Chiroptical Properties of Indolenine Squaraines with a Stereogenic Center at Close Proximity

Abstract

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.

The Impact of Chiral Citronellyl‐Functionalization on Indolenine and Anilino Squaraine Thin Films

“…Since no distinct molecular aggregation could be evidenced for the isotropic mCi‐SQ and bCi‐SQ it is self‐explaining that neither excitonic CD signals, which would arise from chiral molecular aggregation or crystallization, could be detected. This is in line with a study, that compares chiral functionalization at the backbone of indolenine SQ with chiral terminal substitution [26] . This study utilized an acceptor‐core substituted indolenine and covered only colloidal aggregation in solution, however, the chiral ( S )‐citronellyl at the nitrogen atom showed inferior (chiral) aggregation behavior.…”

“…This is in line with a study, that compares chiral functionalization at the backbone of indolenine SQ with chiral terminal substitution. [26] This study utilized an acceptorcore substituted indolenine and covered only colloidal aggregation in solution, however, the chiral (S)-citronellyl at the nitrogen atom showed inferior (chiral) aggregation behavior.…”

“…The chiral groups typically act as steering agent for the packing, but not directly cause the chiroptical response. For indolenine SQs there has also been an approach to integrate the chiral center into the backbone at the indicated position in Figure 1 [26] . Terminal chiral functionalization, such as cyclic pyrrol‐ [27] and prolinol‐derived [28] groups, has proven feasibility for anilino squaraines to induce an excitonic circular dichroic response in aggregated thin film samples.…”

“…For indolenine SQs there has also been an approach to integrate the chiral center into the backbone at the indicated position in Figure 1. [26] Terminal chiral functionalization, such as cyclic pyrrol- [27] and prolinolderived [28] groups, has proven feasibility for anilino squaraines to induce an excitonic circular dichroic response in aggregated thin film samples. Actually, the prolinol moiety successfully generated a largely enhanced CD response, more than a factor of 100 larger than generic chiral organic thin films, solely based on the nature of the excitonic coupling.…”

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

“…To circumvent this problem, a more reactive nucleophile was chosen. As reported in refs, , the alkylation of an oxindole followed by the nucleophilic addition of MeMgBr at the carbonyl group and subsequent acidic elimination of water with HCl constitutes a viable alternative for the synthesis of the quaternary indolium salts. These oxindoles can be easily deprotonated at the nitrogen, therefore generating a significantly more reactive nucleophile compared to the neutral indolenine .…”