Question

1) Provide brief definitions for the following: a) conjugation; b) polymer, c) HOMO and LUMO.

2) Why do conjugated compounds absorb light in the Ultraviolet region? Why do more conjugated compounds absorb at higher wavelengths/lower energies? What is required for organic compounds to absorb in the Visible light region?

Answer 1

1) Provide brief definitions for the following: a) conjugation; b) polymer, c) HOMO and LUMO.

conjugation

In chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals orcarbenium ions may be part of the system. The compound may be cyclic, acyclic, linear or mixed.

Conjugation is the overlap of one p-orbital with another across an intervening sigma bond (in larger atoms d-orbitals can be involved).

A conjugated system has a region of overlapping p-orbitals, bridging the interjacent single bonds. They allow a delocalization of pi electrons across all the adjacent aligned p-orbitals. The pi electrons do not belong to a single bond or atom, but rather to a group of atoms.

polymer

A polymer  is a large molecule, or macromolecule, composed of many repeated subunits. Because of their broad range of properties, both synthetic and natural polymers play an essential and ubiquitous role in everyday life. Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure and function. Polymers, both natural and synthetic, are created via polymerization of many small molecules, known as monomers. Their consequently large molecular mass relative to small molecule compounds produces unique physical properties, including toughness,viscoelasticity, and a tendency to form glasses and semicrystalline structures rather than crystals.

HOMO and LUMO.

HOMO and LUMO are acronyms for highest occupied molecular orbital and lowest unoccupied molecular orbital, respectively. The energy difference between the HOMO and LUMO is termed the HOMO