Host-Guest Complexation of Bisporphyrin Cleft and Electron-Deficient Aromatic Guests

The host-guest complexation of a bisporphyrin cleft with various electron-deficient guest molecules was studied in solution and in the solid-state. X-ray crystal structures of a bisporphyrin cleft with naphthalene dianhydride and 2,4,7-trinitrofluorenone reveal that these guest molecules were located within the bisporphyrin cleft and formed ideal π-π stacking interactions in a host-guest ratio of 1:1. Isothermal titration calorimetry determined the binding constants and thermodynamic parameters for the 1:1 host-guest complexations in 1,2-dichloroethane and toluene. Two types of enthalpy-entropy compensation effects were found: (1) The tightly stacked host-guest structures restrict guest movement within the cleft, which results in significant desolvation with large intrinsic entropies. (2) The loosely bound guests maintain their molecular freedom within the bisporphyrin cleft, which leads to less desolvation with small intrinsic entropies. Chiral guest encapsulation directed the clockwise and anticlockwise twisted conformations of the bisporphyrin units, which induced bisignate CDs.

Persubstituted Triphenylamine Bearing Zinc Porphyrin to Host Endohedral Fullerene, Sc3N@C80: Formation and Excited State Electron Transfer

A persubstituted porphyrin with eight entities of triphenylamines at the β-pyrrole positions of a zinc tetraphenylporphyrin, 1, was newly synthesized and characterized. Due to the severe nonplanar distortion caused by the peripheral, electron rich substituents, the zinc porphyrin was able to comfortably bind a relatively large endohedral fullerene, Sc₃N@C₈₀, to form a new class of donor-acceptor-type host-guest complex. Spectral, computational, and electrochemical studies were systematically performed to evaluate the binding, spatial geometry, and redox properties of the host-guest system. Further, free-energy calculations were performed to seek the thermodynamic feasibility of excited state charge transfer. Finally, transient absorption spectral studies at different time scales were performed to secure evidence and kinetic information on excited state charge transfer leading to the 1^•+:Sc₃N@C₈₀^•- charge separated species. The present unprecedented, highly functionalized material with electron rich substituents carries zinc porphyrin as a photoactive host to large endohedral fullerenes, and its ability to undergo excited state electron transfer opens up new avenues to build photoactive host-guest systems relevant to light energy conversion and optoelectronic applications.

Synthesis of the porphyrin-calix[4]arene conjugates via Pd-catalyzed amination and their evaluation as fluorescent chemosensors

The synthesis of the porphyrin-calix[4]arene conjugates was carried out using the Pd(0)-catalyzed amination of Zn(II) meso-(3-bromophenyl)porphyrinate with bis(3-aminopropoxy)substituted calix[4]arenes (in cone and 1,3-alternate conformations). One of the conjugates was demetalated to give free porphyrin base derivative. The investigation of the fluorescence of the conjugates was studied in the presence of 18 metal perchlorates. The zinc porphyrinate derivatives were found to quench fluorescence in the presence of Cu(II), Al(III) and Cr(III) cations as well as on protonation. Metal-free conjugate was shown to act as a molecular probe for Zn(II), Cu(II) and Cd(II) cations due to strong and different changes of the emission caused by these metals.

A DFT analysis of the ground and charge-transfer excited states of Sc3N@Ih-C80 fullerene coupled with metal-free and zinc-phthalocyanine

Endohedral metallofullerenes and phthalocyanine derivatives are recognized as excellent active materials in organic photovoltaics (OPVs). The tri-metallic nitride endohedral C80 fullerenes have greater absorption coefficients in the visible region and electron-accepting abilities similar to C60, which can allow for higher efficiencies in OPV devices. In this work, we have investigated the ground and charge transfer excited states of two co-facial donor-acceptor (D-A) molecular conjugates formed by the non-covalent coupling of trimetallic nitride endohedral fullerene (Sc3N@Ih-C80) with metal-free (H2Pc) and zinc-phthalocyanine (ZnPc) chromophores using DFT calculations. The charge transfer (CT) excitation energies are calculated using the perturbative delta-SCF method that enforces orthogonality between the ground and excited states. The binding energies calculated using the PBE and DFT-D3 methods indicate that the dispersion effects play an important role in the stabilization of these complexes. The ground state dipole moment of the Sc3N@C80-H2Pc dyad is much larger than that of Sc3N@C80-ZnPc, but this is reversed in the excited state where the dipole moment of Sc3N@C80-ZnPc increases significantly. The lowest few excitation energies in the gas phase for the two complexes are very close, in the range of 1.51-2.66 eV for Sc3N@C80-ZnPc and 1.51-2.71 eV for the Sc3N@C80-H2Pc complex. However, the lower ionization potential and lower exciton binding energy make the Sc3N@C80-ZnPc dyad a better candidate for OPVs as compared to the Sc3N@C80-H2Pc dyad.

Core expanded, 21,23-dithiadiacenaphtho[1,2-c]porphyrin interactions with [60]fullerene

Saddle-shaped 21,23-dithiadiacenaphtho[1,2-c]porphyrin exhibits binding interaction with [60]fullerene in addition to photon absorption bands extending to 1000 nm.

Catching metallic nitride endohedral fullerenes in organic and aqueous media

Two bis-tetraphenylporphyrin tweezers-like conjugates bearing different linkers have been characterized by means of electrochemical and photophysical methods. The thrust of this work was the complexation of these bis-porphyrins with C[Formula: see text] and metallic nitride endohedral fullerenes (MNEF) like Sc3N@C[Formula: see text] in organic and in aqueus media — studied by a combination of steady-state and time-resolved spectroscopies. In the context of charge separation evidence for the formation of the 1a[Formula: see text]-Sc3N@C[Formula: see text] and 1c[Formula: see text]-Sc3N@C[Formula: see text] charge separated states in organic and aqueous media, respectively, came from transient absorption spectroscopy. Multi-wavelength analyses afforded charge separated state lifetimes of around 400 ps.

Synthesis and Photoinduced Electron-Transfer Reactions in a La2 @Ih -C80 -Phenoxazine Conjugate

A newly designed electron donor-acceptor conjugate consisting of an endohedral dimetallofullerene (La₂ @I_h -C₈₀ ) and phenoxazine (POZ) was successfully synthesized under Prato conditions. Our results document that the 1,3-dipolar cycloaddition took place across the [5,6] junction to afford exclusively the corresponding [5,6] cycloadduct. The structure of the conjugate was characterized by means of NMR spectroscopy, absorption spectroscopy, and electrochemical studies. Computational calculations suggest that the electron density of the highest occupied molecular orbital (HOMO) is distributed on the POZ moiety, whereas that of the lowest unoccupied molecular orbital (LUMO) is located at the endohedral La atoms, leading to efficient separation of the HOMO and LUMO in the conjugate. Time-resolved absorption spectroscopic investigations and spectroelectrochemical measurements corroborate the formation of the energetically low-lying (La₂ @I_h -C₈₀ )^.- -(POZ)^.+ radical-ion-pair state by means of ultrafast through-space electron transfer.

A multicomponent molecular approach to artificial photosynthesis – the role of fullerenes and endohedral metallofullerenes

In this review article, we highlight recent advances in the field of solar energy conversion at a molecular level.

Taming C60 fullerene: tuning intramolecular photoinduced electron transfer process with subphthalocyanines

Two subphthalocyanine-C60 conjugates have been prepared by means of the 1,3-dipolar cycloaddition reaction of (perfluoro) or hexa(pentylsulfonyl) electron deficient subphthalocyanines to C60. Comprehensive assays regarding the electronic features - in the ground and excited state - of the resulting conjugates revealed energy and electron transfer processes upon photoexcitation. Most important is the unambiguous evidence - in terms of time-resolved spectroscopy - of an ultrafast oxidative electron transfer evolving from C60 to the photoexcited subphthalocyanines. This is, to the best of our knowledge, the first case of an intramolecular oxidation of C60 within electron donor-acceptor conjugates by means of only photoexcitation.

Supramolecular interaction of fullerenes with a curved π-surface of a monomeric quadruply ring-fused porphyrin

Molecular binding of fullerenes, C60 and C70, with the Zn(II) complex of a monomeric ring-fused porphyrin derivative (2-py) as a host molecule, which has a concave π-conjugated surface, has been confirmed spectroscopically. The structures of associated complexes composed of fullerenes and 2-py were explicitly established by X-ray diffraction analysis. The fullerenes in the 2:1 complexes, which consist of two 2-py molecules and one fullerene molecule, are fully covered by the concave surfaces of the two 2-py molecules in the crystal structure. In contrast, in the crystal structure of the 1:1 complex consisting of one 2-py molecule and one C60 molecule, the C60 molecule formed a π-π stacked pair with a C60 molecule in the neighboring complex using a partial surface, which was uncovered by the 2-py molecule. Additionally, the molecular size of fullerene adopted significantly affects the (1)H NMR spectral changes and the redox properties of 2-py upon the molecular binding.

Enhancing molecular recognition in electron donor-acceptor hybrids via cooperativity

Herein, we report the synthesis of guanidinium bis-porphyrin tweezers 1 and fullerene carboxylate 3, their assembly into a novel supramolecular 1@3 electron donor-acceptor hybrid, and its characterization. In solution, the binding constant affording 1@3 is exceptionally high. 1@3, which features a highly confined topography, builds up from a combination of guanidinium-carboxylate hydrogen bonding and π-π stacking/charge-transfer motifs. The latter is governed by interactions between the electron-donating porphyrin and the electron-accepting fullerene. Importantly, positive cooperativity between the applied binding motifs is corroborated by a number of experimental techniques, such as NMR, absorption, fluorescence, etc. In addition, transient absorption experiments shed light onto electron-transfer processes taking place in the ground state and upon photoexcitation. In fact, porphyrin excitation powers an electron transfer to the fullerene yielding charge separated state lifetimes in the nanosecond regime.

A metallofullerene electron donor that powers an efficient spin flip in a linear electron donor-acceptor conjugate

The dream target of artificial photosynthesis is the realization of long-lived radical ion pair states that power catalytic centers and, consequently, the production of solar fuels. Notably, magnetic field effects, especially internal magnetic field effects, are rarely employed in this context. Here, we report on a linear Lu3N@Ih-C80-PDI electron donor-acceptor conjugate, in which the presence of the Lu3N cluster exerts an appreciable electron nuclear hyperfine coupling on the charge transfer dynamics. As such, a fairly efficient radical ion pair intersystem crossing converts the initially formed singlet radical ion pair state, (1)[(Lu3N@Ih-C80)(•+)-PDI(•-)], to the corresponding triplet radical ion pair state, (3)[(Lu3N@Ih-C80)(•+)-PDI(•-)]. Most notably, the radical ion pair state lifetime of the latter is nearly 1000 times longer than that of the former.

Current status and future developments of endohedral metallofullerenes

Endohedral metallofullerenes (EMFs), a new class of hybrid molecules formed by encapsulation of metallic species inside fullerene cages, exhibit unique properties that differ distinctly from those of empty fullerenes because of the presence of metals and their hybridization effects via electron transfer. This critical review provides a balanced but not an exhaustive summary regarding almost all aspects of EMFs, including the history, the classification, current progress in the synthesis, extraction, isolation, and characterization of EMFs, as well as their physiochemical properties and applications in fields such as electronics, photovoltaics, biomedicine, and materials science. Emphasis is assigned to experimentally obtained results, especially the X-ray crystallographic characterizations of EMFs and their derivatives, rather than theoretical calculations, although the latter has indeed enhanced our knowledge of metal-cage interactions. Finally, perspectives related to future developments and challenges in the research of EMFs are proposed. (381 references).

Molecular assemblies of porphyrins and macrocyclic receptors: recent developments in their synthesis and applications

Metalloporphyrins which form the core of many bioenzymes and natural light harvesting or electron transport systems, exhibit a variety of selective functional properties depending on the state and surroundings with which they exist in biological systems. The specificity and ease with which they function in each of their bio-functions appear to be largely governed by the nature and disposition of the protein globule around the porphyrin reaction center. Synthetic porphyrin frameworks confined within or around a pre-organized molecular entity like the protein network in natural systems have attracted considerable attraction, especially in the field of biomimetic reactions. At the same time a large number of macrocyclic oligomers such as calixarenes, resorcinarenes, spherands, cyclodextrins and crown ethers have been investigated in detail as efficient molecular receptors. These molecular receptors are synthetic host molecules with enclosed interiors, which are designed three dimensionally to ensure strong and precise molecular encapsulation/recognition. Due to their complex structures, enclosed guest molecules reside in an environment isolated from the outside and as a consequence, physical properties and chemical reactions specific to that environment in these guest species can be identified. The facile incorporation of such molecular receptors into the highly photoactive and catalytically efficient porphyrin framework allows for convenient design of useful molecular systems with unique structural and functional properties. Such systems have provided over the years attractive model systems for the study of various biological and chemical processes, and the design of new materials and molecular devices. This review focuses on the recent developments in the synthesis of porphyrin assemblies associated with cyclodextrins, calixarenes and resorcinarenes and their potential applications in the fields of molecular encapsulation/recognition, and chemical catalysis.