Transient Absorption Spectral characterization of Electron Transfer between Fullerenes（C60／C70）and N,N,N’，N’—Tetra（p—methylphenyl）—4,4’—diamino—1,1’—diphenyl Sulphide（TPDAS）

In search of new systems with a photoexcited redox pair which exhibits a strong and stable photoinduced absorption band to understand the photophyscial and photochemical properties of electron transfer between fullernes (C60/C70) and organic donor[N,N,N’，N’-tetra(p-methylphenyl)-4,4’-diamino-1,1’-diphenyl sulphide(TPDAS)],we studied characteristic absorption spectra in the near-IR region obtained from 532nm nanosecond laser flash photolysis of a mixture of the fullerenes (C60/C70) and TPDAS in polar solvents.When fullerenes (C60/C70)were photoexcithed,the rise of the radical anion of fullerenes (C60/C70)with the rapid decay of their excited triplet states were observed in benzonitrile.It can be deduced that the electron transfer reaction does take place from TPDAS to excithed triplet state of rullerens(C60/C70).The rate consants(ket)and quantum yiekls(φet) of this process have been also evaluated.     

Evaluation of Transient Absorption Spectra of N, N, N, N- Tetra - ( p -methylphenyl ) - 4, 4- diamino - 1, 1- diphenyl Ether ( TPDAE ) for Electron Transfer from TPDAE to Fullerenes ( C_(60) / C_(70) ) by Laser Flash Photolysis

Fullerenes C60 and C70 have high electron affinity ( 2.6 - 2.8 ev ) and readily form anions on electronchemical reduction1, which were famous as electron acceptor in photo-excitation because of symmetrical shape, large size, and properties of its p - electron system2. After observation of molecular ferromagnetism3 in the tetrakis (dimethylamino ) ethylene salt of C60 as well as the occurrence of ultra-fast photoinduced electron transfer within the dimethyl aniline - C60 complex4, prompted us…     

Evaluation of Transient Absorption Spectra of N, N, N′, N′- Tetra - ( p -methylphenyl ) - 4, 4′- diamino - 1, 1′- diphenyl Ether ( TPDAE ) for Electron Transfer from TPDAE to Fullerenes ( C60 / C70 ) by Laser Flash Photolysis

Photoinduced electron transfer processes between fullerenes (C60 / C70) and N, N, N′, N′- tetra - ( p-methylphenyl ) - 4, 4′- diamino - 1, 1′- diphenyl ether ( TPDAE ) have been studied by nanosecond laser flash photolysis. Quantum yields and rate constants of electron transfer from TPDAE to excited triplet state of fullerenes (C60 / C70 ) in benzonitrile have been evaluated by observing the transient absorption bands in the near-IR region where the excited triplet state, radical anion of fullerenes ( C60 / C70 ) and radical cations of TPDAE appear.     

富勒烯(C60／C70)与N，N，N’，N’-四-(对甲苯基)-4，4’-二胺-1，1’-二苯硒醚(TPDASe)间光诱导电子转移过程的激光光解研究

富勒烯(C60／C70)与N，N，N’，N’-四-(对甲苯基)-4，4’-二胺-1，1’-二 苯硒醚(TPDASe)间在激光光诱导条件下，发生了分子间的电子转移过程．在可见- 近红外区(600-1200nm)，观测到了TPDASe阳离子自由基、富勒烯(C60／C70)激发三 线态和阴离子自由基，在苯腈溶液中，观测瞬态谱测定了电子从TPDASe转移到富勒 烯(C60／C70)激发三线态的量子转化产率(Φet^T)和电子转移常数(Ket)．     

Photophysical Properties and Photoinduced Electron Transfer between[60]Fullerene—containing Cyclic Sulphoxide [C60—C60H8SO]and Tetrathiafulvalene（TTF） by Laser Flash Photolysis

Photoinduced electron transfer(PET) processes between C60-C6H8SO and Tetrathiafulvalene(TTF) have been studied by nanosecond laser photolysis.Quantrm yiekds(φet) and rate constants of electron transfer(ket) from TTF to excited triplet state of[60] fullerene-containing cyclic sulphoxide in benzonitrile(BN) have been evaluated by observing the transient absorption bands in the NIR region.With the decay of excited triplet state of [60]fullerene-containing cyclic suplhoxide,the rise of radical anion of [60]fullerene-containing cyclic sulphoxinde is observed.     

富勒烯（C_(60)/C_(70)）与三苯基胺间的光诱导电子转移过程研究

用激光光解方法研究了富勒烯（C_(60)/C_(70)）与三苯基胺（TPA）间的光诱 导电子转移过程。在近红外区,观测到TPA阴离子自由基,富勒烯（C_(60)/C_(70) ）激发三线态和阴离子自由基。在苯腈溶液中,利用瞬态谱测定了电子从TPA转移 到富勒烯（C_(60)/C_(70)）激发三线态的量子转化产率（Φ_(et)）和电子转移常 数（k_(et)）。     

Photoinduced Energy and Electron Transfer in Phenylethynyl‐Bridged Zinc Porphyrin–Oligothienylenevinylene–C60 Ensembles

Donor–bridge–acceptor triad (Por‐2TV‐C₆₀) and tetrad molecules ((Por)₂‐2TV‐C₆₀), which incorporated C₆₀ and one or two porphyrin molecules that were covalently linked through a phenylethynyl‐oligothienylenevinylene bridge, were synthesized. Their photodynamics were investigated by fluorescence measurements, and by femto‐ and nanosecond laser flash photolysis. First, photoinduced energy transfer from the porphyrin to the C₆₀ moiety occurred rather than electron transfer, followed by electron transfer from the oligothienylenevinylene to the singlet excited state of the C₆₀ moiety to produce the radical cation of oligothienylenevinylene and the radical anion of C₆₀. Then, back‐electron transfer occurred to afford the triplet excited state of the oligothienylenevinylene moiety rather than the ground state. Thus, the porphyrin units in (Por)‐2TV‐C₆₀ and (Por)₂‐2TV‐C₆₀ acted as efficient photosensitizers for the charge separation between oligothienylenevinylene and C₆₀.     

Highly Efficient Singlet–Singlet Energy Transfer in Light‐Harvesting [60,70]Fullerene–4‐Amino‐1,8‐naphthalimide Dyads

New C₆₀ and C₇₀ fullerene dyads formed with 4‐amino‐1,8‐naphthalimide chromophores have been prepared by the Bingel cyclopropanation reaction. The resulting monoadducts were investigated with respect to their fluorescence properties (quantum yields and lifetimes) to unravel the role of the charge‐transfer naphthalimide chromophore as a light‐absorbing antenna and excited‐singlet‐state sensitizer of fullerenes. The underlying intramolecular singlet–singlet energy transfer (EnT) process was fully characterized and found to proceed quantitatively (Φ_EnT≈1) for all dyads. Thus, these conjugates are of considerable interest for applications in which fullerene excited states have to be created and photonic energy loss should be minimized. In polar solvents (tetrahydrofuran and benzonitrile), fluorescence quenching of the fullerene by electron transfer from the ground‐state aminonaphthalimide was postulated as an additional path.     

Photoinduced electron transfer between chlorophylls (a/b) and fullerenes (C60/C70) studied by laser flash photolysis.

Photoinduced electron-transfer processes in the systems of chlorophylls (Chl) (chlorophyll-a [Chl-a] and chlorophyll-b) and fullerenes (C60/C70) in both polar and non-polar solvents have been investigated with nanosecond laser photolysis technique, observing the transient spectra in the visible/near-IR regions. By the excitation of Chl in benzonitrile (BN) it has been proved that electron transfer takes place from the triplet excited states of Chl to the ground states of C60/C70. By the excitation of C70 in BN electron transfer takes place from the ground states of Chl to the triplet excited state of C70. In both Chl the rate constants and quantum yields for the electron-transfer processes are as high as those of zinc porphyrins and zinc phthalocyanines, indicating that the long alkyl chains of Chl play no role in retarding the electron transfer. The rate constant for the electron-mediating process from the radical anion of C70 to octylviologen dication yielding the octylviologen radical cation was evaluated. The back electron-transfer process from the viologen radical cation to the radical cation of Chl-a takes place in a longer time-scale, indicating that a photosensitized electron-transfer/electron-mediating cycle is achieved.     

Photosynthetic Antenna‐Reaction Center Mimicry with a Covalently Linked Monostyryl Boron‐Dipyrromethene–Aza‐Boron‐Dipyrromethene–C60 Triad

An efficient functional mimic of the photosynthetic antenna‐reaction center has been designed and synthesized. The model contains a near‐infrared‐absorbing aza‐boron‐dipyrromethene (ADP) that is connected to a monostyryl boron‐dipyrromethene (BDP) by a click reaction and to a fullerene (C₆₀) using the Prato reaction. The intramolecular photoinduced energy and electron‐transfer processes of this triad as well as the corresponding dyads BDP‐ADP and ADP‐C₆₀ have been studied with steady‐state and time‐resolved absorption and fluorescence spectroscopic methods in benzonitrile. Upon excitation, the BDP moiety of the triad is significantly quenched due to energy transfer to the ADP core, which subsequently transfers an electron to the fullerene unit. Cyclic and differential pulse voltammetric studies have revealed the redox states of the components, which allow estimation of the energies of the charge‐separated states. Such calculations show that electron transfer from the singlet excited ADP (¹ADP*) to C₆₀ yielding ADP^.+‐C₆₀^.? is energetically favorable. By using femtosecond laser flash photolysis, concrete evidence has been obtained for the occurrence of energy transfer from ¹BDP* to ADP in the dyad BDP‐ADP and electron transfer from ¹ADP* to C₆₀ in the dyad ADP‐C₆₀. Sequential energy and electron transfer have also been clearly observed in the triad BDP‐ADP‐C₆₀. By monitoring the rise of ADP emission, it has been found that the rate of energy transfer is fast (≈10¹¹ s^?1). The dynamics of electron transfer through ¹ADP* has also been studied by monitoring the formation of C₆₀ radical anion at 1000 nm. A fast charge‐separation process from ¹ADP* to C₆₀ has been detected, which gives the relatively long‐lived BDP‐ADP^.+C₆₀^.? with a lifetime of 1.47 ns. As shown by nanosecond transient absorption measurements, the charge‐separated state decays slowly to populate mainly the triplet state of ADP before returning to the ground state. These findings show that the dyads BDP‐ADP and ADP‐C₆₀, and the triad BDP‐ADP‐C₆₀ are interesting artificial analogues that can mimic the antenna and reaction center of the natural photosynthetic systems.     

Modification of Guanine with Photolabile N‐Hydroxypyridine‐2(1H)‐thione: Monomer Synthesis,Oligonucleotide Elaboration,and Photochemical Studies

The photochemistry of N‐hydroxypyridine‐2(1H)‐thione (NHPT), inserted as a photolabile modifier at the 6‐position of 2′‐deoxyguanosine or guanosine, has been evaluated. In particular, 6‐[(1‐oxidopyridin‐2‐yl)sulfanyl]‐ ( 1a ) and 6‐[(pyridin‐2‐yl)sulfanyl]‐2′,6‐dideoxyguanosine ( 2a ), novel photolabile derivatives of the natural nucleosides, were synthesized and characterized. The observed photolysis products of 1a in organic solvents could only be rationalized by assuming a rapid equilibrium with the corresponding 6‐[(2‐thioxopyridin‐1(2H)‐yl)oxy] analogue 3a (Scheme 5). Transient spectroscopy of 1a indicated a strong triplet‐excited state suitable for triplet → triplet energy transfer or singlet‐oxygen generation. The NHPT function was stable enough for (slightly modified) automated solid‐phase oligonucleotide synthesis. The utility of the above compounds is discussed, as well as their potential use in photosensitization of reactive oxygen species in DNA.     

1‐Aryl‐4‐Silylmethyl[60]fullerenes: Synthesis,Properties, and Photovoltaic Performance

The efficient nucleophilic addition of aryl Grignard reagents (aryl=4‐MeOC₆H₄, 4‐Me₂NC₆H₄, Ph, 4‐CF₃C₆H₄, and thienyl) to C₆₀ in the presence of DMSO produced 1,2‐arylhydro[60]fullerenes after acid treatment. The reactions of the anions of these arylhydro[60]fullerenes with either dimethylphenylsilylmethyl iodide or dimethyl(2‐isopropoxyphenyl)silylmethyl iodide yielded the target compounds, 1‐aryl‐4‐silylmethyl[60]fullerenes. The properties and structures of these 1‐aryl‐4‐silylmethyl[60]fullerenes (aryl=4‐MeOC₆H₄, thienyl) were examined by electrochemical studies, X‐ray crystallography, flash‐photolysis time‐resolved microwave‐conductivity (FP‐TRMC) measurements, and electron‐mobility measurements by using a space‐charge‐limited current (SCLC) model. Organic photovoltaic devices with a polymer‐based bulk heterojunction structure and small‐molecule‐based p–n and p–i–n heterojunction configurations were fabricated by using 1‐aryl‐4‐silylmethyl[60]fullerenes as an electron acceptor. The most efficient device exhibited a power‐conversion efficiency of 3.4 % (short‐circuit current density: 8.1 mA/ cm², open‐circuit voltage: 0.69 V, fill factor: 0.59).     

Near‐IR Excitation Transfer and Electron Transfer in a BF2‐Chelated Dipyrromethane–Azadipyrromethane Dyad and Triad

A molecular dyad and triad, comprised of a known photosensitizer, BF₂‐chelated dipyrromethane (BDP), covalently linked to its structural analog and near‐IR emitting sensitizer, BF₂‐chelated tetraarylazadipyrromethane (ADP), have been newly synthesized and the photoinduced energy and electron transfer were examined by femtosecond and nanosecond laser flash photolysis. The structural integrity of the newly synthesized compounds has been established by spectroscopic, electrochemical, and computational methods. The DFT calculations revealed a molecular‐clip‐type structure for the triad, in which the BDP and ADP entities are separated by about 14 Å with a dihedral angle between the fluorophores of around 70°. Differential pulse voltammetry studies have revealed the redox states, allowing estimation of the energies of the charge‐separated states. Such calculations revealed a charge separation from the singlet excited BDP (¹BDP*) to ADP (BDP^.+‐ADP^.?) to be energetically favorable in nonpolar toluene and in polar benzonitrile. In addition, the excitation transfer from the singlet BDP to ADP is also envisioned due to good spectral overlap of the BDP emission and ADP absorption spectra. Femtosecond laser flash photolysis studies provided concrete evidence for the occurrence of energy transfer from ¹BDP* to ADP (in benzonitrile and toluene) and electron transfer from BDP to ¹ADP* (in benzonitrile, but not in toluene). The kinetic study of energy transfer was measured by monitoring the rise of the ADP emission and revealed fast energy transfer (ca. 10¹¹ s^?1) in these molecular systems. The kinetics of electron transfer via ¹ADP*, measured by monitoring the decay of the singlet ADP at λ=820 nm, revealed a relatively fast charge‐separation process from BDP to ¹ADP*. These findings suggest the potential of the examined ADP–BDP molecules to be efficient photosynthetic antenna and reaction center models.     

Mass spectrometric investigation of the aggregation and coalescence of fullerenes in C60-modified poly(N-vinylcarbazole) by UV-laser ablation

The phenomena of aggregation and coalescence of fullerenes in the UV-laser ablation time-of-flight mass spectrometric investigation of C₆₀-modified poly(N-vinylcarbazole) both in the positive and in the negative ion channels have been observed. The results indicate that in C₆₀ chemically modified PVK (C₆₀–PVK) copolymer the nascent fullerene fragments ruptured from main chain can easily coalesce into large fullerenes through collisions, whereas in the C₆₀-doped PVK the aggregation and coalescence of C₆₀ were relative weak due to nonbounding action and incomplete charge transfer behavior between C₆₀ and PVK. Furthermore, the photoinduced electron transfer behavior between C₆₀ and carbazole units in the C₆₀ chemically modified poly(N-vinylcarbazole) in benzonitrile by laser flash photolysis at 355 nm has also been investigated. Efficiency of the anion radical of C₆₀ in copolymer at 1080 nm is higher than that of the C₆₀-doped poly(N-vinylcarbazole) polymers. The formation of a C₆₀ radical anion may be ascribed to photoinduced electron transfer between C₆₀ pendanted on the main chain backbone and the inter-, and intrachain carbazole units in the copolymer. © 1997 John Wiley & Sons, Inc. 35 : 1185–1190, 1997     

Combined Transient Spectra and Semiempirical Calculation of [60]Fullerenes Attached with Piperidinodithiocarboxylate and 7-Chloro-phenazine

[60]Fullerenes attached with piperidinodithiocarboxylate dyad （1） and 7-chloro-1,2,3,4-tetrahydrophenazine （2） were efficiently synthesized through Diels-Atder cycloaddition with dienes. The physical properties of the triplet states of these compounds, in which strong electron acceptor moieties were covalently attached to C60 cores, were investigated by nanosecond laser flash photolysis. The excited triplet states in benzonitrite have been evaluated by observing the transient absorption bands in the near-IR region. The HOMO and LUMO were calculated by semiempirical methods AM1, which could predict the intramolecular photoinduced electron transfer in 1 and 2, and the nanosecond transient absorption spectra observed experimentally in solution were in excellent agreement with the calculated ones.     

Effects of Light Energy and Reducing Agents on C60‐Mediated Photosensitizing Reactions

Many biomolecules contain photoactive reducing agents, such as reduced nicotinamide adenine dinucleotide (NADH) and 6‐thioguanine (6‐TG) incorporated into DNA through drug metabolism. These reducing agents may produce reactive oxygen species under UVA irradiation or act as electron donors in various media. The interactions of C₆₀ fullerenes with biological reductants and light energy, especially via the Type‐I electron‐transfer mechanism, are not fully understood although these factors are often involved in toxicity assessments. The two reductants employed in this work were NADH for aqueous solutions and 6‐TG for organic solvents. Using steady‐state photolysis and electrochemical techniques, we showed that under visible light irradiation, the presence of reducing agents enhanced C₆₀‐mediated Type‐I reactions that generate superoxide anion (O₂^.?) at the expense of singlet oxygen (¹O₂) production. The quantum yield of O₂^.? production upon visible light irradiation of C₆₀ is estimated below 0.2 in dipolar aprotic media, indicating that the majority of triplet C₆₀ deactivate via Type‐II pathway. Upon UVA irradiation, however, both C₆₀ and NADH undergo photochemical reactions to produce O₂^.?, which could lead to a possible synergistic toxicity effects. C₆₀ photosensitization via Type‐I pathway is not observed in the absence of reducing agents.     

Solvent‐Free One‐Step Photochemical Hydroxylation of Benzene Derivatives by the Singlet Excited State of 2,3‐Dichloro‐5,6‐dicyano‐p‐benzoquinone Acting as a Super Oxidant

Photoinduced hydroxylation of neat deaerated benzene to phenol occurred under visible‐light irradiation of 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ), which acts as a super photooxidant in the presence of water. Photocatalytic solvent‐free hydroxylation of benzene derivatives with electron‐withdrawing substituents such as benzonitrile, nitrobenzene, and trifluoromethylbenzene used as neat solvents has been achieved for the first time by using DDQ as a super photooxidant to yield the corresponding phenol derivatives and 2,3‐dichloro‐5,6‐dicyanohydroquinone (DDQH₂) in the presence of water under deaerated conditions. In the presence of dioxygen and tert‐butyl nitrite, the photocatalytic hydroxylation of neat benzene occurred with DDQ as a photocatalyst to produce phenol. The photocatalytic reactions are initiated by oxidation of benzene derivatives with the singlet and triplet excited states of DDQ to form the corresponding radical cations, which associate with benzene derivatives to produce the dimer radical cations, which were detected by the femto‐ and nanosecond laser flash photolysis measurements to clarify the photocatalytic reaction mechanisms. Radical cations of benzene derivatives react with water to yield the OH‐adduct radicals. On the other hand, DDQ ^{. ?} produced by the photoinduced electron transfer from benzene derivatives reacts with the OH‐adduct radicals to yield the corresponding phenol derivatives and DDQH₂. DDQ is recovered by the reaction of DDQH₂ with tert‐butyl nitrite when DDQ acts as a photocatalyst for the hydroxylation of benzene derivatives by dioxygen.     

Photoinduced Triplet–Triplet Energy Transfer in a 2‐Ureido‐4(1H)‐Pyrimidinone‐Bridged,Quadruply Hydrogen‐Bonded Ferrocene–Fullerene Assembly

2‐Ureido‐4(1H)‐pyrimidinone‐bridged ferrocene–fullerene assembly I is designed and synthesized for elaborating the photoinduced electron‐transfer processes in self‐complementary quadruply hydrogen‐bonded modules. Unexpectedly, steady‐state and time‐resolved spectroscopy reveal an inefficient electron‐transfer process from the ferrocene to the singlet or triplet excited state of the fullerene, although the electron‐transfer reactions are thermodynamically feasible. Instead, an effective intra‐assembly triplet–triplet energy‐transfer process is found to be operative in assembly I with a rate constant of 9.2×10⁵ s^?1 and an efficiency of 73 % in CH₂Cl₂ at room temperature.     

Photoinduced electron transfer between C60 and N,N,N′,N′-tetramethylbenzidine (NTMB). Fourier transform electron paramagnetic resonance study

Fourier transform EPR spectroscopy was employed in studying the electron transfer (ET) reaction and the quenching mechanisms of the photoexcited triplet state of C₆₀ as electron acceptor and N,N,N′,N′-tetramethylbenzidine (NTMB) as electron donor in benzonitrile solution. The ET reaction product, the cation radical NTMB^*+, interacts with ^3*C₆₀, leading to photoinduced electron polarization of NTMB^*+ via triplet-doublet mixing mechanism combined with triplet mechanism. The quenching of ^3*C₆₀ and the polarization behavior of NTMB^*+ are discussed.     

Reductive Hydrogenation of Cs‐C70(CF3)8 and C1‐C70(CF3)10

CF₃‐derivatized fullerenes prove once again to be promising scaffolds for regioselective fullerene functionalization: now with the smallest possible addends—hydrogen atoms. Hydrogenation of C_s‐C₇₀(CF₃)₈ and C₁‐C₇₀(CF₃)₁₀ by means of reduction with Zn/Cu couple in the presence of water proceeds regioselectively, yielding only one major isomer of C₇₀(CF₃)₈H₂ and only two for C₇₀(CF₃)₁₀H₂, whose addition patterns are combined in the only abundant isomer of C₇₀(CF₃)₁₀H₄. The observed selectivity is governed by the electronic structure of trifluoromethylated substrates. Interestingly, we discovered that Clar's theory can be utilized to predict the regiochemistry of functionalization, and we look forward to testing it on forthcoming cases of derivatization of pre‐functionalized fullerene building blocks.