Synthesis and photophysical properties of π-extended carboxyl BODIPYs: An experimental and theoretical study

A series of new boron-dipyrromethenes (BODIPYs) bearing 4-carboxyphenyl unit at the meso (8) position of the BODIPY core have been successfully synthesized through the reactions of 2,4-diaryl substituted pyrroles with 4formylbenzoic acid. The palladium-catalyzed Suzuki-Miyaura coupling reactions were performed on the meso unsubstituted BODIPY derivatives with 4-carboxyphenyl boronic acid to obtain distal and proximal substitution. Chemical structures were characterized using high resolution mass spectrometry (HRMS), 1H/13C NMR, and FTIR spectroscopy. The photophysical properties, excited state dynamics, and thermal degradation profiles were investigated in terms of aromatic subunits using electronic absorption/fluorescence measurements, femtosecond transient absorption spectroscopy, and thermogravimetric analysis (TGA), respectively. Experimentally investigated absorption and fluorescence properties, as well as structural features, were clarified using density functional theory (DFT) and electron-hole analysis. The absorption of the compounds ranges from 504 to 594 nm, and their emission ranges from 609 to 640 nm, depending on the aromatic groups at the BODIPY core. Performed pump probe spectroscopy measurements revealed that the excited state lifetime is shortened for 3,5 (proximal) positions compared to 1,7 (distal) positions due to increasing interactions between the molecular orbitals. The local n-pi and pi-pi* excitations from DFT calculations and electron-hole analysis mainly characterize the electronic transitions, leading to limited intramolecular charge transfer to the BODIPY core. Charge transfer is highest at the 3,5-positions, decreases at the meso (8) position, and is lowest at the 1,7-positions of the BODIPY core. The newly developed BODIPYs with carboxyl groups show potential as agents in applications demanding extensive absorption and strong emission characteristics.