LINEARLY EXTENDED PYRYLIUM SALTS (LEPS) AND LINEARLY EXTENDED THIOPYRYLIUM SALTS (LETS) AS ORGANIC SEMICONDUCTORS
Research focused on the development of a new class of n-type organic semiconductors called linearly extended pyrylium salts (LEPS) and linearly extended thiopyrylium salts (LETS). While a lot of progress has been made on p-type organic semiconductors over the last decade, less attention has been paid to n-type organic semiconductors. Pyrylium and thiopyrylium cations are aromatic structures akin to benzene but with one methine carbon (CH) replaced by either a positively charged oxygen or sulfur atom, respectively. This makes these aromatic π-systems very electron-deficient. With pentacene-like backbones, LEPS and LETS compounds combine linearly conjugated π-systems of acenes with the highly electron-deficient nature of pyrylium and thiopyrylium salts. The LEPS and LETS compounds described here were synthesized efficiently using new reactions. Their properties were calculated through high-level computation and studied experimentally by UV-Vis-NIR spectroscopy, cyclic voltammetry, single crystal X-ray diffraction, ESR and variable temperature NMR (VT-NMR) experiments. The LEPS and LETS compounds described here show moderate to high solubilities in organic solvents like acetonitrile and dichloromethane. While they are resistant to oxidation due to their electron-deficient backbones, they are sensitive to nucleophiles like water. LEPS and LETS compounds with a mesityl or 2’,6’-dimethylphenyl substituent have much higher resistance to moisture compared to other LEPS and LETS compounds due to shielding of the reactive site via the o-methyl groups of the phenyl substituents. LEPS and LETS compounds have broad absorptions in the UV-Vis-NIR region and possess small HOMO-LUMO gaps close to that of pentacene. They also exhibited reversible electrochemical behavior including remarkably easy reductions. Single crystal X-ray diffraction studies show that LEPS and LETS compounds form intermolecular face-to-face π-π stacking thin films. Broad and indiscernible NMR signals in the aromatic region and strong ESR signals were detected for LEPS and LETS compounds bearing a mesityl substituent. Weak to moderate ESR signals were also observed for most of other LEPS and LETS compounds. The broad aromatic NMR signals for LETS 106 bearing a mesityl substituent in CD2Cl2 sharpened upon gradually decreasing temperature in VT-NMR experiments. This indicates a switch from a paramagnetic triplet state to a diamagnetic singlet state. We propose that the LEPS and LETS compounds showing strong ESR signals and broadened NMR spectra have paramagnetic, triplet excited states that lie close in energy to their corresponding diamagnetic ground states. In these cases, populating the paramagnetic excited states via the thermal excitation is possible at or near room temperature.
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