s-Dipentacene: Structure, Spectroscopy, and Temperature- and Pressure-Dependent Photochemistry Otto Berg, † Eric L. Chronister,* Tomihiro Yamashita, ‡ and Gary W. Scott* Department of Chemistry, UniVersity of California at RiVerside, RiVerside, California 92521 Robert M. Sweet Biology Department, BrookhaVen National Laboratory, Upton, New York 11973 Joseph Calabrese E.I. DuPont de Nemours & Co., P.O. Box 80228, Wilmington, Delaware 19880-0228 ReceiVed: October 14, 1998; In Final Form: December 30, 1998 We report the synthesis, characterization, spectroscopy, and photochemistry of s-dipentacene (see Figure 1). The symmetric addition product dimer of pentacene (s-dipentacene) is formed upon irradiation of a solution of pentacene. The structure of s-dipentacene was determined by X-ray crystallography. The crystals had a space group symmetry of P1 h with two molecules per unit cell. One molecule in the unit cell was accompanied by two dichloromethane molecules of solvation. Photodecomposition of s-dipentacene, dispersed in a poly- (methyl methacrylate) (PMMA) host matrix, to pentacene was studied. Spectroscopic evidence shows that this photodecomposition proceeds through a trapped intermediate both at low temperature (∼12 K) and at room temperature under high pressure (13 kbar). This intermediate is assigned as a “broken dimer” of two pentacene molecules trapped in the PMMA host. We previously reported similar results in the photolysis of both dianthracene and ditetracene at low temperatures, but this is the first report of broken dimer formation at room temperature under high pressure. 1. Introduction The photodimerization of linear polyacenes was first reported in the synthesis of dianthracene from anthracene in 1866. 1 Later, this synthesis was discussed in more detail. 2 However, the molecular structure of dianthracene was not determined until 1955. 3 Several other homodimers of linear polyacenes, including the syn and anti forms of ditetracene, have also been prepared. 4,5 In addition to these homodimers, mixed dimers and related structural isomers have also been reported. 1-6 One study of the triplet state of pentacene in solution reported observation of a transient dimer of pentacene, but a stable photodimer of pentacene was not prepared under the conditions used in that study. 7 As further noted in that study, 7 an earlier report of the apparent respective photodimerizations of anthracene, tetracene, and pentacene 8 was presumed to be incorrect. The observed products in that even earlier study 8 were most likely to be those of photodecomposition resulting from the short wavelength UV irradiation used in the procedure. The molecular class of aromatic dimers has drawn particular interest because the reverse photochemical decomposition reaction, which reproduces the two original aromatic monomers, is often efficiently and selectively driven by light. In principle, such a pair 9 of reversible photochemical reactions can serve as the basis of selectively erasable optical devices. However, this possibility raises detailed issues of molecular mobility and intermolecular electronic coupling in the condensed phase host. These dimers can serve as precursors of a pair of closely associated monomers, which, after dissociation, may remain trapped together, diffuse apart, or recombine, depending upon the conditions of the host substrate. Matrix-isolated pairs of molecules that interact strongly in the excited electronic state but that are not chemically bound have been identified spec- troscopically as products of the photolysis of dianthracene 10 and ditetracene 11 at low temperature. Crystal structures of dianthracene 12 and anti-ditetracene 13 have been determined. The molecular structures 12,13 of these mol- ecules, as well as those of related photodimers, 14-16 all indicate that the two C-C single bonds joining the two monomer halves of the respective dimer are exceptionally long for such single bonds with lengths on the order of 1.61 to 1.63 Å. Of interest in determining the molecular and crystal structures of s- dipentacene is the comparison of these structures with those of related photodimers. The long C-C single bonds between the two halves of such [4+4] photodimerization products produce a weak linkage between these halves. These bonds can be readily broken by the reVerse photochemical reaction, producing two of the originally linked molecules in close proximity im- mediately following the reaction. In a solid host matrix, photodecomposition of these types of photodimers produces two monomers which, at low temperature, may be constrained to a face-to-face “sandwich” configuration that results in a highly red-shifted fluorescence from the trapped monomer pair. This term, “sandwich dimer”, is generally used for monomer pairs that are constrained by the matrix to remain in such an energetically unfavorable orientation. This orientation † Present Address: Department of Chemistry, Lensfield Road, Cambridge University, Cambridge, England. ‡ Present Address: Oji Paper Co., Immermann Strasse 43, 40210 Dusseldorf, Germany. * To whom correspondence should be addressed. 2451 J. Phys. Chem. A 1999, 103, 2451-2459 10.1021/jp984066g CCC: $18.00 © 1999 American Chemical Society Published on Web 03/09/1999