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.