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Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 3688−3693
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Ion-Pair Dynamics upon Photoinduced Electron Transfer Monitored by Pump−Pump−Probe Spectroscopy Joseph S. Beckwith, Bernhard Lang, Jakob Grilj, and Eric Vauthey* Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, CH-1211 Geneva, Switzerland
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S Supporting Information *
ABSTRACT: The excited-state dynamics of the radical anion of perylene (Pe) generated upon bimolecular photoinduced electron transfer (PET) with a donor was investigated using broadband pump−pump−probe spectroscopy. It was found to depend on the age of the anion, that is, on the time interval between the first pump pulse that triggers PET and the second one that excites the ensuing Pe anion (Pe•−). These differences, observed in acetonitrile but not in tetrahydrofuran, report on the evolution of the PET product from an ion pair to free ions. Two photoinduced charge recombination pathways of the ion pair to the neutral Pe*(S1) + donor state were identified: one occurring in a few picoseconds from Pe•−*(D1) and one taking place within 100−200 fs from Pe•−*(Dn>1). Both processes are sensitive to the interionic distance over different length scales and thus serve as molecular rulers.
P
Scheme 1. Principle of the Pump−Pump−Probe Experiment: Energy-Level Scheme and Pulse Timing
hotoinduced electron transfer (PET) between two neutral closed-shell reactants produces a pair of radical ions that, in polar environments, can either evolve into free ions or undergo geminate charge recombination (CR) back to the neutral ground state.1−7 High free ion yields are desirable for the various practical applications of these reactions.8−11 Radical ions of aromatic molecules are usually easily identified by their electronic absorption in the visible region.12,13 Apart from very few exceptions,14−16 their electronic absorption spectra are not very sensitive to the environment and do not allow paired and free ions to be differentiated. However, the ability to distinguish these species would considerably benefit our understanding of ion-pair dynamics upon PET. Recently, the intense absorption band of the perylene anion produced upon PET was found to shift by ∼60 cm−1 on a time scale consistent with that expected for pair dissociation.17 Previous investigations reported on small frequency shifts observed with a few vibrational bands of ions that were assigned to pair dissociation.18−20 Magnetic field effects can also, in some cases, give rich information about the correlation between paired ions.21−25 However, an unambiguous spectroscopic signature of the close proximity of radical ions generated upon PET is still missing. Here we explore whether the excited-state dynamics of radical ions is sensitive to the presence of the counterion and can report on the evolution of the PET product from paired to free ions. We accomplish this by using electronic pump− pump−probe transient absorption (PPP-TA) spectroscopy (Scheme 1). First, an actinic pump pulse, P1, triggers PET between perylene (Pe) in the S1 state and the electron donor, 4-N-N-trimethylaniline (TMA), and, after a given time delay, Δt12, a second pump pulse, P2, excites the ensuing Pe radical anion (Pe•−). Finally, a white-light pulse, P3, probes the © 2019 American Chemical Society
sample at various times after P2, Δt23. In essence, we perform broadband transient absorption on Pe•− at different times after PET. Previous PPP-TA experiments on the Pe cation pointed to a dependence of its excited-state dynamics on the time after its generation by PET;26 however, because probing was done at Received: May 20, 2019 Accepted: June 13, 2019 Published: June 13, 2019 3688
DOI: 10.1021/acs.jpclett.9b01431 J. Phys. Chem. Lett. 2019, 10, 3688−3693
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The Journal of Physical Chemistry Letters
PPP-TA measurements were performed at eight different time intervals, Δt12, between P1 at 400 nm and P2 at 580 nm. Figure 2 depicts PPP-TA data sets recorded in ACN with Δt12 = 0.25 and 2 ns. They correspond to the difference in the transient absorption measured with and without P2 (See the Supporting Information (SI) for details). The lower panels show evolution-associated difference spectra (EADSs) resulting from a global analysis, assuming a series of successive exponential steps, A → B → C → ..., with increasing time constants. These EADSs and the related time constants cannot be systematically assigned to well-defined species/states and kinetic processes, respectively.29−31 Nevertheless, they give a semiquantitative picture of the spectral dynamics and of the time scales on which they occur. Both sets of transient data show an intense negative band centered at 580 nm due to the bleach of the D5 ← D0 band of Pe•−, which transforms in a few picoseconds into a sawtooth-shaped band that is characteristic of a vibrationally hot-ground-state absorption.32−34 This feature decays, in turn, in a few tens of picoseconds. At short Δt23, two distinct differences can be noticed between the two data sets. At Δt12 = 0.25 ns, (i) a positive red band is visible around 700 nm and (ii) the positive blue band in the 400−530 nm region is significantly weaker than that in the Δt12 = 2 ns data set. From measurements performed at other Δt12 values(Figures S6), one can see that the initial intensity of the red band decreases with increasing Δt12, whereas that of the blue band becomes more pronounced (Figure 3). By comparison, the PP-TA spectra recorded with Pe•̇ − produced electrochemically in ACN (Figure S5) are very similar to the PPP-TA spectra at Δt12 > 1.5 ns; namely, they do not exhibit the red band and show a relatively intense blue band. Finally, the PPP-TA spectra in THF do not present any significant dependence on Δt12 (Figure S7). They resemble those in ACN at short Δt12, but the red band is more pronounced, and the intensity in the blue region is negative. These results suggest that the red band observed in THF at all Δt12 and in ACN at short Δt12 is present only when Pe•− is paired with TMA•+, whereas the positive 400−530 nm band found in ACN at long Δt12 and with the electrochemically produced Pe•− can rather be associated with free or quasifree Pe•−. The very short lifetime of the red band, which can be anticipated from the PPP-TA spectra in THF and in ACN at short Δt12, is confirmed by the global analysis. Indeed, this feature is visible only in first the EADS, A, evolving with a ∼0.2 ps time constant (Figure 2c, Table S1). In addition to the D5 ← D0 bleach ofPe•−, this EADS contains a broad band in the 400−550 nm region, which is positive but hardly visible in ACN and more intense but negative in THF (Figure S10). In the next EADS, B, this feature is replaced by the positive blue band. The remaining EADSs, C and D, can be mostly associated with the hot ground state of Pe•−. Similar EADSs are obtained in ACN at long Δt12 (Figure 2d) and with the electrochemically produced Pe•− (Figure S8), with the exception of EADS A, which does not contain the red band but shows the blue band and thus resembles EADS B. The PP-TA results from the electrochemically produced Pe•− point to a very fast internal conversion from Pe•−* to the vibrationally hot D0 state, as also observed with many other radical ions in solution.35−42 The decay of the lowest excited state, D1, can be related to the 4.7 ps time constant found in the global analysis. The shortest time constant should
only a single wavelength, the origin of this effect could only be hypothesized. Herein we show that the excited-state dynamics of Pe•−is substantially altered when Pe•− is paired with TMA•+ due to the presence of intrapair decay channels that are no longer available when the ions are far apart. This implies that one can use the dynamics of the radical excited states as a kind of molecular ruler. Conventional pump−probe TA (PP-TA) measurements were first performed upon S1 ← S0 excitation of Pe at 400 nm with 1 M TMA in high- and medium-polarity solvents, acetonitrile (ACN) and tetrahydrofuran (THF) (Figure 1a and Figure S3). In both solvents, the early spectra are dominated by the intense Sn ← S1 absorption band around 700 nm.27
Figure 1. (a) Transient absorption spectra recorded at selected time delays after 400 nm excitation of Pe with 1 M TMA in ACN. (b) Time dependence of Pe•−band integral in ACN and THF normalized at 100 ps.
Within 200 ps, this band vanishes completely and is replaced by the D5 ← D0 absorption band of Pe•− around 580 nm.28 The TMA cation band appears as a minor shoulder around 470 nm.13 In ACN, the Pe•− band loses ∼70% of its intensity in 10 ns due to geminate recombination (Figure 1b). Afterward, its decay is minor up to several hundreds of nanoseconds, pointing to negligible CR and to weakly correlated pairs of quasi-free ions. Additionally, the band position upshifts by ∼60 cm−1 (3 to 4 nm) during the first 10 ns, in agreement with a previous report (Figure S4).17 In THF, the ion band decays completely within 100 ns, and its position remains unchanged. These measurements suggest that the pair distribution in ACN evolves considerably during the first few nanoseconds, whereas, in THF, the ions remain paired during their whole lifetime. 3689
DOI: 10.1021/acs.jpclett.9b01431 J. Phys. Chem. Lett. 2019, 10, 3688−3693
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Figure 2. (a,b) Time evolution of the pump−pump−probe signal recorded with Pe and 1 M TMA in ACN at Δt12 = 250 ps and 2 ns. (c,d) Evolution-associated difference spectra obtained from a global analysis of the data in panels a and b assuming a series of successive exponential steps.
Scheme 2. Energy-Level Scheme Illustrating the Deactivation Pathways of Pe•−* in the Pair and as a Free Ion (Grey Box)a
Figure 3. Dependence of the initial red band intensity (relative to the bleach, green) and of the lifetime of Pe•−*(D1) (τB→C, gold) on the time interval between the two pump pulses, Δt12, in ACN and THF. Δt12 = 1.5 ns is the highest delay at which the red band can be resolved in ACN, and the error in its intensity is significantly larger than that at shorter Δt12.
include the relaxation from upper excited states, Dn>1, whereas the longer time constants reflect mostly the cooling of the hot D0 state.32,42,43 The PPP-TA data in ACN at long Δt12 can be interpreted likewise. Scheme 2 reveals that additional excited-state decay pathways are available when Pe•− is paired with TMA•+, namely, recombination back to the neutral reactants in the electronic excited state (CR*). Recombination to the reactants in the electronic ground state can be excluded due to its very large driving force that should make it slower than the recombination of the nonexcited ion pair.7,27,44 Moreover, if operative, this pathway should lead to a permanent bleach of the D5 ← D0 band of Pe•−, contrary to the observation. On the contrary, CR* is fully compatible with the early PPP-TA spectra (EADS A) in THF and ACN at short Δt12: The red band resembles the absorption band of Pe*(S1), whereas
a
Wavy arrows symbolize internal conversion and vibrational relaxation. The energy is referred to the Pe(S0) + D state.
stimulated emission from Pe*(S1) can account for the small intensity of the blue band. The absence of permanent D5 ← D0 bleach is consistent with the repopulation of Pe•− by PET. The decrease in the red band intensity in ACN with increasing Δt12 (Figure 3) can be explained by the evolution of the PET product distribution from ion pairs toward free ions. Global analysis also indicates that the B → C time constant, which mostly reflects the decay of Pe•̇ −*(D1), is the shortest in THF, that is, ∼0.8 ps, and increases from 1.2 to 3.5 ps in ACN when Δt12 is varied from 0.25 to 2 ns (Figure 3, Table S1). This agrees well with the presence of an additional decay 3690
DOI: 10.1021/acs.jpclett.9b01431 J. Phys. Chem. Lett. 2019, 10, 3688−3693
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The Journal of Physical Chemistry Letters pathway of Pe•−*(D1) in the pair that leads to a shortening of its lifetime. As the interionic distance increases, CR*(D1) slows down and is thus less efficient. Taking 4.7 ps as the lifetime of free Pe•−*(D1) results in CR*(D1) time constants of ∼1 ps in THF and growing from 1.6 to ∼15 ps with Δt12 going from 0.25 to 2 ns in ACN. This slowing down of CR*(D1) with increasing Δt12 can be assumed to arise mostly from the distance dependence of the electronic coupling, V(d) ∝ exp(−βd) (see the SI for discussion).44 Consequently, CR*(D1) can be used a molecular ruler. Using the β value of 0.5 Å−1 reported for a similar ion pair in ACN17 allows us to estimate an increase in the ion-pair distance distribution by ∼2.5 Å during the first 2 ns after PET. As discussed in the SI, such a relatively small distance of diffusion in 2 ns can be explained by the Coulomb potential in the pair. The CR*(D1) time constants deduced above cannot account for the presence of the red Pe*(S1) band in the earliest PPP-TA spectra; however, these two observations can be reconciled if we assume that CR* also takes place from the upper excited states of Pe•−, D2−5, in parallel to their ultrafast internal conversion to the longer-lived D1 state. Using the Weller equation,45 the estimated CR* driving force, ΔG*CR, decreases from 1.6 to ∼0.7 eV upon D5 → D1 internal conversion. Therefore, CR*(Dn>1) can be expected to be significantly faster than CR*(D1), assuming similar electronic coupling V at a given distance d. To test this idea, the PPP-TA data in THF were analyzed globally using a target model including two CR* pathways, one from Dn>1 competing with internal conversion to D1 and one from D1 competing with internal conversion to D0 (Scheme S1). Considering that most processes occur on comparable time scales as vibrational relaxation, this approach should be considered on a qualitative basis only. Nevertheless, a good fit with meaningful species-associated difference spectra was obtained, assuming that CR* and internal conversion from Dn>1 occur within the instrument response function (Figure S11). Moreover, the resulting CR*(D1) time constants are in excellent agreement with those estimated above from the lifetime of the D1 state. According to this analysis, the efficiency of CR*(Dn>1) in THF is ∼55%, and the repopulation of the Pe•−(D0)/ TMA•+ pair from Pe*(S1)/TMA occurs with a 0.2 ps time constant (Table S2). Such a short time constant is consistent with those found previously for bimolecular PET in the static regime.46 It also accounts for the very fast decay of the red Pe*(S1) band in THF. This band is only visible because CR*(Dn>1) is fast enough. Comparatively, CR*(D1) with a ∼ 1 ps time constant is too slow to result in an instantaneous population of Pe*(S1) that is sufficiently large to be observable. This target model could also be applied in ACN. The analysis points to a decrease in the CR*(Dn>1) efficiency from ∼20% to ∼0 with increasing Δt12, in agreement with the observed decrease in the red band intensity. It also confirms the parallel slowing down of CR*(D1) discussed above (Table S2). Consequently, both CR* processes can be used as molecular rulers to monitor the temporal evolution of the ionpair distance distribution. This is confirmed by Figure 4, which points to a correlation between the CR*(Dn>1) efficiency and the CR*(D1) time constant obtained from the global target analysis. However, CR*(Dn>1) only reports on very short interionic distances where this process can compete with the ultrafast internal conversion to D1. For example, CR*(Dn>1) is
Figure 4. Dependence of the CR*(D1) time constant (gold), of the efficiency of CR*(Dn>1) (green) on the time interval between the two pump pulses, Δt12, and of the shift of the Pe•− band (purple) on the time delay after 400 nm excitation in ACN, Δt (see Figure S12 for THF).
no longer operative at Δt12 > 1.5 ns, whereas CR*(D1) still takes place, indicating that the ions are paired but not tightly. This figure also reveals that these two quantities at a given Δt12 also correlate with the position of the D5 ← D0 band of Pe•− at the same time delay after 400 nm excitation. This is clear evidence that this transition frequency of Pe•− is also sensitive, although very weakly, to the presence of a counterion. To conclude, this PPP-TA investigation of a bimolecular PET reaction reveals that the excited-state dynamics of the ensuing ions, here Pe•−, is sensitive to the presence of the counterion in close vicinity, as the latter opens intrapair deactivation pathways that are no longer operative for free ions. Because these pathways involve charge-transfer processes, their dynamics are sensitive to the interionic distance and can be used as rulers to monitor the temporal evolution of the distance distribution of the PET product. The length scale of these rulers is determined by the excited-state lifetime of the free ion. Here the ∼5 ps lifetime of Pe•−*(D1) allows us to monitor the ions up to ∼2 ns after their birth; however, significantly longer time scales can, in principle, be explored using radical ions with longer-lived excited states, such as those of naphthalenediimides, perylenediimides, or quinones.47−50 Such experiments, combined with an adequate reaction− diffusion model,7,51,52 should give unprecedented insight into ion-pair dynamics. This approach could also prove powerful to track structural changes occurring after photoinduced intramolecular charge separation, especially in flexible molecules.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b01431. Experimental details, additional data, global analysis, and estimation of the Coulomb potential in the ion pair (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Joseph S. Beckwith: 0000-0003-4726-230X 3691
DOI: 10.1021/acs.jpclett.9b01431 J. Phys. Chem. Lett. 2019, 10, 3688−3693
Letter
The Journal of Physical Chemistry Letters
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Eric Vauthey: 0000-0002-9580-9683 Notes
The authors declare no competing financial interest. All data can be downloaded from http://doi.org/10.5281/ zenodo.2651230.
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ACKNOWLEDGMENTS We thank the Fonds National Suisse de la Recherche Scientifique (project no. 200020-165890) as well as the University of Geneva for financial support.
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