Introduction
1.6 Mechanism of luminescence
In order for luminescence to occur, the first and foremost thing is that the molecule should be in excited state which is obtained by the promotion of an electron from the ground state to an excited state. The excited electron leaves a vacancy in the ground state which is termed as hole. As oppositely charged, the electron and hole undergoes coulombic attraction between them and forms a quasi-particle termed as exciton and the energy that binds the electron-hole pair together is termed as binding energy. The exciton binding energy varies depending on whether the material is organic or inorganic. Unlike their inorganic counterpart, excitons formed in an organic semiconductor is usually are located on one molecule due to the weak intermolecular van-der-Waals interaction. As a result, organic molecule possesses large binding energy in the order of >500 meV as compare to that of inorganic material (<0.1 eV). Since the excitons formed in organic semiconductor are highly localized in the molecule or the polymer unit, it is also called Frenkel exciton to differentiate it from weakly bound excitons in inorganic semiconductors (Mott-Wannier excitons) which are highly delocalized throughout the lattice.
Fig. 1.5 Mott-Wannier excitons (a) and Frenkel exciton (b).
These exciton states are further divided into two different types based on the quantum mechanical selection rule into singlet and triplet states. Depending on the spin orientation of the excited electron, a singlet or a triplet can form when one electron is excited to a higher energy level. In an excited singlet state, the electron is promoted in the same spin orientation as it was in the ground state (paired). In a triplet excited stated, the electron that is promoted has the same spin orientation (parallel) to the other unpaired electron.
Fig.1.6 shows the Jablonski diagram. A Jablonski diagram is the typical representation of the probable electronic states of a molecule and possible processes through which molecules enter and leave the excited state: photon absorption, internal conversion, fluorescence, intersystem crossing, phosphorescence, delayed fluorescence and triplet–triplet transitions. The singlet electronic states are denoted S0, S1, S2, . . . and the triplet states are denoted as T1, T2, and are represented by bold horizontal lines. Each electronic energy states are associated with multiple vibrational levels and are represented by thin horizontal lines. However, due to the large number of possible vibrational energy levels only a portion of them are shown in the diagram.
The first transition in most Jablonski diagrams is the absorption of a photon of certain wavelength that brings a molecule from one of the vibrational level of S0 to one of the vibrational level of S1, S2…. Since at room temperature the majority of molecules laid in the 0-vibrational level of S0, the absorption process is shown to excite a molecule from this level in the diagram. Absorbance is a very fast process and usually occurs within 10-15 seconds. Once the molecule is excited by the process of absorption, there are number of possible de-excitation process that can take place.
Fig. 1.6 Jablonski diagram.
The first de-excitation process is vibrational relaxation and internal conversion. It is a non-radiative process. This process is also very fast and occurs immediately following absorbance between 10-14 and 10-11 seconds. When a molecule is excited to an energy level higher than the lowest vibrational level of the first electronic state, vibrational relaxation (and internal conversion if the singlet excited state is higher than S1) leads the excited molecule towards the 0-vibrational level of the S1 singlet state. From S1, internal conversion to S0 is possible but is less efficient than conversion from S2 to S1, because of the much larger energy gap between S1 and S0).
The second possible de-excitation process by which an excited molecule can come back to ground state is by means of emitting a photon. This is called as fluorescence.
Fluorescence emission generally occurs due to the transition between the first excited state S1 to the ground state S0. Fluorescence is a slow process on the order of 10-9 to 10-7 seconds A third possible de-excitation process from S1 is intersystem crossing where an electron changes its spin multiplicity and moves from excited singlet state S1 towards the excited triplet state T1. It is a non-radiative process and occurs within a time scale of 10-8 to 10-3
T1 S1
Internal Conversion
Absorption
Fluorescence
Phosphorescence Intersystem
Crossing
Vibrational Relaxation Vibrational
Relaxation
Ground State S0
S2
Singlet Excited State
Triplet Excited State
E nergy
seconds. The crossing of an electron from singlet to triplet state is forbidden in principle due to their different spin multiplicity. However, it can be possible if the spin orbit coupling between the orbital magnetic moment and the spin magnetic moment is large enough.
The fourth possible de-excitation pathway is the transition between the excited triplet state T1 to the ground state S0. This is termed as phosphorescence. Usually this is a forbidden process and can only be observed with materials with heavy atoms that possess high spin orbit coupling. In most of the cases the non-radiative de-excitation from the triplet state T1, is predominant over radiative de-excitation called phosphorescence. Phosphorescence is thus very low with a time scale of 10-4 to 10-1 seconds.
Delayed Fluorescence is another possible de-excitation processes through which a molecule can come back to ground state. This can be either thermally activated or due to the triplet-triplet annihilation. In case of thermally activated delayed fluorescence reverse intersystem crossing can happen from T1 to S1 if there is a very small energy difference between them provided that the lifetime of the molecule in the triplet excited state is long enough. This process is thermally activated and therefore the fluorescence increases with the increase in temperature. On the other hand, if the concentration of molecule at the triplet excited state is more, there can be a collision between two molecules in the T1 state. As a result, one of the molecules can gain enough energy to go to the higher triplet excited state and then return to the S1 state, thus leading to a delayed fluorescence emission.