Synthesis and Properties of Covalently Linked Fluorescent Tetrads Containing Two BODIPYs and Two 3‐Pyrrolyl BODIPYs

Abstract

Two covalently linked fluorescent tetrads containing two BODIPY units and two 3‐pyrrolyl BODIPY units have been synthesized over a sequence of steps starting with bis(3‐pyrrolyl BODIPY) as the key precursor. Both covalently linked tetrads 6 and 7 were confirmed by HR‐MS and characterized and studied by 1D and 2D NMR, absorption, cyclic voltammetry, steady state and time‐resolved fluorescence techniques, and also by DFT and TD‐DFT methods. The tetrad 6 exhibited one strong absorption band at 680 nm whereas the tetrad 7 showed strong absorption bands at 510 nm and 648 nm corresponding to BODIPY and 3‐pyrrolyl BODIPY units respectively and the absorption band of tetrad 6 was bathochromically shifted due to effective π‐conjugation in tetrad 6 compared to tetrad 7. The electrochemical studies revealed that tetrads exhibit only two reductions indicating their electron deficient nature. The steady state and time‐resolved fluorescence studies invoked a possibility of singlet‐singlet energy transfer from BODIPY units to 3‐pyrrolyl BODIPY units in one of the tetrads upon selective excitation of BODIPY unit. In this tetrad, the BODIPY unit acts as an energy donor whereas the 3‐pyrrolyl BODIPY unit acts as an energy acceptor. The theoretical studies were corroborated with experimental results.