Acid-Modulated Synthesis of Novel π-Conjugated Microporous Polymers for Efficient Metal-Free Photocatalytic Hydrogen Evolution.

Simple synthetic modifications that tune the molecular structures, thereby the properties of the molecules, are of topical interest. Herein, we report the synthesis of two novel cationic rosaniline-based conjugated microporous polymers (CMPs) from identical monomers via simple acid modulation (Acetic acid and BF3 ⋅ Et2 O). The condensation reaction of rosaniline with 2,4,6-triformylphloroglucinol in acetic acid renders ß-ketoenamine-linked CMP (CMP-A) while changing the acid to BF3 ⋅ Et2 O, the linkages transform to enol and undergoes BF2 -complexation, leading to boranil CMP (CMP-B). BF2 -functionalities in boranil CMP significantly modified the optical and functional properties compared to ß-ketoenamine-linked CMP. The cationic-delocalization along with the extended π-delocalization supported by chromophoric BF2 -groups allow CMP-B to exhibit broad absorption spanning the visible to Near-Infrared region (NIR). The absorption red-edge of CMP-B appears around 1277 nm (optical band gap ∼1.58 eV) while CMP-A displays at 981 nm (optical band gap ∼1.83 eV). Most interestingly, as a photocatalyst, CMP-B catalyzes hydrogen evolution with a superior rate of 252 µmol g-1 over CMP-A (100 µmol g-1 ). It is about 2.5 times higher performance. The transient photocurrent measurements, electrochemical impedance data, and in-depth mott-Schottky analysis demonstrate that the BF2 -group in CMP-B generates photoinduced charge carriers and their migration towards the active sites for photocatalysis. These polymers show significant photocatalytic H2 generation without any supportive metal co-catalyst. The BF2 complexed building blocks are a unique class of metal-free photocatalysts for hydrogen evolution through green and cost-effective approach.

Trans-membrane Fluorescence Enhancement by Carbon Dots: Ionic Interactions and Energy Transfer.

We report on trans-membrane interactions between blue-emitting carbon dots (CDs) and fluorescein. Hydrophobic CDs with a positive surface charge are embedded as-synthesized in the lipophilic sheet of the bilayer membrane of large synthetic phospholipid vesicles. The vesicles are prepared by mixing DOPC phospholipids and lipid molecules that contain anionic fluorescein attached to their hydrophilic head. Due to attractive electrostatic interactions, the CDs and fluorescein conjoin within the vesicle membrane, which leads to photoluminescence enhancement of fluorescein and facilitates trans-membrane energy transfer between the CDs and the dye.

Tracking the Source of Carbon Dot Photoluminescence: Aromatic Domains versus Molecular Fluorophores.

Carbon dots (CDs) are an intriguing fluorescent material; however, due to a plethora of synthesis techniques and precursor materials, there is still significant debate on their structure and the origin of their optical properties. The two most prevalent mechanisms to explain them are based on polycyclic aromatic hydrocarbon domains and small molecular fluorophores, for instance, citrazinic acid. Yet, how these form and whether they can exist simultaneously is still under study. To address this, we vary the hydrothermal synthesis time of CDs obtained from citric acid and ethylenediamine and show that in the initial phase molecular fluorophores, likely 2-pyridone derivatives, account for the blue luminescence of the dots. However, over time, while the overall size of the CDs does not change, aromatic domains form and grow, resulting in a second, faster decay channel at similar wavelengths and also creating additional lower energetic states. Electrophoresis provides further evidence that the ensemble of CDs consists of several subsets with different internal structure and surface charge. The understanding of the formation mechanism enables a control of the chemical origin of these emitters and the ensuing optical properties of the CDs through synthetic means.

Photoluminescence in Carborane-Stilbene Triads: A Structural, Spectroscopic, and Computational Study.

A set of triads in which o- and m-carborane clusters are bonded to two stilbene units through Ccluster -CH2 bonds was synthesized, and their structures were confirmed by X-ray diffraction. A study on the influence of the o- and m- isomers on the absorption and photoluminescence properties of the stilbene units in solution revealed no charge-transfer contributions in the lowest excited state, as confirmed by (TD)DFT calculations. The presence of one or two B-I groups in m-carborane derivatives does not affect the emission properties of the stilbenes in solution, probably due to the rather large distance between the iodo substituents and the fluorophore. Nevertheless, a significant redshift of the photoluminescence (PL) emission maximum in the solid state (thin films and powder samples) compared to solution was observed; this can be traced back to PL sensitization, most probably due to more densely packed stilbene moieties. Remarkably, the PL absolute quantum yields of powder samples are significantly higher than those in solution, and this was attributed to the restricted environment and the aforementioned sensitization. Thus, the bonding of the carborane clusters to two stilbene units preserves their PL behavior in solution, but produces significant changes in the solid state. Furthermore, iodinated species can be considered to be promising precursors for theranostic agents in which both imaging and therapeutic functions could possibly be combined.

Tuning of the electronic and photophysical properties of ladder-type quaterphenyl by selective methylene-bridge fluorination.

The photophysics (spectral positions, band shapes, fluorescence quantum yields and lifetimes) of a series of fluorinated ladder type quaterphenyls L4P and L4P-Fn (n = 2, 4, 6) depend strongly on the degree and position of fluorine, despite the fact that substitution is not performed in the rings but only in methylene-bridges. This is driven by subtle differences in the molecular orbitals (MOs) participating in the electronic transitions, and in the vibronic pattern of the S0 and S1 electronic states as revealed by (TD)DFT calculations. Solid state spectra for n = 0, 2, 4 are similar to those of solution due to small intermolecular interactions as revealed by combined X-ray and (TD)DFT analysis.

Photoswitching and Thermoresponsive Properties of Conjugated Multi-chromophore Nanostructured Materials.

Conjugated multi-chromophore organic nanostructured materials have recently emerged as a new class of functional materials for developing efficient light-harvesting, photosensitization, photocatalysis, and sensor devices because of their unique photophysical and photochemical properties. Here, we demonstrate the formation of various nanostructures (fibers and flakes) related to the molecular arrangement (H-aggregation) of quaterthiophene (QTH) molecules and their influence on the photophysical properties. XRD studies confirm that the fiber structure consists of >95% crystalline material, whereas the flake structure is almost completely amorphous and the microstrain in flake-shaped QTH is significantly higher than that of QTH in solution. The influence of the aggregation of the QTH molecules on their photoswitching and thermoresponsive photoluminescence properties is revealed. Time-resolved anisotropic studies further unveil the relaxation dynamics and restricted chromophore properties of the self-assembled nano/microstructured morphologies. Further investigations should pave the way for the future development of organic electronics, photovoltaics, and light-harvesting systems based on π-conjugated multi-chromophore organic nanostructured materials.

Multichromophoric organic molecules encapsulated in polymer nanoparticles for artificial light harvesting.

We designed a self-assembled multichromophoric organic molecular arrangement inside polymer nanoparticles for light-harvesting antenna materials. The self-assembled molecular arrangement of quaterthiophene molecules was found to be an efficient light-absorbing antenna material, followed by energy transfer to Nile red (NR) dye molecules, which was confined in polymer nanoparticles. The efficiency of the antenna effect was found to be 3.2 and the effective molar extinction coefficient of acceptor dye molecules was found to be enhanced, which indicates an efficient light-harvesting system. Based on this energy-transfer process, tunable photo emission and white light emission has been generated with 14 % quantum yield. Such self-assembled oligothiophene-NR systems encapsulated in polymer nanoparticles may open up new possibilities for fabrication of artificial light harvesting system.

Conjugated polymer P3HT-Au hybrid nanostructures for enhancing photocatalytic activity.

Metal-semiconductor nanostructures have been the subject of great interest, mainly due to their interesting optical properties and their potential applications in light harvesting, photocatalysis and photovoltaic devices. Here, we have designed raspberry type organic-inorganic hybrid nanostructures of the poly-3-hexylthiophene (P3HT)-Au nanoparticle (NP) composite by a simple solution based synthetic method. The electronic interaction of semiconducting P3HT polymer nanoparticles with Au nanoparticles exhibits a bathochromic shift of absorption bands and significant photoluminescence quenching of P3HT nanoparticles in this organic-inorganic hybrid system. The photocatalytic activity of this raspberry type hybrid nanostructure is demonstrated under the visible light irradiation and the degradation efficiency is found to be 90.6%. Such organic-inorganic hybrid nanostructures made of a semiconducting polymer and plasmonic nanoparticles could pave the way for designing new optical based materials for applications in photocatalytic and light harvesting systems.

Single and multistep energy transfer processes within doped polymer nanoparticles.

Herein, we demonstrate the design of multiple fluorophores Coumarin 153 (C153) and Nile Red (NR) encapsulated in semiconducting poly[N-vinylcarbazole] (PVK) polymer nanoparticles (50-70 nm in diameter) by a simple re-precipitation technique, and elucidate their photophysical properties by steady-state and picosecond (ps) time resolved emission spectroscopy. It is interesting to note that multistep cascaded energy transfer occurs from the excited host PVK molecules to NR dye molecules through C153. The energy transfer time constants are found to be 180 ps for PVK→C153, 360 ps for PVK→NR, and 140 ps for the overall energy transfer process from PVK to NR through C153 dye molecules. The multistep energy transfer allows tuning of the wide range emission from 350 nm to 700 nm by changing the relative concentrations of the encapsulated dye molecules. Bright, stable, and white light emission of the dye doped polymer nanoparticles with a quantum yield of 14% is achieved at a particular concentration ratio of the C153 : NR dye. The generation of "cool" white emission in suspension and in the solid state film opens up new possibilities to obtain white light OLEDs based on single nanoparticles.

Solvent-Assisted Structural Modifications of Sulfur Dots Followed by Time-Dependent Emergence of a New Emissive State and Long-Lived Afterglow.

Sulfur dots are a new class of recently developed nonmetallic luminescent nanomaterials with various potential applications. Herein, we synthesized sulfur dots using a mild chemical etching method and then modified the structural features of the as-synthesized sulfur dots using a slow and defined solvent-assisted aggregation process. This increases the particle size and overall crystallinity along with the modifications of the surface functional groups, which eventually show a new emission band at longer wavelengths. Detailed photophysical and temperature-dependent luminescence studies confirmed that the new emissive state evolves due to interparticle interactions in the excited state. Furthermore, the occurrence of a new emissive state in a longer-wavelength region helped reduce the energy gap between the lowest excited singlet state and the lowest excited triplet state in modified sulfur dots, resulting in an aqueous stable room-temperature phosphorescence/afterglow emission through efficient intersystem crossing. This typical efficacious afterglow emission directly shows the potential applicability of structurally modified sulfur dots in encryption devices and can also be potentially effective in light emitting diodes (LED) and sensing devices.

Detection of Hg2+ and F- ions by using fluorescence switching of quantum dots in an Au-cluster-CdTe QD nanocomposite.

A single probe of an Au nanocluster-CdTe quantum dots nanocomposite has been developed by using tripeptide-capped CdTe quantum dots (QD) and bovine serum albumin (BSA) protein-conjugated Au25 nanocluster (NC) for detection of both Hg(2+) ion and F(-) ion. The formation of Au-NC-CdTe QD nanocomposite has been confirmed by TEM, steady state and time resolved spectroscopy, CD and FTIR studies. A significant signal off (74 % PL quenching at 553 nm) phenomenon of this nanocomposite is observed in presence of 6.56×10(-7) M Hg(2+) ion, due to salt-induced aggregation. However, a dramatic PL enhancement (128 %) of the Au-NC-CdTe QD nanocomposite is observed in presence of 8.47×10(-7) M F(-) anion. The calculated limit of detections (LOD) of Hg(2+) ion concentration and F(-) ion concentration are found to be 9 and 117 nM, respectively, which are within the safety range set by the United States Environment Protection Agency. Thus, the simple Au-NC-CdTe QD optical-based sensor is very useful to detect both toxic cations and anions.

Steady state and time resolved spectroscopic study of C-dots-MEH-PPV polymer nanoparticles composites.

Fluorescent carbon dots (C-dots) have been found to be a new class of nanomaterial for potential applications. Herein, polyethylenimine branched (BPEI) functionalized carbon dots (C-dots) are synthesized by changing the synthesis time using a microwave pyrolysis method. The photoluminescence intensity and average decay time of C-dots are found to be increased with increasing the crystallinity of the C-dots. C-dots-MEH-PPV polymer nanoparticles composites are formed by electrostatic interaction between these particles. The intensity of C-dots quenches dramatically with increasing the concentration of MEH-PPV nanoparticles (PNPs) and the intensity of PNPs increases gradually under excitation at 370 nm. This phenomenon may be due to energy transfer from C-dots to PNPs because there is a good spectral overlap between the emission spectra of C-dots and the absorption spectra of PNPs. The drastic photoluminescence quenching and the shortening of the decay time of C-dots in the composites confirms the efficient resonance energy transfer from C-dots to polymer nanoparticles. The energy transfer efficiency (66% to 89%) and rate of energy transfer are found to depend strongly on the time of pyrolysis. These C-dots-polymer composites will open up a way for developing new challenging materials for potential applications.

Role of Noncovalent Interactions in N,P-Functionalized Luminescent Carbon Dots for Ultrasensitive Detection of Moisture in D2O: Boosting Visible-NIR Light Sensitivity.

It is highly desirable to design cost-efficient and eco-friendly fluorometric sensors that can efficiently detect water contamination in D2O and other expensive organic solvents. Herein, we have synthesized N,P-codoped carbon dots (N,P-CDs) from o-phenylene diamine (o-PDA) and H3PO4 through the bottom-up carbonization method. Heteroatom co-doping increases the absorption cross section in the visible-NIR range, followed by the formation of stable emissive states in longer-wavelength regions. We have critically investigated the noncovalent interactions (especially H-bonding interactions) of various surface functional groups with surrounding solvent media through a detailed structure-property correlation. Based on the sensitivity of noncovalent H-bonding interactions to the stability of longer-wavelength emissive domains, we have utilized these N,P-CDs as cost-effective fluorometric sensors of water/moisture contamination in D2O especially under visible-NIR light; the optical sensitivity reaches up to 0.1 volume (%) level. The detailed sensing mechanism has been further supported by a computational study through a simple visualization approach by mapping and analyzing all possible noncovalent interactions between the CDs and the solvent medium.

Single-Atom Ru Catalyst-Decorated CNF(ZnO) Nanocages for Efficient H2 Evolution and CH3OH Production.

The presence of transition-metal single-atom catalysts effectively enhances the interaction between the substrate and reactant molecules, thus exhibiting extraordinary catalytic performance. In this work, we for the first time report a facile synthetic procedure for placing highly dispersed Ru single atoms on stable CNF(ZnO) nanocages. We unravel the atomistic nature of the Ru single atoms in CNF(ZnO)/Ru systems using advanced HAADF-STEM, HRTEM, and XANES analytical methods. Density functional theory calculations further support the presence of ruthenium single-atom sites in the CNF(ZnO)/Ru system. Our work further demonstrates the excellent photocatalytic ability of the CNF(ZnO)/Ru system with respect to H2 production (5.8 mmol g-1 h-1) and reduction of CO2 to CH3OH [249 µmol (g of catalyst)-1] with apparent quantum efficiencies of 3.8% and 1.4% for H2 and CH3OH generation, respectively. Our studies unambiguously demonstrate the presence of atomically dispersed ruthenium sites in CNF(ZnO)/Ru catalysts, which greatly enhance charge transfer, thus facilitating the aforementioned photocatalytic redox reactions.

Energy/hole transfer phenomena in hybrid α-sexithiophene (α-STH) nanoparticle-CdTe quantum-dot nanocomposites.

Considerable attention has been paid to hybrid organic-inorganic nanocomposites for designing new optical materials. Herein, we demonstrate the energy and hole transfer of hybrid hole-transporting α-sexithiophene (α-STH) nanoparticle-CdTe quantum dot (QD) nanocomposites using steady-state and time-resolved spectroscopy. Absorption and photoluminescence studies confirm the loss of planarity of the α-sexithiophene molecule due to the formation of polymer nanoparticles. Upon photoexcitation at 370 nm, a nonradiative energy transfer (73 %) occurs from the hole-transporting α-STH nanoparticles to the CdTe nanoparticles with a rate of energy transfer of 6.13×10(9) s(-1). However, photoluminescence quenching of the CdTe QDs in the presence of the hole-transporting α-STH nanoparticles is observed at 490 nm excitation, which is due to both static-quenching and hole-transfer-based dynamic-quenching phenomena. The calculated hole-transporting rate is 7.13×10(7) s(-1) in the presence of 42×10(-8) M α-STH nanoparticles. Our findings suggest that the interest in α-sexithiophene (α-STH) nanoparticle-CdTe QD hybrid nanocomposites might grow in the coming years because of various potential applications, such as solar cells, optoelectronic devices, and so on.

Spectroscopic investigations on the H-type aggregation of coumarin 153 dye molecules: role of Au nanoparticles and γ-cyclodextrin.

Here, we study the formation of H-type aggregation of coumarin 153 (C153) dye molecule in presence of Au nanoparticles and the removal of dye aggregation in presence of γ-cyclodextrin (CD) due to confinement of dye molecules inside the nanocavity of γ-cyclodextrin (CD) using steady state and time resolved spectroscopy. Blue shifting of absorption band, photoluminescence (PL) band and the enhancement of decay time of C153 dye confirm the formation of H-aggregation. It is found that the concentrations of γ-CD and Au nanoparticles play an important role on H-type aggregation of dye. The rotational relaxation time of free C153 is 0.113 ns and the average relaxation time of C153 dye are 0.275 ns and 0.425 ns for 2 mM and 5 mM γ-CD confined systems, respectively, indicating the anisotropy increases due to confinement of dye. An associated type anisotropy decay of C153 dye is found at 20 mM concentration of CD may be due to formation of nanotubular aggregates of γ-CD.

Specific locations of blue and green-emitting units in dual emissive carbon dots and their reversible emitting properties due to switchable inter-chromophoric interactions.

Carbon dots (CDs) are the unique class of luminescent nanomaterials consist of various chromophoric units heterogeneously distributed throughout the nanoparticle, resulting intriguing multistate emissive properties. Herein, we have critically investigated the specific locations of the blue and green-emitting centers inside dual emissive CDs by steady-state and time-resolved polarized emission study. It is further clarified by a temperature-dependent fluorescence study for both the emitting domains. Results suggest that the blue chromophoric units are located at the interior part of CDs, while green units are mostly at the exterior region. Furthermore, we have investigated the solvent-dependent inter-chromophoric interactions between the two emissive domains by the Time-Resolved Area Normalized Emission Spectroscopy (TRANES). Results suggest that at polar aprotic solvent acetone, time-dependent positive evolution of green-emitting states and negative evolution of blue emissive domains have been observed. This reversible emitting properties evolve due to the excited state energy migration from blue emissive domains to green emissive domains at polar aprotic medium, while in the case of polar protic solvent water, this phenomenon is missing. This switchable inter-chromophoric interaction are correlated further with the inter-particle interactions of CDs.

Insight into the Multistate Emissive N, P-doped Carbon Nano-Onions: Emerging Visible-Light Absorption for Photocatalysis.

Carbon dots (CDs) have become one of the most emerging materials as an alternative solar light-induced photocatalyst in contrast to traditional metal-based systems. However, one of the major challenges is the lack of visible light absorption. Herein, we have fabricated unique N, P-co-doped CDs with a self-assembled onion-like layered structure by using a bottom-up facile synthesis technique from chitosan gel and phosphoric acid as molecular precursors. This typical layered structure of N, P-co-doped carbon nano onions (N, P-CNOs), with an average size of 25-50 nm, displays an enhanced visible light absorption. Detailed structural and elemental characterizations confirm the extensive aromatic domain with P-containing surface functionalities, while electrochemical study clarifies the lowering of band gaps as well as the creation of new electronic states in comparison to the pristine N-CDs. Furthermore, the intrinsic structural features are correlated with the underpinning photophysical processes by steady-state and time-resolved fluorescence spectroscopy. In addition, steady-state polarized emission and thermo-responsive PL properties have been carried out to unveil further the structure-property correlation of N, P-CNOs, and their comparative study with pristine N-CDs at the different excitation wavelengths. Finally, N, P-CNOs exhibit efficient visible-light-induced photocatalysis, and the detailed mechanistic study is carried out by trapping the photogenerated species in an aqueous medium. The prepared N, P-CNOs displayed an excellent visible-light photocatalytic performance over MB dye with a degradation efficiency of 75.8% within 120 min along with a degradation rate constant of ∼0.0109 min-1 . It is concluded that the easy to synthesize and low-cost N, P-CNOs with a unique morphology hold great potential for application in visible-light photocatalysis.

Carbon-based nanomaterials: in the quest of alternative metal-free photocatalysts for solar water splitting.

One of the alarming problems of modern civilization is global warming due to the inevitable rise of CO2 in the environment, mainly because of the excessive use of traditional fossil fuels. The gradual depletion of fossil fuels is another challenge regarding the future energy demand; therefore, alternative renewable energy research is necessary. One of the alternative approaches is the solar fuel generation by means of photocatalytic water splitting and more specifically, hydrogen evolution from water through the reductive half-reaction. Hydrogen is the cleanest fuel and does not produce any greenhouse gas upon direct combustion, or even while acting as a chemical feedstock for other transportable fuel generation. Therefore, it is desirable to produce efficient photocatalysts for solar water splitting. After the discovery of the first photocatalytic water splitting reaction by Fujisima and Honda, several advancements have been made with metal-based inorganic semiconductor photo-catalysts. However, their practical applicability is still under debate considering the environmental sustainability, stability and economical expenses. As a result, it is essential to develop alternate photocatalysts that are environmentally sustainable, cost-effective, stable and highly efficient. The metal-free approach is one of the most promising approaches in this regard. Herein, we discuss the recent developments in carbon-based materials and their hybrids as alternative metal free photocatalysts for solar water splitting. The present discussion includes g-C3N4, two-dimensional graphene/graphene oxides, one-dimensional carbon nanotubes/carbon nanofibers and zero-dimensional graphene QDs/carbon dots. We have focused on the rectification of exciton generation, charge separation and interfacial photochemical processes for photocatalysis, followed by possible optimization pathways of these typical all carbon-based materials. Finally, we have highlighted several fundamental challenges and their possible solutions, as well as the future direction on this particular aspect.

Photobase effect for just-in-time delivery in photocatalytic hydrogen generation.

Carbon dots (CDs) are a promising nanomaterial for photocatalytic applications. However, the mechanism of the photocatalytic processes remains the subject of a debate due to the complex internal structure of the CDs, comprising crystalline and molecular units embedded in an amorphous matrix, rendering the analysis of the charge and energy transfer pathways between the constituent parts very challenging. Here we propose that the photobasic effect, that is the abstraction of a proton from water upon excitation by light, facilitates the photoexcited electron transfer to the proton. We show that the controlled inclusion in CDs of a model photobase, acridine, resembling the molecular moieties found in photocatalytically active CDs, strongly increases hydrogen generation. Ultrafast spectroscopy measurements reveal proton transfer within 30 ps of the excitation. This way, we use a model system to show that the photobasic effect may be contributing to the photocatalytic H2 generation of carbon nanomaterials and suggest that it may be tuned to achieve further improvements. The study demonstrates the critical role of the understanding the dynamics of the CDs in the design of next generation photocatalysts.