S0, S1, and S2 represent the singlet electronic states while the numbers 0, 1, 2 represent the vibrational energy levels associated with the electronic states. T1 depicts the first triplet electronic state. Let us go through the processes shown in Figure 6.1:
Absorbance: S0 state with 0th vibrational level is the state of lowest energy and therefore, the highest populated state. Absorption of a photon of resonant frequency usually results in the population of S1 or S2 electronic states; but usually a higher vibrational state. Transition of electrons from low energy molecular orbital to a high energy molecular orbital through absorption of light is a femtosecond (10-15 s) phenomenon. The electronic transition, therefore, is too quick to allow any significant displacement of the nuclei during transition.
Internal conversion: Apart from few exceptions, the excited fluorophores rapidly relax to the lowest vibrational state of S1 through non-radiative processes. Non-radiative electronic transition from higher energy singlet states to S1 is termed as internal conversion while relaxation of a fluorophore from a higher vibrational level of S1 to the lowest vibration state is termed as vibrational relaxation. The terms ‘internal conversion’ and ‘vibrational relaxation’, however, are often interchangeably used. The timescale of internal conversion/vibrational relaxation is of the order of 10-12 seconds.
Fluorescence: Fluorescence lifetimes are of the order of 10-8 seconds, implying that the internal conversion is mostly complete before fluorescence is observed. Therefore, fluorescence emission is the outcome of fluorophore returning back to the S0 state through S1 → S0 transition emitting a photon. This also explains why emission spectra are usually independent of the excitation wavelength, also known as Kasha’s rule (However, there are exceptions wherein fluorescence is observed from S2 → S1 transition). The S1 → S0 transition, like S0 → S1 transition, typically results in the population of higher energy vibrational states. The molecules then return back to the lowest vibrational state through vibrational relaxation.
Intersystem crossing: Intersystem crossing referes to an isoenergetic non-radiative transition between electronic states of different multiplicities. It is possible that a molecule in a vibrational state of S1 can move to the isoenergetic vibrational state of T1. The molecule then relaxes back to the lowest vibrational state of the triplet state.
Phosphorescence: The molecule in the triplet state, T1, can return back to the S0 state emitting a photon. This process is known as phosphorescence and has time scales of several orders of magnitudes higher than that of fluorescence (10-3 – 10 s).