Molecular Exciton Approach to Anisotropic Absorption and Circular Dichroism II. The Partial Optic Axis and its Application in Molecular Exciton Theory

The first part of the present communication develops the general theory of the partial optic axis, which is an excitation specific structural feature first proposed as an aid to the choice of chromophoric reference points in a molecular exciton approach to optical rotatory power, and discusses its applicability to symmetry analyses in the light of the distinction between the three categories for anisotropic optical rotatory power, i.e. chiral, achiral optically active, and achiral optically inactive molecular structures. The second part of the communication discusses the special role played by the concept of the partial optic axis in the evaluation of the anisotropic chromophoric intensity contributions in a molecular exciton approach, in particular in relation to the use of the chromophoric symmetry for the systematic selection and characterization of the intensity contributions.     

Exciton absorption spectra of optically excited linear molecular aggregates

Exciton absorption spectrum of optically excited linear molecular aggregate is theoretically investigated. The sum rules for the integral intensity of the absorption spectrum are derived. The dipole moments of the optical transitions from the one-exciton states to the two-exciton states are presented. The results obtained indicate an energy increase of the exciton transition after a single excitation of the aggregate. It accounts for the observed short-wavelength shift of the J-band of the pseudoisocyanine (PIC) J-aggregates after their optical excitation. The comparison of the experimental energy of the shift with its theoretical evaluation allows to estimate the number of monomers forming a typical PIC J-aggregate in the solutionN ?20–30.     

Anomalous Lehmann Rotation of Achiral Nematic Liquid Crystal Droplets Trapped under Linearly Polarized Optical Tweezers

Continuous rotation of a cholesteric droplet under the heat gradient was observed by Lehmann in 1900. This phenomenon, the so-called Lehmann effect, consists of unidirectional rotation around the heat flux axis. We investigate this gradient heat effect using infrared laser optical tweezers. By applying single trap linearly polarized optical tweezers onto a radial achiral nematic liquid crystal droplet, trapping of the droplet was performed. However, under a linearly polarized optical trap, instead of stable trapping of the droplet with slightly deformed molecular directors along with a radial hedgehog defect, anomalous continuous rotation of the droplet was observed. Under low power laser trapping, the droplet appeared to rotate clockwise. By continuously increasing the laser power, a stable trap was observed, followed by reverse directional rotation in a higher intensity laser trap. Optical levitation of the droplet in the laser beam caused the heat gradient, and a breaking of the symmetry of the achiral nematic droplet. These two effects together led to the rotation of the droplet under linearly polarized laser trapping, with the sense of rotation depending on laser power.     

Quantitative determination of linear and second-harmonic generation optical effective responses of achiral or chiral materials in planar structures: theory and materials

A theoretical formalism designed to quantify linear optical and second-order nonlinear optical responses of achiral or chiral anisotropic materials in planar structure is presented. In particular, the theory includes linear optical activity that is governed by the gyrotropic components and second-harmonic generation optical rotatory dispersion, the magnitude of which depends on the ratio of chiral and achiral chi((2)) components. Examples are given which reproduce complex interference effects and other subtle optical effects that are encountered in layered structures. Detailed experimental second harmonic generation studies of quartz and dihydrogen phosphate that quantify linear and nonlinear optical activities of these materials are reported.     

Beyond exciton theory: a time-dependent DFT and Franck-Condon study of perylene diimide and its chromophoric dimer

The diimide perylene motif exhibits a dramatic intensity reversal between the 0 --> 0 and 0 --> 1 vibronic bands upon pi-pi stacking; this distinct spectral property has previously been used to measure folding dynamics in covalently bound oligomers and synthetic biological hybrid foldamers. It is also used as a tool to assess organization of the pi-stacking, indicating the presence of H- or J-aggregation. The zeroth-order exciton model, often used to describe the optical properties of chromophoric aggregates, is solely a transition dipole coupling scheme, which ignores the explicit electronic structure of the system as well as vibrational coupling to the electronic transition. We have therefore examined the optical properties of gas-phase perylene tetracarboxylic diimide (PTCDI) and its chromophoric dimer as a function of conformation to relate the excited-state distributions predicted by exciton theory with that of time-dependent density functional theory (TDDFT). Using ground- and excited-state geometries, the Franck-Condon (FC) factors for the lowest energy molecular nature electronic transition have been calculated and the origin of the intensity reversal of 0 --> 0 and 0 --> 1 vibronic bands has been proposed.     

旋光性高分子的旋光能力（下）

在（上）中，我们讨论了结构单元及其远程作用对旋光性高分子旋光能力的影响。由于高分子具有特殊的长链结构，因而链形态对旋光能力的贡献尤其引人注目。     

Atomic partition of the optical rotatory power of methylhydroperoxide

We applied a methodology capable of resolving the optical rotatory power into atomic contributions. The individual atomic contributions to the optical rotatory power and molecular chirality of the methylhydroperoxide are obtained via a canonical transformation of the Hamiltonian by which the electric dipolar moment operator is transformed to the acceleration gauge formalism and the magnetic dipolar moment operator to the torque formalism. The gross atomic isotropic contributions have been evaluated for the carbon, the nonequivalent oxygen, and the nonequivalent hydrogen atoms of methylhydroperoxide, employing a very large Gaussian basis set which is close to the Hartree-Fock limit.     

Exciton Transport in Molecular Semiconductor Crystals for Spin-Optoelectronics Paradigm

Organic semiconductors with long-range exciton diffusion length are highly desirable for optoelectronics but currently remain rare. Here, the estimated diffusion length of singlet excitons (L_D) in 2,6-diphenyl anthracene (DPA) crystals grown by solvent evaporation was shown to be up to approximately 124 nm. These crystals showed a previously unseen parallelogram morphology with layer-by-layer edge-on molecular stacking, isotropic optical waveguiding, radiation rate and non-radiation rate constants of 0.15 and 0.26 ns⁻¹ respectively, as well as good field-effect transistor hole mobility and theoretically computed strong electronic couplings as high as 109 meV. Photoresponse experiments revealed that the photoconductivity of DPA crystals is surprisingly not related to the radiative pathway but associated with rapid exciton diffusion to the crystal surface for charge separation and carrier bimolecular recombination. Taken together, DPA was shown to be a promising semiconducting material for a new organic optoelectronics paradigm.     

Optical activity of chiral thin film and liquid crystal hybrids

Using the glancing angle deposition (GLAD) technique, we have fabricated porous, chiral thin films with optically anisotropic helical microstructures that exhibit optical phenomena such as wavelength specific rotation of linearly polarized light. Initial research has shown that the porosity of the films allows for the addition of nematic liquid crystals (NLCs) to the films for promising applications in dynamically switchable devices, while simultaneously enhancing the optical properties of the film. This study describes the fundamental optical behaviour of LC-filled chiral thin films in relation to material, porosity, structure and thickness. It was found that for SiO₂ films, the addition of NLCs increased the effective rotatory power by two-fold when compared with results from the film without added LCs. The rotatory power of Al₂O₃ and MgF₂ films, while being similarly increased by the addition of LCs, exhibited a reversal in sign, or direction of rotation, for the visible wavelength spectrum investigated. The effects of film porosity and structure were studied by varying the angle of incidence from 83° to 86°; it was found that the greater porosity of the films deposited at larger angles allowed for more filling by the LCs and thus a larger increase in rotatory power. Finally, the addition of LCs was observed to shift the wavelength of peak rotation towards smaller values.     

Optical rotation of achiral compounds

Oriented achiral molecules and crystals with D(2d) symmetry or one of its non-enantiomorphous subgroups, S(4), C(2v), or C(s), can rotate the plane of transmitted polarized light incident in a general direction. This well-established fact of crystal optics is contrary to the teaching of optical activity to students of organic chemistry. This Minireview gives an overview of the measurement and calculation of the chiroptical properties of some achiral compounds and crystals. Methane derivatives with four identical ligands related by reflection symmetry are quintessential optically inactive compounds according to the logic of van't Hoff. Analysis of the optical activity of simple achiral compounds such as H(2)O and NH(3) provides general aspects of chiroptics that are not readily broached when considering chiral compounds exclusively. We show here, through the use of group theoretical arguments, the transformation properties of tensors, and diagrams, why some achiral, acentric compounds are optically active while others are not.     

Frenkel‐exciton decomposition analysis of circular dichroism and circularly polarized luminescence for multichromophoric systems

Recently, a method to calculate the absorption and circular dichroism (CD) spectra based on the exciton coupling has been developed. In this work, the method was utilized for the decomposition of the CD and circularly polarized luminescence (CPL) spectra of a multichromophoric system into chromophore contributions for recently developed through‐space conjugated oligomers. The method which has been implemented using rotatory strength in the velocity form and therefore it is gauge‐invariant, enables us to evaluate the contribution from each chromophoric unit and locally excited state to the CD and CPL spectra of the total system. The excitonic calculations suitably reproduce the full calculations of the system, as well as the experimental results. We demonstrate that the interactions between electric transition dipole moments of adjacent chromophoric units are crucial in the CD and CPL spectra of the multichromophoric systems, while the interactions between electric and magnetic transition dipole moments are not negligible. © 2018 Wiley Periodicals, Inc.     

Optical activity of chiral thin film and liquid crystal hybrids

Using the glancing angle deposition (GLAD) technique, we have fabricated porous, chiral thin films with optically anisotropic helical microstructures that exhibit optical phenomena such as wavelength specific rotation of linearly polarized light. Initial research has shown that the porosity of the films allows for the addition of nematic liquid crystals (NLCs) to the films for promising applications in dynamically switchable devices, while simultaneously enhancing the optical properties of the film. This study describes the fundamental optical behaviour of LC-filled chiral thin films in relation to material, porosity, structure and thickness. It was found that for SiO₂ films, the addition of NLCs increased the effective rotatory power by two-fold when compared with results from the film without added LCs. The rotatory power of Al₂O₃ and MgF₂ films, while being similarly increased by the addition of LCs, exhibited a reversal in sign, or direction of rotation, for the visible wavelength spectrum investigated. The effects of film porosity and structure were studied by varying the angle of incidence from 83° to 86°; it was found that the greater porosity of the films deposited at larger angles allowed for more filling by the LCs and thus a larger increase in rotatory power. Finally, the addition of LCs was observed to shift the wavelength of peak rotation towards smaller values.     

Theory of the absorption and circular dichroism spectra of helical molecular aggregates

A theory of the electronic circular dichroism (CD) and optical rotatory dispersion (ORD) of infinite aggregates exhibiting cylindrical symmetry is presented in which, to the authors' knowledge, for the first time vibrational structure is included explicitly. It is shown that, with the coherent exciton scattering approximation in the Green function approach, the detailed vibrational structure of the aggregate absorption. CD and ORD bands can be calculated from a knowledge of the electronic coupling and the monomer absorption line shape alone. Detailed model calculations for a single helix are made and the results are used to expose the origin of different spectral features. A good reproduction of experimental J-aggregate spectra is obtained, using the same electronic interaction to fit both absorption and CD spectral line shapes. The theory allows some prediction of aggregate geometry to be made, but it is shown that an unambiguous geometrical assignment can only be made where experimental spectra for light of different propagation directions with respect to the cylinder axis are available.     

Reversible light-directed red, green, and blue reflection with thermal stability enabled by a self-organized helical superstructure

Adding external, remote, and dynamic control to self-organized superstructures with desired properties is an important leap necessary in leveraging the fascinating molecular subsystems for employment in applications. Here two novel light-driven dithienylethene chiral molecular switches possessing remarkable changes in helical twisting power during photoisomerization as well as very high helical twisting powers were found to experience photochemically reversible isomerization with thermal stability in both isotropic organic solvents and anisotropic liquid crystal media. When doped into a commercially available achiral liquid crystal host, the chiral switch was able to either immediately induce an optically tunable helical superstructure or retain an achiral photoresponsive liquid crystal phase whose helical superstructure was induced and tuned reversibly upon light irradiation. Moreover, reversible light-directed red, green, and blue reflection colors with thermal stability in a single thin film were demonstrated.     

Disorder-induced exciton localization and violation of optical selection rules in supramolecular nanotubes

Using numerical simulations, we study the effect of disorder on the optical properties of cylindrical aggregates of molecules with strong excitation transfer interactions. The exciton states and the energy transport properties of such molecular nanotubes attract considerable interest for application in artificial light-harvesting systems and energy transport wires. In the absence of disorder, such nanotubes exhibit two optical absorption peaks, resulting from three super-radiant exciton states, one polarized along the axis of the cylinder, the other two (degenerate) polarized perpendicular to this axis. These selection rules, imposed by the cylindrical symmetry, break down in the presence of disorder in the molecular transition energies, due to the fact that the exciton states localize and no longer wrap completely around the tube. We show that the important parameter is the ratio of the exciton localization length and the tube's circumference. When this ratio decreases, the distribution of polarization angles of the exciton states changes from a two-peak structure (at zero and ninety degrees) to a single peak determined by the orientation of individual molecules within the tube. This is also reflected in a qualitative change of the absorption spectrum. The latter agrees with recent experimental findings.     

Optical activity of oriented molecules. comparison of the optical rotatory dispersion and the circular dichroism through the kramers-kronig transform

Circular dichroism and optical rotatory dispersion of anisotropic solutions of testosterone propionate in cholesteryl chloride/cholesteryl laurate have been measured with light propagating parallel to the optical axis of the uniaxial system. Comparison of both effects by the Kramers-Kronig relation excludes falsification by interference with linear birefringence and dichroism effects.     

Isotope effect of optical activity measurements on L-α-alanine

If an optically active organic substance is labelled in the chirality center with another isotopic species (such as¹⁵N for¹⁴N) a pronounced variation of rotatory power is predicted. We tried to varify this idea experimentally on L-α-alanine and found an isotope effect of ORD (optical rotatory dispersion). The magnitude of the rotation is mainly dependent on the pH of the solvent. The ratio of the optical activity alanine-¹⁴N/alanine-¹⁵N is about 1.02. The molecular rotations show a consistently lower ratio but it can be seen that the isotope effect is not only a mass effect.