Hydrogen-bonded solvents blue shifts

Fe(bipy)2(CN)2 and Fe(phen)2(CN)2 in CHCI3 show a red shift of ca. 3 kK as compared with the parent tris-diimine complexes 44). The energy of the ET band is strongly affected by hydrogen-bonding solvents IIS) and, quite dramatically, by protonation or by alkylation (7/4) on the CN nitrogens, which cause a blue-shift of 6—7 AK. Similar effects are observed in Fe(phen)(CN)4 and in Fe(bipy)(CN) 115). 

The colour of indigo depends dramatically upon its physical state and environment for example, the vapour is red but the colour on the fibre is blue. The marked solvatochromism of indigo (Table 6.4) is attributable mainly to hydrogen bonding. A progressive bathochromic shift of the visible absorption band is observed as the solvent polarity... 

The concept of polarity covers all types of solute-solvent interactions (including hydrogen bonding). Therefore, polarity cannot be characterized by a single parameter. Erroneous interpretation may arise from misunderstandings of basic phenomena. For example, a polarity-dependent probe does not unequivocally indicate a hydrophobic environment whenever a blue-shift of the fluorescence spectrum is observed. It should be emphasized again that solvent (or microenvironment) relaxation should be completed during the lifetime of the excited state for a correct interpretation of the shift in the fluorescence spectrum in terms of polarity. 

Effects of Hydrogen Bonding. In a solvatochromic plot of transition energy hv versus solvent polarity [e.g. the f D) function], the effect of hydrogen bonding between the solute and the solvent is an anomalous blue shift or red shift, of which an example is shown in Figure 3.53. This is the... 

Reichardt s dye has thus been used to estimate hydrogen-bond acidity of solvents. The absorption maximum of Reichardt s dye shows a blue shift when the solvent molecule interacts with the dye through a hydrogen bond. The a value (hydrogen-bond acidity) is estimated using ET(30) and it with Eq. (3.5). 

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