Sequential excitation energy transport in stacking multilayers: a

Matt Scott, Chantal McCollum, Sergej Vasil'ev, Cheryl Crozier, George S. Espie, .... P. Karageorgiev, B. Stiller, D. Prescher, B. Dietzel, B. Schulz, ...
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J . Phys. Chem. 1988, 92, 5035-5044

Finally, we call attention to the fact that our study has some obvious limitations, which we mentioned in the Introduction. The first is that the starting point of all the trajectories lies in the region of the saddle point, and second the simplicity of our surfaces cannot fully reproduce the complexity of an a b initio surface. Also, because this is a classical study, we have disregarded effects such as tunneling. Nevertheless, we think that one can capture the

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general trends of four-center elimination reactions working with simple dynamical models. Acknowledgment. We thank Professor E. R. Grant for helpful comments in the early stage ofthis Work. We achowl&Zesupport by a CAICYT Project grant (No. l860/82). Registry No. CH,CF,, 420-46-2; HF, 7664-39-3; CH2CF2, 75-38-7.

Sequential Excitation Energy Transport in Stacking Multilayers: Comparative Study between Photosynthetic Antenna and Langmuir-Blodgett Multilayers Iwao Yamazaki,*,+Naoto Tamai, Tomoko Yamazaki,+ Institute f o r Molecular Science, Myodaiji, Okazaki 444, Japan

Akio Murakami, Mamoru Mimuro, and Yoshihiko Fujita National Institute f o r Basic Biology, Myodaiji, Okazaki 444, Japan (Received: December 7 , 1987)

The electronic excitation transfer via dipoledipole interaction has been studied with two kinds of stacking multilayer architectures: a biological photosynthetic antenna and an artificial Langmuir-Blodgett (LB) film. The layer-to-layer sequential excitation transport was observed by means of a picosecond time-resolved fluorescence spectrophotometer. Commonly to the phycobilin-chlorophyll system of algae and the LB multilayers containing carbazole and three types of cyanine dyes as energy donor and acceptors, the fluorescence is emitted sequentiallyfrom respective layers during 0.5-1 ns, associated with the excitation transfer from the outer surface to the inner layer. The fluorescence rise and decay curves of individual pigments are characterized by a rapid rise in the acceptor but considerably slower decay in the donor. Such time profile of the sequential energy transport can be fitted to an equation of e~p(-2kt’/~)type.

1. Introduction Special attention has recently been paid to the long-distance excitation energy transfer via dipole-dipole interaction’ under restricted molecular geometries.z Recently, theoretical studies have been reported concerning the energy-transfer kinetics under special molecular arrangements of one- and two-dimensional systems and stacking m~ltilayers.~”As for the two-dimensional and stacking multilayer systems, several experimental studies have been performed with Langmuir-Blodgett (LB) films7-12 and surfaces of membranes13-16and solid substrate^'^-*^ on which dye molecules are incorporated. Further studies, especially time-resolved fluorescence studies, are required before a fully consistent picture is available. We are here concerned with the sequential energy transfer in stacking multilayers of biological photosynthetic antenna and artificial LB multilayers in which several pairs of energy donors and acceptors are stacked such that the excitation energy transfer takes place in one direction through several layers. Picosecond time-resolved fluorescence spectra and decay curves are presented, and kinetic behaviors are compared between biological and artificial antenna systems in light of the theoretical treatment. Photosynthetic light-harvesting antenna in plant is a typical example of the sequential energy transfer in which photonic energy absorbed at an outer surface of pigment system is transported sequentially through several chromoproteins to an inner core of the reaction center in an extremely high efficiency near unity. Antenna pigment systems of red and blue-green algae contain an accessory pigment system, phycobilisomes, which are attached on the surface of thylakoid membranes containing chlorophyll systems and reaction centers.21*2zA schematic illustration of the structure of phycobilisome is shown in Figure 1. Phycobilisome consists of three types of chromoproteins, phycobiliproteins: phycoerythrin (PE), phycocyanin (PC), and allophycocyanin Present address: Department of Chemical Process Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060, Japan.

0022-3654/88/2092-5035$01.50/0

(APC). Thus a long-distance (500-700 b;) energy transfer takes place sequentially through these phycobiliproteins and chlorophyll a (Chl a ) toward the reaction center (RC). Recently, dynamical (1) Forster, Th. Z . Naturforsch., A : Astrophys., Phys. Phys. Chem. 1949, 4, 321. (2) Agranovich, V. M.; Galanin, M. D. In Electronic Excitation Energy Transfer in Condensed Matter; North-Holland: New York, 1982. (3) Hauser, M.; Klein, U. K. A.; Gosele, U. Z . Phys. Chem. (Munich) 1976, 101, S255. (4) Zumofen, G.; Blumen, A. J. Chem. Phys. 1982, 76, 3713. (5) Klafter, J.; Blumen, A. J . Chem. Phys. 1984, 80, 875. (6) Baumann, J.; Fayer, M. D. J . Chem. Phys. 1986,85,4087. (7) Biicher, H.; Drexhage, K. H.; Fleck, M.; Kuhn, H.; Mobius, D.; Tillmann, P.; Weigand, J. Mol. Cryst. 1967, 2, 199. (8) Kuhn, H.; Mobius, D.; Biicher, H. In Techniques of Chemistry; Weissberger, A,, Rossiter, B. W., Eds.; Wiley: New York, 1972; Vol. 1, Part 3B, pp 577-702. (9) Leitner, A,; Lippitsch, M. E.; Draxler, S.; Riegler, M.; Aussenegg, F. R. Thin Solid Films 1985, 132, 5 5 . (IO) Yamazaki, T.; Tamai, N.; Yamazaki, I. Chem. Phys. Left.1986,124, 326. ( 1 1 ) Yamazaki, I.; Tamai, N.; Yamazaki, T. J . Phys. Chem. 1987, 91, 3572. (12) Tamai, N.; Yamazaki, T.; Yamazaki, I. J. Phys. Chem. 1987,91,841. (1 3) Fung, B. K.; Stryer, L. Biochemistry 1978, 17, 5241. (14) Tamai, N.; Yamazaki, T.; Yamazaki, I.; Mataga, N. In Ultrafast Phenomena V; Fleming, G. R., Siegman, A. E., Eds.; Springer Series on Chemistry and Physics; Springer-Verlag: Berlin, 1986; Vol. 46, pp 449-453. (15) Tamai, N.; Yamazaki, T.; Yamazaki, I.; Mizuma, A,; Mataga, N. J . Phys. Chem 1987, 91, 3503. (16) Takami, A,; Mataga, N. J . Phys. Chem. 1987, 91, 618. (17) Anfinrud, P.; Crackel, R. L.; Struve, W. S . J . Phys. Chem. 1984,88, 5873. (18) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. J . Phys. Chem. 1986, 90, 5094. (19) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. J . Phys. Chem. 1987, 91, 1423. (20) Alivisatos, A. P.; Arndt, M. F.; Efrima, S.; Waldeck, D. H.; Harris, C. B. J . Chem. Phys. 1987, 86, 6540. (21) Gantt, E. Annu. Rev. Plant Physiol. 1981, 32, 327. (22) Glazer, A. N. Annu. Rev. Biophys. Biophys. Chem. 1985, 14, 47.

0 1988 American Chemical Society

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The Journal of Physical Chemistry, Vol. 92, No. 17, 1988

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aspects of this sequential energy transfer have been studied by several workers by means of a picosecond time-resolved fluorescence s p e c t r o s ~ o p y . ~ ~Many - ~ ~ problems remain unsolved as regards kinetics of the sequential transport. An artificial analogue to the biological antenna of multilayered architecture can be obtained with the Langmuir-Blodgett (LB) technique for successive deposition of monomolecular layers to a solid substrate.* As is shown in Figure lb, one can prepare from this method a stack of monomolecular layers containing chromophoric donor and acceptor molecules with their number densities and interlayer distance being variable in wide range. In 1967, Bucher et al.’ investigated the interlayer energy transfer in the LB multilayer films containing different cyanine dyes and examined the energy-transfer efficiency as a function of distance between donor and acceptor layers on the basis of steady-state fluorescence measurements. Recently, Leitner et aL9 have examined the same problem by using the LB double layers containing a pair of donor layer and acceptor layer, based on the fluorescence decay curve analysis. It was demonstrated that, for low chromophore densities, the donor fluorescence decay curves fit the Forster formula adapted for the two-dimensional case, but they deviate significantly as the chromophore density is increased, owing to the fluorescence quenching by aggregates of chromophores. The purposes of the present study are (1) observation of the sequential energy transport in a picosecond time resolution for the LB multilayers with a construction similar to those studied first by Prof. H. Kuhn and his colleagues;’ (2) comparison of the sequential transport with the biological antenna system of algae; (3) test of an empirical equation for the sequential transport among the biological antenna and the artificial multilayers; and (4) demonstration of a special chromophore array or channel for sequential energy transfer inside phycobilisome rods. In the (23) Porter, G . ; Tredwell, C. J.; Searle, G. F. W.; Barber, J. Biochim. Biophys. Acta 1978, 501, 232. (24) Kobayashi, T.; Degenkolb, E. 0.; Bersohn, R.; Rentzepis, P. M.; MacColl, R.; Berns, D. S . Biochemistry 1979, 18, 5073. (25) Yamazaki, I.; Mimuro, M.; Murao, T.; Yamazaki, T.; Yoshihara, K.; Fujita, Y. Photochem. Photobiol. 1984, 39, 233. (26) Mimuro, M.; Yamazaki, I.; Yamazaki, T.; Fujita, Y . Photochem. Photobiol. 1985, 41, 591. (27) Yamazaki, I.; Tamai, N.; Yamazaki, T.; Mimuro, M.; Fujita, Y. In Ultrafast Phenomena IV; Auston, D. H., Eisenthal, K. B., Eds.; Springer Series on Chemistry and Physics; Springer-Verlag: Berlin, 1984; Vol. 38, pp 490-492. (28) Hanzlik, C. A,; Hancock, L. E.; Knox, R. S.; Guard-Friar, D.; MacColl, R. J . Lumin. 1985, 34, 99. (29) Wendler, J.; John, W.; Scheer, H.; Holzwarth, A. R. Photochem. Photobiol. 1986, 44, 19. (30) Gillbro, T.; Sandstrom, A,; Sundstrom, V.; Wendler, J.; Holzwarth, A. R. Biochim. Biophys. Acta 1985, 808, 52.

present paper, first the sequential energy transfer of the photosynthetic light-harvesting antenna is presented in section 3. Time-resolved fluorescence spectra and decay curves are shown. Corresponding time-resolved data for the LB multilayer films are shown in section 4. Kinetic equations reproducing the sequential energy transfer are discussed in section 5 and applied to both cases of the biological antenna and the artificial LB multilayer film. In the last section, section 6, overall kinetic behaviors are discussed in relation to the structures of molecular organization in which chromophores are distributed.

2. Experimental Section Preparation of Intact Cells of Red Algae and Cyanobacteria. Cells were grown autotrophically in the medium of ASP-23’ for Porphyridium cruentum, the modified Detmer’s medium3, for cyanobacterial strains, or P49,) for Porphyridium aerugineum under the continuous illumination of a weak incandescent light. Cells at the late log-growth phase were used for all measurements. Two kinds of cells different in phycobilin composition were prepared through the chromatic adaptation technique.34 Synechocystis sp. ATCC 27170 was grown under red light (2.5 W/cm2) for establishing the PE-less system and under green light (1 .O W/cm*) for the PE-rich system (hereafter referred to as PC cells and PE cells, respectively). Protein composition in both types of cells were determined to be PE:PC:APC = 80.7 f 4.6:8 1.3 f 4.k35.7 f 1.8 (in units of the number of monomers per R C 11) for PE cells and 1.5 f 0.13530.5 f 4.5:32.7 f 2.0 for PC cells, indicating that the phycobilisomes consist of APC core and PE-PC rods as follows (see Figure 3): in PE cells, one phycobilisome consists of APC core of six hexamers and six PC plus PE rods of two PC and two PE hexamers; and in PC cells, APC core of six hexamers and six PC rods of two h e x a m e r ~ . ~Details ~ . ~ ~ of the sample preparation are described e l s e ~ h e r e . ~ ~ ~ ~ ~ ~ ~ ~ Preparation of LB Multilayer Films. Three types of the LB films consisting of s e q u e n c e s of a donor layer (D) and three acceptor layers (A,, A,, and A,) were prepared: (1) two-layer system of D-AI (hereafter referred to as 2L), (2) three-layer system of D-A,-A, (3L), and (3) four-layer system of D-Al-A,-A3 (4L). The structures of these films are illustrated schematically in a later section. Respective monolayers contain chromophores which were chosen such that the fluorescence band (31) Provasoli, L.; McLaugilin, J. J. A,; Droop, M. R. Arch. Microbiol.

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(32) Watanabe, A. J. Gen. Appl. Microbiol. 1960, 6, 283. (33) Starr, R. C. J . Phycol. 1978, 14, 47. (34) Ohki,K.; Watanabe, M.; Fujita, Y. Plant Cell Physiol. 1982, 23,651. (35) Bryant, D. A,; Guglielmi, G.; Marsac, N. T.; Castets, A. M.; Cohen-Bazire, G. Arch. Microbiol. 1979, 123, 113.

Excitation Energy Transport in Stacking Multilayers of donor overlaps with the absorption band of acceptor and that the Forster energy transfer takes place in a sequence of D-A,-Az-A3. In actual preparation of LB films, six to eight layers were deposited on a quartz plate in the following order: (1) five layers of palmitic acid cadmium salt, (2)monolayer(s) consisting of palmitic acid and small amounts of dye (Al, A2, or A,) and (3) monolayer of carbazole (D), and (4)a monolayer of palmitic acid. An outer layer of palmitic acid prevents the multilayered structure from being destroyed. As pigment molecules for D, A,, Az, and A3 layers, we used, acid (Molecular Probes, respectively, 1 1-(9-~arbazole)undecanoic Co., U.S.A.),1 ,I!-dioctadecyloxocyanine(Nippon Kankoh Shikiso Kenkyusho Co., Okayama, Japan), 1,l'-dioctadecylthiacyanine (Nippon Kankoh Shikiso), l,l'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine (Molecular Probe). Palmitic acid (Wako Chemical Co. Osaka) was purified five times by recrystallization from ethanol. Mixtures of palmitic acid and small amounts of pigment dissolved in chloroform were spread onto the surface of M CdC12. The subphase water subphase containing 3 X conditions were adjusted to be temperature of 17 OC and pH 6.3 by adding N a H C 0 3 buffer solution. Mixed monolayers were deposited on a quartz plate under a constant surface pressure of 2.5 X N m-'. The concentration of pigment molecules in ech layer was 5 mol %. The interlayer distance between donor and acceptor chromophores is 25 8, which corresponds to the hydrocarbon-chain length of arachidic acid molecules. The quartz plates used were precoated with five layers of palmitic acid cadmium salt to minimize the influence of the substrate surface on energy-transfer kinetics. Time-Resolved Fluorescence Spectrophotometer. Fluorescence decay curves and time-resolved spectra were measured with a time-correlated, single-photon counting apparatus equipped with a picosecond laser system as an excitation source.36 The laser system was composed of a synchronously pumped, cavity-dumped dye laser (Spectra Physics 375 and 344s) and a mode-locked argon ion laser (Spectra Physics 171-18). The dye laser was operated with a repetition rate of 800 kHz and a single pulse duration of 6 ps (fwhm). The fluorescence decay curves were obtained with a time-to-amplitude converter and a multichannel pulse-height analyzer. A microchannel-plate photomultiplier (Hamamatsu R1264U or R1564U) was used, which allows us to obtain an instrument response function of 70-ps (R1264U) or 30-ps (R1564U)widths (fwhm) for the scattered laser light.37 The fluorescence lifetime can thus be determined down to 1 ps with an accuracy of f0.2 ps. The time-resolved spectra were obtained with the minimum time difference of 0.8 ps by plotting the fluorescence intensities at particular delay times as a function of wavelength. Analysis of Time-Resolved Fluorescence Spectra. Time-resolved fluorescence spectra of the phycobilin-chlorophyll system are observed as superposition of different fluorescence bands of PE, PC, APC and Chl a. In order to obtain time courses of fluorescence intensities of each phycobilin itself, each of the time-resolved spectra was resolved into components by using the fluorescence band shapes of respective pigment fluorescences. As for the spectra of PE, PC, and APC, we used the spectra of respective phycobiliproteins (hexamers) isolated from Porphyridium cruentum and Anabaena cylindrica by the method previously reported.25 The band shape of Chl a was taken from that of Chlorella pyrenoidosa. In every time-resolved spectrum, the fraction of different components was obtained as a solution of linear equations. The location of fluorescence maximum of the isolated phycobiliprotein is not always identical among different kinds of algae. Therefore, the component spectrum in some cases was shifted within 3 nm to obtain best fitting. The fluorescence decay curves of each phycobilin thus obtained has intensities of 500-2000 in count number at the peak channel. (36) Yamazaki, I.; Tamai, N.; Kume, H.; Tsuchiya, H.; Oba, K. Rev. Sci. Instrum. 1985, 56, 1187.

(37) Koyama, K.; Kume, H. "Application of MCP-PMTs to Time-Correlated Single-Photon Counting and Related Precautions". Technical Note,, Harnamatsu Co., 1987.

The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 5037

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3. Sequential Energy Transfer in Phycobilin-Chlorophyll System of Algal Intact Cells Phycobilisomes in red and blue-green algae are supramolecular units which consist of three kinds of phycobiliproteins, Le., PE, PC, and APC.21,22A phycobiliprotein monomer (molecular weight ca. 10 5 ) contains several open-chain tetrapyrroles (bilins) as chromophores; the number of bilins incorporated in a phycobiliprotein monomer are five in C-PE, three in C-PC, and two in APC. Phycobilisomes consist of APC cores and PC-PE rods, each core and rod being made up of a stack of several disks 60 8,thick and 120 8, in diameter. A schematic illustration is shown in Figures l a and 3. According to an X-ray crystallography of phycocyanin by Schirmer et a1.,38,39the distances between tetrapyrroles are in a range of 20-70 8, depending on sites. During the course of the energy transport from the initially photoexcited phycobiliprotein to RC I and 11, fluorescence is emitted from each of pigments very weakly but it can be used as a probe for studying the kinetics of energy transfer. Figure 2 shows the absorption and fluorescence spectra of respective phycobiliproteins. The band positions and intensities depend on the conformation of tetrapyrroles in protein molecules. The donor fluorescence and acceptor absorption bands overlap with each other between PE and PC, PC and APC, and APC and Chl a. According to Grabowski and Gantt,40the Forster critical transfer distance (R,) between phycobiliproteins in the direction from the outer surface to the inner layer is 63 8, between PE and PC and 61 A between PC and APC, while for the energy transfer of inverse direction, Le., from the inner to outer layers, the Ro value is as small as 13 8, between PC and PE and 44 8, between APC and PC. This indicates that the energy transfer takes place primarily in the direction from the outer to inner cores, since the energy-transfer rate is proportional to Ro6. The stacking structure in phycobilisomes can be regulated through the chromatic adaptation method.34 In the present study, (38) Schirmer, T.; Bode, W.; Huber, R.; Sidler, W.; Zuber, H. J . Mol. Biol. 1985, 184, 257. (39) Schirmer, T.; Huber, R.; Schneider, M.; Bode, W.; Miller, M.; Hackert, M. L. J . Mol. Biol. 1986, 188, 651. (40) Grabowski, J.; Gantt, E. Photochern. Photobiol. 1978, 28, 39.

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obtained experimentally from the spectrum analysis. Solid lines are the best fit curves calculated from eq 19-22. Note that the PE decay curve includes an exponential decay component with a longer lifetime which is omitted in the theoretical curve. Dashed lines are the instrumental response function of scattered laser light. 600 WAVELENGTH (nrn)

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Figure 3. Time-resolved fluorescence spectra of Synechocystis sp. intact cells: (a) PE cells and (b) PC cells. The excitation wavelengths are 540 nm for PE cells and 600 nm for PC cells. Structures of phycobilisomes are also shown schematically for both types of phycobilisomes. One disk unit is a phycobiliprotein hexamer consisting of two trimer disks bound face-to-face.

we prepared two kinds of phycobilisome structures of Synechocystis sp. ATCC 27170: (1) P E cells consisting of rods of two disks of PE and two disks of PC hexamers and APC core, and (2) PC cells consisting of two disks of PC rods and APC core. The structures of these two phycobilisomes are shown in Figure 3 schematically. The time behaviors and the rate constants of energy transfer are compared among these two types of phycobilisomes. Figure 3 shows the time-resolved fluorescence spectra of PC cells and PE cells of Synechocystis sp. ATCC 27170. Each spectrum is normalized to the maximum intensity. In both types of cells, the spectrum is found to be changed with time in the picosecond time scale. In PE cells (Figure 3a), following pulsed excitation, the PE spectrum appears predominantly at 0-51 ps, with a peak at 575 nm, and the PC fluorescence also appears weakly around 645 nm. At 50-300 ps, the spectrum is shifted gradually to the red, indicating an increasing contribution from APC fluorescence around 660 nm. After 500 ps, the Chl a spectrum appears at 684 nm. The spectrum no longer changes after 700 ps. The PC cells (Figure 3b) exhibit the PC spectrum

in the initial time region (0-50 ps) in addition to sequential appearance of the APC and Chl a spectra. The time-resolved spectra were analyzed into components with the method described under Experimental Section to deduce the time courses of fluorescence emitted from each pigment component. Figure 4 shows the results of the analysis. It can be seen for both types of cells commonly that the fluorescence rise time in every pigment is delayed with its delay time becoming longer on going from the outer surface to the inner core of phycobilisome and that in every curve the decay is rather slow in contrast to the fast rise; in a pair of energy donor and acceptor, the donor decay seems not to correspond to the acceptor rise. The former observation provides us with direct evidence of the sequential energy transport in the order of PE-PC-APC-Chl a. The latter observation implies that the kinetic equation of the sequential transport is not a simple exponential form. In our previous study with the red alga and the cyanobacterium, Porphyridium cruentum and Anacystis n i d u l a n ~ , ~ ~it-was ~ ' demonstrated that the sequential time behaviors can be fitted in the first approximation to the decay kinetics of e ~ p ( - 2 k t ' / ~type, ) the exact expressions of which will be given in section 5. As is seen in Figure 4, the same kinetic equations can be fitted for the present cases of the PE and PC cells of Synechocystis sp. The values of the rate constants ( k ) thus obtained are summarized in Table I for different types of phycobilisomes including data from our previous s t ~ d y . * ~ -In~ the ' evaluation of k , the long decay component in PE of PE cells of Synechocystis sp. (Figure 4a) was eliminated under the assumption that the long decaying component of PE arises from chromophores isolated from the energy flow path. The

Excitation Energy Transport in Stacking Multilayers

The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 5039

TABLE I: Rate Constants (k,ps-'/*)O of the Energy Transfer in Phycobilin-Chl a System of Algal Intact Cells PE cells algae PE-PC PC-APC APC-Chl a Chl a-RC PC-APC 0.13 0.056 0.071 Flemyella diplosiphonb 0.11 0.40 0.15 0.043 0.07 Torypothrix tenuisb 0.14 0.22 0.1 1 0.08 0.12 Synechocystis ~ p (540-nm . ~ exc) 0.13 0.22 0.10 0.05 Synechocystis sp.C (600-nm exc) 0.12 0.17 0.29 0.13 0.063 Nostoc sp. Porphyridium cruentum 0.14 0.24 0.27 0.10 0.065 Anabaena cylindrica 0.077 Anabaena uariabilis 0.12 Porphyridium aerugineum average value 0.14 f 0.019 0.29 f 0.070 0.15 f 0.057 0.065 f 0.019 0.087 i 0.023

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Figure 5. Schematic illustration of stacking structures of the LB multilayer films and molecular formulas of the pigment chromophores used in this study. The concentration of pigment chromophores is 5 mol % in each monolayer. The interlayer distance between adjacent layers is approximately 25 A. similar phenomenon was found in PE of'P. ~ r u e n t u m . ~ ~ It is seen from Table I that the k value in each step of the sequential energy transport is nearly constant irrespective of algal species and of phycobilisome structure; the average values are listed in the table. Then it follows that the difference in the global structure of phycobilisome little affects the energy-transfer kinetics; the phycobilisome of P . cruentum is hemispherical, whereas that ~ ~ ~ ~ ,for ~ ~the PC-APC of Nostoc sp. is h e m i d i ~ c o i d a l . ~Except step, the magnitudes of k for the particular step are nearly identical among the PE-cell group (Nostoc sp. and P. cruentum) and the PC-cell group ( A . cylindrica, A . uariabilis, and P. aerugineum). The same trend can be seen among the species prepared from the chromatic adaptation, Le., PE and PC cells in F. diplosiphon, T . tenuis, and Synechocystis sp. Also noteworthy is that the differences in structures of the PE-PC rod little influence the magnitude of k value; the stacking structure is different among these three species (see Table I). However, the PC-APC step (41)

Gantt, E.; Conti, S. F. J . Cell. Biol.

1966, 29, 423.

is unusual: the k values in the PE-cell group (0.29 f 0.07) are larger by a factor of 3 than those of the PC-cell group (0.087 f 0.02). We should note here that, when the PE cells of Synechocystis sp. are irradiated at 600 nm and excited at PC directly, the k value of the PC-APC step decreases from 0.22 to 0.12 and becomes identical with the corresponding value in PC cells of the same species. Note that the P E and PC cells of Synechocystis sp. have the same structure as far as the PC rod and the APC core are concerned. Therefore, it follows that the transfer rate in the PC-APC step is determined depending on whether PC is excited by direct optical excitation or by the energy transfer from PE. This means that there exists a special channel inside the PE-PC-APC stack which causes the energy transport to be very fast. Detailed discussion will be given in section 6.

4. Sequential Energy Transfer in Langmuir-Blodgett Multila yers The LB film is typical of the two-dimensional molecular organization which is prepared by transferring a compact monolayer spread on a water surface onto a substrate.8 One can prepare an artificial analogue of the biological photosynthetic antenna by stacking LB monomolecular layers containing chromophores of energy donor and acceptor with their number densities being variable over a wide range. The multilayer structure of the LB films and the molecular formula of chromophores used in this study are shown in Figure 5 . The chromophores incorporated in multilayers are carbazole in D layer, oxocyanine in A I , thiacyanine in A2, and indocarbocyanine in Aj; the absorption and fluorescence bands of these chromophores are shown in Figure 6. The critical transfer distances (Ro) for the donor and acceptor

5040 The Journal of Physical Chemistry. Vol. 92, No. 17, 1988

W W

m o n olay e rs

a

0 3

-

~

~ __ ? _ _ - .-- ......... _ . _ . "'

400

4' -I

-, ' - . A ....-. - I

_.

/

500

., , ,

600

WAVELENGTH ( n m )

Figure 7. Fluorescence spectra obtained with steady-excitation at 295 nm, for (a) LB multilayers of 1L (D), 2L (D-A,), 3L (D-AI-Az), and 4L (D-A,-A,-A,) and (b) single monolayers of D, A , , Az, and A3.

pairs of D-A,, A,-A,, and A,-A3 are estimated to be approximately 50-70 A, from the spectral overlap of donor fluorescence and acceptor absorption. Thus the sequential energy transfer can be expected to occur in the order of D-A,-A,-A,. From inspection of the steady-excitation fluorescence spectra, one can estimate the efficiency of the sequential excitation transfer. Figure 7a shows the fluorescence spectra obtained with excitation at 295 nm, for the four types of LB multilayers, Le., 1L (D), 2L (D-A,), 3L (D-A,-A,), and 4L (D-A,-A2-A3). The concentration of chromophore is constant ( 5 mol %) throughout different types of multilayers. In lL, only a fluorescence band of D appears with a vibrational structure. In 2L, the intensity of D fluorescence is reduced to about half and the AI fluorescence appears owing to the excitation transfer from D to A, layers. In 3L, the A, band disappears and the A, band appears. Similarly, in 4L, the A2 band reduced to about half and the A, band appears. From the successive relationship with respect to reduction of the fluorescence intensity, we estimated the efficiency of the sequential excitation transfer. Let us denote the efficiency of the excitation transfer @ET by the equations

@ET(D-AI)= P F ( D , ~ L-) @ F ( D , ~ L ) ) / @ F ( D , ~ L(1) )

Yamazaki et al. Figure 8 shows the time-resolved fluorescence spectra of the LB multilayers of 2L, 3L, and 4L. Each spectrum is normalized to the maximum intensity. It is seen that the spectrum changes with time in the picosecond time scale. In 2L, following excitation of the D layer at 295 nm, a fluorescence band of D appears at 350 nm, and then A, band rises after 20 ps. In 3L, the fluorescence bands of D and A, appear in the initial time region, and then A2 band appears at 470 nm after 20 ps. A similar spectral change can be seen in 4L; in the initial time region, the fluorescence bands due to D, A,, and A, appear, and A3 band rises after 50 ps. It can be seen that the fluorescence from the inner layer rises more slowly than those of the outer layers, e.g., A, in 2L, A, in 3L, and A, in 4L. In very short time region after excitation, the fluorescence bands of acceptors appear simultaneously but weakly, e.g., A, band in 2L, 3L, and 4L and A, band in 3L and 4L. This is not a consequence of the direct excitation of these acceptors by 295-nm irradiation but of rapid energy transfer from D to A,-A2, because the ratios of the absorbance of the acceptors to that of the D layer are very small (less than 0.05) at this wavelength. In fact, as shown in Figure 7b, the fluorescence emissions of A,, A2, and A, caused by direct excitation at 295 nm are very weak. Parts a, b, and c of Figure 9 show the fluorescence rise and decay curves of each layer in 2L, 3L, and 4L, respectively. All the decay curve profiles are not simple exponential forms. Qualitatively one can see several features of the sequential excitation transfer similar to the case of the biological antenna. On going from the outer surface to the inner layer, the fluorescence rises more slowly; the rise time is delayed in the order of A,, A,, and A,. In the fluorescence time courses of a pair of energy donor and acceptor, the decay of donor is rather slow in contrast to the fast rise of acceptor. Finally, we should note here that, in every case of 2L, 3L, and 4L, the decay curve of D layer includes a slowly decaying component. Probably this might be regarded as due to the fluorescence from chromophores isolated from the energy flow path. A similar phenomenon has been found in the ; ~ ~fluorescence decay phycobilin antenna system of P. c r u e n t ~ r nthe curve of the outer phycobilin (PE) in phycobilisome (Figure 4a) includes a long decay component which can be analyzed in terms of chromophores isolated from the energy-transfer pathway. All these observations in the steady-excitation and time-resolved fluorescence spectra and the rise and decay curves are very similar to those in the biological antenna system, indicating that the excitation transfer takes place sequentially from the outer to the inner layer of LB multilayer films.

5. Kinetics of Sequential Energy Transfer We now discuss kinetics of the sequential energy transport in stacking multilayers. Electronically excited molecules incorporated in molecular assemblies may undergo various energy relaxation pathways, particularly in artificial molecular assemblies like LB films, liquid crystals, and vesicles. On the other hand, the biological antenna system is designed to be specific for the energy transport without any other path of energy loss. According to our recent s t ~ d y , ~ ~ the ' ~ electronic , ' ~ ~ ' ~ energy relaxation processes in LB films can be summarized as follows:

I , ~ L ) absorption: @ET(Al-A2) = {@F(Ai&) - @ F ( A ~ T ~ L ) ) / @ F ( A (2) @'ET(A~-A~) = (@dAz,3L)- @ F ( A ~ , ~ L ) I / @ F ( A ~(3) ,~L) where @F is the fluorescence quantum yield, and aF(A1,2L),for example, denotes the aFof the A, fluorescence emitted from the 2L system. The @ET values are estimated from Figure 7a to be 0.49, 0.83, and 0.51 for D-A,, A,-A,, and A,-A3 transfer, respectively. Mention should be made here concerning the fluorescences of A,, A,, and A, by direct excitation at the same wavelength of 295 nm. To check this, we measured the fluorescence spectra for the respective monolayers including only D, A,, Az, or A3 with the excitation at 295 nm. The results are shown in Figure 7b. In all cases except for the sample of D layer, the fluorescence is emitted very weakly and the contributions of direct excitation other than the excitation transfer are negligible in the first approximation.

fluorescence:

D* D*

+ hv

-

D

D*

(4)

+ hvD D + Q*

(5)

--+

energy trapping: D* + Q energy migration between donors: D*

+D

-

(6)

D + D*

(7)

energy migration to lower energy sites: (8)

h V"

excimer formation: D*

h vm

+ D -,(D-.D)*

energy transfer to adjacent layer: D* + A

-

(9)

D + A*

(10)

Processes 6-9 occur within a single monolayer and are important

Excitation Energy Transport in Stacking Multilayers

The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 5041

;k ,

400 500 WAVELENGTH (nrn 1

4 00 500 WAVELENGTH ( n m 1

I

400 500 600 WAVELENGTH ( n m )

Figure 8. Time-resolved fluorescence spectra of the LB multilayer films: (a) 2L,D-A,; (b) 3L,D-A,-Az; (c) 4L,D-A,-A2-A3. The excitation wavelength is 295 nm. The structures of these LB films are given in Figure 5.

.

-

..,L i

..,,'., .:,,; '.,,

'.Z.

. .. ..

..........,...

-...

...... .,-, ............. ...... ............ ..........-.. .....

-

"'... ...

I

1

1

I

lf)

1

1.o TIME

1

1.o 2 .o 0 1.o 2.0 (m) T I M E (ns) T I M E (ns) Figure 9. Fluorescence rise and decay curves (a-c) and theoretical best fit curves (d, e) of the LB multilayer films of 2L, 3L,and 4L. The theoretical curves are obtained from eq 19-22 with parameter values listed in Table 11. The decay curves D, A , , Az, and A3 are obtained by monitoring the fluorescence at 350,420,470,and 570 nm, respectively. Theoretical D' curves are the best fit curves for the curves that were obtained from subtracting

0

2.0

0

the long-lifetime exponential decay from the experimental D curves. particularly in LB films. Process 10 represents the interlayer energy transfer between donor and acceptor layers. Figure 10 illustrates schematically the excitation energy transfer pathways in a monolayer and in stacking multilayers. In the present case

of stacking multilayers, the interlayer energy transfer (process 10) should compete with the energy dissipation within a monolayer (processes 6-9). However, one can prepare multilayers, in which the interlayer transfer is the primary process, by adjusting the

5042 The Journal of Physical Chemistry, Vol. 92, No. 17, 1988

Yamazaki et al. G(t) = r exp[-p(t/r)'i3

Figure 10. Schematic illustration of energy transfer under the restricted geometries of (a) a two-dimensional monomolecular layer and (b) a stack of monomolecular layers.

interlayer distance and the chromophore density in each layer. In the present study, the interlayer distance between donor and acceptor layers is adjusted to be 25 8,and the chromophore density in a monolayer to be 5 mol %. From this value of density, the average distance between chromophores within a monolayer is calculated to be 120 8, under an assumption of the random and uniform distribution of chromophores. Note that the Forster radius Ro for the donor and acceptor employed here falls in between 50 and 70 A. Thus we can safely assume that the interlayer transfer (process 10) is the dominant energy relaxation process. In fact, as shown in section 4, the efficiencies of the excitation transfer is more than 50%. According to Hauser et aL3 and Forster,' the donor fluorescence decay functions for the two- and three-dimensional systems, in which acceptor molecules are randomly distributed around a donor, are

- 2gyA'(t/TD)112] (12)

q(t/~)-'/~]

--

-

dN, ( t )/dt = -kl t-l/zNl(t)

(15)

dN2(t)/dt = klt-I/ZNl(t)- k2t-'/2Nz(t)

(16)

dN,(t)/dt = k2t-l/ZN2(t)- k3r-i/2N3(r)

(17)

dN4(t)/dt = k3t-1/2N3(t)- k4t-1/2N4(r)

(18)

where N(t) is the number of the excited molecules at time t for the respective pigments denoted by the suffix, and kl, k2, k,, and k4 are the energy-transfer rate constants in s-'/~. The solution of the differential equations 15-18 can be obtained under the initial conditions of N,(O) = No, N2(0)= N3(0) = N4(0) = 0 as follows: N , ( t ) = No e ~ p ( - 2 k , t ' / ~ ) N2(t) =

(19)

klN0 k2 - kl

-{ e ~ p ( - 2 k ~ t l-/ ~e~p(-2k,t'/~)) ) (20)

k k N e ~ p ( - 2 k ~ t ' /-~e)~ p ( - 2 k , t ' / ~ )

-

N,(t) = -

'

k2 - kl

k3-kl

where

1

e ~ p ( - 2 k ~ t ' /-~e~p(-2k,t'/~) ) k3 - k2

T D is the lifetime of the donor without acceptor; g is the factor determined by the molecular orientation; nA and nA' are the number densities of acceptors in unit area and unit volume, respectively; and Ro is the critical transfer distance where the rate constant for the energy transfer is equal to that for fluorescence by the donor in the absence of acceptors. Equations 11 and 12 both should be considered for the present case of stacking multilayers. The excitation energy transfers from a donor situated in one layer to acceptors in the adjacent layer 25 A away (see Figure 10). In an extreme condition of very low density of chromophores, the interlayer transfer might be approximated with the two-dimensional treatment. On the other hand, in higher density of chromophores the system may behave like the threedimensional system. Baumann and Fayer6 derived the theoretical expressions for the electronic excitation transfer in multilayered structures containing a pair of donor and acceptor under various conditions of interlayer distance, density of molecules, number of stacking layers, and orientational distributions of molecules. The decay of G ( t ) , the probability of an excitation being on the initially excited molecules, which can be probed by the fluorescence of donor molecules, is determined primarily by the surface concentration and the ratio of the layer separation ( d ) and the Forster radius ( R o ) ,u = R o / d . In the interlayer transfer, G ( t ) decays exponentially if the two layers are far apart compared to Ro,and the decay constant depends on the specific conditions. In the other extreme, where the layers are close, the general form of G ( t ) can be expressed as

(14)

where the parameters r, p , and q are specific to the respective situations. In the case that the ratio u is close to unity, the decay profile of G ( t ) is most sensitive to the variations of the layer separation. In the case of multilayer system, at low surface concentration the decay gradually changes from two- to threedimensional character as one increases the number of stacking layers. In 1978, Porter et aLZ3investigated the sequential fluorescence decay kinetics for P.cruentum intact cells and proposed that the fluorescence time behavior of each pigment can be expressed with a hypothetical decay function of exp(-2kt'i2). This type of equation can be derived from the Forster kinetics (eq 12) for the three-dimensional system under certain conditions. If the y value is large enough so that 27 > (kDt)lI2,the exponential term in eq 12 can be approximated by exp(-2ktii2). Now let us suppose the following differential equations for the sequential energy transfer through a pathway of layers 1 2 3 4:

for the 2-D system: p ( t ) = exp[-r/TD - 2gyA(t/TD)1/3] (1 1) for the 3-D system: p ( t ) = exp[-t/TD

-

N4(t) =

kkkN

(21)

e ~ p ( - 2 k , t ' / ~-) e~p(-2k,t'/~)

-

~

(k4 - ki)(k3 - kl)

k2 - kl

l Z 3 O {

e ~ p ( - 2 k ~ t ' /-~exp(-2k3tii2) ) (k3 - k4)(k3 - kl)

+

-

exp(-2k4t'i2) - exp(-2k2N2)

1

(k2 - k4)(k3 - k2) e~p(-2k,t'/~)- e ~ p ( - 2 k ~ t ' / ~ ) (22) (k3 - k4)(k3 - k2)

In the case of the phycobilin-chlorophyll system, eq 19-22 are found to fit the sequence of the fluorescence rise and decay curves as is shown in Figure 4. Also in the LB multilayer films, the sequential rise and decay curves can be fitted approximately with eq 19-22. The best fit curves are shown in parts d, e, and f of Figure 9 for 2L, 3L, and 4L, respectively. For the curves of D layer, two curves are shown: the one (D') is obtained from the simulation calculation by using eq 19; the other (D) is obtained from superpositionof the calculated curve (D') and the decay curve observed with a D monolayer in the absence of any other acceptor layer. The D monolayer in the absence of acceptor layer shows an exponential-like decay with an apparent lifetime of ca. 15 ns.I2 In fact, actual decay curves of D layers in 2L, 3L, and 4L all can be fitted reasonably with a superposition of these two curves (Figure 9). In the actual evaluation of k values, first the curve-fitting calculations for the D curves of 2L-4L were made by varying the parameter values of k l with eq 19. Then the simulation calculations for the A I curves were made by varying

Excitation Energy Transport in Stacking Multilayers

TABLE II: Rate Constants of the Energy Transfer and Fluorescence Rise Times in the LB Multilayer Films 2L 3L 4L layer k, ps-'l2 rdu, ps k, ps-'I2 T " ~ ps , k , p d 2 rriu, ps 0 0.0243 0 0.0243 0 D 0.0243 40 0.211 29 0.211 16 A, 0.0904 0.109 99 0.176 57 '42 0.127 194 '43 4P" 1.2 0.42 0.32

The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 Monomer

5043

T ri mer

ORatio of the preexponential factors of a exp(-kf) to p e~p(-2kt'/~) in the fluorescence decay curves of D layers (see text, section 4). k2 value with eq 20. These sequential curve-fitting calculations were repeated until the k3 and k4 values were determined. The parameter values of the sequential energy transfer thus obtained are summarized in Table 11. The ratios of the preexponential factors of exp(-kt) to exp(-2kt1l2) terms for D curve are also listed in the table. The k values in LB multilayers fall between 0.02 and 0.21 s-I/~. These values are comparable to those in the biological antenna pigments (Table I), indicating that the distribution of chromophores and the critical transfer distance & in the LB films studied are in parallel with those of the phycobilin-chlorophyll system. Interestingly, the k value of D-AI is markedly smaller than the values of the other processes. The smaller value in k must be a consequence of a less effective dipole orientation between D and A I . In D and A I monolayers, long axes of chromophores are in-plane and parallel to substrate surface. However, the electronic transition dipole moments of carbazole and oxocyanine are perpendicular with each other, since the transition dipole moment of carbazole is directed in the short axis.42 On the other hand, the dipole orientations of cyanine chromophores in A,, A2 and A3 are parallel to each other and parallel to the substrate plane. 6. Comparison between Biological Antenna and Artificial Multilayer Films The phenomenological aspects of the sequential energy transfer studied are on the whole parallel between the biological and the artificial systems, with respect to the fluorescence rise and decay times and the rate constant of the energy transfer (see Figure 4 and Tables I and 11). However, the transfer efficiency is higher in the biological antenna than in the LB multilayer; the efficiency in each step is 0.9 in phycobilisomes40whereas it is 0.5-0.8 in LB films. The transfer efficiency is determined predominantly by the branching ratio between the energy transfer to adjacent layer (process 10) and the energy trapping within a layer (process 6). Lower efficiencies in LB multilayers are a consequence of the relatively large density of traps due to dimer and/or higher aggregates of chromophores. In this connection, one should note that the distribution of pigment chromophores is neither uniform nor random in LB monolayers and form an irregular structure, i.e., island structure. Recent studies on time-resolved fluorescence spectroscopy of LB films1h'2*43have demonstrated such an irregular distribution with the effective trap density being much higher than estimated from the actual concentration of chromophores. Even in a low concentration of guest molecule, the island structure makes the effective concentration significantly high and results in forming a large number of dimer and/or higher aggregates on which the excitation energy is trapped.43 In contrast to the artificial LB multilayers, the biological antenna system has a uniform architecture of chromophore distribution, in which tetrapyrroles are distributed with a regular array in polypeptide networks. Recently, the structure of C-phycocyanin of Mastigocladus laminosus has been analyzed by means of an X-ray crystallography with a 0.3-nm r e s ~ l u t i o n : ~Three * ~ ~ ~mnomers, each of which has a crescentlike structure, are oriented (42) Johnson, G . E. J . Chem. Phys. 1974, 78, 1512. (43) Yamazaki, I.; Tamai, N.; Yamazaki, T. In Ultrafast Phenomena V; Fleming, G . R., Siegrnan, A. E., Eds.; Springer Series on Chemistry and, Physics; Springer-Verlag: Berlin, 1986; Vol. 46, pp 444-446.

Figure 11. Schematic representation of position of tetrapyrrole chromophores in C-phycocyanin monomer (left) and trimer (right) reproduced by reference to Figure 11 in ref 38. ~ ( 8 4 and ) @-(84)and @-(l55) mean the chromophores that are covalently bound to the cysteine residue in position 84 or 155 of a- or @subunit, respectively. with a C3 symmetry arrangement around a central axis and form a cyclic trimer with a central hole. Figure 11 shows schematically the structure. The a-chromophore in position 84 of the amino acid sequence is located at the edge of each monomer outside the cyclic trimer. In the @-subunit,the chromophore in position 84 lies at the edge opposite the a-chromophore, but inside the cyclic trimer. The hexameric aggregate takes a form of a face-to-face complementary fitting aggregate of two trimers. In these specific arrays of chromophores, the energy transfer should take place with an extremely high efficiency even in fairly long interchromophore distances; among the energy relaxation processes, processes 5-1 0 shown in section 5, the energy dissipation other than the energy transfer takes place only through the fluorescence emission. The fact that chromophores in phycobiliprotein form a specific array must be related to the observation that the transfer rate in phycobilisomes depends on whether the particular phycobilin is excited by light irradiation or by the energy transfer from the phycobilin-situated outer surface; for example, the energy-transfer rate constant at the PC-APC step is 0.29 ps-lI2 in the sequence of PE-PC-APC of PE cells but it is 0.087 ps-'l2 in PC-APC of PC cells. This means that there exists a specific channel for the energy transfer inside the phycobilisome rod directed toward the core of phycobilisome and RC. When PC in the PE cell or the PC cell is irradiated by 600-nm light, tetrapyrroles outside phycobilisome rods, i.e., ~ ( 8 4 and ) @-(155),are first excited, and then the excitation transfers to the inner chromophores of @-(84) followed by the straightforward transfer along the tetrapyrrole string inside the phycobilisome rod. On the other hand, when the PE cell is irradiated by 560-nm light, PC is excited only with the excitation transfer through this channel. In this case, the PC-APC step should be a faster step in the energy transfer. Probably this channel corresponds to the string of 20-30 tetrapyrroles of a-(84) and @-(84)along the phycobilisome rod. This is consistent with the experimental results of time-resolved fluorescence depolarization of isolated phycobilisomes by Gillbro et aL30 They assigned the fastest fluorescence decay of 10-ps lifetime to the transfer from sensitizing ("s") to fluorescing ("f") chromophores (these chromophores are assigned to a-(84) and p-(l55) for "s" and @-(84) for "f" chromophore^^^) within a hexamer of PC and the 50-ps lifetime component to the energy transfer along phycobilisome rods. The present study has demonstrated that, for both biological antenna and artificial multilayers, the time behavior of the sequential energy transfer can be expressed approximately with empirical equations of e ~ p ( - 2 k t ' / ~type ) (eq 19-22). This form of equation can be derived under an extreme condition from the equation adapted to the three-dimensional system (eq 12). The corresponding equation for the two-dimensional system, Le., e~p(-3ktl/~)-type equation, can be derived from eq 11 in a similar mathematical treatment. It is found, however, that this type of (44) Mirnuro, M.; Fuglistaller, P.; Rumbeli, R.; Zuber, H. Biochim. Biophys. Acta 1986, 848, 155.

J. Phys. Chem. 1988, 92, 5044-5048

5044

equation is not adaptable for both systems; the curve profile of this equation is essentially different from the observed curves (Figures 4 and 9). According to Baumann and Fayer,6 the form of kinetic equations of the interlayer energy transfer is changeable depending on the specific conditions such as density and interlayer distance of guest chromophores involved in layers. Further detailed analyses should be made by varying parameters of the chromo-

phore density, the interlayer distance, and the number of layers. Experimental study along this line is now in progress. Acknowledgment. We acknowledge Professor Saburo Nagakura for his encouragement and fruitful discussion on this work. This work was supported by Ministry of Education, Science and Culture, Grant-in-Aid for Special Project Research No. 621 13002.

Thermal Properties of Tetrakis(alky1thio)tetrathiafulvalenes Zurong Shi,t Toshiaki Enoki,*qt Kenichi Imaeda, Kazuhiko Seki,s Peiji Wu,+ Hiroo Inokuchi, Institute f o r Molecular Science, Okazaki 444, Japan

and Gunzi Saito The Institute for Solid State Physics, The University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan (Received: December 3, 1987)

Thermal properties have been investigated for a series of tetrakis(alky1thio)tetrathiafulvalenes which consist of a TTF ?r system and four alkylthio substitutional groups with n carbon atoms (abbreviated TTC,-TTF’s ( n = 1-18)), by means of differential scanning calorimetry. The linear n dependence of the enthalpy and entropy changes at the melting point, AH, and AS,, is observed, which is consistent with the behavior of flexible molecules reflecting the contribution from the configurational change in alkyl chains of TTC,-TTF’s to the entropy change at the melting point. The n dependence of the melting point, in addition to that of A”, and AS,, suggests that the series of TTC,-TTF‘s are divided into two subgroups depending on n. In one subgroup with smaller n 5 7, the intermolecular interaction between adjacent TTF skeletons dominates the crystal structures, while in the other subgroup with n > 7, van der Waals intermolecular interaction associated with alkyl chain groups works to reduce the interplanar distance between adjacent TTF‘s in the crystal. The reduction in the interplanar distance is also suggested by the investigation of crystal density in the latter subgroup and is considered to cause the good electrical conduction observed in these materials.

Introduction Studies on organic conductors have been widely carried out after a discovery of a perylene-bromine complex with high conductivity.’ Nowadays, organic metals and organic superconductors have been found through superior molecular desigm2 Apart from the above charge-transfer complexes consisting of two components (donor and acceptor) with conjugated a systems, high conductivity will be realized even in one-component organic materials in the presence of columnar structures with effective overlaps of a orbitals between molecules. Tetrakis(alky1thio)tetrathiafulvalenes (abbreviated TTC,-TTF‘s) studied in this work are a series of TTF derivatives with four alkylthio substitutional groups as shown in Figure 1 in which we have observed rather good electrical cond u c t i o ~ It ~ is synthetically easy to extend the length of alkyl chain in TTC,-TTF, and the compounds with n = 1-18 have been obtained before now. Through the measurements of electrical conductivities,crystal structures, and ionization energies for a series of TTC,-TTF’s, we have found a new function named as “molecular fastener effect” in the compounds with long alkyl chains3 The meaning of the fastener effect is that intermolecular interactions between side alkyl chain groups work to reduce the interplanar distance between adjacent a moieties so that high conductivity is realized along the stacking direction of conjugated a systems. In fact, TTC,-TTF and TTClo-TTF have peculiar crystal structures with strong S-S atomic contacts of 3.57 8, between molecules and a short interplanar distance of 3.49 8,,3,4 and show the extraordinarily low ionization energies of 4.7 eV in the solid, suggesting good electrical cond~ction.~TTClo-TTF Permanent address: Institute of Chemistry, Academia Sinica, Beijin, China. *Present address: Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan. Present address: Faculty of Science, Hiroshima Universtiy, Hiroshima 730, Japan

0022-3654/88/2092-5044$01.50/0

and TTCII-TTF reveal the remarkably low resistivities of -lo5 Q cm.‘j In this paper, we present the experimental results of thermal properties and crystal densities of TTC,-TTF‘s to discuss the essence of the molecular fastener effect from the standpoint of thermodynamics and molecular packings, taking into consideration the results of crystal structures, electrical conductivities, and ionization energies in the solid.

Experimental Section The details of the synthetic method of TTC,-TTF ( n = 1-18) were described in the previous paper.’ The samples were purified by column chromatography with silica gel and recrystallization in a mixed solution of hexane and methanol. The purity was examined with thin-layer chromatography. The colors of the samples with n = 1, 3, and 18 were yellow and orange for the others. The thermal properties of TTC,-TTF were investigated by using a differential scanning calorimeter (Du Pont 990) in the temperature range between -120 O C and T, 15 “C, where T, is the melting point. The transition temperatures were determined by the onsets of DSC peaks. The enthalpy and entropy changes

+

(1) Akamatu, H.; Inokuchi, H.; Matsunaga, Y. Nature (London) 1954, 173, 168. ( 2 ) Proceedings of ICSM’86 Synrh. Met. 1987, 17-19. (3) Inokuchi, H.; Saito, G.; Wu, P.; Seki, K.; Tang, T. B.; Mori, T.; Imaeda, K.; Enoki,T.; Higuchi, Y.; Inaka, K.; Yasuoka, N. Chem. Lett. 1986, 1263. (4) Higuchi, Y.; Inaka, K.; Yasuoka, N., private communication. ( 5 ) Seki, K.; Tang, T. B.; Mori, T.; Wu, P.; Saito, G.; Inokuchi, H. J . Chem. SOC.,Faraday Trans. 2 1986, 82, 1067. (6) Imaeda, K.; Enoki, T.; Shi, Z.; Wu, P.; Okada, N.; Yamochi, H.; Saito, G.; Inokuchi, H . Bull. Chem. SOC.Jpn. 1987, 60, 3163. (7) Wu, P.; Saito, G.; Imaeda, K.; Shi, Z . ; Mori, T.; Enoki, T.; Inokuchi, H. Chem. Lett. 1986, 441.

0 1988 American Chemical Society