Charge transfer properties of phthalocyaninato zinc complexes for organic field-effect transistors: Tuning semiconductor nature via peripheral substituents

Density functional theory (DFT) calculations were carried out to investigate the semiconductor performance of a series of phthalocyaninato zinc complexes, namely Zn[Pc(β-OCH₃)₈] (1), ZnPc (2), and Zn[Pc(β-COOCH₃)₈] (3) {[Pc(β-OCH ₃)₈]^2- = dianion of 2,3,9,10,16,17,23,24- octamethoxyphthalocyanine; Pc^2- = dianion of phthalocyanine; [Pc(β-COOCH₃)₈]^2- = dianion of 2,3,9,10,16,17,23,24-octamethoxycarbonylphthalocyanine} for organic field effect transistor (OFET). The effect of peripheral substituents on tuning the nature of phthalocyaninato zinc semiconductor has been clearly revealed. Introduction of eight weak electron-donating methoxy groups onto the peripheral positions of ZnPc (2) leads to a decrease in the hole injection barrier relative to Au electrode and an increase in the electron injection barrier, making compound 1 a better p-type semiconductor material in comparison with 2. In contrast, peripheral methoxycarbonyl substitution depresses the energy level of LUMO and thus induces an increase for the electron affinity (EA) value of ZnPc (2), resulting in the change of semiconductor nature from p-type for ZnPc (2) to n-type for Zn[Pc(β-COOCH₃)₈] (3) due to the improved electron injection ability. The calculated charge transfer mobility for hole is 1.05 cm².V^-1.s^-1 for 1 and 5.33 cm ².V^-1.s^-1 for 2, while that for electron is 0.16 cm².V^-1.s^-1 for 3. The present work should be helpful for designing and preparing novel phthalocyanine semiconductors in particular with good n-type OFET performance.