Pi-electron Delocalization Research Articles

η5-1,2,3-Trisilacyclopentadienyl - A Ligand for Transition Metal Complexes: Rhodium Half-Sandwich and Ruthenium Sandwich

The lithium salt of 1,2,3-trisilacyclopentadienide [eta(5)-((t)Bu(2)MeSi)(3)Si(3)C(2)Et(2)](-) 1(-)*[Li(+)(thf)] reacted with an equivalent amount of Rh(CO)(2)(acac) in toluene to form the half-sandwich (trisilacyclopentadienyl)dicarbonylrhodium complex [eta(5)-((t)Bu(2)MeSi)(3)Si(3)C(2)Et(2)]Rh(CO)(2) 2. Pi-electron delocalization in the trisilacyclopentadienyl ring in 2 was manifested in the shielding of the ring atoms upon complexation, as well as by the observation of (1)J(Rh-C) and (1)J(Rh-Si) coupling constants. In the solid state, cyclic delocalization in 2 was seen in the diagnostic eta(5)-coordination of the heavy cyclopentadienyl ring to the Rh center and the remarkable flattening of the Si(3)C(2) five-membered ring. Reaction of 1(-)*[Li(+)(thf)] with 0.25 equiv of [Cp*RuCl](4) (Cp* = eta(5)-C(5)Me(5)) resulted in the formation of the sandwich complex Cp*Ru[eta(5)-((t)Bu(2)MeSi)(3)Si(3)C(2)Et(2)] 3 as the heavy analogue of ruthenocene.

First-Principles Study of Water Chains Encapsulated in Single-Walled Carbon Nanotube

Water molecules confined inside a single-walled (6, 6) carbon nanotube were investigated using density functional theory. In this narrow-sized carbon nanotube (of about 0.8 nm in diameter), the encapsulated water molecules form chain-like configurations via hydrogen bonding. As compared to the water chains in vacuum, the intramolecular charge transfer in the encapsulated water chain is enhanced and the dipole moment is reduced due to the screening effect of the carbon nanotube. The tube-molecule interaction characterized by the coupling energy is about 0.28 eV per water molecule by local density approximation and 0.1 eV by general gradient approximation; the latter one is close to the results by empirical potentials. Weak coupling between the molecular orbitals of the encapsulated water molecules and the delocalized pi electrons from the carbon nanotube was observed, implying that the tube-water interaction is not a simple effect of geometry confinement. Vibrational analysis revealed some unique hydrogen-bond modes for the water chains as well as red shift of the O-H stretching modes for the encapsulated water molecules with regard to the vacuum frequencies due to the tube-water interaction.

Synthesis, Characterization, and Electro‐Optical Properties of ZnIIComplexes with π‐Conjugated Terpyridine Ligands

A series of nine zinc(II) complexes containing substituted 4'-phenyl-2,2':6',2''-terpyridines as ligands is synthesized and fully characterized. The ground-state structures of four examples are calculated by means of DFT and their structural features are confirmed by experimental Raman spectroscopy. Special focus is placed on the degree of pi-electron delocalization between the terpyridine unit and the attached phenyl moiety. Applying Bader's quantum theory of atoms in molecules (QTAIM) and visualizing the electron-density distribution by intermolecular Deltarho plots reveals an increase in ellipticity-and therefore pi-electron delocalization-for phenylvinyl-substituted derivatives compared to phenylethynyl-substituted ones. Experimentally, this is verified by spectroscopic means, because an increase in ellipticity goes along with a pronounced decrease of the HOMO-LUMO energy band gap. Overall, the lateral pi-conjugated substituents are found to strongly influence the electro-optical properties of the complexes. In solution, the color of emission can be modulated from violet to cyan (425-487 nm) and high quantum yields (Phi(PL) up to 0.60) are observed. Thin solid films of the complexes in a matrix of poly(methyl methacrylate) have been inkjet-printed, and their photophysical behavior (bright emission, Phi(PL) up to 0.30) reveals their potential as new emissive materials for applications in light-emitting devices.

Enhanced Electron‐Transfer Properties of Cofacial Porphyrin Dimers through π–π Interactions

pi-pi assisted: Photoinduced electron transfer from cofacial porphyrin dimers to electron acceptors is prominently accelerated, whereas the back electron transfer is decelerated, relative to the corresponding porphyrin monomer (see figure).The radical cation of zinc tetrapentylporphyrin is dimerized with an excess of the neutral counterpart to form the dimer radical cation in which the unpaired electron is delocalized over both porphyrin rings. The dimeric radical cation exhibits an NIR absorption spectrum characteristic of weak pi-bond formation between the porphyrin rings. When cofacial porphyrin dimers, linked by different spacers, are oxidized such pi-bond formation between the porphyrin rings is also recognized in cyclic voltammetry, and Vis/NIR and ESR spectroscopic measurements. The dynamics of photoinduced electron transfer from the triplet excited states of cofacial porphyrin dimers to a series of electron acceptors were investigated by using laser flash photolysis measurements and compared with the porphyrin monomer. The rates of photoinduced electron-transfer reactions of cofacial porphyrin dimers are prominently accelerated compared with the reference monomer. The driving-force dependence of the rate constants of photoinduced electron-transfer reactions was analyzed in light of the Marcus theory of electron transfer to afford the reorganization energies of electron transfer (lambda). The lambda values of cofacial porphyrin dimers are significantly smaller than those of the porphyrin monomer when compared at the same driving force of the photoinduced electron transfer. The lambda values increase linearly with an increase in the driving force of the photoinduced electron transfer. This is accompanied by an increase in the distance between electron donor and acceptor molecules, where the electron transfer occurs. The enhanced electron-transfer properties of cofacial porphyrin dimers, in relation with the important role of the special pair in the photosynthetic reaction center, result from the smaller reorganization energy (lambda) together with the larger driving force of the photoinduced electron transfer due to the pi-electron delocalization in the dimer radical cations.

Functionalized ferrocenes and ferroceniums: synthesis, crystal structures and electrochemical properties based on carbazole/phenothiazine-ferrocene conjugated molecules

Four new D-pi-D (or D-pi-D-pi-D) complexes 9-ethyl-3-E-((1-ferrocenyl)vinyl)-carbazole (1), 9-ethyl-3,6-E,E-((1,1-diferrocenyl)vinyl)- carbazole(2), 10-ethyl-3-E-((1-ferrocenyl)vinyl)-phenothiazine (3), 10-ethyl- 3,7-E,E- ((1,1-diferrocenyl)vinyl)-phenothiazine (4), have been obtained by solid-phase Wittig reactions and fully characterized. The four complexes were treated with iodine leading to four corresponding [D-pi-A](+) I(3)(-) ferrocenium triiodides (+)I(3)(-) (5), (+)I(3)(-) (6), (+)I(3)(-) (7) and 4(+)I(3)(-) (8), respectively. The results of single crystal X-ray diffraction analysis show that and display better coplanarity between the Fc and carbazole subunits than that between the phenothiazine and ferrocene moieties, which leads to a better pi-electron delocalization. Both the poor pi-electron in [D-pi-A](+) ferroceniums and the heavy iodine atom have a negative effect on the electrochemical properties. The complexes 1-8 undergo multi-step redox processes and show lower oxidation potentials compared with those of ferrocene. Density functional theory (DFT) calculations are performed and the experimental redox properties of the complexes are studied. The eight complexes show MLCT and pi-pi transitions in the UV-visible range in solution, which have been verified by TD-DFT theoretical calculations. In all cases the highest-lying occupied molecular orbitals of the eight complexes are mainly localized on the Fe d orbitals and show a greater Fe proportion than that of alkyl in the complexes, whereas the LUMOs and LUMO+1 are mainly from the pi orbitals of the conjugated vinyl carbazole or phenothiazine unit.

Non-coplanar 9,9-diphenyl-substituted oligofluorenes with large two-photon absorption enhancement

The non-coplanar substituted phenyl rings at the 9-position of a fluorenyl unit can be involved in pi-electron delocalization in which the resulting oligofluorenes exhibit a very large enhancement of two-photon absorption cross sections up to 2559 GM at 710 nm.

Intramolecular dimer radical anions of [3n] cyclophanes: transannular distance dependent stabilization energy

Intramolecular dimer radical anions of [3n]cyclophanes generated by gamma-ray irradiation in low-temperature glassy matrices showed an absorption band attributable to charge resonance around 936-1210 nm, from which the stabilization energy of the dimer radical anions were analyzed on the basis of pi-electron delocalization over two benzene rings and the damping factor (beta) was estimated to be 0.35 A(-1), which is smaller than that of the intramolecular dimer radical cation of [3n]cyclophanes, indicating a effective charge delocalization of the dimer radical anions.

Dual Excited-State Intramolecular Proton Transfer Reaction in 3-Hydroxy-2-(pyridin-2-yl)-4H-chromen-4-one

The synthesis, characterization and fundamental of the dual excited-state proton-transfer properties of 3-hydroxy-2-(pyridin-2-yl)-4H-chromen-4-one (1a) are reported. In the electronic ground state, there exist two competitive hydrogen bonding (HB) isomers for 1a. Conformer 1a(O) reveals a five-membered ring HB structure between O-H and carbonyl oxygen, while conformer 1a(N) possesses a six-membered ring HB formation between O-H and pyridyl nitrogen. In a single crystal, the X-ray crystallography unveils an exclusive formation of conformer 1a(N). In solution such as CH(2)Cl(2), 1a(O) and 1a(N) are in equilibrium, and their respective absorption chromophores are significantly different due to different degrees of hydrogen-bond induced pi electron delocalization. Upon excitation, both conformers 1a(O) and 1a(N) undergo excited-state intramolecular proton transfer (ESIPT) reaction. Following ESIPT, 1a(O) gives rise to a tautomer emission maximized at 534 nm in CH(2)Cl(2). Conversely, due to dominant radiationless quenching processes the tautomer emission for 1a(N) cannot be obtained with a steady-state manner but can be resolved from time-resolved fluorescence. Time resolved fluorescence estimates an equilibrium constant of 27 +/- 5 in favor of 1a(N) in CH(2)Cl(2). Ultrafast ESIPT also takes place for the unique 1a(N) form in the crystal. Due to the prohibition of quenching processes in the solid state, bright tautomer emission maximized at 540 nm is resolved for 1a(N) (Phi(f) approximately 0.3). The interplay between two HB conformers with on(1a(O))/off(1a(N)) character in tautomer emission may find future applications such as the recognition of organic Lewis acid/base in organic solvents.

Syntheses and Crystal Structures of Fused Thiophenes: [7]Helicene and Double Helicene, a D2-Symmetric Dimer of 3,3′-Bis(dithieno[2,3-b:3′,2′-d]thiophene)

The efficient synthesis of (TMS)(2)-[7]helicene (rac-3) and double helicene, a D(2)-symmetric dimer of 3,3'-bis(dithieno[2,3-b:3',2'-d]thiophene) (rac-4) was developed. The crystal structures of 3 and 4 show both strong intermolecular pi-pi interactions and S...S interactions. UV/vis spectra reveal that both 3 and 4 show significant pi-electron delocalization.

Vibrational Raman optical activity of π‐conjugated helical systems: Hexahelicene and heterohelicenes

Helicenes constitute a special class of molecules combining helical conformation with pi-electron delocalization. These confer to helicenes specific chirooptical properties. In this article, we investigate the vibrational signatures thanks to the simulation of vibrational Raman optical activity (VROA) spectra. For that, four representative helicenes: hexahelicene, tetrathia-[7]-helicene, and its pyrrole and furan analogs have been simulated and interpreted using a recently implemented analytical scheme. Helicenes show intense VROA peaks attributed to their pi-conjugated structure and associated with collective vibrational modes. In hexahelicene, the dominant VROA features are due to vibrational modes involving motions of the carbon skeleton and H-wagging, but the intensity finds its source almost exclusively in the former. In the case of the three heterohelicenes, the previous statement is also verified, and on changing the heteroatoms, similar modes presenting comparable atomic contribution patterns have been highlighted, though the vibrational and electronic properties are modified. Some fingerprints could therefore be associated with the helicity of the system. In particular, in forward spectra, most of the VROA bands are positive for left-handed helicenes. Nevertheless, the spectral patterns are quite complex, and no easy rule-of-thumb could distinguish between the different heterohelicenes. Then, considering the fact that most of the contributions originate from the C atoms (group coupling matrices decomposition), it can be concluded that the major role of the heteroatom is restricted to modifying the geometry and the normal modes. At last, the small impact of the gauge-origin on the calculated spectra using a relatively modest basis set (rDPS:3-21G) is demonstrated here in the case of the tetrathia-[7]-helicene molecule presenting a C(2) symmetry. This further demonstrates the adequacy of this basis set for VROA calculations.

Aromaticity: an ab Initio Evaluation of the Properly Cyclic Delocalization Energy and the π-Delocalization Energy Distortivity of Benzene

A complete active space self-consistent field (CASSCF) calculation of the pi system of a conjugated molecule enables one to define optimal valence pi and pi* molecular orbitals (MOs). One may define from them a set of atom-centered orthogonal pi orbitals, one per carbon atom, and the resulting upper multiplet is used to define the pi-electron delocalization energy. This quantity is confirmed to be slightly distortive, i.e., to prefer bond-alternated geometries. One may also define strongly localized bond MOs corresponding to a Kekule structure and then perturb the associated strongly localized single determinant under the effect of the delocalization between the bonds and of the electronic correlation. The third order of perturbation introduces the contribution of the cyclic circulation of the electrons around the benzene ring, i.e. the aromatic energy contribution. Its value is about 1.5 eV. It is antidistortive, but remains important under bond alternation. The cyclic correlation effects are of minor importance.

How DASPMI Reveals Mitochondrial Membrane Potential: Fluorescence Decay Kinetics and Steady-State Anisotropy in Living Cells

Spectroscopic responses of the potentiometric probe 2-(4-(dimethylamino)styryl)-1-methylpyridinium iodide (DASPMI) were investigated in living cells by means of a time- and space-correlated single photon counting technique. Spatially resolved fluorescence decays from single mitochondria or only a very few organelles of XTH2 cells exhibited three-exponential decay kinetics. Based on DASPMI photophysics in a variety of solvents, these lifetimes were attributed to the fluorescence from the locally excited state, intramolecular charge transfer state, and twisted intramolecular charge transfer state. A considerable variation in lifetimes among mitochondria of different morphologies and within single cells was evident, corresponding to high physiological variations within single cells. Considerable shortening of the short lifetime component (τ1) under a high-membrane-potential condition, such as in the presence of ATP and/or substrate, was similar to quenching and a dramatic decrease of lifetime in polar solvents. Under these conditions τ2 and τ3 increased with decreasing contribution. Inhibiting respiration by cyanide resulted in a notable increase in the mean lifetime and a decrease in mitochondrial fluorescence. Increased DASPMI fluorescence under conditions that elevate the mitochondrial membrane potential has been attributed to uptake according to Nernst distributions, delocalization of π-electrons, quenching processes of the methyl pyridinium moiety, and restricted torsional dynamics at the mitochondrial inner membrane. Accordingly, determination of anisotropy in DASPMI-stained mitochondria in living cells revealed a dependence of anisotropy on the membrane potential. The direct influence of the local electric field on the transition dipole moment of the probe and its torsional dynamics monitor changes in mitochondrial energy status within living cells.

Non‐Resonant z‐Scan Characterization of the Third‐Order Nonlinear Optical Properties of Conjugated Poly(thiophene azines)

The nonlinear optical properties of a functionalized poly(thiophene azine), namely, poly(3,4-didodecylthiophene azine), PAZ, at the optical telecommunication wavelength of 1550 nm are investigated by means of the closed-aperture z-scan technique in both thin films and solutions. Values of chi((3))=(2.4+/-0.4)x10(-13) esu, n(2)=(4.0+/-0.7)x10(-15) cm(2) W(-1), and gamma=(4.5+/-0.7)x10(-34) esu are estimated for the third-order (Kerr) susceptibility, the intensity-dependent refractive index, and the molecular second hyperpolarizability of solution samples, respectively. A very small dependence on the polymer chain length is found. Markedly higher values of (4.4+/-1.1)x10(-11) esu, (6.6+/-1.0)x10(-13) cm(2) W(-1), and (5.0+/-0.8)x10(-33) esu are measured for the corresponding quantities in thick (up to 20 mum) polymer films cast on quartz plates. The enhancement of the NLO responses on going from solution to solid samples is attributed to a partially ordered structure and to the presence of interchain interactions leading to greater pi-electron delocalization in the cast polymer films. The results are compared with those previously obtained by using third-harmonic generation (THG), taking into account that those data were measured under conditions of three-photon resonance, whereas our z-scan measurements are fully off-resonance.

Effect of β-Ring Rotation on the Structures and Vibrational Spectra of β-Carotene: Density Functional Theory Analysis

The effect of beta-ring rotation on the structures and vibrational spectroscopic characteristics of beta-carotene, including infrared (IR) intensities and Raman activities, is analyzed using density functional theory. Two stable isomers having Ci symmetry are obtained. The reversion of bond lengths is ascribed to the hyperconjugation effect. The natural bond orbital (NBO) charge analysis suggests that the NBO charges of C5 can be used to estimate the degree of pi-electron delocalization. These structural variations are used to analyze and assign the vibrational spectra. It is concluded that (a) the similar rotational angle dependencies of nu1 and nu2 frequencies justify the contribution of C=C stretch vibrations to the nu2 mode and explain the same conjugation length dependencies of nu1 and nu2 frequencies in polyenes, (b) the nu1 mode can be assigned to the C=C stretching in the central part of polyene chain, whereas beta-rings play an important role in nu2 and IR1 bands, especially for the all-trans isomer, and (c) the transition dipole moment of the calculated IR1 absorption band is relevant to the conjugation degree and the crossing angle between the eigenvectors of the polyene chain and the C5=C6 stretching vibration. This theoretical analysis, together with our previous Raman spectral experiments, suggests that the C6-C7 bond is easier to be twisted than other parts of beta-carotene molecule and so provides an insight into the structures of carotenoids and the properties of binding sites in carotenoproteins.

Interplay of π-Electron Delocalization and Strain in [n](2,7)Pyrenophanes

The geometries of a series of [n](2,7)pyrenophanes (n = 6-12) were optimized at the B3LYP/6-311G** DFT level. The X-ray crystal structures determined for the [9](2,7)- and [10](2,7)pyrenophanes agreed excellently with the computed structures. The degree of nonplanarity of the pyrene moiety depends on the number of CH2 groups in the aliphatic bridge and, as analyzed theoretically, influences the strain energy and the extent of pi-electron delocalization in the pyrene fragment. Various indices, e.g., the relative aromatic stabilization energies (DeltaASE), magnetic susceptibility exaltations (Lambda), nucleus-independent chemical shifts (NICS), and the harmonic oscillator model of aromaticity (HOMA) were used to quantify the change in aromatic character of the pyrene fragment. DeltaASE and relative Lambda values (with respect to planar pyrene) were evaluated by homodesmotic equations comparing the bent pyrene unit with its bent quinoid dimethylene-substituted analog. The bend angle, alpha, DeltaASE, and Lambda were linearly related. The aromaticity decreases smoothly and regularly over a wide range of bending, but the magnitude of the change is not large. The differences between planar pyrene (alpha = 0 degrees) and the most distorted pyrene unit (alpha = 39.7 degrees in [6](2,7)pyrenophane) are only 15.8 kcal/mol (DeltaASE) and 18.8 cgs-ppm (Lambda). Also, the geometry-based HOMA descriptor changes by only 0.07 unit. The local NICS descriptors of aromatic character also correlate very well with the global indices of aromaticity. In line with the known reactivity of pyrenophanes, the variations of NICS(1), a measure of pi-electron delocalization, were largest for the outer, biphenyl-type rings. The strain energies of the pyrene fragments were much larger and varied more than those evaluated for the bridge. Both strain energies were interrelated (correlation coefficient R = 0.979) and depend on the bend angle, alpha.

Accurate Determination of the Structure of Cyclooctatetraene by Femtosecond Rotational Coherence Spectroscopy and ab Initio Calculations

We combine femtosecond time-resolved rotational coherence spectroscopy with high-level ab initio theory to obtain accurate structural information for the nonpolar antiaromatic molecule 1,3,5,7-cyclooctatetraene (C8H8, COT) and its perdeuterated isotopomer COT-d8 (C8D8). We measure the rotational B0 and centrifugal distortion constants D(J), D(JK) of the v = 0 states of COT and COT-d8 to high accuracy, e.g. B0 (COT) = 2710.329(56) MHz, as well as B(v) for the v = 1 states nu6, nu11, nu17, nu22, and nu41/nu42 of COT. The experimental rotational constants are compared to those obtained from calculations at the coupled-cluster with single, double, and perturbative triples [CCSD(T)] level. The latter also take into account vibrational averaging effects of the ground and vibrationally excited states. Combining the experimental and calculated rotational constants with the calculated equilibrium bond lengths and angles allows us to determine accurate equilibrium structure parameters, e.g., r(e) (C-C) = 147.0 +/- 0.05 pm, r(e) (C=C) = 133.7 +/- 0.1 pm, and r(e) (C-H) = 107.9 +/- 0.1 pm. The equilibrium C-C and C=C bond lengths of COT are compared to those of 1,3-butadiene. The expected effect of decreased pi-electron delocalization due to the twisting of adjacent C=C double bonds in COT relative to butadiene is observed for the C-C bonds but not for the C=C bonds.

Charge transport in nanoscale aromatic and antiaromatic systems

Molecules such as benzene, that have six pi electrons cyclically delocalized, have the specialstability that chemists call aromaticity. When electricity passes through a molecular wirecontaining a benzene ring a quinone-like structure is induced, and there is some loss ofaromaticity. The conductance of such systems correlates with the magnitude of this effect.A molecule such as cyclobutadiene with only four cyclically delocalized pi electrons hasspecial instability, called antiaromaticity. Evidence that such antiaromatic systems couldconvey high conductance in appropriate systems is derived from electrochemical studies,which also detect geometric changes in some thiophene-derived nanolength wires.

One‐Electron‐Reduced and ‐Oxidized Stages of Donor‐Substituted 1,1,4,4‐Tetracyanobuta‐1,3‐dienes of Different Molecular Architectures

A series of monomeric and oligomeric donor-substituted 1,1,4,4-tetracyanobuta-1,3-dienes (TCBDs) with various topologies have been synthesized by means of thermal [2+2] cycloaddition between tetracyanoethylene (TCNE) and donor-substituted alkynes, followed by retro-electrocyclization. One-electron-reduced and -oxidized stages of the donor-substituted TCBDs were generated by chemical methods. The obtained radical anions and radical cations were studied by using electron paramagnetic resonance/electron nuclear double resonance (EPR/ENDOR) spectroscopy, supported by density functional theory (DFT) calculations. The extent of pi-electron delocalization in the paramagnetic species was investigated in terms of the EPR parameters. Despite favorable molecular orbital (MO) coefficients, the EPR results suggest that in radical anions the spin and charge are confined to the electron-withdrawing TCBD moieties on the hyperfine EPR timescale. The observed spin localization is presumably caused by an interplay between the nonplanarity of the studied pi systems, limited pi-electron conjugation, and very likely counterion effects. In radical cations, an analogous spin and charge localization confined to the electron-donating N,N-dialkylaniline moieties was found. In this case, an efficient electron delocalization is disabled by small MO coefficients at the joints between the donor and acceptor portions of the studied TCBDs.

Phosphorus Complexes of N-Fused Porphyrin and Its Reduced Derivatives: New Isomers of Porphyrin Stabilized via Coordination

N-fused isophlorin 3 and its tautomeric phlorin forms 4 and 5, the new constitutional isomers of porphyrin which preserve the basic skeleton of their maternal N-fused porphyrin, have been identified in the course of investigation of phosphorus insertion into N-fused porphyrin 2. N-fused porphyrin reacts with PCl3 in toluene yielding phosphorus(V) N-fused isophlorin 3-P wherein the macrocycle acts as a trianionic tridentate ligand. The identical product has been formed in the reaction of N-confused porphyrin 1 and POCl3 or PCl3. The coordinating environment of phosphorus(V) in 3-P as determined by X-ray crystallography resembles a distorted trigonal pyramid with the nitrogen atoms occupying equatorial positions with the oxygen atom lying at the unique apex. Phosphorus(V) is significantly displaced by 0.732(1) A from the N3 plane. The P-N distances are as follows P-N(22) 1.664(2), P-N(23) 1.645(2), and P-N(24) 1.672(2). All P-N(pyrrolic) bond lengths are markedly shorter than the P-N distances in phosphorus porphyrins. 3-P is susceptible to proton addition at the inner C(9) carbon atom, yielding aromatic 4-P. The modified macrocycle acts as a dianionic ligand and allows the efficient 18 pi-electron delocalization pathway. Two stereoisomers affording the syn (4-P syn) and anti (4-P anti) location of the H(9) atom with respect to the oxygen atom of the PO unit have been identified by (1)H NMR. A regioselective reduction of free base N-fused porphyrin 2 with NaBH4 yielded a nonaromatic isomer of 4, that is, N-fused phlorin 5 due to an addition of a hydride to the C(15) carbon and a proton to one of the pyrrolic nitrogens. The isomer 5 reacts with PCl 3 yielding phosphorus(V) fused isophlorin 3-P. Density functional theory has been applied to model the molecular and electronic structure of porphyrin isomers 3, 4, and 5 and their phosphorus(V) complexes.

Nanotechnology-Based Detection of Explosives and Biological Agents Simulants

Nanotechnology based detection of threat agents, such as explosives and biological agents, has been a top research priority at the Center for Chemical Sensors Development at the Department of Chemistry of the University of Puerto Rico-Mayaguez (UPRM). Nanoparticles are of fundamental interest since they possess unique size-dependent properties are quite different from the bulk state. When a bulk metal is reduced in size, its properties begin to change dramatically because the constituent electrons begin to suffer the effects of quantum confinement. One of these important properties deals with the extraordinary enhancement of the intensities of Raman scattering events in chemical systems called surface enhanced Raman scattering (SERS). Until very recently, only aromatic moieties containing strong chromophores or highly delocalized pi electrons would experience such an enhancement, when in close proximity to a silver or gold nanometallic assembly. In other cases, this SERS condition was not sufficient to satisfy the enhanced Raman scattering requirements because of Coulombic repulsions do not allow an intimate contact with the colloidal suspension of nanoparticles. Recent work in the research group includes optimization of particle size, agglomeration rate and ionic strength of the SERS active aqueous colloidal metallic suspensions. Results have led to extend existing benchmarks limits of detection of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> M to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-8</sup> M (10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> g) in DNT and to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> M (10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-19</sup> g) in the case of TNT. Other works include preparation and testing of bimetallic nano-interalloys: Au/Ag and metallic-semiconductor SERS active colloidal substrates: Ag/TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . Group members have prepared silver and gold nanorods and nanolayers in an effort to change the sensing platform: from aqueous media to solventless detection.