The photophysical properties of 2,4,6-triphenylpyrylium (TPP + ) and three para-substituted tritylium ions encapsulated within Y, β, and MCM-41 have been studied. It was found that TPP + adsorbed within MCM-41 or silica only emits fluorescence (λ max 470 nm), whereas when this cation is incorporated within HY and LaY, simultaneous emission of fluorescence and room-temperature phosphorescence (λ max 560 nm) was observed. The fluorescence decay consists of two consecutive first-order processes and is dominated by the fast (0.2-0.7 ns) component. In addition to the prompt fluorescence, delayed emission observable 40 µs after excitation was also detected for the three TPP + samples. Weak fluorescence was observed for the series of tritylium ions embedded within zeolites. The characteristic T-T absorption spectrum of the TPP + triplet excited state has been detected using time-resolved diffuse reflectance. Depending on the zeolite, shifts in the reflectance maximum and changes in the extinction coefficient of the long-wavelength band have been noted. Similar transient spectra have also been obtained for the tritylium samples, which also show long-wavelength bands that are attributed to the corresponding triplet excited state.
The rate constants for quenching of the singlet- and triplet-excited state of acetone and a cyclic azoalkane by the hydrogen donors tributyltin hydride, 1,4-cyclohexadiene, and 2-propanol have been determined by time-resolved spectroscopy. It is concluded, in variance with previous studies, that singlet-excited states are significantly more reactive than triplet-excited states and that the reactivity difference between the two states of different spin multiplicity increases (i) with decreasing reactivity of the hydrogen donor and (ii) with increasing singlet−triplet energy gap of the excited state. This result is corroborated by semiempirical calculations. The relative efficiency for photoreduction by tributyltin hydride, which was determined by monitoring the formation of tributyltin radicals upon flash photolysis, was found to be four times lower for singlet-excited acetone than for the triplet state. The discrepancy between higher reactivity but lower efficiency in the intermolecular interaction of n,π*-excited singlet states with hydrogen donors is attributed to efficient radiationless deactivation, which has been predicted by correlation diagrams as a viable pathway for singlet-excited states.
Laser flash photolysis of a series of substituted styrenes embedded within the cavities of the large pore zeolite NaY leads to the formation of the corresponding styrene radical cation. The reactivity and spectra of these radical cations embedded within NaY are examined and compared to the reactivity of the same radical cations in solution. It is found that for the highly reactive parent styrene radical cation the zeolite framework provides a strong stabilizing effect. For the 4-methoxy-substituted styrene radical cation the zeolite framework plays less of a role in stabilizing the radical cation as compared to the reactivity of the same radical cation in acetonitrile solution. Rigorous analysis of the thermal stability of 4-methoxystyrene, 4-methylstyrene, and anethole in the zeolite micropores was carried out using two sources of NaY zeolite (Aldrich and The PQ Corporation). It was found that the thermal stability was surprisingly dependent on the source of the NaY zeolite. 4-Methoxystyrene, 4-methylstyrene, and anethole were thermally stable in NaY (Aldrich) but rapidly dimerized in NaY (PQ) upon incorporation with dichloromethane. We observed the formation of the same type of dimers not only for 4-methoxystyrene but also for 4-methylstyrene and anethole. In addition, 4-methoxystyrene was incorporated into a series of different acid zeolites (HZSM-5, HMordenite, HBeta, and HY) varying in the shape and size of their micropores where rapid thermal protonation occurs. Dimerization of the thermally formed 4-methoxyphenethyl cation with a neutral molecule of 4-methoxystyrene took place within all the acid zeolites examined. The generation of this secondary 1,3-bis(4-methoxyphenyl)-1-butylium ion was clearly observed in the medium pore ZSM-5. This carbocation was found to be thermally unstable in the acidic environment provided by the four acidic zeolites and underwent a proton and hydride transfer to form the more stable allylic 1,3-bis(4-methoxyphenyl)buten-1-ylium cation. In the large round cavities of HY a competing cyclization reaction took place which led to the formation of the 3-methyl-5-methoxy-1,4-methoxyphenylindanyl cation.
The radical cation 2 is also able to attack the C-N double bond in imines in an intermolecular fashion, a reaction not possible for photochemical 1,3-dipolar cycloaddition.16] The reaction of azirines with imines leads to the formation of the N-substituted imidazoles.The course of the reaction is as follows: Addition of the radical cation 2 to the imine is followed by ring closure to form a dihydroimidazole. Under these reaction conditions the dihydroimidazole is not stable and undergoes subsequent aromatization, so that, even in the crude product, no dihydroimidazole is detected. Huisgen[71 did not find a dihydroimidazole under comparable conditions either. The reaction of azirines with imines offers a ready approach for the synthesis of N-substituted imidazoles with a wide variety of substituents.Since the C-N double bonds of the imine and the azirine compete for reaction, the yields of imidazoles vary rather widely ( Table 1). The remaining products are the azirine
The mobility and location of pyrene within the cavities of the faujasite NaY have been examined using fluorescence and diffuse reflectance techniques. The photophysical properties of pyrene within the zeolite framework show that upon incorporation the pyrene molecules are initially distributed in the outer cavities of the zeolite granules. This leads to a high number of doubly occupied cavities and large excimer emission; this emission shows only 20-25 ps delay, suggesting that excimer-forming molecules are required to undergo only small intracavity motions. With time (days) the distribution of pyrene within the cavities of the zeolite equilibrates and monomer emission dominates the spectra. The time required for this equilibration to take place is shown to be highly dependent on sample preparation. In particular, water and hexane hinder pyrene redistribution, while this process is faster under nitrogen than in samples under vacuum. The detection of delayed fluorescence on the microsecond time scale on freshly prepared samples indicates that there is movement of the pyrene molecules located on the external surface of the zeolite after sample preparation; no delayed fluorescence is observed after 1-2 days.
The xanthenium cation has been spontaneously generated by adsorption of 9-xanthenol within several acidic (HY, HMor, and HZSM5) zeolites. The resulting composites were characterized by diffuse reflectance and luminescence spectroscopy and were found to be stable over an extremely long period of time. Timeresolved diffuse reflectance of these samples at several excitation wavelengths allowed the identification of a 540-nm band to the triplet state of tne cation in the zeolite environment. The triplet state in the zeolite was found to have a much longer lifetime than related triplet xanthenium cations in solution. The observation of the 9-xanthylium radical requires the participation of an electron donor; the question is raised as to whether the zeolite framework can play this role.
Benzyl (4-MeO, 4-Me, and 4-methoxy-1-naphthylmethyl), phenethyl (4-Me2N, 4-MeO, 3,4-(MeO)2, 4-Me, 3-Me, 4-F, 3-MeO, 2,6-Me2, parent, and 4-methoxy-1-naphthylethyl) and cumyl (4-Me2N, 4-MeO, 4-Me, parent) cations have been studied by laser flash photolysis (LFP) in 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP). In most cases styrene or α-methylstyrene precursors were employed for the phenethyl and cumyl ions, the intermediate being obtained by solvent protonation of the excited state. Benzyl cations were generated by photoheterolysis of trimethylammonium and chloride precursors. While a 4-MeO substituent provides sufficient stabilization to permit observation of cations in TFE, cations with less stabilizing substituents usually require the less nucleophilic HFIP. Even in this solvent, the parent benzyl cation is too short-lived (lifetime <20 ns) to be observed. When generated in HFIP, phenethyl cations can be seen to react with unphotolyzed styrene, giving rise to dimer cations that are observed to grow in as the initial phenethyl cation decays. The dimer cations, in common with the oligomer cations seen in cationic styrene polymerization, have a λmax 15-20 nm higher than the monomer and react with both solvent and styrene several orders of magnitude more slowly. This stabilization relative to the phenethyl may reflect an interaction with the aryl group present at the gamma-carbon. Cations 4-MeOC6H4C+(R)-CH3 (R = Me, Et, i-Pr, t-Bu, cyclopropyl, C6H5, 4-MeOC6H4) were generated in TFE via the photoprotonation route. The alkyl series shows that steric effects are important in the decay reaction. The cation with R = cyclopropyl is a factor of 1.5 less reactive than the cation where R = phenyl. Several vinyl cations have also been generated by photoprotonation of phenylacetylenes. ArC+=CH2 has a reactivity very similar to that of its analog ArC+H-CH3, the vinyl cation being slightly (factors of 2-5) shorter-lived. For the various series of cations, including vinyl, substituents in the aryl ring have a consistent effect on the λmax, a shift to higher wavelength relative to hydrogen of 15 nm for 4-Me, 30 nm for 4-MeO, and 50 nm for 4-Me2N.Key words: photogenerated carbocations, carbocation lifetime, styrene, photoprotonation.
The picosecond excited-state dynamics of several derivatives have been investigated using high photon energy excitation combined with picosecond luminescence detection. Instrument response-limited fluorescence (tau(1) approximately equal to 3-5 ps) at 500 nm was observed for all of the complexes, while longer-lived emission (tau(2) > 50 ps), similar in energy, was observed for only some of the complexes. Interestingly, the presence of tau(2) required substitution at the 4,4-positions of the bipyridine ligands and D(3) symmetry for the complex; only the 4,4-substituted homoleptic complexes exhibited tau(2). On the basis of previous assignments of the ultrafast dynamics measured for Ru(bpy)(2+)3 and Ru(dmb)(2+)3, tau(2) has been tentatively ascribed to relaxation from higher electronic or vibrational levels in the triplet manifold having slightly more triplet character than the state responsible for tau(1). However, given that the kinetics for these transition metal complexes are highly dependent on both pump and probe wavelengths and that there is considerable interest in utilizing such complexes for electron transfer in the nonergodic limit, further characterization of the state giving rise to tau(2) is warranted.
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