This Perspective reviews the impact of exciton coupling on the spectroscopic properties of coordination compounds. Exciton coupling features arise in electronic absorption and circular dichroism spectra when chromophores are brought into close spatial proximity, for example by coordination to a metal centre. The analysis of these features can reveal much information such as the geometry of a complex and its absolute configuration. The extension of the exciton coupling model to polynuclear metallosupramolecular arrays is discussed.
The synthesis, characterisation, and TiO2 binding studies of a series of chromophoric complexes of 5-(4-carboxyphenyl)-4,6-dipyrrin (L(b)) are presented. The synthesis of [Ru(bipy)(L(b))2] (bipy = 2,2-bipyridine), [Rh(L(b))3], and [Pd(L(b))2] was achieved by initial coordination of 5-(4-methoxycarbonylphenyl)-4,6-dipyrrin (L(a)) followed by hydrolysis of the ester group. The carboxyl groups that are located on the peripheries of these complexes are able to engage in intermolecular hydrogen bonding interactions in the solid state, as revealed by X-ray crystallography. These groups also allow the complexes to anchor to the surface of TiO2 nanoparticles, as evidenced by colouration of the TiO2 and FT-IR spectroscopy. The ability of these complexes to capture a significant fraction of sunlight and to adhere to TiO2 surfaces renders them viable dyes for photochemical devices such as dye sensitised solar cells.
The first Re(I)-dipyrrinato complexes are reported. Complexes with the general formulas fac-[ReL(CO)(3)Cl](-), fac-[ReL(CO)(3)PR(3)], and [ReL(CO)(2)(PR(3))(PR'(3))] have been prepared, where L is one of a series of meso-aryl dipyrrinato ligands. Access to these complexes proceeds via the reaction of [Re(CO)(5)Cl] with the dipyrrin (LH) to produce fac-[ReL(CO)(3)Cl](-). A subsequent reaction with PR(3) (R = phenyl, butyl) leads to displacement of the chloride ligand to generate fac-[ReL(CO)(3)PR(3)], and further reaction with PR'(3) leads to the displacement of the CO ligand trans to the first PR(3) ligand to give trans(P), cis(C)-[ReL(CO)(2)(PR(3))(PR'(3))]. The structures of the complexes were determined in the solid state by X-ray crystallography and in solution by (1)H NMR spectroscopy. Electronic absorption spectroscopy reveals a prominent band in the visible region at relatively low energy (472-491 nm) for all complexes, which is assigned as a π-π* transition of the dipyrrin chromophore. Weak emission (λ(ex) = 485 nm, quantum yields <0.01) was observed for [ReL(CO)(3)Cl](-) and [ReL(CO)(3)PR(3)] complexes, but no emission was generally evident from the [ReL(CO)(2)(PR(3))(PR'(3))] complexes. On the basis of the large Stokes shift (~6000 cm(-1)), the emission is ascribed to phosphorescence from a triplet excited state. The emission intensity is sensitive to dissolved oxygen and methyl viologen; a Stern-Volmer plot in the latter case gave a straight line. Photochemical ligand substitution reactions of [ReL(CO)(3)PR(3)] were induced by excitation with a 355 nm laser in acetonitrile. [ReL(CO)(2)(PR(3))(CH(3)CN)] is formed as a putative intermediate, which reacts thermally with added PR'(3) to produce [ReL(CO)(2)(PR(3))(PR'(3))] complexes.
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