Synthesis of para-Terphenyl,
an Efficient Laser Dye for Pulsed Operation

Laser Pulse

Background

The word "laser" stands for "light amplification by stimulated emission of radiation". Lasers are possible because of the way light interacts with electrons. Electrons exist at specific energy levels or states characteristic of that particular atom or molecule. The energy levels can be imagined as rings or orbits around a nucleus. Electrons in outer rings are at higher energy levels than those in inner rings. Electrons can be bumped up to higher energy levels by the injection of energy, for example by a flash of light. When an electron drops from an outer to an inner level, "excess" energy is given off as light. The wavelength or color of the emitted light is precisely related to the amount of energy released. Depending on the particular lasing material being used, specific wavelengths of light are absorbed (to energize or excite the electrons) and specific wavelengths are emitted (when the electrons fall back to their initial level). Another characteristic of laser light is that it is coherent. That is, the emitted light waves are in phase with one another and are so nearly parallel that they can travel for long distances without spreading. In contrast, incoherent light from a light bulb diffuses in all directions. Coherence means that laser light can be focused with great precision.

Many different materials can be used as lasers. Some, like the ruby laser, emit short pulses of laser light. Others, like helium-neon gas lasers or liquid dye lasers emit a continuous beam of light. A laser consists of at least three components:

1. a gain medium that can amplify light that passes through it
2. an energy pump source to create a population inversion in the gain medium
3. two mirrors that form a resonator cavity

The gain medium can be solid, liquid, or gas and the pump source can be an electrical discharge, a flashlamp, or another laser. The specific components of a laser vary depending on the gain medium and whether the laser is operated continuously (cw) or pulsed. In a dye laser the gain medium is an organic dye molecule that is dissolved in a solvent. The dye and solvent are circulated through a cell or a jet, and the dye molecules are excited by flashlamps or other lasers. Pulsed dye lasers use a cell and cw dye lasers typically use a jet. The organic dye molecules have broad fluorescence bands and dye lasers are typically tunable over 30 to 80 nm. Dyes exist to cover the near-UV to near-infrared spectral region: 330 - 1020 nm. para-Terphenyl is an efficient laser dye for pulsed operation because it has a high quantum efficiency and a low pump threshold. Its emission is centered at 341 nm and corresponds to the principal fluorescence peak of the molecule.

Schematic Diagram of a Pulsed Dye Laser

 

Schematic Diagram of a Pulsed Dye Laser

Reaction Scheme

 

Reaction Scheme

 

Experimental

I. A mixture of 1.5 g of E,E-1,4-diphenyl-1,3-butadiene (1) and 1.1 g of dimethyl acetylenedicarboxylate (2) in 10 mL of triethylene glycol dimethyl ether (triglyme, caution: discard 90 days after container is opened) was refluxed 30 min. The resulting yellowish solution was cooled and poured into a separatory funnel with 50 mL of tert-butyl methyl ether (see caution above). The organic phase was extracted twice with 50 mL of water to remove the high boiling point solvent, dried over an anhydrous drying agent, and evaporated. The residue was a yellowish oil which solidified on cooling, weight 2.2 g (87%). Recrystallization from methanol afforded colorless prisms of 3, m.p. 97-98 deg.C.

II. A solution of 2 g of 3 in 10 mL of 5% methanolic potassium hydroxide was warmed on a hot plate for about 1 min. A reddish brown color soon disappeared with separation of the isomerized product (4) as a white solid which was collected after cooling, washed with chilled methanol to remove the brown mother liquor, and dried. The yield of colorless product was 1.75 g (88%). Recrystallization from methanol gave fluorescent shiny needles, m.p. 169-170 deg.C.

III. To 1.7 g of the intermediate 4 and 0.7 g of potassium hydroxide were added 5 mL of triethylene glycol. The mixture was stirred with a thermometer and heated, raising the temperature to 140 deg.C in about 5 min. By intermittent heating, the temperature was kept close to 140 deg.C for 5 min longer before the mixture was cooled and diluted with 50 mL of water. It was heated again to boiling and in case there was a small precipitate or the solution was cloudy, pelletized activated charcoal was added and the alkaline solution of 5 was filtered by gravity.

IV. To the alkaline solution of 5 were added 3.4 g of potassium ferricyanide. The mixture was heated on a hot plate with swirling for about 5 min to dissolve the oxidant and to coagulate the white precipitate that soon separated. After filtration, the product was dried to constant weight in a vacuum oven at 100 deg.C. Recrystallization from methanol or dioxane afforded shiny flakes of para-terphenyl (6), m.p. 212 deg.C. The yield was 0.7-0.8 g (ca. 60-70% from 4).

Bibliography

1. Furumoto, H. W.; Ceccon, H. L.; IEEE J. Quantum Electronics 1970, QE-6, 262-268.
2. Fieser, L. F.; Haddadin, M. J.; J. Am. Chem. Soc. 1964, 86, 2392-2395.
3. Williamson, K. L. "Macroscale and Microscale Organic Experiments" 2nd Ed., D. C. Heath and Co: Lexington, 1994, 319-322.

 

Nom : .............................

Prénom : ..........................

Sart-Tilman, le 10 novembre 1999

 

 

Seconde licence en sciences chimiques 1999-2000

Chimie organique

Interrogation de travaux pratiques

 

 

Sur base du schéma réactionnel et des modes opératoires ci-joints, répondez de façon brève, claire et précise aux questions suivantes.
Vos réponses doivent se trouver uniquement dans les espaces prévus à cet effet, sur les faces recto des feuilles. Les faces verso peuvent être utilisées comme brouillons.

 

 

Etape I. Synthèse du composé (3)

1) Représentez la structure plane du E,E-1,4-diphényl-1,3-butadiène (1) dans ses conformations s-cis (1a) et s-trans (1b). (2 points)

Réponse

2) Quelle est la conformation la plus stable? Justifiez votre réponse. (2 points)

Réponse

3) Quelle est la conformation pouvant réagir avec l'acétylènedicarboxylate de diméthyle (2)? Justifiez votre réponse. (2 points)

Réponse

4) En utilisant les réactifs commerciaux suivants, proposez un schéma de synthèse du E,E-1,4-diphényl-1,3-butadiène (1) en trois étapes. L'étape-clé porte un nom spécifique. Mentionnez-le dans votre schéma. (4 points)

Chlorure de benzyle Ph-CH2-Cl

trans-Cinnamaldéhyde Ph-CH=CH-CHO

Méthanolate de sodium CH3ONa

Triphénylphosphine Ph3P

Réponse

5) Pourquoi le mode opératoire recommande-t-il d'écarter les bouteilles de triglyme et de tert-butyl méthyl éther ouvertes depuis plus de 90 jours? Quels contrôle et purification éventuels faudrait-il effectuer avant de passer outre à cette mise en garde? (5 points)

Réponse

Etape II. Synthèse du composé (4)

6) Représentez les structures planes du composé (3) et de son isomère (4). (2 points)

Réponse

7) Quelle est la raison principale de l'isomérisation aisée de (3) en (4)? (2 points)

Réponse

Etape III. Synthèse du dianion (5)

8) Représentez la structure plane du dianion (5). (1 point)

Réponse

9) Dans un mode opératoire plus ancien, la transformation de (4) en (5) était effectuée en chauffant à reflux pendant 4 heures un mélange de (4) et de KOH dans du méthanol. Comparez ces conditions à celles du mode opératoire qui vous est fourni. Commentez les modifications apportées. (4 points)

Réponse

10) Quel(s) avantage(s) pratique(s) voyez-vous à l'utilisation de charbon actif en pastilles plutôt qu'en poudre? (2 points)

Réponse

Etape IV. Synthèse du para-terphényl (6)

11) Commentez la méthode de séchage du para-terphényl brut préconisée par les auteurs. Un simple séchage à l'air aurait-il pu convenir? (4 points)

Réponse

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