J . Phys. Chem. 1984,88, 2351-2357
2351
Resonantly Excited Defect Emission from a Biphenyl Crystal C. Taliani Istituto di Spettroscopia Molecolare, Consiglio Nazionale delle Ricerche, 401 26 Bologna, Italy
and A. Bree* Department of Chemistry, University of British Columbia, Vancouver, B.C., Canada V6T 1 Y6 (Received: June 30, 1983; In Final Form: October 12, 1983)
Fluorescence emission, using narrow-band laser excitation, has been measured from purified biphenyl crystals at temperatures of 4.2 K and lower. The excitation frequency was varied within the range from 600 cm-' above to about 300 cm-' below the first singlet exciton band. Evidence is presented which suggests that the k = 0 levels of the four exciton branches expected for the low-temperature crystal structure are at 33 111 and 33 126 cm-' (A') and 33 126 and 33 139 cm-' (A"). Two chemical impurities were found in trace amounts; one impurity was dibenzofuran, but the other was not identified. As well, X-trap levels were found with an unusually high concentration at about 250 cm-' below the exciton band and were attributed to defect sites at soliton and domain boundaries in the lattice. X-trap emission from solitons dominates at temperatures below 40 K when biphenyl exists only in an incommensurate phase while, to account for a weak emission above 40 K observed by Wakayama, another source of X-traps at domain walls is postulated.
1. Introduction The electronic transition to the first excited singlet state of biphenyl is now fairly well understood. In a crystal at 4.2 K, the pure electronic transition (B3g A, using labels appropriate to the planar molecule) at 33 128.2 cm-' is one-photon (OP) forbidden' and two-photon (TP) allowedS2 The zero-phonon transition is observed as a very weak, sharp line in the O P spectrum, and it is believed that it arises through a magnetic dipole mechanism.' Another unusual feature of this OP spectrum is the appearance of a broad phonon sideband that carries electric dipole intensity by mixing with a higher electronic state. The studies briefly outlined above were carried out on crystals held at temperatures of 4.2 K and lower. Wakayama3has recently reported on the temperature dependence of the lines in the fluorescence spectrum. A previously unrecognized peak at 305.5 nm is strongly temperature dependent. This line is quite prominent in the spectrum at 5.2 K, but its intensity relative to that of the other lines decreases as the temperature is increased to 50.2 K and it vanishes altogether at 70 K. A weak line at about 302.5 nm was tentatively assigned to the pure electronic transition. It is unfortunate that these spectra were measured at low resolution; it was one aim of this work to record the fluorescence near the origin at a higher resolution. Emission spectroscopy has been a useful tool to study energy-relaxation processes in organic solids. Energy transfer is often limited by a localization of the energy at trapping sites such as X-traps.&' An X-trap is a host molecule adjacent to a chemical impurity or physical defect; this molecule has recognizably the same emission spectrum (although displaced) as the bulk molecules. There may be small changes in the molecular force field at the different sites.* In the present study, we excite fluorescence in purified biphenyl crystals using a tunable dye laser. With frequency doubling, narrow-band excitation is possible and we have excited the crystal in the range from 600 cm-' above the first singlet exciton band to 300 cm-' below it. We report here on the energy distribution +
(1) R. M. Hochstrasser, R. D. McAlpine, and J. D. Whiteman, J. Chem. Phys., 58, 5078 (1973). (2) R. M. Hochstrasser and H. N. Sung,J . Chem. Phys., 66,3265 (1977). (3) N. I. Wakayama, Chem. Phys. Lett., 83, 413 (1981). (4) A. Proepstl and H. C. Wolf, Z . Naturforsch., A , Ha, 724 (1963). (5) N. J. Bridge and D. Vincent, J. Chem. SOC.,Faraday Trans. 2, 68, 1522 (1972).
(6) A. Brillante and D. P. Craig, J. Chem. Soc., Faraday Trans. 2, 71, 1457 (1975). (7)'V. A: Lisovenko, M. T. Shpak, and V. G. Antoniuk, Chem. Phys. Lett., 42, 339 (1976). (8) R. M. Hochstrasser and P. N. Prasad, Chem. Phys. Lett., 8, 315 (1971).
0022-3654/84/2088-235 1$01.50/0
of the X-traps below the band and on the effects of some residual chemical impurities. At room temperature, there are two planar biphenyl molecules in a monoclinic unit cell (space group ~ 2 , / a ) . We ~ will refer to this as phase I. At low temperatures, two incommensurate phases have been identified;'O phase I1 is stable at temperatures between 40 and 16 K" (the corresponding temperatures are 38 and 24 K for the fully deuterated species12),and phase I11 is stable below this range and down to at least 1.6 K.13 In phase 11, the wave vectors characterizing the incommensurate modulation are qII = &6aa*&'/,(1 - 6b)b*, where a* and b* denote vectors in reciprocal space. There is a lock-in phase transition along a* in going from I1 to 111, and the modulation in phase I11 is defined 1 6b)b*. The values of 6a and 6b do by the vectors qIrI= k 1 / 2 ( not depend strongly on temperature and lie in the intervals 0.04 < 6a < 0.05 and 0.07 < 6b < 0.085.13
2. Experimental Section The biphenyl used in this study was purified by treatment with maleic anhydride (to remove anthracene-like impurities) and recrystallized from ethyl alcohol followed by repeated vacuum sublimations and 56 passes through a zone refiner. Parallelepipeds with typical dimensions 4 X 6 X 4 mm along the a', b, and c' directions, respectively, were cut from the melt-grown ingots and oriented with respect to the laboratory coordinates with an accuracy of about 3O. We use a', b, and c' to label the principal axes of the optical indicatrix of the biphenyl crystal where, for visible light, a' is 15' from a and c'is 20.4' from c with a'and c' in the ac plane. The crystal was mounted in an immersion cryostat (Pope Scientific) and cooled from room temperature to 4.2 K over a time of at least 1.5 h. In some experiments, the excitation source was a pulsed nitrogen (Laser Elettronica) pumped tunable dye laser which had an oscillator and one stage of amplification. The output from the dye laser was frequency doubled by using an ADP SHG crystal from Lasermetrics. Rhodamine 6G and Rhodamine B dyes were used. The spectral bandwidth of the exciting beam was about 1 cm-'. In later experiments, a more powerful Nd:YAG pumped dye laser system (from Quanta Ray) became available, and so a better spectral resolution in the fluorescence spectra could be obtained. (9) J. Trotter, Acra Crystallogr., 14, 1135 (1961). (10) H. Cailleau, F. Moussa, and J. Mons, Solid State Commun., 31, 521 (1979). (11) A. S. Cullick and R. E. Gerkin, Chem. Phys. Lett., 42, 598 (1976); Chem. Phys., 11, 273 (1977). (12) A. Bree and M. Edelson, Chem. Phys. Leu., 55, 319 (1978). (13) H. Cailleau, F. Moussa, C. M. E. Zeyen, and J. Bouillot, J. Phys. (Orsay, Fr.), 42, 704 (1981).
0 1984 American Chemical Society
2352
The Journal of Physical Chemistry, Vol. 88, No. 11, 1984
Taliani and Bree W A V E N U M B E R Icrn - ' x 32 5
33
33 5
34
\I
I
I
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Figure 1. Experimental setup: f, glass filter (Corning 7-45); c, immersion cryostat; I, focusing and collecting lenses; p, polarizer; s, scrambler. A long focal length lens (f = 850 mm) focused the light from the dye laser onto an ADP SHG crystal. Rotation of the exciting beam was achieved by using a retardation plate (X/2) and by rotating the SHG simultaneously. A quartz flat is used as a beam splitter.
The emission was collected through f/2.5 optics into a Spex 1402 double monochromator and detected with an RCA 31034 cooled photomultiplier whose output was fed into a SO-ohm load. The reference signal was monitored by splitting a small fraction of the frequency-doubled beam onto an RCA 4832 photomultiplier. A schematic diagram of the experimental arrangement is shown in Figure 1. The emission and reference signals were fed into a gated integrator (Evans Associates) and a peak height measuring system,14 respectively, and the outputs were strobed through a 12-bit A/D converter into a Cromemco Z2D microcomputer. The firing of the laser, the scanning of the monochromator, and the integrator aperture time (typically 100-300 ps) were under computer control. The laser fired the desired number of shots (generally between 32 and 99) at each chosen wavelength at the requested repetition rate. The normalized emission was calculated for each shot, and the average emission intensity and standard error were calculated after the given number of shots. All the relevant parameters were stored on a floppy disk and subsequently displayed on a Houston Instruments DP-101 plotter. In what follows, we quote all wavelengths as measured in air while all wavenumbers carry the vacuum correction. 3. Results We have used exciting radiation whose frequency was adjusted to lie either just above or below the origin band of the first excited singlet state; the excited frequencies chosen are indicated in Figure 2 with respect to the absorption profile measured by Hochstrasser et al.' Four different regions of excitation frequency are distinguished, and the four characteristic fluorescence spectra which are generated are given in Figure 3. The upper trace of Figure 3 is obtained when the excitation is to a state within the first excited singlet band system of biphenyl (range A). The next two traces (from the top) of Figure 3 are the result of exciting below the origin band but within about 200 cm-' of it (range B). The sharp lines seen in the upper trace are absent, and only a broad emission is recorded. The next three traces of the figure characterize range C (excitation is from 230 to 290 cm-I below the band) where some new sharp fluorescence lines appear in the region of the spectrum around 310 nm. Range D (excitation frequency more than 290 cm-' below the band) is illustrated by the lowest trace of Figure 3, in which only some weak, broad, and strongly red-shifted fluorescence bands are seen. There are a few sharp lines in Figure 3 (shown as dashed lines) whose frequency shifts with the excitation frequency. These are due to Raman scattering by the biphenyl crystal, and it is easy to show that these peaks reproduce the prominent Raman lines that arise from totally symmetric fundamentals.15 (14) A. Bree, M. Edelson, and C. Taliani, J . Phys. E , 11, 541 (1978). (15) A. Bree, C. Y. Pang, and L. Rabeneck, Spectrochim. Acta, Part A , 27A, 1293 (1971).
305
300 WAVELENGTH inm)
295
Figure 2. Frequencies used to excite the biphenyl fluorescence, indicated as vertical bars, with respect to the absorption profile of a thick ab section
(from ref 1). For an explanation of the excitation ranges A, B, C, and D see the text. WAVENUMBER Icm-fl
32000
33000
31000
0'
4.598
-284
300
1
L
310
320
330
W A V E L E N G T H (nrn)
Figure 3. Biphenyl fluorescence spectra of an a'b section at 4.2 K for
different excitation frequencies indicated by A = (verc,,atlonin cm-I. Dotted lines denote biphenyl Raman lines. An asterisk indicates a
fluorescence line whose shift with respect to the exciting light is constant.
3.1 Excitation into the Singlet State (Range A ) . It is important to have some idea of the extinction depths involved when exciting with light within the first absorption system. Microdensitometer tracings of a photographic plate that records the absorption spectra of an a% section (0.38 mm thick) and a bc'section (1.93 mm thick) are shown in Figure 4. The densitometer (Joyce-Loebl) is linear
Defect Emission from a Biphenyl Crystal
Figure 4. Relative intensities of the B3, origin from microdensitometer tracings of polarized absorption spectra taken from bc' (1.93 mm thick) and a'b (0.38 mm thick) sections. Note that the intensity of the b-polarized origin depends on the face examined.
The Journal of Physical Chemistry, Vol, 88, No. 11, 1984 2353
33 120
33000
52 880
32 760
32 640
32520
WAVENUMBER
Figure 6. Medium-resolution fluorescence in the spectral range between the true and strong false origins excited via two-photon absorption at the B3*origin. Each point is the mean normalized fluorescenceobtained from
32 laser pulses. Lines have been drawn at some peaks as a guide to the eye. TABLE I: Details of the Emission between the True Origin and the Strong False Origin at 32 487 cm-I from a Biphenyl Crystal at 4.2 K Av,~ AV,O u, cm-' cm-' comment v, cm-' cm-' comment 32917, mw 33084, w 27 phonon 194 32 859.4, m 33075, w 36 phonon Y-trapb 32 849.3, w Y-trapd 33 045, m 33 022, w 32 838.5, vw 89 phonon 389 funde 32722, m 33 01 1, w 100 phonon 472 X-trap?" 32 639, vw 32987, mw 545 bZgfund?/ 32 566, mw 140 32971, w Figure 5. The c'-polarized exciton fluorescence of biphenyl at 4.2 K excited at 33 726 cm-' (A = 598 cm-I). The dashed line marks the B3, origin. Frequency differences are in cm''.
in its response to plate optical density, and we estimate the molar extinction coefficient at the origin band to be eo, = 1 and t b = 2 L mol-' cm-' for the a% section and eb = 0.08 and ed = (0.1-0.2) L mol-' cm-I for the bc' section. These values provide a rough scaling for the traces in Figure 4. Figure 4 shows that the origin is at 33 126 cm-' and that there are two other weak lines a t 33 11 1 cm-' in c' polarization and 33 139 cm-I in b polarization. The former, 15 cm-' below the origin, coincides in energy with what Hochstrasser et al.' have identified as being an X-trap and acts as the origin for all the sharp lines observed in the fluorescence spectrum. Previous fluorescence spectra'J6 were excited by broad-band sources (a high-pressure mercury lamp with chemical filters) into the vibronic manifold of higher energy electronic states. The c' polarized fluorescence spectrum excited with narrow-line excitation 598 cm-' above the bottom of the first exciton band is shown in Figure 5. The sharp line at 32487 cm-' is a false origin for which a b,, vibration of the electronic ground state is involved in vibronic coupling with a more highly excited Bl, electronic state. The remaining sharp structure to longer wavelengths can be analyzed1J6in terms of well-known totally symmetric fundamentals of biphenyl (see the deails on Figure 5). (16) A. Bree, M. Edelson, and R. Zwarich, Chem. Phys., 8, 27 (1975). (17) A. Bree and R. Zwarich, J . Raman Spectrosc., 12, 247 (1982).
32 927, mw
184
aEnergy difference from 33 11 1 cm-I, the effective electronicorigin. bThis chemical impurity has not been identified. "This may be the X-trap identified in ref 16. dThis impurity is dibenzofuran, which can occupy one of three different sites in the biphenyl lattice; the frequencies were accurately measured in the excitation profile experiments (see Figure 10). eThere is a line at 390 cm-' in the Raman spectrum of biphenyl at 4.2 K which may mark either an a, or a b,, fundamental of the planar molecule. 'See ref 15. There are two broad bands on the high-energy side of the strong false origin at 33 050 cm-' (fwhm 65 cm-I) and at 32 800 cm-' (fwhm 170 cm-I), and an expanded view of this region is shown in Figure 6. These broad bands appear to be those reported recently by W a k a ~ a m a .If~ this is so, then the weak band labeled c by Wakayama3 and observed here at 33 050 cm-' is not the true electronic origin but is another false origin involving the participation of ground-state phonons in vibronic coupling; this band, then, is the counterpart in fluorescence to the broad absorption shown in Figure 2.' The anisotropy for the band at 33 050 cm-' is IaJb:Zd = 0.8:1:2.2 and for the band at 32 800 cm-' is Ia,:Ib:Id = 0.7:1:2.5. There are some sharper features overlaying the broad bands at 33 050 and 32 800 cm-I shown in Figure 6, and the details of the structure are listed in Table I. We have attempted to account for some of this structure. By resonantly exciting the crystal at each of the sharp peaks and scanning the Spex monochromator to search for a new fluorescence spectrum, we have uncovered the presence of two chemical impurities (Y-traps). A trace amount
2354
The Journal of Physical Chemistry, Vol. 88, No. 11, 1984
Taliani and Bree
0,.= 3 2 8 6 2 C r n '
1028.0
3078.0
3.28.0
ii78.o
12xI.o
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,
WAVELENGTH ~ANGSTROH51
Figure 7. Fluorescence of an a'b face of biphenyl at 4.2 K excited at 33081 cm-' (A = -47 cm-'). This particular polarization was chosen for illustration because the biphenyl Raman lines do not emerge to alter the broad fluorescence contour. The observed frequency differences from the 32 SOO-crn-' band are indicated on the bands.
WRVELENGTH IRNGSTROMSI
Figure 8. Laser line-narrowed fluorescence of a dibenzofuran (DBF) chemical trap from an ak'section of biphenyl at 4.2 K excited at 32 862 cm-t (A = -266 cm-I) with light polarized along b. Frequencies (in cm-')
are indicated as shifts from the exciting frequency. R denotes a biphenyl Raman line.
TABLE II: Biphenyl "Defect Fluorescence" Spectrum Excited below the Energv of the Bottom of the Exciton Band
Au, cm-'
32800 700" 1015
biphenyl ground-state freq defect origin 740
Av, cm-' 1300 1600
biphenyl ground-state freq 1278 1591-1 6 10, doublet
1002
"The fwhm of these bands is about 140 cm-'. (about mol/mol) of dibenzofuran gives rise to a triplet of lines near 32 850 cm-' which are the electronic origins of the spectrum reported in range C. A second, unknown impurity is responsible for the broad line (fwhm 15 cm-') at 33 045 cm-'. The resulting fluorescence spectrum is comprised of a series of sharp lines, whose width is limited by the monochromator slit width, that tune with the exciting frequency as it is scanned across the inhomogeneously broadened origin. The vibrational intervals are 211 (vs), 430 (m), 637 (vw), 873 (m), 1062 (vw), 1069 (vw),1090 (vw), 1222 (mw), and 1519 cm-' (w) with the same polarization as the origin and 586 (vw), 770 (w), 1304 (w), 1403 (mw), 1408 (mw), and 1494 cm-' (mw) with opposite polarization. Possible assignments for the other lines are given in Table I. The broad bands in Figure 5 do not show the structure observed in Figure 6. To account for this difference, we introduce a parameter R defined as the ratio of the intensity under the broad peak at 32 800 cm-' to the intensity under the sharp peak at 32 487 cm-'. The value of R appears to vary from crystal to crystal. For the crystal used to obtain Figure 5 , R was 0.4 while for the crystal used for Figure 6, R was 0.28; for all the crystals used, R was in the range 0.2 C R C 0.4. In Figure 5, the intensity of the broad band at 32 800 cm-' is enhanced and the structure is lost. There is no discernible difference in the fluorescence spectrum when it is produced by OP or by TP excitation into the origin band. For example, the spectrum in Figure 5 was excited at 33 726 cm-' while the spectra in Figure 6 were obtained by TP excitation into the origin band. Evidently, the variation in extinction depth is not important in biphenyl where the O P absorption is so weak. 3.2 Excitation Slightly below the Bottom of the Exciton Band (Range B). The sharp fluorescence spectrum associated with emission from the exciton band or from an X-trap 14 cm-' below the band' has disappeared while a broad emission with the 32 800-cm-' peak as an apparent origin remains. The intensity distribution among the various vibronic bands has the same Franck-Condon envelope as the sharp spectrum; this is seen in Figure 7 once one takes into account the displaced origins in the two spectra. The observed vibrational intervals are listed in Table I1 and compared with the known frequencies of the ground-state fundamentals of biphenyl. We conclude that the broad emission
Uob.0
Figure 9. The a%'-polarizedfluorescence and biphenyl Raman lines at 4.2 K excited parallel to b at 32834 cm-' (A = -294 cm-').
is from biphenyl molecules possibly located at defect sites in the lattice. The broad emission is accompanied by sharp Raman lines15 at 1002, 1278, and 1512 cm-' and the unresolved doublet at 1591-1610 cm-'. Crystals prepared from different lots of purified biphenyl manifest the same broad emission although there is a change in the relative intensity of the Raman compared with that of the broad emission. The fluorescence pulse width was a few nanoseconds for both the sharp and the broad emissions. An accurate measurement of the radiative decay time was limited by the bandwidth of the Model 475A Tektronix oscilloscope used and the laser pulsewidth. However, this decay time is characteristic of prompt fluorescence. 3.3 Excitation in Range C (32896-32838 ern-'). When the exciting frequency is reduced below 32909 cm-', the broad emission at 32 800 cm-' disappears together with its vibronic structure. By exciting at 32 896 cm-', a new spectrum of narrow bands dominates the emission, and this new pattern is accompanied by persistent and prominent biphenyl Raman lines. All the new sharp lines, except a couple of broad bands at 3 1 7 10 and 3 1180 cm-', shift in frequency by the same amount as the change of the exciting light in the same fashion as the Raman lines. A typical spectrum is given in Figure 8. This new spectrum arises from the presence of trace amounts of dibenzofuran impurity in the biphenyl crystal, and this will be the subject of the following paper.'* 3.4 Excitation below 32838 cm-' (Range D ) . The emission spectrum excited at 32 834 cm-' is shown in Figure 9. Evidently, (18) A. Bree,C. Taliani, and R. Zwarich, J. Phys. Chem., following paper in this issue.
The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 2355
Defect Emission from a Biphenyl Crystal
I
Detector set at 32100cm-1
- --irci 32 790
32870
1 1
I
32950
33030
Excitation Frequency (cm-1)
-
_
-
1 .) I
I
33014
Detector set at 32487 cm-1
, 33 OX,
I
33 046
Excitation Frequency (cm-’)
Figure 10. One-photon excitation profile of the defect emission at 4.2 K in c’ polarization monitored at 32 100 cm-’. The onset is at 32 870 cm-’. The line at 32 829 cm-’ marks a biphenyl Raman interval of 739
Figure 11. One-photon excitation profile of the BJg origin at 2 K monitored on the strong false origin at 32 487 cm-I.
cm-’.
temperature. In a first approximation, the s t r ~ c t u r e may ’ ~ be described as containing four molecules in a monoclinic unit cell having space group Pa. The four molecules consist of two crystallographically inequivalent pairs; the inequivalent molecules alternate along the b axis with the two phenyl rings twisted about the long molecular axis (so that the molecule deviates from planarity) in opposite senses. There is an essential doubling of the unit cell dimension along b. In this structure, biphenyl has four factor group components, two with symmetry A’ and two with symmetry A”, which differ in their behavior to reflection in the ac glide plane. The A’ components are active in a’ and c’ polarizations of the O P spectrum and in (ab?, (bb),(c’c?, and ( a t ’ ) TP spectra; the A” components may appear in b polarization of the O P spectrum and in the (a%) and (bc? TP spectra. In the O P spectrum (see Figure 4), there are two weak, sharp lines at 33 11 1 and 33 126 cm-’ in c’ polarization and two lines at 33 126 and 33 139 cm-’ in b polarization. The TP spectrum2 has a sharp line at 33 128 cm-’ (which we take to be the same as our 33 126 cm-I) in both (bc? and (a’c? polarizations. Thus, the four factor group components may be located at A’ (33 111 and 33 126 cm-I) and A” (33 126 and 33 139 cm-’). All the sharp emission come from a level at 33 111 cm-’. This may mark the bottom of the exciton band. In the earlier the sharp lines at 33 111 and 33 139 cm-’ had not been observed in the O P spectrum, and Hochstrasser et al.’, from a careful vibrational analysis of the fluorescence spectrum, had postulated the presence of an X-trap 14 cm-’ below a narrow exciton band at 33 128 cm-’. This provides an alternative explanation for the extra lines in the O P spectrum; there can be no doubt from the TP spectrum2 that an A’ and A” component are near 33 128 cm-l. Perhaps some evidence to support this view may be seen in the excitation spectrum in Figure 11, where the 33 111-cm-’ line is observed as a weak partner to the known origin (observed here at 33 124 cm-I). The difference in the intensities of the two c’ polarized lines in Figures 4 and 11 may be due to the different crystal preparations used for the two samples (a sign of an X-trap), or it may result from the nonlinearities used to record the two different spectra. Although we favor the interpretation of separate exciton branches since it is unusual to observe an X-trap in absorption, we leave this question unanswered. The analysis outlined above is based upon a s t r ~ c t u r e that ’~ is not quite correct. As mentioned in the Introduction, phase I11 is incommensurate, and the unit cell does not quite double along the b crystal axis.I3 If the incommensurate periodicity is close to registry with the host lattice, as here, it can happenZothat the interaction between the two subsystems (Le. the underlying commensurate structure and the modulation that causes the incommensuration) favors the formation of large commensurate regions separated by narrow walls with rapidly varying superlattice phase. The walls which separate the commensurate regions in
the sharp structure on the high-energy side has completely vanished, and so we could observe four more biphenyl Raman lines at 330, 610, 740, and 785 cm-I that were obviously masked in the above spectra. However, broad bands at 3 1 700 and 3 1 180 cm-I remain and are not accounted for. There may be more of these broad bands in the energy range farther from the excitation frequency, but we have chosen not to pursue this spectrum in the belief that it would be difficult to identify the site of this broad emission since it has vibrational intervals unlike those of biphenyl. 3.5 Excitation Profiles. Either the band at 32 800 or 33 050 cm-’ may be acting as the effective origin of the broad emission spectrum shown in Figure 7. To distinguish between these two possibilities, the Spex spectrometer was set at 32 100 cm-’ and the excitation frequency of the doubled output from the dye laser was scanned from about 32750 cm-I to higher energy. The spectrometer was set to the second peak of Figure 7 to avoid the detection of the laser excitation frequency and the strong Raman-active lattice phonons by the RCA 3 1034 photomultiplier. The result of this excitation spectrum is shown in Figure 10. The onset of the excitation of the broad fluorescence occurred at about 32 870 cm-I; the excitation profile rises very steeply and then flattens out and becomes essentially flat. The small peak that appears to the low-energy side of the onset is due to a Raman-active A, fundamental at 739 cm-’. The onset of this excitation coincides with neither of the broad bands listed above as possible origins. Instead, the onset lies on the high-energy wing of the 32 800-cm-’ fluorescence band. We also attempted to measure the TP excitation profile in the spectral region where the X-trap pure electronic absorption is expected. Since the transition is TP allowed in biphenyl, this would have located the X-trap energy levels accurately. However, no fluorescence signal was detected. A second series of excitation profiles were measured in the frequency region near the electronic origin. In this series, the spectrometer was fixed at 32487 cm-I, the frequency of the prominent, sharp, false origin in fluorescence, and the doubled dye laser output was scanned from about 33 100 to 33 160 cm-I. In these experiments, the crystal was held at 2 K. A typical excitation profile, using light polarized parallel to c’, is shown in Figure 11. There is a very weak feature at 33 111 cm-’ which coincides in energy with the X-trap postulated by Hochstrasser et al.’ A stronger band at 33 124 cm-I is taken to be another (less accurate) measurement of the origin band, while the broad feature to higher energy corresponds to the strong phonon-induced OP absorption. 4. Discussion The earlier analysis1J6of the low-temperature fluorescence spectrum was based upon a room-temperature crystal s t r ~ c t u r e . ~ We now set down a description of the number of symmetry types of the factor group components of the biphenyl crystal at low
(19) H. Cailleau and J. L. Baudour, Acta Crystallogr., Sect. E , B35,426 (1979). (20) W. L. McMillan, Phys. Rev. E Solid State, 14, 1496 (1976).
2356 The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 the crystal are called solitons. A soliton wall need not be fixed, but its position within the crystal may fluctuate. Two fluctuation modes have been suggested for biphenyl;21a phason is caused by a fluctuation in the phase of the modulation in space and time while an ampliton is described by a corresponding fluctuation of the modulation amplitude. There is some experimental support for the existence of phasons in b i ~ h e n y l . ' ~ We should emphasize here that there is no experimental evidence for the existence of solitons in biphenyl. Although it is clear that phase I11 is i n c o m m e n ~ u r a t e ,it~ is ~ -not ~ ~ possible to distinguish between a lattice distortion in which the dihedral angle between the two phenyl groups on each molecule varies sinusoidallyZ2along b or whether solitons condense out. However, there is evidence for the presence of p h a s o n ~ in ' ~ the lattice, and so, whatever the nature of the incommensurate distortion, it is mobile in the lattice. Our fluorescence study shows the existence of two different regions in the crystal, and it seems natural to associate the X-traps with the proposed soliton walls. Thus, there is a well-ordered region which gives rise to the sharp fluorescence spectrum and which we identify with the fully commensurate structure between walls and a "defect region" which gives rise to the broad emission and which we associate with the soliton walls themselves. We will return to consider further the possible role played by solitons in biphenyl but turn for the moment to discuss the broad emission bands dominant in the range B of excitation. We recall the following observations: (i) The fluorescence band at 32 800 cm-' shows a Stokes shift from its excitation spectrum which has its onset at 32870 crn-'. (ii) This band shows no evidence of fluorescence line narrowing by site selection; even though the laser line width is less than 1 cm-', the emission remains intrinsically broad. (iii) The polarization ratio for the 33 050-cm-' band, which carries electric dipole intensity through phonon interaction with other more highly excited singlets states, is about the same for the 32 8 0 0 - ~ m band. -~ These three observations imply that there is a high-number density of X-traps near 32 870 cm-l to which the pure electronic transition is forbidden but fluorescence and absorption intensity is induced by a particularly strong electron-phonon interaction. This interaction is so strong that it dominates the 627-cm-' (b2J vibronic interaction which provides the strong false origin in the sharp spectrum from the biphenyl crystal bulk. The complete absence of any fluorescence line narrowing in these experiments may simply be because the excitation is not directly into the (forbidden) origin but instead goes into the broad phonon sidebands which overlap in energy. Because the transition to the X-trap itself is not seen directly, it is not possible to say much about the extent of the energy range over which the X-trap levels are spread. However, when the levels are excited with more than lOO-cm-' excess energy (so that excitation is into the broad phonon sidebands of all X-trap levels), the upper edge of the broad 32 800-cm-' band is at roughly 32900 cm-'. Although this frequency is very difficult to measure accurately, there seems no doubt that it lies above 32 870 cm-' (the onset of the excitation spectrum). These measurements indicate that the X-traps are spread over a range of about 30 cm-' and their number density is much greater at the bottom of this energy range than at the top. Of course, the emission is always to the red of the excitation frequency, but when a site at the top of the X-trap distribution is excited, then it too contributes to the total emission. It is possible to estimate the number of X-trap sites, relative to the number of undistorted bulk sites, in the following way. Provided the absorbance is small, the normalized total fluorescence is given by
Taliani and Bree c is the concentration, in mol L-I, of the absorbing species, and
k is a constant that includes terms like the quantum efficiency of fluorescence, geometric factors concerned with the collection optics, and the crystal thickness. The ratio of these normalized total fluorescence intensities when the crystal is excited into the origin at 33 126 cm-I to that observed from excitation into the X-trap alone (at 33 028 cm-') is 16, and we take this to be the ratio EBc~/cXC~, where the subscripts B and X refer to bulk and X-trap sites, respectively. Thus cx = ( ~ / ~ ~ ) ( C B / ~ X ) C B
where e is the molar extinction coefficient at the exciting frequency,
If ex = tB, then cx = c ~ / 1 6 .However, we have postulated that the intrinsic OP cross section for the phonon sideband at the X-trap is especially large, so that we estimate ex 5 lotBand cx 5 cB/160. This rough estimate shows that the X-trap density is large, and so we look to some naturally occurring phenomenon such as the phase transition in the biphenyl crystal to account for it. Thus, we return to our earlier discussion and ask whether the X-trap can be identified as molecules caught in the jumbled arrangement at the soliton wall. The coherence length ( L ) of the biphenyl molecules in the nearly commensurate phase between the soliton walls is given by20,23L = l/p(6b) where there is a phase shift of 2 r / p across the soliton. We assume a simple one-dimensional model with the walls parallel to ac and uniformly spaced along the b axis. For biphenyl at 4.2 K, 66 is approximately 0.072,24 and so L is about 28 room-temperature unit cell lengths along b if p = 1 or 14 unit cells if (as is likely) p = 2. The estimates provided above show that the soliton density is greater than the X-trap density which, in turn, suggests that biphenyl molecules are not homogeneously disposed in the soliton wall so that X-traps occur only at certain sites. In the interpretation of his results, Wakayama3 had not realized that there were two intermingled and independent spectra in the fluorescence from a biphenyl crystal at low temperature. We believe that the sharp lines represent emission from biphenyl molecules in the bulk (i.e. in the region between soliton walls) as accounted for in the earlier analyses;',l6 the broad bands are derived from biphenyl molecules in disordered regions of the lattice. Will our model account for Wakayama's observations of the temperature dependence of the intensities of the fluorescence lines? Wakayama3 reports that the broad band at 32 800 cm-' appears only at temperatures below 70 K; its intensity relative to that of the sharp lines increases as the temperature is decreased to 25 K, and then its intensity suffers a small decline as the temperature decreases further to 5 K. The density of solitons should be greatest in phase I1 where the lattice is incommensurate along both a and b. Below the lock-in temperature at 16 K," phase I11 is incommensurate only along b with a corresponding decline in the number density of solitons. The simple account given above does not include movement of the solitons nor how soliton formation and their subsequent motion may be impeded by their interaction with phonon^.^'^^^ A more fundamental difficulty lies in the observation that the 32 800-cm-' band is observed weakly at 50 K-well above the transition temperature of 40 K between phases I and 11. We are forced to expand our model to include another source of X-traps whose number does not vary with temperature. These may be at domain walls (rather than at soliton walls). It is expected that the number of such defects may vary from sample to sample according to the conditions and method of its preparation, and this may, in turn, account for the observed variation in R mentioned in section 3.1. The X-trap levels come into effective thermal equilibrium with the exciton band at temperatures above about 40 K; for what it is worth, Wakayama's last two data points at 40 and 50 K give an activation energy of 210 cm-'. Again we emphasize that the evidence favoring the soliton model is derived from spectroscopic results. We have interpreted the
(21) H. Poulet and R. M. Pick, J . Phys. (Orsay, Fr.), 42, 701 (1981); J . Phys. C, 14, 2675 (1981). (22) J. L. Baudour and M. Sanquer, Acta Crystallogr., Sect. B , B39,75 (1983).
(23) P. Bak, Rep. Prog. Phys., 45, 587 (1982). (24) H. Cailleau, F. Moussa, and J. Mons,Solid State Commun., 31, 521 (1979). (25) K. Maki, Phys. Reu. B: Condens. Matter, 26, 4539 (1982).
IF/Io = ktc
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
