Tailoring the Photophysical Properties of a Homoleptic Iron(II) Tetra N-Heterocyclic Carbene Complex by Attaching an Imidazolium Group to the (C∧N∧C) Pincer Ligand─A Comparative Study.

We here report the synthesis of the homoleptic iron(II) N-heterocyclic carbene (NHC) complex [Fe(miHpbmi)2](PF6)4 (miHpbmi = 4-((3-methyl-1H-imidazolium-1-yl)pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) and its electrochemical and photophysical properties. The introduction of the π-electron-withdrawing 3-methyl-1H-imidazol-3-ium-1-yl group into the NHC ligand framework resulted in stabilization of the metal-to-ligand charge transfer (MLCT) state and destabilization of the metal-centered (MC) states. This resulted in an improved excited-state lifetime of 16 ps compared to the 9 ps for the unsubstituted parent compound [Fe(pbmi)2](PF6)2 (pbmi = (pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) as well as a stronger MLCT absorption band extending more toward the red spectral region. However, compared to the carboxylic acid derivative [Fe(cpbmi)2](PF6)2 (cpbmi = 1,1'-(4-carboxypyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)), the excited-state lifetime of [Fe(miHpbmi)2](PF6)4 is the same, but both the extinction and the red shift are more pronounced for the former. Hence, this makes [Fe(miHpbmi)2](PF6)4 a promising pH-insensitive analogue of [Fe(cpbmi)2](PF6)2. Finally, the excited-state dynamics of the title compound [Fe(miHpbmi)2](PF6)4 was investigated in solvents with different viscosities, however, showing very little dependency of the depopulation of the excited states on the properties of the solvent used.

How Rigidity and Conjugation of Bidentate Ligands Affect the Geometry and Photophysics of Iron N-Heterocyclic Complexes: A Comparative Study.

Two iron complexes featuring the bidentate, nonconjugated N-heterocyclic carbene (NHC) 1,1'-methylenebis(3-methylimidazol-2-ylidene) (mbmi) ligand, where the two NHC moieties are separated by a methylene bridge, have been synthesized to exploit the combined influence of geometric and electronic effects on the ground- and excited-state properties of homoleptic FeIII-hexa-NHC [Fe(mbmi)3](PF6)3 and heteroleptic FeII-tetra-NHC [Fe(mbmi)2(bpy)](PF6)2 (bpy = 2,2'-bipyridine) complexes. They are compared to the reported FeIII-hexa-NHC [Fe(btz)3](PF6)3 and FeII-tetra-NHC [Fe(btz)2(bpy)](PF6)2 complexes containing the conjugated, bidentate mesoionic NHC ligand 3,3'-dimethyl-1,1'-bis(p-tolyl)-4,4'-bis(1,2,3-triazol-5-ylidene) (btz). The observed geometries of [Fe(mbmi)3](PF6)3 and [Fe(mbmi)2(bpy)](PF6)2 are evaluated through L-Fe-L bond angles and ligand planarity and compared to those of [Fe(btz)3](PF6)3 and [Fe(btz)2(bpy)](PF6)2. The FeII/FeIII redox couples of [Fe(mbmi)3](PF6)3 (-0.38 V) and [Fe(mbmi)2(bpy)](PF6)2 (-0.057 V, both vs Fc+/0) are less reducing than [Fe(btz)3](PF6)3 and [Fe(btz)2(bpy)](PF6)2. The two complexes show intense absorption bands in the visible region: [Fe(mbmi)3](PF6)3 at 502 nm (ligand-to-metal charge transfer, 2LMCT) and [Fe(mbmi)2(bpy)](PF6)2 at 410 and 616 nm (metal-to-ligand charge transfer, 3MLCT). Lifetimes of 57.3 ps (2LMCT) for [Fe(mbmi)3](PF6)3 and 7.6 ps (3MLCT) for [Fe(mbmi)2(bpy)](PF6)2 were probed and are somewhat shorter than those for [Fe(btz)3](PF6)3 and [Fe(btz)2(bpy)](PF6)2. [Fe(mbmi)3](PF6)3 exhibits photoluminescence at 686 nm (2LMCT) in acetonitrile at room temperature with a quantum yield of (1.2 ± 0.1) × 10-4, compared to (3 ± 0.5) × 10-4 for [Fe(btz)3](PF6)3.

Multifaceted Deactivation Dynamics of Fe(II) N-Heterocyclic Carbene Photosensitizers.

Excited state dynamics of three iron(II) carbene complexes that serve as prototype Earth-abundant photosensitizers were investigated by ultrafast optical spectroscopy. Significant differences in the dynamics between the investigated complexes down to femtosecond time scales are used to characterize fundamental differences in the depopulation of triplet metal-to-ligand charge-transfer (3MLCT) excited states in the presence of energetically accessible triplet metal-centered (3MC) states. Novel insights into the full deactivation cascades of the investigated complexes include evidence of the need to revise the deactivation model for a prominent iron carbene prototype complex, a refined understanding of complex 3MC dynamics, and a quantitative discrimination between activated and barrierless deactivation steps along the 3MLCT → 3MC → 1GS path. Overall, the study provides an improved understanding of photophysical limitations and opportunities for the use of iron(II)-based photosensitizers in photochemical applications.

Competing dynamics of intramolecular deactivation and bimolecular charge transfer processes in luminescent Fe(iii) N-heterocyclic carbene complexes.

Steady state and ultrafast spectroscopy on [FeIII(phtmeimb)2]PF6 (phtmeimb = phenyl(tris(3-methylimidazol-2-ylidene))borate) was performed over a broad range of temperatures. The intramolecular deactivation dynamics of the luminescent doublet ligand-to-metal charge-transfer (2LMCT) state was established based on Arrhenius analysis, indicating the direct deactivation of the 2LMCT state to the doublet ground state as a key limitation to the lifetime. In selected solvent environments photoinduced disproportionation generating short-lived Fe(iv) and Fe(ii) complex pairs that subsequently undergo bimolecular recombination was observed. The forward charge separation process is found to be temperature-independent with a rate of ∼1 ps-1. Subsequent charge recombination takes place in the inverted Marcus region with an effective barrier of 60 meV (483 cm-1). Overall, the photoinduced intermolecular charge separation efficiently outcompetes the intramolecular deactivation over a broad range of temperatures, highlighting the potential of [FeIII(phtmeimb)2]PF6 to perform photocatalytic bimolecular reactions.

Optical cavity-mediated exciton dynamics in photosynthetic light harvesting 2 complexes.

Strong light-matter interaction leads to the formation of hybrid polariton states and alters the photophysical dynamics of organic materials and biological systems without modifying their chemical structure. Here, we experimentally investigated a well-known photosynthetic protein, light harvesting 2 complexes (LH2) from purple bacteria under strong coupling with the light mode of a Fabry-Perot optical microcavity. Using femtosecond pump probe spectroscopy, we analyzed the polariton dynamics of the strongly coupled system and observed a significant prolongation of the excited state lifetime compared with the bare exciton, which can be explained in terms of the exciton reservoir model. Our findings indicate the potential of tuning the dynamic of the whole photosynthetic unit, which contains several light harvesting complexes and reaction centers, with the help of strong exciton-photon coupling, and opening the discussion about possible design strategies of artificial photosynthetic devices.

Photophysical Integrity of the Iron(III) Scorpionate Framework in Iron(III)-NHC Complexes with Long-Lived 2LMCT Excited States.

Fe(III) complexes with N-heterocyclic carbene (NHC) ligands belong to the rare examples of Earth-abundant transition metal complexes with long-lived luminescent charge-transfer excited states that enable applications as photosensitizers for charge separation reactions. We report three new hexa-NHC complexes of this class: [Fe(brphtmeimb)2]PF6 (brphtmeimb = [(4-bromophenyl)tris(3-methylimidazol-2-ylidene)borate]-, [Fe(meophtmeimb)2]PF6 (meophtmeimb = [(4-methoxyphenyl)tris(3-methylimidazol-2-ylidene)borate]-, and [Fe(coohphtmeimb)2]PF6 (coohphtmeimb = [(4-carboxyphenyl)tris(3-methylimidazol-2-ylidene)borate]-. These were derived from the parent complex [Fe(phtmeimb)2]PF6 (phtmeimb = [phenyltris(3-methylimidazol-2-ylidene)borate]- by modification with electron-withdrawing and electron-donating substituents, respectively, at the 4-phenyl position of the ligand framework. All three Fe(III) hexa-NHC complexes were characterized by NMR spectroscopy, high-resolution mass spectroscopy, elemental analysis, single crystal X-ray diffraction analysis, electrochemistry, Mößbauer spectroscopy, electronic spectroscopy, magnetic susceptibility measurements, and quantum chemical calculations. Their ligand-to-metal charge-transfer (2LMCT) excited states feature nanosecond lifetimes (1.6-1.7 ns) and sizable emission quantum yields (1.7-1.9%) through spin-allowed transition to the doublet ground state (2GS), completely in line with the parent complex [Fe(phtmeimb)2]PF6 (2.0 ns and 2.1%). The integrity of the favorable excited state characteristics upon substitution of the ligand framework demonstrates the robustness of the scorpionate motif that tolerates modifications in the 4-phenyl position for applications such as the attachment in molecular or hybrid assemblies.

Borylation in the Second Coordination Sphere of FeII Cyanido Complexes and Its Impact on Their Electronic Structures and Excited-State Dynamics.

Second coordination sphere interactions of cyanido complexes with hydrogen-bonding solvents and Lewis acids are known to influence their electronic structures, whereby the non-labile attachment of B(C6F5)3 resulted in several particularly interesting new compounds lately. Here, we investigate the effects of borylation on the properties of two FeII cyanido complexes in a systematic manner by comparing five different compounds and using a range of experimental techniques. Electrochemical measurements indicate that borylation entails a stabilization of the FeII-based t2g-like orbitals by up to 1.65 eV, and this finding was confirmed by Mössbauer spectroscopy. This change in the electronic structure has a profound impact on the UV-vis absorption properties of the borylated complexes compared to the non-borylated ones, shifting their metal-to-ligand charge transfer (MLCT) absorption bands over a wide range. Ultrafast UV-vis transient absorption spectroscopy provides insight into how borylation affects the excited-state dynamics. The lowest metal-centered (MC) excited states become shorter-lived in the borylated complexes compared to their cyanido analogues by a factor of ∼10, possibly due to changes in outer-sphere reorganization energies associated with their decay to the electronic ground state as a result of B(C6F5)3 attachment at the cyanido N lone pair.

In situpassivation of GaxIn(1-x)P nanowires using radial AlyIn(1-y)P shells grown by MOVPE.

GaxIn(1-x)P nanowires with suitable bandgap (1.35-2.26 eV) ranging from the visible to near-infrared wavelength have great potential in optoelectronic applications. Due to the large surface-to-volume ratio of nanowires, the surface states become a pronounced factor affecting device performance. In this work, we performed a systematic study of GaxIn(1-x)P nanowires' surface passivation, utilizing AlyIn(1-y)P shells grownin situby using a metal-organic vapor phase epitaxy system. Time-resolved photoinduced luminescence and time-resolved THz spectroscopy measurements were performed to study the nanowires' carrier recombination processes. Compared to the bare Ga0.41In0.59P nanowires without shells, the hole and electron lifetime of the nanowires with the Al0.36In0.64P shells are found to be larger by 40 and 1.1 times, respectively, demonstrating effective surface passivation of trap states. When shells with higher Al composition were grown, both lifetimes of free holes and electrons decreased prominently. We attribute the acceleration of PL decay to an increase in the trap states' density due to the formation of defects, including the polycrystalline and oxidized amorphous areas in these samples. Furthermore, in a separate set of samples, we varied the shell thickness. We observed that a certain shell thickness of approximately ∼20 nm is needed for efficient passivation of Ga0.31In0.69P nanowires. The photoconductivity of the sample with a shell thickness of 23 nm decays 10 times slower compared with that of the bare core nanowires. We concluded that both the hole and electron trapping and the overall charge recombination in GaxIn(1-x)P nanowires can be substantially passivated through growing an AlyIn(1-y)P shell with appropriate Al composition and thickness. Therefore, we have developed an effectivein situsurface passivation of GaxIn(1-x)P nanowires by use of AlyIn(1-y)P shells, paving the way to high-performance GaxIn(1-x)P nanowires optoelectronic devices.

Microsecond Photoluminescence and Photoreactivity of a Metal-Centered Excited State in a Hexacarbene-Co(III) Complex.

The photofunctionality of the cobalt-hexacarbene complex [Co(III)(PhB(MeIm)3)2]+ (PhB(MeIm)3 = tris(3-methylimidazolin-2-ylidene)(phenyl)borate) has been investigated by time-resolved optical spectroscopy. The complex displays a weak (Φ âˆ¼ 10-4) but remarkably long-lived (τ ∼ 1 µs) orange photoluminescence at 690 nm in solution at room temperature following excitation with wavelengths shorter than 350 nm. The strongly red-shifted emission is assigned from the spectroscopic evidence and quantum chemical calculations as a rare case of luminescence from a metal-centered state in a 3d6 complex. Singlet oxygen quenching supports the assignment of the emitting state as a triplet metal-centered state and underlines its capability of driving excitation energy transfer processes.

Dye-sensitized solar cells based on Fe N-heterocyclic carbene photosensitizers with improved rod-like push-pull functionality.

A new generation of octahedral iron(ii)-N-heterocyclic carbene (NHC) complexes, employing different tridentate C^N^C ligands, has been designed and synthesized as earth-abundant photosensitizers for dye sensitized solar cells (DSSCs) and related solar energy conversion applications. This work introduces a linearly aligned push-pull design principle that reaches from the ligand having nitrogen-based electron donors, over the Fe(ii) centre, to the ligand having an electron withdrawing carboxylic acid anchor group. A combination of spectroscopy, electrochemistry, and quantum chemical calculations demonstrate the improved molecular excited state properties in terms of a broader absorption spectrum compared to the reference complex, as well as directional charge-transfer displacement of the lowest excited state towards the semiconductor substrate in accordance with the push-pull design. Prototype DSSCs based on one of the new Fe NHC photosensitizers demonstrate a power conversion efficiency exceeding 1% already for a basic DSSC set-up using only the I-/I3 - redox mediator and standard operating conditions, outcompeting the corresponding DSSC based on the homoleptic reference complex. Transient photovoltage measurements confirmed that adding the co-sensitizer chenodeoxycholic acid helped in improving the efficiency by increasing the electron lifetime in TiO2. Time-resolved spectroscopy revealed spectral signatures for successful ultrafast (<100 fs) interfacial electron injection from the heteroleptic dyes to TiO2. However, an ultrafast recombination process results in undesirable fast charge recombination from TiO2 back to the oxidized dye, leaving only 5-10% of the initially excited dyes available to contribute to a current in the DSSC. On slower timescales, time-resolved spectroscopy also found that the recombination dynamics (longer than 40 µs) were significantly slower than the regeneration of the oxidized dye by the redox mediator (6-8 µs). Therefore it is the ultrafast recombination down to fs-timescales, between the oxidized dye and the injected electron, that remains as one of the main bottlenecks to be targeted for achieving further improved solar energy conversion efficiencies in future work.

Carrier Recombination Processes in GaAs Wafers Passivated by Wet Nitridation.

As one of the successful approaches to GaAs surface passivation, wet-chemical nitridation is applied here to relate the effect of surface passivation to carrier recombination processes in bulk GaAs. By combining time-resolved photoluminescence and optical pump-THz probe measurements, we found that surface hole trapping dominates the decay of photoluminescence, while photoconductivity dynamics is limited by surface electron trapping. Compared to untreated sample dynamics, the optimized nitridation reduces hole- and electron-trapping rate by at least 2.6 and 3 times, respectively. Our results indicate that under ambient conditions, recovery of the fast hole trapping due to the oxide regrowth at the deoxidized GaAs surface takes tens of hours, while it is effectively inhibited by surface nitridation. Our study demonstrates that surface nitridation stabilizes the GaAs surface via reduction of both electron- and hole-trapping rates, which results in chemical and electronical passivation of the bulk GaAs surface.

Tracing the Full Bimolecular Photocycle of Iron(III)-Carbene Light Harvesters in Electron-Donating Solvents.

Photoinduced bimolecular charge transfer processes involving the iron(III) N-heterocyclic carbene (FeNHC) photosensitizer [Fe(phtmeimb)2]+ (phtmeimb = phenyltris(3-methyl-imidazolin-2-ylidene)borate) and triethylamine as well as N,N-dimethylaniline donors have been studied using optical spectroscopy. The full photocycle of charge separation and recombination down to ultrashort time scales was studied by investigating the excited-state dynamics up to high quencher concentrations. The unconventional doublet ligand-to-metal charge transfer (2LMCT) photoactive excited state exhibits donor-dependent charge separation rates of up to 1.25 ps-1 that exceed the rates found for typical ruthenium-based systems and are instead more similar to results reported for organic sensitizers. The ultrafast charge transfer probed at high electron donor concentrations outpaces the solvent dynamics and goes beyond the classical Marcus electron transfer regime. Poor photoproduct yields are explained by donor-independent, fast charge recombination with rates of ∼0.2 ps-1, thus inhibiting cage escape and photoproduct formation. This study thus shows that the ultimate bottlenecks for bimolecular photoredox processes involving these FeNHC photosensitizers can only be determined from the ultrafast dynamics of the full photocycle, which is of particular importance when the bimolecular charge transfer processes are not limited by the intrinsic excited-state lifetime of the photosensitizer.

Vibronic coherence contributes to photocurrent generation in organic semiconductor heterojunction diodes.

Charge separation dynamics after the absorption of a photon is a fundamental process relevant both for photosynthetic reaction centers and artificial solar conversion devices. It has been proposed that quantum coherence plays a role in the formation of charge carriers in organic photovoltaics, but experimental proofs have been lacking. Here we report experimental evidence of coherence in the charge separation process in organic donor/acceptor heterojunctions, in the form of low frequency oscillatory signature in the kinetics of the transient absorption and nonlinear two-dimensional photocurrent spectroscopy. The coherence plays a decisive role in the initial ~200 femtoseconds as we observe distinct experimental signatures of coherent photocurrent generation. This coherent process breaks the energy barrier limitation for charge formation, thus competing with excitation energy transfer. The physics may inspire the design of new photovoltaic materials with high device performance, which explore the quantum effects in the next-generation optoelectronic applications.

Effect of hydrogen chloride etching on carrier recombination processes of indium phosphide nanowires.

Introduction of in situ HCl etching to an epitaxial growth process has been shown to suppress radial growth and improve the morphology and optical properties of nanowires. In this paper, we investigate the dynamics of photo-generated charge carriers in a series of indium phosphide nanowires grown with varied HCl fluxes. Time resolved photo-induced luminescence, transient absorption and time resolved terahertz spectroscopy were employed to investigate charge trapping and recombination processes in the nanowires. Since the excitation photons generate charges predominantly in less than a half length of the nanowires, we can selectively assess the charge carrier dynamics at their top and bottom. We found that the photoluminescence decay is dominated by the decay of the mobile hole population due to trapping, which is affected by the HCl etching. The hole trapping rate is in general faster at the top of the nanowires than at the bottom. In contrast, electrons remain highly mobile until they recombine non-radiatively with the trapped holes. The slowest hole trapping as well as the least efficient non-radiative recombination was recorded for etching using the HCl molar fraction of χHCl = 5.4 × 10-5.

Effect of Post-Thermal Annealing on the Performance and Charge Photogeneration Dynamics of PffBT4T-2OD/PC71BM Solar Cells.

In this work, we studied influence of post thermal annealing on the performance and charge photogeneration processes of PffBT4T-2OD/PC71BM solar cells. As-prepared device exhibits a high-power conversion efficiency of 9.5%, much higher than that after thermal annealing. To understand this phenomenon, we studied charge photogeneration processes in these solar cells by means of time resolved spectroscopy. We associate the degradation of solar cell performance with the reduction of exciton dissociation efficiency and with increased bimolecular recombination of photogenerated charges as a result of annealing. We correlate the generation of localized PffBT4T-2OD polarons observed via spectro-electrochemical measurements with enhancement of the bimolecular charge recombination of annealed solar cells.

14.7% Efficiency Organic Photovoltaic Cells Enabled by Active Materials with a Large Electrostatic Potential Difference.

Although significant improvements have been achieved for organic photovoltaic cells (OPVs), the top-performing devices still show power conversion efficiencies far behind those of commercialized solar cells. One of the main reasons is the large driving force required for separating electron-hole pairs. Here, we demonstrate an efficiency of 14.7% in the single-junction OPV by using a new polymer donor PTO2 and a nonfullerene acceptor IT-4F. The device possesses an efficient charge generation at a low driving force. Ultrafast transient absorption measurements probe the formation of loosely bound charge pairs with extended lifetime that impedes the recombination of charge carriers in the blend. The theoretical studies reveal that the molecular electrostatic potential (ESP) between PTO2 and IT-4F is large, and the induced intermolecular electric field may assist the charge generation. The results suggest OPVs have the potential for further improvement by judicious modulation of ESP.

Band-selective dynamics in charge-transfer excited iron carbene complexes.

Ultrafast dynamics of photoinduced charge transfer processes in light-harvesting systems based on Earth-abundant transition metal complexes are of current interest for the development of molecular devices for solar energy conversion applications. A combination of ultrafast spectroscopy and first principles quantum chemical calculations of a recently synthesized iron carbene complex is used to elucidate the ultrafast excited state evolution processes in these systems with particular emphasis on investigating the underlying reasons why these complexes show promise in terms of significantly extended lifetimes of charge transfer excited states. Together, our results challenge the traditional excited state landscape for iron-based light harvesting transition metal complexes through radically different ground and excited state properties in alternative oxidation states. This includes intriguing indications of rich band-selective excited state dynamics on ultrafast timescales that are interpreted in terms of excitation energy dependence for excitations into a manifold of charge-transfer states. Some implications of the observed excited state properties and photoinduced dynamics for the utilization of iron carbene complexes for solar energy conversion applications are finally discussed.

Dimension Engineering of High-Quality InAs Nanostructures on a Wafer Scale.

Low-dimensional narrow-band-gap III-V semiconductors are key building blocks for the next generation of high-performance nanoelectronics, nanophotonics, and quantum devices. Realizing these various applications requires an efficient methodology that enables the material dimensional control during the synthesis process and the mass production of these materials with perfect crystallinity, reproducibility, low cost, and outstanding electronic and optoelectronic properties. Although advances in one- and two-dimensional narrow-band-gap III-V semiconductors synthesis, the progress toward reliable methods that can satisfy all of these requirements has been limited. Here, we demonstrate an approach that provides a precise control of the dimension of InAs from one-dimensional nanowires to wafer-scale free-standing two-dimensional nanosheets, which have a high degree of crystallinity and outstanding electrical and optical properties, using molecular-beam epitaxy by controlling catalyst alloy segregation. In our approach, two-dimensional InAs nanosheets can be obtained directly from one-dimensional InAs nanowires by silver-indium alloy segregation, which is much easier than the previously reported methods, such as the traditional buffering technique and select-area epitaxial growth. Detailed transmission electron microscopy investigations provide solid evidence that the catalyst alloy segregation is the origination of the InAs dimensional transformation from one-dimensional nanowires to two-dimensional nanosheets and even to three-dimensional complex crosses. Using this method, we find that the wafer-scale free-standing InAs nanosheets can be grown on various substrates including Si, MgO, sapphire, GaAs, etc. The InAs nanosheets grown at high temperature are pure-phase single crystals and have a high electron mobility and a long time-resolved terahertz kinetics lifetime. Our work will open up a conceptually new and general technology route toward the effective controlling of the dimension of the low-dimensional III-V semiconductors. It may also enable the low-cost fabrication of free-standing nanosheet-based devices on an industrial scale.

Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime.

Iron's abundance and rich coordination chemistry are potentially appealing features for photochemical applications. However, the photoexcitable charge-transfer states of most iron complexes are limited by picosecond or subpicosecond deactivation through low-lying metal-centered states, resulting in inefficient electron-transfer reactivity and complete lack of photoluminescence. In this study, we show that octahedral coordination of iron(III) by two mono-anionic facial tris-carbene ligands can markedly suppress such deactivation. The resulting complex [Fe(phtmeimb)2]+, where phtmeimb is {phenyl[tris(3-methylimidazol-1-ylidene)]borate}-, exhibits strong, visible, room temperature photoluminescence with a 2.0-nanosecond lifetime and 2% quantum yield via spin-allowed transition from a doublet ligand-to-metal charge-transfer (2LMCT) state to the doublet ground state. Reductive and oxidative electron-transfer reactions were observed for the 2LMCT state of [Fe(phtmeimb)2]+ in bimolecular quenching studies with methylviologen and diphenylamine.