Photochemistry and Photophysics of Coordination Compounds

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Photochemistry and Photophysics of Coordination Compounds: Copper 101 4 Cuprous Clusters 4.1 Cuprous Halide Clusters Cluster compounds contain a group of two or more metal atoms where direct and substantial metal-metal bonding is present. Cuprous halide clusters have been known for about 100 years [142], their general formula is CunXnLm(X =Cl – ,Br – or I – ; L = N or P belonging to an organic molecule). For instance, in solution, mixtures of Cu(I) salts, iodine (I) and pyridine-type molecules (py) are primarily present as tetrahedral clusters Cu4I4py4 and give origin to mononuclear or dinuclear structures only if forced by mass action law under high pyridine concentration. Normally, copper(I) complexes in solution are quite labile towards ligand substitution and the formation of new species is driven by thermodynamic stability rather than kinetic control. Fig. 30 Illustrations of Cu4I4py4 (A), Cu2I2py4 (B), [Cu3(µ-dppm)3(µ3 – η 1 -CΞC-benzo- 15-crown-5)2] + (C), and the repeating unit of a “stairstep” polymer [CuIpy]n (D)redrawn from the structural data. Black circles =copperatoms;grey circles =iodine;white rings = pyridine residues
102 N. Armaroli et al. Cuprous clusters display many structural formats that are characterized by largely different emission behavior (vide infra). The variety of structural motifs and stoichiometries is related to the remarkable flatness of their ground state potential energy surfaces [143]. The most extensive studies carried out on these Cu(I) complexes concern cubane-type clusters of general formula [CuXL]4 [144, 145]. The solid state structural variety observed among cuprous clusters can be illustrated by examining some crystallographic data reported by Ford and co-workers, who found that compounds containing Cu(I), iodine, and pyridine generate different structures depending on the reaction stoichiometry, Fig. 30. For a 1:1:1 Cu:I:L ratio (Fig. 30 A), the most common motif is the tetranuclear “cubane” structure (Cu4I4L4) in which a tetrahedron of copper atoms is included by a larger I4 tetrahedron where each iodine is placed on a triangular face of the Cu4 cluster and the fourth coordination site of each copper is occupied by the ligand (L). For stoichiometry 1:1:2 (Fig. 30 B), the most common structure is an isolated rhombohedron of Cu2I2 with alternating copper and halide atoms. Sometimes, clusters with stoichiometry (1:1:1) exist in more than one crystalline structure. For example (Fig. 30 C) Cu4I4py4 can also give rise to a polymeric “stair” made of an infinite chain of steps [146]. 4.2 Cuprous Iodide Clusters The interest in the luminescence properties of Cu(I) iodide clusters goes back to the pioneering work of Hardt and co-workers [144]. They found that the emission spectra of solid samples of [CuxIy(py)z] are markedly temperaturedependent and defined the term “luminescent thermochromism”. In some cases cuprous iodide clusters exhibit two emission bands termed HE (high energy) and LE (low energy), which sharply change their relative intensities upon temperature variation. As an example, in Fig. 31 are depicted the temperature-dependent emission spectra of Cu4I4(4 – phenylpyridine)4 [147]. The LE band dominates at room temperature, while the HE band is by far the strongest at temperatures below 80 K. The HE band dominating at low temperature has been attributed, on the basis of ab initio calculations and experimental work, to ligand-to-ligand (I – → phenylpyridine) charge transfer states, also indicated as XLCT. The LE emission dominating at room temperature has been assigned to an excited state of mixed halide-to-metal charge transfer (XMCT) and d → s,p metalcentered character which is usually referred to as “cluster-centered” (CC). This term was introduced to highlight that these transitions are localized on the Cu4I4 cluster and are essentially independent on ligand L. The Cu–Cu distance is a fundamental parameter to allow the presence of CC bands and must be shorter than the orbital interaction radius, estimated to be 2.8 ˚A. Ifthe distance between the two metal centers exceeds this critical value the metal
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280 Topics in Current Chemistry Edi
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Photochemistry and Photophysics of
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Volume Editors Prof. Vincenzo Balza
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Topics in Current Chemistry Also Av
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X Preface nation Compounds. The ven
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Contents Photochemistry and Photoph
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258 Author Index Volumes 251-280 Be
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Subject Index Alkyne bridges 148 An
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Subject Index 273 Rh(III) cyclometa
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Photochemistry and Photophysics of Coordination Compounds

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