We individuate a photoinduced electron transfer (PeT) as a quenching mechanism affecting rhodamine B photophysics in solvent. The PeT involves an electron transfer from the carboxylate group to the xanthene ring of rhodamine B. This is finely modulated by the subtle balance of coulombic and non-classical interactions between the carboxyphenyl and xanthene rings, also mediated by the solvent. We propose the use of an electronic density based index, the so called DCT index, as a new tool to assess and quantify the nature of the excited states involved in non-radiative decays near the region of their intersection. In the present case, this analysis allows us to gain insight on the interconversion process from the bright state to the dark state responsible for the quenching of rhodamine B fluorescence. Our findings encourage the use of density based indices to study the processes affecting excited state reactions that are characterized by a drastic change in the excitation nature, in order to rationalize the photophysical behavior of complex molecular systems.
In spite of the detailed information provided by advanced time-resolved spectroscopy, the understanding of the excited-state proton transfer (ESPT) reactivity remains difficult to obtain at molecular level. In this work we studied three photoacids showing different strength: the 8-hydroxy-1,3,6-pyrenetrisulfonate weak photoacid, the N-methyl-6-hydroxyquinolinium strong photoacid and the phenol-carboxyether dipicolinium cyanine (QCy9) superphotoacid, focusing on the intermolecular ESPT toward a solvent molecule or a base molecule in aqueous solution. To this aim, the ground and the first singlet excited-state potential energy surfaces of the three systems were characterized by means of the time-dependent density functional theory and a hybrid implicit/explicit model of the solvent. Main structural and photophysical features of the photoacids were assessed and satisfactorily compared with the experimental data. Energy profiles along the PT coordinate were analyzed in both the electronic states. We reproduced many important features of the photoacidity experimentally observed. The results suggest that the relative strength is mainly due to the different extent of charge transfer caused by the electronic transition in proximity of the acid group. Remarkably, we found that even in the case of the strongest photoacid (QCy9), showing a ESPT rate as rapid as to escape the solvent dynamics control, the PT is modulated and supported by the first solvation shell of the proton-accepting molecule. However, a complete understanding of this fascinating field needs the full account for the electronic and the molecular dynamics in play at different timescales
The detailed knowledge of excited state proton transfer mechanisms in complex environments is of paramount importance in chemistry. However, the definition of an effective reaction coordinate and the understanding of the driving force of the reaction can be difficult from both the experimental and the theoretical points of view. Here we analyzed by theoretical approaches the mechanism and the driving forces of the excited state proton transfer reaction occurring between the 7-hydroxy-4-(trifluoromethyl)coumarin photoacid and the 1-methylimidazole base molecules in toluene solution. In particular, we compared the intrinsic and the dynamical reaction pathways, obtained by integrating the reaction coordinate, and by performing ab initio simulations of molecular dynamics, respectively. Time-dependent density functional theory and polarizable solvation continuum models were adopted to define the excited state potential energy surface. Results were analyzed by means of the D(CT) electronic density based index. Our findings suggest that the reaction coordinate is mainly composed of several intra- and intermolecular modes of the reactants. An analysis of both the intrinsic coordinate and the dynamical results shows that the charge transfer induced by electronic excitation of the coumarin molecule is the main proton transfer driving force. With regards to the methodological validation, the combination of ab initio molecular dynamics with time-dependent density functional theory appears to be feasible and reliable to study excited state proton transfer reactions in the condensed phase.
Detailed mechanistic understanding of multistep chemical reactions triggered by internal conversion via a conical intersection is a challenging task that emphasizes limitations in theoretical and experimental techniques. Hypothesis-driven methodologies (e.g. characterization of critical points and biased molecular dynamics) are commonly employed to explore chemical space and simulate reaction events. In this contribution, we present a discovery-based, hypothesis-free computational approach based on first principles molecular dynamics to discover and refine the switching mechanism of Donor-Acceptor Stenhouse Adducts (DASAs). Using state-of-the-art graphical processing units-enabled electronic structure calculations we performed in total ~2ns of adiabatic and non-adiabatic ab initio molecular dynamics discovering a) critical intermediates that are involved in the open-to-closed transformation, b) several competing pathways which lower the overall switching yield, and c) key elements for future design strategies. Our dynamics describe the natural evolution of both the nuclear and electronic degrees of freedom that govern the interconversion between DASA ground state intermediates exposing significant elements for the future design strategies of molecular switches.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.