Singlet excited state porphyrins

J.R. Bolton In solution most photochemical electron transfer reactions occur from the triplet state because in the collision complex there is a spin inhibition for back electron transfer to the ground state of the dye. Electron transfer from the singlet excited state probably occurs in such systems but the back electron transfer is too effective to allow separation of the electron transfer products from the solvent cage. In our linked compound, the quinone cannot get as close to the porphyrin as in a collision complex, yet it is still close enough for electron transfer to occur from the excited singlet state of the porphyrin Now the back electron transfer is inhibited by the distance and molecular structure between the two ends. Our future work will focus on how to design the linking structure to obtain the most favourable operation as a molecular "photodiode . 

Despite the presence of four bridging groups the quinone is not held rigidly above the plane of the porphyrin. Two channels of electron transfer from the singlet excited state of the porphyrin were found for this compound [145]. They were ascribed to slowly equilibrating introverted and extroverted conformers in which the estimated interplanar porphyrin.quinone separation is respectively, 6.5 and 8.5 A. The faster of the two rate constants is independent of the temperature over the range 80-300 K. 

Titration assays, in which complex formation of 8 7 was probed in the presence of variable concentrations of 7 provided insight into the excited state interactions, namely the deactivation of the singlet excited state of the porphyrin via energy transfer to the fullerene core. In particular, adding increasing amounts of 7 to the SnP solution results in substantial quenching of the porphyrin emission centered at 610 and 665 nm. As illustrated in Fig. 8.18, the quenching is exponential and depends exclusively on the added concentration of 7, which points to the... 

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. ...

A series of chlorophyll-like donor (a chlorin) linked having C60 (chlorin-C60) or porphyrin-C60 dyads with the same short spacer have been synthesized as shown in Schemes 13.1 and 13.2 [39, 40]. The photoinduced electron-transfer dynamics have been reported [39, 40]. A deoxygenated PhCN solution containing ZnCh-C60 gives rise upon a 388-nm laser pulse to a transient absorption maximum at 460 nm due to the singlet excited state of ZnCh [39]. The decay rate constant was determined as 1.0 X 10u s-1, which agrees with the value determined from fluorescence lifetime measurements [39]. This indicates that electron transfer from 1ZnCh to C60 occurs rapidly to form the CS state, ZnCh +-C60 . The CS state has absorption maxima at 790 and 1000 nm due ZnCh+ and C60, ... 

In the natural photosynthetic reaction center, ubiquinones (QA and QB), which are organized in the protein matrix, are used as electron acceptors. Thus, covalently and non-covalently linked porphyrin-quinone dyads constitute one of the most extensively investigated photosynthetic models, in which the fast photoinduced electron transfer from the porphyrin singlet excited state to the quinone occurs to produce the CS state, mimicking well the photo synthetic electron transfer [45-47]. However, the CR rates of the CS state of porphyrin-quinone dyads are also fast and the CS lifetimes are mostly of the order of picoseconds or subnanoseconds in solution [45-47]. A three-dimensional it-compound, C60, is super-... 

Dyes such as erythrosin B [172], eosin [173-177], rose bengal [178,179], rhodamines [180-185], cresyl violet [186-191], thionine [192], chlorophyll a and b [193-198], chlorophyllin [197,199], anthracene-9-carboxylate [200,201], perylene [202,203] 8-hydroxyquinoline [204], porphyrins [205], phthalocyanines [206,207], transition metal cyanides [208,209], Ru(bpy)32+ and its analogs [83,170,210-218], cyanines [169,219-226], squaraines [55,227-230], and phe-nylfluorone [231] which have high extinction coefficients in the visible, are often employed to extend the photoresponse of the semiconductor in photoelectro-chemical systems. Visible light sensitization of platinized Ti02 photocatalyst by surface-coated polymers derivatized with ruthenium tris(bipyridyl) complex has also been attempted [232,233]. Because the singlet excited state of these dyes is short lived it becomes essential to adsorb them on the semiconductor surface with... 

We wish to discuss briefly the photoexcited state of zinc(II) porphyrin. Zinc(II) porphyrin has several singlet excited states that are formed by visible light absorption [62]. Of those singlet excited states, the first and the second, Si and S2, have been investigated well [63]. Energy levels of these excited states are schematically shown in Scheme 27. 

Selective excitation of the Zn porphyrin unit of [2]-rotaxane 111 at 575 nm resulted in photo-induced electron transfer from the singlet excited state ZnP to the ground state AuP+. Two rates of electron transfer could be extracted from the fast kinetic studies, (83 ps)-1 and (770 ps)-1, with 35% and 65% contributions respectively.75 These rates probably correspond to different conformations of the [2]-rotaxane. In any case the electron transfer rates are much slower than those measured for through bond processes in rotaxanes 102 and 107. Related [2]-rotaxanes with Au porphyrin incorporated into, rather than appended to, the macrocyclic component have been synthesized and studied.2411 Similar conclusions could be drawn from the photophysical studies.76... 

In the case of porphyrin systems, there is an agreement that electron injection occurs from the lowest singlet excited state (438, 439). In the tetraruthenated porphyrins, the mechanisms involved in ET are more complex, since both components are responsible for photocurrent generation. As already discussed, from the HOMO and LUMO compositions, the peripheral ruthenium complexes can effectively transfer electronic charge to the porphyrin center via Ru( Jtt) porphyrin MLCT transitions. In addition, the direct interaction between the ZnTPyP core and Ti02 plays an important role in the photoresponse efficiency. 

The photophysical properties of a Ceo-linked phytochlorin (31) [344, 373], a porphyrin analog, are quite different from those of conventional Cgo-linked porphyrins. The phytochlorin-Cfio exciplex is formed in both toluene and benzonitrile via either the singlet excited states of the phytochlorin or the Ceo- The exciplex relaxes directly to the ground state in toluene, whereas it undergoes a conversion to the charge-separated state, followed by the decay to the ground state in benzonitrile. A similar proposal for the exciplex formation has been forwarded by the report on a free-base porphyrin Ceo dyad [374, 375]. 

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