Systems Consisting of Single Molecules

Disperse systems can also be classified on the basis of their aggregation behavior as molecular or micellar (association) systems. Molecular dispersions are composed of single macromolecules distributed uniformly within the medium, e.g., protein and polymer solutions. In micellar systems, the units of the dispersed phase consist of several molecules, which arrange themselves to form aggregates, such as surfactant micelles in aqueous solutions. 

The studies of well defined systems consists of spectroscopic studies of individual molecules and measurements of the rate of catalytic reactions on single crystal surfaces, as well as structure and reactivity of well-defined catalyst models. 

Although site effects are not as prevalent in UV-vis absorption as they are in IR spectra, they do exist and manifest themselves sometimes very clearly in band systems that comprise sharp peaks. An example is the radical cation of all-trans-octatetraene whose first absorption band consists of multiple peaks that can be selectively bleached by highly monochromatic light. The site stmcture can become more evident in laser-induced fluorescence, where excitation of individual sites is possible down to the level of single molecules in favorable cases, but a discussion of this fascinating phenomenon is beyond the scope of this chapter. 

Perhaps the simplest optically controlled switches are single molecules embedded in a solid host matrix. These systems consist of an amorphous, polycrystalline, or crystalline film doped with dilute concentrations of impurity molecules. The most commonly used dopant molecules are fused polycyclic aromatic hydrocarbons and porphyrins. In addition to facile sample preparation, these planar molecules absorb in the visible to near IR regions of the spectrum, possess large extinction coefficients in both the ground and excited states, and have high fluorescence quantum yields. 

Consider the case where we have an open system consisting of a single protein molecule in solution. This system could consist, for example, of a biological molecule in a bath of water held at constant temperature. If this molecule adopts a number of conformational states, its NVT and NPT partition functions are the familiar quantities ... 

Chemistry is primarily concerned not with the properties of single molecules but with periodic trends, homologous series and the like. It is, therefore, important that any method which we apply to the problem of molecular electronic structure depends linearly on the number of electrons in the system being studied. Meaningful comparisons of atoms and molecules of different sizes are then possible. This property has been termed size-consistency1-2. Independent electron models, such as the widely used Hartree-Fock approximation, provide a size-consistent theory of atomic and molecular structure. 

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