Band gap opening of metallic single-walled carbon nanotubes via noncovalent symmetry breaking.

Covalent bonding interactions determine the energy-momentum (E-k) dispersion (band structure) of solid-state materials. Here, we show that noncovalent interactions can modulate the E-k dispersion near the Fermi level of a low-dimensional nanoscale conductor. We demonstrate that low energy band gaps may be opened in metallic carbon nanotubes through polymer wrapping of the nanotube surface at fixed helical periodicity. Electronic spectral, chiro-optic, potentiometric, electronic device, and work function data corroborate that the magnitude of band gap opening depends on the nature of the polymer electronic structure. Polymer dewrapping reverses the conducting-to-semiconducting phase transition, restoring the native metallic carbon nanotube electronic structure. These results address a long-standing challenge to develop carbon nanotube electronic structures that are not realized through disruption of π conjugation, and establish a roadmap for designing and tuning specialized semiconductors that feature band gaps on the order of a few hundred meV.

Twisted molecular wires polarize spin currents at room temperature.

A critical spintronics challenge is to develop molecular wires that render efficiently spin-polarized currents. Interplanar torsional twisting, driven by chiral binucleating ligands in highly conjugated molecular wires, gives rise to large near-infrared rotational strengths. The large scalar product of the electric and magnetic dipole transition moments ([Formula: see text]), which are evident in the low-energy absorptive manifolds of these wires, makes possible enhanced chirality-induced spin selectivity-derived spin polarization. Magnetic-conductive atomic force microscopy experiments and spin-Hall devices demonstrate that these designs point the way to achieve high spin selectivity and large-magnitude spin currents in chiral materials.

All-Carbon Thin-Film Transistors Using Water-Only Printing.

Printing thin-film transistors (TFTs) using nanomaterials is a promising approach for future electronics. Yet, most inks rely on environmentally harmful solvents for solubilizing and postprint processing the nanomaterials. In this work, we demonstrate water-only TFTs printed from all-carbon inks of semiconducting carbon nanotubes (CNTs), conducting graphene, and insulating crystalline nanocellulose (CNC). While suspending these nanomaterials into aqueous inks is readily achieved, printing the inks into thin films of sufficient surface coverage and in multilayer stacks to form TFTs has proven elusive without high temperatures, hazardous chemicals, and/or lengthy postprocessing. Using aerosol jet printing, our approach involves a maximum temperature of 70 °C and no hazardous chemicals─all inks are aqueous and only water is used for processing. An intermittent rinsing technique was utilized to address the surface adhesion challenges that limit film density of printed aqueous CNTs. These findings provide promising steps toward an environmentally friendly realization of thin-film electronics.

Electronic structure and photophysics of a supermolecular iron complex having a long MLCT-state lifetime and panchromatic absorption.

Exploiting earth-abundant iron-based metal complexes as high-performance photosensitizers demands long-lived electronically excited metal-to-ligand charge-transfer (MLCT) states, but these species suffer typically from femtosecond timescale charge-transfer (CT)-state quenching by low-lying nonreactive metal-centered (MC) states. Here, we engineer supermolecular Fe(II) chromophores based on the bis(tridentate-ligand)metal(II)-ethyne-(porphinato)zinc(II) conjugated framework, previously shown to give rise to highly delocalized low-lying 3MLCT states for other Group VIII metal (Ru, Os) complexes. Electronic spectral, potentiometric, and ultrafast pump-probe transient dynamical data demonstrate that a combination of a strong σ-donating tridentate ligand and a (porphinato)zinc(II) moiety with low-lying π*-energy levels, sufficiently destabilize MC states and stabilize supermolecular MLCT states to realize Fe(II) complexes that express 3MLCT state photophysics reminiscent of their heavy-metal analogs. The resulting Fe(II) chromophore archetype, FeNHCPZn, features a highly polarized CT state having a profoundly extended 3MLCT lifetime (160 ps), 3MLCT phosphorescence, and ambient environment stability. Density functional and domain-based local pair natural orbital coupled cluster [DLPNO-CCSD(T)] theory reveal triplet-state wavefunction spatial distributions consistent with electronic spectroscopic and excited-state dynamical data, further underscoring the dramatic Fe metal-to-extended ligand CT character of electronically excited FeNHCPZn. This design further prompts intense panchromatic absorptivity via redistributing high-energy absorptive oscillator strength throughout the visible spectral domain, while maintaining a substantial excited-state oxidation potential for wide-ranging photochemistry--highlighted by the ability of FeNHCPZn to photoinject charges into a SnO2/FTO electrode in a dye-sensitized solar cell (DSSC) architecture. Concepts enumerated herein afford opportunities for replacing traditional rare-metal-based emitters for solar-energy conversion and photoluminescence applications.

Allosteric cooperation in a de novo-designed two-domain protein.

We describe the de novo design of an allosterically regulated protein, which comprises two tightly coupled domains. One domain is based on the DF (Due Ferri in Italian or two-iron in English) family of de novo proteins, which have a diiron cofactor that catalyzes a phenol oxidase reaction, while the second domain is based on PS1 (Porphyrin-binding Sequence), which binds a synthetic Zn-porphyrin (ZnP). The binding of ZnP to the original PS1 protein induces changes in structure and dynamics, which we expected to influence the catalytic rate of a fused DF domain when appropriately coupled. Both DF and PS1 are four-helix bundles, but they have distinct bundle architectures. To achieve tight coupling between the domains, they were connected by four helical linkers using a computational method to discover the most designable connections capable of spanning the two architectures. The resulting protein, DFP1 (Due Ferri Porphyrin), bound the two cofactors in the expected manner. The crystal structure of fully reconstituted DFP1 was also in excellent agreement with the design, and it showed the ZnP cofactor bound over 12 Å from the dimetal center. Next, a substrate-binding cleft leading to the diiron center was introduced into DFP1. The resulting protein acts as an allosterically modulated phenol oxidase. Its Michaelis-Menten parameters were strongly affected by the binding of ZnP, resulting in a fourfold tighter Km and a 7-fold decrease in kcat These studies establish the feasibility of designing allosterically regulated catalytic proteins, entirely from scratch.

Regulating Singlet-Triplet Energy Gaps through Substituent-Driven Modulation of the Exchange and Coulomb Interactions.

Control of the singlet-triplet energy gap (ΔEST) is central to realizing productive energy conversion reactions, photochemical reaction trajectories, and emergent applications that exploit molecular spin physics. Despite this, no systematic methods have been defined to tune ΔEST in simple molecular frameworks, let alone by an approach that also holds chromophore size and electronic structural parameters (such as the HOMO-LUMO gap) constant. Using a combination of molecular design, photophysical and potentiometric experiments, and quantum chemical analyses, we show that the degree of electron-electron repulsion in excited singlet and triplet states may be finely controlled through the substitution pattern of a simple porphyrin absorber, enabling regulation of relative electronically excited singlet and triplet state energies by the designed restriction of the electron-electron Coulomb (J) and exchange (K) interaction magnitudes. This approach modulates the ΔEST magnitude by controlling the densities of state in the occupied and virtual molecular orbital manifolds, natural transition orbital polarization, and the relative contributions of one electron transitions involving select natural transition orbital pairs. This road map, which regulates electron density overlaps in the occupied and virtual states that define the singlet and triplet wave functions of these chromophores, enables new approaches to preserve excitation energy despite intersystem crossing.

Synthetic Control of Exciton Dynamics in Bioinspired Cofacial Porphyrin Dimers.

Understanding how the complex interplay among excitonic interactions, vibronic couplings, and reorganization energy determines coherence-enabled transport mechanisms is a grand challenge with both foundational implications and potential payoffs for energy science. We use a combined experimental and theoretical approach to show how a modest change in structure may be used to modify the exciton delocalization, tune electronic and vibrational coherences, and alter the mechanism of exciton transfer in covalently linked cofacial Zn-porphyrin dimers (meso-beta linked ABm-ß and meso-meso linked AAm-m). While both ABm-ß and AAm-m feature zinc porphyrins linked by a 1,2-phenylene bridge, differences in the interporphyrin connectivity set the lateral shift between macrocycles, reducing electronic coupling in ABm-ß and resulting in a localized exciton. Pump-probe experiments show that the exciton dynamics is faster by almost an order of magnitude in the strongly coupled AAm-m dimer, and two-dimensional electronic spectroscopy (2DES) identifies a vibronic coherence that is absent in ABm-ß. Theoretical studies indicate how the interchromophore interactions in these structures, and their system-bath couplings, influence excitonic delocalization and vibronic coherence-enabled rapid exciton transport dynamics. Real-time path integral calculations reproduce the exciton transfer kinetics observed experimentally and find that the linking-modulated exciton delocalization strongly enhances the contribution of vibronic coherences to the exciton transfer mechanism, and that this coherence accelerates the exciton transfer dynamics. These benchmark molecular design, 2DES, and theoretical studies provide a foundation for directed explorations of nonclassical effects on exciton dynamics in multiporphyrin assemblies.

Engineering opposite electronic polarization of singlet and triplet states increases the yield of high-energy photoproducts.

Efficient photosynthetic energy conversion requires quantitative, light-driven formation of high-energy, charge-separated states. However, energies of high-lying excited states are rarely extracted, in part because the congested density of states in the excited-state manifold leads to rapid deactivation. Conventional photosystem designs promote electron transfer (ET) by polarizing excited donor electron density toward the acceptor ("one-way" ET), a form of positive design. Curiously, negative design strategies that explicitly avoid unwanted side reactions have been underexplored. We report here that electronic polarization of a molecular chromophore can be used as both a positive and negative design element in a light-driven reaction. Intriguingly, prudent engineering of polarized excited states can steer a "U-turn" ET-where the excited electron density of the donor is initially pushed away from the acceptor-to outcompete a conventional one-way ET scheme. We directly compare one-way vs. U-turn ET strategies via a linked donor-acceptor (DA) assembly in which selective optical excitation produces donor excited states polarized either toward or away from the acceptor. Ultrafast spectroscopy of DA pinpoints the importance of realizing donor singlet and triplet excited states that have opposite electronic polarizations to shut down intersystem crossing. These results demonstrate that oppositely polarized electronically excited states can be employed to steer photoexcited states toward useful, high-energy products by routing these excited states away from states that are photosynthetic dead ends.

Mapping hole hopping escape routes in proteins.

A recently proposed oxidative damage protection mechanism in proteins relies on hole hopping escape routes formed by redox-active amino acids. We present a computational tool to identify the dominant charge hopping pathways through these residues based on the mean residence times of the transferring charge along these hopping pathways. The residence times are estimated by combining a kinetic model with well-known rate expressions for the charge-transfer steps in the pathways. We identify the most rapid hole hopping escape routes in cytochrome P450 monooxygenase, cytochrome c peroxidase, and benzylsuccinate synthase (BSS). This theoretical analysis supports the existence of hole hopping chains as a mechanism capable of providing hole escape from protein catalytic sites on biologically relevant timescales. Furthermore, we find that pathways involving the [4Fe4S] cluster as the terminal hole acceptor in BSS are accessible on the millisecond timescale, suggesting a potential protective role of redox-active cofactors for preventing protein oxidative damage.

Excitation energy-dependent photocurrent switching in a single-molecule photodiode.

The direction of electron flow in molecular optoelectronic devices is dictated by charge transfer between a molecular excited state and an underlying conductor or semiconductor. For those devices, controlling the direction and reversibility of electron flow is a major challenge. We describe here a single-molecule photodiode. It is based on an internally conjugated, bichromophoric dyad with chemically linked (porphyrinato)zinc(II) and bis(terpyridyl)ruthenium(II) groups. On nanocrystalline, degenerately doped indium tin oxide electrodes, the dyad exhibits distinct frequency-dependent, charge-transfer characters. Variations in the light source between red-light (∼1.9 eV) and blue-light (∼2.7 eV) excitation for the integrated photodiode result in switching of photocurrents between cathodic and anodic. The origin of the excitation frequency-dependent photocurrents lies in the electronic structure of the chromophore excited states, as shown by the results of theoretical calculations, laser flash photolysis, and steady-state spectrophotometric measurements.

De Novo Design, Solution Characterization, and Crystallographic Structure of an Abiological Mn-Porphyrin-Binding Protein Capable of Stabilizing a Mn(V) Species.

De novo protein design offers the opportunity to test our understanding of how metalloproteins perform difficult transformations. Attaining high-resolution structural information is critical to understanding how such designs function. There have been many successes in the design of porphyrin-binding proteins; however, crystallographic characterization has been elusive, limiting what can be learned from such studies as well as the extension to new functions. Moreover, formation of highly oxidizing high-valent intermediates poses design challenges that have not been previously implemented: (1) purposeful design of substrate/oxidant access to the binding site and (2) limiting deleterious oxidation of the protein scaffold. Here we report the first crystallographically characterized porphyrin-binding protein that was programmed to not only bind a synthetic Mn-porphyrin but also maintain binding site access to form high-valent oxidation states. We explicitly designed a binding site with accessibility to dioxygen units in the open coordination site of the Mn center. In solution, the protein is capable of accessing a high-valent Mn(V)-oxo species which can transfer an O atom to a thioether substrate. The crystallographic structure is within 0.6 Å of the design and indeed contained an aquo ligand with a second water molecule stabilized by hydrogen bonding to a Gln side chain in the active site, offering a structural explanation for the observed reactivity.

Excited-State Dynamics and Nonlinear Optical Properties of Hyperpolarizable Chromophores Based on Conjugated Bis(terpyridyl)Ru(II) and Palladium and Platinum Porphyrinic Components: Impact of Heavy Metals upon Supermolecular Electro-Optic Properties.

A new series of strongly coupled oscillators based upon (porphinato)Pd, (porphinato)Pt, and bis(terpyridyl)ruthenium(II) building blocks is described. These RuPPd, RuPPt, RuPPdRu, and RuPPtRu chromophores feature bis(terpyridyl)Ru(II) moieties connected to the (porphinato)metal unit via an ethyne linker that bridges the 4'-terpyridyl and porphyrin macrocycle meso-carbon positions. Pump-probe transient optical data demonstrate sub-picosecond excited singlet-to-triplet-state relaxation. The relaxed lowest-energy triplet (T1) excited states of these chromophores feature absorption manifolds that span the 800-1200 nm spectral region, microsecond triplet-state lifetimes, and large absorptive extinction coefficients [ε(T1 → Tn) > 4 × 104 M-1 cm-1]. Dynamic hyperpolarizability (ßλ) values were determined from hyper-Rayleigh light scattering (HRS) measurements carried out at several incident irradiation wavelengths over the 800-1500 nm spectral region. Relative to benchmark RuPZn and RuPZnRu chromophores which showed large ßHRS values over the 1200-1600 nm range, RuPPd, RuPPt, RuPPdRu, and RuPPtRu displayed large ßHRS values over the 850-1200 nm region. Generalized Thomas-Kuhn sum (TKS) rules and experimental hyperpolarizability values were utilized to determine excited state-to-excited state transition dipole terms from experimental electronic absorption data and thus assessed frequency-dependent ßλ values, including two- and three-level contributions for both ßzzz and ßxzx tensor components to the RuPPd, RuPPt, RuPPdRu, and RuPPtRu hyperpolarizability spectra. These analyses qualitatively rationalize how the ßzzz and ßxzx tensor elements influence the observed irradiation wavelength-dependent hyperpolarizability magnitudes. The TKS analysis suggests that supermolecules related to RuPPd, RuPPt, RuPPdRu, and RuPPtRu will likely feature intricate dependences of experimentally determined ßHRS values as a function of irradiation wavelength that derive from substantial singlet-triplet mixing, and complex interactions among multiple different ß tensor components that modulate the long wavelength regime of the nonlinear optical response.

Dynamics of charged excitons in electronically and morphologically homogeneous single-walled carbon nanotubes.

The trion, a three-body charge-exciton bound state, offers unique opportunities to simultaneously manipulate charge, spin, and excitation in one-dimensional single-walled carbon nanotubes (SWNTs) at room temperature. Effective exploitation of trion quasi-particles requires fundamental insight into their creation and decay dynamics. Such knowledge, however, remains elusive for SWNT trion states, due to the electronic and morphological heterogeneity of commonly interrogated SWNT samples, and the fact that transient spectroscopic signals uniquely associated with the trion state have not been identified. Here, we prepare length-sorted SWNTs and precisely control charge-carrier-doping densities to determine trion dynamics using femtosecond pump-probe spectroscopy. Identification of the trion transient absorptive hallmark enables us to demonstrate that trions (i) derive from a precursor excitonic state, (ii) are produced via migration of excitons to stationary hole-polaron sites, and (iii) decay in a first-order manner. Importantly, under appropriate carrier-doping densities, exciton-to-trion conversion in SWNTs can approach 100% at ambient temperature. Our findings open up possibilities for exploiting trions in SWNT optoelectronics, ranging from photovoltaics and photodetectors to spintronics.

Topology, Distance, and Orbital Symmetry Effects on Electronic Spin-Spin Couplings in Rigid Molecular Systems: Implications for Long-Distance Spin-Spin Interactions.

Understanding factors that underpin the signs and magnitudes of electron spin-spin couplings in biradicaloids, especially those that are integrated into highly delocalized electronic structures, promises to inform the design of molecular spintronic systems. Using steady-state and variable temperature electron paramagnetic resonance (EPR) spectroscopy, we examine spin dynamics in symmetric, strongly π-conjugated bis[(porphinato)copper] (bis[PCu]) systems and probe the roles played by atom-specific macrocycle spin density, porphyrin-to-porphyrin linkage topology, and orbital symmetry on the magnitudes of electronic spin-spin couplings over substantial Cu-Cu distances. These studies examine the following: (i) meso-to-meso-linked bis[PCu] systems having oligoyne spacers, (ii) meso-to-meso-bridged bis[PCu] arrays in which the PCu centers are separated by a single ethynyl unit or multiple 5,15-diethynyl(porphinato)zinc(II) units, and (iii) the corresponding ß-to-ß-bridged bis[PCu] structures. EPR data show that, for ß-to-ß-bridged systems and meso-to-meso-linked bis[PCu] structures having oligoyne spacers, a through σ-bond coupling mechanism controls the average exchange interaction (Javg). In contrast, PCu centers separated by a single ethynyl or multiple 5,15-diethynyl(porphinato)zinc(II) units display a phenomenological decay of ln[Javg] versus Cu-Cu σ-bond separation number of ∼0.115 per bond, half as large as for these other compositions, congruent with the importance of π-mediated spin-spin coupling. These disparities derive from effects that trace their origin to the nature of the macrocycle-macrocycle linkage topology and the relative energy of the Cu dx2-y2 singly occupied molecular orbital within the frontier orbital manifold of these electronically delocalized structures. This work provides insight into approaches to tune the extent of spin exchange interactions and distance-dependent electronic spin-spin coupling magnitudes in rigid, highly conjugated biradicaloids.

Low-Resistance Molecular Wires Propagate Spin-Polarized Currents.

Spin based properties, applications, and devices are typically related to inorganic ferromagnetic materials. The development of organic materials for spintronic applications has long been encumbered by its reliance on ferromagnetic electrodes for polarized spin injection. The discovery of the chirality-induced spin selectivity (CISS) effect, in which chiral organic molecules serve as spin filters, defines a marked departure from this paradigm because it exploits soft materials, operates at ambient temperature, and eliminates the need for a magnetic electrode. To date, the CISS effect has been explored exclusively in molecular insulators. Here we combine chiral molecules, which serve as spin filters, with molecular wires that despite not being chiral, function to preserve spin polarization. Self-assembled monolayers (SAMs) of right-handed helical (l-proline)8 (Pro8) and corresponding peptides, N-terminal conjugated to (porphinato)zinc or meso-to-meso ethyne-bridged (porphinato)zinc structures (Pro8PZnn), were interrogated via magnetic conducting atomic force microscopy (mC-AFM), spin-dependent electrochemistry, and spin Hall devices that measure the spin polarizability that accompanies the charge polarization. These data show that chiral molecules are not required to transmit spin-polarized currents made possible by the CISS mechanism. Measured Hall voltages for Pro8PZn1-3 substantially exceed that determined for the Pro8 control and increase dramatically as the conjugation length of the achiral PZnn component increases; mC-AFM data underscore that measured spin selectivities increase with an increasing Pro8PZn1-3 N-terminal conjugation. Because of these effects, spin-dependent electrochemical data demonstrate that spin-polarized currents, which trace their genesis to the chiral Pro8 moiety, propagate with no apparent dephasing over the augmented Pro8PZnn length scales, showing that spin currents may be transmitted over molecular distances that greatly exceed the length of the chiral moiety that makes possible the CISS effect.

Quantitative Evaluation of Optical Free Carrier Generation in Semiconducting Single-Walled Carbon Nanotubes.

Gauging free carrier generation (FCG) in optically excited, charge-neutral single-walled carbon nanotubes (SWNTs) has important implications for SWNT-based optoelectronics that rely upon conversion of photons to electrical current. Earlier investigations have largely provided only qualitative insights into optically triggered SWNT FCG, due to the heterogeneous nature of commonly interrogated SWNT samples and the lack of direct, unambiguous spectroscopic signatures that could be used to quantify charges. Here, employing ultrafast pump-probe spectroscopy in conjunction with chirality-enriched, length-sorted, ionic-polymer-wrapped SWNTs, we develop a straightforward approach for quantitatively evaluating the extent of optically driven FCG in SWNTs. Owing to the previously identified trion transient absorptive hallmark (Tr+11 → Tr+nm) and the rapid nature of trion formation dynamics (<1 ps) relative to established free-carrier decay time scales (>ns), we correlate FCG with trion formation dynamics. Experimental determination of the trion absorptive cross section further enables evaluation of the quantum yields for optically driven FCG [Φ(E nn→h ++e -)] as a function of optical excitation energy and medium dielectric strength. We show that (i) E33 excitons give rise to dramatically enhanced Φ(E nn→h ++e -) relative to those derived from E22 and E11 excitons and (ii) Φ(E33→h ++e -) monotonically increases from ∼5% to 18% as the solvent dielectric constant increases from ∼32 to 80. This work highlights the extent to which the nature of the medium and excitation conditions control FCG quantum yields in SWNTs: such studies have the potential to provide new design insights for SWNT-based compositions for optoelectronic applications that include photodetectors and photovoltaics.

Extreme electron polaron spatial delocalization in π-conjugated materials.

The electron polaron, a spin-1/2 excitation, is the fundamental negative charge carrier in π-conjugated organic materials. Large polaron spatial dimensions result from weak electron-lattice coupling and thus identify materials with unusually low barriers for the charge transfer reactions that are central to electronic device applications. Here we demonstrate electron polarons in π-conjugated multiporphyrin arrays that feature vast areal delocalization. This finding is evidenced by concurrent optical and electron spin resonance measurements, coupled with electronic structure calculations that suggest atypically small reorganization energies for one-electron reduction of these materials. Because the electron polaron dimension can be linked to key performance metrics in organic photovoltaics, light-emitting diodes, and a host of other devices, these findings identify conjugated materials with exceptional optical, electronic, and spintronic properties.

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 (eDef-Rutpy) bearing an E1/20/+ potential more than 300 mV more positive than that of any established Ru(II) bis(terpyridyl) derivative, and (ii) an ethyne-bridged eDef-Rutpy-(porphinato)Zn(II) (eDef-RuPZn) supermolecule that affords both panchromatic UV-vis spectral domain absorptivity and a high E1/20/+ potential, comparable to that of Ce(NH4)2(NO3)6 [E1/2(Ce3+/4+) = 1.61 V vs NHE], a strong and versatile ground-state oxidant commonly used in organic functional group transformations. eDef-RuPZn 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 (1E-/* = 1.59 V; 3E-/* = 1.26 V). eDef-RuPZn 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.

Alkyne-Bridged Multi[Copper(II) Porphyrin] Structures: Nuances of Orbital Symmetry in Long-Range, Through-Bond Mediated, Isotropic Spin Exchange Interactions.

Spin and conformational dynamics in symmetric alkyne-bridged multi[copper(II) porphyrin] structures have been studied in toluene solution at variable temperature using steady-state electron paramagnetic resonance (EPR) spectroscopy. Comparison of the dimer EPR spectra to those of Cu porphyrin monomers shows evidence of an isotropic exchange interaction (Javg) in these biradicaloid structures, manifested by a significant line broadening in the dimer spectra. The extent line broadening depends on molecular structure and temperature, suggesting Javg is modulated by conformational dynamics that impact the torsional angle distribution between the porphyrin-porphyrin least-squares planes. Computational simulation of the experimental EPR spectra, using a developed algorithm for J modulation in flexible organic biradicals, supports this hypothesis. Comparison of ethyne and butadiyne alkyne bridges reveals remarkable sensitivity to orbital interactions between the spacer and the metal, reflected in measurements of Javg as a function of temperature. The results suggest orbital symmetry relationships may be more important than recognized in design of optimized molecular spintronic devices.

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.