Chalcogenated polyacenes including organic semiconductors such as tetrathiotetracene, .[.tetraselenotetracene.]., and hexathiopentacene are produced by reaction of a polyacene with elemental sulfur, .[.selenium, and tellurium.]. in the presence of nitrogen-containing hot solvents, preferably alkylated amides. N,N-dimethylformamide is the preferred solvent. The disclosed method of synthesis produces substituted polyacenes of high yield and purity in considerably shorter reaction times than achievable by prior art methods.

The present invention relates to novel methods for preparing 
chalcogen-substituted polyacenes and particularly to methods for preparing 
tetracene and pentacene compounds in which sulfur .[., selenium, or 
tellurium.]. atoms are attached to interior reactive carbon atoms of the 
polyacene nucleus to form heterocyclic rings with the nucleus. 
Certain polyacenes, notably tetrathiotetracene, .[.tetraselenotetracene,.]. 
and hexathiopentacene are known to have low electrical resistivity. 
Tetrathiotetracene, for example, which has the formula: 
##STR1## 
is reported by Matsunaga in J. Chem. Phys., 42, 2248 (1965) as possessing 
a specific resistance of 104.OMEGA. at 15.degree. C when in molded form. 
Accordingly, such compounds are useful as organic semiconductors at room 
temperature. Additionally, these compounds have utility in that they 
readily form ion-radical salts which are themselves organic semiconductors 
having even lower electrical resistivity. Matsunaga U.S. Pat. No. 
3,403,165 discloses such tetrathiotetracene ion-radical salts and their 
semiconductive properties. 
Synthesis of substituted polyacenes has been carried out in the past by 
reacting a polyacene with an elemental chalcogen such as sulfur, selenium, 
and tellurium in the presence of hot solvent such as trichlorobenzene or 
Dowtherm, a eutectic of biphenyl and biphenyl oxide sold by the Dow 
Chemical Company. For example, tetrathiotetracene is ordinarily 
synthesized by reacting tetracene with elemental sulfur in hot 
trichlorobenzene. Synthesis carried out by this method produces 
considerable amounts of undesirable by-products and requires a reaction 
time of from 20 to 24 hours. 
Another method for producing chalcogen-substituted polyacene compounds 
involves the reaction of a halogenated polyacene with an elemental 
chalcogen in the presence of trichlorobenzene. Tetrathiotetracene can be 
prepared according to this method by reacting elemental sulfur with 
5,11-dichlorotetracene in trichlorobenzene. This method requires the 
additional step of halogenating the tetracene. 
Still another method for producing tetrathiotetracene involves heating 
sulfur monochloride and tetracene in trichlorobenzene. Sulfur monochloride 
is a very powerful reagent which must be freshly distilled before use. In 
addition, the reaction is carried out in a current of carbon dioxide and 
tends to produce significant amounts of undesirable by-products. 
Accordingly, the present invention is directed to overcoming the 
deficiencies of the prior art by providing methods for producing 
chalcogen-substituted polyacenes which require relatively short reaction 
times while practically eliminating substantial amounts of undesirable 
by-products. Additionally, the present invention requires no prior 
synthesis of halogenated reactant or use of a strong oxidizing agent. 
Further, the present invention produces high yields of relatively pure 
products which can ordinarily be utilized without further purification. 
It has been discovered that the shortcomings of the prior art methods for 
producing chalcogen-substituted polyacenes, particularly those having the 
general formulas: 
##STR2## 
and 
##STR3## 
wherein X and Y each represent .[.identical atoms selected from the group 
consisting of.]. sulfur .[., selenium, and tellurium,.]. can be overcome 
by reacting an elemental .[.chalcogen selected from the group consisting 
of.]. sulfur .[.selenium,.]. and tellurium with a polyacene such as 
tetracene or pentacene in the presence of a reaction solvent comprising a 
nitrogen-containing organic solvent, preferably an alkylated amide 
maintained at high temperatures, usually at about reflux temperatures. 
Preferred alkylated amides useful in carrying out the method of the 
present invention include N,N-dimethylformamide, N,N-diethylformamide, 
N,N-dimethylacetamide, and N,N,N',N'-tetramethylurea. 
N,N'-dimethylformamide is the solvent most preferred when carrying out the 
method of the present invention. 
Effectiveness of the novel methods of synthesis provided by the present 
invention is demonstrated by the following examples.

EXAMPLE 1 
Tetracene (20 g.) and flowers of sulfur (40 g.) are placed in a flask 
containing 500 ml. of N,N-dimethylformamide. The reaction mixture is 
heated to boiling, and boiling is continued for about 31/2 hours. Small 
additions of N,N-dimethylformamide are made at intervals to replace the 
solvent lost by evaporation. After the reaction is completed the insoluble 
dark green product is separated by filtering while still hot and is 
finally washed with benzene and ligroine. After drying under ambient 
conditions, a yield of 29.3 g. (94.8 percent) of tetrathiotetracene is 
obtained. Comparison of the infrared spectrum of this product with that of 
purified authentic samples of tetrathiotetracene reveals the presence of 
only minor impurities. The product, as prepared above, is then 
successfully utilized without further purification to prepare ion-radical 
derivatives of tetrathiotetracene as described by Matsunaga in U.S. Pat. 
No. 3,403,165. 
Elemental analysis of a typical sample of tetrathiotetracene prepared by 
the method of Example 1 is as follows: 
Theory (percent): C, 61.4; H, 2.3; S, 36.4; Cl, 0; N, .0 
Found (percent): C, 61.1; H, 2.5; S, 36.4; Cl, &lt;.1; N, &lt;.1 
EXAMPLE 2 
Tetracene (0.7 g.) and flowers of sulfur (1.4 g.) are placed in a flask 
containing 30 ml. of N,N,N',N'-tetramethylurea and provided with a reflux 
condenser. The mixture is heated to boiling, and refluxed for about 3 
hours. The insoluble product is then filtered and washed with benzene and 
ligroine. After drying, a yield of 0.96 g. (89 percent) of 
tetrathiotetracene is obtained. 
EXAMPLE 3 
A procedure identical to that of Example 2 is used with the exception that 
1 g. of tetracene, 2 g. of flowers of sulfur, and 50 ml. of 
N,N-dimethylacetamide are used as reagents. The yield of 
tetrathiotetracene is 1.2 g. (78 percent). 
EXAMPLE 4 
A reaction is carried out as described in Example 3 with the exception that 
N,N-diethylformamide is used as solvent, the reaction time is 21/4 hours, 
and the product is recovered after the reaction medium is cooled to room 
temperature. The yield of tetrathiotetracene is 0.93 g. (60 percent). 
EXAMPLE 5 
Pentacene (1 g.) and flowers of sulfur (2 g.) are placed in a flask 
containing 40 ml. of N,N-dimethylformamide and provided with an air-cooled 
reflux condenser. The mixture is heated to boiling, under a blanket of 
nitrogen gas, and refluxed in the dark for about 31/2 hours. The insoluble 
product is then filtered hot and washed with benzene and ligroine. After 
drying in air, a yield of 1.15 g. (68.5 percent) of blue-green 
hexathiopentacene is obtained. 
.[.EXAMPLE 6 
A reaction can be carried out as described in Example 2 with the exception 
that 3.5 g. of selenium are used instead of flowers of sulfur to yield 
tetraselenotetracene..]. 
The invention has been described in detail with particular reference to 
certain preferred embodiments thereof, but it will be understood that 
variations and modifications can be effected within the spirit and scope 
of the invention.