The electronic structure of polydiacetylenes: cyclic versus linear models

The electronic structure of polydiacetylenes is investigated with SCF calculations as a function of backbone structure and chain length. The first dipole-allowed excitation (band gap) is red-shifted for the transformation from polydiacetylene (PDA) to polybutatriene (PBT) for cyclic chains The band gap for linear chains is blue-shifted for short chains, for linear chains containing 36 (or more) carbon atoms the band gap is red-shifted.     

Effects of electron–electron interaction on the band structure in polydiacetylenes

The ground‐state band structure of polydiacetylenes is theoretically studied with the extensional Su–Schriffer–Heeger model supplemented by electron–electron interactions. The results show the following. First, the interval of valence bands (conduction bands) increases because of the electron–electron interactions. Second, the effect of the on‐site Coulomb energy (U) is different from that of the nearest neighbor Coulomb repulsion (V); the competition between U and V shows that U makes the bandwidth narrower and the gap broader, whereas V makes the bandwidth broader and the gap narrower. There is a critical value of U/V. Third, the whole band width (E_w) decreases when the U/V ratio is less than 1.0 and increases when the U/V ratio is greater than 1.0 at V = 2.0 eV. Thus, the ground‐state band structure is sensitive to the U/V ratio. The results also show that electron–electron interactions can play an important role in the band structure of polydiacetylenes. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1656–1661, 2000     

Abrupt dissolution of polydiacetylenes

Polydiacetylenes, where R?? (CH₂)_3,4OCONHCH₂COO(CH₂)_1,3CH₃ dissolve almost instantaneously within a narrow range of temperature (ca. 3°C). The enthalpy of dissolution of these polymers is determined by differential scanning calorimetry to be 25–46kJ/mole of the repeat unit. The abruptness of dissolution is due to breaking of intramolecular H bonds, which appears to be a cooperative phenomenon. Supporting infrared evidence is cited.     

Biosensors and chemosensors based on the optical responses of polydiacetylenes

Polydiacetylenes (PDAs), a family of conjugated polymers, have very unique electrical and optical properties. Upon environmental stimulation, such as by viruses, proteins, DNAs, metal ions, organic molecules etc., the blue PDAs can undergo a colorimetric transition from blue to red, which is accompanied by a fluorescence enhancement. Since the first report on polymerized diacetylene molecules as sensors of influenza virus, the development of efficient sensory systems based on PDAs continues to be of great interest. This tutorial review highlights the recent advances in bio- and chemo-sensors derived from polydiacetylenes.     

Electronic structure of the polydiacetylenes: effect of bond length variations in a model system

SCF Xα calculations are reported for a cyclic molecule whose bonding sequence is a model for the backbone bonding sequence in the polydiacetylenes. The electronic structure of the model molecule, with particular emphasis on the two lowest energy optical transitions, is determined in conjunction with a systematic variation in bonding sequence. Transition energies are in fair agreement with experiment for polydiacetylenes with the acetylene linkage,

, and for the model molecule in solution. The same analysis predicts a substantial red shift on going to the butatriene linkage,

, whereas a small blue-shift is observed experimentally. This qualitative disagreement between theory and experiment is attributed to the inability of the present scheme to represent excitonic excitations and/or the neglect of interchain interactions. The transition energies from the SCF Xα treatment can be satisfactorily represented in a simple Hückel model with the following resonance integral-bond length relationship:β_ij = 67.28 exp (-d_ij/0.424) eV.     

Studies on the electronic structure of conjugated systems

A comprehensive study of the electronic structure of conjugated systems, within the approximation and using a CI SCF procedure, has been undertaken. In this work the common features (method and approximations, semiempirical parameters, details of the program, etc.) are examined and the general characteristics of the results are discussed.

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Zusammenfassung Eine ausführliche Untersuchung der Elektronenstruktur konjugierter Systeme wurde mit Hilfe eines CI SCF Verfahrens innerhalb der normalen -Annäherung durchgeführt. In der vorliegenden Arbeit werden die Grundzüge (wie Methode und Näherungsverfahren, halbempirische Parameter, Einzelheiten des Programms, usw.) besprochen.

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Résumé On a entrepris une étude comprehensive de la structure électronique des systèmes conjugués, dans le cadre de l'approximation et en utilissant la méthode de champ auto-cohérent avec interaction de configurations. Dans le présent travail on examine les charactéristiques générales (méthode et approximations, paramètres sémiempiriques, détails du programme, etc.).

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This work has been supported in part by the National Research Council of Canada.

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Temporary address while on sabbatical leave:Instituto de Quimica Fisica, C.S.I.C., Serrano 119, Madrid 6, Spain.     

Effect of the structure ofα-substituted styrenes on their electronic structure

The relationship between the experimental ionization potentials and energies of the highest occupied MO was analyzed for various conformations of styrene derivatives using the photoelectron spectra of a series of α-substituted styrenes and quantum chemical calculations of their electronic structure. The characteristics of the HOMO of the planar conformations correlate both with the ionization potentials and the reactivities of these molecules. Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of Ukraine, 70 R. Lyuksemburg St., Donetsk, Ukraine 340014, Institute of Technical and Macromolecular Chemistry, Martin Luther University, Halle Wittenberg, Merseburg, Germany D-06217. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 34, No. 1, pp. 36–42, January–February, 1998.     

Synthesis and reversible thermochromism of azobenzene-containing polydiacetylenes

A series of diacetylenic monomers containing azobenzene chromophores with different terminal groups {CH₃-(CH₂)₁₁-CC-CC-(CH₂)₈-COO-(CH₂)₂-O-C₆H₄-NN-C₆H₄-R, where R = NO₂, CN, CH₃ and labeled as 3a, 3b and 3c, respectively} were synthesized and characterized by ¹H NMR. The photo-polymerization of spin-coated films of the monomers was carried out by irradiation at 254 nm under monitoring the optical absorption in the visible region and polymers (P3a-P3c) were obtained as blue films that were no longer soluble in common organic solvents. The thermochromic behavior of the polymers was evaluated with UV, Raman, thermogravimetric analysis, X-ray diffraction and differential scanning calorimetry analyses. It was found from results that all of the polymers were thermally stable and decomposed at temperatures as high as ∼300 °C. By tracing the treatment of samples with heating-cooling cycles, the temperature ranges between the low and the high critical temperature for the red-phase were obtained to be 60-160 °C, 57-162 °C and 50-154 °C for P3a, P3b and P3c, respectively, larger than those reported before for other PDAs. The relationship between characteristics above and the chemical structure of polymers was discussed.     

On the electronic structure of ethidium

The electronic structure of the common intercalating agent ethidium bromide (3,8-diamino-5-ethyl-6-phenylphenanthridinium bromide) is dominated by an interplay of electron donating and withdrawing effects mediated by its nitrogen atoms. X-ray crystallography, UV/Vis and IR absorption, fluorescence emission, and NMR spectroscopy are used to probe the electronic properties of the phenanthridinium "core" of ethidium as well as its exocyclic amines and 6-phenyl groups. Interestingly, despite its positive charge, most of ethidium's aromatic carbon and hydrogen atoms have high electron densities (compared to both 6-phenylphenanthridine and benzene). The data suggest that electron donation by ethidium's exocyclic amines dominates over the electron withdrawing effects of its endocyclic iminium in their combined influence on the electron densities of these atoms. Ethidium's nitrogen atoms are, conversely, electron deficient where the 5-position is the most electropositive, followed by the 3-amino, and lastly the 8-amino group. These results have been used to generate an empirically-based pi-electron density map of ethidium that may prove useful to understanding its nucleic acid binding specificity.     

Tuning the electronic structure of diboradiferrocenes

A series of diboradiferrocenes with different aryl substituents was prepared through reaction of B,B-dichlorodiboradiferrocene with arylcopper and Grignard reagents. The mesityl and pentafluorophenyl derivatives, - and -, were fully characterized by multinuclear NMR, MALDI-TOF mass spectrometry and X-ray crystallography, and their electronic structure was examined by UV-visible spectroscopy and cyclic voltammetry. A comparison of the data for - and - with those of the parent compound - revealed the importance of electronic and steric effects of the substituents on the electronic structure of the compounds and ultimately the degree of electronic interaction between the two ferrocene moieties. An unusually large redox splitting of DeltaE = 703 mV was determined from the cyclic voltammogram of -.     

INDO Studies on the electronic structure of lanthanoid compounds

Expressions needed in INDO calculations for compounds containing 4f elements were derived and an INDO program suitable for lanthanoid compounds was written and tested. Electronic structure of LnF₃ and paramagnetic shift of NMR spectra of LnF were studied.     

Relativistic effects on the electronic structure of allyl mercury

Conclusions Relativistic effects can destabilize the -complexes of allyl mercury compounds and weaken the effects of , conjugation, which leads to a decrease in the first ionization potentials; their effect on the distribution of electron density in the allyl mercury molecule (manifested, in particular, by a decrease in the positive charge on the mercury atom) is almost independent of the conformation of the molecule.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1570–1573, July, 1989.     

Solvate-shell effects on solute-molecule electronic structure

Research Institute for Physical and Organic Chemistry, Rostov University. Translated from Zhurnal Strukturnoi Khimii, Vol. 29, No. 2, pp. 84–88, March–April, 1988     

The colors of polydiacetylenes: a commentary

Polydiacetylenes (PDA) can take several colors, mostly "red" and "blue", and blue-to-red color transitions are usually considered as transitions from a well ordered state to a disordered one. Recent work on single isolated PDA chains shows that this is not correct: red chains can be quasi-perfect quantum wires. The transition is between two different chain conformations, and each may, or may not, be perfectly ordered. Disorder, when it occurs, is a side-product of the transformation.