A Combined Experimental and Theoretical Study on the Circular Dichroism of Staggered and Eclipsed Forms of Dimethoxy[2.2]-, [3.2]-, and [3.3]Pyridinophanes and Their Protonated Forms

Abstract

The circular dichroisms (CDs) of dimethoxy[2.2]-, [3.2]-, and [3.3]pyridinophanes and their protonated forms were investigated experimentally and theoretically. Characteristic multisignate Cotton effects (CEs), typical for planar chiral cyclophane derivatives, were observed. The CD spectral pattern was quite comparable for the staggered forms of [2.2]-, [3.2]-, and [3.3]cyclophanes, but significantly differed for the eclipsed forms. More interestingly, the patterns resembled, but the CE signs were practically opposite between staggered and eclipsed [2.2]pyridinophanes. Upon protonation, the signs of most CEs were inverted in both forms of cyclophanes, due to the reversal of dipole moment in the pyridine against the pyridinium moiety. Such a change in CD spectrum upon protonation was not apparent in [3.2]pyridinophane, and the CD spectral behavior was more complex in [3.3]pyridinophanes. The variation of CD caused by the protonation/deprotonation process was temperature-dependent and hence utilized as a thermal sensor. The protonated forms of the homologous pyridinophanes with different tether lengths in staggered and eclipsed forms served as a model system for systematically studying the cation-π interaction and its effects on chiroptical properties. A steady increase of electronic interaction became apparent for the smaller-sized cyclophanes from the increased excitation energy and electronic coupling element of the charge-transfer (CT) band, while the observed CE at the CT band was a more complex function of the original transition dipole of donor/acceptor pair and linker atoms, as well as the strength of the electronic interaction.

Combined Experimental and Theoretical Studies on Planar Chirality of Partially Overlapped C2-Symmetric [3.3](3,9)Dicarbazolophanes

“…Thus, by lowering the temperature down to −95 °C, the observed CEs in the CD spectra of CZ3 became gradually stronger, particularly at the 1 B transitions (Figure ), but the changes were considerably smaller than those observed for more flexible systems . This contrasts with the known behavior of the typical [3.3]­paracyclophanes (i.e., with benzene unit) where the conformation of the bridging atoms has profound effects on the photophysical properties. ,, Lastly, we also performed a preliminary investigation on the CPL spectra of CZ1 in benzene at 25 °C (Figure ). A considerably intense CPL with luminescence dissymmetry factor ( g lum ) of 0.013 was noticed at around the fluorescence peak of ∼390 nm, with the same signs to the lowest-energy CEs in the CD spectra at 370 nm.…”

“…The pattern of the observed Cotton effects (CEs) for these bands was very complex due to the extensive electronic coupling between the two facing carbazole chromophores. Modest CE strengths (|Δε| = 10–12 and 51–57 M –1 cm –1 , respectively) at these bands were within a typical intensity range for planar chiral cyclophanes. − More interestingly, very intense CEs were notified in their 1 B transitions (|Δε| = 156–216 M –1 cm –1 ), with g abs values being as large as 0.024 at the maximum, which is appreciably larger than the typical values (10 –3 –10 –5 ) reported for the allowed π–π* transitions . These observations can be certainly ascribed to the rigid dicarbazolophane framework where two large carbazole units are stacked in a right orientation and displacement (vide infra), with bridging atoms being in a chairlike conformation.…”

“…Much attention has been paid to cyclophanes of planar chirality in their unique electronic and optical properties, − as well as recent advances in the enantioselective synthesis . Planar chiral cyclophanes have been also employed in thermal , and photochemical asymmetric reactions.…”

Circular Dichroisms of Mono- and Dibromo[2.2]paracyclophanes: A Combined Experimental and Theoretical Study

“…Accordingly, we calculated the optimized geometries for all of the brominated [2.2]­paracyclophanes 1 – 2 at the DFT-D3­(BJ)-TPSS/def2-TZVP level, , which was quite successful indeed, affording the geometrical parameters comparable to those obtained by X-ray crystallography (Table ). Compatibility of the theoretical method of choice with cyclophane structure has also been justified by our recent investigations on pyridinophane systems. − Because crystal structures are not available for some cyclophanes, we employed these DFT-D3-optimized structures for the following theoretical CD investigation. The parent [2.2]­paracyclophane possesses a shallow double-minimum potential along the dihedral-angle coordinate (two minima at Φ = ±12.6°).…”

“…Because both the RI-CC2 and RI-ADC(2) provided essentially the same results and outperformed any of the TD-DFT and SAC-CI methods, we focus on the results obtained by the RI-CC2/def2-TZVPP method here (Figure , right). For better comparison with the experimental spectra, the calculated spectra were scaled to 1/3 for 1 and p - 2 or to 1/5 for m ′- 2 and o ′- 2 , typical corrections frequently employed for other cyclophane systems. − Such scaling against experiment was always observed in this level of calculation, and presumably due, at least in part, to the possible ignorance of molecular dynamic behavior as well as vibronic contributions, but certainly requires further development in theory.…”