The main factors affecting fluorescence

 
The amount of energy absorbed by a compound molecule to undergo a multiplicity invariant transition is less than the energy required to break its weakest chemical bond, and is only the most basic condition for producing fluorescence. The fluorescence of a compound is also affected by a variety of other factors. These factors are mainly:
(1)fluorophores
For a compound to fluoresce, it must have fluorophores in its structure. When these groups are part of a molecule's common system, the compound may produce fluorescence.
(2)fluorochromes
The group that enhances the fluorescence of a compound is called a fluorophore. Fluorescence auxochromes are generally electron-donating substituents.
Such as NH, OH, etc. On the contrary, electron-withdrawing groups such as -COOH and -CN weaken or inhibit the production of fluorescence. This is because electron substituents increase the ability of the compound to give r electrons, and increase the energy level of the highest occupied orbital (HOMO) of the connected unsaturated system, and also decrease the lowest empty orbital (LUMO) energy level of the system. This results in a smaller gap between HOMO and LUMO. Therefore, the energy absorbed by the compound during the transition will decrease, and the transition to the excited state will easily occur, which is conducive to the generation of fluorescence. However, the electron-withdrawing groups always reduce the ability of the compound to give n electrons, and reduce the HOMO energy level of the unsaturated system connected to it, and increase the LUMO energy level, resulting in the widening of the energy gap between HOMO and LUMO. Therefore, the energy required for the transition of the compound will be larger, and the transition to the excited state will be difficult. According to the conditions of fluorescence generation and the same selection rules of light absorption and radiation processes, electron-giving substituents will be favorable to fluorescence generation, and electron-absorbing substituents will be unfavorable to fluorescence generation. For example, the fluorescence of aniline, benzene and benzoic acid decreases successively.

(3) The fluorescence can be enhanced by increasing the gelled ring. Increasing the number of coplanar cohesive rings, especially when the cohesive rings are arranged in linear lines, will facilitate the flow of T electrons in the system, thus reducing the energy required for the transition of the system, which is conducive to the generation of fluorescence. For example, the fluorescence order of the following compounds is:
(4) Increasing molecular rigidity can enhance fluorescence. This is because the increased rigidity will weaken the vibration of the molecule, thus making
The excitation energy of the molecule is not easy to be shaped by heat energy due to vibration. 2. Kinetic a is favorable for dry fluorescence production. For example, the coplanarity of molecules is conducive to increasing the mobility of n electrons within molecules, which is also conducive to the generation of fluorescence. For example, the fluorescence quantum yields of the following compounds are:
Compound (I) has strong rigidity, so the fluorescence quantum yield is high; Trans compound (II) has better planarity than cis-compound (III), so the fluorescence quantum yield is higher than that of Cis-compound (III).
(5) The influence of excited electron configuration. According to Kasha's rule, the fluorescence of substances we observe is emitted from the S state (the first excitation singlet). The first excited singlet (S) of an organic compound usually has two electron configurations. One is S; (r,r*) state, and the other is S,(n, T*) state. Since the superior →t* transition is a permitted process, the radiative transition r*→7 emitting fluorescence is also a permitted process; In contrast, the n→n* transition is a prohibited process, so the radiative transition n*→n that emits fluorescence is also a prohibited process. Therefore, when the electron configuration of S state is (r,T*) state, it is conducive to the generation of fluorescence; When the electron configuration of S state is (n,t*) state, it is not conducive to the generation of fluorescence. For example, the S-state of benzene (r,t*) has a fluorescence quantum yield of 0.2, and the S-state of diphenylone (n, T*) under the same conditions has a $=0.
(6) The heavy atom in the molecule will lead to a decrease in the fluorescence quantum yield, because the heavy atom has the effect of enhancing intersystem transmigration, which will increase the rate constant and quantum yield of intersystem transmigration from the S state to the T state (the first excited triplet state), resulting in a decrease in the fluorescence quantum yield. For example, the fluorescence quantum yield of the following compounds gradually decreases with the increase of heavy atoms in naphthalene molecules.
(7) Increasing the polarity of the solvent is generally conducive to the production of fluorescence. For example, quinoline, pyridine, and pyridine do not fluorescein non-polar solvents, while they all fluorescein polar solvents. The fluorescence quantum yield of 4'-N, n-dimethyl flavone is only 0.007 in cyclohexane and 0.96 in acetonitrile. However, the g of flavonoids in protonic solvent is significantly reduced, which is caused by the fluorescence quenching caused by intramolecular charge transfer.
(8) Reducing the temperature of the system can improve the fluorescence quantum yield. For example, the fluorescence quantum yield of cis-stilbene is =O at 25 ° C, but =0.75 at 77K. This is also because after the temperature is lowered, the thermal vibration of the molecule is reduced, which is not conducive to deactivation in the way of heat dissipation, but is conducive to improving its deactivation in the form of released photons, that is, improving the quantum yield of fluorescence.
(9) Other factors, such as hydrogen bond, adsorption, solvent viscosity increase, etc. can increase the quantum yield of fluorescence.