Directional Energy Migration Research Articles

Luminescence mechanism modeling in Eu3+ doped polymethyl methacrylate by microscopic and macroscopic approaches

In this work, red emission is obtained from polymethylmethacrylate (PMMA) doped with europium ions (Eu3+) under ultraviolet excitation. PMMA:Eu3+ samples are successfully synthesized by free radical polymerization and characterized by X-ray diffraction (DRX) and photoluminescence spectroscopy. DRX patterns show the amorphous nature of PMMA and the crystalline phase of Eu3+. Optical bands related to Eu3+ in the polymer composites are observed in excitation and emission spectra and are attributed to the corresponding electronic transitions. The interaction between Eu3+ ions is studied through the emission spectra of PMMA:Eu3+ samples with different Eu3+ concentrations under an excitation of 395 nm. Luminescent emission response is analyzed theoretically using rate equation models with macroscopic and microscopic approaches. The numerical solution of the model agrees with the experimental results. The rate equation model considers that the interaction between Eu3+ ions is driven by the direct energy migration (Eu3+ → Eu3+). The Förster distance and the ratio of the pumping rate and the radiative and nonradiative relaxation constant of the PMMA:Eu3+ system are calculated, and it is demonstrated that the energy transfer rate and the quantum efficiency for both approaches agree. The results allow to predict the luminescent behavior of the PMMA:Eu3+ system with different concentrations of Eu3+ ions, and the method described here can be applied to other similar systems.

Directional Energy Transfer in Nanocrystals of [Ru(2,2′‐bipyridine)3]­[NaCr(oxalate)3

AbstractSize‐controlled nanocrystals (140 and 670 nm) and microcrystals (2.5 μm) of the three‐dimensional oxalate network [Ru(bpy)3][NaCr(ox)3] (ox = oxalate, bpy = 2,2′‐bipyridine) were prepared by the reverse micelle technique. The photophysical properties of the [Cr(ox)3]3– chromophores in the nanocrystals at low temperatures are significantly different from those of the same chromophore in 4 μm crystallites prepared by fast precipitation. For the latter, the absorption in the region of the R lines of the 4A2→2E transition is characterized by a sharp doublet. For the nanocrystals, the inhomogeneous broadening of the two lines is considerably larger and they have tails on the low‐energy side. The 4 μm crystallites at low temperatures show equally sharp emission from the R1 line, whereas the emission intensity from the nanocrystallites is shifted into the low‐energy tail. Time‐resolved fluorescence line‐narrowing spectra and luminescence decay curves demonstrate that this is due to efficient directional energy migration from the center of the nanocrystals towards the surface.

Directional energy migration in nanoparticles of crystalline metal complexes.

Size-controlled micro- and nanocrystals of a [Ru(bpy)3][NaCr(ox)3] coordination network are prepared using reverse micelles. Compared with the bulk reference, the 2E emission of the Cr(III) ions indicates an efficient directional energy migration toward the surface of the nanocrystals.

Luminescence Concentration Quenching Mechanism in Gd2O3:Eu3+.

Luminescence concentration quenching in Gd2O3:Eu(3+) nanocrystals results from strong interactions among O(2-) ions and Eu(3+) ions. Because all synthesized Gd2O3:Eu(3+) nanocrystals present the same cubic crystalline phase regardless of Eu(3+) concentration, it is possible to study the optical properties as a function of the dopant concentration. The emission intensities and lifetime curves for Gd2O3:Eu(3+) were analyzed by a simple rate equation model to study the interaction between the O(2-) ions and Eu(3+) ions. The rate equation model considers that such interaction is driven by the following energy transfer processes: the direct energy transfer (O(2-) → Eu(3+)), back-transfer (Eu(3+) → O(2-)), and direct energy migration (Eu(3+) → Eu(3+)). The exact solution of this model agrees with the experimental results, luminescence concentration quenching is reproduced and the corresponding energy transfer rates are reported. Quantitative results suggest that the direct energy transfer and direct energy migration processes are the main responsible for the luminescence concentration quenching, whereas the back-transfer process promotes the Eu(3+) emission.

Polydiacetylene‐Peptide 1D Nanomaterials

Polydiacetylenes have received intense attention on account of their well-established chromic alterations that are detectable often by the naked eye, making them ideal for a variety of applications such as biosensory materials. These polymers have been fabricated in a variety of materials platforms including 3D crystals, 2D monolayers, and 0D spherical vesicles; however, 1D morphologies that might be useful for directional energy migration are less common. This article describes the development and current research efforts of protein-based 1D nanowire-like supramolecular assemblies with embedded polydiacetylenes.

Light-Harvesting and Ultrafast Energy Migration in Porphyrin-Based Metal–Organic Frameworks

Given that energy (exciton) migration in natural photosynthesis primarily occurs in highly ordered porphyrin-like pigments (chlorophylls), equally highly ordered porphyrin-based metal-organic frameworks (MOFs) might be expected to exhibit similar behavior, thereby facilitating antenna-like light-harvesting and positioning such materials for use in solar energy conversion schemes. Herein, we report the first example of directional, long-distance energy migration within a MOF. Two MOFs, namely F-MOF and DA-MOF that are composed of two Zn(II) porphyrin struts [5,15-dipyridyl-10,20-bis(pentafluorophenyl)porphinato]zinc(II) and [5,15-bis[4-(pyridyl)ethynyl]-10,20-diphenylporphinato]zinc(II), respectively, were investigated. From fluorescence quenching experiments and theoretical calculations, we find that the photogenerated exciton migrates over a net distance of up to ~45 porphyrin struts within its lifetime in DA-MOF (but only ~3 in F-MOF), with a high anisotropy along a specific direction. The remarkably efficient exciton migration in DA-MOF is attributed to enhanced π-conjugation through the addition of two acetylene moieties in the porphyrin molecule, which leads to greater Q-band absorption intensity and much faster exciton-hopping (energy transfer between adjacent porphyrin struts). The long distance and directional energy migration in DA-MOF suggests promising applications of this compound or related compounds in solar energy conversion schemes as an efficient light-harvesting and energy-transport component.

Tuning the Decay Time of Lanthanide‐Based Near Infrared Luminescence from Micro‐ to Milliseconds through d→f Energy Transfer in Discrete Heterobimetallic Complexes

Inert and optically active pseudo-octahedral Cr(III)N6 and Ru(II)N6 chromophores have been incorporated by self-assembly into heterobimetallic triple-stranded helicates HHH-[CrLnL3]6+ and HHH-[RuLnL3]5+. The crystal structures of [CrLnL(3)](CF(3)SO(3))(6) (Ln=Nd, Eu, Yb, Lu) and [RuLnL3](CF3SO3)5 (Ln=Eu, Lu) demonstrate that the helical structure can accommodate metal ions of different sizes, without sizeable change in the intermetallic MLn distances. These systems are ideally suited for unravelling the molecular factors affecting the intermetallic nd-->4f communication. Visible irradiation of the Cr(III)N6 and Ru(II)N6 chromophores in HHH-[MLnL3]5/6+ (Ln=Nd, Yb, Er; M=Cr, Ru) eventually produces lanthanide-based near infrared (NIR) emission, after directional energy migration within the complexes. Depending on the kinetic regime associated with each specific d-f pair, the NIR luminescence decay times can be tuned from micro- to milliseconds. The origin of this effect, together with its rational control for programming optical functions in discrete heterobimetallic entities, are discussed.

Directional energy migration in an oriented nanometer-scale host/guest composite: semiconducting polymers threaded into mesoporous silica

In this paper, we show that the semiconducting polymer poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) can be incorporated into the channels of an aligned mesoporous silica host. Polarized fluorescence spectroscopy is used to show that more than 80% of the polymer in the composite is aligned by incorporation into the host. Time-resolved transient absorption spectroscopy further indicates that the incorporated chains are isolated from each other, while the unincorporated polymer is aggregated, probably at grain boundaries or surfaces. Control of the fraction of polymer inside versus outside the pores can be achieved by selective oxidation of the unincorporated polymer. Because of the unique nanoscale geometry of this material and the existence of multiple environments, excitations in this composite are funneled from outside the pores down into the aligned, isolated polymer chains inside the pores. Time-resolved stimulated emission spectroscopy is used to follow this process by monitoring the increase in luminesce polarization with time. The results show that control of polymer morphology through host/guest chemistry can be used to direct the motion of excitations and thus to deliver energy to specific regions of a material.

Cyanobacterial Phycobilisomes

Cyanobacterial phycobilisomes harvest light and cause energy migration usually toward photosystem II reaction centers. Energy transfer from phycobilisomes directly to photosystem I may occur under certain light conditions. The phycobilisomes are highly organized complexes of various biliproteins and linker polypeptides. Phycobilisomes are composed of rods and a core. The biliproteins have their bilins (chromophores) arranged to produce rapid and directional energy migration through the phycobilisomes and to chlorophyllain the thylakoid membrane. The modulation of the energy levels of the four chemically different bilins by a variety of influences produces more efficient light harvesting and energy migration. Acclimation of cyanobacterial phycobilisomes to growth light by complementary chromatic adaptation is a complex process that changes the ratio of phycocyanin to phycoerythrin in rods of certain phycobilisomes to improve light harvesting in changing habitats. The linkers govern the assembly of the biliproteins into phycobilisomes, and, even if colorless, in certain cases they have been shown to improve the energy migration process. The Lcmpolypeptide has several functions, including the linker function of determining the organization of the phycobilisome cores. Details of how linkers perform their tasks are still topics of interest. The transfer of excitation energy from bilin to bilin is considered, particularly for monomers and trimers of C-phycocyanin, phycoerythrocyanin, and allophycocyanin. Phycobilisomes are one of the ways cyanobacteria thrive in varying and sometimes extreme habitats. Various biliprotein properties perhaps not related to photosynthesis are considered: the photoreversibility of phycoviolobilin, biophysical studies, and biliproteins in evolution.

Arene-functionalized polyisocyanides: photophysics of well-defined homopolymers and block copolymers for efficient light harvesting

Steady-state and time-resolved fluorescence spectroscopy of the homopolymers and di- and triblock copolymers of 2-naphthylethylisocyanide (7), 9-anthrylethylisocyanide (10), and 2-phenanthrylethylisocyanide (11) exhibit emission from the isolated chromophores, i.e., naphthyl, anthryl, or phenanthryl, upon excitation at 284 or 354 nm. The absence of excimer emission in the fluorescence spectra of all these homopolymers is indicative of a rigid polymeric backbone. Fluorescence quenching in diblock copolymers containing a dimethylaniline donor block and a naphthalene or anthracene acceptor block takes place through directional energy migration to the acceptor–quencher interface. The migrating excited state is then quenched at the interface either by photoinduced electron transfer in the anthracene–dimethylaniline diblock copolymer or by exciplex formation in the naphthalene–dimethylaniline diblock copolymer. Upon incorporation of an intervening block derived from pentamethylphenylethylisocyanide 8, exciplex formation is suppressed in the related triblock copolymer. Transient absorption spectra of this family of di- and triblock copolymers reveal the formation of radical ion pairs, with a lifetime of 1.1 μs in the anthracene–dimethylaniline diblock copolymer. Keywords: electron transfer, energy migration, excimer emission, polyisocyanides, block copolymers.

Energy-transfer dynamics in three light-harvesting mutants of Rhodobacter sphaeroides: a picosecond spectroscopy study

Picosecond absorption spectroscopy has been used to investigate energy-transfer dynamics within the LH1 and LH2 light-harvesting complexes of three mutants of Rhodobacter sphaeroides. We demonstrate that both complexes are inhomogeneous; each contains a specialized pigment pool which absorbs maximally at a longer wavelength. Within LH2 (mutant NF57), Bchl850 transfers energy to Bchl870 in 39 +/- 9 ps; within LH1 (mutants M21 and M2192), energy is transferred from Bchl875 to Bchl896 in 22 +/- 4 and 35 +/- 5 ps, respectively. Examination of the decay of induced absorption anisotropy indicates that each of these specialized pools exists in a state which is highly organized with respect to the remainder of the pigments. Such an arrangement may facilitate and direct energy migration toward the reaction center.