Estudi de l’aromaticitat en l’estat singlet i triplet de compostos de bifenilè fusionats amb anells de sis membres

Abstract

π-Conjugated compounds formed by fused cycles with 4n+2 and 4n π electrons are interesting

for understanding the limits of aromaticity and antiaromaticity. These compounds in the ground

state exhibit intermediate properties between aromatic and antiaromatic systems. However,

their behaviour in the triplet state is not fully understood.

This study explores the rules of aromaticity, specifically Hückel’s and Baird’s rules, along with

Clar's π-sextet theory. The research combines these rules to investigate the triplet state energy

of 25 molecules, which are composed of biphenylene compounds fused with six-membered

rings, using density functional theory (DFT) calculations performed with the Gaussian16

program. To determine the aromaticity of the molecules, the NICS index has been analysed to

identify the presence of diatropic currents, indicative of aromaticity, and paratropic currents,

indicative of antiaromaticity, within the rings of the molecules. Spin density has also been used

to locate unpaired electrons.

The results indicate that molecules with the highest number of Baird rings combined with closedshell Clar π-sextets, with all electrons paired, are particularly stable in the triplet state. However,

steric effects can increase the energy of the molecules, decreasing their aromatic stability in both

electronic states. Additionally, the Radenkovic model used to predict triplet state energies has

been validated, and Leyva-Parra's theory has been applied to analyse how the antiaromatic

character of the central ring in the ground singlet state influences the energy difference between

the singlet (S0) and triplet (T1) states.

In conclusion, this work demonstrates that the combined use of Baird and Clar's rules can help

to better understand the behaviour of compounds containing rings with 4n and 4n+2 π electrons

in their different electronic states, significantly contributing to the theoretical chemistry of

aromatic and antiaromatic compounds