Luminescent Platinum(II) Complexes of N^N^–^N Amido Ligands with Benzannulated N-Heterocyclic Donor Arms: Quinolines Offer Unexpectedly Deeper Red Phosphorescence than Phenanthridines

Abstract

A platform for investigating the impact of π-extension in benzannulated, anionic pincertype N^N-^N-coordinating amido ligands and their Pt(II) complexes is presented. Based on bis(8-quinolinyl)amine, symmetric and asymmetric proligands bearing quinoline or πextended phenanthridine (3,4-benzoquinoline) units are reported, along with their redemitting, phosphorescent Pt(II) complexes of the form (N^N-^N)PtCl. Comparing the photophysical properties of complexes of (quinolinyl)amido ligands with those of πextended (phenanthridinyl)amido analogs revealed a counter-intuitive impact of siteselective benzannulation. Contrary to conventional assumptions regarding π-extension, and in contrast to isoenergetic lowest energy absorption bands and a red shift in fluorescence from the organic proligands, a blue shift of nearly 40 nm in the emission wavelength is observed for Pt(II) complexes with more extended bis(phenanthridinyl) ligand π-systems. Comparing the ground state and triplet excited state structures optimized from DFT and TD-DFT calculations, we trace this effect to a greater rigidity of the benzannulated complexes, resulting in a higher energy emissive triplet state, rather than to a significant perturbation of orbital energies caused by π-extension.

Deep-Red Luminescence from Platinum(II) Complexes of N^N–^N-Amido Ligands with Benzannulated N-Heterocyclic Donor Arms

“…A strong band at around 310 nm -which is a feature of the bis(phenanthridine) L3 R,R series but not of the bis(quinolines) L1 R,R -also appears in the mixed L2 R,R compounds but is proportionately weaker than in L3 R,R , consistent with the presence of one of each type of heterocycle. 38 The L2 H,R compounds (with no substituent on the quinoline) show exactly the same trend (Figure S5), and there is no significant difference in their absorption spectra compared to their respective methylated quinoline analogs L2 Me,R .…”

“…46 For platinum chloride complexes supported by bis(8-quinolinyl)amido ligands (e.g., 1 Me,Me ), bis(phenanthridinyl)amido ligands (3 Me,Me ) and "mixed" analogues (2 Me,Me ) that incorporate one quinoline and one phenanthridine, however, we have discovered that this model does not fully hold. 38 In these compounds, absorption and emission are not affected in the same way by benzannulation; all three complexes show isoenergetic absorption maxima, but emission from the complex with the most extended ligand π-system 3 Me,Me is blueshifted by nearly 40 nm. Similarly exceptional behaviour was observed for phenanthridinyl and quinolinyl derivatives of (P^N) 2 CuX 2 dimers.…”

“…Table 1 compiles room temperature absorption and emission data in dichloromethane solution for all twelve proligands, with a selection of representative spectra shown in Figure 3 (absorption) and Figure 4 (emission). In our initial report, 38 we considered only the dimethylated compounds L1 Me,Me , L2 Me,Me and L3 Me,Me in order to isolate the effect of benzannulation by keeping ring substitution patterns consistent. All three proligands were found to show a strong, lowest energy absorption band centered around 400 nm.…”

Pseudo-octahedral nickel(ii) complexes of strongly absorbing benzannulated pincer-type amido ligands: ligand-based redox and non-Aufbau electronic behaviour

“…15 We have investigated the incorporation of benzannulated phenanthridine (3,4-benzoquinoline) moieties in diarylamido ligands. 16 In addition to producing Pt MCCs that combine strong absorption and red-shied phosphorescence, 17,18 neutral Fe-based MCCs comprised of two such ligands show strong panchromatic absorption and nanosecond charge-transfer lifetimes in their neutral forms, along with a rich redox prole. 19 Similarly strong but narrower absorption bands can also be seen with isoelectronic Co(III) analogues, as well as congeners of redox-silent Zn(II) and Ga(III) ions.…”

[2,2′-Bis(diphenylphosphanyl)-1,1′-binaphthyl-κ2 P,P′]dichloridoplatinum(II) acetonitrile trisolvate

“…Crystals of (racBINAP)PtCl 2 were obtained as a side-product from a reaction mixture of (COD)PtCl 2 and a tridentate, diarylamido-N,N-phenanthridine-based ligand (Mandapati et al, 2019). BINAP was used to construct this ligand via a Pdcross coupling reaction and was not completely removed from the proligand before metalation.…”