Structural and Photophysical Properties of Self-Assembled Porphyrin

(12, 13) Drain and co-workers reported the synthesis, characterization, and stability of porphyrin ... Ethylene glycols (EG) of the best grade availab...
0 downloads 0 Views 1022KB Size
Download PDF
J. Phys. Chem. C 2008, 112, 19209–19216

19209

Structural and Photophysical Properties of Self-Assembled Porphyrin Nanoassemblies Organized by Ethylene Glycol Derivatives Atula S. D. Sandanayaka,*,† Yasuyuki Araki,*,‡ Takehiko Wada,‡ and Taku Hasobe*,†,§ School of Materials Science, Japan AdVanced Institute of Science and Technology (JAIST), Nomi, Ishikawa, 923-1292 Japan, Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira, Sendai, 980-8577 Japan, and PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012 Japan ReceiVed: June 13, 2008; ReVised Manuscript ReceiVed: September 25, 2008

We demonstrate an ability to control the structures and photodynamics of porphyrin-based nanoassemblies via solvent mixture technique. Surfactants such as ethylene glycol (EG) derivatives with different chain lengths are employed to control the sizes and shapes of nanoparticles composed of meso-substituted tetracarboxyphenyl porphyrin [H2P(CO2H)4] in mixed H2O/THF solvent. With increasing the chain lengths, the diameters of H2P(CO2H)4/EG composite nanoparticles systematically increase in the range of 90-350 nm. In contrast with H2P(CO2H)4/EG composite nanoparticles, pristine H2P(CO2H)4 assemblies show long rod-shaped assemblies with micrometer scales. The porphyrin nanoparticles are stable in solution without precipitation for several days. The nanoparticles exhibit the following optical properties: a large bathochromic shift in the absorption spectra and an increase of the fluorescence quenching properties relative to those for the monomer porphyrin solution. The hierarchical clustering of H2P(CO2H)4 molecules within nanoparticles is caused by the hydrogen bonding and π-stacking effects. The efficient fluorescence quenching of H2P(CO2H)4 is mainly due to the effect of singlet-singlet annihilation of H2P(CO2H)4 moieties within nanoassemblies. We further report the quenching processes of the excited triplet states of H2P(CO2H)4 nanoassemblies. Efficient quenching properties of the excited triplet states of H2P(CO2H)4 moieties are observed in the range of lifetime 26-100 ns, which is largely dependent on the sizes of nanoparticles. These quenching processes can be also analyzed by triplet-triplet annihilation theory. Introduction Construction of molecular assemblies based on self-association or aggregation for controlled size and shape is a simple and convenient method to design organized assemblies.1,2 Of particular interest is efficient energy and electron transfer processes,3 which are very related to the molecular assembly in natural photosynthetic system, providing better understanding of photofunctional molecular architectures. This fundamental research of photofunctional organic materials also entails synthetic and photophysical processes of organic molecular assemblies or aggregates with nanometer scales since their optical properties significantly differ from those of monomeric species.1,2,4 As the characteristic wheel-like molecular arrays of chlorophylls are observed in the light-harvesting antenna complex,5 artificial efficient molecular aggregation or assembly has attracted special attention in the field of photochemistry of organic molecules.6 It is well-known that porphyrin assemblies strongly organized by covalent or noncovalent bond show broad absorption properties in the visible region.2c,6 The rich and extensive absorption features of porphyrinoid systems guarantees increased absorption cross-sections and an efficient use of the solar spectrum.7-9 Based on these ideas, porphyrin-appended molecular architectures such as dendrimers and linear and cyclic * To whom correspondence should be addressed. E-mail: t-hasobe@ jaist.ac.jp (T.H.); [email protected] (Y.A.). † JAIST. ‡ Tohoku University. § PRESTO, JST.

oligomers have been proposed for use as light-harvesting antennas due to their large cross-section for light absorption and their capability of directional ultrafast energy transfer within the organized assemblies.2c,6,10,11 In recent years, a wide variety of nanometer-sized selfassembled porphyrins using noncovalent bonding have been reported.12,13 Drain and co-workers reported the synthesis, characterization, and stability of porphyrin nanoparticles with diameters of 20-500 nm.14 In addition to the spherical nanoparticle assemblies of porphyrins, preparation of the anisotropic bar-shaped assemblies of porphyrins (i.e., porphyrin nanorods and nanotubes) is extensively reported.15-19 For example, porphyrin nanorods with several nanometer heights and micrometer lengths have been examined to exhibit remarkable photoconductivity with a rapid turn on/off rate.15 Although various preparation methods of such porphyrin assemblies have been reported, little attention has been drawn toward a systematic investigation on the structural and photochemical properties.11c,d,17c In this study we report an ability to control the structures and photodynamics of meso-tetra(4-carboxyphenyl) porphyrin [H2P(CO2H)4]-based nanoassemblies prepared in polar/nonpolar solvent mixture conditions (H2O/THF) by using surfactants such as ethylene glycols (EG) with different spacer lengths (Figure 1). The sizes and shapes of the nanoparticles are characterized by transmission electron microscopy (TEM), dynamic light scattering (DLS), and X-ray diffraction (XRD) analysis. The steady-state absorption and fluorescence, time-resolved fluorescence, and nanosecond transient absorption measurements are also employed to describe and reveal the photochemical properties of the self-assemblies. A systematic change of

10.1021/jp805202y CCC: $40.75  2008 American Chemical Society Published on Web 11/14/2008

19210 J. Phys. Chem. C, Vol. 112, No. 49, 2008

Sandanayaka et al. Time-Resolved Fluorescence Measurements. The timeresolved fluorescence spectra were measured by single photon counting method using a streakscope (Hamamatsu Photonics, C5680) as a detector and the laser light (Hamamatsu Photonics M10306, laser diode head, 408 nm) as an excitation source. Lifetimes were evaluated with software attached to the equipment. Nanosecond Transient Absorption Measurements. Nanosecond transient absorption measurements were carried out using THG (532 nm) of a Nd:YAG laser (Spectra-Physics, QuantaRay GCR-130, 5 ns fwhm) as an excitation source. For transient absorption spectra in the near-IR region (600-1200 nm) and the time-profiles, monitoring light from a pulsed Xe lamp was detected with a Ge-APD (Hamamatsu Photonics, B2834). For the measurements in the visible region (400-1000 nm), a SiPIN photodiode (Hamamatsu Photonics, S1722-02) was used as a detector. Results and Discussion

Figure 1. Molecular structures of H2P(CO2H)4 and ethylene glycol derivatives with different chain lengths employed in this study.

preparation condition of the nanoassemblies enables us to effectively control the structural and photochemical properties. Therefore, a way to control the structural and photochemical properties in porphyrin nanoassemblies is discussed here. Experimental Section General Information. Ethylene glycols (EG) of the best grade available were purchased from commercial supplier (Tokyo Kasei Kogyo Co. Ltd.) and were used without further purification. The compound of meso-substituted tetracarboxyphenyl porphyrin [H2P(CO2H)4] (Frontier Scientific, Inc.) was used after recrystallization. All solvents were spectroscopic or HPLC grade. All experiments were performed at room temperature. Preparation of Nanoassemblies. All porphyrin nanoparticles were prepared by adding an excess volume of poor solvent (H2O) to H2P(CO2H)4 (0.56 mM) solution in good solvent (THF) mixed with EG surfactant (1.5%, v/v) while vigorously stirring. The final concentration of H2P(CO2H)4 (0.05 mM) in nanoassembled condition is kept constant in the all samples (Supporting Information: S1). Transmission Electron Micrograph Measurements. Transmission electron micrograph (TEM) measurements were recorded by applying a drop of the sample to a copper grid. Images were recorded on a Hitachi H 7100 transmission electron microscope an accelerating voltage of 100 kV for imaging. X-ray Diffraction Measurement. X-ray diffraction (XRD) measurement was carried out with a BRUKER-axs M18XHFSRA using filtered Cu KR radiation. The samples for XRD analysis were prepared by drying suspension liquid over a glass substrate in air. Steady-State Measurements. Steady-state absorption spectra in the visible and near-IR regions were measured by PerkinElmer (Lamda 750) UV-vis-NIR spectrophotometer. Steadystate fluorescence spectra were measured by Perkin-Elmer (LS55) spectrofluorophotometer equipped with a photomultiplier tube having high sensitivity in the 400-800 nm region.

Preparation and Characterization of Porphyrin Nanoassemblies. Stabilizers such as EG are useful for the formation of stable colloidal systems by host-guest solvent method. The four types of EG derivatives with different chain lengths such as triethylene glycol (TriEG), tetraethylene glycol (TetraEG), hexaethylene glycol (HexaEG) and heptaethylene glycol (HeptaEG) are shown in Figure 1. Porphyrin nanoassemblies are prepared by adding an excess volume of poor solvent (H2O: guest) to a H2P(CO2H)4 solution in good solvent (THF: host) mixed with a small volume of EG surfactants while vigorously stirring. In all systems, the final concentrations of H2P(CO2H)4 are fixed as 0.05 mM in H2O/THF (15/1, v/v) containing 1.5% volume of EG derivative. The reference system without EG is also performed under the same condition (H2O/THF ) 15/1, v/v). Transmission electron micrographs (TEM) are employed to examine the sizes and shapes of these porphyrin nanoassemblies as shown in Figure 2. In the case of H2P(CO2H)4/EG composite nanoassemblies (Figure 2A-C), we can see a lot of nanoparticles, which are in sharp contrast with the much larger size of pristine H2P(CO2H)4 rods without EG (Figure 2D). This indicates that H2P(CO2H)4 moieties are effectively organized by EG derivatives to form nanoparticle assemblies. Additionally, with increasing chain lengths of EG surfactants, the average diameters of nanoparticles also increase from ca. 100 to 400 nm. This aggregation trend is also confirmed by dynamic lightscattering (DLS) measurement (Figure 3A-D). In the case of H2P(CO2H)4/EG composites, a similar trend is observed with increasing chain lengths of EG surfactants, giving average sizes of 91, 142, 300, and 342 nm in diameter, respectively, whereas the size of H2P(CO2H)4 rods is around 4800 nm in length. The emphasis is that the sizes of H2P(CO2H)4/EG composite nanoparticles measured from DLS nicely agrees with those from TEM. These observations not only manifest the efficient formation of porphyrin nanoparticles but also give an evidence for controlling sizes and shapes of the nanoassemblies. Thus, based on these TEM and DLS measurements, we have successfully controlled H2P(CO2H)4 nanoparticles by changing chain lengths of EG surfactants. As previously reported in the literature,14 sizes and shapes of these nanoassemblies are largely dependent on the basic structure of porphyrin moiety. In this study, the porphyrin nanoassemblies are likely formed together by both hydrogen bonding and π-stacking interactions. Especially, porphyrin nanoparticles are self-assembled by designed intermolecular interactions or encapsulation by external matrices (i.e., EG

Self-Assembled Porphyrin Nanoassemblies

J. Phys. Chem. C, Vol. 112, No. 49, 2008 19211

Figure 2. TEM images of (A) H2P(CO2H)4/TriEG, (B) H2P(CO2H)4/TetraEG, (C) H2P(CO2H)4/HeptaEG, and (D) pristine H2P(CO2H)4 composite assemblies: [H2P(CO2H)4] ) 0.05 mM in H2O/THF (15/1, v/v).

Figure 3. Size-distributions of (A) H2P(CO2H)4/TriEG, (B) H2P(CO2H)4/ TetraEG, (C) H2P(CO2H)4/HeptaEG, and (D) pristine H2P(CO2H)4 assemblies: 0.05 mM in H2O/THF (15/1, v/v) as estimated by dynamic light-scattering (DLS) measurements.

surfactants). To further examine a substituent effect of hydroxyl groups in EG surfactants on the sizes and shapes of H2P(CO2H)4/ EG nanoassemblies, we have also performed TEM and DLS measurements for the reference system: H2P(CO2H)4 and ethylene glycol diethyl ether (EGDG) composite [H2P(CO2H)4/

EGDE]. H2P(CO2H)4/EGDE composites have nonuniform structures with various diameters (around 400-1000 nm, Supporting Information S2, Figure S2a), whereas H2P(CO2H)4/TriEG composites (Figure 2A) have considerably controlled nanoparticle assemblies with diameters of ∼100 nm.20 On the other hand, we replace H2P(CO2H)4 with 5,10,15,20-tetraphenylporphyrin (H2TPP) and prepare H2TPP/TriEG composite assembly in the same manner. We can clearly see large and nonuniform assemblies with an average size of 396 nm (Supporting Information: Figure S2b) as compared to the uniform assembly of H2P(CO2H)4/TriEG composites (91 nm in diameter). These results ensure that a hydrogen bonding interaction between the -COOH group in H2P(CO2H)4 and the -OH group in EG surfactants play important roles in the organization of H2P(CO2H)4/EG nanoparticles. To further examine hydrogen interactions of H2P(CO2H)4 in nanoassemblies, we measured IR spectra of the nanoparticles (Supporting Information S3).21 The carboxylic CdO stretching band broadly appears at 1700 cm-1 for the monomer: H2P(CO2H)4 (spectrum a), whereas the IR bands become shifted to lower frequencies and split in the case of H2P(CO2H)4 nanoassemblies (1690 cm-1, spectrum b) and H2P(CO2H)4/EG composite nanoparticles (1680 cm-1, spectra c and d). These differences are likely due to hydrogen interaction. The other characteristic band of the dimeric acid species arises from the O-H out-of-plane deformation (wag) vibration,22 which appears as a rather broadband at 950-930 cm-1 (spectra c and d) in contrast with the monomeric form, H2P(CO2H) (spectrum a, 965 cm-1).

19212 J. Phys. Chem. C, Vol. 112, No. 49, 2008

Figure 4. XRD patterns: (a) H2P(CO2H)4 starting material, (b) pristine H2P(CO2H)4 assemblies, (c) H2P(CO2H)4/TriEG, and (d) H2P(CO2H)4/ HeptaEG. The detail pattern analyses were employed using a simulated pattern from the single crystal structure (Supporting Information S4).23,24

X-Ray Diffraction (XRD) Analysis. The internal structures of H2P(CO2H)4 moieties within nanoparticles are investigated by X-ray diffraction (XRD) analysis. It should be noted that the samples are centrifuged at 14 000 rpm and are washed with H2O/THF (15/1, v/v) solvent to remove the EG surfactant. After this treatment, the nanoparticle shapes can be maintained. In Figure 4, the pattern a is obtained from H2P(CO2H)4 bulk starting material. The XRD patterns b, c, and d are derived from the self-assembled pristine H2P(CO2H)4 assemblies, H2P(CO2H)4/ TriEG and H2P(CO2H)4/HeptaEG, respectively. The characteristic peaks in the self-assembled systems, b, c, and d, are approximately comparable to pattern a, which confirms that these H2P(CO2H)4 nanoassemblies consist of pure H2P(CO2H)4 moieties. To further confirm the crystal structure and selfassembled aggregate mode, we have also compared them with theoretically simulated XRD as shown in the Supporting Information S4.19e,23,24 It can be found that the peaks in the XRD patterns of self-assembled H2P(CO2H)4 assemblies are assigned according to the simulated pattern. On the other hand, XRD patterns of H2P(CO2H)4/EG composite nanoassemblies [patterns c and d] show similar sharp and intense diffraction peaks of b axis such as (020),24 which is contrast with a trend of the pristine H2P(CO2H)4 assembly; relatively broad and a multitude of peaks (pattern c). These observations may indicate that H2P(CO2H)4 assemblies surrounded by EG surfactants possess enhanced crystallinities and organized internal structures as compared to the pristine H2P(CO2H)4 assemblies.25 Steady-State Absorption Measurements. The spectroscopic characterization of nanoassemblies was performed by electronic absorption spectroscopy. The characteristic Soret and Q bands of H2P(CO2H)4 monomer are identified at 418, 511, 547, 590, and 646 nm, respectively (spectrum a in Figure 5). On the other hand, in the case of H2P(CO2H)4/EG systems (spectra b and c), the Soret and Q bands become split, broadened, and red-shifted by approximately 20 nm as compared to the corresponding bands of monomer solution (spectrum a). These results suggest that electronic interaction occurs in the ground states of H2P(CO2H)4 nanoparticles. The split Soret band together with the broadened and red-shifted Q-bands mainly suggest the occurrence of J-type interaction in the nanoparticles.26-28 As discussed above, the alignment of H2P(CO2H)4 assembly would be head-to-tail interaction (Supporting Information S4).24 Therefore, the microscopic trend of H2P(CO2H)4 aggregates from

Sandanayaka et al.

Figure 5. Steady-state absorption spectra of (a) 0.05 mM H2P(CO2H)4 monomer solution in THF, (b) H2P(CO2H)4/TriEG, and (c) H2P(CO2H)4/ HeptaEG composite nanoassemblies: [H2P(CO2H)4] ) 0.05 mM in H2O/ THF (15/1, v/v).

absorption spectra approximately agree with the result of internal structures from X-ray crystal analysis (vide supra). The other possibility of the low energy Soret band at 440 nm is diagnostic of exciton coupling of the Soret band.2c,6 These findings not only manifest the efficient formation of nanoassemblies but also give evidence for electronic interaction in nanoassemblies. Steady-State Fluorescence Measurement. Further insight of interactions of the excited states is derived from fluorescence emission studies (spectra a-d in Figure 6). In steady-state fluorescence measurement of the nanoparticles (spectra b-d), the strong quenching of fluorescence intensity (640 and 705 nm) as well as red-shift of spectral broadening is observed as compared to a spectrum of the corresponding monomeric porphyrin solution (spectrum a). These follow that the excitedstate interactions of H2P(CO2H)4 moieties within nanoassemblies. Especially, in small sized nanoparticles [H2P(CO2H)4/ TriEG], the emission intensities at 640 and 705 nm are found to be quenched over 90% (spectrum b) compared to that of the monomeric solution (spectrum a). The steady-state fluorescence quenching properties also increase with a decrease of sizes of nanoparticles, and follow the trend: H2P(CO2H)4/HeptaEG (342 nm) < H2P(CO2H)4/HexaEG (300 nm) < H2P(CO2H)4/TetraEG (142 nm) < H2P(CO2H)4/TriEG (91 nm). These results indicate that the internal molecular interactions of H2P(CO2H)4 moieties become strong with a decrease of the nanoparticle sizes. On the other hand, the spectrum of H2P(CO2H)4/HeptaEG composite (spectrum d) becomes blue-shifted as compared to the corresponding bands of H2P(CO2H)4/TriEG composite (spectrum b). This trend also shows that the blue-shift occurs with a decrease of the quenching efficiency. Although such a red-shift of the 640 and 705 nm peaks in monomer solution is related to the shift of the absorption seen in Figure 5, the quenching of main fluorescence peaks at 640 and 705 nm indicates that adequate association in H2P(CO2H)4 nanoparticles allows intermolecular electronic interactions between the excited singlet state [1H2P*(CO2H)4] within nanoparticles.29 To further support our results and get additional insights into the intermolecular electronic interactions between H2P(CO2H)4 moieties within nanoassemblies, a time-resolved fluorescence study are performed (vide infra). Fluorescence Lifetime Measurement. Additional quantitative electronic interaction on the photoexcited porphyrin within nanoassemblies could be evaluated by time-resolved fluorescence spectroscopy as shown in Figure 7a-f. The fluorescence decays for composite nanoassemblies were examined in H2O/ THF (15/1, v/v) solutions using pulsed 408 nm laser light, which excites the H2P(CO2H)4 moiety (Figure 7). These fluorescence lifetimes were evaluated from a biexponential fitting for the

Self-Assembled Porphyrin Nanoassemblies

Figure 6. Steady-state fluorescence spectra of (a) 0.05 mM H2P(CO2H)4 monomer solution in THF, (b) H2P(CO2H)4/TriEG, (c) H2P(CO2H)4/TetraEG, and (d) H2P(CO2H)4/HeptaEG nanoassemblies: [H2P(CO2H)4] ) 0.05 mM in H2O/THF (15/1, v/v). An inset figure shows the normalized fluorescence spectra. Excitation wavelength is 410 nm.

Figure 7. Time-resolved fluorescence decay profiles of (a) 0.05 mM H2P(CO2H)4 monomer solution in THF, (b) pristine H2P(CO2H)4 assemblies:0.05 mM in H2O/THF (15/1, v/v), (c) H2P(CO2H)4/TriEG, (d) H2P(CO2H)4/TetraEG, (e) H2P(CO2H)4/HexaEG, and (f) H2P(CO2H)4/ HeptaEG nanoassemblies: [H2P(CO2H)4] ) 0.05 mM in H2O/THF (15/ 1, v/v). Excitation wavelength is 408 nm.

J. Phys. Chem. C, Vol. 112, No. 49, 2008 19213 semblies (Table 1), it is evident that the short one is quite related to an interaction between a porphyrin and its nearest neighbor in the assembled formation. The ratios of shorter lifetime species relative to those of longer ones increase with a decrease of average particle sizes. As compared to the fluorescence lifetimes of the rod-shaped pristine H2P(CO2H)4 unit (trace b), such short fluorescence lifetimes of H2P(CO2H)4/EG composites appear to be due to the closely ordered assemblies of H2P(CO2H)4 moieties surrounded by EG surfactant matrices as discussed in XRD measurement section (vide supra). Additionally, photophysical results [fluorescence lifetime (τf), quenching rateconstant (ksq), and average quenching quantum yield (Φsq(ave))] are also shown in Table 1. With decreasing chain lengths of EG surfactants, the particle sizes decrease and quenching property increase, respectively. This probably means that the internal structures of H2P(CO2H)4 moieties within nanoassemblies become closely arranged by short chain length of EG surfactants. The aggregation of porphyrins generally results in the quenching of the fluorescence by concomitant branching factors such as singlet-singlet annihilation30 and energy transfer.10,11,31 To clarify the efficient fluorescence quenching process of H2P(CO2H)4 nanoassemblies, fluorescence decay profile was measured under various laser powers (Supporting Information S5). As examples of H2P(CO2H)4/TriEG nanoparticles, the fluorescence lifetimes have a decreasing trend (i.e., the corresponding fluorescence quenching rate constants have an increasing trend) with increasing laser power (Supporting Information S5). The laser power dependent fluorescence quenching can be explained by means of a singlet-singlet annihilation mechanism.10,11,30 When the laser power is increased, the probability of simultaneously exciting more than one porphyrin moiety within a nanoassembly increases and the possibility of singlet-singlet annihilation arises. In other words, with decreasing laser power, a decrease of the singlet population along with an increase of the ground-state population is observed since one excited porphyrin moiety is deexcited to the ground state. In order to observe the singlet-singlet (S-S) annihilation rate within nanoassemblies, following mathematical equation can be expressed as32

dn ) -kn(t) - γ(t)n(t)2 dt

(1)

where n is the population of the excited state, n (t ) 0) ) n0, and k is the decay rate constant of the excited state. In large aggregates with a high mobility of excitons, the annihilation rate constant, γ(t), can be assumed to be time-independent and the solution to eq 1 yields

n(t) ) Figure 8. Nanosecond transient absorption spectra of H2P(CO2H)4/ TriEG nanoassemblies (H2O/THF, 15/1, v/v) observed by 532 nm laser light (ca. 3 mJ/pulse) irradiation at 0.1 (b) and 1.0 µs (O) in Ar-saturated solution. Inset: time profile at 460 nm. [H2P(CO2H)4] ) 0.05 mM in H2O/THF (15/1, v/v).

respective H2P(CO2H)4/EG nanoassemblies: (traces c-f in Figure 7 and Table 1). The fluorescence lifetime values decrease with a decrease of sizes of nanoparticles and follow the trend: H2P(CO2H)4/HeptaEG > H2P(CO2H)4/HexaEG > H2P(CO2H)4/ TetraEG > H2P(CO2H)4/TriEG. This result is in good agreement with the trend of the steady-state fluorescence experiments (Figure 6). Although two kind of interactions (short and long lifetime species) are observed in the H2P(CO2H)4/EG nanoas-

n0 exp(-kt) 1 + n0γk-1[1 - exp(-kt)]

(2)

At low initial excitation concentrations for excited singlet state, eq 2 can be rewritten thus

(

)

γS 1 1 γ + exp(kSt) ) s(t) s0 kS kS

(3)

where sis the population of the excited singlet state, s (t ) 0) ) s0, and kS is the decay rate of the excited singlet state of H2P(CO2H)4 monomer. The γS is S-S annihilation rate constant. The s0 and γS values are evaluated from the slope and intercept of the linear plot of 1/s(t) vs exp(kSt), respectively (Supporting Information S5). The observed S-S annihilation rate constant (γS) for H2P(CO2H)4/TriEG and H2P(CO2H)4/TetraEG nanoparticles are 8.8 × 108 and 8.3 × 108 mol-1 dm3 s-1,33 respectively. Hence, the efficient fluorescence quenching of

19214 J. Phys. Chem. C, Vol. 112, No. 49, 2008

Sandanayaka et al.

TABLE 1: Nanoparticle Sizes and Fluorescence Lifetime (τf), Quenching Rate-Constant (kqS), Average Quenching Quantum Yield [ΦqS(ave)] via 1H2P*(CO2H)4 of Porphyrin Nanoassemblies at Room Temperature system

average particle size/nm

τf/psa

kSq /s-1b

ΦSq S(ave)b

H2P(CO2H)4/TriEG H2P(CO2H)4/TetraEG H2P(CO2H)4/HexaEG H2P(CO2H)4/HeptaEG

91 142 295 342

190 (80%), 1127 (20%) 258 (65%), 2050 (35%) 550 (57%), 2300 (43%) 620 (55%), 2590 (45%)

4.0 × 109 2.7 × 109 6.0 × 108 4.0 × 108

0.61 0.45 0.19 0.13

a Calculated by kq ) (1/τf)sample - (1/τf)ref and Φq(ave) ) [(1/τf)sample - (1/τf)ref ] × %/ (1/τ)sample. The values of (τf)ref were employed to be 0.82 ns for a actual lifetime of the singlet state of H2P(CO2H)4 in nanoparticle at a low laser density. b Fluorescence decays of 1H2P*(CO2H)4 are fitted with biexponential function and the kSq , and ΦSq (ave), are calculated from the shorter lifetimes with major fraction.

TABLE 2: Absorption Maximum (λmax), Triplet Quenching Rate Constant (kq), Triplet Lifetime (τT0), Quantum Yield of Triplet Quenching (ΦqT), and T-T Annihilation Rate Constants (γT) of Porphyrin Nanoassemblies at Room Temperature sample

λmax/ nm

kq/s-1

H2P(CO2H)4/TriEG H2P(CO2H)4/TetraEG H2P(CO2H)4/HexaEG H2P(CO2H)4/HeptaEG

460 460 460 440

3.9 × 107 2.8 × 107 1.8 × 107 1.5 × 107

τ0T/ns ΦTb q 26 36 55 67

0.35 0.51 0.77 0.83

γT/mol-1 dm3s-1a 6.5 × 1010 5.8 × 1010 4.7 × 1010 3.9 × 1010

a Calculated by eq 2., b calculated by the equation ΦTq ) [0.96 ΦSq (ave)] assuming ΦS of 1H2P*(CO2H)4 of the monomer is 0.96.34a

nanoassemblies indicate that S-S annihilation processes play an important role in the quenching phenomenon. This annihilation process is likely assisted by the strong interaction between H2P(CO2H)4 moieties in the pseudocrystalline structures (vide supra). Further, S-S annihilation rate constants are also trend to decrease with increasing particle sizes. The observed average quenching quantum yield [Φsq(ave)] values were in range from 0.13 to 0.61 for nanoassemblies, which indicate that S-S annihilation via the 1H2P*(CO2H)4 moiety is competitive to the intersystem crossing (ISC) process to 3H2P*(CO2H)4. Nanosecond Transient Absorption Measurement. Eventually, in order to monitor the 3H2P*(CO2H)4 in the nanoassemblies, transient absorption spectra were measured following 532 nm laser light pulse irradiation of H2P(CO2H)4 moiety within nanoassemblies. Upon the excitation of H2P(CO2H)4/TriEG nanoassemblies in H2O/THF (15/1, v/v), transient absorption spectra are observed as shown in Figure 8, where the broad bands appears in 460 and 880 nm regions. A spike at ca. 540 nm may be caused by scattering lights of YAG laser used for excitation of nanoassemblies. The observed broad bands in 460 and 880 regions are attributed to the triplet state of porphyrin [3H2P*(CO2H)4] comparing with transient absorption spectra of H2P(CO2H)4 monomer (Supporting Information S6). An inset of Figure 8 shows the time profiles of the transient absorption. Similar transient spectra are also observed in case of the other nanoassemblies (Supporting Information S7). The observed broad bands in the 460 and 880 nm regions almost decay within 200 ns. These bands decay obey first-order kinetics with a certain range of rate constant 1.5 × 107 to 3.9 × 107 s-1 (Table 2). The triplet lifetimes of the transient species are namely calculated to range from 26 to 67 ns (Table 2), which is infinitely shorter than that of monomer H2P(CO2H)4 condition (40 µs). The generation of 3H2P*(CO2H)4 based on the ISC process from 1H P*(CO H) in nanoassemblies is also possible since the 2 2 4 Φsq(ave) values are less than 0.96 in solutions. Thus, the quantum yield vs the absorbed light can be evaluated from ΦTq ) [0.96 - ΦSq (ave)] (if we can assume ΦISC ) 0.9634 in pure monomer state of H2P(CO2H)4).32 For example, the quantum yield vs the absorbed light for H2P(CO2H)4/TriEG is 0.35 as shown in Table 2. Furthermore, it is in good agreement with observed low intensity of 3H2P*(CO2H)4 peak at 460 nm for nanoassemblies comparing with 3H2P*(CO2H)4 peak at 460 nm of H2P(CO2H)4

monomer under the same laser conditions (Supporting Information S6 and 7). The efficient triplet quenching of H2P(CO2H)4 nanoassembly is due to the aggregation effects of H2P(CO2H)4 moieties such as triplet-triplet annihilation.35 Moreover, the lifetime of triplet state decreases with decreasing particle-sizes (Table 2) since H2P(CO2H)4 moiety strongly interact with the neighbor within a nanoassembly. This trend is also very similar to the case of fluorescence lifetime measurement (vide supra).36 In order to observe the triplet-triplet (T-T) annihilation processes within nanoassemblies, further experiments are conducted. When the laser power was increased at the same concentration of H2P(CO2H)4 nanoassemblies, the initial decay rate increased with changing decay kinetics from first order to a mixed order (first and second order) because of the triplet-triplet annihilation35a,37 as shown in Figure 9 [eq 4].

[3(H2P) * ] + [3(H2P) * ] f [1(H2P) * ] + [H2P]

(4)

For mixed order decay kinetics, a similar eq 3 used for T-T annihilation can be rewritten as

(

)

γT 1 1 γT + exp(kTt) ) c(t) c0 kT kT

(5)

where c is the population of the excited triplet state, c (t ) 0) ) c0, and kT is the decay rate of the excited triplet state of H2P(CO2H)4 monomer. The γT is the T-T annihilation rate constant. The c0 and γT values were evaluated from the slope and intercept of the linear plot of 1/c(t) vs exp(kTt), respectively (Supporting Information S8 and S9). The observed triplet-triplet annihilation rate constants (γT) for nanoassemblies are shown in Table 2. The γT values are dependent on nanoparticle sizes, and evaluated to be 3.9-6.5 × 1010 mol-1 dm3 s-1.38 Thus, we observed quite fast and efficient γT values in the form of aggregates. Conclusions In this study we successfully developed an effective and facile method for self-assembled porphyrin nanoparticles with uniform sizes and shapes. The organized sizes and shapes are efficiently controlled by EG surfactants with different chain lengths. Crystal analyses indicate that H2P(CO2H)4 nanoparticles possess wellordered alignments of H2P(CO2H)4 moieties as compared to the corresponding pristine H2P(CO2H)4 rods. In measurement of steady-state absorption spectra, a broad spectrum derived from aggregation such as J-type aggregation is observed. The efficient fluorescence quenching of H2P(CO2H)4 moieties within a nanoassembly is also probed by steady-state as well as timeresolved fluorescence emission spectroscopy. This fast quenching process is mainly based on singlet-singlet annihilation of H2P(CO2H)4 moieties. After competition processes between fluorescence quenching and intersystem crossing (ISC), the fast quenching of triplet state of H2P(CO2H)4 moieties relative to the monomer solution is observed by nanosecond transient

Self-Assembled Porphyrin Nanoassemblies

Figure 9. Laser-power dependence absorption time profiles of 3(H2P)* (CO2H)4 at 460 nm obtained by 532 nm laser light irradiation of H2P(CO2H)4/TriEG nanoassemblies (H2O/THF, 15/1, v/v) in Arsaturated aqueous solution. [H2P(CO2H)4] ) 0.05 mM in H2O/THF (15/1, v/v). The laser powers are 2.76, 2.87, 3.00, 3.11, 3.23, 3.35, and 3.46 mJ/pulse from the bottom. The respective photon densities are shown in the Supporting Information S8.

absorption measurement because of the triplet-triplet annihilation. Such a systematic investigation of structural and photophysical properties of porphyrin nanoparticles clearly demonstrates the strong promise for size controlled porphyrin nanoassemblies toward nanotechnological applications, especially for applications in electronics, photonics, and light-energy conversion. Acknowledgment. This work was partially supported by Grant-in-Aids for Scientific Research (No. 19710119 to T.H.) and special coordination funds for promoting science and technology from the Ministry of Education, Culture, Sports, Science and Technology, Japan. T.H. also acknowledges Kansai Research Foundation for Technology Promotion. Supporting Information Available: Experimental conditions for nanoparticle preparations (S1), TEM images of H2P(CO2H)4/EGDE and H2TPP/TriEG composite nanoassemblies (S2), IR spectra (S3), crystal packing structures of H2P(CO2H)4 analyzed by reference data (S4), laser-power dependence fluorescence decay profiles (S5), and nanosecond transient absorption spectra (S6-9). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (b) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304. (c) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35, 1154. (d) Horn, D.; Rieger, J. Angew. Chem., Int. Ed. 2001, 40, 4330. (e) Single Organic Nanoparticles; Masuhara, H., Nakanishi, H., Sasaki, K., Eds.; Springer: Berlin, 2003. (f) Kasai, H.; Nalwa, H. S.; Oikawa, H.; Okada, S.; Matsuda, H.; Minami, N.; Kakuda, A.; Ono, K.; Mukoh, A.; Nakanishi, H. Jpn. J. Appl. Phys. 1992, 31, L1132. (g) Tachikawa, T.; Chung, H.-R.; Masuhara, A.; Kasai, H.; Oikawa, H.; Nakanishi, H.; Fujitsuka, M.; Majima, T. J. Am. Chem. Soc. 2006, 128, 15944. (2) (a) Anderson, H. L. Chem. Commun. 1999, 2323. (b) Boyd, P. D. W.; Reed, C. A. Acc. Chem. Res. 2005, 38, 235. (c) Kim, D.; Osuka, A. J. Phys. Chem. A 2003, 107, 8791. (d) Winters, M. U.; Dahlstedt, E.; Blades, H. E.; Wilson, C. J.; Frampton, M. J.; Anderson, H. L.; Albinsson, B. J. Am. Chem. Soc. 2007, 129, 4291. (e) Reek, J. N. H.; Rowan, A. E.; Gelder, R. d.; Beurskens, P. T.; Crossley, M. J.; Feyter, S. D.; Schryver, F. d. R.; Nolte, J. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 361. (f) Tashiro, K.; Aida, T. Chem. Soc. ReV. 2007, 36, 189. (g) Grozema, F. C.; HouarnerRassin, C.; Prins, P.; Siebbeles, L. D. A.; Anderson, H. L. J. Am. Chem. Soc. 2007, 129, 13370. (3) (a) Gust, D.; Moore, T. A. Science 1989, 244, 35. (b) Rybtchinski, B.; Sinks, L. E.; Wasielewski, M. R. J. Am. Chem. Soc. 2004, 126, 12268. (c) Guldi, D. M.; Rahman, G. M. A.; Zerbetto, F.; Prato, M. Acc. Chem. Res. 2005, 38, 871. (d) Hasobe, T.; Saito, K.; Kamat, P. V.; Troiani, V.; Qiu, H.; Solladie´, N.; Kim, K. S.; Park, J. K.; Kim, D.; D’Souza, F.;

J. Phys. Chem. C, Vol. 112, No. 49, 2008 19215 Fukuzumi, S. J. Mater. Chem. 2007, 17, 4160. (e) Hasobe, T.; Imahori, H.; Kamat, P. V.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fujimoto, A.; Hirakawa, T.; Fukuzumi, S. J. Am. Chem. Soc. 2005, 127, 1216. (f) Imahori, H.; Yamada, H.; Guldi, D. M.; Endo, Y.; Shimomura, A.; Kundu, S.; Yamada, K.; Okada, T.; Sakata, Y.; Fukuzumi, S. Angew. Chem., Int. Ed. 2002, 41, 2344. (g) Hasobe, T.; Murata, H.; Kamat, P. V. J. Phys. Chem. C 2007, 111, 16626. (4) (a) Belanger, S. S.; Hupp, J. T. Angew. Chem., Int. Ed. 1999, 38, 2222. (b) Stang, P. J.; Olenyuk, B. Acc. Chem. Res. 1997, 30, 502. (c) Lucia, L. A.; Yui, T.; Sasai, R.; Takagi, S.; Takagi, K.; Yoshida, H.; Whitten, D. G.; Inoue, H. J. Phys. Chem. B 2003, 107, 3789. (d) Marks, T. J. Science 1985, 227, 881. (5) (a) Kuhlbrandt, W. Nature 1995, 374, 497. (b) The Photosynthetic Reaction Center; Deisenhofer, J., Norris, J. R., Eds.; Academic Press: San Diego, 1993. (c) Anoxygenic Photosynthetic Bacteria; Blankenship, R. E., Madigan, M. T., Bauer, C. E., Eds.; Kluwer Academic Publishing: Dordrecht, The Netherlands, 1995. (6) (a) Anderson, H. L. Inorg. Chem. 1994, 33, 972. (b) Rochford, J.; Chu, D.; Hagfeldt, A.; Galoppini, E. J. Am. Chem. Soc. 2007, 129, 4655. (c) Shirakawa, M.; Kawano, S. i.; Fujita, N.; Sada, K.; Shinkai, S. J. Org. Chem. 2003, 68, 5037. (d) Jiang, S.; Liu, M. J. Phys. Chem. B 2004, 108, 2880. (e) Xu, W.; Guo, H.; Akins, D. L. J. Phys. Chem. B 2001, 105, 1543. (f) Hajjaj, F.; Yoon, Z. S.; Yoon, M. C.; Park, J.; Satake, A.; Kim, D.; Kobuke, Y. J. Am. Chem. Soc. 2006, 128, 4612. (g) Washington, I.; Brooks, C.; Turro, N. J.; Nakanishi, K. J. Am. Chem. Soc. 2004, 126, 9892. (7) (a) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26, 198. (b) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40. (c) Gust, D.; Moore, T. A. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 8, pp 153-190. (8) (a) Wagner, R. W.; Lindsey, J. S. Pure Appl. Chem. 1996, 68, 1373. (b) Sazanovich, I. V.; Kirmaier, C.; Hindin, E.; Bocian, D. F.; Linsey, J. S.; Holten, D. J. Am. Chem. Soc. 2004, 126, 2664. (c) In Photoinduced Electron Transfer; Fox, M. A., Channon, M., Eds.; Elsevier: Amsterdam, 1988. (9) (a) Bar-Haim, A.; Klafter, J.; Kopelman, R. J. Am. Chem. Soc. 1997, 119, 6197. (b) Sadamoto, R.; Tomioka, N.; Aida, T. J. Am. Chem. Soc. 1996, 118, 3978. (c) Jiang, D.-L.; Aida, T. J. Am. Chem. Soc. 1998, 120, 10895. (d) Choi, M.-S.; Aida, T.; Yamazaki, T.; Yamazaki, I. Chem. Eur. J. 2002, 8, 2668. (e) Capitosti, G. J.; Guerrero, C. D.; Binkley, D. E., Jr.; Rajesh, C. S.; Modarelli, D. A. J. Org. Chem. 2003, 68, 247. (f) Harth, E. M.; Hecht, S.; Helms, B.; Malmstrom, E. E.; Fre´chet, J. M. J.; Hawker, C. J. J. Am. Chem. Soc. 2002, 124, 3926. (10) (a) Larsen, J.; Bruggemann, B.; Khoury, T.; Sly, J.; Crossley, M. J.; Sundstro¨m, V.; Åkesson, E. J. Phys. Chem. A 2007, 111, 10589. (b) Larsen, J.; Bruggemann, B.; Polivka, T.; Sundstro¨m, V.; Åkesson, E.; Sly, J.; Crossley, M. J. J. Phys. Chem. A 2005, 109, 10654. (c) Hwang, I. W.; Ko, D. M.; Ahn, T. K.; Yoon, Z. S.; Kim, D.; Peng, X.; Aratani, N.; Osuka, A. J. Phys. Chem. B 2005, 109, 8643. (d) Gulbinas, V.; Valkunas, L.; Kuciauskas, D.; Katilius, E.; Liuolia, V.; Zhou, W.; Blankenship, R. E. J. Phys. Chem. 1996, 100, 17950. (e) Lehtivuori, H.; Lemmetyinen, H.; Tkachenko, N. V. J. Am. Chem. Soc. 2006, 128, 16036. (11) (a) Jordens, S.; Belder, G. D.; Lor, M.; Schweitzer, G.; Auweraer, M. V. d.; Weil, T.; Herrmann, A.; Wiesler, U.-M.; Mu¨llen, K.; Schryver, F. C. D. Photochem. Photobiol. Sci. 2003, 2, 1118. (b) Fujitsuka, M.; Hara, M.; Tojo, S.; Okada, A.; Troiani, V.; Solladie´, N.; Majima, T. J. Phys. Chem. B 2005, 109, 33. (c) Maiti, N. C.; Mazumdar, S.; Periasamy, N. J. Phys. Chem. B 1998, 102, 1528. (d) Khairutdinov, R. F.; Serpone, N. J. Phys. Chem. B 1999, 103, 761. (12) (a) Kishida, T.; Fujita, N.; Sada, K.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 7298. (b) Doan, S. C.; Shanmugham, S.; Aston, D. E.; McHale, J. L. J. Am. Chem. Soc. 2005, 127, 5885. (c) Hasobe, T.; Imahori, H.; Fukuzumi, S.; Kamat, P. V. J. Mater. Chem. 2003, 13, 2515. (d) Rotomskis, R.; Augulis, R.; Snitka, V.; Valiokas, R.; Liedberg, B. J. Phys. Chem. B 2004, 108, 2833. (e) Sane, A.; Thies, M. C. J. Phys. Chem. B 2005, 109, 19688. (f) Barber, D. C.; Freitag-Beeston, R. A.; Whitten, D. G. J. Phys. Chem. 1991, 95, 4074. (g) Yuasa, M.; Oyaizu, K.; Yamaguchi, A.; Kuwakado, M. J. Am. Chem. Soc. 2004, 126, 11128. (13) (a) Hatano, T.; Takeuchi, M.; Ikeda, A.; Shinkai, S. Org. Lett. 2003, 5, 1395. (b) Zhang, L.; Yuan, J.; Liu, M. J. Phys. Chem. B 2003, 107, 12768. (c) Maiti, N. C.; Mazumdar, S.; Periasamy, N. J. Phys. Chem. B 1998, 102, 1528. (d) Lensen, M. C.; Takazawa, K.; Elemans, J. A. A. W.; Jeukens, C. R. L. P. N.; Christianen, P. C. M.; Maan, J. C.; Rowan, A. E.; Nolte, R. J. M. Chem. Eur. J. 2004, 10, 831. (e) Jeukens, C. R. L. P. N.; Lensen, M. C.; Wijnen, F. J. P.; Elemans, J. A. A. W.; Christianen, P. C. M.; Rowan, A. E.; Gerritsen, J. W.; Nolte, R. J. M.; Maan, J. C. Nano Lett. 2004, 4, 1401. (14) (a) Drain, C. M.; Smeureanu, G.; Patel, S.; Gong, X.; Garnod, J.; Arijeloyea, J. New J. Chem. 2006, 30, 1834. (b) Xianchang, G.; Tatjana, M.; Chang, X. D.; James, B.; Charles, M. D. J. Am. Chem. Soc. 2002, 124, 14290. (c) Drain, C. M.; Batteas, J. D.; Flynn, G. W.; Milic, T.; Chi, N.; Yablon, D. G.; Sommers, H. Proc. Natl. Acad. Sci., U.S.A. 2002, 99, 6498. (15) (a) Schwab, A. D.; Smith, D. E.; Rich, C. S.; Young, E. R.; Smith, W. F.; dePaula, J. C. J. Phys. Chem. B 2003, 107, 11339. (b) Schwab,

19216 J. Phys. Chem. C, Vol. 112, No. 49, 2008 A. D.; Smith, D. E.; Bond-Watts, B.; D.; Johnston, E.; Hone, J.; Johnson, A. T.; dePaula, J. C.; Smith, W. F. Nano Lett. 2004, 4, 1261. (16) (a) Wang, Z.; Medforth, C. J.; Shelnutt, J. A. J. Am. Chem. Soc. 2004, 126, 15954. (b) Wang, Z.; Medforth, C. J.; Shelnutt, J. A. J. Am. Chem. Soc. 2004, 126, 16720. (17) (a) Hu, J. S.; Guo, Y. G.; Liang, H. P.; Wan, L. J.; Jiang, L. J. Am. Chem. Soc. 2005, 127, 17090. (b) Harada, R.; Matsuda, Y.; Okawa, H.; Kojima, T. Angew. Chem., Int. Ed. 2004, 43, 1825. (c) Kojima, T.; Nakanishi, T.; Harada, R.; Ohkubo, K.; Yamauchi, S.; Fukuzumi, S. Chem. Eur. J. 2007, 13, 8714. (d) Kojima, T.; Harada, R.; Nakanishi, T.; Kaneko, K.; Fukuzumi, S. Chem. Mater. 2007, 19, 51. (e) Hasobe, T.; Fukuzumi, S.; Kamat, P. V. J. Am. Chem. Soc. 2005, 127, 11884. (18) (a) Zhou, Q.; Li, C. M.; Li, J.; Cui, X.; Gervasio, D. J. Phys. Chem. C 2007, 111, 11216. (b) Snitka, V.; Rackaitis, M.; Rodaite, R. Sensor. Actuat. B-Chem. 2005, 109, 159. (c) Matsui, H.; MacCuspie, R. Nano Lett. 2001, 1, 671. (d) Liu, B.; Qian, D.-J.; Chen, M.; Wakayama, T.; Nakamura, C.; Miyake, J. Chem. Commun. 2006, 3175. (e) Li, L.-L.; Yang, C.-J.; Chen, W.-H.; Lin, K.-J. Angew. Chem., Int. Ed. 2003, 42, 1505. (19) (a) Kishida, T.; Fujita, N.; Sada, K.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 7298. (b) Doan, S. C.; Shanmugham, S.; Aston, D. E.; McHale, J. L. J. Am. Chem. Soc. 2005, 127, 5885. (c) Rotomskis, R.; Augulis, R.; Snitka, V.; Valiokas, R.; Liedberg, B. J. Phys. Chem. B 2004, 108, 2833. (d) Okada, S.; Segawa, H. J. Am. Chem. Soc. 2003, 125, 2792. (e) Hasobe, T.; Oki, H.; Sandanayaka, A. S. D.; Murata, H. Chem. Commun. 2008, 724. (f) Hasobe, T.; Sandanayaka, A. S. D.; Wada, T.; Araki, Y. Chem. Commun. 2008, 3372. (20) The other possible reason for the structural difference between EGDE/H2P and TriEG/H2P composites may be also due to the different solubilities between EGDE and TriEG toward a mixed solvent. (21) Broad bands in spectra c and d (around 2890 cm-1) are derived from hydroxyl groups of EG surfactants. (22) Mary, M. B.; Umadevi, M.; Pandiarajan, S.; Ramakrishnan, V. Spectrochim. Acta Part A 2004, 60, 2643. (23) We have searched the published crystal structures of H2P(CO2H)4 including its solvent clathrates. The XRD patterns of H2P(CO2H)4 nanoassemblies are significantly different from ones simulated from the reported crystal structures other than one obtained by Goldberg et al. Therefore, it can be concluded that the H2P(CO2H)4 nanoassemblies have the similar crystal structure described by Goldberg et al. We analyzed by compound 1 in the following reference paper. See the reference paper: George, S.; Lipstman, S.; Muniappan, S.; Goldberg, I. Cryst. Eng. Comm. 2006, 8, 417. (24) The crystal packing pattern of b axis direction is composed of headto-tail interactions of H2P(CO2H)4 moieties as reported in ref 23. Considering the standard set-up of XRD measurement and observed monoclinic unit structure (dihedral angles: 90° in axes a-b and b-c), such head-to-tail type interactions mainly occur in the internal structure. The XRD pattern and detail structures based on the results of the paper in ref 23 are shown in Supporting Information S4. (25) The detail density value of H2P(CO2H)4 molecules in a H2P(CO2H)4/ EG composite within a nanoparticle may be changeable because of the different spacer lengths of surfactants. However, the basic orientation trend appears to be similar based on XRD results. (26) (a) Xu, W.; Guo, H.; Akins, D. L. J. Phys. Chem. B 2001, 105, 1543. (b) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525. (c) Kano, K.; Minamizono, H.; Kitae, T.; Negi, S. J. Phys. Chem. A 1997, 101, 6118. (27) Purrello, R.; Monsu’ Scolaro, L.; Bellacchio, E.; Gurrieri, S.; Romeo, A. Inorg. Chem. 1998, 37, 3647. (28) (a) Drain, C. M.; Nifiatis, F.; Vasenko, A.; Batteas, J. D. Angew. Chem., Int. Ed. 1998, 37, 2344. (b) Drain, C. M.; Lehn, J.-M. J. Chem.

Sandanayaka et al. Soc., Chem. Commun. 1994, 2313. (c) Milic, T. N.; Chi, N.; Yablon, D. G.; Flynn, G. W.; Batteas, J. D.; Drain, C. M. Angew. Chem., Int. Ed. 2002, 42, 2117. (29) Although the similar fluorescence quenching properties relative to the monomeric form are observed in porphyrin aggregated systems (see refs 2c, 11c, d and 19e), the opposite case is also reported. For example, An and coworkers report that fluorescence intensity of organic nanoparticles composed of CN-MBE [1-cyano-trans-1,2-bis-(4′-methylbiphenyl)ethylene] is increased by almost 700 times in the nanoparticles. The enhanced emission in CN-MBE nanoparticles is attributed to the synergetic effect of intramolecular planarization and J-type aggregate formation in nanoparticles. See the reference paper: An, B. K.; Kwon, S. K.; Jung, S. D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410. (30) (a) Lu¨demann, H.-C.; Redmond, R. W.; Hillenkamp, F. Rapid Commun. Mass Spectrom. 2002, 16, 1287. (b) Larsen, J.; Andersson, J.; Polı´vka, T.; Sly, J.; Crossley, M. J.; Sundstro¨m, V.; Åkesson, E. Chem. Phys. Lett. 2005, 403, 205. (c) Larsen, J.; Bru¨ggemann, B.; Polı´vka, T.; Sundstro¨m, V.; Åkesson, E. J. Phys. Chem. A 2005, 109, 10662. (d) Hwang, I. W.; Kamada, T.; Ahn, T. K.; Ko, D. M.; Nakamura, T.; Tsuda, A.; Osuka, A.; Kim, D. J. Am. Chem. Soc. 2004, 126, 16187. (e) Cho, S.; Li, W. S.; Yoon, M. C.; Ahn, T. K.; Jiang, D. L.; Kim, J.; Aida, T.; Kim, D. Chem. Eur. J. 2006, 12, 7576. (31) (a) Sun, S.-S.; Lees, A. J. Coord. Chem. ReV. 2002, 230, 170. (b) Udal’tsov, A. V.; Kazarin, L. A.; Sweshnikov, A. A. J. Mol. Struct. 2001, 562, 227. (c) Okada, S.; Segawa, H. J. Am. Chem. Soc. 2003, 125, 2792. (32) Valkunas, L.; Åkesson, E.; Pullerits, T.; Sundstro¨m, V. Biophys. J. 1996, 70, 2373. (33) For the best fit of the linear plot (Supporting information: Figure S4), the fluorescence lifetime of H2P(CO2H)4 is set to 0.82 ns. (34) (a) Kalyanasundaram, K.; Neumann-spallart, M. J. Phys. Chem. 1982, 86, 5163. (b) Sasabe, H.; Furusho, Y.; Sandanayaka, A. S. D.; Araki, Y.; Kihara, N.; Mizuno, K.; Ogawa, A.; Takata, T.; Ito, O. J. Porphyrins Phthalocyanines 2007, 10, 1346. (c) Sandanayaka, A. S. D.; Watanabe, N.; Ikeshita, K.; Araki, Y.; Kihara, N.; Furusho, Y.; Ito, O.; Takata, T. J. Phys. Chem. B 2005, 109, 2516. (35) (a) Butler, R. P.; Pilling, M. J. J. Chem. Soc., Faraday Trans. 1977, 73, 886. (b) Bergamini, G.; Ceroni, P.; Maestri, M.; Balzani, V.; Lee, S. K.; Vo¨gtle, F. Photochem. Photobiol. Sci. 2004, 3, 898. (c) Olea, A. F.; Worrall, D. R.; Wilkinson, F.; Williams, S. L.; Abdel-Shafi, A. A. Phys. Chem. Chem. Phys. 2002, 4, 161. (36) The degrees of crystallinities in H2P(CO2H)4/EG composite nanoparticles are very similar as discussed in Figure 4. Therefore, the reason for a trend of decreased lifetimes may be attributable to the decreased range of exciton motion with decreasing particle-sizes within a nanoparticle. The similar trend is also reported by Nakanishi and co-workers. See the following paper: Kasai, H.; Kamatani, H.; Yoshikawa, Y.; Okada, S.; Oikawa, H.; Watanabe, A.; Itoh, O.; Nakanishi, H. Chem. Lett. 1997, 26, 1181. (37) Zwicker, E. F.; Grossweiner, L. I. J. Phys. Chem. 1963, 67, 549. (38) The difference between γS and γT may be dependent on the freedom degree of exciton motion of excited singlet or triplet state in the crystalline structures. Although in the short-lived singlet state, excitons move freely within the crystalline structure (free excitons), excitons are likely trapped by some defects because of its relatively long lifetime in the triplet state (trapped excitons). Yokoi and Ohba reported that T-T annihilation between free and trapped triplet excitons occurs much faster due to the long-range interaction comparing to the one between two free excitons. Careful screening of the difference in our system is currently underway. See the reference paper: Yokoi, K.; Ohba, Y. J. Chem. Phys. 1987, 86, 3318.

JP805202Y