Triplet yields

Under optimum conditions electron transfer can produce excited states efficiently. Triplet fluoranthrene was reported to be formed in nearly quantitative yield from reaction of fluoranthrene radical anion with the 10-phenylphenothia2ine radical cation (171), and an 80% triplet yield was indicated for electrochemiluminescence of fluoranthrene by measuring triplet sensiti2ed isomeri2ation of trans- to i j -stilbene (172). 

The apparatus used for picosecond flash spectroscopy on these systems has been described before(8 10). Figure 3a and b show typical transient absorption data obtained on 2-hydroxybenzophenone and the copolymer. Summary of these spectral data are given in Table 3. The transient observed at the shortest delay time (7ps) is the first excited singlet in all systems. The spectral data (at delay times > 50ps) permit placement of upper limits on triplet yields in CH2CI2 for both 2-hydroxy benzophenone itself and the copolymerized chromophore. 

Nanosecond flash kinetic spectroscopy was also carried out on 2-hydroxy benzophenone and the copolymer (11). No transients could be detected in the nanosecond time scale, suggesting that the ground state enol [S (lb) in scheme 1] has a lifetime less than 1 x 10 9 sec. These results strongly imply that processes (3) and (4) are responsible for the deactivation of singlet energy in these systems. A small, non zero triplet yield is postulated in the copolymer both to account for the photodegradation data and the transient spectral data. Triplet... 

Webster S, Padilha L, Przhonska O, PeceliD, HuH, Slominsky Y, Kachkovski A, Tolmachov A, Kurdyukov V, Hagan D, Van Stryland E (2009) Enhancement of triplet yields in cyanine-like molecules. Laser Science XXV, OSA Technical Digest (CD) paper LSTuG2... 

Details of nitrobenzene photochemistry reported by Testa are consistent with the proposal that the lowest triplet excited state is the reactive species. Photoreduction, as measured by disappearance quantum yields of nitrobenzene in 2-propanol is not very efficient = (1.14 0.08) 10 2 iD. On the other hand, the triplet yield of nitro benzene in benzene, as determined by the triplet-counting method of Lamola and Hammond 28) is 0.67 0.10 2). This raises the question of the cause of inefficiency in photoreduction. Whereas Lewis and Kasha 29) report the observation of nitrobenzene phosphorescence, no long-lived emission from carefully purified nitrobenzene could be detected by other authors i4,3o). Unfortunately, the hterature value of Et for nitrobenzene (60 kcal mole i) is thus based on an impurity emission and at best a value between 60 and 66 kcal mole can be envisaged from energy-transfer experiments... 

Quenching studies on the photolysis of 14 (R = C(CH3)3) using 1,3-pentadiene reveal that even at 2.5 mole pentadiene not all excited states are quenched >, thus at present a triplet state as reacting excited state cannot be regarded as proven for the direct irradiations although it seems likely in view of the high triplet yield reported for nitrobenzene. 

Upon irradiation (366 nm) in 2-propanol, 1-nitronaphthalene is inefficiently photoreduced (Ah<10 1 moles i s i) despite its high triplet yield 3). With tributylstannane, a marked increase of the hydrogen abstraction rate (<3 x 10 ... 

Recently, it was shown that in deuterated tetraphenylporphin the fluorescence quantum yield and lifetime, and the triplet yield increase in a similar manner relative to the undeuterated one (24). Such a deuterium isotope effects on the state suggested the participation of Sq internal conversion in the energy dissipating... 

Calibration methods I modified luminol standard II absolute calibration HI Hastings-Weber light standard IV luminol standard V mean value obtained by different determination methods, including chemical titrations VI triplet yield of 0.3 E mol for TMD, obtained with DBA. 

The uncertainty about the formation mechanism is eliminated when such method is used for the determination of the G(5 i) value, which is invariant to the formation route. Such method is the comparison of the yields of those products, which are formed only in one way in the reaction of the Si molecules. Such products come from the singlet decay channel fluorescence photons, products of H2 or alkane elimination. The triplet channel due to the large triplet yield originating from other processes than the Si Tn ISC (e.g., direct triplet excitation, or charge recombination with triplet product) cannot be used for the calculation of the Si yield. The calculations are based on the equation ... 

Norbornene is a good olefin to use as a probe for the triplet-triplet transfer process since the norbornene triplet yields dimers. When carbonyl compounds are irradiated in the presence of norbornene, the ratio of photocycloaddition product to norbornene dimers is dependent upon the n,ir triplet energy of the carbonyl compound. A series of experiments which illustrate this effect is summarized in Table VII. 

Ultraviolet (UV) spectroscopy does not tend to be the method of choice for structure determination, but a list of UV absorptions was given in the review by Knowles <1996CHEC-II(7)489>. Fluorescence properties and triplet yields of [l,2,3]triazolo[4,5-r/ pyridazines in various solvents have been reported <2002JPH83>. These heterocyclic systems were found to be photochemically very stable. In a recent paper, Wierzchowski et al. studied the fluorescence emission properties of 8-azaxanthine ([l,2,3]triazolo[4,5-r/ pyrimidine-5,7-dione) and its A -alkyl derivatives at various pH s <2006JPH276>. For the 8-azaxanthines, an important characteristic of emission spectra in aqueous solutions was the unusually large Stokes shift. Since 8-azaxanthine is a substrate for purine nucleoside phosphorylase II from Escherichia coli, the reaction is now monitored fluorimetrically. The fluorescence properties of [l,2,3]triazolo[4,5-r/ -pyrimidine ribonucleosides were earlier described by Seela et al. <2005HCA751>. 

The effect of singlet quenchers on the sensitizer triplet yield (Ot) is derived similarly. 

Many examples of collisions with changes in local permutational symmetry are known. The Cundall technique31,32,190 used to measure triplet yields to photoexcited benzene involves the transfer of triplet-state excitation energy... 

The photoreduction quantum yields and the triplet yields of 4-aminobenzophenone in several hydrogen donor solvents, ZH, are listed in Table 4.1, with comparison to a benzophenone which has an n-7T lowest triplet state in all these cases. Although dimethylformamide (DMF) is a far better... 

THF tetrahydrofuran 4AB 4-aminobenzophenone DMBP 4,4 -dimethoxybenzophenone R is photoreduction yield and Ot is triplet yield... 

Coulometric generation of the reactants in bulk provides a known number of redox events plus a quantity of ds-stilbene that can be determined chromato-graphically. Since the cis-trans isomerization yields are known from photochemical studies, one can calculate the efficiency with which 3F1 is formed. Triplet yields ranging from 2% to 80% have been recorded for various systems in this way [86,87]. 

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