Meta-Substituted Asymmetric Azobenzenes: Insights into Structure-Property Relationship.

This article presents a comprehensive investigation into the functionalization of methoxyphenylazobenzene using electron-directing groups located at the meta position relative to the azo group. Spectroscopic analysis of meta-functionalized azobenzenes reveals that the incorporation of electron-withdrawing units significantly influences the absorption spectra of both E and Z isomers, while electron-donating functionalities lead to more subtle changes. The thermal relaxation process from Z to E result in almost twice as prolonged for electron-withdrawing functionalized azobenzenes compared to their electron-rich counterparts. Computational analysis contributes a theoretical understanding of the electronic structure and properties of meta-substituted azobenzenes. This combined approach, integrating experimental and computational techniques, yields significant insights into the structure-property relationship of meta-substituted asymmetrical phenolazobenzenes.

Promising Molecular Architectures for Two-Photon Probes in the Diagnosis of α-Synuclein Aggregates.

The abnormal deposition of protein in the brain is the central factor in neurodegenerative disorders (NDs). These detrimental aggregates, stemming from the misfolding and subsequent irregular aggregation of α-synuclein protein, are primarily accountable for conditions such as Parkinson's disease, Alzheimer's disease, and dementia. Two-photon-excited (TPE) probes are a promising tool for the early-stage diagnosis of these pathologies as they provide accurate spatial resolution, minimal intrusion, and the ability for prolonged observation. To identify compounds with the potential to function as diagnostic probes using two-photon techniques, we explore three distinct categories of compounds: Hydroxyl azobenzene (AZO-OH); Dicyano-vinyl bithiophene (DCVBT); and Tetra-amino phthalocyanine (PcZnNH2). The molecules were structurally and optically characterized using a multi-technique approach via UV-vis absorption, Raman spectroscopy, three-dimensional fluorescence mapping (PLE), time-resolved photoluminescence (TRPL), and pump and probe measurements. Furthermore, quantum chemical and molecular docking calculations were performed to provide insights into the photophysical properties of the compounds as well as to assess their affinity with the α-synuclein protein. This innovative approach seeks to enhance the accuracy of in vivo probing, contributing to early Parkinson's disease (PD) detection and ultimately allowing for targeted intervention strategies.

Light-Responsive Oligothiophenes Incorporating Photochromic Torsional Switches.

We present a quaterthiophene and sexithiophene that can reversibly change their effective π-conjugation length through photoexcitation. The reported compounds make use of light-responsive molecular actuators consisting of an azobenzene attached to a bithiophene unit by both direct and linker-assisted bonding. Upon exposure to 350 nm light, the azobenzene undergoes trans-to-cis isomerization, thus mechanically inducing the oligothiophene to assume a planar conformation (extended π-conjugation). Exposure to 254 nm wavelength promotes azobenzene cis-to-trans isomerization, forcing the thiophenic backbones to twist out of planarity (confined π-conjugation). Twisted conformations are also reached by cis-to-trans thermal relaxation at a rate that increases proportionally with the conjugation length of the oligothiophene moiety. The molecular conformations of quaterthiophene and sexithiophene were characterized by using steady-state UV-vis spectroscopy, X-ray crystallography and quantum-chemical modeling. Finally, we tested the proposed light-responsive oligothiophenes in field-effect transistors to probe the photo-induced tuning of their electronic properties.

Elucidating Charge Generation in Green-Solvent Processed Organic Solar Cells.

Organic solar cells have the potential to become the cheapest form of electricity. Rapid increase in the power conversion efficiency of organic solar cells (OSCs) has been achieved with the development of non-fullerene small-molecule acceptors. Next generation photovoltaics based upon environmentally benign "green solvent" processing of organic semiconductors promise a step-change in the adaptability and versatility of solar technologies and promote sustainable development. However, high-performing OSCs are still processed by halogenated (non-environmentally friendly) solvents, so hindering their large-scale manufacture. In this perspective, we discuss the recent progress in developing highly efficient OSCs processed from eco-compatible solvents, and highlight research challenges that should be addressed for the future development of high power conversion efficiencies devices.

Toward oriented surface architectures with three coaxial charge-transporting pathways.

We report a synthetic method to build oriented architectures with three coaxial π-stacks directly on solid surfaces. The approach operates with orthogonal dynamic bonds, disulfides and hydrazones, self-organizing surface-initiated polymerization (SOSIP), and templated stack-exchange (TSE). Compatibility with naphthalenediimides, perylenediimides, squaraines, fullerenes, oligothiophenes, and triphenylamine is confirmed. Compared to photosystems composed of two coaxial channels, the installation of a third channel increases photocurrent generation up to 10 times. Limitations concern giant stack exchangers that fail to enter SOSIP architectures (e.g., phthalocyanines surrounded by three fullerenes), and planar triads that can give folded or interdigitated charge-transfer architectures rather than three coaxial channels. The reported triple-channel surface architectures are as sophisticated as it gets today, the directionality of their construction promises general access to multichannel architectures with multicomponent gradients in each individual channel. The reported approach will allow us to systematically unravel the ultrafast photophysics of molecular dyads and triads in surface architectures, and might become useful to develop conceptually innovative optoelectronic devices.

Cyclodextrin insulation prevents static quenching of conjugated polymer fluorescence at the single molecule level.

Conjugated polymers (CPs) are promising materials for fluorescence imaging application. However, a significant problem in this field is the unexplained abnormally low fluorescence brightness (or number of fluorescence photons detected per one excitation photon) exhibited by most of CP single chains in solid polymer hosts. Here it is shown that this detrimental effect can be fully avoided for short chains of polyfluorene-bis-vinylphenylene (PFBV) embedded in a host polymer matrix of PMMA, if the conjugated backbone is insulated by cyclodextrin rings to form a polyrotaxane (PFBV-Rtx). Fluorescence kinetics and quantum yields are measured for the polymers in liquid solutions, pristine films, and solid PMMA blends. The fluorescence brightness of PFBV-Rtx single chains dispersed in a solid PMMA is very close to that expected for a chain with 100% fluorescence quantum yield, while the unprotected PFBV chains of the same length possess 4 times lower brightness. Despite this, the fluorescence decay kinetics are the same for both polymers, suggesting the presence of static or ultrafast fluorescence quenching in the unprotected polymer. About 80% of an unprotected PFBV chain is estimated to be completely quenched. The hypothesis is that the cyclodextrin rings prevent the quenching by working as 'bumpers' reducing the mechanical forces applied by the host polymer to the conjugated backbone and help retaining its conformational freedom. While providing a recipe for making CP fluorescence bright at the single-molecule level, these results identify a lack of fundamental understanding in the community of the influence of the environment on excited states in conjugated materials.

Self-organizing surface-initiated polymerization, templated self-sorting and templated stack exchange: synthetic methods to build complex systems.

In nature, spectacular function is achieved by highly sophisticated supramolecular architectures. Little is known what we would obtain if we could create complexity with similar precision, because the synthetic methods to do so are not available. This account summarizes recent approaches conceived to improve on this situation. With self-organizing surface-initiated polymerization (SOSIP), charge-transporting stacks can be grown directly on solid substrates with molecular-level precision. The extension to templated self-sorting (SOSIP-TSS) offers a supramolecular approach to multicomponent architectures. A solid theoretical framework for the transcription of information by templated self-sorting has been introduced, intrinsic templation efficiencies up to 97% have been achieved, and the existence of self-repair has been shown. The extension to templated stack exchange (SOSIP-TSE) offers the complementary covalent approach. Compatibility of this robust method with the creation of double-channel architectures with antiparallel two-component gradients has been demonstrated.

Analyzing Fluxional Molecules Using DORI.

The Density Overlap Region Indicator (DORI) is a density-based scalar field that reveals covalent bonding patterns and noncovalent interactions in the same value range. This work goes beyond the traditional static quantum chemistry use of scalar fields and illustrates the suitability of DORI for analyzing geometrical and electronic signatures in highly fluxional molecular systems. Examples include a dithiocyclophane, which possesses multiple local minima with differing extents of π-stacking interactions and a temperature dependent rotation of a molecular rotor, where the descriptor is employed to capture fingerprints of CH-π and π-π interactions. Finally, DORI serves to examine the fluctuating π-conjugation pathway of a photochromic torsional switch (PTS). Attention is also placed on postprocessing the large amount of generated data and juxtaposing DORI with a data-driven low-dimensional representation of the structural landscape.

Self-Assembled Monolayers as Patterning Tool for Organic Electronic Devices.

The patterning of functional materials represents a crucial step for the implementation of organic semiconducting materials into functional devices. Classical patterning techniques such as photolithography or shadow masking exhibit certain limitations in terms of choice of materials, processing techniques and feasibility for large area fabrication. The use of self-assembled monolayers (SAMs) as a patterning tool offers a wide variety of opportunities, from the region-selective deposition of active components to guiding the crystallization direction. Here, we discuss general techniques and mechanisms for SAM-based patterning and show that all necessary components for organic electronic devices, i.e., conducting materials, dielectrics, organic semiconductors, and further functional layers can be patterned with the use of self-assembled monolayers. The advantages and limitations, and potential further applications of patterning approaches based on self-assembled monolayers are critically discussed.

Photochromic Torsional Switch (PTS): a light-driven actuator for the dynamic tuning of π-conjugation extension.

Here we present a molecular architecture that can reversibly change the geometric conformation of its π-system backbone via irradiation with two different wavelengths. The proposed 'molecular actuator' consists of a photoswitchable azobenzene orthogonally connected to a π-conjugated bithiophene by both direct and aliphatic linker-assisted bonding. Upon exposure to 350 nm light, the trans azobenzene moiety isomerizes to its cis form, causing the bithiophene to assume a semiplanar anti conformation (extended π-conjugation). Exposure to 254 nm light promotes the isomerization of the azobenzene unit back to its initial extended trans conformation, thus forcing the bithiophene fragment to twist out of coplanarity (restricted π-conjugation). The molecular conformation of the bithiophene was characterized using steady-state UV-vis and nuclear magnetic resonance spectroscopy, as well as ab initio computations. The proposed molecular design could be envisaged as a π-conjugation modulator, which has potential to be incorporated into extended linear π-systems, i.e. via the terminal α-thiophene positions, and used to tune their optical and electronic properties.

Fluorescence polarization measures energy funneling in single light-harvesting antennas--LH2 vs conjugated polymers.

Numerous approaches have been proposed to mimic natural photosynthesis using artificial antenna systems, such as conjugated polymers (CPs), dendrimers, and J-aggregates. As a result, there is a need to characterize and compare the excitation energy transfer (EET) properties of various natural and artificial antennas. Here we experimentally show that EET in single antennas can be characterized by 2D polarization imaging using the single funnel approximation. This methodology addresses the ability of an individual antenna to transfer its absorbed energy towards a single pool of emissive states, using a single parameter called energy funneling efficiency (ε). We studied individual peripheral antennas of purple bacteria (LH2) and single CP chains of 20 nm length. As expected from a perfect antenna, LH2s showed funneling efficiencies close to unity. In contrast, CPs showed lower average funneling efficiencies, greatly varying from molecule to molecule. Cyclodextrin insulation of the conjugated backbone improves EET, increasing the fraction of CPs possessing ε = 1. Comparison between LH2s and CPs shows the importance of the protection systems and the protein scaffold of LH2, which keep the chromophores in functional form and at such geometrical arrangement that ensures excellent EET.

Excitation polarization provides structural resolution of individual non-blinking nano-objects.

We propose to combine the method of fluorescence intensity centroid localization with rotation of the plane of excitation polarization. Polarized light interacts selectively with differently oriented fluorophores; thus yielding topological information on the nanometer scale, without any need for fluorophore blinking. The method is applicable to photostable individual systems, when most of the traditional super-resolution methods fail. A theoretical study is supported by experiments on 30 nm long cyclodextrin-encapsulated single polyrotaxane conjugated polymer chains.

Inhomogeneous Quenching as a Limit of the Correlation Between Fluorescence Polarization and Conformation of Single Molecules.

The photophysical properties of conjugated polymers (CPs) largely depend on the interactions between the CP and its environment. We present a study of two polymers with identical conjugated backbones, bare and insulated, that showed different fluorescence excitation modulation depth histograms. However, the polarization differences are not related to differences in conformation, as commonly believed, but to the existence of "dark" chromophores in the bare polymer that are statically quenched. This results in inhomogeneous quenching of the polymer chain that breaks the correlation between excitation fluorescence polarization and conjugated polymer chain conformation.

Amplified spontaneous emission in conjugated polyrotaxanes under quasi-CW pumping.

Amplified spontaneous emission under millisecond pulsed excitation on a rotaxane-insulated conjugated polymer is reported. Pump-probe spectroscopy confirms the absence of polaron-pair absorption in the gain spectral region as the main factor contributing to quasi-steady-state amplified spontaneous emission. The low polaron-pair yield in this compound is the likely result of effective backbone encapsulation provided by the threading ring.

Vacuum-deposited planar heterojunction polymer solar cells.

The vacuum thermal evaporation of poly(3-hexylthiophene) (P3HT) for application in photovoltaic cells is demonstrated. Structural changes before and after evaporation are determined using GPC, UV-vis absorption spectroscopy, NMR, and FTIR. GPC showed that the polymer molecular weight is reduced during evaporation, leading to a blue-shift of the absorption spectra. FTIR and NMR were used to examine the change in chemical structure: it was found that conjugation remains mostly intact; however, the conjugation length decreases and side chains dissociate from the backbone. Bilayer heterojunction solar cells were fabricated by sequential deposition of P3HT and C60 and the photovoltaic response measured.