Intrinsic Hyperpolarizability Research Articles

Perpetual Extension of Conjugation of Fluorene-Based Donor–Acceptor Dyads Yield Diminished Nonlinear Optical Response

A fundamental limit of π-conjugation seems to dictate the maximum nonlinear optical (NLO) response of fluorene-based “push–pull” (D−π–A) chromophores SPH1–SPH5 and the precursor aldehyde 4. Comparison of the first hyperpolarizability (βHRS) and intrinsic first hyperpolarizability (βint) values of the fluorene-based chromophores having different lengths and/or pathways of π-conjugation between donor and acceptor units, measured using the hyper-Rayleigh scattering (HRS) technique, revealed that the extension of π-conjugation beyond the limit (the horizontal limit!) does not necessarily improve the NLO response, which is determined by the combined influence of red shift of the intramolecular charge transfer (ICT) bands, optical band gap, and dipole moment of these chromophores. Additionally, the spatial distance between the donor and the acceptor units also constitutes an important attribute influencing the βHRS.

Synthesis and Nonlinear Optical Behavior of Thermally Stable Chromophores Based on 9,9-Dimethyl-9H-fluoren-2-amine: Improving Intrinsic Hyperpolarizability through Modulation of "Push-Pull".

Improvement in the first hyperpolarizability (βHRS) as well as intrinsic hyperpolarizability (βint) of chromophores based on 9,9-dimethyl-9H-fluoren-2-amine through modulation of the conjugation pathway is described. A series of six novel chromophores with "linear" conjugation showed significant enhancement of βHRS as well as βint compared to the counterparts lacking a "linear" conjugation but having an identical combination of donor, acceptor, and the intervening π-conjugated linker. The hyperpolarizability (βHRS as well as βint) values of the new series measured using hyper-Rayleigh scattering exceeded the apparent limit set by the latter set of fluorene-based chromophores. The experimental results are analyzed and interpreted in the context of linear optical properties, single-crystal X-ray analysis, electrochemistry, etc. and corroborated by theoretical studies. We find that modulation of the "push-pull" of the conjugation pathway in these donor-acceptor chromophores compares favourably with the corresponding changes in the optical gaps, transition dipole moments, and dipole moment difference between the ground and excited states.

BaSb(H2PO2)3Cl2: An excellent UV nonlinear optical hypophosphite exhibiting strong second-harmonic generation response

Non π-conjugated tetrahedral groups have emerged as potential basic modules for developing ultraviolet (UV) nonlinear optical materials. However, their intrinsic weak hyperpolarizability and small polarizability anisotropy severely restrict the performance of the designed material. Herein, the first antimony hypophosphite, BaSb(H2PO2)3Cl2, was successfully synthesized via rational design, which exhibits superior nonlinear optical properties including strong second harmonic generation response, appropriate birefringence and wide transmittance, indicating it is a potential UV nonlinear optical material. Remarkably, the title compound exhibits the largest nonlinear optical coefficient (5 × KDP) among all reported antimony-based oxyacid compounds. Detailed theoretical calculations confirmed that its excellent nonlinear optical properties are primarily derived from the ordered [Sb(H2PO2)3Cl2]2- anionic clusters composed of [H2PO2]- pseudo-tetrahedra and [SbO3Cl2]5- distorted polyhedra. This work would extend UV nonlinear optical material system to hypophosphites and lead the further inquiry of new structure-driven functional materials.

Synthesis, linear and non-linear optical properties of “push-pull” chromophores based on 9,9-dimethyl-9H-fluoren-2-amine

Photophysical properties including the first hyperpolarizability of a series of chromophores (SP1-SP5) possessing “push-pull” type conjugation were investigated to evaluate the proficiency as non-linear optically active materials. The compounds are based on the 9,9-dimethyl-9H-fluoren-2-amine platform, which acts as a donor unit and different types of acceptors have been incorporated to design SP1-SP5. The first hyperpolarizability (βHRS) value was in the range of 102 × 10−30 esu to 174 × 10−30 esu, and the trends in the experimental βHRS have been rationalized in terms of the difference in the dipole moment (Δμge) of the ground and excited state, oscillator strength of the contributing absorption bands and the band gaps. The intrinsic efficiency of the non-linear response of these dipolar compounds possessing a single major dipolar hyperpolarizability tensor component βzzz was also determined from the intrinsic hyperpolarizability, a real figure of merit of the non-linear optical response. Comparison of the experimental off-resonant first hyperpolarizability, βHRS, as well as the intrinsic hyperpolarizability of this set of D-π-A chromophores, lacking linear conjugation, revealed advantage over the already reported counterparts based on benzothiazole platform.

Positional isomers of heterocyclic azo dyes: Investigation of NLO properties by Z-scan and correlative DFT studies

We report four azo dyes and their positional isomers having a benzothiazole-pyridone core. Selected heterocyclic emissive azo dyes in this work are analysed for their non-linear optical (NLO) properties. Non-linear absorptive coefficient (β), non-linear refractive index (ƞ2), third-order NLO susceptibility (χ3), and optical limiting (OL) in different solvents are analysed and promising results are obtained using Z-scan technique. Laser damage threshold (LDT) and thermogravimetric analysis (TGA) supports that these dyes can be a good constructive unit in laser assisted devices and several high-temperature NLO processes respectively. Molecular electrostatic potential (MEP), Frontier molecular orbitals (FMOs) are also presented using B3LYP/6–311+ G (d,p) level of theory. Global hybrid functional (GH) and Range separated hybrid functional (RSH) i.e. B3LYP, BHandHLYP and CAM-B3LYP, ωB97, ωB97X, ωB97X-D respectively used for the calculation of NLO parameters. Limits of hyperpolarizabilities and intrinsic hyperpolarizabilities are calculated and all the hyperpolarizabilities of the molecules are within estimated limits. HOMO-LUMO gap have an inverse correlation with the experimental non-linear optical susceptibility (χ3). Inverse or direct correlation with hyperpolarizability with different DFT descriptors such as electrophilicity index (ω), hyperhardness is also presented.

Static- and frequency-dependent NLO properties of dithienothiophene and thienothiophene bridges — A computational investigation

We have explored detailed linear and nonlinear optical properties of push-pull systems bearing thienothiophene and dithienothiophene spacers. By using density functional theory (DFT), frequency-dependent strategies were applied to examine the polarizability ([Formula: see text] and hyperpolarizability ([Formula: see text] and [Formula: see text]. The set of global and range-separated hybrid functionals with different Hartree–Fock (HF) exchange percentage at two basis sets cc-pVDZ and cc-pVTZ were used to evaluate the nonlinear optical (NLO) properties. The observed trends in the absorption maxima supported by perturbation potential analysis; as the absorption maxima increases, the respective amplitude potential decreases. For the investigated compounds, [Formula: see text]-conjugation along with the type of substituted acceptor plays a crucial role in the enhancement of NLO properties. The presence of acceptor group and length of conjugation increase between the D and A group; the first- and second-order intrinsic hyperpolarizability increases, leads to enhanced first- as well as second-order hyperpolarizability. Bond length alternation (BLA)/bond order alteration (BOA) exploration suggested that compounds attain cyanine limit. The trends in NLO properties for investigated compounds are supported by chemical reactivity descriptors, hardness and hyperhardness analysis. The polarizability is linearly correlated with the hyperpolarizability parameters ([Formula: see text] and [Formula: see text] and shows a good regression coefficient by figures of merit analysis.

Investigation of photophysical, structural aspects and nonlinear optical properties of Foron blue SR analogs using Density Functional Theory (DFT)

Abstract Analogs of 2-[(2Z)-2-[[4-(dihexylamino)-2-methylphenyl]methylidene]-1,1-dioxo-1-benzothiophen-3-ylidene]propanedinitrile (Foron blue SR, Disperse Blue 354) are studied in terms of photophysical, structural aspects and its nonlinear optical properties evaluated by using Density Functional Theory (DFT) and Time Dependent Density Functional Theory (TD-DFT). The structures were optimized by using function B3LYP and basis set used was 6-31G(d). Polarizability (\(\alpha \)), first and second order hyperpolarizability (\(\beta \) and \(\gamma \)) was determined by three density functionals B3LYP, CAM-B3LYP and BHandHLYP. B3LYP functional and the basis set 6-311+G(d,p) shows high values of \(\alpha \), \(\beta \) and \(\gamma \). The B3LYP functional gives the highest first order hyperpolarizability value for (Z)-2-(2-(4-(dimethylamino)benzylidene)-1,1-dioxidobenzo[b]thiophen-3(2H)-ylidene)malononitrile (3a) is \(373.31 \times 10^{-30}\) e.s.u and \(486.32 \times 10^{-30}\) e.s.u in ethylacetate and dimethylformamide respectively. Effect of bond length alteration (BLA) and bond order alteration (BOA) on first order hyperpolarizability was evaluated. Intrinsic hyperpolarizability shows the compound (Z)-2-(4-(dimethylamino)benzylidene)benzo[b]thiophen-3(2H)-one 1,1-dioxide (1a) intrinsically good. Perturbation potential is found to decrease as absorption energy deceases and hyperpolarizability increases. Graphic Abstract: Analogs of Foron blue SR which is also known as Disperse Blue 354 are studied in terms of photophysical, structural aspects and its nonlinear optical properties evaluated by using Density Functional Theory (DFT) and Time Dependent Density Functional Theory (TD-DFT). The structures were optimized by using the functional, B3LYP and the basis set used was 6-31G(d). Polarizability (\(\alpha \)), first and second order hyperpolarizability (\(\beta \) and \(\gamma \)) were determined by three density functionals B3LYP, CAM-B3LYP and BHandHLYP. B3LYP and the basis set 6-311+G(d,p) shows high values of \(\alpha \), \(\beta \) and \(\gamma \). The B3LYP functional gives the highest first order hyperpolarizability for (Z)-2-(2-(4-(dimethylamino)benzylidene)-1,1-dioxidobenzo[b]thiophen-3(2H)-ylidene)malononitrile (3a), \(373.31 \times 10^{-30}\) e.s.u and \(486.32 \times 10^{-30}\) e.s.u in ethylacetate and dimethylformamide respectively. Effect of bond length alteration (BLA) and bond order alteration (BOA) on first order hyperpolarizability was evaluated. Intrinsic hyperpolarizability shows that the compound (Z)-2-(4-(dimethylamino)benzylidene)benzo[b]thiophen-3(2H)-one 1,1-dioxide (1a) intrinsically good. Perturbation potential is found to decrease as absorption energy deceases and hyperpolarizability increases.

2‐(1,1‐dioxidobenzo[b]thiophen‐3(2H)‐ylidene)malononitrile (BTD) Based Styryl Chromophores‐ Solvatochomic and Computational Investigation of Linear and NLO properties

AbstractLinear and nonlinear optical properties of the twelve styryl dyes are investigated using density functional theory (DFT). Range‐separated hybrid CAM−B3LYP and global hybrid BHandHLYP estimate high second order hyperpolarizability. Mean polarizability (α0), polarizability anisotropy (Δα), static first‐order hyperpolarizability (β0) and second hyperpolarizability (γ) were found to be overestimated in CAM−B3LYP than in BHandHLYP. As bond length alternation (BLA) increases first order hyperpolarizability increases. Multilinear regression analysis (MLR) shows the solvent basicity, polarizability and dipolarizabilty are responsible for absorption solvatochromism. The ratio of the dipole moments increases as donor strength and conjugation increases which support charge transfer. The figure of merit (FOM) and intrinsic hyperpolarizability found more in chromophores with dialkylamine donors. As the stability indicated by electrophilicity indices decreases the hyperpolarizability increases. Interrelationships between α0, β0 and γ were evaluated and it shows good linear fit. Absorption is red shifted and hyperpolarizability increases with the amplitude of the sine‐shaped potential along the conjugation chain. A high molecular electrostatic potential was observed in dyes containing 2‐piperidinothiophene donor.

Polynomial potentials determined from the energy spectrum and transition dipole moments that give the largest hyperpolarizabilities

We attempt to get a polynomial solution to the inverse problem, that is, to determine the form of the mechanical Hamiltonian when given the energy spectrum and transition dipole moment matrix. Our approach is to determine the potential in the form of a polynomial by finding an approximate solution to the inverse problem, then to determine the hyperpolarizability for that system's Hamiltonian. We find that the largest hyperpolarizabilities approach the apparent limit of previous potential optimization studies, but we do not find real potentials for the parameter values necessary to exceed this apparent limit. We also explore half potentials with positive exponent, which cannot be expressed as a polynomial except for integer powers. This yields a simple closed potential with only one parameter that scans nearly the full range of the intrinsic hyperpolarizability. The limiting case of vanishing exponent yields the largest intrinsic hyperpolarizability.

Applying universal scaling laws to identify the best molecular design paradigms for third-order nonlinear optics

The scaling of the fundamental limits of the second hyperpolarizability is used to define the intrinsic second hyperpolarizability, which aids in identifying material classes with ultralarge nonlinear-optical response per unit of molecular size. The intrinsic nonlinear response is a size-independent metric that we apply to comparing classes of molecular homologues, which are made by adding repeat units to extend their lengths. Several new figures of merit are proposed that quantify not only the intrinsic nonlinear response, but also how the second hyperpolarizability increases with size within a molecular class. Scaling types can be classified into sub-scaling, nominal scaling that follows the theory of limits, and super-scaling behavior. Super-scaling homologues that have large intrinsic nonlinearity are the most promising because they efficiently take advantage of increased size. We apply our approach to data in the literature to identify the best super-scaling molecular paradigms and articulate the important underlying parameters.

Maximizing the hyperpolarizability of 1D potentials with multiple electrons

We optimize the first and second intrinsic hyperpolarizabilities for a 1D piecewise linear potential dressed with Dirac delta functions for $N$ non-interacting electrons. The optimized values fall rapidly for $N>1$, but approach constant values of $\beta_{int}=0.40$, $\gamma_{int}^{+}=0.16$ and $\gamma_{int}^{-}=-0.061$ above $N\gtrsim8$. These apparent bounds are achieved with only 2 parameters with more general potentials achieving no better value. In contrast to previous studies, analysis of the hessian matrices of $\beta_{int}$ and $\gamma_{int}$ taken with respect to these parameters shows that the eigenvectors are well aligned with the basis vectors of the parameter space, indicating that the parametrization was well-chosen. The physical significance of the important parameters is also discussed.

Optical limiting efficiency of an electroactive bis-iminopyridine ligand and its zinc complex

An electroactive based bis-iminopyridine ligand has been synthesized by a condensation reaction between (4-(6,7-dimethyldithio-tetrathiafulvalene)-aniline) with 2,6-diformylpyridine. The complexation of this ligand with ZnCl 2 afforded a tetrahedral neutral Zinc metal complex. optical measurements of these structures have given good results in picosecond regime. The nonlinear absorption of the ligand was significantly enhanced upon complexation with ZnCl 2 . This prompted us to conduct the experiment of optical limiting at wavelength 532nm. The nonlinear absorption properties of these structures were studied and compared to the reference material C 60 . Full Text: PDF References V. Rosso, J. Loicp, Y. Renotte, Y. Lion, Optical non-linearity in Disperse Red 1 dye-doped sol?gel, J. Non-Crystalline Solids 342 (1-3), 140 (2004). CrossRef L. Jiang, Q. Chang, Q.Y. Ouyang, H.B. Liu, Y.X. Wang, X.R. Zhang, et al., Fabrication and nonlinear optical properties of an ultrathin film with acceptor?donor periodically overlapping structure, Chemical Phys, 324(2-3), 556 (2006). CrossRef B. Sahraoui, X. Nguyen Phu, M. Salle, A. Gorgues, Electronic and nuclear contributions to the third-order nonlinear optical susceptibilities of new p-N, N?-dimethylaniline tetrathiafulvalene derivatives, Opt. Lett. 23(23), 1811 (1998). CrossRef A. Apostoluk, D. Chapron, C. Fiorini-Debuisschert, G. Gadret, J.-M. Nunzi, P. Raimond, B. Sahraoui, Quasi-phase-matched gratings printed by all-optical poling in polymer films, Opt. Lett. 27(22), 2028 (2002). CrossRef Ts. Kolev, I.V. Kityk, J. Ebothe, B. Sahraoui, Intrinsic hyperpolarizability of 3-dicyanomethylene-5,5-dimethyl-1-[2-(4-hydroxyphenyl)ethenyl]-cyclohexene nanocrystallites incorporated into the photopolymer matrices, Chem. Phys. Lett. 443(4-6), 309 (2007). CrossRef K. Iliopoulos, A. El-Ghayoury, B. Derkowska, A. Ranganathan, P. Batail, D. Gindre, B. Sahraoui, Effect of the counter cation on the third order nonlinearity in anionic Au dithiolene complexes, Appl.Phys. Lett. 101, 261105 (2012). CrossRef K. Iliopoulos, A. El-Ghayoury, H. El Ouazzani, M. Pranaitis, E. Belhadj, E. Ripaud, M. Mazari, M. Salle, D. Gindre, B. Sahraoui, Nonlinear absorption reversing between an electroactive ligand and its metal complexes, Opt. Expr. 20, 25311 (2012). CrossRef H. El Ouazzani, K. Iliopoulos, M. Pranaitis, O. Krupka, V. Smokal, A. Kolendo, B. Sahraoui, Second- and Third-Order Nonlinearities of Push?Pull Azobenzene Polymers, J. Phys. Chem. B 115, 1944 (2011). CrossRef F. Kajar, J. M. Nunzi, Functional Organic and Polymeric Materials (John Wiley and Sons Ltd., New York, 2000). K. Iliopoulos, I. Guezguez, A.P. Kerasidou, A. El-Ghayoury, D. Branzea, G. Nita, N. Avarvari, H. Belmabrouk, S. Couris, B. Sahraoui, Effect of metal cation complexation on the nonlinear optical response of an electroactive bisiminopyridine ligand, Dyes and Pigments 101, 229 (2014). CrossRef G. Nita, D. Branzea, F. Pop, A. El-Ghayoury, N. Avarvari, Electroactive Bisiminopyridine Ligands: Synthesis and Complexation Studies, Crystals 2, 338 (2012). CrossRef E. Coronado, J.R. Galan-Mascaros, C.J. Gomez-Garcia, V. Laukhin, Coexistence of ferromagnetism and metallic conductivity in a molecule-based layered compound, Nature 408, 447 (2000). CrossRef M. Nihei, N. Takahashi, H. Nishikawa, H. Oshio, Spin-crossover behavior and electrical conduction property in iron(II) complexes with tetrathiafulvalene moieties, Dalton Trans 40, 2154 (2011). CrossRef G.J.P. Britovsek, V.C. Gibson, B.S. Kimberley, P.J. Maddox, S.J. McTavish, G.A. Solan, et al., Novel olefin polymerization catalysts based on iron and cobalt, Chem. Commun. 7, 849 (1998). CrossRef B.L. Small, M. Brookhart, A.M.A. Bennett, Highly Active Iron and Cobalt Catalysts for the Polymerization of Ethylene, J. Am. Chem. Soc. 120, 4049 (1998). CrossRef K. Iliopoulos, S. Couris, U. Hartnagel, A. Hirsch, Nonlinear optical response of water soluble C70 dendrimers, Chem. Phys. Lett. 448, 243 (2007). CrossRef N. Liaros, K. Iliopoulos, M.M. Stylianakis, E. Koudoumas, S. Couris, Optical limiting action of few layered graphene oxide dispersed in different solvents, Opt. Mat. 36, 112 (2014). CrossRef

Cross-conjugation as a Motif for Organic Non-Linear Optical Molecules

ABSTRACTWe studied the effect of a cross-conjugated bridging group (χC) on charge-transfer in a push-pull chromophore system. The hyperpolarizability of such molecules was found to be comparable to that of a fully π-conjugated molecule (πC) with the same donor and acceptor. The cross-conjugated moiety was then applied as a pendant to a fully π-conjugated chromophore containing a tricyanopyrroline acceptor (TCP). The addition of a χC moiety did not alter the intrinsic hyperpolarizability and provides an avenue for extending and aiding πC systems. The molecules were examined by X-ray diffraction (XRD), hyper-Raleigh scattering (HRS) and UV-visible (UV-vis) spectroscopy. Experimental results were compared with the predictions of density functional theory (DFT). Cross-conjugated molecules have comparable β values, relative to πC molecules, due to reduced spatial overlap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Thus, the χC architecture could facilitate independent modification of donor and acceptor strengths while minimizing unfavorable effects on electronic transitions and dipole moments.

Paradox of the many-state catastrophe of fundamental limits and the three-state conjecture

The calculation of the fundamental limits of nonlinear susceptibilities posits that when a quantum system has a nonlinear response at the fundamental limit, only three energy eigenstates contribute to the first and second hyperpolarizability. This is called the three-level ansatz and is the only unproven assumption in the theory of fundamental limits. All calculations that are based on direct solution of the Schrodinger equation yield intrinsic hyperpolarizabilities less than 0.709 and intrinsic second hyperpolarizabilities less than 0.6. In this work, we show that relaxing the three-level ansatz and allowing an arbitrary number of states to contribute leads to divergence of the optimized intrinsic hyperpolarizability in the limit of an infinite number of states - what we call the many-state catastrophe. This is not surprising given that the divergent systems are most likely not derivable from the Schrodinger equation, yet obey the sum rules. The sums rules are the second ingredient in limit theory, and apply also to systems with more general Hamiltonians. These exotic Hamiltonians may not model any real systems found in nature. Indeed, a class of transition moments and energies that come form the sum rules do not have a corresponding Hamiltonian that is expressible in differential form. In this work, we show that the three-level ansatz acts as a constraint that excludes many of the nonphysical Hamiltonians and prevents the intrinsic hyperpolarizability from diverging. We argue that this implies that the true fundamental limit is smaller than previously calculated. Since the three-level ansatz does not lead to the largest possible nonlinear response, contrary to its assertion, we propose the intriguing possibility that the three-level ansatz is true for any system that obeys the Schrodinger equation, yet this assertion may be unprovable.

Optimizing the second hyperpolarizability with minimally parametrized potentials

The dimensionless zero-frequency intrinsic second hyperpolarizability \gamma_{int}=\gamma/4E_{10}^{-5}m^{-2}(e\hbar)^{4} was optimized for a single electron in a 1D well by adjusting the shape of the potential. Optimized potentials were found to have hyperpolarizabilities in the range -0.15\lessapprox\gamma_{int}\lessapprox0.60 ; potentials optimizing gamma were arbitrarily close to the lower bound and were within \sim0.5% of the upper bound. All optimal potentials posses parity symmetry. Analysis of the Hessian of \gamma_{int} around the maximum reveals that effectively only a single parameter, one of those chosen in the piecewise linear representation adopted, is important to obtaining an extremum. Prospects for designing new chromophores based on the design principle here elucidated are discussed.

Universal properties of the optimized off-resonant intrinsic second hyperpolarizability

We seek to identify universal properties shared by all quantum systems with large intrinsic second hyperpolarizability (γint)—an invariant quantity that removes the effects of simple scaling. A large-γint quantum system is generated by varying the shape of a trial potential until γint is optimized. A variety of starting potentials yield a set of systems with distinctly shaped optimized potentials, but are found to share universal properties that separate into classes determined by the magnitude and sign of γint. However, the fact that the best systems are 0.6 times the fundamental limit suggests that exotic Hamiltonians may be required to reach the upper bound. The observed regularity hints at a deeper relationship between optimized systems that may provide useful insights applicable to designing better materials. Being general, this approach applies to any quantum system, including molecules, nanoparticles, or quantum gases.

Computational study of chiral molecules with high intrinsic hyperpolarizabilities

We have investigated the electronic transition, chiroptical properties, and the second-order nonlinear optical (NLO) properties of eight novel chiral diborate compounds and elucidated structure–property relationships from the micromechanism. These compounds show calculated first hyperpolarizabilities (β) ranging from 2738.52 to 83976.45 × 10−33 esu, which means that subtle structural modifications can substantially enhance the first hyperpolarizability. The cooperativity of intramolecular charge transfer and an effective way to enhance the NLO response were also systemically investigated. The linear correlation between the first hyperpolarizability and the inverse of the electronic transition energy suggests that the electronic transition energy plays a key role in determining the NLO response. These compounds have the potential to be excellent second-order NLO materials from the standpoint of the large β values, high transparency and the intrinsic non-centrosymmetry. The electronic transition and chiroptical properties have been assigned and analysed. The main UV–visible absorption features are best described as π → π* transitions. Moreover, the effects of different functionals and basis sets on the first hyperpolarizability were investigated.

Thermally stable ferrocenyl “push–pull” chromophores with tailorable and switchable second-order non-linear optical response: synthesis and structure–property relationship

A combined UV-visible, cyclic voltammetric theoretical and hyper-Rayleigh scattering study has been carried out on a new series of ferrocenyl chromophores (2a–2d and 7–11) to develop a structure–property relationship, by varying the acceptor strength and the conjugation path length between the donor and the acceptor and by disubstitution of the ferrocene moiety. This allowed systematic establishment of the contribution of the conjugation bridge, acceptor strength and substitution on both rings of ferrocene towards non-linear optical response as well as its redox switchability with one of the chromophores (8) having an on/off ratio of 25, the highest for ferrocenyl chromophores, and also thermal stability up to 300 °C. To deduce the participation of the available polarizable electrons towards the overall non-linear optical properties (with electron contribution from both the rings as observed from average bond length calculation from crystal data of 2c and 7 and the absorption studies), respective intrinsic hyperpolarizability (β0int) of the dipolar chromophores was calculated. Interestingly, the shorter chromophore 2c showed exceptionally high non-linear optical response and intrinsic hyperpolarizability compared to its longer counterparts.

Quantum Chemical Evaluation of Ionic Nonlinear Optical Chromophores and Crystals Considering the Counteranion Effects

The direction and the magnitude of the first hyperpolarizability are investigated by the finite field (FF) method for organic nonlinear optical ionic chromophores and crystals with an isotropic counteranion, either a point charge or hexafluorophosphate. Simple model compound 4-aminopyridinium and DAPSH (N,N-dimethylamino-N′-phenyl-4-stilbazolium hexafluorophosphate) crystal with the highest quadratic optical nonlinearities measured to date in organic crystals, as well as stilbazolium derivatives having heteroaromatic and/or bulky aromatic rings, are investigated. Calculations indicate that the extrinsic isotropic point-charges or anions almost have no effect on the direction of the intrinsic first hyperpolarizability of the cation chromophores, and induce a limited increase of the magnitude smaller than ∼15% due to their asymmetric distribution with respect to the pyridinium nitrogen atom. This finding enables one to simply approximate the first hyperpolarizability of ionic crystalline systems having 1D i...

Theoretical studies on the second-order nonlinear optical properties of donor–acceptor-substituted hexaphyrins(1.1.1.1.1.1) and their redox-switchability

The second-order nonlinear optical (NLO) properties of a series of expanded porphyrins, donor–acceptor-substituted hexaphyrins(1.1.1.1.1.1) and their mono- and bis-Au(III) complexes have been calculated based on density functional theory. The result shows that large-sized hexaphyrin molecules have a large static first hyperpolarizability when compared with the relatively small porphyrin molecule. However, the small porphyrin molecule is intrinsically more efficient for enhancement of the intrinsic NLO response according to intrinsic hyperpolarizability analysis. On the other hand, the switchable NLO properties of aromatic-to-antiaromatic conversion caused by two-electron oxidation/reduction processes have also been probed. The result shows that aromatic→antiaromatic reduction on the hexaphyrin core decreases the static first hyperpolarizability and the intrinsic hyperpolarizability. The value of the aromatic complex is ∼25 times larger than that of the antiaromatic complex. This indicates that the hexaphyrin ring plays an important role in the determination of the second-order NLO response, and the aromatic-to-antiaromatic changes on the hexaphyrin ring cause switchable second-order NLO responses.