Understanding the Visible Absorption of Electron Accepting and Donating CNDs.

Carbon nanodots (CNDs) synthesized from citric acid and formyl derivatives, that is, formamide, urea, or N-methylformamide, stand out through their broad-range visible-light absorbance and extraordinary photostability. Despite their potential, their use has thus far been limited to imaging research. This work has now investigated the link between CNDs' photochemical properties and their chemical structure. Electron-rich, yellow carbon nanodots (yCNDs) are obtained with in situ addition of NaOH during the synthesis, whereas otherwise electron-poor, red carbon nanodots (rCNDs) are obtained. These properties originate from the reduced and oxidized dimer of citrazinic acid within the matrix of yCNDs and rCNDs, respectively. Remarkably, yCNDs deposited on TiO2 give a 30% higher photocurrent density of 0.7 mA cm-2 at +0.3 V versus Ag/AgCl under Xe-lamp irradiation (450 nm long-pass filter, 100 mW cm-2 ) than rCNDs. The difference in overall photoelectric performance is due to fundamentally different charge-transfer mechanisms. These depend on either the electron-accepting or the electron-donating nature of the CNDs, as is evident from photoelectrochemical tests with TiO2 and NiO and time-resolved spectroscopic measurements.

Carbon Nanodots for All-in-One Photocatalytic Hydrogen Generation.

Carbon nanodots (CNDs) were photochemically altered to produce dihydrogen under light irradiation. Within the complex structure of CNDs, photo-oxidation takes place at citrazinic acid molecular fluorophore sites. Important is the fact that the resulting CND materials have a dual function. On one hand, they absorb light, and on the other hand, they photo- and electrocatalytically produce dihydrogen from water and seawater, without any external photosensitizer or cocatalyst. Record HER activities of 15.15 and 19.70 mmol(H2) g(catalyst)-1 h-1 were obtained after 1 h of 75 mW/cm2 Xe lamp illumination, from water and seawater, respectively. This impressive performance outweighs the remaining structural uncertainties. A full-fledged physicochemical investigation based on an arsenal of steady-state and time-resolved spectroscopic characterizations together with microscopy enabled a comprehensive look into the reaction mechanism. For an efficient dihydrogen formation, a precatalytic activation by means of reduction with a sacrificial electron donor is imperative.

Global and target analysis of relaxation processes of the collapsed state of P3HT polymer nanoparticles.

Organic-inorganic heterostructure materials have received significant research interest for designing light harvesting devices because of their efficient charge separation. Here, we design organic and inorganic nano-heterostructures using conjugated polymer nanoparticles (PNPs) [poly(3-hexylthiophene-2,5-diyl), P3HT] and Au nanoparticles. We investigate the carrier relaxation processes of this heterostructure at different time scales by ultrafast transient absorption spectroscopy. The lifetime of the singlet state (S1) of the pristine polymer shortens from 480.7 ps to 2.8 ps due to the formation of nanoparticles, and the formation of a delocalized collective state (CLS) is obtained in polymer nanoparticles whose lifetime is found to be 384.6 ps. The hot and ultrafast electron transfers occur from P3HT polymer nanoparticles to Au nanoparticles and the time constants are 253 fs and 37.7 ps, respectively, which are responsible for the efficient charge separation in such heterostructures. Such a fundamental study of relaxation processes of organic-inorganic nano heterostructures is very significant for designing light harvesting systems.

Influence of shape on the carrier relaxation dynamics of CsPbBr3 perovskite nanocrystals.

Lead halide perovskite nanocrystals (NCs) have recently emerged as a new class of functional materials for designing efficient light harvesting systems because of their unique photophysical properties. Here, we report the influence of different shapes on the relaxation dynamics of perovskite nanocrystals. The structural transformation of CsPbBr3 NCs from cubic shape to rod shape occurs by changing the solvent from toluene to dichloromethane (DCM). Rietveld analysis reveals that the crystallinity along with the preferred orientation (PO) of the orthorhombic phase plays a vital role for the unidirectional growth of rod shaped CsPbBr3 NCs in DCM. Time-resolved emission spectroscopy and ultrafast transient absorption spectroscopy are used to understand the photoinduced relaxation processes. Global and target analysis of femto-second transient absorption kinetics has been done to understand the individual excited-state species. The analysis reveals that trap states play an important role in the carrier relaxation dynamics of cubic and rod shaped NCs. The lifetime of the shallow trap (ST) changes from 25 ps to 45 ps and the lifetime of the deep trap (DT) state changes from 163 ps to 303 ps with changing the shape of the nanocrystals from cubic to rod. This work highlights the tuning of the crystal phase, shape and the exciton dynamics of CsPbBr3 NCs that would be beneficial for designing efficient photovoltaic devices.

Light Harvesting and Photocurrent Generation in a Conjugated Polymer Nanoparticle-Reduced Graphene Oxide Composite.

Polymer-graphene nanocomposites are promising candidates for light harvesting applications such as photocatalysis and photovoltaics, where significant charge separation occurs due to photoinduced electron transfer. Much attention has been paid to using reduced graphene oxide (r-GO) as template for anchoring various nanomaterials due to its efficient electron accepting and transport properties. Here, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) nanoparticles are prepared from MEH-PPV polymer and the change in photophysical properties upon formation of polymer nanoparticles (PNPs) from the molecular state are investigated by using steady-state and time-resolved spectroscopy. Nanocomposites are constructed by adding hexadecylamine-functionalized positively charged MEH-PPV PNPs to a solution of negatively charged r-GO. Steady-state and time-resolved spectroscopy are also used to study the electronic interactions between PNPs and r-GO. Ultrafast femtosecond up-conversion and transient absorption spectroscopy unequivocally confirms the electron transfer process from the excited state of MEH-PPV PNPs to r-GO at the interface of the nanocomposite. Analysis reveals that the charge separation time is found to be pulse-width-limited (<100 fs). Due to charge separation in these nanocomposites, an increase (2.6 fold) of photocurrent under visible light illumination is obtained. The fundamental understanding of the charge transfer dynamics affords new opportunities to design efficient light-harvesting systems based on inorganic-organic hybrids.

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.

Tartaric acid-derived chiral carbon nanodots for catalytic enantioselective ring-opening reactions of styrene oxide.

Chiral carbon nanodots (CNDs) were fabricated through the hydrothermal processing of sulfanilic acid and chiral tartaric acid, exhibiting outstanding catalytic performance for the chiral catalysis of the ring-opening reaction. Furthermore, the catalytic mechanism was proposed to understand the link between the chiral structure and the performance of the catalyst.

The Impact of Aggregation of Quaterthiophenes on the Excited State Dynamics.

Oligothiophenes and their aggregates play a dominant role in optoelectronic and light-harvesting applications. Here, we controlled the degree of aggregation of 2,2':5',2″:5'',2‴-quaterthiophene (QTH) to shed light on the impact of the aggregation on the excited state dynamics. QTH aggregation realized the control over the Intersystem Crossing (ISC) rates and, in turn, the formation of triplet excited states via the simple addition of water to QTH solutions in THF. From global target analysis, the time scale was 345.5 ps for ISC for QTHs in THF, but it was 2.33 ns in the case of QTH solutions featuring 70% water. Notably, the excitonic coupling between closely packed QTHs occurred predominantly in the aggregates formed in the presence of large water concentrations. Relaxation dynamics of the resulting QTH-aggregates differed substantially from QTH solutions at lower water content. For example, QTH-aggregates lacked any triplet excited states, and the unusual emission occurs from lower excitonic states from these predominantly H-aggregates.

Ultrafast Relaxation Processes of Conjugated Polymer Nanoparticles in the Presence of Au Nanoparticles.

Considering the importance of conjugated polymer nanoparticles, major emphasis has been given for designing and understanding the energy transfer and charge transfer processes of organic-inorganic hybrids for light harvesting applications. In the present study, we have designed an aqueous solution-based light harvesting system using conjugated polymer nanoparticles (poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], MEH-PPV) and Au nanoparticles. The change in photo-induced processes in the presence of metal nanoparticles are studied by steady-state absorption, time-resolved emission, time-resolved fluorescence up-conversion, ultrafast anisotropy and femtosecond transient absorption spectroscopy. Global and target analysis of transient absorption data validate the creation of a collective delocalized state in polymer nanoparticles, and the time scale for excitation energy funnelling from S1 state to low lying collective delocalized state (CLs ) is 18 ps. Then, the electron transfer from the CLs state to Au NP occurs with a time constant of 150 ps. The 815 ps long lived charge transfer (CT) state signifies the charge transfer from the CLs state of polymer nanoparticles to Au NP. Such basic understanding of relaxation processes in hybrid systems is very important for designing inorganic-organic hybrid light-harvesting systems.

Role of Structural Distortion in Stabilizing Electrosynthesized Blue-Emitting Phosphorene Quantum Dots.

Luminescent phosphorene quantum dots (PQDs) have emerged as fascinating nanomaterials for potential applications in optoelectronics, catalysis, and sensing. Herein, we investigate the structural distortion of black phosphorus (BP) under an applied electric field to yield blue luminescent PQDs [average diameter 8 ± 1.5 nm ( N = 60)]. The electrosynthesized PQDs exhibit photoluminescence emission independent of excitation wavelength with 84% quantum efficiency. Structural distortion that occurred during the transformation of BP to PQDs is confirmed by results obtained during transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Further, using first-principles-based density functional theory, calculations on oxygenated and nonoxygenated PQDs augment the experimental observations that an optimum oxygen content maintains the structural integrity of PQDs, above which the structural robustness of PQDs is drastically diminished.

Photon Harvesting in Conjugated Polymer-Based Functional Nanoparticles.

The design of new generation light-harvesting systems based on conjugated polymer nanoparticles (PNPs) is an emerging field of research to convert solar energy into renewable energy. In this Perspective, we focus on the understanding of the light harvesting processes like exciton dynamics, energy transfer, antenna effect, charge carrier dynamics, and other related processes of conjugated polymer-based functional nanomaterials. Spectroscopic investigations unveil the rotational dynamics of the dye molecules inside of PNPs and exciton dynamics of the self-assembled structures. A detailed understanding of the cascade energy transfer for white light and singlet oxygen generation in multiple fluorophores containing a PNP system by time-resolved spectroscopy is highlighted. Finally, ultrafast spectroscopic investigations provide direct insight into the impacts of electron and hole transfer at the interface in the hybrid materials for photocatalysis and photocurrent generation to construct efficient light-harvesting systems.

Functionalized dye encapsulated polymer nanoparticles attached with a BSA scaffold as efficient antenna materials for artificial light harvesting.

A potential strategy for a new generation light harvesting system is multi-chromophoric donor-acceptor pairs where light energy is absorbed by an antenna complex and subsequently transfers its energy to the acceptor via energy transfer. Here, we design a system of a functionalized polymer nanoparticle-protein scaffold for efficient light harvesting and white light generation where a dye doped polymer nanoparticle acts as a donor and a dye encapsulated BSA protein acts as an acceptor. Analysis reveals that 91.3% energy transfer occurs from the dye doped polymer nanoparticle to the dye encapsulated BSA protein. The antenna effect of this light harvesting system is found to be 31 at a donor to acceptor ratio of 0.82 : 1 which is unprecedented. The enhanced effective molar extinction coefficient of the acceptor dye is potential for the light harvesting system. Bright white light emission with a quantum yield of 14% under single wavelength excitation is obtained by changing the ratio of donor to acceptor. Analysis reveals that the efficient energy transfer in this polymer-protein assembly may open up new possibilities in designing artificial light harvesting systems for future applications.