Linear and Nonlinear Optical Properties of Functional Groups for Conjugated Polymers. Analysis of the Acceptor-Donor Pair Substituents of Benzene: The Case of meta-Nitroaniline.

The nonlinear optical (NLO) properties of meta-nitroaniline (m-NA) are evaluated via Hückel-Agrawal's approximation in a solvent environment. In this context, both the 1B and the intramolecular charge transfer (ICT) electronic transitions are considered. The benzene ring currents on the clockwise or counterclockwise direction and the corresponding Brillouin zone from 0 to π are also considered. Besides, the Bloch equations were applied to a single cell n = 1 defined on the benzene ring. We have considered that the light beam was directed along the ring benzene bonds of m-NA; this topological hypothesis changed the crystal structure to a linear chain and the calculated optical properties were found near the experimental ones. In addition, the Fermi's golden rule was applied to the crystal state and then the calculated refraction index of m-NA had an error of less than 3% of the experimental one. On the other hand, the molar absorptivity ε of m-NA in acetonitrile for the 1B and intramolecular ICT transitions was experimentally determined to be 11 981 and 1192 L mol-1 cm-1, respectively. With this methodology, we found that the change of the charge in the NO2 group has also a strong influence on the linear and NLO properties. In addition, the dipole transition moments, which are originated from the carbon between the carbons joined to NO2 and NH2, are mainly involved in the NLO properties. Thus, the first hyperpolarizability β z was 1.69 × 10-30 esu at λLaser = 1064 nm, 27% of the experimental value. We attribute this difference to the evaluation of the excited dipole moment. If we attribute a separation of charge of 0.1 e in the excited state, the new dipole moment allows for the simulation of the experimental value. Besides, the calculated value of χ(3) for m-NA in a solution of acetonitrile is 2.9 × 10-13 esu at λLaser = 1064 nm, 158% of the experimental value. The discrepancy between these values is attributed to the influence of the electronic correlation effects, that is, because of resonance structures of the aromatic ring and the zwitterionic pair of nitro and aniline. Besides, we have also evaluated the second hyperpolarizability γ, the second-order susceptibility χ(2) of m-NA and their values have similar differences to the experimental values. This type of approach is important because it reduces computing time and gives insight into the molecular causes responsible for linear and NLO properties in this type of functional groups, which can be used as building blocks in more complex polymer systems.