J. Phys. Chem. 1984,88, 4641-4647
4641
Temperature-Dependent Phosphorescence of Deuterium-Substituted Xanthones J. D. Laposat and Richard Bramley* Research School of Chemistry, Australian National University, Canberra, A.C. T. 2601, Australia (Received: February 10, 1984)
Highly resolved phosphorescence spectra have been obtained and analyzed for three deuterated xanthones in n-hexane and n-pentane Shpolskii matrices. The temperature dependences of their multiple phosphorescencehave been interpreted in terms of thermal equilibrium between two close-lying triplets, TI and T,. In addition, the sublevels of TI are widely split, by as much as 17 cm-I; as a result yet a third phosphorescence is detected at low temperatures. Sublevel-selective spin-orbit coupling between TIand T, gives excellent quantitative agreement with the observed sublevel splittings. In n-hexane, TIis of mr* character, while in n-pentane, orbital inverstion takes place, and TI is nr*.
Introduction Spectra of solutes in Shpolskii matrices are a special case at one end of the glassy to dilute mixed crystal continuum. Ideally, the spectroscopist is looking for an isolated molecule or oriented gas spectrum, produced quickly by freezing a dilute solution. For many molecules this is often the easiest route to the oriented gas spectrum and it is generally more successful than searching for a solid host for single-crystal growth, e.g., by Bridgman techniques. A single site in the matrix is preferred but Shpolskii spectra range from this ideal through multiple sites' and solute clustering2 to complete separation of solute and solvent. But even from a single site, multiple emissions can occur; the most obvious example is both fluorescence and phosphorescence from the same molecular site. Less common is phosphorescence from different orbital states of the same molecular sitee3s4 Infrequent, but again well established, is the existence of multiple host phases in one frozen ample.^^^ The other possibility is emission from different molecular conformations in what is otherwise the same site. It may also be possible to place the same solute in the same matrix hole in more than one way. If these are not related by the Shpolskii solvent (crystal) symmetry, then different emissions can potentially be resolved. However, there are still further possibilities and it is these that are the subject of this paper. These cases arise in the following way. When the luminescence comes from triplet states and terminates on a singlet ground state (as in an excited aromatic molecule), the triplet multiplicity itself is a potential source of multiple emission. With microwave optical double resonance techniques it is possible to separate these out, but since optical separation (outside the optical line width) is not achieved by microwave-sized quanta, this is not being considered here. Splitting of this multiplicity arises first from spin-spin dipolar interactions of the unpaired electron^.^ Rarely will splitting from this source approach optical resolution. Spin-orbit interactions involving higher singlet states are an added source of splitting but this again rarely exceeds 1 crn-', although it can in principle and does, particularly for molecules containing second-row elements. Less appreciated is spin-orbit interaction between the sublevels of two close-lying orbitally distinct triplet states.8 Whereas the mechanism cannot be disputed: there seems to be reluctance to accept the consequences of such state mixing, which are, simply put, that such splittings can easily exceed the optical spectral bandwidths by 1 order of magnitude. It is these that have in several cases been interpreted in every way but by this simple mechanism. Some work on the multiple phosphorescences of xanthone has already been reported,'&l2 and the interpretation was advanced that these originate from both T2(a7r*) and TI(3aa*),as well as from sublevels of TI widely split by spin-orbit coupling with T2. We now present data from further systems which convincingly support the earlier contention. Of the four possible optically resolvable phosphorescences, we have observed three repeatedly. On sabbatical leave from McMaster University, Hamilton, Ontario, Canada.
These extra data using deuterated xanthones in alkane solvents are presented here. Deuterated xanthones were used to extend the study since it was known that deuteration can affect %T* and 3nr* state energies differe11t1y.I~ Although protonated xanthone has 3nr* and 3a7r*a mere 11 cm-' apart, in n-pentane, 3a1r*is still lowest. Host deuteration gave state inversion and thus completed the range of possibilities for two close-lying triplet states. Experimental Section Xanthone-ds ( X - d s )was prepared by catalytic deuteration of X-hs (EGA), according to the method of Fischer and Puza.14 A sealed tube containing the catalyst, D,O, and X-h8 was heated at 170 OC for 72 h; the deuterated material was purified by column chromatography, followed by recrystallization. Both mass-spectral and N M R evidence indicated that after three deuteration cycles well over 98% of the product was X-ds. 2,4,5,7-Tetradeuterioxanthone (X-d,) was synthesized from X-hs by the procedure of Werstiuk and Kadai.Is A sealed tube with X-hs in 0.25 M DCl was heated at 280 OC for 2 days; the crude produce was vacuum sublimed. N M R and mass-spectral evidence indicated about 90% of the product was X-d,. 1,3,6,8-Tetradeuterioxanthone( X - d i ) was produced by starting with X-ds in 0.25 M HCl. In this case 250 OC heating yielded the desired material in 92% purity. Spectral-quality n-hexane (Ajax) and n-pentane (EGA) were passed through short columns of alumina. Solutions (5 X lo4 M) of the deuterated xanthones in 4-mm 0.d. Pyrex tubes were subjected to several freeze-pumpthaw cycles and sealed under vacuum. The sample tubes were placed either in conventional helium flow tubes or in a liquid helium immersion cryostat. For achieving temperatures below 4.2 K, the space above the liquid helium could be pumped. The temperature of the sample was measured by a gold-chrome1 thermocouple above 4.2 K, and by vapor pressure determination above the liquid while pumping. The optical spectrometer has already been described.1° In all cases the excitation wavelength was centered at (1) E. V. Shpolskii, Sou. Phys.-Up. (Engl. Trans/.),3, 372 (1960); 5, 522 (1962); 6,411 (1963). (2) E. V. Shpolskii, L. A. Klimova, G. N. Nersesova, and V. I. Glyadkovskii, Opt. Spectrosc., 24, 25 (1968). (3) H. J. Griesser and R. Bramley, Chem. Phys. Lett., 83, 287 (1981). (4) R. E. Connors and W. R.Christian, J. Phys. Chem., 86,1524 (1982). (5) J. D. Spangler and H. Sponer, Spectrochim. Acta, 19, 169 (1963). (6) S. H. Hankin, 0. S. Khalil, and L. Goodman, Chem. Phys. Lett., 63, 11 (1979). (7) S. P. McGlynn, T. Azumi, and M. Kinoshita, "Molecular Spectroscopy of the Triplet State", Prentice-Hall, Englewood Cliffs, NJ, 1969, Chapter 9. (8) M. Batley and R. Bramley, Chem Phys. Lett., 15, 337 (1972). (9) H. F. Hameka in "The Triplet State", A. B. Zahlan, Ed., Cambridge University Press, London, 1967, p 1. (10) H. J. Griesser and R.Bramley, Chem. Phys., 67, 361 (1982). (11) H. J..Griesser and R. Bramley, Chem. Phys., 67, 373 (1982). (12) H. J. Griesser and R. Bramley, Chem. Phys. Lett., 88, 27 (1982). (13) A. DesprCs and E. Migirdicyan, Chem. Phys., 50, 381 (1980). (14) G. Fischer and M. Puza, Synthesis, 218 (1973). (15) N. H. Werstiuk and T. Kadai, Can. J . Chem., 51, 1485 (1973).
0022-3654/84/2088-4641$01,50/00 1984 American Chemical Society
4642
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984
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I
TABLE I: Vibrational Analysis of the Fundamental Region of the Tz(3nna*) So Phosphorescence Spectrum of X-dnin n-Hexane Raman frequency band i&/cm-' Av/cm-' re1 intensity and symmetry' I
II
I1
D
23 1
1 2
II
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
II
17 3 8.6(io6 )cm-'
Figure 1. Temperaturedependence of the 0-0 region of the xanthone-d8 phosphorescence in n-hexane. 3413 A, by using an interference filter. Spectra have not been corrected for the response of the detection system. Wavelengths of the phosphorescence emissions were calibrated by means of a hollow cathode Fe/Ne lamp and are quoted in vacuum wavenumbers. Accuracy of the band positions in the highly resolved spectra is estimated to be f 2 cm-I. Infrared spectra of X-ds in cesium iodide disks at room temperature were obtained by using Perkin-Elmer Model 225 and Hitachi Model F1S-3 infrared spectrometers. Raman spectra of solid and liquid X-ds were recorded by using a Spex Ramalog spectrometer with photon counting detection. Depolarization ratios were measured for the liquid spectra.
Results In both n-hexane and n-pentane Shpolskii matrices, sharp emission spectra were obtained for the deuterated xanthones. X-ds in n-Hexane. In n-hexane, X-ds is incorporated into a single site. The electronic transition is coupled weakly to the lattice, as evidenced by the lack of appreciable phonon bands. The appearance of the phosphorescence spectra varies greatly with temperature. Between about 77 and 10 K, the phosphorescence is dominated by a progression in the carbonyl stretch, which is typical of a %7r* emission. At about 10 K, the relative contribution of this spectrum decreases as a multitude of new bands begins to appear. At 4.2 K the 3n7r* origin band is still evident, but two new origin bands are present. The total emission spectrum at 4.2 K now consists mainly of vibrationally induced bands of m* character. The 0-0 region of the 4.2 K phosphorescence is given in Figure 1. Band 3, at 25 771 cm-I, the origin band of the T2(3n~*) Soemission, is 8 cm-' to higher energy from the onset of T1(37r~*) So emission a t 25 763 cm-I, shown as band 2. As the temperature is further lowered to 2.0 K, the phosphorescence spectrum again changes. Not only has band 3 of Figure 1 disappeared, but also band 2 has vanished. Only band 1 at 25 745 cm-' remains in the origin region; it is 18 cm-' to lower energy from band 2. As was shown for X-hs in n-hexane'O and in n-pentane,12 the growing in of band 1 and the loss of band 2 is due to thermal equilibrium within T1(37ra*),between TI, and the unresolved pair T1, and Tly. Spin-lattice relaxation (SLR) is evidently still efficient at 2.0 K; consequently, T1, is thermally depleted. Since there is no information as to whether T1, or TI, or both are emitting at 2.0 K, this emission is referred to as T1,y(3n7r*) So. Vibrational assignments to the T2(3n7r*) So, T1,(37r7r*) So,and T1,y(37r7r*) So phosphorescences have been carried out by examination of the temperature dependence of the high-resolution spectra. In Tables I and I1 (the latter as supplementary material-see note at end of text regarding supplementary material) are collected transition energies and vibrational frequencies
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Laposa and Bramley
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0 22 1 285 374 513 579 705 786 1036 1168 1322 1380 1580 1602 1651 1673 1675
25771 25 550 25 486 25 397 25 258 25 192 25 066 24 985 24 135 24 603 24 449 24391 24 191 24 169 24 120 24 098 24 095
1 0.03 0.01 0.05 0.02 0.01 0.02 0.07 0.01 0.01 0.03 0.02 0.04 0.08 0.1 0.4 0.8
251 a, 367 al 511 a, 573 782 1031 a] 1164 a] 1314 a, 1374 1571 1592 a] 1658 a] 1664 a] 1669 a,
'Frequency in cm-'. Symmetry from depolarization ratio of Ramad scattering of liquid; where not given, band too weak to allow unambiguous determination. in the fundamental region, and relevant infrared and Raman frequencies for comparison. The relative intensities of the vibronic bands, referred to the 0-0 band, are also given. These are somewhat uncertain due to base-line contributions from neighboring bands. As mentioned above, the T2(3n7r*) So emission has as its main spectral feature a long progression in the 1677-cm-' totally symmetric carbonyl stretch. This mode also forms combinations with many weak a l fundamentals (assuming C,, symmetry). The analysis presented in Table I is in excellent accord with that for T2(3n7r*) So for X-hs in n-hexane," bearing in mind expected deuterium shifts for vibrational frequencies. The T1,(37r7r*) So phosphorescence (see Table I1 in the supplementary material) is composed of a symmetry-allowed part and a weaker vibronically induced part. The vibronic origins at 275,567, and 694 cm-' have progressions of the carbonyl stretching mode built onto them, just as the 0 4 band shows this progression. N o overtones of these nontotally symmetric vibrations are observed. Relative vibronic activity is much weaker than for the corresponding transition for X-hs in n-hexane, where the vibronic origin at 67 1 cm-', for example, was 4 times as intense as the 0-0 band. This diminished vibronic activity is exhibited even though is smaller for X-d8 than for X-h8. The the T2-T1energy gap, UT, bands attributed to T1, Sophosphorescence disappear as the temperature is lowered to 2.0 K. The analysis of T,,,(3mr*) So phosphorescence is also presented in Table I1 (supplementary material). Some weak bands, less than 10%of the intensity of the origin band, are not included. The spectrum contains many nontotally symmetric vibronic origin bands of intensity comparable to the 0-0 band, as was found for X-hs in n-hexane. The dominant vibronic origin is at 695 cm-', like the situation for T1,(37r7r*) So. The other main vibronic bands here, at 154, 308, 805, and 838 cm-I, were not evident in the TI, emission. Only two of the bands below 1000 cm-l are definitely assignable to totally symmetric vibrations. No overtones of nontotally symmetric vibrations are seen. Progressions in the totally symmetric carbonyl stretch are built 011the origin band as well as on the vibronic origins. There is considerably more vibronic activity in the Tlxy So phosphorescence than for the TI, so. X-d, in n-Hexane. The phosphorescence of X-d, in n-hexane also shows marked variations with temperature. Above 20 K only the typical T2(3n7r*) So emission, described above for X-ds, is observed. The 0-0 band at 25 751 cm-I is only slightly blue shifted compared to the corresponding band in X-h, (25 746 cm-'), while it is red shifted by 20 cm-l compared to X-d, (25 771 cm-l). These isotope shifts, and ones reported hereafter, are shown in Figure 2. A detailed vibrational analysis of this emission is given
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Deuterium Effects on Xanthone Phosphorescence
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984 4643
25000 -
25750
25700
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I
3890
n- hexane
n-pentane
I
in n-hexane
I! II
4.2K/
Ii
I Ill
13 9 15 9 (to6 ) c m - '
Figure 3. Temperature dependence of the 0-0 region of the xanthone-d4 phosphorescence in n-hexane.
in Table I11 (supplementary material); it is nearly identical with that presented in Table I, except for only small isotope shifts in the weak bands. Again the carbonyl stretching vibration and its overtones overwhelm the spectrum. As the temperature is lowered to about 10 K, a second band appears in the origin region, at 25 735 cm-', and several new vibronic bands are evident. At 7 K, there are three origin bands, as shown in Figure 3. Band 3 is assigned to the T2(3n7r*) So onset, while band 2 is attributed to the origin of the T1,(37r7r*) So phosphorescence. AETis thus 16 cm-' (27 cm-' in X-hs and only 8 cm-' in X-de). In other words, the effect of partial and complete deuteration of X-hs on 3mr* is larger than on 3n7r*, with the result that the two triplet states approach closer to each other in energy with increasing deuteration. Figure 2 summarizes the situation for the deuterium shifts for xanthones in n-hexane. In Table IV (supplementary material) is found a complete vibrational analysis of the two low-temperature spectra. The three main vibronic bands in the TI,spectrum are at 275,573, and 730 cm-I. These relative vibronic contributions are 2-3 times as prominent for X-d4 when compared to X-ds. Band 1 in Figure 3 is the origin of the third phosphorescence Tl,y(37r1r*) SO,at 25 721 cm-'. The intensity of band 1 and the many vibronic bands accompanying this transition increase
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3874
8
as the temperature is diminished to 4.2 K. The major vibronic bands are at 160, 314, 524, 573, 729, and 808 cm-'. The analysis of the spectrum is very similar to the corresponding Tlxy(37r7r*) So emission in X-d, (compare Tables I1 and IV). Decreasing the temperature from 4.2 to 2 K does not lead to the disappearance of bands 2 and 3, as happened for X-ds. Apparently, SLR in X-d4 in n-hexane is slow at 2.0 K. Earlier results in our laboratory have pointed to high solvent sensitivity of such relaxation phenomena. X-d,' in n-Hexane. The temperature variation of the phosphorescence of X - d i in n-hexane is very similar to that for the 2,4,5,7 isomer (X-d4). At 20 K, the spectrum consists of the typical 3 n ~ * So emission, with an origin a t 25 766 cm-' (band 3 of Figure 4). A long progression in the carbonyl stretching mode is observed. The vibrational analysis of the fundamental region is given in Table V (supplementary material). At 7 K two additional bands are evident in the origin region, at 25 747 and 25 733 cm-'. The relative intensities of all three of these bands change with temperature. By 4.2 K band 1 at 25 733 cm-' is dominant, as shown in Figure 4. Further cooling to 2.0 K results in the loss of bands 2 and 3. In addition to the origin region behavior, as the temperature is decreased from 20 to 2 K, the vibrational detail of the phosphorescence alters dramatically as described above for the other deuterated xanthones. The vibrational analyses of the two phosphorescences based on bands 1 and 2 are contained in Table VI (supplementary material). Band 1 serves as origin for the T1xy(37r7r*) So phosphorescence, with vibronic bands at 156, 311, 617,670, 831, and 947 cm-I. At temperatures between about 10 and 2 K, the T1,(37r7r*) So spectrum, based on origin band 2 of Figure 4, is found with the major vibronic contributions at 289 and 671 cm-'. AET(3n~*37r7r*) is 19 cm-'. X-d4in n-Pentane. The phosphorescence of X-d, in n-pentane likewise is characterized by several spectra whose contributions change with temperature. At 19 K, for example, a band at 25 777 cm-I, denoted as band 2 in Figure 5, serves as the origin for a typical 3n7r* So spectrum, featuring a long progression in the 1675-cm-' carbonyl stretching mode. The analysis of the spectrum is identical with that in n-hexane (Table 111). However, under high resolution, a few new features are obtained at 19 K. First, in the 0-0 region, a band 3 is evident to higher energy, at 25 787 cm-I. This band is assigned to the origin band for T2,(37r7r*) So. As Figure 2 shows, such an attribution fits well with the X-hs larger solvent shifts for 37r7r* than for 3n3**when proceeding from n-hexane to n-pentane. Second, weak bands based on origin band 3 are discernible at 378, 517, 816, and 1675 cm-'. Other bands are too weak and broad to be assigned. Band 3 increases in intensity relative to band 2 as the temperature is increased. As the temperature is lowered to 4.2 K, band 3 and the weak bands based on it have almost completely vanished, while a new origin band 1 at 25 761 cm-' is present. There is much vibrational
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b
2
3890
Figure 4. Temperaturedependence of the 0-0 region of the xanthonedl phosphorescence in n-hexane.
Figure 2. Energies of the lowest 31r1r* and 3n1r*states of xanthone& -d4, -d4', and -dein n-hexane and n-pentane.
7K
3874A 2 0 2 9 (+0118 13 3 19 2 i t 0 61 cm-'
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4644
Laposa and Bramley
The Journal of Physical Chemistry, Vol. 88, No. 20, I984
Dw: H
D
D in n-pentane
4.2 K
l d,LU
4'2K I I
I)
3886
2'0K
' L
3d708 3886 2'0 (io1)A 13 I I i O 6 ) c m "
3870
a
Figure 7. Temperature dependence of the 0-0 region of the xanthone-d8 phosphorescence in n-pentane.
0 0 (io6 ) c m - '
15 9 l f 0 . 6 ) c m - '
Figure 5. Temperature dependence of the 0-0 region of the xanthone-d4 phosphorescence in n-pentane.
2
- -
I
4.2 K
14 K
3
I 12)
Figure 6. Temperature dependence of the 0-0 region of the xanthone-& phosphorescence in n-pentane.
structure in the spectrum accompanying band 1. At 2.0 K, in the origin region band 1 dominates, and only very weak traces of bands 3 and 2 remain. The prominent vibrational features of the 2.0 K spectrum, and their intensities relative to the origin band 1 in parentheses, are at 158 (0.3), 313 (0.7), 525 (0.4), 574 (0.4), 731 (0.9), and 809 (0.7) cm-'. Thus, there is great similarity between this low-temperature spectrum, assigned at T1xy(3na*) So and the Tlxy(3aa*) So spectrum in n-hexane. X-d4' in n-Pentane. The phosphorescence spectra of X-dd in n-pentane exhibit the same temperature behavior as does X-d4 in the same solvent. Figure 6 illustrates the origin region a t 14 K, with band 2 at 25 792 cm-I the origin of the Tl,(3na*) SO phosphorescence. The analysis of the spectrum is very similar to that in n-hexane, found in Table V (supplementary material). At higher energy 25 801 cm-I, a weaker origin band 3 appears that grows in intensity relative to band 2 as the temperature is raised. Band 3 is attributed to the onset of T2,(3ar*) So phosphorescence. As shown in Figure 2, the combination of solvent shifts (n-pentane vs. n-hexane) and isotope shifts on 3n1r* and %?r* results in T2 being of *a* character, while T1 is 3n7r*. The So phosphorescence has only a few discernible T2,(3a?r*) features, among which are the band at about 800 cm-' and the carbonyl stretch near 1670 cm-l. By way of contrast, the most intense vibronic band for X-d,' in n-hexane, %?r* z-sublevel emission, at 671 cm-I, is either weak or absent in n-pentane. The origin region at 14 K also contains a weak band 2', to lower energy of band 2. This band is most likely due to emission from
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a X-d3impurity, the major contaminant in the X-d,' sample. Such an impurity is expected to emit to lower energy than X-d,'. When the X - d l synthesis was carried out under milder conditions, which yielded a lower X-dd isotope purity but no X-d3, the phosphorescence of that sample contained no band 2'. As the temperature is lowered, a third origin, band 1, is observed at 25 775 cm-'. At 4.2 K this band has an intensity about 30% of band 2. At 2.0 K only band 1 remains. The phosphorescence prominent at 2.0 K is assigned to T1xy(3na*) So.Its vibrational structure bears a close resemblance to Tlx7(3a1r*) So for this molecule in n-hexane, even though the nominal nature of Tlxyhas altered. In n-pentane the main vibronic bands for the TI, emission are at 313 (0.7), 668 (0.8), 831 (0.9), and 949 (0.7) cm-I. X-ds in n-Pentane. The temperature variation of the phosphorescence of X-ds in n-pentane is less dramatic than those reported above. From about 70 to 10 K only one phosphorescence is present, with an origin band 2 in Figure 7 at 25 801 cm-'. This spectrum contains the long progression in the carbonyl stretch mode, and its analysis is basically identical with that presented in Table I for X-ds in n-hexane. As the temperature is decreased to 4.2 K, a new origin, band 1, appears at 25 788 cm-l. Many new vibrational features are obvious in the total spectrum at 4.2 K that were absent at 10 K. At 2.0 K, origin band 2 has almost completely disappeared, as well as the rest of the 10 K spectrum. The other bands which were present at 4.2 K remain. Even at 1.8 K, no new third origin band to the red of band 1 is observed. Vibrational analysis of the 2.0 K phosphorescence agrees well with So data for X-de in n-hexane, listed in Table I1 the T1, (supplementary material). Major vibronic bands are at 154 (0.4), 309 (0.6), 696 (l.O), 806 (0.4),and 841 (0.5) cm-I. With state reordering proposed already in proceeding from X-hs to X-d4 and X-d,' in n-pentane, further deuteration to X-d8 should place T2, now *a*, even further away from T1, na*. Thus, we assign band 2 to the origin of T1,(3n7r*) So phosphorescence, and band 1 as the 0-0 band for T1x,,(3na*) So. Although T2 may be thermally populated as the temperature is increased, concomitant spectral broadening and the inherently much weaker oscillator strength for 3?r?r* So combine to preclude observation of this emission.
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Discussion Multiple Phosphorescences in Xanthone. In all previously reported cases based on helium temperature studies, 3?ra* is the lower of the two close-lying triplet states in ~ a n t h o n e . ~ ~ ' ~ ~ ' ~ ~ ~ ~ ~ ' For X-d4,X-dd, and X-d8 in n-pentane, however, state inversion has been achieved, and 3na* is the lower triplet. When mr* is lower, three emissions occur. With na* lower and strong TI, Tz mixing, the third emission, at highest energies, is weak (absent for X - d s ) at temperatures where sharp spectra can be obtained. Our understanding of the phosphorescence of xanthone relies on two interactions. Without spin-orbit coupling no emission (16) A. Chakrabarti and N. Hirota, J . Phys. Chem., 80, 2966 (1976). (17) T. Terada, M. Koyanagi, and Y. Kanda, Chem. Phys. Lett., 72,408 (1980).
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984 4645
Deuterium Effects on Xanthone Phosphorescence
would occur at all. The other interaction is vibronic coupling. Both of these have been discussed in the earlier papers.'*'* The arguments are not repeated here, but the conclusion is that the dominant 3n7r* So emission in xanthone is expected to originate from the z sublevel. Thus, when 3n7r* is the second triplet state, only z-sublevel emission is expected, since the higher energy x and y sublevels (see below) carry a much smaller oscillator strength and also would require thermal population. For X-hs in a variety of matrices, where h7r* was the second triplet state, only z-sublevel emission was observed. Its main spectral characteristics are a strong origin band and a long progression in the carbonyl stretching mode. However, when 3n7r* is the lowest triplet state, and the x and y sublevels are widely split to lower energy due to spin-orbit coupling with appropriate T2 sublevels, thermal depletion of TI, can result in low-temperature phosphorescence from h7r*TI, and TlY.Such multiple emissions from TI 3n7r* have been reported for benzophenones,' but there deviations from planarity were important. In X-hs, no evidence exists for nonplanar distortions from C,, symmetry. For 37r7r* emission of xanthone, three proposed intensity-producing mechanisms for T, So all involve n7r* states. First, in direct spin-orbit coupling, T,(3A2) can mix with the 'A& ha* state. Second, T, can spin-orbit couple to h ? r * states that in turn are vibronically coupled to 'mr*states. Third, T, can vibronically couple to higher triplet states, which are spin-orbit mixed with singlet states. In the T 1 , ( 3 ~ ~ * )So phosphorescence of X-hs, the spectrum contained both totally symmetric fundamentals, interpreted as appearing due to direct spin-orbit coupling, and nontotally symmetric vibronic origins, indicative of the vibronic/spin-orbit pathways. In the case of 37r7r* Tx(3B2)and TJ3B1) for xanthone, direct spin-orbit couplings with higher energy 'B2 Imr* and 'B, ' U T * states are not considered to be important, and vibronic/spin-orbit pathways are invoked. In the T 1 x y ( 3 ~ a * ) So phosphorescence of X-hs in n-hexane and n-pentane, many intense vibronic origin bands were observed. In summary, with 37r7r* lowest, three phosphorescences are So, Tl,(a7r*) So, and observed in xanthone: T2,(n7r*) Tlxy(7r7r*) So. Their oscillator strengths are in the relation T2, >> TI, > Tlxy.As the temperature is lowered, first T,, and then TI, can be depleted. For 3n7r* lowest, three phosphorescences are also seen: Tz,(7r7r*) So, Tl,(n7r*) So, and Tlx,(nx*) So. The strongest of these transitions is from TI,. Even so, the weaker higher energy T2, emission is observable. Only at sufficiently low temperatures, where TI, is significantly depopulated, can Tlxy emission be detected. Spin-Orbit Coupling between Triplets and Zero-Field Splitting. Regardless of the states which are mixed by some small perturbation, spin-orbit coupling here, mixing depends on the state separation. It has so far been entirely satisfactory in the discussion of zero-field (fine-structure) splittings of low-lying triplet states in aromatic carbonyl compounds to restrict discussion to states within, say, 3000 cm-' of each other. This excludes excited states involving u and u* molecular orbitals and in the case of carbonyls restricts discussion to excited states of n7r* and m*orbital symmetries. At the one-electron integral level, the comonent of the spin-orbit coupling which is parallel to the C=O bond mixes the z sublevel of a triplet with a singlet state and the x and y sublevels with the y and x sublevels of a triplet state of different orbital symmetry.18 With xanthone, it is possible to have the lowest 3n7r* and 3 i r ~ * states so close, that z-sublevel mixing with ha* and 'm* states can be ignored. Furthermore, sublevel splitting due to electron spinspin dipolar interctions and Franck-Condon factors can also be disregarded since the effects noted in this study exceed these by at least 1 order of magnitude. The discussion is therefore particularly simple and revolves round the mixing of the x sublevel of 3n7r* with y of 3 ~ 7 r *and vice versa. In this approximation the z sublevels of these two orbital states are unperturbed and represent their electronic origin
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NO T - T
With T - T spin-orbit coup1 ing
spin-orbit coupling
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(18) J. W. Sidman, J . Chem. Phys., 29, 644 (1958).
2Y = E(T2,)-
Figure 8. Effects of triplet-triplet spin-orbit coupling on the zero-field sublevels of the lowest triplet states of xanthone.
positions. The mixing problem for the six sublevels involved can therefore be expressed in the form of a diagonally blocked 6 X 6 matrix of two 1 X 1 and two 2 X 2 submatrices. The matrix has the form
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E(TI,]
Tlx
T2y
Tlz
T2z
T1.y
T2x
T1 x
T
2Y
T1
z
T2z
T
1Y
T2x
where X i s equivalent to (Tlx~Hs,,~T2y) = (TlylHsolTtx) and -Y and Yare the unperturbed energies of TI and T,, respectively. Solution of the characteristic equation of this matrix yields the (twice), -Y, and Y. The energy-level energies *[A? + p]1/2 pattern with spin-orbit coupling included is shown in Figure 8. The TI, and Tlysublevels are depressed, relative to T1,, while T2x and Tzqare raised by a corresponding amount, relative to T2,, If this simple scheme obtains, then provided the spectroscopic origin bands are correctly assigned, the matrix element X should be the same in all cases. There are now seven cases where three of the four optically resolvable states have been observed and assigned and this is sufficient within the approximation being used since Tzxand TZywill be separated identically from T2zas TI, and TI, are from T1,. Table VI1 lists these results. Data were taken from Figures 1 and 3-7 and ref 10 and 12. It can be seen that agreement is excellent with the same matrix element of 20.3 1.7 cm-'.
+
*
4646
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984
TABLE VII: Energy Separations and Spin-Orbit Coupling Matrix Elements for Xanthones
solute x-dg
CnHZn+Z solvQ n=5
X-dg X-d4 X-d4 X-d: X-di X-hg X-hs X-hg
n=6
n=5 n=6
n=5 n=6 n=5 n=6 n=7
zfsb/cm-' 13.1 f 1.2 17.3 f 1.2 15.9 f 1.2 13.9 f 1.2 16.6 f 1.2 13.3 f 1.2 15.1 f 0.6 1 1 .O f 0.6
hETC/cm-'
( 11 )dlCm-'
8.6 f 1.2 10.0 f 1.2 15.9 f 1.2 8.7 f 1.2 19.2 f 1.2 10.8 f 0.6 26.7 f 0.6 71
21.2 f 1.7 20.3 f 1.7 20.4 f 1.7 20.5 f 1.7 20.8 f 1.7 19.8 f 0.9 20.4 k 0.9
QC5H,2= n-pentane, C6HI4= n-hexane, C7HI6= n-heptane. C2Y. dXcalcd.
+ P]I/Z- Y.
[p
The table can now be used in reverse, to predict where states with low transition probabilities lie. In this way, Tlxyfor X-hs in n-heptane can confidently be predicted to lie 5 cm-I below TI,. Similarly, for X-d8 in n-pentane, T,, will be 18-1 9 cm-I higher than the dominating TI,. Phosphorescence of Xanthones in n-Hexane. For the deuterated xanthones in n-hexane, the appearances of the high-temperature phosphorecences and their analyses are in very good agreement with X-h8. The Tz('na*) Sonature of this emission seems well established now.l9 The presence of this emission down to 4.2 K is possible due to the diminished AET compared to X-hs. There So emission was observed below about 5 K. no T2('n7r*) As temperature is lowered, population of T1,(3a7r*) grows at the expense of T2,(3n7r*). The T1,(37r7r*) So phosphorescence in all three deuterated xanthones exhibits similar spectral characteristics as for X-hs in n-hexane. Both totally symmetric and nontotally symmetric vibrations appear in the spectra, but the relative intensities of the corresponding vibronic origins vary through the isotopic series. Although we have not yet completed our vibrational studies for the deuterated xanthones, it seems plausible that for X-h,/X-d,/X-d,l/X-d,, the following bands are related: 123/130/120/694,611/513/611/561,288/215/289/215 crn-'. In T1,(3~7r*) So for the first three of these xanthones, the vibronic origin at 610/513/611 cm-l is the dominant feature, while for X-d8 the 694-cm-' band is most prominent. Also, the intensity relative to the 0-0 band for this vibronic band is smaller for all the deuterated xanthones, compared to X-hs. If we assume that the intensity of the 0-0 band, derived mainly via direct 1000 cm-' to higher energy spin-orbit coupling with h a * S,, from TIZ,does not change significantly with deuteration, then the vibronic contribution to the transition has decreased with deuteration. The main candidates for spin-orbit/vibronic pathways for the vibronic components of T1,(37r7r*) Sohave been outlined previously" and can involve az, and/or b,, and/or b2 vibrations. For X-hs in a KBr disk, infrared data show a weak band at 288 cm-'; its intensity seems high for an a2 assignment, even when taking into account the relaxation of isolated-moleculeforbiddeness due to the perturbing effects of the environment. From the polarized infrared analysis of a polycrystalline film of X-hg," a 295-cm-' band was assigned as bl. In the same study, a band at 673 cm-I was called a bl fundamental, although the experimental data did not allow the determination of the direction of the dipole moment of the transition, as had been possible for several other bl modes. For the deuterated xanthones, the same authors reported infrared results that, unfortunately, had contributions from a mixture of seven deuterium isomers.z1 Our infrared data for X-d8 imbedded in CsI show a very weak band at 696 cm-I; a very weak Raman band appears at this frequency as well. In any event, the identity of the symmetry species of the vibronic origins in the
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(19) R. E. Connors and P. S. Walsh, Chem. Phys. Lett., 52,436 (1977). (20) V. G. Klimenko, E. A. Gastilovich, and D. N. Shigorin, R u m J. Phys. Chem. (Engl. Transl.), 53, 329 (1979). (21) E. A. Gastilovich, V. G. Klimenko, T. S. Kopteva, A. I. Serebryanskaya, I. 0. Shapiro, and D. N. Shigorin, Russ. J . Phys. Chem. (Engl. Transl.), 55, 674 (1981).
Laposa and Bramley
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deuterated xanthone phosphorescence T1,(37rn*) So is still unknown. It is difficult to consider a major perturbing influence from TZz,since the vibronic intensity of T1,(37r~*) Sodecreases as the Tzz-Tlzgap is lowered from X-hs to X-d8. The third, and lowest temperature phosphorescence for the deuterated xanthones has been assigned to T1x,(37r7r*) So. The separation between the origin band for this transition and that involving TI, varies from X-hs to X-ds and increases as AET decreases. Such a pattern is predicted for spin-orbit coupling involving TI and Tz, as discussed above. Recently Connors and Christian4 explained the temperature dependence of the phosphorescences of X-hs and X-ds in n-hexane in terms of thermal equilibrium between the two lowest triplet states T , ( 3 n ~ * )and T1(3~7r*).Their lower resolution results, however, did not detect the splitting between Tz,(3n~*)and T1,(37r7r*) in X-ds, which is 9 cm-I. Our data indicate that the temperature dependence of this phosphorescence is more complicated and that there are three thermally equilibrated emissions, not two. Furthermore, two of the three emissions originate from different sublevels of the same triplet state, which are widely split by spin-orbit coupling between TI and T2. The main feature of the Tlxy(37r~*) So phosphorescences is the prominence of many vibronic origin bands. At this time we do not attempt to relate the vibronic bands from one compound to another, but note that the relative intensities of the vibronic contributions have decreased in the deuterated xanthones. Also most of the vibrational modes active in the T1xy(37r7r*) So transition are absent in T1,(37r?r*) So, as for X-h8. With extensive spin-orbit coupling between T,, and TZy,as well as for TI, and TZx,these sublevels are no longer pure 37r7r* and 'n7r*. Rather, the wave function describing T1 is of the form
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J/T,
= aJ/(37r7r*)+ bJ/(3n7r*)
where
a2
+ bZ = 1
The value of b is obtained from
+
b = { X / [ Y (XZ
+ y2)1/2])a
For X-ds we calculate from Table VI1 that TI, and TI, are 60% 40% 3 n ~ *due , to spin-orbit coupling. In X-d,, the orbitally mixed TI sublevels have 68% %7r* and 32% character, while in X-d,' the mixed natures are 10% 37r7r* and 30%3n7r*. TI,, on the other hand, is not affected by the interaction. Phosphorescence of Xanthones in n-Pentane. The differences encountered between the phosphorescence of the deuterated xanthones in n-pentane, as opposed to n-hexane, stem from the greater shift to higher energy with deuteration for 37r7r* than for 'na*. For n-hexane, the 3 n ~ * - - 3 ~ 7gap r * diminished from 21 (X-h,) to 9 (X-d,) cm-I. That the 11-cm-' spacing between ' n ~ * and 'm* for X-hs in n-pentane actually decreases so that the two states are inverted in the deuterated xanthones is evident from Figure 2. In the partially deuterated xanthones, the 3 n ~ * Soemission is accompanied by a high-energy origin band that decreases in intensity as temperature decreases. To argue that this high-energy Soorigin band, with the dominant band corresponds to a T,, 3n7r* phosphorescence assigned to TZz So, leaves only one emission, rich in vibronic origins, to be accounted for. In such a case Tlly(37r7r*) So would be the obvious choice, by comparison with the n-hexane results. Then one is faced with explaining the absence of T1,(3a7r*) So, which has a greater oscillator strength than Tlxy. Not only are the assignments that we propose, T z , ( 3 r ~ * ) So, T1,(3nr*) So, and T1,,(3na*) So, for X-d, and X-d,' in n-pentane in agreement with the isotope shifts seen in n-hexane for the deuterated compounds, but more importantly, the calculated values for the spin-orbit coupling matrix element mixing TI, and TZyare identical, within experimental error, to values for 3ir7r* and
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J . Phys. Chem. 1984,88, 4647-4649 the four xanthones in n-hexane and X - h , in n-pentane. For X-d, in n-pentane, the inverted triplet states, now T2(3a7r*) and T,(3n7r*), are even further split, and only T 1 , ( 3 n ~ *phos) phorescence is observed until T1, is depopulated sufficiently to allow T,xy(3n7r*) So emission to be observed. We do not detect any T2 emission and consequently are unable to calculate a spin-orbit coupling matrix element. If other assignments for these two emissions are examined, difficulties arise. With a proposal that the emissions correspond So and T,,(3~7r*) So, the observed lowest to T,,(3n7r*) temperature spectrum does not resemble that for TI, emission. Also, if AET is only 13 cm-', then TI, should be widely split, and observable at low temperatures. Such is not the case. If the two emissions are assigned T2z(3n7r*) So and T1x,(37r7r*) So, one must somehow account for Tlz; even if TI, for some unknown reason lost its radiative strength, it should be sandwiched between Tzzand Tlxy.But then any calculated spin-orbit coupling matrix element would not fit. Besides, the deuterium shifts observed for xanthone in n-hexane do not favor these alternative assignments. The prominent features of the Tl,(3n7r*) So phosphorescence of the deuterated xanthones resemble all the other z-sublevel 3n7r* So emissions in other Shpolskii matrices. The appearance of T,x,(3n7r*) So phosphorescence, in all three deuterated xanthones in n-pentane, and the vibrational asssignments are very similar to those seen for the same xanthones in n-hexane, where TI, was %r7r*. This is not surprising, however, if it is remembered that in X-d4 in n-hexane, for example, TlXhas 68% 37r7r* and 32% 3n7r*character, while in the nominal 3n7r*TlXstate in n-pentane, TlXis calculated to have 38% m*character. In other words, the TI"? So phosphorescence appears the same, whether TI is 37r7r* or nT*, since the spin-orbit induced mixed character of Tlxor TI, is not radically different in the two instances, with AET so small. Finally, we note than in n-pentane it is possible to have emissions from a second site, as mentioned in the Introduction, and reported earlier for X-hg.l2 For example, Figure 5 shows the presence of
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4647
a weak band 4 in the phosphorescence of X-d4,whose intensity relative to the major bands in the origin region varied markedly with the cooling method employed. No such effect was seen for n-hexane. Band 4' may be due to X-d, in yet another site, or it may be an impurity emission. In any event, the appearance of such low-intensity bands does not alter any of the main conclusions regarding the emission behavior of xanthone. Concluding Remarks. The temperature-dependent phosphorescences observed for deuterated xanthones in two polycrystalline matrices are interpretable on the basis of emission from three levels. These are the widely split sublevels of T,(optically resolvable into T1, and the nonresolved pair T,, and TI,), and T2 (potentially resolvable but with only T2, observed). Thermal equilibrium among these three levels accounts for the temperature dependences of the emissions. The sublevel separations are accounted for by Tl-T2 spin-orbit coupling. The matrix element for this interaction is the same for four isotopic variants of xanthone in both n-pentane and n-hexane. This constant matrix element must now leave no doubt that our contention is correct. The extent of zfs in the TI state of xanthones depends on the magnitude of AET, which varies not only with solvent, but also with deuteration. This isotope effect alters AET since 37r7r* exhibits a larger blue shift than b 7 r * . In the deuterated xanthones in n-pentane, the shifts are so large so as to render T1 3n7r*.
Acknowledgment. We thank Mr. B. E. Williamson for recording the Raman spectra and are indebeted to Dr. N. H. Werstiuk of McMaster University for suggesting the synthetic route to the partially deuterated xanthones. Registry No. X-d,, 8 1066-40-2; X-d4, 78797-49-6; X-d.,', 91409-20-0.
Supplementary Material Available: Tables 11-VI, giving the vibrational analyses of the fundamental regions of the phosphorescence spectra of deuterated xanthones in n-hexane (TI X-d,, T2X-d4,T, X-d4,T2X-d,', and T1X-d,', respectively) (8 pages). Ordering information is given on any current masthead page.
Core Binding Energies of the Boron Trihaiides, Lewis Acidities of the Boron Trlhaiides, and Heats of Formation of Carbonium Ions David B. Beach and William L. Jolly* Department of Chemistry, University of California, and the Materials and Molecular Research Division, Lawrence Berkeley Laboratory, Berkeley, California 94720 (Received: February 22, 1984)
The core electron binding energies of the boron trihalides have been redetermined. The data are used, in conjunction with literaturevalence ionization potentials, to establish the extent of halogen-boron 7r bonding and, in conjunction with thermodynamic data, to determine the core replacement energy of carbonium ions.
The core binding energies of the boron trihalides were determined with absolute uncertainties of f0.1-0.2eV about 13 years Because of the importance of these data in the study of halogen-boron 7r bonding3 and in the evaluation of the core replacement energy4s5of carbonium ions, we have redetermined the data with probable uncertainties, in most cases, of *0.05 eV. We discuss the pertinence of the results to the trend in Lewis acidity (1) Allison, D. A.; Johansson, G.; Allan, C. J.; Gelius, U.; Siegbahn, H.; Allison, J.; Siegbahn, K. J. Electron Spectrosc. Relat. Phenom. 1972,l. 269. (2) Finn, P.; Jolly, W. L. J. Am. Chem. SOC.1972, 94, 1540. (3) Bassett, P. J.; Lloyd, D. R. J. Chem. SOC.A 1971, 1551. (4) Jolly, W. L.; Gin, C.; Adams, D.B. Chem. phys. Lett. 1977,46,220. ( 5 ) Jolly, W. L.; Gin, C . Int. J. Mass Spectrom. Ion Phys. 1977, 25, 27.
TABLE 1:
coreBinding ~~~~~i~~ (ev)of the B~~~~ Trihalides halogen coreu
B 1s
BF? BC~,
BBr3 B13
E, 202.85 (3)' 199.98 (sj 199.0d 197.92 (5)
fwhmb 1.47 (91 1.71 (20) 1.21 (18)
E, 694.94 (2) 206.84 (3j 76.57 (3) 626.82 (2)
fwhm 1.61 (4) 1.34 (6j 1.48 (8) 1.32 (7)
"Fluorine Is, chlorine 2p3/2,bromine 3dS12.'iodine 3d5/,. *Full width
~ ~ t f ~ ~ ~ ~ ~ ~ ~ h ~
~least-
the boron halidesw and to the estimation of heats of formation of carbonium ions. Of
0022-3654/84/2088-4647$01 .50/0 0 1984 American Chemical Society
~
