Squarylium Dyes with Phenyl and Methyl Substituents at the 3‐Position of Indoline

Squarylium derivatives bearing phenyl and methyl groups at the 3‐position of the indoline ring, which have rarely been reported, were successfully synthesized. Thermogravimetric–differential thermal analysis (TG‐DTA) revealed that all the synthesized derivatives exhibited higher 5% decomposition temperatures ( T d5 ) than the reference squarylium dye (ISQ), which contains two methyl groups at the same position. UV–vis absorption spectroscopy showed that the squarylium dye H‐SQA, bearing one phenyl and one methyl substituent, displayed a maximum absorption wavelength, spectral bandwidth, and molar absorptivity comparable to those of ISQ. A simple photodetector device incorporating H‐SQAs as the photoactive layer was fabricated. Although the external quantum efficiency (EQE) of the device was relatively low, it exhibited a narrow full width at half maximum (FWHM) in its spectral response, indicating selectivity for red light.