Ciro A. Guido

Practical computation of electronic excitation in solution: vertical excitation model

We present a unified treatment of solvatochromic shifts in liquid-phase absorption spectra, and we develop a self-consistent state-specific vertical excitation model (called VEM) for electronic excitation in solution. We discuss several other approaches to calculate vertical excitations in solution as an approximation to VEM. We illustrate these methods by presenting calculations of the solvatochromic shifts of the lowest excited states of several solutes (acetone, acrolein, coumarin 153, indolinedimethine-malononitrile, julolidine-malononitrile, methanal, methylenecyclopropene, and pyridine) in polar and nonpolar solvents (acetonitrile, cyclohexane, dimethyl sulfoxide, methanol, n-hexane, n-pentane, and water) using implicit solvation models combined with configuration interaction based on single excitations and with time-dependent density functional theory.

Planar vs. twisted intramolecular charge transfer mechanism in Nile Red: new hints from theory

Using a time-dependent density functional theory approach and taking into account bulk solvent effects, we investigate the absorption and fluorescence spectra of Nile Red. In particular, we have assessed both the planar and twisted intramolecular charge transfer mechanism by using a panel of exchange correlation functionals including both global and range-separated hybrids, refined solvent models and the simulation of vibronic couplings. It turned out that the appropriate choice of the functional is of prime importance to obtain, not only quantitatively accurate values, but also qualitatively correct evolution of the spectral features with respect to the dihedral angles of the amino group. At the light of this study, the interpretation of the experimental data is critically re-examined and compared to typical dual-fluorescent molecules.

On the TD-DFT accuracy in determining single and double bonds in excited-state structures of organic molecules.

We present an analysis on the behavior of the TD-DFT approach in the determination of excited-state structures with particular attention to single and double bonds. The analysis is based on a direct comparison with the highly correlated CASPT2 ab initio approach. Six DFT exchange-correlation functionals differing in the Hartree-Fock exchange percentage and the type of correlation functional are considered and applied to the study of seven prototype organic molecules characterized by two families of excitations (acrolein, acetone, diazomethane, and propanoic acid anion for n-π* and cis-1,3-butadiene, trans-1,3-butadiene, and pyrrole for π-π*), and three protonated Schiff bases, used as model chromophores for 11-cis retinal. Our analysis allows pinpointing specific correlations between accuracy of the various functionals and category of excitation and/or type of chemical bond involved in the corresponding geometry relaxation. We confirm the role of the long-range correction of the potential to obtain a balanced description of excitation energies and excited-state structures, but we also point out that, for a small system, B3LYP and PBE0 also give results close to CASPT2.

Benchmarking Time-Dependent Density Functional Theory for Excited State Geometries of Organic Molecules in Gas-Phase and in Solution.

We analyze potentials and limits of the Time-Dependent Density Functional Theory (TD-DFT) approach for the determination of excited-state geometries of organic molecules in gas-phase and in solution. Three very popular DFT exchange-correlation functionals, two hybrids (B3LYP and PBE0) and one long-range corrected (CAM-B3LYP), are here investigated, and the results are compared to the correlated RI-CC2 wave function approach. Solvent effects are further analyzed by means of a polarizable continuum model. A total of 15 organic chromophores (including both small molecules and larger push-pull systems) are considered as prototypes of n → π* and π → π* singlet excitations. Our analysis allows to point out specific correlations between the accuracy of the various functionals and the type of excitation and/or the type of chemical bonds involved. We find that while the best ground-state geometries are obtained with PBE0 and B3LYP, CAM-B3LYP yields the most accurate description of electronic and geometrical characteristics of excited states, both in gas-phase and in solution.

Electronic Excitations in Solution: The Interplay between State Specific Approaches and a Time-Dependent Density Functional Theory Description

We critically analyze the performances of continuum solvation models when coupled to time-dependent density functional theory (TD-DFT) to predict solvent effects on both absorption and emission energies of chromophores in solution. Different polarization schemes of the polarizable continuum model (PCM), such as linear response (LR) and three different state specific (SS) approaches, are considered and compared. We show the necessity of introducing a SS model in cases where large electron density rearrangements are involved in the excitations, such as charge-transfer transitions in both twisted and quadrupolar compounds, and underline the very delicate interplay between the selected polarization method and the chosen exchange-correlation functional. This interplay originates in the different descriptions of the transition and ground/excited state multipolar moments by the different functionals. As a result, the choice of both the DFT functional and the solvent polarization scheme has to be consistent with the nature of the studied electronic excitation.

Communication: One third: A new recipe for the PBE0 paradigm

We analyze the performances of the parameter-free hybrid density functional PBE0-1/3 obtained combining the PBE generalized-gradient functional with a predefined amount of exact exchange of 1/3, as recently discussed by Cortona [J. Chem. Phys. 136, 086101 (2012)]. The numerical results that we have obtained for various properties, such as atomization energies (G2-148 dataset), weak interactions (NCB31 dataset), hydrogen-bond length optimizations, and dissociation energies (HB10 dataset), and vertical excitation energies, show an increased performance of PBE0-1/3 with respect to the widely used PBE0. We therefore propose to use one third as the mixing coefficient for the PBE-based hybrid functional.

The Fate of a Zwitterion in Water from ab Initio Molecular Dynamics: Monoethanolamine (MEA)-CO2

Understanding the fundamental reactions accompanying the capture of carbon dioxide in amine solutions is critical for the design of high-performance solvents and requires an accurate modeling of the solute-solvent interaction. As a first step toward this goal, using ab initio molecular dynamics (Car-Parrinello) simulations, we investigate a zwitterionic carbamate, a species long proposed as intermediate in the formation of a stable carbamate, in a dilute aqueous solution. CO2 release and deprotonation are competitive routes for its dissociation and are both characterized by free-energy barriers of 6-8 kcal/mol. Water molecules play a crucial role in both pathways, resulting in large entropic effects. This is especially true in the case of CO2 release, which is accompanied by a strong reorganization of the solvent beyond the first coordination shell, leading to the formation of a water cage entrapping the solute (hydrophobic effect). Our results contrast with the assumptions of implicit solvent models.

Circularly Polarized Luminescence from Axially Chiral BODIPY DYEmers: An Experimental and Computational Study

With our new home-built circularly polarized luminescence (CPL) instrument, we measured fluorescence and CPL spectra of the enantiomeric pairs of two quasi-isomeric BODIPY DYEmers 1 and 2, endowed with axial chirality. The electronic circular dichroism (ECD) and CPL spectra of these atropisomeric dimers are dominated by the exciton coupling between the main π-π* transitions (550-560 nm) of the two BODIPY rings. Compound 1 has strong ECD and CPL spectra (glum =4×10-3 ) well reproduced by TD-DFT and SCS-CC2 (spin-component scaled second-order approximate coupled-cluster) calculations using DFT-optimized ground- and excited-state structures. Compound 2 has weaker ECD and CPL spectra (glum =4×10-4 ), partly due to the mutual cancellation of electric-electric and electric-magnetic exciton couplings, and partly to its conformational freedom. This compound is computationally very challenging. Starting from the optimized excited-state geometries, we predicted the wrong sign for the CPL band of 2 using TD-DFT with the most recommended hybrid and range-separated functionals, whereas SCS-CC2 or a DFT functional with full exact exchange provided the correct sign.

Excited state gradients for a state-specific continuum solvation approach: The vertical excitation model within a Lagrangian TDDFT formulation

The accurate modeling of the environment response is a fundamental challenge for accurately describing the photophysics and photochemistry of molecules both in solution and in more complex embeddings. When large rearrangements of the electron density occur after an electronic transition, state-specific formulations, such as the vertical excitation model, are necessary to achieve a proper modeling of the processes. Such a state-specific model is fundamental not only to obtain accurate energies, but also to follow the geometrical relaxation accompanying the evolution of the excited-states. This study presents the analytical expression of the gradients of the vertical excitation model approach by a Lagrangian formulation in the time dependent density functional theory framework. Representative organic chromophores in solution are used to test the reliability of the implementation and provide comparisons with the linear response description.

Plasmon enhanced light harvesting: multiscale modeling of the FMO protein coupled with gold nanoparticles.

Plasmonic systems, such as metal nanoparticles, are becoming increasingly important in spectroscopies and devices because of their ability to enhance, even by several orders of magnitude, the photophysical properties of neighboring systems. In particular, it has been shown both theoretically and experimentally that combining nanoplasmonic devices with natural light-harvesting proteins substantially increases the fluorescence and absorption properties of the system. This kind of biohybrid device can have important applications in the characterization and design of efficient light-harvesting systems. In the present work, the FMO light-harvesting protein was combined with gold nanoparticles of different sizes, and its photophysical properties were characterized using a multiscale quantum-mechanical classical-polarizable and continuum model (QM/MMPol/PCM). By optimal tuning of the plasmon resonance of the metal nanoparticles, fluorescence enhancements of up to 2 orders of magnitude were observed. Orientation effects were found to be crucial: amplifications by factors of up to 300 were observed for the absorption process, while the radiative decay of the emitting state increased at most by a factor of 10, mostly as a result of poor alignment of the emitting state with the considered metal aggregates. Despite being a limiting factor for high-fluorescence-enhancement devices, the strong orientation dependence may represent an important feature of the natural light-harvesting system that could allow selective enhancement of a specific excited state of the complex.