Excimer fluorescence crystals

Under the (fluid) conditions necessary to promote photoassociation, the excimer fluorescence is rotationally depolarized however, measurements of pyrene crystal fluorescence have confirmed81 the predicted direction of polarization along the major axis. 

In a third type of crystal structure (Type B2, Fig. 13) exhibited by the larger pericondensed hydrocarbons (e.g., coronene and ovalene), the characteristic excimer fluorescence band may originate from a small concentration of... 

Chandross, Longworth, and Visco126 have reported the observation of long-wave structureless emission bands in the vicinity of the electrodes during the ac electrolysis of anthracene, phenanthrene, perylene, and 3,4-benzpyrene in polar solvents such as acetonitrile and dimethylformamide the similarity between the perylene band and the crystal fluorescence spectrum prompted the assignment of these bands to excimer fluorescence originating in the process... 

Excimer fluorescence has been observed in a variety of systems in which intermolecular diffusion does not play a role in excimer formation. Five such systems involving the naphthyl chromophore will be discussed (1) Crystals of aromatic hydrocarbons ... 

Birks 68) has proposed that the only change between the unexcited and excited pyrene pair is a reduction in the interplanar distance from 3.53 to 3.37 A, i.e. that the pyrene excimer is not a completely eclipsed sandwich pair either in solution or in the crystal. This proposal is consistent with the observed similarity of the excimer band position for the crystal and solution environment, and with the emission of excimer fluorescence from the crystal even at 4 K. For naphthalene, the greater separation and the nonparallel structure of nearest-neighbor pairs in the crystal apparently prohibits the formation of the sandwich excimer during the naphthalene singlet monomer lifetime. Thus, no excimer fluorescence is observed from defect-free naphthalene crystals. 

Kawakubo s fluorescence results 86> for methyl- and dimethylnaphthalene solids can be similarly related to the crystal structure. Both 2-and 2,6-substituted naphthalenes retain the same close-packed layer structure as seen in naphthalene. The only effect of the methyl substitution is to increase the crystal dimension along the naphthalene long axis87 . Less is known about the crystal structures of 1- and 1,6-substituted naphthalenes, except that the 1-substituent requires a different packing pattern than naphthalene and that 1- and 1,6-substituted naphthalenes have much lower melting points than the 2-substituted naphthalenes. The absence of sandwich pairs in 2- and 2,6-substituted naphthalene crystals certainly explains the lack of excimer fluorescence in the crystal spectra. Presumably, such pairs are also absent in crystalline 1-methylnaphthylene, but they seem to be present in glassy 1-methyl-naphthalene and in 1,6-dimethylnaphthalene solid. 

Excimer fluorescence has been reported on DSP (a) and CVCC Me crystals54. Further studies are required to correlate the emission spectrum with the crystal structure of these monomers. 

Excimer formation has been studied in polystyrene and poly(a-alkylstyrenes)189 (PS), poly(vinylcarbazole),139>140 poly-(2-vinylnaphthalene), and poly-(4-vinyl-biphenyl).141 For polystyrene films, David et a/.189 showed that the fluorescence yield increased with increasing crystallinity, at both ambient temperature and 77 K. The contribution of excimer fluorescence yield increased in the sequence atactic (0.7) < atactic oriented (0.60) < isotactic amorphous (0.28) < isotactic crystallized (0.01), with normal yields relative to excimer given in parentheses. Similar results were obtained for poly(vinylcarbazole), PVCZ, although the contribution of excimer fluorescence at 77 was independent of crystallinity. The results can be interpreted in terms of electronic energy migration to low-energy defect sites from which excimer emission can occur. In PVCZ copolymers with fumaronitrile (10), diethyl fumarate (11), and diethyl maleate (12) (Scheme 6),... 

Use of a less bulky guest such as ethyl acetate induces a dimeric lattice pattern 65, which exclusively emits excimer fluorescence from the face-to-face dimeric anthracene units [53]. Segregated anthracene (A) and anthraquinone (Q) columns (66) are found in the charge-transfer molecular crystals of adduct 29 2 (anthraquinone) [66]. This is also the case for the 1 1 complex of the diresorcinol derivative of anthracene (29) and the dipyrimidine derivative of anthraquinone (67) as a specific hydrogen-bond donor and an acceptor, respectively [66]. 

Changes to the fluorescence properties of a probe due to stacking effects can also be exploited in piezochromic macroscopic sensors. One such example is based on the liquid crystalline molecule cyano-substituted oligo(p-phenylene vinylene) (OPV), which forms columnar crystal stacks with pronounced n-n interactions, leading to excimer fluorescence [42]. In the solid state, the monomer emission is observed as a structured band in the green (2em = 481 nm). On exposure to external pressure, such as grinding with a pestle and mortar, the emission band is red-shifted (Aem = 548 nm) and the vibrational stracture is lost. The emission properties can be reversed upon heating the sample at 130 °C. The choice of... 

In the same way, Mizoshita and coworkers prepared Nph-bridged PMOs with both crystal-Uke and amorphous walls, and found that Bp-PMO displayed exci-mer fluorescence in both the amorphous and crystal-like forms, while Nph-PMO exhibited excimer fluorescence in its amorphous form and monomeric... 

An example of exciplex formation in the solid state may be afforded by perylene doped crystals of pyrene which emit a green structureless fluorescence in addition to the blue and orange-red excimer bands of pyrene and perylene, respectively. Hochstrasser112 has shown that the energy of the emitting species is consistent with that of a charge transfer complex of pyrene and perylene molecules in a bimolecular unit of the pyrene lattice. 

Since Forster s original work, a large number of aromatic compounds, including benzene, naphthalene, and anthracene, have been found to have concentration-dependent fluorescence spectra under some conditions. Most of these excimers are not as stable a.s the pyrene prototype, and require lower temperatures or higher concentrations to be observed.29 Some crystals also exhibit excimer emission. Crystalline pyrene, for example, has only a single structureless fluorescence band of the same energy as its excimer emission in solution.30... 

The geometrical requirements for excimer emission are stringent. In crystals the molecules must be stacked in columns or arranged in pairs with an interplanar distance of less than 3.5 A.31 Chandross and Dempster carried out an elegant investigation of the geometrical requirements for excimer formation in solution. They studied the fluorescence of compounds 6-10 in methylcyclohexane solution and found that only compounds 7 and 9 exhibit strong emission from an intra-... 

Quenching Excimers and Exciplexes.—By measurements of decay times and fluorescence anisotropy of pyrene and the excimer in cellulose acetate films it has been found that the medium consists of spaces where small pyrene molecules have considerable freedom, Dissado and Walmsley have developed a complete theory of excimer formation and exciton-induced lattice distortion in crystals. Reference is made to data on 9-cyanoanthracene. The spectroscopy of chemically linked dimers of l,3-(l,l -dinaphthyl)propane in a... 

Time resolved synchrotron spectroscopy of excited fluorescence of anthracene single crystals has been published and the time resolved and temperature dependent fluorescence spectra of anthracene and pyrene in both the liquid and crystalline reported states Excimer emission is observed. 

Detailed analyses of intramolecular structures are possible. Comparison of NMR and fluorescence data shows meso- and racemic diastereoisomers are found from 2,4-di(2-pyrenyl)pentane 24 jhe polarization of monomer and excimer of 4,9, disubstituted pyrenes have been measured in nematic liquid crystals 25 Quenching of pyrene fluorescence by alcohols in cyclodextrin inclusion complexes has also been studied in detail 26 Solvent effects on the photophysical properties of pyrene-3-carboxylic acid has been used to measure the pJJ, in different solvents 27 Geminate recombination in excited state proton transfer reactions has been studied with... 

The fluorescence properties of meso-substituted amidoanthracenes have been examined also. Pyrene excimers still attract considerable interest, for example, a picosecond time resolved study has been made of excimer formation in single crystals ... 

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