Solvent, structureless

Molecular Dynamics (MD) is the most fundamental approach to soft-matter simulations. Here the solute particles are immersed in a bath of solvent molecules and Newton s equations of motion are solved numerically. In this case, it is impossible to make the solvent structureless - a structureless solvent would be an ideal gas of point particles, which never reaches thermal equilibrium. Furthermore, the model interaction potentials are stiff and considerable simulation time is spent following the motion of the solvent particles in their local cages. These disadvantages are so severe that nowadays MD is rarely applied to soft-matter systems of the type we are discussing in this article. 

In 1982 Wei et al. [78,79] studied the quenching of N,N-dimethyltoiuidine (DMT) fluorescence by adding the electron-accepting monomer MA or MMA and successfully observed broad and structureless exciplex fluorescences at longer wavelengths in nonpolar solvents for the first time. 

In Eq. (6) Ecav represents the energy necessary to create a cavity in the solvent continuum. Eel and Eydw depict the electrostatic and van-der-Waals interactions between solute and the solvent after the solute is brought into the cavity, respectively. The van-der-Waals interactions divide themselves into dispersion and repulsion interactions (Ed sp, Erep). Specific interactions between solute and solvent such as H-bridges and association can only be considered by additional assumptions because the solvent is characterized as a structureless and polarizable medium by macroscopic constants such as dielectric constant, surface tension and volume extension coefficient. The use of macroscopic physical constants in microscopic processes in progress is an approximation. Additional approximations are inherent to the continuum models since the choice of shape and size of the cavity is arbitrary. Entropic effects are considered neither in the continuum models nor in the supermolecule approximation. Despite these numerous approximations, continuum models were developed which produce suitabel estimations of solvation energies and effects (see Refs. 10-30 in 68)). 

The potential energy function prohibits double occupancy of any site on the 2nnd lattice. In the initial formulation, which was designed for the simulation of infinitely dilute chains in a structureless medium that behaves as a solvent, the remaining part of the potential energy function contains a finite repulsion for sites that are one lattice unit apart, and a finite attraction for sites that are two lattice units apart [153]. The finite interaction energies for these two types of sites were obtained by generalizing the lattice formulation of the second virial coefficient, B2, described by Post and Zimm as [159] ... 

A rigorous solution of this problem was attempted, for example, in the hard sphere approximation by D. Henderson, L. Blum, and others. Here the discussion will be limited to the classical Gouy-Chapman theory, describing conditions between the bulk of the solution and the outer Helmholtz plane and considering the ions as point charges and the solvent as a structureless dielectric of permittivity e. The inner electrical potential 0(1) of the bulk of the solution will be taken as zero and the potential in the outer Helmholtz plane will be denoted as 02. The space charge in the diffuse layer is given by the Poisson equation... 

Exciplexes are complexes of the excited fluorophore molecule (which can be electron donor or acceptor) with the solvent molecule. Like many bimolecular processes, the formation of excimers and exciplexes are diffusion controlled processes. The fluorescence of these complexes is detected at relatively high concentrations of excited species, so a sufficient number of contacts should occur during the excited state lifetime and, hence, the characteristics of the dual emission depend strongly on the temperature and viscosity of solvents. A well-known example of exciplex is an excited state complex of anthracene and /V,/V-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene. Molecules of anthracene in toluene fluoresce at 400 nm with contour having vibronic structure. An addition to the same solution of diethylaniline reveals quenching of anthracene accompanied by appearance of a broad, structureless fluorescence band of the exciplex near 500 nm (Fig. 2 )... 

Anthracene has also been used as an acceptor (Fig. 10). In solution, 26 emits a single fluorescence band that is somewhat structured in nonpolar solvents and becomes broad and structureless with increasing polarity [58]. The strongly hindered molecule 27 also exhibits a similar behavior, but its absorption spectrum is better structured [59]. The rate of formation of a charge transfer state is higher for 27 than for 26. Based on this observation, it appears that the twist around the anthryl-phenyl C-C bond plays a significant role in the fluorescence profile of the probes [60]. Acridines, such as 28, behave similarly to anthracene except that acridine is a better electron acceptor [61]. 

A well-known example of an exciplex is the excited-state complex of anthracene and N,N-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene molecule. In nonpolar solvents such as hexane, the quenching is accompanied by the appearance of a broad structureless emission band of the exciplex at higher wavelengths than anthracene (Figure 4.9). The kinetic scheme is somewhat similar to that of excimer formation. 

Change-transfer complexes of solute-alcohol stoichiometry 1 2 have been reported by Walker, Bednar, and Lumry3 for indole and certain methyl derivatives (M) in mixtures of associating solvents n-butanol and methanol (Q) with n-pentane these authors introduced the term exciplex to describe the emitter of the red-shifted structureless fluorescence band which increases in intensity with the alcohol content of the mixed solvent. The shift of the exciplex band to longer wavelengths as the solvent polarity is increased, described by Eq. (15), confirms the dipolar nature of the complex that must have the structure M+Q2. No emission corresponding to the 1 1 complex is observed in these systems which indicates (but does not prove) that the photo-association involves the alcohol dimer. The complex stoichiometry M+Q determined from (Eqs. 9, 10, and 12)... 

Chandross, Longworth, and Visco126 have reported the observation of long-wave structureless emission bands in the vicinity of the electrodes during the ac electrolysis of anthracene, phenanthrene, perylene, and 3,4-benzpyrene in polar solvents such as acetonitrile and dimethylformamide the similarity between the perylene band and the crystal fluorescence spectrum prompted the assignment of these bands to excimer fluorescence originating in the process... 

Because the solvent molecules are usually of a similar size to the reactants, the assumption that reactants diffuse in a structureless and isotropic continuum is not very satisfactory. Liquids possess short-range order. Solvent molecules are several times more likely to be separated by a distance equal to their diameter than separated by about one and a half diameters. More details are revealed by the radial distribution function [see Figs. 38 (p. 216) and 44 (p. 235)]. This implies that there is an... 

Absorption spectra of peridinin in different solvents are shown in Fig. 2a. In the nonpolar solvent M-hexane, the absorption spectrum exhibits the well-resolved structure of vibrational bands of the strongly allowed S0-S2 transition with the 0-0 peak located at 485 nm. In polar solvents, however, the vibrational structure is lost and the absorption band is significantly wider. In addition, there are also differences between the various polar solvents. Although the loss of vibrational structure is obvious, a hint of shoulder is still preserved in methanol and acetonitrile, but in ethylene glycol and glycerol the absorption spectrum is completely structureless with a broad red tail extending beyond 600 nm. 

The energy spacings between levels associated with solvent dipole reorientations are small, 1-10 cm-1. Since the spacings are well below kBT at room temperature ( 200 cm-1), the contribution of the solvent to the energy 6f activation for electron transfer can be treated classically. The results of classical treatments, where the solvent is modelled as a structureless dielectric continuum, will be discussed in later sections. 

An expression has been derived by Marcus34 and Hush35 for A0 assuming the solvent to be a structureless dielectric continuum characterized by the macroscopic dielectric constants Dop and Ds. D0p and Ds are the optical and static dielectric constants, respectively, and Dop = n2 where n is the index of refraction in the visible spectral region. In the limit that the reactants can be treated as two non-interpenetrating spheres, AQ is given by equation (23). 

For ideal probes the shape of the fluorescence spectrum should be structureless and smooth without vibronic features in a range of solvents of different polarity. Another important observable is the radiative rate constant (Eq. (22))... 

Exceptional fluorescence properties also characterize the ri.s-isomer 38e. Unsubstituted cis-l,2-di-9-anthrylethylene 38a and its monosubstituted derivatives such as 38b are nonfluorescent at room temperature. By contrast, cis-dianthrylethylene 38e does fluoresce with quantum yields of 0.0018, 0.0042, and 0.0064 in cyclohexane, dichloromethane, and acetonitrile, respectively. The emission is structureless (see Figure 18), and is associated with a solvent-independent Stokes shift of about 6000cm-1. As the molecular geometry of 38e is characterized by overlapping anthracene systems [80], the structureless emission may be attributable to an intramolecular excimer state. 

In polar solvents such as chloroform, dichloromethane, acetone, and acetonitrile, the fluorescence quantum yields of 97a-d decrease by varying degrees (see Table 19). Moreover, in the case of the phenyl and acetyl derivatives 97c and 97d, the rather drastic decrease of the structured fluorescence from the locally excited anthracene is associated with the appearance of a structureless, red-shifted emission which is attributable to an intramolecular exciplex. For 97d, in which the electron acceptor properties of the aromatic carbonyl moiety are enhanced by p-acetyl substitution, exciplex emission is dominant even in toluene solution (see Figure 22). 

In polar solvents, the quantum yields for the emission from the locally excited state of anthronyl-anthracenes 98 and 99 decrease drastically (see Tables 20 and 21), and a structureless, red-shifted exciplex emission is observed (see Figure 23). For the parent compound 98a in dichloromethane, for example, the quantum yield of emission from the exciplex state is 0.012, but that of emission from the locally excited state has decreased to 0.00058 (cf. Tables 20 and 22). Thus, intramolecular exciplex formation between the photoexcited anthracene moiety and the aromatic ketone in its electronic ground state represents the major mode of deactivation in polar solvents. 

The structure of a solvated electron depends on the solvent, and it is often difficult to describe accurately. The existence of the solvated electron as a distinct chemical species is however ascertained by its absorption spectrum this is a broad, structureless spectrum which covers the far VIS and NIR regions (Figure 1.5). 

The optical absorption spectra of et consist of one broad structureless band whose position and width also depend on the nature of the solvent. As an example, Fig. 5 presents an optical spectrum of et in a 10 M NaOH + -H20 matrix recorded at 77 K. The nature of the transition responsible for this band can be different in different media. For example, in water-alkaline matrices this band seems to correspond to the transition of an electron to a... 

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