Question

Jablonski diagram (Fig. 18-16): draw & explain in detail absorption, vibrational relaxation, internal conversion, intersystem crossing, fluorescence, phosphorescence

Answer 1

Whenever a molecule absorbs the visible and/or ultraviolet light it acquires sufficient energy to reorganize or break most of the covalent bond in the molecule. Since the relationship E = hc/λ, we see that visible light having longer wavelength (400 to 800 nm) is having less energy (70 to 40 kcal/mole) than that of the light in the accessible shorter wavelength (200 to 400 nm) near ultraviolet region which has high energy (150 to 70 kcal/mole). As a result, ultraviolet light is every so often used to influence photochemical change resulting, change in the electronic configuration of a molecule. This, in turn, leads to an opening geometry of the excited state of the molecule which is usually not the energy minimum. All through the excitation, the spin of the electron remains unaffected because Spin inversion is forbidden by quantum mechanics during excitation.

Precisely after the excitation, a number of phenomenon may happen:

1. Vibronic relaxation: It carries the molecule rapidly into the new energy minimum structure for the excited state.

I.e., Energy is transferred into the solvent system used. Simultaneous electronic and vibrational energy levels of a molecule changes owing to the taken in or giving out a photon of the appropriate energy absorbed.

1. Intersystem crossing leads to the establishment of triplet states by inversion of spin. Yet again, vibrational relaxation helps in reaching the new energy minimum.
2. Emission of an absorbed photon in the form of radiation in the form of light and various ways of returning to the ground state such as luminescence, fluorescence, phosphorescence.
3. Exited state quenching can happen by transferring energy into another appropriate molecule. This is observed usually by collision by diffusion controlled dynamic quenching.
4. Radiationless deactivation: which means molecule will return back to its ground state by vibrational (thermal) deactivation (no light emission) energy. In turn, energy disperses into the solvent or molecular environment. Alternatively, along with this a photochemical reaction may also occur.

In addition to this, the highly energized molecule may possibly return to its initial ground state according to the any of three physical processes below.

1) Releasing its excess energy by emitting luminescent radiation through fluorescence or phosphorescence condition.

2) Transfer can happen between the same molecules Carbon, with which it collides in the right orientation, without emitting light emission.

3) Consequently, because primary step which is light absorption, an atom having an electron (e^-) or molecule might absorb ample amount of energy that it is escaping from the atom or a molecule, which leaves behind the positive M+ ion. This process is called photoionization.

There are more than one of ways the energy obtained by the molecule by absorbing the light can be lost. As shown in the graphical representation - Jablonski diagram (Figure 1), a molecule can lose its energy as heat while returning to its ground state this is known as Internal conversion.

Consecutively, a molecule can also lose its excess energy in the form of light such as Fluorescence, over a short time period.

The third path is Intersystem Crossing to attain triplet state, through which energy is also possible to be lost as light, this is known as phosphorescence, over a longer time period.