In rigid solution

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)...

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... 

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. 

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... 

V. Matee, P. Markovsky, L. Nikolova, and T. Todorov, Temperature dependence of photoinduced anisotropy in rigid solutions of azo dyes, J. Phys. Chem. 96, 3055-3058 (1992). 

One reason for this preoccupation with the physical state concerns the ease with which triplet excimer formation occurs. Phosphorescence from triplet excimers, for example, is common in solid polymeric films but much less common in rigid solutions. For this reason, the interpretation of rate processes involving triplet states tends to be simpler to handle and more susceptible to quantitative treatment for rigid polymer solutions. 

Polymers in Rigid Solution. The emission spectrum of PCVA in 2-methy1tetrahydrofuran (MTHF) at 77 K consists of prominent delayed fluorescence and phosphorescence bands(19). For this reason it was decided to investigate the rate of triplet exciton decay in these rigid solutions and to treat the data in terms of concurrent first and second order processes. For systems in which an equilibrium distribution of potential reactants may be assumed, eq 1 may be employed for data analysis. It is not clear, however, that such a distribution is valid for polymer solutions especially in light of evidence suggesting that T-T annihilations occur principally by intra-coil processes(14-15). 

Figure 2.16 Relative quantum yields of donor (D) and acceptor (A) fluorescence in rigid solutions. The vertical bar indicates the concentration ca = cA...

Thus, in rigid solutions, the critical transfer distance R0 can be determined experimentally either from the observed relative quantum yields of D or A at various concentrations of A (Equation 2.44) or from time-resolved measurement of the fluorescence decay of D (Equation 2.42). The results are in good agreement with those calculated from the Forster Equation 2.37. 

ESR parameters for triplet carbenes15 and nitrenes16 have been summarized, and it has been shown that phenylnitrene is produced predominantly (87-88%) in the singlet state by direct photolysis of phenyl azide in low-temperature matrices.17 The first spectroscopic observation of a singlet nitrene has been reported nanosecond-laser photolysis of 1-azidopyrene gives the S0 nitrene (Amax 450 nm) which has a lifetime of 22 nsec at room temperature (in benzene) and 34 nsec at 77 K in rigid solution. At room temperature it decays to the triplet ground state (Tj, Amax 415 nm) with a rate constant of about 4.4 x 107 sec. Tt is formed directly by biacetyl sensitized photolysis of the azide. The lifetime of the excited triplet (T2) was about 7 nsec. T dimerizes to azopyrene.18... 

The large differences between the S0-Sn absorption spectrum, fluorescence spectrum, and fluorescence decay time of 1,1 -binaphthyl in fluid solutions as compared with those in rigid solutions can be related to a change in the dihedral angle, 6, between the two rings on electronic excitation. The red shift of the fluorescence spectrum demonstrates clearly that after excitation to the Si state,... 

S. Konishi, M. Hoshino, and M. Imamura, Constrained Complexes of Manganese(II) Tetraphenylporphyrin in Rigid Solution, J. Phys. Chem., 86 (1982) 1412. 

This process requires a somewhat unusual experimental condition with the acceptor already present in its triplet state. Nevertheless, with perylene as the donor and phenanthrene-dio the acceptor in rigid solution, this process has been established to occur as long-range transfer . ... 

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