Synthesis, electrochemistry and luminescence of [Pt{4′-(R)trpy}(CN)]+ (R = Ph, o-CH3C6H4, o-ClC6H4 or o-CF3C6H4; trpy = 2,2′:6′,2″-terpyridine): crystal structure of [Pt{4′-(Ph)trpy}(CN)]BF4·CH3CN