π-Extended push-pull azo-pyrrole compounds: Synthesis, solvatochromic properties and determination of optical band gaps

A new family of push-pull biphenyl-azopyrrole compounds 3b-g and 4b-d was efficiently obtained via a Suzuki cross-coupling reaction, between 2-(4’-Iodophenyl-azo)-N-methyl pyrrole (1a) or 3-(4’-Iodophenyl-azo)-1,2,5-trimethyl pyrrole (2a) and 4’-substituted phenyl boronic acids in excellent yields. The influence of π-biphenyl backbone and pyrrole pattern substitution was correlated with their optical properties. Solvatochromic studies via UV-Visible spectrophotometry revealed that inclusion of a 4’-Nitro-biphenyl fragment favors a red-shift of the main absorption band in these azo compounds compared with their non-substituted analogues. The pyrrole pattern substitution and the π-conjugated backbone exhibit a clear influence in their thermal isomerization kinetics at room temperature. In all cases, biphenylazo-pyrrole compounds tend to the formation of J-type aggregates in binary MeOH:H2O solvents. Likewise, optical Band-gaps were estimated by means of electronical absorption spectra and correlated by TD-DFT studies