Abstract
Optical and semiconducting properties of vacuum-deposited thin films of four hydrogen-bonded pigments were studied. Well-known 1,4-diketo-3,6-diphenyl-pyrrolo-[3,4-c]-pyrrole (Ph-DPP) and 1,4-diketo-3,6-(thiophen-2yl)-pyrrolo-[3,4-c]-pyrrole (Th-DPP) were transformed to their 1,4-dithioketo heteroanalogues (Ph-DTPP and ThDTPP) using Lawesson's reagent.
X-ray single crystal structure determination confirmed the presence of CS-HN hydrogen bonds and a similar stack formation in both DTPPs, leading to relatively high decomposition temperatures, about 350 °C. According to density functional theory (DFT) calculations, thionation left HOMO (pi-type) level almost unchanged, considerably destabilized non-bonding orbital (n-orbital) localized on thioketo group, and significantly decreased LUMO (π*-type) energy.
Evolution of HOMO and LUMO energies was confirmed by cyclic voltammetry, establishing LUMO energy at -4.5 eV for both DTPPs, while a signature of high-lying n-orbital was detected through nπ* transition in the far-red/near-infrared area of absorption spectra, interpreted with help of time dependent DFT calculations on monomer and stacked dimer geometry. The lowest optical band-gap among the pigments under study was found to be 1.4 eV for Ph-DTPP thin film.
Field effect mobilities, determined on a bottom-gate top-contact transistor, show DPPs as p-type and DTPPs as n-type semiconductors. The highest electron mobility μ(e) = 0.018 cm(2) V(-1) s(-1) was obtained for Ph-DTPP film in the device with aluminium source and drain electrodes.