3,5-Disubstituted-2-(2’-Pyridylpyrroles) Ir(III) Complexes: Structural and Photophysical Characterization

The synthesis, crystal structure and luminescence properties of three neutral heteroleptic cyclo-metallated Ir(III) complexes of general formula [(ppy)(2)Ir(PyPr-R)], where Hppy = 2-phenylpyridine and wHPyPr-R = 3,5-disubstituted-2-(2'-pyridyl) pyrrole with R = CH3 (1), Ph (2) and CF3, (3), are described. The cyclic voltammetry studies carried out on 1-3 showed oxidation potentials consistent with the nature of the -R substituent groups, indicating that the energy of the HOMO level is significantly affected by the overall electronegativity of the PyPr-R ancillary ligand. Single-crystal X-ray-structural determination was carried out in the case of 2 and 3. While 3 crystallizes in a unique form, two pseudo-polymorphs, a solvated (2a) and non-solvated (2b) specie, have been obtained for compound 2. The supramolecular assembly of the neutral complexes in their crystalline state is mainly directed by pi center dot center dot center dot pi stacking (2a, 2b) and C-H center dot center dot center dot pi interactions (3). The photophysical properties of the HPyPr-R ligands and complexes 1e3 have been investigated in solution and, for 2a, 2b and 3, in their solid crystalline state. All complexes show phosphorescence emission in solution, significantly quenched in presence of dioxygen. In the solid crystalline state, the emission maxima of 2a, 2b and 3 show a significant spectral shift with respect to the solution. For complex 3 it was calculated a bathochromic shift of 1178 cm(-1), while the spectral maxima of complexes 2a and 2b are red-shifted of ca. 3150 cm(-1). Moreover, the average emission lifetime of 2a and 2b crystalline samples is one order of magnitude higher with respect to that of 3 (1.64 and 1.55 mu s, respectively, versus 0.265 mu s). This behaviour accounts for the presence of excimers in the solid sample of 2a and 2b, most likely due to the pi center dot center dot center dot pi stacking interactions involving the pyridine rings of either the cyclometallated and/or the ancyllary ligands. This type of interaction is absent in 3, the presence of the C-H center dot center dot center dot pi interactions being insufficient to produce photophysically observable effects.