Electrochemistry and spectroscopy of intramolecular charge-transfer complexes. p-N,N-Dialkylaminobenzylidenemalononitriles

Abstract

The spectroscopic and electrochemical properties of a series of p-iV^ZV-dialkylaminobenzylidenemalononitriles derivatives (1, 2, and 3) have been studies. These materials exhibit a strong intramolecular charge-transfer absorption. In this series the conformational freedom of the electron-donating group (R2N) was progressively restricted by structurally anchoring the nitrogen atom to the aromatic ring by alkyl chains. Molecular restriction of rotation of the NR2 group had a profound effect on the nonradiative decay rate and the excited state dipole moment. However, it had little effect on the ground state dipole moment. The most pronounced change in dipole moment ( µ) on electronic excitation was observed when the NR2 group was free to rotate. The rate of internal rotation of the dialkylamino group was estimated to be 2 X 10n s'1.

Internal rotation in [p-(dialkylamino)benzylidene]malononitriles

“…The same effect is also observed in solvents of increasing polarity. Lowering the temperature would cause a hypsochromic shift of the fluorescence spectra without affecting the band width (2). The difference in the activation parameters of the two compounds is, therefore, expected to be small and could not be measured by this technique.…”

“…Loutfy (Xerox Research Centre, Canada) and were prepared and purified as described before (2). The deuteriated solvents were from Aldrich Chem.…”

“…1) with interesting photophysical properties (2)(3)(4)(5). Their absorption and fluorescence emission spectra are solvent polarity dependent and their fluorescence quantum yields are solvent viscosity dependent.…”

“…(1 -3) x (2-4). The low quantum yield of emission and fast decay times were attributed to the rotation of the dialkylamino or malononitrile moieties (2)(3)(4). Thus, the rotation of the dialkylamino group in compound 1 was estimated to be of the order of 10" s-' in the excited state (2,3).…”

“…The low quantum yield of emission and fast decay times were attributed to the rotation of the dialkylamino or malononitrile moieties (2)(3)(4). Thus, the rotation of the dialkylamino group in compound 1 was estimated to be of the order of 10" s-' in the excited state (2,3). In order to obtain an estimate of the rate of rotation of both the dialkylamino and the malononitrile moieties, we have investigated the temperature dependence of the dynamic structure of these molecules in the ground state by 'H nmr spectroscopy.…”