Multiporphyrin Systems Research Articles

Weak Affinity Chromatography Using Organic Solvents on Cyanopropyl- and Arene-Modified Silica-Gel Columns—Structural Demands for Host and Guest Moieties and Effect of Solvents

Abstract Weak affinity chromatography (WAC) is effective for isolating target compounds from analogous compounds with similar functional groups. Previously, a few chromatographic behaviors based on WAC using organic solvents were observed in a series of cyclic multiporphyrin systems on cyanopropyl-modified silica gel (CN-MS). Here, three cyclic porphyrin trimers with various rigidity were examined on CN-MS to understand the mechanism of the specific interactions between porphyrin derivatives and functional groups on modified silica gel. In addition to CN-MS, six modified silica-gel columns were tested to compare their retention abilities for a cyclic nickel porphyrin dimer (C4Ni2MsCP2). We examined the cosolvent effects of the pyridine eluents for C4Ni2MsCP2. Apparent dissociation constants of C4Ni2MsCP2 with functional groups on the MS columns and effective amounts of the functional groups were estimated by frontal affinity chromatography (FAC). 1H NMR titrations of acetonitrile and nitrobenzene to C4Ni2MsCP2 were conducted to compare their association constants with movable guest molecules to the dissociation constants with immobilized functional groups obtained in FAC. We found rigidity of cyclic porphyrin derivatives and immobilization of functional groups on silica gel is necessary for significant retentions using WAC. The affinity interaction does not occur at the center of C4Ni2MsCP2, but probably occurs on the surface composed of a bipyridyl moiety and the adjacent edges of the two porphyrins. Polar solvents, such as nitrobenzene, acetonitrile, and methanol, weakened the interaction. Although C4Ni2MsCP2 dissolves well in chloroform, the interactions between C4Ni2MsCP2 and the MS columns are considerably strengthened in the presence of chloroform. The competitiveness of solvents and cosolvents with the interaction of the porphyrin on WAC is independent of the solubility of the analyte.

Quantum nanomagnets in on-surface metal-free porphyrin chains

Unlike classic spins, quantum magnets are spin systems that interact via the exchange interaction and exhibit collective quantum behaviours, such as fractional excitations. Molecular magnetism often stems from d/f-transition metals, but their spin-orbit coupling and crystal field induce a significant magnetic anisotropy, breaking the rotation symmetry of quantum spins. Thus, it is of great importance to build quantum nanomagnets in metal-free systems. Here we have synthesized individual quantum nanomagnets based on metal-free multi-porphyrin systems. Covalent chains of two to five porphyrins were first prepared on Au(111) under ultrahigh vacuum, and hydrogen atoms were then removed from selected carbons using the tip of a scanning tunnelling microscope. The conversion of specific porphyrin units to their radical or biradical state enabled the tuning of intra- and inter-porphyrin magnetic coupling. Characterization of the collective magnetic properties of the resulting chains showed that the constructed S = 1/2 antiferromagnets display a gapped excitation, whereas the S = 1 antiferromagnets exhibit distinct end states between even- and odd-numbered spin chains, consistent with Heisenberg model calculations.

1,1'-Bis(diphenylphosphino)ferrocene Platinum(II) Complexes as a Route to Functionalized Multiporphyrin Systems.

In this study, the cationic complex [PtMe(Me2SO)(dppf)]CF3SO3 (PtFc) (dppf = 1,1′-bis(diphenylphosphino)ferrocene) was exploited as a precursor to functionalize the multi-chromophoric system hexakis(pyridyl-porphyrinato)benzene (1). The final adduct [PtFc]18-1, containing eighteen platinum(II) organometallic [PtMe(dppf)] fragments, was prepared and characterized through UV/Vis absorption, 31P{1H}-NMR spectroscopy, and fluorescence emission. UV/vis and fluorescence titrations confirmed the coordination between the platinum(II) center and all the pyridyl moieties of the peripheral substituent groups of the porphyrin. The drop casting of diluted dichloromethane solution of [PtFc]18-1 onto a glass surface afford micrometer-sized emissive porphyrin rings.

Bis-Porphyrin Structural Motiff for Chirality and Molecular Sensing

Bis-porphyrin structures are widely used as model systems to mimic various natural processes and have numerous practical applications in the research and technoligical fields such as polymer and nano science, light harvesting and nonlinear optics, chemical sensors and catalysis, molecular and chiral recognition, various supramolecular and chiroptical devices, chirogenesis and absolute configuration determination. Several years ago, we have discovered that a simple ethane-bridged bis-porphyrin structural motif (M = Zn, Mg, Cu, Co, 2H), can be effectively served for different application purposes owing to the specific molecular and supramolecular functionalities. In particular, the unique functional property of this bis-porphyrin host is based upon the structural semi-flexibility of covalent linkage between two porphyrin unitsresulting in the environmentally assisted syn-anti conformational switching and tweezer formation depending upon the guest structure, which can be effectively employed in different sensing areas, such as chirogenic probes, chiral recognitions, and chemical sensors .1-13 Further developments and prospects toward new chiral and achiral bis- and multi porphyrin systems on the basis of heterometallic bis- and multi-porphyrin structures will be discussed. Borovkov, Symmetry 2014, 6, 256. Borovkov, Symmetry 2010, 2, 184. Borovkov, Y. Inoue, Eur. J. Org. Chem. 2009, 189. A. Hembury, V. V. Borovkov, Y. Inoue, Chem. Rev. 2008, 108, 1. V. Borovkov, Y. Inoue, Top. Curr. Chem. 2006, 265, 89. V. Borovkov, G. A. Hembury, Y. Inoue, Acc. Chem. Res. 2004, 37, 449.Giancane, V. Borovkov, Y. Inoue, L. Valli, J. Colloid Interface Sci. 2012, 385, 282.Giancane, V. Borovkov, Y. Inoue, S. Conoci, L. Valli,Soft Matter 2013, 9, 2302.Bettini, E. Maglie, R. Pagano, V. Borovkov, Y. Inoue, L. Valli, G. Giancane, Beilstein J. Nanotechnol. 2015, 6, 2154.Colombelli, M.G. Manera, V. Borovkov, G. Giancane, L. Valli, R. Rella, Sensor Actuat. B-Chem. 2017, 246, 1039. Buccolieri, D. Manno, A. Serra, A. Santino, M. Hasan, V. Borovkov, G. Giancane,Sensor Actuat. B-Chem. 2018, 257, 685.Buccolieri, M. Hasan, S. Bettini, V. Bonfrate, L. Salvatore, A.Santino, V. Borovkov, G.,Giancane, Anal. Chem. 2018, 90, 6952.Bettini, R. Pagano, V. Borovkov, G. Giancane , L. Valli, J. Colloid Interface Sci. 2019, 533, 762-770.

Chirality and spatially pre-organized multi-porphyrinoids

We report herein that chiral and enantiopure compounds such nucleosides and peptides can pre-organize multi-porphyrinic systems and influence their properties. The first example given concerns star-shaped mutli-porphyrins with chiral and enantiopure nucleosidic linkers. If the configuration is indeed a star-shaped nanomolecule, it appears that the induced conformation is nothing as expected. The four peripheral Zn(II) porphyrins collapse over the free-base central one, inducing totally different photo-physical properties. Despite a minor expected light energy harvesting behavior, the principal capability of this system is to quench the collected light energy and convert it from radiative to non-radiative de-activation. The second example concerns polypeptides with pendant porphyrins. The peptidic backbone confers to the systems, after a certain degree of oligomerization, a 3[Formula: see text] right handed helical conformation which induces cavities within the multi-porphyrinc architecture, ready to welcome guests and render, for example, the complexation of C[Formula: see text] much easier. We thus have constructed novel organic photovoltaic systems using supramolecular complexes of porphyrin–peptide oligomers with fullerene clusters. The composite cluster OTE/SnO[Formula: see text] electrode prepared with (P(ZnP)[Formula: see text] C[Formula: see text], exhibits an impressive incident photon-to-photocurrent efficiency (IPCE) with values reaching as high as 56%. The power conversion efficiency of the (P(H[Formula: see text]P)[Formula: see text] C[Formula: see text] modified electrode reaches 1.6%, which is 40 times higher than the value (0.043%) of the porphyrin monomer (P(H[Formula: see text]P)[Formula: see text] [Formula: see text] C[Formula: see text] modified electrode. Thus, the organization approach between porphyrins and fullerenes with polypeptide structures is promising, and may make it possible to further improve the light energy conversion properties by using a larger number of porphyrins in a polypeptide unit.

Supramolecular Chirogenesis in Bis- and Multi-Porphyrin Systems: Recent Developments and Further Perspectives

Supramolecular chirogenesis is a fundamentally novel interdisciplinary field of research being a smart combination of supramolecular/nano chemistry and chiral science. Supramolecular chirogenesis deals with the phenomenon of “symmetry breaking” in achiral multi- or unimolecular host systems by chiral guests (and vice versa), and is achieved by the rational application of specific non-covalent interactions via a chirality information transfer mechanism. Recently, we have discovered a novel and very efficient supramolecular chirogenic system based upon an achiral ethane-bridged bis(metallo- or free base octaethylporphyrin).1-6 Upon interaction with the corresponding chiral guests, there are ligand induced structural deformations in the achiral host, resulting in a asymmetric orientation of the two porphyrin units and subsequent pronounced and easily detectable spectral changes monitored by various spectroscopies. Particularly, through-space exciton coupling between the porphyrin electronic transitions induces bisignate Cotton effects in the circular dichroism (CD) spectra. Significantly, the sign of the observed CD couplet is unequivocally correlated with the absolute configuration of the chiral ligand used, allowing us to develop an absolute configuration sensor for enantiopure amines, alcohols, acids. This system possesses a high degree of chiroptical activity, which allows the differentiation of one of the smallest homologous elements of organic chemistry, i.e. the methylene subunit, such as between methyl and ethyl groups attached to the asymmetric carbon; further, it is able to sense a remote chiral center at a position beta to the binding group. It was also found that this system is highly sensitive to various internal and external controlling factors such as the structure of guest molecules, stoichiometry, number of binding sites, solvent, temperature and phase transition. Further developments and prospects toward new chirogenic systems on the basis of heterometallic bis- and multi-porphyrin structures will be discussed. V. Borovkov, Symmetry 2014, 6, 256.V. Borovkov, Symmetry 2010, 2, 184.V. Borovkov, Y. Inoue, Eur. J. Org. Chem. 2009, 189.G. A. Hembury, V. V. Borovkov, Y. Inoue, Chem. Rev. 2008, 108, 1V. V. Borovkov, Y. Inoue, Top. Curr. Chem. 2006, 265, 89.V. V. Borovkov, G. A. Hembury, Y. Inoue, Acc. Chem. Res. 2004, 37, 449.

Design and Synthesis of Porphyrin Oligomers and Porphyrinoids Via Suzuki-Miyaura Reaction

In view of multi-porphyrin arrays and new porphyrinoids, whih has potential applications in optoelectronic devices, sensors, photovoltaic devices, non-linear optical(NLO) materials, photodynamic therapy (PDT) pigments, light-harvesting models, and host-guest chemistry. Both the manipulation of interporphyrinic interaction and design of new structural porphyrinods are often essential to achieve desirable electronic and photophysical properties. Recently, the Osuka group developed a transition metal-catalyzed coupling reaction that has been proven to be very useful to construct multiporphyrinic systems. Till now we have adopted this strategy and prepared many heterocycles bridged β-to-β porphyrin arrays, including direct-bridged β-to-β porphyrin nanorings[ 1 ], earring porphyrins[ 3 ]. The experiment results indicated that the building blocks will remarkably affect the target molecules. As the bridge is thienylene, the main products is cyclic dimer, trimer, when the bridge is pyridylene, there are trace cyclic products are obtained. when the bridge is free base or Zn(II) porphyrin, bigger loop are formed, while Ni(II)porphyrin is used, the products is much simple. As to synthesis of porphyrin nanobarrel, if the porphyrin monomer is free base or Zn(II) porphyrin, no porphyrin barrel is obtained, which means that only ruffled Ni(II)porphyrin monomers can be used to synthesis porphyrin barrels. Synthesis of direct linked porphyrin nanorings is affected by the meso-substituent on porphyrin, when the substituent is aryl, only trimer and twinned pentamer can be prepared, when the substituent is alkyl, tetramer and twinned heptamer can be obtained.The photophysical study revealed that these porphyrin arrays and porphyrinoids exhibit either longer fluorescent lifetime, larger two photon absorption values or Near-infrared absorptions. Some new porphyrin arrays or porphyrinnoids recently have been constructed through Pd-catalyzed cross-coupling reactions (Figure 1), further study on their properties is underway in our lab. Figure 1. Crystals of earring porphyrins and multi-porphyrin arrays

Linearly- and Cross-Conjugated Porphyrin Dimers: Synthesis and Characterization

Acenes consisting of linearly fused benzene rings have been of great interest of study due to their unique electronic properties, in particular, their exceptionally high charge mobility, as well as their close relation to graphene.1-4 Although larger acenes are predicted to have even better electron mobility, larger acenes are not stable and soluble. Much effort has been devoted to the synthesis, stabilization/functionalization and characterization of acenes longer than pentacene in recent years.5-7 The fusion of an acene with two or more chromophores will greatly extend the pi-conjugation, and will likely produce electronically highly interactive multiporphyrinic systems. In this presentation, we present the synthesis and characterization of the pentacene-fused (linearly conjugated) and pentaquinone-fused (cross-conjugated) porphyrin oligomers. These pentacene-fused porphyrins are much more stable than their corresponding pentacene derivatives. The electronic absorption spectra and transit spectroscopy of these porphyrins indicate highly interactive multiple pi-systems. The electronic and optical properties of these i-extended porphyrins were measured using UV-Vis spectroscopy, life-time and steady state fluorescence spectroscopy, and cyclic voltammetry. DFT calculations were also performed for these porphyrins. S. S. Zade, M. Bendikov, J. Phys. Org. Chem. 2012 , 25, 452-461M. Bendikov, F. Wudl, D. F. Perepichka, Chem. Rev. 2004, 104, 4891-4945.Z. Sun, J. S. Wu, Aust. J. Chem. 2011, 64, 519-528J. E. Anthony, Chem. Rev. 2006, 106, 5028-5048J. E. Anthony, Angew. Chem. Int. Ed. 2008, 47, 452-483S. S. Zade, M. Bendikov, J. Phys. Org. Chem. 2012, 25, 452-461J. E. Anthony, A. Facchetti, M. Heeney, S. R. Marder, X. W. Zhan, Adv. Maters 2010, 22, 3876-3892 Figure 1

Towards Electron Transfer Compounds with Rigid Resistor Units

In nature, the antenna chlorophylls in photosynthetic bacteria are arranged as non-covalent macrorings in a spatially defined manner to absorb and transfer solar energy efficiently to the reaction centers. To mimic this, porphyrin assemblies with rigid, well defined geometries are of interest. In the field of biomimetic porphyrins, the strategy for synthetic chemists has become one of designing novel porphyrin scaffolds that exhibit similar arrangements as those found in nature, for use as model compounds in electron transfer studies. For photochemical systems such as these the correct geometry and spatial arrangement of the molecular building block is mandatory, yet this aspect is often neglected in modern tetrapyrrole chemistry. Here we report on the synthesis of tailored multiporphyrin arrays with defined structures and regiochemical arrangement for use in electron transfer studies. This requires the availability of appropriate scaffold molecules and linker units. While many of these are available, there is a notable absence of rigid, non-conjugating units with defined geometry and we have undertaken to develop synthetic chemistry for the use of cubane and triptycene units in this context. The lack of research into aliphatically linked, multiporphyrin systems can be explained by the inherent flexibility of standard sp3 linker units. Cubane, with a virtually inflexible carbon skeleton, overcomes this problem, theoretically allowing it to be used as an isolator in bridged porphyrin systems for electron transfer studies. Furthermore, unlike most linker systems used in organic chemistry, cubane is a saturated hydrocarbon, and thus will also serve as an electronic isolator, preventing through bond communication between substituents, a key goal necessary for electron transfer studies to investigate through space versus through bond ET processes. In addition, the rigid three-dimensional framework of triptycene together with its symmetry, shape persistency and ability to project up to six functional groups in a spatially defined 120° orientation makes it an especially attractive scaffold in modern synthesis. The key goal here is the synthesis of a multichromophoric arrays based on the triptycene scaffold to serve as a key model compound both in light harvesting and energy transfer studies, as well as antenna cores for optical drug delivery systems.

(Invited) Application of Ethane-Bridged Bis-Porphyrin Structural Motif for Effective Supramolecular Chirogenesis, Sensor Development, and Light Harvesting Systems

Porphyrin and multi-porphyrin systems play a vital role in various natural processes and in artificial assemblies resulting in various practical applications in the numerous research and technoligical fields such as catalysis and nonlinear optics, polymer and materials science, light harvesting and different sensors, molecular and chiral recognition, various supramolecular and chiroptical devices, chirogenesis and absolute configuration determination. Recently, it was found that a simple ethane-bridged bis-porphyrin motif, 1 (M = Zn, Mg, Cu, 2H), can be effectively used for various application purposes owing to the specific molecular and supramolecular functionality. In particular, the unique functional property of this bis-porphyrin host is based upon the structural semi-flexibility of ethane linkage resulting in the environmentally assisted syn-to-anti conformational switching (for monodentate guests) and tweezer structure formation (for bidentate guests), which can be effectively employed in different areas.1-10 For example, in the case of monodentate chiral guests there is the unidirectional screw formation in the anti form of 1 (M = Zn, Mg, 2H) upon the corresponding host-guest interaction. The direction of screw governs by the size difference of ligand’s substituents at the chirogenic center resulting in chirality transfer to the bis-porphyrin host and induction of the noticeable (moderate-to-strong) exciton couplet CD signals in the absorption region of porphyrin.1-6 This chirogenic phenomenon was efficiently utilized for determining the absolute configuration of various chiral guests, thus making it possible to apply 1 as effective and universal chirality sensors for different types of organic molecules as in solution and in solid state. To expand further the applicability of these supramolecular systems Langmuir–Schaefer thin-films on the basis of 1 have been prepared and employed as highly selective and sensitive sensors for aromatic amines.7-9 For example, the monometallated copper complex of 1 is able to detect the presence of aniline in aqueous solution at a concentration as low as 1 nM.9 The sensory mechanism includes the corresponding host-guest interaction resulting in the syn-to-anti conformational switching, which can be easily detected by various spectroscopic methods. Another type of functional materials on the basis of the syn-anti flexibility of 1 (M = Zn) was nanoparticles.10 It was found that upon encapsulation into the semiconducting poly (9-vinylcarbazole) (PVK) polymer the conformation is switched to the anti form and the subsequent efficient excited energy transfer (up to 96%) from the PVK host to the energy acceptor, 1, was observed, thus opening further prospects in designing a new class of porphyrin-based functional nanoparticles for the application in effective light harvesting system. Consideration of the 12 Principles of Green Chemistry for supramolecular and chiroptical devices will also be highlighted.11 The authors would like to acknowledge funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement No. 621364 (TUTIC-Green). V. Borovkov, Symmetry 2014, 6, 256.V. Borovkov, Symmetry 2010, 2, 184.V. Borovkov, Y. Inoue, Eur. J. Org. Chem. 2009, 189.G. A. Hembury, V. V. Borovkov, Y. Inoue, Chem. Rev. 2008, 108, 1V. V. Borovkov, Y. Inoue, Top. Curr. Chem. 2006, 265, 89.V. V. Borovkov, G. A. Hembury, Y. Inoue, Acc. Chem. Res. 2004, 37, 449.G. Giancane, V. Borovkov, Y. Inoue, L. Valli, J. Colloid Interface Sci . 2012, 385, 282.G. Giancane, V. Borovkov, Y. Inoue, S. Conoci, L. Valli, Soft Matter 2013, 9, 2302.S. Bettini, E. Maglie, R. Pagano, V. Borovkov, Y. Inoue, L. Valli, G. Giancane, Beilstein J. Nanotechnol. 2015, in press.S. Mandal, S. Bhattacharyya, V. Borovkov, A. Patra, J. Phys. Chem. C 2011, 115, 24029.P. T. Anastas and J. C. Warner, Green Chemistry: Theory and Practice, Oxford University Press, New York, 1998, ch. 1, p. 3.

Electron Spin Relaxation of Hole and Electron Polarons in π-Conjugated Porphyrin Arrays: Spintronic Implications.

Electron spin resonance (ESR) spectroscopic line shape analysis and continuous-wave (CW) progressive microwave power saturation experiments are used to probe the relaxation behavior and the relaxation times of charged excitations (hole and electron polarons) in meso-to-meso ethyne-bridged (porphinato)zinc(II) oligomers (PZnn compounds), which can serve as models for the relevant states generated upon spin injection. The observed ESR line shapes for the PZnn hole polaron ([PZnn](+•)) and electron polaron ([PZnn](-•)) states evolve from Gaussian to more Lorentzian as the oligomer length increases from 1.9 to 7.5 nm, with solution-phase [PZnn](+•) and [PZnn](-•) spin-spin (T2) and spin-lattice (T1) relaxation times at 298 K ranging, respectively, from 40 to 230 ns and 0.2 to 2.3 μs. Notably, these very long relaxation times are preserved in thick films of these species. Because the magnitudes of spin-spin and spin-lattice relaxation times are vital metrics for spin dephasing in quantum computing or for spin-polarized transport in magnetoresistive structures, these results, coupled with the established wire-like transport behavior across metal-dithiol-PZnn-metal junctions, present meso-to-meso ethyne-bridged multiporphyrin systems as leading candidates for ambient-temperature organic spintronic applications.

Synthesis of Highly Fluoroalkyl‐Functionalized Oligoporphyrin Systems

AbstractFour different multiporphyrin systems have been synthesized and characterized. Highly fluoroalkyl‐functionalized porphyrins are the most complex objects so far to have exhibited quantum wave nature. We have functionalized larger oligoporphyrin systems with fluoroalkyl chains to increase their mass and minimize their intermolecular interactions. The to‐some‐extent random substitution of fluorine atoms at the periphery of the oligoporphyrins results in libraries consisting of molecules varying in both the number and spatial distribution of substituents. The mass‐selected individual members of these libraries were designed for quantum interference experiments. To investigate the volatilization nature of the molecules within the library, laser desorption and post‐ionization studies were performed. These studies demonstrated that molecular beams of suitable velocity and ionization cross‐section can be obtained from these libraries. In particular, we present these features for two libraries, based on either a tetrahedrally arranged central porphyrin tetramer or a more planar porphyrin pentamer.

Understanding excitation energy transfer in metalloporphyrin heterodimers with different linkers, bonding structures, and geometries through stimulated X-ray Raman spectroscopy

We present simulations of stimulated X-ray Raman (SXRS) signals from covalently bonded porphyrin heterodimers with different linkers, chemical bonding structures, and geometries. The signals are interpreted in terms of valence electron wavepacket motion. One- and two-color SXRS signals can both mark excitation energy transfer (EET) between the porphyrin monomers. It is shown that the SXRS signals provide a novel window into EET dynamics in multiporphyrin systems, and can be used as a powerful tool to monitor the subtle chemical environment which affects EET.

Cooperative Effect

Cooperative Effect

Structural Factors Determining Photophysical Properties of Directly Linked Zinc(II) Porphyrin Dimers: Linking Position, Dihedral Angle, and Linkage Length

We have investigated the relationship between the photophysical properties and structures of a series of directly linked zinc(II) porphyrin dimers (mmZ2, mbZ2, and bbZ2) using time-resolved spectroscopic measurements and theoretical calculations. Their unique characters such as CT nature and torsional motion are caused by interporphyrin interactions and steric effects, respectively, which can be fully understood in terms of three structural factors: linking position, dihedral angle, and linkage length. The orthogonal geometry and heterolinking of mmZ2 and mbZ2 induce the localized MOs and electron unbalance in the constituent porphyrin units, respectively, and consequently lead to distinct CT characters in spite of their different origin. On the other hand, a small interporphyrin steric hindrance in bbZ2 makes a torsional motion possible around the direct beta-beta' linkage in the excited state, which is correlated with the solvent dependence of the fast S(1) fluorescence decay component. On the basis of this work, we can gain further insight into the effect of individual structural factors on the photophysical properties, which provides a firm basis for further understanding of the photophysical properties mainly determined by the structural factors in multiporphyrin systems.

One- and two-photon absorption of highly conjugated multiporphyrin systems in the two-photon Soret transition region

This study presents a detailed investigation of near-infrared one- and two-photon absorption (TPA) in a series of highly conjugated (porphinato)zinc(II) compounds. The chromophores interrogated include meso-to-meso ethyne-bridged (porphinato)zinc(II) oligomers (PZn(n) species), (porphinato)zinc(II)-spacer-(porphinato)zinc(II) (PZn-Sp-PZn) complexes, PZn(n) structures featuring terminal electron-releasing and -withdrawing substituents, related conjugated arrays in which electron-rich and -poor PZn units alternate, and benchmark PZn monomers. Broadband TPA cross-section measurements were performed ratiometrically using fluorescein as a reference. Superficially, the measurements indicate very large TPA cross-sections (up to approximately 10(4) GM; 1 GM = 1x10(-50) cm(4) s photon(-1)) in the two-photon Soret (or B-band) resonance region. However, a more careful analysis of fluorescence as a function of incident photon flux suggests that significant one-photon absorption is present in the same spectral region for all compounds in the series. TPA cross-sections are extracted for the first time for some of these compounds using a model that includes both one-photon absorption and TPA contributions. Resultant TPA cross-sections are approximately 10 GM. The findings suggest that large TPA cross-sections reported in the Soret resonance region of similar compounds might contain significant contributions from one-photon absorption processes.

Single molecule spectroscopic investigation on various multiporphyrin systems as molecular photonic devices

During the last decade, molecular photonics has been highlighted in the field of materials chemistry, providing the possibility of the construction of photonic devices at individual molecular level. In parallel, well-established single molecule fluorescence spectroscopy enables us to assess the functionality of individual devices in the solid state, as such, to propose a better design for actual utilization. In this Highlight, we discuss our single molecule investigations on the excitation energy transfer processes and the conformational dynamics occurring in various porphyrin arrays with a particular focus on the applicability as photonic materials. For the fabrication of molecular photonic devices in the solid state, the maintenance of structural rigidity is of utmost importance to achieve efficient signal transmission.

The Role of Aromaticity and the π‐Conjugated Framework in Multiporphyrinic Systems as Single‐Molecule Switches

A systematic analysis of electron-transport characteristics for monomer, dimer, and tetramer multiporphyrinic systems is presented, to provide a thorough understanding of the structural dependence of electron transport related to the aromatic nature of the contact structure. Theoretical investigation shows that the electron-transport characteristics can be controlled by manipulating the pi-conjugated framework in the multiporphyrinic systems through the arrangement of the inner hydrogen atoms. The designed pi-conjugated framework assigns the distinct aromaticity on the contact structure, and the large aromatic nature of the contact structure increases conductivity. The feature emerging from this study is that the aromaticity and pi-conjugated framework are important factors that control the electron-transport characteristics in molecular-scale electronic devices, such as single-molecule switches.

Excitonic Coupling in Covalently Linked Multiporphyrin Systems by Matrix Diagonalization

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Ru(ii) and Rh(iii) porphyrin complexes of primary phosphine-substituted porphyrinsDedicated to the memory of Bhaskar G. Maiya.

Primary alkynyl phosphine porphyrins were prepared by AlHCl2 reduction of the corresponding alkynyl phosphonates. Dephosphorylation of the alkyne proved to be a major side reaction. Using LiAlH4 as reducing agent, the alkyne was found to be partially reduced to give the trans-alkenyl phosphine selectively. The primary phosphines coordinate to both ruthenium(II) and rhodium(III) porphyrins and readily form bis-phosphine complexes. The 1H and 31P NMR spectra for the ruthenium complexes show a pattern characteristic of an [AX2]2 spin system with an unusually large 2JPP coupling constant of 620.6 Hz. The IR spectrum of the complex (PAPH2)Ru(CO)(porphyrin) (PAPH2 = phenylacetylenephosphine) indicates weak σ-donor properties of the ligand. In contrast to the corresponding tertiary phosphine complexes, the bis-phosphine complexes with both ruthenium(II) and rhodium(III) porphyrins are more stable than the mono-phosphine complexes, as judged by NMR spectroscopy, and they can also be detected in the gas phase by LDI-TOF MS. In all cases the complexes could not be isolated and they degrade within hours at ambient temperatures when kept in solution. These compounds may therefore not be suitable for the construction of larger multiporphyrin systems, but their accessibility makes it possible to study their coordination behaviour with other transition metals.