1524
J. Phys. Chem. 1982, 86, 1524-1528
&(t$) = [~11/2(O;R)]e-Botz[&1/2(O;R)]T
(6.6)
B O W = OI-~/~(O;R)~~,~(R) [P1-"2(o$)]T
(6.7)
where and where P,(O$)
Figure 8. Rdependent friction kernel for I, in CCI,. The parallel component of the friction kernel in (picosecond)-' is plotted vs. x = R/R, and vs. time in picoseconds. Results are well described by matrix Gaussian fit of eq 6.6.
most of the plotted quantities for x 1 1. This suggests that the SA can often be used to give an inexpensive estimate of the MTGLE parameters. We next turn to the R-dependent friction kernel &(t$). Analysisg shows that only eight distinct nonvanishing components of &(t$) exist. No other rigorous simplifications hold. MD calculationsgshowed, however, that for the present system only the fully diagonal parallel, [&@$)I = [/31(t$)111zz = ..., and perpendicular, [&(t$)11 = [j911(t$)]11zx= ..., components are nonnegligible. The results of the MD calculations may be summarized by the observationgthat B1(t$) may be accurately represented by the matrix Gaussian fit
e ue,2(R)
(6.8)
Equations 6.6-6.8 show that &(t;R)is approximately determined by the short-timescale parameters w,~(R)and uc,4(R).Thus the full matrix chain, including truncation parameters, is approximately determined by ue,2(R), uCl4(R). These parameters, in turn, depend only on equilibrium cage s t r u ~ t u r e .Thus ~ one may build models for the full solvent influence on reagent dynamics which depend only on equilibrium cage structure. This is a valuable result since a basic goal of liquid-state reaction theory is to predict how chemical rates and detailed dynamics vary with changes in solvation shell structure as thermodynamic state, solvent composition, etc. vary. Finally a three-dimensional plot of [/31(t$)]11 is given in Figure 8. The main features of that plot follow from eq 6.5-6.7 and the R dependence of uW2(R)and uc,4(R)(Figure 7). Acknowledgment. Support of this work by the National Science Foundation under Grants CHE 7828043 and CHE 8018285 and by the Purdue NSF-MRL program under Grant CHE 7828063 is gratefully acknowledged. We thank Drs. M. Berkowitz, P. K. Swaminathan, and M. Balk for many helpful discussions. We also thank Dr. David Ceperley for supplying us with the CLAMPS molecular dynamics package.
ARTICLES Origin of the Unusual Triplet-State Properties of Xanthones Robert E. Connors" and Wllliam R. Chrlstlan Department of Chemistry, Worcester Polytechnic Institute, Worcester, Massachusetts 0 1609 (Received: July 20. 1981; In Final Form: November 30, 1981)
Phosphorescence spectra and lifetimes have been measured for xanthone, xanthone-d8, and 3,6-, 2,6-, and 2,7-dimethylxanthonein n-hexane at 4.2 and 77 K and at higher temperatures. The spectra and lifetimes are found to be temperature dependent. We interpret these results in terms of emission from T1(mr*)at low temperatures and thermally populated emission from T2(n7r*)at higher temperatures. This study provides a basis for explaining many of the unusual triplet-state properties that have been reported for xanthones. Introduction The triplet-state propetties of xanthone and its substituted derivatives have been of interest to spectroscopists for a number of years. This attention can be attributed in part to the high sensitivity that these properties show toward solvent and temperature. When dissolved in 3methylpentane (3-MP) at 77 K, xanthone exhibits a relatively short-lived (25 ms) phosphorescence of mixed polarization. The main vibronic activity consists of a strong progression in the carbonyl stretch frequency with polar0022-3654/82/2086-1524$01.25/0
ization along the carbonyl axis (2). The remaining bands in the spectrum are polarized along the long axis of the molecule Cy). When the polar solvent EPA (ethanol-isopentane-ether, 5:2:2) is used, the vibronic features of the phosphorescence spectrum are considerably different from those in 3-MP, and the phosphorescence lifetime is 150 111s. The phosphorescence in EPA is polarized exclusively along the long axis of the molecule. Pownall and Huberl have (1) H. J. Pownall and J. R.Huber, J. Am. Chem. SOC.,93,6429 (1971).
0 1982 American Chemical Society
Triplet-State Properties of Xanthones
I 5. NON-POLAR SOLVENT
POLAR SOLVENT A.
I
Figure 1. Effect of solvent polarity on T, and T, for xanthone at 77 K, (A) as proposed in ref 1 and (B) as proposed in this work.
interpreted these results in terms of a state reordering in going from a nonpolar (3-MP) to polar (EPA) solvent. They have suggested that the TI state of xanthone is %a*) in 3-MP and 3(aa*) in EPA at 77 K. This assignment is consistent with the well-known observation that na* transitions blue shift in polar solvents.2 Their assignment is depicted in Figure 1A. These authors have also found that the triplet-state decay of xanthone in both 3-MP and EPA is nonexponential. A small amount of the long-lived emission that is dominant in EPA is observed in 3-Mp and a small amount of the short-lived species that is observed in 3-MP is found in EPA, resulting in dual phosphorescence. In subsequent work it was shown that the z-polarized bands for xanthone in 3-MP decrease in intensity as the temperature is taken below 77 K.3l4 The phosphorescence spectrum at 2 K is identical with the minor component found in 3-MP at 77 K (dominant in EPA) and decays with a lifetime of 115 ms. In seeking an explanation for these observations, Pownall and Mantulin4 have suggested that two different ground-state conformations exist for xanthone in 3-MP and that the preferred conformation changes with temperature by some unspecified mechanism related to solute-solvent interaction. Pownal16 has examined the triplet-state properties of xanthone and its 3,6-, 2,6-, and 2,7-dimethyl derivatives in 3-MP and 1:l etherethanol glasses a t 77 K. He found that the triplet-state lifetime in 3-MP varied by 1order of magnitude for these compounds. For example, 2,7-dimethylxanthone has a lifetime of 250 ms in 3-MP compared to the 25-me lifetime of xanthone. Considerable solvent effects were also observed for this series in terms of phosphorescence spectra, lifetimes, and polarization. These results were interpreted in terms of quantum-mechanical mixing of na* and aa* character in T,. The relative amount of mixing was postulated to depend on solvent and position of dimethyl substitution. In 1977, Connors and Walsh6 reported the phosphorescence properties of xanthone in n-hexane at 4.2 and 77 K. The 4.2 K phosphorescence spectrum exhibits considerable vibronic activity and a lifetime of 115 ms. The spectrum and the lifetime change dramatically at 77 K. A progression in the carbonyl stretch frequency with an average spacing of 1670 cm-l is the dominant feature of the 77 K spectrum. The emission is short-lived, having a lifetime of 2.5 ms. The emitting species at 4.2 K was (2) M. Kasha in "Light and Life", W. McElroy and B. Glass, Eds., Johns Hopkins University Press, Baltimore, MD, 1955. (3)H. J. Pownall, R. E. Connors, and J. R. Huber, Chem. Phys. Lett., 22,403 (1973). (4)H. J. Pownall and W. W. Mantulin, Mol. Phys., 31, 1393 (1976). (5) H. J. Pownall, Mol. Photochem., 6,425(1974). (6)R.E.Connors and P. S. Walsh, Chem. Phys. Lett., 53,436(1977).
The Journal of Physical Chemistry, Vol. 86, No. 9, 1982 1525
assigned as predominantly 3(aa*)in character, whereas the phosphorescence observed at 77 K was assigned as emission from a 3(na*) state. No attempt was made to assign the origin of this temperature dependence. Approximately 1year later Klimenko, Gastilovich, and S h i g ~ r i reported n~~ similar spectral data. They suggested that the nature of T1changes from na* to aa* for xanthone in n-hexane as the temperature is lowered from 77 to 4.2 K. In a subsequent p~blication'~ they attributed this inversion in orbital character of T1to a change in solute-solvent interaction brought about by a thermally activated change of orientation (hindered rotation) of xanthone molecules within the crystal lattice of the solvent. Most of the previous investigation^'^^-^*^ of the triplet-state properties of xanthones have explicitly assigned the phosphorescence to emission originating from the lowest triplet state T1, in accord with Kasha's rule.6 However, Despres et al.9and Mao and Hirotalo have demonstrated that thermally activated emission from T2 can be observed for certain aromatic carbonyl compounds in mixed crystals. We decided to undertake a study to see whether their observations relate in any way to the unusual triplet-state properties of xanthone and its substituted derivatives. In this paper we report phosphorescence spectra and lifetime data at 4.2 K and higher temperatures for xanthone and its 3,6-, 2,6-, and 2,7-dimethyl derivatives in n-hexane. It is shown that the unusual results mentioned above can be interpreted in terms of emission from two closely spaced triplet states. Under the appropriate conthe ditions of temperature and Tl-T2 energy gap (=,), emission observed from the xanthone compounds can be entirely from Tl(aa*), from T1 and T2with comparable intensity, or almost entirely from T2(na*).
Experimental Section The spectra were recorded with a McPherson Model 218 0.3-m monochromator. Excitation was provided by a 200-W Hg-Xe arc lamp. Phosphorescence decay curves were accumulated on a Tracor Northern Model TN-1505 signal averager. Temperatures above 77 K were obtained by passing cold nitrogen gas over the sample which was situated in a custom-built quartz Dewar system. Xanthone was recrystallized twice from ethanol. The dimethylxanthones were obtained as gifts. Spectral-grade n-hexane (MCB) was used as received. Results and Discussion Temperature Dependence of Phosphorescence Spectra. Figures 2-5 show the phosphorescence spectra of xanthone, 3,6-dimethylxanthone, 2,6-dimethylxanthone, and 2,7-dimethylxanthone in polycrystalline n-hexane at 4.2 K and higher temperatures. The spectral features for each compound change significantly over the range of temperatures shown. It is seen from Figure 2 that the 4.2 K phosphorescence spectrum of xanthone is rich in vibronic structure. A progression in the carbonyl stretch frequency can be identified along with a number of vibrations involving ring modes, including out-of-plane vibrations." The 0-0band is relatively weak compared to the dominant vibronic feature that is found 670 cm-' to the red of the origin. This vibration corresponds to the 673-cm-' mode (7) (a) V. G. Klimenko, E. A. Gastilovich, and D. N. Shigorin, Russ.
J.Phys. Chem. (Engl. Trawl.), 52,1703(1978);(b) ibid., 54,215(1980).
( 8 ) M. Kasha, Discuss. Faraday Soc., 9,14 (1950). (9)A. Despres, E. Migirdicyan, and L. Blanco, Chem. Phys., 14,229 (1976). (10) S. W. Mao and N. Hirota, Mol. Phys., 27,327 (1974). (11)V. G. Klimenko, E. A. Gastilovich, and D. N. Shigorin, Russ. J. Phys. Chem. (Engl. Trawl.), 53,329 (1979). (12)A. Chakrabarti and N. Hirota, J. Phys. Chem., 80, 2966 (1976).
1526
The Journal of Physical Chemistry, Vol. 86, No. 9, 1982
Connors and Christian
ab
4800
4550
4300
4050
3800
A Figure 2. Phosphorescencespectra of xanthone in n-hexane at 4.2 and 77 K.
observed by Klimenko et al." using infrared spectroscopy. They have assigned this fundamental as the out-of-plane bl vibration. A more complete vibrational analysis of the compounds discussed in this paper as well as assignment of the mechanisms that promote conversion between the singlet and triplet manifolds will be given in a future publication. However, it is evident from what has been said already that a vibronic coupling mechanism appears to contribute to the radiative decay of xanthone at 4.2 K. A phosphorescence lifetime of 115 ms was measured at the 0-0 band. As noted in a previous publication from our laboratory,6 xanthone in n-hexane undergoes a dramatic change in emission characteristics when the temperature is raised from 4.2 to 77 K. A progression in the carbonyl stretch frequency with an average spacing of 1670 cm-' dominates the spectrum a t 77 K. The lifetime, measured at the 0-0 band, is approximately 2.5 ms. We interpret these results in terms of emission from T1, which is predominantly 3 ( ~ ~ in * ) character, and thermaly populated emission from T2,which is 3(na*). At 4.2 K only emission from T1 (m*) is observed. As the temperature is increased, T2(na*) becomes populated and emission from this state can be observed. The dominance of T2emission for xanthone at 77 K is consistent with the extremely small A E T value of 29 cm-l, which was determined from the separation between 3(n7r*)and 3(7r7r*) 0-0 bands, and the fact that 3(n7r*) radiative rates are much larger than those for 3(a7r*) states.'O Figure 3 shows the phosphorescence spectrum of 3,6dimethylxanthone in n-hexane at 4.2, 77, and 164 K. As in the xanthone case, the emission spectrum changes considerably with temperature. A t 4.2 K a complex vi-
4700
4450
4200
3950
a Figure 3. Phosphorescence spectra of 3,6dimethylxanthone in nhexane at 4.2, 77, and 164 K. The 0-0 band of the major site at 4.2 K is indicated with an arrow.
bronic pattern is observed (all of the dimethyl derivatives exhibit multiple site structure at 4.2 K which further complicates the spectra). The spectral features and the 44-ms lifetime indicate emission from a state that is predominantly 3(a7r*)in character. As the temperature of the sample is raised, bands corresponding to a progression in the carbonyl stretch appear. At 77 K the phosphorescence spectrum is a combination of emission from Tl(aa*) and T2(n7r*),with T2more dominant. Raising the temperature to 164 K increases the n7r* characteristics of the emission spectrum. A AET value of 55 cm-' is obtained from the spectra. The phosphorescence spectra of 2,6-dimethylxanthone in n-hexane at 4.2,77, and 133 K are presented in Figure 4. The 4.2 K spectrum and decay lifetime of 85 ms are again characteristic of emission from a m*triplet state. At 77 K the only obvious change in the spectrum is thermal broadening. However, raising the sample temperature above 77 K causes emission that is characteristic of na* phosphorescence to appear. The 133 K spectrum clearly shows the presence of the carbonyl stretch progression. The energy gap between T1and T2is measured to be 470 cm-I. The phosphorescence spectra of 2,7-dimethylxanthone in n-hexane at 4.2,77, and 140 K are shown in Figure 5. Similar to the previous xanthone derivatives, the 4.2 K spectral features and 221-ms lifetime clearly indicate emission from a a** triplet state. Raising the temperature to 77 K produces no effect other than thermal broadening. It is seen from the 140 K spectrum that new features do grow to the red of the AT* emission at higher temperatures.
Triplet-State Properties of Xanthones
The Journal of Physical Chemisty, Vol. 86, No. 9, 1982 1527
TABLE I : Phosphorescence 0-0 Band Positions and Lifetimes of Xanthone and 3,6-, 2,6-, and 2,7-Dimethylxanthone Dissolved in n-Hexane
compd xanthone 3,6-dimethylxanthone 2,6-dimethylxanthone 2,7-dimethylxanthone
29 55 470 2 1900
115 44 85 221
2.5 4.3 77 205
1.9 2.6 82 221
cm-I
cm-'
25 714 25 950 25 333 24 677
25 743 26 005 25 803
I
4200
4450
r(77 K calcd),$ ms
AET,
Calculated from eq 1.
k
I
4700
(0,O)mf
4700
r(77 K), ms
T, 0-0,
0-0 band position of the major site at 4.2 K.
I I
cm-'
~ ( 4 . K), 2 ms
TIO-O,a
I
I
4200
4450
I 3950
A
-
Figure 5. Phosphorescence spectra of 2,7dimethylxanthone in n hexane at 4.2, 77, and 140 K. The 0-0 band of the major site at 4.2 K is indicated with an arrow.
I
3950
8
-
Figure 4. Phosporescence spectra of 2,6dimethylxanthone in n hexane at 4.2, 77, and 133 K. The 0-0 band of the major site at 4.2 K is indicated with an arrow.
However, the absence of a progression in the carbonyl stretch frequency makes the assignment of this emission to n?r* phosphorescence tentative. It is possible that some other phenomenon may be responsible for the thermally activated emission, such as delayed thermal fluorescence from SIor emission from 2,7-dimethylxanthone aggregates that have formed in the softened matrix. A lower limit estimate of 1900 cm-' for A E T was obtained by measuring the spacing between the 0-0 band at 4.2 K and the leading edge of the new spectral feature at 140 K. Table I lists the frequencies of the 0-0 bands, A E T values, and phosphorescence lifetimes at 4.2 and 77 K. We have also investigated the phosphorescence properties of xanthone-d,. This compound has a remarkably small A& value of 18 cm-' in n-hexane. Figure 6 shows the 0region at approximately 2 and 4.2 K. It is seen that significant emission from T2(na*) can be observed even at the boiling point of liquid helium. Relationship to Previous Studies. The results presented above provide the basis for a reinterpretation of Pownall
and co-workers' results for the phosphorescence of xanthone in 3-MP (2 and 77 K) and EPA (77 K). In this new interpretation the phosphorescence in 3-MP at 77 K is seen as a superposition of y-polarized emission from T1(m*) and z-polarized bands from T2(na*). The z-polarized bands correspond to a progression in the carbonyl stretch frequency as is characteristic of n?r* emission. When the sample temperature is lowered, the population in T2decreases; ultimately, only ? r ~ *phosphorescence from T1 is observed at 2 K. Measurement of the positions of the 0-0 bands at the two temperatures provides a AET value of -200 cm-' for xanthone in 3-MP. In the polar solvent EPA, the phosphorescence spectrum and lifetime are characteristic of emission from T1(mr*). Apparently, the TI-T2 energy gap is larger in this solvent and little or no thermal populating of T, occurs at 77 K. An increase in the energy gap is consistent with the observation that ?rr*transitions generally red shift and n?r* transitions blue shift in polar solvents. This new interpretation of solvent effects on xanthone's phosphorescence is depicted in Figure 1B. Temperature Dependence of Phosphorescence Lifetimes. If a Boltzmann equilibrium exists between TIand T2,it is a straightforward exercise to demonstrate that the
1528
Connors and Christian
The Journal of Physical Chemisfty, Vol. 86, No. 9, 1982
cqo, n,il*
I
3900
3000
A Figwe 8. phosphdrescencespectra of the 0-0 region for xanthoned8 in n-hexane at 2 and 4.2 K.
observed phosphorescence decay rate shows a temperature dependence approximated by K1 + Kz exp(-&/kV (1) Kobad = 1 + exp(-AET/kT) where K1 is the total decay rate of T1 and K 2 is the total decay rate of T2.10 When one uses the observed phosphorescence decay rate of 40 s-l for xanthone in 3-MP at 77 K, as well as the spectroscopically measured value of AET = 200 cm-l93,*and sets the decay rate measured at liquid-helium temperature equal to K1 (8.7 s-l), a value of K2 = 1.4 X lo3 s-l is calcualted from eq 1. This value of K2 is of the right order of magnitude expected for the decay of an na* triplet state. We can use eq 1 to further test our assignment of the temperature dependence of phosphorescence for xanthone and ita dimethyl-substituted derivatives in n-hexane. To do this, we assume that the K2 values for xanthone and the dimethyl derivatives in n-hexane are the same as the K 2 value calculated for xanthone in 3-MP. This value of K2is used along with the AET values from Table I and the decay rates measured at 4.2 K, which we assume to be equal to K1,to calculate the observed rate of decay at 77 K. The calculated rates have been converted to lifetimes and are listed in the last column of Table I. Considering the assumptions made, the agreement with the experimental values is good. The dual emission observed in 3-MF may be due to the presence of a small amount of impurity in the solvent. Interaction of some of the xanthone molecules in solution with an impurity that causes a significant increase in AET could explain the small amount of long-lived emission observed in 3-MP. The mixed (polar and nonpolar) nature
of EPA provides a natural explanation for the dual emission observed in this solvent.
Concluding Remarks We have presented evidence which shows that the unusual solvent and temperature effects on the triplet-state properties of xanthones can be attributed to a common cause, namely, the near degeneracy of Tl(sa*) and T2(na*) in certain solvents. It should be noted that Chakrabarti and Hirota have measured A E T for xanthone dissolved in crystals using phosphorescence excitation spectroscopy and have obtained an energy gap of approximately 1500 cm-l. On the basis of this relatively large A E T , they conclude that 3(aa*)-3(na*) quantum-mechanical mixing is unimportant in influencing the triplet sublevel decay rates of xanthone. In view of the results presented here, this conclusion should not be considered general until the individual sublevel decay rates have been measured in environments where A E T is known to be small, such as 3-MP and n-hexane. It is true that the overall a** triplet-state lifetime of xanthone in n-hexane appears to be relatively long when compared to other systems with small AET,9 suggesting a small perturbation by the nearby na* triplet state. An ineffectiveness in 3(a7r*)-3(na*)mixing in n-hexane may be related to the rigid planar structure of xanthone. Table I shows that the 4.2 K lifetime of 3,6-dimethylxanthone is considerably shorter than that of xanthone despite the fact that A E T is almost twice as large. This observation suggests that ring methyl groups may promote 3(na*)-3(ax*) interaction for xanthone. Further, the 4.2 K lifetimes of the dimethyl derivatives are found to increase as the AET gap gets larger, consistent with some degree of Tl-T2 interaction for small ST. Of course, one must not overlook the influence of the yet to be determined locations of the lowest ‘(nr*) state for these derivatives. In conclusion, it is our belief that the compounds discussed here represent an interesting series that could provide new information on the effects that closely spaced states have on spectral and dynamic properties. This is particularly the case since the xanthones have rigid planar structures. Previous studies of interstate interaction in aromatic carbonyl compounds have dealt primarily with aldehydes and methyl ketones. These investigations have shown that vibrations not available to the xanthones, such as out-of-plane wagging and torsional vibrations involving the CHO and COCH3 units, are often the most active modes promoting vibronic interaction between aa* and na* states.13 We are continuing our studies along these lines. Acknowuledgment. We thank Dr. Henry J. Pownall for a gift of the substitued xanthone compounds used in this study. We are also grateful to Professor James W. Pavlik for lending us the Dewar system used for measuring spectra at temperatures above 77 K. This research was supported in part by the Research Corp. (13) For a review, see A. J. Duben, L. Goodman, and M. Koyanagi in ‘Excited States”,Vol. 1, E.C. Lim, Ed.,Academic Press, New York, 1974, p 295.
