Acid/base-triggered photophysical and chiroptical switching in a series of helicenoid compounds

International audienceA series of molecules that possess two quinolines, benzoquinolines, or phenanthrolines connected in a chiral fashion by a biaryl junction along with their water-soluble derivatives was developed and characterized. The influence of the structure on the basicity of the nitrogen atoms in two heterocycles was examined and the photophysical and chiroptical switching activity of the compounds upon protonation was studied both experimentally and computationally. The results demonstrated that changes in the electronic structure of the protonated vs. neutral species, promoting a bathochromic shift of dominant electronic transitions and alternation of their character from π-to-π* to charge-transfer-type, when additionally accompanied by the high structural flexibility of a system, leading to changes in conformational preferences upon proton binding, produce particularly pronounced modifications of the spectral properties in acidic medium. The latter combined with reversibility of the read-out make some of the molecules in this series very promising multifunctional pH probes

Modulation of chiroptical and photophysical properties in helicenic rhenium(I) systems: the use of an N‐(aza[6]helicenyl)‐NHC ligand

The photophysical and chiroptical properties of a novel, chiral helicene-NHC−Re(I) complex bearing an N-(aza[6]helicenyl)-benzimidazolylidene ligand are described, showing its ability to emit yellow circularly polarized luminescence. A comparative analysis of this new system with other helicene-Re(I) complexes reported to date illustrates the impact of structural modifications on the emissive and absorptive properties

Synthesis, structural characterization, and chiroptical properties of planarly and axially chiral boranils

International audience2-Amino[2.2]paracyclophane reacts with salicylaldehyde or 2-hydroxyacetophenone to yield imines that then give access to a new series of boranils (8b–d) upon complexation with BF2. These novel boron-containing compounds display both planar and axial chiralities and were examined experimentally and computationally. In particular, their photophysical and chiroptical properties were studied and compared to newly prepared, simpler boranils (9a–d) exhibiting axial chirality only. Less sophisticated chiral architectures were shown to demonstrate overall stronger circularly polarized luminescence (CPL) activity

Tuned range-separated time-dependent density functional theory applied to optical rotation

For range-separated hybrid density functionals, the consequences of using system-speci fi c range-separation parameters ( γ ) in calculations of optical rotations (ORs) are investigated. Computed ORs at three wavelengths are reported for methyloxirane, norbornenone, β -pinene, [6]helicene, [7]helicene, and two derivatives of [6]helicene. The γ parameters are adjusted such that Kohn Sham density functional calculations satisfy the condition ε HOMO ( N ) = IP. For β -pinene, the behavior of the energy as a function of fractional total charge is also tested. For the test set of molecules, comparisons of ORs with available coupled-cluster and experimental data indicate that the γ “ tuning ” leads to improved results for β -pinene and the helicenes and does not do too much harm in other cases

Delocalization error and "functional tuning" in Kohn-Sham calculations of molecular properties

Kohn–Sham theory (KST) is the “workhorse” of numerical quantum chemistry. This is particularly true for first-principles calculations of ground- and excited-state properties for larger systems, including electronic spectra, electronic dynamic and static linear and higher order response properties (including nonlinear optical (NLO) properties), conformational or dynamic averaging of spectra and response properties, or properties that are affected by the coupling of electron and nuclear motion. This Account explores the sometimes dramatic impact of the delocalization error (DE) and possible benefits from the use of long-range corrections (LC) and “tuning” of functionals in KST calculations of molecular ground-state and response properties. Tuning refers to a nonempirical molecule-specific determination of adjustable parameters in functionals to satisfy known exact conditions, for instance, that the energy of the highest occupied molecular orbital (HOMO) should be equal to the negative vertical ionization potential (IP) or that the energy as a function of fractional electron numbers should afford straight-line segments. The presentation is given from the viewpoint of a chemist interested in computations of a variety of molecular optical and spectroscopic properties and of a theoretician developing methods for computing such properties with KST. In recent years, the use of LC functionals, functional tuning, and quantifying the DE explicitly have provided valuable insight regarding the performance of KST for molecular properties. We discuss a number of different molecular properties, with examples from recent studies from our laboratory and related literature. The selected properties probe different aspects of molecular electronic structure. Electric field gradients and hyperfine coupling constants can be exquisitely sensitive to the DE because it affects the ground-state electron density and spin density distributions. For π-conjugated molecules, it is shown how the DE manifests itself either in too strong or too weak delocalization of localized molecular orbitals (LMOs). Optical rotation is an electric–magnetic linear response property that is calculated in a similar fashion as the electric polarizability, but it is more sensitive to approximations and can benefit greatly from tuning and small DE. Hyperpolarizabilities of π-conjugated “push–pull” systems are examples of NLO properties that can be greatly improved by tuning of range-separated exchange (RSE) functionals, in part due to improved charge-transfer excitation energies. On-going work on band gap predictions is also mentioned. The findings may provide clues for future improvements of KST because different molecular properties exhibit varying sensitivity to approximations in the electronic structure model. The utility of analyzing molecular properties and the impact of the DE in terms of LMOs, representing “chemist’s orbitals” such as individual lone pairs and bonds, is highlighted

Does a molecule-specific density functional give an accurate electron density? : the challenging case of the CuCl electric field gradient

In the framework of determining system-specific long-range corrected density functionals, the question is addressed whether such functionals, tuned to satisfy the condition − ε HOMO = IP or other energetic criteria, provide accurate electron densities. A nonempirical physically motivated two-dimensional tuning of range-separated hybrid functionals is proposed and applied to the particularly challenging case of a molecular property that depends directly on the ground-state density: the copper electric field gradient (EFG) in CuCl. From a continuous range of functional parametrizations that closely satisfy − ε HOMO = IP and the correct asymptotic behavior of the potential, the one that best fulfills the straight-line behavior of E ( N ), the energy as a function of a fractional electron number N , was found to provide the most accurate electron density as evidenced by calculated EFGs. The functional also performs well for related Cu systems

Computational analysis of 47/49^{47/49}Ti NMR shifts and electric field gradient tensors of half-titanocene complexes : structure-bonding-property relationships

Metal NMR shielding and electric-field gradient (EFG) tensors are examined by quantum-chemical calculations for a set of 14 titanium(IV) complexes. Benchmarks are performed for titanocene chlorides that have been characterized recently by solid-state NMR experiments, focusing on the dependence of TiIV NMR parameters on the computational model in terms of the choice of the density functional, and considering molecular clusters versus infinite-periodic solid. Nearest-neighbor and long-range effects in the solid state are found to influence NMR parameters in systems without spatially extended ligands. Bulky ligands increase the fraction of local structure and bonding information encoded in the EFG tensors by reducing intermolecular interactions. Next, Ti shielding constants and EFG tensors for a variety of olefin (co)polymerization catalysts are analyzed in terms of contributions from localized molecular orbitals representing Lewis bonds and lone pairs. Direct links between the observed theoretical trends and the local bonding environment around the Ti metal center are found. A specific dependence of the Ti EFG tensors on the exact arrangement and type of surrounding bonds is demonstrated, providing a basis for further studies on solid-supported titanium catalytic systems