© 2020 Wiley-VCH GmbH A 2-naphthol derivative 2 corresponding to the aromatic ring moiety of neocarzinostatin chromophore was found to degrade proteins under photo-irradiation with long-wavelength UV light without any additives under neutral conditions. Structure–activity relationship studies of the derivative revealed that methylation of the hydroxyl group at the C2 position of 2 significantly suppressed its photodegradation ability. Furthermore, a purpose-designed synthetic tumor-related biomarker, a H2O2-activatable photosensitizer 8 possessing a H2O2-responsive arylboronic ester moiety conjugated to the hydroxyl group at the C2 position of 2, showed significantly lower photodegradation ability compared to 2. However, release of the 2 from 8 by reaction with H2O2 regenerated the photodegradation ability. Compound 8 exhibited selective photo-cytotoxicity against high H2O2-expressing cancer cells upon irradiation with long-wavelength UV light.