Unconventional singlet fission materials Ullrich, Tobias; Munz, Dominik; Guldi, Dirk M
Chemical Society reviews,
03/2021, Letnik:
5, Številka:
5
Journal Article
Recenzirano
Singlet fission (SF) is a photophysical downconversion pathway, in which a singlet excitation transforms into two triplet excited states. As such, it constitutes an exciton multiplication generation ...process, which is currently at the focal point for future integration into solar energy conversion devices. Beyond this, various other exciting applications were proposed, including quantum cryptography or organic light emitting diodes. Also, the mechanistic understanding evolved rapidly during the last year. Unfortunately, the number of suitable SF-chromophores is still limited. This is
per se
problematic, considering the wide range of envisaged applicability. With that in mind, we emphasize uncommon SF-scaffolds and outline requirements as well as strategies to expand the chromophore pool of SF-materials.
Singlet fission (SF) is a photophysical downconversion pathway, in which a singlet excitation transforms into two triplet excited states.
Singlet fission (SF) involves the spontaneous splitting of a photoexcited singlet state into a pair of triplets, and it holds great promise toward the realization of more efficient solar cells. ...Although the process of SF has been known since the 1960s, debate regarding the underlying mechanism continues to this day, especially for molecular materials. A number of different chromophores have been synthesized and studied in order to better understand the process of SF. These previous reports have established that pentacene and its derivatives are especially well‐suited for the study of SF, since the energetic requirement E(S1)≥2E(T1) is fulfilled rendering the process exothermic and unidirectional. Dimeric pentacene derivatives, in which individual pentacene chromophores are tethered by a “spacer”, have emerged as the system of choice toward exploring the mechanism of intramolecular singlet fission (iSF). The dimeric structure, and in particular the spacer, allows for controlling and tuning the distance, geometric relationship, and electronic coupling between the two pentacene moieties. This Minireview describes recent advances using pentacene dimers for the investigation of iSF.
Singlet fission: Pentacene derivatives play a crucial role in understanding the mechanism of singlet fission. The synthesis of pentacene dimers to probe specific aspects of singlet fission is reviewed
The fundamental chemical, redox, electrochemical, photoelectrochemical, optical and optoelectronic features of carbon nanotubes are surveyed with particular emphasis on the most relevant applications ...as electron donor/electron acceptor or as electron conductor/hole conductor materials, in solutions and in the solid state. Methods that aim at p- and n-doping as a means to favor hole or electron injection/transport are covered as well (critical review, 208 references).
Graphene-based materials (GBMs), with graphene, their most known member, at the head, constitute a large family of materials which has aroused the interest of scientists working in different research ...fields such as chemistry, physics, or materials science, to mention a few, arguably as no other material before. In this review, we offer a general overview on the most relevant synthetic approaches for the covalent and non-covalent functionalization and characterization of GBMs. Moreover, some representative examples of the incorporation into GBMs of electroactive units such as porphyrins, phthalocyanines, or ferrocene, among others, affording electron donor-acceptor (D-A) hybrids are presented. For the latter systems, the photophysical characterization of their ground- and excited-state features has also been included, paying particular attention to elucidate the fundamental dynamics of the energy transfer and charge separation processes of these hybrids. For some of the presented architectures, their application in solar energy conversion schemes and energy production has been also discussed.
This review offers an overview on the chemical functionalization, characterization and applications of graphene-based materials.
Carbon dots (CDs) are an astonishing class of fluorescent materials with many applications in bioimaging, drug delivery, photovoltaics and photocatalysis due to their outstanding luminescence ...properties and low toxicity. However, the internal CD structure of bottom-up synthesized CDs is still the subject of considerable debate. Unambiguous analysis of the internal CD composition is hampered by the fact that reaction products usually contain mixtures of several CD fractions as well as molecular intermediate and side products. Therefore, purification and careful separation of the various CD fractions is vital for structural analysis and isolation of pure CDs possessing optimized optical properties. In this study, CD solutions were synthesized from citric acid and cysteine
via
a one-pot hydrothermal synthesis. A simple column chromatography unit was used to systematically study the influence of the molar precursor ratios and synthesis conditions (temperature, reaction time) on the CD solution composition. By investigating the structural and optical properties of the chromatographically separated fractions, three different fluorescent species could be identified. Freely floating molecular fluorophores left the column first, followed by highly fluorescent CDs with fluorophores bound to the carbon core, finally followed by low-fluorescent carbon particles without fluorophores. We demonstrate that the CD solution composition and the internal structure of the individual fluorescent components can be clarified
via
chromatographic separation. This information can be further applied to isolate pure CDs with optimized optical properties.
Purification and separation of CD solution into three fluorescent fractions.
The past 25 years have served as a test bed for exploring the chemistry and physics, in general, and the electron transfer chemistry, in particular, of low‐dimensional carbon. Nevertheless, the new ...realm started with the advent of fullerenes, followed in chronological order by carbon nanotubes, and, more recently, by graphene. The major thrust of this Review article is to historically recap the versatility of fullerenes regarding the design, the synthesis, and the tests as an electroactive building block in photosynthetic reaction mimics, photovoltaics, and catalysis.
The main aim of this Review article is to recap the versatility of fullerenes regarding the design, the synthesis, and tests as an electroactive building block in photosynthetic reaction mimics, photovoltaics, and catalysis.
Singlet fission in pentacene dimers Zirzlmeier, Johannes; Lehnherr, Dan; Coto, Pedro B. ...
Proceedings of the National Academy of Sciences - PNAS,
04/2015, Letnik:
112, Številka:
17
Journal Article
Recenzirano
Odprti dostop
Significance In the present work, we show compelling evidence for the unprecedented intramolecular singlet fission at room temperature and in dilute solutions within a set of three different ...regioisomeric pentacene dimers. Pump–probe experiments, which were complemented by theoretical calculations using high-level ab initio multireference perturbation theory methods, corroborate triplet quantum yields as high as 156 ± 5%. To this end, electronic couplings between the two pentacenes in the dimers, by virtue of through-bond or through-space interactions, are decisive in tuning the rates of singlet fission.
Singlet fission (SF) has the potential to supersede the traditional solar energy conversion scheme by means of boosting the photon-to-current conversion efficiencies beyond the 30% Shockley–Queisser limit. Here, we show unambiguous and compelling evidence for unprecedented intramolecular SF within regioisomeric pentacene dimers in room-temperature solutions, with observed triplet quantum yields reaching as high as 156 ± 5%. Whereas previous studies have shown that the collision of a photoexcited chromophore with a ground-state chromophore can give rise to SF, here we demonstrate that the proximity and sufficient coupling through bond or space in pentacene dimers is enough to induce intramolecular SF where two triplets are generated on one molecule.
Carbon nanodots (CNDs) were photochemically altered to produce dihydrogen under light irradiation. Within the complex structure of CNDs, photo-oxidation takes place at citrazinic acid molecular ...fluorophore sites. Important is the fact that the resulting CND materials have a dual function. On one hand, they absorb light, and on the other hand, they photo- and electrocatalytically produce dihydrogen from water and seawater, without any external photosensitizer or cocatalyst. Record HER activities of 15.15 and 19.70 mmol(H2) g(catalyst)−1 h–1 were obtained after 1 h of 75 mW/cm2 Xe lamp illumination, from water and seawater, respectively. This impressive performance outweighs the remaining structural uncertainties. A full-fledged physicochemical investigation based on an arsenal of steady-state and time-resolved spectroscopic characterizations together with microscopy enabled a comprehensive look into the reaction mechanism. For an efficient dihydrogen formation, a precatalytic activation by means of reduction with a sacrificial electron donor is imperative.
Carbon Nanodots for Charge-Transfer Processes Cadranel, Alejandro; Margraf, Johannes T; Strauss, Volker ...
Accounts of chemical research,
04/2019, Letnik:
52, Številka:
4
Journal Article
Recenzirano
Odprti dostop
Conspectus 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. Charge-transfer 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 structure–property 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 charge-transfer 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 ∼1–2 nm. The narrow and excitation-independent 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-of-concept 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-donor–acceptor systems documents the versatility of CNDs as molecular building blocks, whose electronic properties are tunable for the needs of emerging technologies.
Cocrystals in the form of crystalline nanosheets comprised of C70 and (metallo)porphyrins were prepared by using the liquid–liquid interfacial precipitation (LLIP) method where full control over the ...morphologies in the C70/(metallo)porphyrins nanosheets has been accomplished by changing the solvent and the relative molar ratio of fullerene to (metallo)porphyrin. Importantly, the synergy of integrating C70 and (metallo)porphyrins as electron acceptors and donors, respectively, into nanosheets is substantiated in the form of a near-infrared charge-transfer absorption. The presence of the latter, as reflection of ground-state electron donor–acceptor interactions in the nanosheets, in which a sizable redistribution of charge density from the electron-donating (metallo)porphyrins to the electron-accepting C70 occurs, leads to a quantitative quenching of the localized (metallo)porphyrin fluorescence. Going beyond the ground-state characterization, excited-state electron donor–acceptor interactions are the preclusion to a full charge transfer featuring formation of a radical ion pair state, that is, the one-electron reduced fullerene and the one-electron oxidized (metallo)porphyrin.
PDF
