Energy Transport along Conjugated Polymer Chains with Extended Radiative Lifetimes: A Theoretical Study

The ability to improve exciton diffusion lengths is a key issue in optimizing many opto‐electronic devices based on conjugated polymers. On the basis of quantum‐chemical calculations, we investigate a strategy consisting of extending the radiative lifetime of energy carriers through incorporation along the polymer backbone of repeating units with forbidden optical transition. The results obtained for poly(p‐phenylenebutadiyne), PPE, and poly(p‐triphenylenebutadiyne), PTPE, show that the larger number of hops performed by the electronic excitations during their lifetime in PTPE is compensated by the smaller hopping length (associated with the reduced conjugation length), so that similar on‐chain diffusion lengths are predicted in both polymers.     

Theoretical study of the electronic excited states of tetracyanoethylene and its radical anion.

The low-lying electronic states of tetracyanoethylene (TCNE) and its radical anion were studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) basis sets. The results obtained yield a full interpretation of the electronic absorption spectra, explain the spectral changes undergone upon reduction, give support to the occurrence of a bound excited state for the anionic species, and provide valuable information for the rationalization of the experimental data obtained with electron transmission spectroscopy.     

A hybrid ab initio/free electron computational model for conjugated dye molecules: Simple cyanines and oxonols

Justifications developed for the application the free electron model to the π‐orbitals of conjugated molecules suggest that the optical properties of these molecules would be well described by a one‐dimensional free electron model with a potential chosen to reproduce the energy level spacing of the ground state occupied π‐orbitals. Such a hybrid ab initio/free electron modeling approach, where the free electron potential parameters are optimized on a molecule‐by‐molecule basis, is developed, and applied to a series of simple cyanine and oxonol dyes. The ensuing predictions for λ_max, oscillator strengths, and redox properties compare well to available experimental information. Two important strengths of this approach are that no explicit calculations of the excited electronic state are required, and that the ab initio determination of the occupied π‐orbital level spacing considers all the electrons (π and σ) of the entire molecule in a specified geometry, environment, etc. This second characteristic gives the ability to efficiently model modifications of the optical properties of conjugated molecules resulting from chemical and/or physical modifications occuring within and remote to the conjugated region of the molecule. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 943–953, 2000     

Ultrafast Decay of the Excited Singlet States of Thioxanthone by Internal Conversion and Intersystem Crossing

The experimental ultrafast photophysics of thioxanthone in several aprotic organic solvents at room temperature is presented, measured using femtosecond transient absorption together with high‐level ab initio CASPT2 calculations of the singlet‐ and triplet‐state manifolds in the gas phase, including computed state minima and conical intersections, transition energies, oscillator strengths, and spin–orbit coupling terms. The initially populated singlet ππ* state is shown to decay through internal conversion and intersystem crossing processes via intermediate nπ* singlet and triplet states, respectively. Two easily accessible conical intersections explain the favorable internal conversion rates and low fluorescence quantum yields in nonpolar media. The presence of a singlet–triplet crossing near the singlet ππ* minimum and the large spin–orbit coupling terms also rationalize the high intersystem crossing rates. A phenomenological kinetic scheme is proposed that accounts for the decrease in internal conversion and intersystem crossing (i.e. the very large experimental crescendo of the fluorescence quantum yield) with the increase of solvent polarity.     

氧气和CS自由基反应势能面的密度泛函理论研究

应用量子化学从头计算和密度泛函理论(DFT)对O_2和CS自由基的反应进行了研 究。在B3LYP/6-311G~(**)水平上计算出了各物种的优化构型、振动频率和零点振 动能(ZPVE)。各物种的总能量由CCSD（T）/6-311G~(**)//B3LYP/6-311G~(**)给出 ,并对总能量进行了零点能校正。计算结果表明：CS自由基中的C端沿着O_2的双键 中线方向进攻,进行加成反应,反应的第一步放出大量的热量(450 kJ/mol),推动 反应继续进行,从稳定的中间体4(Cs)出发,反应主要通过O的相邻位置的迁移生成 P_1(CO+SO)和P_3(COS+O);通过S的相邻位置的迁移生成了重要的反应复合物 (complex 1),进一步离解为产物P_2(CO_2+S)。计算结果可以很好地解释反应机理 。     

Theoretical estimates of the IR spectrum of water intercalated into kaolinite

Calculations at AM1, PM3, and HF/6‐31G levels of part of the IR spectrum of the water–kaolinite intercalated system based on a 96‐atom cluster of kaolinite with one water molecule are reported. Only the water molecule conformation is optimized. Frequencies and intensities for just the water vibrations and stretchings of four cluster hydroxyls were calculated through partial Hessian matrices and polar tensors obtained by numerical differentiation of energy gradients and dipole moment. The water molecule was found to attach to the cluster mainly through a double hydrogen bond to the siloxane inner surface, partially entering the siloxane ring hexagonal hole. Though the theoretical results predict that the water OH stretching frequencies decrease from the gas‐phase state to the intercalated state, they are still higher than expected with respect to the observed spectrum. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Theoretical Studies on the Intermolecular Interactions of Potentially Primordial Base‐Pair Analogues

Recent experimental studies on the Watson–Crick type base pairing of triazine and aminopyrimidine derivatives suggest that acid/base properties of the constituent bases might be related to the duplex stabilities measured in solution. Herein we use high‐level quantum chemical calculations and molecular dynamics simulations to evaluate the base pairing and stacking interactions of seven selected base pairs, which are common in that they are stabilized by two N? H???O hydrogen bonds separated by one N? H???N hydrogen bond. We show that neither the base pairing nor the base stacking interaction energies correlate with the reported pK_a data of the bases and the melting points of the duplexes. This suggests that the experimentally observed correlation between the melting point data of the duplexes and the pK_a values of the constituent bases is not rooted in the intrinsic base pairing and stacking properties. The physical chemistry origin of the observed experimental correlation thus remains unexplained and requires further investigations. In addition, since our calculations are carried out with extrapolation to the complete basis set of atomic orbitals and with inclusion of higher electron correlation effects, they provide reference data for stacking and base pairing energies of non‐natural bases.     

Femtochemistry of trans‐Azomethane: A Combined Experimental and Theoretical Study

The dissociation dynamics of trans‐azomethane upon excitation to the S₁(n,π*) state with a total energy of 93 kcal mol^?1 is investigated using femtosecond‐resolved mass spectrometry in a molecular beam. The transient signal shows an opposite pump–probe excitation feature for the UV (307 nm) and the visible (615 nm) pulses at the perpendicular polarization in comparison with the signal obtained at the parallel polarization: The one‐photon symmetry‐forbidden process excited by the UV pulse is dominant at the perpendicular polarization, whereas the two‐photon symmetry‐allowed process initiated by the visible pulse prevails at the parallel polarization. At the perpendicular polarization, we found that the two C? N bonds of the molecule break in a stepwise manner, that is, the first C? N bond breaks in ≈70 fs followed by the second one in ≈100 fs, with the intermediate characterized. At the parallel polarization, the first C? N bond cleavage was found to occur in 100 fs with the intensity of the symmetry‐allowed transition being one order of magnitude greater than the intensity of the symmetry‐forbidden transition at the perpendicular polarization. Theoretical calculations using time‐dependent density functional theory (TDDFT) and the complete active space self‐consistent field (CASSCF) method have been carried out to characterize the potential energy surface for the ground state, the low‐lying excited states, and the cationic ground state at various levels of theory. Combining the experimental and theoretical results, we identified the elementary steps in the mechanism: The initial driving force of the ultrafast bond‐breaking process of trans‐azomethane (at the perpendicular polarization) is due to the CNNC torsional motion initiated by the vibronic coupling through an intensity‐borrowing mechanism for the symmetry‐forbidden n–π* transition. Following this torsional motion and the associated molecular symmetry breaking, an S₀/S₁ conical intersection (CI) can be reached at a torsional angle of 93.1° (predicted at the CASSCF(8,7)/cc‐pVDZ level of theory). Funneling through the S₀/S₁ CI could activate the asymmetric C? N stretching motion, which is the key motion for the consecutive C? N bond breakages on the femtosecond time scale.     

Electronic Structures of LixV2O5 (x=0.5 and 1): A Theoretical Study

The electronic properties of α‐Li_xV₂O₅ (x=0.5 and 1) are investigated using first principle calculations based on density functional theory with local density approximation. Different intercalation sites for Li in the V₂O₅ lattices are considered, showing different influences on the electronic structures of Li_xV₂O₅. The lowest total energy is found when Li is only intercalated along the c axis between two bridging oxygen ions of sequential V₂O₅ layers. The intercalation of Li into V₂O₅ does not change the electron transition property of V₂O₅, which is an indirect band gap semiconductor, but leads to a reduction of vanadium ions and an increase of the Fermi level of Li_xV₂O₅ arising from the electron transfer from the Li 2 s orbital to the initially empty conduction band of the V₂O₅ host.     

Influence of Hydrogen Bonds and Nonspecific Interactions on the Spectral and Photophysical Properties of the Excited Singlet States of 4‐Aminophthalimide in Amine Solution

The hydrogen‐bond and nonspecific interaction energies for 4‐aminophthalimide (4‐AP), often used as a probe, in the ground electronic and excited singlet states are determined using ab initio computational methods. It is shown that the 4‐AP molecule can form three relatively strong hydrogen bonds with trimethylamine (TMA) and triethylamine (TEA), which leads to the formation of S₀‐complexes between the solute and solvent molecules. Only two of the hydrogen bonds with the amine group of 4‐AP change significantly their energies upon excitation and deactivation. The theoretical results are necessary to explain the spectral and unusual photophysical properties of 4‐AP in amine solutions.     

Single‐Electron Self‐Exchange between Cage Hydrocarbons and Their Radical Cations in the Gas Phase

We show that the radical cations of adamantane (C₁₀H₁₆^.+, 1 H^.+) and perdeuteroadamantane (C₁₀D₁₆^.+, 1 D^.+) are stable species in the gas phase. The radical cation of adamantylideneadamantane (C₂₀H₂₈^.+, 2 H^.+) is also stable (as in solution). By using the natural ¹³C abundances of the ions, we determine the rate constants for the reversible isergonic single‐electron transfer (SET) processes involving the dyads 1 H^.+/ 1 H, 1 D^.+/ 1 D and 2 H^.+/ 2 H. Rate constants for the reaction 1 H^.++ 1 D? 1 H+ 1 D^.+ are also determined and Marcus’ cross‐term equation is shown to hold in this case. The rate constants for the isergonic processes are extremely high, practically collision‐controlled. Ab initio computations of the electronic coupling (H_DA) and the reorganization energy (λ) allow rationalization of the mechanism of the process and give insights into the possible role of intermediate complexes in the reaction mechanism.     

Global versus Local Aromaticity in Porphyrinoid Macrocycles: Experimental and Theoretical Study of “Imidacene”, an Imidazole‐Containing Analogue of Porphycene

The synthesis, spectroscopic properties, and computational analysis of an imidazole‐based analogue of porphycene are described. The macrocycle, given the trivial name “imidacene”, was prepared by reductive coupling of a diformyl‐substituted 2,2′‐biimidazole using low‐valent titanium, followed by treatment with 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone. Imidacene displays a porphyrin‐like electronic structure, as judged by its ¹H NMR, ¹³C NMR, and UV/Vis spectral characteristics. Despite a cyclic 18 π‐electron pathway, dichloromethane or ethyl acetate solutions of imidacene were found to undergo rapid decomposition, even in the absence of light and air. A series of high‐level theoretical calculations, performed to probe the origin of this instability, revealed that the presence of a delocalized 18 π‐electron pathway in both imidacene and porphycene provides less aromatic stabilization energy than locally aromatic 6 π‐electron heterocycles in their reduced counterparts. That reduction of imidacene occurs on perimeter nitrogen atoms allows it to maintain its planarity and two stabilizing intramolecular hydrogen bonds, thereby distinguishing it from porphycene and, more generally, from porphyrin. Despite the presence of both 18 π‐ and 22 π‐electron pathways in the planar, reduced form of imidacene, aromaticity is evident only in the 6 π‐electron five‐membered rings. Our computational analysis predicts that routine ¹H NMR spectroscopy can be used to distinguish between local and global aromaticity in planar porphyrinoid macrocycles; the difference in the chemical shift for the internal NH protons is expected to be on the order of 19 ppm for these two electronically disparate sets of ostensibly similar compounds.     

A Theoretical Study on the Hydrogen—bonded Dimers of HNCO Molecules

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Theoretical Study on the Complexes of Benzene with Isoelectronic Nitrogen‐Containing Heterocycles

The π–π interactions between benzene and the aromatic nitrogen heterocycles pyridine, pyrimidine, 1,3,5‐triazine, 1,2,3‐triazine, 1,2,4,5‐tetrazine, and 1,2,3,4,5‐pentazine are systematically investigated. The T‐shaped structures of all complexes studied exhibit a contraction of the C? H bond accompanied by a rather large blue shift (40–52 cm^?1) of its stretching frequency, and they are almost isoenergetic with the corresponding displaced‐parallel structures at reliable levels of theory. With increasing number of nitrogen atoms in the heterocycle, the geometries, frequencies, energies, percentage of s character at C, and the electron density in the C? H σ antibonding orbital of the complexes all increase or decrease systematically. Decomposition analysis of the total binding energy showed that for all the complexes, the dispersion energy is the dominant attractive contribution, and a rather large attraction originating from electrostatic contribution is compensated by its exchange counterpart.     

Relationship between Electron Affinity and Half‐Wave Reduction Potential: A Theoretical Study on Cyclic Electron‐Acceptor Compounds

A high‐level ab initio protocol to compute accurate electron affinities and half‐wave reduction potentials is presented and applied for a series of electron‐acceptor compounds with potential interest in organic electronics and redox flow batteries. The comprehensive comparison between the theoretical and experimental electron affinities not only proves the reliability of the theoretical G3(MP2) approach employed but also calls into question certain experimental measurements, which need to be revised. By using the thermodynamic cycle for the one‐electron attachment reaction A+e^?→A^?, theoretical estimates for the first half‐wave reduction potential have been computed along the series of electron‐acceptor systems investigated, with maximum deviations from experiment of only 0.2 V. The precise inspection of the terms contributing to the half‐wave reduction potential shows that the difference in the free energy of solvation between the neutral and the anionic species (ΔΔG_solv) plays a crucial role in accurately estimating the electron‐acceptor properties in solution, and thus it cannot be considered constant even in a family of related compounds. This term, which can be used to explain the occasional lack of correlation between electron affinities and reduction potentials, is rationalized by the (de)localization of the additional electron involved in the reduction process along the π‐conjugated chemical structure.     

Discrimination of Glycine Enantiomers by the Biologically Important Phosphocompounds:A Theoretical Prediction

With the aid of high-level B3LYP and MP2 calculations, three new neutral structures of glycine （iin, ivn and vn, see Fig. 2） were obtained and validated by frequency calculations. The structural and energetic analyses showed that iin, ivn and vn are enantiomers to the previous IIn, IVn and Vn （J. Am. Chem. Soc. 1992, 114, 9568.）, respectively. Owing to the presence of these novel conformers, a redistribution of the populations of glycine conformers is resulted in and causes the remarkable decrease of the most stabilized Ip （from 48% to 38%）. It indicated that the simple glycine molecule can show chirality under certain conditions. The interacting modes of glycine enantiomeric pairs （e.g., ivn and IVn） with PG showed large differences （Fig. 4）; in addition, their interaction energies corrected with basis set superposition errors （BSSE） were calculated to be --66.81 and -46.99 kJ tool^-1, respectively. Accordingly, the glycine enantiomers can be potentially applied to the chiral recognition in biological and pharmaceutical areas.