Paracyclophanes, fluorescence

Diederich and coworkers [10] synthesized so-called dendrophanes (Figure 13.6) containing a paracyclophane core embedded in dendritic poly(ether-amide) shells. X-ray crystal-structure analysis indicated that these dendrimers had an open cavity binding site in the center, suitable for the binding of aromatic guests. NMR and fluorescence titration experiments revealed a site specific binding between these dendrimers and 6-(p-toluidino)naphthalene-2-sulfonate (TNS) with a 1 1 association. Also, the fluorescence spectral shift of TNS, which is... 

The fluorescence and absorption spectra of [2.2]paracyclophane have been investigated and interpreted by a number of authors 23-26). Theoretical calculations of the energy levels with the aid inter alia of an exciton model are in agreement with spectroscopic findings. The methods of calculation are, of course, based on a n approximation and neglect the s skeleton. 

Cyclophanes are naturally suited for MMPI (15b) calculations. The results ofsuch calculations regarding the structures and electronic spectra of the [m] paracyclophanes (n = 5-10) agreed well with the experimental data (169). Attempted X-ray analyses of [2.4]- and [2.5](9,10)-anthracenophanes (46) encountered serious disorder in the ahphatic bridges. MMPI calculations of all possible conformers of these molecules revealed four and six energy minima for 46a and 46b, respectively. Comparison of the calculated C10 C10 distances and bridge conformations with X-ray information unambiguously identified two conformations each for 46a and 46b as the final solutions. These and the calculated structures of photoisomer 47 were highly useful in the interpretation of fluorescence spectra and photoisomerization processes of 46 (170). 

With an interplanar separation of 3.73 A, 4,4 -paracyclophane is the lowest member of the series to exhibit an alkylbenzene absorption spectrum and the broad structureless fluorescence spectrum of this molecule with a peak intensity at 3400 A is by definition an excimer band further separation of the aromatic rings in 4,5 and 6,6 -paracyclophanes restores the fluorescence spectrum to that of the alkylbenzenes. These observations by Rice et al.115 illustrate the critical nature of the interplanar separation in determining the extent of interaction between -electron systems in the ground and excited configurations. 

A broad, structureless fluorescence emission is observed for [2.2], [3.3], and [4.4] paracyclophane, but only structured monomer emission is seen in [4.5] and [6.6] paracyclophane. The fluorescence properties of the [2.3], [2.4], [3.4], [3.6], [4.6], [5.5], and [5.6] paracyclophanes have not been reported, although the latter three would be expected to yield only monomer emission. The UV absorption spectra of all of the above paracyclophanes have been reported, and all [m.n] phanes for which both m and n are 4 have absorption spectra that are identical to 1,4-bis (4 -ethylphenyl)butane, the open-chain analog. The UV absorption spectra of other paracyclophanes become increasingly red-shifted and broadened in the order [3.6], [3.4], [2.4], [3.3], [2.3], and [2.2] paracyclophane. 

Distortion is less in the [3.3] paracyclophane, where the inter-ring separation varies between 3.1 and 3.3 A, and the benzene rings are only bent by 6°. In the crystal, the hydrocarbon links adopt the chair conformation, and the benzene rings are displaced by about 0.5 A from the sandwich structure, although the rings remain parallel. Some ground-state overlap occurs between the rings, since the UV absorbance extends to the red of 305 nm. The peak of the broad fluorescence emission of [3.3] paracyclophane appears at the same position as that of [2.2] paracyclophane. 

The fluorescence bands in [2.2] and [3.3] paracyclophanes should not bethought of as true excimer fluorescence since the ground state in these phanes is not free from interaction. In fact, the low-temperature fluorescence spectrum of [2.2] paracyclophane has been reported to show considerable structure, although this was not observed for [2.2] or [3.3] paracyclophanes at low temperature in a more recent report89a>. 

The inter-ring separation in [4.4] paracyclophane has been calculated to be 3.73 A, assuming normal bond angles and planar benzene rings. At this distance, there is no ground-state overlap, and the UV absorbance does not extend past 280 nm. Nevertheless, the peak of the excimer fluorescence intensity of [4.4] paracyclophane is red-shifted 1900 cm"1 relative to the peak of the solution excimer of toluene at 31,300 cm-1. Neither the excimer lifetime nor the excimer fluorescence response function have been reported for any of the exrimer-forming paracyclophanes, so little is known about the kinetics of excimer formation in these compounds. 

The naphthalenophanes that have been synthesized to date are listed in Table 6, in order of their discovery. The [m.n] isomers for which m,n > 3 have not yet been synthesized. References for the UV absorbance, fluorescence, and other properties of existing naphthalenophanes are given in Table 6. The UV absorption spectra of all the naphthalenophanes are red-shifted and broadened relative to their respective open-chain analogs, similar to the [2.2] and [3.3] paracyclophanes. Moreover, broad and structureless emissions have been observed for the naphthalenophanes in all references cited in Table 6 except one.107) The structural aspects of naphthalenophane photobehavior will be discussed in detail in the following paragraphs. 

The ROMP of [2.2]paracyclophane-l,9-diene (128) yields poly(p-phenylenevinylene) (129) as an insoluble yellow fluorescent powder. Soluble copolymers can be made by the ROMP of 128 in the presence of an excess of cyclopentene387, cycloocta-1,5-diene388 or cyclooctene389. The UV/vis absorption spectra of the copolymers with cyclooctene show separate peaks for sequences of one, two and three p-phenylene-vinylene units at 290, 345 and about 390 nm respectively, with a Bernoullian distribution. The formation of the odd members of this series must involve dissection of the two halves of the original monomer units by secondary metathesis reactions. 

Fluorescence excitation spectrum of jet-cooled (2,2)-paracyclophane in the region 30 800—31 800 cm ... 

Excimer formation is observed quite frequently with aromatic hydrocarbons. Excimer stability is particularly great for pyrene, where the enthalpy of dissociation is A// = 10 kcal/mol (Fbrster and SeidI, 1965). The excimers of aromatic molecules adopt a sandwich structure, and at room temperature, the constituents can rotate relative to each other. The interplanar separation is 300-350 pm and is thus in the same range as the separation of 375 pm between the two benzene planes in 4,4 -paracyclophane (13), which exhibits the typical structureless excimer emission. For the higher homologues, such as 5,5 -paracylophane, an ordinary fluorescence characteristic of p-dialkyl-benzenes is observed (Vala et al., 1965). 

Further confirmation for the necessity of a sandwich configuration was obtained In the paracyclophane series (18-19) in which the 4,4-paracyclophane showed an excimer fluorescence while the 4,5-paracyclophane did not. In the 2,2- and the 3,3-paracyclophane, appreciable Interaction between the chromophores in the ground state is observed in the ultraviolet spectra. Therefore in these systems, the two chromophores are held in a sandwlchllke configuration already in the ground state, they are therefore Irrelevant to check Hirayama s rule. 

It is known that [3.3]paracyclophane, which has the almost highest transannular interaction of the less distorted benzenes (12), has the fluorescence emission at longer wavelength (356 nm) (18) than the excimer of 1,3-diphenylpropane (332 nm). The fluorescence spectrum of the cyclopolymer, poly(St-C3-St), recorded under the same conditions as for [3.3]paracyclophane is illustrated in Figure 1 (20). Both have the fluorescence at the same wavelength, and therefore the polymer is supported to contain [3.3]paracyclophane units as sequence units. The fluorescence emission at 312 nm is ascribed to the residual styryl groups. 

Solutions of both the tetrastyryl[2.2]paracyclophanes 162 and 163, respectively, show intense blue-green fluorescence when exposed to sunlight. In comparison to that of 2,5-dimethyl-1,4-distyrylbenzene the UV/vis spectrum of 163 (R=Ph) shows a bathochromic shift of 41 nm for the longest wavelength absorption Amax= 394 nm) and a significant increase in the extinction coefficients. Excitation of 163 (R=Ph) at 394, 355 or 339 nm leads to fluorescence with a broad, unstructured emission band at 465 nm, which is shifted by 61 nm towards longer wavelengths in comparison to that of 2,5-dimethyl-1,4-distyrylbenzene with a well structured band at 404 nm. The relative quantum yield

[Pg.127]

Ring-opening metathesis polymerization (ROMP) of substituted bicyclo octa-dienes or paracyclophane-enes initiated by Gmbbs molybdenum, tungsten-based carbenes have been used to prepare PPV s [178—181]. The living character of ROMP has been exploited to prepare soluble well-defined precursors, which can be converted into XI. Yu and Turner have used ROMP of tetra octyloxy-substituted paracyclo-phanedienes initiated by reactive ruthenium-based carbenes to prepare monodisperse, soluble yellow fluorescent PPV with an alternating cis-trans microstructure and molecular weights as calculated [178] (Fig. 9.21). 

Photolysis of [2,2]paracyclophane in glassy solvents at 77 K produces a species with two benzyl radicals linked by an ethylene bridge." From studies of excitation spectra and emission lifetime a broad band was observed, which is attributed to intramolecular excimer fluorescence of this radical pair. 

It has also been exploited in the synthesis of phenylenevinylene polymers and copolymers. (Scheme 12.2). In general, these studies focused on the preparation of precursors that could be converted to insoluble phenylenevinylene homopolymer 8 (Thom-Csmyi et a/., 1993 Thorn-Csanyi and Pleug, 1993 Thorn-Csanyi et al., 1994), for example ROMP of (Friend et al, 1999) paracyclophane-1,9-diene 9 gives PPV as an insoluble, yellow fluorescent powder. Soluble phenylenevinylene copolymers have been prepared by ROMP of (Friend et al, 1999) paracyclophane-1,9-diene with cyclopentene, cyclooctene and cyclocta-1,5-diene comonomers (Thorn-Csanyi etal, 1993 Thorn-Csanyi and Pleug, 1993 Thorn-Csanyi et al, 1994). Unfortunately, the incorporation of more than 5% of phenylenevinylene units gives insoluble polymers, limiting the potential application of these materials. 

Bazan and coworkers investigated the emission behavior of [2.2]paracyclophane-based compounds [48-55], They reported two types of emission mechanisms for [2.2]paracyclophane-based compounds, i.e., emission from the monomer state and emission from the phane state (excimer-like emission). The conjugation length of the stacked n-electron system, the extent of overlap, and the orientation between the stacked n-electron systems determine the mechanism. According to the photoluminescence spectra of 13 and 19 (Fig. 5) and their high d>pL (0.82 for 13 and 0.86 for 19), the emission of the [2.2]paracyclophane-based x-stacked polymer occurred from the monomer state. Fluorescence lifetime studies supported this hypothesis. Both fluorescence decay curves of 13 and 19 were a single exponential, and the fluorescence lifetime (r) of the polymer was 1.27 ns (j = 1.14), which was identical to the lifetime of 19 (r= 1.24 ns,1.00) [30]. 

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