Electron Spin Relaxation of Hole and Electron Polarons in π

Feb 19, 2015 - Home · Browse the Journal ... Department of Chemistry, French Family Science Center, Duke ... and [PZnn]–• spin–spin (T2) and spi...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/JPCB

Electron Spin Relaxation of Hole and Electron Polarons in π‑Conjugated Porphyrin Arrays: Spintronic Implications Jeff Rawson,† Paul J. Angiolillo,*,‡ Paul R. Frail,§ Isabella Goodenough,‡ and Michael J. Therien*,† †

Department of Chemistry, French Family Science Center, Duke University, 124 Science Drive, Durham, North Carolina 27708-0346, United States ‡ Department of Physics, Saint Joseph’s University, 5600 City Avenue, Philadelphia, Pennsylvania 19131, United States § Department of Chemistry, The University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104-6323, United States S Supporting Information *

ABSTRACT: 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 meso-to-meso ethyne-bridged (porphinato)zinc(II) oligomers (PZnn compounds), which can serve as models for the relevant states generated upon spin injection. The observed ESR line shapes for the PZnn hole polaron ([PZnn]+•) and electron polaron ([PZnn]−•) states evolve from Gaussian to more Lorentzian as the oligomer length increases from 1.9 to 7.5 nm, with solution-phase [PZnn]+• and [PZnn]−• spin−spin (T2) and spin−lattice (T1) 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-PZnn−metal junctions, present meso-to-meso ethyne-bridged multiporphyrin systems as leading candidates for ambient-temperature organic spintronic applications.

■

INTRODUCTION For over 100 years, information technology has exploited the charge of the electron or its absence (holes), but over the past decade, the electron’s spin has added a new degree of freedom for encoding logic.1,2 This still-nascent field, in which electron spin does duty in electronic devices, has been given the moniker spintronics. Spin-polarized transport, first intimated by Mott in 1936,3 was experimentally verified by the demonstration of magnetic field control over the characteristic current−voltage responses in trilayer architectures, where a ferromagnetic semiconducting Eu chalcogenide layer was sandwiched between two nonmagnetic metal electrodes.4,5 In these structures, it was shown that unpolarized currents that were injected into the ferromagnetic semiconducting layer became polarized. However, it was not until the discovery of giant magnetoresistance (GMR) by Baibich, Binasch, and their co-workers that spintronics evolved from an interesting phenomenon into a blossoming and seductive area of research.6,7 The challenge of maintaining spin-polarized currents has motivated the adoption of organic semiconductors, with their very long spin relaxation times, as spintronic device transport materials.8−12 As further incentive, π-conjugated materials may © XXXX American Chemical Society

offer advantages over inorganic semiconductors in the areas of cost and processability. Pioneering work of Dediu et al. in 2002 demonstrated a 30% negative magnetoresistance (MR) under a modest applied magnetic field of 3400 Oe for sexthiophene thin films between two La0.7Sr0.3MnO3 (LMSO) electrodes.13 Very quickly thereafter, this effect was verified using tris(8hydroxyquinolinato)aluminum (Alq3) as the semiconducting spin transport layer between LMSO and Co electrodes, registering a negative MR of ∼40% at 11 K.14 Very recent work on an organic spin valve employing C60 as the transport layer reported a MR of over 5% at room temperature, with a spin diffusion length of approximately 110 nm.15 Molecular, oligomeric, and polymeric materials that exploit highly polarizable (porphinato)metal components have figured prominently in the development of charge transport materials; porphyrin derivatives, for example, have been engineered to serve as high-mobility single-crystal transistors,16 memory Special Issue: John R. Miller and Marshall D. Newton Festschrift Received: December 9, 2014 Revised: February 18, 2015

A

DOI: 10.1021/jp5122728 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B elements,17 logic gates,18,19 and molecular wires.20,21 Perhaps surprisingly, there have been no corresponding systematic investigations of the spin transport properties of such systems, although a negative MR of 15% at 80 K has been demonstrated in trilayer LMSO/tetraphenylporphyrin/Co devices.22 An exceptional class of π-conjugated materials is comprised of meso-to-meso ethyne-bridged (porphinato)zinc(II) oligomers (PZn n compounds, Chart 1); in these supermolecular

In this work, we use ESR spectroscopic line shape analysis and CW progressive microwave power saturation to probe the relaxation behavior of PZnn charged excitations (hole polarons [PZnn]+• and electron polarons [PZnn]−•) that range in molecular length from 1.9 to 7.5 nm, which can serve as models for the relevant states generated upon spin injection. We note that the observed ESR line shapes for [PZnn]+• and [PZnn]−• evolve from Gaussian to more Lorentzian as the oligomer length increases and that the solution-phase [PZnn]+• and [PZnn]−• spin−spin and spin−lattice relaxation times at 298 K range, respectively, over 40−230 ns and 0.2−2.3 μs. This work further underscores that these very long relaxation times are preserved in thick films. Because these times are vital metrics for spin dephasing in quantum computing or for spin-polarized transport in MR structures, these results, coupled with the established wire-like transport behavior across metal−dithiolPZnn−metal junctions,20 present meso-to-meso ethyne-bridged multiporphyrin systems as leading candidates for ambienttemperature organic spintronic applications.

Chart 1. Neutral PZnE2 and PZnn Conjugated Structures

■

RESULTS AND DISCUSSION Hole and Electron Polaron States in PZnn. Singly charged, paramagnetic [PZnn]+• and [PZnn]−• species can be generated in solution through redox titrations carried out with, respectively, tris(4-bromophenyl)aminium hexachloroantimonate and decamethylcobaltocene reagents.41,42 The ESR spectra of [PZnn]+• evince line widths (ΔBpp values) governed primarily by 14 7 N(I = 1) isotropic hyperfine couplings due to the a2u-derived PZnn highest-occupied molecular orbitals (HOMOs);26,41,43 the distributions of the PZnn lowestoccupied molecular orbitals (LUMOs) ensure that for [PZnn]−•, the principal hyperfine couplings are due to peripheral hydrogen nuclei (I = 1/2).15,42,44−46 In the polaron states of ethynyl-linked porphyrin arrays PZn2−PZn7, the ESR spectra feature hyperfine structure buried within an absorption envelope. The ESR spectra of these PZnn hole41 and electron polaron42 states exhibit line widths congruent with that predicted by Norris for global coherent excitation delocalization,47 where the theoretical line width is given by ΔBpp(Nmer) = (1/N1/2)ΔBpp(monomer), where N represents the number of equivalent sites in the oligomer. The line widths

chromophores, strong interpigment communication provides ground and electronically excited singlet states that are globally delocalized.23−36 Extended polarons have, likewise, been demonstrated in a vast array of π-conjugated oligomers and polymers.37−40 Electron spin resonance (ESR) measurements have shown that for ethyne-extended PZn monomers and PZnn compounds, these elaborations permit predictable control over the spin distribution and orientation in their photoexcited triplet states.32−36 These observations suggest that an investigation of fundamental properties relevant to spin transport in these extraordinary materials may provide insights of value to the emerging field of organic spintronics.

Table 1. ESR Spectroscopic Data, Spin−Lattice Relaxation Times, and Spin−Spin Relaxation Times of Hole and Electron Polaron Species at 298 K ga −•

[PZn1] [PZn2]−• [PZn3]−• [PZn5]−• [PZn7]−•

[PZn1]+• [PZn2]+• [PZn3]+• [PZn4]+• [PZn5]+• [PZn7]+• [PZn5]+• (film)

2.0008 1.9996 2.0003 2.0003 2.0012 ga 2.0030 2.0031 2.0028 2.0040 2.0035 2.0041

ΔBpp (mT)b,c

T2 (ns) g

0.565 0.403 0.317 0.278 ΔBpp (mT)b,d 0.600 0.456 0.322 0.287 0.251 0.235 0.343

T1 (ns)

40 8f 140 ± 30 130 ± 20 230 ± 40

700 1500f 2300 ± 300 3200 ± 400 2300 ± 300

T2 (ns) 7f

T1 (ns) 270f

170 100 100 130 140

± ± ± ± ±

50 25 70 20 20

ρe

g

340 720 180 270 1200

± ± ± ± ±

50 100 30 20 100

2.2 2.1 2.5 2.8 ρe 2.5 2.6 2.8 2.8 2.8 3.8 3.9

g values: ±0.0002 (electron polarons); ±0.0005 (hole polarons). bLine widths: ±0.001 mT. cReference 42. dReference 41. eρ values are ±0.1. Denotes a T2 lower limit based on the envelope line width. gDenotes a T2 based on an individual hyperfine line width.

a f

B

DOI: 10.1021/jp5122728 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B along with g values for hole and electron polarons are given in Table 1. This 1/√N dependence of the experimental ΔBpp line width upon PZnn oligomer length demonstrates unequivocally that the hole and electron polaron states of these species explore the entire molecular dimension (PZn2−PZn7: 1.9−7.5 nm) on time scales commensurate with relevant spin system interactions, which are on the order of or less than 1 MHz. ESR Line Shape Analysis. The ESR spectra of D4h porphyrin anion radicals are broadened by the Jahn−Teller (JT) effect into Gaussian envelopes having ΔBpp ≈ 20 G (2 mT) due to the degeneracy of the 2Eg state.45,46 The X-band ESR spectrum of [PZnE2]−• in THF solvent at 298 K (Figure 1A) reveals that the two ethyne substituents relieve the orbital degeneracy to a degree sufficient to obliterate this effect. The PZnE2 LUMO displays large coefficients on the ethyne carbons and their anchoring meso positions,36 and the [PZnE2]−• hyperfine structure stems predominantly from the macrocycle β-hydrogen atoms, with an observed isotropic coupling constant of 1.74 G (0.174 mT; B3LYP/EPRIII calculations find an average coupling of 2.29 G for these protons). ESR spectra for [PZn2]−• and [PZn7]−• are shown in Figure 1B,C, along with spectral fits to the experimental data; corresponding ESR spectra for [PZn3]−• and [PZn5]−• may be found in the Supporting Information (Figure S1). The ESR line shape is controlled via the interactions of the spin system with other electron and nuclear spins in the environment along with interactions with the lattice (electron−phonon interactions). The line shape broadening derives from homogeneous and inhomogeneous mechanisms. The interactions leading to homogeneous broadening include dipolar interactions between like spins, interaction of the spin system with the radiation field, and an array of motional spin dynamics.48 Those interactions leading to inhomogeneous broadening include the hyperfine interaction, anisotropy broadening (due to both g and hyperfine anisotropy), and dipolar interactions between unlike spins.48 In general, homogeneous broadening leads to Lorentzian line shapes and inhomogeneous broadening to Gaussian profiles. Thus, line shape analysis is a useful tool to interrogate the underlying relaxation behavior of a spin system. If a spin system is broadened through both homogeneous and inhomogeneous mechanisms independently, then the experimental line shape is a convolution of the Lorentzian and Gaussian line shapes, resulting in a Voigt profile. Voigt function line shapes are typically manifest when the spin packet line width, governed by homogeneous mechanisms, becomes comparable to those leading to Gaussian line shapes. The contributions of Gaussian and Lorentzian components to ESR line shapes can be further quantified by examination of the second derivatives of the absorption spectra (Figure 1B,C, insets). We define the ratio of the minimum to the maximum second derivative amplitudes as ρ. For a strictly Gaussian spectral profile, the ρ value is 2.24, while that for a pure Lorentzian profile is 4. The ρ values for hole and electron polarons are provided in Table 1. For [PZn2]−•, the first derivative ESR spectrum is well fit by a Gaussian function, and a ρ = 2.2 further reveals its Gaussian profile; likewise, the line shapes for [PZn3]−• and [PZn5]−• are also strictly Gaussian and feature respective ρ values of 2.1 and 2.5 (Figure S1, Supporting Information). These line shapes are strictly determined by the isotropic nuclear hyperfine coupling. To completely average the nuclear dipolar interaction, the rotational correlation time constant should be approximately 5-fold smaller than the time set by the inverse of the nuclear

Figure 1. (A) X-band ESR spectra of (A) [PZnE2]−•, (B) [PZn2]−•, and (C) [PZn7]−• in THF solution at 298 K. [PZnE2]−• and [PZnn]−• concentrations of ∼50−100 μM; spectra were obtained at a 1 mW microwave power using a modulation amplitude of 1 G (0.1 mT) at 100 kHz. (B) The spectrum for [PZn2]−• is fit (red line) to a Gaussian line shape function, while the spectrum for [PZn7]−• is fit (red lines indicated by arrows) to both Gaussian and Lorentzian line shape functions. The double arrow for [PZnE2]−• indicates the isotropic hyperfine coupling separation of Ao = 1.74 G. The insets show the second derivative of the absorption, d2χ/dB2.

dipolar interactions. Assuming that the magnitude of nuclear dipolar interactions is on the same order as the nuclear isotropic splitting (∼0.1 mT), this gives a frequency of ∼2.8 × 106 Hz and a corresponding time of 3.7 × 10−7 s. Thus, to average the nuclear dipolar interaction, a rotational correlation C

DOI: 10.1021/jp5122728 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

concentrations used in these experiments (50−100 μM), which minimize the extent of dipolar and exchange interactions, the ESR line shape, and in particular the line width, are directly related to an effective hyperfine field, which, in general, is comprised of an isotropic contact interaction and a dipolar component, the latter of which is assumed to average to zero in THF solution at 298 K due to intramolecular spin motional dynamics. These changes in line shape for PZnn hole polarons demonstrate a decreasing hyperfine field as the conjugation length increases,41 as evidenced by the decreasing line width (Table 1); these systems thus exemplify the onset of behavior typified by homogeneous spin relaxation mechanisms as their linear molecular dimension is increased. As seen in the electron polaron line shapes, one can also conclude that intramolecular spin carrier dynamics are becoming a significant line-shapedetermining mechanism. Microwave Power Saturation. In addition to line shape analysis, progressive microwave power saturation experiments provide complementary information giving insight into relaxation mechanisms; such data can be used to determine rates of relaxation due to spin−spin (T2) and spin−lattice (T1) interactions, where T1 is the time required for the longitudinal magnetization to reach thermal equilibrium and T2 is the time that it takes for a spin ensemble initially precessing in phase about the longitudinal field to lose coherence due to precessing frequency fluctuations. The microwave saturation data were collected generally under slow passage conditions (T1T2)1/2 ≪ B1/ωmΔBpp) at 298 K (i.e., the time between successive field modulation cycles is sufficiently long for each spin packet to relax between cycles), where B1 is the microwave magnetic field strength, ωm is the modulation frequency, and ΔBpp is the peakto-peak line width. Saturation data were fitted to a model originally developed by Portis50 and Castner48 and elaborated by Zhidkov51 and Wheeler,52 yielding the microwave field in the rotating plane at half-saturation (B1/2) of the resonance line and a homogeneity factor b, the value of which has extremes at b = 1 for line shapes determined by complete inhomogeneous broadening and b = 3 for homogeneously broadened line shapes. Saturation profiles for these extremes and for cases in between are shown in Figure S2 (Supporting Information). The progressive microwave saturation profile for [PZnE2]−• (Figure S3, Supporting Information), which highlights clear evidence of microwave power saturation, confirms the absence of JT-mediated relaxation channels, which would yield saturation plots that remain linear with the square root of increasing microwave power (∝B1). In particular, the JT interactions provide spin relaxation channels that result in porphyrin radical monoanions that are difficult to saturate in progressive microwave saturation experiments; in the paradigmatic [5,10,15,20-tertaphenyl(porphinato)]zinc(II) (ZnTPP) radical anion, spin relaxation remains unusually fast even at 77 K.46 This unusual spin relaxation behavior has been attributed to the JT-active 2Eg ground state featured in metalloporphyrin monoanions with D4h symmetry. Seth and Bocian, exploiting isotopic substitution along with variable temperature ESR, provide detailed information regarding both static and dynamic JT interactions in monoanionic porphyrin radical species.46 Using an individual hyperfine line of [PZnE2]−•, a lower estimate of the T2 relaxation time at ∼40 ns may be determined using T2 = 2/(31/2γΔBpp) = 65.6 (G ns)/ΔBpp, where γ is the gyromagnetic ratio for the electron and ΔBpp is the peak-to-peak line width. Using this value and a value of B1/2 obtained from the saturation profile (Figure 2B),

time constant of τrot = 71 ns would be required. This constraint is certainly met in THF at 298 K.49 For [PZn7]−•, the first derivative ESR spectrum is not fit well by a Gaussian or a Lorentzian function (Figure 1C), and a ρ value of 2.8 indicates the onset of a Voigt profile (Figure S1C, Supporting Information) resulting from a convolution of Lorentzian character into the spectrum, which reflects a transition to relaxation channels that are becoming more homogeneous in nature (vide supra). At the concentrations used in these studies (50−100 μM), intermolecular separation distances are statistically ∼30−60 nm (300−600 Å); intermolecular dipolar interactions between electron polarons and thus exchange interactions are minimized. This fact, in conjunction with the long spin−lattice relaxation times gleaned from these species, indicate that intramolecular spin carrier dynamics are responsible for the onset of Lorentzian character of the line shape. The ESR spectrum of [PZn1]+• recorded at 298 K (Figure 2A) reveals an overall Gaussian envelope, with a secondderivative analysis giving ρ = 2.5. The ESR spectrum for [PZn7]+• (Figure 2B) evinces a Lorentzian profile, close to the Lorentzian limit with ρ = 3.8. Again, at the molecular

Figure 2. X-band ESR spectra of (A) [PZn1]+• and (B) [PZn7]+• in CH2Cl2 solution at 298 K. [PZnn]+• concentrations of ∼50−100 μM; spectra were obtained at a 1 mW microwave power using a modulation amplitude of 1 G (0.1 mT) at 100 kHz. The spectrum for [PZn1]+• is fit (red line) to a Gaussian line shape function, while the spectrum for [PZn7]+• is fit (red line) to a Lorentzian line shape function. The insets show the second derivative of the absorption, d2χ/dB2. D

DOI: 10.1021/jp5122728 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Figure 3. Progressive microwave saturation measurements of electron polarons (A) [PZn2]−• and (B) [PZn7]−• and hole polarons (C) [PZn1]+• and (D) [PZn7]+• in solution at 298 K. Data were collected using a modulation amplitude of 1 G (0.1 mT) at 100 kHz. The data are fit (solid black line) to the equation [C(B1/B1/2)]/[1 + (B1/B1/2)2]b/2, where B1/2 is the microwave magnetic field strength at half saturation and b is the homogeneity factor. The homogeneity factors for the respective species are [PZn2]−•, b = 1.0; [PZn7]−•, b = 1.7; [PZn1]+•, b = 1.0; [PZn7]+•, b = 1.9.

an upper limit for the T1 relaxation time for [PZnE2]−• can be estimated at ∼700 ns at 298 K using T1 = 1/(T2γ2B21/2), where γ is the magnetogyric ratio for the electron, with B1/2 and T2 as defined above.52 Progressive microwave power saturation profiles for [PZn2]−• and [PZn7]−• are shown in Figure 3A,B; analogous data for [PZn3]−• and [PZn5]−• appear in the Supporting Information (Figure S4A,B). Table 1 summarizes the relevant ESR data for [PZnn]−• and [PZnn]+•. The saturation profile for [PZn2]−• shows a decrease in microwave absorption only at the highest microwave powers (>100 mW). Using the line width at 298 K, the lower limit for the T2 time can be estimated at 8 ns; this provides an upper limit for T1 of 1500 ns. The saturation profile is consistent with the Gaussian line shape of [PZn2]−• and the fact that no power broadening of the line width was observed during the saturation experiment. Consistent with the Voigt ESR line shape for [PZn7]−•, the microwave saturation profile clearly evinces an evolution toward a spin system dominated by homogeneous line broadening decay channels (Figure 3B). The [PZn7]−• line shape and saturation profile changes occur concomitantly with an increase in the spin−spin relaxation time T2 to ∼200 ns and a T1 measuring ∼2000 ns. Similar saturation profiles obtained for [PZn3]−• and [PZn5]−• suggest as well the increasing influence of homogeneous broadening mechanisms (Figure S4A,B, Supporting Information). These data are consistent with recent evidence that shows a decreasing line width and hence a deceasing hyperfine field experienced by the electron polaron with increasing πconjugation length for PZnn electron poalrons.42

Figure 3C,D displays microwave saturation measurements for [PZn1]+• and [PZn7]+•; analogous saturation plots for [PZn3]+• and [PZn5]+• are provided in the Supporting Information (Figure S5A,B). For [PZn1]+•, the saturation at high microwave powers is indicative of an inhomogeneously broadened line shape that is derived from unresolved nuclear hyperfine interactions stemming primarily from the pyrrole nitogens.41 The line width estimated lower limit of T2 is 7 ns, placing an upper limit of 270 ns on T1. In stark contrast, the saturation plot for [PZn7]+• demonstrates that homogeneous broadening mechanisms are the dominant contributors to the line width, and T2 and T1 relaxation times of 130 and 270 ns, respectively, were found. As with the electron polarons, the microwave saturation behavior, which is reflected in the line shape evolution, is consonant with the decreasing hyperfine field seen by the spin system; because dipolar interactions between like spins and exchange interactions are unlikely under these conditions, with [PZnn]+• concentrations in the range of 50−100 μM, the line shape exhibited by [PZn7]+• must derive from intramolecular spin carrier dynamics. The electron and hole polaron states of PZn1−PZn7 show T2 in the range of 100−200 ns and T1 times exceeding 2 μs at 298 K. In general, PZnn hole polarons possess shorter spin−lattice relaxation times, which may be correlated with the earlier onset of Lorentzian character in their respective ESR line shapes (vide supra). Also, in the case of the hole polarons, the homogeneity factor b may have a larger uncertainty due to less data in the region where saturation becomes pronounced. A measure of the spread on the T1 and T2 relaxation times in both cases was estimated using the best-fit homogeneity factors b ± E

DOI: 10.1021/jp5122728 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B 0.1, resulting in changes of the fit’s correlation coefficient to only a few parts in 10000. Hole Polarons in PZn5 Solid Films. ESR measurements at 298 K for PZn5 hole polaron films generated through solvent evaporation in the ESR sample tube reveal that spin relaxation times remain long in the solid state, demonstrating respective T2 and T1 values of 140 and 1200 ns (Table 1). Note that the thin film line shape becomes distinctly Lorentzian (Figure 4A)

diffusion and exchange cannot be dismissed. It is however noteworthy that relaxation times are commensurate with those obtained in solution. Polaron Relaxation Times. The factors that govern spin T1 relaxation are currently disputed, with some evidence that the hyperfine interaction is predominant53−61 and other indications that the spin−orbit interaction is most significant.62−67 In [PZnn]−•, the spin density at the heavy Zn2+ ions and at the nitrogens is nearly zero;42 therefore, both spin−orbit coupling and the hyperfine fields are weak. For [PZnn]+•, the hole wave function is dominated by the a2u-derived orbitals of the individual PZn units of the array;26,41 this brings significant spin density to the vicinity of the heavy zinc ions and I = 1 nitrogens. This distinction may explain the generally longer T1 times manifest for PZnn electron polarons relative to those for PZnn hole polarons for comparably sized oligomers as the PZnn electron polaron wave function manifests negligible Zn2+ ion spin density. While no strict trend is observed, we note that the longer oligomers manifest longer T2 and T1 relaxation times in the electron polarons. This is congruent with the conjugation length-dependent decrease in the average hyperfine field experienced by the electron polaron spin system. In both the hole and electron polaron cases where the line shape evolves into a more Lorentzian profile (certainly the case in the hole polarons), spin relaxation phenomena become more complex and cannot be directly related to decreasing hyperfine fields alone, indicating perhaps a dominant motional line broadening mechanism. While long spin relaxation times are one prerequisite for spintronic applications, high conductivity is also required; this is embodied in the spin diffusion length, Ls = (Dτs)1/2 = [(kT/ q)μτs]1/2, where D is the diffusion coefficient, τs is the relevant spin relaxation time, μ is the mobility, k is Boltzmann’s constant, T is the absolute temperature, and q is the charge on the relevant charge carrier. It has been demonstrated that dark conductivities of amorphous solid-state materials comprised of PZnn lie in the range of 3.8 × 10−5 S/cm, greater than those found for poly(3-hexyl)thiophene (3 × 10−7 S/cm) or crystalline polyacetylene (1 × 10−5 S/cm), and are thus competitive with amorphous silicon.68,69 Recent work on single molecule resistance measurements of meso-to-meso ethynebridged porphyrin wires (PZnn n = 1, 2, 3) connected to gold electrodes suggests that these compositions evince a distancedependent junction resistance following an exponential behavior, R = R0eβL, where R0 is the contact resistance and β is the decay constant for charge transmission across the barrier. A β value of 0.034 Å−1, the lowest β yet determined for thiolterminated single molecules, establishes a quasi-ohmic resistance dependence across metal−dithiol-PZnn−metal junctions20 and defines a new dynamical benchmark for charge transfer mediated by π-conjugated structures at electrode interfaces.70,71 The combination of PZnn experimental data highlighted by single-molecule resistance measurements, bulk-phase conductivity measurements, and ESR studies of PZnn hole and electron polarons underscores the potential of these compositions for spintronics applications. Early studies that have utilized organic semiconductors as spintronic materials have relied, in part, on materials known to possess modest charge diffusion lengths and conductivities; recent studies, in contrast, have examined a host of superior organic semiconducting materials such as CNTs,72,73 graphene,74 rubrene,75 poly(bis(thiophenyl)thienothiophene),76 and triarylamine wires.77 Materials for spintronic devices must

Figure 4. (A) X-band ESR spectra of a [PZn5]+• film at 298 K. The film was generated in a quartz ESR tube via the removal of solvent under vacuum. The spectrum was obtained at a 1 mW microwave power using a modulation amplitude of 1 G (0.1 mT) at 100 kHz. The spectrum for [PZn5]+• is fit (red line) to a Lorentzian line shape function. (B) Progressive microwave saturation measurements of a [PZn5]+• film at 298 K. Data were collected using a modulation amplitude of 1 G (0.1 mT) at 100 kHz. The data are fit (solid black line) to the equation [C(B1/B1/2)]/[1 + (B1/B1/2)2]b/2, where B1/2 is the microwave magnetic field strength at half saturation and the homogeneity factor b is 2.0. The inset shows the line width as a function of the microwave B1 field (∝√P).

with a ρ = 4.0; the fact that ρ = 2.6 for [PZn5]+• in solution at 298 K signals that homogeneous broadening mechanisms are becoming the more prominent relaxation processes. Furthermore, the ESR line width observed for [PZn5]+• films are microwave-power-dependent (Figure 4B inset), increasing from 0.34 mT at 10 μW to 0.52 mT at 200 mW, congruent with relaxation channels becoming more complex in the solid state; given these microwave power saturation and line width behaviors, intermolecular dynamic processes such as spin F

DOI: 10.1021/jp5122728 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B Notes

maintain either injected spin polarization (classical) or phase memory (quantum), as measured by the T1 and T2 relaxation times, and a correspondingly long spin diffusion length.78 In general, because of the strong temperature dependence of T1, organic materials used in magnetoresistive devices have been plagued with a temperature-decaying MR response that typically vanishes above 250 K, and thus, it is unclear whether large spin diffusion lengths can be realized using conventional organic semiconductors at room temperature. The substantial ambient temperature spin relaxation times exemplified by the polaronic species in PZnn, which serve as model systems for injected charge, coupled with the large molecular conductivities evinced by these materials, highlight however the promise of these platforms for use in spintronic devices.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was funded by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, of the U.S. Department of Energy through Grant DESC0001517.

■

■

CONCLUSION The spin relaxation times and concurrent line shape analysis in a systematic series of PZnn electron and hole polarons were studied. Room-temperature microwave saturation profiles determine T2 relaxation times in the range of hundreds of nanoseconds and corresponding T1 relaxation times that exceed several microseconds in the longest polaronic species studied (PZn7−7.5 nm). The observed ESR line shapes for the hole and electron polaron states of PZnn species evolve from Gaussian to more Lorentzian as the oligomer length increases from 1.9 to 7.5 nm, indicating that broadening is dominated by inhomogeneous hyperfine interactions at oligomer lengths that range from 1.9 to 5.2 nm, with homogeneous broadening mechanisms becoming important at longer PZnn oligomer lengths. These mechanisms are likely governed by an array of intramolecular (solution-phase) and intermolecular (solidstate) spin dynamical processes. Note that the magnitudes of these spin relaxation times highlight the diminished hyperfine and weak spin−orbit interactions of these spin systems; further, these spin−lattice relaxation time measurements are consistent with previously acquired ESR line width data for PZnn hole and electron polarons and thus serve to further underscore the extraordinarily weak electron−phonon interactions manifest in these structures. With respect to organic spintronics, the extremely long T1 relaxation times obtained at room temperature are of potential critical importance. The combination of the long relaxation times measured at 298 K for PZnn hole and electron polaron states and single-molecule conductance measurements that establish PZnn compounds as an organic molecular wire performance benchmark,20 coupled with the fact that the spin relaxation length Ls is dependent on the product of the conductivity and the spin−lattice relaxation time, underscores dramatically that PZnn compositions define ideal platforms in which to engineer and explore molecular spintronic functionality.

■

ASSOCIATED CONTENT

S Supporting Information *

Experimental details and additional ESR spectroscopic data. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

(1) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; von Molnár, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Spintronics: A Spin-Based Electronics Vision for the Future. Science 2001, 294, 1488−1495. (2) Ž utić, I.; Fabian, J.; Das Sarma, S. Spintronics: Fundamentals and Applications. Rev. Mod. Phys. 2004, 76, 323−410. (3) Mott, N. F. The Electrical Conductivity of Transition Metals. Proc. R. Soc. London, Ser. A 1936, 153, 699−717. (4) Esaki, L.; Stiles, P. J.; von Molnar, S. Magnetointernal Field Emission in Junctions of Magnetic Insulators. Phys. Rev. Lett. 1967, 19, 852−854. (5) Kasuya, T.; Yanase, A. Anomalous Transport Phenomena in Eu− Chalcogenide Alloys. Rev. Mod. Phys. 1968, 40, 684−696. (6) Baibich, M. N.; Broto, J. M.; Fert, A.; Van Dau, F. N.; Petroff, F.; Etienne, P.; Creuzet, G.; Friederich, A.; Chazelas, J. Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattices. Phys. Rev. Lett. 1988, 61, 2472−2475. (7) Binasch, G.; Grünberg, P.; Saurenbach, F.; Zinn, W. Enhanced Magnetoresistance in Layered Magnetic Structures with Antiferromagnetic Interlayer Exchange. Phys. Rev. B 1989, 39, 4828−4830. (8) Naber, W. J. M.; Faez, S.; van der Wiel, W. G. Organic Spintronics. J. Phys. D: Appl. Phys. 2007, 40, R205−R228. (9) Bergenti, I.; Dediu, V.; Prezioso, M.; Riminucci, A. Organic Spintronics. Philos. Trans. R. Soc. London, Ser. A 2011, 369, 3054− 3068. (10) Sanvito, S. Molecular Spintronics. Chem. Soc. Rev. 2011, 40, 3336−3355. (11) Ehrenfreund, E.; Vardeny, Z. V. Organic Spin-Valves: From Unipolar to Bipolar Devices. Phys. Chem. Chem. Phys. 2013, 15, 7967− 7975. (12) Sun, D.; Ehrenfreund, E.; Vardeny, Z. V. The First Decade of Organic Spintronics Research. Chem. Commun. 2014, 50, 1781−1793. (13) Dediu, V.; Murgia, M.; Matacotta, F. C.; Taliani, C.; Barbanera, S. Room Temperature Spin Polarized Injection in Organic Semiconductor. Solid State Commun. 2002, 122, 181−184. (14) Xiong, Z. H.; Wu, D.; Vardeny, Z. V.; Shi, J. Giant Magnetoresistance in Organic Spin-Valves. Nature 2004, 427, 821− 824. (15) Zhang, X.; Mizukami, S.; Kubota, T.; Ma, Q.; Oogane, M.; Naganuma, H.; Ando, Y.; Miyazaki, T. Observation of a Large SpinDependent Transport Length in Organic Spin Valves at Room Temperature. Nat. Commun. 2013, 4, 1392. (16) Choi, S.; Chae, S. H.; Shin, J.; Kim, Y.; Kim, S.-J.; Choi, D. H.; Lee, S. J. Dramatic Enhancement of Carrier Mobility via Effective Secondary Structural Arrangement Resulting from the Substituents in a Porphyrin Transistor. Chem. Commun. 2013, 49, 3994−3996. (17) Lindsey, J. S.; Bocian, D. F. Molecules for Charge-Based Information Storage. Acc. Chem. Res. 2011, 44, 638−650. (18) Ambroise, A.; Wagner, R. W.; Rao, P. D.; Riggs, J. A.; Hascoat, P.; Diers, J. R.; Seth, J.; Lammi, R. K.; Bocian, D. F.; Holten, D.; et al. Design and Synthesis of Porphyrin-Based Optoelectronic Gates. Chem. Mater. 2001, 13, 1023−1034. (19) de Ruiter, G.; van der Boom, M. E. Orthogonal Addressable Monolayers for Integrating Molecular Logic. Angew. Chem., Int. Ed. 2012, 51, 8598−8601. (20) Li, Z.; Park, T.-H.; Rawson, J.; Therien, M. J.; Borguet, E. QuasiOhmic Single Molecule Charge Transport through Highly Conjugated

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.J.A). *E-mail: [email protected] (M.J.T.). G

DOI: 10.1021/jp5122728 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B meso-to-meso Ethyne-Bridged Porphyrin Wires. Nano Lett. 2012, 12, 2722−2727. (21) Sedghi, G.; Garcia-Suarez, V. M.; Esdaile, L. J.; Anderson, H. L.; Lambert, C. J.; Martin, S.; Bethell, D.; Higgins, S. J.; Elliott, M.; Bennett, N.; et al. Long-range Electron Tunnelling in Oligo-Porphyrin Molecular Wires. Nat. Nanotechnol. 2011, 6, 517−523. (22) Xu, W.; Szulczewski, G. J.; LeClair, P.; Navarrete, I.; Schad, R.; Miao, G.; Guo, H.; Gupta, A. Tunneling Magnetoresistance Observed in La0.67Sr0.33MnO3/Organic Molecule/Co Junctions. Appl. Phys. Lett. 2007, 90, 072506. (23) Lin, V. S.-Y.; DiMagno, S. G.; Therien, M. J. Highly Conjugated, Acetylenyl Bridged Porphyrins: New Models for Light-Harvesting Antenna Systems. Science 1994, 264, 1105−1111. (24) Lin, V. S.-Y.; Therien, M. J. The Role of Porphyrin-to-Porphyrin Linkage Topology in the Extensive Modulation of the Absorptive and Emissive Properties of a Series of Ethynyl- and Butadiynyl-Bridged Bisand Tris(porphinato)zinc Chromophores. Chem.Eur. J. 1995, 1, 645−651. (25) Kumble, R.; Palese, S.; Lin, V. S.-Y.; Therien, M. J.; Hochstrasser, R. M. Ultrafast Dynamics of Highly Conjugated Porphyrin Arrays. J. Am. Chem. Soc. 1998, 120, 11489−11498. (26) Susumu, K.; Therien, M. J. Decoupling Optical and Potentiometric Band Gaps in π-Conjugated Materials. J. Am. Chem. Soc. 2002, 124, 8550−8552. (27) Fletcher, J. T.; Therien, M. J. Strongly Coupled Porphyrin Arrays Featuring Both π-Cofacial and Linear-π-Conjugative Interactions. Inorg. Chem. 2002, 41, 331−341. (28) Rubtsov, I. V.; Susumu, K.; Rubtsov, G. I.; Therien, M. J. Ultrafast Singlet Excited-State Polarization in Electronically Asymmetric Ethyne-Bridged Bis[(porphinato)zinc(II)] Complexes. J. Am. Chem. Soc. 2003, 125, 2687−2696. (29) Duncan, T. V.; Susumu, K.; Sinks, L. E.; Therien, M. J. Exceptional Near-Infrared Fluorescence Quantum Yields and ExcitedState Absorptivity of Highly Conjugated Porphyrin Arrays. J. Am. Chem. Soc. 2006, 128, 9000−9001. (30) Frail, P. R.; Susumu, K.; Huynh, M.; Fong, J.; Kikkawa, J. M.; Therien, M. J. Modulation of Dark Conductivity over a 1 × 10−12 to 1 × 10−5 S/cm Range through Ancillary Group Modification in Amorphous Solids of Ethyne-Bridged (Porphinato)zinc(II) Oligomers. Chem. Mater. 2007, 19, 6062−6064. (31) Duncan, T. V.; Frail, P. R.; Miloradovic, I. R.; Therien, M. J. Excitation of Highly Conjugated (Porphinato)palladium(II) and (Porphinato)platinum(II) Oligomers Produces Long-Lived, Triplet States at Unit Quantum Yield That Absorb Strongly over Broad Spectral Domains of the NIR. J. Phys. Chem. B 2010, 114, 14696− 14702. (32) Angiolillo, P. J.; Lin, V. S.-Y.; Vanderkooi, J. M.; Therien, M. J. EPR Spectroscopy and Photophysics of the Lowest Photoactivated Triplet State of a Series of Highly Conjugated (Porphinato)Zn Arrays. J. Am. Chem. Soc. 1995, 117, 12514−12527. (33) Shediac, R.; Gray, M. H. B.; Uyeda, H. T.; Johnson, R. C.; Hupp, J. T.; Angiolillo, P. J.; Therien, M. J. Singlet and Triplet Excited States of Emissive, Conjugated Bis(porphyrin) Compounds Probed by Optical and EPR Spectroscopic Methods. J. Am. Chem. Soc. 2000, 122, 7017−7033. (34) Angiolillo, P. J.; Susumu, K.; Uyeda, H. T.; Lin, V. S.-Y.; Shediac, R.; Therien, M. J. Trends in Triplet Excitation Delocalization in Highly Conjugated (Porphinato)zinc(II) Arrays Probed by EPR Spectroscopy. Synth. Met. 2001, 116, 247−253. (35) Angiolillo, P. J.; Uyeda, H. T.; Duncan, T. V.; Therien, M. J. Impact of Electronic Asymmetry on Photoexcited Triplet-State Spin Distributions in Conjugated Porphyrin Oligomers Probed via EPR Spectroscopy. J. Phys. Chem. B 2004, 108, 11893−11903. (36) Angiolillo, P. J.; Rawson, J.; Frail, P. R.; Therien, M. J. The Evolution of Spin Distribution in the Photoexcited Triplet State of Ethyne-Elaborated Porphyrins. Chem. Commun. 2013, 49, 9722−9724. (37) Takeda, N.; Miller, J. R. Poly(3-decylthiophene) Radical Anions and Cations in Solution: Single and Multiple Polarons and Their

Delocalization Lengths in Conjugated Polymers. J. Phys. Chem. B 2012, 116, 14715−14723. (38) Zaikowski, L.; Kaur, P.; Gelfond, C.; Selvaggio, E.; Asaoka, S.; Wu, Q.; Chen, H.-C.; Takeda, N.; Cook, A. R.; Yang, A.; et al. Polarons, Bipolarons, and Side-by-Side Polarons in Reduction of Oligofluorenes. J. Am. Chem. Soc. 2012, 134, 10852−10863. (39) Bakalis, J.; Cook, A. R.; Asaoka, S.; Forster, M.; Scherf, U.; Miller, J. R. Polarons, Compressed Polarons, and Bipolarons in Conjugated Polymers. J. Phys. Chem. C 2013, 118, 114−125. (40) Bird, M. J.; Reid, O. G.; Cook, A. R.; Asaoka, S.; Shibano, Y.; Imahori, H.; Rumbles, G.; Miller, J. R. Mobility of Holes in Oligo- and Polyfluorenes of Defined Lengths. J. Phys. Chem. C 2014, 118, 6100− 6109. (41) Susumu, K.; Frail, P. R.; Angiolillo, P. J.; Therien, M. J. Conjugated Chromophore Arrays with Unusually Large Hole Polaron Delocalization Lengths. J. Am. Chem. Soc. 2006, 128, 8380−8381. (42) Rawson, J.; Angiolillo, P. J.; Therien, M. J. Extreme Electron− Polaron Spatial Delocalization in π-Conjugated Materials. Submitted. (43) Fajer, J.; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. πCation Radicals and Dications of Metalloporphyrins. J. Am. Chem. Soc. 1970, 92, 3451−3459. (44) Huber, M.; Fuhs, M. Frontier Orbitals of Porphyrin Electron Donors in Biomimetic Model Compounds  Partial Lifting of Orbital Degeneracy in Asymmetric Porphyrins Studied by EPR on Anion Radicals. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 2057−2064. (45) Pawlik, J.; Gherghel, L.; Karabunarliev, S.; Baumgarten, M. Characterization of Reduced Porphyrinatozinc(II) Complexes by EPR/ENDOR/TRIPLE and Optical Absorption Spectroscopy. Chem. Phys. 1997, 221, 121−133. (46) Seth, J.; Bocian, D. F. Electron Paramagnetic Resonance Studies of Metalloporphyrin Anion Radicals. Effects of Solvent, Counterion, Temperature, and Isotopic Substitution on the Jahn−Teller Active 2Eg Ground State. J. Am. Chem. Soc. 1994, 116, 143−153. (47) Norris, J. R.; Uphaus, R. A.; Crespi, H. L.; Katz, J. J. Electron Spin Resonance of Chlorophyll and the Origin of Signal I in Photosynthesis. Proc. Natl. Acad. Sci. U.S.A. 1971, 68, 625−628. (48) Castner, T. G. Saturation of the Paramagnetic Resonance of a V Center. Phys. Rev. 1959, 115, 1506−1515. (49) Schmidt, R. D.; Kent, C. A.; Concepcion, J. J.; Lin, W.; Meyer, T. J.; Forbes, M. D. E. A Little Spin on the Side: Solvent and Temperature Dependent Paramagnetism in [RuII(bpy)2(phendione)]2+. Dalton Trans. 2014, 43, 17729−17739. (50) Portis, A. M. Electronic Structure of F Centers: Saturation of the Electron Spin Resonance. Phys. Rev. 1953, 91, 1071−1078. (51) Zhidkov, O. P.; Muromtsev, V. I.; Akhvlediani, I. G.; Safronov, S. N.; Kopylov, V. V. Estimation of Relaxation Times and Width of Distribution Function of Inhomogeneously Broadened EPR Lines by Continuous Saturation Technique. Sov. Phys. Solid State 1967, 9, 1403−1409. (52) Wheeler, D. D.; Farach, H. A.; Poole, C. P.; Creswick, R. J. Electron-Spin-Relaxation Times in Se-Doped Potassium Dihydrogen Phosphate Ferroelectric Crystals. Phys. Rev. B 1988, 37, 9703−9709. (53) Sheng, Y.; Nguyen, T. D.; Veeraraghavan, G.; Mermer, Ö .; Wohlgenannt, M.; Qiu, S.; Scherf, U. Hyperfine Interaction and Magnetoresistance in Organic Semiconductors. Phys. Rev. B 2006, 74, 045213. (54) Rolfe, N. J.; Heeney, M.; Wyatt, P. B.; Drew, A. J.; Kreouzis, T.; Gillin, W. P. Elucidating the Role of Hyperfine Interactions on Organic Magnetoresistance Using Deuterated Aluminium tris(8Hydroxyquinoline). Phys. Rev. B 2009, 80, 241201. (55) Schoonus, J. J. H. M.; Lumens, P. G. E.; Wagemans, W.; Kohlhepp, J. T.; Bobbert, P. A.; Swagten, H. J. M.; Koopmans, B. Magnetoresistance in Hybrid Organic Spin Valves at the Onset of Multiple-Step Tunneling. Phys. Rev. Lett. 2009, 103, 146601. (56) Bobbert, P. A.; Nguyen, T. D.; Wagemans, W.; van Oost, F. W. A.; Koopmans, B.; Wohlgenannt, M. Spin Relaxation and Magnetoresistance in Disordered Organic Semiconductors. Synth. Met. 2010, 160, 223−229. H

DOI: 10.1021/jp5122728 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B (57) Nguyen, T. D.; Hukic-Markosian, G.; Wang, F.; Wojcik, L.; Li, X.-G.; Ehrenfreund, E.; Vardeny, Z. V. Isotope Effect in Spin Response of π-Conjugated Polymer Films and Devices. Nat. Mater. 2010, 9, 345−352. (58) Nguyen, T. D.; Hukic-Markosian, G.; Wang, F.; Wojcik, L.; Li, X.-G.; Ehrenfreund, E.; Vardeny, Z. V. The Hyperfine Interaction Role in the Spin Response of π-Conjugated Polymer Films and Spin Valve Devices. Synth. Met. 2011, 161, 598−603. (59) Nguyen, T. D.; Basel, T. P.; Pu, Y.-J.; Li, X. G.; Ehrenfreund, E.; Vardeny, Z. V. Isotope Effect in the Spin Response of Aluminum tris(8-Hydroxyquinoline) Based Devices. Phys. Rev. B 2012, 85, 245437. (60) Majumdar, S.; Majumdar, H. S. Decay in Spin Diffusion Length With Temperature in Organic SemiconductorsAn Insight of Possible Mechanisms. Synth. Met. 2013, 173, 26−30. (61) Yu, Z. G.; Ding, F.; Wang, H. Hyperfine Interaction and Its Effects on Spin Dynamics in Organic Solids. Phys. Rev. B 2013, 87, 205446. (62) Sheng, Y.; Nguyen, T. D.; Veeraraghavan, G.; Mermer, Ö .; Wohlgenannt, M. Effect of Spin−Orbit Coupling on Magnetoresistance in Organic Semiconductors. Phys. Rev. B 2007, 75, 035202. (63) Bandyopadhyay, S. Dominant Spin Relaxation Mechanism in Compound Organic Semiconductors. Phys. Rev. B 2010, 81, 153202. (64) Yu, Z. G. Spin−Orbit Coupling, Spin Relaxation, and Spin Diffusion in Organic Solids. Phys. Rev. Lett. 2011, 106, 106602. (65) Yu, Z. G. Spin−Orbit Coupling and Its Effects in Organic Solids. Phys. Rev. B 2012, 85, 115201. (66) Harmon, N. J.; Flatté, M. E. Distinguishing Spin Relaxation Mechanisms in Organic Semiconductors. Phys. Rev. Lett. 2013, 110, 176602. (67) Nuccio, L.; Willis, M.; Schulz, L.; Fratini, S.; Messina, F.; D’Amico, M.; Pratt, F. L.; Lord, J. S.; McKenzie, I.; Loth, M.; et al. Importance of Spin−Orbit Interaction for the Electron Spin Relaxation in Organic Semiconductors. Phys. Rev. Lett. 2013, 110, 216602. (68) Chiang, C. K.; Fincher, C. R.; Park, Y. W.; Heeger, A. J.; Shirakawa, H.; Louis, E. J.; Gau, S. C.; MacDiarmid, A. G. Electrical Conductivity in Doped Polyacetylene. Phys. Rev. Lett. 1977, 39, 1098− 1101. (69) Yu, G.; Phillips, S. D.; Tomozawa, H.; Heeger, A. J. Subnanosecond Transient Photoconductivity in Poly(3-hexylthiophene). Phys. Rev. B 1990, 42, 3004−3010. (70) Sachs, S. B.; Dudek, S. P.; Hsung, R. P.; Sita, L. R.; Smalley, J. F.; Newton, M. D.; Feldberg, S. W.; Chidsey, C. E. D. Rates of Interfacial Electron Transfer through π-Conjugated Spacers. J. Am. Chem. Soc. 1997, 119, 10563−10564. (71) Sikes, H. D.; Smalley, J. F.; Dudek, S. P.; Cook, A. R.; Newton, M. D.; Chidsey, C. E. D.; Feldberg, S. W. Rapid Electron Tunneling Through Oligophenylenevinylene Bridges. Science 2001, 291, 1519− 1523. (72) Tsukagoshi, K.; Alphenaar, B. W.; Ago, H. Coherent Transport of Electron Spin in a Ferromagnetically Contacted Carbon Nanotube. Nature 1999, 401, 572−574. (73) Hueso, L. E.; Pruneda, J. M.; Ferrari, V.; Burnell, G.; ValdesHerrera, J. P.; Simons, B. D.; Littlewood, P. B.; Artacho, E.; Fert, A.; Mathur, N. D. Transformation of Spin Information into Large Electrical Signals Using Carbon Nanotubes. Nature 2007, 445, 410− 413. (74) Tombros, N.; Jozsa, C.; Popinciuc, M.; Jonkman, H. T.; van Wees, B. J. Electronic Spin Transport and Spin Precession in Single Graphene Layers at Room Temperature. Nature 2007, 448, 571−574. (75) Shim, J. H.; Raman, K. V.; Park, Y. J.; Santos, T. S.; Miao, G. X.; Satpati, B.; Moodera, J. S. Large Spin Diffusion Length in an Amorphous Organic Semiconductor. Phys. Rev. Lett. 2008, 100, 226603. (76) Watanabe, S.; Ando, K.; Kang, K.; Mooser, S.; Vaynzof, Y.; Kurebayashi, H.; Saitoh, E.; Sirringhaus, H. Polaron Spin Current Transport in Organic Semiconductors. Nat. Phys. 2014, 10, 308−313.

(77) Bhattacharya, S.; Akande, A.; Sanvito, S. Spin Transport Properties of Triarylamine-Based Nanowires. Chem. Commun. 2014, 50, 6626−6629. (78) Warner, M.; Din, S.; Tupitsyn, I. S.; Morley, G. W.; Stoneham, A. M.; Gardener, J. A.; Wu, Z.; Fisher, A. J.; Heutz, S.; Kay, C. W. M.; et al. Potential for Spin-Based Information Processing in a Thin-Film Molecular Semiconductor. Nature 2013, 503, 504−508.

I

DOI: 10.1021/jp5122728 J. Phys. Chem. B XXXX, XXX, XXX−XXX