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Solution structure of a trans-opened (10S)-dA adduct of +)-(7S,8R,9S,10R)-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene in a DNA duplex is by Richard Wheeler ( Zephyris ) 2007 under the same license.
In solution, the molecule rapidly relaxes to the lowest vibrational energy level of the electronic state to which it is excited (in this case S 2 ). The kinetically favoured reaction in solution is then internal conversion which shifts the molecule from S 2 to an excited vibrational energy level in S 1 .
Following internal conversion, the molecule loses further energy by vibrational relaxation. Because of internal conversion and vibrational relaxation, most molecules in solution will decay to the lowest vibrational energy level of the lowest singlet electronic state before any radiation is emitted.
the molecule can undergo intersystem crossing which involves and electron spin flip from the singlet state into a triplet state. Following this the molecule decays to the lowest vibrational energy level of the triplet state by vibrational relaxation;
Fluorescence, phosphorescence and internal conversion are competing processes. The fluorescence quantum efficiency and the phosphorescence quantum efficiency are defined as the fraction of molecules which undergo fluorescence and phosphorescence respectively.
The power of fluorescent radiation, F , is proportional to the radiant power of the excitation beam absorbed by the species able to undergo fluorescence:
F = K '( P 0 - P )
where P 0 is the power incident on the sample, P is the power after it traverses a length b of the solution and K ' is a constant which depends upon experimental factors and the quantum efficiency of fluorescence.
Experimentally it is found that fluorescence is favoured in rigid molecules, eg., phenolphthalein and fluorescein are structurally similar as shown below. However, fluorescein shows a far greater fluorescence quantum efficiency because of its rigidity.
Excitation and fluorescence spectra for benzo(a)pyrene in H 2 SO 4 . In the diagram the solid line is the excitation spectrum (the fluorescence signal is measured at 545 nm as the exciting wavelength is varied). The dashed line is the fluorescence spectrum (the exciting wavelength is fixed at 520 nm while the wavelength of collected fluorescence is varied).
Because LSD is active in minute quantities (as little as 50 g taken orally) an extremely sensitive methods of analysis is required. Fluorimetricaly LSD is usually determined in urine from a sample of about 5mL in volume. The sample is made alkaline and the LSD is extracted into an organic phase consisting of n -heptane and amyl alcohol. This is a "clean-up" procedure that removes potential interferents and increases sensitivity. The LSD is then back-extracted into an acid solution and measured directly using and excitation wavelength of 335 nm and a fluorescence wavelength of 435 nm. The limit of detection is approximately 1 ppb: