Excited Molecules In Solution Research Articles

A microscopy technique that images single reaction events in total darkness.

Single photons emitted from individual electrochemically excited molecules in solution can now be detected. The technique can be used to image cells at nanometre resolution, without using an external light source. Electrochemiluminescence enables super-resolution microscopy.

Improved RISM-CASSCF Optimization via State-Average Treatment and Damping for Characterizing Excited Molecules in Solution with Multireference Perturbation Theory.

Determination of excited states of near-infrared (NIR) bioimaging dyes is a challenging theoretical task because of their energy levels with a small gap and the presence of solvation. In the previous study, we showed that the development of the reference interaction site model coupled with the complete active space second-order perturbation theory, the RISM-CASPT2 method, and its extension with the density matrix renormalization group enabled high accuracy prediction of the photochemical properties of bioimaging-related fluorescent molecules in solution (Shimizu et al., J. Chem. Theory Comput. 2018, 14, 5673-5679). This method, however, has a technical issue in convergence of CASSCF optimization, which was encountered when applying the method to a wider variety of systems; thus, practical applications have been hindered. Here, we present an improved scheme of CASSCF optimization with and without the density matrix renormalization group treatment. Detailed derivations and analysis of the second-order orbital optimization scheme with the inclusion of solvation through RISM revealed the requirement of a correction term to the orbital Hessian matrix. As a practical approach, the state-average RISM-CASPT2 method with damping treatment for solvation is presented for improving the convergence of the calculation under reasonable computational cost. The improved scheme allows for performing accurate and numerically stable theoretical analysis of the bioimaging-related excited state with various types of solvation for a P═O-bridged rhodol derivative, which is recently highlighted as a promising photostable NIR dye molecule.

Observing Solvation Dynamics with Simultaneous Femtosecond X-ray Emission Spectroscopy and X-ray Scattering

In liquid phase chemistry dynamic solute-solvent interactions often govern the path, ultimate outcome, and efficiency of chemical reactions. These steps involve many-body movements on subpicosecond time scales and thus ultrafast structural tools capable of capturing both intramolecular electronic and structural changes, and local solvent structural changes are desired. We have studied the intra- and intermolecular dynamics of a model chromophore, aqueous [Fe(bpy)3](2+), with complementary X-ray tools in a single experiment exploiting intense XFEL radiation as a probe. We monitored the ultrafast structural rearrangement of the solute with X-ray emission spectroscopy, thus establishing time zero for the ensuing X-ray diffuse scattering analysis. The simultaneously recorded X-ray diffuse scattering patterns reveal slower subpicosecond dynamics triggered by the intramolecular structural dynamics of the photoexcited solute. By simultaneous combination of both methods only, we can extract new information about the solvation dynamic processes unfolding during the first picosecond (ps). The measured bulk solvent density increase of 0.2% indicates a dramatic change of the solvation shell around each photoexcited solute, confirming previous ab initio molecular dynamics simulations. Structural changes in the aqueous solvent associated with density and temperature changes occur with ∼1 ps time constants, characteristic for structural dynamics in water. This slower time scale of the solvent response allows us to directly observe the structure of the excited solute molecules well before the solvent contributions become dominant.

Electron-Deficient Pyrimidine Adopted in Porphyrin Sensitizers: A Theoretical Interpretation of π-Spacers Leading to Highly Efficient Photo-to-Electric Conversion Performances in Dye-Sensitized Solar Cells

A mass of porphyrin sensitizers have been designed and synthesized for dye-sensitized solar cells in previous works, and almost all of them incorporated an electron-rich system as the π-spacer. We here adopted the electron-deficient pyrimidine as an effective π-spacer and combined a cyanoacrylic acid anchoring group, as such a design yields a more bathochromic shift of the spectral absorption of the dye and results in an improved spectral overlap with the solar spectrum and an enhanced light-harvesting efficiency. The result does tally with the performance of sensitizer adsorbing on a semiconductor. From the electron density difference plots of electron transitions, we found that not all electron transitions could make for the effective electron transfer from donor to acceptor groups, which means the sensitizer performance in dye-sensitized solar cells not only relies on the extrinsic spectral absorption intensity but also depends on the intrinsic character of electron movement related to electron excitation. Moreover, the introduction of electron- deficient pyrimidine could affect the energy levels of excited molecules in solution, further affecting the kinds of electron transfer processes. We presented several novel porphyrin sensitizers for comparison on how the π-spacer and anchoring group influence the optical absorption, electron transfer processes, and regeneration of the oxidized dyes, thereby gaining potential dye-sensitized solar cells with highly efficient photo-to-electric conversion performances.

The entropic enlightenment of organic photochemistry: strategic modifications of intrinsic decay pathways using an information-based approach

Early photochemistry flourished with sunlight plus the experimental and intellectual infrastructure provided by the chemistry of organic compounds. Through the pioneering work of Giacomo Ciamician and Emanuele Paternò, it was shown that photochemical reactions give rise to products that are not accessible by thermal methods, and the green chemistry potential of organic photoreactions was already recognized at the time. Over the last century, the photochemical behavior of many chromophores and functional groups has been well documented in solution. From those studies, it has become clear that applications in organic synthesis suffer from complications arising from competing decay pathways that are intrinsic to those excited states. While there are few opportunities to control the outcome of excited molecules in solution, the potential of organic photochemistry under the influence of highly ordered structures can be appreciated with examples from photobiology. Knowing that nature can synthesize triglycerides with light, CO(2), H(2)O and a few thermal reactions, organic photochemistry should have a great potential and aim high. With that in mind, after exploring the modes of action used by living organisms to take advantage of sunlight, one can identify an approach that relies on entropic factors that result from changes in the information content of the reactant. Analogies with information theory suggest a strategy that may be used to manage chemical information to modify the intrinsic properties of chromophores. Extrapolating from recent examples, it is suggested that an information-based approach to organic photochemistry may result in important advances not only in chemical synthesis and green chemistry, but also in many other applications.

Solvent Effect on the Absorption and Fluorescence Emission Spectra of Some Purine Derivatives: Spectrofluorometric Quantitative Studies

The absorption and emission spectra of six purine derivatives: adenine (I), N(9)-hydroxyethyladenine (II), N(6)-acetyladenine (III), N(6)-isobutyryladenine (IV), guanine (V), and N(2),N(9)-diacetylguanine (VI) have been investigated. The effects of solvent and pH on the positions of λ max (absorption) and λ max (emission) of these compounds were determined. Correlations between the absorption wavelength (λ max ) of these organic compounds and the solvent parameters (D,n,E) or (K,M,N) show that the peak position is affected mainly by specific- and non-specific types of interactions between the solvent and solute. Solvent effects on the electronic absorption band shifts are indicative of the extent of charge reorganization of the solute molecules upon electronic excitation. The Stokes shift (ν abs−ν em) was correlated with the orientation polarizability (Δf) and was found to depend mainly on the dielectric constant and the refractive index of the solvents. This shift reflects the influence of the equilibrium solvent arrangement around the excited solute molecule, which rearranges inertially due to the instantaneous charge redistribution upon radiative deactivation to the electronic ground state. A spectrofluorometric analysis technique was applied for the quantitative analysis of the components of a ternary mixture of compounds (I–III).

Theoretical Studies of Solute Vibrational Energy Relaxation at Liquid Interfaces

Recent advances in the theoretical understanding of solute vibrational energy relaxation at liquid interfaces and surfaces are described. Non-equilibrium molecular dynamics simulations of the relaxation of an initially excited solute molecule are combined with equilibrium force autocorrelation calculations to gain insight into the factors that influence the vibrational relaxation rate. Diatomic and triatomic nonpolar, polar, and ionic solute molecules adsorbed at the liquid/vapor interface of several liquids as well as at the water/CCl(4) liquid/liquid interface are considered. In general, the vibrational relaxation rate is significantly slower (a factor of 3 to 4) at the liquid/vapor and liquid/liquid interface than in the bulk due to the reduced density, which gives rise to a reduced contribution of the repulsive solvent-solute forces on the vibrational mode. The surface effects on the ionic solutes are much smaller (50% or less slower relaxation relative to the bulk). This is due to the fact that ionic solutes at the interface are able to keep part of their solvation shell to a degree that depends on their size. Thus, a significant portion of the repulsive forces is maintained. A high degree of correlation is found between the peak height of the solvent-solute radial distribution function and the vibrational relaxation rate. The relaxation rate at the liquid/liquid interface strongly depends on the location of the solute across the interface and correlates with the change in the density and polarity profile of the interface.

Fast Evaluation of Geometries and Properties of Excited Molecules in Solution: A Tamm-Dancoff Model with Application to 4-Dimethylaminobenzonitrile

We present a method to evaluate ab initio energy, wave function, and gradient of a solvated molecule in an electronically excited state. In particular, this paper extends the Polarizable Continuum Model (PCM) to a specific level of theory for the interpretation of the electronic spectra: configuration interaction (CI) among all singly substituted determinants using a Hartree−Fock reference state. This method, in very wide use, allows investigations of both structures and properties of electronically excited molecules through the evaluation of analytical energy derivatives with respect to various parameters. The most relevant formal aspects on the extension of the theory to solvated systems are discussed, and numerical applications to the study of geometries and one-electron properties of the low-lying excited states of 4-dimethylamino benzonitrile (DMABN) in acetonitrile solution are presented.

Solvent-induced dynamic polarization and vibrational dephasing of electronically excited molecules

In order to account for the extra vibrational Raman bandwidths observed specifically for electronically excited molecules in solution, a new model, the dynamic polarization model, has been introduced. In this model, the vibrational frequency of a highly polarizable electronically excited molecule in solution is stochastically modulated by the solvent-induced polarization. An olefinic C = C double bond is considered as the simplest prototype, for which the carbon-carbon stretch frequency can take values between the unpolarized (C = C) and the completely polarized (C+-C-) frequencies. The two-frequency exchange model is first introduced, followed by more general models. A focus is placed on the asymmetric exchange limit in which the solvent-induced polarization can be treated as a perturbation, so that the effect of the dynamic polarization manifests as a shift of the peak position and an increase in the bandwidth of the original unperturbed C = C stretch vibrational band. The experimental results on S1 tra...

Solvent-induced dynamic polarization and vibrational dephasing of electronically excited molecules

In order to account for the extra vibrational Raman bandwidths observed specifically for electronically excited molecules in solution, a new model, the dynamic polarization model, has been introduced. In this model, the vibrational frequency of a highly polarizable electronically excited molecule in solution is stochastically modulated by the solvent-induced polarization. An olefinic C = C double bond is considered as the simplest prototype, for which the carbon-carbon stretch frequency can take values between the unpolarized (C = C) and the completely polarized (C+-C-) frequencies. The two-frequency exchange model is first introduced, followed by more general models. A focus is placed on the asymmetric exchange limit in which the solvent-induced polarization can be treated as a perturbation, so that the effect of the dynamic polarization manifests as a shift of the peak position and an increase in the bandwidth of the original unperturbed C = C stretch vibrational band. The experimental results on S1 trans-stilbene are discussed, based on the formulae derived.

Non-exponential solvation dynamics of electronically excited 4-aminophthalimide in n-alcohols

Abstract A picosecond study of the non-exponential solvation dynamics of 4-aminophthalimide has been performed in a series of n -alcohols and some polar non-protic solvents. The results suggest single-exponential solvation dynamics in tetrahydrofuran and non-exponential in n -alcohols. The short component observed in n -alcohols is shorter than the longitudinal dielectric relaxation time of the solvent, but the average solvation time is equal to it. The short solvation time is assigned to solvent restructuring in the proximity of the excited solute molecule. Solvent restructuring should be associated with the internal rotations, like hydroxyl rotations, of solvent molecules rather than solvent reorientation. The long component of the correlation function may have its origin in the solute rotational reorientation. The role of the adiabatic intramolecular proton transfer reaction in the solvation dynamics of 4-aminophthalimide is discussed.

Pioneering and Developing Exciplex Chemistry. Part 1. Nonequilibrium Solvation of Excited Dipolar Solute, Its Relaxation Causing Fluorescence Stokes Shifts, and Mechanisms of Photoinduced Electron Transfer between Conjugate .PI.-Electronic Systems Directly Combined by Hydrogen Bond.

Nonequilibrium solvation of excited solute molecules and its relaxation which play roles of crucial importance in the exciplex chemistry are discussed in relation to the mechanism of the fluorescence Stokes shift in polar solution originally given by the Lippert-Mataga equation and also in relation to the classical electron transfer (ET) theory by Marcus. Mechanisms of photoinduced ET which have been elucidated by femtosecond-picosecond laser photolysis studies on the hydrogen bonding (HB) systems in a nonpolar solvent are discussed as an extremely interesting example of ET induced by a specific coupling with a slight proton shift in HB.

Solute–solvent interaction in the photoinduced tautomerization of 7-azaindole in various alcohols and in mixtures of cyclohexane and ethanol

The evolution of the fluorescence of 7-azaindole in several alcohols, in mixtures of cyclohexane and etheanol and in mixtures of 1,4-dioxane and hexafluoropropanol has been investigated by excitation with a 25 ps (u.v.) laser pulse, followed by detection with a streak camera. We show that adiabatic keto–enol tautomerization of the compound in its lowest electronically excited singlet state in these solvents proceeds via double proton transfer in a well defined complex of a single alcohol molecule and the excited solute molecule. Unlike previous authors, we conclude that viscosity and solvent aggregation are not the important factors determining the rate of intramolecular adiabatic excited-state double proton transfer in this compound. Within the series of alcohols the variation of the rate constant for this process can be attributed satisfactorily to the variation in the acidity of the alcohols. A temperature-independent kinetic isotope effect has been observed for the complexes of 7-azaindole with ethanol (OH) and ethanol(OD). Our previous explanation for such kinetic behaviour also applies here. It involves thermally activated isotope independent formation of a specific (cyclic doubly hydrogen bounded) solute–solvent complex from an (open singly hydrogen bonded) precursor complex, followed by thermally unassisted proton transfer in the specific complex.

Solvent and substituent effects on intramolecular charge transfer of selected derivatives of 4-trifluoromethyl-7-aminocoumarin

A comparative study of the fluorescence of selected 4-trifluoromethyl-7-aminocoumarin derivatives has been made in a variety of protic and aprotic solvents. It was found that the fluorescence-decay kinetics were dependent on both the substitution pattern of the aminocoumarins and properties of the solvents used. These dependences are, in general, reminiscent of the corresponding 4-methyl homologues reported previously, but an intriguing difference is found for compounds possessing an unrestricted amine group in polar aprotic solvents. Furthermore, there is a ‘dielectric threshold’ for the solvent-dependence of fluorescence lifetime of non-rigid 4-trifluoromethyl-7-dialkylaminocoumarins, above which their solvent response is markedly reduced. These findings are discussed in connection with the factors affecting formation of a twisted intramolecular charge-transfer (TICT) state and lead to an hypothesis that hydrogen bonding between solvent and excited solute molecule seems not to be a determining factor for TICT state formation in the case of aminocoumarins.

Influence of excitation and solvation on the shift of tautomeric equilibrium. A quantum-mechanical study

Two quantum-mechanical models are proposed to described a shift of tautomeric equilibrium as a result of electronic excitation and change of environment. According to the first n PD MEP model which is used to estimate the relative solvation effect on the stability of tautomers in an excited state, the calculation of the interaction energy between a solvent (simulated by a set of n point dipoles, n PD) and an excited solute molecule is based on the molecular electrostatic potential (MEP) of the corresponding excited state. In the second n PDQ model, a solvent represented by a set of n point dipoles and quadrupoles ( n PDQ) modifies the solute's hamiltonian via an electrostatic interaction contribution. Comparing the results of the calculation for isolated and solvated tautomers, the n PDQ model is used to estimate the influence of electronic excitation on the change of relative stability of tautomers existing in a solution. An application of both models to 2- and 4-oxopyridine predicts a shift of the tautomeric equilibria in their excited states in accordance with experimental evidence.

Identity of intermediates formed on photolysis of CH 2I 2 and CHI 3 in 3-methylpentane at 77 K

Transient intermediates formed on photolysis of CH 2I 2 and CHI 3 in 3MP glass at 77 K have been characterized from their optical absorption and stable products analysis. Variation of stable products yields and intensity of absorption bands in presence of benzene, which is known to lessen solute clustering by molecular complex formation, has indirectly indicated solute aggregation at 77 K and formation of solute cations by energy transfer from excited solute molecules.

Vibrational relaxtion in excited electronic states of aromatic hydrocarbons

Time-resolved experiments are reported Showing kinetic evidence for vibrational relaxation of electronically excited molecules in solution at room temperature. The experiments involve higher electronic states of 3,4,9,10-dibenzpyrene. Data are consistent with slow vibrational relaxation (≈ 15 ps), similar to that for ground state species.

Vibrational relaxation of small molecules in the liquid phase: Liquid nitrogen doped with CO2, CD4, or N2O

The vibrational relaxation times of liquid nitrogen doped with CO2, CD4, or N2O at 77 K were measured. Stimulated Raman scattering was used to excite N2 into the ν=1 level and its concentration as a function of time was monitored by spontaneous anti-Stokes Raman scattering. For these solutions rapid V–V energy transfer occurs between N2 (ν=1) and the near resonant vibrational levels of the solute molecules. On a longer time scale is the relaxation of the vibrationally excited solute molecule by V–T, R energy transfer. Comparison of the nitrogen solution data for V–T,R relaxation of CO2 by N2 and N2O self-relaxation with gas phase results is consistent with the suggestion that isolated binary collisions are responsible for vibrational relaxation in the liquid state. Therefore, for these systems as far as vibrational relaxation processes are concerned the liquid may be treated simply as a high density gas. No direct comparisons between liquid state and gas phase measurements were possible for V–T,R relaxation of vibrationally excited N2O or CD4 by N2. However, extrapolations from available data suggest that there may be a change in the vibrational relaxation mechanisms for these and other systems when the phase is changed from gas to liquid.

Dynamic Behaviors of Excited Molecules in Solution Studied on Laser Spectroscopy

Dynamic Behaviors of Excited Molecules in Solution Studied on Laser Spectroscopy

Dynamic Behaviours of Excited Molecules in Solution Studied on Laser Spectroscopy (2)

Dynamic Behaviours of Excited Molecules in Solution Studied on Laser Spectroscopy (2)