Unusual solvatochromism of the 4,4′-bis(dimethylamino)benzophenone (Michler's ketone)-tetracyanoethene electron donor-acceptor complex

The transition energy of the charge-transfer UV/Vis absorption maxima (νmax, 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, νmax, 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 νmax, 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 (νmax = 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.