Through-Bond Interactions and the Localization of Excited-State Dynamics

Abstract

The influence of through-bond interactions on nonadiabatic excited-state dynamics is investigated by time-resolved photoelectron spectroscopy (TRPES) and ab initio computation. We compare the dynamics of cyclohexa-1,4-diene, which exhibits a through-bond interaction known as homoconjugation (the electronic correlation between nonconjugated double bonds), with the nonconjugated cyclohexene. Each molecule was initially excited to a 3s Rydberg state using a 200 nm femtosecond pump pulse. The TRPES spectra of these molecules display similar structure and time constants on a subpicosecond time scale. Our ab initio calculations show that similar sets of conical intersections (a [1,2]- and [1,3]-hydrogen shift, as well as carbon–carbon bond cleavage) are energetically accessible to both molecules and that the geometry and orbital composition at the minimum energy crossing points to the ground state are directly analogous. These experimental and computational results suggest that the excited-state dynamics of cyclohexa-1,4-diene become localized at a single double bond and that the effects of through-bond interaction, dominant in the absorption spectrum, are absent in the excited-state dynamics. The notion of excited-state dynamics being localized at specific sites within the nuclear framework is analogous to the localization of light absorption by a subsystem within the molecule, designated a chromophore. We propose the utility of the analogous concept, denoted here as a dynamophore.

Initial Processes of Proton Transfer in Salicylideneaniline Studied by Time-Resolved Photoelectron Spectroscopy

“…The apparent lack of structure might indicate a vibrationally hot molecule, as can be the case for a long-lived lower-lying state. (See, for example, the longest-time-constant DAS of cyclohexene in ref 43.) In Figure 7a, it is seen that the fast time constant in SA has distinctively larger amplitude at 370 nm than at 350 and 330 nm (factor of 5:1−10:1).…”

The Dynamophore – Localization of Excited State Dynamics Studied by Time–Resolved Photoelectron Spectroscopy

“…However, dynamics induced by these photons may take place in another, localized region of the molecular framework which is defined as dynamophore [1]. An example is cyclohexa-1,4-diene which is excited along the delocalized -manifold of two hyperconjugated double bonds but whose dynamics takes place only at one of the two sites of the molecule [1] (Fig.…”

“…Ethylene exhibits a characteristic set of motions to return from the excited *-state to the electronic ground state: It twists about its double bond and either pyramidalizes at one of the two carbon atoms or forms a H-bridged structure which can lead to a [1,2]-hydrogen migration [2][3][4]. Other examples of dynamophores are the allyl-group (CH=CH-CH2) which performs an [1,3]-H shift or the cyclohexa- [1,3]-diene group exhibiting a ring opening reaction [5]. The concept of a dynamophore allows for the classification of a wide range of dynamical processes in terms of a small set of prototypical types.…”

“…The concept of a dynamophore allows for the classification of a wide range of dynamical processes in terms of a small set of prototypical types. For example, ethylenic-type pyramidalization results in bond cleavage in the constrained six-membered ring of cyclohea-1,4-diene [1] and a cis-trans isomerization in the case of stilbene [3]. One challenge is to find rules how changes in the local bonding framework of a dynamophore affect these prototypical types (formerly called multidimensional analogues of the Polyani rule [6]).…”

“…This poses the question which of the available dynamophores is and can be accessed. For example whether the presence of an allyl group enforces a [1,3] shift or when this channel might become dominant. In addition, one could ask how the yield of the pathways depends on the excitation wavelength and whether it is possible to control it.…”