Aza-boron-dipyrromethene dyes: TD-DFT benchmarks, spectral analysis and design of original near-IR structures

The excited-state energies of aza-boron-dipyrromethene (Aza-BODIPY) derivatives are investigated with Time-Dependent Density Functional Theory (TD-DFT), with twin goals. On the one hand, a pragmatic, yet efficient, computational protocol is defined in order to reach rapidly semi-quantitative estimates of the lambda(max) of these challenging dyes. It turned out that a PCM-TD-BMK/6-311+G(2d,p)//PCM-PBE0/6-311G(2d,p) approach delivers appropriate lower bounds of the experimental results, despite the inherent limits of the vertical approximation. On the other hand, the method is applied to design new dyes absorbing in the near-IR. The spectral features of ca. 30 new compounds have been simulated in a systematic way, trying to efficiently combine several available synthetic strategies leading to significant bathochromic displacements. A series of dyes absorbing above 850 nm are proposed, illustrating that (relatively) fast theoretical calculations might be a useful pre-screening step preceding synthesis.