Bond Order Alternation Research Articles

Exploring charge transfer effects on linear, non‐linear optical, and dye‐sensitized solar cell properties: A DFT and TD‐DFT investigation of carbazole and aniline‐based dyes

AbstractThe efficacy of aniline and carbazole‐based sensitizers used in dye‐sensitized solar cells (DSSCs) is correlated with the trends in density functional theory (DFT) based charge transfer (CT) and non‐linear optical (NLO) properties since both DSSC and NLO properties highly rely on CT properties. The aniline‐based donors showed a higher degree of planarity over carbazole‐based donor sensitizers. This improved planarity in aniline‐based sensitizers is attributed to efficient charge transfer and higher dipole moment. The efficient CT within the sensitizers is confirmed with bond‐length alternation (BLA), bond order alternation (BOA), and quinoid character (QC). A direct correlation of band‐gap (EG) with μ, ω, η, and Γ is witnessed. Time‐dependent DFT (TD‐DFT) results of vertical excitation demonstrated similar trends that are observed practically. Indeed, we observed a direct correlation between α and β and the experimentally observed DSSC efficiency. DFT and TD‐DFT methods, linear, employed for computing the CT and linear and DSSC properties as implemented in Gaussian 09. NLO properties were simulated through single‐point polar calculations. B3LYP and PBE1PBE functionals were used for geometry optimization. The linear and NLO properties were determined in various functionals, that is, global hybrid and range‐separated hybrid functionals. A polarizable continuum model (PCM) was used for solvation studies. To understand the realistic picture of the dye@TiO2 cluster, dye bound on TiO2 clusters were optimized using a LANL2DZ basis set specifically for the Ti atom.

Dynamic effects on the nonlinear optical properties of donor acceptor stenhouse adducts: insights from combined MD + QM simulations.

The second-order nonlinear optical (NLO) responses of a donor-acceptor stenhouse adduct (DASA) are investigated by using a computational approach combining molecular dynamics simulations and density functional theory (DFT) calculations. Specific force fields for the open and closed photoswitching forms are first parameterized and validated according to the Joyce protocol, in order to finely reproduce the geometrical features and potential energy surfaces of both isomers in chloroform solution. Then, DFT calculations are performed on structural snapshots extracted at regular time steps of the MD trajectories to address the influence of the thermalized conformational dynamics on the NLO responses related to hyper-Rayleigh scattering (HRS) experiments. We show that accounting for the structural dynamics largely enhances the HRS hyperpolarizability (βHRS) compared to DFT calculations considering solely equilibrium geometries, and greatly improves the agreement with experimental measurements. Furthermore, we show that the NLO responses of the NLO-active open form are correlated with the bond order alternation along the triene bridge connecting the donor and acceptor moieties, which is rationalized using simple essential state models.

Structural transition and chemical reactivity of atomic carbon chains.

The sp-hybridized carbon chain (carbyne) is a representative 1D atomic material, whose bonding structure and chemical reactivity have remained a mystery for a century. Here, we report the unexpected alternating bond orders of 1.4 and 2.6 for the most stable carbon chain and the in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) detection of the temperature-dependent reversible change of the bond order alternation. Moreover, we revealed its reactivities with O2, H2, and CO2 at temperatures up to 600 °C and created an end-group-protection strategy to stabilize it. These observations open a new door to the chemistry of atomic materials.

Computational Quantification of the Zwitterionic/Quinoid Ratio of Phenolate Dyes for Their Solvatochromic Prediction

Solvatochromic dyes are utilized in various chemical and biological media as chemical sensors. Unfortunately, there is no simple way to predict the type of solvatochromism based on the structure of the dye alone, which restricts their design and synthesis. The most important family of solvatochromic sensors, pyridinium phenolate dyes, has the strongest solvatochromism. Using a natural population analysis (NPA) of the natural bond orbitals (NBO) of the phenolate group in the frontier molecular orbitals, it is possible to calculate the relative polarity of the ground state and excited state and, thus to develop a model that can predict the three types of solvatochromism observed for this family: negative, positive, and inverted. This methodology has been applied to thirteen representative examples from the literature. Our results demonstrate that the difference in the electron density of the phenolate moiety in the frontier molecular orbitals is a simple and inexpensive theoretical indicator for calculating the relative polarity of the ground and excited states of a representative library of pyridinium phenolate sensors, and thus predicting their solvatochromism. Comparing the results with the bond length alternation (BLA) and bond order alternation (BOA) indices showed that the NPA/NBO method is a better way to predict solvatochromic behavior.

Charge Transfer as Bridging Correlator for DSSC Efficiency and NLO Property

AbstractThe efficacy of dipyridyl and diphenylamine sensitizers in dye‐sensitized solar cells (DSSCs) is linked to trends in DFT‐based charge transfer and non‐linear optical (NLO) properties. We chose DFT and Time‐dependent‐DFT methods. The planarity is improved by replacing the phenyl ring at the donor site with a thiophene ring. Similarly, cyanoacetic acid acceptor improves planarity over rhodamine‐3‐acetic acid acceptor. In all cases in HOMO, the electron density is located at the donor and some part of the spacer, and in LUMO, it is shifted to the acceptor. Bond length alternation, bond order alternation, quinoid character, effective charge transferred distance, effective charge transferred, chemical potential, electrophilicity index, and electron correlation descriptors showed direct relation, while hardness and hyperhardness showed an inverse relation with NLO properties. We observed higher β0 and γ for thiophene‐based sensitizers ranging from 248.81×10−30 to 440.92×10−30 esu and 777.82×10−36 to 1442.69×10−36 esu, respectively. Likewise, more negative values of ΔGinject (−0.630 to −0.752 eV), a smaller value of ΔGreg (0.662 to 0.790 eV), and higher LHE (0.959 to 0.976) are observed for thiophene‐based sensitizers. Also, thiophene‐based sensitizers showed shorter Ti−O bond lengths than their phenyl‐based sensitizers. Photovoltaic parameters are directly related to DSSC efficiency and NLO properties.

Structural Factors Determining the Absorption Spectrum of Channelrhodopsins: A Case Study of the Chimera C1C2.

Channelrhodopsins are photosensitive proteins that trigger flagella motion in single-cell algae and have been successfully utilized in optogenetic applications. In optogenetics, light is used to activate neural cells in living organisms, which can be achieved by exploiting the ion channel signaling of channelrhodopsins. Tailoring channelrhodopsins for such applications includes the tuning of the absorption maximum. In order to establish rational design and to obtain a desired spectral shift, a basic understanding of the absorption spectrum is required. We have studied the chimera C1C2 as a representative of this protein family and the first member with an available crystal structure. For this purpose, we sampled the conformations of C1C2 using quantum mechanical/molecular mechanical molecular dynamics and subjected the resulting snapshots of the trajectory to excitation energy calculations using ADC(2) and simplified time-dependent density functional theory. In contrast to previous reports, we found that different hydrogen-bonding networks-involving the retinal protonated Schiff base, the putative counterions E162 and D292, and water molecules-had only a small impact on the absorption spectrum. However, in the case of deprotonated E162, increasing the distance to the Schiff base hydrogen-bonding partner led to a systematic blue shift. The β-ionone ring rotation was identified as another important contributor. Yet the most important factors were found to be the bond length alternation and bond order alternation that were linearly correlated to the absorption maximum by up to 62 and 82%, respectively. We ascribe this novel insight into the structural basis of the absorption spectrum to our enhanced protein setup that includes membrane embedding as well as long and extensive sampling.

Charge transfer and nonlinear optical properties of anthraquinone D-π-A dyes in relation with the DFT based molecular descriptors and perturbational potential

The static and dynamic NLO properties of Anthraquinone based (D-π-A) dyes are determined by using the Density Functional Theory in ten different functionals. Five global hybrids and five long-range corrected hybrid functionals used based on Hartree-fork exchange contribution. The investigation of vertical excitation is also performed in the equivalent set of functionals. The elaborate study of molecular configuration i.e. intramolecular charge transfer character is analyzed by Frontier Molecular Orbital study, bond length alternation, bond order alternation, molecular electrostatic potential plot, quinoidal character (δr), and electrophilicity index (ω). Benchmarking studies are initiated with the standard deviation method and, the functionals BHHLYP and ωB97 resulted as the suitable functionals for determining the NLO properties of anthraquinone based dyes. The less studied reactivity descriptor hyper-hardness (Γ) and perturbation potential (Vo) is also explored and correlated with the NLO properties. The substitution effect on NLO properties is further confirmed by Hammett constant and excited state Hammett substituent constant.

Theoretical Investigation of Optical and Nonlinear Optical (NLO) Properties of 3‐Azabenzanthrone Analogues : DFT and TD‐DFT Approach.

Abstract3‐Azabenzanthrone based chromophores are investigated to study the effect of different substituents on optical and “non‐linear optical (NLO)” properties by means of density functional theory (DFT) with global hybrid functionals (GHs) ‐ B3LYP and BHandHLYP, range separated hybrid functionals (RSHs) CAM−B3LYP, wB97, wB97X, wB97XD and the basis sets 6–31+G(d), cc‐pVDZ, and cc‐pVTZ. It is observed that the incorporation of an electron withdrawing group at the 1 position and an electron donating group at the 2 and 6 positions of 3‐azabenzanthrone system led to more red shifted absorption maxima and showed better NLO response. Molecule 3 with electron withdrawing ‐CN at the 1 position exhibited the highest absorption wavelength (λ= 590 nm). The geometries, dipole moments, HOMO‐LUMO energy gaps, total hyperpolarizability are calculated to investigate the effect of different substituents on the photophysical and NLO properties of 3‐azabenzanthrone based chromophores. The insertion of ‐CN group at the 1 position of 3‐azabenzanthrone system resulted in higher first order hyperpolarizability (β0). The β0 values of 3, 5, 6 and 12 are found to be the maximum among other remaining molecules as the “Bond Length Alternation (BLA)” and “Bond Order Alternation (BOA)” parameters tend to zero. The charge transfer (CT) properties of these analogues are established using molecular electrostatic potential (MEP) analysis and quinoidal character at the same level of theory.

Substituent effects on linear and nonlinear optical properties of fluorescent (E)-2-(4-halophenyl)-7-arlstyrylimidazo[1,2-A]pyridine: spectroscopic and computational methods

Abstract Effects of alkylamino and bromo substituents on imidazo[1,2-a]pyridines containing donor-π-acceptor type groups were comprehensively investigated for their linear and nonlinear optical properties by solvatochromic and DFT (CAM-B3LYP and BHandHLYP) methods. The difference between the ground and excited dipole moments as well as their ratios obtained by solvatochromic analysis indicate that the excited state is more polar than the ground state for both the bromo and diethyl amino derivative. More than twofold enhancement in the excited state dipole moments was observed as revealed by the difference and ratio of dipole moment upon the introduction of alkylamino donor group and these suggest large intramolecular charge transfer in the dyes. Stabilization energy above 20 kJ/mol was observed for large number of electron donor–acceptor interactions in Natural Bonding Orbital (NBO) analysis. Bond length alternation (BLA) and Bond order alternation (BOA) values tend to zero suggesting a high degree of polarization in the dyes. Enhancement in mean polarizability (α0), first hyperpolarizability (β0) and second static hyperpolarizabilities ( $\bar \gamma$) were observed by the introduction of alkylamino and bromo group in place of chloro in spite of the fact that Hammett constant of chloro and bromo are the same. The dyes have fundamental and intrinsic properties within the Hamiltonian limits. The two-photon absorption cross section value (≈100 GM) is comparable with LDS-698, a commercial TPA dye. This investigation is important for understanding the electronic structure of imidazo[1,2-a]pyridine with active functional groups and extending the potential for optical applications.

Near Length-Independent Conductance in Polymethine Molecular Wires

Polymethine dyes are linear π-conjugated compounds with an odd number of carbons that display a much greater delocalization in comparison to polyenes that have an even number of carbon atoms in their main chain. Herein, we perform scanning tunneling microscope based break-junction measurements on a series of three cyanine dyes of increasing length. We demonstrate, at the single molecule level, that these short chain polymethine systems exhibit a substantially smaller decay in conductance with length (attenuation factor β = 0.04 Å-1) compared to traditional polyenes (β ≈ 0.2 Å-1). Furthermore, we show that by changing solvent we are able to shift the β value, demonstrating a remarkable negative β value, with conductance increasing with molecular length. First principle calculations provide support for the experimentally observed near-uniform length dependent conductance and further suggest that the variations in β with solvent are due to solvent-induced changes in the alignment of the frontier molecular orbitals relative to the Fermi energy of the leads. A simplified Hückel model suggests that the smaller decay in conductance correlates with the smaller degree of bond order alternation present in polymethine compounds compared to polyenes. These findings may enable the design of molecular wires without a length-dependent decay for efficient electron transport at the nanoscale.

NIR emitting new N, N-diethylaniline based NLOphoric D-π-A and D-A′-π-A dyes: Photophysical properties, viscosity sensitivity and DFT studies

Four new N, N-diethylaniline comprising D-π-A and D-A′-π-A type dyes were synthesized and characterized by 1H NMR, 13C NMR, and elemental analysis. The dyes show positive and negative solvatochromism in absorption and emission, especially the compounds (E)− 2-Cyano-3-(5-((E)− 1-cyano-2-(4-(diethylamino)− 2-(hexyloxy) phenyl) vinyl) thiophen-2-yl) acrylic acid (D1) and (E)− 2-Cyano-3-(5-((E)− 4-(diethylamino)− 2-(hexyloxy) styryl) thiophen-2-yl) acrylic acid (D2) having cyanoacrylic acid as an acceptor group show negative emission solvatochromism, and (E)− 2-((5-(1-Cyano-2-(4-(diethylamino)− 2-(hexyloxy) phenyl) vinyl) thiophen-2-yl) methylene) malononitrile (B1) and (E)− 2-((5-(4-(Diethylamino)− 2-(hexyloxy) styryl) thiophen-2-yl) methylene) malononitrile (B2) show positive emission solvatochromism. The D-A′-π-A type dyes (B1 and D1) show viscosity sensitive emission intensity enhancement (6-fold) in far red (FR) to near infra-red (NIR) region in the electromagnetic spectrum. The charge transfer characteristics were established through solvatochromism, polarity graphs, bond length alternation (BLA), bond order alternation (BOA), natural bond order (NBO) analysis and HOMO-LUMO energy gap. Kamlet-Taft and Catalan parameter results confirmed that dipolarity and polarizability factor are responsible for solvatochromism. These dyes exhibit higher first order hyperpolarizability than the urea. Fluorescence lifetimes are more in D-A′-π-A system compared to the D-π-A system. Due to the higher non-linear optical characters, these dyes can be considered as promising candidates for organic NLOphores.

Carbazole based NLOphoric styryl dyes- synthesis and study of photophysical properties by solvatochromism and viscosity sensitivity

We have synthesized push-pull chromophore based on carbazole moiety. The intramolecular charge transfer (ICT) of these dyes were examined by photophysical properties such as absorption, emission and fluorescence quantum yield in varying solvents of different polarities. They show absorption and emission in the range of 390–460 nm and 510–600 nm respectively. Oscillator strengths, transition dipole moments, different solvent polarity functions and frontier molecular orbital (FMO) analysis have been studied for understanding charge transfer from donor to the acceptor moiety. Density Functional Theory (DFT) revealed dyes with phenyl group in conjugation path with an acceptor (Dyes 9c and 10c) exhibited a relatively lower σ2PA cross-section in comparison to their counterparts devoid of conjugation (Dyes 9a, 9b, 10a, and 10b). Viscosity induced emission intensity enhancement has been observed for all dyes at 0–100% PEG-400 in ethanol 9c and 10c are good FMR as compared to the other due to the presence of extra-π-conjugation. Bond length, bond order alternation analysis and non-linear polarizability studies of these dyes were calculated by DFT. The spectral variations were analyzed by means of linear solvation energy relationships concept proposed by Kamlet-Taft and Catalan parameters which indicate that solvent polarizability/dipolarity play an important role as compared to solvent acidity or basicity. Thus, linear and non-linear optical properties of styryls have been determined.

Triphenylamine Derived 3-Acetyl and 3-Benzothiazolyl Bis and Tris Coumarins: Synthesis, Photophysical and DFT Assisted Hyperpolarizability Study

Triphenylamine derived bis- and tris-branched donor-pi-acceptor coumarins with acetyl and benzothiazolyl acceptors are studied for their linear and nonlinear optical properties that originate from their photophysical and molecular structure. Plots of solvent polarities versus the Stokes shift, frontier molecular orbital analysis and Generalised Mulliken Hush analysis have established their strong charge transfer character supported by the strong emission solvatochromism of these chromophores. On the basis of excited state intramolecular charge transfer, the first-, second- and third-order polarizability of these dyes are determined by a solvatochromic method and supported by density functional theory calculations using CAM-B3LYP/6-31g(d). Compared to the acetyl group, the benzothiazolyl group is a strong acceptor, and its corresponding derivatives show enhanced absorption, emission maxima and non-linear optical response. Bond length alternation and bond order alternation analysis reveals that these chromophores approach the cyanine-like framework which is responsible for maximum perturbation to produce high nonlinear optical response. Third order nonlinear susceptibility for dyes 1 and 2 is determined by Z-scan measurement. All of these methods are used to determine the nonlinear optical properties, and thermogravimetric analysis suggests that these chromophores are thermally robust and efficient nonlinear optical materials.

Fluorescent vinyl and styryl coumarins: A comprehensive DFT study of structural, electronic and NLO properties

Nonlinear optical properties of 3-styryl and 3-vinyl coumarin dyes have been investigated with Density Functional Theory (DFT) using global hybrid (GH) and range-separated hybrid (RSH) functionals. The performance of GHs - B3LYP, BHHLYP, PBE0, M06, M06L, M062X, and M06HF and RSHs - CAM-B3LYP, HISSbPBE, HSEH1PBE, wB97, wB97X, and wB97XD in combination with 6-311++G(d,p) basis set has been analyzed. Estimated (hyper) polarizability ( $$\alpha _{0}, \beta _{0})$$ obtained from the GHs - M06, M062X and PBE0 are in agreement with each other. The RSHs - wB97 and wB97X estimate very close values of $$\beta _{0}$$ . The $$\beta _{0}$$ value of 3-styryl and 3-vinyl coumarins reaches the maximum as the bond length alternation and bond order alternation parameters tend to zero. Natural bond orbital analysis shows there is extensive charge transfer in the excited state leading to large value of $$\beta _{0}$$ . The vibrational contribution to $$\alpha _{0}$$ and $$\beta _{0}$$ is significantly less when the donor is methoxy group and acceptor is nitro group in the 3-styryl coumarin. The dye, (E)-7-(diethylamino)-3-(4-nitrostyryl)-2H-chromen-2-one (3c) is found to give the highest NLO response. An increasing electrophilicity originating from the decreased HOMO-LUMO band gap leads to an increase in $$\alpha _{0} $$ and $$\beta _{0}$$ in all the cases. JCSC-D-17-00839R1 Synopsis - The NLO properties of 3-styryl and 3-vinyl coumarin dyes have been investigated thoroughly with the help of DFT using global hybrid (GH) and range-separated hybrid (RSH) functionals. The performance of GHs and RSHs in combination with 6-311++G(d,p) basis set has been analyzed. Calculated BLA/BOA index, NBO analysis, electrophilicity index and HOMO-LUMO gap are good agreement with the $$\alpha _{0}$$ and $$\beta _{0}$$ .

Investigation of NLO Properties of Fluorescent BORICO Dyes: a Comprehensive Experimental and Theoretical Approach.

BORICO dyes with N, N-diethyl as a strong donor and BF2 complexed iminocoumarin six member core as strong acceptor are investigated as an efficient non linear optical chromophores. Extended π-conjugation over iminocoumarin moiety is useful to make ICT character of BORICO dyes more significant and is established on the scale of Generalised Mulliken Hush analysis scale. Bond length alternation and bond order alternation values for three BORICO chromophores estimates the cyanine like framework for optimal non linear optical response. The frontier molecular orbital diagrams obtained from density functional theory calculations shows that there is charge transfer from donor to accepter as well as effective overlap between them making the basis for optimal NLO response of BORICO chromophores. The theoretical values of linear and non linear optical responses for three BORICO NLOphores obtained by using three different functionals B3LYP, CAMB3LYP and BHandHLYP with 6-311+g(d,p) basis set are quite consistent for the values of static dipole moment (μ), linear polarizability (α) and first hyperpolarizability (β). However in case of the γ values calculation, compare to the similar values obtained by CAMB3LYP and BHandHLYP functionals, B3LYP overestimates the same. The vibrational motions play decisive role in the overall non linear optical properties of BORICO chromophores.

Coumarin-Pyrazole Hybrid with Red Shifted ESIPT Emission and AIE Characteristics - a Comprehensive Study.

The newly synthesized three coumarin pyrazole hybrid excited state intramolecular proton transfer (ESIPT) dyes show efficient charge transfer from the pyrazole ring and the coumarin towards the electron withdrawing dicyanovinylene group as revealed from the frontier molecular orbitals. Aggregation induced emission enhancement (AIEE) studies with 2-((3-(4-hydroxy-2-oxo-2H-chromen-3-yl)-1-phenyl-1H-pyrazol-4-yl)methylene) malononitrile showed 9 fold increase in the emission enhancement in 90% DMF-H2O mixture. Lippert-Mataga theory explained the solvatochromic behavior of the dyes in various solvents. The charge transfer characteristics and non-linear optical (NLO) properties have been supported and correlated with bond length alternation, bond order alternation and vibrational spectrum. As values of bond order alternation (BOA) tend to be more towards negative and as the value of α increases β decreases while the values of γ depends on the values of α and β. The observed values of γ are positive which revealed that β contributes significantly. The dyes exhibit linear and NLO properties superior to urea. (E)-2-(3-(2-(3-(4-Hydroxy-2-oxo-2H-chromen-3-yl)-1-phenyl-1H-pyrazol-4-yl)vinyl)-5,5-dimethylcyclohex-2-en-1-ylidene) malononitrile shows enhanced linear and non-linear properties among the three dyes.

Distinguishing the Effects of Bond-Length Alternation versus Bond-Order Alternation on the Nonlinear Optical Properties of π-Conjugated Chromophores.

Understanding the relationships between the molecular nonlinear optical (NLO) properties and the bond-length alternation (BLA) or π-bond-order alternation (BOA) along the molecular backbone of linear π-conjugated systems has proven widely useful in the development of NLO organic chromophores and materials. Here, we examine model polymethines to elucidate the reliability of these relationships. While BLA is solely a measure of molecular geometric structure, BOA includes information pertaining to the electronic structure. As a result, BLA is found to be a good predictor of NLO properties only when optimized geometries are considered, whereas BOA is more broadly applicable. Proper understanding of the distinction between BLA and BOA is critical when designing computational studies of NLO properties, especially for molecules in complex environments or in nonequilibrium geometries.

Ferroelectric states induced by dimer lattice disorder in dimer Mott insulators

We theoretically investigate the dielectric properties of dimer Mott insulators. We adopt the two-dimensional quarter-filled extended Hubbard model for $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$$X$ that includes the coupling with the dimer lattice modes, and numerically calculate the ground state for the bond-length-alternated and disordered lattices. Electric dipole moments in dimers are induced by interdimer bond-order alternation, and electric dipole moments are aligned perpendicular to the direction of bond-order alternation. As a result, the electric polarizations arising from electric dipole moments in dimers are strongly coupled with the dimer lattice modes of interdimer bond-length alternation, and the more stable one of these two ferroelectric states is induced by disorder in dimer lattice. The ferroelectric states arise from the hybridization of in-gap charge excited states to the dimer Mott ground state for the equilibrium lattice, and one (the other) is strongly stabilized near the phase boundary with the metallic phase (in the spin-liquid phase). The present results are consistent with the experimentally observed dielectric anomaly in $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu${}_{2}$(CN)${}_{3}$.

Theoretical investigation of the origin of the large non-linear optical response in acceptor-substituted carotenoids

Correlated semiempirical quantum-chemical calculations are performed to rationalize the origin of the large first- ( α), second- ( β), and third- ( γ) order molecular polarizabilities found in carotenoid-like molecules capped at a single end by an acceptor group. The ground state polarization in the apocarotenal H(-CHCH-) 10CHC(CN) 2, is made to vary via the application of an external homogeneous electric field. The important structural characteristics found for this long polymethine are (i) the relative instability of the cyanine-like (equal-bond length) structures and (ii) the double-step evolution of the dipole moment caused by the transfer of a first and then a second electron towards the acceptor end of the molecule as the electric field increases in the range 10 7–10 8 V/cm. The abruptness of these steps results in very large values of the molecular polarizabilities. The longitudinal components of the molecular polarizabilities are analyzed as a function of the bond-order alternation (BOA) parameter.

Nonlinear optical response in acceptor-substituted carotenoids: A theoretical study

Correlated semi-empirical quantum-chemical calculations are performed to rationalize the origin of the large first- (α), second- (β), and third- (γ) order molecular polarizabilities in carotenoidlike molecules capped at a single end by an acceptor group. The degree of ground-state polarization of the chromophores is made to vary via the application of an external homogeneous electric field in the range 107–108 V/cm. The most interesting feature is a stepwise evolution of the longitudinal component of the dipole moment with field, caused by charge transfer toward the acceptor end of the molecule. Since the steps are abrupt, this results in very large values of the molecular polarizabilities, in accordance with the derivative relationships among the dipole moment and α, β, and γ. The longitudinal components of the molecular polarizabilities are analyzed as a function of both the external field and the bond-order alternation (BOA) parameter. The ability of simple models to describe the evolution of the molecular polarizabilities is assessed.