X-ray crystallography of photoinduced state set stage for studying dynamics with atomic resolution
Ask chemists about the molecule that ushered in the new era of X-ray crystallography studies of short-lived excited-state molecules, and many would point to the nitroprusside ion.
X-ray crystallography had always been a tool for the study of molecules in stasis, their ground-state structures locked in a crystal lattice.
But as X-ray sources grew more sophisticated over the decades, the possibility of using the technique to study the structure of molecules in their excited state was not lost on chemists. They knew they would likely be able to learn immeasurable amounts about how molecules change their structure as they’re about to react or even during their reaction.
The nitroprusside ion was believed to have a photoinduced excited state with a lifetime of more than two hours at low temperatures. In the early 1990s, chemistry professor Philip Coppens, at the University at Buffalo, SUNY, reasoned this state’s long life would make the molecule an ideal candidate for a fledgling excited-state X-ray crystallographic study.
He succeeded, and in a 1994 paper, his team reported the structure, which was hailed as the first excited-state structure obtained by X-ray crystallography (J. Am. Chem. Soc., DOI: 10.1021/ja00091a030).
However, as Coppens tells C&EN, the story isn’t so straightforward. He and his colleagues soon discovered that the photoinduced metastable state of nitroprusside was not, in fact, in an excited electronic state—it was actually the product of a photoinduced rearrangement, in which the NO group had bound differently to the transition metal.
Nevertheless, the structure set the stage for using X-ray crystallography to examine dynamics. New synchrotron facilities, such as the Advanced Photon Source, offered bright, short X-ray pulses, which could capture the microseconds or less timescales of many excited states.
In fact, biochemists were already beginning to take advantage of synchrotron facilities to study the dynamics of “soft crystals,” which are composed of biomacromolecules such as enzymes that have a lot of solvent inside. Such efforts have revealed more global dynamics, such as CO dissociation from myoglobin, but without atomic-scale resolution.
Coppens and others also began using synchrotron facilities to probe atomic-scale dynamics. Finally, in 2002, Coppens’s group reported what they say was the real first X-ray crystallographic excited-state structure of a molecule: the binuclear platinum ion [Pt2(P2O5H2)4]4–, which has an excited state of only 50 microseconds at 17 K (Acta Crystallogr. A, DOI: 10.1107/s0108767301017986).
The Pt-Pt bond contracted by 0.28 Å when excited. A few years later, the Coppens lab discovered a much larger contraction, of 0.85 Å, in the Rh-Rh bond of the [Rh2(1,8-diisocyano-p-menthane)4]2+ ion, which had a lifetime of only 11.7 microseconds (Chem. Commun. 2004, DOI: 10.1039/b409463h).
Since then, atomic-resolution studies of excited-state crystals have proliferated. And new X-ray free-electron lasers, such as the Linac Coherent Light Source at the SLAC National Accelerator Laboratory can produce beams a billion times brighter than traditional synchrotron sources with femtosecond-timescale pulses—promising unprecedented exploration of chemical dynamics.—Elizabeth Wilson