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Unusual solvatochromism of the 4,4′‐bis(dimethylamino)benzophenone (Michler's ketone)–tetracyanoethene electron donor–acceptor complex
Author(s) -
Spange Stefan,
Vilsmeier Elmar,
Adolph Simone,
Fährmann Annett
Publication year - 1999
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/(sici)1099-1395(199907)12:7<547::aid-poc162>3.0.co;2-1
Subject(s) - chemistry , solvatochromism , benzophenone , ketone , photochemistry , electron acceptor , acceptor , electron donor , organic chemistry , solvent , physics , condensed matter physics , catalysis
The transition energy of the charge‐transfer UV/Vis absorption maxima (ν 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, ν max, CT of the EDA complex is quantitatively described by a multiple LSE relationship using the Kamlet–Taft dipolarity/polarizability (π*) and basicity (β) parameters of the solvents. The influence of the two terms β and π* on ν 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 (ν max = 19 800 cm −1 ) 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 π‐complex into the n‐complex is demonstrated for mixtures of DCE with protic solvents and for silica surfaces. Copyright © 1999 John Wiley & Sons, Ltd.