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. Group webpage: https://ravikanthlab.wixsite.com/mysite/.