Strong Acceptor Substituents Research Articles

Relationships between geometrical and electronic structures and optical properties of 1,8-naphthosultam substituents and derivatives: TDDFT study

Geometrical and electronic structure and optical properties of several substituents and derivatives of 1,8-naphthosultam are studied by quantum-chemical DFT and TDDFT. It is found that the substituents –NO2, –CF3, and –N(CH3)2 affect the geometrical and electronic structure the most. It is shown that the Stokes shift is greatest (≈260 nm) for compounds with the strong donor substituent –N(CH3)2, while strong acceptor substituents provide a quite small Stokes shift. The dependence of the Stokes shift on the difference in energies of the frontier orbitals of the ground and excited states of molecules is found. Of the 1,8-naphthosultam substituents considered, compounds with –N(CH3)2 substituent, which emit in the biological window region, can be advised for use in optical bioimaging. The results can be used as a basis for the development and creation of new functional materials and biologically active compounds.

Solid-state deep blue and UV fluorescent dyes based on para-bis(2-thienyl)phenylene

Despite the general rule of strong acceptor substituents having a tendency to quench fluorescence due to molecular stacking, it is shown how tetra-fluorination of the central phenylene unit of para-bis(2-thienyl)phenylene can augment the quantum yields of solid state fluorescent dyes. Another significant part of the present research was the study of the influence of the position of the solubilization alkyl chains position on the fluorescent properties of the abovementioned non- and tetra-fluorinated materials. Tenfold augmentation of quantum yields, depending on the position of the alkyl chains is reported.

Synthesis and optoelectronic characterization of some triphenylamine-based compounds containing strong acceptor substituents

Three novel triphenylamine-based compounds containing strong electron acceptor groups have been synthesized and their comparative photophysical properties are presented. These compounds were obtained by a two-step method: (i) triphenylamine compounds with one, two and three phenylacetylene arms were synthesized by Sonogashira reaction between iodine-substituted triphenylamines and phenylacetylene, followed by (ii) post-modification of these electron-rich alkynes by addition of the strong electron acceptor, tetracyanoethylene. Characterization of all oligomers was made by FTIR, 1H-NMR, UV–vis and fluorescence spectroscopy. A batochromic shifting of the UV and photoluminescence maxima was observed with the increase of the acceptor group number. The electrochemical behavior was studied by cyclic voltammetry. The cyclic voltammograms have evidenced that triphenylamine-phenylacetylene compounds undergo only oxidation processes while compounds modified with tetracyanoethylene show both oxidation and reduction peaks associated with donor and acceptor groups, respectively. The donor–acceptor compounds coordinate metal ions (i.e., Hg2+ and Sn2+) by cyano groups resulting in the decreasing of charge transfer band intensity, and they can be used as chemosensors.

Platinum(IV)‐Mediated Nitrile–Amidoxime Coupling Reactions: Insights into the Mechanism for the Generation of 1,2,4‐Oxadiazoles

AbstractThe nucleophilic addition of amidoximes R′C(NH2)NOH (4: R′=Me, 5: CH2Ph, 6: Ph) to coordinated nitriles in the platinum(IV) complexes trans‐[PtCl4(RCN)2] (1: R=Et, 2: Ph, 3: NMe2) proceeds in a 1:1 molar ratio and leads to the monoaddition products [PtCl4(RCN){HNC(R)ONC(R′)NH2}] (7: R/R′: Et/CH2Ph, 8: Et/Ph, 9: NMe2/CH2Ph, 10: NMe2/Ph). Meanwhile, if the nucleophilic addition proceeds in a 2:1 molar ratio the reaction gives the bisaddition species [PtCl4{HNC(R)ONC(R′)NH2}2] (11: R/R′=Et/Me, 12: Et/CH2Ph, 13: Et/Ph, 14: Ph/Ph, 15: NMe2/Me, 16: NMe2/CH2Ph, 17: NMe2/Ph). All complexes 7–17 bear nitrogen‐bound O‐iminoacylated amidoxime groups. The addition of one equivalent of the corresponding amidoxime to each of 7–10 leads to 12, 13, 16, and 17, respectively. Complex [PtCl4(NCNMe2){HNC(NMe2)ONC(Ph)NH2}] (10), when dissolved in MeNO2, gave mer‐[PtCl3{HNC(NMe2)ONC(Ph)NHC(NMe2)NH}] (18) with the newly formed tridentate ligand derived from an unexpected coupling between two Me2NCN ligands and the N and the O centers of the amidoxime. The O‐imidoylamidoxime compounds R′C(NH2)NOCR(NH) (19–25) were liberated from the corresponding complexes 11–17 by treatment with excess NaCN and these metal‐free species were characterized by 1H and 13C{1H} NMR spectroscopy. The conversion of 19–25 into the 3,5‐substituted 1,2,4‐oxadiazole compounds OaNC(R′)NCb(R)(a–b) (26: R/R′=Me/Et, 27: PhCH2/Et, 28: Ph/Et, 29: Ph/Ph, 30: Me/NMe2, 31: PhCH2/NMe2, 32: Ph/NMe2) occurs at room temperature and the cyclization is promoted by strong acceptor substituents R′.

4,5‐Bis(dialkylamino)‐Substituted Imidazolium Systems: Facile Access to N‐Heterocyclic Carbenes with Self‐Umpolung Option

The first synthesis of 4,5-bis-(dimethylamino)-substituted imidazolium compounds was developed, which is based on the reaction of a 1,2-diamino-1,2-bis(phosphonio)ethene with lithiated formamidines. This represents the first application of this class of ethene derivatives for the preparation of heterocycles. These N-heterocyclic carbene (NHC) precursors show a remarkably reduced basicity and nucleophilicity of their NMe2 groups, which is due to the strong anomeric interactions of the latter with the imidazolium core. According to DFT calculations, these NHCs are capable of self-umpolung if sufficiently strong acceptor substituents are introduced at the carbene center. To test the self-umpolung capabilities of the NHCs, various substituents were attached to the carbene center and the obtained compounds were characterized by single-crystal X-ray analysis as well as quantum chemical computations. Strong acceptor substituents are required to induce self-umpolung, such as in the phosphonio-substituted derivative, for which partial self-umpolung was found. The N,N′-bis(4-dimethylaminophenyl)-substituted imidazolium compound represents a special case, as it incorporates as much as three two-step redox systems within the NHC framework. This will probably result in a high electronic flexibility of the corresponding nucleophilic carbenes, especially when they serve as ligands in transition metal complexes.

Cyclometalated Iridium(III) Complexes Based on Phenyl-Imidazole Ligand

Phenyl-imidazole-based ligands with various substitution patterns have been used as the main ligand for heteroleptic bis-cyclometalated iridium complexes. Two series of complexes have been prepared and their photophysical and electrochemical properties were studied. The phosphorescence emission maxima range from about 490 to 590 nm, that is, from greenish-blue to orange. The first series is of the form Ir(L)2(acac) (L: a phenyl-imidazole based ligand; acac: acetylacetonate). In the first complex, 1a, L is 1,4,5-trimethyl-2-phenyl-1H-imidazole. Then, methyl groups are replaced with phenyl groups and chlorines are grafted on the cyclometalated phenyl ring. The second series is of the form Ir(4,5-dimethyl-1,2-diphenyl-1H-imidazole)2(La) (La: ancillary ligand being acetylacetonate, acac, N,N-dimethylamino-picolinate, NPic, picolinate, Pic, or 2-(diphenylphosphino)acetic acid, P). These series show that modifying the substitution pattern on the ligands can alter the photophysical and electrochemical properties of the complexes. Overall, we show that compared to complexes containing phenyl-pyridine ligands, highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) are more delocalized over the entire main ligand in complexes containing phenyl-imidazole. Contrary to expectations, when chlorine atoms are used as strong acceptor substituents on the orthometalated phenyl, a red shift of the emission is observed. This behavior has been rationalized using theoretical calculations on the excited state of the chloro-substituted complex 3a compared to the model 1a.

Photoinduced orientational ordering in the series of methacrylic azopolymers

Polymethacrylates with side azo fragments containing various end substituents and spacers are synthesized and characterized. Spatial ordering of azo fragments in the series of the synthesized azo polymers under the action of polarized excitation light is studied by the methods of null ellipsometry and polarization spectroscopy. Two types of anisotropy development during photoirradiation are found. In homologs with strong acceptor substituents of azo fragments, biaxial orientational structure can be observed; as the radiation dose is increased, this structure is transformed into a uniaxial structure with a negative order parameter and with an axis parallel to the polarization vector of the excitation light. In this structure, the maximum degree of orientational order is achieved in polymers with high concentrations of azo fragments. In homologs with donor substituents of azo fragments, the initial stages of irradiation likewise lead to the development of biaxial orientation; later, this orientation is transformed into an isotropic distribution. The mode of anisotropy development is controlled by the lifetime of photoisomers of azo fragments.

Subphthalocyanines: Novel Targets for Remarkable Second-Order Optical Nonlinearities

The design and synthesis of new molecular and supramolecular systems for nonlinear optics (NLO) is currently of great scientific and technological interest.1 Until now investigations on second-order nonlinear optical properties of organic molecules have concentrated mainly on one-dimensional dipolar systems. Recently, theoretical and experimental considerations have suggested the possibility of developing new synthetic approaches to molecules with nonlinearities of so-called “multipolar” origin.2-5 In this communication we show that subphthalocyanines (SubPcs), π-conjugated phthalocyanine-related compounds, present very large second-order molecular polarizabilities (â) and emerge as novel targets for second-order nonlinear optical applications. Thus, for example the trinitroSubPc 1, described here for the first time, has shown experimental values of 〈â2〉1/2(2ω) ) 2000 × 10-30 (static 〈â2(0)〉1/2 ) 407 × 10-30 esu) by hyper-Rayleigh scattering (HRS) experiments. These second-order polarizabilities are mostly associated with the octupolar contribution and are comparable to those found in the most efficient linear (or dipolar type) compounds, such as polyenes,6 and to the best octupolar molecules described until now. Phthalocyanines (Pcs) are two-dimensional macrocyclic 18π-electron conjugated systems with a great variety of technological applications.7 In the last years Pcs8 and related analogues9 have emerged also as a novel class of molecular materials for third-order NLO applications. SubPcs 1-3 (Figure 1) are Pc-related macrocyclic compounds formed by three coupled isoindole moieties having a delocalized 14-π-electron system.10 Despite their cone-shaped structure,11 these compounds have an aromatic nature.12 Few axially and peripherally substituted boron SubPcs10,13-15 and dimers of SubPcs12 have been described. Their NLO properties remain unexplored, except for a very recent study on the third-harmonic generation (THG) spectroscopy of an evaporated thin film of SubPc 2,16 where values of o(3), three times larger than those corresponding to Pcs in the same wavelength range, were measured. In that case, the possibility of “cascading mechanisms” related to important second-order NLO properties was suggested. Therefore, the main objective of this work has been to evaluate the potential of these new molecules for second-order NLO applications. In this work we follow a 2D strategy for the design of SubPcbased NLO systems. Thus, besides the nonsubstituted SubPc 2, we prepared SubPc 1, with three surrounding strong acceptor substituents (NO2) forming a trigonal arrangement (Figure 2), and SubPc 3, having three peripheral donor groups (tert-Bu). Compounds 210,16 and 312 were prepared as previously described. Trinitro-SubPc 1 was synthesized for the first time in a similar way, by condensation of 4-nitro-1,2-dicyanobenzene in the pres-