Vibronic effect explains ECD spectral shape and tunability of CPL wavelength of two triarylborane-based [5]helicenes derivatives

Abstract

Substituents and solvent could affect the chiroptical properties of electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) of helicenes. To analyze those effects and unveil all the information carried by the experimental spectra, herein, we report a computational study on optical properties of two optically CPL-active triarylborane-based [5]helicenes derivatives. The pure electronic calculations suggest that the emission wavelength can be tuned by the modification of chemical structure via adjusting the key Molecular Orbital (MO) gaps. However, they alone show expected limitation in reproducing the experimental spectral shape and capturing the fine structures. Thus, more attention is payed on the vibronic effects. To compute vibrationally resolved spectra, the popular, accurate and quite efficient models of Adiabatic Hessian (AH) and Vertical Hessian (VH) are used to build the potential energy surfaces (PESs) in harmonic approximation. Duschinsky mixing, Herzberg-Teller (HT) contribution and temperature effect are considered comprehensively. Cartesian and internal coordinates are compared to express the normal modes since the adopted coordinate can affect the accuracy of the harmonic approximation. Our results show that VH (internal) gives more reliable simulation, predicting with correct sign, reasonable spectral shape, and acceptable spectral width, in very nice agreement with the experiment. We show that HT effect is rather important in capturing the experimental fine structures. What is more, vibronic effect plays a crucial role in revealing the substituent effect on chiroptical properties and solvent effect on CPL maximum frequency.