Singlet transition

As in atoms, the selection rule breaks down as the nuclear charge increases. For example, triplet-singlet transitions are strictly forbidden in FI2 but in CO the a U — transition is observed weakly. 

Absorptions in the UV spectra of thiiranes are observed around 260 nm ( - other transitions are reported in the vacuum UV spectrum, and the calculated lowest singlet transition energies correspond to n - oxirane groups behave as electron withdrawing substituents when attached to aromatic rings as indicated by the UV spectra of 2-arylthiiranes. 

The excitation process may generate an excited molecule in any allowed vibrational state, but tbe excess vibrational energy is rapidly lost, and the excited state species may then emit a photon of frequency Vem, this singlet-singlet transition from the excited to ground state being fluorescence. 

In order to obtain the absolute rate constant Atq, tq must be known. This is a much more difficult measurement, requiring specialized techniques. For singlet-singlet transitions tq is often of the order 10 s. 

The CNDO method has been modified by substitution of semiempirical Coulomb integrals similar to those used in the Pariser-Parr-Pople method, and by the introduction of a new empirical parameter to differentiate resonance integrals between a orbitals and tt orbitals. The CNDO method with this change in parameterization is extended to the calculation of electronic spectra and applied to the isoelectronic compounds benzene, pyridine, pyri-dazine, pyrimidine and pyrazine. The results obtained were refined by a limited Cl calculation, and compared with the best available experimental data. It was found that the agreement was quite satisfactory for both the n TT and n tt singlet transitions. The relative energies of the tt and the lone pair orbitals in pyridine and the diazines are compared and an explanation proposed for the observed orders. Also, the nature of the lone pairs in these compounds is discussed. 

Category 1, Simple Cleavage into Radicals.Aldehydes and ketones absorb in the 230-330-nm region. This is assumed to result from an n—>tt singlet-singlet transition. The excited aldehyde or ketone can then cleave. ... 

Here ho is the kinetic energy and nuclear attraction operator while and 1C are the coulomb and exchange operators, respectively. The coefficients X and Y are solutions of the RPA equations, which for the / singlet transition with excitation energy can be written as... 

That is, in the singlet-singlet transition both Coulomb interaction and exchange interaction are involved. However, when the distance between D and A is large, the exchange term can be ignored, and we can use the multipole expansion for e2/rij, that is,... 

This is the famous Forster relation that expresses the singlet-singlet transition rate constant in terms of the spectral overlap of the emission spectra of D and absorption spectra of A. 

The common element of Schemes 1-3 is that they each postulate direct interaction between the metal center and dioxygen. Although it is not stated explicitly, Eqs. (3) and (11) most likely proceed via an inner-sphere mechanism. Thus, the metal-dioxygen interaction implies spin pairing between the reactants when the metal ion is paramagnetic. As a consequence, the formation of the M-O2 type intermediates circumvents the restriction posed by the triplet to singlet transition which seems to be the major kinetic barrier of autoxidation reactions (5). 

The PL spectra of the PFs show well-resolved structural features with maxima at 420,445, and 475 nm assigned to the 0-0, 0-1, and 0-2 intrachain singlet transition, respectively (the 0-0 transition, the most intense) [247]. Due to the tail emission spectrum of PFs, the thin films emit bright sky-blue light. The QE of the PFs is very high, typically in the range of 40 to 80% and, as shown for PFO 196, it depends substantially on the morphology of the polymer [248]. 

The effects of concentration, rate of freezing, and solvent purity on the luminescence spectrum of dibenzothiophene have been studied.During slow freezing the molecules of the dissolved compound migrate toward the surface of the solvent crystals and thereby decrease the effective concentration. The effect on the triplet-singlet transition of introducing heavy atoms into the system has also been investigated by absorption measurements in benzene and ethyl iodide. ... 

The electronic absorption spectra [25] of Cjq and C q are characterized by several stronger absorptions between 190 and 410 nm as well as by some forbidden transitions in the visible part of the spectrum (Figures 1.22 and 1.23). For the assignment of the transitions has been carried out using the results of theoretical calculations [290, 291]. The absorptions between 190 and 410 nm are due to allowed Tjy- Ag-transitions, whereas those between 410 and 620 nm are due to orbital forbidden singlet-singlet transitions. These latter absorptions in the visible are responsible for the purple color of Cgg and the red color of C g. 

The ultraviolet absorption spectrum of isoxsuprine HCl dissolved in 0.1 N HCl is shown in Figure 3, and was recorded using a Shimadzu UV-VIS model 1601 PC spectrophotometer. The transition characteristic of a hydroxy-substituted phenyl group was found to split into two maxima, with the first being noted at 274.7 nm (Al% 1cm = 66, and molar absorptivity 2230 L/mole cm), and the other at 268.9 nm (Al% 1 cm = 67, and molar absorptivity 2265 L/mole cm). The far more intense singlet->singlet transitions of the phenyl systems are evident in the rapidly rising absorption noted below 240 nm. 

Therefore, singlet-triplet and triplet-singlet transitions are forbidden, whereas singlet-singlet and triplet-triplet transitions are allowed. 

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