Deactivation modes

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. 

Current photochemical research is strongly linked with the study of photophysical behavior of excited particles. Data on photophysical processes (such as luminescence, internal conversion, intersystem crossing, intramolecular energy dissipation) assist photochemists in the identification and interpretation of chemical deactivation modes. Most of the data related to the elementary steps within deactivation of excited particles have been obtained by fast flash techniques in nano-, pico-, and femtosecond time domains. Photophysics is, in general, as rich a branch of science as photochemistry, and both the parts of excited-state research deserve comparable attention and extent. In the present review, some results on photophysics will be mentioned where suitable and necessary. We will restrict our discussion, however, predominantly to photochemical behavior of metallotetrapyrroles. 

It should be pointed out that the nature of the primary photochemical step(s) is still obscured and can depend, even for the same complex, on experimental conditions. Thus, Fen,(Por)N3 converts under irradiation in the solid state at low temperatures [162,163] into FevN(Por) in some solution systems [133] the formation of azidoradicals N3 has been detected by spin-trapping EPR no information on the heterolytic splitting of the Fe-N3 bond yielding NJ anion has been described in the literature (for azido complexes of some other central atoms the photosubstitution of the coordinated N3 ligand is a dominant chemical deactivation mode [1]). In addition, at particular conditions, the... 

Lack of emission from this complex does not appear to be due to any deactivating mode characteristic of the ligand, since the mixed complexes with bpy do indeed emit246-1. Here, the MLCT state lies at much lower energy. 

While the mono-phenyl ligand, tro, is very much like trpy the tri-phenyl ligand, tsite, is quite different in its ruthenium complex. [Ru(tsite)2]2+ has a very intense lowest absorption band191-1, a somewhat higher quantum yield than [Ru(trpy)2]2+ (0.57 vs. 0.48 at 77 K), and is a room temperature emitter (lifetime about 0.2 (is). Unfortunately a room temperature quantum yield has not been reported for comparison with [Ru(bpy)3]2+. Our estimation of the quantum yield at room temperature from the reported lifetime is 0.01, a fraction of that reported for [Ru(bpy)3]2+. Thus, even in [Ru(tsite)2]2+, deactivation modes are more prominent than in [Ru(bpy)3]2+ and the above arguments are most likely applicable. 

The quenching of an excited state of a transition metal complex by chemical reaction can occur, in principle, by means of any of the intermolecular reactions which transition metal complexes are able to undergo. It should be noted, however, that intermolecular excited state reactions can only occur if they are fast enough to compete with the intramolecular deactivation modes of the excited state and with the other quenching processes (Fig. 2). 

The first indication of catalyst deactivation is a significant change in the activity/selectivity of the process. Catalyst deactivation occurs in all processes but it often can be controlled if its causes are understood. This subject is very extensive and the reader is encouraged to seek additional information in references given here.10,11 In the following we will present some of the most common deactivation modes especially for heterogeneous catalysts. These are pictorially shown in cartoon form in Fig. 7.7. 

In addition to the dilution effect described above, the surface area of the oxygen-storage material also figures into a potentially more serious deactivation mode, loss of noble metal by deep encapsulation [19,20], As ceria-zirconia sinters, some of the noble metal particles supported on it may become trapped, either within single grains or at grain boundaries of the dense ceramic, as shown by the TEM micrograph in Fig. 10.9 [20]. 

The schematic of the process cycle controller is shown in Figure 4. The solenoids are in the deactivated mode while the manometer is open to the reaction vessel. When the mercury level drops below electrode E2, the solenoids are activated and the manometer is shut off to the reaction vessel and opened to a source of fresh reactant gas. The rate of the refill, about seconds, can be controlled by the metering valve, C, shown in Figure 1. When the mercury level reaches El, the solenoids are deactivated, the counter is reset, and the manometer is opened to the reaction vessel once again.3... 

In the final deactivation mode reported by the authors, the active acidic sites of the catalyst are poisoned (7 = 145°C, P = 50 bar) by continuous addition of a very dilute solution of pyridine to the reacting mixture over a period of 12 h (see figure 11.10). The catalyst can be reactivated by heating and compressing the reaction mixture to conditions well within the mixture critical region (7 = 250°C, P = 500 bar). Tiltscher and coworkers report that the catalyst poison is precipitated from the product solution as pyridinium chloride. Presumably only a very small amount of pyridinium chloride is needed to deactivate the catalyst since supercritical hexene probably would not be able to solubilize much of this salt. It is surprising, however, that supercritical hexene can overcome the acid-base interactions that are occurring on the catalyst surface and, hence, remove the pyridinium chloride. 

In a previous work (ref. 1), a model which considers simultaneous activation and deactivation processes on a solid catalyst was presented in order to explain the behaviour of numerous catalytic systems for which a maximum in the activity versus time curves had been reported. The results of this work showed that the use of an activation/deactivation mode is required in order to obtain reliable values for the deactivation kinetic parameters whenever activation processes are present simultaneously. 

The tram - cis photoisomerization involves a thermally activated process. In solvents with low viscosity almost identical activation energies have been obtained from the temperature dependences of compare Tables 7 and 12a), This justifies the assumption of a potential barrier for internal rotation in the excited singlet state ( t -> 1 p ), taking into account that t is the only state that fluoresces. If internal rotation is the principal deactivation mode of t in fluid media, a decrease of temperature should reduce the internal rotation and thus decrease , c and simultaneously increase f. This is indeed the case. On decreasing the... 

Chemical reaction is an important quenching mechanism of electronically excited states. Because of the short lifetime (generally less than 1 /xsec) of excited states in fiuid solution at room temperature, quenching by chemical reaction must be very fast if it is to occur. We shall consider here only outer-sphere electron transfer reactions of excited states since these reactions are certainly fast enough to compete with the other deactivation modes. 

COMPOSITION OF THE CARBONACEOUS COMPOUNDS RESPONSIBLE FOR ZEOLITE DEACTIVATION. MODES OF FORMATION... 

Coking, aging and regeneration of zeolites. Ill- Comparison of the deactivation modes of H mordenite, HZSM-5 and HY during n-heptane cracking, J. Catal. 106, 242-250. 

Upon (UV) irradiation, the molecule is converted from its (singlet) ground state to its first excited state. The Jablonski diagram given in Fig. 1 shows that there is more than one possibility to deactivate an excited initiator molecule. This multitude of deactivation modes is quantified in the quantum 5ueld for the primary free radical production, O. O is composed of three parts, which can be assigned to the reactions shown in Scheme 4. 

Of aU the deactivation modes discussed above, hydrothermal deactivation is the most challenging technical hurdle. Significant improvement on the hydrothermal... 

Regeneration - The two first deactivation modes are reversible, i.e. the oxidation of the carbon layer by heating in air and appropriate chemical treatment of the surface (H2, see section 3 CI2, etc.) can free the sites from impurities. Sometimes, regeneration is achieved during the catalytic process. ... 

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