Dual Ratiometric Fluorescence Monitoring of Mechanical Polymer Chain Stretching and Subsequent Strain-Induced Crystallization.

Tracking the behavior of mechanochromic molecules provides valuable insights into force transmission and associated microstructural changes in soft materials under load. Herein, we report a dual ratiometric fluorescence (FL) analysis for monitoring both mechanical polymer chain stretching and strain-induced crystallization (SIC) of polymers. SIC has recently attracted renewed attention as an effective mechanism for improving the mechanical properties of polymers. A polyurethane (PU) film incorporating a trace of a dual-emissive flapping force probe (N-FLAP, 0.008 wt %) exhibited a blue-to-green FL spectral change in a low-stress region (<20 MPa), resulting from conformational planarization of the probe in mechanically stretched polymer chains. More importantly, at higher probe concentrations (∼0.65 wt %), the PU film showed a second spectral change from green to yellow during the SIC growth (20-65 MPa) due to self-absorption of scattered FL in a short wavelength region. The reversibility of these spectral changes was demonstrated by load-unload cycles. With these results in hand, the degrees of the polymer chain stretching and the SIC were quantitatively mapped and monitored by dual ratiometric imaging based on different FL ratios (I525/I470 and I525/I600). Simultaneous analysis of these two mappings revealed a spatiotemporal gap in the distribution of the polymer chain stretching and the SIC. The combinational use of the dual-emissive force probe and the ratiometric FL imaging is a universal approach for the development of soft matter physics.

Environment-sensitive fluorescence of COT-fused perylene bisimide based on symmetry-breaking charge separation.

Flexible and aromatic photofunctional system (FLAP) is composed of flapping rigid aromatic wings fused with a flexible 8π ring at the center such as cyclooctatetraene (COT). A series of FLAP have been actively studied for the interesting dynamic behaviors. Here, we synthesized a new flapping molecule bearing naphtho-perylenebisimide wings (NPBI-FLAP), in which two perylene units are arranged side by side. As a reference compound, we also prepared COT-fused NPBI (NPBI-COT) that contains only single perylene unit. In both compounds, inherent strong fluorescence of the NPBI moiety is almost quenched and the FL lifetime becomes much shortened in highly polar solvents (acetone and DMF). Through the analyses of environment-sensitive fluorescence, electrochemical reduction/oxidation, and femtosecond transient absorption, the fluorescence quenching behavior was attributed to rapid symmetry-breaking charge separation (SB-CS) for NPBI-FLAP and to intramolecular charge transfer (ICT) for NPBI-COT. Most of the excited species of these compounds decay with the bent geometry, which is in contrast with the excited-state planarization behavior of a previously reported COT-fused peryleneimides with the double-headed arrangement of the perylene moieties. These results indicate that changing the fusion manners between COT and other π skeletons offers new functional molecules with distinct dynamics.

Flapping motion as a fluorescent probe for assembly process involving highly viscous liquid-like cluster intermediates during evaporative crystallization.

Fluorescence probes are widely used to assess the molecular environment based on their photo-physical properties. Specifically, flexible and aromatic photo-functional system (FLAP) is unique viscosity probe owing to the excited-state planarization of anthracene wings. We have previously applied fluorescence spectroscopy to monitor the evaporative crystallization of solvents. The fluorescence color and spectral changes, which depend on the aggregation form, enable direct fluorescence visualization during evaporative crystallization. The fluorescence visualization of the liquid-like cluster intermediate proposed in the two-step nucleation model for the nucleation process has been achieved. However, the physical properties of these clusters, especially the viscosity, molecular motion, and intermolecular interactions, are still unclear. In this study, FLAPs are used as probes for local-viscosity changes and space limitations of the liquid-like cluster state during evaporative crystallization by observing the fluorescence-spectral changes and using hyperspectral-camera (HSC) imaging. Green emission originates from the monomer in the solution owing to the free-flapping motion. The fluorescence color turns blue with increasing viscosity under crowding conditions. If the survival time of the liquid-like cluster state is sufficient, crystalline phase (R-phase) formation proceeds via a 2-fold π-stacked array of the V-shaped molecules. It is difficult to form the V-shaped stacked columnar structures in the liquid-like cluster state region, resulting in the deposition of head-to-tail dimer structures, such as the yellow-emissive phase (Y-phase). In the case of the FLAP, the stacking intermediate does not form during solvent evaporation in the liquid-like cluster; rather, it is deposited in an amorphous form that exhibits blue emission (B-phase). These findings suggest that it is important to the maintenance of the survival time of the liquid-like cluster states to organize and rearrange the stacking forms. We have achieved the fluorescence probing of viscosity changes at local molecular motion with solvent depletion during solvent evaporation for the first time.

Ratiometric Flapping Force Probe That Works in Polymer Gels.

Polymer gels have recently attracted attention for their application in flexible devices, where mechanically robust gels are required. While there are many strategies to produce tough gels by suppressing nanoscale stress concentration on specific polymer chains, it is still challenging to directly verify the toughening mechanism at the molecular level. To solve this problem, the use of the flapping molecular force probe (FLAP) is promising because it can evaluate the nanoscale forces transmitted in the polymer chain network by ratiometric analysis of a stress-dependent dual fluorescence. A flexible conformational change of FLAP enables real-time and reversible responses to the nanoscale forces at the low force threshold, which is suitable for quantifying the percentage of the stressed polymer chains before structural damage. However, the previously reported FLAP only showed a negligible response in solvated environments because undesirable spontaneous planarization occurs in the excited state, even without mechanical force. Here, we have developed a new ratiometric force probe that functions in common organogels. Replacement of the anthraceneimide units in the flapping wings with pyreneimide units largely suppresses the excited-state planarization, leading to the force probe function under wet conditions. The FLAP-doped polyurethane organogel reversibly shows a dual-fluorescence response under sub-MPa compression. Moreover, the structurally modified FLAP is also advantageous in the wide dynamic range of its fluorescence response in solvent-free elastomers, enabling clearer ratiometric fluorescence imaging of the molecular-level stress concentration during crack growth in a stretched polyurethane film.

Sterically Frustrated Aromatic Enes with Various Colors Originating from Multiple Folded and Twisted Conformations in Crystal Polymorphs.

Overcrowded ethylenes composed of 10-methyleneanthrone and two bulky aromatic rings contain a twisted carbon-carbon double (C=C) bond as well as a folded anthrone unit. As such, they are unique frustrated aromatic enes (FAEs). Various colored crystals of these FAEs, obtained in different solvents, correspond to multiple metastable conformations of the FAEs with various twist and fold angles of the C=C bond, as well as various dihedral angles of attached aryl units with respect to the C=C bond. The relationships between color and these parameters associated with conformational features around the C=C bond were elucidated in experimental and computational studies. Owing to the fact that they are separated by small energy barriers, the variously colored conformations in the FAE crystal change in response to various external stimuli, such as mechanical grinding, hydrostatic pressure and thermal heating.

On-Surface Synthesis of Porphyrin-Complex Multi-Block Co-Oligomers by Defluorinative Coupling.

On-surface chemical reaction has become a very powerful technique to synthesize nanostructures by linking small molecules in the bottom-up approach. Given the fact that most reactants are simultaneously activated at certain temperatures, a sequential reaction in a controlled way has remained challenging. Here, we present an on-surface synthesis of multi-block co-oligomers from trifluoromethyl (CF3 ) substituted porphyrin metal complexes. The oligomerization on Au(111) is demonstrated with a combination of bond-resolved scanning probe microscopy and density functional theory (DFT) calculations. Even after the first oligomerization of single monomer unit, the termini of the oligomer keep the CF3 group, which can be used as a reactant for further coupling in a sequential order. Consequently, copper, cobalt, and palladium complexes of bisanthracene-fused porphyrin oligomers were linked with each other in a designed order.

Dynamic Polymer Free Volume Monitored by Single-Molecule Spectroscopy of a Dual Fluorescent Flapping Dopant.

Single-molecule spectroscopy (SMS) of a dual fluorescent flapping molecular probe (N-FLAP) enabled real-time nanoscale monitoring of local free volume dynamics in polystyrenes. The SMS study was realized by structural improvement of a previously reported flapping molecule by nitrogen substitution, leading to increased brightness (22 times) of the probe. In a polystyrene thin film at the temperature of 5 K above the glass transition, the spectra of a single N-FLAP molecule undergo frequent jumps between short- and long-wavelength forms, the latter one indicating planarization of the molecule in the excited state. The observed spectral jumps were statistically analyzed to reveal the dynamics of the molecular environment. The analysis together with MD and QM/MM calculations show that the excited-state planarization of the flapping probe occurs only when sufficiently large polymer free volume of more than, at least, 280 Å3 is available close to the molecule, and that such free volume lasts for an average of 1.2 s.

Controlling the S1 Energy Profile by Tuning Excited-State Aromaticity.

The shape of the lowest singlet excited-state (S1) energy profile is of primary importance in photochemistry and related materials science areas. Here we demonstrate a new approach for controlling the shape of the S1 energy profile which relies on tuning the level of excited-state aromaticity (ESA). In a series of fluorescent π-expanded oxepins, the energy decrease accompanying the bent-to-planar conformational change in S1 becomes less pronounced with lower ESA levels. Stabilization energies following from ESA were quantitatively estimated to be 10-20 kcal/mol using photophysical data. Very fast planarization dynamics in S1 was revealed by time-resolved fluorescence spectroscopy. The time constants were estimated to be shorter than 1 ps, regardless of molecular size and level of ESA, indicating barrierless S1 planarization within the oxepin series.

Magnetism of Topological Boundary States Induced by Boron Substitution in Graphene Nanoribbons.

Graphene nanoribbons (GNRs), low-dimensional platforms for carbon-based electronics, show the promising perspective to also incorporate spin polarization in their conjugated electron system. However, magnetism in GNRs is generally associated with localized states around zigzag edges, difficult to fabricate and with high reactivity. Here we demonstrate that magnetism can also be induced away from physical GNR zigzag edges through atomically precise engineering topological defects in its interior. A pair of substitutional boron atoms inserted in the carbon backbone breaks the conjugation of their topological bands and builds two spin-polarized boundary states around them. The spin state was detected in electrical transport measurements through boron-substituted GNRs suspended between the tip and the sample of a scanning tunneling microscope. First-principle simulations find that boron pairs induce a spin 1, which is modified by tuning the spacing between pairs. Our results demonstrate a route to embed spin chains in GNRs, turning them into basic elements of spintronic devices.

Can Anti-Aufbau DFT Calculations Estimate Singlet Excited State Aromaticity? Correspondence on "Dibenzoarsepins: Planarization of 8π-Electron System in the Lowest Singlet Excited State".

The simple anti-aufbau DFT approach for estimating singlet excited state aromaticity suggested in a recent Communication published in this journal is shown to produce incorrect results because it targets a linear combination of the singlet and triplet configurations involving the HOMO and LUMO rather than the first singlet excited state. If the S1 state of a molecule is dominated by the HOMO→LUMO excitation, a comparably simple but theoretically consistent and qualitatively correct approximation to the S1 wavefunction can be achieved by performing a small "two electrons in two orbitals" CASSCF(2,2) calculation which can be followed by the evaluation of magnetic aromaticity criteria such as NICS.

Flapping Peryleneimide as a Fluorogenic Dye with High Photostability and Strong Visible-Light Absorption.

Flapping fluorophores (FLAP) with a flexible 8π ring are rapidly gaining attention as a versatile photofunctional system. Here we report a highly photostable "flapping peryleneimide" with an unprecedented fluorogenic mechanism based on a bent-to-planar conformational change in the S1 excited state. The S1 planarization induces an electronic configurational switch, almost quenching the inherent fluorescence (FL) of the peryleneimide moieties. However, the FL quantum yield is remarkably improved with a prolonged lifetime upon a slight environmental change. This fluorogenic function is realized by sensitive π-conjugation design, as a more π-expanded analogue does not show the planarization dynamics. With strong visible-light absorption, the FL lifetime response synchronized with the flexible flapping motion is useful for the latest optical techniques such as FL lifetime imaging microscopy (FLIM).

Compression of a Flapping Mechanophore Accompanied by Thermal Void Collapse in a Crystalline Phase.

Mechanical control of the molecular energy landscape is an important issue in modern materials science. Mechanophores play a unique role in that the mechanical responses are induced against the activation barrier for intramolecular transformation with the aid of external forces. Here we report an unprecedented activation process of a flexible flapping mechanophore. Namely, thermal void collapse in a crystalline phase triggers mechanophore compression in a definite proportion. Unfavored conformational planarization of the flapping mechanophore is compulsorily induced by packing force, leading to a total energy gain in crystal packing. Fluorescence chromism indicates extended π conjugation resulting from the mechanophore compression, giving rise to an energy transfer from the unpressed to compressed conformers.

Synthesis of Zeolitic Macrocycles Using Site-Selective Condensation of Regioselectively Difunctionalized Cubic Siloxanes.

The designed synthesis of inorganic cyclic compounds is a significant topic because of their many potential applications. In this study, we used a building block approach to synthesize siloxane-based macrocycles that resemble zeolite apertures. We synthesized a regioselectively functionalized cubic octasiloxane having two adjacent corners modified with Si-O-C bonds via the reaction of octa(hydridosilsesquioxane) (H8Si8O12) with 2,2'-( o-phenylenedioxy)diethanol. Hydrolysis and condensation of the Si-O-C bonds yield the cyclic compounds consisting of three, four, and five cage siloxane units. These compounds have more rigid ring structures than conventional cyclic organosiloxanes. Such an approach will lead to the design of a new class of host materials and molecular channels for transport and separation.

Quantum Dots Embedded in Graphene Nanoribbons by Chemical Substitution.

Bottom-up chemical reactions of selected molecular precursors on a gold surface can produce high quality graphene nanoribbons (GNRs). Here, we report on the formation of quantum dots embedded in an armchair GNR by substitutional inclusion of pairs of boron atoms into the GNR backbone. The boron inclusion is achieved through the addition of a small amount of boron substituted precursors during the formation of pristine GNRs. In the pristine region between two boron pairs, the nanoribbons show a discretization of their valence band into confined modes compatible with a Fabry-Perot resonator. Transport simulations of the scattering properties of the boron pairs reveal that they selectively confine the first valence band of the pristine ribbon while allowing an efficient electron transmission of the second one. Such band-dependent electron scattering stems from the symmetry matching between the electronic wave functions of the states from the pristine nanoribbons and those localized at the boron pairs.

Conformational Planarization versus Singlet Fission: Distinct Excited-State Dynamics of Cyclooctatetraene-Fused Acene Dimers.

A set of flapping acene dimers fused with an 8π cyclooctatetraene (COT) ring showed distinct excited-state dynamics in solution. While the anthracene dimer showed a fast V-shaped-to-planar conformational change within 10 ps in the lowest excited singlet state, reminding us of extended Baird aromaticity, the tetracene dimer and the pentacene dimer underwent intramolecular singlet fission (SF) in different manners: A fast and reversible SF with a characteristic delayed fluorescence (FL), and a fast and quantitative SF, respectively. Conformational flexibility of the fused COT linkage plays an important role in these ultrafast dynamics, demonstrating the utility of the flapping molecular series as a versatile platform for designing photofunctional systems.

Structural Monitoring of the Onset of Excited-State Aromaticity in a Liquid Crystal Phase.

Aromaticity of photoexcited molecules is an important concept in organic chemistry. Its theory, Baird's rule for triplet aromaticity since 1972 gives the rationale of photoinduced conformational changes and photochemical reactivities of cyclic π-conjugated systems. However, it is still challenging to monitor the dynamic structural change induced by the excited-state aromaticity, particularly in condensed materials. Here we report direct structural observation of a molecular motion and a subsequent packing deformation accompanied by the excited-state aromaticity. Photoactive liquid crystal (LC) molecules featuring a π-expanded cyclooctatetraene core unit are orientationally ordered but loosely packed in a columnar LC phase, and therefore a photoinduced conformational planarization by the excited-state aromaticity has been successfully observed by time-resolved electron diffractometry and vibrational spectroscopy. The structural change took place in the vicinity of excited molecules, producing a twisted stacking structure. A nanoscale torque driven by the excited-state aromaticity can be used as the working mechanism of new photoresponsive materials.

Cubic Siloxanes with Both Si-H and Si-OtBu Groups for Site-Selective Siloxane Bond Formation.

Cage-type siloxanes have attracted increasing attention as building blocks for silica-based nanomaterials as their corners can be modified with various functional groups. Cubic octasiloxanes incorporating both Si-H and Si-OtBu groups [(tBuO)n H8-n Si8 O12 ; n=1, 2 or 7] have been synthesized by the reaction of octa(hydridosilsesquioxane) (H8 Si8 O12 ) and tert-butyl alcohol in the presence of a Et2 NOH catalyst. The Si-H and Si-OtBu groups are useful for site-selective formation of Si-O-Si linkages without cage structure deterioration. The Si-H group can be selectively hydrolyzed to form a Si-OH group in the presence of Et2 NOH, enabling the formation of the monosilanol compound (tBuO)7 (HO)Si8 O12 . The Si-OH group can be used for either intermolecular condensation to form a dimeric cage compound or silylation to introduce new reaction sites. Additionally, the alkoxy groups of (tBuO)7 HSi8 O12 can be treated with organochlorosilanes in the presence of a BiCl3 catalyst to form Si-O-Si linkages, while the Si-H group remains intact. These results indicate that such bifunctional cage siloxanes allow for stepwise Si-O-Si bond formation to design new siloxane-based nanomaterials.

A Soluble Dynamic Complex Strategy for the Solution-Processed Fabrication of Organic Thin-Film Transistors of a Boron-Containing Polycyclic Aromatic Hydrocarbon.

The solution-processed fabrication of thin films of organic semiconductors enables the production of cost-effective, large-area organic electronic devices under mild conditions. The formation/dissociation of a dynamic B-N coordination bond can be used for the solution-processed fabrication of semiconducting films of polycyclic aromatic hydrocarbon (PAH) materials. The poor solubility of a boron-containing PAH in chloroform, toluene, and chlorobenzene was significantly improved by addition of minor amounts (1 wt % of solvent) of pyridine derivatives, as their coordination to the boron atom suppresses the inherent propensity of the PAHs to form π-stacks. Spin-coating solutions of the thus formed Lewis acid-base complexes resulted in the formation of amorphous thin films, which could be converted into polycrystalline films of the boron-containing PAH upon thermal annealing. Organic thin-film transistors prepared by this solution process displayed typical p-type characteristics.

A Macrocyclic Fluorophore Dimer with Flexible Linkers: Bright Excimer Emission with a Long Fluorescence Lifetime.

Bright fluorescent molecules with long fluorescence lifetimes are important for the development of lifetime-based fluorescence imaging techniques. Herein, a molecular design is described for simultaneously attaining long fluorescence lifetime (τ) and high brightness (ΦF ×ɛ) in a system that features macrocyclic dimerization of fluorescent π-conjugated skeletons with flexible linkers. An alkylene-linked macrocyclic dimer of bis(thienylethynyl)anthracene was found to show excimer emission with a long fluorescence lifetime (τ≈19 ns) in solution, while maintaining high brightness. A comparison with various relevant derivatives revealed that the macrocyclic structure and the length of the alkylene chains play crucial roles in attaining these properties. In vitro time-gated imaging experiments were conducted as a proof-of-principle for the superiority of this macrocyclic fluorophore relative to the commercial fluorescent dye Alexa Fluor 488.

Photodissociation of B-N Lewis adducts: a partially fused trinaphthylborane with dual fluorescence.

The synthesis of a planarized trinaphthylborane with partially fused structure is presented. This compound shows not only high chemical and thermal stability but also sufficient Lewis acidity to form Lewis adducts with pyridine derivatives in solution. The B-N Lewis adducts exhibit unprecedented photodissociation behavior in the excited state, reminiscent of the photogeneration of carbenium ions from triarylmethane leuco dyes. Consequently, these B-N Lewis adducts exhibit dual fluorescence emission arising from the initial tetracoordinate B-N adducts and the photodissociated tricoordinate boranes.