Electronic Excitations in Nonpolar Solvents: Can the Polarizable Continuum Model Accurately Reproduce Solvent Effects?

Solute - solvent repulsion effects on the absorption spectra of anthracene in n-hexane investigated under high pressure

The band positions in the UV-VIS absorption spectra of compressed solution of anthracene in n-hexane significantly depend not only on the dispersive but also on the repulsive solute-solvent interactions, what has so far been omitted. Their strength is determined not only by the solvent polarity but also by Onsager cavity radius changing with pressure. The results obtained for anthracene show that repulsive interactions should be included in the interpretation of barochromic and solvatochromic results of aromatic compounds. We show that the barochromic studies in the liquid solvent can be an alternative to solvatochromic studies, e.g. to determine the polarizability of organic molecules in the electronic excited state. The pressure-induced polarity change in n-hexane exceeds that induced by the exchange of n-alkane solvents between n-pentane and n-hexadecane.

Method to Compute the Solute-Solvent Dispersion Contribution to the Electronic Excitation Energy in Solution

A method formulated within the polarizable continuum model of the solvent and a quantum Monte Carlo treatment of the electronic states of the solute molecule is presented for the calculation of the solute–solvent dispersion contribution to the electronic excitation energy in solution. Variational quantum Monte Carlo is exploited to measure the fluctuations of the electronic electric field of the solute molecule to compute the London’s dispersion forces with the solvent. The method previously applied to the ground state of the solute is here extended to excited states. To perform the Casimir–Polder integration, we introduce a positive parameter Ω whose value is properly chosen for this purpose. We derive a general expression that for Ω = 0 reduces to that already proposed for the ground state. For an excited state, Ω must be less than the first transition electronic energy of the solvent molecule but greater than the transition energy from the ground to excited electronic state of the solute molecule. Benchmark calculations were performed on the n → π* transition for formaldehyde, acrolein, and acetone in six solvents, including water, ethanol, cyclohexane, chloroform, carbon tetrachloride, and toluene, and the π → π* transition of acrolein in cyclohexane. Solvents are characterized by their ionization potential and the refractive index at frequency Ω. In all cases, we found that the dispersion solute–solvent interaction stabilizes the excited state of the solutes leading to red (negative) solvatochromic shifts.

Improvement of theoretical UV-Vis spectra calculations by empirical solvatochromic parameters: Case study of 5-arylazo-3-cyano-1-ethyl-6-hydroxy-4-methyl-2-pyridones

In order to improve the performance of theoretical UV-Vis spectra predictions, a theoretical and experimental study of solvatochromic properties of ten azo pyridone dyes has been performed. For quantitative estimation of intermolecular solvent-solute interactions, a concept of the linear solvation energy relationships has been applied using Kamlet-Taft and Catalán models. Theoretical UV-Vis spectra for all dyes have been calculated using four TD-DFT methods in nine different solvents with the aim to define the most reliable model. Finally, new polylinear equations for more accurate theoretical prediction of UV-Vis maxima are developed using empirical Kamlet-Taft and Catalán solvent parameters as additive corrections for specific and nonspecific solvent-solute interactions.

Quantum Vibrational Spectroscopy of Explicitly Solvated Thymidine in Semiclassical Approximation

In this paper, we demonstrate the possibility to perform spectroscopy simulations of solvated biological species taking into consideration quantum effects and explicit solvation. We achieve this goal by interfacing our recently developed divide-and-conquer approach for semiclassical initial value representation molecular dynamics with the polarizable AMOEBABIO18 force field. The method is applied to the study of solvation of the thymidine nucleoside in two different polar solvents, water and N,N-dimethylformamide. Such systems are made of up to 2476 atoms. Experimental evidence concerning the different behavior of thymidine in the two solvents is well reproduced by our study, even though quantitative estimates are hampered by the limited accuracy of the classical force field employed. Overall, this study shows that semiclassically approximate quantum dynamical studies of explicitly solvated biological systems are both computationally affordable and insightful.

The key to the yellow-to-cyan tuning in the green fluorescent protein family is polarisation

Computational approaches have to date failed to fully capture the large (about 0.4 eV) excitation energy tuning displayed by the nearly identical anionic chromophore in different green fluorescent protein (GFP) variants. Here, we present a thorough comparative study of a set of proteins in this sub-family, including the most red- (phiYFP) and blue-shifted (mTFP0.7) ones. We employ a classical polarisable embedding through induced dipoles and combine it with time-dependent density functional theory and multireference perturbation theory in order to capture both state-specific induction contributions and the coupling of the polarisation of the protein to the chromophore transition density. The obtained results show that only upon inclusion of both these two effects generated by the mutual polarisation between the chromophore and the protein can the full spectral tuning be replicated. We finally discuss how this mutual polarisation affects the correlation between excitation energies, dipole moment variation, and molecular electrostatic field.

Predicting solvatochromic shifts and colours of a solvated organic dye: The example of nile red

The solvatochromic shift, as well as the change in colour of the simple organic dye nile red, is studied in two polar and two non-polar solvents in the context of large-scale time-dependent density-functional theory (TDDFT) calculations treating large parts of the solvent environment from first principles. We show that an explicit solvent representation is vital to resolve absorption peak shifts between nile red in n-hexane and toluene, as well as acetone and ethanol. The origin of the failure of implicit solvent models for these solvents is identified as being due to the strong solute-solvent interactions in form of π-stacking and hydrogen bonding in the case of toluene and ethanol. We furthermore demonstrate that the failures of the computationally inexpensive Perdew-Burke-Ernzerhof (PBE) functional in describing some features of the excited state potential energy surface of the S₁ state of nile red can be corrected for in a straightforward fashion, relying only on a small number of calculations making use of more sophisticated range-separated hybrid functionals. The resulting solvatochromic shifts and predicted colours are in excellent agreement with experiment, showing the computational approach outlined in this work to yield very robust predictions of optical properties of dyes in solution.

Electrostatic versus Resonance Interactions in Photoreceptor Proteins: The Case of Rhodopsin

Light sensing in photoreceptor proteins is subtly modulated by the multiple interactions between the chromophoric unit and its binding pocket. Many theoretical and experimental studies have tried to uncover the fundamental origin of these interactions but reached contradictory conclusions as to whether electrostatics, polarization, or intrinsically quantum effects prevail. Here, we select rhodopsin as a prototypical photoreceptor system to reveal the molecular mechanism underlying these interactions and regulating the spectral tuning. Combining a multireference perturbation method and density functional theory with a classical but atomistic and polarizable embedding scheme, we show that accounting for electrostatics only leads to a qualitatively wrong picture, while a responsive environment can successfully capture both the classical and quantum dominant effects. Several residues are found to tune the excitation by both differentially stabilizing ground and excited states and through nonclassical "inductive resonance" interactions. The results obtained with such a quantum-in-classical model are validated against both experimental data and fully quantum calculations.

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.

Solvent Induced Transformations of n-π* Absorption in Formaldehyde, Acetaldehyde, and Acetone

Absorption spectra of formaldehyde (FA), acetaldehyde (AA), and acetone are compared in the vapor phase, nonpolar, and polar solutions at 295 K. The vibronic n-π* transition of carbonyl chromophore is mainly composed of the overtones of >C═O stretching vibration. A new phenomenon is observed in liquid solutions, consisting of a relative increase of Franck-Condon factors for the second and third harmonics in FA, and the second to fourth replica in AA, with respect to the gas phase. In AA and acetone with poorly resolved vibronic structure, the redistribution of intensities produces a false "solvent shift" of the band maximum between the vapor and nonpolar liquid phase by -250 ± 50 cm(-1). Modification in vibronic coupling can also explain unusual narrowing of the band contour in the solution, reported earlier for acetone (Renge , I. J. Phys. Chem. A 2009, 113, 10678). No detectable shift occurs as a function of solvent polarizability (refractive index function (n(2) - 1)/(n(2) + 2)) in n-alkanes for FA, AA, and acetone, as well as for cyclopentanone and camphor. Incidentally, the bathochromic dispersive shift is almost exactly compensated by a hypsochromic induction shift. The latter is due to the diminishing dipole moment in the excited state of the carbonyl chromophore. Differences in polarizability α and dipole moments μ were estimated for FA (Δα = 0.33 ± 0.1 Å(3)), AA (Δμ = -1.05 ± 0.2 D, Δα = 0.5 ± 0.2 Å(3)), and acetone (Δμ = -1.3 ± 0.2 D, Δα = 0.65 ± 0.2 Å(3)). The increase of α by ∼10% upon excitation is plausible for a weak n-π* transition. By contrast, near doubling of α in the upper state has been reported recently for several ketones, with Δα reaching 10 Å(3) (Catalán, J.; Catalán, J. P. Phys. Chem. Chem. Phys. 2011, 13, 4072). Empirical partitioning of solvent shifts into repulsive-dispersive, induction, dipole-dipole, and hydrogen bonding contributions was proposed to serve as a benchmark in computer chemical calculations.