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

Abstract

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 π‐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.