Molecular Design of A-D-A Electron Acceptors Towards Low Energy Loss for Organic Solar Cells.

Low energy loss is a prerequisite for organic solar cells to achieve high photovoltaic efficiency. Electron-vibration coupling (i. e., intramolecular reorganization energy) plays a crucial role in the photoelectrical conversion and energy loss processes. In this Concept article, we summarize our recent theoretical advances on revealing the energy loss mechanisms at the molecular level of A-D-A electron acceptors. We underline the importance of electron-vibration couplings on reducing the energy loss and describe the effective molecular design strategies towards low energy loss through decreasing the electron-vibration couplings.

Low Polarity-Triggered Basic Hydrolysis of Coumarin as an AND Logic Gate for Broad-Spectrum Cancer Diagnosis.

The ability to accurately diagnose cancer is the cornerstone of early cancer treatment. The mitochondria in cancer cells maintain a higher pH and lower polarity relative to that in normal cells. A probe that reports signals only when both conditions are met may provide a reliable method for cancer detection with reduced false positives. Here, we construct an AND logic gate fluorescent probe using mitochondrial microenvironments as inputs. Utilizing the hydrolysis of a coumarin scaffold, the probe generates fluorescence signals ("ON") only when high pH (>7.0) and low polarity conditions exist simultaneously. Additionally, the higher mitochondrial membrane potential in cancer cells provides an additional level of selectivity because probe has increased affinity for cancer cell mitochondria. These capabilities endow the probe with a high contrast fluorescence diagnosis ability of cancer at cellular and tissue levels (as high as 51.9 fold), which is far exceeding the clinic threshold of 2.0 fold.

Suppressing triplet decay in quinoidal singlet fission materials: the role of molecular planarity and rigidity.

Singlet fission, in which one singlet exciton is split into two triplet excitons, provides the potential to exceed the Shockley-Queisser limit for the power conversion efficiencies of organic solar cells. However, the charge transfer from the triplet state is found to be slow in singlet fission materials, so suppression of the triplet decay is crucial for effective utilization of singlet fission. Here, we first investigated triplet decay for the singlet fission molecular materials of ThBF and TThBF, which are characteristic of twisted and flexible quinoidal backbones. It is found that these compounds show rapid nonradiative decay in the Franck-Condon region and through the T1/S0 crossing point. Interestingly, upon locking the backbone twist by methylene, the LThBF and LTThBF compounds exhibit much higher energy barriers from T1 to the T1/S0 crossing point, vanishing spin-orbit couplings, and decreased reorganization energies due to the planar and rigid structures. Consequently, both the triplet decay pathways are effectively suppressed. Our work reveals the importance of molecular planarity and rigidity in suppressing triplet decay and will be very helpful for full utilization of singlet fission in organic photovoltaics.

Aluminum-Enhanced Fluorescence of Cu8 Nanoclusters: An Effective Method for Sensitive Detection of Fluoride in Aqueous and Bioimaging.

Here, we report glutathione-protected water-soluble copper nanoclusters (Cu8 NCs) with excellent properties of high stability, large Stokes shift, and low toxicity. The molecular formula and structure of the Cu8 NCs were confirmed through good agreement between spectroscopic results and theoretical calculations. The origin of optical properties and chemical stability of the Cu8 NCs was determined through natural bond orbital analysis combined with time-dependent density functional theory calculations. It was found that the fluorescence emission and quantum yields of Cu8 NCs could be effectively enhanced by aluminum(III) ions (Al3+) through the aggregation-induced enhanced emission mechanism. On the basis of the strong reaction of Al3+ with F-, the enhanced fluorescence of the obtained Cu8 NC-Al3+ ensemble could be selectively turned off by fluoride ions (F-), achieving highly sensitive detection of F- in aqueous with a detection limit of 0.16 µM. Also, the fluorescence of Cu8 NC-Al3+ ensemble quenched by F- could be enhanced again upon binding additional Al3+. In addition, the proposed Cu8 NC-Al3+ ensemble with enhanced fluorescence was used for bioimaging in vitro and in vivo. These results indicated that Cu8 NCs are promising materials for sequential sensing and bioimaging in the future.

Insights into N-Heterocyclic Carbene-Catalyzed Oxidative α-C(sp3)-H Activation of Aliphatic Aldehydes and Cascade [2 + 2] Cycloaddition with Ketimines.

Predicting the chemoselectivity of [2 + 2] cyclizations is an important challenge in organic chemistry. Herein, we provided a valuable case for this issue. Density functional theory calculations were performed to systematically study the possible mechanisms and origin of selectivities for the N-heterocyclic carbene (NHC)-catalyzed oxidative α-C(sp3)-H activation of aliphatic aldehydes and the cascade [2 + 2] cycloaddition with ketimines. The [2 + 2] cycloaddition of azolium enolate intermediates to the C═N bond, rather than the C═O bond of ketimine, is revealed to be determined by chemo- and stereoselectivity. By comparing the energy gap between the frontier molecular orbitals (FMOs) of the two reacting parts involved in the [2 + 2] cycloaddition transition states, we propose a new strategy to determine the origin of the reaction chemoselectivity. Moreover, the local nucleophilic index can efficiently predict the active site of ketimines. Further analyses illustrate that NHC can increase the nucleophilicity of aldehydes and the acidity of the α-C(sp3)-H bond, and 3,3',5,5'-tetra- tert-butyl diphenoquinone (DQ) acts as an oxidant and promotes α-C(sp3)-H bond deprotonation. This work is useful not only for understanding the NHC-catalyzed oxidative [2 + 2] annulation but also for developing new applications of the FMO theory in organocatalytic cyclizations.

Deep-Red to Near-Infrared Thermally Activated Delayed Fluorescence in Organic Solid Films and Electroluminescent Devices.

The design and synthesis of highly efficient deep red (DR) and near-infrared (NIR) organic emitting materials with characteristic of thermally activated delayed fluorescence (TADF) still remains a great challenge. A strategy was developed to construct TADF organic solid films with strong DR or NIR emission feature. The triphenylamine (TPA) and quinoxaline-6,7-dicarbonitrile (QCN) were employed as electron donor (D) and acceptor (A), respectively, to synthesize a TADF compound, TPA-QCN. The TPA-QCN molecule with orange-red emission in solution was employed as a dopant to prepare DR and NIR luminescent solid thin films. The high doped concentration and neat films exhibited efficient DR and NIR emissions, respectively. The highly efficient DR and NIR organic light-emitting devices (OLEDs) were fabricated by regulating TPA-QCN dopant concentration in the emitting layers.

Aromatic-Imide-Based Thermally Activated Delayed Fluorescence Materials for Highly Efficient Organic Light-Emitting Diodes.

Aromatic-imide-based thermally activated delayed fluorescence (TADF) materials with a twisted donor-acceptor-donor skeleton were efficiently synthesized and exhibited excellent thermal stability and high photoluminescence quantum yields. The small ΔEST value (<0.1 eV) along with the clear temperature-dependent delayed component of their transient photoluminescence (PL) spectra demonstrated their excellent TADF properties. Moreover, the performance of organic light-emitting diodes in which TADF materials AI-Cz and AI-TBCz were used as dopants were outstanding, with external quantum efficiencies up to 23.2 and 21.1 %, respectively.

Thermally populated "bright" states for wide-range and high temperature sensing in air.

A series of air-stable triarylphosphine oxide-based thermosensitive ratiometric sensors based on local excited (LE) and charge transfer (CT) emissions have been designed and synthesized. The fluorescence intensity of tri(pyren-1-yl)phosphine oxide exhibits a prominent temperature-dependent feature over a wide temperature range due to thermally populated emissive LE states. The wide ratiometric temperature sensing range and high stability/reversibility in an ambient atmosphere indicate their great potentials in facile and practical applications.

Induction of Strong Long-Lived Room-Temperature Phosphorescence of N-Phenyl-2-naphthylamine Molecules by Confinement in a Crystalline Dibromobiphenyl Matrix.

The design and preparation of metal-free organic materials that exhibit room-temperature phosphorescence (RTP) is a very attractive topic owing to potential applications in organic optoelectronic devices. Herein, we present a facile approach to efficient and long-lived organic RTP involving the doping of N-phenylnaphthalen-2-amine (PNA) or its derivatives into a crystalline 4,4'-dibromobiphenyl (DBBP) matrix. The resulting materials showed strong and persistent RTP emission with a quantum efficiency of approximately 20 % and a lifetime of a few to more than 100 milliseconds. Bright white dual emission containing blue fluorescence and yellowish-green RTP from the PNA-doped DBBP crystals was also confirmed by Commission Internationale de l'Eclairage (CIE) coordinates of (x=0.29-0.31, y=0.38-0.41).

Unusual Aggregation-Induced Emission of a Coumarin Derivative as a Result of the Restriction of an Intramolecular Twisting Motion.

Aggregation-induced emission (AIE) is commonly observed for propeller-like luminogens with aromatic rotors and stators. Herein, we report that a coumarin derivative containing a seven-membered aliphatic ring (CD-7) but no rotors showed typical AIE characteristics, whereas its analogue with a five-membered aliphatic ring (CD-5) exhibited an opposite aggregation-caused quenching (ACQ) effect. Experimental and theoretical results revealed that a large aliphatic ring in CD-7 weakens structural rigidity and promotes out-of-plane twisting of the molecular backbone to drastically accelerate nonradiative excited-state decay, thus resulting in poor emission in solution. The restriction of twisting motion in aggregates blocks the nonradiative decay channels and enables CD-7 to fluoresce strongly. The results also show that AIE is a general phenomenon and not peculiar to propeller-like molecules. The AIE and ACQ effects can be switched readily by the modulation of molecular rigidity.

Inhibition of survivin expression to induce the apoptosis of hepatocarcinoma cells by adenovirus-mediated siRNA.

In order to provide more efficient transduction of plasmid siRNA into target cells, develop more susceptible transduction into cancer cell types, and more easily explore application in animal experiments, we examined development of an adenoviral vector-mediated siRNA expression system and inhibition of survivin gene expression to induce the growth and apoptosis of hepatocarcinoma cells. A system of adenoviral vector-mediated siRNA expression was constructed for the survivin gene. Survivin gene expression in HepG2 cells infected with recombinant adenovirus was detected by Western blot and RT-PCR, and apoptotic cells were investigated by FAC. Western blot analysis showed that the infection of adenovirus-mediated siRNA against survivin efficiently inhibited the expression of survivin in hepatocarcinoma cells with an inhibitory rate of 66.32%. Semi-quantitative RT-PCR showed that survivin gene mRNA transcription was reduced by nearly 72.34% with a peak at 72 h. The number of apoptotic cells increased. In conclusion, results demonstrated that this adenovirus-mediated siRNA system could serve as a useful tool for both basic research on the analysis of gene function and cancer therapy applications.