Surface‐Confined Light Harvesting, Energy Transfer, and Amplification of Fluorescence Emission in Chromophore‐Labeled Self‐Assembled Monolayers

Scanning near-field optical microscopy (SNOM) has become a widespread technique due to its promising ability of imaging with sub-micron resolution. Despite being developed over more than one decade, SNOM is still not a mature technique, which can be seen from the large number of recent publications describing instrumentational innovations. However, there are also many applications of near-field microscopy to the observation of thin organic film systems, which are supplementary to other techniques and demonstrate the usefulness of the technique.

Scanning near-field optical microscopy

A phase-separated monolayer of a binary mixture of pyrene-labeled poly(isobutyl methacrylate) (PiBMA-Py) and perylene-labeled poly(octadecyl methacrylate) (PODMA-Pe) was investigated by the fluorescence scanning near-field optical microscope (SNOM), which provided optical images of samples with a high resolution beyond the diffraction limit, by using evanescent field from an aperture smaller than the wavelength of light. Each phase of the PiBMA-Py/PODMA-Pe was selectively imaged as a bright area by choosing the excitation wavelength (325- or 442-nm line of a He−Cd laser). The fluorescence SNOM enabled mapping of the excitation energy transfer efficiency on the monolayer plane. The energy transfer measurement revealed that the phase separation of the PiBMA-Py/PODMA-Pe blend at the air/water interface was almost completed by annealing for 60 min at 40 °C and the phase boundary had a width of a few hundred nanometers.

Phase-Separation Structure of a Monolayer of Binary Polymer Blend Studied by Fluorescence Scanning Near-Field Optical Microscopy

A trinuclear [{RuII(bpy)2(bpy-terpy)}2FeII]6+ complex (I) in which a FeII-bis-terpyridine-like centre is covalently linked to two RuII-tris-bipyridine-like moieties by a bridging bipyridine-terpyridine ligand has been synthesised and characterised Its electrochemical, photophysical and photochemical properties have been investigated in CH3CN and compared with those of mononuclear model complexes The cyclic voltammetry of (I) exhibits, in the positive region, two successive reversible oxidation processes, corresponding to the FeIII/FeII and RuIII/RuII redox couples These systems are clearly separated (ΔE1/2 = 160 mV), demonstrating the lack of an electronic connection between the two subunits The two oxidized forms of the complex, [{RuII(bpy)2(bpy-terpy)}2FeIII]7+ and [{RuIII(bpy)2(terpy-bpy)}2FeIII]9+, obtained after two successive exhaustive electrolyses, are stable (I) is poorly luminescent, indicating that the covalent linkage of the RuII-tris-bipyridine to the FeII-bis-terpyridine subunit leads to a strong quenching of the RuII* excited state by energy transfer to the FeII centre Luminescence lifetime experiments show that the process occurs within 6 ns The nature of the energy transfer process is discussed and an intramolecular energy exchange is proposed as a preferable deactivation pathway Nevertheless this energy transfer can be efficiently quenched by an electron transfer process in the presence of a large excess of the 4-bromophenyl diazonium cation, playing the role of a sacrificial oxidant Finally complete photoinduced oxidation of (I) has been performed by continuous photolysis experiments in the presence of a large excess of this sacrificial oxidant The comparison with a mixture of the corresponding mononuclear model complexes has been made

Photoredox vs. energy transfer in a Ru(II)-Fe(II) supramolecular complex built with an heteroditopic bipyridine-terpyridine ligand.

Langmuir−Blodgett (LB) layers of 16-(9-anthroyloxy)palmitic acid (AA) and C60 mixed with tricosanoic acid have been studied as a model system for photoinduced electron transfer (PET). The PET has been investigated by monitoring the quenching of AA fluorescence in mixed LB films containing different concentrations of C60. Perrin's hard-sphere model adapted for the two-dimensional case provides a good fit to the experimental results, confirming the two-dimensional nature of the system. The critical radius is found to be 11 A.

Photoinduced Electron Transfer between 16-(9-Anthroyloxy)palmitic Acid and Fullerene C60 in Langmuir−Blodgett Films

A water-soluble metal complex dyad, Rh(III)-X-Ru(II), was incorporated in monolayers at the air/ water interface by cospreading with a positively (eicosylamine) or negatively (stearic acid) charged amphiphile. After transfer of the monolayer onto a mica substrate via the Langmuir- Blodgett technique, a domain structure similar to that at the air/water interface was observed by scanning probe microscopy (SPM). With scanning force microscopy (SFM) we found domains with diameters in the micrometer range protruding several angstroms from the interstitial areas. The same structure was imaged with scanning near-field optical microscopy (SNOM) with joint registration of topographic features and ruthenium fluorescence. We present a model for the complex structure of the molecular film compatible with the different microscopy results.

Scanning force and scanning near-field optical microscopy of organized monolayers incorporating a nonamphiphilic metal dyad

The lifetimes of the monomer and the excimer of a pyrene phospholipid in Langmuir−Blodgett films with a counterlayer containing an electron acceptor are independent of the surface density of the acceptor. Only when the acceptor is located within a disk of a critical radius underneath the donor does a photoinduced electron transfer appear. The amount of free-emitting species is then reduced.

Pyrene phospholipid in monolayer assemblies - Photoinduced electron transfer investigated by time-resolved fluorescence emission

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Scanning force and scanning near-field optical microscopy of organized monolayers incorporating a nonamphiphilic metal dyad

Pyrene phospholipid in monolayer assemblies - Photoinduced electron transfer investigated by time-resolved fluorescence emission