The Nature of Electronically Excited States

The filled orbitals of importance in organic photochemistry are therefore of two kinds bonding n MOs (ti), and nonbonding AOs occupied by 

7T 7c Transitions. These involve a simple transfer of an electron from a filled n MO into an empty n MO (or a partly empty n MO in the case of a radical see Fig. 6.1c-e). Note the self-explanatory symbol used to describe the transition. 

n TT Transitions. These likewise involve a transition between an electron occupying a lone-pair AO n and an empty n MO. As we shall see presently, transitions occur with reasonable ease only between orbitals that overlap in space. An n tt transition can therefore occur only if the atom with the AO n forms part of the conjugated system that contributes the TT MO. 

In both cases the electron enters the LUMO, but in the case of A, the HOMO has two electrons in it, whereas in A , it has one only. In forming A from 

FIGURE 6.3. Charge transfer transitions (a) between dissimilar molecules A and B (b) between two similar molecules A. 

Two successive approaches to the understanding of photophysical and photochemical processes and of the nature and properties of electronically excited states in molecules have to be considered (Turro, 1978, 1991)  

The electron shell of molecules is constructed by linear combination of atomic orbitals (LCAO method) of the participating atoms to form bonding ct and n molecular orbitals (MOs), non-bonding n (lone pairs of electrons) and anti-bonding 

An electronically excited state of a molecule is formed by absorption of a photon promoting an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The HOMO-LUMO promotion of an electron is the lowest possible energy transition that occurs between the frontier orbitals of a molecule. This situation is schematically presented in Fig. 3-14. 

The concept of frontier orbitals (Fleming, 1976) is very helpful and illustrative for the understanding of many photochemical reactions. 

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Progress in photochemistry could only be made following progress in spectroscopy and, in particular, the interpretation of spectra in at least semiquantitative terms, but history has shown that this was not enough. The arrival of new methods of analysis which permit determination of small amounts of products, the development of flash photolysis, nuclear magnetic resonance, and electron spin resonances which can yield valuable information about the natures of intermediate excited states, as well as of atoms and radicals, all have permitted the photochemist to approach the truly fundamental problem of photochemistry What is the detailed history of a molecule which absorbs radiation ... 

The nature of the electronic transition related to the 4.9 eV absorption band (the C state) of oxy radical is still unknown. It was experimentally found [65] that the quantum yield of the red luminescence (1.95 eV) is equal to 0.5 + 0.2. Consequently, the nature of this excited state should be so that the transition to the B state will be possible with high probability from this state. This means that the terms corresponding to the C and B states are crossed or converged (come closer to each other) at any arbitrary point of the configurational space. 

On the other hand, molecular mechanics (MM) methods, based on classical concepts, are extremely fast, and are able to handle very large systems, such as entire enzymes, with ease. Some MM methods are also as accurate as the best ab initio methods, particularly for hydrocarbons. Most, but not all, MM methods are parameterized only for ground state systems, and only for common bonding situations. By their nature, they are unable to anticipate unusual bonding situations, the making and breaking of most bonds, the chemistry of electronically excited states of molecules—properties that are fundamentally quantum mechanical in nature. 

The characterization of electronic excited states has attracted much attention in connection with photochemistry. For example, transition metal complexes are characterized by a variety of absorption spectra in the visible and ultraviolet (UV) regions. The absorption spectra essentially give us information about the electronic excited states corresponding to dipole-allowed transitions due to their high symmetries, while some of the data in crystalline fields indicate the existence of several excited states to which dipole transitions are forbidden in the absence of perturbation. Most photochemical reactions of metal complexes, which are occasionally important as homogeneous photocatalytic reactions, involve both allowed and forbidden excited states. Thus, the systematic understanding of the nature of these excited states is essential in designing photochemical reactions. 

A chemical reaction can be viewed as occurring via the formation of an excited state that can be any one of the degrees of freedom of the collection of N atoms. That is, translational, rotational, vibrational, and electronic excitation can lead to a chemical reaction. We often do not need to consider explicitly the quantized nature of rotational and vibrational energies in practical applications because of time scale considerations. For example, when a chemical reaction proceeds via a vibrationally excited state, in which the average lifetime typically is about 3 x 10" where T is in Kelvins... 

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