Synthesis and spectral investigations of a new dyad with spiropyran and fluorescein units: toward information processing at the single molecular level

A new dyad 1 with two spiropyran units as the photochromic acceptors and one fluorescein unit as the fluorescent donor was synthesized and characterized. External inputs (ultraviolet light, visible light, and proton) induce the reversible changes of the structure and, concomitantly, the absorption spectrum of dyad 1 due to the presence of two spiropyran units. Only the absorption spectrum of the ME form of the spiropyran units in dyad 1 has large spectral overlap with the fluorescence spectrum of the fluorescein unit. Thus, the fluorescence intensity of dyad 1 is modulated by reversible conversion among the three states of the photochromic spiropyran units and the fluorescence resonance energy transfer (FRET) between the ME form and the fluorescein unit. Based on the fact that dyad 1 could "read out" three external input signals (ultraviolet light, visible ligh,t and proton) and "write" a compatible specific output signal (fluorescence intensity), dyad 1 described here can be considered to perform an integrated circuit function with one OR and one AND interconnected logic gates. The present results demonstrate an efficient strategy for elaborating and transmitting information at the single molecular level.     

Probing the donor-acceptor proximity on the physicochemical properties of porphyrin-fullerene dyads: "tail-on" and "tail-off" binding approach.

A new approach of probing proximity effects in porphyrin-fullerene dyads by using an axial ligand coordination controlled "tail-on" and "tail-off" binding mechanism is reported. In the newly synthesized porphyrin-fullerene dyads for this purpose, the donor-acceptor proximity is controlled either by temperature variation or by an axial ligand replacement method. In o-dichlorobenzene, 0.1 M (TBA)ClO(4), the synthesized zincporphyrin-fullerene dyads exhibit seven one-electron reversible redox reactions within the accessible potential window of the solvent and the measured electrochemical redox potentials and UV-visible absorption spectra reveal little or no ground-state interactions between the C(60) spheroid and porphyrin pi-system. The proximity effects on the photoinduced charge separation and charge recombination are probed by both steady-state and time-resolved fluorescence techniques. It is observed that in the "tail-off" form the charge-separation efficiency changes to some extent in comparison with the results obtained for the "tail-on" form, suggesting the presence of some through-space interactions between the singlet excited zinc porphyrin and the C(60) moiety in the "tail-off" form. The charge separation rates and efficiencies are evaluated from the fluorescence lifetime studies. The charge separation via the singlet excited states of zinc porphyrin in the studied dyads is also confirmed by the quick rise-decay of the anion radical of the C(60) moiety within 20 ns. Furthermore, a long-lived ion pair with lifetime of about 1000 ns is also observed in the investigated zinc porphyrin-C(60) dyads.     

Photoinduced intramolecular electron-transfer processes in [60]fullerene-(Spacer)-N,N-bis(biphenylyl)aniline dyad in solutions

Intramolecular photoinduced charge-separation and charge-recombination processes in a covalently connected C60-(spacer)-N,N-bis(biphenylyl)aniline (C60-spacer-BBA) dyad, in which the center-to-center distance of the electron acceptor and electron donor is 15 A, have been studied by time-resolved fluorescence and transient absorption methods. The observed low fluorescence intensity and the short fluorescence lifetime of the C60 moiety of the dyad in PhCN and THF indicate that charge separation takes place via the excited singlet state of the C60 moiety at a quite fast rate and a high efficiency. The nanosecond transient absorption spectra in PhCN and THF showed the broad absorption bands at 880 and 1100 nm, which were attributed to C60(*-)-spacer-BBA(*+). The charge-separated state decays with a lifetime of 330-360 ns in PhCN and THF at room temperature. From temperature dependence of the charge-recombination rate constants, the reorganization energy was evaluated to be 0.77-0.87 eV, which indicates that the charge-recombination process is in the inverted region of the Marcus parabola. With lowering temperature, the contribution of charge separation via the excited triplet state of the C60 moiety increases due to an increase in solvation of C60(*-)-spacer-BBA(*+).     

Synthesis and thermal modulation of the fluorescence of a pyrene-arylmaleimide dyad and its Diels-Alder adduct

The fluorescence of a pyrene-arylmaleimide dyad and its Diels-Alder adduct can be thermally modulated through reversible Diels-Alder reaction of arylmaleimide unit with furan. Moreover, the fluorescence intensities of the Diels-Alder adduct of dyad 1 with furan vary linearly with the temperatures of the corresponding solutions, demonstrating that these molecules are interesting compounds for studies of fluorescent molecular thermometers.     

Communicating chemical congregation: a molecular AND logic gate with three chemical inputs as a "lab-on-a-molecule" prototype

We demonstrate the first three-input molecular AND logic gate based on three chemical inputs as a direct way of detecting congregations of chemical species. The AND gate operates in water and responds to Na+, H+, and Zn2+ inputs with an enhanced fluorescence signal when pre-set concentration thresholds are exceeded. Future "lab-on-a-molecule" devices could have application in medicine for rapid disease screening.     

Assembly of a tetrathiafulvalene-anthracene dyad on the surfaces of gold nanoparticles: tuning the excited-state properties of the anthracene unit in the dyad

Due to the unique features of the tetrathiafulvalene (TTF) unit, such as the electron-donating ability and presence of methylthio groups, dyad 1 can be assembled on the surfaces of gold nanoparticles, as indicated by absorption, electrochemical, and fluorescent-spectral studies. Dyad 1 can also be disassembled by the addition of thiols. Assembly of dyad 1 on the surfaces of gold nanoparticles leads to the formation of a triad (A1-D-A2), which in turn modulates the photoinduced electron-transfer process within dyad 1. Accordingly, the fluorescence intensity of dyad 1, after assembly with gold nanoparticles, increases, and the fluorescence lifetime is prolonged. Furthermore, the assembly of dyad 1 on gold nanoparticles facilitates photodimerization of the anthracene units of dyad 1. Both fluorescence and photodimerization are associated with the excited-state behavior of the anthracene unit, thus it may be concluded that the excited-state properties of the anthracene unit can be tuned upon complexation with gold nanoparticles.     

Multi-electron-acceptor dyad and triad systems based on perylene bisimides and fullerenes

Fullerene (C(60)) and 3,4,9,10-perylene tetracarboxylic diimide (PTDCI) were used as building blocks for an electron acceptor dyad (C(60)-PTCDI) and triad (C(60)-PTCDI-C(60)). As the first reduction potentials for C(60) and PTCDI are very close, simultaneous introduction of two or three electrons is possible into the dyad and triad, respectively. Further stepwise electrochemical reduction leads to formation of a series of well-defined anionic species in which electrons associated with the fullerene or the PTDCI components of the molecule can be clearly distinguished. In total, up to four electrons can be reversibly injected into the dyad C(60)-PTCDI and up to six into the triad C(60)-PTCDI-C(60) system. The optical absorption properties in the UV/Vis range are also crucially defined by the distribution of electrons between the acceptor parts, as the injection/removal of electrons causes drastic colour changes in the dyad and the triad systems.     

Synthesis and structural characterization of some new porphyrin-fullerene dyads and their application in photoinduced H2 evolution

The [3 + 2] cycloaddition reaction of C(60) with ethyl isonicotinoylacetate in the presence of piperidine in PhCl at room temperature or in the presence of Mn(OAc)(3) in refluxing PhCl gave the pyridyl-containing dihydrofuran-fused C(60) derivative (4-C(5)H(4)N)C(O)═C(C(60))CO(2)Et (1), whereas the phenyl-containing C(60) derivative PhC(O)═C(C(60))CO(2)Et (2) was similarly prepared by [3 + 2] cycloaddition reaction of C(60) with ethyl benzoylacetate in the presence of piperidine or Mn(OAc)(3). More interestingly, one of the new porphyrin-fullerene dyads, i.e., [4-C(5)H(4)NC(O)═C(C(60))CO(2)Et]·ZnTPPH (3, ZnTPPH = tetraphenylporphyrinozinc), could be prepared by coordination reaction of the pyridyl-containing C(60) derivative 1 with equimolar ZnTPPH in CS(2)/hexane at room temperature. In addition, the β-keto ester-substituted porphyrin derivative H(2)TPPC(O)CH(2)CO(2)Et (4) was prepared by a sequential reaction of HO(2)CCH(2)CO(2)Et with n-BuLi in 1:2 molar ratio followed by treatment with H(2)TPPC(O)Cl in the presence of Et(3)N and then hydrolysis with diluted HCl, whereas the porphyrinozinc derivative ZnTPPC(O)CH(2)CO(2)Et (5) could be prepared by coordination reaction of 4 with Zn(OAc)(2) in refluxing CHCl(3)/MeOH. Particularly interesting is that the second new porphyrin-fullerene dyad H(2)TPPC(O)═C(C(60))CO(2)Et (6) could be prepared by [3 + 2] cycloaddition reaction of 4 with C(60) in the presence of piperidine in PhCl at room temperature. In addition, treatment of 6 with Zn(OAc)(2) in refluxing CHCl(3)/MeOH afforded the third new dyad ZnTPPC(O)═C(C(60))CO(2)Et (7). All the new compounds 1-7 were characterized by elemental analysis and various spectroscopic methods and particularly for 2, 3, and 5 by X-ray crystallography. The five-component system consisting of an electron donor EDTA, dyad 3, an electron mediator methylviologen (MV(2+)), the catalyst colloidal Pt, and a proton source HOAc was proved to be effective for photoinduced H(2) evolution. A possible pathway for such a type of H(2) evolution was proposed.     

Construction and photophysics study of supramolecular complexes composed of three-point binding fullerene-trispyridylporphyrin dyads and zinc porphyrin

A series of novel supramolecular complexes composed of a three-point binding C(60)-trispyridylporphyrin dyad (1) or C(70)-trispyridylporphyrin dyad (2) and zinc tetraphenylporphyrin (ZnP) were constructed by adopting a "covalent-coordinate" bonding approach, composed of three-point binding. The dyads and self-assembled supramolecular triads or pentads formed by coordinating the pyridine groups located on the dyads to ZnP, have been characterized by means of spectral and electrochemical techniques. The formation constants of ZnP-1 and ZnP-2 complexes were calculated as 1.4 × 10(4) M(-1) and 2.0 × 10(4) M(-1), respectively, and the Stern-Volmer quenching constants K(SV) were founded to be 2.9 × 10(4) M(-1) and 5.5 × 10(4) M(-1), respectively, which are much higher than those of other supramolecular complexes such as previously reported ZnP-3 (N-ethyl-2-(4-pyridyl)-3,4-fulleropyrrolidine). The electrochemical investigations of these complexes suggest weak interactions between the constituents in the ground state. The excited states of the complexes were further monitored by time-resolved fluorescence measurements. The results revealed that the presence of the multiple binding point dyads (1 or 2) slightly accelerated the fluorescence decay of ZnP in o-DCB relative to that of the "single-point" bound supramolecular complex ZnP-3. In comparison with 1 and 2, C(70) is suggested as a better electron acceptor relative to C(60). DFT calculations on a model of supramolecular complex ZnP-1 (with one ZnP entity) were performed. The results revealed that the lowest unoccupied molecular orbital (LUMO) is mainly located on the fullerene cage, while the highest occupied molecular orbital (HOMO) is mainly located on the ZnP macrocycle ring, predicting the formation of radical ion pair ZnP(+)˙-H(2)P-C(60)(-)˙ during photo-induced reaction.     

Drastic difference in lifetimes of the charge-separated state of the formanilide-anthraquinone dyad versus the ferrocene-formanilide-anthraquinone triad and their photoelectrochemical properties of the composite films with fullerene clusters

A long-lived charge-separated (CS) state, which can be observed even at 900 mus after laser excitation, has been attained in the formanilide-anthraquinone dyad (FA-AQ) in dimethyl sulfoxide, whereas the CS lifetime is shortened significantly to 20 ps in the ferrocene-formanilide-anthraquinone triad (Fc-FA-AQ). Such a drastic decrease in the CS lifetime by the addition of a ferrocene moiety to the FA-AQ dyad is ascribed to a decrease in the driving force of back electron transfer and an increase in the reorganization energy of electron transfer despite the longer charge-separation distance. The FA-AQ dyad and the Fc-FA-AQ triad have been employed as components of photovoltaic cells, where composite molecular nanoclusters of the FA-AQ dyad or the Fc-FA-AQ triad with fullerene (C60) are assembled onto a SnO2 electrode using an electrophoretic method. The composite films of the Fc-FA-AQ triad exhibit 10 times smaller values of an incident photon-to-photocurrent efficiency (IPCE) as compared with those of the FA-AQ dyad in accordance with a drastic decrease of the CS lifetime by addition of a ferrocene moiety to the FA-AQ dyad.     

Fullerene–Coumarin Dyad as a Selective Metal Receptor: Synthesis,Photophysical Properties,Electrochemistry and Ion Binding Studies

A coumarin derivative with a malonate unit has been synthesized and used for the preparation of a fullerene–coumarin dyad through the Bingel cyclopropanation method. The newly synthesized dyad is soluble in organic solvents and has been fully characterized with traditional spectroscopic techniques. Electronic interactions between the two components of the dyad were probed with the aid of UV/Vis spectroscopy, fluorescence emission, and electrochemistry measurements. Our studies clearly show the presence of electronic interactions between C₆₀ and modified coumarin in the ground state; efficient electron‐transfer quenching of the singlet excited state of the coumarin moiety by the appended fullerene sphere was also observed. Time‐resolved fluorescence measurements revealed lifetimes for the coumarin–C₆₀ dyad at a maximum of 50 ps, while the quantum yield was reaching unity. Additionally, the redox potentials of the C₆₀–coumarin dyad were determined and the energetics of the electron‐transfer processes were evaluated. Finally, after alkaline treatment of C₆₀–coumarin, which resulted in the deprotection of carboxylate units, the dyad was tested as a metal receptor for divalent metal cations; ion competition studies and fluorescence experiments showed binding selectivity for lead ions.     

Synthesis,Photophysical and Electrochemical Properties of Amide‐Linked Phthalocyanine‐Fullerene Dyad

A new amide‐linked phthalocyanine‐fullerene dyad ZnPc‐C₆₀ was synthesized and characterized. The photophysical and electrochemical properties of the ZnPc‐C₆₀ dyad were investigated. The fluorescence spectrum and quantum yield in different solvents showed the occurrence of photoinduced electron transfer (PET) from the singlet excited ZnPc to C₆₀, which was further confirmed by nanosecond transient absorption spectra and cyclic voltammetry data. The free energy change for charge separation (ΔG_CS) was estimated to be exothermic with ?0.51 eV, which favored the formation of charge‐separation state. The PET from ZnPc to C₆₀ in ZnPc‐C₆₀ made the dyad exhibit stronger reverse saturable absorption performance compared with C₆₀ and the control sample in the Z‐scan experiments, which indicated the synergistic effect of two active moieties in the dyad.     

Synthesis of perylene-3,4-mono(dicarboximide)--fullerene C60 dyads as new light-harvesting systems

Fullerene C 60-perylene-3,4-mono(dicarboximide) (C 60-PMI) dyads 1- 3 were synthesized in the search for new light-harvesting systems. The synthetic strategy to the PMI intermediate used a cross-coupling Suzuki reaction for the introduction of a formyl group in the ortho, meta, or para position. Subsequent 1,3-dipolar cycloaddition with C 60 led to the target C 60-PMI dyad. Cyclic voltammetry showed that the first one-electron reduction process unambiguously occurs onto the C 60 moiety and the following two-electron process corresponds to the concomitant second reduction of C 60 and the first reduction of PMI. A quasi-quantitative quenching of fluorescence was shown in dyads 1- 3, and an intramolecular energy transfer was suggested to occur from the PMI to the fullerene moiety. These C 60-PMI dyads constitute good candidates for future photovoltaic applications with expected well-defined roles for both partners, i.e., PMI acting as a light-harvesting antenna and C 60 playing the role of the acceptor in the photoactive layer.     

"Umpolung" photoinduced charge separation in an anion-bound supramolecular complex

A new supramolecular system, consisting of an expanded porphyrin, cyclo[8]pyrrole (C8) and a pyrene carboxylate (Py) is capable not only of electron transfer upon photoexcitation, but results in "umpolung" or a reversal of the predicted charge-separation behavior in that the higher energy radical ion pair, C8.+-Py.- (2.58 eV) is formed as opposed to C8.--Py.+ (1.31 eV). This observation is rationalized in terms of the latter radical ion pair, C8.--Py.+ being in the Marcus inverted region. Transient absorption spectral studies reveal a charge-separated lifetime of 300 micros, which decays to the triplet state of cyclo[8]pyrrole, also an exceedingly long-lived species. The novel features of this noncovalent dyad highlight the potential utility of anion binding in the construction of supramolecular electron transfer systems.     

Coumarin-strapped calix[4]pyrrole: a fluorogenic anion receptor modulated by cation and anion binding

A new strapped calix[4]pyrrole containing a fluorophore as part of the strap has been synthesized and characterized. Association constants with various anions have been determined using both fluorescence titration and isothermal titrations calorimetry (ITC). The two sets of association constants were found to be in good agreement with one another. The fluorescence emission properties of this new receptor could be controlled by addition of Na+ (or H2O) and anions. However, the fluorescence quenching by anions is only observed in the presence of Na+ (or H2O). All the experimental evidence is consistent with the notion that independent PET processes are modulated by separate cation and anion recognition events. As such, this system operates as an elementary logic gate wherein anion and cation concentrations serve as the input and fluorescence intensity changes provide the output.     

The synthesis and fluorescence of a methoxy-phenyl substituted C_60-pyrrolidine derivative

A new 4-methoxyphenyl substituted C60-pyrrolidine derivative, C60(C10H13NO) (1), was prepared and its room-temperature fluorescence was studied. Its fluorescence intensity is three times stronger than that of C60. Its singlet energy was estimated to be 25 kJ/mol lower than that of C60. The fluorescence lifetime was determined to be 2.1+0.3 ns by using the frequency domain method. The fluorescence quenching by concentration and aromatic electron donor: N, N-dimethyl aniline (DMA) was investigated. Data show that its fluorescence could not be quenched by DMA.     

Synthesis,Conformational Interconversion,and Photophysics of Tethered Porphyrin–Fullerene Dyads with Parachute Topology

The synthesis of a porphyrin–fullerene dyad with “parachute” topology is reported. To determine whether the dyad is “flexing” at room temperature, low‐temperature NMR experiments were used. Computational modeling has shown the low‐energy conformation of the dyad to be nonsymmetric. Although, ¹H NMR spectroscopy at room temperature is consistent with a molecule with C_2v symmetry, the spectrum changes on lowering the temperature consistent with “windshield wiper”‐like motion, in which the porphyrin moiety rotates from one side of the C₆₀ sphere to the other. Nanosecond and picosecond fluorescence lifetime experiments show two components contribute to the fluorescence decay, also consistent with the presence of more than one conformer.     

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.     

A versatile standard for bathochromic fluorescence based on intramolecular FRET

A perylene and a terrylene tetracarboxylic bisimide dyad was prepared in which an efficient energy transfer from the former to the latter is observed. The absorption spectrum of this compound covers a broad range. Bathochromic fluorescence with a high quantum yield was obtained independent of excitation wavelengths (λ < 655 nm). The dyad can be recommended for the use of calibrating fluorescence spectrometers, as well as a fluorescence standard in the bathochromic region.     

Tetrathiafulvalene in a perylene-3,4:9,10-bis(dicarboximide)-based dyad: a new reversible fluorescence-redox dependent molecular system

A donor-acceptor dyad system involving tetrathiafulvalene (TTF) as donor attached by a flexible spacer to perylene-3,4:9,10-bis(dicarboximide) (PDI) as acceptor was synthesized and characterized. The strategy used the preliminary synthesis of an unsymmetrical PDI unit bearing an alcohol functionality as anchor group. Single-crystal analysis revealed a highly organized arrangement in which all PDI molecules are packed in a noncentrosymmetrical pattern. It was shown that the fluorescence emission intensity of the TTF-PDI dyad can be reversibly tuned depending on the oxidation states of the TTF unit. This behavior is attributed to peculiar properties of TTF linked to a PDI acceptor, which fluoresces intrinsically. Consequently, this dyad can be considered as a new reversible fluorescence-redox dependent molecular system.