Photoswitching of arylazopyrazoles upon S1 (nπ*) excitation studied by transient absorption spectroscopy and ab initio molecular dynamics

Abstract

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 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 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 τ₁ = 170 ± 10 fs and τ₂ = 1.6 ± 0.1 ps. Our static 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.