The novel (pyridylazo)phenol derivative 8-[4-(2-pyridylazo)phenoxy]octyl disulfide (1) has been designed and synthesized for the formation of a monolayer film on gold-coated optical waveguides via the technique of self-assembly. The photoinduced trans to cis isomerization of the self-assembled monolayer of 1 (1 SAM) and its interaction with transition-metal ions have been studied by spectroscopic (UV-visible and reflection absorption infrared) and electrochemical methods. The 1 SAM in the trans isomeric form provides a bidentate ligand that can chelate metal ions, such as Ni2+ and Co2+, and results in a strong absorption in the UV-visible absorption spectrum at 433 nm, a red shift of 77 nm from the maximum absorption wavelength for the unchelated 1 SAM. Following irradiation with waveguided UV light (? = 365 nm) via the evanescent field, the 1 SAM photoisomerizes to the cis form. No metal chelation was observed with the cis isomer of the 1 SAM. The photoinduced processes are reversible; the cis form SAM is converted to the trans form SAM after visible light irradiation (? = 439 nm), again via the evanescent wave. The trans 1 SAM was used to detect Ni2+ in the range 9.6 × 10-5-2.44 × 10-3 M.