The role of the diyne bond on the excited state deactivation of diyne-bridged 7-azaindoles in solution and solid state

Abstract

In this work we present the synthesis, X-ray diffraction analysis and photophysical characterization of two new diyne bridged azaindole derivatives. DFT and TD-DFT calculations were carried out to deepen the knowledge of the molecular structure and photophysics of diyne compounds. The presence of the diyne group joining two azaindole platforms leads to a low rotational barrier and a large conformational variability in the ground state. The vibrational stretching mode associated to the diyne bridge presents an unusual high reorganization energy and Huang-Rhys factor for a vibrational mode in the high energy region. This vibrational mode has a significant contribution to non-radiative relaxation and seems to be related to the weak fluorescence observed from these compounds in the solvated state or in aggregates. Stretching of the diyne bridge is not totally blocked in π-π stacked aggregates according to our computational studies, which also confirm weak exciton coupling. The interplay of the diyne bridge stretching mode in non-radiative relaxation in aggregates is explained as due to the type of molecular packing (parallel stacking and “head-to-tail” stacking) and leads to overall low fluorescence quantum yield values from both diyne derivatives in the solid state. Thus, the diyne bridge stretching mode acts as a driving force governing the final photophysical properties. This experimental and theoretical study contributes to shed light into the important role that the diyne bond plays in the final electronic and photophysical properties of this family of compounds.