Introduction:
Fluorescence and phosphorescence are both processes of photoluminescence, a form of electronic emission, in which light is absorbed and re-emitted at longer wavelengths. In fluorescence, light is emitted almost instantly, while for phosphorescence, a delay in emission makes a substance appear to glow in the dark. This is a result of transitions between “forbidden” energy levels at a microscopic level. While these processes do occur, they are kinetically unfavorable and so occur at relatively slow rates. The concept is the basis for “glow-in-the-dark” substances, which are activated by exposure to light, which they store as energy. Fluorescence and phosphorescence are closely related in terms of wavelengths, and their electronic energy transitions can be illustrated on a Jablonski diagram. The two processes are important concepts that have numerous practical applications.
Jablonski Diagram:
Electronic energy transitions in fluorescence and phosphorescence can be illustrated by the Jablonski Diagram below. When a fluorophore, part of a molecule that allows the molecule to be fluorescent or phosphorescent, first absorbs a photon of light, it advances from the ground state to a higher vibrational energy level in the first excited state before emitting energy and rapidly relaxing back to the lowest energy level. This is called vibrational relaxation or internal conversion, and usually happens in a picosecond or less. The process occurs much slower in fluorescence, which allows an electron to descend to a stable lowest singlet excited state before falling back to the ground state.
In phosphorescence, an electron undergoes a spin conversion to a forbidden triplet state in a process known as intersystem crossing before emitting a photon of light. Light emitted from the triplet state has a higher wavelength, which means the process occurs at lower energy. In the case of delayed phosphorescence, electrons arrive at the triplet state and ascend up to the lowest singlet excited state before falling back to the ground level.1
Fluorescence:
Fluorescence is when a fluorophore absorbs and re-emits and photon of light in about 10-8 seconds. When a molecule is excited to a higher electronic state, it can return to the ground state by emitting a photon. The time in which a molecule exists in its excited state is around 10-9 to 10-7 seconds, and its decay time is of the same magnitude. During the time that the molecule is in the excited state an internal conversion of energy occurs and the molecule drops to the lowest energy vibrational state. When the molecule then drops down to the ground state, the photon that is emitted is of lower energy and therefore longer wavelength than the absorbed photon.5
It is important to emphasize that within the rotational transitions, there are vibrational transitions which are responsible to the behavior of these fluorescent and phosphorescent compounds. Along with the vibrational transitions, there are two vibrational relaxations: one relaxation from within the excited state and another within the grounds state.
Before an excited molecule in a solution can emit a photon, it will undergo vibrational relaxation, and therefore photon emission will always occur from the lowest vibrational level of an excited state to a vibrationally higher state within its ground state.4
The above figure shows the emission wavelength spectrum for the amino acid tryptophan when it is illuminated with a light of 230 nm. The emission spectrum of a molecule does not tend to change when different wavelengths are used to illuminate the fluorophore. If a higher energy source is used to illuminate the fluorophore then the extra energy involved will just be used up in internal conversions and the emitted light will still be a transition from the lowest singlet state to ground state.
Although the wavelength emission may be the same no matter the light source, the strength of the light emitted might be different. Molecules absorb certain wavelengths of light more easily than other wavelengths of light. Below in Fig. 2 is the excitation spectrum for chloropyll-a and chloropyll-b. It shows that at certain wavelengths such as 425 nm, chlorophyll-a will fluoresce much brighter than cholophyll-b, while at 460 nm chlorophyll-b will fluoresce much brighter than chlorophyll-a. Similarly at 650 nm a will be brighter than b and at 670 nm b will be brighter than a.
Phosphorescence:
To completely address phosphorescence, it is paramount that intersystem crossing is explained. Intersystem crossing is a process that is dependent on the spin of the excited state. As seen in the Jablonski diagram, it is possible for an excited state to transfer from singlet excited states, in which all the electrons in the molecule have their spins paired, to a triplet excited state, in which one set of electron spin has become unpaired due to the excitation.
The mechanism for intersystem crossing involve vibrational coupling between the excited singlet state and a triplet state. According to the Jablonski diagram, the singlet-triplet transition can only occur in the lower vibrational levels of the excited singlet state; but cannot make this transition from the higher vibrational levels of the excited singlet state.
If a molecule is placed in a medium where collisions are minimized, a transition that results in a photon emission between the lowest triplet state and the ground state is observed. This is what creates the phosphorescence. Because this phosphorescent light begins the emission transition from the lowest triplet state, it will have a decay time of approximately equal to the lifetime of the triplet state (10-4 to 10 seconds)4. The spin is “forbidden,” therefore the chances of the transition are much lower than in fluorescence. Because the chances of these transitions are lower, it would take more time in order for this transition to happen. Therefore, phosphorescence is often characterized by an afterglow which is not observed for fluorescence.
Practical Applications:
Fluorescence and phosphorescence both have numerous practical applications. Fluorescence is often used in applications such as industrial and residential lighting (neon and fluorescent lights), as an analytical technique in science, and as a quality and process control method in industry. Fluorescent lights are created by coating the inside of a glass tube with phosphors, converting the lamp's UV rays to visible light. Fluorescent chemicals are also used in common household objects such as eye drops and tooth brighteners.2
Phosphorescence lingers long after the excitation of light is gone, and often appears in the form of glow-in-the-dark materials. It is usually used by the Department of Transportation to attract drivers' attention to road signs, in advertising campaigns to create glowing stickers and other promotional materials, as well as by a number of industries to notify people of potential hazards. Glowing clocks and watches help people tell time in the dark, and emergency doors and stairways are highlighted with phosphorescent paints so people can find their way out in case of power outage.3
References:
Images:
1. Davidson, Michael W. (1998-2010). Jablonski Energy Diagram. Molecular Expressions: Optical Microscopy Primer. Retrieved November 22, 2011 from http://micro.magnet.fsu.edu/primer/java/jablonski/lightandcolor/
2. Spectroscopy Applications. Princeton Instruments. Retrieved November 22, 2011 from http://www.princetoninstruments.com/spectroscopy/fl_lu.aspx
3. Fluorescence and Phosphorescence. (2005-2006). BookRags. Retrieved November 22, 2011 from http://www.bookrags.com/research/fluorescence-and-phosphorescence-wsd/
4. Fluorescence and Phosphorescence. (2006). Retrieved November 22, 2011 from http://www.physik.unibas.ch/Praktikum/VPII/Fluoreszenz/Fluorescence_and_Phosphorescence.pdf
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