Engineering Metal Halide Perovskite Nanocrystals with BODIPY Dyes for Photosensitization and Photocatalytic Applications.

The sensitization of surface-anchored organic dyes on semiconductor nanocrystals through energy transfer mechanisms has received increasing attention owing to their potential applications in photodynamic therapy, photocatalysis, and photon upconversion. Here, we investigate the sensitization mechanisms through visible-light excitation of two nanohybrids based on CsPbBr3 perovskite nanocrystals (NC) functionalized with borondipyrromethene (BODIPY) dyes, specifically 8-(4-carboxyphenyl)-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BDP) and 8-(4-carboxyphenyl)-2,6-diiodo-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (I2-BDP), named as NC@BDP and NC@I2-BDP, respectively. The ability of I2-BDP dyes to extract hot hole carriers from the perovskite nanocrystals is comprehensively investigated by combining steady-state and time-resolved fluorescence as well as femtosecond transient absorption spectroscopy with spectroelectrochemistry and quantum chemical theoretical calculations, which together provide a complete overview of the phenomena that take place in the nanohybrid. Förster resonance energy transfer (FRET) dominates (82%) the photosensitization of the singlet excited state of BDP in the NC@BDP nanohybrid with a rate constant of 3.8 ± 0.2 × 1010 s-1, while charge transfer (64%) mediated by an ultrafast charge transfer rate constant of 1.00 ± 0.08 × 1012 s-1 from hot states and hole transfer from the band edge is found to be mainly responsible for the photosensitization of the triplet excited state of I2-BDP in the NC@I2-BDP nanohybrid. These findings suggest that the NC@I2-BDP nanohybrid is a unique energy transfer photocatalyst for oxidizing α-terpinene to ascaridole through singlet oxygen formation.

Linker-Assisted CdS-TiO2 Nanohybrids as Reusable Visible Light Photocatalysts for the Oxidative Hydroxylation of Arylboronic Acids.

A variety of phenols have been obtained in aqueous media with moderate to excellent chemical yields (≤100%) by using arylboronic acids and esters as substrates, a robust CdS-TiO2 nanohybrid as a heterogeneous photocatalyst, visible light irradiation (467 nm), and an O2-saturated atmosphere. The nanohybrid was prepared through a linker-assisted methodology that uses mercapto alkanoic acids as the organic linkers. The nanohybrid showed improved photocatalytic activity in the hydroxylation of substituted arylboronic acids and phenyl boronic esters compared with that of pristine CdS quantum dots. The nanohybrid can be reused in up to five photocatalytic cycles with ∼90% of its outstanding activity preserved.

Present and Perspectives of Photoactive Porous Composites Based on Semiconductor Nanocrystals and Metal-Organic Frameworks.

This review focuses on the recent developments in synthesis, properties, and applications of a relatively new family of photoactive porous composites, integrated by metal halide perovskite (MHP) nanocrystals and metal-organic frameworks (MOFs). The synergy between the two systems has led to materials (MHP@MOF composites) with new functionalities along with improved properties and phase stability, thus broadening their applications in multiple areas of research such as sensing, light-harvesting solar cells, light-emitting device technology, encryption, and photocatalysis. The state of the art, recent progress, and most promising routes for future research on these photoactive porous composites are presented in the end.

Ruddlesden-Popper Hybrid Lead Bromide Perovskite Nanosheets of Phase Pure n=2: Stabilized Colloids Stored in the Solid State.

Ruddlesden-Popper lead halide perovskite (RP-LHP) nano-nanostructures can be regarded as self-assembled quantum wells or superlattices of 3D perovskites with an intrinsic quantum well thickness of a single or a few (n=2-4) lead halide layers; the quantum wells are separated by organic layers. They can be scaled down to a single quantum well dimension. Here, the preparation of highly (photo)chemical and colloidal stable hybrid LHP nanosheets (NSs) of ca. 7.4 µm lateral size and 2.5 nm quantum well height (thereby presenting a deep blue emission at ca. 440 nm), is reported for the first time. The NSs are close-lying and they even interconnect when deposited on a substrate. Their synthesis is based on the use of the p-toluenesulfonic acid/dodecylamine (pTS/DDA) ligand pair and their (photo)chemical stability and photoluminescence is enhanced by adding EuBr2 nanodots (EuNDs). Strikingly, they can be preserved as a solid and stored for at least one year. The blue emissive colloid can be recovered from the solid as needed by simply dispersing the powder in toluene and then using it to prepare solid films, making them very promising candidates for manufacturing devices.

The Luminescence of CH3 NH3 PbBr3 Perovskite Nanoparticles Crests the Summit and Their Photostability under Wet Conditions is Enhanced.

CH3 NH3 PbBr3 perovskite nanoparticles (PAD ) are prepared with a photoluminescence quantum yield of ≈100% in air atmosphere by using the quasi-spherical shaped 2-adamantylammonium bromide (ADBr) as the only capping ligand. The photostability under wet conditions of this kind of nanoparticles is enhanced by using cucurbit[7]uril-adamantylammonium (AD@CB) host-guest complexes as the capping ligand.

Organic-inorganic and all-inorganic lead halide nanoparticles [Invited].

Organic-inorganic (hybrid) and all-inorganic lead halide perovskites, in particular APbX(3) where A is an organic cation (methylammonium or formamidinium) or cesium cation and X = Cl, Br, I, respectively, are of great interest in photovoltaic devices and as luminescent materials for light-emitting devices. It has recently been demonstrated that they can be prepared not only as nanoparticulate material by using the pores of mesoporous films, but also as colloidal nanoparticles, which exhibit enhanced optical properties with respect to the bulk material. We summarize here the methods reported for their preparation as well as their optical features. Experimental and theoretical studies on this class of materials are ongoing and there is still a demand for enhancing their emissive properties, stability in polar solvents, dispersibility in different media and/or photostability.

Nontemplate synthesis of CH3NH3PbBr3 perovskite nanoparticles.

To date, there is no example in the literature of free, nanometer-sized, organolead halide CH3NH3PbBr3 perovskites. We report here the preparation of 6 nm-sized nanoparticles of this type by a simple and fast method based on the use of an ammonium bromide with a medium-sized chain that keeps the nanoparticles dispersed in a wide range of organic solvents. These nanoparticles can be maintained stable in the solid state as well as in concentrated solutions for more than three months, without requiring a mesoporous material. This makes it possible to prepare homogeneous thin films of these nanoparticles by spin-coating on a quartz substrate. Both the colloidal solution and the thin film emit light within a narrow bandwidth of the visible spectrum and with a high quantum yield (ca. 20%); this could be advantageous in the design of optoelectronic devices.

Sensing chiral drugs by using CdSe/ZnS nanoparticles capped with N-acetyl-L-cysteine methyl ester.

Chiral quantum dots (QDs), differing in their core or shell size and, consequently, in their optical properties, were synthesized by the treatment of commercially available amine-capped quantum dots with methyl ester N-acetyl-L-cysteine (CysP). Interestingly, their colloidal methanol solutions remain stable for several months. Their NMR and IR spectra were in accordance with CysP binding to the QD surface through two anchoring groups; its thiolate (strongly bound) and the carbonyl group of its ester (weaker bound) group, whereas their circular dichroism (CD) spectra showed a new broad redshifted band, suggesting that the attachment to the QD surface modified the conformational equilibrium towards conformer(s) with optical activity in this region. These QDs were sufficiently fluorescent to perform studies of the chiral recognition of drugs, in particular the aryl propionic acids (APAs) ketoprofen (KP), naproxen (NP), flurbiprofen (FP), and ibuprofen (IP). We used different drug concentration ranges, depending on the QD solubility. All the assayed drugs quenched the QD emission in a concentration-dependent mode. Quenching fluorescence assays with the chiral QDs (CS@CysP) showed their extraordinary capacity for the chiral recognition of KP, NP, and FP, and particularly in the case of KP and FP, a remarkable positive allosteric effect was detected for the R enantiomer. By using a drug/CS@CysP molar ratio of 5000:1 and 2500:1, the changes of intensity and the sign of the CD spectrum of the drug evidenced the dissociation of the drug carboxylic group in the presence of the QD.

Photocatalysis Based on Metal Halide Perovskites for Organic Chemical Transformations.

Heterogeneous photocatalysts incorporating metal halide perovskites (MHPs) have garnered significant attention due to their remarkable attributes: strong visible-light absorption, tuneable band energy levels, rapid charge transfer, and defect tolerance. Additionally, the promising optical and electronic properties of MHP nanocrystals can be harnessed for photocatalytic applications through controlled crystal structure engineering, involving composition tuning via metal ion and halide ion variations, dimensional tuning, and surface chemistry modifications. Combination of perovskites with other materials can improve the photoinduced charge separation and charge transfer, building heterostructures with different band alignments, such as type-II, Z-scheme, and Schottky heterojunctions, which can fine-tune redox potentials of the perovskite for photocatalytic organic reactions. This review delves into the activation of organic molecules through charge and energy transfer mechanisms. The review further investigates the impact of crystal engineering on photocatalytic activity, spanning a diverse array of organic transformations, such as C-X bond formation (X = C, N, and O), [2 + 2] and [4 + 2] cycloadditions, substrate isomerization, and asymmetric catalysis. This study provides insights to propel the advancement of metal halide perovskite-based photocatalysts, thereby fostering innovation in organic chemical transformations.

The Dark Side of Lead-Free Metal Halide Nanocrystals: Substituent-Modulated Photocatalytic Activity in Benzyl Bromide Reduction.

We illustrate here the high photocatalytic activity of sustainable lead-free metal halide nanocrystals (NCs), namely, Cs3Sb2Br9 NCs, in the reduction of p-substituted benzyl bromides in the absence of a cocatalyst. The electronic properties of the benzyl bromide substituents and the substrate affinity to the NC surface determine the selectivity in C-C homocoupling under visible light irradiation. This photocatalyst can be reused for at least three cycles and preserves its good performance with a turnover number of ca. 105,000.

Governing the emissive properties of 4-aminobiphenyl-2-pyrimidine push-pull systems via the restricted torsion of N,N-disubstituted amino groups.

Donor-acceptor-substituted biphenyl derivatives are particularly interesting model compounds, which exhibit intramolecular charge transfer because of the extent of charge transfer between both substituents. The connection of a 4-[1,1'-biphenyl]-4-yl-2-pyrimidinyl) moiety to differently disubstituted amino groups at the biphenyl terminal can offer push-pull compounds with distinctive photophysical properties. Herein, we report a comprehensive study of the influence of the torsion angle of the disubstituted amino group on the emissive properties of two pull-push systems: 4-[4-(4-N,N-dimethylaminophenyl)phenyl]-2,6-diphenylpyrimidine (D1) and 4-[4-(4-N,N-diphenylaminophenyl)phenyl]-2,6-diphenylpyrimidine (D2). The torsion angle of the disubstituted amino group, either N,N-dimethyl-amine or N,N-diphenyl-amine, at the biphenyl end governs their emissive properties. A drastic fluorescence quenching occurs in D1 as the solvent polarity increases, whereas D2 maintains its emission independently of the solvent polarity. Theoretical calculations on D1 support the presence of a twisted geometry for the lowest energy, charge-transfer excited state (S1,90), which corresponds to the minimum energy structure in polar solvents and presents a small energy barrier to move from the excited to the ground state, thereby favoring the non-radiative pathway and reducing the fluorescence efficiency. In contrast, this twisted structure is absent in D2 due to the steric hindrance of the phenyl groups attached to the amine group, making the non-radiative decay less favorable. Our findings provide insights into the crucial role of the substituent in the donor moiety of donor-acceptor systems on both the singlet excited state and the intramolecular charge-transfer process.

Photoluminescence enhancement of CdSe quantum dots: a case of organogel-nanoparticle symbiosis.

Highly fluorescent organogels (QD-organogel), prepared by combining a pseudopeptidic macrocycle and different types of CdSe quantum dots (QDs), have been characterized using a battery of optical and microscopic techniques. The results indicate that the presence of the QDs not only does not disrupt the supramolecular organization of the internal fibrillar network of the organogel to a significant extent, but it also decreases the critical concentration of gelator needed to form stable and thermoreversible organogels. Regarding the photophysical properties of the QDs, different trends were observed depending on the presence of a ZnS inorganic shell around the CdSe core. Thus, while the core-shell QDs preserve their photophysical properties in the organogel medium, a high to moderate increase of the fluorescence intensity (up to 528%) and the average lifetime (up to 1.7), respectively, was observed for the core QDs embedded in the organogel. The results are relevant for the development of luminescent organogels based on quantum dots, which have potential applications as advanced hybrid materials in different fields.

Pyrene-functionalized nanoparticles: two independent sensors, the excimer and the monomer.

The high surface-to-volume ratio of nanoparticles has been used to obtain a high local concentration of pyrene units on their periphery, making the formation of both pyrene emissive species possible using amazingly small pyrene concentrations. The sensing properties of model pyrene-functionalized nanoparticles was investigated by using different nitroaromatic compounds [m-nitroaniline and p-nitroaniline] and nitrobenzenes [nitrobenzene, p-nitrotoluene, 2,4-dinitrotoluene, and 2,6-dinitrotoluene]. The hybrid system acts as a dual-fluorescence sensor, in which the decrease of the pyrene emission, induced by the quencher, is hardly reflected in the pyrene excimer emission. The encapsulation capacity of the NPs also plays a key role in their sensitivity to the analyte.

Nitroanilines as quenchers of pyrene fluorescence.

The quenching of pyrene and 1-methylpyrene fluorescence by nitroanilines (NAs), such as 2-, 3-, and 4-nitroaniline (2-NA, 3-NA, and 4-NA, respectively), 4-methyl-3-nitroaniline (4-M-3-NA), 2-methyl-4-nitroaniline (2-M-4-NA), and 4-methyl-3,5-dinitroaniline (4-M-3,5-DNA), are studied in toluene and 1,4-dioxane. Steady-state fluorescence data show the higher efficiency of the 4-NAs as quenchers and fit with a sphere-of-action model. This suggests a 4-NA tendency of being in close proximity to the fluorophore, which could be connected with their high polarity/hyperpolarizability. In addition, emission and excitation spectra evidence the formation of emissive pyrene-NA ground-state complexes in the case of the 4-NAs and, in a minor degree, in the 2-NA. Moreover, time-resolved fluorescence experiments show that increasing amounts of NA decrease the pyrene fluorescence lifetime to a degree that depends on the NA nature and is larger in the less viscous solvent (toluene). Although the NA absorption and the pyrene (Py) emission overlap, we found no evidence of dipole-dipole energy transfer from the pyrene singlet excited state ((1)Py) to the NAs; this could be due to the low NA concentration used in these experiments. Transient absorption spectra show that the formation of the pyrene triplet excited state ((3)Py) is barely affected by the presence of the NAs in spite of their efficiency in (1)Py quenching, suggesting the involvement of (1)Py-NA exciplexes which--after intersystem crossing--decay efficiently into (3)Py.

Further insight into the photostability of the pyrene fluorophore in halogenated solvents.

Pyrene fluorophores of pyrene-functionalized CdSe quantum dots (QD@Py), as well as alkylpyrene and pyrene itself (Py), undergo fast degradation in aerated chloroform under ultraviolet-A (UV-A, 316<λ<400 nm) illumination. Steady-state fluorescence studies of irradiated chloroform solutions of QD@Py show formation of new bands, red-shifted compared to that of the pyrene moiety. Similar behaviour is observed for pyrene and the alkylpyrene system. Column chromatography of the pyrene photolysate in chloroform allowed us to isolate photoproducts arising from pyrene degradation, and to obtain information on the structure of the photoproducts responsible for the emission bands. The most predominant photoproducts were those originating from the reaction of pyrene with dichloromethyl radicals. The phototransformation of QD@Py and the alkylpyrene involves mainly detachment of the alkyl chain from the aromatic ring, induced also by dichloromethyl radicals, and oxidation of the alkyl chain at the benzylic position was detected as well. By contrast, these pyrene systems show a high photostability in aerated dichloromethane. Transient absorption measurements showed formation of both pyrene triplet and pyrene radical cation for all pyrene systems in these halogenated solvents. The yield of pyrene radical cations for Py is higher than for QD@Py and the alkylpyrene. In addition, pyrene radical cations were longer-lived in dichloromethane than in chloroform. The reason for the pyrene photostability in dichloromethane is the different reactivity of chloromethyl and dichloromethyl radicals towards pyrene and oxygen. These studies show that the use of dichloromethane can be a suitable alternative to chloroform when the good solubility properties of these halogenated solvents are needed to dissolve pyrene when this chromophore is used as a fluorescent probe.

Recent Progress in Lanthanide-Doped Inorganic Perovskite Nanocrystals and Nanoheterostructures: A Future Vision of Bioimaging.

All-inorganic lead halide perovskite nanocrystals have great potential in optoelectronics and photovoltaics. However, their biological applications have not been explored much owing to their poor stability and shallow penetration depth of ultraviolet (UV) excitation light into tissues. Interestingly, the combination of all-inorganic halide perovskite nanocrystals (IHP NCs) with nanoparticles consisting of lanthanide-doped matrix (Ln NPs, such as NaYF4:Yb,Er NPs) is stable, near-infrared (NIR) excitable and emission tuneable (up-shifting emission), all of them desirable properties for biological applications. In addition, luminescence in inorganic perovskite nanomaterials has recently been sensitized via lanthanide doping. In this review, we discuss the progress of various Ln-doped all-inorganic halide perovskites (LnIHP). The unique properties of nanoheterostructures based on the interaction between IHP NCs and Ln NPs as well as those of LnIHP NCs are also detailed. Moreover, a systematic discussion of basic principles and mechanisms as well as of the recent advancements in bio-imaging based on these materials are presented. Finally, the challenges and future perspectives of bio-imaging based on NIR-triggered sensitized luminescence of IHP NCs are discussed.

Revisiting the nontemplate approach for the synthesis of highly green emissive hybrid perovskite nanocrystals: platelets or spheres?

The long-standing debate about the morphology of colloidal methylammonium lead bromide perovskites nanocrystals, manufactured by our nontemplate synthetic strategy reported in 2014, is now resolved; specifically, the highest green emissive single nanoplatelets (of up to 93%) with long-term chemical and photochemical stability have been obtained after suitable purification steps.

Size reduction of CdSe/ZnS core-shell quantum dots photosensitized by benzophenone: where does the Cd(0) go?

The size of core-shell CdSe/ZnS quantum dots can be decreased by using the combined action of an n,π* aromatic ketone and UVA light. Energy-dispersive X-ray spectroscopy as well as X-ray photoelectron spectroscopy techniques gave information on the photosensitization mechanism and the eventual destiny of Cd(2+) and Se(2-) core ions. Our data support the electron transfer from the BP ketyl radical to Cd(2+), leading to Cd(0) and H(+), as well as to the recovery of benzophenone. Elemental Cd remains on the core and, eventually, can be oxidized to CdO. In addition, Se(2-) counterions disperse inside the solution mainly attached to protonated amine ligands. The Se(2-) combines with H(+), leading to SeH(2), which is finally oxidized to Se(0) by oxygen. Therefore, quantum dots illumination in the presence of benzophenones brings about a profound nanoparticle reconstruction which takes place after dark storage; this agrees with the drastic quenching of the quantum dot emission detected immediately after illumination as well as the slow recovery in the dark.

Alkoxy-styryl DCDHF fluorophores.

A photostable dicyanomethylenedihydrofuran fluorophore which contains electron-donating alkoxy groups is described. This chromophore is highly environmentally-sensitive, which is a remarkable property for a fluorescent reporter. Its light excitation also enables, in low viscous solvents, the formation of dark states whose radical or triplet nature is ruled out.

The synergy between the CsPbBr3 nanoparticle surface and the organic ligand becomes manifest in a demanding carbon-carbon coupling reaction.

We demonstrate here the suitability of CsPbBr3 nanoparticles as photosensitizers for a demanding photoredox catalytic homo- and cross-coupling of alkyl bromides at room temperature by merely using visible light and an electron donor, thanks to the cooperative action between the nanoparticle surface and organic capping.