Nanosecond Lifetime Standards

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Morris KJ, Roach MS, Xu WY, Demas JN, DeGraff BA (2007) Luminescence lifetime standards for the nanosecond to microsecond range and oxygen quenching of ruthenium(II) complexes. Anal Chem 79 9310-9314... 

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],... 

The two primary reference works on inorganic thermochemistry in aqueous solution are the National Bureau of Standards tables (323) and Bard, Parsons, and Jordan s revision (30) (referred to herein as Standard Potentials) of Latimer s Oxidation Potentials (195). These two works have rather little to say about free radicals. Most inorganic free radicals are transient species in aqueous solution. Assignment of thermodynamic properties to these species requires, nevertheless, that they have sufficient lifetimes to be vibrationally at equilibrium with the solvent. Such equilibration occurs rapidly enough that, on the time scale at which these species are usually observed (nanoseconds to milliseconds), it is appropriate to discuss their thermodynamics. The field is still in its infancy of the various thermodynamic parameters, experiments have primarily yielded free energies and reduction potentials. Enthalpies, entropies, molar volumes, and their derivative functions are available if at all in only a very small subset. 

The major limitation of photoelectric recording is what can be thought of as the nanosecond barrier. This limitation arises because of the intrinsic time response of the electronic devices that must be used to acquire and process the photongenerated cathode current of the PMT or photodiode. All such devices have impedance, and even the best-designed circuitry has stray capacitance of typically 20 pF which, when combined with the 50-Q industry/standard of electronic amplifiers, yields a RC time constant of 1 ns. Hence, instruments that are built up from conventional electronic units will have minimum rise times in the ns region and therefore chemical changes that have lifetimes less than 5 ns, say, will be severely deformed. Of course, other reasons may intervene (e.g. 10-ns-wide laser pulses) that make the instrument response even poorer than implied by the nanosecond barrier. 

In the second test, a number of fluorescent compounds of relatively well known lifetimes in the nanosecond time range (8,9) were used as standards, allowing evaluation of both the instrumental and computational aspects of the measurement. Table I shows the values obtained for 2,3-diphenyloxazole (PPO), anthracene and quinine blsulphate. All chemicals were analytical grade and not further purified before use. Anthracene and PPO were dissolved in cyclohexane, quinine in O.IN 8280 solvents were not degassed. The case of quinine is of interest because of its common use as a standard for fluorescence measurements, despite its complex decay kinetics (10). In agreement with previous work (10) we found satisfactory fits of our deconvolved data to a blexponentlal rather than a single exponential model. 

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