Non-symmetric benzo[b]-fused BODIPYs as a versatile fluorophore platform reaching the NIR: a systematic study of the underlying structure-property relationship

Ten newly synthesized non-symmetric benzo[b]-fused BODIPYs are compared with an extended series of nine related families (23 compounds) to gain insights into their structure-property relationship. The insertion of a fused indole moiety into the dipyrromethene core and various substituents on the proximal aryl including fused aromatic groups, lead to pronounced changes in the properties of compounds 1-10. By taking advantage of this versatile synthetic platform that allows facile substituent modifications and extension of the pi-conjugated system, significant bathochromic shifts in the absorption (lambda(max) = 511-597 nm) and emission (601-757 nm) bands are achieved. Although the oxidation potentials of the compounds vary considerably throughout the series (+1.28-+1.65 V) due to the significant contribution of the aryl function to the HOMO, the reduction remains much more consistent (-0.61 to -0.79 V) as the LUMO resides primarily on the dipyrromethene core with little aryl contribution as calculated by DFT. For example, installation of a dimethylamine substituent in the para position of the aryl group leads to drastic modification of the optoelectronic properties of the absorption (597 nm) and emission (757 nm) maxima. The full electrochemical, photophysical and computational analyses of the compounds along with the structural characterization of compounds 1, 5, 8, and 9 are used to rationalize the potential of this powerful platform.