Crystal structure of r-1, c-2, t-3, t-4-1, 3-bis(4-methylphenyl)-2, 4-bis(4-pyridyl) cyclobutane (DMDPC) was determined by X-ray diffraction method. The crystal is monoclinic, space group is P21/c, with a=1.1376(6), b=1.7379(8), c=1.1590(5)nm, β=106.16(4)ⅲ, Z=4. The cyclobutane ring has a puckered conformation because of repelling of the pyridine ring and benzene ring at the same side of the four-membered ring. It was found that the cyclobutane ring of DMDPC would be opened when irradiated under short wave UV light, trans-1-(4-methylphenyl)-2-(4-Pyridyl)ethene(E-MEP) was reproduced, then some of E-MEP was changed to its cis isomer. The ratio of trans to cis isomer was controlled by the wave length of irradiating light.

Key words: PYRIDINE P, PHOTOCHEMICAL REACTION, INFRARED SPECTROPHOTOMETRY, ELEMENTAL ANALYSIS, X-RAY DIFFRACTION ANALYSIS, MASS SPECTROGRAPHY, ETHYLENE P, PROTON MAGNETIC RESONANCE SPECTROMETRY, CYCLOBUTANE P, PHOTOLYTIC REACTION