Redox properties of hydrogenated quinoline derivatives — inhibitors in oxidation processes of hydrocarbons

Half-wave potentials of the one-electron electrochemical oxidation (E _1/2 ^ox) of hydroquinolines with different degrees of heterocycle hydrogenization as well as containing substituents of various natures in the benzene ring and heterocycle have been measured. Linear correlations betweenE _1/2 ^ox and the values of the Hammett polar -constants form- andp-substituents in dihydroquinolines and related sulfur-containing dithiolthiones were established. The character of the variation ofE _1/2 ^ox in the series of hydroquinolines was found to correlate with the characteristic features of the inhibiting action of these compounds in the liquid-phase oxidation of various hydrocarbons. However, in contrast to phenolic antioxidants for hydroquinolines, there is no dependence of the retardation period onE _1/2 ^oxin the oxidation of hydrocarbons at temperatures higher than 100 °C.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 610–613, April, 1994.     

Effect of Solvent Polarity on the Electronic Structure and Emission Frequencies of Exciplexes of Aromatic Hydrocarbons with Methoxybenzenes and Methylnaphthalenes

The dependence of exciplex emission spectra on the solvent polarity was studied for exciplexes with the Gibbs free energy of excited-state electron transfer, G ^* _et , exceeding –0.1 eV (for pyrene, fluoranthene, 1,12-benzoperylene, and 9-cyanoanthracene with methoxybenzenes or dimethylnaphthalene). These exciplexes showed stronger changes in the spectral shift of exciplex emission and the extent of charge transfer with increasing solvent polarity than the exciplexes having more negative G ^* _et values. The parameters (difference in energy of charge transfer (CT) and locally excited (LE) states in a vacuum, (H ⁰ ₂₂– H ⁰ ₁₁), and the matrix element for electronic coupling of CT and LE states H ₁₂and mrelated to the dipole moment of the CT state and the size of the exciplex) determining the extent of charge transfer, the spectral shift, and other properties of exciplexes were evaluated. The parameters H ₁₂and mfor the exciplexes examined fall in the interval 0.1–0.5 and 1.0–1.7 eV, respectively, and the difference (H ⁰ ₂₂– H ⁰ ₁₁) is proportional to G ^* _et .     

Oligo(p‐phenyleneethynylene) (OPE) Molecular Wires: Synthesis and Length Dependence of Photoinduced Charge Transfer in OPEs with Triarylamine and Diaryloxadiazole End Groups

The systematic synthesis and photophysical, electrochemical and computational studies on an extended series of triphenylamine‐[C?C‐1,4‐C₆H₂(OR)₂]_n‐C?C‐diphenyl‐1,3,4‐oxadiazole dyad molecules (the OR groups are at 2,5‐positions of the para‐phenylene ring and R=C₆H₁₃; n=0–5, compounds 1 , 2 , 3 , 4 and 5 , respectively) are reported. Related molecules with identical end groups, triphenylamine‐C?C‐1,4‐C₆H₂(OR)₂‐C?C‐triphenylamine (R=C₆H₁₃; 6 ) and diphenyl‐1,3,4‐oxadiazole‐[C?C‐C₆H₂(OR)₂]₂‐C?C‐diphenyl‐1,3,4‐oxadiazole (R=C₆H₁₃; 7 ) were also studied. These D–B–A 1 – 5 , D–B–D 6 and A–B–A 7 (D=electron donor, B=bridge, A=electron acceptor) systems were synthesized using palladium‐catalysed cross‐coupling reactions of new p‐phenyleneethynylene building blocks. Steady‐state emission studies on the dyads 1 – 5 reveal a complicated behavior of the emission that is strongly medium dependent. In low polarity solvents the emission is characterized by a sharp high‐energy peak attributed to fluorescence from a locally excited (LE) state. In more polar environments the LE state is effectively quenched by transfer into an intramolecular charge‐transfer (ICT) state. The medium dependence is also observed in the quantum yields (QYs) which are high in cyclohexane and low in acetonitrile, thus also indicating charge‐transfer character. Low‐temperature emission spectra for 2 – 5 in dichloromethane and diethyl ether also reveal two distinct excited states, namely the LE state and the conventional ICT state, depending on solvent and temperature. Hybrid DFT calculations for 1 – 7 establish that the OPE bridge is involved in both frontier orbitals where the bridge character increases as the bridge length increases. Computed TD‐DFT data on 1 – 5 assign the emission maxima in cyclohexane as LE transitions. Each time‐resolved emission measurement on 2 – 7 in cyclohexane and diethyl ether reveals a wavelength dependent bi‐exponential decay of the emission with a fast component in the 5–61 ps range on blue detection and a slower approximately 1 ns phase, independent of detection wavelength. The fast component is attributed to LE fluorescence and this emission component is rate limited and quenched by transfer into an ICT state. The fast LE fluorescence component varies systematically with conjugation length for the series of D–B–A dyads 2 – 5 . An attenuation factor β of 0.15 Å^?1 was determined in accordance with an ICT superexchange mechanism.     

1-萘甲酰苯胺和2-萘甲酰苯胺的分子内电荷转移研究

A series of 1-naphthanilides (1) and 2-naphthanilides (2) with varied substituents at the para- or meta-position of anilino phenyl ring were prepared and their absorption and fluorescence spectra in a nonpolar solvent cyclohexane were investigated. An abnormal long wavelength emission assigned to the charge transfer (CT) state was found for all of the prepared naphthanilides in cyclohexane. A linear free energy correlation between the CT emission energies and the Hammett constants of the substituent was found within series 1 and 2. The value of the linear slope with 1 (0.42 eV) was higher than that with 2 (0.32 eV) being close to that of the substituted benzanilides 3 (0.31 eV) The higher slope value suggested higher charge separation extent in the CT state of 1 than that of 2. It was found that the corresponding linear slope of anilino-substituted benzanilides remained unchanged when para-, meta-, ortho-, or ortho, ortho-methyls were introduced into the anilino moiety, which ruled out the possible contribution of the difference in the steric effect and the electron accepting ability of the naphthoyl acceptor in 1 and 2. Compared with the early reported N-substituted-benzoyl-aminonaphthalene derivatives 4 and 5, it was considered that 1-naphthoyl enhanced the charge transfer in 1 and the proximity of its ^1La and ^1Lb states was suggested to be responsible. It was shown that 1- and/or 2-substituted naphthalene cores acting as either electron acceptor (naphthoyl) or electron donor (aminonaphthalene) were different in not only electron accepting (donating) ability but also shaping the charge transfer pathway.     

The Importance of Excited‐State Energy Alignment for Efficient Exciplex Systems Based on a Study of Phenylpyridinato Boron Derivatives

Understanding excited‐state dynamics is critical for improving the photoluminescence (PL) efficiency of exciplexes. A series of exciplexes based on conventional hole‐transporting materials as donor and newly developed phenylpyridinato boron derivatives as acceptor were investigated. High PL efficiencies were achieved in only some combinations, and a large difference in performance among combinations provided insight into nonradiative processes in exciplex systems. Furthermore, the triplet local excited states (³LE) of each donor and acceptor were found play an important role in triplet exciplex harvesting. Significant contributions from triplets were clearly observed when the charge‐transfer excited states (¹CT and ³CT) and ³LE were ideally aligned. We also demonstrated fine control of relative energy alignment via the concentration to improve the PL efficiency.     

Tetrathiafulvalene Porphyrins

Four tetrathiafulvalene (TTF)‐annulated porphyrins 1 – 4 were synthesized and characterized. All contain a tetraphenylporphyrin (TPP) core onto which four, two, or one TTF subunits were annulated. Absorption and fluorescence spectroscopic studies together with electrochemical investigations reveal that interactions between the porphyrin system and the annulated TTF units take place in solution. The annulation of one or more TTF units to the porphyrin core has a profound effect on the reduction potentials associated with this latter framework, with positive shifts in the range of 0.105 to 0.355 V and 0.200 to 0.370 V for the first and second reduction potential, respectively, compared to the corresponding processes in the model compound TPP, 18 . The redox potentials for the first oxidation of the TTF units are considerably shifted in 4 (ΔE_ox¹=+0.285 V) and 2 (ΔE_ox¹=?0.140 V), whereas for 1 and 3 these potentials remain within the region expected for a normal TTF unit. Considerable changes in the second oxidation potential associated with the TTF subunits were seen for 2 (ΔE_ox¹=?0.085) and 3 (ΔE_ox¹=?0.175). The emission spectra of 1 – 4 revealed that the porphyrin fluorescence is almost quenched in the neutral state of the TTF‐annulated porphyrins, a finding that is consistent with substantial electron transfer taking place from the TTF subunits to the porphyrin core. Oxidation of the TTF unit(s) (TTF→TTF^.+) present in 1 – 4 leads to the emission intensity being restored.     

Luminescence Properties of C‐Diazaborolyl‐ortho‐Carboranes as Donor–Acceptor Systems

Seven derivatives of 1,2‐dicarbadodecaborane (ortho‐carborane, 1,2‐C₂B₁₀H₁₂) with a 1,3‐diethyl‐ or 1,3‐diphenyl‐1,3,2‐benzodiazaborolyl group on one cage carbon atom were synthesized and structurally characterized. Six of these compounds showed remarkable low‐energy fluorescence emissions with large Stokes shifts of 15100–20260 cm^?1 and quantum yields (Φ_F) of up to 65 % in the solid state. The low‐energy fluorescence emission, which was assigned to a charge‐transfer (CT) transition between the cage and the heterocyclic unit, depended on the orientation (torsion angle, ψ) of the diazaborolyl group with respect to the cage C? C bond. In cyclohexane, two compounds exhibited very weak dual fluorescence emissions with Stokes shifts of 15660–18090 cm^?1 for the CT bands and 1960–5540 cm^?1 for the high‐energy bands, which were assigned to local transitions within the benzodiazaborole units (local excitation, LE), whereas four compounds showed only CT bands with Φ_F values between 8–32 %. Two distinct excited singlet‐state (S₁) geometries, denoted S₁(LE) and S₁(CT), were observed computationally for the benzodiazaborolyl‐ortho‐carboranes, the population of which depended on their orientation (ψ). TD‐DFT calculations on these excited state geometries were in accord with their CT and LE emissions. These C‐diazaborolyl‐ortho‐carboranes were viewed as donor–acceptor systems with the diazaborolyl group as the donor and the ortho‐carboranyl group as the acceptor.     

Luminescent Chiral Exciplexes with Sky-Blue and Green Circularly Polarized-Thermally Activated Delayed Fluorescence

Luminescent exciplexes based on a chiral electron donor and achiral acceptors are reported as a new approach to design circularly polarized (CP) and thermally activated delayed fluorescence (TADF) emitters. This strategy results in rather high CP luminescence (CPL) values with g_lum up to 7×10⁻³, one order of magnitude higher in comparison to the CPL signal recorded for the chiral donor alone (g_lum ∼7×10⁻⁴). This increase occurs concomitantly with a CPL sign inversion, as a result of the strong charge-transfer emission character, as experimentally and theoretically rationalized by using a covalent chiral donor-acceptor model. Interestingly, blue, green-yellow and red chiral luminescent exciplexes can be obtained by modifying with the electron accepting character of the achiral unit while keeping the same chiral donor unit. These results bring new (inter)molecular guidelines to obtain simply and efficiently multi-color CP-TADF emitters.     

Low‐oxidation‐potential conducting polymers derived from 3,4‐ethylenedioxythiophene and dialkoxybenzenes

Monomers derived from 3,4‐ethylenedioxythiophene and phenylenes with branched or oligomeric ether dialkoxy substituents were prepared with the Negishi coupling technique. Electrooxidative polymerization led to the corresponding dialkoxy‐substituted 3,4‐ethylenedioxythiophene–phenylene polymers, with extremely low oxidation potentials (E_1/2,p = ?0.16 to ?0.50 V vs Ag/Ag⁺) due to the highly electron‐rich nature of these materials. The polymers were electrochromic, reversibly switching from red to blue upon oxidation, with bandgaps at about 2 eV. The electrochemical behavior of the oligomeric ether‐substituted polymer was investigated in the presence of different metal ions. Films of the polymer exhibited electrochemical recognition for several alkali and alkaline‐earth cations with selectivity in the order Li⁺ > Ba²⁺ > Na⁺ > Mg²⁺. Cyclic voltammetry showed a decrease in the oxidation potential and an improvement in the definition of the voltammetric response, as well as an increase in the overall electroactivity of the polymer films when the concentration of the cations in the medium was increased. These results are discussed in terms of the electrostatic interactions between the complexed cation and the redox center, as well as the diffusion of the ionic species into the polymer matrix. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2164–2178, 2001     

A “classical” trajectory driven nuclear dynamics by a parallelized quantum‐classical approach to a realistic model Hamiltonian of benzene radical cation

We explore the workability of a parallelized algorithm of time‐dependent discrete variable representation (TDDVR) methodology formulated by involving “classical” trajectories on each DOF of a multi‐mode multi‐state Hamiltonian to reproduce the population dynamics, photoabsorption spectra and nuclear dynamics of the benzene radical cation. To perform such dynamics, we have used a realistic model Hamiltonian consists of five lowest electronic states (X²E_1g, B²E_2g, C²A_2u, D²E_1u, and E²B_2u) which are interconnected through several conical intersections with nine vibrational modes. The calculated nuclear dynamics and photoabsorption spectra with the advent of our parallelized TDDVR approach show excellent agreement with the results obtained by multiconfiguration time‐dependent Hartree method and experimental findings, respectively. The major focus of this article is to demonstrate how the “classical” trajectories for the different modes and the “classical” energy functional for those modes on each surface can enlight the time‐dependent feature of nuclear density and its' nodal structure. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Synthesis and Photophysical Properties of Dimethoxybis(3,3,3‐trifluoropropen‐1‐yl)benzenes: Compact Chromophores Exhibiting Violet Fluorescence in the Solid State

Dimethoxybis(3,3,3‐trifluo‐ropropen‐1‐yl)benzenes were prepared through palladium‐catalyzed double cross‐coupling reactions of diiododimethoxybenzenes with CF₃C≡CZnCl, followed by reduction of CF₃C≡C groups with LiAlH₄ or H₂ in the presence of the Lindlar catalyst. The edges of the absorption spectra of 1,2‐(MeO)₂‐4,5‐(CF₃CHC=CH)₂benzenes 1 and 1,3‐(MeO)₂‐4,6‐(CF₃CH=CH)₂benzenes 2 in cyclohexane ranged from 348 to 360 nm, whereas the absorption spectra of 1,4‐(MeO)₂‐2,5‐[(E)‐CF₃CH=CH]₂ benzene ((E)‐ 3 ) ended at 406 nm. These findings indicate that the effective conjugation length of (E)‐ 3 was significantly larger than those of 1 and 2 . Consistently, 1 and 2 in cyclohexane exhibited fluorescence with emission maxima in the UV region, whereas (E)‐ 3 in cyclohexane emitted violet light with an emission maximum at 407 nm. All the fluorescence spectra of 1 – 3 in various solvents redshifted as the solvent polarity increased. The photoluminescence of 1 , E‐1 , Z‐1 , 2 , E‐2 , E‐2H , Z‐2 , E‐3 , E‐3H , Z‐3 in the solid states was also observed with emission maxima in the violet region. It is important to note that the quantum yields of (E)‐ 3 in a neat thin film and in a doped polymer film were 0.37 and 0.49, respectively. Density functional theory calculations suggested that the fluorine atoms contribute to a slight extension of both the HOMOs and the LUMOs, as well as narrowing of the HOMO–LUMO gaps when compared with the corresponding fluorine‐free analogues. In the case of (E)‐ 3 , it is suggested that the HOMO–LUMO transition includes charge transfer from the ethereal oxygen atoms to the C(sp²) CF₃ moieties.     

Oligotriarylamines with a Pyrene Core: A Multicenter Strategy for Enhancing Radical Cation and Dication Stability and Tuning Spin Distribution

Monoamine 1 , diamines 2 – 4 , triamine 5 , and tetraamine 6 have been synthesized by substituting dianisylamino groups at the 1‐, 3‐, 6‐, and/or 8‐positions of pyrene. Diamines 2 – 4 differ in the positions of the amine substituents. No pyrene–pyrene interactions are evident in the single‐crystal packing of 3 , 4 , and 6 . With increasing numbers of amine substituents, the first oxidation potential decreases progressively from the mono‐ to the tetraamine. These compounds show intense charge‐transfer (CT) emission in CH₂Cl₂ at around 530 nm with quantum yields of 48–68 %. Upon stepwise oxidation by electrolysis or chemical oxidation, these compounds were transformed into radical cations 1 ^?+– 6 ^?+ and dications 2 ²⁺– 6 ²⁺, which feature strong visible and near‐infrared absorptions. Time‐dependent density functional theory studies suggested the presence of localized transitions from the pyrene radical cation and aminium radical cation, intervalence CT, and CT between the pyrene and amine moieties. Spectroscopic studies indicated that these radical cations and dications have good stability. Triamine 5 and tetraamine 6 formed efficient CT complexes with tetracyanoquinodimethane in solution. The results of EPR spectroscopy and density functional theory calculations suggested that the dications 2 ²⁺– 4 ²⁺ have a triplet ground state, whereas 5 ²⁺ and 6 ²⁺ have a singlet ground state. The dication of 1,3‐disubstituted diamine 4 exhibits a strong EPR signal.     

Carboxylate–phenolate tautomerism in 5‐[(nitrophenyl)diazenyl]salicylate anions

Aryldiazenyl derivatives of salicylic acid and their salts are used as dyes. In these structures, the carboxylate groups are engaged in short contacts with the cations and in hydrogen bonds with water molecules, if present. If both O atoms of the carboxylate group take part in such interactions, the negative charge is delocalized over the two atoms. In the absence of hydrogen bonds and contacts with cations, the negative charge is localized on one of the O atoms. In the crystal structures of tetramethylammonium 2‐hydroxy‐5‐[(E)‐(4‐nitrophenyl)diazenyl]benzoate and tetramethylammonium 2‐hydroxy‐5‐[(E)‐(2‐nitrophenyl)diazenyl]benzoate, both C₄H₁₂N⁺·C₁₃H₈N₃O₅⁻, all the interactions between the cations and anions are weak, and their effect on the geometry of the anions is negligible. Under these conditions, the 2‐nitro‐substituted anion is an almost pure phenol–carboxylate tautomer, whereas in the 4‐nitro‐substituted anion, the phenolic H atom is shifted towards the carboxylate group, and thus the structure of this anion is intermediate between the phenol–carboxylate and phenolate–carboxylic acid tautomeric forms. The probable formation of such an intermediate form is supported by quantum chemical calculations. Being the characteristic feature of this form, a short distance between the phenolic and carboxylate O atoms is observed in the 4‐nitro‐substituted anion, as well as in the structures of some 3,5‐dinitrosalicylates reported in the literature.     

Near-Infrared Photoactive Aza-BODIPY: Thermally Robust and Photostable Photosensitizer and Efficient Electron Donor

We report herein the synthesis of aza-BODIPY substituted with strongly electron-donating p-(diphenylamino)phenyl substituents (p-Ph₂N−) at 3,5-positions. The presence of p-Ph₂N− groups lowers the energy of the singlet excited state (E_s) to 1.48 eV and induces NIR absorption with λ_abs at 789 nm in THF. The compound studied is weakly emissive with the emission band (λ_f) at 837 nm and with the singlet lifetime (τ_S) equal to 100 ps. Nanosecond laser photolysis experiments of the aza-BODIPY in question revealed T₁→T_n absorption spanning from ca. 350–550 nm with the triplet lifetime (τ_T) equal to 21 μs. By introducing a heavy atom (Br) into the structure of the aza-BODIPY, we managed to turn it into a NIR operating photosensitizer. The photosensitized oxygenation of the model compound–diphenylisobenzofuran (DPBF)-proceedes via Type I and/or Type III mechanism without formation of singlet oxygen (¹O₂). As estimated by CV/DPV measurements, the p-Ph₂N− substituted aza-BODIPYs studied exhibits oxidation processes at relatively low oxidation potentials (E_ox¹), pointing to the very good electron-donating properties of these molecules. Extremely high photostability and thermal robustness up to approximately 300 °C are observed for the p-Ph₂N− substituted aza-BODIPYs.     

Mechanistic Investigation of Phosphate Ester Bond Cleavages of Glycylphosphoserinyltryptophan Radical Cations under Low-Energy Collision-Induced Dissociation

Under the conditions of low-energy collision-induced dissociation (CID), the canonical glycylphosphoserinyltryptophan radical cation having its radical located on the side chain of the tryptophan residue ([G _p SW]^?+) fragments differently from its tautomer with the radical initially generated on the α-carbon atom of the glycine residue ([G^? _p SW]⁺). The dissociation of [G^? _p SW]⁺ is dominated by the neutral loss of H₃PO₄ (98 Da), with backbone cleavage forming the [b₂ – H]^?+/y₁ ⁺ pair as the minor products. In contrast, for [G _p SW]^?+, competitive cleavages along the peptide backbone, such as the formation of [G _p SW – CO₂]^?+ and the [c₂?+?2H]⁺/[z₁ – H]^?+ pair, significantly suppress the loss of neutral H₃PO₄. In this study, we used density functional theory (DFT) to examine the mechanisms for the tautomerizations of [G^? _p SW]⁺ and [G _p SW]^?+ and their dissociation pathways. Our results suggest that the dissociation reactions of these two peptide radical cations are more efficient than their tautomerizations, as supported by Rice–Ramsperger–Kassel–Marcus (RRKM) modeling. We also propose that the loss of H₃PO₄ from both of these two radical cationic tautomers is preferentially charge-driven, similar to the analogous dissociations of even-electron protonated peptides. The distonic radical cationic character of [G^? _p SW]⁺ results in its charge being more mobile, thereby favoring charge-driven loss of H₃PO₄; in contrast, radical-driven pathways are more competitive during the CID of [G _p SW]^?+.

Hydrocarbation of CC Bonds: Quantification of the Nucleophilic Reactivity of Ynamides

Donor‐substituted diarylcarbenium ions Ar₂CH⁺ react with ynamides to give 1‐amido‐substituted allyl cations (α,β‐unsaturated iminium ions). Kinetic studies show that these adducts, which correspond to the addition of a C? H bond across the C?C bond, are formed stepwise with initial formation of keteniminium ions and subsequent 1,3‐hydride shifts. The linear correlations between the second‐order rate constants (lg k₂, 20 °C) with the electrophilicity parameters E of the diarylcarbenium ions allow us to include ynamides in our comprehensive nucleophilicity scale and thus predict potential electrophilic reaction partners.     

Photoconductivity of C60‐doping benzocarbazole‐substituted polysiloxanes on an operating wavelength of 633 nm

A novel polysiloxane having benzo[a]carbazole, benzo[a,i]carbazole, and benzo[a,g]carbazole pendant groups has been synthesized and characterized. We studied the electric‐field‐induced xerographic properties, the photogeneration efficiency, and the photoconductivity of C₆₀‐doping polymeric composites at a wavelength of 633 nm. C₆₀‐doping with benzo[a,g]carbazole‐substituted polysiloxane shows a photogeneration efficiency of 56.55 × 10^?5 and a photoconductivity of 52.2 pS/cm at E = 100 V/µm, which was found to be higher than that of the other benzocarbazole‐substituted polysiloxane composites. Copyright © 2009 John Wiley & Sons, Ltd.     

The structure and decay dynamics of exciplexes derived from dibenzoyl-methanatoboron difluoride and alkylbenzenes in cyclohexane

Dibenzomethanatoboron difluoride (DBMBF2) interacted with alkylbenzenes from its singlet excited state to form exciplexes ranging from weak polarity up to contact radical ion pairs (CRIP); this exciplex series shows the characteristics in the Marcus "normal" region. In cyclohexane these exciplexes gave intense fluorescence spectra and high quantum yields (phi(infinity)ex). The dipole moment of these exciplexes calculated from the solvatochromic shift of the fluorescence maximums (vmax) was used to estimate the coefficient ("a" and "c") of the CT and LE terms in the exciplex wavefunction. On the basis of the measured lifetimes and phi(infinity)ex of these exciplexes, the radiative (k(ex)f) and nonradiative (k(ex)NR) rate constants were calculated. The former k(ex)f were also computed from a semi-empirical approach based on the assumption that the exciplex wavefunction could be adequately described by CT and LE states, and that *DBMBF2 primarily contributes to the probability of exciplex emission. Two results agree with each other with small systematic deviations for those less polar exciplexes. The plots of k(ex)r and k(ex)NR (or their logarithmic value) against the LE contribution (c2) and transition energy gaps (hvmax) afford better correlation than those against -deltaG(-et). This indicates the role played by the LE contribution in generating the stabilization energy (U(s)) in these exciplexes through the A-D+)<==> *AD) resonance interaction; U(s), in turn, modifies -deltaG(-et) to afford the decay driving force hvmax. Also, those plots from k(ex)f values (being determined directly from experiments) show better correlation than those from k(ex)NR. In contrast to the CRIP type exciplexes in the Marcus "inverted" region, these k(ex)f and k(ex)NR increase in the common trend with increasing transition energy gaps. The k(ex)NR plots show less steep slopes and attains more quickly a minimum toward the CRIP region; the latter is identified as the turning point from the "normal" to "inverted" region. Both the attenuation and reversal of the k(ex)NR value with increasing polarity are believed to be generated by the emerging contribution of the intersystem crossing process as an additional nonradiative process, which is induced by the increased spin-orbit coupling in highly polar exciplexes.     

Study of carbonaceous clusters in irradiated polycarbonate with UV–vis spectroscopy

The formation of carbonaceous clusters in ion‐irradiated polymer films was investigated extensively. Information about these clusters may be obtained with ultraviolet–visible (UV–vis) spectroscopy. The optical band gap (E_g), calculated from the absorption edge of the UV spectra of these polymers, can be correlated to the number of carbon atoms (N) in a cluster with the modified Tauc equation. The structure of the cluster is also related to E_g; for example, a six‐membered‐benzene‐ring‐type structure has an E_g of ≈5.3 eV, whereas a buckminsterfullerene‐type structure has an E_g of ≈4.9 eV. These clusters are responsible for the electrical conductivity in these films. In this work, polycarbonate films (20 μm thick) were irradiated with 45‐MeV Li ions at fluences of 1 × 10¹² to 1 × 10¹³ cm⁻² and were characterized with UV–vis spectroscopy and impedance measurements. The E_g values, calculated from the absorption edge in the 280–315‐nm region with the Tauc relation, varied from 4.39 to 4.35 eV for the pristine and various irradiated samples, respectively. The cluster size showed a range of 60–62 carbon atoms per cluster. The sheet conductivity (σ_dc) and loss (tan δ) values of 10⁻¹⁶ Ω⁻¹cm⁻¹ and 10⁻³ for the pristine sample changed to 10⁻¹⁵ Ω⁻¹cm⁻¹ and 10⁻², respectively, for the irradiated samples. This increase in the values of σ_dc and tan δ may be correlated to the increase in the size of the carbonaceous clusters. This study provides insight into the mechanism of electrical conductivity in irradiated polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1589–1594, 2000     

Azo‐Coupling Reactions of Para‐X‐Benzenediazonium Cations with 3‐Ethoxythiophene in Acetonitrile

Kinetic studies for the azo‐coupling reactions of 3‐ethoxythiophene 1 with a series of 4‐X‐substituted diazonium cations 2a‐e (X = OCH₃, CH₃, H, Cl, and NO₂) have been investigated in acetonitrile at 20°C. The second‐order rate constants have been employed to determine the nucleophilicity parameters N and s of the thiophene 1 according the Mayr equation. Thus, the nucleophile‐specific parameters N and s of thiophene 1 have been derived and compared with the reactivities of other C‐nucleophiles in acetonitrile (pyrroles, furan, indoles, etc.). The Yukawa–Tsuno plot resulted in an excellent correlation (R² = 0.9980) with an r value of 0.89, suggesting that the nonlinear Hammett plot observed in the present work is due to resonance demand of the π–electron donor substituent of on the –N₂⁺ moiety. Importantly, using the concept of global electrophilicity (ω) proposed by Parr, we successfully predict the electrophilicity parameters E of seven substituted diazonium cations whose experimental data are available.