Long-Lived Photoinduced Charge Separation in Inclusion Complexes

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Long-Lived Photoinduced Charge Separation in Inclusion Complexes Composed of a PhenothiazineBridged Cyclic Porphyrin Dimer and Fullerenes Takuya Kamimura, Kei Ohkubo, Yuki Kawashima, Shuwa Ozako, Ken-ichi Sakaguchi, Shunichi Fukuzumi, and Fumito Tani J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09147 • Publication Date (Web): 23 Oct 2015 Downloaded from http://pubs.acs.org on October 25, 2015

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The Journal of Physical Chemistry

Long-Lived Photoinduced Charge Separation in Inclusion Complexes Composed of a Phenothiazine-Bridged Cyclic Porphyrin Dimer and Fullerenes

Takuya Kamimura,† Kei Ohkubo,‡,§ Yuki Kawashima,‡ Shuwa Ozako,† Ken-ichi Sakaguchi,† Shunichi Fukuzumi,*,‡,§,|| and Fumito Tani*,†

† Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ‡ Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA and SENTAN, Japan Science and Technology (JST), Suita, Osaka 565-0871, Japan §

Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea

||

Faculty of Science and Engineering, Meijo University, ALCA and SENTAN, Japan Science and Technology Agency (JST), Nagoya, Aichi 468-0073, Japan

Abstract: C60, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), lithium-cation-encapsulated C60 (Li+@C60), and [6,6]-diphenyl-C62-bis(butyric acid methyl ester) (bis-PCBM) were included into a phenothiazine-bridged cyclic free-base porphyrin dimer (H4-Ptz-CPDPy(TEO)) in a polar solvent (benzonitrile) with large association constants of 1.3 × 106, 6.4 × 105, 3.2 × 106 and 2.5 × 105 M–1, respectively. Based on the electrochemical data, the lowest energy levels of the charge-separated (CS) states for the inclusion complexes of H4-Ptz-CPDPy(TEO) with C60, PCBM, Li+@C60, and 1 ACS Paragon Plus Environment


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bis-PCBM

(designated

Li+@C60⊂H4-Ptz-CPDPy(TEO)

as

C60⊂H4-Ptz-CPDPy(TEO), and

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PCBM⊂H4-Ptz-CPDPy(TEO),

bis-PCBM⊂H4-Ptz-CPDPy(TEO))

composed

of

the

phenothiazine donor and fullerene acceptors were determined to be 1.30, 1.40, 0.66, and 1.51 eV, respectively. Both C60⊂H4-Ptz-CPDPy(TEO) and PCBM⊂H4-Ptz-CPDPy(TEO) underwent electron transfer upon photoexcitation of the porphyrin and fullerene chromophores, and the resultant photoinduced CS states comprised the phenothiazine cation and the fullerene anions with lifetimes of 0.71 ms determined by time-resolved transient absorption spectra. Li+@C60⊂H4-Ptz-CPDPy(TEO) also afforded a similar CS state with a lifetime of 0.56 ms. These lifetimes are the longest values ever reported for the CS states of phenothiazine–fullerene complexes in solution. The spin states of these long-lived CS states were assigned to be triplet by ESR spectroscopy. The remarkably long CS lifetimes are attributable mainly to the lower CS energies than the triplet energies of the phenothiazine, fullerenes and porphyrin moieties and the spin-forbidden slow back electron transfer processes. On the other hand, the photoinduced CS state of bis-PCBM⊂H4-Ptz-CPDPy(TEO) was quenched rapidly by fast back electron transfer due to the relatively high CS energy comparable to the triplet energies of the porphyrin and fullerene.

INTRODUCTION The development of efficient systems harnessing solar light as a clean and inexhaustible energy source has been highly required to solve global energy and environmental problems.1-5 The natural photosynthesis,6,7 which is an excellent process for transducing solar energy into chemical resources, has provided profound inspirations for the construction of artificial light-energy conversion systems. In the photosynthetic reaction center containing the elaborate assembly of the redox-active components, the multistep electron transfer (ET) reactions proceed as key steps to afford the 2 ACS Paragon Plus Environment

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long-lived charge-separated (CS) state. Thus, a great deal of attention has been paid to design electron donor-acceptor complexes which attain photoinduced long-lived CS states.8-12 Porphyrins and fullerenes are useful components for the preparation of electron donor-acceptor complexes. Porphyrins, which have a two-dimensional 18π-electron system and strong absorption bands in the UV–visible region, possess sufficient electron-donating ability upon photoexcitation.8-14 Meanwhile, fullerenes, which have a three-dimensional π-electron system, are excellent electron acceptors on account of their suitable reduction potential and small reorganization energy in ET reactions, which is derived from the highly delocalized π-electron system over the curved surface together with the rigid and confined framework.15-18 Thus, numerous porphyrin–fullerene complexes have been explored as artificial mimicking models for CS in the reaction center of photosynthesis.19-27 Porphyrin–fullerene complexes are roughly classified into two groups. One group includes covalently-linked complexes, which have definite distances and orientations between each component in most cases.28-31 However, they have a demerit that the chemical syntheses of covalently-linked complexes are relatively inefficient and costly. The other group embraces noncovalently-linked supramolecular complexes, which are constructed by week interactions such as hydrogen bonds,32,33 Coulomb interactions,34,35 coordination bonds,36-39 and π–π interactions.40-47 They can be readily formed just by simple mixing to give a variety of donor–acceptor systems. However, noncovalent binding between highly π-conjugated compounds such as porphyrins and fullerenes is not strong enough in polar solvents, which are generally used for studies on photoinduced CS reactions.

3 ACS Paragon Plus Environment


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N

R N N

R

N

R

N

M

N N

N

N N

S

S

R N

M

N

N

N

N

N

N N

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N N

N

M

N

R

N

M = H2

N

H 4-Ptz-CPDPy(TEO): R = O O H 4-Ptz-CPDPy(OC3): R = OC3H 7 H 4-Ptz-CPDPy(OC6): R = OC6H13 H 4-Ptz-CPDPy(H): R = H

N

R

R

R

N

M

O

M = H2

H 4-C 4-CPDPy(H): R = H H 4-C 4-CPDPy(OC6): R = OC6H13

N O O TEO

NH N

C60

TEO

N HN

PCBM tBu

tBu

N O

H 2-PorPy

O

TEO =

O

O

O

Li + N O

Li +@C60

S

O

tBu

Ptz(tBuPh)

bis-PCBM (mixture of regioisomers)

Scheme 1. Chemical structures in this study.


tBu

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In order to surmount this problem, we have recently designed and prepared cyclic porphyrin dimers (CPDs) as shown in Scheme 1.48-52 CPDs include fullerenes such as C60,48,50 C70,53 and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)54b within their inner cavities in both solution and solid. We have already reported the supramolecular structures and photodynamics of the resultant 1:1 inclusion complexes.48-54 The inclusion complex of C60 within a butadiyne-bridged cyclic free-base porphyrin dimer without side-chains (C60⊂H4-C4-CPDPy(H)) underwent photoinduced ET from the porphyrin moiety to the included C60.50 However, the lifetime of the resulting CS state (τCS) was very short (0.47 ns) because its estimated energy (1.83 eV) was higher than those of the triplet excited states (ET) of each chromophore (ca. 1.5 eV) to induce the rapid decay of the CS state to the triplet states.55-57 In other words, it can be expected that a lower CS state energy (ECS) than the ET of each chromophore leads to the elongation of τCS. The ECS value of an electron donor–acceptor complex corresponds to the redox potential difference between the oxidation of a donor and the reduction of an acceptor. Therefore, ECS of a donor–acceptor complex can be lowered by increase in the HOMO level of a donor and/or decrease in the LUMO level of an acceptor. Based on this expectation, we have employed lithium-cation-encapsulated C60 (Li+@C60) as an electron acceptor instead of pristine C60.51 It has been reported that Li+@C60 has much lower (by ca. 0.6 eV) LUMO energy level than that of pristine C60 whereas ET of Li+@C60 is almost same as that of pristine C60.58-63 Li+@C60 was also included within a butadiyne-bridged CPD with four hexyloxy side-chains (H4-C4-CPDPy(OC6)) with a association constant (Kassoc) of 1.7 × 105 M–1 in benzonitrile (PhCN) at 25 °C. ECS of the resultant inclusion complex (Li+@C60⊂H4-C4-CPDPy(OC6)) was estimated to be 1.07 eV, which was lower than the ET value of each chromophore. In such a case, Li+@C60⊂H4-C4-CPDPy(OC6) underwent photoinduced charge separation in PhCN with τCS of 0.50



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ms,51 which was 106 times longer than that of C60⊂H4-C4-CPDPy(H) and the longest value ever reported for non-covalent free-base porphyrin–fullerene complexes in solution. We have successfully demonstrated that the use of the stronger electron acceptor brings about the remarkable elongation of τCS of our inclusion complex. Recently, we have prepared a new free-base CPD (H4-Ptz-CPDPy), which has phenothiazine linkers instead of butadiynyl groups.52 Phenothiazine derivative has a rigid aromatic framework comprising three linearly-fused six-membered rings, whereas a moderate flexibility is born of the altering dihedral angle between the two benzene rings from ca. 140° to ca. 160°.64-68 H4-Ptz-CPDPy(OC6) showed stronger affinity for both C60 and C70 in solution than H4-C4-CPDPy(OC6) because of the optimal porphyrin–porphyrin distance (ca. 12.5 Å) provided by the phenothiazine linkers. The crystal structure of the inclusion complex composed of C60 and H4-Ptz-CPDPy(OC3) showed strong π–π interaction between the fullerene and the porphyrins, while there was no obvious π–π interaction between the phenothiazine groups and C60. Moreover, phenothiazine derivative is generally a much stronger electron donor than free-base porphyrins. 69-76 It is expected that the higher HOMO energy of the phenothiazine moiety will result in the lower CS energy of the donor-acceptor pair composed of phenothiazine and fullerene.77 This strategy for lowering CS energy is a reverse way to the previous method applied for the inclusion complex of H4-C4-CPDPy(OC6) and the specially stronger acceptor (Li+@C60). We

report

herein

photodynamics

of

the

inclusion

complexes

composed

of

a

phenothiazine-bridged cyclic free-base porphyrin dimer (H4-Ptz-CPDPy(TEO)) and fullerenes in PhCN. We applied C60 and PCBM as main acceptor fullerenes. PCBM has been widely employed as an electron acceptor of active layers in recent organic photovoltaic devices on account of its good processability and favorable electronic properties.78-85 Additionally, we also used Li+@C60 and


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[6,6]-diphenyl-C62-bis(butyric acid methyl ester) (bis-PCBM) in the present study for comparison. Bis-PCBM is one of the bis-modified C60 derivatives and employed as an electron acceptor of photovoltaic active layers similarly to PCBM with its higher LUMO level than PCBM.80,86-90

EXPERIMENTAL SECTION General Information. Reagents and solvents of best grade available were purchased from commercial suppliers and were used without further purification unless otherwise noted. Lithium ion encapsulated fullerene C60 hexafluorophosphate (Li+@C60·(PF6)–: 96%) provided by Idea International Corp was commercially obtained from Wako Pure Chemicals, Co. Ltd., Japan and used without further purification. [6.6]-Diphenyl-C62-bis(butyric acid methyl ester) (bis-PCBM) was commercially obtained from Sigma-Aldrich Co., as a mixture of regioisomers and used without further purification. Benzonitrile (PhCN) was purified by distillation from P2O5 under reduced pressure after being stirred with K2CO3 overnight. Nuclear magnetic resonance (NMR) spectra were recorded on an AVANCE III 600 spectrometer (Bruker, Germany). The resonance frequencies are 600 and 151 MHz for 1H and 13C, respectively. Chemical shifts were reported as δ values in ppm relative to tetramethylsilane. High-resolution fast atom bombardment mass spectra (HR-FAB-MS) were measured with 3-nitrobenzyl alcohol (NBA) as a matrix and recorded on an LMS-HX-110 spectrometer (JEOL, Japan). Ultraviolet–visible–near-infrared (UV–vis–NIR) absorption and fluorescence spectra were recorded on UV-2500PC and RF-5300PC spectrometers (Shimadzu, Japan), respectively. Phosphorescence spectra were measured on a Fluorolog τ3 (Horiba, Japan) spectrophotometer with a quartz tube (i.d. = 4 mm) at 77 K. Infrared (IR) spectra were recorded on a FTS6000 spectrophotometer (Bio-Rad, USA). Melting point (Mp) measurements were carried out on a MP-S3 7 ACS Paragon Plus Environment


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(Yanaco, Japan) melting point meter. Electrochemical Measurements. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were carried out on an ALS630C (BAS, Japan) electrochemical analyzer in deaerated PhCN containing 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte at room temperature under an Ar atmosphere. A conventional three-electrode cell was used with a platinum working electrode (surface area of 0.3 mm2) and a platinum wire as a counter electrode. The measured potentials were recorded with respect to an Ag/AgNO3 (0.01 M) reference electrode. All potentials (vs Ag+/Ag) were converted to values with respect to that of the Fc+/Fc redox couple, which was measured under the same conditions. Electrolytic absorption spectra were obtained on an UV-2500PC (Shimadzu, Japan) spectrometer with an ALS630C (BAS, Japan) electrochemical analyzer in deaerated PhCN containing 0.1 M TBAPF6 as a supporting electrolyte at room temperature under an Ar atmosphere. The solution was put in a 2.0 mm light-path length quartz cell. A conventional three-electrode cell was used with a platinum mesh working electrode and a platinum wire as a counter electrode. The electrolysis potential was controlled with coulometric technique on an ALS630C electrochemical analyzer. Laser Flash Photolysis. Femtosecond transient absorption spectroscopy experiments were conducted using an ultrafast source: Integra-C (Quantronix Corp.), an optical parametric amplifier: TOPAS (Light Conversion Ltd.) and a commercially available optical detection system: Helios provided by Ultrafast Systems LLC. The source for the pump and probe pulses were derived from the fundamental output of Integra-C (λ = 786 nm, 2 mJ/pulse and fwhm = 130 fs) at a repetition rate of 1 kHz. 75% of the fundamental output of the laser was introduced into a second harmonic generation (SHG) unit: Apollo (Ultrafast Systems) for excitation light generation at λ = 393 nm,


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while the rest of the output was used for white light generation. The laser pulse was focused on a sapphire plate of 3 mm thickness and then white light continuum covering the visible region from λ = 410 nm to 800 nm was generated via self-phase modulation. A variable neutral density filter, an optical aperture, and a pair of polarizer were inserted in the path in order to generate stable white light continuum. Prior to generating the probe continuum, the laser pulse was fed to a delay line that provides an experimental time window of 3.2 ns with a maximum step resolution of 7 fs. In our experiments, a wavelength at λ = 393 nm of SHG output was irradiated at the sample cell with a spot size of 1 mm diameter where it was merged with the white probe pulse in a close angle (< 10°). The probe beam after passing through the 2 mm sample cell was focused on a fibber optic cable that was connected to a CMOS spectrograph for recording the time-resolved spectra (λ = 410–800 nm). Typically, 1500 excitation pulses were averaged for 3 seconds to obtain the transient spectrum at a set delay time. Kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data. All measurements were conducted at room temperature, 295 K. Nanosecond time-resolved transient absorption measurements were carried out using the laser system provided by UNISOKU Co., Ltd. Measurements of nanosecond transient absorption spectrum were performed according to the following procedure. A deaerated solution containing supramolecule was excited by a Panther OPO pumped by a Nd:YAG laser (Continuum SLII-10, 4-6 ns fwhm). The photodynamics was monitored by continuous exposure to a xenon lamp (150 W) as a probe light and a photomultiplier tube (Hamamatsu 2949) as a detector. The solution was oxygenated by nitrogen purging for 15 min prior to measurements. ESR Spectroscopy. ESR spectra were taken with a JES-RE1XE spectrometer (JEOL, Japan) at 4 K. ESR spectra of the CS states of C60•––[H4-Ptz-CPDPy(TEO)]•+, PCBM•––[H4-Ptz-CPDPy(TEO)]•+, and Li+@C60•––[H4-Ptz-CPDPy(TEO)]•+ in frozen PhCN were measured under photoirradiation with a 9 ACS Paragon Plus Environment


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USH-1005D high-pressure Hg lamp (Ushio, Japan) through a water filter focusing at the sample cell in the ESR cavity. The g value was calibrated by using a Mn2+ marker.

RESULTS AND DISCUSSION Supramolecular Formation of Cyclic Porphyrin Dimer with Fullerenes. Previously reported phenothiazine-bridged CPDs with alkoxyl groups52 have poor solubility in polar organic solvents. To improve the solubility, we have designed and prepared a new H4-Ptz-CPDPy with four triethylene N

TEO TEO =

Br

O

O

N

O

CHO

+

(i)

N

6%

TEO

Br

NH N

TEO

N HN

Br

TEO

(ii)

NH

55 %

N

TEO

N HN

tBu

tBu

NH HN

N

N

1

H 2-PorPy

N

TEO N N

N Br

S

N

Br

N

(iii) N

(ii)

M

TEO N

S

S

N

9%

17 %

N

N N

Bpin

N N

S

Bpin

Ptz(tBuPh)

N N TEO

TEO

S tBu

M

N tBu

M = H2

H 4-Ptz-CPDPy(TEO)

Scheme 2. Synthetic pathway for H2-PorPy, Ptz(tBuPh), and H4-Ptz-CPDPy(TEO). (i) TFA, DDQ, CH2Cl2; (ii) p-tert-butylphenylboronic acid, Pd(PPh3)4, Na2CO3, THF/H2O; (iii) Pd(PPh3)4, Cs2CO3, THF/H2O.


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oxide (TEO) chains in place of the alkoxyl groups on the meso-phenyl groups (H4-Ptz-CPDPy(TEO), Scheme 1) according to the reported procedure.52,91 In addition, as reference compounds for H4-Ptz-CPDPy(TEO), a bis-pyridyl-substituted free-base porphyrin monomer and a phenothiazine derivative (H2-PorPy and Ptz(tBu), Scheme 1) have been prepared by palladium-catalyzed Suzuki–Miyaura cross-coupling reaction from the dibrominated porphyrin and phenothiazine monomers, respectively. The synthetic procedures are shown in Scheme 2 and the Experimental Section. The products were successfully characterized by 1H NMR, 13C NMR, 1H–1H COSY, IR, and mass spectroscopies. They have sufficient solubility in PhCN to enable electrochemical and photochemical analysis with C60, PCBM, Li+@C60 and bis-PCBM in solution. Addition of the fullerenes to a PhCN solution of H4-Ptz-CPDPy(TEO) afforded UV–vis absorption spectral changes as shown in Figure 1 and S2 in SI. In all the cases, the Soret absorption bands of H4-Ptz-CPDPy(TEO) were diminished and red-shifted with isosbestic points at ca. 428 nm by the fullerene addition. The Job’s plot upon mixing of bis-PCBM with H4-Ptz-CPDPy(TEO) displayed a typical

signature

pattern

for

the

formation

of

a

1:1

host–guest

complex

(bis-PCBM⊂H4-Ptz-CPDPy(TEO), Figure S3 in SI). This result and our recent studies52,54 indicate that H4-Ptz-CPDPy(TEO) forms 1:1 supramolecular complexes with not only pristine C60 (C60⊂H4-Ptz-CPDPy(TEO))

but

(Li+@C60⊂H4-Ptz-CPDPy(TEO))

also and

PCBM

bis-PCBM

(PCBM⊂H4-Ptz-CPDPy(TEO)), in

PhCN.

Based

on

the

Li+@C60 titration

of

H4-Ptz-CPDPy(TEO) with the fullerenes at 25 °C, the association constants (Kassoc) were determined to be (1.0 ± 0.1) × 106 M–1 for C60⊂H4-Ptz-CPDPy(TEO), (6.4 ± 0.4) × 105 M–1 for PCBM⊂H4-Ptz-CPDPy(TEO), (3.2 ± 0.1) × 106 M–1 for Li+@C60⊂H4-Ptz-CPDPy(TEO), and (2.5 ± 0.5) × 105 M–1 for bis-PCBM⊂H4-Ptz-CPDPy(TEO) by applying a nonlinear curve-fitting method using eq 1 or 2,



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1 + Kassoc (A + X) ! "Abs = "# $

Page 12 of 52

[1 + Kassoc (A + X) ]2 ! 4Kassoc2 AX 2Kassoc

1 + Kassoc (A + X) !

(1)

[1 + Kassoc (A + X) ]2 ! 4Kassoc2 AX

#Abs = "X + (#" ! ") $ 2Kassoc

(2)

where A and X are [H4-Ptz-CPDPy(TEO)]0 and [fullerene]0, respectively; L is the difference of the molar absorptivity between umcomplexed H4-Ptz-CPDPy(TEO) and its inclusion complex; ε is molar absorptivity of the uncomplexed fullerene. Because PCBM and bis-PCBM have non-negligible absorption in the Soret band region of the porphyrin, eq 2 was applied in the titrations of PCBM and bis-PCBM. L and Kassoc were treated as fitting parameters. The resultant Kassoc data are summarized in Table 1. The Kassoc value of C60⊂H4-Ptz-CPDPy(TEO) was larger than that of PCBM⊂H4-Ptz-CPDPy(TEO). Additionally, the Kassoc value of Li+@C60⊂H4-Ptz-CPDPy(TEO) was 3 times larger than that of C60⊂H4-Ptz-CPDPy(TEO). These tendencies are consistent with the results of our previous reports.51,54 On the other hand, the Kassoc value of bis-PCBM⊂H4-Ptz-CPDPy(TEO) was smaller than the half of Kassoc of PCBM⊂H4-Ptz-CPDPy(TEO). In addition, the absorption change (ΔAbs) in the titration of bis-PCBM (Figure S2c and d in SI) was smaller than those of other fullerenes, suggesting that the two side chains of bis-PCBM give steric hindrance against its inclusion and make its π-π interaction with the porphyrin moieties weaker than those of the other fullerenes.


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C60$H4-Ptz-CPDPy(TEO) (a) 0.20 421 ! 423

(b)

[C60]

0.00

0.0 !M 0.2 !M

Kassoc = 1.3 # 106 M–1

0.2

0.10

0.05

–0.02

"Abs. at 421 nm

Absorbance

Absorbance

0.15

2.5 !M 0.1

0.0

400

420

0.00 300

400

500

600

–0.04

440

Wavelength / nm

700

–0.06

800

0.0

0.5

Wavelength / nm

1.5

1.0

[C60],

10–6

2.0

2.5

2.0

2.0

M

PCBM$H4-Ptz-CPDPy(TEO) (c)

421 ! 423

(d)

[PCBM]

0.20

0.00

0.0 !M 0.2 !M

0.15

Kassoc = 6.4 # 105 M–1 –0.02

0.10

0.05

"Abs. at 421 nm

0.2

Absorbance

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.5 !M 0.1

0.0

400

420

440

Wavelength / nm 0.00 300

400

500

600

700

–0.04

–0.06

800

0.0

Wavelength / nm

0.5

1.0

1.5

[PCBM], 10–6 M

Figure 1. Determination of the association constants (Kassoc) of (a,b) C60⊂H4-Ptz-CPDPy(TEO) and (c,d) PCBM⊂H4-Ptz-CPDPy(TEO) by UV–vis absorption spectroscopy in deaerated PhCN at 25 °C. (a,c) UV–vis absorption changes of H4-Ptz-CPDPy(TEO) in the course of titration with (a) C60 and (c) PCBM. [H4-Ptz-CPDPy(TEO)] = 5.0 × 10–7 M. The insets show the Soret band regions. (b,d) Changes in the UV–vis absorbance (ΔAbs) at 421 nm. The curve was fitted by using eq 1 for (b) and eq 2 for (d).



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Page 14 of 52

Table 1. Comparison of the association constants (Kassoc) of the inclusion complexes composed of H4-Ptz-CPDPy(TEO) and the fullerenes in deaerated PhCN at 25 °C

a

fullerene

Kassoca / M–1

Kassocb / M–1

C60

(1.3 ± 0.1) × 106

(1.1 ± 0.2) × 106

PCBM

(6.4 ± 0.5) × 105

(6.4 ± 1.0) × 105

Li+@C60

(3.2 ± 0.1) × 106

bis-PCBM

(2.5 ± 0.5) × 105

Determined from the absorption changes.

b

Determined from the fluorescence changes (λex = 428

nm).


Page 15 of 52

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Upon photoexcitation at the Soret band (428 nm) of H4-Ptz-CPDPy(TEO) in deaerated PhCN, the fluorescence due to the singlet excited state of the porphyrins (1Por*, * denotes the excited state) is observed at λmax = 650 and 713 nm. Noticeable decreases in the fluorescence intensity were observed upon addition of the fullerenes to the solution of H4-Ptz-CPDPy(TEO) as shown in Figure 2, implying that the quenching of 1Por* by the fullerenes via electron or energy transfer occurs in the inclusion complexes. From the plot of the fluorescence intensity changes versus the concentration of the fullerenes,

the

Kassoc

values

were

determined

to

be

(1.1

±

0.2)

×

106

M–1

for

C60⊂H4-Ptz-CPDPy(TEO) and (6.1 ± 1.0) × 105 M–1 for PCBM⊂H4-Ptz-CPDPy(TEO) by applying a nonlinear curve-fitting method using eq 3,

1 + Kassoc (A + X) !

[1 + Kassoc (A + X) ]2 ! 4Kassoc2 AX

#Int = F " 2Kassoc A

(3)

where F is the difference of the fluorescence intensity between uncomplexed H4-Ptz-CPDPy(TEO) and its inclusion complex. The Kassoc values obtained from the fluorescence spectral changes are in good agreement with those determined from the absorption spectral changes within experimental errors (vide supra).



(a)

C60#H4-Ptz-CPDPy(TEO) 650

(b)

1.0

0.0

[C60] 0.0 !M 0.8

–0.1

!Int. at 650 nm

0.2 !M

Intensity

0.6

2.5 !M 713

0.4

0.2

0.0 550

Kassoc = 1.1 " 106 M–1

–0.2

–0.3

–0.4

600

650

700

750

–0.5 0.0

800

0.5

Wavelength / nm (c)

2.0

2.5

2.0

2.5

M

0.0

[PCBM] 0.0 !M 0.8

Kassoc = 6.4 " 105 M–1

–0.1

!Int. at 650 nm

0.2 !M 0.6

2.5 !M 713

0.2

0.0 550

10–6

(d)

1.0

0.4

1.5

1.0

[C60],

PCBM#H4-Ptz-CPDPy(TEO) 650

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 52

–0.2

–0.3

–0.4

600

650

700

750

–0.5 0.0

800

Wavelength / nm

0.5

1.0

[PCBM],

1.5

10–6

M

Figure 2. Determination of the association constants (Kassoc) of (a,b) C60⊂H4-Ptz-CPDPy(TEO) and (c,d) PCBM⊂H4-Ptz-CPDPy(TEO) by fluorescence spectroscopy in deaerated PhCN at 25 °C. λex = 428 nm. (a,c) Fluorescence intensity changes of H4-Ptz-CPDPy(TEO) in the course of titration with (a) C60 and (c) PCBM. [H4-Ptz-CPDPy(TEO)] = 5.0 × 10–7 M. The insets show the Soret band regions. (b,d) Plot of the changes in the fluorescence intensity (ΔInt) at 650 nm versus the concentration of (b) C60 and (d) PCBM. The curves were fitted by using eq 3.


Page 17 of 52

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Energetics of Photoinduced Processes. Cyclic and differential pulse voltammograms (CVs and DPVs, respectively) of the inclusion complexes, uncomplexed H4-Ptz-CPDPy(TEO) and the fullerenes, and the reference compounds in deaerated PhCN are shown in Figure 3 and S7–S9 in SI. The electrochemical data are summarized in Table 2. The comparison of the uncomplexed H4-Ptz-CPDPy(TEO) with the reference compounds H2-PorPy and Ptz(tBu) shows that the reversible first oxidation and the quasi-reversible second oxidation waves of H4-Ptz-CPDPy(TEO) were attributed to the first oxidation of the phenothiazine moieties-Ptz- and the oxidation of the porphyrin parts (Por), respectively (Figure S7 in SI).92 The results clearly indicates that the HOMO energy level of the phenothiazine moieties in the dimer is much higher (by ca. 0.4 eV) than that of the porphyrin choromophores.

(a) C60!H 4-Ptz-CPDPy(TEO)

(b) PCBM!H 4-Ptz-CPDPy(TEO) C60 / C60•–

Ptz •+ / Ptz

PCBM / PCBM •–

Ptz •+ / Ptz

–1.00 0.30 0.93 0.76 0.8

–1.10

1.30 eV 1.0 !A

0.30 0.4

0.91 0.74

–1.00 0.0

Potential / V

–0.4

–0.8

1.40 eV

0.30

1.2

–1.2

vs Fc + /Fc

0.8

1.0 !A

0.30 0.4

–1.10 0.0

–0.4

Potential / V vs

–0.8

–1.2

Fc + /Fc

Figure 3. Cyclic voltammograms (CV) and differential pulse voltammograms (DPV) of H4-Ptz-CPDPy(TEO) with (a) C60 and (b) PCBM in deaerated PhCN with 0.1 M TBAPF6 at room temperature. [C60] = [PCBM] = [H4-Ptz-CPDPy(TEO)] = 0.4 mM. Scan rates: 100 mV s–1 for CV and 4 mV s–1 for DPV



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Page 18 of 52

Table 2. Summary of triplet energies (ET), oxidation and reduction potentials (Eox1/2 and Ered1/2, respectively), and estimated CS energies (ECS) of the inclusion complexes and the reference compounds (the measurement conditions are shown in each caption of the figures and the references) Redox Potential / V vs Fc+/Fc compound

ET / eV

1st Eox1/2

2nd Eox1/2

Ptz(tBuPh)

1.91

0.27

0.89

H2-PorPy

a

1st Ered1/2

ECS / eV Ptz•+–Ful•–a

Por•+–Ful•–b

0.68 0.71c

H4-Ptz-CPDPy(TEO)

1.51

0.27

C60

1.57d

–0.92

PCBM

1.47e

–1.01

Li+@C60

1.53f

–0.23g

bis-PCBM

1.42

–1.12

C60⊂H4-Ptz-CPDPy(TEO)

0.30

0.76c

–1.00

1.30

1.76

PCBM⊂H4-Ptz-CPDPy(TEO)

0.30

0.74c

–1.10

1.40

1.84


0.33

0.75c

–0.33

0.66

1.08

bis-PCBM⊂H4-Ptz-CPDPy(TEO)

0.30

0.74c

–1.21c

1.51

1.95

Calculated from the potential gaps between the 1st Eox1/2 and the 1st Ered1/2. b Calculated from the

potential gaps between the 2nd Eox1/2 and the 1st Ered1/2. c Determined from DPV. d See ref. 95. e See ref. 96. f See ref. 62. g See ref. 61.


Page 19 of 52

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On the other hand, the comparison of the inclusion complexes with the uncomplexed H4-Ptz-CPDPy(TEO) and fullerenes shows that the CVs of the inclusion complexes consist of the oxidation processes of the phenothiazine and porphyrin groups in H4-Ptz-CPDPy(TEO) and the reduction processes of the fullerenes. In addition, the oxidation and reduction potentials of the inclusion complexes showed 0.03–0.06 V anodic shifts and 0.08–0.10 V cathodic shifts, respectively.61 These small anodic shifts of the oxidation potentials and small cathodic shifts of the reduction potentials indicate charge transfer interactions from the phenothiazine and porphyrin moieties to the included fullerenes. The energy levels (ECS) of the expected CS states composed of a phenothiazine radical cation (Ptz•+, • denotes the radical species) and fullerene radical anions (Ful•–, i.e. C60•–, PCBM•–, Li+@C60•–, and bis-PCBM•–) estimated from the potential gaps between their first redox potentials to be 1.30 eV for

C60⊂H4-Ptz-CPDPy(TEO),

1.40

eV

for

PCBM⊂H4-Ptz-CPDPy(TEO),

0.66

eV

for

Li+@C60⊂H4-Ptz-CPDPy(TEO), and 1.51 eV for bis-PCBM⊂H4-Ptz-CPDPy(TEO). These values are lower than those of the CS states comprising a porphyrin radical cation (Por•+) and fullerene radical anions (Ful•–) due to the stronger electron-donating ability of Ptz than Por. In addition, the ECS values of the former ion pairs ([Ptz•+- Ful•–]) are lower than the energy levels of the singlet excited states of Por (1.90 eV) and Ptz (3.07 eV) of H4-Ptz-CPDPy(TEO) and the corresponding fullerenes (1.76-1.99 eV)

62,93,94

(Figure S1 and S6 in SI), implying that the free energy changes of photoinduced

electron transfer affording [Ptz•+- Ful•–] via these singlet excited states are negative (exergonic). The energies of the triplet excited states (ET) for bis-PCBM, Ptz(tBu), and H4-Ptz-CPDPy(TEO) were also determined by phosphorescence spectra in frozen 2-MeTHF/EtI (9/1, v/v) glass at 77 K (Figure S6 in SI). The comparison with ET values for Ptz(tBu) and H4-Ptz-CPDPy(TEO) shows that the phosphorescence of H4-Ptz-CPDPy(TEO) at 823 nm is attributed to its porphyrin moiety. The



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 52

estimated ECS of [Ptz•+- Ful•–] in C60⊂H4-Ptz-CPDPy(TEO), PCBM⊂H4-Ptz-CPDPy(TEO), and Li+@C60⊂H4-Ptz-CPDPy(TEO) are lower than ET of their reference compounds.62,94-96 Thus, the free energy changes of photoinduced electron transfer generating [Ptz•+- Ful•–] states via the triplet excited states are negative (exergonic). These results suggest that the inclusion complexes composed of conventional fullerenes (such as C60 and PCBM) with H4-Ptz-CPDPy(TEO) can afford photoinduced charge-separated states. Additionally, in the case of Li+@C60⊂H4-Ptz-CPDPy(TEO), the estimated ECS of [Por•+-Li+@C60•–] is also lower than ET of its reference compounds.62 In contrast to these cases, however, ECS of bis-PCBM⊂H4-Ptz-CPDPy(TEO) is higher than ET of the reference compounds suggesting no formation of the triplet CS states. Photoinduced Charge Separation. The radical cation of the phenothiazine donor and the radical anions of the fullerene acceptors were investigated by UV-vis-NIR absorption spectroscopy to provide the reference spectra for the analysis of the transient absorption spectra of the inclusion complexes in laser flash photolysis. The absorption spectrum of [Ptz(tBu)]•+ was obtained by the electron-transfer oxidation of Ptz(tBu) with [Fe(bpy)3](PF6)3 in deaerated PhCN (Scheme 3a),97,98 showing a characteristic absorption band at 719 nm (Figure S4a in SI). Similarly, the absorption spectrum

of

PCBM•–

was

taken

by

the

electron-transfer

reduction

of

PCBM

with

1-benzyl-1,4-dihydronicotinamide dimer ((BNA)2) (Scheme 3b),99,100 affording a broad absorption band in 700–1100 nm region with a peak at 1035 nm (Figure S4b in SI). The absorption coefficient of PCBM•– was determined to be 1700 cm–1 M–1 at 1020 nm. Moreover, the absorption spectrum of bis-PCBM•–, which was prepared by the electrochemical reduction of bis-PCBM in deaerated PhCN, was similar to that of PCBM•–, while the maximum wavelength (λmax = 1054 nm) of bis-PCBM•– was slightly red-shifted compared to that of PCBM•– (Figure S5).


Page 21 of 52

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(a)

•+ N

R

S

Ptz(tBuPh)

+

N

[Fe(bpy) 3]3+

R

R

+

S

[Fe(bpy) 3]2+

R

[Ptz(tBuPh)] •+

R = 4-tert-butylphenyl

(b) Bn N

O O

2

+

H 2NOC CONH 2 N Bn

PCBM

(BNA) 2 •– O O

2

CONH 2 +

2

+

N Bn

PCBM •–

BNA +

Scheme 3. (a) Chemical oxidation of Ptz(tBuPh) by using [Fe(bpy)3]3+ as a one-electron oxidant. (b) Chemical reduction of PCBM by using (BNA)2 as a two-electron reductant.



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Page 22 of 52

The photodynamics of the inclusion complexes and the reference compounds were investigated by the transient absorption spectra measured in deaerated PhCN by the use of femtosecond and nanosecond laser flash photolysis. The transient spectra of Ptz(tBu), H4-Ptz-CPDPy(TEO) and bis-PCBM were obtained as references for the inclusion complexes. As for other fullerenes, the previously reported data were used for the analysis of the transient spectra.62,101 The time-resolved transient absorption spectra of Ptz(tBu) and H4-Ptz-CPDPy(TEO) were measured by femtosecond laser flash photolysis (λex = 393 nm) in the time range from 1 to 3000 ps (Figure S10a and c in SI). The spectra of Ptz(tBu) showed a characteristic absorption at 583 nm due to the singlet excited state of Ptz(tBu) (1Ptz(tBu)*) and its fluorescence at 472 nm (Figure S1a in SI). However, no clear absorption due to the triplet excited state of Ptz(tBu) (3Ptz(tBu)*) was detected in the time range from 1 to 3000 ps. The decay rate constant of 1Ptz(tBu)* was determined to be 1.4 × 109 s–1 from the decay time profile at 583 nm (Figure S10b in SI). On the other hand, the time-resolved transient absorption spectrum of uncomplexed H4-Ptz-CPDPy(TEO) at 3 ps after photoexcitation showed a characteristic wide absorption from 500 to 800 nm due to the singlet excited state of its porphyrin moieties (1Por*) as well as the ground state bleaching of its Q-band absorption and fluorescence (Figure S1c in SI).49,102 Additionally, a characteristic absorption around 600 nm due to the singlet excited state of its phenothiazine moieties (1Ptz*) was observed at 3000 ps after photoexcitation. The decay of the absorption at 628 nm was attributed to a combination of two different rate constants (1.9 × 1010 and 3.5 × 109 s–1) (Figure S10d in SI). By comparison with the decay profile of Ptz(tBu), the faster and slower decays were assigned to 1Por* and 1Ptz*, respectively. The decay rate constant of 1Ptz* in H4-Ptz-CPDPy(TEO) was two times larger than that of 1Ptz(tBu)*, suggesting intramolecular energy transfer from 1Ptz* to the porphyrin units in uncomplexed H4-Ptz-CPDPy(TEO). On the other hand, in the time range from 1 to 3500 µs of nanosecond laser


Page 23 of 52

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flash photolysis (λex = 510 nm), the time-resolved transient absorption spectra of free H4-Ptz-CPDPy(TEO) showed characteristic absorptions at 450 nm due to the triplet excited state of its porphyrin moiety (3Por*) (Figure S13c in SI). The kT value of 3Por* was determined to be 1.6 × 103 s–1 from the decay time profile at 450 nm (Figure S13d in SI). The time-resolved transient absorption spectra of free bis-PCBM measured by femtosecond laser flash photolysis (λex = 393 nm) in the time range from 1 to 3000 ps (Figure S11a in SI) showed characteristic absorptions at 892 nm due to the singlet excited state of bis-PCBM (1bis-PCBM*) and 598 nm due to the triplet excited state of bis-PCBM (3bis-PCBM*). The rate constant of the intersystem crossing (ISC, kISC) from 1bis-PCBM* to 3bis-PCBM* was determined to be 8.7 × 108 s–1 from the decay time profile at 892 nm (Figure S11b in SI). On the other hand, in the time range from 1 to 360 µs by nanosecond laser flash photolysis (λex = 355 nm), the time-resolved transient absorption spectra of free bis-PCBM showed characteristic absorptions at 450 and 700 nm due to 3

bis-PCBM* (Figure S13a in SI). The decay rate constant of 3bis-PCBM* (kT) was determined to be

4.9 × 104 s–1 from the decay time profile at 700 nm (Figure S13b in SI). Upon femtosecond laser excitation at 393 nm, the time-resolved transient absorption spectra of C60⊂H4-Ptz-CPDPy(TEO) showed characteristic broad absorptions from 500 to 800 nm due to 1Por* and 1Ptz* in the time range from 3 to 3000 ps (Figure 4a), being similar to the case of free H4-Ptz-CPDPy(TEO) (Figure S11c in SI). The decay rate constants of 1Por* and 1Ptz* were determined to be 4.9 × 1010 and 2.1 × 109 s–1, respectively, from the decay time profile at 628 nm (Figure 4b). Additionally, a broad absorption in NIR region due to the singlet excited state of C60 (1C60*)101 was also observed at 3 ps after the excitation. However, characteristic absorptions due to the radical species (i.e. Ptz•+, Por•+ and C60•–) were not observed clearly in the time range of 3–3000 ps.



C60#H 4-Ptz-CPDPy(TEO) (a)

(b)

8

15

10

3 ps 200 ps 3000 ps

1Por*

(628 nm) | 5 1C

0 400

!Abs628 / 10 –3

!Abs / 10 –3

6

60*

k1 = 4.9 " 10 10 s –1 k2 = 2.1 " 10 9 s –1

4

2

1Ptz*

800

600

1000

0

1200

0

200

100

Wavelength / nm

300

400

Time / ps

PCBM#H 4-Ptz-CPDPy(TEO) (c)

(d)

8

15

10

3 ps 200 ps 3000 ps

1Por*

(628 nm) | 1PCBM*

5

6

!Abs628 / 10 –3

!Abs / 10 –3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 52

k1 = 1.9 " 10 10 s –1 k2 = 2.0 " 10 9 s –1

4

2

0 400

1Ptz*

600

800

1000

0

1200

0

Wavelength / nm

1000

500

1500

Time / ps

Figure 4. Transient absorption spectra of H4-Ptz-CPDPy(TEO) with (a) C60 and (c) PCBM in deaerated PhCN at room temperature taken at 3, 200, and 3000 ps after femtosecond laser excitation at 393 nm. [H4-Ptz-CPDPy(TEO)] = 2.5 × 10–5 M, [C60] = [PCBM] = 5.0 × 10–5 M. (b,d) Decay time profiles at 628 nm


Page 25 of 52

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Similarly to the spectra of C60⊂H4-Ptz-CPDPy(TEO), the time-resolved transient absorption spectra of 1

PCBM⊂H4-Ptz-CPDPy(TEO) showed characteristic broad absorptions due to 1Por* and

Ptz* at 500–800 nm, and 1PCBM* at the NIR region in the time range from 3 to 3000 ps upon

femtosecond laser flash excitation at 393 nm (Figure 4c). The decay rate constants of 1Por* and 1Ptz* were determined to be 1.9 × 1010 and 2.0 × 109 s–1, respectively, from the decay time profile at 628 nm (Figure 4d). However, no characteristic absorptions due to Ptz•+, Por•+ and PCBM•– were observed in this time range (3–3000 ps). Analogously, the time-resolved transient absorption spectra of Li+@C60⊂H4-Ptz-CPDPy(TEO) showed characteristic broad absorptions due to 1Por* and 1Ptz* at 500–800 nm in the time range of 3–3000 ps upon femtosecond laser flash excitation at 393 nm (Figure S12a in SI). No characteristic absorptions due to Ptz•+, Por•+ and Li+@C60•– were observed in this time range. On the other hand, in the case of the inclusion complex of bis-PCBM within H4-Ptz-CPDPy(TEO), the time-resolved transient absorption spectra showed characteristic broad absorptions due to 1Por* and 1Ptz* at 500–800 nm and 1bis-PCBM* at 860 nm in the time range of 3–3000 ps upon femtosecond laser flash photolysis at 393 nm (Figure S13c in SI). The decay rate constants of 1Por* and 1Ptz* were determined to be 1.0 × 1010 and 1.8 × 109 s–1, respectively, from the decay time profile at 628 nm (Figure S12d in SI). The decay rate constant of 1bis-PCBM* were determined to be 2.1 × 109 s–1 from the decay time profile at 860 nm. No characteristic absorptions due to Ptz•+ and Por•+ were clearly observed in this time range (3–3000 ps). However, in contrast to the other inclusion complexes, the transient absorption spectra showed absorption at ca. 1040 nm due to bis-PCBM•– as a shoulder. These results suggest that electron transfer from Ptz to 1bis-PCBM* occurs followed by fast back electron transfer (BET) upon photoexcitation.



C60#H 4-Ptz-CPDPy(TEO) 3C

15

(b)

60*

(740 nm)

(c)

15

740 nm 1080 nm

–6

25

ln(!Abs1080)

(a)

20

5

C60•–

10

!Abs1080 / 10 –4

4 !s 50 !s

!Abs / 10 –3

!Abs / 10 –3

10

5

(1080 nm)

kET = 1.1 " 10 5 s –1

400

–8

15 –10 0

1

800

600

1000

0

1200

0

20

Wavelength / nm

40

60

0

80

2

Time / ms 10

10 mJ/pulse 7 mJ/pulse 5 mJ/pulse

5

0

0

1

2

Time / !s

3

4

5

Time / ms

PCBM#H 4-Ptz-CPDPy(TEO) (e)

12

(f) 710 nm 1020 nm

6

3PCBM*

20

PCBM •–

!Abs1020 / 10 –4

4 !s 50 !s

8

!Abs / 10 –3

(710 nm)

4

4

kET = 1.2 "

10 5

s –1

600

800

1000

–8

15 –10 0

0

1200

0

20

40

60

80

2

10

10 mJ/pulse 7 mJ/pulse 5 mJ/pulse

5

Wavelength / nm

1

Time / ms

2

(1020 nm)

0 400

–6

25

ln(!Abs1020)

(d)

!Abs / 10 –3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 52

0

0

Time / !s

2

4

6

8

Time / ms

Figure 5. Transient absorption spectra of H4-Ptz-CPDPy(TEO) with (a) C60 and (d) PCBM in deaerated PhCN at room temperature taken at 4 and 50 µs after nanosecond laser excitation at 532 nm. [H4-Ptz-CPDPy(TEO)] = 2.5 × 10–5 M, [C60] = [PCBM] = 5.0 × 10–5 M.

(b,e) Rise and (c,f)

decay time profiles at (b,c) 1080 and (e,f) 1020 nm with different laser intensities (5, 7, and 10 mJ/pulse). Inset: first-order plots.


Page 27 of 52

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Table 3. Summary of electron transfer rate constants (kET), CS lifetimes (τCS), CS quantum yields (ΦCS), BET reorganization energies (λ), BET electronic coupling elements (V), BET activation enthalpies (ΔH‡) of the inclusion complexes in deaerated PhCN at room temperature after photoexcitation at 532 nm. compound

kET / s–1

τCS / ms

ΦCS

λ / eV

V / cm–1

ΔH‡ / kcal mol–1


1.1 × 105

0.71

0.026

0.73

1.9 × 10–2

1.6

PCBM⊂H4-Ptz-CPDPy(TEO)

1.2 × 105

0.71

0.25

0.82

1.7 × 10–2

1.5


1.2 × 105

0.56

0.043

In contrast to the results of femtosecond laser flash photolysis in Figure 4a, where no electron transfer was confirmed, the nanosecond laser excitation (532 nm) of C60⊂H4-Ptz-CPDPy(TEO) resulted in appearance of characteristic transient absorption bands at 740 and 1080 nm due to the triplet excited state of C60 (3C60*) and C60•–,99 respectively (Figure 5a). The corresponding counter radical cation (i.e. Ptz•+ and Por•+) was not observed clearly due to the overlap of the absorption. However, the existence of C60•– and the very long lifetime of the CS state (vide infra) suggest that electron transfer from the ground state of Ptz to 3C60* occurs to produce the radical ion pair of Ptz•+ and C60•– by photoexcitation.103 If the porphyrin moiety was an electron donor, the electron transfer from the porphyrin to 3C60* would be endergonic (ΔG>0). The rate constant of the ET from Ptz to 3

C60* was determined to be 1.1 × 105 s–1 from the rise time profile of absorbance at 1080 nm (Figure

5b). The absorbance at 1080 nm due to C60•– in the CS state decayed in first-order kinetics with the same slope irrespective of the difference in the laser intensity (Figure 5c). This clearly indicates that the decay of the CS state occurs via intrasupramolecular BET rather than a bimolecular reaction. The CS lifetime (τCS) was determined to be 0.71 ms from the first-order plots. The CS lifetime is not only 27 ACS Paragon Plus Environment


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longer than that of Li+@C60⊂H4-C4-CPDPy(OC6)51 but also the longest value ever reported for phenothiazine–fullerene complexes in solution.74,75,104,105 Additionally, the quantum yield of the CS state (ΦCS) was estimated to be 0.026 by means of the comparative method with the absorption intensity of the CS state (C60•–: ε1080 = 12000 cm–1 M–1).106-108 The data are summarized in Table 3. Similarly, the nanosecond laser excitation (532 nm) of PCBM⊂H4-Ptz-CPDPy(TEO) afforded characteristic transient absorption bands at 710 and 1020 nm due to 3PCBM* and PCBM•–, respectively (Figure 5c). No clear absorption for the counter cation was observed as in the case of C60⊂H4-Ptz-CPDPy(TEO). However, for the same reason, electron transfer from the ground state of Ptz to 3PCBM* occurs to produce the CS state composed of Ptz•+ and PCBM•–. The rate constant of the electron transfer from Ptz to 3PCBM* was determined to be 1.2 × 105 s–1 from the rise time profile of absorbance at 1020 nm (Figure 5e). The absorbance at 1020 nm due to PCBM•– in the CS state decayed obeying first-order kinetics with the same slope irrespective of the difference in the laser intensity (Figure 5f). This clearly indicates that the decay of the CS state occurs via intrasupramolecular BET rather than a bimolecular reaction similar to C60•––[H4-Ptz-CPDPy(TEO)]•+. The τCS value of PCBM•––[H4-Ptz-CPDPy(TEO)]•+ was determined to be 0.71 ms from the first-order plots. This value was equal to that of C60•––[H4-Ptz-CPDPy(TEO)]•+ even though the estimated ECS of Ptz•+–PCBM•– in PCBM•––[H4-Ptz-CPDPy(TEO)]•+ (1.40 eV) is slightly higher than that of Ptz•+–C60•– in C60•––[H4-Ptz-CPDPy(TEO)]•+ (1.30 eV, Table 2). On the other hand, the ΦCS of PCBM•––[H4-Ptz-CPDPy(TEO)]•+ was estimated to be 0.25 by means of the comparative method with the absorption intensity of the CS state (PCBM•–: ε1020 = 1700 cm–1 M–1, Figure S4b in SI), which was 10 times larger than that of C60•––[H4-Ptz-CPDPy(TEO)]•+. When the included fullerene was replaced by Li+@C60, which has significantly lower LUMO energy level than that of C60 and PCBM (Table 2), nanosecond laser excitation (532 nm) of


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Li+@C60⊂H4-Ptz-CPDPy(TEO) resulted in appearance of characteristic transient absorption bands at 750 and 1035 nm due to 3Li+@C60* and Li+@C60•–, respectively (Figure S14a in SI), showing similar results to the cases of C60⊂H4-Ptz-CPDPy(TEO) and PCBM⊂H4-Ptz-CPDPy(TEO). Although no clear absorption for the cation was observed, these results suggest that electron transfer from the ground state of Ptz and/or Por to the triplet excited state of Li+@C60 (3Li+@C60*) occurs to produce the CS state by photoexcitation. The rate constant of the electron transfer was determined to be 1.2 × 105 s–1 from the rise time profile at 1035 nm (Figure S14b in SI), which was nearly the same as those of C60⊂H4-Ptz-CPDPy(TEO) and PCBM⊂H4-Ptz-CPDPy(TEO). The absorbance at 1035 nm due to Li+@C60•– in the CS state decayed obeying first-order kinetics with the same slope irrespective of the difference in the laser intensity (Figure S15c in SI). This also clearly indicates that the decay of the CS state occurs via intrasupramolecular BET rather than a bimolecular reaction. The τCS value was determined to be 0.56 ms from the first-order plots. This lifetime is shorter than those of C60⊂H4-Ptz-CPDPy(TEO) and PCBM⊂H4-Ptz-CPDPy(TEO), however it is nearly the same as that of Li+@H4-C4-CPDPy(OC6).51 The ΦCS of Li+@C60•––[H4-Ptz-CPDPy(TEO)]•+ was estimated to be 0.043 by means of the comparative method with the absorption intensity of the CS state (Li+@C60•–: ε1035 = 7300 cm–1 M–1).61 In contrast to these three complexes, the time-resolved transient absorption spectra of the inclusion complex bis-PCBM⊂H4-Ptz-CPDPy(TEO) taken at 1–350 µs after nanosecond laser photoexcitation (532 nm) showed only characteristic absorptions at 460 and 720 nm due to 3

bis-PCBM* and no absorption due to the radical ion species (Figure S15a in SI). These results

indicate that no electron transfer from Ptz to

3

bis-PCBM* occurs, because the ECS of

bis-PCBM•––Ptz•+ is higher than the energy level of 3bis-PCBM* (Table 2). The decay rate constant for 3bis-PCBM* was determined to be 2.1 × 105 s–1 from the decay time profile at 690 nm (Figure



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Page 30 of 52

S15b in SI). The formation of the long-lived CS states was also confirmed by using electron spin resonance (ESR) spectroscopy at 4 K with photoirradiation (Figure 6 and S17 in SI). Each ESR spectrum of C60•––[H4-Ptz-CPDPy(TEO)]•+, PCBM•––[H4-Ptz-CPDPy(TEO)]•+, and Li+@C60•––[H4-Ptz-CPDPy(TEO)]•+ showed characteristic signals due to Ptz•+ and the fullerene radical anion. No signal due to Por•+ at g = 2.002109,110 was observed for each inclusion complex. Additionally, in all cases, characteristic triplet signals were observed around g = 4.25. Thus, it was concluded that the inclusion complexes of H4-Ptz-CPDPy(TEO) with C60, PCBM, and Li+@C60 afforded the triplet CS states under photoexcitation.

(a) C60!H 4-Ptz-CPDPy(TEO)

(b) PCBM!H 4-Ptz-CPDPy(TEO)

g = 4.25

g = 4.24

Ptz •+ (g = 2.0671)

Ptz •+ (g = 2.0655)

PCBM •– (g = 2.0027)

C60•– (g = 1.9998)

Figure

6.

ESR

spectra

of

C60•––[H4-Ptz-CPDPy(TEO)]•+

(a)

and

(b)

PCBM•––[H4-Ptz-CPDPy(TEO)]•+ in deaerated PhCN generated by photoirradiation using a high-pressure Hg lamp (1000 W) at 4 K


Page 31 of 52

The temperature dependence of the BET rate constants (kBET) of C60•––[H4-Ptz-CPDPy(TEO)]•+ and PCBM•––[H4-Ptz-CPDPy(TEO)]•+ were examined by using the nanosecond laser photoexcitation at 532 nm in the temperature range of 21–63 °C. The temperature dependence of kBET is predicted by the Marcus equation (eq 4) for nonadiabatic electron transfer,111-113 1/2 ) ! 4! 3 $ (GBET + " 2 && V exp +' kBET = ## 2 + 4! kBT " h " kBT % +*

(

) ,. 2

(4)

. .-

where λ and V are the reorganization energy and the electron coupling term, respectively; –ΔGBET is the driving force for BET; h and kB are the Plank and Boltzmann constants, respectively. The eq 4 can be rewritten as eq 5, ln (kBETT1/2) = ln

2!3/2V2 h("kB

)1/2

10.6

ln(kBET T 1/2 / s –1 K1/2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


#

63 °C

($GBET + ")2 4"k%&

(5)

C60!H 4-Ptz-CPDPy(TEO) PCBM!H 4-Ptz-CPDPy(TEO)

10.4

10.2

10.0 21 °C

9.8

3.0

3.1

3.2

3.3

3.4

T –1 / 10 –3 K –1

Figure 7. Plots of ln(kBET T1/2) vs T–1 for the back electron transfer of (black) C60•––[H4-Ptz-CPDPy(TEO)]•+ and (red) PCBM•––[H4-Ptz-CPDPy(TEO)]•+ after nanosecond laser photoexcitation at 532 nm in deaerated PhCN.



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Page 32 of 52

which predicts a linear correlation between ln(kBET T1/2) versus T–1. The plots of ln (kBETT1/2) versus T–1

for

the

intrasupramolecular

BET

of

C60•––[H4-Ptz-CPDPy(TEO)]•+

and

PCBM•––[H4-Ptz-CPDPy(TEO)]•+ in the temperature range of 21–63 °C gave linear correlations (Figure 7). According to the slope and intercept of the graph shown in Figure 7, the λ and V values were determined to be λ = 0.73 eV and V = 1.9 × 10–2 cm–1 for C60⊂H4-Ptz-CPDPy(TEO), and λ = 0.82 eV and V = 1.7 × 10–2 cm–1 for PCBM⊂H4-Ptz-CPDPy(TEO) (Table 3). These λ values indicate that the BET processes are in the Marcus inverted region, where the kBET value decreases with increasing the driving force.111,112 In addition, the λ values for C60⊂H4-Ptz-CPDPy(TEO) and PCBM⊂H4-Ptz-CPDPy(TEO) were slightly larger than that of Li+@C60⊂H4-C4-CPDPy(OC6) (0.56 eV).51 The possible reason is that the phenothiazine moiety causes a relatively large structure deformation upon the one-electron redox reaction.114 The very small V values result from the spin-forbidden back electron transfer due to the triplet CS states as confirmed by ESR spectroscopy. The Eyring plots of ln (kBET T–1) vs T–1 for the BET in Figure S16 in SI showed linear correlations. The activation enthalpies of the BET (ΔH‡) were determined to be 1.6 kcal mol–1 for C60⊂H4-Ptz-CPDPy(TEO) and 1.5 kcal mol–1 for PCBM⊂H4-Ptz-CPDPy(TEO) from the slopes of the plots. Based on the results described above, the mechanisms of intrasupramolecular photoinduced charge

separation

and

charge

recombination

in


and

PCBM⊂H4-Ptz-CPDPy(TEO) are proposed as shown in Scheme 4, while the mechanisms in the cases of Li+@C60⊂H4-Ptz-CPDPy(TEO) and bis-PCBM⊂H4-Ptz-CPDPy(TEO) are shown in Scheme S1 in SI.


Page 33 of 52

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(a) C60!H 4-Ptz-CPDPy(TEO) 1Por*–C

60–Ptz

1.90 eV

Por–1C60*–Ptz

kET = 4.9 ! 1010 s–1

1.99 eV

Por•+–C60•––Ptz 1.76 eV 3Por*–C

fast BET

Por–3C60*–Ptz

60–Ptz

1.51 eV

1.57 eV 3[Por–C

60

kET = 1.1 ! 105 s–1

•––Ptz•+]

1.30 eV

h"

Por–C60–Ptz

h"

!CS = 0.71 ms kBET = 1.4 ! 103 s–1

(b) PCBM!H 4-Ptz-CPDPy(TEO) 10 –1 1Por*–PCBM–Ptz kET = 1.9 ! 10 s Por•+–PCBM•––Ptz

1.90 eV

kISC = 1.9 ! 1010 s–1

fast BET

1.84 eV

3Por*–PCBM–Ptz

1.76 eV

Por–3PCBM*–Ptz

1.51 eV 3[Por–PCBM•––Ptz•+]

h"

Por–1PCBM*–Ptz

1.47 eV kET = 1.2 ! 105 s–1

1.40 eV

Por–PCBM–Ptz

!CS = 0.71 ms kBET = 1.4 ! 103 s–1

h"

Scheme 4. Proposed energy diagrams for (a) C60⊂H4-Ptz-CPDPy(TEO) and (b) PCBM⊂H4-Ptz-CPDPy(TEO); broken arrow: minor pathway. The singlet and triplet excited states of the phenothiazine moieties (i.e. 3.07 eV for Por–Ful–1Ptz* and 1.91 eV for Por–Ful–3Ptz*) were omitted because these excited states cannot be formed by the photoexcitation at 532 nm (see Table 2 and Figure S1a in SI), which was employed for the observation of the CS states.

In the case of C60⊂H4-Ptz-CPDPy(TEO) (Scheme 4a), the singlet excited states of the porphyrin moiety (1Por*), the phenothiazine moiety (1Ptz*), and the included C60 (1C60*) are generated by the photoexcitation. 1Ptz* undergoes intersystem crossing (ISC) with a rate constant of 2.1 × 109 s–1 and the resultant 3Ptz* decays to the ground state (GS) prior to electron/energy transfer. 1Por* undergoes ET to the GS of C60 with a rate constant of 4.9 × 1010 s–1 to produce the singlet radical pair of



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Por•+–C60•–. However, the resultant charge separated (CS) state is rapidly quenched and generates 3

C60* due to fast back electron transfer (BET), because the energy level of the CS state is much

higher than those of both 3Por* and 3C60*. On the other hand, 1C60* undergoes electron transfer (ET) from the GS of Por and/or ISC. The generated 3C60* by CR of Por•+–C60•– and/or ISC undergoes additional ET from the GS of Ptz with a rate constant of 1.1 × 105 s–1. The resultant triplet CS state (3[Ptz•+–C60•–]) decayed slowly by BET with spin inversion with a rate constant of 1.4 × 103 s–1. However, in this case, 3C60* undergoes not only ET but also energy transfer to Por. This energy transfer is the main reason that the estimated quantum yield of the CS state (ΦCS) is much smaller than that of PCBM⊂H4-Ptz-CPDPy(TEO). In the case of PCBM⊂H4-Ptz-CPDPy(TEO) (Scheme 4b), the generated 1Ptz* undergoes ISC with a rate constant of 2.0 × 109 s–1 and the resultant 3Ptz* decays to the GS prior to electron/energy transfer. On the other hand, the generated 1Por* by the photoexcitation undergoes both ET to the GS of PCBM and ISC to 3Por* with a rate constant of 1.9 × 1010 s–1. The generated 3Por* by the ISC decays to the GS and it may undergoes energy transfer to the GS of PCBM. The generated 1PCBM* by the photoexcitation transforms through ISC to 3PCBM*, which then undergoes ET from the GS of Ptz with a rate constant of 1.2 × 105 s–1. The resultant triplet CS state (3[Ptz•+–PCBM•–]) decayed slowly by BET with spin inversion with a rate constant of 1.4 × 103 s–1, similarly to 3[Ptz•+–C60•–]. In the case of Li+@C60⊂H4-Ptz-CPDPy(TEO) (Scheme S1a in SI), 1Por* forms a CS state (Por•+–Li+@C60•–) as the case of Li+@C60⊂H4-C4-CPDPy(OC6).51 On the other hand, 1Li+@C60* undergoes ISC and the resultant 3Li+@C60* undergoes ET from Ptz and/or Por with a rate constant of 1.2 × 105 s–1. Finally, in the case of bis-PCBM⊂H4-Ptz-CPDPy(TEO) (Scheme S1b in SI), the CS state of Por•+–bis-PCBM•– cannot be formed because its energy level is higher than that of the triplet


Page 35 of 52

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excited state of bis-PCBM. Both generated 1Ptz* and 1Por* undergo ISC with rate constants of 1.8 × 109 and 1.0 × 1010 s–1, respectively, and the resultant 3Ptz* and 3Por* decay to their ground states prior to electron/energy transfer. On the other hand, the singlet excited state of the included bis-PCBM undergoes ET from the GS of Ptz with a rate constant of 2.1 × 109 s–1 and the resultant CS state of Ptz•+–bis-PCBM•– decayed by fast BET to generate 3bis-PCBM*, which decayed to the GS with a rate constant of 2.5 × 105 s–1.

Conclusion: A phenothiazine-bridged free-base porphyrin dimer bearing peripheral four triethylene oxide groups (H4-Ptz-CPDPy(TEO)) included C60, PCBM, Li+@C60 and bis-PCBM with large association constants (Kassoc > 105 M-1) in PhCN. The electrochemical data indicated that the lowest energy levels of the CS states for C60⊂H4-Ptz-CPDPy(TEO), PCBM⊂H4-Ptz-CPDPy(TEO) and Li+@C60⊂H4-Ptz-CPDPy(TEO) are lower than those of the triplet excited states for the porphyrin, fullerene

and

phenothiazine

chromophores.


and

PCBM⊂H4-Ptz-CPDPy(TEO) underwent photoinduced electron transfer from the phenothiazine moiety to the triplet excited state of the included fullerene to afford the CS states composed of the phenothiazine radical cations and fullerene radical anions with the lifetimes of 0.71 ms. Li+@C60⊂H4-Ptz-CPDPy(TEO) also gave the similar photoinduced CS state with the lifetime of 0.56 ms. Moreover, the spin states of these CS states were determined to be triplet by their ESR spectra. The CS lifetimes of C60⊂H4-Ptz-CPDPy(TEO) and PCBM⊂H4-Ptz-CPDPy(TEO) are the longest values ever reported for phenothiazine–fullerene complexes in solution due to the spin-forbidden slow back electron-transfer processes. The phenothiazine bridges in the cyclic dimer play the double roles, that is, making the porphyrin-porphyrin separation (ca. 12.5 Å) suitable for the inclusion of C60



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Page 36 of 52

derivatives and donating electron to the included fullerenes to realize the long-lived CS states. On the other hand, bis-PCBM⊂H4-Ptz-CPDPy(TEO) formed a short-lived CS state because the estimated energy level of its CS state is higher than that of the triplet excited state of the included bis-PCBM.

Associated Content: Supporting Information Absorption and emission spectra, Job plot, electrochemical measurement data, transient absorption spectra, decay and rise time profiles of the transient absorption, temperature dependence of the back electron transfer processes, ESR spectra, theoretical calculation data, synthesis, NMR spectra, references and full author list. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information: Corresponding Author [email protected] [email protected] Notes The authors declare no competing financial interest.

Acknowledgement: This work was supported by Grants-in-Aid (No. 20108009 and 15K05432 to F. T., 20108010 to S. F., 26620154 and 26288037 to K.O.) and JSPS Fellows (25•627 to Y.K. (PD)) from Ministry of Education, Culture, Sports, Science and Technology of Japan and Research Grants to F.T. from 36 ACS Paragon Plus Environment

Page 37 of 52

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Tokuyama Science and Technology Foundation and Iketani Science and Technology Foundation. We acknowledge Prof. T. Shinmyozu of Kyushu University for his kind cooperation on the use of GPC and microbalance instruments. The theoretical calculation was carried out using the computer facilities at Research Institute for Information Technology, Kyushu University.

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Long-Lived Photoinduced Charge Separation in Inclusion Complexes

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