No abstract

A series of cyclometalated platinum(II) complexes have been prepared, [PtL(n)Cl], containing N--C--N-coordinating, terdentate ligands based on 1,3-dipyridylbenzene (HL(1)), incorporating aryl substituents at the central 5 position of the ligand. All of the new complexes are intensely luminescent in a degassed solution at 298 K (phi = 0.46-0.65 in CH(2)Cl(2)) with lifetimes in the microsecond range (7.9-20.5 micros). The introduction of the aryl substituents leads to a red shift in the lowest-energy, intense charge-transfer absorption band compared to [PtL(1)Cl] (401 nm in CH(2)Cl(2)), in the order H < mesityl < 2-pyridyl < 4-tolyl < 4-biphenylyl < 2-thienyl < 4-(dimethylamino)phenyl (431 nm in CH(2)Cl(2)), which correlates with the decreasing order of oxidation potentials. A similar order is also observed in the emission maxima, ranging from 491 nm for [PtL(1)Cl] to 588 nm for the 4-(dimethylamino)phenyl-substituted complex. The emission spectra of all of the complexes, except for the amino-substituted compound, are highly structured in a dilute solution in CH(2)Cl(2), and the emission is assigned to excited states of primarily (3)LC (ligand-centered) character. At higher concentrations, self-quenching accompanied by structureless excimer emission centered at 700 nm is observed, but the aryl groups attenuate the self-quenching compared to the parent compound [PtL(1)Cl], particularly for the most sterically hindered mesityl complex. The introduction of the strongly electron-donating 4-dimethylamino substituent leads to a switch in the nature of the lowest-energy excited state from (3)LC to one of primarily intraligand charge-transfer (ILCT) character in CH(2)Cl(2): this complex displays a structureless and much broader emission band than the other compounds and a high degree of positive solvatochromism. No excimer emission is observed in CH(2)Cl(2), and self-quenching is an order of magnitude lower than that for the other complexes. However, in nonpolar solvents such as CCl(4), the ILCT state is destabilized, such that the (3)LC remains the lowest-energy excited state. Reversible switching between the ILCT and (3)LC states can also be achieved in a CH(2)Cl(2) solution by protonation of the amine, with an accompanying large change in the emission maxima of >100 nm. The X-ray structures of the biphenylyl- and methyl-substituted complexes are reported, together with those of the 2-pyridyl- and mesityl-substituted ligands and the key synthetic intermediate 1-bromo-3,5-di(2-pyridyl)benzene.

This work explores time-resolved emission imaging microscopy (TREM) for noninvasive imaging and mapping of live cells on a hitherto uncharted microsecond time scale. Simple robust molecules for this purpose have long been sought. We have developed highly emissive, synthetically versatile, and photostable platinum(II) complexes that make TREM a practicable reality. fluorescence microscopy ͉ time-resolved luminescence spectroscopy ͉ transition metal complexes ͉ cyclometalation

A series of terdentate cyclometallated PtII complexes with remarkable luminescence properties are used as new phosphorescence‐emitting dopants in a blended host matrix as the emitting layer, resulting in very high electroluminescence efficiencies. Because of the high phosphorescence quantum yields of these Pt complexes and the efficient energy transfer from both singlet and triplet excited states of the host to the emitting guest, external electroluminescence quantum efficiencies as high as 4–16 % photons per carrier and luminous efficiencies of 15–40 cd A–1 are achieved. Moreover, these high efficiency values were maintained over a four‐decade current intensity span with no significant roll‐off. Tuning of the electroluminescence spectra from the yellow to the green‐bluish region of the chromaticity diagram is obtained simply by changing the substituents at the central 5‐position of the cyclometallating ligand.

The neopentyl ester of 1,3-di(2-pyridyl)benzene-5-boronic acid (dpy-B) is a useful intermediate in the divergent synthesis of N;C;N-coordinating, 1,3-di(2-pyridyl)benzene ligands, HL(n), that carry aryl substituents at the 5-position of the central ring. The platinum(ii) complexes, PtL(n)Cl, of several such ligands have been prepared, incorporating pendant anisoles, arylamines, an oxacrown, and an azacrown, all of which are strongly luminescent in solution at 298 K. The emission of the complexes is partially quenched by oxygen, and all of the compounds are very efficient sensitisers of singlet oxygen. The quantum yields of (1)O(2) formation have been measured on the basis of the intensity of the O(2)(1)Delta(g) emission at 1270 nm, and are in the range 0.25-0.65. Density functional theory (DFT) calculations have been carried out that include the effect of the solvent, on the unsubstituted complex PtL(1)Cl and on the derivatives incorporating p-dimethylaminophenyl and phenyl-15-mono-N-azacrown-5 pendants (PtL(9)Cl and PtL(12)Cl respectively). Absorption spectra have been simulated on the basis of the calculated singlet excitations: they closely resemble the experimental spectra. In particular, the DFT successfully accounts for the appearance of low-energy absorption bands that accompany the introduction of the aryl pendants, indicating the participation of the aryl group in the HOMO but not significantly in the LUMO. The calculated lowest energy triplet excitation of PtL(1)Cl is close to the observed 0-0 emission maximum of this complex in solution. Taking together data for this series of complexes and related compounds previously studied, the energies of the lowest-energy spin-allowed absorption bands are shown to correlate approximately linearly with the oxidation peak potential. The emission energies show a similar correlation in toluene, but in CH2Cl2 the value for PtL(9)Cl is anomalously low. The differing emission properties of this complex in the two solvents suggest a switch to a TICT-like state in CH2Cl2 (TICT = twisted intramolecular charge transfer), stabilised in the more polar environment. Transient DC photoconductivity measurements confirm that the dipole moment of the triplet excited state is larger in CH2Cl2 than in toluene. The azacrown PtL(12)Cl displays similar behaviour. Binding of metal ions such as Ca2+ to the azacrown unit of this complex leads to a pronounced blue shift in the emission, which can be readily understood in terms of the large increase in the TICT energy that will accompany the binding of the metal ion to the lone pair of the azacrown nitrogen atom.