Fluorescence decay-time measurements

R. A. Lampert, L. A. Chewter, D. Phillips, D. V. O Connor, A. J. Roberts, andS. R. Meech, Standards for nanoseconds fluorescence decay time measurements, Anal. Chem. 55, 68-73 (1983). 

Most common lock-in amplifiers can be operated at frequencies ranging from a few Hz up to 100 kHz. This fact is important in analyzing the temporal evolution of optical signals for example, fluorescence decay time measurements. Although this particular application of lock-in amplifiers is beyond the scope of this section, it is instructive to mention that this can be done by tuning the relative phase (the time delay) between the signal intensity and the reference signal provided by the chopper. 

This effective dye relaxation time rp is the spontaneous fluorescence decay time shortened by stimulated emission which is more severe the higher the excitation and therefore the higher the population density w j. The dependence of fluorescence decay time on excitation intensity was shown in 34 35>. Thus, fluorescence decay times measured with high intensity laser excitation 3e>37> are often not the true molecular constants of the spontaneous emission rate which can only be measured under low excitation conditions. At the short time scale of modelocking the reorientation of the solvent cage after absorption has occurred plays a certain role 8 > as well as the rotational reorientation of the dye molecules 3M°)... 

Compounds 268a and 269a showed large Stokes shifts in polar solvents. Such large Stokes shifts have been observed in many TICT (twisted intramolecular charge transfer) molecules. Fluorescence decay time measurement indicated that there were two kinds of excited states, fast and slower decaying components, and the latter was the emission from the more polar state. 

Fluorescence decay time measurements have been used to observe... 

The first observations of energy transfer were made in gas phase atomic systems. There have been many publication dealing with similar studies on polyatomic systems in the gas phase, liquid phase and in molecular crystals. Only very little, however, is known about transfer processes and mechanisms when donor and acceptor molecules are adsorbed at the surface of a solid (1-3). In this paper we present some photoacoustic measurements and compare the results with fluorescence decay time measurements. As model substances we have chosen four different donor dyes and two acceptor dyes. The dyes were adsorbed on silica. 

Curve 3. Here the acceptor concentration is 8.71 10 ° mole m A nitrogen pumped dye laser and boxcar averaging were employed for the fluorescence decay time measurements. 

Lampert RA, Chewter LA, Phillips D et al (1983) Standards for nanosecond fluorescence decay time measurements. Anal Chem 55 68-73... 

Although the relation between fluorescence depolarisation and rotational Brownian motion was first identified by Perrin and the development of the theoretical background of the time-resolved fluorescence depolarization experiments was made by Jablonski use of the technique was limited until the advent of improved fluorescence decay time measurements some fifteen years i. An alternative, related technique, involving excitation using a continuous polarised light source, provides only the time average of the correlation function (Eq. 18) and as such, is less useful than the time resolved method. Other disadvantages are that the natural decay time of the chromophore must be determined from a sqrarate experiment and it is necessary to alter the viscosity, and/or temperature of the medium, often withun-... 

The laser pulse initiated processes as observed from sub-picosecond to nanaosecond time range on a series of oligothiophenes in solution started with the increase of the A1 band during the absorption of the pulse energy. The A1 bands are S <- Si absorptions, identified by their decay times which are identical to the fluorescence decay times measured by time resolved fluorescence measurements (Tab. 17.1). The observed induced fluorescence F roughly decays with the same rate constant as A1 if it is not superposed by absorption bands of different time function. 

S. Georghiou, Nordlund, M. Thomas and A. M. Saim, Picosecond fluorescence decay time measurements... 

Fluorescence Decay Time Measurements below the Glass Transition Temperature 491... 

For the investigation of triplet state properties a laser flash photolysis apparatus was used. The excitation source was a Lambda Physik 1 M 50A nitrogen laser which furnished pulses of 3.5 ns half-width and 2 mJ energy. The fluorescence decay times were measured with the phase fluorimeter developed by Hauser et al. (11). 

We are able to measure the ratio p/ F and the fluorescence decay time of the deuterated and undeuterated... 

Every time an excited molecule exits the excited state region by the fluorescence pathway it emits a photon. We can either count the number of photons in a longer time interval (by a steady-state measurement of the fluorescence intensity) or make a time-resolved measurement of the fluorescence decay. These measurements can be done in an ensemble mode or on single molecules—the basic process is the same. The number of photons collected from the donor emission will be depicted by IDA and ID, where we mean the fluorescence intensity of D in the presence (Ida) and absence (ID) of acceptor. All other conditions, other than the presence or absence of acceptor, remain the same. During the same time of the experiment where we have measured the photons emitted by D, many of the excited D molecules have exited from the excited state by a pathway other than fluorescence. Obviously, the number of times a pathway has been chosen as an exit pathway is proportional to the... 

Optical sensors for oxygen measurement are attractive since they can be fast, do not consume oxygen and are not easily poisoned. The most common method adopted in construction is based on quenching of fluorescence from appropriate chemical species. The variation in fluorescence signal (I), or fluorescence decay time (x) with oxygen concentration [O2] is described by Stem-Volmer equation91 ... 

The first photoelectric fhiorimeter was described by Jette and West in 1928. The instrument, which used two photoemissive cells, was employed for studying the quantitative effects of electrolytes upon the fluorescence of a series of substances, including quinine sulfate [5], In 1935, Cohen provides a review of the first photoelectric fluorimeters developed until then and describes his own apparatus using a very simple scheme. With the latter he obtained a typical analytical calibration curve, thus confirming the findings of Desha [33], The sensitivity of these photoelectric instruments was limited, and as a result utilization of the photomultiplier tube, invented by Zworykin and Rajchman in 1939 [34], was an important step forward in the development of suitable and more sensitive fluorometers. The pulse fhiorimeter, which can be used for direct measurements of fluorescence decay times and polarization, was developed around 1950, and was initiated by the commercialization of an adequate photomultiplier [35]. 

Fluorescence Lifetimes. The fluorescence decay times of TIN in a number of solvents (11.14.16.18.19), low-temperature glasses (12.) and in the crystalline form (15.) have been measured previously. Values of the fluorescence lifetime, Tf, of the initially excited form of TIN and TINS in the various solvents investigated in this work are listed in Table III. Values of the radiative and non-radiative rate constants, kf and knr respectively, are also given in this table. A single exponential decay was observed for the room-temperature fluorescence emission of each of the derivatives examined. This indicates that only one excited-state species is responsible for the fluorescence in these systems. 

There are many molecular interactions which influence the fluorescence decay times. The measured fluorescence lifetime r is usually shorter than the radiative lifetime tr because of presence of other decay rates which can be dependent on intramolecular processes and intermolecular interactions (Figure 10.3). The measured fluorescence lifetime (r) is given by the inverse of the total rate of dynamic processes that cause deactivation from the excited (mostly singlet Si) state... 

In the first attempts to overcome the background problem using decay time, the variation of the fluorescence decay time as a function of wavelength across the entire emission profile for a variety of materials have been used (Measures 1985). For a variety of rocks and minerals, it was proved that this information represents a new kind of signature, the so called fluorescence decay spectrum, that possesses considerable discrimination power, being able to characterize the irradiated material with far superior precision than the normal luminescence spectrum (Fig. 7.2). 

Geusic et al. (118) made measurements of the fluorescent lifetime of neodymium in yttrium aluminum garnet (Y3A15012). For neodymium concentrations up to 3 atomic per cent, the measured fluorescent-decay time is approximately 200 jtxsec at both IT and 300°K. Above 6 atomic per cent, a marked decrease in the fluorescent lifetime is observed. They suggest that this is due to neodymium interactions. It is to be noted that yttrium aluminum garnet is a laser material of very exceptional quality. 

Time-Resolved Fluorescence. Emission and excitation monochromators are maintained in a specific wavelength, but the excitation is chopped off and fluorescence decay is measured as a function of time. This kind of spectroscopy is interesting for studying structural changes or different complexation sites. 

The measurement of fluorescence decay times can provide information on a number of fundamental processes. Consider the following kinetic scheme. The molecule AB in its electronic ground state (X) is irradiated by a short, narrow bandwidth pulse of light at a wavelength which corresponds to the transition to the vibronic level (F) t j-, viz. 

It is clear that, by changing the experimental conditions and/or detection wavelength, limiting values can be found for all of the quantities mentioned above from measurements of the fluorescence decay time. The effects of collisional and spontaneous processes can be separated by conventional Stem—Volmer analysis [36]. The concentration, [M], of quenching molecules is varied and the reciprocal of the observed lifetime is plotted against the concentration of M. The quenching rate coefficient is thus obtained from the slope and the intercept gives the rate coefficient for the spontaneous relaxation processes, which is usually the natural lifetime of the excited state. In cases where the experiment cannot be carried out under collision-free conditions, this is the only way to measure the natural lifetime from observation of the fluorescence decay. 

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