Propelling single molecules in a controlled manner along an unmodified surface remains extremely challenging because it requires molecules that can use light, chemical or electrical energy to modulate their interaction with the surface in a way that generates motion. Nature's motor proteins have mastered the art of converting conformational changes into directed motion, and have inspired the design of artificial systems such as DNA walkers and light- and redox-driven molecular motors. But although controlled movement of single molecules along a surface has been reported, the molecules in these examples act as passive elements that either diffuse along a preferential direction with equal probability for forward and backward movement or are dragged by an STM tip. Here we present a molecule with four functional units--our previously reported rotary motors--that undergo continuous and defined conformational changes upon sequential electronic and vibrational excitation. Scanning tunnelling microscopy confirms that activation of the conformational changes of the rotors through inelastic electron tunnelling propels the molecule unidirectionally across a Cu(111) surface. The system can be adapted to follow either linear or random surface trajectories or to remain stationary, by tuning the chirality of the individual motor units. Our design provides a starting point for the exploration of more sophisticated molecular mechanical systems with directionally controlled motion.
Thin film organic lasers represent a new generation of inexpensive, mechanically flexible devices for spectroscopy, optical communications and sensing. For this purpose, it is desired to develop highly efficient, stable, wavelength-tunable and solution-processable organic laser materials. Here we report that carbon-bridged oligo(p-phenylenevinylene)s serve as optimal materials combining all these properties simultaneously at the level required for applications by demonstrating amplified spontaneous emission and distributed feedback laser devices. A series of six compounds, with the repeating unit from 1 to 6, doped into polystyrene films undergo amplified spontaneous emission from 385 to 585 nm with remarkably low threshold and high net gain coefficients, as well as high photostability. The fabricated lasers show narrow linewidth (<0.13 nm) single mode emission at very low thresholds (0.7 kW cm−2), long operational lifetimes (>105 pump pulses for oligomers with three to six repeating units) and wavelength tunability across the visible spectrum (408–591 nm).
as a laser resonator, have found a variety of applications in spectroscopy, [5] optical communications, [6] and sensing. [7][8][9][10] DFB lasers can provide narrow single mode emission (linewidth <1 nm) and require only low pump energy for their operation, i.e., they show a low threshold. The resonator is easily integrated into other devices, and it can be implemented with field-effect-transistor geometry, which promises potential for the development of electrically pumped TFOLs. Moreover, DFB lasers can be mechanically flexible, and their production costs are relatively low. DFB gratings are usually fabricated by electron beam lithography, nanoimprint lithography (NIL), or holographic lithography (HL). [11] A particular advantage of the latter is its capability to produce small structures of different dimensionality over a large area (up to a few cm 2 ) in a simple and low-cost manner, which can be exploited to fabricate wavelength-tunable devices on a single chip.So far, different DFB architectures, with gratings fabricated by various methods, have been reported, [1][2][3][4] whereby efforts have been devoted predominantly to lowering the threshold. The lowest values (<1 kW cm −2 ) have been achieved with lasers whose DFB gratings are engraved on conventional inorganic substrates (e.g., glass or SiO 2 ), onto which the active films are deposited (this configuration will henceforth be denoted as standard; Std). Other studies, aimed at improving device integration, reducing device costs, and achieving mechanical flexibility, have focused either on architectures with gratings imprinted directly on the active film, [12][13][14][15][16][17] or on systems wherein both the active material and the resonator, which is generally located below the active film [18][19][20][21][22] and only in few cases on top of it, [23,24] were processed from solution. Unfortunately, the thresholds of these solution-processed lasers are generally high (>8 kW cm −2 ), except for few exceptions. [19] Finally, several strategies have been proposed in order to accomplish wavelength tunability in a single device. [1][2][3][4] For example, by using multiple gratings (e.g. segmented substrates with a stepped grating period), [25] a wedged-shape active film (i.e. with a continuously variable thickness), [26] mechanical stretching, [27] photoisomerizable azo-polymers, [28] or photochromic molecules doped into the active film. [29] Some works have demonstrated electrical-tuning by combining an elastic DFB laser with an electroactive substrate, [30] or by including a layer contaning a Thin film organic lasers represent attractive light sources for numerous applications. Currently, efforts are devoted to the development of low-cost high-performance and color-tunable devices, whereby both the resonator and the active layer should consist of solution-processable organic materials. Herein, solution-processed distributed-feedback lasers are reported with polymeric resonators on top of active films of perylene orange or carbon-bridged oligo(p-phenylenevi...
The introduction of dibenzocyclohepten-5-ylidene as part of a unidirectional light-driven molecular motor allows a more complete picture of the pathway of thermal helix inversion to be developed. The most stable conformation is similar to that found in related motors in that it has, overall, an anti-folded structure with the substituent at the stereogenic centre adopting an axial orientation. Photochemical cis/trans isomerisation at -40 degrees C results in the formation of an isomer in a syn-folded conformation with the methyl group in an axial orientation. This contrasts with previous studies on related molecular rotary motors. The conformation of the higher energy intermediate typically observed for this class of compound is the anti-folded conformation, in which the methyl group is in an equatorial orientation. This conformation is available through an energetically uphill upper half ring inversion of the observed photochemical product. However, this pathway competes with a second process that leads to the more stable anti-folded conformation in which the methyl group is oriented axially. It has been shown that the conformations and pathways available for second-generation molecular motors can be described by using similar overall geometries. Differences in the metastable high-energy species are attributable to the relative energy and position on the reaction coordinate of the transition states. Kinetic studies on these new molecular motors thus provide important insights into the conformational dynamics of the rotation cycle.
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