CHIRality and luminescence switched ON: a joint theoretical and experimental endeavor – CHIRON

ChirON has explored explore the effect of substituent, chiral class and vibrational coupling onto the chiroptical responses in TMC emitters. To this aim, it has adopted a step-by-step methodology: from minimal structure benchmarks, representative of different types of chirality, to functionalized poly-metallic systems synthesized under the guidance of computational models for attaining remarkable photophysical and chiroptical activities. The powerful strength of ChirON stems from the joint and complementary efforts in state-of-the-art quantum chemistry, synthesis and (chiroptical) photophysical approaches bridging the gap between the fundamental understanding of excited state structures and the design of CPL molecules for next-generation of materials for CPL applications. Whereas well-defined computational strategies are available to interpret and predict luminescence in large, multimetallic, TMCs, deciphering CPL activity is especially challenging because driven by subtle excited-state properties. Besides the modelling of experimental ECD and CPL spectra, theoretical chemistry will contribute at their interpretation providing valuable information about the various chemical and electronic contributions to the chiroptical properties. One step further will be to investigate their correlation with nuclear arrangement and with nuclear motion, thus considering conformational flexibility and vibronic coupling effects. Several aspects have barely been investigated in the context of chiroptically-active TMCs such as vibronic, spin-orbit or excitonic couplings. Another open question was the simultaneous optimization of the PLQY and of the dissymmetry factor gLum that traduce the polarization efficiency (difference between left and right polarized emission over half of the total emission). Is our knowledge based on qualitative concepts developed for organic chromophores valuable for TMCs? This is not always true, especially when the usual S1/T1 model is not valid because of significant nuclear rearrangements in excited states, strong coupling and/or mixing between low-lying excited states. The ultimate goal of ChirOn is to build structure/properties relationships between the molecular architecture, the CPL activity, the PLQY and the dissymmetry factor gLum.

Through the study of a set of chiral Re(I) halide complexes of the type [fac-ReX(CO)3L] (X being Cl or I and L being a N-heterocyclic (NHC) or helicenic-NHC ligands, respectively], a computational protocol was successfully established to compute the targeted properties. The shapes of the absorption and the phosphorescence spectra of the whole set of enantiomers associated with the chosen chiral Re(I) molecules were computed. In particular, besides absorption and emission transition rates for generating ABS and PL spectra, ESD was extended to be able to treat circularly polarized radiative and non-radiative transition rates, providing access to the computation of ECD, CPL spectra, and photoluminescent quantum yield (PLQY) values. The very good agreement between theory and experiment allowed a quantitative electronic structure analysis of all the observed bands.

A novel series of photoactive Re(I) tricarbonyl complexes bearing a pyridyl NHC ligand were investigated. The compounds display long-lived and structured red phosphorescence arising from an excited-state with 3LC/3MLCT/3XLCT character. It has been shown that stronger ECD signal is provided by transitions with larger LC character as illustrated for the iodo complexes when substituting the pyridyl moiety. Polar solvents such as acetone seem to induce significant hypsochromic shift of the optical spectra and large rotatory strengths while SOC effects lead to batochromic shift via singlet/triplet mixing of the low-lying absorbing states. As confirmed experimentally, the potentially luminescent low-lying triplet states undergo important electronic reorganization upon nuclear relaxation. Finally, experimental chiroptical properties for samples in solution show that all the investigated complexes display similar features in both ECD and CPL spectra. Although |glum| values are still moderate, they are only slightly lower than those reported for related helicenic–NHC Re(I) derivatives, highlighting the important role exerted by the partial LC nature (amongst others) of the emissive excited state for achieving CPL emission.

Two enantiopure heterobimetallic Ir(III)-Au(I) complexes were then characterized by means of spectroscopic techniques. For one of the derivatives, the partial structural resolution allowed to unambiguously confirm the absolute configuration of the chirality at metal imparted by the helical arrangement of the tris-chelated coordination sphere. The bimetallic complexes display strikingly different optical and electronic properties compared to the monometallic Ir(III) parental complex. The excited state of the former possesses a largely mixed 3MLCT/3LLCT character that opens for more efficient radiationless deactivation channels, and in turn provides much lower PLQY and shorter-lived excited state lifetime. On the other hand, it may be tentatively accounted for the twofold increase of the gLum factor observed for the bimetallic complexes.

Thanks to the project ChirON a full computational protocol has been established for the computation of the photophysical properties of chiral metallic complexes. Though the current protocol requires time and memory consuming steps to computed the hessian of the excited states which limits the size of the treatable systems. Further developments are required to optimize the protocol. However, systematic investigations of medium sized systems (up to 100 atoms) are already possible. In a short term, we will deepen our analysis on the effect of the excited state nature (Ligand-centered versus metal-to-ligand charge transfer state) and of other parameter on the ECD or CPL properties. We will try to make structural-properties relationship in an attempt to help experimentalists in the design of more efficient CPL emitters complexes. In addition, first attempts of correlation between nature of the emissive excited state, in particular in terms of radiative rate constant and symmetry, and chiroptical features (gabs, gLum) were made that will pave the way for the better understating of design principles for emitters with improved CPL properties.

The interdisciplinary project ChirON aims at disclosing fundamental understandings of the parameters that govern chiroptical properties in phosphorescent transition metal complexes (TMC), Ir(III) and Pt(II), via state-of-the-art computationally-guided rational design of the chiral emitters (either axial, planar and helical): an open pathway for TMC emitters. Furthermore, the project aims at the experimental validation of this approach as well as at the challenging preparation of robust, efficient, enantiomerically pure, mono- and multi-metallic TMC-based circularly polarized light emitters (CPL). These new TMC will be characterized by enhanced emission dissymmetry factors suitable for efficient CPL optoelectronics. Ultimately, the purpose of ChirON is to discover generalized rules for tailoring improved chiroptical properties subsequently applied to low-cost more abundant metals.