Dynamics and Transient Absorption Spectral Signatures of the Single-Wall Carbon Nanotube Electronically Excited Triplet State

We utilize femtosecond-to-microsecond time domain pump–probe transient absorption spectroscopy to interrogate for the first time the electronically excited triplet state of individualized single-wall carbon nanotubes (SWNTs). These studies exploit (6,5) chirality-enriched SWNT samples and poly[2,6-{1,5-bis(3-propoxysulfonic acid sodium salt)}naphthylene]ethynylene (PNES), which helically wraps the nanotube surface with periodic and constant morphology (pitch length = 10 ± 2 nm), providing a self-assembled superstructure that maintains structural homogeneity in multiple solvents. Spectroscopic interrogation of such PNES-SWNT samples in aqueous and DMSO solvents using E₂₂ excitation and a white-light continuum probe enables E₁₁ and E₂₂ spectral evolution to be monitored concomitantly. Such experiments not only reveal classic SWNT singlet exciton relaxation dynamics and transient absorption signatures but also demonstrate spectral evolution consistent with formation of a triplet exciton state. Transient dynamical studies evince that (6,5) SWNTs exhibit rapid S₁→T₁ intersystem crossing (ISC) (τ_ISC ∼20 ps), a sharp T₁→T_n transient absorption signal (λ_max(T₁→T_n) = 1150 nm; full width at half-maximum ≈ 350 cm^–1), and a substantial T₁ excited-state lifetime (τ_es ≈ 15 μs). Consistent with expectations for a triplet exciton state, T₁-state spectral signatures and T₁-state formation and decay dynamics for PNES-SWNTs in aqueous and DMSO solvents, as well as those determined for benchmark sodium cholate suspensions of (6,5) SWNTs, are similar; likewise, studies that probe the ³[(6,5) SWNT]* state in air-saturated solutions demonstrate ³O₂ quenching dynamics reminiscent of those determined for conjugated aromatic hydrocarbon excited triplet states

Engineering High-Potential Photo-oxidants with Panchromatic Absorption

Challenging photochemistry demands high-potential visible-light-absorbing photo-oxidants. We report (i) a highly electron-deficient Ru­(II) complex (<b>eDef-Rutpy</b>) bearing an <i>E</i>_1/2^0/+ potential more than 300 mV more positive than that of any established Ru­(II) bis­(terpyridyl) derivative, and (ii) an ethyne-bridged <b>eDef-Rutpy</b>−(porphinato)­Zn­(II) (<b>eDef-RuPZn</b>) supermolecule that affords both panchromatic UV–vis spectral domain absorptivity and a high <i>E</i>_1/2^0/+ potential, comparable to that of Ce­(NH₄)₂(NO₃)₆ [<i>E</i>_1/2(Ce^3+/4+) = 1.61 V vs NHE], a strong and versatile ground-state oxidant commonly used in organic functional group transformations. <b>eDef-RuPZn</b> exhibits ∼8-fold greater absorptive oscillator strength over the 380–700 nm range relative to conventional Ru­(II) polypyridyl complexes, and impressive excited-state reduction potentials (¹<i>E</i>^–/* = 1.59 V; ³<i>E</i>^–/* = 1.26 V). <b>eDef-RuPZn</b> manifests electronically excited singlet and triplet charge-transfer state lifetimes more than 2 orders of magnitude longer than those typical of conventional Ru­(II) bis­(terpyridyl) chromophores, suggesting new opportunities in light-driven oxidation reactions for energy conversion and photocatalysis

Potentiometric, Electronic, and Transient Absorptive Spectroscopic Properties of Oxidized Single-Walled Carbon Nanotubes Helically Wrapped by Ionic, Semiconducting Polymers in Aqueous and Organic Media

We report the first direct cyclic voltammetric determination of the valence and conduction band energy levels for noncovalently modified (6,5) chirality enriched SWNTs [(6,5) SWNTs] in which an aryleneethynylene polymer monolayer helically wraps the nanotube surface at periodic and constant morphology. Potentiometric properties as well as the steady-state and transient absorption spectroscopic signatures of oxidized (6,5) SWNTs were probed as a function of the electronic structure of the aryleneethynylene polymer that helically wraps the nanotube surface, the solvent dielectric, and nanotube hole polaron concentration. These data: (i) highlight the utility of these polymer-SWNT superstructures in experiments that establish the potentiometric valence and conduction band energy levels of semiconducting carbon nanotubes; (ii) provide a direct measure of the (6,5) SWNT hole polaron delocalization length (2.75 nm); (iii) determine steady-state and transient electronic absorptive spectroscopic signatures that are uniquely associated with the (6,5) SWNT hole polaron state; and (iv) demonstrate that modulation of semiconducting polymer frontier orbital energy levels can drive spectral shifts of SWNT hole polaron transitions as well as regulate SWNT valence and conduction band energies

Fluence-Dependent Singlet Exciton Dynamics in Length-Sorted Chirality-Enriched Single-Walled Carbon Nanotubes

We utilize individualized, length-sorted (6,5)-chirality enriched single-walled carbon nanotubes (SWNTs) having dimensions of 200 and 800 nm, femtosecond transient absorption spectroscopy, and variable excitation fluences that modulate the exciton density per nanotube unit length, to interrogate nanotube exciton/biexciton dynamics. For pump fluences below 30 μJ/cm², transient absorption (TA) spectra of (6,5) SWNTs reveal the instantaneous emergence of the exciton to biexciton transition (E₁₁→E_11,BX) at 1100 nm; in contrast, under excitation fluences exceeding 100 μJ/cm², this TA signal manifests a rise time (τ_rise ∼ 250 fs), indicating that E₁₁ state repopulation is required to produce this signal. Femtosecond transient absorption spectroscopic data acquired over the 900–1400 nm spectral region of the near-infrared (NIR) region for (6,5) SWNTs, as a function of nanotube length and exciton density, reveal that over time delays that exceed 200 fs exciton–exciton interactions do not occur over spatial domains larger than 200 nm. Furthermore, the excitation fluence dependence of the E₁₁→E_11,BX transient absorption signal demonstrates that relaxation of the E₁₁ biexciton state (E_11,BX) gives rise to a substantial E₁₁ state population, as increasing delay times result in a concomitant increase of E₁₁→E_11,BX transition oscillator strength. Numerical simulations based on a three-state model are consistent with a mechanism whereby biexcitons are generated at high excitation fluences via sequential SWNT ground- and E₁₁-state excitation that occurs within the 980 nm excitation pulse duration. These studies that investigate fluence-dependent TA spectral evolution show that SWNT ground→E₁₁ and E₁₁→E_11,BX excitations are coresonant and provide evidence that E_11,BX→E₁₁ relaxation constitutes a significant decay channel for the SWNT biexciton state over delay times that exceed 200 fs, a finding that runs counter to assumptions made in previous analyses of SWNT biexciton dynamical data where exciton–exciton annihilation has been assumed to play a dominant role

Tailoring Porphyrin-Based Electron Accepting Materials for Organic Photovoltaics

The syntheses, potentiometric responses, optical spectra, electronic structural properties, and integration into photovoltaic devices are described for ethyne-bridged isoindigo-(porphinato)­zinc­(II)-isoindigo chromophores built upon either electron-rich 10,20-diaryl porphyrin (Ar-Iso) or electron-deficient 10,20-bis­(perfluoroalkyl)­porphyrin (Rf-Iso) frameworks. These supermolecules evince electrochemical responses that trace their geneses to their respective porphyrinic and isoindigoid subunits. The ethyne linkage motif effectively mixes the comparatively weak isoindigo-derived visible excitations with porphyrinic π–π* states, endowing Ar-Iso and Rf-Iso with high extinction coefficient (ε ∼ 10⁵ M^–1·cm^–1) long-axis polarized absorptions. Ar-Iso and Rf-Iso exhibit total absorptivities per unit mass that greatly exceed that for poly­(3-hexyl)­thiophene (P3HT) over the 375–900 nm wavelength range where solar flux is maximal. Time-dependent density functional theory calculations highlight the delocalized nature of the low energy singlet excited states of these chromophores, demonstrating how coupled oscillator photophysics can yield organic photovoltaic device (OPV) materials having absorptive properties that supersede those of conventional semiconducting polymers. Prototype OPVs crafted from the poly­(3-hexyl)­thiophene (P3HT) donor polymer and these new materials (i) confirm that solar power conversion depends critically upon the driving force for photoinduced hole transfer (HT) from these low-band-gap acceptors, and (ii) underscore the importance of the excited-state reduction potential (<i>E</i>^–/*) parameter as a general design criterion for low-band-gap OPV acceptors. OPVs constructed from Rf-Iso and P3HT define rare examples whereby the acceptor material extends the device operating spectral range into the NIR, and demonstrate for the first time that high oscillator strength porphyrinic chromophores, conventionally utilized as electron donors in OPVs, can also be exploited as electron acceptors

Quasi-Ohmic Single Molecule Charge Transport through Highly Conjugated <i>meso</i>-to-<i>meso</i> Ethyne-Bridged Porphyrin Wires

Understanding and controlling electron transport through functional molecules are of primary importance to the development of molecular scale devices. In this work, the single molecule resistances of <i>meso</i>-to-<i>meso</i> ethyne-bridged (porphinato)­zinc­(II) structures (<b>PZn</b>_{<b><i>n</i></b>} compounds), connected to gold electrodes via (4′-thiophenyl)­ethynyl termini, are determined using scanning tunneling microscopy-based break junction methods. These experiments show that each α,ω-di­[(4′-thiophenyl)­ethynyl]-terminated <b>PZn</b>_{<b><i>n</i></b>} compound (<b>dithiol-PZn</b>_{<b><i>n</i></b>}) manifests a dual molecular conductance. In both the high and low conductance regimes, the measured resistance across these metal–<b>dithiol-PZn</b>_{<b><i>n</i></b>}–metal junctions increases in a near linear fashion with molecule length. These results signal that <i>meso</i>-to-<i>meso</i> ethyne-bridged porphyrin wires afford the lowest β value (β = 0.034 Å^–1) yet determined for thiol-terminated single molecules that manifest a quasi-ohmic resistance dependence across metal–<b>dithiol-PZn</b>_{<b><i>n</i></b>}–metal junctions

Composite Electronic Materials Based on Poly(3,4-propylenedioxythiophene) and Highly Charged Poly(aryleneethynylene)-Wrapped Carbon Nanotubes for Supercapacitors

Supercapacitor charge storage media were fabricated using the semiconducting polymer poly­(3,4-propylenedioxythiophene) (PProDOT) and single-walled carbon nanotubes (SWNTs) that were helically wrapped with ionic, conjugated poly­[2,6-{1,5-bis­(3-propoxysulfonicacidsodiumsalt)}­naphthylene]­ethynylene (PNES). These PNES-wrapped SWNTs (PNES-SWNTs) enable efficient dispersion of individualized nanotubes in a wide range of organic solvents. PNES-SWNT film-modified Pt electrodes were prepared by drop casting PNES-SWNT suspensions in MeOH; high stability, first-generation PProDOT/PNES/SWNT composites were realized via electropolymerization of the ProDOT parent monomer (3,4-propylenedioxythiophene) in a 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide/propylene carbonate solution at the PNES-SWNT-modified electrode. The electrochemical properties of PProDOT and PProDOT/PNES/SWNT single electrodes and devices were examined using cyclic voltammetric methods. The hybrid composites were found to enhance key supercapacitor figures of merit (charge capacity and capacitance) by approximately a factor of 2 relative to those determined for benchmark Type I devices that exploited a classic PProDOT-based electrode material. The charge/discharge stability of the supercapacitors was probed by repeated rounds of cyclic voltammetric evaluation at a minimum depth of discharge of 73%; these experiments demonstrated that the hybrid PProDOT/PNES/SWNT composites retained ∼90% of their initial charge capacity after 21 000 charge/discharge cycles, contrasting analogous data obtained for PProDOT-based devices, which showed only 84% retention of their initial charge capacity

Electron Spin Relaxation of Hole and Electron Polarons in π‑Conjugated Porphyrin Arrays: Spintronic Implications

Electron spin resonance (ESR) spectroscopic line shape analysis and continuous-wave (CW) progressive microwave power saturation experiments are used to probe the relaxation behavior and the relaxation times of charged excitations (hole and electron polarons) in <i>meso</i>-to-<i>meso</i> ethyne-bridged (porphinato)­zinc­(II) oligomers (<b>PZn</b>_{<b><i>n</i></b>} compounds), which can serve as models for the relevant states generated upon spin injection. The observed ESR line shapes for the <b>PZn</b>_{<b><i>n</i></b>} hole polaron (<b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^<b>+•</b>) and electron polaron (<b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^{<b>–•</b>}) states evolve from Gaussian to more Lorentzian as the oligomer length increases from 1.9 to 7.5 nm, with solution-phase <b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^<b>+•</b> and <b>[PZn</b>_{<b><i>n</i></b>}<b>]</b>^{<b>–•</b>} spin–spin (<i>T</i>₂) and spin–lattice (<i>T</i>₁) relaxation times at 298 K ranging, respectively, from 40 to 230 ns and 0.2 to 2.3 μs. Notably, these very long relaxation times are preserved in thick films of these species. Because the magnitudes of spin–spin and spin–lattice relaxation times are vital metrics for spin dephasing in quantum computing or for spin-polarized transport in magnetoresistive structures, these results, coupled with the established wire-like transport behavior across metal–dithiol-<b>PZn</b>_{<b><i>n</i></b>}–metal junctions, present <i>meso</i>-to-<i>meso</i> ethyne-bridged multiporphyrin systems as leading candidates for ambient-temperature organic spintronic applications

Valence Band Dependent Charge Transport in Bulk Molecular Electronic Devices Incorporating Highly Conjugated Multi-[(Porphinato)Metal] Oligomers

Molecular electronics offers the potential to control device functions through the fundamental electronic properties of individual molecules, but realization of such possibilities is typically frustrated when such specialized molecules are integrated into a larger area device. Here we utilize highly conjugated (porphinato)­metal-based oligomers (<b>PM</b>_<b>n</b> structures) as molecular wire components of nanotransfer printed (nTP) molecular junctions; electrical characterization of these “bulk” nTP devices highlights device resistances that depend on <b>PM</b>_<b>n</b> wire length. Device resistance measurements, determined as a function of <b>PM</b>_<b>n</b> molecular length, were utilized to evaluate the magnitude of a phenomenological β corresponding to the resistance decay parameter across the barrier; these data show that the magnitude of this β value is modulated via porphyrin macrocycle central metal atom substitution [β­(<b>PZn</b>_<b>n</b>; 0.065 Å^–1) < β­(<b>PCu</b>_<b>n</b>; 0.132 Å^–1) < β­(<b>PNi</b>_<b>n</b>; 0.176 Å^–1)]. Cyclic voltammetric data, and ultraviolet photoelectron spectroscopic studies carried out at gold surfaces, demonstrate that these nTP device resistances track with the valence band energy levels of the <b>PM</b>_<b>n</b> wire, which were modulated via porphyrin macrocycle central metal atom substitution. This study demonstrates the ability to fabricate “bulk” and scalable electronic devices in which function derives from the electronic properties of discrete single molecules, and underscores how a critical device functionwire resistancemay be straightforwardly engineered by <b>PM</b>_<b>n</b> molecular composition

On the Importance of Electronic Symmetry for Triplet State Delocalization

The influence of electronic symmetry on triplet state delocalization in linear zinc porphyrin oligomers is explored by electron paramagnetic resonance techniques. Using a combination of transient continuous wave and pulse electron nuclear double resonance spectroscopies, it is demonstrated experimentally that complete triplet state delocalization requires the chemical equivalence of all porphyrin units. These results are supported by density functional theory calculations, showing uneven delocalization in a porphyrin dimer in which a terminal ethynyl group renders the two porphyrin units inequivalent. When the conjugation length of the molecule is further increased upon addition of a second terminal ethynyl group that restores the symmetry of the system, the triplet state is again found to be completely delocalized. The observations suggest that electronic symmetry is of greater importance for triplet state delocalization than other frequently invoked factors such as conformational rigidity or fundamental length-scale limitations