2-(2′-Hydroxyphenyl)benzoxazole-containing two-photon-absorbing chromophores as sensors for zinc and hydroxide ions

Three new two-photon-absorbing (2PA) chromophores derived from 2-(2′-hydroxyphenyl)benzoxazole (HPBO), having two-photon absorption cross sections up to 530 GM (δ, expressed in GM; 1 GM = 1 × 10 ^-50 cm⁴ s photon^-1 molecule^-1), were synthesized and investigated as metal ion and pH sensors. These chromophores were designed with an increasing number (0, 1, and 2) of HPBO(s) groups as binding sites for metal ions. Such variations provide a systematic approach to correlate structures with their linear photophysical, sensing, and two-photon absorption properties. The HPBO-containing sensors show unique response for zinc ion. Significant zinc ion stimulated photophysical properties with evident enhancements in fluorescence intensities from both one- and two-photon processes were detected. Moreover, one chromophore having HPBO groups in both terminal ends exhibits significant modulation of fluorescence intensities in response to the phenol group deprotonation in the HPBO moiety. A very large two-photon absorption cross section of 4120 GM, the highest reported for bisphenolate-containing 2PA materials, was measured for the chromophore under basic conditions. The molecular structures have a significant influence on the 2PA properties, which follows a trend of the substitution groups on the HPBO moiety in the order of -O^- > -OZn > -OCH₂R. The present study provides a protocol for elucidating the relationship between the photophysical properties and molecular structures.