Roberto Improta

A state-specific polarizable continuum model time dependent density functional theory method for excited state calculations in solution.

An effective state specific (SS) model for the inclusion of solvent effects in time dependent density functional theory (TD-DFT) computations of excited electronic states has been developed and coded in the framework of the so-called polarizable continuum model (PCM). Different relaxation time regimes can be treated thus giving access to a number of different spectroscopic properties together with solvent relaxation energies of paramount relevance in electron transfer processes. SS and conventional linear response (LR) models have been compared for two benchmark systems (coumarin 153 and formaldehyde in different solvents) and in the limiting simple case of a dipolar solute embedded in a spherical cavity. The results point out the complementarity of LR and SS approaches and the advantages of the latter model especially for polar solvents. The favorable scaling properties of PCM-TD-DFT models in both SS and LR variants and their availability in effective quantum mechanical codes pave the route for the computation of reliable spectroscopic properties of large molecules of technological and/or biological interest in their natural environments.

Effective method to compute Franck-Condon integrals for optical spectra of large molecules in solution

The authors present a new method for the computation of vibrationally resolved optical spectra of large molecules, including the Duschinsky [Acta Physicochim. URSS 7, 551 (1937)] rotation of the normal modes. The method automatically selects the relevant vibronic contributions to the spectrum, independent of their frequency, and it is able to provide fully converged spectra with a quite modest computational time, both in vacuo and in condensed phase. Starting from the rigorous time-dependent expression they discuss indeed in which limits the spectrum of a molecule embedded in a solvent, described as a polarizable continuum, can be computed in a time-independent formalism, defining both nonequilibrium and equilibrium limits. In these cases the polarizable continuum model provides a suitable description of the solvent field. By computing the absorption spectra of anthracene in gas phase and of coumarin C153 in gas phase and cyclohexane, and the phosphorescence spectrum of the unsubstituted coumarin in ethanol they show that the method is fast and efficient.

Toward effective and reliable fluorescence energies in solution by a new state specific polarizable continuum model time dependent density functional theory approach.

A state specific (SS) model for the inclusion of solvent effects in time dependent density functional theory (TD-DFT) computations of emission energies has been developed and coded in the framework of the so called polarizable continuum model (PCM). The new model allows for a rigorous and effective treatment of dynamical solvent effects in the computation of fluorescence and phosphorescence spectra in solution, and it can be used for studying different relaxation time regimes. SS and conventional linear response (LR) models have been compared by computing the emission energies for different benchmark systems (formaldehyde in water and three coumarin derivatives in ethanol). Special attention is given to the influence of dynamical solvation effects on LR geometry optimizations in solution. The results on formaldehyde point out the complementarity of LR and SS approaches and the advantages of the latter model especially for polar solvents and/or weak transitions. The computed emission energies for coumarin derivatives are very close to their experimental counterparts, pointing out the importance of a proper treatment of nonequilibrium solvent effects on both the excited and the ground state energies. The availability of SS-PCM/TD-DFT models for the study of absorption and emission processes allows for a consistent treatment of a number of different spectroscopic properties in solution.

Effective method for the computation of optical spectra of large molecules at finite temperature including the Duschinsky and Herzberg–Teller effect: The Qx band of porphyrin as a case study

The authors extend their recent method for the computation of vibrationally resolved optical spectra of large molecules, including both the Duschinsky rotation and the effect of finite temperature in the framework of the Franck-Condon (FC) approximation, to deal with the more general case of the Herzberg-Teller (HT) model, where also the linear dependence of the transition dipole moment on the nuclear coordinates is taken into account. This generalization allows us to investigate weak and vibronically allowed transitions by far extending the range of application of the method. The calculation of the spectra of sizable molecules is computationally demanding because of the huge number of final vibrational states that must be taken into account, and the inclusion of HT terms further increases the computational burden. The method presented here automatically selects the relevant vibronic contributions to the spectrum, independent of their frequency, and it is able to provide fully converged spectra with a modest computational requirement. The effectiveness of the method is illustrated by computing the HT absorption and fluorescence Q(x) spectra of free-base porphyrin both at T=0 K and at room temperature, performing for the first time an exact treatment of vibrations in harmonic approximation. Q(x) spectra are compared to experiments and FC/HT interferences are analyzed in detail.

Effective method to compute vibrationally resolved optical spectra of large molecules at finite temperature in the gas phase and in solution

The authors present a new method for the computation of vibrationally resolved optical spectra of large molecules, including the Duschinsky rotation of the normal modes and the effect of thermal excitation. The method automatically selects the relevant vibronic contributions to the spectrum, independently of their frequency, and it is able to provide fully converged spectra with moderate computational times, both in vacuo and in solution. By describing the electronic states in the frame of the density functional theory and its time-dependent extension, they computed the room temperature absorption spectra of coumarin C153 and trans-stilbene in cyclohexane and the phosphorescence spectrum of porphyrazine in gas phase, showing that the method is fast and efficient. The comparison with experiment for trans-stilbene and coumarin C153 is very satisfactory, confirming the progress made toward a reliable method for the computation and interpretation for the optical spectra of large molecules.

Absolute pKa determination for carboxylic acids using density functional theory and the polarizable continuum model

The absolute pKas of eight carboxylic acids have been computed using a Born–Haber cycle, the most recent experimental value for the proton solvation energy and completely ab initio structures, energies, and harmonic frequencies of acids and conjugated bases both in vacuo and in aqueous solution. The proton affinities computed by the PBE0 hybrid functional are in very good agreement with experimental values and with the results of the most sophisticate (and expensive) quantum mechanical models (G2, G3, and CBS). The Gibbs energies in aqueous solution have been computed by our last version of the polarizable continuum model, which takes into the proper account escaped charge effects in a very effective computational implementation. The encouraging results obtained for this training set and the linear scaling both of the electronic and solvation models pave the route for the evaluation of reliable pK values for the large systems of interest in biological or material science frameworks.

Quantum Mechanical Studies on the Photophysics and the Photochemistry of Nucleic Acids and Nucleobases

The photophysics and photochemistry of DNA is of great importance due to the potential damage of the genetic code by UV light. Quantum mechanical studies have played a key role in interpretating the results of modern time-resolved pump-probe spectroscopy, and in elucidating the main photoactivated reactive paths. This review provides a concise, complete picture of the computational studies carried out, approximately, in the past decade. We start with an overview of the photophysics of the nucleobases in the gas phase and in solution. We discuss the proposed mechanisms for ultrafast decay to the ground state, that involve conical intersections, consider the role of triplet states, and analyze how the solvent modulates the photophysics. Then we move to larger systems, from dinucleotides to single- and double-stranded oligonucleotides. We focus on the possible role of charge transfer and delocalized or excitonic states in the photophysics of these systems and discuss the main photochemical paths. We finish with an outlook on the current challenges in the field and future directions of research.

Quantum mechanical computations and spectroscopy: from small rigid molecules in the gas phase to large flexible molecules in solution.

Interpretation of structural properties and dynamic behavior of molecules in solution is of fundamental importance to understand their stability, chemical reactivity, and catalytic action. While information can be gained, in principle, by a variety of spectroscopic techniques, the interpretation of the rich indirect information that can be inferred from the analysis of experimental spectra is seldom straightforward because of the subtle interplay of several different effects, whose specific role is not easy to separate and evaluate. In such a complex scenario, theoretical studies can be very helpful at two different levels: (i) supporting and complementing experimental results to determine the structure of the target molecule starting from its spectral properties; (ii) dissecting and evaluating the role of different effects in determining the observed spectroscopic properties. This is the reason why computational spectroscopy is rapidly evolving from a highly specialized research field into a versatile and widespread tool for the assignment of experimental spectra and their interpretation in terms of chemical physical effects. In such a situation, it becomes important that both computationally and experimentally oriented chemists are aware that new methodological advances and integrated computational strategies are available, providing reliable estimates of fundamental spectral parameters not only for relatively small molecules in the gas phase but also for large and flexible molecules in condensed phases. In this Account, we review the most significant methodological contributions from our research group in this field, and by exploiting some recent results of their application to the computation of IR, UV-vis, NMR, and EPR spectral parameters, we discuss the microscopic mechanisms underlying solvent and vibrational effects on the spectral parameters. After reporting some recent achievements for the study of excited states by first principle quantum mechanical approaches, we focus on the treatment of environmental effects by means of mixed discrete-continuum solvent models and on effective methods for computing vibronic contributions to the spectra. We then discuss some new developments, mainly based on time-dependent approaches, allowing us to go beyond the determination of spectroscopic parameters toward the simulation of line widths and shapes. Although further developments are surely needed to improve the accuracy and effectiveness of several items in the proposed approach, we try to show that the first important steps toward a direct comparison between the results obtained in vitro and those obtained in silico have been made, making easier fruitful crossovers among experiments, computations and theoretical models, which would be decisive for a deeper understanding of the spectral behavior associated with complex systems and processes.

Electronic excited states responsible for dimer formation upon UV absorption directly by thymine strands: joint experimental and theoretical study.

The study addresses interconnected issues related to two major types of cycloadditions between adjacent thymines in DNA leading to cyclobutane dimers (T<>Ts) and (6-4) adducts. Experimental results are obtained for the single strand (dT)(20) by steady-state and time-resolved optical spectroscopy, as well as by HPLC coupled to mass spectrometry. Calculations are carried out for the dinucleoside monophosphate in water using the TD-M052X method and including the polarizable continuum model; the reliability of TD-M052X is checked against CASPT2 calculations regarding the behavior of two stacked thymines in the gas phase. It is shown that irradiation at the main absorption band leads to cyclobutane dimers (T<>Ts) and (6-4) adducts via different electronic excited states. T<>Ts are formed via (1)ππ* excitons; [2 + 2] dimerization proceeds along a barrierless path, in line with the constant quantum yield (0.05) with the irradiation wavelength, the contribution of the (3)ππ* state to this reaction being less than 10%. The formation of oxetane, the reaction intermediate leading to (6-4) adducts, occurs via charge transfer excited states involving two stacked thymines, whose fingerprint is detected in the fluorescence spectra; it involves an energy barrier explaining the important decrease in the quantum yield of (6-4) adducts with the irradiation wavelength.

Computing the inhomogeneous broadening of electronic transitions in solution: a first-principle quantum mechanical approach

Starting from Marcuss relationship connecting the inhomogeneous broadening with the solvent reorganization energy and exploiting recent state-specific developments in PCM/TD-DFT calculations, we propose a procedure to estimate the polar broadening of optical transitions. When applied to two representative molecular probes, coumarin C153 and 4-aminophthalimide, in different solvents, our approach provides for the polar broadening values fully consistent with the experimental ones. Thanks to these achievements, for the first time fully ab initio vibrationally resolved absorption spectra in solution are computed, obtaining spectra for coumarin C153 in remarkable agreement with experiments.