Novel Chromogenic Esters of ortho ‐Phenylazonaphthols

We have prepared the photo‐labile benzoic acid esters and the acid‐labile tert ‐butoxycarbonyl esters of Sudan I and Sudan Red B, two representatives of the ortho ‐phenylazonaphthol dye family, exhibiting hydroxyazo–hydrazone tautomerism with the hydrazone as the strongly favored form. The chromophores obtained are “locked” in an exclusive azo configuration and exhibit absorbance spectra which are dramatically blue‐shifted (more than 100 nm) and have strongly reduced extinction coefficients when compared to their parent chromophores. Thus, the esters of the orange dye Sudan I exhibit an absorption maximum in the ultraviolet (UV) regime, and the esters of the red dye Sudan Red B appear yellow. Depending on the nature of the ester moiety, by irradiation with UV light or by exposure to preferentially photochemically released acidic species at elevated temperatures, these esters can be reverted to the parent, highly tinctured Sudan I and Sudan Red B, respectively. Unlike conventional chromogenic systems, these dyes are compatible with melt‐processible polymers and do not require any wet‐chemical development. In polymer films comprised of these chromogenic dyes, color patterns can be produced directly by masked irradiation. We here report on the synthesis, spectral properties, and conversion kinetics of these novel chromogenic dyes, which may be potentially suitable for marking and labeling goods in a lithographic process and for optical data storage and sensing applications.