568
E.A. Dorko, R. Scheps, and S.A. Rice
Comments on the Ultraviolet Spectrum and Photophysical Properties of Trimethylenecyclopropane Ernest A. Dorko, Department of Aeromechanical Engineering, Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio 45433
Richard Scheps, and Stuart A. Rice* The Department of Chemistry and The James Franck Institute, The University of Chicago, Chicago, lliinois 60637 (Received October 12, 1973) Pubifcation costs assisted b y the Air Force Office of Scientific Research
The spectrum and fluorescence of TMCP are analyzed with respect to possible photoproducts.
I. Introduction Recent theoreticall and experimentaP4 studies have considerably extended our understanding of photophysical radiationless processes in large aromatic molecules. In the best studied case, namely benzene2 and several of its deuterated derivative^,^,^ single vibronic level fluorescence lifetime data are in excellent agreement with theoretical calculations which emphasize the importance of frequency changes and shifts of origins of the CH and CC vibrational modes which serve as energy accept0rs.l Thus far, however, the adequacy of the theory has been tested only in the prediction of relative rates of radiationless decay. The far more ambitious task of predicting the absolute rates of radiationless decay depends on accurate evaluation of the electronic matrix element connecting the zeroth-order manifolds lBzu and 3B1u, a calculation which is fraught with uncertainty because of the inaccuracies in currently available molecular wave functions. As part of a study designed to contribute to the data base from which can' be obtained information about the importance of different kinds of vibronic coupling for radiationless processes, we have examined the spectrum and fluorescence of trimethylenecyclopropane (TMCP). TMCP, the first member of the family of radialenes, is a stable isomer of benzene. In contrast with benzene, its lowest electronic transition is allowed and strong while the next two are forbidden. Furthermore, the photolysis of TMCP is expected to lead to fulvene and benzene, isomers more stable than TMCP hence accessible with the energy of excitation in the lowest allowed transition. Thus, a study of the photophysics of TMCP provides, in principle, an opportunity to examine the coupling between competing photochemical and photophysical decay channels, as well as the influence thereon of vibronic coupling. We have been unable to detect fluorescence from TMCP and so have been unable to compare its decay processes with those of benzene. Nevertheless, the absence of fluorescence taken together with available molecular orbital calculations does permit some limited deductions to be made about radiationless decay in TMCP. A more positive outcome of our investigation is the observation of a set of Rydberg transitions from which the ionization potential of TMCP can be estimated. The Journalof PbysicalChemistry, Vol. 78, No. 6, 1974
11. Results The uv absorption spectrum of TMCP is reproduced in Figure 1. Absorption begins near 3430 A. An intense diffuse transition appears with a maximum at 2885 8, and shoulders a t 2990 and possibly 3200 A. Above 3100 A the spectrum displayed shows some absorption due to unremoved second-order light. This contribution is small, however, and its effect may be gauged by the intensity of the structure near 3220 A. In the unpurified compound a weak band was observed at 2515 A, but this did not appear in the purified samples. A second broad transition of lower intensity is observed with a peak at 2235 A and with a shoulder at 2295 A. (This band was originally reporteds to have greater intensity than the 2885-A band. However, the earlier observation was not corrected to account for instrumental anomalies and does not reflect the true intensity relationships.) The absorption at higher energy suggests a Rydberg series with an ionization potential in the vicinity of 9 eV. The results of this spectroscopic investigation are summarized in Tables I and 11. Our suggested classification of the Rydberg bands, shown in Table 11, gives an ionization potential of 9.0 eV and quantum defect 6 = 0.58. The agreement between calculated and observed band positions is excellent, especially when the uncertainties in observed band positions (-100 cm-l) are taken into account. In order to determine if the broadening in the lower energy transitions can be attributed to hot band and sequence congestion an attempt was made to measure the absorption of TMCP at low temperature. Initially, we hoped to use a suitable matrix as a Shpolskii host,6 but difficulty in handling TMCP prevented this and finally we recorded the spectrum of the pure solid at 77°K. The spectrum obtained was not noticeably less diffuse than that of the gas. Although interactions in the pure solid can lead to spectral broadening, and therefore this result is inconclusive, we believe the weight of evidence and conjecture supports the notion that the observed spectral broadening is a result of nonradiative processes. The results of the fluorescence study place an upper limit on the quantum yield for fluorescence. We used A,, 2885 A with relatively large bandpass (3.8 A). After 4 hr of excitation at 5-kHz pulse repetition frequency no fluorescence was observed. Earlier work on chl~roacetylene~ has shown that quantum yields as low as 0.025 can be mea-
569
Photophysical Radiationless Processes in Large Aromatic Molecules cm->
30000
450CC:
a00OO
35000
50000
6CO00
7OooO
80000 1
3300
31W
1
2900
H Figure 1. Absorption spectrum of TMCP vapor at -1 Torr. The region between 1800 and 1900 A was originally recorded on a reduced scale, and was expanded for this figure, hence the line shape in this region is not exact. The resolution is -0.5 A and the cell path length 10 cm.
4000 Figure 2.
3500
3000
2500
Infrared spectrum of TMCP vapor at
-
1800
2000
1600
1400
1200
1000
800
600
Wavenumber !crn-’) 1 mm, cell path 5 cm.
sured using our apparatus, We take this value as an upper limit to the quantum yield of TMCP, although it is reasonable to expect the actual value to be at least an order of magnitude lower. The infrared absorption spectrum we have obtained is shown in Figure 2. The agreement with the work of Rhee and Miller8 is excellent. We observe that the bands proposed by these authors to be impurity bands appear in our spectrum as well. It is interesting to note that although we and they used the same method of synthesis, our purification was more extensive. Yet these “impurity” bands appear with the same relative intensity in both spectra. One infers that these bands may not, after all, be due to impurity. 111. Discussion
The results of this work clearly show that TMCP in its lowest excited state is efficiently depopulated through nonradiative processes. Certainly one such process will be
intersystem crossing. Two triplet manifolds can, in principle, play a role in this crossing. If the theoretical predictiong is accurate, Ti and Tz lie, respectively, -18,000 and 1300 cm-I below SI.We expect that crossing to TI will be inefficient because the energy gap is half an order of magnitude larger than the highest frequency acceptor mode, and the Franck-Condon factor for the isoenergetic crossing must be very small. We also expect that crossing to Ta will be only modestly efficient, probably less efficient than the SI.--.T1 crossing in benzene. Here the limitation on the rate of crossing will be determined by the low density of acceptor modes because of the very small energy energy gap in benzene is -8000 cm-I). gap (the Thus, it seems unreasonable to us that crossing to either TI or Tz can be so efficient that all fluorescence is quenched. Accordingly, we look elsewhere for an explanation of the low quantum yield of fluorescence of TMCP. We expect that the excited state is effectively depopulated via a photochemical pathway. One possibility is The Journal of Physical Chemistry, Vol. 78, No. 6,1974
570
E. A. Dorko, R. Scheps, and S. A. Rice
TABLE I: Transitions in TMCP
TABLE 11: Rydberg Analysis
amax, Aa
Energy, cm-1
Appearance
2885 34,662 (4.3 eV) Broad, intense 1E’ shoulders a t 2990 and 3200 fi 2235 44,743 (5.5 eV) Broad, weaker than 2885-A system 2075 48,193 (6.0 eV) Weak bands 2025 49,383 2005 49,875 1950 51,282 (6.4 eV) Shoulders to ‘E’ continuum 1900 52,632 (6.5 eV) 1857 53,850 (6.7 eV) Intense strucRydberg n = 3 tured system (3R) superposed on continuum 1850 54,054 1835 54,494 1802 55,494 1775 56,338 1752 57,078 1730 57,803 1699 58,858 1675 59,701 1650 60,606 1610 62,112 1580 63,291 (7.8 eV) 1572 63,613 1547 64,641 1542 64,851 1535 65,147 1520 65,789 1498 66,756 (8.3 eV) 1485 67,340 1480 67,568 1472 67,934 1463 68,353 1457 68,634 (8.5 eV) 1448 69,061 1432 69,830 (8.6 eV) 1428 70,028 1423 70,274 Broad bands 1385 72,202 1375 72,727 1357 73,692 a
Bands are uncertain to ir2
due to diffuseness.
which leads to the formation of benzene, fulvene, and dimethylenecyclobutene. Angular momentum is easily conserved for the pictured photochemical intermediate despite the change in moments of inertia. It is at least possible that this mechanism dominates the decay of excited TMCP. We consider now the absorption spectra of TMCP. The ir spectrum is similar enough to that of Rhee and Millers to eliminate the need for any further discussion of it. The electronic absorption spectrum indicates four intraThe Journal of Physical Chemistry, Vol. 78, No. 6, 1974
r=IP--
Assignment
109,737.1 (cm -1) ( n - 6)2
A(obad -
n
Obsd, cm-1
Calcd, cm-1
calcd), cm-1
3 4 5 6 7
53,850 63,291 66,756 68,634 69,830 Not observed
53,774 63,130 66,895 68,777 69,850 70,519
- 76 - 161
8
+139 +143
+20
0.58 cm-1 (9.0 f 0.1 eV)
6 =
IP
=
72,512
=t800
valence transitions and a Rydberg series. From the latter we obtain a first ionization potential of 9.0 eV. This agrees well with the value of 9.07 eV calculated by Simmonsg (corrected to obtain optimum agreement for 1,3butadiene and hence lower by 2.45 eV than the original calculation). The vibrational structure is quite pronounced, especially for u2, the totally symmetric C=C stretch. This is to be expected since the T electrons lie highest in energy, and ionization will be accompanied by a weakening and, hence, elongation of the C=C bond. The Rydberg series is allowed and is either of species E’ or Az”. We note that transitions to all p orbitals and all d orbitals except dt2 are allowed. The value of 6 implies excitation to np orbitals, but we cannot distinguish between npn and npa orbitals. The state of the ion is 2E”. Underlying the “sharp” (15-A full-width at half-maximum) n = 3 Rydberg transition is a broad continuum, which we associate with the SO S4 (IE’) transition, calculated to lie at 7.1 eV and observed at -6.4 eV. It is interesting to note that no interference effects arising from interaction between the Rydberg levels and the intravalence continuum are observed. This observation can be explained if there is only limited configuration interaction between the Rydberg and Sq valence states, or by assuming the Rydberg state has symmetry A2”. This latter suggestion leads to a classification of Rydberg states which bears strong analogy to benzene.1° We associate the low-energy intense transition with SO S1 (lE’), observed at 4.3 eV (ha,,,) and predicted to lie at 4.8 eV (origin). There is little doubt in this assignment. We are left with two unassigned transitions, one at 5.5 eV (humax),the other at 6.0 eV. Two states are predicted to lie in this region, both dipole forbidden and both at -7.1 eV. It is possible that these bands obtain allowed character by vibronic mixing with the E’ states and we would then assign the band to the red as lAz’(S2) and the band to the blue as 1A1’(S3). Unfortunately the band assigned lA2’ has no vibrational structure so that we cannot confirm the assignment. The weaker system is also difficult to assign for the same reason, although we note that the spacing between the first two bands is roughly twice that between the second and third (within the error set by the diffuseness) and this corresponds roughly to the e’ vibration reported by Burr, et al.11 Finally, we remark that several authors have assigned the bands at 5.5 and 6.0 eV to impurities and scattered light (instrumental limitati0ns).~J2 While we cannot conclusively rule out this possibility for the 5.5-eV band, we offer the following information. (1) The 5.5-eV band persisted, with roughly the same relative intensity, through repeated vacuum distillations, so that if
-
-
Photophysical Radiationless Processes in Large Aromatic Molecules
571
10-cm path length Perkin-Elmer infrared cell fitted with it were due to an impurity that impurity must have a two KRS-5 windows. vapor pressure similar to that of TMCP. The expected The vapor absorption spectrum was taken on a McPherimpurities do not satisfy this criterion. In particular, the son 3.0-m coma-corrected vacuum monochromator. This impurity bands which appear in the ir spectrum are most system has been described p r e v i o u ~ l y A . ~ Hinterreger ~ hylikely due to the dimer, which of course will have a lower drogen glow discharge light source was operated at 350 W, vapor pressure. (2) The spectra of the impurities expected and absorption was recorded directly using a dual-beam to be present were taken and none produced the 2235-8, chopping system in conjunction with a logarithmic raabsorption band. (3) We did not observe any further abtiometer. A 0.3-m predisperser was used to reduce stray sorption bands of the proposed impurity in the uv and light. The linear dispersion of the monochromator using a vacuum uv. However, it remains difficult to satisfactorily 1200 line/mm grating blazed at 1500 8, in first order is 2.7 explain the results of Waitkus, et al.,I1 who, using TMCP A/mm. obtained via a different synthesis, did not observe the 5.5-eV band, and the theoretical predictions of S i m m o n ~ , ~ The spectrum of the solid at 77°K was taken on a Cary who does not predict an allowed transition in this region. 14 using a Hanovia 150-W xenon arc lamp. The sample The spectral assignments suggested and the nonradiawas placed in a crude dewar fitted with quartz windows and altered to fit in the Cary 14 sample compartment. tive decay mechanism suggested cannot be considered established at the present time. Experimental work with The compartment was then made light tight and flushed with dry nitrogen to avoid condensation on the dewar winperdeuteriotrimethylenecyclopropane, more sensitive fluodows. rescence experiments, photochemical product analysis, and more accurate molecular orbital calculations are all The fluorescence work was carried out in a manner simnecessary before the matter can be resolved. Further studilar to that used for benzene and using an arrangement ies will be reported at a later time. previously described .3 The infrared spectrum was taken on a standard PerkinIV. Experimental Method Elmer Model 180 infrared spectrometer. TMCP was prepared as previously d e s ~ r i b e dThe . ~ samAcknowledgments. One of us (E. A. D.) would like to ple was passed through a trap containing KOH to absorb HzO and COz and then was subjected to a bulb-to-bulb thank the U. S. Air Force Systems Command for a summer Research Associateship at the University of Chicago distillation a t 250°K (PVap = 0.85 Torr). A complicating under AFSC/AFIT funding. S. A. R. wishes to thank the factor in handling TMCP is its extreme tendency to poDirectorate of Chemical Sciences, Air Force Office of Scilymerize. This called for special considerations in working entific Research, for their support. We also acknowledge with the compound throughout this project. The severest the general support of the Materials Research Laboratory test of purity was the reproducibility of the absorption at the University of Chicago by the National Science spectrum upon repeated fractionation of the TMCP. The Foundat ion. vapor pressure of TMCP was determined a t 256, 263, and 295°K to be 1.5, 2.5, and 23 Torr, respectively. From these References and Notes data the heat of vaporization was determined to be 10.7 D . F. Helier, K. F. Freed, and W. M. Gelbart, J. Chem. Phys., 56, kcal/mol and the boiling point 367°K. 2309 (1972). The sample cell was filled by connecting it to the reserC. S. Parmenter and M. W. Schuyler, Chem. Phys. Lett., 6, 339 voir containing TMCP. The reservoir was immersed in a (1970). K . G. Spears and S. A Rice, J. Chem. Phys., 55, 5561 (1971); A. slurry of appropriate temperature so that TMCP could Abramson, K. G. Spears, and S. A. Rice, ibid., 56, 2291 (1972). vaporize into the test cell to the desired vapor pressure C. Guttman and S. A. Rice, J. Chem. Phys., in press. without excessive polymerization. E. A. Dorko, J. Amer. Chem. SOC.,87,5518(1965). E. V. Shpolskii, Usp. Fiz. Nank., 71, 215 (1960). For a review see Sample cells used were (1) a IO-cm path length LiF S. Leach, Mem. SOC.Roy. Sci. Liege, Collect. 5', 20, 179 (1970). windowed cell modified from the original McPherson deK. Evans, R. Scheps, S. A. Rice, and D. Heiler, J. Chem. SOC., Faraday Trans. 2, 69, 856 (1973). sign to include a stainless steel stopcock valve so the it K. H. Rhee and F. A. Miller, Spectrochlm. Acta, Part A, 27, 1 could be filled on any vacuum line in the laboratory and (1971). moved to the spectrometer. The cover for the stainless H. E.Simmons, Progr. Phys. Org. Chem., 7, 1 (1970). See R. Scheps, D. Florida, and S. A. Rice, J. Chem. Phys., 56, 295 steel dual beam sample chamber was modified to allow (1972); Chem. Phys. Lett., 15,490 (1972). the entire cell assembly to be evacuated. (2) Modified 1J . C. Burr;Jr., E. A. Dorko, and J. A. Merritt, J. Chem. Phys., 45, and 2-mm quartz curvets which fit into a liquid nitrogen 3877 (1966), P. A. Waitkus, E. B. Sanders, L. I . Peterson, and G. W. Griffin, J. optical dewar, (3) a fluorescence cell similar to that used Amer. Chem. SOC., 89, 6318 (1967). in the previously reported studies of benzene, and (4) a R. Scheps, Ph.D. Thesis, University of Chicago, 1973.
The Journalof Physical Chemistry, Vol. 78, No. 6, 1974
