Electron paramagnetic resonance of triplet phenanthrene-d10 and naphthalene-d8 in single-crystal biphenyl from 5–80 K and characteriz

Chemical Ph?rsks 23 (1977) 217-230 0 North-Holland Publishing Company

ELECTRON PARAMAGNETIC RESONANCE OF TRIPLET PHENANTHRENE+, AND NAPHTHALENE.$

IN SINGLE-CRYSTAL BIPHENYL FROM S-SO K

AND CHARACTERIZATION

OF PHASE TRANStTlONS

.

IN BIPHENYL BELOW 80 K

AS. CULLlCK’and Roger E. GERKIN Department of Chemistry, Tile Ohio State University Columbus, Ohio, 43210, USA Received 19 January

1977

Previously reported single discontinuities in the zero-field splittings of triplet phenanthreneil,o and naphthalene-da in biphenyl near 40 K are further charactcrbed. The transition leading to these discontinuities in the mised crystals ip shown to set in at 42.0 + 0.4 K upon lowering the temperature, and to occur over a tenkperature range of a few degree;. Coc!iz~ pureand mixed-crystal biphenyl near the pammagnetic resonance transition temperature were and warming curves measured, and a first order phase transition in pure tiphenyl at 42.4 t 0.3 K is reported, for which AU = 70 cal mole-‘. A second prominent feature in the temperature dependences of the zero-field splittings in these mixed crystals, rvhich was not previously reported, has been shown to occur near 15 K. Although this second feature is only part’kW characterized by the present data, it is tentatively assigned as due to a transition intrinsic to pure biphenyl.

of

I_ Introduction

We have previously reported the existence of single discontinuities of the zero-field splittings of triplet phenanthrened, o and naphthalened% in single crystal biphenyl-h10 which occurred in a small temperature range near 40 K 11) _Those studies revealed that single EPR absorption signals characteristic of these systems near 77 K give way to sets of four prominent EPR absorption signals below 40 K. Although another previous report had suggested the possibility of gradual structural changes occurring in biphenyl over a range of temperatures between 77 K and 15 K [Z], our observation of rather sharp single discontinuities of guest triplet zero-field splittings suggested that there might in fact be an isothermal transition in crystalline biphenyl near 40 K to a phase which persists to 4 K and is possibly characterized by a loss of the molecular inversion center [2,3). We here report subsequent investigations which further characterize the zero-field splitting dicontinuities near 40 K. In addition we report cooling and

warming curves for both pure and mixed-crystal biphenyl which establish an isothermal transition near 42 K in pure biphenyl. Finally, we report the existence of a second set of discontinuities of zero-field splittings near 1.5 K which our subsequent more intensive investigation revealed, and .which may be associated with further structural changes in biphenyl single crystals.

2. Experimental apparatus, samples and methods

The low-field EPR spectrometer and Andonian variable temperature cryostat have been described previously [4,5]. Temperature measurement was effected with a Au (0.07% Fe) versus chromel-P thermocouple attached to the brass resonant cavity. Calibration of the thermocouple has been described in detail [S] and periodic rechecking of it at 3 series of reference temperatures aIlowed temperature measurement to + 0.1 K. Temperature control of the cavity and sample was achieved by regulating proportionally the current through a 20 SLheater in the cryostat according to the

218

AS.

Cullick. R.E. GerkinlEPR

of triplet phenanthrene-dl~

sense and magnitude of the deviation of a second thermocouple’s EMF from a reference voltage or according to the sense and magnitude of the deviation of the voltage developed across a GaAs diode (Lake Shore Cryotronics, Inc.) from a reference voltage. The temperature of the microwave cavity during an individual set of measurements was held fixed to within + 0.1 K. The A-H6 lamp output was filtered by 5 or 3 cm pathlength solution filters containing 45 g Q-1 of CoSO4-7H20, 240 g Q-l of NiSO,.6H,O, and I g Q-i of M8C404 [6] _ No differences in experimental vdues resulted from successive use of the two filters in

several sets of measurements. By use of these filters, the difference between the crystal temperature and the cavity temperature was reduced to < 0.5 K [5]. For the warming and cooling curve determinations, the apparatus consisted primarily of two concentric brass cans. The smaller one, referred to as the sample can, was ftiled either with a powdered sample or a molten sample which was ahowed to cool in the sample can. A thermocouple junction, sometimes attached to a piece of copper foil, was lowered into the sample. The sample can was ftied with He gas before being sealed to a support system, and then was inserted into the outer cylinder which was also sealed to the support system. The entire apparatus was lowered into the Andonian cryostat. The space between the two cyhnders could be evacuated or filled with any desired pressure of He gas up to = 2 atm. In addition, the temperature of the outer can could be controlled to + 0.1 K

in the same way as described for the microwave cavity. Phenanthrene-d10 and naphthalenedg of 98% nominal isotopic purity obtained from Merck, Sharp and Dohme, Ltd. and biphenyl-hIo from Eastman Organic Chemicals were used as received. Biphenyl crystals containing e 0.001 mole fraction phenanthrenedtO or z~ 0.01 mole fraction naphthalene-dg were grown by the Bridgrnan method. The experimental methods used for obtaining the bulk of the splitting data have been described in the previous report [I]. Typically, a sample was cooled from room temperature to = 77 K in a period of = 1 h, further cooled to = 4.5 K in several hours, and cooled from =Z45 to = 30 K in a period of = 5-7 h, including 2-3 h halts at = 40 and = 35 K. Other methods used in subsequent experiments are described below.

3. Treatment

and ncphthrrlene-ds

of data

The basic spin hamiltonian 3C=H-gl~I-S+D$+E(~~

[4] -s;>,

has been used to describe the magnetic resonance data, where A, B, and C designate the principal axes of the tine-structure tensor. Thus, consistent with earlier usage [7], all precision resonance data were leastsquares fitted by expressions of the form

where 4 is c-l times the ith measured frequency for resonance, Hi is the measured field for resonance at the ith frequency, and QIand 7 are parameters. Field magnitudes and orientations were such that this approximation is fully adequate to within 1 X 10M7 cm-l. The values of Ly from the least squares fits of the data were

taken to be the best values of the triplet state zero-field energy level separations divided by hc.

4. Results Figs. 1 and 2 present the temperature dependences of (D-E)Jhc and IZJZi/hc of phenanthrenedro in biphenyl-hlO, respectively, obtained from the data listed in table 1, while figs. 3 and 4 present the temperature dependences of (D-E)fhc and (D + E)/hc of naphthalened in biphenyl-hlo, respectively, obtained from the data listed in table 2. Run-to-run uncertainties are estimated to be = 2 X 10A6 cm-l, typically, although in some cases within-run uncertainties are somewhat larger as indicated by the vertical bars. For convenience, the absorptions below = 40 K are numbered as signals l-8 for each mixed crystal system, in order of decreasing wavenumber at a given temperature, with the exception of the single encircled point at 6 K in fig. 1 which is left unassigned. The smooth curves in figs. 1, 2, and 3 (with the exception of signal 4 in fig. 3) were drawn from calculated third degree polynomial fits to the data in the temperature range in which the data were obtained *. Smooth curves in fig. 4 were drawn * While additional data have been obtained and the quality of the polynomial fits has been improved relative to those given in our previous report, particularly near critical regions of the dependences, it is possible that some timer features are stiU masked or distorted.

AS. C&lick. R.E. Gerkin/EPR

0

10

20

30

40 T, ‘K

30

60

70

of rriplerprzenanrhrene-dr o and naphthlene-ds

219

r 80

Fig. 1. (D-EMrc for pheuanthrene-dro in biphenyl-lrro versus temperature. The smooth curves are best-fit third degree polynomial dependence% Signals 1-4 (below 40 K) in order of decreasing wavenumber. Open circles: signal 3 below 15 K. Encircled point: unassigned signal.

with the aid of a spline. The encircIed points at 38.8 and 40.4 K in figs. 2 and 3, respectively, indicate that a single EPR absorption was observed at each of these temperatures but that the absorption was broadened and exhibited structural features characteristic of two or more partially overlapped signals. 4.1. Phenanthrene-dIO in biphenyl Fig. 5 presents traces of the first derivatives ofD-E resonance absorptions of triplet phenanthrenedio in biphenyl between 42 and 36 K which illustrate the changes in spectral features which occur as the temperature is lowered from = 42 K. A single resonance absorption was observed from 77 K to as low as 41.8 K*. At 41.5 K the first derivative of the single absorption exhibited a broadened low field arm characteristic of two partially overlapped signals. This arm * Reported temperatures of magnetic resonance experiments are the measured cavity temperatures. The difference between the sample temperature and the cavity temperature is taken to be < 0.5 K, consistent with use of the fitter described in the text.

Fig. 2. IZEi/ircfor phenanthrene&ro in biphenylhto versus temperature. The smooth curves are best-fit third degree polynomial dependences. Signals 5-8 (below 40 K) in order of decreasing wavenumber. Open circles: signal 5 below 15 K. Squares: 15 K transition region signals. Encircled point: broadened signal with structural features. appeared to be further broadened at 41.3 K. At 40.8 K two separate, but nonresolvable, signals were observed which separated further in resonant field as the temperature was lowered further in small intervals to 39.6 K. At 38.5 K two absorptions were separated sufficiently in resonant field for determination of each splitting. At 36.6 K signal 3 could be determined, and at 28.2 K separate splittings of four absorptions could be determined. It should be emphasized that the preceding observations in the 36-42 K region were repeatable upon temperature cycling across this region. In this regard, several types of experiments were carried out: (1) fast-cooling (= 30 min) the sample initially from rodm temperature to below 35 K, then raising the temperature slowly to above 42 K; (2) slow-cooling (several hours) the samplefrom. to 35 K with 2 h halts at % 42 and = 38 K; (3) fast-cooling the sampIe from room temperature to 35 K then cycling across the 3S50 K region repeatedly before magnetic resonance ob-

.

‘T&e 1 : Zero+ieM”splittings of phekmthrene-dlo in biphenyl-h&from

E*riimental

Td

series

(0

Zero-field splitting. (wit~kmcertriinty) (cm )

78.0 71.3 66.4 60.5 54.8 51.2 45.3 44.9 42.4 41.8

0.1472231 0.1472508 0.1472684 0.1472883 0.1473040 0.1473117 0.1473227 0.1473277 0.1473267 0.1473255

.

,.

_ <.,:

= 5 to 5‘80 6 Ex~erime+l series

_: : Ta). -(IQ

_-

, ~~~ero&eldsp&ing (wit~u&tilinty) (cm 1 ‘-

~Si&713

signaxI 38.9 34.0 28.2 23.4 18.4 13.9 s&ax 2 3.88 34.0 28.2 23.4 18.4 13.9 9.8 7.7 5.3

(2) (4) (2) (2) (5) (2) (3) (4) (1) (8)

5 1

5 I

6 6 1 7 8 9 10

o.147379 (5) b)

11

0.1474502 (11) b) 0.147527 (5) 0.147557 (2) 0.147602 (1) 0.147619 (1)

1 12

0.147379 (5) b1 0.1474502 (11) b) 0.147494 (2) 0.1475252 (7) 0.1475538 (2) 0.1475819 (3) 0.1476007 (4) 0.1476039 (8) 0.1476095 (12)

B. i2Ellhc transitions 79.4 0.0933197 (3) 74.7 68.7 64.5 59.0 55.0 49.5 44.0 38.8

0.0933422 0.0933652 0.09?3806 0.0933994 0.0934124 0.0934327 0.0934486 0.0934671

(2) (2) (2) (2) (2) (2) (3) (161 d)

10 1 1 1 1

32.0 28.3 23.3 21.7 19.1 17.9 16.3 15.3

0.0935591 o.OY35740 0.0935830 0.0935844 0.0935938 0.0935960 0.0935976 0.0936070

(20) (IO) (IO) (3) (5) (5) (3) (3)

0.1473144 (19) 0.1472891(2) 0.1472834 (6) 0.1472792 (8) 0.1472820 (4) 0.1472787 (7) 0.1472786 (5) 0.1472633 (9) 0.1472599 (18)

28.2 25.1 23.4 18.4 13.8 9.9 5.2

Lkass&ned transition 0.147542 (15) f)

6.5

SignaX5 [continued]

14 14 14 14 14 13 I4 13

Signal 5 13 13 14 I3 I4 14 13 14

.’ 0.1473527 (12) .0.1473620(6) 0.1473766 (4) 0.1473840 (1) 0.1473889 (4) 0.1473926 (8) 0.1474020 (71 0.1474052 (7) 0.1474031 (10) 0.1474054 (5) 0.1473715 (28) 0.1473734 (12) 0.1473743 C33) 0.1473785 (10) 0.1473743 (6) 0.1473797 (I 1)

Signal 4 38.9 33.9

1

13 13 13 13 13 13 13 13 13

36.6 ‘. 34.0 28.2 25.1 23.4 21.4 ‘. 18.7 18.4 16.3 16.0. 14.8 C) 13.8 c) 12.0 c) 9.8 c) 7.7 c) 5.5 c)

14 13 13 13 13

15.0 14.6 13.8 13.6 12.8 12.0 10.7 7.2

0.0936068 0.0935910 0.0935979 0.0936019 0.0935942

.

(4) (3) (4) (14) (3)

0.093597 (5) 0.0935994 (4) 0.0935946 (2)

Siputl6 25.6

0.0935097 (1)

21.7 16.4 12.0 7.2

0.0935189 0.0935263 0.0935347 0.093.5482

”

(2) (4) (4) (5)

.’

Table 1 (continued) Experimental series

Experimental series

Zero-field splitting (wit~mtcertainty) (cm )

(K)

Signal 7 25.6 21.7 17.2 12.1 7.1

14 13 13 13 13 a) b) c) d) e,

Ta)

0.093465 (5) =) 0.0934691 (4) 0.0934707 (3) 0.0934804 (7) 0.0934918 (3)

13 13 13 13 13

splittings

Experimental series

of naphthaleneda

Ta)

in biphenylhto Zero-field splitting (with uncertainty) (cm-‘)

(K)

77.7 14.7 70.0 68.4 65.5 65.3 61.3 58.8 56.6 55.3 49.6 49.4 49.4 45.4 44.6 41.5 40.1

SipI

3

37-4 35.5 30.8 24.8 24.0 20.0 15.5 15.2 10.7 7.4

1

6.8

4 1 1 1 3 1 3 1 5

Si@ai 8 21.7 17.3 12.0 7.1 4.2

0.1149227 (5) 0.1149564 (3) 0.1149953 (3) 0.1150117 (4) 0.1150368 (3) 0.1150416(3) 0.1150767(2) 0.1150986 (2) 0.1151194 (6) 0.1151248 (19) 0.1151740 (5) 0.1151776 (7) 0.1151769 (4) 0.1152086 (9) 0.1152074 (2) 0.1152414 (16) 0.1152356 (5) b)

Experimental

T3

StXkS

6)

0.093425 (I) 0.0934301 (2) -D 0.093463 (2) 0.093466 (2)

of absorption

maxi-

Zero-field splitting (with uncertainty) (cm-‘)

siMa1.2 4 1 3 1 3 1 1

1 4 1

37.2 35.5 30.9 30.5 26.6 24.8 20.0 15.2 10.7 6.8 Signal 3 33.0 30.8 24.8 20.0 15.2 10.7 6.8

1 0.1153526 (17) 0.1153731(10) 0.1154129 (18) 0.1154830 (5) 0.1154807 (13) 0.1155192 (5) 0.1155389 (27) 0.115544 (44) c) 0.1155535 (8) 0.1155656 (38) 0.1155812 (8)

Zero-field splitting (with uncertainty) (cm-’ )

from * 5 to SJ 80 K.

A. (D-E)/hc transitions

: 1

(K)

Temperature is measured cavity temperature. Sample temperature is = < + 0.5 K above this temperature. These sign& are taken to be overlapped signah i and 2. These values were not included in the polynomial fits described in the text. This is a broad resonance absorption with structural features. These signals could not be completely resolved from adjacent signal. Data were obtained by interpolation ma from signal traces. D This signal could not be observed at this temperature.

Table 2 Zero-field

1 2 1 3 2 1 2 3 2 1 1 3 2 2

Ta)

1 4 1 1 1 4 4 1 4 4 1

Signal 4 35.5 33.0 30.8 24.8 20.5 18.1 17.5 15.2 12.2 10.7

6.2

0.1152451 (7) 0.1152498 (15) 0.1152674 (20) 0.1152795 (17) 0.1152913 (24) 0.1153050 (20) 0.1153274 (24) 0.115367~(12) - e)

0.115229 0.115207 0.115 146 0.115159

(15) d) (5) (5) (3)

O.l 151:oe?) - e)

0.115167 0.115143 0.115144 0.115019 0.114962 0.114988 0.114962 0.114946 0.1150080 0,1150070 0.1149819

(5) d) (15) (3) (10) (7) (15) d) (15) d) (4) (34) (4) (11)

222

AS. Cldick, RE_ GerkinfEPR of triplet phenanthrene-d~o and naphthalene-dg

Table 2 (continued) Experimental series

T”) (Ii)

6 6 6 6 6 6 7 6 6

5. (D + &]/he transitions 77.4 0.0839538 72.5 0.0839654 67.5 0.0839791 63.3 0.0839859 57.4 0.0839971 54.1 0.0840007 51.2 0.0840072 48.0 0.0840105 41.4 0.0840234

6 8

Signal 5 36.4 30.3

Zero-field splitting (with uncertainty) (cm-’ )

0.0840733 0.0840799

(4) (8) (2) (5) (4) (17) (12) (5) (10)

(17) (13)

Zero-field splitting (with uncertainty) (cm-‘)

Experimental series

7-a) (K)

6 6 8 8 8 8 7 6

Sig& 36.4 30.0 24.6 20.2 15.0 10.2 10.t 5.6

6 7 7

Signal 7 36.4 31.0 20.0

0.0839828 (60) 0.083966 (26) 0.083919 (26)

7 7 7 6

Signal 8 31.0 20.0 10.1 5.7

0.0838735 0.0837335 0.0837054 0.0836948

6 0.0840264 (3) 0.0840112 (8) 0.0839889 (8) 0.0839621 (22) 0.0839786 (9) 0.0839381 (6) 0.083924 (4) 0.0839152 (8)

(19) (64) (2) (36)

a) b) c) d)

Temperature is measured cavity temperature. Sample temperature is = < + 0.5 K above this temperature. This resonance absorption is broadened and is not included in the polynomial fit. This is an average of two absorptions which merged as a function of time. See text for discussion. These splittings calculated from values of resonant field interpolated from field sweeps and an assumed slope of linear fit.. Not included in polynomial fitting procedure. =) Transition not observed at this temperature.

servations commenced: (4) using a sampIe piece in one experiment which had been cooled to 4 K and then rewarmed to room temperature in a prior experiment. In a final set of experiments the sample was maintained below 4, X Tar a 48 h period to determine whether the multiplet sgr~:;urn observed below 40 K is in fact arising from a stable phase. (Inprevious experiments, the sample remained below 40 K for less than I2 h.) During most of this period, the sample was kept in the 36 to 40 K range, i.e. near but below the transition temperature. At approximately 6 to 10 h intervals, the zero-field splittings of signals 3 and 4 were measured at = 25 and = 35 K. Neither the zerofield splittings of sign& 3 and 4 at 25 K nor the spectral trace at 25 K changed detectably within the 48 h period. At the end of this period, spectral traces analogous to those of fig. 5 were obtained at small temperature intervals between 40 and 42 K. A single

absorption was observed at 41.9 K and zero-field splitting determinations at this temperature and at 5 1.2 K gave values in agreement with values predicted from our earlier results. From table 1, and fig. 2 it can be seen that the magnitude of the largest splitting between adjacent 12EI absorptions is a factor of two smalIer than the largest splitting difference between adjacent D-E signals. Since the multiplet signals spread in energy from a single transition at 42 K, as for D-E, the temperature bad to be lowered to 25.6 K before the splittings of these signals were separately determinable, even though a broad single absorption with structural features was observed at 38.8 K, and signal 5 was resolved from a group of partially overlapped signaIs at 32.0 K. A second prominent feature of fig. 1, which was not delineated by our earlier measurements [I], is the

A.S. Cullick. RE. Gerkin/EPR

of triplet phenanthrenedlo

-0

223

and naphthaleneds

D

10 t

20 1

30

!

50

601

i-2

60 1

T,: Fii. 4. (D + Ej/hc for naphthaleneds in biphenyl-hlo versus temperature. The smooth curves are spYme_fit. Signals 5-8 below 40 K in order of decreasing wavenumber. Opencircles:

data from ref. [ 101.

3. (D-E@ for naphthalene-ds in biphenylhlo versus temperature. The smooth curves are best-fit third degree polynomial fits [except signals 1 (below 15 K) and 41. Signals l-4 below 40 K in order of decreasing wavenumber. Encircled point: broadened signai with structural features. Square: transition region signal. Open circles: data from ref. [lOI.

Fig.

discontinuity of the splittings of signak I and 3 near 15 K. The open circles in fig. 1 represent data from a signal designated as signal 3 (for convenience), although it should be noted that these absorptions may arise from phenanthrene molecules inequivalent to those giving rise to signal 3 above 15 K. Signal 3 was studied intensively from 12 to 17 K during one experimental run, which consisted of splitting determinations and spectral trace recordings at closely spaced temperatures in this range. Between 12.1 and 14.8 K signal 3 exhibited varying degrees of broadening of its low field arm, indicating possible partial overlap with a signal whose zero-field splitting is slightly larger than that of the absorption measured as signal 3 at 14.8 K. Below 12 K, signal 3 exhibited no broadening. Moreover, at 16.2 K no broadening of signal 3 was observed; however, when the temperature was lowered

to 15.7 K, broadening of the high field arm of signal 3 was observed, indicating possible partial overlap with a signal whose zero-field splitting is slightly less than that of the absorption measured as signal 3 at 16.2 K. The discontinuity of the splitting value of signal 3 occurred between 16.2 and 14.8 K-. Zero-field splitting determinations both above and below 15.5 K were made for which the experimental temperature was attained from both lower and higher temperatures. No time dependence of the spectrum was observed in this temperature region. In addition, the splitting value of signal 1 has a discontinuity near I5 K. In one experiment, the value of the splitting of signal 1 was determined only as low as 13.8 K; subsequently, however, in the same experiment in which signal 3 was studied intensively in the 12-17 K range, signal I was not observed at 13.8 K but was observed at 16.5 K. No absorption below 13 K was observed in the region below 13 K predicted by downward extrapolation of the dependence of signal 1 above 15 K. However, a fourth absorption was observed at 6.5 K with a splitting value which was less than that of signal 2. Since signal 1 had a signal-tonoise ratio a factor of five less than that of signal 2, SO that overlap of these signals might not observably affect signal 2, there are two plausible alternatives: (1) the zero-field splitting value of signal 1 passes through

AS.

224

Cullick.

R.E. Gerh-inlEPR

of triplet

phenanrhrene-dlo

and nnphthalene-ds

analogous to that of signal 3. The 12EI absorptions were also studied intensively near 15 K. In a single experiment precision splitting (a)

7 i

-rG

366

(b)

JY 406 N

(cl

-,i\ 35G

Fig. 5. First derivative traces of (D-E)/hc absorptions of phemnthrene&,o in bipheny1-h ,a at 4419 MHz. (a) T = 42.3 K; (‘i) T= 4i.5 K; (c) Tz40.5 K; Cd) T= 37.1 K. a maximum near 13 K and decreases sharply in magnitude between 13 K and 6.5 K, a region in which it is partially overlapped with signal 2; or (2) the zero-field splitting of signal 1 exhibits a discontinuity near 14 K,

measurements from signal 5 were taken in the following temperature order: 23.3, 17.9,15.0, 13.6, 10.7, 12.8, 14.6, 15.3, 13.8, and 19.1 K with splitting values of 0.093583,0.093596,0.093607,0.093602, 0.093S99,0.093594,0.09359 1,0.093607,0.093598, and 0.093594 cm-l, respectively. The splitting increased as the temperature decreased from 23.3 to 15.0 K and then showed a decrease at 13.6 and 10.7 K; however, as the temperature was again increased to 14.8 K, the splitting exhibited a further decrease, which indicated that the splitting value does not pass through a smooth maximum as a function of temperature. The value of the spiitting reached a maximum at 15.0 K, a minimum at 14.6 K and intermediate values at 13.6 and 13.8 K. This suggests a shift of splitting, i.e. a discontinuity in the splitting value analogous to that of signal 3. In addition to the discontinuity near 15 K, the splitting value exhibited time dependence which was observed at 8.5 and 19.1 K. Ten minutes after the temperature was lowered from 13.8 to 8.5 K (the sampIe cavity was at thermal equilibrium), signal 5 had an apparent zero-field splitting of approximately 0.093609 cm-l. However, 20 min later the apparent zero-field splitting was less than 0.093597 cm-l. When the temperature was increased from 8.5 to 19.1 K, two partially overlapped low-field absorptions in a small field range consonant with that of signal 5 were initially observed, but after approximately 20 min only one absorption was present and there was no apparent partial overlap of signal 5 with any other absorption. In order to assign individual multiplet components to sets of equivalent phenanthrene molecules and thus obtain the zero-field parameters for each component, it was necessary to associate each D-E value with a I2El value. Consideration of experimental values of zero-field splittings and spectral traces of D + E multiplet components at 7.5, 16.7, and 20 K and comparison of relative signal-to-noise ratios of multiplet components with those from D-E and 12EI transitions led to assignment of the following signals into transition triples (sets of D-E, \2EI, and D + E transitions arising from sets of equivalent phenanthrene molecules with unique values of D and E): signals 4 and 6, signals 3

A.S. Cullick, R.E. Gerkin/EPR of triplet phenanthrenedl~

and 5, signals 2 and 7, and signals 1 and 8. Signals 3 and 5, which are assigned to the same triple, both exhibited discontinuities in their spIitting values near 15 K. Since signal 8 was assigned to the same set of phenanthrene molecules giving rise to signal 1, a detailed study of signal 8 below 15 K was pursued in order to determine whether its zero-field splitting exhibits a maximum, minimum, or discontinuity near 15 K. No absorption corresponding to signal 8 was observed at 15 or 13 K, which would be expected because of overlap with signal 7, if the dependence has a small slope between 7 and 15 K. If this is in fact the case, either a discontinuity in the splitting value occurs, or the dependence has a large negative slope from = 17 to = 15 K, at which it passes through a sharp inflection. A third possibility is that there is no sharp change in slope and that the curve as drawn approximates the true dependence. The data are not sufficiently definitive for a choice among the above possibilities, but considering the dependence of signal 1 on temperature and the fact that signal 8 was not observed in the 15 K “transition” region, the first possibility seems most likely. When the fitted curves in figs. 1 and 2 are extrapolated to below 2 K, provisional comparison with values from previous reports is possible [S, 93 _However, comparison is complicated by a substantial concentration dependence of the zero-field splittings of phenanthrene [lo], since previous work was done with = 0.01-0.02 mole fraction guest crystals and present work was done with =Z0.00 1 mole fraction guest crystals. Assuming that the concentration dependence of the splittings is the same at 1.6 as at 77 K, corrected values of splittings from the previous work were calculated but are subject to substantial uncertainties (= 40 X low6 cm-l) due to lack of knowledge of actual sample concentrations. Upon extrapolation of our observed dependences to 1.6 K, signals 2,s (below 15 K), 5 (above 15 K), and 8 are consonant with values from the previous work within the estimated [large] uncertainties. The unassigned signal at 6.5 K may also be assigned to one of the earlier values at 1.6 K. HOWever, extrapolation of temperature dependences of signals 3 (both above and below 15 K), 4,6, and 7 yields values at 1.6 K which are not consonant with any observed in the previous work. It should be noted that the extrapolation of the de-

and naphthalene-da

225

pendence of signal 5 (above 15 K) was subject to the assumption that the signals labeled as signal 5 above and below the transition region actually arise from inequivalent types of guest. If this extrapolation is to have physical significance, corresponding “above and below” relative site populations must change greatly between 1.6 and 5 K. 4.2. Naphthalene-dg in bipkenyl SignaI chart traces were obtained for the D-E absorptions of naphthalene at closely spaced tenperatures in the 36-45 K region, with results analogous to those from phenanthrene. In three separate experiments in which three different sample pieces were used, the following results were obtained: (1) The lowest temperature at which a single non-broadened absorption was observed was 42.6,44.3, and 42.0 K for these three experiments. (2) The highest temperature at which there appeared to be any broadening of the low field arm of the first derivative of the single absorption was 41.3,42.5, and 41.5 K for these three experiments, respectiveIy. (3) Just below these temperatures both the low field and high field arms broadened until a splitting into two absorption signals occurred at 40.8,38.6, and 40.0 K for the three experiments, respectively. (4) Further broadening of the high field arm of the high field signal occurred as the temperature was further lowered while simultaneously the two resolved signals separated further in resonant field. A third signal split at 37.7 K from the high field signal. (5) The highest temperature at which two separate zero-field splitting determinations could be obtained was 37.5 K, while three absorptions could be separately measured 2? = 36 K. (6) Signal 1 was the most intense D - E resonance over the etttire temperature range below 40 K, having a signal-to-noise ratio seven times that of the next largest signal at 6 K. For the D + E spectrum (fig. 4) the following results were obtained: (1) the highest temperature at which three absorptions were sufficiently resolved for precision splitting determinations was 36.5 K. (2) Signal 5 was not observed below = 30 K, presumably because of its low signal intensity- (3) Signal 6 was the most intense D + E resonance over the entire range below 40 K, having a signal-to-noise ratio seven times that of the next largest signal at 6 K. Also analogously to the results for phenanthrene,

226

A.S. Cullick, RE. GerkinlEPR

of triplet phenanthrene-dlo

the zero-held splittings of naphthalene as functions of temperature exhibit discontinuities near 1.5 K. During one experiment in which the temperature was lowered to 15.2 K, and at a microwave frequency such that the absorption maximum of signal 1 occurred at a field below 30 G, the signal initially appeared to be broadened and split into two signals as a function of time. These two signals later (= 20 min) began to merge and again to form one resonance signal which possessed a broadened high field arm. The average value of the splittings of the two signals determined separately appears as a square in fig. 3, for which the large uncertainty represents the difference between the average of the splittings and the splitting of each of the signals when they were separated_ More detailed study of signal 1 was undertaken in a subsequent experiment during which spectral traces were obtained at closely spaced temperatures in the 12 to 17 K region. Broadening of the high field arm of signal 1 was observed as high as 17.6 K, which increased-as the temperature was lowered until two partially separated signals emerged at 14.0 K. These signals separated further in resonant field as the temperature was further lowered to 13.5 K, but began to merge again with further lowering of the temperature until at 12.3 K only a single resonance signal was present, which then persisted to 7 K. Upon increasing the temperature, the low field arm of signal 1 was broadened at 12.9 K, was further broadened up to 16.6 K at which two hartially overlapped signals appeared and then these signals merged again by 16.8 K, at which temperature the high field arm of the single signal was broadened. In addition to the apparent transition occurring between 12 and 18 K, there was also an enhancement of signal-to-noise ratio of the first derivative of the single absorption in this range, from = 26 at 17.6 K to = 56 below 12.3 K, reaching = 66 at 7 K. Signal 4 also exhibited the effect of a transition near 15 K, which was even more pronounced than that of signal 1. Between 15.2 K and 12.3 K, the value of the splitting increases by = 6 X lOA cm-l, which results from a zero-field splitting discontinuity analogous to that of signal 3 of phenanthrene. The signal-to-noise ratio of signal 4 also changed substantially in this range: above 18 I( it was = 3, whereas below ! 2.3 K it was Z=10 (measured using the same detection tune constant as for signal 1). Both signals 2 and 3 were observed and their zero-

and naphthalene-ds

field splittings measured at IS.2 K with signal-to-noise ratios of = 8 and * 4, respectively. However, at 12.3 K and below neither was observed within the limits of sensitivity of the spectrometer. Also for the D + E transitions, only two resonance signals were observed at 6 K, signals 6 and 8 with signal-to-noise ratios of = 35 and 5 5, respectively_ The region near 15 K was not studied in detail, but signal 6 seemed to exhibit the effects of a splitting value discontinuity between 15 K and 20 K, and its high field arm was quite broadened at 15 K. Upon comparison ofD -I-E relative signal intensities with D - E relative signal intensities, signals 1 and 6 and signals 4 and 8 may be assigned provisionally to two distinct sets of phenanthrene moIecules. Since by this assignment signal 8 is associated with signal 4, it, too, may be expected to exhibit a zero-field splitting discontinuity near 15 K; however, this was not studied in detail and the smooth curve in fTg.4 indicates only the trend of the dependence. The open circles in fig. 3 and 4 are splitting values obtained at 1.7 K by Scott [S] . Comparison of these data with present results requires extrapolation of the present dependences from at least 5.5 K, and in some cases 15 K, to 1.7 K. Extrapolation of the dependences of signals 1 (above and below 15 K), 2,3,4 (above and below 15 K), and 6 (above and below 15 K) yields vaIues consonant with those of Scott, whereas values at 1.7 K obtained by extrapolation of dependences of signal 5 and 7 do not correspond to any values reported by Scott. Again, physically significant extrapolation from 15 to 1.7 K of dependences which are not observed between 6 and 15 K requires that there be large changes in relative signal intensities between 6 and I.7 K, a matter which is discussed below. 4.3. Cooling and warming curves near 40 K Fig. 6 presents the cooling curve from one experiment on a I7 g sample of pure biphenyl in the apparatus described in section 2. During this experiment, a temperature haIt occurred at 42.242.0 K and persisted for approximately 18 min. Under different temperature conditions for the outer cylinder, another cooling curve exhibited a temperature halt at 42.7 K. Several warming curves were also obtained, for which the sample cylinder was warmed at a low, nearly constant rate by maintaining the outer cylinder at a higher

A.S. CulIick,R.E. Gerkin/EPRof tripletphenanthrene-d,o and naphrhalene-ds

3662. 368

.

374

-

.

221

temperature halt was observed from either sample for either a cooling curve or a warming curve when the experiments were carried out as for pure biphenyl.

. . . 38.1 -

w .

58.7 . .

33.3 :

33.9 T.%

*

4a6-

. .*

41.8 . . . . . . . . ..**~“* -

.

:

41.2 -

424

5. Discussion

:

. . ...=

.

Fig. 6. Caoliig curve of pure biphenyl

from = 45 to = 36 K.

temperature with a nearly constant temperature difference between cylinders. No temperature halt was observed for any of the warming curves, some of which were obtained just preceding or just after the halt had been observed in the cooling curve. Since super-heating of soIid phases in hydrocarbon systems is atypical but not unknown [I 11, an attempt was made to quench the phase transition by rapidly cooling the sample midway through the transition so that the remaining high temperature phase could presumably nucleate the transition during warming; however, this proved unsuccessful_ The heat of transition may be estimated from the heat leak rate out of the sample (for the appropriate temperature differences between sample can and outer can) and the length of the temperature halt. Avalue of 70 cal mole-l for the heat of transition was calculated using an interpolated value of 1.14 cat mole-l deg-1 for brass at 40 K El23 and a value of 8 cal mole-1 deg-1 for the heat capacity of biphenyl at 40 K, assumed upon comparison of the heat capacity of biphenyl at room temperature (40 cal mole-I deg-I) with a number of other organic molecular crystals with the same value at room temperature and a valueof!=8calmole-l deg’lnear40K[ll]. In addition to pure biphenyl, biphenyl containing either x 0.002 mole fraction phenanthrene or = 0.005 mole fraction naphthalene was used as sampIe. No

We have previously reported a single discontinuity near 40 K in the zero-field splittings of naphthalene and phenanthrene in biphenyl single crystals and have proposed that this discontinuity arises from a transition intrinsic to biphenyl [I ]. The present results further characterize this transition as occurring near 42 K in single-crystal bipheny!: (a) there is an isothermal phase transition in pure biphenyl at 42.4 t 0.3 K for which AH= 70 cal mole-t; and (b) there is a marked alteration of the guest triplet magnetic resonance absorptions in dilutely substituted mixed-crystaI bipheny1 which sets in at 42.0 -C0.4 K with decreasing temperature and is complete at 42.0 t 0.4 K with increasing temperature. The fact that the four components of the magnetic resonance absorption multiplets appear initially aS a broadened absorption at 42.0 K and separate in energy over a few degrees with decreasing temperature (rather than appearing simultaneously at a single temperature) is evidence that the transition in the mixed CKYStals occurs over a range of a few degrees rather than isothermally. Thus, for example, the extrapolated splitting value of signal 3 of phenanthrene at 41.5 K is 0.147343 cm-l, which would be measured from an (unobserved) absorption readily distinguishable from the slightly broadened absorption actually measured at 41.5 K for which the splitting value was 0.147326 cm-l; similarly, the extrapolated splitting value of signal 1 of naphthalene at 40.1 K is O-115319 cm-l , which would be measured from an (unobserved) absorption distinguishable and separately measureable from the broadened absorption actually measured at 40.1 K with a splitting value 0.115236 cm-l. Moreover, cooling and warming curves obtained for each of these two mixed crystal systems gave no evidence of an isothermal transition. In our earlier report two quite different limiting cases for the origin of the four different types of guest triplets which appear below x 42 K were introduced: (1) that the host crystal, possibly significantly influenced by presence of guest, undergoes a moderately

228

AS. CMick. RE. GerkinlEPR of triplet phenanthrene-dl~

and naphthnlene-dg

sharp phase transition to a phase in which there are four prdminent inequivaIent guest sites, so that although all host molecules are crystallographically ” equivalent not alI guests are; or (2) that as a result of the transition not all host molecules are crystalIographically equivalent, so that even though all guest molecules are uniquely oriented, there are crystallographically inequivalent guests [ 1J_ Relevant to this consideration is the supposition, based on two lines of evidence, that. the structural change in biphenyl near 42 K may be analogous to that of p-terphenyl near 185 K. p-terphenyl, which is isostructural with biphenyl at room temperature [13,14], having a P 21/c structure with two planar, crysta.llographicaIly equivalent molecules per unit cell, undergoes a transition near 185 K to a structure with four non-planar and inequivalent molecules per unit cell by a rotation (about the long axis of the molecule) of the outer phenyl rings with respect to the central ring [ 15,16]. One line of evidence for the structural analogy is that the amplitude of the libration of biphenyl about its Iong axis, calculMed from X-ray afraction data at 110 K, is unusually large relative to the corresponding amphtude for p-terphenyl in its non-planar low temperature configuration, and that a transition in biphenyl may therefore be expected at some temperature below 110 K [ 171. 4 second line of evidence is that single resonance absorptions which are observed above Z=185 K from triplet 1, 2-benzanthracene-d12 substituted into pterphenyl give way to sets of four resonance absorptions below = 185 K, as do the guest resonance absorptions in biphenyl near 42 K [lS]. Several other reports on biphenylk10 and -dd,, and isotopically mixed biphenyl crystals [2,11,19,20] have indicated

data from biphenyl-h , n and -dt n. The present data do not include any results which might beinterpreted as arising from gradual processes between 42 and 70 K of the type referred to by Friedman et al. However, the “abnorm&’ ? and notably diverse temperature dependences occurring below the sharp 42 K transition region, which may result from a substantial contribution from intermoIecuIar interactions to the total temperature dependences, may indicate such gradual processes. The “abnormal” and diverse dependences in the biphenyl systems oontrast to the “normal” dependences we have observed from the multiplet splittings of triplet 1, 2-benzanthracenedl12 in p-terpheny1 as a result of the p-terphenyl transition [IS], a fact which serves to distinguish the transitions of these two hosts, and which emphasizes the qualitative nature of the structural analogy between them which has been drawn above. Although gradual processes may be occurring in biphenyl below 42 K, the mixedcrystal phase(s) at any particular temperature between 20 and 80 K are thermodynamically stable or at least long-term metastable, since, as detailed in the preceding section, there was no difference in the mixed-crystal magnetic resonance spectra for either mixed system either above or below the 42 K transition upon slow-cooling the sample from room temperature, cycling the temperature repeatedly across the transition temperature, or keeping the sample below the transition temperature for periods up to 48 h. The second prominent transition in biphenyl, that near 15 K, did not exhibit the same type of results for the two mixed systems or even for different signals of a single system, and thus is not characterized defiitiveIy by the present results. This transition also occurs

that in the very low temperature biphenyl structure,

over a small range of temperature, centered near 15 K,

biphenyl is not a centrosymmetric, planar molecule. If the biphenyl change is in fact such that there are crystallographically inequivalent sites in the low temperature phase, limiting case (1) would of course not obtain. It should be noted that although from powder diffraction data at 4.2 K [20] a P21/c structure has been assigned to biphenyl, very subtle changes in the biphenyl structure analogous to those of p-terphenyl may not be resolvabIe by powder diffraction methods. The suggestion that gradual processes may be occurring in biphenyl over the range from 15 to 70 K was made by Friedman et al. [2] on the basis of Raman

rather than at a single temperature. In some instances iittairunent of apparent equilibrium (Le., time-independent behavior of the magnetic resonance spectrum) was observed immediately at a given temperature in the transition region, whereas in other instances nonequilibrium behavior was manifested for from 30 min

_”

*.,

t As in ref. [l], we designate as “normal” temperature dependence the predominant type of dependence observed by us previously in other multiplet systems, namely, at most a small maximum at low temperature followed by an extended interval in which the temperature coefficient of the splitting is negative.

A.,!$ &lli&,

R.E. GerhinlEPR of triplet phenanthrene-d,o

to an hour after setting the sample cavity at a given temperature in the transition region. As a consequence of the 15 K transition, various effects on the magnetic resonance absorptions were observed. Triplet naphthalene gave rise to four D - E and four D + E resonance absorptions above the transition, but only two each below, while triplet phenanthrene gave rise to four D - E and four 12EIabsorptions both above and below the transition. Some signals exhibited discontinuities in their zero-field splittings near 15 K, some signals increased in relative intensity below the transition as other signals disappeared as a result of the transition, and some signals remained apparently unaffected by the transition (though, e.g., signals 2 and 4 for phenanthrene (fig. l), which are drawn as continuous, may exhibit splitting shifts too small to have been detected). However, no single absorptions observed above the transition were transformed into a set of multiple absorptions as a result of the transition. This diverse behavior of the 15 K transition in various experiments and among multiplet components within a single experiment may indicate that the onset of the transition depends upon_properties of the particular crystal sample, such as crystal imperfections, presence of crystallites, or the presence, concentration, and identity of guests. The fact that the transition was observed in the same narrow temperature region in both mixed systems suggests that the transition may be intrinsic to biphenyl, although its character may be significantly influenced by guests. Phosphorescence spectra from biphenyl and mixedcrystal biphenyl near 15 K have been described in two earlier reports, which although much different in de-

and naphthnlenr-d~

229

Between ==1.5 and 6 K no additional prominent features mark the present results. However, the zerofield splittings of some resonance absorptions observed above 15 K but not observed between 6 and 15 K yield values consonant with those from previous work [8,9] when extrapolated to 1.7 K by means of the temperature dependences determined in this research, while others do not. ExtrapoIation of signals observed between 6 and 15 K to 1.7 K also yields values which may or may not be consonant with values reported previously [S,9] _These facts may be rationalized in terms of one of the following aitematives: (a) some guest sites which are relatively well-populated above 15 K still exist below 15 K but are relative!y unpopulated, and then regain population between.6 and 2 K; or (b) as a result of another transition occurring below 6 K, new guest sites are formed some of which yield splitting values fortuitously consonant with those obtained by extrapolation of dependences from higher temperatures. In conclusion, we have been able to characterizepartially two transitions occurring in biphenyl below 80 K by the use of deeply-trapped guest triplets, which are highly sensitive to their crystalline environment and whose resonances are readily distinguishable. It is clear from this investigation and from other reports [2,3,19,20,22,23] that biphenyl at low temperatures is a complex system and that further studies are needed to define its phase behavior. Among obvi-

ous possibilities are extension of studies of the present type to 2 K, high-precision heat capacity measurements from 2 to 80 K, and single-crystal structural studies at selected temperatures between 2 and 80 K.

tail than the present results, also exhibit prominent features near 15 K. Hochstrasser and Small [21] ob-

served a discontinuity at * 14 K in the slope of the relative intensity of phosphorescence emissions from in bitwo inequivalent types of phenanthrened10 phenyl plotted as a function of inverse temperature, and Friedman et al. [22] have observed two inequivalent sites in the phosphorescence spectra of 1% biphenyl-hlo in single-crystal biphenyl-dlo below 15 K, but only a single site at 15 K. Since interpretation of features near 15 K in these previous reports involved not more than two types of sites and the present data describe not less than four, the behavior of dilute mixed-crystal biphenyl, and perhaps of biphenyl itself, is apparently now more fully characterized_

Acknowledgement We thank Professor CA. Hutchison Jr. for making available to us a copy of Dr. G-W. Scott’s thesis.

A.S.C. gratefully acknowledges support from a Phillips Petroleum fellowship during a portion of this research. Computer facilities were provided by the

Ohio State University IRCC.

References [11 A.S. Cullick and RX. Cerkin. Chem. Phys. Lettea 42 (1976) 589.

230

A.S. Cullick, RE. Gerktn/&FR oftripIer phenanthrenedl Oand tiphrhalene-ds

[2] P.S. Friedman,

R. Kopebnan and P.M. Prasad. Cbem. Phys. Letters 24 (1974) 15. (3 J H.C. Brenner, CA. Hutchison Jr. and M.D. Kemple, J. Chem. Phys. 60 (1974) 2180. [4] R.E. Gerkin and ?. Szerenyi, J. Chem. Pbys. 50 (1969) [5J i%erkin and P. Szerenyi, J. Cbem. Phys. 55 (1971) 257s. [6 J hf. Kasba. J. Opt. Sot. Am. 38 (1948) 929. [7] R.E. Gerkm and A.M. Wirier, J. Chem. Phys. 56 (1972) 13.59. [8J G-W. Scott, Ph.D. Thesis, University of Chicago, Chicago, IUinois (1971). (91 CA. Hutchison Jr. and V.H. McCann. J. Chem. Phys. 61 (1974) 820. [IO] A.S. CuUick and R.E. Gerkin. J. Chem. Phys., to be published. [ll J E.F. Westrum and J.P. McCullough. Physics and chemistry of the organic solid state, eds. D. Fox, M. Labes and A. Weissberger (Interscience. New York, 1963). (12 J R. Hultgren, R.L. On and K.K. KeUey, eds., Supplement to selected values of thermodynamic properties ofmetab and alloys, Vol. 3 (American Society for Metals, 1968).

(131 G.B. Robertson.Nature lYl(1961) 593;’ J. Trotter, Acta Cryst. 24 (1961) 1135; A. Hargreaves and S.H. Rizvi, Acta Cryst. 15 (1962) 365. [14] L.W. Picker!, Proc. Roy. Sot. Al42 (1933) 333; .. J. Dejace, Bull. Sot. France Miner. Cryst. 92 (1969) 14; H-M. Rietveld, E.N. Maslen and C.J.B. Clews, Acta Cryst. B26 (1970) 693. (151 G.P. Charbonneau and Y. Detugeard, Acta Cryst. B32 (1976) 1420. [16J J-L. Baudour and G.P. Cbarbonneau, Acta Cryst. B30 (1974) 1379. 1171 J.L. Baudour, Y. Delugeard and H. Cailleau, Acta Cryst. B32 (1976) 150. [ 18 J AS. Culhck and R.E. Gerkin, unpublished results. [19] R.M. Hochstrasser, R.D. McAlpine and J-D. Whiteman, J. Chem. Phys. 58 (1973) 5078. [20] R.M. Hochstmsser, G.W. Scott. A.H. Zewail and H. Fuess, Chem. Phys. ll(l975) 273. [21] R.M. Hochstrasser and G.J. SmaIl, J. Chem. Phys. 48 (1968) 3612. [22 J P.S. Friedman, P.N. Prasad and R. Kopebnan. Chem. Phys. 13 (1976) 121. (231 A. Bree, M. Edebon and R. Zwarich, Chem. Phys. 8 (19 7.5) 27.