Ps In Toluene Research Articles

Regulating the Flow-Driven Translocation of Macromolecules through Nanochannels by Interfacial Physical Adsorption

By using a model system composed of an anodic aluminum oxide nanopore (AAO), poly(benzyl acrylate) (PBAN) adsorbent, and polystyrene (PSN) probe, where N represents the degree of polymerization, this work has explored how the physical adsorption of PBA onto the surface of the AAO nanopore (rp ≈ 10 nm) influences the flow-driven translocation of PS in toluene. In the low flow rate range (Q = 0.02 mL/h), the translocation of PS chains is purely diffusive, and the partially reversible adsorption behavior of PBAN is observed only when N < ∼360, showing a strong chain length dependence. In the partially reversible regime, the interfacial thickness (δ) of the adsorption layer is estimated to be from ∼1.5 to ∼2.9 nm depending on the chain length. In the mixed solutions, when N increases from 0 to 75 and 150 for PBAN, the cut-off size (COS) of the membrane for broadly distributed PSB-285K (Mw/Mn ≈ 2.48) is estimated to decrease from ∼10.0 to ∼8.0 and ∼6.0 nm, respectively; in addition, the introduction of PBA360 can promote the separation of narrowly distributed PS655 and PS1200 by regulating re, whereas in the high flow rate range (Q > 0.02 mL/h), the translocation for PS long chains is based on the mechanism of molecular deformation, and the flow rate-dependent translocation shows a strong dependence on the chain length of PBAN. Namely, in mixed solutions, when N increases from 150 to 360, the critical flow rate (Qc) is found to slightly increase from ∼0.30 to ∼0.45 mL/h for PS1500 translocation; in addition, the cut-off performance of PSB-285K displays dependence both on flow rate and PBAN chain length. Our result has demonstrated a novel strategy to regulate the cut-off performance during the translocation of (bio)macromolecules through nanopores via partially reversible interfacial physical adsorption.

Anthracene-Naphthalenediimide Compact Electron Donor/Acceptor Dyads: Electronic Coupling, Electron Transfer, and Intersystem Crossing.

We attached different electron donors of phenyl, anthryl, and alkylamino moieties, to electron acceptor naphthalenediimide (NDI) to construct compact electron donor/acceptor dyads. The purpose is to study the effect of electron coupling (the magnitude is the matrix element, VDA) on the photophysical properties of UV-vis absorption, fluorescence emission, especially spin-orbit charge transfer intersystem crossing. We found that the magnitude of VDA depends on the electron donating strength of the aryl moieties ( VDA = 0.22-0.55 eV), as well as the molecular conformation, based on steady state and time-resolved transient absorption spectroscopies. We also found that electron coupling does not show the add-up (or synergetic) effect. Solvent polarity-dependent intersystem crossing (ISC) was observed for the dyad/triads and singlet oxygen quantum yield decreases in polar solvents. Femtosecond transient absorption results indicate that the charge separation (CS) for 9-An-NDI-NH occurs on time scale of 0.83 ps (in toluene) or 0.71 ps (in acetonitrile). The charge recombination (CR) process (50 ps in toluene) produces triplet state with ΦISC = 19%. The triplet state lifetime is up to 22 μs. This result indicates that orthogonal geometry for a compact electron donor/acceptor does not lead to efficient ISC via CR. Other factors such as the energy gap between the CS state and triplet state also determine the ISC efficiency.

Ultrafast Ground-State Intramolecular Proton Transfer in Diethylaminohydroxyflavone Resolved with Pump-Dump-Probe Spectroscopy.

4'- N, N-Diethylamino-3-hydroxyflavone (DEAHF), due to excited-state intramolecular proton transfer (ESIPT) reaction, exhibits two solvent-dependent emission bands. Because of the slow formation and fast decay of the ground-state tautomer, its population does not accumulate enough for its detection during the normal photocycle. As a result, the details of the ground-state intramolecular proton-transfer (GSIPT) reaction have remained unknown. The present work uses femtosecond pump-dump-probe spectroscopy to prepare the short-lived ground-state tautomer and track this GSIPT process in solution. By simultaneously measuring femtosecond pump-probe and pump-dump-probe spectra, ultrafast kinetics of the ESIPT and GSIPT reactions are obtained. The GSIPT reaction is shown to be a solvent-dependent irreversible two-state process in two solvents, with estimated time constants of 1.7 ps in toluene and 10 ps in the more polar tetrahydrofuran. These results are of great value in both fully describing the photocycle of this four-level proton transfer molecule and for providing a deeper understanding of dynamical solvent effects on tautomerization.

Tailoring Panchromatic Absorption and Excited-State Dynamics of Tetrapyrrole-Chromophore (Bodipy, Rylene) Arrays-Interplay of Orbital Mixing and Configuration Interaction.

Three sets of tetrapyrrole-chromophore arrays have been examined that exhibit panchromatic absorption across large portions of the near-ultraviolet (NUV) to near-infrared (NIR) spectrum along with favorable excited-state properties for use in solar-energy conversion. The arrays vary the tetrapyrrole (porphyrin, chlorin, bacteriochlorin), chromophore (boron-dipyrrin, perylene, terrylene), and attachment sites (meso-position, β-pyrrole position). In all, seven dyads, one triad, and nine benchmarks in toluene and benzonitrile were studied using steady-state and time-resolved absorption and fluorescence spectroscopy. The results were analyzed with the aid of density functional theory (DFT) and time-dependent DFT calculations. Natural transition orbitals (NTOs) were constructed to assess the net change in electron density associated with each NUV-NIR absorption transition. The porphyrin-perylene dyad P-PMI displays the most even spectral coverage from 400 to 700 nm, with an average ε ∼ 43 000 M-1 cm-1. A significant contributor is a chromophore-induced reduction in the configuration interaction involving the four frontier molecular orbitals of benchmark porphyrins and associated constructive/destructive transition-dipole interference that results in intense (ε ∼ 400 000 M-1 cm-1) NUV and weak (<20 000 M-1 cm-1) visible features. P-PMI has an S1 lifetime (τS) of 4.7 ns in toluene and 1.3 ns in benzonitrile. Bacteriochlorin analogue BC-PMI has more extended spectral coverage (350-750 nm) and τS = 2.8 ns in toluene and 30 ps in benzonitrile. Terrylene analogue P-TMI has intermediate optical characteristics with τS = 310 ps in toluene and 150 ps in benzonitrile. The NTOs for most arrays show that S0 → S1 primarily involves the tetrapyrrole, but for P-TMI the NTOs have electron density delocalized over the two units as a result of extensive orbital mixing. Collectively, the insights obtained should aid the design of tetrapyrrole-based architectures for panchromatic light-harvesting systems for solar-energy conversion.

Photoinitiated long-lived charge separation with near-unity quantum yield in donor-acceptor1-acceptor2 systems for artificial photosynthesis

We report the synthesis and photophysical characterization of isomeric donor-acceptor1-acceptor2 (D-A1-A2) triads each comprising a perylene donor (D) connected by a xylene bridge to a naphthalene-1,8-dicarboximide primary acceptor (NMI, A1) that is, in turn, covalently linked to a naphthalene-1,4:5,8-bis(dicarboximide) secondary acceptor (NDI, A2) through a benzene ring at its ortho, meta or para positions (Per-Xy-NMI-x-NDI). Selective photoexcitation of Per produces both Per+• and NDI‒• simultaneously with time constants of ∼50 ps in benzonitrile and ∼80 ps in toluene with a >98% yield. This is much faster than expected for single-step electron transfer from 1*Per to NDI and is consistent with a two-step mechanism, 1*Per-Xy-NMI-x-NDI → Per+•-Xy-NMI‒•-x-NDI → Per+•-Xy-NMI-x-NDI‒•, in which the second step is much faster than the first step. In addition, preserving long-lived charge separation in these systems is found to depend on inhibiting charge recombination by a triplet radical ion pair recombination pathway.

A time resolved fluorescence and quantum chemical study of the solar cell sensitizer D149

The steady-state spectroscopic properties and ultrafast relaxation dynamics of the very promising indoline-based sensitizer dye D149 are reported in various organic solvents. For the study of relaxation dynamics of both S2 and S1 states, time resolved femtosecond upconversion spectroscopy was used. Quantum chemical calculations of excitation energies and dipole moments at the DFT level as well as using high-level correlated second-order approximate coupled-cluster model with singles and doubles (CC2) have been also performed. The difference between the first excited and ground state dipole moments was found experimentally to be ∼13D in agreement with calculations. According to our time resolved results, the population of the S2 state has shown a rapid internal conversion to S1, taking place within 0.15–0.45 ps, depending on the solvent. This is followed by a slower decay component indicating that some S2 species relax on a long timescale. As the detection is shifted to the transition from the S1 state, the overall decay dynamics is retarded, while a slow fluorescence rise is observed at long wavelengths, indicating a time-dependent spectral shift. The lifetime of S1 was found to depend on the solvent used, being 630 ps in toluene, 720 ps in chloroform, 670 ps in THF, 700 ps in dichloromethane, 220 ps in acetonitrile and 190 ps in EtOH. The time resolved emission spectra as well as the solvent response function, C(t), obtained in the aprotic solvents, have shown that as the solvent polarity increases a more rapid spectral shift is observed. An exception is found in the case of the protic solvent EtOH due to specific H-bonding effects. The time resolved emission spectra in all solvents experience no significant change of shape and width with time, except from THF, where the initially formed Intramolecular Charge Transfer state is observed at early times.

Loading Pentapod Deca(organo)[60]fullerenes with Electron Donors: From Photophysics to Photoelectrochemical Bilayers

A pentapod deca(aryl)[60]fullerene, C(60)(C(6)H(4)CO(2)H)(5)(C(6)H(4)Fc)(5)Me(2) (4; Fc = ferrocenyl), bearing five carboxylic acid and five ferrocenyl groups was synthesized through top and bottom functionalization of [60]fullerene by means of copper-mediated penta-addition reactions. For electrochemical measurements (i.e., E(ox) = 0.08 V, five-electron oxidation of the ferrocenyl groups; E(red) = -1.89 and -2.28 V for the fullerene part vs Fc/Fc(+)), we used an ester-protected compound, C(60)(C(6)H(4)CO(2)Et)(5)(C(6)H(4)Fc)(5)Me(2) (2), and 4 was probed by performing femtosecond flash photolysis experiments in a variety of organic solvents. Importantly, the formation of a radical ion pair state was corroborated with lifetimes of up to 333 ps in toluene. In complementary studies, penta(carboxylic acid)-penta(ferrocenyl) compound 4 was deposited on indium-tin oxide (ITO) electrodes with a surface coverage (i.e., 0.14 nmol/cm(2)) that corresponded to a unique bilayer structure. Decisive for the bilayer motif is the presence of five ferrocenyl groups, which are assembled with a merry-go-round-shaped arrangement on the [60]fullerene. The novel 4/ITO photoelectrode gave rise to a cathodic photocurrent with a 12% quantum yield in the presence of methyl viologen, whereas an anodic photocurrent was generated in the presence of ascorbic acid for a C(60)(C(6)H(4)CO(2)H)(5)(C(6)H(5))(5)Me(2) (5)/ITO photoelectrode. Photophysical investigations revealed that the difference in photocurrent, that is, cathodic versus anodic photocurrents, is related to the nature of the excited state feature in 4 (i.e., charge separated state) and 5 (i.e., triplet excited state). The unique molecular architecture of 4, in combination with its remarkable donor-acceptor properties, validates the use of the pentapod deca(aryl)[60]fullerene in photoelectrochemically active molecular devices.

Electron Donor-Acceptor Interactions in Regioselectively Synthesized exTTF2-C70(CF3)10Dyads

The decakis(trifluoromethyl)fullerene C(1)-C(70)(CF(3))(10), in which the CF(3) groups are arranged on a para(7)-meta-para ribbon of C(6)(CF(3))(2) edge-sharing hexagons, and which has now been prepared in quantities of hundreds of milligrams, was reacted under standard Bingel-Hirsch conditions with a bis-pi-extended tetrathiafulvalene (exTTF) malonate derivative to afford a single exTTF(2)-C(70)(CF(3))(10) regioisomer in 80 % yield based on consumed starting material. The highly soluble hybrid was thoroughly characterized by using 1D (1)H, (13)C, and (19)F NMR, 2D NMR, and UV/Vis spectroscopy; matrix-assisted laser desorption ionization (MALDI) mass spectrometry; and electrochemistry. The cyclic voltammogram of the exTTF(2)-C(70)(CF(3))(10) dyad revealed an irreversible second reduction process, which is indicative of a typical retro-Bingel reaction; whereas the usual phenomenon of exTTF inverted potentials (E1ox>E2ox), resulting in a single, two-electron oxidation process, was also observed. Steady-state and time-resolved photolytic techniques demonstrated that the C(1)-C(70)(CF(3))(10) singlet excited state is subject to a rapid electron-transfer quenching. The resulting charge-separated states were identified by transient absorption spectroscopy, and radical pair lifetimes of the order of 300 ps in toluene were determined. The exTTF(2)-C(70)(CF(3))(10) dyad represents the first example of exploitation of the highly soluble trifluoromethylated fullerenes for the construction of systems able to mimic the photosynthetic process, and is therefore of interest in the search for new materials for photovoltaic applications.

Photoisomerization of Disperse Red 1 Studied with Transient Absorption Spectroscopy and Quantum Chemical Calculations

The photoisomerization of the push-pull substituted azo dye Disperse Red 1 is studied using femtosecond time-resolved absorption spectroscopy and other spectroscopic and computational techniques. In comparison with azobenzene, the pipi* state is more stabilized by the effects of push-pull substitution than the npi* state, but the latter is probably still the lowest in energy. This conclusion is based on the kinetics, anisotropy of the excited state absorption spectrum, the spectra of the ground states, and quantum chemical calculations. The S(1)(npi*) state is formed from the initially excited pipi* state in <0.2 ps, and decays to the ground state with time constants of 0.9 ps in toluene, 0.5 ps in acetonitrile, and 1.4 ps in ethylene glycol. Thermal isomerization transforms the Z isomer produced to the more stable E isomer with time constants of 29 s (toluene), 28 ms (acetonitrile), and 2.7 ms (ethylene glycol). The pathway of photoisomerization is likely to be rotation about the N=N bond. Quantum chemical calculations indicate that along the inversion pathway ground and excited state energy surfaces remain well separated, whereas rotation leads to a region where conical intersections can occur. For the ground-state Z to E isomerization, conclusive evidence is lacking, but inversion is more probably the favored pathway in the push-pull substituted systems than in the parent azobenzene.

Photoinduced charge separation and charge recombination in terthiophene-acetylene-fullerene linked dyads

Charge separation (CS) and recombination (CR) processes in terthiophene-fullerene linked dyads with bridges (3T-br n-C 60-R, bridge-1 (br1) = C C , and bridge-2 (br2) = (CH 2) 5 C C ; R = CN or Me) were investigated by fluorescence up-conversion method and transient absorption measurement in benzonitrile (PhCN) and toluene. With photoexcitation of the 3T moiety in 3T-br n-C 60-R, the CS process takes place fast in the region of (0.14–3.5) × 10 12 s −1 via 13T*-br n-C 60-R. In the case of 3T-br1-C 60-R, the CS process takes place fast via one step before the vibrational relaxation of the 13T* moiety, whereas in the case of 3T-br2-C 60-R, the CS process occurs two steps competing with the vibrational relaxation and after the relaxation. Such difference can be interpreted by the rigidity of the bridges. The 3T +-br n-C 60 −-R states were confirmed by the transient absorption spectra in the (1.0–3.6) × 10 3 ps region. The lifetimes of the radical ion-pair ( τ RIP) were evaluated to be 10–25 ps for 3T +-br1-C 60 −-R in PhCN. In toluene, the τ RIP values became longer ((1.2–4.7) × 10 2 ps) than those in PhCN. In both solvents, the τ RIP values are longer for R = Me than that for R = CN. In the case of 3T-br2-C 60-R in which the methylene chain inserted between 3T and acetylene bridge, the τ RIP values are longer than the corresponding values for of 3T +-br1-C 60 −-R, i.e., (0.91–1.2) × 10 3 ps in PhCN and (1.5–3.6) × 10 3 ps in toluene for R = CN and Me, respectively. It is revealed that the radical ion pairs last for longer in toluene than those in PhCN, suggesting that the CR process in the inverted region of the Marcus parabola, which was also supported by the substituent effect, i.e., unstable 3T +-br n-C 60 −-Me prolongs longer than stable 3T +-br n-C 60 −-CN. Drastic prolongation of τ RIP for 3T +-br2-C 60 −-R indicates that the electronic coupling for CR through the normal methylene group in 3T-br2-C 60-R is smaller than the π-conjugated system in 3T-br1-C 60-R.

Femtosecond studies of exciton dynamics in a novel main chain chiral conjugated poly(arylenevinylene)

The formation and decay dynamics of photogenerated excitons in an optically active poly(arylenevinylene), PAV, in solution have been studied using femtosecond transient absorption spectroscopy. Photoexcitation initially creates hot excitons which quickly (&amp;lt;200 fs) relax geometrically towards the equilibrium position in the excited state. The exciton subsequently decays following a double exponential with time constants of 6.5 and 420 ps in toluene. The decays become faster (5 and 250 ps) in pyridine, indicating a dependence of the relaxation process on the solvent environment. The fast decay is attributed to vibrational relaxation and internal conversion (recombination) of the exciton from the excited to the ground electronic state through tunneling or thermal-activated barrier crossing before thermalization. The slow decay is assigned to conversion of the thermalized exciton to the ground state through both radiative and nonradiative pathways. Anisotropy decay shows a fast component (6 ps in toluene and 10 ps in pyridine) and an offset which persists up to 650 ps. Possible explanations for the fast decay include internal conversion, vibrational relaxation, conformational change, and exciton migration. The offset may decay on a longer time scale through local reorientation of the conjugation segments, exciton migration, or rotational diffusion of the polymer. Comparison to a well-studied system, MEH-PPV [poly(2-methoxy, 5-(2-ethylhexoxy)-p-phenylenevinylene], provides further insight into the relaxation mechanism of photoexcitations in this PAV polymer.

Spectral and Photophysical Properties of Ethylene-Bridged Side-to-Side Porphyrin Dimers. 1. Ground-State Absorption and Fluorescence Study and Calculation of Electronic Structure oftrans-1,2-Bis(meso-octaethylporphyrinyl)ethene

We have studied a double-bond bridged porphyrin dimer, trans-1,2-bis(meso-octaethylporphyrinyl)ethene (tbisOEP), which in solutions exhibits new spectral properties: (i) pronounced absorption bands in addition to the monomer ones are observed in the 480−500 and 600−900 nm regions; (ii) a broad-band fluorescence with a viscosity-dependent intensity and spectral position is detected in the near-IR region (750−1100 nm). An investigation of fluorescence excitation spectra, combined with semiempirical quantum chemical calculations and geometry optimizations suggest that in solution tbisOEP exists in two main conformations, which we name conformers P and U. The ratio of their concentrations was estimated to be approximately 5:1 in toluene at room temperature. The P conformer was found to be responsible for the “usual” monomer-type absorption bands (Soret and Q) in the tbisOEP absorption spectrum, whereas the U conformer is responsible for the additional absorptions in the 480−500 and 600−900 nm regions. The unusual near-IR fluorescence originates from the U conformer and its quantum yield (Φ) is 6 × 10-4 in toluene. Increase of solvent viscosity results in a strong blue-shift of the near-IR emission and increase of its intensity (Φ = 4 × 10-3 in paraffin oil). Both conformers P and U were found to exhibit very short exited-state lifetimes (<10 ps in toluene) which become significantly longer in more viscous solvents. The calculations suggest that the peculiar ground-state spectral properties of the U conformer result from its particular geometrical structure favoring a common conjugation between the π orbitals of the porphyrin rings and the ethylene bond, whereas only excitonic interactions exist in the P conformer. Therefore, the U conformer can be considered as a real supermolecule rather than two interacting separate porphyrin macrocycles. Our results point to the key role of the connecting bridge in the formation of the optical properties of the ethylene-bridged porphyrin dimers.

Dynamic Photophysical Properties of Conformationally Distorted Nickel Porphyrins. 1. Nickel(II) Dodecaphenylporphyrin

Transient absorption measurements with subpicosecond resolution have been performed on nickel(II) dodecaphenylporphyrin (NiDPP), a highly nonplanar tetrapyrrole. Following photoexcitation, NiDPP deactivates by the pathway (π,π*) → (d,d) → ground state, the route proposed previously for planar analogues. However, the (π,π*) state has now been spectrally and kinetically resolved, and evidence is presented that the deactivation proceeds mainly in the singlet manifold. The lifetime of the 1(π,π*) state of NiDPP is about 0.7 ps in all solvents investigated and increases only slightly as the temperature is reduced. The (d,d) state exhibits complex spectral evolution over the following 20 ps or more, which is interpreted in terms of vibrational relaxation and cooling, together with changes in the conformation of the porphyrin macrocycle. The ligand-field excited state decays with a lifetime of about 120 ps in toluene, but this decay time, like the vibrational and conformational dynamics in the (d,d) state, slows...

Brillouin Light Scattering Determination of the Glass Transition in Thin, Freely-Standing Poly(styrene) Films

ABSTRACTWe have used Brillouin light scattering (BLS) to measure the glass transition temperature of thin, freely-standing poly(styrene) (PS) films. The freely-standing films were prepared by spincoating solutions of PS in toluene onto glass substrates, annealing the supported films in vacuum, and then using a water surface transfer technique to place the films across a 3 mm diameter orifice. Ellipsometry measurements of similar floated films transferred to Si(001) wafers allow the determination of the film thicknesses. Atomic force microscopy measurements revealed that the films have an rms roughness of less than 10 A˚. With the freely-standing films placed in an optical furnace, we performed BLS measurements of the films using a high-contrast, multipassed, tandem Fabry-Perot interferometer. We obtained a reliable, reproducible measure of the glass transition temperature, Tg, from the large changes in the frequencies of the thermally-excited, viscoelastic, film-guided waves within the PS films as the films were heated above Tg. BLS results for bulk PS and a 1800 A˚ thick, freely-standing PS film are presented. We find the same glass transition temperature for the 1800 A˚ thick film as the bulk PS sample. This Tg value is the same as that obtained using differential scanning calorimetry (DSC).

PREPARATION AND PHOTOPHYSICAL STUDIES OF PORPHYRIN‐C60 DYADS

Abstract Porphyrin‐C60 dyads in which the two chromophores are linked by a bicyclic bridge have been synthesized using the Diels‐Alder reaction. The porphyin singlet lifetimes of both the zinc (Pzn‐C60) and free base (P‐C60) dyads, determined by time‐resolved fluorescence measurements, are ≦17 ps in toluene. This substantial quenching is due to singlet‐singlet energy transfer to C60 The lifetime of Pzn‐1C60 is ‐5 ps in toluene, whereas the singlet lifetime of an appropriate C60 model compound is 1.2 ns. This quenching is attributed to electron transfer to yield Pznbull;+‐C60bull;‐. In toluene, P‐1C60 is unquenched; the lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an absorption maximum at 700 ran and a lifetime of‐10 μs was detected using transient absorption methods. This state was quenched by oxygen, and is assigned to the C60 triplet. In the more polar benzonitrile, P‐1C60 underoes photoinduced electron transfer to give P•+‐C60bull;‐. The electron transfer rate constant is −2 × 1011 s−1.

Enantiocomplementary preparation of optically pure 2-trimethylsilylethynyl-2-cyclopentenol by homochiralization of racemic precursors: a new route to the key intermediate of 1,25-dihydroxycholecalciferol and vincamine

Treatment of racemic 2-trimethylsilylethynyl-2-cyclopentenol [(±)- 1] with vinyl acetate in the presence of lipase PS in toluene yielded a 1:1 mixture of the unreacted ( S)-alcohol [( S)- 1] and the ( R)-acetate [( R)- 7 which without isolation afforded the chirally homogeneous ( R)-alcohol [( R)- 1] in 73% overall yield on reaction with acetic acid in the same reaction medium in the presence of diisopropyl azodicarboxylate and triphenylphosphine, followed by reductive deacylation of the resulting ( R)-acetate [( R)- 7]. On the other hand, hydrolysis of the racemic acetate [(±)- 7], obtained from [(±)- 1], in a phosphate buffer solution in the presence of lipase PS yielded a 1:1 mixture of the ( R)-alcohol [( R)- 1] and the unreacted ( S)-acetate [( S)- 7] which without separation, furnished the enantiomerically homogeneous ( S)-alcohol [( S)- 1] in 75% overall yield on treatment with acetic acid under the above Mitsunobu conditions, followed by reductive deacylation of the resulting ( S)-acetate [( S)- 7].

Solvent Polarity Dependent Photophysics of a Fixed‐Distance, Symmetric Chlorophyll Dimer. A Model of the Special Pair in Photosynthetic Reaction Centers

AbstractThe synthesis and properties of a fixed‐distance symmetric bis‐pyrochlorophyllide‐a molecule are described. The molecule, 1, consists of two methyl pyrochlorophyllide‐a (MePChlide‐a) moieties which share a common vinyl group at the 2‐position of each macrocycle. The two chlorophylls are trans to one another across the vinyl linkage. The dihedral angle between the plane of each macrocycle and the vinyl linkage is about 50°. The Qy absorption band of this dimer occurs at 689 nm and is essentially independent of solvent. The band is red‐shifted by about 27 run relative to that of MePChlide‐a. The fluorescence maximum of 1 occurs at 722 nm. This is a red‐shift of 49 nm relative to methyl MePChlide‐a. The circular dichroism spectrum of 1 shows strong, nonconservative bands in the red region. The fluorescence quantum yield, fluorescence lifetimes, and picosecond transient absorbance kinetics all exhibit a very strong dependence on solvent polarity. The fluorescence quantum yields of 1 in toluene, butyronitrile, and N, N‐dimethyl formamide (DMF) are 0.18, 0.008, and 0.002, respectively. The bleach of the 689 nm band (following excitation with a 1.5‐ps, 610‐nm laser pulse) recovers with single exponential time constants of 2.51 ns, 100 ps, and 64 ps in toluene, butyronitrile, and DMF, respectively. Possible mechanisms for the polarity‐dependent increase of the nonradiative decay rate of 1 are considered.