We synthesized near-infrared-absorbing anionic heptamethine cyanine (HMC) dyes and investigated their photostability and halochromic properties in detail. Upon changing the methyl groups of the anionic HMC dye to phenyl or perfluorophenyl groups, the maximum absorption wavelengths of the dye solutions were red-shifted and dye photostability in solution under white LED irradiation at 25 °C was improved. The anionic HMC dye featuring methyl groups presented halochromic properties in solution, and the solution color changed from green to yellow in the presence of acids. This is the first reported anionic HMC dye with halochromic properties. Furthermore, we produced a stimuli-responsive filter paper adsorbed with anionic HMC dyes that changed color from green to yellow upon exposure to HCl gas and from yellow to green upon subsequent exposure to NH 3 gas.

A one-step route to a series of perfluoronaphthalene-based donor (D)−acceptor (A)−D fluorescent dyes with various electron-donating groups was developed. The perfluoronaphthalene moiety in the D−A−D dyes served as a good electron-accepting aromatic ring with excellent intramolecular charge transfer properties, as determined by density functional theory calculations and measurements of the fluorescence properties in solution, in poly(methyl methacrylate) (PMMA) films, and in crystal form. Notably, replacing the naphthalene ring with perfluoronaphthalene in the D−A−D dyes carrying the phenothiazine moiety not only stabilized the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels but also reduced the energy band gap to change the emission color from blue to yellow. Among the four synthesized perfluoronaphthalene D−A−D dyes, those bearing diphenylamino groups afforded the best fluorescence quantum yields in Et 2 O solution (0.60) and in PMMA film (0.65) because the propeller structure of the diphenylamino group that acts as a donor substituent effectively suppresses radiation-free deactivation. In contrast, in the crystalline state, the carbazoyl-bearing D−A−D dye provided the best fluorescence quantum yield (0.35) because the radiation-free inactivation was suppressed by π−π F stacking at the donor site, which was confirmed by single-crystal X-ray analysis.

Introduction of a perfluorophenyloxy group at the 5‐position of 3,3‐dibutyl‐2‐methyl‐3H‐indole efficiently promotes aggregation‐induced emission enhancement in both the crystalline solid‐state and in highly concentrated ethanolic solutions of the resultant analog. The phenomenon is attributed to aggregate formation, arising from perfluorophenyl‐perfluorophenyl stacking between the 3H‐indole units, the occurrence of which was deduced from single‐crystal X‐ray analysis of 3,3‐dibutyl‐5‐perfluorophenyloxy‐2‐methyl‐3H‐indole.

The Front Cover shows a molecule that emits almost no fluorescence in the dilute solution but does emit blue light due to the overlap of two perfluorophenyl groups in crystalline states. It is expressed by making the castle glow blue by the overlapping of the two hands. We express gratitude to the workers who are devoted to medical care by illuminating the historic building. More information can be found in the Full Paper by K. Funabiki et al.