An Evanescent-Field-Driven Self-Assembled Molecular Photoswitch for Macrocycle Coordination and Release

Abstract

Two self-assembled monolayer (SAM) films containing the photoswitchable 4-pyridylazophenoxy chromophore have been deposited onto a gold-coated glass substrate. One film contains the chromophore as a single component, 1 SAM, and the other is doped with a nonphotoactive component as a 1:1 mixture, 2 SAM. The reversible photoswitching performances of 1 SAM and 2 SAM via the evanescence field using light of appropriate wavelengths have been investigated by UV spectroscopic and electrochemical monitoring. In principle, the trans-form SAMs present a coordinating surface, the “on” state, that can be switched “off” in the cis form. This has been illustrated by immersing both the as-deposited (trans form) SAMs and the photoswitched (predominantly cis form) SAMs into solutions of cobalt and zinc tetraphenylporphyrin (CoTPP and ZnTPP, respectively) and an octaoctyl-substituted cobalt phthalocyanine. In a further phase of this study, the remote control of binding events at the surface of the SAMs has been demonstrated through evanescent-field-driven photoswitching of trans-form SAMs coordinated at the surfaces with examples of these metallomacrocycles. This photoswitching was undertaken with the constructs immersed in neat toluene, and the macrocycles were released from the surface into the solvent. The release was measured by spectroscopic monitoring of the material remaining on the constructs. The study was extended to develop an in situ release/coordination cycle. Thus, irradiation of a construct of ZnTPP bound to the surface of trans-form 2 SAM using waveguided light at 365 nm releases the macrocycle into a toluene solution of ZnTPP. Further irradiation of the SAM, now in its cis form, with waveguided 439 nm light regenerates the trans form, which recoordinates ZnTPP from the solution. The results demonstrate the potential for using waveguided light to control molecular events within and at the surfaces of SAM constructs

Direct monitoring of opto-mechanical switching of self-assembled monolayer films containing the azobenzene group

“…Previous work has generated some questions about the role of steric hindrance in the cis ↔ trans conversion within a monolayer film [9–10]. If the film is close-packed, there is some evidence that the conversion is restricted.…”

“…Central to the function of such systems are changes in the inter- and intramolecular forces accompanying the transitions. In particular, by virtue of packing into a self-assembled film, steric constraints on the cis ↔ trans conversion, which do not exist in the isolated molecule or bulk disordered films, could dominate the switching [9–10]. Strictly, this steric hindrance requires close packing, thus some slight disorder in the film could be an enabling condition for the isomerization [11].…”

UV/Vis Spectroscopy Studies of the Photoisomerization Kinetics in Self-Assembled Azobenzene-Containing Adlayers

“…The precise reasons for these effects are still under debate. Originally, this was predominantly attributed to the close-packed arrangement of the azobenzene units in the SAM, which impedes trans-cis photoisomerization by steric constrains 22,23 . Another explanation is the electronic quenching of the isomerization, caused by alternative pathways for the deexcitation of the optical excited electronic state, which compete with the photoisomerization reaction.…”

Optical absorption and excitonic coupling in azobenzenes forming self-assembled monolayers: a study based on density functional theory

“…[6][7][8] To use azobenzene switches in SAMs at metal surfaces, it is of importance to decouple individual chromophores from the surface but also from other, neighbouring adsorbates. Otherwise, rapid quenching of photoexcited species by electronic coupling to the metal, steric hindrance by mechanical coupling to neighbours during isomerization, 9,10 or exciton transfer to neighbour molecules by lateral dipole coupling, 11 all tend to suppress switching. Indeed, switching of azobenzenes in densely packed SAMs is often unsuccessful, even when electronic coupling to the surface was minimized by attaching them to isolating linker groups.…”