Picosecond Lifetime Standards

It can be useful to have access to the actual lifetime data. Representative frequency-domain data for tingle-exponential decays are shown in Figuie IL9. All samples were in equiUbrium widi air. Additional frequency-domain data 

AbMipdon and flucnesoeiioe tpectta of (---------) and DBS (--------in cyckAexaoe at 25 C Bottom Abaoiption and fluotcacoice yectra of 

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Rose Bengal can serve as a picosecond lifetime standard at longer wavelengdis (Figuie II.7). Rose Bengal can also be used as a standard for a short rotational correlation time (Figure US). 

J. R. Lakowicz, 1. Gryczynski, G. Laczko, and D. Gloyna, Picosecond fluorescence lifetime standards for frequency- and time-domain fluorescence,./. Fluorescence 1, 87-93 (1991). 

Table 1.3 provides rate constants for the decay of selected carbocations and oxocar-bocations in H2O, TFE, and HFIP. As a general comment, water, methanol, and ethanol are highly reactive solvents where many carbocations that are written as free cations in standard textbooks have very short lifetimes. The diphenylmethyl cation, with two conjugating phenyl groups, has a lifetime in water of only 1 ns. Cations such as the benzyl cation, simple tertiary alkyl cations such as tert-butyl, and oxocarbocations derived from aldehydes and simple glycosides, if they exist at all, have aqueous lifetimes in the picosecond range, and do not form and react in water as free ions. This topic is discussed in more detail in Chapter 2 in this volume. 

To study the excited state one may use transient absorption or time-resolved fluorescence techniques. In both cases, DNA poses many problems. Its steady-state spectra are situated in the near ultraviolet spectral region which is not easily accessible by standard spectroscopic methods. Moreover, DNA and its constituents are characterised by extremely low fluorescence quantum yields (<10 4) which renders fluorescence studies particularly difficult. Based on steady-state measurements, it was estimated that the excited state lifetimes of the monomeric constituents are very short, about a picosecond [1]. Indeed, such an ultrafast deactivation of their excited states may reduce their reactivity something which has been referred to as a "natural protection against photodamage. To what extent the situation is the same for the polymeric DNA molecule is not clear, but longer excited state lifetimes on the nanosecond time scale, possibly of excimer like origin, have been reported [2-4],... 

Note that the small deformation angl, indicative of the vibration amplitudes, are truncated heavily (with the actual percentages written at the top of each standard interval). Note also that the crystal environment has changed the gauche maximum from cos t = 0.5 to cos x w 0.4 (rotation from 180 to 66° or 294 instead of 60° or 300°). The lifetimes of the defects must be very short, in the picosecond range, because of the fast rate of formation documented in Fig. 10. Somewhat longer lifetimes are observed for less restrictive intermolecular forces,... 

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