Excited State and Non-linear Optical Properties of NIR Absorbing β-Thiophene-Fused BF2-Azadipyrromethene Dyes—Computational Investigation

Density functional theory and time-dependent density functional theory computations were used to understand the electronic and photophysical parameters of NIR β-thiophene-fused BF2-azadipyrromethene dyes. The computed data are in good agreement with those obtained experimentally and they provide insights into the origin of red shifted optical spectra compared to the parent aza-BODIPY, low Stokes shift, non-linear optical responses and quantitative description of the singlet–triplet energy gap. The resultant decrease in the HOMO − LUMO energy gap is responsible for the red shift. The possible use as non-linear optical materials is supported by large enhancement in the non-linear optical properties. On the basis of vertical triplet energies, their possible potential therapeutic use as a photosensitizer in photodynamic therapy is proposed. The singlet–triplet energetic gaps suggest that the β-thiophene-fused BF2-azadipyrromethene dyes can act as a sensitizer to produce an efficient generation of singlet oxygen. Their optimal use as an efficient singlet fission materials has been proposed on the basis of excitation energies in the ground, lowest singlet and triplet excited states.