Sub-nanosecond TA Measurements

The time-window of the femtosecond TA setup (0-1.5 ns) does not allow reliable determination of time constant >1 ns. In order to properly monitor charge separation (CS) and the ensuing charge recombination (CR), the measurements were performed with the sub-nanosecond TA setup upon 355 nm excitation. The TA spectra measured at longer time delays for the TAA-tmb-Ru²⁺ dyad in ACN are shown in Figure 9.5. Expectedly, the initial TA spectra are very similar to the later spectra measured with the femtosecond setup and are dominated by the CSS. The TA bands initially increase up to 3-4 ns and then decrease on the 10-30 ns time scale. A very weak residual TA spectrum can still be observed afterward for several hundreds of ns.

The temporal evolution of these spectra could be reproduced using global target analysis assuming a A → B → C → D reaction scheme with 1.5 ns, 10 ns and 560 ns time constants for TAA-tmb-Ru²⁺ in Figure 9.5C. The last step (C → D) had to be introduced to account for the small residual that could arise from some impurity. The time constant of 1.5 ns is attributed to intramolecular electron transfer from TAA to photoexcited Ru(bpy)32+

, and this value is in line with that extracted from the fs-ps studies (1.6 ns) reported above (Figure 9.3C). The time constant of 10 ns is attributed to intramolecular reverse electron transfer from the reduced ruthenium photosensitizer to oxidized TAA⁺ (process D → E); to maintain consistency between Figures 9.3C and Figure 9.5C, the respective species is labeled “D”

in both cases. This value is more accurate than that extracted from the fs-ps

9.3 Transient Absorption

studies in Figure 9.3, while in the studies with higher temporal resolution reported above it was merely possible to determine a lower limit of 5 ns for CR in the TAA-tmb-Ru²⁺ dyad. Finally, the time constant of 560 ns is attributed to an impurity and the respective SADS in Figure 9.5C is labeled with “X”, which corresponds mostly to the ground state but contains some contribution from an impurity that is responsible for the very weak residual.

Figure 9.5 (A) and (B) Transient absorption spectra measured at different time delays after 355 nm excitation of TAA-tmb-Ru²⁺ in ACN; (C) species-associated difference absorption spectra obtained from target analysis assuming sequential exponential steps.

Figure 9.6. As anticipated from the previous measurements between 0-1.5 ns, species-associated difference absorption spectra obtained from target analysis assuming sequential exponential steps.

9.4 Discussion

9.4 Discussion

The key finding of the TA studies is that although the distance between the donor and the acceptor in both cases is the same the nature of the bridge strongly affects the electron transfer rate, namely the intramolecular electron transfer from TAA to ³MLCT-excited Ru(bpy)3

2+ is substantially faster in TAA-tmb-Ru²⁺ (1.5 ns) than in TAA-ph-Ru²⁺ (6.8 ns).

Such difference can be attributed to stronger electronic coupling (𝐻_!") between the TAA and Ru(bpy)3

2+ units in the dyad with the tmb bridging unit. According to superexchange theory,¹⁹¹ the electronic coupling between a donor (D) and an acceptor (A) separated by n identical bridging units (b) is given by:174, 183, 192

     𝐻_!" =ℎ_!"

∆𝜀 ℎ_!!

∆𝜀

!!!

∙ℎ_!"       (9.1)

where ℎ_!", ℎ_!!, and ℎ_!" represent the electronic couplings between the donor and the adjacent bridging unit, between two neighboring bridging units, and between the last bridging unit and the acceptor, respectively, and

∆𝜀 is the so-called tunneling energy gap.^{183, 192} This latter parameter is defined as the energy required to remove an electron from the donor, or hole from the acceptor, depending on whether electron or hole transfer is considered and place it on the bridge.174, 182-183, 193

In practice, ∆𝜀 is often approximated as the difference between the donor oxidation and bridge reduction potentials when considering electron transfer, or as the difference between the acceptor reduction and bridge oxidation potentials when dealing with hole transfer.13, 174, 182-183

In the investigated dyads only a single bridging unit is present (n = 1), and the ℎ_!"

and ℎ_!" terms in equation 9.1 for tmb and ph bridging units should be rather similar. On the other hand, ∆𝜀 is expected to be more affected by going from unsubstituted to substituted phenylene, because the last can be oxidized significantly more easily due to its electron-donating substituents. The relevant reduction potentials for the individual redox-active components of the dyads were extracted from cyclic voltammetry measurements performed by Luisa G. Heinz in the Wenger group (University of Basel) and summarized in Table 9.1.⁴³ The oxidation potentials of tmb and ph are

0.42 V versus Fc⁺/Fc and 2.10 V versus Fc⁺/Fc, respectively (Table 9.1).^194-196 The acceptor reduction potentials are identical in

TAA-tmb-Ru²⁺ and TAA-ph-Ru²⁺ and amount to -1.78 V versus Fc⁺/Fc in the electronic ground state (Table 9.1). In the ³MLCT excited state, they are 0.34 V versus Fc⁺/Fc because the ³MLCT energy is 2.12 eV.¹⁹⁷ Consequently, the estimated tunneling energy gaps are ∆𝜀 = 0.08 eV for TAA-tmb-Ru²⁺ and ∆𝜀

= 1.76 eV for TAA-ph-Ru²⁺. This difference is substantial, and it can explain the difference in time constants for photoinduced electron transfer in the respective two dyads (1.5 vs 6.8 ns).

Table 9.1 Reduction potentials (E⁰) for the individual redox-active components of the two dyads from Figure 9.1^a,43

TAA-tmb-Ru²⁺ TAA-ph-Ru²⁺

Ru(III)/(II) 0.80 0.81

TAA^+/0 0.23 0.23

tmb^+/0 0.42^b,c

ph^+/0 2.10^d,c

bpy^0/- -1.78 -1.78

bpy^0/- −1.99 −2.03 bpy^0/- −2.21 −2.22

aIn ACN with 0.1 M TBAPF6, measured with potential sweep rates of 0.1 V/s. All potentials are given in Volts relative to the Fc⁺/Fc couple. ^bReported in refs 194 and 195 for the 1,2,4,5-tetramethoxybenzene molecule in ACN. ^cConverted from a potential reported in V vs SCE to V vs Fc⁺/Fc according to ref 198. ^dReported in ref 196 for the benzene molecule in ACN.

In the superexchange model for electron transfer, the one-electron reduced or oxidized states of the bridges are never actually populated, but they are merely virtual states that define the height of the tunneling barrier associated with long-range electron transfer.^{174, 199} However, in the TAA-tmb-Ru²⁺ dyad the estimated ∆𝜀 value is very low (0.08 eV). Given the uncertainty associated with its approximation, the possibility of a hole hopping process in which oxidized tmb is formed as a reaction intermediate cannot be excluded a priori. However, the ultrafast time-resolved experiments reported above provide no evidence for the formation of tetramethoxybenzene cation with its characteristic absorption at 450 nm.²⁰⁰

9.5 Conclusion

In any case it seems plausible that the low oxidation potential of tmb compared to ph plays a key role for the kinetics of photoinduced electron transfer.

9.5 Conclusion

In conclusion, we have shown experimentally that the bridge is not always a simple inert linker between a donor and acceptor, and that its energetics nature might strongly influence the charge transport kinetics. It was found that electron transfer from triarylamine to the ³MLCT state of the Ru(bpy)3

2+ species occurs ∼4.5 times faster across the tetramethoxybenzene linker then the unsubstituted phenylene. The faster rate of photoinduced charge transfer in the tetramethoxybenzene-bridged dyad can be understood in the framework of a hole-tunneling model in which the electron-rich tetramethoxybenzene bridge imposes a more shallow barrier than the less electron-rich unsubstituted phenylene spacer. Consequently, fourfold alkoxy substitution might be generally beneficial in more complex molecular wires.

So far alkoxy-substituents are mainly used to improve the solubility of rigid rodlike oligo-p-phenylene vinylene (OPV) and oligo-p-phenylene ethynylene (OPE) wires.186, 201-216

The presented study illustrates how four alkoxy-substituents on a phenylene backbone can have a significant influence on the charge-transfer properties of a molecular wire, and this is relevant in the greater context of a future molecular electronics technology.

Chapter 10

Photoinduced Accumulation of Multiple Electrons ^*

10.1 Introduction

In the previous chapters we have explored single-electron transfer reactions in different systems, however, solar-to-chemical energy conversion in fuel-forming reactions (e.g., H2 from H2O; methanol or methane from CO2) may require multielectron process.^217-229 Such light-driven accumulation of multiple electrons or holes on a given molecular unit is much less straightforward, at least when aiming at charge accumulation without using sacrificial reagents.58, 217, 230-238

At present, only a handful of prior studies have addressed this subject successfully.^239-242

A molecular pentad comprised of a central multielectron donor and two flanking photosensitizer-acceptor moieties was synthesized by Dr.

Annabell G. Bonn in the Wenger group in order to explore the possibility of accumulating two positive charges at the central donor, using visible light as an energy input. The investigated molecular pentad is presented in Figure 10.1 and comprises a central oligotriarylamine (OTA) donor that is easily oxidized up to three times. The OTA multielectron donor is connected to two

* This chapter is adapted from the following publication: Bonn, A. G.;

Yushchenko, O.; Vauthey, E.; Wenger, O. S., Photoinduced Electron Transfer in an Anthraquinone—[Ru(bpy)3]²⁺—Oligotriarylamine—[Ru(bpy)3]²⁺—Anthraquinone Pentad. Inorg. Chem. 2016, 55, 2894-2899.

10.2 Steady-State Spectroscopy

[Ru(bpy)3]²⁺ (bpy = 2,2′-bipyridine) photosensitizers, each of which bears an anthraquinone (AQ) acceptor. In principle, AQ can accept two electrons;

however, here, we aimed to explore whether photoexcitation of both [Ru(bpy)3]²⁺ units could lead to a charge-separated state in which OTA has been oxidized to its dicationic state (OTA²⁺) while both AQ units have been reduced to their monoanionic form (AQ⁻).

Figure 10.1 Structure of the AQ—Ru^II—OTA²⁺—Ru^II—AQ pentad.

10.2 Steady-State Spectroscopy

The steady-state absorption spectrum of the pentad is dominated by a series of bands below 600 nm that are associated with the individual constituents (Figure 10.2).189, 243-244

Although the spectra of the various chromophoric units are overlapping, several observations can be made: 1) the largest contribution of AQ can be found below 280 nm and between 300 and 350 nm (Figure 10.2);²⁴³ 2) the absorption spectrum of the Ru(bpy)3

2+ is dominated by two bands around 290 and 455 nm (Chapter 9);¹⁸⁹ 3) the absorption spectrum of the OTA is characterized by one band around 280 nm that was assigned to the π-π* transition localized on the phenyl rings.²⁴⁴ This implies that 355, 385 and 400 nm irradiation most probably leads to the excitation of a Ru(bpy)3

2+ unit. Partial excitation of AQ at 355 nm should be

N

taken into account.

Figure 10.2 Absorption spectra of the pentad and of AQ in ACN.

10.3 Transient Absorption

10.3.1 Femtosecond TA Measurements

The femtosecond transient absorption (TA) measurements between 0 and 1.5 ns were performed upon 385 and 400 nm excitations by using various pump pulse intensities on the sample.

TA spectra recorded with the pentad upon 400 nm excitation are shown in Figure 10.3. The spectra measured at different pump intensities are similar and will be considered together. This similarity is shown in Figure 10.4, where intensity-normalized time profiles at selected wavelengths recorded at different pump intensities match perfectly.

600 500

400 300

wavelength / nm

Pentad AQ

absorbance

10.3 Transient Absorption

Figure 10.3 Transient absorption spectra at different time delays after 400 nm excitation of the pentad in ACN at various pump intensities (A) 0.4 mJ/cm², (B) 1.6 mJ/cm² and (C) 2.4 mJ/cm².

At early times, the TA spectra are dominated by two positive bands peaking around 405 and 560 nm. The minimum around 470 nm could be due to the bleach of the ground-state absorption. During the first 10 ps, all bands are rising, shift slightly, and the 560 nm band narrows. Afterwards, the 560 nm band rises continuously, while the short-wavelength band shifts to 418 nm. The amplitudes of the positive bands decrease between 100 and 300 ps and increase again afterwards.

Figure 10.4 Transient absorption profiles at the band maxima extracted from the data shown in Figure 10.3 (green: 0.4 mJ/cm²; blue: 1.6 mJ/cm²; red 2.4 mJ/cm²).

The species-associated difference spectra (SADS) of the pentad obtained from target analysis assuming a A → B → C → D scheme are shown in Figure 10.5. These SADS do not exhibit very large differences and their interpretation is not straightforward. By comparison with TA features measured with the individual units, species A could be assigned to the locally-excited (LE) state of AQ,²⁴³ or to the ³MLCT state localized on Ru(bpy)3

2+.¹⁹⁰ On other hand the A, B, C SADS are also quite similar to the spectra reported for the OTA cation,^{43, 172} AQ anion,²⁴⁵ and Ru(bpy)³⁺.⁴³

1.4

10.3 Transient Absorption

Figure 10.5 Species-associated difference spectra obtained from a target analysis assuming an A → B → C → D scheme of the data shown in Figure 10.3 with (A) 0.4 mJ/cm², (B) 1.6 mJ/cm² and (C) 2.4 mJ/cm².

On the other hand, species C is very compatible with UV-vis difference spectra measured after addition of 1 equivalent Cu(ClO4)2 to a solution of the pentad in ACN that cause the formation of AQ⁻—Ru^II—OTA⁺— Ru^II—AQ obtained in the Wenger group.⁴⁴ Moreover, by using the same assign it to the formation of OTA⁺.⁴⁴ The spectrum of state B resembles that

30

of state C but has a smaller amplitude. This suggests that state B might be associated with a reaction intermediate (for example, a charge-separated state with the electron or the hole on the sensitizer). In any case, these data indicate that the quenching of the excited sensitizer (A → B) occurs within a few picoseconds, whereas the AQ⁻—Ru^II—OTA⁺—Ru^II—AQ state (C) is populated within <400 ps.

The TA spectra recorded with the pentad upon 385 nm excitation (Figure 10.6A and C) are relatively similar to those measured upon 400 nm excitation. A small difference was found in the early spectra, mainly in the blue part where some broadening is observed. The measurements were performed at two different pump energies, namely with the low one and the highest possible energy of the pump pulses. In contrast to the measurements upon 400 nm excitation, the 385 nm data show some dependence on the pump intensity. The relative intensity of the early TA spectra (< 25 ps) is significantly larger at high pump intensity.

An intensity dependence of the time evolution is also observed (Figure 10.6B and D). Global target analysis of low pump intensity data yields similar time constants and SADS as those obtained from the 400 nm excitation data. On the other hand, two exponential steps had to be added to reproduce the 8 µJ/cm² pump data (Figure 10.6D). Species B, D and E and their lifetime are close to those found at lower pump energy and should be associated with the same processes. On the other hand, the SADS A to C are all very similar and resemble SADS A in the low intensity measurements.

Thus, one can conclude that at high pump intensity species A undergoes a non-exponential decay. Precise assignment is difficult. Species A most probably corresponds to the the ³MLCT state of Ru(bpy)3

2+, although some contribution from the triplet excited state of AQ cannot be excluded, and the high intensity dynamics could be due to the occurrence of triplet-triplet annihilation (TTA) in pentads with two excitations. Such singlet or triplet annihilation processes are known to be at the origin of pump intensity dependence of the excited-state decay of multichromophoric systems and aggregates.^246-247 Such process is strongly detrimental to efficient accumulative electron transfer, but can be minimized by using sufficiently long excitation pulses, so that the first photoinduced charge separation process occurs before the second sensitizer is excited. This is clearly not

10.3 Transient Absorption

Figure 10.6 Transient absorption spectra at different time delays after 385 nm excitation of the pentad in ACN at various pump intensities (A) 0.4 mJ/cm² and (C) 8 mJ/cm²; (B) and (D) species-associated difference

absorption spectra obtained from target analysis assuming sequential exponential steps

possible with the 100 fs pulses used here for the femtosecond measurements, because quenching of the [Ru(bpy)3]²⁺ in the ³MLCT state occurs within a few picoseconds (step A → step B in Figure 10.6). However, in the sub-nanosecond measurements, pulses are sufficiently longer, and therefore the probability for a pentad to have two excited [Ru(bpy)3]²⁺ units, none of which has yet participated in photoinduced electron transfer should be lower.

10.3.2 Sub-nanosecond TA Measurements

Measurements on a longer time scale were performed with the sub-nanosecond TA setup upon 355 nm excitation. Two sets of measurements were done: 1) pump intensities similar to those used for the femtosecond TA measurements and 2) much higher pump intensities than those achievable with the femtosecond TA setup.

The first set of measurements is shown in Figure 10.7. No significant dependence on the pump intensity can be observed. The TA features are very similar to that found in the femtosecond TA measurements at long time delays. Nevertheless, one significant difference was found in the dynamics of sub-nanosecond TA measurements: the TA intensity decreases continuously to zero in the 300 ps to 4 µs time window, while in the of femtosecond measurements, the TA intensity rises from 300 ps to 1.5 ns.

This could indicate that 355 nm pumping does not lead to the same initial excited-state population than 385 or 400 nm pumping. Both the Ru complexes and the AQ units can be excited at 355 nm.

The temporal evolution of the TA spectra was analyzed by target analysis assuming a scheme with three consecutive exponential steps (Figure 10.8). The resulting SADS do not exhibit very pronounced differences and can be assigned to the same species/state.

10.3 Transient Absorption

Figure 10.8 Species-associated difference spectra obtained from a target analysis assuming an A → B → C → D scheme of the data shown in Figure 10.7 with (A) 0.4 mJ/cm², (B) 1.6 mJ/cm² and (C) 2.4 mJ/cm².

TA spectra measured with 20 mJ/cm² pump intensity are shown in Figure 10.9A. The main difference with the spectra recorded at lower intensities is the presence of long-lived components up to a few hundreds of microseconds, with a spectrum consisting in a very broad band peaking around 400 nm and extending above 700 nm.

The temporal evolution of the TA spectra was analyzed by target analysis assuming five successive steps with the SADS shown in Figure 10.9B. The first SADSs are similar to those found in the lower intensity measurements, whereas SADS D with a 1.2 µs lifetime is characterized by a

30

10.3 Transient Absorption

very broad band. Finally, the SADS E has a very small amplitude and resembles SADS C measured at lower intensity and associated with a shorter lifetime. Assignment of these SADS is not straightforward, especially considering that the actual decay dynamics may be more complicated than a succession of exponential steps. Therefore, the spectra obtained from the target analysis should be considered as evolution-associated difference spectra (EADS) than SADS.³⁶

Figure 10.9 (A) Transient absorption spectra at different time delays after 355 nm excitation of the pentad in ACN with 20 mJ/cm²; (B) species-associated difference absorption spectra obtained from target analysis assuming an A → B → C → D → E → F scheme.

TA spectra recorded at the highest pump intensity (35 mJ/cm²) are depicted in Figure 10.10. They exhibit even more complicated behavior than those measured at 20 mJ/cm². The main feature is the rise on the ~1 µs timescale of a band peaking at 525 nm that is not present in the spectra measured at lower intensity. This band decays within a few µs to a very weak residual spectrum resembling that found at lower intensities. In fact the

absence of the dip around 470 nm in the 20 mJ/cm² spectra could be due to the presence of this 525 band, although at smaller intensity.

Figure 10.10 Transient absorption spectra at different time delays after 355 nm excitation of the pentad in ACN with 35 mJ/cm².

Target analysis assuming a series of successive steps was not possible because some of the resulting SADS had negative amplitude over the whole spectral range. This clearly point to a more complex scheme. The fact that this 525 nm feature is only observed at very high pump intensities and at relatively long time delays suggests that it may arise from a intermolecular process between two initially excited pentads.

Additionally, transient absorption spectra measured after excitation at 532 nm with the pump of 10 mJ/pulse and 60 mJ/pulse, and laser pulses of

∼10 ns duration were recorded in the Wenger group.⁴⁴ Even, it has not been possible to corroborate the presence of OTA²⁺ by exploring the excitation power dependence, it was found some evidence of the formation of AQ⁻— Ru^II—OTA²⁺—Ru^II—AQ⁻ by subtracting the TA spectra measured at lower pump intensity from that measured at high pump intensity that lead to some

∼10 ns duration were recorded in the Wenger group.⁴⁴ Even, it has not been possible to corroborate the presence of OTA²⁺ by exploring the excitation power dependence, it was found some evidence of the formation of AQ⁻— Ru^II—OTA²⁺—Ru^II—AQ⁻ by subtracting the TA spectra measured at lower pump intensity from that measured at high pump intensity that lead to some

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