Metalloporphyrins photochemistry

As in previous years this section consists of a short account of recent developments in metalloporphyrin photochemistry and photophysics and a selective survey of the in vitro photochemistry of chlorophyll and haem. 

A meaningful discussion of porphyrin photochemistry would require an in-depth description of the excited states of metalloporphyrin systems and of energy transfer which is outside the scope of the present Chapter. Porphyrin photochemistry is also the basis of photosynthesis which, in itself, is a massive area of current research endeavor. Readers are therefore directed to authoritative reviews on these aspects (B-75MI30709, B-75MI30710). 

Artificial myoglobins prepared with modified and metalloporphyrins. Biochemistry, 9,2268-2275. Dorr, F. (1971). Polarized light in spectroscopy and photochemistry, in A.A. Lamola (ed.), Creation and Detection of the Excited States, pp. 53-122, Dekker, New York. 

The rates of radiationless transitions between electronic states of porphyrins and their derivatives play a dominant role in their photochemistry because they are the major decay channels of the electronically excited states. Radiative channels, such as fluorescence, rarely exceed 10% of the overall decay rate constant at room temperature. The lifetimes of the lowest electronic states of free-base porph3nins and closed-shell metalloporphyrins vary by more than 10 orders of magnitude with the nature of the substituents. The understanding of such variations is essential to design and control the photochemistry of porphyrins and justifies an incursion on the fundamentals of radiationless transitions. 

Although a number of studies of the photophysics and photochemistry of non-iV-substituted porphyrins and metalloporphyrins have been reported, there are very few studies of N-alkylporphyrins Lavsillee et al. have reported fluorescence spectra of JV-methylporphyrins and their zinc(II) complexes.3,4 There is no report on the lifetime and its correlation with the structure. Ruorescence data for iV-alkylporphyrins and their zinc(II) complexes in degassed acetonitrile are shown in Table 2. 

Considerable interest continues in the photochemistry and photophysics of metalloporphyrins and related species containing zinc and other metals. Details will be found in Part II. The biosynthesis of haem can be partially diverted to produce a zinc protoporphyrin in iron-deficiency anaemia and lead poisoning measurement of fluorescence emission from this species has been recommended... 

Figure 6.2.3 The bond lengths in porphyrins are more or less alternating, feigning polyene character, if the pyrrolic carbon-carbon bonds are included. The inner ring containing the pyrrole and pyrrolenine nitrogen is, however, fully aromatic. The bond lengths are all close to 1.37 A. It is this macrocycle in connection with the electron-donating or -withdrawing power of the central metal ions that determines both the redox and photochemistry of metalloporphyrins. The bond lengths in the center correspond to NH bonds.

Iron porphyrin carbenes and vinylidenes are photoactive and possess a unique photochemistry since the mechanism of the photochemical reaction suggests the Hberation of free carbene species in solution [ 110,111 ]. These free carbenes can react with olefins to form cyclopropanes (Eq. 15). The photochemical generation of the free carbene fragment from a transition metal carbene complex has not been previously observed [112,113]. Although the photochemistry of both Fischer and Schrock-type carbene has been investigated, no examples of homolytic carbene dissociation have yet been foimd. In the case of the metalloporphyrin carbene complexes, the lack of other co-ordinatively labile species and the stability of the resulting fragment both contribute to the reactivity of the iron-carbon double bond. Thus, this photochemical behavior is quite different to that previously observed with other classes of carbene complexes [113,114]. 

The study of emission 1,2-enedithiolates now represents a formidable body of literature, even though the chemistiy, photochemistry, and photophysical properties of metallo-l,2-enedithiolate complexes are not yet as well understood or developed as those of the group metallo-Vlll-diimine and metalloporphyrin complexes. However, recent developments in the synthesis of 1,2-enedithiolates have led to the discovery of room temperature emitters and complexes with useful properties. As new methods allow for the synthesis of yet unknown complexes in this family, the unique and useful properties of these complexes will become even more evident. Many of the heterocyclic-substituted 1,2-enedithiolates now available are dual emitters with a short-lived and analyte-quenchable long-lived excited states. Clearly, these dual emitters will have a unique place in the detection of quenching analytes since selective quenching of Ae long-lived excited state eliminates several problems encountered with luminescence-based sensing. 

Finally, porphyrins and metalloporphyrins have been the focus of countless studies in supramolecular photochemistry over the past several decades. In keeping with the importance of this area to the field of supramolecular photochemistry. Chapter 6 is dedicated to the properties of supramolecular porphyrin and metallo-porphyrin complexes. Takagi and Inoue have put together an excellent chapter that exhaustively explores the fundamental properties of metalloporphyrins. In addition, they describe the supramolecular porphyrin complexes that are constructed from porphyrin units by using covalent and noncovalent approaches. 

Photochemistry and Photophysics of Highly Excited Valence States of Polyatomic Molecules Nonalternant Aromatics, Thioketones, and Metalloporphyrins... 

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