Enhancing near‐infrared photocatalytic activity and upconversion luminescence of BiOCl: Yb3+-Er3+ nanosheets with polypyrrole in-situ modification

Photocatalytic behavior and upconversion (UC) luminescence of rare earth ion (RE) are two opposite photophysical process, but improving the ultimate light conversion efficiency, light absorption enhancement and suppression of excited...

Modulation of Charge Transfer by N-Alkylation to Control Photoluminescence Energy and Quantum Yield

Charge transfer in organic fluorophores is a fundamental photophysical process that can be either beneficial, e.g., facilitating thermally activated delayed fluorescence, or detrimetnal, e.g., mediating emission quenching. <i>N</i>-Alkylation is shown to provide straightforward synthetic control of the charge transfer, emission energy and quantum yield of amine chromophores. We demonstrate this concept using quinine as a model. <i>N</i>-Alkylation causes changes in its emission that mirror those caused by changes in pH (i.e., protonation). Unlike protonation, however, alkylation of quinine’s two N sites is performed in a stepwise manner to give kinetically stable species. This kinetic stability allows us to isolate and characterize an <i>N</i>-alkylated analog of an ‘unnatural’ protonation state that is quaternized selectively at the less basic site, which is inaccessible using acid. These materials expose (i) the through-space charge-transfer excited state of quinine and (ii) the associated loss pathway, while (iii) developing a simple salt that outperforms quinine sulfate as a quantum yield standard. This <i>N</i>-alkylation approach can be applied broadly in the discovery of emissive materials by tuning charge-transfer states.

Modulation of Charge Transfer by N-Alkylation to Control Photoluminescence Energy and Quantum Yield

Charge transfer in organic fluorophores is a fundamental photophysical process that can be either beneficial, e.g., facilitating thermally activated delayed fluorescence, or detrimetnal, e.g., mediating emission quenching. <i>N</i>-Alkylation is shown to provide straightforward synthetic control of the charge transfer, emission energy and quantum yield of amine chromophores. We demonstrate this concept using quinine as a model. <i>N</i>-Alkylation causes changes in its emission that mirror those caused by changes in pH (i.e., protonation). Unlike protonation, however, alkylation of quinine’s two N sites is performed in a stepwise manner to give kinetically stable species. This kinetic stability allows us to isolate and characterize an <i>N</i>-alkylated analog of an ‘unnatural’ protonation state that is quaternized selectively at the less basic site, which is inaccessible using acid. These materials expose (i) the through-space charge-transfer excited state of quinine and (ii) the associated loss pathway, while (iii) developing a simple salt that outperforms quinine sulfate as a quantum yield standard. This <i>N</i>-alkylation approach can be applied broadly in the discovery of emissive materials by tuning charge-transfer states.

Quintet-triplet mixing determines the fate of the multiexciton state produced by singlet fission in a terrylenediimide dimer at room temperature

Singlet fission (SF) is a photophysical process in which one of two adjacent organic molecules absorbs a single photon, resulting in rapid formation of a correlated triplet pair (T1T1) state whose spin dynamics influence the successful generation of uncorrelated triplets (T1). Femtosecond transient visible and near-infrared absorption spectroscopy of a linear terrylene-3,4:11,12-bis(dicarboximide) dimer (TDI2), in which the two TDI molecules are directly linked at one of their imide positions, reveals ultrafast formation of the (T1T1) state. The spin dynamics of the (T1T1) state and the processes leading to uncoupled triplets (T1) were studied at room temperature for TDI2aligned in 4-cyano-4′-pentylbiphenyl (5CB), a nematic liquid crystal. Time-resolved electron paramagnetic resonance spectroscopy shows that the (T1T1) state has mixed5(T1T1) and3(T1T1) character at room temperature. This mixing is magnetic field dependent, resulting in a maximum triplet yield at ∼200 mT. The accessibility of the3(T1T1) state opens a pathway for triplet–triplet annihilation that produces a single uncorrelated T1state. The presence of the5(T1T1) state at room temperature and its relationship with the1(T1T1) and3(T1T1) states emphasize that understanding the relationship among different (T1T1) spin states is critical for ensuring high-yield T1formation from singlet fission.

Photocatalysis with nucleic acids and peptides

In chemical photocatalysis, the photophysical process is coupled to a subsequent chemical reaction. The absorbed light energy contributes to the overall energy balance of the reaction and thereby increases its sustainability. Additionally, oligonucleotides and oligopeptides offer the possibility to control regio- and stereoselectivity as catalysts of organic reactions by providing potential substrate binding sites. We follow this path and want to explore how important substrate binding sites are for photocatalysis. The general concepts of photochemistry and biooligomer catalysis are combined for photochemically active DNAzymes for [2 + 2]-cycloadditions and proline-rich short peptides for nucleophilic additions to styrenes.

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 T m → S n ( 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.