Naoya Kumagai

Substituent-dependent intramolecular hydrogen-bonding topology modulates fluorescence in compact C4N4 fluorophores and enables an acetylation-responsive turn-on probe.

A catalyst-dependent chemoselective photoredox reaction of cyanopyridines and cyanoarenes with N-aryl tertiary amines is described. In competitive reactions under visible-light irradiation, a conventional iridium photocatalyst preferentially promoted coupling of the cyanoarene component, whereas phenothiazine-based catalyst systems, including a DATB–phenothiazine hybrid, favored formation of cyanopyridine-derived products. The cyanopyridine-selective product distribution was observed across several combinations of amines, cyanopyridines, and competing cyanoarenes. These results show that the identity of the photocatalyst can govern the relative substrate preference of closely related cyano(hetero)arene acceptors in photoredox catalysis.

Quinoline-based aromatic oligoamide foldamers composed of anilide (A) and quinoline (Q) units, in which amide-bond formation at the quinoline 8-position provides access to cyclic and acyclic architectures, exhibit distinct folding behaviors. The cyclic series A­(n)­Q­(n) (n = 2–6) was obtained by one-pot cyclooligomerization or by stepwise assembly followed by intramolecular amidation. Structural analyses revealed a pronounced ring-size dependence of conformation and symmetry, ranging from symmetric macrocycles to unsymmetric, partially helical folds. In particular, the hexamer A6Q6 exhibits an unusual two-state conformational equilibrium in solution and was isolated in the solid state as both a partially helical unsymmetric conformer and a highly symmetric S ₆ cubic conformer featuring a hydrophobic internal cavity. In parallel, the corresponding acyclic oligomers NH ₂ -A­(n)­Q­(n)-CO ₂ H (n = 3–6) adopt well-defined helical structures in the solid state and retain compact folded conformations in solution, as supported by NMR, H/D-exchange, and NOE analyses. The helical fold generates a confined cavity comparable in size to that of a 12-membered ring. These findings establish anilide–quinoline hybrids as a versatile platform for size-programmed macrocycles, stable aromatic helices, and functional foldamer architectures.

Compact organic fluorophores are attractive scaffolds for probe design, yet their emission is often compromised by efficient nonradiative decay. Here, we introduce 2,5‐diaminopyrimidine‐based C4N4 as an intrinsically emissive, exceptionally small platform, which remains bright without extended π‐conjugation. Systematic substitution at 4,6‐positions with alkyl, alkenyl, and alkynyl groups reveals robust emission for alkyl and alkynyl derivatives, and a striking bimodality for alkenyls. TDDFT combined with ES/SC‐AFIR mapped the S ₁ → S ₀ internal‐conversion landscape, providing a rationale for substituent‐dependent relaxation behavior. Manipulation at the 5‐position allowed for independent trigger‐responsive emission behavior. This orthogonal design, where the 4,6‐substituted C4N4 core serves as the fluorophore and the 5‐position operates as ON/OFF gate, enabled the development of turn‐on probes for Pd ²⁺ .

Macrocyclic compounds, exhibiting unique properties arising from their large-ring frameworks, have captivated the attention of organic chemists over the last few decades. These molecules serve specific functions across disciplines, including synthetic chemistry, materials science, and biology. Representative molecules in this category, including crown ethers, calix[n]arenes, and porphyrinoids composed of ether/phenol/pyrrole subunits, exhibit unparalleled structural and functional diversity. In contrast, macrocycles exploiting quinoline/quinazoline as key building blocks to construct specific macrocyclic architectures have received limited attention and are much less explored. In this Highlight Review, we summarize our recent advances in oligo-quinoline/quinazoline macrocycles. Emphasis is focused on synthetic strategies, structural features, and emerging applications in organometallic, supramolecular, and photophysical chemistry. We anticipate that these collective results will guide new molecular designs within the broader family of heterocycle-based macrocycles.

We report a new class of nitrogen-containing π-extended heterocycles, DiazaChrysenes (DCs), and their dimeric analogues (dDCs) as metal-free photocatalysts for C-H amination. DCs featuring a quinolino[1,2-c]quinazolin-5-ium core were synthesized via electrophilic amide activation and arylation. The resulting compounds displayed tunable photophysical and redox properties, with CF₃-substituted DCs enabling the C-H amination of arenes with azoles under visible light and aerobic conditions, achieving yields up to 85%. The reaction tolerated a range of arene and azole substrates without requiring metal salts or additives. In contrast, dDCs exhibited diminished activity, likely due to their extended π-curvature. Computational studies, including NCI, ACID, and NICS analyses, revealed locally aromatic but electronically isolated π-systems and deshielded central 8-membered cores. This work highlights the utility of DC scaffolds as synthetically accessible and redox-tunable organic photocatalysts.

TriQuinoline (TQ), a quasi‐planar cyclic quinoline trimer concatenated at the 2,8‐positions in a head‐to‐tail arrangement, strongly captures a proton at the 12‐membered inner cycle and exhibits unusual physicochemical properties, including unexpected water solubility and the ability to engage in complexation with other π‐conjugated molecules. In this study, we designed and synthesized its expanded analog, coined Enlarged‐TriQuinoline (Enl‐TQ), in which three alkyne units are embedded between three quinoline units to acquire an expanded planar 18‐membered ring system. DFT calculations and X‐ray crystallographic analysis revealed its planar architecture with three inwardly facing pyridinic nitrogen atoms. Despite its structural similarity to TQ, Enl‐TQ displayed distinct host capability and photophysical characteristics.

TEtraQuinoline (TEQ) exhibits head-to-tail connectivity of four quinoline units, affording an achiral S₄ symmetric architecture. We re-designed and synthesized iso-TEtraQuinoline (i-TEQ), an inherently chiral D₂-symmetric head-to-head tetramer.