Long‐Lived Circularly Polarized Phosphorescence in Helicene‐NHC Rhenium(I) Complexes: The Influence of Helicene, Halogen, and Stereochemistry on Emission Properties

Abstract

The first enantiopure chiral‐at‐rhenium complexes of the form fac‐ReX(CO)3(:C^N) have been prepared, where :C^N is a helicene‐N‐heterocyclic carbene (NHC) ligand and X=Cl or I. These have complexes show strong changes in the emission characteristics, notably strongly enhanced phosphorescence lifetimes (reaching 0.7 ms) and increased circularly polarized emission (CPL) activity, as compared to their parent chiral models lacking the helicene unit. The halogen along with its position within the dissymmetric stereochemical environment strongly affect the photophysics of the complexes, particularly the phosphorescence quantum yield and lifetime. These results give fresh insight into fine tuning of photophysical and chiroptical properties of Re‐NHC systems.

Modulation of Chiroptical and Photophysical Properties in Helicenic Rhenium(I) Systems: The Use of an N‐(Aza[6]helicenyl)‐NHC Ligand

“… Examples of helicene‐Re(I) complexes reported in the literature ( A and B ) [33,34] and the novel structure described in this work ( C ). Model non‐helicenic systems D with the corresponding A Re / C Re stereodescriptors are also depicted [33] …”

“…Furthermore, the first chiral NHC−Re(I) complexes, A‐I and A‐Cl , were reported by us in 2020, using an N ‐(pyridyl)‐[5]helicene‐imidazolylidene as the : $${C^ \wedge N}$$ ligand ( A‐X in Figure 1). [33] Investigation of the photophysical and chiroptical properties revealed appealing features such as: i) intense electronic circular dichroism (ECD), ii) green circularly polarized luminescence (CPL) with dissymmetry factors, g lum , up to ±5×10 −3 , and iii) long emission lifetimes up to 0.7 ms, which could be tuned by subtle structural modulations including the identity of the halide ligand and the stereochemical environment. The very first example of a helicenic Re(I) complex bearing an $${N^ \wedge N}$$ ‐coordinating [6]helicene‐bipyridine ligand, B in Figure 1, was also prepared by us, in 2015, and found to display CPL in the red spectral region [34] .…”

Beyond Chiral Organic (p-Block) Chromophores for Circularly Polarized Luminescence: The Success of d-Block and f-Block Chiral Complexes

“…More recently, the complex 43 was obtained through the coordination of a helicenic ligand to a Re I metal center with intrinsic achiral coordination sphere (Saleh et al, 2015b). Although this straightforward method provides enantiopure complexes, the associated dissymmetry factors remain weak, and only few further synthesis were performed for CPL application (Gauthier et al, 2020). Finally ligand-centered CPL were casually tuned or switched by a closed shell d-block metal like Ag I or Au I .…”

Strong Coupling of Chiral Frenkel Exciton for Intense, Bisignate Circularly Polarized Luminescence

“…The recent focus put on circularly polarized luminescence (CPL) is driven both by fundamental and applied perspectives [1–3] . Fundamentally, CPL offers a unique way of probing the chiroptical features of molecular excited states, and for understanding how these features emerge from and interact with, the stereochemical structure of chiral molecules [4, 5] . From an applied perspective, the possibility to tailor polarization dissymmetry of light emitting systems contributes to the definition of novel optical technologies, where for instance CPL plays a key role for enhancing the contrast of conventional organic light‐emitting diode displays and thereby their energy efficiency, for the development of chiral lasers, all‐optical information processing strategies, chiral microscopy and for chiral sensing [6–13] …”

“…[1][2][3] Fundamentally, CPL offers a unique way of probing the chiroptical features of molecular excited states, and for understanding how these features emerge from and interact with, the stereochemical structure of chiral molecules. [4,5] From an applied perspective, the possibility to tailor polarization dissymmetry of light emitting systems contributes to the definition of novel optical technologies, where for instance CPL plays a key role for enhancing the contrast of conventional organic light-emitting diode displays and thereby their energy efficiency, for the development of chiral lasers, all-optical information processing strategies, chiral microscopy and for chiral sensing. [6][7][8][9][10][11][12][13] The strength of CPL is determined by the luminescence dissymmetry factor, g lum = 2(I L À I R )/(I L + I R ) where I L and I R are the left-and right-handed emission intensities.…”