Investigations of the Degenerate Intramolecular Charge Exchange in

Mar 31, 2015 - Triarylamines are important hole-transport components in optoelectronic devices. Understanding the factors controlling their intra- and...
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Investigations of the Degenerate Intramolecular Charge Exchange in Symmetric Organic Mixed Valence Compounds: Solvent Dynamics of Bis(triarylamine)paracyclophane Redox Systems Boryana Mladenova,† Daniel R. Kattnig,‡ Conrad Kaiser,§ Julian Schaf̈ er,§ Christoph Lambert,*,§ and Günter Grampp*,† †

Institute of Physical and Theoretical Chemistry, Graz University of Technology, Stremayrgasse 9, A-8010 Graz, Austria Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, U.K. § Institute of Organic Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany ‡

S Supporting Information *

ABSTRACT: Triarylamines are important hole-transport components in optoelectronic devices. Understanding the factors controlling their intra- and intermolecular electron transfer properties is crucial to the application and optimization of organic hole conductors. Here, we report on the degenerate intramolecular electron exchange reactions of several purely organic mixed valence compounds based on the bis(triarylamine)paracyclophane structural unit, which are archetypical molecular wires. Different bridging moieties are compared, and the foremost impact of the solvent environment on the rate of electron transfer is demonstrated. Comparing the rate constants found for many different solvents, we find that surprisingly the electron transfer reaction is limited by the solvent dynamic effect and not strongly impacted by the peculiarities of the bridging moiety, a finding which was not anticipated for this type of long-range, thermally activated intramolecular charge transfer from previous studies. Rate constants are measured by dynamic electron paramagnetic resonance spectroscopy. Our insight was possible using various solvents spanning a wide range of longitudinal relaxation times (0.24 ps ≤ τL ≤ 516 ps) and Pekar factors (0.298 ≤ γ ≤ 0.526).

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INTRODUCTION Solvent effects play an important role, especially in charge transfer reactions, which are typically accompanied by substantial changes of electron density distribution between the initial and final states. The surrounding solvent molecules respond to these changes and can influence both the dynamics and the energetics of the electron transfer reaction to a substantial degree.1−3 The solvent dependency of the energy of activation has emerged from Marcus’ classic works; the dynamic aspect results from application of Kramer’s theory to electron transfer processes and is closely related to the dynamics of reorganization of the solvent polarization. Electron transfer (ET) reactions within linked donor−acceptor compounds in solution have been studied extensively to clarify the role of the solvent reorganization energy in photoinduced electron transfer (PET) reactions,4−10 but to a far lesser extent in thermal intramolecular electron exchange processes. Since the pioneering work of Harriman and Maki11 on several bis(pnitrophenyl) radical anions, it is known that the asymmetrical charge distribution undergoing thermal intramolecular electron transfer (IET) reactions is observable by ESR line broadening effects. More recently, the degenerate electron exchange rate constants in the radical anions of several 1,3-dinitrobenzenes,12−14 benzene-1,3-dicarbaldehyde,12,13 and 2,7-dinitronaphthalenes15 have been measured by dynamic ESR spectros© 2015 American Chemical Society

copy. In a series of impressive papers, Nelsen et al. have investigated the intramolecular electron transfer within numerous σ-bond linked tetraalkylhydrazines and related compounds. 16−24 Since the pioneering work of R. A. Marcus,1−3 it is well-established that electron transfer reactions in solution are energetically and dynamically influenced by the solvents. Only in a few works on short-range, thermally induced ET processes, solvent dynamic effects were considered.25,26 Using the Marcus−Hush two-state model, the inclusion of solvent dynamics in the pre-exponential factor was found to be necessary for more viscous solvents such as benzonitrile or hexamethylphosphoramide (HMPA). In an ESR and ENDOR spectroscopic study, we have recently reported the intramolecular ET rate constants of several bis(triarylamine)paracyclophane radical cations bearing different spacers of different lengths.27 In this paper, we report the solvent dependence of the intramolecular ET rates of these and new, but related organic mixed valence (MV) compounds comprising two equivalent redox moieties of the triphenylamine type linked by variable bridges. Structures of the compounds investigated are shown in Received: February 11, 2015 Revised: March 24, 2015 Published: March 31, 2015 8547

DOI: 10.1021/acs.jpcc.5b01386 J. Phys. Chem. C 2015, 119, 8547−8553

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The Journal of Physical Chemistry C Chart 1

ments, and the evaluation of the rate constants from linebroadening effects are all explained in full in ref 27. Here, the following solvents were used, typically after drying over molecular sieve and distillation (under vacuum where necessary): methylene chloride (MC, Roth, Rotipuran ≥99.8%), o-dichlorobenzene (DCB, Aldrich, Chromasolv ≥99%), acetonitrile (AN, Roth, Rotidry ≥99.9%), butyronitrile (BN, Fluka, ≥99%), nitrobenzene (PhNO2, Riedel-de Haën, ≥99.5%), benzonitrile (PhCN, Fluka, ≥99%), dibutylphthalate (DBP, Aldrich, ≥99%; used as received), dimethylphthalate (DMP, Fluka, ≥98%; used as received), and acetone (Aldrich, ≥99.9%). The radical cations were generated by oxidation with tris(bromophenyl)aminium hexachloroantimonate (Aldrich, technical grade).

Chart 1. In order to avoid confusion and be consistent with the component labeling of our previous study,27 a nonconsecutive numbering scheme has been used here. These compounds may serve as models for low dimensional organic conductors, since they facilitate fast thermal intramolecular electron transfer between the two electrophores through the spacers. The redox active groups are triarylamines which are known for their high radical cation stability and their low inner sphere reorganization energy, factors which promote fast ET processes. These redox centers are either directly attached to cyclophane bridges (I and X) or via ethynylene groups (II, IV, and XI). As bridging groups we use [2,2]cyclophanes and [3,3]cyclophanes. While the π−π distances in both cyclophanes (ca. 3.1 Å for the [2,2]cyclophanes and 3.3 Å for the [3,3]cyclophanes28,29) are smaller than the sum of the van der Waals radii of carbon atoms (3.4 Å), they differ in the type of methylene linkers, ethylene vs propylene, respectively. In the latter case, through-bond interactions can be excluded, because, unlike for the ethylene linker, the CH2−CH2 σ-bond orbitals are not parallel to the benzene π-orbitals in the [3,3]cyclophane. Further variations of the ET transfer pathways are provided by the pseudopara attachment of the redox centers in II vs pseudometa attachment in XI. For all these organic MV radical cations we measured the degenerate intramolecular electron transfer rate constants in various solvents by dynamic ESR spectroscopy to elucidate solvent dynamic effects. No detailed study on the solvent dynamic effects has so far been published for this type of longrange ET reaction. The use of MV compounds for such studies has the clear advantage that thermodynamic considerations can be left aside as we deal with degenerate ET processes, whereas in PET processes ambiguities of the Gibbs energy of the ET event and its solvent dependence might interfere with elucidating the solvent dynamics.

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ELECTRON TRANSFER THEORY Fluctuations of the solvent polarization together with molecular vibrations determine the activation energy and the shape of the free energy surface of the ET precursor and successor. Solvent dynamics strongly influence the kinetics when the rate of barrier crossing becomes comparable to the Debye relaxation time of the solvent, τD. If the charge remains constant during the reaction, τD is replaced by the longitudinal relaxation time τL = (ε∞/εs)τD, where εs denotes the static dielectric constant and ε∞ is the high frequency dielectric constant (ω → ∞) of the solvent; the last quantity is normally approximated by ε∞ ≈ n2 using the reactive index n of the solvent.30 If the solvent dynamic effect is first order, all established theories predict the appearance of the solvent relaxation in the pre-exponential factor of the rate equations.31−37 For barrierless reactions it can also appear in the exponentials of the multiexponential time decay of the reactants.38−40 The solvent dynamics proceed on a time scale comparable to that of the electron transfer on a diffusional random trajectory along a one-dimensional reaction coordinate. Energy is exchanged between the system and the surrounding solvent bath. Weak and strong coupling between reactant and product states is expressed by the electron exchange matrix element V. The Golden Rule description dominates for weak coupling V < RT. For electron self-exchange reactions with ΔG⊖ = 0, the crossover from the V dependence to the solvent dynamic

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EXPERIMENTAL SECTION ESR measurements were performed on a Bruker Elexsys E-500 X-band ESR spectrometer equipped with a digital Bruker temperature control unit. Temperature was kept constant at T = 295 ± 0.5 K. Experimental details concerning the sample preparation and handling, the ESR spectroscopic measure8548

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The Journal of Physical Chemistry C controlled regime is associated with an order-of-magnitude change of the Zusman parameter ξ, eq 1.41

ξ=

4πV 2τL hNLλo

(1)

A rough estimation of the solvent reorganization energy λo, calculated by eq 3, gives values between 45.1 and 95.7 kJ mol−1 for the solvents used in this study. V values obtained from optical analysis of the corresponding intervalence charge transfer bands are in the range 4.1−11.8 kJ mol−1.27 Thus, ξ ≫ 1 and hence a dynamic solvent effect on the electron transfer kinetics is predicted. Detailed reviews of the solvent dependence of electron transfer reactions are published.42−48 Using a simplified version of the rate expression taking into account that the driving force of the degenerate electron exchange reactions of interest is zero (ΔG⊖ = 0), we start from the adiabatic rate expression assuming cusped barrier tops as described in detail by Zusman:41 kex =

1 τL

⎛ ΔG* ⎞ λo ⎟ exp⎜ − ⎝ RT ⎠ 16πRT

(2)

where ΔG* denotes the Gibbs energy of activation and is given by Marcus’ expression: ΔG* = (1/4)(λi + λo). The inner sphere reorganization energy λi describes the changes of bond lengths and angles upon electron transfer, whereas λo, the outer sphere reorganization energy, considers the influence of the solvent. This influence is given by the Pekar factor γ = (1/n2) − (1/εs) and the longitudinal relaxation time τ L . The solvent reorganization energy λo takes into account the shape of the molecule and the electron transfer distance, d, expressed by a geometric factor g(r,d): λo =

e0 2NL g (r , d )γ 4πε0

(3)

g(r,d) varies with the model used for the calculation of λo.49 For intramolecular electron transfer reactions, cavity models are preferred.50,51 For solvent dependent measurements at constant temperature, rearrangement of eqs 2 and 3 leads to the following dependence of the rate constants on the Pekar factor: ln(kexτLγ −1/2) ∼ γ

Figure 1. Experimental (black) and simulated (red) X-band ESR spectra of radical cations I+ to XI+ in DCB at 295 K.

The exchange rate constants were extracted by means of computer simulations employing an approach generalizing Heinzer’s52 original program by allowing the line width to vary with the hyperfine component. The hyperfine coupling constants of radical cation IX+ were previously obtained by ESR and ENDOR spectroscopies (see ref 27). For the purpose of extracting the rate constants of degenerate electron transfer, the dominant proton hyperfine interactions were taken into account, besides those of the nitrogen atoms: Inclusion of the hyperfine interactions of the two ortho-protons of each of the anisyl substituents (4aH,1 = 0.188 mT) and the two orthoprotons of the phenylene substituent of the amines (2aH,2 = 0.159 mT) yielded a reasonable agreement between experimental and simulated ESR spectra, without making unjustified assumptions of the unresolved proton hyperfine structure. The values of the hyperfine coupling constants were assumed constant for the entire set of studied MV components, because they are present in all compounds with the same local environment. No hyperfine interactions to the bridge or the reduced triphenylamine moiety have been considered. The experimental spectra were fitted in the least-squares sense by

(4)

Therefore, plots of ln(kexτLγ−1/2) versus γ should result in linear relations, whereas for an electron transfer without the solvent relaxation time in the pre-exponential factor, ln(ket) vs γ should result in linear dependence.

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RESULTS AND DISCUSSION We have studied the ESR line shape in numerous solvents with differing Pekar factors and longitudinal relaxation times τL to get detailed information on the solvent dependence and the solvent dynamics of the intramolecular, thermal ET reactions of the compounds listed in Chart 1. Note that the temperature dependence of the EPR spectra of compounds I+ and II+ in DCB has been discussed in detail in ref 27. Some additional aspects of the effect of temperature on the spectral shape have been summarized in the Supporting Information. Here, we focus on the solvent dynamic effect at 295 K. For the radical cations I+ to XI+ in o-dichlorobenzene at T = 295 K, typical ESR spectra are shown in Figure 1. The red lines illustrate the simulations of the spectra. 8549

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Figure 2. Solvent dependence of ESR determined rate constants vs Pekar factor for I+ to XI+ at 295 K. Plots show the correlation of ln(kexτLγ−1/2) with γ; insets show the correlation of ln kex with γ.

Table 1. Rate Constants Obtained for I+ to XI+ in Various Solvents Together with the Solvent Relaxation Times and the Corresponding Pekar Factors (T = 295 K) kex/107 s−1 solvent AN acetone BN MC PhNO2 PhCN DCB DBP DMP

τL/ps 53,54

0.24 0.3053 0.4856 1.8055 3.1055 5.8054 6.055,56 51258 51659

γ

I 54,55

0.526 0.49554 0.48355 0.38355 0.38855 0.38954 0.31655 0.29857 0.31755

+

3.6 − 2.4 13 5.4 3.9 14 0.89 0.15

II

+

1.9 2.0 6.3 3.4 2.9 10.3 − −

X+

IV+

XI+

3.0 2.3 2.3 8.9 4.4 3.7 11.9 − −

− − 1.5 7.7 3.4

1.9 − 1.8 8.3 3.0 2.7 10.5 − −

11.0 − −

general, neither the substituent topology (II vs XI) nor the type of the paracyclophane (I vs X and II vs IV) has a strong impact. The effect of the solvent environment always exceeds these structural factors impacting the electronic coupling matrix element. This is not unexpected for the domain of solventcontrolled ET, for which the pre-exponential factor is independent of V, provided that V does not strongly alter the energy of activation. Comparing the ET rate constants presented in Table 1, this appears to indeed be the case for most components. The ET distance, which modulates the outer sphere reorganization energy, gives rise to fast ET rates of I+ and X+ compared to the other components. Furthermore, the ET rates of I+ are slightly larger than that of X+ due to a larger

adjusting the rate constant of degenerate electron exchange and the three line widths associated with the nitrogen hyperfine components of the formally nonexchanging triarylamine moiety. All other parameters including the g-factor and the proton hyperfine coupling constants were fixed. For all investigated systems, plots of ln(kexτLγ−1/2) versus γ are shown in Figure 2. The experimental rate constants obtained together with the solvent longitudinal relaxation times and the Pekar factors are listed in Table 1. Comparing the ET rate constants of the compounds summarized in Chart 1, the following becomes obvious: While compounds I+ and X+ with their short bridges give rise to the largest ET rate constants, kex is surprisingly independent of the identity of the bridge. In 8550

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small insets in Figure 2). This is particularly pronounced for the MV compound I+, for which the most data points are available and where the “normal” correlation suggested by the classical Marcus theory, ln(kex) vs γ, shows strong scatter, whereas a convincing linear dependence results when considering the solvent relaxation. This finding provides strong evidence of the solvent dynamic effect controlling the ET. This holds for all compounds studied irrespective of peculiarities of the bridge and the distance of the two electrophores.

coupling element lowering the ET barrier. For all other components differences in the ET rates are too small to be significant. The solvent dependence of the ESR spectra of radical cation I+ is shown in Figure 3 for butyronitrile, benzonitrile, nitrobenzene, methylene chloride, and o-dichlorobenzene at 295 K.

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CONCLUSION We have employed electron spin resonance spectroscopy to unravel the kinetics of intramolecular electron transfer reactions of purely organic, mixed valence compounds based on bis(triarylamine)paracyclophane radical cations bridged by paracyclophane rings. We have discussed the ET rate constants using classical Marcus theory and the more advanced Zusman model and have found that only the latter can account for their solvent dependence. Our results show that the kinetics of degenerate intramolecular electron transfer reactions within the studied bis(triarylamine)paracyclophane radical cations are dominated by solvent dynamic effects expressed by the dependence on the longitudinal relaxation times. This is a consequence of the large Zusman factor, which in turn is caused by the relatively large electronic coupling V. Furthermore, we could demonstrate that neither the substitution position at the cyclophane (pseudopara vs pseudometa in II+ vs XI+) nor the cyclophane linkers (ethylene vs propylene in I+ vs X+ and II+ vs IX+) have a major impact on the ET rate. Only the distance between the redox centers (I+ vs II+ and X+ vs IV+) shows, as expected, an effect on the ET rates, which however falls short of the solvent dependence. This finding is surprising insofar as long-range ET processes of the type studied here are usually attributed to the perturbation theoretical ET domain, for which the solvent dependence is fully accounted for by that of the outer sphere ET reorganization energy. Our results also demonstrate that thermal ET rate constants may be unsuitable to assess the efficiency of bridging moieties or connection topologies, because of the solvent dynamic effect concealing details of the electronic coupling. Note that solvent dynamic effects of the kind discussed here are expected to also manifest in chemical systems with nonzero driving force of electron transfer such as photoinduced charge separation reactions in asymmetric donor−acceptor dyads. In fact, a few examples of solvent friction in these systems have been published. In analogy to the systems studied here, weak dependencies on V combined with sensitivity of the intramolecular ET to the solvent dynamics in the surroundings of the molecular system have been observed, in particular in viscous solvents.60−63 On the other hand, despite theoretical predictions, no solvent relaxation effect could be observed in ET reactions of porphyrin−quinone cyclophanes occurring close to the activationless regime.64

Figure 3. Experimental (black) and simulated (red) ESR spectra of radical cation I+ in (a) BN, (b) PhCN, (c) PhNO2, (d) MC, and (e) DCB at 295 K.

The solvent dynamic effects on the intramolecular electron transfer kinetics within the radical cations I+ to XI+ is depicted in Figure 2. Nine different solvents were investigated, for which the Pekar factor extends over the large range 0.298 ≤ γ ≤ 0.526, covering the polar and the nonpolar domains. The longitudinal solvent relaxation times also span a large range: 0.24 ps ≤ τL ≤ 516 ps. Only plots containing the longitudinal relaxation time τL in the pre-exponential factor as indicated by eq 2 result in a linear dependence, whereas plots of ln(kex) vs γ do not (see

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ASSOCIATED CONTENT

S Supporting Information *

Additional remarks on the temperature and solvent dependence of the EPR spectra; additional experimental details. This material is available free of charge via the Internet at http:// pubs.acs.org. 8551

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This common project is financed by a bilateral agreement between the German Science Foundation (DFG) and the Austrian Science Fund (FWF) under the Project No. DACHDFG-FWF, I 733-N19.

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DOI: 10.1021/acs.jpcc.5b01386 J. Phys. Chem. C 2015, 119, 8547−8553

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DOI: 10.1021/acs.jpcc.5b01386 J. Phys. Chem. C 2015, 119, 8547−8553