The molecular origin of the photoluminescence of carbon dots (CDs) is not known. This restricts the design of CDs with desired optical properties. We have synthesized CDs starting from carbohydrates by employing a simple synthesis method. We were able to demonstrate that the CDs are composed of aggregated hydroxymethylfurfural (HMF) derivatives. The optical properties of these CDs are quite unique. These CDs exhibit an excitation-independent PL emission maximum in the orange-red region (λ ∼ 590 nm). These CDs also exhibit excitation as well as monitoring wavelength-independent single exponential PL decay. These observations indicate that only one type of chromophore (HMF derivative) is present within the CDs. Several HMF derivatives are aggregated within the CDs; therefore, the aggregated structure cause a large Stokes shift (∼150 nm). By several control experiments, we showed that the same aggregated chromophore unit (HMF derivative), and not the individual fluorophores, is the fluorescing unit. The emission maximum and the single exponential PL lifetime are independent of the polarity of the medium. The existence of a low-lying trap state could be reduced quite significantly. A model has been proposed to explain the interesting steady state and dynamical photoluminescence behaviour of the CDs. As the molecular origin of their photoluminescence is known, CDs with desired optical properties can be designed.

A series of twisted D–π–A type emitters based on the acridine donor unit and CN‐substituted pyridine, pyrimidine, and benzene acceptor units are studied. They not only allow one to systematically probe the influence of different acceptor strengths, but also permit one to intriguingly probe the influence of tunable conformations (twist angles) within the acceptor moieties through controlling the orientation of asymmetric heteroaromatic ring relative to the donor component. Intramolecular charge‐transfer transitions are observed in all these compounds and emission wavelengths are widely tunable from deep blue to yellow not only by the general acceptor strength due to the characters of heteroarene and CN‐substitution pattern but also by the subtle control of in‐acceptor conformation (twist angles). Small triplet‐to‐singlet energy gaps (ΔEST) and significant thermally activated delayed fluorescence (TADF) characteristics are obtained in a series of D–π–A compounds with sufficient acceptor strengths and tunable in‐acceptor conformation, yielding a series of efficient blue‐green to yellow TADF emitters with promisingly high photoluminescence quantum yields of 90%–100%. Highly efficient blue‐green to yellow TADF organic light‐emitting diodes (OLEDs) having external quantum efficiencies of up to 23.1%–31.3% are achieved using these efficient TADF emitters, which are among the most efficient TADF OLEDs ever reported.

Owing to their unique biological, physical, and chemical properties, fluoroalkylated organic substances have attracted significant attention from researchers in a variety of disciplines. Fluoroalkylated compounds are considered particularly important in pharmaceutical chemistry because of their superior lipophilicity, binding selectivity, metabolic stability, and bioavailability to those of their nonfluoroalkylated analogues. We have developed various methods for the synthesis of fluoroalkylated substances that rely on the use of visible-light photoredox catalysis, a powerful preparative tool owing to its environmental benignity and mechanistic versatility in promoting a large number of synthetically important reactions with high levels of selectivity. In this Account, we describe the results of our efforts, which have led to the development of visible-light photocatalytic methods for the introduction of a variety of fluoroalkyl groups (such as, -CF, -CFR, -CHCF, -CF, and -CF) and arylthiofluoroalkyl groups (such as, -CFSPh, -CFSAr, and -CFSAr) to organic substances. In these studies, electron-deficient carbon-centered fluoroalkyl radicals were successfully generated by the appropriate choice of fluoroalkyl source, photocatalyst, additives, and solvent. The redox potentials of the photocatalysts and the fluoroalkyl sources and the choice of sacrificial electron donor or acceptor as the additive affected the photocatalytic pathway, determining whether an oxidative or reductive quenching pathway was operative for the generation of key fluoroalkyl radicals. Notably, we have observed that additives significantly affect the efficiencies and selectivities of these reactions and can even change the outcome of the reaction by playing additional roles during its course. For instance, a tertiary amine as an additive in the reaction medium can act not only as a sacrificial electron donor in photoredox catalysis but also as a hydrogen atom source, an elimination base for dehydrohalogenation of the intermediate, and also a Brønsted base for deprotonation. In the same context, the selection of solvent is also critical since it affects the rate and selectivity of reactions depending upon its polarity and reagent solubilizing ability and plays additional roles in the process, for example, as a hydrogen atom source. By clearly understanding the roles of additives and solvent, we designed several controlled fluoroalkylation reactions where different products were formed selectively from the same starting substrates. In addition, we could exploit one of the most important advantages of radical reactions, that is, the use of unactivated π-systems such as alkenes, alkynes, arenes, and heteroarenes as radical acceptors without prefunctionalization. Furthermore, fluoroalkylation processes under mild room-temperature reaction conditions tolerate various functional groups and are therefore easily applicable to late-stage modifications of highly functionalized advanced intermediates.