Isomeric Effects of Au28(S-c-C6H11)20 Nanoclusters on Photoluminescence: Roles of Electron-Vibration Coupling and Higher Triplet State.

The exploration of near-infrared photoluminescence (PL) from atomically precise nanoclusters is currently a prominent area of interest owing to its importance in both fundamental research and diverse applications. In this work, we investigate the near-infrared (NIR) photoluminescence mechanisms of two structural isomers of atomically precise gold nanoclusters of 28 atoms protected by cyclohexanethiolate (CHT) ligands, i.e., Au28i(CHT)20 and Au28ii(CHT)20. Based on their structures, analysis of 3O2 (triplet oxygen) quenching of the nanocluster triplet states, temperature-dependent photophysical studies, and theoretical calculations, we have elucidated the intricate processes governing the photoluminescence of these isomeric nanoclusters. For Au28i(CHT)20, its emission characteristics are identified as phosphorescence plus thermally activated delayed fluorescence (TADF) with a PL quantum yield (PLQY) of 0.3% in dichloromethane under ambient conditions. In contrast, the Au28ii(CHT)20 isomer exhibits exclusive phosphorescence with a PLQY of 3.7% in dichloromethane under ambient conditions. Theoretical simulations reveal a larger singlet (S1)-triplet (T1) gap in Au28ii than that in Au28i, and the higher T2 state plays a critical role in both isomers' photophysical processes. The insights derived from this investigation not only contribute to a more profound comprehension of the fundamental principles underlying the photoluminescence of atomically precise gold nanoclusters but also provide avenues for tailoring their optical properties for diverse applications.

Tailoring Carbon Tails of Ligands on Au52(SR)32 Nanoclusters Enhances the Near-Infrared Photoluminescence Quantum Yield from 3.8 to 18.3.

One of the important factors that determine the photoluminescence (PL) properties of gold nanoclusters pertain to the surface. In this study, four Au52(SR)32 nanoclusters that feature a series of aromatic thiolate ligands (-SR) with different bulkiness at the para-position are synthesized and investigated. The near-infrared (NIR) photoluminescence (peaks at 900-940 nm) quantum yield (QY) is largely enhanced with a decrease in the ligand's para-bulkiness. Specifically, the Au52(SR)32 capped with the least bulky p-methylbenzenethiolate (p-MBT) exhibits the highest PLQY (18.3% at room temperature in non-degassed dichloromethane), while Au52 with the bulkiest tert-butylbenzenethiolate (TBBT) only gives 3.8%. The large enhancement of QY with fewer methyl groups on the ligands implies a nonradiative decay via the multiphonon process mediated by C-H bonds. Furthermore, single-crystal X-ray diffraction (SCXRD) comparison of Au52(p-MBT)32 and Au52(TBBT)32 reveals that fewer methyl groups at the para-position lead to a stronger interligand π···π stacking on the Au52 core, thus restricting ligand vibrations and rotations. The emission nature is identified to be phosphorescence and thermally activated delayed fluorescence (TADF) based on the PL lifetime, 3O2 quenching, and temperature-dependent PL and absorption studies. The 1O2 generation efficiencies for the four Au52(SR)32 NCs follow the same trend as the observed PL performance. Overall, the highly NIR-luminescent Au52(p-MBT)32 nanocluster and the revealed mechanisms are expected to find future applications.

Single and bi-excitonic characteristics of ligand-modified silicon nanoparticles as demonstrated via single particle photon statistics and plasmonic effects.

Silicon nanoparticles (Si NPs) are of great interest to researchers due to their fluorescence properties, low toxicity, and the low cost of the Si precursor. Recent studies have shown that Si NPs surface-modified with secondary aryl amine ligands emit light at wavelengths ranging from cyan to yellow and with quantum yields of up to 90%. The predominant emitting state in these species has been assigned to a charge-transfer (CT) transition from the ligand to the Si particle as the emission wavelength is determined by the dipolar properties of the ligand rather than the size of the Si core. This contribution focuses on the single-molecule emission properties of Si NPs functionalized with a 1,2,3,4-tetrahydrocarbazole-4-one ligand (Te-On) which have a peak emission wavelength of 550 nm and a quantum yield of 90%. In single-particle dispersed emission spectra, a weak long-wavelength sideband is seen in addition to the dominant yellow emission derived from the CT state. The photon statistical behavior of single Si NPs in the red emission region is consistent with that of a state having collective or bi-excitonic character. In contrast, the yellow emission exhibits predominantly CT character. Deposition of the sample onto a thin gold film causes the CT emission to be quenched whereas that attributed to a bi-exciton state of the Si core is enhanced. These results provide new insights into the mechanism of single-molecule intensity fluctuation in these surface-modified silicon nanoparticles that will benefit proposed applications in biological labeling and as single-photon sources.

Unraveling the Contribution of Residual Monomer to the Emission Spectra of Poly(3-hexylthiophene) Aggregates: Implications for Identifying H- and J-type Coupling.

Poly(3-hexylthiophene) (P3HT) is a well-studied benchmark system for semiconducting polymers used in optoelectronic devices. In these materials, aggregation can improve charge transport efficiency or enhance emission yields depending on the interchain packing. This may be inferred from the absorption and emission spectra when analyzed using exciton coupling models such as the well-known H- and J-coupling model of Kasha. The more recently developed weakly coupled H-aggregate (WCH) model quantifies the degree of disorder via the ratio of the electronic origin intensity to that of the first vibronic band. Here, the underlying assumptions of this approach are tested experimentally for P3HT aggregates formed by solvent poisoning using bulk and single-molecule-based spectroscopic techniques. Specifically, we show that the contribution of residual monomeric chains to the aggregate spectrum must be accounted for to properly assign the spectra as H- or J-type. A modification of the WCH model is introduced to account for multiple emissive species.

Structural distortion and electron redistribution in dual-emitting gold nanoclusters.

Deciphering the complicated excited-state process is critical for the development of luminescent materials with controllable emissions in different applications. Here we report the emergence of a photo-induced structural distortion accompanied by an electron redistribution in a series of gold nanoclusters. Such unexpected slow process of excited-state transformation results in near-infrared dual emission with extended photoluminescent lifetime. We demonstrate that this dual emission exhibits highly sensitive and ratiometric response to solvent polarity, viscosity, temperature and pressure. Thus, a versatile luminescent nano-sensor for multiple environmental parameters is developed based on this strategy. Furthermore, we fully unravel the atomic-scale structural origin of this unexpected excited-state transformation, and demonstrate control over the transition dynamics by tailoring the bi-tetrahedral core structures of gold nanoclusters. Overall, this work provides a substantial advance in the excited-state physical chemistry of luminescent nanoclusters and a general strategy for the rational design of next-generation nano-probes, sensors and switches.

A Mono-cuboctahedral Series of Gold Nanoclusters: Photoluminescence Origin, Large Enhancement, Wide Tunability, and Structure-Property Correlation.

The origin of the near-infrared photoluminescence (PL) from thiolate-protected gold nanoclusters (Au NCs, <2 nm) has long been controversial, and the exact mechanism for the enhancement of quantum yield (QY) in many works remains elusive. Meanwhile, based upon the sole steady-state PL analysis, it is still a major challenge for researchers to map out a definitive relationship between the atomic structure and the PL property and understand how the Au(0) kernel and Au(I)-S surface contribute to the PL of Au NCs. Herein, we provide a paradigm study to address the above critical issues. By using a correlated series of "mono-cuboctahedral kernel" Au NCs and combined analyses of steady-state, temperature-dependence, femtosecond transient absorption, and Stark spectroscopy measurements, we have explicitly mapped out a kernel-origin mechanism and clearly elucidate the surface-structure effect, which establishes a definitive atomic-level structure-emission relationship. A ∼100-fold enhancement of QY is realized via suppression of two effects: (i) the ultrafast kernel relaxation and (ii) the surface vibrations. The new insights into the PL origin, QY enhancement, wavelength tunability, and structure-property relationship constitute a major step toward the fundamental understanding and structural-tailoring-based modulation and enhancement of PL from Au NCs.

Mechanism of Ligand-Controlled Emission in Silicon Nanoparticles.

Although bulk silicon (Si) is known to be a poor emitter, Si nanoparticles (NPs) exhibit size-dependent photoluminescence in the red or near-infrared due to quantum confinement. Recently, it has been shown that surface modification of Si NPs with nitrogen-capped ligands results in bluer emission wavelengths and quantum yields of up to 90%. However, the emission mechanism operating in these surface-modified Si NPs and the factors that determine their emission maxima are still unclear. Here, the emission in these species is shown to arise from a charge-transfer state between the Si surface and the ligand. The energy of this state is linearly correlated to the calculated ground-state dipole moment of the free ligand. This trend can be used in a predictive fashion for the design and synthesis of Si NPs with a broader range of emission wavelengths.

Eliminating Spurious Zero-Efficiency FRET States in Diffusion-Based Single-Molecule Confocal Microscopy.

Single-molecule Förster resonance energy transfer (smFRET) of freely diffusing biomolecules using confocal microscopy is a simple and powerful technique for measuring conformation and dynamics. However, a spurious zero-FRET population can significantly distort the measured histograms and lead to incorrect results, particularly in measurements of intrinsically low-FRET systems. Using a model system consisting of duplex DNAs, we show that there are two important contributions to the zero-FRET state: (1) formation of a dark triplet state of the acceptor dye and (2) the presence of donor-only strands due to incomplete hybridization between donor- and acceptor-labeled strands. The combined strategy of using Trolox as a triplet-state quencher and labeling the same DNA strand with donor and acceptor dyes effectively eliminates the zero-FRET population, even for constructs with intrinsically low FRET efficiencies. This strategy allows us to perform smFRET experiments using a simple confocal microscope with improved accuracy.

Detection of Ultralow Concentrations of Non-emissive Conjugated Polymer Aggregates via Fluorescence Correlation Spectroscopy.

The aggregation of conjugated polymers in common organic solvents is investigated using fluorescence correlation spectroscopy (FCS), burst analysis, and microscopy. Poly(3-hexylthiophene) and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] are both shown to form weakly bonded non-emissive aggregates in toluene that persist even at picomolar concentrations. These aggregates decrease the bulk emission intensity in solution but do not affect the fluorescence spectra or lifetimes, consistent with a static quenching mechanism. Passing the solutions through a syringe filter causes an increase in the number of emitters as measured by FCS, indicating that this process dissociates the aggregates. Films cast from solutions that have been filtered are more uniform and significantly more emissive than those made from unfiltered solutions. These results show that FCS is a highly sensitive probe of non-emissive aggregates in solution that have a deleterious effect on the emission properties and overall quality of spin-cast thin films, even at sub-nanomolar concentrations.

Exciton-Exciton Annihilation as a Probe of Interchain Interactions in PPV-Oligomer Aggregates.

One measure of exciton mobility in an aggregate is the efficiency of exciton-exciton annihilation (EEA). Both exciton mobilities and EEA are enhanced for aggregate morphologies in which the distances between chromophores and their relative orientations are favorable for Förster energy transfer. Here this principle is applied to gauge the strength of interchain interactions in aggregates of two substituted PPV oligomers of 7 (OPPV7) and 13 (OPPV13) phenylene rings. These are models of the semiconducting conjugated polymer MEH-PPV. The aggregates were formed by adding a poor solvent (methanol or water) to the oligomers dissolved in a good solvent. Aggregates formed from the longer-chain oligomer and/or by addition of the more polar solvent showed the largest contribution of EEA in their emission decay dynamics. This was found to correlate with the degree to which the steady-state emission spectrum of the monomer is altered by aggregation. The wavelength dependence of the EEA signal was also shown to be useful in differentiating emission features due to monomeric and aggregated chains when their spectra overlap significantly.

Spectroscopic and MD Study of Dynamic and Structural Heterogeneities in Ionic Liquids.

The structure of ionic liquids (ILs) surrounding solute dyes and the effects of solvent structure on solute diffusion have been investigated using molecular dynamics (MD) and the experimental tools of confocal and fluorescence correlation spectroscopies. Although confocal microscopy and simulations show that the local environment around solutes in ILs is heterogeneous and that the structural heterogeneity is rather long-lived, the local polarity and the diffusion constant were found to be uncorrelated. Moreover, the complex diffusion observed experimentally is not due to the structural heterogeneity of the IL but rather due to the dynamic heterogeneity arising from the viscous glassy nature of the IL environment. MD simulations show that the degree of dynamic heterogeneity depends on the first nonvanishing electric multipole moment of the solute. The dynamics of a cationic solute are the least heterogeneous, whereas those of a solute without an electric multipole moment are the most heterogeneous. This indicates that the length scale over which the solute-solvent interactions occur, and thus the number of solvent degrees of freedom that couple to the solute, are the key factors governing the dynamic heterogeneity of the solute.

Mechanism of Photoinduced Metal-Free Atom Transfer Radical Polymerization: Experimental and Computational Studies.

Photoinduced metal-free atom transfer radical polymerization (ATRP) of methyl methacrylate was investigated using several phenothiazine derivatives and other related compounds as photoredox catalysts. The experiments show that all selected catalysts can be involved in the activation step, but not all of them participated efficiently in the deactivation step. The redox properties and the stability of radical cations derived from the catalysts were evaluated by cyclic voltammetry. Laser flash photolysis (LFP) was used to determine the lifetime and activity of photoexcited catalysts. Kinetic analysis of the activation reaction according to dissociative electron-transfer (DET) theory suggests that the activation occurs only with an excited state of catalyst. Density functional theory (DFT) calculations revealed the structures and stabilities of the radical cation intermediates as well as the reaction energy profiles of deactivation pathways with different photoredox catalysts. Both experiments and calculations suggest that the activation process undergoes a DET mechanism, while an associative electron transfer involving a termolecular encounter (the exact reverse of DET pathway) is favored in the deactivation process. This detailed study provides a deeper understanding of the chemical processes of metal-free ATRP that can aid the design of better catalytic systems. Additionally, this work elucidates several important common pathways involved in synthetically useful organic reactions catalyzed by photoredox catalysts.

Modeling Field-Induced Quenching in Poly(p-phenylene vinylene) Polymers and Oligomers.

Field-induced fluorescence quenching of poly(p-phenylene vinylene) (PPV) oligomers due to nonradiative relaxation through free electron-hole pair (FEHP) states is modeled using singles configuration interaction computations with the intermediate neglect of differential overlap Hamiltonian. The computations find FEHP states with energies that drop linearly with applied field and undergo avoided crossings with the fluorescent state. The coupling between the FEHP and fluorescent state, computed for multiple FEHP states on a variety of oligomer lengths, is found to depend primarily on the field strength required for the state to cross the fluorescent state. The rate of decay to these dark FEHP states is then calculated from Marcus theory, which is modified to take into account dielectric in addition to other bulk measurement considerations. The results predict that individual molecules go from being emissive to fully quenched over a small range of applied field strengths. Phenomenological introduction of inhomogeneous broadening for the energies of the FEHP states leads to a more gradual dependence on applied field. The fluorescence quenching mechanism considered here is found to be important for applied fields above about 1 MV cm(-1), which is similar in magnitude to those present in light-emitting diodes.

The effects of side-chain-induced disorder on the emission spectra and quantum yields of oligothiophene nanoaggregates: a combined experimental and MD-TDDFT study.

Oligomeric thiophenes are commonly used components in organic electronics and solar cells. These molecules stack and/or aggregate readily under the processing conditions used to form thin films for these applications, significantly altering their optical and charge-transport properties. To determine how these effects depend on the substitution pattern of the thiophene main chains, nanoaggregates of three sexithiophene oligomers having different alkyl substitution patterns were formed using solvent-poisoning techniques and studied using steady-state and time-resolved emission spectroscopy. The results indicate the substantial role played by the side-chain substituents in determining the emissive properties of these species. Both the measured spectral changes and their dependence on substitution are well-modeled by combined quantum chemistry and molecular dynamics simulations. The simulations connect the side-chain-induced disorder, which determines the favorable chain-packing configurations within the aggregates, with their measured electronic spectra.

Effects of solvent properties on the spectroscopy and dynamics of alkoxy-substituted PPV oligomer aggregates.

Conjugated systems are frequently studied in their nanoaggregate form to probe the effects of solvent and of film formation on their spectral and dynamical properties. This article focuses on the emission spectra and dynamics of nanoaggregates of alkoxy-substituted PPV oligomers with the goal of interpreting the vibronic emission envelopes observed in these systems (J. Phys. Chem. C2009, 113, 18851-18862). The aggregates are formed by adding a nonsolvent such as methanol (MeOH) or water to a solution of the oligomers in a good solvent such as methyl tetrahydrofuran (MeTHF) or tetrahydrofuran (THF). The emission spectra of aggregates formed using either of these combinations exhibit a vibronic pattern in which the ratio of the intensity of highest-energy band to that of the lower energy peaks depends strongly on the ratio of good to poor solvent. In aggregates formed from MeTHF:MeOH, this was shown to be due to the presence of both aggregate-like and monomer-like emitters forming a "core" and surrounding "shell"-like structure, respectively, within a single aggregate (J. Phys. Chem. C2011, 115, 15607-15616). In support of this model, the monomer-like emission is shown here to be significantly decreased by changing the solvent pair to the more polar THF:water. This suggests that nanoaggregates formed in THF:water contain a much smaller proportion of monomer-like chains than those formed in MeTHF/MeOH, as would be expected from using a more highly polar nonsolvent. Results from bulk steady-state and time-resolved emission measurements as well as fluorescence lifetime imaging microscopy (FLIM) of the aggregates are shown to be consistent with this interpretation.

Fluorescent DNA nanotags featuring covalently attached intercalating dyes: synthesis, antibody conjugation, and intracellular imaging.

We have synthesized fluorescent DNA duplexes featuring multiple thiazole orange (TO) intercalating dyes covalently attached to the DNA via a triazole linkage. The intercalating dyes stabilize the duplex against thermal denaturation and show bright fluorescence in the green region of the spectrum. The emission color can be changed to orange or red by addition of energy-accepting Cy3 or Cy5 dyes attached covalently to the DNA duplex. The dye-modified DNA duplexes were then attached to a secondary antibody for intracellular fluorescence imaging of centrosomes in Drosophila embryos. Bright fluorescent foci were observed at the centrosomes in both the donor (TO) and acceptor (Cy5) channels, because the energy transfer efficiency is moderate. Monitoring the Cy5 emission channel significantly minimized the background signal because of the large shift in emission wavelength allowed by energy transfer.

Wavelength Dependence of the Fluorescence Quenching Efficiency of Nearby Dyes by Gold Nanoclusters and Nanoparticles: The Roles of Spectral Overlap and Particle Size.

The efficiency of the glutathione monolayer-protected gold nanocluster (NC) Au(25) (1.2 nm metal core diameter (d)) in quenching the emission of dyes intercalated into DNA is compared to that of 2 and 4 nm gold nanoparticles (NPs). In all cases, the DNA/dye moieties and the gold particles are not covalently attached but rather form non-covalent ground state complexes. Under these conditions, steady-state measurements reveal that the quenching efficiency of Au(25) is a factor of 10 lower than that of plasmonic 4 nm gold NPs but comparable to that of 2 nm particles which do not show a distinct plasmon band. Nonetheless, significant emission quenching is observed even at very low (nM) concentrations of Au(25). The quenching efficiency of the 4 nm NPs is significantly higher for dyes emitting near the wavelength of the plasmon peak whereas that of the 2 nm gold NPs is well described by the nano-surface energy transfer (NSET) model proposed by the Strouse group (J. Am. Chem. Soc. 127, 3115 2005). Interestingly, for Au(25) the maximum quenching efficiency occurs for dyes emitting in the same wavelength range as that of the 2 and 4 nm NPs (490-560 nm), where it shows no discrete absorption features, rather than for wavelengths coincident with its HOMO-LUMO, intra-band or inter-band transitions. The fluorescence quenching properties of Au(25) NCs are therefore found to be distinct from those of larger NCs and NPs but do not appear to conform to theoretical predictions advanced thus far.

Electric-field-induced fluorescence quenching in polyfluorene, ladder-type polymers, and MEH-PPV: evidence for field effects on internal conversion rates in the low concentration limit.

Electric field-induced fluorescence quenching has been measured for a series of conjugated polymers with applications in organic light-emitting diodes. Electrofluorescence measurements on isolated chains in a glassy matrix at 77 K show that the quenching efficiency for poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) is an order of magnitude larger than that for either a ladder-type polymer (MeLPPP) or polyfluorene (PFH). This effect is explained in terms of the relatively high probability of field-enhanced internal conversion deactivation in MEH-PPV relative to either MeLPPP or PFH. These data, obtained under dilute sample conditions such that chain-chain interactions are minimal, are contrasted with the much higher quenching efficiencies observed in the corresponding polymer films, and several explanations for the differences are considered. In addition, the values of the change in dipole moment and change in polarizability on excitation (|Δµ| and tr(Δα), respectively) are reported, and trends in these values as a function of molecular structure and chain length are discussed.

Electroabsorption of dimers containing MM (M = Mo, W) quadruply bonded units: insights into the electronic structure of neutral coupled redox centers and their relationship with mixed valence ions.

The electroabsorption spectra for the metal-to-ligand charge transfer transition in complexes containing oxalate and terephthalate bridged MM quadruply bonded units, [(MM)(pivalate)(3)](2)-mu(2)-BR, where M = Mo or W and BR = oxalate or terephthalate, are reported. The measured magnitude of the change in dipole moment (|Deltamu|) and the change in polarizability (Deltaalpha) that accompany this electronic transition are found to be small and not to follow the behavior expected on the basis of the two-state model. In addition, the trend in the value of Deltaalpha for the neutral states is mirrored by the trend in the degree of electronic coupling (H(AB)) for the strongly coupled mixed valence states formed by the same complexes in their singly oxidized states.

Fluorescent PNA probes as hybridization labels for biological RNA.

Fluorescent labeling of biological RNA is complicated by the narrow range of nucleoside triphosphates that can be used for biological synthesis (i.e., transcription) as well as the inability to site-specifically incorporate them into long RNA transcripts. Noncovalent strategies for labeling RNA rely on attaching fluorescent dyes to hybridization probes which deliver the dye to a specific region of the RNA through Watson-Crick base pairing. This report demonstrates the use of high-affinity peptide nucleic acid (PNA) probes in labeling mRNA transcripts with thiazole orange donor and Alexa-594 acceptor fluorophores. The PNA probes were targeted to sequences flanking splice sites in a pre-mRNA such that before splicing the PNAs were separated by >300 nucleotides (nts) whereas after splicing the separation decreased to