Time-resolved velocity map imaging of methyl elimination from photoexcited anisole.

Abstract

To date, H-atom elimination from heteroaromatic molecules following UV excitation has been extensively studied, with the focus on key biological molecules such as chromophores of DNA bases and amino acids. Extending these studies to look at elimination of other non-hydride photoproducts is essential in creating a more complete picture of the photochemistry of these biomolecules in the gas-phase. To this effect, CH(3) elimination in anisole has been studied using time-resolved velocity map imaging (TR-VMI) for the first time, providing both time and energy information on the dynamics following photoexcitation at 200 nm. The extra dimension of energy afforded by these measurements has enabled us to address the role of πσ* states in the excited state dynamics of anisole as compared to the hydride counterpart (phenol), providing strong evidence to suggest that only CH(3) fragments eliminated with high kinetic energy are due to direct dissociation involving a (1)πσ* state. These measurements also suggest that indirect mechanisms such as statistical unimolecular decay could be contributing to the dynamics at much longer times.

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Author(s): David J. Hadden, Craig A. Williams, Gareth M. Roberts and
Vasilios G. Stavros
Article Title: Time-resolved velocity map imaging of methyl elimination
from photoexcited anisole
Year of publication: 2011
Link to published article: http://dx.doi.org/10.1039/C0CP02429E
Publisher statement: None

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Time resolved velocity map imaging of methyl elimination
from photoexcited anisole

David J. Hadden, Craig A. Williams, G.M. Roberts and Vasilios G. Stavros*

Department of Chemistry, University of Warwick, Coventry, CV4 7AL, UK

*To whom correspondence should be addressed. E-mail: v.stavros@warwick.ac.uk.


Abstract

To date, H-atom elimination from heteroaromatic molecules following UV excitation
has been extensively studied, with the focus on key biological molecules such as
chromophores of DNA bases and amino acids. Extending these studies to look at
elimination of other non-hydride photoproducts is essential in creating a more
complete picture of the photochemistry of these biomolecules in the gas-phase. To
this effect, CH3 elimination in anisole has been studied using time resolved velocity
map imaging (TR-VMI) for the first time, providing both time and energy information
on the dynamics following photoexcitation at 200 nm. The extra dimension of energy
afforded by these measurements has enabled us to address the role of πσ* states in the
excited state dynamics of anisole as compared to the hydride counterpart (phenol),
providing strong evidence to suggest that only CH3 fragments eliminated with high
kinetic energy are due to direct dissociation involving a
1πσ* state. These
measurements also suggest that indirect mechanisms such as statistical unimolecular
decay could be contributing to the dynamics at much longer times.

Page 3

I. Introduction
Photochemistry is everywhere, playing a vitally important role in our day-to-
day lives. For example our molecular building blocks readily absorb ultraviolet
radiation. However these molecules display a large degree of photostability.1-3 One of
the main reasons for this is that photochemical reactions are effectively quenched
through ultrafast non-radiative processes imparting a high degree of photostability to
these building blocks of life.3-5 In light of this, there have been growing efforts both
from an experimental and theoretical standpoint, aimed at precisely defining
photoresistive pathways potentially operative in some of nature’s most important
molecules. Recently, dissociative 1πσ* states have been implicated as key players in
the photoresistive properties of chromophores of aromatic amino acids and DNA
bases.5 Whilst a number of experiments have been directed at searching for
spectroscopic evidence of 1πσ* states with some success on smaller molecules (e.g.
phenol6-8 and indole,9-11 the chromophores of the amino acids tyrosine and tryptophan
and DNA bases such as adenine12-14), the ever-burgeoning question is to what extent
do 1πσ* states contribute to the photochemistry of a larger range of biological
molecules?
The seminal theoretical work of Domcke and Sobolewski originally suggested
a general decay pathway that may be common to a number of aromatic and
heterocyclic molecules.5,15 Upon UV irradiation, photon energy is deposited into the
molecule through excitation to an optically bright 1ππ* state. In these molecules, an
excited state of 1πσ* character intersects, through conical intersections (CIs), both the
initially excited 1ππ* state and the electronic ground state along an X-H stretch
coordinate where X is typically O or N. Non-radiative decay along this pathway is
predicted to be highly efficient due to the repulsive nature of the 1πσ* state leading to

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H-atom elimination. Since the prediction of the 1πσ* relaxation pathway by ab initio
calculations an increasing number of experiments have been carried out in the gas
phase given the comparison with existing theory which has thus far largely modelled
isolated molecules. The 1πσ* states, however, are optically dark and their potential
energy surfaces (PESs) are dissociative along the X-H coordinate, making them
difficult to detect directly, except through time-resolved photoelectron spectroscopy.
This is often very challenging, especially as the complexity of the photoelectron
spectra increases with increasing molecular size.16 Instead, characteristics of these
states have been exploited for indirect observations. Spectroscopic detection of H-
atom appearance times and velocity distributions are indirect evidence for relaxation
along the 1πσ* state.17
Whilst a flurry of both experiment and theory has been directed towards 1πσ*
states in hydrides, in contrast much less effort has been directed at studying these
dissociative states localized on other coordinates such as X-C. The velocity map ion
imaging (VMI) work by Ashfold and co-workers was the first to identify the role of
1πσ* state induced bond dissociation in non-hydride heteroaromatic systems.18 Their
work on N-methylpyrrole clearly showed the bimodal distribution of CH3
photoproducts following excitation with UV radiation. In keeping with their H-atom
total kinetic energy release (TKER) spectra, the appearance of high and low kinetic
energy (KE) components in their spectra were attributed to direct dissociation along
the 1πσ* state resulting in high KE CH3 fragments and indirect dissociation, in which
highly excited ground state molecules decay, resulting in low KE CH3 fragments. This
work was further complemented by Becucci and co-workers whose combined
experimental and theoretical studies showed the existence of two low-lying
dissociative 1πσ* states localized on the X-CH3 coordinate in the same system.19

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Using multi-mass ion-imaging, Ni and co-workers carried out a series of experiments
on N-methylindole, N-methylpyrrole and anisole, clearly showing the appearance of
either fast or slow CH3 (or both) fragments following photodissociation at 248 nm and
193 nm, to which they once again rationalized these findings by invoking the presence
of a 1πσ* state localized on the X-CH3 coordinate.20 Most recently, Lim and Kim
studied the photodissociation of thioanisole (C6H5S-CH3), observing a striking
dependence on the branching ratio between the ground and excited state C6H5S
fragment (X or A) reaction channels depending on excitation energy.21
Although these measurements have provided significant insight into the
underlying photophysics of these systems, with clear evidence for the active
participation of 1πσ* states, very little is known about the timescales of these
processes which is critical in unravelling the complex interplay between adiabatic and
non-adiabatic dynamics and factors which affect these. In previous studies in phenol-
h6 and phenol-d5, our group has shown that both the high and low KE components of
the H-atom KE release spectra indicated appearance timescales in the order of 100
femtoseconds.8 Whilst this timescale is unsurprising for a direct process in which
dissociation occurs within one vibrational period, what is surprising is the timescale
for the low KE component, given that this process is expected to be much longer, i.e.,
once on the electronic ground state, energy must be localized in the correct mode
before dissociation can occur (statistical unimolecular decay). This has led us to
suggest an alternative decay pathway to generate these low KE H-atoms which is
direct. In doing so, this has prompted the work presented here using time-resolved
velocity map ion imaging (TR-VMI) studies in anisole in order to compare and
contrast the dynamics of this system with its corresponding hydride counterpart (Fig.
1). The work presents strong evidence to suggest that following the same excitation