J. Phys. Chem. 1996, 100, 19289-19291
19289
Phosphorescence of a Stiff cis-Stilbene Analogue, 5H-Dibenzo[a,d]cycloheptene Takeshi Ikeyama,*,† Chizuko Kabuto,‡ and Masaki Sato† Department of Chemistry, Miyagi UniVersity of Education, Sendai 980, Japan, and Instrumental Analysis Center for Chemistry, Faculty of Science, Tohoku UniVersity, Sendai 980, Japan ReceiVed: June 18, 1996; In Final Form: September 12, 1996X
Weak phosphorescence of a stiff cis-stilbene analogue, 5H-dibenzo[a,d]cycloheptene single crystal, has been observed at 77 K. The energy of the T1-S0 transition is 222 kJ mol-1, and the lifetime of the lowest excited triplet state is 14 ms. For interpreting the results, the ground state geometry of the molecule has been determined by the X-ray crystallography. The torsion angles around C(ethylene)dC(ethylene) and C(ethylene)-C(phenyl) are 0° and 32.6°, respectively. The results are discussed and compared with those of cis-stilbene.
Introduction The energy surface of the lowest excited triplet state (T1) of stilbene has long been interesting from several viewpoints, such as nonvertical energy transfer and cis-trans photoisomerization.1 The experimental results of nonvertical energy transfer of cis-stilbene have been interpreted with the mechanism including the torsion around the central ethylenic CdC bond. However, recently, Gorman et al.2 and Caldwell et al.3 pointed out that an almost identical phenomenon is also observed for 2,3-diphenylnorbornene, which has a chromophore identical with cis-stilbene except for the severe restriction of the double-bond twisting. The result clearly indicates the importance of the torsion around the C(ethylene)-C(phenyl) single bond in nonvertical energy transfer, rather than the C(ethylene)dC(ethylene) double bond as has been interpreted. Therefore, investigations on the lowest excited triplet state of stiff stilbene analogues with known rigid geometry are of increasing importance. One of the stiff cis-stilbene analogues, 5H-dibenzo[a,d]cycloheptene (abbreviated hereafter as DBCH), has been investigated by several authors.4,5 Especially, Watkins and Bayrakc¸ eken5 reported several photophysical properties of this molecule including the phosphorescence spectrum. On the phosphorescence spectrum of Watkins et al., however, one important question exists. The energy of the phosphorescence they reported for DBCH (275 kJ mol-1) is much higher than the triplet energy of cis-stilbene (240 kJ mol-1), which is determined by the onset of the singlet-triplet absorption spectrum observed under enhancement with the high-pressure oxygen6,7 or in heavy-atom solvents.8 The large difference in the T1 energies between DBCH and cis-stilbene indicates that the molecular geometry of DBCH might differ very much from that of cis-stilbene, because in stilbene the energy gap between the ground and the lowest excited triplet state is known to depend strongly on the twist angle of the central CdC bond. The problem is important, because if the geometry of DBCH differs so much, DBCH is not a good model for cis-stilbene. The phosphorescence of stilbene is known to be very weak,9,10 though in the rigid analogue it gets somewhat larger.11 To our knowledge, there are no data on the phosphorescence of cisstilbene itself. Therefore, in the present paper, we carefully purified DBCH and observed its phosphorescence spectrum. †
Miyagi University of Education. Tohoku University. X Abstract published in AdVance ACS Abstracts, November 1, 1996. ‡
S0022-3654(96)01774-1 CCC: $12.00
For interpreting the observed phosphorescence spectrum, the ground state geometry of DBCH is also determined by X-ray crystallography. Experimental Section Sample Preparation. 5H-dibenzo[a,d]cycloheptene was synthesized from 5H-dibenzo[a,d]cyclohepten-5-one (Tokyo Kasei) with the Huang-Minlon method.12 The sample was recrystallized from methanol and further purified by the zonemelting method for about 100 passes. For the phosphorescence measurement, a single crystal is grown from the melt with the Bridgman furnace. For the X-ray diffraction measurement, the zone-refined sample was dissolved again in ethanol, and the small crystal is obtained with recrystallization. Phosphorescence Measurements. The neat single crystal of DBCH was excited with light from a high-pressure mercury lamp (500 W) passed through a glass filter (Toshiba UVD33S). The phosphorescence was measured at 77 K by a double spectrometer (Spex 1680) equipped with a cooled photomultiplier tube (Hamamatsu R943-02) employing the photoncounting method (Hamamatsu C2550). Kasha type sectors were used for spectral measurements. For the decay measurements of the phosphorescence the excitation source is replaced with a nitrogen laser (Laser Photonics LN1000). X-ray Experiment for DBCH. The size of the DBCH crystal (colorless, prism) was approximately 0.2 × 0.2 × 0.3 mm. Intensity data were measured on a Rigaku AFC-5R diffractometer using Mo KR radiation (λ ) 0.710 69 Å) and the 2θ/ω scan technique. Crystals of DBCH belong to the orthorhombic space group Pnma (no. 62) with a ) 8.801(2) Å, b ) 19.719(2) Å, c ) 19.674(2) Å, V ) 1059.2(2) Å3, Z ) 4, and Dcalcd ) 1.206 g/cm3. Data were collected at 283 K, and the structure was solved by direct methods and successive D-Fourier method and refined to Rf ) 0.048 and Rw ) 0.062 for 787 reflections with I > 3σ(I). A full description of the structure is provided in the supporting information. Results and Discussion The weak phosphorescence spectrum of the DBCH neat single crystal observed at 77 K is shown in Figure 1. The energy of the shortest wavelength band in the phosphorescence is 538.6 nm (222 kJ mol-1). It differs very much from the result of phosphorescence reported previously; Watkins et al. observed © 1996 American Chemical Society
19290 J. Phys. Chem., Vol. 100, No. 50, 1996
Ikeyama et al.
Figure 1. Phosphorescence spectrum of 5H-dibenzo[a,d]cycloheptene single crystal at 77 K.
the phosphorescence spectrum of DBCH, which has a first peak at 435 nm (275 kJ mol-1). The present result rather coincides well with the origin (240 kJ mol-1) of the low-resolution singlet-triplet absorption spectra of cis-stilbene observed at room temperature in heavy-atom solvents8 or observed under the enhancement with the highpressure oxygen.6,7 Our sample was extremely purified with the zone-melting method, so the influence of the impurity is improbable. Therefore, in view of the triplet energy of cis-stilbene, we conclude that the spectrum that Watkins et al. reported came from an impurity included in the sample or solvent. This, however, does not affect other data and results in the work of Watkins et al. rather than the phosphorescence spectrum, because most of their discussion is based on the analysis of the triplet-triplet absorption. Accordingly, the phosphorescence of DBCH was observed for the first time, and the energy of the lowest excited triplet state is determined to be 222 kJ mol-1. The phosphorescence decay curve observed at 539 nm is approximately a single exponential; the lifetime is about 14 ms. The lifetime is about double that observed in a mixed glassy solvent of isopentane and 3-methylpentane (6:1) at 77 K (7.1 ms) by the decay of triplet-triplet absorption.4 Thus, the lifetime of DBCH shows a large host effect. A similar phenomenon has been reported4,13 also in trans-stilbene, that is, 5.3 ms in isopentane: 3-methylpentane (6:1) glass (6.9 × 106 P), 18 ms in 3-methylpentane glass (9.4 × 1011 P), and 19 ms in neat single crystal. The host dependence in trans-stilbene is interpreted as follows; that is, a viscous solvent reduces the nonradiative deactivation from the triplet state by restricting the torsional rotations. In less viscous solvent the torsional motions of the molecule achieve larger displacements than in a more viscous solvent. The torsional displacement decreases the singlet-triplet energy separation and increases the nonradiative deactivation from the lowest excited triplet state. The observed host dependence of the lifetime of DBCH is explained in the same manner as trans-stilbene, but it is noteworthy that the energy separation in glassy solvents changes also in such a seemingly rigid molecule as DBCH in which the torsional rotations are restricted by the methylene bond. The molecular structure of DBCH is shown in Figure 2. Some coordinates are compared with those of cis-stilbene14 in Table 1. The corresponding torsion angles and bond lengths of DBCH are approximately the same as those of cis-stilbene, except that the molecular symmetry of DBCH is Cs in contrast with C2 of cis-stilbene.14 For example, the central
Figure 2. Two views of molecular structure of 5H-dibenzo[a,d]cycloheptene at 283 K. The atomic numbering, bond lengths (Å), and bond angles (deg) for non-hydrogen atoms are shown in an ORTEP drawing (top). The molecule have a mirror symmetry through C8, H81, and H82.
TABLE 1: Comparison of Structural Parameters for DBCH and cis-Stilbenea
R(C(ethylene)dC(ethylene)) R(C(ethylene)-C(phenyl)) ∠C(ethylene)dC(ethylene)-C(phenyl) C(ethylene)dC(ethylene) torsion C(ethylene)-C(phenyl) torsion a
DBCH (this work)
cis-stilbene (ref 14)
1.335(4) 1.463(3) 128.4 0 32.6
1.334 1.489 129.5 0 43.2
Bond distances are in angstroms and bond angles in degrees.
C(ethylene)dC(ethylene) bond length is 1.335 Å (1.334 Å in cis-stilbene), a double-bond torsion of C(ethylene)dC(ethylene) is 0° (0° in cis-stilbene), and a single-bond torsion of C(ethylene)-C(phenyl) is 32.6° (43.2° in cis-stilbene). In conclusion, from the viewpoint of molecular geometry and the triplet energy, DBCH is confirmed to be a good model for cis-stilbene with rigid structure. Acknowledgment. This research was partially supported by a Grant-in-Aid for Scientific Research No. 07640663 from the Ministry of Education, Science and Culture. This research was also supported in part by The Japan Association of Chemistry. Supporting Information Available: Description of the structure determination, refinement procedure, and tables of X-ray crystallographic data for DBCH, including atomic coordinates and anisotropic thermal parameters (10 pages). Ordering information is given on any current masthead page. References and Notes (1) (a) Waldeck, D. H. Chem. ReV. 1991, 91, 415. (b) Saltiel, J.; D’Agostino, J. T.; Megarity, E. D.; Neuberger, K. R.; Wrighton, M.; Zafiriou, O. C. Org. Photochem. 1973, 3, 1. (c) Saltiel, J.; Marchand, G. R.; Kirkor-Kaminska, E.; Smothers, W. K.; Mueller, W. B.; Charlton, J. L. J. Am. Chem. Soc. 1984, 106, 3144. (2) Gorman, A. A.; Beddoes, R. L.; Hamblett, I.; McNeeney, S. P.; Prescott, A. L.; Unett, D. J. J. Chem. Soc., Chem. Commun. 1991, 963.
Phosphorescence of 5H-Dibenzo[a,d]cycloheptene (3) Caldwell, R. A.; Riley, S. J.; Gorman, A. A.; McNeeney, S. P.; Unett, D. J. J. Am. Chem. Soc. 1992, 114, 4424. (4) Herkstroeter, W. G.; McClure, D. S. J. Am. Chem. Soc. 1968, 90, 4522. (5) Watkins, A. R.; Bayrakc¸ eken, F. J. Lumin. 1980, 21, 239. (6) Evans, D. F. J. Chem. Soc. 1957, 1351. (7) Bylina, A.; Grabowski, Z. R. Trans. Faraday Soc. 1969, 65, 458. (8) Lippert, E. Z. Phys. Chem. (Munich) 1964, 42, 125. (9) (a) Ikeyama, T.; Azumi, T. J. Phys. Chem. 1985, 89, 5332. (b) Ikeyama, T.; Azumi, T. J. Phys. Chem. 1994, 98, 2832.
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Go¨rner, H. J. Phys. Chem. 1989, 93, 1826. Saltiel, J.; Khalil, G.-E.; Schanze, K. Chem. Phys. Lett. 1980, 70, Huang-Minlon. J. Am. Chem. Soc. 1946, 68, 2487. Ikeyama, T.; Azumi, T. J. Phys. Chem. 1988, 92, 1383. Trætteberg, M.; Frantsen, E. B. J. Mol. Struct. (THEOCHEM) 1975,
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