Ultrafast electron diffraction: Excited state structures and chemistries of aromatic carbonyls

Abstract

The photophysics and photochemistry of molecules with complex electronic structures, such as aromatic carbonyls, involve dark structures of radiationless processes. With ultrafast electron diffraction ͑UED͒ of isolated molecular beams it is possible to determine these transient structures, and in this contribution we examine the nature of structural dynamics in two systems, benzaldehyde and acetophenone. Both molecules are seen to undergo a bifurcation upon excitation ͑S 2 ͒. Following femtosecond conversion to S 1 , the bifurcation leads to the formation of molecular dissociation products, benzene and carbon monoxide for benzaldehyde, and benzoyl and methyl radicals for acetophenone, as well as intersystem crossing to the triplet state in both cases. The structure of the triplet state was determined to be "quinoidlike" of * character with the excitation being localized in the phenyl ring. For the chemical channels, the product structures were also determined. The difference in photochemistry between the two species is discussed with respect to the change in large amplitude motion caused by the added methyl group in acetophenone. This discussion is also expanded to compare these results with the prototypical aliphatic carbonyl compounds, acetaldehyde and acetone. From these studies of structural dynamics, experimental and theoretical, we provide a landscape picture for, and the structures involved in, the radiationless pathways which determine the fate of molecules following excitation. For completeness, the UED methodology and the theoretical framework for structure determination are described in this full account of an earlier communication ͓J. S. Feenstra et al., J. Chem. Phys. 123, 221104 ͑2005͔͒.

Distortion dependent intersystem crossing: A femtosecond time-resolved photoelectron spectroscopy study of benzene, toluene, and p-xylene

“…This reasoning is fully consistent with the observation that the lowest triplet states of toluene and p-xylene exhibit quinoidal structures (Figure 2) with the methyl-groups positioned to support the diradicaloid species as found by EPR measurements 49,50 and quantum chemical calculations 48 . Quinoidal triplet structures of benzene derivatives have further been observed by time-resolved electron diffraction experiments by Zewail and coworkers 51 . Triplet energies and the frequency of the modes that could mediate ISC in benzene and its derivatives are therefore expected to be sensitive towards methylation.…”

Ultrafast X-ray scattering: structural dynamics from diatomic to protein molecules

“…Replacing the optical probe with X-ray [38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53] or electron [54][55][56][57] diffraction (scattering) offers a direct route to molecular structures as the diffraction (scattering) effect originates from atomic interferences in real space instead of optical resonances in energy space. In contrast to visible or IR light that is sensitive to electronic or vibrational transitions of specific chromophores, X-rays and electrons are diffracted (or scattered) off all atom-atom pairs and chemical species present in the molecule that direct information on the molecular structure can be retrieved from the measured X-ray diffraction patterns.…”

A general method to describe intersystem crossing dynamics in trajectory surface hopping

“…[31] More recently, also the role of ISC in organic molecules has captured the attention of researchers. Examples of molecules where ISC is important range from aldehydes [37] and small aromatic compounds like benzene, [38] naphthalene, anthracene, and their carbonylic derivatives [39][40][41][42][43][44][45][46][47][48][49][50][51] to nitrocompounds. [43,[52][53][54][55][56][57][58][59][60][61] Furthermore, ISC has been reported for thio-substituted, [62][63][64][65][66][67][68] aza-substituted, [69] bromo-substituted, [70] and canonical nucleobases.…”