Photo-Triggered Chiroptical Switching of Platinum Complexes Bearing Azobenzene Moieties

Abstract

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.