Electronic Communication in Pyrrolo[3,2-b]pyrroles Possessing Sterically Hindered Aromatic Substituents

An investigation of the multicomponent reaction of aldehydes deriving from polycyclic aromatic hydrocarbons with aromatic amines and butane-2,3-dione led to the development of new conditions for the synthesis of tetraaryl-pyrrolo[3,2-b]pyrroles. A major improvement lies in the use of toluene as a co-solvent, enabling both the aldehyde and the initially formed imine to be soluble in the reaction mixture. A variety of aldehydes possessing a naphthalene subunit were transformed into a library of pyrrolopyrroles decorated with large aromatic substituents at the 2 and 5 positions, providing yellow-orange-emitting dyes with large fluorescence quantum yields and large Stokes shifts. We show that there is a significant pi-conjugation between the peripheral polycyclic subunits and the electron-rich core, which translates into pronounced bathochromic shifts in absorption and emission maxima compared to the parent molecule. An analysis of the relationship between the degree of planarization of tetraaryl-pyrrolopyrroles and their photophysical properties is reported. The computational studies, performed with Time-Dependent Density Functional Theory (TD-DFT), have revealed that typically electron-rich polycyclic arenes became electron-acceptors when attached to pyrrolo[3,2-b]pyrrole core. It has also been shown that there is a significant change of geometries of these heterocycles between the ground and the excited states.