INDEX

Perylene diimides and related compounds (naphthalene diimides, anthracene diimides, etc.) are one of the most important classes of organic dyes. Therefore, the prediction and the rationalization of both their transition energies and the particular shape of their absorption and emission spectra is essential to improve their design. Here, we report the simulations of both adiabatic and vibronic signatures of a series of perylene diimide derivatives with a state-of-the-art time-dependent density functional theory (TD-DFT) approach. First, the 0–0 energies have been computed and compared to experimental data. In a second stage, the determination of vibronic shapes has been performed to shed light on the vibrational modes implied in the experimental band topologies. Both anharmonicity and functionnal effects are also discussed. It turns out that theory consistently reproduced 0–0 energies but does not always yield band shapes in perfect match with experiment. In a last stage, new structures are designed, and it is shown that a full push effect is more effective than a push–pull strategy for the present class of molecules

Diogen Laertije - Životi i mišljenja istaknutih filozofa

The vast majority of polyhedral assemblies prepared by combining organic bent ligands and “photophysically innocent” palladium­(II) metal ions are nonemissive. We report here a simple strategy to switch on the luminescence properties of a polyhedral assembly by combining a thermally activated delayed fluorescence (TADF) organic emitter based on a dipyridylcarbazole ligand scaffold with Pd²⁺ ions, giving rise to a luminescent Pd₆L₁₂ molecular cube. The assembly is capable of encapsulating within its cavity up to three molecules per cage of fluorescein, in its neutral lactone form, and up to two molecules of Rose Bengal in its dianionic quinoidal form. Photoinduced electron transfer (PeT) between the photoactive cage and the encapsulated Fluorescein and photoinduced energy transfer (PET) from the cage to encapsulated Rose Bengal have been observed by steady-state and time-resolved emission spectroscopy

Excited-State Dipole and Quadrupole Moments: TD-DFT versus CC2

The accuracies of the excited-state dipole and quadrupole moments obtained by TD-DFT are assessed by considering 16 different exchange-correlation functionals and more than 30 medium and large molecules. Except for excited-state presenting a significant charge-transfer character, a relatively limited dependency on the nature of the functional is found. It also turns out that while DFT ground-state dipole moments tend to be too large, the reverse trend is obtained for their excited-state counterparts, at least when hybrid functionals are used. Consequently, the TD-DFT excess dipole moments are often too small, an error that can be fortuitously corrected for charge-transfer transition by selecting a pure or a hybrid functional containing a small share of exact exchange. This error-cancelation phenomena explains the contradictory conclusions obtained in previous investigations. Overall, the largest correlation between CC2 and TD-DFT excess dipoles is obtained with M06-2X, but at the price of a nearly systematic underestimation of this property by ca. 1 D. For the excess quadrupole moments, the average errors are of the order of 0.2–0.6 D·Å for the set of small aromatic systems treated

What is the Key for Accurate Absorption and Emission Calculations, Energy or Geometry?

Using a hierarchy of wave function methods, namely ADC(2), CC2, CCSD, CCSDR(3), and CC3, we investigate the absorption and emission energies in a set of 24 organic compounds. For all molecules, reference values are determined at the CC3//CC3 or CCSDR(3)//CCSDR(3) levels and the energetic and geometric effects are decomposed considering all possible methodological combinations between the five considered methods. For absorption, it is found that the errors are mainly energy-driven for ADC(2), CC2, and CCSDR(3), but not for CCSD. There is also an error compensation between the errors made on the geometries and transition energies for the two former approaches. For emission, the total errors are significantly larger than for absorption due to the significant increase of the structural component of the error. Therefore, the selection of a very refined method to compute the fluorescence energy will not systematically provide high accuracy if the excited-state geometry is not also optimized at a suitable level of theory. This is further demonstrated using results obtained from TD-DFT and hybrid TD-DFT/wave function protocols. We also found that, compared to full CC3, only CCSDR(3) is able to deliver errors below the 0.1 eV threshold, a statement holding for both absorption (mean absolute error of 0.033 eV) and emission (mean absolute error of 0.066 eV)

Structural and Optical Properties of Subporphyrinoids: A TD-DFT Study

Using <i>ab initio</i> approaches accounting for environmental effects, we investigate the ground- and excited-state properties of four subporphyrinoids: subporphyrin, subporphyrazine, tribenzosubporphyrin, and subphthalocyanine. We first show that the selected level of theory, that is DFT­(PBE0), is able to reproduce the structure and NMR spectra of all compounds. The aromaticity of these four macrocyclic entities are next quantified and it is showed that these bowl-shape induced molecules present very strong aromatic characters. Next we analyze the spectral signatures of all four compounds using an approach going beyond the vertical approximation. The 0–0 energies are reproduced with a mean absolute deviation smaller than 0.1 eV, and the very good agreement obtained between experimental and theoretical band shapes allows us to unravel the vibronic contributions responsible to the specific band shapes of these subporphyrinoids. Finally, we investigate a large series of substituted subporphyrins, demonstrate the quality of the trends that are obtained with theory and design new compounds presenting red-shifted optical bands

Tuning the Spectroscopic Properties of Ratiometric Fluorescent Metal Indicators: Experimental and Computational Studies on Mag-fura‑2 and Analogues

In this joint theoretical and experimental work, we investigate the properties of Mag-fura-2 and seven structurally related fluorescent sensors designed for the ratiometric detection of Mg²⁺ cations. The synthesis of three new compounds is described, and the absorption and emission spectra of all of the sensors in both their free and metal-bound forms are reported. A time-dependent density functional theory approach accounting for hydration effects using a hybrid implicit/explicit model is employed to calculate the absorption and fluorescence emission wavelengths, study the origins of the hypsochromic shift caused by metal binding for all of the sensors in this family, and investigate the auxochromic effects of various modifications of the “fura” core. The metal-free forms of the sensors are shown to undergo a strong intramolecular charge transfer upon light absorption, which is largely suppressed by metal complexation, resulting in predominantly locally excited states upon excitation of the metal complexes. Our computational protocol might aid in the design of new generations of fluorescent sensors with low-energy excitation and enhanced properties for ratiometric imaging of metal cations in biological samples

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

Grafting Spiropyran Molecular Switches on TiO₂: A First-Principles Study

To explore the optoelectronic properties of spiropyran molecular switches adsorbed onto TiO₂ anatase surfaces, we performed a density functional theory (DFT)/time-dependent density functional theory (TD-DFT) study considering the two isomeric forms of the photochromes anchored by both their sides. A comparison between the features of the hybrid and isolated systems is proposed to probe the adsorption effects on both subsystems. This comparison considered, on the one hand, the density of states and the alignment of the energy levels, and, on the other hand, the UV–visible spectra of these systems. We show that several electronic and optical characteristics of the hybrid systems are modulated by the open/closed state of the photochromes. These properties are also modified by the localization of the anchor group on the photochrome

How Adsorption Onto TiO₂ Modifies the Properties of Multiswitchable DTE Systems: Theoretical Insights

In order to best employ multiphotochromes as complex molecular gates, each isomer should ideally have a distinct optical profile to be selectively addressable. In this ab initio DFT and TD-DFT study, we have modeled the electronic and optical properties of a series of dithienylethene (DTE) dimers grafted onto an anatase (101) surface. We seek to investigate how grafting onto a TiO₂ surface modifies the energy levels and UV–visible spectra of the dimers and enhances the asymmetry of the isomers. By extracting information from the density of states, we have qualified the distinct degrees of interaction between the substrate and each isomeric configuration as CO > CC > OC > OO in order of decreasing electronic coupling. We subsequently use this information to interpret the UV–vis spectra computed for the isomers. The results show that the grafted systems present new peaks and shifted <i>S</i>₁ energies compared with the isolated photochrome, suggesting that adsorption onto a TiO₂ surface may induce an asymmetric character in the DTE dyad

Spectral Signatures of Perylene Diimide Derivatives: Insights From Theory

Perylene diimides and related compounds (naphthalene diimides, anthracene diimides, etc.) are one of the most important classes of organic dyes. Therefore, the prediction and the rationalization of both their transition energies and the particular shape of their absorption and emission spectra is essential to improve their design. Here, we report the simulations of both adiabatic and vibronic signatures of a series of perylene diimide derivatives with a state-of-the-art time-dependent density functional theory (TD-DFT) approach. First, the 0–0 energies have been computed and compared to experimental data. In a second stage, the determination of vibronic shapes has been performed to shed light on the vibrational modes implied in the experimental band topologies. Both anharmonicity and functionnal effects are also discussed. It turns out that theory consistently reproduced 0–0 energies but does not always yield band shapes in perfect match with experiment. In a last stage, new structures are designed, and it is shown that a full push effect is more effective than a push–pull strategy for the present class of molecules