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Accidental contamination of substrates and polymer films by organic quantum emitters
摘要： We report the observation of ubiquitous contamination of dielectric substrates and polymethylmethacrylate matrices by organic molecules with optical activity in the visible spectral range. Contamination sites of individual solvent-related fluorophores in thin films of polymethylmethacrylate constitute fluorescence hotspots with quantum emission statistics and quantum yields approaching 30% at cryogenic temperatures. Our findings not only resolve prevalent puzzles in the assignment of spectral features to various nanoemitters on bare dielectric substrates or in polymer matrices, they also identify means for simple and cost-efficient realization of single-photon sources in the visible spectral range.
关键词： contamination of substrate and polymer matrix,organic fluorophores,single photon emitters,single molecule spectroscopy,Photoluminescence and fluorescence spectroscopy
更新于2025-11-25 10:30:42
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Redox-state dependent blinking of single photosystem I trimers at around liquid-nitrogen temperature
摘要： Efficient light harvesting in a photosynthetic antenna system is disturbed by a ragged and fluctuating energy landscape of the antenna pigments in response to the conformation dynamics of the protein. This situation is especially pronounced in Photosystem I (PSI) containing red shifted chlorophylls (red Chls) with the excitation energy much lower than the primary donor. The present study was conducted to clarify light-harvesting dynamics of PSI isolated from Synechocystis sp. PCC6803 by using single-molecule spectroscopy at liquid?nitrogen temperatures. Fluorescence emission at around 720 nm from the red Chls in single PSI trimers was monitored at 80–100 K. Intermittent variations in the emission intensities, so-called blinking, were frequently observed. Its time scale lay in several tens of seconds. The blinking amplitude depended on the redox state of the phylloquinone (A1). Electrochromic shifts of Chls induced by the negative charge on A1 were calculated based on the X-ray crystallographic structure. A Chl molecule, Chl-A839 (numbering according to PDB 5OY0), bound near A1 was found to have a large electrochromic shift. This Chl has strong exciton coupling with neighboring Chl (A838) whose site energy was predicted to be determined by interaction with an arginine residue (ArgF84) [Adolphs et al., 2010]. A possible scenario of the blinking was proposed. Conformational fluctuations of ArgF84 seesaw the excitation-energy of Chl-A838, which perturbs the branching ratio of excitation-energy between the red Chl and the cationic form of P700 as a quencher. The electrochromic shift of Chl-A839 enhances the effect of the conformation dynamics of ArgF84.
关键词： Cryogenic microscope,Phylloquinone,Fluorescence blinking,Site energy,Single-molecule spectroscopy,Electrochromic shift
更新于2025-09-23 15:23:52
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Single Nanocrystal Spectroscopy of Shortwave Infrared Emitters
摘要： Short-wave infrared (SWIR) emitters are at the center of ground-breaking applications in biomedical imaging, next-generation optoelectronic devices, and optical communications. Colloidal nanocrystals based on indium arsenide are some of the most promising SWIR emitters to date. However, the lack of single-particle spectroscopic methods accessible in the SWIR has prevented advances in both nanocrystal synthesis and fundamental characterization of emitters. Here, we demonstrate an implementation of a solution photon correlation Fourier spectroscopy (s-PCFS) experiment utilizing the SWIR sensitivity and time resolution of superconducting nanowire single-photon detectors to extract indium arsenide/cadmium selenide (InAs/CdSe) core/shell single-particle emission linewidths from colloidal nanocrystals emissive from 1.2 to 1.6 μm. We show that the average single InAs/CdSe nanocrystal ?uorescence linewidth is, remarkably, as narrow as 52 meV, similar to what has been observed in some of the most narrowband nanostructured emitters in the visible region. Additionally, the single nanocrystal ?uorescence linewidth increases with increasing shell thickness, suggesting exciton?phonon coupling as the dominant emission line-broadening mechanism in this system. The development of the SWIR s-PCFS technique has enabled measurements of spectral linewidths of colloidal SWIR-emissive NCs in solution and provides a platform to study the single NC spectral characteristics of SWIR emitters.
关键词： nanocrystals,single-molecule spectroscopy,core/shell,indium arsenide,short-wave infrared
更新于2025-09-23 15:21:21
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Fluorescence anisotropy imaging of a polydiacetylene photopolymer film
摘要： UV-illumination of phase-separated surfactant films prepared from mixtures of photopolymerizable 10,12-pentacosadiynoic acid and perfluorotetradecanoic acid results in the formation of fluorescent polydiacetylene fibers and aggregates. In this work, the orientation of polymer strands that comprise the resulting photopolymer structures has been probed using fluorescence anisotropy imaging in combination with defocused single-molecule fluorescence imaging. Imaging experiments indicate the presence of significant fiber-to-fiber heterogeneity, as well as anisotropy within each fiber (or aggregate), with both of these properties changing as a function of film preparation conditions. This anisotropy can be attributed to various alignments of the constituent polymer strands that comprise the larger fibers and aggregates. Intriguingly, when using defocused imaging, fiber images consisted of a series of discrete 'doughnut' fluorescence emission patterns, which exhibited intermittent on–off blinking behavior; both of these properties are characteristic of individual emission transition dipoles (single molecules). Further, all of the individual emission transition dipoles had a uniform orientation with respect to the axis of the fiber, indicating a common orientation of discrete emitters in the larger polymer fiber. The implications of these results for future studies of the electronic properties of conjugated polymers in larger macroscopic systems are noted.
关键词： 12-pentacosadiynoic acid,polydiacetylene,fluorescence anisotropy imaging microscopy,single-molecule spectroscopy,10,Langmuir film
更新于2025-09-19 17:15:36
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Monitoring of Nonadiabatic Effects in Individual Chromophores by Femtosecond Double-Pump Single-Molecule Spectroscopy: A Model Study
摘要： We explore, by theoretical modeling and computer simulations, how nonadiabatic couplings of excited electronic states of a polyatomic chromophore manifest themselves in single-molecule signals on femtosecond timescales. The chromophore is modeled as a system with three electronic states (the ground state and two non-adiabatically coupled excited states) and a Condon-active vibrational mode which, in turn, is coupled to a harmonic oscillator heat bath. For this system, we simulate double-pump single-molecule signals with fluorescence detection for different system-field interaction strengths, from the weak-coupling regime to the strong-coupling regime. While the signals are determined by the coherence of the electronic density matrix in the weak-coupling regime, they are determined by the populations of the electronic density matrix in the strong-coupling regime. As a consequence, the signals in the strong coupling regime allow the monitoring of nonadiabatic electronic population dynamics and are robust with respect to temporal inhomogeneity of the optical gap, while signals in the weak-coupling regime are sensitive to fluctuations of the optical gap and do not contain information on the electronic population dynamics.
关键词： nonadiabatic dynamics,strong-field regime,weak-field regime,single-molecule spectroscopy
更新于2025-09-19 17:15:36
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Spectroscopy and Dynamics of Single Molecules || Quantum dots in single molecule spectroscopy
摘要： In the early-to-mid 1990s, fluorescence from single organic dye molecules, single fluorescent proteins (FPs) and single direct-bandgap semiconductor nanoparticles (also called quantum dots, QDs) were separately observed. There are other luminescent nanomaterial species which have also sometimes been observed (and utilized) at the single particle level, such as small metal clusters, organic polymers/nanomaterials, and lanthanide/actinide ion-doped upconverting nanocrystals, some of which have also been called quantum dots (albeit somewhat inconsistently) in the literature. However, in this book chapter, the primary focus will be on direct-bandgap semiconductor nanoparticles and so we will use the term quantum dots to refer to this specific class of fluorescent nanoparticle. The key criterion for such single molecule/nanoparticle observations (beyond the equipment perspective) is the “brightness” of the fluorophore – the product of extinction coefficient (ε) and the photoluminescence quantum yield (PL QY). It is possible to overcome a low extinction coefficient by increasing the excitation power. However, the background also increases with excitation power so, at some point, the background fluorescent signal may dominate over the signal from the single molecule or particle. Organic dye molecules and fluorescent proteins usually combine a moderate extinction coefficient at their peak absorbance wavelength (λmax) with a moderate-to-high PL QY (with some common dye molecules reaching unity PL QY). QDs exhibit a higher extinction coefficient at or above their band gap energy than organic dyes and FPs do at their λmax (approx. 3–10 fold higher). In the 1990s, PL QYs of QDs were usually below 50%, but nowadays can approach unity, meaning that QDs are often significantly brighter than organic dyes and FPs under similar experimental conditions. This fact, combined with their much higher photostability, narrower emission spectra and broad excitation bands have led to QDs becoming somewhat of a competitor to organic dyes and FPs for certain applications. Of course, the advantages of QDs come with some disadvantages that still render organic dyes and FPs superior for other applications. In particular, the size of QDs, while they can be made to a size comparable to FPs, are larger than organic dyes. Moreover, QDs are rarely monovalent/monofunctional, meaning that conjugates of 1:1 label: target are much more difficult to achieve with QDs than with organic dyes or FPs. Finally, the most common QD compositions contain heavy elements (usually Cd, although some may contain Hg and Pb), and much more care must be taken in their handling and disposal. This latter disadvantage was first mitigated by “shelling” the QD with a lower toxicity material (usually ZnS) and, more recently, by replacing the toxic elements with less toxic ones, such as In, Cu and/or Ag.
关键词： quantum dots,single molecule spectroscopy,semiconductor nanoparticles,photoluminescence,fluorescence
更新于2025-09-19 17:13:59
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Dibenzo[ <i>hi</i> , <i>st</i> ]ovalene as Highly Luminescent Nanographene: Efficient Synthesis via Photochemical Cyclodehydroiodination, Optoelectronic Properties, and Single-Molecule Spectroscopy
摘要： Dibenzo[hi,st]ovalene (DBOV), as a new nanographene, has demonstrated promising optical properties, such as red emission with a high fluorescence quantum yield of 79% and stimulated emission, as well as high thermal stability and photostability, which indicated its promise as a light-emitting and optical gain material. However, the previous synthetic routes required at least 12 steps. This obstructed access to different derivatives, e.g., to obtain crystals suitable for X-ray diffraction analysis and to tune the optoelectronic properties. Here, we report an efficient synthetic pathway to DBOV based on a sequential iodination-benzannulation of bi(naphthylphenyl)diyne, followed by photochemical cyclodehydroiodination (PCDHI). This protocol included a fused bischrysene as a key intermediate and furnished scalable amounts of meso-substituted DBOV derivatives with different substituents. DBOV with 2,6-dimethylphenyl groups could be used for single-crystal X-ray analysis, revealing the precise structure of the DBOV core. The optoelectronic properties of the DBOV derivatives were investigated by UV?vis absorption and fluorescence spectroscopy, cyclic voltammetry, and density functional theory calculations. Single-molecule spectroscopy at room and low temperatures provided novel insights into the photophysics of DBOV embedded in a polymer film. As a result of weak coupling of the optical transitions to the matrix, single-molecule emission spectra at 4.5 K showed narrow vibronic lines. The fluorescence autocorrelation function covering 9 orders of magnitude in time displayed high contrast photon antibunching and bunching, from which the fluorescence decay rate and the triplet population and depopulation rates could be retrieved. Remarkably, the intersystem crossing rate into the triplet state decreased by more than an order of magnitude at low temperature, demonstrating that temperature can be a crucial parameter to boost single photon emission of an aromatic hydrocarbon.
关键词： fluorescence quantum yield,nanographene,single-molecule spectroscopy,optoelectronic properties,photochemical cyclodehydroiodination
更新于2025-09-16 10:30:52
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Matrix-induced Linear Stark Effect of Single Dibenzoterrylene Molecules in 2,3-Dibromonaphthalene Crystal
摘要： Absorption and fluorescence from single molecules can be tuned by applying an external electric field – a phenomenon known as the Stark effect. A linear Stark effect is associated to a lack of centrosymmetry of the guest in the host matrix. Centrosymmetric guests can display a linear Stark effect in disordered matrixes, but the response of individual guest molecules is often relatively weak and non-uniform, with a broad distribution of the Stark coefficients. Here we introduce a novel single-molecule host-guest system, dibenzoterrylene (DBT) in 2,3-dibromonaphthalene (DBN) crystal. Fluorescent DBT molecules show excellent spectral stability with a large linear Stark effect, of the order of 1.5 GHz/(kVcm -1), corresponding to an electric dipole moment change of around 2 D. Remarkably, when the electric field is aligned with the a crystal axis, nearly all DBT molecules show either positive or negative Stark shifts with similar absolute values. These results are consistent with quantum chemistry calculations. Those indicate that DBT substitutes three DBN molecules along the a axis, giving rise to eight equivalent embedding sites, related by the three glide planes of the orthorhombic crystal. The static dipole moment of DBT molecules is created by host-induced breaking of the inversion symmetry. This new host-guest system is promising for applications that require a high sensitivity of fluorescent emitters to electric fields, for example to probe weak electric fields.
关键词： 2,3-dibromonaphthalene,linear Stark effect,single-molecule spectroscopy,symmetry breaking,DFT,dibenzoterrylene,insertion sites,induced dipole moment
更新于2025-09-10 09:29:36