Absorption and fluorescence solvatochromic behaviors of centrosymmetric D-π-D molecules with TTF/dimethylamino electron donors and polyenic π-bridge

A new centrosymmetric D-π-D molecule, TTF-CH=CH-CH=CH-CH=CH-TTF (1, TTF: tetrathiafulvalene) was synthesized and its absorption and fluorescence solvatochromic behaviors were compared with those of Me2NPh-CH=CH-CH=CH-CH=CH-PhNMe2 (2). The lowest-energy absorption band of 1 shifts to blue as the solvent polarity increases. The wavenumber of the maximum peak correlates well with the Onsager polarity function f(ε)−f(n2)=(ε−1)/(2ε+1)−(n2−1)/(2n2+1). Considering the non-dipolar nature of 1 due to its highly symmetric structure, we conclude that the observed negative solvatochromism is related to the quadrupole (solute)-dipole (solvent) electrostatic interaction. For 2, we see the trend of positive solvatochromic shift, although the absorption maximum correlates only very weakly with f(n2). The result suggests that the solvatochromism is mainly related to the dispersion-like interaction. The different absorption solvatochromic behaviors of 1 and 2 can be attributed to the strong quadrupole character of 1 relative to that of 2, resulting from very strong donating ability of TTF. As for the fluorescence, positive solvatochromism is similarly observed for 1 and 2. The emission maxima correlate fairly well with f(ε)−f(n2). Most probably, molecules 1 and 2 experience the excited-state symmetry breaking with possible geometrical relaxation from non-dipolar Frank-Condon states to dipolar, charge transfer emitting states at least in polar solvents.