Formation Of Radical Ion Pair Research Articles

Specificity of Photochemical Energy Conversion in PhotosystemI from the Green Microalga Chlorella ohadii.

Primary excitation energy transfer and charge separation in photosystemI(PSI) from the extremophile desert green alga Chlorella ohadii grown in low light were studied using broadband femtosecond pump-probe spectroscopy in the spectral range from 400 to 850nm and in the time range from 50fs to 500ps. Photochemical reactions were induced by the excitation into the blue and red edges of the chlorophyll Qy absorption band and compared with similar processes in PSI from the cyanobacterium Synechocystissp. PCC6803. When PSI from C.ohadii was excited at 660nm, the processes of energy redistribution in the light-harvesting antenna complex were observed within a time interval of up to 25ps, while formation of the stable radical ion pair P700+A1- was kinetically heterogeneous with characteristic times of 25 and 120ps. When PSI was excited into the red edge of the Qy band at 715nm, primary charge separation reactions occurred within the time range of 7ps in half of the complexes. In the remaining complexes, formation of the radical ion pair P700+A1- was limited by the energy transfer and occurred with a characteristic time of 70ps. Similar photochemical reactions in PSI from Synechocystis6803 were significantly faster: upon excitation at 680nm, formation of the primary radical ion pairs occurred with a time of 3ps in ~30% complexes. Excitation at 720nm resulted in kinetically unresolvable ultrafast primary charge separation in 50% complexes, and subsequent formation of P700+A1- was observed within 25ps. The photodynamics of PSI from C.ohadii was noticeably similar to the excitation energy transfer and charge separation in PSI from the microalga Chlamydomonas reinhardtii; however, the dynamics of energy transfer in C.ohadii PSI also included slower components.

Water modulates the chemiexcitation efficiency of the peroxyoxalate reaction: Enhancement at low and decrease at higher concentrations

Chemiluminescence transformations are widely utilized in analytical and bioanalytical applications. Specifically, the peroxyoxalate system can be employed for sensitive quantification of a wide variety of analytes. Under anhydrous conditions, the peroxyoxalate reaction shows extremely high emission quantum yields, comparable to highly efficient bioluminescence systems. However, in aqueous media, important for bioanalytical applications, the emission efficiency suffers a significant decrease of several orders of magnitudes, although the reason for this loss in efficiency is unknown up to now.In the present work, we show the results of a detailed photophysical study on the effect of water on the chemiexcitation efficiency of the peroxyoxalate reaction using bis(2,4-dinitrophenyl) oxalate as reagent and imidazole as base catalyst in the presence of different chemiluminescence activators. It is shown that in 1,2-dimethoxyethane/water mixtures (DME:H2O), singlet quantum yields increase with increasing the water concentrations up to 0.7 mol/L, followed by a steady decrease at higher water concentrations. The decrease in quantum yields is not predominantly due to concurrent ester hydrolysis, as quantum yields could be corrected for this side reaction. This indicates that the quantum yield decrease is due to the diminished chemiexcitation efficiency in the presence of water. Using different activators, it is shown that chemiexcitation occurs via electron transfer and electron back-transfer steps between a high-energy intermediate and the activator. Additionally, its efficiency is governed by the energetics of the electron back-transfer step, in agreement with the Chemically Initiated Electron Exchange Luminescence (CIEEL) mechanism, also in partially aqueous environment. Lower excitation efficiency in aqueous media may be due to formation of solvent-separated radical ion pairs, inhibiting chemiexcitation.

Homolytic C-H Bond Activation by Phosphine-Quinone-Based Radical Ion Pairs.

Herein, we present the formation of transient radical ion pairs (RIPs) by single-electron transfer (SET) in phosphine-quinone systems and explore their potential for the activation of C-H bonds. PMes3 (Mes=2,4,6-Me3 C6 H2 ) reacts with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) with formation of the P-O bonded zwitterionic adduct Mes3 P-DDQ (1), while the reaction with the sterically more crowded PTip3 (Tip=2,4,6-iPr3 C6 H2 ) afforded C-H bond activation product Tip2 P(H)(2-[CMe2 (DDQ)]-4,6-iPr2 -C6 H2 ) (2). UV/Vis and EPR spectroscopic studies showed that the latter reaction proceeds via initial SET, forming RIP [PTip3 ]⋅+ [DDQ]⋅- , and subsequent homolytic C-H bond activation, which was supported by DFT calculations. The isolation of analogous products, Tip2 P(H)(2-[CMe2 {TCQ-B(C6 F5 )3 }]-4,6-iPr2 -C6 H2 ) (4, TCQ=tetrachloro-1,4-benzoquinone) and Tip2 P(H)(2-[CMe2 {oQtBu -B(C6 F5 )3 }]-4,6-iPr2 -C6 H2 ) (8, oQtBu =3,5-di-tert-butyl-1,2-benzoquinone), from reactions of PTip3 with Lewis-acid activated quinones, TCQ-B(C6 F5 )3 and oQtBu -B(C6 F5 )3 , respectively, further supports the proposed radical mechanism. As such, this study presents key mechanistic insights into the homolytic C-H bond activation by the synergistic action of radical ion pairs.

Homolytic C−H Bond Activation by Phosphine−Quinone‐Based Radical Ion Pairs

AbstractHerein, we present the formation of transient radical ion pairs (RIPs) by single‐electron transfer (SET) in phosphine−quinone systems and explore their potential for the activation of C−H bonds. PMes3 (Mes=2,4,6‐Me3C6H2) reacts with DDQ (2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone) with formation of the P−O bonded zwitterionic adduct Mes3P−DDQ (1), while the reaction with the sterically more crowded PTip3 (Tip=2,4,6‐iPr3C6H2) afforded C−H bond activation product Tip2P(H)(2‐[CMe2(DDQ)]‐4,6‐iPr2‐C6H2) (2). UV/Vis and EPR spectroscopic studies showed that the latter reaction proceeds via initial SET, forming RIP [PTip3]⋅+[DDQ]⋅−, and subsequent homolytic C−H bond activation, which was supported by DFT calculations. The isolation of analogous products, Tip2P(H)(2‐[CMe2{TCQ−B(C6F5)3}]‐4,6‐iPr2‐C6H2) (4, TCQ=tetrachloro‐1,4‐benzoquinone) and Tip2P(H)(2‐[CMe2{oQtBu−B(C6F5)3}]‐4,6‐iPr2‐C6H2) (8, oQtBu=3,5‐di‐tert‐butyl‐1,2‐benzoquinone), from reactions of PTip3 with Lewis‐acid activated quinones, TCQ−B(C6F5)3 and oQtBu−B(C6F5)3, respectively, further supports the proposed radical mechanism. As such, this study presents key mechanistic insights into the homolytic C−H bond activation by the synergistic action of radical ion pairs.

Insights into Single-Electron-Transfer Processes in Frustrated Lewis Pair Chemistry and Related Donor-Acceptor Systems in Main Group Chemistry.

The activation and utilization of substrates mediated by Frustrated Lewis Pairs (FLPs) was initially believed to occur solely via a two-electron, cooperative mechanism. More recently, the occurrence of a single-electron transfer (SET) from the Lewis base to the Lewis acid was observed, indicating that mechanisms that proceed via one-electron-transfer processes are also feasible. As such, SET in FLP systems leads to the formation of radical ion pairs, which have recently been more frequently observed. In this review, we aim to discuss the seminal findings regarding the recently established insights into the SET processes in FLP chemistry as well as highlight examples of this radical formation process. In addition, applications of reported main group radicals will also be reviewed and discussed in the context of the understanding of SET processes in FLP systems.

Self-Assembly Directed Organization of Fullerene-Bisporphyrins into Supramolecular Donut Structures for Excited State Charge Stabilization

Supramolecular structures formed from intermolecular association of repeated units of redox- and photo-active molecular monomers, in solution or in bulk, are of greater significance due to their importance in biology performing various functions making life on earth possible. One such supramolecular organization is ‘photosynthetic antenna-reaction centre’ complex found in green plants and bacteria which is involved in converting sunlight into chemical energy. The core antenna complex in bacteria form concentric rings, LH1 and LH2. LH1 consists of 16 ab protomers with a 68 Å hole in the middle while the LH2 consists of 9 ab protomers. These work in synergy in terms of light capture, transport and generating charge separated states.Several covalent and supramolecular structures containing fullerenes have been described in literature, however, only a few examples where the components are either redox or photochemically amphoteric. This property becomes especially important in light energy harvesting where the presence of an electron donor-acceptor pair is a basic necessity with at least one of them being photo-active to promote photoinduced charge separation leading to the formation of radical ion-pairs. Here, we describe the synthesis, by tether-directed functionalization, of a [60]fullerene e -bisadduct carrying two Zn-porphyrins (see figure inset), its supramolecular organization, and photophysical events. Remarkably, the supramolecular assembly of the present bisporphyrin-C60 forms donuts-shape aggregates (see figure for AFM image), primarily via pi-pi type charge transfer interactions, as demonstrated by means of optical absorption and emission, variable temperature 1H NMR and AFM and supported by theoretical calculations. More importantly, upon photoexcitation, the supramolecular assembly generates long-lived charge separated states of ~1 ms lifetime due to electron/hole delocalization within the supramolecular structure. The unprecedented results of the present study demonstrate the success of supramolecular organization of donor-acceptor pairs in biomimetic light energy harvesting. Figure 1

Peptoid-Conjugated Magnetic Field-Sensitive Exciplex System at High and Low Solvent Polarities.

The magnetic field effect (MFE) in exciplex emission (ExE) has been studied for decades, but it has been observed to occur only in solvents with a limited range of polarity. This limitation is mainly due to the reversible interconversion collapse between two quenching products of the photoinduced electron transfer, the exciplex and magnetic field-sensitive radical ion pair (RIP) beyond that polarity range. In a nonpolar solvent, the formation of RIPs is suppressed, whereas in a polar solvent, the probability of their re-encounter forming the exciplexes decreases. In this study, we developed new exciplex-forming (phenyl-phenanthrene)-(phenyl-N,N-dimethylaniline)-peptoid conjugates (PhD-PCs) to overcome this limitation. The well-defined peptoid structure allows precise control of the distance and the relative orientation between two conjugated moieties. Steady-state and time-resolved spectroscopic data indicate that the PhD-PCs can maintain the reversibility, which allows MFEs in ExE regardless of the solvent polarity. Subtle differences between the ExEs of the PhD-PCs were observed and explained by their exciplex geometries obtained through time-dependent density functional theory (TD-DFT) calculations.

Exclusive triplet electron transfer leading to long-lived radical ion-pair formation in an electron rich platinum porphyrin covalently linked to fullerene dyad.

The formation of a high-energy, long-lived radical ion-pair by electron transfer exclusively from the triplet excited state, is demonstrated in a newly synthesized platinum porphyrin-fullerene dyad, in which the porphyrin ring is modified with three electron rich triphenylamine entities. The spin selectivity of the electron transfer leading to the formation of the radical ion-pair is demonstrated using time-resolved optical and EPR spectroscopic techniques.

Spectral and Kinetic Study of 3-Styrylquinoxalin-2(1 H)-ones Photoreduced by N-Phenylglycine and Amines.

The photoreduction by amines and N-phenylglycine, NPG, of six styrylquinoxalin-2(1 H)-ones derivatives substituted in the styryl moiety, R-SQ, was studied by using flash photolysis. The photoreaction is initiated via a single electron transfer from the electron donor (amines or NPG) to R-SQ excited triplet state, 3R-SQ*, with the formation of a triplet state radical ion pair or a charge transfer exciplex, 3[CRIP/CTE]. These species live longer than the respective 3R-SQ* and have very similar transient spectra. In the presence of NPG, these 3[CRIP/CTE] evolve on μs time scale to the respective hydrogenated radicals, R-SQH•, whose transient spectra and reaction rate constants with NPG are reported. The identity of these hydrogenated radicals was supported by the spectra obtained with the α-H donor triethylamine and previous pulse radiolysis studies in 2-propanol. Our findings allow proposing a radical chain reaction mechanism that explains the observed spectral behavior and rationalizes formation of the main product formed by binding of four PhNHCH2• derived from NPG decarboxylation.

Zwitterionic Inorganic Benzene Valence Isomer with σ-Bonding between Two π-Orbitals

Despite the large number of plausible isomers of benzene (C6H6), only four valence isomers [(CH)6] have been experimentally detected, all of which are nonionic and possess skeletal frameworks built from localized C-C bonds. Herein, we present the isolation of a diazatetraborabenzene analogue of a hypothetical zwitterionic valence isomer of benzene. Therein, two electrons are delocalized over the four boron atoms in the six-membered B4N2 ring, which is a result of the σ-bonding interaction between two odd-electron B-B π-orbitals. Simple treatment with a crown ether leads to the formation of a paramagnetic potassium-doped radical ion pair that exhibits a thermally populated triplet character.

Spin-Correlated Radical Ion Pairs Generated in Liquid Haloalkanes Using High-Energy Radiation.

The probability of formation of spin-correlated secondary radical ion pairs (RIPs) in diluted solutions of charge acceptors in irradiated haloalkanes is believed to be extremely low due to the dissociative attachment of excess electrons to solvent molecules. Contrary to this, it has been found that spin-correlated RIPs can be formed upon irradiation in some liquid chloroalkanes with yield sufficient to observe the recombination fluorescence of the RIP's partners. This allowed the study of primary radical cations (RCs) as well as radical ionic states of molecules dissolved in haloalkanes using the method of time-resolved magnetic field effect (TR MFE) in radiation-induced fluorescence. With this method, the magnetic resonance characteristics of the solvent RCs in a series of liquid haloalkanes were examined for the first time. For the 1,2-dichloroethane RC, the rate of scavenging by solute molecules and the dominant mechanisms of paramagnetic relaxation were determined. Polysulfone and poly(ethyl methacrylate) were used to demonstrate that due to their high dissolving ability, chloroalkanes can be exploited as solvents to study the magnetic resonance characteristics of radical ionic states of polymeric molecules in solutions with the TR MFE method.

20 - Magnetic-Isotope Effects in ATP Hydrolysis Driven by Myosin as Evidence for a Free-Radical Intermediate in the Reaction

Some chemical elements have magnetic and nonmagnetic stable isotopes. The question arises if the magnetic fields of the atomic nuclei impact on efficiency of biocatalysts. Magnesium cation, Mg 2+ , serves as cofactor of ATP hydrolysis catalyzed by muscle myosin. Among three stable isotopes, 24 Mg, 25 Mg, 26 Mg with natural abundance 76.7, 10.13, 11.17 %, only 25 Mg is magnetic (nuclear spin I = 5/2) while 24 Mg and 26 Mg are nonmagnetic (I = 0). The effects of different isotopes of Mg on ATPase activity of isolated myosin were studied. It was found that the rate of the enzymatic ATP hydrolysis with 25 Mg is 2.0–2.5 times higher as compared to the nonmagnetic 24 Mg or 26 Mg. The magnetic isotope effect (MIE) was observed at physiological concentrations, 5 mM, of MgCl 2 . The similar MIE was found with zinc. Among five stable isotopes, 64 Zn, 66 Zn, 67 Zn, 68 Zn, and 70 Zn with natural abundance 48.6, 27.9, 4.1, 18.8, and 0.6 %, only 67 Zn is magnetic (I = 5/2). While Zn 2+ performs the cofactor function less efficiently than Mg 2+ , it was found that the rate of the enzymatic ATP hydrolysis with 67 Zn is 40-50 % higher as compared to the nonmagnetic 64 Zn or 68 Zn. On its own, MIE unambiguously testifies presence of a spin-selective rate-limiting step as the “bottle-neck” that is accelerated by the magnetic field of nuclear spin of 25 Mg or 67 Zn. In free-radical chemistry, MIE is the sufficient demonstration of a free-radical or radical ion pair as intermediate in the reaction under study (Buchachenko, 2013). One can assume that, under the condition of the electron-conformational excitation, there is a transfer of electron density from NH 2 group of Glu459 or OH– group of the bound water molecule to ADP3– or Mg 2+ / Zn 2+ in the active site of the enzyme with formation of the corresponding radical ion pair. Although detailed mechanisms of ability of myosin to perceive the nuclear magnetism require further investigations, the nuclear spin catalysis opens novel ways in free-radical biomedicine based on the stable magnetic isotopes. Supported by RFBR, project no. 14-04-00593.

Unveiling the Nature of Supramolecular Crown Ether-C60 Interactions

Construction of artificial photoactive devices capable of mimicking photosynthesis by transforming light into chemical potential is one of the most sought objectives in the quest for new sources of energy.[1] For this, chemists have developed a wide variety of covalent and supramolecular structures based on donor-acceptor systems where photon energy is used to produce a photoinduced electron-transfer from the donor to the acceptor moiety.[2] As a result, in 2010 our group developed a supramolecular exTTF-B18C6 ether receptor for fullerenes which showed an extraordinary affinity (K a 5.01 × 106 M-1) towards [60]fullerene.[3]This was an outstanding result as previous attempts to complex fullerenes by only one unit of exTTF were unsuccessful. Thus, the presence of crown ethers as recognition patterns seemed to be of paramount importance. In this work, we have further studied this unexpected result and exploited the potentiality of crown ethers as recognition motifs for fullerenes. Therefore, a series of exTTF-(crown ether)2 receptors, designed to host C60, has been prepared. The size of the crown ether and the nature of the heteroatoms have been systematically changed to fine tune the association constants. Electrochemical measurements and transient absorption spectroscopy assisted in corroborating a charge transfer in the ground state and in the excited state leading to the formation of radical ion pairs featuring lifetimes in the range from 12 to 21 ps. To rationalize the nature of the exTTF-(crown ether)2•C60 stabilizing interactions, theoretical calculations have been carried out, prompting to a synergetic interplay of donor-acceptor, π-π, n-π and CH···π interactions, which is the basis for the affinity of our novel receptors towards C60.

Long-lived charge carrier generation in ordered films of a covalent perylenediimide-diketopyrrolopyrrole-perylenediimide molecule.

The photophysics of a covalently linked perylenediimide-diketopyrrolopyrrole-perylenediimide acceptor-donor-acceptor molecule (PDI-DPP-PDI, 1) were investigated and found to be markedly different in solution versus in unannealed and solvent annealed films. Photoexcitation of 1 in toluene results in quantitative charge separation in τ = 3.1 ± 0.2 ps, with charge recombination in τ = 340 ± 10 ps, while in unannealed/disordered films of 1, charge separation occurs in τ < 250 fs, while charge recombination displays a multiexponential decay in ∼6 ns. The absence of long-lived, charge separation in the disordered film suggests that few free charge carriers are generated. In contrast, upon CH2Cl2 vapor annealing films of 1, grazing-incidence X-ray scattering shows that the molecules form a more ordered structure. Photoexcitation of the ordered films results in initial formation of a spin-correlated radical ion pair (electron-hole pair) as indicated by magnetic field effects on the formation of free charge carriers which live for ∼4 μs. This result has significant implications for the design of organic solar cells based on covalent donor-acceptor systems and shows that long-lived, charge-separated states can be achieved by controlling intramolecular charge separation dynamics in well-ordered systems.

Photosynthetic Donor-Acceptor Mimicry Using Near-Infrared Photosensitizers

Supramolecular multi-modular donor-acceptor systems constructed using different photo- and redox-active species have been extensively investigated in the last three decades. With suitably designed and assembled modular systems, these supramolecules have found useful in solar-fuel and solar-electricity generation and in building optoelectronic devices. Traditionally, porphyrins and phthalocyanines have extensively been used as light-harvesting and electron-transfer units in these multi-modular systems performing photoinduced energy- and electron-transfer reactions. Recently, BF2-chelated dipyrromethene (BODIPY) dyes, and their structural analogs have emerged as efficient and tunable light-absorption and luminescent species. The present contribution summarizes our recent progress in the design, synthesis, electrochemistry, and photochemistry of donor-acceptor hybrids derived from BODIPY and azaBODIPY photosensitizers with emphasis on radical ion-pair formation.

Photophysical investigations on supramolecular fullerene/phthalocyanine charge transfer interactions in solution

The photophysical features of non-covalently linked fullerenes C60 and C70 with a designed free-base phthalocyanine, namely, 2,3,9,10,16,17,23,24-octakis-(octyloxy)-29H,31H-phthalocyanine (1) have been investigated employing various spectroscopic tools like UV–vis absorption spectrophotometry, steady state and time resolved fluorescence along with proton NMR measurements in toluene. The ground state interaction between fullerenes and 1 is nicely demonstrated with the appearance of well defined charge transfer absorption bands in the visible region of the electronic spectra. Steady state fluorescence experiment reveals efficient quenching of the excited singlet state of 1 in presence of both C60 and C70. The average values of binding constants for the non-covalent complexes of C60 and C70 with 1 are determined to be ∼18,150 and ∼32,000dm3mol−1, respectively. The magnitude of K suggests that 1 preferentially binds C70 in comparison to C60 although average value of selectivity in binding is measured to be low (∼1.75). Time resolved emission measurements establish photoinduced energy transfer from the excited singlet state of 1 to fullerene in toluene. Measurements of free energy of electron transfer and free energy of radical ion-pair formation elicit that C70/1 complex is stabilized more in comparison to C60/1 complex regarding generation of charge-separated state. Proton NMR studies provide very good support in favor of effective ground state complexation between fullerenes and 1. Semi empirical theoretical calculations on fullerene/1 systems in vacuo substantiate the stronger binding between C70 and 1 in comparison to C60/1 system in terms of heat of formation value of the respective complexes, and determine the orientation of bound guest (here C70) towards the plane of 1 during complexation.

Photophysical and Theoretical Insights on Fullerene/Zincphthalocyanine Supramolecular Interaction in Solution

The present article reports photophysical studies on supramolecular interaction of a zinc phthalocyanine derivative, namely, zinc-2,9,16,23-tetra-tert-butyl phthalocyanine (1) with C(60) and C(70) in solvents having varying polarity, i.e., toluene and 1,2-dichlorobenzene (DCB). The interesting feature of the present work is the observation of charge transfer (CT) absorption bands of the fullerene/1 complexes in DCB. Utilizing the CT transition energy, many important physicochemical parameters like vertical ionization potential of 1, degrees of CT, oscillator strength, transition dipole moment, and resonance energy of interaction have been determined in the present case. The influences of 1 on the UV-vis spectral characteristics of C(60) and C(70) have been explained using a theoretical model that takes into account the interaction between electronic subsystems of 1 with fullerenes. Steady state fluorescence experiment elicits efficient quenching of the fluorescence intensity of 1 in the presence of both C(60) and C(70). The average binding constants of the C(60) and C(70) complexes of 1 (estimated by UV-vis and steady state fluorescence measurements) are determined to be 18,330 dm(3)·mol(-1) (12,595 dm(3)·mol(-1)) and 19,160 dm(3)·mol(-1) (15,292 dm(3)·mol(-1)) in toluene (DCB), respectively. Lifetime experiment yields a larger magnitude of charge separated rate constant for the C(70)/1 species. The faster charge recombination of the fullerene/1 systems observed in more polar solvent results from solvent reorganization energies. Quantum chemical calculations by the ab initio method explore the geometry and electronic structure of the supramolecules and testify the significant redistribution of charge between fullerenes and 1 during fullerene/1 interaction. A variable temperature (13)C NMR study nicely demonstrates that the end-on orientation of C(70) is very much responsible for the low selectivity in binding between C(60)/1 and C(70)/1 systems. Free energy of charge recombination and free energy of radical ion-pair formation signify that electron transfer from the excited 1 to C(60) and C(70) in the C(60)/1 and C(70)/1 complexes, respectively, is an unlikely process. Finally, transient absorption measurements in the visible region establish that energy transfer from (T)C(60*) (and (T)C(70*)) to 1 occurs predominantly in both toluene and DCB, which is subsequently confirmed by the consecutive appearance of the triplet state of 1.

Spectrophotometric studies of charge transfer complex of 8-hydroxyquinoline with 1,4-benzoquinone

The interaction between donor 8-hydroxyquinoline (HQ) and π - acceptor 1, 4-benzoqinone (BQ) has been studied in the solution as well as in the solid state. Infrared spectroscopic data indicate that a charge transfer interaction between the donor and acceptor are due to π → π*/ n → π* transition by the formation of radical ion pairs. In solution state, the formation constant, the extinction coefficient, the free energy and transition energy of the resulting charge transfer complex (CTC) were determined. The optimum experimental conditions, such as time, temperature and solvents for the CTC formation were established.

Spectrophotometric studies of charge transfer complex of 8-hydroxyquinoline with 1,4-benzoquinone

The interaction between donor 8-hydroxyquinoline (HQ) and π - acceptor 1, 4-benzoqinone (BQ) has been studied in the solution as well as in the solid state. Infrared spectroscopic data indicate that a charge transfer interaction between the donor and acceptor are due to π → π*/ n → π* transition by the formation of radical ion pairs. In solution state, the formation constant, the extinction coefficient, the free energy and transition energy of the resulting charge transfer complex (CTC) were determined. The optimum experimental conditions, such as time, temperature and solvents for the CTC formation were established. Key words: 8–Hydroxyquinoline, 1, 4–benzoqinone, the formation constant for the charge transfer complex and infrared spectroscopic.

Charge Transfer Events in Semiconducting Single-Wall Carbon Nanotubes

Electron-donating ferrocene units have been attached to SWNTs, with different degrees of functionalization. By means of a complementary series of novel spectroscopic techniques (i.e., steady-state and time-resolved), we have documented that mutual interactions between semiconducting SWNT and the covalently attached electron donor (i.e., ferrocene) lead, in the event of photoexcitation, to the formation of radical ion pairs. In the accordingly formed radical ion pairs, oxidation of ferrocene and reduction of SWNT were confirmed by spectroelectrochemistry. It is, however, shown that only a few semiconducting SWNTs [i.e., (9,4), (8,6), (8,7), and (9,7)] are susceptible to photoinduced electron transfer processes. These results are of relevant importance for the development of SWNT-based photovoltaics.