Electrooptical Absorption Measurements Research Articles

Figures of merit of push–pull molecules in photorefractive polymers

Push–pull molecules are the most widely used nonlinear optical chromophores incorporated in polymers to generate photorefractivity. They are at the origin of the Pockels and the Kerr electrooptic effects in high-Tg photorefractive polymers and give also rise to the optical birefringence in low-Tg photorefractive polymers. High-Tg and low-Tg figures of merit characterize the efficiency of push–pull molecules to generate phototrefractivity. We present, here, the two-level figures of merit of various push–pull molecules determined by using electrooptical absorption measurements in solution. Analysis of the results within the framework of the two-form two-state model provides useful guidelines for the design of push–pull molecules for photorefractive applications. This model enables us to evaluate the optimum structures as well as the corresponding maximum optical birefringence, Pockels and Kerr electrooptic responses. It also shows how these maxima are related. Tuning a molecular structure to reach an optimum one will not automatically lead to a very large figure of merit. An essential parameter that is usually overlooked is the magnitudes of these maxima. We show that the maxima are affected both by the conjugation length and by the choice of the donor/acceptor substituents. Some of the longest compounds investigated here exhibit very large high-Tg and low-Tg figures of merit.

Highly transparent and birefringent chromophores for organic photorefractive materials

A series of chromophores for application in organic photorefractive (PR) materials is investigated by electro-optical absorption measurements (EOAM). This experimental technique yields information on the transition dipole moment μ ag, the ground-state dipole moment μ g, and the change of the dipole moment upon optical excitation Δ μ within the intense charge transfer (CT) band of the dyes. It is shown that the results of the EOAM experiment allow us to estimate the PR figures-of-merits (FOMs) of the chromophores by either perturbational two-level equations or Kramers–Kronig transformation. In particular, chromophores based on the heterocyclic dihydropyran and dihydropyridine groups in combination with dicyano and cyanocarboxy acceptor units were investigated. These donor–acceptor pairs yield chromophores close to the `cyanine limit' that is characterized by a small dipole difference, but a large ground-state dipole moment and a large polarizability anisotropy. This leads to very high PR FOMs of the new PR chromophores that are demonstrated to be superior to conventional second-order nonlinear optical (NLO) chromophores in situations where the medium has a low glass transition.

Elongated push–pull diphenylpolyenes for nonlinear optics: molecular engineering of quadratic and cubic optical nonlinearities via tuning of intramolecular charge transfer

Push–pull polyenes are of particular interest for nonlinear optics (NLO) as well as model compounds for long-distance intramolecular charge transfer (ICT). In order to tune the ICT phenomenon and control the linear and nonlinear optical properties, we have synthesized and investigated several series of soluble push–pull diphenylpolyenes of increasing length and having various donor (D) and acceptor (A) end groups. Their linear and NLO properties have been studied by performing electro-optical absorption measurements (EOAM) and third-harmonic generation (THG) experiments in solution. Each push–pull molecule exhibits an intense ICT absorption band in the visible characterized by an increase in dipole moment upon excitation (Δ μ). By lengthening the polyenic chain, linear increases in excited-state dipoles are achieved while the ground-state dipole is maintained constant. The polyenic chain thus allows for long-distance intramolecular charge transfer. In addition, tuning of the magnitude of the ICT phenomenon and of the nonlinear responses is achieved by varying the donor and acceptor end groups: increasing the D/A strength leads to concomitant enhancements of Δ μ, quadratic ( β) and cubic ( γ) hyperpolarizabilities values. Giant Δ μ values (up to more than 30 D) and enhanced non-resonant quadratic and cubic NLO molecular responses (i.e. β(0)=500×10 −30 esu and γ(0)=8000×10 −36 esu) have been attained while maintaining suitable solubility, stability and transparency.

Spectral and electrooptical absorption and emission studies on internally hydrogen bonded benzoxazole `double' derivatives: 2,5-bis(benzoxazolyl)hydroquinone (BBHQ) and 3,6-bis(benzoxazolyl)pyrocatechol (BBPC). Single versus double proton transfer in the excited BBPC revisited

Ground and excited state dipole moments and polarizabilities of 2,5-bis(benzoxazolyl)hydroquinone (BBHQ) and 3,6-bis(benzoxazolyl)pyrocatechol (BBPC) are determined by means of electrooptical absorption and emission measurements. BBHQ is found to exhibit a small, while BBPC a large increase of the static polarizability in the Franck–Condon (FC) excited singlet state. The change of the dipole moments upon excitation to the FC state is zero within experimental error. However, both molecules show dipole moments in the fluorescent states of their phototautomers, of about 5 D, the major component being parallel to the long molecular axis. The experimental and theoretical results strongly suggest that the fluorescent species of BBHQ and BBPC are monoketo-tautomers created by single proton transfer in the course of the excited state relaxation. For BBPC this was confirmed by a comparative photophysical study with its monomethoxy-derivative (MeBBPC), in which one active proton is replaced by the CH 3 group. These results lead to the revision of previous conclusions [Grabowska et al., Chem. Phys. Lett. 177 (1991) 17] stating that BBPC undergoes a double proton transfer in the excited state. For BBHQ the previously found single proton transfer mechanism of tautomerization is fully confirmed.

Enhanced nonlinear optical properties and thermal stability of donor-acceptor substituted oligothiophenes

Linear and nonlinear optical properties of a series of novel donor-acceptor substituted α-oligothiophenes were investigated by means of electrooptical absorption measurements (EOAM) and electric field induced second harmonic generation (EFISH). The second-order polarizabilities β(−2ω; ω, ω) were related to dipole changes Δμ ag and transition dipoles μ ag associated with low-lying charge-transfer (CT) excitations by using the perturbational two-level approximation. Systematic variation of the donor and acceptor groups led to compounds with exceptional nonlinearity and thermal stability. Too strong donor/acceptor pairs, however, yielded structures in the charge-resonance (CR) limit with small or vanishing Δμ ag and β.

Intramolecular charge transfer and enhanced quadratic optical non-linearities in push pull polyenes

Push-pull polyenes, which have an electron-donating group (D) and an electron-withdrawing group (A) grafted on opposite ends of a conjugated polyenic chain, are of particular interest as model compounds for long-distance intramolecular charge transfer (ICT), as well as potent non-linear optical chromophores. Several series of push-pull polyenes of increasing length, combining aromatic donor moieties and various acceptor groups, have been prepared and studied. Their linear and non-linear optical properties have been investigated by performing electro-optical absorption measurements (FOAM) and electric-field-induced second-harmonic generation (EFISH) experiments in solution. Each molecule shows a broad and intense ICT absorption band in the visible associated with an increase in the dipole moment (Δμ). Lengthening the polyenic chain linking the D and A groups results in a bathochromic shift of the ICT absorption band and induces a linear increase in the excited state dipole. In contrast, the ground state dipole remains roughly constant. As a result, the longest molecules exhibit huge Δμ values (up to 42 D) as well as markedly enhanced quadratic hyperpolarizabilities (β). In addition, the nature of the end groups has been found to influence strongly both the ICT and optical non-linearities: larger β and Δ μ values, as well as steeper length dependences, are obtained with push-pull phenylpolyenes bearing strong acceptors.

Quadratic hyperpolarizability of push—pull polyenes: a two-form analysis of the length effect

We present a two-form two-state description that generalizes to all push—pull molecules the bond length alternation ( BLA) approach. All relevant parameters in our model can be derived from electrooptical absorption measurements. Within this framework, we analyse the length effect on the quadratic hyperpolarizability in two series of push—pull polyenes.

Intramolecular charge transfer in elongated donor-acceptor conjugated polyenes

Intramolecular charge transfer (ICT) has been investigated by electro-optical absorption measurements for two series of donor-acceptor polyenes of increasing length. Each molecule undergoes an increase in dipole moment upon photoexcitation. The experimental results indicate that lengthening the polyenic chain linking the donor and acceptor end groups results in a bathochromic shift of the ICT absorption band and induces marked increases of both excited state and transition dipoles, whereas the ground-state dipole is relatively unaffected. As a result, marked photoinduced changes in the dipole moment (up to 33 D) were obtained with the longest molecules (up to 27 Å) leading to highly polar excited states. Elongated donor—acceptor polyenes are therefore particularly interesting compounds for long-distance and low-energy intramolecular charge transfer.

Dipole moments of aminophthalimides determined by modified electro-optical absorption and emission measurements

Modified electro-optical absorption and emission methods were used to measure the electric dipole moments of five aminophthalimides in their ground and excited states. The equilibrated ground and excited state dipole moments measured by these methods are in reasonable agreement with those derived from other measurement techniques, confirming the general validity of these methods. A difference is observed between the properties of 3-aminophthalimide (3AP) and 4-amino- N-methylphthalimide (4ANMP) in different solvents. The equilibrated dipole moment of 3AP in its excited state is practically independent of the solvent polarity in contrast with 4ANMP. The possible mechanism of this effect is discussed.

Deviations from Kleinman symmetry of the second-order polarizability tensor in molecules with low-lying perpendicular electronic bands

4-Nitroaniline (pNA) and 3,5-dinitro-aniline (3,5-DNA) were studied by electric field induced second harmonic generation (EFISH) and electrooptical absorption measurements (EOAM) in dioxane solution at T = 298 K. The measurements were carried out for two polarization conditions, the electric field vector of the linearly polarized incident light being parallel or perpendicular to the static electric field. The second-order polarizability tensor of pNA is confirmed to be Kleinman symmetric, while a strong deviation from index permutation symmetry is observed for 3,5-DNA. The latter is quantitatively interpreted by contributions of a low-lying electronic band with transition dipole perpendicular to the symmetry axis. New design possibilities for second-order nonlinear optical materials are pointed out.

Second-order polarizability of donor—acceptor substituted oligothiophenes: substituent variation and conjugation length dependence

Second-order polarizabilities and excited state dipole moments of a number of donor—acceptor substituted α-oligothiophenes have been investigated by use of electric field induced second harmonic generation (EFISH), solvatochromism and electrooptical absorption measurements (EOAM). The experimental results reveal that oligothiophenes are very effective conjugation units offering substantial enhancement of the second-order polarizability with increasing chain length.

Photoinduced charge transfer as revealed by ground and excited state dipole moments

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Photoinduced charge separation and broken symmetry in Franck-Condon excited ππ'* states of tetraphenylpentatetraenes

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Ground- and excited-state dipole moments of some nitroaromatics: Evidence for extensive charge transfer in twisted nitrobenzene systems

Electro-optical absorption measurements have been made on four model nitroaromatics to determine the effect of twisting of the donor–acceptor single bond on the charge-transfer characteristics in the Franck–Condon excited states. Observed ground- and excited-state dipole moments of nitromesitylene, which has been treated experimentally as the nonplanar analogue of planar nitrobenzene, indicate that electronic excitation of twisted nitrobenzene results in a nearly full unit charge transfer from donor (benzene) to the acceptor (nitro) group (Δμ=18.3 D). On the other hand, in planar nitrobenzene and nitronaphthalene the charge transfer is more delocalized over the whole molecular skeleton, resulting in normal changes in dipole moment (Δμ=5–10 D). In the analogous anthracene system, i.e., 9-nitroanthracene, the charge transfer upon electronic excitation is extremely low (Δμ=1.7 D), which is reflected by its very small change in the dipole moment. Therefore, it is evident that the charge-transfer processes in the twisted molecules are quite different for different aromatic ring systems. Simple molecular-orbital calculations satisfactorily explain the reason for such differences on the basis of their highest occupied molecular-orbital (HOMO) and lowest unoccupied molecular-orbital (LUMO) characteristics. Transition moment directions have also been obtained experimentally and compared with the theoretically predicted directions based on the symmetry properties of the HOMO and LUMO. Agreement is found in all cases studied.

Excited state dipole moments and polarizabilities of centrosymmetric and dimeric molecules. I. Model study of a bichromophoric molecule

Refractometric, dielectric and electro-optical absorption measurements are reported for 1-dimethylamino-2,6-dicyano-4-methyl-benzene (I) and 1,4-bis(4′-dimethylamino-3′,5′-dicyanophenyl)bicyclo[2.2.2]octane (II). The evaluation leads to dipole moments and polarizabilities of the ground state as well as the first dipole allowed singlet state. The experimental res excellently substantiate the method of electro-optical absorption measurements in solution. It is shown that the excited dimer wavefunctions of the bichromophoric molecule II localize by solvent induced local site perturbations.

The Determination of Molecular Quantities from Measurements on Macroscopic Systems. I

Abstract For many molecules the determination of molecular quantities, as electric dipole moments or molar absorption coefficients, has to be based on measurements on solutions or other dense phases. The evaluation of such data is rather involved due to the interactions between neighboring molecules, and taking this into account, a general method is presented. Some quantities appropriate for the description of a macroscopic system (bulk quantities) are defined and their properties are discussed, especially partial molar quantities (PMQ's),which are well known from thermodynamics. To any PMQ some corresponding model molar quantities (MMQ's) can be introduced, each MMQ based on a particular molecular model. The MMQ's are related to the interesting molecular quantities and may allow their estimation. The values of PMQ's can be determined uniquely from experimental data, if the investigated system meets just a very few general requirements, whereas the values of the corresponding MMQ's are generally dependent on the chosen model and hence unique only, if a true model is available. As examples, approximate molecular models are introduced for MMQ's corresponding to the Gibbs energy and to quantities convenient for the evaluation of permittivity, optical absorption and electro-optical absorption measurements.

The Determination of Molecular Quantities from Measurements on Macroscopic Systems.III. Electro-optical Absorption Measurements on Michler's Ketone

Abstract Some bulk quantities appropriate for the description of electro-optical absorption measurements on macroscopic systems are defined and their properties are discussed. Based on three molecular models (Lorentz model, Onsager model in spherical approximation and in ellipsoidal approximation) model molar quantities are introduced, which depend on intrinsic properties of the molecule (dipole moments and polarizabilities in the ground and excited state, transition dipole moment and transition polarizability). The relations will be applied for the evaluation of the results of electro-optical absorption measurements on Michler's ketone in cyclohexane in a wavenumber interval near 30 · 105 m-1 . The angles between the dipole moments in the ground and the excited state and the transition dipole moment will be determined; the magnitude of the dipole moment in the corresponding excited state is μaG = 30 · 10-30 Cm. The data show that the symmetry of a solute Michler's ketone molecule most probably corresponds, at least approximately, to the pointgroup Cs.

The Anisotropic Polarizability of a few Aromatic Hydrocarbons

Abstract For sufficiently symmetric molecules without a permanent electric dipole moment, it is shown that refractive indices, Kerr constants and electrooptical absorption measurements in suitable non-polar solvents can be utilized to determine the components αcgx, αgy, αgz of the polarizability tensor of solute molecules in the electronic ground state. Since the required quantities are avail-able for anthracene, tetracene, perylene and bianthryl, an evaluation of the polarizability tensor components is made possible. In solutions where the solvent molecules possess a permanent electric dipole moment or a sufficiently large quadrupole moment, as for example benzene, the fluctuations of the reaction field cause a contribution to the Kerr constant which may be an essential reason for its solvent dependence.

Die Beeinflussung der optischen Absorption von Molekülen durch ein elektrisches Feld

The effect of an external electric field on the optical absorption of molecules in solution allows certain components of the polarizability tensors in the ground state and in excited electronic states to be determined. Electrooptical absorption measurements were performed on some aromatic hydrocarbons (anthracene, tetracene, perylene, bianthryl) and some tetraphenylpolyenes at different temperatures and in different solvents. For the aromatic hydrocarbons, the polarizabilities in the first excited singlet state were found to be 20 to 70 p.c. greater than those in the ground state. The increases are either mainly parallel or mainly perpendicular to the direction of the transition moment of the absorption bands. For the tetraphenylpolyenes, the measurements yielded much greater increases of up to 400 p.c. almost totally in the directions of the transition moments, which are approximately in the directions of the chains. Fluctuation effects of the local electric field were observed, they may be interpreted using the extended model 9