Polypyridine complexes excited states

Electron transfer reactions of the excited states of metal ion complexes continue to attract much attention with interest in solar energy storage and the cleavage of H2O to H2 and 02. ° " ° A useful review of the properties of excited-state polypyridine complexes has appeared. The much studied [ Ru(bipy)3] and related systems remain at the forefront of new work but other metal ion systems are attracting increasing interest (Table 1.3.) When a complex is photochemically activated to form ML, excited state... 

Importantly, it was found [80-82, 311] that interfacial electron transfer from MLCT-excited Ru polypyridine complexes to Ti02 is an ultrafast process, completed in 25-150 fs This groundbreaking discovery implies that the search for new sensitizers need not to be limited to complexes with long-lived excited states. Indeed, [Fe(4,4 -(COOH)2-bpy)2(CN)2], whose MLCT excited state lifetime is only ca 330 ps, was found [304] to act as a sensitizer in a Ti02-based solar cell. In fact, even the classical Gratzel cell [36, 77, 78] would not operate as well as it does, were the interfacial electron transfer not ultrafast, since the [Ru(4,4 -(COOH)2-bpy)2-(NCS)2] sensitizer has an inherent excited state lifetime of only 50 ns. 

Electron injection from MLCT-excited Ru-polypyridine complexes are used to investigate electron transfer along DNA strands, that is to decide whether DNA can behave as a molecular wire [358-360]. In these studies, derivatives of [Ru(phen)2(dppz)] + act as excited-state electron donors and [Rh (phi)2(bpy)] + as a ground-state electron acceptor. Both complexes are anchored at different DNA sites and the rate of Ru —> Rh photoinduced electron transfer is measured. In another study [361], a [Ru (bpy)2(im)(NH2-)] + unit attached to a terminal ribose of a DNA duplex acted as an excited-state oxidant toward a [Ru (NH3)4(py)(NH2-)] " unit attached at the other end. 

The historical development and elementary operating principles of lasers are briefly summarized. An overview of the characteristics and capabilities of various lasers is provided. Selected applications of lasers to spectroscopic and dynamical problems in chemistry, as well as the role of lasers as effectors of chemical reactivity, are discussed. Studies from these laboratories concerning time-resolved resonance Raman spectroscopy of electronically excited states of metal polypyridine complexes are presented, exemplifying applications of modern laser techniques to problems in inorganic chemistry. 

The photochemical and photophysical properties of Ru(bpy)3 and related d° polypyridine complexes have been the subject of intense recent interest (43-49). This is due to their potential in photochemical energy conversion, their intrinsically significant excited state behavior, and their attractive chemical properties. The structures of the excited states of these complexes are clearly of great interest. 

Polypyridine rhodium(III) complexes (RM ) may be reduced by one-electron reductants The reductants which have been successfully employed include Ru(bpy)32+, the luminescent charge-transfer excited state of Ru(bpy)32+ (J, 9)... 

In the case of tetranuclear compounds belonging to the polypyridine family, all four possible energy migration patterns, schematized in Figure 21, have been obtained. Pattern (i) is found for L = bpy, BL = 2,3-dpp, and M = Ru. In such a complex, the three peripheral units are equivalent and Aeir lowest excited state lies at lower energy than the lowest excited state of the central unit.The reverse... 

In dichloromethane solution, the [Ru(bpy)2(l)]2+ complex (Scheme 1) exhibits an absorption band at 455 nm (emax = 10400 M em Figure 5) and an emission band at 619 nm (x = 733 ns, cf> = 0.05, Figure 5, Table 1). These bands can straightforwardly be assigned to spin-allowed and, respectively, spin-forbidden metal-to-ligand-charge-transfer (MLCT) excited states, characteristic of Ru(II) polypyridine complexes[6a,c,e]. 

Prodi examined the photophysical properties of supramolecular assemblies containing polypyridine complexes and pyrene chromophores. Using the complex [(bpy)20s(bpy0bpy)]2+ (Scheme 5) and pyrene-1-carboxylic acid, the authors demonstrated that, upon addition of Zn2+, complex formation occurs in which the Zn2+ links together the free bpy of the Os complex and the carboxyl of the pyrene. Excitation of the Os(II) MLCT absorption results in exclusive emission from the 3MLCT state of the Os complex [84]. 

Fig. 18. Oxidation and reduction potentials in aqueous solution of the lowest excited state of some polypyridine complexes. The M+/M and M+/ M potentials of the Cr complex axe lower limiting values...

As we have seen in Section 8, excited-state electron-transfer reactions can be used for the conversion of light energy into chemical energy. Most of the polypyridine complexes show intense absorption bands in the visible region (see, for examples, Fig. 17) and thus they are particularly suitable for solar energy conversion. 

Several recent studies examined photoinduced ET in dyads featuring transition metal chromophores that are dissimilar to the d6 transition metal polypyridine complexes used in the type 1 and type 2 dyads that have been discussed in the preceding sections. Since the molecular and electronic structure of the excited states involved in these systems is unique from the type 1 and type 2 dyads, results on these systems are discussed separately. 

Among the isocyanide complexes, Ru(bpy) (CNMe)42+ stands out as a remarkable Ru(II) polypyridine photosensitizer in view of (i) its long lifetime in fluid solution, and (ii) its very high excited-state oxidizing power ( e1/2 red v vs SCE)... 

In conclusion, whereas Co(NH3)63+ is useless because it undergoes a fast photodecomposition reaction, the analogous Co(sep)3+ complex may be employed as an electron transfer photosensitizer because of its intrinsic stability in the excited state and in the reduced form. In the same way, one can think to use cage-type polypyridine ligands for Ru complexes, so as to prevent ligand dissociation reactions. 

This part of the photosynthetic chain can be mimicked by ruthenium-tyrosinate and manganese-tyrosinate complexes [80-82], In the excited state the ruthenium-polypyridine complex abstracts one electron from the tyrosinate moiety, yielding... 

While not yet as extensive as the chemistry reported for bipy and phen complexes of ruthenium, the chemistry of rhodium polypyridine complexes (especially their excited states) has generated much recent interest. The claim of Lehn and co-workers805 that excited states of [Rh(bipy)3]3+ are involved in the photoinduced generation of H2(g) from water has sparked renewed efforts to understand the rich excited state chemistry of these complexes. 

Figure 9 Plot of nk as a function of free energy for electron-transfer reactions of Ru-65-c)d 65 labeled with different ruthenium polypyridine complexes. The solid tine shows the theoretical dependence of equation (1) calculated with a reorganizational energy of 0.94 eV and a distance r of 13 A. The open boxes are for the reactions involving the excited state, = ki, and the filed boxes are for the thermal back reactions, ki-i = k2, all determined at 22 °C. (Reprinted with permission from Ref. 55. 1993 American Chemical Society)...

Time-resolved, step-scan FT-IR spectroscopy has been used to monitor the v(CO) frequencies of rhenium(I) carbonyl polypyridine complexes and hence to study the excited-state electronic structures of these systems. The MLCT and IT character in the emissive states of [Re(CO)3(phen)(py-4-Me)]+ and [Re(CO)3(dppz)(PPh3)]+, respectively, has been studied by this technique. The presence of two close-lying states of MLCT and IL character for the complex [Re(CO)3(4,4 - NH2 2-bpy)(py-4-Et)]+ was also confirmed. 

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