Substituted 4,4′-Stilbenoid NCN-Pincer Platinum(II) Complexes. Luminescence and Tuning of the Electronic and NLO Properties and the Application in an OLED

Abstract

A series of 4,4′-disubstituted organic-organometallic stilbenes, i.e., the 4′-substituted stilbenoid-NCN-pincer platinum(II) complexes [PtCl(NCN-R-4)] (NCN-R-4 ) [C 6 H 2 (CH 2 NMe 2 ) 2 -2,6-R-4] -in which R ) C 2 H 2 C 6 H 4 -R′-4′ with R′ ) NPh 2 , NMe 2 , OMe, SiMe 3 H, I, CN, NO 2 ) (1-8), were studied for their electronic, electrochemical, and NLO properties. Complex 7 was also chemically oxidized using Cu(II)Cl 2 , yielding the [Pt(IV)Cl 3 (NCN(C 2 H 2 C 6 H 4 -CN-4′)-4)] complex 11. In contrast to 1 and 7, 11 did not show luminescent properties in solution at room temperature. In the solid state (X-ray crystallography) 11 has an overall bent structure, with the pincer moiety and the cyano group beneath the plane of the central double bond. Stilbenoid pincer complex 1 (R′ ) NPh 2 ) was also studied with UV/vis spectroscopy in a series of different solvents. The compound shows an intense low-energy transition band, appearing at a lower energy (λ max ) 374-379 nm) than 2 (R′ ) NMe 2 ), which contains a stronger donor group. The UV/vis absorption data of a selected series of stilbenoid pincer platinum complexes were interpreted in terms of a qualitative orbital model based on DFT and TD-DFT calculations. The trends observed in the transition energies correlate well with the donor/acceptor properties of the substituents. It was shown that the HOMO-LUMO gap of the stilbenoid pincer platinum compounds decreases when going from an electron-neutral group (R′ ) H) toward an electron-donating (R′ ) NMe 2 ) or -accepting (R′ ) NO 2 ) group. The fluorescence properties of 1 were also investigated using fluorescence and laser spectroscopy, showing that the metal center favors nonradiative decay from the excited state to the ground state. For 7 (R′ ) CN), solid-state 13 C CP/MAS NMR measurements were performed, and a value of 1 J( 13 C, 195 Pt) ) 1003 ( 15 Hz was obtained for the C ipso carbon, reflecting π-contributions in the carbon-metal bond. The cyclic voltammetry study on the complexes revealed an irreversible oxidation from Pt(II) to Pt(IV). In general the oxidation potential is influenced by the electronic character of the R′ substituent. The influence of the para R′ substituent on the hyperpolarizability of the compound was investigated using the femtosecond (frequencyresolved) HRS technique. For 1-7 good zzz values were measured ( zzz ranging from (164-1324) × 10 -30 esu) with the highest value for 6 (R′ ) I). The higher zzz values were found for the compounds that contain a low-energy UV/vis absorption band and/or a large absorption coefficient. Donor-acceptor complexes 7 and 8 were successfully applied in an organic light-emitting diode (OLED). In the electroluminescence spectrum of 7 (R′ ) CN) a low-energy emission band was observed at 643 nm. This band also appeared in the solid-state luminescence spectra of 7, however, only at low temperatures (298-15 K). The emission band was assigned to result from a triplet excited state. The present study shows that the introduction of a NCN-pincer platinum fragment ...

Facile Approaches to Phosphorescent Bis(cyclometalated) Pentafluorophenyl PtIV Complexes: Photophysics and Computational Studies

“…These complexes display highly efficient triplet‐state phosphorescence, primarily determined by the cyclometalating group and the auxiliary ligands, which, in most of the cases, is attributed to 3 LC ( 3 ππ) or admixtures of 3 LC/ 3 MLCT or 3 LLCT/ 3 MLCT depending on the nature of the co‐ligands . By judiciously selecting the metal and ligands, complexes with tuneable emission color covering the visible to near‐infrared region have been designed that have found numerous applications, for example, as dopants in light‐emitting diodes (OLEDs), photocatalysis, biological imaging, optical sensors, nonlinear optical materials, solid‐state light‐emitting electrochemical cells, and photodynamic therapy . In this field, considerable efforts have been made to prepare cyclometalated complexes of d 6 (Ru II , Ir III )– and d 8 (Pt II , Au III ) metal ions having octahedral and square‐planar geometries, respectively, and to study their luminescent properties.…”

“…Later works in the last decade have led to the successful synthesis of a few additional examples of luminescent mono‐, bis‐, and tris(cyclometalated) Pt IV complexes by the oxidation of related Pt II complexes. However, the study of these types of systems is still rather limited . An attractive feature of platinum(IV) complexes is that, with six coordination, they have greater variety of ligand combination than do the four‐coordinate Pt II derivatives.…”

Tuning the Dipolar Second‐Order Nonlinear Optical Properties of Cyclometalated Platinum(II) Complexes with Tridentate N^C^N Binding Ligands

“…[7] It was recently reported [8] that a series of unconventional terpyridine and cyclometalated dipyridylbenzene platinum(II) complexes, with the tridentate ligand carrying a strong electron-acceptor group, display large quadratic hyperpolarizabilities, as measured by the hyper-Rayleigh scattering technique (HRS). [9] These results prompted us to carry out an in-depth investigation of the electronic origin and the tuning of the second-order NLO properties of cyclometalated 1,3-di(2pyridyl)benzene platinum(II) complexes. The complementary use of electric-field-induced second-harmonic (EFISH) generation and harmonic light scattering (HLS) measure-ments demonstrates how the quadratic hyperpolarizability of this appealing family of multifunctional chromophores, characterized by a good transparency throughout much of the visible region, is dominated by an octupolar contribution.…”

“…[8] Indeed, various luminescent 4,4'-stilbenoid-substituted N^C^N platinum(II) complexes show significant quadratic hyperpolarizabilities, as determined by the HRS technique, which are higher than those typical of purely organic 4,4'-disubstituted stilbenes, probably due to the polarization along the CÀPt bond. [9] These results prompted us to carry out an in-depth investigation of the electronic origin and the tuning of the second-order NLO properties of cyclometalated 1,3-di(2pyridyl)benzene platinum(II) complexes. This class of materials is of particular interest for study because of their remarkable luminescent properties.…”

Conjugated compounds in supramolecular informational systems: A review

“…Due to the AIEE effect in low dimensional sys tems, there is a discussion in the literature concerning the mechanism of relaxation of the excited state of chromophore through a twisted state with intramolec ular charge transfer (twisted intramolecular charge transfer (TICT)) and the possibility for realizing such a relaxation pathway in densely packed films [218,255,[263][264][265][266]. A simplified scheme of the formation and nonradiative deactivation of the TICT state is shown in Fig.…”