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.

Noncovalent Liquid Phase Functionalization of 2H-WS2 with PDI: An Energy Conversion Platform with Long-Lived Charge Separation.

Transition metal dichalcogenides are attractive 2D materials in the context of solar energy conversion. Previous investigations have focused predominantly on the properties of these systems. The realization of noncovalent hybrids with, for example, complementary electroactive materials remains underexplored to this date for exfoliated WS2. In this contribution, we explore WS2 by means of exfoliation and integration together with visible light-absorbing and electron-accepting perylene diimides into versatile electron-donor acceptor hybrids. Important is the distinct electron-donating feature of WS2. Detailed spectroscopic investigations of WS2-PDI confirm the electron donor/acceptor nature of the hybrid and indicate that green light photoexcitation leads to the formation of long-lived WS2•+-PDI•- charge-separated states.

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.

Assessing the Photoinduced Electron-Donating Behavior of Carbon Nanodots in Nanoconjugates.

Carbon nanodots (CNDs) undergo electron transfer in different scenarios. Previous studies have mainly focused on the electron-accepting features of CNDs in covalently linked donor-acceptor nanoconjugates. In view of this, we decided to carry out in this study the formation of covalently linked nanoconjugates that feature electron-donating pressure synthesized carbon nanodots (pCNDs) and electron-accepting 11,11,12,12-tetracyano-9,10-anthra-p-quinodimethane (TCAQ): pCND-TCAQ. The stability of the one-electron reduced form of TCAQ renders it the acceptor of choice. Detailed structural and electrochemical investigations allowed the characterization of pCND-TCAQ. Furthermore, investigations regarding intramolecular interactions, by means of steady-state and pump-probe transient absorption spectroscopies, allowed detection and characterization of three excited state species, in general, and the pCND•+-TCAQ•- charge-separated state, in particular.

Symmetry-Breaking Charge-Transfer Chromophore Interactions Supported by Carbon Nanodots.

Carbon dots (CDs) and their derivatives are useful platforms for studying electron-donor/acceptor interactions and dynamics therein. Herein, we couple amorphous CDs with phthalocyanines (Pcs) that act as electron donors with a large extended π-surface and intense absorption across the visible range of the solar spectrum. Investigations of the intercomponent interactions by means of steady-state and pump-probe transient absorption spectroscopy reveal symmetry-breaking charge transfer/separation and recombination dynamics within pairs of phthalocyanines. The CDs facilitate the electronic interactions between the phthalocyanines. Thus, our findings suggest that CDs could be used to support electronic couplings in multichromophoric systems and further increase their applicability in organic electronics, photonics, and artificial photosynthesis.

Ping-Pong Energy Transfer in Covalently Linked Porphyrin-MoS2 Architectures.

Molybdenum disulfide nanosheets covalently modified with porphyrin were prepared and fully characterized. Neither the porphyrin absorption nor its fluorescence was notably affected by covalent linkage to MoS2 . The use of transient absorption spectroscopy showed that a complex ping-pong energy-transfer mechanism, namely from the porphyrin to MoS2 and back to the porphyrin, operated. This study reveals the potential of transition-metal dichalcogenides in photosensitization processes.

Exploring Tetrathiafulvalene-Carbon Nanodot Conjugates in Charge Transfer Reactions.

Carbon nanodots (CNDs) were synthesized using low-cost and biocompatible starting materials such as citric acid/urea, under microwave irradiation, and constant pressure conditions. The obtained pressure-synthesized CNDs (pCNDs) were covalently modified with photo- and electroactive π-extended tetrathiafulvalene (exTTF) by means of a two-step esterification reaction, affording pCND-exTTF. The electronic interactions between the pCNDs and exTTF were investigated in the ground and excited states. Ultrafast pump-probe experiments assisted in corroborating that charge separation governs the deactivation of photoexcited pCND-exTTF. These size-regular structures, as revealed by AFM, are stable electron donor-acceptor conjugates of interest for a better understanding of basic processes such as artificial photosynthesis, catalysis, and photovoltaics, involving readily available fluorescent nanodots.

Charge transfer in graphene quantum dots coupled with tetrathiafulvalenes.

Water-soluble fluorescent graphene quantum dots have been successfully prepared through a top-down approach, that is, starting with graphite, and covalently functionalizing it with π-extended tetrathiafulvalene. Charge transfer investigations reveal noticeably slower charge recombination when compared with exTTF nanoconjugates featuring carbon nanodots, for which a larger presence of trap states is observed.

Fine-tuning the assemblies of carbon nanodots and porphyrins.

We present charge-transfer assemblies of electron accepting, pressure-synthesized carbon nanodots (pCNDs) and an electron donating porphyrin. Amidine derivatization of the porphyrin allows for hydrogen bonding interactions with the carboxyl groups on the surface of pCNDs, which drive the formation of the assembly. Upon photoexcitation, this electron donor-acceptor supramolecular construct features ultrafast charge separation, and subsequent charge recombination in 27 ps.