Efficient energy transfer

As described above, quantum restrictions limit tire contribution of tire free electrons in metals to the heat capacity to a vety small effect. These same electrons dominate the thermal conduction of metals acting as efficient energy transfer media in metallic materials. The contribution of free electrons to thermal transport is very closely related to their role in the transport of electric current tlrrough a metal, and this major effect is described through the Wiedemann-Franz ratio which, in the Lorenz modification, states that... 

Modeling of the reaction center inside the hole of LHl shows that the primary photon acceptor—the special pair of chlorophyll molecules—is located at the same level in the membrane, about 10 A from the periplasmic side, as the 850-nm chlorophyll molecules in LH2, and by analogy the 875-nm chlorophyll molecules of LHl. Furthermore, the orientation of these chlorophyll molecules is such that very rapid energy transfer can take place within a plane parallel to the membrane surface. The position and orientation of the chlorophyll molecules in these rings are thus optimal for efficient energy transfer to the reaction center. 

Langa and co-workers have prepared fullerodendrimers 18 and 19 in which the phenylenevinylene dendritic wedge is connected to a pyrazoHno [60] fullerene core rather than to a fulleropyrrolidine one as for 12-17 (Fig. 9) [44]. Preliminary photophysical investigations suggest that the efficient energy transfer from the excited antenna moiety to the pyrazolino [60] fullerene core is followed by an electron transfer involving the fullerene moiety and the pyrazoHne N atom. 

An extension of this kind of antennae is a first-generation heterometallic den-drimer with appended organic chromophores like pyrenyl units [25,26]. In the tetranuclear species consisting of an Os(II)-based core surrounded by three Ru(II)-based moieties and six pyrenyl units in the periphery, 100% efficient energy transfer is observed to the Os(II) core regardless of the light-absorbing unit. 

In the pentaporphyrin array 46, light absorption by the four peripheral Zn-por-phyrins is followed by efficient energy transfer to the central free base porphyrin [68], as it happens in the polypyridine dendrimers discussed above [53b]. 

AMPLIFIED FLUORESCENCE QUENCHING AND EFFICIENT ENERGY TRANSFER IN LB FILMS... 

It is interesting that statistical copolymers 343, containing m-phenylene linkages that are supposed to interrupt conjugation, showed a PL maximum of 475 nm, similar to 342. Due to efficient energy transfer from the meta- to the para-linked chromophores, the emission maxima did not depend on the ratio of m- and p-divinylbenzenes, unless 100% loading of the meta units was used [420] (Scheme 2.54). 

A very efficient energy transfer (producing emission at 613 nm) was observed in PL spectra of the perylene end-capped polymer 361 in solid films. This material had the highest QE (>60%) among the fluorene- perylene polymers, although the performance of its PLED has not yet been reported [434],... 

Attaching perylene moieties as side groups allows achievement of high concentration without affecting the electronic structure of the polymer backbone. Putting 16% perylene moieties as side chains predictably results in more efficient energy transfer, observed with polymer 360, both in solution and solid state (emission band at 599 nm). Although no PLED device with 360 has been reported, this material showed excellent performance in solar cells (external photovoltaic QE = 7%, in blend with PPV) [434]. 

When an electron-deficient BT unit was incorporated into the backbone of these polymers, an efficient energy transfer resulted in complete fluorescence quenching from the fluorene sites already at BT concentrations as low as 1% (for both neutral and quaternized copolymers, 377 and 378) [440] (Chart 2.93). These macromolecules emit green (544-550 nm, 377) to yellow (555-580 nm, 378) light and can be processed from environment-friendly solvents such as alcohols. The PLED fabricated with these polymers showed high 4>(]over 3 and 1% for 377 and 378, respectively (A1 cathode). 

Blending with dialkoxy-PPV 14 in a device (ITO/PEDOT/polymer blend layer/LiF/Ca) substantially improved the EL efficiency (by about two orders of magnitude). A moderately efficient energy transfer from the higher band-gap PPV (AEL = 650 nm) to PT 468 (AEL = 830 nm) allowed fine-tuning of the emission color by changing the component ratio (Figure 2.32) [569],... 

Q. Chu, Y. Pang, L. Ding, and F.E. Karasz, Green-emitting PPE-PPV hybrid polymers efficient energy transfer across the m-phenylene bridge, Macromolecules, 36 3848-3853, 2003. 

T. Yirgili, D. Lidzey, and D.D.C. Bradley, Efficient energy transfer from blue to red in tetra-porphyrin-doped poly(9,9-dioctylfluorene) light-emitting diodes, Adv. Mater., 12 58-62, 2000. 

Host) (Guest) Poor energy transfer Efficient energy transfer... 

In their follow-up paper, they also demonstrated 100% efficient energy transfer of both singlet and triplet excited states. The device exhibits peak external efficiency and power efficiency of 25 cd/A and 17 lm/W at 0.01 mA/cm2, respectively [343]. Liu demonstrated a high-efficiency red OLED employing DCJTB as a fluorescent dye doped in TPBI with a green phosphorescent Ir(ppy)3 as a sensitizer. A maximum brightness and luminescent efficiency of... 

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