Photochemistry of 2,6-Dicyano-N,N,N',N'-tetramethyl-p-phenylenediamine.

Experimental work in gas phase and solution:

Diploma Thesis of Ulf Rosspeintner (abstract from TUGOnline database);

Diploma Thesis of Markus Weilhofer (abstract from TUGOnline database);

Poster presentations at several conferences;

Paper: in preparation.

Theoretical work in gas phase and solution with the quantum mechanical CASSCF and TDDFT methods:

Diploma Thesis of Stefan Kontur (abstract from TUGOnline database);

Posters and Lectures: Excitation process, computed by CASSCF method;

Paper: in preparation.

Conclusion: CASSCF calculations of the title compound result in three ground state conformations A, B and C with different orientations of the amino-methyl groups on one side of the aromatic ring (structures on the right side in the figure). Conformers B and C are less stable than A (top right structure) by more than 8 kJ/mol, therefore only A can be seen in experimental investigations like NMR, IR spectra.
During excitation, the amino group on one side of the ring rotates (in B and C) and becomes planar (in all conformers) resulting in only two excited state geometries A* and C* (structures on the left side in the figure). This is accompanied by a raise of the dipole moment.
Whereas in the gas phase only C* shows a large dipole moment, in the solvent acetonitril both excited state conformers A* and C* have a very large dipole moment. Such a large dipole moment change can also be seen in the experimental data estimating an excited state dipole moment of around 11 D from the solvatochromic shift. The experimental Stokes shift is in good agreement with the computed difference between the absorption and emission energies.
This large dipole moment change is associated to a large geometry change resulting in an antiquinoid structure with planar amino group in A* and a twisted amino group in C* and to a charge transfer from the NMe2 groups to the aromatic system.
Summarizing the computational results and comparing them to the experimental data, A* represents the locally excited state (LE) and C* is clearly the charge transfer state (CT). The reason why no dual fluorescence can be observed in the experimental flourescence spectra, can be explained by the energetics of the excited state: conformer C* lies by more than 8 eV higher on the energy scale than A*, and the solvation does not bring the LE and CT states closer together. Therefore conformer A* is the only detected emitting state. Its antiquinoid geometry causes its charge transfer character, which is the reason for the measured Stokes shift.