Naphthalene diols: a new class of antioxidants intramolecular hydrogen bonding in catechols,naphthalene diols,and their aryloxyl radicals

1,8-Naphthalenediol, 5, and its 4-methoxy derivative, 6, were found to be potent H-atom transfer (HAT) compounds on the basis of their rate constants for H-atom transfer to the 2,2-di(4-t-octylphenyl)-1-picrylhydrazyl radical (DOPPH*), k(ArOH/DOPPH)*, or as antioxidants during inhibited styrene autoxidation, k(ArOH/ROO)*, initiated with AIBN. The rate constants showed that 5 and 6 are more active HAT compounds than the ortho-diols, catechol, 1, 2,3-naphthalenediol, 2, and 3,5-di-tert-butylcatechol, 3. Compound 6 has almost twice the antioxidant activity, k(ArOH/ROO)* = 6.0 x 10(6) M(-)(1) s(-1), of that of the vitamin E model compound, 2,2,5,7,8-pentamethyl-6-chromanol, 4. Calculations of the O-H bond dissociation enthalpies compared to those of phenols, (deltaBDEs), of 1-6 predict a HAT order of reactivity of 2 < 1 < 3 approximately 4 < 5 < 6 in general agreement with kinetic results. Calculations on the diols show that intramolecular H-bonding stabilizes the radicals formed on H-atom transfer more than it does the parent diols, and this effect contributes to the increased HAT activity of 5 and 6 compared to the activities of the catechols. For example, the increased stabilization due to the intramolecular H-bond of 5 radical over 5 parent of 8.6 kcal/mol was about double that of 2 radical over 2 parent of 4.6 kcal/mol. Linear free energy plots of log k(ArOH/DOPPH)* and log k(ArOH/ROO)* versus deltaBDEs for compounds 1-6 along with available literature values for nonsterically hindered monophenols placed the compounds on common scales. The derived Evans-Polanyi constants from the plots for the two reactions, alpha(DOPPH)* = 0.48 > alpha(ROO)* = 0.32, gave the expected order, since the ROO* reaction is more exothermic than the DOPPH* reaction. Compound 6 is sufficiently reactive to react directly with oxygen, and it lies off the log k(ArOH/ROO)* versus deltaBDE plot.     

Influence of Ligand and Nuclearity on the Cytotoxicity of Cyclometallated C^N^C Platinum(II) Complexes

A series of cyclometallated mono- and di-nuclear platinum(II) complexes and the parent organic ligand, 2,6-diphenylpyridine 1 (HC^N^CH), have been synthesized and characterized. This library of compounds includes [(C^N^C)Pt^II( L )] ( L =dimethylsulfoxide (DMSO) 2 and triphenylphosphine (PPh₃) 3 ) and [((C^N^C)Pt^II)₂( L‘ )] (where L‘ =N-heterocycles (pyrazine (pyr) 4 , 4,4‘-bipyridine (4,4‘-bipy) 5 or diphosphine (1,4-bis(diphenylphosphino)butane (dppb) 6 ). Their cytotoxicity was assessed against four cancerous cell lines and one normal cell line, with results highlighting significantly increased antiproliferative activity for the dinuclear complexes ( 4 – 6 ), when compared to the mononucleated species ( 2 and 3 ). Complex 6 is the most promising candidate, displaying very high selectivity towards cancerous cells, with selectivity index (SI) values >29.5 (A2780) and >11.2 (A2780cisR), and outperforming cisplatin by >4-fold and >18-fold, respectively.     

Ultrafast intersystem crossing and spin dynamics of zinc meso-tetraphenylporphyrin covalently bound to stable radicals

tert-Butylphenylnitroxide (BPNO(?)) and α,γ-bisdiphenylene-β-phenylallyl (BDPA(?)) stable radicals are each attached to zinc meso-tetraphenylporphyrin (ZnTPP) at a fixed distance using one of the ZnTPP phenyl groups. BPNO(?) and BDPA(?) are oriented para (1 and 3, respectively) or meta (2 and 4, respectively) relative to the porphyrin macrocycle. Following photoexcitation of 1-4, transient optical absorption spectroscopy is used to observe excited state quenching of (1)*ZnTPP by the radicals and time-resolved electron paramagnetic resonance (TREPR) spectroscopy is used to monitor the spin dynamics of the paramagnetic product states. The presence of BPNO(?) or BDPA(?) accelerates the intersystem crossing rate of (1)*ZnTPP about 10- to 500-fold in 1-4 depending on the structure compared to that of (1)*ZnTPP itself. In addition, the lifetime of (3)*ZnTPP in 1 is shorter than that of (3)*ZnTPP itself as a result of enhanced intersystem crossing (EISC) from (3)*ZnTPP to the ground state. The TREPR spectra of the three unpaired spins produced within 1 and 2 show spin-polarized excited doublet (D(1)) and quartet (Q) states and subsequent formation of a spin-polarized ground state radical (D(0)). All three signals are absorptive for 1 and emissive for 2. Polarization inversion of the Q state is observed on a tens of nanoseconds time scale in 2, while no polarization inversion is observed for 1. The lack of polarization inversion in 1 is attributed to the short lifetime of the doublet-quartet manifold as a result of the very large exchange interaction. The TREPR spectra of 3 and 4 show ground state radical polarization at X-band (9.5 GHz) at room temperature, but not at 85 K, and similarly no polarization is observed at W-band (94 GHz). No evidence of excited doublet or quartet states is observed, indicating that the exchange interaction is both weak and temperature dependent. These results show that although ultrafast EISC produces (3)*ZnTPP within 1-4, the magnitude of the exchange interactions between the three relevant spins in the resulting (3)*ZnTPP-BPNO(?) and (3)*ZnTPP-BDPA(?) systems dramatically alters their spin dynamics.     

Synthesis and Photophysical Properties of a Series of Biscyclometalated Platinum(II) Complexes on the Basis of a Tridentate 2,6-Diphenylpyridine

The complexes C,C-trans-[Pt(C^N^C)L] ^z [C^N^C is bisdeprotonated 2,6-diphenylpyridinate (dppy^2-); L = CO, Me2SO (dmso), ethylenediamine (en), CN^-, pyrazine (pz), P(C₆F₅)₃, 4,4'-bipyridine (4,4'-bpy)] were synthesized and characterized by ¹H NMR, electronic, and emission spectroscopy. Spectralluminescence properties of the chromophoric group (platinum-cyclometalating ligand) in the C,C-trans-bis-cyclometalated complexes depend on the nature of the noncyclometalating ligand L, which is explained by its donor-acceptor effects on the form of existence of the complex in solution.     

The guanine cation radical: investigation of deprotonation states by ESR and DFT

This work reports ESR studies that identify the favored site of deprotonation of the guanine cation radical (G*+) in an aqueous medium at 77 K. Using ESR and UV-visible spectroscopy, one-electron oxidized guanine is investigated in frozen aqueous D2O solutions of 2'-deoxyguanosine (dGuo) at low temperatures at various pHs at which the guanine cation radical, G*+ (pH 3-5), singly deprotonated species, G(-H)* (pH 7-9), and doubly deprotonated species, G(-2H)*- (pH > 11), are found. C-8-deuteration of dGuo to give 8-D-dGuo removes the major proton hyperfine coupling at C-8. This isolates the anisotropic nitrogen couplings for each of the three species and aids our analyses. These anisotropic nitrogen couplings were assigned to specific nitrogen sites by use of 15N-substituted derivatives at N1, N2, and N3 atoms in dGuo. Both ESR and UV-visible spectra are reported for each of the species: G*+, G(-H)*, and G(-2H)*-. The experimental anisotropic ESR hyperfine couplings are compared to those obtained from DFT calculations for the various tautomers of G(-H)*. Using the B3LYP/6-31G(d) method, the geometries and energies of G*+ and its singly deprotonated state in its two tautomeric forms, G(N1-H)* and G(N2-H)*, were investigated. In a nonhydrated state, G(N2-H)* is found to be more stable than G(N1-H)*, but on hydration with seven water molecules G(N1-H)* is found to be more stable than G(N2-H)*. The theoretically calculated hyperfine coupling constants (HFCCs) of G*+, G(N1-H)*, and G(-2H)*- match the experimentally observed HFCCs best on hydration with seven or more waters. For G(-2H)*-, the hyperfine coupling constant (HFCC) at the exocyclic nitrogen atom (N2) is especially sensitive to the number of hydrating water molecules; good agreement with experiment is not obtained until nine or 10 waters of hydration are included.     

Luminescent organoplatinum(II) complexes with functionalized cyclometalated C^N^C ligands: structures, photophysical properties, and material applications

A series of [(R'-C^N^C-R')Pt(L)] complexes with doubly deprotonated cyclometalated R'-C^N^C-R' ligands (R'-C^N^C-R'=2,6-diphenylpyridine derivatives) functionalized with carbazole, fluorene, or thiophene unit(s) have been synthesized and their photophysical properties studied. The X-ray crystal structures reveal extensive intermolecular π···π and C-H···π interactions between the cyclometalated C^N^C ligands. Compared to previously reported cyclometalated platinum(II) complexes [(C^N^C)Pt(L)], which are non-emissive in solution at room temperature, the carbazole-, fluorene- and thiophene-functionalized [(R'-C^N^C-R')Pt(L)] (L=DMSO 1-9, C≡N-Ar, 1a-9a) complexes are emissive in solution at room temperature with λ(max) at 564-619 nm and Φ=0.02-0.26. The emissions of the [(R'-C^N^C-R')Pt(L)] complexes are attributed to electronic excited states with mixed (3)MLCT and (3)IL character. The carbazole/fluorene/thiophene unit(s) allow the tuning of the electronic properties of the [(R'-C^N^C-R')Pt] moiety, with the emission maxima in a range of 564-619 nm. These are the first examples of organoplatinum(II) complexes bearing doubly deprotonated cyclometalated C^N^C ligands that are emissive in solution at room temperature. In non-degassed DMSO, the emission intensities of 6a-9a are enhanced upon exposure to ambient light. This phenomenon is caused by reacting photogenerated (1)O(2) with a DMSO molecule to form dimethyl sulfone, leading to the removal of dissolved oxygen in solution. Self-assembled nanowires and nanorods are obtained from precipitation of 3a in THF/H(2)O and 8a in DMSO/Et(2)O, respectively. The [(R'-C^N^C-R')Pt(L)] complexes are soluble in common organic solvents with a high thermal stability (>300 °C), rendering them as phosphorescent dopants for organic light-emitting diode (OLEDs) applications. Red OLEDs with CIE coordinates of (0.65±0.01, 0.35±0.01) were fabricated from 7a or 8a. A maximum external efficiency (η(Ext)) of 12.6% was obtained for the device using 8a as emitter.     

A theoretical study on fullerene-dizincocene hybrids

Using the density functional theory, we investigated the possible formation of fullerene-dizincocene hybrids, specifically C(60)*-Zn-Zn-Cp*, C(60)*-Zn-Zn-C(60)*, C(70)*-Zn-Zn-Cp*, and C(70)*-Zn-Zn-C(70)*, where C(60)*, Cp*, and C(70)* represent C(60)(CH(3))(5), C(5)(CH(3))(5), and C(70)(CH(3))(5) radicals. Our calculation shows that these hybrids have HOMO-LUMO gaps which are larger than has been experimentally identified for C(60)*-Fe-Cp. In addition, the strength of the Zn--Zn bonds is similar to that in Cp*-Zn-Zn-Cp* which was also synthesized recently. Furthermore, heterohybrids, C(60)*-Zn-Zn-Cp* and C(70)*-Zn-Zn-Cp* are expected to exist in equilibria with homohybrids, C(60)*-Zn-Zn-C(60)* and C(70)*-Zn-Zn-C(70)*, in which heterohybrids are much more favored. On the other hand, another hybrid involving Sc(3)N@C(68) as a fullerene unit is not highly probable.     

Fragmentation of protonated oligoalanines: Amide bond cleavage and beyond

The fragmentation reactions of the singly-protonated oligoalanines trialanine to hexaalanine have been studied using energy-resolved mass spectrometry in MS(2) and MS(3) experiments. The primary fragmentation reactions are rationalized in terms of the b(x)-y(z) pathway of amide bond cleavage which results in formation of a proton-bound complex of an oxazolone and a peptide/amino acid; on decomposition of this complex the species of higher proton affinity preferentially retains the proton. For protonated pentaalanine and protonated hexaalanine the major primary fragmentation reaction involves cleavage of the C-terminal amide bond to form the appropriate b ion. The lower mass b ions originate largely, if not completely, by further fragmentation of the initially formed b ion. MS(3) energy-resolved experiments clearly show the fragmentation sequence b(n) --> b(n-1) --> b(n-2). A more minor pathway for the alanines involves the sequence b(n) --> a(n) --> b(n-1) --> b(n-2). The a(5) ion formed from hexaalanine loses, in part, NH(3) to begin the sequence of fragmentation reactions a(5) --> a(5)* --> a(4)* --> a(3)* where a(n)* = a(n) - NH(3). The a(3)* ion also is formed from the b(3) ion by the sequence b(3) --> a(3) --> a(3)* with the final step being sufficiently facile that the a(3) ion is not observed with significant intensity in CID mass spectra. A cyclic structure is proposed for the a(3)* ion.     

Singularity of proton transport in salts of orthoperiodic and orthotellurium acids: theoretical modeling using density functional calculations

Density functional B3LYP calculations have been performed to investigate proton transport in orthoperiodic and orthotellurium acids, their salts MIO(6)H(4)(M = Li, Rb, Cs) and CsH(5)TeO(6), dimers of the salt*acid type MIO(6)H(4)*H(5)IO(6)(M = Rb, Cs), CsIO(6)H(4)*H(6)TeO(6), CsHSO(4)*H(6)TeO(6), Cs(2)SO(4)*H(6)TeO(6), and also in double-substituted and binary salts Rb(2)H(3)IO(6) and Rb(4)H(2)I(2)O(10). It has been shown that the energy of salt dimerization is 33-35 kcal mol(-1) and the activation barrier for proton migration between the neighboring octahedrons of the salt*acid --> acid*salt type is calculated to be 3-13 kcal mol(-1). The activation energy of the proton migration along the octahedron, 20-30 kcal mol(-1), is comparable with the barrier for water molecule separation. Quantum-chemical calculations correlate with the results of X-ray and electrochemical studies.     

Palladium(II) Acetylide Complexes with Pincer‐Type Ligands: Photophysical Properties,Intermolecular Interactions,and Photo‐cytotoxicity

Two classes of cationic palladium(II) acetylide complexes containing pincer‐type ligands, 2,2′:6′,2′′‐terpyridine (terpy) and 2,6‐bis(1‐butylimidazol‐2‐ylidenyl)pyridine (C^N^C), were prepared and structurally characterized. Replacing terpy with the strongly σ‐donating C^N^C ligand with two N‐heterocyclic carbene (NHC) units results in the Pd^II acetylide complexes displaying phosphorescence at room temperature and stronger intermolecular interactions in the solid state. X‐ray crystal structures of [Pd(terpy)(C≡CPh)]PF₆ ( 1 ) and [Pd(C^N^C)(C≡CPh)]PF₆ ( 7 ) reveal that the complex cations are arranged in a one‐dimensional stacking structure with pair‐like Pd^II⋅⋅⋅Pd^II contacts of 3.349 Å for 1 and 3.292 Å for 7 . Density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) calculations were used to examine the electronic properties. Comparative studies of the [Pt(L)(C≡CPh)]⁺ analogs by ¹H NMR spectroscopy shed insight on the intermolecular interactions of these Pd^II acetylide complexes. The strong Pd−C_carbene bonds render 7 and its derivative sufficiently stable for investigation of photo‐cytotoxicity under cellular conditions.     

Deep blue mixed-valent PtIIIPtIIIPtII complex [Pt3Br2(mu-pz)6] (pz=pyrazolate) showing valence-detrapping behavior in solution

The oxidation of the pyrazolate bridged cyclic PtII trimer, [Pt3(mu-pz)6] (1), in the presence of bromide ion gave a deep blue mixed-valent Pt(II,III,III) complex, [Pt3Br2(mu-pz)6] (2). The structural analysis of 2 disclosed that the complex has localized Pt--Pt bond. Our theoretical calculations revealed that the HOMO and LUMO of Pt3 (II,III,III) species mainly consists of (dsigma-dsigma) and (dsigma-dsigma)* orbitals, respectively, and the origin of deep blue color of the bromo complex, 2, arises from the (dsigma-dsigma)-->(dsigma-dsigma)* transition. Unique fluxional behavior was observed due to valence-detrapping of 2 in solution. The activation parameters of the valence-detrapping of 2 obtained by Eyring analyses were DeltaH(not equal)=37(2) kJ mol(-1) and DeltaS(not equal)=-67(7) J mol(-1) K(-1).     

NMR transversal relaxivity of aqueous suspensions of particles of Ln(3+)-based zeolite type materials

A series of zeolite-type silicates containing stoichiometric amounts of Ln(3+) ions in the framework (Ln-AV-9 materials), with composition (Na(4)K(2))(Ln(2)Si(16)O(38)).10H(2)O (Ln = Nd, Sm, Eu, Tb, Gd, Dy) has recently been synthesized and characterized. They form paramagnetic microparticles, which as aqueous suspensions have negligible water (1)H longitudinal relaxivities (r(1)) for all Ln(3+) ions studied and quite large transverse relaxivities (r(2)). In this work we further analysed the size distribution of the Ln-AV-9 particles and their r(2)* and r(2) relaxivities. The r(2)* relaxivity effects are explained by the static dephasing regime (SDR) theory. The r(2) relaxivities appear to be strongly dependent on the interval between two consecutive refocusing pulses (tau(CP)) in the train of 180 degrees pulses applied. For long tau(CP) values, the r(2) of the systems saturates at a value, which is always an order of magnitude smaller than r(2)*. These features are explained by a crude model, which takes into account the residual diffusion effect in the static dephasing regime. The large microparticles, although not efficient in T(1) relaxation, are quite effective in enhancing T(2) relaxation, particularly at high magnetic fields. The r(2)* values and the saturation values for r(2) were found to increase linearly with B(0) and mu(0)(2). The largest transversal relaxation rate enhancements were observed for Dy-AV-9 with a saturation value of r(2) of 60 s(-1) mM(-1) and a r(2)* value of 566 s(-1) mM(-1) at 9.4 T and 298 K.     

Photosensitized degradation of dyes in polyoxometalate solutions versus TiO2 dispersions under visible-light irradiation: mechanistic implications

This article examines the photoxidation of a dye (rhodamine-B, RhB) by visible-light irradiation in the presence of a polyoxometalate (12-tungstosilicic acid, H(4)SiW(12)O(40)), and compares it with the analogous process in the presence of TiO(2). The photoreaction processes were examined by UV-visible spectroscopy, fluorescence spectroscopy, high-performance liquid chromatography (HPLC), liquid chromatography/mass spectral techniques (LC-MS), and total organic carbon (TOC) assays in order to identify the intermediates produced. Formation of oxygen species, such as H(2)O(2) and O(2)*-, was also investigated to clarify the details of the reaction pathway. With the use of SiW(12)O(40)(4-) ions as the photocatalyst, the photoreaction leads mainly to N-dealkylation of the chromophore skeleton. In contrast, cleavage of the whole conjugated chromophore structure predominates in the presence of TiO(2). Strong O(2)*-/HO(2)*- ESR signals were detected in the TiO(2) dispersions, whereas only weak ESR signals for the O(2)*- radical ion were seen in the SiW(12)O(40)(4-) solutions during the irradiation period. Experimental results imply that reduction of O(2) occurs by different pathways in the two photocatalytic systems.     

Femtosecond dynamics on 2-(2'-hydroxy-4'-diethylaminophenyl)benzothiazole: solvent polarity in the excited-state proton transfer.

Detailed insights into the excited-state enol(N*)-keto(T*) intramolecular proton transfer (ESIPT) reaction in 2-(2'-hydroxy-4'-diethylaminophenyl)benzothiazole (HABT) have been investigated via steady-state and femtosecond fluorescence upconversion approaches. In cyclohexane, in contrast to the ultrafast rate of ESIPT for the parent 2-(2'-hydroxyphenyl)benzothiazole (>2.9+/-0.3 x 10(13) s(-1)), HABT undergoes a relatively slow rate (approximately 5.4+/-0.5 x 10(11) s(-1)) of ESIPT. In polar aprotic solvents competitive rate of proton transfer and rate of solvent relaxation were resolved in the early dynamics. After reaching the solvation equilibrium in the normal excited state (N(eq)*), ESIPT takes place with an appreciable barrier. The results also show N(eq)*(enol)<-->T(eq)*(keto) equilibrium, which shifts toward N(eq)* as the solvent polarity increases. Temperature-dependent relaxation dynamics further resolved a solvent-induced barrier of 2.12 kcal mol(-1) for the forward reaction in CH(2)Cl(2). The observed spectroscopy and dynamics are rationalized by a significant difference in dipole moment between N(eq)* and T(eq)*, while the dipolar vector for the enol form in the ground state (N) is in between that of N(eq)* and T(eq)*. Upon N-->N* Franck-Condon excitation, ESIPT is energetically favorable, and its rate is competitive with the solvation relaxation process. Upon reaching equilibrium configurations N(eq)* and T(eq)*, forward and/or backward ESIPT takes place with an appreciable solvent polarity induced barrier due to differences in polarization equilibrium between N(eq)* and T(eq)*.     

Spectroscopy and femtosecond dynamics of 7-N,N-diethylamino-3-hydroxyflavone. The correlation of dipole moments among various states to rationalize the excited-state proton transfer reaction

Comprehensive excitation behaviors of 7-N,N-diethylamino-3-hydroxyflavone (I) have been investigated via steady state, temperature-dependent emission, and fluorescence upconversion to probe the excited-state intramolecular proton transfer (PT) reaction. Upon excitation, I undergoes ultrafast (<120 fs), adiabatic type of charge transfer (CT), so that the dipolar vector in the Franck-Condon excited state is much different from that in the ground state. In polar solvents such as CH2Cl2 and CH3CN, early relaxation dynamics clearly reveals the competitive rates between solvent relaxation and PT dynamics. After reaching thermal equilibrium, a relatively slow, solvent-polarity-dependent rate (a few tens of picoseconds(-1)) of PT takes places. Firm support of the early relaxation dynamics is rendered by the spectral temporal evolution, which resolves two distinct bands ascribed to CT and PT emission. The results, in combination with ab initio calculations on the dipolar vectors for various corresponding states, led us to conclude that excited-state normal (N*) and excited proton-transfer tautomer (T*) possesses very different dipole orientation, whereas the dipole orientation of the normal ground state (N) is between that of N* and T*. PT is thus energetically favorable at the Franck-Condon excited N*, and its rate is competitive with respect to the solvent relaxation dynamics induced by CT. Unlike the well-known PT system, 4'-N,N-diethylamino-3-hydroxyflavone, in which equilibrium exists between solvent-equilibrated N(eq)* and T(eq)*, N(eq)* --> T(eq)* PT for I is a highly exergonic, irreversible process in all solvents studied. Further temperature-dependent studies deduce a solvent-polarity-perturbed energy barrier of 3.6 kcal/mol for the N(eq)* --> T(eq)* PT in CH3CN. The proposed dipole-moment-tuning PT mechanism with the associated relaxation dynamics is believed to apply to many PT molecules in polar, aprotic solvents.     

Electronic structure of H2CS3 and H2CS4: an experimental and theoretical study

The HeI photoelectron spectra of H2CS3 and H2CS4 in the gas phase have been obtained for the first time. A complete theoretical study involving the calculation of the ionization energies using orbital valence Green's functional (OVGF) and population analysis was performed. Calculations of cation-radical forms were carried out in order to interpret the main characters of the six highest occupied molecular orbitals (HOMOs). The first vertical ionization potentials are 8.74 and 8.56eV for H2CS3 and H2CS4, and attributed to {9b2(nS(C=S))}-1 and {8a"(3ppi*(S-S), nS)}-1, respectively. Meanwhile, the energy sequence of three types of sulfur 3p lone-pair have been discussed: 3ppi(S-S)*相似文献   

Design and synthesis of enantiopure 1-[1(S)-(2-pyridyl)alkyl]-2(R)-isopropylaziridines, new ligands for asymmetric catalysis

Enantiopure 1-(2-pyridyl)alkyl aziridines were designed as bidentate ligands for asymmetric catalysis. Their synthesis involved the addition of organometallic reagents to the imine prepared from 2-pyridinealdehyde and an enantiopure β-aminoalcohol, followed by cyclisation of the β-aminoalcohol moiety to the aziridine ring. Two such ligands (N–N)* were prepared from (S)-valinol and converted to the complexes (η³-allyl)(N–N)*Pd⁺SbF₆⁻, one of which was characterised by X-ray crystallography. Modest enantioselectivities were achieved in a representative Pd-catalysed allylic substitution reaction.     

Intramolecular electron transfer in cofacially pi-stacked fluorenes: evidence of tunneling

The one-electron reduction of neutral pi-stacked di- and trifluorenes (F-2 and F-3) in HMPA, where ion association is absent, results in the formation of anion radicals in which the odd electron resides predominantly on just one of the external fluorene moieties, as established by EPR spectroscopy. However, in the case of tetrafluorene, introduction of a single electron leads to a kinetically controlled anion radical F-4(int)*- in which the odd electron undergoes rapid exchange between two central fluorene rings, where the anionic charge is partially shielded from solvation due to the presence of external fluorene rings. On a time scale of minutes, anion radical F-4(int)*- converts to a thermodynamically stabilized anion radical F-4(ext)*-, with the electron exhibiting coupling from the protons on an external fluorene moiety. The charge and spin residing on an external moiety allow efficient solvation of the anionic charge. A similar fast exchange of a single electron (probably with the involvement of quantum mechanical tunneling) among three and four internal fluorene moieties is initially observed via EPR spectroscopy in the penta- and hexafluorene derivatives, F-5 and F-6, respectively.     

Interpretation of the electronic spectra of four disilanes

Time-dependent density functional theory (TD-DFT/B3LYP(AC)/cc-pVTZ/cc-pVTZ/6-311G//MP2/cc-pVTZ/cc-pVTZ/6-31G**) has been used to compute vertical excitation energies and oscillator strengths of the six low-lying excited states of four peralkylated disilanes, hexamethyldisilane (1), hexa-tert-butyldisilane (2), 1,6-disila[4.4.4]propellane (3), and 1,7-disila[5.5.5]propellane (4). The results provide an accurate interpretation of the reported UV absorption spectra of 1-4 in solution, and for 1 also in the gas phase up to 62,000 cm(-1). The excellent agreement of the calculated with the available experimental energies and oscillator strengths, and with magnetic circular (MCD) and linear (LD) dichroism, gives us confidence that the method will be useful for dependable interpretation of the electronic spectra of longer oligosilanes. Although the disilane chromophore finds itself in quite different environments in 1-4, its fundamental characteristics remain the same, with one important exception. In all four compounds, the first valence excited state is due to an electron promotion from the sigma(1) HOMO to the pi(1)* orbital, and the second valence excited state to a promotion from the sigma(1) HOMO to the sigma(1)* orbital. Surprisingly, however, it is only in 2, which has an extraordinarily long SiSi bond, that the terminating sigma(1)* orbital is the sigma*(SiSi) antibond, as anticipated, and the sigma sigma* transition has the expected very high oscillator strength. In 1, 3, and 4, the sigma*(SiSi) antibonding orbital is high in energy and does not play any role in low-energy excitations. Instead, the terminating orbital of the sigma(1)sigma(1)* excitation is represented by Si-alkyl antibonds, combined symmetrically with respect to rotation around the SiSi axis and antisymmetrically with respect to operations that interchange the two Si atoms. The common assumption that the characteristic intense sigma sigma* transitions of longer peralkylated oligosilanes extrapolate to the lowest sigma sigma* transition in common peralkylated disilanes is incorrect, and only the weak sigma pi* transitions extrapolate simply.