The influence of the vinyl terminal group on the poly(para-phenylenevinylene) charge transfer integrals.

The charge transport properties of organic semiconductors are one of the foremost limiting factors in technological applications of these materials, which are becoming important competitors with respect to the inorganic semiconductors. In fact, conjugated organic molecules are used at present as active materials in different types of devices. For this reason, the theoretical study of the electron and hole mobility, carried out in order to give hints for the design of new molecules or for the optimization of their supramolecular organization, is a task of great interest. Here, we present the results of a quantum chemical study, in the framework of the Marcus and density functional theories, on the effects of terminal groups (when they directly interact with the pi-conjugated system of the organic semiconductors) on the charge carriers mobility of organic semiconductors. In particular, using a representative oligomer of poly(para-phenylenevinylene) as a model system, we have found that strong effects on the predicted values of the intramolecular transfer integrals as well as on their dependence on the supramolecular organizations occur, when the vinyl moiety (as ending group) is taken into account.

Tuning the photophysical properties of pyrene-based systems: a theoretical study.

Recently new molecular systems based on the pyrene moiety were developed for photovoltaic applications. Here we present the results of a quantum chemical study focused on the effects induced by some different substituents on the electronic properties of pyrene, to obtain general hints for the molecular design of new pyrene-based systems. In particular, a series of electron-donating (hydroxy, amino, acetylamino) and electron-withdrawing (cyano, carbamoyl, formyl, ethynyl, ethenyl) groups were considered. Furthermore, in addition to the single pyrene molecule, two pyrene units linked by ethenylene, ethynylene, 2,5-thienylene, and ethynylene-p-phenylene containing chains of different lengths were taken into account. For all of the model structures presented, the ground state geometries have been optimized using the density functional approach, while the vertical transition energies were calculated using the time-dependent density functional theory. We will show that the tuning of the lowest electronic excitation energy (i.e., the HOMO-LUMO energy gap) as well as the localization of the spatial distributions of the frontier molecular orbitals (i.e., the nature of the electron-hole pair, generated by photon absorption) can be obtained through the analysis of the pyrene frontier molecular orbitals. This approach allows to evaluate the most suitable position of the substituents on the pyrene moiety giving rise to enhanced electronic effects also in function of their electronic nature. In this way, pyrene-structures with tailored electronic properties could be modeled. Our screening shows that promising candidates for photovoltaic applications could be molecular structures formed by two pyrene units joined/linked by a short conjugated bridge containing double or triple bonds (henceforth pyrene-linked dimers). As far as the single pyrene units are considered, the most significant reduction of the transition energy of the lowest optical electronic excitation is obtained with disubstituted pyrenes with push-pull character.