Charge-displacement analysis for excited states

Orbital-free photophysical descriptors to predict directional excitations in metal-based photosensitizers

The development of dye-sensitized solar cells, metalloenzyme photocatalysis or biological labeling heavily relies on the design of metal-based photosensitizes with directional excitations. Directionality is most often predicted by characterizing the excitations manually via canonical frontier orbitals. Although widespread, this traditional approach is, at the very least, cumbersome and subject to personal bias, as well as limited in many cases. Here, we demonstrate how two orbital-free photophysical descriptors allow an easy and straightforward quantification of the degree of directionality in electron excitations using chemical fragments. As proof of concept we scrutinize the effect of 22 chemical modifications on the archetype [Ru(bpy)3]2+ with a new descriptor coined "substituent-induced exciton localization" (SIEL), together with the concept of "excited-electron delocalization length" (EEDL n ). Applied to quantum ensembles of initially excited singlet and the relaxed triplet metal-to-ligand charge-transfer states, the SIEL descriptor allows quantifying how much and whereto the exciton is promoted, as well as anticipating the effect of single modifications, e.g. on C-4 atoms of bpy units of [Ru(bpy)3]2+. The general applicability of SIEL and EEDL n is further established by rationalizing experimental trends through quantification of the directionality of the photoexcitation. We thus demonstrate that SIEL and EEDL descriptors can be synergistically employed to design improved photosensitizers with highly directional and localized electron-transfer transitions.

Charge Displacement Analysis-A Tool to Theoretically Characterize the Charge Transfer Contribution of Halogen Bonds

Theoretical bonding analysis is of prime importance for the deep understanding of the various chemical interactions, covalent or not. Among the various methods that have been developed in the last decades, the analysis of the Charge Displacement function (CD) demonstrated to be useful to reveal the charge transfer effects in many contexts, from weak hydrogen bonds, to the characterization of σ hole interactions, as halogen, chalcogen and pnictogen bonding or even in the decomposition of the metal-ligand bond. Quite often, the CD analysis has also been coupled with experimental techniques, in order to give a complete description of the system under study. In this review, we focus on the use of CD analysis on halogen bonded systems, describing the most relevant literature examples about gas phase and condensed phase systems. Chemical insights will be drawn about the nature of halogen bond, its cooperativity and its influence on metal-ligand bond components.

Density-based descriptors and exciton analyses for visualizing and understanding the electronic structure of excited states

Analysis and interpretation of the electronic structure of excited electronic states are prerequisites for developing a fundamental understanding of photochemistry and optical properties of molecular systems and an everyday task for a computational photochemist. Hence, wavefunction-based and density-based analysis tools have been devised over the last decades, and most recently also a family of quantitative exciton-wavefunction based descriptors has been developed. While the latter represent the main focus of this perspective, they are also discussed in the context of other existing analysis methods. Exciton analysis bridges the gap between the physically intuitive exciton picture and complex quantum-chemical wavefunctions by yielding insightful quantitative descriptors like exciton size, hole and electron size, electron-hole distance and exciton correlation. Thereby, not only a comprehensive characterization of the electronic structure is provided, but moreover, the formalism is automatizable and thus also optimally suited for benchmarking excited-state electronic structure methods.

Statistical analysis of electronic excitation processes: Spatial location, compactness, charge transfer, and electron-hole correlation

We report the development of a set of excited-state analysis tools that are based on the construction of an effective exciton wavefunction and its statistical analysis in terms of spatial multipole moments. This construction does not only enable the quantification of the spatial location and compactness of the individual hole and electron densities but also correlation phenomena can be analyzed, which makes this procedure particularly useful when excitonic or charge-resonance effects are of interest. The methods are first applied to bianthryl with a focus on elucidating charge-resonance interactions. It is shown how these derive from anticorrelations between the electron and hole quasiparticles, and it is discussed how the resulting variations in state characters affect the excited-state absorption spectrum. As a second example, cytosine is chosen. It is illustrated how the various descriptors vary for valence, Rydberg, and core-excited states, and the possibility of using this information for an automatic characterization of state characters is discussed.

Anomalous ligand effect in gold(i)-catalyzed intramolecular hydroamination of alkynes

The ligand electronic effect modifies in entirely unexpected ways the binding mode and effectiveness of gold(i) catalysts for alkyne hydroamination.

A quantitative view of charge transfer in the hydrogen bond: the water dimer case

The hydrogen bond represents a fundamental intermolecular interaction that binds molecules in vapor and liquid water. A crucial and debated aspect of its electronic structure and chemistry is the charge transfer (CT) accompanying it. Much effort has been devoted, in particular, to the study of the smallest prototype system, the water dimer, but even here results and interpretations differ widely. In this paper, we reassess CT in the water dimer by using charge-displacement analysis. Besides a reliable estimate of the amount of CT (14.6 me) that characterizes the system, our study provides an unambiguous context, and very useful bounds, within which CT effects may be evaluated, crucially including the associated energy stabilization.