- Email: [email protected]
Molecular design, photoisomerization and complexation of crown ether styryl dyes
Volume 185, number 5,6
CHEMICAL PHYSICS LETTERS
25 October 199 1
Molecular design, photoisomerization and complexation of crown ether styryl dyes M.V. Alfimov, S.P. Gromov
and I.K. Lednev
Department of Photochemistry, N.N. Semenov Instltufe qf Chemrcal Ph,vsics, USSR Academy oJ.Sc~ences, Kosygm street 4. 117334 Moscow, USSR Received 3 July 199 I
The structure of a new class ofdyes, crown ether styryl dyes, is discussed. Quantitative data on trans-cis- and cis-trans-photoisomerization quantum yields and complexation of the obtained compounds are analyzed. The results on photoinduced and dark complexation of chromogenic 15-crown-5 ether betaine are reported. The formation of an intermolecular coordination bond between a sulfogroup and a metal cation in a crown cavity leading to cis-isomer stabilization has been established.
1. Introduction Photoisomerization of stilbene compounds was studied by us earlier [ 1,2 1. Photoinduced isomerization of diarylethylenes is a classic example of a monomolecular process as the result of which a molecule upon photoexcitation transforms from one stable geometric configuration (trans- or cis-) into another one (cis- or trans-, respectively) [ 31. Since physical and chemical properties of transand cis-isomeric derivatives of ethylene are similar, up to now this process of molecular photoisomerization photocontrol has not found application [ 41. A possibility of construction of photoisomers of different reactivity would mean new prospects for the control of chemical processes. A considerable achievement in creating such systems would be a synthesis of compounds where a change of the molecule’s geometrical configuration under irradiation could lead to a significant change of complexation constant. At present, there are intensive investigations of macroheterocyclic compounds the properties of which are determined by the possibilities of constructing on their basis, polyfunctional compounds capable of the selective complexation of ions or neutral molecules by a host-guest-type [ 5,6]. In this connection, of interest are photoactive compounds which can be constructed on the basis of styryl dyes Elsevier Science Publishers B.V.
containing macroheterocyclic fragment and capable of complexation with metal cations. On the other hand, ionophores containing a crown ether ring and anion group in a spatially accessible position are known to be more efficient receptors of cations of alkaline earth metals [ 71. We have assumed that this conception can be also applied to the synthesis of crown ether styryl dyes, “covered” in c&form by anion groups. It would allow one to obtain chromogenic crown ether capable of “taking off” and “putting on” its “anion cap” upon photo-irradiation, which in its turn provides an opportunity to achieve photocontrol of binding alkaline earth metal cations.
2. Presentation of the molecules We have synthesized indolenine styryl dyes la and lb [ 8,9] and styryl dyes of benzothiazole series 2a and 2b [lo]. Compound lb contains a fragment of crown ether, and perchlorate 2-[2-( 3,4-dimethoxyphenyl)ethenyl]-3H-indolinium la has in a benzene ring two methoxy groups which reproduce in a general way the electron-donor effect of a crown ether substituent on the system of conjugated double bonds of the dye. However, in contrast to crown ether styryl dye lb, compound la must not be noticeably capable of complexation with ions of alkaline and alkaline earth metals. 455
Volume
185,number
CHEMICAL
5,6
PHYSICS LETTERS
OMe
la
Ar :
lb
Ar :
OMe
Polyether macrocycle 15-crown-$ acting as pentadentate ligand upon complexation with metal ions, is built into compounds 2a and 2b. However, in contrast to cation dye 2a, compound 2b is betaine, the sulfogroup of which can form, under certain conditions, a bond with a metal cation supplementing its coordination sphere.
2a R = Et
25 October I99 I
recorded on a Specord M 40 spectrophotometer. The photolysis of la, lb, 2a and 2b solutions was performed in quartz cells with a mercury arc lamp (250 W) as a source of radiation with wavelength 1= 365 or 436 nm. The actinic light intensity was measured by means of a thermoelement. The preparation of solutions and all experiments were carried out under red light in a dark room. The samples’ fluorescence was measured on an Elumin-2M spectrofluorimeter. The quantum yield of fluorescence was determined using fluorescein in a 0.01 N solution of KOH in ethanol, as a standard [ 111. The quantum yield of trans-cis-isomerization 0, was determined under radiation with light of ,I=365 nm by the rate of optical density change in la and lb solutions in water at 436 nm. For the calculations, the following values of molar absorption coefficients at 436 nm were used: et =3.9x 104, r,=O.5X lo4 II mol-’ cm-’ for lb and e,=3.7x104, t,=0.5~10’ P mol-’ cm-’ for la. Qjcwas calculated on the basis of the measured values @,/@,.
2b R : CH2CH2CH2SO;
4. Discussion Consideration of molecular models of the perspective dyes from this series has shown that the length of the sulfopropyl group is evidently enough for the formation in cis-form of a dye of additional coordination bond with a metal cation. The structure of the obtained compounds la, lb, 2a and 2b has been proved by means of ‘H NMR spectroscopy. The data of the elemental analysis correspond to the structures proposed. According to the ‘H NMR data, all styryl dyes obtained exist in a transform. This conclusion can be confidently made on the basis of a high constant of spin-spin coupling, ‘J,,,,, = 15.6-16.7 Hz, for olefinic protons of the dyes obtained.
3. Experimental ‘H NMR spectra have been recorded on a Bruker WM 400 spectrometer in dimethylsulfoxide-d6 and methanol-d, with TMS as a standard. Acetonitrile has been twice distilled over CaHz and PzOs for dehydration. Perchlorates of Na, K, Mg, Ca and Ba were dried in a vacuum at 260°C. Absorption spectra were 456
4.1. Photoisomerization of la and lb Fig. 1 shows absorption and fluoiescence spectra of trans-lb in water. A long-wave absorption band of trans-la and trans-lb has a significant hypsochromic shift with the increase in the solvent polarity. The band maximum for trans-lb is at A=466 nm (6, = 4.1~10~ II mol-’ cm-‘) in chloroform, 447 nm (t,=4.1x10411mol-‘cm-‘)inacetoneand432nm (q=3.9X lo4 I mol-’ cm-‘) in water; for trans-la, respectively, at 11=463 nm (t, =4.1 x lo4 II mol-’ cm-‘), 446 nm (6,=4.0x lo4 ! mol-’ cm-‘) and 431 nm (t,=3.7X lo4 Q mol-’ cm-‘). Upon irradiation of trans-la and trans-lb solutions in water with I= 436 nm, the long-wave absorption decreases with a simultaneous increase in the short-wave ab sorption until a photostationary state is achieved. Upon irradiation of the initial solutions with light of A=365 nm, the absorption spectrum changes up to a transfer into a new photostationary state. In all cases, a pronounced isosbestic point (at A=332 nm for lb and 336 nm for la) and a linear dependence between the absorbance values at two different
Volume 185,number 5,6
CHEMICALPHYSICSLETTERS
7
250
300
350
400
450
500
550
600 650
‘x I llrn
Fig. I. Absorption (-) T= 295 K.
and fluorescence (“‘) spectra of tram-lb and the calculatedabsorption spectrum of cis-lb (---)
wavelength are observed. The spectral changes are, apparently, explained by the occurrence in the solution of the reversible trans-cis-isomerization. After la and lb irradiation in the solvents of moderate polarity (chloroform, acetone), one can observe a fast dark reduction of spectra. This is evidently associated with the occurrence of thermal cis-trans-isomerization. In aqueous solutions, cis-la and lb are stable enough to determine using the Fischer method [ 12 ] the spectra of individual cis-isomers and their absorption coefficients. From the known to c,, @, and Qc were calculated by a standard method. The obtained values of fluorescence and photoisomerization quantum yields are presented in table 1. It is worth noting that the results do not depend on the presence of oxygen in the air with the isomer concentrations used in this work ( 1.2 X 1O- 5 mol P- ’ ).
Table 1 Photochemical data for laand lb at T=295 K in water
Dye
(Y(365 nm)
(Y(436 nm)
0,
@
@f
la lb
0.56 0.58
0.87 0.88
0.47 0.49
0.51 0.50
0.0015 0.002
in water at
The error in the @, and DC measurements is about 20%, and that of @r is about 40%. 4.2. Complexation of 2b with cations qf alkaline earth metals In the acetonitrile solution, compound 2b has in the absorption spectrum an intensive long-wave band with a maximum at A=435 nm (molar absorption coefficient trnax= 3.9x lo4 P mol-’ cm-‘). On the addition to the solution of perchlorates of alkaline Na, K and alkaline earth metals Mg, Ca, Ba, a hypsochromic shift of the long-wave band is observed. With the concentrations of perchlorate C, 2 10m3 M, the absorption band maximum is at A=392 nm (%rxix= 3.69~ lo4 P mol-’ cm-‘), 396 nm (tmax= 3.62~10~ Q mol-’ cm-‘) and 407 nm (cmax= 3.66x10411mol-‘cm-‘)inthecaseofMg’+,Ca’+ and Ba’+, respectively. Similar spectral changes were observed earlier when studying complexation of metal ions with the crown ether not containing styryl chromogene [ 13 1. From the absorption spectra of the solutions with the constant initial concentration of the ligand C0=2.05x 10e5 M and different concentrations of the cation, the degree of the ligand transformation 457
Volume185, number5,6
CHEMICALPHYSICSLETTERS
into the complex form with metal ions was determined according to the formula a=(&-D)/ (Do-D,), where D,, is the absorbance of the ligand initial solution at the fixed wavelength, D, is the absorbance of the solution upon complete complexation, and D is the absorbance of the solution at different values of the cation concentration. When calculating (Y,we made the assumption that the absorption coefficients of the complexes with different content of the bound ligand per mol are the same as the measured wavelength. To diminish systematic measurement error, connected with the accepted assumption, the absorbance of solutions was determined at the wavelength of 460 or 470 nm, where the e of complexes are small as compared to those of ligands. In the experiments on a 2b complexation with Ba ions, the amount of the ligand used at low degrees of transformation exceeded the concentration of the cation nearly twice. Thus, for the given compound, the following scheme of the formation of the complex of the 2L: IM composition can be proposed:
25 October 1991
2.0
T_
1.5
: _
“0 c
1.0
x
0” 0
0.5
Fg. 2. Experimental dependences of the necessary amount of ligand 2b for the total concentration of (0 )Mg'+,(0 )Caz+ and (0) Ba’+.
plexes of a 2 : 1 composition with crown ethers which have small-size cavities [ 141. With a Mg*+ cation, such a complex is apparently not formed.
L+M&LM, LMfLkZ
L2M,
where M is a metal cation and L is a ligand molecule. The following equation corresponds to the scheme presented: C, -= a
ltCck,(l-cr)+C;k,kz(l-a)” k,(l-cx)[1t2c,k,(l-cr)]
.
(1)
Experimental dependences of the ligand concentration decrease, crC, = C, - [L], on the total concentration of Mg*+, Ca*+ and Ba*+ in the solution are presented in fig. 2 by dots. Solid lines represent theoretical dependences corresponding to eq. ( I ) with constants k, =4X lo6 Bmol-‘, k,=Ofor Mg*+; 7X105, 2~10~ R mol-’ for Ca*+; and 6x10’, 1.5 X lo5 B mol-’ for Ba’+. Good agreement of the experimental data with the results of the calculation confirms the validity of the chosen scheme of complexation. In addition, the decrease in the value of k, in the order of Mg2+ > Cal+ x Ba*+ correlates with the value of the long-wave absorption band shift in the complex spectra. The increase of the value of k2 in the order Mg*+ < Ca2+ i Ba*+ corresponds to the idea that the cations of large size tend to form com458
4.3. Photoinduced formation of intramolecular coordination bond in 2b complexes with Mg2+, Ca’+ and Ba*+ cations Compound 2b, like a crown ether indolenine styryl dye lb, can be subjected to a reversible photochemical reaction of trans-cis-isomerization around the central double C=C bond. The absorption band of cis-2b in acetonitrile was calculated by means of the Fischer method [ 131 using the spectra of trans-isomer and photostationary states obtained from the irradiation of a 2b solution with light of wavelength 365 and 436 nm. In fig. 3, trans- and cis-isomer absorption spectra of 2b are presented. The distance between the maxima of the long-wave absorption bands of these isomers is only 15 nm, which does not allow the complete transformation of the trans-isomer into a cisform through direct photoisomerization. A similar situation is characteristic for the majority of photochromic compounds [ 41. Another situation is observed in the case of trans3b complexation with alkaline earth metals when the concentration of the latter is 10m3 M. The irradiation of solutions with
Volume 185,number 5.6
CHEMICALPHYSICSLETTERS
(Tram-Zb)M”
54, nm
Fig. 3. (a) Absorption spectra of trans-2b (-.-,-), cis-2b (- - -) and complexes of trans-2b ( “‘1 and cis-lb (-) with Mg’+at [Mg(ClO,),] = 1x 10-j M. (b) Absorption spectra of complexes of cis-2b with Mg’* at [Mg(Cl0,),]=3~ 10-j M (.,.), 0.5 M (-_) and a cis3a and Mg’+ complex at [Mg(CIO,),]=3xlO-3M (---).
light (i=436 nm) leads to a photostationary state including > 99% of cis-isomer. Cis-isomer absorption spectra of the complexes have a long-wave maximum at A=321 nm (c=8.96>( lo3 P mol-’ cm-‘), 325 nm (c=9.51 X lo3 P mol-’ cm-‘) and 338 nm (e=8.05x103 II mol-’ cm-’ for Mg’+, Ca’+ and Ba’+, respectively. Fig. 3 shows an example of cisand trans-isomer absorption spectra of a 2b with Mg’+ complex. A strong hypsochromic shift of 70 nm of the cisisomer spectra of the complexes in respect to the spectra of trans-isomers can be apparently explained by the formation in a cis-form of a coordination bond between a sulfogroup and a metal cation, located in a crown cavity, according to the scheme:
25 October 199I
(Cis-2b]M2*
For the formation of such a bond, a certain twist of the fragments of the cis-2b molecule with the change of the planar structure is necessary, which in analogy with the arylethylene case [ 15 1, must be accompanied by a long-wave maximum hypsochromic shift in the absorption spectrum as the result of the destruction of x-system conjugation. With the increase in the concentration of magnesium perchlorate in a cis-isomer.solution of the 2b complex with Mg2+ (C,> lo-* M), the absorption in the long-wave portion of the spectrum gradually increases, and at C, > 0.2 M the spectrum takes an entirely different shape. The change of the spectrum shape is attributed to the destruction of the intramolecular coordination bond due to the association of the SOi group with an additional cation of Mg2+. Fig. 3 shows the absorption spectra of a cis-2b complex with Mg*+ at concentrations of Mg ( Clod)? at 3 x 10e3 and 0.5 M, as well as of a cis-2a complex with Mg’+ at [Mg(C10,)2]=3x 10e3 M. In compound 2a, an ethyl group instead of a sulfopropyl one is present; therefore, the formation of an intramolecular coordination bond is impossible. The fact, that the spectrum (cis-2a)MgZt at Cr,,=3x 10e3 M and the spectrum (cis-2b ) Mg2+ at CM= 0.5 M practically coincide, confirms the assumption on the destruction of the intramolecular bond in (cis-2b)Mg2+ at high concentrations of Mg(C104)*.
5. Conclusion Thus, in the moderately polar solvents (acetonitrile, acetone), the compounds la, lb, 2a and 2b are unstable in a cis-form. With the addition of alkaline and alkaline earth metal perchlorates (Na, Ca, Mg and others) to the solution of the compounds under investigation in these solvents, a hypsochromic shift of the long-wave absorption of more than 30 nm is observed in the case of lb and 2a and in the case of 459
Volume
185,number 5,6
CHEMICAL PHYSICS LETTERS
la, the shift is 2-3 nm. It is associated with a complexation of crown ether styryl dyes with metal cations. A formation of the intramolecular complex between a metal cation and a sulfogroup located in a cis-2b crown cavity has been established which leads to the stabilization of a new configuration of a cisisomer with a shorter-wave absorption. As a result, the complexes of 2b with alkaline metal cations investigated in the present work serve as a very rare example of photochromic systems with a strong hypsochromic shift of a long-wave absorption upon trans-cis-isomerization.
References [ I ] M.V. Altimov, V.F. Razumov, A.G. Rochinsky, V.N. Listvan and Yu.B. Sheck, Chem. Phys. Letters 101 (1983) 593.
[ 2] V.A. Sazhnikov, M. Rakhmatov and M.V. Alfimov, Dokl. Akad. Nauk SSSR 241 ( 1978) 1378. [3]Yu.B. Sheck and M.V. Allimov, Usp. Nauchn. Fotogr. 19 (1978) 216.
460
25 October 1991
[4] A.V. Eltsov, ed.. Organic photochromes (Khimia, Leningrad, 1982). [ 51 J.M. Lchn, Angew. Chem. Intern. Ed. 27 ( 1988) 92. [6] F. Vijgtle and E. Weber, eds.. Host-guest complex chemistry macrocycles (Springer, Berlin, 1985). [ 71 W. Wierenga, B.R. Evans and J.A. Woltersom, J. Am. Chem. sot. 101 (1979) 1334. [S] S.P. Gromov and Yu.G. Bundel, Dokl. Akad. Nauk SSSR 281 (1985) 585. [9] S.P. Gromov, M.V. Fomina,E.N. Ushakov, I.K. Lednevand M.V. Alfimov, Dokl. Adad. Nauk SSSR 3 14 ( 1990) I 135. [lO]S.P. Gromov, O.A. Fedorova, E.N. Ushakov, O.B. Stanislavsky, I.K. Lednev and M.V. Alfimov, Dokl. Akad. NaukSSSR 317 (1991) 1134. [ 1 I ] H. G&ten and D. Schulte-Frohinde, Tethrahedron Letters 41 (1970) 3567. [ 121 E. Fischer. J. Phys. Chem. 71 (1967) 3704. [ 131 H.G. Lijhr and F. Fogtle, Accounts Chem. Res. 18 (1985) 65. [ 141 U.Takaki and J. Smid, J. Am. Chem. Sot. 96 ( 1974) 1085. [IS] Yu.B. Sheck, N.P. Kovalenko and M.V. Altimov, J. Luminescence I5 ( 1977) 157.
CHEMICAL PHYSICS LETTERS
25 October 199 1
Molecular design, photoisomerization and complexation of crown ether styryl dyes M.V. Alfimov, S.P. Gromov
and I.K. Lednev
Department of Photochemistry, N.N. Semenov Instltufe qf Chemrcal Ph,vsics, USSR Academy oJ.Sc~ences, Kosygm street 4. 117334 Moscow, USSR Received 3 July 199 I
The structure of a new class ofdyes, crown ether styryl dyes, is discussed. Quantitative data on trans-cis- and cis-trans-photoisomerization quantum yields and complexation of the obtained compounds are analyzed. The results on photoinduced and dark complexation of chromogenic 15-crown-5 ether betaine are reported. The formation of an intermolecular coordination bond between a sulfogroup and a metal cation in a crown cavity leading to cis-isomer stabilization has been established.
1. Introduction Photoisomerization of stilbene compounds was studied by us earlier [ 1,2 1. Photoinduced isomerization of diarylethylenes is a classic example of a monomolecular process as the result of which a molecule upon photoexcitation transforms from one stable geometric configuration (trans- or cis-) into another one (cis- or trans-, respectively) [ 31. Since physical and chemical properties of transand cis-isomeric derivatives of ethylene are similar, up to now this process of molecular photoisomerization photocontrol has not found application [ 41. A possibility of construction of photoisomers of different reactivity would mean new prospects for the control of chemical processes. A considerable achievement in creating such systems would be a synthesis of compounds where a change of the molecule’s geometrical configuration under irradiation could lead to a significant change of complexation constant. At present, there are intensive investigations of macroheterocyclic compounds the properties of which are determined by the possibilities of constructing on their basis, polyfunctional compounds capable of the selective complexation of ions or neutral molecules by a host-guest-type [ 5,6]. In this connection, of interest are photoactive compounds which can be constructed on the basis of styryl dyes Elsevier Science Publishers B.V.
containing macroheterocyclic fragment and capable of complexation with metal cations. On the other hand, ionophores containing a crown ether ring and anion group in a spatially accessible position are known to be more efficient receptors of cations of alkaline earth metals [ 71. We have assumed that this conception can be also applied to the synthesis of crown ether styryl dyes, “covered” in c&form by anion groups. It would allow one to obtain chromogenic crown ether capable of “taking off” and “putting on” its “anion cap” upon photo-irradiation, which in its turn provides an opportunity to achieve photocontrol of binding alkaline earth metal cations.
2. Presentation of the molecules We have synthesized indolenine styryl dyes la and lb [ 8,9] and styryl dyes of benzothiazole series 2a and 2b [lo]. Compound lb contains a fragment of crown ether, and perchlorate 2-[2-( 3,4-dimethoxyphenyl)ethenyl]-3H-indolinium la has in a benzene ring two methoxy groups which reproduce in a general way the electron-donor effect of a crown ether substituent on the system of conjugated double bonds of the dye. However, in contrast to crown ether styryl dye lb, compound la must not be noticeably capable of complexation with ions of alkaline and alkaline earth metals. 455
Volume
185,number
CHEMICAL
5,6
PHYSICS LETTERS
OMe
la
Ar :
lb
Ar :
OMe
Polyether macrocycle 15-crown-$ acting as pentadentate ligand upon complexation with metal ions, is built into compounds 2a and 2b. However, in contrast to cation dye 2a, compound 2b is betaine, the sulfogroup of which can form, under certain conditions, a bond with a metal cation supplementing its coordination sphere.
2a R = Et
25 October I99 I
recorded on a Specord M 40 spectrophotometer. The photolysis of la, lb, 2a and 2b solutions was performed in quartz cells with a mercury arc lamp (250 W) as a source of radiation with wavelength 1= 365 or 436 nm. The actinic light intensity was measured by means of a thermoelement. The preparation of solutions and all experiments were carried out under red light in a dark room. The samples’ fluorescence was measured on an Elumin-2M spectrofluorimeter. The quantum yield of fluorescence was determined using fluorescein in a 0.01 N solution of KOH in ethanol, as a standard [ 111. The quantum yield of trans-cis-isomerization 0, was determined under radiation with light of ,I=365 nm by the rate of optical density change in la and lb solutions in water at 436 nm. For the calculations, the following values of molar absorption coefficients at 436 nm were used: et =3.9x 104, r,=O.5X lo4 II mol-’ cm-’ for lb and e,=3.7x104, t,=0.5~10’ P mol-’ cm-’ for la. Qjcwas calculated on the basis of the measured values @,/@,.
2b R : CH2CH2CH2SO;
4. Discussion Consideration of molecular models of the perspective dyes from this series has shown that the length of the sulfopropyl group is evidently enough for the formation in cis-form of a dye of additional coordination bond with a metal cation. The structure of the obtained compounds la, lb, 2a and 2b has been proved by means of ‘H NMR spectroscopy. The data of the elemental analysis correspond to the structures proposed. According to the ‘H NMR data, all styryl dyes obtained exist in a transform. This conclusion can be confidently made on the basis of a high constant of spin-spin coupling, ‘J,,,,, = 15.6-16.7 Hz, for olefinic protons of the dyes obtained.
3. Experimental ‘H NMR spectra have been recorded on a Bruker WM 400 spectrometer in dimethylsulfoxide-d6 and methanol-d, with TMS as a standard. Acetonitrile has been twice distilled over CaHz and PzOs for dehydration. Perchlorates of Na, K, Mg, Ca and Ba were dried in a vacuum at 260°C. Absorption spectra were 456
4.1. Photoisomerization of la and lb Fig. 1 shows absorption and fluoiescence spectra of trans-lb in water. A long-wave absorption band of trans-la and trans-lb has a significant hypsochromic shift with the increase in the solvent polarity. The band maximum for trans-lb is at A=466 nm (6, = 4.1~10~ II mol-’ cm-‘) in chloroform, 447 nm (t,=4.1x10411mol-‘cm-‘)inacetoneand432nm (q=3.9X lo4 I mol-’ cm-‘) in water; for trans-la, respectively, at 11=463 nm (t, =4.1 x lo4 II mol-’ cm-‘), 446 nm (6,=4.0x lo4 ! mol-’ cm-‘) and 431 nm (t,=3.7X lo4 Q mol-’ cm-‘). Upon irradiation of trans-la and trans-lb solutions in water with I= 436 nm, the long-wave absorption decreases with a simultaneous increase in the short-wave ab sorption until a photostationary state is achieved. Upon irradiation of the initial solutions with light of A=365 nm, the absorption spectrum changes up to a transfer into a new photostationary state. In all cases, a pronounced isosbestic point (at A=332 nm for lb and 336 nm for la) and a linear dependence between the absorbance values at two different
Volume 185,number 5,6
CHEMICALPHYSICSLETTERS
7
250
300
350
400
450
500
550
600 650
‘x I llrn
Fig. I. Absorption (-) T= 295 K.
and fluorescence (“‘) spectra of tram-lb and the calculatedabsorption spectrum of cis-lb (---)
wavelength are observed. The spectral changes are, apparently, explained by the occurrence in the solution of the reversible trans-cis-isomerization. After la and lb irradiation in the solvents of moderate polarity (chloroform, acetone), one can observe a fast dark reduction of spectra. This is evidently associated with the occurrence of thermal cis-trans-isomerization. In aqueous solutions, cis-la and lb are stable enough to determine using the Fischer method [ 12 ] the spectra of individual cis-isomers and their absorption coefficients. From the known to c,, @, and Qc were calculated by a standard method. The obtained values of fluorescence and photoisomerization quantum yields are presented in table 1. It is worth noting that the results do not depend on the presence of oxygen in the air with the isomer concentrations used in this work ( 1.2 X 1O- 5 mol P- ’ ).
Table 1 Photochemical data for laand lb at T=295 K in water
Dye
(Y(365 nm)
(Y(436 nm)
0,
@
@f
la lb
0.56 0.58
0.87 0.88
0.47 0.49
0.51 0.50
0.0015 0.002
in water at
The error in the @, and DC measurements is about 20%, and that of @r is about 40%. 4.2. Complexation of 2b with cations qf alkaline earth metals In the acetonitrile solution, compound 2b has in the absorption spectrum an intensive long-wave band with a maximum at A=435 nm (molar absorption coefficient trnax= 3.9x lo4 P mol-’ cm-‘). On the addition to the solution of perchlorates of alkaline Na, K and alkaline earth metals Mg, Ca, Ba, a hypsochromic shift of the long-wave band is observed. With the concentrations of perchlorate C, 2 10m3 M, the absorption band maximum is at A=392 nm (%rxix= 3.69~ lo4 P mol-’ cm-‘), 396 nm (tmax= 3.62~10~ Q mol-’ cm-‘) and 407 nm (cmax= 3.66x10411mol-‘cm-‘)inthecaseofMg’+,Ca’+ and Ba’+, respectively. Similar spectral changes were observed earlier when studying complexation of metal ions with the crown ether not containing styryl chromogene [ 13 1. From the absorption spectra of the solutions with the constant initial concentration of the ligand C0=2.05x 10e5 M and different concentrations of the cation, the degree of the ligand transformation 457
Volume185, number5,6
CHEMICALPHYSICSLETTERS
into the complex form with metal ions was determined according to the formula a=(&-D)/ (Do-D,), where D,, is the absorbance of the ligand initial solution at the fixed wavelength, D, is the absorbance of the solution upon complete complexation, and D is the absorbance of the solution at different values of the cation concentration. When calculating (Y,we made the assumption that the absorption coefficients of the complexes with different content of the bound ligand per mol are the same as the measured wavelength. To diminish systematic measurement error, connected with the accepted assumption, the absorbance of solutions was determined at the wavelength of 460 or 470 nm, where the e of complexes are small as compared to those of ligands. In the experiments on a 2b complexation with Ba ions, the amount of the ligand used at low degrees of transformation exceeded the concentration of the cation nearly twice. Thus, for the given compound, the following scheme of the formation of the complex of the 2L: IM composition can be proposed:
25 October 1991
2.0
T_
1.5
: _
“0 c
1.0
x
0” 0
0.5
Fg. 2. Experimental dependences of the necessary amount of ligand 2b for the total concentration of (0 )Mg'+,(0 )Caz+ and (0) Ba’+.
plexes of a 2 : 1 composition with crown ethers which have small-size cavities [ 141. With a Mg*+ cation, such a complex is apparently not formed.
L+M&LM, LMfLkZ
L2M,
where M is a metal cation and L is a ligand molecule. The following equation corresponds to the scheme presented: C, -= a
ltCck,(l-cr)+C;k,kz(l-a)” k,(l-cx)[1t2c,k,(l-cr)]
.
(1)
Experimental dependences of the ligand concentration decrease, crC, = C, - [L], on the total concentration of Mg*+, Ca*+ and Ba*+ in the solution are presented in fig. 2 by dots. Solid lines represent theoretical dependences corresponding to eq. ( I ) with constants k, =4X lo6 Bmol-‘, k,=Ofor Mg*+; 7X105, 2~10~ R mol-’ for Ca*+; and 6x10’, 1.5 X lo5 B mol-’ for Ba’+. Good agreement of the experimental data with the results of the calculation confirms the validity of the chosen scheme of complexation. In addition, the decrease in the value of k, in the order of Mg2+ > Cal+ x Ba*+ correlates with the value of the long-wave absorption band shift in the complex spectra. The increase of the value of k2 in the order Mg*+ < Ca2+ i Ba*+ corresponds to the idea that the cations of large size tend to form com458
4.3. Photoinduced formation of intramolecular coordination bond in 2b complexes with Mg2+, Ca’+ and Ba*+ cations Compound 2b, like a crown ether indolenine styryl dye lb, can be subjected to a reversible photochemical reaction of trans-cis-isomerization around the central double C=C bond. The absorption band of cis-2b in acetonitrile was calculated by means of the Fischer method [ 131 using the spectra of trans-isomer and photostationary states obtained from the irradiation of a 2b solution with light of wavelength 365 and 436 nm. In fig. 3, trans- and cis-isomer absorption spectra of 2b are presented. The distance between the maxima of the long-wave absorption bands of these isomers is only 15 nm, which does not allow the complete transformation of the trans-isomer into a cisform through direct photoisomerization. A similar situation is characteristic for the majority of photochromic compounds [ 41. Another situation is observed in the case of trans3b complexation with alkaline earth metals when the concentration of the latter is 10m3 M. The irradiation of solutions with
Volume 185,number 5.6
CHEMICALPHYSICSLETTERS
(Tram-Zb)M”
54, nm
Fig. 3. (a) Absorption spectra of trans-2b (-.-,-), cis-2b (- - -) and complexes of trans-2b ( “‘1 and cis-lb (-) with Mg’+at [Mg(ClO,),] = 1x 10-j M. (b) Absorption spectra of complexes of cis-2b with Mg’* at [Mg(Cl0,),]=3~ 10-j M (.,.), 0.5 M (-_) and a cis3a and Mg’+ complex at [Mg(CIO,),]=3xlO-3M (---).
light (i=436 nm) leads to a photostationary state including > 99% of cis-isomer. Cis-isomer absorption spectra of the complexes have a long-wave maximum at A=321 nm (c=8.96>( lo3 P mol-’ cm-‘), 325 nm (c=9.51 X lo3 P mol-’ cm-‘) and 338 nm (e=8.05x103 II mol-’ cm-’ for Mg’+, Ca’+ and Ba’+, respectively. Fig. 3 shows an example of cisand trans-isomer absorption spectra of a 2b with Mg’+ complex. A strong hypsochromic shift of 70 nm of the cisisomer spectra of the complexes in respect to the spectra of trans-isomers can be apparently explained by the formation in a cis-form of a coordination bond between a sulfogroup and a metal cation, located in a crown cavity, according to the scheme:
25 October 199I
(Cis-2b]M2*
For the formation of such a bond, a certain twist of the fragments of the cis-2b molecule with the change of the planar structure is necessary, which in analogy with the arylethylene case [ 15 1, must be accompanied by a long-wave maximum hypsochromic shift in the absorption spectrum as the result of the destruction of x-system conjugation. With the increase in the concentration of magnesium perchlorate in a cis-isomer.solution of the 2b complex with Mg2+ (C,> lo-* M), the absorption in the long-wave portion of the spectrum gradually increases, and at C, > 0.2 M the spectrum takes an entirely different shape. The change of the spectrum shape is attributed to the destruction of the intramolecular coordination bond due to the association of the SOi group with an additional cation of Mg2+. Fig. 3 shows the absorption spectra of a cis-2b complex with Mg*+ at concentrations of Mg ( Clod)? at 3 x 10e3 and 0.5 M, as well as of a cis-2a complex with Mg’+ at [Mg(C10,)2]=3x 10e3 M. In compound 2a, an ethyl group instead of a sulfopropyl one is present; therefore, the formation of an intramolecular coordination bond is impossible. The fact, that the spectrum (cis-2a)MgZt at Cr,,=3x 10e3 M and the spectrum (cis-2b ) Mg2+ at CM= 0.5 M practically coincide, confirms the assumption on the destruction of the intramolecular bond in (cis-2b)Mg2+ at high concentrations of Mg(C104)*.
5. Conclusion Thus, in the moderately polar solvents (acetonitrile, acetone), the compounds la, lb, 2a and 2b are unstable in a cis-form. With the addition of alkaline and alkaline earth metal perchlorates (Na, Ca, Mg and others) to the solution of the compounds under investigation in these solvents, a hypsochromic shift of the long-wave absorption of more than 30 nm is observed in the case of lb and 2a and in the case of 459
Volume
185,number 5,6
CHEMICAL PHYSICS LETTERS
la, the shift is 2-3 nm. It is associated with a complexation of crown ether styryl dyes with metal cations. A formation of the intramolecular complex between a metal cation and a sulfogroup located in a cis-2b crown cavity has been established which leads to the stabilization of a new configuration of a cisisomer with a shorter-wave absorption. As a result, the complexes of 2b with alkaline metal cations investigated in the present work serve as a very rare example of photochromic systems with a strong hypsochromic shift of a long-wave absorption upon trans-cis-isomerization.
References [ I ] M.V. Altimov, V.F. Razumov, A.G. Rochinsky, V.N. Listvan and Yu.B. Sheck, Chem. Phys. Letters 101 (1983) 593.
[ 2] V.A. Sazhnikov, M. Rakhmatov and M.V. Alfimov, Dokl. Akad. Nauk SSSR 241 ( 1978) 1378. [3]Yu.B. Sheck and M.V. Allimov, Usp. Nauchn. Fotogr. 19 (1978) 216.
460
25 October 1991
[4] A.V. Eltsov, ed.. Organic photochromes (Khimia, Leningrad, 1982). [ 51 J.M. Lchn, Angew. Chem. Intern. Ed. 27 ( 1988) 92. [6] F. Vijgtle and E. Weber, eds.. Host-guest complex chemistry macrocycles (Springer, Berlin, 1985). [ 71 W. Wierenga, B.R. Evans and J.A. Woltersom, J. Am. Chem. sot. 101 (1979) 1334. [S] S.P. Gromov and Yu.G. Bundel, Dokl. Akad. Nauk SSSR 281 (1985) 585. [9] S.P. Gromov, M.V. Fomina,E.N. Ushakov, I.K. Lednevand M.V. Alfimov, Dokl. Adad. Nauk SSSR 3 14 ( 1990) I 135. [lO]S.P. Gromov, O.A. Fedorova, E.N. Ushakov, O.B. Stanislavsky, I.K. Lednev and M.V. Alfimov, Dokl. Akad. NaukSSSR 317 (1991) 1134. [ 1 I ] H. G&ten and D. Schulte-Frohinde, Tethrahedron Letters 41 (1970) 3567. [ 121 E. Fischer. J. Phys. Chem. 71 (1967) 3704. [ 131 H.G. Lijhr and F. Fogtle, Accounts Chem. Res. 18 (1985) 65. [ 141 U.Takaki and J. Smid, J. Am. Chem. Sot. 96 ( 1974) 1085. [IS] Yu.B. Sheck, N.P. Kovalenko and M.V. Altimov, J. Luminescence I5 ( 1977) 157.