Study on the photophysics of red-emitting meso-substituted α,α′-Dithiophene BODIPYs and exploration of their application potential

Abstract

The basic photophysics and fluorescence anisotropy behaviors of meso-substituted dimethylaniline (DMA), triphenylamine (TPA), and N-phenylcarbazole α,α′-dithiophene BODIPYs: BD4, BD5, and BD6, and meso–(N-phenylcarbazole) BODIPY BD7 have been studied in detail. The application potential of these molecules as sensing and imaging tools is explored. BD4 displays significantly high emission intensity in nonpolar solvents and negligibly low intensity in polar media. In contrast, the fluorescence of BD5 gets heavily quenched in all of the solvents due to its free-rotating phenyl rings. The absorption and emission spectral patterns of BD6 and BD7 look similar, although the maxima differ by 125 nm. The blue shift in absorption and emission peaks of BD7, compared to those of BD4, BD5, and BD6, is due to the absence of thiophene substitution. Additionally, BD7 displays twisted intramolecular charge transfer (TICT) and aggregate-induced emission (AIE) properties. The compound BD6 exhibits an elevated quantum yield in the range of 80–90% in all the solvents, probably due to its rigid conformation and the absence of excited-state relaxation. The results obtained from the experiments were rationalized using DFT calculations. The fluorescence anisotropies of the compounds were recorded in glycerol with varying temperatures. All the compounds show higher sensitivity toward the change in anisotropy with temperature than the commercially available DPH probe. The compound BD6 displays a remarkably high value of fluorescence anisotropy around 0.37. The fluorescence anisotropy values of the dye obtained from the steady-state, time-dependent, and theoretical studies are well in agreement. Since the compounds BD4, BD6, and BD7 exhibited versatile photophysical properties and high sensitivity toward anisotropy, their probing/sensing behaviors were studied in dimyristoylphosphatidylcholine (DMPC) liposomes. The compounds could successfully monitor the phase changes in DMPC small unilamellar vesicles (SUVs) with temperature. The fluorescence microscopy images of the dyes in DMPC giant unilamellar vesicles (GUVs) reveal that the dye molecules could permeate the lipid membranes, resulting in bright images. BD4 and BD6 were used in cell inhibition and cellular uptake studies of human breast cancer cells. BD4 displays a high IC50 value of 15.5 μM. The least cytotoxicity and more distinct images at lower concentrations of BD4 make it a better option for bioimaging. BD6 is found to display a low IC50 value of 1.52 μM, making it a potential candidate for photodynamic therapy (PDT).