Electrooptical Absorption Measurements Research Articles

Polarity and strong sensitivity to external electric field in azacrown oligophenylenevinylene

Complex study of quadrupolar azacrown dye (E,E)-5,5́-Bis[2-(4-(4′,7′,10′,13′,16′-pentaoxa-1 azacyclooctadecyl)phenyl)ethenyl]-2,2́-bipyridine 1 was performed. Electronic spectra of absorption and fluorescence in different solvents exhibit strong solvatochromism. Electrooptical absorption measurements (EOAM) were performed to determine the electric dipole moments. These measurements gave large values of dipole moments in the ground μg and Franck–Condon excited state μeFC equal to 6.8 ± 0.14C m and 39.3 ± 0.3C m, respectively. Furthermore, the results of EOAM suggest the existence two conformers in the ground state with close energies of electronic transitions.Density functional theory (DFT) calculations directly show that the shape of this molecule is not planar in the ground state and also allows the existence of two stable conformers with close energies. They appeared due to different orientations of the left and right pyridine fragments of the solute.The energies, electric dipole moments and dependences of dipole moments on the strength of applied electric field were calculated for found stable conformers of 1. DFT calculations with TD / B3LYP / 3-21G and cc-pVDZ (Time Depend) approach show that external electric field increases dramatically the dipole moments of the solute under study. The higher field intensity the larger the excited electric dipole in the range intensities from zero to ∼ 2.8·× 10 9 V/m.

Solvatochromy and symmetry breaking in two quadrupolar oligophenylenevinylenes

Electrooptical absorption measurements (EOAM), solvatochromic dependences and quantum chemical simulations testify to large dipole moments change of two quadrupolar oligophenylenevinylenes upon transition to Franck–Condon excited state μeFC. The values of the dipole moments μg and μeFC are in the range [(4.2 – 4.9)1030] C m and (30.8 – 47.0)1030C m, respectively. The relations of dipole moments in the ground and excited states determined by EOAM correlate well with results obtained via the solvatochromic method. Calculations carried out by density functional theory (DFT) show that optimized configuration of the ground state of these molecules is not planar. The results from all methods applied unequivocally show the structural symmetry breaking in the studied compounds.

Specific orientation of dipole moments in azocrown cetocyanine dyes determined by electrooptical absorption measurements (EOAM)

From electrooptical absorption measurements (EOAM) follows that the dipole moments of azacrown ketocyanine dyes 2,5-di{(E)-1-[4-(4,7,10,13-tetraoxa-1-azacyclopentadecyl)phenyl]methylidene}-1-cyclopentanone (Compound 1) and (E)-1-(2-hydroxy-4methoxyphenyl)-3-[4-(4,7,10,13-tetraoxa-1-azacyclopentadecyl)phenyl]-2-propen-1-one (Compound 2) in the equilibrium ground state μg and the change of dipole moments upon transition to the excited Franck–Condon state Δaμ are large. This alteration causes a significant long-wavelength shift of the absorption and emission spectra, as well as large fluorescence Stokes shift with increasing polarity of the solvent.From the DFT calculations follows that the terminal groups of the dyes are located outside the plane of the central part of the molecules by the torsion angles that allow good conjugation of the nitrogen atoms lone pairs with π-system of the molecule. The transition dipole moment of Compound 1 ma is perpendicular to the dipole moment μg in the equilibrium ground state and for Compound 2 ma is parallel to μg. This is due to the differences in the geometric structure of the dyes.The distribution of fluorescence lifetime of Compound 1 significantly depends on the polarity of the solvent. Thus, ketocyanine dyes with azacrown cycles can serve not only as ion indicators but also as fluorescent probes for studying the polarity of the environment.

Structural symmetry breaking in octupolar tetrastyrylpyrazines and their dipole moments in equilibrium ground and Franck-Condon excited state

From electrooptical absorption measurements (EOAM) follows that the dipole moment of octupolar tetrastyrylpyrazines in the equilibrium ground state is large and that the change of dipole moments upon transition to the excited Franck–Condon state is significant. Obtained results unambiguously testify to structural symmetry breaking in the studied octupolar tetrastyrylpyrazines. Molecular mechanics and semi-empirical calculations evidence that tetrastyrylpyrazines have non-planar configurations. The non-planar geometry of the molecules causes large μg values and significant change of dipole moment Δaμ after excitation.Due to large μg and Δaμ values the maxima of the first absorption band and fluorescence band of the studied tetrastyrylpyrazines in various solvents shift to the long-wavelength region with an increasing solvent polarity. Both absorption and fluorescence spectra of the dyes do not change the shape during bathochromic shift in the solvents. This phenomenon evidences about the defining role of universal van der Waals interactions of dyes with the environment, namely dispersive, orientational, and inductive nature.

Electrooptical Absorption Measurements (EOAM) Testify Existence of two Conformers of Prodan and Laurdan with Different Dipole Moments in Equilibrium Ground and Franck-Condon Excited State.

The results from the electrooptical absorption measurements (EOAM) on the equilibrium ground and excited Franck-Condon state dipole moments of Prodan and Laurdan in 1,4-dioxane are presented. As follows from experiments Prodan and Laurdan in the equilibrium ground and excited Franck-Condon state have two conformers with considerably different dipole moments. The electrical dipole moments and the transition dipole moment, obtained from the short-wavelength region of the absorption spectrum are parallel. The electrical dipole moments measured at the long-wavelength spectral region are parallel to each other but not parallel to the transition dipole moment m a. The angle θ between the transition dipole moment m a and the dipole moment in the equilibrium ground state μ g of the long-wavelength conformer is about 30(0) for both probes. Obtained results evidence that donor-acceptor pairs of the short-wavelength and long-wavelength conformers are not located on the same axis. Two low-energy conformers of Prodan have been found by density functional theory (DFT) calculations, differing in the orientation of the carbonyl group towards the naphthalene system.

Electro-optical properties from CC2 Calculations: A comparison between theoretical and experimental results

The coupled cluster CC2 method has been employed for the computation of dipole moments and static first order hyperpolarizability of a large size push–pull molecule with an extended π electron system. The results are critically compared with the outcomes of electro-optical absorption measurements. CC2 ground and excited state dipole moments are slightly overestimated with respect to the experimental ones, but their differences are well reproduced; good agreement has also been found for absorption wavelengths. Sum of state computations of the static first hyperpolarizability highlight the importance of at least two excited states.

Molecular hyperpolarizabilities of push–pull chromophores: A comparison between theoretical and experimental results

Electric dipole moments and static first order hyperpolarizabilities of two push–pull molecules with an extended π electron systems have been evaluated at different computational levels and compared with the results of electro-optical absorption measurements, based on the two state model. Calculations show that: (i) the dipole moments of such elongated systems depend significantly on conformation, a thorough conformational search is necessary for a meaningful comparison between theoretical and experimental results; (ii) DFT methods, in particular CAM-B3LYP and M05-2X, yield dipole moments which compare well with those obtained by post Hartree–Fock methods (MP2) and by EOA measurements; (iii) theoretical first order hyperpolarizabilities are largely underestimated, both by MP2 and DFT methods, possibly because of the failure of two state model used in electro-optical measurements.

Localized excitation effect on dipole moments of oligophenylenevinylenes in their excited Franck–Condon state

The results from electrooptical absorption measurements (EOAM) on the ground and excited Franck–Condon state dipole moments of oligophenylenevinylenes in 1,4-dioxane and cyclohexane are presented. The dipole moments of quadrupolar dyes in the ground state are not equal zero. This means that one of the functional end groups of these molecules is not on the axis or on the plane with the central part of the molecule. Such a conclusion is confirmed by the quantum-chemical and semi-empirical recalculations. Owing to disorder in the ground-state geometry of the dyes the π–π conjugated system is located on mostly the plane part of the molecule and in the main this part absorbs the light. The localized excitation effect causes the large change of the dipole moments of quadrupolar molecules upon excitation to the Franck–Condon excited state. Due to an efficient intramolecular charge transfer the new oligophenylenevinylenes must have significant non-linear optical properties.

Nonlinear optical properties of pyridinium-betaines of squaric acid: Experimental and theoretical study

The linear and nonlinear optical properties of six pyridinium-betaines of squaric acid were characterized by means of electro-optical absorption measurements (EOAM) in dioxane solution. All chromophores studied exhibit intense absorption bands in the visible region within 372–441 nm, accompanied with decrease in dipole moment upon excitation. The static hyperpolarizabilities of the NLO-phores studied depend strongly on the substituent in the pyridinium ring and increase significantly going from donor to acceptor substituent. Specifically, the noncentrosymmetrically crystallizing 4-benzoyl compound showed the highest static hyperpolarizability. These empirical trends were supported and verified by quantum-chemical calculations (DFT and ab initio RHF). Bulk crystal properties such as decomposition points within the high temperature range 480–665 K and improved thermal stability in melt demonstrate the advantage of these materials for nonlinear optical applications.

Effect of Polarity and Structural Design on Molecular Photorefractive Properties of Heteroaromatic‐Based Push–Pull Dyes

A combined experimental (optical and electro-optical absorption measurements) and computational (ab initio RHF and DFT) approach has been used to investigate the molecular low- and high-T(g) photorefractive (PR) performances of neutral and zwitterionic heteroaromatic dipolar chromophores in terms of structural and solvent-polarity effects. We have found that the nature of the building units (donor, acceptor, and spacer) and the polarity of the surrounding medium strongly affect all the relevant ground-state and nonlinear optical properties involved in the PR activity, that is, the dipole moment, the polarizability anisotropy, and first hyperpolarizability of the electronic ground-state. The variation of these properties is in turn transferred to molecular low- and high-T(g) PR figures of merit. It is shown that PR molecular performance not only relies on a proper choice of structural components but varies by orders of magnitude as a function of the medium polarity, and this suggests that a combination of molecular design and host-matrix engineering is required for optimized performances of PR materials.

Molecular Stark-effect spectroscopy of Prodan and Laurdan in different solvents and electric dipole moments in their equilibrated ground and Franck–Condon excited state

The results from electrooptical absorption measurements (EOAM) on the ground and excited Franck–Condon state dipole moments of Prodan and Laurdan in 1,4-dioxane and cyclohexane are presented. The ground and excited Franck–Condon state electric dipole moments as well as the respective transition moment of both probes are parallel. The electric dipole moments of Prodan and Laurdan in the ground state in cyclohexane and 1,4-dioxane have values within the range (15.7–16.5) × 10 −30 C m. On optical excitation the dipole moments increase by (42.1–49.5) × 10 −30 C m. The obtained results are compared with the values of the dipole moments of Prodan and Laurdan determined by other methods.

Electrochemistry and Photophysics of Donor‐Substituted Triarylboranes: Symmetry Breaking in Ground and Excited State

We synthesized a series of amino substituted triarylboranes (TABs) 1-3 by copper(I)-catalyzed cross-coupling reactions. The title compounds were investigated by means of cyclic voltammetry (CV) and UV-visible absorption and fluorescence spectroscopy. Electrochemical oxidation of tris(4-carbazolyl-2,6-dimethylphenyl)borane (3) leads to the formation of an electroactive polymer film on the electrode surface. The charge-transfer (CT) absorption band of all three TABs shows a pronounced negative solvatochromism, while the emission is positively solvatochromic. By combining Jortner's theory, AM1 computations, and electrooptical absorption measurements (EOAM), this unexpected behavior was shown to be due to a dipole inversion upon S0-->S1 excitation. Furthermore, polarized steady-state fluorescence spectroscopy and EOAM prove that the ground-state geometry of 3 is of lower symmetry than D3 and that the excitation energy can be transferred from one subchromophore to another within the lifetime of the excited state. Exciton-coupling theory was used to quantitatively analyze this excitation transfer.

Photochemical Charge Separation within Aromatic Hydrazines and the Effect of Excited-State Intervalence in Dihydrazines

Photolysis into the longest wavelength absorption band of 2-tert-butyl-2,3-diazabicyclo[2.2.2]oct-3-yl hydrazine (Hy) substituted naphthalenes causes aryl group reduction electron transfer to give (+)Hy-Ar(-). Electrooptical absorption measurements characterize the charge separation properties from these bands. Emission studies demonstrate that the separation between absorption and emission maxima for symmetrically disubstituted compounds is smaller than that for monosubstituted compounds, which is attributed to excited-state intervalence. The excited-state diabatic surfaces may be described as a Hy(+)-NA(- )-Hy(0), Hy(0)-NA(-)-Hy(+) pair, for which electronic interaction produces a double minimum that qualitatively resembles that in the ground state of the disubstituted intervalence radical cations.

Dipole Moments of 4?-Aminoflavonol Fluorescent Probes in Different Solvents

Electrooptical absorption measurements (EOAM) were used to measure the dipole moments of the normal form of 4'-(dimethylamino)-3-hydroxyflavone (FME), and 4' N-(15-azacrown-5)-3-hydroxyflavone (FCR) in 1,4-dioxane, toluene, and cyclohexane. With these probes excited-state intramolecular proton transfer (ESIPT) takes place. For comparison, the dipole moments of 4'-(dimethylamino)-3-metoxyflavone (FME3ME), for which ESIPT is lacking, were measured, too. For all three probes the ground (micro(g)) and excited Franck-Condon state (micro(e)FC) electrical dipole moments are parallel to each other and also parallel to the transition dipole moment. The electrical dipole moments of FCR, FME, and FME3ME in their ground state have values within the range (12.0-17.7) x 10(-30) C m. Upon optical excitation, the dipole moments increase by (41.9-52.9) x 10(-30) C m. The value of the change of the dipole moment vector delta(a)micro with excitation to the Franck-Condon state and the value of the vector micro(e)FC for FCR and FME are practically independent on the solvent polarity. From this point of view and due to large values of the dipole moments FCR and FME are very promising probes for the investigation of the distribution of the local polarity in biological systems using site-selective excitation of the different sites. Our steady-state fluorescence studies on FME and FCR have demonstrated a high spectral sensitivity of the normal form to such solvent characteristics as polarity.

Off-diagonal second-order polarizability of N,N′-dihexyl-1,3-diamino-4,6-dinitrobenzene

The second-order polarizability components β zzz , β zyy and β yzy = β yyz of N,N′-dihexyl-1,3-diamino-4,6-dinitrobenzene (DHADNB) have been determined by combination of polarization dependent electric-field-induced second-harmonic generation (EFISHG) and hyper-Rayleigh scattering (HRS) experiments in dioxane solution at T=298 K. Dipole moments of the first three optically allowed excited singlet-states were obtained from electrooptical absorption measurements (EOAM) in methylcyclohexane solution. The results show that the second-order polarizability of this two-dimensional (2D) non-linear optical (NLO) chromophor is completely dominated by the off-diagonal components β zyy and β yzy = β yyz , whereas the contribution of β zzz is nearly insignificant. Independent quantum chemical calculations confirm these findings. Although DHADNB is a dipolar molecule, non-dipolar three-level contributions to β are found to be much larger than dipolar two-level contributions.

A distinctive example of the cooperative interplay of structure and environment in tuning of intramolecular charge transfer in second-order nonlinear optical chromophores.

The strongly enhanced cooperative influence of medium polarity and organic structural design on the first hyperpolarizability beta of a novel family of highly polarizable azinium-(CH=CH-thienyl)-dicyanomethanido chromophores 1-3 is described. The dyes can be efficiently synthesized by regioselective protonation/alkylation of the corresponding bidentate anion precursors. Consecutive annelation of the pyridyl ring of 1 (pyridine-->quinoline-->acridine) and medium polarity effects are responsible for an extraordinarily variable range of intramolecular charge transfer (ICT), leading to a large set of pi-electron distribution patterns. Accordingly, systems with remarkably different zwitterionic/quinoid character in the ground and excited states present beta values in a broad range, eventually switching from negative to positive. Our investigation is based on a combination of experimental (UV/Vis spectroscopy, multinuclear NMR spectroscopy, and electrooptical absorption measurements) and computational (ab initio) approaches. It is shown that: 1) beta and mubeta are dramatically influenced, even by orders of magnitude, by a complex, non-monotonic interplay of structure and medium action, which in turn affects molecular ICT and bond length alternation (BLA), 2) the computations, validated by different experimental data, are to be recommended as an extremely useful tool in the search for a greatly improved set of molecular nonlinear optical (NLO) responses (in the case of 1-3 they show that such conditions may be attained only in a narrow and limited range of dielectric constants in which the annelation effect operates most efficiently), and 3) the search for the most favorable molecular NLO response of a highly polarizable chromophore both in solution and in solid matrices should simultaneously take into account not only the molecular design supplemented by annelation effects but also the polarity of the medium.

Dipole moments of 4′-aminoflavonols determined using electro-optical absorption measurements or molecular Stark-effect spectroscopy

The electro-optical absorption measurements (EOAM) were used to measure the dipole moments of the normal form of 4′-(methoxy)-3-hydroxyflavone (FOM), 4′-(dimethylamino)-3-hydroxyflavone (FME), and 4′- N-(15-azacrown-5)-3-hydroxyflavone (FCR). For these probes the excited state intramolecular proton transfer (ESIPT) takes place. For comparison, the dipole moments of 4′-(dimethylamino)-3-methoxyflavone (FME3ME), for which ESIPT is lacking, were measured. In the case of FCR, FME, and FME3ME the equilibrated ground ( μ g) and excited Franck–Condon state ( μ e FC ) electrical dipole moments are parallel to each other and also parallel to the transition dipole moment. The electrical dipole moments of FCR, FME, and FME3ME in their ground state have values within the range (14.1–17.7)×10 −30 C m. Upon optical excitation, the dipole moments increase by (47.1–50.4)×10 −30 C m. Among the studied flavonols, μ g and μ e FC decrease in the order FME3ME>FCR>FME. This effect can be explained by the existence of an intramolecular H-bond for FCR and FME, which partly compensates a negative charge on the carbonyl oxygen in the ground state and thus diminishes the electron-donating ability of the amino-group in the excited Franck–Condon state. For FOM the equilibrated ground and excited Franck–Condon state dipole moments of the normal form have values of 9.4×10 −30 C m and 26.3×10 −30 C m, respectively. The smaller values of the dipole moments for FOM, in comparison with other flavonols, is caused by the absence in FOM of a strong electron-donating group, such as the amino-group. The dipole moments are not parallel and the angles between the different dipole moments are in the range 4–55°.

Dimerization of merocyanine dyes. Structural and energetic characterization of dipolar dye aggregates and implications for nonlinear optical materials.

Aggregation of polar merocyanine dyes has been identified as an important problem in the fabrication of organic materials for photonic applications. In this work, a series of merocyanine dyes is synthesized, and their aggregation is investigated by a combination of several experimental techniques to reveal structure-property relationships. These studies provide clear evidence for the formation of centrosymmetric dimers for all investigated merocyanines in concentrated solution and in the solid state. The thermodynamics of dimerization in liquid solution is studied by concentration-dependent permittivity measurements, UV-vis spectroscopy, and electrooptical absorption experiments. A centrosymmetric dimer structure with antiparallel ordering of the dipole moments is observed in solution by 2D NMR spectroscopy as well as in the solid state by X-ray crystallography and interpreted in terms of dipolar and pi-pi interactions. The optical properties of the dimer aggregates are satisfactorily explained by an excitonic coupling model. The effect of an external electric field on the dimerization equilibrium is considered and quantitatively determined by electrooptical absorption measurements. Implications of the observed findings on the design of nonlinear optical and photorefractive materials are discussed.

Donor-acceptor oligoenes with a locked all-trans conformation: synthesis and linear and nonlinear optical properties.

A general synthetic approach to variously polarised merocyanines and a cyanine with enhanced thermal and (photo)chemical stability by a locked all-trans conformation (derived from a rigidified hexatriene unit and a variety of common donor and acceptor groups) is presented as well as a systematic study of their (non)linear optical properties. Apart from the UV/Vis absorption and fluorescence behaviour, the ground- and excited-state dipoles, the first-, second- and third-order molecular polarisabilities were determined by electro-optical absorption measurements (EOAM) and degenerate four-wave mixing (DFWM) techniques in solution. Large values for the second- and third-order polarisability up to beta(0)=461x10(-50) C V(-2) m(3) (1242x10(-30) esu) and [gamma(LL)]=183x10(-60) C V(-3) m(4) (15x10(-34) esu) were found. The linear and nonlinear optical properties were related to the ground-state polarisation and the resonance structure of the chromophores. In order to reveal the influence of the length of the polymethinic chain (number of pi electrons within the chromophore), some lower homologues shortened by one C=C (double) bond were also taken into account. The unexpectedly high gamma values of some of the merocyanines cannot be explained by a two-level model. Molecular vibrational third- order polarisabilities (calculated from absolute Raman intensities in solution) were qualitatively correlated to the DFWM results. Furthermore, the dependence of the (13)C NMR chemical shifts of the polymethinic carbons within the merocyanines upon ground-state polarisation was investigated and compared to those within a corresponding cyanine.

Ground- and First-Excited-Singlet-State Electric Dipole Moments of Some Photochromic Spirobenzopyrans in Their Spiropyran and Merocyanine Form

The dipole moments of four photochromic derivatives of indolinospirobenzopyran have been studied in their spiro and their open merocyanine form, in dioxane. Determinations have been done by electrooptical absorption measurements (EOAM). To prepare either a pure spiropyran or a pure merocyanine form solution, the experimental setup had to be modified by addition of a second light beam. In this way thermal equilibration between the spiropyran and the merocyanine form and switching from one form to the other induced by the measuring light beam could be avoided. The ground-state dipole moment of the spiro form amounts to (10−15) × 10-30 C m and increases strongly to about 60 × 10-30 C m with the excitation to the first excited singlet state. The ground-state dipole moment of the merocyanine form is much larger and amounts to (50−60) × 10-30 C m. In contrast to the spiro form it decreases to about 40 × 10-30 C m, when exciting to the first excited singlet state of the merocyanine form.