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Department of Chemistry

Prof. Jan R.R. Verlet

Personal web page

Professor in the Department of Chemistry
Telephone: +44 (0) 191 33 42159

(email at j.r.r.verlet@durham.ac.uk)

Research Interests

Our research aims to develop a mechanistic understanding of how anions cope with high levels of excess energy. For example, how does DNA dispose of the energy it gains following the absorption of UV radiation or how does an electron acceptor hold on to an incoming electron? These are fundamental questions central to the understanding of how energy can be harvested and how fundamental chemical processes work. Such an understanding will in turn enable us to predict and control the outcome of certain reactions. As these questions probe the nature of interactions on a molecular level, our methodologies to probe such process must also be on such a level. To this end, we perform time-resolved methods on isolated and solvated anions.

Following excitation of a molecular anion, the energy can be converted into a directed mechanical action via a molecular transformation such as isomerisation or to heat by populating highly excited vibrational modes in lower-lying electronically excited states. Both processes require the motion of atoms within the molecule. To study how the energy is transferred to the nuclear dynamics, we monitor the excited state dynamics on the time-scale of the motion of the atoms – that is on a femtosecond timescale (10-15 seconds). Using laser pulses of femtosecond (10-15 seconds) duration, the atoms in a molecule are effectively frozen – similar in concept to the speed of a shutter in a camera. This allows us to monitor in real time molecular processes such as vibrations and non-radiative decay. Our specific interest is in anions. 

Dynamics of biological chromophores

In biology, chemistry and technology, there are many chromophores (molecules or parts of molecules that absorb light) that perform a specific action upon the absorption of photons. In nature, many of these are anions and by isolating the chromophore and studying its dynamics, we can understand the intrinsic properties which in turn allow us to understand how the environment alters these dynamics. As recent examples, we have studied the spectroscopy and dynamics of: the chromophore of the green fluorescent protein (GFP) which is a key fluorescent marker in biology [1]; and nucleotides which are the building blocks of DNA [2].

Ultrafast spectroscopy of radical anions

Many processes in nature and chemistry involve the transfer of an electron. A common electron accepting molecule is a quinone-derivative. Indeed, many of these have been shown to undergo electron transfer processes at rates that exceed theoretical predictions. One explanation for this is the electron transfer into electronically excited states of the anion radical of the quinone. We have recently developed a methodology based on time-resolved photoelectron spectroscopy to study these processes. In para-benzoquinone, excited states of the radical anion that lie above the detachment threshold can decay back to the ground state on the timescale of a single symmetric vibration of the quinone (~20 fs) [3]. Hence, as soon as an electron is attached into these short lived states, the excess energy can be rapidly converted into internal vibrational energy of the system, which can then be dissipated to the surroundings.

Photoelectron imaging of polyanions: Riding the repulsive Coulomb barrier

How does an electron leave a polyanion? Unlike anions, neutrals or cations, photodetachment of an electron from a polyanion will leave an anionic species behind. This means that when the photoelectron leaves, it experiences the Coulomb potential of the remaining anion which “guides” the photoelectron in a specific direct. It also means that the electron can tunnel through the barrier [4]. We can measure this directionality using photoelectron imaging methods and have shown that one can use this to learn about electron tunnelling through the barrier as well as a tool to monitor large-amplitude motion in molecular system. This is qualitatively shown in the figure on the right where then changes in photoelectron anisotropy signify the rotation of a molecular dianion. Initially, the molecule is aligned and photoelectrons are guided in the direction perpendicular to the molecular frame. As time evolves and the molecule rotates, the connection between lab and molecular frame is lost and the image is isotropic [5].

Hydrated electrons in clusters and at surfaces

Arguably the most fundamental anion is the hydrated electron – and electron bound within a cavity formed in water. But what happens if we bring this electron to the surface of water? In the gas-phase, an excess electron can bind to the surface of a water cluster [6] and we perform experiments to understand the nature of the electron in such clusters and its reactivity. We have also extended this to the ambient water/air interface. Using time-resolved second-harmonic generation spectroscopy, we have shown that hydrated electron can reside at the water/air interface, but remain below the dividing surface, within the first nanometer [7]. Currently, we are extending these studies to gain full quantitative understanding of how an electron solvates at aqueous interfaces and to extend this to other interfaces. We are also developing new gas-phase methods to study electrons at the interface of aqueous nano-particles.

References

  1. C. R. S. Mooney, D. A. Horke, A. S. Chatterley, A. Simperler, H. H. Fielding and J. R. R. Verlet, “Taking the green fluorescence out of the protein: dynamics of the isolated GFP chromophore anion.”, Chem. Sci. 4, 921 (2013)
  2. A. S. Chatterley, A. S. Johns, V. G. Stavros and J. R. R. Verlet, “Base-specific ionization of deprotonated nucleotides by resonance enhanced two-photon detachment”, J. Phys. Chem. A 117, 5299 (2013) 
  3. D. A. Horke, Q. Li, L. Blancafort and J. R. R. Verlet, “Ultrafast above-threshold dynamics of the radical anion of a prototypical quinone electron-acceptor”, Nature Chemistry 5, 711 (2013)
  4. D. A. Horke, A. S. Chatterley and J. R. R. Verlet, “Effect of internal energy on the repulsive Coulomb barrier of polyanions”, Phys. Rev. Lett. 108, 083003 (2012) 
  5. D. A. Horke, A. S. Chatterley and J. R. R. Verlet, “Femtosecond photoelectron imaging of aligned polyanions: Probing molecular dynamics through the electron-anion” , J. Phys. Chem. Lett. 3,834 (2012)
  6. J. R. R. Verlet, A. E. Bragg, A. Kammrath, O. Cheshnovsky and D.M. Neumark, “Observation of large water-cluster anions with surface-bound excess electrons”, Science 307, 93 (2005)
  7.  D. M. Sagar, C. D. Bain and J. R. R. Verlet , “Hydrated electrons at the water/air interface”, J. Am. Chem. Soc. 132, 6917 (2010) 

Publications

Journal Article