Photo-Triggered Chiroptical Switching of Platinum Complexes Bearing Azobenzene Moieties

l-Alanine-derived optically
active trans-platinum complexes bearing azobenzene
moieties, l-trans-Pt­[P­(Ph)2-1,4-C6H4-CONH-C*H­(CH3)-CONH-1,4-C6H4-NN-Ph]2(CCPh)2 (L-2a-5t) and l-cis-Pt­[P­(Ph)2-1,4-C6H4-CONH-C*H­(CH2Ph)-CONH-1,4-C6H4-NN-Ph]2(CCPh)2 (L-2a-5c) were
synthesized by the ligand exchange reaction of trans-Pt­(PPh3)2(CCPh)2 and cis-Pt­(cod)­(CCPh)2, respectively, with l-P­(Ph)2-1,4-C6H4-CONH-C*H­(CH2Ph)-CONH-1,4-C6H4-NN-Ph. The trans-azobenzene moieties of the complexes isomerized to
the cis form upon UV-light irradiation and returned
to the trans form upon visible-light irradiation. L-2a-5t displayed weak circular
dichroism (CD) signals irrespective of the trans and cis forms of the azobenzene moieties. On the other hand, L-2a-5c with trans-azobenzene moieties displayed strong CD signals, whose intensity
decreased upon UV-light irradiation, accompanied by an increase in
CD signals based on cis-azobenzene moieties. DFT
calculations of the complexes indicated that intramolecular hydrogen
bonds form between the amide moieties of two ligands in the cis-platinum complexes, resulting in retention of conformation.
A noncovalent interaction plot analysis also indicated that the π–π
interaction between the benzene rings of the ligands also contributes
to conformational maintenance. The d-alanine-derived counterparts, D-2a-5t and D-2a-5c, exhibited mirror-image CD spectroscopic
patterns in comparison to those of L-2a-5t and L-2a-5c. Solution-state IR absorption
spectroscopic peaks of CO stretching and N–H bending
vibrations of L-2a-5c were
observed broadly at ca. 10 cm–1 lower and higher
than those of P­(Ph)2-1,4-C6H4-CONH-C*H­(CH3)-CONH-1,4-C6H4-NN-Ph (L-2a), respectively, thus supporting the formation of intramolecular
hydrogen bonding at the amide groups.