Ion-pair reorganization regulates reactivity in photoredox catalysts.

Cyclometalated and polypyridyl complexes of d6 metals are promising photoredox catalysts, using light to drive reactions with high kinetic or thermodynamic barriers via the generation of reactive radical intermediates. However, while tuning of their redox potentials, absorption energy, excited-state lifetime and quantum yield are well-known criteria for modifying activity, other factors could be important. Here we show that dynamic ion-pair reorganization controls the reactivity of a photoredox catalyst, [Ir[dF(CF3)ppy]2(dtbpy)]X. Time-resolved dielectric-loss experiments show how counter-ion identity influences excited-state charge distribution, evincing large differences in both the ground- and excited-state dipole moment depending on whether X is a small associating anion (PF6-) that forms a contact-ion pair versus a large one that either dissociates or forms a solvent-separated pair (BArF4-). These differences correlate with the reactivity of the photocatalyst toward both reductive and oxidative electron transfer, amounting to a 4-fold change in selectivity toward oxidation versus reduction. These results suggest that ion pairing could be an underappreciated factor that modulates reactivity in ionic photoredox catalysts.

The effect of broken conjugation on the excited state: ether linkage in the cyano-substituted poly(p-phenylene vinylene) conjugated polymer poly(2,5,2',5'-tetrahexyloxy-8,7'-dicyano-di-p-phenylene vinylene).

We investigate the effect of broken conjugation on the excited state dynamics of excimers in cyano-substituted phenylene-vinylene polymers. We compare previous studies on the well-characterized poly(2,5,2',5'-tetrahexyloxy-8,7'-dicyano-di-p-phenylene vinylene) (CN-PPV) with poly[oxa-1,4-phenylene-1,2-(1-cyano)-ethenylene-2,5-dioctyloxy-1,4-phenylene-1,2-(2-cyano)-ethenylene-1,4-phenylene] (CN-ether-PPV), in which the conjugation is disrupted by the insertion of an oxygen atom within the polymer backbone. Despite the broken conjugation, the spectroscopic behavior of the two materials is similar, indicating that the cyano group dominates the photophysics in these materials. The emission in CN-ether-PPV is due to a single-chain exciton in solution and due to an interchain excimer in thin film, as previously reported for CN-PPV; however, the excimer absorption and emission in thin film are blueshifted by approximately 0.2 eV relative to CN-PPV, implying that the excimer in CN-ether-PPV is less stable. Furthermore, substitution of an ether group along the chain results in decay times in both solution and film that are twice as long than in CN-PPV due to the broken conjugation which restricts the exciton within a conjugation segment and reduces its access to internal quenching sites. These properties result in a decay time of 14 ns for CN-ether-PPV film, one of the longest decay times observed in a conjugated polymer film. The long lifetime indicates a large exciton diffusion length, making these species particularly vulnerable to quenching by other materials. This work has implications for the design of conjugated polymers for efficient optoelectronic devices, such as photovoltaics.

Sensing isothermal changes in the lateral pressure in model membranes using di-pyrenyl phosphatidylcholine.

In this work we present data from a homologous series of di-pyrenyl phosphatidylcholine (dipyPC) probes which can sense lateral pressure variations in the chain region of the amphiphilic membrane (lateral pressures are tangential to the interface). The dipyPC has pyrene moieties attached to the ends of equal length acyl chains on a phosphatidylcholine molecule. Ultraviolet stimulation produces both monomer and excimer fluorescence from pyrene. At low dilutions of dipyPC in model membranes the excimer signal is entirely intra-molecular and since it depends on the frequency with which the pyrene moieties are brought into close proximity, the relative intensity of the excimer to monomer signal, eta, is a measure of the pressure. We synthesised or purchased dipyPC probes with the pyrene moieties attached to acyl chains having 4, 6, 8 and 10 carbon atoms and then measured eta in fully hydrated bilayers composed of dioleoylphosphatidylcholine and dioleoylphosphatidylethanolamine (DOPC and DOPE respectively). Although the resolution of our measurements of lateral pressure as a function of distance into the monolayer was limited, we did observe a dip in the excimer signal in the region of the DOPC/DOPE cis double bond. As we isothermally increased the DOPE composition, and hence the desire for interfacial curvature, we observed, as expected, that the net excimer signal increased. However this net increase was apparently brought about by a transfer of pressure from the region around the glycerol backbone to the region near the chain ends, with the lateral pressure dropping above the cis double bond but increasing at a greater rate beyond the double bond.

Comparative photophysical study of disulfonated aluminum phthalocyanine in unilamellar vesicles and leukemic K562 cells.

The photophysical properties of cis-disulfonated aluminum phthalocyanine (AlPcS2) in unilamellar vesicles (liposomes) of DL-alpha-dipalmitoyl-phosphatidylcholine have been measured. Both the fluorescence and triplet quantum yields decreased with increasing sensitizer concentration. The time-resolved fluorescence decays, analyzed by both the sum of exponentials and decay time distribution analyses, are compared with those reported for AlPcS2 in leukemic K562 cells. Information on the photodynamic transport and localization mechanism has been obtained by drawing correlations between the two systems, indicating active transport of the phthalocyanine into tumor cells involving lysosomal accumulation.

Time-resolved fluorescence spectroscopy and intracellular imaging of disulphonated aluminium phthalocyanine.

Spectroscopic studies were carried out on the photosensitizer disulphonated aluminium phthalocyanine (AlS2Pc) which has prospective applications in photodynamic therapy. The fluorescence lifetimes of AlS2Pc were measured in a range of model systems and cultured leukaemic cells using laser excitation and time-correlated, single-photon-counting detection. In an investigation of non-covalent protein binding, we studied AlS2Pc in the presence of human serum albumin (HSA) in 0.1 M phosphate-buffered saline at pH 7.4. On addition of excess concentrations of HSA, small red shifts in the fluorescence and absorbance spectra were observed, together with an increase in fluorescence polarization anisotropy, consistent with binding of the phthalocyanine. Fluorescence decays could be resolved into two lifetimes for bound AlS2Pc with a dominant component of 5.5 ns and a minor component of 1 ns. Fluorescence imaging and time-resolved microfluorometry were carried out on intracellular AlS2Pc using leukaemic K562 cells. Microscopic imaging with a charge-coupled device (CCD) camera revealed that AlS2Pc fluorescence predominated in a discrete perinuclear region which was then probed selectively by a focused laser spot for fluorescence lifetime measurements. Bi-exponential decays with lifetime components of 6.1 and 2.2 ns were observed. On irradiation at 633 nm, the fluorescence intensity increased initially and subsequently declined due to photodegradation.