A　novel　metalloviologen　complex　{[Cd(PhSQ)(HTBC)(H2O)(2)]center　dot　1.25H(2)O}n　(1)　based　on　N-(4-sulfophenyl)-4,4＇-bipyridinium　(PhSQ)　and　monoprotonated　trimesic　acid　(HTBC)　has　been　prepared　and　structurally　characterized.　Complex　1　has　been　characterized　by　elemental　analysis,　IR,　thermogravimetric　analyses,　UV-Vis　spectra　and　X-ray　crystallography.　The　coordination　geometry　of　Cd(II)　ion　is　a　pentagonal　bipyramid.　Complex　1　displays　a　zigzag　chain　along　the　b　axis,　and　such　chains　are　extended　into　a　2D　layer　supramolecular　network　through　hydrogen　bonding　and　pi...pi　interactions.　Moreover,　complex　1　exhibits　reversible　photochromic　and　switchable　luminescent　behaviors　via　UV　light　irradiation　at　room　temperature　induced　by　the　photogenerated　radicals　via　a　photo-induced　electron　transfer　(PET)　mechanism.