- Email: [email protected]
Excited states and electrochromism of radical cation of the carotenoid astaxanthin
18 September 1998
Chemical Physics Letters 294 Ž1998. 351–356
Excited states and electrochromism of radical cation of the carotenoid astaxanthin Stanisław Krawczyk
)
Institute of Physics, Maria Curie-Skłodowska UniÕersity, pl. M.C. Skłodowskiej 1, 20-031 Lublin, Poland Received 5 June 1998
Abstract Radical cations of the carotenoid astaxanthin were generated by chemical oxidation with FeŽCl. 3 , and their absorption and electroabsorption ŽStark. spectra at temperatures about 150 K were recorded in the spectral range from 5900 to 26000 cmy1 Ž380 to 1700 nm., covering two absorptive electronic transitions from D 0 Žground. to D1 and D 2 excited states. The changes in static polarizability are negative and equal y40 " 10 A3 for D 0 ™ D 1 and y105 " 15 A3 for D 0 ™ D 2 , pointing that dominant contribution to polarizabilities results from the coupling of D 1 and D 2 with the ground state. An approximate localization of the next excited state with ground-state parity is estimated based on arguments from perturbation theory. q 1998 Published by Elsevier Science B.V. All rights reserved.
1. Introduction Carotenoids are a class of substituted linear conjugated hydrocarbons Žpolyenes. that gained significant interest in different areas of molecular spectroscopy, concerning not only the properties of neutral carotenoid molecules but also of their ions. This interest stems mainly from the significant role of carotenoids in photosynthesis, where they serve as light-harvesting pigments and photoprotectors w1,2x. Normally, carotenoids function in these two roles in the form of neutral molecules. However, several years ago, the formation and accumulation of bcarotene radical cations in Photosystem II reaction centers, induced by photoaccumulated primary electron donor radical P680 Øq was detected w3,4x. Also the nonradiative energy transfer from carotenoid to )
E-mail: [email protected]
chlorophyll is thought to proceed through the electron exchange mechanism w5x, which detailed description requires the knowledge of electronic states of carotenoid ions w6x. Carotenoids serve also as model systems in the search for strongly nonlinear optical materials w7,8x, which leads to deeper understanding of the electronic structure of conjugated systems and offers a prospect of practical applications. Here, the modification of bond order alternation is the factor of primary importance and this is achieved by polarizing the polyene chain by electron donor and acceptor substituents covalently bound at the chain ends w9x. The third area of research where carotenoids and polyenes serve as model molecular structures comprises the studies on the mechanism of electrical conductivity in conjugated polymers, especially in polyacetylene w10,11x. In the latter case, the primary species leading to electronic conduction are the radical cation or radical
0009-2614r98r$ - see front matter q 1998 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 8 5 7 - 4
352
S. Krawczykr Chemical Physics Letters 294 (1998) 351–356
anion states Žpolarons. in conjugated ŽCH. x chains. The electronic states pertinent to these species appear in absorption spectra in the IR range, and one of the very few ways of their spectroscopic identification is the comparison with spectra of radical cations of shorter conjugated chains w12x. However, due to the low chemical stability of radical cations, only the absorption spectra, sometimes missing the lowest-energy transition have been available for a long time w13x. Attempts to stabilize radical cations produced by oxidation of polyenes with FeCl 3 or SbCl 5 were reported to lead directly to dications w12x. Only recently the preparation of relatively stable carotenoid cation radicals and dications by chemical oxidation or electrolysis in CH 2 Cl 2 at y708C was reported w14,15x, making these species available also for further spectroscopic characterization by resonance Raman spectroscopy w16x. Quite similar to this is the development of studies on polyene anions, where pure absorption and resonance Raman spectra are becoming available w17x. In a previous paper w18x, we described the formation and characterized the absorption and electroabsorption spectra of radical cation and dication forms of two carotenoids canthaxanthin and astaxanthin in fluorinated alcohols at low temperature. Only the second Žhigher-energy. near-infrared electronic transition in radical cation was examined, revealing a very conspicuous case of negative change in molecular polarizability in this transition. Since the polarizability depends on the energetic ordering of states and on transition moments, this source of information is wider exploited in this work, both experimentally and with some reference to theory.
2. Materials and methods Puriss. grade 2,2,2-trifluoroethanol ŽFET. and anhydrous FeCl 3 were obtained from Fluka and 1,1,1,3,3,3-hexafluoroisopropanol ŽHFP . from ABCR. Crystalline all-trans astaxanthin Ž3,3X-dihydroxy-b,b-carotene-4,4X-dione. was a gift from Professor George Britton of Liverpool University. Since astaxanthin is 3,3X-dihydroxy derivative of canthaxanthin, it is spectroscopically identical with the latter, but its cations are chemically more stable w18x. Absorption and electroabsorption spectra were
recorded with a home-built spectrometer equipped with either a silicon photodiode or with InGaAs photodiode ŽHamamatsu G5125. as light detector, which allowed for recording the spectra between 380 and 1700 nm. Quadratic electroabsorption Žsecondorder Stark effect. was measured by applying a 920 Hz sinusoidal voltage to the thin Ž0.1 mm. sample in frozen glassy solvent, and detecting the electric field-induced modulation of transmitted light intensity with the lock-in amplifier at the second harmonic frequency, simultaneously with the DC transmission. According to theory w19,20x, for molecules in a solution the changes of absorbance due to an external electric field with intensity F are related to the absorption spectrum AŽ Õ . through three additive contributions proportional to the zeroth, first and second derivatives of absorption:
ž
D A s aŽ 0 . A q aŽ 1 . Õ
d Ž ArÕ . dÕ
2
=F , Ž1.
q aŽ 2 . Õ
d 2 Ž ArÕ . dÕ 2
/ Ž 1.
Ž2.
with a and a proportional, respectively, to the molecular polarizability change D a and to the square of permanent dipole moment change D m , and aŽ0. reflecting the transition moment polarizability. This formula was used in the least-squares procedure for fitting the recorded D A spectra. 3. Results and discussion The near-infrared absorption spectrum of the radical cation of astaxanthin in HFP:FETs 2:1 at low temperature is shown in Fig. 1. The two bands at 7810 cmy1 Ž1280 nm. and at 12000 cmy1 Ž833 nm. correspond to D 0 ™ D 1 and D 0 ™ D 2 electronic transitions between the doublet states D 0 Žground. and D 1 and D 2 Žexcited., with relative intensities similar to the spectra of radical cations of canthaxanthin w14x and a ,v-diŽtert-butyl.-polyenes w12x. Their optical and electrooptical characteristics are summarized in Table 1. Both these bands are shifted toward higher energy compared to the absorption bands of canthaxanthin radical cation in CH 2 Cl 2 at y708C, which are at 7630 cmy1 Ž1310 nm. and 11270 cmy1 Ž887 nm. w14x. It has been shown in our previous study w18x that the ; 700 cmy1 upshift of the D 0 ™ D 2
S. Krawczykr Chemical Physics Letters 294 (1998) 351–356
353
Fig. 1. Absorption spectrum of astaxanthin cation radical in HFP:FET s1:1 at temperature 152 K.
transition in canthaxanthin radical cation results from solvatochromism. Similar explanation is applicable here to the D 0 ™ D 1 transition. The Bayliss shifts of D 0 ™ D 1 and D 0 ™ D 2 bands between the two solvents should fulfil the relation: D n 01
s
D n 02
< M01 < 2 < M02 < 2
,
Ž 2.
where Mi k denote dipole transition moments. With D n 02 s 700 cmy1 for D 0 ™ D 2 and the values of transition moments taken from Table 1 one obtains D n 01 s 150 cmy1 . This in good agreement with the experimental 180 cmy1 difference of D 0 ™ D 1 transition energies in CH 2 Cl 2 and in HFPrFET. The electroabsorption spectra of both transitions are presented in Figs. 2 and 3. The shapes of both electroabsorption spectra were found to be independent of the angle between the electric vector of light and the applied electric field F; this confirms, to the accuracy of our experiments, that the spectra recorded result from two similarly polarized and energetically well-separated electronic transitions in the spectral region investigated. For both transitions the main polarizability axes were found to be essentially colinear with transition moments.
Fig. 2. Absorption ŽA. and electroabsorption ŽB. spectra of astaxanthin cation radical, corresponding to the D 0 ™D 2 transition. In panel ŽB., the square points are experimental data obtained with r.m.s. electric field strength F s95000 Vrcm, and the thick continuous line is the fitting curve. Fit components proportional to the absorption spectrum, its first and second derivatives are shown with the continuous, dashed and dotted lines, respectively.
Good fits to electroabsorption spectrum within the D 0 ™ D 2 transition obtainable with only the firstand second-derivative combination, such as the one presented in the previous paper w18x, can be slightly improved by adding a small negative term proportional to the absorption intensity Žcf. Table 1.. The main feature of fits to D A spectra corresponding to both electronic transitions is the contribution from the first derivative of absorption but with negative sign, unambiguously pointing to the electric field-in-
Table 1 Spectroscopic characteristics of the two electronic transitions in astaxanthin cation radical Electronic transition
Wavenumber Žcmy1 .
Transition dipole ŽDebye.
Da ˚ 3. ŽA
Dm ŽDebye.
D 0 ™ D1 D0 ™ D2
7810 12000
7.45 16.0
y40 " 10 y105 " 15
1.8 " 0.3 1.5 " 0.2
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S. Krawczykr Chemical Physics Letters 294 (1998) 351–356
Fig. 3. Absorption ŽA. and electroabsorption ŽB. spectra of astaxanthin cation radical, corresponding to the D 0 ™ D1 transition. Electric field strength F s107000 Vrcm. The data in panel ŽB. are presented analogously to Fig. 2B.
duced ‘repulsion’ of D 1 and D 2 states toward higher energies from the ground state ŽD 0 .. Fig. 2B presents the experimental D A spectrum for D 0 ™ D 2 and the fit with three components. Although the fit is very good within the Ž0–0. vibronic transition in the range 11000–12500 cmy1 , it exhibits systematic deviations from the experimental points in the 13000– 14000 cmy1 region covered by the Ž0–1. vibronic band. The sign of these deviations on both sides of the interception point at ; 13400 cmy1 , very close to the Ž0–1. band center, indicates a smaller absolute value of negative polarizability change in this vibronic transition than needed to fit the entire D 0 ™ D 2 spectrum. Similar but more remarkably expressed in this inhomogeneity in the D 0 ™ D 1 transition, where also the vibronic activity is slightly more extensive. An inspection into the absorption spectrum in Figs. 1 and 3A shows that the Ž0–0. band centered at 7800 cmy1 is unsymmetrical. The second derivative points
to the presence of a vibronic transition about 500 cmy1 toward higher wavenumbers. It accompanies also the vibronic transitions comprising the much broader band about 1400 cmy1 from Ž0–0.. The fitting curve in Fig. 3B is based on least-squares minimization procedure in the range 6500–8500 cmy1 . The apparent deviations from experimental data on the high-energy side show that, due to differences in electrooptical parameters of individual vibronic transitions, a single set of electrooptical parameters is not sufficient to describe absorption changes within the entire electronic transition D 0 ™ D 1. This observation points to a significant variability of electrooptical parameters along the spectrum, depending on the vibrational state Žand possibly on the quantum number. reached on electronic excitation. Similar observations pointing to vibrational dependence of electrooptical parameters were already reported for neutral carotenoids, both symmetrical w21x and donor–acceptor substituted w9x. This variability can make the electrooptical parameters derived from the fits somewhat ambiguous, but in practice this uncertainty affects mainly the relative contribution of the zeroth-and the second-derivative terms, while the first-derivative term, related to polarizability, varies by - 10% and thus remains almost independent of the fitting range and number of components. A remarkable feature of the polarizability changes for both electronic transitions are their negative values. According to the second-order perturbation theory w19,20x, the one-dimensional polarizability of a given electronic state is represented by the expression:
ai s 2 Ý j
<² i < m < j :< 2 E j y Ei
,
Ž 3.
where m is the dipole moment operator. It follows from Eq. Ž3. that the polarizability of the ground state is always positive and that the negative polarizability Žequivalent to an increase of energy in an electric field. is possible for D 1 and D 2 if their electric field-induced interaction with higher-energy states is weaker than the interaction with the ground state. The considerations in the following will be simplified by considering the D 1 and D 2 polarizabilities as a result of their electric field-induced coupling
S. Krawczykr Chemical Physics Letters 294 (1998) 351–356
with the ground state D 0 and with only one additional excited state Dx , which must be of the ground-state symmetry in order to have a nonzero dipole matrix elements Žtransition moments. with D 1 and D 2 . These symmetry requirements limit the number of relevant matrix elements in Eq. Ž3., which leads to the following expressions for polarizabilities of D 0 , D1 and D 2 states:
a0 s 2 a1 s 2 a2 s 2
ž ž ž
2 M01
q
E10 2 M01
2 M02
E20 q
M12x
E10
E x1
2 M02
M22x
q
E20
Ex 2
/ / /
,
Ž 4.
,
Ž 5.
.
Ž 6.
Here, Mi k is the dipole transition moment between ith and k th states, and Ei k is the energy difference: Ei k s Ei y Ek . By pairwise subtraction we obtain the expressions for experimentally determined polarizability changes in electronic transitions: D a 01 ' a 1 y a 0 s y4 D a 02 ' a 2 y a 0 s y4
2 M01
E10 2 M02
E20
q2 q2
M12x E x1 M22x Ex 2
y2 y2
2 M02
E20 2 M01
E10
, Ž 7. .
Ž 8. Substituting the experimentally determined transition moments M01 , M02 , transition energies E01 , E02 and polarizability changes D a 01 and D a 02 from Table 1 into Eqs. Ž7. and Ž8. we obtain the following values for the contributions of other excited electronic states Žrepresented by Dx . to the polarizabilities of D 1 and D2 : M12x E x1 M22x Ex 2
˚3 , s 160 A ˚3 . s 200 A
Both these values are positive and this result is very important for further band assignment. It indicates that Dx must be located at an energy higher than D 1 and D 2 in order to partly diminish the negative polarizabilities acquired by the latter two through their coupling with the ground state.
355
The physical significance of the two remaining terms in Eq. Ž1., related to field-induced modulation of absorption intensity Ž aŽ0. . and to permanent dipole moments Ž aŽ2. . is more difficult to interpret. This difficulty concerns first of all a reasonable explanation of non-negligible values of D m Žcf. Table 1. in this symmetrical molecule. Usually, it is tentatively ascribed to the polarization due to statistical fields generated by solvent molecules. This interpretation does not agree with experimental observations on neutral carotenoids w21x and the origin of the effects leading to the third term in Eq. Ž1. still await explanation. The changes in absorption intensities reflected by the coefficient aŽ0. in Eq. Ž1. can be calculated by taking into account the electric field-induced mixing of states of different symmetry Žcalculations not quoted here.. However, the calculated values of aŽ0. for both electronic transitions differ from experimental results by several times. Different values for the energy of the state Dx must be assumed to quantitatively account for the two experimental values of aŽ0. and for the lack of observable intensity of D 0 ™ Dx transition. Remarkably, however, the required values of D 0 ™ Dx transition energies range from 14000 cmy1 to 21000 cmy1 , i.e. above D 1 and D 2 , in agreement with the conclusion concerning polarizabilities Žvide supra.. According to qualitative considerations based on simple MO theory w12x the states D 1 and D 2 result from configuration mixing and are, respectively, the sum and the difference of two singly-excited configurations corresponding to the nearly isoenergetic promotions of an electron from HOMOy 1 to HOMO and from HOMO to HOMOq 1. This picture explains the large intensity of the D 0 ™ D 2 transition and the weakness of D 0 ™ D 1 transition as due to, respectively, summing up and partial cancellation of two components in the dipole transition moments. In the radical cation of astaxanthin, D 0 , D 1 and D 2 are, respectively, the 12A u , 12 B g and 2 2 B g states. Recent theoretical study on radical cations of polyene series from butadiene to decapentaene w22x, consistent with this picture, revealed the more complicated structure of D 1 and D 2 resulting from more extensive CI mixing, and predicted a next excited state above D 2 to be of ground-state symmetry Ži.e., 2 2A u in astaxanthin radical cation.. Thus, this 2 2A u
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S. Krawczykr Chemical Physics Letters 294 (1998) 351–356
state potentially could be the Dx state with considerable positive contribution to D a 01 and D a 02 . However, the trend apparent in the calculations w22x indicates that 2 2A u could go lower in energy, below D 2 in radical cations of polyenes significantly longer than decapentaene, and in this respect 2 2A u could be an analogue of the low-energy 2 1A g state in neutral polyenes and carotenoids, which remains invisible in single-photon absorption spectra. In the case of the presence of 2 2A u between closely lying D 1 and D 2 , the transition moments between 2 2A u and D 1 and D 2 shall be very small for the field-induced intensity borrowing by 2 2A u to be undetectable. However, it cannot be excluded that the remarkable variability of electrooptical parameters within the D 0 ™ D 1 absorption band are due to just this effect. The other possibility is the localization of 2 2A u below D 1 , which makes 2 2A u the lowest excited state. In these two cases, the excited state Dx that provides important components to the polarizabilities of D 1 and D 2 shall be sought as another 2A u state at higher energy. An argument concerning the ordering of electronic states in polyene radical cations seems to emerge from the well-known fact that the electronic structure and spectra of polyene cations and anions are closely analogous w12,17x. The strong fluorescence of radical anions of tetradesmethyl-b-carotene w17x, with the double bond number Ž11. similar to that of astaxanthin examined here Ž13., suggests that the lowering of the 2 2A u state is not the case in radical anion and possibly not the case also in radical cations, and Dx located above D 1 and D 2 could be identified as 2 2A u . Fluorescence studies seem to be decisive in solving this question.
Acknowledgements The author wishes to thank Professor George Britton of Liverpool University for the sample of
astaxanthin. This work was supported by grant No. 6P04A 044 11 from the Polish Committee for Scientific Research.
References w1x D. Siefermann-Harms, Biochim. Biophys. Acta 811 Ž1985. 325. w2x Y. Koyama, M. Kuki, P.O. Andersson, T. Gillbro, Photochem. Photobiol. 63 Ž1996. 243. w3x P. Mathis, A. Rutherford, Biochim. Biophys. Acta 767 Ž1984. 217. w4x J.O. de Las Rivas, A. Telfer, J. Barber, Biochim. Biophys. Acta 1142 Ž1993. 155. w5x D.L. Dexter, J. Chem. Phys. 21 Ž1953. 836. w6x G.D. Scholes, R.D. Harcourt, G.R. Fleming, J. Phys. Chem. B 101 Ž1997. 7302. w7x S.R. Marder, C.B. Gorman, F. Meyers, J.W. Perry, G. Bourhill, J.-L. Bredas, B.M. Pierce, Science 265 Ž1994. 632. w8x C.B. Gorman, S.R. Marder, Proc. Natl. Acad. Sci. USA 90 Ž1993. 11297. w9x G.U. Bublitz, R. Ortiz, S.R. Marder, S.G. Boxer, J. Am. Chem. Soc. 119 Ž1997. 3365. w10x S. Roth, H. Bleier, Adv. Phys. 36 Ž1987. 2385. w11x A.J. Heeger, S. Kivelson, J.R. Schrieffer, W.-P. Su, Rev. Mod. Phys. 60 Ž1988. 781. w12x T. Bally, K. Roth, W. Tang, R.R. Schrock, K. Knoll, L.Y. Park, J. Am. Chem. Soc. 114 Ž1992. 2440. w13x J. Lafferty, A. Roach, R.S. Sinclair, T.G. Truscott, E.J. Land, J. Chem. Soc., Faraday Trans. 1 Ž73. Ž1977. 416. w14x J.A. Jeevarajan, C.C. Wei, A.S. Jeevarajan, L.D. Kispert, J. Phys. Chem. 100 Ž1996. 5637. w15x G. Gao, C.C. Wei, A.S. Jeevarajan, L.D. Kispert, J. Phys. Chem. 100 Ž1996. 5362. w16x A.S. Jeevarajan, L.D. Kispert, G. Chumanov, C. Zhou, T.M. Cotton, Chem. Phys. Lett. 259 Ž1996. 515. w17x J.-Y. Kim, Y. Furukawa, M. Tasumi, Chem. Phys. Lett. 276 Ž1997. 418. w18x S. Krawczyk, Chem. Phys. 230 Ž1998. 297. w19x W. Liptay, B. Dumbacher, H. Weisenberger, Z. Naturforsch. 23a Ž1968. 1601. w20x W. Liptay, Ber. Bunsenges. Phys. Chem. 80 Ž1976. 207. w21x S. Krawczyk, A. Daniluk, Chem. Phys. Lett. 236 Ž1995. 431. w22x Y. Kawashima, K. Nakayama, H. Nakano, K. Hirao, Chem. Phys. Lett. 267 Ž1997. 82.
Chemical Physics Letters 294 Ž1998. 351–356
Excited states and electrochromism of radical cation of the carotenoid astaxanthin Stanisław Krawczyk
)
Institute of Physics, Maria Curie-Skłodowska UniÕersity, pl. M.C. Skłodowskiej 1, 20-031 Lublin, Poland Received 5 June 1998
Abstract Radical cations of the carotenoid astaxanthin were generated by chemical oxidation with FeŽCl. 3 , and their absorption and electroabsorption ŽStark. spectra at temperatures about 150 K were recorded in the spectral range from 5900 to 26000 cmy1 Ž380 to 1700 nm., covering two absorptive electronic transitions from D 0 Žground. to D1 and D 2 excited states. The changes in static polarizability are negative and equal y40 " 10 A3 for D 0 ™ D 1 and y105 " 15 A3 for D 0 ™ D 2 , pointing that dominant contribution to polarizabilities results from the coupling of D 1 and D 2 with the ground state. An approximate localization of the next excited state with ground-state parity is estimated based on arguments from perturbation theory. q 1998 Published by Elsevier Science B.V. All rights reserved.
1. Introduction Carotenoids are a class of substituted linear conjugated hydrocarbons Žpolyenes. that gained significant interest in different areas of molecular spectroscopy, concerning not only the properties of neutral carotenoid molecules but also of their ions. This interest stems mainly from the significant role of carotenoids in photosynthesis, where they serve as light-harvesting pigments and photoprotectors w1,2x. Normally, carotenoids function in these two roles in the form of neutral molecules. However, several years ago, the formation and accumulation of bcarotene radical cations in Photosystem II reaction centers, induced by photoaccumulated primary electron donor radical P680 Øq was detected w3,4x. Also the nonradiative energy transfer from carotenoid to )
E-mail: [email protected]
chlorophyll is thought to proceed through the electron exchange mechanism w5x, which detailed description requires the knowledge of electronic states of carotenoid ions w6x. Carotenoids serve also as model systems in the search for strongly nonlinear optical materials w7,8x, which leads to deeper understanding of the electronic structure of conjugated systems and offers a prospect of practical applications. Here, the modification of bond order alternation is the factor of primary importance and this is achieved by polarizing the polyene chain by electron donor and acceptor substituents covalently bound at the chain ends w9x. The third area of research where carotenoids and polyenes serve as model molecular structures comprises the studies on the mechanism of electrical conductivity in conjugated polymers, especially in polyacetylene w10,11x. In the latter case, the primary species leading to electronic conduction are the radical cation or radical
0009-2614r98r$ - see front matter q 1998 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 8 5 7 - 4
352
S. Krawczykr Chemical Physics Letters 294 (1998) 351–356
anion states Žpolarons. in conjugated ŽCH. x chains. The electronic states pertinent to these species appear in absorption spectra in the IR range, and one of the very few ways of their spectroscopic identification is the comparison with spectra of radical cations of shorter conjugated chains w12x. However, due to the low chemical stability of radical cations, only the absorption spectra, sometimes missing the lowest-energy transition have been available for a long time w13x. Attempts to stabilize radical cations produced by oxidation of polyenes with FeCl 3 or SbCl 5 were reported to lead directly to dications w12x. Only recently the preparation of relatively stable carotenoid cation radicals and dications by chemical oxidation or electrolysis in CH 2 Cl 2 at y708C was reported w14,15x, making these species available also for further spectroscopic characterization by resonance Raman spectroscopy w16x. Quite similar to this is the development of studies on polyene anions, where pure absorption and resonance Raman spectra are becoming available w17x. In a previous paper w18x, we described the formation and characterized the absorption and electroabsorption spectra of radical cation and dication forms of two carotenoids canthaxanthin and astaxanthin in fluorinated alcohols at low temperature. Only the second Žhigher-energy. near-infrared electronic transition in radical cation was examined, revealing a very conspicuous case of negative change in molecular polarizability in this transition. Since the polarizability depends on the energetic ordering of states and on transition moments, this source of information is wider exploited in this work, both experimentally and with some reference to theory.
2. Materials and methods Puriss. grade 2,2,2-trifluoroethanol ŽFET. and anhydrous FeCl 3 were obtained from Fluka and 1,1,1,3,3,3-hexafluoroisopropanol ŽHFP . from ABCR. Crystalline all-trans astaxanthin Ž3,3X-dihydroxy-b,b-carotene-4,4X-dione. was a gift from Professor George Britton of Liverpool University. Since astaxanthin is 3,3X-dihydroxy derivative of canthaxanthin, it is spectroscopically identical with the latter, but its cations are chemically more stable w18x. Absorption and electroabsorption spectra were
recorded with a home-built spectrometer equipped with either a silicon photodiode or with InGaAs photodiode ŽHamamatsu G5125. as light detector, which allowed for recording the spectra between 380 and 1700 nm. Quadratic electroabsorption Žsecondorder Stark effect. was measured by applying a 920 Hz sinusoidal voltage to the thin Ž0.1 mm. sample in frozen glassy solvent, and detecting the electric field-induced modulation of transmitted light intensity with the lock-in amplifier at the second harmonic frequency, simultaneously with the DC transmission. According to theory w19,20x, for molecules in a solution the changes of absorbance due to an external electric field with intensity F are related to the absorption spectrum AŽ Õ . through three additive contributions proportional to the zeroth, first and second derivatives of absorption:
ž
D A s aŽ 0 . A q aŽ 1 . Õ
d Ž ArÕ . dÕ
2
=F , Ž1.
q aŽ 2 . Õ
d 2 Ž ArÕ . dÕ 2
/ Ž 1.
Ž2.
with a and a proportional, respectively, to the molecular polarizability change D a and to the square of permanent dipole moment change D m , and aŽ0. reflecting the transition moment polarizability. This formula was used in the least-squares procedure for fitting the recorded D A spectra. 3. Results and discussion The near-infrared absorption spectrum of the radical cation of astaxanthin in HFP:FETs 2:1 at low temperature is shown in Fig. 1. The two bands at 7810 cmy1 Ž1280 nm. and at 12000 cmy1 Ž833 nm. correspond to D 0 ™ D 1 and D 0 ™ D 2 electronic transitions between the doublet states D 0 Žground. and D 1 and D 2 Žexcited., with relative intensities similar to the spectra of radical cations of canthaxanthin w14x and a ,v-diŽtert-butyl.-polyenes w12x. Their optical and electrooptical characteristics are summarized in Table 1. Both these bands are shifted toward higher energy compared to the absorption bands of canthaxanthin radical cation in CH 2 Cl 2 at y708C, which are at 7630 cmy1 Ž1310 nm. and 11270 cmy1 Ž887 nm. w14x. It has been shown in our previous study w18x that the ; 700 cmy1 upshift of the D 0 ™ D 2
S. Krawczykr Chemical Physics Letters 294 (1998) 351–356
353
Fig. 1. Absorption spectrum of astaxanthin cation radical in HFP:FET s1:1 at temperature 152 K.
transition in canthaxanthin radical cation results from solvatochromism. Similar explanation is applicable here to the D 0 ™ D 1 transition. The Bayliss shifts of D 0 ™ D 1 and D 0 ™ D 2 bands between the two solvents should fulfil the relation: D n 01
s
D n 02
< M01 < 2 < M02 < 2
,
Ž 2.
where Mi k denote dipole transition moments. With D n 02 s 700 cmy1 for D 0 ™ D 2 and the values of transition moments taken from Table 1 one obtains D n 01 s 150 cmy1 . This in good agreement with the experimental 180 cmy1 difference of D 0 ™ D 1 transition energies in CH 2 Cl 2 and in HFPrFET. The electroabsorption spectra of both transitions are presented in Figs. 2 and 3. The shapes of both electroabsorption spectra were found to be independent of the angle between the electric vector of light and the applied electric field F; this confirms, to the accuracy of our experiments, that the spectra recorded result from two similarly polarized and energetically well-separated electronic transitions in the spectral region investigated. For both transitions the main polarizability axes were found to be essentially colinear with transition moments.
Fig. 2. Absorption ŽA. and electroabsorption ŽB. spectra of astaxanthin cation radical, corresponding to the D 0 ™D 2 transition. In panel ŽB., the square points are experimental data obtained with r.m.s. electric field strength F s95000 Vrcm, and the thick continuous line is the fitting curve. Fit components proportional to the absorption spectrum, its first and second derivatives are shown with the continuous, dashed and dotted lines, respectively.
Good fits to electroabsorption spectrum within the D 0 ™ D 2 transition obtainable with only the firstand second-derivative combination, such as the one presented in the previous paper w18x, can be slightly improved by adding a small negative term proportional to the absorption intensity Žcf. Table 1.. The main feature of fits to D A spectra corresponding to both electronic transitions is the contribution from the first derivative of absorption but with negative sign, unambiguously pointing to the electric field-in-
Table 1 Spectroscopic characteristics of the two electronic transitions in astaxanthin cation radical Electronic transition
Wavenumber Žcmy1 .
Transition dipole ŽDebye.
Da ˚ 3. ŽA
Dm ŽDebye.
D 0 ™ D1 D0 ™ D2
7810 12000
7.45 16.0
y40 " 10 y105 " 15
1.8 " 0.3 1.5 " 0.2
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S. Krawczykr Chemical Physics Letters 294 (1998) 351–356
Fig. 3. Absorption ŽA. and electroabsorption ŽB. spectra of astaxanthin cation radical, corresponding to the D 0 ™ D1 transition. Electric field strength F s107000 Vrcm. The data in panel ŽB. are presented analogously to Fig. 2B.
duced ‘repulsion’ of D 1 and D 2 states toward higher energies from the ground state ŽD 0 .. Fig. 2B presents the experimental D A spectrum for D 0 ™ D 2 and the fit with three components. Although the fit is very good within the Ž0–0. vibronic transition in the range 11000–12500 cmy1 , it exhibits systematic deviations from the experimental points in the 13000– 14000 cmy1 region covered by the Ž0–1. vibronic band. The sign of these deviations on both sides of the interception point at ; 13400 cmy1 , very close to the Ž0–1. band center, indicates a smaller absolute value of negative polarizability change in this vibronic transition than needed to fit the entire D 0 ™ D 2 spectrum. Similar but more remarkably expressed in this inhomogeneity in the D 0 ™ D 1 transition, where also the vibronic activity is slightly more extensive. An inspection into the absorption spectrum in Figs. 1 and 3A shows that the Ž0–0. band centered at 7800 cmy1 is unsymmetrical. The second derivative points
to the presence of a vibronic transition about 500 cmy1 toward higher wavenumbers. It accompanies also the vibronic transitions comprising the much broader band about 1400 cmy1 from Ž0–0.. The fitting curve in Fig. 3B is based on least-squares minimization procedure in the range 6500–8500 cmy1 . The apparent deviations from experimental data on the high-energy side show that, due to differences in electrooptical parameters of individual vibronic transitions, a single set of electrooptical parameters is not sufficient to describe absorption changes within the entire electronic transition D 0 ™ D 1. This observation points to a significant variability of electrooptical parameters along the spectrum, depending on the vibrational state Žand possibly on the quantum number. reached on electronic excitation. Similar observations pointing to vibrational dependence of electrooptical parameters were already reported for neutral carotenoids, both symmetrical w21x and donor–acceptor substituted w9x. This variability can make the electrooptical parameters derived from the fits somewhat ambiguous, but in practice this uncertainty affects mainly the relative contribution of the zeroth-and the second-derivative terms, while the first-derivative term, related to polarizability, varies by - 10% and thus remains almost independent of the fitting range and number of components. A remarkable feature of the polarizability changes for both electronic transitions are their negative values. According to the second-order perturbation theory w19,20x, the one-dimensional polarizability of a given electronic state is represented by the expression:
ai s 2 Ý j
<² i < m < j :< 2 E j y Ei
,
Ž 3.
where m is the dipole moment operator. It follows from Eq. Ž3. that the polarizability of the ground state is always positive and that the negative polarizability Žequivalent to an increase of energy in an electric field. is possible for D 1 and D 2 if their electric field-induced interaction with higher-energy states is weaker than the interaction with the ground state. The considerations in the following will be simplified by considering the D 1 and D 2 polarizabilities as a result of their electric field-induced coupling
S. Krawczykr Chemical Physics Letters 294 (1998) 351–356
with the ground state D 0 and with only one additional excited state Dx , which must be of the ground-state symmetry in order to have a nonzero dipole matrix elements Žtransition moments. with D 1 and D 2 . These symmetry requirements limit the number of relevant matrix elements in Eq. Ž3., which leads to the following expressions for polarizabilities of D 0 , D1 and D 2 states:
a0 s 2 a1 s 2 a2 s 2
ž ž ž
2 M01
q
E10 2 M01
2 M02
E20 q
M12x
E10
E x1
2 M02
M22x
q
E20
Ex 2
/ / /
,
Ž 4.
,
Ž 5.
.
Ž 6.
Here, Mi k is the dipole transition moment between ith and k th states, and Ei k is the energy difference: Ei k s Ei y Ek . By pairwise subtraction we obtain the expressions for experimentally determined polarizability changes in electronic transitions: D a 01 ' a 1 y a 0 s y4 D a 02 ' a 2 y a 0 s y4
2 M01
E10 2 M02
E20
q2 q2
M12x E x1 M22x Ex 2
y2 y2
2 M02
E20 2 M01
E10
, Ž 7. .
Ž 8. Substituting the experimentally determined transition moments M01 , M02 , transition energies E01 , E02 and polarizability changes D a 01 and D a 02 from Table 1 into Eqs. Ž7. and Ž8. we obtain the following values for the contributions of other excited electronic states Žrepresented by Dx . to the polarizabilities of D 1 and D2 : M12x E x1 M22x Ex 2
˚3 , s 160 A ˚3 . s 200 A
Both these values are positive and this result is very important for further band assignment. It indicates that Dx must be located at an energy higher than D 1 and D 2 in order to partly diminish the negative polarizabilities acquired by the latter two through their coupling with the ground state.
355
The physical significance of the two remaining terms in Eq. Ž1., related to field-induced modulation of absorption intensity Ž aŽ0. . and to permanent dipole moments Ž aŽ2. . is more difficult to interpret. This difficulty concerns first of all a reasonable explanation of non-negligible values of D m Žcf. Table 1. in this symmetrical molecule. Usually, it is tentatively ascribed to the polarization due to statistical fields generated by solvent molecules. This interpretation does not agree with experimental observations on neutral carotenoids w21x and the origin of the effects leading to the third term in Eq. Ž1. still await explanation. The changes in absorption intensities reflected by the coefficient aŽ0. in Eq. Ž1. can be calculated by taking into account the electric field-induced mixing of states of different symmetry Žcalculations not quoted here.. However, the calculated values of aŽ0. for both electronic transitions differ from experimental results by several times. Different values for the energy of the state Dx must be assumed to quantitatively account for the two experimental values of aŽ0. and for the lack of observable intensity of D 0 ™ Dx transition. Remarkably, however, the required values of D 0 ™ Dx transition energies range from 14000 cmy1 to 21000 cmy1 , i.e. above D 1 and D 2 , in agreement with the conclusion concerning polarizabilities Žvide supra.. According to qualitative considerations based on simple MO theory w12x the states D 1 and D 2 result from configuration mixing and are, respectively, the sum and the difference of two singly-excited configurations corresponding to the nearly isoenergetic promotions of an electron from HOMOy 1 to HOMO and from HOMO to HOMOq 1. This picture explains the large intensity of the D 0 ™ D 2 transition and the weakness of D 0 ™ D 1 transition as due to, respectively, summing up and partial cancellation of two components in the dipole transition moments. In the radical cation of astaxanthin, D 0 , D 1 and D 2 are, respectively, the 12A u , 12 B g and 2 2 B g states. Recent theoretical study on radical cations of polyene series from butadiene to decapentaene w22x, consistent with this picture, revealed the more complicated structure of D 1 and D 2 resulting from more extensive CI mixing, and predicted a next excited state above D 2 to be of ground-state symmetry Ži.e., 2 2A u in astaxanthin radical cation.. Thus, this 2 2A u
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state potentially could be the Dx state with considerable positive contribution to D a 01 and D a 02 . However, the trend apparent in the calculations w22x indicates that 2 2A u could go lower in energy, below D 2 in radical cations of polyenes significantly longer than decapentaene, and in this respect 2 2A u could be an analogue of the low-energy 2 1A g state in neutral polyenes and carotenoids, which remains invisible in single-photon absorption spectra. In the case of the presence of 2 2A u between closely lying D 1 and D 2 , the transition moments between 2 2A u and D 1 and D 2 shall be very small for the field-induced intensity borrowing by 2 2A u to be undetectable. However, it cannot be excluded that the remarkable variability of electrooptical parameters within the D 0 ™ D 1 absorption band are due to just this effect. The other possibility is the localization of 2 2A u below D 1 , which makes 2 2A u the lowest excited state. In these two cases, the excited state Dx that provides important components to the polarizabilities of D 1 and D 2 shall be sought as another 2A u state at higher energy. An argument concerning the ordering of electronic states in polyene radical cations seems to emerge from the well-known fact that the electronic structure and spectra of polyene cations and anions are closely analogous w12,17x. The strong fluorescence of radical anions of tetradesmethyl-b-carotene w17x, with the double bond number Ž11. similar to that of astaxanthin examined here Ž13., suggests that the lowering of the 2 2A u state is not the case in radical anion and possibly not the case also in radical cations, and Dx located above D 1 and D 2 could be identified as 2 2A u . Fluorescence studies seem to be decisive in solving this question.
Acknowledgements The author wishes to thank Professor George Britton of Liverpool University for the sample of
astaxanthin. This work was supported by grant No. 6P04A 044 11 from the Polish Committee for Scientific Research.
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