Screening electronic communication through ortho-, meta- and para-substituted linkers separating subphthalocyanines and C60

We have prepared two complementary series of SubPc-C(60) (SubPc=subphthalocyanine) electron/energy donor-acceptor systems, in which the two constituents are linked through ortho-, meta-, or para-substituted phenoxy spacers. In one of the series (1 a) the SubPc units bear iodine atoms, while in the other series (1 b) diphenylamino groups are linked to the SubPc macrocycles. The iodine atoms and diphenylamino groups both influence the resulting oxidation potentials of the electron-donating SubPc. They also modulate the outcome of excited state interactions, namely, energy and/or charge transfer. In addition, we have studied the impact that the substitution pattern in the phenoxy spacer exerts onto intramolecular processes in the ground and excited states. Although some of these processes are governed by the spatial separation between both components, the different electronic coupling through ortho-, meta-, or para- connections also plays decisive roles in some cases.     

Designing Cascades of Electron Transfer Processes in Multicomponent Graphene Conjugates

A novel family of nanocarbon-based materials was designed, synthesized, and probed within the context of charge-transfer cascades. We integrated electron-donating ferrocenes with light-harvesting/electron-donating (metallo)porphyrins and electron-accepting graphene nanoplates (GNP) into multicomponent conjugates. To control the rate of charge flow between the individual building blocks, we bridged them via oligo-p-phenyleneethynylenes of variable lengths by β-linkages and the Prato–Maggini reaction. With steady-state absorption, fluorescence, Raman, and XPS measurements we realized the basic physico-chemical characterization of the photo- and redox-active components and the multicomponent conjugates. Going beyond this, we performed transient absorption measurements and corroborated by single wavelength and target analyses that the selective (metallo)porphyrin photoexcitation triggers a cascade of charge transfer events, that is, charge separation, charge shift, and charge recombination, to enable the directed charge flow. The net result is a few nanosecond-lived charge-separated state featuring a GNP-delocalized electron and a one-electron oxidized ferrocenium.     

A charge-transfer challenge: combining fullerenes and metalloporphyrins in aqueous environments

A series of truly water-soluble C(60)/porphyrin electron donor-acceptor conjugates has been synthesized to serve as powerful mimics of photosynthetic reaction centers. To this end, the overall water-solubility of the conjugates was achieved by adding hydrophilic dendrimers of different generations to the porphyrin moiety. An important variable is the metal center of the porphyrin; we examined zinc(II), copper(II), cobalt(II), nickel(II), iron(III), and manganese(III). The first insights into electronic communication between the electron donors and the electron acceptors came from electrochemical assays, which clearly indicate that the redox processes centered either on C(60) or the porphyrins are mutually affected. Absorption measurements, however, revealed that the electronic communication in terms of, for example, charge-transfer features, remains spectroscopically invisible. The polar environment that water provides is likely to be a cause of the lack of detection. Despite this, transient absorption measurements confirm that intramolecular charge separation processes in the excited state lead to rapid deactivation of the excited states and, in turn, afford the formation of radical ion pair states in all of the investigated cases. Most importantly, the lifetimes of the radical ion pairs were found to depend strongly on several aspects. The nature of the coordinated metal center and the type of dendrimer have a profound impact on the lifetime. It has been revealed that the nature/electronic configuration of the metal centers is decisive in powering a charge recombination that either reinstates the ground state or any given multiplet excited state. Conversely, the equilibrium of two opposing forces in the dendrimers, that is, the interactions between their hydrophilic regions and the solvent and the electronic communication between their hydrophobic regions and the porphyrin and/or fullerene, is the key to tuning the lifetimes.     

Dendronizing and Metalating trans‐2 C60 Tetraaryl Porphyrins—A Versatile Approach Toward Water‐Soluble Donor–Acceptor Conjugates

We have realized for the first time a series of truly water‐soluble and tightly coupled porphyrin/C₆₀ electron‐donor–acceptor conjugates in which the charge separation and charge recombination dynamics are controlled by modifying the nature of the dendrimer and/or the choice of the central metal atom.     

Engaging Copper(III) Corrole as an Electron Acceptor: Photoinduced Charge Separation in Zinc Porphyrin–Copper Corrole Donor–Acceptor Conjugates

An electron‐deficient copper(III) corrole was utilized for the construction of donor–acceptor conjugates with zinc(II) porphyrin (ZnP) as a singlet excited state electron donor, and the occurrence of photoinduced charge separation was demonstrated by using transient pump–probe spectroscopic techniques. In these conjugates, the number of copper corrole units was varied from 1 to 2 or 4 units while maintaining a single ZnP entity to observe the effect of corrole multiplicity in facilitating the charge‐separation process. The conjugates and control compounds were electrochemically and spectroelectrochemically characterized. Computational studies revealed ground state geometries of the compounds and the electron‐deficient nature of the copper(III) corrole. An energy level diagram was established to predict the photochemical events by using optical, emission, electrochemical, and computational data. The occurrence of charge separation from singlet excited zinc porphyrin and charge recombination to yield directly the ground state species were evident from the diagram. Femtosecond transient absorption spectroscopy studies provided spectral evidence of charge separation in the form of the zinc porphyrin radical cation and copper(II) corrole species as products. Rates of charge separation in the conjugates were found to be of the order of 10¹⁰ s^?1 and increased with increasing multiplicity of copper(III) corrole entities. The present study demonstrates the importance of copper(III) corrole as an electron acceptor in building model photosynthetic systems.     

Design, syntheses, and studies of supramolecular porphyrin-fullerene conjugates, using bis-18-crown-6 appended porphyrins and pyridine or alkyl ammonium functionalized fullerenes

Photoinduced electron-transfer processes in cis and trans functionalized bis-18-crown-6 porphyrin self-assembled with fullerene functionalized with pyridine or alkylammonium cation entities are reported. The structural integrity of the newly formed supramolecular conjugates was accomplished by optical absorption and emission, electron spray ionization mass, electrochemistry, and semiempirical PM3 calculations. A 1:2 stoichiometry of the supramolecular porphyrin:fullerene conjugates was deduced from these studies. The conjugates revealed stable "two-point"' binding involving metal-ligand coordination and alkylammonium cation-crown ether binding or only the latter type of binding depending upon the functionality of the fullerene and metal ion in the porphyrin cavity. The effect of the variation on free energy changes of charge separation and the charge recombination was achieved by varying the metal ion in the porphyrin cavity. The charge-separation rates (k(CS)) determined from the picosecond time-resolved emission studies were generally higher for the cis bis-crown functionalized porphyrins than those of the corresponding trans ones. A comparison of the k(CS) values reported earlier for 1:1 porphyrin-fullerene conjugates with a similar self-assembly mechanism suggested that employing a higher number of acceptor entities improves the electron-transfer rates. The calculated charge-recombination rates (k(CR)) were 2-3 orders of magnitude smaller than the k(CS) values, suggesting the occurrence of the charge recombination process in the Marcus inverted region. The lifetimes of the radical ion pair (tau(RIP)) ranged between 46 and 233 ns indicating charge stabilization in the studied conjugates.     

Dendronized Fullerene–Porphyrin Conjugates in ortho,meta, and para Positions: A Charge‐Transfer Assay

The physicochemical characterization, that is, ground and excited state, of a new series of dendronized porphyrin/fullerene electron donor–acceptor conjugates in nonaqueous and aqueous environments is reported. In contrast to previous work, we detail the charge‐separation and charge‐recombination dynamics in zinc and copper metalloporphyrins as a function of first‐ and second‐generation dendrons as well as a function of ortho, meta, and para substitution. Both have an appreciable impact on the microenvironments of the redox‐active constituents, namely the porphyrins and the fullerenes. As a matter of fact, the resulting charge‐transfer dynamics were considerably impacted by the interplay between the associated forces that reach from dendron‐induced shielding to dipole–charge interactions.     

A new reaction sequence involving palladium-catalyzed unsymmetrical aryl coupling

Biphenylylalkenes containing an ortho substituent in one ring and a different ortho-, meta-, or para-substituent in the other are prepared in satisfactory yield from an ortho-substituted aryl iodide, an ortho-, meta-, or para-substituted aryl bromide, a terminal olefin, and a base in DMF under the joint catalytic action of palladium(0) and norbornene.     

The role of electronic coupling in linear porphyrin arrays probed by single-molecule fluorescence spectroscopy

Single-molecule photophysical properties of two families of linear porphyrin arrays have been investigated by single-molecule fluorescence detection techniques. Butadiyne-linked arrays (Z(N)B) with extensive π-conjugation perform as photostable one-quantum systems. This demonstration has been suggested by the long-lasting initial emissive state and subsequent discrete one-step photobleaching in the fluorescence intensity trajectories (FITs). As in the behavior of a one-quantum system, Z(N)B shows anti-bunching data in the coincidence measurements. On the other hand, in directly-linked arrays (Z(N)) with strong dipole coupling, each porphyrin moiety keeps individual character in photobleaching dynamics. The stepwise photobleachings in the FITs account for this explanation. Most of the FITs of Z(N) do not carry momentary cessation of fluorescence emission, which has been explained by the strongly bound electron-hole pair of Frenkel exciton that suppresses charge transfer between the molecule and surrounding polymers. These results give insight into the influences of interchromophorinc interactions between porphyrin moieties in the multiporphyrin arrays on their fluorescence dynamics at the single-molecule level.     

The effects of solute-solvent electrostatic interactions on solvation dynamics in supercritical CO2

We present here the results of molecular-dynamics simulation of solvation dynamics in supercritical CO(2) at a temperature of about 1.05T(c), where T(c) is the critical temperature, and at a series of densities ranging from 0.4 to 2.0 of the critical density rho(c). We focus on electrostatic solvation dynamics, representing the electronic excitation of the chromophore as a change in its charge distribution from a quadrupolar-symmetry ground state to a dipolar excited state. Two perturbations are considered, corresponding to different magnitudes of solute excited-state dipoles, denoted as d5 and d8. The d8 solute is more attractive, leading to a larger enhancement in CO(2) clustering upon solute electronic excitation. This has a large impact on solvation dynamics, especially at densities below rho(c). At these densities, solvation dynamics is much slower for the d8 than for the d5 solute. For both solutes, solvation dynamics becomes faster at densities above rho(c) at which solvent clustering diminishes. We show that the slowest solvation time scale is associated with solvent clustering and we relate it to solute-solvent mutual translational diffusion and the extent of change in effective local density resulting from solute electronic excitation.     

Ultrafast Photophysics of Regioisomeric Triphenylamine Dyes Having Dipolar D-π-A-A and Octupolar D-(π-A-A)3 Character with a Benzothiazole Unit in Matched or Mismatched Orientation

Benzothiazole is among prominent electron-withdrawing heteroarene moieties used in a variety of π-conjugated molecules. Its relative orientation with respect to the principal dipole vector(s) of chromophores derived thereof is crucial, affecting photophysical and nonlinear optical properties. Here we compare the photophysics and ultrafast dynamics of dipolar and octupolar molecules comprising a triphenylamine electron-donating core, ethynylene π-conjugated linker(s) and benzothiazole acceptor(s) having the matched or mismatched orientation (with respect to the direction of intramolecular charge transfer), while a carbaldehyde group is attached as an auxiliary acceptor. Among chromophores without the auxiliary acceptor, stronger fluorescence solvatochromism and faster excited state dynamics are exhibited for the derivatives with the mismatched geometry. On the contrary, introduction of the auxiliary acceptor to the benzothiazole unit enhances the intramolecular charge transfer ICT (featuring ultrafast dynamics of the excited state) for the matched geometry. The data confirm the crucial role of the relative orientation of asymmetric heteroaromatic unit (regioisomeric effect) in dipolar as well as in multipolar molecules in tuning linear and nonlinear optical properties as well as excited state dynamics.     

反相高效液相色谱法分离2-(氟苯基)-5-甲基苯并恶唑和2-(氯苯基)-5-甲基苯并恶唑的位置异构体

基于商品化的普通色谱柱建立了2-(氟苯基)-5-甲基苯并恶唑和2-(氯苯基)-5-甲基苯并恶唑邻、间、对位置异构体的分离检测方法。色谱柱为Inertsil ODS-SP C₁₈(250 mm×4.6 mm, 5 μm),以乙腈(A)和水(B)为流动相,在60%A~80%A间线性梯度洗脱15 min,流速为1.5 mL/min,柱温40℃,检测波长为310 nm。在质量浓度为2~200 mg/L范围内,2-(氟苯基)-5-甲基苯并恶唑邻、间、对位的异构体、2-(氯苯基)-5-甲基苯并恶唑邻、间、对位的异构体具有良好的线性关系,6种化合物的检出限(S/N=3)依次为0.0307、0.0293、0.0315、0.0226、0.0237、0.0226 mg/L。该法既为5-甲基苯并恶唑与氟苯或氯苯碳氢活化偶联反应制备的异构体混合物提供了一个快速检测的方法,又为2-芳基苯并恶唑类异构体的分离检测提供了参考。     

Ab initio description of photoabsorption and electron transfer in a doubly-linked porphyrin-fullerene dyad

Structure, photoabsorption and excited states of two representative conformations obtained from molecular dynamics (MD) simulations of a doubly-linked porphyrin-fullerene dyad DHD6ee are studied by using both DFT and wavefunction based methods. Charge transfer from the donor (porphyrin) to the acceptor (fullerene) and the relaxation of the excited state are of special interest. The results obtained with LDA, GGA, and hybrid functionals (SVWN, PBE, and B3LYP, respectively) are analyzed with emphasis on the performance of used functionals as well as from the point of view of their comparison with wavefunction based methods (CCS, CIS(D), and CC2). Characteristics of the MD structures are retained in DFT optimization. The relative orientation of porphyrin and fullerene is significantly influencing the MO energies, the charge transfer (CT) in the ground state of the dyad and the excitation of ground state CT complex (g-CTC). At the same time, the excitation to the locally excited state of porphyrin is only little influenced by the orientation or cc distance. TD-DFT underestimates the excitation energy of the CT state, however for some cases (with relatively short donor-acceptor separations), the use of a hybrid functional like B3LYP alleviates the problem. Wavefunction based methods and CC2 in particular appear to overestimate the CT excitation energies but the inclusion of proper solvation models can significantly improve the results.     

Functionalized arylzinc compounds in ethereal solvent: direct synthesis from aryl iodides and zinc powder and application to Pd-catalyzed reaction with allylic halides

Arylzinc compounds, ArZnX, were conveniently prepared in high yields by the reaction of zinc powder with aryl iodides, which contain electron-withdrawing groups such as CO(2)CH(3), CN, Br, or CF(3) at the ortho-, meta- or para-position, or electron-donating groups such as CH(3), OCH(3), or H, at 70 degrees C in THF, at 100 degrees C, or at 130 degrees C in diglyme, respectively. Pd(dba)(2) exhibited an outstanding efficient catalytic effect for the cross-coupling of these ethereal solutions of ArZnX with allylic halides to afford a variety of functionalized allylbenzenes in high yields; the reactions were mostly carried out at 0 degrees C for 5-30 min in the presence of 5 mol % of catalyst. The conversion of the aryl iodides to allylbenzenes via two reactions could be accomplished in one pot.     

Formation of Benzyl Oxazole, A Competitive Path with the Classical Bishler-Napieralski Reaction

1,2,3,4-tetrahydroisoquinoline-3-carboxylic acids (TIC) are of considerable interest due to their biological activity and as an important structural element in several important alkaloids and other medically useful products1. Moreover, TIC is a phenylalanine analogue in which the dihedral angle is limited to a very small range because of its bicyclic nature2. In connection with the design of topographically constrained peptides, TIC has been utilized in several instance as a replacement o…     

Effect of conformational heterogeneity on excitation energy transfer efficiency in directly meso-meso linked Zn(II) porphyrin arrays

We have investigated the overall excitation energy relaxation dynamics in linear porphyrin arrays as well as the energy transport phenomena by attaching an energy acceptor to one end of a linear porphyrin array by using steady state and time-resolved spectroscopic measurements. We have revealed that the solvation dynamics as well as the conformational dynamics contributes significantly to the energy relaxation processes of linear porphyrin arrays. Consequently, long porphyrin arrays no longer serve as good energy transmission elements in donor-acceptor linked systems due to conformational heterogeneities which provide the non-radiative deactivation channels as energy quenchers.     

Effect of configuration and conformation on the spin multiplicity in xylylene type biradicals

The singlet-triplet splitting energy gap ΔEs-T= Es - ET is calculated for the ortho-, meta-, and para-xylylenes and their heteroatomic analogous by means of AM1-CI approach. It is shownthat when the radical centers R(R=H2C- ,H2N - or HN'- ) are twisted sufficiently far out ofconjugation with the benzene ring, ΔEs-T tends to zero or is negative, i.e. ortho-, meta-, and para-phenylenes turn into weak ferromagnetic or antiferromagnetic coupling unit, while they are strong ferromagnetic (meta-isomers) or antiferromagnetic (ortho-, para-isomers) coupling units under planar conformation. It is suggested that serious twisted conformation is not recommended candidate for the design of novel high-spin molecules with stable high-spin ground states by ortho-or para-phenylene coupling unit.     

气相色谱法测定磷酸三甲苯酯异构体含量

建立测定磷酸三甲苯酯临、间、对位异构体含量的气相色谱方法。通过优化色谱分析条件,如降低初始柱箱温度和程序升温速度,可较好分离开TCP的10种同分异构体。选用弱极性的AB–5MS色谱柱,设置柱箱初始温度为120℃,升温速度为3℃/min,终温为300℃。使用相应的纯物质,对其中的临-临-临,间-间-间,对-对-对进行了定量分析。三种异构体在各自的线性范围内均呈良好的线性,线性相关系数大于0.999,检出限为0.162 7~0.168 8 mg/L,测定结果的相对标准偏差小于5%(n=6)。该方法检出限低,重现性好,适合于磷酸三甲苯酯异构体含量的检测。     

Sequence and linker dependent chiral dimerization of DNA–porphyrin conjugates

Circular dichroism (CD), UV–vis absorption, fluorescence, and resonance light scattering (RLS) spectroscopies were used to elucidate the role of the DNA sequence, linkers between DNA and porphyrin, and metal in the porphyrin coordination center on the self-assembly of DNA–porphyrin conjugates. A series of eight non-self-complementary DNA–porphyrin conjugates have been synthesized with zinc and free-base porphyrins covalently attached to the short ODNs (A₈ or T₈) via amide or phosphate linker. A small structural modification (e.g., amide linker replaced by the phosphate linker) showed a dramatic effect on the aggregation properties of DNA–porphyrin conjugates and greatly altered their spectroscopic properties. At low ionic strength, porphyrin aggregation was not observed for any conjugate. An increase in the ionic strength caused two out of eight conjugates to form chiral porphyrin dimers.     

Charge-Transfer in Panchromatic Porphyrin-Tetracyanobuta-1,3-Diene-Donor Conjugates: Switching the Role of Porphyrin in the Charge Separation Process

Using a combination of cycloaddition-retroelectrocyclization reaction, free-base and zinc porphyrins (H₂P and ZnP) are decorated at their β-pyrrole positions with strong charge transfer complexes, viz., tetracyanobuta-1,3-diene (TCBD)-phenothiazine ( 3 and 4 ) or TCBD-aniline ( 7 and 8 ), novel class of push-pull systems. The physico-chemical properties of these compounds (MP-Donor and MP-TCBD-Donor) have been investigated using a range of electrochemical, spectroelectrochemical, DFT as well as steady-state and time-resolved spectroscopic techniques. Ground-state charge transfer interactions between the porphyrin and the electron-withdrawing TCBD directly attached to the porphyrin π-system extended the absorption features well into the near-infrared region. To visualize the photo-events, energy level diagrams with the help of free-energy calculations have been established. Switching the role of porphyrin from the initial electron acceptor to electron donor was possible to envision. Occurrence of photoinduced charge separation has been established by complementary transient absorption spectral studies followed by global and target data analyses. Better charge stabilization in H₂P derived over ZnP derived conjugates, and in phenothiazine derived over aniline derived conjugates has been possible to establish. These findings highlight the importance of the nature of porphyrins and second electron donor in governing the ground and excited state charge transfer events in closely positioned donor-acceptor conjugates.