Octupolar Molecules Research Articles

One- and two-photon absorption properties of octupolar molecules with tetrahedral structure

We have theoretically examined two series of tetrahedral octupolar molecules that exhibit large two-photon absorption cross-sections in the visible region. The density functional theory B3LYP/6-31G* associated with ZINDO–SOS methods has been used to calculate the molecular geometries, electronic structure, one-photon and two-photon absorption properties. The computational results are in good agreement with the available experimental and theoretical values. It is found that the molecular conjugation length and the one-photon absorption intensity are quite strongly correlated factors, and a corresponding correlation for large two-photon absorption cross-section is equally strong. A most important role for altering the position of one-photon absorption is played by the substitution of the conjugated bridged N N for CH CH. The center with stronger electron-accepting ability is favorable for the efficient increase of two-photon absorption (TPA) cross-section in the higher energy region. While the molecules centered by C atom all exhibit large TPA cross section in a fairly wide range and this property may lead tetrahedral octupolar molecules centered by C atom to more nonlinear optical applications.

First hyperpolarizabilities of dipolar, bis-dipolar, and octupolar molecules

A series of dipolar ( 1), bis-dipolar ( 2), and octupolar molecules ( 3) containing 1, 2, and 6 dipolar units within a molecule has been synthesized and their hyperpolarizabilities were analyzed. The β HRS increases in the order, 1 < 2 < 3. The ‘monomeric’ β zzz increases by approximately twofold from 1 to 2, whereas β zzz of 2 and β xxx of 3 are similar. Noteworthy is the parallel increase in the hyperpolarizability tensor with the λ max.

A new multi-branched two-photon photopolymerization initiator: synthesis and non-linear optical properties of a 1,3,5-triazine-based octupolar molecule

A new multi-branched two-photon photopolymerization initiator, tris-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]-vinyl}-phenyl)-amine (abbreviated to TPVPA) has been synthesized and characterized. The experimental results confirmed that TPVPA is a good two-photon absorbing chromophore and operative two-photon photopolymerization initiator. The calculated two-photon absorption cross-section of TPVPA is 42.4 × 10 −50 cm 4 s photon −1. A microstructure using TPVPA as initiator has been fabricated under irradiation of a Ti:sapphire femtosecond laser at 830 nm. The possible photopolymerization mechanism is discussed.

First hyperpolarizabilities of ionic octupolar molecules: structure–function relationships and solvent effects

We present a quantum-chemical analysis of the first hyperpolarizabilities of a series of ionic octupolar molecules and comparison of their characteristics with corresponding neutral molecules. The molecular geometries are obtained via BL3YP/6-31G (d,p) optimization including SCRF/PCM approach, while the dynamic NLO properties are calculated with the ZINDO/CV method including solvent effects. The effect of donor or acceptor substitution and elongation of the conjugation path length are established to demonstrate the engineering guidelines for enhancing molecular optical non-linearities. The solvent induced effect on the NLO properties are studied using ZINDO/CV/SCRF method. It was found that solvents play a significant role on the first hyperpolarizabilities of ionic octupolar molecules.

Theoretical investigation on the one- and two-photon absorption properties of multi-branched oligomers with truxenone center and fluorene branches

A comparative study of electronic structure, one-photon absorption and two-photon absorption (TPA) properties of octupolar oligofluorenes and their dipolar counterparts is presented by using AM1 and ZINDO-SOS methods. The investigation of the structure–property relationship of these materials has indicated differences between octupolar oligofluorenes and dipolar ones. The results indicate that the conjugated chain length importantly affects the magnitude of TPA cross-section. The dipolar molecules reach a saturation limit for energy gap between HOMO and LUMO versus the number of fluorene rings more quickly than octupolar systems. The maximum TPA cross-section increases in proportion to N α ( α = 3.16 and 2.77), where N = 2–6 is the number of fluorene moieties in dipolar molecules and each arm of octupolar analogs. Because of the weak coupling among the branches of the octupole, an increase by a factor of less than 3 of the TPA cross-section of the octupolar molecule with respect to the dipolar molecule occurs.

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.

Theoretical studies of one- and two-photon absorptions for a series of three-coordinate compounds with different centers (B, Al, N and P) and distyrylbenzenes as branching chromophores

Abstract One- and two-photon absorption properties of a series of octupolar molecules with different centers have been theoretically studied with the ZINDO-SOS method. The optimized geometries are obtained by using the ab initio HF/3-21G(d) method, which has been testified good enough to predict the correct geometries for our investigated octupolar molecules. Different centers have important effect on the geometries, one- and two-photon absorptions. The boron (B) and aluminum (Al) act as acceptor centers, while the nitrogen (N) and phosphorus (P) act as donor centers. The calculated results indicate that our investigated molecules are all good candidates for the TPA materials from the view point of transparency-nonlinearity, but Al(DSB)3 and P(DSB)3 are better than B(DSB)3 and N(DSB)3. To illustrate the results, the frontier orbitals and net charge changes are analyzed theoretically.

Calculations on the octupolar molecules with enhanced two-photon absorption cross sections based on the Zn (II) and Cu (I) as centers.

The equilibrium geometries, electronic structures, as well as one- and two-photon absorption cross sections of a series of octupolar chromophores with Zn(2+) or Cu(+) as coordinate centers and 4,4'-bis(dibutylaminostyryl)-[2,2']-bis(bipyridyl) as ligands have been determined by using B3LYP/6-31G and ZINDO methods. These molecules are designed by controlled combination of two or three bipyridyl ligands with the metal centers. The results show that Zn(2+) is an effective template for the design of octupolar structures which enable it to form tetrahedral and octahedral coordinated complexes; while Cu(+) only exists in a tetrahedral coordinated complex, comparing the tetrahedral complex with Zn(2+) as the center with that of Cu(+) as the center, it is found that the complex with the Cu(+) center is a better two-photon absorption material than the former as far as the transparency/nonlinearity is concerned. Furthermore, for the same metal center of Zn(2+), both one- and two-photon absorptions of the tetrahedral complex are redshifted relative to those of the octahedral complex, is attributed to the spiroconjugation effect in the tetrahedral complex. Our theoretical findings are consistent with recent experimental observations and provide an important foundation for the design of improved transparency-nonlinearity two-photon absorption materials.

Ultrafast Exciton Dynamics in a Branched Molecule Investigated by Time-Resolved Fluorescence, Transient Absorption, and Three-Pulse Photon Echo Peak Shift Measurements

The excited-state dynamics of a branched octupolar molecule (tris-4,4‘,4‘ ‘-(4-nitrophenilethynyl)triphenylamine (T-NPTPA)) with charge-transfer character is investigated by complementary measurements of time-resolved fluorescence, transient absorption, and three-pulse photon echo peak shift. The data are compared with those obtained for the linear molecule dimethylnitroaminotolane (DMNAT) representing a linear building block of the octupolar system. Time-resolved fluorescence as well as transient absorption experiments showed the presence of an intermediate C3 symmetry state for the trimer with a depolarization time faster than ∼50 fs. Three-pulse photon echo peak shift measurements for the trimer revealed a large initial fast component of the peak shift decay. The initial peak shift during the first 100 fs was found to be much higher for the trimer as compared to the linear building block. This is in accordance with the time-resolved anisotropy results suggesting a different character of the initial excitation of the trimer. The higher peak shift value for the trimer suggests a smaller total coupling of electronic transition frequency to the nuclear motions, which may be the result of the delocalized character of this initial excited state. A residual peak shift (∼2 fs) at long population times was also observed and was interpreted as an indication of the residual static inhomogeneity (inhomogeneous broadening) in the system.

Two-photon absorption properties of star-shaped molecules containing peripheral diarylthienylamines

Two-photon absorption (TPA) properties of several novel donor–acceptor–donor (D–A–D) quadrupolar and octupolar star-shaped molecules containing peripheral diarylthienylamines were investigated and are discussed in this Letter. The effects of donor–acceptor strength, conjugation length, and molecular symmetry on the effective TPA cross-section of these novel molecules were studied. It was found that incorporation of a triazine acceptor center in octupolar molecules can effectively enhance TPA response. This provides a strategy for designing novel octupolar molecules with a large TPA response but without red-shifting their linear absorption wavelengths.

Two-photon-absorption properties of multipolar molecules studied using femtosecond z-scan method

Two-photon-absorption (TPA) properties of multipolar molecules were studied using femtosecond z-scan method. We measured the z-scan curves at 800 nm, 1 kHz using 150 fs-laser pulse. The z-scan curves were measured for two kinds of multipolar molecules: three different quadrupole molecules (Q1, Q2, Q3) and three different octupolar molecules (O1, O2, O3). Each multipolar molecule has same molecular skeleton but the lengths of the dipolar subunits or the donor strengths are different each other. Their TPA cross-sections were obtained from the magnitudes of the transmittance dip in the measured z-scan curves: 233 (Q1), 127 (Q2), 164 (Q3) and 381 (O1), 230 (O2), 85 (O3) cm4/(photon/s) for quadrupole and octupolar molecules, respectively. These results are ascribed to the fact that TPA cross-sections of the multipolar molecules are raised by increasing the number of the alternating double bonds in the dipolar subunits or by enlarging the electron-donating strength of the substituent.

Octupolar Dendrimers with Large First Hyperpolarizability

AbstractA series of 1,3,5‐tricyano‐2,4,6‐tris(styryl)benzene derivatives (1,2) and dendrimers (3) have been synthesized and their first hyperpolarizabilities (β) were determined. Whereas the λmax values of 1–3 are similar, the oscillator strength increases with the molecular size. For all compounds, the Stokes shifts are significantly larger when NAr2 is used as the donor. The β(0) values of 1–3 range from 252 to 507 × 10–30 esu and increases with the number of octupolar units in the molecule. When the octupolar molecules are linked to the 1,3,5‐position of the benzene ring by flexible ether linkages, the β(0) value increases by 1.7 fold. On the other hand, octupolar monodendron based on the nitrogen atom core and 1,3,5‐tricyano‐2,4,6‐tris(p‐diarylaminostyryl)benzene moieties at the periphery exhibits β(0) = 507 × 10–30 esu, which is one of the largest β(0) values reported for the octupolar molecules.

Toward Highly Active Two-Photon Absorbing Liquids. Synthesis and Characterization of 1,3,5-Triazine-Based Octupolar Molecules

Three novel two-photon absorbing (TPA) chromophores with 1,3,5-triazine as the π-electron deficient core, dialkylfluorene as aromatic bridges, and diphenylamino groups as the electron-donating end-groups were prepared. Designated as AF-450 (2,4,6-tris[7-(diphenylamino)-9,9-didecylfluoren-2-yl]-1,3,5-triazine), AF-455 (2,4,6-tris[9,9-bis(3,7-dimethyloctyl)-7-(diphenylamino)-fluoren-2yl]-1,3,5-triazine), and AF-457 (2,4,6-tris[(7-(diphenylamino)-9,9-diprop-2-enylfluoren-2-yl]-1,3,5-triazine), their overall molecular structure and local symmetry (D3h) are similar to those of previously reported three-armed AF-350 (N,N,N-tris[4-{7-(2-benzothiazolyl)-9,9-diethylfluoren-2-yl}phenyl]amine) and AF-380 (N,N,N,-tris[7-(2-benzothiazolyl)-9,9-diethylfluoren-2-yl]amine). Among the family of AFX chromophores previously reported by us, AF-450 possesses one of the largest effective TPA cross-sections (σ2‘ = 39 500 × 10-50 cm4-sec/photon-molecule, or 39 500 GM) as determined by nonlinear transmission method in the nanosecond regime at 800 nm. In contrast, AF-455, a mixture of stereoisomers with the same chemical formula as AF-450, is a glassy material that becomes fluid (molasses-like) upon heating at 70−80 °C and has noticeably smaller effective σ2‘ value (33 300 GM). AF-457 (σ2‘ = 27 800 GM) with six allyl side groups was prepared as a precursor toward the synthesis of a TPA liquid. The intrinsic TPA cross-sections of these chromophores were also determined as a function of excitation wavelengths via a femtosecond white-light continuum generation and direct degenerate-TPA measurement technique. At the TPA peaks ∼779 nm, their σ2‘ values are 216, 214, and 199 GM (±15%) for AF-450, AF-455, and AF-457, in that order. They are in the same trend as the nanosecond values, albeit two orders of magnitude lower.

Propeller-Shaped Octupolar Molecules Derived from Triphenylbenzene for Nonlinear Optics: Synthesis and Optical Studies

Nanoscale propeller-shaped conjugated molecules based on a central triphenylbenzene core bearing three oligomeric phenylene−vinylene branches having either electron-donating or electron-withdrawing peripheral groups have been synthesized. These octupolar derivatives show definite solvatochromic behavior (in absorption and even more in emission), which reveals a multidimensional intramolecular charge transfer (MDICT) taking place between the center and the periphery of the molecules. Correlations between the solvatochromism magnitude and the nonlinear responses (first-order hyperpolarizabilities β) of such derivatives have been established. Accordingly, boosting of the MDICT phenomenon (by means of elongation of the branches and benefit from appropriate peripheral groups) allowed achievement of very high hyperpolarizabilities while maintaining wide transparency in the visible region (up to ||β|| = 810 × 10-30 esu for λmax= 377 nm).

Coupling Octupoles in Crystals: The Case of the 1,3,5-Trinitrobenzene−Triphenylene 1:1 Molecular Co-Crystal

A new supramolecular crystal engineering approach toward quadratic nonlinear optics is inferred from an acentric cocrystalline template made of two different bi-dimensional octupolar molecules: trinitrobenzene (TNB) and triphenylene (TP). The resulting 2-D octupolar cocrystalline lattice is proposed as a generic template toward multipolar crystalline engineering studies. X-ray diffraction data evidence parallel stacks with alternating vertically overlapping TNB and TP units, leading to a significant π−π intermolecular charge transfer which cannot be accounted for by an oriented gas model.

Z-Scan Method Using the Flat-Topped Beam and Its Application to the Evaluation of the Two-Photon-Absorbing Material

It was studied the method to evaluate the two-photon-absorbing material from the measurement of the z-scan curve using flat-topped (FT) beam. Theoretically, I obtained the analytical expression for the focused beam profile of FT beam by using the Rayleigh diffraction formula, and calculated the transmittance dip as a function of two-photon absorption (TPA) coefficient. It was found that the TPA coefficient could be obtained from the magnitude of the transmittance dip in the z-scan curve with the FT beam. Experimentally, the z-scan curves were measured for three different octupolar molecules (O1, O2 and O3). Each molecule has same molecular skeleton but the lengths of the dipolar subunits or the donor strengths are different each other. The z-scan measurements were performed using FT beam in tetrahydrofuran solutions at 800 nm in the femtosecond regime. Their TPA cross-sections were obtained from the magnitudes of the transmittance dip in the measured z-scan curves: 25, 38 and 117 cm4/(photon/s) for O1, O2 and O3, respectively. These results are ascribed to the fact that TPA cross-sections of the octupolar molecules are raised by increasing the number of the alternating double bonds in the dipolar subunits or by enlarging the electron-donating strength of the substituent.

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.

Synthesis and Nonlinear Optical Properties of 1,3,5‐Tricyano‐2,4,6‐tris[2‐(thiophen‐2‐yl)vinyl]benzene Derivatives.

1,3,5-Tricyano-2,4,6-tris[2-(thiophen-2-yl)vinyl]benzene derivatives have been synthesized as a new type of octupolar nonlinear optical molecules and their linear and nonlinear optical properties were determined. The λ m a x values increased with the conjugation length and as the conjugation bridge was changed from phenyl to thienyl group. For phenyl and thiophene derivatives, the β value increased with conjugation length. In contrast, the b values of the thiophene derivatives were much smaller than the corresponding phenyl derivatives.

Improved transparency--nonlinearity trade-off with boroxine-based octupolar molecules.

A series of octupolar molecules derived from the boroxine framework were designed and their optical nonlinearities were investigated by performing harmonic light scattering experiments in solution; the molecules were found to combine excellent transparency in the near UV-visible region (lambdamax < or = 280 nm) and significant first-order hyperpolarisabilities (up to beta(0) = 56 x 10(-30) esu).

Synthesis and Nonlinear Optical Properties of 1,3,5-Tricyano-2,4,6-tris[2-(thiophen-2-yl)vinyl]benzene Derivatives

1,3,5-Tricyano-2,4,6-tris[2-(thiophen-2-yl)vinyl]benzene derivatives have been synthesized as a new type of octupolar nonlinear optical molecules and their linear and nonlinear optical properties were determined. The λmax values increa sed with the conjugation length and as the conjugation bridge was changed from phenyl to thienyl group. For phenyl and thiophene derivatives, the b value increased with conjugation length. In contrast, the <TEX>$\beta$</TEX> values of the thiophene derivatives were much smaller than the corresponding phenyl derivatives.