UV−Vis, NMR, and Time-Resolved Spectroscopy Analysis of Photoisomerization Behavior of Three- and Six-Azobenzene-Bound Tris(bipyridine)cobalt Complexes

Abstract

The photoisomerization properties of tris(bipyridine)cobalt complexes containing six or three azobenzene moieties, namely, [Co(II)(dmAB)3](BF4)2 [dmAB = 4,4'-bis[3''-(4'''-tolylazo)phenyl]-2,2'-bipyridine], [Co(III)(dmAB)3](BF4)3, [Co(II)(mAB)3](BF4)2 [mAB = 4-[3' '-(4' ''-tolylazo)phenyl]-2,2'-bipyridine], and [Co(III)(dmAB)3](BF4)3, derived from the effect of gathering azobenzenes in one molecule and the effect of the cobalt(II) or cobalt(III) ion were investigated using UV-vis absorption spectroscopy, femtosecond transient spectroscopy, and 1H NMR spectroscopy. In the photostationary state of these four complexes, nearly 50% of the trans-azobenzene moieties of the Co(II) complexes were converted to the cis isomer, and nearly 10% of the trans-azobenzene moieties of the Co(III) complexes isomerized to the cis isomer, implying that the cis isomer ratio in the photostationary state upon irradiation at 365 nm is controlled not by the number of azobenzene moieties in one molecule but rather by the oxidation state of the cobalt ions. The femtosecond transient absorption spectra of the ligands and the complexes suggested that the photoexcited states of the azobenzene moieties in the Co(III) complexes were strongly deactivated by electron transfer from the azobenzene moiety to the cobalt center to form an azobenzene radical cation and a Co(II) center. The cooperation among the photochemical structural changes of six azobenzene moieties in [Co(II)(dmAB)3](BF4)2 was investigated with 1H NMR spectroscopy. The time-course change in the 1H NMR signals of the methyl protons indicated that each azobenzene moiety in [Co(II)(dmAB)3](BF4)2 isomerized to a cis isomer with a random probability of 50% and without interactions among the azobenzene moieties.

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

“…[23][24][25] Various methods for suppressing AB transǞcis photoisomerization have also been reported. By introducing a redox active metal ion such as iron or cobalt coordinated onto an AB-functionalized ligand, metal oxidation state changes shift the AB absorption maximum (λ max ), which causes both isomers to absorb strongly at the same wavelength.…”

Induction of E/Z isomerization in a pendant metal-bound azobenzene: a synthetic, spectroscopic and theoretical study

“…[1][2][3][4] Azobenzene has been appended to a number of chromophores, including transition metal complexes, in an attempt to couple the E/Z isomerization process to an external electronic transition. [5][6][7][8][9][10][11][12][13] Transition metal compounds that exhibit metal to ligand charge transfer transitions (MLCT), as seen in many d 6 metal complexes, are ideal for transferring energy to an external group. [14][15][16][17][18] In this report, we present a study of two Re(CO) 3 diimine complexes with a pendant azobenzene group.…”

Molecular docking, theoretical calculations and potentiometric studies of some azo phenols

“…The dyes have been most widely used in fields such as dying textile fibers, biomedical studies, advanced applications in organic synthesis and high technology areas like lasers, liquid crystalline displays, electrooptical devices and ink-jet printer [7]. The presence of\ \N_N\ \ group can lead to increase the solubility of low valent metal oxidation states due to its π acidity and presence of low lying azo centered π* molecular orbitals [8].…”