Investigating the excited state optical properties and origin of large stokes shift in Benz[c,d]indole N-Heteroarene BF2 dyes with ab initio tools.

Density Functional Theory and Time Dependent Density Functional Theory computations reveal experimental trends accurately and provide an insight into the origin of red shifted optical spectra, large Stokes shift, nonlinear optical responses and quantitative description of the singlet-triplet energy gap of recently synthesized highly fluorescent membrane permeable benz[c,d]indole N-heteroarene BF2 rigidified BBN and BBC dyes. Our results benchmark the importance of the DFT functionals and polarization functions using the accurate basis set for the calculations of excitation energies, band gaps, dipole moment, non-linear optical properties and geometrical parameters. The anomalous increase in dipole moment in the excited state and pronounced geometric distortion are found to be responsible for the increased Stokes shift in BBN dyes. The resultant decrease in the HOMO-LUMO energy gap was found to be responsible for the red shift in BBC dyes. Enhanced nonlinear optical properties and inverse relationship between the Stokes shift and static hyperpolarizability were found in these dyes. Their possible potential therapeutic use as a photosensitizer in photodynamic therapy is proposed on the basis of vertical triplet energies resulting in the range of 0.78 to 0.88eV. The singlet-triplet energetic gaps suggest that the simple variation of the N-heteroarenes or their post functionalization can lead to the efficient generation of singlet oxygen. Further, we compared these new class of dyes with conventional aza-BODIPY and BODIPY dyes, thereby confirming their use as alternative promising candidates for nonlinear optics, triplet sensitizers, and imaging.