Thioxobimanes

Dioxobimanes, colloquially known as bimanes, are a well-established family of N-heterobicyclic compounds that share a characteristic core structure, 1,5-diazabicyclo[3.3.0]octadienedione, bearing two endocyclic carbonyl groups. By sequentially thionating these carbonyls in the syn and anti isomers of the known (Me,Me)dioxobimane, we were able to synthesize a series of thioxobimanes, representing the first heavy-chalcogenide bimane variants. These new compounds were extensively characterized spectroscopically and crystallographically, and their aromaticity was probed computationally. Their potential role as ligands for transition metals was demonstrated by synthesizing a representative gold(I)-thioxobimane complex.

Unravelling the Turn-On Fluorescence Mechanism of a Fluorescein-Based Probe in GABAA Receptors

GABAA (γ-aminobutyric acid type A) receptors are ligand-gated ion channels mediating fast inhibitory transmission in the mammalian brain. Here we report the molecular and electronic mechanism governing the turn-on emission of a fluorescein-based imaging probe able to target the human GABAA receptor. Multiscale calculations evidence a drastic conformational change of the probe from folded in solution to extended upon binding to the receptor. Intramolecular ππ-stacking interactions present in the folded probe are responsible for quenching fluorescence in solution. In contrast, unfolding within the GABAA receptor changes the nature of the bright excited state triggering emission. Remarkably, this turn-on effect only manifests for the dianionic prototropic form of the imaging probe, which is found to be the strongest binder to the GABAA receptor. This study is expected to assist the design of new photoactivatable screening tools for allosteric modulators of the GABAA receptor.

Unravelling the Turn‐On Fluorescence Mechanism of a Fluorescein‐Based Probe in GABA A Receptors

GABA_A (γ‐aminobutyric acid type A) receptors are ligand‐gated ion channels mediating fast inhibitory transmission in the mammalian brain. Here we report the molecular and electronic mechanism governing the turn‐on emission of a fluorescein‐based imaging probe able to target the human GABA_A receptor. Multiscale calculations evidence a drastic conformational change of the probe from folded in solution to extended upon binding to the receptor. Intramolecular ππ‐stacking interactions present in the folded probe are responsible for quenching fluorescence in solution. In contrast, unfolding within the GABA_A receptor changes the nature of the bright excited state triggering emission. Remarkably, this turn‐on effect only manifests for the dianionic prototropic form of the imaging probe, which is found to be the strongest binder to the GABA_A receptor. This study is expected to assist the design of new photoactivatable screening tools for allosteric modulators of the GABA_A receptor.

Radiative lifetime of a BODIPY dye as calculated by TDDFT and EOM-CCSD methods: solvent and vibronic effects

The radiative emission lifetime and associated S₁ excited state properties of a BODIPY dye are investigated with TDDFT and EOM-CCSD calculations. The effects of a solvent are described with the polarizable continuum model using the linear response (LR) approach as well as state-specific methods. The Franck-Condon (FC), Herzberg-Teller (HT) and Duschinsky vibronic effects are evaluated for the absorption and emission spectra, and for the radiative lifetime. The transition energies, spectra shapes and radiative lifetime are assessed with respect to experimental results. It is found that the TDDFT transition energies are overestimated by about 0.4-0.5 eV, whereas EOM-CCSD improves the vertical emission energy by about 0.1 eV in comparison to TDDFT. The solvatochromic and Stokes shifts are better reproduced by the state-specific solvation methods, which show that these methods are more suited than the LR model to describe the solvent effects on the BODIPY dye. The vibronic effects lead to an increase of the radiative lifetime of about 0.4 to 1.0 ns depending on the theoretical approach, which highlights the importance of such effects. Moreover, the HT effects are negligible on both the spectra and lifetime, which demonstrates that the FC approximation is accurate for the BODIPY dye. Finally, the comparison with experimental data shows that the radiative lifetimes predicted by EOM-CCSD and TDDFT have comparable accuracy.

Unveiling the role of short-range exact-like exchange in the optimally tuned range-separated hybrids for fluorescence lifetime modeling

We propose and validate several variants of the optimally tuned range-separated hybrid functionals (OT-RSHs) including different density functional approximations for predicting the fluorescence lifetimes of different categories of fluorophores within the time-dependent density functional theory (TD-DFT) framework using both the polarizable continuum and state-specific solvation models. Our main idea originates from performing the optimal tuning in the presence of a contribution of the exact-like exchange at the short-range part, which, in turn, leads to the small values of the range-separation parameter, and computing the fluorescence lifetimes using the models including no or small portions of the short-range exact-like exchange. Particular attention is also paid to the influence of the geometries of emitters on fluorescence lifetime computations. It is shown that our developed OT-RSHs along with the polarizable continuum model can be considered as the promising candidates within the TD-DFT framework for the prediction of fluorescence lifetimes for various fluorophores. We find that the proposed models not only outperform their standard counterparts but also provide reliable data better than or comparable to the conventional hybrid functionals with both the fixed and interelectronic distance-dependent exact-like exchanges. Furthermore, it is also revealed that when the excited state geometries come into play, more accurate descriptions of the fluorescence lifetimes can be achieved. Hopefully, our findings can give impetus for future developments of OT-RSHs for computational modeling of other characteristics in fluorescence spectroscopy as well as for verification of the related experimental observations.

Ab Initio Prediction of Fluorescence Lifetimes Involving Solvent Environments by Means of COSMO and Vibrational Broadening

The fluorescence lifetime is a key property of fluorophores that can be utilized for microenvironment probing, analyte sensing, and multiplexing as well as barcoding applications. For the rational design of lifetime probes and barcodes, theoretical methods have been developed to enable the ab initio prediction of this parameter, which depends strongly on interactions with solvent molecules and other chemical species in the emitteŕs immediate environment. In this work, we investigate how a conductor-like screening model (COSMO) can account for variations in fluorescence lifetimes that are caused by such fluorophore-solvent interactions. Therefore, we calculate vibrationally broadened fluorescence spectra using the nuclear ensemble method to obtain distorted molecular geometries to sample the electronic transitions with time-dependent density functional theory (TDDFT). The influence of the solvent on fluorescence lifetimes is accounted for with COSMO. For example, for 4-hydroxythiazole fluorophore containing different heteroatoms and acidic and basic moieties in aprotic and protic solvents of varying polarity, this approach was compared to experimentally determined lifetimes in the same solvents. Our results demonstrate a good correlation between theoretically predicted and experimentally measured fluorescence lifetimes except for the polar solvents ethanol and acetonitrile that can specifically interact with the heteroatoms and the carboxylic acid of the thiazole derivative.

Using non-empirically tuned range-separated functionals with simulated emission bands to model fluorescence lifetimes

Fluorescence lifetimes were evaluated using TD-DFT under different approximations for the emitting molecule and various exchange-correlation functionals, such as B3LYP, BMK, CAM-B3LYP, LC-BLYP, M06, M06-2X, M11, PBE0, ωB97, ωB97X, LC-BLYP*, and ωB97X* where the range-separation parameters in the last two functionals were tuned in a non-empirical fashion. Changes in the optimised molecular geometries between the ground and electronically excited states were found to affect the quality of the calculated lifetimes significantly, while the inclusion of vibronic features led to further improvements over the assumption of a vertical electronic transition. The LC-BLYP* functional was found to return the most accurate fluorescence lifetimes with unsigned errors that are mostly within 1.5 ns of experimental values.

Computational modelling of singlet excitation energy transfer: a DFT/TD-DFT study of the ground and excited state properties of a syn bimane dimer system using non-empirically tuned range-separated functionals

Range-tuned DFT/TD-DFT improves predicted properties for the sequence of events leading to excitation energy transfer (EET) in bimanes.