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A Density Functional Theory Investigation of Charge Mobility in Titanyl-Phthalocyanines and Their Tailored Peripherally Substituted Complexes

Jeffrey Roshan de Lile and Su Zhou

Our recent theoretical simulations demonstrated titanyl-phthalocyanines can be used as oxygen reduction catalyst in polymer electrolyte membrane fuel cell (PEMFC). However, the origin of high electron transfer ability to spontaneously reduce peroxide in chlorine substituted singlet complex and triplet state Ti(II)Pc complexes remain elusive. Thus we performed density functional theory calculations to study Ti(IV)Pc and their tailored peripheral substituted complexes as a representative compound of titanyl-phthalocyanines for charge mobility, reorganization energies and electronic coupling. In addition, oxo(phthalocyaninato)titanium(IV) (TiOPc) convex and concave compounds are investigated to benchmark the method. Higher charge mobility and reorganization energy associated with electron transfer of TiOPc are predicted with reasonable accuracy. Based on the results, reorganization energies of triplet state Ti(II)Pc and their tailored peripheral substituted complexes are compared with Ti(IV)Pc complexes in order to understand the charge mobility. Chlorine substituted complex demonstrates higher electron hopping rate due to higher electronic coupling in comparison to other halogens. Similarly, weak electron-donating methyl group increases the electron transport rate. Moreover, halogen substituted Ti(II)Pc complexes elucidate lower reorganization energies. The lowest reorganization energy is predicted for chlorine substituted Ti(II)Pc complex, which is 0.09 eV. Therefore, higher electronic coupling and lower reorganization energies can be considered as the origin of higher electron transfer ability of these complexes. Furthermore, increase electron hopping rate due to weak electron-donating substituents provide a method to produce efficient n-channel organic field-effect transistors with higher electron mobility.