Journal of Porphyrins and Phthalocyanines J. Porphyrins Phthalocyanines 2006; 10: ###-###

PROOFS JPP061461_L_osamu

CONTENTS

hν

###

Ar

N

Zn

N

N

N

N

Ar

O

N

N

Zn

N

Ar

N

a ra t

ion

O

O H 3C N

DABCO

O

sep

Zinc porphyrin–fullerene–zinc porphyrin (ZnP-C60-ZnP) triad, in which two ZnP and C60 moieties are linked by flexible bonds, aiming a working model of the photosynthetic antenna-reaction centre, has been newly synthesized and its photophysical properties have been investigated by both time-resolved emission and transient absorption techniques.

N

rge-

Mohamed E. El-Khouly, Jun Hasegawa, Atsuya Momotake, Mikio Sasaki, Yasuyuki Araki, Osamu Ito* and Tatsuo Arai*

Ar

Ar

cha

Intramolecular photoinduced processes of newly synthesized dual zinc porphyrin-fullerene triad with flexible linkers

Ar

(ZnP-C60-ZnP)

Journal of Porphyrins and Phthalocyanines J. Porphyrins Phthalocyanines 2006; 10: 1-##

PROOFS JPP061461_L_osamu

Published at http://www.u-bourgogne.fr/jpp/

N

M N

N

Intramolecular photoinduced processes of newly synthesized dual zinc porphyrin-fullerene triad with flexible linkers Mohamed E. El-Khoulya,b, Jun Hasegawac, Atsuya Momotaked, Mikio Sasakia, Yasuyuki Arakia, Osamu Ito*a∏ and Tatsuo Arai*c Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan b Department of Chemistry, Faculty of Education, Tanta University, Kafr El-Sheikh, Egypt c Graduate School of Pure and Applied Sciences, University of Tsukuba, Tennodai, Tsukuba 305-8571, Japan d Research Facility Centre for Science and Technology, University of Tsukuba, Tennodai, Tsukuba 305-8571, Japan a

Received 24 June 2006 Accepted 12 January 2007 ABSTRACT: Zinc porphyrin-fullerene-zinc porphyrin triad, in which two zinc porphyrin (ZnP) moieties and a fullerene (C60) moiety are linked by flexible bonds and which is intended to be a working model of the photosynthetic antenna-reaction centre, has been newly synthesized. Its photophysical properties have been investigated by both time-resolved emission and transient absorption techniques. Excitation of the zinc porphyrin moiety of the triad induced charge separation, generating the radical ion pair, in which the electron localizes on the C60 moiety and the hole localizes on the zinc porphyrin moiety. In polar solvents, the charge-separated states decayed with lifetimes of 300600 ns returning to the ground state. Compared with ZnP-C60 dyad, ZnP-C60-ZnP triad showed longer lifetimes for the radical ion pair due to the conformation of the two ZnP moieties. The effects of the coordinating reagents on the zinc atom have been studied, with the expectation of conformational change of the two ZnP moieties with respect to C60. Copyright © 2006 Society of Porphyrins & Phthalocyanines. KEYWORDS: photoinduced electron transfer, fullerene, zinc porphyrin.

INTRODUCTION Development of relatively simple donor-acceptor models designed to mimic the events of the photosynthetic reaction centre has been one of the important goals of chemistry [1, 2]. Porphyrin (MP) and fullerene (C60) are among the most frequently employed compounds in artificial photosynthetic models. Studies on photoinduced processes in the linked MP-C60 systems have witnessed a rapid growth in the past decade [3, 4]. The rich redox electrochemical properties, highly delocalized πsystems and absorption extending over most of the ∏SPP full

member in good standing

*Correspondence to: Osamu Ito, email: [email protected]. ac.jp and Tatsuo Arai, email: [email protected] Copyright © 2006 Society of Porphyrins & Phthalocyanines

visible region render porphyrin and fullerene excellent building blocks for the construction of artificial photosynthetic systems and molecular photovoltaic devices [5, 6]. Furthermore, the small reorganization energies for the widely diffuse electron on the C60 radical anion accelerate the initial charge-separation (CS) process and decelerate the exothermic chargerecombination (CR) process [7]. The spatial arrangements of MP-C60 molecules are interesting subjects, because the rates of electron transfer depend considerably on the distance and relative orientation between MP-donor and C60acceptor. Here, we newly synthesized C60 triad connected with two ZnP moieties by –O–(CH2)4– O– flexible linkers (abbreviated as ZnP-C60-ZnP in Scheme 1), which may be of interest from the point

2

PROOFS JPP061461_L_osamu of view of molecular topology, because ZnP-C60-ZnP may allow substantial degrees of conformational flexibility. The flexibility of spacer bonds may be one of the important factors which affect the photophysical properties of the triad and it is quite useful to make the comparison with the previously reported rigid systems [8]. The two ZnP moieties in the studied triad are asymmetric with respect to the C60 sphere (see Scheme 1), which may provide a new aspect of the photophysical properties, compared to the previously reported C60 triad which is connected with two symmetrical ZnP moieties [8, 9]. ZnP-C60 dyad with the same –O–(CH2)4–O– flexible linker was also prepared as a reference. Steady-state absorption and fluorescence, time-resolved fluorescence and absorption spectral studies were systematically performed in order to reveal the photoinduced intramolecular processes of both compounds in a wide variety of solvents such as dimethylformamide (DMF), benzonitrile (PhCN), dichlorobenzene (DCB), and toluene (TN). The addition of pyridine (Py) and diazabicyclo[2.2.2]octane (DABCO), which are capable of coordinating to the zinc atoms of the ZnP moieties, will induce changes in the electronic properties of ZnP and in the conformations of the two appended ZnP moieties with respect to C60, which

may affect electron transfer properties [8, 10].

EXPERIMENTAL General remarks All materials for synthesis were reagent grade. Solvents for measurements were spectroscopic grade. Octylviologen (OV2+) phosphofulorite was prepared from the commercially available bromide, and 1-benzyl-1,4-benzyldihydronicotinamide (BDNA) was also commercially available. Steady-state UV-vis absorption and fluorescence spectra were measured with a JASCO model V570 DS and Shimadzu RF-5300 PC, respectively. The redox values were measured by the differential pulse voltammetry (DPV) technique using a BAS CV-50W Voltammetric Analyzer. A platinum disk and wire were used as a working electrode and counter electrode, respectively. An Ag/AgCl electrode was used as a reference electrode. All measurements were carried out in benzonitrile containing 0.1 M of t-Bu4NClO4. The picosecond time-resolved fluorescence spectra were measured by a single-photon counting method using a 392-400 nm light of Ti:sapphire laser (SpectraPhysics, fwhm = 1.5 ps) as an excitation source and

N

O

NH O

O

HO O

+

N

N H N H N

a

N

Br

TPP-OH

1

N

O

O

O

O

Br

+ TPP-OH

Br

4

ZnP-C60

3

O

N

N

HN

N NH

O O

a

b O

N NH HN N

5

O

N

N H N H N

O

O N

2

NH

O

c

O

N H N H N

N

N

O

b

O

N N Zn N

HN

O

N

O

N N Zn N N

HN N

O

c

O

O

N N N Zn N

O

N

6

ZnP-C60-ZnP

Scheme 1. Synthetic routes. (a) K2CO3, DMF, 18 h, 60 °C; (b) C60, sarcosine, 24 h, reflux; (c) Zn(OAc)2.H2O, 24 h, 40 °C Copyright © 2006 Society of Porphyrins & Phthalocyanines

J. Porphyrins Phthalocyanines 2006; 10: ##-##

PROOFS JPP061461_L_osamu a streakscope (Hamamatsu Photonics) as a detector. The nanosecond transient absorption spectra were measured by means of laser-flash photolysis; 532 nm light from Nd:YAG laser (Spectra-Physics, fwhm = 6 ns) was used as an excitation source. Monitoring light from a pulsed Xe-lamp in the near-IR region (6001100 nm) was detected with Ge-avalanche photodiode module (Hamamatsu Photonics) [11]. All the sample solutions in quartz cell (1 × 1 cm) were deaerated by bubbling argon gas. Synthesis Scheme 1 depicts the synthetic procedures for ZnPC60 and ZnP-C60-ZnP. ZnP-C60 was prepared starting from the coupling reaction of 1 [12] with TPP-OH to give aldehyde 2, followed by condensing 2 with C60 in the presence of sarcosine for fulleropyrrolidine synthesis. The free-base porphyrin-C60 dyad 3, thus obtained, was treated with Zn(OAc)2 to give ZnP-C60 [13]. Synthesis of ZnP-C60-ZnP was similar to that for ZnP-C60, but the starting material, 4, was used instead. 4 was newly prepared from 3,5-dihydroxybenzaldehyde with 1,4-dibromobutane. Both ZnPC60 dyad and ZnP-C60-ZnP triad were prepared according to the procedure below, described in Scheme 1. Synthesis of 1. To a solution of 3-hydroxybenzaldehyde (504 mg, 4.13 mmol) and 1,4-dibromobutane (4.52 g, 20.9 mmol) in anhydrous DMF (30 ml) was added finely powdered K2CO3 (794 mg, 5.74 mmol). After stirring for 15 h at room temperature, the reaction mixture was filtered and evaporated. The residue was purified by column chromatography (SiO2, hexanes/ ethyl acetate = 5/1) to give 1 (766 mg, 72%) as a pale yellow oil. 1H NMR (400 MHz, CDCl3): δ, ppm 9.98 (s, 1H, CHO), 7.46 (m, 2H, ArH), 7.38 (m, 1H, ArH), 7.17 (m, 1H, ArH), 4.06 (t, 2H, OCH H2), 3.50 (t, 2H, CH H2Br), 2.10-1.95 (m, 4H, CH2CH2). Synthesis of 2. To a solution of TPP-OH (245 mg, 0.389 mmol) and K2CO3 (87.0 mg, 0.624 mmol) in anhydrous DMF, was added drop wise a solution of 1 (102 mg, 0.40 mmol) in DMF (5 ml) at room temperature. After stirring for 18 h at 60 °C, DMF was removed by evaporation. The residue was purified by column chromatography (SiO2, chloroform) to give 2 (142 mg, 44%) as a purple powder. 1H NMR (400 MHz, CDCl3): δ, ppm 10.0 (s, 1H, CHO), 8.958.83 (m, 8H, ArH), 8.20 (m, 6H, ArH), 8.12 (m, 2H, ArH), 7.76 (m, 9H, ArH), 7.47 (m, 2H, ArH), 7.24 (m, 2H, ArH), 4.22 (m, 4H, OCH2 × 2), 2.18 (m, 4H, CH2CH2), -2.76 (s, 2H, porphyrin-NH). Synthesis of 3. A solution of 2 (51.0 mg, 63.2 μmol), C60 (702 mg, 0.975 mmol), and sarcosine (88.4 mg, 0.992 mmol) in toluene (250 ml) was refluxed for 24 h. After evaporation, the resulting mixture was purified by column chromatography (SiO2, eluting 0-100% chloroform in toluene) to give 3 (14.0 mg, Copyright © 2006 Society of Porphyrins & Phthalocyanines

XXXXXXXXXXXXXXX

3

9.8%) as a brown solid. 1H NMR (400 MHz, CDCl3): δ, ppm 8.83 (m, 8H, ArH), 8.21 (m, 6H, ArH), 8.04 (m, 2H, ArH), 7.77 (m, 9H, ArH), 7.18 (m, 2H, ArH), 6.94 (m, 1H, ArH), 4.82 (d, J = 8.4 Hz, 1H, pyrrolidine), 4.78 (m, 1H, pyrrolidine), 4.33-4.20 (m, 4H, OCH2), 4.08 (m, 1H, pyrrolidine), 2.80 (s, 3H, Nmethyl), 2.20-2.07 (m, 4H, CH2CH2), -2.80 (m, 2H, porphyrin-NH). ZnP-C60 dyad. To a solution of 3 (120 mg, 77.2 μmol) in dichloromethane (30 mL), was added dropwise, a solution of Zn(OAc)2.H2O (90.0 mg, 0.391 mmol) in methanol (10 mL). After stirring for 24 h at room temperature, the mixed solvent was evaporated. The resulting mixture was purified by column chromatography (SiO2, toluene) to give ZnP-C60 (49.8 mg, 40%) as a dark purple solid. 1H NMR (400 MHz, CDCl3): δ, ppm 8.92 (m, 8H, ArH), 8.21 (m, 6H, ArH), 8.04 (m, 2H, ArH), 7.77 (m, 9H, ArH), 7.18 (m, 4H, ArH), 6.94 (m, 1H, ArH), 5.30 (s, 1H, pyrrolidine), 4.70 (d, J = 9.2 Hz, 1H, pyrrolidine), 4.37-4.19 (m, 4H, OCH2), 3.48 (d, J = 9.2 Hz, 1H, pyrrolidine), 2.76 (s, 3H, N-methyl), 2.22-2.07 (m, 4H, CH2CH2). 13C NMR (500 MHz, CDCl3): δ, ppm 158.44, 155.10, 152.24, 150.55, 150.21, 150.13, 146.56, 146.53, 146.29, 145.70, 145.54, 145.50, 145.34, 145.16, 145.04, 144.70, 144.60, 144.50, 144.45, 144.39, 143.91, 142.93, 142.62, 142.35, 141.95, 141.81, 141.41, 141.29, 141.22, 141.15, 140.83, 139.14, 139.09, 139.02, 138.51, 136.21, 135.85, 135.45, 135.14, 134.42, 134.38, 132.14, 131.93, 127.45, 126.57, 121.13, 121.07, 121.00, 112.90, 68.51, 67.68, 67.59, 39.98, 31.59, 23.92, 25.54, 22.66, 14.12. Found C, 86.46; H, 3.16; N, 4.14, C117H45N5O2Zn. Requires C, 86.85; H, 2.80; N, 4.33. MS (MALDI-TOF): m/z calcd. for C117H45N5O2Zn: 1614.3. Found: 1616.5. Synthesis of 4. To a solution of 1,4-dibromobutane (15.7 g, 72.7 mmol) and finely powdered K2CO3 (1.24 g, 9.19 mmol) in anhydrous DMF (30 mL) was added dropwise, a solution of 3,5-dihydroxybenzaldehyde (500 mg, 3.62 mmol) in DMF (10 mL). After stirring for 18 h at room temperature, the reaction mixture was filtered and evaporated. The residue was purified by column chromatography (SiO2, hexanes/ethyl acetate = 10/1) to give 4 (794 mg, 54%) as a pale yellow solid. 1H NMR (400 MHz, CDCl3): δ, ppm 9.89 (s, 1H, CHO), 7.00 (d, 2H, ArH), 6.69 (t, 1H, ArH), 4.03 (t, 4H, OCH2), 3.49 (t, 4H, CH2Br), 1.92-2.12 (m, 8H, CH2CH2). Synthesis of 5. To a solution of TPP-OH (339 mg, 0.537 mmol) and K2CO3 (238 mg, 1.72 mmol) in anhydrous DMF, (25 mL) was added drop wise, a solution of 4 (101 mg, 0.247 mmol) in DMF (5 mL) at room temperature. After stirring for 18 h at 50 °C, DMF was removed by evaporation. After concentrating in vacuo, the residue was purified by column chromatography (SiO2, toluene) to give 5 (104 mg, 23%) as a purple powder. 1H NMR (400 J. Porphyrins Phthalocyanines 2006; 10: ##-##

4

PROOFS JPP061461_L_osamu MHz, CDCl3): δ, ppm 9.97 (s, 1H, CHO), 8.83 (m, 16H, ArH), 8.19 (m, 12H, ArH), 8.09 (m, 4H, ArH), 7.75 (m, 18H, ArH), 7.22 (m, 4H, ArH), 7.19 (m, 2H, ArH), 6.87 (m, 1H, ArH), 4.26 (s, 8H, OCH2), 2.16 (m, 8H, CH2CH2), -2.77 (s, 4H, porphyrin-NH). Synthesis of 6. A solution of 5 (50.6 mg, 33.6 μmol), C60 (127 mg, 0.176 mmol), and sarcosine (16.2 mg, 0.182 mmol) in toluene (80 mL) was refluxed for 24 h. After evaporation, the resulting mixture was purified by column chromatography (SiO2, toluene/ chloroform = 1/1) to give 6 (51.0 mg, 67%) as a purple solid. 1H NMR (400 MHz, CDCl3): δ, ppm 8.80 (m, 16H, ArH), 8.17 (m, 12H, ArH), 7.98 (m, 4H, ArH), 7.72 (m, 18H, ArH), 7.18 (m, 4H, ArH), 6.52 (m, 1H, ArH), 4.62 (d, J = 9.6 Hz, 1H, pyrrolidine), 4.56 (s, 1H, pyrrolidine), 4.23-4.10 (m, 8H, OCH2), 3.84 (d, J = 9.6 Hz, 1H, pyrrolidine), 2.74 (s, 3H, N-methyl), 2.12-2.00 (m, 8H, CH2CH2), -2.83 (m, 4H, porphyrinNH). MS (MALDI-TOF): m/z calcd. for C165H83N9O4: 2253.7. Found: 2254.7. ZnP-C60-ZnP triad. To a solution of 3 (36.0 mg, 16.0 μmol) in dichloromethane (15 mL) was added dropwise, a solution of Zn(OAc)2.H2O (420 mg, 1.84 mmol) in methanol (10 mL). After stirring for 20 h at 40 °C, the solvent was evaporated. The resulting mixture was purified by column chromatography (SiO2, chloroform) to give ZnP-C60-ZnP (25 mg, 65%) as a reddish brown solid. 1H NMR (400 MHz, CDCl3): δ, ppm 8.89 (m, 16H, ArH), 8.18 (m, 12H, ArH), 7.98 (m, 4H, ArH), 7.72 (m, 18H, ArH), 7.17 (m, 4H, ArH), 6.51 (m, 1H, ArH), 4.51 (d, J = 9.6 Hz, 1H, pyrrolidine), 4.46 (s, 1H, pyrrolidine), 4.23-4.10 (m, 8H, OCH2), 3.74 (d, J = 9.6 Hz, 1H, pyrrolidine), 2.73 (s, 3H, N-methyl), 2.12-2.00 (m, 8H, CH2CH2). 13C NMR (500 MHz, CDCl3): δ, ppm 158.31, 155.19, 153.08, 152.60, 152.27, 150.17, 150.09, 146.30, 146.25, 146.04, 145.55, 145.28,

145.05, 144.89, 144.77, 144.73, 144.52, 144.29, 144.20, 144.16, 144.09, 144.03, 143.89, 143.81, 143.68, 143.54, 143.35, 143.01, 142.95, 142.59, 142.10, 141.58, 141.44, 141.22, 141.18, 141.12, 141.01, 140.95, 140.86, 140.82, 140.70, 140.57, 140.28, 139.94, 139.24, 138.92, 138.88, 138.81, 138.73, 135.57, 135.42, 135.23, 134.45, 134.41, 132.06, 131.85, 127.39, 126.50, 121.00, 120.96, 120.90, 112.86, 101.52, 68.72, 67.66, 67.59, 39.98, 31.60, 25.92, 25.42, 22.66, 14.13. Found C, 81.96; H, 3.77; N, 5.02, C165H79N9O4Zn2. Requires C, 83.19; H, 3.34; N, 5.29; MALDI-TOF MS: m/z calcd. for C165H79N9O4Zn2: 2315.0. Found: 2315.6.

RESULTS AND DISCUSSION Optimized structures and MO calculations In order to evaluate optimized structures, the molecular dynamics (MD) calculations of H2P-C60H2P and H2P-C60 were performed using MM3 force field modified in MacroModel™ Package assuming vacuum as medium [14]; here, free-base porphyrin (H2P) was employed instead of ZnP, because of the absence of the parameters of the Zn atom. The optimized structures are selected among the various structures having low total energies; typical structures among them are shown in Fig. 1, in which both compounds adopt folded conformations. The H2P moieties approach around the C60 sphere, probably because of favorable π-π interactions between H2P and C60. From these structures, the centre-centre distances (dCC) between the H2P and C60 moieties of H2P-C60-H2P are estimated 7.3 and 11.2 Å; the latter dCC was almost the same as that of H2P-C60. In triad, when one of two H2P moieties approaches the C60

Fig. 1. The HOMO and LUMO of ZnP-C60-ZnP (Left side) and ZnP-C60 (Right side); optimized structures of these molecules were calculated for H2P in stead of ZnP Copyright © 2006 Society of Porphyrins & Phthalocyanines

J. Porphyrins Phthalocyanines 2006; 10: ##-##

PROOFS JPP061461_L_osamu sphere, another H2P must go to the opposite side of the C60 sphere with a different dCC value, because of the asymmetric structure of the linkage attached to one side of the pyrrodino group. Molecular orbitals of ZnP-C60-ZnP and ZnP-C60 were calculated using the density functional method (DFT) at B3LYP/3-21G level [15], assuming that ZnP-C60-ZnP and ZnP-C60 would take the same optimized structures as H2P-C60-H2P and H2P-C60. As shown in Fig. 1, the electron density of the highest occupied molecular orbital (HOMO) for ZnP-C60 was found to be located on the ZnP entity, while the electron density of the lowest unoccupied molecular orbital (LUMO) was localized on the C60 spheroid. These results suggest that the charge transfer from ZnP to C60 yields the radical ion-pairs. For triad, the charge-separated state like as ZnP•+-C60•--ZnP was suggested, although the HOMO and HOMO-1 localizing on each ZnP moiety are almost degenerated.

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5

and the C60 moiety [17]. The emission spectra were measured using 520 nm light to avoid the excitation of an intense Soret band, because signal saturation can usually be observed due to excessive light absorption by an intense band. The fluorescence intensities of the ZnP moiety of the triad in toluene significantly decreased relative to ZnPreference in toluene as shown in Fig. 3(a); suggesting quenching of the 1ZnP* moiety by the attached C60 entity even in toluene. Similar quenching of the 1ZnP* emission was observed in benzonitrile accompanied

Electrochemical studies Using differential pulse voltammetry, the first oxidation potential of the ZnP moiety (E Eox) was recorded at 0.42 V vs Ag/Ag+ in benzonitrile, while the first reduction potential of the C60 moiety (E Ered) was recorded at -0.83 V for both compounds. These redox values are in agreement with the earlier reported values for the ZnP and pyrrolidino-C60 derivatives [8]. The driving forces for the charge-separation (-ΔGCS) and charge-recombination (-ΔGCR) were evaluated according to the Rehm-Weller equations [16] as listed in Tables 1 and 2. The negative ΔGCS values in the studied polar solvents render charge separation from both the excited singlet states of the ZnP (1ZnP*) and C60 (1C60*) moieties. In toluene, although the ΔG values may include considerable estimation errors, the ΔG G values in the Tables indicate that the energy levels of the charge-separated states are lying between those of 1ZnP* and 1C60*.

Fig. 2. Steady-state absorption spectra of ZnP-C60-ZnP, ZnP-C60 and ZnP-reference in PhCN. Concentrations were maintained at 1.0 × 10-6 M

Steady-state absorption and fluorescence studies The absorption spectrum of zinc tetraphenylporphyrin (ZnP-reference) is characterized by a strong Soret band (428 nm) and a series of weaker Q-bands (560 and 602 nm) in benzonitrile. The electronic absorption spectra of ZnP-C60-ZnP and ZnP-C60 showed the Soret band at 430 nm as shown in Fig. 2, which slightly red-shifted compared with the ZnP-reference, suggesting weak interaction of the ZnP moiety with the C60 moiety. The weak absorptions of the C60 moiety in the shorter wavelength region than 650 nm are hidden in the huge absorptions of ZnP; at 700 nm a weak band of the C60 moiety appeared as shown in the inset of Fig. 2. A further weak band extending to 800 nm was observed, which is probably due to weak charge-transfer interaction between the ZnP moiety Copyright © 2006 Society of Porphyrins & Phthalocyanines

Fig. 3. Steady-state fluorescence spectra of: (Left panel) ZnP-C60-ZnP in toluene and PhCN along with ZnP-reference in TN. (Right panel) ZnP, ZnP-C60 and ZnP-C60-ZnP in PhCN; λex = 520 nm and concentrations were maintained at 3.0 × 10-6 M J. Porphyrins Phthalocyanines 2006; 10: ##-##

6

PROOFS JPP061461_L_osamu by the red-shift of the fluorescence peak. Compared with ZnP-C60, twice the fluorescence intensity of ZnP-C60-ZnP would be expected, when the solutions of these molecules with the same concentrations were subjected to the same excitation light power. However, the observed fluorescence intensity of ZnP-C60-ZnP was less than twice of that of ZnP-C60 as shown in Fig. 3(b), indicating that quenching of the 1ZnP* moiety by the C60 entity of ZnP-C60-ZnP is more efficient than that of ZnP-C60 by a factor of ca. 20%, when the fluorescence intensities were evaluated on the basis of the ZnP-reference. This finding suggests the higher light-harvesting ability of ZnP-C60-ZnP triad than that of ZnP-C60 dyad [18].

value [3]. The fluorescence time-profile of the 1ZnP* moiety of ZnP-C60-ZnP in each solvent exhibited biexponential decay, which gave major short and minor long lifetimes (τfZnP) as summarized in Table 1. The longer lifetime component is only a small portion, which may be attributed to the presence of extended conformation which appeared in the MD calculations. In all solvents, the τfZnP values are shorter than the

Time-resolved emission and transient absorption measurements via 1ZnP* The fluorescence-time profiles of the 1ZnP* moiety monitored at 600-700 nm excited by 400 nm laser light are shown in Fig. 4. The fluorescence time-profile of the ZnP-reference in toluene exhibited a mono-exponential decay with a lifetime (τf0ZnP) of 2.1 ns, which is in good agreement with the reported

Fig. 4. Fluorescence decay profiles of ZnP-reference in TN and ZnP-C60-ZnP in TN, PhCN and DMF monitored in the 600-700 nm region; λex = 400 nm. Shadow is laser pulse profile

Table 1. Free-energy changes of CS (-ΔGCS) via 1ZnP*, fluorescence lifetimes (τf), rate constants (kCSZnP) and quantum yields (ΦCSZnP) for CS via 1ZnP* of ZnP-C60-ZnP and ZnP-C60 Compound ZnP-C60-ZnP

Solvent

-ΔGCS, eV a

τfZnP, ps(fraction)b

kCSZnP, s-1 c

ΦCSZnP c

DMF

0.91

450 (50%)

1.8 × 109

0.80

PhCN

0.90

490 (64%)

1.6 × 109

0.77

DCB

0.84

630 (70%)

1.1 × 109

0.71

0.86

520 (74%)

1.5 × 10

0.76

0.86

480 (74%)

9

1.6 × 10

0.78

TN

0.28

540 (70%)

1.4 × 109

0.75

TN-Py

0.27

430 (70%)

1.9 × 109

0.80

DMF

0.90

500 (73%)

1.6 × 109

0.77

PhCN

0.89

590 (75%)

1.2 × 109

0.73

DCB

0.78

740 (94%)

0.9 × 10

0.66

0.79

680 (100%)

9

1.0 × 10

0.69

DCB-DABCO d

0.79

620 (100%)

1.2 × 109

0.70

TN

0.20

630 (94%)

1.1 × 109

0.71

TN-Py

0.18

510 (100%)

1.5 × 10

0.77

DCB-Py

d

DCB-DABCO

ZnP-C60

DCB-Py

d

d

9

9

9

The -ΔGCS values were calculated according to equations; -ΔGCS = ΔE E00 - e (E Eox + Ered) - ΔGS; where ΔE E00 is the 0-0 transition energy (2.08 eV for 1ZnP*); Eox (ZnP) = 0.42 V and Ered (C60)= -0.83 V vs Ag/Ag+ in PhCN. ΔGS refers to static energy calculated by -ΔGS = e2/ (4πε0εRdCC) in PhCN, while in other solvents, -ΔGS = -(e2/(4πε0))[(1/(2R+) + 1/(2R-) - (1/d dCC)/εS - (1/(2R+) + 1/(2R-))/εR), where R+ and R- are radii of the radical cation and radical anion evaluated from the optimized structure; εR and εS refer to solvent dielectric constants for electrochemistry and electron-transfer, respectively. b Goodness-of-fit parameters (χ2) were 1.00-1.17 and slow lifetimes were ca. 2000 ps. c The kCSZnP and ΦCSZnP were evaluated by kCSZnP = (1/ (1/ττf ZnP) - (1/ (1/τf0ZnP) and ΦCSZnP = kq/ (1/ (1/τfZnP).d Py and DABCO (10 equivalents to the ZnP moiety). a

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PROOFS JPP061461_L_osamu τf0ZnP value, suggesting that the fluorescence of the 1 ZnP* moiety is quenched by the attached C60 entity.

7

shown in Scheme 2. DABCO may induce a drastic change in the distance between the ZnP moieties and the C60 sphere, making a scissor-type structure. Indeed, upon addition of a small excess of Py and DABCO in dichlorobenzene, which induced redshifts in the absorption and fluorescence peaks due to the coordination to the zinc atoms (SI), slight shortenings of the τfZnP values were observed as listed in Table 1, which suggests that Py and DABCO tend to detach the ZnP moiety from the C60 sphere [8, 9c]. DABCO showed a slightly stronger effect than Py, supporting the scissor-type structure by the DABCO coordination. The characteristic transient absorption band of the C60•- moiety at 1020 nm was employed to determine the rates of the charge-recombination process (kkCR)

In polar solvents. The nanosecond transient absorption spectra, which was obtained by 532 nm laser light excitation (Fig. 5(a)), exhibited the characteristic absorption peaks of the fulleropyrrolidine radical anion, (C60•-) at 1020 nm and the ZnP radical cation (ZnP•+) at 660 nm; clear evidence for the occurrence of photoinduced charge-separation from 1ZnP* to C60. Therefore, we assigned the shorter τfZnP values in polar solvents to the charge-separation process, because the generation of the radical ion pairs (ZnP•+-C60•- and ZnP•+-C60•--ZnP) was confirmed. The rates (kkCSZnP) and quantum-yields (ΦCSZnP) of the charge-separation process were estimated from shorter τfZnP values as listed in Table 1. It was found that, the kCSZnP values increase with the solvent polarity; toluene < benzonitrile < dimethylformamide, although the values in dichlorobenzene deviate from this order. Slightly higher rates and yields of the charge-separation process for ZnP-C60-ZnP triad compared to those of ZnP-C60 dyad were obtained, which can be explained by the shorter distance between the C60 sphere and one of two ZnP moieties of ZnP-C60-ZnP, than that of ZnP-C60. In general, the kCSZnP values of both compounds are comparable with the reported ZnP-C60 molecules with flexible polyether chains [8, 9] and slower than those in the reported doubly-linked faceto-face ZnP-C60 systems [19]. This difference can be rationalized in terms of the topological factors such as proximity and rigidity. Upon addition of pyridine (Py), a slight increase in the distance between the ZnP moieties and the C60 sphere can be expected, as

Fig. 5. Transient absorption spectra observed by 532 nm laser light excitation of ZnP-C60-ZnP (1.0 × 10-4 M) in Arsaturated PhCN. Inset: time profile Ar

Ar

N

O

Ar

N

Zn N

O

N

N

H3C

Ar

N

N

Zn N

Ar

N N

Ar

N Ar

N

N N

N Zn N

O

O

Ar

Py

(ZnP-C60)

O

Ar

O

H3C N

DABCO

(ZnP-C60-ZnP)

Scheme 2. Possible structures in the presence of Py and DABCO Copyright © 2006 Society of Porphyrins & Phthalocyanines

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PROOFS JPP061461_L_osamu of ZnP•+-C60•--ZnP and ZnP•+-C60•-, since each decay of the C60•- was well-fitted by a single-exponential function (inset of Fig. 5). As listed in Table 2, the lifetimes of the radical ion pairs (τRIP) of ZnP-C60ZnP were evaluated as 310 ns (dimethylformamide), 370 ns (benzonitrile), and 680 ns (dichlorobenzene), which suggests that the charge-recombination process occurs in the inverted region of the Marcus parabola [20]. Indeed, the calculated -ΔGCR values for the ZnP•+-C60•--ZnP (> 1 eV in Table 2) are larger than the reported reorganization energies less than ca. 0.7 eV [7]. Interestingly, the τRIP value for ZnP•+-C60•-ZnP in each solvent is slightly longer than the τRIP value for ZnP•+-C60•- in the same solvent. Although the opposite tendency would be anticipated from the optimized structures in Fig. 1, it is considered that ZnP•+-C60•--ZnP and ZnP•+-C60•- may have different structures from the neutral molecules. Upon addition of DABCO in dichlorobenzene, slight shortenings of the τRIP values were observed as listed in Table 2, suggesting that DABCO accelerates the chargerecombination process. It is notable, that the τRIP values in dichlorobenzene in the present study were longer than those of the previously reported dyads and triads, which were shorter than 20 ns in dichlorobenzene [10]. Since dichlorobenzene is a border-line solvent between polar benzonitrile and non-polar toluene, slight structural changes of the dyad and triad may induce the drastic variations of τRIP values. The charge-recombination rates in polar solvents including dichlorobenzene are slower than the charge-separation rates by about

3 orders, suggesting the usefulness of ZnP-C60-ZnP (and ZnP-C60) as photosynthetic models.

Fig. 6. (Left panel) Time resolved fluorescence spectra of ZnP-C60-ZnP at 0.1 and 1.0 ns time intervals in TN (λex = 400 nm). (Right panel) Fluorescence decay profiles of ZnPC60-ZnP monitored in 700-800 nm region in TN and PhCN (λex = 392 nm). Concentrations were maintained at 5.0 × 10-5 M

Table 2. Free-energy changes (-ΔGCR) and rate constants (kkCR) for CR and lifetimes of radical ion pairs (τRIP) of ZnP-C60-ZnP and ZnP-C60 in various solvents Compound ZnP-C60-ZnP

Solvent

-ΔGCR / eV a

kCR / s-1

DMF

1.17

3.2 × 10

310

PhCN

1.18

2.7 × 106

370

DCB

1.25

1.4 × 106

680

1.23

2.8 × 10

360

TN

1.80

8

2.0 × 10

< 10

TN-DABCO b

1.78

1.7 × 106

590

DMF

1.18

3.7 × 106

270

PhCN

1.19

3.1 × 10

320

DCB

1.28

6

1.5 × 10

650

DCB-DABCO b

1.27

2.0 × 106

500

TN

1.88

2.0 × 108

< 10

1.87

1.7 × 10

590

DCB-DABCO

ZnP-C60

TN-DABCO a

τRIP / ns 6

b

b

6

6

6

-ΔGCR = e (Eox - Ered) + ΔGS. b DABCO (10 equivalents to the ZnP moiety).

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PROOFS JPP061461_L_osamu In toluene. As shown in Fig. 6(a), the time-resolved fluorescence spectrum at 1.0 ns of ZnP-C60ZnP exhibited a new fluorescence peak at 720 nm due to the 1C60* moiety, in addition to the fluorescence peaks of the 1ZnP* moiety at 648 nm, although the peak at 720 nm was not appreciable in the spectrum, at 0.1 ns. However, the emission of 1C60* was not observed in polar solvents. Similar phenomena were observed for ZnP-C60. This observation suggests that the C60 emission emerges with the decay of the 1 ZnP* moiety in toluene. Since the energy level of the charge-separated state in toluene is lower than 1ZnP* and higher than 1C60*, the rapid decay of 1ZnP* may be attributed to the charge-separation process, after which the charge-recombination process gave rise to the 1C60* moiety [21]. Since the fluorescence of the 1 C60* moiety decayed with a lifetime (1300 ps) in toluene as shown in Fig. 6(b), which is the same as that of pristine C60, the charge-separation process via the 1C60* moiety does not take place in toluene. Upon addition of Py in toluene, slight shortenings of the τf values of the 1ZnP* moiety were observed, similar to those in dichlorobenzene (Table 1). The nanosecond transient spectra of both compounds in toluene showed mainly the absorption band of the 3C60* moiety at 700 nm, along with the weak band of the C60•- moiety at 1020 nm, as shown in Fig. 7(a). The time profile of the C60•- moiety showed quick decay followed by slow decay. The quick decay may be attributed to the rapid chargerecombination process with a rate constant of 2 × 108 s-1, whereas the slow decay part at 1020 nm may be ascribed to the decay of the absorption tail of the 3C60* moiety, which may be generated via the intersystem crossing (ISC) from the 1C60* moiety. By combining with the fluorescence data such as the short fluorescence lifetime of the 1ZnP* moiety and the unchanged fluorescence lifetime of the 1C60* moiety, the predominant observation of the 3C60* moiety in the transient spectra supports the charge-separation/ charge-recombination processes as reported in some other types of porphyrin-fullerene systems [22]. Upon addition of DABCO to toluene, the transient absorption spectra changed drastically, showing clearly the C60•- moiety in the charge-separated state, which is prolonged for more than 1020 ns as shown in Fig. 7(b). The lifetimes of radical ion-pairs are listed in Table 2, suggesting that DABCO very much decelerated the charge-recombination process. Such drastic changes induced by the addition of DABCO were the first observation in toluene, because a similar tendency was not observed in dichlorobenzene for previously reported systems [10]. Even in non-polar solvent, additions of small amounts of DABCO significantly increase the ratios of kCS/kkCR, which increase the usefulness of both compounds as photosynthetic models. Copyright © 2006 Society of Porphyrins & Phthalocyanines

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Fig. 7. Transient absorption spectra observed by 532 nm laser light excitation of ZnP-C60-ZnP (1.0 × 10-4 M) (Left panel) in Ar-saturated toluene and (Right panel) DABCO (1.0 × 10-3 M) in toluene. Inset: time profile

Time-resolved fluorescence via 1C60* Figure 6(b) shows the fluorescence time-profile of the 1C60* moiety in the 700-800 nm region of ZnP-C60ZnP observed in benzonitrile with laser light at 392 nm, which excited the C60 to ZnP moieties in a ratio of 43:57 as evaluated from the absorption spectrum. The time profile shows two-component decay, giving a major short lifetime as 300 ps for ZnP-C60-ZnP. Similarly, for ZnP-C60, a major short lifetime was evaluated to be 330 ps in benzonitrile. These findings indicate that the charge-separation process takes place via the 1C60* moiety to yield RIP in polar solvents such as benzonitrile. The rates (kkCSC60) and quantum yields (ΦCSC60) for ZnP-C60-ZnP in benzonitrile were evaluated as 2.6 × 109 s-1 and 0.76, respectively, which are slightly higher that those of ZnP-C60 (kkCSC60 = 2.2 × 109 s-1 and ΦCSC60 = 0.73). This tendency is the same as that observed via the 1ZnP* moiety. The chargeseparation process of ZnP-C60-ZnP triad via the 1C60* moiety was confirmed by recording the nanosecond transient spectra while applying 355 nm laser light in benzonitrile where the lifetime τRIP was evaluated as 330 ns, which was in good agreement with the τRIP value obtained by 532 nm laser. J. Porphyrins Phthalocyanines 2006; 10: ##-##

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PROOFS JPP061461_L_osamu Energy diagrams An energy diagram is shown in Fig. 8, in which the energy levels are cited from the ΔGCS and -ΔGCR values listed in Tables 1 and 2. In polar solvents, the charge-separated states are lower than the energy levels of both the 1ZnP* and 1C60* moieties, which makes the efficient charge-separation process possible, as confirmed by the combination of the fluorescence quenching with the transient absorptions of the C60•- moiety in addition to the ZnP•+ moiety. In toluene, the charge-separation process takes place only via the 1ZnP* moiety, shown Fig. 8. Energy level diagram in polar and non-polar solvents

as a dotted line in Fig. 8. Furthermore, rapid chargerecombination takes place yielding the 1C60* moiety, as the spike-like time profile at 1020 nm suggested; then, the 1C60* moiety decayed yielding 3C60* via an intersystem crossing process. Although the chargerecombination process in toluene takes place in the deep inverted region of the Marcus parabola, the rapid charge-recombination process suggests that the radical cation and radical anion in the charge-separation state are not fully separated in toluene. Since the energy levels are not appreciably changed by the addition of Py and DABCO, drastic prolongations of the lifetimes of the radical ion pairs are difficult to explain, only on the basis of this energy diagram. Electron mediating systems

Fig. 9. (Left panel) Steady-state absorption spectral changes of ZnP-C60-ZnP (3.0 × 10-6 M) in the presence of OV2+ ((0.0 - 4.88) × 10-3 M) and BDNA (2.0 × 10-3 M) in Ar-saturated PhCN after repeated 532 nm laser light irradiations up to maximal absorbance. (Right panel) Time profiles of ZnPC60-ZnP (5.0 × 10-5 M) in the presence of OV2+ ((0.0 - 4.88) × 10-3 M) at 1020 nm and 610 nm observed by the transient absorption method using 532 nm light pulse in Ar-saturated PhCN Copyright © 2006 Society of Porphyrins & Phthalocyanines

The electron-mediating processes of ZnP•+-C60•-ZnP were confirmed using octhylviologen dication (OV2+) as an electron mediator. An accumulation of characteristic absorption bands of the radical cation OV•+ (λmax = 610 and 400 nm) [22] was observed in benzonitrile with the repeated 532 nm laser light irradiation as shown in Fig. 9(a) upon further addition of 1,4-benzyldihydronicotinamide (BDNA). This observation suggests that an intermolecular electronmediating process takes place from C60•- to OV2+, since this process is exothermic by ca. 0.21 eV, on comparison between the Ered values of C60 and OV2+. During the accumulation of OV•+, the consumption of BDNA was observed at 375 nm in Fig. 9(a), suggesting that the hole-shift from ZnP•+ to BDNA takes place, because this process is exothermic by 0.10 eV comparing Eox values of ZnP and BDNA (E Eox = 0.32 V) [23]. After the hole shift, the radical cation of BDNA usually changes irreversibly to 1-benzyl-nicotinamidiniumion (BNA+), which has less electronaccepting ability. Then, the same amount of OV•+ as the consumed BDNA, accumulates [23]. By the transient absorption measurements, the J. Porphyrins Phthalocyanines 2006; 10: ##-##

PROOFS JPP061461_L_osamu increases of the rising absorption band of OV•+ at 610 nm were observed with concomitant decays of the C60•- moiety (Fig. 9(b)), which confirms the electronmediating process from the C60•- moiety to OV2+, in addition to the direct electron transfer from the 3 ZnP* moiety to OV2+ [24]. From the pseudo-firstorder relation, the bimolecular electron-mediating rate-constant (kkmd) was evaluated as 1.2 × 1010 s-1 in benzonitrile. By the long, time-scale measurements, slow decays of OV•+ were observed due to the back electron transfer between positively charged species, OV•+ and ZnP•+. The back electron-transfer rate was further suppressed by the addition of BDNA, which may finally lead to the accumulation of the OV•+ concentration. These findings serve as clear evidence for the photosensitized, charge-separation process of both compounds and successive, electron-mediating processes, resulting in the accumulation of the electron mediator, OV•+.

2.

3.

CONCLUSION We reported here the newly synthesized ZnP-C60ZnP, in which C60 is connected with ZnP via long flexible bonds to exhibit an example of a working model of the photosynthetic antenna-reaction centre. Efficient charge-separation processes via 1ZnP* and also via 1C60* were confirmed for ZnP-C60ZnP, which can be supported by the presumed structures using molecular dynamic calculations, which suggest that one of the two ZnP moieties and C60 moiety are in close proximity. Compared with ZnPC60 dyad, ZnP-C60-ZnP triad shows higher chargeseparation efficiency and a longer lifetime of the radical ion-pairs. The charge-recombination rates in polar solvents are slower than the charge-separation rates by about 3 orders, suggesting the usefulness, as a photosynthetic model, of ZnP-C60-ZnP. In nonpolar solvent, additions of small amounts of Py and DABCO very much increase the ratios of kCS/kkCR. The photosensitized charge-separation/electron-mediating/hole-shifting processes of ZnP-C60-ZnP and ZnP-C60 were confirmed in the presence of electronmediator and hole-shifter. These observations afford a guiding principle for more efficient molecular systems for further study.

4.

5.

6.

Acknowledgements This work was also supported by a Grant-in-Aid for Scientific Research on Priory Area (417) from the Ministry of Education, Culture, Sports, Science, and Technology of Japanese Government.

REFERENCES 1. a) Fox MA and Chanon M. In Photoinduced Copyright © 2006 Society of Porphyrins & Phthalocyanines

7.

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11

Electron Transfer, Part D, Elsevier: Amsterdam, 1988. b) Gust D and Moore TA. Top. Curr. Chem. 1991; 159: 103. c) Wasielewski MR. Chem. Rev. 1992; 92: 435. d) McLendon G and Hake R. Chem. Rev. 1992; 92: 48. e) Deisenhofer J and Norris JR. In The Photosynthetic Reaction Centre, Academic Press: San Diego, 1993. a) Paddon-Row MN. Acc. Chem. Res. 1994; 27: 18. b) Petty MC, Bryce MR and Bloor D. In Introduction of Molecular Electronics, Oxford University Press: New York, 1995. c) Piotrowiak P. Chem. Soc. Rev. 1999; 2: 143. d) Verhoeven JW. Adv. Chem. Phys. 1999; 106: 603. e) Sun L, Hammarstrom L, Akermark B and Styring S. Chem. Soc. Rev. 2001; 30: 36. f) Balazani V. In Electron Transfer in Chemistry, Wiley-VCH: Weinheim, 2001. a) Kalyanasundaram K. In Photochemistry of Polypyridine and Porphyrin Complexes, Academic Press: London, 1992. b) Kadish KM, Smith KM and Guilard R. In The Porphyrin Handbook, Vol. 8, Academic Press: San Diego, CA, 2000. a) Hirsch A. In Fullerene and related Structures, Springer: Berlin, 1999; pp 199. b) Kadish KM and Ruoff RS. In Fullerenes Chemistry, Physics, and Technology, John Wiley & Sons: New York, 2000. c) Meijer MD, van Klink GPM and van Koten G. Coord. Chem. Rev. 2002; 230: 141. d) Nalwa HS. In Handbook of Photochemistry and Photobiology, Vol. 2, American Science Publisher: CA, 2003; pp 111. e) Verhoeven JW, J. Photochem. Photobiol. C 2006; 7: 40. a) Kuciauska D, Lin S, Seely GR, Moore AL, Moore TA, Gust D, Drovetskaya T, Reed CA and Boyd PDW. J. Phys. Chem. 1996; 100: 15926. b) Armaroli N, Marconi G, Echegoyen L, Bourgeois JP and Diederich F. Chem. Eur. J. 2000; 6: 1629. c) Reed CA and Bolskar RD. Chem. Rev. 2000; 100: 1075. d) Guldi DM, Chem. Soc. Rev. 2002; 31: 22. e) Bell TDM, Ghiggino KP, Jolliffe KA, Ranasinghe MG, Langford SJ, Shephard MJ and Paddon-Row MN. J. Phys. Chem. A 2002; 106: 10079. a) Guldi DM and Martín N. In Synthesis to Optoelectronic Properties, Kluwer Academic Publishers: Norwell, 2002. b) Imahori H, Mori Y and Matano Y. J. Photochem. Photobiol. C 2003; 4; 51. c) El-Khouly ME, Ito O, Smith PM and DʼSouza F. J. Photochem. Photobiol. C 2004; 4: 79. a) Imahori H, Hagiwara K, Akiyama T, Aoki M, Taniguchi S, Okada T, Shirakawa M and Sakata Y. Chem. Phys. Lett. 1996; 263: 545. b) Imahori H, Tamaki K, Guldi DM, Luo C, Fujitsuka M, Ito O, Sakata Y and Fukuzumi S. J. Am. Chem. Soc. 2001; 123: 2607. c) Yamazaki M, Araki Y, J. Porphyrins Phthalocyanines 2006; 10: ##-##

12

PROOFS JPP061461_L_osamu

8. 9.

10.

11. 12. 13. 14.

15.

16. 17.

18. 19.

Fujitsuka M and Ito O. J. Phys. Chem. A 2001; 105: 8615. Sandanayaka ASD, Ikeshita K, Araki Y, Kihara N, Furasho Y, Takata T and Ito O. J. Mater. Chem. 2005; 15: 2276. a) Da Ros T, Prato M, Guldi DM, Ruzzi M and Pasimeni L. Chem. Eur. J. 2001; 7: 816; b) Guldi DM, Luo C, Da Ros T, Prato M, Dietel E and Hirsch A. Chem. Commun. 2000; 375. c) Guldi DM, Luo C, Swartz A, Scheloske M and Hirsch A. Chem. Commun. 2001; 1066. d) Diederich F and Lopez MG. Chem. Soc. Rev. 1999; 28: 263; e) Piotrowiak P. Chem. Soc. Rev. 1999; 28: 143. f) Yin G, Xu D and Xu Z. Chem. Phys. Lett. 2002; 365: 232. g) Guldi DM, Imahori H, Tamaki K, Kashiwagi Y, Yamada H, Sakata Y and Fukuzumi S. J. Phys. Chem. A 2004; 108: 541. h) Schuster DI, MacMahon S, Guldi DM, Echegoyen L and Braslavsky SE. Tetrahedron 2006; 62: 1928. a) DʼSouza F, Gadde S, Zandler ME, Klyov A, El-Khouly ME, Fujitsuka M and Ito O. J. Phys. Chem. A 2002; 106: 12393. b) El-Khouly ME, Araki Y, Ito O, Gadde S, McCarty AL, Karr PA, Zandlerb ME and DʼSouza F. Phys. Chem. Chem. Phys. 2005; 7: 3163. Matsumoto K, Fujitsuka M, Sato T, Onodera S and Ito O. J. Phys. Chem. B 2000; 104: 11632. Poradosu E, Gazit A, Reuveni H and Levitzki A. Bioorg. Med. Chem. 1999; 9: 1727. Prato M, Maggini M, Giacometti C, Scorrano G, Sandona G and Farnia G. Tetrahedron 1996; 52: 5221. a) Mohamadi F, Richards NGJ, Guida WC, Liskamp R, Lipton M, Gaufield C, Chang G, Hendrickson T and Still WC. J. Comput. Chem. 1990; 11: 440. b) Brown ST, Rienstra-Kiracofe JC and Schaefer HF. J. Phys. Chem. A 1999; 103: 4065. Gaussion package. For Actual calculations, see for example; DʼSouza F, Deviprasad GR, Zandler ME, Hoang VT, Arkady K, VanStipdonk KM, Perera A, El-Khouly ME, Fujitsuka M and Ito O. J. Phys. Chem. A 2002; 106: 3243. Rehm D and Weller A. Isr. J. Chem. 1970; 8: 259. a) Watanabe N, Kihara N, Furusho Y, Takata T, Araki Y and Ito O. Angew. Chem. Int. Ed. 2003; 42: 681. b) Sandanayaka ASD, Watanabe N, Ikeshita K, Araki Y, Kihara N, Furusho, Y, Ito O and Takata T. J. Phys. Chem. A 2004; 109: 2516. Choi MS, Aida T, Luo X, Araki Y and Ito O. Angew. Chem., Int. Ed. 2001; 40: 3194. a) Frey W, Klann R, Laermer F, Elaesser T, Baumann E, Futscer M and Staab HA. Phys. Chem. Lett. 1992; 190: 567. b) Schuster DI,

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20.

21. 22. 23. 24.

Cheng P, Wilson SR, Prokhorenko V, Katterie M, Holzwarth AR, Braslavsky SE, Klihm G, Williams RM and Luo C. J. Am. Chem. Soc. 1999; 121: 11599. c) Chukharev V, Tkachenko NV, Efimov A, Guldi DM, Hirsch A, Scheloske M and Lemmetyinen H. J. Phys. Chem. B 2004; 108: 16377. a) Marcus RA. J. Chem. Phys. 1965; 43: 679. b) Marcus RA and Sutin N. Biochim. Biophys. Acta. 1985; 811: 265. c) Marcus RA. Angew. Chem., Int. Ed. Eng. 1993; 32: 111. Imahori H, El-Khouly ME, Fujitsuka M, Ito O, Sakata Y and Fukuzumi S. J. Phys. Chem. A 2001; 105: 325. Konishi T, Fujitsuka M, Ito O, Toba Y and Usui Y. J. Phys. Chem. A 1999; 103: 9938. Fukuzumi S, Suenobu T, Patz M, Hirasaka T, Itoh S, Fijitsuka M and Ito O. J. Am. Chem. Soc. 1998; 120: 8068. Xiao S, El-Khouly ME, Li Y, Gan Z, Liu H, Jiang L, Araki Y, Ito O and Zhu D. J. Phys. Chem. B 2005; 109: 365.

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