Article pubs.acs.org/JPCA
Donor-Linked Di(perylene bisimide)s: Arrays Exhibiting Fast Electron Transfer for Photosynthesis Mimics Yishi Wu, Yonggang Zhen, Zhaohui Wang,* and Hongbing Fu* Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, People's Republic of China S Supporting Information *
ABSTRACT: The first example of donor-linked di(perylene bisimide)s is reported. UV−vis absorption spectra of these newly synthesized dyads showed intense absorption across the entire visible region, demonstrating their excellent light-harvesting activities. The severe fluorescence quenching event probed by steady-state fluorescence spectroscopy and the free-energy calculations suggested the possibility of electron transfer (ET) in these arrays upon photoexcitation. Further femtosecond transient absorption spectra clarified that the fluorescence quenching was due to fast intramolecular ET. The rate of the charge separation (CS) was found to be as high as 1012 s−1 in CH2Cl2. It was suggested that the large ET driving forces, strong donor−acceptor electronic coupling, and relatively small reorganization energy of diPBI accounted for the rapid ET process in a synergic manner. The fate of the generated radical ion pair depended on the solvent used. Rapid charge recombination to ground state occurred for the dyads in polar CH2Cl2 and for diPBI-TPA in nonpolar toluene. However, sufficient 3diPBI* population was attained via efficient spin−orbit coupled intersystem crossing from the charge-separated state for diPBI-PdTPP in toluene. These photophysical properties are interpreted as the cooperation between thermodynamic feasibility and kinetic manipulation.
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INTRODUCTION The conversion of solar energy into photochemical energy in natural photosynthesis systems proceeds via a sequence of wellorganized, highly efficient, directional, and specific electrontransfer processes between the electron donor and acceptor entities which are arranged through noncovalent incorporation into a well-defined protein matrix.1 The mimicry of these complexes has prompted the development of artificial photosynthetic ensembles.2−4 Elegantly designed covalently linked donor−acceptor (D-A) systems are actively considered as models for the investigation of photoinduced electron-transfer events.5−27 In the last decades, the efforts to increase the list of available building blocks adding new ones with specific absorption, emission, redox, and excited-state properties have produced a wide variety of compounds which can be usefully employed in the engineering of smart multicomponent structures to perform light-driving actions.28−33 The porphyrinoid compounds (including porphyrins, phthalocyanines, chlorines, and coreexpanded prophyrins) have been the most frequently used as electron-donating building blocks in the mimics of photosynthesis.8−10,34−37 Thiophene,38−40 triphenylamine (TPA),41−44 and tetrathiafulvalene (TTF)45−51 have also been examined in these models although to less extent. As the choice of the electron acceptor, aromatic imides (for example, perylene bisimide)52 and fullerene53 have dominated this area of research in recent years. Perylene-3,4:9,10-tetracarboxylic bisimides (PBI) chromophores exhibit large absorption cross © 2013 American Chemical Society
section in the visible region, excellent electron mobility, high electron affinity, and good thermal/photochemical stability. The advantages of employing fullerenes as viable electronaccepting building blocks in donor−acceptor systems are their small ET reorganization energy and three-dimensional rigid structures. However, considering the poor spectral matching with solar energy distribution of fullerenes, the development of new electron-accepting materials is of crucial importance. Di(perylene bisimide)s (diPBIs), the triply linked PBI dimers via bay fusion, have been readily available very recently.54−56 These polycyclic aromatic hydrocarbons (PAHs) possess extended π-conjugated rigid structures, and exhibit extremely large absorption coefficient across the whole visible region and strong electron-accepting ability. Recently, Lv et al. have fabricated single crystal transistors based on di(perylene bisimide) compound, which exhibit charge carrier mobility as high as 4.65 cm2 V−1 s−1 with excellent air stability.56 These prominent characters suggest that covalently attaching a wellknown electron donor might lead to dyads with novel photochemical and photophysical properties. This has been explored in the present study by covalently linking diPBI to either palladium tetraphenylporphyrin (PdTPP) or triphenylamine (TPA) to form a new series of donor−acceptor dyads (i.e., diPBI-PdTPP and diPBI-TPA). In the newly synthesized Received: November 2, 2012 Revised: February 7, 2013 Published: February 7, 2013 1712
dx.doi.org/10.1021/jp310838w | J. Phys. Chem. A 2013, 117, 1712−1720
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Scheme 1. Synthesis of Donor-diPBI Linked Dyads
the supporting electrolyte. CH2Cl2 was dried over calcium hydride and degassed prior to measurement. Quantum Chemical Calculations. The theoretical calculations were performed by the DFT B3LYP/3-21G method with the GAUSSIAN-03 software package. The optimized geometries were generated by using the GaussView software. Fluorescence Lifetime Measurements. The ps time-resolved fluorescence apparatus is described here: The excitation laser pulses (615 nm) were supplied by an optical parametric amplifier (OPA-800CF, Spectra Physics), which was pumped by a regenerative amplifier (Spitfire, Spectra Physics). The excitation energy at the sample was ∼100 nJ/pulse. Fluorescence collected with the 90°-geometry was dispersed by a polychromator (250is, Chromex) and detected with a streak camera (C5680, Hamamatsu Photonics). The spectral resolution was 0.2 nm, and the temporal resolution was about 20 ps on the measured delay-time-range setting. Femtosecond Transient Absorption Spectroscopy. A Ti:sapphire femtosecond laser system provided laser pulses for the femtosecond transient absorption measurements. A regenerative amplifier (Spitfire, Spectra Physics) seeded with a mode-locked Ti:sapphire laser (Tsunami, Spectra Physics) delivered laser pulses at 800 nm (120 fs, 1 kHz), which were then divided into two components by using a 9:1 beam splitter. The major component was sent to an optical parametric amplifier (OPA-800CF, Spectra Physics) to generate the pump pulses (615 nm, 130 fs, 1 kHz). The minor component was further attenuated and focused into a 3-mm sapphire plate to generate the probe pulses. A band-pass filter (SPF-750, CVI) was inserted into the probe beam to select visible probe (640− 790 nm). The time delay between the pump and probe beams was regulated through a computer-controlled motorized translation stage in the probe beam. A magic-angle scheme was adopted in the pump−probe measurement. The temporal resolution between the pump and the probe pulses was determined to be ∼150 fs (fwhm). The transmitted light was detected by a CMOS linear image sensor (S8377-512Q, Hamamatsu). The excitation pulsed energy was 0.2 μJ/pulse as measured at the rotating sample cell (optical path length 1 mm). A typical absorbance of 0.4−0.8 at the excitation wavelength was used. The stability of the solutions was spectrophotometrically checked before and after each experiment. Analysis of the kinetic traces derived from time-resolved spectra was performed individually by using nonlinear leastsquares fitting to a general sum-of-exponentials function after
dyads, the diPBI entities fulfill two functions. First, they absorb photon energy as sensitizers. Second, the linkage at the bay-area of the expanded aromatic core provides efficient electronic communication between diPBI and the donors, promoting fast electron-transfer reactions. As demonstrated here, the dyads undergo extremely fast photoinduced electron transfer between the donor and diPBI moieties within hundreds of femtoseconds. To the best of our knowledge, the obtained electrontransfer rate, which is as high as 1012 s−1, is among the highest values in the reported intramolecular electron transfer processes in synthetic donor−acceptor linked arrays.16−27,39,40,57−59
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EXPERIMENTAL SECTIONS Materials. All chemicals were purchased from commercial suppliers and used without further purification unless otherwise specified. DMSO was freshly distilled from CaH2. The 2CldiPBI compound was prepared according to known procedure.55 Characterization. 1H NMR and 13C NMR spectra were recorded in deuterated solvents on a Bruker DMX 300 NMR spectrometer, a Bruker ADVANCE 400 NMR spectrometer, and a Bruker ADVANCE 600 NMR spectrometer. 1H NMR chemical shifts are reported in ppm downfield from tetramethylsilane (TMS) reference, using the residual protonated solvent as an internal standard. Mass spectra (MALDITOF-MS) were determined on a Bruker BIFLEX III mass spectrometer. Steady-State Spectral Measurements. The UV−visible absorption spectra were measured on a Perkin-Elmer Lambda 35 spectrometer with a scanning speed of 480 nm/min and a slit width of 1 nm. The fluorescence emission spectroscopy was performed on a Hitachi F-4500 fluorescence spectrophotometer. The fluorescence quantum yield was determined with a dilute solution (A < 0.10) by the comparative method, using NdiPBI as a standard (ΦF = 0.033 in CH2Cl2).60 Solvents for the measurements were of chromatographic grade and were used as received. Electrochemistry. Cyclic voltammograms (CVs) were recorded on a Zahner IM6e electrochemical workstation, using glassy carbon discs as the working electrode, Pt wire as the counter electrode, Ag/AgCl electrode as the reference electrode, and ferrocene/ferrocenium as an internal potential marker. Tetrabutylammonium hexafluorophosphate (TBAPF6; 0.1 M) dissolved in CH2Cl2 (HPLC grade) was employed as 1713
dx.doi.org/10.1021/jp310838w | J. Phys. Chem. A 2013, 117, 1712−1720
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Figure 1. Optimized frontier orbitals of diPBI-PdTPP (a, b) and diPBI-TPA (c, d) obtained by the density functional method at the B3LYP/3-21G level.
Figure 2. Normalized steady-state absorption spectra (a) and fluorescence spectra (b) of N-diPBI (black), diPBI-PdTPP (green), and diPBI-TPA (red) in CH2Cl2 solutions. The excitation wavelength was 615 nm, at which the absorbance was adjusted to be 0.1.
at the B3LYP/3-21G level. The frontier molecular orbitals (Figure 1) and the optimized geometries (Figure S1) are shown. DiPBI-PdTPP adopts a nearly orthogonal conformation (110.2°) due to the steric hindrance between the porphyrin macrocycle and diPBI core. The edge-to-edge distance (ree) between the bay-region carbon of diPBI and the meso-phenyl carbon of PdTPP is estimated to be 2.6 Å, displaying an extremely short donor−acceptor distance. The same ree value was obtained for diPBI-TPA. In the optimized structure, the highest occupied molecular orbital (HOMO) for diPBI-PdTPP was located on the PdTPP macrocycle, while the lowest unoccupied molecular orbital (LUMO) was fully spread over the diPBI entities. As for diPBI-TPA, the majority of its HOMO was located on the TPA unit, while the LUMO was entirely delocalized over the diPBI unit. Importantly, the phenyl group that connected the donors and the acceptors had a small fraction of charge density in both HOMO and LUMO for the dyads, indicating significant interaction between the donor and acceptor moieties. The distribution of the HOMO and LUMO also suggested that PdTPP or TPA acts as the electron donor and the diPBI acts as the electron acceptor, respectively. Steady-State Absorption and Fluorescence Spectra. The ground-state absorption spectra of diPBI-PdTPP, diPBITPA, and the reference compound N-diPBI were recorded in
deconvolution of instrument response function (IRF). All the spectroscopic measurements were carried out at room temperature if there is no further notification.
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RESULTS AND DISCUSSION Synthesis and Characterization. Using a stoichiometric amount of Pd(OAc)2 as the reagent, P(Cy)3 as the ligand, and KOt-Bu as the base, the Buchwald−Hartwig coupling of 2CldiPBI with aminotriphenylamine (NH2TPA) proceeded in toluence to afford bay-functionalized diPBI derivatives diPBITPA at 110 °C in 67.2% yield. However, in the case of aminotetratphenylporphyrin (NH2TPP), the porphyrin unit was attached partly by palladium to give diPBI-PdTPP conjugate as well as diPBI-TPP in the same reaction conditions. DiPBI-TPP was difficult to separate from diPBI-PdTPP but transformed easily into diPBI-PdTPP in the presence of KOAc and Pd(OAc)2 in total yield of 34.6% (Scheme 1). The compounds diPBI-PdTPP and diPBI-TPA show good solubility in common organic solvents such as dichloromethane, chloroform, toluene, and tetrahydrofuran. Their structures are unambiguously identified by 1H NMR, 13C NMR spectroscopy, and MALDI-TOF (see the Supporting Information). DFT Calculations. To visualize the molecular geometry and electronic structure of the new synthesized dyads, computational studies were performed by the density functional theory 1714
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V) and diPBI-TPA (−0.23 V), respectively. The former oxidation potential was ascribed to the PdTPP (1.11 V) or TPA (1.04 V) while the latter one was assignable to the diPBI unit. The first oxidation of PdTPP or TPA and the first reduction of diPBI were used to evaluate the driving forces for the photoinduced charge separation (CS) and charge recombination (CR) processes in the dyads according to Rehm and Weller’s approach:62
CH2Cl2 and shown in Figure 2. The N-diPBI compound (Chart S1) possessed intense and extremely broad absorption bands peaked at 414, 614, and 671 nm, in good agreement with previous studies.60 These absorption bands were red-shifted by ∼3 nm in the dyad of diPBI-PdTPP, together with the typical features at ∼417 and 523 nm assignable to PdTPP. However, an opposite trend with the blue-shift of 3 nm was observed for the broadened low-energy band in diPBI-TPA compound. In addition, a long tail above 700 nm was also shown for diPBITPA, verifying the aforementioned strong interactions indicated by MO calculations. Generally, these donor-linked systems displayed remarkably broad absorption spectra with high extinction coefficients (Figure 1a), representing excellent light-absorbing abilities of these compounds. Upon selective photoexcitation of diPBI moiety at 615 nm, steady-state fluorescence spectra of the diPBI derivatives were recorded in dichloromethane (Figure 1b). The absorbance at 615 nm for all the samples was adjusted to match. In sharp contrast to N-diPBI, the 683-nm emission band, correlated with the S0 ← S1 transition of the diPBI unit, was found to be fully quenched in the dyads (over 99%). The fluorescence quantum yields (ΦF) of the dyads were measured to be
