Dehua Hu

Reverse intersystem crossing from upper triplet levels to excited singlet: a ‘hot excition’ path for organic light-emitting diodes

Since researches on the fate of highly excited triplet states demonstrated the existence of reverse intersystem crossing (RISC) from upper triplet levels to singlet manifold in naphthalene, quinoline, isoquinoline, etc. in the 1960s, this unique photophysical process was then found and identified in some other aromatic materials. However, the early investigations mainly focus on exploring the mechanism of this photophysical process; no incorporation of specific application was implemented. Until recently, our group innovatively used this ‘sleeping’ photophysical process to enhance the efficiency of fluorescent organic light-emitting diodes by simultaneously harvesting singlet and triplet excitons. Efforts are devoted to developing materials with high photoluminescence efficiency and effective RISC through appropriate molecular design in a series of donor–acceptor material systems. The experimental and theoretical results indicate that these materials exhibit hybridized local and charge-transfer excited state, which achieve a combination of the high radiation from local excited state and the high Tm→Sn (m≥2, n≥1) conversion along charge-transfer excited state. As expected, the devices exhibited favourable external quantum efficiency and low roll-off, and especially an exciton utilization efficiency exceeding the limit of 25%. Considering the significant progress made in organic light-emitting diodes with this photophysical process, we review the relevant mechanism and material systems, as well as our design principle in materials and device application.

Preparation and electronic characteristics of anionic perylene bisimide films

Neutral perylene bisimides (PBI) are well-known n-type organic semiconductors, with number of challenging electronic properties in their neutral and reduced states. We report the characteristic electronic properties of PBI anionic films. We unexpectedly discovered that pristine PBI dianion film showed p-type character, while oxidized dianion film (dominant neutral state with few radical anions) showed normal n-type semiconductor character based on Seebeck effect measurements. Both kinds of films exhibit high electrical conductivity with a potential for thermoelectric applications. The mechanism of polarity reversal is proposed.

Pyridal[2,1,3]thiadiazole as strong electron-withdrawing and less sterically-hindered acceptor for highly efficient donor–acceptor type NIR materials

Pyridal[2,1,3]thiadiazole (PT) is an analogue of benzo[1,2,5]thiadiazole (BT), which is a common acceptor in charge transfer (CT) chromophores. Compared with BT, PT is more electron-deficient and less sterically hindered. Herein we demonstrate the effect of using PT as proto-acceptor in the design of D–A type NIR emitting materials. The compound p-TPA-PT-CN was synthesized, which combines PT with the donor triphenylamine (TPA) and cyano-substituent (CN), and shows NIR emission at ∼700 nm with a high PL efficiency of 0.3 in both solution and solid state. The strong D–A interaction within p-TPA-PT-CN results in a low energy charge-transfer state. The pyridine nitrogen of PT reduces the steric hindrance between PT and the adjacent TPA donor segment, resulting in increased planarity and strong mixing (hybridization) of CT and π–π states (HLCT). Such strong mixing contributes towards a higher luminous efficiency. The OLEDs fabricated with p-TPA-PT-CN as the emitting layer exhibited an EQE of 1.47% and Lmax = 640 cd m−2, which are amongst the highest values reported for NIR-OLED devices.

Aggregation enhanced pure violet emission of a spiral meta-polyfluorene by supramolecular control of excimer formation

In π-conjugated optoelectronic functional materials, the excimer in the aggregate state drew much attention because of undesired low energy emission and fluorescence quenching. We report enhanced pure violet emission in meta-polyfluorene by restrained excimer formation in double-helix-like interchain entangled aggregates. It will be beneficial for optoelectric application of conjugated polymers, especially in the processing of film fabrication for high efficiency devices.

Anomalous Effect of Intramolecular Charge Transfer on the Light Emitting Properties of BODIPY

A new way to approaching highly emissive BODIPY dyes in both solution and solid state is achieved by introducing intramolecular charge transfer (ICT). A hybrid excited state that shows behaviors of both the ICT excited state and local excited state is discovered to be beneficial in avoiding the disturbance from the rotation vibration of the flexible phenyl substituents. Thus, the nonradiative transition process is suppressed, and the fluorescence efficiency goes up. By modification of the excited state, we realize emission enhancement of crystalline BODIPY derivatives with a PLQY from 0.08 to 0.27, and the compound expresses a good potential application prospect in organic semiconductors.