Exploring the π-System of the (Aza-)BOIMPY Scaffold: Electron-Rich Pyrrole Moieties Working in Concert with Electron-Depleted Meso-Positions.

The recently introduced (Aza-)BOIMPY scaffold (bis(borondifluoride)-8-imidazodipyrromethene) is equipped with electron-rich β-alkoxy/α-aryl-substituted pyrrole moieties to saturate the highly electron-depleted, benzimidazole- and tetrazole-bearing meso-positions of the (Aza-)BOIMPY core. The established push-pull system gives rise to a series of compact, red- and near-infrared absorbing dyes and fluorophores with inverted high polarity compared to the native borondipyrromethene motif. The extent of conjugation of the pyrrolic α-aryl fragments with the dipyrrin core is studied by density functional theory (DFT) computations and correlated with observed red-shifts, vibronic progressions, and emission properties. In the course of the stepwise red-shift induced by more compact, electron-rich α-aryl groups, quantum yields of fluorescence decrease dramatically despite mitigated excited state relaxations. Time-dependent DFT (TDDFT) computations reveal the modes of geometrical relaxations upon excitation and their influence on fluorescence quantum yields; they also provide background information for further structural refinements.