Professor
                               Department of Chemistry
                                     Temple University


   
    Education:

    B.S., 1994, National and Kapodistrian University of  Athens, Greece;
    Ph.D., 2000, The Ohio State University;
    Postdoctoral fellow, 2000-2003, Johns Hopkins University.

    2005 NSF CAREER Award

 

    Research Interest:

   * Conical intersections and nonadiabatic effects
   * Photophysical behavior of DNA/RNA bases
   * QM/MM methodology for studying solvation in excited states
   * Understanding laser control of molecular dynamics

 

 

                                               

Department of Chemistry
Temple University
13th and Norris Streets
Philadelphia PA 19122

Voice: 215-204-7703 
Fax: 215-204-1532

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Research Interests


Conical intersections and nonadiabatic effects



Most chemical processes follow the Born-Oppenheimer (adiabatic) approximation, in which the nuclei move on a single electronic potential energy surface (PES). However there are important processes where this approximation breaks down. These nonadiabatic events play an important role in essential processes in nature such as photosynthesis, vision, charge transfer and photochemistry. Nonadiabatic processes are facilitated by the close proximity of two or more PES where radiationless transitions between the surfaces can occur with higher probability. The efficiency for radiationless transitions increases in the extreme case when two PES become degenerate forming conical intersections. Through our work, as well as others, it has also been shown that three states can also become degenerate forming three-state conical intersections. Thus many ultrafast nonradiative transitions occur through conical intersections. Theoretical developments have enabled the efficient study of conical intersections in small systems. The focus of our group is to extend these studies to more complicated systems, particularly of biological interest, in an effort to understand the underlying mechanisms of photoinitiated nonadiabatic processes and their potential implications.




Photophysical behavior of DNA/RNA bases /Designing more efficient fluorescent probes




The photophysical and photochemical behavior of the nucleobases is of particular importance since they are the chromophores in DNA and RNA. The excited states of the nucleobases are extremely short-lived, having lifetimes of the order of femtoseconds or picoseconds. This property has been associated with their photostability in ultraviolet (UV) radiation from the sun. When UV radiation is absorbed by DNA photochemistry can occur which may lead to DNA damage and photocarcinogenesis. Efficient dissipation of the absorbed energy prevents extensive photodamage. We are investigating the radiationless decay mechanisms that can explain the ultrafast lifetimes and fluorescence quenching in these systems. Two- and three-state conical intersections are present that can facilitate nonadiabatic transitions between the different electronic states of these molecules. 

Understanding the mechanism for fluorescence quenching can have important applications as well, since it enables us to design molecules that will not have this property, and they will instead fluoresce.

In recent work we have found exciting new insight into the excited state dynamics of pi-stacked nucleobases. These results may help us understand the more complex dynamics in single and double stranded DNA. Furthermore, we have been able to explain how quenching of the fluorescence occurs in some fluorescent probes when pi-stacked with other bases. These knowledge can help design more efficient fluorescent probes.




 


Developing methods for excited states and conical intersections

QM/MM with MRCI: Although the majority of quantum mechanical studies on photoinitiated processes are carried out in vacuo, the incorporation of environmental perturbations is important for a more realistic modeling of these processes. The effect of solvent in photophysics and photochemistry can be very important, and methods that account for these effects are essential. We have developed a simple scheme that incorporates the most important electrostatic solvation effects into the ab initio studies of excited states. In this scheme the solvent structure is obtained by molecular dynamics simulations using classical force fields. An average electrostatic potential is derived from these simulations where thousands of snapshots are included, and then is introduced into the quantum mechanical multireference configuration interaction (MRCI) calculations for the solute. This is necessary since an MRCI calculation at every step is impractical. The methodology has been used to examine solvatochromic shifts on vertical absorption maxima and along photophysical pathways with very promising results.

Efficient approach to MRCI: Multireference Configuration Interaction (MRCI) is a very useful tool in studying excited states, dissociation of molecules, and chemical systems with multireference character. We are testing approaches to reduce the computational cost of this method. We have found that using natural orbitals obtained from a high multiplicity state in a subsequent MRCI can give accurate results for excited states while reducing the cost substantially. This is very encouraging for being able to study photophysical properties of larger systems.





Understanding laser control of molecular dynamics


We are working in collaboration with Prof. Thomas Weinacht on a project aimed at understanding the underlying mechanisms involved in the dynamics and in laser control of basic chemical reactions, such as fragmentation and isomerization. Intense, shaped, ultrafast laser pulses are used to follow and control molecular dynamics and high level ab initio calculations to interpret the dynamics and guide the control. We are applying the techniques and understanding we have developed to dissociative ionization pump-probe spectroscopy and pulse shape spectroscopy.  Close collaboration between theory and experiment is the major element in this project and this interaction has already lead to detailed understanding of several key processes.  Theoretical calculations are used to examine the excited state dynamics of neutral systems, while calculations on the ion are used to find which fragmentation patterns after the probe which lead to the final signal. Key element is also the ionization process in strong laser fields, for which we have gained unique perspectives in recent work with other collaborators.


Development of methods and applications for electron-induced reactions


Electron-induced reactions are important in a myriad of chemical and biological process (including human physiology, cosmology and medicine). Electron attachment to a closed-shell neutral molecule usually represents a net thermodynamic destabilization - leading to a metastable anion resonance that is heavily coupled to the continuum. States within the continuum are unstable with respect to electron detachment – thereby driving the system towards the parent ground state neutral + electron at an infinite separation. Despite this finite electron detachment lifetime, experiments have observed a diverse range of electron-induced reactions following irradiation with electrons with well-defined kinetic energies. The clear competition between such electron-induced reactions and electron detachment is of particular interest to our group but is computational challenging. For example, the resonance/continuum coupling means that the associated electronic wavefunction is continuous. The continuous wavefunction is therefore non-square integrable and thus cannot be modelling with mainstream electronic structure theory. Our group is therefore interested in modifying existing electronic structure methods and developing complex Hamiltonian methods that ‘stabilize’ a given anion resonance position. Using such developments, we are particularly interested in the nuclear motions that drive an electron-induced reactions. Such methods were recently used to map the potential energy profiles associated with the dissociative electron attachment of Uracil observed experimentally (see figure). The interaction of low energy electrons with nucleobases is of particular importance because it can lead to biological damage.