Strong Charge-transfer Complex Research Articles

Thermoplastic polyimide with good comprehensive performance based on carbazole groups and short flexible linkages

AbstractThermoplastic polyimides (TPIs) can meet the requirements of advanced electronic packaging such as flexible copper clad laminate (FCCL) applied under extreme conditions, attributed to their excellent thermal stability, mechanical properties, electrical insulation and chemical resistance. With the increasing demands for electronic devices, such as applications in high‐frequency communication, it is highly desirable to develop high‐performance TPI with good thermoplasticity, thermal stability, mechanical properties and dielectric properties. However, there is a trade‐off between thermoplasticity and other properties. Herein, we introduce an effective strategy to afford TPIs with good performance by copolymerization of a diamine monomer consisted of a coplanar aromatic carbazole group and a short flexible linkage methylene group. On the one hand, short flexible linkages help improve the flexibility of polymer chains and are not too much to enable small‐scale molecular motions below Tg. On the other hand, coplanar aromatic donor groups benefit to strong charge transfer complex (CTC) interaction and π‐π stacking interaction. Based on this strategy, the resultant TPI possesses good thermoplasticity with a proper Tg of 348 °C. Meanwhile, it preserves a low coefficient thermal expansion of 48 ppm K−1, low dielectric constant/dielectric loss factor of 3.27/0.0073 at 10 GHz, and tensile strength of ca. 78 MPa.

The Synergistic Effect of Triazine and Phosphaphenanthrene Units on the Physico-Chemical Behavior of Polyimides.

With the aim to develop polymers with appealing, multifunctional characteristics, a series of polyimides were designed by anchoring 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) units on the main polymer chains containing 1,3,5-triazine and several flexible moieties, such as ether, hexafluoroisopropylidene, or isopropylidene. A detailed study was conducted to establish structure-property correlations, with a focus on the synergistic effectiveness of triazine and DOPO moieties on the overall features of polyimides. The results evidenced good solubility of the polymers in organic solvents, their amorphous nature with short-range regular-packed polymer chains, and high thermal stability with no glass transition temperature below 300 °C. Spectrophotometric measurements revealed the existence of a strong charge transfer complex in these polymers that led to a "black" appearance, which generated broad absorption bands spanning on the overall visible range. Nevertheless, these polymers displayed green light emission associated with 1,3,5-triazine emitter. The electrochemical characteristics of the polyimides in solid state demonstrated their strong n-type doping character induced by three different structural elements with electron-acceptance capability. The useful properties of these polyimides, including optical, thermal, electrochemical, aesthetics, and opaqueness, endow them with several possible applications in the microelectronic field, such as protecting layers for the inner circuits against UV light deterioration.

The Importance of Electrostatics and Polarization for Noncovalent Interactions: Ionic Hydrogen Bonds vs Ionic Halogen Bonds

A series of 26 hydrogen-bonded complexes between Br− and halogen, oxygen and sulfur hydrogen-bond (HB) donors is investigated at the M06-2X/6–311 + G(2df,2p) level of theory. Analysis using a model in which Br− is replaced by a point charge shows that the interaction energy ({Delta E}_{Int}) of the complexes is accurately reproduced by the scaled interaction energy with the point charge ({Delta E}_{Int}^{PC}).This is demonstrated by {Delta E}_{Int}=0.86{Delta E}_{Int}^{PC} with a correlation coefficient, R2 =0.999. The only outlier is (Br-H-Br)−, which generally is classified as a strong charge-transfer complex with covalent character rather than a HB complex. {Delta E}_{Int}^{PC} can be divided rigorously into an electrostatic contribution ({Delta E}_{ES}^{PC}) and a polarization contribution ({Delta E}_{Pol}^{PC}).Within the set of HB complexes investigated, the former varies between -7.2 and -32.7 kcal mol−1, whereas the latter varies between -1.6 and -11.5 kcal mol−1. Compared to our previous study of halogen-bonded (XB) complexes between Br− and C–Br XB donors, the electrostatic contribution is generally stronger and the polarization contribution is generally weaker in the HB complexes. However, for both types of bonding, the variation in interaction strength can be reproduced accurately without invoking a charge-transfer term. For the Br−···HF complex, the importance of charge penetration on the variation of the interaction energy with intermolecular distance is investigated. It is shown that the repulsive character of {Delta E}_{Int} at short distances in this complex to a large extent can be attributed to charge penetration.

Ultrafast Charge-Transfer in Panchromatic, Push-Pull Porphyrin-Tetracyanobuta-1,3-Diene-Donor Conjugates

Light-induced electron transfer is a key step in solar energy conversion, and a process with myriad relevance in the fields of artificial photosynthesis, photocatalysis, photoconductivity, and molecular photovoltaics. In a donor-acceptor, D-A system, light absorption by a chromophore results in the formation of an excited state species, followed by a charge transfer between an electron donor (D) and an electron acceptor (A) leading to a charge-separated D .+ -A .- species.The optimization of these processes and comprehending their mutual interplay to ultimately achieve charge-separated species is of paramount importance for the development of efficient systems in solar light conversion schemes. As part of our continued effort in this field of research,1-3 using a combination of cycloaddition-retroelectrocyclization reaction, free-base and zinc porphyrins (H2P and ZnP) are decorated at their b-pyrrole positions with strong charge-transfer complexes, viz., tetracyanobuta-1,3-diene (TCBD)-DONOR as a novel class of push-pull systems.4 The physicochemical properties of these compounds have been investigated using a range of electrochemical, spectroelectrochemical, DFT as well as steady-state and time-resolved spectroscopic techniques. Charge transfer interactions between the porphyrin and the electron-withdrawing TCBD directly attached to the porphyrin p-system extended the absorption well into the near-infrared region. The occurrence of photoinduced charge transfer and separation has been established by complementary transient absorption spectral studies followed by global and target data analyses. The role of central metal in the porphyrin cavity and the nature of the donor in governing these photo events will be discussed. Khan, Y. Jang, Y. Patil, R. Misra, and F. D’Souza, Photoinduced Charge Separation Prompted Intervalence Charge Transfer in a Bis(thienyl)diketopyrrolopyrrole Bridged Donor-TCBD Push-Pull System (hot article), Angew. Chem. Int. Ed. 2021, 60, 20518–20527.S. Yadav, A. Z. Alsaleh,R. Misra and F. D’Souza, ‘Excited State Charge Transfer Leading to Charge Stabilization via an Electron Exchange Mechanism in Symmetric, Central Triphenylamine Derived, Dimethylaminophenyl-Tetracyanobutadiene Donor-Acceptor Conjugates, Chem. Sci. 2021, 12, 1109-1120.Pinjari, A. Z. Alsaleh,Y. Patil, R. Misra and F. D’Souza, Interfacing High-Energy Charge Transfer States to a Near-IR Sensitizer for Efficient Electron Transfer upon Near-IR Irradiation’ Angew. Chem. Int. Ed. 2020, 59, 23697-23705.Sekaran, A. Dawson, Y. Jang, K. V. MohanSingh, R. Misra and F. D’Souza, Charge-Transfer in Panchromatic Porphyrin-Tetracyanobuta-1,3-diene-Donor Conjugates: Switching the Role of Porphyrin in the Charge Separation Process, Chem. Eur. J. 2021, 27, 14335-14344 Figure 1

Why do ammonium salt/phenol-based deep eutectic solvents show low viscosity?

• Glycolic acid (GA) DESs exhibit higher activation energy compared with phenolics DESs. • A strong charge transfer complex was found between GA and ammonium salt. • Hydrogen bonds of GA DES are partially covalent and partially electrostatic. • Interaction network stability of GA DES is more robust owing to strong covalency. • This work paves the way for rational design of novel DES with low viscosity. The viscosity of deep eutectic solvents (DESs) plays an important role in determining how they are used industrially. In order to gain a deeper insight into the parameters which affect the viscosity of ionic DES, a series of systems composed of ammonium salts and two types of representative donors were prepared and characterized. They were investigated by quantum-chemistry calculations and molecular dynamics simulations. The viscosity of phenol/4-methylphenol-based system is much lower than that of glycolic acid-based system. Moreover, DESs containing glycolic acid exhibit higher activation energy values compared with DESs containing phenolics. It was found the existence of a strong charge transfer complex between glycolic acid and ammonium salt, thus suggesting its vital role in the fluidity difference of studied mixtures. The hydrogen bonds of glycolic acid-based system are partially covalent and partially electrostatic, manifested via atoms in molecules (AIM) analysis. Additionally, Cl − ⋯HO phenolic hydroxyl is expected to be less covalent than Cl − ⋯HO carboxyl , which is also identified by lower delocalization index in the AIM basin. The interaction network stability of glycolic acid-based DES is more robust than that of phenolics-based one due to the strong covalency of hydrogen bond. This is the main reason that ammonium salt/phenol-based DESs show low viscosity. This work gives new perspectives on more rational design of novel DES with low viscosity.

Charge-Transfer in Panchromatic Porphyrin-Tetracyanobuta-1,3-Diene-Donor Conjugates: Switching the Role of Porphyrin in the Charge Separation Process.

Using a combination of cycloaddition-retroelectrocyclization reaction, free-base and zinc porphyrins (H2 P and ZnP) are decorated at their β-pyrrole positions with strong charge transfer complexes, viz., tetracyanobuta-1,3-diene (TCBD)-phenothiazine (3 and 4) or TCBD-aniline (7 and 8), novel class of push-pull systems. The physico-chemical properties of these compounds (MP-Donor and MP-TCBD-Donor) have been investigated using a range of electrochemical, spectroelectrochemical, DFT as well as steady-state and time-resolved spectroscopic techniques. Ground-state charge transfer interactions between the porphyrin and the electron-withdrawing TCBD directly attached to the porphyrin π-system extended the absorption features well into the near-infrared region. To visualize the photo-events, energy level diagrams with the help of free-energy calculations have been established. Switching the role of porphyrin from the initial electron acceptor to electron donor was possible to envision. Occurrence of photoinduced charge separation has been established by complementary transient absorption spectral studies followed by global and target data analyses. Better charge stabilization in H2 P derived over ZnP derived conjugates, and in phenothiazine derived over aniline derived conjugates has been possible to establish. These findings highlight the importance of the nature of porphyrins and second electron donor in governing the ground and excited state charge transfer events in closely positioned donor-acceptor conjugates.

Polyimides containing a novel bisbenzoxazole with high T g and low CTE.

A novel diamine named (2,2′-bibenzoxazole)-5,5′-diamine (DBOA) and derived polyimides (PIs) were successfully synthesized. The rigid, linear, symmetrical molecular structure and the strong charge transfer complex (CTC) were considered to be the reasons for the improved molecular packing and enhanced thermal properties of the polymers. These DBOA based PIs exhibited a higher glass transition temperature (Tg) and lower coefficient of thermal expansion (CTE) than traditional benzoxazole (BOA) based PIs. Meanwhile, the PI derived from DBOA and BPDA (3,3′,4,4′-biphenyltetracarboxylic dianhydride) exhibited high Tg (395 °C) and low CTE (8.9 ppm per °C), and is expected to be applied in organic light-emitting diode (OLED) displays.

Multiple Charge Transfer Dynamics in Colloidal CsPbBr3 Perovskite Quantum Dots Sensitized Molecular Adsorbate

Ultrafast charge-transfer (CT) dynamics has been verified in CsPbBr3 (CPB) quantum dot (QD)–4,5-dibromofluorescein (DBF) composite materials, which form a strong CT complex in the ground state and can absorb more photons in the red region of the solar spectrum. Cyclic voltammetry and steady state luminescence studies suggest that the conduction (CB) and valence bands (VB) of CPB lie, respectively, below the LUMO and the HOMO of the DBF molecule. Steady state and time-resolved luminescence measurements with selective photoexcitation reveal the photoexcited hole transfer from CPB QDs to the DBF molecule, which is thermodynamically viable. Additionally, a red-shifted PL band was detected upon excitation of the CT complex that has been attributed to CT luminescence. Femtosecond transient absorption measurements have been performed to measure the hole transfer and direct electron transfer processes in the composite system and have been measured to be 1–1.25 ps and <100 fs, respectively. Dual behavior of the DB...

Manipulating the Surface Chemistry of Quantum Dots for Sensitive Ratiometric Fluorescence Detection of Sulfur Dioxide.

Herein, we report a novel approach to the rapid visual detection of gaseous sulfur dioxide (SO2) by manipulating the surface chemistry of 3-aminopropyltriethoxysilane (APTS)-modified quantum dots (QDs) using fluorescent coumarin-3-carboxylic acid (CCA) for specific reaction with SO2. The CCA molecules are attached to the surface amino groups of the QDs through electrostatic attraction, thus the fluorescence of CCA is greatly suppressed because of the formation of an ion-pair complex between the ATPS-modified QDs and CCA. Such an interaction is vulnerable to SO2 because SO2 can readily react with surface amino groups to form strong charge-transfer complexes and subsequently release the strongly fluorescent CCA molecules. The mechanism has been carefully verified through a series of control experiments. Upon exposure to different amounts of SO2, the fluorescent color of the nanoparticle-based sensor displays continuously changes from red to blue. Most importantly, the approach owns high selectivity for SO2 and a tolerance of interference, which enables the sensor to detect SO2 in a practical application. Using this fluorescence-based sensing method, we have achieved a visual detection limit of 6 ppb for gaseous SO2.

Ultrafast Hole- and Electron-Transfer Dynamics in CdS–Dibromofluorescein (DBF) Supersensitized Quantum Dot Solar Cell Materials

Ultrafast charge-transfer (CT) dynamics has been demonstrated in CdS quantum dot (QD)–4′,5′-dibromofluorescein (DBF) composite materials, which form a strong CT complex in the ground state. Charge separation in the CdS–DBF composite was found to take place in three different pathways, by transferring the photoexcited hole of CdS to DBF, electron injection from photoexcited DBF to the CdS QD, and direct electron transfer from the HOMO of DBF to the conduction band of the CdS QD. CT dynamics was monitored by direct detection of the DBF cation radical and electron in the QD in the transient absorption spectra. Electron injection and the electron-transfer process are found to be pulse-width-limited (<100 fs); however, the hole-transfer time was measured to be 800 fs. Charge recombination dynamics has been found to be very slow, confirming spatial charge separation in the CdS–DBF supersensitized quantum dot system. Grand charge separation process suggests that the CdS–DBF supersensitized quantum dot system can be used as superior materials for quantum dot solar cells (QDSCs).

Human Augmenter of Liver Regeneration: Probing the Catalytic Mechanism of a Flavin-Dependent Sulfhydryl Oxidase

Augmenter of liver regeneration is a member of the ERV family of small flavin-dependent sulfhydryl oxidases that contain a redox-active CxxC disulfide bond in redox communication with the isoalloxazine ring of bound FAD. These enzymes catalyze the oxidation of thiol substrates with the reduction of molecular oxygen to hydrogen peroxide. This work studies the catalytic mechanism of the short, cytokine form of augmenter of liver regeneration (sfALR) using model thiol substrates of the enzyme. The redox potential of the proximal disulfide in sfALR was found to be approximately 57 mV more reducing than the flavin chromophore, in agreement with titration experiments. Rapid reaction studies show that dithiothreitol (DTT) generates a transient mixed disulfide intermediate with sfALR signaled by a weak charge-transfer interaction between the thiolate of C145 and the oxidized flavin. The subsequent transfer of reducing equivalents to the flavin ring is relatively slow, with a limiting apparent rate constant of 12.4 s(-1). However, reoxidation of the reduced flavin by molecular oxygen is even slower (2.3 s(-1) at air saturation) and thus largely limits turnover at 5 mM DTT. The nature of the charge-transfer complexes observed with DTT was explored using a range of simple monothiols to mimic the initial nucleophilic attack on the proximal disulfide. While β-mercaptoethanol is a very poor substrate of sfALR (∼0.3 min(-1) at 100 mM thiol), it rapidly generates a mixed disulfide intermediate allowing the thiolate of C145 to form a strong charge-transfer complex with the flavin. Unlike the other monothiols tested, glutathione is unable to form charge-transfer complexes and is an undetectable substrate of the oxidase. These data are rationalized on the basis of the stringent steric requirements for thiol-disulfide exchange reactions. The inability of the relatively bulky glutathione to attain the in-line geometry required for efficient disulfide exchange in sfALR may be physiologically important in preventing the oxidase from catalyzing the potentially harmful oxidation of intracellular glutathione.

Structure and Intermolecular Vibrations of Perylene·trans-1,2-Dichloroethene, a Weak Charge-Transfer Complex

The vibronic spectra of strong charge-transfer complexes are often congested or diffuse and therefore difficult to analyze. We present the spectra of the π-stacked complex perylene trans-1,2-dichloroethene, which is in the limit of weak charge transfer, the electronic excitation remaining largely confined to the perylene moiety. The complex is formed in a supersonic jet, and its S0 ↔ S1 spectra are investigated by two-color resonant two-photon ionization (2C-R2PI) and fluorescence spectroscopies. Under optimized conditions, vibrationally cold (T(vib) ≈ 9 K) and well resolved spectra are obtained. These are dominated by vibrational progressions in the “hindered-rotation” Rc intermolecular vibration with very low frequencies of 11 (S0) and 13 cm(–1) (S1). The intermolecular Tz stretch and the Ra and Rb bend vibrations are also observed. The normally symmetry-forbidden intramolecular 1a(u) “twisting” vibration of perylene also appears, showing that the π- stacking interaction deforms the perylene moiety, lowering its local symmetry from D2h to D2. We calculate the structure and vibrations of this complex using six different density functional theory (DFT) methods (CAM-B3LYP, BH&HLYP, B97-D3, ωB97X-D, M06, and M06-2X) and compare the results to those calculated by correlated wave function methods (SCS-MP2 and SCS-CC2). The structures and vibrational frequencies predicted with the CAM-B3LYP and BH&HLYP methods disagree with the other calculations and with experiment. The other four DFT and the ab initio methods all predict a π-stacked “centered” structure with nearly coplanar perylene and dichloroethene moieties and intermolecular binding energies of D(e) = −20.8 to −26.1 kJ/mol. The 000 band of the S0 → S1 transition is red-shifted by δν = −301 cm(–1) relative to that of perylene, implying that the D(e) increases by 3.6 kJ/mol or 15% upon electronic excitation. The intermolecular vibrational frequencies are assigned to the calculated Rc, Tz, Ra, and Rb vibrations by comparing to the observed/calculated frequencies and S0 ↔ S1 Franck–Condon factors. Of the three TD-DFT methods tested, the hybrid-meta-GGA functional M06-2X shows the best agreement with the experimental electronic transition energies, spectral shifts, and vibronic spectra, closely followed by the ωB97X-D functional, while the M06 functional gives inferior results.

Ultrafast Electron-Transfer and -Trapping Dynamics in the Inter-Band-Gap States of ZrO2 Nanoparticles Sensitized by Baicalein

Charge-transfer dynamics of baicalein (BIC) adsorbed on TiO2 and ZrO2 nanoparticles have been studied by steady-state and time-resolved emission spectroscopy and femtosecond transient absorption spectroscopy. Steady-state absorption and emission studies indicate that the BIC molecule forms a charge-transfer (CT) complex with both TiO2 and ZrO2 nanoparticles through its pyrogallol moiety and quinone moiety, respectively. On exciting the CT complex a new charge-transfer emission band was detected only in the BIC–ZrO2 system not from the BIC–TiO2 system. On photoexcitation of BIC–TiO2 and BIC–ZrO2 systems by an ultrafast laser pulse, electron injection into the nanoparticles has been confirmed for both systems by direct detection of the electron in the nanoparticle and cation radical of BIC (BIC•+) in the transient spectra. Ultrafast transient decay kinetics of the BIC•+ cation radical suggests that back electron transfer (ET) dynamics is multiexponential on both nanoparticle surfaces. Due to the fact that the first excited state (S1) state lies below the conduction band edge of ZrO2, our measurements suggest that in the case of ZrO2 electrons are directly injected into the surface states and getting trapped in different surface states with a time constant of 900 fs before slow recombination with BIC cation radical. From the above investigation we suggest that electron injection into the surface states of nanoparticles is only feasible or facilitated when the adsorbate forms a strong CT complex with the nanoparticles.

A complementary disk-shaped π electron donor–acceptor pair with high binding affinity

Hexaazatriphenylene triimides (HAT) have been shown to be a novel class of disk-shaped π electron acceptors that pair with donors with complementary shape and electron demands, such as triphenylene (TP) derivatives. The donor–acceptor (DA) pair forms a strong charge-transfer complex in CH2Cl2 with an association constant of 2.6 × 104 M−1, which is remarkable for a recognition system that is solely based on electrostatic interactions between two π systems. NMR studies, along with molecular modelling, have revealed a complementary charge distribution and an “eclipsed” conformation in the DA complex. The strong DA interaction results in extended alternating DA stacks in the thin film and mesophases.

On the Nature of Stabilization in Weak, Medium, and Strong Charge-Transfer Complexes: CCSD(T)/CBS and SAPT Calculations

Weak, medium, and strong charge-transfer (CT) complexes containing various electron donors (C(2)H(4), C(2)H(2), NH(3), NMe(3), HCN, H(2)O) and acceptors (F(2), Cl(2), BH(3), SO(2)) were investigated at the CCSD(T)/complete basis set (CBS) limit. The nature of the stabilization for these CT complexes was evaluated on the basis of perturbative NBO calculations and DFT-SAPT/CBS calculations. The structure of all of the complexes was determined by the counterpoise-corrected gradient optimization performed at the MP2/cc-pVTZ level, and most of complexes possess a linear-like contact structure. The total stabilization energies lie between 1 and 55 kcal/mol and the strongest complexes contain BH(3) as an electron acceptor. When ordering the electron donors and electron acceptors on the basis of these energies, we obtain the same order as that based on the perturbative E2 charge-transfer energies, which provides evidence that the charge-transfer term is the dominant energy contribution. The CCSD(T) correction term, defined as the difference between the CCSD(T) and MP2 interaction energies, is mostly small, which allows the investigation of the CT complexes of this type at the "cheap" MP2/CBS level. In the case of weak and medium CT complexes (with stabilization energy smaller than about 15 kcal/mol), the dominant stabilization originates in the electrostatic term; the dispersion as well as induction and δ(HF) terms covering the CT energy contribution are, however, important as well. For strong CT complexes, induction energy is the second (after electrostatic) most important energy term. The role of the induction and δ(HF) terms is unique and characteristic for CT complexes. For all CT complexes, the CCSD(T)/CBS and DFT-SAPT/CBS stabilization energies are comparable, and surprisingly, it is true even for very strong CT complexes with stabilization energy close to 50 kcal/mol characteristic by substantial charge transfer (more than 0.3 e). It is thus possible to conclude that perturbative DFT-SAPT analysis is robust enough to be applied even for dative-like complexes with substantial charge transfer.

Organic photosensitizers with catechol groups for dye-sensitized photovoltaics

We have carried out a study on a number of organic dyes to identify photosensitizers for dye-sensitized photovoltaic applications with well matched redox potentials and good absorption characteristics. Triphenylmethyne, anthraquinone and xanthene dyes with catechol groups were chosen as the most promising dyes for fabrication of nanocrystalline TiO 2 based dye-sensitized photovoltaic (DSPV) cells using a liquid electrolyte. From our study, a xanthene dye, 9-phenyl-2,3,7-trihydroxy-6-fluorone is found to be efficient photosensitizer in DSPV cells compared to other dyes when a liquid electrolyte is used. Redox potentials, absorption and photovoltaic properties of these dyes are presented. Two possible ways of electron transfer, namely direct charge transfer via formation of strong charge transfer complexes and the conventional excited state electron injection through the HOMO–LUMO energy level matching are discussed.

Dynamics of Interfacial Electron Transfer from Photoexcited Quinizarin (Qz) into the Conduction Band of TiO2 and Surface States of ZrO2 Nanoparticles

Electron injection and back-electron-transfer (BET) dynamics of quinizarin (Qz) adsorbed on TiO2 and ZrO2 nanoparticles has been studied by femtosecond transient absorption spectroscopy in the visible and near-IR region. A good fraction of Qz forms a charge-transfer (CT) complex while being adsorbed on the TiO2 or ZrO2 nanoparticles surface. Following photoexcitation of Qz/TiO2 and Qz/ZrO2 systems, electron injection into the nanoparticles has been confirmed for both the systems by direct detection of electron in the nanoparticle and cation radical of Qz (Qz.+). The dynamics of BET from TiO2 and ZrO2 to the parent cation has been measured by monitoring the decay kinetics of Qz•+ and electron in the nanoparticles and it is found to be multiexponential. As S1 state of Qz lies below the conduction band edge of ZrO2 so electron injection from S1 state into the nanoparticle is not thermodynamically possible. However, the detection of Qz•+ as well as injected electrons in the case of Qz/ZrO2 system confirms that electron injection also takes place in ZrO2. We have attributed this to the injection into surface states of ZrO2 nanoparticles. It has been observed that electron injection takes place in <50 fs and the majority of the injected electrons come back to the parent cation with a time constant of <1 ps for both the systems. We have observed multiphasic recombination dynamics with time constants ranging from ∼600 fs to the pico-, nano-, and microsecond time scale for both Qz/TiO2 and Qz/ZrO2 systems. Our investigation has revealed that electron injection into the surface states of nanoparticles is possible or facilitated when the adsorbed sensitizer molecule forms a strong CT complex with the semiconductor nanoparticles.

Selective preparation of polycyclic aromatic hydrocarbons. Part 5.1 Bromination of 2,7-di-tert-butylpyrene and conversion into pyrenoquinones and their pyrenoquinhydrones

T. Yamato, M. Fujimoto, A. Miyazawa and K. Matsuo, J. Chem. Soc., Perkin Trans. 1, 1997, 1201 DOI: 10.1039/A606200H

Molecular interaction of phenosafranin with surfactants and its photogalvanic effect

The spectrophotometric studies of phenosafranin (PSF) dye in aqueous solutions containing three different types of surfactants such as CTAB (cationic), SLS (anionic), and Triton X-100 (neutral) show that PSF forms a 1:1 molecular complex with Triton X-100 and SLS, and there is no interaction of PSF with CTAB. The thermodynamic and spectrophotometric properties of these complexes suggest that PSF forms a strong charge-transfer complex with Triton X-100 whereas the interaction of PSF with SLS is coulombic in nature. The photogalvanic and photoconductivity effects in PSF-surfactants aqueous systems have been studied using platinum electrodes for both chambers, illuminated and dark, at room temperature (25°C); the PSF + Triton X-100 system only generates photovoltage and photoconductivity and it is almost reversible for a number of cycles.

Charge-transfer derivatization in fast-atom-bombardment mass spectrometry

The formation of charge-transfer complexes as derivatization reactions for fast-atom- bombardment (f.a.b.) mass spectrometry has been investigated. The donor N, N, N′, N′- tetramethyl-1,4-phenylenediamine (TMPD) and the acceptor 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) were studied. The f.a.b, spectrum of this complex in glycerol first yielded the cation radical, TMPD + at m/z 164. However, with time the dominant ion becomes (TMPD+H) +. Results suggest that although a strong charge-transfer complex is formed, protonation of the donor molecule occurs whenever possible. In dimethylsulfoxide, initial f.a.b, spectra contain (TMPD+H) +, but as solvent evaporates, this ion is supplanted by the cation radical. Increased abundances of the cation radicals for charge- transfer complexes are observed in the absence of solvent or with the use of aprotic solvents. Characteristic ultraviolet/visible spectra of charge-transfer complexes clarify the competing processes of electron and proton transfer, and their time dependence. Charge- transfer derivatization is used to increase signals for the donor cation radicals of anthracene/picric acid, pyrene/picric acid, and indole/trinitrofluorenone.