Conformation-dependent degradation of thermally activated delayed fluorescence materials bearing cycloamino donors

Heavy Atom Engineering-Mediated Conformational Diversification to Construct Aggregation-Induced NIR-II Emission Luminogens for Cancer Phototheranostics

Chromophores with emission in the second near-infrared (NIR-II) window have captivated much interest by taking advantage of the reduced light scattering and bio-autofluorescence in this region. However, those conventional construction approaches to NIR-II chromophores inevitably suffer from some unpractically limitations, as well as from undiversified molecular skeletons and stereotyped design philosophy. A concise strategy is reported for developing an NIR-II chromophore, PSeD, through heavy chalcogen atom engineering-induced conformational diversification. Compared with the quasi-axis conformer, the emission peak of the quasi-equatorial counterpart is significantly redshifted to the NIR-II region, and the NIR-II emission behaviour is also contributed by the long-range ordered J-aggregates resulting from heavy atom selenium-driven intermolecular interactions. Moreover, the presence of selenium atoms enables PSeD to possess high photothermal conversion efficiency, eventually endowing it with unprecedented performance on NIR-II fluorescence imaging-guided phototherapy.

Dual Charge-Transfer Emission in Chalcone-Based Donor-π-Acceptor System and the Modulation of Down-Conversion of Triplet Exciton with the Polarity of the Medium

Thermally activated delayed fluorescence (TADF) has recently emerged as a promising process with significant potential to advance organic light-emitting diodes (OLEDs) for display applications. The donor-acceptor system is a well-known molecular arrangement exhibiting TADF properties. However, our investigation into the chalcone-based donor-π-acceptor (D-π-A) system (SKG1) reveals that the en-one bridging unit in chalcone plays a crucial role in the reverse intersystem crossing (rISC) process and may be responsible for the existence of two conformational isomers. In stark contrast with the conventional endothermic TADF process, the designed molecule follows a down-converted cold rISC pathway that also from a higher-lying triplet (Tn) state to the lowest singlet (S1) state (in toluene) with remarkably short delayed fluorescence lifetime of 350 ns. Additionally, this rISC process is found to be sensitive to the polarity of the medium. The UV-vis-NIR transient absorption spectroscopy reveals an ultrafast intersystem crossing (ISC) process within <100 ps and the involvement of higher lying triplet state in rISC process. This comprehensive research deepens the understanding of the rISC mechanism and paves the way for developing next-generation OLED materials using D-π-A-based delayed emitters.

Light Induced Proton Coupled Charge Transfer Triggers Counterion Directional Translocation

We demonstrate directed translocation of ClO4 - anions from cationic to neutral binding site along the synthetized BPym-OH dye molecule that exhibits coupled excited-state intramolecular proton-transfer (ESIPT) and charge-transfer (CT) reaction (PCCT). The results of steady-state and time-resolved spectroscopy together with computer simulation and modeling show that in low polar toluene the excited-state redistribution of electronic charge enhanced by ESIPT generates the driving force, which is much stronger than by CT reaction itself and provides more informative gigantic shifts of fluorescence spectra signaling on ultrafast ion motion. The associated with ion translocation red-shifted fluorescence band (at 750 nm, extending to near-IR region) appears at the time ~83 ps as a result of electrochromic modulation of PCCT reaction. It occurs at substantial delay to PCCT that displayed fluorescence band at 640 nm and risetime of <200 fs. Thus, it becomes possible to visualize the manifestations of light-triggered ion translocation and of its driving force by fluorescence techniques and to separate them in time and energy domains.

Organic Photoredox Catalysts Exhibiting Long Excited-State Lifetimes

Organic photoredox catalysts with a long excited-state lifetime have emerged as promising alternatives to transition-metal-complex photocatalysts. This paper explains the effectiveness of using long-lifetime photoredox catalysts for organic transformations, focusing on the structures and photophysics that enable long excited-state lifetimes. The electrochemical potentials of the reported organic, long-lifetime photocatalysts are compiled and compared with those of the representative Ir(III)- and Ru(II)-based catalysts. This paper closes by providing recent demonstrations of the synthetic utility of the organic catalysts.

1 Introduction

2 Molecular Structure and Photophysics

3 Photoredox Catalysis Performance

4 Catalysis Mediated by Long-Lifetime Organic Photocatalysts

4.1 Photoredox Catalytic Generation of a Radical Species and its Addition to Alkenes

4.2 Photoredox Catalytic Generation of a Radical Species and its Addition to Arenes

4.3 Photoredox Catalytic Generation of a Radical Species and its Addition to Imines

4.4 Photoredox Catalytic Generation of a Radical Species and its Addition to Substrates Having C≡X Bonds (X=C, N)

4.5 Photoredox Catalytic Generation of a Radical Species and its Bond Formation with Transition Metals

4.6 Miscellaneous Reactions of Radical Species Generated by Photoredox Catalysis

5 Conclusions

Approaching the Spin-Statistical Limit in Visible-to-Ultraviolet Photon Upconversion

Triplet-triplet annihilation photon upconversion (TTA-UC) is a process in which triplet excitons combine to form emissive singlets and holds great promise in biological applications and for improving the spectral match in solar energy conversion. While high TTA-UC quantum yields have been reported for, for example, red-to-green TTA-UC systems, there are only a few examples of visible-to-ultraviolet (UV) transformations in which the quantum yield reaches 10%. In this study, we investigate the performance of six annihilators when paired with the sensitizer 2,3,5,6-tetra(9H-carbazol-9-yl)benzonitrile (4CzBN), a purely organic compound that exhibits thermally activated delayed fluorescence. We report a record-setting internal TTA-UC quantum yield (ΦUC,g) of 16.8% (out of a 50% maximum) for 1,4-bis((triisopropylsilyl)ethynyl)naphthalene, demonstrating the first example of a visible-to-UV TTA-UC system approaching the classical spin-statistical limit of 20%. Three other annihilators, of which 2,5-diphenylfuran has never been used for TTA-UC previously, also showed impressive performances with ΦUC,g above 12%. In addition, a new method to determine the rate constant of TTA is proposed, in which only time-resolved emission measurements are needed, circumventing the need for more challenging transient absorption measurements. The results reported herein represent an important step toward highly efficient visible-to-UV TTA-UC systems that hold great potential for driving high-energy photochemical reactions.

Acceleration of Reverse Intersystem Crossing using Different Types of Charge Transfer States

There is a need to boost the rate constant of reverse intersystem crossing (k_RISC ) in thermally activated delayed fluorescence (TADF) materials for applications to organic light-emitting diodes. Recently, energy level matching of the locally excited state (LE) and charge transfer state (CT) has been reported to enhance k_RISC . In this study, we conceptually demonstrate that k_RISC can be improved even between CT states without LE states, through the use of different types of CT states. On the basis of this concept, we design a new compound, named DMAC-bPmT, where two phenyl groups of a well-known TADF material DMAC-TRZ are substituted by pyrimidine groups. Theoretical calculations indicated that the energy levels of the different CT states of DMAC-bPmT are very close and enhanced spin orbit coupling may be expected between them. As predicted, DMAC-bPmT experimentally exhibited a k_RISC three times as high as that of DMAC-TRZ.

Modeling Electron-Transfer Degradation of Organic Light-Emitting Devices

The operational lifetime of organic light-emitting devices (OLEDs) is governed primarily by the intrinsic degradation of the materials. Therefore, a chemical model capable of predicting the operational stability is highly important. Here, a degradation model for OLEDs that exhibit thermally activated delayed fluorescence (TADF) is constructed and validated. The degradation model involves Langevin recombination of charge carriers on hosts, followed by the generation of a polaron pair through reductive electron transfer from a dopant to a host exciton as the initiation steps. The polarons undergo spontaneous decomposition, which competes with ultrafast recovery of the intact materials through charge recombination. Electrical and spectroscopic investigations provide information about the kinetics of each step in the operation and degradation of the devices, thereby enabling the building of mass balances for the key species in the emitting layers. Numerical solutions enable predictions of temporal decreases of the dopant concentration in various TADF emitting layers. The simulation results are in good agreement with experimental operational stabilities. This research disentangles the chemical processes in intrinsic electron-transfer degradation, and provides a useful foundation for improving the longevity of OLEDs.