Synthesis and Characterization of a Pyromellitic Diimide-Based Polymer with C- and N-Main Chain Links: Matrix for Solution-Processable n-Channel Field-Effect Transistors

A highly soluble pyromellitic diimide-based polymer was obtained through imidization polymerization. The novel architecture features diimide subunits linked alternately at 3,6 and <i>N</i>,<i>N</i>′ positions. The polymer is highly transparent in the near-ultraviolet–visible regions. Smooth and uniform thin-films were obtained through spin-coating even after blending the polymer with PCBM in 1:9 polymer/PCBM weight ratio. While the polymer itself has modest electron mobility in typical bottom-gate top-contact OFETs, an electron mobility of 3 × 10^–3 cm² V^–1 s^–1 was achieved for the blend, which increased to 10^–2 cm² V^–1 s^–1 on exposure to propylamine. Thus, polyimides are demonstrated as promising binder materials for solution-processable n-channel semiconductor blends, of which very few examples are known

Ultrafast Interfacial Charge-Transfer Dynamics in a Donor-π-Acceptor Chromophore Sensitized TiO₂ Nanocomposite

The dynamics of interfacial charge transfer across (<i>E</i>)-3-(5-((4-(9H-carbazol-9-yl)­phenyl)­ethynyl)­thiophen-2-yl)-2-cyanoacrylic acid (CT-CA) and TiO₂ nanocomposites was studied with femtosecond transient absorption, fluorescence upconversion, and molecular quantum dynamics simulations. The investigated dye, CT-CA is a push–pull chromophore that has an intramolecular charge-transfer (ICT) excited state and binds strongly with the surface of TiO₂ nanoparticles. Ultrafast transient absorption and fluorescence measurements, in both solution and thin film samples, were carried out to probe the dynamics of electron injection and charge recombination. Multiexponential electron injection with time constants of <150 fs, 850 fs, and 8.5 ps were observed from femtosecond fluorescence measurements in solution and on thin films. Femtosecond transient absorption measurements show similar multiexponential electron injection and confirm that the picosecond electron injection component arises from the excited ICT state of the CT-CA/TiO₂ complex. Quantum dynamics calculations also show the presence of a slow component (30%) in the electron injection dynamics although most of the electron injection (70%) takes place in less than 20 fs. The slow component of electron injection, from the local ICT state, is attributed to the energetic position of the excited state, which is close to, or slightly below, the conduction band edge. In addition, the transient bleach of CT-CA on the TiO₂ surface is shifted to longer wavelengths when compared to its absorption spectrum and the transient bleach is further shifted to longer wavelengths with charge recombination. These features are attributed to transient Stark shifts that arise from the local electric fields generated at the dye/TiO₂ interface due to charge-transfer interactions