Photoexcited Dye Molecules Research Articles

Factors that Impact Photochemical Cage Escape Yields.

The utilization of visible light to mediate chemical reactions in fluid solutions has applications that range from solar fuel production to medicine and organic synthesis. These reactions are typically initiated by electron transfer between a photoexcited dye molecule (a photosensitizer) and a redox-active quencher to yield radical pairs that are intimately associated within a solvent cage. Many of these radicals undergo rapid thermodynamically favored "geminate" recombination and do not diffuse out of the solvent cage that surrounds them. Those that do escape the cage are useful reagents that may undergo subsequent reactions important to the above-mentioned applications. The cage escape process and the factors that determine the yields remain poorly understood despite decades of research motivated by their practical and fundamental importance. Herein, state-of-the-art research on light-induced electron transfer and cage escape that has appeared since the seminal 1972 review by J. P. Lorand entitled "The Cage Effect" is reviewed. This review also provides some background for those new to the field and discusses the cage escape process of both homolytic bond photodissociation and bimolecular light induced electron transfer reactions. The review concludes with some key goals and directions for future research that promise to elevate this very vibrant field to even greater heights.

Co-exposed crystal facets optimizing photocatalytic activity and photosensitization ability: A case of facile, fast and scalable synthesis of BiOBr round nanosheets

BiOBr round nanosheets were obtained by a facile, fast and scalable synthesis methodology in ambient conditions. The as-prepared sheet-shaped three-dimensional nanostructured BiOBr exposes reactive {001} surface facets, as well as {110} and {200} lateral facets. The average diameter is only 80–100 nm, and the thickness ∼20 nm. BiOBr not only exhibits excellent photocatalytic activity in degradation of Bisphenol A, outperforming hydrothermally synthesized BiOBr and other important visible-light-responsive photocatalysts such as BiOI, Bi2WO6, BiVO4, P25, g-C3N4 or Cu2(OH)PO4, it also possesses extraordinary activity in degradation of photoexcited dye molecules. A high reactive {001} facets exposure is demonstrated beneficial for photocatalytic activity in degradation of Bisphenol A. At the same time, the exposure of {110} and {200} facets also benefits photo-excited dye degradation ability, which is likely related to favorable adsorption on sites associated with these facets. The co-exposed facets optimizing for multi-properties endow the easily obtained BiOBr nanocrystals potential application in dye-sensitized solar cells, light-emitting diodes, or optoelectronic devices. Also, we believe our finding improve the ability of creating advanced nanosized structures for multifunctional devices.

Photoexcited Intramolecular Charge Transfer in Dye Sensitizers: Predictive In Silico Screening for Dye-Sensitized Solar Cell Devices.

Efficient photoinduced intramolecular charge transfer (ICT) from donor to acceptor in dye molecules is the functional basis and key property in the working of a dye-sensitized solar cell (DSSC). To understand the ICT process in photoexcited dye molecules, we analyze the electronic properties and structural parameters of a chosen set of experimentally synthesized donor–acceptor (D–A) and donor−π-spacer−acceptor (D−π–A) type dye molecules in their ground, excited, and cationic states. The correlation between structural modification and charge redistribution in different parts of the molecule helps to identify the extent of π-conjugation and spatial rearrangement of electron density localization along the molecular skeleton. We find that prominent twisting of several groups and the resulting molecular bond rearrangements in larger parts of the molecule promote efficient donor to acceptor ICT, such as in D–A type ADEKA1 and C275 dyes. Thus, based on the modest computation of structural and electronic properties of dye molecules in their respective ground, excited, and cationic states, we identify the desired structural changes that facilitate tunable intramolecular charge transfer to highlight a simple and direct prescription to screen out probable efficient dye molecules among many samples. Our approach complements recent experimental evidence of capturing the structural view of the excited-state charge transfer in molecules.

Tuning the Color Palette of Semi-Transparent Solar Cells via Lateral π-Extension of Polycyclic Heteroaromatics of Donor–Acceptor Dyes

Durable solar cells with tunable color and diaphaneity are very promising for building integrated photovoltaic applications. In this paper we employ donor–acceptor organic dyes U3, U4, U5, and R6 featured by polycyclic heteroaromatics 6,12-dihydroindeno[1,2-b]indeno[2′,1′:4,5]thieno[2,3-d]thiophene (IT2), 7,15-dihydrobenzo[6′,7′]indeno[2′,1′:4,5]thieno[3,2-b]benzo[6,7]indeno[2,1-d]thiophene (BIT2), 7,15-dihydrophenaleno[1,2-b]phenaleno[2′,1′:4,5]thieno[2,3-d]thiophene (PT2), and 9,19-dihydrobenzo[1′,10′]phenanthro[3′,4′:4,5]thieno[3,2-b]benzo[1,10]phenanthro[3,4-d]thiophene (BPT2) to fabricate semitransparent dye-sensitized solar cells (DSSCs). The U3-, U4-, U5-, and R6-based cells are goldenrod, crimson, red, and sapphire blue, with power conversion efficiencies of 3.5%, 8.2%, 7.6%, and 10.1% at the AM1.5G conditions, respectively. Density functional theory calculations reveal that lateral π-extension of the polycyclic heteroaromatic brings forth a downward displacement of lowest unoccupied molecular orbital, affording a high-molar-extinction-coefficient, low-energy-gap blue dye. Femtosecond fluorescence decay measurements of dyed titania and alumina films unravel the electron injection yields of photoexcited dye molecules, which are well correlated with the maximal values of external quantum efficiencies of DSSCs. After 1000 h full sunlight soaking at 60 °C, the red and blue DSSCs exhibit stable photocurrents, owing to the strong bonding and photochemical stability of dye molecules adsorbed on the surface of titania as well as the retention of close-to-unity electron collection yield.

Light-triggered dissociation of self-assembled β-amyloid aggregates into small, nontoxic fragments by ruthenium (II) complex

Alzheimer's disease is the most common progressive neurodegenerative disease, affecting more than 13% of the population over age 65. Over the last decades, researchers have focused on understanding the mechanism of amyloid formation, the hallmark of various amyloid diseases including Alzheimer's and Parkinson's. In this paper, we successfully demonstrate the dissociation of β-Amyloid (Aβ) aggregates into small, less-amyloidic fragments by photoexcited [Ru(bpy)3]2+ through destabilization of β-sheet secondary structure. We validated the light-triggered dissociation of amyloid structure using multiple analytical tools. Furthermore, we confirmed that photoexcited [Ru(bpy)3]2+ reduces cytotoxicity of Aβ aggregates. Our work should open a new horizon in the study of Alzheimer's amyloid aggregation by showing the potential of photoexcited dye molecules as an alternative therapeutic strategy for treating Alzheimer's disease in future.

Analyzing Spin Selectivity in DNA-Mediated Charge Transfer via Fluorescence Microscopy.

Understanding spin-selective interactions between electrons and chiral molecules is critical to elucidating the significance of electron spin in biological processes and to assessing the potential of chiral assemblies for organic spintronics applications. Here, we use fluorescence microscopy to visualize the effects of spin-dependent charge transport in self-assembled monolayers of double-stranded DNA on ferromagnetic substrates. Patterned DNA arrays provide background regions for every measurement to enable quantification of substrate magnetization-dependent fluorescence due to the chiral-induced spin selectivity effect. Fluorescence quenching of photoexcited dye molecules bound within DNA duplexes is dependent upon the rate of charge separation/recombination upon photoexcitation and the efficiency of DNA-mediated charge transfer to the surface. The latter process is modulated using an external magnetic field to switch the magnetization orientation of the underlying ferromagnetic substrates. We discuss our results in the context of the current literature on the chiral-induced spin selectivity effect across various systems.

Revealing the Origin of Fast Electron Transfer in TiO2-Based Dye-Sensitized Solar Cells.

In dye-sensitized solar cells (DSCs), the electron transfer from photoexcited dye molecules to semiconductor substrates remains a major bottleneck. Replacing TiO2 with ZnO is expected to enhance the efficiency of DSCs, owing to the latter possesses a much larger electron mobility, but similar bandgap and band positions as TiO2 remain. However, the record efficiency of ZnO-based DSCs is only 7% compared with 13% of TiO2-based DSCs due to the even slower electron-transfer rate in ZnO-based DSCs, which becomes a long-standing puzzle. Here, we computationally investigate the electron transfer from the dye molecule into ZnO and TiO2, respectively, by performing the first-principles calculations within the frame of the Marcus theory. The predicted electron-transfer rate in the TiO2-based DSC is about 1.15 × 10(9) s(-1), a factor of 15 faster than that of the ZnO-based DSC, which is in good agreement with experimental data. We find that the much larger density of states of the TiO2 compared with ZnO near the conduction band edge is the dominant factor, which is responsible for the faster electron-transfer rate in TiO2-based DSCs. These denser states provide additional efficient channels for the electron transfer. We also provide design principles to boost the efficiency of DSCs through surface engineering of high mobility photoanode semiconductors.

Functional Mode Hot Electron Transfer Theory

Charge carriers that have been driven out of thermal equilibrium by their excessive vibrational energies are termed hot carriers. A theory has been developed to model the injection of these vibrationally excited electrons by explicitly accounting for the time-dependent thermal relaxation of the electron-transfer driving vibrational mode, as ascertained using functional mode analysis. Specifically, the thermal relaxation rate of the driving mode is first determined through the so-called frozen-phonon approach after which the energy-dependent line shape function is revisited to include memory effects for the vibrational quanta within the framework of Fermi’s golden rule. As shown by the numerical simulations of a 6-methyl-azulene-2-carboxylic acid dye molecule bound to an anatase TiO2[101] surface, our new theory not only yields persistently faster electron injection rates with higher incident photon energy but also exhibits a sharp increase when the vibrational quanta of the photoexcited dye molecule chang...

The effect of pulsed laser radiation on polyelectrolyte capsule shells modified with fluorescent dyes

Polyelectrolyte capsules containing rhodamine 6G and fluorescein isothiocyanate in their shells are obtained by successive adsorption on spherical microscopic CaCO3 particles followed by the dissolution of the latter. Suspensions of the capsules are irradiated with a laser operating at a wavelength corresponding to the absorption bands of the dyes, and it is shown that shell modification with the selected dyes promotes photosensitized disruption of these structures. The mechanism proposed for this disruption is realized via energy transfer from photoexcited dye molecules to the polymer matrix. Therewith, the dye-modified capsules are disrupted due to their nonuniform local heating.

Characterizing the Solvated Structure of Photoexcited [Os(terpy)₂](2+) with X-ray Transient Absorption Spectroscopy and DFT Calculations.

Characterizing the geometric and electronic structures of individual photoexcited dye molecules in solution is an important step towards understanding the interfacial properties of photo-active electrodes. The broad family of “red sensitizers” based on osmium(II) polypyridyl compounds often undergoes small photo-induced structural changes which are challenging to characterize. In this work, X-ray transient absorption spectroscopy with picosecond temporal resolution is employed to determine the geometric and electronic structures of the photoexcited triplet state of [Os(terpy)2]2+ (terpy: 2,2′:6′,2″-terpyridine) solvated in methanol. From the EXAFS analysis, the structural changes can be characterized by a slight overall expansion of the first coordination shell [OsN6]. DFT calculations supports the XTA results. They also provide additional information about the nature of the molecular orbitals that contribute to the optical spectrum (with TD-DFT) and the near-edge region of the X-ray spectra.

Lasing Emission from Plasmonic Nanodome Arrays

Plasmonic nano‐dome arrays (PNDA) have been previously studied as chemical and biological sensors. The present study reports large‐area plasmonic lasing from the PNDA structures with liquid‐phase gain materials to provide optical gain. The PDNA structure consists of a 2D array of nanoposts, fabricated inexpensively using a nanoreplica molding process, with silicon dioxide and gold thin‐film coatings to produce the dome‐shaped surface profile. Covered with a dye‐doped solution, the PDNA structures can generate lasing emission when they are optically pumped above the lasing threshold. The lasing emission is enabled by energy coupling from the photoexcited dye molecules to the hybridized plasmon mode supported by the PDNA oscillator. Two PDNA lasers with array periods of 400 and 450 nm are fabricated and characterized. Plasmonic resonant modes of the PDNA lasers are numerically studied to illustrate the underlying lasing mechanism. The emission characteristics, including lasing wavelength, linewidth, threshold, beam divergence, and radiation angle, are presented in the paper. The PNDA laser represents a new path for plasmonic lasing in the near‐infrared region with potential applications that include integrated photonic circuits, optical communications, and high‐performance biosensors.

Monitoring Ultrafast Chemical Dynamics by Time-Domain X-ray Photo- and Auger-Electron Spectroscopy.

The directed flow of charge and energy is at the heart of all chemical processes. Extraordinary efforts are underway to monitor and understand the concerted motion of electrons and nuclei with ever increasing spatial and temporal sensitivity. The element specificity, chemical sensitivity, and temporal resolution of ultrafast X-ray spectroscopy techniques hold great promise to provide new insight into the fundamental interactions underlying chemical dynamics in systems ranging from isolated molecules to application-like devices. Here, we focus on the potential of ultrafast X-ray spectroscopy techniques based on the detection of photo- and Auger electrons to provide new fundamental insight into photochemical processes of systems with various degrees of complexity. Isolated nucleobases provide an excellent testing ground for our most fundamental understanding of intramolecular coupling between electrons and nuclei beyond the traditionally applied Born-Oppenheimer approximation. Ultrafast electronic relaxation dynamics enabled by the breakdown of this approximation is the major component of the nucleobase photoprotection mechanisms. Transient X-ray induced Auger electron spectroscopy on photoexcited thymine molecules provides atomic-site specific details of the extremely efficient coupling that converts potentially bond changing ultraviolet photon energy into benign heat. In particular, the time-dependent spectral shift of a specific Auger band is sensitive to the length of a single bond within the molecule. The X-ray induced Auger transients show evidence for an electronic transition out of the initially excited state within only ∼200 fs in contrast to theoretically predicted picosecond population trapping behind a reaction barrier. Photoinduced charge transfer dynamics between transition metal complexes and semiconductor nanostructures are of central importance for many emerging energy and climate relevant technologies. Numerous demonstrations of photovoltaic and photocatalytic activity have been performed based on the combination of strong light absorption in dye molecules with charge separation and transport in adjacent semiconductor nanostructures. However, a fundamental understanding of the enabling and limiting dynamics on critical atomic length- and time scales is often still lacking. Femtosecond time-resolved X-ray photoelectron spectroscopy is employed to gain a better understanding of a short-lived intermediate that may be linked to the unexpectedly limited performance of ZnO based dye-sensitized solar cells by delaying the generation of free charge carriers. The transient spectra strongly suggest that photoexcited dye molecules attached to ZnO nanocrystals inject their charges into the substrate within less than 1 ps but the electrons are then temporarily trapped at the surface of the semiconductor in direct vicinity of the injecting molecules. The experiments are extended to monitor the electronic response of the semiconductor substrate to the collective injection from a monolayer of dye molecules and the subsequent electron-ion recombination dynamics. The results indicate some qualitative similarities but quantitative differences between the recombination dynamics at molecule-semiconductor interfaces and previously studied bulk-surface electron-hole recombination dynamics in photoexcited semiconductors.

DC-field-assisted grating formation and nonlinear diffractions in methyl-red dye-doped blue phase liquid crystals.

We report the observation of enhanced nonlinear optical responses of methyl-red-doped blue-phase liquid crystals by application of a DC field. We have observed strong multi-order nonlinear grating diffractions characterized by a nonlinear index coefficient n(2)∼0.5 cm(2)/W using unfocused CW laser power of ∼1 mW and a DC field of a few V/μm. The underlying mechanisms are crystalline lattice and director axis reorientations by torques exerted by the DC field and photo-excited dye molecules.

Grain Boundary Controlled Electron Mobility in Polycrystalline Titanium Dioxide

Grain Boundary Controlled Electron Mobility in Polycrystalline Titanium Dioxide

Ultrafast Spectroscopy and Numerical Analysis of Inhomogeneous Electron Injection in Dye-Sensitized Solar Cells: Effect of Electrolyte

Ultrafast electron injection from a photo-excited dye molecule to semiconductor electrode is one of the most important reactions in the dye-sensitized solar cell (DSSC). The reaction occurs in the wide time region from femtoseconds to nanoseconds with a inhomogeneous manner.[1,2] Since the origin of such inhomogeneity has not been understood well, there still is many problems to improve the photo conversion efficiency of DSSC.[2]Here, we propose a quantitative reaction model of the electron injection dynamics to discuss the origin of the inhomogeneity. Our experimental results of transient absorption spectroscopy (TAS) were successfully analyzed by the model.Dye sensitized TiO2 films with Ru-complex (N719) were prepared. The fs-TAS and ns-TAS measurements were carried out for the films in acetonitrile-based electrolytes. The results were numerically analyzed as a sum of non-adiabatic electron transfer reactions, which supported our experimental finding that the inhomogeneous energy distribution of the energy gap between the donor and acceptor states (Δ) was present.[2] Fig.1 shows the results of population time profile of injected electron in DSSC. The profiles are measured by fs-TAS at <1 ns and sub-nsTAS >1 ns. The simulation result based on our model showed a good agreement with the experimental result. Furthermore, the effects of electrolyte with different potential controlling ion (Li+) concentrations and the electron concentration in semiconductor electrode were investigated experimentally and the obtained electron injection kinetics were well explained by the numerical simulation, indicating that the model is reasonable.In some cases, however, the model does not seem to be easily applicable. For example, when we measured electron injection kinetics from excited N719 to TiO2 in the presence of ionic liquid (IL) as surrounding solvent, less efficiency was observed for some ILs with higher viscosity (> 30 cP). This suggests that not only the driving force but also restriction of molecular reorientation of the sensitizer may affect the reaction.Some other examples of electron injection dynamics relating to the effect of electrolytes and solvents will be shown and the mechanism will be discussed [3, 4]. Acknowledgement: A part of this work is supported by NEDO Japan. Fig.1Population time profile of injected electron in DSSC. Solid line shows a simulation result.

Experimental observation of sub-femtosecond charge transfer in a model water splitting dye-sensitized solar cell

Charge transfer from photoexcited dye molecules to a semiconductor substrate forms the basis of dye sensitized solar cells (DSCs); the overall effectiveness of a DSC device is critically dependent upon the efficiency of this process due to competition with other de-excitation channels. In this paper, we experimentally derive timescales for the charge transfer process in model water splitting DSCs. We studied two organometallic dye complexes adsorbed onto a rutile TiO(2)(110) substrate, the dye molecules were deposited in ultra-high vacuum using electrospray deposition. DFT simulations were used to calculate the spatial distribution of orbitals relevant to the charge transfer process. The core-hole clock implementation of resonant photoemission spectroscopy was used to determine upper limits on charge transfer timescales for previously unoccupied orbitals, which were found to be in the low-femtosecond regime apart from one orbital found to have an upper limit in the sub-femtosecond regime.

Modified light emission from emitters coupled to long-range guided modes in strongly absorbing layers

We demonstrate the near-field coupling and energy transfer between photoexcited dye molecules and guided modes in layers of strongly absorbing dielectrics. The dye molecules decay by exciting long-range guided modes (LRGMs) in a thin layer of chalcogenide glass. These modes can exist in spite of the very large absorption of the material forming the layer. The LRGMs are detected by coupling then out to free space radiation through a prism in the Krestschmann configuration. By calculating the dissipated power of a dipole, representing a dye molecule, in the vicinity of the absorbing thin film, we show that there is a large probability of decay exciting LRGMs. This probability can reach 35% for perpendicularly oriented dipoles. The demonstration of the excitation of LRGMs in thin films of absorbing dielectrics by near-field coupling of excited molecules opens the possibility to compensate for the losses in the propagation of these modes.

Effect of Polymeric p-Type Semiconductor on Photovoltaic Properties in Dye-Sensitized Solar Cell

A hole formed in photo-excited dye molecule under illumination is transferred to cathode with iodine ion in electrolyte solution in the dye-sensitized solar cell developed by M. Grätzel and the conductivity of the iodine ion affects conversion efficiency of the cell. In this work, effect of the polymeric p-type semiconductor on the hole transferability is investigated. PEDOT:PSS and P3HT were used as the p-type semiconductors. Dye-sensitized solar cells were composed of aluminium plate, Nb-doped TiO2 layer adsorbed with TCNQ, the electrolyte solution containing iodine ions, and ITO glass plate. In a solid-type cell substituted the electrolyte solution with the p-type semiconductor, photocurrent was not generated. The photocurrent was generated in the solar cell by introducing the electrolyte solution containing iodine ions between the ITO glass and the p-type semiconductor formed on the dye-adsorbed Nb-doped-TiO2 layer or between the dye-adsorbed Nb-doped-TiO2 layer and the p-type semiconductor formed on the ITO glass. The conversion efficiency increased with the introduction of the p-type semiconductor such as PEDOT:PSS, compared only with the electrolyte solution. The p-type semiconductor containing iodine ions promote the hole transfer of iodine ion, compared with that in the electrolyte solution.

Engineering of Push‐Pull Thiophene Dyes to Enhance Light Absorption and Modulate Charge Recombination in Mesoscopic Solar Cells

AbstractThe elaborate selection of diverse π‐conjugated segments which bridge the electron donors and acceptors in organic push‐pull dyes can not only tune the molecular energy‐levels but also impact the interfacial energetics and kinetics of dye‐sensitized solar cells (DSCs). In this paper, a series of triphenylamine‐cyanoacrylic acid photosensitizers is reported with TT, EDOT‐BT, EDOT‐CPDT, and CPDT‐EDOT (herein TT, EDOT, BT, and CPDT denote terthiophene, ethylenedioxythiophene, bithiophene, and cyclopentadithiophene, respectively) as the π‐linkers, and the dye‐structure correlated photocurrent and photovoltage features of DSCs based on a cobalt electrolyte are scrutinized via analyzing light absorption and multichannel charge transfer kinetics. Both stepwise incorporation of more electron‐rich blocks and rational modulation of connection order of dissimilar segments can result in a negative movement of ground‐state redox potential and a red‐shift of the absorption peak. While these styles of reducing energy‐gap do not exert too much influence on the electron injection from photoexcited dye molecules to titania, the dyestuff employing the EDOT‐BT linker presents a faster interfacial charge recombination and a slower dye regeneration, accounting for its inferior cell efficiency of 5.3% compared to that of 9.4% at the AM1.5G conditions achieved by the CPDT‐EDOT dye.

Plasmonic Bowtie Nanolaser Arrays

Plasmonic lasers exploit strong electromagnetic field confinement at dimensions well below the diffraction limit. However, lasing from an electromagnetic hot spot supported by discrete, coupled metal nanoparticles (NPs) has not been explicitly demonstrated to date. We present a new design for a room-temperature nanolaser based on three-dimensional (3D) Au bowtie NPs supported by an organic gain material. The extreme field compression, and thus ultrasmall mode volume, within the bowtie gaps produced laser oscillations at the localized plasmon resonance gap mode of the 3D bowties. Transient absorption measurements confirmed ultrafast resonant energy transfer between photoexcited dye molecules and gap plasmons on the picosecond time scale. These plasmonic nanolasers are anticipated to be readily integrated into Si-based photonic devices, all-optical circuits, and nanoscale biosensors.