Ground- and excited-state pinched cone equilibria in calix[4]arenes bearing two perylene bisimide dyes

We report on a series of bis-chromophoric compounds o2c, g2c, and r2c, afforded by linking two identical orange, green, or red perylene bisimide (PBI) units, respectively, through a calix[4]arene spacer unit. The PBI units are characterized by their increasing sterical demand from a planar conformation, which is orange (o) colored, via the slightly distorted greenish (g) colored form to the strongly distorted derivative, which is red (r) colored. An equilibrium between the two possible pinched cone conformations of the calix[4]arene unit is observed for all three compounds, with one conformation showing a π-stacked sandwich arrangement of the PBI units and the second revealing a nonstacked conformation. The amount of the π-stacked conformation in the equilibrium enhances upon decreasing the steric encumbering of the respective PBI unit as well as upon lowering the solvent polarity, thus increasing "-" interactions as well as electronic coupling between the PBI units. Accordingly, the presence of the π-stacked calix[4]arene conformation is most pronounced for compound o2c in methylcyclohexane (MCH), CCl