SCIENCE
Instrument Tracks Photolysis From Picoseconds to Seconds Powerful spectrophotometer follows reaction kinetics continuously, helps correlate excited-state photophysics with photochemical reactions A spectrophotometer that can fol low kinetics of photolytic reactions continuously from picoseconds to seconds following excitation has been built by chemists at Brookhaven National Laboratory. The in strument lets them correlate the photophysics of excited states with results of photochemical reactions. The Brookhaven researchers have demonstrated the power of the in strument by following the sequence of excited states that arises from pho tolysis of certain metal complexes. Their findings raise the possibility that chemists can use the high en ergies of various excited states to initiate multielectron chemical re actions in other molecules. Such a development would extend the use of metal complexes to carry out pho tochemical as well as thermal re actions. The instrument was developed at the Upton, N.Y., lab over the past 15 years by physical chemist Thom as L. Netzel, aided recently by inor ganic chemist Jay R. Winkler [J. Am. Chem. Soc, 108, 4451 (1986)]. Netzel has since moved to Amoco Corp/s research center in Naperville, 111. Winkler has succeeded him at Brook haven. The two scientists collaborated with inorganic chemistry professor Daniel G. Nocera of Michigan State University to investigate photolysis of such complexes as tetrachlorotetrakis(tributylphosphine)dimolybdenum(IV). The work was supported
by the Department of Energy, AMAX Foundation, Research Corp., and the Camille & Henry Dreyfus Founda tion. In the past, chemists have relied on luminescence, the light emitted by excited states on decay to ground state, to detect excited states. For the dimolybdenum complex, for ex ample, only one excited state was seen, which had a lifetime of nine nanoseconds. By using absorption spectropho tometry from picoseconds out to mi croseconds continuously, the three collaborators saw the combined de cay of both the nine-nanosecond excited state and another with a lifetime of 90 nanoseconds. They suggest that the nine-nanosecond intermediate gives rise to the 90nanosecond one, and that the 90nanosecond species does not lumi nesce on decay. That finding encourages Nocera to think that the high energy of the 90-nanosecond intermediate may initiate multielectron oxidation or reduction reactions in organic com pounds. That's because the per turbed, electron-rich bond between the two molybdenum atoms also of fers opportunities for donation or acceptance of electrons. Nocera will study that possibility in future work at Michigan State. In the ground state, the two mo l y b d e n u m atoms are q u a d r u p l y bonded by one σ-, two π-, and one Δ-bond, formed by overlap of d-orbitals. The nine-nanosecond inter mediate forms by a A-to-Δ* elec tronic transition. The structure of the 90-nanosecond species is un known. Nocera and Winkler will continue their collaboration to discover the structure. Winkler will add a multi channel detector to the Brookhaven instrument to make it as versatile
Netzel: incremental
improvements
in recording UV-visible spectra of intermediates as it is in following their kinetics. At Amoco, Netzel will build infrared spectrophotometry into his new instrument for even more detailed structural studies. Netzel emphasizes that develop ment of the Brookhaven instrument came from many incremental im provements rather than from one "breakthrough." Through the years, he shortened the firing time of the laser, improved detection, and per fected the optics. Data points are collected by photolyzing the sample solution with a laser shot and detecting the absorp tion of a probe beam after a fixed time lapse. Netzel devised two probe systems, one each for time lapses shorter and longer than 10 nano seconds. For time lapses from 10 nanoseconds to one second, the elec tronics of a digitizer are fast enough to record the signal from a probe beam from a xenon arc lamp. July 28, 1986 C&EN
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