Near‐neutral pH sensing by azoheteroarene dyes

We serendipitously discovered a novel series of azoheteroarene dyes capable of detecting pH variations in near‐neutral solutions. These dyes feature thiazole, thiadiazole, triazole, pyrazole, or benzothiazole heteroaryls linked to hydroxyphenyl azo groups. They exhibit distinctive light absorption properties in aqueous solutions and show notable color changes in a narrow pH range, visible to the naked eye. Both experimental data and quantum calculations suggest a plausible mechanism for their function as pH indicators. Additionally, these azoheteroarene dyes effectively monitor pH in complex environments, and we show their use for detecting the pH change of culture medium containing growing cancer cells.