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DFT calculations were performed on covalently linked porphyrin–quinone and porphyrin–hydroquinone dyads, and quinhydrone (1:1 quinone–hydroquinone) paired self-assembled supramolecular complexes. The frontier HOMO's and LUMO's of the B3LYP/3-21G(*) and PBEPBE/6-31G(d) optimized structures in the ground state were utilized to probe the site of electron transfer (porphyrin or quinone centered redox reactions) and the results were compared to the experimentally determined site of electron transfer. These studies revealed good agreement between the experimental results and computational predictions. The computed HOMO–LUMO energy gap also followed the trends of the electrochemical results. In the optimized structure of quinhydrone supramolecules, the quinone–hydroquinone entities were stacked on one another with an inter-planar distance of ∼3.1 A. The calculated O–H–O distances suggested the presence of hydrogen bonds. The prediction of the structure and site of electron transfer suggests the usefulness of DFT calculations for probing the self-assembly process and redox behavior of structurally complex molecular and supramolecular systems. The B3LYP/3-21G(*) calculated association energy (the energy of the quinhydrone complex relative to the sum of the energies of the individual entities) for the quinhydrone formation was found to range between −7 and −15 kcal/mol, suggesting quinhydrone as a viable self-assembly mechanism to form supramolecular complexes. The B3LYP/3-21G(*) results were further improved by the PBEPBE/6-31G* method with solvation. Calculations performed at larger basis sets including 6-31G*, 6-31+G*, 6-31++G*, 6-311G*, 6-311G (d,p), and 6-311G (2df, dp) either at HF, BLYP, B3LYP, B3PW91, PBEPBE and MPW1PW91 methods also predicted negative association energies for the simple quinone:hydroquinone (quinhydrone) formation, with smaller association energies at the higher basis sets presumably due to reduced BSSE. Adding the basis set superposition error (BSSE), computed by counterpoise method, over-corrected the B3LYP/3-21G(*) association energy, but adding 50% of BSSE converged rapidly to the large basis set result. Larger association energies were obtained for the porphyrin–quinhydrone assemblies using PBEPBE/6-31G* calculations, presumably due to larger intermolecular ‘dispersion’ forces predicted with the PBEPBE method.

The first example of a porphyrin-quinonoid donor–acceptor triad featuring (tetraphenylporphinato)zinc(II) moieties covalently attached to an oxoporphyrinogen through its macrocyclic nitrogen atoms is reported. This arrangement of chromophores results in an interesting interplay between the electron-donating zinc–porphyrin(s) and the electron/energy accepting oxoporphyrinogen. The optical absorption of the triad reveals features corresponding to both the donor and acceptor entities. The geometry and electronic structure of the triad deduced from B3LYP/3-21G(*) calculations reveal an absence of inter-chromophoric interactions and localization of the HOMO on one zinc–porphyrin group and the LUMO on the oxoporphyrinogen scaffold. The electrochemical redox states of the triad were established from a comparative electrochemistry of the triad and the reference compounds. Both steady-state and time-resolved emission studies revealed quenching of the singlet excited state of zinc–porphyrin in the triad, and the free-energy calculations performed using Weller’s approach indicate the possibility of electron transfer from the singlet excited zinc–porphyrin group to the oxoporphyrinogen in polar solvents. Time-resolved fluorescence studies reveal excited state energy transfer from zinc–porphyrin to oxoporphyrinogen in nonpolar solvents, while nanosecond transient absorption studies combined with time-resolved fluorescence studies in polar solvents are indicative of the occurrence of photoinduced charge separation from the singlet excited zinc–porphyrin to the oxoporphyrinogen. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Alkylation of 5,10,15,20-tetrakis(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadienylidene)porphyrinogen at its macrocyclic nitrogen atoms results in four multiply N-substituted products, which were isolated and characterized. The electrochemical and spectroelectrochemical properties of these compounds were determined. Molecular structures of the N-alkylated compounds are influenced strongly by intermolecular π-π stacking interaction between naphthyl groups leading to formation of dimers or stacked arrays of the macrocycle depending on multiplicity of substituents. The redox and spectral properties of the compounds are related to the increasing multiplicity of N-substitution. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Spectroscopic, redox, and electron transfer reactions of a self-assembled donor-acceptor dyad formed by axial coordination of magnesium meso-tetraphenylporphyrin (MgTPP) and fulleropyrrolidine appended with an imidazole coordinating ligand (C(60)Im) were investigated. Spectroscopic studies revealed the formation of a 1:1 C(60)Im:MgTPP supramolecular complex, and the anticipated 1:2 complex could not be observed because of the needed large amounts of the axial coordinating ligand. The formation constant, K(1), for the 1:1 complex was found to be (1.5 +/- 0.3) x 10(4) M(-1), suggesting fairly stable complex formation. The geometric and electronic structures of the dyads were probed by ab initio B3LYP/3-21G() methods. The majority of the highest occupied frontier molecular orbital (HOMO) was found to be located on the MgTPP entity, while the lowest unoccupied molecular orbital (LUMO) was on the fullerene entity, suggesting that the charge-separated state of the supramolecular complex is C(60)Im(*-):MgTPP(*+). Redox titrations involving MgTPP and C(60)Im allowed accurate determination of the oxidation and reduction potentials of the donor and acceptor entities in the supramolecular complex. These studies revealed more difficult oxidation, by about 100 mV, for MgTPP in the pentacoordinated C(60)Im:MgTPP compared to pristine MgTPP in o-dichlorobenzene. A total of six one-electron redox processes corresponding to the oxidation and reduction of the zinc porphyrin ring and the reduction of fullerene entities was observed within the accessible potential window of the solvent. The excited state events were monitored by both steady state and time-resolved emission as well as transient absorption techniques. In o-dichlorobenzene, upon coordination of C(60)Im to MgTPP, the main quenching pathway involved electron transfer from the singlet excited MgTPP to the C(60)Im moiety. The rate of forward electron transfer, k(CS), calculated from the picosecond time-resolved emission studies was found to be 1.1 x 10(10) s(-1) with a quantum yield, Phi(CS), of 0.99, indicating fast and efficient charge separation. The rate of charge recombination, k(CR), evaluated from nanosecond transient absorption studies, was found to be 8.3 x 10(7) s(-1). A comparison between k(CS) and k(CR) suggested an excellent opportunity to utilize the charge-separated state for further electron-mediating processes.

Supramolecular triads have been constructed by using covalently linked zinc porphyrin−ferrocene(s) dyads, self-assembled via axial coordination to either pyridine- or imidazole-appended fulleropyrrolidine. These triads were characterized by optical absorption, computational, and electrochemical methods. The calculated binding constants (K) revealed stable complexation and suggested the existence of intermolecular interactions between the ferrocene and fullerene entities. Accordingly, the optimized geometry obtained by ab initio B3LYP/3-21G(*) methods revealed closely spaced ferrocene and fullerene entities in the studied triads. Photoinduced charge-separation and charge-recombination processes were examined in the dyads and triads by means of time-resolved transient absorption and fluorescence lifetime measurements. In the case of zinc porphyrin−ferrocene(s) dyads, upon photoexcitation, efficient (ΦCS = 0.98) to moderate (ΦCS = 0.54) amounts of electron transfer from the ferrocene to the singlet excited z...

Without written forms, signed languages do not permit the type of textual record available to speakers of English and other written languages. Deaf signers have generally relied on the language of the dominant hearing culture for this purpose. Because of their visual-gestural modality, signed languages present a unique set of challenges for developing written forms. These issues are considered from a behavioral perspective, and two sign language notation systems, Stokoe Notation and Sutton SignWriting, are described.

Degeneration of the articulating surfaces and pain associated with temporomandibular joint (TMJ) dysfunction are the primary symptoms of TMJ disorders (TMDs), where normal life activities such as eating, talking, and even sleeping may be drastically impaired [1–3]. To accelerate the discovery of effective therapeutic interventions for the treatment of TMD pain, we have been establishing a novel non-invasive approach for objectively assessing the presence of joint hypersensitivity. Our approach to identify chronic joint pain is based on evidence that all of the etiological factors associated with TMD pain implicate remodeling and degeneration of the joint in response to alterations in motion and loading. The injury model used for this study was a reversible, mechanical model through splint placement on the molars. It is hypothesized that arthrokinematic analysis will identify a specific pattern of functional changes that constitute a signature for the presence of irreversible damage.Copyright © 2011 by ASME

Supramolecular triads composed of fullerene (C60) as primary electron acceptor, zinc porphyrin (ZnP) as primary electron donor, and either a ferrocene (Fc), or N,N-dimethylaminophenyl (DMA), or N,N-diphenylaminophenyl (DPA) entity as a second electron donor were constructed via a ‘two-point’ binding motif involving axial coordination and hydrogen bonding. The B3LYP/3-21G(*) optimized structures revealed disposition of the three entities of the triads in a triangular fashion. The redox behavior of the different components was studied using cyclic voltammetry in o-dichlorobenzene containing 0.1 M (n-C4H9)4NClO4. The oxidation potentials of the second electron donor followed the trend: Fc

Photoinduced electron transfer in intramolecularly interacting free-base porphyrin bearing one or four 18-crown-6 ether units at different positions of the porphyrin macrocycle periphery and pristine fullerene was investigated in polar benzonitrile and nonpolar o-dichlorobenzene and toluene solvents. Owing to the presence of two modes of binding, stable dyads were obtained in which the binding constants, K, were found to range between 4.2 x 10(3) and 10.4 x 10(3) M(-1) from fluorescence quenching data depending upon the location and number of crown ether entities on the porphyrin macrocycle and the solvent. Computational studies using the B3LYP/3-21G() method were employed to arrive at the geometry and electronic structure of the intramolecular dyads. The energetics of the redox states of the dyads were established from cyclic voltammetric studies. Under the intramolecular conditions, both the steady-state and time-resolved emission studies revealed efficient quenching of the singlet excited free-base porphyrin in these dyads, and the measured rates of charge separation, k(CS), were found to be in the 10(8)-10(9) s(-1) range. Nanosecond transient absorption studies were performed to characterize the electron-transfer products and to evaluate the charge-recombination rates. Shifting of the electron-transfer pathway from the intra- to intermolecular route was achieved by complexing potassium ions to the crown ether cavity(ies) in benzonitrile. This cation complexation weakened the intramolecular interactions between fullerene and the crown ether appended free-base porphyrin supramolecules, and under these conditions, intermolecular type interactions were mainly observed. Reversible inter- to intramolecular electron transfer was also accomplished by extracting the potassium ions of the complex with the addition of 18-crown-6. The present study nicely demonstrates the application of supramolecular methodology to control the excited-state electron-transfer path in donor-acceptor dyads.

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.

A covalently linked magnesium porphyrin-fullerene (MgPo-C60) dyad was synthesized and its spectral, electrochemical, molecular orbital, and photophysical properties were investigated and the results were compared to the earlier reported zinc porphyrin-fullerene (ZnPo-C60) dyad. The ab initio B3LYP/3-21G(*) computed geometry and electronic structure of the dyad predicted that the HOMO and LUMO are mainly localized on the MgP and C60 units, respectively. In o-dichlorobenzene containing 0.1 M (n-Bu)4NClO4, the synthesized dyad exhibited six one-electron reversible redox reactions within the potential window of the solvent. The oxidation and reduction potentials of the MgP and C60 units indicate stabilization of the charge-separated state. The emission, monitored by both steady-state and time-resolved techniques, revealed efficient quenching of the singlet excited state of the MgP and C60 units. The quenching pathway of the singlet excited MgP moiety involved energy transfer to the appended C60 moiety, generating the singlet excited C60 moiety, from which subsequent charge-separation occurred. The charge recombination rates, k(CR), evaluated from nanosecond transient absorption studies, were found to be 2-3 orders of magnitude smaller than the charge separation rate, k(CS). In o-dichlorobenzene, the lifetime of the radical ion-pair, MgPo*+-C60*-, was found to be 520 ns which is longer than that of ZnPo*+-C60*- indicating better charge stabilization in MgPo-C60. Additional prolongation of the lifetime of MgPo*+-C60*- was achieved by coordinating nitrogenous axial ligands. The solvent effect in controlling the rates of forward and reverse electron transfer is also investigated.

Two types of structurally well-defined, self-assembled zinc porphyrin-fullerene conjugates were formed by "two-point" binding strategies to probe the effect of axial ligation or pi-pi-type interactions on the photochemical charge stabilization in the supramolecular dyads. To achieve this, meso-tetraphenylporphyrin was functionalized to possess one or four [18]crown-6 moieties at different locations on the porphyrin macrocycle while fullerene was functionalized to possess an alkyl ammonium cation, and a pyridine or phenyl entities. As a result of the crown ether-ammonium cation complexation, and zinc-pyridine coordination or pi-pi-type interactions, stable zinc porphyrin-fullerene conjugates with defined distance and orientation were formed. Evidence for the zinc-pyridine complexation or pi-pi-type interactions was obtained from the spectral and computational studies. Steady-state and time-resolved emission studies revealed efficient quenching of the zinc-porphyrin singlet excited state in these dyads, and the measured rates of charge separation, k(CS) were found to be slightly better in the case of the dyads held by axial coordination and crown ether-cation complexation. Nanosecond transient absorption studies provided evidence for the electron transfer reactions, and these studies also revealed charge stabilization in these dyads. The lifetimes of the radical ion pairs were found to depend upon the type of porphyrins utilized to form the dyads, that is, porphyrin possessing the crown ether moiety at the ortho position of one of the phenyl rings yielded prolonged charge stabilized states. Addition of pyridine to the supramolecular dyads eliminated the zinc-pyridine coordination or pi-pi-type interactions of the "two-point" bound systems due to the formation of a new zinc-pyridine axial bond thus giving a unique opportunity to probe the effect of axial coordination or pi-pi interactions on k(CS) and k(CR). Under these conditions, the measured electron transfer rates revealed faster k(CS) and slower k(CR) as compared to those obtained in the absence of added pyridine. The evaluated lifetimes of the radical ion-pairs were found to be hundreds of nanoseconds and were longer in the presence of pyridine.

The structures and electrochemistry of N-benzylated meso-tetrakis (3,5-di-tert-butyl-4-oxo-cyclohexa-2,5-dienylidene) porphyrinogens have been investigated. Structural determinations reveal the isomeric identity of the products obtained from the N-alkylation of the parent meso-tetra (oxo-cyclohexadienylidene) porphyrinogen. The compounds are subject to increased macrocyclic deformations upon increasing N-substitution culminating in the tetra-N-benzyl derivative, which has a buckling superimposed on the already highly puckered macrocycle. The electrochemical analyses emphasize the electron deficiency of the N-benzylated meso-tetra(oxo-cyclohexadienylidene) porphyrinogens and indicate that they can be considered as quinones conjugated via the unsaturated tetrapyrrolic macrocycle. The N-benzylated compounds studied form stable and well-defined pi-cation radical and pi-anion radical species because of their highly conjugated nature. Ab initio molecular orbital calculations at the B3LYP/3-21G() level confirmed the high degree of conjugation between tetrapyrrole and meso substituents and also gave good agreement between calculated and experimentally determined HOMO-LUMO band gap energies.

The first example of a working model of the photosynthetic antenna-reaction center complex, constructed via self-assembled supramolecular methodology, is reported. For this, a supramolecular triad is assembled by axially coordinating imidazole-appended fulleropyrrolidine to the zinc center of a covalently linked zinc porphyrin-boron dipyrrin dyad. Selective excitation of the boron dipyrrin moiety in the boron dipyrrin-zinc porphyrin dyad resulted in efficient energy transfer (k(ENT)(singlet) = 9.2 x 10(9) s(-)(1); Phi(ENT)(singlet) = 0.83) creating singlet excited zinc porphyrin. Upon forming the supramolecular triad, the excited zinc porphyrin resulted in efficient electron transfer to the coordinated fullerenes, resulting in a charge-separated state (k(cs)(singlet) = 4.7 x 10(9) s(-)(1); Phi(CS)(singlet) = 0.9). The observed energy transfer followed by electron transfer in the present supramolecular triad mimics the events of natural photosynthesis. Here, the boron dipyrrin acts as antenna chlorophyll that absorbs light energy and transports spatially to the photosynthetic reaction center, while the electron transfer from the excited zinc porphyrin to fullerene mimics the primary events of the reaction center where conversion of the electronic excitation energy to chemical energy in the form of charge separation takes place. The important feature of the present model system is its relative "simplicity" because of the utilized supramolecular approach to mimic rather complex "combined antenna-reaction center" events of photosynthesis.

A one-step synthesis of bis-pyrrolidine functionalized fullerene-dibenzo[18]crown-6 conjugate and its metal cation complexation to the crown ether entity dependent redox behavior is reported.

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The cover picture shows an example of an oxoporphyrinogen molecule substituted at macrocyclic nitrogen atoms with zinc–porphyrins (oxoporphyrinogen gets zinc–porphyrin “wings”). The electrochemical and photochemical behaviour of this unprecedented tetrapyrrolic triad has been investigated using physical and computational methods. Evidence for the energy transfer and charge separation between the zinc–porphyrin “wings” and the quinonoid oxoporphyrinogen scaffold is discussed in the article by J. P. Hill, O. Ito, F. D'Souza et al. on p. 595 ff.

The oxidation state of the phenol-substituted porphyrin TDtBHPP is coupled with its structure so that its 2-electron oxidation leads to a coplanarization of the molecule and a substantial stabilization of its surface self-assembled structures adsorbed at metal substrates.

The cover picture shows the X-ray crystal structures of the compounds obtained by the alkylation of a conjugated porphyrinogen precursor at its macrocyclic nitrogen atoms with 2-(methylenenaphthyl) groups. The presence of the N-alkyl groups introduces intermolecular π−π stacking interactions which culminate in a 1-dimensional stacked array for the fully N-substituted derivative. The N-alkylation enhances the ability of the compounds to form anion radical and cation radical species and permits “tuning” of the electrochemical properties of the core porphyrinogen. Details are discussed in the article by J. P. Hill et al. on p. 2893 ff.