When exposed to light at a specific wavelength, azobenzene and its derivatives experience a transformation from trans form to cis form through isomerization. Due to its ability to change color upon illumination, azobenzene finds extensive use in various molecular devices and functional materials. However, despite significant researches focused on practical applications, there are still ongoing investigations into the underlying mechanisms governing azobenzene’s photochemical reactions and isomerization. In this study, we employ femtosecond stimulated Raman spectroscopy (FSRS), and transient absorption spectroscopy, in conjunction with quantum chemical calculations, to elucidate the ultrafast isomerization dynamics of an azobenzene derivative known as trans-AZOTIPS ((E)-1-phenyl-2-((triisopropylsilyl)ethynyl)diazene). The results demonstrate that upon photoexcitation, rapid isomerization occurs along the C−N=N bonds via the singlet excited state S1 to hot ground state (S0*) state transition. Additionally, we explore the impact of solvent viscosity on the isomerization process and find that the duration of isomerization remains unaffected by variations in solvent viscosity. These results suggest that the isomerization pathway involves a volume-conserving motion known as “hula twist”. After that, the vibrational cooling process is obtained in S0 state.

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