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Design, Synthesis and Properties of ‘Photochromic Torsional Switches’ (PTS)
No full textThe geometrical arrangement of the p-orbitals in organic semiconductors plays a pivotal role for the optoelectronic properties of the resulting bulk materials.[1] Control over the π-bond geometry, e.g. the planarity, of an extended conjugated system offers the possibility to modulate the effective conjugation length of a π-system, thus, allowing for the tuning of optical and electronic properties.[1,2] A promising way to reversibly modulate the orientation of the p-orbitals in a conjugated strucrure is to incorporate photochromic segments onto the ‘backbone’ of the π-system. Attempts to use photochromic molecules as monomer units in a polymer chain have shown that the photo-reversibility efficiency decreases inversely with the enhancement of the π-conjugation.[3] In the present work we report on a novel molecular architecture, referred to as a ‘photochromic torsional switch’ (PTS), which can overcome the limits of todays photochromic dyes towards their incorporation into extended π-system. The aforementioned molecular structure consists of a polymerizable bithiophene unit able to mechanically change its π-system planarity in response to a photochromic isomerization of a laterally attached azobenzene unit. In the dark and upon exposure of visible light, the azobenzene moiety assumes its extended trans conformation, thus, forcing the bithiophene backbone to twist out of coplanarity (dihedral angle from 50° to 68°). By contrast, exposure to UV light results in isomerization of the azobenzene unit to the cis conformation, which allows the bithiophene fragment to assume a planar, π-conjugated conformation (dihedral angles from 150° to 168°). The PTS architectures, proposed in this work, represent a new generation of photochromic dyes that can allow for the preparation of ‘conjugated photochromic polymers’, and help to gain deeper understanding of the correlation between molecular conformation and optoelectronic properties of π-conjugated macromolecules
Coaxial Multichromophoric Photosystems
No full textHere we report preparationofmultichromophoric coaxial charge-transportarchitectures consisting of phthalocyanine(Pc) andperylenediimide (PDI).ArationallydesignedPccontainingstructurallysupportingpeptide side-chains, hydrazides, polymerizable disulfidesand diphosphonate anchoringgroupswascovalentlyboundontothe ITOsurface.Self-organizing surface-initiated polymerization(SOSIP)was usedto buildverticallyaligned Pcassembly(p-typechannel)covalentlylinkedbydisulfide bonds.Hydrazoneexchange of benzaldehydes withPDIaldehydes introduces an outerverticalelectron-transportingpathway(n-type channel).[1]The proposedbottom-upapproach affords formation of well-organized verticalcharge-transportcoaxial architectures which have largeinterestinthe optoelectronicsfield
Inhomogeneous Quenching as a Limit of the Correlation Between Fluorescence Polarization and Conformation of Single Molecules
No full textThe photophysical properties of conjugated polymers (CPs) largely depend on the interactions between the CP and its environment. We present a study of two polymers with identical conjugated backbones, bare and insulated, that showed different fluorescence excitation modulation depth histograms. However, the polarization differences are not related to differences in conformation, as commonly believed, but to the existence of "dark" chromophores in the bare polymer that are statically quenched. This results in inhomogeneous quenching of the polymer chain that breaks the correlation between excitation fluorescence polarization and conjugated polymer chain conformation
Regioselectivity in arene-catalyzed reductive lithiation of acetals of chlorobenzaldehydes
No full textThe regioselectivity of arene-catalyzed reductive lithiation of acetals of chlorobenzaldehydes strongly depends on the form of lithium metal employed as a reducing agent. According to previous findings, naphthalene catalyzed reductions run in the presence of lithium powder (high Na content) led to competitive metalations of both aromatic carbon-chlorine and benzylic carbon-oxygen bonds. At variance with these results, naphthalene catalyzed reductions run in the presence of lithium wire (either high or low Na content) led to highly regioselective metalation of aromatic carbon-chlorine bonds. These results disclose new possibilities of selective applications of arene-catalyzed reductive lithiation reactions. (c) 2005 Elsevier Ltd. All rights reserved
Excitation polarization provides structural resolution of individual non-blinking nano-objects.
No full textWe propose to combine the method of fluorescence intensity centroid localization with rotation of the plane of excitation polarization. Polarized light interacts selectively with differently oriented fluorophores; thus yielding topological information on the nanometer scale, without any need for fluorophore blinking. The method is applicable to photostable individual systems, when most of the traditional super-resolution methods fail. A theoretical study is supported by experiments on 30 nm long cyclodextrin-encapsulated single polyrotaxane conjugated polymer chains
Self-Assembled Monolayers as Patterning Tool for Organic Electronic Devices
No full textThe patterning of functional materials represents a crucial step for the implementation of organic semiconducting materials into functional devices. Classical patterning techniques such as photolithography or shadow masking exhibit certain limitations in terms of choice of materials, processing techniques and feasibility for large area fabrication. The use of self-assembled monolayers (SAMs) as a patterning tool offers a wide variety of opportunities, from the region-selective deposition of active components to guiding the crystallization direction. Here, we discuss general techniques and mechanisms for SAM-based patterning and show that all necessary components for organic electronic devices, i.e., conducting materials, dielectrics, organic semiconductors, and further functional layers can be patterned with the use of self-assembled monolayers. The advantages and limitations, and potential further applications of patterning approaches based on self-assembled monolayers are critically discussed
Double-channel photosystems with antiparallel redox gradients: templated stack exchange with porphyrins and phthalocyanines
Get PDFWe report the synthesis of multicomponent surface architectures composed of phthalocyanines (Pc), porphyrins (TPP) and naphthalenediimides (NDI). Naphthalenediimide stacks are grown first by self-organizing surface initiated disulfide-exchange polymerization (SOSIP). An oriented redox gradient driving electrons toward the surface is applied by growing electron-richer NDI stacks on top of poorer ones. Lateral stacks of porphyrins and phthalocyanines are then added by templated stack exchange (TSE). A three-component gradient is constructed to drive the holes away from the solid surface. Antiparallel gradients are found to minimize charge recombination during photocurrent generation. Templates used for stack exchange also serve as hole barriers, whereas their size has surprisingly little importance. These results demonstrate the compatibility of SOSIP-TSE technology with porphyrins and phthalocyanines, confirm the importance of oriented antiparallel gradients to minimize charge recombination, and show that electronics rather than the size matter to template stack exchange
Toward Oriented Surface Architectures with Three Coaxial Charge-Transporting Pathways
No full textWe report a synthetic method to build oriented architectures with three coaxial pi-stacks directly on solid surfaces. The approach operates with orthogonal dynamic bonds, disulfides and hydrazones, self-organizing surface-initiated polymerization (SOSIP), and templated stack-exchange (TSE). Compatibility with naphthale-nediimides, perylenediimides, squaraines, fullerenes, oligothiophenes, and triphenylamine is confirmed. Compared to photosystems composed of two coaxial channels, the installation of a third channel increases photocurrent generation up to 10 times. Limitations concern giant stack exchangers that fail to enter SOSIP architectures (e.g., phthalocyanines surrounded by three fullerenes), and planar triads that can give folded or interdigitated charge-transfer architectures rather than three coaxial channels. The reported triple-channel surface architectures are as sophisticated as it gets today, the directionality of their construction promises general access to multichannel architectures with multicomponent gradients in each individual channel. The reported approach will allow us to systematically unravel the ultrafast photophysics of molecular dyads and triads in surface architectures, and might become useful to develop conceptually innovative optoelectronic devices
Ultrafast all-optical switching and laser action in rotaxane
No full textLinear and non-linear optical properties of conjugated polymers are often masked by the inter-chain network in solid state. The formation of aggregates may trap excitons, reduce oscillator strength and modify relaxation processes. The control of the inter-chain interaction is the main reason for developing “threaded” polymers, where supra-molecular encapsulation should reduce aggregation. Here, we investigate the influence of the encapsulation with β-cyclodextrin (β-CD) macrocycles on the photophysics of the polyfluorene-alt-biphenylene (PFBP) using femtosecond non-linear spectroscopy. Upon threading we observe enhancement of the stimulated emission (SE) in the visible range and reduction of the charge absorption. These phenomena are ascribed to the reduced inter-chain interaction. In more isolated chains the dynamics of intra and inter-chain charge states are distinguished. In addition, we performed three-beam experiments in which a first pulse (pump) creates singlet excited states; a second (push) pulse re-excite the singlet state and a broadband probe pulse detects the induced changes in transmission. This technique shows: (i) charges are generated from higher lying singlet states also in isolated chains (ii) ultrafast optical gain switching is possible in threaded chain. Finally, we demonstrate that ASE occurs in films of threaded polymers and lasing can be achieved with much lower threshold than the neat polymer chain in the DFB configuration. All our findings point out the potential role of rotaxanes in photonics, as amplifiers and reopen the route to the electrically pumped organic lasers and all-optical logic devices
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