Electron Localization Dynamics in the Triplet Excited State of [Ru(bpy)3]2+ in Aqueous Solution

Hybrid DFT/classical molecular dynamics of the long‐lived triplet excited state of [Ru(bpy)₃]²⁺ (bpy=2,2′‐bipyridine) in aqueous solution is used to investigate the solvent‐mediated electron localization and dynamics in the triplet metal‐to‐ligand charge‐transfer (MLCT) state. Our studies reveal a solvent‐induced breaking of the coordination symmetry with consequent localization of the photoexcited electron on one or two bipyridine units for the entire length of our simulation, which amounts to several picoseconds. Frequent electronic “hops” between the ligands constituting the pair are observed with a characteristic time of approximately half a picosecond.     

Impact of Ion-Pairing Effects on Linear and Nonlinear Photophysical Properties of Polymethine Dyes**

The two-photon absorption (2PA) and photophysics of heptamethine dyes featuring cyanine or dipolar electronic structures have been compared for the first time. The perfectly delocalized cyanine system is classically characterized by a two-photon transition matching the vibronic component of its lower energy absorption band. The dipolar species is generated by ion-pairing with a hard counterion in a non-dissociating solvent and displays significant modifications oft he optical properties, including a significant hypochromic shift of absorption, weaker emission and 2PA matching the lower energy transition, thus revealing symmetry breaking within the polymethine electronic structure.     

Calculation of EPR g tensors for transition-metal complexes based on multiconfigurational perturbation theory (CASPT2).

The computation of the electronic g tensor by two multireference methods is presented and applied to a selection of molecules including CN, BO, AlO, GaO, InO, ZnH, ZnF, O(2), H(2)O(+), O(3) (-), and H(2)CO(+) (group A) as well as TiF(3), CuCl(4) (2-), Cu(NH(3))(4) (2+), and a series of d(1)-MOX(4) (n-) compounds, with M=V, Cr, Mo, Tc, W, Re and X=F, Cl, Br (group B). Two approaches are considered, namely, one in which spin-orbit coupling and the Zeeman effect are included using second-order perturbation theory and another one in which the Zeeman effect is added through first-order degenerate perturbation theory within the ground-state Kramers doublet. The two methods have been implemented into the MOLCAS quantum chemistry software package. The results obtained for the molecules in group A are in good agreement with experiment and with previously reported calculated g values. The results for the molecules in group B vary. While the g values for the d(1) systems are superior to previous theoretical results, those obtained for the d(9) systems are too large compared to the experimental values.     

The Low‐Lying Excited States of 2,2′‐Bithiophene: A Theoretical Analysis

The low‐energy regions of the singlet→singlet, singlet→triplet, and triplet→triplet electronic spectra of 2,2′‐bithiophene are studied using multiconfigurational second‐order perturbation theory (CASPT2) and extended atomic natural orbitals (ANO) basis sets. The computed vertical, adiabatic, and emission transition energies are in agreement with the available experimental data. The two lowest singlet excited states, 1¹B_u and 2¹B_u, are computed to be degenerate, a novel feature of the system to be borne in mind during the rationalization of its photophysics. As regards the observed high triplet quantum yield of the molecule, it is concluded that the triplet states 2³A_g and 2³B_u, separated about 0.4 eV from the two lowest singlet excited states, can be populated by intersystem crossing from nonplanar singlet states.     

Using End Groups to Tune the Linear and Nonlinear Optical Properties of Bis(dioxaborine)‐Terminated Polymethine Dyes

Six anionic pentamethine dyes with different 2,2‐difluoro‐4‐aryl‐1,3,2(2 H)‐dioxaborin‐6‐yl termini were synthesized and isolated as tetra‐n‐octylammonium salts with a variety of aryl groups appended to increase conjugation beyond the dioxaborine termini. The increased conjugation was expected to decrease the energy of the lowest‐lying excited state, and increase the transition dipole moment linking this state to the ground state, which would be anticipated to result in an increase in the real part of the third‐order polarizability, Re(γ). UV/Vis‐NIR absorption spectroscopy indicates that the absorption maxima in DMSO vary from 691 to 761 nm, with the longest wavelength transitions observed for a derivative where the aryl group is 4‐nitrophenyl. Closed‐aperture Z‐scan measurements at 1.3 μm in DMSO indicate that Re(γ) varies from ?2.9×10^?33 to ?5.4×10^?33 esu in these systems. The largest magnitude of Re(γ) was observed for a dye with E‐4‐styrylphenyl aryl groups. This result can be rationalized using a two‐state expression which relates Re(γ) to the energy and transition dipole moment of the transition from the ground state to the lowest‐lying excited state. A nonamethine analogue of this compound was also synthesized and exhibits a slightly larger Re(γ) with respect to a previously reported bis(dioxaborine)‐terminated nonamethine. The extension of conjugation beyond the dioxaborine termini seems to result in an overall increase in Re(γ). However, the effects are smaller than those found by increasing conjugation in the polymethine bridge due to reduced participation of terminal groups in the HOMO.     

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     

Are water–aromatic complexes always stabilized due to π–H interactions? LMP2 study

Eight complexes of various aromatic molecules with water have been studied theoretically at the local Møller–Plesset 2nd order theory (LMP2)/aug‐cc‐pVTZ(‐f)//LMP2/6‐31+G* level of theory. Two types of complexes can be formed, depending on the electronic structure of aromatic molecules. Donor hydrocarbons form A‐type complexes, while aromatics bearing electron‐withdrawing substituents form B‐type complexes. A‐type complexes are stabilized due to π–H interactions with the OH bond pointing to the aromatic molecule plane, while B‐type complexes have geometry with the oxygen atom pointing to the aromatic molecule plane stabilized by the interaction of highest occupied molecular orbital (HOMO) of water molecule with π* orbitals of the aromatics. It has been found that a (? HOMO–lowest unoccupied molecular orbital (LUMO)/2 value of aromatic molecule, which can be called “molecular electronegativity,” is useful to predict the type of complex formed by aromatic molecule and water. Aromatic hydrocarbons with “molecular electronegativity” of <0.15 tend to form A‐type complexes, while aromatic molecules with “molecular electronegativity” of <0.15 a.u. form B‐type complexes. The binding energy of water–aromatic complexes undergoes a minimum in the area of switching from A‐type to B type complexes, which can be rationalize in terms of frontier orbital interactions. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

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.     

Adsorbed States of Hydrogen on Platinum: A New Perspective

The interaction of hydrogen with platinum is enormously important in many areas of catalysis. The most significant of these are in polymer electrolyte membrane fuel cells (PEMFC), in which carbon-supported platinum is used to dissociate hydrogen gas at the anode. The nature of adsorbed hydrogen on platinum has been studied for many years on single-crystal surfaces, on high-surface area-platinum metal (Raney platinum and platinum black), and on supported catalysts. Many forms of vibrational spectroscopy have played a key role in these studies, however, there is still no clear consensus as to the assignment of the spectra. In this work, ab initio molecular dynamics (AIMD) and lattice dynamics were used to study a 1.1 nm nanoparticle, Pt₄₄H₈₀. The results were compared to new inelastic neutron scattering spectra of hydrogen on platinum black and of a carbon-supported platinum fuel cell catalyst and an assignment scheme that rationalises all previous data is proposed.     

Electronic Structure of the Ground and Excited States of β‐Carboline

Coupled‐cluster calculations are used to compute the energy of conversion between the neutral and the zwitterionic forms of β‐carboline. The stability of the different species is discussed in terms of charge separation and aromatic character, which is related to magnetic criteria. By means of a linear response formalism the vertical excitation energies and oscillator strengths of the lowest singlet states of both structures as well as of the cationic species are determined. General agreement of the relative position and intensity of the different peaks with experimental data is achieved, but the overall spectra are slightly displaced because of solvent effects.     

On the kinetic mechanism of the hydrogen abstraction reactions of the hydroxyl radical with CH3CF2Cl and CH3CFCl2: A dual level direct dynamics study

By means of the dual‐level direct dynamics method, the mechanisms of the reactions, CH₃CF₂Cl + OH → products (R1) and CH₃CFCl₂ + OH → products (R2), are studied over a wide temperature range 200–2000 K. The optimized geometries and frequencies of the stationary points are calculated at the MP2/6‐311G(d,p) level, and then the energy profiles of the reactions are refined with the interpolated single‐point energy method at the G3(MP2) level. The canonical variational transition‐state theory with the small‐curvature tunneling (SCT) correction method is used to calculate the rate constants. For the title reactions, three reaction channels are identified and the H‐abstraction channel is the major pathway. The results indicate that F substitution has a significant (reductive) effect on hydrochlorofluorocarbon reactivity. Also, for all H‐abstraction reaction channels the variational effect is small and the SCT effect is only important in the lower temperature range on the rate constants calculation. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

First principles computation of lattice energies of organic solids: the benzene crystal

We provide a first-principles methodology to obtain converged results for the lattice energy of crystals of small, neutral organic molecules. In particular, we determine the lattice energy of crystalline benzene using an additive system based on the individual interaction energies of benzene dimers. Enthalpy corrections are estimated so that the lattice energy can be directly compared to the experimentally determined sublimation energy. Our best estimate of the sublimation energy is 49.4 kJ mol(-1), just over the typical experimentally reported values of 43-47 kJ mol(-1). Our results underscore the necessity of using highly correlated electronic structure methods to determine thermodynamic properties within chemical accuracy. The first coordination sphere contributes about 90 % of the total lattice energy, and the second coordination sphere contributes the remaining 10 %. Three-body interactions are determined to be negligible.     

Kinetic mechanism of the hydrogen abstraction reactions of the chlorine atoms with CH3CF2Cl and CH3CFCl2: a dual level direct dynamics study

The mechanisms of the reactions: CH(3)CFCl(2) + Cl (R1) and CH(3)CF(2)Cl + Cl (R2) are studied over a wide temperature range (200-3000 K) using the dual-level direct dynamics method. The minimum energy path calculation is carried out at the MP2/6-311G(d,p) and B3LYP/6-311G(d,p) levels, and energetic information is further refined by the G3(MP2) theory. The H-abstraction from the out-of-plane for (R1) is the major reaction channel, while the in-plane H-abstraction is the predominant route of (R2). The canonical variational transition-state theory (CVT) with the small-curvature tunneling (SCT) correction method is used to calculate the rate constants. Using group-balanced isodesmic reactions and hydrogenation reactions as working chemical reactions, the standard enthalpies of formation for CH(3)CFCl(2), CH(3)CF(2)Cl, CH(2)CFCl(2), and CH(2)CF(2)Cl are evaluated at the CCSD(T)/6-311 + G(3df,2p)//MP2/6-311G(d,p) level of theory. The results indicate that the substitution of fluorine atom for the chlorine atom leads to a decrease in the C-H bond reactivity with a small increase in reaction enthalpies. Also, for all reaction pathways the variational effect is small and the SCT effect is only important in the lower temperature range on the rate constants.     

A Study of Excitation Delocalization/Localization in Multibranched Chromophores by Using Fluorescence Excitation Anisotropy Spectroscopy

We describe a simple approach to study the excitation localization/delocalization in multibranched chromophores by using fluorescence excitation anisotropy spectroscopy at room temperature. As examples, the electronic excitations in three different multibranched chromophores (dimers) are investigated. For a weakly coupled dimer, fluorescence anisotropy is independent of excitation wavelength, due to localized excitation as well as the degenerate electronic excited states. In contrast, in the case of a strongly coupled dimer, owing to excitonic splitting, a redistribution of the excitation energy is demonstrated by the dependence of anisotropy spectra on the excitation wavelength, which leads to significant deviation from the anisotropy signal of localized excitation. In particular, based on the law of additivity for anisotropy, the degree of delocalized excitation can be simply estimated for a given dimer.     

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.