Photoswitching of arylazopyrazoles upon S 1 (nπ*) excitation studied by transient absorption spectroscopy and ab initio molecular dynamics.

Arylazopyrazoles (AAPs) are an important class of molecular photoswitches with high photostationary states (PSS) and long thermal lifetimes. The ultrafast photoisomerization of four water-soluble arylazopyrazoles, all of them featuring an ortho -dimethylated pyrazole ring, is studied by narrowband femtosecond transient absorption spectroscopy and ab initio molecular dynamics simulations. Upon S 1 (nπ*) photoexcitation of the planar E -isomers ( E -AAPs), excited-state bi-exponential decays with time constants τ 1 in the 220-440 fs range and τ 2 in the 1.4-1.8 ps range are observed, comparable to those reported for azobenzene (AB). This is indicative of the same photoisomerization mechanism as has been reported for ABs. In contrast to the planar E -AAPs, a twisted E -AAP with two methyl groups in ortho -position of the phenyl ring displays faster initial photoswitching with τ 1 = 170 ± 10 fs and τ 2 = 1.6 ± 0.1 ps. Our static DFT calculations and ab initio molecular dynamics simulations of E -AAPs on the S 0 and S 1 potential energy surfaces suggest that twisted E -isomer azo photoswitches exhibit faster initial photoisomerization dynamics out of the Franck-Condon region due to a weaker π-coordination of the central CNNC unit to the aromatic ligands.