Benzene-Templated Model Systems for Photosynthetic Antenna−Reaction Center Function

Hexabenzocoronene functionalized with antiaromatic S- and Se-core-modified porphyrins (isophlorins): comparison with the dyad with regular porphyrin

The important and perspective molecular building blocks composed of hexaphenylbenzenes (HPBs) or their oxidized derivatives, hexa-peri-hexabenzocoronenes (HBCs), and metalloporphyrins have recently received significant attention of the researchers. In this study, motivated by recent findings, we have addressed the modifications of structures and properties of HBC-porphyrin compounds by using instead of aromatic porphyrins antiaromatic 20π isophlorin derivatives of thiophene or selenophene. We have reported the first comparative computational investigation of the following systems: (i) HBC with one non-metallated aromatic porphyrin, P(N4H2), unit, HBC-P(N4H2), (ii) HBC with one S-core-modified antiaromatic porphyrin (S-isophlorin), PS4, unit, HBC-PS4, and (iii) HBC with one Se-core-modified antiaromatic porphyrin (Se-isophlorin), PSe4, unit, HBC-PSe4. The study has been done employing the B3LYP/6-31G* approach (in the gas phase and in the implicit solvents, benzene and dichloromethane), and comparison with the B3LYP/6-31G** and B3LYP/6-311G* approaches was performed, where relevant. The effects of the core-modified antiaromatic isophlorins on the structures, electronic, and other properties, potentially including reactivity, of the whole building block HBC-isophlorin have been shown to be quite pronounced and to be noticeably stronger than the effects of the original aromatic non-metallated porphyrin. Thus, we have demonstrated theoretically that the complete porphyrin core-modification with other elements, this time with S and Se leading to the formation of the antiaromatic isophlorins, should be considered as a promising way for modifying and tuning structures, electronic properties and reactivity of the hexabenzocoronene-porphyrin(s) building blocks.

Diels-Alder Cycloaddition with CO, CO2, SO2, or N2 Extrusion: A Powerful Tool for Material Chemistry

Phenyl, naphthyl, polyarylphenyl, coronene, and other aromatic and polyaromatic moieties primarily influence the final materials' properties. One of the synthetic tools used to implement (hetero)aromatic moieties into final structures is Diels-Alder cycloaddition (DAC), typically combined with Scholl dehydrocondensation. Substituted 2-pyranones, 1,1-dioxothiophenes, and, especially, 1,3-cyclopentadienones are valuable substrates for [4 + 2] cycloaddition, leading to multisubstituted derivatives of benzene, naphthalene, and other aromatics. Cycloadditions of dienes can be carried out with extrusion of carbon dioxide, carbon oxide, or sulphur dioxide. When pyranones, dioxothiophenes, or cyclopentadienones and DA cycloaddition are aided with acetylenes including masked ones, conjugated or isolated diynes, or polyynes and arynes, aromatic systems are obtained. This review covers the development and the current state of knowledge regarding thermal DA cycloaddition of dienes mentioned above and dienophiles leading to (hetero)aromatics via CO, CO2, or SO2 extrusion. Particular attention was paid to the role that introduced aromatic moieties play in designing molecular structures with expected properties. Undoubtedly, the DAC variants described in this review, combined with other modern synthetic tools, constitute a convenient and efficient way of obtaining functionalized nanomaterials, continually showing the potential to impact materials sciences and new technologies in the nearest future.

Donor-acceptor conjugates derived from cobalt porphyrin and fullerene via metal-ligand axial coordination: Formation and excited state charge separation

Two types of cobalt porphyrins, viz., meso-tetrakis(tolylporphyrinato)cobalt(II), (TTP)Co (1), and meso-tetrakis(triphenylamino porphyrinato)cobalt(II), [(TPA)4P]Co, (2) were self-assembled via metal-ligand axial coordination of phenyl imidazole functionalized fulleropyrrolidine, ImC[Formula: see text] to form a new series of donor–acceptor constructs. A 1:2 complex formation with ImC[Formula: see text] was established in the case of (TTP)Co while for [(TPA)4P]Co only a 1:1 complex was possible to positively identify. The binding constants [Formula: see text] and [Formula: see text] for step-wise addition of ImC[Formula: see text] to (TTP)Co were found to be 1.07 × 105 and 3.20 × 104 M[Formula: see text], respectively. For [(TPA)4P]Co:ImC[Formula: see text], the measured [Formula: see text] values was found to be 6.48 × 104 M[Formula: see text], slightly smaller than that observed for (TTP)Co. Although both cobalt porphyrins were non-fluorescent, they were able to quench the fluorescence of ImC[Formula: see text] indicating occurrence of excited state events in the supramolecular donor-acceptor complexes. Electrochemistry coupled with spectroelectrochemistry, revealed the formation of cobalt(III) porphyrin cation instead of a cobalt(II) porphyrin radical cation, as the main product, during oxidation of phenyl imidazole coordinated cobalt porphyrin. With the help of computational and electrochemical results, an energy level diagram was constructed to witness excited state photo-events. Competitive energy and electron transfer from excited CoP to coordinated ImC[Formula: see text], and electron transfer from Im1C[Formula: see text]* to cobalt(II) porphyrin resulting into the formation of PCo[Formula: see text]:ImC[Formula: see text] charge separated state was possible to envision from the energy diagram. Finally, using femtosecond transient absorption spectroscopy and data analysis by Glotaran, it was possible to establish sequential occurrence of energy transfer and charge separation processes. The lifetime of the final charge separated state was [Formula: see text] 2 ns. A slightly better charge stabilization was observed in the case of [(TPA)4P]Co:ImC[Formula: see text] due to the presence of electron rich, peripheral triphenylamine substituents on the cobalt porphyrin.

Multiple-Porphyrin Functionalized Hexabenzocoronenes

Porphyrin-hexabenzocoronene architectures serve as good model compounds to study light-harvesting systems. Herein, the synthesis of porphyrin functionalized hexa-peri-hexabenzocoronenes (HBCs), in which one or more porphyrins are covalently linked to a central HBC core, is presented. A series of hexaphenylbenzenes (HPBs) was prepared and reacted under oxidative coupling conditions. The transformation to the respective HBC derivatives worked well with mono- and tri-porphyrin-substituted HPBs. However, if more porphyrins are attached to the HPB core, Scholl oxidations are hampered or completely suppressed. Hence, a change of the synthetic strategy was necessary to first preform the HBC core, followed by the introduction of the porphyrins. All products were fully characterized, including, if possible, single-crystal XRD. UV/Vis absorption spectra of porphyrin-HBCs showed, depending on the number of porphyrins as well as with respect to the substitution pattern, variations in their spectral features with strong distortions of the porphyrins' B-band.

Porphyrin-Hexaphenylbenzene Conjugates via Mixed Cyclotrimerization Reactions

Mixed cyclotrimerization reactions of diarylacetylenes (tolans) were applied to generate a library of multiple porphyrin-hexaphenylbenzene (HPB) architectures. Successful reactions, which could be influenced by the ratio of tolan starting materials, were conducted using dicobaltoctacarbonyl as a catalyst. Separation of the reaction products was performed by chromatographic and crystallization techniques. The physical properties were investigated with respect to the number of porphyrins per HPB and their substitution pattern.

Charge-separation in panchromatic, vertically positioned bis(donor styryl)BODIPY-aluminum(iii) porphyrin-fullerene supramolecular triads

Three, broad band capturing, vertically aligned supramolecular triads, R2-BDP-AlPorF3←Im-C60 [R = H, styryl (C2H2-Ph), C2H2-TPA (TPA = triphenylamine); ← = coordinate bond], have been constructed using BODIPY derivative (BDP, BDP-Ph2 or BDP-TPA2), 5,10,15,20-tetrakis(3,4,5-trifluorophenyl)aluminum(iii) porphyrin (AlPorF3) and fullerene (C60) entities. The C60 and BDP units are bound to the Al center on the opposite faces of the porphyrin: the BDP derivative through a covalent axial bond using a benzoate spacer and the C60 through a coordination bond via an appended imidazole. Owing to the bis-styryl functionality on BDP, the constructed dyads and triads exhibited panchromatic light capture. Due to the diverse absorption and redox properties of the selected entities, it was possible to demonstrate excitation wavelength dependent photochemical events. In the case of the BDP-AlPorF3 dyad, selective excitation of BDP resulted in singlet-singlet energy transfer to AlPorF3 (kEnT = 1.0 × 1010 s-1). On the other hand, excitation of the AlPorF3 entity in the BDP-AlPorF3←Im-C60 triad revealed charge separation leading to the BDP-(AlPorF3)˙+-(C60)˙- charge separated state (kCS = 2.43 × 109 s-1). In the case of the Ph2-BDP-AlPorF3 dyad, energy transfer from 1AlPorF3* to 1(Ph2-BDP)* was witnessed (kEnT = 1.0 × 1010 s-1); however, upon assembling the supramolecular triad, (Ph2-BDP)-AlPorF3←Im-C60, electron transfer from 1AlPorF3* to C60 (kCS = 3.35 × 109 s-1), followed by hole shift (kHS = 1.00 × 109 s-1) to Ph2-BDP, was witnessed. Finally, in the case of the TPA2-BDP-AlPorF3←Im-C60 triad, only electron transfer leading to the (TPA2-BDP)˙+-AlPorF3←Im-(C60)˙- charge separated state, and no energy transfer, was observed. The facile oxidation of Ph2-BDP and TPA2-BDP compared to AlPorF3 in the latter two triads facilitated charge separation through either an electron migration or hole transfer mechanism depending on the initial excitation. The charge-separated states in these triads persisted for about 20 ns.

Energy redistribution dynamics in triarylamine–triarylborane containing hexaarylbenzenes

Hexaarylbenzenes with triarylamine donors and triarylborane acceptors show rapid energy transfer (∼3 ps) as proved by pump–probe spectroscopy with fs-time resolution.

Axially assembled photosynthetic reaction center mimics composed of tetrathiafulvalene, aluminum(III) porphyrin and fullerene entities

The distance dependence of sequential electron transfer has been studied in six, vertical, linear supramolecular triads, (TTF-Ph(n)-py → AlPor-Ph(m)-C60, n = 0, 1 and m = 1, 2, 3), constructed using tetrathiafulvalene (TTF), aluminum(III) porphyrin (AlPor) and fullerene (C60) entities. The C60 and TTF units are bound to the Al center on opposite faces of the porphyrin; the C60 through a covalent axial bond using a benzoate spacer, and the TTF through a coordination bond via an appended pyridine. Time-resolved optical and EPR spectroscopic methods and computational studies are used to demonstrate that excitation of the porphyrin leads to step-wise, sequential electron transfer (ET) between TTF and C60, and to study the electron transfer rates and exchange coupling between the components of the triads as a function of the bridge lengths. Femtosecond transient absorption studies show that the rates of charge separation, k(CS) are in the range of 10(9)-10(11) s(-1), depending on the length of the bridges. The lifetimes of the charge-separated state TTF˙(+)-C₆₀˙⁻ obtained from transient absorbance experiments and the singlet lifetimes of the radical pairs obtained by time-resolved EPR are in good agreement with each other and range from 60-130 ns in the triads. The time-resolved EPR data also show that population of the triplet sublevels of the charge-separated state in the presence of a magnetic field leads to much longer lifetimes of >1 μs. The data show that a modest stabilization of the charge separation lifetime occurs in the triads. The attenuation factor β = 0.36 Å(-1) obtained from the exchange coupling values between TTF˙(+) and C₆₀˙⁻ is consistent with values reported in the literature for oligophenylene bridged TTF-C60 conjugates. The singlet charge recombination lifetime shows a much weaker dependence on the distance between the donor and acceptor, suggesting that a simple superexchange model is not sufficient to describe the back reaction.

Multiporphyrin arrays with π-π interchromophore interactions

A recently reported synthetic method has been employed to prepare several arrays of free base and zinc porphyrins. In the arrays, the porphyrins are arranged around a central benzene ring. The lack of aryl rings in the linkages to the central benzene ring, coupled with the presence of only one meso-aryl substituent on each porphyrin, allows strong electronic interactions between the porphyrin macrocycles. In arrays containing two or six porphyrins, a variety of evidence indicates that the porphyrins exist as twist-stacked dimers reminiscent of the special pairs of bacteriochlorophylls found in some photosynthetic bacteria. These dimers feature van der Waals contact between the macrocycles, and demonstrate excitonic splitting due to π-π interactions. The excitonic effects split and blue-shift the Soret absorptions, and slightly broaden the Q-band absorptions and shift them to longer wavelengths. The interactions also lower the first oxidation potentials by ca. 100 mV, and the arrays show evidence for delocalization of the radical cation over both porphyrins in the dimer. The arrays demonstrate singlet-singlet energy transfer among the chromophores. Arrays of this type will be good models for some aspects of the interactions of photosynthetic pigments, including those of reaction center special pairs and possibly quantum coherence effects. They can also be useful in artificial photosynthetic constructs.

On the relation of energy and electron transfer in multidimensional chromophores based on polychlorinated triphenylmethyl radicals and triarylamines

Chromophores with many donors and acceptors show electron transfer which is identical to energy transfer.

Artificial photosynthetic antenna-reaction center complexes based on a hexaphenylbenzene core

A hexaphenylbenzene scaffold has been used to organize the components of artificial photosynthetic antennas and antenna-reaction center mimics that feature bis(phenylethynyl)anthracene antenna moieties and porphyrin-fullerene charge-separation units. The five bis(phenylethynyl)anthracene chromophores absorb in the spectral region around 430-480 nm, where porphyrins have low extinction coefficients but solar irradiance is maximal. The hexaphenylbenzene core was built up by the well-known Diels-Alder reaction of diarylacetylenes with substituted tetraphenylcyclopentadienones. The latter were in turn prepared by condensation of substituted benzils and dibenzyl ketones, allowing flexibility in the design of the substitution pattern on the core. The spacing between the various chromophores is suitable for rapid singlet-singlet energy transfer among antenna moieties and the porphyrin, and the relatively rigid structure of the hexaphenylbenzene limits conformational heterogeneity that could reduce the efficiency of energy and electron transfer. NMR studies reveal a high barrier to rotation of the porphyirn plane relative to the hexaphenylbenzene.

Supramolecular tetrad of subphthalocyanine-triphenylamine-zinc porphyrin coordinated to fullerene as an "antenna-reaction-center" mimic: formation of a long-lived charge-separated state in nonpolar solvent

We report here the formation of a long-lived charge-separated state of a self-assembled donor-acceptor tetrad, formed by axial coordination of a fulleropyrrolidine appended with an imidazole coordinating ligand (C(60)Im) to the zinc center of a subphthalocyanine-triphenylamine-zinc porphyrin (SubPc-TPA-ZnP), as a charge-stabilizing antenna reaction center mimic in toluene. The subphthalocyanine and triphenylamine entities, with their high-energy singlet states, act as an energy-transferring antenna unit to produce a singlet zinc porphyrin. The formation constant for the self-assembled tetrad was determined to be 1.0 x 10(4) M(-1), suggesting a moderately stable complex formation. The geometric and electronic structures of the covalently linked SubPc-TPA-ZnP triad and self-assembled SubPc-TPA-ZnP:C(60)Im tetrad were examined by using an ab initio B3LYP/6-31G method. The majority of the highest occupied frontier molecular orbital was found over the ZnP and TPA entities, whereas the lowest unoccupied molecular orbital was located over the fullerene entity, suggesting the formation of the radical-ion pair (SubPc-TPA-ZnP(*+):C(60)Im(*-)). The redox measurements revealed that the energy level of the radical-ion pair in toluene is located lower than that of the singlet and triplet states of the zinc porphyrin and fullerene entities. The femtosecond transient absorption measurements revealed fast charge separation from the singlet porphyrin to the coordinated C(60)Im with a lifetime of 1.1 ns. Interestingly, slow charge recombination (1.6 x 10(5) s(-1)) and the long lifetime of the charge-separated state (6.6 micros) were obtained in toluene by utilizing the nanosecond transient measurements.

Multiantenna artificial photosynthetic reaction center complex

In order to ensure efficient utilization of the solar spectrum, photosynthetic organisms use a variety of antenna chromophores to absorb light and transfer excitation to a reaction center, where photoinduced charge separation occurs. Reported here is a synthetic molecular heptad that features two bis(phenylethynyl)anthracene and two borondipyrromethene antennas linked to a hexaphenylbenzene core that also bears two zinc porphyrins. A fullerene electron acceptor self-assembles to both porhyrins via dative bonds. Excitation energy is transferred very efficiently from all four antennas to the porphyrins. Singlet-singlet energy transfer occurs both directly and by a stepwise funnel-like pathway wherein excitation moves down a thermodynamic gradient. The porphyrin excited states donate an electron to the fullerene with a time constant of 3 ps to generate a charge-separated state with a lifetime of 230 ps. The overall quantum yield is close to unity. In the absence of the fullerene, the porphyrin excited singlet state donates an electron to a borondipyrromethene on a slower time scale. This molecule demonstrates that by incorporating antennas, it is possible for a molecular system to harvest efficiently light throughout the visible from ultraviolet wavelengths out to approximately 650 nm.

Self-regulation of photoinduced electron transfer by a molecular nonlinear transducer

Organisms must adapt to survive, necessitating regulation of molecular and subcellular processes. Green plant photosynthesis responds to potentially damaging light levels by downregulating the fraction of excitation energy that drives electron transfer. Achieving adaptive, self-regulating behaviour in synthetic molecules is a critical challenge that must be met if the promises of nanotechnology are to be realized. Here we report a molecular pentad consisting of two light-gathering antennas, a porphyrin electron donor, a fullerene electron acceptor and a photochromic control moiety. At low white-light levels, the molecule undergoes photoinduced electron transfer with a quantum yield of 82%. As the light intensity increases, photoisomerization of the photochrome leads to quenching of the porphyrin excited state, reducing the quantum yield to as low as 27%. This self-regulating molecule modifies its function according to the level of environmental light, mimicking the non-photochemical quenching mechanism for photoprotection found in plants.

Multi-Triphenylamine-Substituted Porphyrin-Fullerene Conjugates as Charge Stabilizing “Antenna−Reaction Center” Mimics

A new concept of charge stabilization via delocalization of the pi-cation radical species over the donor macrocycle substituents in a relatively simple donor-acceptor bearing multimodular conjugates is reported. The newly synthesized multimodular systems were composed of three covalently linked triphenylamine entities at the meso position of the porphyrin ring and one fulleropyrrolidine at the fourth meso position. The triphenylamine entities were expected to act as energy transferring antenna units and to enhance the electron donating ability of both free-base and zinc(II) porphyrin derivatives of these pentads. Appreciable electronic interactions between the meso-substituted triphenylamine entities and the porphyrin pi-system were observed, and as a consequence, these moieties acted together as an electron-donor while the fullerene moiety acted as an electron-acceptor in the multimodular conjugates. In agreement with the spectral and electrochemical results, the computational studies performed by the DFT B3LYP/3-21G(*) method revealed delocalization of the frontier highest occupied molecular orbital (HOMO) over the triphenylamine entities in addition to the porphyrin macrocycle. Free-energy calculations suggested that the light-induced processes from the singlet excited state of porphyrins are exothermic in the investigated multimodular conjugates. The occurrence of photoinduced charge-separation and charge-recombination processes was confirmed by the combination of time-resolved fluorescence and nanosecond transient absorption spectral measurements. Charge-separated states, on the order of a few microseconds, were observed as a result of the delocalization of the pi-cation radical species over the porphyrin macrocycle and the meso-substituted triphenylamine entities. The present study successfully demonstrates a novel approach of charge-stabilization in donor-acceptor multimodular conjugates.

Photoinduced Electron Transfer in a Hexaphenylbenzene-based Self-assembled Porphyrin-fullerene Triad

A hexaphenylbenzene-based zinc porphyrin dyad forms a 1:1 complex with a fullerene bearing two pyridyl groups via coordination of the pyridyl nitrogens with the zinc atoms. The fullerene is symmetrically located between the two zinc porphyrins. The binding constant for the complex is 7.3 x 10(4) M(-1) in 1,2-difluorobenzene. Photoinduced electron transfer from a porphyrin first excited singlet state to the fullerene occurs with a time constant of 3 ps, and the resulting charge-separated state has a lifetime of 230 ps. This self-assembled construct should form a basis for the construction of more elaborate model photosynthetic antenna-reaction center systems.

Energy and Photoinduced Electron Transfer in a Wheel-Shaped Artificial Photosynthetic Antenna-Reaction Center Complex

Functional mimics of a photosynthetic antenna-reaction center complex comprising five bis(phenylethynyl)anthracene antenna moieties and a porphyrin-fullerene dyad organized by a central hexaphenylbenzene core have been prepared and studied spectroscopically. The molecules successfully integrate singlet-singlet energy transfer and photoinduced electron transfer. Energy transfer from the five antennas to the porphyrin occurs on the picosecond time scale with a quantum yield of 1.0. Comparisons with model compounds and theory suggest that the Förster mechanism plays a major role in the extremely rapid energy transfer, which occurs at rates comparable to those seen in some photosynthetic antenna systems. A through-bond, electron exchange mechanism also contributes. The porphyrin first excited singlet state donates an electron to the attached fullerene to yield a P(*+)-C(60)(*-) charge-separated state, which has a lifetime of several nanoseconds. The quantum yield of charge separation based on light absorbed by the antenna chromophores is 80% for the free base molecule and 96% for the zinc analogue.

Dendrimers Made of Porphyrin Cores and Carbazole Chromophores as Peripheral Units. Absorption Spectra, Luminescence Properties, and Oxidation Behavior

Luminescent and redox-active porphyrin-based dendrimers of first and second generation have been synthesized, and their absorption spectra, photophysical properties, and oxidation behavior have been investigated, together with those of the corresponding aldehyde carbazole precursors. All the dendrimers contain a porphyrin core and carbazole-based chromophores as branches. The structural formulas of the new species are represented in Figures 1 and 2, with the corresponding schematizations. The absorption spectra of the aldehyde carbazole precursors A1-A6 in dichloromethane exhibit intense transitions in the UV region, centered on the carbazole and benzaldehyde subunits. The lowest-energy absorption bands receive contribution from charge-transfer transitions. Compounds A1-A6 are luminescent at room temperature in fluid solution; such a luminescence is attributed to twisted intramolecular charge-transfer excited states. The luminescence at 77 K in a rigid matrix is blue-shifted with respect to room-temperature emission and is assigned to locally excited states. Absorption spectra of the porphyrin-cored dendrimers P1-P6 appear additive as they are constituted by visible bands due to porphyrin absorption and bands in the UV region due to transitions centered on the carbazole-based branches. Emission spectra of P1-P6 both at 77 K and at room temperature are typical of porphyrin species and independent of excitation wavelength, indicating that the light collected by the peripheral chromophores is quantitatively transferred to the core. All the compounds exhibit a rich oxidation behavior in 1,2-dichloroethane solution, with reversible processes centered on the different carbazole subunits. Interaction between the different carbazole centers depends on the size of the spacer interposed.

Role of intramolecular vibrations in long-range electron transfer between pheophytin and ubiquinone in bacterial photosynthetic reaction centers

The dynamics of the elementary electron transfer step between pheophytin and primary ubiquinone in bacterial photosynthetic reaction centers is investigated by using a discrete state approach, including only the intramolecular normal modes of vibration of the two redox partners. The whole set of normal coordinates of the acceptor and donor groups have been employed in the computations of the Hamiltonian matrix, to reliably account both for shifts and mixing of the normal coordinates, and for changes in vibrational frequencies upon ET. It is shown that intramolecular modes provide not only a discrete set of states more strongly coupled to the initial state but also a quasicontinuum of weakly coupled states, which account for the spreading of the wave packet after ET. The computed transition probabilities are sufficiently high for asserting that electron transfer from bacteriopheophytin to the primary quinone can occur via tunneling solely promoted by intramolecular modes; the transition times, computed for different values of the electronic energy difference and coupling term, are of the same order of magnitude (10(2) ps) of the observed one.

Photoinduced Electron Transfer in Double-Bridged Porphyrin−Fullerene Triads

Electron and energy transfer reactions of porphyrin-porphyrin-fullerene triads (P2P1C) with controllable sandwich-like structures have been studied using spectroscopic and electrochemical methods. The stabile, stacked structure of the molecules was achieved applying a two-linker strategy developed previously for porphyrin-fullerene dyads. Different triad structures with altered linker positions, linker lengths, and center atoms of the porphyrin rings were studied. The final charge-separated (CS) state and the different transient states of the reactions have been identified and energies of the states estimated based on the experimental results. In particular, a complete CS state P2(+) P1C- was achieved in a zinc porphyrin-free-base porphyrin-fullerene triad (ZnP2t9P1C) in both polar (benzonitrile) and nonpolar (toluene) solvents. The lifetime of this state was longer living in the nonpolar solvent. An outstanding feature of the ZnP2t9P1C triad is the extremely fast formation of the final CS state, P2(+) P1C-. This state is formed after primary excitation of either zinc porphyrin or free-base porphyrin chromophores in less than 200 fs. Although the intermediate steps between the locally excited states and the final CS state were not time-resolved for this compound, the process is clearly multistep and the fastest ever observed for porphyrin-based compounds.

C60−exTTF−C60 Dumbbells: Cooperative Effects Stemming from Two C60s on the Radical Ion Pair Stabilization

[structure: see text] The presence of a second C(60) cage in C(60)-exTTF-C(60) triads [exTTF = 9,10-bis(1,3-dithiol-2-ylidene)-9,10-anthraquinone] has beneficial effects on the stabilization of the radical ion pair formed upon irradiation in comparison with the related C(60)-exTTF dyad. Although C(60)-exTTF-C(60) ensembles show no electronic interaction between the electroactive units in the ground state, their irradiation leads to C(60)(*)(-)-exTTF(*)(+)-C(60) species with lifetimes on the order of 600 ns in benzonitrile; these lifetimes are twice those determined for the analogous C(60)-exTTF dyad.