Acidity control by photoswitching of intramolecular NH center dot center dot center dot O hydrogen bond using E/Z photoisomerization of cinnamate framework was achieved. According to the photoisomerization, the distance between amide NH and carboxylic oxygen was disgregated, and an intramolecular NH center dot center dot center dot O hydrogen bond formed in E carboxylate was interrupted in Z compound in DMSO-d(6) solution. The pK(a) value of Z carboxylic acid was increased in E compound in DMSO solution.