Visible light absorbing materials, also known as visible light absorbing dyes, are one of the functional dyes that have been studied more in the field of dye chemistry in recent years. Such dyes can be used as sensitizing dyes, photochromic compounds for erasable optical discs, secondary write optical storage materials, photosensitizers in photodynamic therapy, laser protection absorbing dyes, and infrared absorbing dyes for electrophotography, etc. Wide application market and huge application potential.
At present, with the continuous expansion of laser applications in military and civilian fields, the demand for laser protection has become increasingly prominent. The laser protection wavelength is shifted from the traditional visible light region to the near-infrared light region. From the perspective of technical feasibility and economy, organic absorbing dyes are the main way to achieve wide-spectrum protection of near-infrared lasers. The current main technical requirements for this type of near-infrared laser protective dyes include: near-infrared (700~1400mm) broadband strong absorption (lge≥4), no or weak absorption in the visible light region, good light, thermal and chemical stability, and organic The matrix material has good compatibility and low toxic and side effects on the human body. In addition, this type of dye can also be used as near-infrared stealth materials, near-infrared laser absorbers and stealth materials. In the long research history of cyanine dyes, the most important use is to use in silver halide photographic emulsion systems to enhance its photosensitive performance. In the early 19th century, it was known that by adjusting the number of vinyl groups to control the length of the conjugated main chain, the maximum absorption wavelength (mx) of the cyanine dye was changed. In addition, by modifying or changing the structure of this type of dyes, the m and absorption intensity can also be changed. Therefore, the research on cyanine dyes has been the most active in the research of near-infrared ⊙ dyes in recent years.
Cyanine dyes can be mainly divided into polymethylcyanine dyes, squaraine and crotonic acid cyanine dyes, and octocyanine dyes. The cyanine dye (1) containing polymethyl groups can absorb in a wide range of wavelengths, approximately 3401400nm. In compound (1), R1 and R2 represent heteroatomic aromatic ring substituents, and their electrical absorption or c-supplying properties strongly affect Amax. The greater the basicity of the parent nucleus heterocycle in the main chain, the greater the λmx redshift; the introduction of different substituents on the heterocycle, benzene nucleus or carbon chain can also change the λmx of this type of dye. When an alkyl group is attached to the nitrogen atom, changing the alkyl group can adjust the solubility and affect the aggregation behavior of dye molecules. For example, the introduction of long-chain alkyl groups in cyanine dyes can reduce or eliminate the crystallization caused by aggregation. Among the cyanine dyes containing polymethyl groups, carotenoids (2) are one of the most important ones, and their maximum absorption wavelength Amax=580~700nm. These compounds are derivatives with a linear skeleton composed of eight isoprene units, and Amak has a red shift with the increase of conjugated double bonds. Taking advantage of its light absorption properties, these compounds can also be used in near-infrared sensitizing materials, optical disc recording materials and so on. Because of their broadband absorption characteristics in the visible light region, their application in laser protection will seriously affect the visibility of protective equipment.
Generally speaking, the polymethylcyanine dye Amax=700~900nm, the molar absorption coefficient is also larger, and it is a better type of near-infrared absorbing dye. However, most of these dyes have poor light stability, and with the increase of the number of vinyl groups and the basicity of the heterocyclic ring of the parent nucleus, although λmx has a corresponding red shift, the light and heat stability of the dye will decrease significantly. At present, the focus of research is how to make the mx of this type of dyes produce a larger red shift, without affecting or minimizing the impact on its photothermal stability. Usually a "bridging chain" is added to the main chain of the polymethyl group to make the molecule rigid, which can significantly improve its stability. Squaraine (3) and crotonocyanine dyes (4) can be regarded as the products after the corresponding methine group is replaced by squaraine or crotonic acid, which can be roughly divided into symmetrical and asymmetrical types. When R and R2 are the same, it is symmetrical, R1 and R2 are not the same or R and R2 are ortho-position, it is asymmetrical, these two groups can be alkyl, alkoxy, cycloalkyl, aromatic groups, containing Heteroatomic cycloalkyl groups or aromatic groups, etc., while the oxygen on the central ring can be sulfur, etc. The introduction of squaric acid or croconic acid groups can make the λmx of the cyanine dye produce a larger red shift. For ordinary methine cyanine dyes, the introduction of crotonic acid groups makes the λmx red shift of the dye much greater than the introduction of squaraine groups. The order of the half-width of these dyes at Amx is crotonocyanine>cyanine>squaraine cyanine.
Since Cohen et al. first reported the synthesis of a new compound squaric acid in 1959, the research of this type of dyes (5) The structure of catenary cyanine dyes has been developed very quickly. There are now many patents and documents reporting its new application value and possible possibilities. Applied research direction and so on. Squaraine dyes have excellent optical properties, high absorption strength, good light and thermal stability and chemical stability, and their absorption band can be adjusted in the wavelength range of 500-1400m or even higher by changing the structure of the side chain substituents. What is interesting is that some of these dyes have reduced or even disappeared fluorescence in the solid state. Therefore, these dyes have certain application potential for laser protection and laser stealth at specific wavelengths in the near-infrared. However, this type of dye has a big defect. The dye molecules will form different aggregation states in different media, which will have a great impact on the spectral performance. Different aggregation states make the dye λm shift larger, or make the dye form a wider absorption band without obvious maximum absorption peak. The aggregation behavior of squaraine dyes and their compatibility with organic matrix materials are greatly affected by the molecular structure. For example, the introduction of long-chain alkyl groups in the dye molecules can not only improve the solubility of the dye, but also improve the dye and the organic matrix. Compatibility. Moreover, the spectral properties and solubility properties of these dyes may also be affected by other factors. Therefore, in order to be specifically applied to laser protection and laser stealth, there is still a lot of research work to be carried out for this kind of dyes, such as changing different chromophores and changing the substituents in molecules of the same type. The R~R15 groups in the cyanine dye (5) can be hydrogen atoms, halogen atoms, single-bonded organic groups (such as alkyl groups, alkoxy groups, etc.), etc., and Z can be heteroatoms such as nitrogen and oxygen. They usually have strong absorption in the wavelength range of 750~900nm, are easily soluble in organic solvents, and have good light stability. However, the seven-membered ring in this type of dye is difficult to synthesize, difficult to modify, and the absorption wavelength can be adjusted in a small range.