Quantum yield solvent polarity, effect

Additionally, note that the polarity of the solvent significantly affects not only the positions of absorption and fluorescence spectra but also the fluorescence quantum yields. The largest difference in quantum yield is observed for G19 (eight times larger in toluene) [86]. The effect of solvent polarity on quantum yield and fluorescence lifetime was investigated in mixtures of toluene and ACN (polarity range 0.013-0.306). Polarity dependent quantum yield and lifetime measurements are presented in Fig. 22. 

DDQ ( red = 0.52 V). It is noteworthy that the strong medium effects (i.e., solvent polarity and added -Bu4N+PFproduct distribution (in Scheme 5) are observed both in thermal reaction with DDQ and photochemical reaction with chloranil. Moreover, the photochemical efficiencies for dehydro-silylation and oxidative addition in Scheme 5 are completely independent of the reaction media - as confirmed by the similar quantum yields (d> = 0.85 for the disappearance of cyclohexanone enol silyl ether) in nonpolar dichloromethane (with and without added salt) and in highly polar acetonitrile. Such observations strongly suggest the similarity of the reactive intermediates in thermal and photochemical transformation of the [ESE, quinone] complex despite changes in the reaction media. 

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

Montalti and co-workers studied dansyl [27] and pyrene [28] derivatives and found the fluorescence quantum yields and excited-state lifetime of these two dyes increased in DDSNs. They attributed the enhancements to the shielding effect from the quenchers or polar solvent in the suspension. Their studies also demonstrated that the lifetime of the doped dye molecules was also dependent on the size of the DDSNs. Small DDSNs had a larger population of the short-living moieties that were more sensitive to the environment outside the DDSN. In contrast, the large DDSN had a larger population of the long-living moieties that were not sensitive to the environment. 

More important, all of the 4 -substituted trans-9-styrylanthracenes 69b-k undergo photochemical trans- cis isomerization. The effect of solvent polarity is obvious from the greatly enhanced quantum yields of trans-+cis isomerization observed for most substituted 9-styrylanthracenes in acetonitrile solution. In some cases, the cis-isomers are actually favored at the photostationary state (see Table 10). 

The effects of substitution and solvent polarity on the fluorescence properties of trans-9-styrylanthracenes 69a-k in terms of Stokes shift and fluorescence quantum yields have been summarized in Table 15. The fluorescence quantum yields in cyclohexane solution generally are about 0.5, exceptions with lower quantum yields (0.27) being the N,N-dimethylamino and nitro derivatives. For nonpolar substituted trons-9-styrylanthracenes in acetonitrile solution, the quantum yields are of the same order of magnitude as in cyclohexane. By contrast, the fluorescence quantum yields for trans-9-styrylanthracenes substituted by polar groups are drastically reduced in acetonitrile, as would be expected for bichromophoric excited state species of polar character (cf. Section III.B). 

From a systematic study of bichromophoric compounds 97-99, the importance of substituents and solvent polarity in intramolecular deactivation processes of photoexcited anthracenes by nonconjugatively tethered, and spatially separated, aromatic ketones in their electronic ground state is apparent. For 97a-d, in which the electron acceptor properties of the aromatic ketone moiety have been varied by appropriate p-substitution of the phenyl ring (R is methoxy, H, phenyl, and acetyl, respectively), the longest-wavelength absorption maximum band lies at 388 nm, i.e., any ground state effects of substitution are not detectable by UV spectroscopy. Also, the fluorescence spectra of 97a-d in cyclohexane are all related to the absorption spectra by mirror symmetry. However, the fluorescence quantum yields for 97a-d in cyclohexane dramatically are substituent dependent (see Table 19), ranging from 0.20 for the methoxy derivative to 0.00059 for the acetyl compound [33,109],... 

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... 

In other polar solvents such as alcohols and acetonitrile (CH3CN) the ejected electron can be trapped as a solvated electron , shared between several solvent molecules, or as a negative ion by attachment to a solvent molecule. Many aromatic molecules such as naphthalene, anthracene, etc., undergo such photoionizations with low quantum yields. The ions eventually recombine on a time-scale of microseconds, and there is no overall chemical effect (Figure 4.6). 

Conjugated dienes have also been reported to yield [2+2] cycloadducts with 1,2-diphenylcyclobutene (7) (107), diphenyl-vinylene carbonate (10) (108), and a-phenylcinnamonitrile (54) (109). The effect of solvent polarity on the relative quantum... 

The reactions of - -t and 7 with secondary aliphatic amines are proposed to occur via formation of a nonfluorescent singlet ex-ciplex which yields a dialkylaminyl-l,2-diphenylethyl radical pair. The stereoselective formation of 69 indicates that radical pair combination is exclusively an in-cage process which competes effectively with rotation of the 1,2-diphenylcyclobutyl radical. The limiting quantum yields for the formation of 68 and 69 in nonpolar solvent are 0.14 and 0.16, respectively. Unlike the reaction of It with tertiary aliphatic amines, the quantum yield for the formation of 68 decreases with increasing solvent polarity (113). 

Fluorescent compounds are sensitive to changes in their chemical environment. Alterations in media pH, buffer components, solvent polarity, or dissolved oxygen can affect and quench the quantum yield of a fluorescent probe (Bright, 1988). The presence of absorbing components in solution that absorb light at or near the excitation wavelength of the fluorophore will have the effect of decreasing luminescence. In addition, noncovalent interactions of the probe with other components in solution can inhibit rotational freedom and quench fluorescence. 

On the basis of these observations, Bryce-Smith et al. [115] introduced a rule stating that for addition to benzene, Pmeta when 9.6 eV < IP (alkene) <8.65 eV. They concluded that if this rule is correct, ortho addition of ethylenes to Si benzene necessarily involves an element of charge transfer to or from the ethylene. Indeed, a marked effect of polar solvents (methanol or acetonitrile) in promoting the ortho addition of benzene to ethyl vinyl ether and tetramethylethene was observed (portho increased by 20-50%, whereas cpmeta was unaffected. One exception to this rule was found by Heine and Hartmann [10], who discovered that vinylene carbonate (IP = 10.08 eV) undergoes mainly meta photocycloaddition to benzene, accompanied by some para addition. Bryce-Smith and Gilbert [46] commented that their rule referred to quantum yields and not chemical yields, whereas no quantum yields were given for the vinylene carbonate additions. Moreover, quantum yield measurements should be made at low conversions because most ortho cycloadducts are photolabile. 

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