Photoinduced electron transfer of zinc porphyrin-oligo(thienylenevinylene)-fullerene[60] triads; thienylenevinylenes as efficient molecular wires.

Two novel donor-bridge-acceptor arrays (ZnP-nTV-C60) with zinc porphyrin (ZnP) and fullerene (C60), covalently connected by oligo(thienylenevinylene) (nTV) molecular wires (n = 3 and 8; ), have been prepared in a multistep convergent manner. The influence of the nTV-length on the electrochemical and electronic properties of the ZnP-nTV-C60 triads has been revealed. Interestingly, an efficient photoinduced electron transfer process occurs in both triads with formation of intermediate radical-ion pairs (namely, ZnP˙(+)-nTV-C60˙(-) and ZnP-nTV˙(+)-C60˙(-)) as confirmed by the nanosecond transient absorption measurements in the visible and NIR regions. In polar and nonpolar solvents, the rate constants of charge-separation processes (kCS) via(1)ZnP*-nTV-C60 were found to decrease from ca. 1.2 × 10(10) s(-1) for n = 3 (RDA = 20 Å) to (5-7) × 10(9) s(-1) for n = 8 (RDA = 60 Å) on the basis of fluorescence lifetime measurements of the ZnP moiety. From these data, together with those previously obtained ones for n = 4 in the related ZnP-nTV-C60 systems, a low attenuation coefficient was evaluated for the nTV molecular wires.

Study on the association and phase separation behavior of surfactants and promethazine hydrochloride: impact of ammonium electrolytes.

In the current study, the association and phase separation of cationic tetradecyltrimethylammonium bromide (TTAB) and nonionic Triton X-100 (TX-100) surfactants with promethazine hydrochloride (PMH) were investigated in aqueous ammonium-based solutions. The micellization nature of the TTAB and PMH drug mixture was examined by evaluating critical micelle concentration (CMC) and counterion binding extent (ß) at different salt contents and temperatures (298.15-323.15 K). Micelle formation in the TTAB + PMH mixture was enhanced in the presence of ammonium salts, whereas the process was delayed with an increase in temperature in the respective salt solution. With an increase in salt content, the cloud point (CP) of the TX-100 + PMH mixture decreased, which revealed that the respective progression occurred through the salting out phenomenon. In micellization and clouding processes, the changes in free energies ΔG0m and ΔG0c were found to be negative and positive, respectively, demonstrating that the corresponding processes are spontaneous and non-spontaneous. Standard enthalpies (ΔH0m/ΔH0c) and standard entropies (ΔS0m/ΔS0c) for the association and clouding processes, respectively, were also calculated and discussed. The core forces amid TTAB/TX-100 and PMH in the manifestation of electrolytes are dipole-dipole and hydrophobic forces among the employed components according to the values for ΔH0m/ΔH0c and ΔS0m/ΔS0c, respectively.

Photoinduced energy and electron transfer in phenylethynyl-bridged zinc porphyrin-oligothienylenevinylene-C60 ensembles.

Donor-bridge-acceptor triad (Por-2TV-C(60)) and tetrad molecules ((Por)(2)-2TV-C(60)), which incorporated C(60) and one or two porphyrin molecules that were covalently linked through a phenylethynyl-oligothienylenevinylene bridge, were synthesized. Their photodynamics were investigated by fluorescence measurements, and by femto- and nanosecond laser flash photolysis. First, photoinduced energy transfer from the porphyrin to the C(60) moiety occurred rather than electron transfer, followed by electron transfer from the oligothienylenevinylene to the singlet excited state of the C(60) moiety to produce the radical cation of oligothienylenevinylene and the radical anion of C(60). Then, back-electron transfer occurred to afford the triplet excited state of the oligothienylenevinylene moiety rather than the ground state. Thus, the porphyrin units in (Por)-2TV-C(60) and (Por)(2)-2TV-C(60) acted as efficient photosensitizers for the charge separation between oligothienylenevinylene and C(60).

Photoinduced charge-separation and charge-recombination processes of fullerene[60] dyads covalently connected with phenothiazine and its trimer.

Photoinduced charge-separation and charge-recombination processes of fullerene[60] dyads covalently connected with phenothiazine and its trimer (PTZ n -C 60, n = 1 and 3) with a short amide linkage were investigated. A time-resolved fluorescence study provided evidence of charge separation via the excited singlet state of a C 60 moiety ( (1)C 60*), which displayed high efficiencies in various solvents; Phi (S) CS (quantum yield of charge separation via (1)C 60*) = 0.59 (toluene) to 0.87 (DMF) for PTZ 1-C 60 and 0.78 (toluene) to 0.91 (DMF) for PTZ 3-C 60. The transient absorption measurement with a 6 ns time resolution in the visible and near-IR regions showed evidence of the generation of radical ion pairs in relatively polar solvents for both dyads. In nonpolar toluene, only PTZ 1- (3)C 60* was observed for PTZ 1-C 60, whereas PTZ 3- (3)C 60* as well as the radical ion pair state in equilibrium were observed for PTZ 3-C 60. The radical ion pairs had relatively long lifetimes: 60 (DMF) to 910 ns ( o-dichlorobenzene) for (PTZ) 1 (*+)-C 60 (*-) and 230 (PhCN) to 380 ns ( o-dichlorobenzene) for (PTZ) 3 (*+)-C 60 (*-). The small reorganization energy (lambda) and the electronic coupling element (| V|) were estimated by the temperature dependence of the charge-recombination rates, i.e., lambda = 0.53 eV and | V| = 1.6 cm (-1) for (PTZ) 3 (*+)-C 60 (*-).

Photoinduced electron transfer in a Watson-Crick base-paired, 2-aminopurine:uracil-C60 hydrogen bonding conjugate.

A fluorescent reporter molecule, 2-aminopurine was self-assembled via Watson-Crick base-pairing to a uracil appended fullerene to form a donor-acceptor conjugate; efficient photoinduced charge separation was confirmed by time-resolved emission and transient absorption spectral studies.

High effectiveness of oligothienylenevinylene as molecular wires in Zn-porphyrin and C60 connected systems.

Photoinduced energy transfer and electron transfer processes have been found between the excited singlet state of Zn-porphyrin and C(60) via an oligothienylenevinylene bridge depending on the length of the oligothiophene and solvent polarity.

Design and studies on supramolecular ferrocene-porphyrin-fullerene constructs for generating long-lived charge separated states.

Supramolecular ferrocene-porphyrin-fullerene constructs, in which covalently linked ferrocene-porphyrin-crown ether compounds were self-assembled with alkylammonium cation functionalized fullerenes, have been designed to achieve stepwise electron transfer and hole shift to generate long-lived charge separated states. The adopted crown ether-alkylammonium cation binding strategy resulted in stable conjugates as revealed by computational studies performed by the DFT B3LYP/3-21G(*) method in addition to the binding constants obtained from fluorescence quenching studies. The free-energy changes for charge-separation and charge-recombination were varied by the choice of different metal ions in the porphyrin cavity. Free-energy calculations suggested that the light-induced electron-transfer processes from the singlet excited state of porphyrins to be exothermic in all of the investigated supramolecular dyads and triads. Photoinduced charge-separation and charge-recombination processes have been confirmed by the combination of the time-resolved fluorescence and nanosecond transient absorption spectral measurements. In case of the triads, the charge-recombination processes of the radical anion of the fullerene moiety take place in two steps, viz., a direct charge recombination from the porphyrin cation radical and a slower step involving distant charge recombination from the ferrocene cation moiety. The rates of charge recombination for the second route were found to be an order of magnitude slower than the former route, thus fulfilling the condition for charge migration to generate long-lived charge-separated states in supramolecular systems.

Multi-triphenylamine-substituted porphyrin-fullerene conjugates as charge stabilizing "antenna-reaction center" mimics.

A new concept of charge stabilization via delocalization of the pi-cation radical species over the donor macrocycle substituents in a relatively simple donor-acceptor bearing multimodular conjugates is reported. The newly synthesized multimodular systems were composed of three covalently linked triphenylamine entities at the meso position of the porphyrin ring and one fulleropyrrolidine at the fourth meso position. The triphenylamine entities were expected to act as energy transferring antenna units and to enhance the electron donating ability of both free-base and zinc(II) porphyrin derivatives of these pentads. Appreciable electronic interactions between the meso-substituted triphenylamine entities and the porphyrin pi-system were observed, and as a consequence, these moieties acted together as an electron-donor while the fullerene moiety acted as an electron-acceptor in the multimodular conjugates. In agreement with the spectral and electrochemical results, the computational studies performed by the DFT B3LYP/3-21G(*) method revealed delocalization of the frontier highest occupied molecular orbital (HOMO) over the triphenylamine entities in addition to the porphyrin macrocycle. Free-energy calculations suggested that the light-induced processes from the singlet excited state of porphyrins are exothermic in the investigated multimodular conjugates. The occurrence of photoinduced charge-separation and charge-recombination processes was confirmed by the combination of time-resolved fluorescence and nanosecond transient absorption spectral measurements. Charge-separated states, on the order of a few microseconds, were observed as a result of the delocalization of the pi-cation radical species over the porphyrin macrocycle and the meso-substituted triphenylamine entities. The present study successfully demonstrates a novel approach of charge-stabilization in donor-acceptor multimodular conjugates.

Synthesis and photoinduced intramolecular processes of fulleropyrrolidine-oligothienylenevinylene-ferrocene triads.

Two new triads based on N-methylfulleropyrolidine, oligothienylenevinylenes (nTV) and ferrocene (Fc), namely C(60)-nTV-Fc (n=2, 4) have been synthesized. A HOMO-LUMO gap as low as 1.09-1.11 eV was experimentally determined by cyclic voltammetry. In both polar and nonpolar solvents, photoinduced charge-separation (CS) processes in C(60)-nTV-Fc predominantly take place from the singlet excited states of C(60) and nTV; this result was indicated by steady and time-resolved emission spectroscopy. In the case of C(60)-4TV-Fc, the CS state was indicated by the nanosecond transient absorption spectra. In C(60)-2TV-Fc, although the CS process was also confirmed by the fluorescence quenching in nonpolar and polar solvents, the lifetimes of the CS states were shorter than those of C(60)-4TV-Fc. It was revealed that the introduction of Fc donor moiety at the end of the longer nTV chain in the C(60)-nTV dyad systems effectively increases the CS efficiency and the lifetimes of CS states.