Switching from sky blue to deep green fluorescent Zn(II) complexes for OLEDs applications

Three new Zn(II) 4'-aryl substituted 2,2':6', 2'-terpyridine (terpy) complexes of the type [Zn(Ln)(2)]NO3 complexes (where, L-1 = 4'-(4-methylphenyl)-2,2':6', 2'-terpyridine, L-2 = 4'-(3,4,5-trimethoxyphenyl)-2,2':6', 2'-terpyridine, and L-3 = 4'-(2-thiophenyl)-2,2':6', 2'-terpyridine) were successfully synthesized and photophysical characterizations were done for OLEDs applications. The UV-visible absorption spectra of the complexes exhibited an intense absorption band in the range of 240-380 nm owing to a spin-allowed intraligand (pi-pi*) charge transition. The addition of an electron-donating aryl substituent into the terpy ligand within the complexes induced a significant red-shift of the spectrum. A solid state photoluminescence (PL) analysis revealed a transition from sky blue to green, while the complexes in the DMSO solution which possessed the most emissions appeared sky to deep blue, respectively. A cyclic voltammetry investigation of the complexes revealed a quasi-reversible reduction where the estimated electrochemical data concurred with theoretical calculations. Moreover, the complexes showed an optical band gap of 3.40-3.51 eV and a HOMO level from -6.64 to -6.47 eV. In addition, all the complexes depicted an excellent thermal stability. Hence, the presented complexes are promising candidates for blue and green fluorescent OLEDs application as electron transport as well as electroluminescent material.