Circularly Polarized Luminescence Signal Inversion in Aza-[7]Helicene Derivatives: Theoretical Insight into Pyridine Nitrogen Position and Protonation Effects.

Designing molecules that can produce controllable circularly polarized luminescence (CPL) and achieve CPL signal inversion is a challenge. While CPL switches can be achieved by modifying chiral molecules' structures or using external stimuli (e.g., concentration, temperature, solvent, and pH), a quantitative framework for modulating CPL signals, especially for inversion, remains absent. Herein, a theoretical approach combining density functional theory (DFT), time-dependent DFT, and thermal vibration correlation function theory to investigate the effects of pyridine nitrogen positions and protonation on the CPL performance of the aza-[7]helicene skeleton is presented. The findings show that protonation markedly narrows the energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), enhancing electronic properties and optoelectronic potential. It also induces redshifts in fluorescence and CPL signal reversals, modulating optical properties and decay pathways. The HOMO-LUMO transition is the main driver of spectral changes, with charge separation in protonated forms due to the pyridine group's electron-withdrawing effect. The position of pyridine nitrogen and protonation state influences chiroptical parameters, altering CPL signals without changing molecular configurations, thus impacting optoelectronic applications. Herein, insights into the structure-property relationship of aza-[7]helicenes derivatives and their protonated forms, guiding the rational design of helicenes featuring pH-triggered CPL switches, controllable CPL signals, and superior optoelectronics properties are offered.