Energetics and structural consequences of axial ligand coordination in nonplanar nickel porphyrins

The effects of ruffling on the axial ligation properties of a series of nickel(II) tetra(alkyl)porphyrins have been investigated with UV-visible absorption spectroscopy, resonance Raman spectroscopy, X-ray crystallography, classical molecular mechanics calculations, and normal-coordinate structural decomposition analysis. For the modestly nonplanar porphyrins, porphyrin ruffling is found to cause a decrease in binding affinity for pyrrolidine and piperidine, mainly caused by a decrease in the binding constant for addition of the first axial ligand; ligand binding is completely inhibited for the more nonplanar porphyrins. The lowered affinity, resulting from the large energies required to expand the core and flatten the porphyrin to accommodate the large high-spin nickel(II) ion, has implications for nickel porphyrin-based molecular devices and the function of heme proteins and methyl-coenzyme M reductase.     

Use of Low Frequency Raman Bands to Identify Non-planar Deformation of Ni(II) Meso-tetraphenylporphyrin Induced by Axial Ligands

It is important to identify non-planar deformations of porphyrin macrocycle in metallo-porphyrin proteins due to their functional relevance. The relationship between non-planar deformations of porphyrin macrocycle and low frequency Raman spectral bands of Ni(II) meso-tetraphenyl porphyrin (NiTPP), with different coordination numbers, was studied by density functional theory (DFT), normal coordinate structural decomposition method and Raman experiments. The results show that the crystal of four-coordinate NiTPP has two major kinds of non-planar deformations: ruffling and saddling. The non-planar deformations of ruffling and saddling for NiTPP are 1.473 ? and 0.493 ? determined by DFT calculation. The ruffling and saddling deformations can be identified by using the low frequency Raman characteristic peaks (\begin{document}$ \gamma_{12} $\end{document}, \begin{document}$ \gamma_{13} $\end{document}) and (\begin{document}$ \gamma_{16} $\end{document}, \begin{document}$ \gamma_{17} $\end{document}), respectively. When four-coordinate NiTPP is transformed to the six-coordinate bis(pyrrolidine) NiTPP (NiTPP(Pyr)\begin{document}$ _2 $\end{document}), the large non-planar distortion of the porphyrin macrocycle almost disappears, with the non-planar deformation of saddling only about 0.213 ? estimated by DFT calculation. Experimentally, we can make use of the characteristic peaks of low frequency Raman spectra to identify the saddling deformation beyond 0.25 ?.     

Origin of the red shifts in the optical absorption bands of nonplanar tetraalkylporphyrins

The view that the large red shifts seen in the UV-visible absorption bands of peripherally crowded nonplanar porphyrins are the result of nonplanar deformations of the macrocycle has recently been challenged by the suggestion that the red shifts arise from substituent-induced changes in the macrocycle bond lengths and bond angles, termed in-plane nuclear reorganization (IPNR). We have analyzed the contributions to the UV-visible band shifts in a series of nickel or zinc meso-tetraalkylporphyrins to establish the origins of the red shifts in these ruffled porphyrins. Structures were obtained using a molecular mechanics force field optimized for porphyrins, and the nonplanar deformations were quantified by using normal-coordinate structural decomposition (NSD). Transition energies were calculated by the INDO/S semiempirical method. These computational studies demonstrate conclusively that the large Soret band red shifts ( approximately 40 nm) seen for very nonplanar meso-tetra(tert-butyl)porphyrin compared to meso-tetra(methyl)porphyrin are primarily the result of nonplanar deformations and not IPNR. Strikingly, nonplanar deformations along the high-frequency 2B(1u) and 3B(1u) normal coordinates of the macrocycle are shown to contribute significantly to the observed red shifts, even though these deformations are an order of magnitude smaller than the observed ruffling (1B(1u)) deformation. Other structural and electronic influences on the UV-visible band shifts are discussed and problems with the recent studies are examined (e.g., the systematic underestimation of the 2B(1u) and 3B(1u) modes in artificially constrained porphyrin structures that leads to a mistaken attribution of the red shift to IPNR). The effect of nonplanar deformations on the UV-visible absorption bands is then probed experimentally with a series of novel bridled nickel chiroporphyrins. In these compounds, the substituent effect is essentially invariant and the amount of nonplanar deformation decreases as the length of the straps connecting adjacent meso-cyclopropyl substituents decreases (the opposite of the effect observed for conventional strapped porphyrins). Several spectroscopic markers for nonplanarity (UV-visible bands, resonance Raman lines, and (1)H NMR resonances) are found to correlate with time-averaged deformations obtained from an NSD analysis of molecular dynamics snapshot structures. These results suggest that UV-visible band shifts of tetrapyrroles in proteins are potentially useful indicators of changes in nonplanarity provided other structural and electronic factors can be eliminated.     

Porphyrin distortion from resonance Raman intensities of out-of-plane modes: Computation and modeling of N-methylmesoporphyrin, a ferrochelatase transition state analog

Resonance Raman spectra of porphyrins are computed with DFT/CIS methodology to monitor out-of-plane distortions. A framework is established for assessing protein-induced distortion of porphyrin bound to a ferrochealatase antibody. Tests on undistorted porphyrins give good agreement with the experimental intensity pattern of in-plane modes for free-base porphine (FBP) and mesorporphyrin IX free base (MP). The computed spectrum of N-methylmesoporphyrin (NMP), in which the methyl-substituted pyrrole ring is tilted 32 degrees from the mean porpyrin plane, also gives reasonable agreement with experiment and reveals activation of out-of-plane (oop) vibrational modes. To model oop distortions systematically, an artificial molecule, FBP-X8, was constructed, in which the H atoms attached to the FBP pyrrole Cb atoms are replaced by heavy substituents, as in physiological porphyrins. Oop mode enhancements are computed for FBP-X8 by displacing it along the canonical distortion coordinates: doming, saddling, ruffling and waving. When FBP-X8 is constrained to the NMP porphyrin geometry, normal coordinate decomposition reveals significant contributions also from modes higher in frequency than the canonical modes, and they contribute importantly to the computed RR intensities. NMP-constrained FBP-X8 gives a reasonable facsimile of the NMP RR spectrum, but better results are obtained with a full computation of MP, constrained in the same way; thus the physiological substituents have significant influence on the RR spectra, over and above their effective masses. Attention focuses on a mode analogous to gamma 15 in 4-fold symmetric porphyrins, which is a kind of saddling mode. This mode is selectively enhanced in NMP-constrained MP; a corresponding RR band is induced upon binding MP to a ferrochelatase antibody.     

Conformational control of oxidation sites, spin states and orbital occupancy in nickel porphyrins

Ni(II) porphyrin π cation radicals are known to undergo an internal electronic isomerization to L₂Ni(III) cations upon complexation with ligands (L). Additional examples of the Ni(II) to Ni(III) conversion are presented for flexible, 'planar' NiOEP (2,3,7,8,12,13,17,18-octaethylporphyrin) and NiT(Pr)P (5,10,15,20-tetra-n-propylporphyrin) in which the Ni(III) orbital occupancy, d _z2 or d _x2-y2, is determined by the ligand field strength of the axial ligands (pyridine, imidazole, or cyanide). In contrast to these results, the nonplanar NiOETPP (2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin), which is easily oxidized because of its saddle-shape, yields a complex postulated to be a high spin Ni(II) π cation radical, based on crystallographic and optical data for (imidazole)2NiOETPP⁺ClO^4-, in which the electron of high spin Ni(II) in the d _x2-y2 orbital is antiferromagnetically coupled to the unpaired electron of the porphyrin radical leaving one electron in the Ni(II) d _z2 orbital, i.e. a pseudo Ni(III). The sterically encumbered, nonplanar NiT(t-Bu)P (5,10,15,20-tetra-tertiary-butylporphyrin) yields Ni(III) complexes when ligated by pyridine, imidazole or cyanide, but in all cases only the Ni(III) d _z2 orbital is occupied as evidenced by EPR spectroscopy. This anomalous chemistry is attributed to the fact that the macrocycle of NiT(t-Bu)P is so sterically constrained that it cannot readily expand to accommodate the longer equatorial Ni—N distances required by population of the d _x2-y2 orbital in Ni(III) or high spin Ni(II). Further support for this postulate derives from NiD(t-Bu)P (5,10-di-tertiary-butylporphyrin) which is less sterically constrained and in which the Ni(III) d _x2-y2 orbital is indeed occupied upon complexation with cyanide. These results thus illustrate the significant effects that the conformations, plasticity or rigidity of Ni porphyrin macrocycles can have on sites of oxidation (metal or porphyrin), spin states (low spin Ni(III) or high spin Ni(II)), and orbital occupancies (d _z2 or d _x2-y2 in Ni(III)).     

On the negligible impact of ruffling on the electronic spectra of porphine, tetramethylporphyrin, and perfluoroalkylporphyrins.

In the first part of this paper, the syntheses, structural characterization, molecular modeling, and electronic spectra for planar and nonplanar perfluoroalkylated porphyrins, (R(f))(4)P's, are reported. These studies demonstrate that the intrinsic substituent effect of the perfluoroalkyl group on the long-wavelength electronic spectrum of porphyrins is substantial, and similar (in magnitude) to that of a phenyl ring. Moreover, it is shown that out-of-plane distortion of (R(f))(4)P's has a negligible impact on their electronic spectra. These data bolster the findings of our earlier work and demonstrate that nonplanarity of (R(f))(4)P's does not result in a red-shift in their optical spectra. In the second part of this paper, time-dependent density functional spectral calculations (B3LYP/6-311G/TD) for porphine, 5,10,15,20-tetrakis(trifluoromethyl)porphyrin, and 5,10,15,20-tetramethylporphyrin in a variety of ruffled conformations are reported. The results of these studies indicate that (1) substantial ruffling of porphyrins has a negligible effect upon their electronic spectra, (2) similarly small effects upon electronic spectra are predicted if electron-withdrawing or electron-releasing groups decorate the porphyrin periphery, (3) for sterically encumbered porphyrins, ruffling can actually result in hypsochromic shifts in various absorption bands, and (4) the bulk of the red-shift commonly thought to be due to nonplanar distortion actually arises from other substituent effects. These results pose serious challenges to previous theoretical and empirical studies that have sought to find a cause-and-effect relationship between nonplanarity and electronic spectra in porphyrins.     

Resonance Raman studies of beta-substituted porphyrin systems with unusual electronic absorption properties.

Zn(II) and Cu(II) porphyrins with beta-conjugated barbiturate functional groups have low-energy electronic transitions which are unusual in that there are two strong bands in the Soret region. Resonance excitation of the two bands shows that each has features characteristic of both the porphyrin and barbiturate groups, with some perturbation to these features caused by the interaction of the two chromophores. The resonance Raman (RR) spectrum (lambda(exc)=413.1 nm) of the 412 nm band shows two bands at 1722 and 1743 cm(-1) attributable to C==O stretches in the substituent. Changes in frequency of porphyrin core modes due to the differing metal centres are reproduced by density functional theory calculations. The Q band RR spectra show modes with anomalous polarization which may be attributed to A(2g) modes, however no overtone or combination bands are observed.     

Nonplanar distortions of bis-base low-spin iron(II)-porphyrinates: absorption and resonance Raman investigations of cross-trans-linked iron(II)-basket-handle porphyrin complexes

Electronic absorption and Soret-excited resonance Raman (RR) spectra are reported for bis-N-alkylimidazole and bis-pyridine complexes of various cross-trans-linked iron(II)-"basket-handle" porphyrins (Fe(II)-BHP) in methylene chloride. These compounds enable us to characterize the spectroscopic properties of ruffled six-coordinated low-spin Fe(II)-porphyrin complexes. The visible absorption spectra show that the Q and B bands are progressively red-shifted when the handles are shortened and/or when the steric hindrance of the axial ligands is increased. This effect is accompanied by both a decrease in RR frequency of the nu(2) mode and an increase in frequency of the nu(8) and nu(s)(Fe-ligand(2)) modes. More precisely, an inverse linear correlation is found between the frequencies of the nu(2) and nu(8) modes. For each ligation state, the positions of the absorption bands are also linearly correlated with the frequency of the nu(2) or nu(8) mode. All of these spectroscopic data reveal that the degree of ruffling of the Fe(II)-BHP complexes is increased by the N-methylimidazole --> pyridine axial substitutions, presumably because the mutual steric strains between the axial ligand rings, the porphyrin macrocycle and the porphyrin handles are increased. The present study provides a first basis for discerning ruffled conformations from planar and other nonplanar structures in ferrous heme proteins.     

The importance of vibronic perturbations in ferrocytochrome c spectra: a reevaluation of spectral properties based on low-temperature optical absorption, resonance Raman, and molecular-dynamics simulations

We have measured and analyzed the low-temperature (T=10 K) absorption spectrum of reduced horse heart and yeast cytochrome c. Both spectra show split and asymmetric Q(0) and Q(upsilon) bands. The spectra were first decomposed into the individual split vibronic sidebands assignable to B(1g) (nu15) and A(2g) (nu19, nu21, and nu22) Herzberg-Teller active modes due to their strong intensity in resonance Raman spectra acquired with Q(0) and Q(upsilon) excitations. The measured band splittings and asymmetries cannot be rationalized solely in terms of electronic perturbations of the heme macrocycle. On the contrary, they clearly point to the importance of considering not only electronic perturbations but vibronic perturbations as well. The former are most likely due to the heterogeneity of the electric field produced by charged side chains in the protein environment, whereas the latter reflect a perturbation potential due to multiple heme-protein interactions, which deform the heme structure in the ground and excited states. Additional information about vibronic perturbations and the associated ground-state deformations are inferred from the depolarization ratios of resonance Raman bands. The results of our analysis indicate that the heme group in yeast cytochrome c is more nonplanar and more distorted along a B(2g) coordinate than in horse heart cytochrome c. This conclusion is supported by normal structural decomposition calculations performed on the heme extracted from molecular-dynamic simulations of the two investigated proteins. Interestingly, the latter are somewhat different from the respective deformations obtained from the x-ray structures.     

Raman and infrared spectral study of meso-sulfonatophenyl substituted porphyrins (TPPSn,n = 1, 2A, 2O, 3, 4)

Raman and IR spectra of the free base p-sulfonatophenyl and phenyl meso-substituted porphyrins [5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS4); 5,10,15-tris(4-sulfonatophenyl)-20-phenyl-porphyrin (TPPS3); 5,10-bis(4-sulfonatophenyl)-15,20-diphenylporphyrin (TPPS2A); 5,15-bis(4-sulfonatophenyl)-10,20-diphenylporphyrin (TPPS2O); and 5-(4-sulfonatophenyl)-10, 15,20-trisphenylporphyrin (TPPS1)] and their N-diprotonated derivatives (porphyrin diacids) were studied. The Raman spectra of the deuterated analogues of these porphyrins, in which the central hydrogen atoms were substituted with deuterium, were also measured. The observed vibrational bands were assigned on the basis of the deuteration shifts and compared with the structural analogues of these compounds. In IR spectra of the free-base porphyrins, the p-sulfonation of phenyl groups results in evident alteration for the phenyl modes and the porphyrin skeleton modes that are strongly coupled with phenyl vibrations. While the p-sulfonation of phenyl groups causes only slight changes for the high-frequency Raman bands (> 850 cm(-1)), dramatic shifts and band splitting were observed in the low-frequency region (< 500 cm(-1)) of Raman spectra. The observed differences of low-frequency Raman spectra were attributed to the alteration of the structure of the porphyrin ring, especially the CalphaCmCalpha bond-angles, by different meso-sulfonatophenyl substitutions. In addition, different packing style of TPPSn molecules in the aggregates is also responsible for the alteration of the vibrational spectra of the aggregated TPPSn.     

Fingerprinting Petroporphyrin Structures with Vibrational Spectroscopy. 4. Resonance Raman Spectra of Nickel(II) Cycloalkanoporphyrins: Structural Effects Due To Exocyclic Ring Size

Nickel(II) complexes of cycloalkanoporphyrins (CAPs) bearing a saturated carbon ring of varying size between pyrrole C(beta) and methine bridge carbon atoms are widespread in crude oil and related organic rich sediments. We have synthesized a series of NiCAPs containing meso,beta-ethano (NiCAP5), meso,beta-propano (NiCAP6), and meso,beta-butano (NiCAP7) groups and applied UV-visible absorption and resonance Raman (RR) spectroscopies to investigate the effects of the exocyclic ring size on the porphyrin structure and to establish vibrational CAP marker frequencies for petroporphyrins in fossil fuels. The RR spectra of NiCAPs, excited at or near porphyrin Soret ( approximately 400 nm) and Q (510-580 nm) bands are informative and display a rich array of skeletal and alkyl substituent modes. High-frequency (1300-1700 cm(-)(1)) structure-sensitive RR bands shift down considerably (up to 24 cm(-)(1)) with increasing size of the exocyclic ring, implicating increased nonplanar distortions of the tetrapyrrole macrocycle. Unlike in other petroporphyrins studied thus far, etio- and tetrahydrobenzoporphyrins, out-of-plane distortions of the porphyrin imposed by the meso,beta-cycloalkano ring are also sufficient to destroy the center of symmetry of the porphyrin pi-system and produce significant enhancement of the IR-active E(u) skeletal modes in the Q-band-excited RR spectra. The UV-visible absorption spectra also vary with the size of the exocyclic ring; both the Soret and Q bands progressively red shift as the cycloalkano chain becomes longer, implying a destabilization of the two highest occupied pi orbitals in NiCAP6 and NiCAP7. In addition, the size of the exocyclic ring in NiCAPs can be readily determined from the frequency of the approximately 900 cm(-)(1) marker band and the characteristic patterns of skeletal and substituent bands in the 700-1200 and nu(4) ( approximately 1380 cm(-)(1)) regions.     

Structural, optical, and photophysical properties of nickel(II) alkylthioporphyrins: insights from experimental and DFT/TDDFT studies

The ground- and excited-state properties of a Ni(II) porphyrin bearing peripheral alkylthio group, NiOMTP (OMTP = 2,3,7,8,12,13,17,18-octakis methylthio porphyrinate) have been investigated by steady-state and time-resolved absorption spectrometry and DFT/TDDFT theoretical methods. Several conformations corresponding to different deformations of the porphyrin core and to different orientations of the alkylthio groups have been theoretically explored. The nearly degenerate, purely ruffled D(2d) and hybrid (ruffled with a modest degree of saddling) D(2) conformations, both characterized by an up-down (ud) orientation of the vicinal methylthio groups are by far the preferred conformations in the "gas phase". In contrast to NiOEP, it is the orientation of the peripheral substituents rather than the type and degree of distortions of the porphyrin core that determines the stability of the NiOMTP conformers. The ground-state electronic absorption spectra of NiOMTP exhibit significant changes compared to its parent NiP and beta-alkylated analogues, such as NiOEP, resulting in a considerable red shift of the B and the Q bands, intensification and broadening of the Q band, and additional weak absorptions in the region between the Q and B bands. These spectral changes can be understood in terms of the electronic effects of the methylthio groups with nonplanar distortions of the porphyrin ring playing a very minor role. Transient absorption measurements with sub-picosecond resolution performed in toluene and TDDFT calculations reveal that following photoexcitation, NiOMTP deactivates by the pathway 1(pi,pi) --> 3(d(z2),d(x2-y2))--> ground state. The (d,d) state exhibits complex spectral evolution over ca. 8 ps, interpreted in terms of vibrational relaxation and cooling. The cold ligand-field excited state decays with a lifetime of 320 ps. At variance with the highly distorted nickel porphyrins but similar to the planar analogues, the (d,d) spectrum of NiOMTP has transient absorption bands immediately to the red of the bleaching of the ground-state Q and B bands.     

Scanning tunneling microscopy, orbital-mediated tunneling spectroscopy, and ultraviolet photoelectron spectroscopy of metal(II) tetraphenylporphyrins deposited from vapor.

Thin films of vapor-deposited Ni(II) and Co(II) complexes of tetraphenylporphyrin (NiTPP and CoTPP) were studied supported on gold and embedded in Al-Al(2)O(3)-MTPP-Pb tunnel diodes, where M = Ni or Co. Thin films deposited onto polycrystalline gold were analyzed by ultraviolet photoelectron spectroscopy (UPS) using He I radiation. Scanning tunneling microscopy (STM) and orbital-mediated tunneling spectroscopy (STM-OMTS) were performed on submonolayer films of CoTPP and NiTPP supported on Au(111). Inelastic electron tunneling spectroscopy (IETS) and OMTS were measured in conventional tunnel diode structures. The highest occupied pi molecular orbital of the porphyrin ring was seen in both STM-OMTS and UPS at about 6.4 eV below the vacuum level. The lowest unoccupied pi molecular orbital of the porphyrin ring was observed by STM-OMTS and by IETS-OMTS to be located near 3.4 eV below the vacuum level. The OMTS spectra of CoTPP had a band near 5.2 eV (below the vacuum level) that was attributed to transient oxidation of the central Co(II) ion. That is, it is due to electron OMT via the half-filled d(z)(2) orbital present in Co(II) of CoTPP. The NiTPP OMTS spectra show no such band, consistent with the known difficulty of oxidation of the Ni(II) ion. The STM-based OMTS allowed these two porphyrin complexes to be easily distinguished. The present work is the first report of the observation of STM-OMTS, tunnel junction OMTS, and UPS of the same compounds. Scanning tunneling microscope-based orbital-mediated tunneling provides more information than UPS or tunnel junction-based OMTS and does so with molecular-scale resolution.     

Photochemical hole burning in macromolecular complexes

Photochemical hole burning (PHB) of free-base porphyrins in host polymers as a site-selective spectroscopy reveals the existence of vibronic structure in a porphyrin Q-band peak and low-energy excitation modes of host polymers. A new mechanism of photon-gated PHB by two-color sensitization of photoreactive polymers with a zinc porphyrin is also presented.     

Three Short Stories about Hexaarylbenzene–Porphyrin Scaffolds

A feasible two‐step synthesis and characterization of a full series of hexaarylbenzene (HAB) substituted porphyrins and tetrabenzoporphyrins is presented. Key steps represent the microwave‐assisted porphyrin condensation and the statistical Diels–Alder reaction to the desired HAB‐porphyrins. Regarding their applications, they proved to be easily accessible and effective high molecular mass calibrants for (MA)LDI mass spectrometry. The free‐base and zinc(II) porphyrin systems, as well as the respective tetrabenzoporphyrins, demonstrate in solid state experiments strong red‐ and near‐infrared‐light emission and are potentially interesting for the application in “truly organic” light‐emitting devices. Lastly, they represent facile precursors to large polycyclic aromatic hydrocarbon (PAH) substituted porphyrins. We prepared the first tetra‐hexa‐peri‐hexabenzocoronene substituted porphyrin, which represents the largest prepared PAH‐porphyrin conjugate to date.     

四-（4-吡啶基）卟啉二酸聚集体的共振拉曼光谱

研究了近激子吸收带激发下四-（4-吡啶基）卟啉二酸（H8TPyP^6＋）聚集体的共振拉曼光谱。测量了H8TPyP^6＋单体和聚集体的紫外可见吸收谱和共振光散射光谱．在氘代位移的基础上结合相关体系振动光谱研究,对测得的H8TPyP^6＋单体和聚集体的拉曼谱带进行了指认．聚集体的形成导致H8TPyP^6＋的卟啉环CC／CN面内伸缩振动向低波数方向位移2-6cm^-1,而卟啉环鞍形面外振动带向高波数方向位移12cm^-1．基于拉曼谱带的强度和频率变化分析了聚集引起的H8TPyP^6＋分了内结构变化和分子间氢键作用．     

Influence of electronic and structural effects on the oxidative behavior of nickel porphyrins

With the aim of better understanding the electronic and structural factors which govern electron-transfer processes in porphyrins, the electrochemistry of 29 nickel(II) porphyrins has been examined in dichloromethane containing either 0.1 M tetra-n-butylammonium perchlorate (TBAP) or tetra-n-butylammonium hexafluorophosphate (TBAPF(6)) as supporting electrolyte. Half-wave potentials for the first oxidation and first reduction are only weakly dependent on the supporting electrolyte, but E(1/2) for the second oxidation varies considerably with the type of supporting electrolyte. E(1/2) values for the first reduction to give a porphyrin pi-anion radical are effected in large part by the electronic properties of the porphyrin macrocycle substituents, while half-wave potentials for the first oxidation to give a pi-cation radical are affected by the substituents as well as by nonplanar deformations of the porphyrin macrocycle. The potential difference between the first and second oxidations (Delta/Ox(2) - Ox(1)/) is highly variable among the 29 investigated compounds and ranges from 0 mV (two overlapped oxidations) to 460 mV depending on the macrocycle substituents and the anion of the supporting electrolyte. The magnitude of Delta/Ox(2) - Ox(1)/ is generally smaller for compounds with very electron-withdrawing substituents and when TBAP is used as the supporting electrolyte. This behavior is best explained in terms of differences in the binding strengths of anions from the supporting electrolyte (ClO(4)(-) or PF(6)(-)) to the doubly oxidized species. A closer analysis suggests two factors which are important in modulating Delta/Ox(2) - Ox(1)/ and thus the binding affinity of the anion to the porphyrin dication. One is the type of pi-cation radical (a proxy for the charge distribution in the dication), and the other is the conformation of the porphyrin macrocycle (either planar or nonplanar). These findings imply that the redox behavior of porphyrins can be selectively tuned to display separate or overlapped oxidation processes.     

Spectroscopic characterization of structural isomers of naphthalene: (E)- and (Z)-phenylvinylacetylene

Near-pure samples of (E)-phenylvinylacetylene ((E)-PVA) and (Z)-phenylvinylacetylene ((Z)-PVA) were synthesized, and their ultraviolet spectroscopy was studied under jet-cooled conditions. The fluorescence excitation and UV-UV holeburning (UVHB) spectra of both isomers were recorded. The S0-S1 origin of (E)-PVA occurs at 33,578 cm(-1), whereas that for (Z)-PVA occurs at 33,838 cm(-1), 260 cm(-1) above that for (E)-PVA. The present study focuses primary attention on the vibronic spectroscopy of (E)-PVA. Single vibronic level fluorescence spectra of many prominent bands in the first 1200 cm(-1) of the S0-S1 excitation spectrum of (E)-PVA were recorded, including several hot bands involving low-frequency out-of-plane vibrations. Much of the ground-state vibronic structure observed in these spectra was assigned by comparison with styrene and trans-beta-methylstyrene, assisted by calculations at the DFT B3LYP/6-311++G(d,p) level of theory. Both S0 and S1 states of (E)-PVA are shown to be planar, with intensity appearing only in even overtones of out-of-plane vibrations. Due to its longer conjugated side chain compared with that of its parent styrene, (E)-PVA supports extensive Duschinsky mixing among the four lowest-frequency out-of-plane modes (nu45-nu48), increasing the complexity of this mixing relative to that of styrene. Identification of the v' = 0-3 levels of nu48, the lowest frequency torsion, provided a means of determining the 1D torsional potential for hindered rotation about the C(ph)-C(vinyl) bond. Vibronic transitions due to (Z)-PVA were first identified as small vibronic bands that did not appear in the UVHB spectrum recorded with the hole-burn laser fixed on the S0-S1 origin of (E)-PVA. The LIF and UVHB spectra of a synthesized sample of (Z)-PVA confirmed this assignment.     

A theoretical approach to the influence of the macrocycle conformation on the molecular electronic structure in Mg-porphyrins

Nonplanar saddled (sad) ruffled (ruf) and domed (dom) conformations of the Mg-porphyrin (MgP) macrocycle in several degrees of deformation have been computed. These symmetrical distortion modes were induced in unsubstituted macrocycle using molecular definitions for calculations which permits us to achieve a systematical variation of the nonplanarity varying only a convenient geometrical parameter of the molecule. Series of nonplanar macrocycles like those synthesized in previous works employing peripheral substitutions are obtained. The procedure here used to induce deformations gives the possibility of investigating the modulator role of the out-of-plane distortions on the geometry and electronic properties of the porphyrin avoiding additional influences due to the substituents or the surrounding protein scaffolding.