Aromatic solvents, excited states

Scavenging Studies. The solvent excited states produced by radiolysis of aromatic liquids could be produced directly, or formed, via charge neutralisation of solvent ions. The low oscillator strengths of the first and second excited states of benzene, toluene and p-xylene preclude direct excitation into these states. However, the third excited state could be excited with a yield as high as unity. 

In aromatics, both singlet and triplet excited states of the solvent are produced in comparable yields, with some contribution from direct excitation. 

In alicyclic hydrocarbon solvents with aromatic solutes, energy transfer (vide infra) is unimportant and probably all excited solute states are formed on neutralization of solute cations with solute anions, which are formed in the first place by charge migration and scavenging in competition with electron solvent-cation recombination. The yields of naphthalene singlet and triplet excited states at 10 mM concentration solution are comparable and increase in the order cyclopentane, cyclohexane, cyclooctane, and decalin as solvents. Further, the yields of these... 

With many aromatic hydrocarbons as solutes, excited state yields in alkane solutions are nearly equally divided between singlets and triplets, and these yields increase with solute concentration until -0.1 M (Salmon, 1976 Thomas et al., 1968). In these systems, both the solute anion and the solute excited state yields increase similarly with solute concentration. With anthracene as a solute, the rate of growth of anthracene triplet matches that of the decay of the anthracene anion. With aromatic solvents, on the other hand, solute ions play... 

Alkenes, alkynes, and arenes become stronger bases in the singlet excited state. As a result, photoprotonation can occur under much more weakly acidic conditions than required in the ground state. Excited styrenes and phenylacetylenes, for example, are protonated by the solvent in 2,2,2-trifluoroethanol (TEE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), giving rise to phenethyl and a-arylvinylcations that can be observed by LFP (see Eq. 15). In a similar manner, benzenium ions can be observed by photoprotonation of electron-rich aromatics in FIFIP. " Equation 16 provides an example where the orientation of the protonation is different in the excited state from that of the ground state. 

In aromatic hydrocarbons, short-lived ](it, it ) is the lowest excited state and energy gap between (n, it ) and 8(it, it ) states is large. Both these factors are conducive to fluorescence emission and in general aromatic hydrocarbons are good fluorescer. Sometimes, the prediction may not come true if a higher triplet state T2 is available near the St state such as in anthracene. In such cases, fluorescence and phosphorescence both are observed at low temperatures in suitable solvent medium specially when S, and T, are states of different symmetry type. Some data correlating AEst and 4[Pg.148]

Contact between the energetic /3 particles and S in the ground state results in transfer of energy and conversion of an S molecule into an excited state, S. Aromatic solvents are most often used because their electrons are easily promoted to an excited state orbital (see discussion of fluorescence, Chapter 5). The / particle after one collision still has sufficient energy to excite several more solvent molecules. The excited solvent molecules normally return to the ground state by emission of a photon, S ---> S + hv. Photons... 

In polar solvents, the quantum yields for the emission from the locally excited state of anthronyl-anthracenes 98 and 99 decrease drastically (see Tables 20 and 21), and a structureless, red-shifted exciplex emission is observed (see Figure 23). For the parent compound 98a in dichloromethane, for example, the quantum yield of emission from the exciplex state is 0.012, but that of emission from the locally excited state has decreased to 0.00058 (cf. Tables 20 and 22). Thus, intramolecular exciplex formation between the photoexcited anthracene moiety and the aromatic ketone in its electronic ground state represents the major mode of deactivation in polar solvents. 

The cation radical can undergo deprotonation to yield an allyl radical or nucleophilic attack by the solvent to produce a methoxyalkyl radical. Coupling of these radicals with the aromatic radical anion produces acyclic adducts. As an alternative, the anion radical can be protonated, ultimately giving reduction product. Thus, the degree of charge separation within the excited state complex dramatically influences the observable chemistry. 

In charge transfer to solvent transition the bound electron in the spectroscopically observed first excited state interacts strongly with the solvent oriented in the field of the ion. Will the formation of e aq occur also when excitation results in an internal molecular transition For instance, on illumination will aqueous solutions of aromatic molecules evolve H2 through preceding e aq formation ... 

CTTS transitions, in which the oriented solvent participates are absent in the spectrum of the aromatic solutes but the lifetime of the excited state, as shown by the existence of fluorescence is longer, probably 10 9 sec. During this extended lifetime, sufficient reorganization of the solvent may occur to enable the excited electron to be trapped, and to allow for the first relative diffusive displacement of the gemini, which is necessary for the observed kinetics to develop. The temperature ef-... 

---