Development of excitation power-responsive anti-stokes emission wavelength switching and their energy saving induced by localized surface plasmon resonance.

We designed an external stimulus-responsive anti-Stokes emission switching using dual-annihilator-based triplet-triplet annihilation upconversion systems. This system, which was constructed by incorporating a palladium porphyrin derivative as a sensitizer and 9,10-diphenylanthracene (DPA) and 9,10-bis(triisopropylsilyl)ethynylanthracene (TIPS) as annihilators into polymer thin films, produced TIPS- and DPA-based anti-Stokes emission under low and high excitation powers, respectively. The mechanism involves the following: under low excitation power, triplet energy transfer from triplet-excited PdOEP to DPA is induced, followed by relay to TIPS. This results in the generation of triplet-excited TIPS, and the subsequent triplet-triplet annihilation between them produces TIPS-based anti-Stokes emission. Conversely, under high excitation power, the high-density triplet-excited DPA, generated through triplet energy transfer from PdOEP, undergoes triplet-triplet annihilation among themselves, resulting in the generation of DPA-based anti-Stokes emission. Additionally, we achieved energy savings by reducing the required excitation power for switching through the utilization of plasmonic metal nanoparticles. The strong local electromagnetic fields associated with the localized surface plasmon resonance of metal nanoparticles enhance the photoexcitation efficiency of PdOEP, subsequently increasing the density of triplet-excited DPA. As a result, anti-Stokes emission switching becomes feasible at lower excitation powers.

Optimizing the Distance between Upconversion Thin Films and Silver Nanoprisms for the Design of a High-Performance Plasmonic Triplet-Triplet Annihilation Upconversion System.

While the distance dependence of metal-enhanced fluorescence has been extensively studied for composite systems comprising fluorophores and metal nanoparticles, the corresponding distance dependence of triplet-triplet annihilation upconversion (TTA-UC) systems remains unexplored. Herein, we investigated the influence of the spatial distance between Ag nanoprisms (AgPRs) and TTA-UC thin films consisting of a palladium octaethylporphyrin (PdOEP) sensitizer and a 9,10-diphenylanthracene (DPA) emitter, aiming at enhancing the upconverted (UC) emission as efficiently as possible. Results indicated that the optimal distance for the examined system was significantly longer (12.6 nm) than those of typical metal-enhanced fluorescence systems (about 2 nm). We demonstrated that the UC emission enhancement factor can be expressed as a product including factors of the PdOEP photoexcitation rate, triplet-triplet energy transfer (TTET) efficiency from PdOEP to DPA, triplet excited DPA lifetime, and fluorescence efficiency of singlet excited DPA. We discovered that the AgPRs play a beneficial role in enhancing the PdOEP photoexcitation, whereas they exert detrimental effects on the other three factors. Among these three factors, quenching contributions by the decrease of the triplet excited DPA lifetime and DPA fluorescence efficiency were significant, making these the primary and secondary factors, respectively, for the UC emission quenching, particularly at short distances. These results demonstrate that the characteristic distance dependence of the UC emission enhancement is determined by the competing effects of beneficial PdOEP photoexcitation enhancement and the detrimental localized surface plasmon (and/or AgPR)-induced nonradiative decay of the triplet- and singlet excited DPA molecules. The findings offer valuable guidelines for the design of high-performance plasmonic TTA-UC systems.

Determination Method of Diffusion Coefficient in a Neat Redox-Active Ionic Liquid at a Microdisk Electrode in the Domains Ranging from the Steady-State to Potentiodynamic Near-Steady-State.

In redox-active ionic liquids (RAILs), either or both of the constituent ions are redox-active. Because of the high concentration of the ions, RAILs exhibit not only ion conduction but also electron conduction through the bimolecular electron self-exchange reaction. Because neat RAILs do not contain any supporting electrolyte, migration of the redox active ions results in enhancement or diminishment of the redox current at an electrode. To treat the migration effect for electrochemical analysis, a limiting current correction was theoretically derived by Oldham, Hyk, and Stojek (Oldham, K. J. Electroanal. Chem. 1992, 337, 91-126; Hyk, W.; Stojek, Z. Anal. Chem. 2002, 747, 4805-4813) for the steady-state voltammetry. Although steady-state voltammetry is a robust method in electrochemistry, the actual measurement is time-consuming and cannot be always made because of the instability of the electrochemical system. To overcome the problem, we propose the use of cyclic voltammetry to evaluate the diffusion coefficient of the redox-active ion that constitutes RAIL. The peak currents were analyzed by the purely diffusional framework of the Aoki-Matsuda-Osteryoung equation (Aoki, K.; Akimoto, K.; Tokuda, K.; Matsuda, H.; Osteryoung, J. J. Electroanal. Chem. 1984, 171, 219-230.) in the range from several mV s-1 to several ten mV s-1, and the migration correction to the near-steady-state limiting current was applied on the basis of the Oldham-Hyk-Stojek theory to scale the diffusion coefficient. As an example of RAILs, [FcC6ImC1][TFSI], which exhibits charge increase reaction with the same sign (S+ - e- ⇌ P2+), was used and the cyclic voltammograms were recorded at various sizes of the microdisk electrodes and various scan rates. The peak currents obeyed the Aoki-Matsuda-Osteryoung equation with the scaled diffusion coefficient, which has the same value as determined by the steady-state voltammogram. Our approach can be used to evaluate the diffusion coefficient of redox-active ions that constitute the RAIL with the charge increase reaction with the same sign.

Photothermal therapeutic ability of copper open-shell nanostructures that are effective in the second biological transparency window based on symmetry breaking-induced plasmonic properties.

In this study, a photothermal therapy agent that works efficiently in the second biological transparency window was developed based on the localized surface plasmon (LSP) resonance of symmetry-broken open-shell nanostructures of low-cost Cu (CuOSNs). The strong LSP resonance and superior photothermal conversion ability in the second biological transparency window were achieved by generating the dipolar bonding mode due to the plasmon hybridization between the nanoshell dipole and the nanohole dipole at the opening edge in CuOSNs derived from the symmetry breaking of a Cu nanoshell. Oxidative dissolution of CuOSNs in water was significantly suppressed by successive coating with the self-assembled monolayer of 16-mercaptohexadecanoic acid and a thin silica layer. Furthermore, the stability in phosphate buffered saline, which models the biological environment, was attained by further coating the nanoparticles with polyethylene glycol. It was demonstrated from in vitro cell tests using HeLa cells that the cytotoxicity of CuOSNs was effectively suppressed by the surface protection. The viability of HeLa cells incubated with CuOSNs was decreased under the irradiation of low intensity 1060 nm laser with increasing number of CuOSNs. These results demonstrate that low-cost symmetry-broken Cu-based nanostructures can act as an excellent photothermal therapy agent in the second biological transparency window.

Plasmonic Metal Nanostructures Meet Triplet-Triplet Annihilation-Based Photon Upconversion Systems: Performance Improvements and Application Trends.

Improving the performance of upconversion systems based on triplet-triplet annihilation (TTA-UC) can have far-reaching implications for various fields, including solar devices, nano-bioimaging, and nanotherapy. This review focuses on the use of localized surface plasmon (LSP) resonance of metal nanostructures to enhance the performance of TTA-UC systems and explores their potential applications. After introducing the basic driving mechanism of TTA-UC and typical sensitizers used in these systems, we discuss recent studies that have utilized new sensitizers with distinct characteristics. Furthermore, we confirm that the enhancement in upconverted emission can be explained, at least in part, by the mechanism of "metal-enhanced fluorescence", which is attributed to LSP resonance-induced fluorescence enhancement. Next, we describe selected experiments that demonstrate the enhancement in upconverted emission in plasmonic TTA-UC systems, as well as the emerging trends in their application. We present specific examples of studies in which the enhancement in upconverted emission has significantly improved the performance of photocatalysts under both sunlight and indoor lighting. Additionally, we discuss the potential for future developments in plasmonic TTA-UC systems.

Modulation Technique of Localized Surface Plasmon Resonance of Palladium Nanospheres by Coating with Titanium Dioxide Shell for Application to Photothermal Therapy Agent.

Although plasmonic palladium (Pd) nanospheres are thermodynamically stable and have high photothermal conversion due to the free and bound electron coupling associated with the intrinsic high interband transition, they have not attracted attention as a photothermal conversion material for next-generation photothermal cancer therapy. This is because the Pd nanospheres generate the localized surface plasmon resonance (LSPR) intrinsically in the ultraviolet region, which is far away from the biological transparent window (750-900 nm). In this study, we controlled the LSP wavelength of Pd nanospheres by coating with high refractive index TiO2 shells taking advantage of the Pd LSPR which is highly sensitive to changes in the local refractive index around the nanospheres. Our calculations indicated that the absorption cross section at 808 nm (corresponding to the wavelength used for photothermal treatment) was increased by 4.5 times by redshifting the LSPR and increasing the extinction intensity associated with the coating with TiO2 shell. Experiments confirmed the theoretical prediction in that the LSPR of the synthesized Pd nanospheres with a diameter of 81 nm was significantly redshifted by coating with amorphous TiO2 shell, resulting in significant large extinction intensity at 808 nm. The photothermal conversion efficiency was estimated to be 50%. In vitro cell tests, HeLa cells incubated with 100-300 µg/mL TiO2-coated Pd nanospheres were efficiently killed by irradiating 808 nm laser (1.8 W) even though the nanospheres with the same concentrations showed little cytotoxicity. These results indicate that the Pd nanospheres coated with high refractive index shells can be promising as a photothermal therapy agent.

Performance Improvement of Triplet-Triplet Annihilation-Based Upconversion Solid Films through Plasmon-Induced Backward Scattering of Periodic Arrays of Ag and Al.

The performance improvement of solid-state triplet-triplet annihilation-based photon upconversion (TTA-UC) systems is required for the application to various solar devices. The performance can be improved by making use of the local strong electric field generated through the excitation of localized surface plasmon (LSP) resonance of metal nanostructures. However, since the improvement is effective only within the limited nanospace around nanoparticles (i.e., the near-field effect), a methodology for improving the performance over a wider spatial region is desirable. In this study, a significant improvement in the threshold light excitation intensity (Ith) (77% decrease) as the figure of merit and the upconverted emission intensity (6.3 times enhancement) in a solid-state TTA-UC film with a thickness of 3 µm was achieved by stacking the film with periodic Ag half-shell arrays. The highest-enhanced upconverted emission was obtained by tuning the diffuse reflectance peak, which results from the excitation of LSP resonance of the Ag half-shell arrays, to overlap well with the photoexcitation peak of the sensitizer in the TTA-UC film. The intensity of the enhanced upconverted emission was independent of the distance between the lower edge of the TTA-UC film and the surface of half-shell arrays in the nanometer order. These results suggest that the performance improvement was attributed to the photoexcitation enhancement of the sensitizer by elongating the excitation light path length inside the TTA-UC film, which was achieved through a strong backward scattering of the incident light based on the LSP resonance excitation (i.e., the far-field effect). In addition, the upconverted emission was improved using half-shell arrays comprising low-cost Al, although the enhancement factor was 3.5, which was lower than that of Ag half-shell arrays. The lower enhancement may be attributed to a decrease in the backward scattering of the excitation light owing to the intrinsic strong interband transition of Al at long visible wavelengths.

A redox-active ionic liquid manifesting charge-transfer interaction between a viologen and carbazole and its effect on the viscosity, ionic conductivity, and redox process of the viologen.

Redox-active ionic liquids (RAILs) are gaining attention as a material that can create a wide range of functions. We herein propose a charge-transfer (CT) RAIL by mixing two RAILs, specifically a carbazole-based ionic liquid ([CzC4ImC1][TFSI]) as a donor and a viologen-based ionic liquid ([C4VC7][TFSI]2) as an acceptor. We investigated the effect of CT interaction on the physicochemical properties of the CT ionic liquid (CT-IL) using the results of temperature-dependent measurements of UV-vis absorption, viscosity, and ionic conductivity as well as cyclic voltammograms. We employed the Walden analysis and the Grunberg-Nissan model to elucidate the effect of the CT interaction on the viscosity and ionic conductivity. The CT interaction reduces the viscosity by reducing the electrostatic attraction between the dicationic viologen and TFSI anion. It also reduces the ionic conductivity by the CT association of the dicationic viologen and carbazole. The electrochemically reversible responses of the viologens in [C4VC7][TFSI]2 and CT-IL are consistent with the Nernstian and the interacting two-redox site models. Notably, the transport and electrochemical properties are modulated by CT interaction, leading to unique features that are not present in individual component ILs. The inclusion of CT interaction in RAILs thus provides a powerful means to expand the scope of functionalized ionic liquids.

Redox of Viologen for Powering and Coloring.

Viologen is among the most attractive and easiest-to-use organic redox active group in many functional molecular assemblies. It plays crucial roles as an electron transfer mediator in the artificial photo-energy conversion systems and electron-transfer protein assemblies and as a building block of supramolecules. Its features include electrochemically reversible redox activity and stability. Strong blue color and tendency to dimerization of the one-electron reduced form, viologen mono-radical mono-cation, are remarkable. In this Account, we describe the use of viologen to give a powered movement of small molecules and motion of millimetre-sized macroscopic soft-matters and the use of viologen ionic liquid as electrochromic materials. Attractivities of the use of viologen units for powering and coloring are demonstrated and discussed. In particular, we highlight driving of mechanical movements by π-π stacking dimerization, incorporation in a hydrogel to attain highly deformable material, induction of 2D phase transformation, and sharp color change of very thin ionic liquid layer in a compartment-less electrochromic display.

Combined Use of Anisotropic Silver Nanoprisms with Different Aspect Ratios for Multi-Mode Plasmon-Exciton Coupling.

Absorption enhancement based on interaction between the localized surface plasmon (LSP) and molecular exciton is one of the most important phenomena for the development of high-performance solar devices. In this study, hybrids of plasmonic metal nanoparticles and dye molecules have been developed, which exhibit enhanced absorption at precisely tuned wavelengths in a visible region. The hybrids consist of a porphyrin derivative, which has four absorption peaks (Q-bands) in a range of 500-700 nm, and triangular silver nanoprisms (AgPRs), which are developed by us to exhibit precisely tuned LSP resonance wavelengths. Absorption enhancement over the whole Q-band range is induced by the combined use of three kinds of AgPRs of different aspect ratios. Furthermore, the quantitative evaluation of absorption enhancement based on the LSP-based fluorescence enhancement phenomenon has demonstrated that efficient absorption enhancement can be effected at multiple wavelengths.

Plasmonic Silver Nanoprism-Induced Emissive Mode Control between Fluorescence and Phosphorescence of a Phosphorescent Palladium Porphyrin Derivative.

We have succeeded in significantly enhancing fluorescence from intrinsically phosphorescent palladium octaethylporphyrin (Pd-porphyrin) that has an intersystem crossing efficiency of ∼1 by using silver nanoprisms (AgPRs). This was achieved by controlling the wavelength of the localized surface plasmon (LSP) resonance of AgPRs and the distance between the Pd-porphyrin molecules and the AgPR surfaces. In addition to enhancing phosphorescence by spectrally overlapping the phosphorescence band with the LSP resonance band, tuning the LSP wavelength to approximately 520 nm led to the appearance of a new emission band around the wavelength corresponding to the fluorescent radiation. The appearance of fluorescence suggests that the nonradiative energy transfer from the singlet excited state of Pd-porphyrin to the LSP of AgPRs overcame the ultrafast intramolecular intersystem crossing to the triplet excited state, manifesting the spectral properties of the singlet excited state of Pd-porphyrin. The fluorescence nature of this radiation was strongly supported by lifetime measurements of the hybrids of Pd-porphyrin and AgPRs. Furthermore, the dependence of the emissive intensities on the distance between the Pd-porphyrin molecules and the AgPR surfaces showed interesting opposite trends. The fluorescence intensity was increased as the distance between the molecules and the AgPRs was decreased from 10.5 to 1 nm, while the phosphorescence intensity was decreased, which indicates that the LSP-induced fluorescence radiation process from Pd-porphyrin near the AgPRs outweighed the quenching by the AgPRs, even though the phosphorescence significantly suffered quenching.

Electrochromism of Ferrocene- and Viologen-Based Redox-Active Ionic Liquids Composite.

Redox-active ionic liquids (RAILs) require no other additional reagents such as solvent and supporting electrolyte for electrochemical reactions under undiluted condition. Viologen-based RAILs are one of the electrochromic (EC) ionic liquids with sharp color contrast and high chemical stability. An operation of an EC cell requires two electroactive elements, an EC material and a charge compensating material. In this study, an equimolar composite of a viologen-based RAIL as the EC material and a ferrocene-based RAIL as the charge compensation material, was synthesized and applied to an EC cell. The EC cell with the composite RAIL of as high concentration as 0.92 M each redox species showed good coloration efficiency (91.4 cm2 C-1 at 540 nm on 1.0 V). The coloration process of the EC cell was diffusion-limited process. The current and absorbance of the EC cell reached constant values at large enough bias voltage because of the charge recombination between reduced viologens and oxidized ferrocenes. The recombination affected rapid color erasing process. Almost no deterioration of the composite RAIL was found by 1H NMR after 13 000 potential cycle durability experiment.

Driving Quick and Large Amplitude Contraction of Viologen-Incorporated Poly-l-Lysine-Based Hydrogel by Reduction.

A glutaraldehyde-cross-linked poly-l-lysine-based hydrogel with pendant viologens was synthesized with various [viologen unit]/[Lys unit] ratios. The hydrogel with the ratio of 25% was extensively characterized. Characterization of the hydrogel revealed that (i) water content reaches 95.8%, (ii) unbound viologen unit is absent, and (iii) lyophilized gel, showing porous structure, is rewettable. The hydrogel in contact with a gold, glassy carbon, pyrolytic graphite, or ITO electrode was electroactive, showing voltammetric responses involving diffusion process. Occurrence of electron transfer between viologen sites was verified by ESR measurements. The electroreflectance spectrum demonstrated significant dimerization of one-electron-reduced forms, viologen radical cations. Most remarkably, reduction of the hydrogel by dithionite showed a 93% volume decrease, being near to the water content, at 100 s and finally 97% contraction at 380 s. The initial rate of contraction was 0.02 s-1, and the swelling weight ratio was over 17. The contraction rate was much faster than that observed when adding of redox-inactive salts in the outer medium. Recovery by reoxidation was relatively sluggish even when O2-saturated water was used. The quick and large-amplitude contraction with removal of water from the gel body should be the consequence of permeability of reductant, interchain π-stacking of one-electron-reduced forms (viologen monoradical monocation), desolvation of viologen upon reduction, electron hopping between viologen units and its acceleration with contraction, and effective osmotic pressure difference.

Electrochromism of a bipolar reversible redox-active ferrocene-viologen linked ionic liquid.

A ferrocene-viologen linked "bipolar" type redox-active ionic liquid ([FcC11VC1][TFSI]2) was synthesized as an electrochromic (EC) material that functions without any other additives: solvents, supporting electrolytes and sacrificial agents. The efficiency of a prototype symmetrical EC cell was 70 cm2 C-1 at 1.0 V. The EC process was stable even after over 10 000 potential cycles.

Efficient Photocurrent Enhancement from Porphyrin Molecules on Plasmonic Copper Arrays: Beneficial Utilization of Copper Nanoanntenae on Plasmonic Photoelectric Conversion Systems.

We demonstrated the usefulness of Cu light-harvesting plasmonic nanoantennae for the development of inexpensive and efficient artificial organic photoelectric conversion systems. The systems consisted of the stacked structures of layers of porphyrin as a dye molecule, oxidation-suppressing layers, and plasmonic Cu arrayed electrodes. To accurately evaluate the effect of Cu nanoantenna on the porphyrin photocurrent, the production of Cu2O by the spontaneous oxidation of the electrode surfaces, which can act as a photoexcited species under visible light irradiation, was effectively suppressed by inserting the ultrathin linking layers consisting of 16-mercaptohexadecanoic acid, titanium oxide, and poly(vinyl alcohol) between the electrode surface and porphyrin molecules. The reflection spectra in an aqueous environment of the arrayed electrodes, which were prepared by thermally depositing Cu on two-dimensional colloidal crystals of silica with diameters of 160, 260, and 330 nm, showed clear reflection dips at 596, 703, and 762 nm, respectively, which are attributed to the excitation of localized surface plasmon resonance (LSPR). While the first dip lies within the wavelengths where the imaginary part of the Cu dielectric function is moderately large, the latter two dips lie within a region of a quite small imaginary part. Consequently, the LSPR excited at the red region provided a particularly large enhancement of porphyrin photocurrent at the Q-band (ca. 59-fold), compared to that on a Cu planar electrode. These results strongly suggest that the plasmonic Cu nanoantennae contribute to the substantial improvement of photoelectric conversion efficiency at the wavelengths, where the imaginary part of the dielectric function is small.

Refractive index susceptibility of the plasmonic palladium nanoparticle: potential as the third plasmonic sensing material.

We demonstrate that Pd nanospheres exhibit much higher susceptibility of the localized surface plasmon resonance (LSPR) peak to medium refractive index changes than commonly used plasmonic sensing materials such as Au and Ag. The susceptibility of spherical Au nanoparticle-core/Pd-shell nanospheres (Au/PdNSs, ca. 73 nm in diameter) was found to be 4.9 and 2.5 times higher, respectively, than those of Au (AuNSs) and Ag nanospheres (AgNSs) having similar diameters. The experimental finding was theoretically substantiated using the Mie exact solution. We also showed from a quasi-static (QS) approximation framework that the high susceptibility of Pd LSPR originates from the smaller dispersion of the real part of its dielectric function than those of Au and Ag LSPR around the resonant wavelength. We conclude that the Pd nanoparticle is a promising candidate of "the third plasmonic sensing material" following Au and Ag to be used in ultrahigh-sensitive LSPR sensors.

Metal-enhanced fluorescence platforms based on plasmonic ordered copper arrays: wavelength dependence of quenching and enhancement effects.

Ordered arrays of copper nanostructures were fabricated and modified with porphyrin molecules in order to evaluate fluorescence enhancement due to the localized surface plasmon resonance. The nanostructures were prepared by thermally depositing copper on the upper hemispheres of two-dimensional silica colloidal crystals. The wavelength at which the surface plasmon resonance of the nanostructures was generated was tuned to a longer wavelength than the interband transition region of copper (>590 nm) by controlling the diameter of the underlying silica particles. Immobilization of porphyrin monolayers onto the nanostructures was achieved via self-assembly of 16-mercaptohexadecanoic acid, which also suppressed the oxidation of the copper surface. The maximum fluorescence enhancement of porphyrin by a factor of 89.2 was achieved as compared with that on a planar Cu plate (CuP) due to the generation of the surface plasmon resonance. Furthermore, it was found that while the fluorescence from the porphyrin was quenched within the interband transition region, it was efficiently enhanced at longer wavelengths. It was demonstrated that the enhancement induced by the proximity of the fluorophore to the nanostructures was enough to overcome the highly efficient quenching effects of the metal. From these results, it is speculated that the surface plasmon resonance of copper has tremendous potential for practical use as high functional plasmonic sensor and devices.