Excited-state Electron-transfer Processes Research Articles

Ultrafast charge transfer dynamics of Rhodamine B with graphene oxide.

Graphene and its derivatives, due to their two-dimensional carbon nanostructures, have provided new opportunities to fortify organic dye-based photovoltaic and photocatalytic assemblies. In this article, we employed organic dyes Rhodamine B (RdB) and graphene oxide (GO) [or reduced graphene oxide (RGO)] to assemble the composite materials RdB-GO and RdB-RGO. It was found that both GO and RGO could strongly quench the fluorescence (FL) intensity of RdB. The mechanisms of FL quenching in both RdB-GO and RdB-RGO have been investigated by using femtosecond fluorescence up-conversion and transient absorption spectroscopy. When RdB was anchored on the GO (or RGO) surface, the solvent relaxation component 3.1ps of pure RdB disappeared; instead, the ultrafast excited state electron transfer process (1-2ps) in RdB-GO and RdB-RGO was found and originated from the LUMO of RdB to the conduction band of GO (or RGO). We further reveal that the energy level change caused by GO reduction makes the energy levels of RGO closer to those of RdB, resulting in the electron transfer being more effective in RdB-RGO. Therefore, the RdB-RGO composite materials may have higher application values for dye-sensitized solar cells.

Highly Efficient Luminescence from Charge-Transfer Gold Nanoclusters Enabled by Lewis Acid.

Understanding the complicated intramolecular charge transfer (ICT) behaviors of nanomaterials is crucial to the development of high-quality nanoluminophores for various applications. However, the ICT process in molecule-like metal nanoclusters has been rarely explored. Herein, a proton binding-induced enhanced ICT state is discovered in 6-aza-2-thiothymine-protected gold nanoclusters (ATT-AuNCs). Such an excited-state electron transfer process gives rise to the weakened and red-shifted photoluminescence of these nanoclusters. By the joint use of this newfound ICT mechanism and a restriction of intramolecular motion (RIM) strategy, a red shift in the emission maxima of 30 nm with 27.5-fold higher fluorescence quantum efficiency is achieved after introducing rare-earth scandium ion (Sc3+) into ATT-AuNCs. Furthermore, it is found that upon the addition of Sc3+, the photoinduced electron transfer (PET) rate from ATT-AuNCs to minocycline is largely accelerated by forming a donor-bridge-acceptor structure. This paper offers a simple method to modulate the luminescent properties of metal nanoclusters for the rational design of next-generation sensing platforms.

Collective Effects of Band Offset and Wave Function Dimensionality on Impeding Electron Transfer from 2D to Organic Crystals.

Excited-state electron transfer (ET) across molecules/transition metal dichalcogenide crystal (TMDC) interfaces is a critical process for the functioning of various organic/TMDC hybrid optoelectronic devices. Therefore, it is important to understand the fundamental factors that can facilitate or limit the ET rate. Here it is found that an undesirable combination of the interfacial band offset and the spatial dimensionality of the delocalized electron wave function can significantly slow down the ET process. Specifically, it is found that whereas the ET rate from TMDCs (MoS2 and WSe2) to fullerenes is relative insensitive to the band offset, the ET rate from TMDCs to perylene molecules can be reduced by an order of magnitude when the band offset is large. For the perylene crystal, the sensitivity of the ET rate on the band offset is explained by the 1D nature of the electronic wave function, which limits the availability of states with the appropriate energy to accept the electron.

Phosphorescent iridium(III) complex based photoluminescence sensor for sensitive and selective detection of picric acid

The sensitive and selective detection of picric acid is of great importance to the society safety and human health. In this paper, a novel blue emissive iridium(III) complex FIrPicOMPBHCz containing carbazole moieties has been successfully synthesized and applied in photoluminescence (PL) sensing towards nitroaromatic explosives (e.g. PA, TNB, DNP, TNT, DNT, NP, NBA, NB) detection. The PL emission of FIrPicOMPBHCz could be quenched by the explosives with different extents. Among these, the quenching constant KSV towards PA is the highest, which is 1.9 × 104 M−1, with one to two orders of magnitude higher than that of other nitroaromatics, thus demonstrating a selective PL detection of PA. The sensing mechanism is based on the excited state electron transfer process, by analyzing the UV–vis absorption, excitation and PL spectra and cyclic voltammogram results. In addition, the paper strip tests indicate a great potential of FIrPicOMPBHCz as a solid PL sensor for PA.

Improvement of Photostability and NIR Activity of Cyanine Dye through Nanohybrid Formation: Key Information from Ultrafast Dynamical Studies.

Near-infrared (NIR) light harvesting has enormous importance for different potential applications in the modern era of research. Some NIR cyanine dyes such as IR820 have achieved great success in energy harvesting and cancer therapy. However, their action is limited for low photostability, considerable thermal degradation, short circulation times, and nonspecific biodistribution. Our present study is an attempt to overcome such limitations by attaching a model cyanine dye IR820 with ZnO nanoparticles. We prepared an IR820-ZnO nanohybrid and characterized it using microscopic and optical spectroscopic tools. Thermogravimetric analysis depicted greater thermal stability of the IR820-ZnO nanohybrid compared to free dye. We explored the enhancement in the photostability of IR820 upon nanohybrid formation. We detected generation of photoinduced reactive oxygen species (ROS) such as superoxide, singlet oxygen, and so forth using appropriate molecular probes. The formation of IR820-ZnO nanohybrid reduced production of photoinduced singlet oxygen. However, it revealed an alternative trend in overall ROS formation (increases total ROS) under red light illumination. To correlate the enhanced photostability of IR820 on the ZnO surface, we explored excited-state dynamical processes at the interface in nanohybrids. We illustrated the photoinduced excited-state electron-transfer process from the lowest unoccupied molecular orbital of IR820 to the conduction band of ZnO. This photoelectron-transfer process enhances the production of ROS and decreases the formation of singlet oxygen that altogether leads to improvement in photostability and overall activity. A quencher of singlet oxygen sodium azide (NaN3) was used to further confirm the direct association of singlet oxygen generation with the photostability issue of IR820. Also, ZnO is able to deliver the dye selectively in acidic environment, which suggests its diseased site-specific targeted activity. Our results provide promising improvement for potential use of IR820 through formation of a nanohybrid that could be translated for other NIR cyanine dyes.

Evaluating Electronic Couplings for Excited State Charge Transfer Based on Maximum Occupation Method ΔSCF Quasi-Adiabatic States.

Electronic couplings of charge-transfer states with the ground state and localized excited states at the donor/acceptor interface are crucial parameters for controlling the dynamics of exciton dissociation and charge recombination processes in organic solar cells. Here we propose a quasi-adiabatic state approach to evaluate electronic couplings through combining maximum occupation method (mom)-ΔSCF and state diabatization schemes. Compared with time-dependent density functional theory (TDDFT) using global hybrid functional, mom-ΔSCF is superior to estimate the excitation energies of charge-transfer states; moreover it can also provide good excited electronic state for property calculation. Our approach is hence reliable to evaluate electronic couplings for excited state electron transfer processes, which is demonstrated by calculations on a typical organic photovoltaic system, oligothiophene/perylenediimide complex.

Highly sensitive iridium(iii) complex-based phosphorescent probe for thiophenol detection.

A cyclometalated iridium(iii) complex (Ir-DNBS) was designed and synthesized as a high-performance phosphorescent thiophenol probe. Ir-DNBS displays a distinct phosphorescence "off-on" response toward thiophenol with high selectivity, high sensitivity (detection limit: 2.5 nM) and fast response (10 min). It is noteworthy that the signaling phosphore of Ir-DNBS possesses relatively high photoluminescence quantum efficiency (ΦPL = 0.21) together with relatively long lifetime (τ = 2.07 μs), indicative of its potential in achieving high temporal resolution. Ir-DNBS is also applicable to the detection of thiophenol in actual water samples with high recovery rate. Photophysical and spectral characterization results revealed that the probing mechanism of Ir-DNBS toward thiophenol lies in the thiolate-mediated cleavage reaction, resulting in suppressed photo-induced excited state electron transfer process in the reaction product.

Three‐dimensional structure, binding, and spectroscopic characteristics of the monoclonal antibody 43.1 directed to the carboxyphenyl moiety of fluorescein

Unlike other known anti-fluorescein antibodies, the monoclonal antibody 43.1 is directed toward the fluorescein's carboxyl phenyl moiety. It demonstrates a very high affinity (KD ∼ 70 pM) and a fast association rate (kon ∼ 2 × 10(7) M(-1 ) s(-1) ). The three-dimensional structure of the Fab 43.1-fluorescein complex was resolved at 2.4 Å resolution. The antibody binding site is exclusively assembled by the CDR loops. It is comprised of a 14 Å groove-shaped entrance leading to a 9 Å by 7 Å binding pocket. The highly polar binding pocket complementary encloses the fluorescein's carboxyphenyl moiety and tightly fixes it by multiple hydrogen bonds. The fluorescein's xanthene ring is embedded in the more hydrophobic groove and stacked between the side chains of Tyr37L and of Arg99H providing conditions for an excited state electron transfer process. In comparison to fluorescein, the absorption spectrum of the complex in the visible region is shifted to the "red" by 23 nm. The complex demonstrates a very weak fluorescence (Φc = 0.0018) with two short lifetime components: 0.03 ns (47%) and 0.8 ns (24%), which reflects a 99.8% fluorescein emission quenching effect upon complex formation. The antibody 43.1 binds fluorescein with remarkable affinity, fast association rate, and strongly quenches its emission. Therefore, it may present a practical interest in applications such as molecular sensors and switches.

Quantum Dot Surface Chemistry: Ligand Effects and Electron Transfer Reactions

With the increased interest in quantum dot sensitized solar cells (QDSCs) there comes a need to better understand how surface modification of quantum dots (QDs) can affect the excited state dynamics of QDs, electron transfer at the QD–metal oxide (MO) interface, and overall photoconversion efficiency of QDSCs. We have monitored the surface modification of solution based QDs via the steady state absorption and emission characteristics of colloidal CdSe passivated with β-alanine (β-Ala). The trap-remediating nature of the β-Ala molecule, arising from the Lewis basicity of the amine group, is realized from the hypsochromic shifts seen in excitonic absorption and emission bands as well as an increase in fluorescence quantum yield. Transient absorption measurements of CdSe–TiO2 films prepared with and without β-Ala as a linker molecule further reveal the role of the surface modifier in influencing excited state electron transfer processes. Electron transfer at this interface was found to be dependent on the me...

Bimolecular Electron Transfers That Follow a Sandros−Boltzmann Dependence on Free Energy

Rate constants (k) for exergonic and endergonic electron-transfer reactions of equilibrating radical cations (A(•+) + B ⇌ A + B(•+)) in acetonitrile could be fit well by a simple Sandros-Boltzmann (SB) function of the reaction free energy (ΔG) having a plateau with a limiting rate constant k(lim) in the exergonic region, followed, near the thermoneutral point, by a steep drop in log k vs ΔG with a slope of 1/RT. Similar behavior was observed for another charge shift reaction, the electron-transfer quenching of excited pyrylium cations (P(+)*) by neutral donors (P(+)* + D → P(•) + D(•+)). In this case, SB dependence was observed when the logarithm of the quenching constant (log k(q)) was plotted vs ΔG + s, where the shift term, s, equals +0.08 eV and ΔG is the free energy change for the net reaction (E(redox) - E(excit)). The shift term is attributed to partial desolvation of the radical cation in the product encounter pair (P(•)/D(•+)), which raises its free energy relative to the free species. Remarkably, electron-transfer quenching of neutral reactants (A* + D → A(•-) + D(•+)) using excited cyanoaromatic acceptors and aromatic hydrocarbon donors was also found to follow an SB dependence of log k(q) on ΔG, with a positive s, +0.06 eV. This positive shift contrasts with the long-accepted prediction of a negative value, -0.06 eV, for the free energy of an A(•-)/D(•+) encounter pair relative to the free radical ions. That prediction incorporated only a Coulombic stabilization of the A(•-)/D(•+) encounter pair relative to the free radical ions. In contrast, the results presented here show that the positive value of s indicates a decrease in solvent stabilization of the A(•-)/D(•+) encounter pair, which outweighs Coulombic stabilization in acetonitrile. These quenching reactions are proposed to proceed via rapidly interconverting encounter pairs with an exciplex as intermediate, A*/D ⇌ exciplex ⇌ A(•-)/D(•+). Weak exciplex fluorescence was observed in each case. For several reactions in the endergonic region, rate constants for the reversible formation and decay of the exciplexes were determined using time-correlated single-photon counting. The quenching constants derived from the transient kinetics agreed well with those from the conventional Stern-Volmer plots. For excited-state electron-transfer processes, caution is required in correlating quenching constants vs reaction free energies when ΔG exceeds ∼+0.1 eV. Beyond this point, additional exciplex deactivation pathways-fluorescence, intersystem crossing, and nonradiative decay-are likely to dominate, resulting in a change in mechanism.

Effects of Photoinduced Electron Transfer on the Rational Design of Molecular Fluorescence Switch

Aiming at exploring the relationship between the spacer and fluorescence switch properties, we synthesized a series of new photoactive triads, in which one perylenetetracarboxylic diimide unit acting as the electron acceptor was attached to two ferrocene moieties through different spacers. This kind of donor-spacer-acceptor structure allows for tuning one of the key factors that governs photoinduced electron transfer, the distance between the electron donor and acceptor units. The excited-state electron-transfer processes were monitored by both steady-state and time-resolved emission as well as transient absorption techniques. It was found that fluorescence intensity of the solution of all triads 1-3 can be reversibly modulated by the electrochemical oxidation and reduction sequentially. More importantly, as the length of the spacer between the donor and acceptor increases, the background fluorescence increased proportionally, but the contrast ratio of the fluorescence decreases. Together these two factors determine the assay sensitivity, and therefore this work is helpful to provide a basis for the rational design of fluorescence switch by optimizing these factors above.

Synthesis and Photoinduced Electron‐Transfer Process of a Novel Triphenylamine‐Substituted Polyfluorene–C60 Triad

The photoinduced electron-transfer process of a newly prepared, soluble, pi-conjugated poly[9,9-bis(4-diphenylaminophenyl)-2,7-fluorene] (PDPAF), covalently bridged, C60 triad (C60-PDPAF-C60) is described. The molecular orbital calculations revealed that the majority of the highest occupied molecular orbital (HOMO) is located on the polyfluorene entity, while the lowest unoccupied molecular orbitals (LUMO) are found to be entirely on the C60 entity. The excited-state electron-transfer processes were monitored by both steady-state and time-resolved emission as well as by transient absorption techniques in toluene and benzonitrile. By excitation of the polyfluorene moiety, fluorescence quenching of the singlet excited state of polyfluorene moiety was observed. The nanosecond transient spectra in near-IR region revealed the charge-separation process from the polyfluorene moieties to the C60 moiety through the excited singlet states of polyfluorene. The lifetimes of the charge separated states were evaluated to be 20-50 ns, depending on the solvent polarity.

Intramolecular energy transfer in a tetra-coumarin perylene system: influence of solvent and bridging unit on electronic properties

The synthesis and characterisation of a novel coumarin donor-perylene bisimide acceptor light-harvesting system is reported, in which an energy-transfer efficiency of >99% is achieved. Comparison of the excited-state properties of the donor-acceptor system with model compounds revealed that although the photophysical properties of the perylene bisimide acceptor unit are affected considerably by the nature of the substituent at the imide positions and the solvent employed, through-bond interaction between the donor and acceptor units is negligible. Energy transfer in the present system can be described as occurring via a through-space energy-transfer mechanism. Careful consideration of the redox properties of the donor relative to the acceptor units allows for avoidance of potentially deleterious excited-state electron-transfer processes.

Electron-Acceptor-Induced Isomerization of Aryl [6,5] Open Fulleroids to [6,6] Closed Methanofullerenes and the Electrochemical Evaluation of Their Free Energy Difference

The thermal and photochemical rearrangements of a series of aryl substituted [6,5] open fulleroids to [6,6] closed methanofullerenes are accelerated in the presence in of electron acceptors. These [6,5] open fulleroids, facilitated by electron acceptors, rearrange thermally by a zwitteronic-type intermediate, while the photochemical reactions proceed via an excited-state electron-transfer process. The oxidation potentials of these [6,5] open fulleroids and their corresponding [6,6] closed methanofullerenes isomers have been evaluated. The free energy difference between the [6,5] open fulleroids and their corresponding [6,6] closed isomers have been estimated from the difference in their oxidation potentials.

Ultrafast Energy-Electron Transfer Cascade in a Multichromophoric Light-Harvesting Molecular Square

A molecular square with dimensions of about 4 nm, incorporating sixteen pyrene chromophores attached to four ditopic bay-functionalized perylene bisimide chromophores, has been synthesized by coordination to four Pt(II) phosphine corner units and fully characterized via NMR spectroscopy and ESI-FTICR mass spectrometry. Steady-state and time-resolved emission as well as femtosecond transient absorption studies reveal the presence of a highly efficient (>90%) and fast photoinduced energy transfer (k(en) approximately equal to 5.0 x 10(9) s(-1)) from the pyrene to the perylene bisimide chromophores and a very fast and efficient electron transfer (>94%, k(et) approximately equal to 5 x 10(11) up to 43 x 10(11) s(-1)). Spectrotemporal parametrization indicates upper excited-state electron-transfer processes, various energy and electron-transfer pathways, and chromophoric heterogeneity. Temperature-dependent time-resolved emission spectroscopy has shown that the acceptor emission lifetime increases with decreasing temperature from which an electron-transfer barrier is obtained. The extremely fast electron-transfer processes (substantially faster and more efficient than in the free ligand) that are normally only observed in solid materials, together with the closely packed structure of 20 chromophoric units, indicate that we can consider the molecular square as a monodisperse nanoaggregate: a molecularly defined ensemble of chromophores that partly behaves like a solid material.

Dyads and Triads Containing Perylenetetracarboxylic Diimide and Porphyrin: Efficient Photoinduced Electron Transfer Elicited via Both Excited Singlet States

Synthesis, characterizations, and photophysical properties of new photoactive dyads and triads containing perylenetetracarboxylic diimide (PIm) and porphyrin (free-base porphyrin (H(2)P) and zinc porphyrin (ZnP)), in which both entities were connected with a short ether bond, were examined with the aim of using these systems for molecular photonics. The porphyrin(P)-PIm systems absorbed strongly across the visible region, which greatly matched the solar spectrum. The geometric and electronic structures of the dyads and triads were probed using density function theory method at the B3LYP/3-21G level. It was revealed that the majority of the highest-occupied molecular orbital was located on the porphyrin entity, while the lowest-unoccupied molecular orbitals were entirely on the PIm entity. The excited-state electron-transfer processes were monitored by both steady-state and time-resolved emission as well as transient-absorption techniques in polar solvent benzonitrile. Upon excitation of the P (H(2)P and ZnP) moieties, efficient fluorescence quenching of the P moiety was observed, suggesting that the main quenching paths involved charge separation from the excited singlet porphyrin ((1)P) to the PIm moiety. Upon excitation of the PIm moiety, fluorescence quenching of the (1)PIm moiety was also observed. The nanosecond transience of spectra in near-IR region revealed the charge separation process from the P moieties to the PIm moiety via their excited singlet states. The lifetimes of the charge-separated states were evaluated to be 7-14 ns, depending on the solvent polarity. Photosensitized electron mediation systems were also revealed in the presence of methyl viologen and sacrificial electron donor.

Studies on intra-supramolecular and intermolecular electron-transfer processes between zinc naphthalocyanine and imidazole-appended fullerene.

Spectroscopic, computational, redox, and photochemical behavior of a self-assembled donor-acceptor dyad formed by axial coordination of zinc naphthalocyanine, ZnNc, and fulleropyrrolidine bearing an imidazole coordinating ligand (2-(4'-imidazolylphenyl)-fulleropyrrolidine, C60Im) was investigated in noncoordinating solvents, toluene and o-dichlorobenzene, and the results were compared to the intermolecular electron transfer processes in a coordinating solvent, benzonitrile. The optical absorption and ab initio B3 LYP/3-21G(*) computational studies revealed self-assembled supramolecular 1:1 dyad formation between the ZnNc and C60Im entities. In the optimized structure, the HOMO was found to be entirely located on the ZnNc entity while the LUMO was found to be entirely on the fullerene entity. Cyclic voltammetry studies of the dyad exhibited a total of seven one-electron redox processes in o-dichlorobenzene, with 0.1 M tetrabutylammonium perchlorate. The excited-state electron-transfer processes were monitored by both optical-emission and transient-absorption techniques. Direct evidence for the radical-ion-pair (C60Im.-:ZnNC.+) formation was obtained from picosecond transient-absorption spectral studies, which indicated charge separation from the singlet-excited ZnNc to the C60Im moiety. The calculated rates of charge separation and charge recombination were 1.4 x 10(10) s-1 and 5.3 x 10(7) s-1 in toluene and 8.9 x 10(9) s-1 and 9.2 x 10(7) s-1 in o-dichlorobenzene, respectively. In benzonitrile, intermolecular electron transfer from the excited triplet state of ZnNc to C60Im occurs and the second-order rate constant (kqtriplet) for this quenching process was 5.3 x 10(8)M-1s-1.

Intramolecular shielding of charge-transfer excited states of surfactant active rhenium(I) photosensitizers

Excited-state electron-transfer processes involving organized assemblies are currently under active study. Such systems show promise for increasing the efficiency of energy-storage reactions. In particular, surfactant-active analogues of Ru(bpy)/sub 3//sup 2 +/ (bpy = 2,2'-bipyridine) with long hydrophobic alkyl chains have been used to modify sensitizer chemistry and photophysics. Reported here is an investigation of a new series of surfactant-active Re(I) photosensitizers: fac-((bpy)Re/sup 1/(CO)/sub 3/L)/sup +/ (L = NC(CH/sub 2/)/sub n/CH/sub 3/, n = 0,2,5,6,7,9,10,13,17). These systems exhibit intense d-..pi..* metal to ligand charge-transfer (MLCT) emissions in fluid solutions similar to the parent acetonitrile complex (n = 0). A dramatic example of an intramolecular perturbation of CT excited-state properties by the electronically passive alkyl chain is examined.

PHOTOOXIDATION AND SINGLET OXYGEN SENSITIZATION BY PROTOPORPHYRIN IX AND ITS PHOTOOXIDATION PRODUCTS

AbstractProtoporphyrin IX and its various ester derivatives have been previously shown to undergo self‐sensitized photooxygenation to yield hydroxyaldehydes (photoprotoporphyrin) and mono‐ and diformyl deuteroporphyrin derivatives. In the present study the photoreactions of these products in the presence of oxygen have been investigated. All of the photooxidation products are themselves good sensitizers of singlet oxygen. In addition spin trapping experiments indicate these products can produce superoxide in low‐to‐moderate efficiency by an excited state electron transfer process. The photo‐products themselves are somewhat more stable to photooxidation than protoporphyrin IX itself. The two monoformyl‐monovinyl deuteroporphyrins have been found to undergo further photooxidation at the vinyl groups to yield primarily monoformyl hydroxyaldehydes in a reaction mainly involving singlet oxygen analogous to the initial reaction of protoporphyrin IX.

Photosensitization of oxygen-bridged dicobalt(III) cations by the tris(2,2'-bipyridine)ruthenium(II) excited state. Luminescence quenching and product quantum yields

Mechanisms of quenching of the excited state of Ru(bpy)/sub 3//sup 2 +/ by superoxo-bridged dinuclear cobalt(III) complexes are investigated by using luminescence quenching studies and by product quantum yield analysis. Bimolecular quenching constants are of the order of 10/sup 9/ M/sup -1/ s/sup -1/, which is somewhat higher than the value expected on formal charge basis. Formation of Ru(bpy)/sub 3//sup 3 +/ and the cobaltous ion as products of photosensitization although in low yields is explained to occur in an energy-transfer pathway. The energy-transfer pathway accounts for about 10% of the quenching events whereas the excited-state electron-transfer process accounts for 90% of the quenching. The dark redox reaction which follows the energy-transfer pathway leads to the products formed in steady photolysis studies.