A series of four indolenine squaraines bearing a chiral center at the 3-position of the indolenine moiety, with either an n-propyl or a phenyl group alongside a methyl group, were synthesized and obtained in a high purity of ≥98% for the desired stereoisomer. The indolenine precursors with a phenyl group attached at the chiral center were asymmetrically synthesized using a pericyclic-reaction cascade and obtained in a high ee of 98%, whereas the ones with an n-propyl group were prepared by kinetic resolution through asymmetric hydrogenation, resulting in an ee of up to 98%. X-ray crystallography revealed a slightly twisted geometry for the phenyl-substituted cisoid squaraine derivative, whereas the n-propyl-substituted derivative possessed the expected planar geometry. Variation of the substitution also influenced the optical properties, where the introduction of phenyl groups caused a progressive red-shift and reduction in squared transition moments, as well as reduced fluorescence quantum yields, Stokes shifts, and fluorescence lifetimes. All of the investigated compounds exhibited strong ECD signals, with Δε values of up to 24 M −1 cm −1 for the HOMO−LUMO transition. DFT calculations indicated that this was due to both large electric and magnetic transition moments, although the two vectors were mutually almost orthogonal.

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.

A squaraine heterotriad consisting of three different covalently linked squaraine chromophores was synthesized, and its absorption spectra were interpreted in terms of Kasha's exciton coupling theory. Using the exciton couplings derived from model dyads (ca. 700 cm −1 ) as the input, we were able to predict the exciton state energies of the heterotriad. Transient absorption spectroscopy with femtosecond time resolution showed that excitation of the highest exciton state populates a state mainly localized at one terminal squaraine chromophore, and energy transfer to the lowest exciton state localized at the other terminal squaraine occurs within 30 fs. Field-induced surface hopping dynamics simulations support the assumption of ultrafast energy transfer. Moreover, they show the close relationship between internal conversion and energy transfer in the intermediate to weak coupling regime. The latter is a consequence of excitation localization caused by molecular vibrations.

Experimental and theoretical foundations for femtosecond time-resolved circular dichroism (TRCD) spectroscopy of excitonic systems are presented. In this method, the system is pumped with linearly polarized light and the signal...