Exploiting the intramolecular charge transfer direction to amplify the BODIPY probe response

The high-performance fluorescent probes hinge on their intramolecular charge transfer (ICT) pathways. The unidirectional planarized intramolecular charge transfer (PLICT) and bidirectional twisted intramolecular charge shuttle (TICS) are two pivotal mechanisms. However, leveraging charge transfer direction to amplify probe response remains understudied. Here, using boron-dipyrromethene (BODIPY) derivatives 3 and 7 as fluorescent probes, we found that the ICT direction from unidirectional PLICT to bidirectional TICS leads to significant optical and functional differences. This induces a 116 nm blue shift in absorption, converts fluorescence from dual-peak to single-peak emission, and an approximately eightfold shortened excited-state lifetime (31 ps for the TICS state vs 241 ps for the PLICT state). The TICS-based probe 7 demonstrates markedly enhanced sensitivity in discriminating cancer cells (A549) from normal cells (Beas-2B) in bioimaging applications. Our study results open an enlightening avenue for boosting the performance of fluorescent probes.