Factors affecting the luminescent properties of fluorescent environmental protection pigments

When a wavelength of light is used to illuminate a substance, the substance can emit a longer wavelength of light than the irradiation wavelength in a very short time, that is, the substance will emit a longer wavelength of light after absorbing a shorter wavelength of ultraviolet or visible light. The generation of fluorescence starts from the lowest vibrational level of the first electron singlet excited state, which is the radiation released when the substance is deactivated from the excited state to the low energy state with the same multiplicity, and has nothing to do with which energy level the fluorescent substance was originally excited to. The most basic condition for a compound to be able to produce fluorescence is that the energy absorbed by it to undergo a multiplicity invariant transition is less than the energy required to break the weakest chemical bond.
4.1.3.1 Wavelength of excitation light
A necessary condition of fluorescence process is that the wavelength of excitation light is higher than the wavelength of fluorescence, that is, the energy of excitation light is higher than the minimum excitation energy of valence electrons. Because the molecule has to absorb enough energy to jump above the first excited state in order to emit waste light. After the molecule absorbs light energy, the electron transitions to the first or second excited state, and loses part of the energy back to the first excited state due to vibration relaxation and heat dissipation, and then emits fluorescence, therefore, the fluorescence wavelength emitted by the fluorescent material is generally longer than the wavelength of the excited light, that is, redshift occurs, known as Stokes shift.
4.1.3.2 Molecular Structure of organic compounds
Only a small part of the known large number of organic compounds can emit strong fluorescence, their fluorescence intensity is closely related to their structure, to understand the relationship between fluorescence and structure, it is necessary to understand the type of molecular light absorption, and the competition of various processes after molecular light absorption. Strong fluorescent organic compounds have the following characteristics:
Has a large common r bond structure; A rigid plane structure;
The lowest excited state S of a single heavy electron is T*→→T type; ④ Substituents are electron-donating substituents.
(1 Common fluorescent effect of organic substances, all contain common double bond system, the larger the common bond system, the more easily the delocalized large T bond electrons excited, the easier the fluorescence is generated and the peak shift to the direction of long wave. Most fluorescent substances have aromatic or heterocyclic rings, usually larger than 1 benzene ring. There must be fluorophores in the structure of the compound such as =C=O, one N=O, one N one N, -C-n one, =C-S, etc. When these groups are part of a molecular conjugate system, the compound may produce fluorescence. For example, the q of tea =0.29, (=310nm; q1 of anthracene =0.46, =400nm.
(2) The rigidity and coplanarity of fluorescent materials with rigid planar structure are improved, which is conducive to fluorescence emission. For example, fluorescent yellow (fluorescein) is flat
The fluorescence is very strong, and the fluorescence efficiency in 0.1mol/L NaOH solution can reach 0.92. Phenolphthalein, on the other hand, does not produce fluorescence because it has no oxygen bridge and its molecules do not easily remain flat. For example, the fluorescence efficiency of Sue is close to 1, while that of biphenyl is only 0.20(Figure 4-3).
(3) Substituent effect, substituents in the molecular structure have a certain influence on the generation of fluorescence, compounds with high fluorescence quantum yield should have fluorescence emitting groups in their molecules, chromophore refers to the molecular structure of the valence electron energy level within the range of excitation light energy, and has a large light absorption coefficient, which is the main influence factor to determine the fluorescence color and efficiency. In addition, the fluorescence cochromophore is connected in the molecule, which can improve the fluorescence quantum yield and change the absorption wavelength. For example, when the structure of a compound contains an electron-donating substituent - HN2, NHR,
When one NR, one OH, one OR, one CN, etc., and these groups are part of the molecular copulation system, p →x copulation is generated, then the compound may produce more obvious fluorescence. Electron absorbing substituents weaken fluorescence and strengthen phosphorescence, such as nitro and the most common one N one N- have a strong blocking and quenching effect on fluorescence. Azo dyes rarely show significant fluorescence.
(4) The position of substituents the ortho-substituents and para-substituents on the aromatic ring can enhance fluorescence.
The meta-substituents weaken the fluorescence, -CN substituents
Except. With the increase of common system, the influence of substituents will be weakened correspondingly.
(5) Heavy atom substitution effect The so-called heavy atom substitution generally refers to halogen (CI, Br and I) substitution. The fluorescence intensity of aromatic monster decreases with the increase of halogen atomic weight. After the fluorophores replace the heavy atoms, the fluorescence decreases and the phosphorescence increases. This is because heavy atoms strengthen the coupling of electron spin orbits in the fluorescent body, and the intersystem crossover of S→T is enhanced.
Generally speaking, for aromatic compounds, the fluorescence quantum yield can be improved by increasing the number of dense rings, increasing the degree of molecular coactivity and increasing the molecular rigidity.

4.1.3.3 Impact of Environmental Factors

(1) Influence of Luoji If the fluorescence process occurs in a solution, the polarity of the solution has an influence on the fluorescence process. Many illuminants are very sensitive to the polarity of the solvent environment in which they are located, and the fluorescence effect is related to the solvent used. Especially those containing polar substituents on the aromatic ring. Different additives will significantly change the position and intensity of fluorescence harmonic. For example, the change of solvent polarity may lead to the harmonic red shift (often accompanied by the decrease of fluorescence quantum yield) or blue shift of luminescence light. Changing from a non-polar or low-polar solvent to a polar solvent can produce a dark (reddish) effect, and the order of the dark effect is lipid rib meridian < aromatic meridian < lipid < alcohol < amide. Generally, the fluorescence intensity increases with the polarity of the solution, which can produce a longer wavelength fluorescence effect.
Solvent polarity effect can be simply divided into general solvent effect and special solvent effect. The former refers to the influence of refractive index and dielectric constant of solvent.
Ring, the latter refers to the special chemical interaction between phosphor and solvent molecules. Such as hydrogen bond formation and coordination. The general solvent effect is ubiquitous. The special solvent effect depends on the chemical structure of solvent and phosphor, and the shift value of fluorescence spectrum caused by it is often large.
Lippert equation is commonly used as the equation of general solvent effect. Diploe-diploe interaction (dipole-induced dipole-induced dipole) between phosphor and solvent molecules depends on the polarization of solvent molecules and solute molecules. This interaction affects the energy level difference between the ground state and the excited state. The energy level difference is proportional to the refractive index n (reflective index) and dielectric constant e( dielectric con-stant) of the solvent, which is described by Lippert equation as follows: V is suitable for aprotic solvents, and V is the wave number of absorption and emission respectively; H is Planck constant; C is the speed of light; P and p' are the dipole moments of the excited state and the ground state of the phosphor, respectively; A is the radius of the solvent cavity where the phosphor is located. F=(e-1)/(2e+1)-(n2-1)/(2n2+1) is called the degree of polarization. △f increases, the wave number difference increases; With the increase of n, the wave number difference decreases; As e increases, the wave number difference increases.
The special solvent effect of hydrogen bonding in excited States often decreases the fluorescence quantum yield. For example, the quantum yield of 5- hydroxyquinoline in polar ethyl ether, acetonitrile, dioxane and other solvents decreases compared with that in alkane, especially the intramolecular hydrogen bonding, which makes the fluorescence quantum yield decrease even more. Two quinoline derivatives, 8- hydroxyquinoline and 5- hydroxyquinoline, have the same absorption spectra, but the quantum yield of 8- hydroxyquinoline is 1/100 of that of 5- hydroxyquinoline. Because 8- hydroxyquindox is in the acceptor solvent, intramolecular hydrogen bonds may be converted into intermolecular hydrogen bonds, and the quantum yield increases. When determining molecules containing functional groups of-OH,-COOH,-NR and-SR, we should try to choose solvents that do not have strong hydrogen bonding with substituents.
(2) Influence of pH value. When the fluorescent substance is weak acid or weak base, the change of pH value of the solution will affect its fluorescence intensity and fluorescence spectrum. The fluorescence spectra of most aromatic compounds containing acidic or basic groups are very sensitive to solvents and hydrogen bonding ability. Molecules and ions of weak acids or bases can be regarded as different types, with their own special fluorescence spectra and quantum yields. Sometimes, the two types of fluorescent substances, the co-acid base and the co-acid base, both emit fluorescence and overlap each other, which is similar to the isoelectric point with equal absorption point [4].
(3) Influence of temperature Temperature has a significant influence on fluorescence intensity and fluorescence spectrum, and the fluorescence intensity mainly affects the fluorescence quantum yield. In normal circumstances
In this case, with the decrease of temperature, the fluorescence quantum yield and fluorescence intensity of fluorescent substance solution will increase. The fluorescence process is very sensitive to temperature, and the fluorescence intensity and wavelength of materials will change at different temperatures.
When the temperature of sodium fluorescein in ethanol is below 0℃, the quantum yield increases by 3% for every 10℃ decrease in temperature, and reaches 100% after cooling to-80℃. At this time, both quenching and internal energy conversion disappear. Temperature rise is one of the main reasons for the decrease of fluorescence intensity, and it is the internal energy conversion of molecules. When the excited molecules receive additional heat energy and move to the intersection point C along the excitation potential energy curve, they are converted to the potential energy curve of the ground state, so that the excitation energy is converted into the vibration energy of the ground state, and then the vibration energy is lost through vibration relaxation. The relationship curve between fluorescence intensity of solution and temperature can be expressed by the following formula. E is the additional excitation heat energy that must be obtained when the excitation molecule is transferred to the ground state curve.
Temperature changes affect the viscosity of the medium, and with the decrease of temperature, the viscosity increases, thus reducing the collision quenching probability between molecules and solvent molecules; it can also affect the rotation or twisting speed of molecules themselves and affect anisotropy.
(4) Influence of concentration: Fluorescent dyes can only emit fluorescence in dilute solution. With the increase of concentration, the fluorescence intensity increases, and it weakens when it reaches a certain concentration or above.
In addition, there are the effects of surfactants.