Excited state lifetimes

At still shorter time scales other techniques can be used to detenuiue excited-state lifetimes, but perhaps not as precisely. Streak cameras can be used to measure faster changes in light intensity. Probably the most iisellil teclmiques are pump-probe methods where one intense laser pulse is used to excite a sample and a weaker pulse, delayed by a known amount of time, is used to probe changes in absorption or other properties caused by the excitation. At short time scales the delay is readily adjusted by varying the path length travelled by the beams, letting the speed of light set the delay. 

The interpretation of emission spectra is somewhat different but similar to that of absorption spectra. The intensity observed m a typical emission spectrum is a complicated fiinction of the excitation conditions which detennine the number of excited states produced, quenching processes which compete with emission, and the efficiency of the detection system. The quantities of theoretical interest which replace the integrated intensity of absorption spectroscopy are the rate constant for spontaneous emission and the related excited-state lifetime. 

Demas J N 1983 Excited State Lifetime Measurements (New York Aoademio)... 

Data on duorescence, phosphorescence, excited-state lifetimes, transient absorption spectra, and dye lasers are tabulated in Ref. 16. The main nonduorescent process in cyanine dyes is the radiationless deactivation Sj — Sg. Maximum singlet-triplet interconversion ( 52 ) methanol for carbocyanines is about 3% (maxLgrp > 0.03), and the sum [Lpj + st] I than 0.10. 

Quantum well interface roughness Carrier or doping density Electron temperature Rotational relaxation times Viscosity Relative quantity Molecular weight Polymer conformation Radiative efficiency Surface damage Excited state lifetime Impurity or defect concentration... 

Pulsed method. Using a pulsed or modulated excitation light source instead of constant illumination allows investigation of the time dependence of emission polarization. In the case of pulsed excitation, the measured quantity is the time decay of fluorescent emission polarized parallel and perpendicular to the excitation plane of polarization. Emitted light polarized parallel to the excitation plane decays faster than the excited state lifetime because the molecule is rotating its emission dipole away from the polarization plane of measurement. Emitted light polarized perpendicular to the excitation plane decays more slowly because the emission dipole moment is rotating towards the plane of measurement. 

The value of tan A depends upon the modulation frequency, the excited state lifetime, and the rate of rotation. The value decreases to zero when the rotation period is either longer or shorter than the excited state lifetime and is a maximum when the two times are comparable in magnitude. Tan A also increases as the modulation frequency increases. For spherical rotators, the measured value of tan A for a given modulation frequency and excited state lifetime allows the rotational rate to be calculated from... 

Demas, J.N. Excited State Lifetime Measurements Academic New York, 1983 ... 

Figure 11. Experimental He 12(8,v ) and Ne 12(8,v ) excited-state lifetimes, circles and squares, plotted as a function of vibrational quanta, v. The values taken from line width [72] and time-domain [73] measurements are shown as solid and open symbols, respectively.

The nuclear y-resonance effect in ° Ni was first observed in 1960 by Obenshain and Wegener [2]. However, the practical application to the study of nickel compounds was hampered for several years by (1) the lack of a suitable single-line source, (2) the poor resolution of the overlapping broad hyperfine lines due to the short excited state lifetime, and (3) the difficulties in producing and handling the short-lived Mossbauer sources containing the Co and Cu parent nuclides, respectively. 

Upon absorption of UV radiation from sunlight the bases can proceed through photochemical reactions that can lead to photodamage in the nucleic acids. Photochemical reactions do occur in the bases, with thymidine dimerization being a primary result, but at low rates. The bases are quite stable to photochemical damage, having efficient ways to dissipate the harmful electronic energy, as indicated by their ultrashort excited state lifetimes. It had been known for years that the excited states were short lived, and that fluorescence quantum yields are very low for all bases [4, 81, 82], Femtosecond laser spectroscopy has, in recent years, enabled a much... 

All the nucleic acid bases absorb UV radiation, as seen in Tables 11-1, 11-2, 11-3, 11-4, and 11-5, making them vulnerable to the UV radiation of sunlight, since the energy of the photons absorbed could lead to photochemical reactions. As already mentioned above, the excited state lifetimes of the natural nucleobases, and their nucleotides, and nucleosides are very short, indicating that ultrafast radiationless decay to the ground state takes place [6], The mechanism for nonradiative decay in all the nucleobases has been investigated with quantum mechanical methods. Below we summarize these studies for each base and make an effort to find common mechanisms if they exist. 

As discussed earlier, thymine is very similar to uracil in its excited states pattern. This is also true for its radiationless decay mechanism except from the fact that the excited state lifetime in thymine is somewhat longer than in uracil. Theoretically the mechanism for radiationless decay has been studied using CASPT2 electronic structure methods [150, 152],... 

The photophysical properties of adenine have intrigued chemists from early on. Broo studied adenine and 2-aminopurine (2AP) in order to understand their differences in photophysical properties. Adenine like all natural nucleobases has very short excited state lifetimes and low quantum yields of fluorescence, while 2AP, which differs from adenine in the position of the amino group, has long lifetimes and strong fluorescence, making it a very useful fluorescent probe. In Broo s work it was observed that the first excited state is a nn at vertical excitation but crosses with an nn state which becomes the Si state adiabatically at the minimum. The large out-of-plane distortion on the nn state opens up a deactivation channel in adenine compared to 2AP. In 2AP, on the other hand, the Si state always has a 7T7r character. 

It has been shown that photoexcitation of the guanine-cytosine (G-C) base pair leads to proton transfer [231], Watson-Crick (WC) base pairs have excited state lifetimes much shorter than other non-WC base pairs indicating once again that the natural occurring WC base pairs are more photostable than other alternative configurations [115, 118, 232-235], Much work has been done in the gas phase where many different base pair isomers exist. The ultrafast relaxation of the WC base pair has also been confirmed in solution using fluorescence up-conversion measurements [117]. 

Peon J, Zewail AH (2001) DNA/RNA nucleotides and nucleosides direct measurement of excited-state lifetimes by femtosecond fluorescence up-conversion. Chem Phys Lett 348 255... 

Santoro F, Barone V, Gustavsson T, Improta R (2006) Solvent effect on the singlet excited-state lifetimes of nucleic acid bases a computational study of 5-fluorouracil and uracil in acetonitrile and water. J Am Chem Soc 128 16312-16322... 

Sobolewski AL, Domcke W, Hattig C (2005) Tautomeric selectivity of the excited-state lifetime of guanine/cytosine base pairs The role of electron-driven proton-transfer processes. Proc Natl Acad Sci USA 102 17903-17906... 

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