Absorption and lifetime

TABLE 1.2 Absorption and Lifetime (l/k) Data for Selected Qninone Methides... 

Wan s group investigated a number of phenyl-substituted hydroxybiphenylbenzyl alcohols in the hope that the a-phenyl quinone methides photogenerated from them might show enhanced absorption and lifetimes, and thus be easier to characterize by LFP.37,38 They were successfully able to photogenerate and characterize quinone... 

An electronically excited molecule may, under some conditions, absorb another quantum and be raised to a higher excited state. Usually the population of excited species is so low that the probability of this occurrence is very slight. However, in recent years the technique of flash photolysis has been developed, which allows us to investigate the absorption properties of excited states. An extremely high intensity laser, which has approximately one million times the power of a conventional spectroscopic lamp, is turned on for a tiny fraction of a second, and a large population of excited species is produced. Immediately after this photolysis flash is turned off, a low-power spectroscopic flash may be turned on and the absorption spectrum of the already-excited system determined. By varying the delay between photolysis and spectroscopic flashes, much can be learned about the absorption and lifetime of singlet and triplet excited states. 

The transition dipole moment, and therefore the oscillator strengths, are sensitive to the inclusion of the amino acid, conformation, method, and basis sets. This makes accurate calculations involving the oscillator strengths difficult to obtain. The oscillator strengths of the amino acids qualitatively show their order of absorptivity and lifetimes, but do not agree well with the experimental lifetimes [4, 5, 35]. The sensitivity of these properties on the conformers and the method contributes to the poor agreement with experiment. 

Because most carbenes are so reactive, it is often difficult to prove that they are actually present in a given reaction. The lifetime of formylcarbene was measured by transient absorption and transient grating spectroscopy to be 0.15-0.73 ns in dichloromethane. In many instances where a carbene is apparently produced by an a elimination or by disintegration of a double-bond compound, there is evidence that no free carbene is actually involved. The neutral term carbenoid is used where it is known that a free carbene is not present or in cases where there is doubt. a-Halo organometallic compounds (R2CXM) are often called carbenoids because they readily give a elimination reactions (e.g., see 12-37). ° ... 

The use of emission (fluorescence and phosphorescence) as welt as absorption spectroscopy. From these spectra the presence of as well as the energy and lifetime of singlet and triplet excited states can often be calculated. 

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 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. 

The lifetimes, molar absorptivities, and oscillator strengths for various transitions are summarized in Table 1.3. 

Dialkylanthracene-containing squaraine dyes 17 show intense absorption and emission in the NIR region (720-810 nm) [74]. They are compatible with aqueous environments and show substantial enhancement of quantum yields and fluorescence lifetimes in hydrophobic and micellar media, suggesting that these dyes can be potentially useful as fluorescent probes in biological applications, e.g., for imaging of hydrophobic domains such as cell membranes. 

Additionally, note that the polarity of the solvent significantly affects not only the positions of absorption and fluorescence spectra but also the fluorescence quantum yields. The largest difference in quantum yield is observed for G19 (eight times larger in toluene) [86]. The effect of solvent polarity on quantum yield and fluorescence lifetime was investigated in mixtures of toluene and ACN (polarity range 0.013-0.306). Polarity dependent quantum yield and lifetime measurements are presented in Fig. 22. 

It is seen that the fluorescence quantum yield and lifetime of G19 gradually decreases with increasing solvent polarity. For example, the insertion of 20% ACN by volume into toluene leads to a decrease of a factor of two. Based on these results we can conclude that G19 is very sensitive to solvent polarity and can be used as an efficient probe to test the polarity of its microenvironment. A reverse trend of the absorption peak at 1 1 mixture of ACN and toluene (50%T in Fig. 22b) corresponds to a change of the sign of due to a transition from a polyene-like structure in nonpolar toluene to a polymethine-like structure in polar ACN. 

Experiment [80], however, requires that the red-shifted fluorescence comes from molecules that are in an environment quite different from the average, behaving as if in a nonpolar environment (long wavelength, long lifetime). The red-shifted component, therefore, appears to be from chromophores that are embedded more deeply into the membrane. In the case of Aladan, the absorption and fluorescence wavelengths are both longer in the more water-exposed protein sites, but the lifetime is shorter in water [53]. 

Those organometallic thexi states which have been detected have involved compounds where the quantum yield for photodissociation is very low. Time-resolved uv-visible absorption and emission studies have been made on W(CO)5L and W(CO)4L species (L = acetylpyridine, L = o-phenanthroline) (54), but, as in the case of intermediates, these studies provided lifetimes but no structural information. 

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