Luminescence lifetimes

Luminescence lifetime spectroscopy. In addition to the nanosecond lifetime measurements that are now rather routine, lifetime measurements on a femtosecond time scale are being attained with the intensity correlation method (124), which is an indirect technique for investigating the dynamics of excited states in the time frame of the laser pulse itself. The sample is excited with two laser pulse trains of equal amplitude and frequencies nl and n2 and the time-integrated luminescence at the difference frequency (nl - n2 ) is measured as a function of the relative pulse delay. Hochstrasser (125) has measured inertial motions of rotating molecules in condensed phases on time scales shorter than the collision time, allowing insight into relaxation processes following molecular collisions. 

Luminescence lifetimes are measured by analyzing the rate of emission decay after pulsed excitation or by analyzing the phase shift and demodulation of emission from chromophores excited by an amplitude-modulated light source. Improvements in this type of instrumentation now allow luminescence lifetimes to be routinely measured accurately to nanosecond resolution, and there are increasing reports of picosecond resolution. In addition, several individual lifetimes can be resolved from a mixture of chromophores, allowing identification of different components that might have almost identical absorption and emission features. 

The luminescence lifetimes of ARRR, ASSS, ARRS, and ASSR diastereomers (188), R = Ph, show that the symmetric forms are longer lived by a factor of two than the less symmetric species and are solvent dependent.348 The X-ray structure of R = Me confirms the fac arrangement of the Si/N ligands.349... 

Ultraviolet absorption spectra were obtained from a Cary 118C Spectrophotometer. Luminescence measurements were obtained from a Perkin-Elmer Model MPF-3 Fluorescence Spectrophotometer equipped with Corrected Spectra, Phosphorescence and Front Surface Accessories. A Tektronix Model 510N Storage Oscilloscope was used for luminescence lifetime measurements. Fiber irradiation photolyses were carried out in a Rayonet Type RS Model RPR-208 Preparative Photochemical Reactor equipped with a MGR-100 Merry-go-Round assembly. 

Photophysical investigations performed on chirally resolved forms of compounds 26 and 27 [63 b, 66] showed that the diastereoisomeric forms do not exhibit any significant difference at room temperature. However, in a glass at low temperature, the luminescence lifetimes of the heterochiral diastereoisomers were slightly shorter than those of the homochiral forms [66]. 

A strong decrease in relaxivity (from 12.8mM-1s-1 to 2mM-1s-1) between pH 6 and 11 has been reported for a positively charged macrocyclic Gdm complex (Scheme 10), which was explained by the successive deprotonation of the coordinated water molecules.167 Luminescence lifetime measurements of a Yb111 analogue proved that the complex possesses three bound waters at pH 5.5. Above pH 11, a di-oxo-bridged dimer is formed that has no more bound water or OH groups. 

Figure 7 Luminescent lanthanide complexes with representative luminescence lifetimes, and hydration states (derived from luminescence measurements) where appropriate.

Flowever, there is a trade-off in using near-IR emissive lanthanides, in that luminescence lifetimes are shorter, and quantum yields lower, compared to complexes of Tb and Eu. This arises because the near-IR emissive lanthanides are quenched by lower harmonics of the O-H oscillator, increasing the Franck-Condon overlap with the metal excited state. For neodymium, matters are further complicated by the manifold of available metal-centered excited states, which leads to particularly effective quenching by C-H oscillators. Thus, complexes in which there are few C-H oscillators close to the metal are desirable if the luminescence lifetime is to be optimized (e.g. 44).76 97-101... 

According to Ludwig (1968), there is a some similarity between UV- and high-energy-induced luminescence in liquids. In many cases (e.g., p-ter-phenyl in benzene), the luminescence decay times are similar and the quenching kinetics is also about the same. However, when a mM solution of p-terphenyl in cyclohexane was irradiated with a 1-ns pulse of 30-KeV X-rays, a long tail in the luminescence decay curve was obtained this tail is absent in the UV case. This has been explained in terms of excited states produced by ion neutralization, which make a certain contribution in the radiolysis case but not in the UV case (cf. Sect. 4.3). Note that the decay times obtained from the initial part of the decay are the same in the UV- and radiation-induced cases. Table 4.3 presents a brief list of luminescence lifetimes and quantum yields. 

TABLE 4.3 Luminescence Lifetimes and Quantum Yields (qy) of Some Selected Compounds... 

The luminescence lifetime may also be extracted from phase-sensitive detection of the modulated emission that originates from modulated continuous excitation of the indicator dye (equation 9)2 ... 

The commercialization of inexpensive robust LED and laser diode sources down to the uv region (370 nm) and cheaper fast electronics has boosted the application of luminescence lifetime-based sensors, using both the pump-and-probe and phase-sensitive techniques. The latter has found wider application in marketed optosensors since cheaper and more simple acquisition and data processing electronics are required due to the limited bandwidth of the sinusoidal tone(s) used for the luminophore excitation. Advantages of luminescence lifetime sensing also include the linearity of the Stem-Volmer plot, regardless the static or dynamic nature of the quenching mechanism (equation 10) ... 

A similarly wide choice of lumophores can be found within photophysics and photochemistry.191 The excitation (or absorption) and emission bands of luminescence come in a variety of wavelength positions, intensities and shapes. Another parameter, which is gaining in popularity among designers, is the luminescence lifetime. Time-resolved observation is a neat way of dissecting out the response of the luminescent device from the emissive noise of real-life matrices. Photostability of lumophores is a parameter which perhaps deserves more attention as more and more demanding applications are being tackled. 

Enhanced environmental stability was recently demonstrated for PLED with PFO 196/gold nanoparticle (5-10 nm) nanocomposite-emitting layer [337]. In addition, the gold nanoparticle-doped PLEDs (1.5 x 10-5 volume fraction of Au) demonstrated improved luminescent lifetime and 2-3 times higher QE, compared with pure PFO-based PLED. 

G. Liebsch, I. Klimant, B. Frank, G. Holst and O.S. Wolfbeis, Luminescence lifetime imaging of oxygen, pH, and carbon dioxide distribution using optical sensors, Appl. Spectrosc., 54(4) (2000) 548-559. 

Because Si — Ti absorption has a very small molar absorption coefficient, we would expect (because of the inverse relation between 8 and T0) the Ti state to have a much greater luminescent lifetime than the same molecules in the Si state. As a result of this longer lifetime, the Ti state is particularly susceptible to quenching, such that phosphorescence in fluid solution is not readily observed as the Ti state is quenched before emission can occur. This quenching in solution involves the diffusion together of either two Ti molecules or the Ti molecule and a dissolved oxygen molecule or some impurity molecule. In order to observe phosphorescence it is necessary to reduce or prevent the diffusion processes. The techniques most often used are ... 

The fluorescence decay kinetics of exemplary chosen QDs and small organic dyes are compared in Fig. 2. The size of the fluorescence parameter luminescence lifetime is determined by the electronic nature of the transitions involved. As a rule... 

Medlycott EA, Hanan GS (2005) Designing tridentate ligands for ruthenium(II) complexes with prolonged room temperature luminescence lifetimes. Chem Soc Rev 34 133-142... 

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... 

Cajlakovic M, Bizzarri A, Ribitsch V (2006) Luminescence lifetime-based carbon dioxide optical sensor for clinical applications. Anal Chim Acta 573-574 57-64... 

Figure 9.2. Transient luminescence of a sensor as a function of the concentration of a [Parameter]. In the figure knr = jl (Eq, (9,12) with a = 0). Increasing values of [Parameter] result in shorter luminescence lifetimes. kr = Vw, k , = V a in units of time-1. The curves in the figure are determined by Eq. (9.3).

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