Amine quenching

Irradiation in the presence of MDEA completely inhibits the formation of products. The amine quenches the fluorescence of Eosin with a rate constant of 8 x 108 M-1s-1 and quenches the Eosin triplet with a rate two orders of magnitude lower. A summary of rate constants for the decay of the triplet is presented in Table 8. In addition to the reactions shown in Scheme 3, with Am = (V-methyl diethanolamine, the rate constants for reaction of PDO with Eosin triplet and semioxidized Eosin radical in aqueous solution (Eqs. 19 and 20) are included in the table. 

The fluorescence of 3-t (113-117) and 3-7 (118) is quenched by secondary and tertiary amines. Rate constants for quenching of It by tertiary amines increase with decreasing amine ionization or oxidation potential (Table 11), as expected for the formation of a charge-transfer stabilized exciplex in which the amine serves as the electron donor. Electron transfer quenching in nonpolar solvent is calculated to be exothermic for amines with E 2 < 1 34 V. Thus, it is not surprising that secondary and tertiary amines quench 3-t with rate constants which approach or even exceed the rate of diffusion. The inefficient quenching of It and 3-7 by primary amines is consistent with their higher oxidation potentials. 

Quenching of excited Cr(bpy)j+, Ru(bpy)j+ and Ir(Me2phen)2ClJ by aromatic amines and methoxybenzenes Quenching of excited Cr(bpy)2+ and Ir(Me2phen)2CI by aliphatic amines Quenching of excited Ru(bpy),+ by aromatic amines ... 

Photoreduction by amines differs from photoreduction by alcohols in two respects quantum yields are always lower than maximal and rate constants for amine quenching of triplet ketones are very large. These two facts led Cohen 153> and Davidson 154> to suggest that amines react with excited carbonyl compounds by electron transfer followed by proton transfer from the amine radical cation. 

Although all amines quench triplet ketones rapidly, the efficiency with which the interaction produces radicals varies considerably. 

The dependence of kp on the type of a-hydrogen is not known, nor can variations in ke be predicted easily. Mataga has studied amine quenching of triplet benzophenone by flash spectroscopy. N,N-dialkylanilines are the only amines which actually yield radical ions, and then only in polar solvents 157). He suggests two competing decay modes of the exciplex. Monoalkylanilines and tertiary aliphatic amines in any solvent and dialkylanilines in nonpolar solvents yield only radicals, presumably from the exciplex. Even though the oxidation potentials of tertiary aliphatic amines are so low that they quench triplet ketones at rates... 

Second, it is conceivable that amines quench triplet ketones before spin-lattice relaxation takes place within the three triplet sublevels 159>, which are populated unevenly 160>. In that event, radicals can be produced with their electron spins polarized. The CIDEP phenomenon 161>, whereby EPR emission is observed upon irradiation of ketones and very reactive substrates, may involve this mechanism. In fact, certain CIDNP observations may depend on rapid quenching of spin-polarized triplets 162>. 

There is really no solid evidence regarding the relative reactivities of n,n and n,7t triplets in CT quenching. The rate constants for tertiary amine quenching of triplet naphthyl ketones are in the 10s—106 M 1 sec-1 range, as compared to values greater than 10 M-1 sec-1 for quenching of triplet benzophenone, acetophenone, etc. A good deal of this difference must reflect the low triplet excitation... 

With ketones in the long-lived singlet state (e.g. TX, Ac, MAc), the singlet charge-transfer complex generated by amine quenching is expressed mainly as a back electron transfer and deactivation rather than proton transfer and initiation. Therefore, such a reaction course is a loss in efficiency of the photopolymerization. 

By adding triethylamine to this reaction mixture, no photo-oxidation took place, in accordance with the evidence31 that tertiary amines quench singlet oxygen. On the other hand, the yield improved to about 70% for 24 on adding pyridine. However, the production of 19c and 22 was not... 

Benzene is photoreduced by primary, secondary and tertiary amines (Bellas, et al., 1977) and the various products have been identified. The reaction with tertiary amines is accelerated by the addition of small amounts of protic solvents and use of CH3OD leads to deuterium incorporation. These findings are strongly indicative of the participation of radical ions. Primary and secondary amines quench the fluorescence of benzene but whether or not this leads to radical ions is not known. Product studies have been made of the... 

The photoamination of aromatic hydrocarbons and arylalkenes by ammonia and primary aliphatic amines occurs via photoinduced electron transfer in the presence of m-dicyanobenzene (Scheme 15) [56-58]. In this photoreaction, secondary amines are less reactive than ammonia and primary amines, and the high concentration of the aminated products retards the photoreaction. This is due to the fact that the oxidation potentials of the aminated products produced by the photoreaction are usually much lower than those of the starting ammonia and primary amines. The secondary amines quench the reactive species much faster than the primary amines. Yasuda extended this photoreaction to intramolecular cyclization reactions [59],... 

The conclusion that most amine quenching occurs at the pre-equilibrium limit and therefore rate constants will depend, in part, on the equilibrium constant for exciplex formation, which in turn depends on the amine electron-donating ability, raises the possibility of an alternative mechanism. Here formation of the exciplex would simply facilitate the electronic to vibrational energy transfer discussed for solvent quenching where kA (cf. Eq. (21)) replaces fcisc in Eq. (34). This would be much more effective than solvent quenching where interaction simply involves encounter complexation. That such a mechanism does not operate is demonstrated in Figure 6 which shows a plot [82] of the first-order constant for decay of 02( Ag) luminescence in benzene as a function of DABCO and DABCO-2HI2 [83], There is clearly no isotope effect and the mechanism of Eq. (31) appears very firmly established. A similar conclusion has been drawn from recent work [84] which shows that, as expected, hydrazines also quench 02(1Ag) via the same mechanism. The hydrazine 4 is a particularly efficient quencher with kq values in benzene and acetonitrile about twice those of strychnine. 

Fetterolf and Offen [26] studied the effect of pressure on the reductive quenching of Ru(bpy) by several aromatic amines. Quenching by N,N-dimethylaniline (DMA) in CH3CN resulted in AV values between +1.3 and +2.9 cm mol while values of 9 to 13 cm mol were reported for the quenching by the free base benzidine and N,N,N, N -tetra-methylbenzidine (TMB) in CH3CN and n-BuOH as solvents. These pressure effects can be interpreted in terms of the following mechanism... 

It is well known that amines quench the triplet excited state of ketones, with or without resulting photoreduction of the ketone. Quenching may occur by com-plexation, electron transfer, or hydrogen transfer. For example, Scheme 5... 

EnoUzation. Phenylmanganese chloride deprotonates ketones in the presence of a catalytic amount of an amine. Quenching with an anhydride gives enol esters. 

An observed fluorescence dependence on pH is in keeping with an intramolecular amine quenching mechanism. Protonation of an amine group in... 

We attribute the effect of the amine at C-6 as deriving from the extent of dissociation of the ammonium salt at C-6, and it is an equilibrium effect which depends on the relative concentrations of the protonated phenolate group and the dissociated salt form. Strongly basic amines remove the proton completely and form the ammonium salt. Weakly basic amines do not cause complete ionization of the C-6 phenol. This same effect has been observed for monomeric models in solution. (We can ignore free amine quenching because the concentration of the amine is very low.)... 

Stoddart and co-workers synthesized a monopyrene-functionalized pil-lar[5]arene (H5.19) to detect the complexation of alkyldiamines at low concentrations. When all ldiamines were added to the solution of H5.19, fluorescence quenching of the pyrene moiety in H5.19 took place because primary amines quench the pyrene fluorescence through photoinduced... 

Scheme 25 Amines quench the fluorescence of most unsubstituted aromatic systems. Blue and red are used to depict the relative Stake s shift in wavelength rather than the observed colour.

Due to a fast and efficient intersystem crossing process (cf. Table 3.2) most ketones perform mainly triplet photochemistry [25,73], In Table 3.6, the photoreactivity of triplet-excited acetone, biacetyl, and benzophenone is compared with the triplet-excited azoaUcane DBH-T. The data for both chromophores follow similar trends. Namely, dienes and amines quench with quite high rate constants, while ethers and aromatic compounds react rather inefficiently. 

Interestingly, a change of the reactivity of singlet-excited DBO with sulfides upon going to the gas phase has been documented. In solution, amines quench DBO fluorescence faster than sulfides as expected, based on electron donor strength. This order of reactivity is inverted in the gas phase, where sulfides quench faster than amines [55,65]. 

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