Michael R. Topp

Professor of Chemistry
Physical Chemistry, Chemical Physics

Office: 249 N
Lab: 240 N
Phone: (215) 898-4859
Email: mrtsasupennedu

Jump to: Research Statement | Education and Academic History | Selected Publications

Research Statement
Conformational Relaxation in Isolated Molecular Clusters

When molecules become electronically excited, the rearrangement of electronic charge can precipitate many types of relaxation processes. To probe details of such events, one can employ isolated molecular clusters consisting of only a few hydrogen-bonded molecules. One particularly interesting case is the cluster involving a Coumarin 151 molecule bonded to a water dimer. Two different structures have been identified in the ground state, as shown below.

Recent experiments have shown that the species shown on the left here is unstable in the excited state, and relaxes to that shown on the right on a picosecond or nanosecond time scale depending on the available energy. This corresponds to movement of the water dimer by ~10Å from one side of the molecule to another, following and yet the activation energy is only 60 cm-1. Such conformational changes have been studied by a combination of fluorescence and infrared double resonance techniques in conjunction with ionization and mass resolution.

Hydrogen-Bonded Molecular Dimers

Hydrogen-bonded dimers present important opportunities to study short-range intermolecular interactions, including modification of the electronic structure, and corelated proton or hydrogen atom transfer. Molecules such as the dimer of 4-Amino-N-methylphthalimide, shown here, reveal dramatic changes in their infrared spectra between the ground and excited states. The simple ground-state ground-state infrared spectrum reflects the high symmetry of the ground state. On the other hand, the much more complex excited-state spectrum shows evidence for a loss of symmetry resulting from changes in the acid-base properties of NH2 and >C=O groups, which may result in proton transfer across the intermolecular hydrogen bonds. These types of strongly bonded dimers are different from many other dimer systems studied so far, because both their electronic and vibrational spectra are highly structured, despite the large increase in binding energy upon electronic excitation. Femtosecond-domain experiments are planned, to follow in real time the changes in vibrational spectra, which will provide further insights into the reasons for their complexity in the excited state.

Ultrafast Electronic Relaxation of Hydrogen-Bonded Molecules Studied by Femtosecond Pump-Probe Spectroscopy

Femtosecond pump-probe experiments allow us to observe the time evolution of the first events following pulsed laser excitation, including motions in the first coordination shell of a hydrogen-bonded molecule in fluid solution. New pump-probe experiments involving the detection of ultrashort-lived fluorescence have explored spectroscopic changes of aminophthalimide molecules in hydrogen-bonding solvents on a time scale more than 10 times faster than existing data for fluorescence Stokes shifts. Both fluorescence upconversion and pump-probe methods are being used to investigate ultrafast energy transfer processes in complex molecules, in collaboration with the Regional Laser and Biomedical Technology Laboratories at Penn.

Education and Academic History

Selected Publications
139. Syntheses, Photophysical Properties, and Applications of Through-bond Energy Transfer Cassettes for DNA Sequencing, G.-S. Jiao, L. H. Thoresen, T.G. Kim, W. C. Haaland, M. R. Topp, R.M. Hochstrasser, M. L. Metzker, and K. Burgess, Chem. Eur. J. 12 7816-7826 (2006).

138. Femtosecond-domain Dynamical Studies of Energy-Transfer Cassettes, T.G. Kim, M.R. Topp, R.M. Hochstrasser, J.C. Castro, A. Loudet, J. G.-S. Jiao, and K. Burgess, Femtochemistry VII: Fundamental Ultrafast Processes in Chemistry, Physics, and Biology; Castleman, A. W., Jr., Kimble, M. L., Eds.; Elsevier: Amsterdam, 128-131 (2006).

137. Correlations of structure and rates of energy transfer for through-bond energy-transfer cassettes: T.G. Kim, J.C. Castro, A. Loudet, J.G.S. Jiao, M.R. Topp, R.M. Hochstrasser, K. Burgess, M.R. Topp, J. Phys. Chem. A. 110, 20-27 (2006).

136. How are the abbreviations of the periodic table determined?: M.R. Topp, Sci. Am. 292, 106 (2005).

135. Ultrafast Excited-State Deprotonation and Electron Transfer in Hydroxyquinoline Derivatives, T.G. Kim and M. R. Topp, J. Phys. Chem. A108, 10060-10065 (2004).

134. Solvent Effects on the Fluorescence Depolarization Rates of Coumarins in Solution: The Likely Influence of Site-Selective Hydrogen Bonding, T.G. Kim and M.R. Topp, J. Phys. Chem. A 108, 7653-7659 (2004).

133. A Blue-to-red Energy-transfer Thymidine Analog that Functions in DNA, G.-S. Jiao, T.G. Kim, M.R. Topp, and K. Burgess, Org. Lett., 6, 1701-1704 (2004).

132. Anthracene-BODIPY® Cassettes: Syntheses and Fluorescence Energy Transfer, C.-W. Wan, A. Burghart, J. Chen, F. Bergström, L.B.-Å. Johansson, M.F. Wolford, T.G. Kim, M.R. Topp, R.M. Hochstrasser and K. Burgess, Chem. Eur. J., 9, 1-12 (2003).

131. Ultrashort-lived Excited States of Aminophthalimides in Fluid Solution, T.G. Kim, M.F. Wolford and M.R. Topp, Photochem. Photobiol. Sci., 2, 576 - 584 (2003).

130. Infrared-Optical Double-Resonance Measurements of Hydrogen-Bonded Interactions in Clusters involving Aminophthalimides, Y. Chen and M.R. Topp, Chem. Phys. 283, 249-268 (2002).

129. Infrared Spectroscopy of Jet-Cooled, Electronically Excited Clusters of Coumarin 151: Excited-State Interactions and Conformational Relaxation, Y. Chen, P.M. Palmer and M.R. Topp, Int. J. Mass Spectr. 220, 231-251 (2002).

128. Excited State Interactions in the Doubly Hydrogen-Bonded Jet-Cooled Dimers of 4-Aminophthalimide and 4-Amino-N-methylphthalimide, Y. Chen and M.R. Topp, Chem. Phys. Lett. 355, 270-278 (2002).

127. Conformational Barrier Crossing in Electronically Excited Coumarin Clusters Studied by Infrared-Optical Double-resonance Spectroscopy, Y. Chen and M.R. Topp, Chem. Phys. Lett. 337, 284-292 (2001).

126. Perylene/Water Clusters: Some Different Trends in Hydrogen-Bonded Structure Induced by a Large Aromatic Template, P.M. Palmer, Y. Chen and M.R. Topp, Chem. Phys. Lett. 325, 568-576 (2000).

125. Structural Differences among Methanol Clusters (n=1-4) Hydrogen-Bonded to Coumarin 151, P.M. Palmer, Y. Chen and M.R. Topp, Chem. Phys. Lett. 321, 62-70 (2000).

123. Identification of Dual Conformers of Coumarin 153 under Jet-Cooled Conditions, B.A. Pryor, P.M. Palmer, Y. Chen and M.R. Topp, Chem. Phys. Lett. 299, 536-544 (1999).

122. Electronic Spectroscopy of Jet-Cooled Anthracene/(H2O)n Clusters (n=1-16): Comparisons of Inhomogeneous Structure, P.M. Palmer and M.R. Topp, Chem. Phys. 239, 65-81 (1998).

120. Spectroscopy of Jet-Cooled Water Complexes with Coumarin 151: Observation of Vibronically Induced Conformational Barrier Crossing, B.A. Pryor, P.M. Palmer, P.M. Andrews, M.B. Berger and M.R. Topp, J. Phys. Chem., A102, 3284-3292 (1998).

119.Spectroscopic Properties of Jet-Cooled Methanol Clusters, n=1-6, Attached to Perylene, P.M. Palmer and M.R.Topp, Chem. Phys. Lett., 286, 113-120 (1998).

116. Rotational Coherence Measurements of van der Waals Complexes of Perylene with Water and Alcohols, P.M. Andrews, B.A. Pryor, M.B. Berger, P.M. Palmer, and M.R. Topp, J. Phys. Chem. A101, 6222-6232 (1997).