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

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.