Multiphoton Excitation of CsPbBr3 Perovskite Quantum Dots (PQDs): How Many Electrons Can One PQD Donate to Multiple Molecular Acceptors?

Metastable multiexcitonic states (MESs) of semiconductor quantum dots can be involved in multielectron transfer reactions, which opens new perspectives in nanomaterials-based optoelectronic applications. Herein, we demonstrate the generation of a MES in CsPbBr3 perovskite quantum dots (PQDs) and its dissociation dynamics through multiple electron transfers to molecular electron acceptors, anthraquinones (AQs), bound to the PQD surface by a carboxylic anchor. As many as 14 excitons are produced at an excitation density of roughly 220 µJ cm-2 without detectable PQD degradation. Addition of AQ results in the formation of PQD-AQ hybrids with excess of AQs (PQD:AQ ≈ 1:20), which opens the possibility of multielectron transfer acts from MES to AQs. We found that the electron transfer saturates after roughly five transfer acts and that the first electron transfer (ET) time constant is as short as 1 ps. However, each ET increases the Coulomb potential barrier for the next ET, which decreases the rate of ET, resulting in a saturation after five ETs.

Charge transfer dynamics in CsPbBr3 perovskite quantum dots-anthraquinone/fullerene (C60) hybrids.

An advantage of colloidal quantum dots, particularly perovskite quantum dots (PQDs), as photoactive components is that they easily form complexes with functional organic molecules, which results in hybrids with enriched photophysical properties. Herein, we demonstrate the formation of stable ground state complexes of CsPbBr3 PQD with two widely used molecular electron acceptors, fullerene (C60) and anthraquinone, (AQ) which contain carboxylic anchor groups. Dynamics of the photo-induced electron transfer in the hybrids were compared. The use of carboxylic groups for binding results in stable complex formation and their photophysical properties depend on the ratio of components but not the absolute concentrations (up to micromolar concentrations). Time-resolved transient absorption (TA) spectroscopy shows that in both cases, a charge separated (CS) state is formed. Data analysis was aimed to evaluate the CS time constant in ideal one-to-one complexes and was found to be in the range of 30-190 ps. The CS state of PQD-AQ complexes recombines directly to the ground state in roughly one microsecond. Recombination of the CS state of PQD-C60 is more complex and points to strong inhomogeneity of these complexes. Majority of the CS states relax by first forming the C60 triplet state.

Photoinduced Energy Transfer in ZnCdSeS Quantum Dot-Phthalocyanines Hybrids.

In this article, interaction between ZnCdSeS quantum dot (QD) and phthalocyanines with variable linker has been reported. Steady-state and time-resolved spectroscopic investigation reveals that only photoinduced energy transfer occurs from QD to phthalocyanines. To evaluate quantitatively the energy transfer, the Poisson statistics of QD-dye complex formation was used in the analysis of steady-state and time-resolved emission quenching, which allows to estimate the energy transfer rate constant for an ideal one-to-one complex. The measured rate constants are compared to the rates evaluated based on the classic Förster theory, which shows roughly 1 nm discrepancy in the energy transfer distance estimation, or one order in magnitude discrepancy in the transfer rate constants.

Photon Harvesting in Sunscreen-Based Functional Nanoparticles.

The ultraviolet light component in the solar spectrum is known to cause several harmful effects, such as allergy, skin ageing, and skin cancer. Thus, current research attention has been paid to the design and fundamental understanding of sunscreen-based materials. One of the most abundantly used sunscreen molecules is Avobenzone (AB), which exhibits two tautomers. Here, we highlight the preparation of spherically shaped nanoparticles from the sunscreen molecule AB as well as from sunscreen-molecule-encapsulated polymer nanoparticles in aqueous media and study their fundamental photophysical properties by steady-state and time-resolved spectroscopy. Steady-state studies confirm that the AB molecule is in the keto and enol forms in tetrahydrofuran, whereas the enol form is stable in the case of both AB nanoparticles and AB-encapsulated poly(methyl methacrylate) (PMMA) nanoparticles. Thus, the keto-enol transformation of AB molecules is restricted to a nanoenvironment. An enhancement of photostability in both the nanoparticle and PMMA-encapsulated forms under UV light irradiation is observed. The efficient excited energy transfer (60 %) from AB to porphyrin molecules opens up further prospects in potential applications as light-harvesting systems.

Surfactant-assisted porphyrin based hierarchical nano/micro assemblies and their efficient photocatalytic behavior.

In this report, we have demonstrated the synthesis of surfactant-assisted different morphologies of meso-tetra(4-carboxyphenyl)porphyrin assemblies (spherical to flower shaped). These nano/micro assemblies are well characterized by scanning electron microscopy and X-ray diffraction. The formation of assemblies is driven by noncovalent interactions such as hydrophobic-hydrophobic and aromatic π-π stacking between the molecules. The steady state and time-resolved spectroscopic investigation reveal that different assemblies are formed by virtue of special supramolecular organizations. The photocatalytic activities of different assemblies have been demonstrated with an organic pollutant Rhodamine B dye under the visible light irradiation. Such porphyrin based assemblies could pave the way for designing new optical based materials for the applications in photocatalytic, photovoltaic, and light harvesting system.

Photophysical properties of Au-CdTe hybrid nanostructures of varying sizes and shapes.

We design well-defined metal-semiconductor nanostructures using thiol-functionalized CdTe quantum dots (QDs)/quantum rods (QRs) with bovine serum albumin (BSA) protein-conjugated Au nanoparticles (NPs)/nanorods (NRs) in aqueous solution. The main focus of this article is to address the impacts of size and shape on the photophysical properties, including radiative and nonradiative decay processes and energy transfers, of Au-CdTe hybrid nanostructures. The red shifting of the plasmonic band and the strong photoluminescence (PL) quenching reveal a strong interaction between plasmons and excitons in these Au-CdTe hybrid nanostructures. The PL quenching of CdTe QDs varies from 40 to 86 % by changing the size and shape of the Au NPs. The radiative as well as the nonradiative decay rates of the CdTe QDs/QRs are found to be affected in the presence of both Au NPs and NRs. A significant change in the nonradiative decay rate from 4.72×10(6) to 3.92×10(10) s(-1) is obtained for Au NR-conjugated CdTe QDs. It is seen that the sizes and shapes of the Au NPs have a pronounced effect on the distance-dependent energy transfer. Such metal-semiconductor hybrid nanostructures should have great potentials for nonlinear optical properties, photovoltaic devices, and chemical sensors.

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.