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Thermally activated and light-induced metal ion complexation of 5′-(hydroxy)spiroindolinonaphthooxazines in polar solvents
Polyhedron 23 (2004) 3147–3153 www.elsevier.com/locate/poly
Thermally activated and light-induced metal ion complexation of 5 0-(hydroxy)spiroindolinonaphthooxazines in polar solvents Stela Minkovska a, Monika Fedieva b, Bojana Jeliazkova
b,*
, Todor Deligeorgiev
b
a
b
Institute of Catalysis, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Department of Chemistry, Sofia University, 1, James Bourchier ave., 1164 Sofia, Bulgaria Received 22 July 2004; accepted 29 September 2004 Available online 5 November 2004
Abstract Chelation with Al(III), Fe(II) or Cu(II) of the open photomerocyanine form obtained under steady irradiation of spiroindolinonaphthooxazines, with a hydroxyl group at the 5 0 position in the naphthooxazine moiety, induces a slight hypsochromic shift of its visible absorption band and increases the lifetime of this form, slowing down its thermal bleaching in the dark (rate constant 103 s1). Complexation with Al(III), Fe(II) or Cu(II) allows the spiroindolinonaphthooxazines to isomerize to their open coloured form even under dark conditions giving a complex spectroscopically identical to the photoinduced product. The activation energy of thermal complexation is independent of the metal ion which implies the ring opening as the rate determining step. 2004 Elsevier Ltd. All rights reserved. Keywords: Absorption spectra; Complexation; Kinetics; Photochromism; Spiroindolinonaphthooxazines
1. Introduction When a metal ion coordinating moiety is combined with a photosensitive spiropyran or spirooxazine, light may induce metal binding and metal binding may influence photochromism [1–4]. This approach has been mainly reported for spiropyranes [1–12] and only in a few cases for spirooxazines [13–17]. A conceptually attractive possibility for spironaphthooxazines is the modification of the naphthooxazine moiety to enable chelation by metal ions and preferentially stabilize the open merocyanine form. Previous demonstrations of light-induced metal binding have relied upon substitution at the 5 0 position to create a nascent bidentate chelator that becomes active after photoinduced ring opening [15–17]. In this paper we report the effect of chelation with metal ions on the irradiation of two spiroin*
Corresponding author. Tel.: +359 2 8161347; fax: +359 2 9625438. E-mail address: [email protected]fia.bg (B. Jeliazkova).
0277-5387/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2004.09.022
dolinonaphthooxazines recently synthesized in our laboratory, [18] with an -OH substituent at the 5 0 position in the naphtooxazine moiety (Scheme 1) as a potential chelating functional group. The effect of chelation on the thermal SO MC equilibrium between the coloured merocyanine (MC) and the colourless spiro (SO) form is also discussed. The thermal equilibrium between the closed and opened form in Scheme 1 also depends on solvent polarity [18,19], since polar solvents promote the formation of the coloured form at room temperature in the absence of light. The equilibrium between both forms is strongly displaced upon irradiation to the side of the open-chain coloured photomerocyanine which spontaneously converts to the colourless spiroform to reach thermal equilibrium immediately after removing the light. Here, we report our results on the possibility to stabilize the coloured MC form towards thermal reversion by complexation with selected metal ions in appropriate solvents.
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S. Minkovska et al. / Polyhedron 23 (2004) 3147–3153
4
H3C
2'
5 2 6 7
1
H3C
CH3
3
N
3'
10'
N
8'
O HO
CH3 N
hv kT
7'
4'
5'
R1
9'
N O
6'
OH
R1
(SO)
(MC) Scheme 1.
2. Experimental 2.1. Materials The photochromic molecules were two spiro[indolinonaphthooxazines]: 1,3,3-trimethyl-5 0 -hydroxyspiro(indoline-2,3 0 -[3H]naphth[2,1-b][1,4]oxazine) (1) and 1-butyl-3,3-dimethyl-5 0 -hydroxyspiro(indoline-2,3 0 -[3H] naphth[2,1-b][1,4]oxazine) (2). The compounds were recently prepared and characterized in our laboratory [18,19]. The p.a. grade metal salts were obtained from Fluka and were used without further purification. The solvents were used after distillation. H3 C
CH3
N 1: R = CH3
N
O
2: R = C4H9
R HO
2.2. Instrumentation Absorption spectra were recorded on a SPECORD UV–Vis (Carl Zeiss, Jena) spectrophotometer using quartz cells. For absorption measurements at varying temperatures the reaction cell was enclosed in a thermostatic water jacket placed inside the spectrophotometer sample chamber. A 250 W medium-pressure mercury lamp was used for producing the coloured form. Photochemical reactions were carried out in the spectrophotometric quartz cell with a homogeneously spread light on the cell window to avoid stirring [20,21]. Measurements were made on aerated solutions. EPR spectra were taken at 77 K on an X-band BRUKER B-ER 420 spectrometer using 100 kHz modulation of the magnetic field. 2.3. Kinetics measurements The effects of metal ions on the SO MC process were studied in 1:1 aqueous/acetone or aqueous/ethanol
mixtures at a constant spironaphthooxazine concentration of 5 · 105 M and varying the ratio between the SO and the metal ion from 2:1 to 1:20. The kinetics of the thermal colouration in the presence of metal ions was measured at different temperatures in the range 20–50 C, ca. 30 min after having set the temperature control in order to allow the initial solutions to reach the appropriate temperature in separate vessels. The absorbance was measured at the wavelength of maximum absorption (kmax) directly in the sample cell at 30 s intervals after the insertion of 10 equiv. of aqueous metal ion solution in ethanol solutions of 1 or 2. First order build up rate constants were obtained from the linear dependences of ln A versus time. The activation energies along with the frequency factors of thermal ring opening were determined from Arrhenius plots. In the photocolouration experiment the cuvette was irradiated for several seconds after thermal equilibration of the reaction mixture containing SO and the metal ion in 1:1 aqueous/ethanol. Trial and error experiments enabled the optimum exposure time to be determined for maximum build-up of the complex. The ring-closure reaction after photocolouration was monitored directly after the removal of the light, scanning at the wavelength of maximum absorption of the complex between the MC form and the metal ion (kcompl) at 15 s intervals over a period of 30–50 min at room temperature. First order rate constants of thermal re-equilibration were obtained from the linear ln A versus time descending curves. by extrapolation of the obtained ln A/t plots to zero time, the absorbance Ao of the complexes at t = 0 was related to their ‘‘colourabilities’’ [21–24] using the expression Ao/cSOb in which cSO was the initial concentration of SO and b was the optical path length.
3. Results and discussion The colourless spiro compounds 1 and 2 give no complexes with the metal ions. However, their merocyanines form complexes with Cu(II), Fe(II) and Al(III) in polar solvents, while Cr(III), Mn(II), Ni(II), Co(II), Ba(II) and Mg(II) do not affect the equilibrium between SO and MC in Scheme 1.
S. Minkovska et al. / Polyhedron 23 (2004) 3147–3153
3149
metal salt [25] induces irreversible photo-degradation without any complexation, indicating that the nitrogen atom at the 1 0 position in the naphthooxazine moiety takes no part in chelation.
3.1. Absorption spectra Visible spectra of photomerocyanines, obtained upon UV illumination of 1 or 2 in polar solvents [19], are characterized by an absorption in the range 600– 620 nm which remains unchanged upon UV illumination in the presence of Cr(III), Mn(II), Ni(II), Co(II), Ba(II) and Mg(II). Irradiation of 1 and 2 in the presence of 1 equiv. of Cu(II), Fe(II) or Al(III) generates a new photo-reversible absorption band in the range 540–600 nm, with band intensity dependent on the [M(II),(III)]/ [SO] ratio and highest effectiveness obtained at 10 equiv. of added metal ion. Less effective is irradiation in the presence of Zn(II) and Pb(II), with absorption bands appearing between 600 and 592 nm. The observed kmax values (Table 1) are slightly blue-shifted and the same is observed at a 10-fold molar excess of M(II),(III). It is obvious that UV irradiation of SO in the presence of the appropriate metal ion yields a MC–M(II),(III) complex and this process is considered as photoinduced complexation. Thermal coordination to the metal ion also takes place, giving a complex which is spectroscopically identical to the photoinduced product. The colouration obtained in the dark becomes more intense upon UV illumination and reverses back to its initial level after removing of the light. The time needed for re-establishing equilibria is sensitive to the metal ion and is substantially prolonged compared to the non-complexed merocyanine. Photoirradiation of the parent unsubstituted spiroindolinonaphthooxazine (Scheme 2) in the presence of a
3.2. EPR spectra Fig. 1 shows a typical EPR spectrum of the photoinduced MC–Cu(II) complex in ethanol immediately frozen at 77 K after irradiation. The spin-Hamiltonian parameters: Ai = 112 ± 2 G, gi = 2.317 ± 0.002 and g^ = 2.010 ± 0.002 indicate a strong anisotropy, consistent with the Jahn–Teller deformation of the molecule to a square-planar geometry [26]. The isotropic value go = 2.112 ± 0.002, calculated by using the gi and g^ parameters, seems to be the average of the g-values [27,28] of bis(acetylacetonato)Cu(II) with the chromophore CuO4 (go = 2.219 ± 0.002) and bis(8quinolinolato)Cu(II) with the chromophore CuO2N2 (go = 2.105 ± 0.002). Since the unpaired electron occupies a ÔgroupÕ orbital with the participation of Cu and four donor atoms, these results indicate an averaged covalence within the chromophore of MC–Cu(II) as compared to CuO4 and CuO2N2. Following the Ôadditive ruleÕ [28,29] we suggest the chromophore CuO3N for MC–Cu(II), i.e. the formation of a mixed-ligand complex (3 in Scheme 3) with one chelate ligand and two ethanol molecules coordinated in the xy-plane. For comparison, the spin-Hamiltonian parameters of the EPR spectrum (77 K) of an ethanol solution of CuCl2 are Ai = 115 ± 2 G, gi = 2.26 and g^ = 2.06 [30].
Table 1 Absorption maxima of the complexes obtained upon UV illumination of 5 · 105 M solutions of 1 and 2 in the presence of 1 equiv. of metal ion SO
Solvent
kcompl (nm) None*
Cu(II)
Al(III)
Fe(II)
1
EtOH/H2O CH3COCH3/H2O
610 600
595 590
585 580
546 540
2
EtOH/H2O CH3COCH3/H2O
620 600
600 590
580 580
546 540
*
From [19].
4
H 3C
2'
5 2 6 7
1
H3C
CH3
3
N
3'
10'
N
9' 8'
O
7'
4'
5'
CH3 N
hv kT
N O
6'
R1
R1 Scheme 2.
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S. Minkovska et al. / Polyhedron 23 (2004) 3147–3153
By analogy with MC–Cu(II), for the cases of higher metal ion concentration than that of SO, we suggest one merocyanine and four ethanol molecules as ligands for Fe(II) or Al(III) in Scheme 3. The spectral and kinetic data (Figs. 1–5) also indicate this possibility for [SO] [M(II),(III)]. JobÕs [31], which is commonly used for determination of the stoichiometry, cannot be applied to our case since the ligand MC concentration cannot be varied independently of [M(II),(III)].
2600
3100
3600
Magnetic Field, Gauss
Typical spectral changes obtained in a few minutes after mixing 1 · 104 M ethanol solutions of 1 or 2 with 0.5–10 equiv. of aqueous Al(III), Fe(II) or Cu(II) are given in Fig. 2(a) and (b). The observed changes correspond to the thermal conversion of SO to a MC–Al(III) complex as indicated by the position of the visible absorption. It seems that chelation affects the thermal equilibrium SO MC and promotes the formation of the open form at room temperature in the absence of light, according to one of the following pathways [32]:
Fig. 1. Typical EPR spectrum of Cu(II)-MC in ethanol, 77 K.
H 3C
CH 3
N N
O
R
3.3. Thermal colouration and decolouration in the presence of metal ions
O Cu II (solv)2
SO MC þ MðIIÞ; ðIIIÞ ! MC MðIIÞ; ðIIIÞ
ðaÞ
(3)
SO þ MðIIÞ; ðIIIÞ SO MðIIÞ; ðIIIÞ ! MC MðIIÞ; ðIIIÞ H 3C
H 3C
CH 3
N
N N
O
O II
R
Fe (solv)4
(4) Scheme 3.
ðbÞ
CH 3
N
SO ! MC þ MðIIÞ; ðIIIÞ MC MðIIÞ; ðIIIÞ O
R
O AlIII (solv)4
(5)
ðcÞ
Fig. 2(a) and (b) depicts only a slow increase of intensity of the MC–Al(III) absorption band of 2, getting a saturation after 15–20 min in the dark. The concentration of the metal ion has no effect on the rate of chelation, but the time for saturation and the amount of the obtained
Fig. 2. Visible absorption spectra of an ethanol solution of 2 taken every 2 min in the dark after insertion of: 0.5 (a) and 10 (b) equiv. of aqueous Al(III); cSO = 5 · 105 M.
S. Minkovska et al. / Polyhedron 23 (2004) 3147–3153
3151
Fig. 3. (a) Kinetic runs at kcompl of 5 · 105 M ethanol solution of 2 taken in the dark after insertion of aqueous: Al(III) 1, 2 and 10 equiv. (plots 1–3) or Cu(II) 10 equiv. (plot 4) at 20 C; (b,c) ln A vs. time dependence.
Fig. 4. (a) Plots of ln A vs. time at 585 nm obtained in the dark after insertion of 10 equiv. of aqueous Al(III) in 5 · 105 M ethanol solution of 1 at 20 (1), 25 (2), 30 (3), 35 (4) and 40 C (5); (b) Arrhenius plots of: (1) MC–Al(III) using the data of (a); (2) MC–Cu(II) and (3) MC–Fe(II) under the same experimental conditions.
Fig. 5. Absorbance changes at 600 nm following 15 s UV illumination of an ethanol solution of 1 after insertion of: (1) 0.5 or (2) 10 equiv. of Cu(II) in ethanol and (3) 10 equiv. of aqueous Cu(II). Insets: ln A vs. time plots, initial (0.5–6 min) and final (10–70 min).
complex are higher at a larger metal ion excess. After reaching a saturation, the colouration slowly bleaches with longer reaction times (Fig. 3(a)). Kinetic plots (1)–(3) of MC–Al(III) and (4) of MC– Cu(II) in Fig. 3(a) are typical of the colouration and decolouration processes of all studied systems. The plots
of the linear dependences of ln A versus time at kcompl (Fig. 3(b)), analysed by the least squares method, give kc values of ca. (1–2) · 103 s1 at 20 C for the rate constants of chelation of all complexes, without any effect of the metal ion. It seems from these data that route (c) is the reaction pathway of chelation since for (a) and (b) the rate constants will depend on the nature of the metal ions, while for (c) metal ions react readily with the coloured form MC as it is formed thermally from SO. The first order build-up kinetics of MC–Al(III) studied in the range 20–40 C in 1:1 aqueous/ethanol (Fig. 4(a)) gives the plot (1) of log kc versus 1/T in Fig. 4(b) and similar kinetics runs of MC–Fe(II) and MC–Cu(II) under the same experimental conditions give the plots (2) and (3). From the linear dependences in Fig. 4(b), the activation energies (Ea) of chelation and the pre-exponential factors are derived. The pre-exponential factors for 3–5 vary from 1012 to 1014 s1, but the values for Ea are 27–30 kJ mol1 showing no effect of the metal ion, as the process of thermal ring opening of the spiroform is the rate limiting step of chelation. Thermal
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degradation of the complexes (Fig. 3(c)) is also a first order reaction with a metal-dependant rate constant kd of ca. (5–9) · 105 s1. 3.4. Effect of chelation on the photocolouration and on the thermal re-equilibration of photomerocyanines In the absence of metal ion, the first order decay of the photomerocyanines of 1 and 2 occurs with kr 0.1 s1 [19] and their relaxation time in acetone and ethanol is sMC–SO 10 s. Taking into account that the colouring and fading times are important characteristics of the photochromic systems, we have studied the effect of chelation on the kinetics of photochromic transformations of spironaphthooxazines. In contrast to the lack of effect of Cr(III), Mn(II), Ni(II), Co(II), Ba(II) or Mg(II) on the decolouration of the free MC forms of 1 and 2, Cu(II), Fe(II) and Al(III) have a significant effect on the ring closure reaction and the light enhanced colouration disappears much slower after illumination. A typical picture of bleaching after irradiation of 1 and 2 in the presence of Al(III), Cu(II) or Fe(II) is depicted in Figs. 5 and 6. Kinetic runs in Fig. 5 followed at kcompl immediately after illumination of ethanol (plots 1 and 2) or 1:1 aqueous/ethanol (plot 3) solutions indicate that the photoinduced MC–Cu(II) complex of 1 undergoes spontaneous thermal decolouration which is first order in the concentration of the starting complex, as the plots of ln A versus time (initial inset in Fig. 5) are linear (q > 0.98) about 50% of reaction completion and similar changes are observed in Fig. 6 for the MC– Cu(II) complex of 2. The strongly descending part of the curves associated with the thermal spironaphthooxazine ring closure, analysed by the least squares method, give metal-dependant values of the re-equilibration rate constant kr ranging from 1.5 to 2.6 · 103 s1 at 20 C.
Fig. 6. Absorbance changes at 600 nm following 15 s UV illumination of an ethanol solution of 2 after insertion of: (1) 0.5 or (2) 1 equiv. of aqueous Cu(II). Insets: ln A vs. time plots, initial (1–5 min) and final (10–180 min).
Relaxation times (scom–SO) of 5–10 min were obtained using the expression s = 1/kr. Data obtained about all the studied systems suggest that complexation with Cu(II), Fe(II) and Al(III) strongly inhibits thermal reversion of MC to SO. From the absorbances Ao obtained by extrapolation to t = 0 of the room temperature linearly descending kinetic curves ln A/t of spironaphthooxazine ring closure immediately after UV irradiation (Figs. 5 and 6) are calculated ‘‘colourability’’ [21–24] values up to 34 000. Kinetic runs at larger reaction times in Figs. 5 and 6 are much slower. The rate constants calculated by the least squares method from the final ln A versus time linear plots ca. 5 · 105 s1 for 1 and 1 · 105 s1 for 2 in 1:1 aqueous/ethanol (up to 1 · 104 s1 in ethanol) are near to the obtained kd values of thermal degradation. These observations suggest two parallel reactions of the photoinduced complexes proceeding simultaneously (Scheme 4): thermal relaxation (kr ca. 103 s1) approaching the steady state concentration of the complex C in polar solvents. thermal degradation (kd ca. 105 s1) of the complex yielding the product D. Special experiments were carried out to check the reversibility of the photocolouration. Kinetic runs in Fig. 7(a) depict the increasing intensity with time of the MC–Cu(II) absorption band at 600 nm (spectra 1– 5), reaching a saturation (spectra 4–5) on keeping its 1:1 aqueous/ethanol solution in the dark. The consequent increase in intensity upon 10 and 30 s UV illuminations is given in Fig. 7(b) (spectra 6–7) together with the kinetic runs of thermal decolouration on keeping the irradiated sample for 0.5, 1, 2 and 3 h in the dark (spectra 8–11) due to the parallel relaxation and degradation processes in Scheme 4. Spectrum 12 in Fig. 7(b), obtained after a further 20 s photoirradiation of the final solution, clearly indicates a reversible photocolouration of SO, remaining unaffected by degradation. Similar changes are obtained with MC–Fe(II) and MC–Al(III).
Scheme 4.
S. Minkovska et al. / Polyhedron 23 (2004) 3147–3153
3153
Fig. 7. Visible absorption spectra taken after insertion of 2 equiv. of aqueous Cu(II) into a 5 · 105 M ethanol solution of 1: (a) – every 2 min in the dark (1–5); (b) – (5) upon consecutive 10 and 30 s UV illuminations (6–7); (7) kept for 0.5, 1, 2 and 3 h in the dark (8–11); (11) after a new 20 s UV illumination (12).
4. Conclusions Coordination of Al(III), Fe(II) and Cu(II) with the open MC form of two 5 0 -(hydroxy)spiroindolinonaphthooxazines takes place both thermally (slowly) and photochemically in ethanol, acetone and their 1:1 water mixtures giving a reversibly photochromic complex. The thermal build-up reaction (103 s1) is independent of the metal ion thus implying that the ring opening is the rate determining step. Chelation induces a 10– 45 nm hypsochromic shift of the visible absorption band of the photomerocyanine and drastically slows down its thermal re-equilibration in the dark. References [1] N.Y.C. Chu, in: H. Durr, H. Bouas-Laurent (Eds.), Photochromism: Molecules and Systems, Elsevier, Amsterdam, 1990, p. 879. [2] R. Guglielmetti, in: H. Durr, H. Bouas-Laurent (Eds.), Photochromism: Molecules and Systems, Elsevier, Amsterdam, 1990, p. 855. [3] I. Willner, B. Willner in: H. Morrison (Ed.), Bioorganic Photochemistry, vol. 2, Biological Applications of Photochemical Switches, New York, 1993, p. 1. [4] J. D. Winkler, K. DeShayes, B. Shao, in: H. Morrison (Ed.), Bioorganic Photochemistry, vol. 2, Biological Applications of Photochemical Switches, New York, 1993, p. 167. [5] J.D. Phillips, A. Mueller, F. Przystal, J. Am. Chem. Soc. 87 (1965) 4020. [6] L.D. Taylor, J. Nicholson, K.B. Davis, Tetrahedron Lett. (1967) 1585. [7] M. Le Baccon, R. Guglielmetti, J. Chem. Res. (S) (1979) 154. [8] J.-W. Zhou, Y.-T. Li, X.-Q. Song, J. Photochem. Photobiol., A: Chem. 87 (1995) 37. [9] H. Go¨rner, A.K. Chibisov, J. Chem. Soc., Faraday Trans. 94 (1998) 2557. [10] J.T.C. Wojtyk, P.M. Kazmaier, E. Buncel, Chem. Commun. (1998) 1703.
[11] G.E. Collins, L.-S. Choi, K.J. Ewing, V. Michelet, C.M. Bowen, J.D. Winkler, Chem. Commun. (1999) 321. [12] L.S. Atabekyan, A.K. Chibisov, High Energy Chem. 32 (1998) 30. [13] H. Nishikiori, R. Sasai, N. Arai, K. Takadi, Chem. Lett. (2000) 1142. [14] M.J. Preigh, Fu-Tyan Lin, K.Z. Ismail, J. Chem. Soc., Chem. Commun. (1995) 2091. [15] T. Tamaki, Y. Kawanishi, T. Sehi, M. Sakuragi, K. Ichimura, T. Yamaguchi, J. Photopolymer Sci. Technol. 3 (1990) 85. [16] T. Tamaki, K. Ichimura, J. Chem. Soc., Chem. Commun. (1989) 1447. [17] H. Oda, Chem. Express 8 (1993) 629. [18] T. Deligeorgiev, S. Minkovska, B. Jeliazkova, S. Rakovsky, Dyes Pigments 53 (2002) 101. [19] S. Minkovska, B. Jeliazkova, E. Borisova, L. Avramov, T. Deligeorgiev, J. Photochem. Photobiol., A: Chem. 163 (2004) 121. [20] F. Wilkinson, J. Hobley, M. Naftaly, J. Chem. Soc., Faraday Trans. 88 (1992) 1511. [21] G. Favaro, V. Malatesta, U. Mazzucato, G. Ottavi, A. Romani, J. Photochem. Photobiol., A: Chem. 87 (1995) 235. [22] S. Kawauchi, H. Yoshida, N. Yamashina, M. Ohira, S. Saeda, M. Irie, Bull. Chem. Soc. Jpn. 63 (1990) 267. [23] J.-L. Pozzo, A. Samat, R. Guglielmetti, D. de Keukeleire, J. Chem. Soc., Perkin Trans. 2 (1993) 1327. [24] V. Pimienta, C. Frout_e, M.H. Deniel, D. Lavabre, R. Guglielmetti, J.C. Micheau, J. Photochem. Photobiol., A: Chem. 122 (1999) 199. [25] S. Minkovska, K. Kolev, B. Jeliazkova, T. Deligeorgiev, Dyes Pigments 39 (1998) 25. [26] B.J. Hathaway, D.E. Billig, Coord. Chem. Rev. 5 (1970) 143. [27] H.A. Kuska, M.T. Rogers, in: E.T. Kaiser, L. Kevan (Eds.), Radical Ions, Wiley, New York, 1968. [28] N.D. Yordanov, V. Alexiev, Commun. Dep. Chem. Bulg. Acad. Sci. 16 (1983) 214. [29] K. Wuetrich, Helv. Chim. Acta 48 (1965) 1012. [30] N.D. Yordanov, V. Terziev, D. Shopov, Compt. rendue Acad. Sci. Bulg. 27 (1974) 1529. [31] F.R. Hartley, C. Burgess, R.M. Alcock, Solution Equilibria, Wiley, New York, 1980. [32] J.-W. Zhou, Y.-T. Li, X.-Q. Song, J. Photochem. Photobiol., A: Chem. 87 (1995) 37.
Thermally activated and light-induced metal ion complexation of 5 0-(hydroxy)spiroindolinonaphthooxazines in polar solvents Stela Minkovska a, Monika Fedieva b, Bojana Jeliazkova
b,*
, Todor Deligeorgiev
b
a
b
Institute of Catalysis, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Department of Chemistry, Sofia University, 1, James Bourchier ave., 1164 Sofia, Bulgaria Received 22 July 2004; accepted 29 September 2004 Available online 5 November 2004
Abstract Chelation with Al(III), Fe(II) or Cu(II) of the open photomerocyanine form obtained under steady irradiation of spiroindolinonaphthooxazines, with a hydroxyl group at the 5 0 position in the naphthooxazine moiety, induces a slight hypsochromic shift of its visible absorption band and increases the lifetime of this form, slowing down its thermal bleaching in the dark (rate constant 103 s1). Complexation with Al(III), Fe(II) or Cu(II) allows the spiroindolinonaphthooxazines to isomerize to their open coloured form even under dark conditions giving a complex spectroscopically identical to the photoinduced product. The activation energy of thermal complexation is independent of the metal ion which implies the ring opening as the rate determining step. 2004 Elsevier Ltd. All rights reserved. Keywords: Absorption spectra; Complexation; Kinetics; Photochromism; Spiroindolinonaphthooxazines
1. Introduction When a metal ion coordinating moiety is combined with a photosensitive spiropyran or spirooxazine, light may induce metal binding and metal binding may influence photochromism [1–4]. This approach has been mainly reported for spiropyranes [1–12] and only in a few cases for spirooxazines [13–17]. A conceptually attractive possibility for spironaphthooxazines is the modification of the naphthooxazine moiety to enable chelation by metal ions and preferentially stabilize the open merocyanine form. Previous demonstrations of light-induced metal binding have relied upon substitution at the 5 0 position to create a nascent bidentate chelator that becomes active after photoinduced ring opening [15–17]. In this paper we report the effect of chelation with metal ions on the irradiation of two spiroin*
Corresponding author. Tel.: +359 2 8161347; fax: +359 2 9625438. E-mail address: [email protected]fia.bg (B. Jeliazkova).
0277-5387/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2004.09.022
dolinonaphthooxazines recently synthesized in our laboratory, [18] with an -OH substituent at the 5 0 position in the naphtooxazine moiety (Scheme 1) as a potential chelating functional group. The effect of chelation on the thermal SO MC equilibrium between the coloured merocyanine (MC) and the colourless spiro (SO) form is also discussed. The thermal equilibrium between the closed and opened form in Scheme 1 also depends on solvent polarity [18,19], since polar solvents promote the formation of the coloured form at room temperature in the absence of light. The equilibrium between both forms is strongly displaced upon irradiation to the side of the open-chain coloured photomerocyanine which spontaneously converts to the colourless spiroform to reach thermal equilibrium immediately after removing the light. Here, we report our results on the possibility to stabilize the coloured MC form towards thermal reversion by complexation with selected metal ions in appropriate solvents.
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4
H3C
2'
5 2 6 7
1
H3C
CH3
3
N
3'
10'
N
8'
O HO
CH3 N
hv kT
7'
4'
5'
R1
9'
N O
6'
OH
R1
(SO)
(MC) Scheme 1.
2. Experimental 2.1. Materials The photochromic molecules were two spiro[indolinonaphthooxazines]: 1,3,3-trimethyl-5 0 -hydroxyspiro(indoline-2,3 0 -[3H]naphth[2,1-b][1,4]oxazine) (1) and 1-butyl-3,3-dimethyl-5 0 -hydroxyspiro(indoline-2,3 0 -[3H] naphth[2,1-b][1,4]oxazine) (2). The compounds were recently prepared and characterized in our laboratory [18,19]. The p.a. grade metal salts were obtained from Fluka and were used without further purification. The solvents were used after distillation. H3 C
CH3
N 1: R = CH3
N
O
2: R = C4H9
R HO
2.2. Instrumentation Absorption spectra were recorded on a SPECORD UV–Vis (Carl Zeiss, Jena) spectrophotometer using quartz cells. For absorption measurements at varying temperatures the reaction cell was enclosed in a thermostatic water jacket placed inside the spectrophotometer sample chamber. A 250 W medium-pressure mercury lamp was used for producing the coloured form. Photochemical reactions were carried out in the spectrophotometric quartz cell with a homogeneously spread light on the cell window to avoid stirring [20,21]. Measurements were made on aerated solutions. EPR spectra were taken at 77 K on an X-band BRUKER B-ER 420 spectrometer using 100 kHz modulation of the magnetic field. 2.3. Kinetics measurements The effects of metal ions on the SO MC process were studied in 1:1 aqueous/acetone or aqueous/ethanol
mixtures at a constant spironaphthooxazine concentration of 5 · 105 M and varying the ratio between the SO and the metal ion from 2:1 to 1:20. The kinetics of the thermal colouration in the presence of metal ions was measured at different temperatures in the range 20–50 C, ca. 30 min after having set the temperature control in order to allow the initial solutions to reach the appropriate temperature in separate vessels. The absorbance was measured at the wavelength of maximum absorption (kmax) directly in the sample cell at 30 s intervals after the insertion of 10 equiv. of aqueous metal ion solution in ethanol solutions of 1 or 2. First order build up rate constants were obtained from the linear dependences of ln A versus time. The activation energies along with the frequency factors of thermal ring opening were determined from Arrhenius plots. In the photocolouration experiment the cuvette was irradiated for several seconds after thermal equilibration of the reaction mixture containing SO and the metal ion in 1:1 aqueous/ethanol. Trial and error experiments enabled the optimum exposure time to be determined for maximum build-up of the complex. The ring-closure reaction after photocolouration was monitored directly after the removal of the light, scanning at the wavelength of maximum absorption of the complex between the MC form and the metal ion (kcompl) at 15 s intervals over a period of 30–50 min at room temperature. First order rate constants of thermal re-equilibration were obtained from the linear ln A versus time descending curves. by extrapolation of the obtained ln A/t plots to zero time, the absorbance Ao of the complexes at t = 0 was related to their ‘‘colourabilities’’ [21–24] using the expression Ao/cSOb in which cSO was the initial concentration of SO and b was the optical path length.
3. Results and discussion The colourless spiro compounds 1 and 2 give no complexes with the metal ions. However, their merocyanines form complexes with Cu(II), Fe(II) and Al(III) in polar solvents, while Cr(III), Mn(II), Ni(II), Co(II), Ba(II) and Mg(II) do not affect the equilibrium between SO and MC in Scheme 1.
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metal salt [25] induces irreversible photo-degradation without any complexation, indicating that the nitrogen atom at the 1 0 position in the naphthooxazine moiety takes no part in chelation.
3.1. Absorption spectra Visible spectra of photomerocyanines, obtained upon UV illumination of 1 or 2 in polar solvents [19], are characterized by an absorption in the range 600– 620 nm which remains unchanged upon UV illumination in the presence of Cr(III), Mn(II), Ni(II), Co(II), Ba(II) and Mg(II). Irradiation of 1 and 2 in the presence of 1 equiv. of Cu(II), Fe(II) or Al(III) generates a new photo-reversible absorption band in the range 540–600 nm, with band intensity dependent on the [M(II),(III)]/ [SO] ratio and highest effectiveness obtained at 10 equiv. of added metal ion. Less effective is irradiation in the presence of Zn(II) and Pb(II), with absorption bands appearing between 600 and 592 nm. The observed kmax values (Table 1) are slightly blue-shifted and the same is observed at a 10-fold molar excess of M(II),(III). It is obvious that UV irradiation of SO in the presence of the appropriate metal ion yields a MC–M(II),(III) complex and this process is considered as photoinduced complexation. Thermal coordination to the metal ion also takes place, giving a complex which is spectroscopically identical to the photoinduced product. The colouration obtained in the dark becomes more intense upon UV illumination and reverses back to its initial level after removing of the light. The time needed for re-establishing equilibria is sensitive to the metal ion and is substantially prolonged compared to the non-complexed merocyanine. Photoirradiation of the parent unsubstituted spiroindolinonaphthooxazine (Scheme 2) in the presence of a
3.2. EPR spectra Fig. 1 shows a typical EPR spectrum of the photoinduced MC–Cu(II) complex in ethanol immediately frozen at 77 K after irradiation. The spin-Hamiltonian parameters: Ai = 112 ± 2 G, gi = 2.317 ± 0.002 and g^ = 2.010 ± 0.002 indicate a strong anisotropy, consistent with the Jahn–Teller deformation of the molecule to a square-planar geometry [26]. The isotropic value go = 2.112 ± 0.002, calculated by using the gi and g^ parameters, seems to be the average of the g-values [27,28] of bis(acetylacetonato)Cu(II) with the chromophore CuO4 (go = 2.219 ± 0.002) and bis(8quinolinolato)Cu(II) with the chromophore CuO2N2 (go = 2.105 ± 0.002). Since the unpaired electron occupies a ÔgroupÕ orbital with the participation of Cu and four donor atoms, these results indicate an averaged covalence within the chromophore of MC–Cu(II) as compared to CuO4 and CuO2N2. Following the Ôadditive ruleÕ [28,29] we suggest the chromophore CuO3N for MC–Cu(II), i.e. the formation of a mixed-ligand complex (3 in Scheme 3) with one chelate ligand and two ethanol molecules coordinated in the xy-plane. For comparison, the spin-Hamiltonian parameters of the EPR spectrum (77 K) of an ethanol solution of CuCl2 are Ai = 115 ± 2 G, gi = 2.26 and g^ = 2.06 [30].
Table 1 Absorption maxima of the complexes obtained upon UV illumination of 5 · 105 M solutions of 1 and 2 in the presence of 1 equiv. of metal ion SO
Solvent
kcompl (nm) None*
Cu(II)
Al(III)
Fe(II)
1
EtOH/H2O CH3COCH3/H2O
610 600
595 590
585 580
546 540
2
EtOH/H2O CH3COCH3/H2O
620 600
600 590
580 580
546 540
*
From [19].
4
H 3C
2'
5 2 6 7
1
H3C
CH3
3
N
3'
10'
N
9' 8'
O
7'
4'
5'
CH3 N
hv kT
N O
6'
R1
R1 Scheme 2.
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By analogy with MC–Cu(II), for the cases of higher metal ion concentration than that of SO, we suggest one merocyanine and four ethanol molecules as ligands for Fe(II) or Al(III) in Scheme 3. The spectral and kinetic data (Figs. 1–5) also indicate this possibility for [SO] [M(II),(III)]. JobÕs [31], which is commonly used for determination of the stoichiometry, cannot be applied to our case since the ligand MC concentration cannot be varied independently of [M(II),(III)].
2600
3100
3600
Magnetic Field, Gauss
Typical spectral changes obtained in a few minutes after mixing 1 · 104 M ethanol solutions of 1 or 2 with 0.5–10 equiv. of aqueous Al(III), Fe(II) or Cu(II) are given in Fig. 2(a) and (b). The observed changes correspond to the thermal conversion of SO to a MC–Al(III) complex as indicated by the position of the visible absorption. It seems that chelation affects the thermal equilibrium SO MC and promotes the formation of the open form at room temperature in the absence of light, according to one of the following pathways [32]:
Fig. 1. Typical EPR spectrum of Cu(II)-MC in ethanol, 77 K.
H 3C
CH 3
N N
O
R
3.3. Thermal colouration and decolouration in the presence of metal ions
O Cu II (solv)2
SO MC þ MðIIÞ; ðIIIÞ ! MC MðIIÞ; ðIIIÞ
ðaÞ
(3)
SO þ MðIIÞ; ðIIIÞ SO MðIIÞ; ðIIIÞ ! MC MðIIÞ; ðIIIÞ H 3C
H 3C
CH 3
N
N N
O
O II
R
Fe (solv)4
(4) Scheme 3.
ðbÞ
CH 3
N
SO ! MC þ MðIIÞ; ðIIIÞ MC MðIIÞ; ðIIIÞ O
R
O AlIII (solv)4
(5)
ðcÞ
Fig. 2(a) and (b) depicts only a slow increase of intensity of the MC–Al(III) absorption band of 2, getting a saturation after 15–20 min in the dark. The concentration of the metal ion has no effect on the rate of chelation, but the time for saturation and the amount of the obtained
Fig. 2. Visible absorption spectra of an ethanol solution of 2 taken every 2 min in the dark after insertion of: 0.5 (a) and 10 (b) equiv. of aqueous Al(III); cSO = 5 · 105 M.
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Fig. 3. (a) Kinetic runs at kcompl of 5 · 105 M ethanol solution of 2 taken in the dark after insertion of aqueous: Al(III) 1, 2 and 10 equiv. (plots 1–3) or Cu(II) 10 equiv. (plot 4) at 20 C; (b,c) ln A vs. time dependence.
Fig. 4. (a) Plots of ln A vs. time at 585 nm obtained in the dark after insertion of 10 equiv. of aqueous Al(III) in 5 · 105 M ethanol solution of 1 at 20 (1), 25 (2), 30 (3), 35 (4) and 40 C (5); (b) Arrhenius plots of: (1) MC–Al(III) using the data of (a); (2) MC–Cu(II) and (3) MC–Fe(II) under the same experimental conditions.
Fig. 5. Absorbance changes at 600 nm following 15 s UV illumination of an ethanol solution of 1 after insertion of: (1) 0.5 or (2) 10 equiv. of Cu(II) in ethanol and (3) 10 equiv. of aqueous Cu(II). Insets: ln A vs. time plots, initial (0.5–6 min) and final (10–70 min).
complex are higher at a larger metal ion excess. After reaching a saturation, the colouration slowly bleaches with longer reaction times (Fig. 3(a)). Kinetic plots (1)–(3) of MC–Al(III) and (4) of MC– Cu(II) in Fig. 3(a) are typical of the colouration and decolouration processes of all studied systems. The plots
of the linear dependences of ln A versus time at kcompl (Fig. 3(b)), analysed by the least squares method, give kc values of ca. (1–2) · 103 s1 at 20 C for the rate constants of chelation of all complexes, without any effect of the metal ion. It seems from these data that route (c) is the reaction pathway of chelation since for (a) and (b) the rate constants will depend on the nature of the metal ions, while for (c) metal ions react readily with the coloured form MC as it is formed thermally from SO. The first order build-up kinetics of MC–Al(III) studied in the range 20–40 C in 1:1 aqueous/ethanol (Fig. 4(a)) gives the plot (1) of log kc versus 1/T in Fig. 4(b) and similar kinetics runs of MC–Fe(II) and MC–Cu(II) under the same experimental conditions give the plots (2) and (3). From the linear dependences in Fig. 4(b), the activation energies (Ea) of chelation and the pre-exponential factors are derived. The pre-exponential factors for 3–5 vary from 1012 to 1014 s1, but the values for Ea are 27–30 kJ mol1 showing no effect of the metal ion, as the process of thermal ring opening of the spiroform is the rate limiting step of chelation. Thermal
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degradation of the complexes (Fig. 3(c)) is also a first order reaction with a metal-dependant rate constant kd of ca. (5–9) · 105 s1. 3.4. Effect of chelation on the photocolouration and on the thermal re-equilibration of photomerocyanines In the absence of metal ion, the first order decay of the photomerocyanines of 1 and 2 occurs with kr 0.1 s1 [19] and their relaxation time in acetone and ethanol is sMC–SO 10 s. Taking into account that the colouring and fading times are important characteristics of the photochromic systems, we have studied the effect of chelation on the kinetics of photochromic transformations of spironaphthooxazines. In contrast to the lack of effect of Cr(III), Mn(II), Ni(II), Co(II), Ba(II) or Mg(II) on the decolouration of the free MC forms of 1 and 2, Cu(II), Fe(II) and Al(III) have a significant effect on the ring closure reaction and the light enhanced colouration disappears much slower after illumination. A typical picture of bleaching after irradiation of 1 and 2 in the presence of Al(III), Cu(II) or Fe(II) is depicted in Figs. 5 and 6. Kinetic runs in Fig. 5 followed at kcompl immediately after illumination of ethanol (plots 1 and 2) or 1:1 aqueous/ethanol (plot 3) solutions indicate that the photoinduced MC–Cu(II) complex of 1 undergoes spontaneous thermal decolouration which is first order in the concentration of the starting complex, as the plots of ln A versus time (initial inset in Fig. 5) are linear (q > 0.98) about 50% of reaction completion and similar changes are observed in Fig. 6 for the MC– Cu(II) complex of 2. The strongly descending part of the curves associated with the thermal spironaphthooxazine ring closure, analysed by the least squares method, give metal-dependant values of the re-equilibration rate constant kr ranging from 1.5 to 2.6 · 103 s1 at 20 C.
Fig. 6. Absorbance changes at 600 nm following 15 s UV illumination of an ethanol solution of 2 after insertion of: (1) 0.5 or (2) 1 equiv. of aqueous Cu(II). Insets: ln A vs. time plots, initial (1–5 min) and final (10–180 min).
Relaxation times (scom–SO) of 5–10 min were obtained using the expression s = 1/kr. Data obtained about all the studied systems suggest that complexation with Cu(II), Fe(II) and Al(III) strongly inhibits thermal reversion of MC to SO. From the absorbances Ao obtained by extrapolation to t = 0 of the room temperature linearly descending kinetic curves ln A/t of spironaphthooxazine ring closure immediately after UV irradiation (Figs. 5 and 6) are calculated ‘‘colourability’’ [21–24] values up to 34 000. Kinetic runs at larger reaction times in Figs. 5 and 6 are much slower. The rate constants calculated by the least squares method from the final ln A versus time linear plots ca. 5 · 105 s1 for 1 and 1 · 105 s1 for 2 in 1:1 aqueous/ethanol (up to 1 · 104 s1 in ethanol) are near to the obtained kd values of thermal degradation. These observations suggest two parallel reactions of the photoinduced complexes proceeding simultaneously (Scheme 4): thermal relaxation (kr ca. 103 s1) approaching the steady state concentration of the complex C in polar solvents. thermal degradation (kd ca. 105 s1) of the complex yielding the product D. Special experiments were carried out to check the reversibility of the photocolouration. Kinetic runs in Fig. 7(a) depict the increasing intensity with time of the MC–Cu(II) absorption band at 600 nm (spectra 1– 5), reaching a saturation (spectra 4–5) on keeping its 1:1 aqueous/ethanol solution in the dark. The consequent increase in intensity upon 10 and 30 s UV illuminations is given in Fig. 7(b) (spectra 6–7) together with the kinetic runs of thermal decolouration on keeping the irradiated sample for 0.5, 1, 2 and 3 h in the dark (spectra 8–11) due to the parallel relaxation and degradation processes in Scheme 4. Spectrum 12 in Fig. 7(b), obtained after a further 20 s photoirradiation of the final solution, clearly indicates a reversible photocolouration of SO, remaining unaffected by degradation. Similar changes are obtained with MC–Fe(II) and MC–Al(III).
Scheme 4.
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Fig. 7. Visible absorption spectra taken after insertion of 2 equiv. of aqueous Cu(II) into a 5 · 105 M ethanol solution of 1: (a) – every 2 min in the dark (1–5); (b) – (5) upon consecutive 10 and 30 s UV illuminations (6–7); (7) kept for 0.5, 1, 2 and 3 h in the dark (8–11); (11) after a new 20 s UV illumination (12).
4. Conclusions Coordination of Al(III), Fe(II) and Cu(II) with the open MC form of two 5 0 -(hydroxy)spiroindolinonaphthooxazines takes place both thermally (slowly) and photochemically in ethanol, acetone and their 1:1 water mixtures giving a reversibly photochromic complex. The thermal build-up reaction (103 s1) is independent of the metal ion thus implying that the ring opening is the rate determining step. Chelation induces a 10– 45 nm hypsochromic shift of the visible absorption band of the photomerocyanine and drastically slows down its thermal re-equilibration in the dark. References [1] N.Y.C. Chu, in: H. Durr, H. Bouas-Laurent (Eds.), Photochromism: Molecules and Systems, Elsevier, Amsterdam, 1990, p. 879. [2] R. Guglielmetti, in: H. Durr, H. Bouas-Laurent (Eds.), Photochromism: Molecules and Systems, Elsevier, Amsterdam, 1990, p. 855. [3] I. Willner, B. Willner in: H. Morrison (Ed.), Bioorganic Photochemistry, vol. 2, Biological Applications of Photochemical Switches, New York, 1993, p. 1. [4] J. D. Winkler, K. DeShayes, B. Shao, in: H. Morrison (Ed.), Bioorganic Photochemistry, vol. 2, Biological Applications of Photochemical Switches, New York, 1993, p. 167. [5] J.D. Phillips, A. Mueller, F. Przystal, J. Am. Chem. Soc. 87 (1965) 4020. [6] L.D. Taylor, J. Nicholson, K.B. Davis, Tetrahedron Lett. (1967) 1585. [7] M. Le Baccon, R. Guglielmetti, J. Chem. Res. (S) (1979) 154. [8] J.-W. Zhou, Y.-T. Li, X.-Q. Song, J. Photochem. Photobiol., A: Chem. 87 (1995) 37. [9] H. Go¨rner, A.K. Chibisov, J. Chem. Soc., Faraday Trans. 94 (1998) 2557. [10] J.T.C. Wojtyk, P.M. Kazmaier, E. Buncel, Chem. Commun. (1998) 1703.
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