Interaction-dependent photophysical behavior of two-component ensembles arising from conjugated polyelectrolytes and platinum(II) complexes of different supramolecular architectures

Abstract

Recent works have shown that cationic alkynylplatinum(II) 2,6-bis(benzimidazol-2′-yl)pyridine (bzimpy) complexes possess the capability of undergoing electrostatic assembly with anionic conjugated polyelectrolytes (CPEs) to form two-component supramolecular ensembles. However, previous studies on the FRET properties of CPEs were mainly focused on and restricted to rigid and extended π-conjugated polymers, while little discussions have been made on those with flexible linkers or interrupted π-conjugation. Moreover, the mechanism for the reciprocal interactions between the CPE and platinum(II) complexes in the formation of the ensemble is essentially less explored. Therefore, in order to have a more in-depth understanding and to provide further insights into these processes, a CPE with interrupted π-conjugation, poly(phenylene ethynylene sulfonate) with a diphenyl ether unit (bPPE-SO3–), has been employed to induce the formation of supramolecular aggregates of alkynylplatinum(II) bzimpy complexes in aqueous buffer solution. The formation of the two-component CPE–platinum(II) complex ensembles via electrostatic, Pt∙∙∙Pt and π–π stacking interactions has been investigated by UV-vis absorption, steady-state emission, resonance light scattering, and time-resolved emission experiments. Interestingly, the reciprocal interactions, energy transfer processes and the resultant photophysical properties of the two-component ensemble comprising bPPE-SO3– and [Pt(bzimpy-Et){C≡CC6H4(CH2NMe3-4)}]Cl2 (1) have been found to be distinctively different when compared to those formed by 1 with a structurally similar CPE, PPE-SO3–. The conformational flexibility and geometrical features imparted by the –O– linker of bPPE-SO3– has empowered it with the capability in forming strong bPPE-SO3––1 aggregates, as evidenced by the large Stern–Volmer constants in the presence of 1 at low concentrations. Due to the combination of a relatively large Fӧrster radius, high triplet state energy of bPPE-SO3– as well as large HOMO–LUMO energy gap, photo-induced charge transfer (PCT) and Dexter triplet energy back transfer (TEBT) quenching of the emission of platinum(II) complexes would be effectively suppressed. This would facilitate efficient Fӧrster resonance energy transfer (FRET) process between bPPE-SO3– and aggregate form of complex molecules of 1, leading to the emergence of triplet metal–metal-to-ligand charge transfer (3MMLCT) emission. In stark contrast, the weaker PPE-SO3––1 aggregates that possibly exhibit more efficient PCT and Dexter TEBT quenching would lead to the absence of 3MMLCT emission in the PPE-SO3––1 ensemble. It is anticipated that these results would open up a new avenue and provide new strategies for the construction of two-component CPE–metal complex ensembles with optimized FRET efficiency. Moreover, the mechanisms for the reciprocal interactions between the CPEs and the platinum(II) complexes have been found to be dependent on their intrinsic structural and geometrical features as well as their extrinsic supramolecular architectures. In particular, bPPE-SO3– is found to exhibit a multiple-step emission spectral response with an increasing concentration of 1. Similar observations have not been encountered before in the meta-linked poly(phenylene ethynylene) containing alanine pendant (mPPE-Ala) counterpart. The result has been rationalized by the structural non-coplanarity of bPPE-SO3–, at which the twisted –O– linker in the bPPE backbone would weaken the intramolecular hydrophobic and π–π stacking interactions and thus, become more susceptible towards the perturbations brought about by the microenvironmental changes. In comparison, the strong intramolecular hydrophobic and π–π stacking interactions between the adjacent gyrations of mPPE-Ala would strengthen its helical conformation, making it less responsive towards external stimuli. The current study demonstrates the versatility of this class of two-component stimuli-responsive CPE-platinum(II) complex ensembles in diverse applications. It is envisaged that the above results should emerge as a prime driving force for initiating further studies in exploring their potential sensing or imaging applications by providing valuable information on the photophysical properties of the ensembles.