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

Abstract

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 Δε 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.

Time-resolved circular dichroism of excitonic systems: theory and experiment on an exemplary squaraine polymer

“…4d). 9 Interestingly, a strong lowenergy maximum around 850 nm appears in the CD spectrum that is barely visible in the linear absorption spectrum (Fig. 5).…”

“…6 in ref. 9), where it is assumed to be in a pure zigzag random coil conformation. Finally, we note that the absorption spectrum can be tted equally well by assuming that the zigzag random coil and helix conformations are covalently connected, e.g., by considering a polymer made of 24 chromophores (instead of 16 considered so far), where the rst 19 are in a squeezed helix conformation and the remaining ve chromophores constitute the zigzag random coil fraction (Fig.…”

Polymer‐Based Circularly Polarized Luminescent Materials

“…More recently, Selby et al reported four squaraine polymers carrying chiral branched alkyl pendants (Figure 9B), which were acquired by Yamamoto homocoupling reaction. [130] The steric demand and stereogenic center of the pendants were adjusted to afford structural perturbation. The polymers took a homohelical structure in some solvents.…”

“…Reproduced with permission. [ 130 ] Copyright 2022, American Chemical Society. C) Illustration of the CPL‐active cellulose derivatives bearing pyrene‐based side chains.…”

Chiroptical Properties of Planar Benzobisthiazole-Bridged Squaraine Dimers

“…Chiral organic dyes are used in organic solar cells, as chiroptical switches, , molecular sensors, catalysts for photochemical reactions, photodetectors, or in organic light emitting diodes (OLEDs). , A plethora of chiral chromophores, such as merocyanines, BODIPYs, perylene diimides, − phthalocyanines, porphyrins, , and metal complexes, , is known. Also, chiral squaraine dyes have been developed for the above-mentioned applications. − These dyes possess outstanding optical properties, such as a narrow absorption band in the red to near-infrared spectral region, and high photochemical stability and fluorescence quantum yields . A strategy to further enhance the chiroptical properties of dyes is to use excitonically coupled axially chiral chromophore dimers. ,− Since the first description of the exciton coupling theory by Kasha, , exciton coupling effects have extensively been used to tune the optical properties of organic dye assemblies. − A chromophore dimer represents the smallest and therefore simplest coupled system, which is best suited to investigate the influence of, for example, distance , and mutual orientation , of the two chromophores, or their chemical heterogeneity ,, and effects of vibronic coupling − on the optical properties of the dimer .…”