Donor End Groups Research Articles

Spectroscopic and Theoretical Study of Push−Pull Chromophores Containing Thiophene-Based Quinonoid Structures as Electron Spacers

Donor−acceptor chromophores containing three different types of thiophene-based electron spacers and the same donor (1,3-dithiol-2-ylidene) and acceptor (dicyanomethylene) end groups have been investigated by infrared and vis-near-IR absorption spectroscopies with the aim of elucidating the ability of the heteroquinonoid spacers as electron transmitters. Density functional theory calculations have been carried out, both within the standard and the time-dependent formalisms, to assign the most relevant electronic and infrared features of these chromophores and to assess useful information about their molecular structures. Both theoretical calculations and vibrational spectra demonstrate the occurrence of a sizable intramolecular charge transfer from the electron-donor unit to the electron-acceptor group in the ground state. The optical properties of the chromophores are strongly influenced by the electron spacer. The intense optical absorptions recorded in the visible mainly correspond to the π−π* excitation of the oligothienoquinonoid bridge. As an additional merit of these molecular materials, their infrared spectra recorded at various temperatures between −170 and +180 °C reveal, at the molecular level, their high thermal stability, what has importance for their potential use in molecular electronics and optoelectronic devices.

Vibrational and quantum-chemical study of push-pull chromophores for second-order nonlinear optics from rigidified thiophene-based pi-conjugating spacers.

Two types of push-pull chromophores built around thiophene-based pi-conjugating spacers rigidified by either covalent bonds or noncovalent intramolecular interactions have been analysed by means of IR and Raman spectroscopical measurements in the solid state as well as in a variety of solvents. Comparison of the Raman features of NLO-phores based on a covalently rigidified dithienylene (DTE) spacer with those of their open chain DTE analogues shows that the bridging of the central double bond of DTE with the nearest beta-positions of the thienyl units through two ethylene bridges significantly improves the intramolecular charge transfer. This also occurs for NLO-phores based on a 2,2'-bi(3,4-ethylenedioxythiophene) (BEDOT) spacer as compared with their corresponding parent compounds based on an unsubstituted bithiophene (BT) spacer. For NLO-phores based on a BEDOT spacer, noncovalent intramolecular interactions between sulfur and oxygen atoms are responsible for the rigidification of the spacer. The Raman spectra of these NLO-phores obtained in the form of solutes in dilute solutions reveal two different behaviours: i) chromophores based on covalently bridged or open chain DTE spacers display Raman spectral profiles in solution quite similar to those of the corresponding solids, with a very little dependence on the polarity of the solvent, while ii) larger spectral changes are noticed for NLO-phores built around BEDOT or BT spacers on going from solids to solutions. In the second case, spectral changes must be ascribed not solely to conformational distortions of the donor and acceptor end groups with respect to the pi-conjugated backbone mean-square-plane (as for the DTE-based NLO-phores) but also to distortions of the thienyl units of the pi-conjugating spacer from coplanarity. The insertion of vinylenic bridges between the thienyl units of the pi-conjugating spacer and between the spacer and the donor and acceptor end groups is a suitable strategy to reach a fairly large intramolecular charge transfer both in polar and nonpolar solvents. Density functional theory (DFT) calculations have been carried out to assign the relevant electronic and vibrational features and to derive useful information about the molecular structure of these NLO-phores.

The linear and nonlinear optical properties of organometallic chromophores derived from ferrocene, [Fe 2(η 5-C 5H 5) 2(CO) 2(μ-CO)(μ-CCH 3)] +[BF 4] − and terthienyl spacers. Crystal structure of 2-[( E)-2-ferrocenylethenyl]-5-(2-thienyl)thiophene

Several chromophores based on ferrocene, [Fe 2(η 5-C 5H 5) 2(CO) 2(μ-CO)(μ-CCH 3)] + and terthiophene or dithienylbenzo[ c]thiophene have been synthesized. Both thienyls have been shown to act as good donor end groups in their own right but contrary to our expectations the benzo[ c]thiophene merocyanine has a lower hyperpolarizablity (344×10 −30 esu) than its thienyl congener (650×10 −30 esu). The structure of 2-[( E)-2-ferrocenylethenyl]-5-(2-thienyl)thiophene has been determined by single crystal X-ray diffraction. The terminal thienyl (thp) group is disordered over two orientations about the bithienyl C(sp 2)C(sp 2) bond such that the two sulfur atoms in either thp ring are positioned trans/ cis in the ratio 0.628(4):0.372(4). The integrity of the alkenyl CC remains with a bond length of 1.332(2) Å and the CC lengths adjacent are 1.453(4)/1.441(4) Å. Twisting of the rings from planarity occurs along the Fc–CC–(thp)–(thp) axis: the angle which the ethenylthiophene CCC 4H 2S moiety makes with the C 5H 4 is 11.2(2)°, and with the major orientation of the disordered terminal thienyl group, 16(1)°.

Structure−Property Relationships for Two-Photon Absorbing Chromophores: Bis-Donor Diphenylpolyene and Bis(styryl)benzene Derivatives

The two-photon absorption properties of a series of bis dialkylamino- or diarylamino-substituted diphenylpolyenes and bis(styryl)benzenes have been investigated. Two-photon absorption cross sections, δ, as large as 1420 × 10-50 cm4 s/photon-molecule have been observed for molecules with this general bis-donor structure. The effect of the type and length of the conjugated chain and of dialkylamino or diarylamino substitution on the position and magnitude of the peak two-photon absorptivity is reported. The transition dipole moments for the transitions between the ground state and the first excited singlet state (Mge) and between the first and second excited singlet states (Mee‘) have been estimated using experimental data from the one- and two-photon spectra. It was found that increases in chain length result mainly in an increase in Mge, whereas the addition of donor end groups or going from diphenylpolyene- to phenylene-vinylene-type bridges leads primarily to an increase in Mee‘. The trends in the energy of the lowest excited singlet states and in the transition moments for the diphenylpolyene series as a function of chain length are in agreement with those calculated by quantum mechanical methods. These results furnish a link between structural features in these classes of molecules and the electronic dipole couplings and state energies that control the strength of the two-photon absorption. In bis(aminophenyl)polyenes containing up to four double bonds (m) the lowest excited singlet state is a Bu state, as opposed to the case of simple polyenes and diphenylpolyenes, for which it is an Ag state for m > 2. The relationship of the state ordering in these systems with the observed values of the radiative and nonradiative decay rates is also discussed.

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.

Stark Spectroscopy of Donor/Acceptor Substituted Polyenes

The electronic structure of donor/acceptor polyenes (merocyanines) is studied by means of electroabsorption (Stark) spectroscopy. Molecules belonging to this class exhibit an extreme variability of their linear and nonlinear optical properties. Depending on the nature of the donor and acceptor end groups and the polarity of their solvent environment they can adopt ground state molecular structures between limits with mostly localized double bonds (polyene-like) or fully delocalized double bonds (cyanine-like). By comparing the results obtained for the change in dipole moment, Δμ, and change in polarizability, Δα, upon optical excitation of 12 donor/acceptor polyenes with theoretical predictions, each molecule's location in between these limits can be assigned. The results demonstrate that this position can be understood by considering only the relative electron donating and accepting strengths of the end groups. The solvent dependence of the electronic structure is also studied by measuring Stark spectra for two compounds in different frozen solvents. The results are consistent with a change of the molecule's ground state toward more dipolar structures in polar solvents. Taken together, the donor, acceptor, and solvent dependences of the electronic structure of donor/acceptor polyenes can be described by a simple model based on the gradual change of the molecule’s ground state between polyene-like and cyanine-like structures.

Two-band tight-binding model for push–pull polyenes

We propose a two-band model for the description of the electronic structure of push–pull polyenes in order to analyze in simple ways their potentially useful electronic and optical features. The polyene part is described by two (valence and conduction) bands, which are coupled with two tight-binding frontier orbitals representing the donor and acceptor end groups. In this model, the ground state consists of the one-electron states of the (π) valence band hybridized with the donor orbitals, while the charge-transfer excited state is described as an excitation from the highest occupied molecular orbital to the lowest unoccupied molecular orbital, the latter being a hybridized one-electron state between the (π*) conduction band and the acceptor orbital. It is shown, by the Green’s function method, that the electron localizations (the partial density of states) at the end groups are determined by three factors; (1) the unperturbed energy levels of the frontier orbitals, (2) the density of states of the unperturbed polyene bands, and (3) the coupling constants between the π (π*) band and the donor (acceptor) orbital. Based on the results, a simple description is provided for the characteristic nonlinear optical responses and the intramolecular adiabatic charge-transfer mechanism of push–pull polyenes. In order to estimate the magnitude of the coupling constants, we compare the analytical results from the model with numerical calculations based on an established semiempirical method. This kind of modeling provides guidance for the design of functional 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.

Chain-length dependence of the quadratic hyperpolarizability of push-pull polyenes and carotenoids. Effect of end groups and conjugation path

Numerous push-pull polyenes and carotenoids of increasing length and bearing various donor and acceptor end groups have been synthesized in order to investigate the chain-length dependence of their quadratic hyperpolarizability β. The variation of β was studied using the electric-field-induced second-harmonic (EFISH) generation technique. In each series of homologous compounds, the lengthening of the polyenic conjugation path results in a sharp increase in β with the number n of double bonds of the polyenic chain. This behaviour leads to very large values of β for the longest molecules whose length reaches 25 Å. A steeper increase of β was observed when the end groups are weaker electron donors and electron acceptors. Moreover, β- kn a relationships have been established, the a exponent depending on the end groups. Finally, the effect of the introduction of a triple bond in the middle of the polyenic linker has been studied. The replacement of a double bond by a triple bond in the conjugation path leads to smaller β and to a blue shift of the charge transfer absorption band. However, this phenomenon is modulated by the nature of the end groups. Therefore, in some cases, the presence of the triple bond may advantageously result in a gain of transparency with almost no decrease in β.

Ultrafast vibrational predissociation of hydrogen bonds: Mode selective infrared photochemistry in liquids

Using intense ultrashort excitation pulses in the infrared (IR) tuned to the CH or oligomeric OH absorption bands of ethanol dissolved in CCl4, vibrational predissociation and partial reassociation of hydrogen bonds are observed on the picosecond time scale. Induced absorption at the OH stretching frequency 3500 cm−1 of proton donor end groups serves as a spectroscopic probe of the broken H bridges and is studied as a function of time by the help of delayed, tunable interrogation pulses. Different values for the effective dissociation rates and total quantum yields provide strong evidence for mode specifity of the IR photodissociation in the liquid at ambient temperature. The experimental results on vibrational population decay and energy transfer rates suggest a V–V transfer mechanism for CH to OH modes for the vibrational predissociation after CH excitation.

ChemInform Abstract: OPEN‐CHAIN POLYETHERS. INFLUENCE OF AROMATIC DONOR END GROUPS ON THERMODYNAMICS AND KINETICS OF ALKALI METAL ION COMPLEX FORMATION

Chemischer InformationsdienstVolume 10, Issue 34 Article ChemInform Abstract: OPEN-CHAIN POLYETHERS. INFLUENCE OF AROMATIC DONOR END GROUPS ON THERMODYNAMICS AND KINETICS OF ALKALI METAL ION COMPLEX FORMATION B. TUEMMLER, B. TUEMMLERSearch for more papers by this authorG. MAASS, G. MAASSSearch for more papers by this authorF. VOEGTLE, F. VOEGTLESearch for more papers by this authorH. SIEGER, H. SIEGERSearch for more papers by this authorU. HEIMANN, U. HEIMANNSearch for more papers by this authorE. WEBER, E. WEBERSearch for more papers by this author B. TUEMMLER, B. TUEMMLERSearch for more papers by this authorG. MAASS, G. MAASSSearch for more papers by this authorF. VOEGTLE, F. VOEGTLESearch for more papers by this authorH. SIEGER, H. SIEGERSearch for more papers by this authorU. HEIMANN, U. HEIMANNSearch for more papers by this authorE. WEBER, E. WEBERSearch for more papers by this author First published: August 21, 1979 https://doi.org/10.1002/chin.197934109Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume10, Issue34August 21, 1979 RelatedInformation

Open-chain polyethers. Influence of aromatic donor end groups on thermodynamics and kinetics of alkali metal ion complex formation

Open-chain polyethers. Influence of aromatic donor end groups on thermodynamics and kinetics of alkali metal ion complex formation

Complexes of Short‐Chain Oligo(ethylene Glycol Ethers) Bearing only one Rigid Donor End Group

Crystalline complexes of alkali metal ions are formed not only by crown ethers and “noncyclic crown ethers” bearing two donor end groups but also by neutral ligand such as (1) and (2) having only one donor end group. The oligo(ethylene glycol ether) chain may be very short (n ≥ 1, i.e. with only four donor atoms per ligand).