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Intrazeolite Photochemistry. 14. Photochemistry of r,ω-Diphenyl Allyl Cations within Zeolites Hermenegildo Garcı´a,*,†,§ Sara Garcı´a,‡ Julia Pe´ rez-Prieto,*,†,⊥ and J. C. Scaiano*,† Department of Chemistry, UniVersity of Ottawa, Ottawa, Canada K1N 6N5, and Instituto de Tecnologı´a Quı´mica, UniVersidad Polite´ cnica de Valencia, Apartado 22012, 46071 Valencia, Spain ReceiVed: March 8, 1996; In Final Form: August 2, 1996X
1,3-Diphenylpropenylium (DP3+) and 1,5-diphenylpentadienylium (DP5+) ions have been generated within acid ZSM-5 and mordenite zeolites as persistent species by adsorption of R,ω-disubstituted R,ωdiphenylalkanes. It was found that besides R,ω-diacetates, R,ω-dichlorides and R,ω-diols are also convenient precursors. Irradiation of these allylic cations embedded within zeolites led to cis-trans isomerization as the only observable process. Molecular modeling suggests that there is enough space in the intersection of the channels in HZSM-5 to accommodate both the planar E,E-DP3+ and the nonplanar E,Z-DP3+ stereoisomers. Fluorescence emissions follow a near-first-order decay, with a dominant short and a minor long component; the latter having a lifetime close to that reported for DP5+ in solution at low temperatures. Z-E isomerization led to a blue-shift in the absorption spectra together with an increase in the apparent extinction coefficient, while emission maxima also appeared at longer wavelengths exhibiting distinctive decays. Evidence for the triplet excited states has been obtained by time-resolved diffuse reflectance spectroscopy. Estimated lifetimes obtained from the best first-order fit are in the tens of microseconds range. Weak delayed emission associated to the triplet excited state has also been observed.
Introduction Allyl cations are among the most important organic reaction intermediates and a large extent of our present understanding of organic reactions mechanisms is based on their behavior.1 Because of the simplicity of the system, propenylium ion has been subjected to many theoretical and experimental studies.2 It has been observed in solution that although aryl caps at the ends of the conjugate system provide some stabilization compared to the unsubstituted simplest allyl cation, 1,3diphenylpropenylium (DP3+) cation could be characterized only in FSO3H/SO2 solution at -78 °C.3 Even under these extreme conditions a rapid cyclization to the corresponding indanyl ions takes place.4,5 However, the stability of conjugated polyenylium ions increases with the number of CdC double bonds, and thus 1,5-diphenylpentadienylium (DP5+) ion is a persistent species in CH2Cl2 solution at temperatures below -10 °C.6,7 Recently it has been of interest to establish if it is possible to obtain the parent unsubstituted allyl cation as a persistent species within medium-pore zeolites. Previous assignments of 13C NMR signals to this carbenium ion were later found to be due to propanal, the main reaction product.8,9 In a preliminary report,10 we have been able to characterize by diffuse-reflectance and IR spectroscopies R,ω-diphenyl-substituted allylic cations for the first time as stable, persistent species within HZSM-5. Apparently, the phenyl rings with a size very close to the HZSM-5 channels are “capping” the allyl cation, acting like stoppers and protecting the positive centers from attack by external nucleophilic reagents. In previous and accompanying papers in this series,11,12 we have shown that zeolites are very convenient polar hosts that not only stabilize carbenium ions but also are capable of †
University of Ottawa. Universidad Polite´cnica de Valencia. § On leave from the Universidad Polite ´ cnica de Valencia. ⊥ On leave from the Universidad de Valencia, 46100 Burjasot, Valencia, Spain. X Abstract published in AdVance ACS Abstracts, October 15, 1996. ‡
S0022-3654(96)00731-9 CCC: $12.00
Figure 1. (A) Diffuse reflectance spectra of DP3+-HZSM5 before (curve a) and after (curve b) 2 h of 350 nm irradiation. (B) DP5+HZSM-5 before (curve a) and after 2 h of 350 nm irradiation (curve b).
© 1996 American Chemical Society
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Figure 2. Molecular modeling of stereoisomers of DP3+ (see text).
SCHEME 1: Preparation of DP3+ and DP5+ Incorporated Inside HZSM-5 and HMor
photophysical properties of DP3+ and DP5+ incorporated within the acid zeolites HZSM-5 and Hmordenite (HMor). Results and Discussion
SCHEME 2: Possible Stereoisomers of DP3+
controlling and modifying their photochemical properties. By incorporation within zeolites we have been able to detect roomtemperature fluorescence and characterize the T-T absorption spectra of triplet excited states of carbenium ions. As a general feature, triplet lifetimes of cations incorporated within the zeolite voids are much longer than in solution, following the pattern found for triplet excited states of other aromatic compounds.13-15 Fluorescence of R,ω-diphenyl allyl cations7 and anions16-19 has been previously studied in solution. It has been found that the singlet lifetime decreases as the temperature or the number of CdC double bonds increase. Intersystem crossing to reach the triplet is negligible in solution and no detection of triplet excited states has been reported. We report here on the
Preparation of the samples was initially carried out starting from the corresponding diacetates, 1 and 2.10 We have found that it is also possible to generate these carbenium ions by stirring a CH2Cl2 solution of the corresponding diols or dichlorosubstituted precursors for 12 h in the presence of the zeolite at room temperature (Scheme 1). Even formation of DP3+ and DP5+ can be conveniently achieved by grinding in a mortar the corresponding precursor and dehydrated zeolite, allowing the powder to evolve under anhydrous conditions. No detectable changes in the diffuse reflectance spectra of our samples of DP3+ and DP5+ in ZMS-5 or DP5+ in mordenite were observed for periods of several months. The stability of this sample is so remarkable that the HZSM-5 samples can be even suspended in aqueous solution during days without any noticeable disappearance of the cation. We did not study DP3+ within Hmordenite due to the comparatively much lower stability of this sample.10,20 Photochemical Z-E Isomerization of Allyl Cations. Steadystate irradiations were carried out in order to assess if there is some bleaching of the allyl cations. Instead, visual inspection revealed that the intensity of the colors increased and a change from orange to red (DP3+) and from purple to blue (DP5+) was observed. Figure 1 compares the diffuse reflectance (DR) spectra of the samples before and after 2 h irradiation. These changes are evidence that a cis-trans isomerization of the allyl
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Garcı´a et al.
Figure 3. Docking of E,E- and E,Z-DP3+ inside the straight channels (5.4 × 5.6 Å2) of a pentasil zeolite, such as HZSM-5. Note the different planarity between the two stereoisomers. The lines connecting the allyl hydrogens with the framework indicate overlapping of van der Waals atomic radii.
moiety is taking place upon irradiation (Scheme 2). Irradiation for extended periods did not lead to any appreciable decrease of the absorption. NMR studies of DP3+ in FSO3H/SO2 solutions have shown that while the barrier for free rotation of the Ph-C bonds is low, both C1-C2 and C2-C3 bonds can have cis and trans configurations.3 Therefore, 3 stereoisomers for DP3+ (see Scheme 2) and 10 for DP5+ are possible. In fact, up to now the search for room-temperature stable cis-trans isomers has been unsuccessfully pursued in solution,7 and only time-resolved techniques have given some indication of the photochemical formation of cis-trans DP3+ analogues.21 However, it is wellknown that anionic analogues of DP3+ and DP5+ undergo photochemical cis-trans isomerization and the corresponding
stereoisomers can be easily distinguished by the shift in their absorption spectra. This behavior is well-known to be general for related stilbene-like systems.22 In view of our results, the previous failures to observe cistrans isomerization in solution have to be attributed to the strong acidity of the medium required to generate DP3+ or DP5+ rather than to an intrinsic thermal instability of Z-E stereoisomers. Thus, a fast acid-catalyzed cis-trans isomerization back to the more stable all-E isomer could be taking place under strong acid conditions. One point of concern was to establish if spatial constraints imposed by the zeolite framework allow isomerization to take place. Shape-selective absorption in HZSM-5 is well documented.23 Thus, it has been found that while trans-stilbene can
Intrazeolite Photochemistry. 14
Figure 4. Normalized emission spectra of DP3+-HZSM-5 (λex 490 nm) and DP5+-HZSM-5 (λex 515 nm).
be readily adsorbed in pentasil zeolites, the bulkier cis isomers cannot penetrate through the micropores.24,25 However, we would like to note that the situation here is different because even if the pore openings are too small to allow incorporation from a solution, once adsorption of the less bulky all-E isomer has taken place in the interior of the pentasil lattice there are much larger voids in the intersection of straight and sinusoidal channels that should be able to accommodate the other isomers.26 To address this question, we performed molecular modeling of optimized E,E and E,Z-DP3+ stereoisomers. We noticed that while isomer E,E is planar, interaction of ortho phenyl hydrogens with syn hydrogen in the allyl moiety prevents E,Z-DP3+ from reaching planarity (Figure 2). Docking of both DP3+ stereoisomers within the channels of HZSM-5 shows that overlapping of E,Z-DP3+ with the framework is higher than for E,E-DP3+ as expected but that there is certainly enough room for the isomerization if the guest is located in the intersection of the channels (Figure 3). Thus, these intracavity isomerizations provide another example of ship-in-a bottle synthesis.27-29 Fluorescence Emission. It has been reported that DP3+ and DP5+ are weakly fluorescent in solution.7,30 We have been able to observe luminescence in all our samples containing DP3+ or DP5+ (Figure 4). The position of the maxima show a solvatochromic shift with respect to the wavelength measured in CH2Cl2 solution. However, what was even more remarkable was that in the case of DP5+, besides the maximum at 575 nm, there is a shoulder at 615 nm that is absent in the emission spectrum reported in solution. Excitation spectra obtained monitoring the emission at 575 and 615 nm established that there are two different emitting species that match the spectrum of E,E and E,Z stereoisomers shown in Figure 1B. This shift in the emission was more evident when the fluorescence spectra of DP5+-HZSM-5 before and after irradiation were compared. A similar effect was observed for DP3+-HZSM-5 (Figure 5). The decays of DP3+ and DP5+ incorporated inside zeolites were almost single monoexponential as has been reported in solution.7 However, a minor (0.1%) slower component of the fluorescence emission was also observed. Similar twocomponent first-order decays have also been observed for the emission of pyrene and other species within zeolites.13,31 This feature may be related to the presence of two populations of emitting species associated to a different environments and/or interaction with charge-balancing cations within the zeolite framework. Owing to its instability, no data are available in the literature reporting the fluorescence lifetime of DP3+ in solution, while
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Figure 5. Normalized emission spectra of DP3+-HZSM-5 before (curve a) and after (curve b) 2 h of irradiation.
Figure 6. (A) Fluorescence decay of DP3+-HZSM-5 before irradiation (4) and after 2 h lamp irradiation (O); (B) emission decay of DP5+ in HZSM-5 (4) and within HMor (O).
the lifetime of DP5+ exhibits a sigmoid dependence with the temperature.7 We observed that the lifetimes of stereoisomeric of DP3+ and DP5+ within HZSM-5 or mordenite are remarkably similar, the slower component having a lifetime close to that reported for DP5+ at 0 °C.7 However, the following observations were made: (i) the singlet lifetime of DP3+ was somewhat shorter than DP5+, thus agreeing with the expected influence of the chain length;7 (ii) the fluorescence decay of E,E was faster than that of the E,Z isomer of DP3+; the difference may be related to the E,E T E,Z interconversion efficiences in the excited state; (iii) emission of DP5+ in HZSM-5 is shorter lived than in mordenite. Some selected decay traces are shown in Figure 6. An interesting feature of the luminescence experiments is the observation of some delayed fluorescence emission 20 µs after excitation. Since 20 µs of delay time is close to the 10 µs gating
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Figure 7. Transient diffuse reflectance (∆J/J0) of the samples DP3+HZSM-5 (A) and DP5+-HZSM-5 (B) recorded 12.1 and 6.4 µs after 355 nm excitation. The band at 340 nm in spectrum A probably corresponds to scattered light from the excitation source.
limit of our spectrofluorimeter, control experiments were performed using original, nondoped zeolites to establish that the emission was not an artifact; these controls suggest that the observationseven if borderline within the detection limitssis not due to experimental artifacts. Similar delayed fluorescence has also been observed for other strongly fluorescent cations within zeolites.32 Two alternative mechanisms can be envisioned to explain this delayed emission, either thermal population of the singlet from the triplet excited state or triplet-triplet annihilation. Whatever the operating mechanism may be, the involvement of longer lived triplet excited states has to be invoked.33,34 In contrast, intersystem crossing in solution has been found to be negligible.7 Detection of DP3+ and DP5+ Triplet Excited States. To establish if there is some transient that could be assigned to the allyl cation triplet excited state we carried out nanosecond laser flash experiments employing time-resolved diffuse reflectance techniques.35 For a series of R,ω-diphenyl allylic cations in solution it has been established that intersystem crossing does not contribute to the decay of the singlet excited states to any appreciable extent.7 The transient diffuse reflectance spectra observed for some of our samples after 355 nm excitation are presented in Figure 7. An identical spectrum was obtained for DP5+-HZSM5 using a dye laser operating a 420 nm for excitation. These transients are very long lived (see Figure 8 and Table 1) and no appreciable oxygen quenching could be observed. This insensitivity to oxygen is not surprising taking into account the tight fit of these cations within the channels of the zeolites. As already mentioned, even water cannot attack these cations when they are embedded within pentasil zeolites. Steady-state 350 nm irradiation does not lead to any bleaching of these cations even on prolonged irradiation. The only process
Garcı´a et al.
Figure 8. Decay of T-T absorption of triplet excited state of DP3+HZSM-5 (A, monitored 580 nm) and DP5+-HZSM-5 (B, monitored 610 nm).
that we have observed is the cis-trans isomerization and the accompanying color changes. It has been suggested that in the case of anionic analogues DP3- and DP5-, isomerization takes place through the intermediacy of the corresponding allyl radical generated by electron ejection following excitation.36,37 It has been proposed that the same mechanism may operate for the isomerization of DP3+ and DP5+.7 Absorption spectra of R,ωdiphenyl allyl radicals in solution have not been reported yet although theoretical calculations have predicted their absorption maximum around 340 nm.38 However, our transient absorptions are very far from this region. Another possible mechanism, commonly observed for cis-trans olefin isomerization and that is known to operate for stilbene and related aryl-substituted CdC double bonds, involves triplet excited states.22 In addition, the estimated lifetime obtained by the best first-order decay fitting is in the range generally observed for these transients within zeolites.12 Therefore, it is reasonable to assign our absorption to triplet excited states that besides product formation may also explain the observation of delayed emission. Conclusions By using zeolites as hosts, it has been possible to study the photochemical properties of two R,ω-diphenyl allylic cations. We have found that the behavior of DP3+ and DP5+ within zeolites follows the common pattern of other aryl substituted olefinic compounds. Thus, cis-trans isomerization is the main photochemical process that probably involves the intermediacy of the triplet excited state. We have recently generated 1,3diphenylpropenyl radical (DP3•) in solution and observed a characteristic strong absorption band at 360 nm.39 Therefore, the formation of allyl radicals from the excited R,ω-diphenyl allyl cations incorporated within zeolites by electron transfer
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TABLE 1: Relevant Photophysical Values of DP3+ and DP5+ within Zeolites DP3+-HZSM-5 absorption λmax (nm) λmax (nm) after irradiationa fluorescence λmax (nm) λmax (nm) after irradiationa τS (ns)b T-T absorption λmax (nm) τT (µs)c
DP5+-HZSM-5
495 530
517 545
535 545 0.13 (99.9), 0.69 (0.1) 590 53
575 615 0.12 (99.9), 0.46 (0.1) 610 99
DP5+-HMor 545 555 565 610 0.19 (99.9), 0.67 (0.1) 620 >100
a After 2 h lamp irradiation. b The number between brackets indicates the relative percent contribution of each first-order component; errors do not exceed 5% for components contributing at least 30%. c (10%.
from reducing sites in the zeolite appears unlikely. The different Z-E stereoisomers exhibit shifts in their absorption and emission maxima, as well as changes in the extinction coefficients.40 Triplet lifetimes of tens of microseconds are long enough to allow the observation of delayed emission. Experimental Section Samples of zeolites HZSM-5 (Si/Al 26.5) and HMor (Si/Al 10) were obtained as previously described.41 1,3-Diphenylpropanediol was obtained by NaBH4 reduction in ethanol of commercial 1,3-diphenyl-1,3-propanedione (Aldrich). Synthesis of 1,5-diphenyl-1,5-pentanediol has been reported previously.42 Acetylation of R,ω-diphenyl-substituted diols was accomplished by stirring at room temperature a solution of acetyl chloride in CH2Cl2 in the presence of catalytic amounts of 4-(N,Ndimethylamino)pyridine. The R,ω-diphenyl-R,ω-dichloroalkanes were synthesized from the corresponding diols using a concentrated HCl aqueous solution.42 Incorporation of organic compounds (40 mg) within dehydrated zeolites (1.0 g, pretreated by baking at 550 °C overnight) was carried out at room temperature using CH2Cl2 (50 mL) as solvent. The suspension was stirred for 12 h and then centrifuged, and the zeolite resuspended with fresh CH2Cl2 (10 mL), stirred for 30 min, and centrifuged. The washing cycle was repeated twice. Alternatively, the adsorption was carried out in a drybox by mixing dehydrated zeolite and the organic compound in a mortar and letting the sample evolve for 1 week. Diffuse reflectance measurements were carried out in a Cary 1E spectrophotometer using an integrating sphere setup. Kubelka-Munk equations F(R) were plotted against wavelength. Fluorescence and delayed fluorescence (20 µs delay, 1 ms gate) were measured in a Perkin-Elmer LS-50 spectrofluorimeter using a front-face attachment. Control experiments for delayed fluorescence were carried out using HY zeolite. Fluorescence decays were measured using a Hamamatsu C-4334 streakscope that allows simultaneous spectral and time resolution using the output of a Continuum PY-61 picosecond Nd:YAG laser (355 nm,
