Rigid solutes

Typical singlet lifetimes are measured in nanoseconds while triplet lifetimes of organic molecules in rigid solutions are usually measured in milliseconds or even seconds. In liquid media where drfifiision is rapid the triplet states are usually quenched, often by tire nearly iibiqitoiis molecular oxygen. Because of that, phosphorescence is seldom observed in liquid solutions. In the spectroscopy of molecules the tenn fluorescence is now usually used to refer to emission from an excited singlet state and phosphorescence to emission from a triplet state, regardless of the actual lifetimes. 

One of the first explanations for the afterglow of organic compounds in rigid solutions after exposure to UV light was offered by Perrin.<12) Perrin postulated that the excited molecules could undergo a transition to a metastable state of lower energy. Emission from this state was thought to be... 

The inhomogeneous broadening effect will be apparent in practically all cases, and the character of this broadening may be both stationary (in rigid solutions or when time of relaxation xr is less than lifetime of fluorescence x, or xr>t) and dynamic in nature. Inhomogeneous broadening affects all spectral characteristics of organic molecules in solutions. 

Table 5.2 The effect of heavy atoms on transitions between states in rigid solution (77K)...

Azobisisobutyronitrile, 182, reacts thermally or photochemically to give the intermediate 183, which leads, in inert solvents, to combination products 184 and 185, and disproportionation products 186 and 187. The parent compound is dimorphic, and both crystal forms behave similarly on photolysis, yielding 95% disproportionation and 5% 184. In contrast, in both fluid and rigid solution the disproportionation products form only 5% of the total. The cage effect in the solid is almost quantitative. 

Tryptophan at 77 K in rigid solution has a phosphorescence quantum yield of 0.17(20) and a lifetime of 6 s. These values at 77 K are relatively invariant from protein to protein and do not vary significantly between buried and exposed tryptophans.(21,22) If one assumes that the intersystem crossing yield is a constant, a calculation of the quantum yield of indole phosphorescence can be roughly estimated from the lifetimes. The phosphorescence yield is related to lifetime by... 

Helene110 says that thymidine fluorescence from aqueous solutions at 77°K of a thymidine-adenosine mixture (as well as the identical fluorescence from rigid solutions of G + T, AcC + T or C + T) is owing to a triplet energy transfer to the thymidine moiety. The order of triplet-state energies among the purines and pyrimidines is given as... 

The experimental data show that for photo-colourability in either the crystal or rigid solution there is required an ortho- hydroxyl group which is hydrogen bonded to the nitrogen of the same molecule. There is no direct evidence as to the quinoid nature of the coloured form— we have only the suggestive analogy with the azonaphthols and the relative behaviour of the anils from 2 3- and l 2-hydroxynaphth-aldehydes. 

In the second category, polymers with the repeat unit [CH2— CRR —(CH2)m CRR —CH2], where R = phenyl, m = 3-6 or 10, and R = hydrogen or methyl, were synthesized by Richards et al.143). The fluorescence of the R = H compounds 144) and the R = CH3 compounds 2S 145) were studied in fluid and rigid solution and in pure films. Although no spectra were given for the R = H compounds, these were stated 144) to have no excimer emission at 330 nm in fluid solution nor in pure films. A similar report was made25) for the R = CH3 compounds in 2-methyltetrahydrofiiran solution at room temperature, and in such solutions to which methanol had been added to the point of opalescence. These results were confirmed for the R = CH3 compounds in solution, and the spectra of pure films did not show significant amounts of excimer fluorescence at 330 nm t4S). However, an extraneous emission at 310 nm in the film spectra made quantitative measurement of the 330 nm excimer band impossible. 

As noted earlier, the limiting lifetime of pyrene excimer fluorescence from concentrated solutions in PS and PMMA glasses was found to be the same as that of pyrene in cyclohexane solution. There have been no similar studies of naphthyl compounds in rigid glasses. Values of k and Q for the [2,6]-naphthalenophanes have not yet been determined for any solvent system. The bis(2-naphthyl) compounds have not been quantitatively characterized in rigid matrices, probably because excimer fluorescence is weak and difficult to detect under such conditions. Given such limited data, it can only be assumed that the values of QD and kD of 2-naphthyl excimers remain the same in rigid solution as in fluid solution. 

Excimer fluorescence from polychromophoric compounds in rigid systems, while easy to detect, is difficult to interpret. The transient response of the excimer can be empirically characterized by the limiting lifetime T p. In the absence of processes which convert D to M, this limiting lifetime is the reciprocal of k . We will examine xa]D for PS, P2VN, and other aromatic polymers to see if there is any difference between fluid and rigid solution at room temperature. 

Results reported above clearly demonstrate that the impact of solvation on molecular geometry of molecules is small and produces only a small increase in the dipole moment (around 6% in water), which leads to a parallel increase in the computed solvation free energy (see Figure 4.3). Note that such an increase can be easily corrected during the parameterization of continuum models, suggesting that gas phase geometries can be safely used to reproduce solvation of many small quasi-rigid solutes. 

The emission spectrum of 9 TCP has been studied revealing a CT fluorescence both in the solid state and in nonpolar rigid solution at - 196 C, but not in fluid solution (74BCJ442). The CT complex 9 TCP in the solid state shows exclusively a CT fluorescence, the emission from the component ions being completely quenched. 

Let us briefly mention here that proton hyperfine splittings have been observed in the high-field ESR spectra of the naphthalene triplet (33,40) in single crystals. Recently Grivet (43) has succeeded in observing hyperfine splittings in the intermediate field (AMg=2) line of the triplet of partially deuterated naphthalene in rigid solution with a conventional spectrometer. 

Kavun and Buchachenko (247) have found that the "primary" amino radicals derived from thioamines in the presence of hydroperoxide are very stable and are not readily converted into "secondary" nitroxide radicals. Kommandeur and Wiersma (107) studied the photodecomposition of tetraphenylhydrazine in rigid solution at 77°K, and the observed ESR spectrum was assigned to the dimer of the diphenyl amino radical (248). Shida et al. (249) have also studied the photodimer of tetraphenylhydrazine and the photochromic dimer of triphenylimidazolyl at low temperature by optical and ESR techniques. These authors noted that photolysis of these compounds induces the homolysis while y-irradiation leads to heterolytic dissociation. Blinder et al. (250) reported the ESR spectrum of the monomer tetraphenylpyrryl radical in... 

As has been discussed previously, it became apparent during the negotiation of the CWC that, because of uncertainty about the number of facilities that would be declared under Schedules 1, 2 and 3 and as DOC facilities, as well as their relative risk to the object and purpose of the CWC, it would be impractical to attempt to develop rigid solutions in the Convention text. The nature of the practical verification problems involved would become apparent only in the course of implementation of the CWC. See Mathews, Intention of Article VI . 

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