Coumarin dyes are important components of fluorescent dyes. The unmodified coumarin dye is colorless flake or powdery crystal. It is barely insoluble in cold water, but can be dissolved in hot water. Molecular formula can be expressed as C9H6O2 and the relative molecular weight is 146.15. Melting point is 68~70oC and the boiling point is 298oC. Relative density is 0.9350 g/mL, which is less dense than water. It should be noted that this substance is carcinogenic. Coumarin dyes are found naturally in some plants such as black bean, wild vanilla and the orchid, which do not make edible commonly, but smoking and external use are allowed.
Figure 1. The structural formula of unmodified coumarin.
The emission region of coumarin dyes is mainly in the short-wave region (400-520nm). Coumarin dyes have excellent fluorescence properties, such as perfect photostability, high fluorescence emission intensity, large Stokes shift and good two-photon properties. Coumarin dyes can be used in the field of fluorescent detection, photoelectric materials and dye laser.
- Fluorescent Detection: Coumarin dyes can be used as fluorescent dyes in the field of fluorescence detection technology. Coumarin dyes can be designed as organic small molecule fluorescent probe with the advantages of low toxicity, high sensitivity and excellent selectivity. Meanwhile, fluorescent probes are highly selective and sensitive to the detection of small biological molecules, which is in line with the needs of the current medical demands and has been constantly applied to the biomedical field.
Figure 2. An example of coumarin dye used as a fluorescent probe.
- Photoelectric Materials: Coumarin dyes are ideal donor dyes because of their rigid coplanar structure and good electron donor capacity. Coumarin dyes modified with appropriate substituents can effectively sense visible light and have a high photoelectric conversion rate. Therefore, they can be used in solar cells. As we all know, solar energy is a kind of renewable and clean energy. The effective use of solar energy can effectively alleviate environmental pollution and energy shortage.
Figure 3. An example of coumarin dye used as a photoelectric material.
- Dye Laser: Traditional lasers can only produce particular wavelength light, which limits their applications. To solve this problem, tunable wavelength dye lasers are researched. Coumarin dyes are ideal candidates due to high fluorescence efficiency and large Stokes shift. In addition, with the increase of pH of dye solution, the laser wavelength can be redshifted, which can realize tunable wavelength.
Figure 4. An example of coumarin dye used as a dye laser.
Coumarin is readily modified to obtain many derivatives. According to the types of substituents, coumarin dyes can be divided into four groups: simple coumarin dyes, furocoumarins, pyranocoumarins and the others.
- Simple coumarin dyes: This type of coumarin dyes is modified by substituents only on the benzene ring.
- Furocoumarins: The isopentenyl group on the nucleus of coumarin often reacts with orthotopic phenolic hydroxyl group (7-OH) to obtain furan or pyran ring.
- Pyranocoumarins: Coumarin C-6 or C-8 isopentenyl groups are cyclized with orthotopic phenolic hydroxyl group to form 2,2- dimethyl-α-pyran ring structure.
- The others: Bishydroxycoumarin, and the others.
Under the action of sodium acetate, salicylaldehyde reacts with acetic anhydride to obtain coumarin in one step.
- Bugaenko D I, Karchava A V, Yunusova Z A, et al. Fluorescent probes on the basis of coumarin derivatives for determining biogenic thiols and thiophenols[J]. Chemistry of Heterocyclic Compounds, 2019,55(6),483-489.
- Bo Liu,* Ran Wang, Weijun Mi, et al. Novel branched coumarindyes for dye-sensitized solar cells: significant improvement in photovoltaic performance by simple structure modification[J]. Journal of Materials Chemistry, 2012,22, 15379-15387.
- I. Esnal, G. Duran-Sampedro, A. R. Agarrabeitia, et al. Coumarin–BODIPY hybrids by heteroatom linkage: versatile, tunable and photostable dye lasers for UV irradiation. Physical Chemistry Chemical Physics. 2015,17, 8239-8247.