Using non-equilibrium Green's function method combined with first-principles density functional theory, the photoswitching of conductivities of 15,16-dinitrile dihydropyrene/cyclophanediene (DDP/CPD) molecular wires with three typical adsorption sites are investigated. A 4-6 times conduction enhancement is predicted for the closed-ring DDP over the open-ring CPD. The main effect on the conduction enhancement is discussed in terms of effective conjugation length and HOMO-LUMO gap. DDP/CPD is one of good candidates for solid-state optical switches due to its unique advantages, i.e., minimal conformational change during a switching cycle and complete conversion from one isomer to the other, and may have some future application. (C) 2008 Elsevier B.V. All rights reserved.