The design and characterization of thermally activated delayed fluorescence (TADF) materials for optoelectronic applications represents an active area of recent research in organoelectronics. Noble metal-free TADF molecules offer unique optical and electronic properties arising from the efficient transition and interconversion between the lowest singlet (S1) and triplet (T1) excited states. Their ability to harvest triplet excitons for fluorescence through facilitated reverse intersystem crossing (T1→S1) could directly impact their properties and performances, which is attractive for a wide variety of low-cost optoelectronic devices. TADF-based organic light-emitting diodes, oxygen, and temperature sensors show significantly upgraded device performances that are comparable to the ones of traditional rare-metal complexes. Here we present an overview of the quick development in TADF mechanisms, materials, and applications. Fundamental principles on design strategies of TADF materials and the common relationship between the molecular structures and optoelectronic properties for diverse research topics and a survey of recent progress in the development of TADF materials, with a particular emphasis on their different types of metal-organic complexes, D-A molecules, and fullerenes, are highlighted. The success in the breakthrough of the theoretical and technical challenges that arise in developing high-performance TADF materials may pave the way to shape the future of organoelectronics.

Thermally Activated Delayed Fluorescence Materials Towards the Breakthrough of Organoelectronics

Pyrene-based materials for organic electronics.

https://www.ks.uiuc.edu/Publications/Papers/PDF/RITZ2000/RITZ2000.pdf

A Model for Photoreceptor-Based Magnetoreception in Birds

Photoinduced Electron Transfer in Solution: Exciplex and Radical Ion Pair Formation Free Enthalpies and their Solvent Dependence

Organic light-emitting diodes (OLEDs) have their performance limited by the number of emissive singlet states created upon charge recombination (25%). Recently, a novel strategy has been proposed, based on thermally activated up-conversion of triplet to singlet states, yielding delayed fluorescence (TADF), which greatly enhances electroluminescence. The energy barrier for this reverse intersystem crossing mechanism is proportional to the exchange energy (ΔEST ) between the singlet and triplet states; therefore, materials with intramolecular charge transfer (ICT) states, where it is known that the exchange energy is small, are perfect candidates. However, here it is shown that triplet states can be harvested with 100% efficiency via TADF, even in materials with ΔEST of more than 20 kT (where k is the Boltzmann constant and T is the temperature) at room temperature. The key role played by lone pair electrons in achieving this high efficiency in a series of ICT molecules is elucidated. The results show the complex photophysics of efficient TADF materials and give clear guidelines for designing new emitters.

Triplet harvesting with 100% efficiency by way of thermally activated delayed fluorescence in charge transfer OLED emitters.

Magnetic Field Dependence of the Geminate Recombination of Radical Ion Pairs in Polar Solvents

Pairs of radical ions, 2A–+2D+, generated in acetonitrile by nanosecond laser flashes have been investigated in the presence of external magnetic fields. The pairs are produced in the overall singlet state via photoinduced electron transfer between electron-donor (D, dimethylaniline) and electron-acceptor (A, pyrene) molecules and they recombine geminately by back electron transfer to form the molecular triplet state of pyrene.The results obtained with both freely diffusing systems and polymethylene-linked compounds (A—[CH2]n—D, n= 6–12) can be interpreted quantitatively on the basis of the assumption that the spin realignment in the initial radical ion pair is governed by the hyperfine interaction, ΔEhfi, between the unpaired electron spin and the nuclear spins in each radical and by the exchange interaction, J(n), of the radical spins in the pair. The latter increases with decreasing chain length, n.The differences in behaviour with respect to geminate triplet formation of the linked compounds with short (n⩽ 6), medium (6 < n < 12) and long (n 12) polymethylene chains are discussed.

Magnetic-field effects on geminate radical-pair recombination

A quantitative interpretation of the magnetic field effect on hyperfine-coupling-induced triplet fromation from radical ion pairs

The magnetic field dependence of the geminate recombination triplet yield of radical ion pairs generated via photoinduced electron transfer in polar solvents is investigated for the systems pyrene/N,N‐dimethylaniline (Py/DMA), pyrene/3,5‐dimethoxy‐N,N‐dimethylaniline (Py/DMDMA), and the perdeuterated system Py‐d10/DMA‐d11. The magnetic field dependence characterized through its B1/2 value is found to be dependent on the sum of the hyperfine coupling constants in the radical pair in agreement with previous theoretical predictions. A drastic reduction of the B1/2 value is observed with the perdeuterated system. By means of measurements of the radical ion and triplet absorption signals with nanosecond time resolution, the influence of the solvent on the geminate singlet and triplet recombination yields is investigated. Complementary measurements of exciplex lifetimes and quantum yields are carried out in a series of solvents with different polarities in order to determine the rate constants of fluorescence e...

H. Staerk

Papers

Magnetic Field Dependence of the Geminate Recombination of Radical Ion Pairs in Polar Solvents

Magnetic-field effects on geminate radical-pair recombination

A quantitative interpretation of the magnetic field effect on hyperfine-coupling-induced triplet fromation from radical ion pairs

Solvent, isotope, and magnetic field effects in the geminate recombination of radical ion pairs

Magnetic field dependence of intramolecular exciplex formation in polymethyelene-linked A–D systems