Octupolar Compounds Research Articles

Effect of Branching on the Delayed Fluorescence and Phosphorescence of Simple Borylated Arylamines.

A donor-π-acceptor strategy is being well exploited in several fields in view of their robust optical properties. However, the impact of branching in quadrupolar [A-(π-D)2] and octupolar [A-(π-D)3] molecules in comparison to parent dipolar (A-π-D) molecules on the delayed fluorescence and phosphorescence properties is seldom explored. We have presented herein the distinct and contrasting optical properties of a tridurylborane core bearing -NH2 (1-3) and -NMe2 (4-6) donor moieties, wherein the number of donors is increased systematically. Because of propeller molecular architecture, the donor and acceptor are weakly coupled, and the frontier molecular orbitals are spatially localized. All of the compounds show delayed fluorescence under ambient conditions and persistent phosphorescence at low temperature. Solvent-dependent studies and temperature-dependent luminescence measurements established that quadrupolar (2 and 5) and octupolar (3 and 6) compounds underwent symmetry breaking in the excited state. Curiously, delayed fluorescence and phosphorescence spectra are found to be blue-shifted and follow the same trend as the fluorescence upon an increase in the branches. The highest quantum yield was observed for dipolar compounds. Besides, the phosphorescence lifetime decreases with an increase in the number of branches. These interesting experimental observations are further supported by quantum-mechanical calculations.

Hyper-Rayleigh scattering studies of a three-photon fluorescent non-linear optical molecule at 1500 nm

A new octupolar compound 2,4,6-tris{4-[4-(dibutylamino)phenylethynyl]phenyl}-1,3,5-triazine was synthesized and characterized. The first hyperpolarizability β of it has been investigated by Hyper-Rayleigh scattering (HRS) measurements. A facile data processing method was proposed to eliminate the influence of the relatively weak three-photo-absorption induced fluorescence (3PF) irradiated by laser beam at λ = 1500 nm. The values of β(1500) and β0(1500) are close to those obtained at λ = 1600 nm laser beam at which no multi-photon-absorption induced fluorescence is detected, validating this method. This method is useful for the investigation of the first hyperpolarizability β with weak absorption induced fluorescence.

Photophysical and Protonation Time Resolved Studies of Donor-Acceptor Branched Systems With Pyridine Acceptors.

A comparative study of the photophysical properties of octupolar pyridyl-terminated triphenylamine molecule, with its quadrupolar and dipolar analogues, by means of ambient and low temperature steady state spectroscopy and femtosecond to nanosecond time-resolved fluorescence spectroscopy is reported. The push-pull molecules bear triphenylamine electron donating core, pyridine peripheral electron acceptors, and acetylene π-bridge. The samples were studied in solvents of varying polarity and also upon addition of small amounts of acetic acid to induce protonation of the pyridine group. All samples exhibit significant positive fluorescence solvatochromism as well as a relaxation of their excited state to a solvent relaxed intramolecular charge transfer state on the picosecond time scale. For the octupolar compound, excited state relaxation occurs simultaneously with excitation energy hopping among the branches. The hopping time is solvent polarity controlled since it becomes slower as the polarity increases. The experimental hopping times are compared to those predicted by Förster and Fermi formulations. The samples are capable of emitting broadband light covering almost the whole visible spectrum by careful control of protonation. Energy transfer from the neutral toward the protonated species on the 1 ps time scale is revealed.

Octupolar organometallic Pt(II) NCN-pincer complexes; Synthesis, electronic, photophysical, and NLO properties

A series of organometallic octupolar 1,3,5-substituted-2,4,6-styryl-benzene complexes was synthesized by post-modification of parent [PtCl(NCN-CHO-4)], i.e. 1,3,5-tri-R-2,4,6-tris[(4-(PtCl)(3,5-bis[(dimethylamino)methyl]styryl)]benzene (NCN = [C6H2(CH2NMe2)2-2,6]– in which R = OMe, H, Br (1–3). Their synthesis involved a triple Horner-Wadsworth-Emmons reaction of [PtCl(NCN-CHO-4)] with the appropriate tris[(diethoxyphosphoryl)]methyl]benzene derivative. The 195Pt{1H} NMR chemical shift reflects the electronic properties of the π-system to which it is connected. The UV/Vis bands of the octupolar platinum complexes are only slightly red-shifted (by 5–12 nm) with respect to those of corresponding stilbenoid Pt-Cl pincer compounds (i.e., the separate branches), suggesting that there is only a limited electronic interaction between these branches. The fluorescence Stokes shift, quantum yields and lifetimes of 1–3 also are of the same order of magnitude as those of stilbenoid Pt-Cl pincer compounds, indicating that a dipolar excited state is formed, which is localized on one of the three branches. The hyper-Rayleigh scattering technique revealed hyperpolarizabilites βHRS of 430, 870 and 183 × 10−30 esu for 1, 2 and 3, respectively, which are among the highest for transition metal complexes. The highest value was found for the compound lacking a donor or acceptor group at the central core, lending support to the idea that dispersion overwhelms charge transfer in determining the magnitude of the first hyperpolarizability in octupolar compounds.

Ultrafast responses of dipolar and octupolar compounds with dipicolinate as an electron acceptor

Two dipolar compounds with dipicolinate as electron acceptor group named trans-dimethyl-4-[4’-(N,N-dimethylamino)-styry1]-pyridin-2,6-dicarboxylate (M-1), trans-dimethyl-4-[4’-(N,N-diphenylamino)-styry1]-pyridin-2,6-dicarboxylate (P-1) as well as a P-1 based multi-branched octupolar compound {4-[(E)-2-(2,6-dimethoxycarbonylpyridin-4-yl) vinyl]}-N,N-bis{4-[(E)-2-(2,6-dimethoxycarbonylpyridin-4-yl)vinylphenyl]}aniline (P-3) with intense two-photon fluorescence emission properties are systematically investigated by using steady-state absorption and fluorescence spectroscopy, Z-scan, and two-photon excited fluorescence (TPF) method. The two-photon absorption cross section of octupolar compound P-3 in THF solution is determined to be 376 GM, which is approximately 12 times greater than that of dipolar counterpart P-1 (32 GM). Transient absorption spectroscopy is employed to investigate the excited state dynamics of the dipolar and octupolar compounds. The formation and relaxation lifetimes of the intra-molecular charge transfer (ICT) state are determined to be in the ranges of several picoseconds and several-hundreds of picoseconds, respectively, for all the three compounds in THF solutions. An extended π-conjugated system and increased intra-molecular cooperative effect are responsible for the observed large two-photon absorption character.

Density functional theory study of pure octupolar porphyrazines with second-order nonlinear optical activity

Delicately designed pure octupolar molecular compounds, tetra(triptycene)porphyrazine and its skeleton expanded derivatives have been studied on the basis of density functional theory. The pure octupolar contribution with better isotropy to the second-order nonlinear optical response for these compounds is revealed, with further investigation to the molecular size effects, as well as peripheral polarized substituent influences to the nonlinear optical response.

Dispersion Overwhelms Charge Transfer in Determining the Magnitude of the First Hyperpolarizability in Triindole Octupoles

A comprehensive series of systematically functionalized C3-symmetric hexakis (para-substituted) triindoles has been studied for its linear and second-order nonlinear optical properties. The carbazole-derived triindole central core is electron rich and electron donating. The peripheric substitution pattern, resulting in a donor–donor or a donor–acceptor charge transfer, is reflected in the UV–vis absorption spectra, where a lower energy charge-transfer band is observed for the donor–acceptor pattern. On-resonance all compounds exhibit a strongly enhanced second-order nonlinear optical response, critically dependent on the particular wavelength but showing no clear correlation with the charge-transfer character imparted by the peripheral substituents. Nonetheless, extremely large values are obtained: we measured the highest value ever reported for octupolar compounds in transparent conditions on-resonance. Off-resonance significantly smaller values are found, which are very similar for all compounds and show no correlation with the charge-transfer character as well. Both observations have been unambiguously confirmed by (linear and nonlinear) spectroelectrochemistry on a donor–donor structure, effectively transforming this to an acceptor–donor structure (by oxidizing the donor triindole core to an electron-accepting triindole-based cation radical). The strong wavelength dependence of the first hyperpolarizability values around resonance is clearly shown to be overwhelmed by dispersion effects and not to be determined by the charge-transfer pattern in these octupolar materials. This finding provides insight for independent tuning of the linear absorptive properties, determined by the charge-transfer pattern, and the second-order nonlinear polarizability, not determined by this pattern but strongly dispersive.

Simultaneous Control of Emission Localization and Two-Photon Absorption Efficiency in Dissymmetrical Chromophores

The aim of the present work is to demonstrate that combined spatial tuning of fluorescence and two-photon absorption (TPA) properties of multipolar chromophores can be achieved by introduction of slight electronic chemical dissymmetry. In that perspective, two model series of structurally related chromophores have been designed and investigated. One is based on rod-like quadrupolar chromophores bearing either two identical or different electron-donating (D) end groups and the other on three-branched octupolar chromophores built from a trigonal donating moiety bearing identical or different acceptor (A) peripheral groups. The influence of the electronic dissymmetry is investigated by combined experimental and theoretical studies of the linear and nonlinear optical properties of dissymmetrical chromophores compared to their symmetrical counterparts. In both types of systems (i.e., quadrupoles and octupoles), experiments and theory reveal that excitation is essentially delocalized and that excitation involves synchronized charge redistribution (i.e., concerted intramolecular charge transfer) between the different D and A moieties within the multipolar structure. In contrast, the emission stems only from a particular dipolar subunit bearing the strongest D or A moiety due to fast excitation localization after excitation, prior to emission. Hence, control of emission characteristics (polarization and emission spectrum), can be achieved, in addition to localization, by controlled introduction of electronic dissymmetry (i.e., replacement of one of the D or A end-groups by a slightly stronger D' or A' unit). Interestingly, slight dissymmetrical functionalization of both quadrupolar and octupolar compounds does not lead to significant loss in TPA responses and can even be beneficial due to the spectral broadening and peak position tuning that it allows. This study thus reveals an original molecular engineering route allowing TPA enhancement in multipolar structures, due to concerted core-to-periphery or periphery-to-core intramolecular charge redistribution upon excitation, while providing for control of emission localization. Such a route could be extended to more intricate (dendritic) and multipolar (3D) systems.

The Synthesis and One‐ and Two‐Photon Optical Properties of Dipolar, Quadrupolar and Octupolar Donor–Acceptor Molecules Containing Dimesitylboryl Groups

Two series of related donor-acceptor conjugated dipolar, pseudo-quadrupolar (V-shaped) and octupolar molecular systems based on the p-dimesitylborylphenylethynylaniline core, namely, 4-(4-dimesitylborylphenylethynyl)-N,N-dimethylaniline, 4-[4-(4-dimesitylborylphenylethynyl)phenylethynyl]-N,N-dimethylaniline, 3,6-bis(4-dimesitylborylphenylethynyl)-N-n-butylcarbazole and tris[4-(4-dimesitylborylphenylethynyl)phenyl]amine, and on the E-p-dimesitylborylethenylaniline motif, namely, E-4-dimesitylborylethenyl-N,N-di(4-tolyl)aniline, 3,6-bis(E-dimesitylborylethenyl)-N-n-butylcarbazole and tris(E-4-dimesitylborylethenylphenyl)amine have been synthesised by palladium-catalyzed cross-coupling and hydroboration routes, respectively. Their absorption and emission maxima, fluorescence lifetimes and quantum yields have been obtained and their two-photon absorption (TPA) spectra and TPA cross-sections have been examined. Of these systems, the octupolar compound tris(E-4-dimesitylborylethenylphenyl)amine has been shown to exhibit the largest TPA cross-section among the two series of approximately 1000 GM at 740 nm. Its TPA performance is comparable to those of other triphenylamine-based octupoles of similar size. The combination of such large TPA cross-sections and high emission quantum yields, up to 0.94, make these systems attractive for applications involving two-photon excited fluorescence (TPEF).

Synthesis of an octupolar compound and its biological effects on serum albumin

A special rigid planar structural octupolar molecule titled 2,4,6-tris(p-methylstyryl)-s-triazine (TMT) was synthesized and its interaction with bovine serum albumin (BSA) were studied by molecular spectroscopy. The quenching mechanism of fluorescence of BSA by TMT was discussed. The thermodynamic parameters DeltaH(theta), DeltaG(theta), DeltaS(theta) at different temperatures were calculated and the results indicate hydrogen bond forces played major role in the reaction and the reaction was mainly enthalpy-driven. The distance r between donor (BSA) and acceptor (TMT) was obtained according to Förster theory of non-radiation energy transfer. Circular dichroism (CD) spectra, synchronous and three-dimensional fluorescence spectra were used to investigate the structural change of BSA molecules with addition of TMT, the result indicates that the alpha-helical structures of BSA molecules reduced in the presence of TMT. Sketch map of the interaction process was analyzed at molecular level.

Effect of Branching on Two‐Photon Absorption in Triphenylbenzene Derivatives

The photophysical and linear and nonlinear spectral properties of octupolar compounds with a triphenylbenzene core are investigated and compared with properties of corresponding dipolar branches. A correlation is found between the solvatochromic behavior and the two-photon absorption cross section. Moreover, the nature of the core is found to be responsible for the nature of the coupling between branches; in the studied case only (weak) electrostatic interactions are effective, while other cores, like the triphenylamine moiety, are able to promote coherent coupling between the branches, leading to strongly nonadditive properties.

A comparative study of the two-photon absorption properties of a new three-branched molecule—sumanene 3O derivative and relative molecules

Abstract Using density-functional theory and ZINDO-SOS method, we carry out a comparative study of the two-photon absorption (TPA) properties of a new octupolar compound—sumanene 3O derivative and relative molecules. The results indicate that the TPA cross sections of the octupolar compounds increase with the electron-accepting ability of the center. Sumanene 3O is a stronger electron-acceptor than 1,3,5-tri(nitro)benzene. Compared with the conventional octupolar compounds, the sumanene 3O derivative, which has the largest TPA cross section among the studied molecules here, is a more promising TPA material.

A comparative study of the two-photon absorption properties of new three-branched molecules – Sumanene and triphenylene derivatives

Using density-functional theory and ZINDO-SOS method, we carry out a comparative study of the two-photon absorption (TPA) properties of sumanene and triphenylene derivatives. The results indicate that sumanene and triphenylene derivatives have relatively large TPA cross-sections and the high transparency. And the TPA cross-section increases as strength of donor/acceptor and conjugation length increase. In comparison with classical octupolar compounds, there exists stronger interaction between the branches of sumanene and triphenylene derivatives. Compared with triphenylene derivatives, the bowl-shaped structure of sumanene segment has an important effect on TPA of sumanene derivatives. It blocks electron transitions on the interior of sumanene segment and from the peripheric groups to molecular center.

Supramolecular associations of anionic and cationic octupoles

Abstract In this communication, we describe the first attempts for supramolecular associations between cationic and anionic octupolar derivatives. To this end, the synthesis and second-order non-linear optical (NLO) activity of new classes of anionic octupolar compounds will be reported and we will try to estimate the effect of the anion/cation association on the NLO activity of the resulting supramolecule. To cite this article: C. Feuvrie et al., C. R. Chimie 8 (2005) .

One- and two-photon absorption of three-coordinate compounds with different centers (B,Al,N) and a 2,2′-dipyridylnitrogen functional group

A series of three-coordinate octupolar compounds with varied centers (boron, aluminum, and nitrogen), which exhibit very large effective two-photon absorption cross sections have been theoretically studied. The ground state geometries and electronic structures are obtained using the density functional theory with the B3LYP functional and 6-31G(d) basis set, and the results are comparable to the available experimental determinations. Based on the correct geometrical and electronic structures, the one- and two-photon absorptions are predicted by the ZINDO-SOS method. Among these compounds, the boron (B) and aluminum (Al) centers act as acceptors, while the nitrogen center acts as donor according to the net charge changes during the excitation. It is found that (i) the compounds with boron and aluminum centers show two large two-photon absorption peaks, while the molecule with nitrogen center show only one two-photon absorption peak; (ii) the cross sections of the molecules with B or Al as centers are larger than that of the molecule with nitrogen as center; furthermore, the two-photon absorption cross section of the molecule with Al center is larger than that of the molecule with B center, from this point of view, our theoretical prediction provides for the experiment a good new candidate with large two-photon absorption cross section for further research; (iii) lengthening the conjugation bridge by inserting a benzene ring on the organoborane compounds (forming the investigated molecule B-2) enhances the two-photon absorption cross section, and keeping good transparency at the same time.

Theoretical Design of Substituted Tetrathia‐[7]‐Helicenes with Large Second‐Order Nonlinear Optical Responses

Second-order nonlinear optical (NLO) materials have attracted a lot of interest because of foreseeable applications in optoelectronic technology. In particular, organic compounds are attractive for this purpose because of their large and fast responses combined with the flexibility of organic synthesis which hastens the design of new chromophores. The first generation of NLO organic chromophores consists of one-dimensional (1D) p-conjugated systems end-capped with donor (D) and acceptor (A) moieties. Experimental and theoretical work has shown during the last decades how to choose the D/A strengths and p segments for optimizing the second-order NLO responses. Later, two-dimensional (2D) octupolar molecules such as 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) were shown to exhibit second-order NLO responses that can be as large as those of their dipolar analogues. In addition, with these 2D structures, some of the limitations of the 1D dipolar structures were displaced, if not overcome. Indeed, 1D compounds suffer from the transparency–efficiency tradeoff, which is difficult to achieve because the desirable increase in b(the first hyperpolarizability) is accompanied by a bathochromatic shift of the electronic transitions. Moreover, their dipolar nature and the predominance of dipole–dipole intermolecular interactions favor the formation of centrosymmetric crystal arrangements which exhibit no second harmonic generation (SHG) responses. In addition, phase-matching conditions cause only a fraction of the microscopic response to be preserved at the macroscopic scale. On the other hand, octupolar compounds present an improved nonlinearity/transparency tradeoff, enable non-centrosymmetric crystal structures owing to their lack of permanent dipole moment, and present large offdiagonal b tensor components that can give rise to large macroscopic SHG responses.

Frequency up-conversion of s-triazine derivatives via two photon absorption and second-harmonic generation

A three-branched octupolar compound, 2,4,6-tris[2-(4-N,N-diethylamino)phenylethenyl]-1,3,5-s-triazine (compound III), as well as its one- and two-branched analogues (compounds I and II respectively) have been synthesized. The structures of I and II have been determined by X-ray diffraction. The linear and nonlinear spectra of these compounds regularly vary: (1) the spectral positions (λmax values) of the linear absorption, the single-photon excited fluorescence (SPEF) and the two-photon excited fluorescence (TPEF) show regular red-shifts when the branch number increases from 1 to 3; (2) the spectral intensities, including the linear absorbance emax and the two-photon absorption cross-section σ, the SPEF and the TPEF intensities are all increased significantly when going from I to III. The values of emax/MW and σ/MW (of compounds I, II and III) vary with the ratios of 1.0 : 1.3 : 1.7 and 1.0 : 1.4 : 1.3, respectively. All the photophysical data indicate that by incorporating more branches into the triazine ring, the linear and nonlinear optical responses have been significantly enhanced, and that the spectral behaviour of II is closer to that of III, while more different from that of I. The single-branched compound I crystallizes in the noncentrosymmetric polar space group Cc. Its crystalline powder sample shows remarkable macro second-order nonlinear response with a SHG intensity of 46.8 times that of urea under a fundamental picosecond Nd : YAG laser beam (1064 nm). An important factor of such a large SHG intensity is that all the molecular dipoles take the same-oriented packing style that makes the crystal maximally polarized, fulfilling the optimization in the sense of crystal engineering.

Syntheses, structures and second-order nonlinear optical properties of octupolar compounds: 2,4,6-tri-substituted s-triazine

Three compounds with three p-substituted styryls attaching to the central s-triazine have been synthesized and two crystals determined to be in non-centrosymmetric Cmc2 1 and Pmn2 1 space groups, respectively. All of them in solid state are second harmonic generation (SHG) active and two of them show powder SHG intensities one order of magnitude that of urea.

A comparative study of the two-photon absorption properties of a new octupolar molecule – truxenone derivative and relative molecules

Using density-functional theory and ZINDO methods, we carry out a comparative study of the two-photon absorption (TPA) properties of a new octupolar compound – truxenone derivative and relative molecules. The results indicate that the octupolar compounds have significantly increased the TPA cross-section of molecules and at the same time maintain the high transparency. The TPA cross-section value increases as the electron-accepting ability of the core in octupolar chromophores increases. It is notable that the truxenone derivative has more predominant TPA properties than the conventional octupolar compounds and so worth investigating further.

Role of dimensionality on the two-photon absorption response of conjugated molecules: The case of octupolar compounds

A comparative study of the two-photon absorption (TPA) properties of octupolar compounds and their dipolar one-dimensional counterparts is presented on the basis of correlated quantum-chemical calculations. The roles of dimensionality and symmetry are first discussed on the basis of a simple exciton picture where the ground-state and excited-state wavefunctions of three-arm octupolar systems are built from a linear combination of the corresponding single-arm wavefunctions. This model predicts a factor of 3 increase in the TPA cross section in the limiting case of three independent charge-transfer pathways. When taking into account the full chemical structures of representative octupolar molecules, the results of the calculations indicate that a much larger enhancement associated with an increase in dimensionality and delocalization can be achieved when the core of the chromophore allows significant electronic coupling among the individual arms. These theoretical predictions are in agreement with the experimental determination of the TPA cross sections for crystal violet and the related compound, brilliant green, and suggest new strategies for the design of conjugated materials with large TPA cross sections.