Localised to intraligand charge-transfer states in cyclometalated platinum complexes: an experimental and theoretical study into the influence of electron-rich pendants and modulation of excited states by ion binding

Abstract

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.

Novel N^C^N-cyclometallated platinum complexes with acetylide co-ligands as efficient phosphors for OLEDs

“…1) show intense bands in the region 260-330 nm, which can be assigned to intraligand 1 p-p* transitions of cyclometallated 1,3-di(2-pyridyl)benzene 21 and acetylide 18 ligands, and less intense bands at 330-450 nm, corresponding to transitions of mixed charge-transfer/ligandcentred character. 28 Although the substitution of the chloride ligand by the phenylacetylide does not change the spectral profile substantially, the individual bands in this region become rather less wellresolved, and there is evidence of a tail to longer wavelength in the phenyl acetylide complexes. This might tentatively be attributed to the introduction of a p C^C / p* N^C^N ligand-toligand charge-transfer (LLCT) transition, by analogy with other Pt(II) acetylide complexes, 10,11,18 and as suggested by the observed trend in oxidation potentials discussed above, which indicate that the HOMO is raised in energy upon introduction of the acetylide.…”

“…All four complexes show a similar reduction peak around À2.3 V. The lack of significant variation with the co-ligand or central aryl substituent is consistent with the lowest unoccupied molecular orbital (LUMO) being dominated by the pyridyl rings, with little contribution expected from the cyclometallating ring or co-ligand. 28 On the other hand, the introduction of the phenylacetylide co-ligand is seen to shift the oxidation potential significantly to less positive potentials for both pairs of complexes, suggesting that the strongly s-donating acetylide ligand raises the energy of the highest occupied molecular orbital (HOMO). Density functional theory calculations have previously suggested that the HOMO in [Pt(N^C^N) Cl] complexes is spread over the metal, the co-ligand, and the cyclometallating aryl ring.…”

Tuning the dipolar second-order nonlinear optical properties of 5-π-delocalized-donor-1,3-di(2-pyridyl)benzenes, related cyclometallated platinum(ii) complexes and methylated salts

“…Typically in complexes such as Pt(dpyb)Cl, the bands in the 350 -400 nm region have ε values of around 7000 M , with no counterparts in the proligands, and are due to metal--to--ligand and intra--ligand charge--transfer transitions that are introduced upon cyclometallation. 30,47,48 Such transitions will necessarily be present in the current complexes, but they are evidently superimposed by intense ligand--centred transitions. This leads to unusually high ε values in this region.…”

Cyclometallated platinum(ii) complexes of 1,3-di(2-pyridyl)benzenes for solution-processable WOLEDs exploiting monomer and excimer phosphorescence

“…Meanwhile, the change in the monomeric co-ligand from chloride in MePtCl to isothiocyanate in MePtNCS is seen to have essentially no significant effect, probably reflecting the rather small participation of the co-ligand in such complexes, 27 and the similar positions of the NCS and Cl ligands in the spectrochemical series.…”

“…Thus, timedependent density functional theory calculations on HPtCl have revealed that the T 1 state has predominantly HOMO-LUMO character. 27 The HOMO is based primarily on the aryl ring, the metal ion and the chloride co-ligand, whilst the LUMO is predominantly localised on the pyridyl rings. The inductive electron-withdrawing effect of F atoms, irrespective of whether they be meta or para to the metal, should lower the energy of both HOMO and LUMO, as found, for example, with iridium (III) complexes containing related fluorine-substituted terdentate ligands.…”