1,721,027 research outputs found
ECD exciton chirality method today: a modern tool for determining absolute configurations
No full textThe application of the exciton chirality method (ECM) to interpret electronic circular dichroism (ECD) spectra is a well‐established and still popular approach to assign the absolute configuration (AC) of natural products, chiral organic compounds, and organometallic species. The method applies to compounds containing at least two chromophores with electric dipole allowed transitions (e.g., π–π* transitions). The exciton chirality rule correlates the sign of an exciton couplet (two ECD bands with opposite sign and similar intensity) with the overall molecular stereochemistry, including the AC. A correct application of the ECM requires three main prerequisites: (a) the knowledge of the molecular conformation, (b) the knowledge of the directions of the electric transition moments (TDMs), and (c) the assumption that the exciton coupling mechanism must be the major source of the observed ECD signals. All these prerequisites can be easily verified by means of quantum‐mechanical (QM) calculations. In the present review, we shortly introduce the general principles that underpin the use of the ECM for configurational assignments and survey its applications, both classic ones and some reported in the recent literature. Based on these examples, we will stress the advantages of the ECM but also the key requisites for its correct application. Additionally, we will discuss the dependence of the couplet sign on geometrical parameters (angles α,β,γ between TDMs), which can be helpful for discerning the sign of exciton chirality in ambiguous situations. Finally, we will present a molecular orbital (MO) description of the exciton coupling phenomenon
Chiroptical Properties in Thin Films of π-Conjugated Systems
Full text linkChiral π-conjugated molecules provide new materials with outstanding features for current and perspective applications, especially in the field of optoelectronic devices. In thin films, processes such as charge conduction, light absorption, and emission are governed not only by the structure of the individual molecules but also by their supramolecular structures and intermolecular interactions to a large extent. Electronic circular dichroism, ECD, and its emission counterpart, circularly polarized luminescence, CPL, provide tools for studying aggregated states and the key properties to be sought for designing innovative devices. In this review, we shall present a comprehensive coverage of chiroptical properties measured on thin films of organic π-conjugated molecules. In the first part, we shall discuss some general concepts of ECD, CPL, and other chiroptical spectroscopies, with a focus on their applications to thin film samples. In the following, we will overview the existing literature on chiral π-conjugated systems whose thin films have been characterized by ECD and/or CPL, as well other chiroptical spectroscopies. Special emphasis will be put on systems with large dissymmetry factors (gabs and glum) and on the application of ECD and CPL to derive structural information on aggregated states
The "case of Two Compounds with Similar Configuration but Nearly Mirror Image CD Spectra" Refuted. Reassignment of the Absolute Configuration of N-Formyl-3′,4′-dihydrospiro[indan-1,2′(1′H)-pyridine]
No full textIn 1997, Sandström and co-workers reported the case of two chiral spiro compounds with very similar skeletons but showing almost mirror-image electronic circular dichroism (ECD) spectra for the corresponding absolute configuration. The paper has been often cited as a proof and good educational example of the pronounced sensitivity of ECD toward molecular conformation, and a clear warning against the use of ECD spectral correlations to assign absolute configurations. Although both concepts remain valid, they are not exemplified by the quoted paper. We demonstrate that the original configurational assignment of one compound was wrong and revise it by using TDDFT calculations. The main reason for the observed failure is the use of the matrix method, a popular approach to predict ECD spectra of compounds which can be treated with an independent system approximation (ISA), including proteins. Using a modern version of the matrix method, we demonstrate that the ISA is not valid for the title compound. Even in the absence of apparent conjugation between the component chromophores, the validity of the ISA should never be taken for granted and the effective extent of orbital overlap should always be verified
A simple dimeric model accounts for the vibronic ECD spectra of chiral polythiophenes in their aggregated states
No full textAggregates of chiral polythiophenes (PTs) show strong electronic circular dichroism (ECD) spectra with a unique vibronic structure. A computationally fast procedure, exploiting the quantum dynamics of the exciton-coupled electronic states of simple oligothiophene dimers as models of PT aggregates, is able to reproduce experimental vibronic ECD spectra of aggregated phases
Circularly polarized light at the mirror: Caveats and opportunities
Full text linkMoving from the simple concept that reflection onto a mirror surface changes the handedness of circularly polarized light, we describe what happens to the emergent polarization in two different cases after reflection on a back mirror. In the first case, a regular emitter is taken into account, where reflection has the effect to destroy the emergent polarization. In the second case, we show what could happen when a hypothetical apparently non-reciprocal emitting material undergoes a similar experiment. These simple concepts have important implications in the design of efficient circularly polarized emitting devices
Magnetic Circularly Polarized Luminescence of Organic Compounds
No full textAchiral purely organic molecules can show selectivity towards circularly polarized light in emission in the presence of a magnetic field. This phenomenon is called magnetic circularly polarized luminescence (MCPL). Recently a few examples of MCPL from organic molecules have appeared in the literature. Through this technique, interesting photophysical information can be inferred and, moreover, a few technological applications can be devised based on this principle. This short review has the purpose to give a general introduction to this recent field of research and some critical insights on the reported examples.Magnetic circularly polarized luminescence (MCPL) is the phenomenon through which non-chiral compound emit circularly polarized light under a magnetic field. Potentialities and limitations of MCPL from organic compounds are discussed.imag
Tunable Excimer Circularly Polarized Luminescence in Isohexide Derivatives from Renewable Resources
No full textOrganic compounds showing circularly polarized luminescence (CPL) are at the forefront of novel applications and technologies. Here we show the synthesis and chiroptical properties of pyrene and perylene derivatives of inexpensive chiral scaffolds: isomannide and isosorbide. Low-intensity ECD spectra were obtained, suggesting the absence of chromophore interaction in the ground state, except in the case of isomannide bis-perylenecarboxylate, whose ECD spectrum showed a positive exciton couplet. All isomannide derivatives, with the only exception of the one containing a pyrenecarboxylate and a perylenecarboxylate, exhibited excimer CPL spectra, whereas isosorbide derivatives did not show any CPL. Isomannide derivatives bearing two pyrenecarboxylate or two pyrenylacetate groups showed positive CPL emission with dissymmetry factors up to 10−2, which depends on the conformational freedom of the appended units. The CPL sign, Stokes shift and order of magnitude of dissymmetry factor were reproduced by excited-state calculations on a representative compound. Interestingly, the mixed derivative containing pyrenic units with different spacing from the isomannide scaffold showed an oppositely signed excimer band with respect to the homo-substituted derivatives
The shape of the electronic circular dichroism spectrum of (2,6-dimethylphenyl)(phenyl)methanol: Interplay between conformational equilibria and vibronic effects
No full textComparison between chiroptical spectra and theoretical predictions is the method of choice for the assignment of the absolute configuration of chiral compounds in solution. Here we report the case of an apparently simple biarylcarbinol, whose electronic circular dichroism (ECD) in the 1Lb region exhibits a peculiar alternation of negative and positive bands. Adopting Density Functional Theory, and describing solvent effects with implicit methods, we found three stable conformers in ethanol, each of them with two close lying states corresponding to similar local 1Lb excitations on the two phenyls. We computed the corresponding vibronic ECD spectra in harmonic approximation, including Duschinsky mixings as well as both Franck Condon (FC) and Herzberg Teller (HT) effects. Exploiting a recently developed mixed quantum/classical method, we further investigated the contribution of the vibronic spectra of out-of-equilibrium structures along the interconversion path connecting the different conformers. In this way, we achieved a reasonable agreement with experiment and attributed the alternating signs of the bands to the existence of different conformers. The remaining discrepancies with experiment indicate that specific solute-solvent interactions modulate the relative conformers' stabilities, calling for new methods able to combine Molecular Dynamics explorations and vibronic calculations. Moreover, the poor performance of HT approaches and the existence of two closely-lying states suggest the necessity of an improved fully-nonadiabatic vibronic approach. These findings demonstrate that even for such a simple system as the biarylcarbinol investigated here, a full reproduction of the fine details of the ECD spectrum requires the development of new improved methods
Symmetry-Dependent Vibrational Circular Dichroism Enhancement in Co(II) Salicylaldiminato Complexes
Full text linkChiral coordination compounds of Co(II) and other open-shell metal complexes display enhanced vibrational circular dichroism (VCD) spectra associated with the existence of low-lying excited states (LLESs). In addition to the enhancement, a series of Co(II) salicylaldiminato complexes exhibits an almost monosignate pattern of VCD bands, a unique feature if compared with the usual alternation of positive and negative signals. Frequency and excited-state calculations reveal that VCD enhancement and sign reversal selectively affect the normal modes of B symmetry of the C 2 -symmetric pseudotetrahedral species thanks to their combination with one or more LLES having the same B symmetry. This proves the strict relation between VCD enhancement and monosignate appearance and demonstrates an unprecedented symmetry dependence of the two phenomena
Impact and amplification of chirality in the aggregation of leucine-appended poly(p-phenylene ethynylene) (PPE)
Full text linkA leucine-appended poly(p-phenyleneethynylene) (PPE) was prepared in enantiomeric stereoregular (L-1 and D-1) and stereorandom (rac-1) forms. The solution aggregates of L-1, D-1, rac-1, and mixtures of L-1/D-1, were characterized by absorption, electronic circular dichroism and emission spectra. Both rac-1 and L-1/D-1 mixtures are more prone to aggregate than L-1 and D-1. Upon aggregating, the enantiomeric mixtures manifest an apparent majority-rules effect, which is mostly due to the greater tendency to form heterochiral aggregates with respect to homochiral ones. The impact of chirality on the aggregation behaviour of the aminoacid-appended PPE is demonstrated
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