Enhancement of phosphorescence

Some solvents containing heavy atoms can induce enhancement of phosphorescence at the expense of fluorescence, e.g. ethyl iodide, nitro-methane, CS2 (external heavy atom effect). Irreversible conversion to ionic or radical products is often observed. Hence the system changes with time and the process should be classed a photochemical reaction distinct from the reversible quenching reactions discussed above. For example for anthracene and carbon tetrachloride ... 

For reasons clear from the introduction, enhancement of phosphorescence is a particularly attractive application of plasmonics to OLED technology. Since carriers are injected into an OLED from separate contracts, their spins are uncorrelated and spin statistics dictate preferential formation of triplet excited states. Since these are generally poor emitters at ambient temperature, metallic enhancement of the phosphorescent rate would be desirable. Moreover, triplet states are typically long-lived and prone to oxidation reactions so that reduction of the triplet lifetime could potentially improve stability of the phosphors. PtOEP is a model phosphor and its application to electroluminescence was pioneered by the groups of Forrest and Thomson (46-47). We have investigated plasmonic enhancement of the PtOEP phosphorescence on silver surfaces prepared using the Tollens reaction. Dilute PtOEP in a polymer binder was spin cast onto substrates with various densities of nanotextured silver and assumed to deposit conformally, the spin speed being used to control the approximate thickness of the overlayer. 

Meerheim, R., Scholz, S., Schwartz, G. et al. 2008. Efficiency and lifetime enhancement of phosphorescent organic devices. Proc. SPIE 6999 699917-1. 

Silver also has been demonstrated to be reactive in solution systems. Thus, silver perchlorate has been shown to influence the photochemical reactivity of stilbene in acetonitrile and methanol. The fluorescence of the stilbene is quenched on addition of the perchlorate and this is good evidence for the enhancement of the So-Ti crossing induced by the heavy ion Ag+. It seems likely that an Ag+/stilbene complex is formed. The perturbation of the system is better in methanol than in acetonitrile. However, cis.trans isomerism of the stilbene is reduced within the excited Ag+/stilbene complex since it is difficult for the geometrical isomerism to occur. Enhanced isomerism is observed with the Ag+/azobenzene system. In this complex there are steiic problems encountered in the nitrogen rehybridization process that is operative in the isomerism . Enhanced So-T crossing is also seen in the Ag+/1 1 complex with tryptophan where the fluorescence is quenched and there is a threefold increase in phosphorescence . Complexes between Ag+ and polynucleotides and DNA cause quenching of the fluorescence. Enhancement of phosphorescence and a 20-fold increase in the dimerization of thymine moieties has also been observed when silver ions are added to the reaction system . ... 

Schulz, G.L, Chen, X., Chen, S.-A., Holdcroft, S., 2006. Enhancement of phosphorescence of Ir complexes bound to conjugated polymers increasing the triplet level of the main chain. Macromolecules 39, 9157-9165. 

MCR 58] McRae E.G., Kasha M., Enhancement of phosphorescence ability upon aggregation of e molecules , Journal of Chemical Physics, vol. 28 pp. Ill-Ill, 1958. 

Stabilization of populated triplet states and enhancement of phosphorescence... 

Of particular interest in the application of cyclodextrins is the enhancement of luminescence from molecules when they are present in a cyclodextrin cavity. Polynuclear aromatic hydrocarbons show virtually no phosphorescence in solution. If, however, these compounds in solution are encapsulated with 1,2-dibromoethane (enhances intersystem crossing by increasing spin-orbit coupling external heavy atom effect) in the cavities of P-cyclodextrin and nitrogen gas passed, intense phosphorescence emission occurs at room temperature. Cyclodextrins form complexes with guest molecules, which fit into the cavity so that the microenvironment around the guest molecule is different from that in... 

In dianions of xanthene dyes although halosubstituents decrease the quantum efficiency of fluorescence, phosphorescence efficiency is not increased proportionately. The phosphorescence lifetime decreases with fa. It is suggested that in these dyes, besides enhancement of Sx intersystem crossing rates, S nonradiative transition is promoted by heavy atom... 

Wilkinson has recently described a novel approach.113 It has long been known that solvents containing heavy atoms markedly quench the fluorescence of aromatic hydrocarbons, and it has been shown that this effect arises from an enhancement of the rate of intersystem crossing. Thus the ratio of phosphorescence to fluorescence for naphthalene irradiated at 77°K can be increased more than a hundredfold upon addition of some isopropyl iodide to the solvent.114 The same effect has been noted upon changing from hydrocarbon glasses to frozen krypton and xenon matrices.115 Wilkinson found that the decrease in fluorescence intensity from irradiated solutions of anthracene and some of its derivatives upon addition of bromobenzene is attended by an increase in T-T absorption intensity.116 The fluorescence quenching follows the Stern-Volmer law ... 

The photochemistry of 6-arylpropiophenones has been examined in Silicalite and other zeolites The channel structure of Silicalite prevents intramolecular quenching by the 6-phenyl group in 6-phenylpropiophenone and leads to a dramatic enhancement of the phosphorescence. 

Surfactants were found to induce phosphorescence from 1-bromonaphthalene in aerated aqueous solutions of (3-CD [22], In the case of the 1 1 inclusion-complex formation between 7-cyclodextrin and acenaphthene in aqueous solution, bromoalcohols such as 2,3-dibromopropane-l-ol or 2-bromoethanol induced a decrease of the fluorescence and an enhancement of room-temperature phosphorescence of acenaphthene [23],... 

Polymers and oligomers that are platinum-acetylide based have important optical and optoelectronic applications because the lowest triplet excited state involves a mixture of Ptn (dji) orbitals and organic n-conjugated orbitals that enhance the phosphorescence efficiency. Batista and Martin studied the low-lying... 

Cyclodextrins have been used for enhancing the phosphorescence of guests by encapsulation [12]. de Silva et al. employed cyclodextrins as transparent shields to protect the phosphor molecule sterically from contact with its environment (especially O2) while allowing access to photons. Sensing remains viable because the proton receptor module is not enveloped (Figure 9) [13]. The authors used this system as a phosphorescent PET (photoinduced electron transfer) chemosensor for... 

The phosphorescence of diamagnetic metalloporphyrins can be easily observed at 77 K. This is consistent with slower radiationless rates, and hence with the absence of metastable cis NH intermediates, but the heavy-atom effect exerted by the central metal ion can also contribute to enhance the phosphorescence rate. For example, the phosphorescence lifetimes in the series MgTPP, ZnTPP, and PdTPP decrease in the order 45, 26, and... 

Pan, S. L., and Rothberg, L. J. (2005). Enhancement of platinum octaethyl porphyrin phosphorescence near nanotextured silver surfaces. Journal of the American Chemical Society 127 6087-6094. 

Enhanced fluorescence, or MEF, is a result of both a net system absorption and plasmon coupling and subsequently efficient emission, but to date, it has not been possible to quantify the relative contributions of enhanced emission and net increase in the system absorption to the MEF phenomena.(23) Due to the increase in the population of the singlet excited state or net system absorption, the very presence of MEP has also suggests an increase in the population of the triplet state.(23) The presence of Metal-Enhanced Fluorescence, Phosphorescence, Metal-Enhanced singlet oxygen and superoxide anion radical generation in the same system is an effect of the enhanced absorption and emission effects of the fluorophores near-to silver, although these processes are effectively competitive and ultimately provide a route for deactivation of electronic excited states. 

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