Shigeru Yamago

Radial π‐conjugation provides a unique platform for extending electronic delocalization in cyclic molecular architectures. Although radial π‐conjugation is relatively robust against static disorders in the ground state, both theoretical and experimental observations indicate that excited states undergo localization through exciton self‐trapping once nanorings exceed a critical size. However, direct observation of the ultrafast structural dynamics underlying exciton localization remains a major challenge. Here, we employ ultrafast time‐resolved impulsive stimulated Raman spectroscopy (TR‐ISRS) to track the radial π‐conjugation during excited‐state structural relaxation. Using [n]cycloparaphenylene ([n]CPP) as a model nanoring system, we show that intrinsic structural metrics—ring strain and curvature—dictate the ultrafast nuclear dynamics. Comparative TR‐ISRS analysis along two distinct nonadiabatic transition pathways provides direct vibrational evidence for exciton self‐trapping, a process previously inferred only by ultrafast electronic spectroscopies. Furthermore, we demonstrate that the electron‐transfer (ET) dynamics of photoexcited [n]CPP are governed by the character of the resulting excitonic state: delocalized excitons promote ultrafast, near‐quantitative ET, whereas self‐trapped excitons substantially suppress the ET rate. These findings establish a direct mechanistic link among radial π‐conjugation, ultrafast structural dynamics, and electron‐transfer processes, and offer design principles of functional cyclic π‐conjugated materials.

Radial π‐conjugation provides a unique platform for extending electronic delocalization in cyclic molecular architectures. Although radial π‐conjugation is relatively robust against static disorders in the ground state, both theoretical and experimental observations indicate that excited states undergo localization through exciton self‐trapping once nanorings exceed a critical size. However, direct observation of the ultrafast structural dynamics underlying exciton localization remains a major challenge. Here, we employ ultrafast time‐resolved impulsive stimulated Raman spectroscopy (TR‐ISRS) to track the radial π‐conjugation during excited‐state structural relaxation. Using [n]cycloparaphenylene ([n]CPP) as a model nanoring system, we show that intrinsic structural metrics—ring strain and curvature—dictate the ultrafast nuclear dynamics. Comparative TR‐ISRS analysis along two distinct nonadiabatic transition pathways provides direct vibrational evidence for exciton self‐trapping, a process previously inferred only by ultrafast electronic spectroscopies. Furthermore, we demonstrate that the electron‐transfer (ET) dynamics of photoexcited [n]CPP are governed by the character of the resulting excitonic state: delocalized excitons promote ultrafast, near‐quantitative ET, whereas self‐trapped excitons substantially suppress the ET rate. These findings establish a direct mechanistic link among radial π‐conjugation, ultrafast structural dynamics, and electron‐transfer processes, and offer design principles of functional cyclic π‐conjugated materials.

The termination mechanism in the radical polymerization of chloroprene (Cp), a representative conjugated diene monomer, was investigated using structurally well-defined polychloroprene (PCp) end radicals generated by photoactivation of organotellurium dormant species. Product analyses revealed that PCp end radicals terminate almost exclusively by combination (Comb), with >98% selectivity even under highly viscous conditions, in sharp contrast to styrene-derived radicals but similar to isoprene-derived radicals. The results indicate that viscosity-independent Comb termination is a characteristic feature of diene-derived radicals. This mechanistic insight was further exploited for polymer synthesis. The Cp-mediated radical coupling reactions of macro-chain transfer agents prepared via organotellurium-mediated radical polymerization proceeded with high coupling efficiency, affording well-defined coupled polymers bearing approximately two Cp units at the junction. In addition, AB diblock copolymers containing a PCp segment were efficiently converted into symmetric ABA triblock copolymers by photoinduced coupling. These findings provide valuable insights into the molecular design of advanced polymer materials by exploiting Cp as both a polymerizable monomer and an efficient radical coupling agent.

The ortho-nitrobenzyl (oNB) group is among the most widely used photodegradable units, serving in diverse fields, including its use as a protective group in organic synthesis, as a stimulus in photoresponsive and photodegradable polymer materials, and in caged molecules in biology. Despite its extensive application, however, little is known about the fate of the photodegraded fragments of the oNB group and their potential impact on the system in which they are used. Here, we isolated o-nitrosobenzaldehyde (2), a proposed primary photoadduct of the oNB group, and investigated its stability and reactivity. Compound 2 rapidly decomposed under both thermal and photochemical conditions, forming oligomers bearing azo- and amide-linkages. The structure of a trimer containing both linkages was unambiguously determined by single-crystal X-ray diffraction. We further found that dienes effectively trapped the nitroso group in 2 by the hetero-Diels–Alder reaction, thereby minimizing undesired oligomerization. This trapping strategy, in combination with a microflow reaction system, significantly enhances the photodegradation efficiency of the oNB-containing polymers. These findings shed new light on the photochemistry of oNB groups and provide a rational basis for the design and implementation of oNB groups in advanced functional systems.

Carbenes are divalent carbon species that have attracted significant research interest due to their ability to form two new bonds and their potential applications as ligands in catalysis and in the development of magnetic materials. In this study, we report singlet ground-state diarylcarbenes, embedded within a unique curved π-conjugated cycloparaphenylene (CPP) framework and composed solely of carbon and hydrogen atoms. We also examined the Möbius and Hückel aromaticity of these CPP-embedded carbenes.

Polymer additives are commonly used in lubricants, yet previous studies have predominantly focused on those with linear chain structures. Generally, low viscosity lubricants are effective in the hydrodynamic lubrication regime, but they lead to increased friction and wear in the boundary lubrication regime. In this study, hyperbranched poly(octadecyl acrylate)s (HBPs) were proposed as a novel class of lubricant additives, and their performance was compared to that of linear polymers. HBPs with controlled macromolecular structures were synthesized by the organotellurium-mediated radical polymerization and evaluated as additives in a poly(α-olefin) (PAO) base oil. Viscosity measurements showed that HBP solutions maintained low fluid resistance comparable to PAO itself, unlike linear polymers, which increased in viscosity. Resonance shear measurements demonstrated superior nanoscopic lubrication efficiency for HBPs, HBPs > linear polymers > PAO, and macroscopic friction tests confirmed a significant reduction in the coefficient of friction in the boundary lubrication regime. These results revealed that HBPs offered effective lubrication for both the hydrodynamic and boundary lubrication regimes.

Ortho-nitrobenzyl (oNB) group is among the most widely used photodegradable units, serving in diverse fields, including its use as a protective group in organic synthesis, as a stimulus in photo-responsive and photo-degradable polymer materials, and in caged molecules in biology. Despite its extensive application, however, little is known about the fate of the photodegraded fragments of the oNB group and their potential impact on the system in which they are used. Here, we isolated ortho-nitrosobenzaldehyde (2), a proposed primary photoadduct of the oNB group, and investigated its stability and reactivity. Compound 2 rapidly decomposed under both thermal and photochemical conditions, forming oligomers bearing azo and amide linkages. The structure of a trimer containing both linkages was unambiguously determined by single-crystal X-ray diffraction. We further found that dienes effectively trapped the nitroso group in 2 by the hetero Diels-Alder reaction, thereby minimizing undesired oligomerization. This trapping strategy, in combination with a microflow reaction system, significantly enhances the photodegradation efficiency of oNB-containing polymers. These findings shed new light on the photochemistry of oNB groups and provide a rational basis for the design and implementation of oNB groups in advanced functional systems.

The development of hyperbranched polymers (HBPs) with controlled structures remains a significant challenge in polymer chemistry. A one‐step synthesis of HBPs has been previously reported with controlled molecular weight and low dispersity by organotellurium‐mediated radical polymerization (TERP) in the presence of an “evolmer,” which induces controlled branch structure. Although indirect evidence supports the structural control of these HBPs, direct visualization of their branch architecture has been lacking. In this study, the use of atomic force microscopy (AFM) is demonstrated to directly visualize individual HBP molecules synthesized by this method using octadecyl acrylate (ODA) as an acrylate monomer. Two HBP samples with different branch molecular weights with a theoretical dendritic generation ( G _n ) of 4 are synthesized and analyzed on highly oriented pyrolytic graphite (HOPG) substrates. High‐resolution AFM phase imaging successfully reveals dendritic branch structures with 16 branches, closely matching the theoretical values of 17 for a G _n = 4. These results provide direct evidence of the controlled dendritic architecture of HBPs synthesized by this method. This study thus represents a critical step toward advancing structurally defined HBPs for materials applications.

Dendritic hyperbranched poly(methyl acrylate)s (HBPMAs) with controlled molecular weight and low dispersity (Đ ≤ 2) were synthesized by radical copolymerization of MA and a branch-inducing monomer, evolmer 2b, in the presence of organotellurium chain transfer agent (CTA) 1, by targeting theoretical generation (G_n) of 3-5 and branch molecular weight (M_BL) of 0.25M_e-2.0M_e, with M_e (= 11,000) being the entanglement molecular weight of PMA. The control of the self-similar branch layer structure in HBPMAs thus synthesized was examined by measuring their linear viscoelastic moduli (G* = G' + iG″). For all G_ns having M_BL = 0.25M_e, the storage modulus G' was lower than the loss modulus G″ at all ω examined. This unentangled feature was observed even when the portion of the chain spanning the two outmost ends of the branches had a molecular weight of 2.0M_e, possibly because of a lack of deep penetration among those G_n chains. In contrast, for M_BL ≥ M_e, G' was higher than G″ in a given range of ω, and this range became wider and the relaxation mode distribution therein became broader on an increase of M_BL, which is characteristic of entangled branch polymers. For the entangled HBPMA chains having the same M_BL but different G_n, hierarchically self-similar relaxation based on the branch retraction mechanism was confirmed for the G' and G″ data: The data for higher G_n agreed with the data for lower G_n after a shift along the ω axis according to this self-similarity. For the unentangled BPMA chains of different G_n, the G' and G″ data obeyed the Rouse-type relationship based on the self-similarity but without the hierarchy. These results support good control of the dendritic branch structures of the HBPMAs synthesized with the current method.