Helicene Appended Benzothiadiazoles as Chiral Emitters.

A homologous series of 4,7-bis(aryl) substituted benzothiadiazole (BTD) compounds, containing the helicenic derivatives bis([4]helicene), bis([5]helicene) and bis([6]helicene), have been prepared upon a double Suzuki coupling between 3,6-bis(pinacolyl-borane)-BTD and the corresponding bromo-aryl precursors. The single crystal X-ray structure of the bis([4]helicene) compound shows the existence of both helicities (M) and (P) on the same molecule. All the compounds of the series are highly emissive in solution, with quantum yields of the emission ranging from 50 to 91 %. The enantiopure compounds (M,M) and (P,P) for the BTD-bis([6]helicene) have been prepared from the corresponding enantiopure 2-bromo-[6]helicene precursors. Their chiroptical properties have been investigated in correlation with density functional theory (DFT) calculations, which allowed to confidently assign the absolute configuration of the helicene arms and to characterize the different electronic transitions, including the low energy charge transfer excitation from helicenes to BTD. The enantiomerically pure fluorophores (M,M)- and (P,P)-BTD-bis([6]helicene), which exist in solution as two main conformers, according to the DFT calculations, show CPL activity in solution, with glum factors of ≈1.7×10-3 at λem=525 nm, and also in the solid state, with glum factors of ≈1.2×10-3 in spite of the strong decrease of the quantum efficiency.

Compact Basis Sets for Optical Rotation Calculations.

In this work, we present two compact basis sets optimized for the calculation of specific rotation: augD-3-21G and augT3-3-21G. They are obtained by combining the standard 3-21G basis set with the diffuse functions of aug-cc-pVDZ and aug-cc-pVTZ, respectively, followed by a reoptimization of the exponents of the diffuse functions. The exponent optimization is based on minimization of the root-mean-square relative error (RMSE) of the specific rotation computed at 589.3 nm (the sodium D line, [α]D) with CAM-B3LYP compared with the corresponding calculations using the full correlation-consistent basis sets. The training set comprises 21 chiral molecules with |[α]D| > 50 deg dm-1 (g/mL)-1. For augT3-3-21G, the functions with the highest angular momentum are neglected, so that augD-3-21G and augT3-3-21G are of the same size. The exponents are optimized for four common elements in chiral organic molecules (H, C, N, and O), while the original exponents are maintained for other elements. Tests are conducted on the training set with CAM-B3LYP at 450 and 633 nm and with B3LYP at 589.3 nm; furthermore, a similar comparison is performed on a control set containing 30 more chiral molecules. A comparison with the optical rotatory prediction (ORP) basis set is also presented. The results show that the new compact basis sets are able to reproduce the calculations with the full Dunning basis sets remarkably well, and definitely better than before reoptimization of the exponents, with relative mean unsigned errors of around 4%. More significantly, augT3-3-21G is either of similar quality or better than aug-cc-pVDZ in reproducing the values obtained with aug-cc-pVTZ, even though augT3-3-21G is smaller than aug-cc-pVDZ. The larger ORP basis set outperforms both augT3-3-21G and aug-cc-pVDZ, but it requires a considerably larger computational effort. In summary, augT3-3-21G provides results that are in very good agreement with those obtained using aug-cc-pVTZ, but approximately 20 times faster, and it may be used for quick and reliable calculations of specific rotation of large chiral molecules.

A molecular orbital selection approach for fast calculations of specific rotation with density functional theory.

In this work, we describe a simple approach to select the most important molecular orbitals (MOs) to compute the optical rotation tensor through linear response (LR) Kohn-Sham density functional theory (KS-DFT). Taking advantage of the iterative nature of the algorithms commonly used to solve the LR equations, we select the MOs with contributions to the guess perturbed density that are larger than a certain threshold and solve the LR equations with the selected MOs only. We propose two criteria for the selection, and two definitions of the selection threshold. We then test the approach with two functionals (B3LYP and CAM-B3LYP) and two basis sets (aug-cc-pVDZ and aug-cc-pVTZ) on a set of 51 organic molecules with specific rotation spanning five orders of magnitude, 100 -104 deg (dm-1 (g/mL)-1 ). We show that this approach indeed can provide very accurate values of specific rotation with estimated speedup that ranges from 2 to 8× with the most conservative selection criterion, and up to 20 to 30× with the intermediate criterion.

Configuration Space Analysis of the Specific Rotation of Helicenes.

In this work, we present an analysis of a series of helicene molecules to determine the driving forces for their large specific rotation, [α]ω, and probe the effects of functionalization. The analysis is done in the configuration space of the molecular orbitals (MOs), and it allows us to decompose [α]ω into the component transition electric and magnetic dipoles from single MO excitations. We find that [α]ω for helicene molecules may be described by three sets of transitions based on the orientation of the magnetic dipole with respect to the helical axis: parallel, orthogonal, or tilted. The transitions with the magnetic dipole parallel to the helical axis, corresponding to a delocalized motion of the electron along the body of the helix, provide the largest contributions and determine the sign and magnitude of [α]ω. Functionalization has a complex effect on [α]ω, which is dependent on the number of substituent groups and their electron directing strength. Furthermore, we test the [α]ω decomposition analysis using localized MOs (Boys and Pipek-Mezey). We show that localization schemes may be useful to simplify the interpretation of the [α]ω decomposition, but they are best used when the electronic transitions involve relatively small chromophoric groups.

Comparison of measured and predicted specific optical rotation in gas and solution phases: A test for the polarizable continuum model of solvation.

A comparative theoretical and experimental study of dispersive optical activity is presented for a set of small, rigid organic molecules in gas and solution phases. Target species were chosen to facilitate wavelength-resolved measurements of specific rotation in rarefied vapors and in organic solvents having different polarities, while avoiding complications due to conformational flexibility. Calculations were performed with two density functionals (B3LYP and CAM-B3LYP) and with the coupled-cluster singles and doubles (CCSD) ansatz, and solvent effects were included through use of the polarizable continuum model (PCM). Across the various theoretical methods surveyed, CCSD with the modified velocity gauge provided the best overall performance for both isolated and solvated conditions. Zero-point vibrational corrections to equilibrium calculations of chiroptical response tended to improve agreement with gas-phase experiments, but the quality of performance realized for solutions varied markedly. Direct comparison of measured and predicted specific-rotation suggests that PCM, in general, is not able to reproduce attendant solvent shifts (neither between gas and solution phases nor among solvents) and fares better in estimating actual medium-dependent values of this property (although the error is rather system dependent). Thus, more elaborate solvation models seem necessary for a proper theoretical description of solvation in dispersive optical activity.

Triggering Emission with the Helical Turn in Thiadiazole-Helicenes.

Introduction of heterocycles into the helical skeleton of helicenes allows modulation of their redox, chiroptical, and photophysical properties. This paper describes the straightforward preparation and structural characterization by single-crystal X-ray diffraction of thiadiazole-[7]helicene, which was resolved into M and P enantiomers by chiral HPLC, together with its S-shaped double [4]helicene isomer, as well as the smaller congeners thiadiazole-[5]helicene and benzothiadiazole-anthracene. A copper(II) complex with two thiadiazole-[5]helicene ligands was structurally characterized, and it shows the presence of both M and P isomers coordinated to the metal center. The emission properties of the heterohelicenes are highly dependent on the helical turn, as the [7]- and [5]helicene are poorly emissive, whereas their isomers, that is, the S-shaped double [4]helicene and thiadiazole-benzanthracene, are luminescent, with quantum efficiencies of 5.4 and 6.5 %, respectively. DFT calculations suggest quenching of the luminescence of enantiopure [7]helicenes through an intersystem-crossing mechanism arising from the relaxed excited S1 state.