Understanding the modulation of the electronic coupling in donor–acceptor systems connected through an aliphatic bridge is crucial from a fundamental point of view as well as for the development of organic electronics. In this work, we present a first-principles approach for the calculation of the electronic coupling (or transfer integrals) in such systems via a block-diagonalization of the Fock/Kohn–Sham matrix of the supersystem, followed by a projection on the basis of the fragment orbitals of the donor and acceptor groups. The strength of the approach is that the bridge is shared by the donor and acceptor blocks in the diagonalization step, so that through-space and through-bond couplings are obtained simultaneously. The method is applied to two test sets: a series of fused-ring bridged systems and G(T)nG DNA oligomers. The results for the first set are compared to experiment and show an average error lower than 10%. For the DNA set, we show that the coupling may be significantly larger (and the decay with length slower) when the entire backbone is included.

Femtosecond stimulated Raman scattering (FSRS) probes the structural dynamics of molecules in electronically excited states by following the evolution of the vibrational spectrum. Interpreting the dynamics requires accurate assignments to connect the vibrational bands with specific nuclear motions of an excited molecule. However, the assignment of FSRS signals is often complicated by mode-specific resonance enhancement effects that are difficult to calculate for molecules in electronically excited states. We present benchmark results for a series of eight aryl-substituted thiophene derivatives to show that calculated off-resonance Raman spectra can be used to assign experimental bands on the basis of a comparison of structurally similar compounds and careful consideration of the resonance condition. Importantly, we show that direct comparison with the off-resonant calculations can lead to incorrect assignments of the experimental spectrum if the resonance condition is neglected. These results highlight the importance of resonance enhancement effects in assigning FSRS spectra.

AbstractIntroduction 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.

In this work, we benchmark the equation of motion coupled cluster with single and double excitations (EOM-CCSD) method combined with the polarizable continuum model (PCM) for the calculation of electronic excitation energies of solvated molecules. EOM-CCSD is one of the most accurate methods for computing one-electron excitation energies, and accounting for the solvent effect on this property is a key challenge. PCM is one of the most widely employed solvation models due to its adaptability to virtually any solute and its efficient implementation with density functional theory methods (DFT). Our goal in this work is to evaluate the reliability of EOM-CCSD-PCM, especially compared to time-dependent DFT-PCM (TDDFT-PCM). Comparisons between calculated and experimental excitation energies show that EOM-CCSD-PCM consistently overestimates experimental results by 0.4–0.5 eV, which is larger than the expected EOM-CCSD error in vacuo. We attribute this decrease in accuracy to the approximated solvation model. Thus, we investigate a particularly important source of error: the lack of H-bonding interactions in PCM. We show that this issue can be addressed by computing an energy shift, ΔHB, from bare-PCM to microsolvation + PCM at DFT level. Our results show that such a shift is independent of the functional used, contrary to the absolute value of the excitation energy. Hence, we suggest an efficient protocol where the EOM-CCSD-PCM transition energy is corrected by ΔHB(DFT), which consistently improves the agreement with the experimental measurements.

Understanding the modulation of the electronic coupling in molecular crystals and two-dimensional materials is crucial from a fundamental point of view as well as for the development of organic electronics. In this work, we present a first-principles quantum-mechanical method for the calculation of the electronic coupling (or transfer integrals) between fragments or layers, using density functional theory with periodic boundary conditions (DFT-PBC) within the Γ-point approximation. This method is applied to two periodic systems: crystalline pentacene and a bilayer graphene film. For the former system, we find that the inclusion of the solid environment affects the interfragment electronic couplings, with changes of the order of 10%. However, we confirm the qualitative trends obtained with the “isolated molecular dimer” model. For the graphene film, we show how the interlayer coupling changes with the relative position of two π-stacked layers. Interestingly, we find that particle–particle coupling is large even for configurations that are not perfectly stacked.

Quantum mechanical (QM) calculations of electronic couplings provide great insights for the study of resonance energy transfer (RET). However, most of these calculations rely on approximate QM methods due to the computational limitations imposed by the size of typical donor–acceptor systems. In this work, we present a novel implementation that allows computing electronic couplings at the coupled cluster singles and doubles (CCSD) level of theory. Solvent effects are also taken into account through the polarizable continuum model (PCM). As a test case, we use a dimer of indole, a common model system for tryptophan, which is routinely used as an intrinsic fluorophore in Förster resonance energy transfer studies. We consider two bright π → π* states, one of which has charge transfer character. Lastly, the results are compared with those obtained by applying TD-DFT in combination with one of the most popular density functionals, B3LYP.

We present a method to analyze the origin of molecular optical activity in terms of orbital contributions and rotatory strength in configuration space. The method uses quantities already available at completion of standard linear-response calculations of specific rotation and requires minimal manipulation. Preliminary application to (1S,4S)-norbornenone and (P)-2,3-pentadiene shows that only a few orbitals (6 and 4, respectively) contribute significantly to the specific rotation and can be used directly for a qualitative interpretation of this fundamental property.

In this work, we study the difference in specific rotation of the stable conformers of two test chiral molecules: (S)-(+)-2-carene and (R)-3-methylcyclopentanone. We perform the analysis of the specific rotation in terms of rotational strength in configuration space, S̃k, which provides information about the contribution of occupied-virtual molecular orbital pairs to this property. We show that, although a considerable number of excited configurations contribute to the total value of the specific rotation, only a limited number of configurations are necessary to explain the different sign and magnitude of the rotation between different conformers. The results in this work thus offer a promising picture for our ability to better understand and possibly predict the value of specific rotation of chiral molecules.