Unusually Long-Wavelength Emissions of Cyclopropanes: New Insight into C-C Bond Homolysis.

Bond homolysis (BHo) is a fundamental concept in chemical-bonding phenomena. To date, research studies on the BHo concept have provided crucial information for understanding the nature of chemical bonding and reactions. Two potential-energy minima, a σ-bonding isomer and a singlet-diradical isomer, have been known to exist in carbon-carbon BHo. Herein, a third isomer, that is, a puckered singlet diradical exhibiting unstructured long-wavelength fluorescence beyond 460 nm, was first observed in the excited states of 1,4-diarylbicyclo[2.1.0]pentane derivatives. The careful selection of appropriate substituents in the bicyclic structures enabled direct spectral detection. State-of-the-art ab initio quantum chemical calculations quantitatively reproduced the experimental observations. This new finding provides new insight into carbon-carbon bond-breaking and -forming processes.

Effects of Internal Electron-Withdrawing Moieties in D-A-π-A Organic Sensitizers on Photophysical Properties for DSSCs: A Computational Study.

D-A-π-A dyes differ from the traditional D-π-A framework having several merits in dye-sensitized solar cell (DSSC) applications. With regard to D-π-A dyes, D-A-π-A dyes red-shift absorption spectra and show particular photostability. Nevertheless, the effects of internal acceptor on the charge transfer (CT) probability are unclear. We employed density functional theory (DFT), time-dependent DFT (TD-DFT), and TD-DFT molecular dynamics (MD) simulations to investigate the effects of internal acceptor on the photophysical properties of D-A-π-A dyes on DSSCs. Our calculations show the absorption bands of D-A-π-A dyes with strong electron-withdrawing internal acceptors exhibiting significant characteristics of dual CT; the excited electron density is transferred to the internal and terminal acceptors simultaneously. Particularly, the internal acceptor traps a significant amount of electron density upon photoexcitation. The TD-DFT MD simulations at 300 K show that only a small amount of excited electron density is pushing and pulling between the internal acceptor and terminal acceptor moieties; the thermal energy is not high enough to drive the electron density from the internal acceptor to the terminal acceptor. Our study reveals the nature of CT bands of D-A-π-A dyes providing a theoretical basis for further rational engineering.

First-Principle Characterization of the Adsorption Configurations of Cyanoacrylic Dyes on TiO2 Film for Dye-Sensitized Solar Cells.

The loading of sensitizers on a semiconductor is crucial for determining the light-harvesting efficiency of dye-sensitized solar cells (DSSCs). The interfacial properties of dyes adsorbed on a TiO2 film, such as adsorption configurations and adsorption energy, can influence the total amount of dye sensitizers that loads and the stability of a DSSC device. Therefore, it is important to characterize the adsorption properties of sensitizers on TiO2 films atomically and electronically to ensure rational structure-based dye design for high-performance DSSCs. Due to the complex properties of interfacial dyes, previous works on the identification of adsorption configurations of dyes on TiO2 have sometimes been controversial, in particular, the essential IR band assignments. In this study, we employed density functional theory to investigate the adsorption energies, geometries, and vibrational frequencies of various adsorption configurations of 2-cyano-3-(thiophen-2-yl)acrylic acid adsorbed on TiO2. We performed a comparative assignment of the calculated vibrational peaks of tridentate and bidentate configurations to the experimental FT-IR spectra simultaneously. Our work backs up the coexistence of tridentate and bidentate bridging configurations, first proposed by Meng and co-workers. Moreover, our comparative IR mode assignments provide clues for further studies of the interfacial properties of dyes adsorbed on TiO2. Study of the transformation mechanisms between tridentate and bidentate modes suggests that the bidentate bridging configuration is a kinetically trapped adsorption mode and the tridentate configuration is thermodynamically the most stable one. Finally, we investigated the photophysical properties of a D-π-A dye in tridentate and bidentate adsorption configurations.

Geometrical effects of phospholipid olefinic bonds on the structure and dynamics of membranes: A molecular dynamics study.

The trans isomers of fatty acids are found in human adipose tissue. These isomers have been linked with deleterious health effects (e.g., coronary artery disease). In this study, we performed molecular dynamics simulations to investigate the structures and dynamic properties of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) and 1-palmitoyl-2-elaidoyl sn-glycero-3-phosphatidylcholine (PEPC) lipid bilayers. The geometry of the olefinic bond and membrane packing effects significantly influenced the conformations and dynamics of the two C-C single bonds adjacent to the olefinic bond. For the PEPC lipid, the two C-C single bonds adjacent to the olefinic bond adopted mainly nonplanar skew-trans and planar cis-trans motifs; although the cis conformation featured relatively strong steric repulsion, it was stabilized through membrane packing because its planar structure is more suitable for membrane packing. Moreover, membrane packing effects stabilized the planar transition state for conformational conversion to a greater extent than they did with the nonplanar transition state, thereby affecting the dynamics of conformational conversion. The rotational motions of the first neighboring C-C single bonds were much faster than those of typical saturated C-C single bonds; in contrast, the rotational motions of the second neighboring C-C single bonds were significantly slower than those of typical saturated torsion angles. The packing of PEPC lipids is superior to that of POPC lipids, leading to a smaller area per lipid, a higher order parameter and a smaller diffusion coefficient. The distinct properties of POPC and PEPC lipids result in PEPC lipids forming microdomains within a POPC matrix.

Synthesis of the decalin core of codinaeopsin via an intramolecular Diels-Alder reaction.

We describe the synthesis of the decalin core of codinaeopsin (1), a tryptophan-polyketide hybrid natural product with promising antimalarial activity (IC50 4.7 µM, against Plasmodium falciparum), via an intramolecular Diels-Alder (IMDA) reaction. A convergent synthesis was developed to prepare the precursors for the IMDA reaction in 10 steps. The exo cycloadducts were derived from thermal, IMDA reactions of the substrates containing a Weinreb amide or ester conjugated dienophile, and the endo adducts were from Lewis acid promoted reactions of the substrates with a formyl group. Both exo and endo products of the IMDA were exclusively isolated and characterized by NMR spectroscopy. One endo cycloadduct was further confirmed with X-ray crystallography. Theoretical calculations reveal the influence of the substituents of the decalin core on the IMDA process.

TD-DFT study of the excited-state potential energy surfaces of 2-(2'-hydroxyphenyl)benzimidazole and its amino derivatives.

In this study, we used TD-PBE0 calculations to investigate the first singlet excited state (S(1)) behavior of 2-(2'-hydroxyphenyl)benzimidazole (HBI) and its amino derivatives. We employed the potential energy surfaces (PESs) at the S(1) state covering the normal syn, tautomeric (S(1)-T(syn)), and intramolecular charge-transfer (S(1)-T(ICT)) states in ethanol and cyclohexane to investigate the reaction mechanisms, including excited-state intramolecular proton transfer (ESIPT) and intramolecular charge-transfer (ICT) processes. Two new S(1)-T(ICT) states, stable in ethanol and cyclohexane, were found for HBI and its amino derivatives; they are twisted and pyramidalized. The flat PES of the ICT process makes the S(1)-T(ICT) states accessible. The S(1)-T(ICT) state is effective for radiationless relaxation, which is responsible for quenching the fluorescence of the S(1)-T(syn) state. In contrast to the situation encountered conventionally, the S(1)-T(ICT) state does not possess a critically larger dipole moment than its precursor, S(1)-T(syn) state; hence, it is not particularly stable in polar solvents. On the basis of the detailed PESs, we rationalize various experimental observations complementing previous studies and provide insight to understand the excited-state reaction mechanisms of HBI and its amino derivatives.