Enhanced quantum efficiency of dicyano BODIPYs over difluoro BODIPYs: A DFT approach

The modified BODIPY (pyrromethene) dyes with cyano (-CN), substituting usual fluorine (-F), at the 4-position may give enhanced photochemical stability and quantum yield of fluorescence (QYF) when compared to the corresponding fluoro derivative. We have investigated and discussed comparatively the structural parameters of the ground (S₀) and excited (S₁) state geometries of a few widely used BODIPY dyes, both 4-cyano and 4-fluoro pyrromethene (PM), through DFT and TD-DFT studies and established the reasons for improved QYF of the cyano derivatives. The electrophilicity index indicates the stability of the excited geometry of the cyanated BODIPY and the calculated transition dipole moments reveal a longer lifetime of the excited state (S₁) for the cyano BODIPY. The higher singlet and triplet gap in cyanated BODIPY accounts for higher QYF due to restricted transition from singlet to triplet state. This suggests that a population inversion, crucial for laser action, is easier to build up for cyano derivative than for the commonly used fluoro analogue while other gain/loss parameters of the PM dye lasers remain similar. Hence, 4-cyano derivatives of the PM dyes may be useful for more efficient lasing applications. The non-linear optical properties of the PM dyes are also investigated.