Light- and pH-driven electron transfer in the pyranine–methylviologen system

Femtosecond excited state dynamics of a stilbene–viologen charge transfer complex assembled via host–guest interaction

The ultrafast photoinduced electron transfer in a supramolecular charge transfer complex was studied.

Probing electron transfer dynamics of pyranine with reduced graphene oxide

A stable reduced graphene oxide (rGO) was prepared and characterized by X-ray diffraction (XRD) and laser Raman spectroscopy. Steady state and time-resolved fluorescence quenching studies have been carried out to elucidate the process of electron transfer from excited pyranine (POH) into the rGO dispersion. POH adsorbed strongly on rGO dispersion with an apparent association constant of 33.4 (mg ml)(-1), and its fluorescence emission was quenched with an apparent association constant of 33.7 (mg ml)(-1). Picosecond lifetime measurements gave the rate constant for the electron transfer process from the excited singlet state of POH into the rGO dispersion as 8.8 × 10(9) s(-1). Laser flash photolysis studies demonstrated the formation of radicals for the evidence of electron transfer between POH and rGO.

Study of cavity size and nature of bridging units on recognition of nucleotides by cyclophanes

We synthesized a few novel cyclophanes CP-1 to CP-4 containing anthracene units linked together through different bridging and spacer groups and have investigated their interactions with various nucleosides and nucleotides. Of these systems, CP-1 and CP-3 showed selectivity for 5'-GTP and 5'-ATP as compared to other nucleotides and nucleosides, whereas negligible selectivity was observed with CP-2 and CP-4. Interestingly, CP-1, CP-2 and CP-3 exhibited significant binding interactions with the fluorescent indicator, 8-hydroxy-1,3,6-pyrene trisulfonate (HPTS), resulting in the formation of non-fluorescent complexes. Titration of these complexes with nucleosides and nucleotides resulted in the displacement of HPTS, leading to the revival of its fluorescence intensity. It was observed that 5'-GTP induced the maximum displacement of HPTS from the complex [CP-1·HPTS] with an overall fluorescence enhancement of ca. 150-fold, while 5'-ATP induced ca. 45-fold. Although the displacement of HPTS from the complexes [CP-2·HPTS] and [CP-3·HPTS] was found to be similar to that of [CP-1·HPTS], these complexes showed lesser selectivity and sensitivity. In contrast, negligible displacement of HPTS was observed from the complex [CP-4·HPTS] under similar conditions. These results indicate that CP-1, having a well-defined cavity and good electron acceptor (viologen), is capable of forming selective and stable complexes. Though CP-2 and CP-3 retain the good electron acceptor (viologen), their reduced aromatic surface and larger cavity, respectively, resulted in lesser sensitivity. In contrast, CP-4 having a large cavity and a poor acceptor (1,2-bis(pyridin-4-yl)ethene) showed negligible selectivity, thereby indicating the importance of cavity size, bridging unit and aromatic surface on biomolecular recognition properties of cyclophanes.

Aggregation of amphiphilic pyranines in water: facile micelle formation in the presence of methylviologen

Four amphiphilic pyranines in which the acidic hydrogen of pyranine was replaced by an octyl, dodecyl, hexadecyl, and eicosyl group, POCn (n=8, 12, 16, and 20), were prepared, and their spectroscopic properties and aggregation behaviors in water were investigated. The critical micelle concentration (cmc) of the amphiphilic pyranines was found to be relatively large even in POC20 having a long hydrophobic alkyl chain (ca. 3x10(-3) M). 1H NMR studies revealed that POC16 and POC20 exist in the compact structure with the pyranine nucleus wrapped with a long methylene chain in water. As in the case of parent pyranine, the addition of methylviologen (MV2+) to an aqueous solution of POCn resulted in the absorption spectral change and efficient quenching of POCn fluorescence. In the case of POC8 and POC12, these spectral changes induced by the MV2+ addition were thoroughly explained in terms of the formation of the electrostatic complex POCn/MV2+ with a complexation constant of approximately 3x10(4) M(-1). On the other hand, an unexpectedly large dependence of the absorption spectral change, as well as fluorescence quenching, on the total concentration of MV2+ was observed in POC16 and POC20. The Stern-Volmer plot for quenching of the fluorescence of POC16 and POC20 gave a curve deviating largely upward from a straight line. The plot was successfully analyzed by the equation induced by assuming the aggregate formation of the complex POCn/MV2+, which revealed the considerably small cmc values of the complexes, 3.6x10(-7) and 3.6x10(-8) M for POC16/MV2+ and POC20/MV2+, respectively. Experimental evidence in support of the aggregate formation was obtained by 1H NMR and dynamic light scattering studies.

A supramolecular ON-OFF-ON fluorescence assay for selective recognition of GTP

With the objective of developing small molecule based receptors for nucleosides and nucleotides, interactions of a cyclic donor-acceptor conjugate 1 with adenosine, AMP, ADP, CTP, UTP, ITP, ATP, and GTP have been investigated by absorption, steady-state, and time-resolved fluorescence, cyclic voltammetry (CV), NMR, and fluorescence indicator displacement techniques. Titration of 1 with the fluorescent indicator, 8-hydroxy-1,3,6-pyrene trisulfonate (HPTS), resulted in nearly complete fluorescence quenching of HPTS, along with 25% hypochromicity in its absorption spectrum. Benesi-Hildebrand analysis gave a 1:1 stoichiometry for the complex between the receptor 1 and HPTS with an association constant (Kass) of 4.66 x 104 M-1 in buffer. The driving force for such a complexation was evaluated to be the synergistic effects of pi-stacking and electrostatic interactions inside the cavity as confirmed by the effect of ionic strength, temperature, and the negative results obtained with the model compound 2. Titration of the nonfluorescent complex [1.HPTS] with various nucleosides and nucleotides resulted in revival of fluorescence of the indicator, HPTS. It was observed that GTP induces maximum displacement of HPTS from the complex [1.HPTS] with an overall fluorescence enhancement of ca. 150-fold. The addition of adenosine, AMP, ADP, CTP, and UTP showed negligible changes, whereas ca. 45- and 50-fold enhancement was observed with ATP and ITP, respectively. The competitive displacement of the indicator by various analytes is found to be in the order GTP (buffer) approximately GTP (biofluid) > ITP approximately ATP > UTP > CTP approximately ADP approximately AMP approximately Ade. By virtue of having a better pi-electron cloud, GTP undergoes effective electronic, pi-stacking, and electrostatic interactions inside the cavity and forms a stable complex with the receptor 1. The uniqueness of this assay is that it differentiates GTP from ATP and other nucleotides and signals the event through a visual "turn on" fluorescence mechanism in buffer as well as in biological fluids.

Spectroscopy of photoinduced charge-transfer reactions between tetrasulfonated aluminium phthalocyanine and methyl viologen

Interactions between tetrasulfonated aluminium phthalocyanine (AlPcTS4-) and methyl viologen (Mv2+) have been studied in water and ethanol solutions using several experimental techniques. UV-visible absorption and fluorescence spectroscopies show that ion-pair complexes occur in ethanol, disappearing in more polar environments such as water. Time-resolved fluorescence spectroscopy (picosecond timescale) reveals the existence of several emissive species in ethanol solutions, of which one of the components is attributed to the charge-transfer complex (AlPcTS4-)(Mv2+)2, another to higher-order aggregates and yet another to the isolated AlPcTS4- molecule. The AlPcTS4- emission is quenched by Mv2+, leading to transient diffusion in the fluorescence decay kinetics. On the other hand, the emissive complex has an exponential decay with a relatively long lifetime (above 1 ns). Time-resolved absorption measurements did not reveal the existence of radicals in aqueous solution, even on the picosecond timescale. The spectra reveal the presence of excited singlet state AlPcTS4-, which decays via the triplet excited state back to the ground state.