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 π−π 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 4, and toluene, and to a lesser extent for compound g2c in MCH. Almost no π-stacked conformation is found for the most sterically demanding red system r2c . Molecular modeling studies at the force-field level show that the π-stacked conformations of the least sterically hindered dimmer o2c are lower in energy than those of the nonstacked ones in a nonpolar environment, whereas, in polar media, these stabilities are inversed. A quantitative analysis of the excited-state properties has been obtained by UV/vis absorption, steady-state and time-resolved emission, and femtosecond transient absorption spectroscopy. Global and target analysis of the femtosecond transient absorption data is used for probing ground-state populations with excited-state dynamics. The o2c molecules in the π-stacked conformation display a distinctly different decay behavior relative to the molecules in the nonstacked conformation. The former population decays via an excimer state, with the latter population decays through a charge separation process involving the calix[4]arene as a donor.