Tetrathiafulvalene-1,3,5-triazines as (Multi)Donor–Acceptor Systems with Tunable Charge Transfer: Structural, Photophysical, and Theoretical Investigations

Palladium-catalyzed cross-coupling
reactions between chlorinated
1,3,5-triazines (TZ) and tetrathiafulvalene (TTF) trimethyltin derivatives
afford mono- and C3 symmetric tris­(TTF)-triazines
as donor–acceptor compounds in which the intramolecular charge
transfer (ICT) is modulated by the substitution scheme on TTF and
TZ and by chemical or electrochemical oxidation. The TTF-TZ-Cl2 and (SMe)2TTF-TZ-Cl2 derivatives show
fully planar structures in the solid state as a consequence of the
conjugation between the two units. Electrochemical and photophysical
investigations, supported by theoretical calculations, clearly demonstrate
that the lowest excited state can be ascribed to the intramolecular
charge transfer (ICT) π­(TTF)→π*­(TZ) transition.
The tris­(TTF) compound [(SMe)2TTF]3-TZ shows
fluorescence when excited in the ICT band, and the emission is quenched
upon oxidation. The radical cations TTF+• are easily
observed in all of the cases through chemical and electrochemical
oxidation by steady-state absorption experiments. In the case of [(SMe)2TTF]3-TZ, a low energy band at 5000 cm–1, corresponding to a coupling between TTF+• and
TTF units, is observed. A crystalline radical cation salt with the
TTF-TZ-Cl2 donor and PF6– anion,
prepared by electrocrystallization, is described.