Accurate Excited-State Geometries: A CASPT2 and Coupled-Cluster Reference Database for Small Molecules

We present an investigation of the excited-state structural parameters determined for a large set of small compounds with the dual goals of defining reference values for further works and assessing the quality of the geometries obtained with relatively cheap computational approaches. In the first stage, we compare the excited-state geometries obtained with ADC(2), CC2, CCSD, CCSDR(3), CC3, and CASPT2 and large atomic basis sets. It is found that CASPT2 and CC3 results are generally in very good agreement with one another (typical differences of ca. 3 × 10^–3 Å) when all electrons are correlated and when the aug-cc-pVTZ atomic basis set is employed with both methods. In a second stage, a statistical analysis reveals that, on the one hand, the excited-state (ES) bond lengths are much more sensitive to the selected level of theory than their ground-state (GS) counterparts and, on the other hand, that CCSDR(3) is probably the most cost-effective method delivering accurate structures. Indeed, CCSD tends to provide too compact multiple bond lengths on an almost systematic basis, whereas both CC2 and ADC(2) tend to exaggerate these bond distances, with more erratic error patterns, especially for the latter method. The deviations are particularly marked for the polarized CO and CN bonds, as well as for the puckering angle in formaldehyde homologues. In the last part of this contribution, we provide a series of CCSDR(3) GS and ES geometries of medium-sized molecules to be used as references in further investigations

Unveiling Solvents Effect on Excited-State Polarizabilities with the Corrected Linear-Response Model

Aiming to assess the environmental effects on the dipole moments and polarizabilities of electronically excited-states, we have applied a combined Polarizable Continuum Model/Time-Dependent Density Functional Theory (PCM/TD-DFT) approach on six representative chromophores. For the first time, we compare polarizabilities obtained with gas phase, linear-response and corrected linear response continuum models and we also investigate the relative importance of direct (electronic) and indirect (geometric) environmental contributions for these properties. It is shown that the solvent effects on excited-state polarizabilities tend to be large and can often, but not always, be captured with the computationally efficient linear-response formalism

Calculations of <i>n</i>→π* Transition Energies: Comparisons Between TD-DFT, ADC, CC, CASPT2, and BSE/<i>GW</i> Descriptions

Using a large panel of theoretical approaches, namely, CC2, CCSD, CCSDR(3), CC3, ADC(2), ADC(3), CASPT2, time-dependent density functional theory (TD-DFT), and BSE/ev<i>GW</i>, the two latter combined with different exchange-correlation functionals, we investigate the lowest singlet transition in 23 <i>n</i>→π* compounds based on the nitroso, thiocarbonyl, carbonyl, and diazo chromophores. First, for 16 small derivatives we compare the transition energies provided by the different wave function approaches to define theoretical best estimates. For this set, it surprisingly turned out that ADC(2) offers a better match with CC3 than ADC(3). Next, we use 10 functionals belonging to the “LYP” and “M06” families and compare the TD-DFT and the BSE/ev<i>GW</i> descriptions. The BSE/ev<i>GW</i> results are less sensitive than their TD-DFT counterparts to the selected functional, especially in the M06 series. Nevertheless, BSE/ev<i>GW</i> delivers larger errors than TD-CAM-B3LYP, which provides extremely accurate results in the present case, especially when the Tamm–Dancoff approximation is applied. In addition, we show that, among the different starting points for BSE/ev<i>GW</i> calculations, M06-2X eigenstates stand as the most appropriate. Finally, we confirm that the trends observed on the small compounds pertain in larger molecules

Synthesis and Photophysical Properties of Novel Donor–Acceptor <i>N</i>‑(Pyridin-2-yl)-Substituted Benzo(thio)amides and Their Difluoroboranyl Derivatives

The unprecedented <i>N</i>-pyridin-2-yl substituted benzo­(thio)­amides were prepared and subsequently converted into the cyclic difluoroboranyl (BF₂) derivatives. Mass spectrometry, multinuclear NMR, IR, and elemental analysis confirmed the structure of these compounds. UV/vis and fluorescence spectroscopy as well as first-principle calculations were used to study their properties. For the first time, the influence of both the O/S replacement and presence/absence of the BF₂ moiety on the photophysical properties of compounds exhibiting charge transfer properties were examined experimentally and theoretically. We show that the sulfur-containing compound has a much smaller emission quantum yield than its oxygen counterpart. The fluorescence quantum yield is much higher upon formation of the difluoroboranyl complex