Low-Lying ππ* Excited States in Five-Membered Ring Heterocycles: A Continuing Challenge
Chaiyaporn Lakmuang1,2, Thierry Tran3, Antonio Prlj1
1Division of Physical Chemistry, Ruđer Bošković Institute, Bijenička 54, 10000Zagreb, Croatia.
The Journal of Physical Chemistry. A
|August 6, 2026
Summary
Accurately calculating excited states in conjugated molecules is challenging due to diverse electronic characters. This study analyzes five-member-ring heterocycles, offering insights into electronic correlation and valence-Rydberg mixing.
Area of Science:
- Computational Quantum Chemistry
- Theoretical Spectroscopy
- Electronic Excited States
Background:
- Accurate quantum chemical calculations of low-lying ππ* excited states in conjugated systems are notoriously difficult.
- Excited states exhibit diverse electronic characters (e.g., bright/dark, ionic/covalent, singly/doubly excited), requiring balanced theoretical descriptions.
- Five-member-ring heterocycles like furan, pyrrole, and thiophene present specific challenges due to their electronic structure.
Purpose of the Study:
- To analyze the origins of diverse excited-state characters in five-member-ring heterocycles.
- To investigate the role of electronic correlation in achieving accurate excited-state descriptions.
- To explore the phenomenon of valence-Rydberg mixing in excited states.
Main Methods:
- Analysis from both molecular orbital and valence bond theory perspectives.
- Application of pseudosymmetry arguments to bridge theoretical viewpoints.
- Comparison with ππ* excited states of cis-butadiene and benzene model systems.
- Evaluation of standard time-dependent density functional theory (TD-DFT) approximations.
- Utilizing algebraic diagrammatic construction (ADC) methods with systematic correlation improvements.
Main Results:
- Diverse electronic characters of low-lying ππ* states in furan, pyrrole, and thiophene are elucidated.
- The critical importance of electronic correlation for accurate excited-state calculations is highlighted.
- Limitations of commonly used electronic-structure approximations are demonstrated.
- Results from TD-DFT and various ADC methods are presented, showcasing varying levels of accuracy.
Conclusions:
- Achieving high accuracy for excited states often requires sophisticated methods beyond standard approximations due to complex electronic correlation.
- The study provides a deeper understanding of the electronic origins of excited-state properties in heterocyclic systems.
- Valence-Rydberg mixing in excited states remains a significant area for further investigation in quantum chemistry.
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