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International audience; Electrons and protons are the main actors in play in proton coupled electron transfer (PCET) reactions, which are fundamental in many biological (i.e., photosynthesis and enzymatic reactions) and electrochemical processes. The mechanism, energetics and kinetics of PCET reactions are strongly controlled by the coupling between the transferred electrons and protons. Concerted PCET reactions are classified according to the electronical adiabaticity degree of the process. To discriminate among different mechanisms, we propose a new analysis based on the use of electron density based indexes. We choose, as test case, the 3-Methylphenoxyl/phenol system in two different conformations to show how the proposed analysis is a suitable tool to discriminate between the different degree of adiabaticity of PCET processes. The very low computational cost of this procedure is extremely promising to analyze and provide evidences of PCET mechanisms ruling the reactivity of many biological and catalytic systems. K E Y W O R D S density based indexes, density functional theory, electronic adiabaticity degree, non adiabatic coupling, proton coupled electron transfer

Advances in time resolved vibrational spectroscopy techniques provided a new stimulus for understanding the transient molecular dynamics triggered by the electronic excitation. The detailed interpretation of such time dependent spectroscopic signals is a challenging task from both experimental and theoretical points of view. We simulated and analyzed the transient photorelaxation of the pyranine photoacid in aqueous solution, with special focus on structural parameters and low frequency skeleton modes that are possibly preparatory for the photoreaction occurring at later time, as suggested by experimental spectroscopic studies. To this aim, we adopted an accurate computational protocol that combines excited state ab-initio molecular dynamics within a hybrid quantum mechanical/molecular mechanics framework and a time resolved vibrational analysis based on the wavelet transform. According to our results, the main nuclear relaxation on the excited potential energy surface is completed in about 500 fs, in agreement with experimental data. The rearrangement of C-C bonds occurs according to a complex vibrational dynamics, showing oscillatory patterns that are out of phase and modulated by modes below 200 cm-1. We also analyzed in both the ground and the excited state the evolution of some structural parameters involved in excited state proton transfer reaction, namely those involving the pyranine and the water molecule hydrogen bonded to the phenolic O-H group. Both the hydrogen bond distance and the intermolecular orientation are optimized in the excited state, resulting in a tighter proton donor-acceptor couple. Indeed, we found evidence that collective low frequency skeleton modes, such as the out of plane wagging at 108 cm-1and the deformation at 280 cm-1are photoactivated since the ultrafast part of the relaxation and modulate the pyranine-water molecule rearrangement, favouring the step preparatory for the photoreactivity.

We simulated for the first time the oxygen-oxygen bond formation in a synthetic calcium-tetra manganese complex recently developed by Zhang and co-workers. In spite of promising structural similarities to the native oxygen evolving complex (OEC) in Photosystem II, several uncertainties on the mimic stability in water and on its potential catalytic activity still persist. Here, we characterized at density functional theory level the electronic and structural features of the Sn states of the complex, along with the oxygen-oxygen bond formation reaction, proposing a reasonable model for the hydrate complex. As a main finding, both the synthetic compound and the natural OEC show very close energetic barriers for the oxo-oxyl coupling process, suggesting that key electronic features of the natural OEC reactivity are well reproduced. This result strongly encourages the use of this synthetic complex in combination with other molecular assemblies for the design of successful artificial leaves.

The formation of polaron pairs is one of the important photophysical processes that take place after the excitation in semiconducting organic polymers. First-principles Ehrenfest excited-state dynamics is a unique tool to investigate ultrafast photoinduced charge carrier dynamics and related nonequilibrium processes involving correlated electron-nuclear dynamics. In this work the formation of polaron pairs and their dynamical evolution in an oligomer of seven thiophene units is investigated with a combined approach of first-principles exciton-nuclear dynamics and wavelet analysis. The real-time formation of a polaron pair can be observed in the dipole evolution during the excited-state dynamics. The possible driving force of the polaron pair formation is investigated through qualitative correlation between the structural dynamics and the dipole evolution. The time-dependent characteristics and spectroscopic consequences of the polaron pair formation are probed using the wavelet analysis.

The detailed knowledge of excited state proton transfer mechanisms in complex environments is of paramount importance in chemistry. However, the definition of an effective reaction coordinate and the understanding of the driving force of the reaction can be difficult from both the experimental and the theoretical points of view. Here we analyzed by theoretical approaches the mechanism and the driving forces of the excited state proton transfer reaction occurring between the 7-hydroxy-4-(trifluoromethyl)coumarin photoacid and the 1-methylimidazole base molecules in toluene solution. In particular, we compared the intrinsic and the dynamical reaction pathways, obtained by integrating the reaction coordinate, and by performing ab initio simulations of molecular dynamics, respectively. Time-dependent density functional theory and polarizable solvation continuum models were adopted to define the excited state potential energy surface. Results were analyzed by means of the D(CT) electronic density based index. Our findings suggest that the reaction coordinate is mainly composed of several intra- and intermolecular modes of the reactants. An analysis of both the intrinsic coordinate and the dynamical results shows that the charge transfer induced by electronic excitation of the coumarin molecule is the main proton transfer driving force. With regards to the methodological validation, the combination of ab initio molecular dynamics with time-dependent density functional theory appears to be feasible and reliable to study excited state proton transfer reactions in the condensed phase.

The mechanism of base to base intermolecular proton shuttling occurring in the excited state proton transfer reaction between 7-hydroxy-4-(trifluoromethyl)coumarin (CouOH) and concentrated 1-methylimidazole base (1-MeId) in toluene solution is disclosed here by means of a computational approach based on Density Functional Theory (DFT) and Time Dependent DFT (TD-DFT). These methods allow us to characterize both the ground and excited state potential energy surfaces along the proton shuttling coordinate, and to assess the nature of the emitting species in the presence of an excess of 1-MeId. As a result, the tautomerism of CouOH is found to be photo-activated and, from a mechanistic point of view, the calculations clearly show that the overall driving force of the entire shuttling is the coumarin photoacidity, which is responsible for both the first proton transfer event and the strengthening of the following chain mechanism of base to base proton hopping.

In-chain donor/acceptor block copolymers comprised of alternating electron rich/poor moieties are emerging as promising semiconducting chromophores for use in organic photovoltaic devices. The mobilities of charge carriers in these materials are experimentally probed using gated organic field-effect transistors to quantify electron and hole mobilities, but a mechanistic understanding of the relevant charge diffusion pathways is lacking. To elucidate the mechanisms of electron and hole transport following excitation to optically accessible low-lying valence states, we utilize mean-field quantum electronic dynamics in the TDDFT formalism to explicitly track the evolution of these photo-accessible states. From the orbital pathway traversed in the dynamics, p- and n-type conductivities can be distinguished. The electronic dynamics of the studied polymers show the time-resolved transitions between the initial photoexcited state, a tightly-bound excitonic state that is dark to the ground state, and a partially charge separated state indicated by long-lived, out-of-phase charge oscillations along the polymer backbone. The frequency of these charge oscillations yields an insight into the characteristic mobilities of charge carriers in these materials. When the barycenters of the electron and hole densities are followed during the dynamics, a pseudo-classical picture for the translation of charge carrier densities along the polymer backbone emerges that clarifies a crucial aspect in the design of efficient organic photovoltaic materials.

The excited state proton transfer (ESPT) reaction taking place between 7-hydroxy-4-(trifluorometyl)coumarin and 1-methylimidazole, recently experimentally characterized, has been here considered as a case study to illustrate the possibility of using theoretical approaches rooted in density functional theory (DFT) and time-dependent DFT (TD-DFT) for the description of complex reactions occurring at the excited state. In particular, beside identifying all stable species occurring at the ground and excited state during the ESPT reaction, a quantitative characterization of their photophysical properties, such as absorption and emission, is obtained by properly including solvent effects. More interestingly, a computational protocol enabling one to locate possible reaction pathways for the ESPT is here proposed. This protocol is based on the use of density based indices purposely developed to characterize the properties of vertical and relaxed excited states which allow one to discriminate the most favorable reaction paths on potential energy surfaces that are in the case of ESPT intrinsically very flat and difficult to characterize based on sole energy criteria, thus opening a new scenario for the description of photoinduced proton transfer reactions.

In this study, we analyze the photophysics of the N-methyl-6-oxyquinolinium betaine (MQ) solvatochromic dye in aqueous solution, focussing on the important structural rearrangement of its first solvation shell following the electronic excitation, which is characterized by a strong charge transfer. To this aim, we compare the results provided by ground- and excited-state ab-initio molecular dynamics with that of full QM calculations on clusters including a small number of solvent molecules (from 1 to 4), taking into account bulk solvent effects by the Polarizable Continuum Model. The two methods agree in predicting that while in the ground electronic state between three and four water molecules are strongly coordinated to the oxygen atom of MQ, in the excited state two water molecules are present in the first solvation layer of the MQ oxygen. Vertical excitation and emission energies computed on the structures provided by the two approaches allow for estimates of the Stokes shift consistent with the experimental results. On the ground of the present results, some general considerations on the advantages and limitations of the dynamical and static approaches in describing a photoactivated process in solution are proposed.

In spite of the detailed information provided by advanced time-resolved spectroscopy, the understanding of the excited-state proton transfer (ESPT) reactivity remains difficult to obtain at molecular level. In this work we studied three photoacids showing different strength: the 8-hydroxy-1,3,6-pyrenetrisulfonate weak photoacid, the N-methyl-6-hydroxyquinolinium strong photoacid and the phenol-carboxyether dipicolinium cyanine (QCy9) superphotoacid, focusing on the intermolecular ESPT toward a solvent molecule or a base molecule in aqueous solution. To this aim, the ground and the first singlet excited-state potential energy surfaces of the three systems were characterized by means of the time-dependent density functional theory and a hybrid implicit/explicit model of the solvent. Main structural and photophysical features of the photoacids were assessed and satisfactorily compared with the experimental data. Energy profiles along the PT coordinate were analyzed in both the electronic states. We reproduced many important features of the photoacidity experimentally observed. The results suggest that the relative strength is mainly due to the different extent of charge transfer caused by the electronic transition in proximity of the acid group. Remarkably, we found that even in the case of the strongest photoacid (QCy9), showing a ESPT rate as rapid as to escape the solvent dynamics control, the PT is modulated and supported by the first solvation shell of the proton-accepting molecule. However, a complete understanding of this fascinating field needs the full account for the electronic and the molecular dynamics in play at different timescales.

In this work, the GLOB model, an effective and reliable computational approach well suited for ab initio and QM/MM molecular dynamics simulations of complex molecular systems in solution, has been applied to study two representative open-shell systems, the cobalt(II) ion and the glycine radical in aqueous solution, with special reference to their structural and magnetic properties. The main structural features of the solvent cage around the cobalt ion and the hydrogen bonding patterns around the neutral and zwitterionic forms of the glycine radical have been investigated in some detail. The general good agreement with experiments supports the use of the present model to investigate more challenging and biological/technological relevant open-shell systems.

A recently developed extended Lagrangian model employing localized basis functions and nonperiodic boundary conditions (GLOB/ADMP) was applied to the radicals issuing from the homolytic breaking of the C(alpha)-H(alpha) bond of glycine in aqueous solution at different pH values. Although the modifications of the structure and the magnetic properties of these species induced by the solvent are qualitatively reproduced by a static discrete-continuum model, magnetic parameters are further tuned by short-time dynamical effects (solute vibrations and solvent librations). The results delivered by GLOB/ADMP simulations for both hyperfine tensors and g-tensors are in remarkable agreement with their experimental counterparts, allowing a reliable disentanglement of the overall observables into well-defined contributions. The dominant role of out-of-plane vibrations in determining hyperfine splittings is confirmed and quantified, together with the remarkable sensitivity of the gyromagnetic tensor to bond lengths and valence angles defining the NC(alpha)C' moiety. Together with their specific interest for the title radical, our results suggest some interesting trends for other biologically significant radicals and point out the need of extending magneto-structural relationships to dynamical aspects.

The excited state intramolecular proton transfer (ESIPT) reaction taking place within 2-(2-hydroxyphenyl) benzoxazole (HBT) and two recently experimentally characterised napthalimide derivatives – known as N-1 and N-4 – has been investigated in order to identify and test a possible protocol for the description and complete mechanistic and electronic characterisation of the reaction at the excited state. This protocol is based on density functional theory (DFT), time-dependent density functional theory (TD-DFT) and a recently proposed electron density based index (DCT). This method is able to identify all stable species involved in the reaction; discriminate between possible reaction pathways over potential energy surfaces (PES), which are intrinsically very flat and difficult to characterise, and quantitatively measure the excited state charge transfer character throughout the reaction. The photophysical properties of the molecules (i.e. absorption and emission wavelength) are also quantitatively determined via the implicit inclusion of solvent effects in the case of toluene and, more polar, tetrahydrofuran (THF). The accuracy obtained with this protocol then opens up the possibility of the ab initio design of molecules exhibiting ESIPT for tailored applications such as highly selective molecular sensors.

We individuate a photoinduced electron transfer (PeT) as a quenching mechanism affecting rhodamine B photophysics in solvent. The PeT involves an electron transfer from the carboxylate group to the xanthene ring of rhodamine B. This is finely modulated by the subtle balance of coulombic and non-classical interactions between the carboxyphenyl and xanthene rings, also mediated by the solvent. We propose the use of an electronic density based index, the so called DCT index, as a new tool to assess and quantify the nature of the excited states involved in non-radiative decays near the region of their intersection. In the present case, this analysis allows us to gain insight on the interconversion process from the bright state to the dark state responsible for the quenching of rhodamine B fluorescence. Our findings encourage the use of density based indices to study the processes affecting excited state reactions that are characterized by a drastic change in the excitation nature, in order to rationalize the photophysical behavior of complex molecular systems.

Most spectroscopic parameters are influenced by nuclear dynamics and by the chemical environment. However, proper inclusion of these effects still represents a challenge in computational spectroscopy studies. In many cases, a route coupling satisfactory accuracy with reasonable computational costs consists in the integration of DFT-based methods to compute spectroscopic parameters, with ab initio molecular dynamics simulations to sample from the classical phase space of the system. Here, we discuss the application of this approach in two case studies of remarkable practical interest, namely the simulation of the absorption spectrum of 9-methyladenine, an adenine nucleoside model; and the prediction of electron spin resonance parameters for nitroxyl radicals, the prototypical spin probes. In both cases, the accuracy of the results increases significantly when the subtle interplay of intra-molecular dynamics and solvent effects is introduced.

In this work, we present an effective and flexible computational approach, which is the result of an on-going development conducted in our group, for simulating complex solute–solvent systems and computing relevant spectroscopic observables. Such an approach is based on QM/MM molecular dynamics techniques using non-periodic conductor boundary conditions and localized basis sets, combined with a posteriori high-level quantum mechanical methods for the calculation of spectroscopic parameters. As illustrative applications, we report structural and spectroscopic analyses of acetone, acrolein and glycine radical in aqueous solutions, where solvent effects on the NMR chemical shifts, UV absorption spectrum and EPR hyperfine coupling constants, respectively, are investigated and favorably compared to experimental measurements. In particular, it will be shown the importance of including dynamical effects in order to reproduce experimental data accurately. Moreover, we present an infrared analysis of formamide in both gas phase and acetonitrile from first-principle molecular dynamics simulations.

We present a computational method, exploiting some features of the polarizable continuum model (PCM) to describe heterogeneous media; it belongs to the family of electrostatic embedding mixed methods, such as the more common quantum-mechanical (QM)/molecular mechanics approaches, with the electrostatic long range effects accounted for by a polarized continuum instead of atomic point charges. Provided effective dielectric constants are determined for the various parts of the system, the method is much faster than its atomistic counterpart, and allows for high-level QM calculations on the fragment of interest, using all the highly efficient computational tools developed for homogeneous PCM. Two case studies (the calculation of the pKa of solvent exposed acidic residues in a model protein, and the calculation of the electron spin resonance spectrum of a typical spin probe partially embedded in a membrane) are analyzed in some detail, to illustrate the application of the method to complex systems.

We describe the building blocks of an effective model employing localized basis functions and non-periodic boundary conditions for the treatment of solvent effects by ab initio molecular dynamics. After sketching the construction of a well sound hybrid potential in a first-principle dynamical framework, we validate the model for a diluted aqueous solution of chloride ion with special reference to the behaviour of different properties with the number of explicit water molecules. The reliability of the results and their stability well below 100 water molecules point out the interest of this new tool in the study of solvent effects.

The hyperfine parameters of a number of representative free radicals have been computed by post‐Hartree–Fock and density functional approaches including averaging effects from large amplitude vibrations and solvent effects through a recent implementation of the polarizable continuum model. Our results show that fully ab initio hyperfine splittings are accurate enough to back the interpretation of experimental data and to allow an unbiased judgement of the role played by electronic, vibrational, and environmental effects in determining the observed value. The very good results obtained by a density functional approach including some Hartree–Fock exchange both for intrinsic values and for solvent shifts pave the route for the investigation of large biologically significant radicals in their natural aqueous medium.