Primary step photophysical

E. Lippert, Photophysical primary steps In solutions of aromatic compounds , Arc. Chem. Res., 3, 1970, 74. 

The absorption of light by a substance causes the formation of excited-state molecules. This excitation is followed by various elementary transformations which eventually lead to the deactivation or to the disappearance of those excited molecules. The absorption of light as well as each one of the elementary transformations of the original molecule in an excited state is a primary step. Specifically, a primary step may be (a) a transformation of the excited molecule into a different chemical species, as in steps 24, 15, and 14 of Figure 1, or (b) a radiative or nonradiative transition between different energy levels of the molecule, e.g., steps 02, 21, 22, 23, 13, 11, and 16 of Figure 1. Those corresponding to (a) are photochemical primary steps, while those of (b) are photophysical primary steps. 

Even so, the distinction between the two is sometimes a more subtle matter. Thus, in a photoisomerization a common excited state intermediate may undergo a transformation to either of the two isomeric cis-trans species of a planar ground-state molecule. These two transformations are virtually identical in nature, yet one leads back to the original species and is therefore a photophysical primary step, e.g., internal conversion or intersystem crossing, while the other leads to the chemically distinct isomer and should be called a photochemical primary step. As another example, the distinction between the formation of an excimer and of a photodimer lies in the instability and stability, respectively, of the dimeric species in the ground state. Excimer formation is usually considered as photophysical and photodimer formation as photochemical. These examples show that the classification of steps as photochemical and photophysical is in some cases arbitrary. 

Among the more important photophysical primary steps are internal conversion, intersystem crossing, phosphorescence, fluorescence, vibrational and rotational... 

In principle, in almost all photophysical primary steps, considered strictly as elementary transformations, an excited state or the ground state is formed with vibrational (and to a less important extent, rotational) energy in excess of the amount it would have at equilibrium. 

However, especially for the nonradiative transitions to the ground state, steps 22 and 13, and especially in the gas phase, vibrational equilibration might have to be considered to represent the true final photophysical primary steps, as in Figure 2(b). 

Final photophysical primary steps and photochemical primary steps cannot be followed by any other primary steps, but only by physical and/or chemical transformations of molecules other than the original excited molecules, as will be discussed in Sections III.A.3. and III.C. 

As we have seen above, a photochemical primary step can itself be followed only by transformations of other than the original molecules. It follows, therefore, that a photochemical primary step can only be part of a primary process if it is the last step in the sequence. Such a primary process, ending on a photochemical primary step, is a photochemical primary process. A primary process which does not end with a photochemical primary step (and thus does not contain any such step in the sequence) is a photophysical primary process. Those which end with a final photophysical primary step are final photophysical primary processes. In Figure 1, sequences 02-24, 02-23-15, and 02-23-14 represent photochemical primary processes, while sequences 02 (absorption alone or absorption followed by vibrational relaxation as appropriate to the medium),... 

FINAL PHOTOPHYSICAL PRIMARY STEP A photophysical primary step which converts the excited molecule to the fully equilibrated ground state. 

PHOTOPHYSICAL PRIMARY STEP an elementary transformation of the originally excited species without any chemical change. 

With the aid of CIDNP spectroscopy it was shown that the primary step of these reactions is cleavage of the X—R bond, which upon direct excitation occurs both from the singlet and the triplet state. By combining the results of CIDNP experiments and photophysical measurements, rate constants of singlet and triplet reactions could be determined. In these systems, cleavage is again faster from the singlet state. 

Apart from the principles of photophysics and photochemical primary steps of reactions on which a number of books have been published in the past, the principles of photokinetics will be necessary for advanced students to gain a detailed understanding of quantitative considerations of photochemical reactions. For the same reason, researchers as well as professionals in industry, university, or governmental institutions will refer to a book of this type. It provides a concise treatment of photokinetics, its principles, its methods, and also gives a wide range of possible applications as well as a formal description of a large number of possible mechanisms of reactions. This will enable the readers either to find an example for their own application or to set up their own formalism. 

For a photoexcited molecule, the time allowed for a reaction to occur is of the order of the lifetime of the particular excited state, or less when the reaction step must compete with other photophysical processes. The photoreaction can be unimolecular such as photodissociation and photo isomerization or may need another molecule, usually unexcited, of the same or different kind and hence bimolectdar. If the primary processes generate free radicals, they may lead to secondary processes in the dark. 

The present article reviews the photochemical deactivation modes and properties of electronically excited metallotetrapyrroles. Of the wide variety of complexes possessing a tetrapyrrole ligand and their highly structured systems, the subject of this survey is mainly synthetic complexes of porphyrins, chlorins, corrins, phthalocyanines, and naphthalocyanines. All known types of photochemical reactions of excited metallotetrapyrroles are classified. As criteria for the classification, both the nature of the primary photochemical step and the net overall chemical change, are taken. Each of the classes is exemplified by several recent results, and discussed. The data on exciplex and excimer formation processes involving excited metallotetrapyrroles are included. Various branches of practical utilization of the photochemical and photophysical properties of tetrapyrrole complexes are shown. Motives for further development and perspectives in photochemistry of metallotetrapyrroles are evaluated. 

In all experiments described in this work only extremely low concentrations of intermediates are considered. This is due to our interest which is primarily focussed on the most important initial steps of the polymerization reaction, which are characteristic of the overall polymerization reaction mechanism. Consequently only low final polymer conversion is exp>ected and, therefore, complications arising from the interaction between the intermediate oligomer states can be neglected. It will be shown that the low temperature conventional optical absorption and ESR spectroscopy are powerful spectroscopic methods which yield a wealth of information concerning structural and dynamical aspects of the intermediate states in the photopolymerization reaction of diacetylene crystals. Therefore, this contribution will center on the photochemical and photophysical primary and secondary processes of this... 

The excited-state deactivation pathway of PYP model chromophores is foimd to depend strongly on the substituent adjacent to the carbonyl group. The photoisomerization reaction of the deprotonated p-coumaric acid (pCA ) and of its amide analogue (pCNT) in solution does not show any spectroscopically detectable intermediate, which is quite different from PYP. On the contrary, the phenyl thioester derivative pCT exhibits a photophysical behavior in solution surprisingly close to that of the protein during its initial deactivation step. This study highlights the determining role of the thioester bond in the primary molecular events in PYP. 

As in the case of thermal reactions, the reaction scheme introduced in Section 2.1.1.1 can be used to set up the differential equations. However, the degrees of advancement are primed, since the number of steps can be reduced as will be demonstrated by use of the Bodenstein hypothesis. In the last column of this scheme, the number of moles of light quanta are written for a photochemical step, which are absorbed by the reactant starting this photochemical step. According to this assumption and the different photophysical relaxation processes discussed in Section 1.3 the primary exited molecule A completely deactivates into the lowest level of vibrational energy of the first exited singlet state. Three further steps are possible ... 

In photosynthetic proteins, the primary charge separation and the sequence of electron transfer reactions can be utilized in the photosignal generation in electrochemical cells. The signal is the result of a combination of photophysical, photochemical and electrochemical events. The first is related to elearonic excitation followed by charge separation, the second deals with leaaions of excited molecules and finally, the electrochemical step involves a charge transfer at the interface between the electrolyte and the elearode. 

Therefore, as in the case of parent phenyl azide 47 and its simple derivatives, the photochemistry of polynuclear aromatic azide, especially that of naphthyl azides 79 and 80, is now well understood. Specifically, the dynamics of the primary photophysical processes as well as the subsequent photochemical steps have been directly investigated using a variety of modem and conventional experimental techniques and compntational chemistry. It is clear now, that the difference between the photochemistry of phenyl azide (and its simple derivative) and polynuclear aromatic azide is caused mainly by the difference in the thermodynamics of the singlet nitrene rearrangement to azinine type species. 

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