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

Vibrational Circular Dichroism Spectroscopy of Chiral Molecular Crystals: Insights from Theory

Chirality is a curious phenomenon that appears in various forms. While the concept of molecular (RS-)chirality is ubiquitous in chemistry, there are also more intricate forms of structural chirality. One of them is the enantiomorphism of crystals, especially molecular crystals, that describes the lack of mirror symmetry in the unit cell. Its relation to molecular chirality is not obvious, but still an open question, which can be addressed with chiroptical tools. Vibrational circular dichroism (VCD) denotes chiral infrared (IR) spectroscopy that is susceptible to both, the molecular as well as the intermolecular space by means of vibrational transitions. When carried out in the solid state, VCD delivers a very rich set of non-local contributions that are determined by crystal packing and collective motion. Since its discovery in the 1970s, VCD has become the method of choice for the determination of absolute configurations, but its applicability reaches beyond towards the study of different crystal forms and polymorphism. This brief review summarises the theoretical concepts of crystal chirality and how computations of solid-state VCD can shed light into the intimate connection of chiral structure and vibrational optical activity.

How Crystal Symmetry Dictates Non-Local Vibrational Circular Dichroism in the Solid State

Solid-State Vibrational Circular Dichroism (VCD) can be used to determine the absolute structure of chiral crystals, but its interpretation remains a challenge in modern spectroscopy. In this work, we investigate the effect of a twofold screw axis on the solid-state VCD spectrum in a combined experimental and theoretical analysis of P2₁ crystals of (S)-(+)-1-indanol. Even though the space group is achiral, a single proper symmetry operation has an important impact on the VCD spectrum, which reflects the supramolecular chirality of the crystal. Distinguishing between contributions originating from molecular chirality and from chiral crystal packing, we find that while IR absorption hardly depends on the symmetry of the space group, the situation is different for VCD, where completely new non-local patterns emerge. Understanding the two underlying mechanisms, namely gauge transport and direct coupling, will help to use VCD to distinguish polymorphic forms.

Exploration of chromophores for a VCD couplet in a spectrally transparent infrared region for biomolecules

Interactions of two chromophores such as carbonyl groups yield a strong VCD couplet that reflects the molecular structures. The use of VCD couplets for biomacromolecular structural studies has been hampered by severe signal overlap caused by numerous functional groups that originally exist in biomacromolecules. Nitrile, isonitrile, alkyne, and azido groups show characteristic IR absorption in the 2300-2000 cm^-1 region, where biomolecules do not strongly absorb. We herein examined the usefulness of these functional groups as chromophores to observe a strong VCD couplet that can be readily interpreted using theoretical calculations. Studies on a chiral binaphthyl scaffold possessing two identical chromophores showed that nitrile and isonitrile groups generate moderately-strong but complex VCD signals due to anharmonic contributions. The nature of their anharmonic VCD patterns is discussed by comparison with the VCD spectrum of a mono-chromophoric molecule and by anharmonic DFT calculations. On the other hand, through studies on diazido binaphthyl and diazido monosaccharide, we demonstrated that the azido group is more promising for structural analysis of larger molecules due to its simple, strong VCD couplet whose spectral patterns are readily predicted by harmonic DFT calculations.

The important role of non-covalent interactions for the vibrational circular dichroism of lactic acid in aqueous solution

We present a computational study of vibrational circular dichroism (VCD) in solutions of (S)-lactic acid, relying on ab initio molecular dynamics (AIMD) and full solvation with bulk water. We discuss the effect of the hydrogen bond network on the aggregation behaviour of the acid: while aggregates of the solute represent conditions encountered in a weakly interacting solvent, the presence of water drastically interferes with the clusters - more strongly than originally anticipated. For both scenarios we computed the VCD spectra by means of nuclear velocity perturbation theory (NVPT). The comparison with experimental data allows us to establish a VCD-structure relationship that includes the solvent network around the chiral solute. We suggest that fundamental modes with strong polarisation such as the carbonyl stretching vibration can borrow VCD from the chirally restructured solvent cage, which extends the common explanatory models of VCD generation in aqueous solution.

Vibrational Circular Dichroism Detects Symmetry Breaking due to Conformational Mobility in C2-Symmetry Chiral Molecules and Provides Further Insight into Inter-Chromophoric Interactions

Bicyclo[3.3.1]nonane-2,6-dione (1) and bicyclo[3.3.1]nona-3,7-diene-2,6-dione (2) have been examined by vibrational circular dichroism (VCD), which, as for most C2-symmetric systems, exhibits strong VCD signals. In the case of 2, VCD signals are stronger and sharper with several bisignate doublets; for 1, signals are less intense and broader. The VCD and IR spectra are excellently predicted by DFT calculations: only one conformer is present for 2, while for 1, three main conformers, related through concerted skeleton torsional motions are present (two of them being interchanged by C2-rotation). The VCD spectrum shows specific features for the different conformers, such that correct population factors are crucial for reproducing experimental data. Also, the TD-DFT prediction of ECD (electronic circular dichroism) spectra is good. By comparing the spectroscopic signature of the two molecules (both VCD and ECD) and by careful analysis of the theoretical results, the role of the C=C double bond in compound (2) is evidenced. The double bond contributes toward enhancing the CD response both electronically and vibrationally.

Synthesis and Functions of Oligomeric and Multidentate Dipyrrin Derivatives and their Complexes

The dipyrrin–metal complexes and especially the boron complex 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) have recently attracted considerable attention because of their interesting properties and possible applications. We have developed two unique and useful ways to extend versatility and usefulness of the dipyrrin complexes. The first one is the linear and macrocyclic oligomerization of the BODIPY units. These arrangements of the B–F moieties of the oligomerized BODIPY units provide sophisticated functions, such as unique recognition ability toward cationic guest, associated with changes in the photophysical properties by utilizing unprecedented interactions between the B–F and a cationic species. The second one is introduction of additional ligating moieties into the dipyrrin skeleton. The multidentate N2Ox dipyrrin ligands thus obtained form a variety of complexes with 13 and 14 group elements, which are difficult to synthesize using the original N2 dipyrrin derivatives. Interestingly, these unique complexes exhibit novel structures, properties, and functions such as guest recognition, stimuli-responsive structural conversion, switching of the optical properties, excellent stability of the neutral radicals, etc. We believe that these multifunctional dipyrrin complexes will advance the basic chemistry of the dipyrrin complexes and develop their applications in the materials and medicinal chemistry fields.

1 Introduction

2 Linear Oligomers of Boron–Dipyrrin Complexes

3 Cyclic Oligomers of Boron–Dipyrrin Complexes

4 A Cyclic Oligomer of Zinc–Dipyrrin Complexes

5 Group 13 Element Complexes of N2Ox Dipyrrins

6 Chiral N2 and N2Ox Dipyrrin Complexes

7 Group 14 Element Complexes of N2O2 Dipyrrins

8 Other N2O2 Dipyrrin Complexes with Unique Properties and Functions

9 Conclusion

Theoretical Investigation of the Circularly Polarized Luminescence of a Chiral Boron Dipyrromethene (BODIPY) Dye

Over the last decade, molecules capable of emitting circularly polarized light have attracted growing attention for potential technological and biological applications. The efficiency of such devices depend on multiple parameters, in particular the magnitude and wavelength of the peak of emitted light, and also on the dissymmetry factor for chiral applications. In light of these considerations, molecular systems with tunable optical properties, preferably in the visible spectral region, are particularly appealing. This is the case of boron dipyrromethene (BODIPY) dyes, which exhibit large molecular absorption coefficients, have high fluorescence yields, are very stable, both thermally and photochemically, and can be easily functionalized. The latter property has been extensively exploited in the literature to produce chromophores with a wide range of optical properties. Nevertheless, only a few chiral BODIPYs have been synthetized and investigated so far. Using a recently reported axially chiral BODIPY derivative where an axially chiral BINOL unit has been attached to the chromophore unit, we present a comprehensive computational protocol to predict and interpret the one-photon absorption and emission spectra, together with their chiroptical counterparts. From the physico-chemical properties of this molecule, it will be possible to understand the origin of the circularly polarized luminescence better, thus helping to fine-tune the properties of interest. The sensitivity of such processes require accurate results, which can be achieved through a proper account of the vibrational structure in optical spectra. Methodologies to compute vibrationally-resolved electronic spectra can now be applied on relatively large chromophores, such as BODIPYs, but require more extensive computational protocols. For this reason, particular attention is paid in the description of the different steps of the protocol, and the potential pitfalls. Finally, we show how, by means of appropriate tools and approaches, data from intermediate steps of the simulation of the final spectra can be used to obtain further insights into the properties of the molecular system under investigation and the origin of the visible bands.

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

Dirhodium complexes bearing N-substituted chiral amino acid ligands are investigated. These complexes have an unusual twisted paddlewheel structure, showing inherent chirality. We would like to demonstrate that parallel application of chiroptical spectroscopic methods (ECD and VCD) and NMR spectroscopy combined with quantum chemical calculations constitutes a powerful tool to determine the configuration of the complexes unequivocally. Two chiroptical methods are needed to determine the absolute configuration: ECD for the coordinated nitrogen atom and VCD for the rhodium core. A quick to use NMR method is also presented: Upon the coordination of small molecules in the axial position, the relative configuration of both the rhodium core and the nitrogen atom can be determined simultaneously by studying spatial proximities provided by 1D NOE spectra.

GUI Implementation of VCDtools, A Program to Analyze Computed Vibrational Circular Dichroism Spectra

As computing power increases, vibrational circular dichroism (VCD) calculations on molecules of larger sizes and complexities become possible. At the same time, the spectra resulting from these computations become increasingly more cumbersome to analyze. Here, we describe the GUI implementation into the Amsterdam Density Functional (ADF) software package of VCDtools, a toolbox that provides a user-friendly means to analyze VCD spectra. Key features are the use of the generalized coupled oscillator analysis methods, as well as an easy visualization of the atomic electric and magnetic transition dipole moments which together provide detailed insight in the origin of the VCD intensity. Using several prototypical examples we demonstrate the functionalities of the program. In particular, we show how the spectra can be analyzed to detect differences between theory and experiment arising from large-amplitude motions or incorrect molecular structures and, most importantly, how the program can be used to prevent incorrect enantiomeric assignments.

Highly enantioselective "inherently chiral" electroactive materials based on a 2,2'-biindole atropisomeric scaffold

Chiral oligothiophene monomers with C 2 symmetry, based on 3,3'-bithiophene atropisomeric cores with high racemization barriers, have recently been shown to provide excellent chiral starting materials with high electroactivity for the easy preparation of enantiopure electroactive films endowed with powerful chirality manifestations. We now introduce an inherently chiral monomer based on a 2,2'-biindole core, as the prototype of a new inherently chiral monomer family, whose properties could be modulable through functionalization of the pyrrolic N atoms. By fast, regular electrooligomerization the new monomer yields inherently chiral films with high, reversible electroactivity and, above all, impressive enantioselectivity towards very different chiral probes, some of pharmaceutical interest, as general-scope electrode surfaces. Such results, while opening the way to a new, attractive inherently chiral selector class, nicely confirm the general validity of the inherent chirality strategy for chiral electrochemistry. Furthermore, the enantioselectivity of the new selectors not only holds with electroactive chiral probes, but also with circularly polarized light components as well as electron spins, resulting in good chiroptical and spin filter performances, which suggests fascinating correlations between the three contexts.

Stereoselective interactions as manifested by vibrational circular dichroism spectra: the interplay between chiral metal complexes co-adsorbed in a montmorillonite clay

Solid state VCD is applied for intercalated metal complexes.

Electronic and vibrational exciton coupling in oxidized trianglimines

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.

Solvent-Sensitive Emitting Urea-Bridged bis-BODIPYs: Ready Access by a One-Pot Tandem Staudinger/Aza-Wittig Ureation

Herein we describe the synthesis, and computationally aided photophysical characterization of a new set of urea-bridged bis-BODIPY derivatives. These new dyads are efficiently obtained by a one-pot tandem Staudinger/aza-Wittig ureation protocol, from easily accessible meso-phenyl ortho-azidomethyl BODIPYs. These symmetric bis-BODIPYs outstand by a high absorption probability and excellent fluorescence and laser emission in less polar media. Nevertheless, this emission ability decreases in more polar media, which is ascribed to a light-induced charge-transfer from the urea spacer to the dipyrrin core, a process that can be modulated by appropriate changes in the substitution pattern of the BODIPY core. Furthermore, this ureation protocol can also be employed for the direct conjugation of our BODIPY-azides to amine-containing compounds, thus providing access to fluorescent non-symmetric ureas.