Triadic Intramolecular Charge Transfer Compound of Tetrathiafulvalene Exhibiting Multicolor Solvatochromism

An A−D−A triad compound comprising a tetrathiafulvalene (TTF) moiety and two pyridyl groups, py−TTF−py (1), has been studied in view of intramolecular charge transfer (ICT). The compound shows a sharp and multicolor change in different solvents and at different pH values, exhibiting good solvatochromism. The results of electronic absorption spectroscopy, cyclic voltammetry, and theoretical calculations confirm that there exists predominantly a monoprotonated A−D−A triad that displays a strong ICT effect, which is tunable as a function of pH. The equilibrium of protonation of 1 has been further studied by means of pH titration, and the result confidently supports the conclusion that only one equivalent H+ combines with py−TTF−py in dilute solution, even when excessive acid is added. However, unlike the state in dilute solution, the crystal structure of the protonated 1 is a diprotonated A−D−A triad, (1·2H+)·(CF3SO3−)2·H2O (2), that has been structurally characterized.