Espectroscopia de absorção de agregados moleculares com potencial aplicação em optoeletrônica orgânica

Abstract

Understanding the optical and electronic properties of organic semiconductors is of fundamental importance to improve the performance of optoelectronic devices, facilitating the design of new materials. The present work investigated the effects of molecular aggregation upon the absorption spectrum in aromatic organic systems, including compounds with applications in organic optoelectronics (benzene, naphthalene, perylene, and perylene diimide [PDI]). Spectra of benzene molecular aggregates and crystal were studied at the TDDFT level. Two methods were employed: an analytic continuation procedure with square-integrable functions (L2), and the Liouville-Lanczos approach with plane wave basis functions. For the naphthalene, perylene, and PDI crystals, GW–BSE calculations were also performed. The Liouville-Lanczos approach adequately described the shapes of the spectra below the first ionization potential. On the other hand, the analytic continuation properly reproduced the region above the ionization threshold. The parallel displaced (PD) dimer presented the intense allowed transition 1A1g → 1E1u close to 7 eV in an energy slightly below that obtained for the tilted-T (TT) dimer. The absorption spectrum of the solid showed a typical redshift along with a small increase in the bandwidth. The vibronic coupling effects on the benzene spectra indicated that the aggregation, together with small distortions compared to the equilibrium geometry in the gas phase, is already capable of promoting the forbidden transitions 1A1g → 1B2u and 1A1g → 1B1u. However, geometry distortions along the inducing modes of the selected forbidden transitions are able to augment even more band intensities. Specifically, the vibronic coupling is more relevant for the transition 1A1g → 1B1u than the interactions inside the clusters. Absorption spectra of naphthalene, perylene, and PDI show progressively smaller gaps. The calculations for monomers and dimers of these systems at the PBE/PW level agree well with the experiments, but the crystal band gaps were underestimated. This issue was overcome at the GW–BSE level. This study shed light on the optical, electronic, and vibrational properties of relevant aromatic aggregates.