J. Phys. Chem. 1984, 88, 49-52 quenching mechanism have revealed that the excited states having intramolecular charge-transfer character are efficiently quenched by electrophilic attack of proton.20 Shizuka et al.21investigated the proton-induced quenching of the excited singlet and triplet states of 9,9-bianthryl and reported that the quenching rate constants are 1.5 X lo9 and 1.2 X lo6 M-I s-I in the excited singlet and the triplet states, respectively. It appears that the quenching rate constant of the triplet state is extremely smaller than that of the excited singlet state. The present study has shown that the quenching of the triplet TPPTi=O takes place by a diffusion-controlled process. Anal(17) Stevens, C. G.; Strickler, S. J. J . Am. Chem. SOC.1973, 95, 3922-8. (18) Tsutsumi, K.; Shizuka, H. Chem. Phys. Lett. 1977, 52, 485-8. (19) Shizuka, H.; Tsutsumi, K.; Takeuchi, H.; Tanaka, I. Chem. Phys. Lett. 1979, 62, 408-12. (20) Tobita, S.; Shizuka, H. Chem. Phys. Lett. 1980, 75, 140-4. (21) Shizuka, H.; Ishii, Y . ;Morita, T. Chem. Phys. Lelt. 1977,51,40-4.
49
ogously with the case of aromatics, the quenching mechanism is considered to be due to the electrophilic protonation as represented by EtOH2+ TPPTi+-OH3* EtOH TPPTi=03*
+
--
TPPTi+-OH3* TPPTi+-OH
+ EtOH
+
TPPTi+-OH
TPPTi=O
+ EtOH2+
where the lifetime of the triplet TPPTi+-OH, TPPTi+-OH3*, is assumed to be very short because of the fact that no transient ascribable to TPPTi+-OH3*is observed during the proton-induced decay of TPPTi=03*. The efficient quenching of TPPTi=03* by protons suggests that the Ti=O bond is polarized like Ti6+= O*-, which may give access for a proton to the oxygen atom of TPPTi=O. Registry No. TPPTi=O, 58384-89-7; TPPTi=O(EtOH), 87764-098; EtOH, 64-17-5; HCI, 7647-01-0; TPPTiC-OH, 87764-10-1.
Dielectric and Electron Spin Resonance Spectroscopic Studies of Glassy Crystalline States of Organic Compounds’ R. Parthasarathy, K. J. Rao, and C. N. R. Rao* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-56001 2, India (Received: January 13, 1983; In Final Form: March 30, 1983)
Dielectric studies of the glassy crystalline states of cyclohexanol, cyclohexanone, and camphor obtained by supercooling the plastic crystalline phase demonstrate the presence of characteristic a-and p-relaxations. The parameters of the a-relaxation fit the Vogel-Tammann-Fulcher (VTF) equation. ESR spin-probe studies of the glassy crystalline phase of cyclohexanol show that there is a marked decrease in the correlation time above the glasslike transition temperature. The present studies suggest the similarity between glassy crystals having long-range orientational disorder and glasses which are known to be translationally disordered.
Introduction It is well-known that glasses are formed by supercooling viscous liquids. It is interesting to ponder whether one may not form “glasses” wherein disorder, other than positional, is frozen in. Plastic crystals would be appropriate materials for such an enquiry and earlier indicates that orientationally disordered glasses or “glassy crystals” may indeed be formed by supercooling the plastic crystalline phase. Glassy crystals also seem to show glasslike transitions2-6 occurring at a temperature T i similar to glass transitions at Tg. Calorimetric studies6 of glassy crystals have shown that an ideal glass (-like) transition temperature, To,may be described in these systems just as in the case of bona fide glasses. The variation of excess entropy with temperature is known to be similar in glasses as well as in glassy crystals6 so that it seems reasonable to expect that a viable model of the glass transition can be easily extended to explain glasslike transitions in materials which may be positionally ordered. Dielectric studies of glassy crystals have shown the presence of‘ multiple relaxations, those at temperatures above T i being (1) Contribution No. 198 from the Solid State and Structural Chemistry Unit. (2) H. Suga and S. Seki, J. Non-Cryst. Solids, 16, 171 (1974). (3) H. Suga and S. Seki, Furuduy Discuss. Cfiem. SOC.,69 (1980). (4) H. Adachi, H. Suga, and S. Seki, Bull. Chem. SOC.Jpn., 41, 1073 (1971). (5) K. Adachi, H. Suga, S. Seki, S. Kubota, S. Yamaguchi, 0.Yono, and Y. Wada, Mol. Cryst. Liq. Cryst., 18, 345 (1972). (6) G.P. Johari, Philos. Mug. Part B, 41, 41 (1981).
0022-3654/84/2088-0049$01.50/0
called a-relaxations and those below T i being called p-rela~ations.~ a-Relaxations have been associated with the activation of intermolecular modes, and @-relaxationswith that of intramolecular modes.5 In cyclohexanol, however, the fl-relaxation has been attributed to hydrogen-bond f l i ~ p i n g .In ~ order to elucidate the details of the glassy crystalline state we have carried out dielectric investigations on cyclohexanone and dl-camphor and have also reexamined the behavior of cyclohexanol. An ESR spin-probe study has been carried out on cyclohexanol, which is the first such study of the glassy crystalline state. We find that @-relaxations are likely to be found generally in glassy crystals. The present , at a ESR study shows that the correlation times, T ~ decrease temperature Tk( Tk> T i ) as the glassy crystal is warmed. Such a behavior is reminiscent of the bona fide glasses studied earlier in this l a b ~ r a t o r y . We ~ have also explored the use of the recently developed cluster model8 of the glass transition in order to explain our results. Experimental Section Samples of cyclohexanol and cyclohexanone were purified by fractional distillation while camphor was repeatedly sublimed before use. Dielectric measurements on liquids were carried out by using an annular cell while those of camphor employed a three-terminal setup. A General Radio null detector was used along with a GR 1615 A bridge for these measurements. The (7) R. Parthasarathy, K. J. Rao, and C. N. R. Rao, J . Phys. Chem., 85, 3085 (1981). (8) K. J. Rao and C. N. R. Rao, Muter. Res. Bull., 17, 1337 (1982).
0 1984 American Chemical Society
50 The Journal of Physical Chemistry, Vol. 88, No. 1, 1984
Parthasarathy et al.
TABLE I : Thermodynamic Data Related t o Glassy Crystals _ _ _ _ _ _ _ _ _ _ l l l l l l _ l _ ~ ~ -
-_I__---
To TcpC -- ASp,/AScp 5.5 136 0.11 9 65 4.7 222 0.14 20 14 5 6.7 d d d d 52 5.72 e 258.4 e e 120 22 7.6 5 camphor 11.76 449 35.2 203 0.33 246 140 6 cyclohexanol 5.96 29 8 33.2 24 5 0.18 53 120 7 cis- 1,2-dimethylcyclohexane 7.56 22 0 35.2 170 0.21 50 82 a In J mol-’ K-’. T,, = TF = temperature of fusion (plastic crystal to liquid transition). Tcp = crystal t o plastic crystal transition temperature. Substance does not crystallize. e This has two transitions and we have therefore not given the data. All temperatures in Kelvin. f ATpl = Tpl - Tcp.
no. ---____--__ compd 1 2,3-dimethylbutane 2 cyclohexanone 3 2,3- and 2,2-dimethylbutanes 4 cycloheptanol
TPlb 145 24 2 170 280
Aspla
__I____
I
AScpC 47.7 34.5
-_l_l__
I
I
I
I
150
200
I
2 50
I
L
T (K)
Figure 1. Plot of dielectric constant against temperature for cyclohexanol at the following frequencies (in kHz): (A)0.5,(B) 1, (C) 50, (D) 100. Inset: the relaxation region on an expanded scale.
I
I
I
100
150
200 T
1 2 50
(K)
Figure 2. Plot of dielectric constant against temperature for cyclohexanone at the following frequencies (in kHz): (A)0.5, (B)5, (C) 50, (D) 100. Inset: the relaxation region on an expanded scale.
samples were first quenched to 120 K and were then gradually allowed to warm up. Measurements were made on the heating run and no attempt was made to hold the temperature constant since the large thermal mass of the cell kept drift down to a convenient level. A Varian E-109 ESR spectrometer was used for the spectroscopic study and the sample was quenched in situ. The spin probe, 2,2,6,6-tetramethyl-4-oxopiperidinyl-l-oxy (Tempone) was added to cyclohexanol at a concentration of less than 0.4 mM.
Results and Discussion In Table I we list some of the compounds that are supposed to form glassy crystals, when the plastically crystalline states are quenched to low temperatures. Unsymmetrical molecular shapes and the existence of intermolecular forces such as hydrogen bonding seem to favor formation of the glassy crystalline phase. This phase seems to be stabilized in compounds where the ratio liquid transition (AS,,) of the entropy change a t the plastic to that at the crystal plastic transition (AS,) is less than -0.2. Evidently, highly disordered plastic crystals are easily supercooled, in obvious analogy to glass formation in viscous liquids. Dielectric Studies. Figures 1-3 show the variation of the dielectric constant, E’, as a function of temperature for cyclohexanol, cyclohexanone, and camphor. Dispersions in dielectric constant are seen for cyclohexanone and camphor while cyclohexanol shows only a single relaxation. There is also a steep rise in e’ in each case at a temperature closely matching the known plastic crystal transition t e m p e r a t ~ r e . ~Figures 4-6 crystal show the variation of dielectric loss, tan 6, as a function of temperature for the three compounds. Again a pair of relaxations is seen in cyclohexanone and camphor (Figures 5 and 6). A pair of relaxations is seen in cyclohexanol as well, but the relaxation
-
-
-
(9) J. N. Sherwood, “The Plastically Crystalline State”, Wiley, London, 1979.
1
1
I
150
I 200 Temperature ( K )
I
2 50
Figure 3. Plot of dielectric constant against temperature for camphor at the following frequencies (in kHz): (A,circles) 0.5,(B, triangles) 1, ( C , crosses) 10, (D, squares) 50, (E, dots) 100.
-
a t higher temperature may be related to a supercooled plastic crystal metastable nonrotor crystal transition which is. known to occur in this compound.s Plotting up against TI,where T denotes the temperature at which the loss is a maximum at a , obtain Arrhenius plots (insets to Figures 5 and frequency F ~we 6) for the low-temperature relaxations. The relaxations at higher temperatures yield plots (insets to Figures 4-6) which are described by the Vogel-Tammann-Fulcher (VTF) e q u a t i ~ n ~ ~ ’ ~
x = xo exp[(B)/(T - Tdl
(1)
The Journal of Physical Chemistry, Vol. 88, No. 1, 1984 51
Glassy Crystalline States of Organic Compounds
A
I
130
230
180 T (K)
Figure 4. Plot of tan 6 (on a semilog scale) against temperature for cyclohexanol at the following frequencies (in kHz):(A) 0.5, (B) 1, (C) I for the main 5 , (D) 50, (E) 100. Inset: plot of log up against T relaxation. A
mc
d
I
I
200
150
I 250
T (K)
Figure 6. Semilog plot of tan 6 against temperature for camphor at the following frequencies (in kHz): (A)1, (B) 10, ( C ) 50, and (D) 100. Inset: plots of up against TI for the low-temperature ((3-) and hightemperature (a-)relaxations.
K:
*b I
.Q
5
c
1
150
200 T(K)
Figure 5. Semilog plot of tan 6 against temperature for cyclohexanone at the following frequencies (in kHz): (A) 0.5, (B) 1, (C) 50, (D) 100.
Inset: plots of up against T 1for the low-temperature (p-) and hightemperature (a-)relaxations. where x, in this case, is up. B and To are constants and xo is weakly temperature dependent. In keeping with the terminology used for glasses,]’ we call the relaxation that yields the Arrhenius plot the @-relaxation, and the other the a-relaxation. The broadening of the peak at higher frequencies as a consequence of the VTF relation, however, gives rise to some uncertainty in peak temperatures. We would have expected the curves for aand p-relaxations shown in the insets to Figures 5 and 6 to converge at high temperature. The curves for cyclohexanone indeed exhibit such behavior. Camphor (Figure 6) does not display this convergence for reasons that are not entirely clear; divergent log (10) K. J. Rao, Bull. Mater. Sei., 1, 181 (1979). (11) J. Wong and C. A. Angell, “Glass: Structure by Spectroscopy”, Marcel Dekker, New York, 1976.
1
I
I
100
200
300
Tp[-To
(K)
Figure 7. Plot of AS,, against Tpl- To for some glassy crystals. The numbers refer to the compounds listed in Table I.
upvs. 1 / T plots for the a- and p-relaxations have also been seen
in glassy crystalline dimethylb~tane.~ However, even in the plastic crystalline phase, camphor does show a rather anomalous behavior.12 From Figures 1-3 we see that supercooled plastic crystals generally crystallize at some temperature beyond the a-relaxation, the crystal so formed later transforming to the plastic crystal. This differs from the report of Adachi et al.,5 who could maintain supercooled plastic crystals of cyclohexanol up to the melting point. Examination of Figures 4-6 shows that tan 6 decreases abruptly just beyond the a-relaxation, again perhaps due to the crystal(12) J. G. Aston in “Physics and Chemistry of the Organic Solid State”, B. Fox, M. Labes, and A. Weisgberger, Eds., Interscience, New York, 1963.
52 The Journal of Physical Chemistry, Vol. 88, No. 1, 1984
Parthasarathy et al.
lllG
-7.5
$ 0 d
-8.5
I
I
I
5.2
6.2
7.2
1031~( K - l j
Figure 9. Variation of the ESR correlation times, T ~ against , temperature for cyclohexanol. Figure 8. ESR spectra of Tempone in cyclohexanol at the temperatures shown in Kelvin.
lization of the supercooled plastic crystal. The results discussed above as also the results of Adachi et aL5 suggest that @-relaxationsare quite generally found in the glassy crystalline state. Consequently, it seems reasonable to ascribe these relaxations to the nature of amorphous packing rather than to the activation of specific intramolecular modes. Such a description is considered to be valid for glasses by Haddad and Goldstein13 and we surmise that it may be equally applicable in the context of glassy crystals. Attempts have been made5 to understand the dielectric behavior of glassy crystals in terms of various models. We have attempted to interpret our results using the cluster model of the glass transitions since this model yields an insight into the nature of packing in glassy crystals as well. According to the cluster model,* glassy crystals may be regarded as an ensemble of “clusters” dispersed in intercluster “tissue material”. X-ray scattering experiments have indeed revealed the existence of strong short-range orientational correlations in glassy crystah3 Orientational correlations would be much stronger in the clusters. Such rotation may enable molecules to explore lower energy configurations, tending ultimately to the crystallization of the supercooled plastic crystal into the nonrotor crystal. It has been suggested recently that the VTF equation (eq 1) describes the growth of the correlation length,14with Tobeing that temperature at which the correlation length is large enough to suppress relaxations on the longest experimental time scales. This offers an alternative interpretation of Toto that of the configurational entropy model, where To represents the temperature at which the configurational entropy van is he^.'^ In terms of the cluster model, Towould represent that temperature at which the clusters have achieved their maximum (self-limiting) dimensions. The values of Toobtained by fitting the a-relaxation parameters to eq 1 are 120, 145, and 140 K for cyclohexanol, cyclohexanone, and camphor, respectively. In the case of cyclohexanol, the value of Toobtained by us is in fair agreement with those obtained from earlier dielectric5 and calorimetric4 studies. In view of the rather limited frequency range accessible to us and also the rather broad peaks in the high-frequency tan 6 - T plots, we shall not attach undue importance to the value of To. In any event, using data from Table I, we find that ASplvaries quite linearly with the plastic range (Tpl- To) (Figure 7). This relationship has been demonstrated by Ganguly et a1.16 in the case of plastic crystals, and it is interesting that it holds for the supercooled state as well.
E S R Spin-Probe Studies. Figure 8 shows the spectra of Tempone in cyclohexanol at selected temperatures. It is evident that g-factor anisotropy is small and that A,, is the largest component of the hyperfine splitting tensor. These features were also observed earlier in organic g l a ~ s e s .Using ~ the method due to McConnell,17 one can calculate correlation (tumbling) times, T,; a plot of log T , vs. T1is shown in Figure 9 for cyclohexanol. At the lowest temperatures, 7, is nearly temperature independent. As the glassy crystal is warmed, T , decreases at a temperature Tk(-160 K) which is greater than the Tg) of cyclohexano15(148 K) before increasing again. We can understand this behavior in terms of the cluster model as follows. It appears that clusters melt completely at Tkgiving rise to the abrupt decrease in T , with further increase in temperature. Thus, Tk marks the temperature at which rotational motion commences, in close analogy with gla~ses.~~’* It seems likely that the glasslike transition at T l is essentially due to the “congelation” of clusters formed in the supercooled plastic crystal at Tk.19 The increase in 7, seen at temperatures well above Tkcould be due to the transformation of the plastic crystal to the nonrotor crystalline phase. Our dielectric measurements indicate that (Iand a-relaxations are quite generally found in orientationally disordered materials. The @-relaxationoccurs below Tg) while the a-relaxation occurs above this temperature at the frequencies used in this study. The a-relaxation parameters follow the Vogel-Tammann-Fulcher relationship, which may imply that short-range orientational correlations increase as the supercooled plastic crystal transforms to the glassy crystal. The ESR correlation time (of the spin probe) shows a marked decrease at Tk( > T i ) as the glassy crystal is warmed. These phenomena are substantially similar to those observed in glasses and lend credence to the possibility that orientationally ordered clusters may exist in positionally ordered materials. Consequently, it seems likely that the cluster model of the glass transition can be extended to glassy crystals. &Relaxations, in this model, could correspond to motion in the tissue material while a-relaxations could arise from molecular rotation in the clusters. To in the Vogel-Tamman-Fulcher equation would represent that temperature at which the self-limiting growth of clusters is complete. Furthermore, Tk,below which the correlation times are nearly temperature independent, seems to mark the temperature at which cluster growth commences in the supercooled plastic crystal (or where the rotational motion is arrested). Acknowledgment. We are grateful to Dr. V. Dhar of the Department of Physics for assistance in recording the ESR spectra. Registry No. Cyclohexanol, 108-93-0; cyclohexanone, 108-94-1; camphor, 76-22-2; Tempone, 2896-70-0.
(13) J. Haddad and M. Goldstein, J . Non-Cryst. Solids, 30, 1 (1978). (14) M. Shablakh, R. M. Hill, and L. A. Dissado, J. Chem. SOC.,Faraday Trans. 2, 78, 625 (1982). (15) C . A. Angel1 and D. L. Smith, J . Phys. Chem., 86, 3845 (1982). (16) S. Ganguly, J. R. Fernandes, and C: N. R. Rao, Adu. Mol. Relaxation Interact. Processes, 20, 149 (1981).
(17) T. J. Stone, T. Buckman, F. L. Nordio, and H. M. McConnell, Proc. Narl. Acad. Sci. U.S.A.,64, 1010 (1965). (18) J. I. Spielberg and E. Gelerinter, J . Chem. Phys., 77, 2159 (1982). (19) M . Hoare and J. R. Barker in “The Structure of Non-Crystalline Materials”, P. H . Gaskell, Ed., Taylor and Francis, London, 1976.
