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

We report oil 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 pi-stacked sandwich arrangement of the PBI units and the second revealing a nonstacked conformation. The amount of the pi-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 pi-pi interactions as well as electronic coupling between the PBI units. Accordingly, the presence of the pi-stacked calix[4]arene conformation is most pronounced for compound o2c in methylcyclohexane (MCH), CCl4, and toluene, and to a lesser extent for compound g2c in MCH. Almost no pi-stacked conformation is found for the most sterically demanding red system r2c. Molecular modeling studies at the force-field level show that the pi-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 pi-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.