Nonalternant Polycyclic Aromatic Hydrocarbons Research Articles

Phenyl migrations in dehydroaromatic compounds. A new mechanistic link between alternant and nonalternant hydrocarbons at high temperatures

Flash vacuum pyrolysis of benzo[b]biphenylene, an alternant polycyclic aromatic hydrocarbon (PAH), gives fluoranthene, a nonalternant PAH, as the major product at 1100 degrees C in the gas phase. The most reasonable mechanism to explain this isomerization involves equilibrating diradicals of 2-phenylnaphthalene that rearrange by the net migration of a phenyl group to give equilibrating diradicals of 1-phenylnaphthalene, one isomer of which then cyclizes to fluoranthene.

High temperature behavior of alternant and nonalternant polycyclic aromatic hydrocarbons

Three different flash vacuum pyrolysis (FVP) set-ups were employed for the study of the thermal behavior of alternant and nonalternant polyclic aromatic hydrocarbons (PAH). The 1,2-switch of carbon atoms in the thermal automerizations of PAH, previously observed in benzene, pyrene and benz[ a]anthracene, has been additionally observed for [1- 13C]anthracene, [6- 13C]benzo[ c]phenanthrene and [1- 13C]- and [4- 13C]phenanthrenes at temperatures between 900 and 1100°C. Bay region PAH (chrysene, benz[ a]anthracene, phenanthrene, picene, perylene, benzo[ ghi]perylene, benzo[ e]pyrene, and benz[ a,c]-anthracene) were discovered to undergo an intriguing transformation when heated at 1100°C; the transformation is believed to be triggered by cyclodehydrogenation across the bay region. Thermal formation of five-membered rings was used to synthesize a series of new nonalternant hydrocarbons. When the thermal closure of a five-membered ring is energetically unfavorable, migration of an ethynyl group from one benzene ring to another along the edge of a PAH (e.g. anthracene) can take place.

Selective fluorescence quenching of polycyclic aromatic hydrocarbons by aliphatic amines

The ability of primary, secondary, and tertiary mono- and diamines to serve as fluorescence quenchers for polycyclic aromatic hydrocarbons (PAHs) has been evaluated in acetonitrile and methanol. In general, the efficiency of quenching increases with the electron-donating ability of the amine and with the number of amine groups. The selectivity for nonalternant PAHs relative to the alternant isomers appears to decrease as the quenching efficiency increases. Nevertheless, all of the amines exhibit greater selectivity than 1,2,4-trimethoxybenzene, a previously characterized selective quencher for nonalternant PAHs. The experimentally measured rate constants for dynamic quenching show good agreement to those predicted by Rehm-Weller electron-transfer theory in the solvent acetonitrile. In contrast, the rate constants in methanol are anomalously low and do not conform well to Rehm-Weller theory. Presumably this is due to explicit hydrogen bonding between the solvent and quencher, which decreases the concentration of the free amine that is available for fluorescence quenching.

Spectrochemical investigations in dendritic media: evaluation of nitromethane as a selective fluorescence quenching agent in aqueous carboxylate-terminated polyamido amine (PAMAM) dendrimers

Alternant and nonalternant polycyclic aromatic hydrocarbons (PAHs) are employed to compare carboxylate-terminated polyamido amine (PAMAM) dendrimers to typical anionic micelles. Nitromethane is a known, selective, quenching agent of alternant PAHs. However, recent studies by Acree and co-workers have found that nitromethane will also quench the fluorescence emission intensity of nonalternant PAHs in the presence of anionic surfactants above the critical micelle concentration. The quenching of alternant and nonalternant PAHs by nitromethane is used to compare dendritic ‘unimolecular micelles’ to traditional micelles. Experimental results indicate that the PAHs’ association with these dendrimers does not appear to occur in the ‘palisade’ region, as seen in traditional micelles, but rather deeper within the dendrimer structure. Solvent polarity probe studies and quenching in the absence of nitromethane also support this conclusion. Due to the fact that the PAHs do not reside near the negatively charged surface groups, the nitromethane selective quenching rule is obeyed in carboxylate-terminated PAMAM dendrimers (10 mM in surface groups).

Spectroscopic Investigations in Molecularly Organized Solvent Media. Part 4. Effect of Cosurfactant On the Ability of the Cetylpyridinium Cation to Selectively Quench Fluorescence Emission of Alternant Versus Nonalternant Polycyclic Aromatic Hydrocarbons

Fluorescence behavior is reported for 12 alternant and 15 nonalternant polycyclic aromatic hydrocarbons (PAHs) dissolved in aqueous micellar cetyltrimethylammonium bromide (CTAB) + cetylpyridinium chloride (CPC), Brij-35 + CPC and sodium dodecyl-sulfate (SDS) + CPC mixed surfactant solvent media. the three cosurfactants studied contain a cationic, nonionic and anionic headgroup, respectively. Experimental measurements indicate that the cetylpyridinium cation selectively quenches fluorescence emission of alternant PAHs. Emission intensities of nonalternant PAHs, with a few noted exceptions, remain essentially constant, irrespective of both CPC concentration and cosurfactant headgroup charge.

Rapid and Accurate Determination of Stern—Volmer Quenching Constants

In this work, a novel system has been designed, characterized, and validated for the determination of fluorescence quenching constants. Capillary flow injection methods are used to automate the preparation and mixing of the fluorophore and quencher solutions. Because of the small diameter of the capillary (75–200 μm), fluorescence measurements can be made without corrections for primary and secondary absorbance effects. The fluorescence spectrometer is equipped with a charge-coupled device (CCD) that has a detection limit of 3.0 × 10−9 M (2.3 ppb) and a linear dynamic range of 105 for integration times of 0.01–10 s. This spectrometer has a 300 nm spectral range with 1 nm resolution, allowing the fluorescence quenching constants to be calculated at single wavelengths or over integrated wavelength ranges. This system was validated by comparison to traditional methods for the determination of Stern–Volmer constants for alternant and nonalternant polycyclic aromatic hydrocarbons with nitromethane and triethylamine.

Examination of dodecylpyridinium chloride as a potentially selective fluorescence quenching agent for discriminating between alternant versus nonalternant polycyclic aromatic hydrocarbons

Fluorescence behavior is reported for 13 alternant and 12 nonalternant polycyclic aromatic hydrocarbons (PAHs) dissolved in aqueous micellar cetyltrimethylammonium chloride (CTAC)+dodecylpyridinium chloride (DDPC) and sodium dodecylsulfate (SDS)+DDPC mixed surfactant solvent media. Experimental measurements indicate that the dodecylpyridinium cation selectively quenches fluorescence emission of alternant PAHs. Emission intensities of nonalternant PAHs, with a few noted exceptions, essentially remain constant, irrespective of both DDPC concentration and cosurfactant headgroup charge.

Classification of Select Aceanthrylenes, Acephenanthrylenes and Dicyclopentapyrenes as Alternant versus Nonalternant Polycyclic Aromatic Hydrocarbons on the Basis of Their Fluorescence Quenching Behavior in the Presence of Nitromethane and Cetylpyridinium Cation Selective Quenching Agents

Fluorescence behavior of dicyclopenta[cd, fg]pyrene, dicyclopenta[cd, jk]pyrene and dicyclopenta[cd, mn]pyrene is reported in the presence of nitromethane and cetylpyridinium (CPy+) cation quenching agents. Nitromethane and CPy+ are known to selectively quench fluorescence emission of alternant polycyclic aromatic hydrocarbons (PAHs). Emission intensities of nonalternant PAHs, with a few noted exceptions, are unaffected. Experimental measurements show that nitromethane and CPy+ effectively quenched fluorescence emission of the three dicyclopentapyrenes, thus suggesting that the perimeter C = C double bonds in the two cyclopenta-rings are alkenic, rather than aromatic in nature. Also reported are the fluorescence properties of 14 aceanthrylene and acephenanthrylene derivatives, as well as several structurally related compounds, to further document the cetylpyridinium cation's quenching selectivity.

Spectroscopic investigations in molecularly organized solvent media. Part 5. Fluorescence behavior of polycyclic aromatic hydrocarbons dissolved in cetylpyridinium chloride+zwitterionic and cetyltrimethylammonium chloride+zwitterionic mixed surfactant systems

Applicability of the cetylpyridinium (CPy +) cation as a selective fluorescence quenching agent for discriminating between alternant versus nonalternant polycyclic aromatic hydrocarbons (PAHs) is examined for 25 representative solutes dissolved in two aqueous micellar cetylpyridinium chloride (CPC)+zwitterionic surfactant solvent media. Experimental results show that the CPy + cation effectively quenched fluorescence emission of all 10 alternant PAHs studied despite the presence of strong intramicellar coulombic interactions. Emission intensities of the 15 nonalternant PAHs also decreased upon addition of CPC to the zwitterionic surfactant solutions. Reduction in emission intensities for the nonalternant PAHs is rationalized in terms of changes in micellar structure caused by the coulombic interactions, rather than from loss of quenching selectivity by the CPy + cation.

The Sphol'skii fluorescence spectrum of cyclopenta[c,d]pyrene. Novel evidence for anomalous S 2-emission

The high resolution Sphol'skii fluorescence spectrum of the non-alternant polycyclic aromatic hydrocarbon cyclopenta[c,d]pyrene (CPP, 1) in an n-hexane matrix at cryogenic temperature is reported. Two distinct narrow spectral lines at 378.0 and 379.6 nm representing two sites were found, whereas in n-octane no Sphol'skii effect was observed. In accordance with reported data on the fluorescence of CPP ( 1) these lines have to be assigned to anomalous S 2-emission.

Ab Initio Study of Polycyclic Aromatic Hydrocarbons in Their Ground and Excited States

Both ground- and excited-state ab initio calculations are reported for four polycyclic aromatic hydrocarbons (PAHs): pyrene, benzo(a)pyrene, fluoranthene, and benzo(b)fluoranthene. Ground-state geometries, IR frequencies, excitation energies, changes in excited-state electron distributions, excited-state geometries, and excited-state frequencies are calculated and quantitatively compared to experimental data. Overall agreement with experiment is quite good, with the largest discrepancies occurring when predicting the excitation energies of the molecules. Changes in electron density correlate with changes in the excited-state geometry, with all PAHs lengthening along their axis of polarization upon excitation. These calculations are also used to examine characteristic differences between the alternant (pyrene and benzo(a)pyrene) and nonalternant (fluoranthene and benzo(b)fluoranthene) PAHs. Relative to their alternant isomers, nonalternant PAHs tend to possess higher ground-state energies, lower relative excitation energies, and greater changes in their excited-state electron densities and geometries.

Spectroscopic investigations in molecularly organized solvent media. Part 3. Examination of the nitromethane selective quenching rule in aqueous anionic + cationic and anionic + nonionic mixed surfactant solutions

Applicability of the nitromethane selective quenching rule for discriminating between alternant versus nonalternant polycyclic aromatic hydrocarbons (PAHs) is examined for 20 representative PAH solutes dissolved in micellar sodium dodecylsulfate (SDS) + cetyltrimethylammonium bromide (CTAB), SDS + dodecyltrimethylammonium bromide (DTAB), SDS + Brij-35, and SDS + sodium octanoate (SO) mixed surfactant solvent media. Experimental results show that nitromethane quenched fluorescence of all 8 alternant PAHs studied in the four different solvent systems. Unexpected quenching behavior was observed, however, in the case of nonalternant PAHs. Nitromethane quenched fluorescence emission of nonalternant PAHs dissolved in the SDS + SO solvent media, which is contrary to the selective quenching rule. In the case of the mixed anionic + cationic surfactant solvent media, nitromethane quenching selectivity was restored at concentration ratios of approximately 4 : 1 (anionic:cationic) or less.

Examination of the nitromethane selective quenching rule in micellar anionic sodium dodecylbenzenesulfonate and micellar cationic dodecylethyldimethylammonium bromide solvent media

Applicability of the nitromethane selective quenching rule for discriminating between alternant versus nonalternant polycyclic aromatic hydrocarbons (PHAs) is examined for 38 representative PAH solutes dissolved in micellar sodium dodecylbenzenesulfonate (SDBS) and in micellar dodecylethyldimethyl-ammonium bromide (DEDAB) solvent media. Experimental results show that nitromethane quenched fluorescence of only the 20 alternant PAHs in the cationic DEDAB surfactant solvent media as expected. Emission intensities of nonalternant PAHs, except for the few borderline cases noted in previous work, were unaffected by nitromethane addition. Unexpected quenching behavior was observed, however, in the case of nonalternant PAHs dissolved in micellar sodium dodecylbenzenesulfonate solvent media. Nitromethane quenched fluorescence emission of all 18 nonalternant PAHs studied in the SDBS solvent media, which is contrary to the selective quenching rule.

Cetylpyridinium chloride micelles as a selective fluorescence quenching solvent media for discriminating between alternant versus nonalternant polycyclic aromatic hydrocarbons

Fluorescence behavior of 41 polycyclic aromatic hydrocarbons (PAHs) dissolved in aqueous micellar cetyltrimethylammonium chloride (CTAC) solvent media and in five different cetyltrimethylammonium chloride + cetylpyridinium chloride (CPC) surfactant mixtures is reported. Experimental fluorescence measurements reveal that CPC is a selective fluorescence quenching agent for alternant PAHs. The cetylpyridinium ion effectively quenched emission intensities of the 21 alternant PAHs studied. Emission intensities of nonalternant PAHs, with a few noted exceptions, were unaffected by the presence of CPC in the mixed cationic surfactant micelles.

Spectrofluorometric Analysis of Aromatic Compounds: Review of Applicability of Nitromethane as a Selective Fluorescence Quenching Agent for Identification of Alternant vs. Nonalternant Polycyclic Aromatic Hydrocarbons

Applicability of nitromethane as a selective fluorescence quenching agent for discriminating between alternant versus nonalternant polycyclic aromatic compounds is critically reviewed. Experimental data for 170 different polycyclic aromatic hydrocarbons (PAHs) and polycyclic aromatic nitrogen hetero-atoms (PANHs) dissolved in nonelectrolyte solvents and in aqueous micellar solvent media are presented. Specific items examined include the basic quenching mechanism and determination of rate constants, chemical and instrumental artifacts associated with accurate determination of fluorescence emission intensities, and the effect that solvent polarity, surfactant headgroup charge and electron donating and electron withdrawing substituents have in regards to fluorescence emission quenching by nitromethane.

Spectroscopic investigations in molecularly organized solvent media. Part 2: Examination of the nitromethane selective quenching rule at different ‘effective’ micellar surface charge densities

Applicability of nitromethane as a selective quenching agent for discriminating between alternant versus nonalternant polycyclic aromatic hydrocarbons (PAHs) is examined for select solutes dissolved in micellar sodium dodecylsulfate (SDS) + 1-pentanol, sodium dodecylsulfate + 1-hexanol and cetyltrimethylammonium bromide (CTAB) + 1-pentanol solvent media. Results of measurements show that nitromethane quenched fluorescence emission of only the 6 alternant PAHs in the cationic CTAB + 1-pentanol solvent media as expected. Emission intensities of nonalternant PAHs, except for two exceptions noted previously, were unaffected by nitromethane addition. Unexpected quenching behavior was observed, however, in the case of nonalternant PAHs dissolved in micellar anionic SDS + 1-pentanol and micellar anionic SDS + 1-hexanol. Nitromethane quenched fluorescence emission of all nonalternant PAHs studied in these two anionic surfactant systems, which is contrary to the selective quenching rule. The reduced emission quenching that is observed at increasing alcohol cosolvent concentration is rationalized in terms of the decreased micellar surface charge density that occurs when more alcohol molecules are incorporated into the anionic micelle.

Spectroscopic properties of polycyclic aromatic compounds Part 6. The nitromethane selective quenching rule visited in aqueous micellar zwitterionic surfactant solvent media

Applicability of the nitromethane selective quenching rule for discriminating between alternant versus nonalternant polycyclic aromatic hydrocarbons (PAHs) is examined for 58 representative PAH solutes dissolved in micellar N-hexadecyl- N, N-dimethyl-3-ammonio-1-propanesulfonate and in micellar N-dodecyl- N, N-dimethyl-3-ammonio-1-propanesulfonate solvent media. Results of measurements show that zwitterionic surfactants can be considered, for the most part, as providing a polar solubilizing media as far as the nitromethane selective quenching rule is concerned. Nonalternant PAHs that contain electron donating methoxy- and hydroxy-functional groups (and methyl-groups to a much lesser extent) are noted exceptions.

Spectrochemical investigations of fluorescence quenching agents: Part 5. Effect of surfactants on the ability of nitromethane to selectively quench fluorescence emission of alternant PAHs

Applicability of the nitromethane selective quenching rule for discriminating between alternant vs. nonalternant polycyclic aromatic hydrocarbons (PAHs) is examined for 18 representative PAH solutes dissolved in micellar cetyltrimethylammonium chloride (CTACl), micellar dodecyltrimethylammonium bromide (DTAB), micellar Brij-35 and micellar sodium octanoate (SO) solvent media. Experimental results show that nitromethane quenched fluorescence emission of only the 10 alternant PAHs in the two cationic (CTAC1 and DTAB) and nonionic Brij-35 surfactant solvent media as expected. Emission intensities of nonalternant PAHs, except for the few exceptions noted previously, were unaffected by nitromethane addition. Unexpected quenching behavior was observed, however, in the case of nonalternant PAHs dissolved in micellar sodium octanoate solvent media. Nitromethane quenched fluorescence emission of all nonalternant PAHs studied in the SO solvent media, which is contrary to the selective quenching rule.

Spectroscopic Properties of Polycyclic Aromatic Compounds. Part 5. The Nitromethane Selective Quenching Rule Revisited in Aqueous Micellar Solvent Media

Applicability of the nitromethane selective quenching rule for discriminating between alternant versus nonalternant polycyclic aromatic hydrocarbons (PAHs) is examined for 40 representative PAH solutes dissolved in micellar tetradecyltrimethylammonium broresults (TTAB) and micellar sodium dodecanoate (SDD) solvent media. Experimental results show that nitromethane quenched flourescence emission of only the 19 alternant PAHs in the cationic (TTAB) solvent media as expected. Emission intensities of nonalter-exceptions PAHs, except for dibenzo[b, k]flouranthene, dibenz[a, e]acephenanthrylene and two exceptions noted previously, were unaffected by nitromethane addition. Unexpected quenching behavior was observed, however, in the case of nonalternant PAHs dissolved in micellar sodium dodecanoate solvent media. Nitromethane quenched fluorescence emission of nonalternant PAHs studied in the SDD solvent media, which is contrary to the selective quenching rule.

Spectrochemical investigations in molecularly organized solvent media: Evaluation of nitromethane as a selective fluorescence quenching agent for alternant PAHs dissolved in micellar solvent media

Applicability of the nitromethane selective quenching rule for discriminating between alternant versus nonalternant polycyclic aromatic hydrocarbons (PAHs) is examined for 29 representative PAH solutes dissolved in micellar cetyltrimethylammonium bromide (CTAB) and micellar sodium dodecylsulfate (SDS) solvent media. Experimental results show that nitromethane quenched fluorescence emission of only the 17 alternant PAHs in the cationic CTAB surfactant solvent media as expected. Emission intensities of nonalternant PAHs, except for the few exceptions noted previously, were unaffected by nitromethane addition. Unexpected quenching behavior was observed, however, in the case of nonalternant PAHs dissolved in micellar SDS solvent media. Nitromethane quenched fluorescence emission of nonalternant PAHs studied in the SDS solvent media, which is contrary to the selective quenching rule.