Delayed fluorescence lifetimes

It is of interest to note that the electron-transfer time from O to Qa was indirectly estimated to be a few hundred picoseconds by Shuvalov et. al in 1980 and by Karukstis and Sauer in 1983. Shuvalov et. aid" reasoned that since the amplitude of AA and delayed fluorescence (lifetime 4 ns) due to the [P680" O ] was decreased by a factor of at least 10 when Qa is present in the oxidized state, the electron-transfer time from [P680+ O ] to Qa would be expected to be < 0.4 ns. 

Thus, the delayed fluorescence lifetime is essentially equal to that of T for PVCA and to T for PIVN. The latter is easy to prove since excimer phosphorescence nd delayed fluorescence lifetimes are the same for PIVN. For PVCA at 77 Kt x. but no direct and independent measure of the lifetime of T Tn PyCA has been accomplished at this time. 

Delayed fluorescence lifetimes for these two polymers indicate that there is more than one type of excimeric species present. It had been pointed out by Siebrand(18) that when this situation obtains and when the lifetime of mobile the exciton is less than that of the... 

Emission of light due to an allowed electronic transition between excited and ground states having the same spin multiplicity, usually singlet. Lifetimes for such transitions are typically around 10 s. Originally it was believed that the onset of fluorescence was instantaneous (within 10 to lO-" s) with the onset of radiation but the discovery of delayed fluorescence (16), which arises from thermal excitation from the lowest triplet state to the first excited singlet state and has a lifetime comparable to that for phosphorescence, makes this an invalid criterion. Specialized terms such as photoluminescence, cathodoluminescence, anodoluminescence, radioluminescence, and Xray fluorescence sometimes are used to indicate the type of exciting radiation. 

The variable delay can be as simple as an RC network. Often the variable delay line is calibrated directly in terms of lifetime units (nanoseconds). When the reference and comparison signals are in phase the fluorescence lifetimes can simply be read off the calibrated variable delay. 

The first observations of P-type delayed fluorescence arose from the photoluminescence of organic vapors.<15) It was reported that phenanthrene, anthracene, perylene, and pyrene vapors all exhibited two-component emission spectra. One of these was found to have a short lifetime characteristic of prompt fluorescence while the other was much longer lived. For phenanthrene it was observed that the ratio of the intensity of the longer lived emission to that of the total emission increased with increasing phenanthrene vapor... 

By measuring the intensities and lifetimes of delayed fluorescence of both solutions in the same apparatus [(/ ) = (Ia)2] we obtain... 

A linear plot indicates that the luminescence decay is exponential. The slope of the line gives kt, and rt can be calculated as above. The lifetime obtained by measuring the decay of P-type delayed fluorescence is equal to one-half the lifetime of the triplet state (see Section 5.2). Since in fluid solution at room temperature phosphorescence is generally much weaker than delayed fluorescence, the measurement of delayed fluorescence decay offers a convenient method for determining the lifetime of triplets at room temperature. 

In TG methods, the fluorescence emission is detected in two or more time-gates each delayed by a different time relative to the excitation pulse (see Fig. 3.3). In the case of a detection scheme equipped with two time-gates, the ratio of the signals acquired in the two time-gates is a measure of the fluorescence lifetime. For a decay that exhibits only a single exponent, the fluorescence lifetime is given by ... 

Flash photolysis studies with absorption or delayed fluorescence detection were performed to compare the binding of ground and excited state guests with DNA.113,136 The triplet lifetimes for 5 and 6 were shown to be lengthened in the presence of DNA.136 The decays were mono-exponential with the exception of the high excitation flux conditions where the triplet-triplet annihilation process, a bimo-lecular reaction, contributed to the decay. The residence time for the excited guest was estimated to be shorter than for the ground state, but no precise values for the rate constants were reported. However, the estimated equilibrium constants for the... 

Temporal characteristics at early stages were elucidated by measuring fluorescence intensity with the gate time of 1.74 ns as a function of the delay time. Compared to the laser pulse, the time where the maximum intensity is attained shifts to the early stage as the laser fluence becomes high. Of course, we could not find out any decay component with intrinsic fluorescence lifetime of 17 and 35 ns. It is concluded that an Si - Si annihilation occurs quite efficiently during the pulse width. 

In certain compounds a weak emission has been observed with the same spectral characteristics (wavelengths and relative intensities) as fluorescence, but with a lifetime more characteristic of phosphorescence. Two mechanisms are used to account for delayed fluorescence. 

P-type delayed fluorescence is so called because it was first observed in pyrene. The fluorescence emission from a number of aromatic hydrocarbons shows two components with identical emission spectra. One component decays at the rate of normal fluorescence and the other has a lifetime approximately half that of phosphorescence. The implication of triplet species in the mechanism is given by the fact that the delayed emission can be induced by triplet sensitisers. The accepted mechanism is ... 

The intensity of the delayed fluorescence emission from eosin decreases as the temperature is lowered and this indicates that an energy barrier is involved. Since the delayed fluorescence is spectrally identical to normal fluorescence, emission must occur from the lowest vibrational level of Si. However, the fact that the lifetime is characteristic of phosphorescence implies that the excitation originates from T,. The explanation of this requires a small Si-Ti energy gap, where T, is initially populated by intersystem crossing from Si. Ti to Si intersystem crossing then occurs by thermal activation. 

Triplet-triplet annihilation In concentrated solutions, a collision between two molecules in the Ti state can provide enough energy to allow one of them to return to the Si state. Such a triplet-triplet annihilation thus leads to a delayed fluorescence emission (also called delayed fluorescence of P-type because it was observed for the first time with pyrene). The decay time constant of the delayed fluorescence process is half the lifetime of the triplet state in dilute solution, and the intensity has a characteristic quadratic dependence with excitation light intensity. 

Monitoring of phosphorescence or delayed fluorescence enables us to study much slower phenomena. Examples of lifetimes are given in Table 3.1. 

Prior to describing the possible applications of laser-diode fluorometry, it is important to understand the two methods now used to measure fluorescence lifetimes these being the time-domain (Tl)/4 5 24 and frequency-domain (FD) or phase-modulation methods.(25) In TD fluorometry, the sample is excited by a pulse of light followed by measurement of the time-dependent intensity. In FD fluorometry, the sample is excited with amplitude-modulated light. The lifetime can be found from the phase angle delay and demodulation of the emission relative to the modulated incident light. We do not wish to fuel the debate of TD versus FD methods, but it is clear that phase and modulation measurements can be performed with simple and low cost instrumentation, and can provide excellent accuracy with short data acquisition times. 

A kinetic technique for determining a fluorophore s excited state lifetime by using a light source whose intensity is modulated sinusoidally at a certain frequency, such that the intensity of the fluorescence emission likewise varies sinusoidally but with an added delay from the finite relaxation constant for fluorescence decay. The period of the sinusoidal modulation is chosen to be in the neighborhood of the magnitude of the fluorescence lifetime. 

The time-resolved emission spectra (TRES) and fluorescence lifetimes, ti, of the fluorene derivatives were measured in liquid solutions at room temperature with a PTI QuantaMaster spectrofluorimeter with 0.1 ns temporal resolution [20]. At this resolution, all investigated fluorenes exhibited TRES which were coincident with the corresponding steady-state fluorescence spectra. As an example, TRES for compounds 3 and 11 in hexane, THE, and ACN are presented in Eig. 8 for different nanosecond delays 0 ns (curves 2,4,6) and 5 ns, which modeled the steady-state condition (curves 3,5,7). No differences in the fluorescence spectra for these two delays were observed, indicating that all relaxation processes in the first excited state Si are sufficiently fast for fluorene molecifles and did not exceed the time resolution of the PTI system ( 0.1 ns). 

A simplified instrument for the measurement of fluorescent lifetimes using the stroboscopic method has been described by Brown (67). The major virtue of this system is that it makes use of a Tektronix oscilloscope to obtain all the necessary trigger pulses, including a trigger of continuously variable delay. Since most laboratories are equipped with a good oscilloscope, the need to purchase expensive trigger-delay apparatus is thus eliminated. 

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