Density functional theory study on the semiconducting properties of metal phthalocyanine compounds: Effect of axially coordinated ligand

To investigate the effect of axially coordinated ligand(s) on the semidconducting properties of metal phthalocyanine complexes, density functional theory (DFT) calculations were carried out in terms of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy, ionization energy (IE), electronic affinity (EA), and reorganization energy (X) of F₂SnPc, Cl₂SnPc, I₂SnPc, OSnPc, OVPc, and Cl₂TiPc. For the purpose of comparative studies, calculation on SnPc without axially coordinated ligand has also been conducted. The electronic couplings (V) and the charge transfer mobilities for the electron of metal phthalocyanine compounds with reported single crystal structures for Cl ₂SnPc, I₂SnPc, and Cl₂TiPc are also calculated. Comparison of the calculated results of SnPc with F₂SnPc, Cl ₂SnPc, I₂SnPc, and OSnPc indicates that introduction of axially coordinated ligand(s) obviously lowers the HOMO and LUMO energies of metal phthalocyanine complexes but does not change their energy difference, which results in an increase in their electronic affinity and ionization energy for metal phthalocyanine complexes containing axially coordinated ligand(s). This result is responsible for the decrease in the electron injection barrier and increase in the hole injection barrier of metal phthalocyanine complexes containing axially coordinated ligand(s) in comparison with metal phthalocyanine complexes without axially coordinated ligand, leading to the change in the nature of semiconductivity from p-type for SnPc to n-type for F₂SnPc, Cl₂SnPc, I₂SnPc, and OSnPc. Because of the smaller electronegativity of V^IV than that of Sn^IV, OVPc is revealed to display p-type semiconductivity in terms of electronic affinity (EA_V). In contrast, Cl₂TiPc is revealed to show n-type semiconductivity because of its large electronic affinity (EA_V). The present work, representing the first theoretical effort toward understanding the effect of axially coordinated ligand(s) on the semiconducting properties of metal phthalocyanine complexes, will be helpful for designing and preparing novel phthalocyanine semidconductors with good organic field effect transistor (OFET) performance.