Synthesis and properties of air-stable n-channel semiconductors based on MEH-PPV derivatives containing benzo[c]cinnoline moieties

Abstract

In this study we synthesized three novel poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) derivatives that differ in terms of their ratios of heterocyclic benzo[c]cinnoline moieties: P50 containing only MEH-PPV/benzo[c]cinnolinevinylene (BZCV) alternating segments and P25 and P10 containing both MEH-PPV/BZCV alternating segments and MEH-PPV block segments. UV-Vis and PL spectra of these polymers revealed values of λ^UV_max and λ^PL_max that ranged from 440 to 489 and from 492 to 551 nm, respectively; these wavelengths blue-shifted upon incorporating additional benzo[c]cinnoline moieties. We also observed energy transfer from the MEH-PPV/BZCV alternating segments to the MEH-PPV block segments. P50, which featured an alternating structure in its main chain, displayed a solvatochromic effect, emitting green light in non-polar solvents and yellow light in polar solvents. The HOMO and LUMO energy levels of these polymers, measured using cyclic voltammetry, ranged from −5.11 to −5.62 and from −3.08 to −3.31 eV, respectively. The incorporation of electron-withdrawing benzo[c]cinnoline moieties enhanced the electron affinity and improved the oxidative stability of the polymers. A bottom-gate, top-contact organic field-effect transistor (OFET) based on P50 exhibited n-channel behavior under ambient conditions, with an electron mobility of 7.8 × 10⁻³ cm² V⁻¹ s⁻¹ and an on/off ratio greater than 10⁴. No significant variation in electron mobility can be observed after this OEFT was stored under ambient conditions up to 30 days. To the best of our knowledge, this is the first time that air-stable n-channel MEH-PPV derivatives have ever been reported. It indicated that p-type MEH-PPV can be transformed into n-type materials upon incorporation of benzo[c]cinnoline moieties.