Capturing a photoexcited molecular structure through time-domain X-ray absorption fine structure

The determination of the structure of transient molecules, such as photoexc ited states, in disordered media (such as in solution) usually requires met hods with high temporal resolution. The transient molecular structure of a reaction intermediate produced by photoexcitation of NiTPP-L-2 (NiTPP, nick eltetraphenylporphyrin; L, piperidine) in solution was determined by x-ray absorption fine structure (XAFS) data obtained on a 14-nanosecond time scal e from a third-generation synchrotron source. The XAFS measurements confirm that photoexcitation Leads to the rapid removal of both axial Ligands to p roduce a transient square-planar intermediate, NiTPP, with a lifetime of 28 nanoseconds. The transient structure of the photodissociated intermediate is nearly identical to that of the ground state NiTPP, suggesting that the intermediate adopts the same structure as the ground state in a noncoordina ting solvent before it recombines with two Ligands to form the more stable octahedrally coordinated NiTPP-L-2.