J. Phys. Chem. 1984, 88, 2357-2360 sharp and the broad fluorescence spectra observed in this work as characterizing two different regions in the crystal, the commensurate and soliton regions, respectively. Earlier, rather puzzling results may now be given an explanation with the recognition that, at the soliton, the molecules approach planarity: (i) Triplet biphenyl-hIomolecules are planar in a biphenyl-dlo host crystal at 1.9 K (phase III).26 We propose that the soliton becomes effectively immobilized in the lattice near the biphenyl-hlo excitation sites. While sites at the soliton represent positions of high potential energy for molecules in their ground electronic state, this may not be true for excited molecules where the bond order is expected to increase for the central C C bond; Hutchison and Kemple26have reviewed the structural effects that this changing bond order may have, and this idea is also consistent with the soliton having X-trap properties even in the pure crystal. (ii) The fluorescence spectrum of a biphenyl-hIoguest in a biphenyl-d,, host shows a doubling of each sharp spectral line.' We propose (26) C. A. Hutchison and M. D. Kemple, J . Chem. Phys., 71,866 (1977); 74, 192 (1981).
2357
that this sharp fluorescence originates from the commensurate lattice between solitons where there are indeed two crystallographically inequivalent sites. The marked difference in behavior in (i) and (ii) above is associated with the very different lifetimes of the electronic states involved. Because both transitions are forbidden, the intermolecular interaction is weak and only about a third of the singlet excitons reach X-traps within their lifetime (of about 1 ns) while virtually all triplet excitons are trapped within the triplet lifetime of more than 1 s. Finally, a comment about the broad emission excited in range D should be made. Since one expects more-or-less sharp emission from a chemical impurity, we make the tentative suggestion that this broad emission is from weakly bound excimers at the few defect sites in the lattice where they are preformed. Acknowledgment. We are grateful to the International Collaboration Program of the C.N.R. Italy and to the Natural Sciences and Engineering Research Council of Canada for grants which supported this work. Registry No. Biphenyl, 92-52-4; dibenzofuran, 132-64-9
Laser-Excited Fluorescence from Dibenzofuran in a Biphenyl Host C. Taliani, Istituto di Spettroscopia Molecolare, Consiglio Nazionale delle Richerche, 40126 Bologna, Italy
A. Bree,* Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada V6T I Y6
and R. Zwarich Department of Chemistry, University of Petroleum and Minerals, Dhahran, Saudi Arabia (Received: June 30, 1983; In Final Form: October 24, 1983)
The fluorescence spectrum of dibenzofuran in a biphenyl host lattice at 4.2 K is reported. The fluorescence, excited by using light of frequency well below the biphenyl exciton band, is comprised of closely spaced triplets because the dibenzofuran molecules occupy three different sites in the biphenyl lattice. The spectrum also contains many strong single lines due to Raman scattering from biphenyl; the Raman and fluorescence spectra are of comparable intensity since the dibenzofuran is present at a concentration of 0.25 ppm.
Introduction In the preceding paper,' dibenzofuran (DBF) in trace amounts was found as an impurity in biphenyl crystals grown from the melt by using a Bridgman furnace. A sharp fluorescence spectrum was measured by holding the crystal at 4.2 K or lower and resonantly exciting the fluorescence from DBF with a frequency-doubled dye laser. In the present paper, we give a detailed analysis of this spectrum with some emphasis placed on attempts at fluorescence line narrowing. Experimental Section
Although no special precautions were taken to remove DBF from the biphenyl used, it is worth pointing out (in retrospect) that the trace of impurity discovered may have been introduced (1) C. Taliani and A. Bree, J . Phys. Chem., preceding paper in this issue.
0022-3654/84/2088-2357$01.50/0
by the method of sample preparation. The biphenyl was zone refined and the sample grown from the melt in a glass tube that was evacuated to about only 1 torr. In this way, molten biphenyl was exposed to oxygen at a reduced pressure for an extended time (about 30 h in the Bridgman furnace). The extended growing time was necessary for the preparation of large single crystals. The charge of biphenyl used for the crystal growth was about 2 g. The excitation source was a Nd:YAG pumped dye laser system supplied by Quanta Ray and operated at 20 Hz. The output energy from the frequency-doubled dye laser beam was about 3 mJ in each pulse; this light excited the sample as a parallel beam with a diameter of about 5 mm. The manufacturer quotes values for the laser line width of 0.25 cm-I without an etalon and 0.05 cm-' with an etalon inserted in the dye laser, and these were approximately the values measured interferometrically in the output beam. With the frequency doubled, these figures become 0.5 and 0.1 cm-I, respectively. 0 1984 American Chemical Society
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The Journal of Physical Chemistry, Vol. 88, No, 11, 1984
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32 000
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Taliani et al.
32 600
32 000
31 800
31 400
32400
31 200
, 31 600
32 200
31 800
, 31 400
I
31 200
Wavenumber ( cm-1)
Wavenumber ( cm-I)
Figure 1. The b-polarized emission spectrum from a single biphenyl crystal at 4.2 K containing a trace of DBF. The excitation frequency was 32 859 cm-I.
Figure 2. The d-polarized emission from a single biphenyl crystal at 4.2
A Spex Model 1403 double monochromator, fitted with a holographic grating with 1800 grooves/mm, was used in second order to record the emission spectra; under these conditions, the manfacturer claims an optimum resolution of 0.3 cm-I although, for most of our spectra, the resolution was limited by the slit width.
this source are shown in the last column of Table I for comparison. Two lines, at 32486.8 and 32505.1 cm-', did not shift when the excitation frequency was varied. These lines represent the two strong false origins in the fluorescence spectrum of b i ~ h e n y l . ~ We assume that they are weakly present in this spectrum due to two-photon absorption into a very highly excited state followed by a rapid relaxation to the vibrationless first excited singlet state. Other, very weak lines at 31 485 and 31 214 cm-' mark the strong Franck-Condon activity of the 1002- and 1277-cm-' a, fundamentals built on the 32487-cm-' false origin. The decay time of all of the peaks in the spectrum was equal to or less than 3 ns, a limitation arising from the laser pulse shape and the bandwidth of the detecting electrofiics. This is consistent with them arising from either fluorescence or Raman scattering. The vibrational analysis and the polarization of the fluorescence bands are in excellent agreement with values reported earlier4 for DBF; the literature values for the vibrational frequencies and their symmetries are given in the final column of Table I. It is on this basis that the identification of the impurity was made. Two points about the analysis of the DBF fluorescence spectrum given in Table I should be brought out. In an earlier assignment4 of the phosphorescence spectrum of DBF in n-heptane at 4.2 K, it was not clear whether a 1214-cm-' interval should be recognized as a bz fundamental or combination. In the present spectrum, the line at 31 647 crn-', with a polarization opposite to that of the origin, is too strong to be a combination band and, instead, is taken
Results and Discussion Figures 1 and 2 provide an overview of the emission spectrum observed from a bephenyl crystal at 4.2 K when it is excited at 32 859 cm-I, which is 267 cm-' below the biphenyl exciton band.' DBF occupies three different sites in the biphenyl lattice at 32 838.5 cm-' (site A), 32 849.3 cm-l (site B), and 32 859.4 cm-' (site C), and so the fluorescence from DBF is comprised of a series of characteristic triplets. For example, the first triplet of lines shown at the top left-hand side of Figure 1 represents fluorescence to a 217-cm-' ground-state mode having a, symmetry. The excitation frequency used coincides with the transition energy of the impurity molecules in site C, and so there is an intensity enhancement of the high-energy component of the triplet of bands that is typical of DBF. A listing and an analysis of the observed frequencies are given in Table I. However, many of the features in the spectra are single lines whose absolute frequency varied with the excitation energy. These arise from Raman scattering from the biphenyl host material. The spectra shown in the figures were recorded with the double monochromator set to a spectral band-pass of 1 cm-I. An independent measurement of the Raman spectrum of biphenyl at 4.2 K with 514.5-nm excitation and using a Spex Ramalog also at a resolution of 1 cm-I was available.* The data obtained from (2) A. Bree and M. Edelson, unpublished work.
K containing a trace amount of DBF. The excitation frequency was 32 859 cm-'.
(3) R. M. Hochstrasser, R. D. McAlpine, and J. D. Whiteman, J. Chem. Phys,, 58, 5078 (1973); A. Bree, M. Edelson, and R.Zwarich, Chem. Phys., 8, 27 (1975). (4) A. Bree, V. V. B. Vilkos, and R. Zwarich, J . Mol. Specrrosc., 4, 124, 135 (1973).
The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 2359
Dibenzofuran in a Biphenyl Host
TABLE I: Emission Spectrum from DBF in Biphenyl at 4.2 K Excited at 32 859 cm-' Y,' cm-l A Y ,cm-' ~ moleculeC cornmen t 32641 ms 218 DBFK)) 218 a, 32632 w 217 32621 w DBF(A) 217 B 336 a, 32 522 mw 337 fluorescence from the 32505 w B exciton band 32487 m B 390 a,, or blge 32469 w 390 DBF(C) 425 a, 32434 w 425 DBF(C) 2 X 218 32423 w 436 B 547 b2g 32311 vw 548 DBF(C) 557 32 302 m 556 b2 556 32 293 vw DBF(B) 558 32281 vw DBF(A) 611 bj, B 611 32248 m DBF(C) 616 b2 615 32244 w DBF(C) 660 32 199 m 659 659 DBF(B) 32 190 vw DBF(A) 661 32 178 vw B 739 32 120 ms 739 a, 784 785 32075 mw B 790 b2g B 791 32068 w 826 a,,f B 826 32033 vw DBF(C) 851 a l 852 32007 w B 858 32001 w 858 bigC 218 + 660 878 31981 mw DBF(C) B 974 31 885 vw g 980 31 879 vw B g 1002 31 857 vs B 1002 a, 1011 DBF(C) 1010 a l 31 848 m 1041 1041 a, B 31 818 ms 1102 a, 31 758 w 1101 DBF(C) 31 746 w 1113 DBF(C) 1114 b2 31 713 mw B 1146 1146 1167 b2g 1168 B 31691 m 1192 DBF(C) 31 667 m 1193 b2 1191 31658 w DBF(B) 1212 DBF(C) 1212 bz, see text 31 647 s 1232 557 + 660 + 15 31 627 mw DBF(C) 1244 31 615 ms 1242 a l 1243 31 606 vw DBF(B) 1275 1277 ag B 31 584 vs 1311 DBF(C) 31 548 m 1309 DBF(B) 31 540 w 1308 a l 1311 DBF(A) 31 528 w 1326 DBF(C) 31 533 vw 2 X 660 + 6 1337 DBF(C) 218 + 1114 + 5 31 522 vw 31 485 w B see text 31 397 mw 1462 DBF(C) 218 + 1242 + 2 1489 DBF(C) 1489 a l 31 370 w 1516 1515 a, 31 343 ms B 1527 31 332 vw DBF(C) 218 + 1311 - 1 1589 31 270 vs B 1589 1599 B 31 260 vs 1598 1610 31 249 vs 1609 B 1619 31 240 ms 1618 B 1633 1633 a, DBF(C) 31 226 w see text B 31 214 vw 1671 660 + 1011 DBF(C) 31 188 w
Bt
t
1
1
t
t
"The relative intensity, taken as the stronger component, is included to aid the correlation with the lines in Figure 1. AY measures the energy separation from the excitation frequency (for a Raman interval of biphenyl) or from the origin band of DBF at site A, B, or C. CBis used to denote an interval of the biphenyl host, and DBF(1) stands for a DBF molecule at site I. dThe biphenyl Raman intervals are for a crystal at 4.2 K2 while the DBF vibrational intervals are for a solution or crystal at 300 KG4eSee A. Bree and R. Zwarich, J . Raman Spectrosc., 12, 247 (1982). 'This assignment is suggested by the normal-coordinate analysis carried out by R. M. Barrett and D. Steele (R. M. Barrett and D. Steele, J . Mol. Struct., 11, 105 (1972)). gThere are weak lines in the Raman spectrum of biphenyl at 4.2 K at 974 and 980 cm-' that have not been assigned.2
I
I
Figure 3. Microdensitometer tracings of photographic plates which record the polarized fluorescence (upper) and absorption (lower) spectra at the origin band of DBF in biphenyl at 4.2 K. The three sites A, B, and C are marked on the b-polarized absorption spectrum.
as the missing4 b2 fundamental of DBF. The combination (557 660) is seen here as a moderately weak line at 31 627 cm-I; however, the fundamental and combination, initially nearly degenerate, have suffered a strong Fermi resonance displacing the combination by about 15 cm-'. The second point concerns the assignment of the triplet derived from the 1310-cm-' a, fundamental. The site splittings observed here are not the same as for the other vibrations, and this provides further evidence that molecular force fields may be differently perturbed at the different sites as suggested by Hochstrasser and Pra~ad.~ Figure 3 shows the fluorescence (upper traces) and the absorption spectrum (lower traces) from a 0.193-cm-thick biphenyl crystal in the region of the origin band of the DBF impurity. The crystal was at 4.2 K, and the spectra were recorded photographically. The three sites are clearly seen, and the transition Ale4 is b polarized, consistent with its assignment as A, An estimate of the impurity concentration may be made in the following way. The absorbance due to DBF at site C was estimated to be 0.12. The molar extinction coefficient is 6700 L mol-' cm-' in b polarization of a pure DBF crystal at room temperature, and the fwhm is4 about 350 cm-'. If one assumes that the total absorption is the same in the biphenyl host (where the fwhm is about 3 cm-' at 4.2 K), then the molar extinction coefficient at site C is about 300 000 L mol-' cm-l in b polarization. The ratio of impurity molecules/host molecules is then about 2.5 X lo-' mol/mol. In the Experimental Section, it was suggested that the DBF impurity may have been produced by the purification procedure. The biphenyl crystals were grown from the melt in an evacuated glass tube. The free volume in the tube was about 4 cm3, and it was evacuated only to 1 torr. The amount of oxygen enclosed was about 4 X lo-* mol. If the oxidation mechanism is such that one oxygen molecule produces two molecules of DBF (by ring closure of biphenyl molecules), then only a 4% yield would give the impurity concentration estimated above. Since the melt was slowly lowered from the Bridgman furnace over a period of about 30 h, it seems likely that the method of sample preparation led to the impurity formation. A series of fluorescence spectra taken near the prominent 218-cm-l mode are shown as Figure 4. The spectra differ in the choice of the excitation frequency. For example, the spectrum labeled "b" was excited by using light at 32 833.6 cm-I; the excitation frequency was increased through the series to "q" which
+
-
( 5 ) R. M. Hochstrasser and P. N. Prasad, Chem. Phys. Left., 8, 315 (1971).
2360 The Journal of”Physical Chemistry, Vol. 88, No. 11, 1984
Taliani et al.
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’.
a
32650
32630 W o J e -urn b e r ’c m - I
32613
Figure 4. The effect of varying the excitation frequency
t
on the fluorescence spectrum near the 218-cm-’ triplet. The values for yo (in cm-’) are as follows: a, 32828.0; b, 32 833.6, c, 32 835.8; d, 32839.0; e, 32841.2; f, 32842.2; g, 32844.4; h, 32846.5; i, 32849.8;j, 32850.8; k, 32851.9; 1, 32854.1;m, 32855.2; n, 32859.0;0,32860.6; p, 32862.7; q, 32 875 7.
was excited at 32 875.7 cm-’. The intensity of each member of the triplet is enhanced as the doubled dye laser frequency comes into resonance with the corresponding member of the origin triplet. Apparently, there are more than three peaks in the spectra, and the features rise and fall in ways that are not always obvious. To aid in the interpretation of Figure 4, it is helpful to consider Figure 5 which shows excitation profiles from the three sites occupied by DBF in the biphenyl lattice at 1.8 K. In Figure 5 , the Spex monochromator was set at 32 621 cm-l (site A), 32 632 cm-I (site B), or 32 641 cm-I (site C) to monitor separately the fluorescence of the 217-cm-’ mode from each site as the frequency of the dye laser was scanned through the region of the origin band. These scans delineate the phonon sidebands active in the excited state. It is interesting to notice that the sideband structure is quite different for the three sites; that is, the DBF molecules at each site see a different environment which, in turn, gives rise to a different electron-phonon coupling. Similar effects arising from a site-dependent electron-phonon coupling have been observed earlier in other systems, such as for an anthracene guest molecule in a p-terphenyl host.6 The results displayed in Figure 4 can now be accounted for in the following way. In curve c, molecules in site A alone were excited and the fluorescence spectrum appears as a single line. As the exciting frequency shifts to the blue to come into resonance with DBF in sites B and C, molecules at the lower energy sites can absorb light into their phonon sidebands and the fluorescence spectrum becomes more complex. The intensities of emission from ( 6 ) G . J. Small, J . Chem. Phys., 52, 656 (1970).
32 840
(yo)
32 860
32 880
Wavenurnber ( cm-1) Figure 5. Fluorescence excitation spectra of DBF in biphenyl at 1.8 K monitored at 32621 cm-’ (site A), 32632 cm-’ (site B), and 32641 cm-’
(site C). the different sites depend on the relative absorption cross sections at the chosen excitation frequency given in Figure 5. The complexity of the spectrum shown in curve o of Figure 4, for example, indicates that DBF probably occupies more than the three principal sites in the biphenyl lattice. The line widths of the main peaks in the excitation profiles shown in Figure 5 are 3.4, 4.3, and 4.6 cm-’ for sites A, B, and C, respectively. These values are the inhomogeneous line widths as are also measured in the absorption spectrum of Figure 3. In an attempt to measure a homogeneous line width, an etalon was placed in the dye laser (the output line width was reduced to about 0.1 cm-I) and the line-narrowed fluorescence was measured with the Spex double monochromator; with excitation at 32 859 cm-’ (site C) the measured fwhm was 1.8 cm-’, while with excitation at 32 839 cm-’ (site A) the fwhm was 2.4 cm-’. The measured line width of the laser under the same conditions was 0.6 cm-’. We conclude that the values quoted above are the fully linenarrowed widths for the two sites. Although this site-selective excitation experiment has achieved some line narrowing, we cannot claim to have reached the homogeneous line width. It is still necessary to deconvolute the measured line shape with (i) the distribution function of the impurity centers over the laser line width and (ii) the band-pass characteristics of the double monochromator. These corrections are expected to reduce the homogeneous width to less that half the measured width.
Acknowledgment. This work was supported by the International Collaboration Program of the C.N.R. Italy and the Natural Sciences and Engineering Research Council of Canada. Registry No. Dibenzofuran, 132-64-9;biphenyl, 92-52-4.
