Water‐soluble donor–acceptor‐type fluorophore 15Nap‐Cl having two trifluoromethyl groups and a Cl group on a 1,5‐aminonaphthyridine framework was prepared. Fluorophore 15Nap‐Cl showed strong solvatochromic fluorescence, and, as the solvent polarity increased, a bathochromic shift was observed accompanied by an increase in the fluorescence quantum yield. In addition, in the presence of amines such as ethylamine, diethylamine, and aniline, further considerable bathochromic shifts in the fluorescence were observed. Density functional calculations identified the source of the fluorescence behavior as exciplex formation between 15‐Nap‐Cl and the corresponding amine. The fluorescence behavior was exploited to fabricate a sensor that can identify various primary, secondary, and tertiary amines.

Two fluorophores consisting of tricyclic amidine derivatives (DHIm and DHPy) were prepared as selective turn-on probes for acids, which were triggered by an aromaticity enhancement. Both amidine derivatives were expanded rings prepared by condensed reactions between the corresponding dibromoalkanes and an aminonaphthyridine analogue. In X-ray analyses, DHIm, in which the dihydroimidazole ring was condensed into aminonaphthyridine, showed high planarity, compared to DHPy, with condensed dihydropyrimidine. The fluorescence properties of DHIm exhibited a higher quantum yield than DHPy due to the difference in planarity. Under acidic conditions, such as in the presence of H+ and M­(II), protonations and complexations occurred, exhibiting a higher quantum yield than the neutral DHX (X = Im or Py). The nucleus-independent chemical shift values from the density functional theory calculations suggested that the protonations and complexations caused an enhancement of the aromaticity within the frameworks. These aromaticity changes led to intense fluorescence, and DHX behaved as a selective turn-on probe for acids and metal ions. Interestingly, this fluorescence turn-on system triggered by the aromaticity-based enhancement is not a typical system, such as the photoinduced electron transfer, aggregation-induced enhanced emission, and twisted intramolecular charge transfer systems, but is classified as a novel turn-on system.

Small chiral organic molecules with CD properties are in high demanded due to their potential use in promising electronic and biological applications. Herein, we reveal a system in which the oxidation of a phosphino group to the corresponding phosphine oxide on the inner rim of a helicene derivative induces a CPL response. Laterally π-extended 7,8dihydro[5]helicenes bearing phosphine and phosphine oxide groups on their inner helical rims (i. e., the C1 position) were synthesized, and their helical structures were unambiguously determined by X-ray crystallography. The photophysical (UV/ visible and emission) and chiroptical properties of these compounds were investigated in various solvents. Despite their structural similarities, phosphine oxide showed a significantly better CPL response than phosphine, with a high dissymmetry factor for emission (j g lum j = (1.3-1.9) × 10 À 3 ) that can be attributed to structural changes in the interior of the helicene helix.

The crystal structures and thermal profiles of three crystal polymorphs of tricyclic imidazonaphthyridine derivatives (INA) were investigated. The aniline derivative INA−AN, in which a 4-anilino moiety was introduced at position 9 of the imidazonaphthyridine framework, yielded a nonplanar curved structure between imidazonaphthyridine and aniline. Three crystals (INA−AN-α (I4 1 /a with Z′ = 1) as an emissive polymorph, as well as INA−AN-β1 and INA−AN-β2 (Pc with Z′ = 4) as nonemissive polymorphs) were obtained. The polymorphs comprised multiple-hydrogen-bond networks. INA−AN-β1 and -β2 crystallized into different polymorphs via recrystallization: INA−AN-β1 and -β2 were obtained from the solvent, as well as via melting and recrystallization processes. Notably, the crystal structures and parameters of INA−AN-β1 and -β2 largely corresponded, although their thermal behaviors were significantly different: INA−AN-β1 underwent a thermal crystal transformation, yielding INA−AN-α at ∼185 °C, while INA−AN-β2 readily melted at ∼190 °C. These distinct profiles indicated that the subtle distinct molecular arrangements, as well as crystal packings, might account for the different thermal profiles of INA−INA-β polymorphs. Interestingly, in addition to INA−AN-β polymorphs, an imidazonaphthyridine framework bearing an N,N-dimethylamine moiety, INA−DMA, crystallized in the P1̅ space group with Z′ = 4, indicating that the INA framework containing an aromatic ring afforded high-Z′ crystal structures.

Invited for the cover of this issue are the groups of Kazuteru Usui and Satoru Karasawa at Showa Pharmaceutical University, and Yoshitane Imai at Kindai University. The image depicts how a phosphine‐oxide‐bearing helicene exhibits markedly enhanced CPL response in the excited state compared with that of one with a corresponding phosphine. Read the full text of the article at 10.1002/chem.202202922.