Pi-conjugated systems

We now wish to describe qualitative trends in pi-conjugated systems in terms of the localized NBO donor-acceptor stabilizations (3.109) and NRT bond orders. For this purpose we consider the representative group of molecules 1-16 shown in Fig. 3.37. Numerical bond orders and conjugative stabilization energies pertaining to these molecules are gathered in Table 3.19. 

Schenning, A. P. H. J. Meijer, E. W. Supramolecular electronics nanowires from self-assembled pi-conjugated systems. Chem. Corrmun. 2005, 3245-3258. 

Nakamura, Akira, Norikazu Ueyama, and Kizashi Yam-aguchi. Organometallic Conjugation Structures, Reactions, and Functions of D-D and D-[Pi] Conjugated Systems. Springer Series in Chemical Physics, vol. 73. New York Springer, 2002. 

FJ.M. Hoeben, P. Jonkheijm, E.W. Meijer, and A. Schenning, About supramolecular assem-bUes of pi-conjugated systems, Chem. Rev., 105, 1491-1546 (2005). 

I. in, K.W., Ming, S.L., Zhen, S.J., Zhao, Y., Lu, B.Y., Xu, J.K., 2015. Molecular design of DBT/DBF hybrid thiophenes pi-conjugated systems and comparative study of their electropolymerization and optoelectronic properties from comonomers to electrochromic polymers. Polym. Chem. 6,4575-4587. 

Blanchard, R, A. Cappon, E. Levillain, Y. Nicolas, P. Frere, and J. Roncali. 2002. Thieno[3,4-b]-l,4-oxathiane An unsymmetrical sulfur analogue of 3,4-ethylenedioxythiophene (EDOT) as a building block for linear pi-conjugated systems. Org Lett 4 607-609. 

Hoeben FJM, Jonkheijm P, Meijer EW, Schenning A. About supramolecular assemblies of pi-conjugated systems. Chem Rev 2005 105 1491-1546. 

Fullerene is a condensed ring aromatic compound, possessing extended pi-conjugated systems, which has been applied in various research fields, especially in biology. Ogoshi and co-workers have demonstrated that... 

Roncali J (1997) Synthetic principles for bandgap control in linear pi-conjugated systems. 

Roncali J (2007) Molecular engineering of the band gap of pi-conjugated systems facing technological applications. Macromol Rapid Commnn 28 1761... 

Rissler J (2004) Effective conjugation length of pi-conjugated systems. Chem Phys Lett 395 92... 

Korzdorfer T, Sears IS, Sutton C, Bredas JL (2011) Long-range corrected hybrid functionals for pi-conjugated systems dependence of the range-separation parameter on conjugation length. J Chem Phys 135 204107... 

Hissler M, Lescop C, Reau R (2007) Functional phosphorus-based pi-conjugated systems structural diversity without multistep synthesis. Pure Appl Chem 79 201... 

Nohra B, Graule S, Lescop C, Reau R (2006) Mimicking [2,2]paracyclophane topology molecular clips for the coordination-driven cofacial assembly of pi-conjugated systems. J Am Chem Soc 128 3520... 

Yao Y, Shen W, Nohra B, Lescop C, Reau R (2010) Coordination-driven hierarchical organization of pi-conjugated systems from molecular to supramolecular pi-stacked assemblies. Chem Eur J 16 7143... 

Hay C, Hissler M, Fischmeister C, Rault-Berthelot J, Toupet L, Nyulaszi L, Rdau R (2001) Phosphole-containing pi-conjugated systems from model molecules to polymer films on electrodes. Chem Eur J 7 4222... 

In Section I the sensitivity of conjugated systems to possible photo-dissociation (PD) was mentioned. If this PD is conducted in the ionization chamber of a mass spectrometer, a PDPI/MS (photo-dissociation, photo-ionization/mass spectrometry) can be measured7. The examination of the PDPI/MS spectra of hexa-1,3-, 1,4-, 1,5- and 2,4-dienes using 9.68 and 10.49 eV photo-ionization (PI) is summarized in Table 1. 

Since the discovery of doped polyacetylene, a range of polymer-intense semiconductor devices have been studied including normal transistors and field-effect transistors (FETs), and photodiodes and light-emitting diodes (LEDs). Like conductive polymers, these materials obtain their properties due to their electronic nature, specifically the presence of conjugated pi-bonding systems. 

Inner electrons Sigma electrons n and pi electrons Conjugated systems Highly conjugated systems Vibrational and rotational Rotational... 

X(3) values for a number of amorphous polysilanes as measured by the Maker fringe technique are reported in Table I. These values are quite large for a sigma-bonded system as predicted by the highly polarizable nature of the polymer backbone and the intense oa electronic transition in the ultraviolet. In some cases, the numbers are as large as those reported for a number of pi conjugated polymers... 

The paralocalization energies can also be estimated14 by the simple PMO method. To a first approximation, the energy (PiS) required to remove two atoms simultaneously from a conjugated system is given by ... 

As we saw for the formate ion in Figure 3.15, resonance structures are just different ways of arranging pi bonds and electrons among a series of adjacent p orbitals that form a conjugated system. 

Because of its nonplanar geometry, cyclooctatetraene is not antiaromatic and its hydrogens appear at 5.75 8, a value typical for alkenes. However, the triple bonds of the compound called benzo-l,5-cyclooctadiene-3,7-diyne force this molecule to assume a nearly planar geometry. The pi system of its eight-membered ring contains eight electrons. (Only two of the electrons of each triple bond are part of the conjugated system.)... 

Understanding the Energies and Nodal Properties of the Pi MOs of a Conjugated System... 

Show the energies and nodal properties of the pi MOs of a small conjugated system, whether it is composed of an even or odd number of orbitals. (Problem 22.25)... 

Lewis structures are inadequate to represent delocalized molecules such as buta-1,3-diene. To represent the bonding in conjugated systems accurately, we must consider molecular orbitals that represent the entire conjugated pi system, and not just one bond at a time. 

This lowest-energy orbital is exceptionally stable for two reasons There are three bonding interactions, and the electrons are delocalized over four nuclei. This orbital helps to illustrate why the conjugated system is more stable than two isolated double bonds. It also shows some pi-bond character between C2 and C3, which lowers the energy of the planar conformation and helps to explain the short C2—C3 bond length. 

Pattern of molecular orbitals in a cyclic conjugated system. In a cyclic conjugated system, the lowest-lying MO is filled with two electrons. Each of the additional shells consists of two degenerate MOs, with space for four electrons. If a molecule has AN+2) pi electrons, it will have a filled shell. If it has (47V) electrons, there will be two unpaired electrons in two degenerate orbitals. 

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