Turning-On of Coumarin Phosphorescence in Acetylacetonato Platinum Complexes of Cyclometalated Pyridyl-Substituted Coumarins

Abstract

Two pyridine-functionalized coumarins differing with respect to the site of pyridine attachment to the coumarin dye (3 in L1 or 7 in L2) and with respect to the presence (L1) or absence (L2) of a peripheral NMe2 donor were prepared and used as cyclometalating ligands towards the Pt(acac) fragment. X-ray crystal structures of complexes 1 and 2 show strong intermolecular interactions by π-stacking and short Pt•••Pt or C-H•••O hydrogen bonding that result in the formation of sheetlike packing patterns. The NMe2 donor substituent has a profound influence on the absorption and emission properties of the free coumarin dyes; L1 emits strongly while L2 is only weakly emissive. On binding to Pt(acac) the strong fluorescence of L1 is partially quenched while coumarin phosphorescence is observed from cyclometalated L1 and L2. The ligand-centered nature of the LUMO was confirmed by IR spectroelectrochemistry while the assignment of the phosphorescence emission as ligand-based rests on the vibrational structuring, the negligible solvatochromism, the small temperature-induced Stokes shifts on cooling to 77 K, the emission lifetimes, and strong oxygen quenching. (TD-)DFT calculations confirm our experimental results and provide an assignment of the electronic transitions and the spin density distributions in the T1 state. OPEN ACCESS Inorganics 2015, 3 56 Keywords: platinum; emission; coumarin; crystal structure; quantum chemical calculations Chart 1. Structures of complexes 1 and 2. Scheme 1. Synthesis of ligand L2.

Accessing the Long-Lived Triplet Excited States in Transition-Metal Complexes: Molecular Design Rationales and Applications

“…A recent study showed that the N donor for the N–Pt bond exerts a significant influence on the photophysical properties of cyclometalated C^N Pt(II) acac complexes (Figure ). For example, using pyridine instead of benzothiazole led to a reduced phosphorescence quantum yield (21%) and phosphorescence lifetime (12.4 μs) for the coumarin‐containing cyclometalated Pt(II) complexes . Thus, further investigations on the elusive photophysical properties of the Pt(II) complexes are desired.…”

Functional Materials Based on Cyclometalated Platinum(II) β-Diketonate Complexes: A Review of Structure–Property Relationships and Applications

“…A minor change in the functional group or position has a remarkable effect on their PL properties [ 44 ]. It has been demonstrated that the topology/connectivity of metal to ligand influences the photophysical properties, and this has been exploited to realize long-lived 3 IL or dual (fluorescence and phosphorescence) emission [ 45 , 46 ] and charge transfer (CT) [ 47 ]. Fluorinated substituents are well known for altering the electron distribution and providing steric protection around the metal center.…”

Phase‐Dependent Long Persistent Phosphorescence in Coumarin‐Phosphine‐Based Coinage Metal Complexes

“…However, by changing the substituents in the coumarin structure, the HOMO–LUMO energy gap as well as the character of excited states [e. g., (n,π*) vs. (π,π*)] can be modulated, strongly affecting optical and PL properties [27–28] . Furthermore, several coumarin‐based platinum, ruthenium and iridium phosphorescent complexes have been reported, demonstrating a PL lifetime in the range of microseconds [29–34] …”

“…[27][28] Furthermore, several coumarin-based platinum, ruthenium and iridium phosphorescent complexes have been reported, demonstrating a PL lifetime in the range of microseconds. [29][30][31][32][33][34] To the best of our knowledge, only a handful of studies have dealt with the synthesis of coinage metal complexes containing coumarin in the ligand backbone, [35][36][37][38] and only in few works, the photophysical properties of the respective complexes have been explored. [39,40] Herein, we describe the synthetic approach to a novel coumarin-functionalized diphosphine ligand.…”