Fluorescence Amplification of Unsaturated Oxazolones Using Palladium: Photophysical and Computational Studies

Abstract

Weakly fluorescent (Z)-4-arylidene-5-(4H)-oxazolones (1), ΦPL < 0.1%, containing a variety of conjugated aromatic fragments and/or charged arylidene moieties, have been orthopalladated by reaction with Pd­(OAc)2. The resulting dinuclear complexes (2) have the oxazolone ligands bonded as a C^N-chelate, restricting intramolecular motions involving the oxazolone. From 2, a variety of mononuclear derivatives, such as [Pd­(C^N-oxazolone)­(O2CCF3)­(py)] (3), [Pd­(C^N-oxazolone)­(py)2]­(ClO4) (4), [Pd­(C^N-oxazolone)­(Cl)­(py)] (5), and [Pd­(C^N-oxazolone)­(X)­(NHC)] (6, 7), have been prepared and fully characterized. Most of complexes 3–6 are strongly fluorescent in solution in the range of wavelengths from green to yellow, with values of ΦPL up to 28% (4h), which are among the highest values of quantum yield ever reported for organometallic Pd complexes with bidentate ligands. This means that the introduction of the Pd in the oxazolone scaffold produces in some cases an amplification of the fluorescence of several orders of magnitude from the free ligand 1 to complexes 3–6. Systematic variations of the substituents of the oxazolones and the ancillary ligands show that the wavelength of emission is tuned by the nature of the oxazolone, while the quantum yield is deeply influenced by the change of ligands. TD-DFT studies of complexes 3–6 show a direct correlation between the participation of the Pd orbitals in the HOMO and the loss of emission through non-radiative pathways. This model allows the understanding of the amplification of the fluorescence and the future rational design of new organopalladium systems with improved properties.

Orthopalladated imidazolones and thiazolones: synthesis, photophysical properties and photochemical reactivity

“…Thus, the imidazolone moiety in 4c and thiazolone groups in 11a and 11b deviate significantly from planarity, adopting a U-shaped form, and similarly, they largely deviate from coplanarity with respect to the square-planar metal coordination environments. Both types of distortions are analogous to those recently described in mononuclear complexes derived from oxazolones, 22,24 suggesting that they are inherent to this type of structural skeleton and not dependent on the nuclearity of the resulting complexes or the ancillary ligands of palladium. It is noteworthy that the free imidazolone ligands exhibit remarkable coplanarity of the arylidene and heterocycle rings, 51–55 which is lost upon metal incorporation.…”

“…The change of ligands attached to the Pd atom has a significant influence on Φ PL , similar to that determined in oxazolones. 24 In the present case, the complex 9e with the acetylacetonate group as the ancillary ligand exhibits the lowest Φ PL (2%), followed by complex 5e with TFA and pyridine (5%), quite similar to that of complex 7e with TFA and Cl (7%). As in related complexes with palladated oxazolones, 24 the presence of two pyridines in cis arrangement as ancillary ligands for the Pd (complex 8e ) provides the highest performance, reflected in a quantum yield value as high as 15%.…”

“…58 As for the photophysical properties, palladacycles typically exhibit low-efficient emission, only detected at low temperatures, due to the thermal deactivation of the excited state to the low-lying metal-centered orbitals (MC; d-d*) which experience non-radiative decay. 25,[59][60][61] On the grounds of previous studies performed in oxazolones where fluorescence amplification was achieved by using Pd as an intramolecular lock, 24 we have investigated here the emissive properties of orthopalladated analogs with imidazolones 5e-9e, which result to be fluorescent. Photophysical data of 2a-2e and 5e-9e including their absorption, emission maxima, and emission quantum yields (Φ PL ) in solution at room temperature are summarized in Table 1.…”

“…Regarding their photophysical properties, the incorporation of a metal such as palladium as an intramolecular lock between the 4-arylidene ring and the heterocycle 20 allows for the amplification of the fluorescence in solution of certain substrates by several orders of magnitude (Fig. 1, past work), [21][22][23][24][25] because well-characterized non-radiative deactivation pathways, such as the hulatwist or Z-E isomerizations, are restricted. [26][27][28][29][30] Regarding photochemical reactivity, the formation of dinuclear palladacycles is the basis for the building of molecular templates that enable a stereoselective [2 + 2] photocycloaddition in solution, resulting in diaminotruxillic-type cyclobutanes as single diastereoisomers, [31][32][33][34] in sharp contrast to what occurs with free-state oxazolones.…”

Chimeric GFP–uracil based molecular rotor fluorophores

“…While the imidazolinone moiety is part of natural GFP chromophore, unsaturated azlactone analogues were previously reported to show fluorescence increase when the hula twist was suppressed by locking the chromophore using palladium metal. 20 The variety of uracil–GFP chimeras examined is given in Fig. 3.…”