Energy transfer booster: how a leaving group controls the excited state pathway within a caging BASHY-BODIPY dyad

Photocages are powerful tools for spatiotemporal control of molecule release or biological activity. However, many photocages are unsuitable for biological experiments since they are mostly activated by harmful ultraviolet (UV) light and often lack a sufficient optical readout. Thus, there is a high demand for near infrared (NIR) and/or two-photon activatable photocages with a characteristic readout. In this report, we will study a supramolecular, covalently linked energy-transfer dyad based on a BASHY fluorophore serving as a two-photon antenna for a poorly two-photon absorbing BODIPY photocage. The herein investigated systems, with and without a leaving group (LG), show different excitation energy transfer (EET) efficiencies and therefore differ in their fluorescence properties. To understand the molecular basis for these significant differences, detailed spectroscopic and theoretical analyses were employed from ultrafast transient absorption spectroscopy to excited-state electronic structure calculations and quantum dynamical modelling. The result of our comprehensive study reveals the pivotal role of the LG as an EET booster through specific pathway guidance. In contrast, without the LG, the EET efficiency is reduced and the excitation energy predominantly dissipates within the BASHY chromophore. The present study highlights that LGs can actively contribute to optimizing the properties of dyad based systems and offers new design principles for monitoring uncaging via an intrinsic fluorescence readout.