This adds a driving force for the electrons to hop towards the drain and the circuit to run. If simultaneously there is a voltage applied between the source and drain electrodes, a sort of energetic slant is added to the LUMOs. When we apply a voltage to the gate, it introduces the excess electrons into the system in a mechanism similar to a capacitor (there is a dielectric material between the gate electrode and the organic semiconductor). It acts like a switch that turns the system on. How did these electrons get there? In the OFET there is a gate electrode. LUMO here is referring to the neutral molecule, not the one with an excess electron, which is really an anion whose molecular orbitals are perturbed relative to the neutral molecule by the presence of the excess electron. "Excess" electrons hop between the LUMOs of adjacent molecules. In the particular case of an n-type (the majority of carriers that move around are electrons) OFET, you can imagine a set of nearly iso-energetic LUMO orbitals which have some amount of Gaussian disorder due to the disordered nature adjacent molecular units. Rather than charge carriers traveling in bands (inorganic materials), the carriers in an organic material hop between the localized states, which to a good approximation are very similar to the frontier orbitals (HOMO and LUMO) of the isolated molecules. On the other hand, organic materials tend to be disordered, which leads to poor orbital overlap between neighboring molecules, and electronic states to be localized on individual molecules. The bands are highly delocalized electronic states that arise from the ordered nature the material. In an inorganic material we have the idea of the valence band, where holes (missing electron) can move around, and the conduction band (excited / excess electron) and move around. You can think of the HOMOs as playing the role of a valence band, and the LUMOs playing the role of a conduction band, but there are some really important differences. Substituents on the diene or dienophile change the energy of the FMOs and polarise their attached π bond, introducing a rate-increasing hard-hard (Coulombic) factor into the reaction and enhancing the regioselectivity (Figure 2.7).įigure 2.3 FMO interaction diagram for 1,3-butadiene and ethene.While the band gap is an important idea, particularly in solar cells / light emitting diodes, which absorb / emit light, the HOMO and LUMO play an important role in how charge carriers (electrons and / or holes) move around in organic semiconductors. The interaction increases if the ψ 2– ψ* gap is made smaller, usually by using a dienophile with an electron attractor group Y. For 1,3-butadiene reacting with ethene (Figure 2.3), the HOMO-LUMO interactions are relatively weak since the interacting orbitals are not close in energy (Δ E is small). The soft-soft interaction of filled with empty orbitals is the major attractive force in the Diels-Alder transition structure, which is stabilised through two HOMO-LUMO interactions: The filled ψ 2 of the diene interacts with the unfilled ψ* of the dienophile, while the empty ψ 3 of the diene interacts with the filled π of the dienophile. Kurt Alder (1902–1958)Ĥ Matching the reacting components: Effects of donor, acceptor and conjugating substituents Figure 2.1 depicts the cycloaddition using both line structures and orbitals note that the symmetry of the interacting orbitals permits synchronous bond formation while the p orbitals of the pi bonds become the sigma bonds of the product. The six-membered transition structure for the process resembles a folded cyclohexane (boat conformation). The net reaction involves the loss of two pi bonds and the gain of two sigma bonds, which makes it about 167 kJ/mol (40 kcal/mol) exothermic. For example, if the dienophile bears two groups with a cis relationship, the two groups will remain cis in the product. The process is very useful because it generates four new sp 3 carbons stereoselectively, the geometry of both starting molecules being conserved within the product (the reaction is stereospecific). The archetypal Diels-Alder reaction is a cycloaddition in which a diene (the 4 π component) combines with a dienophile (the 2 π component) to produce a 6-membered ring. Matching the reacting components: Effects of donor, acceptor and conjugating substituentsġ Thermal concerted cycloaddition.Stereoselectivity: Preference for endo transition structure.Sigmatropic rearrangements Sigmatropic rearrangements.Electrocyclic reactions Electrocyclic reactions.
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