What Is Fluorene?
Fluorene is an aromatic hydrocarbon that is a shiny, colorless, plate-like crystal. It is obtained by distillation of coal tar and is insoluble in water, but soluble in ether and benzene. Fluorene, which usually contains impurities, tends to fluoresce. In particular, alcohol solutions are characterized by a weak violet fluorescence.
Fluorenone can also be obtained by oxidation. Fluorenone is used as an intermediate in the manufacture of dyes and is characterized by the high reactivity of the methylene group at position 9 in its chemical structure. The methylene group reacts with aldehydes and carboxylic esters in the presence of a base to produce ethylidene and ketone derivatives.
Uses of Fluorene
Fluorene is used as a raw material for synthesizing various chemicals and as a light-emitting layer and charge transport layer in organic EL devices (organic electronic devices).
Fluorenone is a compound used as a raw material for electrophotographic photoconductors, functional resins, and organic intermediates. Various derivatives are attracting attention in fields such as photoreceptors, optical functional materials, functional resins, organic intermediates, and organic EL.
When introduced into resins, they provide unparalleled resin properties, such as the effect of both high refractive index and low birefringence, as well as enhanced heat resistance and semiconductor-related materials.
It is also used as a raw material for fluorescent dyes and dyestuffs. In particular, fluorescein, a dye that emits green to yellow fluorescence, is widely used in biochemical analysis. in the synthesis of aromatic compounds such as phthalic acid, acetic acid esters, and amines. It is also known to be used as a fuel additive for environmental protection due to its effectiveness in removing sulfur compounds from fuels.
Properties of Fluorene
Fluorene is a colorless solid with a melting point of 116-118°C, a boiling point of 295°C, and a density of 1.17 g/cm3 (20°C). Its solubility in water is 0.005 g/100mL. Although no health hazards have been reported, inhalation may cause respiratory irritation, headache, dizziness, and other symptoms.
1. Physical Properties
It is a clear, colorless, crystalline solid with an aromatic quality. It is insoluble in water, but soluble in alcohol and organic solvents such as benzene and toluene. It has low surface activity with water and is not suitable as a surfactant.
2. Chemical Properties
Fluorene is an aromatic hydrocarbon, which is more reactive than benzene rings, and is used in organic synthetic reactions. Fluorene is aromatic and has a conjugated system because the benzene rings are connected by π-bonds. When excited by light, it emits its fluorescence. Therefore, fluorene is used as a raw material for fluorescent dyes and as a luminescent material.
It is chemically stable, stable to light and air, and has a strong aromatic property and a unique fragrance. It has a high melting and boiling point, is resistant to heat, is soluble in organic solvents, and is luminescent due to its ability to absorb light.
Other Information on Fluorene
1. Synthesis of Fluorene
There are many synthetic methods for fluorene, but the most common are as follows:
Diels-Alder reaction
The Diels-Alder reaction is a reaction between a diene and an enone to form cyclohexadiene. Using this reaction, cyclohexadiene can be ring-opened to yield fluorene.
Friedel-Crafts Reaction
The Friedel-Crafts reaction is widely used in the synthesis of aromatic compounds. Using this reaction, the benzene ring is acylated with a Lewis acid such as aluminum chloride, followed by a dehydrogenation reaction to yield fluorene.
Cycloaddition
Cycloaddition is a reaction in which compounds with double bonds react with each other to form cyclic compounds. Using this reaction, diphenylbutene, a precursor of fluorene, is formed, and fluorene can be obtained by further oxidation reaction.
Fluorene can be produced by pyrolysis of aromatic hydrocarbons at high temperatures. This method is relatively simple and can synthesize fluorene in large quantities, but requires purification because the product may contain many impurities.
Fluorene derivatives can also be synthesized from diiodobiphenyls and other dihalobiphenyls by cross-coupling reactions in the presence of a palladium catalyst.
2. Reaction Mechanism of Synthesis of Fluorene Derivatives
Fluorene derivatives are formed by the following reaction mechanism.
Oxidative addition occurs, in which 0-valent Pd is oxidized to divalent Pd, followed by a metal exchange reaction with copper acetylide to combine acetylene, and cyclization proceeds by repeating the reductive elimination cross-coupling reaction twice, in which divalent Pd is reduced to 0-valent and eliminated.