Functionalization of Supramolecular Polymers by Dynamic Covalent Boroxine Chemistry.

Molecular scaffolds that enable the combinatorial synthesis of new supramolecular building blocks are promising targets for the construction of functional molecular systems. Here, we report a supramolecular scaffold based on boroxine that enables the formation of chiral and ordered 1D supramolecular polymers, which can be easily functionalized for circularly polarized luminescence. The boroxine monomers are quantitatively synthesized in situ, both in bulk and in solution, from boronic acid precursors and cooperatively polymerize into 1D helical aggregates stabilized by threefold hydrogen-bonding and π-π stacking. We then demonstrate amplification of asymmetry in the co-assembly of chiral/achiral monomers and the co-condensation of chiral/achiral precursors in classical and in situ sergeant-and-soldiers experiments, respectively, showing fast boronic acid exchange reactions occurring in the system. Remarkably, co-condensation of pyrene boronic acid with a hydrogen-bonding chiral boronic acid results in chiral pyrene aggregation with circularly polarized excimer emission and g-values in the order of 10-3. Yet, the electron deficiency of boron in boroxine makes them chemically addressable by nucleophiles, but also sensitive to hydrolysis. With this sensitivity in mind, we provide first insights into the prospects offered by boroxine-based supramolecular polymers to make chemically addressable, functional, and adaptive systems.

Polaritons in a Polycrystalline Layer of Non-fullerene Acceptor.

Non-fullerene acceptor molecules developed for organic solar cells feature a very intense absorption band in the near-infrared. In the solid phase, the strong interaction between light and the transition dipole moment for molecular excitation should induce formation of polaritons. The reflection spectra for polycrystalline films of a non-fullerene acceptor with a thienothienopyrrolo-thienothienoindole core of the so-called Y6 type indeed show a signature of polaritons. A local minimum in the middle of the reflection band is associated with the allowed molecular transition. The minimum in reflection allows efficient entry of light into the solid, resulting in a local maximum in external quantum efficiency of a photovoltaic cell made of the pure acceptor.

The Exciton Model for Molecular Materials: Past, Present and Future?

In assemblies of identical molecules or chromophores, electronic excitations can be described as excitons, bound electron-hole pairs that can move from site to site as a pair in a coherent manner. The understanding of excitons is crucial when trying to engineer favorable photophysical properties through structuring organic molecular matter. In recent decades, limitations of the concept of an exciton have become clear. The exciton can hybridize with phonon and photons. To clarify these issues, the exciton is discussed within the broader context of the gauge properties of the electromagnetic force.

Circular depolarization spectroscopy: A new tool to study photo-imprinting of chirality.

When irradiating a molecular material containing photo-isomerizable groups with pure circularly polarized light, a particular handedness may get imprinted into the material. To study the mechanism and kinetics of this process in situ and operando, we have developed a new chiroptical tool where the circular polarization of the incident circularly polarized light is monitored after transmission through the photoactive layer. Practical limits to the resolution and sensitivity of the measurements as well as its calibration are discussed. To aid interpretation of experimental results, we present kinetic Monte Carlo simulations on a model for the active material involving photo-induced reorientation of molecules in a cholesteric organization. The simulations support the interpretation of a transient minimum in the degree of circular polarization of the transmitted light in terms of a nematic transient state during photo-inversion of a cholesteric organization in the molecular material.

Circularly Polarized Light Probes Excited-State Delocalization in Rectangular Ladder-type Pentaphenyl Helices.

Ladder-type pentaphenyl chromophores have a rigid, planar π-system and show bright fluorescence featuring pronounced vibrational structure. Such moieties are ideal for studying interchromophoric interactions and delocalization of electronic excitations. We report the synthesis of helical polymers with a rigid square structure based on spiro-linked ladder-type pentaphenyl units. The variation of circular dichroism with increasing chain length provides direct evidence for delocalization of electronic excitations over at least 10 monomeric units. The change in the degree of circular polarization of the fluorescence across the vibronic side bands shows that vibrational motion can localize the excitation dynamically to almost one single unit through breakdown of the Born-Oppenheimer approximation. The dynamic conversion between delocalized and localized excited states provides a new paradigm for interpreting circular dichroism in helical polymers such as proteins and polynucleic acids.

Nonequilibrium site distribution governs charge-transfer electroluminescence at disordered organic heterointerfaces.

The interface between electron-donating (D) and electron-accepting (A) materials in organic photovoltaic (OPV) devices is commonly probed by charge-transfer (CT) electroluminescence (EL) measurements to estimate the CT energy, which critically relates to device open-circuit voltage. It is generally assumed that during CT-EL injected charges recombine at close-to-equilibrium energies in their respective density of states (DOS). Here, we explicitly quantify that CT-EL instead originates from higher-energy DOS site distributions significantly above DOS equilibrium energies. To demonstrate this, we have developed a quantitative and experimentally calibrated model for CT-EL at organic D/A heterointerfaces, which simultaneously accounts for the charge transport physics in an energetically disordered DOS and the Franck-Condon broadening. The 0-0 CT-EL transition lineshape is numerically calculated using measured energetic disorder values as input to 3-dimensional kinetic Monte Carlo simulations. We account for vibrational CT-EL overtones by selectively measuring the dominant vibrational phonon-mode energy governing CT luminescence at the D/A interface using fluorescence line-narrowing spectroscopy. Our model numerically reproduces the measured CT-EL spectra and their bias dependence and reveals the higher-lying manifold of DOS sites responsible for CT-EL. Lowest-energy CT states are situated ∼180 to 570 meV below the 0-0 CT-EL transition, enabling photogenerated carrier thermalization to these low-lying DOS sites when the OPV device is operated as a solar cell rather than as a light-emitting diode. Nonequilibrium site distribution rationalizes the experimentally observed weak current-density dependence of CT-EL and poses fundamental questions on reciprocity relations relating light emission to photovoltaic action and regarding minimal attainable photovoltaic energy conversion losses in OPV devices.

Programming and Dynamic Control of the Circular Polarization of Luminescence from an Achiral Fluorescent Dye in a Liquid Crystal Host by Molecular Motors.

Circular polarized light is utilized in communication and display technologies and a major challenge is to develop systems that can be switched between left and right circular polarized luminescence with high degrees of polarization and enable multiple addressable stable states. Luminescent dyes in Liquid Crystal (LC) cholesteric phases are attractive systems to generate, amplify and modulate circularly polarized luminescence (CPL). In the present study, we employ light-driven molecular motors as photo-controlled chiral dopants in LCs to switch the handedness of the LC and the circular polarization of luminescence from an achiral dye embedded in the mesogenic material. Tuning of the color of the CPL and the retention time of the photoprogrammed helicity is demonstrated making use of a variety of motors and dyes. The flexibility offered by the design based on inherently chiral unidirectional rotary motors provides full control over CPL non-invasively by light, opening possibilities for pixilated displays with externally addressable polarization.

Photo-Imprinting of the Helical Organization in Liquid-Crystal Networks Using Achiral Monomers and Circularly Polarized Light.

Control over molecular motion is facilitated in materials with highly ordered nanoscale structures. Here we report on the fabrication of cholesteric liquid-crystal networks by circularly polarized light irradiation, without the need for chiral dopant or plasticizer. The polymer network is obtained by photopolymerization of a smectic achiral diacrylate mesogen consisting of an azobenzene core and discrete oligodimethylsiloxane tails. The synchronous helical photoalignment and photopolymerization originate from the cooperative movement of the mesogens ordered in well-defined responsive structures, together with the flexibility of the oligodimethylsiloxane blocks. The resulting thin films show excellent thermal stability and light-induced memory features with reversible responses. Additionally, we demonstrate the fabrication of photo-patterned films of liquid-crystal networks with opposite helical senses. These findings provide a new method to make light-controllable chiroptical materials with exciting applications in optics and photonics.

Robust Angular Anisotropy of Circularly Polarized Luminescence from a Single Twisted-Bipolar Polymeric Microsphere.

It has long been surmised that the circular polarization of luminescence (CPL) emitted by a chiral molecule or a molecular assembly should vary with the direction in which the photon is emitted. Despite its potential utility, this anisotropic CPL has not yet been demonstrated at the level of single molecules or supramolecular assemblies. Here we show that conjugated polymers bearing chiral side chains self-assemble into solid microspheres with a twisted bipolar interior, which are formed via liquid-liquid phase separation and subsequent condensation into a cholesteric lyotropic liquid crystalline mesophase. The resultant microspheres, when dispersed in methanol, exhibit CPL with a glum value as high as 0.23. The microspheres are mechanically robust enough to be handled with a microneedle under ambient conditions, allowing comprehensive examination of the angular anisotropy of CPL. The single microsphere is found to exhibit distinct angularly anisotropic birefringence and CPL with glum up to ∼0.5 in the equatorial plane, which is 2.5-fold greater than that along the polar axis. Such optically anisotropic solid materials are important for the application to next-generation microlight-emitting and visualizing devices as well as for fundamental optics studies of chiral light-matter interaction.

Helicity Control in the Aggregation of Achiral Squaraine Dyes in Solution and Thin Films.

Squaraine dyes are well known for their strong absorption in the visible regime. Reports on chiral squaraine dyes are, however, scarce. To address this gap, we here report two novel chiral squaraine dyes and their achiral counterparts. The presented dyes are aggregated in solution and in thin films. A detailed chiroptical study shows that thin films formed by co-assembling the chiral dye with its achiral counterpart exhibit exceptional photophysical properties. The circular dichroism (CD) of the co-assembled structures reaches a maximum when just 25 % of the chiral dye are present in the mixture. The solid structures with the highest relative CD effect are achieved when the chiral dye is used solely as a director, rather than the structural component. The chiroptical data are further supported by selected spin-filtering measurements using mc-AFM. These findings provide a promising platform for investigating the relationship between the dissymmetry of a supramolecular structure and emerging material properties rather than a comparison between a chiral molecular structure and an achiral counterpart.

Long-Lived Charge-Transfer State from B-N Frustrated Lewis Pairs Enchained in Supramolecular Copolymers.

The field of supramolecular polymers is rapidly expanding; however, the exploitation of these systems as functional materials is still elusive. To become competitive, supramolecular polymers must display microstructural order and the emergence of new properties upon copolymerization. To tackle this, a greater understanding of the relationship between monomers' design and polymer microstructure is required as well as a set of functional monomers that efficiently interact with one another to synergistically generate new properties upon copolymerization. Here, we present the first implementation of frustrated Lewis pairs into supramolecular copolymers. Two supramolecular copolymers based on π-conjugated O-bridged triphenylborane and two different triphenylamines display the formation of B-N pairs within the supramolecular chain. The remarkably long lifetime and the circularly polarized nature of the resulting photoluminescence emission highlight the possibility to obtain an intermolecular B-N charge transfer. These results are proposed to be the consequences of the enchainment of B-N frustrated Lewis pairs within 1D supramolecular aggregates. Although it is challenging to obtain a precise molecular picture of the copolymer microstructure, the formation of random blocklike copolymers could be deduced from a combination of optical spectroscopic techniques and theoretical simulation.

An efficient zero-order description of the fine structure in the infrared reflection band of cubic ionic crystals and the phonon-polariton dispersion using Lorentz gauge.

The reflection of infrared light by ionic crystals with cubic symmetry such as lithium fluoride, LiF, is analyzed in terms of phonon-polaritons. In contrast to the conventional view on phonon-polaritons that uses the Coulomb gauge and assumes a purely local dielectric response of the material, we here develop an alternative description making use of the Lorentz gauge. This involves retarded interactions between charges, implying a non-local response of the material to electromagnetic radiation. The resulting new phonon-polariton dispersion relation features polaritons with negative group velocity in the frequency range in between the transverse (ωT) and longitudinal frequency (ωL). By contrast, the conventional description predicts, in zero order, the absence of any propagating polaritons in the frequency interval between ωT and ωL. The new dispersion relation provides an efficient, zero-order description of the fine structure within the reststrahlen band of LiF. The local minimum near the middle of the reflectance band is due to excitation of a phonon-polariton whose energy and momentum matches that of the incoming photon. The Lorentz gauge description can also describe off-normal reflection and accounts for the experimentally observed widening of the reflection band with increasing angle of incidence.

Pathway Complexity in the Enantioselective Self-Assembly of Functional Carbonyl-Bridged Triarylamine Trisamides.

Functional supramolecular systems like carbonyl-bridged triarylamine (CBT) trisamides are known for their long-range energy transport at room temperature. Understanding the complex self-assembly processes of this system allows for control over generated structures using controlled supramolecular polymerization. Here, we present two novel CBT trisamides with (S)- or (R)-chiral side chains which show a two-pathway self-assembly behavior in solution. Depending on the thermal profile during the self-assembly process, two different stable states are obtained under otherwise identical conditions. A kinetically trapped state A is reached upon cooling to 7 °C, via a proposed isodesmic process. In addition, there is a thermodynamically stable state B at 7 °C that is induced by first undercooling to -5 °C, via a nucleation-elongation mechanism. In both cases, helical supramolecular aggregates comprising H-aggregated CBTs are formed. Additionally, controlled supramolecular polymerization was achieved by mixing the two different states (A and B) from the same enantiomer, leading to a conversion of the kinetically trapped state to the thermodynamically stable state. This process is highly enantioselective, as no conversion is observed if the two states consist of opposite enantiomers. We thus show the importance and opportunities emerging from understanding the pathway complexity of functional supramolecular systems.

Reflection of light by anisotropic molecular crystals including exciton-polaritons and spatial dispersion.

A theory for the reflection of light by molecular crystals is described, which reproduces the minimum within the reflection band that is observed experimentally. The minimum in reflection is related to the excitation of polaritons in the crystal. The theory involves reformulation of the boundary conditions for electromagnetic waves at the interface between vacuum and material. The material is modeled by a cubic lattice of oriented Lorentz oscillators. By requiring uniformity of gauge of the electromagnetic potential across the interface between vacuum and the dipole lattice, the need for additional boundary conditions is obviated. The frequency separation between the maxima in reflectance on both sides of the minimum allows for the extraction of a plasma frequency. The plasma frequencies extracted from reflection spectra are compared to the plasma frequencies calculated directly from structural data on the crystals and the oscillator strengths of the constituent molecules. A good agreement between extracted and calculated plasma frequency is obtained for a set of 11 dye molecules.

Reflection and extinction of light by self-assembled monolayers of a quinque-thiophene derivative: A coherent scattering approach.

Scattering matrix theory is used to describe resonant optical properties of molecular monolayers. Three types of coupling are included: exciton-exciton, exciton-photon, and exciton-phonon coupling. We use the K-matrix formalism, developed originally to describe neutron scattering spectra in nuclear physics to compute the scattering of polaritons by phonons. This perturbation approach takes into account the three couplings and allows one to go beyond molecular exciton theory without the need of introducing additional boundary conditions for the polariton. We demonstrate that reflection, absorption, and extinction of light by 2D self-assembled monolayers of molecules containing quinque-thiophene chromophoric groups can be calculated. The extracted coherence length of the Frenkel exciton is discussed.

Stimuli-Responsive Nanostructured Viologen-Siloxane Materials for Controllable Conductivity.

Spontaneous phase separation is a promising strategy for the development of novel electronic materials, as the resulting well-defined morphologies generally exhibit enhanced conductivity. Making these structures adaptive to external stimuli is challenging, yet crucial as multistate reconfigurable switching is essential for neuromorphic materials. Here, a modular and scalable approach is presented to obtain switchable phase-separated viologen-siloxane nanostructures with sub-5 nm features. The domain spacing, morphology, and conductivity of these materials can be tuned by ion exchange, repeated pulsed photoirradiation and electric stimulation. Counterion exchange triggers a postsynthetic modification in domain spacing of up to 10%. Additionally, in some cases, 2D to 1D order-order transitions are observed with the latter exhibiting a sevenfold decrease in conductivity with respect to their 2D lamellar counterparts. Moreover, the combination of the viologen core with tetraphenylborate counterions enables reversible and in situ reduction upon light irradiation. This light-driven reduction provides access to a continuum of conducting states, reminiscent of long-term potentiation. The repeated voltage sweeps improve the nanostructures alignment, leading to increased conductivity in a learning effect. Overall, these results highlight the adaptivity of phase-separated nanostructures for the next generation of organic electronics, with exciting applications in smart sensors and neuromorphic devices.

Optical imaging as an expansion of nuclear medicine: Cerenkov-based luminescence vs fluorescence-based luminescence.

Integration of optical imaging technologies can further strengthen the field of radioguided surgery. Rather than using two separate chemical entities to achieve this extension, hybrid imaging agents can be used that contain both radionuclear and optical properties. Two types of such hybrid imaging agents are available: (1) hybrid imaging agents generated by Cerenkov luminescence imaging (CLI) of ß-emitters and (2) hybrid imaging agents that contain both a radioactive moiety and a fluorescent dye. One major challenge clinicians are now facing is to determine the potential value of these approaches. With this tutorial review we intend to clarify the differences between the two approaches and highlight the clinical potential of hybrid imaging during image-guided surgery applications.

Intramolecular excimer formation between 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione chromophoric groups linked by a flexible alkyl spacer.

Bichromophoric molecules containing two 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP) moieties linked via aliphatic spacers of different length are synthesized. Optical absorption spectroscopy indicates that the molecules adopt an extended conformation in solution. Fluorescence spectroscopy shows that photons are emitted from the locally excited singlet state in an extended conformation. In sufficiently polar solvents, quenching of fluorescence occurs and fluorescence quantum yield (ΦF) and fluorescence lifetime (τF) measurements indicate formation of an intramolecular excimer as the quenching mechanism. The redox potentials of the molecules and the solvent polarity dependence of the quenching are consistent with significant charge-transfer character of the excimer state. Photoinduced absorption measurements show enhanced intersystem crossing to the triplet state in polar solvents. Results indicate that in donor-acceptor π-conjugated materials involving the DPP moiety, excimer-like interchain polaron pair excited states could play an important role in the photophysics because of their close proximity in energy to the lowest singlet excited state.

Synthesis and optical properties of pyrrolo[3,2-b]pyrrole-2,5(1H,4H)-dione (iDPP)-based molecules.

We describe the synthesis and photophysical properties of a series of derivatives of pyrrolo[3,2-b]pyrrole-2,5(1H,4H)-dione-3,6-diyl (iDPP) linked to two oligothiophenes of variable length (nT). The iso-DPP-oligothiophenes (iDPPnTs) differ from the common pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-3,6-diyl-oligothiophene analogues (DPPnTs) by a different orientation of the two lactam rings in the bicyclic iDPP unit compared to DPP. In contrast to the highly fluorescent DPPnTs, the new isomeric iDPPnTs exhibit only very weak fluorescence. We demonstrate with the help of quantum-chemical calculations that this can be attributed to a different symmetry of the lowest excited state in iDPPnT (A in C2 symmetry) compared to DPPnTs (B) and the corresponding loss in oscillator strength of the lowest energy transition. Upon extending the oligothiophene moiety in the iDPPnTs molecules, the charge transfer character of the lowest A excited state becomes more pronounced. This tends to preclude high fluorescence quantum yields even in extended iDPPnTs systems.

Semi-Transparent, Chiral Organic Photodiodes with Incident Direction-Dependent Selectivity for Circularly Polarized Light.

Detection of the circular polarization of light is possible using chiral semiconductors, yet the mechanisms remain poorly understood. Semi-transparent chiral photodiodes allow for a simple experiment to investigate the basis of their selectivity: changing the side from which the diode is illuminated. A reversal of circular selectivity is observed in photocurrent generation when changing the direction of illumination on organic, bulk-heterojunction cells. The change in selectivity can be explained by a space-charge limitation on the collection of photocarriers in combination with preferential absorption of one of the circular polarizations of near-infrared light by the chiral non-fullerene acceptor. The space-charge limitation is supported by detailed measurements of frequency and intensity dependence of dc and ac photocurrents.