Chemistry

Property and function of chemical and biological systems are dictated by molecular structures. The central focus of our research is the determination of the structures of organic molecules by quantum chemistry calculations and rotationally resolved electronic spectroscopy. The structures so obtained are used to understand the photophysical properties of the molecular system that are deduced from frequency- and time-domain laser spectroscopies in supersonic molecular beams. Current research centers on the following three major areas:

Structure and Excited-State Dynamics of Aromatic Clusters

As the species formed due to intermolecular interactions, the geometrical structures of aromatic clusters generated by free jet expansion provide important information concerning the nature of the forces between aromatic molecules and understanding the cluster's other properties. The inset displays the most stable structure of naphthalene and the two minimum energy forms of anthracene dimer that are obtained from our ab initio calculations. These structures, which are consistent with those based on the analyses of the electronic spectra, can account for the observation that the naphthalene trimer has substantial activation barrier for forming an excimer (dimeric species which are stable only in the excited electronic state), and that there are two dimer conformers of anthracene which exhibit different absorption spectra as well as different excimers.

Experiments involving high resolution rotationally resolved electronic spectroscopy are currently under way to deduce precise equilibrium structure of these and other aromatic clusters.

Intramolecular Charge Transfer in Bichromophoric Molecules

Relating electronic excitation, and relaxation, of dimeric species to those of the individual chromophore is a long-standing goal, which has important relevance to photochemical properties of molecular aggregates, polymers, and organized molecular assemblies, including the photosynthetic pigments.

Our concerted experimental and computational study of electronic excitation in bichromophoric aromatic molecules has demonstrated that the most noteworthy spectroscopic consequence of interchromoiety interaction is the appearance of the interchromophore charge transfer (CT) absorption band in the near infrared. The intensity of this CT absorption band, as well as the efficiency of photoionization, is strongly dependent on the interchromophore separation and orientation.

For a definite assignment of the CT transition, it is essential to demonstrate ion-pair character of the terminal state of the CT absorption, and to correlate the CT transition energies with the corresponding redox potentials of the bichromophoric molecules. Experiments along these lines are presently in progress.

Conformation and Symmetry Control of Electronic Relaxation

Radiationless transition, a process whereby a molecule makes a transition from one electronic state to another of lower energy without emission of radiation, plays a central role in photophysics and photochemistry of polyatomic molecules. The nonradiative processes are expected to depend strongly on conformation as well as symmetry of the initial quantum state relative to those of the final state. A good example of this is the highly varying photophysical properties (fluorescence and phosphorescence) of a homologous series of thiocarbonyl compounds we have observed.

The very different emission characteristics of the four-thiocarbonyl compounds can be rationalized on the basis of the presence or absence of the promoting mode of a 2 symmetry and the extent of S1 (lowest excited singlet) and T1 (lowest triplet) nonplanarity at the thiocarbonyl center. Another dramatic example of symmetry effect in radiationless transition is our discovery of the strong torsional symmetry dependence of S T1 radiationless transition in jet-cooled acetaldehyde. Thus, e methyl torsional sublevels of S1 undergo much more efficient S T1 nonradiative transition than the corresponding a torsional sublevels. We have attributed this remarkable torsional symmetry dependence to the torsion-rotation interactions leading to symmetry selective S1-T1 coupling.

We are presently carrying out ultra high resolution and ultrafast laser studies of the radiationless transitions in aliphatic carbonyls and diazabenzenes for the purpose of quantitatively testing existing theories of electronic relaxation.

Selected Publications

T. Fujiwara, E.C. Lim, R.H. Judge and D.C. Moule. An Optical-Optical Double Resonance Probe of the Lewest Triplet State of Jet-Cooled Thiophosgene: Rovibronic Structures and Electronic Relaxation. J. Chem. Phys. 124, 124301/1-124301/12 2006.

T. Fujiwara, E. C. Lim, and D. C. Moule, Symmetry Segregation of the Vibronic Levels within the S1 „] S0 System of Thiophosgene, Cl2CS, by Optical-Optical Double Resonance Spectroscopy, J. Chem. Phys. 126, 144304/1-144304/7 2007.

M. Z. Zgierski, T. Fujiwara, W. G. Kofron, and E. C. Lim, Highly Effective Quenching of the Ultrafast Radiationless Decay of Photoexcited Pyrimidine Bases by Covalent Modification: Photophysics of 5,6-Trimethylenecytosine and 5,6-Trimethyleneuracil, Phys. Chem. Chem. Phys., 95, 3206-3209 2007.

"Time-resolved EPR and Optical Studies of Intermoiety Interactions in the Lowest Triplet State of L-Shaped Dimers of Naphthalene: Conformation Dependence of Excitation Exchange Interaction," Terazima, M.; Cai, J.; and Lim, E.C.; J. Phys. Chem. A. ,2000, 104, 1662-1669.

"Electronic Spectra and Photophysics of the Two Stable Conformers of Anthracene Dimer: Evaluation of an Ab Initio Structure Prediction," Gonzalez, C., and Lim, E.C., Chem. Phys. Lett., 2000, 322, 382-388.

"Electronic States, Excimers, and Triplet Decay," East, A.L.L. and Lim, , E. C, J. Chem. Phys., 2000, 113, 8781 - 8994.

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Last modified: November 05 2007 16:11:35