413 research outputs found
Molecular wires:impact of Ï-conjugation and implementation of molecular bottlenecks
In this review we highlight recent progress in the field of photochemically and thermally induced electron transport through molecular bridges as integrative parts of electron donorâbridgeâacceptor conjugates. The major emphasis is hereby on the design and the modular composition of the bridges. To this end, we will demonstrate that control over attenuation factors and reorganization energies, on one hand, as well as electronic and electronâvibration couplings, on the other hand, enables tuning electron transport over distances as short as 3.5 Ă
and as large as 50 Ă
by up to nine orders of magnitude. In terms of electron transport, the maximum extreme is given by carbon-bridged oligo-p-phenylenevinylenes of different lengths, while a zinc tetraphenylporphyrin free base tetraphenylporphyrin dyad constitutes the minimum extreme
A push-pull unsymmetrical subphthalocyanine dimer
Unsymmetrical subphthalocyanine fused dimers have been prepared from appropriate ortho-dinitrile SubPc precursors. In particular, either electron-donating or electron-accepting substituents have been introduced on each SubPc constituent unit, resulting in unprecedented pushâpull Ï-extended curved aromatic macrocycles. From fluorescence experiments in solvents of different polarity we conclude a dual fluorescence, namely a delocalized singlet excited state (1.73 eV) and a polarized charge transfer state (<1.7 eV). Pump probe experiments corroborate the dual nature of the fluorescence. On one hand, the delocalized singlet excited state gives rise to a several nanosecond lasting intersystem crossing yielding the corresponding triplet excited state. On the other hand, the polarized charge transfer state deactivates within a few picosesonds. Visualization of the charge transfer state was accomplished by means of molecular modeling with a slight polarization of the HOMO towards the electron donor and of the LUMO towards the electron acceptor
The electronic structure of Amorphous Carbon Nanodots
We
have studied hydrogen-passivated amorphous carbon nanostructures
with semiempirical molecular orbital theory in order to provide an
understanding of the factors that affect their electronic properties.
Amorphous structures were first constructed using periodic calculations
in a melt/quench protocol. Pure periodic amorphous carbon structures
and their counterparts doped with nitrogen and/or oxygen feature large
electronic band gaps. Surprisingly, descriptors such as the elemental
composition and the number of sp<sup>3</sup>-atoms only influence
the electronic structure weakly. Instead, the exact topology of the
sp<sup>2</sup>-network in terms of effective conjugation defines the
band gap. Amorphous carbon nanodots of different structures and sizes
were cut out of the periodic structures. Our calculations predict
the occurrence of localized electronic surface states, which give
rise to interesting effects such as amphoteric reactivity and predicted
optical band gaps in the near-UV/visible range. Optical and electronic
gaps display a dependence on particle size similar to that of inorganic
colloidal quantum dots
Fullerene van der waals Oligomers as electron traps
Density functional theory calculations indicate that van der Waals fullerene dimers and larger oligomers can form interstitial electron traps in which the electrons are even more strongly bound than in isolated fullerene radical anions. The fullerenes behave like super atoms , and the interstitial electron traps represent one-electron intermolecular Ï-bonds. Spectroelectrochemical measurements on a bis-fullerene-substituted peptide provide experimental support. The proposed deep electron traps are relevant for all organic electronics applications in which non-covalently linked fullerenes in van der Waals contact with one another serve as n-type semiconductors
Cyclopenta[hi]aceanthrylene decorated with multiple and long alkoxy chains: physicochemical properties and single-walled carbon nanotubes exfoliation capability
A rod-like cyclopenta[hi]aceanthrylene (CPA) derivative bearing three dodecyloxy chains at each of its two terminal positions was prepared. Spectroscopic (i.e., steady-state absorption and fluorescence) and electrochemical studies carried out with this polycyclic aromatic hydrocarbon (PAH) derivative showed an intense absorption through the entire UV-vis spectral range, weak fluorescence, small HOMO-LUMO gap, and excellent electron accepting capability. Transient absorption spectroscopy (TAS) revealed the formation of singlet and triplet excited states; the latter was, however, only observed in the presence of a triplet sensitizer. The exfoliation capability of this lipophilic CPA towards single-walled carbon nanotubes (SWCNTs) in THF was also investigated. On one hand, transmission electron microscopy (TEM) pointed to an efficient debundling of SWCNTs by the CPA derivative by means of non-covalent interactions. On the other hand, important differences in the ground and excited state features of the uncomplexed and SWCNT-complexed CPA were revealed by Raman and TASFinancial support from the âSolar Energy goes Hybridâ Initiative of the Bavarian Ministry for Science, Culture and Education (SolTech), Comunidad de Madrid (FOTOCARBON), and Spanish MICINN (CTQ2017-85393-P) is acknowledge
Carbon Nanodots for Charge-Transfer Processes
In recent years, carbon nanodots (CNDs) have emerged as an environmentally friendly, biocompatible, and inexpensive class of material, whose features sparked interest for a wide range of applications. Most notable is their photoactivity, as exemplified by their strong luminescence. Consequently, CNDs are currently being investigated as active components in photocatalysis, sensing, and optoelectronics. Chargetransfer interactions are common to all these areas. It is therefore essential to be able to fine-tune both the electronic structure of CNDs and the electronic communication in CND-based functional materials. The complex, but not completely deciphered, structure of CNDs necessitates, however, a multifaceted strategy to investigate their fundamental electronic structure and to establish structureâproperty relationships. Such investigations require a combination of spectroscopic methods, such as ultrafast transient absorption and fluorescence up-conversion techniques, electrochemistry, and modeling of CNDs, both in the absence and presence of other photoactive materials. Only a sound understanding of the dynamics of charge transfer, charge shift, charge transport, etc., with and without light makes much-needed improvements in, for example, photocatalytic processes, in which CNDs are used as either photosensitizers or catalytic centers, possible. This Account addresses the structural, photophysical, and electrochemical properties of CNDs, in general, and the chargetransfer chemistry of CNDs, in particular. Pressure-synthesized CNDs (pCNDs), for which citric acid and urea are used as inexpensive and biobased precursor materials, lie at the center of attention. A simple microwave-assisted thermolytic reaction, performed in sealed vessels, yields pCNDs with a fairly homogeneous size distribution of âŒ1â2 nm. The narrow and excitationindependent photoluminescence of pCNDs contrasts with that seen in CNDs synthesized by other techniques, making pCNDs optimal for in-depth physicochemical analyses. The atomistic and electronic structures of CNDs were also analyzed by quantum chemical modeling approaches that led to a range of possible structures, ranging from heavily functionalized, graphene-like structures to disordered amorphous particles containing small sp2 domains. Both the electron-accepting and -donating performances of CNDs make the charge-transfer chemistry of CNDs rather versatile. Both covalent and noncovalent synthetic approaches have been explored, resulting in architectures of various sizes. CNDs, for example, have been combined with molecular materials ranging from electron-donating porphyrins and extended tetrathiafulvalenes to electron-accepting perylendiimides, or nanocarbon materials such as polymer-wrapped single-walled carbon nanotubes. In every case, charge-separated states formed as part of the reaction cascades initiated by photoexcitation. Charge-transfer assemblies including CNDs have also played a role in technological applications: for example, a proof-ofconcept dye-sensitized solar cell was designed and tested, in which CNDs were adsorbed on the surface of mesoporous anatase TiO2. The wide range of reported electron-donorâacceptor systems documents the versatility of CNDs as molecular building blocks, whose electronic properties are tunable for the needs of emerging technologies.Fil: Cadranel, Alejandro. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de QuĂmica InorgĂĄnica, AnalĂtica y QuĂmica FĂsica; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Ciudad Universitaria. Instituto de QuĂmica, FĂsica de los Materiales, Medioambiente y EnergĂa. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de QuĂmica, FĂsica de los Materiales, Medioambiente y EnergĂa; ArgentinaFil: Margraf, Johannes T.. Technische Universitat MĂŒnchen; AlemaniaFil: Strauss, Volker. No especifĂca;Fil: Clark, Timothy. Universitat Erlangen-Nuremberg; AlemaniaFil: Guldi, Dirk M.. Universitat Erlangen-Nuremberg; Alemani
Ultrasound-induced transformation of fluorescent organic nanoparticles from a molecular rotor into rhomboidal nanocrystals with enhanced emission
Fluorescent organic nanoparticles (FONs) based on aggregation-induced emission (AIE) are receiving increasing attention owing to their simple preparation, enhancedoptical properties, and a wide range of applications. Therefore, finding simple methods to tune the structural and emissive properties of FONs is highly desirable. In this context, we discuss the preparation of highly emissive, amorphous AIE spherical nanoparticles based on a structurally-simple molecular rotor and their sonochemicaltransformation into rhomboidal nanocrystals. Interestingly, the ultrasound-induced modification of the morphology is accompanied by a remarkable enhancement in the stability and emission of the resulting nanocrystals. Detailed characterization of both spherical and rhomboidal nanoparticles was carried out by means of several microscopic, crystallographic, and spectroscopic techniques as well as quantum mechanical calculations. In a nutshell, this work provides a unique example of the ultrasound-induced switching of morphology, stability, and emission in FONsFinancial support from Spanish Ministry of Economy and Competitivity, MICINN (CTQ-2011-24187/BQU), MIUR through FIRB program (contract no. RBFR10DAK6), ERC Advanced Grant 2012 (number 320951
Determination of the Attenuation Factor () in Hybrid Covalent/Non-Covalent Molecular Wires
We have established for the first time the molecular wire behaviour in a new set of hybrid covalent/supramolecular porphyrinfullerene structures, in which hydrogen-bond interactions and pphenylene oligomers of different length act as highly efficient molecular wires exhibiting a remarkably low attenuation factor ( = 0.07 ± 0.01 Ă
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