J . Phys. Chem. 1990, 94, 3387-3391
3387
spectra of metal liquidlike silver films (MELLF) formed at water dichlorocomethane interfaces.6 The absorption maximum, 387 nm, indicates the presence of 200-300-A particles.’ Heating the silver particulate film on the quartz substrate at 300 O C for 5 and 15 min did not alter the position of the interband transition, but shifted the absorption maximum to longer wavelengths (Figure 3). Annealing apparently increases the particle sizes. Scanning tunneling microscopy of the silver particulate film on HOPG revealed the presence of particles with heights of 100-200 A and average and short and long axes of 200 and 300 A, respectively (Figure 4). Most significantly, particle-particle connectivity is clearly seen in the micrographs. The electron diffraction pattern of the silver particulate film on a grid (Figure 5 ) has been analyzed for a face-centered cubic polycrystalline structure with lattice constants of 4.08 A and hkl indices of ( 1 1 I), (200), (220), (31 l ) , (222), (400), (31 l ) , and (402). Resistivity ( p ) of the silver particulate films (on quartz) was measured by a multimeter connected to two silver electrodes
pressed on to the film at a distance of 0.2 cm (L) to be 40 R (R). Taking the cross section of the silver particulate film to be 2.25 x cm2 gave
(5) Beaglehole, D.; Hunderi, 0.Phys. Reu. B. 1970, 2, 309-321. Hunderi, 0.;Beaglehole, D. Phys. Rev. B 1970, 2, 321-329. Hunderi, 0.J . Phys. (Fr.) Colloq. (SUPPI.) 1977, 11, C5-89. (6) Yogev, D.; Efrima, S . J . Phys. Chem. 1988, 92, 5754-5760. (7) Kerker, M. The Scattering of Light and Other Electromagnetic Radiation; Academic Press: New York, 1969.
Acknowledgment. Support of this research by a grant from the National Science Foundation is gratefully acknowledged.
p
= R-A = 40 R 2’25 lo” cm2 = 4.5 x 10-4 L 0.2 cm
Q
cm
which is some 2 orders of magnitude larger resistivity than that of bulk silver (1.586 X 10” R cm).8 Conclusion
Application of a potential across monolayers having silver counterions resulted in the two-dimensional formation of a silver film containing 250 8, diameter interconnected silver particles which could be deposited on solid substrates. Ultrathin films containing interconnected, roughened metal particles provide unique opportunities for catalysis, electron transfer, nonlinear and surface-enhanced optics, and spectroscopy. Development of some of these applications will be the subject of future communications from our laboratories.
( 8 ) Handbook of Chemistry and Physics, 67th ed.; CRC Press: Boca Raton, FL, 1986-1987; p F120.
Reinvestigation of the Electronic Spectrum of the Phenylnitrene Radical David W. Cullin, Lian Yu, James M. Williamson, Matthew S. Platz, and Terry A. Miller* Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 4321 0 (Received: January 25, 1990)
The origin band of the electronic transition long attributed to gaseous triplet phenylnitrene has been investigated by using a very high resolution pulsed laser to interrogate molecules cooled in a supersonic jet. The observed rotational and spin structure of this band is inconsistent with simple ideas of triplet phenylnitrene’s structure and spectra. Details of these inconsistencies are presented. A possible alternative carrier for the spectrum is presented.
Introduction
Nitrenes are among the most interesting and important intermediates in chemistry. The simplest arylnitrene is phenylnitrene, C,H5N. Its gas-phase electronic spectrum was first reported by Porter and Wardl in the late 1960s. They flash photolyzed o-chloroaniline and phenyl isocyanate and observed an absorption ban$ with an origin at 368 nm which they attributed to the A 3B2-X 3A2 transition of the phenylnitrene radical. Since the original work of Porter and Ward, several have used this gas-phase UV band system to monitor the dynamics of phenylnitrene in the gas phase. Recently the first jet-cooled spectrum of phenylnitrene has been reported by Ozawa et aL5 While the low temperature of the jet simplifies the vibronic structure of the electronic transition by largely eliminating hot bands, there is no question that in all the previous gas-phase work the same band system has been observed, and by and large, it has been produced by photolyzing the same precursor molecules. ( I ) Porter, G.; Ward, B. Proc. R . SOC.London, A 1968, 303, 139. (2) Lehmann, P. A.; Berry, R. S. J . Am. Chem. SOC.1973, 95, 8614. (3) Hancock, G.; McKendrick, K. G. Chem. Phys. Lett. 1986,127, 125. (4) Hancock, G.; McKendrick, K. G. J . Chem. SOC.,Faraday Trans. 2 1987, 83 (11). 2011. (5) Ozawa, K.; Ishida, T.; Fuke, K.; Kaya, K. Chem. Phys. Lett. 1988,150, 249.
0022-3654/90/2094-3387$02.50/0
There have also been a number of studies of phenylnitrene in condensed phases. A number of years ago, an EPR matrix spectrum was attributed to the phenylnitrene radical formed by the photolysis of neat phenyl azide.6 This spectrum provided ample evidence that the ground state of the radical was a triplet and yielded a measure’ of its spin-spin interaction parameters, D 1 .O cm-I, E 0 cm-I. Photolyzed samples of phenyl azide in rigid organic glasses two relatively broad UV absorptions, with maxima at 303 and 368 nm. The latter band has generally been assumed to correspond to the previously discussed electronic transition in the gas phase. Flash photolysis of phenyl azide in fluid solution also produces a broad UV absorption in this region which has been associatedI0 with triplet phenylnitrene, but more recently this assignment has been called into question.” Rare gas matrix IR studies have shown that other products are
-
-
(6) Smolinsky, G.; Wassermann, E.; Yager, W. A. J . Am. Chem. SOC. 1962, 84, 3162. (7) Coope, J. A. R.; Farmer, J. B.; Gardner, C. L.; McDowell, C. A. J. Chem. Phys. 1965, 42, 54. (8) Reiser, A.; Bowes, G.; Horne, P. J. Trans. Faraday SOC.1966, 62, 3162. (9) Leyva, E.; Platz, M. S . ; Persy, G.; Wirz, J. J. Am. Chem. SOC.1986, 108, 3783. (10) Feilchenfeld, N. B.; Waddell, W. H. Chem. Phys. Lett. 1983, 98, 190. ( 1 1 ) Schrock, A.; Schuster, G. B. J. Am. Chem. SOC.1984, 106, 5228.
0 1990 American Chemical Society
3388 The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 formed in this photolysis and that formation of triplet phenylnitrene is not necessarily a dominant path.I2 It is obvious though that at least some triplet phenylnitrene is formed in an organic glass matrix as evidenced by the presence of the EPR signal corresponding to triplet phenylnitrene. Furthermore the intensity of the triplet EPR signal has been correlated9 with UV absorption bands at 303 and 368 nm. Investigations of a different nature have recently studied the electronic structure of phenylnitrene by photodetachment of an electron from the anion.I3 These studies suggest that the lowest singlet state lies 1500 cm-l above the ground triplet level. Recently we have observed and analyzed rotational and spin fine structure in the electronic spectra of several aromatic organic radicals, namely, C5H5,I4C5D5,ISand the radical ions, C6H3F3+ and C6F6+.l6 This was accomplished by producing very cold radicals in a free jet expansion and observing their laser-induced fluorescence with a very high resolution pulse amplified, Ar+ pumped CW ring dye laser. Upon reading the report of Ozawa et al., we decided to try to obtain a rotationally resolved spectrum of the 0; band of the phenylnitrene radical. We hoped that such a spectrum would yield information about the radical’s geometry, especially the first measurement of its C-N bond length and additional inforFation on the spin-spin interactions, particularly in the excited A state. In this paper we report the rotationally resolved 0; band of this transition. However, we find the interpretation of the molecular parameters characterizing our spectrum very difficult to understand in terms of the expected geometric and spin structure of phenylnitrene. We present our observations and analysis and speculate upon the possible interpretations.
-
Experimental Section The experimental arrangement has been described elsewhere’”I6 and will only be summarized here. Briefly, phenylnitrene was produced in a supersonic free jet expansion by 193 nm (ArF) photolysis of suitable precursor molecules. The precursors were entrained in the jet flow by passing high-pressure helium gas over the liquid sample. The starting materials used were phenyl azide, o-chloroaniline, and phenyl isocyanate. The latter two were obtained from Aldrich Chemical Company and were used without further purification. Phenyl azide was synthesized by treatment of phenyl hydrazine with first nitrous acid and then sodium a2ide.l’ I n all cases the precursor liquids needed to be heated to as high as 120 “ C to increase their partial pressures in the jet. After expansion, the precursors were photolyzed at the throat of the jet and probed downstream after sufficient cooling (typical rotational temperatures were 1-3 K). The heart of the probe laser system was a Coherent 699-29 Autoscan CW ring dye laser (specified line width of = I MHz), pumped by an Ar+ laser (Coherent Innova loo), operating with LDS 722 laser dye obtained from Exciton Chemical Company. The output of the ring laser was then seeded into a pulse amplifier (Lambda Physik FL 2003) pumped by a 308-nm (XeCI) excimer laser (Lambda Physik EMG 103 MSG). The resulting pulse amplified beam (line width of =lo0 MHz) was doubled by using the D crystal of an Inrad doubler (Autotracker 11) to generate the necessary near-UV wavelengths. The laser-induced fluorescence was collected and imaged onto an optical slit to reduce the Doppler width by limiting the fluorescence viewed by the photomultiplier tube from molecules with off-axis velocity components. Relative frequency calibration was accomplished by simultaneously recording the fringes of an external etalon along with the LIF spectrum. The absolute frequency was arrived at using the Coherent 699-29 wavemeter corrected for its known error which was (12) Chapman, 0. L.; LeRoux, J. P. J . Am. Chem. SOC.1978, 100, 282. ( I 3 ) Drzaic, P. S.; Brauman, J. 1. J . Am. Chem. SOC.1984, 106, 3443. (14) Yu, L.; Foster, S. C.; Williamson, J. M.; Heaven, M. C . ; Miller, T. A . J . Phys. Chem. 1988, 92, 4263. (15) Yu, L.; Williamson. J. M.; Miller, T. A. Chem. Phys. Letr. 1989, 162, 43 1 (16) Yu, L.; Foster, S. C.; Williamson, J. M.; Miller, T.A. J . Chem. Phys., in press. (17) Lindsay, R. 0.;Allen, C. F. H. Org. Synth. 1955, Collect. 3 , 710.
Letters previously measured in a nearby spectral region where calibration was possible against the I, atlas.’*
Results and Analysis A. Electronic Structure. It has been traditional to discuss the near-UV electronic transition of phenylnitrene by referring to the more thoroughly studied analogous transition in the benzyl radical, C6H5CH2.The usefulness of this analogy in the present case is only strengthened by the fact that the rotational constants of the two near-prolate tops, C ~ H S C and H ~ C6H5N,are expected to be similar. The ground state of benzyl is 2B2arising from the unpaired electron in the nonbonding 3b2 orbital. Since benzyl is an odd alternant hydrocarbon, there is more than one lowest energy one-electron excitation. Experimentally, two closely spaced excited states, 2A2and 2B2,are known from electronic transitions in the blue region of the spectrum. Electric dipole selection rules require the electronic transition 2A2-X 2B2to be a b-type (Ior AKa = f l , f 3 , f5, ...) band, while the transition 2B2-X 2B2is an a-type (/I or AKa = 0, f 2 , f4,...) band. Contour analyses by the Orsay group of the rotational structure of the electronic transitionsI9 have determined the lowest excited benzyl state to be 2A2with the 2 2B2 state a few hundred cm-’ higher in energy. This result is illustrated on the left-hand side of Figure 1. Triplet phenylnitrene’s electronic structure can be viewed analogously to that of benzyl’s. One of the “triplet electrons” occupies a b2 orbital, probably largely localized on the N, but certainly also interacting with the ring T-structure as in benzyl. The second “triplet electron” occupies the in-plane b, p orbital and is probably largely a spectator in the electronic transition. By multiplying the symmetry of the bl electron and the symmetries of the benzyl states, we obtain the electronic states of phenylnitrene on the right-hand side of Figure 1. Pairing up the electrons in the p orbitals on N gives rise to the two lowest singlet states whose positions were recently reported.I3 B. Rotational and Fine Structure. The effective Hamiltonian, including both rotational kinetic energy an$ elec_tronic spin interactions, can be written for either the X or A state in the following fashion:
% = 7fLfR
+ %SLfR + %sS
(1)
I n the above SfR is the usual asymmetric rotor Hamiltonian %;t~
= ANz2 + BN,Z
+ CN:
(2)
where A, B, and Care the usual rotational constants. Higher order distortion terms have been neglected as they are likely to be unimportant at the low temperatures of this experiment. One would expect the values of the rotational constants of C6H5Nto be much like those of benzyl, A 5 G H z > B C = 2 GHz. 7fsRgives the spin rotation interaction. Based upon sparse data from other polyatomic molecules with S # 0 in nondegenerate electronic states, the spin rotation constants are likely less than I GHz.14 This magnitude is not negligible compared to our experimental resolution. Nonetheless, considering the presence of the much larger spin-spin i n t e r a ~ t i o n it, ~will at most have a minor effect on the spectrum. For this analysis, we will ignore the effects of 7fSR. The remaining term W S sdescribes the interaction of the unpaired spins in the triplet state. In the principal axis system of the spin tensor (which we will assume coincides with that of the inertial tensor), H s scan be written i ; :
%ss = 1/,D(3SZ2- S2)
i ; :
+ E(S,Z - Sy2)
(3)
Fortunately there has been considerable work on the ESR spectrum of phenylnitrene in condensed phases, which yield values of D = 30 GHz and E = 0.637 While these values may be slightly affected by environmental perturbations, it would be very sur( I 8) Gerstenkorn, S.; Luc, P. Atlas du Spectre cfabsorption de la molecule d’lode; Centre National de la Recherche Scientifique: Paris, 1978. (19) Cossart-Magos, C.; Goetz, W. J . Mol. Spectrosc. 1986, 115, 366.
-
The Journal of Physical Chemistry, Vol. 94, No. 9, I990 3389
Letters 2 B2-
0;
2
Type b ( I Transition 2
Benzyl
Phenyl Nitrene
Figure 1. Schematic diagram showing correlation of electronic transitions between benzyl- and phenylnitrene radical.
prising if they were substantially different in the gas-phase molecule. In order to obtain a semiquantitative idea of the energy levels, it is necessary to choose a basis set in which the dominant terms of 7t are diagonal. W e note that the spin-spin interaction parameterized by D is likely the largest single term. Therefore, we adopt a basis set in which the projection, E, of S along the inertial z axis is a good quantum number. This basis set quickly gives as eigenvalues of H S s , - 2 0 1 3 (2 = 0) and 0 1 3 (2 = f l ) . At 1 K, typical of our lowest rotational temperatures in the jet, about 50% of the radicals will be in the Z = 0 state with the rest roughly equally divided between E = f 1. As the temperature is increased, all the E levels will tend toward equal population. To complete our basis set, we must multiply by eigenfunctions of the asymmetric rotor Hamiltonian, (2). Thus, the expectation value of 7t will be simply D (E2 - 2 / 3 ) plus the asymmetric rotor levels. In this approximation, the allowed electronic spectrum will consist of three independent asymmetric rotor spectra whose relative intensities depend upon the spin temperature in the jet. This picture will break now due to the spin uncoupling term, J$,, etc. of 7tR connecting different I: states, derived from the substitution of Jxy, - s,,, for NxyJin ( 2 ) . However, in the jet where only the lowest rotational levels are populated, neglect of this perturbation should be a reasonable approximation, at least for the X state, where we know that D >> A, B, C.
Results The upper trace in Figure 2 shows a low-resolution scan of the vibronic structure of the electronic transition attributed to phenylnitrene. Except for some very minor differences, probably involving vibrational hot bands, this spectrum is essentially identical with the excitation spectrum of phenylnitrene published recently by Ozawa et al.,s and both jet spectra are entirely consistent with the flash photolysis spectrum of Porter and Ward.’ The lower trace in Figure 2 shows a much higher resolution scan of the 0; band taken with our pulse-amplified C W dye laser system. As Figure 2 illustrates, the AN = 0, f l branches are clearly visible as are the lines in each branch. There is, however, further incompletely resolved structure in each of the lines of these branches. In order to assign the rotationally resolved spectrum, it is useful to examine the rotational structure of both a- and b-type transitions. Figure 3 shows a- and b-type rotational structures simulated for the origin band of the similar benzyl radical (spin splittings in benzyl are negligible). By comparing Figures 2 and 3, it is clear that the a-type structure is qualitatively similar to the observed spectrum while the b-type structure is obviously not. Referencing Figure 1, this observaJion would imply that the phenylnitrene transition is A 3A2 X 3A2.This would mean that the ordering of the two upper electronic states has been reversed in going from benzyl- to phenylnitrene. While perhaps mildy surprising, considering the closeness of the states in benzyl, the result is hardly implausible. The analysis of the experimental spectrum in terms of an a-type transition as shown in the lower trace of Figure 3 is then relatively straightforward. The AN = f l (R and P branch) lines can be viewed as initially arising from the normal rotational branches
-
20
80
60
40
GHZ Figure 2. Upper trace: low-resolution vibronic structure of the electronic transition. Lower trace: high-resolution spectrum of the origin band.
-30
-lo
GHr
lo
30
Figure 3. Comparison of a- and b-type band structure using the rotational constants of the benzyl radical origin band.
of the K , = 0 stack. Transitions from the K , = 1 state occur a t nearly the same frequency but are split by the K-type doubling. This accounts for the significant triplet splitting of these R and P lines. Higher K, stacks, nearly unsplit and with sharply diminishing thermal populations, add more rotational transitions to each “line” and thereby increase the overall spectral congestion. The central region of the spectrum are those transitions for which AN = 0 (Q branch). All of these transitions originate from states other than K, = 0 and so only arise from asymmetry split states. Table I lists the assigned transitions and their calculated frequencies using th,e simp_le asymmetric rotor Hamiltonian, ( 2 ) . Table I I lists the X and A rotational constants obtained from this fit. B and C in both states are well determined. The difference in the A rota_tional _constants is likewise well determined but independent X- and A-state values of these parameters much less so. Since we are dealing with a-type transitions, the correlation between A‘and A”is only broken by the asymmetric rotor terms coupling K, and K, f 2 states which are relatively small as the molecule approaches a prolate symmetric top.
Discussion The original primary motivation for performing high-resolution work on the spectrum attributed to the phenylnitrene radical was to obtain information about its structure. Obviously for a molecule so complex, very extensive isotopic substitution would be required
3390 The Journal of Physical Chemistry, Vol. 94, No. 9, 1990 TABLE I: Observed and Predicted Transition Frequencies" N' 5 5 5 4 4 4 3 3 3 2 2 2 5 1 4 3 2 I I 2 3 0 4 I
K,'
K,'
1
4 5 5 3 4 4 2 3 3 l 2 2 4 1 3 2 1 O
0 1 1
0 1
I 0 I l 0 I 1 0 1 1 1 l 1 1 1
1
2 3 0 4
0 I 1
I I
0 1
2 2 2 3 3 3 4 4 4
1
1 1
0 2 2 I 3 3 2 4 4 3
0 1 1
0 I I 0 1
Kc
N" 4 4 4 3 3 3 2 2 2 l 1 l 5 0 4 3 2 l I 2 3
K,"
1
0
1
4 2 2 2 3 3 3 4 4 4 5
1
3 2 2 1 3 3 2 4 4 3 5 5 4
1
0 1 1
0 I I 0 I I 0 l 1 0 1 1 1 1 1 1 1
I 0 1 1
0 1 1
0 1 1
5
0
5
1
3 4 4 2 3 3 1 2 2 O 1 I 5 0 4 3 2 1 0 1 2
obs, GHz calc. GHz 18.130 17.033 16.421 14.616 13.863 13.229 10.964 10.471 10.017 7.346 7.01 3 6.702 5.104 3.563 3.423 2.009 0.956 0.326 -0.382 -1.227 -2.420 -3.559 -4.089 -6.920 -7.204 -7.488 -10.284 -10.695 -11.387 -13.787 -14.51! -15.396 -17.146 -17.998 -19.317
18.117 17.092 16.449 14.606 13.821 13.237 11.037 10.467 9.984 7.410 7.038 6.692 5.051 3.545 3.369 2.021 1.010 0.336 -0.413 -1.241 -2.481 -3.582 -4.131 -6.813 -7,190 -7.596 -10.256 -10.804 -11.455 -13.718 -14.410 -15.347 -17.197 -17.994 -19.267
obs calc, GHz 0.013 -0.059 -0.029 0.010 0.042 -0.008 -0.073 0.004 0.033 -0.064 -0.025
0.010 0.053 0.018
0.055 -0.012 -0.054
-0.010 0.031 0.0 14 0.060 0.023 0.042 -0.107 -0.015 0.108 -0.028 0.109 0.068 -0.069 -0.101 -0.049 0.051 -0.004 -0.050
a Assignments of the rotational transitions observed for the origin band. The measured frequencies are shown along with the corresponding values calculated with the rotational constants of Table 11. The line positions are reported in relation to the band origin, which is taken to be zero. The origin itself is found to be at 27 139.453 (21) cm-' (814.18359 (63) THz).
TABLE 11: Determined Rotational Constants (GHz)" A" B" C"
9.60 (1.54) 1.9828 (31) 1.5995 (28)
A' B' C'
9.58 (1.54) 1.9555 (29) 1.5884 (29)
Results of the least-squares analysis of the rotationally resolved band origin transition. The above constants are determined from the assignment of 35 individual rotational lines. The numbers quoted in parentheses are the l u error limits. The overall standard deviation of the fit is 53 MHz. As discussed, the AA value (A" - A ? is better determined than either A"or A ' individually. AA has been determined to be 0.020 (22) GHz. to obtain a complete structure from measured rotational constants. Nonetheless, our three observed constants for a single isotopomer plus a little "chemical intuition" can place relatively severe restrictions on the geometry. Little variation in the bond lengths and angles of the phenyl group from ordinary benzene would be expected in phenylnitrene. Similarly, there is no plausible reason for the molecule's C, symmetry to be destroyed. Thus effectively the only geometric unknown for phenylnitrene is RC-N, the C-N bond length, and our three rotational parameters should determine that redundantly. Table 111 gives the experimental rotational constants for the ground state of the molecule and compares them with the best (in the least-squares sense) rotational constants calculated for phenylnitrene by varying RC+. As Table I11 shows, the agreement is rather poor. It is possible that some distortion of the phenyl geometry would decrease this discrepancy, but to eliminate it entirely would clearly require a quite anomalous phenyl structure. Even worse, the "best" calculated rotational constants require Rc+ = 2.231 8,. This is truly an incredible result considering RC-Y
Letters TABLE 111: Comparison of Experimentally Determined Rotational Constants (in GHz) with Those Calculated for Reasonable Geometries of Phenylnitrene (C,H5N) and Cyanocyclopentadienyl (C5H4CN) Radicals CAHCN" CrHaCNb exDt CY., A" 5.696 8.950 9.60 (1.54) B"
C"
2.097 1.533
1.966 1.612
1.9828 (31) 1.5995 (28)
Constants calculated assuming planar regular benzene ring23 with A, RC+ = 1.080 A, and RC+ = 2.231 A. bConstants calculated assuming planar cyclopentadienyl ringi7 with Rc_c(ring) = 1.420 A. RC+ = 1.094 A. RcX = 1.389 A, and Rc-v = Rc_c(ring) = 1.395 1.175 A.
values for amines (and methylnitrene) are approximately 1.4 A.Zo Additionally, we find another anomaly with the observed spectrum. As mentioned earlier, there are three spin levels which should give rise to at least two ( 2 = 0 and f l ) distinct asymmetric rotor spectra whose intensity should vary as we vary the temperature of the jet. These spectra _wouldbe separated by the difference in D (neglecting E ) in the X and A states, which would likely amount to a significant fraction of its value, e.g., 10 GHz. Such a separation would be clearly visible at our resolution. Despite this fact, all attempts to identify spectra of other spin components (or the weaker A 2 # 0 transitions) as a function of jet temperature have failed. While the _negative result could be explained if the D values in the X and A states were so similar that transitions from different 2 levels were unresolved, with our resolution this explanation would required D's within about 1 % of one another. Thus our observations force us to the following conclusions. Either the rotational spectrum of phenylnitrene is very unusual-due to strange spin uncoupling effects or peculiar ge?metry and remarkably coincidental spin structure in the A and X state-or the carrier of the gas-phase spectrum attributed time and time again to phenylnitrene indeed belongs to some other species. While we believe more experiments and analysis need to be done, we presently lean toward the latter interpretation. If this hypothesis is true, then the most important question to be answered is the carrier's identity. While we are not yet certain, based upon the available evidence, we speculate that a reasonable candidate for the carrier is the cyanocyclopentadienyl (or cyclopentadienecarbonitrile) radical, C5H4CN,which is, within one hydrogen, isomeric with phenylnitrene. At the moment, our strongest argument for this proposal is 2-fold. (i) The cyclopentadienyl and halogen-substituted cyclopentadienyl radicals have absorption and emission spectra in this spectral region.' (ii) The rotational structure of the spectrum is consistent with the expected geometry of this radical. As Table I11 shows, by assuming the previously determined cyclopentadienyl ring geometry and varying the RmN and RC-pN bond distances, values for all three observed rotational constants can be reproduced to nearly experimental precision (slight distortion of the ring should remove any residual discrepancies). Most importantly, the values found for RC----N= 1.175 8, and RC-N = 1.389 8, are not inconsistent with other similar molecules, e g , benzonitrile,z' C,H,CN, Rc-N = 1.156 A and RC-CN = 1.444 8,. Furthermore, the small spin-rotation splitting of the doublet state of C5H4CN would likely be unresolvable at our r e s ~ l u t i o n ,explaining '~ the lack of spin structure in the spectrum. Finally, it has been shown that gas-phase pyrolysis of aryl azides leads to extrusion of molecular nitrogen and then ring contraction to form the cyanocyclopentadienyl radical .*2-24 Conclusions A high-resolution spectrum of the origin band of the electronic transition long attributed to triplet phenylnitrene has been ob(20) Carrick, P. G.; Brazier, C. R.; Bernath. P. F.; Engelking, P. C. J . Am. Chem. Soc. 1987, 109, 5100. (21) Casado, J.; Nygaard, L.; Sorensen, G.0. J . Mol. Spectrosc. 1971. 8. 21 I . (22) Wentrup, C.; Crow, W. D. Tetrahedron 1970, 26. 4375. (23) Wentrup, C. Tetrahedron 1974, 30, 1301. (24) Crow, W . D.; Paddon-Row, M . N . Aust. J . Chem. 1975. 28, 1755.
J . Phys. Chem. 1990, 94, 3391-3393
3391
transition, previously assumed to be of the same origin, has been used extensively to monitor triplet phenylnitrene in condensed phases. A negative conclusion regarding the assignment of the gas-phase spectrum to phenylnitrene cannot automatically be carried over to the condensed-phase spectrum. Nonetheless, if such a conclusion is forthcoming in the gas phase, careful reinvestigation of the condensed-phase work would also be in order.
tained. The rotational and spin structure revealed by this experiment is difficult to reconcile with that expected from phenylnitrene. It will be necessary to perform further experiments to see if these anomalies can be explained. However, at this point, serious consideration must be given to the possibility that the carrier of the spectrum is not phenylnitrene. An alternative candidate for the carrier is the cyanocyclopentadienyl radical. Future investigations involving alternative precursors and the dispersed fluorescence spectrum are planned to try to unambiguously resolve this issue. If it is proven that phenylnitrene is not the carrier of the spectrum, the previous gas-phase work involving this spectrum, and its use as a monitor of phenylnitrene chemistry and dynamics, will have to be completely reevaluated. A similar electronic
Acknowledgment. We gratefully acknowledge the support of this work by the National Science Foundation via Grant CHE85-07537. We also recognize Dr. Elisa Leyva for synthesizing the large amounts of phenyl azide used throughout these studies. Registry No. Phenylnitrene, 2655-25-6.
Triplet-State Photophysics of Naphthalene and a,o-Diphenylpolyenes Included in Heavy-Cation-Exchanged Zeolitest V. Ramamurthy,* Jonathan V. Caspar,* David R. Corbin, Central Research and Development Department,l E . I . du Pont de Nemours and Company, Experimental Station, P.O. Box 80328, Wilmington, Delaware 19880-0328
Bruce D. Schlyer, and August H. Maki* Department of Chemistry, University of California, Davis, California 95616 (Received: January 23, 1990)
Heavy-cation-exchanged zeolites offer a powerful new medium for observing external heavy-atom perturbation of organic triplet states. In this Letter we report the use of optically detected magnetic resonance measurements to determine the geometric mode of interaction between the perturbing heavy atom and naphthalene within the X- and Y-type faujasites exchanged with K', Rb', and Cs'. Building upon these results, we demonstrate the use of the heavy-cation-exchanged zeolites as spectroscopic matrices enabling the detection of triplet phosphorescence from a series of all-trans-a,w-diphenylpolyenesfor which phosphorescence has not been previously observed.
Introduction
a variety of guests ( e g , naphthalene, anthracene, pyrene, phenanthrene, chrysene, acenaphthene, t e t r a ~ e n e ) . ~The . ~ principal restriction on the effect is that the aromatic guest must be small enough to fit through the ca. 8-A-diameter windows of the zeolite supercages, thus demonstrating that the effect is not due to aromatics adsorbed on the exterior surfaces of the zeolites.
Since the discovery by Kasha' that spin-orbit coupling due to an external heavy atom can significantly perturb the photophysical properties of the lowest excited triplet states of organic chromophores, this effect has been the object of extensive investigation in a variety of different media including solution, low-temperature glasses, and crystals.2 Previously, we reported that faujasite and pentasil zeolites exchanged with heavy cations ( e g , Rb+, Cs+, TI+) represent an important new and general medium for observation of these effects and that the magnitude of the external - ~ this Letter, we heavy-atom effect is large in these ~ n e d i a . ~ In report the use of optically detected magnetic resonance (ODMR) measurements to determine the geometric mode of interaction between the perturbing heavy atom and naphthalene within the zeolite cavity. Building upon these results, we demonstrate the use of the heavy-atom-exchanged zeolites as spectroscopic matrices enabling the detection of triplet phosphorescence from a series of all-rrans-a,w-diphenylpolyenesfor which phosphorescence has not been previously observed. Our earlier work d e m ~ n s t r a t e denhanced ~,~ intersystem crossing and phosphorescence yields and decreased singlet and triplet excited state lifetimes for naphthalene included in heavy-cationexchanged X-type faujasite zeolites due to an external heavy-atom perturbation. Briefly, the shape and position of the naphthalene phosphorescence spectrum were observed to be only weakly dependent upon the identity of the perturbing zeolite cation while vs In (E2) ( T is~ the triplet lifetime and is the a plot of In (iT) spin-orbit coupling constant of the heavy atom) was linear with unit slope as expected for the heavy- atom effect.' The effect was shown to be general for aromatics included within faujasites, making possible the facile observation of phosphorescence from
Results and Discussion Given the generality of the external heavy-atom effect in zeolites, we presumed that the formation of weak complexes between the exchangeable zeolite cations and the included organic guests was responsible for keeping the cation-chromophore distance short, resulting in large heavy-atom perturbations as the effect is known to have a strong distance dependence.s While the location of small aromatic guests such as benzene or toluene within X- and Y-type faujasites has recently been determined by neutron diffraction methodsg-l3 and detailed IR ~pectroscopy,'"'~ ( I ) Kasha, M. J . Chem. Phys. 1952, 20, 71-74. (2) McGlynn, S. P.; Azumi, T.;Kinoshita, M. Molecular Spectroscopy of the Triplet State; Prentice-Hall: Englewood Cliffs, N J , 1969. (3) Ramamurthy, V.; Caspar, J . V.: Corbin, D. R. Tetrahedron Lett. 1990, 31, 1097-1 100. (4) Caspar, J . V.; Ramamurthy, V.; Corbin, D. R. Coord. Chem. Reu. 1990, 97, 225-236. ( 5 ) Ramamurthy, V.; Caspar, J. V.; Corbin, D. R.; Eaton, D. F. J . Photochem. Photobiol., A 1989, 50, 157-161. (6) Ramamurthy, V.; Caspar, J . V.; Corbin, D. R.; Eaton, D. F.; Kauffman, J . S.; Dybowski. C. J . Photochem. Photobiol. A , in press. (7) McClure, D. S. J . Chem. Phys. 1949, 17, 905-913. (8) Chandra, A.; Turro, N.; Lyons, A.; Stone, P. J . Am. Chem. Soc. 1978, 100, 4964-4968. (9) Czjzek, M.;Vogt, T.;Fuess, H. Angew. Chem., Int. Ed. Engl. 1989, 28, 770-772. (IO) Jobic, H.; Renouprez, A,; Fitch, A. N.; Lauter. H. J . J . Chem. Soc., Faraday Trans. I 1987, 83, 3199-3205
'Part of the series "Modification of Photochemical Reactivity by Zeolites". *Contribution No. 5386.
0022-3654/90/2094-339l$02.50/0 I
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Q 1990 American Chemical Societv
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