•P3AT/CNT sensors with different alkyl side chain lengths in P3AT were tested as alkane vapor sensors.•P3DT/CNT show sensitivity to n-dodecane vapor at ppb level.•P3AT/CNT with alkyl side chain length similar to the length of the alkane analyte gives a better response.•Sensor array based on P3AT/CNT sensors were able to distinguish the lengths of certain alkanes at various concentrations.In general, alkane vapors are difficult to detect due to their non-reactive nature at room temperature. Here, we show chemiresistive sensors made of carbon nanotubes (CNTs) noncovalently functionalized with three kinds of poly(3-alkylthiophene) (P3AT), namely, poly(3-butylthiophene) (P3BT), poly(3-octylthiophene-2,5-diyl) (P3OT) and poly(3-dodecylthiophene-2,5-diyl) (P3DT). We compared the responses of sensors composed of these materials to four linear alkanes, hexane, octane, decane and dodecane. The results show that sensors with CNTs functionalized with P3ATs that have alkyl side chains with length similar to the length of the analyte alkane produced a bigger response than the case in which the lengths are different. Based on this response trend, a sensor array was made, which can distinguish different sizes of linear alkane vapors. This work facilitates the future design of CNT-based sensor arrays for distinguishing analytes with similar physical and chemical properties.Download high-res image (127KB)Download full-size image

The research on donor–acceptor (D–A) cocrystalline structures based on organic semiconductive molecules has drawn great attention due to their unique optical and electronic properties. Among the building block molecules, derivatives of perylene-3,4,9,10-tetracarboxylic-3,4,9,10-diimides (PTCDI) are of particular interest as these molecules form high performance n-type semiconductors with strong air stability. However, the cocrystal of PTCDIs remains challenging to fabricate, and only few D–A cocrystals of PTCDIs have been reported. Herein, we report a successful molecular self-assembly design for the PTCDI cocrystal with a donor molecule, coronene. Within the triclinic cell of the cocrystal, the PTCDI and coronene molecules achieved 1 : 1 alternated π–π stacking. The cocrystal showed clear red-shifted absorption and photoluminescence bands, implying the strong charge transfer interaction between coronene and PTCDI. Additionally, the cofacial π–π stacking between coronene and PTCDI planes favors strong one-dimensional self-assembly, leading to the formation of microsized fibril cocrystals and arrays. This presented cocrystal design strategy helps to explore new D–A cocrystalline structures, particularly with one-dimensional morphology control.

We report a chemiresistive sensor approach based on a TTF–TCNQ charge transfer material, which can real-time detect and distinguish the vapors of alkyl amine and aromatic amine species under ambient conditions, based on the dramatic difference in the kinetics of the electric current recovery processes after the exposure of the two amine species.

•PIx-NCN as a Z-scheme heterojunction is built to accelerate charge separation.•PIs change H2O2 generation from single-channel to two-channel.•Photogenerated electrons and holes are separated into two different phases.•The built system helps spatially isolate oxidation and reduction reaction sites.Hydrogen peroxide is a promising solar fuel and widely used in many industrial processes. Here we report on a new basis for clean energy storage, generating H2O2 from H2O and O2 by organic photocatalysis. In this study, we construct an all-solid-state Z-scheme heterojunction (PIx-NCN) by assembling perylene imides (PI) on g-C3N4 nanosheets (NCN), where x is a percentage weight ratio of PI to NCN. PIx-NCN exhibits significant enhancement in photocatalytic production H2O2, and the maximum enhancement was observed for PI5.0-NCN. It was shown that PI can change H2O2 generation from single-channel to two-channel. Specifically, photoexcitation of the PI moieties transfers their conduction band electrons to the valence band of NCN, resulting in enhanced charge separation. Thus, more electrons are available to reduce O2, producing more H2O2. More importantly, the holes in the valence band of PI moiety have more positive potential (2.08 V) than those of NCN (1.63 V), which can oxidize OH− to form OH (1.99 V) and transform to H2O2 via the second channel. Therefore, for PIx-NCN, the photogenerated electrons and holes can be separated into two different phases, helping spatially isolate the oxidation and reduction reaction sites, and thus minimizing the catalytic deactivation.Download high-res image (89KB)Download full-size image

•We report on a simple method of synthesizing ultrathin nanosheets of g-C3N4 and the application as photocatalyst for NO removal.•Ultrathin nanostructure and abundant surface carbon vacancy, could promote its visible light absorption, and favor the separation and transfer of photogenerated charge carriers as well as strong chemisorption of NO.•Carbon vacancy-modified nanosheet structure g-C3N4 (Ns-g-C3N4) can efficiently and selectively reduce NO to N2 under visible light.•The surface carbon vacancy of Ns-g-C3N4 shift the adsorption structure of NO from CNO for the bulk counterpart to CvON.Photocatalytic oxidation has recently been recognized as an attractive technology for NO removal, in which the main products are NO2 or HNO3. However, these products may cause secondary pollution and deactivation of the involved photocatalysts. In this study, we demonstrate that carbon vacancy-modified nanosheet structure g-C3N4 (Ns-g-C3N4) can efficiently and selectively reduce NO to N2 under visible light. Since N2 is a green gas and can easily desorb from the active sites, the problems such as secondary pollution and catalyst deactivation are largely avoided. It was found that two structural characters of Ns-g-C3N4, ultrathin nanostructure and abundant surface defect sites, could promote its visible light absorption, and favor the separation and transfer of photogenerated charge carriers as well as strong chemisorption of NO, leading to high photoreactivity. Meanwhile, the surface defects of Ns-g-C3N4 shift the adsorption structure of NO from CNO for the bulk counterpart to CvON (adsorbed at the carbon vacancy site, Cv), eventually resulting in its high selectivity of converting NO to N2. The present study underlines the impetus of utilizing surface defect structure to regulate photocatalytic reaction pathway.Download high-res image (142KB)Download full-size imageWe report on a simple method of synthesizing ultrathin nanosheets of g-C3N4 and the application as photocatalyst for converting NO to N2. Ultrathin nanostructure and abundant surface carbon vacancies, could promote its visible light absorption, and favor the separation and transfer of photogenerated charge carriers as well as strong chemisorption of NO, leading to high photoreactivity. Meanwhile, the surface carbon vacancies of Ns-g-C3N4 shift the adsorption structure of NO from CNO for the bulk counterpart to CvON, eventually resulting in its high selectivity of converting NO to N2.

For photocatalytic removal of nitric oxide (NO), two major issues need to be addressed: incomplete oxidation of NO and deactivation of the photocatalyst. In this study, we aimed to solve these two problems by constructing an all-solid-state Z-scheme heterojunction (PI-g-C3N4) consisting of g-C3N4 surface modified with perylene imides (PI). PI-g-C3N4 exhibits significant enhancement in photocatalytic activity (in comparison to pristine g-C3N4) when examined for NO removal. More importantly, the Z-scheme charge separation within PI-g-C3N4 populates electrons and holes into the increased energy levels, thereby enabling direct reduction of O2 to H2O2 and direct oxidation of NO to NO2. H2O2 can further oxidize NO2 to NO3– ion at a different location (via diffusion), thus alleviating the deactivation of the catalyst. The results presented may shed light on the design of visible photocatalysts with tunable reactivity for application in solar energy conversion and environmental sustainability.Keywords: g-C3N4; molecular oxygen activation; nitric oxide removal; PTCDI; Z-scheme

Intrinsically low electrical conductivity of organic semiconductors hinders their further development into practical electronic devices. Herein, we report on an efficient chemical self-doping to increase the conductivity through one-dimensional stacking arrangement of electron donor–acceptor (D–A) molecules. The D–A molecule employed was a 1-methylpiperidine-substituted perylene tetracarboxylic diimide (MP-PTCDI), of which the methylpiperidine moiety is a strong electron donor, and can form a charge transfer complex with PTCDI (acting as the acceptor), generating anionic radical of PTCDI as evidenced in molecular solutions. Upon self-assembling into nanoribbons through columnar π–π stacking, the intermolecular charge transfer interaction between methylpiperidine and PTCDI would be enhanced, and the electrons generated are delocalized along the π–π stacking of PTCDIs, leading to enhancement in conductivity. The conductive fiber materials thus produced can potentially be used as chemiresistive sensor for vapor detection of electron deficient chemicals such as hydrogen peroxide, taking advantage of the large surface area of nanofibers. As a major component of improvised explosives, hydrogen peroxide remains a critical signature chemical for public safety screening and monitoring.Keywords: chemiresistive sensor; electrical conductivity; nanoribbon; organic semiconductor; self-doping

The detection of alkane vapors has strong implications for safety, health, and the environment. Alkanes are notoriously difficult to detect because of their chemical inertness at room temperature. Herein, we introduce a tunable photoinduced charge transfer strategy to selectively detect alkane vapors under ambient condition. A unique donor–acceptor nanofibril composite comprising a compatible interface was fabricated, which is preferential for alkane adsorption. Then the enhanced adsorption disrupts the charge transfer across the interface and decreases the photocurrent, enabling the design of alkane gas sensor. We demonstrate a critical relationship between the tunable donor–acceptor interface and alkane response. The composite sensor is able to provide specific distinction between different alkanes based on their kinetics of the response profiles, and outstanding general selectivity against the common polar solvents. The work described herein may provide a basis for a new type of sensing material for detecting inert chemicals at room temperature.Keywords: alkane detection; interfacial engineering; nanofiber composite; photoinduced charge transfer; tunable D−A interface

In this work, we use time-resolved photoluminescence (PL) spectroscopy, microscopy, and current measurements to characterize the slow transient responses of methylammonium lead triiodide (MAPbI3) on a lateral interdigitated electrode device. By systematically varying the applied bias magnitude and electrode polarity, we observed distinct reversible and irreversible PL transient responses in the form of spectrally and spatially resolved PL quenching occurring over a range of 0.5–100 s. When the simultaneous current and the PL measurements were correlated, the reversible responses, present under all electric fields, were attributed to charge trapping, whereas the irreversible response, occurring above a nominal electric field between 1 and 5 kV cm–1, was attributed to ion migration. Thus, these results indicate that the slow transient response, and therefore hysteretic behavior, in MAPbI3 devices is a complex response with contributions from both charge trapping and ion migration.

Perylene tetracarboxylic diimide (PTCDI) derivatives have been extensively investigated for one-dimensional (1D) self-assembly and their applications in optoelectronic devices. Our study on self-assembled PTCDI nanofiber materials revealed a persistent photoconductivity (PPC) effect, which is sustained conductivity after illumination is terminated. A comprehensive understanding of the PPC effect in PTCDI nanofibril materials will enable us to explore and enhance their optoelectronic applications. Here, we have investigated the PPC effect in the nanofibers assembled from 1-methylpiperidine-substituted perylene tetracarboxylic diimide (MP-PTCDI) with respect to the PPC relaxation at different temperatures, illumination power densities, molar amount, and morphology of the PTCDI film deposited on the interdigitated electrodes. The photocurrent relaxation was also performed on several other PTCDI nanofiber materials for comparative study. We conclude that the significant PPC effect in MP-PTCDI nanofibers can be attributed to the electrical potential fluctuations caused by the structure defects, which thus hinder the recombination of charge carriers. This study may provide new design rules for fabrication of molecular semiconductor materials with strong PPC in order to approach high efficiency of photovoltaics and photocatalysis.

Self-assembly of π-conjugate molecules often leads to formation of well-defined nanofibril structures dominated by the columnar π–π stacking between the molecular planes. These nanofibril materials have drawn increasing interest in the research frontiers of nanomaterials and nanotechnology, as the nanofibers demonstrate one-dimensionally enhanced exciton and charge diffusion along the long axis, and present great potential for varying optoelectronic applications, such as sensors, optics, photovoltaics, and photocatalysis. However, poor electrical conductivity remains a technical drawback for these nanomaterials. To address this problem, we have developed a series of nanofiber structures modified with different donor–acceptor (D–A) interfaces that are tunable for maximizing the photoinduced charge separation, thus leading to increase in the electrical conductivity. The D–A interface can be constructed with covalent linker or noncovalent interaction (e.g., hydrophobic interdigitation between alkyl chains). The noncovalent method is generally more flexible for molecular design and solution processing, making it more adaptable to be applied to other fibril nanomaterials such as carbon nanotubes. In this Account, we will discuss our recent discoveries in these research fields, aiming to provide deep insight into the enabling photoconductivity of nanofibril materials, and the dependence on interface structure.The photoconductivity generated with the nanofibril material is proportional to the charge carriers density, which in turn is determined by the kinetics balance of the three competitive charge transfer processes: (1) the photoinduced electron transfer from D to A (also referred to as exciton dissociation), generating majority charge carrier located in the nanofiber; (2) the back electron transfer; and (3) the charge delocalization along the nanofiber mediated by the π–π stacking interaction. The relative rates of these charge transfer processes can be tuned by the molecular structure and nanoscale interface engineering. As a result, maximal photoconductivity can be achieved for different D–A nanofibril composites. The photoconductive nanomaterials thus obtained demonstrate unique features and functions when employed in photochemiresistor sensors, photovoltaics and photocatalysis, all taking advantages of the large, open interface of nanofibril structure. Upon deposition onto a substrate, the intertwined nanofibers form networks with porosity in nanometer scale. The porous structure enables three-dimensional diffusion of molecules (analytes in sensor or reactants in catalysis), facilitating the interfacial chemical interactions. For carbon nanotubes, the completely exposed π-conjugation facilitates the surface modification through π–π stacking in conjunction with D–A interaction. Depending on the electronic energy levels of D and A parts, appropriate band alignment can be achieved, thus producing an electric field across the interface. Presence of such an electric field enhances the charge separation, which may lead to design of new type of photovoltaic system using carbon nanotube composite.

Low dose detection of γ radiation remains critical for radiology therapy and nuclear security. We report herein on a novel dual-band fluorescence sensor system based on a molecule, 4-(1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenylaniline (PI-DPA), which can be dissolved into halogenated solvents to enable expedient detection of γ radiation. The limit of detection was projected down to 0.006 Gy. Exposure to γ radiation decomposes CHCl3 into small radicals, which then combine to produce HCl. Strong interaction of HCl with the imidazole group of PI-DPA converts it into a PI-DPA–HCl adduct, which self-assembles into nanofibers, quenching the fluorescence of the pristine PI-DPA molecule, while producing new fluorescent emission at longer wavelength. Such dual-band emission response provides improved sensing reliability compared to single band response. Systematic investigations based on acid titration, 1H NMR spectral measurements and time-course SEM imaging suggest that the observed new fluorescence band is due to π–π stacking of the PI-DPA–HCl adduct, which is facilitated by the formation of hydrogen bonded cluster units. The nanofibers exhibited high and reversible photoconductivity. Combining with the sensitive fluorescence response, the photoconductive nanofibers will enable development of a multimode stimuli-responsive sensor system that is suited for small, low cost dosimetry of γ radiation with improved sensitivity and detection reliability.

High-performance chemiresistive sensors were made using a porous thin film of single-walled carbon nanotubes (CNTs) coated with a carbazolylethynylene (Tg-Car) oligomer for trace vapor detection of nitroaromatic explosives. The sensors detect low concentrations of 4-nitrotoluene (NT), 2,4,6-trinitrotoluene (TNT), and 2,4-dinitrotoluene (DNT) vapors at ppb to ppt levels. The sensors also show high selectivity to NT from other common organic reagents at significantly higher vapor concentrations. Furthermore, by using Tg-Car/CNT sensors and uncoated CNT sensors in parallel, differential sensing of NT, TNT, and DNT vapors was achieved. This work provides a methodology to create selective CNT-based sensors and sensor arrays.Keywords: carbazole oligomer; carbon nanotube; chemiresistive vapor sensor; differential sensing; nitroaromatic explosive; thin film;

Development of low cost, easy-to-use chemical sensor systems for low dose detection of γ radiation remains highly desired for medical radiation therapy and nuclear security monitoring. We report herein on a new fluorescence sensor molecule, 4,4′-di(1H-phenanthro[9,10-d]imidazol-2-yl)biphenyl (DPI-BP), which can be dissolved into halogenated solvents (e.g., CHCl3, CH2Cl2) to enable instant detection of γ radiation down to the 0.01 Gy level. The sensing mechanism is primarily based on radiation induced fluorescence quenching of DPI-BP. Pristine DPI-BP is strongly fluorescent in halogenated solvents. When exposed to γ radiation, the halogenated solvents decompose into various radicals, including hydrogen and chlorine, which then combine to produce hydrochloric acid (HCl). This strong acid interacts with the imidazole group of DPI-BP to convert it into a DPI-BP/HCl adduct. The DPI-BP/HCl adduct possesses a more planar configuration than DPI-BP, enhancing the π–π stacking and thus molecular aggregation. The strong molecular fluorescence of DPI-BP gets quenched upon aggregation, due to the π–π stacking interaction (forming forbidden low-energy excitonic transition). Interestingly the quenched fluorescence can be recovered simply by adding base (e.g., NaOH) into the solution to dissociate the DPI-BP/HCl adduct. Such sensing mechanism was supported by systematic investigations based on HCl titration and dynamic light scattering measurements. To further confirm that the aggregation caused fluorescence quenching, a half size analogue of DPI-BP, 2-phenyl-1H-phenanthro[9,10-d]imidazole (PI-Ph), was synthesized and investigated in comparison with the observations of DPI-BP. PI-Ph shares the same imidazole conjugation structure with DPI-BP and is expected to bind the same way with HCl. However, PI-Ph did not show fluorescence quenching upon binding with HCl likely due to the smaller π-conjugation structure, which can hardly enforce the π–π stacking assembly. Combining the low detection limit, fast and reversible fluorescence quenching response, and low cost of halogenated solvent composites, the sensor system presented may lead to the development of new, simple chemical dosimetry for low dose detection of γ radiation.

Trace vapor detection of hydrogen peroxide (H2O2) represents a practical approach to nondestructive detection of peroxide-based explosives, including liquid mixtures of H2O2 and fuels and energetic peroxide derivatives, such as triacetone triperoxide (TATP), diacetone diperoxide (DADP), and hexamethylene triperoxide diamine (HMTD). Development of a simple chemical sensor system that responds to H2O2 vapor with high reliability and sufficient sensitivity (reactivity) remains a challenge. We report a fluorescence ratiometric sensor molecule, diethyl 2,5-bis((((4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl)oxy)carbonyl)amino)terephthalate (DAT-B), for H2O2 that can be fabricated into an expedient, reliable, and sensitive sensor system suitable for trace vapor detection of H2O2. DAT-B is fluorescent in the blue region, with an emission maximum at 500 nm in the solid state. Upon reaction with H2O2, DAT-B is converted to an electronic “push–pull” structure, diethyl 2,5-diaminoterephthalate (DAT-N), which has an emission peak at a longer wavelength centered at 574 nm. Such H2O2-mediated oxidation of aryl boronates can be accelerated through the addition of an organic base such as tetrabutylammonium hydroxide (TBAH), resulting in a response time of less than 0.5 s under 1 ppm of H2O2 vapor. The strong overlap between the absorption band of DAT-N and the emission band of DAT-B enables efficient Förster resonance energy transfer (FRET), thus allowing further enhancement of the sensing efficiency of H2O2 vapor. The detection limit of a drop-cast DAT-B/TBAH film was projected to be 7.7 ppb. By combining high sensitivity and selectivity, the reported sensor system may find broad application in vapor detection of peroxide-based explosives and relevant chemical reagents through its fabrication into easy-to-use, cost-effective kits.Keywords: fluorescent sensor; FRET; hydrogen peroxide; vapor detection;

We report the observation of anomalous high photovoltages, which are significantly higher than the energy level offset between the highest occupied molecular orbital (HOMO) of the electron donor (D) and the lowest unoccupied molecular orbital (LUMO) of the acceptor (A), in single trunk shish kebab-like organic p–n junction nanostructures. Creation of such high photovoltages is likely due to the special intermolecular orientation in the unique structure.

A series of novel nanocomposite structures have been fabricated by in situ deposition of TiO2 layers and/or a co-catalyst (Pt) on one-dimensional (1D) self-assembled nanofibers of perylene diimide derivatives (PDIs). The PDI molecules were functionalized with dodecyl and/or phenylamino groups to compare the effect of nanofiber morphology and intramolecular charge transfer on the photocatalytic performance. Under visible-light irradiation (λ > 420 nm), hydrogen production for all composite systems has been detected through photocatalytic water splitting in aqueous solutions with sacrificial reagent methanol or triethanolamine, proving the applicability of organic nanofibers in the photocatalytic system. Compared to the well-defined nanofibril morphology obtained from dodecyl-substituted PDI molecules, donor–accepter type PDIs with electron-rich phenylamino moieties attached show much improved photocatalytic activity due to efficient inter- and intra-molecular charge transfer. This work provides insight into the role of molecular design and nanomorphology of organic semiconductor materials in the field of photocatalysis.

High dark electrical conductivity was obtained for a p-type organic nanofibril material simply through a one-step surface doping. The nanofibril composite thus fabricated has been proven robust under ambient conditions. The high conductivity, combined with the intrinsic large surface area of the nanofibers, enables development of chemiresistor sensors for trace vapor detection of amines, with detection limit down to sub-parts per billion range.Keywords: high conductivity; organic nanofiber; p-type materials; sensors; surface doping; vapor detection;

A fluorescence turn-on sensor molecule (C6NIB) has been synthesized and fabricated into a porous matrix to enable trace vapor detection of hydrogen peroxide. The detection limit was projected to be below 5 ppb.

A water soluble perylene diimide molecule has been fabricated into nanofibers via a pH triggered hydrogelation route. The one-dimensional self-assembly is dominated by the intermolecular π–π stacking interactions in concert with the hydrogen bonding between the carboxylic acid side chains. The anisotropic electronic and optical properties observed for the nanofibers are consistent with the one-dimensional intermolecular π–π arrangement.

•Polymer solar cells with enhanced morphology stability were prepared without resorting to complicated synthetic methods.•The simple method utilizes the spatial confinement of preformed P3HT nanofiber networks on PCBM crystallization.•Our work provides a “one-step” way to directly prepare efficient polymer solar cells with thermally stable morphology.A stable morphology in the photoactive layer is a prerequisite for increasing the lifetime of organic solar cells. Intense research efforts focusing on this research topic have typically resorted to complicated synthetic methods to reach this goal. Herein, the authors present a facile approach to directly achieve efficient polymer solar cells with a remarkably enhanced thermally stable morphology by constructing densely distributed poly(3-hexylthiophene) (P3HT) nanofibers in the pristine composite films with PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) from solution without any post treatments. Controlled experiments reveal that the presence of numerous preformed P3HT nanofibers in the pristine films, with much larger size than P3HT and PCBM molecules, provides a fixed and rigid network to spatially confine the diffusion of PCBM molecules during thermal annealing, thus preventing the formation of large-scale PCBM crystals. This simple method represents a “one-step” way to prepare high performance photovoltaic devices with thermally stable morphologies and no necessary post treatments.Graphical abstract

Development of simple, cost-effective, and sensitive fluorescence-based sensors for explosives implies broad applications in homeland security, military operations, and environmental and industrial safety control. However, the reported fluorescence sensory materials (e.g., polymers) usually respond to a class of analytes (e.g., nitroaromatics), rather than a single specific target. Hence, the selective detection of trace amounts of trinitrotoluene (TNT) still remains a big challenge for fluorescence-based sensors. Here we report the selective detection of TNT vapor using the nanoporous fibers fabricated by self-assembly of carbazole-based macrocyclic molecules. The nanoporosity allows for time-dependent diffusion of TNT molecules inside the material, resulting in further fluorescence quenching of the material after removal from the TNT vapor source. Under the same testing conditions, other common nitroaromatic explosives and oxidizing reagents did not demonstrate this postexposure fluorescence quenching; rather, a recovery of fluorescence was observed. The postexposure fluorescence quenching as well as the sensitivity is further enhanced by lowering the highest occupied molecular orbital (HOMO) level of the nanofiber building blocks. This in turn reduces the affinity for oxygen, thus allocating more interaction sites for TNT. Our results present a simple and novel way to achieve detection selectivity for TNT by creating nanoporosity and tuning molecular electronic structure, which when combined may be applied to other fluorescence sensor materials for selective detection of vapor analytes.

Large area uniform nanofibers have been fabricated from a hexameric arylene–ethynylene macrocycle (1) through in situ self-assembly on a glass substrate during solvent evaporation. The fibril morphology is controlled by the solvophilic core of 1, in conjunction with the interfacial interactions between the side chains of 1 and the substrate.

One-dimensional nanostructures are self-assembled from an amphiphilic arylene-ethynylene macrocycle (AEM) in solution phase. The morphology and size of the nanostructures are controlled by simply changing the temperature, reversibly switching between monomolecular cross-sectioned nanofibers and large bundles. At elevated temperature in aqueous solutions, the tri(ethylene glycol) (Tg) side chains of the AEM become effectively more hydrophobic, thus facilitating intermolecular association through side chain interactions. The enhanced intermolecular association causes the ultrathin nanofibers to be bundled, forming an opaque dispersion in solution. The reported observation provides a simple molecular design rule that may be applicable to other macrocycle molecules for use in temperature-controlled assembly regarding both size and morphology.

Charge transport is studied in single-molecule junctions formed with a 1,7-pyrrolidine-substituted 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) molecular block using an electrochemical gate. Compared to an unsubstituted-PTCDI block, spectroscopic and electrochemical measurements indicate a reduction in the highest occupied (HOMO)–lowest unoccupied (LUMO) molecular orbital energy gap associated with the electron donor character of the substituents. The small HOMO–LUMO energy gap allows for switching between electron- and hole-dominated charge transports as a function of gate voltage, thus demonstrating a single-molecule ambipolar field-effect transistor. Both the unsubstituted and substituted molecules display similar n-type behaviors, indicating that they share the same n-type conduction mechanism. However, the substituted-PTCDI block shows a peak in the source–drain current vs gate voltage characteristics for the p-type transport, which is attributed to a two-step incoherent transport via the HOMO of the molecule.Keywords: ambipolar FET; electrochemical gate; PTCDI; single-molecule junction; STM

Fluorescent nanoscale coordination polymers with cubic morphology and long range ordered structure were fabricated and exhibited efficient sensing for both nitroaromatic explosive and nitromethane due to large surface area to volume ratio and strong binding affinity to explosive molecules.

Vapor detection of hydrogen peroxide still remains challenging for conventional sensing techniques, though such vapor detection implies important applications in various practical areas, including locating IEDs. We report herein a new colorimetric sensor system that can detect hydrogen peroxide vapor down to parts per billion level. The sensory materials are based on the cellulose microfibril network of paper towels, which provide a tunable interface for modification with Ti(IV) oxo complexes for binding and reacting with H2O2. The Ti(IV)-peroxide bond thus formed turns the complex from colorless to bright yellow with an absorption maximum around 400 nm. Such complexation-induced color change is exclusively selective for hydrogen peroxide, with no color change observed in the presence of water, oxygen, common organic reagents or other chelating reagents. This paper-based sensor material is disposable and one-time use, representing a cheap, simple approach to detect peroxide vapors. The reported sensor system also proves the technical feasibility of developing enhanced colorimetric sensing using nanofibril materials that will provide plenty of room to enlarge the surface area (by shrinking the fiber size), so as to enhance the surface interaction with gas phase.Keywords: colorimetric sensor; hydrogen peroxide; paper-based; vapor detection

Intrinsic graphene is a semimetal or zero bandgap semiconductor, which hinders its applications for nanoelectronics. To develop high-performance nanodevices with graphene, it is necessary to open the bandgap and precisely control the charge carrier type and density. In this perspective, we focus on tailoring the electronic properties of graphene by noncovalent stacking with aromatic molecules through π–π interaction. Different types of molecules (functioning as either an electron donor or acceptor when stacked with graphene) as reported in recent literature are presented regarding surface patterning, bandgap engineering, surface doping, as well as applications in nanodevices, particularly the field-effect transistors (FETs). On the basis of the current progress along this research line, future issues and challenges are also briefly discussed.

Foldamers are synthetic and designable oligomers that adopt a conformationally ordered state in selected solvents. We found that oligo(m-phenylene ethynylene)s, which are single-stranded foldamers, can be made to reversibly disperse and release single-walled carbon nanotubes (SWCNTs) simply by changing the solvent, consistent with a change from an unfolded state to a folded state. Using absorption spectroscopy, atomic force microscopy, Raman spectroscopy, and electrical measurements, we observed that the foldamer-dispersed SWCNTs are individually well-dispersed and have a strong interfacial interaction with the foldamers. In contrast, the released SWCNTs appeared to be free of foldamers. Under illumination, transistors based on the foldamer-dispersed SWCNTs demonstrated significant photoresponse, apparently due to photoinduced charge transfer between the foldamers and SWCNTs. The reported nanocomposites may open an alternative way of developing optoelectronic devices or sensors based on carbon nanotubes.

Photoconductive organic materials have gained increasing interest in various optoelectronics, such as sensors, photodetectors, and photovoltaics. However, the availability of such materials is very limited due to their intrinsic low charge carrier density and mobility. Here, we present a simple approach based on nanofibril heterojunction to achieve high photoconductivity with fast photoresponse, that is, interfacial engineering of electron donor (D) coating onto acceptor (A) nanofibers via optimization of hydrophobic interaction between long alkyl side-chains. Such nanofibril heterojunctions possess two prominent features that are critical for efficient photocurrent generation: the nanofibers both create a large D/A interface for increased charge separation and act as long-range transport pathways for photogenerated charge carriers toward the electrodes, and the alkyl groups employed not only enable effective surface adsorption of D molecules on the nanofibers for effective electron-transfer communication, but also spatially separate the photogenerated charge carriers to prevent their recombination. The reported approach represents a simple, adaptable method that allows for the development and optimization of photoconductive organic materials.

Well-defined ultrathin nanoribbons have been fabricated from an amphiphilic electron donor−acceptor (D−A) supramolecule comprising perylene tetracarboxylic diimide as the backbone scaffold to enforce the one-dimensional intermolecular assembly via strong π-stacking. These nanoribbons demonstrated high photoconductivity upon illumination with white light. The high photoconductivity thus obtained is likely due to the optimal molecular design that enables a good kinetic balance between the two competitive processes, the intramolecular charge recombination (between D and A) and the intermolecular charge transport along the nanoribbon. The photoconduction response has also proven to be prompt and reproducible with the light turning on and off. The photogenerated electrons within the nanoribbon can be efficiently trapped by the adsorbed oxygen molecules or other oxidizing species, leading to depletion of the charge carriers (and thus the electrical conductivity) of the nanoribbon, as typically observed for n-type semiconductor materials as applied in chemiresistors. Combination of this sensitive modulation of conductivity with the unique features intrinsic to the nanoribbon morphology (large surface area and continuous nanoporosity when deposited on a substrate to form a fibril film) enables efficient vapor sensing of nitro-based explosives.

Organic fluorescent nanofibrils were fabricated from a linear carbazole trimer and employed for expedient detection of nitroaromatic explosives (DNT and TNT) and highly volatile nitroaliphatic explosives (nitromethane).

High photoconductivity of p-type nanofibers fabricated from a reducing tetracyclic macromolecule was achieved though a simple photodoping process under ambient conditions, which, together with the intrinsic high surface area and porosity of the nanofibers when deposited on a substrate, enables application in electrical vapor sensing of organic amines.

A series of perylene tetracarboxylic monoimides substituted with cycloalkanes were synthesized through a one-step reaction between cycloalkyl amines and the parent perylene dianhydride. The reaction demonstrates high selectivity for the production of monoimides with no formation of diimides. The high reaction selectivity is primarily due to the insolubility of the monoimides in the reaction medium, which in turn causes rapid precipitation of the products, shifting the reaction equilibrium to the right.

Well-defined single-crystalline nanobelts with strong fluorescence were fabricated from a perylene tetracarboxylic diimide molecule modified with specific side-chains that afford flip-flap stacking, rather than the common translated stacking, between the molecules along the long axis of the nanobelt. The nanobelts thus fabricated possess highly polarized, self-waveguided emission, making them ideal candidates for application in nanolasers and other angle-dependent optical nanodevices.

The fluorescence sensing of amine vapor was largely enhanced upon using ultrathin nanofibers, which were fabricated from N-(1-hexylheptyl)perylene-3,4,9,10-tetracarboxyl-3,4-anhydride-9,10-imide by a new self-assembly approach.

In general, fabrication of well-defined organic nanowires or nanobelts with controllable size and morphology is not as advanced as for their inorganic counterparts. Whereas inorganic nanowires are widely exploited in optoelectronic nanodevices, there remains considerable untapped potential in the one-dimensional (1D) organic materials. This Account describes our recent progress and discoveries in the field of 1D self-assembly of planar π-conjugated molecules and their application in various nanodevices including the optical and electrical sensors. The Account is aimed at providing new insights into how to combine elements of molecular design and engineering with materials fabrication to achieve properties and functions that are desirable for nanoscale optoelectronic applications. The goal of our research program is to advance the knowledge and develop a deeper understanding in the frontier area of 1D organic nanomaterials, for which several basic questions will be addressed: (1) How can one control and optimize the molecular arrangement by modifying the molecular structure? (2) What processing factors affect self-assembly and the final morphology of the fabricated nanomaterials; how can these factors be controlled to achieve the desired 1D nanomaterials, for example, nanowires or nanobelts? (3) How do the optoelectronic properties (e.g., emission, exciton migration, and charge transport) of the assembled materials depend on the molecular arrangement and the intermolecular interactions? (4) How can the inherent optoelectronic properties of the nanomaterials be correlated with applications in sensing, switching, and other types of optoelectronic devices? The results presented demonstrate the feasibility of controlling the morphology and molecular organization of 1D organic nanomaterials. Two types of molecules have been employed to explore the 1D self-assembly and the application in optoelectronic sensing: one is perylene tetracarboxylic diimide (PTCDI, n-type) and the other is arylene ethynylene macrocycle (AEM, p-type). The materials described in this project are uniquely multifunctional, combining the properties of nanoporosity, efficient exciton migration and charge transport, and strong interfacial interaction with the guest (target) molecules. We see this combination as enabling a range of important technological applications that demand tightly coupled interaction between matter, photons, and charge. Such applications may include optical sensing, electrical sensing, and polarized emission. Particularly, the well-defined nanowires fabricated in this study represent unique systems for investigating the dimensional confinement of the optoelectronic properties of organic semiconductors, such as linearly polarized emission, dimensionally confined exciton migration, and optimal π-electronic coupling (favorable for charge transport). Combination of these properties will make the 1D self-assembly ideal for many orientation-sensitive applications, such as polarized light-emitting diodes and flat panel displays.

A new type of fluorescence sensory material with high sensitivity, selectivity, and photostability has been developed for vapor probing of organic amines. The sensory material is primarily based on well-defined nanofibers fabricated from an n-type organic semiconductor molecule, N-(1-hexylheptyl)perylene-3,4,9,10-tetracarboxyl-3,4-anhydride-9,10-imide. Upon deposition onto a substrate, the entangled nanofibers form a meshlike, highly porous film, which enables expedient diffusion of gaseous analyte molecules within the film matrix, leading to milliseconds response for the vapor sensing.

Low dose detection of γ radiation remains critical for radiology therapy and nuclear security. We report herein on a novel dual-band fluorescence sensor system based on a molecule, 4-(1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenylaniline (PI-DPA), which can be dissolved into halogenated solvents to enable expedient detection of γ radiation. The limit of detection was projected down to 0.006 Gy. Exposure to γ radiation decomposes CHCl3 into small radicals, which then combine to produce HCl. Strong interaction of HCl with the imidazole group of PI-DPA converts it into a PI-DPA–HCl adduct, which self-assembles into nanofibers, quenching the fluorescence of the pristine PI-DPA molecule, while producing new fluorescent emission at longer wavelength. Such dual-band emission response provides improved sensing reliability compared to single band response. Systematic investigations based on acid titration, 1H NMR spectral measurements and time-course SEM imaging suggest that the observed new fluorescence band is due to π–π stacking of the PI-DPA–HCl adduct, which is facilitated by the formation of hydrogen bonded cluster units. The nanofibers exhibited high and reversible photoconductivity. Combining with the sensitive fluorescence response, the photoconductive nanofibers will enable development of a multimode stimuli-responsive sensor system that is suited for small, low cost dosimetry of γ radiation with improved sensitivity and detection reliability.

A liquid to solid phase transition of methylammonium lead triiodide (MAPbI3) under methylamine (MA) atmosphere at elevated temperatures was discovered, and used to form high quality and uniform thin films containing large, low defect crystal grains tens of microns in size.

We report a chemiresistive sensor approach based on a TTF–TCNQ charge transfer material, which can real-time detect and distinguish the vapors of alkyl amine and aromatic amine species under ambient conditions, based on the dramatic difference in the kinetics of the electric current recovery processes after the exposure of the two amine species.

A water soluble perylene diimide molecule has been fabricated into nanofibers via a pH triggered hydrogelation route. The one-dimensional self-assembly is dominated by the intermolecular π–π stacking interactions in concert with the hydrogen bonding between the carboxylic acid side chains. The anisotropic electronic and optical properties observed for the nanofibers are consistent with the one-dimensional intermolecular π–π arrangement.

A fluorescence turn-on sensor molecule (C6NIB) has been synthesized and fabricated into a porous matrix to enable trace vapor detection of hydrogen peroxide. The detection limit was projected to be below 5 ppb.

Large area uniform nanofibers have been fabricated from a hexameric arylene–ethynylene macrocycle (1) through in situ self-assembly on a glass substrate during solvent evaporation. The fibril morphology is controlled by the solvophilic core of 1, in conjunction with the interfacial interactions between the side chains of 1 and the substrate.

Fluorescent nanoscale coordination polymers with cubic morphology and long range ordered structure were fabricated and exhibited efficient sensing for both nitroaromatic explosive and nitromethane due to large surface area to volume ratio and strong binding affinity to explosive molecules.

The fluorescence sensing of amine vapor was largely enhanced upon using ultrathin nanofibers, which were fabricated from N-(1-hexylheptyl)perylene-3,4,9,10-tetracarboxyl-3,4-anhydride-9,10-imide by a new self-assembly approach.

Organic fluorescent nanofibrils were fabricated from a linear carbazole trimer and employed for expedient detection of nitroaromatic explosives (DNT and TNT) and highly volatile nitroaliphatic explosives (nitromethane).

High photoconductivity of p-type nanofibers fabricated from a reducing tetracyclic macromolecule was achieved though a simple photodoping process under ambient conditions, which, together with the intrinsic high surface area and porosity of the nanofibers when deposited on a substrate, enables application in electrical vapor sensing of organic amines.