Generation mechanism of fluorescence

(1) Molecular energy levels and transitions. Each substance molecule has a series of closely spaced energy levels, which are called electronic energy levels. The molecular energy level is more complex than the atomic energy level, and there are a series of vibration and rotation energy levels in each electronic energy level. When a substance is irradiated by light, it may partially or completely absorb the energy emitted by people. In this process, the energy of photons is transferred to matter molecules, so the transition of electrons from lower energy level to higher energy level occurs, which is extremely
Fast, it takes about 10-15s, and the absorbed photon energy is equal to the energy difference between the two involved energy levels. At room temperature, most molecules are in the lowest vibration energy level of the ground state, that is, the ground state. Molecules in the ground state are excited after absorbing energy, which is called electron excited molecules.
(2) The multiplicity (also called multiplicity) of electron excited state is expressed by M=2S+1, and s is the algebraic sum of electron spin quantum numbers (its value is 0 or 1). Most molecules contain even-numbered electrons. In the ground state, these electrons exist in pairs in each atom or molecular orbit, with opposite spin directions. The net spin of electrons is equal to zero: S=1/2+ (1/2)=0, and its multiplicity m = 2s11 = 1 (M=2S the magnetic quantum number). Therefore, the molecule is diamagnetic and its energy level is not affected by the external magnetic field. The ground state of most organic molecules is singlet, which is represented by the symbol S.
When one of a pair of electrons in the ground state absorbs energy, the molecule is still in the excited singlet state if its spin direction does not change during the transition to a higher energy level. If the spin direction of electrons is also changed during the transition, then the molecule has two electrons with unpaired spins. The net spin of electrons is not equal to zero, but equal to 1(S=1/2+1/2=1), and its multiplicity M=2S+1=3. The molecule is in the excited triplet state, which is represented by the symbol T. Compared with the triplet, the excited triplet has lower energy than the corresponding excited singlet. The ground state, the first-and second electron excited singlets are represented by S, St and S; the first and second electron excited triplets are represented by Ti(1st excited triplet state) and T.

(3) Energy transfer path from excited state to ground state When an electron is in an excited state, it is unstable. When it returns to the ground state, various photophysical processes can occur in the molecule to lose energy, including the excitation process of the molecule, radiation transition (luminescence), non-radiation transition and vibration relaxation. There are many ways to inactivate the excited state and return from the excited state to the ground state, and the way with the fastest speed and the shortest lifetime of the excited state is dominant. Deactivation pathway is as follows: Deactivation process of non-radiation transition, as a result, electron excitation energy is converted into vibration energy or rotation energy. Comprises the following processes.
Internal conversion (IC) refers to the non-radiative transition process between two electron levels in the same multiplet. Such as S2→Sj, T:→T, these two processes are completed in 10-13~10-11s.
Intersystem crossing (ISC) refers to the nonradiative transition between two electron energy levels with different multiplets, the nonradiative transition between overlapping rotational energy levels, and the change of electron spin state. Such as S→T, t → S.
(3) vibrational relaxation (VR) the transition from high vibrational energy level to low adjacent vibrational energy level in the form of heat energy exchange within the same electronic energy level. The time of vibration relaxation is 10~14~10-12s.
(4) external conversion (EC) excites non-radiative transition of energy by interaction between molecules and solvents or other molecules. External conversion will weaken or "quench" fluorescence or phosphorescence.

In the deactivation process of radiation transition, photon emission occurs, accompanied by fluorescence or phosphorescence. According to Jablonsky's light energy dissipation diagram, the energy absorbed by molecules can be dissipated through various paths, and the fluorescence process is only one of them. Fluorescence emission is the radiation (mostly S-+$, transition) released by electrons from the lowest vibrational energy level of the first excited singlet state to the low energy state with the same multiplicity. The common electronic transitions in luminescence analysis are
N-T* and T-n* transitions have different energy level properties of singlet or triplet states. Generally, the absorption coefficient of-pico * transition is large, the fluorescence is strong, and the yield of intersystem crossover is small; N-T * transition is opposite. N→T*Emax <100, average life span 10-7 ~ 107 5s, "* * Gx≥10i, average life span 10 ~ 9 ~ 10' s. Form *…→*T is the main transition type of organic compounds to produce fluorescence. The transition of excited state properties of molecules is shown in Figure 4-I..
Fluorescence emission is the inverse process of light absorption. Since the energy of emitted fluorescence is less than the energy absorbed by molecules, the emission wavelength of fluorescence is longer than the absorption wavelength. The fluorescence spectrum has a mirror image relationship with the absorption spectrum, but the fluorescence harmonic is always redshifted from the corresponding absorption spectrum, which is called Stokes shift.
Is the difference between the maximum excitation wavelength and the maximum emission wavelength. There are two main reasons for Stokes displacement; First, the excited molecule that transits to the excited state with high vibrational energy level will firstly undergo vibration relaxation at a faster speed, losing part of its energy and reaching the zero vibrational energy level; second, the configuration of the excited molecule will be further adjusted soon to reach the stable state at a lower energy level, which will lose part of its energy, both of which will lead to the energy of the fluorescence quantum being lower than that of the absorbed photon.