Hydrazone Based pH‐Responsive Configurational Molecular Rotary Switches Containing a New Conjugated π‐Electronic Framework

A pyrido‐pyrimidinone heterocycle derived pH‐activated, hydrazone based molecular switch and its switching mechanism are reported here. The introduction of a new conjugated p electronic framework (enone part in the molecule) allowed the system to undergo hydrazone‐azo tautomerization , followed by E‐Z isomerization through rotation around the partial C‐N single bond. Under acidic pH, protonation took place selectively on pyrimidinone nitrogen, and that turned the molecule to the corresponding Z‐H+ configuration. On the other hand, addition of triethylamine in the same solution reverted the molecule back to its original E‐configuration. Extensive proton‐NMR, and detailed X‐ray crystallography coupled with UV‐vis spectroscopy were carried out to establish the switching phenomena. Theoretical DFT calculations leading to free energy diagrams, transition states, and intermediates shed light on the mechanism behind the switching phenomenon.