Direct Observation of Hole Shift and ... - ACS Publications

Article pubs.acs.org/JPCA

Direct Observation of Hole Shift and Characterization of Spin States in Radical Ion Pairs Generated from Photoinduced Electron Transfer of (Phenothiazine)n−Anthraquinone (n = 1, 3) Dyads Ayumu Karimata,† Shuichi Suzuki,† Masatoshi Kozaki,† Kenshi Kimoto,‡ Koichi Nozaki,*,‡ Hironori Matsushita,§ Noriaki Ikeda,*,§ Kimio Akiyama,*,∥ Daisuke Kosumi,⊥ Hideki Hashimoto,⊥ and Keiji Okada*,† †

Department of Chemistry, Graduate School of Science and ⊥Advanced Research Institute for Natural Science and Technology (OCARINA), Osaka City University, Sumiyoshi-ku, Osaka, 558-8585, Japan ‡ Department of Chemistry, Graduate School of Science and Engineering, University of Toyama, Gofuku, Toyama 930-8555, Japan § Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Sakyo-ku, Kyoto 606-8585, Japan ∥ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan S Supporting Information *

ABSTRACT: Photoinduced intramolecular electron transfer of dyad PTZ3-PTZ2-PTZ1-B-AQ consisting of phenothiazine trimer (PTZ3-PTZ2-PTZ1), bicyclo[2.2.2]octane (B), and anthraquinone (AQ) was investigated. After excitation (∼20 ps) of the AQ moiety in THF, a metastable radical ion pair (RIP) PTZ3-PTZ2-PTZ1+-B-AQ− appeared at ∼620 nm. From 500 ps to 6 ns the spectrum changed to a new absorption (∼950 nm), which was assigned to the holeshifted stable RIP state PTZ3-PTZ2+-PTZ1-B-AQ−. The time constant of the hole-shift process was determined to be 6.0 ns. The hole-shifted RIP state had a lifetime (τ) of 250 ns and was characterized by spin-polarized signals as a spin-correlated radical pair (SCRP) by means of timeresolved ESR. These results were compared with those for the phenothiazine monomer analog PTZ-B-AQ, which also produced the RIP state PTZ+-B-AQ− with τ = 1.9 μs. Time-resolved ESR showed an all emission signal pattern showing the triplet mechanism of PTZ-B-3AQ* → 3[PTZ+B-AQ−]. The origin of the difference in the lifetimes between the trimer and the monomer RIP states was discussed from various points of view, including free energy difference in the RIP states, reorganization energy difference in the charge recombination process, and the spin-state difference. Of these, the spin-state difference effect provided the most reasonable explanation.

■

2.5 × 1012 s−1).10 To date, various donor-linked anthraquinone (D-AQ) systems have been reported.10−20 Most of them are conformationally flexible;13−18 some of them include metal ions (Ru(II)(bpy)313,20 and TiO214). The lifetimes of their RIP states range from a few picoseconds12 to ∼10 μs14 in solution and to several tens of seconds15 in the solid state. In many cases, the spin states of the observed RIPs have not been clarified. Furthermore, Fukuzumi and co-workers reported on a very short-linked D-AQ system, N-phenyl-9,10-anthraquinone2-carboxamide, which had a surprisingly long charge-transfer (CT) lifetime of >900 μs in DMSO.21,22 Shortly after that report, Verhoeven, Harriman, and co-workers studied in detail the same compound in the same solvent, wherein the lifetime of the CT state was found to be 130 ns. They also proposed important experimental criteria for identification of unusually long-lived CT and RIP states.10 To identify typical lifetimes for RIP states generated from a conformationally rigid system we

INTRODUCTION Photoinduced electron transfer of D−A systems (D, electron donors; A, electron acceptor) provides fundamental insight for conversion of light energy to electric and/or chemical energies in a manner that mimics photosynthesis, in which fast and efficient charge separation (CS) and slow charge recombination (CR) are primary essential processes. Various studies, including multistep electron transfer of multichromophoric systems, have been conducted.1,2 However, they do not always form longlived radical ion pair (RIP) states with high efficiency. The most promising strategy is a spin-control approach using a chromophore that undergoes rapid intersystem crossing.3 Recently, electron donor−acceptor systems linked by metal (Ru, Ir, and Pt) complexes have garnered attention because of their ultrafast intersystem crossing to the triplet state, from which CS occurs to form long-lived RIP states.4−9 This principle is also applicable to organic compounds with fast intersystem crossing. 9,10-Anthraquinone (AQ) undergoes very rapid intersystem crossing (within 0.4 ps for 2-substituted anthraquinone; kisc = © 2014 American Chemical Society

Received: September 24, 2014 Published: October 27, 2014 11262

dx.doi.org/10.1021/jp509643q | J. Phys. Chem. A 2014, 118, 11262−11271

The Journal of Physical Chemistry A

Article

phenothiazine ring (plane A/plane B ≈ 3°, Scheme 1), which indicates that the radical cation is mainly localized in the central ring (PTZ3-PTZ2+-PTZ1) and conjugates with the sp2 orbitals on the nitrogen atoms in the outer-folded phenothiazine rings (plane A/plane C ≈ 11°; B/D ≈ 5°, Scheme 1).27 Thus, PTZ3+ exhibits a unique intramolecular CT absorption at ∼950 nm (ε ≈ 3 × 104 M−1·cm−1). A fast geometrical change in PTZ3+ was suggested by its reversible redox behavior, provided that the geometry of the adsorbed state on an electrode is similar to that of the neutral molecule (cyclic voltammogram of PTZ3+ in Figure S1, Supporting Information). In this study, we investigated the photoinduced electron transfer of phenothiazine trimer-bicyclo[2.2.2]octane (B)anthraquinone dyad (PTZ3-B-AQ or PTZ3-PTZ2-PTZ1-BAQ, Scheme 2) and a phenothiazine monomer dyad analog (PTZ-B-AQ, Scheme 2). Excitation of the anthraquinone chromophore produces a 3AQ* state via rapid intersystem crossing.10 Then, the intramolecular electron transfer in this rigid dyad occurs in through-bond fashion3,28 via PTZ3-PTZ2PTZ1+-B-AQ− and PTZ3-PTZ2+-PTZ1-B-AQ−. The initially formed RIP state would be a metastable state and exhibit an absorption similar to that of the phenothiazine monomer radical cation (500−600 nm). The hole-shifted PTZ3-PTZ2+PTZ1-B-AQ− dyad would show a strong absorption at ∼950 nm because of the delocalized structure of PTZ3-PTZ2+PTZ1, where the p orbitals of the central PTZ2 cation are conjugated with the nitrogen p orbitals on the outer PTZ1,3 rings (see PTZ3+ in Scheme 1). Transient absorption studies succeeded to detect both these RIP states (vide infra). The hole-shift process would cause several changes in the character of the RIP states: the stability of the donor radical cation, the center(+)-to-center(−) distance, ΔGRIP, the reorganization energy and the electronic interaction in the CR process, and also the spin-state difference in these RIP states. All these factors should be reflected on the CR rate (the reciprocal of the lifetime) in these RIP states: We found that the CR rate of PTZ3-PTZ2+-PTZ1-B-AQ− was considerably higher (∼8 times in THF) than that of PTZ+-B-AQ− (vide infra). The higher CR rate of the trimer RIP state was discussed in terms of Marcus parameters (reorganization energy and the free energy differences in the RIP states) and their spin-state difference.

studied 10PTZ-B-AQ (10PTZ, 10-phenylphenothizine as a donor) dyad, where the donor and acceptor are separated by ∼19 Å (center-to-center distance) using a rigid bicyclo[2.2.2]octane spacer (B).23 The lifetime of 10PTZ+-B-AQ− was determined to be 1.0 μs and is considered to be long enough to investigate the hole-shift process in the phenothiazine trimer radical cation (vide infra). Phenothiazine (PTZ) is a good electron donor with a butterfly structure (folding angle is 30−40° along the S−N line).24,25 Oxidation of PTZ provides a planar radical cation that absorbs in the visible region (500−600 nm; ε ≈ 104 M−1· cm−1).26,27 We have previously shown that the phenothiazine trimer radical cation (PTZ3+) has a considerably different structure compared to its neutral molecule. In contrast to the phenothiazine trimer neutral state (PTZ3 or PTZ3-PTZ2PTZ1), which has a perpendicular geometry between three phenothiazine rings (PTZ3-PTZ2-PTZ1, plane A/plane B ≈ 31°, Scheme 1), PTZ3+ has a planar form in the central Scheme 1. Structures of PTZ3 and Crystal Structures of Neutral PTZ3 and Radical Cation PTZ3+

Scheme 2. Synthetic Scheme for PTZ3-B-AQ and Structures of Other Related Compounds

11263



■

Article

RESULTS AND DISCUSSION Materials. PTZ3-B-AQ was synthesized through sequential Suzuki−Miyaura coupling reactions as outlined in Scheme 2: A mixture of 3-(4,4,5,5,-tetramethyl-1,3,2-dioxaborolan-2-yl)(phenothiazine trimer) (1) and 1,4-bis(4-bromophenyl)bicyclo[2.2.2]octane (2) was heated in the presence of Pd(OAc)2−Cs2CO3 in DMF-toluene to produce coupling product 3, which was further reacted with 2-(4,4,5,5teramethyl-1,3,2-dioxaborolan-2-yl)-9,10-anthraquinone (4) in the presence of Pd(PPh3)4−K2CO3 in DMF to produce PTZ3B-AQ in 36% yield in the two steps. These procedures are described in the Experimental Section. The preparation of 1 is described in the Supporting Information. PTZ-B-AQ was synthesized from compounds 2, 5, and 4 by similar procedures. Absorption Spectra. The absorption spectrum of PTZ3-BAQ in THF resembled the spectral summation of reference compounds 7 and 8 (Figure 1), which suggests negligible

AQ, PTZ3-B-AQ, and the model compounds (6, 7, and 8) were measured in CH2Cl2 (Table 1). PTZ3-B-AQ and 7 showed Table 1. Redox Potentials,a Center(+)-to-Center(−) Distance in CS States, and ΔGCS from the Triplet of Anthraquinone Moietyb compds

Eox11/2/Va

Ered11/2/Va

R /Å

ΔGRIP/eVb

ΔGCS/eVc

PTZ-B-AQ PTZ3-B-AQ 6 7 8

+0.23 +0.10 +0.23 +0.10

−1.39 −1.38

21.6 26.2

1.59 1.46

−0.75 −0.88

−1.39

a

V vs Fc/Fc+ measured in CH2Cl2 in the presence 0.1 M nBu4NClO4 using glassy carbon as a working electrode and a Pt wire as a counter electrode with a sweep rate of 100 mV/s. bThe ΔGRIP value was calculated using R (center-to-center distance) with reference to the ground state. cThe ΔGCS value was calculated using the value of ET = 2.34 eV for 3AQ*.

lower oxidation potentials than the monomer analogues (PTZB-AQ and 6) because of the conjugated PTZ3-PTZ2+-PTZ1 structure (see PTZ3+ in Scheme 1). The free energy difference (ΔGRIP) between the RIP state and the ground state and the free energy difference in the charge separation (ΔGCS) from the 3 AQ* state (ET = 2.34 eV) were calculated according to eqs 1 and 2, respectively ΔG RIP = e[Eox (D) − Ered(A)] − 14.4/(εR ) + (14.4/r )(1/ε − 1/εref ) ΔGCS = ΔG RIP − E T(3AQ*)

Figure 1. Absorption spectra of PTZ3-B-AQ (black line) and reference compounds 7 (green dotted line), and 8 (red dotted line), and phosphorescence spectrum of 8 at 77 K in THF under an inert atomosphere (inset).

(1) (2)

where ε is the solvent dielectric constant (ε = 7.58 for THF, εref = 8.93 for CH2Cl2), r represents the effective radii that are assumed to be 5 Å31 in both D+ and A−, and R is the center(+)to-center(−)32 separation distance in the RIP states. The molecular model for PTZ3-PTZ2+-PTZ1-B-AQ− was constructed by taking into consideration the X-ray structural data of PTZ3+,27 which has a flat central phenothiazine ring (PTZ2) conjugated with the nitrogen atoms in the outer phenothiazine rings (PTZ1 and PTZ3). The center-to-center distances in the RIP states were approximated to be ∼22 Å for PTZ+-B-AQ− and ∼26 Å for PTZ3+-B-AQ− (Scheme 3). As described in the Introduction, the photoinduced electron transfer of PTZnB- 3 AQ* (n = 1, 3) occurs via an electron-exchange mechanism3,28,32,33 and the sequential electron transfer via 3 [PTZ3-PTZ2-PTZ1+-B-AQ−] and 3[PTZ3-PTZ2+-PTZ1-BAQ−] is expected to occur. The first RIP state would have an energy level similar to that of PTZ+-B-AQ− (ΔGRIP = 1.59 eV) and undergoes the hole-shift process to produce the stable hole-shifted RIP state (ΔGRIP = 1.46 eV), which would be slightly higher (by ∼0.02 eV) if the center-to-center throughbond distance (∼35 Å) is used for the calculation. Scheme 3 also illustrates the HOMOs and LUMOs of the neutral PTZ-BAQ and PTZ3-B-AQ as models of their RIP states.34 PTZ3-BAQ has a HOMO that is essentially localized on the central ring (PTZ2) and the two nitrogen atoms in the outer phenothiazine rings (PTZ1 and PTZ3). This feature is in good agreement with the radical cation structure in the central ring and the large intramolecular CT absorption at ∼950 nm.27 The CS processes from PTZn-B-3AQ* (n = 1, 3) were calculated to be largely

electronic interaction between PTZ3 and AQ chromophores in the ground state. We employed a 355 nm laser light as the excitation source in the transient absorption studies. The AQ chromophore in PTZ3-B-AQ absorbs ∼33% of the incident light at 355 nm for PTZ3-B-AQ (∼48% for PTZ-B-AQ). Judging from the longest edge of absorption, the lowest locally excited singlet state ( 1 LE) is on the anthraquinone chromophore (∼420 nm; ES ≈ 2.95 eV), which can be assigned to an extended π−π* absorption overlapped with an n−π* absorption in the 2-phenyl-AQ moiety.29,30 The excited singlet state of anthraquinone (1AQ*) undergoes ultrafast intersystem crossing to afford 3AQ*, from which a rapid electron transfer occurs to produce RIP states (vide infra). The initially formed excited singlet state of phenothiazines (1PTZ*) would undergo rapid intramolecular energy transfer and/or electron transfer to the anthraquinone moiety. As shown in the section Transient Absorption Studies, the initial species observed for picosecond laser photolysis (∼20 ps after laser pulse) is assigned to the charge-separated states. The triplet energy (ET) of anthraquinone was determined to be 2.34 eV from the phosphorescence spectrum of reference compound 8 (Figure 1, inset). Redox Potentials and Free-Energy Change in Photoinduced Electron Transfer. In order to estimate the stabilities of the RIP states, the redox potentials of PTZ-B11264



Article

Scheme 3. Molecular Models of RIP States and Frontier Orbitals of PTZ-B-AQ and PTZ3-B-AQ

exergonic (ΔGCS = −0.75 eV for PTZ+-B-AQ− and −0.88 eV for PTZ3-PTZ2+-PTZ1-B-AQ−). Electrochemical Oxidation and Reduction of Model Compounds. To assist in the assignment of the transient absorption, we examined the absorption spectra of the model radical ions generated by an electrochemical method. We monitored the absorption spectra using a thin-layer cell during electrochemical oxidation of 6 and 7, which provided approximate absorptions for PTZ+ and PTZ3+, with applied voltages of +0.4 and +0.3 V vs Fc/Fc+ in CH2Cl2, respectively (Figure S2a,b, Supporting Information). Similarly, electrochemical reduction of 8 was performed as a model study for generation of AQ− with an applied voltage of −1.6 V vs Fc/Fc+ in DMF (Figure S2c, Supporting Information). Figure 2 shows

in Figure 3a. Immediately after excitation (∼20 ps), a broad absorption at ∼620 nm appeared. Although this absorption is similar to the T−T absorption of 3AQ* (Figure S3, Supporting Information),23 the absorption maximum and intensity at 620 nm did not change within the time region from 20 ps to 6 ns. This transient species has a small hump at ∼450 nm as shown in the nanosecond transient spectra (Figure 3b). The T−T absorption of 3AQ* from model compound 8 exhibited no absorption in the 450 nm region (Figure S3, Supporting Information), which indicates that the observed absorption is ascribed to the RIP state PTZ+-B-AQ− (Figure 2a, black line). The small difference in the wavelengths of the absorption maxima in Figures 2 and 3 would be due to the different structures between PTZ+-B-AQ− and the model compounds in different solvents under electrolytic conditions. The small 450 nm hump with bleaching at less than 400 nm can be explained by the absorption of PTZ+ (Figure 2a, red line); this observation supports that the observed 620 nm absorption can be ascribed to the RIP state PTZ+-B-AQ−. This assignment is corroborated by the time-resolved ESR study in which a radical pair is observed (vide infra). Thus, the transient absorption studies indicate a very rapid (time constant < 20 ps) transformation of PTZ-B-3AQ* → PTZ+-B-AQ−, which is compatible with the ultrafast intersystem crossing (0.4 ps) of the AQ chromophores.10 This rapid CS is also compatible with the large driving force from the PTZ-B-3AQ* state (ΔGCS = −0.75 eV in Table 1). The RIP state PTZ+-B-AQ− decays slowly during 73−6400 ns with a lifetime of 1.9 μs (Figure 3b). Excitation of PTZ3-B-AQ in THF produced a similar transient absorption (λmax ≈ 620 nm) within 20−100 ps after the laser pulse (Figure 4a). The 620 nm absorption is assigned to PTZ3-PTZ2-PTZ1+-B-AQ−, in which PTZ1+ should have a similar absorption to the phenothiazine monomer RIP state (620 nm in Figure 3a) because of the perpendicular relation between PTZ1+ and PTZ2. Interestingly, the absorption at 620 nm was slightly blue shifted with a decrease in the intensity in the 100 ps to 6 ns time region. Simultaneously, a new absorption at 950 nm developed (Figure 4a). These spectral changes clearly indicate the occurrence of the hole-shift process, PTZ3-PTZ2-PTZ1+-B-AQ− → PTZ3-PTZ2+-PTZ1B-AQ−, and can be interpreted as follows: (1) the decreased intensity of 620 nm absorption can be explained by the disappearance of the PTZ1+ absorption (630 nm in Figure 2a, red line) to produce blue-shifted overlapped absorptions of PTZ3+ and AQ− (594 nm in Figure 2b); (2) the newly appeared absorption at 950 nm can be ascribed to formation of PTZ3-PTZ2+-PTZ1-B-AQ− (Figure 2b, red line). The time constant of the hole-shift process was determined to be 6 ns (khole‑shift = 1.7 × 108 s−1) by monitoring the development of the 950 nm absorption. PTZ3-PTZ2+-PTZ1-B-AQ− decayed

Figure 2. Absorption spectra of (a) 6+ (red, in CH2Cl2) and 8− (green, in DMF) and their summation (black, simulation for PTZ+-B-AQ−) and (b) 7+ (red, in CH2Cl2) and 8− (green, in DMF) and their summation (black, simulation for PTZ3+-B-AQ−).

the absorption spectra of the model compound radical ions (6+, 7+, and 8−) and their summation spectra for estimation of the absorptions of the RIP states, PTZn+-B-AQ− (n = 1, 3). Transient Absorption Studies. Excitation of PTZ-B-AQ in a THF solution with a picosecond laser pulse (355 nm; fwhm 17 ps) produced the transient absorption spectra shown 11265



Article

Figure 3. Transient absorption spectra for PTZ-B-AQ from (a) 20 ps to 6 ns and (b) 13 to 6400 ns after the laser pulse (355 nm) in THF, and (c) time-resolved ESR spectrum for PTZ-B-AQ measured at 500 ns after the laser pulse (355 nm) in THF: (solid line) observed ESR spectrum, (dotted line) simulation spectrum using the parameters described in the text, (A) absorption direction, (E) emission direction in an integrated mode ([PTZB-AQ] = 7.6 × 10−5 (a), 3.8 × 10−5 (b), and 4.0 × 10−4 M (c)).

Figure 4. Transient absorption spectra for PTZ3-B-AQ from (a) 20 ps to 6 ns and (b) 13 to 900 ns after the laser pulse (355 nm) in THF, and (c) time-resolved ESR spectrum for PTZ3-B-AQ measured at 500 nm after the laser pulse (355 nm) in THF: (solid line) observed ESR spectrum, (dotted line) simulation spectrum using the parameters described in the text, (A) absorption direction, (E) emission direction ([PTZ3-B-AQ] = 5.2 × 10−5 (a), 2.6 × 10−5 (b), and 4.0 × 10−4 M (c)).

in the 43−900 ns range, and the lifetime of the RIP state was determined to be 250 ns (Figure 4b). We found considerably different lifetimes of the RIP states between PTZ-B-AQ and PTZ3-B-AQ: τ = 1.9 μs for PTZ+-BAQ− and τ = 250 ns for PTZ3-PTZ2+-PTZ1-B-AQ− in THF. A similar trend was also observed in PhCN: τ = 1.4 μs for PTZ+-B-AQ− and τ = 140 ns for PTZ3-PTZ2+-PTZ1-B-AQ− (Figure S4, Supporting Information). We also determined the efficiencies of the formations of RIP states from the molar absorptivities of the radical ions and the number of photons (at the 355 nm excitation), which were estimated using [Ru(bpy)3]2+ as a chemical actinometer according to our previously reported procedure.35 The quantum yields for formation of RIP states were determined to be Φ = 0.78 (THF) and 0.70 (PhCN) for PTZ+-B-AQ− (monitored at 620 nm) and Φ = 0.36 (THF) and 0.22 (PhCN) for PTZ3-PTZ2+-PTZ1-B-AQ− (monitored at 950 nm). During the hole-shift process, a large geometrical change occurs from the metastable PTZ3-PTZ2-PTZ1+-B-AQ−, wherein the planar PTZ1+ ring is attached almost orthogonally36 to the benzo ring in PTZ2, to the most stable PTZ3PTZ2+-PTZ1-B-AQ−, wherein the planar PTZ2+ ring is conjugated with the nitrogen atoms in the PTZ3 and PTZ1 rings. The observed hole-shift process is comparable but slightly faster than that from chemically generated Ru(bpy)33+ to the directly bound bulky tetramethoxybenzene (τhole‑shift = 24 ns) in the related system.37

In the present hole-shift process, the bond rotation around the C(in PTZ2)−N(in PTZ1 or PTZ3) bond is essential. In general, the rates of bond rotations are strongly dependent on the molecular structures. The time constant of the internal bond rotation of ethane is reported to be ultrafast with τrot = ∼12 ps at room temperature in solution.38 The flipping of 2,2′trimethylenebiphenyl requires τrot = ∼10 μs.39,40 The rotation of the phenyl group in 10-phenylanthracene derivatives occurs on a much longer time scale, τrot = ∼2 s at 80 °C.41 The rate of rotation of the phenyl ring in 10-phenylphenothiazine should be much faster than that of 10-phenylanthracene because the steric repulsions between the ortho protons in the phenyl group and the peri-positioned protons in the phenothiazine can be quite diminished by the pyramidalization of the nitrogen atom. The rate constant of phenyl rotation in 10-phenylphenothiazine has not been determined experimentally, although dynamic NMR of phenothiazine derivatives at low temperatures has been conducted.42,43 The observed hole-shift process (τhole‑shift = 6 ns) in the present study can serve as an upper limit in the time constant for phenyl rotation in 10phenylphenothiazines. Time-Resolved ESR Studies. To obtain detailed information on the observed RIP states, we applied the time-resolved ESR technique for the RIP states generated from PTZ-B-AQ (Figure 3c) and PTZ3-B-AQ (Figure 4c) in THF. Excitation of PTZ-B-AQ at room temperature produced spin-polarized signals (Figure 3c) monitored at 500 ns after the laser pulse 11266



Article

thus, −ΔGRIP corresponds to the driving force of the CR process. Assuming that the reorganization energy of CR of PTZn+-BAQ− (n = 1, 3) is invariant for the donor structures PTZ and PTZ3, the higher CR rate (by ∼8 times; kCR = 4.0 × 106 s−1, τRIP = 250 ns) of PTZ3-PTZ2+-PTZ1-B-AQ− compared to that of PTZ+-B-AQ− (kCR = 5.3 × 105 s−1, τRIP = 1.9 μs) cannot be explained by the small energy difference (ΔΔGRIP = 0.13 eV) between the driving forces in the Marcus normal or inverted regions, in the latter a small enhancement of the CR rate in the trimer system may be expected. Furthermore, the electronic interaction (electron matrix element VDA)55−57 is worthy of comment. In particular, the VDA value in PTZ3-PTZ2+-PTZ1-B-AQ− should be smaller than that of PTZ+-B-AQ− because of the longer distance between the spin centers in the RIP states. This effect would reduce the CR rate of PTZ3-PTZ2+-PTZ1-B-AQ−. Thus, the electric effect of PTZ3-PTZ2+-PTZ1-B-AQ− is opposite to the effect in the observed CR process. The CR process of the trimer system (PTZ3-PTZ2+-PTZ1B-AQ− → PTZ3-PTZ2-PTZ1-B-AQ) involves a much larger geometrical change than that (PTZ+-B-AQ− → PTZ-B-AQ) of the monomer system; this results in a larger reorganization energy for the trimer system. We herein estimate the reorganization energies of CR of PTZn+-B-AQ− (n = 1, 3) using density functional theory (DFT). In order to estimate the energy of the RIP states (ΔERIP), geometrical optimizations for both the singlet ground state and the triplet excited state of PTZ3-PTZ2-PTZ1-B-AQ were performed using the UB3LYP/ 6-31G level of theory, in which the SCRF58 calculations were achieved using a polarizable continuum model (PCM)58 under the solvent (THF) field. The spin-density map of the optimized triplet state clearly shows that the calculated state is the expected RIP state (Figure S7). Thus, ΔERIP of 3[PTZ3PTZ2+-PTZ1-B-AQ−] in THF was calculated to be 1.45 eV above the fully optimized ground state (1GS); this finding is in excellent agreement with the experimental ΔGRIP value (1.46 eV in THF, in Table 1). The internal reorganization energy (λi) for the CR process can be obtained as the energy difference between the fully optimized ground state and the ground state at the 3RIP state geometry; this was estimated to be 0.70 eV. The solvent reorganization energy (λS) can be expressed using two spheres in a dielectric continuum model as follows56,57

(355 nm). The polarized signals consist of three emissive signals split by ∼0.34 mT. These features are very similar to the spin polarization pattern observed for 10PTZ+-B-AQ− in our previous paper23 and can be assigned to the RIP state PTZ+-BAQ−. The simulation can be achieved using the following ESR parameters: for PTZ+ g = 2.0055, aN(1N) = 0.66 mT, aH1 (1H) = 0.24 mT, aH2 (2H) = 0.11 mT; for AQ− g = 2.0045, aH1 (3H) = 0.098 mT, aH2 (4H) = 0.025 mT, |J| > 1.5 mT) (Figure S5, Supporting Information). The J value could not be precisely determined because the simulated three-line shape with nearly equal intensities was insensibly altered when |J| > 1.5 mT. The observed all-emission pattern can be explained by the triplet mechanism44,45 for generation of the RIP state, PTZ-B-3AQ* → 3[PTZ+-B-AQ−], which is a pure triplet state, and deactivation to the singlet ground state is forbidden and slow. Similar excitation of PTZ3-B-AQ produced a totally different spin-polarization pattern (Figure 4c). The observed AEAEAE pattern (A (absorption) in the lowest field and E (emission) in the highest field have low intensities and difficult to see) is characteristic for a spin-correlated radical-pair (SCRP) state.9,46−49 The observed phase pattern was reproducible by assuming a triplet precursor (PTZ3-B-3AQ*) of the SCRP state (PTZ3-PTZ2+-PTZ1-B-AQ−) using J = +0.8 ± 0.2 mT with other simulation parameters (1/T1 106 s−1 and 1/T2 5 × 107 s−1 for PTZ3+, 1/T1 106 s−1and 1/T2 2 × 107 s−1 for AQ−, S−T dephasing rate constant = 8 × 107 s−1, recombination rate constant = 4.0 × 106 s−1 from τRIP(PTZ3-PTZ2+-PTZ1-B-AQ− ) = 250 ns) and the following ESR parameters: for PTZ3+ g = 2.0041, aN(1N) = 0.665 mT, aH (2H) = 0.110 mT; for AQ− g = 2.0045, aH1 (3H) = 0.098 mT, aH2 (4H) = 0.025 mT (Figure S6, Supporting Information). In general, the SCRP states are produced when the exchange interaction in the radical pair becomes as small as the electron− nuclear hyperfine interactions (a few mT order). Because of the small energy gap between the singlet and the triplet states, mixing of these states occurs to generate four magnetic states (S0 − δT0, T0 + δS0, T−1, and T+1 for positive J), in which the energy gap between T0 + δS0 and S0 − δT0 is defined as 2J and the dephasing transformation of these states is facilitated by the hyperfine interactions. Recently, Wasielewski and co-workers investigated photoinduced electron transfer using a series of perylene diimide (PDI)-PTZ dyads with phenylene linkage (the center-to-center distance between PDI and PTZ is ca. 15− 25 Å).50−54 The 1PDI*-PTZ state produces the 1[PDI−-PTZ+] state, which experiences fast hyperfine-induced radical pair intersystem crossing (RP-ISC) to give 3[PDI−-PTZ+], and vice versa, to give the SCRP states, from which the spin-sorted CR processes 3[PDI−-PTZ+] → 3PDI*-PTZ and 1[PDI−-PTZ+] → PDI−PTZ (ground state) occur. In the present case, PTZ3B-3AQ*-derived hole-shifted 3[PTZ3-PTZ2+-PTZ1-B-AQ−] is selectively produced, which undergoes rapid RP-ISC to give 1 [PTZ3-PTZ2+-PTZ1-B-AQ−]. In our system, no fastdeactivation process from 3[PTZ3-PTZ2+-PTZ1-B-AQ−] is present. The 1[PTZ3-PTZ2+-PTZ1-B-AQ−] state undergoes the spin-allowed fast deactivation to the ground state. Lifetimes, Marcus Theory, and Spin-State Effects in PTZ+-B-AQ− and PTZ3-PTZ2+-PTZ1-B-AQ− Systems. We determined the following important rate parameters: τRIP = 1.9 μs, ΔGRIP = 1.59 eV, Rcenter‑to‑center = 22 Å for PTZ+-B-AQ−, τRIP = 250 ns, ΔGRIP = 1.46 eV, Rcenter‑to‑center = 26 Å for PTZ3PTZ2+-PTZ1-B-AQ−. The ΔGRIP value is equal to the energy difference between the RIP and the ground states (Table 1);

⎛1 1 ⎞⎛ 1 1 1⎞ + − ⎟ λS = 14.4(Δe)2 ⎜ 2 − ⎟⎜ ⎝n ε ⎠⎝ 2rA 2rD R⎠

(3)

where Δe is the number of transferred electrons, n is the refractive index, ε is the dielectric constant, and rA and rD are the radius of the acceptor radical anion and the donor radical cation moieties, respectively. The λS value was calculated to be 1.07 eV using the following parameters: n = 1.41, ε = 7.58 for THF, rA = 3.6 Å for AQ−, rD = 5.3 Å for PTZ3-PTZ2+-PTZ1, and R = 26.2 Å. The total reorganization energy (λ = λi + λS) was calculated to be 1.77 eV. A similar approach was employed to calculate CR of the monomer system. The ΔERIP value was determined to be 1.75 eV in THF, which is slightly higher than ΔGRIP estimated from redox potentials (1.59 eV in THF, in Table 1). In addition, λi and λS were estimated to be 0.40 and 1.21 eV, respectively. The parameters used for the latter are n = 1.41, ε = 7.58 for THF, rA = 3.6 Å for AQ−, rD = 4.0 Å for PTZ, and R = 21.6 Å. λ was calculated to be 1.61 eV for the monomer system. 11267



■

Thus, CR of PTZ3+-B-AQ− → PTZ3-B-AQ occurs in a normal region with λ = 1.77 eV at ΔGRIP = 1.46 eV; PTZ+-BAQ− → PTZ-B-AQ occurs in an almost top region with λ = 1.61 eV at ΔGRIP = 1.59 eV. However, such reorganization and free energy effects cannot explain the faster CR rate observed for the trimer system. During the above energy calculations we also calculated the energy gaps (ΔERIP-ST) between the 3RIP (lower) and the 1RIP (higher) states in PTZn-B-AQ (n = 1, 3) using TD-UB3LYP/6311G* calculations on the 3RIP geometries. The ΔERIP-ST values were roughly as small as a mT order for both the monomer and the trimer systems; the monomer RIP state was found to be larger by 1 order of magnitude than that of the trimer RIP state.59 Interestingly, the trimer system with the smaller ST gap undergoes faster CR. In addition to Marcus effects, we considered the spin-state effect. PTZ+-B-AQ− has a longer lifetime (τRIP = 1.9 μs) with R = 22 Å and |J| > 1.5 mT (where J is the exchange interaction expressing 2J as the energy gap between the singlet and the triplet states in the RIP states; a plus sign indicates a system with a lower triplet energy state). All emission signals of PTZ+B-AQ− observed in the time-resolved ESR indicate the triplet mechanism,44,45 where the RIP state, generated from 3AQ (PTZ-B-3AQ* → 3[PTZ+-B-AQ−]), has a pure triplet character. Therefore, the decaying process from 3[PTZ+-BAQ−] to the singlet ground state is spin forbidden and slow. However, if the distance between the ion pairs is extended (∼26 Å) for PTZ3-PTZ2+-PTZ1-B-AQ−, the exchange interaction decreases to the values close to an order of hyperfine interaction (J ≈ +0.8 mT). In such a condition, singlet−triplet mixing occurs to produce SCRP states: first, PTZ3-B-3AQ*-derived 3[PTZ3-PTZ2+-PTZ1-B-AQ−] undergoes rapid RP-ISC to produce 1[PTZ3-PTZ2+-PTZ1-B-AQ−]. Then, these states become an equilibrated mixture through a readily available hyperfine interaction. In contrast to pure triplet RIP state 3[PTZ+-B-AQ−] for the monomer system, the trimer RIP state can deactivate to the ground state through hyperfineinduced 1[PTZ3-PTZ2+-PTZ1-B-AQ−]. In summary, we directly observed the hole-shift process of PTZ3-PTZ2-PTZ1+-B-AQ− → PTZ3-PTZ2+-PTZ1-B-AQ− and determined the rate constant of khole‑shift = 1.7 × 108 s−1 (τ = 6.0 ns). Furthermore, considerably different lifetimes were observed between PTZ+-B-AQ− (τ = 1.9 μs) and PTZ3PTZ2+-PTZ1-B-AQ− (250 ns) in THF (PTZ+-B-AQ− (1.4 μs) and PTZ3-PTZ2+-PTZ1-B-AQ− (140 ns) in PhCN). To rationalize the difference in the lifetimes, we considered various factors including the free energy difference between the RIP and the ground states, reorganization energy effects, and the electronic effect on CR in the Marcus parabola. However, a clear explanation could not be obtained using these approaches. The spin-state difference was then considered based on timeresolved ESR experiments exhibiting the triplet radical pair for PTZ+-B-AQ− and the SCRP for PTZ3-PTZ2+-PTZ1-B-AQ−. The produced radical pair [PTZ+-B-AQ−] showed a clear triplet state with |J| > 1.5 mT, and the decay from 3[PTZ+-BAQ−] to the ground state PTZ-B-AQ is a spin forbidden and slow process. In contrast, the SCRP state of PTZ3-PTZ2+PTZ1-B-AQ− (with J = +0.8 ± 0.2 mT) showed a high S ↔ T dephasing rate induced by hyperfine interactions, and deactivation to the ground state readily occurs though the 1 [PTZ3-PTZ2+-PTZ1-B-AQ−] state.

Article

EXPERIMENTAL SECTION

Preparations of 1 and PTZ-B-AQ are described in the Supporting Information. 3-{4-[4-(4-Bromophenyl)bicyclo[2.2.2]oct-1-yl]phenyl}-10′-phenyl-10′H-10,3′:7′,10″-terphenothiazine (3). Compound 1 (200 mg, 0.251 mmol), compound 2 (211 mg, 0.502 mmol), palladium(II) acetate (5.68 mg, 25.3 μmol), and cesium carbonate (235 mg, 0.721 mmol) were placed into a two-necked 50 mL round-bottomed flask. Anhydrous DMF (10 mL) was added into the flask, and the mixture was stirred at 110 °C for 0.5 h under a N2 atmosphere. After cooling to room temperature, the mixture was filtered through a Celite pad. The Celite pad was washed with CH2Cl2, and the filtrate was combined and concentrated under reduced pressure. The crude product was purified by alumina column chromatography using an eluent of hexane−CH2Cl2 (5:1 v/v) to afford 3 (154 mg, 61%) as a yellow solid. 3: C62H46BrN3S3; MW 1009.15; yellow powder; mp 222−224 °C; 1H NMR (400 MHz, DMSO-d6) δ (ppm) 7.77 (t, J = 7.3 Hz, 2H), 7.67−7.62 (m, 3H), 7.50−7.47 (m, 3H), 7.41−7.22 (m, 9H), 7.07−7.03 (m, 5H), 6.99−6.95 (m, 3H), 6.87−6.83 (m, 3H), 6.40−6.37 (m, 2H), 6.31−6.27 (m, 4H), 1.91 (s, 6H), 1.90 (s, 6H); IR (KBr/cm−1) 3061, 2937, 2916, 2860, 1593, 1578, 1491, 1464, 1441, 1306, 1259, 1232, 812, 745, 700; MS (FAB+) = 1007 [M+]; HRMS (FAB+) found m/z 1007.2047, calcd for C62H46BrN3S3 m/z 1007.2037. 2-(4-{4-[4-(10′-Phenyl-10′H-10,3′:7′,10″-terphenothiaz in-3- yl)phe nyl ]bi cycl o[2.2.2]oc t- 1-yl }ph eny l)anthracene-9,10-dione (PTZ3-B-AQ). Compound 3 (152 mg, 0.151 mmol), tetrakis(triphenylphosphino)palladium(0) (18.0 mg, 15.6 μmol), compound 4 (51.0 mg, 0.153 mmol), and potassium carbonate (113 mg, 0.347 mmol) were placed into a two-necked 50 mL round-bottomed flask. Anhydrous DMF (5 mL) and anhydrous toluene (5 mL) were added into the flask, and the mixture was stirred at 120 °C for 3 h under a N2 atmosphere. After cooling to room temperature, the mixture was filtered through a Celite pad. The Celite pad was washed with CH2Cl2, and the filtrate was combined and concentrated under reduced pressure. The crude product was purified by alumina column chromatography using an eluent of hexane− CH2Cl2 (2:1 v/v) to afford PTZ3-B-AQ (101 mg, 59%) as a yellow solid. PTZ3-B-AQ: C76H53N3O2S3; MW 1136.43; yellow powder; mp 233−235 °C; 1H NMR (400 MHz, DMSO-d6) δ (ppm) 8.43 (s, 1H), 8.31−8.24 (m, 4H), 7.97−7.95 (m, 2H), 7.83−7.76 (m, 4H), 7.67−7.57 (m, 5H), 7.51 (d, J = 8.4 Hz, 2H), 7.42 (d, J = 8.8 Hz, 2H), 7.33 (s, 1H), 7.26−7.23 (m, 3H), 7.07−6.96 (m, 8H), 6.87−6.84 (m, 3H), 6.40−6.27 (m, 6H), 1.96 (s, 12H); IR (KBr/cm−1) 3061, 2939, 2914, 2860, 1674, 1593, 1491, 1466, 1443, 1306, 997, 932, 814, 745, 712, 702; MS (FAB+) = 1136.6 [M+]; HRMS (FAB+) found m/z 1135.3307, calcd for C76H53N3O2S3 m/z 1135.3299. Anal. Calcd for C76H53N3O2S3: C, 80.32; H, 4.70; N, 3.70. Found: C, 79.90; H, 4.61; N, 3.70. Laser Photolysis. Nanosecond time-resolved difference absorption spectra were obtained using the third harmonic of a Q-switched Nd3+:YAG laser (Continuum Surelite I-10, λ = 355 nm). Sample solutions in a 1 cm quartz cell were deaerated by bubbling with argon for 5 min. White light from an intensified Xe-arc lamp was passed through the photoexcited sample solutions and monochromated using a grating monochromator (H-10 infrared model, Jobin Yvon). The intensity of the monochromated light was measured using a Si avalanche photodiode (S5139, Hamamatsu), amplified through a wide11268



Article

(5) Flamigni, L.; Collin, J.-P.; Sauvage, J.-P. Iridium Terpyridine Complexes as Functional Assembling Units in Arrays for the Conversion of Light Energy. Acc. Chem. Res. 2008, 41, 857−871. (6) Flamigni, L.; Baranoff, E.; Collin, J.-P.; Sauvage, J.-P. A Triad Based on an Iridium(III) Bisterpyridine Complex Leading to a ChargeSeparated State with a 120-μs Lifetime at Room Temperature. Chem.Eur. J. 2006, 12, 6592−6606. (7) Chakraborty, S.; Wadas, T. J.; Hester, H.; Schmehl, R.; Eisenberg, R. Platinum Chromophore-Based Systems for Photoinduced Charge Separation: A Molecular Design Approach for Artificial Photosynthesis. Inorg. Chem. 2005, 44, 6865−6878. (8) McGarrah, J. E.; Kim, Y.-J.; Hissler, M.; Eisenberg, R. Toward a Molecular Photochemical Device: A Triad for Photoinduced Charge Separation Based on a Platinum Diimine Bis(acetylide) Chromophore. Inorg. Chem. 2001, 40, 4510−4511. (9) Suzuki, S.; Sugimura, R.; Kozaki, M.; Keyaki, K.; Nozaki, K.; Ikeda, N.; Akiyama, K.; Okada, K. Highly Efficient Photoproduction of Charge-Separated States in Donor−Acceptor-Linked Bis(acetylide) Platinum Complexes. J. Am. Chem. Soc. 2009, 131, 10374−10375. (10) van Ramesdonk, H. J.; Bakker, B. H.; Groeneveld, M. M.; Verhoeven, J. W.; Allen, B. D.; Rostron, J. P.; Harriman, A. Ultrafast Intersystem Crossing in 9,10-Anthraquinones and Intramolecular Charge Separation in an Anthraquinone-Based Dyad. J. Phys. Chem. A 2006, 110, 13145−13150. (11) Schuster, G. B. Long-Range Charge Transfer in DNA: Transient Structural Distortions Control the Distance Dependence. Acc. Chem. Res. 2000, 33, 253−260. (12) Lewis, F. D.; Thazhathveetil, A. K.; Zeidan, T. A.; Vura-Weis, J.; Wasielewski, M. R. Dynamics of Ultrafast Singlet and Triplet Charge Transfer in Anthraquinone−DNA Conjugates. J. Am. Chem. Soc. 2010, 132, 444−445. (13) Striplin, D. R.; Reece, S. Y.; McCafferty, D. G.; Wall, C. G.; Friesen, D. A.; Erickson, B. W.; Meyer, T. J. Solvent Dependence of Intramolecular Electron Transfer in a Helical Oligoproline Assembly. J. Am. Chem. Soc. 2004, 126, 5282−5291. (14) Zhang, H.; Zhou, Y.; Zhang, M.; Shen, T.; Li, Y.; Zhu, D. Photoinduced Charge Separation across Colloidal TiO 2 and Fluorescein Derivatives. J. Phys. Chem. B 2002, 106, 9597−9603. (15) Luzzati, S.; Scharber, M.; Catellani, M.; Giacalone, F.; Segura, J. L.; Martin, N.; Neugebauer, H.; Sariciftci, N. S. Long-Lived Photoinduced Charges in Donor−Acceptor Anthraquinone-Substituted Thiophene Copolymers. J. Phys. Chem. B 2006, 110, 5351−5358. (16) Gómez, R.; Blanco, R.; Veldman, D.; Segura, J. L.; Janssen, R. A. J. Synthesis and Photophysical Properties of Conjugated Polymers with Pendant 9,10-Anthraquinone Units. J. Phys. Chem. B 2008, 112, 4953−4960. (17) Li, L.; Shen, S.; Yu, Q.; Zhou, Q.; Xu, H. Photoinduced Electron Transfer and Charge Separation in Anthraquinone-substituted Porphyrin−Phthalocyanine Heterodimer. J. Chem. Soc., Chem. Commun. 1991, 619−620. (18) Tkachenko, N. V.; Tauber, A. Y.; Grandell, D.; Hynninen, P. H.; Lemmetyinen, H. Light-Induced Electron Transfer in Pyropheophytin−Anthraquinone and Phytochlorin−Anthraquinone Dyads: Influence of Conformational Exchange. J. Phys. Chem. A 1999, 103, 3646− 3656. (19) Gouloumis, A.; González-Rodríguez, D.; Vázquez, P.; Torres, T.; Liu, S.; Echegoyen, L.; Ramey, J.; Hug, G. L.; Guldi, D. M. Control Over Charge Separation in Phthalocyanine−Anthraquinone Conjugates as a Function of the Aggregation Status. J. Am. Chem. Soc. 2006, 128, 12674−12684. (20) Hankache, J.; Wenger, O. S. Microsecond Charge Recombination in a Linear Triarylamine−Ru(bpy)32+−Anthraquinone Triad. Chem. Commun. 2011, 47, 10145−10147. (21) Okamoto, K.; Hasobe, T.; Tkachenko, N. V.; Lemmetyinen, H.; Kamat, P. V.; Fukuzumi, S. Drastic Difference in Lifetimes of the Charge-Separated State of the Formanilide−Anthraquinone Dyad versus the Ferrocene−Formanilide−Anthraquinone Triad and Their Photoelectrochemical Properties of the Composite Films with Fullerene Clusters. J. Phys. Chem. A 2005, 109, 4662−4670.

band amplifier (DC-500 MHz, CLC110), and accumulated on a digitizing oscilloscope (HP 54520, Hewlett-Packard). For measurements of picosecond time-resolved difference spectra, a sample solution in a quartz cell (1 cm length) was excited with the third-harmonic pulses of a mode-locked Nd3+:YAG laser (Continuum PY61C-10, λ = 355 nm). Transient absorption spectra in the time range from −20 ps to 6 ns were acquired using continuum pulses generated by focusing the fundamental laser pulse into a flowing H2O/D2O mixture (1:1 by volume). Time-Resolved ESR Study. Time-resolved ESR (TRESR) measurements were performed using an X-band EPR spectrometer (Varian E109E) without field modulation as reported elsewhere.60 A Nd:YAG laser (Quanta-ray INDI-40, 355 nm) was used as the light pulse source. Sample solutions prepared at 4 × 10−4 M were degassed on a vacuum line by several freeze−pump−thaw cycles. Simulation of the polarized spectra was performed using the Matlab-based simulation tool box Easyspin61 or calculated from the equation of the motion of the radical-pair spin density in Liouville space according to the formulation of Maeda et al.61−63

■

ASSOCIATED CONTENT

* Supporting Information S

Complete citation of refs 31 and 34; additional experimental procedures, cyclic voltammograms of PTZn-B-AQ, UV−vis absorptions of electrochemically generated model radical ions, transient absorption of model compound 8, transient absorption of RIP states for PTZn-B-AQ in PhCN, ESR spectra of PTZ3+·ClO4− and Na+·AQ− and PTZ3+·PF6−· 0.5CH3CN, and spin density maps of PTZn+-B-AQ−. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

[email protected]. [email protected]. [email protected]. [email protected].

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This study was supported by grants (no. 26288041 for K.O., no. 24550062 for S.S.) from JSPS. We thank to Dr. Kiminori Maeda (Associate Professor in Saitama University, Graduate School of Science and Engineering) for assistance with analysis of the time-resolved EPR spectra.

■

REFERENCES

(1) Kavarnos, G. J. Fundamentals of Photoinduced Electron Transfer; Wiley: New York, 1993. (2) Wasielewski, M. R. Energy, Charge, and Spin Transport in Molecules and Self-Assembled Nanostructures Inspired by Photosynthesis. J. Org. Chem. 2006, 71, 5051−5066. (3) Verhoeven, J. W. On the Role of Spin Correlation in the Formation, Decay, and Detection of Long-lived, Intramolecular Charge-transfer States. J. Photochem. Photobiol. C 2006, 7, 40−60. (4) Borgström, M.; Shaikh, N.; Johansson, O.; Anderlund, M. F.; Styring, S.; Åkermark, B.; Magnuson, A.; Hammarström, L. Light Induced Manganese Oxidation and Long-Lived Charge Separation in a Mn2II,II−RuII(bpy)3−Acceptor Triad. J. Am. Chem. Soc. 2005, 127, 17504−17515. 11269



Article

(22) Okamoto, K.; Araki, Y.; Ito, O.; Fukuzumi, S. A Dramatic Elongation of the Lifetime of Charge-Separated State by Complexation with Yttrium Triflate in Ferrocene−Anthraquinone Linked Dyad. J. Am. Chem. Soc. 2004, 126, 56−57. (23) Karimata, A.; Kawauchi, H.; Suzuki, S.; Kozaki, M.; Ikeda, N.; Keyaki, K.; Nozaki, K.; Akiyama, K.; Okada, K. Photoinduced Charge Separation of 10-Phenyl-10H-phenothiazine−2-Phenylanthraquinone Dyad Bridged by Bicyclo[2.2.2]octane. Chem. Lett. 2013, 42, 794−796. (24) McDowell, J. J. H. The Crystal and Molecular Structure of Phenothiazine. Acta Crystallogr. 1976, B32, 5−10. (25) Klein, C. C.; Conrad, J. M., III; Morris, S. Structure of NPhenylphenothiazine, C18H13NS. Acta Crystallogr. 1985, C41, 1202− 1204. (26) Wagner, E.; Filipek, S.; Kalinowski, M. K. Visible Absorption Spectra of the Phenothiazine Radical Cation and Its 10-Substituted Derivatives. Monatsh. Chem. 1988, 119, 929−932. (27) Okamoto, T.; Kuratsu, M.; Kozaki, M.; Hirotsu, K.; Ichimura, A.; Matsushita, T.; Okada, K. Remarkable Structure Deformation in Phenothiazine Trimer Radical Cation. Org. Lett. 2004, 6, 3493−3496. (28) Oevering, H.; Paddon-Raw, M. N.; Heppener, M.; Oliver, A. M.; Cotsaris, E.; Verhoeven, J. W.; Hush, N. S. Long-range Photoinduced Through-bond Electron Transfer and Radiative Recombination via Rigid Nonconjugated Bridges: Distance and Solvent Dependence. J. Am. Chem. Soc. 1987, 109, 3258−3269. (29) Shcheglova, N. A.; Shigorin, D. N.; Dokunikhin, N. S. The Luminescence and Absorption Spectra of α- and β-Alkyl and -Arylderivatives of Anthraquinone in Solution. I. Absorption Spectra of αand β-Alkyl and -Aryl-derivatives of Anthraquinone. Russ. J. Phys. Chem. 1965, 39, 1622−1623. (30) Val’kova, G. A.; Shigorin, D. N.; Shcheglova, N. A. The Photochemical Properties of Molecules with Lowest Tπ,π* or T(π,l),π* States. Russ. J. Phys. Chem. 1978, 52, 631−633. (31) Lor, M.; Viaene, L.; Pilot, R.; Fron, E.; Jordens, S.; Schweitzer, G.; Weil, T.; Müllen, K.; Verhoeven, J. W.; Van der Auweraer, M.; et al. J. Phys. Chem. B 2004, 108, 10721−10731. (32) Moser, C. C.; Page, C. C.; Farid, R.; Dutton, P. L. Biological electron transfer. J. Bioenerg. Biomembr. 1995, 27, 263−274. (33) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Modern Molecular Photochemistry of Organic Molecules; University Science Books: Califormia, 2010. Chapter 7, pp 383−481. (34) Calculation was achieved using Gaussian 09 (B3LYP/3-21G): Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision C.01; Gaussian, Inc.: Wallingford, CT, 2009. (35) Torieda, H.; Nozaki, K.; Yoshimura, A.; Ohno, T. Low Quantum Yields of Relaxed Electron Transfer Products of Moderately Coupled Ruthenium(II)−Cobalt(III) Compounds on the Subpicosecond Laser Excitation. J. Phys. Chem. A 2004, 108, 4819−4829. (36) Sergiy, D. S.; Rosokha, V.; Kochi, J. K. Donor−Acceptor (Electronic) Coupling in the Precursor Complex to Organic Electron Transfer: Intermolecular and Intramolecular Self-Exchange between Phenothiazine Redox Centers. J. Am. Chem. Soc. 2004, 126, 1388− 1401. (37) Walther, M. E.; Wenger, O. S. Hole Tunneling and Hopping in a Ru(bpy)32+-Phenothiazine Dyad with a Bridge Derived from oligo-pPhenylene. Inorg. Chem. 2011, 50, 10901−108907. (38) Zheng, J.; Kwak, K.; Xie, J.; Fayer, M. D. Ultrafast CarbonCarbon Single-Bond Rotational Isomerization in Room-Temperature Solution. Science 2006, 313, 1951−1955. (39) Müllen, K.; Heinz, W.; Klärner, F.-G.; Roth, W. R.; Kindermann, I.; Adamczak, O.; Wette, M.; Lex, J. Inversions barrier en Ortho,ortho′-verbrückter Biphenyle. Chem. Ber. 1990, 123, 2349− 2371. (40) Rotzler, J.; Gsellinger, H.; Bihlmeier, A.; Gantenbein, M.; Vonlanthen, D.; Häussinger, D.; Klopper, W.; Mayor, M. Atropisomerization of Di-para-substituted Propyl-bridged Biphenyl Cyclophanes. Org. Biomol. Chem. 2013, 11, 110−118.

(41) Nikitin, K.; Mü ller-Bunz, H.; Ortin, Y.; Muldoon, J.; McGlinchey, M. J. Restricted Rotation in 9-Phenyl-anthracenes: A Prediction Fulfilled. Org. Lett. 2011, 13, 256−259. (42) Fronza, G.; Mondelli, R. 13C NMR of N-Heterocycles. Conformation of Phenothiazines and 2,3-Diazaphenothiazines. J. Magn. Reson. 1976, 23, 437−454. (43) Soltz, B. A.; Corey, J. Y.; Larsen, D. W. Molecular Motion in Chlorpromazine and Chlorpromazine Hydrochloride. J. Phys. Chem. 1979, 83, 2162−2166. (44) Atkins, P. W.; Buchanan, I. C.; Gurd, R. C.; McLauchlan, K. A.; Simpson, A. F. Emission Electron Spin Resonance Spectra from Free Radicals in Solution. Chem. Commun. 1970, 513−514. (45) Wong, S. K.; Wan, J. K. S. Photochemically Induced Dynamic Electron Spin Polarization. 1,4-Naphthosemiquinone Radical in 2Propanol. J. Am. Chem. Soc. 1972, 94, 7197−7198. (46) Closs, G. L.; Forbes, M. D. E.; Norris, J. R., Jr. Spin-polarized Electron Paramagnetic Resonance Spectra of Radical Pairs in Micelles: Observation of Electron Spin-spin Interactions. J. Phys. Chem. 1987, 91, 3592−3599. (47) Hore, P. J. In Advanced EPR in Biology and Biochemistry; Hoff, A. J., Ed.; Elsevier: Amsterdam, 1989; pp 405−440. (48) Buckley, C. D.; Hunter, D. A.; Hore, P. J.; McLauchlan, K. A. Electron Spin Resonance of Spin-correlated Radical Pairs. Chem. Phys. Lett. 1987, 135, 307−311. (49) Leavanon, H.; Galili, T.; Regev, A.; Wiederrecht, G. P.; Svec, W. A.; Wasielewski, M. R. Determination of the Energy Levels of Radical Pair States in Photosynthetic Models Oriented in Liquid Crystals with Time-Resolved Electron Paramagnetic Resonance. J. Am. Chem. Soc. 1998, 120, 6366−6378. (50) Wasielewski, M. R. Energy, Charge, and Spin Transport in Molecules and Self-Assembled Nanostructures Inspired by Photosynthesis. J. Org. Chem. 2006, 71, 5051−5066. (51) Weiss, E. A.; Tauber, M. J.; Kelley, R. F.; Ahrens, M. J.; Ratner, M. A.; Wasielewski, M. R. Conformationally Gated Switching between Superexchange and Hopping within Oligo-p-phenylene-Based Molecular Wires. J. Am. Chem. Soc. 2005, 127, 11842−11850. (52) Dance, Z. E. X.; Mi, Q.; McCamant, D. W.; Ahrens, M. J.; Ratner, M. A.; Wasielewski, M. R. Time-Resolved EPR Studies of Photogenerated Radical Ion Pairs Separated by p-Phenylene Oligomers and of Triplet States Resulting from Charge Recombination. J. Phys. Chem. B 2006, 110, 25163−25173. (53) Weiss, E. A.; Ahrens, M. J.; Sinks, L. E.; Gusev, A. V.; Ratner, M. A.; Wasielewski, M. R. Making a Molecular Wire: Charge and Spin Transport through para-Phenylene Oligomers. J. Am. Chem. Soc. 2004, 126, 5577−5584. (54) Miura, T.; Scott, A. M.; Wasielewski, M. R. Making a Molecular Wire: Charge and Spin Transport through para-Phenylene Oligomers. J. Phys. Chem. C 2010, 114, 20370−20379. (55) Closs, G. L.; Miller, J. R. Intramolecular Long-Distance Electron Transfer in Organic Molecules. Science 1988, 240, 440−447. (56) Marcus, R. A.; Sutin, N. Electron transfers in chemistry and biology. Biochim. Biophys. Acta 1985, 811, 265−322. (57) Sumida, J. P.; Liddle, P. A.; Lin, S.; Macpherson, A. N.; Seely, G. R.; Moore, A. L.; Moore, T. A.; Gust, D. Contrasting Photoinduced Electron-Transfer Properties of Two Closely Related, Rigidly Linked Porphyrin−Quinone Dyads. J. Phys. Chem. A 1998, 102, 5512−5519. (58) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum Mechanical Continuum Solvation Models. Chem. Rev. 2005, 105, 2999−3093. (59) The ΔERIP‑ST values were determined to be 2.6 × 10−5 (220 mT) and 1.1 × 10−6 eV (9.5 mT) for the monomer and trimer systems, respectively. (60) Akiyama, K.; Tero-Kubota, S.; Ikoma, T.; Ikegami, Y. Spin Polarization Conservation during Intramolecular Triplet-Triplet Energy Transfer Studied by Time-Resolved EPR Spectroscopy. J. Am. Chem. Soc. 1994, 116, 5324−5327. (61) Stoll, S.; Schweiger, A. EasySpin, a Comprehensive Software Package for Spectral Simulation and Analysis in EPR. J. Magn. Reson. 2006, 178, 42−55. 11270



Article

(62) Fukuju, T.; Yashiro, H.; Maeda, K.; Murai, H. Real-time Observation of the Singlet−Triplet Dephasing Effect on the Spin Dynamics of the Spin-correlated Radical Pair Formed in the Photolysis of Tetraphenylhydrazine in a Micelle. Chem. Phys. Lett. 1999, 304, 173−179. (63) Maeda, K.; Robinson, A. J.; Henbest, K. B.; Hogben, H. J.; Biskup, T.; Ahmad, M.; Schleicher, E.; Weber, S.; Timmel, C. R.; Hore, P. J. Magnetically Sensitive Light-Induced Reactions in Cryptochrome Are Consistent with its Proposed Role as a Magnetoreceptor. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 4774−4779.

11271


Direct Observation of Hole Shift and ... - ACS Publications

Recommend Documents