Triplet states of aromatic

Until recently the lifetimes of the triplet states of aromatic hydrocarbons in fluid solution at room temperature had been investigated exclusively by the technique of flash absorption spectroscopy. The lifetimes reported for many hydrocarbons, e.g., anthracene or phenan-threne, had been below 1 msec, and it had been assumed that radiationless conversion processes were so rapid under these conditions that the competing radiative triplet-singlet transition would be too slow to per-... 

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.

We have provided experimental evidence demonstrating the very different behaviour of triplet states of aromatic carbonyls in lignin and in dilute solution. These differences result, we believe, from the very restricted motion within the lignin structure relative to fluid solution. By using oxygen saturated samples rather than air saturated samples, we have been able to.establish for the first time that dynamic quenching by oxygen occurs within TMP. 

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... 

Quenching of triplet states of aromatic hydrocarbons by quinones Benzene ( 22 0)... 

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. 

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... 

Wilkinson and Schroeder (1979) have shown that the triplet states of aromatic hydrocarbons are quenched by quinones, the efficiency of quenching being related to the electron affinity of the quinone and the ionisation potential of the triplet hydrocarbon (Schroeder and Wilkinson, 1979). It was concluded that the quenching did not involve full electron transfer in nonpolar solvents. Photolysis experiments have shown that in propionitrile tetrachloro-benzo-l,4-quinone reacts with naphthalene to give radical ions (Gschwind and Haselbach, 1979). The naphthalene radical cation reacts with naphthalene to give a detectable intermediate. 

Twisted non-charge transfer excited triplet states of aromatic aldehydes and ketones are revealed by resonance Raman scattering and transient dielectric loss interconversion between two populated triplet states in the case of 2-naphthaldehyde and 2-acetophenone in cyclohexane. Solvent polarity effects on the nlT and nn states of 1,2-naphthoquinone and 9,10-phenanthrenequinone examined... 

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

Effects of stacking interactions on the photoexcited triplet state of aromatic residues such as Trp are found over and above the external HAE described above. These effects are observed in protein/peptide-nucleic acid interactions in the absence of heavy atom derivatization of the nucleobases. Binding of Eco SSB to poly(dT) produces profound effects on the... 

It is well established that o grgen quenches both excited singlet and triplet states of aromatic compounds. Thus, indole derivatives in solution at room temperature display a ratio for the rate constant Icnk/I qp of approximately two (48, 49). However, the quenching of alkaline phosphatase phosphorescence by oxygen proceeds at a rate kqp = 1.2 x lO s (47). The magnitude of this rate... 

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]. 

A laser study of the prototropic equilibrium of triplet benzophenone (BP) has been reported [111]. Acid-base properties in the triplet state of aromatic ketones in HjO-CHjCN (4 1) mixtures have been studied by means of nanosecond laser flash photolysis. The acidity constants p /T) in the triplet state are determined by means of the T T, absorbance titration curve, the Ware plot, and the Rayner-Wyatt plot, whose values agree well among them, showing that the acid-base equilibrium in the... 

Tj state of aromatic ketones is established during the triplet lifetime [112], The acid-base reaction in the triplet state of aromatic ketones CA ) can be accounted for by Scheme 2.5, where and denote the Ufetimes of A (in the absence of protons)... 

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. 

Metz, F., Friedrich, S., and Hohlneicher, G. (1972) What is the leading mechanism for the nonradiative decay of the lowest triplet state of aromatic hydrocarbons Chem. Phys. Lett, 16, 353. 

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