The Flip in the aromaticity of Pentalene and Naphthalene: The Effect of Dimerization and Spin States

Abstract

The photophysical and photochemical properties of the organic molecules are governed mainly by the aromaticity in their excited state. Using the density functional theoretical methods at CAM-B3LYP/6-311+G(d,p) level, the present study investigates the aromaticity of pentalene, naphthalene and their respective dimers for 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 are employed to assess aromaticity. To understand the effect of molecular arrangement on aromaticity, four different orientations of dimer viz. parallel, slipped parallel, crossed and T-shaped are explored. The antiaromatic nature of pentalene in its ground singlet state switches to aromatic nature in excited state, in agreement with Hückel's rule for ground state and Baird's rules for 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 for the dimer in the parallel orientation for both types of molecules in singlet and triplet states. The slipped, crossed, and T-shaped dimers display more complex aromaticity patterns. The MICD and NICS analyses reveal considerable alteration in the electron delocalization and induced current patterns depending on orientations and spin states, highlighting the importance of tuning aromaticity for potential applications in optoelectronics, photovoltaics, and spintronics.