Present Time-dependent Density Functional Theory Research Articles

Calculations of the light absorption spectra of porphyrinoid chromophores for dye-sensitized solar cells.

Solar power is a strong alternative to the currently used fossil fuels in order to satisfy the world's energy needs. Among them, dye-sensitized solar cells (DSSC) represent a low-cost option. Efficient and cheap dyes are currently needed to make DSSCs competitive. Computational chemistry can be used to guide the design of new light-absorbing chromophores. Here, we have computationally studied the lowest excited states of ZnPBAT, which is a recently synthesized porphyrinoid chromophore with high light-absorption efficiency. The calculations have been performed at ab initio correlated levels of theory employing second-order coupled clusters (CC2) and algebraic diagrammatic construction using second order (ADC(2)) methods and by performing density functional theory (DFT) calculations using the time-dependent DFT (TDDFT) approach for excitation energies. The ultraviolet-visible (UV-vis) spectrum calculated at the ADC(2) and CC2 levels agrees well with the experimental one. The calculations show that ZnPBAT has six electronic transitions in the visible range of the absorption spectrum. The ab initio correlated calculations and previously reported experimental data have been used to assess the performance of several well-known density functionals that have been employed in the present TDDFT study. Solvent effects have been estimated by using the conductor-like screening model (COSMO). The influence of the addition of a TiO2 cluster to the chromophore systems has also been investigated. The results indicate that both CAM-B3LYP and Becke's "half-and-half" (BHLYP) density functionals are appropriate for the studies of excitation energies in the blue range of the visible spectrum for these kinds of porphyrinoid chromophores, whereas the excitation energies of the Q band calculated at the ab initio correlated level are more accurate than those obtained in the present TDDFT calculations. The inclusion of solvent effects has a modest influence on the spectrum of the protonated form of the studied chromophores, whereas solvent models are crucial when studying the absorption spectrum of the anionic chromophore. The calculated UV-vis spectrum for the chromophore anion is not significantly affected by attaching a TiO2 cluster to it.

Mechanism of Co-C bond photolysis in the base-on form of methylcobalamin.

A mechanism of Co-C bond photodissociation in the base-on form of the methylcobalamin cofactor (MeCbl) has been investigated employing time-dependent density functional theory (TD-DFT), in which the key step involves singlet radical pair generation from the first electronically excited state (S1). The corresponding potential energy surface of the S1 state was constructed as a function of Co-C and Co-Naxial bond distances, and two possible photodissociation pathways were identified on the basis of energetic grounds. These pathways are distinguished by whether the Co-C bond (path A) or Co-Naxial bond (path B) elongates first. Although the final intermediate of both pathways is the same (namely a ligand field (LF) state responsible for Co-C dissociation), the reaction coordinates associated with paths A and B are different. The photolysis of MeCbl is wavelength-dependent, and present TD-DFT analysis indicates that excitation in the visible α/β band (520 nm) can be associated with path A, whereas excitation in the near-UV region (400 nm) is associated with path B. The possibility of intersystem crossing, and internal conversion to the ground state along path B are also discussed. The mechanism proposed in this study reconciles existing experimental data with previous theoretical calculations addressing the possible involvement of a repulsive triplet state.

Theoretical Study of Metallophilic Interactions and Excited States of Heterobimetallic d10−d8 Complexes with Bridging Ligands: The Tuning of Electronic Spectroscopy

A series of heterobimetallic d10−d8 complexes, [AuIM′(R1)2(R2)2] (M′ = PtII, AuIII; R1 = CN−, C≡CH−; R2 = PH2CH2PH2, CH2PH2CH2−), were explored using ab initio methods and time-dependent density functional theory (TD-DFT). In the ground states, d10−d8 distances were calculated at 2.88−3.00 Å with associated vibrational frequencies of 102−114 cm−1 by the MP2 method; energies of metallophilic interactions were estimated to be in the range 11.0−18.2 and 7.03−9.14 kcal/mol at the MP2 and CCSD levels, respectively. With the reference complex [AuIPtII(CN)2(PH2CH2PH2)2]+ (1), the variation of the d8 heterometal (M′), the coordination environment of the d8 metal (R1), and the bridging ligand (R2) changes the electronic structures and thus further tunes the electronic spectroscopy of these complexes. The known experimental absorption spectra were well reproduced by the present TD-DFT calculations. Upon excitation, UMP2 revealed the metal−metal interactions in their triplet excited states were strengthened. The corresponding phosphorescent emissions were attributed to the combination of metal-centered (MC) transition, metal-to-metal charge transfer (MM′CT), and ligand-to-metal charge transfer (LMCT). The MC and MM′CT transitions are responsible for the excited metal−metal contraction. The contraction in the AuI−AuIII complex is larger than that in corresponding AuI−PtII complex due to greater participation of MM′CT transitions.

Mixed time-dependent density-functional theory/classical trajectory surface hopping study of oxirane photochemistry

We present a mixed time-dependent density-functional theory (TDDFT)/classical trajectory surface hopping (SH) study of the photochemical ring opening in oxirane. Previous preparatory work limited to the symmetric CC ring-opening pathways of oxirane concluded that the Tamm-Dancoff approximation (TDA) is important for improving the performance of TDDFT away from the equilibrium geometry. This observation is supported by the present TDDFT TDA/SH calculations which successfully confirm the main experimentally derived Gomer-Noyes mechanism for the photochemical CO ring opening of oxirane and, in addition, provide important state-specific information not easily accessible from experiments. In particular, we find that, while one of the lowest two excited states is photochemically relatively inert, excitation into the other excited state leads predominantly to rapid ring opening, cyclic-C(2)H(4)O-->(*)CH(2)CH(2)O(*). This is followed by hopping to the electronic ground state where hot (4000 K) dynamics leads to further reactions, namely, (*)CH(2)CH(2)O()-->CH(3)CHO-->(*)CH(3)+(*)CHO and CH(4)+CO. We note that, in the dynamics, we are not limited to following minimum energy pathways and several surface hops may actually be needed before products are finally reached. The performance of different functionals is then assessed by comparison of TDDFT and diffusion Monte Carlo potential energy curves along a typical TDDFT TDA/SH reaction path. Finally, although true (S(0),S(1)) conical intersections are expected to be absent in adiabatic TDDFT, we show that the TDDFT TDA is able to approximate a conical intersection in this system.

Time-dependent density functional theory study of cobalt corrinoids: Electronically excited states of methylcobalamin

Time-dependent density functional theory (TDDFT) has been applied to the analysis of the electronic spectra of methylcobalamin (MeCbl) and its derivative in which the trans axial base was replaced by a water molecule (MeCbi[Single Bond]H(2)O). The latter corresponds to the situation encountered in strongly acidic solutions. The study primarily focuses on the accuracy of two functionals, the hybrid B3LYP and the gradient corrected BP86, in dealing with the electronic excitations. The high resolution crystal structure of MeCbl was the source of the initial coordinates. To generate the initial structures, the full MeCbl was simplified by replacing the corrin side chains by H atoms. The vertical excitation energies, together with the corresponding oscillator strengths, were calculated at the optimized BP86 and B3LYP structures of the ground electronic state of the complexes. The NBO analysis shows that the B3LYP functional gives a bonding description of the ground state as a more polarized covalent bond compared to that given by BP86. The latter functional has more covalent bonding and is thus more appropriate for modeling the axial bonding properties. To validate the accuracy of the present TDDFT analysis, the computed excitations were directly compared to the absorption spectra of MeCbl. In order to obtain a reliable agreement between experiment and theory, the two-parameter scaling technique was introduced, which compensates differently the low-energy and high-energy excitations. Electronic excitations strongly depend on the choice of the functional. Transitions involving corrin pi-->pi(*) excitations are better described by the B3LYP functional while transitions associated with metal-to-ligand (dpi-->pi(*)d) excitations are better described by BP86. These differences can be associated with the different bonding descriptions obtained by B3LYP and BP86.

Strong Solvent Imprint

The optical rotation of polarized light caused by chiral molecules dissolved in solution is usually attributed to the interaction of the light field with the electronic distribution of the solute. However, the solvent shell around the molecule can also exert strong effects; for example, the sign of the optical rotation of (S)-methyloxirane switches from positive to negative when the solvent is changed from water to benzene. Previous theoretical work has shown that the interaction of (S)-methyloxirane with water accounts for most of the optical rotary dispersion (ORD) in aqueous media. Mukhopadhyay et al. now present time-dependent density functional theory calculations of ORD for Monte Carlo simulations of (S)- and (R)-methyloxirane solvated with benzene. They show that for an explicit solvent model, the dissymmetric benzene solvent shell, rather than the solute itself, dominates the ORD. This effect was not seen with implicit solvent molecules or with an achiral solute (ethylene oxide). — PDS

The Role of the β-Ionone Ring in the Photochemical Reaction of Rhodopsin

Time-dependent density functional theory (TDDFT) calculations on the photoabsorption process of the 11-cis retinal protonated Schiff base (PSB) chromophore show that the Franck-Condon relaxation of the first excited state of the chromophore involves a torsional twist motion of the beta-ionone ring relative to the conjugated retinyl chain. For the ground state, the beta-ionone ring and the retinyl chain of the free retinal PSB chromophore form a -40 degrees dihedral angle as compared to -94 degrees for the first excited state. The double bonds of the retinal are shorter for the fully optimized structure of the excited state than for the ground state suggesting a higher cis-trans isomerization barrier for the excited state than for the ground state. According to the present TDDFT calculations, the excitation of the retinal PSB chromophore does not primarily lead to a reaction along the cis-trans torsional coordinate at the C11-C12 bond. The activation of the isomerization center seems to occur at a later stage of the photo reaction. The results obtained at the TDDFT level are supported by second-order Møller-Plesset (MP2) and approximate singles and doubles-coupled cluster (CC2) calculations on retinal chromophore models; the MP2 and CC2 calculations yield for them qualitatively the same ground state and excited-state structures as obtained in the density functional theory and TDDFT calculations.

Time-dependent density-functional theory for strong-field multiphoton processes: Application to the study of the role of dynamical electron correlation in multiple high-order harmonic generation

We present a self-interaction-free time-dependent density-functional theory (TDDFT) for nonperturbative treatment of multiphoton processes of many-electron atomic systems in intense laser fields. The theory is based on the extension of the time-dependent Kohn-Sham formalism. The time-dependent exchange-correlation potential with proper short-and long-range behavior is constructed by means of the time-dependent optimized effective potential (TDOEP) method and the incorporation of an explicit self-interaction correction (SIC) term. The resulting TDOEP-SIC equations are structurally similar to the time-dependent Wartree-Fock equations, but include the many-body effects through an orbital-independent single-particle local time-dependent exchange-correlation potential. We also introduce a generalized pseudospectral lime-propagation method, allowing optimal spatial grid discretization, for accurate and efficient numerical solution of the TDOEP-SIC equations. The theory is applied to the study of the role of dynamical electron correlation on the multiple high-order harmonic generation (HHG) processes of He atoms in intense laser fields. We also perform a detailed study of the mechanisms responsible for the production of the higher harmonics in He atoms observed in a recent experiment that cannot be explained by the single-active-electron model. We found that both the dynamical electron correlation and the He+ ion are important to the generation of the observed higher harmonics. The present TDDFT is thus capable of providing a unified and self-consistent dynamical picture of the HHG processes. [S1050-2947(98)04601-0].