Triplet states of aromatic molecules

Several cases have been reported of sensitized chemical reaction from the triplet states of aromatic molecules. Anthracene triplets formed by triplet energy transfer from coronene react with carbon tetrachloride to yield, among other products, 9-chloroanthracene and hydrogen chloride,214 the same reaction which occurs upon direct excitation.215... 

Fig. 13. Absorption spectra of triplet states of aromatic molecules in acetone solution (from ref.

The important energy transfer process from the triplet state of aromatic molecules to molecular oxygen giving singlet molecular oxygen (" Ag) has been elegantly demonstrated in the gas-phase ESR studied by Kearns and... 

E. C. Lim. Photoassociation in the lowest triplet-state of aromatic molecules - triplet excimers and exdplexes. Pure Appl. Chem., 65(8/1659-1664, 1993. 

The esr-spectroscopy is one of the most important techniques to detect and study photochemically excited (paramagnetic) triplet states of aromatic molecules or of molecules of biological interest. By his technique, very small changes in the electron distribution caused by inter- and intramolecular interactions (e.g. substitution effects, solvent effects, complex formation) may be detected [69]. 

Quenching of triplet states of aromatic hydrocarbons and carbonyl compounds by inorganic anions (I-, Br , NOj, Cl-) Quenching of excited aromatic molecules by aromatic hydrocarbons, nitriles, methoxy- and amino-aromatics Quenching of excited aromatic molecules by methoxy and amino-aromatics Quenching of excited cyanoanthracenes, by aromatic hydrocarbons, methoxy-aromatics and sulfides... 

Response theory describes the S-T transition probabilities in unsaturated hydrocarbons quite well more than 99 % of the So - Xi transition intensity is out-of-plane polarized in agreement with experiment for aromatics in ethylene, butadiene and naphthalene the y spin-sublevel of the T state is the most active one, where y is the long in-plane axis of the molecules [134,132]. The main difference between the triplet states of aromatic and aliphatic compounds is the lack of phosphorescence for the latter. We have related this to the fact that polyenes also lack fluorescence (or have very weak fluorescence). This have been explained from the effective quenching of singlet excited (tr r ) states, which is an inherent property for the short polyenes. Our results suggest that this situation also prevails for the lowest triplet states. 

Experimentally, there is little to suggest that large anharmonicities may be expected for the low-lying electronic states of the aromatic and aza-aromatic molecules, at least on the evidence of the extensive sequence structure and the well-developed progressions observed for the lowest excited singlet and triplet states of many molecules. 

The study of ionic states of aromatic molecules has dealt largely with the radical anions of these molecules in polar liquids, and more recently to a lesser extent with aromatic cations. The study of electronically excited states has been concerned principally with the triplet state in both non-polar and polar liquids, and to a lesser extent with the singlet state. The direct observation of these reactive species has provided some understanding of fundamental phenomena in radiation chemistry such as the extent of charge separation in polar liquids and the persistence of this charge separation into the chemical stage of events, the mode of formation and yield of both ionic and electronically excited... 

Up to the present, liquid crystalline solvents have been used (a) to determine the anisotropies of both the g-factor and the hyperfine interaction in ground state doublet (free) radicals [65] and (b) to study the dipole—dipole- and the electron exchange-interaction in species containing two unpaired electrons (biradicals and ground state triplets) [66, 67, 68]. More recently liquid crystals have been used extensively for studies of the anisotropic properties of photochemically excited triplet states in aromatic molecules and molecular complexes [60, 69]. 

As has been explained earlier, the mechanism of lype II photoinitiators is based on the phenomenon that triplet states of aromatic ketones readily abstract hydrogen atoms from hydrogen donors, such as tertiary amines. Regarding macrophotoinitiators, there are examples with the ketone and the amine bound to polymer molecules. 

When Hammond and co-workers(59) found that the intersystem crossing quantum yield for aromatic ketones was unity (see Chapter 3) it was a short but very important step to realize that these compounds should be ideal triplet sensitizers. Thus one can excite the triplet state of molecules that otherwise would be formed inefficiently, if at all, by intersystem crossing. This idea resulted in a number of papers in the early 1960 s from the Hammond group on this topic. It is not possible in this short section to survey this area, but a few of the early studies are indicated by the following reactions ... 

As shown, for example, by calculations of the structural index A, the aromaticity of alumobenzene (112) is still lower than that in borabenzene. Calculations (MNDO) have shown the ground state of this molecule to be, in contrast to benzene, triplet (3B,). Even so, the 67r-electron structure of (112) is more stable than the 47r-electronic one. However, the difference between their energies (36.6 kcal/mol) is nearly twice as small as that for the corresponding structures of borabenzene. 

The most widely used nomenclature in the field of luminescence of aromatic molecules is that proposed by Birks. 3) The photophysical species and processes in this scheme which are encountered in rigid systems of aryl vinyl polymers are described in Tables 1-4. Triplet excimers have been omitted since it will be shown later that all triplet states play minor roles in the room-temperature, air saturated P2VN blends studied recently 2). 

Consider, now, radiationless transitions in pure molecular crystals of aromatic molecules. At the very outset we must realize that crystal field effects may lead to the inversion of the order of the triplet and singlet exciton levels relative to the ordering of the corresponding molecular states.12 The Davydov tight binding formulation of exciton theory leads to the following representation for the manifold of optically accessible (k = 0) energy levels in a pure molecular crystal 138... 

Most of the fairly extensive photochemistry of aromatic compounds has not been studied in sufficient detail to permit disentanglement of singlet and triplet mechanisms. Theoretical calculations indicate that the pattern of substituent effects on side-chain reactions of excited disubstituted benzenes should be quite different from that observed in the ground states of the molecules. One problem associated with these predictions is the question of whether or not they are appropriate for triplets as well as for corresponding singlet excited states. Consider the following system ... 

Because of the relatively slow rates of unimolecular reactions of excited acetone in solution at room temperature, acetone makes a convenient solvent-sensitizer for photosensitizatioh studies, provided that the substrate does not undergo competing chemical reactions with triplet acetone. A recent study of the effects of high-energy radiation on dilute acetone solutions of polynuclear aromatic molecules revealed that the triplet states of these compounds were being formed at close to the diffusion-controlled rate by collision with some pre-... 

Triplet states for naphthalene, anthracene, and other aromatic compounds had been identified by absorption spectroscopy mainly with the aid of flash photolysis by G. Porter and his co-workers.22 Although a triplet state of benzene had been identified in a glassy matrix and had been associated with a long-lived emission of 10 sec or more duration,5 no evidence for the existence of this state by spectroscopic means had been produced until recently.23 Thus it has been known for some time that benzene in a glassy matrix when irradiated at wavelengths around 2500 A produces molecules which cross over to a triplet state with a relatively high probability. 

The lifetimes and kinetic behavior of many triplet states of molecules, particularly of aromatic compounds, have been studied by Porter and his coworkers. The identification of the absorption spectra of methyl radicals, of HCO radicals and of NH2 radicals, mainly in Herzberg s laboratory at Ottawa, has permitted flash photolysis to be used directly to measure the rates of radical reactions. 

If two strongly associated aromatic molecules undergo charge-transfer to the D+A- state, one would expect some coupling between the electrons to give a triplet level, unless, subsequently, the ions separated. Thus if the complex existed as D+A-, a major doublet would be expected, comparable with that found for the Ams = 1 transitions for the triplet state of naphthalene (Hutchison and Mangum, 1958). 

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