Carles Curutchet

Beyond Förster Resonance Energy Transfer in Biological and Nanoscale Systems

After photoexcitation, energy absorbed by a molecule can be transferred efficiently over a distance of up to several tens of angstroms to another molecule by the process of resonance energy transfer, RET (also commonly known as electronic energy transfer, EET). Examples of where RET is observed include natural and artificial antennae for the capture and energy conversion of light, amplification of fluorescence-based sensors, optimization of organic light-emitting diodes, and the measurement of structure in biological systems (FRET). Forster theory has proven to be very successful at estimating the rate of RET in many donor-acceptor systems, but it has also been of interest to discover when this theory does not work. By identifying these cases, researchers have been able to obtain, sometimes surprising, insights into excited-state dynamics in complex systems. In this article, we consider various ways that electronic energy transfer is promoted by mechanisms beyond those explicitly considered in Forster RET theory. First, we recount the important situations when the electronic coupling is not accurately calculated by the dipole-dipole approximation. Second, we examine the related problem of how to describe solvent screening when the dipole approximation fails. Third, there are situations where we need to be careful about the separability of electronic coupling and spectral overlap factors. For example, when the donors and/or acceptors are molecular aggregates rather than individual molecules, then RET occurs between molecular exciton states and we must invoke generalized Forster theory (GFT). In even more complicated cases, involving the intermediate regime of electronic energy transfer, we should consider carefully nonequilibrium processes and coherences and how bath modes can be shared. Lastly, we discuss how information is obscured by various forms of energetic disorder in ensemble measurements and we outline how single molecule experiments continue to be important in these instances.

Photosynthetic Light-Harvesting Is Tuned by the Heterogeneous Polarizable Environment of the Protein

In photosynthesis, special antenna proteins that contain multiple light-absorbing molecules (chromophores) are able to capture sunlight and transfer the excitation energy to reaction centers with almost 100% quantum efficiencies. The critical role of the protein scaffold in holding the appropriate arrangement of the chromophores is well established and can be intuitively understood given the need to keep optimal dipole-dipole interactions between the energy-transferring chromophores, as described by Förster theory more than 60 years ago. However, the question whether the protein structure can also play an active role by tuning such dipole-dipole interactions has not been answered so far, its effect being rather crudely described by simple screening factors related to the refractive index properties of the system. Here, we present a combined quantum chemical/molecular mechanical approach to compute electronic couplings that accounts for the heterogeneous dielectric nature of the protein-solvent environment in atomic detail. We apply the method to study the effect of dielectric heterogeneity in the energy migration properties of the PE545 principal light-harvesting antenna of the cryptomonad Rhodomonas CS24. We find that dielectric heterogeneity can profoundly tune by a factor up to ∼4 the energy migration rates between chromophore sites compared to the average continuum dielectric view that has historically been assumed. Our results indicate that engineering of the local dielectric environment can potentially be used to optimize artificial light-harvesting antenna systems.

Solvation in octanol: parametrization of the continuum MST model

This study reports the parametrization of the HF/6‐31G(d) version of the MST continuum model for n‐octanol. Following our previous studies related to the MST parametrization for water, chloroform, and carbon tetrachloride, a detailed exploration of the definition of the solute/solvent interface has been performed. To this end, we have exploited the results obtained from free energy calculations coupled to Monte Carlo simulations, and those derived from the QM/MM analysis of solvent‐induced dipoles for selected solutes. The atomic hardness parameters have been determined by fitting to the experimental free energies of solvation in octanol. The final MST model is able to reproduce the experimental free energy of solvation for 62 compounds and the octanol/water partition coefficient (log Pow) for 75 compounds with a root‐mean‐square deviation of 0.6 kcal/mol and 0.4 (in units of log P), respectively. The model has been further verified by calculating the octanol/water partition coefficient for a set of 27 drugs, which were not considered in the parametrization set. A good agreement is found between predicted and experimental values of log Po/w, as noted in a root‐mean‐square deviation of 0.75 units of log P.

Quantum Chemical Studies of Light Harvesting

The design of optimal light-harvesting (supra)molecular systems and materials is one of the most challenging frontiers of science. Theoretical methods and computational models play a fundamental role in this difficult task, as they allow the establishment of structural blueprints inspired by natural photosynthetic organisms that can be applied to the design of novel artificial light-harvesting devices. Among theoretical strategies, the application of quantum chemical tools represents an important reality that has already reached an evident degree of maturity, although it still has to show its real potentials. This Review presents an overview of the state of the art of this strategy, showing the actual fields of applicability but also indicating its current limitations, which need to be solved in future developments.

A Subsystem TDDFT Approach for Solvent Screening Effects on Excitation Energy Transfer Couplings.

We present a QM/QM approach for the calculation of solvent screening effects on excitation-energy transfer (EET) couplings. The method employs a subsystem time-dependent density-functional theory formalism [J. Chem. Phys. 2007, 126, 134116] and explicitly includes solvent excited states to account for the environmental response. It is investigated how the efficiency of these calculations can be enhanced in order to treat systems with very large solvation shells while fully including the environmental response. In particular, we introduce a criterion to select solvent excited states according to their approximate contribution weight to the environmental polarization. As a model system, we investigate the perylene diimide dimer in a water cluster in comparison to a recent polarizable QM/MM method for EET couplings in the condensed phase [J. Chem. Theory Comput. 2009, 5, 1838]. A good overall agreement in the description of the solvent screening is found. Deviations can be observed for the effect of the closest water molecules, whereas the screening introduced by outer solvation shells is very similar in both methods. Our results can thus be helpful to determine at which distance from a chromophore environmental response effects may safely be approximated by classical models.

Electrostatic component of solvation: Comparison of SCRF continuum models

We report a systematic comparison of the electrostatic contributions to the free energy of solvation from three different kinds of quantum mechanical self‐consistent reaction field (SCRF) methods. We also compare the liquid‐phase dipole moments as a measure of the solutes response to the reaction field of the solvent. In particular, we compare these quantities for the generalized Born model as implemented in the SM5.42R method, the multipolar expansion model developed at Nancy, and the MST version of the polarizable continuum model. All calculations are carried out at the HF/6‐31G(d) level. The effects of various choices of solute cavities and representations of the charge density are examined. The test set consists of 18 molecules containing prototypical polar groups, and three different values of the dielectric permittivity are considered.

Toward a Unified Modeling of Environment and Bridge-Mediated Contributions to Electronic Energy Transfer: A Fully Polarizable QM/MM/PCM Approach

Recent studies have unveiled the similar nature of solvent (screening) effects and bridge-mediated contributions to electronic energy transfer, both related to the bridge/solvent polarizability properties. Here, we exploit the similarity of such contributions to develop a fully polarizable mixed QM/discrete/continuum model aimed at studying electronic energy transfer processes in supramolecular systems. In the model, the definition of the three regions is completely flexible and allows us to explore the possibility to describe bridge-mediated contributions by using a polarizable MM description of the linker. In addition, we show that the classical MMPol description of the bridge can be complemented either with an analogous atomistic or with a continuum description of the solvent. Advantages and drawbacks of the model are finally presented and discussed with respect to the system under study.

Continuum solvation models: Dissecting the free energy of solvation

The most usual self-consistent reaction field (SCRF) continuum models for the description of solvation within the quantum mechanical (QM) framework are reviewed, trying to emphasize their common roots as well as the inherent approximations assumed in the calculation of the free energy of solvation. Particular attention is also paid to the specific features involved in the development of current state-of-the-art QM SCRF continuum models. This is used to discuss the need to maintain a close correspondence between each SCRF formalism and the specific details entailing its parametrization, as well as the need to be cautious in analyzing the balance between electrostatic and non-electrostatic contributions to the solvation free energy between different SCRF models. Finally, special emphasis is given to the post-processing of the free energy of solvation to derive parameters providing a compact picture of the ability of a molecule to interact with different solvents, which can be of particular interest in biopharmaceutical studies.

The role of the environment in electronic energy transfer: a molecular modeling perspective

The key role of the environment in electronic energy transfer has been underscored in recent experimental and theoretical studies. In this perspective, we provide an overview of novel quantum-mechanical methodologies aimed at describing environment effects in energy transfers. The techniques described include continuum dielectric and atomistic descriptions of the surroundings. We discuss the advantages and limitations of each technique, as well as the main insights that have emerged from their application to solvated dyads and photosynthetic pigment-protein complexes. We finally highlight the aspects that still need to be solved in order to provide a full theoretical route to the study of energy transfer phenomena in complex environments.

Excitation Dynamics in Phycoerythrin 545: Modeling of Steady-State Spectra and Transient Absorption with Modified Redfield Theory

We model the spectra and excitation dynamics in the phycobiliprotein antenna complex PE545 isolated from the unicellular photosynthetic cryptophyte algae Rhodomonas CS24. The excitonic couplings between the eight bilins are calculated using the CIS/6-31G method. The site energies are extracted from a simultaneous fit of the absorption, circular dichroism, fluorescence, and excitation anisotropy spectra together with the transient absorption kinetics using the modified Redfield approach. Quantitative fit of the data enables us to assign the eight exciton components of the spectra and build up the energy transfer picture including pathways and timescales of energy relaxation, thus allowing a visualization of excitation dynamics within the complex.