Singlet absorption

NMR Spectra - The proton NMR spectrum of poly(N-pheny1-3,4-dimethy-lenepyrroline) (VII) had three singlet absorptions at 6 2.56, 4.81 and 7.60 respectively (Figure 10). The integration of these peaks showed a ratio of 4 4 5. The presence of exocyclic olefinic protons was not observed, indicating that 1,4- addition was predominant in the polymerization with little or no 1,2 addition taking place. 

The absorption bands measured by the flash spectrographic method are often assigned by (a) comparison with known singlet-singlet absorption spectra, (b) comparison of the lifetime of the species responsible for the absorption with the phosphorescence lifetime, (c) comparison with calculated energies and intensities of the various possible absorptions by semi-empirical molecular orbital methods, and (d) comparison with published triplet absorption spectra and decay kinetics of model compounds. 

These selection rules are affected by molecular vibrations, since vibrations distort the symmetry of a molecule in both electronic states. Therefore, an otherwise forbidden transition may be (weakly) allowed. An example is found in the lowest singlet-singlet absorption in benzene at 260 nm. Finally, the Franck-Condon principle restricts the nature of allowed transitions. A large number of calculated Franck-Condon factors are now available for diatomic molecules. 

In some aromatic molecules that have a high degree of symmetry, i.e. with a minimum D2h symmetry (e.g. benzene, triphenylene, naphthalene, pyrene, coronene), the first singlet absorption (So —> Si) may be symmetry forbidden61 and the corresponding oscillator strength is weak. The intensities of the various forbidden vibronic bands are highly sensitive to solvent polarity (Ham effect). In polar solvents, the intensity of the 0-0 band increases at the expense of the others. 

Fig. 8. The c axis polarized second singlet-singlet absorption of As naphthalene in a durene host crystal. The center of the 0-0 band is near 34,550 cm 1 and that of the a (9) addition is near 35,050 cm 1. The fine structure, as narrow as 1 cm"1 in some crystals, is attributed to interference between the second excited singlet state and vibrational additions to the first which have coupling symmetry. The isotopes 1,4, 5,8 — dt, 2,3, 6,7 — d4, and dg-naphthalene produce entirely different structures. For a given isotope, the major structural features change as the separation between the two excited singlets is varied by host-induced shifts, but remain rather similar for hosts that produce the same separation.

The proton nmr resonances of the N—H protons of amides are different from any we have discussed so far. Generally, these will appear at room temperature as a broad singlet absorption, which may turn into a broad triplet at higher temperatures. A typical example is propanamide (Figure 24-2). 

For example, the singlet-triplet transitions in ethylenic compounds generally have tmax <3C 1. The fact that spin-forbidden transitions can be observed at all shows that the transition moment, f electric dipole operator. This operator also contains small terms such as quadrupole operators and spin-orbit operators. The latter is the part of any dynamical operator which couples orbital and spin angular moments this term is responsible for the appearance of weak triplet — singlet absorption spectra. 

In fact, it was possible to prove the formation of the radical ion pair state by transient absorption spectroscopy. Particularly, at the expense of the vanishing H2P/ZnP singlet absorption new features with maxima in the 600-700 nm range as well as at 480 nm grow in. These maxima correspond to the one-electron oxidized 7t-radical cations of H2P (H2P +) and ZnP (ZnP +). Additionally, in the near-infrared region the spectral signatures of the one-electron reduced anion of Ceo are discernible at 1000 nm (Fig. 9.23). 

In comparison to the reference compounds, the transient absorption features of 21a,b and 22a,b disclose a rather fast deactivation of the ZnP singlet-singlet absorption. However, at the conclusion of the singlet decay the recorded transient spectra lack any triplet excited state signatures. Instead, the singlet-singlet... 

The state of spin alignment in the lowest triplet state can be prepared (47) by a number of other methods in addition to its preparation by singlet-singlet absorption, e.g., by ground-state singlet+excited triplet-state absorption, excitation by electron scattering, and... 

The major product formed by intramolecular aldol cyclization of 2,5-hejitanedione (Problem 23.30) has two singlet absorptions in the... 

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