Bodipy-squaraine triads: Preparation and study of the intramolecular energy transfer, charge separation and intersystem crossing

Two triads (BDP-SQ and Styryl-BDP-SQ) were prepared with Bodipy, styrylBodipy and Squaraine (SQ) units. SQ shows unexpected efficient intersystem crossing (ISC. ?T = 50%), which is attributed to S1->T1 transition. In the two triads, the Förster Resonance Energy Transfer (FRET) direction, as well as the spatial localization of the T1 state, was judiciously tuned. The cascade photophysical properties of the triads were studied with steady-state and time-resolved optical spectroscopies, as well as with electrochemical characterization and theoretical computations. We show that triplet state was produced in triad BDP-SQ upon photoexcitation, but in Styryl-BDP-SQ the fast FRET and the charge separation (CS) processes compete with the ISC of the SQ unit, and no triplet state was formed upon photoexcitation. The singlet energy transfer kinetics were found to be 1.6 and 0.6 ps, respectively and are solvent polarity dependent. Charge transfer was confirmed with ultrafast transient absorption spectroscopy.