Ultrafast Dynamics of Nonrigid Zinc-Porphyrin Arrays Mimicking the Photosynthetic "Special Pair".

Conjugated porphyrin arrays are heavily investigated as efficient molecular systems for photosynthesis and photocatalysis. Recently, a series of one-, two-, and six-zinc-porphyrin arrays, noncovalently linked through benzene-based hubs, have been synthesized with the aim of mimicking the structure and function of the bacteriochlorophyll "special pair" in photosynthetic reaction centers. The excitonically coupled porphyrin subunits are expected to activate additional excited state relaxation channels with respect to the monomer. Here, we unveil the appearance of such supramolecular electronic interactions using ultrafast transient absorption spectroscopy with sub-25 fs time resolution. Upon photoexcitation of the Soret band, we resolve energy trapping within ∼150 fs in a delocalized dark excitonic manifold. Moreover, excitonic interactions promote an additional fast internal conversion from the Q-band to the ground state with an efficiency of up to 60% in the hexamer. These relaxation pathways appear to be common loss channels that limit the lifetime of the exciton states in noncovalently bound molecular aggregates.

Metal-free organic sensitizers for use in water-splitting dye-sensitized photoelectrochemical cells.

Solar fuel generation requires the efficient capture and conversion of visible light. In both natural and artificial systems, molecular sensitizers can be tuned to capture, convert, and transfer visible light energy. We demonstrate that a series of metal-free porphyrins can drive photoelectrochemical water splitting under broadband and red light (λ > 590 nm) illumination in a dye-sensitized TiO2 solar cell. We report the synthesis, spectral, and electrochemical properties of the sensitizers. Despite slow recombination of photoinjected electrons with oxidized porphyrins, photocurrents are low because of low injection yields and slow electron self-exchange between oxidized porphyrins. The free-base porphyrins are stable under conditions of water photoelectrolysis and in some cases photovoltages in excess of 1 V are observed.

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.

Artificial photosynthetic reaction center with a coumarin-based antenna system.

In photosynthesis, sunlight is absorbed mainly by antenna chromophores that transfer singlet excitation energy to reaction centers for conversion to useful electrochemical energy. Antennas may likewise be useful in artificial photosynthetic systems that use sunlight to make fuels or electricity. Here, we report the synthesis and spectroscopic properties of a molecular hexad comprising two porphyrin moieties and four coumarin antenna chromophores, all organized by a central hexaphenylbenzene core. Light absorbed by any of the coumarins is transferred to a porphyrin on the 1-10 ps time scale, depending on the site of initial excitation. The quantum yield of singlet energy transfer is 1.0. The energy transfer rate constants are consistent with transfer by the Förster dipole-dipole mechanism. A pyridyl-bearing fullerene moiety self-assembles to the form of the hexad containing zinc porphyrins to yield an antenna-reaction center complex. In the resulting heptad, energy transfer to the porphyrins is followed by photoinduced electron transfer to the fullerene with a time constant of 3 ps. The resulting P(•+)-C60(•-) charge-separated state is formed with an overall quantum yield of 1.0 and decays with a time constant of 230 ps in 1,2-difluorobenzene as the solvent.

Base-catalyzed direct conversion of dipyrromethanes to 1,9-dicarbinols: a [2 + 2] approach for porphyrins.

A variant of the MacDonald approach was devised for the synthesis of porphyrins. A new base-catalyzed one-step synthesis of 1,9-dipyrromethane-dicarbinols was achieved by Friedel-Crafts alkylation of dipyrromethanes using commercially available ethyl glyoxylate solution in toluene. This method avoids the use of acid chlorides, Grignard reagents, borohydride reductions, and acidic conditions. The [2 + 2] condensation of dipyrromethanedicarbinols and dipyrromethanes yielded 5,15-di(ethoxycarbonyl)porphyrins.

Mimicking the role of the antenna in photosynthetic photoprotection.

One mechanism used by plants to protect against damage from excess sunlight is called nonphotochemical quenching (NPQ). Triggered by low pH in the thylakoid lumen, NPQ leads to conversion of excess excitation energy in the antenna system to heat before it can initiate production of harmful chemical species by photosynthetic reaction centers. Here we report a synthetic hexad molecule that functionally mimics the role of the antenna in NPQ. When the hexad is dissolved in an organic solvent, five zinc porphyrin antenna moieties absorb light, exchange excitation energy, and ultimately decay by normal photophysical processes. Their excited-state lifetimes are long enough to permit harvesting of the excitation energy for photoinduced charge separation or other work. However, when acid is added, a pH-sensitive dye moiety is converted to a form that rapidly quenches the first excited singlet states of all five porphyrins, converting the excitation energy to heat and rendering the porphyrins kinetically incompetent to readily perform useful photochemistry.

Photochemical "triode" molecular signal transducer.

A molecular "hexad" in which five bis(phenylethynyl)anthracene (BPEA) fluorophores and a dithienylethene photochrome are organized by a central hexaphenylbenzene unit has been prepared. Singlet-singlet energy transfer among the BPEA units occurs on the 0.4 and 60 ps time scales, and when the dithienylethene is in the open form, the BPEA units fluoresce in the 515 nm region with a quantum yield near unity. When the dithienylethene is photoisomerized by UV light to the closed form, which absorbs in the 500-700 nm region, the closed isomer strongly quenches all of the excited singlet states of BPEA via energy transfer, causing the fluorescence quantum yield to drop to near zero. This photochemical behavior permits the hexad to function in a manner analogous to a triode vacuum tube or transistor. When a solution of the hexad is irradiated with steady-state light at 350 nm and with red light (>610 nm) of modulated intensity, the BPEA fluorescence excited by the 350 nm light is modulated accordingly. The fluorescence corresponds to the output of a triode tube or transistor and the modulated red light to the grid signal of the tube or gate voltage of the transistor. Frequency modulation, amplitude modulation, and phase modulation are all observed. The unusual ability to modulate intense, shorter-wavelength fluorescence with longer-wavelength light could be useful for the detection of fluorescence from probe molecules without interference from other emitters in biomolecular or nanotechnological applications.

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.

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.

Conductance of a biomolecular wire.

Carotenoids (Car) act as "wires" that discharge unwanted electrons in the reaction center of higher plants. One step in this "side-path" electron conduction is thought to be mediated by Car oxidation. We have carried out direct measurements of the conductance of single-Car molecules under potential control in a membrane-mimicking environment, and we found that when Car are oxidized conductance is enhanced and the electronic decay constant (beta) is decreased. However, the neutral molecule may already be conductive enough to account for observed electron transfer rates.

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.

Electronic decay constant of carotenoid polyenes from single-molecule measurements.

The conductance of carotenoid polyenes chemically bound at each end to gold contacts has been measured for single molecules containing 5, 7, 9, and 11 carbon-carbon double bonds in conjugation. The electronic decay constant, beta, is determined to be 0.22 +/- 0.04 A-1, in close agreement with the value obtained from first principles simulations (0.22 +/- 0.01 A-1). The absolute values of the molecular conductance are within a factor of 3 of those calculated from first principles. The small value of beta demonstrates that conductivity drops off only slowly with chain length, confirming that carotenoid conjugated chains are relatively good molecular "wires".

Molecule-based photonically switched half-adder.

A molecule-based binary half-adder with optical inputs and outputs has been demonstrated. The half-adder consists of two photochromic organic molecules in solution and a third-harmonic-generating crystal. One substance acts as an AND Boolean logic gate and the other as an XOR gate. Inputs are laser pulses at 1064 or 532 nm that initiate photoisomerization reactions. Outputs are the optical absorbance of a fullerene radical anion (AND gate) and fluorescence of a porphyrin (XOR gate). The system carries out binary addition based on the laser input pulses. Half-adders in combination are capable of carrying out all mathematical operations necessary for digital computing.

Synthesis and characterization of beta-trifluoromethyl-meso-tetraphenylporphyrins.

Beta-trifluoromethyl-meso-tetraphenylporphyrins were synthesized to investigate the electronic and steric effects of the trifluoromethyl groups on the macrocycle. Preparation of these novel porphyrins was carried out by copper-assisted trifluoromethylation of beta-tetrabromo-meso-tetraphenylporphyrin metal complexes and in situ generated CF3Cu. For comparison, the beta-methyl analogues were also prepared. Analysis of beta-trifluoromethylporphyrins by UV-vis, NMR, and cyclic voltammetry (CV) showed that the electron-withdrawing effects of the trifluoromethyl groups on the antipodal pyrroles required the macrocycle to take a fixed 18pi-electron pathway. UV-vis, CV, and molecular modeling studies suggest that the novel porphyrins are distorted following introduction of trifluoromethyl groups onto the pyrrolic beta-position of meso-tetraphenylporphyrin. The pK(a) difference of beta-tetrakis(trifluoromethyl)-meso-tetraphenylporphyrin from that of DBU in CH2Cl2, obtained by spectrophotometric titration, affirms that it is one of the most electron-deficient porphyrins so far prepared.

Synthesis, crystal structures, and redox potentials of 2,3,12,13-tetrasubstituted 5,10,15,20-tetraphenylporphyrin zinc(II) complexes.

Zinc(II) complexes of antipodal beta-tetrasubstituted meso-tetraphenylporphyrin with trifluoromethyl (Zn(TPP(CF(3))(4)) (1a)), bromine (Zn(TPPBr(4)) (2a)), and methyl groups (Zn(TPP(CH(3))(4)) (3a)) were synthesized in order to examine the steric and the electronic effects of trifluoromethyl groups on the macrocycle. The analysis of X-ray crystal structures of the five-coordinate complexes Zn(TPP(CF(3))(4))(EtOH)(3) (1b), Zn(TPPBr(4))(MeOH)(DMF) (2b), and Zn(TPP(CH(3))(4))(THF)(1.6)(CHCl(3))(0.4) (3b) revealed distorted macrocyclic cores where significant differences in the Zn-N distance between the beta-substituted and the non-beta-substituted side were observed. The difference was significant in 1b due to the strong steric interactions among the peripheral substituents and the electronic effects of trifluoromethyl groups. The macrocycles of 1b-3b are saddle-distorted and slightly ruffled due to the five-coordination of zinc(II) and the peripheral substitution. Distortion of the macrocycles of 2b and 3b were modest. On the other hand, distortion in 1b was severe due to the peripheral strain. Cyclic voltammetric measurements of the four-coordinate complexes Zn(TPP) and 1a-3a were performed and their redox potentials were analyzed together with previously reported potentials of Zn(TPP(CN)(4)). The oxidation potential of 1a did not gain as much as expected from the electron-withdrawing effect of the four trifluoromethyl groups. The HOMO-LUMO gap of 1a was very small (1.5 V) and cannot just be explained by macrocyclic distortion. The magnitude of this gap is very similar to that of Zn(TPP(CN)(4)). Compound 2a also exhibited a modest gap contraction. Compound 3a was easier to oxidize and harder to reduce than Zn(TPP), even though the HOMO-LUMO gap of 3a was similar to that of Zn(TPP).