Cryptochirality in 2,2′‐Coupled BODIPY DYEmers

Abstract

The two chromophores in 2,2′-coupled BODIPY DYEmers are linearly arranged and possess a configurationally stable axis. Their successful enantiomeric resolution was shown by HPLC-UV and HPLC–ECD on-line measurements. In accordance with theoretical predictions, the main UV/Vis signal of the dimers is redshifted in comparison with that of the monomers, and no exciton couplets are observed in the electronic circular dichroism (ECD) spectra. Moreover, the experimental ECD spectra have a very low intensity over the wavelength region from 300 to 800 nm and the linear BODIPY DYEmers thus show cryptochirality. The reason for this phenomenon is the peculiar molecular arrangement together with the conformational behavior of the dimers. For each compound two different conformations are possible around the biaryl axis that exhibit nearly mirror-image ECD spectra. As a result, the overall ECD curves show only weak signals dominated by vibronic coupling effects. Thus, the method commonly used to determine absolute configurations by TDDFT calculations is not reliable for linear BODIPY dimers and only VCD investigations (experimental in combination with DFT calculations) could unambiguously determine the absolute configurations of the 2,2′-coupled BODIPY DYEmers

Vibrational Optical Activity of BODIPY Dimers: The Role of Magnetic–Electric Coupling in Vibrational Excitons

“…In fact, all the prerequisites for a correct application of the model appear to be met in the present case. In particular: we have two symmetric systems allowing for degenerate exciton coupling, that is, the two “local” vibrations have equal frequencies (diagonal force constants); the two interacting units do not perturb much each other in their electrical response, in fact, for the relevant normal modes, the total electric dipole transition moments are approximated by the sum of the moments of the two separate units (see Tables and and discussion below), i.e., the total dipole strength is conserved; the relevant normal modes are associated with large electric dipole transition moments, i.e., the corresponding IR bands are strong; this is necessary for the two units to interact through a dipole–dipole term; the relevant normal modes give rise to intense bisignate doublets, with small frequency separation and nearly conservative, i.e., with similar band integral; because of the 1,1′ or 3,3′ coupling in compounds 1 and 2 , the dipole transition moments of the two BODIPY units are far from a parallel/antiparallel orientation: this situation, encountered, for instance, in 2,2′-coupled BODIPY dimers, would lead to a vanishing exciton coupling contribution, so other terms would dominate the rotational strengths. Moreover, a little deviation from this condition may generate opposite results; ,, such deviations are difficult to calculate with precision and can be accessible through thermal energy fluctuations. …”

“…(d) the relevant normal modes give rise to intense bisignate doublets, with small frequency separation and nearly conservative, i.e., with similar band integral; (e) because of the 1,1′ or 3,3′ coupling in compounds 1 and 2, the dipole transition moments of the two BODIPY units are far from a parallel/antiparallel orientation: 28 this situation, encountered, for instance, in 2,2′-coupled BODIPY dimers, 30 would lead to a vanishing exciton coupling contribution, so other terms would dominate the rotational strengths. Moreover, a little deviation from this condition may generate opposite results; 16,18,30 such deviations are difficult to calculate with precision and can be accessible through thermal energy fluctuations. Thus, the two couplets observed at 1250 and 1650 cm −1 seem to fulfill all the requirements for the application of the VE treatment.…”

ECD exciton chirality method today: a modern tool for determining absolute configurations

“…Recent applications of ECM to biaryl compounds include bis(indole) alkaloids 63 and BODIPY dimers, 64 , 65 though these latter are complicated by the fact that the major π–π* transition around 500 nm is also magnetically dipole allowed.…”

How and How Much Molecular Conformation Affects Electronic Circular Dichroism: The Case of 1,1-Diarylcarbinols

“…The dependence of ECD on conformational factors has been discussed in several instances, especially in combination with ECD quantum-chemical calculations [ 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. In fact, it is easy to demonstrate “in silico” that different conformations of the same molecule exhibit different chiroptical responses.…”