Crystal Structure offac−Ir(ppy)3and Emission Properties under Ambient Conditions and at High Pressure

Solution and refinement of the crystal structure of fac−Ir(ppy)3 is severely hampered by systematic twinning and pseudo-symmetry. fac−Ir(ppy)3 crystallizes in the centrosymmetric space group P3̄ as has been deduced from single-crystal structure refinement and investigations of the second harmonic generation (SHG) of fac−Ir(ppy)3 powder as compared to two standard materials. The topology of the molecular packing of fac−Ir(ppy)3 is identical to the packing observed for [Ru(bpy)3]0, however, the site symmetry of all Ir(ppy)3 molecules is necessarily lowered from D3 to C3. Packing motifs with intermolecular "π−π interactions" of T-shaped and "shifted π stack" geometry are realized. The systematic twinning leads to the occurrence of crystalline domains with rigorously alternating chirality within the bulk of the domains but with homochiral fac-Ir(ppy)3 contacts at the domain interfaces. These differences in packing motifs are displayed in the emission spectra and in the high-pressure-induced shifts of the emission. The emission maximum of the bulk material at 18 350 cm-1 (545 nm) and of the domain interfaces at 19 700 cm-1 (507 nm) experience for p < 25 kbar and T = 295 K red shifts of Δν̄/Δp = −(12 ± 2) cm-1/kbar, and −(22 ± 4) cm-1/kbar, respectively.