A Simple Central Seven-Membered BOPYIN: Synthesis, Structural, Spectroscopic Properties, and Cellular Imaging...

A two-step, one-flask synthesis of central seven-membered borondifluoride-3,3-dimethyl-2-[2-(2-pyrrolyl)ethenyl] indole (BOPYIN) ligands has been developed by using the unexplored 3,3-dimethyl-2-[2-(2-pyrrolyl)ethenyl] indole. The simple synthetic approach has enabled modification of the electronic structure by changing the substituents on the indole unit. X-ray analysis indicated that conformations of the seven-membered BF₂ complex including BOPYIN and diazaborepin differ from that of the five- and six-membered organoboron complexes. Interestingly, the bond angle of the N⋅⋅⋅B-N bond increases with the number of atoms in the core ring, based on Baeyer strain theory. These unsymmetric BOPYIN derivatives have excellent photophysical properties, including high fluorescence quantum yields, except for BOPYIN-4 in the solution state, large Stokes shifts, and good molar absorptivity. The dipole moment of BOPYIN-3 in the first excited singlet state and ground state was demonstrated by a linear Lippert-Mataga plot. The absorption and emission spectra were not mirror images for BOPYIN-1-3 and 5, in contradiction to Kasha's rule, as determined by TDDFT. The synthesized BOPYINs have been shown to be biocompatible fluorophores in cell bioimaging.