Conjugated Polyelectrolyte-Induced Self-Assembly of Alkynylplatinum(II) 2,6-Bis(benzimidazol-2'-yl)pyridine Complexes

Water-soluble cationic alkynylplatinum(II) 2,6-bis(benzimidazol-2'-yl)pyridine (bzimpy) complexes have been demonstrated to undergo supramolecular assembly with anionic polyelectrolytes in aqueous buffer solution. Metal-metal-to-ligand charge transfer (MMLCT) absorptions and triplet MMLCT ((3) MMLCT) emissions have been found in UV/Vis absorption and emission spectra of the electrostatic assembly of the complexes with non-conjugated polyelectrolytes, driven by Pt⋅⋅⋅Pt and π-π interactions among the complex molecules. Interestingly, the two-component ensemble formed by [Pt(bzimpy-Et){CCC6 H4 (CH2 NMe3 -4)}]Cl2 (1) with para-linked conjugated polyelectrolyte (CPE), PPE-SO3 (-) , shows significantly different photophysical properties from that of the ensemble formed by 1 with meta-linked CPE, mPPE-Ala. The helical conformation of mPPE-Ala allows the formation of strong mPPE-Ala-1 aggregates with Pt⋅⋅⋅Pt, electrostatic, and π-π interactions, as revealed by the large Stern-Volmer constant at low concentrations of 1. Together with the reasonably large Förster radius, large HOMO-LUMO gap and high triplet state energy of mPPE-Ala to minimize both photo-induced charge transfer (PCT) and Dexter triplet energy back-transfer (TEBT) quenching of the emission of 1, efficient Förster resonance energy transfer (FRET) from mPPE-Ala to aggregated 1 molecules and strong (3) MMLCT emission have been found, while the less strong PPE-SO3 (-) -1 aggregates and probably more efficient PCT and Dexter TEBT quenching would account for the lack of (3) MMLCT emission in the PPE-SO3 (-) -1 ensemble.