Highly Photostable Near-Infrared Fluorescent pH Indicators and Sensors Based on BF₂-Chelated Tetraarylazadipyrromethene Dyes

In this study, a series of new BF(2)-chelated tetraarylazadipyrromethane dyes are synthesized and are shown to be suitable for the preparation of on/off photoinduced electron transfer modulated fluorescent sensors. The new indicators are noncovalently entrapped in polyurethane hydrogel D4 and feature absorption maxima in the range 660-710 nm and fluorescence emission maxima at 680-740 nm. Indicators have high molar absorption coefficients of ~80,000 M(-1) cm(-1), good quantum yields (up to 20%), excellent photostability and low cross-sensitivity to the ionic strength. pK(a) values of indicators are determined from absorbance and fluorescence measurements and range from 7 to 11, depending on the substitution pattern of electron-donating and -withdrawing functionalities. Therefore, the new indicators are suitable for exploitation and adaptation in a diverse range of analytical applications. Apparent pK(a) values in sensor films derived from fluorescence data show 0.5-1 pH units lower values in comparison with those derived from the absorption data due to Förster resonance energy transfer from protonated to deprotonated form. A dual-lifetime referenced sensor is prepared, and application for monitoring of pH in corals is demonstrated.