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Excitation Energy Migration Processes in Self-Assembled Porphyrin Boxes Constructed by Conjugated Porphyrin Dimers† Pyosang Kim,‡ Jong Min Lim,‡ Min-Chul Yoon,‡ Junko Aimi,§ Takuzo Aida,§ Akihiko Tsuda,*,| and Dongho Kim*,‡ Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea, Department of Chemistry, Graduate School of Science, Kobe UniVersity, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan, and Department of Chemistry and Biotechnology, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ReceiVed: April 26, 2010; ReVised Manuscript ReceiVed: June 7, 2010
meso-Pyridine-appended alkynylene-bridged zinc(II) porphyrin dimers D2 and D4 assemble spontaneously, in noncoordinating solvents such as toluene, into tetrameric porphyrin boxes B2 and B4, respectively. Interestingly, the formation of Bn from Dn leads to the two kinds of self-assembled porphyrin boxes constructed by planar and orthogonal conformers, respectively. Excitation energy migration processes within these assemblies have been investigated in detail by using both steady-state and time-resolved spectroscopic methods. The pump-power dependence on the femtosecond transient absorption decay profiles is directly associated with the excitation energy migration process within the Bn boxes, where the exciton-exciton annihilation time is well-described in terms of the Fo¨ster-type incoherent energy hopping model. Consequently, the excitation energy hopping rates in porphyrin boxes constructed by planar and orthogonal conformers have been estimated to be (∼1.2 ps)-1 and (∼1 ps)-1, respectively. Furthermore, the porphyrin boxes constructed by orthogonal conformers show additional slow excitation energy hopping rates of (∼12 ps)-1. Overall, the dihedral angle in the constituent dimers is a key factor to control the energy transfer efficiency for the fabrication of artificial light-harvesting complexes using conjugated porphyrin dimer systems. I. Introduction Recently, there have been continuous attempts to mimic the natural photosynthetic system for the realization of highly efficient molecular photonic and electronic devices such as molecular wires, switches, transistors, and artificial lightharvesting apparatus in order to exploit the high efficiency of natural photosynthesis.1,2 Especially, particular attention has been focused on the construction of artificial light-harvesting complexes to investigate the fundamental mechanism of the excitation energy hopping (EEH) processes as mimicries of the natural photosynthetic antenna (LH1 and LH2) and to develop photofunctional materials for use in organic photovoltaics that must be capable of transferring excitation energy efficiently,3 because photogenerated excitons must travel relatively long distances to reach the interface at which charge carriers are generated.4 In this regard, much research has been directed toward the construction of various multidimensional porphyrin arrays for the fabrication of artificial light-harvesting complexes. The reason for much interest in usages of porphyrins as building block pigments stems from their desirable characteristics, such as rigid planar geometry, high stability, intense electronic absorption, and relatively easy synthesis and modification, as well as structural similarity to chlorophylls and bacteriochlorophylls of natural light-harvesting complexes.5 † This work is dedicated with respect and affection to the late Prof. Chi Sun Hahn, an inspiring teacher and mentor. * Corresponding authors. E-mail:
[email protected] (D.K.); tsuda@ harbor.kobe-u.ac.jp (A.T.). ‡ Yonsei University. § University of Tokyo. | Kobe University.
To prepare porphyrin-based molecular assemblies for an artificial light-harvesting antenna system, various strategies have been illustrated, such as covalently and noncovalently as well as dendritically linked arrays.6-8 Among them, a synthetic approach to utilize supramolecular chemistry using noncovalent self-assembly of molecular units has largely been in the spotlight, because it provides versatility in molecular networking in multidimensional space to increase the efficiency in light harvesting. Above all, since nature has optimized light harvesting in the photosynthetic organisms using noncovalent interactions, the self-assembly approach has been increasingly pursued as mimicries of LH1 and LH2 complexes. In addition, since arrangement and magnitude of transition dipole moments in building block elements significantly affect the efficiency of EEH processes, various structures of self-assembled porphyrin complexes have been synthesized hitherto using this strategy and simultaneously the capability of EEH processes in such complexes has been investigated. The representative cyclic complexes are the giant cyclic porphyrin arrays developed by Kobuke et al., in which five slipped cofacial zinc(II) diporphyrin complexes, constructed by the complementary coordination of imidazolyl to zinc(II) metal, are connected together through 1,3phenylene linkage.9 This system exhibits similar structures to the natural light-harvesting complex, resulting in well-defined organization of transition dipole moments between energyhopping units in this complex, which leads to the EEH times of 8.0 and 5.3 ps in five or six slipped cofacial zinc(II) diporphyrin complexes. One example of square-type complexes is a self-assembled large porphyrin square prepared by Maeda et al., using meso-triazole-appended L-shaped meso-meso linked zinc(II) triporphyrins as a building block element.10 This square structure leads to a linear orientation of transition dipole
10.1021/jp103767m 2010 American Chemical Society Published on Web 06/30/2010
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SCHEME 1: Di- And Tetraalkynylene-Bridged Porphyrin Dimers (D2 and D4) and Their Self-Assembled Box Complexes (B2 and B4) Made by Planar and Orthogonal Conformers, Respectively
moments between hopping units, resulting in the efficient EEH time of 8.8 ps. Another complex is self-assembled porphyrin boxes with four constituent subunits, meso-meso directly linked zinc(II) porphyrin dimers by using meso-pyridyl substituents. Hwang et al. has obtained the efficient EEH time of 48 ps among four adjacent pigments by assuming the number of hopping sites of N ) 4.11 According to this result, the excitation energy migration processes within porphyrin boxes are well-described by the Fo¨ster-type incoherent energy-hopping model by their perpendicular orientations of transition dipole moments among hopping sites. Finally, Kelly et al. has recently reported that a supramolecular architecture in which the combination of butadiyne-linked porphyrins with triethynylpyridylbenzene results in the reversible formation of trigonal, prismatic assemblies.12 This prismatic assembly reveals the EEH time of 4.8 ps between the macrocyclic dimers within the prism. Especially, in spite of the relatively long distance of ∼17 Å and H-type coupling between the two transition dipole moments of hopping units, this prismatic assembly has shown faster EEH time than that of the three complexes mentioned above, due to the larger magnitude of transition dipole moments in hopping sites. As illustrated in the prismatic assembly system, alkynylenebridged porphyrin oligomers are highly attractive as building block elements for artificial light-harvesting complex. The advantage of using alkynylene-bridged porphyrin dimers as hopping units arises from alkynylene linkers providing extended π-conjugation between porphyrin units, resulting in new, lowenergy, and strong electronic transitions of Q-bands, which ensures enhanced solar spectral absorption and efficient energy transfer process. According to previous reports, however, it has been inferred that these linkers give rise to rotational conformational heterogeneity, which can diminish the population of
molecules having the desired low-energy transition caused by a distribution of dihedral angles between porphyrin units.13 To reveal the effect of dihedral angle between porphyrin units in alkynylene-bridged porphyrin dimers on the EEH processes, we have prepared a series of self-assembled zinc(II) porphyrin boxes Bn such as B2 and B4 which are composed of di- and tetraalkynylene-bridged porphyrin dimers (D2 and D4), respectively (Scheme 1).14 Here, Dn molecules dissolved in strongly coordinating pyridine solvent have been taken as a reference of free alkynylene-bridged porphyrin dimer, and those dissolved in noncoordinating toluene solvent have been used for the characterization of Bn (Scheme 1). As mentioned above, owing to free rotation of these dimers, Bn also show two kinds of self-assembled porphyrin boxes constructed by planar or orthogonal conformers, respectively (Scheme 1). Therefore, the excitation energy migration processes within B2 and B4 have been comparatively investigated by the different length of linkers and dihedral angle between porphyrin moieties in alkynylenebridged porphyrin dimers, which significantly affect the π-conjugation between porphyrin units, using steady-state absorption/ emission, fluorescence lifetime, and femtosecond transient absorption (TA) measurements. Especially, S1-S1 exciton-exciton annihilation processes in pump-power-dependent TA decay profiles have identified and quantified the EEH processes in B2 and B4. Finally, we have analyzed the effect of dihedral angle in conjugated porphyrin dimers on the EEH processes by calculating the Fo¨ster-type energy transfer rates. II. Experimental Methods Sample Preparation. Conjugated porphyrin dimers D2 and D4 were prepared according to the reported methods.14 HPLCgrade toluene and pyridine solvents were purchased from Sigma-
Energy Migration in Porphyrins Boxes Aldrich and used without further purification. All measurements carried out at room temperature. Especially, at room temperature, we could observe a slow equilibrium process between B2 constructed by planar and orthogonal conformers in the absorption spectra in contrast with that of B4. Absorption spectral changes were detected after 12 h, 3 days, and 1 week as soon as the dimers were dissolved in toluene at room temperature, showing a very slow conformational change leading to the formation of porphyrin boxes composed of orthogonal conformers. Therefore, since B2 complex evidently shows the two porphyrin boxes constructed by planar and orthogonal conformers during 12 h after being dissolved in toluene, we have measured the steady-state fluorescence excitation anisotropy spectra, time-resolved fluorescence decay, and femtosecond TA decay under this condition. Steady-State Absorption and Fluorescence Spectra. Absorption spectra were obtained by using UV-vis-NIR spectrometer (Varian, Cary5000), and steady-state fluorescence excitation spectra were measured by a Hitachi model F-2500 fluorescence spectrophotometer at room temperature. Steadystate excitation anisotropy spectra were obtained by changing the fluorescence detection polarization either parallel or perpendicular to the polarization of the excitation light. The excitation anisotropy spectra then were calculated as follows
r)
EVV - GEVH EVV + 2GEVH
where EVV (or EVH) is the fluorescence excitation spectrum when the excitation light detected is the vertically (or horizontally) polarized portion of the fluorescence, denoting that the first and second subscripts represent excitation and detection polarization, respectively. The factor G is defined by IHV/IHH, which is equal to the ratio of the sensitivities of the detection system for vertically and horizontally polarized light. Nanosecond Time-Resolved Fluorescence Decay Measurements. Time-resolved fluorescence lifetime experiments were performed by the time-correlated single-photon-counting (TCSPC) technique. As an excitation light source, we used a homemade cavity-dumped Ti:sapphire oscillator, which provides a high repetition rate (200-400 kHz) of ultrashort pulses [100 fs at full width at half-maximum (fwhm)] pumped by a continuous wave (cw) Nd-YVO4 laser (Coherent, Verdi). The output pulse of the oscillator was frequency-doubled by a 1 mm thickness of a second harmonic crystal (β-barium borate, BBO, CASIX). The fluorescence was collected by a microchannel plate photomultiplier (MCP-PMT, Hamamatsu, R3809U-51) with a thermoelectric cooler (Hamamatsu, C4878) connected to a TCSPC board (Becker&Hickel SPC-130). The overall instrumental response function was about 25 ps (fwhm). A vertically polarized pump pulse by a Glan-laser polarizer irradiated samples, and a sheet polarizer, set at an angle complementary to the magic angle (54.7°), was placed in the fluorescence collection path to obtain polarization-independent fluorescence decays. Femtosecond Transient Absorption Measurements. The femtosecond time-resolved transient absorption (TA) spectrometer consisted of a Ti:sapphire regenerative amplifier system (Quantronix, Integra-C) operating at 1 kHz repetition rate and an optical detection system. The generated visible Noncolinear Optical Parametric Amplifier (NOPA) pulses had a pulse width of ∼100 fs and an average power of 1 mW in the range 500-700 nm, which were used as pump pulses. White light
J. Phys. Chem. B, Vol. 114, No. 28, 2010 9159 continuum (WLC) probe pulses were generated using a sapphire window (2 mm of thickness) by focusing of small portion of the fundamental 800 nm pulses which was picked off by a quartz plate before entering to the NOPA. The time delay between pump and probe beams was carefully controlled by making the pump beam travel along a variable optical delay (Newport, ILS250). Intensities of the spectrally dispersed WLC probe pulses are monitored by miniature spectrograph (OceanOptics, USB2000+). To obtain the time-resolved transient absorption difference signal (∆A) at a specific time, the pump pulses were chopped at 25 Hz and absorption spectra intensities were saved alternately with or without pump pulse. Typically, 6000 pulses excite samples to obtain the TA spectra at a particular delay time. The polarization angle between pump and probe beam was set at the magic angle (54.7°) in order to prevent polarization-dependent signals. Cross-correlation fwhm in pump-probe experiments was less than 200 fs and the chirp of WLC probe pulses was measured to be 800 fs in the 400-800 nm region. To minimize chirp, all reflection optics in probe beam path and 2 mm path length of quartz cell were used. Computational Methods. Quantum mechanical calculations were carried out with the Gaussian03 program suit.15 Geometry optimizations were performed by density functional theory (DFT) method with Becke’s three-parameter hybrid exchange functional and the Lee-Yang-Parr correlation functional (B3LYP), employing a basis set consisting of 6-31G(d) for all atoms in D2 and D4. Calculation of the Transition Dipole Moment for Emission. The magnitude of the transition dipole moment for emission, |µ|2 (in C · m), was calculated from the following relationship16
krad )
16π3υ03 3ε0hc3
f|2 n3 |µ
where ν0 is the average emission frequency (in s-1), c the speed of light, and ε0 the vacuum permittivity. III. Results and Discussion Steady-State Spectroscopic Measurements. Figure 1 displays the absorption and fluorescence spectra of Dn in pyridine and Bn in toluene, and their peak positions are tabulated in Table 1. Zinc(II) porphyrin monomers have two distinct absorption bands that correspond to intense B and relatively weak Q bands appearing at 400 and 550 nm, respectively.17 Connecting two porphyrin units through alkynylene-linkers in D2 and D4 give rise to electronic transitions depending on the dihedral angle between porphyrin units, which was observed previously for butadiyne-linked porphyrin dimers; planar conformation of D2 shows a high-energy B-band at around 425 nm, a low-energy B-band at 486 nm, and the lowest Q-band at 693 nm, resulting from enhanced π-conjugation between porphyrin units through the alkynylene bridge. As the number of triple bonds increases from two (D2) to four (D4), the splitting energy in B-bands, which correspond to energy differences between high- and lowenergy B-bands, becomes slightly smaller, followed by a blueshifted lowest Q-band at 674 nm. On the other hand, orthogonal conformation of D2 displays only one B-band at 453 nm and the lowest Q-band at 634 nm. In addition, an increase in bridge length from D2 to D4 leads to red-shifted B- and lowest Q-bands at 466 and 656 nm, respectively. As shown in the lowest Q-bands of planar conformer, since the increased bridge length
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Figure 1. Steady-state absorption and fluorescence spectra in pyridine (left) and toluene (right). The fluorescence spectra with red and blue lines were obtained by photoexcitation at different wavelengths indicated by red and blue arrows, respectively.
TABLE 1: Photophysical Parameters for Dn and Bn absorption (nm) sample D2 D4 B2 B4
B-band 453, 466, 447, 459,
486 492 482 487
Q-band 572, 578, 577, 581,
ΦFb
fluorescence (nm) 634, 656, 625, 638,
orthogonala
693 674 684 665
-c 630 642 c
planara
orthogonala
planara
703 686 690 671
-c 0.05 0.07
0.15 0.14 0.05 0.06
c
a
Excitation wavelength was B-band of orthogonal and planar conformers, respectively. b Fluorescence quantum yields were measured relative to H2TPP (0.11 in toluene). c The fluorescence spectrum of orthogonal conformer cannot be resolved from that of planar conformer.
brings about weak π-conjugation through alkynylene-bridge between porphyrin units, the opposite shift in the lowest Q-bands in orthogonal conformer by changing the bridge length seems to be caused by nearly zero π-conjugation between porphyrin units. On the other hand, the absorption spectra of B2 and B4 show similar electronic transitions in both B- and Q-bands and, especially, sharper Q-bands compared to their corresponding D2 and D4 through the formation of self-assembled boxes resulting in the locking of conformational flexibilities. In contrast to D2 and D4, the lowest Q-bands of B2 and B4 exhibit a band shift to blue in the order of B2 < B4, regardless of structural conformation, resulting from H-type exciton coupling between transition dipole moments along the long molecular axis of conjugated porphyrin dimer. The fluorescence spectra of D2 and D4 show different spectral features depending on the excitation wavelengths (Figure 1 and Table 1). While the selective excitation of planar conformers (blue arrow) yields considerably narrow fluorescence spectra (blue line), as shown in Figure 1, the excitation of orthogonal conformers (red arrow) yields the fluorescence spectra (red line) accompanied by structureless bands which do not appear by selective excitation of planar conformers. It is noteworthy that the excitation of orthogonal conformers results in the fluorescence spectra mostly originating from planar conformers, reflecting the conformational changes from orthogonal to planar conformers in the excited states. In contrast to the fluorescence spectra of D2 and D4, the fluorescence spectra of B2 and B4 clearly show two unique spectra arising from planar and orthogonal conformers, respectively, by selective excitation of two conformers, confirming that the formation of box complexes blocks the conformational changes in the excited states. That is, B2 and B4 show relatively sharp fluorescence bands (red lines) at 630 and 642 nm, respectively, when the boxes composed of orthogonal conformers are selectively excited. To confirm the existence of two types of boxes composed of orthogonal and planar conformers, the fluorescence excitation spectra were recorded at the emission wavelengths of planar
(dotted lines in Figure 2a-d) and orthogonal (dotted line in Figure 2e,f) conformers. While the excitation spectra of the emission at 790 (B2) and 750 nm (B4) produced the spectra resembling the ground-state absorption spectra with intensified lowest Q-band, the excitation spectra of the emission at 630 (B2) and 640 (B4) nm produced totally different spectra that are similar to that of porphyrin monomer. These observations clearly indicate that the origin of the emission peak in the steadystate spectra is due to the existence of distinctive spectroscopic species such as planar and orthogonal conformers. The steady-state fluorescence excitation anisotropy spectra were comparatively measured for Dn and Bn (solid lines in Figure 2). The polarization anisotropy measurement is informative for the excitation energy migration process, because the energy migration between the same molecular units with different orientations provides a depolarization channel. In our previous work, self-assembled porphyrin boxes with four constituent subunits, meso-meso directly linked zinc(II) porphyrin dimers, showed decreased anisotropy values over the entire absorption region as compared to that of free dimer, because of energy migration process among transition dipole moments perpendicular to each other.11 In the cases of D2 and D4 measured in emission wavelengths of planar conformers, while the low-energy B-bands and lowest Q-bands exhibit positive anisotropy values, the high-energy B-bands show nearly zero value, indicating that the direction of absorbing transition dipole moments between the high-energy B-bands and lowenergy B-bands and lowest Q-bands are perpendicular to each other. These results are consistent with the previous work that the lowest Q-band of planar conformation has well-defined transition dipole moment oriented along the long axis of dimer.18 However, the fluorescence excitation anisotropy spectra of B2 and B4 constructed by planar conformers (Figure 2c,d) do not reveal significantly decreased anisotropy values relative to those of D2 and D4 (Figure 2a,b) but slightly increased anisotropy values over the entire absorption region arising from increased molecular volume through the formation of porphyrin box
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Figure 2. Steady-state fluorescence excitation anisotropy spectra (solid line) and excitation spectra (dotted line) of Dn in pyridine and Bn in toluene. These spectra were obtained at the emission maxima of planar (a-d) and orthogonal conformers (e and f), respectively.
Figure 3. Fluorescence decay profiles of Dn in pyridine and Bn in toluene. The sample was excited at 450 (D2), 460 (D4), and 420 nm (Bn), and the emission was monitored at the wavelength of around 700 nm for planar conformers (blue line) and around 650 nm for orthogonal conformers (red line).
complexes. These results indicate that the transition dipole moments of lowest Q-bands are parallel to each other, which does not lead to a change in the anisotropy values arising from energy migration between hopping units. On the other hand, the absolute anisotropy values of porphyrin boxes composed of orthogonal conformers (Figure 2e,f) over the B- and lowest Q-band regions are slightly decreased as compared to those in the low-energy B- and lowest Q-bands of porphyrin boxes composed of orthogonal conformers. These results suggest that porphyrin boxes constructed by orthogonal conformers show the excitation energy migration processes among transition dipole moments perpendicular to each other, in contrast with those constructed by planar conformers. Time-Resolved Fluorescence Decay. To investigate the excited-state dynamics of D2, D4 and B2, B4, we have performed time-resolved fluorescence decay measurements using the TCSPC technique. The fluorescence temporal profiles of D2, D4 and B2, B4 monitored at the emission wavelengths corresponding to planar and orthogonal conformers, respectively, with photoexcitation of B-band at 450 (D2), 460 (D4), and 420
nm (Bn) are depicted in Figure 3. Although Dn and Bn were photoexcited at higher electronic S2 states, the observed decay times can be regarded as the S1 state lifetimes, since the internal conversion process from S2 to S1 state is beyond the temporal resolution of our TCSPC setup. The fluorescence lifetimes of free conjugated porphyrin dimers D2 and D4 were necessary to fit the resulting fluorescence decay curves. The fluorescence temporal profiles at 650 (D2) and 660 nm (D4) were dominated by a fast decay, ∼120 ps, whereas the fluorescence temporal profile at 700 nm (D2 and D4) exhibited a corresponding rise time, ∼200 ps, and a subsequent slow decay. These results are indicative of the torsional dynamics of the macrocycles, as the constituent monomer units rotate around the alkynylene-bridge, from orthogonal into planar conformation in the excited state. Similar features were also observed in butadiyne-linked porphyrin dimers.13 On the other hand, the S1 fluorescence decay profiles of B2 and B4 exhibit monoexponential behaviors in both porphyrin boxes constructed by orthogonal and planar conformers, reinforcing that conformational changes between
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Figure 4. Power-dependent transient absorption decay profiles of Bn in toluene constructed by planar (left) and orthogonal conformers (right).
TABLE 2: Fluorescence Lifetimes, Radiative and Nonradiative Decay Rates, and Emissive Transition Dipole Moment of Dn and Bn fluorescence lifetime (ns)
kr/108 (s-1)
knr/108 (s-1)
µ/D (C · m)
sample
orthogonal
planar
orthogonal
planar
orthogonal
planar
orthogonal
planar
D2 D4 B2 B4
0.12/0.20/1.58 1.14
0.12/1.08 0.17/0.68 1.33 0.85
3.8 6.1
13.8 20 3.1 7.3
5.5 8.1
7.9 13 7.1 11
3.0 3.8
6.6 7.7 3.0 4.5
orthogonal and planar conformers are believed to be completely blocked in self-assembled porphyrin boxes. Pump-Power-Dependent Transient Absorption Decay Measurements. To reveal the fast EEH processes within B2 and B4, pump-power-dependent femtosecond transient absorption (TA) decay profiles were measured, where the excitations at 650 nm in B2 and B4 were employed to photoexcite selectively the planar conformers, and the excitations at 620 nm in B2 and B4 to photoexcite selectively the orthogonal conformers. Since porphyrin boxes are directly excited to the S1 state, we expect that the energy relaxation dynamics of S2-S1 internal conversion is eliminated. The TA decay profiles monitored at the bleaching signals probed at the lowest Q-bands of planar and orthogonal conformers were exponentially fitted by using three decay components (τ1, τ2, and τ3), where the slowest decay components τ3 were fixed as the τ values in the TCSPC measurements (Figure 4 and Table 3). When the pump power was increased, the relative contributions of the fast τ1 and τ2 components of porphyrin boxes were enhanced as compared to that of τ3. The τ2 components of porphyrin boxes constructed by planar conformers, and τ1 components of porphyrin boxes composed of orthogonal conformers are not significantly changed with an increase in the bridge length. Especially, B2 and B4 constructed by orthogonal conformers revealed the component with τ2 ) 10 ps, which was not detected in porphyrin boxes constructed by planar conformers. The pumppower dependence on the TA decay profile is a strong indication of S1-S1 exciton-exciton annihilation processes. In addition, according to the description of the natural light-harvesting systems (LH1 and LH2), S1-S1 exciton-exciton annihilation processes are directly indicative of the Fo¨ster-type incoherent
TABLE 3: Power-Dependent Transient Absorption Decay Parameters for Bn Constructed by Planar and Orthogonal Conformers fitted decay time sample B2
a
B4a B2b B4b
b
pump power (µW)
τ1 (ps)
τ2 (ps)
350 180 75 350 180 75 350 180 75 350 180 75
< 0.1 (61%) < 0.1 (51%) < 0.1 (40%) < 0.1 (55%) < 0.1 (44%) < 0.1 (35%) 0.7 (34%) 0.8 (27%) 0.8 (12%) 0.7 (42%) 0.8 (37%) 0.8 (15%)
1.1 (6%) 1.2 (3%) 1.1 (6%) 1.0 (3%) 10 (17%) 10 (9%) 10 (19%) 10 (14%) 10 (8%)
τ3 (ns) 1.33 1.33 1.33 0.85 0.85 0.85 1.58 1.58 1.58 1.14 1.14 1.14
(35%) (46%) (60%) (39%) (53%) (65%) (49%) (64%) (88%) (39%) (49%) (77%)
a TA decay profile of boxes constructed by planar conformers. TA decay profile of boxes constructed by orthogonal conformers.
energy transfer from an excited donor to an adjacent excited acceptor resulting in the formation of a doubly excited state which quickly relaxes to the singly excited state.19,20 The τ1 components (∼100 fs) of porphyrin boxes composed of planar conformers are expected to be associated with higher order annihilation processes, which are frequently observed in multichromophoric systems or are the result of nonlinear decay processes contributed by the increased polarizability resulting from the extended conjugation of coplanar macrocycles.12 We have estimated the excitation energy migration processes within B2 and B4 in terms of the S1-S1 exciton-exciton
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SCHEME 2: Energy-Hopping Model of Bn Constructed by Planar and Orthogonal Conformers, Respectively
annihilation. Assuming this annihilation is the result of random energy transfer between adjacent dimeric chromophores, the time constants for energy hopping within porphyrin boxes, τh, can be derived from the annihilation lifetime, τa, using the following formula
τa )
τh 8 sin2(π/2N)
developed by Trinkunas21 for cyclic arrays composed of N hopping sites. In the case of porphyrin boxes comprising planar conformers, N ) 4 and the measured τ2 components reveal the EEH times of B2 and B4 as ∼1.2 ps. Because the τ2 component related to this process does not become slower as the distance between two porphyrin units within dimers changes, we can directly assign this annihilation component as the energyhopping time among four building blocks. In a similar manner, the τ1 components of B2 and B4 constructed by orthogonal conformers also can be related to the EEH time of ∼1 ps among four constituents. Moreover, additional slow τ2 components of B2 and B4 constructed by orthogonal conformers reveal the EEH times of 12 ps by assuming N ) 4. To confirm the EEH model described in Scheme 2, the Fo¨stertype energy transfer rate was calculated by using the following equation:22
KT(r) )
(
QDκ2 9000(ln 10) τDr6 128π5Nn4
)∫ ∞
FD(λ) εA(λ) λ4 dλ
0
where κ2 is the orientation factor, QD is the fluorescence quantum yield of the donor in the absence of acceptor, n is the refractive index of the medium, N is Avogadro’s number, r is the distance between donor and acceptor, and τD is the lifetime of the donor in the absence of acceptor, FD(λ) is the corrected fluorescence intensity of the donor normalized to unity, and εA(λ) is the extinction coefficient of the acceptor at λ, which is typically in the unit of M-1 cm-1. On the basis of previously reported selfassembled porphyrin boxes constructed by meso-meso directly linked zinc(II) porphyrin dimers and self-assembled porphyrin
square prepared by zinc(II) porphyrin monomers, we can estimate the distance between hopping units as 10 Å in B2 and B4. Using this distance and κ2 ) 1, the Fo¨ster energy transfer rates of B2 and B4 composed of planar conformers were calculated to be (0.5 ps)-1 and (0.3 ps)-1 in toluene, respectively. In the same condition, B2 and B4 constructed by orthogonal conformers show the calculated Fo¨ster energy transfer rates of (0.8 ps)-1 and (0.3 ps)-1, respectively. These calculated values are in a relatively good agreement with the experimental hopping times, suggesting that the assumption of N ) 4 is reasonable. To reveal the EEH mechanism related to additional slow component of porphyrin boxes constructed by orthogonal conformers, the Fo¨ster energy transfer rates between porphyrin monomer units within dimers are first considered and calculated to be (1.3 ps)-1 and (3.5 ps)-1 by using κ2 ) 4 and the distance of 13.53 Å (B2) and 18.67 Å (B4), which were obtained by the calculated optimized geometries of D2 and D4 at B3LYP/631G (d) level. In the case of this EEH model, the measured annihilation times of 10 ps represents directly the energyhopping time because it has only two energy-hopping sites. Meanwhile, we also have considered the previous study on selfassembled zinc(II) porphyrin square describing the EEH time of 26 ps among porphyrin monomers.23 On the basis of this EEH time, we can estimate that the emissive transition dipole moments of B2 and B4 are 1.8 and 2.2 times larger than that of porphyrin square, respectively, which results in the EEH times of 14 ps (B2) and 11 ps (B4). These indirectly obtained EEH times are well-matched with the experimental results, 12 ps in B2 and B4 as compared to the EEH rates between porphyrin monomer units within dimers. This result suggests that slower EEH times of porphyrin boxes constructed by orthogonal conformers follow the EEH mechanism of porphyrin square complex in which the excitation energy migration processes occur among exciton localized four porphyrin monomer units having transition dipole moments over the rectangular cycle on the xy-plane (green arrow). This perpendicular arrangement of transition dipole moments leads to slower EEH times than a parallel arrangement, which also shows decreased steady-state anisotropy values in going from porphyrin boxes constructed by planar conformers to those by orthogonal conformers. Collectively, considering the steady-state spectroscopic mea-
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surements, the dihedral angle of 90° provides relatively weak through-bond π-interaction between porphyrin subunits, which induces exciton state localized on porphyrin monomers. This feature results in additional slower EEH process in porphyrin boxes constructed by orthogonal conformers which was not detected in porphyrin boxes composed of planar conformers having fully delocalized exciton state. Consequently, these characteristics indicate that the dihedral angle between porphyrin units plays a key role in the energy-hopping processes for the fabrication of artificial light-harvesting complexes using conjugated porphyrin dimer systems. IV. Conclusions In this work, we have investigated the excitation energy migration dynamics in the lowest singlet excited state of selfassembled porphyrin box complexes B2 and B4 composed of di- and tetraalkynylene-bridged porphyrin dimers. Through S1-S1 exciton-exciton annihilation processes, we have evaluated the EEH rate of ∼1.2 ps in porphyrin boxes composed of planar conformers. Especially, the EEH rates of B2 and B4 constructed by orthogonal conformers were obtained as ∼12 ps, as well as shorter EEH rates of ∼1 ps. On the basis of the analysis of the calculated Fo¨ster energy transfer rates, it is suggested that the dihedral angle of 90° causes the additional EEH processes among exciton states localized on porphyrin monomer units within orthogonal dimers. Thus, our investigation demonstrates that the dihedral angle between porphyrin units is an important factor to control the excitation energy migration processes in the fabrication of artificial light-harvesting complexes using conjugated porphyrin dimer systems. Acknowledgment. This research was financially supported by the Star Faculty and WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (R32-2008000-10217-0) and Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Korea. (D.K.). The theoretical calculation was performed using the supercomputing resource of the Korea Institute of Science and Technology Information. P.K. and J.M.L. acknowledge the Seoul Science Fellowship. References and Notes (1) (a) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (b) de silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97, 1515. (c) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40. (2) (a) Holten, D.; Boican, D. F.; Lindsey, J. S. Acc. Chem. Res. 2002, 35, 57. (b) Szacilowski, K.; Macyk, W.; Drzewiecka-matuszek, A.; Brindell, M.; Stochel, G. Chem. ReV. 2005, 105, 2647. (c) Wasielewski, M. R. Acc. Chem. Res. 2009, 34, 40. (3) (a) Kim, D.; Osuka, A. J. Phys. Chem. A. 2003, 107, 8791. (b) Kim, D.; Osuka, A. Acc. Chem. Res. 2004, 37, 735. (c) Balaban, T. S. Acc. Chem. Res. 2005, 38, 612. (d) Satake, A.; Kobuke, Y. Org. Biomol. Chem. 2007, 5, 1679.
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