Electronic and vibrational exciton coupling in oxidized trianglimines

Abstract

Readily available chiral trianglimine and their (poly)oxygenated congeners represent a unique class of macrocyclic rigid compounds optimal for testing electronic and vibrational circular dichroism exciton chirality methods. Electronic and vibrational circular dichroism spectra of such trianglimines are strongly affected by polar substituents in macrocycle skeletons. Double substitution by OH groups in each aromatic fragment of the macrocycle causes sign reversal of the exciton couplet in the region of the strongest UV absorption. On the other hand, electronic circular dichroism spectrum of the macrocycle having 2 methoxy groups shows 2 exciton couplets-the long-wavelength positive and the second of the negative sign, observed at the shorter wavelengths. VCD spectra of macrocyclic imines show vibrational exciton couplets in the region of strong C=N stretches. The signs of these couplets are positive and the opposite of the diamine chirality. For trianglimine macrocycles the interpretation of VCD spectra in terms of excitons is much more convincing than for electronic circular dichroism spectra. By contrast, trans-1,2-diaminocyclohexane-based vicinal diimines, being a one-third of the respective macrocycle, do not exhibit any vibrational exciton effect. Experimental data were confronted with DFT calculations. We observed good-to-excellent agreement between experimental and computed data.

Sterically Crowded Trianglimines—Synthesis, Structure, Solid‐State Self‐Assembly, and Unexpected Chiroptical Properties

“…The amplitude of exciton couplet measured for 7 is more or less of the same magnitude as of the basic non‐substituted trianglimine ( 12 , R=R′=H). However, the formation of new chromophoric systems in 8 and 9 leads to unexpectedly large Cotton effects, which are about seven times larger than those measured for 12 . The presence of a non‐planar, thus, inherently chiral, p ‐terphenyl chromophore in 10 and 11 might strongly affect the overall ECD spectrum of the respective macrocycle.…”

“…The negative exciton couplet is associated with exciton interactions between EDTMs polarized along the long axis of the chromophore. The sign of this couplet reflects the negative angle between EDTMs and is consistent with the negative torsion angle N‐C*‐C*‐N in the DACH molecule . For the model compound 7′ , this electronic transition appeared at 263 nm and is preceded by a lower energy transition of low oscillator strength, which appeared at 282 nm (Figure ).…”

“…The EDTMs are distorted from the main axis of the molecule, and they are responsible for the generation of lower energy CEs, which appeared as positive/negative couplets. Therefore, analysis of the lower energy region of the ECD spectrum of 7 provides information about the conformation of the aromatic linker …”

“…To date, there are only a few examples of trianglimines with one or two small and/or polar groups in each of the aromatic rings , . In spite of the strong effect of these substituents on the properties of the given macrocycle, it is hard to consider these cases as trianglimines grown up‐ and/or downwards.…”

Twisted paddlewheel rhodium complexes: Contribution of central and axial chirality to ECD, VCD, and NMR spectra

“…Recently, Taniguchi and Monde expanded the exciton chirality concept on vibrational circular dichroism (VCDEC method) . In the last few years, several experimental and theoretical studies deal with the applicability of the VCDEC method …”