Emission from Zeolite-Occluded Manganese-Diimine Complexes

Manganese complexes of 2,2'-bipyridine (bpy) and 1,10-phenantroline (phen) have been synthesised in the supercages of cubic NaX and NaY and in the hypercages of the hexagonal NaEMT faujasites. The coordination and redox chemistry were studied with ESCA, CA, FT-IR, FT-Raman, diffuse reflectance and emission techniques. FT-IR/FT-Raman shows cis coordination for all complexes and a high Mn-N stretching frequency in the phen complexes as a result of steric constraints imposed by the ligand. [Mn(bpy)₂ ]²⁺ in the different zeolites shows metal-to-ligand charge transfer (MLCT; at 495 nm); for [Mn(phen)₂ ]²⁺ -NaY the MLCt is broadened owing to complex distortion. On MLCT excitation [Mn(bpy)₂ ]²⁺ complexes show an ipsochromic shift in the emission and an increase in quantum yield with increasing steric restrictions imposed by the zeolite. The ipsochromic shift of the emission band of [Mn(phen)₂ ]²⁺ in NaY results from the combined effect of the ligand field (this suggests emission from a CT state) and of coordinative distortion. The key factor influencing the emission properties is found to be the overall matrix-induced complex distortion. Cation stabilisation of the ligand anion affects emission indirectly. The decay times for [Mn-(bpy)₂ ]²⁺ -NaY are in the millisecond range (7.5-11.5 ms). A proposed model for excitation and emission properties of zeolite-occluded Mn^II complexes involves excitation of a quartet CT state, intersystem crossing and subsequent emission. The enhanced stability of the coordination sphere in the zeolite allows complexes to luminesce from a CT state, which is not detected in solution. The zeolite behaves as a supramolecular cryptating agent, protecting complexes from photodissociation.