Investigation of excited state electron-transfer reactions. A search for other associative processes

The present investigations were carried out to reveal the nature of the photoinduced electron-transfer (ET) process within the electron donors 1,2,3,4-tetrahydroquinoline (THQ) and 1-methyl-1,2,3,4-tetrahydroquinoline (THMe), and widely used acceptor tetracyanoquinodimethane (TCNQ) in the highly polar solvent acetonitrile (ACN) at 300 K. Observations of considerable overlapping between the emission spectrum of the donor molecules studied in the present investigation and the electronic absorption spectrum of the acceptor TCNQ, coupled to a high negative value of ΔG [the energy gap between the locally excited (LE) and radical ion pair (RIP) states] when one of the chromophores is excited, indicate the possibility of concurrent occurrence of the two processes, e.g. energy and electron transfer. Surprisingly even when the donor chromophore is photoexcited, no spectral manifestation of energy transfer was observed, though both steady state and time resolved (in the time domain of nanosecond order) spectroscopic measurements strongly suggest the occurrence of a highly exothermic ET reaction within the present donor—acceptor systems. Furthermore such ET reactions have been suggested to occur between donor and acceptor separated by a large distance ( ∼ 7 Å), and quenching of fluorescence emission of donor molecules is caused primarily due to outer sphere ET reactions with the acceptor. Measured electron transfer rates (k_ET) were found to be of much lower value ( ∼ 10⁷s⁻¹). It is demonstrated that loose structure of the transient geminate ion pair complex is formed due to the encounter between excited acceptor (or donor) and unexcited donor (or acceptor), and due to this structural property, a stable anionic species (TCNQ⁻ ion) is produced due to the rapid dissociation (probably in the picosecond time domain) of this excited complex. It is hinted that synthesis of biochromophoric systems in which the present donor and acceptor chromophore would be linked by a polymethylene type (σ-type) spacer might be useful in building good photoconducting materials.     

Non-radiative depletion of the excited electronic states of 9-cyanoanthracene in presence of tetrahydronaphthols

Both steady state and time resolved spectroscopic measurements reveal that the prime process involved in quenching mechanism of the lowest excited singlet (S1) and triplet (T1) states of the well known electron acceptor 9-Cyanoanthracene (9CNA) in presence of 5,6,7,8-tetrahydro-1-naphthol (TH1N) or 5,6,7,8-tetrahydro-2-naphthol (TH2N) is H-bonding interaction. It has been confirmed that the fluorescence of 9CNA is not at all affected in presence of 5,6,7,8-tetrahydro-2-methoxy naphthalene (TH2MN) both in non-polar n-heptane (NH) and highly polar acetonitrile (ACN) media. This indicates that the H-bonding interaction is crucial for the occurrence of the quenching phenomenon observed in the present investigations with TH1N (or TH2N) donors and 9CNA acceptor. In ACN solvent both contact ion-pair (CIP) and solvent-separated (or dissociated) ions are formed due to intermolecular H-bonding interactions in the excited electronic states (both singlet and triplet). In NH environment due to stronger H-bonding interactions, the large proton shift within excited charge transfer (CT) or ion-pair complex, 1 or 3(D+-H...A-), causes the formation of the neutral radical, 3(D+H-A)*, due to the complete detachment of the H-atom. It is hinted that both TH1N and TH2N due to their excellent H-bonding ability could be used as antioxidants.     

Photophysical properties of 5-hydroxyindole (5HI): Laser flash photolysis study

Steady state fluorescence emission and transient absorption spectra of 9-fluorenone (9FL) were measured in the presence of 5-hydroxyindole (5HI) in highly polar acetonitrile (ACN) environment at ambient temperature. Cyclic voltammetry measurements demonstrate that ground state 5HI as a donor could take part in highly exothermic electron transfer (ET) reactions with excited 9FL, which should serve as electron acceptor. From the transient absorption measurements it is inferred that in geminate ion-pair (GIP) (or contact ion pair), formed initially due to photoinduced ET, the decay of this contact ion-pair occurs not only through ion recombination (back electron transfer to ground state of reactants), but through the other processes also such as proton-transfer (hydrogen abstraction) from radical cation to anion and separation of ion-pair producing the free ions. From the computed reorganisation energy parameter (λ) and experimentally observed -‡ _ET ⁰ values it is hinted that there is a possibility that highly exothermic forward electron transfer reactions in the singlet stateS ₁ occur, within present reacting systems, in Marcus inverted region. Back transfer seems to follow the same path. Investigations with similar other reacting systems are underway.     

Studies on unimolecular and bimolecular photoprocesses of a newly synthesized selenium compound, 7-chloro-2-phenyl-9H-[1]-benzopyrano[3,2-b]-selenophene-9-one (SeP).

Using steady state/time resolved spectroscopic and electrochemical techniques the spectroscopic and photophysical studies were made on a novel synthesized selenophene compound SeP in nonpolar methylcyclohexane (MCH), polar aprotic acetonitrile (ACN) and polar protic ethanol (EtOH) solvents at the ambient temperature as well as at 77 K. Both from the studies on unimolecular and bimolecular photoprocesses this selenophene compound was found to possess several electronic levels, 1Bb, 1La, 1Lb (all are of pi pi* nature and 1Lb is hidden within 1La band envelop like the characteristics of most of the acenes) and 1(nO pi*) state arising due to carbonyl oxygen atom. In polar ACN environment this nO pi* state disappears because it moves within the envelop of intense 1La band due to large destabilization. Large overlapping of different band systems within the 1La band of SeP was confirmed from the observed depolarization effect. The lack of phosphorescence of SeP both in MCH and EtOH rigid glassy matrix at 77 K has been inferred due to large vibronic interactions between closely lying triplets of the corresponding 1nO pi* and 1Lb states. From the bimolecular investigations, it reveals that SeP acts as a good electron donor in presence of the well known electron acceptor 9 cyanoanthracene (9CNA). Transient absorption spectra measured by laser flash photolysis technique demonstrate the formation of ion-pair when the acceptor is excited. From the analysis of the fluorescence quenching data it seemingly indicates that the major contribution in the diminution of the fluorescence intensity of the acceptor 9CNA in presence of SeP is not only due to the photoinduced electron transfer (ET) but also originates from static type (instantaneous) quenching processes along with external heavy atom effect. The possibility of occurrence of photoinduced ET reaction in Marcus inverted region is hinted.     

Synthesis and studies on spectroscopic as well as electron donating properties of the two alkoxy benzo[b]thiophenes

Synthesis, characterization, steady state and time resolved, using time correlated single photon counting as well as laser flash photolysis techniques, spectroscopic investigations were made for two alkoxy benzo[b]thiophene molecules: 5-methoxy benzo[b]thiophene (5MBT) and 5-methoxymethyl benzo[b]thiophene (5MMBT). In both non-polar n-heptane (NH) and polar acetonitrile (ACN) solvents and at ambient temperature the electronic absorption spectra of these thiophenes exhibit different band systems whose assignments were made from the measurements of the steady state excitation polarization spectra. Steady state fluorescence spectra of these molecules in the different polarity solvents show the presence of non-specific interactions. From the redox properties of the benzothiophenes, measured by cyclic voltammetry, their electron donating properties were observed in the presence of the well-known electron acceptor 9cyanoanthracene (9CNA). Further, detailed studies by laser flash photolysis techniques show that ion-recombination mechanism predominates after the initial excitation of the acceptor moiety using the third harmonic of Nd:YAG laser. This recombination together with the external heavy atom effect (the donor containing 'sulphur' atom) appears to be responsible for the formation of the triplet of the monomeric acceptor 9CNA. From the steady state experiments it is shown that both in non-polar NH and highly polar ACN the quenching in the fluorescence emission of 9CNA in the presence of the benzothiophene donors is brought about primarily by the external heavy atom effect and in ACN, although the presence of the photoinduced ET reaction is confirmed, this process seems, from the observed bimolecular dynamic quenching rate, kq to be significantly masked by the external heavy atom effect.     

Photophysics of intramolecular electron donor—acceptor systems; N-carbazolyl(CH2)ntetrachlorophthalimide

In systems N-carbazolyl—(CH₂)_n—tetrachlorophthalimide (n = 2, 3, 4, 7) virtually complete quenching of the carbazole fluorescence via an intramolecular electron-transfer mechanism occurs, with concomitant formation of an emissive polar excited state. The rate of formation of this polar excited state is found to exceed considerably those reported for formation of intramolecular exciplex states in other non-conjugated bichromophoric systems with a less pronounced electron donor—acceptor character.     

An ab initio description of the excited states of the reaction O(3P, 1D) + H2 → OH(2Π, 2Σ+) + H. An attempt to describe several potential energy surfaces with constant accuracy

A new aspects of the role of the solvent mode in the photoinduced electron-transfer process of electron donor and acceptor system in polar solvents has been exploited. Taking into account the important fact that the vibrational frequency of the solvent mode in the initial neutral state of the reactants is considerably smaller than that in the final ionic state, we have derived a new formula for the energy-gap dependence of the electron-transfer rate. In this formulation, the activation energy is greatly reduced and the electron-transfer rate is almost independent of the energy gap over a wide down-hill energy region. This qualitative feature explains the experimental results for the relation between the bimolecular quenching rate constant k_w and the standard free-energy change ΔG° associated with electron transfer in the “anomalous region”.     

Photoinduced intramolecular charge transfer phenomena in 5-(4-dimethylamino-phenyl)-penta-2,4-dienoic acid

A donor acceptor substituted aromatic system 5-(4-dimethylamino-phenyl)-penta-2,4-dienoic acid (DMAPPDA) has been synthesized and its spectral properties have been explored on the basis of steady state absorption and fluorescence spectroscopy. Spectral features point largely towards a possible occurrence of photoinduced intramolecular charge transfer process from the donor NMe2 group to the acceptor acid group. Solvent dependency of the large Stokes' shifted emission band and the calculated large excited state dipole moment support the polar character of the charge transfer excited state. Quantum yield calculations and effect of addition of acid and base on the steady state spectra were also performed to further scrutinize the excited state CT character.     

Theoretical investigation of charge transfer excitation and charge recombination in acenaphthylene–tetracyanoethylene complex

Ab initio calculations were performed to investigate the charge separation and charge recombination processes in the photoinduced electron transfer reaction between tetracyanoethylene and acenaphthylene. The excited states of the charge‐balanced electron donor–acceptor complex and the singlet state of ion pair complex were studied by employing configuration interaction singles method. The equilibrium geometry of electron donor–acceptor complex was obtained by the second‐order Møller–Plesset method, with the interaction energy corrected by the counterpoise method. The theoretical study of ground state and excited states of electron donor–acceptor complex in this work reveals that the S₁ and S₂ states of the electron donor–acceptor complexes are excited charge transfer states, and charge transfer absorptions that corresponds to the S₀ → S₁ and S₀ → S₂ transitions arise from π–π* excitations. The charge recombination in the ion pair complex will produce the charge‐balanced ground state or excited triplet state. According to the generalized Mulliken–Hush model, the electron coupling matrix elements of the charge separation process and the charge recombination process were obtained. Based on the continuum model, charge transfer absorption and charge transfer emission in the polar solvent of 1,2‐dichloroethane were investigated. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 23–35, 2003     

Solvent-promoted chemiluminescent decomposition of a bicyclic dioxetane bearing a 4-(benzothiazol-2-yl)-3-hydroxyphenyl moiety

An aprotic polar solvent such as N-methylpyrrolidone (NMP) promoted the thermal decomposition of bicyclic dioxetane bearing a 4-(benzothiazol-2-yl)-3-hydroxyphenyl moiety 1 without the addition of any base. This solvent-promoted decomposition (SPD) gave light as effectively as the base-induced decomposition (BID) in an aprotic polar solvent. SPD caused intramolecular CT-induced chemiluminescence similar to BID, but, in contrast to BID, SPD proceeded through a pathway with a large negative entropy of activation. Dioxetane 1 was also shown to give light due to excited state intramolecular proton transfer (ESIPT) upon heating in p-xylene.     

Primary electron-transfer dynamics in 2-phenylindole-9-cyanoanthracene system. A comparative study with 2-methylindole

Electrochemical measurements by cyclic voltammetry predict the possibility of occurrence of photoinduced electron-transfer (PET) reactions between the ground state of 2-phenylindole (2PI) (electron donor) and the excited singlet of 9-cyanoanthracene (9CNA) molecule acting as an electron acceptor. However, 2PI should be expected to behave as a relatively weaker electron donating agent than the structurally related donor 2-methylindole (2MI) as it possesses higher oxidation potential value. Both steady-state and time-resolved spectroscopic measurements in the polar acetonitrile (ACN) and ethanol (EtOH) solvents show that the fluorescence quenching phenomenon of 9CNA in presence of 2PI is primarily due to the involvement of dynamic process which in high probability should be PET. Nevertheless, in less polar tetrahydrofuran (THF) medium, the quenching of 9CNA results from the combined effect of dynamic and static modes. The transient absorption spectra, measured by using nanosecond laser flash photolysis, of 9CNA in presence of 2PI exhibit the signature of the bands of the anionic species of 9CNA, cation of the donor 2PI and the contact neutral radical. Observations of the transient absorption at the different delays infer that ion-recombination mechanism is responsible for production of the monomeric triplets of both 9CNA and 2PI. From the transient absorption decays in ACN medium, it has been demonstrated that the diffusional separation of ions from geminate ion-pair is facilitated in the case of 2MI-9CNA pair whereas for 2PI-9CNA system the energy wasting charge recombination dominates over the process of charge dissociation. From the above observations, the possibility of developing much potential photosynthetic model compounds with the donor 2MI, rather than with the other donor 2PI molecule has been hinted.     

Formation of Highly Efficient,Long-Lived Charge Separated States in Star-Shaped Ferrocene-Diketopyrrolopyrrole-Triphenylamine Donor–Acceptor–Donor Conjugates

The significance of multiple number of donor–acceptor entities on a central electron donor in a star-shaped molecular system in improving light energy harvesting ability is reported. For this, donor–acceptor–donor type conjugates comprised up to three entities ferrocenyl (Fc)-diketopyrrolopyrrole (DPP) onto a central triphenylamine (TPA), ( 4 – 6 ) by the Pd-catalyzed Sonogashira cross-coupling reactions have been newly synthesized and characterized. Donor–acceptor conjugates possessing diketopyrrolopyrrole (1 to 3 entities) onto the central triphenylamine, ( 1 – 3 ) served as reference dyads while monomeric DPP and Fc-DPP served as control compounds. Both DPP and Fc-DPP carrying conjugates exhibited red-shifted absorption compared to their respective control compounds revealing existence of ground state interactions. Furthermore, DPP fluorescence in 4 – 6 was found to be quantitatively quenched while for 1 – 3 , this property varied between 73–65 % suggesting occurrence moderate amounts of excited state events. The electrochemical investigations exhibited an additional low potential oxidation in the case of Fc-DPP-TPA based derivatives ( 4 – 6 ) owing to the presence of ferrocene unit(s). This was in addition to DPP and TPA redox peaks. Using spectral, electrochemical and computational studies, Gibbs free-energy calculations were performed to visualize excited state charge separation (ΔG_CS) in these donor–acceptor conjugates as a function of different number of Fc-DPP entities. Formation of Fc⁺-DPP^.−-TPA charge separated states (CSS) in the case of 4 – 6 was evident. Using spectroelectrochemical studies, spectrum of CSS was deduced. Finally, femtosecond transient absorption spectral studies were performed to gather information on excited state charge separation. Increasing the number of Fc-DPP entities in 4 – 6 improved charge separation rates. Surprisingly, lifetime of the charge separated state, Fc⁺-DPP^.−-TPA was found to persist longer with an increase in the number of Fc-DPP entities in 4 – 6 as compared to Fc-DPP-control and simple DPP derived donor–acceptor conjugates in literature. This unprecedented result has been attributed to subtle changes in ΔG_CS and ΔG_CR and the associated electron coupling between different entities.     

Mechanism of Exciplex Decay: The Quantum Yields and the Rate Constants of Radical Ion Formation from Exciplexes with Partial Charge Transfer

The dynamics of exciplex and radical ion formation was studied in donor–acceptor systems with G ^* _et > –0.1 eV. It was shown that the quenching of excited singlet states of aromatic molecules by electron donors in polar solvents led to the formation of radical ions via exciplex dissociation resulting to complete charge separation. Intersystem crossing and internal conversion into the ground state (back electron transfer) compete with this process. The quantum yields and the rate constants of the radical ion formation were measured.     

Luminescent Quadrupolar Borazine Oligomers: Synthesis,Photophysics, and Two‐Photon Absorption Properties

A set of monodisperse bent donor–acceptor–donor‐type conjugated borazine oligomers, BnNn+1 (n=1–4), incorporating electron‐rich triarylamine donor and electron‐deficient triarylborane acceptor units has been prepared through an iterative synthetic approach that takes advantage of highly selective silicon–boron and tin–boron exchange reactions. The effect of chain elongation on the electrochemical, one‐ and two‐photon properties and excited‐state photodynamics has been investigated. Strong intramolecular charge transfer (ICT) from the arylamine donors to boryl‐centered acceptor sites results in emissions with high quantum yields (Φ_fl>0.5) in the range of 400–500 nm. Solvatochromic effects lead to solvent shifts as large as ～70 nm for the shortest member (n=1) and gradually decrease with chain elongation. The oligomers exhibit strong two‐photon absorption (2PA) in the visible spectral region with 2PA cross sections as large as 1410 GM (n=4), and broadband excited‐state absorption (ESA) attributed to long‐lived singlet–singlet and radical cation/anion absorption. The excited‐state dynamics also show sensitivity to the solvent environment. Electrochemical observations and DFT calculations (B3LYP/6‐31G*) reveal spatially separated HOMO and LUMO levels resulting in highly fluorescent oligomers with strong ICT character. The BnNn+1 oligomers have been used to demonstrate the detection of cyanide anions with association constants of log K>7.     

Spectroscopic studies of multiple charge transfer complexes of p-toluidine with π-acceptor picric acid in different polar solvents

The charge transfer complexes of the donor p-toluidine with π-acceptor picric acid have been studied spectrophotometrically in various solvents such as acetone, ethanol, and methanol at room temperature using absorption spectrophotometer. The results indicate that formation of CTC in less polar solvent is high. The stoichiometry of the complex was found to be 1: 1 ratio by straight line method between donor and acceptor with maximum absorption bands. The data are discussed in terms of formation constant (K _CT), molar extinction coefficient (?_CT), standard free energy (ΔG°), oscillator strength (f), transition dipole moment (μ_EN), resonance energy (R _N) and ionization potential (I _D). The results indicate that the formation constant (K _CT) for the complex were shown to be dependent upon the nature of electron acceptor, donor and polarity of solvents which were used.     

Synthesis of OMS Materials and Investigation of Their Acceptor–Donor Characteristics

Three ordered mesoporous siliceous (OMS) materials known as MCM41s—unmodified MCM-41C16 (“C16”), and two MCM41s with different surface functionalities: MCM-41C16-SH (“C16-SH”) and MCM-41C16-NH₂ (“C16-NH₂”)—were synthesized and studied by inverse gas chromatography in order to determine their acceptor–donor properties. The specific retention volumes of nonpolar and polar probes that were chromatographed on these ordered mesoporous silica adsorbents were evaluated under infinite dilution conditions. Two methods were employed to calculate the standard free energy of adsorption, ΔG _ads, of each chromatographed probe on the basis its specific retention volume. These ΔG _ads values were then employed to estimate the van der Waals contribution and the specific contribution of the free surface energy for each MCM41. DN values (donor numbers, based on the Gutmann scale) and AN* values (acceptor numbers, based on the Riddle–Fowkes scale) were employed to determine the values of parameters that characterize the ability of the MCM41s to act as electron acceptors (parameter: K _A) and donors (parameter: K _D). Considering the different compositions of the probes, each of which has different acceptor–donor properties, a new chromatographic test to supplement the Grob test is suggested.     

Synthesis and properties of π-conjugated donor–acceptor macrocycles derived from phenanthrylene building blocks

Phenanthrylene-ethynylidene macrocycles combining electron donor and electron acceptor subunits in their shape-persistent fully conjugated core were synthesized. The donor subunits consisted of two 9,10-dialkoxyphenanthrenes linked either with 1,2-ethynylidene or 2,5-thienylene bridge. The acceptors were 9,10-phenanthroquinone and dibenzoquinoxaline and dibenzophenazines derived from it. Solvatochromic photoluminescence from intramolecular-charge-transfer (ICT) excited state was observed mainly in non-polar solvents. In more polar solvents, the excited states favor non-radiative relaxation. DFT calculated HOMO/LUMO energies of the macrocycles correlate well with spectroscopic and electrochemical data. In the series of substituted dibenzophenazine acceptors a good correlation with Hammett substituent constants σ_p^– was found.     

Auxiliary Methoxy Aided Triphenylamine and Dicyanoisophorone Based Flurophores with Viscosity and Polarity Sensitive Intramolecular Charge Transfer

Novel, multibranched “triphenylamine based donor with added auxiliary methoxy donor and dicyanovinyl acceptor” based fluorescent molecules are developed. The dicyanoisophorone moiety is used as a configurationally locked polyene system for π-conjugation linking between donor and acceptor, to control the unnecessary intramolecular rotations in the molecule, which can to act as a rotor. The synthesized dyes show good fluorescent molecular rotor properties and strong emission solvatochromism. Auxiliary methoxy donors shift both the absorption and emission maxima towards longer wavelengths compared to known analogues, along with increased Stokes shifts. Fluorescent molecular rotor properties of the dyes are influenced by a local excited state to twisted intramolecular charge transfer state transition, which is discussed in terms of emission solvatochromism and Lippert–Mataga, Weller and Rettig polarity plots. Three different viscous solvent systems i.e., paraffin oil–dichloromethane, polyethylene glycol-400–dichloromethane and polyethylene glycol-400–N,N-dimethylformamide are used to investigate the sensitivity of rotors towards the viscosity of the environment. A maximum 16-fold enhancement in emission intensity and 0.616 × value is achieved for rotor Dye-3. The polarity effect of a binary viscous solvent system, by the virtue of intramolecular charge transfer, on the viscosity sensing properties of rotors is explained by constructing the Weller and Rettig’s plots for different viscous systems.     

Encapsulation of TCNQ and the Acridinium Ion within a Bisporphyrin Cavity: Synthesis,Structure, and Photophysical and HOMO–LUMO‐Gap‐Mediated Electron‐Transfer Properties

The encapsulation of tetracyanoquinodimethane (TCNQ) and fluorescent probe acridinium ions (AcH⁺) by diethylpyrrole‐bridged bisporphyrin (H₄DEP) was used to investigate the structural and spectroscopic changes within the bisporphyrin cavity upon substrate binding. X‐ray diffraction studies of the bisporphyrin host (H₄DEP) and the encapsulated host–guest complexes (H₄DEP ? TCNQ and [H₄DEP ? AcH]ClO₄) are reported. Negative and positive shifts of the reduction and oxidation potentials, respectively, indicated that it was difficult to reduce/oxidize the encapsulated complexes. The emission intensities of bisporphyrin, upon excitation at 560 nm, were quenched by about 65 % and 95 % in H₄DEP ? TCNQ and [H₄DEP ? AcH]ClO₄, respectively, owing to photoinduced electron transfer from the excited state of the bisporphyrin to TCNQ/AcH⁺; this result was also supported by DFT calculations. Moreover, the fluorescence intensity of encapsulated AcH⁺ (excited at 340 nm) was also remarkably quenched compared to the free ions, owing to photoinduced singlet‐to‐singlet energy transfer from AcH⁺ to bisporphyrin. Thus, AcH⁺ acted as both an acceptor and a donor, depending on which part of the chromophore was excited in the host–guest complex. The electrochemically evaluated HOMO–LUMO gap was 0.71 and 1.42 eV in H₄DEP ? TCNQ and [H₄DEP ? AcH]ClO₄, respectively, whilst the gap was 2.12 eV in H₄DEP. The extremely low HOMO–LUMO gap in H₄DEP ? TCNQ led to facile electron transfer from the host to the guest, which was manifested in the lowering of the CN stretching frequency (in the solid state) in the IR spectra, a strong radical signal in the EPR spectra at 77 K, and also the presence of low‐energy bands in the UV/Vis spectra (in the solution phase). Such an efficient transfer was only possible when the donor and acceptor moieties were in close proximity to one another.     

Dynamics of Photoinduced Electron‐Transfer Processes in Fullerene‐Based Dyads: Effects of Varying the Donor Strength

Two classes of fullerene‐based donor–bridge–acceptor (D–B–A) systems containing donors of varying oxidation potentials have been synthesized. These systems include fullerenes linked to heteroaromatic donor groups (phenothiazine/phenoxazine) as well as substituted anilines (p‐anisidine/p‐toluidine). In contrast to the model compound, an efficient intramolecular electron transfer is observed from the fullerene singlet excited state in polar solvents. An increase in the rate constant and quantum yield of charge separation (k_cs and Φ_cs) has been observed for both classes of dyads, with decrease in the oxidation potentials of the donor groups. This observation indicates that the rates of the forward electron transfer fall in the normal region of the Marcus curve. The long‐lived charge separation enabled the characterization of electron transfer products, namely, the radical cation of the donor and radical anion of the pyrrolidinofullerene, by using nanosecond transient absorption spectroscopy. The small reorganization energy (λ) of C₆₀ coupled with large negative free energy changes (‐ΔG°) for the back electron transfer places the back electron process in the inverted region of Marcus curve, thereby stabilizing the electron transfer products.