Redox-Active Chiroptical Switching in Mono- and Bis-Iron Ethynylcarbo[6]helicenes Studied by Electronic and Vibrational Circular Dichroism and Resonance Raman Optical Activity

A Perspective on the Simulation of Electronic Circular Dichroism and Circularly Polarized Luminescence Spectra in Chiral Solid Materials

Chiral materials have shown tremendous potential for many technological applications, such as optoelectronics, sensing, magnetism, information technology, and imaging. Characterization of these materials is mostly based on chiroptical spectroscopies, such as electronic circular dichroism (ECD) and circularly polarized luminescence (CPL). These experimental measurements would greatly benefit from theoretical simulations for interpretation of the spectra as well as predictions on new materials. While ECD and CPL simulations are well established for molecular systems, they are not for materials. In this Perspective, we describe the theoretical quantities necessary to simulate ECD and CPL spectra in oriented systems. Then, we discuss the approximate strategies currently used to perform these calculations, what computational machinery is already available to develop more general approaches, and some of the open challenges for the simulation of ECD and CPL spectra in solid materials. When methods that are as reliable and computationally efficient as those for molecules are developed, these simulations will provide invaluable insight and guidance for the rational design of optically active materials.

UV-vis and ECD spectroelectrochemistry of atropisomeric naphthalenediimide derivative

The UV-vis and ECD spectroelectrochemistry (SEC) of a chiral binaphthalenylamine derivative of the N-butyl naphthalenediimide (NDIB-NH₂) enantiomers were applied to measure UV-vis and ECD spectra of NDIB-NH₂ radicals and dianion formed in the reduction and oxidation processes observed in cyclic voltammetry (CV). The CV curves and EPR spectroelectrochemistry enabled us to establish conditions at which a radical-anion [NDIB-NH₂]̇^.-, a dianion [NDIB-NH₂]^2-, and a radical-cation [NDIB-NH₂]̇^.+ are formed. The DFT restricted open-shell CAM-B3LYP-D3/def2TZVP/PCM calculations demonstrated that in the radical-anion [NDIB-NH₂]̇^.-, spin is spread over the NDI system while in the radical-cation [NDIB-NH₂]̇⁺ it is spread over the aminonaphthalene moiety. The UV-vis spectra of radical-anion and dianion show the most significant changes in the 400-800 nm range. In that range, the ECD spectra varied with the change of electrode potential more than the UV-vis did and enabled the identification of a new ECD band of [NDIB-NH₂]̇^.- at ca. 400 nm hidden in the background in the UV spectra at -1000 mV. A broad structured ECD pattern with a maximum at ca. 530 nm was observed for [NDIB-NH₂]̇^.- (-1000 mV), while a single smooth ECD band of [NDIB-NH₂]^2- was located at 520 nm (-1750 mV). For the first time, an isosbestic point (455 nm) was found in ECD spectroelectrochemical measurements for the radical-cation [NDIB-NH₂]̇^.+ in equilibrium with the NDIB-NH₂ neutral form. The TD-DFT CAM-B3LYP-D3/6-31G** calculations combined with the hybrid (explicit combined with implicit) solvation model fairly well reproduced the UV-vis and ECD SEC of neutral and redox forms of NDIB-NH₂ but the ECD spectrum of [NDIB-NH₂]̇^.+ above 390 nm.

Comparative Study of Vibrational Raman Optical Activity with Different Time-Dependent Density Functional Approximations: The VROA36 Database

A new database, VROA36, is introduced to investigate the performance of computational approaches for vibrational Raman optical activity (VROA) calculations. The database is composed of 36 molecules with known experimental VROA spectra. It includes 93 conformers. Normal modes calculated with B3LYP-D3(BJ)/def2-TZVP are used to compute the VROA spectra with four functionals, B3LYP-D3(BJ), ωB97X-D, M11, and optimally tuned LC-PBE, as well as several basis sets. SimROA indices and frequency scaling factors are used to compare calculated spectra with each other and with experimental data. The four functionals perform equally well independently of the basis set and usually achieve good agreement with the experimental data. For molecules in near- or at-resonance conditions, the inclusion of a complex (damped) linear response approach is important to obtain physically meaningful VROA intensities. The use of any of the tested functional approximations with the def2-SVPD Gaussian-type basis set, or a basis of similar flexibility, can be recommended for efficient and reliable theoretical VROA studies.

Measurement and Theory of Resonance Raman Optical Activity for Gases, Liquids, and Aggregates. What It Tells about Molecules

Resonance Raman Optical Activity (RROA) appeared as a natural extension of the nonresonance branch. It combines the structural sensitivity of chiroptical spectroscopy with the signal enhancement coming from the resonance of molecular electronic transitions with the excitation laser light. However, the idea has been hampered by many technical and theoretical problems that are being clarified only in recent years. We provide the theoretical basis and several examples documenting the problems, achievements, and potential of RROA, in particular in biomolecular studies.

Recognition of the True and False Resonance Raman Optical Activity

Resonance Raman optical activity (RROA) possesses all aspects of a sensitive tool for molecular detection, but its measurement remains challenging. We demonstrate that reliable recording of RROA of chiral colorful compounds is possible, but only after considering the effect of the electronic circular dichroism (ECD) on the ROA spectra induced by the dissolved chiral compound. We show RROA for a number of model vitamin B12 derivatives that are chemically similar but exhibit distinctively different spectroscopic behavior. The ECD/ROA effect is proportional to the concentration and dependent on the optical pathlength of the light propagating through the sample. It can severely alter relative band intensities and signs in the natural RROA spectra. The spectra analyses are supported by computational modeling based on density functional theory. Neglecting the ECD effect during ROA measurement can lead to misinterpretation of the recorded spectra and erroneous conclusions about the molecular structure.

Enantioenriched Ruthenium-Tris-Bipyridine Complexes Bearing One Helical Bipyridine Ligand: Access to Fused Multihelicenic Systems and Chiroptical Redox Switches

The synthesis and photophysical and chiroptical properties of novel aza[n]helicenes (6a-d, 10a,b, n = 4-7) substituted with one or two 2-pyridyl groups are described. The preparation was performed via an adapted Mallory reaction using aromatic imines as precursors. The obtained novel class of helical 2,2'-bipyridine ligands was then coordinated to Ru(bipy)₂²⁺ units, thus affording the first diastereomerically and enantiomerically pure [RuL(bipy)₂]²⁺ (11a,c, L = 6a,c) or [Ru₂L'(bipy)₄]⁴⁺ (12, L' = 10b) complexes. The topology and stereochemistry of these novel metal-based helical architectures were studied in detail, notably using X-ray crystallography. Interestingly, the coordination to ruthenium(II) enabled the preparation of fused multihelical systems incorporating aza- and ruthena-helicenes within the same scaffold. The photophysical, chiroptical, and redox properties of these complexes were examined in detail, and efficient redox-triggered chiroptical switching activity was evidenced.

Synthesis and Properties of Partially Saturated Fluorenyl-Derived [n]Helicenes Featuring an Overcrowded Alkene

The straightforward, multigram-scale synthesis of the partially saturated H₆ -fluoreno[n]helicenes (n=5 or 7) featuring a central, overcrowded alkene is described. The key cyclization step was based on an intramolecular McMurry reaction from the corresponding 1,5-diketones. Chiral stationary phase HPLC analysis and isomer separation indicate that each helicenic compound is constituted of three diastereoisomers at room temperature, i. e. the configurationally stable (R,R,P)/(S,S,M) pair of enantiomers and an apparently achiral compound resulting from the rapid interconversion between the (R,S,P) and (S,R,M) enantiomers. The partially saturated H₆ -fluoreno[n]helicenes are oxidatively aromatized to give an efficient access to the corresponding fluoreno[n]helicenes. The chiroptical properties (vibrational and electronic circular dichroism) of the chiral, enantiopure compounds have been measured and analyzed by quantum-chemical calculations, confirming their helicoidal nature.

Influence of Positional Isomerism on the Chiroptical Properties of Functional Aromatic Oligoamide Foldamers

A series of functionalized quinoline-based aromatic oligoamide foldamers were prepared in their two enantiomeric forms, comprising an enantiopure terminal camphanyl chiral inducer, which governed the adjacent (P-/M-) helical-handedness. Hierarchical chirality transfer was further investigated in chromophore-appended variants via a range of electronic and vibrational spectroscopic techniques, including circularly polarized luminescence, vibrational circular dichroism and fluorescence. Intense total and polarized photoluminescence (up to Φ_lum =0.39, g_lum =1.5×10^-3 ) was observed in the visible region from these modular multicomponent architectures and a significant influence of positional isomerism was evidenced. The optimal position of a fluorophore substituent on the quinoline hexamers was determined as being position 2 over position 6, as stronger chiroptical features were systematically observed with the 2-positioned derivatives.

Origins of Optical Activity in an Oxo-Helicene: Experimental and Computational Studies

Helicenes are known to provide extremely strong optical activity. Prediction of the properties of helicenes may facilitate their design and synthesis for analytical or materials sciences. On a model 7,12,17-trioxa[11]helicene molecule, experimental results from multiple spectroscopic techniques are analyzed on the basis of density functional theory (DFT) simulations to test computational methodology and analyze the origins of chirality. Infrared (IR), vibrational circular dichroism (VCD), electronic circular dichroism (ECD), magnetic circular dichroism (MCD), and Raman optical activity (ROA, computations only) spectra are compared. Large dissymmetry factors are predicted both for vibrational (ROA/Raman ∼ VCD/IR ∼ 10^-3) and electronic (ECD/Abs ∼10^-2) optical activity, which could be verified experimentally except for ROA. Largest VCD signals come from a strong vibrational coupling of the C-H in-plane and out-of-plane bending modes in stacked helicene rings. The sum-over-states (SOS) approach appeared convenient for simulation of MCD spectra. Our results demonstrated that selected computational methods can be successfully used for reliable modeling of spectral and chiroptical properties of large helicenes. In particular, they can be used for guiding rational design of strongly chiral chromophores.

NWChem: Past, present, and future

Specialized computational chemistry packages have permanently reshaped the landscape of chemical and materials science by providing tools to support and guide experimental efforts and for the prediction of atomistic and electronic properties. In this regard, electronic structure packages have played a special role by using first-principle-driven methodologies to model complex chemical and materials processes. Over the past few decades, the rapid development of computing technologies and the tremendous increase in computational power have offered a unique chance to study complex transformations using sophisticated and predictive many-body techniques that describe correlated behavior of electrons in molecular and condensed phase systems at different levels of theory. In enabling these simulations, novel parallel algorithms have been able to take advantage of computational resources to address the polynomial scaling of electronic structure methods. In this paper, we briefly review the NWChem computational chemistry suite, including its history, design principles, parallel tools, current capabilities, outreach, and outlook.

Supramolecular chiroptical switches

Recent progress in chiroptical switches including on/off, amplification, and inversion of the chiral signals such as ECD and CPL in supramolecular assemblies is shown.

Time-Dependent Formulation of Resonance Raman Optical Activity Spectroscopy

In this work, we extend the theoretical framework recently developed for the simulation of resonance Raman (RR) spectra of medium-to-large sized systems to its chiral counterpart, namely, resonance Raman optical activity (RROA). The theory is based on a time-dependent (TD) formulation, with the transition tensors obtained as half-Fourier transforms of the appropriate cross-correlation functions. The implementation has been kept as general as possible, supporting adiabatic and vertical models for the PES representation, both in Cartesian and internal coordinates, with the possible inclusion of Herzberg-Teller (HT) effects. Thanks to the integration of this TD-RROA procedure within a general-purpose quantum-chemistry program, both solvation and leading anharmonicity effects can be included in an effective way. The implementation is validated on one of the smallest chiral molecule (methyloxirane). Practical applications are illustrated with three medium-size organic molecules (naproxen-OCD₃, quinidine and 2-Br-hexahelicene), whose simulated spectra are compared to the corresponding experimental data.