Insights into the Optical Properties of Triarylboranes with Strongly Electron‐Accepting Bis(fluoromesityl)boryl Groups: when Theory Meets Experiment

Houmam Belaidi,Camille Latouche,Abdou Boucekkine,Todd B Marder,Florian Rauch,Jean‐François Halet,Zuolun Zhang

doi:10.1002/cptc.201900256

Abstract

AbstractThe photophysical properties (absorption, fluorescence and phosphorescence) of a series of triarylboranes of the form 4‐D−C6H4−B(Ar)2 (D=tBu or NPh2; Ar=mesityl (Mes) or 2,4,6‐tris(trifluoromethylphenyl (Fmes)) were analyzed theoretically using state‐of‐the‐art DFT and TD‐DFT methods. Simulated emission spectra and computed decay rate constants are in very good agreement with the experimental data. Unrestricted electronic computations including vibronic contributions explain the unusual optical behavior of 4‐tBu−C6H4−B(Fmes)2 2, which shows both fluorescence and phosphorescence at nearly identical energies (at 77 K in a frozen glass). Analysis of the main normal modes responsible for the phosphorescence vibrational fine structure indicates that the bulky tert‐butyl group tethered to the phenyl ring is strongly involved. Interestingly, in THF solvent, the computed energies of the singlet and triplet excited states are very similar for compound 2 only, which may explain why 2 shows phosphorescence in contrast to the other members of the series.

Highlights

Due to their high Lewis acidity, these compounds need to be protected from nucleophilic attack
Simulated fluorescence spectra and computed decay rate constants are in very good agreement with the available experimental data for compounds 2–4
Unrestricted electronic computations coupled with vibronic contributions allowed us to

Summary

Introduction

Due to their high Lewis acidity, these compounds need to be protected from nucleophilic attack. This paper reports state-of-the-art density functional theory (DFT) computations which offer additional insight into the optical and electronic properties of compounds 1–4. Simulation of the optical spectra was 45 obtained using the VMS software.[74] The phosphorescence spec trum of 2 was simulated including vibronic coupling within the Adiabatic Hessian (AH) model This model, which explicitly includes mode-mixing between fundamental and excited states, reproduces accurately the optical properties of molecular systems.[75,76,77,78,79,80] To ensure a sufficient spectrum progression (> 90 %), normal modes with the lowest energies were neglected in the vibronic treatment

Ground-State Molecular Structures

Absorption Properties

Emission – Fluorescence

Radiative Decays and Fluorescence Lifetimes

Emission – Phosphorescence

Conclusion