An evanescent-field-driven self-assembled molecular photoswitch for macrocycle coordination and release

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.