Triplet state in aromatic

From the materials just mentioned earlier, one can conclude that mutual meta orientation (meta through a benzene) of the spin-bearing moieties is an indispensable condition for the existence of triplet states in aromatic di- or tri-(cation-radical)s. However, in fact, these systems have both singlet and triplet forms, and the questions are about what is the difference in the corresponding energy and which state is more stable. Stability of the polyion-polyradicals is also a very important factor, especially in the sense of practical application. Let us consider several relevant examples. 

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]. 

In a series of experiments [229, 230], a dramatic increase in the lifetime of the triplet state in aromatic hydrocarbons was measured when they were completely deuterated. At 77 K, naphthalene in a durene solid solution has a 2.5 s lifetime (inverse of the... 

In contrast to aromatic hydrocarbons, heavy-atom substitution onto carbonyl and heterocyclic molecules appears to have little effect on radiative and nonradiative intercombinational transitions. Wagner(138) has shown that as determined by the type II photoelimination, aliphatic ketones (n -> it excited states) are not sensitive to external heavy-atom perturbation. As seen previously in our discussion of type II photoelimination, aliphatic ketones undergo this cleavage from both the excited singlet and triplet states (in... 

Kearns, D. R. Determination of the assignment of triplet states in cata-condensed aromatic hydrocarbons. J. Chem. Phys. 36, 1608 (1962). 

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-... 

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

The mechanism of this reaction shows that excitation of the substrate gave an n,n triplet state, but this excited state was unable to dissociate the carbon-iodine bond. This was demonstrated by showing that the n,n triplet state, when sensitized by chrysene, did not produce coupling products. Probably, the reaction occurred in an excited a,a triplet state mainly localized on the carbon-iodine bond, and the interaction between this triplet state and aromatic compounds led to homolytic cleavage of the C-I bond with the formation of both a 5-thienyl radical and a complex between the aromatic compound and the halogen atom. The formation of this complex was demonstrated by the presence of a short-lived transient with Amax = 510 nm, showing a second-order decay kinetics and a half-life of ca. 0.4 (is in laser flash photolysis. The thienyl radical thus formed... 

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. 

For most carbonyl compounds, we expect to have two near-lying excited states in the triplet manifold, which are either n-n or tc-tc in character. The n-7T states frequently show radical-like behavior. Benzophenone is an example of such a molecule which has an n-7t triplet state in which we see occurrences of hydrogen abstraction and very efficient intersystem crossing. When the lowest state is the 71-71 state, largely centered on the aromatic part of the molecule, as in the case of p-methoxyacetophenone, the reactivity decreases significantly (8,9). With the nature of lignin and the nature of the model we have chosen, we are mostly interested in molecules which have this type of behavior. 

The involvement of an olefin triplet state in an electron transfer induced olefin isomerization was first recognized during the (electron transfer) quenching of aromatic hydrocarbon excited states by Z- or E-l,2-dicyanoethylene. Taylor... 

Phthalimides. Like their aromatic ketone counterparts, TV-alkyl phthalimides participate in hydrogen atom abstraction reactions to form a large variety of heterocyclic compounds. However, despite many similarities between phthalimides and aromatic ketones, there are some important differences. In contrast to aromatic ketones, electronically excited phthalimides are not transformed quantitatively into the triplet state, and thus they may react from both the singlet and the triplet state. In addition, phtalimides are much more inclined to photoinduced single-electron-transfer (PET) reactions [24,25]. (For details see Sec. 3.3.2.1.)... 

McGlynn Mid Boggus describe the phenomenon thus absorption in the charge transfer bMid is followed either by the converse emission or by intersystem crossing (according to Kasha [124]) to a dissociative level of the complex which yields the aromatic in its first excited triplet state. The aromatic hydrocarbon then phosphoresces. 

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 question of whether it, it triplets of aromatic ketones have some intrinsic chemical property has been a controversial subject in organic photochemistry (130). Recently, the correlation of structure and reactivity of some excited triplet biphenyl ketones was approached by a combined esr, CIDEP, and CIDNP study (129). The low-temperature esr confirmed the assignment of the ir, ir triplet as the lowest triplet state in these biphenyl ketones. 

There can be a substantial difference in the reactivity of n,n and n,n excited triplet states in a-cleavage. These differences are experimentally characterized by Tj (triplet lifetime) or (the dissociation rate constant) for a-cleavage. In general, aromatic ketones with n,n lowest triplet states undergo much faster a-cleavage and have shorter triplet lifetimes than ketones with n,n states. The latter may be dubbed inefficient PIs. ... 

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... 

The role of the triplet state in biological and biochemical systems continues to receive wide attention. Photoexcited triplet states of prophins and derivatives (389), prophyrins (390), aromatic amino acids (390), aromatic amines (391), and monoanionic thymine (392) have been observed at 77°K. Shiga and Piette (393) confirmed the interpretation of ESR data on triplet-state excitation in proteins by a simultaneous phosphorescence study. 

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. 

In order to demonstrate this effect to best advantage it was necessary to choose a PVCA sample having a relatively low molecular weight. In this way Interference of the phosphorescence emission by delayed fluorescence is minimized. These are provacative results because they indicate that there may be no well defined lowest triplet state in vinyl aromatic polymers unless special steric or electronic effects are present which nullify inter-chromophore interactions. On the other hand, they may provide an additional tool with which to investigate rates of energy migration in polymers and in some polymer/dopant systems as well. 

At the same time we must have in mind that for the majority of aromatic molecular crystals such as, for example, naphthalene, the lowest molecular electron excited state is a triplet state. In these circumstances triplet excitons exhibit a number of specific properties, which has been used for their study. 

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. 

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