QCforever is a wrapper designed to automatically and simultaneously calculate various physical quantities using quantum chemical (QC) calculation software for blackbox optimization in chemical space. We have updated it to QCforever2 to search the conformation and optimize density functional parameters for a more accurate and reliable evaluation of an input molecule. In blackbox optimization, QCforever2 can work as compactly arranged surrogate models for costly chemical experiments. QCforever2 is the future of QC calculations and would be a good companion for chemical laboratories, providing more reliable search and exploitation in the chemical space.

Excited-state aromaticity is one of the most widely applied concepts in the field of chemistry, often used as a rational guideline for predicting conformational changes of cyclic π-conjugated systems induced by photoexcitation. Yet, the details of the relationship between the corresponding photoinduced electronic and structural dynamics have remained unclear. In this work, we applied femtosecond transient absorption and time-resolved time-domain Raman spectroscopies to track the nonequilibrium planarization dynamics of a cyclooctatetraene (COT) derivative associated with the excited-state aromaticity. In the femtosecond time-resolved Raman data, the bent-to-planar structural change was clearly captured as a continuous peak shift of the marker band, which was unambiguously identified with 13C labeling. Our findings show that the planarization occurs after a significant change in the electronic structure, suggesting that the system first becomes aromatic, followed by a conformational change. This work provides a unique framework for understanding the excited-state aromaticity from a dynamical aspect.

Molecular design of dual-fluorescent probes requires precise adjustment of the energy levels of two excited states and the energy barrier between them. While the hybridized local and charge-transfer (HLCT) state has been recently focused as an important excited state for high emission efficiency with a tunable energy level, a dual emission involving the HLCT state has been only achieved with the excited-state intramolecular proton transfer (ESIPT) system. Here, a series of dual-fluorescent molecules involving an HLCT excited state with the excited-state conjugation enhancement (ESCE) motif is presented as the first case. The energy level of the HLCT state has been adjusted by changing substituents and solvents, separately from the ESCE energy level. The HLCT-ESCE molecular design with tunable fluorescence properties proposes a new strategy for the development of advanced fluorescent probes.

Recently, the chemistry of supramolecular adhesives has rapidly progressed in materials science and tissue engineering. Many structural motifs with a variety of non-covalent interactions have been proposed for advanced adhesive properties. Here, we propose a new materials class of liquid crystalline pressure-sensitive adhesive (LC-PSA). Without the aid of hydrogen bonding or Coulomb force, a cyclooctatetraene(COT)-fused electron-deficient dipyridophenazine dimer (dppz-FLAP) forms a tight twofold columnar-stacking based on its V-shaped molecular structure (π-π distance: 3.32 Å). With the dppz-FLAP core as a mesogen of liquid crystal, a high shear LC-PSA bearing a well-defined packing structure in a hexagonal columnar phase has been developed. Both hydrophobic and hydrophilic glass substrates can be easily bonded at room temperature by simply pressing a flake sample of the molecular adhesive between the substrates. Tensile shear strengths reached approximately 1 MPa for glass, SUS and Fe substrates based on dispersion interaction with significant ductility, while the easy peelability on a PET tape was confirmed. The rigid columnar structure formed by the shape-assisted assembly results in the high cohesive force of the material, while the soft liquid crystalline properties provide sufficient fluidity as a PSA. Viscoelastic analysis revealed a unique position of the LC-PSA (G′~10^7 Pa, G′′~10^6 Pa) compared with conventional PSAs. The concept of LC-PSA based on the rigid/soft hybridization and hydrogen-bond-free molecular engineering extends the potential of supramolecular adhesives and functional small-molecule materials.

Cyclooctatetraene (COT) is predicted to have a planar aromatic triplet state. Using femtosecond-to-microsecond transient absorption spectroscopy, we show that acene chromophore-fused COT dyes also have a planar COT ring in their triplet states, although they are not aromatic.

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. Only 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 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 spatio-temporal 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.

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 modification of a previously reported flapping molecule by nitrogen substitution, leading to 20 times increased brightness 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 spectral jumps were statistically analyzed to reveal the dynamics of molecular environment. The analysis together with MD and QM/MM calculations demonstrate that the planarization of the flapping probe occurs only when sufficiently large polymer free volume of more than, at least, 200 Å^3 is available close to the molecule, and that such free volume lasts for an average of 1.17 seconds.

New synthetic protocols to the nitrogen-embedded flapping molecules have been developed. Gram-scale synthesis of a key precursor, tetraamine of dibenzo[a,e]cyclooctatetraene has been established for designing flapping quinoxaline and flapping phenazineimide. The impact of the nitrogen substitution on the photophysical properties and the viscosity-probing function has been investigated in comparison with the reported flapping anthraceneimide.