Rhodium complexes luminescence

Rh(phi)2(phen)]3+ is a particularly suitable luminescence quencher for our investigations of electron-transfer reactions on DNA. Its electronic properties are favorable for electron transfer, and this rhodium complex is primarily sequence neutral, so that nearly random binding of the donor and acceptor is expected. Moreover, the photocleavage reaction actually allows us to identify the positions of binding of the acceptor to the DNA double helix. 

Ruthenium complexes used to lead research in photochemistry of metal compounds, but rhodium complexes have recently overtaken them as the key target compounds due to their applications in OLEDs. This is a lively and ever-changing field for example, over 90% of luminescent iridum(III) complexes have been reported only in the six years to the beginning of 2009. With their luminescence tuneable through ligand choice, iridium complexes are firm candidates for optical display applications. 

Only two rhodium complexes, 38 and 39, have been employed for cell imaging and their behavior resembles that of their iridium counterparts. In particular, the monocationic rhodiimi(III) cyclometalated, [Rh(pba)2(bpy-Et)] complex 38 (Scheme 11.7) showed weak emission in fluid solutions at 298 K but more intense luminescence in glass matrix at 77 K [110]. The emission is attributed to a triplet intraligand ( IL)(3t -> 7t )(pba) excited state. This complex was internalized in HeLa cells after incubation at 37 °C for 24 h and found localized in the cytoplasm, without significant uptake in the nucleus. 

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)... 

It is also useful to consider the luminescence from metallated oligonucleotides in the presence of noncovalent metallointercalator. Adding one equivalent of free [Ru(phen)2(dppz)]2+ to the ruthenium-modified duplex doubles the intensity in luminescence, consistent with independent intercalation by the two species. As described earlier, steady-state luminescence reaches saturation at approximately three times the luminescence of the ruthenium-modified duplex when two equivalents of [Ru(phen)2(dppz)]2+ have been added. It is not surprising, then, that addition of a stoichiometric amount of [Rh(phi)2(phen)]3+ to the ruthenium-modified duplex leads to substantial but not complete quenching of the ruthenium emission. Statistically, some duplexes will accommodate two rhodium(III) complexes, leaving a few ruthenium-modified duplexes unoccupied and therefore unquenched. Thus, complete quenching is observed only when the acceptor is covalently bound to the same duplex as the donor. 

Hydride Complexes of the Transition Metals Iridium Organometallic Chemistry Luminescence Macrocyclic Ligands Rhodium Inorganic Coordination Chemistry. 

Watts and coworkers reported the luminescence properties of cyclometalated iridium(III) and rhodium(III) complexes (see Cyclometalation). The dichloro-bridged dimers [M2(N C)4Cl2] (M = Ir, Rh HN C = Hppy, Hbzq) displayed intense emission with structural features in EtOH/MeOH/CH2Cl2 (4 1 1 v v) glass at 77 K. The emission of the rhodium(III) dimers was assigned to an lE excited state... 

Reactions of the dichloro-bridged rhodium(ni) and iridium(III) dimers with diimine ligands resulted iu the formation of the luminescent mononuclear complexes [M(N C)2(N N)]+ (M = Ir, Rh = ppy, bzq N N = bpy, phen). The electronic absorption and emission spectra of these complexes are shown in Figure 8. The rhodium(III) complexes displayed very long-lived emission in... 

Rhodium(III) and Iridium(III) Complexes, in contrast to the b py and phen containing complexes of ruthenium(II), which invariably show a CT (tt -> d) type emission, the rhodium(III) analogs display either a metal localized (d -> d) or ligand localized luminescence (64,179-182). The tris com-... 

Brensted Base Quenching. The trans-Rh(cyclam)(CN)2+ ion (cyclam = 1,4,8,11-tetraazacyclotetradecane) displays luminescence from a ligand field excited state (3LF ) at room temperature, in an aqueous solution with a lifetime (8.1 /is) [53] several orders of magnitude longer than generally observed for rhodium(III) amine complexes [54]. As was observed for some other Rh(III) amines, the 3LF emission from trans-Rh(cyclam)(CN)2 is quenched by OH- in solution (Eq. (13)), a process attributed to amine deprotonation [55],... 

In contrast to common luminescent rhodium (III) systems, the complexes [Rh(ppy)2(TAP)]+ (36) and [Rh(ppy)2(HAT)]+ (37) showed structureless emission spectra in fluid solutions at room temperature and in rigid glass at 77 K. In addition to the observation of the irreversible oxidation wave, the emission was assigned to an SBLCT (cr(Rh-C) — 7t (TAP or HAT)) excited state. The iridium(III) complex [Ir(ppy)2(HAT)] + displayed dual emissions in 77 K glass, which was assigned to excited states of MLCT An (Ir) — ... 

A second pertinent feature of the decay kinetics of complexes that display charge-transfer luminescence is the exponentiality of the observed transients at all temperatures reached — 1.5 K). This behavior is in stark contrast to that exhibited by organic systems at low temperatures. For the latter, single exponential decays are maintained to 10°K, but they are replaced by complicated kinetics at lower temperatures (2). Nonexponential decays have also been observed at low temperature for tris complexes of rhodium (III) that exhibit states lowest (14),... 

However, the luminescence measurements show quenching of fluorescence in the trimer, which is attributed to a photo-induced electron transfer from the axial ruthenium(II) porphyrin to the excited state of the basal tin porphyrin. Not only ruthenium(II) porphyrins, but also rhodium(III) porphyrins can easily be incorporated into the arrays with the same strategy [33]. Again, the isonicotinic acid is first reacted with the bis-hydroxy tin porphyrin to give the bis-isonicotinic acid complex. Addition of two equivalents of rhodium(III) porphyrin readily yields the trimeric array of the composition Rh-Sn-Rh. The X-ray structure of this complex, which is shown in Fig. 34c, shows that the ligands on the tin center (carboxylates) are in an off-direction which is close to orthogonal to the porphyrin plane, and the three porphyrins adopt a near coplanar arrangement. The tin porphyrin is tilted by about 8.6° with respect to the rhodium(III) porphyrins. 

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