The flip in the aromaticity of pentalene and naphthalene: the effect of dimerization and spin states

Abstract

The photophysical and photochemical properties of organic molecules are mainly governed by the aromaticity in their excited states. Using the density functional theoretical methods at the CAM-B3LYP/6-311+G(d,p) level, the present study investigated the aromaticity of pentalene, naphthalene and their respective dimers for their ground and excited states. The nucleus-independent chemical shift (NICS), magnetically induced current density (MICD), harmonic oscillator model of aromaticity (HOMA), and spin density distributions were employed to assess the aromaticity. To understand the effect of molecular arrangement on aromaticity, four different orientations of dimers, viz. parallel, slipped parallel, crossed and T-shaped, were explored. The antiaromatic nature of pentalene in its ground singlet state switched to aromatic nature in its excited state in agreement with Hückel's rule for the ground state and Baird's rule for the excited state. This holds true for naphthalene, which shows aromaticity in the ground singlet state and anti-aromaticity in the triplet excited state. Interestingly, there is a reversal in the aromatic nature due to π–π stacking interactions existing in the dimer in the parallel orientation for both types of molecules in their singlet and triplet states. The slipped, crossed, and T-shaped dimers displayed more complex aromaticity patterns. The MICD and NICS analyses revealed considerable alteration in the electron delocalization and induced current patterns depending on the orientations and spin states, highlighting the importance of tuning aromaticity for potential applications in optoelectronics, photovoltaics, and spintronics.