Different Mechanisms for Hole and Electron Transfer along Identical

May 14, 2014 - electron transfer are similar, we observed striking differences in the distance dependence and absolute magnitude of the rates of these...
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Different Mechanisms for Hole and Electron Transfer along Identical Molecular Bridges: The Importance of the Initial State Delocalization Natalie Gorczak, Simge Tarkuç, Nicolas Renaud, Arjan J. Houtepen, Rienk Eelkema, Laurens D. A. Siebbeles, and Ferdinand C. Grozema* Department of Chemical Engineering, Delft University of Technology, Julianalaan 136, Delft 2629 BL, The Netherlands S Supporting Information *

ABSTRACT: We report measurements of hole and electron transfer along identical oligo-pphenylene molecular bridges of increasing length. Although the injection barriers for hole and electron transfer are similar, we observed striking differences in the distance dependence and absolute magnitude of the rates of these two processes. Electron transfer is characterized by an almost distance-independent, fast charge-transfer rate. Hole transfer presents a much slower rate that decreases significantly with the length of the bridge. Time-dependent density functional calculations show that the observed differences can be explained by the delocalization of the respective initial excitation. The evaluation of the initial state is therefore essential when comparing charge-transfer rates between different donor−bridge−acceptor systems.

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Several fields would greatly benefit from a direct comparison of photoinduced electron and hole transfer through the same molecular bridge. One example is charge transfer in DNA. Motivated by the relevance of hole transfer for the DNA damage8,9 and electron transfer for the DNA repair10 mechanism, both electron and hole transfer have been independently measured along various DNA hairpins.11−14 While striking differences between the two types of charge carriers were observed, the different experimental conditions during the measurements make a direct comparison between hole and electron transfer difficult. Another example where a direct comparison would be beneficial is the investigation of effects of cross-conjugation on rates of charge transfer. Ricks et al.15 have recently reported electron-transfer rate constants that are one order of magnitude lower through a cross-conjugated than through a linearly conjugated bridge. This large difference was attributed to quantum interference effects that occur in cross-conjugated bridges,16,17 although other potentially differing factors between the linearly and cross-conjugated bridges, for example, bridge energetics, molecular conformation, or initial state delocalization, were not taken into consideration. Because quantum interference effects should be identical for the two types of charge carriers, measurements of both hole and electron transfer rates through the same bridges could clarify the role of quantum interference in these systems. The distance dependence of electron or hole transfer along a variety of molecular bridges has been previously measured experimentally using transient absorption spectroscopy. These studies have revealed, in some cases, a crossover from a strong to a weak distance dependence upon lengthening the bridge.

INTRODUCTION Understanding charge transfer through molecular bridges is of fundamental interest: not only because this mechanism is at the center of perspective technologies, such as molecular electronics1,2 or artificial solar energy conversion,3,4 but also due to the importance of charge-transfer processes in biochemical reactions.5,6 Donor−bridge−acceptor (DBA) molecules offer an interesting platform to systematically study photoinduced electron and hole transfer through molecular bridges and to understand the key parameters that govern these processes. Transient absorption spectroscopy is widely applied in such studies, where either the electron-donor is excited, initiating electron transfer, or the electron acceptor is excited, initiating hole transfer. Although charge separation involving both hole and electron transfer has been investigated independently in separate DBA systems, only a few measurements of these two distinct processes through the same molecular bridge and under the same experimental conditions have been reported. Following the seminal work of Johnson et al.7 on charge shift along saturated molecular bridges, it is generally assumed that both electron and hole transfer through the same bridge occur via identical mechanisms and thus exhibit similar distance dependence of their respective transfer rates. We report measurements of photoinduced hole and electron transfer along a series of well-defined conjugated molecular bridges. A key difference with the previous studies is that we deal with a charge-separation reaction rather than a charge shift reaction in the work of Johnson et al.7 This means that the energetics of the transfer reaction are strongly determined by Coulomb interactions between electron and hole in this case. These measurements allow a direct comparison of charge and electron transfer in a charge-separation reaction and yield a conclusion that is strikingly different from the one obtained for charge shift.7 © XXXX American Chemical Society

Received: January 24, 2014 Revised: May 13, 2014

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This abrupt change in distance dependence is typically interpreted in terms of two distinct charge transport regimes: (1) the superexchange mechanism, where the charge tunnels from the donor to the acceptor through the energy barrier constituted by the bridge molecule and (2) a thermally activated hopping mechanism where the charge hops between neighboring units of the bridge to reach the acceptor.18−21 The crossover from superexchange to hopping can be attributed in many cases to a lowering of the energy barrier with increasing number of bridge units caused by the extended conjugation of the bridge.18,22 A particular focus has therefore been put on the impact of this energy barrier on the overall charge-transfer rates.23−27 When dominated by the superexchange mechanism, the charge-transfer rate constant kCT shows a strong exponential decay with the distance between the donor and acceptor, d:28 k CT ∝ e−βd

Figure 1. Chemical structures of the DBA systems 1−3 where the compound name stands for the number of phenyl units n and the donor and acceptor reference compounds SNSref and PDIref.

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The decay is characterized by the falloff parameter β with a value >0.2 Å−1;29 while lower values might hint at a hopping mechanism. Because β is largely determined by the energy difference between the donor/acceptor and bridge units, a larger energy difference results in a stronger distance dependence of kCT. The energy difference is generally approximated by the Gibbs free-energy barrier for electron or hole injection ΔGe/h that is calculated from the oxidation and reduction potentials of the isolated donor, bridge, and acceptor units.22,23,25 It is important here to distinguish the situation of actual injection of a charge into the bridge and the situation where the bridge acts as a tunneling barrier. These two situations are characterized by a different (dielectric) response of the environment, as is discussed in more detail later. The approach of using experimental oxidation and reduction potentials for the individual nonconnected subsystems neglects possible shifts of energy levels due to the covalent bond between neighboring units and assumes that the initial excitation is fully localized on the donor or acceptor. These approximations are rather crude and may not be valid in all cases. Nevertheless, a correlation between ΔGe/h and β is usually experimentally observed. For example, Hanss and Wenger30 showed that using a different donor moiety with the exact same molecular bridge can significantly reduce ΔGe/h and consequently yields a weaker distance dependence of the charge-transfer rate. A systematic study of such energy barrier alteration was particularly successful using DNA hairpins as molecular bridges for hole transfer.11 We present measurements of both hole and electron transfer along the same molecular bridges depicted in Figure 1 and under the same experimental conditions. This study offers the first direct comparison between photoinduced electron- and hole-transfer mechanisms and provides important information about their respective rates. These DBA molecules are composed of a thiophene derivative (SNS) as electron donor and a perylene derivative (PDI) as acceptor connected by poligophenylene bridges (p-Phn) where n = 1−3. The chargetransfer rate constants were determined upon selective pulsed laser excitation of the donor or the acceptor by femtosecond transient absorption spectroscopy using global and target analysis. We observe a striking difference between hole and electron transfer in these systems, in terms of both absolute rate constant and distance dependence. The distance dependence of hole transfer falls in the superexchange regime, while the electron-transfer rate constants are one to two orders of

magnitude faster and almost independent of distance. An analysis of the Gibbs injection barriers ΔGe/h indicates that the weak distance dependence of electron transfer cannot be attributed to a hopping mechanism. Instead, we identify a substantial delocalization of the initial excited state onto the bridge as the origin for the small β by performing timedependent density functional theory (DFT) electronic structure calculations. Moreover, because of considerable shifts of the bridge energy levels when connecting the bridge to the donor and acceptor, we question the validity of the common approach to estimate ΔGe/h using energetic parameters for the nonconnected subunits.

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RESULTS AND DISCUSSION Absorbance Spectra. The absorbance spectra of 1−3 and of the donor and acceptor reference compounds SNSref and PDIref (see Figure 1) are shown in Figure 2. The reference

Figure 2. Ground-state absorbance spectra of 1−3 and of the donor and acceptor reference compounds PDIref and SNSref.

compound PDIref is substituted with two branched alkyl chains for solubility reasons. It is known that the electronic properties are not significantly affected by substitution on the imide nitrogen.31 The spectra of 1−3 overlap with the spectrum of PDIref in the range of 450−550 nm, indicating that the electronic coupling between PDI and Phn or SNS is very small. The similarity of the spectra around 350 nm between 1−3 and SNSref also suggests that there is no significant coupling between SNS and Phn or PDI. However, 1 shows a stronger absorption compared with 2 and 3, which is likely due to interaction between SNS and Ph or PDI for the shortest bridge. The phenylene bridges absorb below 300 nm B

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Figure 3. (a) Difference absorbance spectra of 3 after excitation of PDI at 0, 600, and 3000 ps. (b) Target analysis reveals the difference absorbance spectra corresponding to hot PDI*, PDI*, and PDI−•. The spectrum of PDIref is displayed for comparison. (c) Population profiles visualize the formation of PDI−• that cannot be deducted directly from the kinetic trace at 710 nm due to the overlapping photoinduced absorption bands of PDI−• and PDI*.

Figure 4. Difference absorbance spectra of 3 following excitation at 350 nm at 0, 10, and 3000 ps (a), the fitted SAS (b), and population profiles (c). The kinetic trace at 710 nm is also shown in panel c.

with a red shift in absorption with increasing n resulting from the linear conjugation of the phenyl units. The absorption appears at the blue edge of the spectra for 2 and 3. Figure 2 demonstrates that the absorption of SNS, Phn, and PDI is sufficiently separated to allow for selective excitation of acceptor and donor. This enables the study of both hole and electron transfer along the same bridge. The excitation of PDI at 527 nm generates a hole in the HOMO of PDI that can subsequently transfer to the HOMO of SNS. Excitation at 350 nm leads to excitation of an electron from the SNS HOMO to

its LUMO. Subsequently, this electron can transfer to the LUMO of PDI, resulting in the same charge-separated state but via a different route. It is important to note that light of 350 nm is also partially absorbed by PDI. Therefore, parallel to the process of electron transfer, hole transfer is also observed after excitation at 350 nm. Hole- and Electron-Transfer Rates. Hole-transfer rate constants were determined from target analysis of transient absorption data following excitation of PDI at 527 nm, as described in the Experimental and Theoretical Methodology C

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section. Figure 3a shows the evolution of the difference absorbance (ΔOD) spectra observed during hole transfer in 3. Immediately after excitation of PDI (Figure 3a), the spectrum is identical to the ΔOD spectrum of PDIref (see Figure 3b) with a typical bleach at 490 and 530 nm, stimulated emission around 580 nm, and two broad absorption bands at 690 and 850 nm. After 600 ps, the spectrum exhibits additional sharp absorption features at 710 and 800 nm that are typical for PDI−•.32 After 3 ns, the spectrum corresponds entirely to that of PDI−•. Because of the overlapping photoinduced absorption of PDI* and PDI−•, charge-transfer rates could not be determined from the transient absorption at a single wavelength but had to be determined by target analysis. (See Figure 3c.) The data sets of 1−3 were analyzed by fitting to a sequential kinetic model in which the initial PDI* (hot PDI*) relaxes within the first picoseconds due to solvent reorganization or interactions between the charge and molecular vibration modes. PDI* subsequently undergoes hole transfer leading to the formation of SNS+•−Phn−PDI−•. This fast initial relaxation effect has been show previously for PDI even for nearly nondipolar solvent such as toluene.33 The corresponding species-associated spectra (SAS) are shown in Figure 3b with their respective population profiles in Figure 3c for 3. No signatures of SNS+• were observed because these are out of the spectral range of the probe light. We exclude the formation of Ph+n because no photoinduced hole transfer in Phn-PDI was observed according to previous work by Weiss et al.22 Electron-transfer rates were obtained by the change in absorbance after excitation of SNS at 350 nm. Figure 4a shows the transient absorption spectra of 3. Immediately after excitation, the spectrum contains features of two species: a broad absorption band from 490 to 580 nm resembling the SNS singlet (compare to SNSref in Figure 4b) and the ΔOD spectrum of PDI*. The presence of PDI* is due to absorption by PDI that accounts for ∼30% of the total absorption at 350 nm. Therefore, both initial states and the subsequent electronand hole-transfer processes were included in the target analysis of the transient absorption data for excitation at 350 nm. Hot PDI* is omitted in the hole-transfer scheme for simplicity; this does not lead to significantly worse global fits. In the parallel electron-transfer process, SNS* undergoes electron transfer, leading to the formation of SNS+•−Phn−PDI−•. As a consequence of the parallel electron- and hole-transfer processes, the population of PDI−• rises in two stages: initially by electron transfer from SNS** and subsequently by hole transfer from PDI**. This can be seen in the kinetic trace at 710 nm (Figure 4c) that exhibits a fast increase during the first 10 ps as a result of electron transfer. Thereafter, it continues to rise as a result of hole transfer on the nanosecond time scale similar to that following PDI excitation. The SAS of PDI* and PDI−• resulting from data analysis for excitation at 350 nm (Figure 4b) agree with the SAS resulting from PDI excitation. Although the SAS of SNS* could not be distentangled completely from the contribution of PDI*, they resemble the ΔOD spectrum of SNSref. The strength of the applied analysis is shown by the fact that the rate constants of hole transfer determined from excitation at 527 and 350 nm are similar for each sample (Table 1). The hole- and electron-transfer rate constants are listed in Table 1 and plotted on a logarithmic scale versus the throughspace donor−acceptor distance in Figure 5. There are two clear distinctions between electron and hole transfer. The electrontransfer rates are at least one order of magnitude larger than

Table 1. Electron- (kET) and Hole-Transfer (kHT) Rate Constants Determined by Target Analysis of Transient Absorption Following Excitation at 350 or 527 nm ex. at 350 nm −1

1 2 3

ex. at 527 nm −1

kET (ps )

kHT (ps )

kHT (ps−1)

0.35 0.25 0.17

0.028 0.0025 0.0010

0.042 0.0024 0.0010

Figure 5. Logarithmic plots of the electron- (blue squares) and holetransfer (black squares) rate constants versus the donor−acceptor distance for 1−3. The error bars fall within the size of the symbols. The solid lines correspond to an exponential distance dependence with decay parameter β (eq 1).

hole-transfer rates, which could be due to a larger driving force for electron than for hole transfer. Even more interesting is the strikingly differing distance dependence of the rates. The strong distance dependence with β = 0.42 Å−1 of hole transfer is characteristic for a superexchange mechanism through phenylene bridges.34 The much weaker distance dependence of electron transfer is surprising for two reasons. First, the small β of 0.09 Å−1 is not consistent with a superexchange mechanism29 and may indicate that electron transfer occurs via of a hopping mechanism. However, a hopping mechanism is usually not observed for such short phenylene bridges.22,24,35 Second, the large difference between electron and hole transfer suggests different transfer mechanisms. A possible origin for this distinct behavior could be that the energy of the electron originating from the SNS is substantially closer to the LUMO of Phn than the energy of the hole in PDI to the HOMO of Phn. Therefore, we examine in the following section the height of the injection barriers for hole and electron transfer. Energetics. As discussed in the Introduction, the distance dependence of charge transfer is largely determined by the injection barrier. Therefore, we consider whether the large difference in distance dependence for hole and electron transfer can be explained by a significantly smaller injection barrier for electron compared with hole transfer. Electron injection barriers were estimated for the three compounds, as the Gibbs energy of photoinduced charge transfer for the radical ion pairs SNS+•−Ph−• n with respect to the excitation energy Eex, according to the Rehm−Weller equation36 ΔG = Eox − Ered − + D

⎛ 1 ⎛1 e2 1 ⎞ + e 2⎜⎜ ⎟ ⎜ − εox ⎠ dε ⎝ 2rox ⎝ ε

1 ⎛1 1 ⎞⎞ ⎜ − ⎟⎟⎟ − Eex εred ⎠⎠ 2rred ⎝ ε

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with the elementary charge e. Barriers for hole injection were calculated using eq 2 for the radical ion pairs PDI−•−Ph+• n . The oxidation and reduction potentials of the electron donating and accepting unit Eox and Ered were obtained from cyclic voltammetry and are listed in Table 2 along with the radii of

However, as mentioned in the Introduction, the distance dependence of kCT is dominated by the injection barrier. Using eq 2, we obtained hole injection barriers of 1.7, 0.5, and 0.2 eV for 1−3. Surprisingly, the injection barriers for electron transfer are on the same order of magnitude, namely, 1.8, 0.6, and 0.3 eV. All of these values correspond to the situation where a charge is actually injected into the bridge and were calculated using the full (static) dielectric constant of the solvent (toluene). Because toluene is an almost nondipolar solvent, the difference between the static and optical dielectric constant is very small (2.38 vs 2.24). When using the optical dielectric constant, corresponding to the bridge acting as a virtual intermediate, the injection barrier for hole injection in 1 changes from 1.7 to 1.8 eV. The most important conclusion from these calculations is the fact that injection barriers for electron and hole transfer are very similar, suggesting that both transfer processes are likely to follow the same mechanism. This, however, contradicts the experimental observation of β = 0.09 Å−1 for electron transfer, suggesting a hopping mechanism, and β = 0.42 Å−1 for hole transfer, suggesting a superexchange mechanism. In particular, the similar injection barriers imply similar decay parameters β for both processes. The differences found experimentally between electron and hole transfer thus cannot be explained by the injection barriers calculated using eq 2. Charge Distribution of the Initial State. To understand the difference between hole and electron transfer measured for compounds 1−3, we have computed the excitation spectra of 1−3 using time-dependent DFT (M06-2X/DZP) in the gas phase. According to the TD-DFT calculations, the first two excited states with significant oscillator strength (0.9 and 0.5) are located at 2.73 and 4.16 eV for 1−3. These values are systematically larger than the experimentally observed ones (2.35 and 3.54 eV). We attribute this discrepancy to stabilization by the solvent that is not taken into account in the calculations. The lower energy excited state corresponds to the HOMO−LUMO transition (weight of >0.95) on the PDI fragment. The calculated energy of the lowest excited state of 1−3 matches the one of an isolated PDI molecule (2.74 eV). This agreement is also in line with the overlapping experimental absorbance spectra of 1−3 and PDIref in Figure 2. The higher energy excited-state corresponds to SNS excitation. In contrast with PDI excitation, this excited state in 1−3 is 0.09 eV lower than the excitation of the isolated SNS molecule. Moreover, the SNS excitation in 1−3 consists of mainly two transitions (both with weight of ∼0.5): the HOMO−LUMO transition on the SNS fragment and a transition from the HOMO on the SNS fragment to the LUMO on the Phn fragment. The subtle red shift of ∼6 nm in the calculated energy of the SNS excitation in

Table 2. Oxidation and Reduction Potentials and Radical Ions Radii of SNS, PDI, and Phn Eox (V vs SCE) SNS PDI Ph Ph2 Ph3

0.96

Ered (V vs SCE)

rox/red (Å)a

−0.70c −3.35d,e −2.68d,e −2.40d,e

3.8 7.6 1.6 3.8 5.8

b

2.40c,d 1.85c,d 1.60c,d

a

Taken from ref 37 where the radius of SNS is approximated by the radius of phenothiazine. bACN as solvent. cDCM as solvent. dTaken from ref 38. eDMA as solvent.

the radical ions rox and rred that were taken from literature. It should be noted that in the Rehm−Weller equation the assumption is made that the donor and acceptor are spherical, and this is usually not the case. Therefore, the values obtained in this way should be considered as a rough estimate, not a precise calculation. The center-to-center distance between the radical ions dD−B and dA−B was determined from the DFT structure calculations (Table 3). ε is the static dielectric constant of the solvent that Table 3. Distance between the Radical Ions of Donor and Bridge, Acceptor and Bridge, and Donor and Acceptor 1 2 3

dD−B (Å)

dA−B (Å)

dD−A (Å)

4.0 6.1 8.3

8.5 10.7 12.8

12.5 16.8 21.1

was used in the transient absorption spectroscopy measurements (toluene with ε = 2.38). εox and εred denote the solvent used in the cyclic voltammetry measurements of the respective electron donating and accepting unit (DCM with ε = 8.9, ACN with ε = 37.5, and DMA with ε = 37.8). The energies of the excited states Eex were obtained from the maxima of the absorbance spectra: Eex = 3.54 eV for donor excitation and Eex = 2.35 eV for acceptor excitation. The injection barrier and driving force for hole and electron transfer are represented in Figure 6a,b, respectively. As seen in these Figures, the driving force for electron transfer is 10 times larger than the one for hole transfer. This could explain why electron transfer is much faster than hole transfer in 1−3.

Figure 6. Gibbs energies for the states relevant to the (a) hole- and (b) electron-transfer pathways with respect to the excited states. E

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inconsistent with a tunneling process. We attribute this behavior to a modification of the electronic structure of the bridge induced by the covalent bonding to the donor and acceptor moieties. As a consequence, the bridge energy levels shift down in energy, whereby the LUMO is most strongly affected. The shift of the LUMO diminishes the electron injection barrier; the shift of the HOMO increases the hole injection barrier. Our results thus disclose the failure of the common approach of estimating the barrier from the redox potentials of the isolated units. Moreover, we have demonstrated the importance of the charge-transfer character of the initial excitation that is usually disregarded but crucially affects the charge-transfer rate. The careful examination of the initial excitation is particularly important when comparing transfer rates between various DBA systems. When relating observed differences in transfer rates to the electronic structure of the bridge, one should exclude that these do not stem from differences in the initial state delocalization.

1−3 with respect to the isolated SNS molecule cannot be clearly observed in the experimental absorbance spectra. The change in charge distribution upon excitation to the second allowed excited state is shown in Figure 7. The

Figure 7. Charge distribution over the SNS, the individual phenyl units, and the PDI of the excited state that corresponds to excitation of the SNS unit as compared with the ground state for 1−3.

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EXPERIMENTAL AND THEORETICAL METHODOLOGY Sample Preparation and Optical and Electrochemical Characterization. DBA series were synthesized with 2,5di(thiophen-2-yl)-1H-pyrrol-1-yl (SNS) as donor, the perylenediimide derivative (tridecan-7-yl)anthra[2,1,9-def:6,5,10d’e’f’]diisoquinoline-1,3,8,10(2H,9H)-tetraone (PDI) as acceptor, and p-oligophenylene bridges (p-Phn) where n = 1−3. The synthesis and characterization of 1−3, PDIref, and SNSref is described in the Supporting Information. All steady-state absorption spectroscopy measurements were performed in toluene of spectroscopic grade using a PerkinElmer Lambda 40 spectrophotometer. Cyclic voltammetry on SNSref was carried out in acetonitrile; DCM was used in the measurements on PDIref. Details of the electrochemistry can be found in the Supporting Information. Transient Absorption Spectroscopy. Pump−probe transient absorption measurements were performed using a tunable Yb:KGW laser system comprising a Yb:KGW laser (1028 nm) operating at 5 kHz with a pulse duration of 200 fs (PHAROS-SP-06-200, Light Conversion) and an optical parametric amplifier (ORPHEUS-PO15F5HNP1, Light Conversion). White-light continuum pulses, generated by focusing the fundamental on a sapphire crystal, were used as probe. A transient absorption spectrometer (HELIOS, Ultrafast Systems) was employed for data acquisition in a spectral window of 490−910 nm and a time window of 3.3 ns. The samples were placed in a 2 mm path length quartz cuvette using toluene of spectroscopic grade as solvent. To prevent aggregation of the compounds, we kept the concentration at ∼6 μM, corresponding to an absorbance at 527 nm of 0.1. The samples were excited at 350 (excitation of SNS) or 527 nm (excitation of PDI) with pulses of 0.2 μJ and a 200 μm spotsize in quasiparallel pump−probe geometry. During the experiments, the samples were stirred continuously with a magnetic stirrer. The 2-D data were analyzed with global and target analysis utilizing the open source software Glotaran42 a graphical user interface for the R package TIMP.43 TIMP is based on spectrotemporal parametrization, assuming that the timedependent spectra are linear combinations of difference absorption spectra of various species with their respective population profiles.44 A parametrized Gaussian instrument response function accounting for dispersion and the coherent artifact is taken into account. Target analysis was applied with a

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difference of multipole-derived atomic charges in the excited state with respect to the ground-state was summed for the donor, acceptor, and the individual phenyl units. A clear chargetransfer character with approximately uniform charge delocalization over SNS and Phn is observed for 1−3. The degree of bridge population decreases only slightly with increasing number of phenyl units. The charge-transfer state extends on the PDI for 1. However, the population on the PDI is negligible for 2 and 3. This can explain the discrepancy observed experimentally between the absorbance spectra of 1 and of 2 to 3 between 300 and 400 nm in Figure 2. The finding that the charge is initially evenly distributed over the SNS and the entire Phn provides an intuitive explanation for the observed weak distance dependence of electron transfer and the larger rates overall. The calculations show that the real through-space distance between the radical ion pairs needs to be redefined. More importantly, the electron-transfer process cannot be described by the superexchange mechanism as the electron transfer is not mediated by the bridge but originates from it to some extent. The substantial charge-transfer character for SNS excitation was not expected, especially not for 1, where the estimated electron injection barrier is almost 2 eV. However, as previously demonstrated by Senthilkumar et al.,40 fragment energies are strongly affected by their local environment. Our calculations show that in the present case this shift amounts to ∼1 eV. Therefore, the energy difference between the LUMO of Phn SNS decreases, which gives rise to the observed delocalization of the SNS excitation. Such a modification of site energies is not considered by common approaches that might consequently miss the initial state delocalization that has a large effect on charge transfer. The effect of initial delocalization on the charge-transfer dynamics has been previously discussed by Skourtis and Nitzan.41 The present results experimentally confirm this theoretical prediction.

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CONCLUSIONS We have shown that hole transfer through the widely studied poligophenylene bridges occurs via the superexchange mechanism of hole tunneling for n ≤ 3, which conforms with widespread literature.22,24,35 Electron transfer through the same bridges on the contrary exhibits barely any distance dependence, despite the equally high injection barriers. This is F

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compartmental kinetic scheme modeling the photophysical processes after 350 or 527 nm excitation. This analysis yields SAS, that is, difference absorbance spectra of the excited or radical ion states, with their population profiles. Molecular and Electronic Structure Calculations. All calculations were performed using the Amsterdam Density Functional software.45 The molecular structures were determined from ground-state geometry optimization of donorbridge and acceptor-bridge systems at the DFT level of theory using the meta-hybrid functional M06-2X by Zhao and Truhlar46 with the DZP basis. The alkyl chains attached to the PDI unit were replaced by hydrogen atoms in all calculations. The initial states corresponding to excitation at 350 and 527 nm were derived from excited-state calculations using timedependent DFT using the same functional and basis set as in the ground-state calculations. The M06-2X functional is used because it performs reasonably well for both local valence excitations and charge-transfer excitations.46,47 The charge distribution in the excited states was determined from multipole-derived charges at the quadrupole level.39

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ASSOCIATED CONTENT

S Supporting Information *

Details of the synthesis and characterization of 1−3 and cyclic voltammetry on SNSref and PDIref. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by The Netherlands Organization for Scientific Research (NWO) through a VIDI grant. The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. 240299.

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REFERENCES

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