Suppression of blinking in single CsPbBr3 perovskite nanocrystals through surface ligand exchange.

Photoluminescence blinking in individual semiconducting and perovskite quantum dots reflects reduced emission quantum yield and represents an obstacle towards quantum dot applications. One of the origins of blinking is the presence of surface structural defects that can function as charge traps. To reduce the defects the surface can be modified by, e.g., covering with ligands that are more strongly bound to the surface. Here, we report exchange of ligands on the CsPbBr3 perovskite nanocrystal surface and the effect of the exchange on photoluminescence blinking. Replacement of the oleic acid and oleylamine ligands which are used in the synthesis process with quaternary amine ligands leads to substantial increase of photoluminescence quantum yield. On single particle level this is reflected by significantly improved blinking characteristics. Statistical analysis using the probability density function shows that the ligand exchange leads to longer duration of ON-times and shorter OFF-times, as well as to the presence of a higher fraction of ON-time intervals. These characteristics are not affected by sample aging within three weeks. On the contrary, storage of the samples in solution for one-to-two weeks leads to further improvement of the ON-time interval fraction statistics.

Air-stable mixed cation lead halide perovskite films and microscopic study of their degradation process.

We report the preparation and nanoscale photophysical characterization of mixed cation perovskite films of the composition MA1-xFAxPbI3, with x = 0, 0.3 and 0.5. Films with x = 0.5 and 0.3 prepared in air using ethyl acetate as an antisolvent in a one-step spin-coating process are compositionally stable in ambient air for more than a year, in contrast to films prepared using a chlorobenzene antisolvent. The onset of degradation of the films near the film edges was monitored using in situ photoluminescence (PL) spectroscopy. The PL spectra of the degradation products are consistent with the PL spectra of 2D perovskite sheets of varying thicknesses. Morphologically, aging of the films brings about coalescing of the film grain structure into larger crystal grains. Furthermore, monitoring of the time traces of PL from individual nanoscale locations in the films (PL blinking) reveals that aging of the films does not change the extent of dynamic PL quenching or affect the observed long-range charge diffusion on the order of micrometers.

Thermally-assisted photosensitized emission in a trivalent terbium complex.

Luminescent lanthanide complexes containing effective photosensitizers are promising materials for use in displays and sensors. The photosensitizer design strategy has been studied for developing the lanthanide-based luminophores. Herein, we demonstrate a photosensitizer design using dinuclear luminescent lanthanide complex, which exhibits thermally-assisted photosensitized emission. The lanthanide complex comprised Tb(III) ions, six tetramethylheptanedionates, and phosphine oxide bridge containing a phenanthrene frameworks. The phenanthrene ligand and Tb(III) ions are the energy donor (photosensitizer) and acceptor (emission center) parts, respectively. The energy-donating level of the ligand (lowest excited triplet (T1) level = 19,850 cm-1) is lower than the emitting level of the Tb(III) ion (5D4 level = 20,500 cm-1). The long-lived T1 state of the energy-donating ligands promoted an efficient thermally-assisted photosensitized emission of the Tb(III) acceptor (5D4 level), resulting in a pure-green colored emission with a high photosensitized emission quantum yield (73%).

Simultaneous Force and Fluorescence Spectroscopy on Single Chains of Polyfluorene: Effect of Intra-Chain Aggregate Coupling.

Conjugated polymer chains in compact conformations or in films exhibit spectral features that can be attributed to interactions between individual conjugated segments of the chain, including formation of aggregates or excimers. Here, we use atomic force microscopy (AFM) on single chains of the conjugated polymer polyfluorene (PFO) to control the intersegment interactions by mechanically unfolding the chain. Simultaneously with the force spectroscopy we monitor fluorescence from the single PFO chains using a fluorescence microscope. We found that mechanical stretching of the chain causes disappearance of the green emission band. This observation provides evidence that the green emission originates from an intrachain aggregated state on the self-folded chain, which is decoupled by the stretching. In addition, the stretching upon laser irradiation leads to the appearance of additional features in the force spectra, small force peaks in the initial stages of the unfolding. These features are attributed to a combination of excitonic and van der Waals coupling of a ground-state intrachain aggregate.

Dynamic molecular conformational change leading to energy transfer in F8-5% BSP copolymer revealed by single-molecule spectroscopy.

Polyfluorene-based copolymers such as poly(9,9-dioctylfluorene)-alt-5% [bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine] (F8-5% BSP) are efficient blue-emitting polymers with various electronic phases: F8 blue-emitting glassy phase, F8 ordered more red-emitting ß-phase, and F8/BSP charge transfer (CT) state. Polymer light-emitting device performance and color purity can be significantly improved by forming ß-phase segments. However, the role of the ß-phase on energy transfer (ET) among glassy F8, ß-phase, and F8/BSP CT state is unclear. Herein, we identify dynamic molecular conformation-controlled ET from locally excited states to either the CT state or ß-phase in light-emitting copolymers. By conducting single-molecule spectroscopy for single F8-5% BSP chains, we find inefficient intra-chain ET from glassy segments to the CT state, while efficient ET from the glassy to the ß-phase. Spontaneous and reversible CT on-off emission is observed both in the presence and absence of the ß-phase. The density functional theory calculations reveal the origin of the on-chain CT state and indicate this CT emission on-off switching behavior could be related to molecule torsional motion between BSP and F8 units. The population of the CT state by ET can be increased via through-space interaction between the F8 block and the BSP unit on a self-folded chain. Temperature-dependent single-molecule spectroscopy confirms such interaction showing a gradual increase in intensity of the CT emission with the temperature. Based on these observations, we propose the dynamic molecular motion-induced conformation change as the origin of the glassy-to-CT ET, and thermal energy may provide the activation for such a change to enhance the ET from glassy or ß-phases to the CT state.

Enhancement of the Photocurrent of a Single Photosystem I Complex by the Localized Plasmon of a Gold Nanorod.

A combination of conductive atomic force microscopy (AFM) and confocal fluorescence microscopy was used to measure photocurrents passing through single trimeric photosytem I (PSI) complexes located in the vicinity of single gold nanorods (AuNRs). Simultaneous excitation of PSI and of the AuNR longitudinal plasmon mode and detection of photocurrents from individual PSI in relation to the position of single AuNRs enable insight into plasmon-induced phenomena that are otherwise inaccessible in ensemble experiments. We have observed photocurrent enhancement by the localized plasmons by a factor of 2.9 on average, with maximum enhancement values of up to 8. Selective excitation of the longitudinal plasmon modes by the polarization of the excitation laser enables controllable switch-on of the photocurrent enhancement. The dependence of the extent of enhancement on the distance between PSI and AuNRs indicates that, apart from the enhancement of absorption, there is an additional enhancement mechanism affecting directly the electron transport process. The present study provides deeper insight into the molecular mechanisms of plasmon-enhanced photocurrents, not only in PSI but also potentially in other systems as well.

Dynamic Polymer Free Volume Monitored by Single-Molecule Spectroscopy of a Dual Fluorescent Flapping Dopant.

Single-molecule spectroscopy (SMS) of a dual fluorescent flapping molecular probe (N-FLAP) enabled real-time nanoscale monitoring of local free volume dynamics in polystyrenes. The SMS study was realized by structural improvement of a previously reported flapping molecule by nitrogen substitution, leading to increased brightness (22 times) of the probe. In a polystyrene thin film at the temperature of 5 K above the glass transition, the spectra of a single N-FLAP molecule undergo frequent jumps between short- and long-wavelength forms, the latter one indicating planarization of the molecule in the excited state. The observed spectral jumps were statistically analyzed to reveal the dynamics of the molecular environment. The analysis together with MD and QM/MM calculations show that the excited-state planarization of the flapping probe occurs only when sufficiently large polymer free volume of more than, at least, 280 Å3 is available close to the molecule, and that such free volume lasts for an average of 1.2 s.

Real-Time Monitoring of Formation and Dynamics of Intra- and Interchain Phases in Single Molecules of Polyfluorene.

Poly(9,9-dioctylfluorene) (PFO) is one of the most important conjugated polymer materials, exhibiting outstanding photophysical and electrical properties. PFO is also known for a diversity of morphological phases determined by conformational states of the main chain. Our goal in this work is to address some of the key questions on formation and dynamics of one such conformation, the ß-phase, by following in real time the evolution of fluorescence spectra of single PFO chains. The PFO is dispersed in a thin polystyrene film, and the spectra are monitored during the process of solvent vapor annealing with toluene. We confirm unambiguously that the PFO ß-phase segments are formed on a true single-chain level at room temperature in the solvent-softened polystyrene. We further find that the formation of the ß-phase is a dynamic and reversible process occurring on the order of seconds, leading to repeated spontaneous transitions between the glassy and ß-phase segments during the annealing. Comparison of PFO with two largely different molecular weights (Mw) shows that chains with lower Mw form the ß-phase segments much faster. For the high Mw PFO chains, a detailed Franck-Condon analysis of the ß-phase spectra shows a large distribution of the Huang-Rhys factor, S, and even dynamic changes of this factor occurring on a single chain. Such dynamics are likely a manifestation of changing coherence length of the exciton. Further, for the high Mw PFO chains we observe an additional conformational state, a crystalline γ-phase. The γ-phase formation is also a spontaneous reversible process in the solvent-softened matrix. The phase can form from both the ß-phase and the glassy phase, and the formation requires high Mw to enable intersegment interactions in a self-folded chain.

Toward accurate measurement of the intrinsic quantum yield of lanthanide complexes with back energy transfer.

Trivalent lanthanide complexes are an important class of luminescent material characterized by their strong absorption of light by the organic ligands and subsequent energy transfer to the lanthanide ion, realizing intense luminescence from the ion. With this mechanism of luminescence, the total quantum yield of a lanthanide complex is the product of the energy transfer efficiency from the ligand to the lanthanide ion and the "intrinsic" quantum yield of the lanthanide ion itself. The "absolute" method in measuring the quantum yield uses an integrating sphere, and this method can be used for measuring both the total and the intrinsic quantum yields. The presence of back energy transfer (the reverse process of energy transfer) adds complication to this by affecting both the dynamics of the excited state of the ligands and the lanthanide ion. Herein, we theoretically derive an equation that shows that in the presence of back energy transfer the intrinsic quantum yield may differ depending on whether it is determined from the measurement through excitation of the ligands or the lanthanide directly. The value measured by direct lanthanide excitation could decrease to 20% or less of the actual value when back energy transfer is prominent. Several previously reported Tb(iii) complexes are within the range to be cautious. This report shows that the "absolute" method for measuring the lanthanide ion-centered quantum yield may not be suitable in the presence of back energy transfer by principle. We also provide a possible workaround in the case that several approximations and assumptions can be made.

Perfluorophenyl-Directed Giant Porphyrin J-Aggregates.

Quadrupolar interactions of porphyrin bearing two pentafluorophenylethynyl terminals (1) drove the formation of a successive one-dimensional staircase structure, i.e., J-aggregates, to yield millimeter-length needles with a single-crystalline character in methylcyclohexane solution. In contrast, π-stacked interactions of porphyrin bearing two nonfluorinated phenyl terminals (2) formed no aggregates in solution. A spin-cast film of 1 also showed bathochromic shift of the Soret and Q bands, indicating the formation of J-aggregates. The molecular arrangement of the J-aggregates was revealed by microbeam glazing-incidence wide-angle X-ray diffraction (GIWAXD), and was in good agreement with the optimized structure generated by density functional theory (DFT) calculations.

Fully Conjugated Porphyrin Glass: Collective Light-Harvesting Antenna for Near-Infrared Fluorescence beyond 1 µm.

Expanded π-systems with a narrow highest occupied molecular orbital-lowest unoccupied molecular orbital band gap encounter deactivation of excitons due to the "energy gap law" and undesired aggregation. This dilemma generally thwarts the near-infrared (NIR) luminescence of organic π-systems. A sophisticated cofacially stacked π-system is known to involve exponentially tailed disorder, which displays exceptionally red-shifted fluorescence even as only a marginal emission component. Enhancement of the tail-state fluorescence might be advantageous to achieve NIR photoluminescence with an expected collective light-harvesting antenna effect as follows: (i) efficient light-harvesting capacity due to intense electronic absorption, (ii) a long-distance exciton migration into the tail state based on a high spatial density of the chromophore site, and (iii) substantial transmission of NIR emission to circumvent the inner filter effect. Suppression of aggregation-induced quenching of fluorescence could realize collective light-harvesting antenna for NIR-luminescence materials. This study discloses an enhanced tail-state NIR fluorescence of a self-standing porphyrin film at 1138 nm with a moderate quantum efficiency based on a fully π-conjugated porphyrin that adopts an amorphous form, called "porphyrin glass".

Luminescent Mechanochromic 9-Anthryl Gold(I) Isocyanide Complex with an Emission Maximum at 900 nm after Mechanical Stimulation.

Upon mechanical stimulation, 9-anthryl gold(I) isocyanide complex 3 exhibited a bathochromic shift of its emission color from the visible to the infrared (IR) region, which is unprecedented in its magnitude. Prior to exposure to the mechanical stimulus, the polymorphs 3α and 3ß exhibit emission wavelength maxima (λem,max) at 448 and 710 nm, respectively. Upon grinding, the λem,max of 3αground and 3ßground are bathochromically shifted to 900 nm, i.e., Δλem,max (3α) = 452 nm or 1.39 eV. Polymorphs 3α and 3ß thus represent the first examples of mechanochromic luminescent materials with λem,max in the IR region.

Development of Ion-Conductive and Vapoluminescent Porous Coordination Polymers Composed of Ruthenium(II) Metalloligand.

We synthesized two new porous coordination polymers (PCPs) {Ln7(OH)5[Ru(dcbpy)3]4·4nH2O} (Ln7-Ru4; Ln = Ce, Nd) composed of the luminescent ruthenium(II) metalloligand [Ru(4,4'-dcbpy)3]4- ([4Ru]; 4,4'-dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) and lanthanide ions Ln3+ (Ln = Ce, Nd). These two PCPs Ln7-Ru4 are isomorphous with the previously reported PCP La7-Ru4, and the lattice constants (a, c, and unit cell volume V) changed systematically according to the lanthanide contraction. All three Ln7-Ru4 compounds have OH- anion containing porous structures and a large number of hydrate water molecules within the pores, resulting in moderate ion conductivities (10-6-10-7 S cm-1) at 90% relative humidity (RH) and 298 K. In contrast, the structural transformation of Ln7-Ru4 associated with water-vapor adsorption/desorption strongly depends on the lanthanide ion; the Ln7-Ru4 compounds with larger Ln3+ ions recover the original porous structure at lower relative humidities (RH). A similar trend was observed for the ion conduction activation energy, suggesting that the bridging Ln3+ ion plays an important role in the formation of the ion-conductive pathways. La7-Ru4 and Ce7-Ru4 exhibit vapochromic luminescence associated with water vapor adsorption/desorption, arising from the 3MLCT emission of [4Ru]. This vapochromic behavior is also affected by the replacement of the Ln3+ ion; the vapochromic shift of Ce7-Ru4 was observed at RH values (near 100% RH) higher than that of La7-Ru4. 3MLCT emissions of the [4Ru] metalloligand in Nd7-Ru4 were barely observable in the visible region, but sharp emission bands characteristic of 4f-4f transitions of the Nd3+ ion were observed in the near-infrared (NIR) region (arising from the 1MLCT transition of [4Ru]), suggesting the transfer of energy from the [4Ru] 3MLCT excited state to the 4f-4f transition state of the Nd3+ ions.

Critical Role of Energy Transfer Between Terbium Ions for Suppression of Back Energy Transfer in Nonanuclear Terbium Clusters.

Lanthanide (Ln(III)) complexes form an important class of highly efficient luminescent materials showing characteristic line emission after efficient light absorption by the surrounding ligands. The efficiency is however lowered by back energy transfer from Ln(III) ion to the ligands, especially at higher temperatures. Here we report a new strategy to reduce back energy transfer losses. Nonanuclear lanthanide clusters containing terbium and gadolinium ions, TbnGd9-n clusters ([TbnGd9-n(µ-OH)10(butylsalicylate)16]+NO3-, n = 0, 1, 2, 5, 8, 9), were synthesized to investigate the effect of energy transfer between Tb(III) ions on back energy transfer. The photophysical properties of TbnGd9-n clusters were studied by steady-state and time-resolved spectroscopic techniques and revealed a longer emission lifetime with increasing number of Tb(III) ions in TbnGd9-n clusters. A kinetic analysis of temperature dependence of the emission lifetime show that the energy transfer between Tb(III) ions competes with back energy transfer. The experimental results are in agreement with a theoretical rate equation model that confirms the role of energy transfer between Tb(III) ions in reducing back energy transfer losses. The results provide a new strategy in molecular design for improving the luminescence efficiency in lanthanide complexes which is important for potential applications as luminescent materials.

Effective photosensitized energy transfer of nonanuclear terbium clusters using methyl salicylate derivatives.

The photophysical properties of the novel nonanuclear Tb(III) clusters Tb-L1 and Tb-L2 involving the ligands methyl 4-methylsalicylate (L1) and methyl 5-methylsalicylate (L2) are reported. The position of the methyl group has an effect on their photophysical properties. The prepared nonanuclear Tb(III) clusters were identified by fast atom bombardment mass spectrometry and powder X-ray diffraction. Characteristic photophysical properties, including photoluminescence spectra, emission lifetimes, and emission quantum yields, were determined. The emission quantum yield of Tb-L1 (Φ(ππ*) = 31%) was found to be 13 times larger than that of Tb-L2 (Φ(ππ*) = 2.4%). The photophysical characterization and DFT calculations reveal the effect of the methyl group on the electronic structure of methylsalicylate ligand. In this study, the photophysical properties of the nonanuclear Tb(III) clusters are discussed in relation to the methyl group on the aromatic ring of the methylsalicylate ligand.