Solid-state and solution photoluminescence of platinum(ii) complexes with 4′-substituted terpyridine ligands – structural, spectroscopic and electrochemical studies

Abstract

Platinum(II) complexes with the formula [PtCl(L)]CF3O3S (where L = 4′-(4-chlorophenyl)-2,2′:6′,2′′-terpyridine (L1), 4′-(4-bromophenyl)-2,2′:6′,2′′-terpyridine (L2), and 4′-(benzothiophene-2-yl)-2,2′:6′,2′′-terpyridine (L3)) were synthesized from precursor [Pt(PhCN)2Cl2] and characterized by FT-IR, NMR, UV-vis spectroscopy, X-ray crystallography and electrochemical studies. The emission properties of the coordination compounds were studied in solution and in the solid state at ambient temperature and in methanol : ethanol frozen glass matrices at 77 K. The quantum yields of fluorescence, lifetimes and the nature of the excited states were described on the basis of the molecular properties of the compounds. To elucidate the structural, spectroscopic and bonding properties of the obtained compounds, calculations at the DFT level were undertaken. Coordination compounds are emissive in both solution and solid states. Emission energies and the character of excited states in solution are governed by the donor–acceptor properties of the 4′ substituent of the terpyridine core. The emission properties in the solid state are attributed to the aggregation induced phenomenon. Moreover, the compounds [PtCl(L1)]CF3O3S and [PtCl(L2)]CF3O3S display a bathochromic shifted emission corresponding to J-aggregation (head-to-head Pt⋯Pt interactions), while in the case of compound [PtCl(L3)]CF3O3S a hypsochromic shifted emission compared to that in solution is characteristic of the H-aggregation process (head-to-tail Pt⋯π interaction). The coordination compounds display a strong rigidochromic effect with decay times changing from the ps/ns scale at ambient conditions to the μs domain in frozen glass matrices.