1,720,991 research outputs found
“Computational studies of environmental effects and their interplay with experiment”
No full textNowadays, computational techniques have become useful interpretative and predictive tools to investigate environmental effects on properties and processes in supramolecular systems of increasing complexity. The purpose of this chapter is to show the capabilities of such techniques, focussing particularly on the simulation of spectroscopic properties, since they allow a direct comparison between calculated and experimental data. Moreover, the computation of the spectroscopic response permits an analysis of the relationship between the nuclear and electronic structure of the molecular probes and the interactions with the environment. These ideas are illustrated with case studies involving different spectroscopic techniques and various molecular and environmental systems
Excitation Energy Transfer in Donor-Bridge-Acceptor Systems: A Combined Quantum-Mechanical/Classical Analysis of the Role of the Bridge and the Solvent
No full textDissecting the Nature of Exciton Interactions in Ethyne-Linked Tetraarylporphyrin Arrays
No full textWe investigate how electronic energy transfer in a series of three ethyne- linked zinc- and free base tetraarylporphyrin dimers is tuned by the type of linker and by substitution on the porphyrin rings. We use time-dependent density functional theory (TD-DFT) combined with a recently developed fully polarizable QM/MM/PCM method. This allows us to dissect the bridge-mediated contributions to energy transfer in terms of superexchange (through-bond) interactions and Coulomb (through-space) terms mediated by the polarizability of the bridge. We explore the effects of the substituents and of the bridge-chromophore mutual orientation on these contributions. We find that bridge-mediated superexchange contributions largely boost energy transfer between the porphyrin units. When the effect of the solvent is also considered through the polarizable continuum model (PCM), we find good agreement with the through-bond versus through-space contributions determined experimentally, thus indicating the need to properly include both solvent and bridge effects in the study of energy transfer in bridged molecular dyads
Low energy electron collisions with small molecular clusters
No full textWe have developed a Multiple Scattering technique to treat elastic electron collisions with small molecular clusters. The method produces cross sections in good agreement with more accurate ab initio results for several geometries of the water dimer
Quantum mechanical study of the solvent-dependence of electronic energy transfer rates in a Bodipy closely-spaced dyad
No full textTheabilityofFo ̈rstertheorytodescribeelectronicenergytransferrates,andtheirsolvent-dependence, have been studied theoretically in a series of 15 solvents of varying degrees of polarity for a rigid closely-spaced dyad, constituted by two boron dipyrromethene dyes, which was recently studied experimentally by Harriman & Ziessel, Photochem. Photobiol. Sci., 2010, 9, 960. We use time-dependent density functional theory calculations coupled to the polarizable continuum model to analyse the solvent-dependence of the spectroscopic and energy transfer properties of the system. This methodology allows us to examine the impact of the solvent on both electronic (solvent screening) and structural (dipole separation and orientation) factors by consistently incorporating solvent effects in the determination of molecular geometries, transition densities, and electronic couplings. In addition, we analyse the impact of bridge-mediated contributions to the electronic interaction between the dyes. We are therefore able to assess whether a Fo ̈ rster-type point-dipole approximation is valid for the molecular system studied
Shaping excitons in light-harvesting proteins through nanoplasmonics
No full textNanoplasmonics has been used to enhance molecular spectroscopic signals, with exquisite spatial resolution down to the sub-molecular scale. By means of a rigorous, state-of-the-art multiscale model based on a quantum chemical description, here we show that optimally tuned tip-shaped metal nanoparticles can selectively excite localized regions of typically coherent systems, eventually narrowing down to probing one single pigment. The well-known major light-harvesting complex LH2 of purple bacteria has been investigated because of its unique properties, as it presents both high and weak delocalization among subclusters of pigments. This finding opens the way to the direct spectroscopic investigation of quantum-based processes, such as the quantum diffusion of the excitation among the chromophores, and their external manipulation
A Multiple-Scattering Approach to Electron Collisions with Small Molecular Clusters
No full textWe present a method based on multiple-scattering to determine elastic cross sections for electron collisions with molecular clusters. The method is based on the calculation of accurate collisional information for the molecules constituting the cluster that is then combined to obtain a cross section for interaction with the whole system. Themethod provides a computationally cost-effectiveway of treating low energy electron scattering from (homogeneous and heterogeneous) molecular clusters and aggregates.Results for (H2O)n (n D 2,5) and (HCOOH)2 are presented; the cross sections agree well with more accurate ab initio data
Control of Coherences and Optical Responses of Pigment-Protein Complexes by Plasmonic Nanoantennae
No full textThe key for light-harvesting in pigment-protein complexes are molecular excitons, delocalized excited states comprising a superposition of excitations at different molecular sites. There is experimental evidence that the optical response due to such excitons can be largely affected by plasmonic nanoantennae. Here we employ a multiscale approach combining time-dependent density functional theory and polarizable classical models to study the optical behavior of the LH2 complex present in bacteria when interacting with a gold nanorod. The simulation not only reproduces the experiments but also explains their molecular origin. By tuning the chromophoric unit and selectively switching on/off the excitonic interactions, as well as by exploring different setups, we clearly show that the dramatic enhancement in the optical response, unexpectedly, is not accompanied by changes in the coherences. Instead polarization effects are dominant. These results can be used to design an optimal control of the light-harvesting process through plasmonic nanoantennae
Geometry Optimization in Polarizable QM/MM Models: The Induced Dipole Formulation
No full textWe present the mathematical derivation and the computational implementation of the analytical geometry derivatives for a polarizable QM/MM model (QM/MMPol). In the adopted QM/MMPol model, the focused part is treated at QM level of theory, while the remaining part (the environment) is described classically as a set of fixed charges and induced dipoles. The implementation is performed within the ONIOM procedure, resulting in a polarizable embedding scheme, which can be applied to solvated and embedded systems and combined with different polarizable force fields available in the literature. Two test cases characterized by strong hydrogen-bond and dipole−dipole interactions, respectively, are used to validate the method with respect to the nonpolarizable one. Finally, an application to geometry optimization of the chromophore of Rhodopsin is presented to investigate the impact of including mutual polarization between the QM and the classical parts in conjugated systems
Theoretical Investigation of the Mechanism and Dynamics of Intramolecular Coherent Resonance Energy Transfer in Soft Molecules: A Case Study of Dithia-anthracenophane
No full textA computational study is conducted on dithia-anthracenophane (DTA), for which there is experimental evidence for coherent resonance energy transfer dynamics, and on dimethylanthracene (DMA), a molecule representing the energy donor and the acceptor in DTA. Electronic excitation energies are
calculated by configuration interaction singles (CIS) and time-dependent density functional theory (TDDFT)
methods and are compared to experimental ones. Electronic coupling constants are calculated between
two DMAs embedded into the ground-state structure of DTA employing methods based on transition densities. The resulting values of electronic coupling provide a more consistent interpretation of experiments than those based on one-half the level spacing of DTA excitation energies. Solvation effects are studied based on the polarizable continuum model (PCM). Solvent-induced polarization and screening effects are shown to make opposite contributions, and the net electronic coupling is little different from the value in a vacuum. The likelihood of coherent population transfer is assessed on the basis of a recently developed theory of coherent resonance energy transfer. The time scale of bath is shown to have an important role in sustaining the quantum coherence. The combination of quantum chemical and dynamical data suggests that the electronic coupling in DTA is in the range of 50-100 cm-1. The presence of oscillatory excitation population dynamics can be understood from the picture of polaronic excitation moderately dressed with dispersive vibrational modes. The effect of torsional modulation on the excitation energies of DTA and electronic coupling is examined on the basis of optimized structures with the torsional angle constrained. The result suggests that inelastic effect due to torsional motion cannot be disregarded in DTA
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