Theoretical Insights into the Impact of Pyrrole and Imidazole Substituents on the BODIPY Chromophore

Abstract

This paper concerns the in silico studies of the influence of heterocyclic substituents as well as their protonated and deprotonated forms on the spectral characteristics of BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes. Computational studies were carried out in order to reveal the most effective method of modeling of the spectral features of fluorescent BODIPY dyes. To perform these studies, the pyrrole and imidazole derivatives of BODIPY dyes were selected, and their spectral features were investigated with DFT and TD-DFT calculations. The calculations showed that the deprotonation of the substituents leads to a bathochromic shift of the calculated absorption wavelength, while the protonation (imidazole derivative) brings about a hypsochromic shift with respect to the neutral form of the dye. The calculated spectral characteristics, considering the influence of the solvent polarity (PCM model), were correlated with the ETN solvatochromic parameter. These correlations show that the increase in the solvent polarity causes a hypsochromic shift of the calculated absorption and emission wavelengths, whereas the bathochromic shift of the wavelengths is observed for the protonated form.

Keywords: BODIPY dye; DFT; TD-DFT; deprotonation; protonation.

Figure 1

Structures of the studied molecules.

Figure 2

The optimized structures of conformers and relative energy (ΔE = E_min − E_i) of studied dyes calculated with M062x/6-31+G(d,p) and TD-M062x/6-31+G(d,p) methods for full optimization parameters of the molecule at ground and excited states, respectively.

Figure 3

Non-covalent interactions (NCI) plots and RDG scatter plots of studied BODIPY dyes. The region of the non-covalent repulsion between the substituent and the BODIPY core fragment are marked as black circles.

Figure 4

Dependencies of absorption (filled circles) and emission (open circles) wavelengths positions (obtained by the SS approach) on the ${\mathrm{E}}_{\mathrm{T}}^{\mathrm{N}}$ solvatochromic parameter for neutral (black circles) and deprotonated (green circles) forms of studied dyes.

Figure 5

Dependency of the calculated Stokes’ shift ($∆\overline{\mathsf{\nu }}$) on ${\mathrm{E}}_{\mathrm{T}}^{\mathrm{N}}$ solvatochromic parameter for neutral (black circles) and protonated (green circles) forms of studied dyes.

Figure 6

Energy levels and isosurfaces of selected isomers of dyes computed with M062x/6-31+G(d,p) method.