Phthalocyanine–C60 Fused Conjugates Exhibiting Molecular Orbital Interactions Depending on the Solvent Polarity

New covalently C₆₀‐connected zinc phthalocyanine (ZnPc) derivatives have been synthesized by utilizing successive cycloaddition reactions of C₆₀ with a ZnPc derivative containing a pyridazine moiety employing Komatsu’s method in reaction of C₆₀ with phthalazine. The UV/Vis absorption spectrum of the fused conjugate ( 5 ) shows red shifts from the corresponding absorption of ZnPc derivative ( 8 ), indicating interactions between the ZnPc and C₆₀ moieties. The DFT calculations under non‐polar medium predict that the HOMO and LUMO of 5 localize on the ZnPc moiety, whereas LUMO+1 localizes on the C₆₀ moiety, which reasonably explain the magnetic circular dichroism (MCD) and absorption spectra in toluene. Electrochemical redox potentials of 5 in polar solvents indicate the first‐oxidation potential arises from the ZnPc moiety, whereas the first reduction potential is associated with the C₆₀ moiety, suggesting the LUMO localizes on the C₆₀ moiety in polar solvent. This reversal of the LUMO is supported by the ZnPc‐fluorescence quenching with a nearby C₆₀ moiety in benzonitrile, which leads to the charge‐separation via the excited singlet state of the ZnPc moiety. In toluene on the other hand, such a ZnPc‐fluorescence quenching owing to the photoinduced charge separation is not observed as predicted by the DFT‐calculated LUMO on the ZnPc moiety.     

Synthesis and Photoinduced Intramolecular Processes of Light‐Harvesting Silicon Phthalocyanine–Naphthalenediimide–Fullerene Connected Systems

Fast moving : A new pentad (see figure) composed of silicon phthalocyanine (SiPc), as electron donor, that is connected with two units of naphthalenediimide (NDI) and fullerene C₆₀, as electron acceptors, undergoes fast and efficient charge‐separation processes via the NDI and SiPc singlet excited states.

     

Facile Synthesis and Photophysical Properties of Sphere–Square Shape Amphiphiles Based on Porphyrin–[60]Fullerene Conjugates

Molecules constructed from a combination of zero‐dimensional ([60]fullerene (C₆₀)) and two‐dimensional (porphyrin (Por)) nanobuilding blocks represent an intriguing category of sphere–square “shape amphiphiles”. These sphere–square shape amphiphiles possess interesting optoelectronic properties. To efficiently synthesize a large variety of C₆₀–Por shape amphiphiles, a facile route based on Steglich esterification was developed. The synthetic strategy enables the preparation of hydroxy‐functionalized Por precursors ( 9 , 10 , 11 , 12 ) with high purity in a one‐pot procedure. All of the C₆₀–Por shape amphiphiles ( 1 , 2 , 3 , 4 , 5 ) can be readily synthesized in good yields through subsequent Steglich esterification with a highly soluble carboxylic acid derivative of methanofullerene ( 13 ). Photophysical studies indicated weak electronic coupling between the C₆₀ and Por moieties and suggest an edge‐to‐face alignment for the moieties. The fluorescence of electronically excited Por portions of each amphiphile was efficiently quenched, which was indicative of electron transfer from ¹Por to the C₆₀ group(s). Increasing the number of C₆₀ groups on the shape amphiphiles led to more pronounced quenching of the Por fluorescence, which indicated the potential for more effective generation of charge‐separated species, C₆₀^?.Por^+., from the photoexcited C₆₀–Por shape amphiphiles.     

Lanthanide(III) Bis(phthalocyaninato)–[60]Fullerene Dyads: Synthesis,Characterization, and Photophysical Properties

A novel series of double‐decker lanthanide(III) bis(phthalocyaninato)–C₆₀ dyads [Ln^III(Pc)(Pc′)]–C₆₀ (M=Sm, Eu, Lu; Pc=phthalocyanine) ( 1 a – c ) have been synthesized from unsymmetrically functionalized heteroleptic sandwich complexes [Ln^III(Pc)(Pc′)] (Ln=Sm, Eu, Lu) 3 a – c and fulleropyrrolidine carboxylic acid 2 . The sandwich complexes 3 a – c were obtained by means of a stepwise procedure from unsymmetrically substituted free‐base phthalocyanine 5 , which was first transformed into the monophthalocyaninato intermediate [Ln^III(acac)(Pc)] and further reacted with 1,2‐dicyanobenzene in the presence of 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU). ¹H NMR spectra of the bis(phthalocyaninato) complexes 3 a – c and dyads 1 a – c were obtained by adding hydrazine hydrate to solutions of the complexes in [D₇]DMF, a treatment that converts the free radical double‐deckers into the protonated species, that is, [Ln^III(Pc)(Pc′)H] and [Ln^III(Pc)(Pc′)H]–C₆₀. The electronic absorption spectra of 3 a – c and 1 a – c in THF exhibit typical transitions of free‐radical sandwich complexes. In the case of dyads 1 a – c , the spectra display the absorption bands of both constituents, but no evidence of ground‐state interactions could be appreciated. When the UV/Vis spectra of 3 a – c and 1 a – c were recorded in DMF, typical features of the reduced forms were observed. Cyclic voltammetry studies for 3 a – c and 1 a – c were performed in THF. The electrochemical behavior of dyads 1 a – c is almost the exact sum of the behavior of the components, namely the double‐decker [Ln^III(Pc)(Pc′)] and the C₆₀ fullerene, thus confirming the lack of ground‐state interactions between the electroactive units. Photophysical studies on dyads 1 a – c indicate that only after irradiation at 387 nm, which excites both C₆₀ and [Ln^III(Pc)(Pc′)] components, a photoinduced electron transfer from the [Ln^III(Pc)(Pc′)] to C₆₀ occurs.     

Benzo[c]thiophene–C60 Diadduct: An Electron Acceptor for p–n Junction Organic Solar Cells Harvesting Visible to Near‐IR Light

We synthesized a new 56‐π‐electron fullerene derivative through a Diels–Alder cycloaddition of benzo[c]thiophene that featured a relatively low temperature, closer to stoichiometric use of the diene, and easy product purification. The 56‐π‐electron benzo[c]thiophene diadduct ( BTCDA ) has a LUMO energy level of 0.09 to 0.18 eV higher than that of 58‐π‐electron fullerenes, and therefore, the BTCDA ‐based organic photovoltaic device exhibited a higher open‐circuit voltage and power‐conversion efficiency (PCE). When used with a binary‐donor system, including visible‐light‐harvesting tetrabenzoporphyrin ( BP ) and near‐IR‐harvesting titanyl phthalocyanine ( TiOPc ), the device had a PCE that was 1.5–3 times higher (2.8 %) than that for devices with BP or TiOPc alone because the binary‐donor device can utilize light between λ=350 and 950 nm.     

High‐Potential Perfluorinated Phthalocyanine–Fullerene Dyads for Generation of High‐Energy Charge‐Separated States: Formation and Photoinduced Electron‐Transfer Studies

High oxidation potential perfluorinated zinc phthalocyanines (ZnF_nPcs) are synthesised and their spectroscopic, redox, and light‐induced electron‐transfer properties investigated systematically by forming donor–acceptor dyads through metal–ligand axial coordination of fullerene (C₆₀) derivatives. Absorption and fluorescence spectral studies reveal efficient binding of the pyridine‐ (Py) and phenylimidazole‐functionalised fullerene (C₆₀Im) derivatives to the zinc centre of the F_nPcs. The determined binding constants, K, in o‐dichlorobenzene for the 1:1 complexes are in the order of 10⁴ to 10⁵ M ^?1; nearly an order of magnitude higher than that observed for the dyad formed from zinc phthalocyanine (ZnPc) lacking fluorine substituents. The geometry and electronic structure of the dyads are determined by using the B3LYP/6‐31G* method. The HOMO and LUMO levels are located on the Pc and C₆₀ entities, respectively; this suggests the formation of ZnF_nPc^.+–C₆₀Im^.? and ZnF_nPc^.+–C₆₀Py^.? (n=0, 8 or 16) intra‐supramolecular charge‐separated states during electron transfer. Electrochemical studies on the ZnPc–C₆₀ dyads enable accurate determination of their oxidation and reduction potentials and the energy of the charge‐separated states. The energy of the charge‐separated state for dyads composed of ZnF_nPc is higher than that of normal ZnPc–C₆₀ dyads and reveals their significance in harvesting higher amounts of light energy. Evidence for charge separation in the dyads is secured from femtosecond transient absorption studies in nonpolar toluene. Kinetic evaluation of the cation and anion radical ion peaks reveals ultrafast charge separation and charge recombination in dyads composed of perfluorinated phthalocyanine and fullerene; this implies their significance in solar‐energy harvesting and optoelectronic device building applications.     

Synthesis and Atropisomerism of Cascaded Tetraphenylporphyrin–[60]Fullerene Hybrids

Flexible, linked dendritic tetraphenylporphyrin (TPP)–fullerene hybrids were synthesized. They were designed to gain insight into and mimic the primary events in the natural photosynthetic reaction center. These multiporphyrin moieties are based on a light‐harvesting concept. Moreover, they incorporate multiple redox components aligned along a redox gradient. Newkome‐type dendrons were added to these TPP–fullerene hybrids. In principle they can mediate pH‐dependent water solubility, which, however, could not be observed in this case. A protecting‐group strategy using tert‐butyldiphenylsilyl groups allows convergent synthesis of the dendritic compounds. The dendritic multiporphyrins were synthesized separately and can be used as individual building blocks. Atropisomerism was observed in the dendritic compounds, and single atropisomers could be assigned to the corresponding peaks of a characteristic pattern in the NMR spectra. Deprotection of the Newkome‐type dendrons was shown to be feasible under mild conditions that leave the redox gradient intact.     

Tunable Electronic Properties of a Proton‐Responsive N,N‐Dimethylaminoazobenzene Fullerene (C60) Dyad

A basic N,N‐dimethylaminoazobenzene–fullerene (C₆₀) dyad molecular skeleton is modelled and synthesized. In spite of the myriad use of azobenzene as a photo‐ and electrochromic moiety, the idea presented herein is to adopt a conceptually different path by using it as a bridge in a donor–bridge–acceptor single‐molecular skeleton, connecting the electron acceptor N‐methylfulleropyrrolidine with an electron donor N,N‐dimethylaniline. Addition of trifluoroacetic acid (TFA) results in a drastic colour change of the dyad from yellow to pink in dichloromethane (DCM). The structure of the protonated species are established from electronic spectroscopy and time‐dependent density functional theory (TD‐DFT) calculations. UV/Vis spectroscopic investigations reveal the disappearance of the 409 nm ¹(π→π*) transition with appearance of new features at 520 and 540 nm, attributed to protonated β and α nitrogens, respectively, along with a finite weight of the C₆₀ pyrrolidinic nitrogen. Calculations reveal intermixing of n_(N?N)→π*_(N?N) and charge transfer (CT) transitions in the neutral dyad, whereas, the n_(N?N)→π*_(N?N) transition in the protonated dyad is buried under the dominant ¹(π →π*) feature and is red‐shifted upon Gaussian deconvolution. The experimental binding constants involved in the protonation of N,N‐dimethylanilineazobenzene and the dyad imply an almost equal probability of existence of both α‐ and β‐protonated forms. Larger binding constants for the protonated dyads imply more stable dyad complexes than for the donor counterparts. One of the most significant findings upon protonation resulted in frontier molecular orbital (FMO) switching with the dyad LUMO located on the donor part, evidenced from electrochemical investigations. The appearance of a new peak, prior to the first reduction potential of N‐methylfulleropyrrolidine, clearly indicates location of the first incoming electron on the donor‐centred LUMO of the dyad, corroborated by unrestricted DFT calculations performed on the monoanions of the protonated dyad. The protonation of the basic azo nitrogens thus enables a rational control over the energetics and location of the FMOs, indispensable for electron transport across molecular junctions in realizing futuristic current switching devices.     

Light‐Harvesting Phthalocyanine–Diketopyrrolopyrrole Derivatives: Synthesis,Spectroscopic, Electrochemical,and Photochemical Studies

A new family of light‐harvesting zinc phthalocyanine (ZnPc)–diketopyrrolopyrrole (DPP) hybrids have been synthesized and characterized. The absorption spectral measurements showed that the major absorptions of DPP (450–600 nm) are complementary to those of zinc phthalocyanine (300–400 and 600–700 nm). Therefore, the designed hybrids absorb over a broad range in the visible region. The geometric and electronic structures of the dyads were probed by initio B3LYP/6‐311G methods. The majority of the HOMOs were found to be located on the ZnPc, while the majority of the LUMOs were on the DPP units. The DPP units serve as the antenna, which upon excitation undergo efficient singlet–singlet energy transfer to the attached ZnPc units. The formed singlet ZnPc, in turn, donates its electron to the electron‐deficient DPP forming the low‐lying radical ion pairs ZnPc^.+–DPP^.? (energy=1.44–1.56 eV as calculated from the electrochemical measurements). The excited‐state events were confirmed by using a transient absorption technique in the picosecond–microsecond time range, as well as a time‐resolved emission technique. The rates of energy transfer from the singlet DPP to ZnPc were found to be extremely fast >10¹⁰ s^?1, while the rates of electron transfer from the singlet excited state of ZnPc to DPP were found to be 3.7–6.6×10⁹ s^?1.     

Synthesis and Fluorescence Properties of a Novel Supramolecular Complex Containing [60]Fullerene Moiety

Abstract

A new crown ether styryl dye containing [60]Fullerene moiety has been synthesized, and characterized by FT-IR, NMR and Mass spectra. Fluorescence spectra of the compound have also been studied. The results indicate a strong hypsochromic shift from 548 nm to 526 nm is observed on cation complexation, and the intensity of peaks at 504 nm, 548 nm and 590 nm decrease as the metal cation concentration is increased at 77K.     

Synthesis of New Porphyrin–Fullerene Dyads Capable of Forming Charge‐Separated States on a Microsecond Lifetime Scale

A series of covalently linked axially symmetric porphyrin–fullerene dyads with a rigid pyrrolo[3,4‐c]pyrrolic linker enabling a fixed and orthogonal arrangement of the chromophores has been synthesized and studied by means of transient absorption spectroscopy and cyclic voltammetry. The lifetime of the charge‐separated state has been found to depend on the substituents on the porphyrin core, reaching up to 4 μs for a species with meso‐(p‐MeOC₆H₄) substituents. The ground and excited electronic states of model compounds have been calculated at the DFT and TD‐DFT B3LYP(6‐31G(d)) levels of theory and analyzed with regard to the effect of the substituent on the stabilization of the charge‐separated state in the porphyrin–fullerene ensemble with a view to explaining the observed dependence.     

Extension of N‐Heteroacenes through a Four‐Membered Ring

The synthesis of novel π‐extended N‐heteroacenes, which have a large tetraazaacene subunit and a quinoxaline subunit connected through a four‐membered ring, is reported. They were studied with experimental and computational methods in comparison to the corresponding tetraazaacenes. As found from the DFT calculation, the four‐membered ring is a better linker than a five‐membered ring or a C?C single bond to extend N‐heteroacenes for a new design of n‐type semiconductors in terms of the spatial delocalization and energy level of LUMO as well as the reorganization energy. In solution‐processed thin film transistors, the π‐extended N‐heteroacenes are found to function as n‐type semiconductors with field effect mobility of up to 0.02 cm² V^?1 s^?1 under ambient conditions.     

A Carbon Nano‐Onion–Ferrocene Donor–Acceptor System: Synthesis,Characterization and Properties

Don't cry! The attachment of ferrocene moieties on the surface of carbon nano‐onions influences the electrochemical properties of these moieties and the photophysical properties of the carbon nano‐onions (see figure). Quantum chemical calculations confirm that the spectral properties of carbon nano‐onions depend on their size and the degree of functionalisation.

     

Facile Separation,Spectroscopic Identification,and Electrochemical Properties of Higher Trifluoromethylated Derivatives of [70]Fullerene

We survey the structure and electronic properties of the family of higher trifluoromethylated C₇₀(CF₃)_n molecules with n=14, 16, 18, and 20. Twenty‐two available compounds, of which thirteen are newly obtained and characterized, demonstrate the broad diversity of π‐system topologies, which enabled us to study the interplay between the CF₃ addition pattern and the electronic properties. UV/Vis spectroscopic and cyclic voltammetric studies demonstrate the importance of the exact addition pattern rather than the plain number of addends. Of particular interest is the skew pentagonal pyramid (SPP) addition pattern, which enables formation of closed‐shell cyclopentadienyl anions C₇₀(CF₃)_{n? 1} ^? through CF₃ detachment upon electron transfer. A detailed study of the process is presented for a SPP‐C₇₀(CF₃)₁₆ where potentiostatic electrolysis at the second reduction potential gives C₇₀(CF₃)₁₅^? oxidizable to a persistent C₇₀(CF₃)₁₅^· radical. Together with the literature data for the lower C₇₀(CF₃)_n compounds with n=2–12, the present results show good correlation between the experimental boundary level positions and the DFT predictions. The compounds turn out to be electron acceptor molecular semiconductors with experimental LUMO energies and HOMO–LUMO gaps within the ranges of ?4.3 to ?3.7 eV and 1.6 to 3.3 eV, respectively, depending on the shape of the conjugated fragments. The HOMO levels fall within the range of ?5.6 to ?6.9 eV and show linear correlation with the number of addends.     

Ultrafast Photoinduced Charge Separation Leading to High‐Energy Radical Ion‐Pairs in Directly Linked Corrole–C60 and Triphenylamine–Corrole‐C60 Donor–Acceptor Conjugates

Closely positioned donor–acceptor pairs facilitate electron‐ and energy‐transfer events, relevant to light energy conversion. Here, a triad system TPACor‐C₆₀ , possessing a free‐base corrole as central unit that linked the energy donor triphenylamine ( TPA ) at the meso position and an electron acceptor fullerene (C₆₀) at the β‐pyrrole position was newly synthesized, as were the component dyads TPA‐Cor and Cor‐C₆₀ . Spectroscopic, electrochemical, and DFT studies confirmed the molecular integrity and existence of a moderate level of intramolecular interactions between the components. Steady‐state fluorescence studies showed efficient energy transfer from ¹ TPA* to the corrole and subsequent electron transfer from ¹corrole* to fullerene. Further studies involving femtosecond and nanosecond laser flash photolysis confirmed electron transfer to be the quenching mechanism of corrole emission, in which the electron‐transfer products, the corrole radical cation ( Cor^?+ in Cor‐C₆₀ and TPA‐Cor^?+ in TPACor‐C₆₀ ) and fullerene radical anion (C₆₀^??), could be spectrally characterized. Owing to the close proximity of the donor and acceptor entities in the dyad and triad, the rate of charge separation, k_CS, was found to be about 10¹¹ s^?1, suggesting the occurrence of an ultrafast charge‐separation process. Interestingly, although an order of magnitude slower than k_CS, the rate of charge recombination, k_CR, was also found to be rapid (k_CR≈10¹⁰ s^?1), and both processes followed the solvent polarity trend DMF>benzonitrile>THF>toluene. The charge‐separated species relaxed directly to the ground state in polar solvents while in toluene, formation of ³corrole* was observed, thus implying that the energy of the charge‐separated state in a nonpolar solvent is higher than the energy of ³corrole* being about 1.52 eV. That is, ultrafast formation of a high‐energy charge‐separated state in toluene has been achieved in these closely spaced corrole–fullerene donor–acceptor conjugates.     

Efficient Synthesis of C2v‐Symmetrical Pentakisadducts of C60 as Versatile Building Blocks for Fullerene Architectures that Involve a Mixed Octahedral Addition Pattern

We report here on the selective synthesis of fullerene pentakisadducts 3 with an incomplete octahedral addition pattern by means of mixed [5:1]hexakisadducts 1 that involve an isoxazoline moiety as a protection group. The isoxazoline addend can be cleanly cleaved by irradiation with light. By using this protection–deprotection strategy, a variety of fullerene pentakisadducts 3 were synthesized in 29–44 % overall yield without the need of HPLC purification. This novel photolytic deprotection of 1 can be explained by an initial electron transfer that leads to a biradical, which can easily eliminate the isoxazoline added. The very efficient and straightforward syntheses of the bisfullerene 4 and the globular hexakisadduct 7 , each of which involves mixed octahedral addition patterns, clearly demonstrate the advantage of fullerene pentakisadducts 3 as suitable precursors for the construction of highly functional and complex [5:1]hexakisadduct architectures. Complete structural characterization of all new compounds was carried out by MALDI mass spectrometry, UV/Vis, FTIR, ¹H NMR and ¹³C NMR spectroscopy, as well as X‐ray diffraction.