The realization of molecule-based miniature devices with advanced functions requires the development of new and efficient approaches for combining molecular building blocks into desired functional structures, ideally with these structures supported on suitable substrates 1-4. Supramolecular aggregation occurs spontaneously and can lead to controlled structures if selective and directional non-covalent interactions are exploited. But such selective supramolecular assembly has yielded almost exclusively crystals or dissolved structures 5; the self-assembly of absorbed molecules into larger structures 6-8, in contrast, has not yet been directed by controlling selective intermolecular interactions. Here we report the formation of surface-supported supramolecular structures whose size and aggregation pattern are rationally controlled by tuning the non-covalent interactions between individual absorbed molecules. Using low-temperature scanning tunnelling microscopy, we show that substituted porphyrin molecules adsorbed on a gold surface form monomers, trimers, tetramers or extended wire-like structures. We find that each structure corresponds in a predictable fashion to the geometric and chemical nature of the porphyrin substituents that mediate the interactions between individual adsorbed molecules. Our findings suggest that careful placement of functional groups that are able to participate in directed non-covalent interactions will allow the rational design and construction of a wide range of supramolecular architectures absorbed to surfaces.

Intrinsic molecular fluorescence from porphyrin molecules on Au(100) has been realized by using a nanoscale multimonolayer decoupling approach with nanoprobe excitation in the tunneling regime. The molecular origin of luminescence is established by the observed well-defined vibrationally resolved fluorescence spectra. The molecules fluoresce at low "turn-on" voltages for both bias polarities, suggesting an excitation mechanism via hot electron injection from either tip or substrate. The excited molecules decay radiatively through Franck-Condon pi(*)-pi transitions.

Saddle-shaped deformation of planar porphyrin molecules is accomplished by rotations of four phenyl-based substituents, which results from optimum adsorption onto Au(111) surface. The nonplanar macrocyclic conformation is clearly visualized by using low-temperature scanning tunneling microscopy and confirmed by molecular orbital calculations. Inside of the supramolecular molecular islands, we find that two different orientations of the nonplanar porphyrins are randomly distributed. An orientational ordering is obtained after short thermal excitations, which should be associated with steric intermolecular interactions between substituents.

The possibility of controlling light emission and propagation by exploiting periodic structures of dielectric media has attracted interest in the last decade. These photonic-bandgap materials, so-called photonic crystals, have generated considerable interest due to their wide applicability in optoelectronic and microwave devices.[1] In particular, emission control and lasing action in optically active photonic crystals can offer new applications for low-threshold lasers from small-size devices. [2][3][4] Currently an intensive effort is underway in molecular crystallography to develop photonic-bandgap materials with lattice parameters comparable to the wavelengths from visible to infrared light. This approach involves using liquid-crystalline materials that naturally form helical structures with a helical pitch in the optical-wavelength range.[5] The relevant optical property of the cholesteric phases of liquid crystals is the selective reflection of light over a range of wavelengths, that is, the photonic stop band. Previous studies have readily demonstrated that the lasing action of cholesteric liquid crystals can be attributed to the band-edge effect of the photonic stop band. These studies further explore mechanically, electrically, and chemically tunable photonic-stop-band responses [6][7][8] and a defect mode for a low-laser-threshold application. [9][10][11] Cholesteric liquid crystals can be regarded as one-dimensional photonic crystals, whereas the liquid-crystalline blue phases are three-dimensional cubic structures with lattice periods of several hundred nanometers, which give rise to selective Bragg reflections.[12] Therefore, probing light confinement in the blue phases and using them as novel molecularly assembled photonic crystals is of great interest. Although selective light reflections in the blue phases have already been studied for quite some time, [12][13][14] such three-dimensional extensions in molecular self-assembly are normally much more difficult to produce. A practical limitation of blue phases is their narrow temperature occupation (∼ 1-2°C) at the transition between the isotropic and cholesteric phases. The potential photonic application of the blue phases has been recently demonstrated by measuring the laser emission in three dimensions.[15]However, lasing action was still limited to a very narrow temperature range. Therefore, improving the temperature stability has been required for practical application of blue phases. [16,17] In this study, we describe the preparation of polymer-stabilized blue phases and the demonstration of laser emission attributed to the photonic effect of the blue-phase photonic crystal. The polymer network that forms in the blue phase leads to restriction of the deformation of the photonic crystal in a wide temperature range. We confirm the thermal stability of the polymer-stabilized blue phase by measuring laser emission over a wide range of temperature above 35°C. Pulsed excitation gives rise to laser emission with the low threshold excitation energy of a...

This letter describes the electrical control of structure and lasing in the photonic bandgaps of cholesteric liquid crystals (CLCs). Photoexcitation of dye-doped CLC cells with a linearly polarized laser gives rise to laser emission at the edge(s) of the chiral photonic band gap. Applying voltages to the optically pumped CLC cells enables reversible switching of the laser action as a result of the structural changes in the chiral photonic band gap.