Overview of Research in the Drucker Group

 

    In the broadest terms, the aim of our research is to contribute to an understanding of photochemical reaction mechanisms. This is a complex problem that is being attacked from many different perspectives within the chemistry community. In the future, it is likely to be computational chemistry that provides the most detailed information about photochemical pathways. The experimental work in our molecular spectroscopy laboratory is designed to support the vigorous ongoing effort of the computational chemists to simulate photochemical events accurately and efficiently.

 

    What underlies the success (or failure) of any photochemical dynamics simulation is the accuracy of the computed excited-state potential-energy surface. This has steadily risen over the past decade, but still does not quite match the accuracy of ground-state calculations. Benchmark data from experiments are required for evaluating the excited-state calculations. This imperative has shaped my research program, which is focused on electronic spectroscopy. Our central experimental approach is laser absorption spectroscopy, conducted on low-pressure gas phase samples. Our work [1,2,3,4,5] has focused on small organic molecules that contain photochemically active functional groups or chromophores. Spectroscopic parameters available from our studies provide a rigorous test of computed excited-state properties. In broader terms, the spectra reveal changes in structure and dynamics that accompany photoexcitation and which can activate the molecule toward chemical transformation.


    The small, stable organic molecules we study have singlet ground-state configurations (S0); the singlet excited states (S1, S2,) are most accessible spectroscopically. However we are particularly interested in the lowest-energy triplet states because of their importance in photochemistry. Several combined factors make the triplet species (designated as Tn, n=1,2, ) uniquely suited to act as photochemical intermediates. For one, the low-lying triplet states may be readily populated in a solution-phase photochemical environment. This occurs via rapid nonradiative relaxation (intersystem crossing) from an initially photoexcited S1 state. The triplet yield can be significant, because S1 Tn intersystem crossing is often the fastest decay process when the S1 state is prepared with an appropriate amount of vibrational excitation [6,7,8,9,10]. Once the excited system reaches the triplet surface, it is especially prone to chemical reaction, due to a diradical electronic structure and slow radiative decay rate. Within this chain of events, the ultimate return to the ground state typically occurs via T1  S0 surface hopping. The shape of the T1 potential surface can thus strongly influence ground-state product formation.

 

    We have exploited the highly sensitive cavity ringdown (CRD) method [4,11,12,13,14] to detect the nominally spin-forbidden Tn S0 transitions in the gas phase. In the CRD technique, one monitors the leakage of a laser light pulse out of an optical cavity that is bounded by highly reflective mirrors. The decay function is I0 exp(-kt), where I0 is the initial pulse intensity, and k is the ringdown rate constant. In the presence of an absorbing sample, k increases when the laser is on resonance. The CRD absorption spectrum is recorded as the fitted value of k vs. wavelength. The very high sensitivity comes about because the measurement of k is almost completely immune to fluctuations in I0, the laser source intensity. Moreover, the very high reflectivity available for CRD mirrors leads to long ringdown times, high precision of the fitted k values, and correspondingly low noise in the CRD spectrum.

REFERENCES CITED

[1]
N. R. Pillsbury, J. Choo, J. Laane, and S. Drucker. Lowest n,p* triplet state of of 2-cyclopenten-1-one: Cavity ringdown absorption spectrum and ring-bending potential-energy function. J. Phys. Chem. A, 107:10648, 2003.
[2]
J. Choo, S. Kim, S. Drucker, and J. Laane. Density functional calculations, structure, and vibrational frequencies of 2-cyclopenten-1-one in its S0, S1(n,p*), T1(n,p*), and T2 (p,p*) states. J. Phys. Chem. A, 107:10648, 2003.
[3]
E. J. Gilles, J. Choo, D. Autrey, M. Rishard, S. Drucker, and J. Laane. Ultraviolet cavity ringdown spectra of 2-cyclohexen-1-one and its potential energy function and structure for the electronic ground state. Can. J. Chem., 82:867, 2004.
[4]
S. Drucker, J. L. Van Zanten, N. D. Gagnon, E. J. Gilles, and N. R. Pillsbury. Triplet excited states probed by cavity ringdown spectroscopy. J. Molec. Struct., 692:1, 2004.
[5]
N. R. Pillsbury and S. Drucker. Ultraviolet spectrum of vinylamine. J. Mol. Spectrosc., 224:188, 2004.
[6]
R. P. Wayne. Principles and Applications of Photochemistry, page 129. Oxford University Press, Oxford, 1988.
[7]
M. Reguero, M. Olivucci, F. Bernardi, and M. A. Robb. Excited-state potential surface crossings in acrolein-a model for understanding the photochemistry and photophysics of a,b-enones. J. Am. Chem. Soc., 116:2103, 1994.
[8]
M. Reguero, F. Bernardi, M. Olivucci, and M. A. Robb. A model study of the mechanism of the type B (di-p-methane) and lumiketone rearrangement in rotaionally constrained a,b enones. J. Org. Chem., 62:6897, 1997.
[9]
E. García-Expósito, M. J. Bearpark, R. M. Ortu[(n)\tilde]o, V. Branchadell, M. A. Robb, and S. J. Wilsey. The T1 3(p-p*)/S0 intersections and triplet lifetimes of cyclic a,b-enones. J. Org. Chem., 66:8811, 2001.
[10]
H. E. Zimmerman and E. E. Nesterov. An experimental and theoretical study of the type C enone rearrangement: Mechanistic and exploratory organic photochemistry. J. Am. Chem. Soc., 125:5422, 2003.
[11]
A. O'Keefe and D. A. G. Deacon. Cavity ring-down optical spectrometer for absorption measurements using pulsed laser sources. Rev. Sci. Instrum., 59:2544, 1988.
[12]
J. J. Scherer, J. B. Paul, A. O'Keefe, and R. J. Saykally. Cavity ringdown laser absorption spectroscopy: History, development, and application to pulsed molecular beams. Chem. Rev., 97:25, 1997.
[13]
M. D. Wheeler, S. M. Newman, A. J. Orr-Ewing, and M. N. R. Ashfold. Cavity ring-down spectroscopy. J. Chem. Soc., Faraday Trans., 94:337, 1998.
[14]
G. Berden, R. Peeters, and G. Meijer. Cavity ring-down spectroscopy: Experimental schemes and applications. Int. Rev. Phys. Chem., 19:565, 2000.



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