The first example of ab initio calculations of f-f transitions for the case of [Eu(DOTP)]5− complex-experiment versus theoryElectronic supplementary information (ESI) available: Table S1. The assignments of the selected vibrations in IR spectra of I and II, Table S2. Theoretical energies (in cm−1) 2S+1LJ states of IIEu1 and IIEu2 obtained within CASSCF/CASPT2/RASSI calculations. Table S3. Cartesian coordinates (in Å) of the DFT optimized geometry of {K4[Eu(DOTP)]}− cluster representing IEu1 and

Crystal structures and photophysical properties (IR and UV-vis-NIR) of two compounds, [C(NH 2 ) 3 ] 5 [Eu(DOTP)]·12.5H 2 O and K 5 [Eu(DOTP)]·11H 2 O (DOTP = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis (methylenephosphonic acid)), were determined. The DOTP ligand is bonded to Eu 3+ via four O and four N atoms, filling thus eight coordination sites of Eu 3+ . The experimental structures of two [K 4 Eu(DOTP)] − clusters were used as a starting point for theoretical ab initio calculations based on a multireference wavefunction approach. Positions of the energy levels of the 4f 6 configuration of the Eu 3+ ion have been calculated and compared with those derived from the experimental spectra. This enabled us to tentatively assign energy levels of the Eu 3+ ion. The relationship between calculated energies of excited states and Eu-N and Eu-O bond lengths was discussed with respect to the nephelauxetic effect. Structure, IR and UV-vis-NIR spectra of the [Eu(DOTP)] 5− complex in single crystals were studied experimentally. Ab initio calculations of the excited states of the [Eu(DOTP)] 5− complex provide new insights into the interpretation of the observed f-f electronic transitions spectra.