The first solution-processable n-type phthalocyaninato copper semiconductor: tuning the semiconducting nature via peripheral electron-withdrawing octyloxycarbonyl substituents

Abstract

A series of unsymmetrical phthalocyaninato copper complexes simultaneously incorporating electron-withdrawing and electron-donating substituents at the phthalocyanine periphery Cu{Pc(15C5)[(COOC₈H₁₇)₆]} (2), Cu{Pc[(adj-15C5)₂][(COOC₈H₁₇)₄]} (3), Cu{Pc[(opp-15C5)₂][(COOC₈H₁₇)₄]} (4), Cu{Pc(15C5)₃[(COOC₈H₁₇)₂]} (5) were prepared and isolated. For comparative studies, symmetrical analogues including 2,3,9,10,16,17,24,25-octakis(octyloxycarbonyl)phthalocyaninato copper complex Cu[Pc(COOC₈H₁₇)₈] (1) and 2,3,9,10,16,17,24,25-tetrakis(15-crown-5)phthalocyaninato copper complex Cu[Pc(15C5)₄] (6) were also prepared. Their electrochemistry was studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). With the help of a solution-based self-assembly process, these compounds were fabricated into organic field effect transistors (OFETs) with top contact configuration on hexamethyldisilazane (HMDS)-treated SiO₂/Si substrate. In line with the electrochemical investigation results, a p-type OFET with a carrier mobility (for holes) of 0.06 cm² V⁻¹ s⁻¹ was shown for Cu[Pc(15C5)₄] (6) with electron-donating 15-crown-5 as the sole type of peripheral substituent. In contrast, n-type devices with a carrier mobility (for electrons) of 6.7 × 10⁻⁶–1.6 × 10⁻⁴ cm² V⁻¹ s⁻¹ were achieved for 1–5 with electron-withdrawing octyloxycarbonyl substituents at the peripheral positions of phthalocyanine ring, indicating the significant effect of electron-withdrawing octyloxycarbonyl substituents on tuning the nature of phthalocyanine organic semiconductors. The present results represent the first example of solution-processed n-type phthalocyanine-based OFET devices.