Role of Centrosymmetry in the Photophysics of Molecular Aggregates

TL;DR

This paper investigates how centrosymmetry influences the optical properties of molecular aggregates, revealing that symmetry governs transition rules and is linked to ultralong organic phosphorescence, with broad applicability to many organic crystals.

Contribution

It introduces a group theory-based model to analyze the role of centrosymmetry in the photophysics of molecular aggregates, addressing a gap in previous exciton models.

Findings

Symmetry determines optical selection rules in centrosymmetric aggregates.

Symmetry-forbidden transitions are linked to ultralong phosphorescence.

Over 50% of organic crystals are centrosymmetric, broadening the model's relevance.

Abstract

To understand the photophysics of molecular aggregates, exciton model of J- and H-aggregate has been extensively utilized. However, it lacks consideration of crystal symmetry. Although discrete molecules may lack symmetry, their aggregates can exhibit a high degree of symmetry. Herein, we utilized group theory to study the optical properties of centrosymmetric molecular aggregates, showing that their optical selection rules (transition dipole moment and spin-orbit coupling) are determined by the symmetry of singlet and triplet excited states and the intermolecular orbital overlap. Symmetry-forbidden electronic transitions are closely related to ultralong organic phosphorescence. Our model's scope is broad, as over 50% of organic crystals belong to centrosymmetric space groups according to Cambridge Structural Database.