Excited states, TRIR spectroscopy

Besides TRIR, it is also possible to employ excited-state resonance Raman spectroscopy (TR or TR2) to characterize excited-states of [Re(L)(CO)3(N,N)] 1 complexes [25, 27, 37, 48, 79, 81, 82], The spectra show bands due to vibrations of the N,N ligand and provide information on its structural changes upon excitation. Picosecond TR3 spectra of Re complexes give very weak signals [37], Measurements on the ns timescale are more informative. In the case of CT and TL states of [Re(py) (CO)3(dppz)]+ and the 3LLCT state of [Re(py-azacrown)(CO)3(bpy)]+ [16, 27], complementary vibrational information was provided by TR3 and TRIR measured in the fingerprint region. 

Time-resolved Infrared spectroscopy (TRIR), a combination of UV flash photolysis and fast IR spectroscopy (ns), has been outstandingly successful in identifying reactive intermediates [5] and excited states [6] of metal carbonyl complexes in solution at room temperature. We have used infrared spectroscopy to probe the mechanism of photo-17] and electrochemical [8] catalytic reduction of COj. We have used TRIR to study organometallic reactions in supercritical fluids on a nanosecond time-scale [9-10]. 

The IR and Raman spectra of MLnI2, where M = Ni, n = 4 M = Zn or Cd, n = 2, L = m-methylaniline, gave quite detailed ligand mode vibrational assignments.197 The IR spectra of platinum(II) complexes in carbamide and carbamide-halide melts show the formation of Pt(NH3)42+ on dissolution of (NH4)2[PtCl4], as well as Pt(NH3)X3 in the presence of NH4+X, where X = Cl or Br.198 Picosecond-scale TRIR spectroscopy was used to probe the dynamics of the lowest excited state of Pt(bipy)(4-CN-C6F4-S)2.199... 

Time-resolved infrared spectroscopy (TRIR) has been outstandingly successful in identifying reactive intermediates and excited states of both metal carbonyl [68,69] and organic complexes in solution [70-72]. Some time ago, the potential of TRIR for the elucidation of photochemical reactions in SCFs was demonstrated [73]. TRIR is particularly suited to probe metal carbonyl reactions in SCFs because v(CO) IR bands are relatively narrow so that several different species can be easily detected. Until now, TRIR measurements have largely been performed using tunable IR lasers as the IR source and this has restricted the application of TRIR to the specialist laboratory [68]. However, recent developments in step-scan FTIR spectroscopy promise to open up TRIR to the wider scientific community [74]. 

NN = polypyridyl ligands and L = stilbene-like ligands, in acetonitrile solution and in poly(methyl methacrylate) (PMMA) polymer film exhibited hypsochromic shifts as the medium rigidity increases due to the luminescence rigidochromic effect. Time-resolved IR (TRIR) spectroscopy, in combination with other techniques, characterized the excited-state electronic properties of the fac-[Re(CO)3(phen) (bpe)]PFa complex, where bpe is l,2-bis(4-pyridyl)ethylene. 

An understanding of the structure and properties of excited states in the photochemistry of coordination compounds is of fundamental importance. Furthermore, monitoring electron and energy transfer is vital for a wide range of applications. TRIR spectroscopy often provides key information on the excited states of coordination compounds, in particular those containing CO or CN ligands, since these groups act as direct probes of the electron density at the metal center. 

The excited states of coordination compounds that do not contain CO or CN ligands have also been investigated using TRIR by probing peripheral organic j/(CO) reporter groups, such as carboxylic acids, esters, and amides, on substituted diimine ligands. For example, step scan FTIR was used to probe the ester /(CO) bands in the MLCT states of [Ru(bpy)2(4,4 -(COOEt)2-bpy)] + and [Ru(bpy)2(4-COOEt-4 -CH3-bpy)] " ". Such an approach has been used with ps-TRIR spectroscopy to probe the excited state dynamics in the Pt chromophore, Pt(4,4 -(COOEt)2-bpy)Cl2. The lifetime of the MLCT excited state was found to be 8.7 ps. ... 

These questions have been recently addressed using ultrafast time-resolved infrared spectroscopy (Fig 3.14), on the example of stretching vibration v(CN) in the ground state, at 2239 cm is clearly seen. Notably, the IR bands in the excited state are much broader, and more intense than... 

The value of vCO in (36) is 1907 cm ie. very high for a bridging carbonyl -ascribed to the semibridging nature of the bonding. TRIR spectroscopy of transition metal carbonyls, e.g. W(CO)5py, showed that MLCT excited states gave broad vCO bands due to solvent-solute interactions. ... 

The lack of excited state involvement for 21 is in contrast to the behavior in systems that have substantial equilibrium concentrations of the syn conformer. To examine the effect that conformation has on ketene growth kinetics, 4-diazo-3-isochromanone (22), ° a cyclic analogue (phenyl version) of 21 that is locked in the syn conformation, was also recently studied by TRIR spectroscopy. In this case, a carbene IR band is observed at 1686 cm that decays with a lifetime of 526 50 ns in Freon-113. The ketene IR band at 2116 cm, however, in dramatic contrast to the data observed with acyclic diazocarbonyl 21, is produced faster than the experimental time resolution. Ketene 24, therefore, is formed entirely from a non-carbene source, presumably the excited state of 22 (Scheme 8). In agreement with this hypothesis, oxygen and methanol quench carbene 23, but they leave the initial intensity of the ketene IR band unaffected. These... 

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