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Optical and magnetic resonance properties of the triplet state of biphenyl-h10 in biphenyl-d10 and 4.2 K neutron diffraction of biphenyl-d10
Chemical Physics 11 (1975) 273-279 D North-Holland Publishing Company
OPTICAL AND MAGNETIC RESONANCE PROPERTiES OF THE TRIPLET STATE OF BIF’HENYL-fq, IN BIPHENYL-d,,, AND 4.2K NEUTRON DIFFRACTION OF BIPHENYL+,* R.M. HOCHSTRASSER, G.W. SCOTT** and A.H. ZEWAIL*** Deparrment of Chemistry,and Laboratory for Rcsenrch on the Structure of Jfatter, Ut~iversity of Pennsyylvania, Philadelphia, Pa. 19174, USA and H. FUESS Institut Ma.x YOULaue - Paul Langesin. 38042 Grenoble. France Received
13 June 1975
The phosphorescence spectrum of 2% proto in deutcrobiphcnyl dots not exhibit the splittings that were seen previously in the fluorescence spectra [R.M. Hochstrasser, R.D. McAlpine and J.D. Whitcman, J. Chem. Phys. 58 (1973) 50781. Ihc zeroficld magnetic resonance (optically detected) showed a structure that is most likely not due to hyperline cffscts: Two or three transitions wc~e observed in the ID + El mG ID .- El reions. In the ID +El region the largest splitting is 4.2 t 0.3 MHz while the splitting between the two strongest peaks is 2.0 f 0.3 MHz. A neutron diffraction study of a CtzDto crystalline powder at 4.2 K could be indexed on the basis of cell parameters u = 7.760, b = 5.544, c = 0.42 (A), P =?3.6O and space group WI /a. It is supgested that this structure is disordered, thereby accounting for the optical, Raman and magnetic resonance experimental results, but the exact nature of the disorder has not been determined.
1. Introduction Since the optical work by McAlpine [l] and by Hochstrasrer, McAlpine and ‘niteman [2] (HMW) exposed the existence of unexpected small splittings in the fluorescence spectra of dilute mixed crystals of proto in deuterobiphenyl, there has been considerabie activity and conjecture regarding the detailed structure of biphenyl crystals at low temperature. I-IMW found that each line of the fluorescence spectrum and the O-O absorption spectrum was split by ca. 10 cm-l into two nearly equally intense lines, and some weaker satellites, reminiscent of the site splitting that had been seen in the optical spectra (absorption, fluorescence, phosphorescence) of chemical impurities in bi* This research was supported by an N.I.H. grant GM12592 and in part by the LRSM at the University of Pennsylvania. ** Chemistry Department. University of California, Riverside, California 92502. *** Chemistry Department, University of California, Berkeley, California 94720.
phenyl crystals at low temperatures [3]. Although HMW did not report the details of the phosphorescence observed for proto in deutero crystals they made an extensive but unsuccessful search for a splitting analogous to that observed in fluorescence. HMW concluded upon the possibility that biphenyl crystals may undergo a structural modification upon cooling to 4.2 K. Subsequently two additional reports have appeared: Raman spectra of biphenyl have been shown to vary with temperature in such a manner that suggests a slight continuous change in crystal structure [4] ; The ENDOR of the lowest triplet state of protobiphenyl in a deuterobipheny1 host has exhibited additional transitions [S] over those expected on the basis of a structure having one-half a molecule of biphenyl per asymmetric unit. Any multiple site hypothesis would require than in principle the singlet-triplet transitions also be multiple and in this note we describe briefly our phosphorescence spectra and the optically detected magnetic resonance at zero field ot the lowest triplet state.
274
R.M. Hochstrasser
et al/Triplet
In addition to this k-; report on some preliminary neutr& diffraction data referring to a crystalline powder of perdeuterobiphenyl at 4.2 K ‘and 300 K. The main problem to be addressed is the nature of the structure of the-crystal and of.the photoexcited states in the mixed crystal at low temperatures.
2. Experimental
procedures
and results
Z 1. I%ocedures Just as in HMk we have used perprotobiphenyl and pzrdeuterobiphenyl that have been chromatographed, treated with potassium, and zone rsfmed. The crystals were grown in a Bridgman furnace. The phosphorescence spectra were photographed at ca. lo5 resolution on a 2 m Czerny-Turner fitted with an echelle grating, and with a lower resolution Eagle spectrograph. The zero field magnetic resonance spectra were taken in several ways (to be described) but each method involved detestion of the phosphorescence with an EMI 62565 photomultiplier either after dispersion using a Spex :;.75 m Czerny-Turner monochromator or istilation with the 3 mm Schott filters GG435 and BG38. In some experiments the output of the photomultiplier was accumulated with a signal averager ‘(NS575) using a time constant of 50 ms. The microwave sweep o.=sillatcn (HP 86908 with 8699B) was adjusted to repetitively =x’eep at 200 KHz/s through the microwave range of interest. The beginning and end frequencies of the sweep were measured with a frequency counter (EIP Inc. 3508-03) and the frequencies of tie peaks in the microwave spectrum were obtained by linear interpolation. In other experiments the output of the photomultiplier was amplified chart recorded with a time constant of 2 s and a sweep rate of 80 kHz/s 2.2. Z?ie pfzqsphorescence The phos‘phorescence of a perdeuterobiphcnyl crystal containing 2% perprotobiphenyl is shown in fig. i. The inscri shows the O-O transition at high resolution tc consist of a single peak (i.e,, no splitting such as -was seen in the fluorescen&e)‘having a width df c&3.3 +I’~. The slight asymmeiry-is noi tinder: stood,.& it may-simply indik&a characteristic of
state ofbiphetlyl-h
10 in biphenyld10
the inhomogeneous distribution of molecules in this crystal. Each main peak in the phosphorescence is attributable to a ground state fundamental vibration [6] or a combination of these vibrations: No repetitions occur, so there is no indication at the present resolution ofa site splitting such as was seen in fluorescence [2]. The origin is located at 4316.1 A (23 161 cm-l) and the spectrum consists of the totally symmetric vibrations at 334 (m), 743 (m), 1002 (ms), 1035 (mw), 1276 (s) and 1595 (s) cm-l _ The doublets in the spectrum starting around 0 + 1600 cm-l are due to the splitting between combinations containing the 1595 mode and those containing the comparably strong 334 + 1276 = 1610 cm-kombination. During some experiments on the temperature dependence of the magnetic resonance (see below) we noticed that the phosphorescence becomes much more complex and many new lines appear when the tempe:;ture is raised much above 4.2 K. 2.3. The optically detected magnetic resonance (ODMR) The ODMR at zero magnetic field in the region of 3229 MHz consistently appeared as at least a doublet independent of whether the light detected was confined to that at the wavelength of the O-O band (3/4 m Czemy-Turner: 2 mm slits) or whether the whole phosphorescence was being detected. A typical spectrum corresponding to a single chart recorded microwave sweep (10 MHz in 100 s) at 1.5 K, detecting on the O-O band, showed peaks at 3229.3 MHz of strength 1 .O and 3227.1 MHz of strength 0.4. More accurate values were obtained by broad band phosphorescence detection and signal averaging. In this case three peaks were observed (relative intensities in parentheses) at: 3228.4 MHz (1.0); 3226.6 MHz (0.6); and 3224.2 MHz (0.1). The ID +El region of triplet biphenyl ODMR at zero field should occur at 3209 MHz according to Mispelter [7]. In the region of the ID-El transition (- 353 1 MHz) we similarly observed three peaks at: 3461 .O MHz (0.5); 3463.5 MHz (1 .O); and 3465.3 MHz (0.4). The estimated frequency measurement error is L 0.3 MHz, but some frequencies and intensities may be subject to uncertainty due to adiabatic fast passage effects oq ihe spectra. What is certain is that in the 3225 MHz region the largejt splitting is at least 4’.2 kO.3 MHz while the splitting between the two strongest peaks is 2.0 i 0.3 MHz. The ODMR spectrum
. .
:
275
R.M. Hochstrasser et al./Tripler rrate of biphenyl-h 1,~in biphenyl-dlo
BIPHENYL 2%
(C,,H,,
ISOTOPIC
) MIXED
1
PHOSPHORESCENCE CRYSTAL
I-O
I
I
4800
4700
I
I
4600
4500
I
I
4400
I
4300
X,(81
Fig. 1. The phosphorescence assignments
spectrum
of biphcnyl-h
10 (2%) in biphenyl-d
1o at 1.6 K. The numbering
of the vibrations
follows
the
given in ref. [6].
at 3225 MHz was studied with a crystal both at 1.6 K and at 4.2 K and no differences in the relative intensities of the peaks were detected.
to index it in the same space group WI/a) and refme new cell parameters. These new parameters and their standard deviations are given in table 1 together with low temperature X-ray values of Kozhin and Mirskoya
2.4. Z+e neu troll diffraction
191. Both the X-ray results between 77 K and room temperature and the neutron data at 4.2 K show a pronounced contraction in the a-axis direction which is nearly perpendicular to the molecular planes: .The 4.2 K values obtained from a limited number of well resolved powder peaks are certainly less precise than the X-ray results on a single crystal but they seem to indicate that the change in the unit cell is smooth. The integrated intensities at 3’00 K agree Fairly well with calculated values based on Trotter’s [S] structural model without any further refinement for the positional and thermal parameters. The agreement for the low temperature pattein is not so good (agti without reftiing) but does not indicate that a structural transition has occuried.
Neutron powder diffraction patterns were taken at 300K and 4.2 K for C:,D,, on the DIA facility at the HFR (Grenoble). The wavelength used was 1.5 10 A obtained with a Ce monochromater. The 300 K pattern was readily indexed using the unit ceil parameters previously determined by X-rays [S] . That is, space group P2t/a, n = 8.12, b = 5.64, c = 0.42 (a), fl= 95.4O. It was shown by HMW [2] that there are no significant differences in the structures of C12H10 and C,,D,, at room temperature - a confirmation of lhis fact was later reported by Brenner et al. [S]. The low temperature data could not be indexed on the basis of these cell parameters, but it was possible
. . ._.
:
276 Table 1 IALice constants
of biphcnyl
as function
of tcmpemture
(estimated
rtandxd
deviations
in parentheses)
2-W
a
b
C
0 (deg)
Method
Rcfcrcnce
CIZHIO
294
8.167(4)
5.661(4)
9.449 141
95.24 (0.2)
X-ray
ClZJIO
220
8.013 (4)
5.620 (4)
9.442(4)
96.28CO.2)
X-ray xal
71
7.906 (4)
5.610(4)
9.436(4)
96.48tO.2)
X-ray
7.760 (61
5.544 (7)
9.42 (4)
93.6lCO.5)
Ncu tr. powder
Koshin and Mirskoya 191 Koshin and Mirskoya [9] Koshin and hlirskoya [9] prescn t work
ClzH IO
4.2
ClZDlO
3. Discusion The
Xd
resonance transitions, corresponding to the two triplet components under discussion, are not equal but more than two components are observed just as in the optical spectra of the singlet. The observed ratio of intensities in the zero field ODMR spectrum is not caused by a Boltzmann distribution being attained during the excited state lifetime {the phosphorescence lifetime is ca. lo6 times that for fluorescence) because the optically detected magnetic resonance is the same at 1.6 K and 4.2 K. A possible reason for the difference in the re!ative intensities of the doublet components seen in absorption (a measure of the r:lative amounts of each) and in the zero field ODMR is that differences in the non-radiative processes, such as intersystem crossing (S1 + Tf and/or T, + So) and/or triplet radiative processes (T, + So), are introduced by the different environmental perturbations. For the case of mixed crystals of phenanthrene in biphenyl the singlet aad triplet spectra exhibited site splitting but the components did not have the same relative intensity in the two spectra [3] _In that case it was found that the intersystem crossing was not equally efficient for the two principal sites [3, lo]. The fractional change in the phosphorescence signal expected at a microwave resonance is intimately connected with the populating rates, nonradiative and radiative rates, so no reliable assessment of the source of the intensity difference can be made on the basis of a single ODMR relative intensity measurement. A key experiment consists of studying the ODMR of C12HIo in C12D,o subsequent to the excitation of just one of the singlet transitions, but our attempts to do this have failed -. due to a lack of light intensity within such a narrow frequency range in the region of 3000 A. Ultimately it should be possible to use a frequency doubled dye. laser for this type ofexperiment. netic
observation
of multiplets
in the zero
field
mag-
subject to the fiJIlowing assumptions, that the photoexcited C,2D.Lo crystal contains two or more nonequivalent triplet CIZHIo molecules. While we &mot. defmi’:ely rule out proton hyperfine interactions as the cause of the various peaks in the ODMR it appears extremely unlikely that the hyperfine levels could be distributed over an energy region exceeding - 1.5 MHz. A model calculation using Mispelter’s hyperfiie tensor [7], and assuming that the largest effectis arise from the electron spin density at carbon atom number 4, yields for the case of two protons (4 and 4’) a total hyperfine width of 1.3 MHz: The lines we observe have widths of ca. 1.5 -2.0 MHz, sur.h as might be expected if unresolved hyperfine splitting were rhe source of linewidth. The phosphorescence spectrum displays only one component within the limitations ti;josed by the ca. 3 IXII-~ spectral Xnewidth. This linewidth is due to inhomogeneity of the sample and may consist of overlapping spectra - corresponding to two (or more) in.homogeneously broadecdd phosphorescence transitionsfrom the two (or more) nonequivalent triplet C,,H,, molecules whose presence was inferred from the optically detected zero field magnetic resonance. The.splitting in the phosphorescence spectrtim would have to be less than ti. 1 cm-l in order for this interpretation to be valid. The observed splitting between the two strongest lines of the fluoreicence and. absorp tion of C12H,, in C12Dlo at 1.6 K and 4.2 K is 10 cm-l. Furthermore-the intensities of.these two,&rongest lines of.the.O-0 singlet transition ate about equal. Clearly the relative stkngths of the two strongest magnetic
xal
resonance
implies,
: .‘.
,: .
.
. ..
:
.-
.,
” .
_....~
.,.
.’
.
278 f&
RAW.Hohtrasser
et al/Triplet
state of bipJlenyI-Jr,a in biphet?vl_dlo
2 Number of observed tnnsitions
System
from:
fluorescence
phosphorescence
triplet magnetic reScInnnce
neat C12H10 nest C,zD,o
1 -
-
-
CIZHIO in CIZDIO phenanthrcnc in CLlHlo[3]
4(1:1:0.1:0.1)
--
pyrenc in C1zH1o [3 J anthracene in C12H,o 13)
other
1a) 2b)
4(1.7:1:0.1:0.1) 3 c) (0.8:1:0.5) 2c)
3d)
2 151
131
4(2 intense: _ -
:c)(l.*%J) [3] 3C)(O.8:1:0.5) -
2 weak) [I21 _
‘a) From absorption spectra in HXlW and high resoltution 2-photon ztbsorption [ 181. b, Discussed in HMW: see ref. [ll. c) This is not a specification of the number of separate centers for excitation: It refers to those spectral transitions that were observed at the limited spectral resolution used in the experiment. In some experiments (see ref. [3] ) the spectral ener_qy resolution was less than the expected inhomogeneous linewidth, thus additional transitions may actunlly be observable. Thus in ref. 131 the system phenanthrene in biphenyl was known to display only two peaks but more transitions are observable when a higher resolution specuometer is used [ZO]. d) In the present work we have observed as many as three transitions in the zero-field ODMR cxperimcnt involving CrzHlo in its triplet state.
chanism dominates and therefore that the twisting not more than a degree or so [ 171.
is
3.3. Site spliiting numerology Important
to understanding
the structure
of bi-
phenyl molecules in crystalline biphenyl is the actual number of transitions observed in the various physical measurements. For example, as described in HMW, there are two prominent transitions and two satellites (i.e., much weaker lines) seen in the single crystal optical spectra of C,,H,h in C,,D,,.,As mentioned above various numbers of peaks up to a maximum of three were observed in olir zero-field ODMR spectra. In table 2 we have collect&I some of the available data relating to fluorescence, phosphorescence, two-photon absorption, zero-field ODMR, and EPR of photoexcited states of neat and +x&d crystal studies involving biphenyl. A general observation is that different peysical measurements have usually-yielded at least two prevalent transitions and often one but not more than -two other-much.weaker ?ran~itions;.The high resolution optical study by Prasad and Hochstrasser of phetia&hrene in bipheoyl ji9 ] suggtisted that- the peaks in the dptical spectrum - usually chosen to in-didate sites - are actually-peaks of a continuous dikibution rep+nting a.range. of transition energies Thus to assign a “number” -td the sits multiplicity may not he
too meaningful other than in broad-band excitation experiments. Surely with ultra-sharp laser excitation a wider range of physical parameters would be exposed: Hence the use of the term “prev,alent sites” indicating molecules from the inhomogeneous distribution having the most probable transition energies. It seems reasonable as a first guess to assume that the site shifts giving rise to the multiplet spectra observed by various spectroscopic techniques are caused by nearest neighbor
interactions.
In that case the num-
ber of different sites, say for a perdeutero molecule in a disordered structure, can be calculated readily and each given a statistical weight. A working model might exclude even near neighbour interactions along tb.20, b and c axes but include inteactions between interchange equivalent molecules. Such a model gives rise to five sites with relative statistical weights 1:2:2:2:4, and given that.the energies of some of these confiiurations will be measureably different, the experimental results are consistent with this model.
4. Conclusions me phosphorescence of C12Hi3 in C12D,, at i.S K displays no “site” splitting, such as was seen pieviously in the fluorescence [2]. However the ODkR, both for the’O-O.transition &d the total phosphorescence, ex-
:. ,_ : .,
., :
_’
~-,
:
.,
:
:
:._
..
..
hibits more microwave transitions than expected, again consistent with there being more than one phosphorescent center not spectrally resolved in the optical spectrum. The neutron powder diffraction data indicates a significant contraction of the a-axis length on cooling +D,O to 4.2 K from 300 K but does not indicate that a phase transition has occurred. Whatever the changes in structure, if any, that are occurring, it is clear they are very slight indeed. A disordered structure of very slightly non-centrosymmetric biphenyl molecules is consistent with the known experiments on biphenyl-12 1o molecules in biphenyl-dlO, and on the structure of biphenyl-dlO. Brenner et al. [S] suggested disorder as one of the possibilities for their observation of multiple ENDOR transitions. The heterogeneous effects mentioned above require experimental exploration. The biphenyl crystal is unusually deformable at 300 K and it is conceivable that considerable macroscopic heterogeneity can be introduced by what would be otherwise normal procedures.
5. Note A very recent paper by Bree et al. [20] reports fluorescence polarization data for biphenyl that are consistent with there being surface related biphenyl molecules that are substantially misoriented compared with those in the bulk of the C,,Ht,-, crystal. These authors have also reported the mixed crystal phosphorecence with which we are basically in agreement.
Acknowledgement We thank Professors R. Kopelman, P.N. Prasad and Dr. Friedman (University of Michigan) for providing us with an advance copy of their results on the’ phosphorescence of biphenyl in mixed crystals. R.M.H. wishes to express his gratitude to Professor
:
H.J. Kahane and Dr. H.P. Trommsdorff
(Universitk de Grenoble) for making possible part of this research. References [l]
R.D. hlcAlpinc, Ph.D. Dissertation, Pennsylvania (1968).
(21 R.hI. Hochstmsscr,
University
of
R.D. McAIpine and J.D. Whiteman, J. Chem. Phys. $8 (1973) 5078. [3] RM. Hochstrnsscr 3ndG.J. Small. J. Chcm. Phys. 48 (1968) 3612. [4] P.S. Friedman. R. Kopclman and P.N. Prasad, Chem. Phys. Lcttcrs 24 (1974) IS. [S] H.C. Brenncr, CA. Hutchison. Jr. and M.D. Kemplc, J. Chcm. Phys. 60 (1974) 2180. [6] G. Zcrbi and S. Sandroni, Spectrochin~. Acta A24 (1968) 483; A24 (1968) 511; A26 (1970) 1951. [7] J. MispeItcr,Chcm. Phys. Letters 10 (1971) 539. [S] J. Trotter, Acta Cryst. 14 (1961) 1135; A. Hargreavcs and S.H. Rizvi, Actn Cryst. 15 (1962) 365. 191 V.&f. Koshin and K.V. Mirskoya, Sov. Phys. Cryst. 14 (1970) 938. lo] G.J. Small, Ph. D. Dissertation, University of Pennsylvania (1967). see also ref. [3]_ 111 A. Almcnningcn and 0. Bastiansen, Klp. Norskc Sclsk. Skriftcr 4 (1958). 121I.L. Messagcr, hl. Snnquer. J.L. Baudour and J. Meinnel, unpublished results: CNRS report No. 015 (1974) 1. [ I3 ] C.A. Hutchison, Jr. and V.H. McCann, J. Chcm. Phys. 6 1 (1974) 820. 1141 E.V. Shpolskii ond L.A. Klinova, 91111.Acad. Sci. USSR Phys. Ser. 20 (195G) 428, and rclcrences therein. [lS] E.J. Bowen and B. Brocklehurst, J. Ghan. Sot. (London) (1954) 3875. [16] If the electric dipole transition moment caused by crystal lield miring were very much skew to the molecular principal axes, then an altcmativc description of the observed polarization behavior might bc found. [ 171 Assuming that the exciton-like partners in the twisted form have relative intensities I(B,,)/~(B$ 3 5 X 10’ (measured) and that as little as 70% of I(t),& derives from the magnetic dipole mechanism [2] that tits the experimental data, then the elcctic dipole intensity ratio might bc as much as - 1.7 X 103. This coiresponds to approximately 1.7’ of twist. [ 181 R.M. Hochstrasser, H.-N. Sung and J.E. Wcssel, J. Chcm. Phys. 58 (1973) 10.4694. [ 191 R.M. Hochstrasser and P.N. Pmsad, Chcm. Phys. Letters 8 (1971) 315. (201 A. Bree, M. Ed&on-and R.+Swnrich, Chem. Phys. 8 (1975) 27.
OPTICAL AND MAGNETIC RESONANCE PROPERTiES OF THE TRIPLET STATE OF BIF’HENYL-fq, IN BIPHENYL-d,,, AND 4.2K NEUTRON DIFFRACTION OF BIPHENYL+,* R.M. HOCHSTRASSER, G.W. SCOTT** and A.H. ZEWAIL*** Deparrment of Chemistry,and Laboratory for Rcsenrch on the Structure of Jfatter, Ut~iversity of Pennsyylvania, Philadelphia, Pa. 19174, USA and H. FUESS Institut Ma.x YOULaue - Paul Langesin. 38042 Grenoble. France Received
13 June 1975
The phosphorescence spectrum of 2% proto in deutcrobiphcnyl dots not exhibit the splittings that were seen previously in the fluorescence spectra [R.M. Hochstrasser, R.D. McAlpine and J.D. Whitcman, J. Chem. Phys. 58 (1973) 50781. Ihc zeroficld magnetic resonance (optically detected) showed a structure that is most likely not due to hyperline cffscts: Two or three transitions wc~e observed in the ID + El mG ID .- El reions. In the ID +El region the largest splitting is 4.2 t 0.3 MHz while the splitting between the two strongest peaks is 2.0 f 0.3 MHz. A neutron diffraction study of a CtzDto crystalline powder at 4.2 K could be indexed on the basis of cell parameters u = 7.760, b = 5.544, c = 0.42 (A), P =?3.6O and space group WI /a. It is supgested that this structure is disordered, thereby accounting for the optical, Raman and magnetic resonance experimental results, but the exact nature of the disorder has not been determined.
1. Introduction Since the optical work by McAlpine [l] and by Hochstrasrer, McAlpine and ‘niteman [2] (HMW) exposed the existence of unexpected small splittings in the fluorescence spectra of dilute mixed crystals of proto in deuterobiphenyl, there has been considerabie activity and conjecture regarding the detailed structure of biphenyl crystals at low temperature. I-IMW found that each line of the fluorescence spectrum and the O-O absorption spectrum was split by ca. 10 cm-l into two nearly equally intense lines, and some weaker satellites, reminiscent of the site splitting that had been seen in the optical spectra (absorption, fluorescence, phosphorescence) of chemical impurities in bi* This research was supported by an N.I.H. grant GM12592 and in part by the LRSM at the University of Pennsylvania. ** Chemistry Department. University of California, Riverside, California 92502. *** Chemistry Department, University of California, Berkeley, California 94720.
phenyl crystals at low temperatures [3]. Although HMW did not report the details of the phosphorescence observed for proto in deutero crystals they made an extensive but unsuccessful search for a splitting analogous to that observed in fluorescence. HMW concluded upon the possibility that biphenyl crystals may undergo a structural modification upon cooling to 4.2 K. Subsequently two additional reports have appeared: Raman spectra of biphenyl have been shown to vary with temperature in such a manner that suggests a slight continuous change in crystal structure [4] ; The ENDOR of the lowest triplet state of protobiphenyl in a deuterobipheny1 host has exhibited additional transitions [S] over those expected on the basis of a structure having one-half a molecule of biphenyl per asymmetric unit. Any multiple site hypothesis would require than in principle the singlet-triplet transitions also be multiple and in this note we describe briefly our phosphorescence spectra and the optically detected magnetic resonance at zero field ot the lowest triplet state.
274
R.M. Hochstrasser
et al/Triplet
In addition to this k-; report on some preliminary neutr& diffraction data referring to a crystalline powder of perdeuterobiphenyl at 4.2 K ‘and 300 K. The main problem to be addressed is the nature of the structure of the-crystal and of.the photoexcited states in the mixed crystal at low temperatures.
2. Experimental
procedures
and results
Z 1. I%ocedures Just as in HMk we have used perprotobiphenyl and pzrdeuterobiphenyl that have been chromatographed, treated with potassium, and zone rsfmed. The crystals were grown in a Bridgman furnace. The phosphorescence spectra were photographed at ca. lo5 resolution on a 2 m Czerny-Turner fitted with an echelle grating, and with a lower resolution Eagle spectrograph. The zero field magnetic resonance spectra were taken in several ways (to be described) but each method involved detestion of the phosphorescence with an EMI 62565 photomultiplier either after dispersion using a Spex :;.75 m Czerny-Turner monochromator or istilation with the 3 mm Schott filters GG435 and BG38. In some experiments the output of the photomultiplier was accumulated with a signal averager ‘(NS575) using a time constant of 50 ms. The microwave sweep o.=sillatcn (HP 86908 with 8699B) was adjusted to repetitively =x’eep at 200 KHz/s through the microwave range of interest. The beginning and end frequencies of the sweep were measured with a frequency counter (EIP Inc. 3508-03) and the frequencies of tie peaks in the microwave spectrum were obtained by linear interpolation. In other experiments the output of the photomultiplier was amplified chart recorded with a time constant of 2 s and a sweep rate of 80 kHz/s 2.2. Z?ie pfzqsphorescence The phos‘phorescence of a perdeuterobiphcnyl crystal containing 2% perprotobiphenyl is shown in fig. i. The inscri shows the O-O transition at high resolution tc consist of a single peak (i.e,, no splitting such as -was seen in the fluorescen&e)‘having a width df c&3.3 +I’~. The slight asymmeiry-is noi tinder: stood,.& it may-simply indik&a characteristic of
state ofbiphetlyl-h
10 in biphenyld10
the inhomogeneous distribution of molecules in this crystal. Each main peak in the phosphorescence is attributable to a ground state fundamental vibration [6] or a combination of these vibrations: No repetitions occur, so there is no indication at the present resolution ofa site splitting such as was seen in fluorescence [2]. The origin is located at 4316.1 A (23 161 cm-l) and the spectrum consists of the totally symmetric vibrations at 334 (m), 743 (m), 1002 (ms), 1035 (mw), 1276 (s) and 1595 (s) cm-l _ The doublets in the spectrum starting around 0 + 1600 cm-l are due to the splitting between combinations containing the 1595 mode and those containing the comparably strong 334 + 1276 = 1610 cm-kombination. During some experiments on the temperature dependence of the magnetic resonance (see below) we noticed that the phosphorescence becomes much more complex and many new lines appear when the tempe:;ture is raised much above 4.2 K. 2.3. The optically detected magnetic resonance (ODMR) The ODMR at zero magnetic field in the region of 3229 MHz consistently appeared as at least a doublet independent of whether the light detected was confined to that at the wavelength of the O-O band (3/4 m Czemy-Turner: 2 mm slits) or whether the whole phosphorescence was being detected. A typical spectrum corresponding to a single chart recorded microwave sweep (10 MHz in 100 s) at 1.5 K, detecting on the O-O band, showed peaks at 3229.3 MHz of strength 1 .O and 3227.1 MHz of strength 0.4. More accurate values were obtained by broad band phosphorescence detection and signal averaging. In this case three peaks were observed (relative intensities in parentheses) at: 3228.4 MHz (1.0); 3226.6 MHz (0.6); and 3224.2 MHz (0.1). The ID +El region of triplet biphenyl ODMR at zero field should occur at 3209 MHz according to Mispelter [7]. In the region of the ID-El transition (- 353 1 MHz) we similarly observed three peaks at: 3461 .O MHz (0.5); 3463.5 MHz (1 .O); and 3465.3 MHz (0.4). The estimated frequency measurement error is L 0.3 MHz, but some frequencies and intensities may be subject to uncertainty due to adiabatic fast passage effects oq ihe spectra. What is certain is that in the 3225 MHz region the largejt splitting is at least 4’.2 kO.3 MHz while the splitting between the two strongest peaks is 2.0 i 0.3 MHz. The ODMR spectrum
. .
:
275
R.M. Hochstrasser et al./Tripler rrate of biphenyl-h 1,~in biphenyl-dlo
BIPHENYL 2%
(C,,H,,
ISOTOPIC
) MIXED
1
PHOSPHORESCENCE CRYSTAL
I-O
I
I
4800
4700
I
I
4600
4500
I
I
4400
I
4300
X,(81
Fig. 1. The phosphorescence assignments
spectrum
of biphcnyl-h
10 (2%) in biphenyl-d
1o at 1.6 K. The numbering
of the vibrations
follows
the
given in ref. [6].
at 3225 MHz was studied with a crystal both at 1.6 K and at 4.2 K and no differences in the relative intensities of the peaks were detected.
to index it in the same space group WI/a) and refme new cell parameters. These new parameters and their standard deviations are given in table 1 together with low temperature X-ray values of Kozhin and Mirskoya
2.4. Z+e neu troll diffraction
191. Both the X-ray results between 77 K and room temperature and the neutron data at 4.2 K show a pronounced contraction in the a-axis direction which is nearly perpendicular to the molecular planes: .The 4.2 K values obtained from a limited number of well resolved powder peaks are certainly less precise than the X-ray results on a single crystal but they seem to indicate that the change in the unit cell is smooth. The integrated intensities at 3’00 K agree Fairly well with calculated values based on Trotter’s [S] structural model without any further refinement for the positional and thermal parameters. The agreement for the low temperature pattein is not so good (agti without reftiing) but does not indicate that a structural transition has occuried.
Neutron powder diffraction patterns were taken at 300K and 4.2 K for C:,D,, on the DIA facility at the HFR (Grenoble). The wavelength used was 1.5 10 A obtained with a Ce monochromater. The 300 K pattern was readily indexed using the unit ceil parameters previously determined by X-rays [S] . That is, space group P2t/a, n = 8.12, b = 5.64, c = 0.42 (a), fl= 95.4O. It was shown by HMW [2] that there are no significant differences in the structures of C12H10 and C,,D,, at room temperature - a confirmation of lhis fact was later reported by Brenner et al. [S]. The low temperature data could not be indexed on the basis of these cell parameters, but it was possible
. . ._.
:
276 Table 1 IALice constants
of biphcnyl
as function
of tcmpemture
(estimated
rtandxd
deviations
in parentheses)
2-W
a
b
C
0 (deg)
Method
Rcfcrcnce
CIZHIO
294
8.167(4)
5.661(4)
9.449 141
95.24 (0.2)
X-ray
ClZJIO
220
8.013 (4)
5.620 (4)
9.442(4)
96.28CO.2)
X-ray xal
71
7.906 (4)
5.610(4)
9.436(4)
96.48tO.2)
X-ray
7.760 (61
5.544 (7)
9.42 (4)
93.6lCO.5)
Ncu tr. powder
Koshin and Mirskoya 191 Koshin and Mirskoya [9] Koshin and hlirskoya [9] prescn t work
ClzH IO
4.2
ClZDlO
3. Discusion The
Xd
resonance transitions, corresponding to the two triplet components under discussion, are not equal but more than two components are observed just as in the optical spectra of the singlet. The observed ratio of intensities in the zero field ODMR spectrum is not caused by a Boltzmann distribution being attained during the excited state lifetime {the phosphorescence lifetime is ca. lo6 times that for fluorescence) because the optically detected magnetic resonance is the same at 1.6 K and 4.2 K. A possible reason for the difference in the re!ative intensities of the doublet components seen in absorption (a measure of the r:lative amounts of each) and in the zero field ODMR is that differences in the non-radiative processes, such as intersystem crossing (S1 + Tf and/or T, + So) and/or triplet radiative processes (T, + So), are introduced by the different environmental perturbations. For the case of mixed crystals of phenanthrene in biphenyl the singlet aad triplet spectra exhibited site splitting but the components did not have the same relative intensity in the two spectra [3] _In that case it was found that the intersystem crossing was not equally efficient for the two principal sites [3, lo]. The fractional change in the phosphorescence signal expected at a microwave resonance is intimately connected with the populating rates, nonradiative and radiative rates, so no reliable assessment of the source of the intensity difference can be made on the basis of a single ODMR relative intensity measurement. A key experiment consists of studying the ODMR of C12HIo in C12D,o subsequent to the excitation of just one of the singlet transitions, but our attempts to do this have failed -. due to a lack of light intensity within such a narrow frequency range in the region of 3000 A. Ultimately it should be possible to use a frequency doubled dye. laser for this type ofexperiment. netic
observation
of multiplets
in the zero
field
mag-
subject to the fiJIlowing assumptions, that the photoexcited C,2D.Lo crystal contains two or more nonequivalent triplet CIZHIo molecules. While we &mot. defmi’:ely rule out proton hyperfine interactions as the cause of the various peaks in the ODMR it appears extremely unlikely that the hyperfine levels could be distributed over an energy region exceeding - 1.5 MHz. A model calculation using Mispelter’s hyperfiie tensor [7], and assuming that the largest effectis arise from the electron spin density at carbon atom number 4, yields for the case of two protons (4 and 4’) a total hyperfine width of 1.3 MHz: The lines we observe have widths of ca. 1.5 -2.0 MHz, sur.h as might be expected if unresolved hyperfine splitting were rhe source of linewidth. The phosphorescence spectrum displays only one component within the limitations ti;josed by the ca. 3 IXII-~ spectral Xnewidth. This linewidth is due to inhomogeneity of the sample and may consist of overlapping spectra - corresponding to two (or more) in.homogeneously broadecdd phosphorescence transitionsfrom the two (or more) nonequivalent triplet C,,H,, molecules whose presence was inferred from the optically detected zero field magnetic resonance. The.splitting in the phosphorescence spectrtim would have to be less than ti. 1 cm-l in order for this interpretation to be valid. The observed splitting between the two strongest lines of the fluoreicence and. absorp tion of C12H,, in C12Dlo at 1.6 K and 4.2 K is 10 cm-l. Furthermore-the intensities of.these two,&rongest lines of.the.O-0 singlet transition ate about equal. Clearly the relative stkngths of the two strongest magnetic
xal
resonance
implies,
: .‘.
,: .
.
. ..
:
.-
.,
” .
_....~
.,.
.’
.
278 f&
RAW.Hohtrasser
et al/Triplet
state of bipJlenyI-Jr,a in biphet?vl_dlo
2 Number of observed tnnsitions
System
from:
fluorescence
phosphorescence
triplet magnetic reScInnnce
neat C12H10 nest C,zD,o
1 -
-
-
CIZHIO in CIZDIO phenanthrcnc in CLlHlo[3]
4(1:1:0.1:0.1)
--
pyrenc in C1zH1o [3 J anthracene in C12H,o 13)
other
1a) 2b)
4(1.7:1:0.1:0.1) 3 c) (0.8:1:0.5) 2c)
3d)
2 151
131
4(2 intense: _ -
:c)(l.*%J) [3] 3C)(O.8:1:0.5) -
2 weak) [I21 _
‘a) From absorption spectra in HXlW and high resoltution 2-photon ztbsorption [ 181. b, Discussed in HMW: see ref. [ll. c) This is not a specification of the number of separate centers for excitation: It refers to those spectral transitions that were observed at the limited spectral resolution used in the experiment. In some experiments (see ref. [3] ) the spectral ener_qy resolution was less than the expected inhomogeneous linewidth, thus additional transitions may actunlly be observable. Thus in ref. 131 the system phenanthrene in biphenyl was known to display only two peaks but more transitions are observable when a higher resolution specuometer is used [ZO]. d) In the present work we have observed as many as three transitions in the zero-field ODMR cxperimcnt involving CrzHlo in its triplet state.
chanism dominates and therefore that the twisting not more than a degree or so [ 171.
is
3.3. Site spliiting numerology Important
to understanding
the structure
of bi-
phenyl molecules in crystalline biphenyl is the actual number of transitions observed in the various physical measurements. For example, as described in HMW, there are two prominent transitions and two satellites (i.e., much weaker lines) seen in the single crystal optical spectra of C,,H,h in C,,D,,.,As mentioned above various numbers of peaks up to a maximum of three were observed in olir zero-field ODMR spectra. In table 2 we have collect&I some of the available data relating to fluorescence, phosphorescence, two-photon absorption, zero-field ODMR, and EPR of photoexcited states of neat and +x&d crystal studies involving biphenyl. A general observation is that different peysical measurements have usually-yielded at least two prevalent transitions and often one but not more than -two other-much.weaker ?ran~itions;.The high resolution optical study by Prasad and Hochstrasser of phetia&hrene in bipheoyl ji9 ] suggtisted that- the peaks in the dptical spectrum - usually chosen to in-didate sites - are actually-peaks of a continuous dikibution rep+nting a.range. of transition energies Thus to assign a “number” -td the sits multiplicity may not he
too meaningful other than in broad-band excitation experiments. Surely with ultra-sharp laser excitation a wider range of physical parameters would be exposed: Hence the use of the term “prev,alent sites” indicating molecules from the inhomogeneous distribution having the most probable transition energies. It seems reasonable as a first guess to assume that the site shifts giving rise to the multiplet spectra observed by various spectroscopic techniques are caused by nearest neighbor
interactions.
In that case the num-
ber of different sites, say for a perdeutero molecule in a disordered structure, can be calculated readily and each given a statistical weight. A working model might exclude even near neighbour interactions along tb.20, b and c axes but include inteactions between interchange equivalent molecules. Such a model gives rise to five sites with relative statistical weights 1:2:2:2:4, and given that.the energies of some of these confiiurations will be measureably different, the experimental results are consistent with this model.
4. Conclusions me phosphorescence of C12Hi3 in C12D,, at i.S K displays no “site” splitting, such as was seen pieviously in the fluorescence [2]. However the ODkR, both for the’O-O.transition &d the total phosphorescence, ex-
:. ,_ : .,
., :
_’
~-,
:
.,
:
:
:._
..
..
hibits more microwave transitions than expected, again consistent with there being more than one phosphorescent center not spectrally resolved in the optical spectrum. The neutron powder diffraction data indicates a significant contraction of the a-axis length on cooling +D,O to 4.2 K from 300 K but does not indicate that a phase transition has occurred. Whatever the changes in structure, if any, that are occurring, it is clear they are very slight indeed. A disordered structure of very slightly non-centrosymmetric biphenyl molecules is consistent with the known experiments on biphenyl-12 1o molecules in biphenyl-dlO, and on the structure of biphenyl-dlO. Brenner et al. [S] suggested disorder as one of the possibilities for their observation of multiple ENDOR transitions. The heterogeneous effects mentioned above require experimental exploration. The biphenyl crystal is unusually deformable at 300 K and it is conceivable that considerable macroscopic heterogeneity can be introduced by what would be otherwise normal procedures.
5. Note A very recent paper by Bree et al. [20] reports fluorescence polarization data for biphenyl that are consistent with there being surface related biphenyl molecules that are substantially misoriented compared with those in the bulk of the C,,Ht,-, crystal. These authors have also reported the mixed crystal phosphorecence with which we are basically in agreement.
Acknowledgement We thank Professors R. Kopelman, P.N. Prasad and Dr. Friedman (University of Michigan) for providing us with an advance copy of their results on the’ phosphorescence of biphenyl in mixed crystals. R.M.H. wishes to express his gratitude to Professor
:
H.J. Kahane and Dr. H.P. Trommsdorff
(Universitk de Grenoble) for making possible part of this research. References [l]
R.D. hlcAlpinc, Ph.D. Dissertation, Pennsylvania (1968).
(21 R.hI. Hochstmsscr,
University
of
R.D. McAIpine and J.D. Whiteman, J. Chem. Phys. $8 (1973) 5078. [3] RM. Hochstrnsscr 3ndG.J. Small. J. Chcm. Phys. 48 (1968) 3612. [4] P.S. Friedman. R. Kopclman and P.N. Prasad, Chem. Phys. Lcttcrs 24 (1974) IS. [S] H.C. Brenncr, CA. Hutchison. Jr. and M.D. Kemplc, J. Chcm. Phys. 60 (1974) 2180. [6] G. Zcrbi and S. Sandroni, Spectrochin~. Acta A24 (1968) 483; A24 (1968) 511; A26 (1970) 1951. [7] J. MispeItcr,Chcm. Phys. Letters 10 (1971) 539. [S] J. Trotter, Acta Cryst. 14 (1961) 1135; A. Hargreavcs and S.H. Rizvi, Actn Cryst. 15 (1962) 365. 191 V.&f. Koshin and K.V. Mirskoya, Sov. Phys. Cryst. 14 (1970) 938. lo] G.J. Small, Ph. D. Dissertation, University of Pennsylvania (1967). see also ref. [3]_ 111 A. Almcnningcn and 0. Bastiansen, Klp. Norskc Sclsk. Skriftcr 4 (1958). 121I.L. Messagcr, hl. Snnquer. J.L. Baudour and J. Meinnel, unpublished results: CNRS report No. 015 (1974) 1. [ I3 ] C.A. Hutchison, Jr. and V.H. McCann, J. Chcm. Phys. 6 1 (1974) 820. 1141 E.V. Shpolskii ond L.A. Klinova, 91111.Acad. Sci. USSR Phys. Ser. 20 (195G) 428, and rclcrences therein. [lS] E.J. Bowen and B. Brocklehurst, J. Ghan. Sot. (London) (1954) 3875. [16] If the electric dipole transition moment caused by crystal lield miring were very much skew to the molecular principal axes, then an altcmativc description of the observed polarization behavior might bc found. [ 171 Assuming that the exciton-like partners in the twisted form have relative intensities I(B,,)/~(B$ 3 5 X 10’ (measured) and that as little as 70% of I(t),& derives from the magnetic dipole mechanism [2] that tits the experimental data, then the elcctic dipole intensity ratio might bc as much as - 1.7 X 103. This coiresponds to approximately 1.7’ of twist. [ 181 R.M. Hochstrasser, H.-N. Sung and J.E. Wcssel, J. Chcm. Phys. 58 (1973) 10.4694. [ 191 R.M. Hochstrasser and P.N. Pmsad, Chcm. Phys. Letters 8 (1971) 315. (201 A. Bree, M. Ed&on-and R.+Swnrich, Chem. Phys. 8 (1975) 27.