Noble metal nanoclusters protected with carboranes, a 12-vertex, nearly icosahedral boron–carbon framework system, have received immense attention due to their different physicochemical properties. We have synthesized ortho-carborane-1,2-dithiol (CBDT) and triphenylphosphine (TPP) coprotected [Ag42(CBDT)15(TPP)4]2– (shortly Ag42) using a ligand-exchange induced structural transformation reaction starting from [Ag18H16(TPP)10]2+ (shortly Ag18). The formation of Ag42 was confirmed using UV–vis absorption spectroscopy, mass spectrometry, transmission electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, and multinuclear magnetic resonance spectroscopy. Multiple UV–vis optical absorption features, which exhibit characteristic patterns, confirmed its molecular nature. Ag42 is the highest nuclearity silver nanocluster protected with carboranes reported so far. Although these clusters are thermally stable up to 200 °C in their solid state, light-irradiation of its solutions in dichloromethane results in its structural conversion to [Ag14(CBDT)6(TPP)6] (shortly Ag14). Single crystal X-ray diffraction of Ag14 exhibits Ag8–Ag6 core–shell structure of this nanocluster. Other spectroscopic and microscopic studies also confirm the formation of Ag14. Time-dependent mass spectrometry revealed that this light-activated intercluster conversion went through two sets of intermediate clusters. The first set of intermediates, [Ag37(CBDT)12(TPP)4]3– and [Ag35(CBDT)8(TPP)4]2– were formed after 8 h of light irradiation, and the second set comprised of [Ag30(CBDT)8(TPP)4]2–, [Ag26(CBDT)11(TPP)4]2–, and [Ag26(CBDT)7(TPP)7]2– were formed after 16 h of irradiation. After 24 h, the conversion to Ag14 was complete. Density functional theory calculations reveal that the kernel-centered excited state molecular orbitals of Ag42 are responsible for light-activated transformation. Interestingly, Ag42 showed near-infrared emission at 980 nm (1.26 eV) with a lifetime of >1.5 μs, indicating phosphorescence, while Ag14 shows red luminescence at 626 nm (1.98 eV) with a lifetime of 550 ps, indicating fluorescence. Femtosecond and nanosecond transient absorption showed the transitions between their electronic energy levels and associated carrier dynamics. Formation of the stable excited states of Ag42 is shown to be responsible for the core transformation.

Chiral hybrid metal halide perovskites provide hopes of combining chirality induced by the organic sublattice with optoelectronic properties arising from the inorganic sublattice. The field is still in its infancy, with material space mainly focused on two-dimensional hybrid lead halide perovskites. Here we report a zero-dimensional Pb-free perovskite derivative structure with chemical composition [(R-/S-MBA)4Bi2I10], where MBA stands for methylbenzylammonium. Single-crystal X-ray diffraction data show that the enantiomerically pure R-MBA and S-MBA induce chirality in the Bi–I inorganic sublattice. Consequently, chiroptical properties like circular dichroism (CD) is observed for the excitonic transitions of (R-/S-MBA)4Bi2I10. Temperature-dependent (7–300 K) photoluminescence shows excitonic and shallow-defect emissions, indicating fewer deep-defect trap states. Further ultrafast exciton many-body interactions studied by transient absorption spectroscopy reveal Stark effect, band gap renormalization, and shallow-defect states absorption near the band-edge. The material design, structure, and optical properties reported here will be useful to develop next-generation Pb-free perovskites for chiral optoelectronics.

Nonresonant optical driving of confined semiconductors can open up exciting opportunities for experimentally realizing strongly interacting photon-dressed (Floquet) states through the optical Stark effect (OSE) for coherent modulation of the exciton state. Here we report the first room-temperature observation of the Floquet biexciton-mediated anomalous coherent excitonic OSE in CsPbBr3 quantum dots (QDs). Remarkably, the strong exciton–biexciton interaction leads to a coherent red shift and splitting of the exciton resonance as a function of the drive photon frequency, similar to Autler–Townes splitting in atomic and molecular systems. The large biexciton binding energy of ∼71 meV and exciton–biexciton transition dipole moment of ∼25 D facilitate the hallmark observations, even at large detuning energies of >300 meV. This is accompanied by an unusual crossover from linear to nonlinear fluence dependence of the OSE as a function of the drive photon frequency. Our findings reveal crucial information on the unexplored many-body coherent interacting regime, making perovskite QDs suitable for room temperature quantum devices.

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Superstructures made by assemblies of metal nanoclusters (NCs) have gained interest due to their atomic precision and exciting photophysical properties. Although there are some reports of cluster-assembled materials of NCs protected with thiols, the preparation of stable thiol-free analogs is largely unexplored due to the poor stability of such structures. Herein, we report the synthesis of phosphine-protected alloy NCs of silver with varying gold doping and superstructures of such systems. We show that alloying of phosphine-protected silver clusters with gold results in comparatively more stable clusters than weakly ligated hydride-and phosphine-coprotected silver clusters. Two new Ag− Au alloy cluster series, [Ag 11−x Au x (DPPB) 5 Cl 5 O 2 ] 2+ , where x = 1− 10 (Ag 11−x Au x in short), and [Ag 15−x Au x (DPPP) 6 Cl 5 ] 2+ , where x = 1−6 (Ag 15−x Au x in short), have been synthesized using two different phosphines, 1,4-bis(diphenylphosphino)butane (DPPB) and 1,3-bis(diphenylphosphino)propane (DPPP), respectively. These alloy clusters possess aggregation-induced emission (AIE) property, which was unexplored till now for phosphine-protected silver clusters. A visibly nonluminescent methanol solution of these clusters showed strong red luminescence in the presence of water due to the formation of cluster-assembled spherical hollow superstructures without any template. A solvophobic effect along with π•••π and C−H•••π interactions in the ligand shell make the alloy NCs assemble compactly within the hollow spheres. The assembly makes them highly emitting due to the restriction of intramolecular motion. The emissive states of the alloy clusters show a many-fold increase in lifetime in the presence of water. Femtosecond transient absorption studies revealed the lifetime of the excited-state charge carriers in their monomeric and aggregated states. Apart from enriching the limited family of phosphine-protected silver alloy NCs, this work also provides a new strategy to build a controlled assembly of NCs with tailored luminescence. These materials could be new phosphors for applications in composites, sensors, thin films, and photonic materials.

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ReS2, a layered transition metal dichalcogenide (TMD) with reduced crystal symmetry exhibiting unique anisotropic and layer-independent properties, holds great potential for optoelectronic and photonic applications. Despite a flurry of research activities in the third-order nonlinear optical response of TMDs, tuning those properties in a completely reversible manner in real time is a challenge and remains largely unexplored. Here, we experimentally demonstrate band edge carrier-induced sign reversal of the ultrafast third-order nonlinear optical response in few-layer (4–8) ReS2 nanoflakes. In particular, saturable absorption observed before hot carrier thermalization (<0.3 ps) is tuned to reverse saturable absorption (RSA) after the carrier thermalized (>0.6 ps) at the band edge and defects using a single-color pump–probe intensity scan (I-scan) technique. RSA in our experiment is due to the two-step two single-photon absorption of the long-lived (∼1000s of ps from our ultrafast transient absorption) carriers at the band edges and defects. Motivated by the results, a liquid cell-based high-performance few-layer ReS2 optical limiter is fabricated with a remarkable 0.1 GW/cm 2 onset threshold and 0.64 limiting differential transmittance better than the other optical-limiting materials. These results offer a direction to manipulate the nonlinear optical response of materials which otherwise requires a large electric field, high intensity, or efficient charge transfer between donor and acceptor pairs.

We report the synthesis, structural characterization, and photophysical properties of a propeller-shaped Ag 21 nanomolecule with six rotary arms, protected with m-carborane-9-thiol (MCT) and triphenylphosphine (TPP) ligands. Structural analysis reveals that the nanomolecule has an Ag 13 central icosahedral core with six directly connected silver atoms and two more silver atoms connected through three Ag−S−Ag bridging motifs. While 12 MCT ligands protect the core through metal−thiolate bonds in a 3−6−3-layered fashion, two TPP ligands solely protect the two bridging silver atoms. Interestingly, the rotational orientation of a silver sulfide staple motif is opposite to the orientation of carborane ligands, resembling the existence of a bidirectional rotational orientation in the nanomolecule. Careful analysis reveals that the orientation of carborane ligands on the cluster's surface resembles an assembly of double rotors. The zero circular dichroism signal indicates its achiral nature in solution. There are multiple absorption peaks in its UV−vis absorption spectrum, characteristic of a quantized electronic structure. The spectrum appears as a fingerprint for the cluster. High-resolution electrospray ionization mass spectrometry proves the structure and composition of the nanocluster in solution, and systematic fragmentation of the molecular ion starts with the loss of surface-bound ligands with increasing collision energy. Its multiple optical absorption features are in good agreement with the theoretically calculated spectrum. The cluster shows a narrow near-IR emission at 814 nm. The Ag 21 nanomolecule is thermally stable at ambient conditions up to 100 °C. However, white-light illumination (lamp power = 120−160 W) shows photosensitivity, and this induces structural distortion, as confirmed by changes in the Raman and electronic absorption spectra. Femtosecond and nanosecond transient absorption studies reveal an exceptionally stable excited state having a lifetime of 3.26 ± 0.02 μs for the carriers, spread over a broad wavelength region of 520−650 nm. The formation of core-centered long-lived carriers in the excited state is responsible for the observed light-activated structural distortion.