Pyrenyl radicals

The absorption band (Amax 488 nm) in the transient spectrum corresponds to the pyrene radical ion (Py ) [155, 156], while the band at Amax 400 nm is assigned to the absorption of the 1-hydro-1-pyrenyl radical (Py ) [157, 158]. Steady-state photolysis of pyrene in the presence of TEA leads to its disappearance, and addition of vinyl monomers decreases the rate of pyrene photoreduction. The photobleaching process follows first-order kinetics. Encinas et al. [154] suggest that the photoinitiation of polymerization by pyrene-TEA is catalyzed by the pyrene radical ion. 

Figure 9. ENDOR spectrum of 2H-benzo[cd]pyrenyl radical measured at 400 K. (Reproduced with permission from reference 25. Copyright 1985 Wiley.)...

A mechanism to describe the high-energy, radiolysis-initiated attachment of pyrene molecules to PE chains has been proposed by Biscogho and Thomas. Its principal claim, that preformed carbon-centered PE radicals or cations add to ground-state pyrene molecules, is supported by transient absorption spectra. Signals ascribed to electronically excited singlet and triplet states ( Py , Py ), and radical ions (Py, Py ) of pyrene were detected initially after initiation of reactions by a pulse of 0.4 MeV electrons, but decayed to undetectable levels within 4 msec. However, polymer-centered radicals remained 4 msec after pulsed excitation and are presumed to decay according to step 19 of Scheme 3. Another transient absorption, consistent with a PE-bound 1-pyrenyl radical (RPyH ), persisted up to 3 sec after the excitation pulse. 

The DNA bases most easily reduced are T and C, the reduction potentials of which are very similar [26]. It is therefore expected that excess electron migration through DNA occurs via a hopping mechanism involving all base pairs (C-G and T-A) and the radical anions C and T as stepping stones. We focused our work on 5-pyrenyl-2 -deoxyuridine (Py-dU) and 5-pyrenyl-2 -deoxycytidine (Py-dC) as nucleoside models for ET in DNA. Photoexcitation of the pyrenyl group results in... 

The positive charge of solvent radical-cations transfers to solute molecules in halogenated hydrocarbons such as chloroform and dichloroethane. However, only few studies have been made on the radical cations of polymers in solution. Tanaka et al. observed the dimer cation of the biphenyl group or the pyrenyl group of the polymers in the pulse radiolysis of PVB and PVP in 1,2-dichloroethane [49]. The absence of the monomeric cation is due to the rapid intramolecular dimerization of the radical cations of the side groups of the polymers. Irie et al. observed two kinds of intramolecular dimer cations in the... 

Other examples of generating free radicals utilized in our laboratory are the consecutive two-photon ionization of the aromatic pyrene residue in the benzo[a] pyrene derivative 7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene (B[a]PT) or the covalent adducts derived from the reactions of racemic anti-r7,t8-dihydroxy-t9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (B[a]PDE) with the N2-exocyclic amino groups of guanine or adenine bases in DNA [43-47]. The selective excitation of the pyrenyl residue of B[a]PT by intense 355-nm nanosecond laser pulses yields free radical products by a two-photon mechanism ... 

In a follow-up paper from Kuwana s group [167], an o-quinone derivative attached to a larger aromatic derivative (4-(2-(l-pyrenyl)vinyl)catechol) that strongly adsorbed on graphite was studied. When the mediator was kept in its reduced state (catechol), it was virtually stable for a long term. When kept in its oxidized state (o-quinone), some minor deactivation occurred with time, whereas when kept at a potential close to the E°, a much more rapid loss of electroactivity was noticed, explained by abortive side reactions caused by an initial reaction between catechol and o-quinone. In the presence of NADH, an even much faster deactivation process occurs, presumably caused by an intermediate semiquinone assumed to react with a radical intermediate of the coenzyme, NADH +, to give an inactive compound, which poisons and blocks off the surface... 

Poly(l-vinylpyrene) (46a) has been synthesized using a Ziegler-Natta catalyst system [311-313], and it photoconductive properties have been studied in detail. The hole transport characteristics of (46a) are similar to those of PVK [11]. The polymer exhibits a carrier mobility of 7.5 x 10 crn /Y s at a field of 2 X 10 V/cm. Poly(A(-(l-pyrenyl)acetyl ethyleni-mine) (46b) has been obtained by cationic isomerization polymerization of 2-(l-pyrenyl)methyl-2-oxazoline, and poly(methyl-2-(l-pyrenyl)acetamido-propenoate) (46c) has been synthesized by free-radical polymerization of the corresponding monomer [314]. In contrast to the corresponding carbazole-containing polymers (23a,b), excimer formation is the major... 

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