- Email: [email protected]
Electroabsorption spectra of carotenoid isomers: Conformational modulation of polarizability vs. induced dipole moments
Chemical Physics 326 (2006) 465–470 www.elsevier.com/locate/chemphys
Electroabsorption spectra of carotenoid isomers: Conformational modulation of polarizability vs. induced dipole moments Stanisław Krawczyk *, Beata Jazurek, Rafał Luchowski, Dariusz Wia˛cek Institute of Physics, Maria Curie-Skłodowska University, Pl. M. Curie-Sklodowskiej 1, 20-031 Lublin, Poland Received 18 November 2005; accepted 6 March 2006 Available online 10 March 2006
Abstract Electroabsorption spectra of all-trans, 13-cis and 15-cis isomers of carotenoids violaxanthin and b-carotene frozen in organic solvents were analysed in terms of changes in permanent dipole moment, Dl, and in the linear polarizability, Da, on electronic excitation. The spectral range investigated covered the two carotenoid absorption bands in the VIS and UV, known to originate from differently oriented transition dipole moments. In contrast with the collinearity of the apparent Dl with Da in the lowest-energy allowed (VIS) transition þ þ 1A g ! 1Bu , the axis of the largest polarizability change in the UV transition 1Ag ! 1Ag (‘‘cis band’’) was found to make a large angle with the transition moment, while the direction of Dl appears to be much closer to it. These data support the view that Dl’s inferred from electrochromic spectra of carotenoids are apparent and are not induced by the local matrix field in the solvent cavity, but merely result from conformational modulation of molecular polarizability. 2006 Elsevier B.V. All rights reserved. Keywords: Electroabsorption; Carotenoid; Isomer; Polarizability; Dipole moment
1. Introduction Among several classes of biological pigments, carotenoids are known mainly for their important role in the process of biological photosynthesis. This includes the absorption of light and subsequent transfer of energy to the chlorophylls toward the reaction centers, quenching of triplet excitations in chlorophylls, and deactivation of electronically excited and thus highly reactive singlet oxygen. In all these processes, the properties of carotenoid molecules in their electronically excited states are of primary importance. For instance, the isomeric 15-cis form is usually present in the reaction centers as optimized for quenching of chlorophyll triplet states [1,2], while differently perturbed all-trans forms are optimized for non-radiative energy transfer in antenna systems [3]. Thanks to the *
Corresponding author. Tel.: +48 81 537 6253; fax: +48 81 537 6191. E-mail address: [email protected] (S. Krawczyk).
0301-0104/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2006.03.006
large polarizability of their linear conjugated chains, carotenoids present in photosynthetic membranes are also sensitive indicators of the transmembrane electric field generated by photosynthetic charge transport [4–6]. The latter function is closely related to the electrochromic properties of carotenoids, i.e., the occurence of a large molecular polarizability and dipole moment differences between the electronic ground and excited states [7–11], which define their quadratic and linear electrochromic response. Besides, carotenoids are prototype molecules to study the non-linear optical properties of conjugated linear chains [12,13] and their electronic conductivity [14]. For studies in these directions the knowledge of electronic states ordering, their symmetry, transition moment vectors, electronic charge distribution and the susceptibility of these properties to structural perturbation and to external fields is of key importance. The field-induced coupling of electronic states, which is important for optical non-linearity [12,15], depends on their spatial symmetry [3].
466
S. Krawczyk et al. / Chemical Physics 326 (2006) 465–470
Among the disputable questions is still the occurence and origin of the substantial dipole moment change on electronic excitation of carotenoid molecules possessing a center of symmetry [7–9,16]. A straightforward explanation would indicate the inductive effect of the random electric fields generated by the solvent. However, the apparent dipoles do not exhibit a clear correlation with solvent polarity and instead were found to be correlated with the inhomogeneous broadening of the vibronic transitions in absorption spectra [16]. We postulated a deformationinduced polarizability modulation as the source of the observed second derivative term in carotenoid electroabsorption spectra and we have shown that such effect can quantitatively account for a substantial part of the observed apparent difference dipole. Here, we investigate the electrooptical parameters of geometrical isomers of carotenoids bcarotene and violaxanthin with, respectively, 11 and 9 double bonds (see Scheme 1), in the spectral range comprising two allowed lowest energy transitions, and analyse their directional properties with the aim of establishing the origin of their apparent dipole moments. 2. Methods
mers were identified according to their order in the chromatogram, UV/VIS absorbance ratio, and by recording their resonance Raman spectra in the 900–1600 cm1 range under 476.5 nm (violaxanthin) or 488 nm (b-carotene) excitation at temperature 150 K and comparing them with literature data for b-carotene [17–19]. The low temperature absorption and the field-induced electroabsorption spectra were measured for 0.08 mm thick samples between transparent ITO electrodes on glass or quartz plates, placed in a flow cryostat and subjected to 1000 V sinusoidal voltage, as described previously [16]. The spectrum of electric field-induced absorption change, DA(m), was recorded as a function of the wavenumber and fitted with the linear combination of the first and second derivatives of the absorption spectrum DA ¼ a1 m
d ðA=mÞ d 2 ðA=mÞ þ a2 m dm dm2
ð1Þ
with the fit coefficients given by the theory [22,23] in the form: 2 ðfF Þ 5 3 1 2 TrðDaÞ þ ð3 cos v 1Þ pðDaÞp TrðDaÞ a1 ¼ 2 2 15hc 2 ð2Þ
All-trans b-carotene purchased from Sigma was isomerized with iodine under illumination, or thermally by keeping a concentrated solution in benzene for 1 h at 100 C. The isomers were separated by HPLC using a 10 · 400 mm lime column, following the procedures described in the literature [17–19]. Violaxanthin was isolated from pansy flowers (Viola tricolor) and, after saponification, the isomers were separated by HPLC on a C-30 column [20,21]. The isomers were stored frozen at 35 C in benzene solution. The iso-
2
a2 ¼
ðfF Þ jDlj2 5 þ ð3 cos2 v 1Þð3 cos2 d 1Þ 30h2 c2
ð3Þ
In Eqs. (1) and (2), v is the experimentally variable angle between the electric vector of light and the vector of applied electric field F taken together with the local field factor f, d is the angle between Dl and the transition moment, and p is a unit vector in the direction of the dipole transition moment. As is appropriate for carotenoids [7,11,16]
Scheme 1. Molecular structure of b-carotene in the all-trans and 15-cis configurations. In the molecule of violaxanthin, the terminal double bonds are not present.
S. Krawczyk et al. / Chemical Physics 326 (2006) 465–470
467
and for many other chromophores, the fitting term proportional to the absorption spectrum itself is negligible and has been neglected in Eq. (1). The data from electroabsorption measurements are insufficient to fully determine the difference polarizability tensor Da. One possibility to characterize the molecular polarizability stems from Eq. (2) by calculating Tr(Da) using a1 from experimental data at v = 54.7, i.e., at the magic angle, when the second term in Eq. (2) vanishes. Alternatively, for the lowest allowed transition in all-trans carotenoids, the tensor Da can be reduced to a scalar Da corresponding to the polarizability along a single axis directed at an angle c to the transition moment [9,16]; in such case Eq. (2) takes a simplified form: 2
a1 ¼
ðfF Þ Da 3 cos2 c 1 cos2 v þ 2 cos2 c 10hc
ð4Þ
The quantities Tr(Da) and Da have equal numerical values albeit they differ in their physical meaning. Within the assumption of a single axis of polarizability, the values of c and then Da can be determined using the linear dependence of the ratio a1(v)/a1(v = p/2) on cos2(v) which follows from Eq. (4): a1 ð vÞ 3 cos2 c 1 ¼1þ cos2 v a1 ðv ¼ p=2Þ 2 cos2 c
ð5Þ
An expression analogous to Eq. (5) holds also for a2, the coefficient at the second derivative, for which the ratio a2(v)/a2(v = p/2) has similar form with the angle d (cf. Eq. (3)) in place of c. 3. Results and discussion The absorption and electroabsorption spectra for 13-cis violaxanthin are shown in Fig. 1A and B, respectively. The two absorption bands, one in the VIS region and the other in UV (the ‘‘cis band’’ between 29,000 and 33,000 cm1) show sharp vibrational structure which is highly helpful for obtaining reliable estimates of Da and Dl and their orientations within the molecular skeleton. The next allowed electronic transition starts at 36,000 cm1 and has less resolved vibronic structure, which makes its electroabsorption signal unreliably weak for the purposes of this work (not shown here). The spectra are shown for two angles of incidence of p-polarized light onto the sample, associated with an even increase in intensity along the absorption spectrum but different effects in the VIS and UV electroabsorption signals. The difference in electroabsorption for the VIS transition is, according to Eq. (4), associated with a value of c less than the magic angle 54.7, and with c larger than the magic angle for the UV transition. The quality of the fit in the UV is illustrated in Fig. 2. Although the electroabsorption spectrum is dominated by the contribution from the first derivative which is directly related to Da, the second derivative component formally results in permanent dipole change of about 2 D (cf. Table 1) for the transition in UV, comparable to that for the VIS transition in
Fig. 1. Absorption (A) and electroabsorption (B) spectra of 13-cis isomer of violaxanthin in a glassy solvent composed of isopropanol, diethyl ether and tert-butyl ether in 1:3:3 proportions by volume. Dashed lines are for spectra with v = p/2, continuous lines are for magic angle. Sample temperature 80 K, applied field intensity F = 109,600 V/cm (r.m.s.). Insets: fragments of absorption and electroabsorption spectra of 15-cis bcarotene in ethylbenzene/toluene 1:1 (v/v) at 100 K and 100,000 V/cm.
both cis isomers and in the all-trans violaxanthin. For bcarotene, smaller values of Dl occur in the VIS transition in both isomers, but a large Dl 5D appears in the UV, like for the VIS transition in the all-trans isomer [9,16]. In the spectra of b-carotene, the cis band lies more close to the higher vibronic bands of the VIS transition; due to this, the electroabsorption signal above 25,000 cm1 may contain a contribution from the lower-energy transition as indicated by the wavy features at 25,000 cm1 in Fig. 1B (inset). This interference, although weak throughout the whole cis band of b-carotene, may have noticeable effect on the fits, perturbing their angular dependence and making the estimated angles more similar to those for the VIS transition. This spectral overlap may be the cause for generally larger values of d for UV transition in cis isomers of b-carotene than in cis violaxanthin (cf. Table 1). The orientations of the vector Dl and the polarizability axes with respect to the transition moment were obtained from the slopes of the linear plots based on Eq. (5) (and its analogue for the second derivative coefficient a2, see Section 2), analogous to those for 13-cis isomer of violaxanthin shown in Fig. 3. The slope of such plot can vary between 2 and 0.5 for the values of angle c or d ranging, respectively, from zero to p/2, and is zero for the magic angle 54.7. As can be seen in Fig. 3, only for the fit coefficient a1 in the UV transition is the slope negative, meaning a large value of the angle c. In contrast with this
468
S. Krawczyk et al. / Chemical Physics 326 (2006) 465–470
Fig. 3. Angular dependence of the fit coefficients a1 and a2 (cf. Eq. (1)) for 13-cis violaxanthin.
Fig. 2. Fits to electroabsorption spectra of 13-cis violaxanthin in the region of the cis transition. A – absorption, B – electroabsorption spectrum (points) and the fit (line), C – fit components (scaled): the first (continuous line) and second (dashed line) derivatives of absorption.
Table 1 Summary of electroabsorption data for the all-trans and cis isomers of violaxanthin and b-carotene Violaxanthin a
VIS ˚ 3) Da (A c () Dl (D) d ()
Carotene
all-trans
13-cis
15-cis
all-transa
13-cis
15-cis
1050 17 3.3 17
1000 10 3.6 11
950 15 3.6 11
1280 18 5.2 17
950 14 3.50 45
770 17 3.75 19
750 68 1.95 (12 ± 8)
700 74 2.25 (10 ± 10)
– – – –
540 70 4.9 <54.7
410 69 6.1 <30
UV (cis band) ˚ 3) – Da (A c () – Dl (D) – d () –
The sets denoted by VIS and UV refer to the lowest allowed transition þ (1A g ! 1Bu ) and the transition that becomes allowed in cis-isomers (1Ag ! 1Aþ g ) [1,19]. a New data merged with those from Ref. [9].
remains the coefficient a2 which slope indicates the direction of Dl close to the transition moment. As mentioned above, the assumption underlying Eqs. (4) and (5) applies well to the VIS transition in all-trans carotenoids and is justified by the clear angular dependence of the fit parameter a1 in Eq. (1). This would not be the case if there were two or three perpendicular components of Da with comparable values. The ratio a1(v)/a1(p/2) plotted vs. cos2(v) has a large positive slope which translates into a small angle c 15o between the Da axis and the transition moment. This result is intuitively clear in the light of the very elongated shape of the molecule and seems to indicate the maximum polarizability exactly along the axis of the molecule, consistently with the oblique transition moment orientation at closely similar angle 15 to the molecular axis in unsubstituted polyenes [24,25]. It is well known from crystallography [26] and from computational studies [27] that the presence of methyl side groups in carotenoids makes the conjugated chain significantly s-shaped. Our results indicate that this curvature does not introduce significant off-axis component into the polarizability tensor Da of carotenoids. In the UV transition associated with the 15-cis or 13-cis configurations, the transition moment is oriented more or less exactly along the Z-axis shown in Scheme 1. Within the framework of a single polarizability axis, a large angle c around 70 (see Table 1) between the transition moment and the axis of Da is obtained. However, the assumption which is well substantiated for all-trans form, may be not well fulfilled for isomers and one can expect substantial polarizability components along the Z-axis as well. Assuming the main axes of polarizability tensor to be along the Y and Z directions in the coordinate system shown in Scheme 1, for the two non-zero polarizability components Daz and Day we obtain using Eq. (2): a1 ð vÞ 2Daz Day ¼1þ cos2 v a1 ðv ¼ p=2Þ 2Day þ Daz
ð6Þ
S. Krawczyk et al. / Chemical Physics 326 (2006) 465–470
Eq. (6) applied to the plots depicted in Fig. 3 gives the ratio Daz/Day=0.19 ± 0.07, meaning that Day is larger than Daz about five times. Thus, in both approximations the estimates of the tensor Da confirm a much larger component perpendicular to the transition moment in the UV than the component parallel to it. This has an important implication as to the origin of the second derivative component in the electroabsorption spectra and to the apparent change in dipole moment which formally follows from that data. If the local matrix field had more or less isotropic distribution of field intensity, then the induced Dly would make a small angle with the Y-axis; however, what is observed is just a reverse situation, as reflected by the values of the angle d for the UV transition quoted in Table 1. To check the interpretation of experimental results, we performed semiempirical calculations of singlet excited states for a model of violaxanthin chromophore consisting of nine conjugated double bonds in the all-trans and central cis configurations. Each geometry was optimized in the ground state with AM1 and 420 double-determinant singly excited configurations were used in CI. Both AM1 and INDO/S methods reproduced very well the transition energies and predicted small ground state dipole not larger than 0.5 D for the cis isomer. The dipole moment changes for the UV cis transition were 2.6 D (INDO/S) and 4.1 D (AM1), both in the Z-axis direction indicated in Scheme 1. The similarity of this result with experimental data is remarkable. It indicates that the experimental |Dl| of 2 D, which makes a small angle d with the ‘‘cis’’ transition moment in violaxanthin, results merely from the real charge redistribution in the Z direction perpendicular to the axis of largest polarizability (Y). Small experimental value of d for cis isomers of violaxanthin indicates that there may be only small induced dipole moments along the Y-axis. For both carotenoids, a decrease in polarizability is observed for the VIS transition in the order alltrans > 13-cis > 15-cis, and is similar for the UV transition (cf. Table 1). These differences reflect most probably the changes in molecular size down the long axis. Also the calculations for the all-trans and central-cis isomers of violaxanthin show a 20% decrease in the oscillator strength on isomerization. This result explains the smaller values of Da in cis isomers, since the polarizability components can be expressed as transition moment products [22,23]. Due to less resolved vibrational structure in absorption spectra of b-carotene, the directions ascribed to Dl for its isomers are less clear than for violaxanthin. However, this weaker accuracy does not invalidate the main conclusions of this work which, supported more strongly by the results for violaxanthin isomers, show that: (i) the orientation of the axis of the largest polarizability in both cis isomers of both pigments examined is at an angle with respect to the transition moment of the ‘‘cis transition’’ clearly larger than the magic angle and (ii) the directions of dipole moment differences make an angle with the transition moment less than the magic angle. Also, despite of the smaller value of polarizability change in the UV transition in the isomers of b-car-
469
otene than in violaxanthin, the difference dipoles observed for b-carotene are significantly larger (5–6 D) than in for violaxanthin (2 D). It would be difficult to reconcile such values of difference dipoles and their directional properties with the inductive effect of the matrix field on molecular polarizability. It is also remarkable that the apparent Dl’s in b-carotene isomers are 2.5 times larger than in violaxanthin, although the conjugated chains of these molecules differ by only two double bonds. Irrespective of the origin of Dl in carotenoids, these experimental data are in odds with the picture assuming the inductive effect of the matrix electric field as the basic source of the second derivative component in their electroabsorption spectra. The isomerization has stronger effect on molecular polarizability in b-carotene than in violaxanthin. This difference is seen in both the VIS and the UV transitions, for which a 10% decrease occurs on going from all-trans to 15-cis form in violaxanthin, compared to 40% decrease in b-carotene. Also the polarizability differences between UV and VIS transitions are larger in b-carotene. This set of data indicates a generally larger susceptibility of electrooptical parameters to structural distortions in b-carotene than in violaxanthin. This and other features mentioned above are consistent with the effect of polarizability modulation by molecular distortions postulated in our previous work for the VIS transition in all-trans carotenoids [16]. As a matter of fact, extending conclusions based on the þ UV transition 1A g ! 1Ag to the one in the VIS range þ 1Ag ! 1Bu [1,28] may be limited by the fact that the final states in both transitions are of different symmetry and thus the transition energies and respective polarizabilities may differ in their sensitivity to the distortions of molecular geometry. However, the intensity distribution among vibrational subbands in the VIS and UV absorption spectra of cis violaxanthin indicates involvement of similar frequencies as in the all-trans isomer, albeit with slightly different Franck–Condon factors, which suggests similar coupling of the electron density redistribution with the changes in molecular geometry. In conclusion, our experimental observations point to the lack of the inductive effect of the matrix field in carotenoids. Their analysis indicates features of electrooptical parameters compatible with the conformational modulation of polarizability correlated with the inhomogeneous broadening in carotenoid spectra [16], proposed as the main mechanism leading to the second derivative term in electroabsorption spectra of carotenoids. Acknowledgement This work was supported by research funds from KBN administered by UMCS. References [1] R. Fujii, K. Furuichi, J.-P. Zhang, H. Nagae, H. Hashimoto, Y. Koyama, J. Phys. Chem. A 106 (2002) 2410.
470
S. Krawczyk et al. / Chemical Physics 326 (2006) 465–470
[2] Y. Koyama, J. Photochem. Photobiol. B: Biology 9 (1991) 265. [3] T. Ritz, A. Damjanovic´, K. Schulten, J.-P. Zhang, Y. Koyama, Photosynth. Res. 66 (2000) 125. [4] M.G. Goodwin, J.B. Jackson, Biochim. Biophys. Acta 1144 (1993) 191. [5] K. Matsuura, K. Shimada, Biochim. Biophys. Acta 1140 (1993) 293. [6] M. Haumann, W. Junge, in: D.R. Ort, C.F. Yocum (Eds.), Advances in Photosynthesis, Kluwer, 1996, p. 165. [7] D.S. Gottfried, M.A. Steffen, S.G. Boxer, Biochim. Biophys. Acta 1059 (1991) 76. [8] D.S. Gottfried, M.A. Steffen, S.G. Boxer, Science 251 (1991) 662. [9] S. Krawczyk, A. Daniluk, Chem. Phys. Lett. 236 (1995) 431. [10] S. Krawczyk, Z. Krupa, W. Maksymiec, Biochim. Biophys. Acta 1143 (1993) 273. [11] K. Yanagi, A.T. Gardiner, R.J. Cogdell, H. Hashimoto, Phys. Rev. B71 (2005), art. No. 195118. [12] J.B.v. Beek, F. Kajzar, A.C. Albrecht, J. Chem. Phys. 95 (1991) 6400. [13] S.R. Marder, W.E. Torruellas, M. Blanchard-Desce, V. Ricci, G.I. Stegeman, S. Gilmour, J.-L. Bredas, J. Li, G.U. Bublitz, S.G. Boxer, Science 276 (1997) 1233. [14] G.K. Ramachandran, J.K. Tomfohr, J. Li, O.F. Sankey, X. Zarate, A. Primak, Y. Terazono, T.A. Moore, A.L. Moore, D. Gust, L.A. Nagahara, S.M. Lindsay, J. Phys. Chem. B 107 (2003) 6162.
[15] F. Rohlfing, D.D.C. Bradley, Chem. Phys. Lett. 277 (1997) 406. [16] S. Krawczyk, D. Olszo´wka, Chem. Phys. 265 (2001) 335. [17] K. Tsukida, K. Saiki, T. Takii, Y. Koyama, J. Chromatogr. 245 (1982) 359. [18] Y. Koyama, I. Takatsuka, M. Nakata, M. Tasumi, J. Raman Spectr. 19 (1988) 37. [19] R. Fujii, C.-H. Chen, T. Mizoguchi, Y. Koyama, Spectrochim. Acta A 54 (1998) 727. [20] D. Niedz´wiedzki, Z. Krupa, W.I. Gruszecki, J. Photochem. Photobiol. B: Biol. 78 (2005) 109. [21] D. Niedz´wiedzki, W.I. Gruszecki, Colloids Surf. B: Biointerf. 28 (2003) 27. [22] W. Liptay, Z. Naturforsch. 20 (1965) 272. [23] W. Liptay, in: C. Lim (Ed.), Excited States, vol. 1, Academic Press, 1974, p. 129. [24] R.R. Birge, M.Z. Zgierski, L. Serrano-Andres, B.S. Hudson, J. Phys. Chem. A 103 (1999) 2251. [25] Q. Shang, X. Dou, B.S. Hudson, Nature 352 (1991) 703. [26] C. Sterling, Acta Cryst. 17 (1964) 1224. [27] R.E. Connors, D.S. Burns, R. Farhoosh, H.A. Frank, J. Phys. Chem. 97 (1993) 9351. [28] Y. Koyama, M. Kuki, P.O. Andersson, T. Gillbro, Photochem. Photobiol. 63 (1996) 243.
Electroabsorption spectra of carotenoid isomers: Conformational modulation of polarizability vs. induced dipole moments Stanisław Krawczyk *, Beata Jazurek, Rafał Luchowski, Dariusz Wia˛cek Institute of Physics, Maria Curie-Skłodowska University, Pl. M. Curie-Sklodowskiej 1, 20-031 Lublin, Poland Received 18 November 2005; accepted 6 March 2006 Available online 10 March 2006
Abstract Electroabsorption spectra of all-trans, 13-cis and 15-cis isomers of carotenoids violaxanthin and b-carotene frozen in organic solvents were analysed in terms of changes in permanent dipole moment, Dl, and in the linear polarizability, Da, on electronic excitation. The spectral range investigated covered the two carotenoid absorption bands in the VIS and UV, known to originate from differently oriented transition dipole moments. In contrast with the collinearity of the apparent Dl with Da in the lowest-energy allowed (VIS) transition þ þ 1A g ! 1Bu , the axis of the largest polarizability change in the UV transition 1Ag ! 1Ag (‘‘cis band’’) was found to make a large angle with the transition moment, while the direction of Dl appears to be much closer to it. These data support the view that Dl’s inferred from electrochromic spectra of carotenoids are apparent and are not induced by the local matrix field in the solvent cavity, but merely result from conformational modulation of molecular polarizability. 2006 Elsevier B.V. All rights reserved. Keywords: Electroabsorption; Carotenoid; Isomer; Polarizability; Dipole moment
1. Introduction Among several classes of biological pigments, carotenoids are known mainly for their important role in the process of biological photosynthesis. This includes the absorption of light and subsequent transfer of energy to the chlorophylls toward the reaction centers, quenching of triplet excitations in chlorophylls, and deactivation of electronically excited and thus highly reactive singlet oxygen. In all these processes, the properties of carotenoid molecules in their electronically excited states are of primary importance. For instance, the isomeric 15-cis form is usually present in the reaction centers as optimized for quenching of chlorophyll triplet states [1,2], while differently perturbed all-trans forms are optimized for non-radiative energy transfer in antenna systems [3]. Thanks to the *
Corresponding author. Tel.: +48 81 537 6253; fax: +48 81 537 6191. E-mail address: [email protected] (S. Krawczyk).
0301-0104/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2006.03.006
large polarizability of their linear conjugated chains, carotenoids present in photosynthetic membranes are also sensitive indicators of the transmembrane electric field generated by photosynthetic charge transport [4–6]. The latter function is closely related to the electrochromic properties of carotenoids, i.e., the occurence of a large molecular polarizability and dipole moment differences between the electronic ground and excited states [7–11], which define their quadratic and linear electrochromic response. Besides, carotenoids are prototype molecules to study the non-linear optical properties of conjugated linear chains [12,13] and their electronic conductivity [14]. For studies in these directions the knowledge of electronic states ordering, their symmetry, transition moment vectors, electronic charge distribution and the susceptibility of these properties to structural perturbation and to external fields is of key importance. The field-induced coupling of electronic states, which is important for optical non-linearity [12,15], depends on their spatial symmetry [3].
466
S. Krawczyk et al. / Chemical Physics 326 (2006) 465–470
Among the disputable questions is still the occurence and origin of the substantial dipole moment change on electronic excitation of carotenoid molecules possessing a center of symmetry [7–9,16]. A straightforward explanation would indicate the inductive effect of the random electric fields generated by the solvent. However, the apparent dipoles do not exhibit a clear correlation with solvent polarity and instead were found to be correlated with the inhomogeneous broadening of the vibronic transitions in absorption spectra [16]. We postulated a deformationinduced polarizability modulation as the source of the observed second derivative term in carotenoid electroabsorption spectra and we have shown that such effect can quantitatively account for a substantial part of the observed apparent difference dipole. Here, we investigate the electrooptical parameters of geometrical isomers of carotenoids bcarotene and violaxanthin with, respectively, 11 and 9 double bonds (see Scheme 1), in the spectral range comprising two allowed lowest energy transitions, and analyse their directional properties with the aim of establishing the origin of their apparent dipole moments. 2. Methods
mers were identified according to their order in the chromatogram, UV/VIS absorbance ratio, and by recording their resonance Raman spectra in the 900–1600 cm1 range under 476.5 nm (violaxanthin) or 488 nm (b-carotene) excitation at temperature 150 K and comparing them with literature data for b-carotene [17–19]. The low temperature absorption and the field-induced electroabsorption spectra were measured for 0.08 mm thick samples between transparent ITO electrodes on glass or quartz plates, placed in a flow cryostat and subjected to 1000 V sinusoidal voltage, as described previously [16]. The spectrum of electric field-induced absorption change, DA(m), was recorded as a function of the wavenumber and fitted with the linear combination of the first and second derivatives of the absorption spectrum DA ¼ a1 m
d ðA=mÞ d 2 ðA=mÞ þ a2 m dm dm2
ð1Þ
with the fit coefficients given by the theory [22,23] in the form: 2 ðfF Þ 5 3 1 2 TrðDaÞ þ ð3 cos v 1Þ pðDaÞp TrðDaÞ a1 ¼ 2 2 15hc 2 ð2Þ
All-trans b-carotene purchased from Sigma was isomerized with iodine under illumination, or thermally by keeping a concentrated solution in benzene for 1 h at 100 C. The isomers were separated by HPLC using a 10 · 400 mm lime column, following the procedures described in the literature [17–19]. Violaxanthin was isolated from pansy flowers (Viola tricolor) and, after saponification, the isomers were separated by HPLC on a C-30 column [20,21]. The isomers were stored frozen at 35 C in benzene solution. The iso-
2
a2 ¼
ðfF Þ jDlj2 5 þ ð3 cos2 v 1Þð3 cos2 d 1Þ 30h2 c2
ð3Þ
In Eqs. (1) and (2), v is the experimentally variable angle between the electric vector of light and the vector of applied electric field F taken together with the local field factor f, d is the angle between Dl and the transition moment, and p is a unit vector in the direction of the dipole transition moment. As is appropriate for carotenoids [7,11,16]
Scheme 1. Molecular structure of b-carotene in the all-trans and 15-cis configurations. In the molecule of violaxanthin, the terminal double bonds are not present.
S. Krawczyk et al. / Chemical Physics 326 (2006) 465–470
467
and for many other chromophores, the fitting term proportional to the absorption spectrum itself is negligible and has been neglected in Eq. (1). The data from electroabsorption measurements are insufficient to fully determine the difference polarizability tensor Da. One possibility to characterize the molecular polarizability stems from Eq. (2) by calculating Tr(Da) using a1 from experimental data at v = 54.7, i.e., at the magic angle, when the second term in Eq. (2) vanishes. Alternatively, for the lowest allowed transition in all-trans carotenoids, the tensor Da can be reduced to a scalar Da corresponding to the polarizability along a single axis directed at an angle c to the transition moment [9,16]; in such case Eq. (2) takes a simplified form: 2
a1 ¼
ðfF Þ Da 3 cos2 c 1 cos2 v þ 2 cos2 c 10hc
ð4Þ
The quantities Tr(Da) and Da have equal numerical values albeit they differ in their physical meaning. Within the assumption of a single axis of polarizability, the values of c and then Da can be determined using the linear dependence of the ratio a1(v)/a1(v = p/2) on cos2(v) which follows from Eq. (4): a1 ð vÞ 3 cos2 c 1 ¼1þ cos2 v a1 ðv ¼ p=2Þ 2 cos2 c
ð5Þ
An expression analogous to Eq. (5) holds also for a2, the coefficient at the second derivative, for which the ratio a2(v)/a2(v = p/2) has similar form with the angle d (cf. Eq. (3)) in place of c. 3. Results and discussion The absorption and electroabsorption spectra for 13-cis violaxanthin are shown in Fig. 1A and B, respectively. The two absorption bands, one in the VIS region and the other in UV (the ‘‘cis band’’ between 29,000 and 33,000 cm1) show sharp vibrational structure which is highly helpful for obtaining reliable estimates of Da and Dl and their orientations within the molecular skeleton. The next allowed electronic transition starts at 36,000 cm1 and has less resolved vibronic structure, which makes its electroabsorption signal unreliably weak for the purposes of this work (not shown here). The spectra are shown for two angles of incidence of p-polarized light onto the sample, associated with an even increase in intensity along the absorption spectrum but different effects in the VIS and UV electroabsorption signals. The difference in electroabsorption for the VIS transition is, according to Eq. (4), associated with a value of c less than the magic angle 54.7, and with c larger than the magic angle for the UV transition. The quality of the fit in the UV is illustrated in Fig. 2. Although the electroabsorption spectrum is dominated by the contribution from the first derivative which is directly related to Da, the second derivative component formally results in permanent dipole change of about 2 D (cf. Table 1) for the transition in UV, comparable to that for the VIS transition in
Fig. 1. Absorption (A) and electroabsorption (B) spectra of 13-cis isomer of violaxanthin in a glassy solvent composed of isopropanol, diethyl ether and tert-butyl ether in 1:3:3 proportions by volume. Dashed lines are for spectra with v = p/2, continuous lines are for magic angle. Sample temperature 80 K, applied field intensity F = 109,600 V/cm (r.m.s.). Insets: fragments of absorption and electroabsorption spectra of 15-cis bcarotene in ethylbenzene/toluene 1:1 (v/v) at 100 K and 100,000 V/cm.
both cis isomers and in the all-trans violaxanthin. For bcarotene, smaller values of Dl occur in the VIS transition in both isomers, but a large Dl 5D appears in the UV, like for the VIS transition in the all-trans isomer [9,16]. In the spectra of b-carotene, the cis band lies more close to the higher vibronic bands of the VIS transition; due to this, the electroabsorption signal above 25,000 cm1 may contain a contribution from the lower-energy transition as indicated by the wavy features at 25,000 cm1 in Fig. 1B (inset). This interference, although weak throughout the whole cis band of b-carotene, may have noticeable effect on the fits, perturbing their angular dependence and making the estimated angles more similar to those for the VIS transition. This spectral overlap may be the cause for generally larger values of d for UV transition in cis isomers of b-carotene than in cis violaxanthin (cf. Table 1). The orientations of the vector Dl and the polarizability axes with respect to the transition moment were obtained from the slopes of the linear plots based on Eq. (5) (and its analogue for the second derivative coefficient a2, see Section 2), analogous to those for 13-cis isomer of violaxanthin shown in Fig. 3. The slope of such plot can vary between 2 and 0.5 for the values of angle c or d ranging, respectively, from zero to p/2, and is zero for the magic angle 54.7. As can be seen in Fig. 3, only for the fit coefficient a1 in the UV transition is the slope negative, meaning a large value of the angle c. In contrast with this
468
S. Krawczyk et al. / Chemical Physics 326 (2006) 465–470
Fig. 3. Angular dependence of the fit coefficients a1 and a2 (cf. Eq. (1)) for 13-cis violaxanthin.
Fig. 2. Fits to electroabsorption spectra of 13-cis violaxanthin in the region of the cis transition. A – absorption, B – electroabsorption spectrum (points) and the fit (line), C – fit components (scaled): the first (continuous line) and second (dashed line) derivatives of absorption.
Table 1 Summary of electroabsorption data for the all-trans and cis isomers of violaxanthin and b-carotene Violaxanthin a
VIS ˚ 3) Da (A c () Dl (D) d ()
Carotene
all-trans
13-cis
15-cis
all-transa
13-cis
15-cis
1050 17 3.3 17
1000 10 3.6 11
950 15 3.6 11
1280 18 5.2 17
950 14 3.50 45
770 17 3.75 19
750 68 1.95 (12 ± 8)
700 74 2.25 (10 ± 10)
– – – –
540 70 4.9 <54.7
410 69 6.1 <30
UV (cis band) ˚ 3) – Da (A c () – Dl (D) – d () –
The sets denoted by VIS and UV refer to the lowest allowed transition þ (1A g ! 1Bu ) and the transition that becomes allowed in cis-isomers (1Ag ! 1Aþ g ) [1,19]. a New data merged with those from Ref. [9].
remains the coefficient a2 which slope indicates the direction of Dl close to the transition moment. As mentioned above, the assumption underlying Eqs. (4) and (5) applies well to the VIS transition in all-trans carotenoids and is justified by the clear angular dependence of the fit parameter a1 in Eq. (1). This would not be the case if there were two or three perpendicular components of Da with comparable values. The ratio a1(v)/a1(p/2) plotted vs. cos2(v) has a large positive slope which translates into a small angle c 15o between the Da axis and the transition moment. This result is intuitively clear in the light of the very elongated shape of the molecule and seems to indicate the maximum polarizability exactly along the axis of the molecule, consistently with the oblique transition moment orientation at closely similar angle 15 to the molecular axis in unsubstituted polyenes [24,25]. It is well known from crystallography [26] and from computational studies [27] that the presence of methyl side groups in carotenoids makes the conjugated chain significantly s-shaped. Our results indicate that this curvature does not introduce significant off-axis component into the polarizability tensor Da of carotenoids. In the UV transition associated with the 15-cis or 13-cis configurations, the transition moment is oriented more or less exactly along the Z-axis shown in Scheme 1. Within the framework of a single polarizability axis, a large angle c around 70 (see Table 1) between the transition moment and the axis of Da is obtained. However, the assumption which is well substantiated for all-trans form, may be not well fulfilled for isomers and one can expect substantial polarizability components along the Z-axis as well. Assuming the main axes of polarizability tensor to be along the Y and Z directions in the coordinate system shown in Scheme 1, for the two non-zero polarizability components Daz and Day we obtain using Eq. (2): a1 ð vÞ 2Daz Day ¼1þ cos2 v a1 ðv ¼ p=2Þ 2Day þ Daz
ð6Þ
S. Krawczyk et al. / Chemical Physics 326 (2006) 465–470
Eq. (6) applied to the plots depicted in Fig. 3 gives the ratio Daz/Day=0.19 ± 0.07, meaning that Day is larger than Daz about five times. Thus, in both approximations the estimates of the tensor Da confirm a much larger component perpendicular to the transition moment in the UV than the component parallel to it. This has an important implication as to the origin of the second derivative component in the electroabsorption spectra and to the apparent change in dipole moment which formally follows from that data. If the local matrix field had more or less isotropic distribution of field intensity, then the induced Dly would make a small angle with the Y-axis; however, what is observed is just a reverse situation, as reflected by the values of the angle d for the UV transition quoted in Table 1. To check the interpretation of experimental results, we performed semiempirical calculations of singlet excited states for a model of violaxanthin chromophore consisting of nine conjugated double bonds in the all-trans and central cis configurations. Each geometry was optimized in the ground state with AM1 and 420 double-determinant singly excited configurations were used in CI. Both AM1 and INDO/S methods reproduced very well the transition energies and predicted small ground state dipole not larger than 0.5 D for the cis isomer. The dipole moment changes for the UV cis transition were 2.6 D (INDO/S) and 4.1 D (AM1), both in the Z-axis direction indicated in Scheme 1. The similarity of this result with experimental data is remarkable. It indicates that the experimental |Dl| of 2 D, which makes a small angle d with the ‘‘cis’’ transition moment in violaxanthin, results merely from the real charge redistribution in the Z direction perpendicular to the axis of largest polarizability (Y). Small experimental value of d for cis isomers of violaxanthin indicates that there may be only small induced dipole moments along the Y-axis. For both carotenoids, a decrease in polarizability is observed for the VIS transition in the order alltrans > 13-cis > 15-cis, and is similar for the UV transition (cf. Table 1). These differences reflect most probably the changes in molecular size down the long axis. Also the calculations for the all-trans and central-cis isomers of violaxanthin show a 20% decrease in the oscillator strength on isomerization. This result explains the smaller values of Da in cis isomers, since the polarizability components can be expressed as transition moment products [22,23]. Due to less resolved vibrational structure in absorption spectra of b-carotene, the directions ascribed to Dl for its isomers are less clear than for violaxanthin. However, this weaker accuracy does not invalidate the main conclusions of this work which, supported more strongly by the results for violaxanthin isomers, show that: (i) the orientation of the axis of the largest polarizability in both cis isomers of both pigments examined is at an angle with respect to the transition moment of the ‘‘cis transition’’ clearly larger than the magic angle and (ii) the directions of dipole moment differences make an angle with the transition moment less than the magic angle. Also, despite of the smaller value of polarizability change in the UV transition in the isomers of b-car-
469
otene than in violaxanthin, the difference dipoles observed for b-carotene are significantly larger (5–6 D) than in for violaxanthin (2 D). It would be difficult to reconcile such values of difference dipoles and their directional properties with the inductive effect of the matrix field on molecular polarizability. It is also remarkable that the apparent Dl’s in b-carotene isomers are 2.5 times larger than in violaxanthin, although the conjugated chains of these molecules differ by only two double bonds. Irrespective of the origin of Dl in carotenoids, these experimental data are in odds with the picture assuming the inductive effect of the matrix electric field as the basic source of the second derivative component in their electroabsorption spectra. The isomerization has stronger effect on molecular polarizability in b-carotene than in violaxanthin. This difference is seen in both the VIS and the UV transitions, for which a 10% decrease occurs on going from all-trans to 15-cis form in violaxanthin, compared to 40% decrease in b-carotene. Also the polarizability differences between UV and VIS transitions are larger in b-carotene. This set of data indicates a generally larger susceptibility of electrooptical parameters to structural distortions in b-carotene than in violaxanthin. This and other features mentioned above are consistent with the effect of polarizability modulation by molecular distortions postulated in our previous work for the VIS transition in all-trans carotenoids [16]. As a matter of fact, extending conclusions based on the þ UV transition 1A g ! 1Ag to the one in the VIS range þ 1Ag ! 1Bu [1,28] may be limited by the fact that the final states in both transitions are of different symmetry and thus the transition energies and respective polarizabilities may differ in their sensitivity to the distortions of molecular geometry. However, the intensity distribution among vibrational subbands in the VIS and UV absorption spectra of cis violaxanthin indicates involvement of similar frequencies as in the all-trans isomer, albeit with slightly different Franck–Condon factors, which suggests similar coupling of the electron density redistribution with the changes in molecular geometry. In conclusion, our experimental observations point to the lack of the inductive effect of the matrix field in carotenoids. Their analysis indicates features of electrooptical parameters compatible with the conformational modulation of polarizability correlated with the inhomogeneous broadening in carotenoid spectra [16], proposed as the main mechanism leading to the second derivative term in electroabsorption spectra of carotenoids. Acknowledgement This work was supported by research funds from KBN administered by UMCS. References [1] R. Fujii, K. Furuichi, J.-P. Zhang, H. Nagae, H. Hashimoto, Y. Koyama, J. Phys. Chem. A 106 (2002) 2410.
470
S. Krawczyk et al. / Chemical Physics 326 (2006) 465–470
[2] Y. Koyama, J. Photochem. Photobiol. B: Biology 9 (1991) 265. [3] T. Ritz, A. Damjanovic´, K. Schulten, J.-P. Zhang, Y. Koyama, Photosynth. Res. 66 (2000) 125. [4] M.G. Goodwin, J.B. Jackson, Biochim. Biophys. Acta 1144 (1993) 191. [5] K. Matsuura, K. Shimada, Biochim. Biophys. Acta 1140 (1993) 293. [6] M. Haumann, W. Junge, in: D.R. Ort, C.F. Yocum (Eds.), Advances in Photosynthesis, Kluwer, 1996, p. 165. [7] D.S. Gottfried, M.A. Steffen, S.G. Boxer, Biochim. Biophys. Acta 1059 (1991) 76. [8] D.S. Gottfried, M.A. Steffen, S.G. Boxer, Science 251 (1991) 662. [9] S. Krawczyk, A. Daniluk, Chem. Phys. Lett. 236 (1995) 431. [10] S. Krawczyk, Z. Krupa, W. Maksymiec, Biochim. Biophys. Acta 1143 (1993) 273. [11] K. Yanagi, A.T. Gardiner, R.J. Cogdell, H. Hashimoto, Phys. Rev. B71 (2005), art. No. 195118. [12] J.B.v. Beek, F. Kajzar, A.C. Albrecht, J. Chem. Phys. 95 (1991) 6400. [13] S.R. Marder, W.E. Torruellas, M. Blanchard-Desce, V. Ricci, G.I. Stegeman, S. Gilmour, J.-L. Bredas, J. Li, G.U. Bublitz, S.G. Boxer, Science 276 (1997) 1233. [14] G.K. Ramachandran, J.K. Tomfohr, J. Li, O.F. Sankey, X. Zarate, A. Primak, Y. Terazono, T.A. Moore, A.L. Moore, D. Gust, L.A. Nagahara, S.M. Lindsay, J. Phys. Chem. B 107 (2003) 6162.
[15] F. Rohlfing, D.D.C. Bradley, Chem. Phys. Lett. 277 (1997) 406. [16] S. Krawczyk, D. Olszo´wka, Chem. Phys. 265 (2001) 335. [17] K. Tsukida, K. Saiki, T. Takii, Y. Koyama, J. Chromatogr. 245 (1982) 359. [18] Y. Koyama, I. Takatsuka, M. Nakata, M. Tasumi, J. Raman Spectr. 19 (1988) 37. [19] R. Fujii, C.-H. Chen, T. Mizoguchi, Y. Koyama, Spectrochim. Acta A 54 (1998) 727. [20] D. Niedz´wiedzki, Z. Krupa, W.I. Gruszecki, J. Photochem. Photobiol. B: Biol. 78 (2005) 109. [21] D. Niedz´wiedzki, W.I. Gruszecki, Colloids Surf. B: Biointerf. 28 (2003) 27. [22] W. Liptay, Z. Naturforsch. 20 (1965) 272. [23] W. Liptay, in: C. Lim (Ed.), Excited States, vol. 1, Academic Press, 1974, p. 129. [24] R.R. Birge, M.Z. Zgierski, L. Serrano-Andres, B.S. Hudson, J. Phys. Chem. A 103 (1999) 2251. [25] Q. Shang, X. Dou, B.S. Hudson, Nature 352 (1991) 703. [26] C. Sterling, Acta Cryst. 17 (1964) 1224. [27] R.E. Connors, D.S. Burns, R. Farhoosh, H.A. Frank, J. Phys. Chem. 97 (1993) 9351. [28] Y. Koyama, M. Kuki, P.O. Andersson, T. Gillbro, Photochem. Photobiol. 63 (1996) 243.