J. Phys. Chem. B 2001, 105, 4441-4445
4441
Morphological Dependence of Radiative and Nonradiative Relaxation Energy Balance in Photoexcited Aryl Ether Dendrimers as Observed by Fluorescent and Thermal Lens Spectroscopies Yuki Wakabayashi,† Manabu Tokeshi,‡ Akihide Hibara,† Dong-Lin Jiang,§,| Takuzo Aida,§,| and Takehiko Kitamori*,†,‡,| Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, Integrated Chemistry Project, Kanagawa Academy of Science and Technology, Sakado, Takatsu, Kawasaki 213-0012, Japan, Department of Chemistry and Biochemistry, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and Precursory Research for Embryonic Science and Technology 21, Japan Science and Technology Corporation, Japan ReceiVed: December 11, 2000; In Final Form: February 22, 2001
Radiative and nonradiative relaxation processes after excitation by ultraviolet light were measured for o-, m-, and p-aryl ether dendrimers of fourth generation (o-, m-, and p-Ar(L4)2) by using fluorescent and thermal lens spectroscopies. Samples were dissolved in CH2Cl2 to provide concentrations of a constant absorbance at excitation light wavelength (244 nm). Regarding the nonradiative process, we investigated thermal lens signal dependence on the light modulation frequency to determine the nonradiative relaxation rate. When frequency ranged from 4 to 200 Hz, the thermal lens signal became larger in the order p-, o-, m-Ar(L4)2, while the fluorescent intensity became larger in the order p-, m-, o-Ar(L4)2. We transformed these results into an energy balance of the radiative and nonradiative relaxation processes. Our analysis showed that 50% of the excitation energy was not released from p-Ar(L4)2 for a 100-ms order. Next, the measured fluorescence decay times of the three Ar(L4)2 were obtained as 1.7 ns which revealed that the anomalous properties of dendrimers did not originate in long-lived electronic excitation states, but in long-term storage of internal energy. To explain this phenomenon, a novel mechanism for intramolecular energy storage with nonergodic energy transfer should be considered. Last, we proposed that the nonlinear conjugated oscillator model of Fermi-Pasta-Ulam theory would be suitable for the intramolecular energy storage.
1. Introduction Dendrimers, which have a three-dimensional tree-like structure constructed from repeating monomer units, were first synthesized by Tomalia et al.1 in 1985. Recently, dendrimers have been paid much attention because of their anomalous photoproperties. In our previous report,2 we found IR light (6.3 µm, 0.2 eV, 1597 cm-1) could induce the cis-trans isomerization of the azobenzene core located at the center of an aryl ether dendrimer. Since the isomerization reaction rate was proportional to the fifth power of the photon flux, we considered that the isomerization was induced by a 5-photon process. The isomerization required more than 0.8 eV, and the 5-photon process seemed to give a good explanation. However, it would be difficult to consider a simultaneous 5-photon absorption because the IR radiation source was a simple incoherent Nichrome light. In other reports,3,4 we determined the absorption cross section of the azo-dendrimer was 3.1 × 10-19 cm2/ molecule based on strict IR photon flux measurements, and we concluded that the azo-dendrimer never absorbed five photons simultaneously. * Corresponding author. E-mail:
[email protected]. Fax: +81-3-5841-6039. † Department of Applied Chemistry, The University of Tokyo. ‡ Kanagawa Academy of Science and Technology. § Department of Chemistry and Biochemistry, The University of Tokyo. | PREST, JST.
One possible explanation for the 5-photon process is a combination of sequential absorption and intramolecular energy storage. Verification of the model can be done by measuring quantum yields of radiative and nonradiative energy relaxation processes and examining whether part of the absorbed energy is stored in the molecule or not. However, measurements of the quantum yields of the radiative and nonradiative processes for 6.3-µm-photon absorption are very difficult because of technical problems with light sources, photodetectors, and background fluctuations. Then, another approach is necessary to investigate the energy relaxation processes. Since the 6.3µm-photon absorption is assigned to a vibrational distortion mode of aromatic rings, we considered that aromatic ring excitation is the key process of the phenomenon. We presumed that the energy storage is also induced by an electronic excitation of aromatic rings, and measurement of relative quantum yields of the radiative and nonradiative processes for ultraviolet-photon absorption is possible by using fluorescence and thermal lens spectroscopies. In this report, we obtained relative quantum yields of the radiative and nonradiative processes following ultraviolet-photon (5.08 eV) absorption by using fluorescence and thermal lens spectroscopies. In this investigation, a photochemically inactive group should be used as the core unit of the dendrimers because a photochemically active group such as the azobenzene group consumes the absorbed energy and complicates the quantum
10.1021/jp004448y CCC: $20.00 © 2001 American Chemical Society Published on Web 04/25/2001
4442 J. Phys. Chem. B, Vol. 105, No. 19, 2001
Figure 1. Molecular structures of (a) o-, (b) m-, and (c) p-Ar(L4)2.
yield analysis. Therefore, we used a dimethoxy benzene group as the core unit. To examine dependences of the dendrimers photoproperties on their morphologies, three structural isomers of aryl ether dendrimers were used. The modulation frequency of thermal lens spectroscopy was varied from 4 to 200 Hz, and total energy release yield for each modulation frequency was calculated. In addition, we measured time-resolved fluorescence responses to confirm whether the dendrimers store the absorbed energy as electronic excitation states. Finally, a possible mechanism for anomalous photoproperties of the dendrimers was also discussed.
Wakabayashi et al. zene unit. While o- and m-Ar(L4)2 have an asymmetric structure about the core units which results in a conical morphology, p-Ar(L4)2 has a symmetric structure which results in a spherical morphology. CH2Cl2 (Wako, ∞-Pure) was used as solvent. Sample solutions are prepared around concentrations of 1 × 10-6 M so that their absorbance was equivalent at the excitation light wavelength (244 nm). In the absorption, fluorescence, and photothermal spectroscopic measurements, we used a homemade cell, which had a quartz glass cuvette for optical measurement and a round-bottomed reservoir for sample preparation. The two components were thermally bonded to a boro-silicated glass tube. The cell was connected to a vacuum line by using a threeway stopcock. First, solutions were introduced into the reservoir, which was immersed in liquid nitrogen to freeze the solutions, followed by degassing under vacuum. After several cycles of freezing and degassing, the solutions were transferred to the cuvette, and the cell was filled with pure nitrogen gas. Apparatus. Absorption and fluorescence spectra were obtained by using absorption (JASCO) and fluorescence (JASCO) spectrometers, respectively. Thermal lens response was measured with a standard double-beam setup.6 Intracavity frequencydoubled emission of an Ar+ laser (Coherent, Innova 300 FReD), which had a wavelength of 244 nm (5.08 eV) and an output power of 5 mW, was used as an excitation beam. A He-Ne laser (Uniphase, 1177P), which had a wavelength of 632.8 nm and an output power of 7 mW, was used as a probe laser. The excitation beam was mechanically modulated with an optical chopper and was coaxially aligned with the probe beam. The coaxial beams were focused in the sample cell, which had an optical length of 10 mm, with a convex lens having a focal length of 100 mm. To obtain a better signal-to-noise ratio, excitation and probe intensities were reduced to set their power at the sample cell as 0.05 and 0.10 mW, respectively, by using neutral density filters. The thermal lens effect induced by the excitation beam was measured by detecting the probe beam intensity after passing though an interference filter and a pinhole by using a photodiode. The photodiode output was fed into a lock-in amplifier (NF Electronic Instruments, LI-575), and the synchronous signal with the modulation frequency was recorded. The time constant of the lock-in amplifier was set to 1.25 s. Nanosecond time-resolved fluorescence responses of the dendrimers were measured with excitations by 266 and 280 nm pulses from a high-pressure hydrogen arc lamp and with a timeresolved spectrometer (Horiba, NAES-1100). The hydrogen lamp was operated with the charging pressure of 1 MPa, impressed voltage of 25 kV, and repetition rate of 3.7 kHz. The excitation slit width was set at 14 nm. The time-resolved response for each dendrimer was measured and averaged for 2 h. All optical measurements were done at room temperature (296 K). 3. Results and Discussion
2. Experimental Section Samples. Three dendrimers which have the dimethoxybenzene group as the core unit and no photochemically active group were synthesized as in our previous work.5 According to the structural isomerism of the core units, we named the dendrimers o-, m-, and p-Ar(L4)2, where Ar and L4 mean aryl ether and four layers, respectively. Molecular structures of o-, m-, and p-aryl ether dendrimers (o-, m-, and p-Ar(L4)2) are shown in Figure 1. Each dendrimer has two branches which have four layers (L4) of the aryl ether dendrimer structure. Ortho, meta, and para show structural isomerism of the core dimethoxyben-
We first investigated radiative and nonradiative relaxation processes at a single modulation frequency, 100 Hz, for the thermal lens measurement to examine whether the energy balance analysis indicated the intramolecular long-term energy storage. We prepared a spherical (p-Ar(L4)2) and two conical (o- and m-Ar(L4)2) aryl ether dendrimers. Since the anomalous isomerization was observed for symmetric spherical dendrimers and was not observed for asymmetric conical ones in the previous report, the two conical dendrimers were assumed to have no anomalous photoproperties and were used as reference samples. When there is no photochemical reaction, the sum of
Energy Balance in Photoexcited Aryl Ether Dendrimers
J. Phys. Chem. B, Vol. 105, No. 19, 2001 4443
the radiative energy yield ηr and the nonradiative energy yield ηnr should be unity,
ηr + ηr ) 1
(1)
In our experiments, ηr was estimated only by fluorescence intensity because phosphorescence or another radiative process was not observed. ηnr was measured by thermal lens spectroscopy. When energy storage occurs in our model, the sum of ηr and ηnr should be less than unity. Neither the fluorescence nor thermal lens spectroscopies give information about absolute energy yields, they tell only about relative ones. To obtain the relative quantum yield, we assumed that o- and m-Ar(L4)2 did not store absorbed energy, and we determined correction factors for the fluorescence (Ef) and thermal lens signal (Etl) intensities from the following simultaneous equations:
REf,o + βEtl,o ) 1 REf,m + βEtl,m ) 1
(2)
where R and β are correction factors for the fluorescence and thermal lens intensities and subscripts o and m on Ef and Etl mean o- and m-Ar(L4)2, respectively. Thus, the factors, R and β, can be used to discuss total energy balance of p-Ar(L4) quantitatively. Absorption spectra of o-, m-, and p-Ar(L4)2 are shown in Figure 2a. Each spectrum had absorption peaks at wavelengths around 240 and 280 nm. By changing solvent polarity, these peaks could be assigned to the π-π* transition of aromatic rings (not shown). Since relative quantum yields were measured with an excitation wavelength of 244 nm, concentrations of the solutions were adjusted around 10-6 M to equalize absorbance at 244 nm. The molar absorption coefficient at 244 nm was measured as 54 200 cm-1‚M-1. Fluorescence and thermal lens responses for the normalized photon absorption are shown in Figure 2b,c. The fluorescence intensity was quenched in the order of o-, m-, p-Ar(L4)2 and the thermal lens intensity decreased in the order of m-, o-, p-Ar(L4)2. By using eq 2, the correction factors were calculated, and the energy quantum yields for p-Ar(L4)2 were obtained. The obtained quantum yields are presented in Table 1. Since the thermal lens signals were measured with the excitation modulation frequency of 100 Hz, the signal intensities could be treated as released nonradiative energy within 10 ms after excitation. Therefore, the result that the sum of ηr and ηnr for p-Ar(L4)2 was less than unity, as shown in Table 1, indicated p-Ar(L4)2 did not release the absorbed energy for 10 ms at least. Possible uncertainties in the experimental technique were fluorescence quenching by dissolved oxygen and occurrence of a bond-breaking reaction by the ultraviolet laser irradiation. The former was eliminated by our diligent degassing. To examine the latter, after the measurements, samples were analyzed by MALDI-TOF mass spectroscopy, and no decomposed fragments were detected. Therefore, we concluded that the missing 53% of the yield observed in our experiments corresponded to the long-term energy storage by p-Ar(L4)2. To investigate the storage time, we next looked at the energy balance dependence on the excitation modulation frequency in the thermal lens measurements. The inverse of the frequency could be treated as measurement time for heat release of the dendrimers. The modulation frequency dependence of the thermal lens signal of o-, m-, p-Ar(L4)2 and solvent in the range of 4-200 Hz are shown in Figure 3. The intensities of the thermal lens signal decreased as the modulation frequency increased. It is well-known that the frequency characteristic of
Figure 2. Absorption (a) and fluorescence (b) spectra, and intensity of thermal lens effect (c) measured for the CH2Cl2 solution of o-, m-, and p-Ar(L4)2. Each sample was prepared so that the absorption was constant at 244 nm. Fluorescence spectra were measured by excitation at 244 nm. The thermal lens signal intensities were measured under the following conditions: wavelength of the excitation, 244 nm; light modulation frequency, 100 Hz; time constant of lock-in amplifier, 1.25 s; and irradiation time of excitation laser, 60 s.
TABLE 1: Radiative and Nonradiative Energy Yields for o-, m-, and p-Ar(L4)2 for Excitation Modulation Frequency at 100 Hz
o-Ar(L4)2 m-Ar(L4)2 p-Ar(L4)2
radiative energy yield ηr() REf)
nonradiative energy yield ηnr() βEtl)
1 - (ηr + ηnr)
0.58 0.31 0.16
0.42 0.69 0.31
0.53
the thermal lens signal is inversely proportional to the modulation frequency. Our result had the same frequency characteristic. As shown in Figure 3, the intensities of the thermal lens signal always decreased as o-, m-, p-Ar(L4)2 in the range of 4-200 Hz. The modulation dependence of energy balance of p-Ar(L4)2 is shown in Figure 4. The deficiency in relaxation energy was kept around 50% in the range of 8-200 Hz, that is, for measurement times of 5-125 ms. For the modulation frequency
4444 J. Phys. Chem. B, Vol. 105, No. 19, 2001
Wakabayashi et al. TABLE 2: Fluorescence Decay Times of o-, m-, and p-Ar(L4)2 Solutions and Solvent (CH2Cl2)
Figure 3. Modulation frequency dependence of the thermal lens signal of (×) o-, (b) m-, and ([) p-Ar(L4)2.
Figure 4. Modulation dependence of energy balance of p-Ar(L4)2 in the range of 4-200 Hz.
of 4 Hz, the degree of confidence was low due to a limitation in the guaranteed performance of the lock-in amplifier. Although long-lived intermediate states for metal complexes, supramolecules, and light-harvesting proteins are well-known,7-9 their origins are charge separation states such as metal-ligand charge transfer (MLCT) states, (triplet) ion-pair states, and so on. If the long-lived charge separation states of the dendrimers are present, the radiation relaxation must be detected by the present steady-state fluorescent measurement. To directly confirm whether the dendrimers store the absorbed energy as electronic excitation states, we measured nanosecond timeresolved fluorescence responses for the o-, m-, and p-Ar(L4)2 solutions and solvent (CH2Cl2). The responses for the Ar(L4)2 solutions required double exponential function fitting, where decay times of τ1 and τ2 were fitted, while response for the solvent could be fitted with a single-exponential function. The obtained decay times are summarized in Table 2. The τ2 was measured as constant for all samples and assigned to fluorescence of the solvent. Since the τ1 was not observed for the solvent, it should correspond to fluorescence due to Ar(L4)2. The values of τ1 for the Ar(L4)2 solutions were also constant around 1.7 ns, which is a usual value for relaxation time of electronic excitation states with the aromatic π-π* excitation. These results indicate that Ar(L4)2 does not store the energy as electronic excitation states; rather, they suggest that absorbed
266 nm
280 nm
excitation wavelength
τ1 (ns)
τ2 (ns)
τ1 (ns)
τ2 (ns)
o-Ar(L4)2 m-Ar(L4)2 p-Ar(L4)2 CH2Cl2
1.3 1.8 1.7 -
9.2 10.5 11.8 9.0
1.7 1.7 1.7 -
10.1 10.9 10.1 11.5
light energy absorbed by the aromatic ring is quickly converted to internal energy. From the energy balance analysis, a part of the internal energy is stored in p-Ar(L4)2 for longer than 125 ms. Several theoretical studies of the dendrimer molecules have been reported concerning their photochemical properties.10-18 However, these approaches are limited to the theoretical explanation of a high efficiency energy transfer in the dendrimer molecules, and there are no reports on the long-term storage of internal energy. Explanation of the long-term intramolecular energy storage of the spherical dendrimer must be considered a hitherto unknown kind of energy storage mechanism, which may work in the anomalous isomerization of the azo-containing dendrimer (L5AZO) by IR irradiation as described previously.2 The cis-trans isomerization of the azo unit by a 5-IR-photon absorption was only observed for the large-sized spherical dendrimer, not for small-sized or conical ones. The most unaccountable phenomenon is energy consumption by the isomerization reaction of the L5AZO. The reaction rate is proportional to the fifth power of the photon flux and the activation energy of the isomerization reaction is equivalent to 5-IR-photon energy. These results mean that five photons are sequentially absorbed by L5AZO and the photon energy is consumed instantaneously by the isomerization reaction without dissipation. More recently, we have found that the oxygen bondbreaking reaction in a large-sized spherical aryl ether dendrimer was induced by 11 infrared photons at 6.26 µm.19 This phenomenon was also observed for only the spherical shape. To explain these nonergodic properties, we focused on the structural characteristics of the dendrimers. From 1H NMR pulse relaxation times analysis, the spherical dendrimers such as p-Ar(L4)2 and L5AZO are seen to have a unique structure like a raw egg, that is, a rigid outer shell and a soft inner yolk and white structure.2 Although screening and quasi-adiabatic effects from the solvent due to the rigid outer structure of p-Ar(L4)2 seem reasonable for the anomalous phenomena, the nonergodic properties cannot be explained by this hypothesis because the temperature rise is equivalent to energy dissipation and dispersion. Another possibility is that the absorbed energy is stored by nonlinear oscillator dynamics. When the aromatic group is considered to be one particle, L5AZO and p-Ar(L4)2 have 62 and 31 particles connected by a methoxy group, -CH2-O-. As mentioned above, the shell of L5AZO and p-Ar(L4)2 is rigid and can be regarded as spherical fixed ends of the threedimensional oscillator system. When the oscillator system has nonlinearlity, nonergodic and soliton-like wave propagation can be induced. Since the soliton-like propagation can transfer the internal energy without dissipation and dispersion and can be synchronized in the system, this hypothesis seems to be valid for anomalous properties of the dendrimers. Originally, this idea was proposed by Fermi, Pasta, and Ulam in 1974.20 They studied a one-dimensional dynamical system of 64 particles with forces between neighbors containing nonlinear terms. If the analysis can be extended to a three-dimensional and branched system, a model similar to that from the Fermi-Pasta-Ulam theory may be applied to our system. Of course, there are some uncertainties
Energy Balance in Photoexcited Aryl Ether Dendrimers in the hypothesis. For example, molecular vibration energy should be converted to internal energy just after photon absorption, and internal energy also should be converted to chemical energy when it is consumed. 4. Conclusions We measured morphological dependence of radiative and nonradiative relaxation energy balance in photoexcited aryl ether dendrimers on time scales of 4-250 ms. Our results showed that p-Ar(L4)2 stored a half of the excitation energy for at least 250 ms. In addition, we measured fluorescence decay times for o-, m-, and p-Ar(L4)2, where the decay time was measured as 1.7 ns for all samples. The results indicated that the absorbed energy was not stored as electronic excited states, but as internal energy. To explain this long-term intramolecular energy storage, we proposed the novel mechanism including nonlinear conjugated oscillator and chemical/dynamical energy conversion models. We expect these results and mechanism will enhance understanding and promote not only studies of anomalous properties of dendrimers but also new chemistry fields such as development of energy storage materials and chemical/dynamical energy conversion systems. References and Notes (1) Tomalia, D. A.; Baker, H.; Dewald, J. R.; Hall, M.; Kallos, G.; Martin, S.; Ryder J.; Smith, P. Polym. J. 1985, 17, 117.
J. Phys. Chem. B, Vol. 105, No. 19, 2001 4445 (2) Jiang, D.-L.; Aida, T. Nature 1997, 388, 454. (3) Wakabayashi, Y.; Tokeshi, M.; Jiang, D.-L.; Aida T.; Kitamori, T. J. Lumin. 1999, 83-84, 313. (4) Wakabayashi, Y.; Tokeshi, M.; Hibara, A.; Jiang, D.-L.; Aida, T.; Kitamori, T. Anal. Sci. 2001, 16, 1323. (5) Tomoyose, Y.; Jiang, D.-L.; Jin, R.-H.; Aida, T.; Yamashita, T.; Horie, K.; Yashima E.; Okamoto, Y. Macromolecules 1996, 29, 5236. (6) Many reviews on thermal lens spectroscopy have been published. See, for expample: (a) Franko, M.; Tran, C. D. ReV. Sci. Instrum. 1996, 67, 1. (b) Bialkowski, S. E. Photothermal Spectroscopy Methods for Chemical Analysis; John Wiley & Sons: New York, 1996. (7) See, for example: Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26, 198. (8) See, for example: Campagna, S.; Denti, G.; Serroni, S.; Ciano, M.; Juris, A.; Balzani, V. Inorg. Chem. 1992, 31, 2982. (9) See, for example: Deisenhofer, J.; Michel, H. In Photochemical Energy ConVersion; Norris, J. R., Jr., Meisel, D., Eds.; Elsevier: New York, 1989; p 232. (10) Koplmann, R.; Shortreed, M.; Shi, Z.-Y.; Tan, W.; Xu, Z.; Moore, J. S.; Bar-Haim, A.; Klafter, J. Phys. ReV. Lett. 1997, 78, 1239. (11) Bar-Haim, A.; Klafter, J.; Koplmann, R. J. Am. Chem. Soc. 1997, 119, 6197. (12) Bar-Haim, A.; Klafter, J. J. Phys. Chem. B 1998, 102, 1662. (13) Bar-Haim, A.; Klafter J. J. Lumin. 1998, 76-77, 197. (14) Tretiak, S.; Chernyak, V.; Mukamel, S. J. Phys. Chem. B 1998, 102, 3310. (15) Poliakov, E. Y.; Chernyak, V.; Tretiak, S.; Mukamel, S. J. Chem. Phys. 1999, 110, 8161. (16) Chernyak, V.; Poliakov, E. Y.; Tretiak, S.; Mukamel, S. J. Chem. Phys. 1999, 111, 4158. (17) Harigaya, K. Chem. Phys. Lett. 1999, 300, 33. (18) Harigaya, K. Phys. Chem. Chem. Phys. 1999, 1, 1687. (19) Jiang, D.-L.; Kitamori, T.; Aida, T. Unpublished data. (20) Fermi, E.; Pasta, J.; Ulam, S. Lect. Appl. Math. 1974, 15, 143.
