Generalized Born Solvation Model SM12

We present a new self-consistent reaction-field implicit solvation model that employs the generalized Born approximation for the bulk electrostatic contribution to the free energy of solvation. The new solvation model (SM) is called SM12 (where ″12″ stands for 2012), and it is available with two sets of parameters, SM12CM5 and SM12ESP. The SM12CM5 parametrization is based on CM5 partial atomic charges, and the SM12ESP parametrization is based on charges derived from a quantum-mechanically calculated electrostatic potential (ESP) (in particular, we consider ChElPG and Merz-Kollman-Singh charges). The model was parametrized over 10 combinations of theoretical levels including the 6-31G(d) and MG3S basis sets and the B3LYP, mPW1PW, M06-L, M06, and M06-2X density functionals against 2979 reference experimental data. The reference data include 2503 solvation free energies and 144 transfer free energies of neutral solutes composed of H, C, N, O, F, Si, P, S, Cl, Br, and I in water and in 90 organic solvents as well as 332 solvation free energies of singly charged anions and cations in acetonitrile, dimethyl sulfoxide, methanol, and water. The advantages of the new solvation model over our previous generalized Born model (SM8) and all other previous generalized Born solvation models are (i) like the SMD model based on electron density distributions, it may be applied with a single set of parameters with arbitrary extended basis sets, whereas the SM8 model involves CM4 or CM4M charges that become unstable for extended basis sets, (ii) it is parametrized against a more diverse training sets than any previous solvation model, and (iii) it is defined for the entire periodic table.

Effect of Molecular Symmetry on the Spectra and Dynamics of the Intramolecular Charge Transfer (ICT) state of peridinin

The spectroscopic properties and dynamics of the excited states of two different synthetic analogues of peridinin were investigated as a function of solvent polarity using steady-state absorption, fluorescence, and ultrafast time-resolved optical spectroscopy. The analogues are denoted S-1- and S-2-peridinin and differ from naturally occurring peridinin in the location of the lactone ring and its associated carbonyl group, known to be obligatory for the observation of a solvent dependence of the lifetime of the S(1) state of carotenoids. Relative to peridinin, S-1- and S-2-peridinin have their lactone rings two and four carbons more toward the center of the π-electron system of conjugated carbon-carbon double bonds, respectively. The present experimental results show that as the polarity of the solvent increases, the steady-state spectra of the molecules broaden, and the lowest excited state lifetime of S-1-peridinin changes from ∼155 to ∼17 ps which is similar to the magnitude of the effect reported for peridinin. The solvent-induced change in the lowest excited state lifetime of S-2-peridinin is much smaller and changes only from ∼90 to ∼67 ps as the solvent polarity is increased. These results are interpreted in terms of an intramolecular charge transfer (ICT) state that is formed readily in peridinin and S-1-peridinin, but not in S-2-peridinin. Quantum mechanical computations reveal the critical factors required for the formation of the ICT state and the associated solvent-modulated effects on the spectra and dynamics of these molecules and other carbonyl-containing carotenoids and polyenes. The factors are the magnitude and orientation of the ground- and excited-state dipole moments which must be suitable to generate sufficient mixing of the lowest two excited singlet states.

First-principles modeling of dye-sensitized solar cells: challenges and perspectives

Since dye-sensitized solar cells (DSSCs) appeared as a promising inexpensive alternative to the traditional silicon-based solar cells, DSSCs have attracted a considerable amount of experimental and theoretical interest. In contrast with silicon-based solar cells, DSSCs use different components for the light-harvesting and transport functions, which allow researchers to fine-tune each material and, under ideal conditions, to optimize their overall performance in assembled devices. Because of the variety of elementary components present in these cells and their multiple possible combinations, this task presents experimental challenges. The photoconversion efficiencies obtained up to this point are still low, despite the significant experimental efforts spent in their optimization. The development of a low-cost and efficient computational protocol that could qualitatively (or even quantitatively) identify the promising semiconductors, dyes, and electrolytes, as well as their assembly, could save substantial experimental time and resources. In this Account, we describe our computational approach that allows us to understand and predict the different elementary mechanisms involved in DSSC working principles. We use this computational framework to propose an in silico route for the ab initio design of these materials. Our approach relies on a unique density functional theory (DFT) based model, which allows for an accurate and balanced treatment of electronic and spectroscopic properties in different phases (such as gas, solution, or interfaces) and avoids or minimizes spurious computational effects. Using this tool, we reproduced and predicted the properties of the isolated components of the DSSC assemblies. We accessed the microscopic measurable characteristics of the cells such as the short circuit current (J(sc)) or the open circuit voltage (V(oc)), which define the overall photoconversion efficiency of the cell. The absence of empirical or material-related parameters in our approach should allow for its wide application to the optimization of existing devices or the design of new ones.

TD-DFT Assessment of Functionals for Optical 0-0 Transitions in Solvated Dyes

Using TD-DFT, we performed simulations of the adiabatic energies of 40 fluorescent molecules for which the experimental 0-0 energies in condensed phase are available. We used six hybrid functionals (B3LYP, PBE0, M06, M06-2X, CAM-B3LYP, and LC-PBE) that have been shown to provide accurate transition energies in previous TD-DFT assessments, selected two diffuse-containing basis sets, and applied the most recent models for estimating bulk solvation effects. In each case, the correction arising from the difference of zero-point vibrational energies between the ground and the excited states has been consistently determined. Basis set effects have also been carefully studied. It turned out that PBE0 and M06 are the most effective functionals in terms of average deviation (mean absolute error of 0.22-0.23 eV). However, both the M06-2X global hybrid that contains more exact exchange and the CAM-B3LYP range-separated hybrid significantly improve the consistency of the prediction for a relatively negligible degradation of the average error. In addition, we assessed (1) the cross-structure/spectra relationships, (2) the importance of solvent effects, and (3) the differences between adiabatic and vertical energies.