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
The Journal of Physical Chemistry, Vol. 92, No. 1 7 , 1988 5045
Thermal Properties of TTC,-TTF's
TABLE I: Glass Transition Temperature ( T & Crystallization Transition Temperature ( T c ) ,Solid-Solid Phase Transition Temperature ( Ts), Melting Point (T,,,),Enthalpy Change at Melting Point (AH,,,), Entropy Change at Melting Point (AS,,,), Observed Value of Density (dobsd), and Calculated Value of Density (dmld)for TTC,-ITF
compd TTC 1-TTF TTC,-TTF
T,, OC
Tc, OC
-30.6 -50.1
TTC,-TTF TTC,-TTF
-59.3 -62.1
TTCS-TTF
-62.4
TTC6-TT F TTC7-TTF
-64.0 -6 1.O
-11.2 -23.7 -4.2 -27.5 -23.4 -5.0 -37.7 -12.7 -28.9 -41.7 -0.4
TTCs-TTF TTCg-TT F TTC lo-TTF TTCII-TTF TTCl,-TTF TTCl3-TTF TTC 14-TTF TTCl S-TTF TTCl6-TTF TTC IT-TT F TTCIS-TTF
T,,OC
32.2 39.8 54.6 37.2 56.2 66.3 79.8 81.7 81.2
T,,,, OC
AH,, kJ mol-'
AS,, J K-' mol-I
dobad,g cm-3
96.5 70.6
32.0 41.2
86.6 123
1.572 1.440
30.4 24.6
34.4 68.2
115 230
1.350 1.308
32.2
59.4
195
1.240
28.6 44.0
59.0 77.8
196 246
1.195 1.174
47.6 56.8 59.4 63.6 68.5 72.7 76.5 79.3 83.8 84.1 85.0
85.8 106 138 143 124 172 162 192 214 208 216
267 322 414 423 364 498 464 544 602 582 607
1.146 1.144 1.145 1.130 1.107 1.115 1.091 1.100 1.092 1.082 1.074
d C a l ~g , 1.568 1.446
1.242
1.149 1.148
R=CnH2n+i Figure 1. Molecular structure of TTC,-TTF.
-
at the melting point, AH, and AS,, were calibrated with mercury as a standard sample. The crystal densities were investigated by a density gradient method in an aqueous solution of phosphoric acid with a calibrated density ball made of glass. The positions of glass balls and samples floating in the solution with density gradient were measured accurately by a cathetometer. The solution temperature was maintained at 20 OC in an air-conditioned room.
Results and Discussion Thermal Properties. The investigation of thermal properties by means of differential scanning calorimetry (DSC) gives information about the melting points T, depending on the number of carbon atoms n of an alkyl chain in TTC,-TTF. Solid-solid phase transitions T , were observed just below T , for the compounds with n I8. A glass transition Tgwas observed which was accompanied with one or two crystallization transitions Tc with an exothermic change just above T g in a heating run for the compounds with n I7, where the solidsolid transition was absent. The glassy state in the compounds with 2 I n I4 was realized regardless of the cooling conditions below the melting point in which the cooling rates were 1 and 25 OC/min for a slow and a rapid cooling, respectively, while only the rapid cooling could give the glassy state for the compounds with n = 1 and 5 I n I 7. Table I and Figure 2 summarize the n dependence of T,, T,, Tg, and T,. The enthalpy and entropy changes at T,, AH,, and AS, are also presented in Table I. According to these results, the series of TTC,-TTF's are phenomenologically divided into two subgroups. One subgroup with small n ( n I4-7) is characterized by the decrease in T, with n and the presence of the glassy state. On the contrary, for the other subgroup with large n ( n 1 8), T, increases in an alternate manner, accompanied by a solid-solid transition just below T,. (Though a solid-solid transition is not observed for the compounds with n = 13 and 15 in our DSC experiment, recent observation under a polarized microscope with a hot stage suggests the presence of a solidsolid transition in these compounds.8) This obvious difference of thermal properties
1
1
-
c
0
5
n
10
15
Figure 2. Carbon number dependence of glass transition temperature crystallization transition temperature (Tc),solidsolid phase transition temperature (T,), and melting point (T,) for TTC,-TTF.
(T!),
between the compounds with n I 7 and the compounds with n 1 8 can be related to the relative length of an alkyl chain which becomes nearly equal to that of a tetrathieTTF ( c & ) moiety at n = 8. Now we analyze the results of the melting behavior of TTC,-TTF depending on n to clarify the contributions of the central CsSs moiety and the side group of alkyl chains to the thermal properties. The alkyl chain groups in a TTC,-TTF molecule are considered to be flexible parts similar to n-alkanes. The enthalpy and entropy changes at a melting point, AH, and AS,, for flexible molecules such as n-alkane with n carbon atoms can be expressed in the equations9 AH, = Ho + nH (1) where n is the carbon number, S, is positional entropy, So, is is configurational entropy which orientational entropy, and Smnf is the most relevant to flexible molecules. The former two terms S, and So, do not depend on the number of carbon atoms, while (8) Miyajima, S.; Abe, I.; Chiba, T.; Saito, G.; Inokuchi, H., private
communication.
(9) Ubbelohde, A. R. The Molten Stare of Matter, Melting and Crystal Structure; Wiley: Chichester, 1978.
5046 The Journal of Physical Chemistry, Vol. 92, No. 17, 1988
Shi et al.
TABLE 11: Values of HmSmH , S . and lim-m T , for TTC,-TTF and n-Alkanes with Even Number and Odd Number of C Atoms compd H,, kJ mol-l So, J K-' mol-' H , kJ mol-' S, J K-I mol-' limws T,,,, "C
TTC.-TTF n-alkane (even) n-alkane (odd)
6.02 -1 1.7' -1 1.2'
43.1 10.0' 5.02b
2.97" 4.08' 3.01'
8.05" 10.8' 7.66b
95 105 120
"The value per one alkyl chain. 'See ref 11.
2ook 100
1 ' " ' " " ~ ' ' " ~ " ' 1
r/
,
,
,
,
,
,
,
,
,
I
*
,
, ,
,
1
1
1
,
~,
100
0 0
0
0
o o o
0
6001
I
4 00
2oo~o,;~o~;I ~
00
5 n
io
, , , ,
,
,
,I
-20 Oi
- 4 0 1 ! > , L l
0
1
10
5
1
1
1
,
1
1
15
n
Figure 4. Comparison of the behavior of melting point with n between
the experimental result (solid line) and the calculated result (dashed line) from T,,, = AH,,,/ASs, using experimental relations of AH,,, and AS,,, with n.
15
Figure 3. Carbon number dependence of enthalpy and entropy changes at melting point, AH,,, and AS',,,, for TTC,-TTF. The solid lines represent the lines drawn by the least-squares method.
Sconf includes a term proportional to n since each carbon atom contributes to a configurational change. Therefore, we replace (2) with the equation AS, = So + nS (3) Figure 3 shows the carbon number dependence of AH,,, and AS,,, for TTC,-TTF, where AH, and AS,,, have linear relations with n which are consistent with the behavior of flexible molecules. The least-squares fittings to the experimental results give the following equations: AH,,,(kJ mol-') = 6.02 + 11.8n and AS,,,(J K-I mol-]) = 43.1 + 32.2n. At first, we discuss the first term, So, in (3). The typical magnitude of positional entropy can be estimated with the melting behavior of plastic crystals consisting of rigid molecules. Just below the melting point, the rigid molecules in a plastic crystal are rotating freely, so that only a change in the positional entropy S, contributes to the melting process since So, = 0 and Smnf = 0. The empirical results of the change in S, give the estimation of S, 5 15 J K-' m01-I.~ In the case of n-alkanes, the changes in the enthalpy and entropy at melting points have linear relations with n, as shown in Table 11. The terms So independent of the number of carbon atoms are estimated at 10.0 and 5.02 J K-' mol-' for the n-alkanes with even and odd numbers of carbon atoms, respectively. Therefore, Sois considered not to include the change in the orientational and configurational entropies so much in the melting of n-alkanes. On the other hand, for TTC,-TTF's, So = 43.1 J K-' mol-' is greatly larger than that of the plastic crystals or the n-alkanes. This means that Sois associated not only with S, but also with So, and SmnPThe TTF skeleton in the TTC,-TTF's has a planar structure, and the bondings between the T T F part and the outer sulfur atoms are supposed to be flexible, taking into account the existence of two kinds of molecular configurations, boat form for n = 1-3 and chair form for n L 4.6 The contributions of So,and Smnf are ascribed to the anisotropy and flexibility of the C.438 moiety in the TTC,-TTF molecules. Next, we compare the linear term S of the TTC,-TTF's with that of the n-alkanes. A TTC,-TTF molecule involves four equivalent alkyl chains, so that S / 4 = 8.05 J K-I mol-' should be taken as the change in the configurational entropy in an alkyl chain. Since each carbon atom has three possible configurations in an alkyl chain, the linear term of the configurational entropy
is estimated at 9.13 J K-l mol-], according to the equation nS = R In 3" using Boltzmann relation. In the case of the n-alkanes with even n which structure is triclinic, S is evaluated to be 10.8 J K-I mol-I, which suggests that the melting generates completely random configurations in the alkyl chains in molten state. The n-alkanes with odd n have smaller configurational entropy (S = 7.66 J K-I mol-]) than those with even n. The solid phases just below the melting points in these compounds have hexagonal crystal structures where the alkyl chains are rotating freely along the long axis of the molecule. This has been considered to require some amount of the configurational entropy in the n-alkanes with odd n.'O In the case of the TTC,-TTF's, the configurational entropy (8.05 J K-' mol-') is also smaller than that of the n-alkanes with even n or nS/4 = R In 3,. Since the one end of the alkyl chain is bound to the CSS8 moiety in the TTC,-TTF molecule, the configurational change at the melting point will not be completely developed so that the magnitude of the change in the configurational entropy is supposed to be smaller than the estimation from nS = 4 R In 3". Next, we discuss about the behavior of the melting points changing with the number of carbon atoms n after the analysis of the melting properties for n-paraffin by Broadhurst.'* According to (1) and (3), the melting point T,,, of flexible molecules is given by Ho + n H T,,, = So nS
+
~
H d H - so/s = "S( l +
n
+ So/S
(4)
T, obeys a hyperbolic function of n withasymptotic lines of T , = H/S and n = -So/S. The shape of the function depends on
the difference in magnitudes between the enthalpy ratio of Ho/H and entropy ratio of SOIS.In the case of HoIH> SOIS,T,,, shows a hyperbolic curve decreasing with n, while in the case of HoIH < So/S,T , shows an increasing curve with n. The experimental result of T , for TTC,-TTF is found to include the above two cases, as shown in Figure 4. Namely, for n I4, T,,,steeply decreases as n decreases, while it gradually increases with n for larger n ( n > 5). (This behavior of T , with n resembles that of 1- or 2-nalkylnaphthalenes which consists of an aromatic part and an (10) McClure, D. W. J. Chem. Phys. 1968, 49, 1830. ( 1 1) Broadhurst, M. G. J. Res. Natl. Bur. Srand., Sect. A 1962,66A, 241. (12) Broadhurst, M. G. J . Chem. Phys. 1962, 36, 2578.
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 5047
Thermal Properties of TTC,-TTF's aliphatic part similar to the present TTC,-TTF system.13) The former corresponds to the case of H o / H > So/S, while the latter to the case of H o / H < So/S. If all contribution to entropy,, ,S SOT, and S,, are consumed in the melting process and molecular conformations change to random coils in liquid state, the entropy ratio So/S will be assumed to change hardly with n, as a first approximation. Thus, the enthalpy ratio H o / H should decrease with the increase of n to give rise to the two cases, though the decrement is small since the experimental results shown in Figure 3 suggest that the quantities Ho, H , So,and S are approximately constant. This means that the role of intermolecular interactions associated with alkyl chains at melting is relatively important for large n (n > 5 ) in comparison with the region with small n (n 5 4). The importance of the intermolecular interactions between alkyl chains in the region with large n can be understood by means of the experimental results of crystal structures and densities of TTC,,-TTF, as discussed in the next section. The melting point T , as a function of n can be estimated by using ( l ) , (3), and (4) and the values of Ho, So,H , and S in the region with large n, as shown in Figure 4. The estimation is in good agreement with the behavior of the observed T , for the compounds with long alkyl chains. The melting point at infinite n (n a) is evaluated at 95 OC from limpT , = limp(AH,/A,S,). The magnitude of limp- T , is about the same with that for the n-alkanes with even or odd n (105 or 120 OC, respectively) shown in Table 11. Finally, we discuss the glass transitions and the solidsolid phase transitions. The glass transitions take place in the compounds with short alkyl chains (n 5 7), where intermolecular interactions associated with tetrathio-TTF moieties importantly contribute to their crystal structures,6 which are not so favorable to the intermolecular interactions related to the alkyl chain parts. This means that the alkyl chains in these compounds with small n possess more free volume than those in the compounds with large n (n 2 8). The presence of a glassy state requires free volume (effective vacancy) to give room for configurational changes in the alkyl chains. Therefore, the glass transition in the compounds with short alkyl chain is supposed to be caused by the quenching of configurational randomness in the alkyl chains. The compounds with long alkyl chains (n 1 8) have solid-solid transitions just below melting points. According to the thermal properties and the crystal structures: the behaviors in the compounds with long alkyl chains resemble to those of n-alkanes. n-Alkanes with odd number of carbon atoms have solid-solid transitions just below melting points, similar to the present case.14 The solid-solid transitions in the n-alkanes are ascribed to the onset of nearly free rotation of alkyl chains along the long axes of the molecules. In the case of TTC,-TTF, though the origin of the solid-solid transitions is not clear from the results in the present experiment, the rotation of alkyl chains may be one of the possibilities to explain the transitions. Densities. X-ray analyses for TTC,,-TTF compounds show that crystal structures are quite different between the compounds with n I3 and the compounds with n I The density measurements will give some information concerning the difference in molecular packing for TTC,-TTF compounds. Table I sum) the densities by the density marizes the observed values ( d O M of gradient method and the calculated values (daId) from crystal data for TTC,-TTF. The excellently good agreement between these values assures the accuracy of the present experiment. Here, we try to treat the densities by means of a phenomenological analysis. The molecular weight consists of two terms: a contribution from the c& moiety ( M o )and that from four alkyl chains with 4n carbon atoms (approximated to be nM). An effective volume of a TTC,-TTF molecule is also considered to be described in terms of the above two contributions (Vo + nV).
-
4.4915
(13) Anderson, D. G.; Smith, J. C.; Rallings, R. J. J . Chem. SOC.1953, 443. (14) Piesczek, W.; Strobl, G. R.; Malzahn, K. Acta Crystallogr., Sect. E Struct. Crystallogr. Cryst. Chem. 1974, 830, 1278. ( 1 5 ) Katayama, C.; Honda, M.; Kumagai, H.; Tanaka, J.; Saito, G.; Inokuchi, H. Bull. Chem. SOC.Jpn. 1985, 58, 2272.
1.51
\
\
1.4: o\
3
5
\
1,3:\\%\\
0 v
p
1,2-
'
--&>,
1.1 -
""\ V-kXa,:
1.0I , , , , I I , , I I
_1/;,
-1-: % U
I , ,
, ,
I
I
,,,
I
,
2
0
0
5
10
15
n
Figure 6. Correlation between ( d - 0.96)-'and carbon number n. The solid lines represent the lines drawn by the least-squares method.
Therefore, the density of the TTC, - TTF crystal can be expressed by the equation Mo nM d= NWO +
+
nv
M o / M - vo/v n + vo/v
= - (M 1+ NV
where N is Avogadro's number. According to (9,the density for TTC,-TTF is expected to change in a hyperbolic function of n with asymptotic lines of d = M / N V and n = -Vo/V, and is extrapolated to the density of polyethylene (0.96)16 at infinite n. Figure 5 shows the n dependence of the densities for TTC,-TTF, where the densities are varying from 1S 7 2 of n = 1 to 1.074 of n = 18 in a nearly hyperbolic curve. If the TTC,-TTF crystal has similar molecular packing independent of n, the density should obey a simple hyperbolic function. However, a careful investigation of the present result reveals a superposition of two kinds of hyperbolic curves as shown by solid and dashed lines in Figure 5 . This fact suggests a difference in the manner of molecular packing between the compounds with n 2 8 and the compounds with n < 8. The modification of ( 5 ) to the equation 1
n + vo/v M Mo/M-Vo/V
- N_V
d-M/NV-
(6)
clarifies this difference in the molecular packing, as illustrated in Figure 6. In (6), the value of M / N V denotes the density of polyethylene and l/(d - M / N V ) is expressed by a linear function of n, if the manner of the molecular packing does not depend on n. Figure 6 shows an inflection point of a linear line at n rrr 8 which suggests the presence of two regions of n corresponding to a different manner of the molecular packing from each other. As the value of M o / M is known, the slope of the straight line gives the value of Vo/V. V o / Vis estimated to be 4.0 in n < 8 and 2.9 (16) The density of polyethylene made by the high-pressure method.
Shi et al.
5048 The Journal of Physical Chemistry, Vol. 92, No. 17, 1988
in n I 8. Vo/Vindicates the ratio of the effective volumes between the c6sS moiety and four alkyl groups. The smaller value of Vo/V in n I 8 than in n < 8 means that the volume of a c6sS moiety is effectively depressed for the compounds with long alkyl chains. The close packing of c6sS moieties is considered to bring about the reduction in the interplanar distance between adjacent TTF skeletons, which is consistent with the crystal structure analysis6 Namely, TTC9-TTF and TTClo-TTF possess a remarkably short interplanar distance of 3.49 A, while the interplanar distance of TTCS-TTF is 3.73 A, which is longer than that of TTC9-TTF or TTC,,-TTF. The reduction in the interplanar distance for the compounds with long alkyl chains is considered to be caused by intermolecular van der Waals interaction through long alkyl chains taking into account the result of thermal properties as shown in Figure 3. The strong intermolecular interaction due to the reduction of interplanar distance gives novel electronic properties, Le., the low ionization energies for the compounds with n I 8 and the remarkably low resistivities for TTCIo-TTF and TTCII-TTF.’7 These can be accomplished by the overlapping of adjacent T electron systems generated by the reduction in the interplanar distance between adjacent TTF‘s. Therefore, the function named a molecular fastener effect is attributable to the peculiar structure of TTC,-TTF molecules consisting of an aromatic moiety and alkyl chains, judging from the present results of thermal properties and crystal densities.
Conclusion Thermal properties of tetrakis(alky1thio)tetrathiafulvalene (abbreviated as TTC,-TTF) with the number of carbon atoms n = 1-18 in each alkyl group have been investigated by differential scanning calorimetry. The crystal densities of these compounds were also measured by the density gradient method. The experimental results on the thermal properties suggest that the series of TTC,-TTF are divided into two subgroups: the compounds with n I7 and the compounds with n 2 8. For the former subgroup with glass transition, melting point (T,) decreases with n, while T , for the latter subgroup increases alternately with n accompanied by a solid-solid transition just below T,. It is interesting that the length of the side alkyl chain becomes nearly equal to that of the central tetrathio-TTF (c6sS) moiety at the boundary between these two subgroups. Enthalpy and entropy changes at T,, AH,, and AS, have linear relations with n, where (17) From conductivity measurement, low resistivity is observed even at n = 4, so that the molecular fastener effect is considered to act partly around n 4. See ref 6.
-
+
+
AH,(kJ mol-’) = 6.02 11.8n and AS,(J K-I mol-’) = 43.1 32.2n. The linear n dependences of AH, and AS, are consistent with the behavior of flexible molecules where the configurational entropy change in alkyl chains contributes to the entropy change at T,. The behavior of T , as a function of n can be qualitatively explained by the ratio of the enthalpy changes between the tetrathio-TTF (c6sS) moiety and four alkyl chains. In the region of small n ( n I4) with the decrease in T,, intermolecular interactions by the c6sS moieties are dominant in the crystal structure, while in the region of large n ( n > 5 ) with the increase of T,, intermolecular interactions associated with alkyl chains play an important role. The experimental results of the crystal densities also suggest the existence of the two subgroups, as well as the result of thermal properties. The ratio between the volume of a c& moiety ( Vo)and the volume of four side alkyl chains has an abrupt change at the boundary between the two subgroups. Namely, the ratio (Vo/V= 2.9) is smaller in the compounds with long alkyl chains ( n I 8) than the ratio (Vo/V = 4.0) in those with short alkyl chains ( n C 8). The crystal structure analysis shows that the interplanar distance between adjacent TTF planar skeletons is reduced in the compounds with long alkyl chains. For example, the interplanar distance in TTCIo-TTF is 3.49 A, which is remarkably shorter than that of TTC5-TTF (3.73 A). Taking into account the thermal properties of the TTC,-TTF series, the experimental results of the densities and the crystal structures are considered to suggest that the van der Waals intermolecular interactions associated with the side alkyl groups act to reduce the interplanar distance between adjacent T T F skeletons in the compounds with long alkyl chains (n I 8). The previous paper6 reported the remarkable enhancement of electrical conduction in the compounds with long alkyl chains, in comparison with that in the compounds with short alkyl chains. The effective overlapping of ?r electrons between adjacent c6sS conjugated systems is considered to be realized through the reduction in the interplanar distance in the case of the compounds with long alkyl chains. Therefore, the van der Waals intermolecular interactions associated with side alkyl chains play an important role to generate good electrical conduction for the compounds with long alkyl chains. This function of the alkyl chains can be regarded as a “molecular fastener effect”. Acknowledgment. We thank Dr. S. Miyajima at Nihon University and Prof. M. Oguni at Tokyo Institute of Technology for their valuable discussions. W e also thank S. Bandow of Institute for Molecular Science for his helpful advice on DSC measurements.
