Indoles electronic absorption band

The longer wavelength electronic absorption band of indole appears in the near-UV region with molar extinction coefficients of several thousand in the range 255-295 nm. The band possesses characteristic structure with maxima or shoulders observable at 287, 278, 276, 270, and 266 nm when ethanol is the solvent [2d, 7]. It has been noted that the spectrum is very... 

The long wavelength absorption band of indole consists of two electronic transitions 1La and b, whose transition moments are almost perpendicular (more precisely, they are oriented at —38° and 56° to the long molecular axis, respectively). Figure B5.2.1 shows the excitation spectrum and the excitation polarization spectrum in propylene glycol at —58 °C. 

Annelation on to a benzene ring increases considerably the complexity of the spectra, and indole has absorptions at 216 (4.54), 266 sh (3.76), 270 (3.77), 276 (3.76), 278 (3.76) and 287 (3.68) nm in ethanol solution. Because of the widespread occurrence of the indole ring system in nature and the sensitivity of absorption band position and intensity to substitution type, considerable use has been made of electronic spectroscopy in the past for structure identification. An extensive tabulation of data, primarily for monosubstituted derivatives, is available (71PMH(3)67,p.94). As expected, whereas the effects of alkylation are comparatively slight, introduction of groups capable of mesomeric interaction with the indole it -system may cause profound changes in the appearance of the spectrum representative examples are given in Table 24. 

Luminescent ruthenium(II) polypyridine indole complexes such as [Ru (bpy)2(bpy-indole)]2+ (37) and their indole-free counterparts have been synthesised and characterised [77]. The ruthenium(II) indole complexes display typical MLCT (djt(Ru) tt (N N)) absorption bands, and intense and long-lived orange-red 3MLCT (djt(Ru) -> Ti (bpy-indolc)) luminescence upon visible-light irradiation in fluid solutions at 298 K and in alcohol glass at 77 K. In contrast to the rhenium(I) indole complexes, the indole moiety does not quench the emission of the ruthenium(II) polypyridine complexes because the excited complexes are not sufficiently oxidising to initiate electron-transfer reactions. Emission titrations show that the luminescence intensities of the ruthenium(II) indole complexes are only increased by ca. 1.38- to... 

The UV absorption spectrum of pergolide mesylate is omsistent with the absorption pattern for compounds with an indole chromophore. The electronic transition of the indole chromophore of the pergolide mesylate molecule is observed as follows the absorption bands at 280 and 274 nm are... 

---