Systematic Modulation of Hydrogen Bond Donors in Aminoazobenzene Derivatives Provides Further Evidence for the Concerted Inversion Photoisomerization Pathway

A series of aminoazobenzene derivatives structurally related to AzoAMP‐1 have been prepared and characterized by using a variety of analytical techniques. AzoAMP‐1 is based on 2,2′‐diaminoazobenzene with N‐methylpyridine groups. The new derivatives all contain a hydrogen bond between the aniline hydrogen atom and the azo group, as well as a separate pendant functional group that could contribute an additional hydrogen‐bond acceptor to an intramolecular network. A combination of photoisomerization studies and NMR spectroscopic and X‐ray crystallographic investigations suggest that AzoAMP‐1 possesses a unique structure that prevents isomerization through the concerted inversion pathway, which cannot be reproduced with other types or arrangements of substituents. Only AzoAMQ, which contains a similar quinolone heterocycle in place of the pyridine group of AzoAMP‐1, displayed somewhat similar photochemistry. Noncovalent interactions or electronic effects interrupt photoisomerization in certain azobenzene derivatives. Disrupting these processes could provide new molecular switches. Hydrogen bonding can prevent photoisomerization by blocking the concerted inversion mechanism, however, only select groups are capable of doing so. The scope of this behavior was elucidated in the current study.