Polydiacetylene Oligomers Research Articles

Spectral shape of photoluminescence in luminescent π conjugated polymers

In polyacetylene and polydiacetylene oligomers, photoluminescence (PL) is induced from the dipole-forbidden state by vibronic coupling. The assumption that the PL in polythiophene (PT) and poly( p-phenylene vinylene) (PPV) also includes this induced component, consistently explains many experimental results. In this work, we focus on the temperature dependence of the time-integrated and time-resolved PL spectra of a PT derivative and unsubstituted PPV, and demonstrate the validity of the above model for their temperature dependence. Applying this model to the hitherto unresolved drastic PL spectral change of unsubstituted PPV, we propose that the spectral change and the spectral shape itself can be understood as resulting from a contribution to the PL from the 2 A g state coupled with a change in the ordering of the 2 A g and the 1 B U states at low temperatures.

Ultrafast optical Kerr effect measurements of third-order nonlinearities in cross-conjugated iso-polydiacetylene oligomers

We present a study on the ultrafast third-order nonlinearities of a novel series of iso-polydiacetylene oligomers (iso-PDAs). Unlike polydiacetylenes that contain a linearly-conjugated backbone, iso-PDAs have a backbone that is cross-conjugated. A new Kerr-gate technique, differential optical Kerr effect (DOKE) detection, is used to measure third-order nonlinear susceptibilities, χ(3), and second hyperpolarizabilities, γ, of monomer, dimer, trimer, pentamer, and heptamer samples in tetrahydrofuran (THF) solutions. A linear increase in γ as a function of the number of repeat units is observed for all samples except the monomer, suggesting that the fixed-length linearly conjugated segments dominate the electronic polarizability. An added increase to the oligomer nonlinearities due to communication along the cross-conjugated path is not observed. The largest nonlinearity was observed in the heptamer sample, displaying a second hyperpolarizability relative to the THF solvent of γheptamer/γTHF=181±9. In addition, an interesting feature arising out of the signal decay tail is present in the samples but absent in our THF solvent reference.

Excited-State Molecular Dynamics Simulations of Conjugated Oligomers Using the Electronic Density Matrix

Vibrational motions in optically excited polyacetylene and polydiacetylene oligomers are calculated using potential surfaces obtained from the collective electronic oscillators (CEO) technique. The role of the effective conjugation coordinate (ECC) in the relaxation processes following an impulsive vertical excitation from the ground state is demonstrated. Real-space analysis of the electronic transition density matrices shows the charge and bond-order redistribution taking place upon photoexcitation.

Shear-induced mechanochromism in polydiacetylene monolayers

We use atomic force microscopy to actuate and characterize the nanoscale “mechanochromism” of polydiacetylene monolayers on atomically-flat silicon oxide substrates. We find explicit evidence that the irreversible blue-to-red transformation is caused by shear forces exerted normal to the polydiacetylene polymer backbone. The anisotropic probe-induced transformation is characterized by a significant change in the tilt orientation of the side chains with respect to the surface normal. We discuss preliminary molecular dynamics simulations and electronic structure calculations on twelve-unit polydiacetylene oligomers that allow us to correlate the transformation with bond-angle changes in the conjugated polymer backbone.

Excited states of polydiacetylene oligomers

The peculiar electronic properties of polydiacetylenes are largely determined by the nature of their electronic excited states. This paper reports the results of a detailed quantum chemical study of the excited states of their shortest oligomers.

Iterative Synthesis and Properties of Cross-Conjugated iso-Polydiacetylene Oligomers

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTIterative Synthesis and Properties of Cross-Conjugated iso-Polydiacetylene OligomersYuming Zhao and Rik R. TykwinskiView Author Information Department of Chemistry, University of Alberta Edmonton, Alberta, T6G 2G2 Canada Cite this: J. Am. Chem. Soc. 1999, 121, 2, 458–459Publication Date (Web):January 5, 1999Publication History Received15 September 1998Published online5 January 1999Published inissue 1 January 1999https://doi.org/10.1021/ja983288pCopyright © 1999 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views539Altmetric-Citations59LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (45 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Cross coupling reaction,Delocalization,Electronic absorption,Molecular properties,Oligomers Get e-Alerts

Nonlinear optical and electroabsorption spectra of polydiacetylene crystals and films

Vibronic structure of nonlinear optical (NLO) coefficients is developed within the Condon approximation, displaced harmonic oscillators, and crude adiabatic states. The displacements of backbone modes of conjugated polymers are taken from vibrational data on the ground and 1B excited state. NLO resonances are modeled by three excitations and transition moments taken from Pariser–Parr–Pople (PPP) theory and optimized to polydiacetylene (PDA) spectra in crystals and films, with blue-shifted 1B exciton. The joint analysis of third-harmonic-generation, two-photon absorption, and nondegenerate four-wave-mixing spectra of PDA crystals and films shows weak two-photon absorption to 2A below 1B, leading to overlapping resonances in the THG spectrum, strong two-photon absorption to an nA state some 35% above 1B, and weak Raman resonances in nondegenerate FWM spectra. The full π-π* spectrum contributes to Stark shifts and field-induced transitions, as shown by PPP results for PDA oligomers. The Stark shift dominates high-resolution electroabsorption (EA) spectra of PDA crystals below 10 K. The close correspondence between EA and the first-derivative I′(ω) of the linear absorption above the 1B exciton in PDA crystals provides an experimental separation of vibrational and electronic contributions that limits any even-parity state in this 0.5 eV interval. An oscillator-strength sum rule is applied to the convergence of PDA oligomers with increasing length, N, and the crystal oscillator strengths are obtained without adjustable parameters. The sum rule for the 1B exciton implies large transition moments to higher-energy Ag states, whose locations in recent models are contrasted to PPP results. Joint analysis of NLO and EA spectra clarifies when a few electronic excitations are sufficient, distinguishes between vibrational and electronic contributions, and supports similar π-electron interactions in conjugated molecules and polymers.

Localized π electron states in polydiacetylene oligomers

It is shown that the particular bond alternation pattern of short chain polydiacetylene intermediates gives rise to the occurrence of two nonbonding and nearly degenerate π molecular orbitals (MOs) which are localized at the chain ends of the longer oligomers. These orbitals evolve with increasing chain length from the highest bonding and the lowest antibonding π MOs leading to a loss of the closed shell structure of the π system, which is characterized by an increasing admixture of a doubly excited configuration to the ground configuration. We present a Hartree-Fock scheme which optimizes the coefficients of this configurational mixing and the MO coefficients simultaneously. The configuration interaction (CI) calculation is reformulated in terms of a simple two-site Hubbard model in a localized basis.

ChemInform Abstract: Electronic Structure of Diradical and Dicarbene Intermediates in Short‐ Chain Polydiacetylene Oligomers

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Electronic structure of diradical and dicarbene intermediates in short-chain polydiacetylene oligomers

Semiempirical molecular orbital (MO) calculations on short-chain (n=2 to n=8 monomer units) diradical and dicarbene intermediates obtained in the solid state polymerization reaction of diacetylene monomer crystals have been performed. With increasing chain length a transition from the diradicals with two unpaired electrons to the dicarbenes with four unpaired electrons is found in agreement with experimental results. The energetic ordering of the low-lying multiplets (singlet and triplet for the diradicals; singlet, triplet, and quintet for the dicarbenes) is discussed in terms of a model Hamiltonian containing a Hubbard term which takes into account the electron transfer between the radical electrons on different chain ends, and a Heisenberg term describing the exchange interaction of radical electrons on the same chain end in the case of the dicarbenes. All intermediates have a singlet ground state. The small energy gaps to the higher triplet and quintet states decrease rapidly with increasing chain length due to a decrease in the ratio t/U, with t and U being the transfer integral and the on-site Coulomb repulsion of the Hubbard model, respectively. The fine structure arising from the magnetic dipolar interaction of the unpaired electrons is discussed.

Coupled-Hartree—Fock calculations of the third-order hyperpolarizabilities of substituted polydiacetylenes

Coupled-Hartree—Fock calculations of the static third-order hyperpolarizabilities (γ) in the chain direction for regular polydiacetylene and several donor (D)—acceptor (A) substituted polydiacetylene oligomers through C 42H 24 are carried out at the level of the INDO approximation. The modes of the variations in γ with the increasing chain length are investigated for the different types of polydiacetylene systems. The γ density analysis is carried out to elucidate the spatial characteristics of γ. Results for sufficiently long oligomers are extrapolated to an infinity of the chain length to predict the intrinsic γ values per repeating unit of the polydiacetylenic chains.

Theory of the radical-electron configurations and optical excitations in short-chain polydiacetylene oligomers.

The chain-length-dependent radical-electron structure of short polydiacetylene chains is calculated using a configuration model. The butatriene-to-acetylene transition and a change from diradicals to dicarbenes as a function of chain length is described. By introduction of coupled electronic two-level systems a one-dimensional Frenkel exciton theory is applied for an explanation of the chain-length-dependent uv-absorption spectra. The theoretical results are in good agreement with the experimental observations.

Long polyene and polydiacetylene oligomers: Pariser–Parr–Pople investigation of the geometric and electronic structures in the first 1B u excited state

We present Pariser–Parr–Pople (PPP) calculations on long polyene and polydiacetylene oligomers containing up to 32 carbon atoms. Working at both PPP restricted Hartree–Fock and PPP configuration interaction with single excitations levels, we study the excitation towards the 1 1B+u state. We report, as a function of chain length, the evolution of the transition energies and the modifications in geometry and atomic charges along the backbones. Comparison is made between the two series of oligomers. In the longer molecules, localization of the excitation is found, in agreement with the excitonic character of the transition to the 1 1B+u state.