Biosensors, which harness the unique specific binding properties of biomaterials such as proteis,... more Biosensors, which harness the unique specific binding properties of biomaterials such as proteis, are increasingly recognized as a powerful tool for chemical sensing. In this context, the detection of nucleic acids DNA and RNA will take on ever more importance for screening as the fields of genomics and diagnostics advance. The same properties that allow molecular recognition–strong, specific non-covalent interactions–also enable bottom-up assembly of sensing architectures. Here, we take advantage of such interactions for the self-assembly of field-effect transistors from semi-conducting single-walled carbon nanotubes selectively dispersed by DNA-block-copolymers and anchored to the electrodes through DNA hybridization. These transistors can sensitively detect the hybridization of complementary target DNA strands through transduction of the chemical recognition event into electrical doping, achieving an analyte sensitivity of 10 fM. Such ultra-sensitive electrical-based detection removes the need for DNA amplification and offers a new route to nucleic acids diagnostics.
The presence of energetically low-lying triplet states is a hallmark of organic semiconductors. E... more The presence of energetically low-lying triplet states is a hallmark of organic semiconductors. Even though they present a wealth of interesting photophysical properties, these optically dark states significantly limit optoelectronic device performance. Recent advances in emissive charge-transfer molecules have pioneered routes to reduce the energy gap between triplets and " bright " singlets, allowing thermal population exchange between them and eliminating a significant loss channel in devices. In conjugated polymers, this gap has proved resistant to modification. Here, we introduce a general approach to reduce the singlet−triplet energy gap in fully conjugated polymers, using a donor−orthogonal acceptor motif to spatially separate electron and hole wave functions. This new generation of conjugated polymers allows for a greatly reduced exchange energy, enhancing triplet formation and enabling thermally activated delayed fluorescence. We find that the mechanisms of both processes are driven by excited-state mixing between π−π*and charge-transfer states, affording new insight into reverse intersystem crossing.
In this work, we have rationally designed and synthesized a novel thiophene-diketopyrrolopyrrole ... more In this work, we have rationally designed and synthesized a novel thiophene-diketopyrrolopyrrole (TDPP)-vinyl-based dimer. We have investigated the optical and electronic properties and have probed the photophysical dynamics using transient absorption to investigate the possibility of singlet exciton fission. These revealed extremely rapid decay to the ground state (<50 ps), which we confirm is due to intramolecular excitonic processes rather than large-scale conformational change enabled by the vinyl linker. In all cases, the main excited state appears to be " dark " , suggesting rapid internal conversion into a dark 2A g-type singlet state. We found no evidence of triplet formation in TDPP-V-TDPP under direct photoexcitation. This may be a consequence of significant singlet stabilization in the dimer, bringing it below the energy needed to form two triplets. Our studies on this model compound set valuable lessons for design of novel triplet-forming materials and highlight the need for more broadly applicable design principles.
Understanding the mechanism of singlet exciton fission, in which a singlet exciton separates into... more Understanding the mechanism of singlet exciton fission, in which a singlet exciton separates into a pair of triplet excitons, is crucial to the development of new chromophores for efficient fission-sensitized solar cells. The challenge of controlling molecular packing and energy levels in the solid state precludes clear determination of the singlet fission pathway. Here, we circumvent this difficulty by utilizing covalent dimers of pentacene with two types of side groups. We report rapid and efficient intramolecular singlet fission in both molecules, in one case via a virtual charge-transfer state and in the other via a distinct charge-transfer intermediate. The singlet fission pathway is governed by the energy gap between singlet and charge-transfer states, which change dynamically with molecular geometry but are primarily set by the side group. These results clearly establish the role of charge-transfer states in singlet fission and highlight the importance of solubilizing groups to optimize excited-state photophysics.
1 wileyonlinelibrary.com proceed on ultrafast (≈100 fs) time scales, allowing it to out-compete o... more 1 wileyonlinelibrary.com proceed on ultrafast (≈100 fs) time scales, allowing it to out-compete other decay channels and achieve high effi ciencies. [ 3 ] The essential condition for effi cient SEF is the energetic alignment of the singlet and triplet states, such that 2 E (T 1) ≤ E (S 1). A recent combined theoretical and experimental study of SEF rates in a range of acene solids has demonstrated that the rate of SEF is also greatly affected by the strength of intermolecular coupling within the fi lm. [ 4 ] In the canonical system, pentacene, triplet pair formation is exo-thermic and the intermolecular coupling is strong, resulting in SEF with an 80 fs time constant and nearly 200% yield. [ 5 ] Though most experimental studies of SEF have involved crystalline, polycrystalline or amorphous solids, the most basic unit capable of SEF is a pair of chromophores. Indeed, it was recently demonstrated in concentrated solutions of TIPS-pentacene that singlet fi ssion can proceed at high efficiency through bimolecular diffusional interactions. [ 6 ] However , early attempts to directly control the interaction between chromophores through the use of covalent dimers have not been as successful. The most notable systems in this regard are tetracene and 1,3-diphenylisobenzofuran. These materials are found to exhibit effi cient SEF in the solid state, but their covalent dimers achieved triplet yields of only a few percent. In both of these studies, [ 7 ] the two SEF chromophores were joined by a range of linkers to modify the strength of the electronic coupling between them, with the aim of tuning the rate and effi ciency of SEF. The impact was subtle, and it thus remains unclear why covalent dimers have proved ineffi cient to date. Current models suggest that dimers should be asymmetric or contain signifi cant cofacial interaction between chromophores to attain high triplet yields. [ 2,8 ] Interestingly, a recent study of pentacene dimers separated by a phenyl spacer unit achieved triplet yields above 100% in spite of using the same symmetric bonding motifs of the earlier tetracene dimers. [ 9 ] In this work, we report highly effi cient intramolecular SEF in a new type of covalent dimer, with triplet yields of up to 192 ± 3%. The molecule used in this study, 13,13′-bis(mesityl)-6,6′-dipentacenyl (DP-Mes, Figure 1 a), consists of two pen-tacenes directly bonded through a single C C bond with two bulky mesityl groups at the meso-positions. The geometry of the dimer, with two nearly orthogonal pentacene cores, is unlike Fast and highly effi cient intramolecular singlet exciton fi ssion in a pentacene dimer, consisting of two covalently attached, nearly orthogonal pentacene units is reported. Fission to triplet excitons from this ground state geometry occurs within 1 ps in isolated molecules in solution and dispersed solid matrices. The process exhibits a sensitivity to environmental polarity and competes with geometric relaxation in the singlet state, while subsequent triplet decay is strongly dependent on conformational freedom. The near orthogonal arrangement of the pentacene units is unlike any structure currently proposed for effi cient singlet exciton fi ssion and may lead to new molecular design rules.
Singlet exciton fission allows the fast and efficient generation of two spin triplet states from ... more Singlet exciton fission allows the fast and efficient generation of two spin triplet states from one photoexcited singlet. It has the potential to improve organic photovoltaics, enabling efficient coupling to the blue to ultraviolet region of the solar spectrum to capture the energy generally lost as waste heat. However, many questions remain about the underlying fission mechanism. The relation between intermolecular geometry and singlet fission rate and yield is poorly understood and remains one of the most significant barriers to the design of new singlet fission sensitizers. Here we explore the structure−property relationship and examine the mechanism of singlet fission in aggregates of astaxanthin, a small polyene. We isolate five distinct supramolecular structures of astaxanthin generated through self-assembly in solution. Each is capable of undergoing intermolecular singlet fission, with rates of triplet generation and annihilation that can be correlated with intermolecular coupling strength. In contrast with the conventional model of singlet fission in linear molecules, we demonstrate that no intermediate states are involved in the triplet formation: instead, singlet fission occurs directly from the initial 1B u photoexcited state on ultrafast time scales. This result demands a re-evaluation of current theories of polyene photophysics and highlights the robustness of carotenoid singlet fission.
Solution-based studies of singlet exciton fission have provided valuable insight to this spin-all... more Solution-based studies of singlet exciton fission have provided valuable insight to this spin-allowed process in organic chromophores, whereby a photogenerated spin-singlet exciton splits into two spin-triplet excitons on separate molecules. Here we review the most significant experimental contributions made regarding fission in solution, in both intra-and intermolecular systems. Intramolecular fission allows a clearer examination of the molecular excited states involved in triplet formation, and the ability to control inter-chromophore structure offers a route to directly investigate the role of molecular coupling. In diffusional, intermolecular systems the conformational freedom and slower timescales of fission reveal the nature of intermediate states.
Singlet exciton fission is the spin-conserving transformation of one spin-singlet exciton into tw... more Singlet exciton fission is the spin-conserving transformation of one spin-singlet exciton into two spin-triplet excitons. This exciton multiplication mechanism offers an attractive route to solar cells that circumvent the single-junction Shockley–Queisser limit. Most theoretical descriptions of singlet fission invoke an intermediate state of a pair of spin-triplet excitons coupled into an overall spin-singlet configuration, but such a state has never been optically observed. In solution, we show that the dynamics of fission are diffusion limited and enable the isolation of an intermediate species. In concentrated solutions of bis(triisopropylsilylethynyl)[TIPS]— tetracene we find rapid (<100 ps) formation of excimers and a slower (∼10 ns) break up of the excimer to two triplet exciton-bearing free molecules. These excimers are spectroscopically distinct from singlet and triplet excitons, yet possess both singlet and triplet characteristics, enabling identification as a triplet pair state. We find that this triplet pair state is significantly stabilized relative to free triplet excitons, and that it plays a critical role in the efficient endo-thermic singlet fission process. singlet fission | photochemistry | TIPS–tetracene | triplet | excimer
Singlet exciton fission is the process in organic semiconductors through which a spin-singlet exc... more Singlet exciton fission is the process in organic semiconductors through which a spin-singlet exciton converts into a pair of spin-triplet excitons residing on diierent chromophores, entangled in an overall spin-zero state. For some systems, singlet fission has been shown to occur on the 100 fs timescale and with a 200% quantum yield, but the mechanism of this process remains uncertain. Here we study a model singlet fission system, TIPS-pentacene, using ultrafast vibronic spectroscopy. We observe that vibrational coherence in the initially photogenerated singlet state is transferred to the triplet state and show that this behaviour is eeectively identical to ultrafast internal conversion for polyenes in solution. This similarity in vibronic dynamics suggests that both multi-molecular singlet fission and single-molecular internal conversion are mediated by the same underlying relaxation processes, based on strong coupling between nuclear and electronic degrees of freedom. In its most eecient form this leads to a conical intersection between the coupled electronic states. S inglet exciton fission has long commanded interest as an exceptionally fast channel to generate triplet excitons in organic materials. At the heart of the process is the coupling of the final triplet pair to the initially photoexcited singlet state, which ensures conservation of spin 1. In systems where singlet fission is exergonic, it can be rapid and highly efficient. For instance, thin films of pentacene and TIPS-pentacene exhibit triplet formation with a time constant of 80 fs and quantum yields of 200% (refs 2,3). Current interest in this phenomenon is driven by its potential to circumvent the Shockley–Queisser limit for single-junction solar cells. By converting high-energy photons into two low-energy excited states, singlet fission offers a means to overcome thermalization losses. Devices based on pentacene, a fission sensitizer, have demonstrated external quantum efficiencies of 129%, the highest for any photovoltaic technology to date 4. Current theoretical descriptions of singlet fission are framed by the kinetic model proposed by Johnson and Merrifield in 1970 (ref. 5), which established the role of a triplet pair (TT) coupled into an overall singlet state as an intermediate to triplet formation, but the details of the ultrafast fission process are subject to debate. Most theoretical studies of the canonical system, pentacene, focus on the low-lying electronic states and their composition by TT, intermolecular charge-transfer and monomolecular singlet configurations 6–9. The relative strengths of the direct electronic couplings between these states then determine the overall fission mechanism, proposals for which can be roughly divided into two classes. In the 'direct' model, the TT state is formed by electronic coupling between the initial singlet and the TT manifolds, and singlet-to-triplet conversion is accomplished through an avoided crossing or a conical intersection 9–11. Alternatively, the 'mediated' model proposes that the initial singlet couples much more strongly to virtual charge-transfer configurations 6,7 and, in some descriptions, may even directly populate charge-transfer states 1,8. These charge-transfer configurations, whether real or virtual, in turn have strong coupling to TT and enable singlet fission. The limit of very strong coupling between the singlet and this intermediate state results in the 'coherent' model, where the photoexcited singlet state and TT are linked by electronic coherence 12. A recent survey of acene derivatives 3 suggests that two of these mechanisms, direct and virtual mediated coupling, may contribute to determining the rate of singlet fission in the acenes, with the weight of mediated coupling dependent on the contribution of charge-transfer character to the first excited state. In contrast to the rigorous theoretical debate, there is little experimental data available that helps our understanding of the mechanism behind singlet fission. Standard experimental techniques such as transient absorption or photoluminescence spectroscopy enable direct monitoring of singlet and triplet exciton populations and the kinetics of their interconversion 2,13–17. These methods, however, are generally insensitive to the nature of the coupling between states. More sophisticated spectroscopic approaches are therefore required to build a clear picture of such underlying dynamics. At the same time, there is a growing awareness of the central role of vibronic coupling in a range of ultrafast photophysical processes, from photosynthesis to charge separation in organic heterojunctions 18–21. Many ultrafast (<200 fs) photophysical processes, such as internal conversion in isolated molecules, have been studied experimentally and theoretically in detail and are generally accepted to involve conical intersections between the electronic states induced by strong coupling between vibrational and electronic degrees of freedom 22,23. The ultrafast timescales for fission point to the importance of understanding the associated nuclear dynamics that mediate the conversion of singlets to triplets, but no experimental information on the underlying vibronic coupling is at present available. To probe the involved nuclear dynamics and thereby provide new insight into the mechanism of ultrafast singlet fission, we approach the problem with a recently developed experimental technique that uses impulsively generated vibrational coherence as a probe of vibronic coupling 24. We study thin films of TIPS-pentacene (Fig. 1a), a solution-processable pentacene derivative that undergoes efficient fission on sub-100-fs
Singlet exciton fission, the spin-conserving process that produces two triplet excited states fro... more Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley–Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight.
Singlet exciton fission is a spin-allowed process to generate two triplet excitons from a single ... more Singlet exciton fission is a spin-allowed process to generate two triplet excitons from a single absorbed photon. This phenomenon offers great potential in organic photo-voltaics, but the mechanism remains poorly understood. Most reports to date have addressed intermolecular fission within small-molecular crystals. However, through appropriate chemical design chromophores capable of intramolecular fission can also be produced. Here we directly observe sub-100 fs activated singlet fission in a semiconducting poly-(thienylenevinylene). We demonstrate that fission proceeds directly from the initial 1B u exciton, contrary to current models that involve the lower-lying 2A g exciton. In solution, the generated triplet pairs rapidly recombine and decay through the 2A g state. In films, exciton diffusion breaks this symmetry and we observe long-lived triplets which form charge-transfer states in photovoltaic blends.
DNA-polymer conjugates have been recognized as versatile functional materials in many different f... more DNA-polymer conjugates have been recognized as versatile functional materials in many different fields ranging from nanotechnology to diagnostics and biomedicine. They combine the favorable properties of nucleic acids and synthetic polymers. Moreover, joining both structures with covalent bonds to form bioorganic hybrids allows for the tuning of specific properties or even the possibility of evolving completely new functions. One important class of this type of material is amphiphilic DNA block copolymers, which, due to microphase separation, can spontaneously adopt nanosized micelle morphologies with a hydrophobic core and a DNA corona. These DNA nano-objects have been explored as vehicles for targeted gene and drug delivery, and also as programmable nanoreactors for organic reactions. Key to the successful realization of these potential applications is that (1) DNA block copolymer conjugates can be fabricated in a fully automated fashion by employing a DNA synthesizer; (2) hydrophobic compounds can be loaded within their interior; and (3) they can be site-specifically functionalized by a convenient nucleic acid hybridization procedure. This chapter aims to broaden the range of biodiagnostic and biomedical applications of these materials by providing a comprehensive outline of the preparation and characterization of multifunctional DNA-polymer nanoparticles. The first reports of DNA-polymer hybrids go back to the late 1980s. In the earliest example, antisense oligonucleotides (ODNs) covalently attached to a poly(l-lysine) backbone were investigated for their ability to inhibit the synthesis of vesicular stomatitis virus proteins; and indeed it was found that these conjugates acted as antiviral agents (1). With this as a starting point, various applications
Virus capsids (VCs) or virus-like particles are a relatively new class of natural biomaterials wi... more Virus capsids (VCs) or virus-like particles are a relatively new class of natural biomaterials with great potential for materials science and nanotechnology. They form precisely defined stable cage structures, permit coat protein (CP) manipulation through mutagenesis or chemical modification, 1 and can be easily produced. Such exceptional characteristics make them particularly strong candidates for applications in biomedicine. 2-7 The natural container-like properties of viruses, as well as their ability to specifically target individual cells, have been attractive for gene delivery and are now being harnessed for therapeutic delivery. While some VCs have been investigated and chemically modified to probe targeting behavior, 8 scant work has been dedicated to loading these nano-containers. 9 An excellent model system in this regard is the Cowpea Chlorotic Mottle Virus (CCMV). As with other VCs, CCMV evolved to encapsulate and transport RNA; in its natural state it consists of 180 identical CPs, which self-assemble around the central RNA into 28 nm icosahedral particles. These can be described according to the Caspar and Klug T (triangulation) number as T) 3 particles. 10 It has been shown that such capsids can also be readily made to self-assemble around large polyanions, resulting in smaller T) 1 icosahedral particles of 18 nm. 11,12 CCMV is unique in that capsid assembly can be induced in acidic conditions even in the absence of nucleic acids, allowing the possibility of loading more diverse cargos such as enzymes. 13 Nonetheless, the porous walls of the shell severely complicate the loading and retention of small molecules. A still more severe limit is imposed by solvent incompatibilities, which prevent the loading of hydrophobic drugs except through covalent modification of the protein or complexation with polyanions. 14 The success of engineered virus nanoparticles as delivery vehicles will hinge in large part on the resolution of these issues and the development of efficient loading strategies for small molecules and macromolecular entities. We report here a strategy for the facile self-assembly and loading of CCMV capsids using DNA amphiphiles. These structures aggregate into micelles with a hydrophobic core and an anionic DNA corona. The negatively charged particles induce capsid formation, allowing the entrapment of a large number of small oligonucleotides (ODNs) as a constituent part of the micellar template. Furthermore, preloading of the micelles with hydrophobic entities in the core or hydrophilic entities by sequence-specific hybridization enables encapsulation of various small molecules inside VCs. Two classes of DNA amphiphiles known to self-assemble into larger aggregates should be distinguished, though representatives of both were investigated in this study. The first class consists of low molecular weight hydrophobic molecules that are attached to ODNs. 15 Here a lipid-DNA 11mer (UU11) containing two 5-dodec-1-ynyluracil nucleobases at the 5′-end was synthesized (Scheme S1). The other class of DNA amphiphiles involves linear DNA block copolymers (DBCs) in which a nucleic acid sequence is covalently connected to a hydrophobic organic polymer via complementary end groups. 16 This study employed two DBCs containing polypropylene oxide blocks of the same molecular weight (M W) 6800 g/mol) but different lengths of ODNs, namely an 11mer (P11) and a 22mer (P22) sequence. See Figure S2 for the DNA sequences and key chemical structures. All three amphiphiles formed micellar structures at room temperature, and in some cases the micelles were additionally loaded with model cargo molecules (SI.5). Particle sizes were characterized by dynamic light scattering (DLS) and AFM, and all fell in the range 7-11 nm (Figures S4 and S5). 15 To assess the ability of these DNA particles to template VC formation both components were combined through a simple mixing procedure. In most experiments DNA amphiphiles were mixed with CP in a 1:2.3 molar ratio at pH 7.5 and incubated for 30 min at 4 °C. It should be stressed that under these conditions VC formation can only be attributed to the organizing role of the micelles. The resulting materials were isolated by fast protein liquid chromatography (FPLC). Transmission electron microscopy (TEM) analysis revealed successful envelopment of all DNA micelle species by the CCMV capsid protein. Particles eluted at 1.3-1.4 mL (Figure
We report the formation and characterisation of easily functional-isable mixed micelles with DNA/... more We report the formation and characterisation of easily functional-isable mixed micelles with DNA/PEO corona and PPO core which can be loaded with hydrophobic molecules and stabilised by the formation of a cross-linked semi-interpenetrating network. Furthermore, the corona is functionalised by hybridisation either with dye-modified complementary DNA, with demonstrable distance control, or with DNA-labelled gold nanoparticles. Amphiphilic block copolymers have long been the subject of intense study for their supramolecular aggregation properties, especially with regard to potential pharmaceutical applications. 1,2 One family of these materials, the Pluronict block copolymers, composed of poly(ethyleneoxide) (PEO) and poly(propyleneoxide) (PPO) blocks with triblock structure PEO n –PPO m –PEO n , have received particular attention for their tunable, thermally responsive aggregation behavior. The micellisation and gelation behaviour of Pluronic block copolymers has been described elsewhere; 3 here it is sufficient to note that the process is governed by a critical micellisation concentration (CMC) and critical micellisation temperature (CMT) specific to each Pluronic species. In part due to the known biocompatibility of PEO chains, Pluronic block copolymers are strong candidates for biomedical applications, both for the bioactivity of the unimers and the possibility to load hydrophobic drugs into the micelle core. 4 The micelles can be cross-linked either at the periphery of the corona 5 or within the core 6 to stabilise them against dilution and low temperature. The primary limitation to Pluronic-based drug delivery is the difficulty of targeting—Pluronic micelles can only be equipped with targeting moieties through chemical modification of the terminal hydroxy groups of the polymer. 7 Another class of amphiphilic block copolymers with great biomedical potential is the DNA block copolymers (DBCs), consisting of single-or double-stranded DNA covalently attached to polymer units. 8 In aqueous media, amphiphilic DBCs also self-assemble into aggregates or micelles with a hydrophobic core and a hydrophilic DNA corona, which can be used for addressable functionalisation through simple hybridisation with complementary DNA (cDNA) covalently linked to the desired moiety. 9,10 A powerful potential application for this technique within the pharmaceutical sphere is targeting, as has been demonstrated with tumor cells. 11 In vivo applications are limited, though, by the instability of the micelles against dilution. Another potential drawback of pure DBC micelles for drug delivery might be the strong stimulation of immune responses, due to the high local DNA concentration, surrounding these aggregates with PEO chains would potentially implement a desirable kind of ''stealth'' function. We report here on the combination of these two classes of materials (Fig. 1), represented by DNA-b-PPO (22PPO), a PPO block (M W = 6800 g mol À1) covalently connected to the 5 0-end of a 22-base single-stranded DNA (5 0-CCT CGC TCT GCT AAT CCT GTT A-3 0), and Pluronic F127. The latter was selected for the comparable sizes of the hydrophobic block and micellar structure to those of 22PPO. 12,13 These aggregates combine the potential for intramicellar cross-linking of Pluronic with the facile functionalisability of DBCs, allowing the formation of stable micelles with easy targeting capabilities and thereby addressing two of the current drawbacks of the individual components. It is also expected that the PEO corona should shield the DNA backbone and thus improve immunocompatibility. The resulting mixed micelles were characterised with UV/Vis and fluorescence spectroscopy, atomic force microscopy (AFM), Fo¨rster resonance energy transfer (FRET) and transmission electron microscopy (TEM). Before forming blend micelles, we investigated the stabilisation of the individual components through UV-induced cross-linking of pentaerythrytol tetraacrylate (PETA) using a pyrene solubilization method. 14 F127 and 22PPO were prepared and stabilised as per reported procedures, 6,15 and stored overnight at 4 1C. Only the stabilised F127 samples retained well-resolved Fig. 1 Schematic of the mixed micelle architecture and chemical structures of the polymeric components. (A) PEO block of Pluronic. (B) DNA block of DBC. (C) PPO blocks of Pluronic or DBC. (D) and (E) Probes at 5 0-and 3 0-ends of the complementary DNA, respectively. (F) Hydrophobic compound loaded into the hydrophobic core. (G) Cross-linked nanodomains of PETA in the core.
Biosensors, which harness the unique specific binding properties of biomaterials such as proteis,... more Biosensors, which harness the unique specific binding properties of biomaterials such as proteis, are increasingly recognized as a powerful tool for chemical sensing. In this context, the detection of nucleic acids DNA and RNA will take on ever more importance for screening as the fields of genomics and diagnostics advance. The same properties that allow molecular recognition–strong, specific non-covalent interactions–also enable bottom-up assembly of sensing architectures. Here, we take advantage of such interactions for the self-assembly of field-effect transistors from semi-conducting single-walled carbon nanotubes selectively dispersed by DNA-block-copolymers and anchored to the electrodes through DNA hybridization. These transistors can sensitively detect the hybridization of complementary target DNA strands through transduction of the chemical recognition event into electrical doping, achieving an analyte sensitivity of 10 fM. Such ultra-sensitive electrical-based detection removes the need for DNA amplification and offers a new route to nucleic acids diagnostics.
The presence of energetically low-lying triplet states is a hallmark of organic semiconductors. E... more The presence of energetically low-lying triplet states is a hallmark of organic semiconductors. Even though they present a wealth of interesting photophysical properties, these optically dark states significantly limit optoelectronic device performance. Recent advances in emissive charge-transfer molecules have pioneered routes to reduce the energy gap between triplets and " bright " singlets, allowing thermal population exchange between them and eliminating a significant loss channel in devices. In conjugated polymers, this gap has proved resistant to modification. Here, we introduce a general approach to reduce the singlet−triplet energy gap in fully conjugated polymers, using a donor−orthogonal acceptor motif to spatially separate electron and hole wave functions. This new generation of conjugated polymers allows for a greatly reduced exchange energy, enhancing triplet formation and enabling thermally activated delayed fluorescence. We find that the mechanisms of both processes are driven by excited-state mixing between π−π*and charge-transfer states, affording new insight into reverse intersystem crossing.
In this work, we have rationally designed and synthesized a novel thiophene-diketopyrrolopyrrole ... more In this work, we have rationally designed and synthesized a novel thiophene-diketopyrrolopyrrole (TDPP)-vinyl-based dimer. We have investigated the optical and electronic properties and have probed the photophysical dynamics using transient absorption to investigate the possibility of singlet exciton fission. These revealed extremely rapid decay to the ground state (<50 ps), which we confirm is due to intramolecular excitonic processes rather than large-scale conformational change enabled by the vinyl linker. In all cases, the main excited state appears to be " dark " , suggesting rapid internal conversion into a dark 2A g-type singlet state. We found no evidence of triplet formation in TDPP-V-TDPP under direct photoexcitation. This may be a consequence of significant singlet stabilization in the dimer, bringing it below the energy needed to form two triplets. Our studies on this model compound set valuable lessons for design of novel triplet-forming materials and highlight the need for more broadly applicable design principles.
Understanding the mechanism of singlet exciton fission, in which a singlet exciton separates into... more Understanding the mechanism of singlet exciton fission, in which a singlet exciton separates into a pair of triplet excitons, is crucial to the development of new chromophores for efficient fission-sensitized solar cells. The challenge of controlling molecular packing and energy levels in the solid state precludes clear determination of the singlet fission pathway. Here, we circumvent this difficulty by utilizing covalent dimers of pentacene with two types of side groups. We report rapid and efficient intramolecular singlet fission in both molecules, in one case via a virtual charge-transfer state and in the other via a distinct charge-transfer intermediate. The singlet fission pathway is governed by the energy gap between singlet and charge-transfer states, which change dynamically with molecular geometry but are primarily set by the side group. These results clearly establish the role of charge-transfer states in singlet fission and highlight the importance of solubilizing groups to optimize excited-state photophysics.
1 wileyonlinelibrary.com proceed on ultrafast (≈100 fs) time scales, allowing it to out-compete o... more 1 wileyonlinelibrary.com proceed on ultrafast (≈100 fs) time scales, allowing it to out-compete other decay channels and achieve high effi ciencies. [ 3 ] The essential condition for effi cient SEF is the energetic alignment of the singlet and triplet states, such that 2 E (T 1) ≤ E (S 1). A recent combined theoretical and experimental study of SEF rates in a range of acene solids has demonstrated that the rate of SEF is also greatly affected by the strength of intermolecular coupling within the fi lm. [ 4 ] In the canonical system, pentacene, triplet pair formation is exo-thermic and the intermolecular coupling is strong, resulting in SEF with an 80 fs time constant and nearly 200% yield. [ 5 ] Though most experimental studies of SEF have involved crystalline, polycrystalline or amorphous solids, the most basic unit capable of SEF is a pair of chromophores. Indeed, it was recently demonstrated in concentrated solutions of TIPS-pentacene that singlet fi ssion can proceed at high efficiency through bimolecular diffusional interactions. [ 6 ] However , early attempts to directly control the interaction between chromophores through the use of covalent dimers have not been as successful. The most notable systems in this regard are tetracene and 1,3-diphenylisobenzofuran. These materials are found to exhibit effi cient SEF in the solid state, but their covalent dimers achieved triplet yields of only a few percent. In both of these studies, [ 7 ] the two SEF chromophores were joined by a range of linkers to modify the strength of the electronic coupling between them, with the aim of tuning the rate and effi ciency of SEF. The impact was subtle, and it thus remains unclear why covalent dimers have proved ineffi cient to date. Current models suggest that dimers should be asymmetric or contain signifi cant cofacial interaction between chromophores to attain high triplet yields. [ 2,8 ] Interestingly, a recent study of pentacene dimers separated by a phenyl spacer unit achieved triplet yields above 100% in spite of using the same symmetric bonding motifs of the earlier tetracene dimers. [ 9 ] In this work, we report highly effi cient intramolecular SEF in a new type of covalent dimer, with triplet yields of up to 192 ± 3%. The molecule used in this study, 13,13′-bis(mesityl)-6,6′-dipentacenyl (DP-Mes, Figure 1 a), consists of two pen-tacenes directly bonded through a single C C bond with two bulky mesityl groups at the meso-positions. The geometry of the dimer, with two nearly orthogonal pentacene cores, is unlike Fast and highly effi cient intramolecular singlet exciton fi ssion in a pentacene dimer, consisting of two covalently attached, nearly orthogonal pentacene units is reported. Fission to triplet excitons from this ground state geometry occurs within 1 ps in isolated molecules in solution and dispersed solid matrices. The process exhibits a sensitivity to environmental polarity and competes with geometric relaxation in the singlet state, while subsequent triplet decay is strongly dependent on conformational freedom. The near orthogonal arrangement of the pentacene units is unlike any structure currently proposed for effi cient singlet exciton fi ssion and may lead to new molecular design rules.
Singlet exciton fission allows the fast and efficient generation of two spin triplet states from ... more Singlet exciton fission allows the fast and efficient generation of two spin triplet states from one photoexcited singlet. It has the potential to improve organic photovoltaics, enabling efficient coupling to the blue to ultraviolet region of the solar spectrum to capture the energy generally lost as waste heat. However, many questions remain about the underlying fission mechanism. The relation between intermolecular geometry and singlet fission rate and yield is poorly understood and remains one of the most significant barriers to the design of new singlet fission sensitizers. Here we explore the structure−property relationship and examine the mechanism of singlet fission in aggregates of astaxanthin, a small polyene. We isolate five distinct supramolecular structures of astaxanthin generated through self-assembly in solution. Each is capable of undergoing intermolecular singlet fission, with rates of triplet generation and annihilation that can be correlated with intermolecular coupling strength. In contrast with the conventional model of singlet fission in linear molecules, we demonstrate that no intermediate states are involved in the triplet formation: instead, singlet fission occurs directly from the initial 1B u photoexcited state on ultrafast time scales. This result demands a re-evaluation of current theories of polyene photophysics and highlights the robustness of carotenoid singlet fission.
Solution-based studies of singlet exciton fission have provided valuable insight to this spin-all... more Solution-based studies of singlet exciton fission have provided valuable insight to this spin-allowed process in organic chromophores, whereby a photogenerated spin-singlet exciton splits into two spin-triplet excitons on separate molecules. Here we review the most significant experimental contributions made regarding fission in solution, in both intra-and intermolecular systems. Intramolecular fission allows a clearer examination of the molecular excited states involved in triplet formation, and the ability to control inter-chromophore structure offers a route to directly investigate the role of molecular coupling. In diffusional, intermolecular systems the conformational freedom and slower timescales of fission reveal the nature of intermediate states.
Singlet exciton fission is the spin-conserving transformation of one spin-singlet exciton into tw... more Singlet exciton fission is the spin-conserving transformation of one spin-singlet exciton into two spin-triplet excitons. This exciton multiplication mechanism offers an attractive route to solar cells that circumvent the single-junction Shockley–Queisser limit. Most theoretical descriptions of singlet fission invoke an intermediate state of a pair of spin-triplet excitons coupled into an overall spin-singlet configuration, but such a state has never been optically observed. In solution, we show that the dynamics of fission are diffusion limited and enable the isolation of an intermediate species. In concentrated solutions of bis(triisopropylsilylethynyl)[TIPS]— tetracene we find rapid (<100 ps) formation of excimers and a slower (∼10 ns) break up of the excimer to two triplet exciton-bearing free molecules. These excimers are spectroscopically distinct from singlet and triplet excitons, yet possess both singlet and triplet characteristics, enabling identification as a triplet pair state. We find that this triplet pair state is significantly stabilized relative to free triplet excitons, and that it plays a critical role in the efficient endo-thermic singlet fission process. singlet fission | photochemistry | TIPS–tetracene | triplet | excimer
Singlet exciton fission is the process in organic semiconductors through which a spin-singlet exc... more Singlet exciton fission is the process in organic semiconductors through which a spin-singlet exciton converts into a pair of spin-triplet excitons residing on diierent chromophores, entangled in an overall spin-zero state. For some systems, singlet fission has been shown to occur on the 100 fs timescale and with a 200% quantum yield, but the mechanism of this process remains uncertain. Here we study a model singlet fission system, TIPS-pentacene, using ultrafast vibronic spectroscopy. We observe that vibrational coherence in the initially photogenerated singlet state is transferred to the triplet state and show that this behaviour is eeectively identical to ultrafast internal conversion for polyenes in solution. This similarity in vibronic dynamics suggests that both multi-molecular singlet fission and single-molecular internal conversion are mediated by the same underlying relaxation processes, based on strong coupling between nuclear and electronic degrees of freedom. In its most eecient form this leads to a conical intersection between the coupled electronic states. S inglet exciton fission has long commanded interest as an exceptionally fast channel to generate triplet excitons in organic materials. At the heart of the process is the coupling of the final triplet pair to the initially photoexcited singlet state, which ensures conservation of spin 1. In systems where singlet fission is exergonic, it can be rapid and highly efficient. For instance, thin films of pentacene and TIPS-pentacene exhibit triplet formation with a time constant of 80 fs and quantum yields of 200% (refs 2,3). Current interest in this phenomenon is driven by its potential to circumvent the Shockley–Queisser limit for single-junction solar cells. By converting high-energy photons into two low-energy excited states, singlet fission offers a means to overcome thermalization losses. Devices based on pentacene, a fission sensitizer, have demonstrated external quantum efficiencies of 129%, the highest for any photovoltaic technology to date 4. Current theoretical descriptions of singlet fission are framed by the kinetic model proposed by Johnson and Merrifield in 1970 (ref. 5), which established the role of a triplet pair (TT) coupled into an overall singlet state as an intermediate to triplet formation, but the details of the ultrafast fission process are subject to debate. Most theoretical studies of the canonical system, pentacene, focus on the low-lying electronic states and their composition by TT, intermolecular charge-transfer and monomolecular singlet configurations 6–9. The relative strengths of the direct electronic couplings between these states then determine the overall fission mechanism, proposals for which can be roughly divided into two classes. In the 'direct' model, the TT state is formed by electronic coupling between the initial singlet and the TT manifolds, and singlet-to-triplet conversion is accomplished through an avoided crossing or a conical intersection 9–11. Alternatively, the 'mediated' model proposes that the initial singlet couples much more strongly to virtual charge-transfer configurations 6,7 and, in some descriptions, may even directly populate charge-transfer states 1,8. These charge-transfer configurations, whether real or virtual, in turn have strong coupling to TT and enable singlet fission. The limit of very strong coupling between the singlet and this intermediate state results in the 'coherent' model, where the photoexcited singlet state and TT are linked by electronic coherence 12. A recent survey of acene derivatives 3 suggests that two of these mechanisms, direct and virtual mediated coupling, may contribute to determining the rate of singlet fission in the acenes, with the weight of mediated coupling dependent on the contribution of charge-transfer character to the first excited state. In contrast to the rigorous theoretical debate, there is little experimental data available that helps our understanding of the mechanism behind singlet fission. Standard experimental techniques such as transient absorption or photoluminescence spectroscopy enable direct monitoring of singlet and triplet exciton populations and the kinetics of their interconversion 2,13–17. These methods, however, are generally insensitive to the nature of the coupling between states. More sophisticated spectroscopic approaches are therefore required to build a clear picture of such underlying dynamics. At the same time, there is a growing awareness of the central role of vibronic coupling in a range of ultrafast photophysical processes, from photosynthesis to charge separation in organic heterojunctions 18–21. Many ultrafast (<200 fs) photophysical processes, such as internal conversion in isolated molecules, have been studied experimentally and theoretically in detail and are generally accepted to involve conical intersections between the electronic states induced by strong coupling between vibrational and electronic degrees of freedom 22,23. The ultrafast timescales for fission point to the importance of understanding the associated nuclear dynamics that mediate the conversion of singlets to triplets, but no experimental information on the underlying vibronic coupling is at present available. To probe the involved nuclear dynamics and thereby provide new insight into the mechanism of ultrafast singlet fission, we approach the problem with a recently developed experimental technique that uses impulsively generated vibrational coherence as a probe of vibronic coupling 24. We study thin films of TIPS-pentacene (Fig. 1a), a solution-processable pentacene derivative that undergoes efficient fission on sub-100-fs
Singlet exciton fission, the spin-conserving process that produces two triplet excited states fro... more Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley–Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight.
Singlet exciton fission is a spin-allowed process to generate two triplet excitons from a single ... more Singlet exciton fission is a spin-allowed process to generate two triplet excitons from a single absorbed photon. This phenomenon offers great potential in organic photo-voltaics, but the mechanism remains poorly understood. Most reports to date have addressed intermolecular fission within small-molecular crystals. However, through appropriate chemical design chromophores capable of intramolecular fission can also be produced. Here we directly observe sub-100 fs activated singlet fission in a semiconducting poly-(thienylenevinylene). We demonstrate that fission proceeds directly from the initial 1B u exciton, contrary to current models that involve the lower-lying 2A g exciton. In solution, the generated triplet pairs rapidly recombine and decay through the 2A g state. In films, exciton diffusion breaks this symmetry and we observe long-lived triplets which form charge-transfer states in photovoltaic blends.
DNA-polymer conjugates have been recognized as versatile functional materials in many different f... more DNA-polymer conjugates have been recognized as versatile functional materials in many different fields ranging from nanotechnology to diagnostics and biomedicine. They combine the favorable properties of nucleic acids and synthetic polymers. Moreover, joining both structures with covalent bonds to form bioorganic hybrids allows for the tuning of specific properties or even the possibility of evolving completely new functions. One important class of this type of material is amphiphilic DNA block copolymers, which, due to microphase separation, can spontaneously adopt nanosized micelle morphologies with a hydrophobic core and a DNA corona. These DNA nano-objects have been explored as vehicles for targeted gene and drug delivery, and also as programmable nanoreactors for organic reactions. Key to the successful realization of these potential applications is that (1) DNA block copolymer conjugates can be fabricated in a fully automated fashion by employing a DNA synthesizer; (2) hydrophobic compounds can be loaded within their interior; and (3) they can be site-specifically functionalized by a convenient nucleic acid hybridization procedure. This chapter aims to broaden the range of biodiagnostic and biomedical applications of these materials by providing a comprehensive outline of the preparation and characterization of multifunctional DNA-polymer nanoparticles. The first reports of DNA-polymer hybrids go back to the late 1980s. In the earliest example, antisense oligonucleotides (ODNs) covalently attached to a poly(l-lysine) backbone were investigated for their ability to inhibit the synthesis of vesicular stomatitis virus proteins; and indeed it was found that these conjugates acted as antiviral agents (1). With this as a starting point, various applications
Virus capsids (VCs) or virus-like particles are a relatively new class of natural biomaterials wi... more Virus capsids (VCs) or virus-like particles are a relatively new class of natural biomaterials with great potential for materials science and nanotechnology. They form precisely defined stable cage structures, permit coat protein (CP) manipulation through mutagenesis or chemical modification, 1 and can be easily produced. Such exceptional characteristics make them particularly strong candidates for applications in biomedicine. 2-7 The natural container-like properties of viruses, as well as their ability to specifically target individual cells, have been attractive for gene delivery and are now being harnessed for therapeutic delivery. While some VCs have been investigated and chemically modified to probe targeting behavior, 8 scant work has been dedicated to loading these nano-containers. 9 An excellent model system in this regard is the Cowpea Chlorotic Mottle Virus (CCMV). As with other VCs, CCMV evolved to encapsulate and transport RNA; in its natural state it consists of 180 identical CPs, which self-assemble around the central RNA into 28 nm icosahedral particles. These can be described according to the Caspar and Klug T (triangulation) number as T) 3 particles. 10 It has been shown that such capsids can also be readily made to self-assemble around large polyanions, resulting in smaller T) 1 icosahedral particles of 18 nm. 11,12 CCMV is unique in that capsid assembly can be induced in acidic conditions even in the absence of nucleic acids, allowing the possibility of loading more diverse cargos such as enzymes. 13 Nonetheless, the porous walls of the shell severely complicate the loading and retention of small molecules. A still more severe limit is imposed by solvent incompatibilities, which prevent the loading of hydrophobic drugs except through covalent modification of the protein or complexation with polyanions. 14 The success of engineered virus nanoparticles as delivery vehicles will hinge in large part on the resolution of these issues and the development of efficient loading strategies for small molecules and macromolecular entities. We report here a strategy for the facile self-assembly and loading of CCMV capsids using DNA amphiphiles. These structures aggregate into micelles with a hydrophobic core and an anionic DNA corona. The negatively charged particles induce capsid formation, allowing the entrapment of a large number of small oligonucleotides (ODNs) as a constituent part of the micellar template. Furthermore, preloading of the micelles with hydrophobic entities in the core or hydrophilic entities by sequence-specific hybridization enables encapsulation of various small molecules inside VCs. Two classes of DNA amphiphiles known to self-assemble into larger aggregates should be distinguished, though representatives of both were investigated in this study. The first class consists of low molecular weight hydrophobic molecules that are attached to ODNs. 15 Here a lipid-DNA 11mer (UU11) containing two 5-dodec-1-ynyluracil nucleobases at the 5′-end was synthesized (Scheme S1). The other class of DNA amphiphiles involves linear DNA block copolymers (DBCs) in which a nucleic acid sequence is covalently connected to a hydrophobic organic polymer via complementary end groups. 16 This study employed two DBCs containing polypropylene oxide blocks of the same molecular weight (M W) 6800 g/mol) but different lengths of ODNs, namely an 11mer (P11) and a 22mer (P22) sequence. See Figure S2 for the DNA sequences and key chemical structures. All three amphiphiles formed micellar structures at room temperature, and in some cases the micelles were additionally loaded with model cargo molecules (SI.5). Particle sizes were characterized by dynamic light scattering (DLS) and AFM, and all fell in the range 7-11 nm (Figures S4 and S5). 15 To assess the ability of these DNA particles to template VC formation both components were combined through a simple mixing procedure. In most experiments DNA amphiphiles were mixed with CP in a 1:2.3 molar ratio at pH 7.5 and incubated for 30 min at 4 °C. It should be stressed that under these conditions VC formation can only be attributed to the organizing role of the micelles. The resulting materials were isolated by fast protein liquid chromatography (FPLC). Transmission electron microscopy (TEM) analysis revealed successful envelopment of all DNA micelle species by the CCMV capsid protein. Particles eluted at 1.3-1.4 mL (Figure
We report the formation and characterisation of easily functional-isable mixed micelles with DNA/... more We report the formation and characterisation of easily functional-isable mixed micelles with DNA/PEO corona and PPO core which can be loaded with hydrophobic molecules and stabilised by the formation of a cross-linked semi-interpenetrating network. Furthermore, the corona is functionalised by hybridisation either with dye-modified complementary DNA, with demonstrable distance control, or with DNA-labelled gold nanoparticles. Amphiphilic block copolymers have long been the subject of intense study for their supramolecular aggregation properties, especially with regard to potential pharmaceutical applications. 1,2 One family of these materials, the Pluronict block copolymers, composed of poly(ethyleneoxide) (PEO) and poly(propyleneoxide) (PPO) blocks with triblock structure PEO n –PPO m –PEO n , have received particular attention for their tunable, thermally responsive aggregation behavior. The micellisation and gelation behaviour of Pluronic block copolymers has been described elsewhere; 3 here it is sufficient to note that the process is governed by a critical micellisation concentration (CMC) and critical micellisation temperature (CMT) specific to each Pluronic species. In part due to the known biocompatibility of PEO chains, Pluronic block copolymers are strong candidates for biomedical applications, both for the bioactivity of the unimers and the possibility to load hydrophobic drugs into the micelle core. 4 The micelles can be cross-linked either at the periphery of the corona 5 or within the core 6 to stabilise them against dilution and low temperature. The primary limitation to Pluronic-based drug delivery is the difficulty of targeting—Pluronic micelles can only be equipped with targeting moieties through chemical modification of the terminal hydroxy groups of the polymer. 7 Another class of amphiphilic block copolymers with great biomedical potential is the DNA block copolymers (DBCs), consisting of single-or double-stranded DNA covalently attached to polymer units. 8 In aqueous media, amphiphilic DBCs also self-assemble into aggregates or micelles with a hydrophobic core and a hydrophilic DNA corona, which can be used for addressable functionalisation through simple hybridisation with complementary DNA (cDNA) covalently linked to the desired moiety. 9,10 A powerful potential application for this technique within the pharmaceutical sphere is targeting, as has been demonstrated with tumor cells. 11 In vivo applications are limited, though, by the instability of the micelles against dilution. Another potential drawback of pure DBC micelles for drug delivery might be the strong stimulation of immune responses, due to the high local DNA concentration, surrounding these aggregates with PEO chains would potentially implement a desirable kind of ''stealth'' function. We report here on the combination of these two classes of materials (Fig. 1), represented by DNA-b-PPO (22PPO), a PPO block (M W = 6800 g mol À1) covalently connected to the 5 0-end of a 22-base single-stranded DNA (5 0-CCT CGC TCT GCT AAT CCT GTT A-3 0), and Pluronic F127. The latter was selected for the comparable sizes of the hydrophobic block and micellar structure to those of 22PPO. 12,13 These aggregates combine the potential for intramicellar cross-linking of Pluronic with the facile functionalisability of DBCs, allowing the formation of stable micelles with easy targeting capabilities and thereby addressing two of the current drawbacks of the individual components. It is also expected that the PEO corona should shield the DNA backbone and thus improve immunocompatibility. The resulting mixed micelles were characterised with UV/Vis and fluorescence spectroscopy, atomic force microscopy (AFM), Fo¨rster resonance energy transfer (FRET) and transmission electron microscopy (TEM). Before forming blend micelles, we investigated the stabilisation of the individual components through UV-induced cross-linking of pentaerythrytol tetraacrylate (PETA) using a pyrene solubilization method. 14 F127 and 22PPO were prepared and stabilised as per reported procedures, 6,15 and stored overnight at 4 1C. Only the stabilised F127 samples retained well-resolved Fig. 1 Schematic of the mixed micelle architecture and chemical structures of the polymeric components. (A) PEO block of Pluronic. (B) DNA block of DBC. (C) PPO blocks of Pluronic or DBC. (D) and (E) Probes at 5 0-and 3 0-ends of the complementary DNA, respectively. (F) Hydrophobic compound loaded into the hydrophobic core. (G) Cross-linked nanodomains of PETA in the core.
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Papers by Andrew J Musser