Lifetime, emission

Jablonski (48-49) developed a theory in 1935 in which he presented the now standard Jablonski diagram" of singlet and triplet state energy levels that is used to explain excitation and emission processes in luminescence. He also related the fluorescence lifetimes of the perpendicular and parallel polarization components of emission to the fluorophore emission lifetime and rate of rotation. In the same year, Szymanowski (50) measured apparent lifetimes for the perpendicular and parallel polarization components of fluorescein in viscous solutions with a phase fluorometer. It was shown later by Spencer and Weber (51) that phase shift methods do not give correct values for polarized lifetimes because the theory does not include the dependence on modulation frequency. 

Theory. If two or more fluorophores with different emission lifetimes contribute to the same broad, unresolved emission spectrum, their separate emission spectra often can be resolved by the technique of phase-resolved fluorometry. In this method the excitation light is modulated sinusoidally, usually in the radio-frequency range, and the emission is analyzed with a phase sensitive detector. The emission appears as a sinusoidally modulated signal, shifted in phase from the excitation modulation and partially demodulated by an amount dependent on the lifetime of the fluorophore excited state (5, Chapter 4). The detector phase can be adjusted to be exactly out-of-phase with the emission from any one fluorophore, so that the contribution to the total spectrum from that fluorophore is suppressed. For a sample with two fluorophores, suppressing the emission from one fluorophore leaves a spectrum caused only by the other, which then can be directly recorded. With more than two flurophores the problem is more complicated but a number of techniques for deconvoluting the complex emission curve have been developed making use of several modulation frequencies and measurement phase angles (79). 

This result is further supported by the very short 130 + 10 ps lifetime combined with the remarkably high quantum yield of 0.43, both observed upon excitation at 335 nm, near the long-wavelength edge of the absorption band. These values combine to a radiative lifetime of 300 ps, which corresponds (33a) to an oscillator strength of 1.8. Similarly short emission lifetimes have been observed for other poly(di-n-alkylsilanes) di-n-pentyl, 200 ps, di-n-decyl, 150 ps. The average oscillator... 

The electronic absorption and emission spectra and emission lifetimes of [Ir(/x-L)(CO)2]2 (L = pz, mpz and dmpz) have been determined.529 The intense low-energy absorption band around 400 nm is assigned to a d/2 > pz electronic transition. The three complexes all emit around 740 nm at 300 K and 670 nm at 77 K. The dimer excited states are stabilized relative to monomer levels by strong metal-metal bonding. 

Luminescent Ir complexes of diphosphine and diphosphinite calix[4]arene show emission maxima at 619 nm and 597 nm, respectively, at 77 K. The emission lifetimes are perturbed by addition of Li+, Na+, and U022+. 

Both chloro ligands of [Pt2Cl2(/u-dppm)2] are replaced by quinoline, 1-methylimidazole, and 4-t-butylpyridine, but only one is replaced by 2,4,6-trimethylpyridine.109 All of these platinum(I) dimers and pt2(PPh3)2(/i-dppm)2] display solid-state luminescence at room temperature and long emission lifetimes at 77 K.109... 

Energy transfer by the trivial mechanism is characterized by (a) change in the donor emission spectrum (inner filter effect), (b) invariance of the donor emission lifetime, and (c) lack of dependence upon viscosity of the medium. 

D. MacKenzie assisted with the (ButtN)2MgXllf emission lifetime measurements. Research at the California Institute of Technology was supported by National Science Foundation Grants CHE78-10530 and CHE81-20419. This is Contribution No. 6703 from the Arthur Amos Noyes Laboratory. 

Emission lifetimes span the range of nano-seconds to hours. How wide a range is this compared to the time-span of recorded history on Earth On the scale of useful chemical synthesis kinetics ... 

Figure 13.9 Emission lifetimes of Ru(bpy>32+ (20 /xM) monitored in air saturated aqueous solutions of PAMAM dendrimers (20 fiM) as a function of generation. Adapted from ref. 23...

For both organic dyes and QDs, bioconjugation often leads to a decrease in fluorescence quantum yield and thus typically also in emission lifetime. Parameters that can affect label fluorescence are the chemical nature and the length of the spacer and, at least for organic dyes, the type of neighboring biomolecules like oligonucleotides or amino acids in the bioconjugated form. 

In order to avoid complications caused by excitation energy transfer between tryptophan residues, most investigations have been performed with proteins containing one tryptophan residue per molecule. When studying protein solutions, there are difficulties in separating the effects of rotation of entire protein molecules and of the chromophores themselves relative to their environment in the protein matrix. It is usually assumed that intramolecular motions are more rapid and manifest themselves as short-lived components of anisotropy decay curves or in depolarization at short emission lifetimes. 

EXAMPLE 5.6 The low-temperature emission lifetime of a very low concentration ofCr ions in ruby (AfOs Cr) is about 4 ms. For a 1%... 

Emission lifetime provides a powerful selectivity dimension as each fluorophore has a unique decay rate. While laboratory fluorescent lifetime measurement systems are widely available and the concept of lifetime LIF sensing has been demonstrated,commercial LIF lifetime sensors are not yet available on the market for PAT and field applications to monitor organic analytes or bioagents. Once available, lifetime LIF sensors could revolutionize fluorescent based monitoring by affording superior detection merits along with sufficient recognition power. 

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