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Acetone-induced triboluminescence of triphenylphosphine
JOURNAL OF
Journal of Luminescence 51(1992) 323—334 North-Holland
Acetone-induced triboluminescence of triphenyiphosphine Martin B. Hocking and Jeffrey I. Zink a
a
Francies W. VandervoortMaarschalk
a
John McKiernan
* b
b
Department of Chemist,y, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6, Canada of Chemist,y, University of California, Los Angeles, CA 90024-1569, USA
b Department
Received 1 April 1991 Revised 24 August 1991 Accepted 10 November 1991
The triboluminescence of triphenylphosphine has been found to be acetone-dependent and is positively correlated with acetone content, crystal size, and negatively correlated with the age of the crystal. There is no measurable difference in the unit cell dimensions of triboluminescent triphenylphosphine crystallized from acetone and those of inactive material from cyclohexane. Similar emission results are obtained from crystals obtained from saturated solutions in some, but not all carbonyl-containing solvents, and from methanol and ethanol but not diethyl ether or ethyl acetate. Inactive material was also obtained from saturated solutions in benzene, cyclohexane or chloroform. The triboluminescence and photoluminescence maxima and bandwidths were the same, within the error limits of the experimental method and instrumentation used, for triphenylphosphine crystallized from acetone, acetonitrile, or cyclohexanone. Among the other highly arylated heteroorganics tested in measures to extend this correlation, triphenylamine and 1,2,5-triphenylphosphole were triboluminescent as purchased or prepared and when recrystallized from acetone, cyclohexane and several other solvents. Tritylamine, triphenyiphosphine oxide, and triphenylphosphine sulfide were not triboluminescent as purchased or prepared, or when re-crystallized from acetone or cyclohexane.
1. Introduction Triboluminescence, or the emission of light on the crushing or fracturing of crystals, is a mechanical energy transfer phenomenon that has been known to occur with sugars and certain minerals for many years [1,2]. Previous studies have determined the principal mechanistic details which give rise to the emission. The two most common spectral origins of triboluminescence are luminescence from the excited states of molecules of which the crystals are composed, or nitrogen emission from the air incorporated into or surrounding the crystal [3]. Sometimes a combina*
Presented at the 74th Canadian Chemical Conference Hamilton, Ontario, 2—6 June 1991, and at the Thirteenth International Congress of Heterocyclic Chemistry, Corvallis, Oregon, 11—16 August 1991.
0022-2313/92/$05.00 © 1992
—
tion of both origins is observed [4]. The formation of intense electric fields at the apex of propagating cracks during crystal fracture is generally considered to provide the energy required to initiate either of these emission processes. Surveys of triboluminescence from molecular crystals show a strong correlation with non-centrosymmetric space groups [5—8], a relationship which has been confirmed by the study of polymorphs [4]. Involvement of local dopants in the emission process has also been proposed [4,9]. During an earlier screening phase of a study of the triboluminescence of 1,2,5-triphenylphosphole, which is a particularly intense example visible in subdued daylight, samples of triphenylphosphine were also tested [10]. It was found that the phosphine, when tested as supplied from several commercial sources, did not triboluminesce. This result was at variance with prior re-
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.
324
M.B. Hocking et a!.
/ Acetone-induced triboluminescence of triphenyiphosphine
ports [8,11]. Further tests demonstrated that recrystallization of the triphenylphosphine from acetone produced triboluminescent material, in agreement with the procedures of the prior reports, which also satisfied the screening requirements of our earlier study. This paper describes the details of experiments conducted to more comprehensively delineate the physical requirements, scope, and originating mechanism of this novel, solvent-induced triboluminescence emission. The experiments conducted here were designed to determine whether any or all of polymorphism, local crystal structure defects, and/or the presence of solvent in the crystal were responsible for the observed triboluminescence from triphenyiphosphine. The extent to which this influence might extend to and be correlated with other highly arylated hetero-organics and to other carbonyl-, and oxygen-containing solvents is also briefly examined.
2. Experimental Commercial samples of triphenylphosphine were obtained from the following sources: Aldrich, 99%, No. T.8440-9; BDH, 98%, No. 30537, lot 645130; Fisons, 98% mm.; Koch-Light Labs. Ltd. (Colnbrook, Bucks), No. 5537p, “pure”; and M&T Chemicals Inc., Lot AVVAH534K. The other triboluminescence candidate compounds were all purchased from Aldrich, cxcept for triphenylphosphine oxide, which was prepared from triphenylphosphine as previously described [101. Solvents were used as supplied from the following sources: acetone, technical grade, Van Waters and Rogers (VWR, Vancouver) and ACS reagent grade from Aldrich; 2-butanone, Fisher, lot 855634; 3-pentanone, cyclohexanone, C10,2180, lot LA, and propionaldehyde, all 99 + % from Aldrich; 2,4-pentanedione PX 215, lot P 5103 from Matheson, Coleman and Bell; methanol and ethanol (95%) were VWR technical grades; chloroform, PIN 1888, BDH Omnisolve; ethyl acetate, ACS Reagcnt Grade. Deuteriochloroform, 99.8 at% D, No. 15-182-3, and dideuteriodichloromethane, 99.9 at% D, were used as
supplied by Aldrich and Isotech. Inc. (Matheson), respectively. Benzene and diethyl ether were from Canlab, and were redistilled from sodium before use. 2.1. Observation procedure and variables
crystallization
Triboluminescence tests were conducted in a photographic darkroom by direct observation, after allowing several minutes for dark adaptation of vision. For each test a small sample of the candidate material was crushed by a Pyrex glass rod on the concave face of an 8 cm Pyrex watch glass resting on a white background, and on white bond paper crushed by the convex face of the watch glass. In an unlit photographic darkroom several samples of triboluminescent crystals of triphenylphosphine were dropped into a dewar of liquid nitrogen to observe any resulting emission. Samples of triphenylphosphine from each of the suppliers were first tested for triboluminescence as supplied. They were each then crystallized by cooling of hot saturated solutions without permitting evaporation (the “normal” fashion) from acetone and/or other solvents, filtered from the mother liquor, and dried by various techniques before being tested again for triboluminescence. Microscopically-small crystals were prepared by rapid rotary evaporation of a saturated solution of triphenylphosphine in acetone using reduced pressure, or by rapid chilling of a solution supersaturated at the boiling point. 2.2. Acetone content and unit cell dimensions Triphenylphosphine was crystallized from acetone, dried for 5 days loosely covered under ambient conditions, and further dried for 24 h at room temperature and 0.1 mm Hg pressure. A 49 mg portion of this still triboluminescent material was dissolved in 0.7 ml dideuteriodichioromethane and its 250 MHz 1H NMR spectrum, after 66 scans, recorded on a Bruker WM 250 instrument. For the more numerous phosphine samples requiring determination of acetone content in
MB. Hocking et a!.
/ Acetone-induced triboluminescence of triphenylphosphine
connection with time series trends, deuteriochloroform was used as the primary solvent with a trace of nitromethane added (1 drop in 50 ml CDCI3) as an internal shift and integration reference. Light chloroform to nitromethane to water relative intensity ratios at 5 7.24,4.30 and ca. 1.53 of this seeded solvent were usually about 1.0:2.6:1.3. This was measured for each new batch of standardized solvent before use for acetone content determinations. Solvent content for triphenylphosphine crystallized from acetonitrile was determined in the same way as the acetone determinations except that the relative intensity for the acetonitrile methyl at 2.02 5 was taken and phosphine to acetonitrile mole ratio calculated by normalizing for the 15 : 3 ratio of hydrogens. Unit cell dimensions were determined on suitable crystals selected from a sample of triphenylphosphine crystallized from acetone (tnboluminescent) and from a sample crystallized from cyclohexane (non-triboluminescent). Measurements were made from film records taken from these crystals on Precession, and Weissenberg cameras. Each of the same two crystals were also aligned on a Picker 4-circle diffractometer, with five-centered reflections, in order to obtain more accurate unit cell dimensions. 2.3. Triboluminescence and photoluminescence spectra Triboluminescence for determination of emission frequency range was generated by crushing the sample of test material against the walls of a 3 dram glass vial with a 3 mm brass rod. To record a spectrum the vial was clamped about 4 cm from the entrance slit of an iSA model HR320 0.32 m monochromator. A typical slit setting was 500 p.m and no focusing elements were used between the sample and the slit. The emission was detected and accumulated by an EG and G PARC model 1420 optical multichannel analyzer using 5 to 25 s collection times depending on the intensity of the emission from the sample. Mercury lines from a fluorescent lamp were used for calibration,
325
Photoluminescence of the neat sample was stimulated by the 308 nm line from a Lambda Physik EMG-201 XeCl excimer laser running at 20 Hz. The sample was pressed into the cupped end of an aluminum rod and positioned and the emission spectrum recorded as in the triboluminescence experiments. Typically, a slit width of 100 p.m was used to accumulate 100 scans of 50 ms exposure each. 2.4. Experiments with related compounds, procedures Samples of tritylamine, triphenyiphosphine oxide, triphenylphosphine sulfide, triphenylamine, and 1,2,5-triphenylphosphole were crystallized from acetone and one or more additional solvents by cooling hot saturated solutions without permitting evaporation. Samples of the original crystals and the newly crystallized material were tested for triboluminescence after filtration from the mother liquors using a variety of drying procedures.
3. Results The triboluminescence emissions of the materials under test here were inherently low which required photographic darkroom conditions for observation. Even the very low level lighting from a standard 15 W frosted glass incandescent bulb behind a Kodak green OA safelight filter was sufficient to completely mask the direct observation of the phosphine triboluminescence emissions being screened. 3.1. Effect of purity and solvent content of triphenyiphosphine Triboluminescence was not detected from any of the commercial samples of triphenylphosphine as supplied. But one recrystallization from acetone followed by brief drying open to the air was sufficient to produce triboluminescence on crushing from all the commercial materials. If the triphenylphosphine crystals were still moist with acetone, or once cursorily dried, were then re-
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/ Acetone-induced triboluminescence of triphenylphosphine
Table I Triphenylphosphine triboluminescence correlated with crystal preparation conditions. Solvent
Crystal preparation details
Crystal drying conditions
Triboluminescence
from saturated solution ~
moist with acetone ambient, ca. 30 mm ambient 24 h 0.1 mm Hg, room temp., 24 h ambient, ca. 30 mm for each ambient, ca. 30 mm.
no yes yes yes yes, all three yes
ambient, ca. 30 mm.
no
Acetone variations
acetone
‘~
acetone acetone acetone, then cyclohexane
recrystallized from fresh acetone once, twice, and thrice; each filtered slow partial evaporation of saturated solution under ambient conditions sequential recrystallizations from hot saturated solutions by cooling, filtered
acetone, then cyclohexane, then acetone cyclohexane
sequential recrystallizations saturated solution ~
ambient, ca. 30 mm. ambient, ca. 30 mm.
yes no
Other carbony!-containing so!t ents methyl ethyl ketone (2-butanone) diethyl ketone (3-pentanone) cyclohexanone ‘~ propionaldehyde C) 2,4-pentanedione
saturated saturated saturated saturated saturated
solution ~ solution ~ solution h) solution h) solution h)
ambient, ca. ambient, Ca. ambient, Ca. ambient, Ca. ambient, ca.
30 30 30 30 30
mm. mm. mm. mm. mm.
yes yes yes yes no
Other oxygen-containing sob ‘ents methanol ethanol ethyl acetate diethyl ether
saturated saturated saturated saturated
solution solution solution solution
ambient, ca. ambient, Ca. ambient, Ca. ambient, Ca.
30 30 30 30
mm. mm. mm. mm.
yes yes no no
Miscellaneous so/cents acetonitrile benzene chloroform dichloromethane
saturated solution ~ saturated solution b) saturated solution h) saturated solution h)
ambient, Ca. 30 ambient, Ca. 30 ambient, Ca. 30 ambient, Ca. 30
mm. mm. mm. mm.
yes no no no
“~
h) C)
~
1,) ‘~ “~
Identical results were obtained whether Technical, or Reagent Grade acetone was used (see section 2). By cooling of hot saturated solution without allowing evaporation, followed by filtration of crystals. Samples held in closed vials for several months turned yellow.
moistened with acetone, the triboluminescence was lost (table 1). Newly filtered crystals which were allowed to dry under ambient conditions for short lengths of time or which were placed under a reduced pressure of 0.1 mm Hg at room ternperature for 24 h, still showed triboluminescence. Saturated solutions of triphenyiphosphine in acetone which were crystallized by slow partial evaporation or by slow complete evaporation of the solvent gave crystals with triboluminescence behaviour indistinguishable by eye from that of crystals obtained by cooling hot saturated solu-
tions (the normal procedure). Concentration of the combined mother liquors from several normal crystallizations followed by cooling, which would have been expected to concentrate any impurities in the later fractions of triphenyiphosphine crystals obtained, also gave material indistinguishable in luminescence properties from normally crystallized material. Crystals of acetone-induced triboluminescent triphenylphosphine did not show any visible emission when dropped into liquid nitrogen in the darkroom.
MB. Hocking et a!.
/ Acetone-induced tribo!uminescence
Triphenylphosphine crystallized from acetone lost its triboluminescence if this material was crystallized again from cyclohexane or chloroform. Recrystallization of either of these samples yet again from acetone restored triboluminescence. A selection of other crystallizing solvents were also tested for their effect on the triboluminescence properties of triphenylphosphine. Results of these tests are given in table 1. The acetone content was determined by high frequency NMR measurements, which enabled estimation of the mole ratio of triphenylphosphine to acetone in the acetone-crystallized material when dried under various conditions (table 2). The spectrum for entry 1, for example, showed the presence of sharp singlets (without expansion) at 7.36 5 (aromatic H, Ph3P); 5.32 5 (HDCCI2, residual in CH2CI2); and 2.14 5 ((CH3)2C0). Computer recorded relative intensities were 502.04 for aromatic hydrogens: 1.25 for acetone, giving a 405: 1.00 proton ratio, The mean measured peak height ratio for the same two components from two different gain settings gave a 480: 1.00, aromatic hydrogen to acetone proton ratio. Normalizing these values for their respective 15 : 6 hydrogen contents gave the recorded triphenyiphosphine to acetone mole ratio estimates of 160: 1 to 190: 1. A variety of experimen-
of triphenylphosphine
327
tal tests established that the instrumentally recorded relative intensity values gave the most reliable triphenylphosphine to solvent mole ratios. With recognition of the integration limitations of pulsed NMR this method gave a mole ratio range from 36: 1 to 160: 1, triphenylphosphine to acetone, for crystals which were triboluminescent, and 260: 1 for crystals which were not. The effect on the relative intensity ratios of halving or doubling the number of NMR scans for a single phosphine sample caused a variability of ±5% (R = 0.83). Triplicating an NMR experiment on the same sample using the same number of scans gave a smaller variability of ±4%. At no time was the light chloroform, which was resolved from the phosphine multiplet at 5 7.25—7.67, more than i% of the relative intensity of the triphenylphosphine multiplet. The preliminary survey results plus the tests of the NMR approach outlined above provided the validation of the relative intensity ratio method which was used for a more closely regulated series of tests on the effects of elapsed time and/or solvent content on triboluminescence. Whilst the individual mole ratio results were somewhat scattered because “of the interaction of additional variables (crystal size, etc.), the fading and the eventual loss of triboluminescence correlated well to both elapsed time and loss of
Table 2 Correlation of triboluminescence to mole ratios of triphenylphosphmne to solvent in the crystal. Triphenylphosphine sample Type
Mole ratio, Ph3P to solvent, via NMR Solvent
Medium crystals, vacuum dried 20 hours acetone Large crystals, air dried acetone Large crystals, air dried acetonitrile Large crystals, air dried acetone Small crystals, air dried acetone Large crystals b) vacuum dried 20 h, then stored acetone Medium crystals C) vacuum dried 20 h, then stored acetone a) Measured at 0.25 months from crystallization. b) 2.5—3.5 mm on an edge. c) 0.5—0.7 mm on an edge.
Age (months)
0.25 0.25, 0.5, 1.0 0.25, 0.5, 1.0 8 8
Triboluminescent +ve +ve +ve + ye —ye
Integration
—
13: 1 a) 120:1 a) 69: 1 410:1
Relative intensities
Peak heights
160 6.4 57 36 260
190 5.3 54 30 320
:1 :1 :1 :1 :1
a) a)
:1 : 1’~ :1 a) :1 :1
18
weak +ve
—
—
—
18
+ve
—
—
—
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/ Acetone-induced triboluminescence of tripheny/phosphine
Table 3 Triboluminescence of triphenylphosphine (TPP) crystallized from acetone correlated with subsequent treatment measured). Age of sample (days)
0 5 13 19 23 40 53 65 110
5) C)
a)
(nm: not
Sample air-dried, stored In capped vial
Open to air
TPP: acetone mole ratio
TPP: acetone mole ratio
Triboluminescence C)
TPP: acetone mole ratio
Triboluminescence C)
TPP: acetone mole ratio
Triboluminescence C)
nm 22 30 39 49 43 71 23
nm
48 43 50 46 40 44 63 130 110
+ + + +++
nm 45 43 58 85 100 120 93
nm
22 33 27 33 67 62 55 130 106
Triboluminescence C) + + + +++ +++ +++
+++ +++ + + +
0
Sample dried in vacuo 5), stored In capped vial Open to air
+++ +++ +++ +++ ++ +
0
—
+++ +++
+++ +++
+ +
0
+
—
+++ +++
+ +
0 —
Four samples for time-series tests were obtained by subsampling a single 13.3 g crop of medium sized (2.5 X 2.5 x 1.0 mm) crystals of triphenylphosphmne freshly crystallized from a filtered solution in warm acetone. Details are given in experimental section. Closed storage was in a polyethylene snap-capped vial. Storage “open to air” was with a loosely suspended watch glass above the sample to prevent entry of dust. At Ca. 3 mm Hg, 26 h. Triboluminescence relative intensities: + + +, strong; + +, medium; +, weak; 0, none, all observed by dark-adapted vision in photographic darkroom.
d) Result from a large crystal. Not included in slope, intercept calculations.
solvent (table 3). Both solvent loss and loss of triboluminescence occurred more rapidly if the crystal subsample, which was otherwise identical, was stored open to the air rather than in a capped vial, Crystallization of triphenylphosphine from a selected range of aldehydic and ketonic monocarbonyl-containing solvents uniformly conferred tnboluminescence on samples of triphenyiphosphine, but no emission was observe4 when it was crystallized from 2,4-pentanedione (table 1). Crystallization from methanol and ethanol also produced triboluminescent material, but ethyl acetate and diethyl ether did not. Use of acetonitrile as crystallization solvent conferred triboluminescence whereas inactive material was ob-
not show triboluminescence after 30 mm or 24 h drying time under ambient conditions. Intermediate-sized crystals (angular spars, ca. 1 x 1 >< 4 mm) were triboluminescent after 30 mm drying time and for some weeks afterwards, but lost the effect after 8 months standing in polyethylene snapcapped glass vials under ambient conditions. Very large crystals of approximate dimensions 4 x 10 x 10 mm showed triboluminescence both after drying for 30 mm in the open air and after standing in capped vials for 8 months. Unit cell dimensions were determined by careful film measurements, and alignment on a diffractometer. These showed that crystals grown from acetone or from cyclohexane were both monoclinic, type P21 /c, and had identical gross
tamed from benzene or chloroform,
unit cell dimensions within, experimental error (table 4).
3.2. Effect of crystal size and comparison of unit cells Rapid crystallization of triphenylphosphine from acetone yielded microscopically-sized prisms of less than 0.5 mm in any dimension which did
3.3. Triboluminescence and emission spectra
photoluminescence
Four of the triboluminescent samples of triphenylphosphine were selected for detailed emis-
M.B. Hocking et a!. Table 4 Cell parameters from film measurements acetone and from cyclohexane. Cell parameter
a)
/ Acetone-induced tribo/uminescence of tripheny!phosphine
and actual unit cell dimensions
TPP crystallized from Acetone
Cyclohexane
b)
329
from triphenylphosphmne (TPP) crystallized from
TPP crystallized from Acetone
Cyclohexane
d-spacing 8.475 15.142
8.491 15.037
From film measurements a b From diffractometer a b c
p a) h) C)
Zero precession distance, A 10.936 10.916 6.121 6.164
Unit cell dimensions Published values C) 11.421A 11.413A 11.413A 15.023A 15.027A 15.032A 8.498 A 8.514 A 8.500 A 92°54’ 92°48’ 92°53’ X-ray film exposed on Precession and Weissenberg cameras. Obtained by alignment on a diffractometer, with five-centered reflections. Details from ref. [12]. Neither crystallization procedure nor solvent(s) used were specified.
sion measurements. The triphenylphosphine samples crystallized from acetone, acetonitrile, and cyclohexanone all gave very similar triboluminescence emission maxima, although the last two samples had triboluminescence emission intensities too weak to allow precise measurement of the frequency width at half height (table 5). Whilst the triboluminescence of the sample crystallized from propionaldehyde was visually and instrumentally detectable, it was too weak to enable either the maximum or the frequency range to be accurately determined, Strong photoluminescence was obtained from
all triphenyiphosphine samples which permitted accurate maxima and frequency distribution information to be collected. For the samples crystallized from acetone and cyclohexanone the photoluminescence maxima appeared at slightly shorter wavelengths than the triboluminescence whereas for the sample crystallized from acetonitrile the photoluminescence maximum was at slightly longer wavelengths. The frequency distribution of the two types of emissions appeared to be the same, within experimental error, for the acetone-crystallized sample of triphenylphosphine.
Table 5 Comparison of the room temperature photoluminescence and triboluminescence emission spectra of triphenylphosphmne (TPP) crystallized from various solvents a) Crystallization solvent
Emission maximum (nm)
Width at half maximum height (nm)
Photolum.
Tribolum.
Photolum.
Tribolum.
452±10 445±20 450 ±20
85±4 98±4 72 ±4 96±4
88±10 80±20 80 ±20
TPP/acetone 443±5 TPP/acetonitrile 458±5 TPP/cyclohexanone 446 ±5 TPP/propionaldehyde 456±5 a) Uncertainty in maxima are as specified. 5)
Could not be accurately determined from spectrum.
5)
5)
330
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/ Acetone-induced triboluminescence
3.4. Triboluminescence induction in other highly arylated hetero-organics Tritylamine, triphenyiphosphine oxide, and triphenylphosphine sulfide were tested both before and after crystallization from acetone, None of these six samples showed any triboluminescence. Triphenylamine, however, was triboluminescent both as supplied, and when recrystallized from acetone, cyclohexane or ethyl acetate, but lost this property when recrystallized from nitromethane. When a further sample of triphenylamine was newly crystallized from acetonitrile and tested for triboluminescence it was inactive, However, after standing open to the air for 9 days triboluminescence was restored. Recrystallization of samples of 1,2,5-triphenylphosphole from acetone, chloroform, dichloromethane, or ethyl acetate produced triboluminescent material from all solvents after short drying periods open to the air.
4. Discussion As far as can be determined only the group of Stranski and Wolff have devoted significant attention to the phenomenon of temporary triboluminescence. After an initial delineation of characteristics they found that the temporary triboluminescence of arsenic trioxide, “arsenolith”, only occurred if the crystals were prepared in a specific way [13,14] and that crystal size and age were both factors which affected these emissions [15]. It was concluded that the temporary triboluminescence of arsenic trioxide could be attributed to lattice defects which changed over time [14,161. The loss of triboluminescence from newly crystallized uranyl nitrate was found to take 70 h at 60°Cand more than 190 h at 45°Cin Stranski’s tests [17]. Salicylic acid crystallized from alcohol, as the sole organic example, was found to lose its triboluminescence on standing at room temperature for four months. By X-ray or by UV methods they were unable to detect any changed products or decomposition fragments with these two examples that might have explained the observed re-
of tripheny!phosphine
sults, and so concluded that there were too many possible factors to offer any mechanistic hypothesis. Relevant to the influence of solvent, Hurt’s group found that the relative intensity of triboluminescent europium complexes was related to the choice of recrystallization solvent among other factors [19]. However, no particular solvent was necessary to obtain triboluminescence nor did any solvent “switch off” triboluminescence. We have examined the novel example of the temporary triboluminescence of triphenylphosphine [10] to test a number of possible operating factors. The results of these experiments allow the formation of a mechanistic hypothesis to explain the process. 4.1. Triphenylphosphine purity and solvent effects The presence of impurities in commercial saccharm has been previously noted to be responsible for the triboluminescence of this material, probably by serving as dopants [4]. Consequently the possibility that trace impurities trapped in the crystals of commercial samples of triphenyiphosphine might have stimulated the observed triboluminescence was tested. None of the purchased samples showed triboluminescence as received, nor did any sample when recrystallized two or three times, as long as the last crystallization was from cyclohexane. If second and third crops of triphenylphosphine crystals were taken from the mother liquor of a cyclohexane crystallization, or if the final mother liquor was allowed to evaporate to dryness leaving residual triphenylphosphine crystals, none of these samples showed triboluminescence. Thus, the origin of the emission from triphenylphosphine crystallized from acetone is unlikely to originate from a phosphine impurity doping phenomenon. Originally non-triboluminescent triphenyiphosphine samples from several different suppliers each became triboluminescent on recrystallization from acetone. Triboluminescence was also maintained if these once recrystallized materials were crystallized from acetone once or twice more. These results tend to further rule out any influence of a trace impurity in the triph-
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/ Acetone-induced triboluminescence
enylphosphine, and rather, suggest that the acetone is somehow involved. This could have been via one or more impurities present in the acetone itself. However, crystallization tests using several widely differing lots of technical acetone and one sample of reagent grade material all produced triboluminescent crystals from triphenyiphosphine, which would suggest that, this possibility is unlikely. That the observed effect may be due to residual acetone trapped in the matrix of so-crystallized triphenylphosphine was suggested by the loss of triboluminescence on subsequent recrystallization of these active crystals from cyclohexane or chloroform. Confirmation of this suggestion was obtained by restored triboluminescence on recrystallization of these phosphine samples yet again from acetone. Liquid nitrogen has been used as a non-destructive method for the introduction of strain to cause triboluminescence emission [18]. This method, which has the potential to enable the determination of triboluminescence of a particular crystal and then to measure the solvent content of the same crystal failed to yield any emission with active samples of triphenylphosphine crystallized from acetone. If emission had been achieved by this thermal shock method an experimental procedure would have been available which could have decreased the number of vanables and hence the variability of the mole ratio data of table 4. Additionally, it could have enabled repeated triboluminescence emission from the same crystal without the need for intervening reciystallizations. It is tempting to suggest that the triboluminescence of triphenylphosphine when crystallized from acetone, 2-butanone, 3-pentanone, cyclohexanone, and propionaldehyde is related somehow to the presence of the polar carbonyl function, or at least a polar function since ethanol and methanol also conferred triboluminescence. The presence of two polar functions evidently interferes with this interaction since crystallization from 2,4-pentanedione, or ethyl acetate, yielded inactive material. At no time did solvents such as benzene, chloroform, and cyclohexane ever confer tribolumi-
of triphenylphosphine
331
nescence on triphenylphosphine. From this group there is no published suggestion that either cyclohexane or chloroform are inhibitors of triboluminescence [10,19]. It has been stated that dichloromethane could inhibit triboluminescence via fluorescence quenching [20,21]. Whilst this was the case for the europium complexes cited, the triboluminescence of 1,2,5-triphenylphosphole was found to be unaffected when a small sample was crystallized from dichloromethane (see also ref. [10]). The solvent-related results suggest that to confer triboluminescence the crystallization solvent used must have an exchangeable proton or other electronegative element, e.g., carbonyl or nitrile, so placed that it can cause occasional disruptions in the local structure in the developing triphenylphosphine crystals, as they form. When a crack moves through these local disorder sites, then triboluminescence occurs. This mechanism for the introduction of disorder has been proposed for the preparation of triboluminescent lanthanide complexes [21], and has also been invoked to explain the triboluminescence of 9anthryl carbinols [19]. Non-polar solvents such as cyclohexane and benzene also may be occluded in developing crystals as they form but the lack of any polar element in these solvents means that this occurs at locations in the crystal ineffective for triboluminescence, likely near the phenyl rings. Use of less polar solvents such as chloroform or dichloromethane results in a similar outcome. In these circumstances any solvent which is occluded in the crystal occurs at sites where no triboluminescence is conferred. 4.2. Crystal size and unit cell considerations Experiments which sought to explore the role of triphenylphosphine crystal size demonstrated that microscopic crystals from acetone showed no triboluminescence, either after short or longer term drying. The larger the crystals the longer the time that they continued to show triboluminescence on long term storage. These results correlate well with the time series experiments conducted with a near-constant crystal size in which
332
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/ Acetone-induced tribo/uminescence of tripheny/phosphine
it was found that solvent loss was positively correlated with loss of triboluminescence. Both outcomes are consistent with the interpretation that, as solvent is lost from the crystal defect sites occupied by the solvent molecules, the defects gradually anneal or relax, in so doing removing both the defect and the triboluminescence emission of the candidate crystal. However, neither the solvent-related nor the crystal size-related information allows differentiation of whether the solvent alone, or the crystal imperfections alone or both factors are necessary to confer triboluminescence. Structural and spectroscopic information is required to answer these questions. Polymorphism has been shown previously to be a factor in the observed triboluminescent and non-triboluminescent forms of diphenylcarbodiphosphorane [4]. Accordingly, film measurements and unit cell parameters determined by alignment on a diffractometer were undertaken to determine if this could be a factor here. It was not. The same crystal type and identical unit cell dimensions, within experimental error, were obtamed both for triboluminescent crystals recovered from acetone and for the inactive crystals obtained from cyclohexane solutions. Not only was the same polymorph obtained from the two solvents, but evidently at the 1: 45 or lower mole ratios of acetone to triphenylphosphine measured in the solvent-related experiments, the low ratio of acetone incorporation is having no other measurable effect on the gross crystal structure. It is still possible, however, that occasional local imperfections in structure induced by acetone and not observable by ordinary X-ray structure determination methods could still be present. There are precedents for these kinds of imperfections to be responsible for triboluminescence [8,20]. Additional spectroscopic information assisted in settling the residual questions. 4.3. Triboluminescence and photoluminescence spectra The similarity of the data obtained from the triboluminescence and photoluminescence emission measurements for samples of triphenylphosphine crystallized from different solvents
strengthens the case for the role of the triphenylphosphine itself rather than the solvent in both types of emission. The triboluminescence emission maximum of the acetone-crystallized sample is close to the photoluminescence maximum of acetone in an EPA (5 : 5 : 2 ether: isopentane : ethyl alcohol) glass at 77 K [23]. However, this is almost certainly fortuitous since the acetonitrile-crystallized sample shows roughly the same photoluminescence emission parameters, and neat acetonitrile shows no photoluminescence emission in this wavelength region. Therefore, the triboluminescence emission is probably related to the formation of specific types of imperfection in the triphenylphosphine crystal which in turn is related to the type of solvent used in the crystallization but is not unique to a particular solvent. Apparently crystallization of triphenylphosphine from benzene, cyclohexane, or chloroform either does not introduce these crystal imperfections, or if it does the imperfections are not of the type or location which confers triboluminescence. 4.4. Tests of solvent influence in other heteroorganics Experiments with acetone crystallization and triboluminescence testing of other arylated analogs of triphenylphosphine, tritylamine, triphenylphosphine oxide, and triphenylphosphine sulfide confirmed that the acetone induction of tnboluminescence in triphenyiphosphine was not a general phenomenon. Crystallization of the other generally brightly triboluminescent compounds triphenylamine and 1,2,5-triphenylphosphole from cyclohexane, chloroform, or acetone did not affect their emissions on crushing. During the course of these experiments, however, it was noticed that although triphenylamine was triboluminescent as received, and (eventually) from every solvent from which it was crystallized, when freshly crystallized from acetonitrile, ethyl acetate, or nitromethane it lost its triboluminescence. After standing for 9 days open to the air the triboluminescence was restored, essentially amounting to a solvent effect opposite to that shown by triphenylphosphine. This type of
MB. Hocking et a!.
/ Acetone-induced triboluminescence of triphenylphosphine
solvent effect has been previously noted by Sweeting for the complex tetrakis(dibenzoylmethanato) europate crystallized from dichloromethane [20]. As crystallized, this complex did not show triboluminescence, But after 18 h at 50°Cor several months standing at room temperature tniboluminescence was restored. In this case, however, markedly different space group parameters were measured for the active form crystallized from methanol as opposed to the (initially) inactive form crystallized from dichioromethane,
5. Conclusions The triboluminescence of triphenylphosphine crystallized from acetone has been shown to extend to several other carbonyl-containing solvents, and to ethanol and methanol, and to be “turned off” by crystallization from benzene or cyclohexane. The observed activity or non-activity does not relate to different polymorphs since active and inactive forms of tniphenylphosphine have identical unit cell parameters.
removes a large proportion of the occluded solvent but does not “switch off” the triboluminescence, which is what would be expected if the emission originated from the trapped solvent molecules, But the triboluminescence of the triphenylphosphine, previously pumped or not, fades over approximately the same period of time. During this period, a sufficient proportion of the initially formed defects are able to anneal, or relax to remove the defect, in so doing causing the gradual loss of tniboluminescence. Our results suggest that the relaxation rate of the solvent-induced defects are the nate-determining factor for the loss of triboluminescence, not the rate of solvent loss. Finally, these results signal the need for more attention to be directed towards the choice of solvent and the influence that this factor might
I,,,,,.
50000
40000 30000
Acetone at 77 K shows a photo-stimulated
emission and enylphosphine triboluminescence [23]. similar So itto ofisthe emission possible that active emission triphfrom acetone is contributed tophotoluminescence the But triboluminescence of profile the triphenylphosphine. not required. The triboluminescence ofit is triphenylphosphine crystallized from acetonitrile or cyclohexanone gave the same maxima and bandwidths, though weaker, as the acetone-crystallized material. Acetonitrile shows no photoemission in the visible region, so the emission from the first of these two samples can only be from the tniphenylphosphine. This combined evidence, plus the triphenylphosphine and acetone purity tests confirm that solvent- or dopant-induced emission is not the explanation for the observed activity. The available data suggest, therefore, that the originating mechanism for the active triphenylphosphine arises from crystal defects in the triphenylphosphine brought about by crystallization from certain solvents. Exposure of the crystals to low pressures for a short period of time
333
20OOO~
10000 340 50000
420
460
500
540
580
4000
20000
0
__________________________________________ 340
380
420
460
WAVELENGTH, nm
500
540
580
Fig. 1. Triboluminescence (noisy lines) and photoluminescence (smoother lines) spectra triphenylphosphine crystallized from acetone (upper pair)ofand acetonitrile (lower pair) all measured at room temperature. Exciting wavelength for the photoluminescence spectra was 308 nm.
334
MB. Hocking et a!.
/ Acetone-induced triboluminescence of tripheny!phosphine
have on triboluminescence emissions in future studies of this phenomenon. Solvent choice has not previously been of significant concern. Acknowledgements The authors are grateful to the Government of British Columbia and to the University of Victona for financial support of this work. They also thank K. Beveridge for unit cell measurements. References [1] [2] [3] [4]
A.J. Walton, Adv. Phys. 26 (1977) 887. B.P. Chandra, NucI. Tracks 10 (1985) 225. J.I. Sink, Naturwiss. 68 (1981) 507. G.E. Hardy, W.C. Kaska, B.P. Chandra and J.I. Zink, J. Am. Chem. Soc. 103 (1981) 1074. [5] B.P. Chandra and J.I. Zink, Inorg. Chem. 19 (1980) 3098. [6] B.P. Chandra and J.I. Zink, Int. J. Phys. Chem. Solids 42 (1981) 529. [7] E. Leyrer, F. Zimmerman, J.I. Zink and G. Gliemann, Inorg. Chem. 24 (1985) 102. [8] B.P. Chandra and J.I. Zink, J. Lumin. 23 (1981) 363.
191
PA. Thiessen and K. Meyer, Naturwiss. 57 (1970) 423.
[10] MB. Hocking, D.M. Preston and J.I. Zink, J. Lumin. 43 (1989) 309. 1111 B.P. Chandra and J.I. Zink, J. Phys. Chem. 86 (1982) 4138. [12] J.J. Daly, J. Chem. Soc. (1964) 3799. [13] IN. Stranski, E. Strauss and G. Wolff, Z. Elektrochem. 55 (1951) 633. [14] G. Wolff, G. Gross and IN. Stranski, Z. Elektrochem. 56 (1952) 420. [15] G.Gross, IN. Stranski and G. Wolff, Z. Elektrochem. 59 [16] G. Wolff, I. Schönewald and IN. Stranski, Z. Krist. 106 (1954) 146. [17] G. Wolff, G. Gross and IN. Stranski, Z. Elektrochem. 59 (1955) 341. [18] J.I. Zink and W.C. Kaska, J. Amer. Chem. Soc. 95 (1973) 7510. [19] CR. Hurt, N. McAvoy, S. Bjorklund and N. Filipescu, Nature 212 (1966) 179. [20] L.M. Sweeting and AL. Rheingold, J. Phys. Chem. 92 (1988) 5648. [21] L.M. Sweeting and AL. Rheingold, J Am. Chem. Soc. 109 (1987) 2652. [22] AL. Rheingold and W. King, Inorg. Chem. 28 (1989) 1715. [23] W.D. Chandler and L. Goodman, J. Mol. Spectrosc. 36 (1970) 141.
Journal of Luminescence 51(1992) 323—334 North-Holland
Acetone-induced triboluminescence of triphenyiphosphine Martin B. Hocking and Jeffrey I. Zink a
a
Francies W. VandervoortMaarschalk
a
John McKiernan
* b
b
Department of Chemist,y, University of Victoria, P.O. Box 3055, Victoria, BC V8W 3P6, Canada of Chemist,y, University of California, Los Angeles, CA 90024-1569, USA
b Department
Received 1 April 1991 Revised 24 August 1991 Accepted 10 November 1991
The triboluminescence of triphenylphosphine has been found to be acetone-dependent and is positively correlated with acetone content, crystal size, and negatively correlated with the age of the crystal. There is no measurable difference in the unit cell dimensions of triboluminescent triphenylphosphine crystallized from acetone and those of inactive material from cyclohexane. Similar emission results are obtained from crystals obtained from saturated solutions in some, but not all carbonyl-containing solvents, and from methanol and ethanol but not diethyl ether or ethyl acetate. Inactive material was also obtained from saturated solutions in benzene, cyclohexane or chloroform. The triboluminescence and photoluminescence maxima and bandwidths were the same, within the error limits of the experimental method and instrumentation used, for triphenylphosphine crystallized from acetone, acetonitrile, or cyclohexanone. Among the other highly arylated heteroorganics tested in measures to extend this correlation, triphenylamine and 1,2,5-triphenylphosphole were triboluminescent as purchased or prepared and when recrystallized from acetone, cyclohexane and several other solvents. Tritylamine, triphenyiphosphine oxide, and triphenylphosphine sulfide were not triboluminescent as purchased or prepared, or when re-crystallized from acetone or cyclohexane.
1. Introduction Triboluminescence, or the emission of light on the crushing or fracturing of crystals, is a mechanical energy transfer phenomenon that has been known to occur with sugars and certain minerals for many years [1,2]. Previous studies have determined the principal mechanistic details which give rise to the emission. The two most common spectral origins of triboluminescence are luminescence from the excited states of molecules of which the crystals are composed, or nitrogen emission from the air incorporated into or surrounding the crystal [3]. Sometimes a combina*
Presented at the 74th Canadian Chemical Conference Hamilton, Ontario, 2—6 June 1991, and at the Thirteenth International Congress of Heterocyclic Chemistry, Corvallis, Oregon, 11—16 August 1991.
0022-2313/92/$05.00 © 1992
—
tion of both origins is observed [4]. The formation of intense electric fields at the apex of propagating cracks during crystal fracture is generally considered to provide the energy required to initiate either of these emission processes. Surveys of triboluminescence from molecular crystals show a strong correlation with non-centrosymmetric space groups [5—8], a relationship which has been confirmed by the study of polymorphs [4]. Involvement of local dopants in the emission process has also been proposed [4,9]. During an earlier screening phase of a study of the triboluminescence of 1,2,5-triphenylphosphole, which is a particularly intense example visible in subdued daylight, samples of triphenylphosphine were also tested [10]. It was found that the phosphine, when tested as supplied from several commercial sources, did not triboluminesce. This result was at variance with prior re-
Elsevier Science Publishers B.V. All rights reserved
.
324
M.B. Hocking et a!.
/ Acetone-induced triboluminescence of triphenyiphosphine
ports [8,11]. Further tests demonstrated that recrystallization of the triphenylphosphine from acetone produced triboluminescent material, in agreement with the procedures of the prior reports, which also satisfied the screening requirements of our earlier study. This paper describes the details of experiments conducted to more comprehensively delineate the physical requirements, scope, and originating mechanism of this novel, solvent-induced triboluminescence emission. The experiments conducted here were designed to determine whether any or all of polymorphism, local crystal structure defects, and/or the presence of solvent in the crystal were responsible for the observed triboluminescence from triphenyiphosphine. The extent to which this influence might extend to and be correlated with other highly arylated hetero-organics and to other carbonyl-, and oxygen-containing solvents is also briefly examined.
2. Experimental Commercial samples of triphenylphosphine were obtained from the following sources: Aldrich, 99%, No. T.8440-9; BDH, 98%, No. 30537, lot 645130; Fisons, 98% mm.; Koch-Light Labs. Ltd. (Colnbrook, Bucks), No. 5537p, “pure”; and M&T Chemicals Inc., Lot AVVAH534K. The other triboluminescence candidate compounds were all purchased from Aldrich, cxcept for triphenylphosphine oxide, which was prepared from triphenylphosphine as previously described [101. Solvents were used as supplied from the following sources: acetone, technical grade, Van Waters and Rogers (VWR, Vancouver) and ACS reagent grade from Aldrich; 2-butanone, Fisher, lot 855634; 3-pentanone, cyclohexanone, C10,2180, lot LA, and propionaldehyde, all 99 + % from Aldrich; 2,4-pentanedione PX 215, lot P 5103 from Matheson, Coleman and Bell; methanol and ethanol (95%) were VWR technical grades; chloroform, PIN 1888, BDH Omnisolve; ethyl acetate, ACS Reagcnt Grade. Deuteriochloroform, 99.8 at% D, No. 15-182-3, and dideuteriodichloromethane, 99.9 at% D, were used as
supplied by Aldrich and Isotech. Inc. (Matheson), respectively. Benzene and diethyl ether were from Canlab, and were redistilled from sodium before use. 2.1. Observation procedure and variables
crystallization
Triboluminescence tests were conducted in a photographic darkroom by direct observation, after allowing several minutes for dark adaptation of vision. For each test a small sample of the candidate material was crushed by a Pyrex glass rod on the concave face of an 8 cm Pyrex watch glass resting on a white background, and on white bond paper crushed by the convex face of the watch glass. In an unlit photographic darkroom several samples of triboluminescent crystals of triphenylphosphine were dropped into a dewar of liquid nitrogen to observe any resulting emission. Samples of triphenylphosphine from each of the suppliers were first tested for triboluminescence as supplied. They were each then crystallized by cooling of hot saturated solutions without permitting evaporation (the “normal” fashion) from acetone and/or other solvents, filtered from the mother liquor, and dried by various techniques before being tested again for triboluminescence. Microscopically-small crystals were prepared by rapid rotary evaporation of a saturated solution of triphenylphosphine in acetone using reduced pressure, or by rapid chilling of a solution supersaturated at the boiling point. 2.2. Acetone content and unit cell dimensions Triphenylphosphine was crystallized from acetone, dried for 5 days loosely covered under ambient conditions, and further dried for 24 h at room temperature and 0.1 mm Hg pressure. A 49 mg portion of this still triboluminescent material was dissolved in 0.7 ml dideuteriodichioromethane and its 250 MHz 1H NMR spectrum, after 66 scans, recorded on a Bruker WM 250 instrument. For the more numerous phosphine samples requiring determination of acetone content in
MB. Hocking et a!.
/ Acetone-induced triboluminescence of triphenylphosphine
connection with time series trends, deuteriochloroform was used as the primary solvent with a trace of nitromethane added (1 drop in 50 ml CDCI3) as an internal shift and integration reference. Light chloroform to nitromethane to water relative intensity ratios at 5 7.24,4.30 and ca. 1.53 of this seeded solvent were usually about 1.0:2.6:1.3. This was measured for each new batch of standardized solvent before use for acetone content determinations. Solvent content for triphenylphosphine crystallized from acetonitrile was determined in the same way as the acetone determinations except that the relative intensity for the acetonitrile methyl at 2.02 5 was taken and phosphine to acetonitrile mole ratio calculated by normalizing for the 15 : 3 ratio of hydrogens. Unit cell dimensions were determined on suitable crystals selected from a sample of triphenylphosphine crystallized from acetone (tnboluminescent) and from a sample crystallized from cyclohexane (non-triboluminescent). Measurements were made from film records taken from these crystals on Precession, and Weissenberg cameras. Each of the same two crystals were also aligned on a Picker 4-circle diffractometer, with five-centered reflections, in order to obtain more accurate unit cell dimensions. 2.3. Triboluminescence and photoluminescence spectra Triboluminescence for determination of emission frequency range was generated by crushing the sample of test material against the walls of a 3 dram glass vial with a 3 mm brass rod. To record a spectrum the vial was clamped about 4 cm from the entrance slit of an iSA model HR320 0.32 m monochromator. A typical slit setting was 500 p.m and no focusing elements were used between the sample and the slit. The emission was detected and accumulated by an EG and G PARC model 1420 optical multichannel analyzer using 5 to 25 s collection times depending on the intensity of the emission from the sample. Mercury lines from a fluorescent lamp were used for calibration,
325
Photoluminescence of the neat sample was stimulated by the 308 nm line from a Lambda Physik EMG-201 XeCl excimer laser running at 20 Hz. The sample was pressed into the cupped end of an aluminum rod and positioned and the emission spectrum recorded as in the triboluminescence experiments. Typically, a slit width of 100 p.m was used to accumulate 100 scans of 50 ms exposure each. 2.4. Experiments with related compounds, procedures Samples of tritylamine, triphenyiphosphine oxide, triphenylphosphine sulfide, triphenylamine, and 1,2,5-triphenylphosphole were crystallized from acetone and one or more additional solvents by cooling hot saturated solutions without permitting evaporation. Samples of the original crystals and the newly crystallized material were tested for triboluminescence after filtration from the mother liquors using a variety of drying procedures.
3. Results The triboluminescence emissions of the materials under test here were inherently low which required photographic darkroom conditions for observation. Even the very low level lighting from a standard 15 W frosted glass incandescent bulb behind a Kodak green OA safelight filter was sufficient to completely mask the direct observation of the phosphine triboluminescence emissions being screened. 3.1. Effect of purity and solvent content of triphenyiphosphine Triboluminescence was not detected from any of the commercial samples of triphenylphosphine as supplied. But one recrystallization from acetone followed by brief drying open to the air was sufficient to produce triboluminescence on crushing from all the commercial materials. If the triphenylphosphine crystals were still moist with acetone, or once cursorily dried, were then re-
326
M.B. Hocking et a!.
/ Acetone-induced triboluminescence of triphenylphosphine
Table I Triphenylphosphine triboluminescence correlated with crystal preparation conditions. Solvent
Crystal preparation details
Crystal drying conditions
Triboluminescence
from saturated solution ~
moist with acetone ambient, ca. 30 mm ambient 24 h 0.1 mm Hg, room temp., 24 h ambient, ca. 30 mm for each ambient, ca. 30 mm.
no yes yes yes yes, all three yes
ambient, ca. 30 mm.
no
Acetone variations
acetone
‘~
acetone acetone acetone, then cyclohexane
recrystallized from fresh acetone once, twice, and thrice; each filtered slow partial evaporation of saturated solution under ambient conditions sequential recrystallizations from hot saturated solutions by cooling, filtered
acetone, then cyclohexane, then acetone cyclohexane
sequential recrystallizations saturated solution ~
ambient, ca. 30 mm. ambient, ca. 30 mm.
yes no
Other carbony!-containing so!t ents methyl ethyl ketone (2-butanone) diethyl ketone (3-pentanone) cyclohexanone ‘~ propionaldehyde C) 2,4-pentanedione
saturated saturated saturated saturated saturated
solution ~ solution ~ solution h) solution h) solution h)
ambient, ca. ambient, Ca. ambient, Ca. ambient, Ca. ambient, ca.
30 30 30 30 30
mm. mm. mm. mm. mm.
yes yes yes yes no
Other oxygen-containing sob ‘ents methanol ethanol ethyl acetate diethyl ether
saturated saturated saturated saturated
solution solution solution solution
ambient, ca. ambient, Ca. ambient, Ca. ambient, Ca.
30 30 30 30
mm. mm. mm. mm.
yes yes no no
Miscellaneous so/cents acetonitrile benzene chloroform dichloromethane
saturated solution ~ saturated solution b) saturated solution h) saturated solution h)
ambient, Ca. 30 ambient, Ca. 30 ambient, Ca. 30 ambient, Ca. 30
mm. mm. mm. mm.
yes no no no
“~
h) C)
~
1,) ‘~ “~
Identical results were obtained whether Technical, or Reagent Grade acetone was used (see section 2). By cooling of hot saturated solution without allowing evaporation, followed by filtration of crystals. Samples held in closed vials for several months turned yellow.
moistened with acetone, the triboluminescence was lost (table 1). Newly filtered crystals which were allowed to dry under ambient conditions for short lengths of time or which were placed under a reduced pressure of 0.1 mm Hg at room ternperature for 24 h, still showed triboluminescence. Saturated solutions of triphenyiphosphine in acetone which were crystallized by slow partial evaporation or by slow complete evaporation of the solvent gave crystals with triboluminescence behaviour indistinguishable by eye from that of crystals obtained by cooling hot saturated solu-
tions (the normal procedure). Concentration of the combined mother liquors from several normal crystallizations followed by cooling, which would have been expected to concentrate any impurities in the later fractions of triphenyiphosphine crystals obtained, also gave material indistinguishable in luminescence properties from normally crystallized material. Crystals of acetone-induced triboluminescent triphenylphosphine did not show any visible emission when dropped into liquid nitrogen in the darkroom.
MB. Hocking et a!.
/ Acetone-induced tribo!uminescence
Triphenylphosphine crystallized from acetone lost its triboluminescence if this material was crystallized again from cyclohexane or chloroform. Recrystallization of either of these samples yet again from acetone restored triboluminescence. A selection of other crystallizing solvents were also tested for their effect on the triboluminescence properties of triphenylphosphine. Results of these tests are given in table 1. The acetone content was determined by high frequency NMR measurements, which enabled estimation of the mole ratio of triphenylphosphine to acetone in the acetone-crystallized material when dried under various conditions (table 2). The spectrum for entry 1, for example, showed the presence of sharp singlets (without expansion) at 7.36 5 (aromatic H, Ph3P); 5.32 5 (HDCCI2, residual in CH2CI2); and 2.14 5 ((CH3)2C0). Computer recorded relative intensities were 502.04 for aromatic hydrogens: 1.25 for acetone, giving a 405: 1.00 proton ratio, The mean measured peak height ratio for the same two components from two different gain settings gave a 480: 1.00, aromatic hydrogen to acetone proton ratio. Normalizing these values for their respective 15 : 6 hydrogen contents gave the recorded triphenyiphosphine to acetone mole ratio estimates of 160: 1 to 190: 1. A variety of experimen-
of triphenylphosphine
327
tal tests established that the instrumentally recorded relative intensity values gave the most reliable triphenylphosphine to solvent mole ratios. With recognition of the integration limitations of pulsed NMR this method gave a mole ratio range from 36: 1 to 160: 1, triphenylphosphine to acetone, for crystals which were triboluminescent, and 260: 1 for crystals which were not. The effect on the relative intensity ratios of halving or doubling the number of NMR scans for a single phosphine sample caused a variability of ±5% (R = 0.83). Triplicating an NMR experiment on the same sample using the same number of scans gave a smaller variability of ±4%. At no time was the light chloroform, which was resolved from the phosphine multiplet at 5 7.25—7.67, more than i% of the relative intensity of the triphenylphosphine multiplet. The preliminary survey results plus the tests of the NMR approach outlined above provided the validation of the relative intensity ratio method which was used for a more closely regulated series of tests on the effects of elapsed time and/or solvent content on triboluminescence. Whilst the individual mole ratio results were somewhat scattered because “of the interaction of additional variables (crystal size, etc.), the fading and the eventual loss of triboluminescence correlated well to both elapsed time and loss of
Table 2 Correlation of triboluminescence to mole ratios of triphenylphosphmne to solvent in the crystal. Triphenylphosphine sample Type
Mole ratio, Ph3P to solvent, via NMR Solvent
Medium crystals, vacuum dried 20 hours acetone Large crystals, air dried acetone Large crystals, air dried acetonitrile Large crystals, air dried acetone Small crystals, air dried acetone Large crystals b) vacuum dried 20 h, then stored acetone Medium crystals C) vacuum dried 20 h, then stored acetone a) Measured at 0.25 months from crystallization. b) 2.5—3.5 mm on an edge. c) 0.5—0.7 mm on an edge.
Age (months)
0.25 0.25, 0.5, 1.0 0.25, 0.5, 1.0 8 8
Triboluminescent +ve +ve +ve + ye —ye
Integration
—
13: 1 a) 120:1 a) 69: 1 410:1
Relative intensities
Peak heights
160 6.4 57 36 260
190 5.3 54 30 320
:1 :1 :1 :1 :1
a) a)
:1 : 1’~ :1 a) :1 :1
18
weak +ve
—
—
—
18
+ve
—
—
—
328
MB. Hocking et a!.
/ Acetone-induced triboluminescence of tripheny/phosphine
Table 3 Triboluminescence of triphenylphosphine (TPP) crystallized from acetone correlated with subsequent treatment measured). Age of sample (days)
0 5 13 19 23 40 53 65 110
5) C)
a)
(nm: not
Sample air-dried, stored In capped vial
Open to air
TPP: acetone mole ratio
TPP: acetone mole ratio
Triboluminescence C)
TPP: acetone mole ratio
Triboluminescence C)
TPP: acetone mole ratio
Triboluminescence C)
nm 22 30 39 49 43 71 23
nm
48 43 50 46 40 44 63 130 110
+ + + +++
nm 45 43 58 85 100 120 93
nm
22 33 27 33 67 62 55 130 106
Triboluminescence C) + + + +++ +++ +++
+++ +++ + + +
0
Sample dried in vacuo 5), stored In capped vial Open to air
+++ +++ +++ +++ ++ +
0
—
+++ +++
+++ +++
+ +
0
+
—
+++ +++
+ +
0 —
Four samples for time-series tests were obtained by subsampling a single 13.3 g crop of medium sized (2.5 X 2.5 x 1.0 mm) crystals of triphenylphosphmne freshly crystallized from a filtered solution in warm acetone. Details are given in experimental section. Closed storage was in a polyethylene snap-capped vial. Storage “open to air” was with a loosely suspended watch glass above the sample to prevent entry of dust. At Ca. 3 mm Hg, 26 h. Triboluminescence relative intensities: + + +, strong; + +, medium; +, weak; 0, none, all observed by dark-adapted vision in photographic darkroom.
d) Result from a large crystal. Not included in slope, intercept calculations.
solvent (table 3). Both solvent loss and loss of triboluminescence occurred more rapidly if the crystal subsample, which was otherwise identical, was stored open to the air rather than in a capped vial, Crystallization of triphenylphosphine from a selected range of aldehydic and ketonic monocarbonyl-containing solvents uniformly conferred tnboluminescence on samples of triphenyiphosphine, but no emission was observe4 when it was crystallized from 2,4-pentanedione (table 1). Crystallization from methanol and ethanol also produced triboluminescent material, but ethyl acetate and diethyl ether did not. Use of acetonitrile as crystallization solvent conferred triboluminescence whereas inactive material was ob-
not show triboluminescence after 30 mm or 24 h drying time under ambient conditions. Intermediate-sized crystals (angular spars, ca. 1 x 1 >< 4 mm) were triboluminescent after 30 mm drying time and for some weeks afterwards, but lost the effect after 8 months standing in polyethylene snapcapped glass vials under ambient conditions. Very large crystals of approximate dimensions 4 x 10 x 10 mm showed triboluminescence both after drying for 30 mm in the open air and after standing in capped vials for 8 months. Unit cell dimensions were determined by careful film measurements, and alignment on a diffractometer. These showed that crystals grown from acetone or from cyclohexane were both monoclinic, type P21 /c, and had identical gross
tamed from benzene or chloroform,
unit cell dimensions within, experimental error (table 4).
3.2. Effect of crystal size and comparison of unit cells Rapid crystallization of triphenylphosphine from acetone yielded microscopically-sized prisms of less than 0.5 mm in any dimension which did
3.3. Triboluminescence and emission spectra
photoluminescence
Four of the triboluminescent samples of triphenylphosphine were selected for detailed emis-
M.B. Hocking et a!. Table 4 Cell parameters from film measurements acetone and from cyclohexane. Cell parameter
a)
/ Acetone-induced tribo/uminescence of tripheny!phosphine
and actual unit cell dimensions
TPP crystallized from Acetone
Cyclohexane
b)
329
from triphenylphosphmne (TPP) crystallized from
TPP crystallized from Acetone
Cyclohexane
d-spacing 8.475 15.142
8.491 15.037
From film measurements a b From diffractometer a b c
p a) h) C)
Zero precession distance, A 10.936 10.916 6.121 6.164
Unit cell dimensions Published values C) 11.421A 11.413A 11.413A 15.023A 15.027A 15.032A 8.498 A 8.514 A 8.500 A 92°54’ 92°48’ 92°53’ X-ray film exposed on Precession and Weissenberg cameras. Obtained by alignment on a diffractometer, with five-centered reflections. Details from ref. [12]. Neither crystallization procedure nor solvent(s) used were specified.
sion measurements. The triphenylphosphine samples crystallized from acetone, acetonitrile, and cyclohexanone all gave very similar triboluminescence emission maxima, although the last two samples had triboluminescence emission intensities too weak to allow precise measurement of the frequency width at half height (table 5). Whilst the triboluminescence of the sample crystallized from propionaldehyde was visually and instrumentally detectable, it was too weak to enable either the maximum or the frequency range to be accurately determined, Strong photoluminescence was obtained from
all triphenyiphosphine samples which permitted accurate maxima and frequency distribution information to be collected. For the samples crystallized from acetone and cyclohexanone the photoluminescence maxima appeared at slightly shorter wavelengths than the triboluminescence whereas for the sample crystallized from acetonitrile the photoluminescence maximum was at slightly longer wavelengths. The frequency distribution of the two types of emissions appeared to be the same, within experimental error, for the acetone-crystallized sample of triphenylphosphine.
Table 5 Comparison of the room temperature photoluminescence and triboluminescence emission spectra of triphenylphosphmne (TPP) crystallized from various solvents a) Crystallization solvent
Emission maximum (nm)
Width at half maximum height (nm)
Photolum.
Tribolum.
Photolum.
Tribolum.
452±10 445±20 450 ±20
85±4 98±4 72 ±4 96±4
88±10 80±20 80 ±20
TPP/acetone 443±5 TPP/acetonitrile 458±5 TPP/cyclohexanone 446 ±5 TPP/propionaldehyde 456±5 a) Uncertainty in maxima are as specified. 5)
Could not be accurately determined from spectrum.
5)
5)
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3.4. Triboluminescence induction in other highly arylated hetero-organics Tritylamine, triphenyiphosphine oxide, and triphenylphosphine sulfide were tested both before and after crystallization from acetone, None of these six samples showed any triboluminescence. Triphenylamine, however, was triboluminescent both as supplied, and when recrystallized from acetone, cyclohexane or ethyl acetate, but lost this property when recrystallized from nitromethane. When a further sample of triphenylamine was newly crystallized from acetonitrile and tested for triboluminescence it was inactive, However, after standing open to the air for 9 days triboluminescence was restored. Recrystallization of samples of 1,2,5-triphenylphosphole from acetone, chloroform, dichloromethane, or ethyl acetate produced triboluminescent material from all solvents after short drying periods open to the air.
4. Discussion As far as can be determined only the group of Stranski and Wolff have devoted significant attention to the phenomenon of temporary triboluminescence. After an initial delineation of characteristics they found that the temporary triboluminescence of arsenic trioxide, “arsenolith”, only occurred if the crystals were prepared in a specific way [13,14] and that crystal size and age were both factors which affected these emissions [15]. It was concluded that the temporary triboluminescence of arsenic trioxide could be attributed to lattice defects which changed over time [14,161. The loss of triboluminescence from newly crystallized uranyl nitrate was found to take 70 h at 60°Cand more than 190 h at 45°Cin Stranski’s tests [17]. Salicylic acid crystallized from alcohol, as the sole organic example, was found to lose its triboluminescence on standing at room temperature for four months. By X-ray or by UV methods they were unable to detect any changed products or decomposition fragments with these two examples that might have explained the observed re-
of tripheny!phosphine
sults, and so concluded that there were too many possible factors to offer any mechanistic hypothesis. Relevant to the influence of solvent, Hurt’s group found that the relative intensity of triboluminescent europium complexes was related to the choice of recrystallization solvent among other factors [19]. However, no particular solvent was necessary to obtain triboluminescence nor did any solvent “switch off” triboluminescence. We have examined the novel example of the temporary triboluminescence of triphenylphosphine [10] to test a number of possible operating factors. The results of these experiments allow the formation of a mechanistic hypothesis to explain the process. 4.1. Triphenylphosphine purity and solvent effects The presence of impurities in commercial saccharm has been previously noted to be responsible for the triboluminescence of this material, probably by serving as dopants [4]. Consequently the possibility that trace impurities trapped in the crystals of commercial samples of triphenyiphosphine might have stimulated the observed triboluminescence was tested. None of the purchased samples showed triboluminescence as received, nor did any sample when recrystallized two or three times, as long as the last crystallization was from cyclohexane. If second and third crops of triphenylphosphine crystals were taken from the mother liquor of a cyclohexane crystallization, or if the final mother liquor was allowed to evaporate to dryness leaving residual triphenylphosphine crystals, none of these samples showed triboluminescence. Thus, the origin of the emission from triphenylphosphine crystallized from acetone is unlikely to originate from a phosphine impurity doping phenomenon. Originally non-triboluminescent triphenyiphosphine samples from several different suppliers each became triboluminescent on recrystallization from acetone. Triboluminescence was also maintained if these once recrystallized materials were crystallized from acetone once or twice more. These results tend to further rule out any influence of a trace impurity in the triph-
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/ Acetone-induced triboluminescence
enylphosphine, and rather, suggest that the acetone is somehow involved. This could have been via one or more impurities present in the acetone itself. However, crystallization tests using several widely differing lots of technical acetone and one sample of reagent grade material all produced triboluminescent crystals from triphenyiphosphine, which would suggest that, this possibility is unlikely. That the observed effect may be due to residual acetone trapped in the matrix of so-crystallized triphenylphosphine was suggested by the loss of triboluminescence on subsequent recrystallization of these active crystals from cyclohexane or chloroform. Confirmation of this suggestion was obtained by restored triboluminescence on recrystallization of these phosphine samples yet again from acetone. Liquid nitrogen has been used as a non-destructive method for the introduction of strain to cause triboluminescence emission [18]. This method, which has the potential to enable the determination of triboluminescence of a particular crystal and then to measure the solvent content of the same crystal failed to yield any emission with active samples of triphenylphosphine crystallized from acetone. If emission had been achieved by this thermal shock method an experimental procedure would have been available which could have decreased the number of vanables and hence the variability of the mole ratio data of table 4. Additionally, it could have enabled repeated triboluminescence emission from the same crystal without the need for intervening reciystallizations. It is tempting to suggest that the triboluminescence of triphenylphosphine when crystallized from acetone, 2-butanone, 3-pentanone, cyclohexanone, and propionaldehyde is related somehow to the presence of the polar carbonyl function, or at least a polar function since ethanol and methanol also conferred triboluminescence. The presence of two polar functions evidently interferes with this interaction since crystallization from 2,4-pentanedione, or ethyl acetate, yielded inactive material. At no time did solvents such as benzene, chloroform, and cyclohexane ever confer tribolumi-
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331
nescence on triphenylphosphine. From this group there is no published suggestion that either cyclohexane or chloroform are inhibitors of triboluminescence [10,19]. It has been stated that dichloromethane could inhibit triboluminescence via fluorescence quenching [20,21]. Whilst this was the case for the europium complexes cited, the triboluminescence of 1,2,5-triphenylphosphole was found to be unaffected when a small sample was crystallized from dichloromethane (see also ref. [10]). The solvent-related results suggest that to confer triboluminescence the crystallization solvent used must have an exchangeable proton or other electronegative element, e.g., carbonyl or nitrile, so placed that it can cause occasional disruptions in the local structure in the developing triphenylphosphine crystals, as they form. When a crack moves through these local disorder sites, then triboluminescence occurs. This mechanism for the introduction of disorder has been proposed for the preparation of triboluminescent lanthanide complexes [21], and has also been invoked to explain the triboluminescence of 9anthryl carbinols [19]. Non-polar solvents such as cyclohexane and benzene also may be occluded in developing crystals as they form but the lack of any polar element in these solvents means that this occurs at locations in the crystal ineffective for triboluminescence, likely near the phenyl rings. Use of less polar solvents such as chloroform or dichloromethane results in a similar outcome. In these circumstances any solvent which is occluded in the crystal occurs at sites where no triboluminescence is conferred. 4.2. Crystal size and unit cell considerations Experiments which sought to explore the role of triphenylphosphine crystal size demonstrated that microscopic crystals from acetone showed no triboluminescence, either after short or longer term drying. The larger the crystals the longer the time that they continued to show triboluminescence on long term storage. These results correlate well with the time series experiments conducted with a near-constant crystal size in which
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it was found that solvent loss was positively correlated with loss of triboluminescence. Both outcomes are consistent with the interpretation that, as solvent is lost from the crystal defect sites occupied by the solvent molecules, the defects gradually anneal or relax, in so doing removing both the defect and the triboluminescence emission of the candidate crystal. However, neither the solvent-related nor the crystal size-related information allows differentiation of whether the solvent alone, or the crystal imperfections alone or both factors are necessary to confer triboluminescence. Structural and spectroscopic information is required to answer these questions. Polymorphism has been shown previously to be a factor in the observed triboluminescent and non-triboluminescent forms of diphenylcarbodiphosphorane [4]. Accordingly, film measurements and unit cell parameters determined by alignment on a diffractometer were undertaken to determine if this could be a factor here. It was not. The same crystal type and identical unit cell dimensions, within experimental error, were obtamed both for triboluminescent crystals recovered from acetone and for the inactive crystals obtained from cyclohexane solutions. Not only was the same polymorph obtained from the two solvents, but evidently at the 1: 45 or lower mole ratios of acetone to triphenylphosphine measured in the solvent-related experiments, the low ratio of acetone incorporation is having no other measurable effect on the gross crystal structure. It is still possible, however, that occasional local imperfections in structure induced by acetone and not observable by ordinary X-ray structure determination methods could still be present. There are precedents for these kinds of imperfections to be responsible for triboluminescence [8,20]. Additional spectroscopic information assisted in settling the residual questions. 4.3. Triboluminescence and photoluminescence spectra The similarity of the data obtained from the triboluminescence and photoluminescence emission measurements for samples of triphenylphosphine crystallized from different solvents
strengthens the case for the role of the triphenylphosphine itself rather than the solvent in both types of emission. The triboluminescence emission maximum of the acetone-crystallized sample is close to the photoluminescence maximum of acetone in an EPA (5 : 5 : 2 ether: isopentane : ethyl alcohol) glass at 77 K [23]. However, this is almost certainly fortuitous since the acetonitrile-crystallized sample shows roughly the same photoluminescence emission parameters, and neat acetonitrile shows no photoluminescence emission in this wavelength region. Therefore, the triboluminescence emission is probably related to the formation of specific types of imperfection in the triphenylphosphine crystal which in turn is related to the type of solvent used in the crystallization but is not unique to a particular solvent. Apparently crystallization of triphenylphosphine from benzene, cyclohexane, or chloroform either does not introduce these crystal imperfections, or if it does the imperfections are not of the type or location which confers triboluminescence. 4.4. Tests of solvent influence in other heteroorganics Experiments with acetone crystallization and triboluminescence testing of other arylated analogs of triphenylphosphine, tritylamine, triphenylphosphine oxide, and triphenylphosphine sulfide confirmed that the acetone induction of tnboluminescence in triphenyiphosphine was not a general phenomenon. Crystallization of the other generally brightly triboluminescent compounds triphenylamine and 1,2,5-triphenylphosphole from cyclohexane, chloroform, or acetone did not affect their emissions on crushing. During the course of these experiments, however, it was noticed that although triphenylamine was triboluminescent as received, and (eventually) from every solvent from which it was crystallized, when freshly crystallized from acetonitrile, ethyl acetate, or nitromethane it lost its triboluminescence. After standing for 9 days open to the air the triboluminescence was restored, essentially amounting to a solvent effect opposite to that shown by triphenylphosphine. This type of
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/ Acetone-induced triboluminescence of triphenylphosphine
solvent effect has been previously noted by Sweeting for the complex tetrakis(dibenzoylmethanato) europate crystallized from dichloromethane [20]. As crystallized, this complex did not show triboluminescence, But after 18 h at 50°Cor several months standing at room temperature tniboluminescence was restored. In this case, however, markedly different space group parameters were measured for the active form crystallized from methanol as opposed to the (initially) inactive form crystallized from dichioromethane,
5. Conclusions The triboluminescence of triphenylphosphine crystallized from acetone has been shown to extend to several other carbonyl-containing solvents, and to ethanol and methanol, and to be “turned off” by crystallization from benzene or cyclohexane. The observed activity or non-activity does not relate to different polymorphs since active and inactive forms of tniphenylphosphine have identical unit cell parameters.
removes a large proportion of the occluded solvent but does not “switch off” the triboluminescence, which is what would be expected if the emission originated from the trapped solvent molecules, But the triboluminescence of the triphenylphosphine, previously pumped or not, fades over approximately the same period of time. During this period, a sufficient proportion of the initially formed defects are able to anneal, or relax to remove the defect, in so doing causing the gradual loss of tniboluminescence. Our results suggest that the relaxation rate of the solvent-induced defects are the nate-determining factor for the loss of triboluminescence, not the rate of solvent loss. Finally, these results signal the need for more attention to be directed towards the choice of solvent and the influence that this factor might
I,,,,,.
50000
40000 30000
Acetone at 77 K shows a photo-stimulated
emission and enylphosphine triboluminescence [23]. similar So itto ofisthe emission possible that active emission triphfrom acetone is contributed tophotoluminescence the But triboluminescence of profile the triphenylphosphine. not required. The triboluminescence ofit is triphenylphosphine crystallized from acetonitrile or cyclohexanone gave the same maxima and bandwidths, though weaker, as the acetone-crystallized material. Acetonitrile shows no photoemission in the visible region, so the emission from the first of these two samples can only be from the tniphenylphosphine. This combined evidence, plus the triphenylphosphine and acetone purity tests confirm that solvent- or dopant-induced emission is not the explanation for the observed activity. The available data suggest, therefore, that the originating mechanism for the active triphenylphosphine arises from crystal defects in the triphenylphosphine brought about by crystallization from certain solvents. Exposure of the crystals to low pressures for a short period of time
333
20OOO~
10000 340 50000
420
460
500
540
580
4000
20000
0
__________________________________________ 340
380
420
460
WAVELENGTH, nm
500
540
580
Fig. 1. Triboluminescence (noisy lines) and photoluminescence (smoother lines) spectra triphenylphosphine crystallized from acetone (upper pair)ofand acetonitrile (lower pair) all measured at room temperature. Exciting wavelength for the photoluminescence spectra was 308 nm.
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have on triboluminescence emissions in future studies of this phenomenon. Solvent choice has not previously been of significant concern. Acknowledgements The authors are grateful to the Government of British Columbia and to the University of Victona for financial support of this work. They also thank K. Beveridge for unit cell measurements. References [1] [2] [3] [4]
A.J. Walton, Adv. Phys. 26 (1977) 887. B.P. Chandra, NucI. Tracks 10 (1985) 225. J.I. Sink, Naturwiss. 68 (1981) 507. G.E. Hardy, W.C. Kaska, B.P. Chandra and J.I. Zink, J. Am. Chem. Soc. 103 (1981) 1074. [5] B.P. Chandra and J.I. Zink, Inorg. Chem. 19 (1980) 3098. [6] B.P. Chandra and J.I. Zink, Int. J. Phys. Chem. Solids 42 (1981) 529. [7] E. Leyrer, F. Zimmerman, J.I. Zink and G. Gliemann, Inorg. Chem. 24 (1985) 102. [8] B.P. Chandra and J.I. Zink, J. Lumin. 23 (1981) 363.
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PA. Thiessen and K. Meyer, Naturwiss. 57 (1970) 423.
[10] MB. Hocking, D.M. Preston and J.I. Zink, J. Lumin. 43 (1989) 309. 1111 B.P. Chandra and J.I. Zink, J. Phys. Chem. 86 (1982) 4138. [12] J.J. Daly, J. Chem. Soc. (1964) 3799. [13] IN. Stranski, E. Strauss and G. Wolff, Z. Elektrochem. 55 (1951) 633. [14] G. Wolff, G. Gross and IN. Stranski, Z. Elektrochem. 56 (1952) 420. [15] G.Gross, IN. Stranski and G. Wolff, Z. Elektrochem. 59 [16] G. Wolff, I. Schönewald and IN. Stranski, Z. Krist. 106 (1954) 146. [17] G. Wolff, G. Gross and IN. Stranski, Z. Elektrochem. 59 (1955) 341. [18] J.I. Zink and W.C. Kaska, J. Amer. Chem. Soc. 95 (1973) 7510. [19] CR. Hurt, N. McAvoy, S. Bjorklund and N. Filipescu, Nature 212 (1966) 179. [20] L.M. Sweeting and AL. Rheingold, J. Phys. Chem. 92 (1988) 5648. [21] L.M. Sweeting and AL. Rheingold, J Am. Chem. Soc. 109 (1987) 2652. [22] AL. Rheingold and W. King, Inorg. Chem. 28 (1989) 1715. [23] W.D. Chandler and L. Goodman, J. Mol. Spectrosc. 36 (1970) 141.