Electron Localization Function in Excited States: The Case of the Ultrafast Proton Transfer of the Salicylidene Methylamine

Elucidating the modulatory effects of heteroatoms and substituent groups on ESIPT dynamics and optical properties in HBBX derivants

The benzo furan-extended 2-(2'-hydroxybenzofuranyl) benzazole derivants (HBBX) have attracted widespread attention for their excellent photophysical properties. Some subtle structural alterations have been proven to influence the spectral characteristics significantly. However, a comprehensive understanding of the influence of structural regulation is still lacking. Here, the effects of heteroatoms and substituent groups on the excited state intramolecular proton transfer (ESIPT) process and photophysical properties of HBBX derivants are investigated by the density functional theory/time-dependent density functional theory (DFT/TDDFT). Our simulation results show that heteroatoms and substituent groups significantly change the hydrogen bond strength of HBBX derivants. To the heteroatoms, the HBBX substituted by the N atom (HBBI) has the lowest ESIPT energy barrier in the first excited state, and the substitution of tBu group (HBBO-1) can cause the maximum energy barrier reduction to the substituent groups. Moreover, the absorption, emission spectra and hole-electron analysis are consistent with the experimental results, and the simulation results show that HBBI has the lowest degree of intramolecular charge transfer and highest fluorescence intensity in all HBBX derivants. Our work will provide the comprehensive insight into the effects of heteroatoms and substituent groups on ESIPT processes and photophysical properties for the design of fluorescent materials.

In silico design of bio-marker detection fluorescent probes

Fluorescent probes capable of sensing the biological medium are of utmost importance in medical diagnostics. However, the optical spectrum of such probes needs to be tuned with care for compatibility with living tissues. More specifically, fluorescent bioprobes must be adjusted so as to avoid light interference with pigments (e.g. hemoglobin), tissue photodamage, scattering of the emitted light, and autofluorescence. This leads to two important conditions on the optical spectrum of the probes. On the one hand, the emission wavelength must be in an optical window of 650 to 950 nm. On the other hand, the Stokes shift must be large, ideally greater than 150 nm. In this paper, we showcase the in-silico design of potential fluorescent biomarkers fulfilling these two conditions by means of heteroatomic substitution and conjugation on a 1,2,4-triazole core initially far away from biological standards.

Calculation of the ELF in the excited state with single-determinant methods

Since its first definition, back in 1990, the electron localization function (ELF) has settled as one of the most commonly employed techniques to characterize the nature of the chemical bond in real space. Although most of the work using the ELF has focused on the study of ground-state chemical reactivity, a growing interest has blossomed to apply these techniques to the nearly unexplored realm of excited states and photochemistry. Since accurate excited electronic states usually require to account appropriately for electron correlation, the standard single-determinant ELF formulation cannot be blindly applied to them, and it is necessary to turn to correlated ELF descriptions based on the two-particle density matrix (2-PDM). The latter requires costly wavefunction approaches, unaffordable for most of the systems of current photochemical interest. Here, we compare the exact, 2-PDM-based ELF results with those of approximate 2-PDM reconstructions taken from reduced density matrix functional theory. Our approach is put to the test in a wide variety of representative scenarios, such as those provided by the lowest-lying excited electronic states of simple diatomic and polyatomic molecules. Altogether, our results suggest that even approximate 2-PDMs are able to accurately reproduce, on a general basis, the topological and statistical features of the ELF scalar field, paving the way toward the application of cost-effective methodologies, such as time-dependent-Hartree-Fock or time-dependent density functional theory, in the accurate description of the chemical bonding in excited states of photochemical relevance.

A Comprehensive Study of N-Butyl-1H-Benzimidazole

Imidazole derivatives have found wide application in organic and medicinal chemistry. In particular, benzimidazoles have proven biological activity as antiviral, antimicrobial, and antitumor agents. In this work, we experimentally and theoretically investigated N-Butyl-1H-benzimidazole. It has been shown that the presence of a butyl substituent in the N position does not significantly affect the conjugation and structural organization of benzimidazole. The optimized molecular parameters were performed by the DFT/B3LYP method with 6-311++G(d,p) basis set. This level of theory shows excellent concurrence with the experimental data. The non-covalent interactions that existed within our compound N-Butyl-1H-benzimidazole were also analyzed by the AIM, RDG, ELF, and LOL topological methods. The color shades of the ELF and LOL maps confirm the presence of bonding and non-bonding electrons in N-Butyl-1H-benzimidazole. From DFT calculations, various methods such as molecular electrostatic potential (MEP), Fukui functions, Mulliken atomic charges, and frontier molecular orbital (HOMO-LUMO) were characterized. Furthermore, UV-Vis absorption and natural bond orbital (NBO) analysis were calculated. It is shown that the experimental and theoretical spectra of N-Butyl-1H-benzimidazole have a peak at 248 nm; in addition, the experimental spectrum has a peak near 295 nm. The NBO method shows that the delocalization of the aσ-electron from σ (C1-C2) is distributed into antibonding σ* (C1-C6), σ* (C1-N26), and σ* (C6-H11), which leads to stabilization energies of 4.63, 0.86, and 2.42 KJ/mol, respectively. Spectroscopic investigations of N-Butyl-1H-benzimidazole were carried out experimentally and theoretically to find FTIR vibrational spectra.

Photochemically Induced 1,3-Butadiene Ring-Closure from the Topological Analysis of the Electron Localization Function Viewpoint

The electronic rearrangement featuring the photochemically-induced 1,3-cis-butadiene is discussed within a bonding evolution theory (BET) perspective based on the topological analysis of the electron localization function and Thom's catastrophe theory. The process involves the vertical singlet-singlet excitation S₀ →S₂ , and the subsequent deactivation implying the S₂ /S₁ and S₁ /S₀ conical intersection regions. BET results reveal that the new CC bond is finally formed on the S₀ surface, as also recently found in the photochemical addition of two ethylenes [Phys. Chem. Chem. Phys. 23, 20598, (2021)].

On Electron Pair Rearrangements in Photochemical Reactions: 1,3-Cyclohexadiene Ring Opening

1,3-Cyclohexadiene ring opening has been studied within the bonding evolution theory (BET) framework. We have focused on describing for the first time the electron pair rearrangements leading to the cis-1,3,5-hexatriene (HT) product from CHD. The nature of bonding in this process begins with the weakening of the double bonds in the Franck-Condon region. Along the 1¹B surface, the C-C sigma bond weakens. Meanwhile, its density redistributes toward the whole CHD ring, mainly over double bonds. Breaking of this bond occurs on the 2¹A surface due to the symmetrical splitting of pair density from this region. This density redistributes toward the reaction center once the pericyclic minimum is reached. The formation of the double bonds that characterize HT occurs gradually in the ground state. However, near the 2¹A/1¹A intersection, these bonds are partially established.

Unraveling the Bonding Nature Along the Photochemically Activated Paterno-Büchi Reaction Mechanism

The photochemically activated Paterno-Büchi reaction mechanism following the singlet excited-state reaction path was analyzed based on a bonding evolution framework. The electronic rearrangements, which describe the mechanism of oxetane formation via carbon-oxygen attack (C-O), comprises of the electronic activation of formaldehyde and accumulation of pairing density on the O once the reaction system is approaching the conical intersection point. Our theoretical evidence based on the ELF topology shows that the C-O bond is formed in the ground-state surface (via C-O attack) returning from the S₁ surface accompanied by 1,4-singlet diradical formation. Subsequently, the reaction center is fully activated near the transition state (TS), and the ring-closure (yielding oxetane) involves the C-C bond formation after the TS. For the carbon-carbon attack (C-C), both reactants (formaldehyde and ethylene) are activated, leading to C-C bond formation in the S₁ excited state before reaching the conical intersection region. Finally, the C-O formation occurs in the ground-state surface, resulting from the pair density flowing primarily from the C to O atom.

On the nature of bonding in the photochemical addition of two ethylenes: C-C bond formation in the excited state?

In this work, the 2s + 2s (face-to-face) prototypical example of a photochemical reaction has been re-examined to characterize the evolution of chemical bonding. The analysis of the electron localization function (as an indirect measure of the Pauli principle) along the minimum energy path provides strong evidence supporting that CC bond formation occurs not in the excited state but in the ground electronic state after crossing the rhombohedral S₁/S₀ conical intersection.

From fluorogens to fluorophores by elucidation and suppression of ultrafast excited state processes of a Schiff base

Strategies have been explored for developing strongly fluorescent species out of a weakly fluorescent Schiff base, 2-(((pyridin-2-ylmethyl)imino)methyl)phenol (salampy). The locally excited enolic state of salampy undergoes an intramolecular proton transfer with a time constant of ca. 200 fs. The emissive cis keto state thus formed decays completely within 50 ps. Its fast decay and miniscule fluorescence quantum yield are attributed to efficient non-radiative channels associated with conformational relaxation. The anionic form, salampy^-, has a significantly longer fluorescence lifetime of 800 ps. Its emissive state evolves in tens of picoseconds, from the locally excited state, by solvent and conformational relaxation. Both the neutral and anionic forms have a fluorescence lifetime of about 6 ns at 77 K, a temperature at which all activated nonradiative channels are blocked. This lifetime is similar to that obtained at room temperature, upon rigidification of the anion by complexation with Zn²⁺. Two such complexes have been studied. The first is binuclear, with acetate bridge between the two Zn²⁺ ions. The second, with ClO₄^- as the counterion, is mononuclear with two salampy ligands ligating the metal ion. Unlike a previous report on a different Schiff base, in which the ligands are π-stacked in its dimeric Zn²⁺ complex, no additional nonradiative deactivation pathway opens up in the Zn complexes of salampy, which are devoid of such stacking. The complex of salampy with Al³⁺ has an even longer fluorescence lifetime of 9 ns, indicating a greater degree of rigidification and consequent suppression of nonradiative processes.

Are There Only Fold Catastrophes in the Diels-Alder Reaction Between Ethylene and 1,3-Butadiene?

This work revisits the topological characterization of the Diels-Alder reaction between 1,3-butadiene and ethylene. In contrast to the currently accepted rationalization, we here provide strong evidence in support of a representation in terms of seven structural stability domains separated by a sequence of 10 elementary catastrophes, but all are only of the fold type (F and F^†), that is, C₄H₆ + C₂H₄:1-7-[FF]F[F^†F^†][F^†F^†][FF]F^†-0: C₆H₁₀. Such an unexpected finding provides fundamental new insights opening simplifying perspectives concerning the rationalization of the CC bond formation in pericyclic reactions in terms of the simplest Thom's elementary catastrophe, namely, the one-(state) variable, one-(control) parameter function.

Substituent effect on ESIPT and hydrogen bond mechanism of N-(8-Quinolyl) salicylaldimine: A detailed theoretical exploration

The effects of substituent on excited-state intramolecular proton transfer (ESIPT) and hydrogen bonding of N-(8-Quinolyl) salicylaldimine (QS) have been studied by theoretical calculation with DFT and TDDFT. The representative electron-withdrawing nitryl and electron-donating methoxyl were selected to analyze the effects on geometries, intramolecular hydrogen bond interaction, absorption/fluorescence spectra, and the ESIPT process. The configurations of the three molecules (QS, QS-OMe and QS-NO₂) were optimized in the ground and excited states. The structure parameters, infrared spectra, hydrogen bond interactions, frontier molecular orbitals, absorption/fluorescence spectra, and potential curves have cross-validated the current results. The results show that the introduction of substituent results in a bathochromic-shift of the absorption and fluorescence spectra with large Stokes shift, and is more beneficial to the ESIPT process. The current work will be beneficial to the improvement of ESIPT properties and deepen understanding of the mechanism of ESIPT process.