Light Absorption and Electronically-excited States

We can digress at this point to consider two laws of photochemistry thal feature in most introductions to the subject. First, the Grotthuss-Draper law states, in essence, that only light absorbed by a molecule can be effective in bringing about chemical change. This may seem obvious when considered at the molecular level—if the photon energy is not made available to the molecule by absorption, an electronically excited state cannot be produced, and no photochemical change can result. The law s importance is in its practical... 

As briefly introduced earlier, both the electronic ground state and electronic excited state have vibrational levels. Actually, a vibrationally and electronically excited molecule can be generated by absorption of a photon in the UV visible range. In order to excite only the vibrational state of a molecule, IR light is needed (lower energy photons). When both vibrations and electronic levels are excited, the term vibronic excitation is used (contraction of vibration and electronic). The jump from one state to another (either vibrational, electronic or vibronic) is called a transition. 

This concept is illustrated, for example, in Fig. 1.1 [9]. In the case of a photochemical stimulation, a system A B, consisting of two units ( indicates any type of bond that keeps the units together), can be defined a supramolecular species if light absorption leads to excited states that are substantially localized on either A or B, or causes an electron transfer firom A to B (or viceversa). By contrast, when the excited states are substantially delocalized on the entire system, the species can be better considered as a large molecule. Similarly (Fig. 1.1), oxidation and reduction of a supramolecular species can substantially be described... 

The absorption by a molecule of a photon of light, with energy E, equal to the difference between energy states, can promote the transition of an electron from the ground electronic state to a vibrationaUy and electronically excited state. This is illustrated in Figure 1.3. The amount of incident radiation absorbed is proportional to the number of molecules (molecular concentration) in the path (photons cm- ), and is expressed in Beer and Lambert s Law (Eq. [1.6]) ... 

For a triplet to result, two electrons must have different orbitals and the same spin (the Pauli principle forbids having two unpaired electrons in the same orbital since all four quantum numbers would be the same). We saw in Chapter 1 that upon light absorption an electron is promoted from one orbital to a higher orbital. If we were to excite directly from the ground state singlet to the triplet state we would have to simultaneously change orbitals and electronic spins. Since this process is relatively improbable, direct absorption to the triplet state is seldom observed (later in this chapter,... 

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