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D. P. Singh et al.
occurs with a rate constant k isc and is known as intersystem crossing having the
lifetime scale of 10
−10 to 10
−8 s. All singlet excited states S n exist with corresponding excited triplet states, T n . The energy of excited triplet state is always
lower than that of the singlet excited state. The triplet excited state is energetically
unstable.
• Non-radiative relaxations: Some part of the absorbed photonic energy is dissipated
as heat. This type of energy is non-radiative and takes place with a rate constant
k i .
• Quenching: Excited molecules can also transfer a part of their energy to neighbouring molecules with a rate constant k q (in the case of collisional quenching
which occurs when an excited fluorophore comes in contact with a molecule or
atom that facilitates non-radiative transitions to the ground state), or with a rate
constant k t , if energy transfer occurs within a close proximity.
Step 3: Transition of excited molecule form T 1 to S o via below mentioned processes:
• Phosphorescence: Emission of photons with a rate constant k p and lifetime scale
of 10
−3 to 100 s.
• Dissipation of nonradiative energy having a rate constant k
i .
• Transfer of energy to other molecules by collision with a rate constant k
q or with
a rate constant k
t , for the energy transfer occurring at a certain distance.
In a given material, chromophores are the molecules that absorb some particular wavelengths of visible light. A chromophore that emits photons is termed as a
fluorophore. All chromophores do not necessarily show fluorescence. In this situation, the absorbed energy is dissipated within the surrounding in the form of heat,
collisional energy due to solvent and energy transfer with neighbouring molecules.
Fluorophores render the repetition of the excitation/emission cycles from hundreds
to thousands of times before obtaining the ‘photobleached’ status which terminates
the property of fluorescence.
Singlet and triplet states: In the singlet state, all electronic spins are paired
in the molecular electronic states and the electronic energy levels do not split
under the exposure of the magnetic field. A singlet or a triplet state can be
formed when an electron is excited to a higher energy level. In an excited
singlet state, the electron possesses the same spin orientation as it has in the
ground state; whereas, in a triplet excited state, the excited electrons have the
same parallel spin orientation to the other unpaired electron present at the
ground state. The singlet, doublet, and triplet states are determined using the
equation for multiplicity, 2S + 1, where S is the total spin angular momentum.
In the ground state, electrons are diamagnetic; whereas, they are paramagnetic
in the triplet state. This difference in the spin state makes the transition from a
singlet to a triplet (or triplet to singlet) forbidden in comparison to the singletto-singlet transitions. This singlet to triplet (or reverse) transition involves a
change in the electronic state. Due to this reason, the lifetime of the triplet state
D. P. Singh et al.
occurs with a rate constant k isc and is known as intersystem crossing having the
lifetime scale of 10
−10 to 10
−8 s. All singlet excited states S n exist with corresponding excited triplet states, T n . The energy of excited triplet state is always
lower than that of the singlet excited state. The triplet excited state is energetically
unstable.
• Non-radiative relaxations: Some part of the absorbed photonic energy is dissipated
as heat. This type of energy is non-radiative and takes place with a rate constant
k i .
• Quenching: Excited molecules can also transfer a part of their energy to neighbouring molecules with a rate constant k q (in the case of collisional quenching
which occurs when an excited fluorophore comes in contact with a molecule or
atom that facilitates non-radiative transitions to the ground state), or with a rate
constant k t , if energy transfer occurs within a close proximity.
Step 3: Transition of excited molecule form T 1 to S o via below mentioned processes:
• Phosphorescence: Emission of photons with a rate constant k p and lifetime scale
of 10
−3 to 100 s.
• Dissipation of nonradiative energy having a rate constant k
i .
• Transfer of energy to other molecules by collision with a rate constant k
q or with
a rate constant k
t , for the energy transfer occurring at a certain distance.
In a given material, chromophores are the molecules that absorb some particular wavelengths of visible light. A chromophore that emits photons is termed as a
fluorophore. All chromophores do not necessarily show fluorescence. In this situation, the absorbed energy is dissipated within the surrounding in the form of heat,
collisional energy due to solvent and energy transfer with neighbouring molecules.
Fluorophores render the repetition of the excitation/emission cycles from hundreds
to thousands of times before obtaining the ‘photobleached’ status which terminates
the property of fluorescence.
Singlet and triplet states: In the singlet state, all electronic spins are paired
in the molecular electronic states and the electronic energy levels do not split
under the exposure of the magnetic field. A singlet or a triplet state can be
formed when an electron is excited to a higher energy level. In an excited
singlet state, the electron possesses the same spin orientation as it has in the
ground state; whereas, in a triplet excited state, the excited electrons have the
same parallel spin orientation to the other unpaired electron present at the
ground state. The singlet, doublet, and triplet states are determined using the
equation for multiplicity, 2S + 1, where S is the total spin angular momentum.
In the ground state, electrons are diamagnetic; whereas, they are paramagnetic
in the triplet state. This difference in the spin state makes the transition from a
singlet to a triplet (or triplet to singlet) forbidden in comparison to the singletto-singlet transitions. This singlet to triplet (or reverse) transition involves a
change in the electronic state. Due to this reason, the lifetime of the triplet state