Theoretical study of metal‐free organic dyes based on different configurations for efficient dye‐sensitized solar cells

Considering different solar dyes configuration, four novel metal‐free organic dyes based on phenoxazine as electron donor, thiophene and cyanovinylene linkers as the ‐conjugation bridge and cyanoacrylic acid as electron acceptor were designed to optimize open circuit voltage and short circuit current parameters and theoretically inspected. Density functional theory and time‐dependent density functional theory calculations were used to study frontier molecular orbital energy states of the dyes and their optical absorption spectra. The results indicated that D2‐4 dyes can be suitable candidates as sensitizers for application in dye sensitized solar cells and among these three dyes, D3 showed a broader and more bathochromically shifted absorption band compared to the others. The dye also showed the highest molar extinction coefficient. This work suggests optimizing the configuration of metal‐free organic dyes based on simple D‐ ‐A configuration containing alkyl chain as substitution, starburst conformation, and symmetric double D‐ ‐A chains would produce good photovoltaic properties.     

Time‐dependent density functional theory study of electronic spectrum property for carbazolyl–pyridinyl alternating copolymers

Three A‐B‐type fluorescent copolymers comprised of alternating carbazolyl and pyridinyl units, poly[(2,7‐(N‐(2‐ethylhexyl)carbazolyl)‐alt‐(3,5‐pyridinyl))](PEHCP‐35), poly[(2,7‐(N‐(2‐ethylhexyl)carbazolyl)‐alt‐(2,6‐pyridinyl))] (PEHCP‐26) and poly[(2,7‐(N‐(2‐ethyl‐hexyl)carbazolyl)‐alt‐(2,5‐pyridinyl))] (PEHCP‐25), are studied by means of the density functional theory (DFT/B3LYP/6‐31G). Based on the optimized geometries, the optical properties are calculated by employing time‐dependent density functional theory (TD‐DFT). The bandgaps and optical properties are saturated quickly in PEHCP‐35 and PEHCP‐26. It is known from experiment that PEHCP‐25 is actually an oligomer with a polymerization degree of 4. So the tetramers of PEHCP‐35, PEHCP‐26, and PEHCP‐25 are adopted to study the electronic and optical properties, and the calculated results are in close agreement with experiment. The calculated bandgaps of copolymers obtained from two ways, i.e., HOMO–LUMO gaps and the lowest excitation energies, decrease in the following order PEHCP‐35 > PEHCP‐26 > PEHCP‐25, the same trend as the data obtained from the edge of the electric band but different from the electrochemically obtained data from experiment (PEHCP‐25 > PEHCP‐26 > PEHCP‐35). The outcomes showed that, when excited, a charge transfer from carbazolyl unit to pyridinyl unit occurs, and the lumophor is mainly carbazolyl units. The UV absorption and emission wavelengths both exhibit bathochromic shifts: PEHCP‐35 < PEHCP‐26 < PEHCP‐25. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Time‐dependent local‐density approximation in real time: Application to conjugated molecules

The time‐dependent local‐density approximation (TDLDA) is applied to the optical response of conjugated carbon molecules in the energy range of 0–30 eV, with calculations given for carbon chains, polyenes, retinal, benzene, and C₆₀. The major feature of the spectra, the collective π–π* transition, is seen at energies ranging from below 2 to 7 eV and is reproduced by the theory to a few tenths of an electron volt with a good account of systematic trends. However, there is some indication that TDLDA predicts too much fragmentation of the strength function in large molecules. Transition strengths are reproduced with a typical accuracy of 20%. The theory also predicts a broad absorption peak in the range of 15–25 eV, and this feature agrees with experiment in the one case where quantitative data is available (benzene). ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 55–66, 1999     

Domain‐Averaged Exchange‐Correlation Energies as a Physical Underpinning for Chemical Graphs

A novel solution to the problem of assigning a molecular graph to a collection of nuclei (i.e. how to draw a molecular structure) is presented. Molecules are universally understood as a set of nuclei linked by bonds, but establishing which nuclei are bonded and which are not is still an empirical matter. Our approach borrows techniques from quantum chemical topology, which showed for the first time the construction of chemical graphs from wave functions, shifting the focus on energetics. This new focus resolves issues surrounding previous topological analyses, in which domain‐averaged exchange‐correlation energies (V_xc), quantities defined in real space between each possible atom pair, hold the key. Exponential decay of V_xc in non‐metallic systems as the intercenter distance increases guarantees a well‐defined hierarchy for all possible V_xc values in a molecule. Herein, we show that extracting the set of atom pairs that display the largest V_xc values in the hierarchy is equivalent to retrieving the molecular graph itself. Notably, domain‐averaged exchange‐correlation energies are transferable, and they can be used to calculate bond strengths. Fine‐grained details resulted to be related to simple stereoelectronic effects. These ideas are demonstrated in a set of simple pilot molecules.     

Synthesis of poly(2‐oxazoline)s with side chain methyl ester functionalities: Detailed understanding of living copolymerization behavior of methyl ester containing monomers with 2‐alkyl‐2‐oxazolines

Poly(2‐oxazoline)s with methyl ester functionalized side chains are interesting as they can undergo a direct amidation reaction or can be hydrolyzed to the carboxylic acid, making them versatile functional polymers for conjugation. In this work, detailed studies on the homo‐ and copolymerization kinetics of two methyl ester functionalized 2‐oxazoline monomers with 2‐methyl‐2‐oxazoline, 2‐ethyl‐2‐oxazoline, and 2‐n‐propyl‐2‐oxazoline are reported. The homopolymerization of the methyl ester functionalized monomers is found to be faster compared to the alkyl monomers, while copolymerization unexpectedly reveals that the methyl ester containing monomers significantly accelerate the polymerization. A computational study confirms that methyl ester groups increase the electrophilicity of the living chain end, even if they are not directly attached to the terminal residue. Moreover, the electrophilicity of the living chain end is found to be more important than the nucleophilicity of the monomer in determining the rate of propagation. However, the monomer nucleophilicity can be correlated with the different rates of incorporation when two monomers compete for the same chain end, that is, in copolymerizations. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2649–2661     

Gradient‐driven molecule construction: An inverse approach applied to the design of small‐molecule fixating catalysts

Rational design of molecules and materials usually requires extensive screening of molecular structures for the desired property. The inverse approach to deduce a structure for a predefined property would be highly desirable, but is, unfortunately, not well defined. However, feasible strategies for such an inverse design process may be successfully developed for specific purposes. We discuss options for calculating “jacket” potentials that fulfill a predefined target requirement—a concept that we recently introduced (Weymuth and Reiher, MRS Proceedings 2013, 1524, DOI:10.1557/opl.2012.1764). We consider the case of small‐molecule activating transition metal catalysts. As a target requirement we choose the vanishing geometry gradients on all atoms of a subsystem consisting of a metal center binding the small molecule to be activated. The jacket potential can be represented within a full quantum model or by a sequence of approximations of which a field of electrostatic point charges is the simplest. In a second step, the jacket potential needs to be replaced by a chemically viable chelate‐ligand structure for which the geometry gradients on all of its atoms are also required to vanish. To analyze the feasibility of this approach, we dissect a known dinitrogen‐fixating catalyst to study possible design strategies that must eventually produce the known catalyst. © 2014 Wiley Periodicals, Inc.     

尿酸分子互变异构体平面构象的理论研究

使用半经验量子化学中的AM1方法、从头计算Hartree-Fock理论(在3-21G*水平)和密度泛函理论中的B3LYP方法(使用6-31G(d)基组),研究了尿酸分子的所有35种互变异构体。计算结果表明,三羰基互变异构体是所有异构体中能量最低的,其次为单羟基异构体和双羟基异构体,而含有三羟基的互变异构体相对能量最高。随着羟基数的增加, C-N键的平均键长从1.395逐渐缩短到1.351,而CC键的平均键长基本保持不变(1.400~1.406)。     

A quantitative analysis of light‐driven charge transfer processes using voronoi partitioning of time dependent DFT‐derived electron densities

An analytical method is presented that provides quantitative insight into light‐driven electron density rearrangement using the output of standard time‐dependent density functional theory (TD‐DFT) computations on molecular compounds. Using final and initial electron densities for photochemical processes, the subtraction of summed electron density in each atom‐centered Voronoi polyhedron yields the electronic charge difference, Q ^VECD. This subtractive method can also be used with Bader, Mulliken and Hirshfeld charges. A validation study shows Q ^VECD to have the most consistent performance across basis sets and good conservation of charge between electronic states. Besides vertical transitions, relaxation processes can be investigated as well. Significant electron transfer is computed for isomerization on the excited state energy surface of azobenzene. A number of linear anilinepyridinium donor‐bridge‐acceptor chromophores was examined using Q ^VECD to unravel the influence of its pi‐conjugated bridge on charge separation. Finally, the usefulness of the presented method as a tool in optimizing charge transfer is shown for a homologous series of organometallic pigments. The presented work allows facile calculation of a novel, relevant quantity describing charge transfer processes at the atomic level. © 2017 The Authors Journal of Computational Chemistry Published by Wiley Periodicals, Inc.     

Tuning the Protein‐Induced Absorption Shifts of Retinal in Engineered Rhodopsin Mimics

Rational design of light‐capturing properties requires understanding the molecular and electronic structure of chromophores in their native chemical or biological environment. We employ here large‐scale quantum chemical calculations to study the light‐capturing properties of retinal in recently designed human cellular retinol binding protein II (hCRBPII) variants (Wang et al. Science, 2012 , 338, 1340–1343). Our calculations show that these proteins absorb across a large part of the visible spectrum by combined polarization and electrostatic effects. These effects stabilize the ground or excited state energy levels of the retinal by perturbing the Schiff‐base or β‐ionone moieties of the chromophore, which in turn modulates the amount of charge transfer within the molecule. Based on the predicted tuning principles, we design putative in silico mutations that further shift the absorption properties of retinal in hCRBPII towards the ultraviolet and infrared regions of the spectrum.     

Analytical excited state forces for the time-dependent density-functional tight-binding method

An analytical formulation for the geometrical derivatives of excitation energies within the time-dependent density-functional tight-binding (TD-DFTB) method is presented. The derivation is based on the auxiliary functional approach proposed in [Furche and Ahlrichs, J Chem Phys 2002, 117, 7433]. To validate the quality of the potential energy surfaces provided by the method, adiabatic excitation energies, excited state geometries, and harmonic vibrational frequencies were calculated for a test set of molecules in excited states of different symmetry and multiplicity. According to the results, the TD-DFTB scheme surpasses the performance of configuration interaction singles and the random phase approximation but has a lower quality than ab initio time-dependent density-functional theory. As a consequence of the special form of the approximations made in TD-DFTB, the scaling exponent of the method can be reduced to three, similar to the ground state. The low scaling prefactor and the satisfactory accuracy of the method makes TD-DFTB especially suitable for molecular dynamics simulations of dozens of atoms as well as for the computation of luminescence spectra of systems containing hundreds of atoms.     

Theoretical insight into the photodeactivation pathway of the tetradentate Pt (II) complex with different inductive substituents

Density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) both were used to explore the impacts of different inductive substituents on the photophysical properties, radiative/nonradiative processes and photodeactivation mechanism for the Pt (II) complex with novel spiro‐arranged tetradentate ligand. Spectrum simulations show that the electron donor methoxyl (‐OCH₃) group can cause the emission wavelength to red‐shift but have little effect on the absorption spectrum. In the simulation of the radiative decay process for the tetradentate Pt (II) complex, the singlet‐triplet splitting energy is reduced by the introduction of substituents with strong electron‐releasing capability (i.e., from the original trifluoromethyl (‐CF₃) group to ‐OCH₃ group), accompanied with a lower radiative rate constant (k_r). The analyses of non‐radiative decay processes show that the substitution of ‐OCH₃ group on azole rings reduces the energy barriers of thermally activated non‐radiative photodeactivation pathway, which in turn increases the temperature‐dependent non‐radiative rate constants (k_nr(T)). In addition, the substitution of ‐CF₃ by ‐OCH₃ group slightly weakens molecular rigidity and enhances the Huang‐Rhys factor, but decreases the SOC between the triplex excited (T) state and the ground (S₀) state. Thereby, the two complexes may have the similar temperature‐independent non‐radiative rate constant (k_nr’). This work offers theoretical guidance for the design and optimization of the efficient organic light emitting diode (OLED) materials based on the structure of tetradentate Pt (II) complexes.     

Quantum chemical modeling of 1,1‐proton transfer reaction catalyzed by a cofactor‐independent α‐methylacyl‐CoA racemase

α‐Methylacyl‐CoA racemases (AMACR) are essential enzymes for branched‐chain lipids and drugs metabolism. AMACR catalyzes the chiral inversion of (2R) and (2S)‐methylacyl‐CoA esters in both directions. In this study, we investigated the catalytic mechanism of Mycobacterium tuberculosis (MCR) α‐methylacyl‐CoA racemase by using the density functional theory with the hybrid functional B3LYP. Our calculations elucidate and support the mechanism proposed by Prasenjit Bhaumik. His126 and Asp156 serve as the acid/base‐pair residues in the 1,1‐proton transfer catalytic reaction. From the optimized structures, it can be seen that an enolate intermediate is formed and the possibility of forming a ketene or a carbanion intermediate is excluded. By comparing the energy barriers, we could consider that the deprotonation step is the rate‐determined step in the invert direction from (S)‐ to (R)‐enantiomer. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

TDDFT Study of the Electronic Structure,Absorption and Emission Spectra of the Light Emitters of the Amazing Firefly Bioluminescence and Solvation Effects on the Spectra

The ground and excited state properties of luciferin (LH₂) and oxyluciferin (OxyLH₂), the bioluminescent chemicals in the firefly, have been characterized using density functional theory (DFT) and time dependent DFT (TDDFT) methods. The effects of solvation on the electronic absorption and emission spectra of luciferin and oxyluciferin were predicted with a self‐consistent isodensity polarized continuum model of the solvent using TDDFT. The S₀→S₁ vertical excitation energies in the gas phase and in water were obtained. Optimizations of the excited state geometries permitted the first predictions of the fluorescence spectra for these biologically important molecules. Shifts in both of the absorption and emission spectra on proceeding from the gas phase to aqueous solution were also predicted.     

Synthesis of fluoro‐substituted silole‐containing conjugated materials

2,5‐Dihydroxyboryl‐1,1‐dimethyl‐3,4‐bis(3‐fluorophenyl)‐silole ( 2a ) was prepared in 40% overall yield by reaction between 3‐fluorophenyl‐acetylene and dichlorodimethylsilane to yield bis[2(3‐fluorophenyl)ethynyl]dimethylsilane ( 1a ), which subsequently undergoes a reductive cyclization reaction using an excess of lithium naphthalenide. The fluoro substituted silole was applied as a co‐monomer in the Suzuki polycondensation reaction with 2,7‐dibromo‐9,9‐dioctyl‐fluorene. An oligomer ( 3a ) with a degree of polymerization of 6 was prepared and compared with an oligomer without fluoro substitution on the silole ( 3b ), with a degree of polymerization of 4. The new oligomers were spin coated onto glass slides and showed weak green photoluminescence (PL) in the solid state. Cyclic voltammetry, visible absorption spectroscopy, and density functional theory calculations showed that the fluoro substituents were sufficiently electron withdrawing to lower both the highest occupied molecular orbital and the lowest unoccupied molecular orbital in the oligomer. Two further alternating co‐oligomers were prepared from 2,5‐dihydroxyboryl‐1,1‐dimethyl‐3,4‐bis(phenyl)‐silole ( 2b ) and 1,3‐dibromo‐5‐fluoro‐benzene ( 4a ) or 1,3‐dibromobenzene ( 4b ). These oligomers both had degrees of polymerization of 8 and showed green PL in the solid state. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5116–5125, 2009     

Theoretical study on a chemosensor for fluoride anion‐based on a urea derivative

The sensing mechanism of the N‐Phenyl‐N′‐(3‐quinolinyl)urea (PQU) chemosensor for fluoride anion has been investigated by density functional theory/time‐dependent density function theory. The double intermolecular hydrogen bonds are formed between the three anions (X^??F^?, AcO^?, Cl^?) and the urea fragment of PQU. In the S₀ states, the H_b? X^? hydrogen bonds are slightly stronger than the H_a? X^? hydrogen bonds and the fluoride‐induced deprotonation occurs at the N? H_b position rather than at the N? H_a position. Consequently, the absorption peaks, including an intramolecular charge transfer transition and a ππ* transition, are significantly red‐shifted. Thermodynamic calculations confirm that the deprotonation in the ground state is favorable in energy only when excess fluoride anion exists. Along with the S₀ → S₁ transition, the H_a? X^? hydrogen bonds strengthen and the H_b? X^? hydrogen bonds weaken. However, the emission spectra of [PQU‐H_b]^?, instead of [PQU‐H_a]^?, are observed upon addition of fluoride anion. © 2013 Wiley Periodicals, Inc.