Π-conjugated Substituents Research Articles

Two-photon absorption properties of simple neutral Ir(III) complexes.

A series of neutral Ir(2-phenylpyridine)3 derivatives substituted on the para-position of the pyridyl ligands with a π-conjugated substituent possessing different donor abilities has been prepared. Their two-photon absorption properties have been determined using the Z-scan technique. Such simple iridium(III) neutral complexes, which are easy to synthesize, show good two-photon absorption activity, with relevant TPA cross sections (the best is 750 GM), giving rise to multifunctional chromophores, since they present also high second-order NLO properties.

Tuning the resonance-assisted hydrogen bond (RAHB) of malonaldehyde using π-conjugated substituents and presentation of its energy decomposition

Position of substituents (R1 = H and R2, R3 = H, F, CnHn+1) on malonaldehyde has been changed to amplify its intramolecular hydrogen bonding (IHB) interaction. Results show that π-conjugated substituents at R3 position cause increase of IHB energies of the substituted malonaldehydes. Then, amplification of the IHB interaction in malonaldehyde with C20H21 group at R3 position was verified. Thus, polyethylene chains influence on resonance-assisted hydrogen bond (RAHB) of malonaldehyde and can form malonaldehyde-polyene materials with improved properties. Then, hydrogen bond (HB) interaction of formaldehyde with the substituted malonaldehydes without RAHB ring was studied and HB interaction energies calculated. The HB interaction energies have linear relation with intermolecular hydrogen bond length (dHB). A procedure was employed to estimate IHB energies of the substituted malonaldehydes using the mentioned relation and intramolecular hydrogen bond length (dIHB). Relation between geometrical parameters, spectroscopic data, topological properties, and molecular orbital descriptors with the estimated IHB energies were surveyed. Moreover, IHB energies of the current system were estimated using some quantum chemistry methods to validate the data and find more consistent method with the proposed viewpoint. Results indicated that the estimated IHB energies using proposed procedure have good correlations with other methods. Also, energy decomposition of the IHB interactions in the substituted malonaldehydes was performed using a new method and results showed that electrostatic term has major contribution in the IHB energies.

Elaborate Design of d8-d10 Heteronuclear Phosphors for Ultrahigh-Efficiency Solution-Processed Organic Light-Emitting Diodes.

Highly soluble d8-d10 heteronuclear phosphors afford an alternative approach to achieve high-efficiency organic light-emitting diodes (OLEDs) through a solution process. In this work, four highly phosphorescent d8-d10 heteronuclear complexes with significant Pt-Au interactions were prepared. By judicious selection of sterically hindered and π-conjugated substituents in triphosphine ligands, the phosphorescence is dramatically promoted through effectively prohibiting nonradiative thermal relaxation with an efficiency of 0.94-0.99 in doping films. Exploiting highly emissive Pt-Au complexes as phosphorescent dopants, ultrahigh-efficiency solution-processed OLEDs were attained. The peak current efficiency, power efficiency, and external quantum efficiency are 96.2 cd A-1, 65.0 lm W-1, and 26.4% for the green-emitting PtAu2 phosphor and 68.6 cd A-1, 42.5 lm W-1, and 25.1% for the orange-emitting Pt2Au phosphor, which represent the state-of-art for solution-processed OLEDs based on non-iridium phosphors.

联三吡啶铂(Ⅱ)配合物的非线性吸收和光限幅（特邀）

The reported work in 2003-2019 on the reverse saturable absorption (RSA) or two-photon absorption (TPA) and/or optical limiting (OPL) of platinum(II) terpyridine complexes was summarized in this minireview. Photophysical properties, including the ground-state absorption (GSA), excited-state absorption (ESA), excited-state lifetimes, and the quantum yields of triplet excited-state formation, RSA/OPL at 532 nm for ns laser pulses, TPA characteristics in the near-IR spectral regions, and the structure-property correlations were reviewed. This paper is composed of four sections. First, the current status of OPL materials and devices, the general requirements for reverse saturable absorbers and two-photon absorbing materials, and the different types and characteristics of square-planar platinum(II) complexes were briefly introduced. Then the photophysics and RSA/OPL of six series of Pt(Ⅱ) terpyridine-analogous complexes and the structure-property correlations were discussed. Following it the TPA of five series of Pt(Ⅱ) terpyridine complexes and the impacts of structural variations on the TPA cross sections (σ2) were reviewed. Finally, brief conclusions were drawn based on the reported studies. A general trend discovered was that the charge transfer absorption band(s) and the ESA can be readily tuned by substituents on the acetylide or the terpyridine ligand. Introducing electron-donating substituent to the acetylide or terpyridine ligand or improving the coplanarity between the aromatic substituent and the terpyridine ligand red-shifted the ground-state charge-transfer absorption band(s) at the price of decreasing/quenching the triplet ESA, which consequently reduced the RSA/OPL at 532 nm. Extending the π-conjugation of the terpyridine ligand dramatically improved the σ2 values of the Pt(Ⅱ) terpyridine complexes. Incorporation of electron-withdrawing π-conjugated aromatic substituent restrained the GSA to < 500 nm while keeping a long-lived triplet excited state with broadband ESA in the visible spectral regions and moderately strong TPA in the NIR regions. This approach could provide a solution for developing broadband OPL materials.

Solid-state polymerization of 1,4-bis(hexatriynyl)benzene derivatives

Two 1,4-bis(hexatriynyl)benzene derivatives with urethane groups, i.e., 10,10′-(1,4-phenylene)bis(5,7,9-decatriynyl N-(butoxycarbonylmethyl)carbamate) 1 and its perfluorophenylene derivative 2, were synthesized, and their solid-state photopolymerization was investigated. Upon UV irradiation, both of them showed excitonic absorption characteristic of polydiacetylene (PDA). In particular, 2 showed an absorption maximum at 743 nm, which was approximately 100 nm longer than that of conventional PDAs because of the effective conjugation between the polymer backbone and π-conjugated substituents. The conversion of 1 to the corresponding polymer was quite low. On the other hand, more than half of the hexatriynyl moieties of 2 were found to polymerize, indicating that most of the monomers were converted to the polymer upon prolonged UV irradiation.

Steric/π-Electronic Insulation of the carbo-Benzene Ring: Dramatic Effects of tert-Butyl versus Phenyl Crowns on Geometric, Chromophoric, Redox, and Magnetic Properties.

Hexa-tert-butyl-carbo-benzene (C18 tBu6 ) and three phenylated counterparts (C18 tBum Ph6-m ; m=4, 2) have been synthesized. The peralkylated version (m=6) provides experimental access to intrinsic features of the insulated C18 core independently from the influence of π-conjugated substituent. Over the series, structural, spectroscopic, and electrochemical properties are compared with those of the hexaphenylated reference (m=0). Anchoring tBu substituents at the C18 macrocycle is shown to enhance stability and solubility, and to dramatically modify UV/Vis absorption and redox properties. Whereas all carbo-benzenes reported previously were obtained as dark-reddish/greenish solids, crystals and solutions of C18 tBu6 happen to be yellow (λmax =379 vs. 472 nm for C18 Ph6 ). In comparison to C18 Ph6 , the reduction of C18 tBu6 remains reversible, but occurs at twice as high an absolute potential (E1/2 =-1.36 vs. -0.72 V). Systematic XRD analyses and DFT calculations show that the C18 ring symmetry is the nearest to D6h for m=6, which indicates a maximum geometric aromaticity. According to calculated nucleus-independent chemical shifts (NICS), the macrocyclic magnetic aromaticity is also maximum for C18 tBu6 : NICS(0)=-17.2 ppm versus (-18.0±0.1) ppm for the theoretical references C18 H6 and C18 F6 , and -13.5 ppm for C18 Ph6 . Accurate correlations of NICS(0) with experimentally recorded or calculated maximum UV/Vis absorption wavelengths, λmax , and chemical hardness, η=ELUMO -EHOMO , are evidenced.

Assessment of Front-Substituted Zwitterionic Cyanine Polymethines for All-Optical Switching Applications

Cyanine dyes in dilute solution have demonstrated the required third-order nonlinear optical properties necessary for all-optical switching (AOS) applications. However, in the solid state, interactions between the cyanine and its counterion typically induce significant geometric and electronic changes. When the counterion localizes toward one end of the cyanine, symmetry is broken and the figure-of-merit for AOS is dramatically reduced. Here, we use quantum-chemical methods to assess zwitterionic cyanines based on the heptamethine thiopyrylium dye as a means to eliminate untethered counterion interactions. In these zwitterionic systems, rigid π-conjugated substituents oppositely charged to the cyanine backbone are added to the thiopyrylium central carbon atom. For substituents that are unable to retain their anionic charge, and thus have a lower degree of zwitterionic character, electronic coupling between the substituent and cyanine backbone leads to profound geometry modifications and the thiopyrylium e...

Positioned substituent effect on self-assembly behaviors of perylene diimide derivatives on graphite

Three kinds of π-conjugated perylenetetracarboxylic diimide (PTCDI)-based organic functional derivatives, i.e., N,N′-di(2-ethylhexyl)-3,4,9,10-perylene diimide (molecule 1), N,N′-di(2-ethylhexyl)-1,7-dithien-2-yl-3,4,9,10-perylene diimide (molecule 2), N,N′-di(2-ethylhexyl)-1,7-di(2-bromothien-5-yl)-perylene-3,4,9,10-perylene diimide (molecule 3), have been prepared and investigated by using a scanning tunneling microscopy under ambient conditions. All of them can form long-ranged ordered self-assembled monolayer (SAM) with two different phase structures on highly oriented pyrolitic graphite (HOPG) surface. Coexistence of the two phases in the formed SAMs depends strongly on cooperation of various interactions including the molecule-substrate and molecule-molecule. Orientation of the molecular perylene cores primarily governs the molecular orientation in the SAMs by π-π interaction of the molecule-substrate through uttermost matching the graphite surface lattice. At the same time, the branched alkyl-chain substituents at N-sites can adjust intermolecular distances through conformation change while cyclic π-conjugated substituents at bay-positions lead to molecular rotation movement on HOPG, driven by the decreased molecule-substrate interaction caused by distortion of the perylene cores and the intermolecular interaction among the adjacent bay-substituents with rotation conformation change. In addition, the lateral forces from rotation conformation change of the bay-position substituents can also be strong enough to keep the orientation of the perylene cores unchanged. DFT calculation results further reveal the formation mechanisms for the three molecular self-assembly systems. These findings distinctly show that the SAM structures based on the π-conjugated PTCDI-based organic functional derivative molecules can be precisely modulated through introducing suitable substituents at both N- and bay-positions on the PTCDI core.

Activation of the SN2 Reaction by Adjacent π Systems: The Critical Role of Electrostatic Interactions and of Dissociative Character.

The activation of the SN2 reaction by π systems is well documented in textbooks. It has been shown previously that this is not primarily due to classical (hyper)conjugative effects. Instead, π-conjugated substituents enhance favorable substrate-nucleophile electrostatic interactions, with electron-withdrawing groups (EWG) on the sp(2) system leading to even stronger activation. Herein we report computational and experimental results which show that this activation by sp(2) EWG-substitution only occurs in a fairly limited number of cases, when the nucleophile involves strong electrostatic interactions (usually strongly basic negatively charged nucleophiles). In other cases, where bond breaking is more advanced than bond making at the transition state, electrophile-nucleophile electrostatic interactions are less important. In such cases, (hyper)conjugative electronic effects determine the reactivity, and EWG-substitution leads to decreased reactivity. The basicity of the nucleophile as well as solvent effects can help to determine which of these two regimes occurs for a given electrophile.

The barriers to internal rotation of benzaldehyde and benzoyl fluoride: "reconciliation" between theory and experiment.

This work is devoted to investigation of the known problem of the discrepancy between the experimental and theoretical values for the barrier to internal rotation of the molecules comprised of a benzene ring and a π-conjugated substituent, such as benzaldehyde and benzoyl fluoride. The possible reasons of such discrepancy are considered. As a result the conclusion was drawn that the origin of the problem is incorrectness of the assignments of the torsional levels higher than first, both for benzaldehyde and benzoyl fluoride. In addition the significant kinematic interaction between torsional vibration and out-of-plane CHO deformation was found for benzaldehyde.

Photochromic hybrid containing in situ-generated benzyl viologen and novel trinuclear [Bi3Cl14]5-: improved photoresponsive behavior by the π···π interactions and size effect of inorganic oligomer.

Two new member of (V)((2n+2)/2)[Bi(2n)Cl(8n+2)] series hybrids, (BzV)2[Bi2Cl10] (1) and (BzV)5[Bi3Cl14]2·(C6H5CH2)2O (2) (where BzV(2+) = N,N'-dibenzyl-4,4'-bipyridinium and (C6H5CH2)2O = dibenzyl ether) have been obtained, and compound 2 contains an unprecedented discrete trimer [Bi3Cl14](5-) counterion. The novel in situ-synthesized symmetric viologen cation with aromatic groups on both sides of 4,4'-bipy would provide more opportunities to create π···π interactions to optimize the photochromic property of the hybrid, and different bismuthated-halide oligomers enable us to discuss the size effect in this series of compounds. Both 1 and 2 are photochromic, and their photoresponsive rate is faster than that of reported viologen-metal halide hybrids. Experimental and theoretical data illustrated that the size of the inorganic oligomer can significantly influence the photoresponsive rate of the viologen dication, and the π···π interaction behaves as not only a powerful factor to stabilize the viologen monocation radical but also the second electron-transfer pathway, from a π-conjugated substituent to a viologen cation, for the photochromic process.

Effects of Extended π-Conjugation in Phenanthroline (N∧N) and Phenylpyridine (C∧N) Ligands on the Photophysics and Reverse Saturable Absorption of Cationic Heteroleptic Iridium(III) Complexes

Five new iridium(III) complexes (3,8-R-Phen)Ir(2-(3-R′-phenyl)pyridine)2 (1, R = fluoren-2-yl, R′ = H; 2, R = 7-benzothiazolylfluoren-2-yl, R′ = H; 3, R = H, R′ = fluoren-2-yl; 4, R = H, R′ = 7-benzothiazolylfluoren-2-yl; 5, R = R′ = 7-benzothiazolylfluoren-2-yl) with fluoren-2-yl or 7-benzothiazolylfluoren-2-yl substituent on the 2-phenylpyridine (C∧N) and/or phenanthroline (N∧N) ligands were synthesized and characterized. Their photophysical properties were investigated systematically via UV–vis absorption, emission, and transient difference absorption spectroscopy. Time-dependent density functional theory (TDDFT) calculations were performed to complement the experimental data and aid in our understanding of the characters of optical transitions. In addition, reverse saturable absorption was demonstrated at 532 nm for all complexes using nanosecond laser pulses. All complexes possess a weak low-energy tail that is attributed to 1,3MLCT (metal-to-ligand charge transfer)/1,3LLCT (ligand-to-ligand charge transfer) transitions. The major absorption bands below 475 nm for 1–5 arise from the 1π,π* and intraligand (1ILCT) transitions within the N∧N or C∧N ligands. Attachment of fluoren-2-yl or 7-benzothiazolylfluoren-2-yl substituents on N∧N ligand results in stronger red-shifts of the main absorption band to 400 and 408 nm for 1 and 2, respectively, as compared to 321 and 361 nm in complexes 3 and 4 with the same substituents on the C∧N ligands. In contrast, the 1,3MLCT/1,3LLCT transitions in 1 and 2 are just slightly red-shifted as compared to those in 3 and 4. The emission of complexes 1 and 2 is attributed to the N∧N ligand-centered 3π,π* state with some admixture of 3MLCT/3ILCT/3LLCT characters for 1. In contrast, the emission of 3 and 4 emanates exclusively from the 3MLCT/3LLCT states. For complex 5, which contains 7-benzothiazolylfluoren-2-yl substituents on both the C∧N and the N∧N ligands, the emission predominantly arises from the 3MLCT/3LLCT states with a small portion of N∧N ligand-localized 3π,π* character. All complexes exhibit broadband triplet excited-state absorption in the visible to the near-IR region, with the major absorption bands bathochromically shifted in 1 and 2 as compared to those in 3 and 4. The stronger excited-state absorption leads to dramatic reverse saturable absorption (RSA) at 532 nm for nanosecond laser pulses. The RSA strength decreases as 5 ≈ 2 > 4 ≈ 1 > 3, which is primarily determined by the ratio of the triplet excited-state absorption cross section relative to that of the ground state. Extended π-conjugation in the N∧N ligand obviously increases the RSA of complexes 1, 2, and 5 in comparison to those with π-conjugated substituents only on the C∧N ligands (3 and 4).

Investigation on CD Inversion at Visible Region Caused by a Tilt of the π-Conjugated Substituent: Theoretical and Experimental Approaches by Using an Asymmetric Framework of Diarylethene Annulated Isomer

A substituent effect of asymmetric diarylethene annulated isomer on their chiroptical properties was investigated by means of theoretical and experimental approaches. The absolute configuration of the annulated isomer was determined by X-ray structural analysis and DFT calculation. The TD-DFT calculation successfully reproduced not only the sign but also the shape and magnitude of experimental CD spectrum by considering the Boltzmann-weighted average of four atrop-isomers. A fragment decomposition (FD) analysis of rotatory strength clearly revealed a noteworthy effect; the tilting motion concomitant with the rotating motion of the substituent affects the sign and magnitude of CD signals. It was found that even when the absolute structure of the chiral core moiety does not change, the slight motion of the substituent can trigger the inversion of the CD signal.

Luminescent Polymer Consisting of 9,12‐Linked o‐Carborane

The synthesis of novel luminescent polymer containing p-phenylene-ethynylene and 9,12-linked o-carborane units alternately in the main chain is reported. The obtained polymer exhibits intense blue photoluminescence, providing the first insights into the optical properties of a 9,12-disubstituted o-carborane dye. π-Conjugated substituent at 9 and/or 12-positions in o-carborane is electrically independent, and both the HOMO and the LUMO levels slightly increase, whereas LUMO of the π-conjugated substituent at 1 and/or 2-positions in o-carborane decrease. Thus, it is deduced that polymers consisting of the 9,12-linked o-carborane unit are able to be applied as light-emitting materials.

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.

Helix-sense-selective Polymerization of 3,5-Bis(hydroxymethyl)phenylacetylene Connected with a Rigid and π-Conjugated Substituent

Abstract An achiral 3,5-bis(hydroxymethyl)phenylacetylene connected with a rigid and π-conjugated substituent was polymerized using a rhodium (Rh) complex catalyst in the presence of chiral 1-phenylethylamines. The circular dichroism (CD) spectra of the polymers indicated that an excess of one-handed helical polyacetylene backbone was induced by helix-sense-selective polymerization under the asymmetric conditions despite the achiral monomer. The split-type-induced CD signals at 300 nm were attributed to the chromophore including the conjugated side group since the signal appeared at a longer wavelength than the poly[3,5-bis(hydroxymethyl)phenylacetylene] without conjugated substituent.

Improved Photochromic Properties on Viologen-Based Inorganic–Organic Hybrids by Using π-Conjugated Substituents as Electron Donors and Stabilizers

A series of inorganic-organic hybrid compounds L(2)(Bi(2)Cl(10)) (L = HMV(2+) = N-proton-N'-methyl-4,4'-bipyridinium for 1, L = HBzV(2+) = N-proton-N'-benzyl-4,4'-bipyridinium for 2, and L = HPeV(2+) = N-proton-N'-phenethyl-4,4'-bipyridinium for 3) have been successfully synthesized by an in situ solvothermal reaction. Compounds 1-3, with the same metal halide as anions but different asymmetric viologen molecules as cations, are ideal model compounds for investigating the detailed effect of different photochromically active molecules on the photochromic properties of the hybrids. Compound 1 shows no photochromic behavior, but compounds 2 and 3 possess photochromism and show a faster photoresponse rate than other reported viologen metal halide hybrids. Studies on the relationship between the structure and photochromic behavior clearly reveal that π-conjugated substituents could be used to improve the photoresponsibility and enrich the developed color efficiently and that the π···π interaction among organic components may not only be a powerful factor to stabilize the viologen monocation radical but also act as the second path of electron transfer from the π-conjugated substituent to the viologen cation for the photochromic process, which significantly influences the photochromic properties.

Rotational Barriers in Azobenzene and Azonaphthalene

Theoretical predictions of rotational barriers for pi-conjugated substituents of aromatic rings sometimes significantly overestimate the corresponding experimental values. In this work, the rotational barriers in benzaldehyde, azobenzene, and azonaphthalene are studied by DFT calculations employing a variety of exchange-correlation functionals and basis sets. The results for benzaldehyde and azobenzene agree with previously published theoretical values. For azonaphthalene, 10 unique minima and corresponding rotational barriers have been found. The ability to distinguish minima connected by rotational barriers opens an opportunity for a detailed experimental study of rotational barrier heights in substituted aromatics.

Theoretical Investigation on the Origin of the CD Signal Reversal for the Closed-ring Isomer of Diarylethene with Peripherical π-Conjugated Substituents

Abstract Theoretical investigation was performed on the CD signal of the closed-ring isomer of diarylethene with an extended π-conjugated substituent. The result showed that the different contribution of the substituent to the transition electronic and magnetic dipole moments induces CD signal reversal. The obtained finding can be applied to general dyes with a chiral core and extended π-conjugated substituents.

Synthesis, structures, optical properties, and TD-DFT studies of donor-π-conjugated dipicolinic acid/ester/amide ligands

A series of 2,6- and 4-functionalized dipicolinic acid, ester, or amide featuring π-conjugated substituents such as donor-(phenyl or fluorenyl)-acetylene groups have been synthesized. Four crystallographic structures are reported. The influence of the substituent position, the nature of the donor group, and the conjugated backbone as well as the role of the pyridinic side arms on the absorption and emission properties are studied and discussed on the basis of TD-DFT calculations.