Metal centered ligand field excited states: Their roles in the design and performance of transition metal based photochemical molecular devices

The study of the photochemistry and photophysics of octahedral and pseudo-octahedral Cr(III) complexes has a rich history. An initial discussion is devoted to a general appraisal of the state of these two subjects up to December 1998, after providing a framework of state energy levels and radiative and non-radiative relaxation processes relevant to Cr(III) systems. The remaining sections cover some of the more active areas in the Cr(III) field, such as ultrafast dynamics, photosubstitution, thermally activated excited state relaxation, energy transfer, and photoactivated redox processes (both intermolecular and intramolecular). Each of these sections begins with an overview of the subject area, and then one or more representative papers from the recent literature are selected for more detailed discussion.

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Photochemistry and Photophysics of Coordination Compounds: Chromium

Alkynyl complexes of the type [M(cyclam)(CCR)2]OTf (where cyclam = 1,4,8,11-tetraazacyclotetradecane; M = Rh(III) or Cr(III); and R = phenyl, 4-methylphenyl, 4-trifluoromethylphenyl, 4-fluorophenyl, 1-naphthalenyl, 9-phenanthrenyl, and cyclohexyl) were prepared in 49% to 93% yield using a one-pot synthesis involving the addition of 2 equiv of RCCH and 4 equiv of BuLi to the appropriate [M(cyclam)(OTf)2]OTf complex in THF. The cis and trans isomers of the alkynyl complexes were separated using solubility differences, and the stereochemistry was characterized using infrared spectroscopy of the CH2 rocking and NH bending region. All of the trans-[M(cyclam)(CCR)2]OTf complexes exhibit strong Raman bands between 2071 and 2109 cm–1, ascribed to νs(C≡C). The stretching frequencies for the Cr(III) complexes are 21–28 cm–1 lower than for the analogous Rh(III) complexes, a result that can be interpreted in terms of the alkynyl ligands acting as π-donors. UV–vis spectra of the Cr(III) and Rh(III) complexes are domin...

Synthesis of cis and trans bis-alkynyl complexes of Cr(III) and Rh(III) supported by a tetradentate macrocyclic amine: a spectroscopic investigation of the M(III)-alkynyl interaction.

Arylethynylchromium(III) complexes of the form trans-[Cr(cyclam)(CCC(6)H(4)R)(2)]OTf (where cyclam = 1,4,8,11-tetraazacyclotetradecane, R = H, CH(3), or CF(3) in the para position, and OTf = trifluoromethanesulfonate) have been prepared and characterized by IR spectroscopy and X-ray diffraction. The complexes are emissive with excited-state lifetimes in a deoxygenated fluid solution between 200 and 300 micros.

Emissive chromium(III) complexes with substituted arylethynyl ligands

This chapter reviews the literature published during 2006 on macrocyclic coordination chemistry. Porphyrin ligands and supramolecular chemistry are not covered in this review as there is insufficient space and adequate coverage elsewhere, although a few examples may be included where particularly relevant. In general, the focus is on the coordination properties and reactivity of the compounds, although some applications will also be discussed, particularly in a brief section at the end of the chapter. Some representations of X-ray structures have been included to show particularly interesting coordination geometries, ion encapsulation or ligand orientation.

Macrocyclic coordination chemistry

The synthesis and characterization of several Cr(III) complexes of the constrained macrocyclic ligand 1,11-C3-cyclam (1,4,8,11-tetraazabicyclo[9.3.3]heptadecane) is reported. Only trans complexes are formed, and the structure of trans-[Cr(1,11-C3-cyclam)Cl2]PF6 is presented. The chemical and photophysical behavior of the 1,11-C3-cyclam complexes are compared with those of the corresponding cyclam (1,4,8,11 tetraazacyclotetradecane) and 1,4-C2-cyclam (1,4,8,11-tetraazabicyclo[10.2.2]hexadecane) complexes. The aquation rate of trans-[Cr(1,11-C3-cyclam)Cl2]+ is similar to that of the corresponding 1,4-C2-cyclam complex and is more than 5 orders of magnitude faster than the cyclam counterpart. A monotonic increase in the extinction coefficient is observed on going from the cyclam complexes to the 1,11-C3-cyclam complexes to the 1,4-C2-cyclam complexes, and this is related to the degree of centrosymmetry in each complex. The trans-[Cr(1,11-C3-cyclam)(CN)2]+ complex is a weak emitter in aqueous solution with a room-temperature emission maximum at 724 nm (tau=23 micros). Like the corresponding 1,4-C2-cyclam complex (tau=0.24 micros), the 1,11-C3-cyclam complex shows no deuterium-isotope effect in room-temperature solution. This is in marked contrast to the corresponding cyclam complex which has an emission lifetime of 335 micros and a significant deuterium isotope effect in room-temperature solution. Low temperature (77K) data are also presented in an attempt to understand the differences in photophysical behavior.

Effects of steric constraint on chromium(III) complexes of tetraazamacrocycles, 2. Comparison of the chemistry and photobehavior of the trans-dichloro- and trans-dicyano- complexes of cyclam, 1,4-C2-cyclam, and 1,11-C3-cyclam.

Kimberley Wright-Garcia

Papers

Effects of steric constraint on chromium(III) complexes of tetraazamacrocycles, 2. Comparison of the chemistry and photobehavior of the trans-dichloro- and trans-dicyano- complexes of cyclam, 1,4-C2-cyclam, and 1,11-C3-cyclam.