Yield triplet pyrene

With the aid of ns-laser flash photolysis techniques, they found that the yield of triplet pyrene ( A ) had been decreased by magnetic flelds less than 50 mT and that the yield of the pyrene anion (A ) had been increaded by them as shown in Fig. 6-8. 

Previous studies (27) showed that the quenching of pyrene fluorescence by DMA is via an exciplex process. The transient absorption spectra of pyrene and DMA have been measured (12) by laser flash photolysis. The relative ratio of the quantum yield of excited triplet pyrene, < ) j to the quantum yield of anion pyrene < ) is estimated as 3.3 in PA-I8K2, and 1.3 in SDS micellar system. Mataga and coworkers (28, 29) indicated that the yields of pyrene anion are related to the dielectric constants of the solvent. In polar solvents the ionic photodissociation process, is dominant among the decay process... 

In contrast to 6-azidobenzo[/)]thiophene, which yields only benzo[i]thiophen-6-amine (9 %) and JVh,Ar(1-diethylbenzo[/)]thiopheiie-6,7-diamine (25 % bp 175-177 C/0.7 Torr), 6-azido-2,3-dibromobcnzojhjthiophene (1 a, R = R2 = Br) on irradiation in diethylamine in the presence of pyrene, a triplet nitrene quenching agent, yields a mixture of 2,3-dibromo-./V6,./V6-diethyl-benzo[5]thiophene-6,7-diamine (2a, R1 = R2 = Br 13%) and the 8W-thieno[2,3-r]azepine 3a.14<1 Likewise, methyl 6-azidobenzo[6]thiophene-2-carboxylate (lb, R1 = C02Me R2 = H) yields the thienoazepine ester 3b.147... 

The photochemical reaction can also proceed via the triplet state and in this case no cyclization is observed. Especially when acetophenone is added as a triplet sensitizer, 41 is not formed. Remarkable is the observation that in the presence of anthracene or pyrene as triplet quencher, the yield of the cyclization product 41 was not enhanced and that nitrene insertion into CH bonds of anthracene or pyrene was observed. When the photochemical cyclization reaction was performed with the tosyl azide derivative 42a or the azido nitrile derivative 42b (Scheme 6), only low yields of the tricyclic amide 41 (32% from 42a, 9% from 42b, respectively) were obtained <2001JCS(PI)2476>. 

Quantum yields for isomerization of t-1 to c-1 via intersystem crossing are similar to those obtained in triplet sensitization, indicating that t is formed with near unit efficiency. Similar results were reported for the interaction of t with inorganic anion electron donors (127b). Rate constants for intersystem crossing of the t-1-ethyldiisopropylamine and triethylamine exciplexes obtained from the measured exciplex lifetimes are 5 x 107 s"1 (116), similar to the values for the exciplexes of pyrene (128) and naphthalene (121) with triethylamine. Larger intersystem crossing rate constants are observed for the exciplexes of t-1 with the bicyclic amines l-azabicyclo[2.2.2]-... 

Electron transfer can be accomplished by quenching of a micelle trapped chromophore by ions capable of ion pairing with the micelle surface. For example, excited N-methylphenothiazine in sodium dodecylsulfate (SDS) micelles can exchange electrons with Cu(II). The photogenerated Cu(I) is rapidly displaced by Cu(II) from the aqueous phase so that intramicellar recombination is averted, Fig. 5 (266). Similarly, the quantum yield for formation of the pyrene radical cation via electron transfer to Cu(II) increases with micellar complexation from 0.25 at 0.05 M SDS to 0.60 at 0.8 M SDS (267). The electron transfer quenching of triplet thionine by aniline is also accelerated in reverse micelles by this mechanism (268). 

The formation of triplets via excimers is still an area which is little explored. Medinger and Wilkinson (1966) have shown that pyrene excimer formation leads to a decrease in the quantum yield of triplet formation. A similar result has been obtained for benz(a)anthracene (Heinzelmann and Labhart, 1969). On the other hand, benzene excimer formation leads to an increased triplet yield (Cundall et al., 1972 Hentz and Thibault, 1973). There has been a recent theoretical study of intersystem crossing in molecular pairs (Bowman and Norris, 1978). 

Figure 2b. Yield of solute triplets in p-xylene. Naphthalene 30 C ( ) napthalene 100 C ( o ) pyrene ( ) calculated for naphthalene ( ). Concentration units are M/liter. In these calculations (J), is taken as 0.56, while is 0.8 for naphthalene, and ) 2 taken as 3 x 10 M sec". ...

Werner et al. measured solvent, isotope, and magnetic field effects in the geminate recombination of radical ion pairs [23. 26]. They found similar MFEs in reaction (6-21) in acetonitrile (ACN), dimethylformamide (DMF), ethanol, and 2-propanol. Their typical results on the magnetic field dependence of the pyrene triplet yield in acetonitrile are shown in Fig. 6-9. This figure shows that the yield of each reaction decreases with increasing B from 0 T, but that the increase is saturated at 65 mT. Such MFEs can be explained by the HFCM, where the S-T conversion rate in (6-21c) is reduced by magnetic fields. The Bm value of the HFCM can be expressed by Eqs (6-11) and (6-12). 

Fig. 6-9. Magnetic field dependence of the pyrene triplet yield (<1>t(-B)/ Ot (0)) in acetonitrile as derived from delayed fluorescence measurements ...

The mechanism of reversible triplet-triplet energy transfer is used in a lightharvesting array constructed around a the Ru(bpy)3 " core and bearing one or three pyrenyl units in the periphery (27) [192], UV excitation of these systems yields a strong visible MLCT-based emission, with a lifetime of 2.9 and 9.0 ps respectively for the mono- and the tris-pyrene system. The equilibrium constant between the MLCT and the pyrene triplet, and therefore the triplet lifetimes, are modulated by the fractional contribution of each individual triplet molecule involved in the equilibrium. This approach allows for a synthetic control of the luminescence lifetime by simply changing the number of pyrenyl units. 

Furthermore, like styrylpyrene (5b), pyrene-substituted imine (12) undergoes mutual isomerization between Z and E isomers via the triplet excited state, and the quantum yield for the Z -> E isomerization increases with Z-isomer concentration by way of the dual mechanism. 

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