Nature of Color Diversity in Phenylenevinylene-Based Polymorphs

In this work we present a joint experimental and theoretical study of the light emitting properties of (E,E)-2,5-dimethoxy-1,4-bis[2-(4-carboxylatestyryl)]benzene, a phenylenevinylene (PV, from now on) derivative in the solid state. Careful crystal growth under different conditions of this PV luminophore yields three different crystals with dissimilar color emissions with maxima ranging between 504 and 565 nm. Single crystal structure elucidations indicate that they correspond to a DMF solvate, denoted 2 center dot DMF (yellow) and two conformational polymorphs named 2 alpha (yellow) and 2 beta (orange). Analysis of the close contacts and the framework topologies help us to rationalize the different colors observed, not only on the basis of different intramolecular conformations but also of the different intermolecular interactions observed. Particularly, the presence of different C-H center dot center dot center dot pi and pi...pi interactions in 2 alpha and in 2 beta polymorphs, respectively, give rise to different supramolecular frameworks with hex and bcu topologies. Theoretical studies reinforce the hypothesis that cooperative pi...pi interactions in the crystal lattice are responsible for the red-shifted color observed in the polymorph 2 beta. In fact, in contrast to what is observed with the polymorph 2 alpha, as we incorporate more monomers in the calculations of the optical properties of polymorph 2 beta, the estimated energy for the vertical excitation from the ground state to the first excited state moves to lower values.