A comparative study on photophysics of meso-substituted mono- and bis-BODIPY carbazoles

Abstract

A comparative study of the photophysics of mono- and bis-BODIPY (BDP) substituted N-butylcarbazoles (CBZ): CBZ-BDP and CBZ-bisBDP has been carried out. The longest wavelength electronic absorption band at ∼495 nm corresponds to BODIPY moiety and does not exhibit solvatochromism. The absorbance value of CBZ-bisBDP is twice that of CBZ-BDP due to the presence of two BODIPY chromophores in the former. For both the molecules, fluorescence spectra show locally excited (LE) emission in non-polar solvents and red-shifted broad Intramolecular Charge Transfer (ICT) emission in polar solvents. CBZ-BDP shows a larger red-shift and more significant intensity reduction of the ICT band than the bis-conjugated molecule. Theoretical calculations reveal that the Highest Occupied Molecular Orbitals (HOMO) of the two molecules are localized primarily on the carbazole unit. In contrast, the Lowest Unoccupied Molecular Orbitals (LUMO) are mainly concentrated on BODIPY unit. This suggests that excited state charge transfer in both the fluorophores is from the carbazole unit to the BODIPY unit. The S1 states of both CBZ-BDP and CBZ-bisBDP are Twisted Intramolecular Charge Transfer (TICT) states. The temperature-dependent study in ethyl acetate has shown an increase in emission intensity with temperature accompanied by a blue shift of emission spectrum in CBZ-BDP. In contrast, CBZ-bisBDP displayed a decrease in intensity along with blue shift. Besides, CBZ-bisBDP showed an additional LE emission whose emission intensity gets enhanced with increasing temperature. The Critical Micelle Concentration (CMC) of sodium deoxycholate (NaDC) bile salt in aqueous medium was determined by using CBZ-BDP as a fluorescent probe. Three independent parameters: absorbance, fluorescence intensity and anisotropy of the molecule were employed to determine the CMC value. Cell imaging studies using confocal fluorescence microscopy showed that, unlike the mono-BODIPY, CBZ-bisBDP could penetrate the cells and reveal the cytoplasmic distribution in Hela cells. The IC50 values obtained via MTT assay indicate that the cytotoxicity against cancer cells is significant in CBZ-bisBDP compared to the mono-BODIPY.