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The way in which a molecule luminesces can be used to identify it using spectroscopy. There are two kinds of photoluminescence that can be exhibited by molecules, which occur when their electrons relax from an excited, higher energy state.
In this article, we look at the two types of photoluminescence, fluorescence and phosphorescence, and how their luminescent mechanisms differ.
Both fluorescence and phosphorescence occur when molecule-bound electrons absorb photons which causes them to excite and enter a higher molecular energy state. Following excitation, the electrons then relax and lose energy. This results in a return to the ground state and the emission of a photon - this is photoluminescence. Whether it is fluorescence or phosphorescence that is exhibited depends on the molecule that is excited and the mechanism of relaxation to the ground state.
In photoluminescence, the promotion of electrons to a higher orbital, followed by its subsequent decay to the ground state has no effect on the electronic state of the molecule. Most molecules have an even number of electrons (which are paired in opposite spins) and electron promotion does not generally have any perturbing effect. The mechanism of both fluorescence and phosphorescence is often depicted using Jablonski diagrams.
The Mechanism of Fluorescence
Molecules which can fluoresce contain fluorophores, which are regions of electronic structure that exhibit fluorescence. At room temperature, these molecules are usually in their lowest energy, ground-state. There are many different vibrational levels within the ground state, and molecules in their un-excited form adopt the lowest vibrational level.
When exposed to UV/Vis light the fluorophore will absorb photons. Upon absorption, photons make the molecule's electrons adopt a higher molecular and vibrational state. This is usually the first or second excited singlet state, termed S1 and S2, respectively.
The molecule now has additional energy to lose, some of which can be lost as kinetic energy. The extra energy causes the molecules to collide with the other molecules in a sample. These collisions cause the molecules to lose their vibrational energy - these energy losses are known as non-radiative transitions. Once some of the energy is lost, the molecule returns to the lowest vibrational level of the excited state. Once this point is reached no more energy can be lost kinetically and, to return to the ground state, the molecule must relax via fluorescence.
The S1 or S2 electron returns to the ground state by ejecting the extra energy as a photon. As some energy has been lost the molecule’s energy levels return to the ground state (to any of the ground state vibrational levels). The return to the ground state causes a photon to be ejected from the molecule. Due to the earlier, non-transitive energy losses, this photon is of a different wavelength as the photon that excited the molecule. This is characteristic of fluorescence.
A diagram illustrating the mechanism of fluorescence - photon absorption causes an excitation, which is then followed by non-radiative (kinetic) and radiative (fluorescence) relaxations. Image Credit: Wikipedia.
Because fluorescence always occurs as a transition from the lowest energy excited state to the ground state, the shape of a fluorescence emission spectrum is always the same, regardless of the wavelength of light used to excite the molecule. Additionally, there is no perturbation of the molecular shape under excitation, so the distribution of vibrational levels in the excited and ground states are very similar. The spin direction of the electrons is also preserved in fluorescence.
It is not only molecules which can fluoresce. Atoms exhibit a phenomenon known as resonance fluorescence. There are no vibrational levels in atoms, therefore they can’t exhibit the same type of fluorescence that a molecule can. Additionally, the wavelength emitted by atoms is the same as the incident wavelength. It is a phenomenon often seen in quantum dots.
The Mechanism of Phosphorescence
Phosphorescence follows the same initial excitation mechanism. That is, a monochromatic beam of light is fired towards a molecule causing it to go from the ground state to either the S1 or S2 excited singlet states. It is in the return to the ground state that it is very different.
Unlike in fluorescence, the spin of the excited (and promoted) electron can be reversed. This creates a scenario where two electrons are no longer paired, and the molecule has two electrons of the same spin in different electronic orbitals. Following quantum mechanical laws, these electrons can exist in 3 states. This gives rise to an excited triple triplet state if the spin is reversed, and a singlet state if the spin is not reversed.
Upon decay, phosphorescence doesn’t return immediately to the ground state. Instead, it transitions through a metastable state, known as T1. This transition is known as an intersystem crossing, the metastable state occurs because the decay of an excited triplet state is forbidden in molecules with even numbers.
The decay via the T1 state is much slower, as it is technically forbidden, and the molecule is only considered to be back at the ground state when all the energy has been released. Phosphorescence is most commonly seen with heavier molecules and many are crystalline in nature, as this confers enough stability for them to cope with the forbidden transition.
A diagram comparing fluorescent and phosphorescent relaxations. Phosphorescence occurs via an intersystem crossing into a triplet state, which then relaxes slowly to the ground state, emitting photons in the process. Image Source: IBS
The Differences Between Fluorescence and Phosphorescence
Despite them being similar in nature, there are differences. The key mechanistic difference is that fluorescence emits light immediately after photon adsorption, whereas there is a delay with phosphorescence.
Additionally, once the excitation source is removed from a fluorescing molecule, the light emission stops. In comparison, the emission stays for a while once the excitation source is removed in phosphorescence. Their visible appearance is also slightly different. Fluorescence gives an immediate flash, whereas phosphorescence is a lower ‘glow in the dark’ appearance.
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