Ultrafast Intramolecular Photoinduced Energy Transfer Events in Benzothiazole–Borondipyrromethene Donor–Acceptor Dyads

Benzothiazole (BTZ)–boron dipyrromethene (BODIPY)-based dyads, Dyad 1 and Dyad 2, containing BTZ as light energy absorbing and transferring antenna while BODIPY as an energy acceptor, linked together with ethoxy and ester spacers, respectively, have been synthesized, and photoinduced energy transfer (PEnT) events occurring within these systems were studied. Both of the dyads were characterized by ¹H NMR, MALDI-MS, UV–visible, steady-state fluorescence, and femtosecond transient absorption spectroscopic as well as electrochemical methods. A comparison of the absorption spectra of the dyads with their reference compounds (i.e., BTZ-OMe and BODIPY-OMe) revealed minimal ground-state interactions between chromophores in Dyad 1 while a more pronounced effect in case of Dyad 2. Electrochemical and computational studies revealed that the LUMO of both of the dyads is located on BODIPY, indicating that the reduction of BODIPY moiety is easier as compared to that of BTZ moiety. Selective excitation on BTZ in the dyads at ∼300 nm resulted in quenching of the BTZ emission followed by the appearance of BODIPY emission showing efficient energy transfer from singlet excited BTZ (¹BTZ*) to BODIPY. The photoinduced energy transfer phenomenon was observed in four different solvents of varying polarity. The driving forces of energy transfer (ΔGEN) in both of the dyads were found to be exothermic, and, in case of Dyad 1, they followed the trend toluene > DCB ≈ DMF > acetonitrile. Transient absorption studies performed in polar solvents such as acetonitrile revealed an efficient photoinduced energy transfer (∼95%) from ¹BTZ* to BODIPY (kEN = 6.17 × 10¹² s–¹ for Dyad 1 and 2.5 × 10¹² s–¹ for Dyad 2 in acetonitrile) creating singlet excited BODIPY (¹BTZ*-BODIPY → BTZ-¹BODIPY*), indicating the quenched pathway is exclusively a PEnT process.