Optical Properties of Macrocyclic Chiral Molecules: The Limitations of Ring Size Increase

Chiral macrocyclic molecules are extensively investigated as potential candidates to develop organic emitters exhibiting circularly polarized luminescence (CPL) with large dissymmetry factors (<i>g</i>). Here, based on time-dependent density functional theory calculations, we investigate the relationship between macrocycle size and chiral properties. Our results underline that the rotatory strength (<i>R</i>) of the transition to the first excited state (S₀ → S₁) increases linearly with the macrocycle loop area. While this evolution could promote high <i>g</i> values in the case of very large rings, it is found that the increase in system size can lead to energetic quasi-degeneracy of several low-lying transitions. In large macrocycles, among those transitions, it is the slightly higher-energy transitions possessing large oscillator strengths but small <i>g</i> values that come to dominate over the S₀ → S₁ transition. Also, the corresponding decrease in energy spacing among these lowest excited states can trigger a broken symmetry of the S₁-state geometry via a pseudo Jahn-Teller effect. Overall, our results highlight that in large macrocycles the CPL can gain in intensity but this occurs at the expense of the <i>g</i> value. Thus, it is critical that the interaction of the S₀ → S₁ transition with higher-energy states be carefully considered when designing large-size CPL emitters.