Near-IR absorption

One of the earliest series of metal complexes which showed strong, redox-dependent near-IR absorptions is the well-known set of square-planar bis-dithiolene complexes of Ni, Pd, and Pt (Scheme 4). Extensive delocalization between metal and ligand orbitals in these non-innocent systems means that assignment of oxidation states is problematic, but does result in intense electronic transitions. These complexes have two reversible redox processes connecting the neutral, monoanionic, and dianionic species. 

Some particular change is taking place in the nature of the near-IR absorption at —0.2 V,... 

Figure 3. Energy schemata of transitions involving vibrational states (a excitation of 1st vibrational state - mid-IR absorption b excitation of overtone vibrations - near-IR absorptions c elastic scattering - Rayleigh lines d Raman scattering - Stokes lines e Raman scattering - Anti-Stokes lines f fluorescence).

Nonetheless, near-IR is the most widely used IR technique. Less intense water absorptions permit to increase the sampling volume to compensate, to some extent, for the lower near-IR absorption coefficients and the inferior specificity of the absorption bands can for many applications be overcome by application of advanced chemometric methods. Miniaturised light sources, various sensor probes, in particular based on transmission or transflectance layouts, and detectors for this spectral range are available at competitive prices, as are (telecommunications) glass or quartz fibres. 

Fig. 2 Jablonski energy level diagram illustrating possible transitions, where solid lines represent absorption processes and dotted lines represent scattering processes. Key A, IR absorption B, near-IR absorption of an overtone C, Rayleigh scattering D, Stokes Raman transition and E, anti-Stokes Raman transition. S0 is the singlet ground state, S, the lowest singlet excited state, and v represents vibrational energy levels within each electronic state.

IR and visible near-IR absorption, NMR, ESR and X-ray diffraction. It was emphasized that solid-state high-resolution 13C NMR spectroscopy is a powerful tool for the structure analysis of polydiacetylenes. THY is particularly attractive because of its potential for forming the polydiacetylene ladder polymer. 

Figure 1. The near IR absorption spectrum of anhydrous liquid hydrofluoric acid with a few of the many possible IR wavelengths obtainable from a neodymium-doped glass laser superimposed.

Deoxvhemerythrin. Henrerythrin appears to retain its triply bridged crae structure in the deoxy form. A low (3.9 A) resoluticm difference electron density map of deoxyHr vs. metHr fi- n X-ray diffraction suggests that the iron atoms move slightly further apart in deoxyHr, but remain five and six coordinate, respectively (29). Confirmation of the iron coordination comes from near-IR absorption and circular dichroism spectra (40,46,47). Based on model high-spin ferrous complexes, the six coordinate iron is expected to give two of the three observed transitions near 10000 cm- while the five-coordinate iron accounts for the d-d transition at ca. 5000 cm-. ... 

Its one-electron oxidation (Fe Fe ) product has a comproportionation constant of 10 and near IR absorption band with 940 = 27,000, both consistent with Robin and Day334,335 jjj valence character7 The bis-triazacyclononane ligand (142), btacn,... 

Figure 14.1. Visible-near-IR absorption spectrum of poly(methylmethacrylate) (PMMA) dissolved in...

Figure 14.12. Visible-near-IR absorption spectra of 10FEDA/4FMPD and 6FDA/TFDB polyimide films.

There are a large number of spectroscopic reports involving Erythrosin, particularly from Russian laboratories. The near-IR absorption spectra of both Eosin and Erythrosin triplets and their dianion radical cations have been reported [236], the pK s of both species measured [237], and the influence on radiative and radiationless transitions by external heavy atoms such as iodide reported [238]. The nonradiative decay of Erythrosin in water at room temperature has also been studied by photoacoustic spectroscopy [239],... 

Near IR absorption minutes 2 x Id1 L — cross section uncertain... 

It has been found that whereas Cu11 porphyrins luminesce, the Ag" complexes do not. By an examination of electronic absorption spectra, emission spectra, redox potentials and near-IR absorption data, it was proposed that this could be rationalized on the basis of the energy of the big (dx2 yi) orbital. Extended Hiickel molecular orbital calculations predicted that the d-n transition would be above the lowest (n, jt ) levels for Cu11 but below them in the Agn complexes. The near-IR absorptions found for Ag11 were attributed to CT transitions.554... 

The photolytic flash must have enough energy to prepare, in a very short time, a detectable concentration of transient species. The lowest detectable concentration depends on the probe technique, and here the methods of UV/VIS/near IR absorption and emission spectroscopy are the best. Their drawback is that they provide very little structural information about the nature of the transient species. IR and Raman spectra are much more informative, but they present many problems in fast reaction kinetics because of the weakness of the signals. 

Figure 1 shows the schematic chemical structure of the two dithiolenes reported here. The near IR absorption peaks and linear loss at 1064 nm are listed in Table I below. 

Very recently, Zuo et al. (368) reported third-order NLO properties of a compound previously studied for its strong near-IR absorption, namely, ( -Bu4N)[Ni(phdt)21 (Section IV.A and 12 in Scheme 20), as well as compounds previously studied for their electrical conductivity, ( -Bu4N)[Ni(dddt)2] and ( -Bu4N)[Ni(pddt)21 (pddt2- = [C5H6S4]2, 6,7-dihydro-5//-1,4-dithiepinin-2,3-dithiolato) (Section II and Scheme 5). 

Fig. 7. (A) Absorption and (B) electroabsorption spectra of [ (NH3)5Ru 2(M-4,4 -bpy)]5+ aty = 90° (—)andy = 54.7° (—tare shown. Fifty percent glycerol/H20,77 K,Fe t = 4 x 105 V/cm. The near-IR absorption has been magnified 5x, (Reproduced with permission from Ref. (18). Copyright [1991] American Chemical Society.)...

Figure 11 Polarized room temperature near-IR absorption spectra of a Blastochloris viridis bacterial reaction center single crystal of 200 xm thickness. Spectra were taken with the crystal encapsulated and preserved in a thin-walled capillary while illuminated with a 633nm HeNe laser. The spectral band seen near 1300nm is of the photogenerated bacteriochlorophyll special pair cation P+. The amplitude of this peak allows the extent of photoconversion of the sample and its kinetics to be directly monitored (Drawn from data in Baxter et al. )...

Figure 4.31 Redox potentials E and wavenumber i of the near-IR absorption maxima of the sandwich complexes M(OEP)2 as functions of the ionic radii r of the trivalent central metal M [43c]. (Reprinted with permission from J.W. Buchler, and B. Scharbert, Metal complexes with tetrapyrrole ligands. 50. Redox potentials of sandwichlike metal bis(octaethylporphyrinates) and their correlation with ring-ring distances, Journal of the American Chemical Society, 110, no. 13, 4272-4276, 1988. 1988 American Chemical Society.)...

Abramczyk H, Kroh J. (1992) Near-IR absorption spectrum of the solvated electron in alcohols and deuterated water. Radiat Phys Chem 39 99-104. 

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