Anthracene chromophores

The signs of the exciton-split Cotton effect reflect the absolute configuration of the molecule if the direction of the transition moment in the chromophore is established. For example, chiral 9,9 -spirobifluorene derivative 1 has R configuration as indicated by a strong positive CD couplet (A = +1111.7) due to the LBb band of the anthracene chromophores, the electric transition moment of which is polarized along the long axis of the chromophore (Figure 14)100... 

The chemical deactivation of photoexcited anthracenes by dimerization usually proceeds by 4re + 4re cycloaddition [8]. However, exceptions to this rule have become known in recent years [8], and a multitude of steps, including the formation of metastable intermediates such as excimers, may actually be involved in a seemingly simple photochemical reaction such as the dimerization of 9-methylanthracene [9, 10]. Moreover, substitution of the anthracene chromophore may affect and alter its excited state properties in a profound manner for a variety of reasons. For example, in 9-tert-butylanthracene the aromatic ring system is geometrically distorted [11,12] and, consequently, photoexcitation results in the formation of the terf-butyl-substituted Dewar anthracene [13-15], The analogous photochemical isomerization of decamethylanthracene [16] probably is attributable to similar deviations from molecular planarity. 

When the anthracene chromophore is substituted at the 9-position by a re-system, coplanarity of the two re-systems will be impaired for steric reasons, and so will be electronic conjugation. For example, in ground state 9-phenylanthracene the angle between the plane of the anthracene and that of the phenyl group is about 60° [17,18]. Likewise, in 9-acetylanthracene, the... 

In numerous investigations during the past two decades, the excited state properties of various types of linked anthracenes such as l,2-di-9-anthrylethane have been assessed in terms of fluorescence and isomerization quantum yields [8,42], Although details of the mechanism of the isomerization by cycloaddition may still be a subject of discussion [43-45], effects of substitution of the anthracene chromophore are clearly noticeable... 

For both cis- and Irans-dianthrylethylenes, the degree of deconjugation, which parallels the degree of deviation from coplanarity of the ethylene and anthracene ir-systems, is borne out in the shape of the electronic absorption spectra. Thus, the long-wavelength absorption of cis-dianthrylethylene 38a is characterized by the fine structure pattern which is typical of the anthracene chromophore, while the bathochromically shifted spectrum of the transisomer 39a is virtually structureless (see Figure 7). Substitution of the ethylene double bond is absorption spectroscopically noticeable for the cis-isomers 38b-f by distortion of the anthracene absorption. The absorption... 

The effects of substituents and solvent polarity on the luminescence properties also have been evaluated for of a series of bichromophoric anthronyl-substituted anthracenes 98 and 99. It can be concluded from the quantum yield data summarized in Table 20 for spiro-substituted compounds 98a e that, dependent on solvent polarity, two different modes of intramolecular interactions between the electronically excited anthracene chromophore and the ground state ketone typically are operative, and both types of interaction result in fluorescence quenching. In nonpolar solvents, fluorescence quenching apparently involves endothermic intramolecular... 

Photoexcitation of lepidopterene in solution also gives rise to a structured emission of low intensity around 400 nm. This emission is attributable to the deactivation of the locally excited state of the E rotamer A, formed mainly by inadvertent direct excitation of the ground state cycloreversion product 114 [131]. The absorption and emission spectra of 114 are typical of the anthracene chromophore (see Figure 33). Selective excitation of 114, experimentally possible because of the suitable ground state [L]/[A] equilibrium ratio, gives rise to locally excited A, which in cyclohexane solution at room temperature has a fluorescence quantum yield of 0.84 [131]. The adiabatic conversion of A into E is difficult to detect because it proceeds at 298 K... 

G. D. Scholes, K. P. Ghiggino, A. M. Oliver and M. N. Paddon-Row, Intramolecular electronic energy transfer between rigidly linked naphthalene and anthracene chromophores, J. Phys. Chem., 97 (1993) 11871-11876. 

Fig. 14.16 Alkylethynylanthracenes (24 and 25), showing overlap of alkyl substituents with the anthracene chromophore of adjacent molecules in the crystal.

Fig. 14.24 1,4,5,8-Tetrakis(l-butoxy)anthracene (34) and packing showing anthracene chromophore sitting atop alkyl chains of adjacent molecules in the crystal.

Micelles also influence the dimerization of substituted anthracenes53). For example, 9-hydroxy-anthracene (68) when irradiated in SDS yields a 30-60% increase in the HH/HT ratio observed in homogeneous solutions (Scheme XXIV). Since the HH dimer is equally unstable in both micellar solutions and homogeneous solutions, the increase in the ratio is attributed to the proximity of the hydroxyl groups to the Stern layer and the orientation of the anthracene chromophores toward the micelle core thereby increasing the initial yield of the HH dimer. 

The ionic pair of [Co(AMMEsar)] + cation with an anthracene carboxylate anion (A-Co(III)) was used as both a photosensitizer and an ETA in the photodecomposition of water to produce hydrogen [387]. The photoreduction of encapsulated cobalt(III) ion to cobalt(II) ion occurs on excitation of anthracene chromophore (v< 25 000 cm-i). The A-Co(III) complex shows almost no fluorescence (0<2x 10 ), whereas the A-Co(II) complex produces specific violet fluorescence (Fig. 66). The cobalt(II) complex is formed in the presence of EDTA on light irradiation of the A-Co(III) solution (v> 25 641 cm i). The visible band at 21 276 cm- disappeared, and violet fluorescence was observed. The quantum yield of cobalt(II) complex formation was... 

Two cobalt cage complexes with covalently linked anthracene chromophore (Scheme 124) have been studied as coupled photosensitizers (anthracene substituent) and electron relays (cage fragment) to produce hydrogen [305]. 

Figure 11 CD and UV spectra of (6R,15R)-(+)-6,15-dihydro-6,15-ethanonaphtho[2,3-c]pentaphene 3 in dioxane/EtOH and chemical correlation between compound 3 and related compounds, where the most ideal chiral cage compound 3 with two anthracene chromophores shows intense exciton-coupled CD Cotton effects, establishing the consistency between X-ray Bijvoet and CD exciton chirality methods. Redrawn from N. Harada K. Nakanishi, Circular Dichroic Spectroscopy - Exciton Coupling in Organic Stereochemistry, University Science Books Mill Valley, CA, and Oxford University Press Oxford, 1983.

Vance and Czamik have also developed a new chemosensor for pyrophosphate based on an anthracene chromophore (Figure 91) [118]. This compound exhibits low fluorescence. However, upon binding pyrophosphate, the fluorescence increases considerably. It has been shown by Vance and Czamik that inorganic pyrophosphatase can be assayed in real time using this chemosensor since the inorganic phosphate produced does not affect the fluorescence of this molecule. 

In this paper, we report measurements of the orientation autocorrelation function of a backbone bond in dilute solutions of anthracene-labeled polyisoprene. The anthracene chromophore was covalently bonded into the chain such that the transition dipole for the lowest electronic excited state lies along the chain backbone. This assures that only backbone motions are detected. 

Fractionation of the resulting products allowed polyisoprene chains containing exactly one anthracene chromophore in the chain center to be isolated. The labeled polymer was Mn = 10,800 and M /Mn =... 

Because the substituted benzene chromophores absorb in the 205- to 280-nm range and have low e values, the solvents used must be transparent down to at least 250 nm. This requirement is unnecessary for the more conjugated naphthalene and anthracene chromophores. The usual polar, nucleophilic solvents that have been used to observe ions and ion-derived products are various alcohols, water, or water mixed with a cosolvent such as dioxane for solubility reasons. Recently, and particularly for the observation of the intermediate carbocations by laser flash photolysis (LFP) methods, the strongly ionizing (high Tore values) but weakly nucleophilic Oow N values) alcohols, 2,2,2-trifluoroethanol (TFE) and l,l,l,3,3,3,-hexafluoro-2-propanol (HFIP), have been more commonly used. A limited list of polar solvents and their properties is given in Table 2 [23,24]. 

Solutions of 24 and 26 exhibit an intense fluorescence. The quantum yield for 24 (< = 0.8, excitation at 388 nm) is substantially increased in comparison with that of the parent anthracene chromophore [27]. Oxygen readily cycloadds across the 9,10-positions of 24, when solutions are exposed to air and light [23]. 

Similarly, Desvergne et al. exploited the importance of close proximity of anthracene chromophores by linking the two anthracene chromophores with an oxymethyleneoxy (e.g., A-X-A, see Fig. 6.22) tether [61]. The dynamics and... 

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