Electric Dipole Transition Moments Research Articles

Longitudinal Extension of Double π-Helix Enables Near-Infrared Amplified Dissymmetry and Chiroptical Response.

Near-infrared (NIR) circularly polarized light absorbing or emitting holds great promise for highly sensitive and precise bioimaging, biosensing, and photodetectors. Aiming at designing NIR chiral molecular systems with amplified dissymmetry and robust chiroptical response, herein, we present a series of double π-helical dimers with longitudinally extended π-entwined substructures via Ullmann or Yamamoto homocoupling reactions. Circular dichroism (CD) spectra revealed an approximate linear bathochromic shift with the rising number of naphthalene subunits, indicating a red to NIR chiroptical response. Particularly, the terrylene diimide-entwined dimers exhibited the strongest CD intensities, with the maximal |Δε| reaching up to 393 M-1 cm-1 at 666 nm for th-TDI[2]; and a record-high chiroptical response (|ΔΔε|) between the neutral and dianionic species of 520 M-1 cm-1 at 833 nm for th-TDI[2]Cl was achieved upon further reduction to its dianionic state. Time-dependent density functional theory (TDDFT) calculations suggested that the pronounced intensification of the CD spectra originated from a simultaneous enhancement of both electric (μ) and magnetic (m) transition dipole moments, ultimately leading to an overall increase in the rotatory strength (R). Notably, the circularly polarized luminescence (CPL) brightness (BCPL) reached 77 M-1 cm-1 for th-TDI[2]Cl, among the highest values reported for NIR-CPL emitters. Furthermore, all chiral dianions exhibited excellent air stability under ambient conditions with half-life times of up to 10 days in N-methylpyrrolidone (NMP), which is significant for future biological applications and chiroptic switches.

Quantum Size-Driven Spectral Variations in Pillar[n]arene Systems: A Density Functional Theory and Wave Function Assessment.

This study explores the quantum size effects on the optical properties of pillar[n]arene (n = 5, 6, 7, 8) utilizing density functional theory (DFT) and wave function analysis. The mechanisms of electron transitions in one-photon absorption (OPA) and two-photon absorption (TPA) spectra are investigated, alongside the calculation of electron circular dichroism (ECD) for these systems. Transition Density Matrix (TDM) and electron-hole pair density maps are employed to study the electron excitation characteristics, unveiling a notable size dependency. Analysis of the transition electric dipole moment (TEDM) and the transition magnetic dipole moment (TMDM) reveals the electromagnetic interaction mechanism within pillar[n]arene. Raman spectra computations further elucidate vibrational modes, while interactions with external environments are studied using electrostatic potential (ESP) analysis, and electron delocalization is assessed under an external magnetic field, providing insights into the magnetically induced current phenomena within these supramolecular structures. The thermal stability of pillar[n]arene was investigated by ab initio molecular dynamics (AIMD).

Achieving Strong Circularly Polarized Luminescence through Cascade Cationic Insertion in Lead-free Hybrid Metal Halides.

Generating circularly polarized luminescence (CPL) with simultaneous high photoluminescence quantum yield (PLQY) and dissymmetry factor (glum) is difficult due to usually unmatched electric transition dipole moment (μ) and magnetic transition dipole moment (m) of materials. Herein we tackle this issue by playing a "cascade cationic insertion" trick to achieve strong CPL (with PLQY of ~100 %) in lead-free metal halides with high glum values reaching -2.3×10-2 without using any chiral inducers. Achiral solvents of hydrochloric acid (HCl) and N, N-dimethylformamide (DMF) infiltrate the crystal lattice via asymmetric hydrogen bonding, distorting the perovskite structure to induce the "intrinsic" chirality. Surprisingly, additional insertion of Cs+ cation to substitute partial (CH3)2NH2 + transforms the chiral space group to achiral but the crystal maintains chiroptical activity. Further doping of Sb3+ stimulates strong photoluminescence as a result of self-trapped excitons (STEs) formation without disturbing the crystal framework. The chiral perovskites of indium-antimony chlorides embedded on LEDs chips demonstrate promising potential as CPL emitters. Our work presents rare cases of chiroptical activity of highly luminescent perovskites from only achiral building blocks via spontaneous resolution as a result of symmetry breaking.

Achieving Strong Circularly Polarized Luminescence through Cascade Cationic Insertion in Lead‐free Hybrid Metal Halides

AbstractGenerating circularly polarized luminescence (CPL) with simultaneous high photoluminescence quantum yield (PLQY) and dissymmetry factor (glum) is difficult due to usually unmatched electric transition dipole moment (μ) and magnetic transition dipole moment (m) of materials. Herein we tackle this issue by playing a “cascade cationic insertion” trick to achieve strong CPL (with PLQY of ~100 %) in lead‐free metal halides with high glum values reaching −2.3×10−2 without using any chiral inducers. Achiral solvents of hydrochloric acid (HCl) and N, N‐dimethylformamide (DMF) infiltrate the crystal lattice via asymmetric hydrogen bonding, distorting the perovskite structure to induce the “intrinsic” chirality. Surprisingly, additional insertion of Cs+ cation to substitute partial (CH3)2NH2+ transforms the chiral space group to achiral but the crystal maintains chiroptical activity. Further doping of Sb3+ stimulates strong photoluminescence as a result of self‐trapped excitons (STEs) formation without disturbing the crystal framework. The chiral perovskites of indium‐antimony chlorides embedded on LEDs chips demonstrate promising potential as CPL emitters. Our work presents rare cases of chiroptical activity of highly luminescent perovskites from only achiral building blocks via spontaneous resolution as a result of symmetry breaking.

Enhancing Chiroptical Responses in Helical Nanographenes via Geometric Engineering of Double [7]Helicenes.

Helical nanographenes with high quantum yields and strong chiroptical responses are pivotal for developing circularly polarized luminescence (CPL) materials. Here, we present the successful synthesis of novel π-extended double [7]helicenes (ED7Hs) where two helicene units are fused at the meta- or para-position of the middle benzene ring, respectively, as the structural isomers of the reported ortho-fused ED7H. The structural geometry of these ED7Hs is clearly characterized by single-crystal X-ray analysis. Notably, this class of ED7Hs exhibits bright luminescence with high quantum yields exceeding 40 %. Through geometric regulation of two embedded [7]helicene units from ortho-, meta- to para-position, these ED7Hs display exceptional amplification in chiroptical responses. This enhancement is evident in a remarkable approximate fivefold increase in the absorbance and luminescence dissymmetry factors (gabs and glum), respectively, along with a boosted CPL brightness up to 176 M-1 cm-1, surpassing the performance of most helicene-based chiral NGs. Furthermore, DFT calculations elucidate that the geometric adjustment of two [7]helicene units allows the precise alignment of electric and magnetic transition dipole moments, leading to the observed enhancement of their chiroptical responses. This study offers an effective strategy for magnifying the CPL performance in chiral NGs, promoting their expanded application as CPL emitters.

Transition properties of the spin- forbidden a4Π-X2Π systems in the NO, PO and PS molecules

The potential energy curves (PECs) for six low-lying states corresponding to the first dissociation limit (4Su+3Pg) of the NO, PO, and PS molecules are calculated by the multireference configuration interaction (MRCI) method. The X2Π -a4Π band is the focus of this work. By introducing the spin-orbit coupling (SOC) with Breit-Pauli operator via state-interacting technique, the PECs and wavefunctions compositions of Ω states are obtained. Based on the Λ-S states composition of the corresponding Ω states and the calculated electric-dipole transition moments between Λ-S state, we present intensity of the spin-forbidden quartet-doublet bands. Spectroscopic constants for all sub-bands of the a24Π3/2, a34Π1/2, a44Π1/2→X12Π1/2 and a14Π5/2, a24Π3/2, a34Π1/2, a44Π1/2→X22Π3/2 systems of the NO, PO, and PS are estimated. A set of transition parameters of these sub-bands are given. The present results are supposed to provide a significant supplement for the spin-forbidden a4Π→X2Π band origins of the astrophysically important NO, PO, and PS molecules.

System of metastable volume-localized electronic states in positively charged semiconductor single-wall carbon nanotubes

The volume-localized electronic states (SAMOs) with a maximum of their electron wave functions located in the cavity of nanomaterials have been experimentally and theoretically demonstrated in a fullerene. The existence of SAMOs in single-wall carbon nanotubes (SWCNTs) was also predicted theoretically. In the present paper, these volume states in semiconductor SWCNTs were theoretically investigated using numerical quantum modeling based on density functional theory (DFT). It is shown that the well appears in the center of the tube, whose depth increases with increasing positive charge, since the total potential of a positively charged structure can be represented as the sum of the Coulomb potential and the potential of the atoms of the tube wall. In this context, in addition to the well-studied surface-localized states, states localized in the volume of the cylinder also occur. Using the components of the electric transition dipole moment, the lifetime of the volume states was preliminarily estimated in comparison to the lifetime of the ordinary surface states.

Verstärkung von chiroptischen Eigenschaften in helikalen Nanographenen durch geometrische Anpassung von Doppel‐[7]Helicenen

AbstractHelikale Nanographene (NG) mit hohen Quantenausbeuten und starken chiroptischen Antworten sind entscheidend für die Entwicklung von lumineszenten Materialien mit zirkularer Polarisation (engl. circular polarized luminescence, CPL). Hier berichten wir von der erfolgreichen Synthese neuartiger π‐erweiterter Doppel‐[7]Helicene (ED7Hs), bei denen sich zwei Helicen‐Einheiten meta bzw. para am zentralen Benzenring befinden. Sie sind damit strukturelle Isomere des bekannten ortho‐ED7H. Die strukturelle Integrität und Geometrie der ED7Hs wurde eindeutig durch Röntgen‐Einkristalldiffraktometrie aufgeklärt. Die ED7Hs besitzen eine ausgeprägte Fluoreszenz mit Quantenausbeuten über 40 %. Durch Variation der relativen Positionen der Doppel‐[7]Helicene am zentralen Benzenring (ortho, meta, para) kann eine außergewöhnliche Verstärkung der chiroptischen Eigenschaften registriert werden. Diese Verbesserung zeigt sich in einem etwa fünffachen Anstieg der Absorbanz und Lumineszenz‐Dissymmetriefaktoren (gabs und glum) sowie einer erhöhten CPL‐Helligkeit von bis zu 176 M−1 cm−1, was die Leistung der meisten helicenbasierten chiralen NGs übertrifft. Des weiteren zeigen DFT‐Simulationen, dass die relative geometrische Lage der [7]Helicene eine präzise Anpassung der elektrischen und magnetischen Übergangsdipolmomente zur Folge hat, welche die beobachteten Verstärkungen der chiroptischen Eigenschaften erklären. Diese Studie liefert eine effektive Strategie zur Verstärkung der CPL‐Eigenschaften chiraler Nanographene, welche ihre Anwendbarkeit als CPL‐Emitter verbessert.

The Physical Mechanism of Linear and Nonlinear Optical Properties of Nanographene-Induced Chiral Inversion.

Based on density functional theory (DFT) and wave function analysis, the ultraviolet and visible spectrophotometry (UV-Vis) spectra and Raman spectra of 1-meso and 1-rac obtained by the chiral separation of chiral nanographenes are theoretically investigated. The electron excitation properties of 1-meso and 1-rac are studied by means of transition density matrix (TDM) and charge density difference (CDD) diagrams. The intermolecular interaction is discussed based on an independent gradient model based on Hirshfeld partition (IGMH). The interaction of 1-meso and 1-rac with the external environment is studied using the electrostatic potential (ESP), and the electron delocalization degree of 1-meso and 1-rac is studied based on the magnetically induced current under the external magnetic field. Through the chiral separation of 1-rac, two enantiomers, 1-(P, P) and 1-(M, M), were obtained. The electrical-magnetic interaction of the molecule is revealed by analyzing the electron circular dichroism (ECD) spectra of 1-meso, 1-(P, P) and 1-(M, M), the transition electric dipole moment (TEDM) and the transition magnetic dipole moment (TMDM). It is found that 1-(P, P) and 1-(M, M) have opposite chiral properties due to the inversion of the structure.

Physical insight into optical spectroscopy of chiral covalent organic pillars as molecular nanotubes

In this paper, we theoretically investigate chiral covalent organic pillars synthesized by connecting rim-desymmetrized macrocycle, stimulated by recent experimental synthesis [Nat. Synth. 2023, 2, 395–402]. Transition densities reveal physical mechanism of spectral intensity in one-photon absorption (OPA) and two-photon absorption (TPA), and solvent effect strongly influence the spectral profile and intensity in TPA, but little influence the spectrum in OPA. The intra-unit and inter-unit charge transfer are visualized in five phenylenediamine linkers. The chirality of covalent organic pillars is revealed by electronic circular dichroism (ECD) spectroscopy, and interpreted by the visualized transition electric and magnetic dipole moments. Molecular vibrational information is demonstrated by normal and resonance Raman spectra and vibrational modes. The visualization of static and dynamic (hyper) polarization can reveal the physical mechanisms of polarization dependent normal and resonance Raman scattering. Our results provide deeper understanding on the physical mechanism of chiral covalent organic pillars as molecular nanotubes.

Structurally Diverse Pyrene-decorated Planar Chiral [2,2]Paracyclophanes with Tunable Circularly Polarized Luminescence between Monomer and Excimer.

We reported the synthesis of a series of structurally diverse CPL-active molecules, in which pyrene units were installed to chiral pm/po-[2,2]PCP scaffolds either with or without a triple bond spacer for pm/po-PCP-P1 and pm/po-PCP-P2, respectively. The X-ray crystallographic analyses revealed that these pyrene-based [2,2]PCP derivatives exhibited diverse structures and crystal packings in the solid phases. The pyrene-based [2,2]PCP derivatives exhibit various (chir)optical properties in organic solutions, depending on their respective structures. In a mixture of dioxane and water, pm/po-PCP-P1 emit green excimer fluorescence, whereas pm/po-PCP-P2 emit blue one. The chiroptical investigation demonstrated that Rp-pm-PCP-P1 and Rp-pm-PCP-P2 exhibited completely opposite CD and CPL signals even they possess the same chiral Rp-[2,2]PCP core. The same argument also holds for other chiral pyrene-based [2,2]PCP derivatives. The theoretical calculation revealed that these unusual phenomena were attributed to different orientation between transition electric dipole moments and the magnetic dipole moments originating from the presence or absence of a triple bond spacer. These pyrene-based [2,2]PCP derivatives display various colours and fluorescence emissions in the solid state and PMMA films, possibly due to the different packings as observed in the crystal structure. Moreover, these compounds also can interact with perylene diimide through π-π interactions, leading to near-white fluorescence.

Sum of electric and magnetic transition dipole moments – A new molecular descriptor for explaining enantiomeric enrichment of chiral amino acids by l- and r-circularly polarized light

One of the big scientific questions is why all amino acids constituting biomolecules have the l-configuration in nature. Enantiomeric enrichment of chiral amino acids by left- and right-circularly polarized light (l- and r-CPL) could be part of the answer. Previous studies directly measured the enantiomeric excesses of amino acids initiated by l- and r-CPL. This study found well-known molecular properties for circular dichroism: electric transition dipole moment, magnetic transition dipole moment, rotational strength, and anisotropy factor could not explain those enantiomeric excess data. By means of DFT/TDDFT (B3LYP-D3BJ) with a basis set (def2-TZVPD), this study discovered that a new molecular descriptor: the sum of electric and magnetic transition dipole moments could provide an explanation of those enantiomeric excess data. In addition, this discovery may provide new theoretical insights into photochemical reactions initiated by l- and r-CPL and assist in establishing fundamental chemistry for the abiotic origin of life.

A donor-acceptor cage for circularly polarized TADF emission.

Herein, we report the first example of chiral donor-acceptor cage DA-2 displaying efficient circularly polarized thermally activated delayed fluorescence (CP-TADF) with |glum| values up to 2.1 × 10-3 and PLQY of 32%. A small ΔEST of 0.051 eV and quasi-parallel (θ = 6°) transition electric and magnetic dipole moments were realized from the through-space charge transfer interaction between the parallelly aligned donor and acceptor in DA-2. This D-A cage configuration has provided a novel design strategy for discovering potential efficient CP-TADF emitters.

Ab initio calculations of the spectra and lifetimes of the lead dimer.

Owing to the key role of the lead dimer (Pb2) as a heavy element benchmark for the Group IV-A dimers the assignment of its spectroscopic properties and chemical bonding is an important undertaking. To meet this demand, the present work provides comprehensive and detailed information on electronic structure and properties comprising a wide set of Pb2 states. Calculations are performed by a high-level ab initio approach. Firstly, the potential energy curves (PECs) of 19 Λ-S states as well as those of 24 ungerade Ω states are calculated by utilizing the multi-reference configuration interaction plus Davidson correction (MRCI + Q) method taking into account core-valence correlation (CV) and spin-orbit coupling (SOC) effect, where Ω is a quantum number of the total (Λ + S) angular momentum projection. Secondly, interactions between the bound F3Σ-u, 23Σ+u states and repulsive 15Πu state induced by strong SOC are discussed based on the PECs analysis and calculated SOC matrix, which also indicates that the F3Σ-u state is predissociative. Thirdly, based on the calculated electric dipole transition moments and energy gaps between the 0+u(III), F0+u(II), C0+u(I) and X0+g states, the intense absorption bands of Pb2 due to these transitions are interpreted. Our results indicate that the trends in intensity of absorption spectra (F0+u(II), C0+u(I) ← X0+g) in the range of 12 600-13 600 and 22 200-23 800 cm-1 are consistent with the previously observed spectra of Pb2 in the qualitatively similar regions (15 200-16 200 and 19 800-21 800 cm-1). Finally, the calculated intensity of the weak magnetic-dipole transitions from the singlet excited b1Σ+g and a1Δg states to the triplet ground X3Σ-g state and their electric quadrupole components are presented for the Pb2 molecule in terms of SOC perturbations for the calculated Ω states expressed in Λ-S state notation. Based on our theoretical assignment, we predict that the weak emission a1Δg2 → X3Σ-g1 bands could be observed experimentally. The present work provides comprehensive electronic structure information and sheds new light on the absorption and emission spectra of the Pb2 dimer.

Sensitivity of radio-frequency electric field sensor based on Rydberg Stark effect

Rydberg atoms hold special attraction in electric applications due to their large transition electric dipole moments and huge polarization, which leads to a strong response of atom to electric fields. In radio-frequency (RF) fields, the Rydberg levels are AC Stark shift and splitting, which can realize the study of high-sensitivity electric field sensor of Rydberg atoms. In this work, we use the simpler Shirley’s time-independent Floquet Hamiltonian model to calculate the AC Stark energy spectrum of Cs Rydberg atoms. This model can reduce the basic Hamiltonian into such a Hamiltonian that includes only those Rydberg states that have direct dipole-allowed transitions with the target state, thereby significantly improving the speed of computation. The accuracy of the calculation is proved by fitting with the calculated frequency shift of DC Stark energy levels in the weak fields, and the polarizability of 60D<sub>5/2</sub> and 70D<sub>5/2</sub> Rydberg atomic states are obtained by fitting with the measured ion spectra of DC Stark Cs ultra-cold Rydberg atoms in magneto-optical trap. In addition, we calculate the AC Stark shift of Cs Rydberg atom <inline-formula><tex-math id="M2">\begin{document}$ \left| {60{{\text{D}}_{5/2}},{m_j} = 1/2} \right\rangle $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240162_M2.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240162_M2.png"/></alternatives></inline-formula> state in electric fields with different frequencies with <i>ε</i> = 100 mV/m. Rydberg atoms provide a structured spectrum of sensitivity to electric fields due to strong resonant interaction and off-resonant interaction with many dipole-allowed transitions to nearby Rydberg states. This kind of the frequency response structure is of significance to a broadband sensor. And we calculate the sensitivity and the scaling of the signal-to-noise ratio (SNR), <i>β</i>, varying with detuning from the <inline-formula><tex-math id="M3">\begin{document}$ \left| {60{{\text{D}}_{5/2}}} \right\rangle \to \left| {61{{\text{P}}_{3/2}}} \right\rangle $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240162_M3.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20240162_M3.png"/></alternatives></inline-formula> transition. The value of <i>β</i> allows one to use the result for any Rydberg state sensor to determine the SNR for any <i>Ε</i> in a 1 s measurement. Therefrom, Rydberg sensor can preferentially detect many RF frequencies spreading across its carrier spectral range without modification while effectively rejecting large portions where the atom response is significantly weaker, and the signal depends primarily on the detuning of the RF field to the nearest resonance which does not convey the RF frequency directly.

Can Magnetic Dipole Transition Moment Be Engineered?

AbstractThe development of chiral compounds with enhanced chiroptical properties is an important challenge to improve device applications. To that end, an optimization of the electric and magnetic dipole transition moments of the molecule is necessary. Nevertheless, the relationship between chemical structure and such quantum mechanical properties is not always clear. That is the case of magnetic dipole transition moment (m) for which no general trends for its optimization have been suggested. In this work we propose a general rationalization for improving the magnitude of m in different families of chiral compounds. Performing a clustering analysis of hundreds of transitions, we have been able to identify a single group in which |m| value is maximized along the helix axis. More interestingly, we have found an accurate linear relationship (up to R2=0.994) between the maximum value of this parameter and the area of the inner cavity of the helix, thus resembling classical behavior of solenoids. This research provides a tool for the rationalized synthesis of compounds with improved chiroptical responses.

Can Magnetic Dipole Transition Moment Be Engineered?

The development of chiral compounds with enhanced chiroptical properties is an important challenge to improve device applications. To that end, an optimization of the electric and magnetic dipole transition moments of the molecule is necessary. Nevertheless, the relationship between chemical structure and such quantum mechanical properties is not always clear. That is the case of magnetic dipole transition moment (m) for which no general trends for its optimization have been suggested. In this work we propose a general rationalization for improving the magnitude of m in different families of chiral compounds. Performing a clustering analysis of hundreds of transitions, we have been able to identify a single group in which |m| value is maximized along the helix axis. More interestingly, we have found an accurate linear relationship (up to R2 =0.994) between the maximum value of this parameter and the area of the inner cavity of the helix, thus resembling classical behavior of solenoids. This research provides a tool for the rationalized synthesis of compounds with improved chiroptical responses.

Macrocyclization-induce emission enhancement characteristics of benzothiadiazole-based macrocycles: A theoretical study

We present a comprehensive investigation into the molecular structures in the ground state and the first singlet excited state, intramolecular and intermolecular interactions, as well as the emission properties of monomers, dimers and macrocycles using density functional theory and time-dependent density functional theory. Our study aims to unveil the macrocyclization-induce emission enhancement principle for benzothiadiazole-based macrocycles. Compared with the typical π-π stacking of monomers, macrocycles exhibit the tighter stacking arrangement due to the presence of strong intramolecular and intermolecular hydrogen bond interactions and the rigid triangular geometry of macrocycle, which is conducive to the enhancement of emissions efficiency. The calculated oscillator strength of macrocycles in the first excited state is much higher than that of dimers composed of two monomers. Meanwhile, the large transition electric dipole moment is beneficial for enhancing the emission efficiency. Additionally, our study reveals an emission wavelength red-shift for macrocycles after “CH”/N substitution in the acceptor moiety, and the transition mode from LUMO to HOMO can be mainly attributed to intramolecular charge transition characters from acceptor to donor fragments. Our theoretical study might provide valuable insights for the design of innovative luminescent macrocycles with high emission efficiency.

Chiral Benzene Triimide (BTI) Radical Anions for Probing the Interplay of Unpaired Electron Spin and Chirality.

Herein a series of chiral BTI radical anions bearing different chiral substituents were efficiently prepared by chemical reduction. X-ray crystallography revealed finely-tuned packing and helix assemblies of the radicals by the size of chiral substituents in crystalline state. In accordance with the crystalline-state packing, the powder ESR spectra indicate that 4 a- ⋅CoCp2 + and 4 c- ⋅CoCp2 + π-dimers exhibit thermally excited triplet states arising from strong spin-spin interactions, while discrete 4 b- ⋅CoCp2 + shows a broad doublet-state signal reflecting weak spin-spin interactions. The interplay between the unpaired electron spin and chiral substituents was studied by UV-Vis-NIR spectra, electronic circular dichroism (ECD) and TD DFT calculations. Different NIR absorptions of the radicals attributing to isolated SOMO→LUMO+1 (~889 nm) transitions were recorded. The emergence of Cotton effects (CEs) at the NIR region for 4 c- ⋅CoCp2 + radical enantiomers suggest the interplay between chirality and unpaired electron spin. The origin of the different circularly polarized light absorptions regarding SOMO derived transitions (around 880 nm) was attributed to chiral substitutes regulated electric and magnetic transition dipole moments of the unpaired electron participated transition.

Chiroptical Properties of Planar Benzobisthiazole-Bridged Squaraine Dimers.

Five chiral squaraine dimers were synthesized by fusing chiral indolenine semisquaraines with three different benzobisthiazole bridges. The thereby created squaraine dimers show a strong splitting of the lowest energy absorption bands caused by exciton coupling. The intensities of the two exciton transitions and the energetic splitting depend on the angle of the two squaraine moieties within the chromophore dimer. The electric circular dichroism spectra of the dimers show intense Cotton effects whose sign depends on the used squaraine chromophores. Sizable anisotropies gabs of up to 2.6 × 10-3 could be obtained. TD-DFT calculations were used to partition the rotational strength into the three Rosenfeld terms where the electric-magnetic coupling turned out to be the dominant contribution while the exciton chirality term is much smaller. This is because the chromophore dimers are essentially planar but the angle between the electric transition dipole moment of one squaraine and the magnetic transition dipole moment of the other squaraine strongly deviates from 90°, which makes the dot product between the two moment vectors and, thus, the rotational strength substantial.