radiative lifetime of S-1 -> S-0 due to n(p(C)(O))pi-pi*,p(P) character facilitated the forward ISC process. The environment, Phenac's number and Ln(III) size affect the T-1 energy similar to 0.1 eV. The calculated (Phenac -> Eu-III) energy transfer rates suggested the most likely T-1 -> D-5(1)/D-5(0) channels for a population of emitting metal-centered states. The small Phi of Eu-III and larger luminescence of Tb-III were discussed and explained. The importance of the P=O and C=O groups for the coordination ability and absorption properties of Phenac has been elucidated.