Scanning Maxwell stress microscopy of photo-induced charge separation in A–S–D triad monolayers

Photo-induced charge separation in mixed Langmuir–Blodgett (LB) films containing oriented A–S–D triads and inert matrix molecules was studied by using scanning Maxwell stress microcopy. A, S, and D stand for electron acceptor, sensitizer, and electron donor moieties of the triad, respectively. The change in surface potentials upon photo-excitation corresponded well with creation and disappearance of photo-induced dipole moments in the LB films due to the photo-induced charge separation in the oriented A–S–D triads followed by charge recombination. The observed lifetime of the separated charges in the LB films, however, was longer by several orders of magnitude than that of isolated A–S–D triads in solution observed by transient absorption spectroscopy. The magnitude of the photo-induced surface potentials, i.e. the content of the separated charges with the longer lifetime decreased drastically with the decrease in the concentration of A–S–D triads in the mixed LB films. Therefore, we concluded that the separated charges with the longer lifetime were created by lateral diffusion of photo-produced anion (electrons) and cation radicals (holes) among neighboring A and D moieties, respectively, in the LB films. We also found that addition of a second donor (D′) monolayer next to a layer of D end moieties of the mixed A–S–D monolayer in multilayered LB films enhanced the photo-induced charge separation with the long lifetime. When light harvesting (H) molecules acting as antenna was mixed with the A–S–D triads in place of the inert matrix molecules, the photo-induced charge separation was observed under irradiation of photons, which were absorbed by the H moieties.