p-Type Organic Semiconductors With an Annulated Thiazole: Photochemical Synthesis and Theoretically High Hole...

Two dithienonaphthothiazole derivatives, b-DNT and l-DNT, were synthesized via photoinduced 6π-electrocyclization-dehydrogenation of the bithiophene-substituted benzothiazole 1. The direct photoirradiation reaction of 1 in the presence of molecular iodine (I₂) and propylene oxide preferentially forms b-DNT, but under photoinduced electron-transfer conditions using p-chloranil as the photosensitizer, b-DNT is generated exclusively. Investigations using nanosecond absorption spectroscopy in conjunction with laser flash photolysis, along with density-functional theory calculations, led to the identification of potential intermediates in these photochemical processes. X-ray crystallographic analysis showed that both b-DNT and l-DNT crystallize in a slipped π-stacking packing motif. PBEPBE/6-31G* level calculations suggest that crystals of these substances have large charge-transfer integrals (t) between neighboring molecules that are π-stacked along the b-axis. The t value for l-DNT (101 meV) is nearly twofold larger than that for b-DNT (54 meV), which is attributed to the existence of more extensive π-overlap in the former. Based on the Marcus hopping model, theoretical hole mobility along the π-stacking direction in l-DNT is predicted to be as high as 4.0 cm² V^-1 s^{- 1}. These results suggest that, owing to their excellent charge transport properties, crystals of b-DNT and l-DNT have the potential of serving as p-type organic semiconductors.