Electron density distribution studies on ZnS4N2 chromophore in solution and solid phases: XPS and cyclic voltammetric studies on 1,10-phenanthroline and 2,2′-bipyridine adducts of bis(piperidinecarbodithioato-S,S′)zinc(II). Single crystal X-ray structure of (2,2′-bipyridine)bis-(piperidinecarbodithi