Effects of the π-conjugation length of bipyridyl ligand on the photophysical properties of binuclear organotin(IV) complexes: Synthesis and characterization of dimethyltin(IV) complexes with bipyridyl

The dimethyltin(IV) complexes of rigid bipyridyl-based ligands, 4,4′-bipyridine (4,4′-bipy) and 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (4-bpdb), were prepared and characterized by 1H, 13C and 119Sn NMR, IR, and UV spectroscopies, single-crystal X-ray diffraction and elemental analysis. The prepared complexes were used as green and yellow-orange electroluminescence dopant materials to fabricate organic light-emitting diodes (OLEDs) with a general configuration of ITO/PEDOT:PSS(90 nm)/PVK:PBD:tin-complexes (80 nm)/Al(200 nm). The effects of long and rigid bipyridyl-based ligands on the electroluminescence properties of the complexes were investigated. The maximum emission peaks showed that a good correlation exists between the π-conjugation length of bipyridyl ligands and emission wavelength. Replacing 4-bpdb with the long π-conjugation length, 4,4′-bipy, resulted in red shifts in both absorption and fluorescence spectra and moderately enhanced the photoluminescence intensity and the fluorescence quantum yield. Furthermore, complex of dimethyltin(IV) with 4-bpdb ligand was doped into a PVK:PBD blend as the host in two different concentrations. It was found that the high concentration of dopant caused a trap effect in the current voltage and led to a self-quenching of emission and reduced the efficiency of the device.