Chromophores pyridine

The general features of scaevodimerine B were very similar to those of 71 ([ ]D +39°, C21H23N3O4), but the UV spectrum (222,283 nm) indicated that only pyridine chromophores were present. The NMR spectrum dis-... 

In this section on the heterospin system, we will discuss three subjects 1) the magnetic coupling between the metal ion and organic spins through pyridine chromophores, 2) the photochemical formation of ferri- and ferrimagnetic chain (ID), and 3) the attempt to create frozen solution magnets (a carbene-based magnet). 

PCTA is a tetraazamacrocyclic ligand bearing a pyridine chromophore and three carboxylic functions. It offers seven potential donor atoms able to coordinate a lanthanide. The stability constant of 20.3 for (Eu )PCTA is acceptable in order to work in the presence of competing ions or ligands [101]. A well known complex 37 is based on a phosphonate equivalent of DOTA Tb(III)-3,6,9-tris(methylene phosphonic acid -butyl ester)-3,6,9,15-tetraazabicyclo[9.3.1]-pentadeca-l (15),ll,13-triene (Tb-PCTMB) [102,103] k azx. = 270nm, 8259 = 3,000cm X (H2O) = 4.98 ms [104] tot (H2O) = 0.51. As one could expect from a single pyridine chromophore the absorption is rather low and the absorption falls below the workable window (300-340 nm) Amax = 269 nm, 6259 = 4,600 cm ... 

One strategy to keep the water solubility of these complexes is to introduce in this para position polyoxyethylene arms fitted by different functions, such as alcohol, antine, or carboxylic acid. This later can be coupled to a second chromophore, such as a coumarin, with a higher excitation wavelength, which enables a double sensitization of the lanthanide ions (dominated by the efficient sensitization of the pyridine chromophore under 300 nm and by the sole absorption, and hence sensitization, of the comnarin moiety above 300 nm). ... 

In the 1988-1999 period, almost all absolute kinetic studies of carbenic reactions employed LFP with UV detection. Carbenes that contain a UV chromophore (e.g., PhCCl) are easily observed, and their decay kinetics during reaction can be readily followed by LFP.11 However, alkyl, alkylhalo, and alkylacyloxycarbenes are generally transparent in the most useful UV region. To follow their kinetics, Jackson et al. made use of the ylide method, 12 in which the laser-generated carbene (2) is competitively captured by (e.g.) pyridine, forming a chromophoric ylide (3, cf. Scheme 1). The observed pseudo first order rate constants (kobs) for the growth of ylide 3 at various concentrations of pyridine are monitored by UV spectroscopy, and obey Eq. 1. 

MP >260°C, [a]D —40°), which is also a natural product from T. coffeoides (67). Its UV spectrum [ max 227(19200), 260(17700), and 335(13200) in acid solution] indicated a 2-acylindole chromophore perturbed by the presence of a carbonyl group in position 6. Its IR spectrum exhibited two carbonyl absorptions at 1670 and 1615 cm-1 for the carbonyl functions located at positions 3 and 6, respectively, while in the H-NMR spectrum (pyridine-d5) the aromatic proton H-9 was shifted downfield by the neighboring C-6 carbonyl. 

The assumption that AOM parameters are transferable from one system to another is one which deserves close scrutiny. For example, the negative value of for pyridine found in [Cr(py)4X2]+ depends on the assumption that the ex parameters for the halides are the same as in [Cr(NH3)4X2 ]+ (58). However, there is some reason to believe that the AOM parameters for halides may differ considerably from one compound to another in Cr(III)N4X2 chromophores, even when the equatorial ligands are closely related. Thus Barton and Slade (57) found ea for F to range between 7.2 kK and 9.0 kK, and ew between 1.7 kK and 2.5 kK, in a series of CrN4Xj systems where the equatorial ligands were saturated amines. This could account for the negative value of en obtained for pyridine, which seems inconsistent with the positive values found for Fe(II), Co(II) and Ni(II) complexes (48, 66,153,154). 

In an effort to prepare bidentate boranes as colorimetric anion sensors, the incorporation of chromophoric boron moieties has also received attention. Reaction of lO-bromo-9-thia-lO-boranthracene 33 with dimesityl-1,8-naphthalenediylborate 26 affords diborane 34 (Scheme 14). This bright yellow diborane is soluble in chloroform, THF and pyridine. It has been fully characterized but its X-ray crystal structure could not be determined experimentally. Its structure was computationally optimized using DFT methods (B3LYP, 6-31 + G for the boron and sulfur... 

Given the availability of a suitable chromophore, the use of light offers the mildest possible thermal environment in which to effect a synthetic transformation insofar as photoinduced processes may be activated at temperatures well below 0 °C. Not unexpectedly then, one finds the photosynthetic procedure to be, thus far, the only one which has been successfully applied to the construction of the relatively sensitive parent (unrestricted) 77- excessive heteroannulenes as shown in Scheme 9a. Similarly, exposure of the tetracyclic azides depicted in Scheme 9b to low-pressure mercury irradiation leads to deazotation and the formation of the pyridine-like 14- and 18-membered parents (49) and (75). 

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