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 P (1) over bar 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 (similar to 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.