Photoinduced Electron Transfer and Charge-Recombination in 2

ARTICLE pubs.acs.org/JPCC

Photoinduced Electron Transfer and Charge-Recombination in 2-Ureido-4[1H]-Pyrimidinone Quadruple Hydrogen-Bonded PorphyrinFullerene Assemblies Mao-Lin Yu,† Su-Min Wang,† Ke Feng,† Tony Khoury,‡ Maxwell J. Crossley,*,‡ Yang Fan ,§ Jian-Ping Zhang,*,§ Chen-Ho Tung,† and Li-Zhu Wu*,† †

Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry & Graduate University, The Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ School of Chemistry, The University of Sydney, NSW 2006, Australia § Department of Chemistry, Renmin University of China, Beijing 100872, P. R. China

bS Supporting Information ABSTRACT: 2-Ureido-4[1H]-pyrimidinone-bridged porphyrin fullerene dyad assemblies III were designed and synthesized to investigate the intra-assembly photoinduced electron transfer (PET) via the quadruple complementary hydrogen bonds. Steady-state and time-resolved spectroscopy demonstrate that upon excitation of the porphyrin, electron transfer to the fullerene occurs with rate constants (quantum efficiency) of 1.6 108 s1 (60%) and 4.2 108 s1 (44%) for assembly I and II, respectively, in a CH2Cl2 solution at room temperature. More importantly, the rate of charge recombination was found to be rather slow with a lifetime of charge separation (CS) up to 9.8 μs for assembly I and 4.0 μs for assembly II. Because the relatively rigid assemblies prevent the porphyrin and fullerene from any intra-assembly collisions either through solvent or through space mechanisms, the quadruple complementary hydrogen bonds play crucial roles in mediating the intra-assembly PET process. This is, to the best of our knowledge, the first direct evidence for the PET process via 2-ureido-4[1H]-pyrimidinone complementary quadruple hydrogen-bonded systems.

’ INTRODUCTION Photosynthesis is a natural energy-conversion system that converts solar energy into chemical energy; the primary processes of which are a cascade of photoinduced excitation energy transfer and subsequent electron transfer (ET) between the electron donor and acceptor to attain a charge-separated (CS) state.1 The success and importance of photosynthesis have inspired researchers to study ET processes for developing artificial molecular systems for solar energy conversion. Over the past decades, a host of donoracceptor systems have been constructed, which upon photoexcitation, give rise to long-lived CS states with high quantum yields.214 In this context, the investigation of the PET process, wherein electron donor and acceptor are assembled via hydrogen-bonding interaction, has attracted much interest because ET reactions in biological photosynthesis in nature are regulated through a network of hydrogen bonds.8 These systems have been extended to the donoracceptor assemblies linked by two point hydrogen bonds,710 triple hydrogen bonds,11,12 or multiple hydrogen bonds.13,14 Therien et al.,7 for example, demonstrated that the PET process via the carboxylic acid/hydrogen-bond interface is more efficient than those provided by comparable σ- or r 2011 American Chemical Society

π-bonding networks. Guldi et al.8 revealed a remarkable level of electronic communication in amidiniumcarboxylate saltbridged porphyrinfullerene pairs with the formation of the longest CS state, ∼10 μs, ever reported for a hydrogen-bonded assembly in solution at room temperature. Sessler et al.11 studied the hydrogen-bond-mediated PET via WatsonCrick base-pairing and achieved the CS lifetime up to 2.02 μs. Very recently, Hirsch et al.14 made use of the Hamilton-receptor/ cyanuric acid binding motif to link porphyrin and fullerene at distances from 12.2 to 19.1 Å for the PET occurrence. The 2-ureido-4[1H]-pyrimidinone module developed by Meijer and Sijbesma15 represents a fascinating self-complementary quadruple hydrogen-bonding module. Owing to its great binding strength and directionality, this module has shown extensive applications in assembling and disassembling supramolecular systems.16,17 Recently, the quadruple complementary module has also been utilized to create hydrogen-bonded donoracceptor dyads.1820 However, only the singletsinglet Received: August 15, 2011 Revised: October 20, 2011 Published: October 22, 2011 23634

dx.doi.org/10.1021/jp207852j | J. Phys. Chem. C 2011, 115, 23634–23641

The Journal of Physical Chemistry C

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Scheme 1. 2-Ureido-4[1H]-Pyrimidinone Quadruple Complementary Hydrogen-Bond Interfaced PorphyrinFullerene Assemblies I (1a 3 2) and II (1b 3 2)

energy transfer process in several cases18,19 and the triplet triplet energy transfer process in one case20 were observed. It is surprising to see that in no case was direct evidence for the PET process in the complementary quadruple hydrogen-bonded system obtained, even for the energetically possible ET reactions.18,21 One may suspect whether 2-ureido-4[1H]-pyrimidinone quadruple complementary hydrogen-bonds could be used as a conduit for the PET process. In the present work, we wish to report the synthesis and PET processes of two relatively rigid 2-ureido-4[1H]-pyrimidinonebridged porphyrinfullerene assemblies III (Scheme 1). With directionality and rigidity of the hydrogen-bonded module, the porphyrin and fullerene directly attached to the hydrogenbinding unit in assemblies III are projected in the opposite direction within the assembly and, at the same time, inhibit intraassembly collisions between the donor and acceptor via the through-space mechanism. Moreover, the high dimerization constant and long pre-exchange lifetime of the 2-ureido-4[1H]pyrimidinone quadruple hydrogen-binding unit would enhance the intra-assembly donoracceptor interaction, thereby avoiding any intermolecular diffusion encounter between the electron donor and acceptor even at low concentration. As for the electronic donor and acceptor, the porphyrin and fullerene were used owing to their rich and well-understood electrochemical and spectroscopic properties.26 Benefiting from the unique giant, spherical structure and symmetry, fullerene requires small reorganization energy in ET reactions, which should accelerate forward ET and slow down charge recombination (CR) resulting in the formation of long-lived CS states, as evidenced in a number of porphyrinfullerene dyads.26 This expectation was found indeed to be the case. Upon excitation of the porphyrin, the intraassembly PET can take place in 2-ureido-4[1H]-pyrimidinonebridged porphyrinfullerene assemblies I and II efficiently with the formation of long-lived CS states up to 9.8 and 4.0 μs, respectively.

’ EXPERIMENTAL SECTION General. NMR spectra were recorded on a Bruker Advance DPX 400 MHz instrument using TMS as an internal standard. UV/vis spectra were obtained using a Shimadzu 1601PC spectrophotometer. Fluorescence spectra were recorded on a Hitachi 4500 fluorescence spectrophotometer. Mass spectra were obtained on Bruker APEX II spectrometers. Elemental analyses were performed on a Carlo Erba 1106 elemental analyzer. Timeresolved fluorescence measurements were run on a FL-900 Edinburgh-Analytical-Instrument. Nanosecond transient absorption spectra in the visible region were performed on a

LP-920 pumpprobe spectroscopic setup (Edinburgh). Pulsed optical excitation at 532 nm (∼10 ns, 10 Hz) was provided by a Nd:YAG laser (Continuum Surelite II); the probe light source was a pulse xenon lamp. For submicrosecond time-resolved absorption spectroscopy in the near-infrared (NIR) region, the excitation laser pulses at 532 nm (2 mJ/pulse, 7 ns, 10 Hz) were obtained by an optical parametric oscillator (OPO, Quanta-Ray MOPO-SL, Spectra Physics, Mountain View, CA, USA) driven by a Nd:YAG pumped laser (Quanta-Ray PRO-230; Spectra Physics). Kinetics at individual NIR wavelengths were recorded with a xenon lamp probe (CW, 350W) and a photodiode detector (S8890-02, Hamamatsu, Hamamatsu City, Japan). The optical path length of the flow cuvette used for laser flash photolysis was 1 cm. The anaerobic condition was achieved by bubbling the solution with high-purity argon for 30 min. Electrochemical experiments were performed on a computer-controlled Princeton Applied Research model 283 Potentistat/Galvanostat in dry THF or CH2Cl2 with a Pt wire auxiliary electrode, Ag/ AgNO3 nonaqueous reference electrode, and glassy carbon working electrode. The inner solution of the Ag/AgNO3 nonaqueous reference electrode was a mixture of 0.1 M nBu4NPF6 and 10 mM AgNO3 in CH3CN. All solutions were purged with argon prior to measurements and contained ca. 0.1 M nBu4NPF6 as the supporting electrolyte. A ferrocene/ferricenium couple was used as the internal reference. The computational calculations were performed by B3LYP/def2-SVP with RIJCOSX using the ORCA2.8 package. Materials and Reagents. Compounds 3,22 4,4b and 58 were prepared according to the literature methods. Anhydrous solvents were treated according to standard procedures. Other reagents and solvents were used as received without further purification. 2-(5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)-22-(40 -phenyl)1 2 H-imidazo[4,5-b]porphyrin)uredio-6-(1-undecyl)-4[1H]pyrimidinone (1a). Compound 4 (547 mg, 0.368 mmol) and Et3N (0.06 mL, 0.438 mmol) in dry toluene (10 mL) were stirred under an argon atmosphere until they were completely dissolved. Diphenylphosphoryl azide (DPPA) (0.1 mL, 0.458 mmol) was added to the mixture, which was then heated at 40 °C for 1 h and 80 °C for 4 h. At this point, compound 3 (100 mg, 0.38 mmol) was added, and the mixture was then stirred at 80 °C for an additional 16 h. The solvent was removed by evaporation, and the resultant residue was thoroughly washed with cold methanol and then subjected to the purification by chromatography on silica gel (CH2Cl2/MeOH, 100:1) and recrystallization from CH3OH/CHCl3 to afford the product 1a as a dark red solid in 78% yield. 1H NMR (CDCl3, δ ppm): 2.79 (br, 2H), 0.87 (t, J = 4.4 Hz, 3H), 1.26 (m, 16H), 1.53 (s, 36H), 1.56 (s, 36H), 23635

dx.doi.org/10.1021/jp207852j |J. Phys. Chem. C 2011, 115, 23634–23641


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Scheme 2. Synthetic Routes to Compounds 1a, 1b, and 2 for Assemblies III

1.76 (m, 2H), 2.60 (t, J = 4.4 Hz, 2H), 6.08 (s, 1H), 7.757.83 (m, 6H), 7.88 (s, 1H), 8.078.17 (m, 9H), 8.41 (s, 1H), 8.86 (m, 2H), 8.979.01 (m, 4H), 12.38 (s, 1H), 12.42 (s, 1H), 13.10 (s, 1H). 13C NMR (CDCl3, δ ppm): 173.33, 154.80, 153.18, 151.07, 148.86, 148.79, 148.72, 142.44, 141.84, 141.42, 139.99, 129.84, 129.74, 129.32, 127.55, 126.04, 122.48, 122.14, 121.96, 121.10, 120.68, 120.47, 119.33, 115.54, 106.47, 35.58, 35.29, 35.20, 32.05, 31.91, 29.85, 29.71, 29.65, 29.42, 22.77, 14.21. MS (MALDITOF) m/z: 1486.2 [M+]. Anal. Calcd (%) for C99H124N10O2: C 80.01, H 8.41, N 9.43. Found: C 80.49, H 8.25, N 9.23. 2-(5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)-22-(40 -phenyl)1 2 H-imidazo[4,5-b]porphyrinato)zinc(II))uredio-6-(1-undecyl)4[1H]-pyrimidinone (1b). Free base porphyrin 1a (25 mg, 0.0168 mmol) and zinc(II) acetate dihydrate (19.7 mg, 0.0897 mmol) were dissolved in dichloromethane (20 mL) and methanol (5 mL). The mixture was then stirred in the dark for 2 h. The solvent was then removed under a vacuum, and the residue was purified by chromatography over silica. The major band was collected by removal of the solvent, and the residue was recrystallized from a chloroformmethanol solution to afford 1b (26 mg, 99%) as purple red crystals. 1H NMR (CDCl3, δ ppm): 0.87 (t, J = 4.2 Hz, 3H), 1.26 (m, 16H), 1.54 (s, 36H), 1.56 (s, 36H), 1.77 (m, 2H), 2.60 (m, 2H), 6.09 (s, 1H), 7.807.83 (m, 6H), 7.90 (s, 1H), 8.078.17 (m, 9H), 8.56 (s, 1H), 9.00 (s, 2H), 9.06 (m, 4H), 12.40 (br, 2H), 13.09 (s, 1H). 13C NMR (CDCl3, δ ppm): 173.32, 154.78, 153.19, 153.01, 152.09, 151.59, 150.94, 149.93, 149.34, 148.62, 142.20, 140.65, 139.38, 131.82, 129.69, 127.52, 126.23, 123.55, 121.78, 120.90, 120.76, 116.63, 106.33, 35.58, 35.32, 35.21, 32.10, 31.94, 29.86, 29.72, 29.42, 29.13, 22.77, 14.21. MS (MALDI-TOF) m/z: 1548.8 [M+]. Anal. Calcd

(%) for C99H122N10O2Zn: C 76.74, H 7.94, N 9.04. Found: C 77.49, H 8.02, N 8.84. 2-(4-(1-Octyl-3,4-fullerene-2-pyrrolidino)uredio-6-(1-undecyl)-4[1H]-pyrimidinone (2). The synthetic procedure for 2 was essentially the same as that for compound 1a; 57% yield. 1H NMR (CDCl3, δ ppm): 0.870.92 (m, 6H), 1.251.42 (m, 28H), 1.91 (m, 2H), 2.48 (m, 3H), 3.20 (m, 1H), 4.10 (d, J = 9.2 Hz, 1H), 5.03 (s, 1H), 5.09 (d, J = 9.8 Hz, 1H), 5.92 (s, 1H), 7.78 (br, 4H), 12.25 (s, 2H), 13.02 (s, 1H). 13C NMR (CDCl3, δ ppm): 172.58, 154.52, 154.20, 153.56, 152.45, 147.23, 146.77, 146.43, 146.34, 146.26, 146.16, 146.09, 146.03, 145.86, 145.74, 145.53, 145.47, 145.32, 145.21, 145.08, 144.66, 144.63, 144.35, 143.08, 142.95, 142.61, 142.50, 142.27, 142.24, 142.09, 141.98, 141.93, 141.86, 141.62, 141.49, 140.14, 140.12, 139.93, 139.54, 138.54, 136.87, 136.65, 135.72, 129.87, 120.14, 106.16, 82.27, 68.79, 66.83, 41.96, 32.80, 32.19, 32.13, 29.98, 29.84, 29.71, 29.63, 29.59, 29.49, 29.13, 28.58, 27.85, 27.27, 27.08, 23.03, 14.40. MS (MALDI-TOF) m/z: 1258.5 [M+], 1281.6 [M+ + Na], 1297.6 [M+ + K]. Anal. Calcd (%) for C92H51N5O2: C 87.81, H 4.08, N 5.57. Found: C 88.43, H 4.03, N 5.42.

’ RESULTS AND DISCUSSION The preparation of key monomers for assemblies III is summarized in Scheme 2. 2-Uredio-pyrimidinones of 1a, 1b, and 2 were readily synthesized in good chemical yields by a one pot treatment of compound 3 with isocyanate, which was generated via a Curtius rearrangement23 from the corresponding carboxylic acid (4 or 5)4b,8 and diphenylphosphorylazide (DPPA). Metalation of 1a with zinc(II) acetate dehydrate gave 1b quantitatively. 23636



Figure 1. Partial 400 MHz 1H NMR spectra of the dimers in CDCl3: 2 3 2 (top); 1:1 mixture of 1a and 2 (middle); 1a 3 1a (bottom).

Figure 2. (a) Absorption spectra of 1a 3 1a, 2 3 2, 1:1 mixture of 1a 3 1a, and 2 3 2 in a CH2Cl2 solution ([1a] = [2] = 5.0 μM). (b) Fluorescence spectra of 1a 3 1a in the absence and presence of 2 3 2 in CH2Cl2 (λex = 420 nm; [1a] = 5.0 μM). 1

H NMR spectra indicated that compounds 1 and 2 exist as respective homodimers 1 3 1 and 2 3 2 in CDCl3 (Figure 1; Scheme S1, Supporting Information; and Figure S1, Supporting Information). The large downfield shifts in NH protons provided evidence of the involvement of strong hydrogen bonding. As shown in Figure 1, mixing 1 equiv of 1a with 1 equiv of 2 in CDCl3 resulted in partial dissociation of homodimers 1a 3 1a and 2 3 2. At the same time, the new set of signals of Ha-Hf labeled with asterisks at downfield suggested the formation of heterodimer 1a 3 2, namely assembly I. Considering the reversibility and the similar binding constant for homo- and heterodimers, we inferred that 1a 3 1a, 1a 3 2, and 2 3 2 existed in a ratio of 1:2:1 in the0 1:1 mixture of 1a and 2 by integration of H4 peaks for 1a 3 1a, H4 peaks for 2 3 2, and Hg-Hh peaks for 1a 3 2. A similar investigation was also performed for assembly II (Figure S1, Supporting Information). The absorption spectra of 1 and 2, as well as that of a 1:1 mixture of these two compounds in CH2Cl2, are shown in Figure 2a and Figure S2 (Supporting Information). Compound 1a 3 1a displays a Soret-band at 421 nm and Q bands at 519, 553, 589, and 648 nm, while the characteristic absorption peaks of 2 3 2 are at 433 and 705 nm, respectively. The absorption spectrum of their mixture resembles the superposition of the spectra of its components, suggesting no significant electronic interaction between the porphyrin and the fullerene motifs in the ground state. Because the molar extinction coefficient of 1a 3 1a is much

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larger than that of 2 3 2 in the visible region, the light was mainly absorbed by the porphyrin electron donor when the mixture of 1 3 1 and 2 3 2 was irradiated at 420 nm (or 532 nm), while only a small amount of the fullerene was excited. An initial observation for the intra-assembly singlet PET processes in assemblies I and II came from steady-state fluorescent quenching measurements. The fluorescence of 1 3 1 in the CH2Cl2 solution shows the characteristic emission at 650 and 707 nm for the free base porphyrin 1a 3 1a and at 592 and 644 nm for the zinc porphyrin 1b 3 1b, respectively. Progressive addition of 2 3 2 into the CH2Cl2 solution of 1 3 1 resulted in the fluorescence intensity of the porphyrins in 1 3 1 being quenched dramatically (Figure 2b; Figure S2b, Supporting Information). The controlled experiments under the same conditions showed no change in the fluorescence character of 1 3 1 upon either the addition of the reference compound of 5 (Figure S3, Supporting Information) or the replacement of the hydrogen bond promoting solvent of CH2Cl2 with a hydrogen bond disrupting solvent of DMF (Figure S4, Supporting Information), thus excluding any possible inter-assembly interactions between the donor and acceptor in the excited state. The time-resolved fluorescence measurement further evidenced the intra-assembly interaction in the excited state (Table 1). Analysis of the decay showed that the fluorescence of 1a 3 1a in CH2Cl2 was monoexponential with a lifetime τ1 of 8.82 ns (1.86 ns for 1b 3 1b), while in the presence of 2 3 2, the fluorescence kinetics had to be described by a biexponential model function shown in eq 1. The longer lifetime (τ1) was identical to that of 1a 3 1a (or 1b 3 1b), while the shorter lifetimes (τ2) were found to be 3.58 ns (1.05 ns). Clearly, the longer lifetimes are derived from the homodimers 1 3 1, while the shorter lifetimes are from the heterodimers 1a 3 2 (assembly I) and 1b 3 2 (assembly II). IðtÞ ¼ A1 expð t=τ1 Þ þ A2 expð t=τ2 Þ

ð1Þ

In the biexponential model function for fitting the fluorescence kinetics (eq 1), the pre-exponential coefficients A1 and A2, respectively, reflect the relative contribution of the longer and the shorter lived components to the fluorescence intensity I(t); their ratio relies on the relative concentrations of 1 3 1 and 2 3 2. When equivalent 1 3 1 and 2 3 2 were mixed together, the ratio of A1/A2 was equal to unity. As 1 3 1 involves two porphyrin chromophores, the concentration ratio of 1 3 1 to 1 3 2 should be 2:1, which is in line with that determined by a 1H NMR measurement (Figure 1). The lifetimes were independent to the concentrations of 1 3 1 and 2 3 2, suggesting the intra-assembly interaction between the porphyrin and fullerene of 1 3 2 in the excited state. According to eqs 2 and 3, the quenching constant (kET) and quenching efficiency (ΦET) were determined to be kET = 1.6 108 s1 and ΦET = 60% for assembly I (1a 3 2) and kET = 4.2 108 s1 and ΦET = 44% for assembly II (1b 3 2), respectively. ð2Þ kET ¼ ð1=τ2 Þ ð1=τ1 Þ ΦET ¼ 1 τ2 =τ1

ð3Þ

Time-resolved absorption spectroscopy provided direct evidence on the PET processes in assemblies I and II. Upon laser pulsing at 532 nm, a strong negative bleach of the ground state absorption of the porphyrin at ∼420 nm and characteristic absorptions at 470 nm were seen immediately (Figure S5, Supporting Information); the decay of which throughout the 23637



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Table 1. Photophysical Properties of Assemblies I (1a 3 2) and II (1b 3 2) in CH2Cl2 at Room Temperature assembly

λabs (nm)

λem (nm)

τ1 (ns)

1a 3 1a

421, 519, 553, 589, 648

650, 707

8.82

1a 3 2

421, 519, 553, 589, 648

650, 707

8.82

1b 3 1b

426, 514, 550, 587

592, 644

1.86

1b 3 2

426, 514, 550, 587

592, 644

1.86

232

433, 705

710, 800

1.65

Figure 3. Transient absorption spectra at indicated delay times (a) and the time profile of 1000 nm (b) for the mixture of 1b 3 1b and 2 3 2 ([1b] = 20 μM; [2] = 50 μM) in argon-saturated CH2Cl2 upon a laser pulsed at 532 nm.

absorption region and the recovery of the bleach occurred on the same time scale and could be well described by a monoexponential function. The lifetimes of 21.8 and 15.4 μs for 1a 3 1a and 1b 3 1b are consistent with that of the triplet excited state of porphyrin derivatives in the literature.14 Figure 3a shows the transient spectra of the mixture of 1 3 1 and 2 3 2 in a degassed CH2Cl2 solution. When 2 3 2 was introduced, excited state absorptions in the region of 600800 nm and ∼1000 nm were clearly observed in addition to the strong triplet-state absorption of the porphyrin at 470 nm. Following from other spectroscopic studies of porphyrin and fullerene derivatives,26 the transient species absorbing over 600800 nm is ascribed to the porphyrin radical cation in assembly II, and the weak absorption at ∼1000 nm, however, is attributed to the reduced acceptor counterpart, the fullerene radical anion. Interestingly, the decay of the fullerene radical anion was rather slow with a lifetime of 4.0 μs in the hydrogen-bonded assembly II (Figure 3b), although the rise grew so fast that it was comparable to the duration of the laser pulse (∼10 ns), i.e., out of the present time resolution. The rate of CR (kCR) derived from the decay phase of the fullerene radical anion was determined as 2.5 105 s1, which is in phase with that of the porphyrin radical cation at 670 nm (Figure S6, Supporting Information). The kinetics at 470 nm probing the triplet excited state of the porphyrin showed a decay time constant of 15.6 μs, close to that of 1b 3 1b alone (Figure S7, Supporting Information). The controlled experiments under the same conditions showed no change in the transient absorption character of 1b 3 1b upon the addition of the reference compound of 5 (fullerene acid without 2-ureido-4[1H]-pyrimidinone bridge) (Figure S8, Supporting Information), indicating the intra-assembly interaction between the donor and acceptor in the excited state. Similarly, the fullerene radical anion was also detected at ∼1000 nm with a long-lived CS up to 9.8 μs in assembly I in CH2Cl2 solution (Figure S9, Supporting Information). Since the composition of the mixture excited is 1 3 1 and 1 3 2, the signal of the triplet-state absorption of the porphyrin at 470 nm should be originated from 1 3 1 in the mixture of 1 3 1 and 2 3 2. It is known that 2-ureido-4[1H]-pyrimidinones are able

τ2 (ns)

τCS (μs)

kET (s1)

kCR/ (s1)

Φ (%)

3.58

9.8

1.6 108

1.0 105

60

1.05

4.0

4.2 108

2.5 105

44

to dimerize via a strong quadruple hydrogen bond array. The dissociation rate constant and lifetime of the quadruple hydrogen bond 2-ureido-4[1H]-pyrimidinone dimers were estimated as approximately 8.5 s1 at 300 K in CDCl3, i.e., hydrogen-bonded dimers with a pre-exchange lifetime of 120 ms,15b this value is much larger than that of the CS state obtained in this work. Given the high dimerization constant and long pre-exchange lifetime, the intermolecular recombination between the different dimers should not be dominant if it occurs. To shed light on the intra-assembly PET and CR processes mediated by the self-complementary quadruple hydrogen bonds, we estimated the free-energy change ΔG in assembly I (1a 3 2) and II (1b 3 2). These estimates were based on the first oxidative potential of 1 3 1, the reductive potential of 2 3 2, and the excitedstate energy of 1 3 1 (1.91 eV for 1a 3 1a and 2.09 eV for 1b 3 1b), with corrections for ion-pair salvation and Coulombic energies (Table 2). The geometrical structure and distance between electron donor porphyrin 1 3 1 and electron acceptor fullerene 2 3 2 were optimized by using DFT calculations that were based on B3LYP/def2-SVP with RIJCOSX using the ORCA2.8 package. The calculations were performed on simplified models with 3,5-Me2C6H3 instead of 3,5-tBu2C6H3 groups on the porphyrin rings and methyl substituents instead of the undecyl and octyl groups on the 4[1H]-pyrimidinone and pyrrolidine rings, respectively. As shown in Figure 4, the hydrogen-bonded interface was parallel, and the porphyrin and fullerene chromophores were located at each side of the hydrogen-bonded interface plane. The extended conformation was found with the lowest energy, and the center-to-center distances (R) separating the donor and acceptor were 28.0 and 27.7 Å for assembly I (1a 3 2) and II (1b 3 2), respectively. Moreover, in the optimized conformations, the highest occupied molecular orbital (HOMO) was mainly located on the porphyrin, and the lowest unoccupied molecular orbital (LUMO) was mainly on the fullerene. On the basis of the above studies, the free energy changes were found to be exothermic with values of 0.57 eV (assembly I), 0.87 eV (assembly II) for ΔGET, 1.34 eV (assembly I), and 1.22 eV (assembly II) for ΔGCR, where r+ and r were taken as 5.3 Å and 5.0 Å (evaluated by the DFT calculations), respectively. Since internal reorganization energies (λi) of porphyrin and fullerene are very small, the solvent reorganization energy (λs) should approximate to the reorganization energy (λ).4 Calculation revealed that the reorganization energy λ of assembly I and II that is closely related to the donoracceptor distance (R) is similar (Table 1). The negative ΔGET values indicate that the PET process occurs near the top region of the Marcus parabola because the reorganization energy is close to the values of ΔGET. However, the ΔGCR values suggest that the CR process occurs in the Marcus inverted region. It is well-known that natural photosynthesis in the purple bacterial reaction centers employs a multistep ET strategy to realize the efficient conversion of light energy into chemical 23638



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Table 2. Free Energy Changes of the PET and CR Processes in Assemblies I (1a 3 2) and II (1b 3 2) in CH2Cl2 assembly

R (Å)a

28.0

+0.34c

1.12

+0.22

1b 3 1b 1b 3 2 232

Ered (Vbvs Fc0/+)

+0.34c

1a 3 1a 1a 3 2

Eox (Vbvs Fc0/+)

27.7

+0.22

ES00 (eV)d

ET00 (eV)e

1.91

1.37

1.91

1.37

2.09

1.36

1.12

2.09

1.36

1.12

1.75

1.52

ΔGET(eV)f

ΔGCR(eV)f

λ (eV)f

HAD (cm1)g

0.57

1.34

0.87

1.26

0.87

1.22

0.87

1.23

a The value was directly obtained from DFT calculations. b CV in 0.1 M nBu4NPF6 as the supporting electrolyte; scan rate, 100 mV/s; reference electrode, 0.1 M Ag/AgNO3 in THF. c Irreversible peak. d Calculated as the average of the energy of the (00) band in the absorption and the emission spectra. e From ref 7b. f ΔGET = Eox Ered E00 e2/(4πε0)[(1/(2r+) + 1/(2r))/εref (1/(2r+) + 1/(2r) 1/R)/εs]; ΔGCR = E00 + ΔGET; λ ≈ λs = e2/(4πε0)[(1/(2r+) + 1/(2r) 1/R)][1/n2 1/εs], where r+ and r are the radii of the oxidized donor and the reduced acceptor, respectively; R is the distance separating the donor and acceptor; ε0, εs, and εref refer to the vacuum permittivity, the dielectric constants of CH2Cl2, and THF, respectively; n is the refractive index of the solvent; and E00 is the energy of the lowest excited state.24 g The value was obtained from the semiclassical 2 1/2 2 [(ΔGET + λ)2/4λkBT] Marcus theory expression: kET = (π/(p λkBT)) |HDA| e , where kB is Boltzmann's constant, p is Planck’s constant divided by 2π, and T is the temperature.

Figure 4. DFT B3LYP/def2-SVP optimized structures and molecular orbital schemes of assembly I (1a 3 2, top; HOMO, left; LUMO, right) and II (1b 3 2, bottom; HOMO, left; LUMO, right).

Scheme 3. Schematic Energy Diagram for the PET and CS Processes in Assembly I (LHS) and Assembly II (RHS) in CH2Cl2

energy.1 Though the quantum efficiency of the production of the CS state is unity, the energy conversion efficiency (40%) is not perfect. In the case of assembly I, light absorbed by the porphyrin creates its singlet excited state (1H2P*), which lies at ∼1.91 eV above the ground state (Scheme 3). The PET reaction takes place from the excited porphyrin of assembly I to the fullerene unit (C60) located ∼30 Å distant and yields the final products of

the porphyrin radical cation (H2P• +) and fullerene radical anion (C60• ) that lies 1.34 eV above the ground state. This means that the excitation energy of the porphyrin loses only 0.57 eV to reach the CS state of assembly I under the PET conditions; i.e., the energy conversion efficiency is as high as 70%. Similarly, the PET and CS processes in assembly II are analogous to that of assembly I with the energy conversion efficiency of 58%. The long-lived CS 23639


The Journal of Physical Chemistry C state achieved implies that the 2-ureido-4[1H]-pyrimidinone quadruple complementary hydrogen-bond interface is a promising conduit for PET processes. According to the Marcus theory24 for the nonadiabatic electron transfer for weakly interacting donoracceptor systems, the electron coupling HAD between the porphyrin donor and the fullerene acceptor in assembly I and II was evaluated straightforwardly (Table 2) and is comparable to those observed by Miller and Closs in covalent-linked dyads.25 The fast ET (kET = 1.6 or 4.2 108 s1) from the excited porphyrin to fullerene but the slow CR (kCR = 1.0 or 2.5 105 s1), as well as the high excitation conversion efficiency are advantageous for effective light-energy conversion.

’ CONCLUSIONS In summary, we demonstrated for the first time that the intraassembly PET process can take place via the 2-ureido-4[1H]pyrimidinone-bridged porphyrinfullerene assembly. Steadystate and time-resolved spectroscopy demonstrate that upon excitation of the porphyrin the electron transfer to the fullerene occurs with rate constants (quantum efficiency) of 1.6 108 s1 (60%) and 4.2 108 s1 (44%) for assemblies I and II, respectively, and then giving rise to a long-lived CS state with a lifetime up to 9.8 μs for assembly I (1a 3 2) and 4.0 μs for assembly II (1b 3 2). Because the relatively rigid assemblies prevent the porphyrin and fullerene from any intra-assembly collisions either through solvent or through space mechanisms, the quadruple complementary hydrogen bonds play crucial roles in mediating the intra-assembly PET and CR processes. This finding opens the door not only to the development of new assembling systems to illustrate the fundamental principles governing the PET and CR processes through hydrogen-bonded pathways but also to the design of artificial photosynthetic systems to achieve long-lived CS for solar energy conversion. ’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed spectroscopic spectra of 1 and 2 and their assemblies. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (L.-Z.W.); [email protected]. edu.au (M.J.C.); [email protected] (J.-P.Z.).

’ ACKNOWLEDGMENT We are grateful for financial support from the Ministry of Science and Technology of China (2009CB22008 and 2007CB808004), National Natural Science Foundation of China (20732007, 21002108, 20933010, 91027041, and 21090343), Solar Energy Initiative of the Knowledge Innovation Program of the Chinese Academy of Sciences (KGCXZ-YW-389), and the Bureau for Basic Research of the Chinese Academy of Sciences. We also thank the Australian Research Council for partial funding of this research (DP0773847). ’ REFERENCES (1) (a) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2009, 42, 1890–1898. (b) Magnuson, A.; Anderlund, M.; Johansson, O.;

ARTICLE

Lindblad, P.; Lomoth, R.; Polivka, T.; Ott, S.; Stensj€ o, K.; Styring, S.; Sundstr€om, V.; Hammarstr€om, L. Acc. Chem. Res. 2009, 42, 1899–1909. (c) Youngblood, W. J.; Anna Lee, S.-H.; Maeda, K.; Mallouk, T. E. Acc. Chem. Res. 2009, 42, 1966–1973. (d) Balzani, V.; Credi, A.; Venturi, M. ChemSusChem 2008, 1, 26–58. (2) (a) Kuciauskas, D.; Lin, S.; Seely, G. R.; Moore, A. L.; Moore, T. A.; Gust, D.; Drovetskaya, T.; Reed, C. A.; Boyd, P. D. W. J. Phys. Chem. 1996, 100, 15926–15932. (b) Kuciauskas, D.; Liddell, P. A.; Lin, S.; Johnson, T. E.; Weghorn, S. J.; Lindsey, J. S.; Moore, A. L.; Moore, T. A.; Gust, D. J. Am. Chem. Soc. 1999, 121, 8604–8614. (c) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40–48. (3) (a) Bell, T. D. M.; Smith, T. A.; Ghiggino, K. P.; Ranasinghe, M. G.; Shephard, M. J.; Paddon-Row, M. N. Chem. Phys. Lett. 1997, 268, 223–228. (b) Bell, T. D. M.; Ghiggino, K. P.; Jolliffe, K. A.; Ranasinghe, M. G.; Langford, S. J.; Shephard, M. J.; Paddon-Row, M. N. J. Phys. Chem. A 2002, 106, 10079–10088. (c) Lokan, N. R.; Paddon-Row, M. N.; Koeberg, M.; Verhoeven, J. W. J. Am. Chem. Soc. 2000, 122, 5075–5081. (4) (a) Imahori, H.; Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.; Ito, O.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 2607–2617. (b) Kashiwagi, Y.; Ohkubo, K.; McDonald, J. A.; Blake, I. M.; Crossley, M. J.; Ariki, A.; Ito, O.; Imahori, H.; Fukuzumi, S. Org. Lett. 2003, 5, 2719–2721. (c) Curiel, D.; Ohkubo, K.; Reimers, J. R.; Fukuzumi, S.; Crossley, M. J. Phys. Chem. Chem. Phys. 2007, 9, 5260–5266.(d) Lee, S.-H.; Larsen, A. G.; Ohkubo, K.; Cai, Z.-L.; Reimers, J. R.; Fukuzumi, S.; Crossley, M. J. Chem. Sci. 2011, DOI: 10.1039/C1SC00614B. (5) (a) Guldi, D. M. Chem. Soc. Rev. 2002, 31, 22–36. (b) MateoAlonso, A.; Ehli, C.; Guldi, D. M.; Prato, M. J. Am. Chem. Soc. 2008, 130, 14938–14939. (c) Megiatto, J. D., Jr.; Schuster, D. I.; Abwandner, S.; Rojas, G.; Guldi, D. M. J. Am. Chem. Soc. 2010, 132, 3847–3861. (6) (a) D’ Souza, F.; Smith, P. M.; Zandler, M. E.; McCarty, A. L.; Itou, M.; Araki, Y.; Ito, O. J. Am. Chem. Soc. 2004, 126, 7898–7907. (b) D’ Souza, F.; Chitta, R.; Gadde, S.; Shafiqul Islam, D. M.; Schumacher, A. L.; Zandler, M. E.; Araki, Y.; Ito, O. J. Phys. Chem. B 2006, 110, 25240–25250. (c) Kavakka, J. S.; Heikkinen, S.; Kilpel€ainen, I.; Tkachenko, N. V.; Helaja, J. Chem. Commun. 2009, 758–760. (7) de Rege, P. J.; Williams, S. A.; Therien, M. J. Science 1995, 269, 1409–1413. (8) (a) Segura, M.; Sanchez, L.; de Mendoza, J.; Martin, N.; Guldi, D. M. J. Am. Chem. Soc. 2003, 125, 15093–15100. (b) Sanchez, L.; Sierra, M.; Martín, N.; Myles, A. J.; Dale, T. J.; Rebek, J., Jr.; Seitz, W.; Guldi, D. M. Angew. Chem., Int. Ed. 2006, 45, 4637–4641. (9) (a) Turro, C.; Chang, C. K.; Leroi, G. E.; Cukier, R. I.; Nocera, D. G. J. Am. Chem. Soc. 1992, 114, 4013–4015. (b) Deng, Y.; Roberts, J. A.; Peng, S.-M.; Chang, C. K.; Nocera, D. G. Angew. Chem., Int. Ed. 1997, 36, 2124–2127. (c) Kirby, J. P.; Roberts, J. A.; Nocera, D. G. J. Am. Chem. Soc. 1997, 119, 9230–9236. (d) Young, E. R.; Rosenthal, J.; Hodgkiss, J. M.; Nocera, D. G. J. Am. Chem. Soc. 2009, 131, 7678–7684. (10) (a) Prasad, E.; Gopidas, K. R. J. Am. Chem. Soc. 2000, 122, 3191–3196. (b) Myles, A. J.; Branda, N. R. J. Am. Chem. Soc. 2001, 123, 177–178. (c) Smitha, M. A.; Prasad, E.; Gopidas, K. R. J. Am. Chem. Soc. 2001, 123, 1159–1165. (11) (a) Sessler, J. L.; Wang, B.; Harriman, A. J. Am. Chem. Soc. 1993, 115, 10418–10419. (b) Sessler, J. L.; Wang, B.; Harriman, A. J. Am. Chem. Soc. 1995, 117, 704–714. (c) Sessler, J. L.; Sathiosatham, M.; Brown, C. T.; Rhodes, T. A.; Wiederrecht, G. J. Am. Chem. Soc. 2001, 123, 3655–3660. (d) Sessler, J. L.; Jayawickramarajah, J.; Gouloumis, A.; Torres, T.; Guldi, D. M.; Maldonado, S.; Stevenson, K. J. Chem. Commun. 2005, 1892–1894. (e) Torres, T.; Gouloumis, A.; SanchezGarcia, D.; Jayawickramarajah, J.; Seitz, W.; Guldi, D. M.; Sessler, J. L. Chem. Commun. 2007, 292–294. (f) Lin, Z.; Lawrence, C. M.; Xiao, D.; Kireev, V. V.; Skourtis, S. S.; Sessler, J. L.; Beratan, D. N.; Rubtsov, I. V. J. Am. Chem. Soc. 2009, 131, 18060–18062. (12) (a) Schenning, A. P. H. J.; v. Herrikhuyzen, J.; Jonkheijm, P.; Chen, Z.; Wurthner, F.; Meijer, E. W. J. Am. Chem. Soc. 2002, 124, 10252–10253. (b) Gadde, S.; Islam, D.-M. S.; Wijesinghe, C. A.; Subbaiyan, N. K.; Zandler, M. E.; Araki, Y.; Ito, O.; D’ Souza, F. J. Phys. Chem. C. 2007, 111, 12500–12503. 23640



ARTICLE

(13) (a) Tecilla, P.; Dixon., R. P.; Slobodkin, G.; Alavi, D. S.; Waldeck, D. H.; Hamilton, A. D. J. Am. Chem. Soc. 1990, 112, 9408–9410. (b) Ghaddar, T. H.; Castner, E. W.; Isied, S. S. J. Am. Chem. Soc. 2000, 122, 1233–1234. (c) McClenaghan, N. D.; Grote, Z.; Darriet, K.; Zimine, M.; Williams, R. M.; De Cola, L.; Bassani, D. M. Org. Lett. 2005, 7, 807–810. (14) (a) Wessendorf, F.; Gnichwitz, J.-F.; Sarova, G. H.; Hager, K.; Hartnagel, U.; Guldi, D. M.; Hirsch, A. J. Am. Chem. Soc. 2007, 129, 16057–16071. (b) Wessendorf, F.; Grimm, B.; Guldi, D. M.; Hirsch, A. J. Am. Chem. Soc. 2010, 132, 10786–10795. (15) (a) Beijer, F. H.; Sijbesma, R. P.; Kooijman, H.; Spek, A. L.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120, 6761–6769. (b) Sontjens, S. H. M.; Sijbesma, R. P.; van Genderen, M. H. P.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 7487–7493. (c) Sijbesma, R. P.; Meijer, E. W. Chem. Commun. 2003, 5–16. (16) (a) Folmer, B. J. B.; Sijbesma, R. P.; Versteegen, R. M.; van der Rijt, J. A. J.; Meijer, E. W. Adv. Mater. 2000, 12, 874–878. (b) Sanchez, L.; Rispens, M. T.; Hummelen, J. C. Angew Chem., Int. Ed. 2002, 41, 838–840. (17) (a) Wang, X.-Z.; Li, X.-Q.; Shao, X.-B.; Zhao, X.; Deng, P.; Jiang, X.-K.; L, Z.-T.; Chen, Y.-Q. Chem.—Eur. J. 2003, 9, 2904–2913. (b) Zhao, Y.-P.; Zhao, C.-C.; Wu, L. -Z.; Zhang, L.-P.; Tung, C.-H.; Pan, Y.-J. J. Org. Chem. 2006, 71, 2143–2146. (18) (a) Beckers, E. H. A.; van Hal, P. A.; Schenning, A. P. H. J.; El-ghayoury, A.; Peeters, E.; Rispens, M. T.; Hummelen, J. C.; Meijer, E. W.; Janssen, R. A. J. J. Mater. Chem. 2002, 12, 2054–2060. (b) Neuteboom, E. E.; Beckers, E. H. A.; Meskers, S. C. J.; Meijer, E. W.; Janssen, R. A. J. Org. Biomol. Chem 2003, 1, 198–203. (19) (a) Zhao, C.-C.; Tong, Q.-X.; Li, Z.-T.; Wu, L.-Z.; Zhang, L.-P.; Tung, C.-H. Tetrahedron Lett. 2004, 45, 6807–6811. (b) Dudek, S. P.; Pouderoijen, M.; Abbel, R.; Schenning, A. P. H. J.; Meijer, E. W. J. Am. Chem. Soc. 2005, 127, 11763–11768. (20) Wang, S.-M.; Yu, M.-L.; Ding, J.; Tung, C.-H.; Wu, L.-Z. J. Phys. Chem. A 2008, 112, 3865–3869. (21) Pichlmaier, M.; Winter, R. F.; Zabel, M.; Zalis, S. J. Am. Chem. Soc. 2009, 131, 4892–4903. (22) Keizer, H. M.; Gonzalez, J. J.; Segura, M.; Prados, P.; Sijbesma, R. P.; Meijer, E. W.; de Mendoza, J. Chem.—Eur. J. 2005, 11, 4602–4608. (23) Ninomiya, K.; Shioiri, T.; Yamada, S. Tetrahedron 1974, 30, 2151–2157. (24) Marcus, R. A.; Sutin, M. Biochim. Biophys. Acta 1985, 811, 266–273. (25) Paulson, B. P.; Miller, J. R.; Gan, W. X.; Closs, G. J. Am. Chem. Soc. 2005, 127, 4860–4868.

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