Synthesis, photophysics, and reverse saturable absorption of platinum complexes bearing extended π-conjugated C^N^N ligands

Abstract

The synthesis of ligands 1-L–6-L that feature the 6-[9,9-di(2-ethylhexyl)-7-R-9H-fluoren-2-yl]-2,2′-bipyridine (C^N^N) core (R = 4-R′-phenylethynyl with R′ = NO₂, benzothiazol-2-yl (BTZ), H and OCH₃ or R = 4′-BTZ-phenyl or BTZ) and their platinum complexes 1–6 were reported in this paper. The photophysical properties of these ligands and the Pt(II) complexes, including the UV-vis absorption spectra, emission characteristics at room temperature and at 77 K, and the triplet transient difference absorption spectra, were systematically investigated in order to understand the effects of the substituent at the 4-position of the 1-ethynylphenyl component and the extension of π-conjugation between the C^N^N core and the BTZ substituent. Reverse saturable absorption (RSA) of complexes 1–6 was demonstrated at 532 nm using 4.1 ns laser pulses. The UV-vis absorption spectra of 1-L–6-L are featured by strong ¹π,π* transitions in the blue spectral region, and the absorption bands are effectively red-shifted by substitution at the 4-position of the ethynylphenyl motif and by the extended π-conjugation of the linkage. A similar effect was observed for the fluorescence spectra of these ligands in CH₂Cl₂ at room temperature, but the nature of the fluorescence varies from ¹π,π* fluorescence in 3-L and 6-L, to intraligand charge transfer (¹ILCT) fluorescence in 1-L, 2-L and 5-L; while 4-L possesses mixed ¹π,π*/¹ILCT characters. All ligands exhibit moderate triplet transient absorption (TA) in the visible spectral region, with substitution at the 4-position of the ethynylphenyl component broadening of the TA bands, while extended π-conjugation of the linkage inducing red-shifts of the TA bands. For Pt(II) complexes 1–6, their UV-vis absorption spectra constitute red-shifted ¹π,π* transitions and low-energy metal-to-ligand charge transfer (¹MLCT/¹ILCT) tails. The emission of these complexes at room temperature in CH₂Cl₂ predominantly originates from the C^N^N core localized ³π,π* state, probably mixed with minor ³MLCT character. 4-Position substitution and extended π-conjugation on the ligands exert a negligible effect on the shape and energy of the emission spectra. Similar to their respective ligands, 1–6 all exhibit broader and red-shifted TA spectra with respect to their ligands and both the 4-position substitution and extended π-conjugation bathochromically shift the TA band maxima. The nonlinear transmission experiments carried out for 1–6 at 532 nm reveal that all complexes exhibit strong reverse saturable absorption (RSA), and the degree of RSA follows this trend: 6 <4 <5 <2 ≤3 <1. The RSA performance is efficiently improved by electron-withdrawing substituents (NO₂ and BTZ) and by extending the π-conjugation; while electron-donating substituent (OCH₃ in 4) decreases the RSA at 532 nm.