Electronic excitations singlet states

The electronically excited singlet states of uracil, and how they can lead to efficient radiationless decay to the ground state, were initially investigated using MRCI methods by Matsika [147] and later with other methods that agree with the MRCI results in the more general features [92, 126, 128, 148-150], The discussion in this section describes MRCI results for free uracil in detail, along with the studies that have been... 

Figure 1 Relative positions of the potential energy (E) surfaces of the electronic states involved in a hypothetical chemiluminescent reaction as a function of intemuclear separation (r). P and P represent the ground and lowest electronically excited singlet states of the product of the reaction, respectively. R represents the ground electronic state of the reactant. AH is the enthalpy of the dark reaction while AHa is its enthalpy of activation. AH is the enthalpy of activation of the photoreaction, hv denotes the emission of chemiluminescence.

Figure 4.17 shows the energetics of these pathways, including the possible formation of electronically excited singlet states of Oz. The threshold for (18a) is 585.5 nm (Johnston et al., 1996). While reaction (18b) is close to thermoneutral overall, there is a substantial energy barrier to the dissociation, 47.3 + 0.8 kcal mol-1 the threshold observed for this reaction is 594.5 nm (Johnston et al., 1996). 

In 1963, E. J. Bowen published his classic review The Photochemistry of Aromatic Hydrocarbon Solutions, in which he described two major reaction pathways for PAHs irradiated in organic solvents photodimerization and photooxidation mediated by the addition of singlet molecular oxygen, 02 ) (or simply 102), to a PAH (e.g., anthracene). For details on the spectroscopy and photochemistry of this lowest electronically excited singlet state of molecular oxygen, see Chapter 4.A, the monograph by Wayne (1988), and his review article (1994). For compilations of quantum yields of formation and of rate constants for the decay and reactions of 02( A), see Wilkinson et al., 1993 and 1995, respectively. 

Wilkinson, F., W. P. Helman, and A. B. Ross, Quantum Yields for the Photosensitized Formation of the Lowest Electronically Excited Singlet State of Molecular Oxygen in Solution, J. Phys. Chem. Ref. Data, 22, 113-262 (1993). 

Calculations of PN are more challenging than that of PN because it is, of course, an excited state of phenylnitrene. The first two electronically excited singlet states of PN are both of Ai symmetry and are calculated to be at 1610 and 765 nm. Neither of these transitions have been detected, since both of these states have zero oscillator strength due to symmetry considerations, and they lie outside the wavelength range accessible to our spectrometer. ... 

The fluorescence and phosphorescence spectra of a complex molecule are generally discussed by reference to an energy level diagram such as that shown in Figure 1. Absorption of light raises the molecule from the ground state to one of the upper electronically excited singlet states. At... 

A. Reaction with Electronically Excited Singlet States. 264... 

The earliest indication that NO could quench electronically excited singlet states was given by Kondratiev and Siskin,265 who observed the fluorescence quenching by NO of rhodamine and benzoflavin adsorbed on silica gel. 

Wilkinson F, Helman WP, Ross AB (1995) Rate constants for the decay and reactions of the lowest electronically excited singlet state of molecular oxygen in solution. An expanded and revised compilation. J Phys Chem Ref Data 24 663-1021... 

In a pair of complementary studies, Archer et al. (9,10) have made observations that agree with the previous studies but differ somewhat in interpretation. Their work interprets the decomposition mechanism in terms of several species of electronically excited singlet states. These studies agree with the previously discussed studies, if the molecular products formed via process 47 are assumed to be due to "singlet" decomposition only and if 4>isc = 1 - I CO The yield of intersystem... 

Pressure Dependence. That the precursor for reaction (10) is identical whether the reactant is 1,3-cyclohexadiene or 1,3,5-hexatriene is best illustrated by the Stern-Volmer plot for hydrogen formation (Fig. 3). The data for both molecules fit the same straight line. It is unlikely that (rans-1,3,5-hexatriene would give the same electronically excited singlet state as m-hexatriene or cyclohexadiene since the short life of this state would be unfavorable for rotational processes. But it is... 

The hydrogen migration reaction to give 1,2,4-hexatriene is observed in the gas phase as well as in solution. It is not quenched by an increase in pressure, nor is it observed during pyrolysis of 1,3,5-hexatriene. It has hence been taken to be a reaction which proceeds from the electronically excited singlet state. Based on exactly the same reasoning, the process... 

The sequence of steps involved in fluorescence is described in Fig. 2. The initial absorption takes the molecule or ion pair from the ground electronic singlet state (So) to the electronically excited singlet state (5i), and the resulting absorption (excitation) spectrum should resemble that showm in Fig. 3a. 

Fluorescence and phosphorescence are emission processes which originate directly or indirectly (see 5 section ll.B) from the electronically excited singlet state and triplet state, respectively, produced by charge-transfer processes (Eqs. 1 and 2). Many publications deal with such charge-transfer transitions by diffuse reflectance spectroscopy (DRS) (2-6) showing the link between the latter technique and photoluminescence. It is worthwhile to recall that the emergence of the coordination chemistry of solid-state anions, namely, of surface lattice oxide ions, has almost entirely been based on the results of both photoluminescence and DRS analyses (7, 66). For some catalytic systems, vibrational structures can be detected (see Section IV.B) with an associated vibrational constant, which may be determined directly and independently by IR or Raman spectroscopy, evidencing the relation between these spectroscopies and photoluminescence (33, 34). 

---