Sulfur-bridged polycyclic aromatic compounds

Carbon Printed

Vol. 21. No. 4. pp. 337-343. in Great Britain.

1983 0

SULFUR-BRIDGED POLYCYCLIC COMPOUNDSt

0008&6223/83 1983 Pergamon

$3.00 + .oO Press Ltd.

AROMATIC

H. AKAMAT~Jand H. INOKUCHI Institute for Molecular Science, Myodaiji, Okazaki 444, Japan and

M KINOSHITA Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan (Received 25 January 1983)

Abstract-When polycyclic aromatic hydrocarbons are heattreated with an excess amount of sulfur in evacuated closed tubing, they react with sulfur, liberating hydrogen sulfide; after removing unreacted sulfur, dark-colored compounds in the form of an amorphous solid are obtained. These compounds possess semiconductive properties; the electrical resistivity is greatly reduced from that of the original hydrocarbons. The ESR absorption of these compounds is characterized by the large g-value, which is due to sulfur-centered unpaired electrons. From these findings, with the results of chemical analysis, we can assume that the polycyclic aromatic nuclei crosslink with each other through the sulfur-bridges, which are good electron transport paths, and yield the aromaticity of the molecule. The sulfur-bridged polycyclic aromatic hydrocarbons can be considered as the structural model of chars and cokes. The ESR studies of a violanthrene-sulfur compound were carried out during its heat-treatment up to 1ClOO”C.

1. INTRODUCTION When an organic compound is heattreated, its molecules are partly decomposed and, simultaneously, aromatic rings are formed, which successively combine with each other to make a larger plane of condensed aromatic nuclei. This process of aromatic condensation results in the formation of carbons. One of the authors (H. A.)[11 has suggested that the condensed polycyclic aromatic compounds can be considered as a model for molecules of chars or carbon blacks. However, in carbons their molecules will not be free but are presumably crosslinked with each other by disorganized carbon atoms or impurity atoms. This is in particular presumable for the charstructure which is formed by the carbonization process which occurs in the solid phase. The crosslinking in char or coke is an important factor to characterize its physical and chemical properties, and also for further growth of crystallites in the carbonization process up to the formation of graphite. However, because of its complexity, the nature of the crosslinkage in char has been rarely investigated, with the of a series of papers by Prof. exception Mrozowski[2-71 on ESR studies of chars. When a polycyclic aromatic hydrocarbon is heattreated with sulfur, it reacts with the sulfur, liberating hydrogen sulfide, and changes to a dark-colored solid. It is assumed that in this compound the condensed aromatic nuclei of hydrocarbon molecules remain intact and are linked to each other by the bridges of sulfur atoms, which results in a char-like structure.

tThis work was carried out at the Department of Chemistry, University of Tokyo. CAR Vol. 21. No. 4-b

The sulfur-bridged aromatic compounds are characterized by their electrical conductivity which is remarkably improved in comparison with their parent compounds; they also show a strong ESR absorption with a large g-value, 2.0042.01. We have studied these sulfur-bridged aromatic compounds quite a long time ago. This work was partly reported at symposia in 1960[8] and 1964[9]. The topic dealt with is still an interesting theme from the current point of view that aromatic sulfur compounds have received considerable attention in recent years. This paper, a review of our past work, consists of three sections; preparation of materials, semiconductive character and structural model, and ESR absorption of violanthrene-sulfur and its char. 2. PREPARATIONOF MATERIALS Violanthrene (400 mg) was heattreated with an excess amount of sulfur (2 g) in a sealed Pyrex glass tube, after being degassed by pumping, flashed with argon and finally evacuated to 10e5Torr. The heat treatment proceeded in two steps. At first, the mixture was heated at 250°C for 3 hr; the tube was then opened in order to release the gases produced. In the second step, the sample was heated using the same procedure but to a higher temperature, up to 500°C. The unreacted sulfur was removed first by extraction with carbon disulfide using a Soxhlet extractor for 24 hr and then by sublimation in a vacuum at 250°C. The dark-colored compounds obtained are like a char powder in appearance. Chemical analysis showed that the sulfur content in the compounds increased with the heat treatment temperature (HTT); however it did not increase above a HTT of 400°C as shown in Table 1; indicating that a limit had been reached. 337

H. AUMATUet al.

338

Table 1. Sulfur content and electrical conduction in heattreated violanthrene with sulfur Heattreatment

250

320

400

450

500

23,4

35.0

40.0

40.2

40.0

5.6

8.3

9.7

9.7

9.7

1.3 x104

7.9 x103

1.7 x103

5.4 x102

temperature/% Sulfur cantent In wt. % Number of S atoms Per vlolonthrenemolecule Electricalreslstlvltv

( P15.,p cm) Activationenergy

0.20

_

(E/eV)

0.19

0.16

0.14

Table 2. Chemical analysis of heattreated compounds with sulfur (HTT 450°C) atomic ratio

wt. x

Parent comaound

s

C

H

s/c

H/C

Anthracene

Cl4HlO

49.9

44.8

0.3

S-8/14

1 I l/14

Nanhthacene

LA2

50.2

48.0

0.4

7.1/18

l-8/18

Perylene

CzoH12

5132

44.8

0.6

8.6120

3.1120

Pyranthrene

C30”1,

43.8

42.8

0.3

11.5/30

2.8/30

Violanthrene

C34”1,

40.2

52.9

0.8

9 I7134

6-O/34

Violanthrone

C34H1602

38.2

51.4

0.4

9.5734

2.9/34

Experimental

error:

0.27

8 for

sulfur

Examination by X-ray diffraction showed that a number of diffraction lines were found for the compound of HTT 250°C but they almost disappeared for the compound of HTT 320°C. The diffraction pattern of the compounds of HTT above 400°C is an amorphous one with only a halo at the spacing of about 3.5 A. The measurement of electrical conduction (Table 1) showed that, as will be described in the next section, these compounds have a semiconductive character. In the same procedure as for violanthrene, other polycyclic aromatic hydrocarbons including anthracene, naphthacene, perylene, pyranthrene and violanthrone (see Fig. 1) were heat treated with sulfur in an evacuated closed tube. After heating to 450°C dark-colored compounds in the form of amorphous solids were obtained. The results of chemical analysis for these compounds are summarized in Table 2. This indicates that most of the hydrogen of a hydrocarbon molecule were replaced by sulfur. When one can assumet that the condensed aromatic nuclei of a parent hydro-

tThis assumption is encouraged by the recent studies of I. C. Lewis and R. A. Greinke, J. Polym. Sci., Polym. C/tern. Ed. 20, 1119 (1982).

and

0.2

% for

hydrogen.

@ onthracene

naphthacene

perylene

d

0

6

CzJ!z?& rwranthrene

violonthrene

violanthrone

Fig. 1. Polycyclic aromatic compounds.

carbon remain intact, keeping its original skeleton after the heat treatment, the atomic ratio of sulfur to carbon content (S/C) will be reasonably interpreted by assuming that the sulfur atoms make bridges for crosslinking among the condensed aromatic nuclei of the parent compound. Thus these heat treated compounds may have the char-like structure, although in the ordinary chars their condensed aromatic nuclei are linked to each other by disorganized carbon atoms instead of sulfur atoms. The sulfur-bridged polycyclic aromatic compounds can be considered as a model for chars in respect to their structure.

Sulfur-bridgedpolycyclic 3. SEMICONDUCTIVE CHARACTER AND STRUCTURAL MODEL

The electrical conduction of the sulfur-bridged polycychc aromatic compounds has a semiconductive character. The preparation of specimens for the conductivity measurements was carried out in two ways. In one of them, the compounds were sealed, to eliminate the contact resistance among char-like powders, into a phenolic-resin tube, 2 mm dia., with polymerization under pressure of thermal 1600 kg/cm2. This preparation was applied to the heat treated violanthrene with sulfur at the various temperatures shown in Table 1. In the other case, the electrical conduction was measured with the specimens compressed in a quartz tube, 2 mm dia., under pressure up to 116 kg/cm2. No intrinsic difference between these two specimens was found. The semiconductive character of these compounds, observed from the temperature (T) dependence of the electrical resistivity (p); p = ~,,exp(E/kT) where E is the activation energy, is summarized in Tables 1 and 3. The polycyclic aromatic compounds are characterized by the degree of aromatic condensation of their molecules. They show a semiconductivity which increases with increasing number of aromatic rings, or with the extent of the n-electron conjugation system. The electrical resistivity plYc ranges from the order of 10”Qcm for perylene to the order of 10iORcm for violanthrone, while anthracene is still in the region of insulator. In comparison with these aromatic compounds, the following characteristics can be seen from Tables 1 and 3 for the sulfur-bridged aromatic compounds; (i) they show a fairly high semiconductivity being remarkably improved from that of their parent compounds, (ii) the effect is quite independent of the degree of aromatic condensation of the parent compounds, but (iii) it depends on the sulfur content. These observations suggest that the sulfur-bridges between the parent hydrocarbon molecules, (Fig. 2 shows the proposed model) should be good electron transport paths presumably extending the rr-electron conjugation system.

Table 3. Semiconductive

character,

Comoound

/S ‘S

o#

s,

S’

s s s

/S ‘S

9# s I

s I

s,

S’

s I

Fig. 2. A hypothetical structural model for sulfur-bridged polycyclic aromatic compounds, represented by anthracenesulfur.

The ESR observation supports this assumption. All of these sulfur-bridged aromatic compounds show a strong ESR absorption whose spin concentration and g-value are summarized in Table 3. The common characteristic for the ESR absorption of these compounds is the large g-value in comparison with the g-value, 2.002-2.003, usually observed for ordinary chars. These large g-values are.undoubtedly of the sulfur-centered free radicals, or of the unpaired electrons of mobile n-character residing on sulfur atoms[ lo]. The large g-value is reminiscent of the studies reported by Mrozowski[4], and also by Blayden and Patrick[l 11, who have sometimes observed large gvalues for their specimens of sulfur-carbons. From these above-mentioned findings, we assume that bridging sulfur yields the aromaticity of the compound: in 1960, we proposed that this structure be called a “thio-polycychc aromatic structure”[b]. This assumption was supported by the existence of a coplanar structure of a compound involving bridging sulfur. Thio-thiophthen (Fig. 3) was first discovered

spin concentration compounds Reslstivity ~~s~o/ocrn

339

aromatic compounds

and g-value

Activation energy E/eV

of sulfur-bridged

Spin concentration oer 4

s-value

Anthracene-S

1.0 x104

0.09

1.9~lo1g

2.0097

Naphthacene-S

1.4 x104

0.08

5.0~101g

2.0083

Perylene-S

2.0 x109

0.29

5.9x101s

2.0056

Pyranthrene-S

5.0x103

0.11

Violanthrene-S

1.1.104

0.19

2.9x101'

2.0067

Violonthrone-S

4.0 x10*

0.08

4.3x1o1g

2,0099

aromatic

H. AKAMATU et

340

m-7 HF

Thio-thiophthen

TTN

Fig. 3. The coplanar structure of thio-thiophthen and tetrathionaphthacene. by an Italian group [12] who showed that its molecule is planar which confers the aromatic character to this compound. Another example is tetrathionaphthacene (TTN). The unusual electronic behavior of TTN gives some support for this assumption of aromaticity. The electrical resistivity of TTN in vacua is very low as compared with the ordinary polycyclic aromatic hydrocarbons: 1.3 x lo6 R cm at room temperature for a surface type cell and 4.2 x 10” n cm for a sandwich_type[l3]. Its ionization potential in the gaseous state (6.07 eV) is considerably lower than that of naphthacene (6.89 eV), and in the solid state (4.4eV) it is lower than that of graphite (4.7 eV)[14]. 4.ESR ABSORPTION OF VIOLANTHRENESULFUR AND ITS CHAR

The violanthrene-sulfur compounds prepared by heattreatment at the various temperatures shown in Table 1 are finely pulverized by an agate-mortar and examined by an Hitachi MPS-1 ESR spectrometer. The results are summarized in Table 4. The violanthrene-sulfur compounds prepared at HTT 250°C and 320°C exhibit almost the same feature. The ESR signal is very intense and consists of a symmetrical single absorption with a line-width of about 6 G between the points of maximum slopes (AH,,,,,). The signal intensity is not affected by air to an observable extent, but the line-width becomes a little broader by the introduction of air. The spin concentrations of 7.5 x lOI and 1.0 x 10” spins/g correspond to the situation in

al.

which there is an average of one unpaired electron per 15-20 molecular skeletons of violanthrene. The parent violanthrene itself exhibits a weak ESR signal, probably due to impurities, corresponding to a spin concentration of 7.5 x 10” spins/g. When violanthrene is heat-treated below 550°C in an inert atmosphere, the number of unpaired electrons originally present is known to decrease[2]. The high spin concentrations observed here therefore indicate that a number of spin centers are newly formed during the reaction with sulfur. Replacement of a number of hydrogen atoms by sulfur atoms takes place during the heattreatment, but some of the spin centers formed by the removal of the peripheral hydrogen atoms may be left uncombined with sulfur. The unpaired electrons thus formed seem to be mobile, because the somewhat large g-values observed are characteristic of unpaired electrons residing on a sulfur atom for some portion of time. Such a large g-value shift from the free spin value is quite commonly observed in organic radicals containing sulfur atoms, owing to the large spin-orbit coupling of p-orbitals of a sulfur atom[lO, 15, 161. It is therefore very likely that each unpaired electron in these sulfur-bridged compounds is stabilized by delocalization over at most 15-20 molecular skeletons of violanthrene on the average. A polycrystalline organic radical containing sulfur atoms usually gives rise to an ESR absorption with a structure characteristic of g-value anisotropy. However, the compounds studied here show a symmetrical absorption with the line shape very close to the Lorentzian. The g-value anisotropy seems to be averaged out by exchange interactions that are increased by electron delocalization over several violanthrene skeletons bridged by the sulfur atoms. Even though there is exchange narrowing, the line-width of about 6G is rather broad. This may be due to an enhancement of spin-lattice relaxation through the spin-orbit mechanism owing to the involvement of the sulfur atoms in the electron delocalization. On the other hand, the violanthrene-sulfur compounds prepared at higher HTT (400,450 and SOOOC)

Table 4. g-value, line-width and spin concentration of violanthrene-sulfur compounds

temperature/Y

Llnewidth(AH,,,/G)

s-value

Heattreatment in air

in vat.

in air

in vat.

(I

6.1

5.7

250

2.0072

320

2.0071 2.0071

Spin concentration

Per 9

7.5x 1ol9

6bg

[; 3

1.0 x 1020

400

-2.006b

C

450

-2.006b

c

b

[;,5

3 xa 1019

c

b

[;,5

9 x 1oL9

500

2,00684

a) not measured, b)asymmetric signal, c)two signals of g= 2.007 and g= 2.003, d) nearly symmetric signal after prolonged exposure to air, see Fig. 4(b).

Sulfur-bridged

polycyclic

show two ESR signals overlapping each other in a vacuum of cu. 10 ’ Torr as shown in Fig. 4(a). One of the signals is rather broad with a g-value of about 2.007; the same feature as for the compounds of HTT 250 and 320°C. However, the other signal is sharp and gives a g-value of about 2.003. The intensity of this sharp signal is approx. 20-25x of the total intensity. Mrozowski[4] also observed a similar double line structure in a char obtained from a sulfur-rich rubber (400 < HTT < 600°C) and explained it as a result of an inhomogeneous mixture of two kinds of chars. The double line signal of our sample is also explained in a similar manner, because the two signals behave quite independently to exposure to air. When air is introduced, the sharp signal becomes broad and coalesces with the broad one, resulting in an asymmetric absorption. If the sample is kept in air for a long time (a few days), the sharp signal disappears almost completely, leaving the broad nearly symmetric absorption as shown in Fig. 4(b). It is to be noted that the two signals can be observed even in air, when the sample is not pulverized. From these observations, we may conclude that the compounds prepared at relatively high HTT contain two kinds of spin centers. One is related to the sulfur atoms crosslinking the parent polycyclic aromatic compounds and is spread over the a-electron conjugation systems involving the sulfur atoms. The other exhibits the feature characteristic of spin centers usually found during carbonization processes of various hydrocarbons. Therefore, it may be said that a small portion of violanthrene is subject to the usual carbonization reaction when treated with sulfur at an elevated temperature for a long time.? The violanthrene-sulfur compounds prepared at HTT 320 and 500°C have been further heattreated at about 50°C temperature intervals up to 1000°C in a vacuum of cu. 10 5Torr. A finely ground sample (2-3 mg) sealed in a quartz ESR sample tube was placed in an electric furnace pre-heated to a temperature about 100°C below the heat treatment temperature. The temperature was raised to the treatment temperature within 7-10 min and kept there for 3 min. The sample was then cooled to room temperature (23°C) evacuated without exposure to air and measured by the ESR spectrometer. The results for the compound prepared at HTT 320°C are shown in Fig. 5. The ESR absorption first increases in intensity, reaches a maximum at HTT 520°C and then decreases rather rapidly. This behavior is similar to that usually found in the carbonization of various hydrocarbons, although the HTT where absorption becomes most intense seems to be somewhat low. The shape of the absorption line remains symmetric over the whole HTT range, but the line-width gradually decreases up to HTT 600°C tit has been shown[2] that violanthrene is hardly decomposed until HTT 55O”C, when the soaking time is short enough (I min).

aromatic

(a)

compounds

IN

341

VACUO

10

(b)

IN

G

AIR “L

----y/j

L1OG\

Fig. 4. ESR absorption of violanthrene-sulfur (HTT 500°C) (a) in a vacuum and (b) after prolonged exposure to air.

and then increases very rapidly. The g-value also changes gradually to 2.0047 for the sampie of HTT 700°C. The broad line-width and large g-value are characteristic of the present char and are reminiscent of those reported by Mrozowski for a rubber with high sulfur content [4]. The initial increase of the intensity is explained by a dehydrogenation reaction accompanied by the condensation of violanthrene skeletons to form sulfurbridged clusters. The rapid decrease of the intensity above a HTT of 520°C suggests that dehydrogenation has almost been completed and the condensation reaction becomes dominant. The condensation at this stage may also involve a reorientation of the clusters, resulting in a pairing off of the spin centers. It is to be noted that the bridging sulfur atoms are also involved in the condensation reaction, because the g-value remains large relative to the free electron value and the line-width is still broad in comparison with that of ordinary chars. The gradual change of the g-value may be attributed to the shortening of the average time spent on the sulfur atoms by unpaired electrons due to (i) the loss of a small number of sulfur atoms, (ii) the growth of the n-electron conjugation system as a result of cluster formation, or (iii) a combination of the two. The results of further heattreatment of the compound prepared at HTT 500°C are shown in Fig. 6. As mentioned above, this compound gives rise to the double line absorption. The broad signal behaves similarly to that of the compound prepared at HTT 320°C though the HTT where the absorption becomes most intense is somewhat lower, probably because the compound has already been subjected to heattreatment at 500°C. The sharp signal shows a behavior usually found in a variety of chars. However, the reason why these two behave independently even at HTT’s as high as 7OG3OO”C is in question.

H. AKAMATU er al.

342

Starting

material

/

lN=I.O

x Id0

-7’” spin+)

+j ..:

j

/Q

9

g-value

300

400

500

600

HEAT-TREATMENT

700

TEMPERATURE

800

-

2.0080

-

2.0070

-

2.0060

-

2.0050

-

2.0040

900

/‘C

Fig. 5. Variations of ESR absorption intensity, line-width and g-value of violanthrene-sulfur (prepared at 320°C) with HTT.

150-

’

I

I

I

I

-@

Sharp

a

Broad Absorption

I-

Absorption

In conclusion, it seems to be. advantageous to use sulfur-bridged polycyclic aromatic compounds as a starting material for a study of carbonization by ESR. The peripheral hydrogen atoms are easily replaced by sulfur without damaging the aromatic ring system itself, if the compound is prepared at a relatively low HTT. In this case the aromatic ring systems are already present, and the disorganized carbon atoms in the normal char are replaced by sulfur atoms which label the unpaired electrons by shifting the g-value from the free electron value. This g-value shift and possibly the broad line-width characteristic of the spin centers residing on sulfur atoms serve as a good parameter to follow what happens in carbonization reactions. REFJBENCFS

500

600

700

HEAT- TREATMENT

800

900

1000

TEMPERATURE /‘C

Fig. 6. Variation of ESR absorption intensity of violanthrene-sulfur (prepared at SOOT) with HTT.

1. H. Akamatu and K. Nagamatsu, J. Colloid Sci. 2, 593 (1947). 2. H. Akamatu, S. Mrozowski and D. C. Wobschall, Proc. 3rd ConJ on Carbon, p. 135. Pergamon Press, Oxford (1959). 3. S. Mrozowski, Proc. 4rh Conf. on Carbon, p. 271. Pergamon Press, Oxford (1960). 4. S. Mrozowski, Proc. 5th Co@ on Carbon, Vol. 2, p. 79. Pergamon Press, Oxford (1963). 5. S. Mrozowski and A. Gutsze, Carbon 15, 335 (1977).

343

Sulfur-bridged polycyclic aromatic compounds 6. S. Mrozowski, Carbon 19, 365 (1981). 7. S. Mrozowski. Carbon 20. 303 (1982). 8. H. Akamatu and H. Inokuchi, iymp&ium on Electrical Conductivity in Organic Solids (Edited by H. Kallmann and M. Silver), p. 277. Wiley, New York (1961). 9. H. Akamatu, Y. Maruyama, M. Kinoshita and H. Inokuchi, Symposium on Curbon, Tokyo, July 1964. 10. Y. Kurita and W. Gordy, J. Chem. Phys. 34, 282, 1285 (1961); W. Gordy, Theory and Applications of ESR, pp. 339-353. Wiley-Interscience, New York (1980). 11. H. E. Blayden and J. W. Patrick, Fuel 49, 257 (1970). 12. S. Be& M. Mammi and C. Garbuglio, Nature 182,247

(1858); S. Bezzi, C. Garbuglio, M. Mammi and G. Traverso. Gaz. Chim. Italiana 88. 1226 (1958). 13. Y. Matsunaga, J. Chem. Phys. ‘42, 22‘48 (i965); H. Inokuchi, M. Kochi and Y. Harada, Bull. Chem. Sot. Japan 40, 2695 (1967). 14. N. Sato, K. Seki and H. Inokuchi, J. Chem. Sot. Faraday Trans. (J. Chem. Sot., Faraday Trans. 2, 27, 1621 (1981)). 15. H. E. Redford and F. 0. Rice, J. Chem. Phys. 33, 774

(1960). 16. M. Kinoshita, Bull. Chem. Sot. Japan 35, 1137 (1962).

Professor Mrozowski foresaw the importance of carbon research as early as 1945, and he has consistently exerted himself for the pioneering work in the field of science and technology of carbon, Professor Mrozowski originated carbon conferences mostly by his personal effort. The Carbon Conference held in Buffalo 1957 was a truly first international conference and a memorable event in the history of carbon research. He organized this conference including the symposium “from benzene to graphite”, which indicated what the scope of carbon research should be. The success of this conference encouraged worldwide carbon research and technology; and led to the publication of this international journal devoted to carbon research. Since Carbon started in 1962, Professor Mrozowski had been the Editor-in-Chief; for 20 years he executed this important task. I express my personal gratitude for his long years service. H. AKAMATU