Porphyrin triplet

A rather important aspeet that should be eonsidered is that interfaeial quenching of dyes does not neeessarily imply an eleetron-transfer step. Indeed, many photoehemieal reactions involving anthracene oeeur via energy transfer rather than ET [128]. A way to discern between both kinds of meehanisms is via monitoring the accumulation of photoproducts at the interfaee. Eor instance, heterogeneous quenehing of water-soluble porphyrins by TCNQ at the water-toluene interfaee showed a elear accumulation of the radical TCNQ under illumination [129]. This system was also analyzed within the framework of the exeited-state diffusion model where time-resolved absorption of the porphyrin triplet state provided a quenehing rate eonstant of the order of 92M ems. ... 

An interesting variation on this theme has been reported by Lehn and coworkers. In 1986, they reported the synthesis of macrocycle 33, which consists of a zinc porphyrin bearing two linked cyclic binding subunits [87]. It was later found that addition of silver triflate to a solution of 33 in methanol resulted in the incorporation of a silver ion in each of the binding subunits [88], Thus, the complex may be represented as Ag+-P-Ag+. The porphyrin fluorescence of the silver complex was quenched, and transient absorption studies demonstrated that the porphyrin singlet state was quenched with a rate constant of 5.0 x 109 s 1 to yield a charge separated state Ag°-P+-Ag+. Some quenching of the porphyrin triplet... 

Table 9.4 Solvent dependent driving forces for charge separation (CS) out of the porphyrin singlet excited state and charge recombination (CR) to the ground state/porphyrin triplet excited state calculated after the dielectric continuum model (dielectric constant e toluene 2.4 THF 7.6 oDCB 9.8, benzonitrile 24.9). The case, where charge recombination to the porphyrin triplet state is prohibited, is assigned as n.p. ...

The first experiments in this direction were carried out on the triads 244+ and 254+ (Fig. 8), by exciting preferentially the metal centre around 340-355 nm. Excitation at this wavelength region produces to a predominant extent the excited state localized on the iridium complex unit, the ligand centered triplets PH2 - 3Ir - PAu or PZn - 3Ir - PAu [48]. Energy transfer to the porphyrin triplets dominates the deactivation of PH2 - 3Ir - PAu in 244+, with rate constants of 2.9 x 1010 s 1 for the transfer to the gold porphyrin localized excited state and ca. 1011 s 1 to the free base porphyrin localized excited state, respectively (Scheme 9). 

It decays with a lifetime of 570 ps due to reverse electron transfer (Figure 24d). The longer-lived product (trace 1.5 ns) is the triplet excited state of the Zn porphyrin subunit, obtained in ca. 15 % yield by energy transfer from the Au porphyrin triplet. It decays with a Ufetime of 1.3 ns, possibly by intramolecular electron transfer to the appended Au porphyrin. 

Excitation of Cu.20 with a 30-ps laser pulse at 532 nm, where the Au porphyrin subunit absorbs about 80 % of the total incident photons, resulted in formation of the Au porphyrin triplet excited state. This species decayed within the laser pulse to... 

Cu.20, which has the lowest B. On the other hand, the Au porphyrin triplet is more likely to react via hole transfer through the HOMO of the spacer. However, there is a small decrease of the energy gap (B+) upon coordination of a copper(I) cation to the bridging dpp moiety, which is nevertheless accompanied by an increase in the rate of electron transfer. This rate acceleration is modest by compari-... 

There was a marked difference in the rate of triplet energy transfer for 24 and 26. In a benzene solution of 24, the carotenoid triplet species had a rise time of ca. 2/is and decayed in ca. lO s. Concomitant with the rise of the carotenoid triplet absorbance at 550 nm, the porphyrin triplet absorbance at 440 nm decayed with a time constant of 2/js. There was no appreciable change in these parameters when 24 was dissolved in a rigid plastic matrix [73]. For 26 the triplet energy transfer was much faster. In 1981, we reported it as faster than 30 ns, which was the limit of our instrumentation [91]. Measurements with greater time resolution were made in 1983, but it remained difficult to separate the carotenoid triplet rise time from the instrument response time [73]. In any case, under conditions ranging from solution to solid plastic to a glass at 10 K, the rise time of triplet carotene in 26 was < 5 ns. 

Ag) to a nearby porphyrin triplet, forming the excited singlet state of the porphyrin. No such reaction has been seen in solution phase. 

A normal emission/absorption/emission/absorption pattern that did not vary in the time interval 0.7-5 ps was observed for the radical pairs from 35 and 36, while the spectrum from 34 was initially totally absorptive (0.4-0.6ps), then rapidly changed to emissive/absorptive (1.9-2.1 ps), and eventually became totally emissive (4.6-5 ps).37 It is suggested that for ZP4V, due to the smaller spacer chain, the time elapsed between laser excitation and radical-pair formation was shorter than for ZP6V and ZP8V and the spin polarization in the porphyrin triplet was retained to a larger extent before electron transfer took place.37... 

Andres, G. O., Cabrerizo, F. M., Martinez Junza, V., Braslavsky, S. E., A Large Entropic Term Due to Water Rearrangement is Concomitant with the Photoproduction of Anionic Free base Porphyrin Triplet States in Aqueous Solutions, Photochem. Photobiol. 2007, 83, 503 510. 

In the study of protein electron transfer (ET), radiolytic and photochemical techniques have indeed proven highly complementary. Between them, these techniques provide a range of reaction types and reaction free energies [cf. Zn porphyrin triplets (F° - 0.8 V) versus Fe porphyrins ( ° - 0 V)], Of particular interest in the current study is the different dynamic range(s) of the techniques. Photochemistry is subject to a natural time window set by the excited state lifetime only reactions faster than the excited state decay can be observed. Conversely, the bimolecular nature of radiolysis sets an upper hmit on the observed rates that is often determined by the rate of electron capture. 

One of the most prominent energy transfer processes involving porphyrin triplets involves the conversion of triplet oxygen to singlet oxygen ... 

Pulse radiolysis of porphyrins in benzene permitted the observation of porphyrin triplet states and provided a more accurate method than laser photolysis to determine their molar absorption coefficients. ... 

H. Levanon and P. Neta, Electron Transfer Reactions of Chlorophyll a and Porphyrin Triplets with Radicals in Aqueous Micellar Solutions, J. Phys. Chem., 86 (1982) 4532. 

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