Nanosecond Transient Absorption Spectroscopy Research Articles

Transient Absorption of N719 and its Electron Transfer Kinetics on ZnO Nanoparticles Surface

The transient absorption and emission spectra of ruthenium complex sensitizer N719 under excitation in different solvents were studied. Isopropanol was found to stabilize the singlet excited state of N719. An emission band centered at 460 nm resulting from the singlet excited state of N719 was observed at 1 ns time delay, which is much longer than the reported observation time of the singlet excited state of N719. The triplet excited state of N719 undergoes photophysical decay in acetonitile and isopropanol with lifetimes of around 40 ns, while it may encounter photochemical reactions in water resulting in long living intermediate. The sensitizer was introduced to the surface of three types of ZnO nanoparticles with different morphology, which were used as the electron acceptors upon photoexcitation. The electron transfer dynamics between sensitized N719 and ZnO interface both in the presence and absence of electrolyte were studied by time-correlated single photon counting technique, nanosecond transient absorption and emission spectroscopies. It was revealed that the electrolyte has a significant impact upon the electron transfer dynamics at the N719-ZnO interface. In the absence of electrolyte, the electron transfer process at the N719-ZnO interface is dependent on the depth of defects in ZnO nanoparticles. Conversely, in the presence of electrolyte, ZnO defects show no impacts on the electron transfer process at the N719-ZnO interface and effective electron injection happens similarly from the excited N719 to ZnO in spite of ZnO particle sizes.

Photoinduced electron transfer within a zinc porphyrin-cyclobis(paraquat-p-phenylene) donor-acceptor dyad.

Understanding the mechanism of efficient photoinduced electron-transfer processes is essential for developing molecular systems for artificial photosynthesis. Towards this goal, we describe the synthesis of a donor-acceptor dyad comprising a zinc porphyrin donor and a tetracationic cyclobis(paraquat-p-phenylene) (CBPQT(4+) ) acceptor. The X-ray crystal structure of the dyad reveals the formation of a dimeric motif through the intermolecular coordination between the triazole nitrogen and the central Zn metal of two adjacent units of the dyad. Photoinduced electron transfer within the dyad in MeCN was investigated by femtosecond and nanosecond transient absorption spectroscopy, as well as by transient EPR spectroscopy. Photoexcitation of the dyad produced a weakly coupled ZnP(+.) -CBPQT(3+.) spin-correlated radical-ion pair having a τ=146 ns lifetime and a spin-spin exchange interaction of only 0.23 mT. The long radical-ion-pair lifetime results from weak donor-acceptor electronic coupling as a consequence of having nine bonds between the donor and the acceptor, and the reduction in reorganization energy for electron transfer caused by charge dispersal over both paraquat units within CBPQT(3+.) .

Time-Resolved Spectroscopic and Density Functional Theory Study of the Photochemistry of Irgacure-2959 in an Aqueous Solution

The photocleavage reaction mechanism of 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure-2959) was investigated using femtosecond (fs) and nanosecond (ns) transient absorption (-TA) spectroscopy and also picosecond (ps) and nanosecond (ns) time-resolved resonance Raman (-TR(3)) spectroscopy experiments in a water-rich (volume ratio of acetonitrile/water = 3:7) solution. TA spectroscopy was used to study the dynamics of the benzoyl radical growth and decay as well as to investigate the radical quenching process by the radical scavenger methyl acrylate. ps- and ns-TR(3) spectroscopies were employed to monitor the formation of the benzoyl radical and also to characterize its electronic and structural properties. The fs-TA experiments results indicate that the Irgacure-2959 lowest lying excited singlet state S1 underwent efficient intersystem crossing (ISC) to convert into its triplet state with a time constant of 4 ps. Subsequently, this triplet species dissociated into the benzoyl and alkyl radicals with a corresponding maximum absorption band at 415 nm. The TR(3) results in conjunction with results from DFT calculations confirmed that Irgacure-2959 cleaved into the benzoyl and alkyl radicals at a fast rate on the tens of picosecond time scale.

Synthesis and photoinduced electron transfer in platinum(II) bis(N-(4-ethynylphenyl)carbazole)bipyridine fullerene complexes.

Platinum(ii) bis(N-(4-ethynylphenyl)carbazole)bipyridine fullerene complexes, (Cbz)2-Pt(bpy)-C60 and ((t)BuCbz)2-Pt(bpy)-C60, were synthesized. Their photophysical properties were studied by electronic absorption and emission spectroscopy and the origin of the transitions was supported by computational studies. The electrochemical properties were also studied and the free energies for charge-separation and charge-recombination processes were evaluated. The photoinduced electron transfer reactions in the triads were investigated by femtosecond and nanosecond transient absorption spectroscopy. In dichloromethane, both triads undergo ultrafast charge separation from the (3)MLCT/LLCT excited state within 300 fs to yield their respective triplet charge-separated (CS) states, namely (Cbz)2˙(+)-Pt(bpy)-C60˙(-) and ((t)BuCbz)2˙(+)-Pt(bpy)-C60˙(-), and the CS states would undergo charge recombination to give the (3)C60* state, which subsequently decays to the ground state in 22-28 μs.

Photophysics and non-linear absorption of Au(I) and Pt(II) acetylide complexes of a thienyl-carbazole chromophore.

In order to understand the photophysics and non-linear optical properties of carbazole containing π-conjugated oligomers of the type ET-Cbz-TE (E = ethynylene, T = 2,5-thienylene, Cbz = 3,6-carbazole), a detailed investigation was carried out on a series of oligomers that feature Au(i) or Pt(ii) acetylide "end groups", as well as a Pt(ii)-acetylide linked polymer (CBZ-Au-1 and CBZ-Pt-1, CBZ-Poly-Pt). These organometallic chromophores were characterized by UV-visible absorption and variable temperature photoluminescence spectroscopy, nanosecond transient absorption spectroscopy, open aperture nanosecond z-scan and two photon absorption (2PA) spectroscopy. The Au(i) and Pt(ii) oligomers and polymer exhibit weak fluorescence in fluid solution at room temperature. Efficient phosphorescence is observed from the Pt(ii) systems below 150 K in a solvent glass; however, the Au(i) oligomer exhibits only weak phosphorescence at 77 K. Taken together, the emission results indicate that the intersystem crossing efficiency for the Pt(ii) chromophores is greater than for the Au(i) oligomer. Nonetheless, nanosecond transient absorption indicates that direct excitation affords moderately long-lived triplet states for all of the chromophores. Open aperture z-scan measurement shows effective optical attenuation can be achieved by using these materials. The 2PA cross section in the degenerate S0→S1 transition region was in the range 10-100 GM, and increased monotonically toward shorter wavelengths, reaching 800-1000 GM at 550 nm.

Mono‐ and Bis(pyrrolo)tetrathiafulvalene Derivatives Tethered to C60: Synthesis, Photophysical Studies, and Self‐Assembled Monolayers

A series of mono- (MPTTF) and bis(pyrrolo)tetrathiafulvalene (BPTTF) derivatives tethered to one or two C60 moieties was synthesized and characterized. The synthetic strategy for these dumbbell-shaped compounds was based on a 1,3-dipolar cycloaddition reaction between aldehyde-functionalized MPTTF/BPTTF derivatives, two different tailor-made amino acids, and C60. Electronic communication between the MPTTF/BPTTF units and the C60 moieties was studied by a variety of techniques including cyclic voltammetry and absorption spectroscopy. These solution-based studies indicated no observable electronic communication between the MPTTF/BPTTF units and the C60 moieties. In addition, femtosecond and nanosecond transient absorption spectroscopy revealed, rather surprisingly, that no charge transfer from the MPTTF/BPTTF units to the C60 moieties takes place on excitation of the fullerene moiety. Finally, it was shown that the MPTTF-C60 and C60-BPTTF-C60 dyad and triad molecules formed self-assembled monolayers on a Au(111) surface by anchoring to C60.

Design and Synthesis of Bipyridine Platinum(II) Bisalkynyl Fullerene Donor–Chromophore–Acceptor Triads with Ultrafast Charge Separation

Donor-chromophore-acceptor triads, (PTZ)2-Pt(bpy)-C60 and ((t)BuPTZ)2-Pt(bpy)-C60, along with their model compound, (Ph)2-Pt(bpy)-C60, have been synthesized and characterized; their photophysical and electrochemical properties have been studied, and the origin of the absorption and emission properties has been supported by computational studies. The photoinduced electron transfer reactions have been investigated using the femtosecond and nanosecond transient absorption spectroscopy. In dichloromethane, (Ph)2-Pt(bpy)-C60 shows ultrafast triplet-triplet energy transfer from the (3)MLCT/LLCT excited state within 4 ps to give the (3)C60* state, while in (PTZ)2-Pt(bpy)-C60 and ((t)BuPTZ)2-Pt(bpy)-C60, charge-separated state forms within 400 fs from the (3)MLCT/LLCT excited state with efficiency of over 0.90, and the total efficiency with the contribution of (3)C60* is estimated to be 0.99. Although the forward electron transfer reactions are very rapid, the charge-separated state recombines to the singlet ground state at a time of hundreds of nanoseconds because of the difference in spin multiplicity between the charge-separated state and the ground state.

Long-Lived Charge Separated State in NiO-Based p-Type Dye-Sensitized Solar Cells with Simple Cyclometalated Iridium Complexes.

Three new cyclometalated iridium complexes were prepared and investigated on nanocrystalline NiO cathodes. Nanosecond transient absorption spectroscopy experiments show they present a surprisingly slow geminate charge recombination upon excitation on NiO, representing thus the first examples of simple sensitizers with such feature. These complexes were used in dye-sensitized solar cells using nanocrystalline NiO film as semiconductor. The long-lived charge separated state of these Ir complexes make them compatible with other redox mediators than I3(-)/I(-), such as a cobalt electrolyte and enable to reach significantly high open circuit voltage.

Photoinduced Electron Transfer Reactions of Hexarhenium(III) Cluster: Oxidative Quenching of [Re6S8Cl6]4− Emission by Electron Acceptors

The photoredox ability of a hexarhenium(III) cluster, [Re6(μ 3-S)8Cl6]4−, in acetonitrile was examined on the basis of the emission quenching experiments of the cluster by a series of neutral organic electron acceptors (A). The photoinduced electron transfer quenching rate constants and the relevant activation parameters demonstrated that the hexarhenium(III) cluster acted as a useful photoredox sensitizer towards various A and the system gave a high charge separation yield owing to electrostatic repulsion between the product ions: [Re6(μ 3-S)8Cl6]3− and A −. These results were proved explicitly by nanosecond transient absorption spectroscopy.

Molecular Dyads Comprising Metalloporphyrin and Alkynylplatinum(II) Polypyridine Terminal Groups for Use as a Sensitizer in Dye‐Sensitized Solar Cells

A new class of molecular dyads comprising metalloporphyrin-linked alkynylplatinum(II) polypyridine complexes with carboxylic acids as anchoring groups has been designed and synthesized. These complexes can sensitize nanocrystalline TiO2 in dye-sensitized solar cell (DSSC) studies. The photophysical, electrochemical, and luminescence properties of the complexes were studied and their excited-state properties were investigated by nanosecond transient absorption spectroscopy, with the charge-separated [Por(.-)-{(C≡C)Pt(tBu3 tpy)}(.+)] state observed upon excitation. Excited-state redox potentials were determined; the electrochemical data supports the capability of the complexes to inject an electron into the conduction band of TiO2 . The complexes sensitize nanocrystalline TiO2 and exhibited photovoltaic properties, as characterized by current-voltage measurements under illumination of air mass 1.5 G sunlight (100 mWcm(-2)). A DSSC based on one of the complexes showed a short-circuit photocurrent of 10.1 mAcm(-2), an open-circuit voltage of 0.64 V, and a fill factor of 0.52, giving an overall power conversion efficiency of 3.4 %.

Deprotonation Kinetics of 1-Methylguanine After One-Electron Oxidation

Among the four natural DNA bases, guanine (G) is the most sensitive to oxidation due to its lowest oxidation potential. When G base is oxidized to guanine cation radical (G(+center dot)), it will deprotonate from both the imino proton N-1-H and the amino proton N-2-H. According to the pKa values for N-1-H and N-2-H deprotonation, the main deprotonation site in G base is N-1-H which would interfere with the N-2-H deprotonation, making the kinetics of N-2-H deprotonation difficult to be measured. Herein, the N-2-H deprotonation kinetics is investigated using 1-methylguanosine (mG), where N-1-H is substituted by methyl group to avoid the N-1-H deprotonation and N-9-H is substituted by ribose to ensure enough solubility of methylguanine in water, by nanosecond transient absorption (ns-TA) spectroscopy. By 355 nm photolysis of Na2S2O8, the highly oxidizing radical SO4-center dot is generated, which will oxidize mG to mG(+center dot) instantaneously. The time-resolved absorption spectra obtained for reaction of mG with SO4-center dot exhibits transient absorptions for mG(N-2-H)(center dot) featured by absorption band at 600 nm, indicating that the mG(+center dot) deprotonation product is mG(N-2-H)(center dot) and the deprotonation site is therefore validated to be N-2-H. The mG concentration dependence of mG(N-2-H)(center dot) formation rate constant is assessed through changing the mG concentration from 0.25 mmol.L-1 to 5 mmol.L-1. The concentration dependence experiment reveals that the rate-limiting step to form mG(N-2-H)(center dot) is the bimolecular reaction of mG with SO4-center dot when mG concentration is lower than 2 mmol.L-1 and the bimolecular reaction rate constant to form mG(+center dot) is (3.7 +/- 0.1)x10(9) L.mol(-1).s(-1); when mG concentration is above 2 mmol.L-1, the rate-limiting step to form mG(N-2-H)(center dot) is the first-order mG(+center dot) deprotonation and the N-2-H deprotonation rate constant is (7.1 +/- 0.2) x 10(6) s(-1). Furthermore, the N-2-H deprotonation rate constant is measured at different temperatures varying from 278 K to 298 K. According to Arrhenius equation, the activation energy barrier for the N-2-H deprotonation is determined to be 19.9 +/- 1.0 kJ.mol(-1). These results can provide valuable kinetic information on the oxidative damage of DNA.

Two-Electron HCl to H2 Photocycle Promoted by Ni(II) Polypyridyl Halide Complexes

Photochemical HX splitting requires the management of two protons and the execution of multielectron photoreactions. Herein, we report a photoinduced two-electron reduction of a polypyridyl Ni(II) chloride complex that provides a route to H2 evolution from HCl. The excited states of Ni complexes are too short to participate directly in HX activation, and hence, the excited state of a photoredox mediator is exploited for the activation of HX at the Ni(II) center. Nanosecond transient absorption (TA) spectroscopy has revealed that the excited state of the polypyridine results in a photoreduced radical that is capable of mediating HX activation by producing a Ni(I) center by halogen-atom abstraction. Disproportionation of the photogenerated Ni(I) intermediate affords Ni(II) and Ni(0) complexes. The Ni(0) center is capable of reacting with HX to produce H2 and the polypyridyl Ni(II) dichloride, closing the photocycle for H2 generation from HCl.

Revealing the Relationship between Semiconductor Electronic Structure and Electron Transfer Dynamics at Metal Oxide–Chromophore Interfaces

Interfacial charge recombination dynamics in nanocrystalline SnO2 and TiO2 thin films sensitized with phosphonate-derivatized ruthenium chromophores (Ru(bpy)2(4,4′-(PO3H2)2bpy)]2+, RuP) have been investigated in aqueous media by nanosecond transient absorption spectroscopy. Back electron transfer (BET) rates for RuP–SnO2 were observed to be 2–3 times greater than for RuP–TiO2. Additionally, rates of charge recombination for RuP–TiO2 show a significant pH dependence, while only a subtle influence of pH is observed for BET in RuP–SnO2. Cyclic voltammetry measurements indicate the exponential distribution of intra-band-gap trap states varies with pH for both SnO2 and TiO2 nanocrystalline thin films. BET rates for RuP–SnO2 and RuP–TiO2 are correlated with the distribution, identity, and occupation of localized trap states within the nanocrystalline metal oxide films, which are pH specific. Recombination between injected electrons and oxidized chromophores is influenced by the identity of metal oxide localized...

Photovoltaic performance and charge recombination dynamics of P3HT/PCBM blend heterojunction

We prepared the polymer solar cell based on poly(3-hexylthiophene)(P3HT)/fullerene derivative PCBM(PCBM=[6,6]-phenyl-C61-butyric acid methyl ester) heterojunction and investigated the irradiation intensity-dependent charge recombination dynamics of heterojunction employing nanosecond transient absorption spectroscopy with bias light so as to simulate the photophysical process in heterojunction when the photovoltaic device is on operation. The experimental data exhibit that the yield of free charges gradually decreases and the loss of mobile carriers originated from bimolecular recombination simultaneously increases as the irradiation intensity gradually enhances. This indicates that the polymer solar cell is much suitably used at a low irradiation intensity.

Time-Resolved Spectroscopic and Imaging Studies on Laser Ablation of Molecular Systems: From Mechanistic Study to Bio/Nano Applications

Abstract Studies on laser ablation of polymer films, molecular crystals in solution, protein solution, and culture media containing living cells are summarized and considered. Dynamics and mechanism of laser ablation were systematically studied by utilizing time-resolved spectroscopy and imaging; femtosecond–nanosecond transient absorption and emission spectroscopy, nanosecond shadowgraphy, nanosecond–nanometer interferometry, femtosecond surface light scattering imaging. It was confirmed by integrating both data that primary processes of laser ablation can be well understood in the framework of Jablonski diagram. For nanosecond laser ablation of doped polymers, it was demonstrated that cyclic multiphoton absorption is an efficient photothermal conversion process leading to photothermal ablation. For femtosecond laser ablation of dye films, transient pressure mechanism was proposed indicating photomechanical ablation. As applications of laser ablation, nanoparticle preparation, protein crystallization, and manipulation of living cells are presented. Laser ablation of molecular crystals in poor solvent gives small fragments whose size are in a few tens nm. The fabricated nanocolloids are stable without adding detergents and their size was the smallest as nanoparticles produced by the top-down-method. Multiphoton laser ablation of water generates local impulsive force due to bubble formation, shockwave propagation, and local convection flow. The force triggers molecular and protein crystallization in their supersaturated solutions, whose mechanisms are described and considered. The impulsive force is also very useful for manipulating living cells and its high potential was confirmed by examining cell functions such as division, differentiation, death, and migration. Finally summary and future plan are presented.

Intramolecular RET Enhanced Visible Light-Absorbing Bodipy Organic Triplet Photosensitizers and Application in Photooxidation and Triplet–Triplet Annihilation Upconversion

Resonance energy transfer (RET) was used for the first time to enhance the visible light absorption of triplet photosensitizers. The intramolecular energy donor (boron-dipyrromethene, Bodipy) and acceptor (iodo-Bodipy) show different absorption bands in visible region, thus the visible absorption was enhanced as compared to the monochromophore triplet photosensitizers (e.g., iodo-Bodipy). Fluorescence quenching and excitation spectra indicate that the singlet energy transfer is efficient for the dyad triplet photosensitizers. Nanosecond time-resolved transient absorption spectroscopy has confirmed that the triplet excited states of the dyads are distributed on both the energy donor and acceptor, which is the result of forward singlet energy transfer from the energy donor to the energy acceptor and in turn the backward triplet energy transfer. This 'ping-pong' energy transfer was never reported for organic molecular arrays, and so it is useful to study the energy level of organic chromophores. The triplet photosensitizers were used for singlet oxygen ((1)O2) mediated photooxidation of 1,5-dihydroxylnaphthalene to produce juglone. The visible light absorption of the new visible light-absorbing triplet photosensitizers are higher than the conventional monochromophore based triplet photosensitizers, as a result, the (1)O2 photosensitizing ability is improved with the new triplet photosensitizers. Triplet-triplet annihilation upconversion with these triplet photosensitizers was also studied. Our results are useful to design the triplet photosensitizers showing strong visible light absorbance and for their applications in photocatalysis and photodynamic therapy.

Luminescent Cyclometalated Alkynylplatinum(II) Complexes with a Tridentate Pyridine‐Based N‐Heterocyclic Carbene Ligand: Synthesis, Characterization, Electrochemistry, Photophysics, and Computational Studies

A new class of luminescent alkynylplatinum(II) complexes with a tridentate pyridine-based N-heterocyclic carbene (2,6-bis(1-butylimidazol-2-ylidenyl)pyridine) ligand, [Pt(II)(C^N^C)(C≡CR)][PF6], and their chloroplatinum(II) precursor complex, [Pt(II)(C^N^C)Cl][PF6], have been synthesized and characterized. One of the alkynylplatinum(II) complexes has also been structurally characterized by X-ray crystallography. The electrochemistry, electronic absorption and luminescence properties of the complexes have been studied. Nanosecond transient absorption (TA) spectroscopy has also been performed to probe the nature of the excited state. The origin of the absorption and emission properties has been supported by computational studies.

Photoinduced Charge Separation of 10-Phenyl-10H-phenothiazine–2-Phenylanthraquinone Dyad Bridged by Bicyclo[2.2.2]octane

Abstract Photoinduced charge separation of a phenothiazine–anthraquinone dyad 1 with a rigid bicyclo[2.2.2]octane spacer was investigated by means of picosecond and nanosecond transient absorption and time-resolved ESR spectroscopies. Irradiation of the anthraquinone chromophore in 1 produced the charge-separated (CS) state in THF. Time constants of the formation and the decay of the CS state were determined to be 1.3 ns and 1.0 µs, respectively. Time-resolved ESR showed spin-polarized all-emission signals due to the formation of the CS state via the triplet mechanism.

(Invited) Exciton Fission and Interfacial Charge Transfer in Rubrene/Fullerene Mixed Film Revealed by Femtosecond and Nanosecond Transient Absorption Spectroscopy

not Available.

Polymerization Inhibition by Triplet State Absorption for Nanoscale Lithography

In the field of photopolymerization, multi-photon lithography1 stands out as a mask-free tool for fabricating three-dimensional structures inside a photocurable resin that is transparent at the wavelength used. This approach enables a great deal of versatility compared with single photon absorption lithography where the thickness of the polymerized parts is limited by the penetration depth of the light in the resin2 and therefore a layer-by-layer approach is necessary to fabricate 3D structures.3 The possibility of producing 3D, free-standing, and arbitrarily complex architectures opens up novel applications in various research fields, e.g., photonic crystals, metamaterials, and biological scaffold fabrication.4–10