K. Alimi

Theoretical study of the alkoxyls groups effect on PPV-ether excited states, a relationship with femtosecond decay

Based on Density Functional Theory (DFT), the effect of alkoxyl substituents groups on the excited states of new PPV derivative (PPV-ether) are studied. Thus differences on the relaxation energy, the atomic charge distribution and the conjugation length at their excited state are interpreted on the pathways of the nature on the emissive species. Photoluminescence and optical spectra of the corresponding modeling structures are carried out. Furthermore, the life time at the excited state are emphasized either theoretically from oscillator strength or experimentally from time resolved photoluminescence.

Theoretical study of phenylene–thiophene oligomers: Structure–properties relationship

Theoretical results including geometrical characteristics, electronic structures, photophysical parameters (lowest excitation energies, electron affinities (EAs), ionization potentials (IPs), maximum of absorption and emission) and vibrational modes of some new oligomers, based on phenylene-thiophene motives, are investigated using Density Functional Theory DFT/B3LYP/6-31G(d) approach. The electronic and optical properties of phenylene-based derivatives can be tuned through the insertion of thiophene in the main oligomers backbone as well as the addition of alkoxy-substituent groups on 2 and 5 positions on phenylene groups. It can be noticed, that different conformational behaviors and steric effects take place. Then, an increase in conjugation length induces a decrease in the gaps energy and a bathochromic shift of absorption/emission spectra. Based on these computed results, which are consistent with the available experimental data, the correlation structure-properties is better understood, where these nanostructures show a great potential for opto-electronic devices.

Unraveling the real structures of solution-based and surface-bound poly(3-hexylthiophene) (P3HT) oligomers: a combined theoretical and experimental study

While the crystalline structure for regio-regular poly(3-hexylthiophene) (P3HT) in thin films is well established, the conformation of P3HT chains in solution has received less attention. Nevertheless, the control of this in-solution structure can be used for managing the structure-processing relationship, which can be used to improve the optoelectronic behavior and thus the efficiency of devices exploiting electroactive polymers. In the current study, we report a combined theoretical and experimental study of P3HT and a series of oligomers, both in the solid state and in solution. (3HT)n oligomers were simulated in a variety of planar and non-planar conformations by means of density functional theory (DFT) and time-dependent DFT (TDDFT), comparing results for various functionals with and without dispersion correction in order to evaluate the role of intermediate and long-range effects. Our calculations show that regio-regular P3HT chains adopt a twisted conformation in solution (dihedral angle of about 40°), which contrasts with the well-established planar (θ = 0°) conformation when deposited onto a substrate, due to inter-chain interactions. Determining the Raman spectra, electronic gaps, quasi-particle energies and optical spectra, a good agreement between experimental and simulated optical absorption spectra was obtained for the in-solution case. This study will help to promote the development of alternative strategies for controlling the optoelectronic features of conjugated polymers and polymer blends by exploiting the in-solution structure.

New bridged oligofuran for optoelectronic applications.

Based on density functional theory (DFT) calculations, we have investigated the structural and optoelectronic properties of oligofuran (OFu)-bridged systems via useful electron donating groups (>S, >CH2, >SiH2 and >NH) and electron accepting ones (>CC(CN)2, >CO, >CS and >CCH2). The results were then discussed and compared with those obtained with the correspondingunbridged form. It was found that the optical band gap of OFu decreases significantly when it is bridged by >NH group arranged through an alternating way with >CS or >CC(CN)2 group, which gives bridged polyfuran (PFu) with desirable opto-electronic properties. Further, an intra-molecular charge transfer for the systems was undertaken in support of time-dependent DFT (TD-DFT) and semi-empirical ZINDO calculations. In this frame, we have shown that >CC(CN)2 and >S bridging groups leads to a new oligomer possessing favorable optoelectronic parameter for its use as an active layer in organic photovoltaic cells.

New polymer-matrix nanocomposites based on SWCNTs and PVK-PPV copolymer: Synthesis, functionalization and characterization

An original nanocomposite material was prepared by in-situ grafting of a PVK-PPV copolymer onto SWCNTs. The new composites were elaborated with various SWCNTs weight concentrations into homemade PVK-PPV copolymer. Different experimental analyses were performed to investigate their morphological features and their optical and vibrational spectroscopy behaviors as a function of the concentration of carbon nanotubes. Π-staking and covalent interactions and functionalization process between copolymer and SWCNT undergo dramatic effect in vibrational and photoemissives properties of copolymer matrix. Transmission (TEM) and scanning electron microscopy (SEM) were used. Then, resonant Raman scattering and steady-state and time-resolved photoluminescence (PL) and (TRPL) measurements were used to study the evolution of the spectroscopic characteristics and optical properties of PVK-PPV/SWCNTs composites. As results, we have observed PL quenching effect and a decrease of average life time. These indicated the processing of a charge transfer that leads to exciton dissociations to the SWCNTs matrix. Such results, also supported by modelling based on DFT method, have given a strong evidence of the functionalization and the charge transfer between the SWCNTs and the PVK-PPV copolymer and predicted a structure-properties correlation.

Theoretical investigation of oligomer structure and optoelectronic properties for [4-(methoxyphenyl)acetonitrile]n (n=1-5)

AbstractExperimental measurement on oligomeric 4-(methoxyphenyl)acetonitrile (OMPA) synthesized via an electrochemical method indicated that the average chain length for OMPA was around 5 units (5-MPA) [J Mol Struct 1031:186 (2013)], but did not provide enough information to completely characterize the chemical structure of the molecule. Nevertheless a possible structure was proposed on the basis of 13C NMR and the spin density hypothesis for radical polymerization. A more complete validation of the resultant structure is needed to show the extent to which the structure is consistent with a variety of measured properties. This is done here for the infrared (IR), ultraviolet–visible (UV–vis), and photoluminescence (PL) spectra of 5-MPA which are found to agree reasonably well with the experimentally measured spectra of OMPA. Electronic structure information regarding the highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO) energies, ionization potentials and electron affinities, as well as optical properties (UV–vis, PL) is also provided. Graphical AbstractComparison of theoretical and experimental UV-visible absorption and photoluminescence spectra of oligomeric 4-(methoxyphenyl)acetonitrile