Molecular Self‐Doping Controls Luminescence of Pure Organic Single Crystals

Abstract

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.

Cooperative Behaviors in Amplified Emission from Single Microcrystals of Thiophene/Phenylene Co‐Oligomers toward Organic Polariton Laser

“…The PLE spectrum measured at λ em = 493 nm corresponding to the 0–2 emission shows a broad band absorption at λ < 470 nm with a weak shoulder around 450 nm. It is known that the TPCO crystals include unintentional impurities, in particular, longer oligomer byproducts act as a doped acceptor which gives rise to red‐shifted emission based on Förster resonance energy transfer (FRET) . One may assign the weak absorption shoulder to such a self‐dopant; however, no red‐shifted band is visible in the fluorescence spectrum.…”

Enhanced Energy Transfer in Doped Bifluorene Single Crystals: Prospects for Organic Lasers

“…The observed inverse temperature dependence of exciton transport rate is also an attribute of rigid molecular structures, which lead to lower torsional disorder in crystal. Opposite behavior showing temperature activated energy transport was previously observed in bifluorene and thiophene–phenylene co‐oligomer single crystals featuring similar J‐type molecular packing . Evidently, linkers between fragments enabling torsional degrees of freedom contributed to broad distribution of states evidenced by the featureless ground state absorption in the crystals.…”

Clarification of the Molecular Doping Mechanism in Organic Single‐Crystalline Semiconductors and their Application in Color‐Tunable Light‐Emitting Devices

“…By taking advantage of this host–guest system, molecular doping technique is employed in organic single‐crystalline semiconductors to combine high mobility and the enhanced light‐emitting properties. It allows to increase the photoluminescence (PL) internal quantum efficiency through suppressing luminescence quenching of the H ‐ aggregate ‐like molecular dipole–dipole interaction . Wang and co‐workers have prepared the tetracene‐ and pentacene‐doped oligo( p ‐phenylenevinylene) crystals which exhibit high luminescent efficiency and color‐tunable emission, and further elucidated their thermodynamics characteristics and charge transport properties .…”

“…They also found that thiophene/phenylene co‐oligomer‐doped oligo( p ‐phenylenevinylene) crystals possess tunable light‐amplification characteristics with an extremely low‐threshold amplified spontaneous emission . Recently, Parashchuk and co‐workers even introduced a novel concept of self‐doping by the synthesis of byproduct in thiophene/phenylene co‐oligomer crystals which doubled the PL efficiency . In addition, research on steady‐state and time‐resolved fluorescence measurements indicated that Förster energy transfer is the dominant process in the doped crystals .…”

“…However, the pace of understanding the doping mechanism of the basic stacking modes between host and guest molecules in organic single‐crystalline semiconductors has lagged far behind the rapid progress in research on their applications . It is generally believed that doping molecules are dispersed throughout the host crystal lattice by strong molecular interactions during crystal growth, and that their potential to replace the original host molecules in the crystal lattice is high.…”