(a) Molecule X receives excitation and emits fluorescence. If the excitation wav
ID: 822832 • Letter: #
Question
(a) Molecule X receives excitation and emits fluorescence. If the excitation wavelength is 320 nm, which of the following could be the emission wavelength: 270 nm, 290 nm, 320 nm, 350 nm? Explain your result.
(b) How the following factors affect fluorescence quantum efficiency: increasing structural rigidity, decreasing temperature, increasing sample concentration.
(c) Which method is more sensitive: fluorescence or absorption spectroscopy? Explain your result.
(d) Explain the difference between fluorescence and phosphorescence.
Explanation / Answer
a) possible emission wavlength can be 270nm and 290nm as thier energy is more than the excitation energy
b)Structural Rigidity
Collisional deactivation is a major fluorescence quenching mechanism. Therefore, molecules possessing rigid structures are better fluorescers than others which lack rigidity. This explains why fluorene is an excellent fluorescer while biphenyl is a weak one.
Temperature
As temperature is increased, the translational, rotational and vibrational motions of molecules increase. This increases the possibilities of collisions and lead to collisional deactivation and quenching of fluorescence. Therefore, it is always wise to conduct fluorometric measurements at low temperatures.
increasing sample concentration.: Fluorescence signal is directly proportional to concentration. This is different
from relation between absorbance and concentration which is logarithmic.
The linear correlation between fluorescence and concentration is only true
when the absorbance is less than 0.05.
c)This isn't the technical way of describing it, but my analytical chemistry teacher described it this way. If you are in a room with 1000 lights on. If you turn 1 light off, the difference is not very noticeable. (akin to spectrometric measurements.) If you are in a dark room, on the other hand, and someone turns on 1 light, the difference is quite pronounced. (akin to fluorescence)
d)Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. However, when the absorbed electromagnetic radiation is intense, it is possible for one electron to absorb two photons; this two-photon absorption can lead to emission of radiation having a shorter wavelength than the absorbed radiation.
Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum mechanics. As these transitions occur less often in certain materials, absorbed radiation may be re-emitted at a lower intensity for up to several hours.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.