Effect of conjugation path length on quadratic nonlinear optical properties of monomer and aggregates of zwitterionic merocyanine dyes

We present a quantum-chemical analysis of the conjugation path length effect on first hyperpolarizabilities of a series of zwitterionic merocyanine dyes whose synthesis has been reported earlier (J. Am. Chem. Soc.2002, 124, 9431, J. Am. Chem. Soc. 1997, 119, 3144). The effect of the conjugation path lengths is evaluated to demonstrate the engineering guidelines for enhancing molecular optical nonlinearity. The first hyperpolarizabilities are calculated for extended conjugated monomer and H and J type aggregates of merocyanine dyes, to provide insight into the intermolecular interactions and the relationship between structural and collective nonlinear optical properties. The molecular geometries for monomers are obtained via B3LYP/6-31G(d,p) level optimization including the SCRF/PCM approach, and the dynamic nonlinear optical (NLO) properties for monomer and aggregates are calculated with the ZINDO/CV method, including solvent effects. It is found that the chain length dependence of the first nonlinearity peaks at n = 6 and then it starts changing slowly for monomer and aggregates of zwitterionic merocyanine dyes. It is concluded that an excellent NLO response in solution might vanish when the active chromophore forms higher H aggregates. The importance of our results on the design of electrooptic materials has been discussed.     

Influence of the central metal ion on nonlinear optical and two-photon absorption properties of push-pull transition metal porphyrins

We present a quantum-chemical analysis of the central metal ion's effect on first hyperpolarizabilities and two-photon absorption (TPA) cross sections at the infrared region of a series of push-pull porphyrins whose synthesis and NLO properties have been reported earlier (J. Am. Chem. Soc. 2005, 127, 9710). The molecular geometries are obtained via the B3LYP/6-31G(d,p) level optimization including SCRF/PCM approach, and the NLO and TPA properties are calculated with the ZINDO/CV method including solvent effects. It is found that the CT transition between the metal ion's d orbital and the macrocycle pi orbitals plays an important role on NLO and TPA properties of metal porphyrins. Our data suggest a new approach to enhance TPA properties of porphyrin materials. We also present a quantum-chemical analysis on porphyrin dimers and trimers to understand the relationship between structural and collective NLO properties. It has been observed that beta values can be improved about an order of magnitude and TPA properties can be enhanced by 2 orders of magnitude by the formation of a trimer. The importance of our results with respect to the design of photonic and photodynamic therapy materials have been discussed.     

Nonlinear optical properties of zwitterionic merocyanine aggregates: role of intermolecular interaction and solvent polarity

We present a time-dependent quantum-chemical analysis on merocyanine aggregates to understand the insight of the intermolecular interactions and to find the relationship between structural and collective nonlinear optical properties. The first hyperpolarizabilities are evaluated for monomer and aggregates of a series of zwitterionic merocyanine dyes, whose synthesis and formation of H and J type aggregates in solvents are reported recently in the literature (J. Am. Chem. Soc. 2002, 124, 9431). The molecular geometries are obtained via B3LYP/6-31G (hybrid density-functional theory) optimization including PCM approach, while the dynamic NLO properties are calculated with the TD-DFT/SOS and ZINDO/CV method including solvent effects. It has been observed that the first hyperpolarizability changes tremendously as monomers undergo aggregation, and the magnitude of first hyperpolarizabilities highly depends on the nature of the aggregates. It is found that solvents play a remarkable role on the structure and first hyperpolarizabilities of merocyanine monomers and aggregates. Changing the solvent from low to high dielectric causes not only an increase in magnitude of beta but also a change in sign, therefore passing through zero at intermediate dielectric. The importance of our results on the design of electrooptic materials have been discussed.     

Outstanding Quadratic to Septic Optical Nonlinearity at Dipolar Alkynylmetal-Porphyrin Hybrids

Porphyrins are important macrocycles with applications in several areas including therapy, catalysis, and sensing. Strong nonlinear optical (NLO) responses are the key to fully exploiting the potential of these biocompatible molecules. We herein report that certain metal-alkynyl donor/nitro acceptor-functionalized porphyrins are attractive candidates for NLO applications. We show that specific examples exhibit record quadratic optical nonlinearity, exceptional two-photon absorption, and outstanding three-photon absorption, and we report the first porphyrins that exhibit four-photon absorption. The two-, three-, and four-photon absorption maxima are found at the corresponding multiples of linear absorption bands that time-dependent density functional theory assigns as admixtures of porphyrin-localized π*←π and donor-porphyrin to porphyrin-acceptor charge-transfer transitions.     

Electrochemical, spectroelectrochemical, and molecular quadratic and cubic nonlinear optical properties of alkynylruthenium dendrimers

A combination of cyclic voltammetry (CV), UV-vis-NIR spectroscopy and spectroelectrochemistry, hyper-Rayleigh scattering (HRS) [including depolarization studies], Z-scan and degenerate four-wave mixing (DFWM) [including studies employing an optically transparent thin-layer electrochemical (OTTLE) cell to effect electrochemical switching of nonlinearity], pump-probe, and electroabsorption (EA) measurements have been used to comprehensively investigate the electronic, linear optical, and nonlinear optical (NLO) properties of nanoscopic pi-delocalizable electron-rich alkynylruthenium dendrimers, their precursor dendrons, and their linear analogues. CV, UV-vis-NIR spectroscopy, and UV-vis-NIR spectroelectrochemistry reveal that the reversible metal-centered oxidation processes in these complexes are accompanied by strong linear optical changes, "switching on" low-energy absorption bands, the frequency of which is tunable by ligand replacement. HRS studies at 1064 nm employing nanosecond pulses reveal large nonlinearities for these formally octupolar dendrimers; depolarization measurements are consistent with lack of coplanarity upon pi-framework extension through the metal. EA studies at 350-800 nm in a poly(methyl methacrylate) matrix are consistent with the important transitions having a charge-transfer exciton character that increases markedly on introduction of peripheral polarizing substituent. Time-resolved pump-probe studies employing 55 ps, 527 nm pulses reveal absorption saturation, the longest excited-state lifetime being observed for the dendrimer. Z-scan studies at 800 nm employing femtosecond pulses reveal strong two-photon absorption that increases significantly on progression from linear complex to zero- and then first-generation dendrimer with no loss of optical transparency. Both refractive and absorptive nonlinearity for selected alkynylruthenium dendrimers have been reversibly "switched" by employing the Z-scan technique at 800 and 1180 nm and 100-150 fs pulses, together with a specially modified OTTLE cell, complementary femtosecond time-resolved DFWM and transient absorption studies at 800 nm suggesting that the NLO effects originate in picosecond time scale processes.     

具有大的双光子吸收截面的四羧酸二萘嵌苯衍生物的理论研究

用密度泛涵方法和ZINDO方法,从理论上研究了一系列四羧酸二萘嵌苯的衍生物(PTCDS)。对其平衡几何、电子结构和单双光子吸收性质进行了详细的计算研究。结果表明,此系列衍生物的双光子吸收截面主要由其主体结构(二萘嵌苯)决定。△EH-Lgap (HOMO与LUMO间的能隙)的减小,HOMO轨道能量的增大,分子主轴两端取代基给电子强度的增强,共轭长度的增大,分子刚性的增强都有利于此系列化合物双光子吸收截面的增大。并设计了比文献[22]报道的分子C有更大的双光子吸收截面的两个分子D1,D2。     

Synthesis, spectroscopic and nonlinear optical properties of multiple [60]fullerene-oligo(p-phenylene ethynylene) hybrids

A series of multiple [60]fullerene terminated oligo(p-phenylene ethynylene) (OPE) hybrid compounds has been synthesized through a newly developed in situ ethynylation method. Structural and magnetic shielding properties of the highly unsaturated carbon-rich C(60) and OPE scaffolds were characterized by 1D and 2D NMR spectroscopic analyses. Electronic interactions between the [60]fullerenes and the OPE backbones were investigated by UV/Vis spectroscopic and cyclic voltammetry (CV) experiments. Our studies clearly show that although the multiple [60]fullerene groups are connected via pi-conjugated OPE frameworks, they present diminutive electronic interactions in the ground state, and the electronic behavior of the [60]fullerene cages are only affected by the OPE backbones through modest inductive effects. Interestingly, sizable third-order nonlinear optical (NLO) responses (gamma) and enhanced two-photon absorption (TPA) cross-sections (sigma((2))) were determined for the multifullerene-OPE hybrid 31 relative to its OPE precursor from differential optical Kerr effect (DOKE) experiments. Such enhanced NLO performance is presumably due to the occurrence of periconjugation and/or charge transfer effects in the excited state. In addition, comparatively strong excited-state absorption was observed and characterized for OPE pentamer 12. Thus, the use of such fullerene-derivatized conjugated oligomers aids the quest for molecules with large third-order NLO and TPA properties.     

取代基对二噻吩并噻吩衍生物的双光子吸收性质的影响

利用ZINDO/SOS方法, 从理论上研究了对称和不对称取代两种情况下, 取代基对二噻吩并噻吩衍生物单双光子吸收性质的影响. 结果表明, 所设计的噻吩类分子具有较大的双光子吸收截面, 且双受体取代比双给体取代更有利于增大分子的双光子吸收截面. 同时发现, 此类分子受体取代可以显著增加波长较短的双光子峰附近的双光子吸收, 而给体取代则可以改善波长较长吸收峰附近的双光子吸收.     

Triaryl-1,3,5-triazinane-2,4,6-triones (Isocyanurates) Peripherally Functionalized by Donor Groups: Synthesis and Study of Their Linear and Nonlinear Optical Properties

The linear optical (LO) and nonlinear optical (NLO) properties of a series of isocyanurates functionalized by donor arms at the periphery are reported herein. These octupolar derivatives were obtained in a straightforward way from commercial isocyanate derivatives and were fully characterized. Although several of these compounds are known, those that exhibited the largest NLO activities are all new compounds. In terms of second-order activity, several of these derivatives exhibit remarkable activity/transparency tradeoffs. In terms of third-order activity, the longer derivatives with the stronger donor groups (X=NH(2) , NMe(2) , or NPh(2) ) were shown to possess significant two-photon absorption cross sections. These strongly luminescent derivatives exhibit two-photon absorption cross sections up to 410?GM. DFT computations were also conducted to unravel their electronic structures and to rationalize their NLO properties. To our knowledge, the present study is the first concerned with the nonlinear optical properties of these original cyclotrimers.     

Two-photon absorption cross sections: an investigation of solvent effects. Theoretical studies on formaldehyde and water

The effects of a solvent on the two-photon absorption of microsolvated formaldehyde and liquid water have been studied using hybrid coupled-cluster/molecular mechanics (CC/MM) response theory. Both water and formaldehyde were considered solvated in water, where the solvent water molecules were described within the framework of molecular mechanics. Prior to the CC/MM calculations, molecular dynamics simulations were performed on the water/formaldehyde and water/water aggregates and many configurations were generated. By carrying out CC/MM response calculations on the individual configurations, it was possible to obtain statistically averaged results for both the excitation energies and two-photon absorption cross sections. For liquid water, the comparison between one- and two-photon absorption spectra is in good agreement with the experimental data available in the literature. In particular, the lowest energy transition occurring in the one-photon absorption spectrum of water only occurs with a relatively small strength in the two-photon absorption spectrum. This result is important for the interpretation of two-photon absorption data as these results show that in the absence of selection rules that determine which transitions are forbidden, the spectral profile of the two-photon absorption spectrum can be significantly different from the spectral profile of the one-photon absorption spectrum.     

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.     

Theoretical prediction of one- and two-photon absorption properties of N-annulated quaterrylenes as near-infrared chromophores

Graphene nanoribbons (GNRs) have attracted increasing attention due to high potentiality in nanoelectronics. In the present study, quantum-chemical calculations of structural and nonlinear optical properties have been first carried out for the nanoelectronical materials, a new series of ladder-type N-annulated quaterrylenes and their imide chromophores. The effects of the solvent, terminal groups, the number of N-annulated bridges, and π-conjugated length are discussed in detail. The solvent effect is significant on the one-photon absorption (OPA). Moreover, the OPA and two-photon absorption (TPA) properties of the two series of DI and N-MI molecules show a clear solvent dependence, which is attributed to the carboximide substitution featuring larger polarization. Introducing electron-donating groups and dicarboximides and increasing the conjugated length lead to red-shifts of the OPA, emission, and TPA spectra, lower emission lifetimes, and enhanced TPA cross sections (δ(max)), but further extension of the conjugated framework does not always promote an increase of δ(max). The changing trends of δ(max) can be explained by the transition moment and the intramolecular charge transfer. All N-annulated quaterrylene and their imide derivatives possess small energy gaps, intense near-infrared absorption and emission, and large δ(max), which are important for use as two-photon fluorescent labeling materials.     

Theoretical investigation of one- and two-photon absorption properties of platinum acetylide chromophores

In this paper, we have theoretically investigated bis((4-phenylethynyl)phenyl) ethynyl)bis(trimethylphosphine)platinum(II) (PE2) and its analogs three platinum acetylide complexes (1-3) that feature highly pi-conjugated ligands (alkynyl-dimethylfluorene substituted with electron-donating or -withdrawing moieties). The geometrical and electronic structures are calculated at the ECP60MWB//6-31G*(H, C, P, N, S) basis set level by the density functional theory (DFT) method; one-photon absorption properties have been calculated by using time-dependent DFT (TDDFT) and Zerner's intermediate neglect of differential overlap (ZINDO) methods, and two-photon absorption (TPA) properties are obtained with the ZINDO/sum-over-states method. The values of beta(sp) and beta(d) for Pt are adjusted to -1 eV and -28.5 eV, respectively, to make one-photon absorption spectra calculated by ZINDO closest to the experimental data and TDDFT results. The calculated results indicate that all molecules in this work (involving cis isomers of molecules 1-3) take on two TPA peaks in the 600-800 nm region. The peak at 700-750 nm should not be simply attributed to the appearance of noncentrosymmetric cis isomers in solution, although trans and cis isomers adhere to a different selection rule. Every TPA peak results from its transition character. Molecules 1-3 show greater two-photon absorption strength compared with PE2 and retain good transparency.     

Effects of the structure of the branches on the two-photon absorption properties for the multi-branched molecules with nitrogen (N) as coupling center

In order to investigate the effects of the structure of branches on the TPA properties for multi-branched molecules, the TPA cross section is calculated by using ZINDO/SOS method. The investigated mole- cules have different branches (chomorfores based on stilbene, dithienothiophene and flourene) with nitrogen(N) as coupling center. The results show that the cooperative enhancement in multi-branched molecules depends on the structures of the branches and the structures of branches play an important role in the enhancement of the TPA cross section. The designed molecules with stilbene and dithie- nothiophene as branched possess relatively larger two-photon absorption cross sections.     

二吡唑铝化合物双光子吸收性质的理论研究

应用密度泛函理论(DFT)和半经验的ZINDO方法对二吡唑铝化合物的单、双光子吸收(OPA、TPA)性质进行了研究.结果表明,铝氮烷杂环化合物具有好的双光子吸收性质,其双光子最大吸收截面值(δmax)可达到2860.1 GM(1 GM=10-50 cm4·s·photon-1).在中心、共轭桥和末端引入强的吸电子基团可调谐单、双光子吸收光谱,实现在不同波长范围的双光子吸收;利用三态公式分析了分子的双光子吸收截面变化的内在原因;铝氮烷杂环化合物与其相应的硼化合物相比,表现出类似的单、双光子吸收性质,但一定程度上可增大双光子吸收截面.     

A Relative Study on Two-photon Absorption Properties of C60 and C70

We have theoretically investigated the one- and two-photon absorption properties of C60 and C70 using the ZINDO method. From the results it is suggested that the one-photon absorption spectra are in agreement with the experimental observations. It is found that the maximum TPA cross section of C70 is more than twice that of C60,which is consistent with the experimental results. A notable point is that the TPA process of C60 is different from that of C70 as well as other ordinary conjugated molecules.     

Molecular design and theoretical investigation into one-and two-photon absorption properties of two series of cyclometalated platinum(Ⅱ) complexes

We have theoretically investigated two series of cyclometalated Pt(Ⅱ) complexes,a series [Pt(C,N,N) Cl] and b series [Pt(C,N,Npyrazolyl) Cl].The geometrical and electronic structures are calculated at the ECP60MWB//6-31G*(H,C,Cl,N,S) basis set level using DFT method;one-photon absorption(OPA) properties are calculated by using both TDDFT and ZINDO methods and two-photon absorption(TPA) properties are obtained with the ZINDO/SOS method.The resonance integrals parameters(βsp and βd) for Pt are adjusted to -1 and -28.5 eV,respectively,to make max OPA wavelength calculated by ZINDO closest to the experimental data and TDDFT results.The calculated results indicate the molecule 2b([Pt(Cnaphthyl,N,Npyrazolyl) Cl]) has the biggest potential as outstanding TPA materials because(i) the TPA properties of b series are more outstanding in IR wavelength range,the molecules in b series have good transparencies and possess 1-pyrazolyl-NH that is also available for another metal coordination(e.g.,dimerization) and chemical interactions;(ii) when C is Cnaphthyl in the C,N,N ligand of cyclometalated Pt(Ⅱ) complexes,the molecules have the best conjugation effect and the best TPA properties.     

Solvent Effects on Two-Photon Absorption of Alkyne and Alkene π-bridging Chromophores

The present work concerns the study of solvent effects on the geometrical structures, as well as one- and two-photon absorption (TPA) processes, for two series of alkyne and alkene π-bridging molecules, within the framework of the polarization continuum model. Particular emphasis was put on the characterization of solvent effects on the molecular geometrical structures and geometric distortion, which were measured by the bond-length-alternation parameter. The π centres in the compounds are seen to play a decisive role in increasing the TPA cross section and nonlinear optical properties. All studied molecules have relatively strong TPA characteristics, while the alkyne π-bridging ones yield larger TPA cross sections.     

Theoretical study of one- and two-photon absorption properties of pyrene derivatives

The geometrical structure, electronic structure, one-photon absorption (OPA) properties of pyrene and its derivatives have been theoretically studied by using density functional theory (DFT) method and Zerner’s intermediate neglect of differential overlap (ZINDO) methods, and their two-photon absorption (TPA) properties are calculated by the ZINDO/sum-over-states method. The results show that introducing donor groups to pyrene molecule, increasing the number of donor groups, extending the conjugated length, or forming circular conjugated dimer can increase the oscillator strength (f) in the TPA process and ultimately result in extremely large TPA cross-sections and strong OPA around 400 nm of pyrene derivatives. All these results give us some basic principles to design pyrene derivatives with large TPA cross-sections. This shed light into the significance of the pyrene derivatives as promising fluorescent probes in biochemistry when they were linked to some special recognizing groups.     

Quadratic and cubic nonlinear optical properties of salts of diquat-based chromophores with diphenylamino substituents

A series of chromophoric salts has been prepared in which 4-(diphenylamino)phenyl (Dpap) electron donor groups are connected to electron-accepting diquaternized 2,2'-bipyridyl (diquat) units. The main aim is to combine large quadratic and cubic nonlinear optical (NLO) effects in potentially redox-switchable molecules with 2D structures. The chromophores have been characterized as their PF(6)(-) salts by using various techniques including electronic absorption spectroscopy and cyclic voltammetry. The visible absorption spectra are dominated by intense π → π* intramolecular charge-transfer (ICT) bands, and all of the compounds show two reversible or quasireversible diquat-based reductions and partially reversible Dpap oxidations. Single crystal X-ray structures have been obtained for one salt and for the precursor compound (E)-4-(diphenylamino)cinnamaldehyde, both of which adopt centrosymmetric space groups. First hyperpolarizabilities β have been measured by using hyper-Rayleigh scattering (HRS) with a 800 nm laser, and Stark (electroabsorption) spectroscopy of the ICT bands affords estimated static first hyperpolarizabilities β(0). The directly and indirectly derived β values are large and generally increased substantially for the bis-Dpap derivatives when compared with their monosubstituted analogues. Polarized HRS studies show that the NLO responses of the disubstituted species are dominated by "off-diagonal" β(zyy) components. Lengthening the diquaternizing alkyl unit lowers the electron-acceptor strength and therefore increases the ICT energies and decreases the E(1/2) values for diquat reduction. However, compensating increases in the ICT intensity prevent significant decreases in the Stark-based β(0) responses. Cubic NLO properties have been measured by using the Z-scan technique over a wavelength range of 520-1600 nm, revealing relatively high two-photon absorption cross-sections of up to 730 GM at 620 nm for one of the disubstituted chromophores.