florescence

1. Name: Arfan Kashif
College Roll#: 1624
Uni.Roll#: 25602
Class: B.S Chemistry (2016-2020)
Semester: 8th
Morning
Topic: Florescence & Phosphorescence
Species
Subject: Luminescence Spectroscopy
Submitted To: Dr.Moin Ud Din Sb
Govt. Postgraduate College of Science, Samanabad
Faisalabaad

2. Contents:
1. History of Florescence
2. Introduction
3. Definition of Florescence
4. Principle of Florescence & Phosphorescence
 Jablonski Diagram
 Singlet State
 Triplet State
 Internal Conversion
 Vibrational Relaxation
 Florescence
 Inter-System Crossing
 Phosphorescence
5. Basic Differences b/w Florescence & Phosphorescence

3. 1.History of Flourescence:
Fluorescence was first discovered in 1845 by Fredrick W. Herschel. He
discovered that UV light can excite a quinine solution (e.g. tonic water) to
emit blue light. British scientist Sir George G. Stokes further studied this
discovery, and he observed that fluorescence emission from an object
represents a longer wavelength than the UV light that originally excited the
object.
Many decades later, in the early 1900s, the first uses of fluorophores in
biological investigations were performed to stain tissues, bacteria, and other
pathogens. This was later developed into fluorescence microscopy by Carl
Zeisi and Carl Reichert.
Fluorescence labeling was achieved by Ellinger and Hirt in the early 1940s.
The cloning of green fluorescent protein (GFP) was achieved in the early
1990s and was easily applied to fluorescence microscopy.
2.Introduction:
The excited molecules relax back to ground state by using different processes.
In some of the processes, the absorbed energy is given off as heat in the
surroundings and in some it is emitted as radiations.
When such a relaxation is accompanied by emission of radiation, it is called
luminiscence.
There are two types of Luminiscence
1. Flourescence
2. Phosphorescence

4. 3.Definition of Florescence:
“Fluorescence is the emission of visible light by a substance that has
absorbed light of a different wavelength. The emitted photon has a longer
wavelength.”
4.Principle of Florescence & Phosphorescence
When a molecule absorbs electromagnetic energy, this energy lost as heat, as
the molecule is deactivated via collisional processes. Some molecule lose
only part of the energy via collisions, and then the electron drops back to the
ground state by emitting a photon of lower energy than was absorbed.This
phenomenon is called florescence.
 Jablonski Diagram:
This Jablonski diagram explain following terms which help us to understand
how florescence and phosphorescence occurs.

5.  Singlet State:
The ground state is usually a singlet state (S0), with all electrons paired.
Electrons that occupy the same molecular orbital must be paired, that is, have
opposite spins.
 Triplet State:
If electrons have same spin, they are “unpaired” and the molecule is in triplet
state.
Singlet and Triplet states refer to the multiplicity of the molecule.
The process leading to the emission of a photon begins with absorption of a
photon (a process that takes 10-15
s) by the fluorophore, resulting in an
electronic transition to a higher energy state.
The spacing of the vibrational levels and rotational levels in these higher
electonic states give rise to absorption spectrum of the molecule.
 Internal Conversion:
Internal conversion is a non-radiative and isoenergetic transition between two
electronic states of the same spin multiplicity.Internal conversion occurs
because of the overlap of vibrational and electronic energy states .
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. Internal conversion occurs in the same time frame as vibrational
relaxation, therefore, is a very likely way for molecules to dissipate energy
from light perturbation.

6.  Vibrational Relaxation:
It is thought that excited molecule passes from the vibrational level of higher
electronic state S1 to a high vibrational level of S1 that is isoenergetic with
original excited state. Collision with solvent at this pointrapidly removes
excess of energy from higher vibrational level of S1; this process is called
vibrational relaxation.
These energy deggradation processes occur rapidly @ 10-12
s.
 Florescence:
Once the molecule reaches the first excited state, internal conversion to the
ground state is a comparatively slow process. Thus, decay of the first excited
state by emission of a photon can effectively compete with other decay
process. This emission process is called Florescence.
Generally, florescence emission occurs very rapidly after excitation (10-6
to
10-9
s).So, it is not possible for the eye to perceive florescence emission after
removal of the excitation source. Because florescence occurs from lowest
excited state.
The intensity of emitted radiation, will be proportional to the intensity of
incident radiation.
 Inter-system Crossing:
While the molecule is in the excited state, it is possible form one electron to
reverse its spin, and the molecule transfers to a lower energy triplet state by a
process called intersystem crossing.
 Phosphorescence:
Through the process of internal conversion and vibrational relaxation, the
molecule then rapidly attains the lowest vibrational level of the first excited

7. triplet (T1). From here, the molecule can return to the ground state S0 by
emission of a photon. This emission is referred to as Phosphorescence.
“A process in which energy of light absorbed by a substance is released
relatively slowly in the form of light.”
The phosphorescence emission took place from a triplet excited states.
Its average lifetime is from 10-5
to >10+3
sec.
5.Basic Difference b/w Florescence & Phosphorescence
Florescence Phosphorescence
 It is absorption of energy by
atoms or molecules followed by
immediate emission of light or
electromagnetic radiation.
 Emission of light suddenly stops
on removal of source of
excitation.
 In florescence,the excited atom
has comparatively short life time
before its transition to low energy
state.
 Examples:
Gemstones Florescence including
gypsum,talc,
Jelly fish,chlorophyll extract,vitamins
 It is absorption of energy by
atoms or molecules followed by
delayed emission of light or
electromagnetic radiation.
 Emission of light remains for
some time even after removal of
source of excitation.
 In phosphorescence,the excited
atom has comparatively long life
time before its transition to low
energy state.
 Examples:
Glow of clock dial,toys or in bulbs
after switching off the light in the
room.
Phosphorescence material in sign
board illuminate during night.

8. THE END