TA Spectra Research Articles

Ultrafast Photoelimination of Nitrogen from Upper Excited States of Diazoalkanes and the Fate of Carbenes Formed in the Reaction

The photochemical reactivity of diphenyldiazomethane 1 and phenyl 1- and 2-adamantyl diazomethanes 2 and 3, respectively, was investigated by transient absorption spectroscopy (TA). Photoelimination of N2 upon UV excitation takes place in the anti-Kasha ultrafast photochemical reaction from the upper excited singlet states to deliver singlet carbenes, which were, in the case of 1 and 2, detected by fs-TA. The reactivity of the carbenes differs with respect to the substituent at the carbene center. The singlet car-1 in a nonpolar solvent delivers the triplet carbene by intersystem crossing (ISC). Singlet car-2 does not undergo ISC but reacts in the intermolecular insertion reactions into C-H bonds. Car-3 has an α-C-H bond next to the carbene center and reacts rapidly in the intramolecular C-H insertion reaction to deliver alkene, precluding its detection by fs-TA. However, the isolation of ketone photoproducts from 3 is highly indicative of triplet car-3's intermediate formation. The TA spectra from the S1-S3 states of 1-3 were computed using time-dependent density functional theory, while the multiconfigurational perturbation theory to the second order was used for the absorption spectra of the corresponding singlet and triplet carbenes. The modeled and measured spectra are in good agreement, and the computations corroborate the assignments of the key short-lived intermediates.

Synthesis and Exciton Dynamics of a One-Dimensional Organic Copper Halide Perovskite

Low-dimensional copper halides have recently received increasing attention, owing to their non-toxicity, low-cost, and efficient broadband emission with large Stokes shift. Herein, we report a novel one-dimensional organic copper halide PEACuI2 (PEA = C6H5CH2CH2NH3+), which exhibits broad green emission originating from self-trapped excitons (STEs) with a peak of 525 nm and large Stokes shifts of 215 nm. The dynamics and multi-body interactions of STEs were investigated using femtosecond transient absorption (fs TA) spectroscopy at different pump intensities. Interestingly, under high STE density, we observed the absorption signals of self-trapped biexcitons (STBs) formed by binding two STEs in the fs TA spectra. The hot STBs undergo cooling process with a rate constant of 3.57 ps, yielding relaxed STBs. Furthermore, a trap-associated recombination of STBs with a time constant of 64.98 ps is unveiled. Our results offer direct evidence of the occurrence of STBs and provide new insights into the many body interactions of STEs.

Exciton Cooling in 2D Perovskite Nanoplatelets: Rationalized Carrier-Induced Stark and Phonon Bottleneck Effects.

Using femtosecond transient absorption (fs-TA), we investigate the hot exciton relaxation dynamics in strongly confined lead iodide perovskite nanoplatelets (NPLs). The large quantum and dielectric confinement leads to discrete excitonic transitions and strong Stark features in the TA spectra. This prevents the use of conventional relaxation analysis methods extracting the carrier temperature or measuring the buildup of the band-edge bleaching. Instead, we show that the TA spectral line shape near the band-edge reflects the state of the system, which can be used to probe the exciton cooling dynamics. The ultrafast hot exciton relaxation in one- to three- monolayer-thick NPLs confirms the absence of intrinsic phonon bottleneck. However, excitation fluence-dependent measurements reveal a hot phonon bottleneck effect, which is found to be independent of the nature of the internal cations but strongly affected by the ligands and/or sample surface state. Together, these results suggest a role of the surface ligands in the cooling process.

Charge-carrier dynamics of trivalent-metal alloyed halide double-perovskite nanocrystals

The charge-carrier dynamics of lead-free halide double-perovskite Cs2AgBi1−xSbxBr6 (0 ≤ x ≤ 0.75) nanocrystals (NCs) are studied by using femtosecond transient absorption technique. The ground-state bleaching decay signal can be fitted by using a triple-exponential function with lifetimes of τ1 = 0.8 ps (ultrafast component), τ2 = 64.4 ps (fast component), and τ3 = 4.3 ns (slow component) in Cs2AgBiBr6 NCs. Pump-fluence dependent TA spectra indicate that the ultrafast and fast component account for the majority of the total decay process and which can be assigned to the charge-carrier trapping processes. The slow component is due to the exciton geminate recombination because of the large exciton binding energy. Furthermore, we find that the charge-carrier trapping processes account for larger proportion with higher Sb content in Cs2AgBi1−xSbxBr6 NCs, which may be due to the increased carrier-phonon coupling strength with Sb incorporating. Besides, we find the absorption cross-section decreases with increasing Sb content. The in-depth understanding of the charge-carrier dynamics is beneficial for further developing lead-free double-perovskite NCs with improved optoelectronic properties.

Synthesis, characterization and third-order nonlinear optical behaviour of three novel ethyne-linked donor/acceptor chromophores

Three novel ethyne-linked donor/acceptor chromophores, BAB, BAS and SAS were synthesized. The results of BAB in femtosecond and nanosecond Z-scan at 532 nm reveal that the transition from saturable absorption (SA) to reverse saturable absorption (RSA), while that of BAS and SAS display RSA. The femtosecond Z-scan results for all compounds at 600 nm–800 nm indicate the obvious RSA, which can be mainly attributed to the two-photon absorption (TPA). The TA results demonstrate that all compounds display strong excited-state absorption and long lifetime, and the evolution of TA spectra for these compounds reveals the relaxation process from the local excited-state (LE) to charge transfer state (CTS). With different conjugation extent and intramolecular charge transfer (ICT), the nonlinear response and excited-state dynamics of these chromophores could be dramatically modulated. The threshold of BAS/DCM solution excited at 532 nm with nanosecond pulse was 0.631 J/cm2. The results indicate that these compounds may be the potential candidates for future application in optical limiting.

Switching resonance character within merocyanine stacks and its impact on excited-state dynamics

Summary In this study, the optical properties and excited-state dynamics of the unique self-assembled donor-acceptor (DA) merocyanine dye stacks from dimer up to octamer, prepared via dipole-dipole interactions, are reported in terms of coherent exciton dynamics and formation of an excimer-like state. Our findings are based on the steady-state absorption/emission, time-resolved fluorescence, and transient absorption (anisotropy) measurements, including wavepacket analysis and quantum mechanical calculations. Coherent exciton of torsional motions-restricted dye stacks rapidly localizes into the weakly emissive excimer-like state, by shortening the inter-moiety distance and changing the bond-length alternation pattern. The inner merocyanine moiety, having two neighboring units, has a reversed resonance character (non-polar (N) Z) in the ground state. This difference has led to two conclusions: (1) tetramers and octamers exhibit different features of excimer-like state than the dimer, and (2) octamers exhibit slower localization dynamics due to the enhanced homogeneity (six inner-moieties) compared with tetramers (two inner moieties).

Tunable Non-linear Refraction Properties and Ultrafast Excited State Dynamics of Dicyanomethylene Dihydrofuran Derivative.

The third order non-linear optical response of a dicyanomethylene dihydrofuran compound (DCDHF-2V) was investigated using a Z-scan technique in picosecond and nanosecond time regimes. The results show that DCDHF-2V has excellent excited state non-linear refraction properties on both time regimes, and the non-linear refraction index is also solvent-dependent in the nanosecond regime. The excited state relaxation dynamics of DCDHF-2V were demystified via femtosecond transient absorption spectroscopy. The TA spectra reveal that the solvent viscosities have a substantial impact on the excited state relaxation of DCDHF-2V. The exotic photophysical phenomena in DCDHF-2V reported herein can shed new light on future development of small organic non-linear optical materials with large non-linear coefficients and fast response.

Inter-Ligand Energy Transfer Process in an Ir-Complex with Expanding π-Conjugated Ligand.

The inter-ligand energy transfer (ILET) process in heteroleptic iridium complex, [Ir(dfppy)2 (bpy-Im2 )]+ , where dfppy=2-(2,4-difluorophenyl)pyridine and bpy-Im2 =4,4'-bis(1,2-diphenyl-1H-benzo[d]imidazole)-2,2',-bipyridine, was investigated using a femtosecond transient absorption (fs-TA) spectroscopic technique. The photophysical properties of [Ir(dfppy)2 (bpy-Im2 )]+ with significantly expanding π-conjugated ligand are compared to those of [Ir(dfppy)2 (bpy)]+ (bpy=2,2'-bipyridine) and a free bpy-Im2 ligand. The emission spectrum of [Ir(dfppy)2 (bpy-Im2 )]+ shows no shift upon changing the solvent polarity, whereas the free ligand bpy-Im2 showed bathochromic fluorescence shifts with increasing solvent polarity, which is attributed to intramolecular charge transfer (ICT). The unique photophysical properties of [Ir(dfppy)2 (bpy-Im2 )]+ are due to the fast ILET process from 3 MLCTdfppy to 3 MLCT/3 LCbpy-Im2 , resulting in the phosphorescence emission originating from 3 MLCT/3 LCbpy-Im2 . On the other hand, the TA bands of bpy-Im2 are observed at 540 and 480 nm, corresponding to the singlet and triplet manifolds, respectively. In contrast, the TA spectrum of [Ir(dfppy)2 (bpy-Im2 )]+ showes broad bands centered at 420 and 600 nm, attributed to the transitions from 3 MLCTdfppy and 3 MLCT/3 LCbpy-Im2 , respectively. Time-resolved spectroscopic results confirm the efficient ILET dynamics from 3 MLCTdfppy to 3 MLCT/3 LCbpy-Im2 in [Ir(dfppy)2 (bpy-Im2 )]+ . From the relaxation times determined by singular value decomposition analysis and simple sequential kinetic model, we infer that the ILET process from 3 MLCTdfppy to 3 MLCT/3 LCbpy-Im2 occurs with a time constant of ca. 4 ps. The presented results in this study show that the introduction of an expanding π-conjugated ligand can lead to the efficient ILET dynamics for improving the OLED performance.

Ultrafast carrier relaxation in SnSe (x=1, 2) thin films observed using femtosecond time-resolved transient absorption spectroscopy

Abstract In this study, SnSex (x = 1, 2) films are prepared using pulsed laser deposition (PLD) and characterized using X-ray diffraction spectroscopy (XRD) and X-ray photoelectron spectroscopy (XPS). The crystal structure and valence state of SnSex films can be observed using XRD patterns and XPS spectra. The carrier dynamics are investigated using femtosecond time-resolved TA spectroscopy. The carrier relaxation process is strongly influenced by thin film stoichiometry. The TA spectra observed in the SnSe and SnSe2 films associated with photoinduced absorption, free carrier absorption, and state filling effects. The carrier ultrafast dynamics of two films are well fitted using a tri-exponential decay model with fast (several picoseconds), medium (tens of picoseconds), and slow (several nanoseconds) components. These components are associated with carrier scattering, Auger recombination, and diffusion, respectively. The component with medium lifetime (Auger recombination) in the SnSe2 film is faster than that of SnSe film because defect state trapping. The slow relaxation component in SnSe2 film, on the other hand, is much longer than that of SnSe film. This is also due to trapping by defect states in SnSe2, which affects the diffusion of the photo-induced carriers. We investigated the carrier relaxation processes in SnSex (x = 1, 2) films and offered an insight in the ultrafast dynamic mechanism in the two tin selenides.

Photodetachment and Electron Dynamics in 1-Butyl-1-methyl-pyrrolidinium Dicyanamide.

The ultrafast transient absorption spectrum of 1-butyl-1-methyl-pyrrolidinium dicyanamide, [Pyr1,4+][DCA-], was measured in the visible and near-infrared (IR) spectral regions. Excitation of the liquid at 4.6 eV created initially delocalized and highly reactive electrons that either geminately recombined (69%) or localized onto a cavity with a time constant of ∼300 fs. Electron localization was reflected in the evolution of the TA spectrum and the time-dependent loss of reactivity with a dichloromethane quencher. The delocalized initial state and spectrum of the free electrons were consistent with computational predictions by Xu and Margulis [ J. Phys. Chem. B, 2015, 119, 532-542] on excess electrons in [Pyr1,4+][DCA-]. The computational study considered two possible localization mechanisms for excess electrons, localization on ions, and localization on cavities. In the case of photogenerated electron-hole pairs, the results presented here demonstrate localization to cavities as the dominant channel. Following localization onto a cavity, the free electrons underwent solvation and loss of reactivity with the quencher with rates that slowed in time. The dynamics were similar to an analogous prior study on the related liquid [Pyr1,x+][NTf2-]. One significant difference was the larger yield of free electrons from photoexcitation of [Pyr1,4+][DCA-]. This was found to primarily reflect more efficient localization onto cavities rather than a slower geminate recombination rate.

The MMCT excited state of a localized mixed valence cyanido-bridged RuII-Ru-RuII complex.

To investigate MMCT excited states of MV complexes, two symmetrical tetranuclear cyanido-bridged localized MV complexes RuIICNRuIII,III2NCRuII have been designed and synthesized. The ultrafast time-resolved transient absorption (TA) spectroscopy experiment reveals that the MMCT rate of 1 and 2 is 0.18 × 1014 s-1 (τ = 5.7 × 10-14 s) and 0.29 × 1013 s-1 (τ = 3.46 × 10-13 s), respectively, which suggests that the MMCT rate or the lifetime of the MMCT excited state may be controlled by a slight change of the substituent group on the metal center.

R-FREDHOLM THEORY IN BANACH ALGEBRAS

AbstractHarte (1982, Math. Z. 179, 431–436) initiated the study of Fredholm theory relative to a unital homomorphism T: A → B between unital Banach algebras A and B based on the following notions: an element a ∈ A is called Fredholm if 0 is not in the spectrum of Ta, while a is Weyl (Browder) if there exist (commuting) elements b and c in A with a = b + c such that 0 is not in the spectrum of b and c is in the null space of T. We introduce and investigate the concepts of r-Fredholm, r-Weyl and r-Browder elements, where 0 in these definitions is replaced by the spectral radii of a and b, respectively.

Tectono-Magmatic Cycles and Geodynamic Settings of Ore-Bearing System Formation in the Southern Cis-Argun Region

The ore-bearing geological structural units of the southern Cis-Argun region are considered in the context of varying geodynamic regimes related to the Proterozoic, Caledonian, and Hercynian tectono-magmatic cycles, as well as during the Late Mesozoic within-plate tectono-magmatic activity, which give rise to the formation of subalkaline igneous rocks of the Shakhtama Complex with Au, Cu–Mo, Pb–Zn–Ag metallogenic specialization; volcano-plutonic complexes of calderas with Mo–U, Pb–Zn, and fluorite ores; and rare-metal granite of the Kukulbei Complex with a Sn–W–Li–Ta spectrum of mineralization. The comparative geochemical characteristics inherent to Mesozoic ore-bearing felsic igneous rocks are considered, as well as geodynamic settings of ore-bearing fluido-magmatic systems, taking into consideration new data on geochemistry of bimodal trachybasalt–trachydacite series and rhyolite of the Turga Series, which fill the Strel’tsovka Caldera, whose trend of evolution is defined as a reference for geological history of the studied territory. The geodynamic conditions, phase composition, and geochemistry of rocks along with metallogenic specialization of Mesozoic volcano-plutonic complexes of southern Cis-Argun region are close to those of the Great Khingan Belt in northeastern China and eastern Mongolia.

Electron Transfer from Triplet State of TIPS-Pentacene Generated by Singlet Fission Processes to CH3NH3PbI3 Perovskite.

To reveal the applicability of singlet fission processes in perovskite solar cell, we investigated electron transfer from TIPS-pentacene to CH3NH3PbI3 (MAPbI3) perovskite in film phase. Through the observation of the shorter fluorescence lifetime in TIPS-pentacene/MAPbI3 perovskite bilayer film (5 ns) compared with pristine MAPbI3 perovskite film (20 ns), we verified electron-transfer processes between TIPS-pentacene and MAPbI3 perovskite. Furthermore, the observation of singlet fission processes, a faster decay rate, TIPS-pentacene cations, and the analysis of kinetic profiles of the intensity ratio between 500 and 525 nm in the TA spectra of the TIPS-pentacene/MAPbI3 perovskite bilayer film indicate that electron transfer occurs from triplet state of TIPS-pentacene generated by singlet fission processes to MAPbI3 perovskite conduction band. We believe that our results can provide useful information on the design of solar cells sensitized by singlet fission processes and pave the way for new types of perovskite solar cells.

Tectonomagmatic cycles and geodynamic conditions of formation of the ore-bearing systems in the Southern Argun’ Region

The evolution of the geological structure in the Southern Argun’ Region is studied in terms of changing geodynamic conditions of the Proterozoic, Caledonian, and Variscan Tectonomagmatic Cycles, which also under Mesozoic tectonomagmatic activation led to the formation of latite igneous rocks enriched in Au, Cu–Mo, Pb–Zn–Ag, volcanic and plutonic complexes of the caldera structures with Mo–U, Pb–Zn, and fluorite ores, and rare-metal granites with a Sn–W–Li–Ta spectrum.

Near-infrared-emitting heteroleptic cationic iridium complexes derived from 2,3-diphenylbenzo[g]quinoxaline as in vitro theranostic photodynamic therapy agents.

Five heteroleptic cationic iridium complexes with a π-expansive cyclometalating 2,3-diphenylbenzo[g]quinoxaline (dpbq) ligand (C^N ligand) and different diimine ligands (N^N ligands) (i.e. 2,2'-bipyridine (bpy, 1), phenanthroline (phen, 2), 2-(2-pyridinyl)quinoline (pqu, 3), 2,2'-bisquinoline (bqu, 4), and 2-(quinolin-2-yl)quinoxaline (quqo, 5)) were synthesized and characterized. The lowest-energy singlet electronic transitions (S1 states) were mainly dpbq ligand-centred 1ILCT (intraligand charge transfer)/1MLCT (metal to ligand charge transfer) transitions mixed with some 1π,π* transitions for complexes 1-4 with increased contributions from 1LLCT (ligand to ligand charge transfer) in 3 and 4. For complex 5, the S1 state was switched to the 1LLCT/1MLCT transitions. All five complexes displayed weak near-infrared (NIR) phosphorescence, with maximal emission output spanning 700-1400 nm and quantum yields being on the order of 10-3. The triplet state absorptions of 1-4 all resembled that of the [Ir(dpbq)2Cl]2 dimer with lifetimes of ca. 400 ns, while the TA spectrum of 5 possessed the characteristics of both the quqo ligand and the [Ir(dpbq)2Cl]2 dimer with a bi-exponential decay of ca. 5 μs and 400 ns. While the photophysics of these complexes differ slightly, their theranostic photodynamic therapy (PDT) effects varied drastically. All of the complexes were biologically active toward melanoma cells. Complexes 2 and 3 were the most cytotoxic, with 230-340 nM activity and selectivity factors for melanoma cells over normal skin fibroblasts of 34 to 40 fold. Complexes 2, 3, and 5 became very potent cytotoxins with light activation, with EC50 values as low as 12-18 nM. This potent nanomolar light-triggered activity combined with a lower dark toxicity resulted in 5 having a phototherapeutic index (PI) margin of almost 275. The bpy coligand led to the least amount of dark toxicity of 1, while phen and pqu produced cytotoxic but selective complexes 2 and 3. The quqo coligand produced the most potent complex 5 for in vitro PDT, both in terms of photocytotoxicity and PI. All Ir(iii) complexes exhibited very bright NIR phosphorescence in melanoma cells. The wide range of cytotoxicity and photocytotoxicity effects within a relatively small class of complexes highlights the importance of the identity of the coligand in the biological activity of the π-expansive biscyclometalated Ir(iii) complexes, and their bright NIR emission in live cells demonstrates their potential as theranostic PDT agents.

Elucidating the Electronic Cross-Talk Dynamics across the Heterointerface of Janus CdSe/PbSe Nanocrystals

CdSe/PbSe Janus heteronanocrystals (HNCs) were synthesized in one pot, and the underlying reaction mechanism along with the epitaxy at the hexagonal CdSe–cubic PbSe heterojunction were investigated. During the initial stages of reaction, unusually large CdSe nanocrystals were formed due to rapid growth in the presence of Pb-oleate which cation exchanged asymmetrically to form the Janus structures. Distinct PbSe and CdSe domains were visualized after sufficient growth as seen from the high-resolution transmission electron microscopic images with a unique rock salt PbSe and wurtzite CdSe interface. The core Pb–Se bonds were differentiated from interfacial Pb–Se bonds through the X-ray photoelectron spectroscopy measurements. Transient absorption spectroscopy of the Janus NCs revealed intriguing spectroscopic signatures in both the spectral and time domains as manifested by the early population of higher excitonic states upon pulsed laser excitation along with broad TA spectra rich in higher excitonic states...

Ultrafast photo-induced nuclear relaxation of a conformationally disordered conjugated polymer probed with transient absorption and femtosecond stimulated Raman spectroscopies.

A combination of transient absorption (TAS) and femtosecond stimulated Raman (FSRS) spectroscopies were used to interrogate the photo-induced nuclear relaxation dynamics of poly(3-cyclohexyl,4-methylthiophene) (PCMT). The large difference in inter-ring dihedral angles of ground and excited-state PCMT make it an ideal candidate for studying large-amplitude vibrational relaxation associated with exciton trapping. Spectral shifting in the S1 TA spectra on sub-ps timescales (110 ± 20 and 800 ± 100 fs) is similar to spectroscopic signatures of excited-state relaxation observed with related photoexcited conjugated polymers and which have been attributed to exciton localization and a combination of resonant energy transfer and torsional relaxation, respectively. Measurements made with both techniques reveal fast PCMT S1 decay and triplet formation (τS1 = 25-32 ps), which is similar to the excited-state dynamics of short oligothiophenes and highly twisted polyconjugated molecules. On ultrafast timescales FSRS of S1 PCMT offers a new perspective on the nuclear dynamics that underlie localization of excitons in photoexcited conjugated polymers: Spectral dynamics in the C=C stretching region (1400-1600 cm(-1)) include a red-shift of the in-phase C=C stretching frequency, as well as a change in the relative intensity of in-phase and out-of-phase stretch intensities on a timescale of ∼100 fs. Both changes indicate an ultrafast vibrational distortion that increases the conjugation length in the region of the localized excitation and are consistent with exciton self-localization or trapping. Wavelength-dependent excited-state FSRS measurements further demonstrate that the C=C stretching frequency provides a useful spectroscopic handle for interrogating the degree of delocalization in excited conjugated polymers given the selectivity achieved via resonance enhancement.

Probing excited state charge transfer dynamics in a heteroleptic ruthenium complex

Dynamics of metal to ligand charge transfer in the excited states of ruthenium polypyridyl complexes, which have shown promise as materials for artificial solar energy harvesting, has been of immense interest recently. Mixed ligand complexes are especially important for broader absorption in the visible region. Dynamics of ultrafast vibrational energy relaxation and inter-ligand charge transfer processes in the excited states of a heteroleptic ruthenium complex, [Ru(bpy)2(pap)](ClO4)2 (where bpy is 2,2'-bipyridine and pap is 2-(phenylazo)pyridine) have been investigated using femtosecond to nanosecond time-resolved transient absorption spectroscopic techniques. A good agreement between the TA spectrum of the lowest excited (3)MLCT state of [Ru(bpy)2(pap)](ClO4)2 complex and the anion radical spectrum of the pap ligand, which has been generated using the pulse radiolysis technique, confirmed the charge localization at the pap ligand. While the lifetime of the inter-ligand charge transfer from the bpy to the pap ligand in the (3)MLCT state is about 2.5 ps, vibrational cooling of the pap-localized(3)MLCT state occurs over a much longer time scale with a lifetime of about 35 ps. Ultrafast charge localization dynamics observed here may have important consequences in artificial solar energy harvesting systems, which employ heteroleptic ruthenium complexes.

High- and Mid-Valent Tantalum and Mono(peralkylcyclopentadienyl)tantalum Complexes of the Bicyclic Guanidinate Hexahydropyrimidopyrimidinate

Ta(hpp)Cl4, Ta(hpp)2Cl3, and (η-C5Me4R)Ta(hpp)Cl3 (R = Me, Et; hpp = anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine, hppH) are readily formed in good to excellent yields by addition of (hpp)SiMe3 to TaCl5, Ta(hpp)Cl4, or (η-C5Me4R)TaCl4, respectively, in dichloromethane. Ta(hpp)Cl4 in solution exhibits three CH2 multiplets in the proton NMR spectrum, consistent with a mirror plane that bisects the hpp ligand. The six-coordinate, yellow Ta(hpp)Cl4 possesses a distorted octahedral solid-state structure with chelating hpp and Ta–N distances of 2.038(1) and 2.059(1) Å. Ta(hpp)2Cl3 is fluxional in solution by proton NMR spectroscopy, with three CH2 multiplets at room temperature that split into additional multiplets at −50 °C, consistent with the six multiplets expected from the solid-state structure. In the solid state, bright yellow Ta(hpp)2Cl3 has an approximately pentagonal bipyramidal structure with both hpp chelates coordinated in the pentagonal plane. Ta–N distances are 2.114(3) and 2.110(3) Å for one hpp ligand and 2.112(3) and 2.117(3) Å for the other. The proton and carbon-13 solution NMR spectra of (η-C5Me4R)Ta(hpp)Cl3 are consistent with a static solution structure at room temperature, with the two coordinated nitrogens in different chemical environments as shown by six different methylene resonances. The solid-state structure of bright pinkish-red (η-C5Me5)Ta(hpp)Cl3 is a four-legged piano stool with axial ligation, with one hpp nitrogen coordinated in the square plane and one nitrogen in the axial position opposite the cyclopentadienyl centroid. Addition of (hpp)SiMe3 to the organoditantalum(III) complex (η-C5Me4R)2Ta2(μ-Cl)4 or Na/Hg reduction of (η-C5Me4R)Ta(hpp)Cl3 in benzene yields the first mid-valent tantalum hpp adduct, paramagnetic red (η-C5Me4R)Ta(hpp)Cl2. This Ta(IV) four-legged piano stool structure in the solid state has an hpp chelate with Ta–N distances of 2.118(3) and 2.091(3) Å. Intraligand bond distances and angles for the hpp ligands, proton and carbon-13 NMR solution spectra, EPR spectra, and UV–vis spectra for the new Ta(V) and Ta(IV) hpp complexes are discussed.