7736
J. Phys. Chem. B 1997, 101, 7736-7743
Photoisomerization of Azobenzocrown Ethers. Effect of Complexation of Alkaline Earth Metal Ions Ruriko Tahara, Tatsuya Morozumi, and Hiroshi Nakamura* DiVision of Material Science, Graduate School of EnVironmental Earth Science, Hokkaido UniVersity, Sapporo 060, Japan
Masatsugu Shimomura Molecular DeVice Laboratory, Research Institute for Electronic Science, Hokkaido UniVersity, Sapporo 060, Japan ReceiVed: January 7, 1997; In Final Form: July 15, 1997X
The effects of complexed metal ions on trans-cis photoisomerization reactions of two azobenzene compounds having a crown ether ring, 1a (small ring) and 1b (large ring) were investigated. The photoisomerization of 2,2′-dimethoxyazobenzene (2) was compared as a control. Anomalous increase in the quantum yields of the photoisomerization of 1a and 1b were observed by complexation with alkaline earth metal ions in both cases of photo irradiation at nfπ* and πfπ* absorption bands, except for some cases. These increase was attributed to the interaction between coordinated metal ions and oxygen atoms on 2,2′-position and/or nitrogen atoms of the azobenzene moiety. The dependence of the quantum yields on the ring size of the crown ether was also discussed.
Introduction Azobenzenes are well-known compounds that undergo reversible isomerization reaction on the irradiation of ultraviolet or visible light, and their isomerization rates are usually fast.1-7 Thus, numbers of their analogues of photofunctionalized compounds, such as a molecular switch controlled by light, have been investigated.8-10 For example, an electrochemical approach has been proposed for actinometric measurements by using a difference between absorption wavelengths of these isomers.11 Molecular switch for two functional groups of azobenzene and anthraquinone was reported to be controlled by light and electronic current.12 Azobenzene moiety was impregnated into the crown ethers in order to control the complexation with alkali metal ions and extraction property of the ions by trans-cis isomerization.13-15 However, these photoisomerization properties are mainly determined by substituents of the compounds. The subtituent on the azobenzene group should be properly selected in order to change the isomerizating conditions, such as irradiation wavelength, isomerization rate, and thermal stability of the isomer. Some other external conditions could be applicable to control the isomerization. For example, Shinkai et al. have reported that the thermal stability of the cis isomer of the azobenzene type crown ether is affected by the complexation with alkali metal ions.13-15 Thermal isomerization could be also controlled by the protonation on the nitrogen atom in the azobenzene moiety.16 However, the control of the photoisomerization by the external conditions such as complexation has been seldom reported. In our previous papers,17,18 syntheses of several azobenzene type crown ether compounds (1a and 1b) and their complexing properties with alkali metal ions have been reported. Recently, we found that the complexation abilities of 1a or 1b with alkaline earth metal ions are stronger than those with alkali metal ions and that their photoisomerization reactions can be controlled * Corresponding author. E-mail:
[email protected]. X Abstract published in AdVance ACS Abstracts, September 1, 1997.
S1089-5647(97)00100-4 CCC: $14.00
by the presence of alkaline earth metal ions. In this paper, the complexation properties of cis and trans isomers of 1a, 1b, and 2 with alkaline earth metal ions and detailed kinetics of the photoisomerization reactions and their reverse reaction were reported. Experimental Section Reagents. Syntheses of the compounds 1a, 1b, and 2 used in this experiment were reported previously.18 Acetonitrile used for a solvent was purified by distillation, twice on phosphorus pentoxide and once on calcium hydride.
Apparatus. Absorption spectra were recorded by a SHIMADZU UV2200 spectrophotometer. 1H NMR spectra were obtained by a JOEL EX-400 1H NMR spectrometer. Photoirradiations were carried out by a USHIO 500 W Xenon short arc lamp after passing through a Jovin Ybon UV-10 monochrometer (bandwidth: 16 nm). The light intensity was monitored by photocurrent of a photodiode (Hamamatsu S1336 BQ). Measurement of Complex Formation Constants. Various amounts of alkaline earth metal perchlorate ([M2+] ) 1.0 × 10-6 to 1.0 × 10-5 M) were added to a 1.0 × 10-5 M acetonitrile solution of azobenzocrown ether (1a, 1b), and the resulting spectral changes were measured by using a standard 1 cm quartz cell. Absorbances at a certain wavelength that gave the largest absorbance difference were treated by a nonlinear © 1997 American Chemical Society
Photoisomerization of Azobenzocrown Ethers
J. Phys. Chem. B, Vol. 101, No. 39, 1997 7737
least-squares method (Marquardt method),20 and complex formation constants were evaluated. To prevent the effects of photoisomerization, these experiments were carried out in the dark. Measurement of Photoisomerization Rate. An acetonitrile solution of azobenzocrown ether (∼2 × 10-6 M) in a standard 1 cm quartz cell was irradiated by monochromated UV or visible light in the presence or absence of alkaline earth metal perchlorates (1 × 10-2 M for 1a, 1 × 10-4 M for 1b) and Mg(ClO4)2 (3 × 10-2 M). The absorbance of azobenzocrown ethers at the irradiation wavelength was made below 0.02. The absorption spectra were recorded at intervals of 1 min until 5 min, 2 min until 10 min, and 3 min in the rest. The ratio of the amount of cis and trans isomers at the photostationary state was determined by the absorption spectra and/or peak areas of the 1H NMR spectra. In the 1H NMR method, the ratios were obtained from peak areas of the aromatic protons at the photostationary state in the same conditions as the former measurements except for the azobenzocrown ether concentrations: [1a] or [1b] ) 1 × 10-4 M. The sample tube was rotated throughout the measurement in order to keep a uniform light irradiation. In the case of absorption spectra, the ratios were obtained by spectrum fitting of the observed spectra at the photostationary state and numerically added spectra of cis and trans isomers. Then, the photoisomerization rates and absolute quantum yields were calculated on the basis of first-order kinetics as described in the Appendix. Results UV Absorption Spectra. The UV absorption spectra of trans isomers of 1a, 1b, and 2 are obtained from the solution of the corresponding pure trans isomers of these compounds in the dark. However, cis isomers were not obtained in pure form. Therefore, the UV absorption spectra of cis isomers were obtained by subtracting the spectra of the pure trans isomers from those of the mixtures of trans and cis isomers, where the ratio of the isomers was estimated from the peak areas of 1H NMR spectrum. Thus obtained spectra of 1a, 1b, and 2 were shown in parts a, b, and c of Figure 1, respectively. Addition of alkaline earth metal perchlorate to the solution of trans isomers of 1a and 1b caused the spectral changes that depend on the concentration of the salts due to the metal complex formations of trans isomers of 1a or 1b. Since isosbestic points were observed in any case (Table 2), the formation of a 1:1 complex of 1a or 1b with the metal ions is confirmed. These spectra under the presence of excess amount of alkaline earth metal salts in the solution were summarized in Figure 1a,b. The spectrum of 2, however, did not change by the addition of excess amounts of alkaline earth metal perchlorates, showing no interaction with alkaline earth metal ions. Figure 1a,b shows that absorption spectra of free compounds and the alkaline earth metal complexes of 1a and 1b consist of several absorption bands. To determine the spectral shift and absorption coefficients in each transition band, these spectra in 300-500 nm region were resolved to three components (two πfπ* transitions and one nfπ* transition) (Figure 2). The resolution was conducted by assuming the Gaussian shape with a nonlinear curve-fitting method (Marquardt method).20 Thus obtained data are listed in Table 1. Complex Formation Constants. In the previous paper,18 we have already reported the complex formation constants obtained from spectral change of 1a and 1b in the complexation with alkali metal ions. However, the detailed data with alkaline earth metal ions have not been reported.
Figure 1. Absorption spectra of (a) 1a (5 × 10-5 M), (b) 1b (1 × 10-4 M), and (c) 2 (1 × 10-4 M) and their alkaline earth metal ion complexes in acetonitrile. [M2+] ) 1 × 10-1 M.
The spectral change of 1a or 1b in Figure 1a,b was small for cis isomer rich solution of 1a or 1b at salt concentrations less than 10-2 M. This shows that the complex formation of cis isomers with alkaline earth metal ion is weak. The formation constants (K) defined by eq 1 for 1:1 complex (ligand:metal ion) were calculated according to the method previously reported.18
7738 J. Phys. Chem. B, Vol. 101, No. 39, 1997
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TABLE 1: Resolved Absorption Band of 1a, 1b, and 2, and Their Complexes πfπ*(short) cation 1a (trans) Mg2+ Ca2+ Sr2+ Ba2+ 1a (cis) 1b (trans) Mg2+ Ca2+ Sr2+ Ba2+ 1b (cis) 2 (trans) 2 (cis) a
πfπ*(long)
nfπ*
λ/nm
∆νa/cm-1
λ/nm
∆νa/cm-1
λ/nm
∆νa/cm-1
300 329 331 325 324 280 288 305 307 308 305 276 311 277
3730 3440 3960 3790 3510 1560 3140 3710 3620 3440 3370 1590 4040 1820
10970 18200 11700 10910 11490 1630 9150 8230 12530 10980 12210 2670 6820 2170
351 370 374 373 371 310 337 351 350 351 351 308 372 313
1730 1500 1560 1680 1700 3680 2090 1470 1390 1400 1490 3300 2000 3090
9400 5030 4620 6550 8300 4090 7880 2870 4500 4220 5860 3750 7660 3140
461 418 444 450 453 436 439 424 424 427 433 433 469 434
2070 1370 1530 1440 1280 1850 2040 1810 1680 1800 1880 1980 1390 1860
980 7320 1120 1050 830 1560 2830 2210 3340 3200 3510 1610 990 1010
Half-width of the peak.
TABLE 2: Complex Formation Constants (log10 K) of Azobenzocrown Ethers for Alkali and Alkaline Earth Metal Ions (25 °C)a
a
cation
log10 K
Mg2+ Ca2+ Sr2+ Ba2+ Li+ Na+ K+
5.00 5.15 4.91 4.61 4.00b 3.69b 3.15b
trans 1a λobs/nm 327 327 327 327
λiso/nm
log10 K
298, 432 304, 418 302.5, 472 304.5, 449
< 3.00 6.90 7.13 6.67 3.41b 3.43b 3.37b
trans 1b λobs/nm 309 309 304
λiso/nm
ionic radiusc/ Å
278.5, 453 283, 462 284.5, 470
0.86 1.14 1.32 1.49 0.90 1.16 1.52
λobs is a wavelength for calculation of the complex formation constant. λiso is a wavelength of the isosbestic point. b Reference 18. c Reference
19.
Gex ) Ggnd + ∆E ) Ggnd + hν
(2)
where ∆E is an excitation energy, Gex and Ggnd are the free energy of the excited and ground states, respectively, in the free or the complexed form, and ν is frequency of the absorbed light. A photon energy of fluorescence light is usually used for calculation of ∆E. However, 1a and 1b have no fluorescence at all as many azobenzene compounds. A photon energy of absorbed light (hν) was used for ∆E in this study. The free energy changes at the complexation of 1a or 1b with alkaline earth metal ions in a ground state are calculated by the following thermodynamical relation:
∆Ggnd ) -RT loge K Figure 2. Spectral resolution of 1b to three transition bands (Gaussian shape).
K ) [ML]/([M2+][L]) M2+
(1)
Here, and L represents metal ion and ligand (i.e., azobenezocrown ether), respectively. Since the solution of trans-1a or 1b became a mixture of trans and cis isomers under the room light (the ratio were 1:1), the actual concentration of trans-1a or 1b was used for this calculation. Some constants (K < 103 M-1) could not be evaluated due to too low accuracy. Table 2 lists the complex formation constants of trans-1a and 1b. The analogous compound 2 did not form complexes with any alkaline earth metal ions at concentrations less than 10-2 M. Those values for the Mg2+ complex of trans-1b can not be determined accurately due to a weak complexing ability. Energy Levels of the Excited State of the Complexes. The free energies of the excited states of 1a, 1b, and their complexes are calculated by the following equation
(3)
The free energy changes at the complexation in an excited state can be also calculated by following equation:
∆Gex ) Gex(complex) - Gex(free) ) ∆Ggnd - hν1 + hν2
(4)
Here, ν1 and ν2 are frequencies of the absorbed lights of free and complexed compounds, respectively. Figure 3 shows these calculated free energy levels in the ground state and the excited state. Photoisomerization of Azobenzocrown Ether Compounds. The UV and visible light irradiations of 1a, 1b, and 2 caused both isomerization from trans to cis and Vice Versa (Figure 4) depending on which was a starting compound. Since the several (∼10 times) repetition of forward and reverse photoisomerizations did not give any degradations, these isomerizations were reversible in this experiment. Both absolute quantum yields
Photoisomerization of Azobenzocrown Ethers
J. Phys. Chem. B, Vol. 101, No. 39, 1997 7739
Figure 5. Time dependence of absorbance of 1a and its Ca2+ complex by the irradiation at the nfπ* absorption band (460 and 446 nm for free 1a and Ca2+ complex, respectively).
TABLE 3: Quantum Yields of Isomerization between cis and trans Forms of Azobenzocrown Ethers by Irradiation at nfπ* Transition Bands (25 °C)a compound
cation λ/nmb
1a Mg2+ Ca2+ Sr2+ Ba2+ 1b Mg2+ Ca2+ Sr2+ Ba2+ 2 azobenzened
460 418 446 446 453 440 423 421 427 439 472 439
trans
Φt-c
cis
940 9560 1920 1470 940 2800 2300 3470 3240 3420 1000
0.21 0.35 0.45 0.51 0.60 0.27 0.67 0.35 0.34 0.32 0.60 0.31
1160 3800 1200 1600 1200 1610 1540 1525 1565 1610 600
Φc-t trans:cisc 0.61 0.29 0.96 0.67 0.68 0.58 1.0 0.64 0.79 0.61 0.65 0.46
0.78:0.22 0.25:0.75 0.57:0.43 0.59:0.41 0.59:0.41 0.55:0.45 0.50:0.50 0.45:0.55 0.53:0.47 0.47:0.53 0.44:0.56
a Under this experimental condition ([M2+] ) 1 × 10-2 M for 1a, 1 × 10-4 M for 1b, [Mg2+] ) 3 × 10-2 M), and more than 98% of trans 1a or 1b existed as a metal complex. b Irradiation wavelength. c The trans:cis ratio at the photostationary state. d Reference 6.
Figure 3. Energy levels of ground and excited states of 1a and 1b and their metal ion complexes: (a) nfπ* and (b) πfπ* (long wavelength) of 1a; (c) nfπ* and (d) πfπ* (long wavelength) of 1b.
Figure 4. Photoisomerization of cis-1a by irradiation at the nfπ* absorption band (460 nm). [1a] ) 1.4 × 10-5 M, 25 °C.
(Φt-c and Φc-t) can be obtained as described in the Appendix section even in the cases where pure isomers are not used, and
the present measurements were started from mixtures of trans and cis isomers in convenience (cis-rich or trans-rich solution). The time dependence of the absorbance of 1a solution (started with cis-rich solution) at the nfπ* transition band irradiation is shown in Figure 5, in which the time dependence under the presence of Ca2+ is also represented for comparison. Although nfπ* transition bands of trans and cis isomers were close to each other (∆λ ) 4-25 nm), the irradiation wavelengths were set to the absorption maximums of trans isomers. The molar ratios of cis and trans isomers in the photostationary state under these light irradiations were determined from peak areas of 1H NMR spectra. From Figure 5, both absolute quantum yields, Φt-c and Φc-t, of 1a were evaluated (Appendix). The quantum yields for 1b and 2 were similarly obtained. Results are listed in Table 3. Table 4 shows the quantum yields corresponding to irradiation at the πfπ* transition band. Here, the cis isomer does not have πfπ* absorption bands at longer wavelengths of near to that of trans isomers (∼350 nm). The irradiation wavelengths of Table 4 were set to the center of the longer band of the πfπ* transition, which was obtained by spectral resolution as described in the previous section. Since these irradiation wavelengths were far from the πfπ* transition of the cis isomers, the molar coefficients of the cis isomer were small, and the accuracy of the Φc-t values for these cases were not as good. Alkaline earth metal ions in the solution of trans-1a or 1b form stable complexes. When trans isomers of 1a or 1b existed as a complex more than 98% under the presence of excess
7740 J. Phys. Chem. B, Vol. 101, No. 39, 1997
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TABLE 4: Quantum Yields of Isomerization between cis and trans Isomers of Azobenzocrown Ethers by Irradiation at πfπ* Transition Bands (25 °C)a compound
cation λ/nmb
1a Mg2+ Ca2+ Sr2+ Ba2+ 1b Mg2+ Ca2+ Sr2+ Ba2+ 2 azobenzened
353 369 374 373 370 339 335 349 351 351 371 317
trans
Φt-c
cis
12400 14710 11430 11590 13070 9180 7020 9950 8480 6560 9700
0.25 0.17 0.59 0.53 0.39 0.26 0.41 0.37 0.33 0.32 0.46 0.15
1870 1050 1260 2890 3560 3260 3510 2500 2380 2380 420
Φc-t trans:cisc 0.18 0.13 0.05 0.11 0.13 0.19 0.50 0.33 0.46 0.46 0.52 0.21
0.10:0.90 0.05:0.95 0.01:0.99 0.05:0.95 0.08:0.92 0.20:0.80 0.38:0.62 0.18:0.82 0.28:0.72 0.32:0.68 0.05:0.95
a Under this experimental condition ([M2+]) 1 × 10-2 M for 1a, 1 × 10-4 M for 1b, [Mg2+] ) 3 × 10-2 M), and more than 98% of trans 1a or 1b existed as a metal complex. b Irradiation wavelength. c The trans:cis ratio at the photostationary state. d Reference 6.
amount of alkaline earth metal ion ([M2+] ) 10-2 M for 1a and 10-4 M for 1b), the observed Φt-c’s were those of the complexes of 1a or 1b. At lower concentrations of the salts ([M2+] e 10-4 M), the cis isomer hardly formed the complex. The obtained Φc-t was that for free cis isomer. However, at relatively high concentrations of the salts ([M2+] g 10-2 M), a comparable amount of the complex of cis isomer of 1a with alkaline earth metal ions was formed as shown from the 1H NMR spectra. The Φc-t values for 1a in the presence of metal ions are average values of Φc-t of free and complexed species. Discussion Absorption Spectra. Absorption spectra of cis and trans isomers of 1a, 1b, and 2 are shown in parts a, b, and c of Figure 1, respectively. These compounds in trans form reveal absorption peaks at 300-400 and ca. 450 nm due to πfπ* and nfπ* transitions, respectively.1 In the case of usual azobenzene compounds, the absorption coefficient of the nfπ* transition peak of the cis isomer is larger than that of the trans isomer. However, the absorption coefficient of the nfπ* transition peak (∼440 nm) of the cis isomers of 1b and 2 is less than those of the trans isomer. This should be mainly due to the repulsion between the etherial ortho-substituents and the lone pairs of azo nitrogen atoms in trans isomers, which causes a distortion of azobenzene plane, and partial cancellation of forbidden nfπ* transition. In the case of crown ethers with a small ring such as 1a, usual nfπ* transitions for cis and trans isomers were observed, but the absorption maximum was shifted to longer wavelength. This reason is not so clear, but it may be due to the strain of the small crown ether ring, which makes the distorted azobenzene moiety more planar. Figure 1a,b also shows absorption spectra of the alkaline earth metal complexes of 1a and 1b. The resolution gives wavelength data listed in Table 1. The band at longer wavelength around 460 nm due to the nfπ* transition of the azo group shifted to shorter wavelength by the complexation with alkaline earth metal ions. The magnitude of the shift reflects the interaction between the azo group and metal ions and indicates the stabilization of the ground state of the lone pair electron (n) of the azo group and/or the destabilization of the excited nπ* level by the positive charge of the metal ions. Figure 3a,c shows the energy-level changes by the complexation and indicate that the spectral shifts are attributed to the destabilization of the excited states probably caused by the decrease of the lone pair electron (n) density at the excitation. The magnitudes of these shifts (Table 1) were in the order of Mg2+ > Ca2+ > Sr+2 >
Ba2+ for 1a and Ca2+ = Mg2+ > Sr2+ > Ba2+ for 1b, indicating the order of interaction energies between these metal ions and the azo group of 1a or 1b. However, these orders are not in parallel to those of the complex formation constants (Table 2). Especially, Mg2+ does not have as strong of a complexing ability with 1a and 1b but shows quite a large lower shift for 1a (∆λ ) 43 nm), being larger than that of Ca2+, and a smaller one for 1b (∆λ ) 15 nm), the same as that of Ca2+. This result indicates that the order of complexing abilities with alkaline earth metal ions with 1a and 1b are mainly dominated by the oxygen atoms of the crown ether moiety. On the other hand, the πfπ* transition band (long wavelength) of 1a and 1b usually shifted to longer wavelength by the complexation. The order of the shifts was almost the same as that for nfπ* transition, except for Mg2+. The dependence on the metal ions was smaller than that for nfπ* transition. Since the complexation of crown ethers is taken to stabilize the ground state of the molecule, this phenomenon shows a larger stabilization of the ππ* excited level at the complexation than ground states (Figure 3b,d). The amounts of this stabilization at the excitation were ca. 20 kJ mol-1 for 1a and ca. 14 kJ mol-1 for 1b. This stabilization would be caused by the increase of the electron density on the 2,2′-substituted oxygen or azo nitrogen atoms at the excitation to the ππ* state, and this ππ* excited state was stabilized by positive charge of the cation in the complex. The reason for the difference between the amounts of the stabilization of 1a and 1b is not clear, because structures of these complexes in the ground and excited states are hardly determined. Complex Formation Property. The complex formation constants listed in Table 2 show that the complex formation constants of trans-1a with alkaline earth metal ions are 10100 times larger than those with alkali metal ions, and those of trans-1b are still 103-104 times larger than those for alkali metal ions, except for the Mg2+ case. This is due to the larger surface charge densities of alkaline earth metal ions, giving rise to a stronger electrostatic interaction between ligand oxygen and the metal ions than those of alkali metal ions. In the case of trans1a with a smaller poly(oxyethylene) ring, the strongest complexing ability was observed for Ca2+. From CPK model consideration, the hole radius of trans-1a is 0.85-0.90 Å, and the ionic radius of Ca2+ is 1.14 Å in usual coordination number.19 On the other hand, in the case of trans-1b with a larger ring, the complexing ability for Sr2+ was strongest. In this case, hole radius is 1.25-1.30 Å, and the ionic radius of Sr2+ is 1.32 Å. Furthermore, trans-1b is a more flexible molecule due to the longer poly(oxyethylene) ring and usually shows a stronger complexing ability than does trans-1a. These results show that the largest complexing constant is due to the size fitting between the hole and ion, as observed in the complexation of many crown ethers and cations.21 Photoisomerization of Azobenzocrown Ether Compounds. On the photoisomerization of azobenzene,1,2 many investigators have reported that the quantum yield from cis to trans isomer (Φc-t) is larger than Φt-c on the excitations at both nfπ* and πfπ* bands and the quantum yield of the isomerization between trans and cis isomer (Φt-c) by excitation is smaller at the πfπ* than at the nfπ* transition band. As seen from Tables 3 (nfπ*) and 4 (πfπ*), in the present case of 1a and 1b, the results at the nfπ* band irradiation reproduce Φc-t > Φt-c as in the azobenzene case, while the results at the πfπ* band irradiation were reverse to that in the nfπ* case (i.e., Φc-t < Φt-c). However, it must be noticed that the accuracy of Φc-t values in the πfπ* case is low due to a small cis at these experiments. In the case of 2, Φc-t was close to Φt-c
Photoisomerization of Azobenzocrown Ethers
Figure 6. Schematic representation of interaction between azobenzene moiety and coordinated metal ion.
value at both nfπ* and πfπ* irradiation. As mentioned before, compound 2 is a unique compound, and the substituents at the 2,2′-position of azobenzene might cause this anomalous result. In the case of 1a and 1b, the quantum yield, Φt-c, at πfπ* excitation is close to Φt-c at nfπ* excitation. This is not the case at unsubstituted azobenzene and suggests that the isomerization from ππ* and nπ* excited states of the trans isomer takes place via a different pathway. Rotational and inversion processes can be likely ones for ππ* and nπ*, respectively, which were also proposed for azobenzene by calculation5 and experiment.6 However, a possibility of internal conversion from ππ* to nπ* excited state with a high efficiency cannot be excluded. The quantum yields from trans to cis isomer were in the order of Φt-c(2) > Φt-c(1a,1b) > Φt-c(azobenzene) for πfπ* excitation and Φt-c(2) > Φt-c(azobenzene) > Φt-c(1a,1b) for nfπ* excitation. These different substituent effects in two cases also suggest that the isomerization path from trans to cis isomer of 1a, 1b, and 2 at πfπ* excitation does not occur via the nπ* excited state or a resembled state1,2 but via a direct isomerization path from the ππ* excited state as mentioned above. This result may be attributed to the nonplanar structures of 1a, 1b, or 2 due to the substituents at the 2,2′-position, as suggested from absorption spectra (Vide ante). Effect of Alkaline Earth Metal Ions. The quantum yields for trans to cis isomerization at both nfπ* and πfπ* excitations increased when 1a and 1b were complexed with any alkaline earth metal ions except for the Mg2+ complex of 1a. Since the complexation with metal ion usually stabilizes the structure of the ligand molecule, the thermal isomerization or photoisomerization rate is decreased.13,15 However, the present results show the increases of the quantum yields by the complexation. When the πfπ* transition bands were excited, the quantum yields Φt-c of 1a were in the order of Ca2+ ≈ Sr2+ > Ba2+ > Mg2+ (Table 4). Since this order is parallel to the complex formation constants for these ions and also to the shifts of the πfπ* absorption bands except for Mg2+, the cause of the increase of quantum yield should be caused by the formation of a chelating bond between the metal ion and oxygen and/or nitrogen atoms on the azobenzene moiety (Figure 6). However, the cavity size of trans-1a (0.85-0.90 Å in radius) is smaller than those of the ions (radii of these metal ions are 1.14, 1.32, and 1.49 Å for Ca2+, Sr2+ and Ba2+, respectively),19 and the ideal fitting in size is not expected. However the relatively large formation constants (Table 2) reveal the existence of the strong chelating interactions between the metal ion and the crown ether, which are strengthened by πfπ* excitation (∼20 kJ mol-1, see Figure 3b). These chelating bonds will cause molecular strain even in the transient or excited singlet state and promote the isomerization from trans to cis. On the other hand, when the nfπ* transition bands of 1a were excited, the quantum yields Φt-c were in the order of Ca2+
J. Phys. Chem. B, Vol. 101, No. 39, 1997 7741 < Sr2+ < Ba2+ (Table 3). The Φt-c values were still larger than that of free 1a. The order of Φt-c is just in the order of the stability energy of the excited state (Figure 3a). The chelating bond also promotes the isomerization from trans to cis in the nπ* excited singlet state as in the above case. The quantum yields of the isomerization from cis to trans (Φc-t) of 1a at nfπ* irradiation were not so much affected by the addition of Sr2+ or Ba2+. This is attributed to too small amount of these metal complexes, because of the weak complex formation abilities of cis-1a. Ca2+ and Mg2+ may form a comparable amount of complexes as expected from Φc-t’s affected by the addition of a large amount of these metal ions (3 × 10-2 M). However, the correct Φc-t values were not obtained because stability constants of these complexes were not known, and the discussion of these effects remains untreated in the present study. The quantum yields of the isomerization from trans to cis of 1b by excitation at πfπ* transition bands were also increased by the presence of alkaline earth metal ions, and the difference among the metal ions were smaller than that in the case of 1a. Except for Mg2+, the quantum yield was in the order of Ca2+ > Sr2+ > Ba2+. Though the order was not the same as that of the stability of the excited state, the difference was small. The reason for these increases is probably the same as that in the case of 1a. The increase of quantum yield by complexation was also smaller than that in the 1a case, probably because of the smaller stability energy (14 kJ mol-1) than that in the 1a case (Figure 3d). These trends were also observed for the quantum yields at nfπ* excitation except for Mg2+. In the Mg2+ case, the nfπ* band irradiation gave higher quantum yield than the other metal ions, even though the stability at the excited state of the complex was relatively small (-∆Gex < 8.4 kJ mol-1, Figure 3c). Other factors must be taken into account. These results can be also explained from potential curves as follows: The potential energy curves for isomerization of azobenzene were calculated, and the isomerization paths via inversion and rotation process were proposed.5 These potential curves cannot be adopted to 1a and 1b directly, but the shape may resemble those of 1a and 1b. As shown in Figure 3, the complexation with metal ions stabilized the ππ* state of the trans isomer and destabilized the nπ* state, relative to the ground state. Thus, the position of the peak top on the potential curve of the inversion process in the ground state shifted to the trans form, and that of the saddle points on the nπ* potential curves of the inversion process shifted to the cis form (Figure 7). This effect increased the branching ratio to the cis isomer from both excited states in the inversion process, and Φt-c was increased. However, the energy level of the excited cis isomer was unknown. The increase of Φc-t could not be explained. The quantum yields of the isomerization from cis to trans (Φc-t) of 1b by the irradiation at nfπ* bands were also increased by the addition of metal ions. The complex formation constants of cis-1b with alkaline earth metal ions are relatively low (log10 K < 3), and only a small amount of cis-1b will exist as a metal complex at the experimental conditions. Here, we only mention two possible explanations for this increase: (i) The photoisomerization of complexed cis-1b to the trans form is quite fast, and the metal ion acts as a catalyst, and (ii) the thermal isomerization of complexed cis-1b was assisted by metal ion as a catalyst. Details will require further examination including the characterization of complexed cis-1b. In many cases of nfπ* band irradiation in the presence of metal ions, the sums of Φt-c and Φc-t exceeded 1.0. This suggests that the isomerization processes from trans to cis and
7742 J. Phys. Chem. B, Vol. 101, No. 39, 1997
Tahara et al.
Figure 8. Photoisomerization path of azobenzocrown ether.
Figure 7. Schematic representation of the S0, S1, and S2 potential energy curves of free azobenzocrown ethers (s) adopted from the results of calculations reported in refs 5b and 6, and predicted curves of its alkaline earth metal complex (- - -).
from cis to trans isomers of the metal complexes do not proceed via a common excited state which has been suggested for unsubstituted azobenzene.1,2,5 The effects of metal ions on the photoisomerization of 2 were not observed even if the concentration was more than 1 × 10-2 M. This is due to weak complexation ability of 2.
where I0 is intensity of light (count of photons per second and per square centimeter) and cis and trans are molar absorption coefficients of cis and trans isomers, respectively. T and Abs are transmittance and absorbance of the sample solution in the cell at a given irradiation wavelength, respectively. At the condition of Abs < 0.02, Abs can be approximated to (1 T)/2.303. Thus, the eqs 5 and 6 are simplified, and the concentration of the isomers (Ccis and Ctrans) excited to the S1 or S2 level per second are given as follows
Ccis ) 1000Ecis/NA ) 2.303 × 103I0cis [Lcis]/NA
(7)
Ctrans ) 1000Etrans/NA ) 2.303 × 103 I0trans [Ltrans]/NA (8)
Conclusion The quantum yields of photoisomerization were examined on two azobenzene compounds with a poly(oxyethylene) ring. It becomes clear that the photoisomerization behavior of these compounds is controlled by means of the complexation with metal ions and/or the change of the irradiation band. These compounds can be used for the construction of a molecular switch by light, which can be controlled by external conditions such as metal ions or wavelength of light. Appendix Evaluation of Efficiency of Photoisomerization. Since photoisomerization rates depend on the intensity of light and also on the absorption coefficient of the compound at a given irradiation wavelength, the rates should be normalized by the count of photons absorbed by the molecule. The normalized rate is identical to the absolute quantum yield of the isomerization. Although the method to obtain the quantum yields has been reported only briefly in several literature sources,1,2 no details were described on the effects such as the consideration of reverse reaction. Several reaction mechanisms have been proposed for the photoisomerization. However, in the presence of metal salts the excited state of the trans isomer should be different from that of cis isomer, because the trans isomer easily forms and the cis isomer hardly forms a complex. Thus, the present work assumed the essential scheme of Figure 8 for the photoisomerization. According to Figure 8, the absolute quantum yields for the respective isomerizations are evaluated as follows. When cis and trans isomers of the molecules (Lcis and Ltrans, respectively) exist in a sample solution, the amounts of photons absorbed by cis and trans isomers (Ecis and Etrans) are given as follows
where NA is Avogadro’s number and the unit of concentration is expressed as M (mol dm-3). Differential equations for the isomerization kinetics are given as follows
d[Lcis]/dt ) -2.303 × 103I0 cis [Lcis]/NA + k1[Lcis*] + k4[Ltrans*] (9) d[Ltrans]/dt ) -2.303 × 103I0trans [Ltrans]/NA + k3 [Ltrans*] + k2 [Lcis*] (10) d[Lcis*]/dt ) 2.303 × 103I0cis [Lcis]/NA - (k1 + k2)[Lcis*] (11) d[Ltrans*]/dt ) 2.303 × 103I0 trans [Ltrans]/NA (k3 + k4)[Ltrans*] (12) where, k1, k2, k3, and k4 are rate constants of each step represented in Figure 8, and asterisks show excited species. Since the lifetimes of excited states are quite short,7 the concentrations of the excited species are usually negligible. Thus, the derivatives of eqs 11 and 12 are equated to 0, and the time dependence of the concentrations of the isomers are derived as follows
[Lcis] ) Φt-c trans [Lt]/A + B exp(-ARt)
(13)
[Ltrans] ) Φc-t cis [Lt]/A + B′ exp(-ARt)
(14)
where
Ecis ) I0 (1 - T)cis [Lcis]/Abs
(5)
[Lt] ) [Lcis] + [Ltrans]
Etrans ) I0 (1 - T)trans [Ltrans]/Abs
(6)
Φt-c ) k4/(k3 + k4)
Photoisomerization of Azobenzocrown Ethers
J. Phys. Chem. B, Vol. 101, No. 39, 1997 7743 Supporting Information Available: 1H-NMR spectra of 1a, 1b, and 2 and their complexes (3 pages). Ordering information is given on any current masthead page.
Φc-t ) k2/(k1 + k2) R ) 2.303 × 103I0/NA
References and Notes
A ) Φt-ctrans + Φc-tcis B ) [Lcis]t)0 - [Lcis]t)∞ B′ ) [Ltrans]t)0 - [Ltrans]t)∞ ) -B and
[Lcis]t)∞ ) Φt-ctrans [Lt]/A [Ltrans]t)∞ ) Φc-tcis [Lt]/A Here, [Lcis]t)∞ and [Ltrans]t)∞ are the concentrations of respective isomers in the photostationary state. From above equations, the time dependence of the absorbance of the sample solution at a monitoring wavelength, Absobs, are given as:
Absobs ) (cisobs transΦt-c + transobs cisΦc-t)[Lt]/A + (cisobs - transobs)B exp(-ARt) (15) Then, AR values are evaluated from the time dependence of the Absobs, experimentally, using the nonlinear least-squares curve-fitting method (Marquardt method).20 Since R is obtained from the light intensity for irradiation, the averaged quantum yield, A, can be obtained. Therefore, the absolute quantum yields of the photoisomerization of the both isomers are evaluated from following equations:
Φc-t ) A[Ltrans]t)∞/(cis [Lt])
(16)
Φt-c ) A[Lcis]t)∞/(trans [Lt])
(17)
The values of Φc-t and Φt-c are summarized in Tables 3 and 4.
(1) Zimmerman, G.; Chow, L. Y.; Pail, U. J. J. Am. Chem. Soc. 1958, 80, 3528. (2) Yamashita, S.; Ono, H.; Toyama, O. Bull. Chem. Soc. Jpn. 1962, 35, 1849. (3) Ronayette, J.; Arnaud, R.; Lebourgeois, P.; Lemaire, J. Can. J. Chem. 1974, 52, 1848. (4) Morgante, C. G.; Struve, W. S. Chem. Phys. Lett. 1979, 68, 267. (5) (a) Monti, S.; Gardini, E.; Bortolus, P.; Amouyal, E. Chem. Phys. Lett. 1981, 77, 115. (b) Monti, S.; Gardini, E.; Palmieri. P. Chem. Phys. 1982, 71, 87. (c) Bortolus, P.; Monti, S. J. Phys. Chem. 1979, 83, 648. (6) Rau, H.; Lu¨ddecke, E. J. Am. Chem. Soc. 1982, 104, 1616. (7) Lednev, I. K.; Ye, T.; Hester, R. E.; Moore, J. N. J. Phys. Chem. 1996, 100, 13338. (8) Sasaki, T.; Ikeda, T.; Ichimura, K. Macromolecules 1993, 26, 151. (9) Tamaoki, N.; Yoshimura, S.; Yamaoka, T. Thin Solid Films 1992, 221, 132. (10) Anzai, J.; Sakasegawa, S.; Takemura, T.; Osa, T. Mat. Sci. Eng. C2 1994, 102. (11) Liu, Z.; Morigaki, K.; Hashimoto, K.; Fujishima, A. Anal. Chem. 1992, 64, 134. (12) Shaika, T.; Iyoda, T.; Honda, K.; Shimidzu, T. J. Chem. Soc., Perkin Trans. 2 1993, 1181. (13) Shinkai, S.; Minami, T.; Kusano, Y.; Manabe, O. J. Am. Chem. Soc. 1983, 105, 1851. (14) Shinkai, S.; Nakaji, T.; Ogawa, T.; Shigematsu, K.; Manabe, O. J. Am. Chem. Soc. 1981, 103, 111. (15) Shinkai, S.; Ogawa, T.; Kusano, Y.; Manabe, O.; Kunikawa, K.; Goto, T.; Matsuda, T. J. Am. Chem. Soc. 1982, 104, 1960. (16) Sanches, A.; de Rossi, R. H. J. Org. Chem. 1993, 58, 2094; Ibid. 1995, 60, 2974. (17) Aoki, S.; Shiga, M.; Tazaki, M.; Nakamura, H.; Takagi, M.; Ueno, K. Chem. Lett. 1981, 1583. (18) Shiga, M.; Nakamura, H.; Takagi, M.; Ueno, K. Bull. Chem. Soc. Jpn. 1984, 57, 412. (19) Shannon, R. D.; Prewitt, C. T. Acta Crystallogr. 1969, B25, 925. (20) Marquardt, D. W. J. Soc. Ind. Appl. Math. 1963, 11, 431. (21) Takagi, M.; Nakamura, H. J. Coord. Chem. 1986, 15, 53 and references cited therein.
