Claudia Dragonetti

Cyclometallated platinum(II) complexes of 1,3-di(2-pyridyl)benzenes: tuning excimer emission from red to near-infrared for NIR-OLEDs

The Pt(II) complex N^C2^N-1,3-di(2-pyridyl)benzene platinum chloride (PtL1Cl) is known to display efficient triplet luminescence in the green region of the spectrum, and to form an unusually emissive excimer that emits around 690 nm. In this contribution, the introduction of trifluoromethyl groups into either the 4- or 5-position of the pyridyl rings of the ligand is shown to lead to a red-shift in the excimer band, moving it into the near infra-red (NIR) region. The new ligands, synthesised by either Suzuki or Stille cross-coupling methods, are 1,3-bis(4-(trifluoromethyl)pyridin-2-yl)benzene HL27, 1,3-bis(4-(trifluoromethyl)pyridin-2-yl)-4,6-difluorobenzene HL28, and 1,3-bis(5-(trifluoromethyl)pyridin-2-yl)-4,6-difluorobenzene HL29, from which the corresponding Pt(II) complexes PtLnCl have been prepared. The monomer and excimer emission energies in solution are compared with those of PtL1Cl and PtL22Cl {HL22 = 1,3-di(2-pyridyl)-4,6-difluorobenzene}. The order for the monomer can be rationalised in terms of the stabilising effects of the F atoms and the CF3 groups on the HOMO and LUMO respectively. The order of excimer emission proves to be subtly different, but the most red-shifted complex in both cases is PtL27Cl. The electroluminescence of neat films of the complexes as emitting layers in OLEDs displays uniquely excimer-like emission, extending well into the technologically important NIR region.

Coordination and Organometallic Complexes as Second-Order Nonlinear Optical Molecular Materials

Coordination and organometallic complexes with second-order nonlinear optical (NLO) properties have attracted increasing attention as potential molecular building block materials for optical communications, optical data processing and storage, or electrooptical devices. In particular, they can offer additional flexibility, when compared to organic chromophores, due to the presence of metal–ligand charge-transfer transitions, usually at relatively low-energy and of high intensity, tunable by virtue of the nature, oxidation state, and coordination sphere of the metal center. This chapter presents an overview of the main classes of second-order NLO coordination and organometallic complexes with various ligands such as substituted amines, pyridines, stilbazoles, chelating ligands (bipyridines, phenanthrolines, terpyridines, Schiff bases), alkynyl, vinylidene, and cyclometallated ligands, macrocyclic ligands (porphyrins and phthalocyanines), metallocene derivatives, and chromophores with two metal centers. The coverage, mainly from 2000 up to now, is focused on NLO properties measured at molecular level from solution studies, as well as on NLO properties of bulk materials.

Cyclometalated IrIII Complexes with Substituted 1,10‐Phenanthrolines: A New Class of Efficient Cationic Organometallic Second‐Order NLO Chromophores

Cyclometalated cationic Ir(III) complexes with substituted 1,10-phenanthrolines (1,10-phen), such as [Ir(ppy)(2)(5-R-1,10-phen)]Y (ppy=cyclometalated 2-phenylpyridine; R=NO(2), H, Me, NMe(2); Y(-)=PF(6) (-), C(12)H(25)SO(3) (-), I(-)) and [Ir(ppy)(2)(4-R,7-R-1,10-phen)]Y (R=Me, Ph) are characterized by a significant second-order optical non linearity (measured by the electrical field induced second harmonic generation (EFISH) technique). This nonlinearity is controlled by MLCT processes from the cyclometalated Ir(III), acting as a donor push system, to pi* orbitals of the phenanthroline, acting as an acceptor pull system. Substitution of cyclometalated 2-phenylpyridine by the more pi delocalized 2-phenylquinoline (pq) or benzo[h]quinoline (bzq) or by the sulfur-containing 4,5-diphenyl-2-methyl-thiazole (dpmf) does not significantly affect the mubeta absolute value, which instead is affected by the nature of the R substituents on the phenanthroline, the higher value being associated with the electron-withdrawing NO(2) group. By using a combined experimental (the EFISH technique and (1)H and (19)F PGSE NMR spectroscopy) and theoretical (DFT, time-dependent-DFT (TDDFT), sum over states (SOS) approach) investigation, evidence is obtained that ion pairing, which is controlled by the nature of the counterion and by the concentration, may significantly affect the mubeta values of these cationic NLO chromophores. In CH(2)Cl(2), concentration-dependent high absolute values of mubeta are obtained for [Ir(ppy)(2)(5-NO(2)-1,10-phen)]Y if Y is a weakly interacting anion, such as PF(6) (-), whereas with a counterion, such as C(12)H(25)SO(3) (-) or I(-), which form tight ion-pairs, the absolute value of mubeta is lower and quite independent of the concentration. This mubeta trend is partially due to the perturbation of the counterion on the LUMO pi* levels of the phenanthroline. The correlation between the mubeta value and dilution shows that the effect of concentration is a factor that must be taken into careful consideration.

From red to near infra-red OLEDs: the remarkable effect of changing from X = –Cl to –NCS in a cyclometallated [Pt(N∧C∧N)X] complex {N∧C∧N = 5-mesityl-1,3-di-(2-pyridyl)benzene}

[PtL(6)X] {X = Cl or NCS and L(6) = 5-mesityl-1,3-di(2-pyridyl)-benzene} display similar luminescence in solution but, in the solid state, the packing of the molecules is different, with short PtPt interactions for X = NCS, leading to a red-shifted emission band. The effect has been used to generate OLEDs that emit squarely in the NIR region (855 nm).

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

Two cyclometallated platinum(II) complexes, N^C^N-5-fluoro-1,3-di(2-pyridyl)benzene platinum(II) chloride, FPtCl, and N^C^N-5-methyl-1,3-di(2-pyridyl)benzene platinum(II) isothiocyanate, MePtNCS, have been synthesized and characterized. Both complexes are highly efficient phosphorescent green emitters which can also display excimer emission in the red region. They have been studied as triplet emitters in solution-processed, multilayer organic light-emitting diodes (OLEDs), together with the known complex of 5-methyl-1,3-di(2-pyridyl)benzene, MePtCl, for comparison. The trend in efficiencies of the OLEDs prepared correlates with the charge-trapping properties of the complexes. The most efficiently emitting complex, FPtCl, was used as the dopant in a solution-processed white OLED, employing monomer and excimer emission.

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

Two new cyclometallated platinum(II) complexes have been prepared that incorporate a terdentate N^C^N-coordinating ligand and a monodentate acetylide co-ligand. The complexes, namely [PtL3–CC–C6H3F2] and [PtL6–CC–C6H3F2] (where HL3 = 5-methyl-1,3-di(2-pyridyl)benzene; HL6 = 5-mesityl-1,3-di(2-pyridyl)benzene; H–CC–C6H3F2 = 3,5-difluorophenylacetylene), were prepared by ligand metathesis from the corresponding chloro complex PtLnCl. Both of the new complexes are intensely luminescent in solution, displaying quantum yields superior to PtLnCl. OLEDs have been prepared using the new compounds as phosphorescent emitters. Although both lead to efficient devices, the best electroluminescence quantum efficiencies are obtained with the derivative of HL6, having the mesityl group on the cyclometallated phenyl ring. The superior performance with this complex can be rationalised in terms of the greater steric hindrance that serves to reduce aggregate-induced quenching.

Thiocyanate-Free Ruthenium(II) Sensitizer with a Pyrid-2-yltetrazolate Ligand for Dye-Sensitized Solar Cells

The synthesis of the new complex [Ru(Tetrazpy)(dcbpy)2]Cl is reported, along with its spectroscopical, electrochemical, and theoretical characterization. The first dye-sensitized solar cell device with this complex has been prepared, leading to a 3% of conversion efficiency, promising data considering the simplicity of the Tetrazpy ligand.

Linear and nonlinear optical properties of tris-cyclometalated phenylpyridine Ir(III) complexes incorporating π-conjugated substituents.

The synthesis, luminescence, and nonlinear optical properties of a new series of Ir(ppy)3 (ppy = 2-phenylpyridine) complexes incorporating π-extended vinyl-aryl substituents at the para positions of their pyridine rings are reported. Appropriate substitution of the pyridyl rings allows the tuning of the luminescence properties and the second-order nonlinear optical response of this unusual family of three-dimensional chromophores. Theoretical calculations were performed to evaluate the dipole moments, to gain insight into the electronic structure, and to rationalize the observed optical properties of the investigated complexes.

Platinum(II) complexes with cyclometallated 5-π-delocalized-donor-1,3-di(2-pyridyl)benzene ligands as efficient phosphors for NIR-OLEDs

Two new pincer proligands, namely 5-(p-(N,N-diphenylamino)phenylethynyl)-1,3-di(2-pyridyl)benzene (HL1) and trans-5-(p-(N,N-diphenylamino)styryl-1,3-di(2-pyridyl)benzene (HL2) were prepared together with their N^C^N-coordinated cyclometallated platinum(II) complexes PtL1X (X = Cl, NCS) and PtL2Cl. Both ligands are intensely luminescent in solution (quantum yields > 0.8). PtL1X complexes display high quantum yields in solution whereas that of PtL2Cl is very low due to the ease with which trans to cis isomerisation of the diphenylaminostyryl CC bond occurs. Distinct sets of emission bands attributable to the cis and trans forms are observable in glass at 77 K, the assignments being supported by TD-DFT calculations. Organic light-emitting diodes (OLEDs) have been prepared using the new compounds as phosphorescent emitters. Remarkably, despite the inferior quantum yield of PtL2Cl in solution, the best electroluminescence quantum efficiencies are obtained with this complex, which emerges as an excellent candidate for the preparation of NIR-OLEDs.

Thiocyanate-free cyclometalated ruthenium sensitizers for solar cells based on heteroaromatic-substituted 2-arylpyridines

The first examples of thiocyanate-free thiophene-substituted Ru(II) cyclometalated complexes, based on thiophene-derived 2-(2,4-difluorophenyl)pyridine ligands, are presented and investigated as photosensitizers in DSCs. Upon thiophene substitution the complexes presented enhanced optical properties compared to the reference dye with no thiophene substitution. DSCs based on the dithienyl-derived dye showed power conversion efficiencies up to 5.7%, more than twice that containing the complex without the thiophene substitution.