Polymeric Indolenine-Squaraine Foldamers with a Preferred Helix Twist Sense and Their Chiroptical Absorption and Emission Properties

A series of dicyanomethylene-substituted cisoid indolenine squaraine polymers (X-n up to 65) bearing chiral side chains were prepared via Yamamoto homocoupling of the respective dibrominated monomers and subsequent fractionation by preparative gel permeation chromatography. In this series, the steric demand of the alkyl side chains and the position of the stereogenic center were systematically varied in order to create a different degree of structural perturbation. While S-3,7-dimethyloctanol as the starting material for the chiral side chain was commercially available, the analogous compounds R-2,7-dimethyloctanol and R-2-ethyl-7-methyloctanol bearing the chiral center at the 2-position were synthesized using Evans asymmetric alkylation as the key synthetic step. Absorption spectroscopy of the polymers revealed a solvent-dependent excitonic H- or J-type behavior, which is caused by a helix (H) or random coil (J) structure. For some solvents, circular dichroism (CD) spectroscopy of the polymers further showed a homohelical structure, with Delta epsilon values of up to 396 M-1 cm(-1) and g(abs) values of up to 0.0786 cgs, where an influence of the type of side chains on the homohelicity was apparent. Temperature-dependent absorption spectroscopy in PhCN allowed the measurement of the thermodynamic parameters of the helix folding, which revealed an increase in negative enthalpy and entropy when increasing the steric demand of the side chains. Comparison of temperature-dependent CD and absorption data allowed us to assess the degree of homohelicity, which was found to be nearly complete for all polymers in PhCN. Fluorescence spectroscopy of the random coil structures in toluene and CHCl3 further showed the typical emission from the lowest-lying excitonic state according to Kasha's rule. In toluene, emission was observed from the lowest energy state of the helix fraction of the polymer bearing 3,7-dimethyloctyl side chains, where circular polarized luminescence spectroscopy gave a large g(lum) value of 0.053 cgs.