Substituent effect on the excited state dynamics of bistable photochromic hydrazones

Hydrazones are among the most promising newly developed photochromic molecules, often exhibiting very high isomerization quantum yields, long and tunable half-lives and excellent photostability. It has recently been demonstrated that substitutions on the hydrazone core with electron donating (EDG) or withdrawing (EWG) groups on either the rotor or stator moiety can greatly influence their photoswitching properties, in some cases conferring new and interesting properties onto them, such as emission toggling and two-photon activity. Here, we analyze the photodynamic behavior of three representative substituted hydrazones in depth, using ultrafast visible and infrared transient absorption spectroscopies, and perform density functional theory calculations to interpret their steady-state spectra. In particular, we focus on systems having an EDG and/or EWG substituent on the stator and rotor moieties, whose principal effect is to red shift the absorption spectra of the Z and E isomers relative to the parent unsubstituted hydrazone. Our results highlight the effect that substituents have on the isomerization and emission mechanisms of the hydrazones, and subsequently on their spectroscopic properties. Moreover, these studies elaborate on how the substituents and solvent polarity play important roles in altering the isomerization quantum yield.