Drugs - a study in post-primary schools situated outside Dublin 1981.

The article reports on a survey of a random sample of pupils attending post-primary schools outside Dublin city and county. The survey of 5,408 students from 16 schools was conducted between September and December 1981. Information on the availability, use and knowledge of illicit drugs was gathered by means of self administered questionnaires. Some of the survey results were compared with those from a study of Irish rural post-primary school-children dating from 1970/71, and another carried out among Dublin post-primary school-children in 1981. A three- to eight-fold increase in the numbers of respondents who said they had taken a drug was found when the results were compared to the 1970/71 survey, and these students occurred with one-third to one-half the frequency to the Dublin 1980/81 survey. The authors suggested that their findings exposed the need to improve the drugs education available in schools

NON A PRIORI ANALYSIS OF FLUORESCENCE DECAY SURFACES OF EXCITED-STATE PROCESSES .2. INTERMOLECULAR EXCIMER FORMATION OF PYRENE

The so-called global compartmental approach to analyze the kinetics of excited-state processes (Ameloot et al., preceding paper) is tested on simulated and real fluorescence data of the excimer formation of pyrene in a nonviscous solvent. The identifiability problem of a bicompartmental system is verified. The results of the bicompartmental analysis are compared to the results of the classical analysis. The following rate constant values for the excited-state processes of pyrene in isooctane at room temperature are obtained by the bicompartmental analysis: k01 = 2.17 X 10(6) s-1, k21 = 7.2 X 10(9) s-1 M-1, k02 = 14.8 X 10(6) s-1, and k12 = 5.7 X 10(6) s-1. The evidence for the nonexistence of ground-state dimers of pyrene in isooctane for concentrations lower than 1.5 X 10(-3) M is demonstrated. The species associated emission spectra (SAEMS) of the monomer and the excimer emission are calculated. It is shown that the unsubstituted intermolecular pyrene excimer is emitting at 375 nm

Disorder in charge-transport in doped polymers

Charge transport in molecular doped polymers has important applications in xerography, as well as being a theoretical challenge. Its investigation by determination of transient photocurrents is reviewed, with experiments and pertinent theories, such as hopping in a Gaussian density of states and the small polaron model, being outlined. The temporal features of the photocurrents, the field and temperature dependence of the charge carrier mobilities, and the influence of molecular properties—and what can be inferred from these—are discussed. Copyright © 1994 Verlag GmbH & Co. KGaA, Weinheimstatus: publishe

SPECIES-ASSOCIATED SPECTRA AND UPPER AND LOWER BOUNDS ON THE RATE CONSTANTS OF REVERSIBLE INTRAMOLECULAR 2-STATE EXCITED-STATE PROCESSS WITH ADDED QUENCHER - GLOBAL COMPARTMENTAL ANALYSIS OF THE FLUORESCENCE DECAY SURFACE

This paper explains how, in the absence of any a priori information, upper and lower bounds can be specified for the four rate constants describing the kinetics of a reversible intramolecular two-state excited-state process. It is further shown that the steady-state spectrum can be decomposed into unique species-associated spectra. It is demonstrated theoretically that if the fluorescence decay surface includes at least one set of decay traces measured at a minimum of three different quencher concentrations, it is possible to specify limits on the rate constants and to construct the species-associated spectra. The two quenching rate constants must be different. This new analysis method allows one to distinguish reversible from irreversible intramolecular two-state excited-state processes. Practically, the bounds on the rate constants can be obtained by performing a series of global compartmental analyses of a fluorescence decay surface in which one of the rate constants is held fixed at different preset values. Computer-generated fluorescence decay surfaces are used to illustrate how the rate constant bounds are specified and how a steady-state spectrum is decomposed into its two species-associated spectra. This analysis method will be useful for understanding the photophysical behavior of tryptophan, tryptophyl polypeptides, and proteins. Furthermore, using this approach, the kinetics of intramolecular electron transfer and excimer and exciplex formation are now accessible

SPECIES-ASSOCIATED SPECTRA AND UPPER AND LOWER BOUNDS ON THE RATE CONSTANTS OF REVERSIBLE INTRAMOLECULAR 2-STATE EXCITED-STATE PROCESSS WITH ADDED QUENCHER - GLOBAL COMPARTMENTAL ANALYSIS OF THE FLUORESCENCE DECAY SURFACE

This paper explains how, in the absence of any a priori information, upper and lower bounds can be specified for the four rate constants describing the kinetics of a reversible intramolecular two-state excited-state process. It is further shown that the steady-state spectrum can be decomposed into unique species-associated spectra. It is demonstrated theoretically that if the fluorescence decay surface includes at least one set of decay traces measured at a minimum of three different quencher concentrations, it is possible to specify limits on the rate constants and to construct the species-associated spectra. The two quenching rate constants must be different. This new analysis method allows one to distinguish reversible from irreversible intramolecular two-state excited-state processes. Practically, the bounds on the rate constants can be obtained by performing a series of global compartmental analyses of a fluorescence decay surface in which one of the rate constants is held fixed at different preset values. Computer-generated fluorescence decay surfaces are used to illustrate how the rate constant bounds are specified and how a steady-state spectrum is decomposed into its two species-associated spectra. This analysis method will be useful for understanding the photophysical behavior of tryptophan, tryptophyl polypeptides, and proteins. Furthermore, using this approach, the kinetics of intramolecular electron transfer and excimer and exciplex formation are now accessible

GLOBAL ANALYSIS OF UNMATCHED POLARIZED FLUORESCENCE DECAY CURVES

The performance of the simultaneous analysis of unmatched polarized fluorescence decay curves collected under various experimental conditions, including reference convolution, is investigated systematically for a number of nonassociative (homogeneous) and associative (heterogeneous) model cases with zero limiting anisotropy at long times. The study has been carried out using an improved and rapidly converging algorithm based on the constraint for equal area under the observed and fitted curves. The efficiency of changing experimental conditions such as the orientation of the emission polarizer, the emission/excitation wavelength, and the timing calibration is discussed with respect to the parameter recovery and the power to discriminate between alternative models for the fluorescence anisotropy. When the decay of the total fluorescence and the anisotropy are monoexponential, the combination of decay traces collected at the two extreme orientations of the emission polarizer is sufficient for the parameter recovery when the rotational relaxation time is not too much different from the fluorescence relaxation time and is larger than the timing calibration of the channels. For a single compound system with a multiexponential r(t) the use of different excitation wavelengths is recommended. Different timing calibrations have to be utilized when the relaxation times in the anisotropy decay are widely different. For a mixture of compounds the best results are obtained by using different excitation/emission wavelengths. As an example of the latter a mixture of resorufine and cresyl violet in 1-propanol at 20-degrees-C has been analyzed using reference convolution

Global analysis of unmatched polarized fluorescence decay curves of systems exhibiting a residual anisotropy at long times

A previous study of the performance of the global, i.e., simultaneous, analysis of unmatched polarized decay traces [Crutzen, M.; et al. J. Phys. Chem. 1993, 97, 8133] is extended for the time-dependent anisotropy r(t), given by r(t) = beta exp(-t/Phi) + r infinity. The evaluation was done with computer-generated and experimentally collected data. Two different schemes of analysis of unmatched polarized decay traces were considered. In scheme A the decay curves measured with the analyzer set at 0 degrees, 54.7 degrees, and 90 degrees with respect to the polarization of the incident light were globally analyzed, while only the traces at 0 degrees and 90 degrees were taken into account in scheme B. Data were simulated for various Phi/tau ratios (0.1, 1, 3, and 10). The value obtained for the fluorescence lifetime tau was accurate and precise for all cases considered. For Phi/tau = 0.1, 1, and 3 (tau = 5 ns) the parameters beta and Phi, were well recovered. The accuracy and precision were very similar to the results obtained with matched decay traces. The recovery of r infinity was case dependent. Scheme A performed somewhat better than scheme B. When r(0) was fixed to the true value, the analysis with schemes A and B yielded the same accuracy and precision as the analysis of the matched curves. For Phi/tau = 10, the parameter estimates for the anisotropy parameters were inaccurate and suffered from very high uncertainty. No improvement was obtained when r(0) and the matching factors were kept fixed to the true value. This is in contrast with the results obtained for r(t) without an r infinity term, where Phi/tau can even be much higher when r(0) or the matching factors are kept fixed. There was some improvement when the signal to noise ratio was increased. When the decay of the total fluorescence was biexponential, scheme A performed much better than scheme B. The best results were obtained when G or r(0) was kept fixed. It was found that with respect to the normalization of the polarized decay traces the fundamental anisotropy r(0) and the matching factor G contain the same information. In general, the best accuracy and precision in the parameter recovery were obtained when a predetermined value for G or r(0) was used in the analysis. The performance of the analyses without the information on r(0) or G depended more on the Phi/tau ratio than for the analyses where this information was used. Fixing of both G and r(0) to the proper value might lead to an improved precision. It has to be emphasized that the use of incorrect values for G (with r(0) freely adjustable) or r(0) (with G freely adjustable) could lead to good data fits, although the parameter estimates are incorrect. A small change in r(0) can lead to a large change in Phi. Therefore, in general it can be recommended that either G or r(0) be fixed, but not both. Alternatively, the comparison of the results obtained with a freely adjustable G and a fixed G can provide a check for the data analysis