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 (g). 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 (R) of the transition to the first excited state (S₀ → S₁) increases linearly with the macrocycle loop area. While this evolution could promote high g 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 g 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 g 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.