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Letter
Magnetical Control of the Charge-Separated State Lifetime Realized by Covalent Attachment of a Platinum Complex Tomoaki Miura, Dai Fujiwara, Kimio Akiyama, Takafumi Horikoshi, Shuichi Suzuki, Masatoshi Kozaki, Keiji Okada, and Tadaaki Ikoma J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02887 • Publication Date (Web): 13 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017
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The Journal of Physical Chemistry Letters
Magnetical Control of the Charge-Separated State Lifetime Realized by Covalent Attachment of a Platinum Complex Tomoaki Miura,*,† Dai Fujiwara,† Kimio Akiyama,‡ Takafumi Horikoshi,§ Shuichi Suzuki,§,¶ Masatoshi Kozaki,§ Keiji Okada,§ and Tadaaki Ikoma*, †, || † Department of Chemistry, Faculty of Science, Niigata University, 2-8050 Ikarashi, Nishi-ku, Niigata 950-2181, Japan. ‡ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Aoba-ku, Sendai 980-8577, Japan. §Department of Chemistry, Graduate School of Science, Osaka City University, Sugimoto Sumiyoshi-ku, Osaka 558-8585, Japan. || Center for Instrumental Analysis, Niigata University, 2-8050 Ikarashi, Nishi-ku, Niigata 950-2181, Japan. Supporting Information Placeholder ABSTRACT: Dynamics of the photo-generated charge separated (CS) state is studied for a newly synthesized molecular triad, in which the donor (D) dimethoxytriphenylamine, 1,3-bis(2pyridylimino)isoindolate platinum (BPIPt) and the acceptor (A) naphthaldiimide are linked with triethynylbenzene unit (BPIPtDA). Photoexcitation of BPIPt gives rise to generation of a longlived (~4 s) CS state BPIPt-D+A−, of which the lifetime is considerably increased by an applied magnetic field of 270 mT. The positive magnetic field effect (MFE) is in contrast to the negative MFE for the reference DA molecule, which indicates successful switching of the initial spin state of the CS state from singlet to triplet. Simulations of MFE and time-resolved electron paramagnetic resonance show that spin-selective charge recombination and spin relaxation are unaffected by attachment of BPIPt. The minimum impact of the heavy atom substitution on the electronic and magnetic properties has been realized by the small electronic coupling mediated by the rigid meta-triethynylbenzene.
tronic/magnetic properties of the CS state is still an open question, because the heavy metals are “embedded” in the D or A molecule by itself or the bridge unit connecting the D and A. In this study we demonstrate control of only the initial spin state of the CS state by attaching a Pt complex on the bridge moiety of a D-A linked molecule with a minimum impact on its electronic/magnetic properties. A molecular triad, in which dimethoxytriphenylamine (MTA = D), 1,3-bis(2pyridylimino)isoindolate platinum (BPIPt)11 and naphthaldiimide (NDI = A) are linked with tri-ethynylbenzene unit (BPIPt-DA), has been synthesized. Photoinduced electron and spin dynamics is compared with that of a reference molecule DA, which does not possess the BPIPt moiety. R
D
O
MeO N
It is of much importance to create molecular systems that exhibit photo-generated long-lived charge separated (CS) states for realization of artificial photosynthesis.1-3 Numbers of donor (D)– acceptor (A) linked molecules have been designed focusing on structural and electronic properties of D, A, and intervening “bridge” molecular units so that the rate of charge recombination (CR) of the CS states is minimized.4 An alternative strategy to the conventional approach is to control the electron spin dynamics of the CS states; namely, spincontrol. CS and CR proceed spin-selectively due to the conservation of spin angular momentum. It has previously been demonstrated that the singlet-triplet intersystem crossing (ISC) in the CS state (or radical ion pair state) can be a bottleneck in the CR process if the exchange coupling 2J = ES − ET, where ES and ET are spin state energies for the singlet and triplet CS states, is sufficiently large.5 The initial spin state of the CS state is also important factor. Heavy metal complexes are known to yield excited triplet states very rapidly due to a strong spin–orbit coupling (SOC). Many D–A dyads or D–Chromophore –A triads, in which a heavy-metal complex functions as the D, chromophore or A, have been synthesized.6-10 In those systems, the triplet sublevels of the CS state are selectively populated upon photoexcitation of the metal complex, so that the CR requires ISC in the CS state.6-7 However, the effect of introducing the heavy-atom on the elec-
A
N O
O N N
OMe
BPIPt-DA: R=
DA-ethynyl:
C8H17
Pt N
R=
N DA:
N
O
N
H R=
C CH
BPIPt-DA was prepared through the Cu(I)-catalyzed coupling reaction of 1,3-bis(2-pyridylimino)isoindolate platinum(II) chloride (BPIPt-Cl) with DA-ethynyl in the presence of an excess amount of trimethylamine in dichloromethane (Supporting Information, SI). All the spectroscopic measurements (see SI for detail) were conducted at room temperature. For measurements of nanosecond transient absorption (nsTA), nsTA-detected magnetic field effect (MFE) and time-resolved electron paramagnetic resonance (trEPR), the solution samples were made with vacuum-distilled or deoxygenated tetrahydrofuran (THF, Wako co. ltd) and were carefully degassed with several freeze-pump-thaw cycles. The UV-visible absorption spectrum of BPIPPt-DA in THF (Figure S1) can be explained by superposition of BPIPt complex, D and A, suggesting negligible electronic interactions between the three chromophores in the ground state. Energy levels of the triplet state of BPIPt (3BPIPt*, ET(BPIPt)) and that of the CS state
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Figure 1 shows nsTA spectra of BPIPt-DA (2.0 × 10−5 M) in THF following selective photoexcitation of the mixed metal and ligand to ligand charge transfer (MMLL’CT) band of the BPIPt moiety by the second harmonics of a Nd:YAG laser (wavelength of 532 nm, energy of 2 mJ/pulse and duration of ~5 ns).12 The spectrum at the earliest time peaks around 420 nm, which is similar to those observed for 1,3-bis(2-pyridylimino)isoindolate platinum(II) phenylacetylide (BPIPt-Ph) (Figure S4). Thus the earlytime spectrum is assigned to the excited triplet state of BPIPt moiety (3BPIPt*), which is formed from the excited singlet state (1BPIPt*) by the very fast SOC-induced ISC.12 Upon decay of the 3 BPIPt*, the peaks at 470 nm and 760 nm grow within a few hundreds of nanosecond accompanying isosbestic points at ~450 nm and ~780 nm. The absorption peaks at 470 nm and 760 nm are assigned to the radical anion of NDI (A−) and the radical cation of MTA (D+), respectively. Observation of the two radical ions clearly indicates formation of the CS state BPIPt-D+A− most likely from 3BPIPt*. After the generation of the final CS state, D+ and A− signals decay simultaneously in microsecond timescale due to charge recombination (CR).
time constant of 4.3 s, latter of which is attributed to CR to the ground state at 0 mT (CR(0 mT) = 4.3 s, Figure 2a). It turns out that the CR(0 mT) is almost the same with that for the reference DA molecule. However, situation is totally different at 270 mT. The CS state for BPIPt-DA exhibits a similar rise time (777 ± 6.1 ns) with that at 0 mT, but considerably prolonged single decay time of CR(270 mT) = 9.6 s, which is in contrast to the negative MFE on the major decay component for DA. 0.025
(a)
0.020 0.015
BPIPt-DA 0 mT 270 mT
0.010 0.005 0.000
A
(BPIPt-D+A− or D+A−, ECS) are estimated from the analysis of the phosphorescence spectrum and electrochemical measurements (SI) as ET(BPIPt) = ~2.2 eV, ECS = 1.19 eV in THF.
0.12
(b)
DA 0 mT 270 mT
0.08 0.04 0.00
0.015
0
A
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20
30
Figure 2. Nanosecond transient absorption kinetics of the CS state (A−) for BPIPt-DA (a) and DA (b) in THF with (red) and without (blue) the applied magnetic field of 270 mT. Black dashed lines indicate simulations based on the kinetic model described in the text.
0.005 0.000 400
500
600
700
800
Wavelength / nm
Figure 1. nsTA spectra of BPIPt-DA in THF observed at 400 ns (red), 800 ns (orange), 1.5 s (green) and 6 s (blue) in the absence of applied magnetic field. The rise times for D+ and A− are 731 ± 17 ns and 767 ± 3.4 ns, respectively, which are almost the same with the decay time of 3 BPIPt* (761 ± 10 ns) as shown in Figure 2 and Figure S5. Thus it is considered that the final CS state BPIPt-D+A− is generated from 3 BPIPt* via very short-lived intermediate CS states BPIPt−-D+A and/or BPIPt+-DA−. The reaction pathway to generate BPIPtD+A− is shown as 1
10
Time / s
0.010
10 ns BPIPt*-DA 3 BPIPt*-DA
BPIPt -D A or BPIPt BPIPt-D A .
~760 ns 760 ns
3
3
3
+
+
+
-DA
The reference molecule DA in THF also exhibits the CS state upon selective photoexcitation of NDI (A) moiety by the third harmonics of the Nd:YAG laser (wavelength of 355 nm, energy of 1 mJ/pulse and duration of ~ 6 ns) as shown in Figure S6. NsTA kinetics for the CS states of BPIPt-DA and DA observed at 470 nm (A−) are shown in Figure 2. The CS state signal for the DA molecule rises within the instrument response (~ 10 ns) and decays with a time constant of 4.0 s, which is attributed to CR to the ground state (CR(0 mT) = 4.0 s), as shown in Figure 2b. At an applied magnetic field (B) of 270 mT, the CS state exhibits two decay components, a major component with a shorter lifetime of 1.5 s (70%) and a minor component with a longer lifetime of 8.8 s (30%). On the other hand, the CS state of BPIPt-DA rises with the 767 ± 3.4 ns, time constant, and subsequently decays with a
The observation of the opposite MFE clearly indicates successful switching of the precursor spin state of the CS state by introducing the Pt complex as explained as follows. In BPIPt-DA, the CS state is populated from 3BPIPt* conserving the triplet spin multiplicity during the stepwise CS, so that CR to the singlet ground state requires the singlet–triplet (S–T) ISC in the CS state. In DA, on the other hand, the CS state is populated in the singlet manifold from 1A*, and CR occurs in the same spin manifold. Since the S–T ISC in the CS state is less efficient in higher magnetic fields, as discussed in detail in the following paragraphs, CR of the triplet-born CS state is inhibited by the magnetic field, whereas that of the singlet-born CS state is promoted.13 The positive MFE on the CS state lifetime realized by the spin-control is of great advantage in terms of photon-energy conversion. It is also noteworthy that the amplitude of MFE is larger for BPIPt-DA (up to ~ 30% increase relative to the initial yield) compared to that for DA (~ 15% decrease). The enhancement of MFE is important for the earth magnetic field sensing using photochemical systems as seen in birds’ magnetophotoreception.14 Figure 3 shows the magnetically affected reaction yield (MARY) spectra obtained by plotting MFE on TA signal as defined by equation 1, as a function of B.
MFE
A( B, t ) A(0, t ) 100 % . A(0, t )
(1)
MARY spectra correspond to the field dependence of the ISC efficiency of the CS-state until the delay time after laser flash (t).15 Both BPIPt-DA and DA (Figure S7) exhibit saturation feature above ~10 mT at the early times, whereas gradual increase at higher fields is observed at the late times. This feature is frequently observed in long-lived radical pair systems as micellar sys-
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tems.16-17 The ISC at the early times is characterized by the hyperfine (HF) interaction, of which the magnitude is ~10−3 cm−1 for typical organic systems; the angular frequency of HF-induced coherent mixing is HF ~1 × 108 s−1.13 At the later times, field dependent spin relaxation of ~106 s-1 also contributes to the ISC efficiency resulting in the change of MARY spectral shape.18 It should be noted that dip of the 2J-resonance in MARY, which is frequently observed in D-A linked systems with large 2J values,5, 16, 19-20 does not appear for BPIPt-DA and DA. The lack of 2Jresonance indicates that 2J of these molecules is smaller than the HF interaction (a few tens of millitesla).
MFE (%)
150
100
BPIPt-DA 2 s 10 s
50
0 0
50
100
150
200
250
Magnetic Field / mT
Figure 3. Magnetic field dependence of the MFE for BPIPt-DA (Time-resolved MARY spectra). Figure 4 shows the trEPR spectrum of BPIPt-DA in THF (5×10−4 M). The observed antiphase pattern with emission (E)/absorption (A) of microwave is characteristic of spin correlated radical pairs.21-22 The spectrum can be simulated assuming the triplet-precursor and a negative 2J with HF couplings consistent with the reported values for D+ and A− radicals.6 The effect of CR from the singlet state with a rate constant of kCRS, on the spin polarization is also taken into account. From the simulation, 2J = −0.17 mT and kCRS = 8 × 105 s−1 are obtained. The very small 2J is consistent with the MARY spectra without the resonance feature. The 2J value, which is known to be proportional to the square of D-A electronic coupling (VDA),23 is considerably smaller than those reported for similar D-A systems linked by pphenylene or p-phenylethynylene units having similar throughbond D-A distances (|2J| ~ 1 mT at ~3 nm).19-20 This would be due to the smaller VDA mediated by the meta-substituted benzene than that for para-substituted molecules. Namely, meta-position of the benzene moiety has less electron/hole density than ortho- and para-positions in the virtual bridge-reduced/oxidized state, so that the substitution with ethynyl group does not interfere the major electron/hole pathway.
A
"
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The Journal of Physical Chemistry Letters excitation. A and E indicate absorption and stimulated emission of microwave, respectively. TrEPR spectrum of the reference DA molecule is also observed with A/E antiphase polarization, which is opposite to that for BPIPt-DA (Figure S9). This observation indicates that we have successfully switched the CS-state spin state without changing the sign of 2J. Very sharp HF structure around the center field is observed for BPIPt-DA and DA, whereas such a structure is not observed for previously reported MTA-Pt complex-NDI triad.6 Pt atom embedded in the bridge moiety of the previous triad is likely to induce transverse spin relaxation due to SOC. Such a SOC effect has successfully been turned off in the present triad by the meta-linkage. From the MARY and trEPR measurements, one can depict the spin dependent dynamics for the CS state as shown in SCHEME 1. At 0 mT, coherent ISC between the S and triply degenerated T states is efficiently induced by the HF interaction because of small 2J. The observed CR rate of ~106 s−1 is much smaller than HF of ~1 × 108 s−1 indicating that the spin state quickly reaches a steady state where the four spin states are almost equally populated, which is followed by the slow CR from the S state. At high magnetic fields, on the other hand, the T+1 and T−1 sublevels are energetically separated from the S and T0 states due to the Zeeman interaction. The HF-induced S-T0 mixing is still active even at the high fields, but the T±1 states cannot mix with the S and T0 states due to the large energy gap. In such a condition, the interconversion between the mixed S-T0 states and the energeticallyisolated T±1 states is dominantly governed by the incoherent spin relaxation in microsecond timescale. Since the coherent mixings between the degenerated states are much faster than the CR and the spin relaxation, one can approximate the coherent mixings as kinetic interconversion with a rate of ~108 s−1. The MFEs on the CS state kinetics of BPIPt-DA and DA are simulated based on this kinetic model assuming the triplet and singlet excited states as the precursor state, respectively (Figure 2, See SI for detail).5, 24 kCRS and the spin relaxation rate at 270 mT (krlx(270 mT)) are estimated from the simulation.
SCHEME 1. Spin-dependent CS state dynamics for BPIPtDA and DA
a)
3
BPIPt*-DA
kT S
T
CS state (B < HFC) kCRS Ground State
Experiment
E
1
D A*
b)
3
1
BPIPt*-DA
D A*
Simulation
T+1 S 334
335
336
337
338
339
T0 T−1
340
Magnetic Field / mT
CS state (B >> HFC)
Figure 4. Experimental (red) and simulated (black) trEPR spectra of spin polarized BPIPt-D+A− observed at 1.5 s after the laser
kCRS Ground State
a) At low magnetic fields. b) At high magnetic fields
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kT
HF krlx
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Optimized parameters for BPIPt-DA are kCRS = 9 × 105 s−1 and krlx(270 mT) = 1.4 × 105 s−1, whereas those for DA are kCRS = 1.0 × 106 s−1 and krlx (270 mT) = 1.6 × 105 s−1. The obtained kCRS for BPIPt-DA is in good agreement with that obtained from the trEPR simulation. It is noteworthy that the kCRS and krlx(270 mT) are nearly unaffected by attachment of BPIPt. This fact demonstrates that we have successfully switched the initial spin state of the CS state by attaching the Pt complex with minimum impact on the electronic and magnetic properties of the CS state. Matching of the kCRS values for BPIPt-DA and DA indicates that the electronic interaction between D+ and A− is not affected by introducing BPIPt on the bridge moiety. The small electronic coupling mediated by meta-substituted benzene ring again would be the reason for negligible substitution effect of BPIPt on VDA. The agreement of the krlx(270 mT) for BPIPt-DA and DA indicates that the SOC of the covalently attached Pt complex does not affect the spin state of the CS state. SOC-induced spin relaxation requires a strong electronic coupling between a heavy atom and a radical molecule, which is absent in BPIPt-DA molecule because of the rigid meta-linkage over the long distance. The MFE on the CR kinetics shown in Figure 2 is explained according to SCHEME 1 with the obtained rate constants. The apparent CR time constants for BPIPt-DA and DA are both ~ 4 s at 0 mT, which is about 4 times longer than 1/kCRS; namely, observed CR rate is kCRS/4 for both the molecules. This is due to the fact that the CS states are in the weak-coupling regime (2J
