Most current XTA studies are on the transient structures of excited states that are directly generated by photons through ground to excited state transitions. In solar energy conversion processes, such as photocatalysis, the most interesting structures to be determined are often those resulting from the excited state, such as a photocatalyst-substrate complex and other intermediate structures produced in subsequent transformations. In order to find the structure/property correlations of the catalysts, a detailed knowledge and direct observation of reaction trajectories is necessary.
As illustrated by Figure 12.11, a photoinitiated chemical reaction starts from the light absorption/excited state formation, followed by its transformation to other intermediates and finally the product. It is only possible to probe the structures of these transient species if they have synchronized or in-phase actions. When a laser pulse with 1013 14 photons and a bandwidth much smaller than the vibronic level separation strikes a sample, it creates a sub-set of excited state molecules with similar energetics, or more intuitively, at the same point of the multi-dimensional potential landscape (Figure 12.11). This sub-set of the excited state population initially with coherent vibrational motions will proceed to pass multiple potential barriers and then to different intermediate states. As the barrier crossing occurs, the coherence of molecular motions will be lost because the probability of barrier crossing is highly dependent upon the trajectory. A slight deviation of the trajectory could result in success or failure of the barrier crossing. Owing to the nature of the reaction dynamics understood so far, capturing the transition state structures beyond the first barrier crossing will be very challenging, the timescales for the barrier crossing (i. e., a few to a
* » ”
//\ “ 4.
у / ЛЛ 4 Ч N
J * Vs. > 4
Jf / "
і " [Cu(ll)dmpJ*3
/ / —————— [Cu(l)dppJ*
J ‘ – – ‘ [Cu(ll)dppJ*2
Solvent ligation coordinate 3 93 8.99 9.00 9.01
Figure 12.10 (A) MLCT state structural dynamics of [Cu(I)(dmp)2]+ + hv-
[Cu(II)(dmp)(dmp)~]+ as functions of two key reaction coordinates, the angle between the two ligand planes and the solvent ligation distance; (B) structural dependence of XANES spectra for [Cu(I)(dmp)2]+ and [Cu(I)(dpp)2]+ (dpp = 2,9-dipenyl-1,10-phenan – throline) and their corresponding Cu(II) species generated by bulk electrolysis. The significant differences in the Cu(I) and Cu(II) spectra provide bases for monitoring the oxidation state and geometry during photoinduced interfacial charge transfer in DSSC composed of the derivatives of these complexes.
few tens of fs) is many orders of magnitude shorter than the average time for the molecular trajectory from one thermally equilibrated intermediate to another (e. g., ns-ms). In order to deal with multiple transient species on multiple timescales and gradual dephasing of the coherent motions, the signal-to-noise ratio of the data must be high in order to implement principle component
analysis or singular value decomposition in data analyses. Meanwhile, rephasing molecules at a certain time after the excitation remains a challenge.
Advances in XTA enable direct comparisons between transient structures obtained from experimental observations and theoretical calculations, which simultaneously provide guidance and pose challenges for theoretical modeling. A few requirements in theoretical modeling that only reflect the authors’ opinion are briefly outlined below.