Photoswitching of arylazopyrazoles upon S1 (nπ*) excitation studied by transient absorption 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 calculations. Upon S1 (nπ*) photoexcitation of the planar E-isomers (E-AAPs), excited-state bi-exponential decays with time constants τ1 in the 220–440 fs range and τ2 in the 1.4–1.8 ps range are observed, comparable to those reported for azobenzene (AB). This is indicative of the same bottleneck for completing the photoisomerization which was 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 τ1=170 ± 10 fs and τ2=1.6 ± 0.1 ps. Our DFT calculations and ab-initio molecular dynamics simulations of E-AAPs on the S0 and S1 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.