Switching between H- and J-type electronic coupling in single conjugated polymer aggregates

Hierarchical Self-Assembly of J-Aggregated 1,2-Bis(2-(benzyloxy)benzylidene) Hydrazine@2β-Cyclodextrin into Left-Handed Superhelix and Its External Stimuli-Responsive Unwinding

Induction of chirality to nanosized superstructures from hierarchical self-assembly of achiral monomeric units is an important area to understand the natural chiral amplification and evolution of life processes. We report herein that the complexation of salicylaldehyde azine, 1,2-bis(2-(benzyloxy)benzylidene)hydrazine (BSAZ), with β-cyclodextrin (β-CD) in aqueous solution results in the formation of a slipped J-aggregate (θ < 54.7°) that aggregates further into a left-handed superhelix through sterical constraints triggered by the hydrophobic effect. The structure of the monomeric BSAZ@2β-CD was elucidated by ultraviolet-visible (UV-vis), Fourier transform infrared spectroscopy (FT-IR), mass, powder X-ray diffraction (PXRD), and 1H, 13C, and 13C CP/MAS nuclear magnetic resonance (NMR) spectroscopy. The size, shape, and morphology of the self-aggregated hierarchy were evidenced by dynamic light scattering (DLS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) studies. The system showed excellent aggregation induced circular dichroism (AICD) with a negative Cotton effect and a high fluorescence quantum yield of 0.33 at 620 nm in a poor solvent, water, because of the formation of a higher order excimer (N ≈ 46). The helical superstructure showed responsiveness under 254 nm UV light irradiation. Light irradiation slowly unwinds the supercoiled structure into a single strand, as was visualized by a SEM image taken after 15 min of continuous light irradiation. The excellent solvatochromic effect and the control over the formed hierarchical morphology show how a supramolecular approach tailored by noncovalent interactions can develop chiral superstructures from completely achiral molecular building blocks that would have a considerable practical value in chiroptics, templates, and chiral sensing.

Collective Interactions of Quantum-Confined Excitons in Halide Perovskite Nanocrystal Superlattices

Collective optical properties can emerge from an ordered ensemble of emitters due to interactions between the individual units. Superlattices of halide perovskite nanocrystals exhibit collective light emission, influenced by dipole-dipole interactions between simultaneously excited nanocrystals. This coupling changes both the emission energy and rate compared to the emission of uncoupled nanocrystals. We demonstrate how quantum confinement governs the nature of the coupling between the nanocrystals in the ensemble. The extent of confinement is modified by controlling the nanocrystal size or by compositional control over the Bohr radius. In superlattices made of weakly confined nanocrystals, the collective emission is red-shifted with a faster emission rate, showing the key characteristics of superfluorescence. In contrast, the collective emission of stronger quantum-confined nanocrystals is blue-shifted with a slower emission rate. Both types of collective emission exhibit correlative multiphoton emission bursts, showing distinct photon bunching emission statistics. The quantum confinement changes the preferred alignment of transition dipoles within the nanocrystal and switches the relative dipole orientation between neighbors, resulting in opposite collective optical behaviors. Our results extend these collective effects to relatively high temperatures and provide a better understanding of exciton interactions and collective emission phenomena at the solid state.

Unveiling emissive H-aggregates of benzocoronenediimide, their photophysics and ultrafast exciton dynamics

H- and J-aggregates of many molecules can be considered ordered mesoscopic structures that behave like a single entity. This is due to coherent electronic coupling between electronic excitations of aggregated molecules, resulting in distinct electronic properties compared to the monomer. H-aggregates are generally non-emissive and, due to this property, they are considered unfit for optoelectronics applications, but they have found applications in organic light-emitting transistors. Herein, we designed t-butyl-substituted benzocoronenediimide (t-But-BCDI) forming rare emissive H-aggregates. The tertiary butyl groups are placed to inhibit the formation of strong aggregates. Photophysical studies showed that t-But-BCDI forms H-aggregates in a concentrated solution in a THF/CHCl3 mixture. A blue shift in absorption along with a decrease in the A0-0/A0-1 ratio and red-shifted weaker emission are observed for the aggregate compared to the monomer. Ultrafast transient absorption studies revealed biphasic relaxation with lifetimes of 150 (±10) fs and 13 (±2) ps, which are attributed to a higher-to-lower state transition and vibrational cooling, respectively. The transient spectral signature suggests the Frenkel-type (localized to a monomer) character of the exciton. Faster evolution at the tens of picosecond timescale suggests relaxation of the exciton state within the H-type exciton band. An extraordinarily long emission lifetime from the H-aggregated state is observed.

Preference for excited state imine-amine isomerisation over enol-keto isomerisation: spectroscopic exploration and quantum chemical calculations

In this article, the photophysical properties of ethyl-3-(benzo[D]thiazol-2-yl)-5-chloro-2-hydroxybenzoate (EBTCH) and ethyl-3-(benzo[D]oxazol-2-yl)-5-chloro-2-hydroxybenzoate (EBOCH) have been explored spectroscopically along with quantum chemical calculations. From a structural viewpoint, both molecules have two proton acceptor sites (thiazole/oxazole N atom and ester O atom) and a common proton donor site (phenolic -OH) connected by a six-membered H-bonding ring capable of both imine-amine and enol-keto photoisomerisation. Steady state absorption and emission spectra and time-resolved fluorescence characteristics of EBTCH and EBOCH and a comparison with the spectral data of controlled compounds 2-(benzo[D]thiazol-2-yl)-4-chlorophenol (BTCP), 2-(benzo[D]oxazol-2-yl)-4-chlorophenol (BOCP) and ethyl 5-chloro-2-hydroxybenzoate (ECHB) support the preference for imine-amine isomerisation over enol-keto isomerization in the excited state. The computed structural parameters for the ground and excited states for both the molecules from the density functional theory (DFT) calculations are found to be in favour of the imine-amine isomerization process. Theoretical potential energy curves along the proton transfer coordinates of EBTCH and EBOCH support the existence of both isomers in the ground state (S0). However, the formation of a tautomeric form involving imine-amine isomerisation obtained through asymmetric double-well potential is favoured over the enol-keto isomerisation in the excited state (S1) due to lower barrier energy.

Organic BODIPY Based Gels: Optical, Electrochemical and Self-Assembly Properties

Two novel BODIPY dyes, BOC3 and BC12, were synthesized with variable alkyl chains at terminal amide functional units. BC12, featuring a longer alkyl chain (-C12H25), formed a gel compared to BOC3, which has a shorter alkyl chain (< C->CH2OCH3), due to supra molecular self-assembly in film. Both dyes exhibited absorption peaks around 530 nm in the visible region, with a red shift of about 30 nm in the film state, essential for organic electronic applications. Concentration variation studies revealed π-π stacking/aggregates in the solid state causing red shifts in absorption and emission. BC12 exhibited more significant red shifts in film compared to its solution state due to supra molecular self-assembly. Electronic structure analysis using density functional theories (BMK and O3LYP) showed better correlation with absorption using the O3LYP method. Both dyes displayed quasi-irreversible oxidation and reduction couples with suitable HOMO (5.46 eV) and LUMO (3.32 eV) energy levels for organic electronic applications. Transient photoluminescence studies indicated a longer lifetime for BC12 (5.28 ns) than BOC3 (4.50 ns), suggesting π-π aggregation and supra molecular self-assembly. BC12's gelation, attributed to its long alkyl chain and two-dimensional motifs of the BODIPY core, forms spherical-shaped nano networks. These findings underscore the potential of molecularly tuned dyes with alkyl chains for nano-sized self-assembly in organic electronic devices. Red shifts were observed due to combination of aggregation, stacking and columnar meso-phase formation in supramolecular assembly. Absorption spectra of dyes in toluene with various concentrations showed the formation of Aggregation/π-π stacking might be due to head to tailing interactions.

Solution and solid-state fluorescence emission from cyanostyrene molecules with multiple nitrogen atoms

A design strategy has been proposed to utilize structure-driven solution and solid-state fluorescence emission of polynitrogen atoms. The strategy uses benzimidazole as the electron donor and pyridine as the electron acceptor to construct D-A-type cyanopyridine ethylene molecules. Theoretical calculations reveal that compound 1 has energy-close isomers in dilute solutions, with planar conformation in S0 and S1 states, reducing molecular motion and thus enhancing radiation efficiency (quantum yield up to 42.7%). Conversely, the distorted cyanobenzene structure reduces the quenching effect of π-π stacking alignment, and hydrogen bonding between molecules limits molecular vibration and rotation, ultimately leading to strong emission in the solid state (quantum yield up to 27.4%). These dual-state luminescence systems have wide-ranging potential applications in information encryption and temperature sensors.

Achiral Plasmonic Antennas Enhance Differential Absorption To Increase Preferential Detection of Chiral Single Molecules

Plasmonic antennas increase the photon flux in their vicinity, which can lead to plasmon-enhanced fluorescence for molecules near these nanostructures. Here, we combine plasmon-coupled fluorescence and fluorescence-detected circular dichroism to build a specific and sensitive detection strategy for chiral single molecules. Electromagnetic simulations indicate that a two-dimensional gold nanoparticle dimer antenna enhances the electric field and optical chirality of a plane wave in its near field. Furthermore, this optical chirality enhancement can be tuned based on the polarization of the incident electric field, such that enhancing the optical chirality via these antennas will increase the differential absorption of parity-inverted fields. We measured the fluorescence from single molecules of chiral absorbers-Cy5 J-dimers assembled in double-stranded DNA backbones-and achieved increased detectability of these right-handed molecules near achiral gold nanoparticle dimer antennas under right circularly polarized illumination. This strategy offers a new approach to distinguishing weakly fluorescent enantiomers.

The Influence of Thickness and Spectral Properties of Green Color-Emitting Polymer Thin Films on Their Implementation in Wearable PLED Applications

A systematic investigation of optical, electrochemical, photophysical, and electrooptical properties of printable green color-emitting polymer (poly(9,9-dioctylfluorene-alt-bithiophene)) (F8T2) and spiro-copolymer (SPG-01T) was conducted to explore their potentiality as an emissive layer for wearable polymer light-emitting diode (PLED) applications. We compared the two photoactive polymers in terms of their spectral characteristics and color purity, as these are the most critical factors for wearable lighting sources and optical sensors. Low-cost, solution-based methods and facile architecture were applied to produce rigid and flexible light-emitting devices with high luminance efficiencies. Emission bandwidths, color coordinates, operational characteristics, and luminance were also derived to evaluate the device's stability. The tuning of emission's spectral features by layer thickness variation was realized and was correlated with the interplay between H-aggregates and J-aggregates formations for both conjugated polymers. Finally, we applied the functional green light-emitting PLED devices based on the two studied materials for the detection of Rhodamine 6G. It was determined that the optical detection of the R6G photoluminescence is heavily influenced by the emission spectrum characteristics of the PLED and changes in the thickness of the active layer.

J-Aggregate Promoting NIR-II Emission for Fluorescence/Photoacoustic Imaging-Guided Phototherapy

J-aggregate is a promising strategy to enhance second near-infrared window (NIR-II) emission, while the controlled synthesis of J-aggregated NIR-II dyes is a huge challenge because of the lack of molecular design principle. Herein, bulk spiro[fluorene-9,9'-xanthene] functionalized benzobisthiadiazole-based NIR-II dyes (named BSFX-BBT and OSFX-BBT) are synthesized with different alkyl chains. The weak repulsion interaction between the donor and acceptor units and the S…N secondary interactions make the dyes to adopt a co-planar molecular conformation and display a peak absorption >880 nm in solution. Importantly, BSFX-BBT can form a desiring J-aggregate in the condensed state, and femtosecond transient absorption spectra reveal that the excited states of J-aggregate are the radiative states, and J-aggregate can facilitate stimulated emission. Consequently, the J-aggregated nanoparticles (NPs) display a peak emission at 1124 nm with a high relative quantum yield of 0.81%. The efficient NIR-II emission, good photothermal effect, and biocompatibility make the J-aggregated NPs demonstrate efficient antitumor efficacy via fluorescence/photoacoustic imaging-guided phototherapy. The paradigm illustrates that tuning the aggregate states of NIR-II dye via spiro-functionalized strategy is an effective approach to enhance photo-theranostic performance.

Decoding Excimer Formation in Covalent-Organic Frameworks Induced by Morphology and Ring Torsion

A thorough and quantitative understanding of the fate of excitons in covalent-organic frameworks (COFs) after photoexcitation is essential for their augmented optoelectronic and photocatalytic applications via precise structure tuning. The synthesis of a library of COFs having identical chemical backbone with impeded conjugation, but varied morphology and surface topography to study the effect of these physical properties on the photophysics of the materials is herein reported. The variation of crystallite size and surface topography substantified different aggregation pattern in the COFs, which leads to disparities in their photoexcitation and relaxation properties. Depending on aggregation, an inverse correlation between bulk luminescence decay time and exciton binding energy of the materials is perceived. Further transient absorption spectroscopic analysis confirms the presence of highly localized, immobile, Frenkel excitons (of diameter 0.3-0.5 nm) via an absence of annihilation at high density, most likely induced by structural torsion of the COF skeletons, which in turn preferentially relaxes via long-lived (nanosecond to microsecond) excimer formation (in femtosecond scale) over direct emission. These insights underpin the importance of structural and topological design of COFs for their targeted use in photocatalysis.

Solvent Geometry Regulated J- and H-Type Aggregates of Photoswitchable Organogelator: Phase-Selective Thixotropic Gelation and Oil Spill Recovery

We demonstrate supramolecular polymerization and formation of 1D nanofiber of azobenzene based organogelator (AZO-4) in cyclic hydrocarbon solvents (toluene and methylcyclohexane). The AZO-4 exhibits J- and H-type aggregates in toluene: MCH (9 : 1) and MCH: toluene (9 : 1) respectively. The type of aggregate was governed by the geometry of the solvents used in the self-assembly process. The J-type aggregates with high thermal stability in toluene is due to the enhanced interaction of AZO-4 π- surface with the toluene π-surface, whereas H-aggregate with moderate thermal stability in MCH was due to the interruption of the cyclic hydrocarbon in van der Waals interactions of peripheral chains of AZO-4 molecule. The light induced reversible photoisomerization is observed for both J- and H-aggregates. The macroscopic property revealed spontaneous and strong gelation in toluene preferably due to the strong interactions of the AZO-4 nanofibers with the toluene solvent molecules compared to the MCH. The rheological measurements revealed thixotropic nature of the gels by step-strain experiments at room temperature. The thermodynamic parameter (ΔHm) of gel-to-sol transition was determined for all the gels to get more insight into the gelation property. Furthermore, the phase selective gelation property was extended to the oil spill recovery application using diesel/water and petrol/water mixture.

The Soft Nanodots as Fluorescent Probes for Cell Imaging: Analysis of Cell and Spheroid Penetration Behavior of Single Chain Polymer Dots

This study describes the formation, size control, and penetration behavior of polymer nanodots (Pdots) consisting of single or few chain polythiophene-based conjugated polyelectrolytes (CPEs) via nanophase separation between good solvent and poor solvent of CPE. Though the chain singularity may be associated with dilution nanophase separation suggests that molecules of a good solvent create a thermodynamically driven solvation layer surrounding the CPEs and thereby separating the single chains even in their poor solvents. This statement is therefore corroborated with emission intensity/lifetime, particle size, and scattering intensity of polyelectrolyte in good and poor solvents. Regarding the augmented features, Pdots are implemented into cell imaging studies to understand the nuclear penetration and to differentiate the invasive characteristics of breast cancer cells. The python based red, green, blue (RGB) color analysis   depicts that Pdots have more nuclear penetration ability in triple negative breast cancer cells due to the different nuclear morphology in shape and composition and Pdots have penetrated cell membrane as well as extracellular matrix in spheroid models. The current Pdot protocol and its utilization in cancer cell imaging are holding great promise for gene/drug delivery to target cancer cells by explicitly achieving the very first priority of nuclear intake.

Estimation of Environmental Effects and Response Time in Gas-Phase Explosives Detection Using Photoluminescence Quenching Method

Detecting the presence of explosives is important to protect human lives during military conflicts and peacetime. Gas-phase detection of explosives can make use of the change of material properties, which can be sensitive to environmental conditions such as temperature and humidity. This paper describes a remote-controlled automatic shutter method for the environmental impact assessment of photoluminescence (PL) sensors under near-open conditions. Utilizing the remote-sensing method, we obtained environmental effects without being exposed to sensing vapor molecules and explained how PL intensity was influenced by the temperature, humidity, and exposure time. We also developed a theoretical model including the effect of exciton diffusion for PL quenching, which worked well under limited molecular diffusions. Incomplete recovery of PL intensity or the degradation effect was considered as an additional factor in the model.

A Chirally Locked Bis-perylene Diimide Macrocycle: Consequences for Chiral Self-Assembly and Circularly Polarized Luminescence

Macrocycles containing chiral organic dyes are highly valuable for the development of supramolecular circularly polarized luminescent (CPL) materials, where a preorganized chiral framework is conducive to directing π-π self-assembly and delivering a strong and persistent CPL signal. Here, perylene diimides (PDIs) are an excellent choice for the organic dye component because, alongside their tunable photophysical and self-assembly properties, functionalization of the PDI's core yields a twisted, chiral π-system, capable of CPL. However, configurationally stable PDI-based macrocycles are rare, and those that are also capable of π-π self-assembly beyond dimers are unprecedented, both of which are advantageous for robust self-assembled chiroptical materials. In this work, we report the first bay-connected bis-PDI macrocycle that is configurationally stable (ΔG⧧ > 155 kJ mol-1). We use this chirally locked macrocycle to uncover new knowledge of chiral PDI self-assembly and to perform new quantitative CPL imaging of the resulting single-crystal materials. As such, we discover that the chirality of a 1,7-disubstituted PDI provides a rational route to designing H-, J- and concomitant H- and J-type self-assembled materials, important arrangements for optimizing (chir)optical and charge/energy transport properties. Indeed, we reveal that CPL is amplified in the single crystals of our chiral macrocycle by quantifying the degree of emitted light circular polarization from such materials for the first time using CPL-Laser Scanning Confocal Microscopy.

Path Integral over Equivalence Classes for Quantum Dynamics with Static Disorder

An efficient, fully quantum mechanical, real-time path integral method for including the effects of static disorder in the dynamics of systems coupled to common or local harmonic baths is presented. Rather than performing a large number of demanding calculations for different realizations of the system Hamiltonian, the influence of the bath is captured through a single evaluation of the path sum by grouping the system paths into equivalence classes of fixed system amplitudes. The method is illustrated with several analytical and numerical examples that show a variety of nontrivial effects arising from the interplay among coherence, dissipation, thermal fluctuations, and geometric phases.

Tuning the Aggregates of Thiophene-based Trimers by Methyl Side-chain Engineering for Photocatalytic Hydrogen Evolution

Organic π-conjugated semiconductors (OCSs) have recently emerged as a promising alternative to traditional inorganic materials for photocatalysis. However, the aggregation of OCSs in photocatalytic aqueous solution caused by self-assembly, which closely relates to the photocatalytic activity, has not yet been studied. Here, the relationship between the aggregation of 4,7-Bis(thiophen-2-yl) benzothiadiazole (TBT) and the photocatalytic activity was systematically investigated by introducing and varying the position of methyl side chains on the two peripheral thiophene units. Experimental and theoretical results indicated that the introduction of -CH3 group at the 3-position of TBT resulted in the smallest size and best crystallinity of aggregates compared to that of TBT, 4- and 5-positions. As a result, TBT-3 exhibited an excellent photocatalytic activity towards H2 evolution, ascribed to the shorten charge carrier transport distance and solid long-range order. These results suggest the important role of aggregation behavior of OCSs for efficient photocatalysis.

Julolidine based red emitting ESIPT/AIE active material showing luminescence beyond excimer emission: An "on-off" emission response to Cu2

A new julolidine-fluorene based excited state intramolecular proton transfer (ESIPT)/aggregate induced emission (AIE) active Schiff-base (JDF) has been synthesized and evaluated for its photophysical properties in solution and aggregated/solid states. The correlation between the emission behavior and the solid state crystal packing structure revealed the interplay of ESIPT coupled excimer reaction occurring in the solid state, which is one of the rare examples reported so far. For a comprehensive comparison, we synthesized a non-ESIPT methyl derivative (JDF-Me) of JDF capable of showing excimer emission only in the solid state. Further, JDF exhibits normal as well as keto emission in solution, upon addition of water, its poor solvent, that promotes aggregation, the fluorescence emission shows the preponderance of the excimer band in the low energy region. It was also interesting to note that in the solid state (thin films), JDF shows emission beyond the excimer emission, which is wavelength dependent. This is attributed to the formation of diverse clusters leading to the extended delocalization beyond excimers, and represents a clustering-triggered emission ascribing bright red color to the solid JDF. Such mélange of emission characteristics of JDF are responsible for the multicolor emission covering a broad range of electromagnetic spectrum, which is demonstrated by the confocal microscopy images of the JDF recorded in different states. Further, in its aggregated state, JDF recognized Cu²⁺ ions, selectively, manifested in the form of emission quenching via the interaction of Cu²⁺ ions with the oxygen and nitrogen atoms of JDF inhibiting the excimer formation.

Bioinspired porphyrin-peptide supramolecular assemblies and their applications

Inspired by the hierarchical chiral assembly of porphyrin-proteins in photosynthetic systems, the hierarchical self-assembly of porphyrin-amino acids/peptides provides a novel strategy for constructing functional materials. How to artificially simulate the assembly of porphyrins, proteins, and other cofactors in the photosynthesis system to obtain persistent strong light capture, charge separation and catalytic reactions has become an important concern in the construction of biomimetic photosynthesis systems. This paper summarizes the different assembly strategies adopted in recent years, the effects of driving forces on self-assembly, and the application of porphyrin-peptides in catalysis and biomedicine, and briefly discusses the challenges and prospects for future research.

Influence of Side Chain Interdigitation on Strain and Charge Mobility of Planar Indacenodithiophene Copolymers

Indacenodithiophene (IDT) copolymers are a class of conjugated polymers that have limited long-range order and high hole mobilities, which makes them promising candidates for use in deformable electronic devices. Key to their high hole mobilities is the coplanar monomer repeat units within the backbone. Poly(indacenodithiophene-benzothiadiazole) (PIDT_C16-BT) and poly(indacenodithiophene-thiapyrollodione) (PIDT_C16-TPD_C1) are two IDT copolymers with planar backbones, but they are brittle at low molecular weight and have unsuitably high elastic moduli. Substitution of the hexadecane (C₁₆) side chains of the IDT monomer with isocane (C₂₀) side chains was performed to generate a new BT-containing IDT copolymer: PIDT_C20-BT. Substitution of the methyl (C₁) side chain on the TPD monomer for an octyl (C₈) and 6-ethylundecane (C_13B) afford two new TPD-containing IDT copolymers named PIDT_C16-TPD_C8 and PIDT_C16-TPD_C13B, respectively. Both PIDT_C16-TPD_C8 and PIDT_C16-TPD_C13B are relatively well deformable, have a low yield strain, and display significantly reduced elastic moduli. These mechanical properties manifest themselves because the lengthened side chains extending from the TPD-monomer inhibit precise intermolecular ordering. In PIDT_C16-BT, PIDT_C20-BT and PIDT_C16-TPD_C1 side chain ordering can occur because the side chains are only present on the IDT subunit, but this results in brittle thin films. In contrast, PIDT_C16-TPD_C8 and PIDT_C16-TPD_C13B have disordered side chains, which seems to lead to low hole mobilities. These results suggest that disrupting the interdigitation in IDT copolymers through comonomer side chain extension leads to more ductile thin films with lower elastic moduli, but decreased hole mobility because of altered local order in the respective thin films. Our work, thus, highlights the trade-off between molecular packing structure for deformable electronic materials and provides guidance for designing new conjugated polymers for stretchable electronics.

Stimuli-Responsive Aggregation-Induced Emission (AIE)-Active Polymers for Biomedical Applications

At high concentration or in the aggregated state, most of the traditional luminophores suffer from the general aggregation-caused quenching (ACQ) effect, which significantly limits their biomedical applications. On the contrary, a few fluorophores exhibit an aggregation-induced emission (AIE) feature which is just the opposite of ACQ. The luminophores with aggregation-induced emission (AIEgens) have exhibited noteworthy advantages to get tunable emission, excellent photostability, and biocompatibility. Incorporating AIEgens into polymer design has yielded diversified polymer systems with fascinating photophysical characteristics. Again, stimuli-responsive polymers are capable of undergoing chemical and/or physical property changes on receiving signals from single or multiple stimuli. The combination of the AIE property and stimuli responses in a single polymer platform provides a feasible and effective strategy for the development of smart polymers with promising biomedical applications. Herein, the advancements in stimuli-responsive polymers with AIE characteristics for biomedical applications are summarized. AIE-active polymers are first categorized into conventional π-π conjugated and nonconventional fluorophore systems and then subdivided based on various stimuli, such as pH, redox, enzyme, reactive oxygen species (ROS), and temperature. In each section, the design strategies of the smart polymers and their biomedical applications, including bioimaging, cancer theranostics, gene delivery, and antimicrobial examples, are introduced. The current challenges and future perspectives of this field are also stated at the end of this review article.

The Pursuit of Shortwave Infrared-Emitting Nanoparticles with Bright Fluorescence through Molecular Design and Excited-State Engineering of Molecular Aggregates

Shortwave infrared (SWIR) fluorescence detection gradually becomes a pivotal real-time imaging modality, allowing one to elucidate biological complexity in deep tissues with subcellular resolution. The key challenge for the further growth of this imaging modality is the design of new brighter biocompatible fluorescent probes. This review summarizes the recent progress in the development of organic-based nanomaterials with an emphasis on new strategies that extend the fluorescence wavelength from the near-infrared to the SWIR spectral range and amplify the fluorescence brightness. We first introduce the most representative molecular design strategies to obtain near-infrared-SWIR wavelength fluorescence emission from small organic molecules. We then discuss how the formation of nanoparticles based on small organic molecules contributes to the improvement of fluorescence brightness and the shift of fluorescence to SWIR, with a special emphasis on the excited-state engineering of molecular probes in an aggregate state and spatial packing of the molecules in nanoparticles. We build our discussion based on a historical perspective on the photophysics of molecular aggregates. We extend this discussion to nanoparticles made of conjugated polymers and discuss how fluorescence characteristics could be improved by molecular design and chain conformation of the polymer molecules in nanoparticles. We conclude the article with future directions necessary to expand this imaging modality to wider bioimaging applications including single-particle deep tissue imaging. Issues related to the characterization of SWIR fluorophores, including fluorescence quantum yield unification, are also mentioned.

Not All Aggregates Are Made the Same: Distinct Structures of Solution Aggregates Drastically Modulate Assembly Pathways, Morphology, and Electronic Properties of Conjugated Polymers

Tuning structures of solution-state aggregation and aggregation-mediated assembly pathways of conjugated polymers is crucial for optimizing their solid-state morphology and charge-transport property. However, it remains challenging to unravel and control the exact structures of solution aggregates, let alone to modulate assembly pathways in a controlled fashion. Herein, aggregate structures of an isoindigo-bithiophene-based polymer (PII-2T) are modulated by tuning selectivity of the solvent toward the side chain versus the backbone, which leads to three distinct assembly pathways: direct crystallization from side-chain-associated amorphous aggregates, chiral liquid crystal (LC)-mediated assembly from semicrystalline aggregates with side-chain and backbone stacking, and random agglomeration from backbone-stacked semicrystalline aggregates. Importantly, it is demonstrated that the amorphous solution aggregates, compared with semicrystalline ones, lead to significantly improved alignment and reduced paracrystalline disorder in the solid state due to direct crystallization during the meniscus-guided coating process. Alignment quantified by the dichroic ratio is enhanced by up to 14-fold, and the charge-carrier mobility increases by a maximum of 20-fold in films printed from amorphous aggregates compared to those from semicrystalline aggregates. This work shows that by tuning the precise structure of solution aggregates, the assembly pathways and the resulting thin-film morphology and device properties can be drastically tuned.

Recent Structural Engineering of Polymer Semiconductors Incorporating Hydrogen Bonds

Highly planar, extended π-electron organic conjugated polymers have been increasingly attractive for achieving high-mobility organic semiconductors. In addition to the conventional strategy to construct rigid backbone by covalent bonds, hydrogen bond has been employed extensively to increase the planarity and rigidity of polymer via intramolecular noncovalent interactions. This review provides a general summary of high-mobility semiconducting polymers incorporating hydrogen bonds in field-effect transistors over recent years. The structural engineering of the hydrogen bond-containing building blocks and the discussion of theoretical simulation, microstructural characterization, and device performance are covered. Additionally, the effects of the introduction of hydrogen bond on self-healing, stretchability, chemical sensitivity, and mechanical properties are also discussed. The review aims to help and inspire design of new high-mobility conjugated polymers with superiority of mechanical flexibility by incorporation of hydrogen bond for the application in flexible electronics.

Sulfur-bridged chromophores for photofunctional materials: using sulfur oxidation state to tune electronic and structural properties

The use of a heteroatom, such as sulfur, as a linker or bridge, in π-conjugated materials has advantages over purely carbon-based ones due to the accessibility of higher oxidation states as a result of hypervalence. Materials containing a sulfide bridge (S) can be systemically oxidized into sulfoxides (SO) and sulfones (SO2), each of which can then influence how a material interacts with light, playing a large role in dictating the photophysical and sometimes photochemical properties. In this perspective, we summarize the progress that our group and others have made, showing how oxidation of a sulfur bridge in symmetric bichromophoric dimers and in diimine ligands can influence the excited state behavior in organic π-conjugated materials and metal complexes.

Vibrations Responsible for Luminescence from HJ-Aggregates of Conjugated Polymers Identified by Cryogenic Spectroscopy of Single Nanoparticles

A single polymer chain can be thought of as a covalently bound J-aggregate, where the microscopic transition-dipole moments line up to emit in phase. Packing polymer chains into a bulk film can result in the opposite effect, inducing H-type coupling between chains. Cofacial transition-dipole moments oscillate out of phase, canceling each other out, so that the lowest-energy excited state turns dark. H-aggregates of conjugated polymers can, in principle, be coaxed into emitting light by mixing purely electronic and vibronic transitions. However, it is challenging to characterize this electron-phonon coupling experimentally. In a bulk film, many different conformations exist with varying degrees of intrachain J-type and interchain H-type coupling strengths, giving rise to broad and featureless aggregate absorption and emission spectra. Even if single nanoparticles consisting of only a few single chains are grown in a controlled fashion, the luminescence spectra remain broad, owing to the underlying molecular dynamics and structural heterogeneity at room temperature. At cryogenic temperatures, emission from H-type aggregates should be suppressed because, in the absence of thermal energy, internal conversion drives the aggregate to the lowest-energy dark state. At the same time, electronic and vibronic transitions narrow substantially, facilitating the attribution of spectral signatures to distinct vibrational modes. We demonstrate how to distinguish signatures of interchain H-type aggregate species from those of intramolecular J-type coupling. Whereas all dominant vibronic modes revealed in the photoluminescence (PL) and surface-enhanced resonance Raman scattering spectra of a single chromophore within a single polymer chain are identified in the J-type aggregate luminescence spectra, they are not all present at once in the H-type spectra. Universal spectral features are found for the luminescence from strongly HJ-coupled chains, clearly resolving the vibrations responsible for the nonadiabatic excited-state molecular dynamics that enable light emission. We discuss the possible combinations of vibrational modes responsible for H-type aggregate PL and demonstrate that only one, mainly the lowest energy one, of the three dominant vibrational modes contributes to the 0-1 transition, whereas combinations of all three are found in the 0-2 transition. From this analysis, we can distinguish between energy shifts due to either J-type intrachain coupling or H-type interchain interactions, offering a means to directly discriminate between structural and energetic disorder.

A Perspective on Polythiophenes as Conformation Dependent Optical Reporters for Label-Free Bioanalytics

Poly(3-alkylthiophene) (PT)-based conjugated polyelectrolytes (CPEs) constitute an important class of responsive polymers with excellent optical properties. The electrostatic interactions between PTs and target analytes trigger complexation and concomitant conformational changes of the PT backbones that produce distinct optical responses. These conformation-induced optical responses of the PTs enable them to be utilized as reporters for detection of various analytes by employing simple UV-vis spectrophotometry or the naked eye. Numerous PTs with unique pendant groups have been synthesized to tailor their interactions with analytes such as nucleotides, ions, surfactants, proteins, and bacterial and viral pathogens. In this perspective, we discuss PT-target analyte complexation for bioanalytical applications and highlight recent advancements in point-of-care and field deployable assays. Subsequently, we highlight a few areas of critical importance for future applications of PTs as reporters, including (i) design and synthesis of specific PTs to advance the understanding of the mechanisms of interaction with target analytes, (ii) using arrays of PTs and linear discriminant analysis for selective and specific detection of target analytes, (iii) translation of conventional homogeneous solution-based assays into heterogeneous membrane-based assay formats, and finally (iv) the potential of using PT as an alternative to conjugated polymer nanoparticles and dots in bioimaging.

A Perylenediimide-Based Zinc-Coordination Polymer for Photosensitized Singlet-Oxygen Generation

In the face of anthropogenic global warming the design and synthesis of materials, which enable energy transfer processes using sunlight as an energy source, are of high interest. Perylenediimides are a highly absorbing class of chromophores suitable for sunlight absorption and conversion. Therefore, metal–organic frameworks (MOFs) and coordination polymers (CPs) with incorporated organic perylene chromophores are highly interesting materials both for applied, but also fundamental, photophysical research. MOFs/CPs have the advantage of a modular adjustability of interchromophoric distances and angles, and the choice of metal nodes can be used to further tune the material towards the desired photophysical properties. In the present paper, we present a study using a reported organic perylenediimide (PDI) chromophore (H2tpdb) as a linker to be incorporated into coordination polymer and test towards applicability within the photochemical 1O2 generation. In detail, a novel zinc 2D -coordination polymer Zn(tpdb)(DMF)3 is reported, which is synthesized using a solvothermal synthesis with Zn(NO3)2 and a ditopic organic perylene linker. Both the linker and Zn-CP are fully characterized, including SC-XRD, showing a strong aggregation of tightly packed chromophores in the solid state. The photophysical properties are examined and discussed, including the observed shifts within the absorption spectra of the CP are compared to the linker in solution. These shifts are mainly attributed to the for PDIs known H-type aggregation and an additional charge transfer in the framework structure, causing a limited quantum yield of the emission. Finally, the photosensitization of triplet oxygen to singlet oxygen using 1,3-diphenylisobenzofurane (DBPF) as a trapping agent is investigated both for the free linker and the Zn-CP, showing that the perylene chromophore is an efficient photosensitizer and its activity can, in principle, be retained after its incorporation in the coordination polymer.

Gadolinium and Polythiophene Functionalized Polyurea Polymer Dots as Fluoro-Magnetic Nanoprobes

A rapid and one-pot synthesis of poly 3-thiopheneacetic acid (PTAA) functionalized polyurea polymer dots (Pdots) using polyethyleneimine and isophorone diisocyanate is reported. The one-pot mini-emulsion polymerization technique yielded Pdots with an average diameter of ~20 nm. The size, shape, and concentration of the surface functional groups could be controlled by altering the synthesis parameters such as ultrasonication time, concentration of the surfactant, and crosslinking agent, and the types of isocyanates utilized for the synthesis. Colloidal properties of Pdots were characterized using dynamic light scattering and zeta potential measurements. The spherical geometry of Pdots was confirmed by scanning electron microscopy. The Pdots were post-functionalized by 1,4,7,10 tetraazacyclododecane-1,4,7,10-tetraacetic acid for chelating gadolinium nanoparticles (Gd³⁺) that provide magnetic properties to the Pdots. Thus, the synthesized Pdots possess fluorescent and magnetic properties, imparted by PTAA and Gd³⁺, respectively. Fluorescence spectroscopy and microscopy revealed that the synthesized dual-functional Gd³⁺-Pdots exhibited detectable fluorescent signals even at lower concentrations. Magnetic levitation experiments indicated that the Gd³⁺-Pdots could be easily manipulated via an external magnetic field. These findings illustrate that the dua- functional Gd³⁺-Pdots could be potentially utilized as fluorescent reporters that can be magnetically manipulated for bioimaging applications.

4D Microspectroscopy Explores Orientation and Aggregations in π-Conjugated Polymer Films Prepared by Brush Printing

The orientation of polymers improves their mechanical, electrical, and optical properties, but aggregates alter these properties. Unfortunately, there is no definitive way to control aggregates and quantify orientations by distinguishing between polymer chains and aggregates. Herein, we show 4D microspectroscopy to examine brush-printed oriented films of π-conjugated polymers. Three-dimensional (x-y-z) and 1D (photon energy) components based on polarized-fluorescence spectra and film thickness at identical positions were measured. Stochastic analysis of 4D data for a brush-printed OLED film (30 × 30 μm², 900 pixels) distinguished orientations of polymer chains and their aggregates with a 1 μm x-y resolution and a z-range of 20-1800 nm. Polymer chains in thin regions (t < 50 nm) were oriented parallel to the brush-printing direction, whereas aggregates were oriented perpendicularly in thicker regions (t > 1000 nm). This difference was attributed to shear stress, uneven thickness, and capillary forces. The generality of the 4D method was also examined using conventional drop casting.

Molecular Encapsulation of Naphthalene Diimide (NDI) Based π-Conjugated Polymers: A Tool for Understanding Photoluminescence

Conjugated polymers are an important class of chromophores for optoelectronic devices. Understanding and controlling their excited state properties, in particular, radiative and non-radiative recombination processes are among the greatest challenges that must be overcome. We report the synthesis and characterization of a molecularly encapsulated naphthalene diimide-based polymer, one of the most successfully used motifs, and explore its structural and optical properties. The molecular encapsulation enables a detailed understanding of the effect of interpolymer interactions. We reveal that the non-encapsulated analogue P(NDI-2OD-T) undergoes aggregation enhanced emission; an effect that is suppressed upon encapsulation due to an increasing π-interchain stacking distance. This suggests that decreasing π-stacking distances may be an attractive method to enhance the radiative properties of conjugated polymers in contrast to the current paradigm where it is viewed as a source of optical quenching.

Polymerization-Induced Helicity Inversion Driven by Stacking Modes and Self-Assembly Pathway Differentiation

Regulating the mutual stacking arrangements is of great interest for understanding the origin of chirality at different hierarchical levels in nature. Different from molecular level chirality, the control and manipulation of hierarchical chirality in polymer systems is limited to the use of external factors as the energetically demanding switching stimulus. Herein, the first self-assembly strategy of polymerization-induced helicity inversion (PIHI), in which the controlled packing and dynamic stereomutation of azobenzene (Azo) building blocks are realized by in situ polymerization without any external stimulus, is reported. A multiple helicity inversion and intriguing helix-helix transition of polymeric supramolecular nanofibers occurs during polymerization, which is collectively confirmed to be mediated by the transition between functionality-oriented π-π stacking, H-, and J-aggregation. The studies further reveal that helicity inversion proceeds through a delicate interplay of the thermodynamically and kinetically controlled, pathway-dependent interconversion process, which should provide new insight into the origin and handedness control of helical nanostructures with desired chirality.

A soft on/off switch based on the electrochemically reversible H-J interconversion of a floating porphyrin membrane

Soft molecular assemblies that respond reversibly to external stimuli are attractive materials as on/off switches, in optoelectronic, memory and sensor technologies. In this Edge Article, we present the reversible structural rearrangement of a soft porphyrin membrane under an electrical potential stimulus in the absence of solid-state architectures. The free-floating porphyrin membrane lies at the interface between immiscible aqueous and organic electrolyte solutions and is formed through interfacial self-assembly of zinc(ii) meso-tetrakis(4-carboxyphenyl)porphyrins (ZnPor). A potential difference between the two immiscible electrolyte solutions induces the intercalation of bis(triphenylphosphoranylidene)ammonium cations from the organic electrolyte that exchange with protons in the porphyrin membrane. In situ UV/vis absorbance spectroscopy shows that this ionic intercalation and exchange induces a structural interconversion of the individual porphyrin molecules in the membrane from an H- to a J-type molecular configuration. These structural rearrangements are reversible over 30 potential cycles. In situ polarisation-modulation fluorescence spectroscopy further provides clear evidence of structural interconversion of the porphyrin membrane, as intercalation of the organic electrolyte cations significantly affects the latter's emissive properties. By adjusting the pH of the aqueous phase, additional control of the electrochemically reversible structural interconversion can be achieved, with total suppression at pH 3.

Nanotube Template-Directed Formation of Strongly Coupled Dye Aggregates with Tunable Exciton Fluorescence Controlled by Switching between J- and H-Type Electronic Coupling

Strongly coupled dye aggregates with tailored exciton properties may find their use in developing artificial light-harvesting and optoelectronic devices. Here, we report the control of tubular pseudoisocyanine (PIC) dye J- and H-aggregate formation with tunable exciton fluorescence using lithocholic acid (LCA) as a template. The LCA-templated PIC J-aggregate nanotubes formed at a higher LCA/PIC molar ratio (≥5:1) exhibit a sharp, red-shifted absorption band (at 555 nm), intense fluorescence (at 565 nm), and shorter lifetime (200 ps), all indicating their strong superradiance properties. In contrast, the H-aggregate nanotubes formed at a lower LCA/PIC molar ratio (2:1) exhibit a significantly blue-shifted absorption band (at 420 nm) and highly red-shifted fluorescence emission (at 600 nm) with enhanced lifetime (4.40 ns). The controlled switching of the optical properties of the PIC dye aggregates achieved by controlling the LCA/PIC molar ratio could serve as an important guideline for the design of organic photo-functional materials.

Unraveling the Contribution of Residual Monomer to the Emission Spectra of Poly(3-hexylthiophene) Aggregates: Implications for Identifying H- and J-type Coupling

Poly(3-hexylthiophene) (P3HT) is a well-studied benchmark system for semiconducting polymers used in optoelectronic devices. In these materials, aggregation can improve charge transport efficiency or enhance emission yields depending on the interchain packing. This may be inferred from the absorption and emission spectra when analyzed using exciton coupling models such as the well-known H- and J-coupling model of Kasha. The more recently developed weakly coupled H-aggregate (WCH) model quantifies the degree of disorder via the ratio of the electronic origin intensity to that of the first vibronic band. Here, the underlying assumptions of this approach are tested experimentally for P3HT aggregates formed by solvent poisoning using bulk and single-molecule-based spectroscopic techniques. Specifically, we show that the contribution of residual monomeric chains to the aggregate spectrum must be accounted for to properly assign the spectra as H- or J-type. A modification of the WCH model is introduced to account for multiple emissive species.

Nanopatterns of molecular spoked wheels as giant homologues of benzene tricarboxylic acids

Molecular spoked wheels with D_3h and C_s symmetry are synthesized by Vollhardt trimerization of C_2v-symmetric dumbbell structures with central acetylene units and subsequent intramolecular ring closure. Scanning tunneling microscopy of the D_3h-symmetric species at the solid/liquid interface on graphite reveals triporous chiral honeycomb nanopatterns in which the alkoxy side chains dominate the packing over the carboxylic acid groups, which remain unpaired. In contrast, C_s-symmetric isomers partially allow for pairing of the carboxylic acids, which therefore act as a probe for the reduced alkoxy chain nanopattern stabilization. This observation also reflects the adsorbate substrate symmetry mismatch, which leads to an increase of nanopattern complexity and unexpected templating of alkoxy side chains along the graphite armchair directions. State-of-the-art GFN-FF calculations confirm the overall structure of this packing and attribute the unusual side-chain orientation to a steric constraint in a confined environment. These calculations go far beyond conventional simple space-filling models and are therefore particularly suitable for this special case of molecular packing.

Scanning tunneling microscopy investigations of phenylene-based molecular spoked wheels with D_3h and C_s symmetries on graphite show the competitive or complementary effects of carboxylic acid groups and alkoxy chains on the nanopattern formation.

Bright Frenkel Excitons in Molecular Crystals: A Survey

We computed the optical properties of a large set of molecular crystals (∼2200 structures) composed of molecules whose lowest excited states are strongly coupled and generate wide excitonic bands. Such bands are classified in terms of their dimensionality (1-, 2-, and 3-dimensional), the position of the optically allowed state in relation with the excitonic density of states, and the presence of Davydov splitting. The survey confirms that one-dimensional aggregates are rare in molecular crystals highlighting the need to go beyond the simple low-dimensional models. Furthermore, this large set of data is used to search for technologically interesting and less common properties. For instance, we considered the largest excitonic bandwidth that is achievable within known molecular crystals and identified materials with strong super-radiant states. Finally, we explored the possibility that strong excitonic coupling can be used to generate emissive states in the near-infrared region in materials formed by molecules with bright visible absorption and we could identify the maximum allowable red shift in this material class. These insights with the associated searchable database provide practical guidelines for designing materials with interesting optical properties.

Carbocycle-Based Organogelators: Influence of Chirality and Structural Features on Their Supramolecular Arrangements and Properties

The rational design and engineer of organogel-based smart materials and stimuli-responsive materials with tuned properties requires the control of the non-covalent forces driving the hierarchical self-assembly. Chirality, as well as cis/trans relative configuration, also plays a crucial role promoting the morphology and characteristics of the aggregates. Cycloalkane derivatives can provide chiral chemical platforms allowing the incorporation of functional groups and hydrophobic structural units able for a convenient molecular stacking leading to gels. Restriction of the conformational freedom imposed by the ring strain is also a contributing issue that can be modulated by the inclusion of flexible segments. In addition, donor/acceptor moieties can also be incorporated favoring the interactions with light or with charged species. This review offers a perspective on the abilities and properties of carbocycle-based organogelators starting from simple cycloalkane derivatives, which were the key to establish the basis for an effective self-assembling, to sophisticated polycyclic compounds with manifold properties and applications.

Picosecond time-resolved photon antibunching measures nanoscale exciton motion and the true number of chromophores

The particle-like nature of light becomes evident in the photon statistics of fluorescence from single quantum systems as photon antibunching. In multichromophoric systems, exciton diffusion and subsequent annihilation occurs. These processes also yield photon antibunching but cannot be interpreted reliably. Here we develop picosecond time-resolved antibunching to identify and decode such processes. We use this method to measure the true number of chromophores on well-defined multichromophoric DNA-origami structures, and precisely determine the distance-dependent rates of annihilation between excitons. Further, this allows us to measure exciton diffusion in mesoscopic H- and J-type conjugated-polymer aggregates. We distinguish between one-dimensional intra-chain and three-dimensional inter-chain exciton diffusion at different times after excitation and determine the disorder-dependent diffusion lengths. Our method provides a powerful lens through which excitons can be studied at the single-particle level, enabling the rational design of improved excitonic probes such as ultra-bright fluorescent nanoparticles and materials for optoelectronic devices.

Controlling Solar Hydrogen Production by Organizing Porphyrins

In this study, a highly efficient photocatalytic H₂ production system is developed by employing porphyrins as photocatalysts. Palladium and platinum tetracarboxyporphyrins (PdTCP and PtTCP) are adsorbed or coadsorbed onto TiO₂ nanoparticles (NPs), which act as the electron transport medium and as a scaffold that promotes the self-organization of the porphyrinoids. The self-organization of PdTCP and PtTCP, forming H- and J-aggregates, respectively, is the key element for H₂ evolution, as in the absence of TiO₂ NPs no catalytic activity is detected. Notably, J-aggregated PtTCPs are more efficient for H₂ production than H-aggregated PdTCPs. In this approach, a single porphyrin, which self-organizes onto TiO₂ NPs, acts as the light harvester and simultaneously as the catalyst, whereas TiO₂ serves as the electron transport medium. Importantly, the concurrent adsorption of PdTCP and PtTCP onto TiO₂ NPs results in the most efficient catalytic system, giving a turnover number of 22,733 and 30.2 mmol(H₂ ) g(cat)^-1 .

Open-Shell and Closed-Shell Quinoid-Aromatic Conjugated Polymers: Unusual Spin Magnetic and High Charge Transport Properties

While quinoidal moieties are considered as emerging platforms showing efficient charge transport and interesting open-shell diradical characteristics, whether these properties could be changed by extension to the conjugated polymer structure remains as a fundamental question. Here, we developed and characterized two conjugated polymers incorporating quinoids with different lengths, which have a stable close- and open-shell diradical character, respectively, namely, poly(quinoidal thiophene-thienylene vinylene) (PQuT-TV) and poly(quinoidal bithiophene-thienylene vinylene) (PQuBT-TV). A longer length of a quinoidal core led to enhanced diradical characteristics. Therefore, the longer core length of QuBT was favorable for the formation of an open-shell diradical structure in its monomer and in the quinoidal polymer. PQuBT-TV exhibited high spin characteristics observed by the strong ESR signal, a low band gap, and improved electrochemical stability. On the other hand, as QuT maintained a closed-shell quinoid structure, PQuT-TV exhibited high backbone coplanarity and strong intermolecular interaction, which was beneficial for charge transport and led to high hole mobility (up to 2.40 cm² V^-1 s^-1) in organic field-effect transistors. This work successfully demonstrated how the control of the closed/open-shell character of quinoidal building blocks changes charge transport and spin properties of quinoidal conjugated polymers via quinoid-aromatic interconversion.

Beyond p-Hexaphenylenes: Synthesis of Unsubstituted p-Nonaphenylene by a Precursor Protocol

The synthesis of unsubstituted oligo‐para‐phenylenes (OPP) exceeding para‐hexaphenylene—in the literature often referred to as p‐sexiphenyl—has long remained elusive due to their insolubility. We report the first preparation of unsubstituted para‐nonaphenylenes (9PPs) by extending our precursor route to poly‐para‐phenylenes (PPP) to a discrete oligomer. Two geometric isomers of methoxylated syn‐ and anti‐cyclohexadienylenes were synthesized, from which 9PP was obtained via thermal aromatization in thin films. 9PP was characterized via optical, infrared and solid‐state ¹³C NMR spectroscopy as well as atomic force microscopy and mass spectrometry, and compared to polymeric analogues. Due to the lack of substitution, para‐nonaphenylene, irrespective of the precursor isomer employed, displays pronounced aggregation in the solid state. Intermolecular excitonic coupling leads to formation of H‐type aggregates, red‐shifting emission of the films to greenish. 9PP allows to study the structure–property relationship of para‐phenylene oligomers and polymers, especially since the optical properties of PPP depend on the molecular shape of the precursor.

Design and Synthesis of Annulated Benzothiadiazoles via Dithiolate Formation for Ambipolar Organic Semiconductors

Substituted 2,1,3-benzothiadiazole (BTD) is a widely used electron acceptor unit for functional organic semiconductors. Difluorination or annulation on the 5,6-position of the benzene ring is among the most adapted chemical modifications to tune the electronic properties, though each sees its own limitations in regulating the frontier orbital levels. Herein, a hitherto unreported 5,6-annulated BTD acceptor, denoted as ssBTD, is designed and synthesized by incorporating an electron-withdrawing 2-(1,3-dithiol-2-ylidene)malononitrile moiety via aromatic nucleophilic substitution of the 5,6-difluoroBTD (ffBTD) precursor. Unlike the other reported BTD annulation strategies, this modification leads to the simultaneous decrease in both frontier orbital energies, a welcoming feature for photovoltaic applications. Incorporation of ssBTD into conjugated polymers results in materials boasting broad light absorption, dramatic solvatochromic and thermochromic responses (>100 nm shift and a band gap difference of ∼0.28 eV), and improved crystallinity in the solid state. Such physical properties are in accordance with the combined electron-withdrawing effect and significantly increased polarity associated with the ssBTD unit, as revealed by detailed theoretical studies. Furthermore, the thiolated ssBTD imbues the polymer with ambipolar charge transport property, in contrast to the ffBTD-based polymer, which transports holes only. While the low mobilities (10^-4 to 10^-5 cm² V^-1 s^-1) could be further optimized, detailed studies validate that the thioannulated BTD is a versatile electron-accepting unit for the design of functional stimuli-responsive optoelectronic materials.

Aggregation induced emission behavior in oleylamine acetone system and its application to get improved photocurrent from In2S3 quantum dots

Blue emission giving nanoscale molecular clusters of Oleylamine-Acetone system was formed by an aging assisted hydrogen bond formation between the interacting molecular systems, at room temperature. The as-formed nanoscale molecular clusters were found to be self-assembled into flower-like aggregates and shifted the emission wavelength to red colour depicting an exciton delocalization in the aggregate system. Interestingly aging process has also produced imine type binding between Oleylamine and Acetone due to the condensation reaction. The experimental conditions and formation mechanism of hydrogen bond assisted Oleylamine-Acetone molecular aggregates and imine bond assisted Oleylamine-Acetone is elaborated in this paper in a systematic experimental approach with suitable theory. Finally we have introduced this Acetone assisted aging process in In2S3 QD system prepared with Oleylamine as functional molecules. It was found that the aging process has detached Oleylamine from QD surface and as a consequence In2S3 QD embedded Oleylamine-Acetone aggregates was obtained. When this In2S3 QD embedded molecular cluster system was used as an active layer in a photo conductor device then a maximum photo current value of the order of milli Ampere was obtained. The surfactant molecules normally inhibit the charge transport between QD systems and as a result it is always problematic to have the functional molecules in the QD based transport devices. Our approach has a solution to this problem and the present paper discusses the outcome of the results in detail.

An NIR-II-Emissive Photosensitizer for Hypoxia-Tolerant Photodynamic Theranostics

As a common feature in a majority of malignant tumors, hypoxia has become the Achilles' heel of photodynamic therapy (PDT). The development of type-I photosensitizers that show hypoxia-tolerant PDT efficiency provides a straightforward way to address this issue. However, type-I PDT materials have rarely been discovered. Herein, a π-conjugated molecule with A-D-A configuration, COi6-4Cl, is reported. The H₂ O-dispersible nanoparticle of COi6-4Cl can be activated by an 880 nm laser, and displays hypoxia-tolerant type I/II combined PDT capability, and more notably, a high NIR-II fluorescence with a quantum yield over 5%. Moreover, COi6-4Cl shows a negligible photothermal conversion effect. The non-radiative decay of COi6-4Cl is suppressed in the dispersed and aggregated state due to the restricted molecular vibrations and distinct intermolecular steric hindrance induced by its four bulky side chains. These features make COi6-4Cl a distinguished single-NIR-wavelength-activated phototheranostic material, which performs well in NIR-II fluorescence-guided PDT treatment and shows an enhanced in vivo anti-tumor efficiency over the clinically approved Chlorin e6, by the equal stresses on hypoxia-tolerant anti-tumor therapy and deep-penetration imaging. Therefore, the great potential of COi6-4Cl in precise PDT cancer therapy against hypoxia challenges is demonstrated.