Enhanced singlet oxygen generation from a porphyrin-rhodamine B dyad by two-photon excitation through resonance energy transfer.

Mitochondrial-targeting photosensitizers have been associated with effective photodynamic responses. However, most photosensitizers absorb light between 400 and 700 nm, where light penetration through tissues is limited. Two-photon excitation is a rational approach to improve light penetration through tissues. In this report, the two-photon photophysical properties of a porphyrin-rhodamine B conjugate (TPP-Rh), previously demonstrated to target the mitochondria, were evaluated. The properties studied included: two-photon absorption (TPA) cross sections (σ2 ); resonance energy transfer (RET) kinetics and dynamics; and singlet oxygen generation. The conjugation of Rh B to TPP-OH approximately doubled the σ2 of TPP-Rh at 800 nm (40 ± 4 GM) compared with the parent porphyrin, TPP-OH (16 ± 4 GM). Furthermore, the rate of DPBF oxidation by singlet oxygen generated from TPP-Rh was twice as fast compared with that from TPP-OH (73 % versus 33% in 10 min) following two-photon excitation at 800 nm. In addition, a significantly stronger luminescence signal was detected from TPP-Rh, than from TPP-OH at 1270 nm, following two-photon excitation. This study indicates that conjugating photosensitizers to Rh B could provide greater TPA at the near-infrared range in addition to preferential mitochondrial accumulation for improved photodynamic responses.

Theoretical study of two-photon absorption properties and up-conversion efficiency of new symmetric organic π-conjugated molecules for photovoltaic devices.

Organic material with high intensity of two-photon absorption (TPA) induced fluorescence can be used as the frequency up-converter materials for improving efficiency of the solar cells. In this work, the organic molecular structures were designed by symmetrically grafting two elongated conjugated linkers on a conjugated core and then adding donor groups at both terminals. Fluorene derivatives as a core acceptor, phenylethynyl as the conjugated linker and diphenylamino or methyl-9H-carbozole as the donor end groups were selected. Quantum mechanical modeling techniques were applied to investigate the molecular electronic structure and properties. Absorption properties of these novel π-conjugated organic molecules were studied. The TPA cross-sections of these derivatives were calculated using few-states models, respectively. The effects of donor and acceptor groups on the TPA behaviors of these designed molecules were investigated. The up-conversion efficiency of designed molecules was also calculated.

Linkage dependence of intramolecular fluorescence quenching process in porphyrin-appended mixed (phthalocyaninato)(porphyrinato) yttrium(III) double-decker complexes.

Three novel mixed (phthalocyaninato)(porphyrinato) yttrium double-decker complexes appended with one metal-free porphyrin chromophore at the para, meta, and ortho position, respectively, of one meso-phenyl group of the porphyrin ligand in the double-decker unit through ester linkage, 3-5, have been designed, synthesized, and spectroscopically characterized. The photophysical properties of these three isomeric tetrapyrrole triads were comparatively investigated along with the model compounds metal-free tetrakis(4-butyl)porphyrin H(2)TBPP (1) and mixed [1,4,8,11,15,18,22,25-octakis(butyloxyl)phthalocyaninato][tetrakis(4-butyl)porphyrinato] yttrium double-decker complex HY[Pc(α-OC(4)H(9))(8)](TBPP) (2) by steady-state and transient spectroscopic methods. The fluorescence of the metal-free porphyrin moiety attached through ester linkage at the meta and ortho position of one meso-phenyl group of porphyrin ligand in the double-decker unit in triads 4 and 5 is effectively quenched by the double-decker unit, which takes place in several hundred femtoseconds. However, the fluorescence of the metal-free porphyrin moiety attached at the para position of one meso-phenyl group of porphyrin ligand in the double-decker unit in triad 3 is only partially quenched, clearly revealing the effect of the position of porphyrin-substituent on the photophysical properties of the triads. Furthermore, the molecular structures of 3-5 were simulated using density functional theory calculations. It was found that the relative orientation between the metal-free porphyrin moiety and the double-decker unit for compound 3 is crossed, while those for compounds 4 and 5 are open- and closed-shellfish-like, respectively, which is suggested to be responsible for their different intramolecular fluorescent quenching efficiency.

Excitation-dependent fluorescence of triphenylamine-substituted tridentate pyridyl ruthenium complexes.

Two polypyridyl ruthenium complexes, bis[4-(N,N'-diphenylamino)phenyl-2,2':6',2''-terpyridine]ruthenium(II) (1) and bis[4'-(4-{2-[4-(N,N'-diphenylamino)phenyl]ethylene}phenyl)-2, 2':6',2''-terpyridine]ruthenium(II) (2), have been synthesized. They possess an extended conjugation and strongly coupled electronic states. The features of these compounds were carefully studied from several respects. Steady-state spectroscopy showed that the two compounds had strong excitation dependent emission behaviors caused by mixing features of different electronic states. Femtosecond fluorescence upconversion spectroscopy was also used to investigate the fluorescence dynamics of the compounds. An ultrafast relaxation time of approximately 100 fs of the (1)MLCT (metal-to-ligand charge-transfer) states, which may originate from an ultrafast intersystem crossing to form (3)MLCT states, was found in both samples. However, thermal populated states and vibration associated excited state interactions were suggested for 1 with excitation at wavelengths below 400 nm, whereas vibrational energy redistribution with a time scale of few picoseconds was observed in the extended conjugated system of 2. These compounds will have potential application in both artificial photosynthesis systems and photovoltaic devices.

High frequency dielectric response in a branched phthalocyanine.

We describe the dielectric effects in a novel branched phthalocyanine system. The synthesis and characterization of the hyperbranched structure are provided. The dielectric constant was approximately 45 over many decades of frequency, and the dispersion was small up to 1 MHz. The losses experienced in this novel material were very small (close to 0.001) at high frequency. The mechanism of this novel effect in the branched structure involves a delocalized polaronic state which takes advantage of the strong intramolecular interactions in the system.

Ultrafast energy migration in chromophore shell-metal nanoparticle assemblies.

A multifunctional ligand-coated nanoparticle system containing approximately 2000 highly two-photon absorptive chromophores has been investigated by means of steady-state and femtosecond time-resolved spectroscopy. This system with a high local concentration of chromophores showed remarkably low self-quenching and a high fluorescence quantum yield, which is important for a variety of two-photon sensing and imaging applications. We have observed evidence for ultrafast energy migration in these chromophore shell-metal nanoparticle systems. Time-resolved experiments also showed non-zero residual anisotropy after the initial fast decay, which can be interpreted as due to the formation of the specific domains on the metal surfaces. This investigation opens new avenues toward the development of multi-chromophoric efficient TPA fluorescence sensing/imaging systems with large numbers of chromophores per one metal particle nanoparticle.

Up-converted emission in a series of phenylazomethine dendrimers with a porphyrin core.

The nonlinear optical and time-resolved properties of a series of phenylazomethine-porphyrin dendrimers are reported. The linear optical properties were also investigated, and the efficiency of the energy transfer process was obtained. Measurements were also carried out with the basic building-block molecules. The process of frequency up-converted emission was observed in these porphyrin dendrimers. The mechanism for this effect is investigated and related to the process of "hot-band" absorption in the phenylazomethine-porphyrin system. Time-resolved measurements also suggested efficient intramolecular vibrational energy redistribution in these systems. These properties suggest that the porphyrin dendrimers may also have applications in light harvesting of low-frequency photons, as well as in sensors.