A Molecular Reaction Cycle With a Solvatochromic Merocyanine Dye An Experiment in Photochemistry, Kinetics, and Catalysis M. H. Abdel-Kader Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt U. Steiner Fakultat fur Chemie, Universitat Konstanz D-7750 Konstanz, Federal Republic of Germany
Merocyanines of the stilbazolium betaine type, e.g., 1methyl-stilbazolium 4'-olatel (see formula I), in the following abbreviated as M, are characterized hy their extreme solvatochromic properties ( I ) ,i.e., their absorption spectra exhibit very large solvent effects which allow the polarity of a solvent to be judged by the color of the solution (2). For this reason M has been suggested as an interesting preparation for students in THIS JOURNAL (3).
In addition to its solvatochromy the compound M exhibits some remarkable chemical and ~hotochemicalm o ~ e r t i e s( 4 ) making it suitable to illustratesome important concepts in ~ h v s i c achemistry l hv s i m ~ l experiments. e The resonance B t r k t u r i Ih ripresents an essentially stilbene-like structure, sugeesting the possibility of cisltrans isomerism known for many stabene derivatives (51, so that one should expect merocyanines of this type to exist likewise in cis and trans configurations. Recently we have identified the isomer M,i, (4) which represents the first example of a cis-stilhazoliumbetaine stable enough to be amenable to both spectral and photochemical characterization. However, M,,, cannot be obtained from Mt,,,, directly. Instead one has to make use of the protonated form MH+,,,,,, which is even more stilbene-like since resonance structure Ib is stabilized bv the protonation. MH+,,,,, may be transformed to MH+,is photochemically. Deprotonation of MH+,, yields the cis-betaine M,i* Thermally as well as photochemically, M,i8 can be completely reverted to the trans isomer Mt,,,,. The protolytic, photochemical, andlor thermal reactions described above can be combined to a complete molecular reaction cycle (Fig. 11, which is a one way cycle, however, since M, ,,,, is irreversible. the step M,;, I n this paper we describe three experiments with the merocyanine M, suitable as an integrated laboratory experience for undereraduates. 1) A simple experiment, demonstrating the complete molecular cvcle comoosed of nhotochemical, thermal. and protolytic reaction steps. This molecular reaction cycle exhibits interesting aspects with relation to a model discussed in the literature for the mechanism of a light driven proton pump (6, 7).
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2) A kinetic study of the thermal cis trans isomerization of the form M, which is a simple first-order reaction and is very sensitive to changes in temperature and solvent. This reaction is ideally suited for demonstrating the obvious significance of the activation parameters in Eyring's absolute rate theory ( 8 ) . 3) An experiment, demonstrating the mechanism of base catalysis for thermal isomerization of MHfCi, MH+tran*
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Experimental The merocyanine dye M may be synthesized as descrihed ~ ~ . I . H I S JOURNAL (3).It is obtained as Mb,,, (5). The reaction cycle depicted in Figure 1 is carried out in a l-cm spectrophotnmeter cuvette, starting with a 2 X M solution of the dye (Mt,,,) in 10W4Maqueous HCI, prepared freshly in the dark. The first absorption spectrum is measured at this stage. Then the euvette
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Journal of Chemical Education
Figure 1. Absorption spectra representing the four stages of the reaction cycle. A: Spectrum of a 2 X l O V Maqueous solution of M, prepared in the dark. with 10-"HCI, 6:Aner irradiating solution A with light of 366 nm until reaching the photostationary state. C: Afler addition of 1 drop of 1 NNaOH to the cuvetie, D: After irradiating with light of 436 nm until reaching the photostationary state (volume of the cuvene 4 ml, optical pathlength 1 cm).
is exposed t o normal daylight or light from a halogen lamp. T h e progress oi'the conversion can he followed by measuring the absorb^ ance a t different stages oi'the reaction until the photostationary state is reached (approximately 2 hr a t normal daylight). Addition of one drop of 1N NaOH a t room temperature yields a mixture of M,i, and Mv,,, with the same ratio as the photostationary state composition before the deprotonation. T h e cisltrans ratio of this mixture, if kept in the dark, is nut changed significantly lor 30 min, so there is enough time t o record the absorption spectrum. T h e cis fraction is then isomerized t o Mi,,,, either photochemically or thermally. T o follow the nhotochemical isomerization the solution must not Ire keol above room temoerature. T o measure the thermal isomerization. the reaction should he aceeleraled by heating in the dark (at 2 3 ' ~ ,tin is 260 min, a t 60°C only 1.2 min). After the cis-trans isomerization of M has been completed, MH+t,,,, is regenerated hy adding 2 drops of 1N HCI. This completes the reaction cycle. All the reactions described above can be followed by recording the ahsorption spectra as shown in Figure I T o measure the rate constant ofthermal cis trans isomerization of M , a thermostated cuvette holder is required. An irradiated acidic solution (HCI 1 0 - W ) , which is thermally stable u p to 10O0C, is heated t o the required temperature. A drop of 1 N NaOH is added lo the euvette, and after mixing, the isomerization is followed hy measuring the change of absorption a t a fixed wavelength, preferentially al 440 nm. This procedure is repeated a t several temperatures between 20 and fiODCt o determine the activation parameters. To pursue the hase catalyzed thermal isomerization MHfCi, MH+t,,,,, builec solutions are used instead of NaOH. An irradiated 4 X 1 0 - W slightly acidic solution (8 X 1 0 - W HCI) of the dye is heated t o the required temperature and then diluted to the half concentration with a given buffer solution, which has been previously adjusted in temperature t o the irradiated solution. T h e rate af isomerization is measured as herore.
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Fig. 3) according to the equation:
The rate constant according to Eyring's theory (8) is given by:
where k is Boltzmann's constant, h is Planck's constant, AS# is the entropy of activation, and AH+ the enthalpy of activation. For liquid phase reactions E + and AH+ are related by:
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Resulls and Discussion
Reaction cycle. The spectral change in going from a pure MH+t,,,, solution to the photostationary state MH+,,,,,/ MHt,j, is shown inFigure 1(A- B). It should be noted that A of irthe nhotostationarv state denends on the waveleneth u radiation. It is, however, not kery sensitive to A if A > 300 nm. Denrolonation. which is achieved hv addition of NaOH. transforms MH+,~,to M,j, and MH+,,,,, to M, ,,,, where the cisitrans conformation is Dreserved in the nrotolvtic reaction. Therefore the cisitrans ratio of M obtained immediately after addition of NaOH (spectrum C) is the same as in the photostationary state of MH+. Subsequent irradiation of this solution transforms M,,, to M,,,,, with an increase of the main absorption hand (spectrum D). The composition of the photostationary state obtained hy irradiation with any wavelength above 254 nm corresponds to a pure Mt,,,, solution. On acidifying this solution one obtains the original unirradiated MH+l,,,, solution (spectrum A). It can be seen from the reactions considered above that the cis-betaine M,i, constitutes an essential link of a complete photochemical, thermal, and protolytic reaction cycle. Since Mt,,,, is irreversible, the cycle can be trathe step M,i, versed in one direction only. This peculiarity is of special interest with r e s ~ e cto t a model for a molecular lieht-driven proton pump (if. Fig. 2). This was recently suggested by Schulten and Tavan 161 to e x ~ l a i nthe role of the Schiff hase
Figure 2. A model of a molecular light-driven proton pump according to (6). The merocyanine molecules are part of a closed membrane. By the reaction sequence . . . 1. 2.3.4, 1 . . . protons are accepted from carriers inside the vesicle, shifted across the membrane by the photochemical trans 9 cis i~omerizationand accepted by carriers outside the vesicle. The working direction of the pump is determined by the irreversibilityof step 4.
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Halobacterium halohium. Thermal isomerization. In contrast to MH+,is which is thermally stable up to 100°C, M,i, may be isomerized thermally. As expected, this is a first-order reaction with strong temperature~dependence.Let A, he the absorbance observed a t the beainnina of the reaction, A, a t time t and A., a t infinity, corresponding to complete conversion to the trans isomer. The difference (A, - A l ) is proportional to the concentration of cis isomer a t any time t, and the integrated first-order rate equation is given by: h,t = In-
A , - AA,-A-
(1)
Figure 3. Arrhenius plot for the rate constant of thermal isomerization M,,, M .",,
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Number 2
February 1983
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The following values are obtained for the thermal cis isomerization of M:
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trans
k, = 7 X 10'6 s s '
These activation parameters are particularly useful in order to demonstrate that even though the activation energy is
Figure 5. Apparent rate constant k,,, of isomerization of MH+,,, as a function cu of protolytic dissociation ( T = 40°C).
of the degree
course of the isomerization (cf. Fig. 4). While the planar state is very polar due to the strong contribution of resonance structure Ib, the dipole momentmust be considerably smaller in the 90" twisted configuration where resonance structure Ia should be dominant. Polar solvent molecules which initially are highly oriented in the strong dipole field of the planar configuration can reorient rather freely in the activated twisted configuration. I t is of interest to note that the observed entropy of activation corresponds to the entropy of melting about three moles of water. Base catalysis. MH+ cannot be cisltrans isomerized thermally. From the spectra in Figure 1,however, it can be inferred that, by base catalysis, a thermal isomerization MH+,i, MH+,,,,, becomes feasible through the reaction steps MH+,,
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i;! M , i , M, ,,,, e M H + t ., This offers the possibility of demonstrating the role of a base catalyst in reducing the energy barrier for M H + , i , isomerization. For a given p H a fraction a of M H + , i , is dissociated. The value of a can be measured directly from the absorption spectrum or calculated using the relation (9):
The apparent rate constant for thermal isomerization is then given by:
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k c being the rate constant of the isomerization M , i , Mt,,,,. The validity of this relation is shown in Figure 5 where the apparent rate constant k,,, is plotted against a. Acknowledgment
We would like to thank Professor H. E. A. Kramer for support and for critically reading the manuscript. We also wish to thank the students Thomas Zwing and Axel Ganzhorn for assistance with the base catalysis measurements. Literature Cited (1)
Rrooker, L. G. S., Keyes. G . H . and Heaeltine. D. W.. J. Amer Chem S o r , 73,5350
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1,9511 ~,
I21 Brooke?. L. G. S,Craic,A. C..Hereltine, D. W.,Jenkins.P. W.. and 1.incaln. L. L.. J.
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" . , . , I \ 1 ,...I, I.,. I .:. h : . I .\ .. I I , ( - 1 . ' I - I, . I . . I , I I,..., 1.1 I , . . 5% \: ..' . \ , , is) Byrinq, H., Chrm RPU, 17.66 (19361. iY) Albrrt. A,. and Serjeant. E. P., "kmization Constanb of Acida and Baaes;Metht~en Co. Ltd.,London,John Wiiey& Sons Inc.,New York. 1962, p. 9. I
Figure 4. Interpretation of the positive enlropy of activation for thermal isomMm, Since the dipole moment of M isdrastically decreased eriration of M,,. in the 90' twisted state there is a corresponding increase of the disorder of the solvent shell.
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Modern Industrial Spectroscopy
T h e "roeram inch~deshasic theoretical considerations. hands-on instrumental trainine. and the interoretation of results.
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