Evidence for Two Separate One-Electron Transfer Events in Excited Fulleropyrrolidine Dyads Containing Tetrathiafulvalene (TTF)

Excited state dynamics of a spontaneously generated TTF radical cation inside a water-soluble nanocage

Supramolecular cavities have been traditionally used to stabilize reactive redox intermediates. Recently with the success of multiple new photoredox catalytic strategies that use supramolecular cages, there is a growing demand for photogeneration strategies of diverse reactive intermediates inside confined spaces, which will drive enzyme-like catalysis in real time. Here we report the excited state dynamics of a redox-active TTF radical cation and its corresponding dimethyl-derivative DiMeTTF inside a confined supramolecular cavity. We prepare the radical cation by spontaneous oxidation of neutral TTF upon incarceration inside a water-soluble nanocage Pd₆L₄¹²⁺, and characterize it with a combination of resonance Raman and electron paramagnetic resonance spectroscopy. Using broadband transient absorption spectroscopy, we demonstrate that the confined native TTF radical cation and its dimethyl derivative upon photoexcitation rapidly de-excite to form the hot ground state, thereby inhibiting further oxidation to a TTF⁺² dication. We discuss our results in the context of excited state crossings of the radical cation potentials as well as modifying the cage energetics to generate a stable dication. Our work has important implications for the usage of such radical cations for photoactivated catalysis.

Sc3N@Ih-C80 based donor–acceptor conjugate: role of thiophene spacer in promoting ultrafast excited state charge separation

Photoinduced charge separation and dark charge recombination occurring within picoseconds is observed in newly synthesized triphenylamine–thiophene-Sc₃N@I_h-C₈₀ and triphenylamine–thiophene-C₆₀ conjugates.

Recent Advances on Nitrofluorene Derivatives: Versatile Electron Acceptors to Create Dyes Absorbing from the Visible to the Near and Far Infrared Region

Push–pull dyes absorbing in the visible range have been extensively studied so that a variety of structures have already been synthesized and reported in the literature. Conversely, dyes absorbing in the near and far infrared region are more scarce and this particularity relies on the following points: difficulty of purification, presence of side-reaction during synthesis, low availability of starting materials, and low reaction yields. Over the years, several strategies such as the elongation of the π-conjugated spacer or the improvement of the electron-donating and accepting ability of both donors and acceptors connected via a conjugated or an aliphatic spacer have been examined to red-shift the absorption spectra of well-established visible dyes. However, this strategy is not sufficient, and the shift often remains limited. A promising alternative consists in identifying a molecule further used as an electron-accepting group and already presenting an absorption band in the near infrared region and to capitalize on its absorption to design near and far infrared absorbing dyes. This is the case with poly(nitro)fluorenes that already exhibit such a contribution in the near infrared region. In this review, an overview of the different dyes elaborated with poly(nitro)fluorenes is presented. The different applications where these different dyes have been used are also detailed.

Conformational Impact on Energy Storage Efficiency of Subphthalocyanine-Fullerene Hybrids

Hybrid molecules involving subphthalocyanine and Buckminsterfullerene derivatives are interesting candidates as heavy metal free triplet sensitizers. Subphthalocyanine efficiently absorbs visible photons and transfer the singlet excited state energy to the Buckminsterfullerene where intersystem crossing produces triplet states in high yield. Thus, far the efficiency of the triplet-generating photophysics in these systems has been hampered by back energy transfer to the subphthalocyanine triplet state resulting in loss of excitation energy. Herein an efficient strategy is realized to avoid loss of triplet energy by back energy transfer. A hybrid molecule based on subphthalocyanine and Buckminsterfullerene is presented in which dispersion-induced π-π interactions result in a molecular geometry where highly efficient through-space singlet excited state energy transfer takes place in one direction, whereas energy flow in the opposite direction via the triplet manifold is blocked by lack of orbital overlap. The approach opens for a new class of heavy-metal-free triplet sensitizers of particular relevance to the fields of photodynamic therapy and noncoherent photon upconversion.

Electron and excitation energy transfers in covalently linked donor–acceptor dyads: mechanisms and dynamics revealed using quantum chemistry

A quantum chemical strategy is developed to study photoinduced electron and energy transfer processes in covalently linked dyads. The strategy is applied to two zinc and free-based porphyrin–naphthalenediimide dyads in solution.

Stabilising the lowest energy charge-separated state in a {metal chromophore - fullerene} assembly: a tuneable panchromatic absorbing donor-acceptor triad

Photoreduction of fullerene and the consequent stabilisation of a charge-separated state in a donor-acceptor assembly have been achieved, overcoming the common problem of a fullerene-based triplet state being an energy sink that prevents charge-separation. A route to incorporate a C60-fullerene electron acceptor moiety into a catecholate-Pt(ii)-diimine photoactive dyad, which contains an unusually strong electron donor, 3,5-di-tert-butylcatecholate, has been developed. The synthetic methodology is based on the formation of the aldehyde functionalised bipyridine-Pt(ii)-3,5-di-tert-butylcatechol dyad which is then added to the fullerene cage via a Prato cycloaddition reaction. The resultant product is the first example of a fullerene-diimine-Pt-catecholate donor-acceptor triad, C60bpy-Pt-cat. The triad exhibits an intense solvatochromic absorption band in the visible region due to catechol-to-diimine charge-transfer, which, together with fullerene-based transitions, provides efficient and tuneable light harvesting of the majority of the UV/visible spectral range. Cyclic voltammetry, EPR and UV/vis/IR spectroelectrochemistry reveal redox behaviour with a wealth of reversible reduction and oxidation processes forming multiply charged species and storing multiple redox equivalents. Ultrafast transient absorption and time resolved infrared spectroscopy, supported by molecular modelling, reveal the formation of a charge-separated state [C60˙-bpy-Pt-cat˙+] with a lifetime of ∼890 ps. The formation of cat˙+ in the excited state is evidenced directly by characteristic absorption bands in the 400-500 nm region, while the formation of C60˙- was confirmed directly by time-resolved infrared spectroscopy, TRIR. An IR-spectroelectrochemical study of the mono-reduced building block (C60-bpy)PtCl2, revealed a characteristic C60˙- vibrational feature at 1530 cm-1, which was also detected in the TRIR spectra. This combination of experiments offers the first direct IR-identification of C60˙- species in solution, and paves the way towards the application of transient infrared spectroscopy to the study of light-induced charge-separation in C60-containing assemblies, as well as fullerene films and fullerene/polymer blends in various OPV devices. Identification of the unique vibrational signature of a C60-anion provides a new way to follow photoinduced processes in fullerene-containing assemblies by means of time-resolved vibrational spectroscopy, as demonstrated for the fullerene-transition metal chromophore assembly with the lowest energy charge-separated excited state.

Donor-acceptor type co-crystals of arylthio-substituted tetrathiafulvalenes and fullerenes

A series of donor-acceptor type co-crystals of fullerene (as the acceptor) and arylthio-substituted tetrathiafulvalene derivatives (Ar-S-TTF, as the donor) were prepared and their structural features were thoroughly investigated. The formation of co-crystals relies on the flexibility of Ar-S-TTF and the size matches between Ar-S-TTF and fullerene. Regarding their compositions, the studied co-crystals can be divided into two types, where types I and II have donor:acceptor ratios of 1:1 and 1:2, respectively. Multiple intermolecular interactions are observed between the donor and acceptor, which act to stabilize the structures of the resulting co-crystals. In the type I co-crystals, the fullerene molecule is surrounded by four Ar-S-TTF molecules, that is, two Ar-S-TTF molecules form a sandwich structure with one fullerene molecule and the other two Ar-S-TTF molecules interact with the fullerene molecule along their lateral axes. In the type II co-crystals, one fullerene molecule has the donor-acceptor mode similar to that in type I, whereas the other fullerene molecule is substantially surrounded by the aryl groups on Ar-S-TTF molecules and the solvent molecules.

Femtosecond time-resolved electronic relaxation dynamics in tetrathiafulvalene

In the present paper, the ultrafast electronic relaxation of tetrathiafulvalene (TTF) initiated around 4 eV is studied by femtosecond time-resolved velocity-map imaging. The goal is to investigate the broad double structure observed in the absorption spectrum at this energy. By monitoring the transients of the parent cation and its fragments and by varying the pump and the probe wavelengths, two internal conversions and intramolecular vibrational relaxation are detected both on the order of a few hundred of femtoseconds. Photoelectron images permit the assignment of a dark electronic state involved in the relaxation. In addition, the formation of the dimer of TTF has been observed.

Fulleropyrrolidine molecular dumbbells act as multi-electron-acceptor triads. Spectroscopic, electrochemical, computational and morphological characterizations

Three novel fulleropyrrolidine dumbbells consisting three electron acceptor moieties joined by alkyl linker displaying tunable electrical and morphological properties were synthesized and characterized.

Comparing femtosecond multiphoton dissociative ionization of tetrathiafulvene with imaging photoelectron photoion coincidence spectroscopy

In this paper we describe femtosecond photoionization and the imaging photoelectron photoion coincidence spectroscopy of tetrathiafulvene, TTF. Femtosecond photoionization of TTF results in the absorption of up to twelve 808 nm photons leading to ion internal energies up to 12.1 eV as deduced from the photoelectron spectrum. Within this internal energy a variety of dissociation channels are accessible. In order to disentangle the complex ionic dissociation, we utilized the imaging photoelectron photoion coincidence (iPEPICO) technique. Above the dissociation threshold, iPEPICO results show that the molecular ion (m/z = 204) dissociates into seven product ions, six of which compete in a 1.0 eV internal energy window and are formed with the same appearance energy. Ab initio calculations are reported on the possible fragment ion structures of five dissociation channels as well as trajectories showing the loss of C2H2 and C2H2S from high internal energy TTF cations. A three-channel dissociation model is used to fit the PEPICO data in which two dissociation channels are treated as simple dissociations (one with a reverse barrier), while the rest involve a shared barrier. The two lower energy dissociation channels, m/z = 146 and the channel leading to m/z = 178, 171, 159, 140, and 127, have E0 values of 2.77 ± 0.10 and 2.38 ± 0.10 eV, respectively, and are characterized by ΔS(‡)(600 K) values of -9 ± 6 and 1 ± 6 J K(-1) mol(-1), respectively. Competing with them at higher internal energy is the cleavage of the central bond to form the m/z = 102 fragment ion, with an E0 value of 3.65 ± 0.10 eV and ΔS(‡)(600 K) = 83 ± 10 J K(-1) mol(-1).

Graphene as a quencher of electronic excited states of photochemical probes

Graphene sheets quench the singlet and triplet excited states of a series of six photochemical probes including pyrene, acridine orange, tris(2,2́-bipyridyl)ruthenium(II) dichloride, methylene blue, meso-tetrakis(phenylsulphonate)porphyrin, and 5,10,15,20-tetraphenyl-21H,28H-porphine zinc. It was found that Stern-Volmer fluorescence quenching can fit to one or two different quenching regimes depending on the probe. In addition, the quenching can be either static or dynamic depending on the fluorophore. The occurrence of several quenching regimes has been interpreted considering that quenching arises from the crowding of the fluorophore on both graphene faces, or site isolation on the graphene sheets. Laser flash photolysis has shown that the triplet lifetime of the probes generally decreases due to graphene quenching and that no new transients appear except in the case of methylene blue, where a new absorption spectrum characterized by a continuous absorption band is observed and attributed to graphene radical ion. This spectroscopic evidence suggests that the most general quenching mechanism is energy transfer from the singlet or triplet excited state of the dye to graphene. This raises the issue of determining the energy of the electronic excited states of graphene.

Electronic communication in tetrathiafulvalene (TTF)/C60 systems: toward molecular solar energy conversion materials?

The covalent connection of the electron acceptor C 60 to p-quinonoid pi-extended tetrathiafulvalenes (exTTFs) has allowed for the preparation of new photo- and electroactive conjugates able to act as artificial photosynthetic systems and active molecular materials in organic photovoltaics. The gain of aromaticity undergone by the pi-extended TTF unit in the oxidation process results in highly stabilized radical ion pairs, namely, C 60 (*-)/exTTF (*+). Lifetimes for such charge-separated states, ranging from a few nanoseconds to hundreds of microseconds, have been achieved by rationally modifying the nature of the chemical spacers. These long-lived radical pairs are called to play an important role for the conversion of sunlight into chemical or electrical power.

Contrasting Photodynamics between C60-Dithiapyrene and C60-Pyrene Dyads

The photodynamics of a C60-dithiapyrene donor-acceptor conjugate were compared with the corresponding C60-pyrene conjugate. The photoinduced charge separation and subsequent charge recombination processes were examined by time-resolved fluorescence measurements on the picosecond timescale and transient absorption measurements on the picosecond and microsecond timescales with detection in the visible and near-infrared regions. We have observed quite long lifetimes (i.e., up to 1.01 ns) for the photogenerated charge-separated state in a C60-dithiapyrene dyad without the need for i) a long spacer between the two moieties, or ii) a gain in aromaticity in the radical ion pair.

Photoinduced Electron Transfer on Aqueous Carbon Nanohorn–Pyrene–Tetrathiafulvalene Architectures

Water-soluble carbon-nanohorn-tetrathiafulvalene (CNH-TTF) nanoensembles were prepared by utilizing positively charged pyrene as an assembly medium and characterized by spectroscopy and electron microscopy. Electronic interactions within the nanoensemble were probed by optical spectroscopy, indicating electron transfer between the TTF units and CNHs after light illumination.

A photoactive molecular triad as a nanoscale power supply for a supramolecular machine

A tetrathiafulvalene-porphyrin-fullerene (TTF-P-C(60)) molecular triad, which generates electrical current by harnessing light energy when self-assembled onto gold electrodes, has been developed. The triad, composed of three unique electroactive components, namely, 1) an electron-donating TTF unit, 2) a chromophoric porphyrin unit, and 3) an electron-accepting C(60) unit, has been synthesized in a modular fashion. A disulfide-based anchoring group was tagged to the TTF end of the molecule in order to allow its self-assembly on gold surfaces. The surface coverage by the triad in a self-assembled monolayer (SAM) was estimated to be 1.4 nm(2) per molecule, a density which is consistent with hexagonal close-packing of the spherical C(60) component (diameter approximately 1 nm). In a closed electronic circuit, a triad-SAM functionalized working-electrode generates a switchable photocurrent of approximately 1.5 microA cm(-2) when irradiated with a 413 nm Kr-ion laser, a wavelength which is close to the porphyrin chromophore's absorption maximum peak at 420 nm. The electrical energy generated by the triad at the expense of the light energy is ultimately exploited to drive a supramolecular machine in the form of a [2]pseudorotaxane comprised of a pi-electron-deficient tetracationic cyclobis(paraquat-p-phenylene) (CBPQT(4+)) cyclophane and a pi-electron-rich 1,5-bis[(2-hydroxyethoxy) ethoxy]naphthalene (BHEEN) thread. The redox-induced dethreading of the CBPQT(4+) cyclophane from the BHEEN thread can be monitored by measuring the increase in the fluorescence intensity of the BHEEN unit. A gradual increase in the fluorescence intensity of the BHEEN unit concomitant with the photocurrent generation, even at a potential (0 V) much lower than that required (-300 mV) for the direct reduction of the CBPQT(4+) unit, confirms that the dethreading process is driven by the photocurrent generated by the triad-SAM.

Novel Reactions of [60]Fullerene with Amino Acid Esters and Carbon Disulfide

[reaction: see text] Novel reactions of C60 with amino acid ester hydrochlorides and CS2 in the presence of Et3N affording fullerene derivatives 2 and 3 containing biologically active amino acids, thioamide, and thiourea units have been investigated. The thioamide groups in compounds 2 are sensitive to moisture and can easily be hydrolyzed to amide groups.

Photoinduced Charge Separation and Charge Recombination in the [60]Fullerene−Diphenylbenzothiadiazole−Triphenylamine Triad: Role of Diphenylbenzothiadiazole as Bridge

Photoinduced electron-transfer processes of the newly synthesized [60]fullerene-diphenylbenzothiadiazole-triphenylamine (C60-PBTDP-TPA) triad in polar and nonpolar solvents have been studied by using time-resolved transient absorption and fluorescence measurements from picosecond to microsecond regions. By fluorescence lifetime measurements in picosecond time regions, excitation of the charge-transfer transition of the PBTDP-TPA moiety in C60-PBTDP-TPA induces energy transfer to the C60 moiety generating 1C60*-PBTDP-TPA, competitively with charge separation generating C60*--PBTDP-TPA*+. From 1C60*-PBTDP-TPA, which is generated directly and indirectly, charge separation occurs generating C60*--PBTDP-TPA*+ in polar solvents. The C60*--PBTDP-TPA*+ formed via the singlet excited states decayed within a few nanoseconds as revealed by the picosecond transient absorption spectra. In the nanosecond time region, C60*--PBTDP-TPA*+ is produced slowly, probably via 3C60*-PBTDP-TPA. Lifetimes of such slowly generated C60*--PBTDP-TPA*+ were longer than 1 micros, which are the longest values among the C60-bridge-TPA triad systems reported hitherto at room temperature. Roles of the PBTDP-TPA moiety with twisted intermolecular charge-transfer character playing as energy donor and electron donor in addition to the bridge have been disclosed.

Hydrogen-Bonding Motifs in Fullerene Chemistry

The combination of fullerenes and hydrogen-bonding motifs is a new interdisciplinary field in which weak intermolecular forces allow modulation of one-, two-, and three-dimensional fullerene-based architectures and control of their function. This Minireview aims to extend the scope of fullerene chemistry to a truly supramolecular level from which unprecedented architectures may evolve. It is shown that electronic communication in C(60)-based hydrogen-bonded donor-acceptor ensembles is at least as strong as that found in covalently connected systems and that hydrogen-bonding fullerene chemistry is a versatile concept for the construction of functional ensembles.

Photoinduced Charge Separation and Charge Recombination in [60]Fullerene−Ethylcarbazole and [60]Fullerene−Triphenylamines in Polar Solvents

Molecules of C60 covalently connected with N-ethylcarbazole (EtCz) and triphenylamine (TPA) have been synthesized. Photoinduced electron transfer in C60-EtCz and C60-TPA has been studied in polar and nonpolar solvents using time-resolved transient absorption and fluorescence measurements. From the fluorescence lifetimes, the excited singlet state of the C60 moiety (1C60) of C60-TPA generates predominantly C60*--TPA*+, which decays quickly to the ground state within 6 ns even in polar solvents. In the case of C60-EtCz, on the other hand, about half of the 1C60 moiety generates short-lived C60*--EtCz*+, while the other half of the 1C60 moiety is transferred to the 3C60 moiety via intersystem crossing in dimethylformamide, in which the energy level of C60*--EtCz*+ is lower than that of 3C60. Thus, the charge separation takes place via 3C60 generating C60*--EtCz*+, having a lifetime as long as 300 ns, probably because of the triplet spin character of C60*--EtCz*+. A special property of the EtCz moiety to stabilize the hole in the charge-separated state was revealed.

Photoinduced Electron Transfer Competitive with Energy Transfer of the Excited Triplet State of [60]Fullerene to Ferrocene Derivatives Revealed by Combination of Transient Absorption and Thermal Lens Measurements

The quenching processes of the exited triplet state of fullerene (3C60) by ferrocene (Fc) derivatives have been observed by the transient absorption spectroscopy and thermal lens methods. Although 3C60 was efficiently quenched by Fc in the rate close to the diffusion controlled limit, the quantum yields (phi(et)) for the generation of the radical anion of C60 (C60*-) via 3C60 were quite low even in polar solvents; nevertheless, the free-energy changes (deltaG(et)) of electron transfer from Fc to 3C60 are sufficiently negative. In benzonitrile (BN), the phi(et) value for unsubstitued Fc was less than 0.1. The thermal lens method indicates that energy transfer from 3C60 to Fc takes place efficiently, suggesting that the excited triplet energy level of Fc was lower than that of 3C60. Therefore, energy transfer from 3C60 to ferrocene decreases the electron-transfer process from ferrocene to 3C60. To increase the participation of electron transfer, introduction of electron-donor substituents to Fc (phi(et) = 0.46 for decamethylferrocene in BN) and an increase in solvent polarity (phi(et) = 0.58 in BN:DMF (1:2) for decamethylferrocene) were effective.

NMR Spectrometric Studies of Complexation of [60]Fullerene with Series of Anisoles

Detailed (1)H and (13)C NMR spectrometric studies have been carried out to gain insight into the nature of molecular interactions of the electron donor-acceptor (EDA) complexes of [60]fullerene with a series of anisoles, namely, anisole, m-bromoanisole, and p-bromoanisole. [60]Fullerene has been shown to form 1:1 adducts with the above series of anisoles. Formation constants (K) for all the complexes have been determined from the systematic variation of the NMR chemical shifts of specific protons of the anisoles in the presence of [60]fullerene. The K values of [60]fullerene/anisole, [60]fullerene/m-bromoanisole, and [60]fullerene/ p-bromoanisole complexes yield good estimates of the Hammett rho constant for the complexation reaction. To the best of our knowledge, this paper reports for the first time a very fruitful technique by which the concentrations of EDA complexes can be estimated from systematic variations of the (13)C NMR signal.