CH391L/S12/Pigments: Difference between revisions
| Line 14: | Line 14: | ||
Fluorescence can be generally described as an electron of a molecule relaxes to its ground state by emitting a photon of light after its excitation from a higher energy quantum state. The most commonly seen example of fluorescence occurs when ultraviolet region of energy is absorbed, which is invisible to human eye, and the emitted light is in the visible region, and shows the color (As can be seen of a TLC plate before staining under short UV). The process begins with excitation, relaxation (dropping to a relatively lower energy state), and shines fluorescence from that "relatively lower energy state". There are several kinds of relaxations: 1) non-radioactive: which means non-fluore/phosphorescnece, non-pigment, pure heat is released. 2) vibrational: returned to [http://en.wikipedia.org/wiki/Maxwell-Boltzmann_distribution Maxwell–Boltzmann distribution]. As in infrared/Raman process (relates to structure biology/chemistry). 3)Intersystem crossing: which means relaxation time is consumed upon "intersystem crossing", and emission of light is slow and delayed. Thus you see relatively long lasting phosphorescence. Accurately, it means total spin angular momentum of a molecule equals to one (triplet state), and thus it only takes a forbidden or non-preferred route to relax, since it is non-preferred, so there is the probability that one out of a million molecules would like to take this route, so that the overall "decaying" is very slowly. I personally see this from this example: if you get each of the four wheels of your car run (each with singlet state), it runs (fast relaxation: fluorescence); if your get a boot on three of them (triplet state, all to one), it still runs by only one wheel, but very very slowly (decaying: phosphoerescence). | Fluorescence can be generally described as an electron of a molecule relaxes to its ground state by emitting a photon of light after its excitation from a higher energy quantum state. The most commonly seen example of fluorescence occurs when ultraviolet region of energy is absorbed, which is invisible to human eye, and the emitted light is in the visible region, and shows the color (As can be seen of a TLC plate before staining under short UV). The process begins with excitation, relaxation (dropping to a relatively lower energy state), and shines fluorescence from that "relatively lower energy state". There are several kinds of relaxations: 1) non-radioactive: which means non-fluore/phosphorescnece, non-pigment, pure heat is released. 2) vibrational: returned to [http://en.wikipedia.org/wiki/Maxwell-Boltzmann_distribution Maxwell–Boltzmann distribution]. As in infrared/Raman process (relates to structure biology/chemistry). 3)Intersystem crossing: which means relaxation time is consumed upon "intersystem crossing", and emission of light is slow and delayed. Thus you see relatively long lasting phosphorescence. Accurately, it means total spin angular momentum of a molecule equals to one (triplet state), and thus it only takes a forbidden or non-preferred route to relax, since it is non-preferred, so there is the probability that one out of a million molecules would like to take this route, so that the overall "decaying" is very slowly. I personally see this from this example: if you get each of the four wheels of your car run (each with singlet state), it runs (fast relaxation: fluorescence); if your get a boot on three of them (triplet state, all to one), it still runs by only one wheel, but very very slowly (decaying: phosphoerescence). | ||
=Biochromes and their uses= | |||
For a short history, pigments such as iron oxides have been used since prehistoric times. This is one of the naturally occurring pigment (Besides synthetic one after Industrial Revolution). The other one is the biological pigments or simply, biochromes. There are two kinds for this: plant pigment and animal pigment. | For a short history, pigments such as iron oxides have been used since prehistoric times. This is one of the naturally occurring pigment (Besides synthetic one after Industrial Revolution). The other one is the biological pigments or simply, biochromes. There are two kinds for this: plant pigment and animal pigment. | ||
=Plant pigment= | =Plant pigment= | ||
Revision as of 14:32, 18 March 2012
Concepts and physical basis of pigment
Pigment is the substance showing the color. Specifically, "substance" means a certain kind/kinds of molecule(s), for pigment, it is usually the chemical bond of a conjugating system; "showing" means selectively absorbing certain wavelength out of the background light. And the color of the pigment we see is the net effect of all others remaining on the background. This is different and often compared to fluorescence or phosphorescence. Whereas pigment shows color by simple subtraction, these two adds new light for emission.
→"absorbing"
Electrons in molecules exist on certain energy levels. They try to exist at the lowest possible energy level as possible as they can. However, they can be boosted to higher level by "absorbing" the energy that fits the level difference. This is exactly what pigment does. They absorb the light that has right amount of energy that fits such energy level difference. The remaining nonabsorbed ones are perceived by us to "see" the color of pigment.
→"conjugating system"
Conjugating system generally means the delocalized electrons within overlapping p-orbitals. Since the electron orbitals are overlapping with each other, so single electron that presumably assigned to one atom can be shared to some extent by other atoms, which lowers the systemic/molecular energy. To make a figurative comparison (as how I understand it), suppose you have a balloon in hand. That is the molecule we have, and the air in it are the "electrons". And suppose you have a big balloon, that in another way means it gets high tension on its rubber surface---even a squeeze may cause it to burst---which means high "energy" of your balloon "molecule". However, if there is another space, or simply another small bag (overlapping p-orbital) that connects with this "molecule", you would see that the air (electrons) in this big balloon (molecule) is transferred to the small bag, and the big balloon now shrinks to a smaller size (lower energy). Since conjugating system usually possess lower energy, it is commonly seen as the "receiver" for the light energy, and thus performs as pigment.
→"fluorescence and phosphorescence"
Fluorescence can be generally described as an electron of a molecule relaxes to its ground state by emitting a photon of light after its excitation from a higher energy quantum state. The most commonly seen example of fluorescence occurs when ultraviolet region of energy is absorbed, which is invisible to human eye, and the emitted light is in the visible region, and shows the color (As can be seen of a TLC plate before staining under short UV). The process begins with excitation, relaxation (dropping to a relatively lower energy state), and shines fluorescence from that "relatively lower energy state". There are several kinds of relaxations: 1) non-radioactive: which means non-fluore/phosphorescnece, non-pigment, pure heat is released. 2) vibrational: returned to Maxwell–Boltzmann distribution. As in infrared/Raman process (relates to structure biology/chemistry). 3)Intersystem crossing: which means relaxation time is consumed upon "intersystem crossing", and emission of light is slow and delayed. Thus you see relatively long lasting phosphorescence. Accurately, it means total spin angular momentum of a molecule equals to one (triplet state), and thus it only takes a forbidden or non-preferred route to relax, since it is non-preferred, so there is the probability that one out of a million molecules would like to take this route, so that the overall "decaying" is very slowly. I personally see this from this example: if you get each of the four wheels of your car run (each with singlet state), it runs (fast relaxation: fluorescence); if your get a boot on three of them (triplet state, all to one), it still runs by only one wheel, but very very slowly (decaying: phosphoerescence).
Biochromes and their uses
For a short history, pigments such as iron oxides have been used since prehistoric times. This is one of the naturally occurring pigment (Besides synthetic one after Industrial Revolution). The other one is the biological pigments or simply, biochromes. There are two kinds for this: plant pigment and animal pigment.
