Strategic molecular design to engineer the electron affinity of self-assembling organic semiconductors

Development of electron-accepting (n-type) semiconductors used in organic photovoltaic cells and field effect transistors has been an area of research with less advancement compared to their electron-donating (p-type) counterparts. Currently, the highest performing n-type semiconductor is a fullerene-based derivative (PCBM) with a favorable ELUMO of -4.08 eV. However, PCBM has limited absorption in the visible region and fixed electron affinity. This work focuses on the development of self-assembling n-type materials with controllable electronic properties by strategically lowering ELUMO to a level comparable to PCBM. Molecular design follows an acceptor-acceptor'-acceptor (A-A'-A) configuration; with A being two 2,3-dioctyloxyphenazine substituents connected to A' with a C-C triple bond. A' was altered to increase the electron deficiency using benzothiadiazole (BTD), naphthalene diimide (NDI), and perylene-tetracarboxylic diimide (PTCDI). Based on this molecular design, four new n-type materials (BTD-P, NDI-P-1, NDI-P-2, PTCDI-P ) were successfully synthesized with low ELUMO values of -3.34 eV, -3.90 eV, -3.90, and -3.97 eV, respectively. Photophysical, thermal, and electrochemical properties were studied using UV-Visible absorption and fluorescence emission spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and cyclic voltammetry. Theoretical evaluations were conducted to understand the experimental electronic properties. Charge-transfer (CT) was also used to test the accepting properties of the title molecules when paired with a pyrene donor. Successful CT results were seen using NDI-P-1, which were confirmed through UV-Vis and fluorescence spectroscopy. The morphology of the CT complex was studied with polarized optical microscopy (POM). Additionally, fluorescence resonance energy transfer (FRET) through organogelation was studied with BTD-P as a donor with NDI-P-2 as an acceptor. It was found that FRET was efficient even at low acceptor concentration of 5mole%. FRET results were characterized with fluorescence spectroscopy and POM.