Photophysics of lanthanide double-decker compounds with mixed octaethylporphyrinato and 2,3-naphthalocyaninato ligands

Ground state electronic absorption and luminescence spectra for MIII(OEP)(Nc) (M = La, Eu, Lu; OEP = 2,3,7,8,12,13,17,18-octaethylporphyrinate; Nc = 2,3-naphthalocyaninate) and Ce(OEP)(Nc) are recorded and comparatively studied. Strong π-π interactions are present in these highly π-conjugated systems. The central metal ions seem to play no direct role either in the absorption spectra or in the emissions, but have great influences on the properties of the complexes revealed by both absorption and emission spectra through tuning the ring-to-ring separation and thus ring-to-ring interaction. The absorption bands of the complexes MIII(OEP)(Nc) are dependent on the lanthanide ionic radius, red- or blue-shifted, respectively (depending on the electronic transition nature), along with the lanthanide contraction. Ce(OEP)(Nc) displays particularly different electronic absorption features in comparison with the other three tervalent metal complexes. The fast deactivation process for the sandwich double-decker complexes is mainly caused by the additional π-π interactions and electronic transition states arising from the coupling of the two macrocycles. The lanthanide ions in the complexes also facilitate the deactivation processes through heavy atom enhancement of intersystem crossing rate, or opening new pathways by ligand-field or ring-metal charge transfer states lying below the normally emissive (π,π*) states.