L. Moggi

Rate Constants for the Quenching of Excited States of Metal Complexes in Fluid Solution

The rate constants for the quenching of the excited states of metal ions and complexes in homogeneous fluid solution are reported in this compilation. Values of kq for dynamic, collisional processes between excited species and quenchers have been critically evaluated, and are presented with the following information, among others, from the original publications, when available: description of the solution medium, temperature at which kq was determined, experimental method, range of quencher concentration used, lifetime of the excited state in the absence of quencher, activation parameters, quenching mechanism. Data collection is complete through the end of 1986, and covers the coordination compounds of 26 metals, including the ions and complexes of the inner‐ and outer‐transition metals, and porphyrin complexes of nontransition metals. Data for 261 excited states quenched by more than 400 inorganic quenchers and 600 organic quenchers have been extracted from almost 500 publications. The introduction to th...

Photochemical reactions of hexachloroiridate(IV) Ion

Abstract The photochemical behaviour of IrCl62− has been investigated in acid solutions both in the presence and absence of Cl− ions. The results show that the complex is little photosensitive and that the nature and the quantum yield of its photoreactions depend on the type of excited state reached upon irradiation. Irradiation with 254-nm light (π → eg ligand-to-metal charge transfer transitions) causes two parallel photoreactions, yielding IrCl5(H2O)2− and IrCl5(H2O)−; the quantum yields of the two processes (but not their sum) depend on Cl− concentration, the formation of IrCl5(H2O)2− being favoured by the increase of [Cl−]. Irradiation with 313- or 365-nm light (which presumably correspond to t2g → eg ligand field transitions) yields IrCl5(H2O)− in the absence of Cl− and IrCl63− in 1.2 F Cl−. Finally, irradiation with 433- or 495-nm light (π → t2g ligand-to-metal charge transfer transitions) does not cause any appreciable reaction. As far as the irradiation at 254 nm is concerned, a photolysis mechanism is proposed which accounts for the detailed experimental results.

Photochemistry of coordination compounds: a glance at past, present, and future

Abstract After an induction period lasted up to the early 1960s, the photochemistry of coordination compounds has exhibited an autocatalytic development for about two decades and continues to be a rapidly growing field. In the past fifteen years there has been a progressive displacement of interest from intramolecular photoreactions (e.g., ligand photosubstitution and photoredox decomposition) towards intermolecular processes (e.g., energy and electron transfer). Presently there is a growing interest towards the study of supramolecular systems, and it seems likely to expect that in the next few years there will be a strong development in investigations concerning photonic molecular devices (i.e., assemblies of molecular components capable of performing valuable light-induced functions).

From a molecular to a supramolecular photochemistry

Abstract Following a current trend of chemical research, photochemical investigations are moving from molecular to supramolecular species. Some of the results obtained by the authors with supramolecular species containing metal complexes are briefly reviewed, with particular emphasis on (i) cage-type complexes, (ii) host-guest systems, (iii) metal catenates, and (iv) oligonuclear metal complexes.

Photochemistry of the nitrotetraammineamidoplatinum(IV) ion

Abstract The photochemical behavior of aqueous solutions of Pt(NH 3 ) 4 (NH 2 )(NO 2 ) 2+ was investigated, using 254 and 313 nm light. Spectrophotometric measurements showed that, besides the photoisomerization reaction to the nitrito isomer Pt(NH 3 ) 4 (NH 2 )(ONO) 2+ (ϕ 254 = 0.11; ϕ 313 =0.29), NO 2 − photoaquation and/or an oxidation-reduction reaction also occurred (ϕ 254 =0.54; ϕ 313 =0.23). The relative importance of the last two processes and the nature of the oxidation-reduction reaction was then investigated by means of chemical analyses carried out on solutions irradiated with 254 nm light. Some results concerning the thermal and photochemical behavior of Pt(NH 3 ) 4 (NH 2 )(ONO) 2+ in its solid state and in aqueous solutions have also been obtained.

Photochemistry of the Co(Sep)3+I− system: evaluation of the quantum yield for the photochemical redox reaction of the ion pair

Abstract Spectrophotometric measurements in the region of the ion pair charge transfer band of the Co(Sepulchrate) 3+ I − system were used to assess the role played by the adducts between the complex cation and iodide anion in the photochemical behaviour of the system. No clear evidence was obtained for the presence or absence of 1:2 or higher adducts and the association constant and molar absorptivity of the 1:1 adduct (ion pair) could not be determined with a satisfactory accuracy. Nevertheless, reliable values were evaluated for the fraction of the exciting light absorbed by the ion pair in the photochemical experiments, and these values were used to correct the previously reported quantum yields, which were based on the total amount of light absorbed by the solutions. The “true” quantum yields for the ion pair are discussed in relation to the proposed reaction mechanism.

Energy and Electron Transfer in the Field of Transition Metal Complexes

Abstract The bimolecular quenching of an excited state molecule in fluid solution can occur in three fundamental ways:1–3 (i) chemical reaction between the excited state and the quencher, (ii) electronic energy transfer from the excited state to the quencher, or (iii) deactivation of the excited state by some catalytic action of the quencher. The intimate nature of the quenching mechanism is often difficult to elucidate for a single excited state-quencher couple. More useful information can generally be obtained from correlations of data concerning homogeneous families of excited states and/or quenchers.

ELECTRON TRANSFER BETWEEN FE(CN)63- AND IODIDE PROMOTED BY SUPERCOMPLEXATION WITH A POLYAMMONIUM MACROCYCLE

Some new properties promoted by the formation of a supercomplex between iron hexacyanometallates and the polyazamacrocycle [32]aneN 8 (1,5,9,13,17,21,25,29-octaazacyclodotriacontane) are described. In the presence of the polyazamacrocycle, thermal and photoinduced electron transfer from iodide to Fe(CN) 6 3- were observed in moderately acidic media. The thermal reaction is slow (k obs = 8.9 × 10 -4 s -1 , at 25 °C) and proceeds to an equilibrium (K = 7 M -2 , at 25 °C). The reaction is almost isoergonic, with favorable enthalpy and unfavorable entropy changes (ΔG° = -4.8 kJ mol -1 , ΔH° = -160 kJ mol -1 , ΔS° = -0.54 kJ mol -1 K -1 ). A photoinduced electron-transfer process, leading to additional iodide oxidation, was observed upon flash irradiation of equilibrated solutions. Following the photoinduced process, the system reverts to the thermal equilibrium in the dark. The promoting role of the macrocycle is thermodynamic for the thermal process (anodic shift in the Fe II/III potential upon supercomplex formation) and kinetic for the photoinduced process [formation of ion-paired species between hexacyanoferrate(III) and iodide upon supercomplex formation]. The thermal reaction is reversible in basic media (where the macrocycle deprotonates and supercomplex formation is prevented), providing an example of on/off switching by pH changes of an electron-transfer reaction.

Charge effects on the photochemistry of the Co(EDTA)−·I− system in the presence of polyammonium macrocyclic receptors

Abstract The effects of polyammonium macrocycles on the spectroscopic and photochemical properties of the Co(EDTA)−·I− ion-pair have been investigated. The addition of a macrocycle to aqueous solutions containing Co(EDTA)− and I− causes an increase of the absorbance in the region of the ion-pair charge-transfer band, as well as an increase of the quantum yield for the intramolecular photooxidation reduction of the ion-pair. Both these effects are mainly, if not only, due to an increase of the association constant between Co(EDTA)− and I−, caused by the positive charge of the macrocycle bound to the complex. On the contrary no change was observed on the intrinsic photoreactivity of the excited ion-pair. This last result is discussed in comparison with the effects already observed on the ligand photodissociation of MC excited states of Co(III) cyanide complexes.

Reactivity of trans-β-styrylnaphthalene photosensitized by tris(2,2'-bipyridine) chromium (III)

Abstract On photosensitization by tris(2,2′-bipyridine)chromium(III), trans -β-styrylnaphthalene undergoes geometrical isomerization and (in the presence of dioxygen) oxidation to benzaldehyde and naphthaldehyde. Whereas oxidation seems to be due to attack by singlet oxygen, isomerization originates from a cationic radical of the olefin, formed in an electron transfer to be excited complex. The reactivity of such a radical is compared with those of triplet excited states and anionic radicals of stilbene derivatives.