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In the last past twenty years, research on luminescent platinum (II) complexes has been intensively developed for useful application such as organic light emitting diodes (OLEDs). More recently, new photoluminescent complexes based on diazine ligands (pyrimidine, pyrazine, pyridazine, quinazoline and quinoxaline) have been developed in this context. This review will summarize the photophysical properties of most of the phosphorescent diazine Pt(II) complexes described in the literature and compare the results to pyridine analogues whenever possible. Based on the emission color, and the photoluminescence quantum yield (PLQY) values, the relationship between structure modification, and photophysical properties are highlighted. Tuning of emission color, quantum yields in solution and solid state and, for some complexes, aggregation induced emission (AIE) or thermally activated delayed fluorescence (TADF) properties are described. When emitting OLEDs have been built from diazine Pt(II) complexes, the external quantum efficiency (EQE) values and luminance for different emission wavelengths and in some cases, chromaticity coordinates obtained from devices, are given. Finally, this review highlights the growing interest in studies of new luminescent diazine Pt(II) complexes for OLED applications. [ABSTRACT FROM AUTHOR]

A series of four push‐pull compounds based on a 4‐phenylquinazoline scaffold were designed. Three of these compounds exhibit dual state emission with intense fluorescence in solution and solid state. Their emission properties are sensible to environement stimuli with intense emission solvatochromism and halochromism. In case of methoxy derivative, both neutral and protonated form are luminescent and white light emission can be observed by controlled partial protonation. [ABSTRACT FROM AUTHOR]

A pyrimidine chromophore bearing an acridan fragment was synthesized and its photophysical properties were studied. In solution, this compound is characterized by an important positive emission solvatochromism with a shift of 5800 cm−1 between nonpolar heptane and dichloromethane (DCM) associated with large Stokes shifts (up to 9100 cm−1 in DCM). Mono‐exponential fluorescence decays are observed in heptane whereas more complicated bi‐ or three‐exponential decays are observed in more polar solvents due to an interplay between locally excited and charge transfer excited state. Additionally, an aggregation‐induced enhanced emission process was demonstrated in THF/water mixtures. At low temperature (77 K), in a polymethylmethacrylate (PMMA) thin film, the presence of an accessible triplet state (T1) was demonstrated, which was not observed in solution. Finally, we show that it is possible to protonate the chromophore in thin film leading to panchromatic dual emission [ABSTRACT FROM AUTHOR]

A series of four V-shaped methylpyrimidinium salts bearing diphenylamino-electron-donating groups appended at various pyrimidine positions were designed. These chromophores were obtained by regioselective N(1) monomethylation by methyl methanesulfonate of the pyrimidine core. Linear optical properties were studied experimentally and electronic properties were further completed by (TD)-DFT calculations. The second-order nonlinear optical (NLO) properties were also studied using electric field induced second harmonic generation (EFISH) method in chloroform, and all pyrimidinium salts exhibited μβ0>1000×10-48 esu. The 2,4-disubstituted pyrimidinium core is preferred over 4,6-disubstitution as it enhances the NLO response and increases the dipole moment. (E,E)-2,4-Bis[4-(diphenylamino)styryl]-1-methylpyrimidin-1-ium methanesulfonate appears to be the best NLO-phore in chloroform in the series (μβ0=2500×10-48 esu) and a figure of merit μβ0/MW= 3.4 10-48 esumolg-1. [ABSTRACT FROM AUTHOR]

A series of five platinum (II) complexes based on 2‐phenylpyrimidine ligands have been designed. Pyridine and chloride were used as auxiliary ligands. These complexes exhibit a slightly distorted square‐planar geometry. The nature and position of substituent on the phenyl ring was thoroughly studied. The presence of an electron‐donating substituent on the phenyl ring in the para position of the platinum atom leads to a red shift of the lowest‐energy absorption band, which corresponds to the HOMO→LUMO transition and permits to obtain phosphorescence in deoxygenated CH2Cl2 solution. All compounds are emissive in the solid state with a significant redshift for complexes bearing electron‐donating substituent on the phenyl ring of phenylpyrimidine ligand. [ABSTRACT FROM AUTHOR]

Six pyrimidine‐based push–pull systems substituted at positions C2 and C4/6 with phenylacridan and styryl moieties, employing methoxy or N,N‐diphenylamino donors, have been designed and synthesized through cross‐coupling and Knoevenagel reactions. X‐ray analysis confirmed that the molecular structure featured the acridan moiety arranged perpendicularly to the residual π system. Photophysical studies revealed significant differences between the methoxy and N,N‐diphenylamino chromophores. Solvatochromic studies revealed that the methoxy derivatives showed dual emission in polar solvents. Time‐resolved spectroscopy revealed that the higher energy band involved very fast (<80 ps) fluorescence, whereas the lower energy one included long components (≈30 ns) due to long‐lived intramolecular charge‐transfer fluorescence. In contrast to N,N‐diphenylamino chromophores, the methoxy derivatives also showed aggregation‐induced emission in mixtures of THF/water, as well as dual emission in thin films, covering almost the whole visible spectrum with corresponding chromaticity coordinates not far from that of pure white light. These properties render the methoxy derivatives as very promising organic materials for white organic light‐emitting diodes. [ABSTRACT FROM AUTHOR]

A series of chromophores bearing 4‐cyanoquinazoline, 2‐(4‐cyanophenyl)quinazoline or 2‐(4‐trifluorophenyl)quinazoline electron–acceptor (A) and 5‐(4‐aminophenyl)thiophen‐2‐yl or 4‐aminophenyl electron‐donor (D) units has been designed. The influence of the electron‐withdrawing substituent on the pyrimidine core as well as the nature of the amino electron donating group has been studied by cyclic voltammetry, UV/Vis and emission spectroscopy. Whereas 2‐(4‐cyanophenyl)quinazoline and 2‐(4‐trifluorophenyl)quinazoline derivatives are highly luminescent in chloroform solution, 4‐cyanoquinazolines are poorly emissive. Interestingly all compounds are luminescent in the solid state with the emission ranging from blue to red. The second order nonlinear optical properties were studied using electric field induced second harmonic generation (EFISH) method. Quantum‐chemical calculations corroborate the aforementioned experimental results. [ABSTRACT FROM AUTHOR]

The photophysical properties of a series of conjugated push–pull (iso)quinolines were studied. The compounds were synthesized by well‐established and straightforward methodologies. The materials exhibited not only emission solvatochromism in a variety of nonpolar solvents, but also tunable halochromism. Some of the compounds remained moderately luminescent after protonation and had a red emissive form, which was used to obtain white‐light emission, both in solution and in thin films, by controlled protonation of the initially blue–green‐emitting materials. This methodology has potential applications in the fabrication of white organic light‐emitting diodes with two forms of a single emitter in equilibrium. [ABSTRACT FROM AUTHOR]

Conjugated push‐pull molecules that incorporate nitrogen heterocycles as electron‐withdrawing units are interesting materials because of their luminescence properties. These chromophores can be easily and reversibly protonated at the nitrogen atom of the heterocyclic ring and this can cause dramatic color changes. White and multi‐color photoluminescence both in solution and in the solid state can be obtained by an accurate control of the amount of acid. Thus, with a suitable design these compounds have potential applications in the development of colorimetric pH sensors and the fabrication of OLEDs based on only one material. We provide here a brief overview of our collaborative efforts made in this area. [ABSTRACT FROM AUTHOR]

Efficient synthetic routes that combine different palladium‐catalyzed cross‐coupling reactions have been developed for the preparation of a new family of push‐pull derivatives in which pyridine was used as the acceptor group and different para‐substituted diphenylamines as the donor groups. All compounds showed absorption in the UV/Vis region and blue‐green emission with high quantum yields. Significant red shifts were observed in the absorption and fluorescence emission maxima on increasing the electron‐donating ability of the substituents or on incorporating a π‐conjugated linker. This finding can be explained on the basis of enhanced intramolecular charge transfer (ICT). Strong emission solvatochromism confirmed the formation of an intramolecular charge‐separated emitting state. The HOMO‐LUMO energy gaps have been estimated by experimental electrochemical measurements and the results were interpreted with the aid of DFT calculations. The thermal behavior of all materials has also been studied by differential scanning calorimetry. A series of push‐pull chromophores with a pyridine electron‐withdrawing fragment and methoxy‐, 9H‐carbazol‐9‐yl‐, and 5‐methoxythiophen‐2‐yl‐substituted diphenylamino groups as electron‐donating units was designed. [ABSTRACT FROM AUTHOR]

Eighteen push-pull pyrazine fluorophores are described. Methoxy, dimethylamino, and diphenylamino groups were used as electron-donating groups (EDGs), and six π-conjugated systems consisting of multiple bonds and 1,4-phenylene moieties were used to connect the EDGs to the pyrazine core. The chromophores were obtained by Knoevenagel condensation and palladium-catalyzed cross-coupling reactions starting from commercially available 2-methyl- or 2-chloropyrazine. The optical absorption and emission properties of these chromophores were studied in different solvents. Some proved to be highly emissive and exhibit strong emission solvatochromism, which suggests a highly polar emitting state, characteristic of compounds that undergo intramolecular charge transfer (ICT). The influences of the various electron-donating groups and π-conjugated systems on the emission and ICT have been carefully studied. [ABSTRACT FROM AUTHOR]