Effect of the external electric field on photoisomerization dynamics of diphenylpolyenes doped in a PMMA polymer film has been examined by using electromodulation spectroscopy in absorption and emission. A significant decrease in fluorescence intensity is found in the presence of an external electric field, suggesting that trans-to-cis isomerization of diphenylpolyenes is accelerated by an electric field in the S, state. The present results support the zwitter ionic model for the photoisomerization pathway of diphenylpolyenes. Photoisomerization of diphenylpolyenes is also shown to occur in a PMMA film. (C) 2004 Elsevier B.V. All rights reserved.