Unusual solvatochromism of the 4,4′-bis(dimethylamino)benzophenone (Michler's ketone)–tetracyanoethene electron donor–acceptor complex
✍ Scribed by Stefan Spange; Elmar Vilsmeier; Simone Adolph; Annett Fährmann
- Publisher
- John Wiley and Sons
- Year
- 1999
- Tongue
- English
- Weight
- 128 KB
- Volume
- 12
- Category
- Article
- ISSN
- 0894-3230
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✦ Synopsis
The transition energy of the charge-transfer UV/Vis absorption maxima (# max, CT ) and the structure of the Michler's ketone (MK)-tetracyanoethene (TCNE) electron donor-acceptor (EDA) complex are remarkably solvent dependent. The UV/Vis spectra of the MK-TCNE complex were measured in 20 non-protic and seven protic solvents. In non-protic solvents, # max, CT of the EDA complex is quantitatively described by a multiple LSE relationship using the Kamlet-Taft dipolarity/polarizability (p*) and basicity (b) parameters of the solvents. The influence of the two terms b and p* on # max, CT is opposite, indicating a qualitatively different solvent-induced stabilization of the electronic ground and excited states of the complex. As expected, increasing dipolarity/ dipolarizability of the solvent causes a bathochromic band shift (positive solvatochromism), whereas the basicity of solvents is responsible for a hypsochromic band shift due to specific solvation of the TCNE site. In protic solvents, the complex formation is associated with the formation of an ionic species (n max = 19 800 cm À1 ) derived from Michler's ketone due to coordination of TCNE at the carbonyl oxygen of MK (called an n-complex). The solvent-induced switching of the p-complex into the n-complex is demonstrated for mixtures of DCE with protic solvents and for silica surfaces.