New Perspectives on Delocalization Pathways in Aromatic Molecular Chameleons.

Area of Science:

• * Theoretical and computational chemistry
• * Quantum chemistry
• * Organic chemistry

Background:

• * Polycyclic compounds, termed "aromatic chameleons," possess adaptable π-electron systems enabling aromaticity in both ground (S₀) and excited triplet (T₁) states.
• * Traditional magnetic descriptors for aromaticity are often insufficient for these systems due to superimposed local magnetic responses from fused rings.
• * Understanding excited-state aromaticity is critical for predicting and designing novel organic materials with specific electronic properties.

Purpose of the Study:

• * To comprehensively analyze the magnetically induced current density in polycyclic aromatic chameleons.
• * To investigate the aromaticity of these compounds in both the ground (S₀) and lowest triplet (T₁) states.
• * To correlate electronic and geometric descriptors with excited-state aromaticity and singlet-triplet energy gaps.

Main Methods:

• * Computational analysis of magnetically induced current density.
• * Application of advanced electronic and geometric descriptors for aromaticity assessment.
• * Investigation of π-electron delocalization pathways in polycyclic systems.

Main Results:

• * In the S₀ state, fragments exhibit (anti)aromaticity consistent with resonant structures involving aromatic benzenoids and an antiaromatic central ring.
• * A global, diatropic ring current is identified in the T₁ state, indicating global aromaticity.
• * Advanced descriptors confirm features of π-electron delocalization in fused 4n π-electron ring systems.
• * An inverse correlation was found between the singlet-triplet energy difference and the central ring's antiaromaticity in the S₀ state.

Conclusions:

• * Aromatic chameleons display distinct aromaticity profiles in ground and excited states, driven by their unique π-electron arrangements.
• * Advanced descriptors are essential for accurately evaluating excited-state aromaticity in complex polycyclic systems.
• * The interplay between central ring antiaromaticity and singlet-triplet energy gap offers insights into tuning excited-state properties.