Organic optoelectronics calls for materials combining bright luminescence and efficient charge transport. The former is readily achieved in isolated molecules, while the latter requires strong molecular aggregation, which usually quenches luminescence. This hurdle is generally resolved by doping the host material with highly luminescent molecules collecting the excitation energy from the host. Here, a novel concept of molecular self-doping is introduced in which a higher luminescent dopant emerges as a minute-amount byproduct during the host material synthesis. As a one-stage process, self-doping is more advantageous than widely used external doping. The concept is proved on thiophene-phenylene cooligomers (TPCO) consisting of four (host) and six (dopant) conjugated rings. It is shown that <1% self-doping doubles the photoluminescence in the TPCO single crystals, while not affecting much their charge transport properties. The Monte-Carlo modeling of photoluminescence dynamics reveals that host-dopant energy transfer is controlled by both excitonic transport in the host and host-dopant Förster resonant energy transfer. The self-doping concept is further broadened to a variety of conjugated oligomers synthesized via Suzuki, Kumada, and Stille crosscoupling reactions. It is concluded that self-doping combined with improved excitonic transport and host-dopant energy transfer is a promising route to highly luminescent semiconducting organic single crystals for optoelectronics.

Solution-grown single crystals of furan/phenylene co-oligomer combine efficient charge transport properties and high fluorescence efficiency.

Single crystals of furan/phenylene co-oligomers are among the most promising highly-emissive materials for applications in various optoelectronic devices. In this work, we synthesized and studied furan/phenylene co-oligomers with the same conjugated core 1,4-bisIJ5-phenylfuran-2-yl)benzene and methyl substituents at pand m-positions of the terminal phenyls. The effect of substituents on the crystal packing, charge transport and luminescence of the single crystals was studied. Compared to the unsubstituted compound, the methyl-substituted co-oligomers demonstrated improved thermostability and enhanced photoluminescence, which we assign to J-aggregation resulting from the strong inclination of the molecules against the main crystal facet. The charge mobility in single crystal organic field-effect transistors decreased upon the inclination of the molecules. We conclude that the molecular tilt angle, intermolecular distances and interactions in crystals of heteroaryl-containing linear conjugated oligomers can be controlled by the introduction of end methyl groups in the appropriate positions.

Long-range exciton diffusion facilitates efficient exciton harvesting in a lightly-doped organic semiconductor crystal.