6240
J. Phys. Chem. 1993,97, 62404248
Molecular Design for Nonpolymieric Organic Dye Glasses with Thermal Stability: Relations between Thermodynamic Parameters and Amorphous Properties Katsuyuki Naito' and Purim Miura Advanced Research Laboratory, Research and Development Center, Toshiba Corp., 1 Komukai- Toshiba-cho, Saiwai-kv, Kawasaki, 21 0, Japan Received: February 12, 1993 The molecular structures of low-molecular-weight organic compounds and their amorphous properties have been investigated to obtain a design rule for uniform amorphous films with high thermal stability. The glass transition temperature (Y',/K), maximum crystal-growth velocity (MCV/m s-I), and maximum crystal-growth temperature (T,,,/K) are key parameters for characterizing the amorphous properties of organic materials. Some quantitative relaticins between these parameters and thermodynamic parameters were examined from both theoretical and experimental viewpoints. The equation for Tgof various aromatic compounds expressed as T, = a - bCASe,m/N has been found to be nearly established, where is the sum of the entropies of fusion and of phase trrtnsitions between Tg and the melting point (Tm/K), N is the number of heavy atoms per molecule except hydrogen atoms, and a and b are constants. The relation can be successfully explained by using the Adam-Gibss theory on the viscosity of supercooled liquids. It has also been found that MCVfor aromatic compounds nearly followed the equation log(MCV) = c- d N / ( TmCAHw,,), where c and dare constants and CAH,,,,,, is the sum of the enthalpies of fusion and of phase transitions between Tc,, and Tm. This can be explained by a potential barrier model for molecular diffusion both at a crystal/supercooled liquid interface and in a bulk supercooled liquid. Consequently, molecules preferably used for amorphous films should have a symmetric globular structure with a large molecular weight and small intermolecular cohesion. According to these findings, it has been revealed that high Tg and Tc,maxand low MCVyield stable organic glasses with high thermal stability.
Introduction Organic molecules have attracted much attention in relation to molecular electronicsdue to thevarieties of optical and electrical properties.' Ultrathin films of organic molecules formed by the Langmuir-Blodgett methodZand the molecular beam epitaxy method,3 etc., have recently been investigated. Optical and/or electric devices using organic thin films controlled by solid electrodes require film homogeneity to avoid undesired electric short circuits, although the device characteristics are substantially affected by various types of carrier traps within the films. Monocrystalline ultrathin films without any ritructural imperfections are thus considered to be ideal. Howewer, it is difficult to form monocrystalline thin films of complex organic functional molecules with thicknesses less than 100 nm. Moreover, polycrystalline films prepared by conventional methods always have grain boundaries of various sizes, which are causes of serious structural defectsand deepcarrier traps. An alternative approach to form uniform organic films is amorphous (glass) thin films. This is because the difficulties in fabricating thin films of 10 nm or less or in controlling localized molecular aggregations over the whole surface of the films have been overcome. Organic electroluminescent(EL) devices reported by Tang et ala4and by other researcherssseemto be good examplesof devices in which noncrystallized amorphous dye films; play important roles. The devices were composed by layered 'dye films of 1050-nm thicknesses sandwiched between electrodes. Electroluminescence is observed when charged carriers injected from the electrodes are combined within a light-emitting dye layer. The carrier injection, transport, and recombination properties are markedly dependent on the film qualities. Therefore, film uniformity and homogeneity are required for highly efficient EL performance. The dye layers have usually been constructed by conventional physical vapor deposition, and therefore amorphous dye compounds used in the devices should have: glass transition temperatures (T6) at least higher than room temperature. However, their thermal stability is still insufficient. As a result, the layered dye films are frequently damaged during evaporation
of the metal electrodes and also during driving of the devices at high electric current densities. Similar situations can also be found for other layered organic films, such as photovoltaic cells and organic photoconductors (OX)used in xerography. Therefore, amorphous organic materials with a high thermal stability have recently become of vital importance from the application viewpoint, Many studies have been carried out on glasses that include organic materials.6.' However, only limited classes of nonpolymeric amorphous dye molecules with T, higher than room temperature, such as 1,3,5-trinaphthylbenzene( T , = 342 K),8 are known. Most of the organic dye amorphous compounds reported previously have a rather low T,, and efforts to synthesize new materials with an improved thermal stability have not been extensive. Most recently, Shirota et al. have reported several 'star-burst" molecules with a high T,, yielding homogeneous glasses.9.z9.3z.33However, the relations between their molecular structures and amorphous properties have not yet been well clarified. The stability of amorphous phases is usually elucidated by the crystal-growthvelocity (CV),which is a kinetic parameter related to the transition from noncrystalline (amorphous) to crystalline phases. The crystal-growthvelocity is known to be a bell-shaped function of temperature, having a maximum (MCV) at Tc,- a little lower than the melting point (T,).lo High Tc,- and low MCV are thus required for stable amorphous states of organic materials in addition to high T,. Accordingly, the amorphous parameters of dye molecules with a higher thermal stability are characterized not only by thermodynamic parameters but also by kinetic parameters. In this paper, the relations between the amorphousparameters and thermodynamicparameters concerned with phase transitions are described. They have been investigated both theoretically and experimentally. Molecular structural factors have been interrelated with the obtainedthermodynamicvariables. Optimal structures of amorphous molecular materials with high T, and Tc,- and with low MCV are also discussed. Q 1993 American Chemical Society
Molecular Design for Nonpolymeric Organic Dye Glasses
-
X.
Aaq3
The Journal of Physical Chemistry, Vol. 97, No. 23, 1993 6241
OXD-so
U DIAMINE nPAE
OXD-S1 OXD-SP OXD-S3
CH:
BNlBPC TDP
0XD-M OXD-S5 OXPSB OXD-S7
CH3-
I
X1-
I MTDATA
OXD-sa
OXD-3 C s H s O G
oxnse
OXW
d_gCaH6 OXD-5 -CH=CH-
CH3-
Figure 1. Molecular structurw of high-Ts aromatic compounds.
Experimental Section Materials. Figure 1 shows the high-T, aromatic compounds used in this study, most of which have been applied to organic EL devices45 and/or OPC. OXD-S compounds (OXD-SO -49) (Figure 1) werenewly synthesized by the authors. Their synthetic methods and analytical data have been reported elsewhere. Diamine was kindly supplied by Prof. Kyo Abe (Osaka City University).'' TPD was purchased from Sogo Pharmaceutical Co. Ltd. (Japan). These compounds were purified by recrystallization and/or by sublimation. Reagent grades of chalcone, phenyl saliciate, and piperonal were purchased from Tokyo Kasei (Japan) and were used without any purification. Thermal Analysis. Thermal properties were determined by differential scanning calorimetry (DSC) using a Du Pont 990 thermal analyzer. Samples of 5-20 mg in polycrystalline form were put in aluminum pans and heated at a scan rate of 5 deg/ min under a nitrogen flow of 130 mL/min. A typical DSC chart is shown in Figure 2. The temperatures of phase transitions, including the melting (Tm)and crystal-crystal transitions (Tw), and their enthalpy changes were measured. Indium metal was used as a standard. After melting, the samples were then cooled at the same rate to observe the crystallization points ( T 4 . The compounds which underwent crystallization in the process were cooled rapidly to form glasses by inserting them in their fused states into liquid nitrogen or by putting them on a cooled aluminum block. The resulting glasses were heated again under the same condition to measure Tg and Tcl.
General View of Amorpboua Properties Tgand Thermodynamic Parameters. Regarding glass transition, a widely accepted concept is that transition takes place when the viscosity (TJ)of the relevant supercooled liquid reaches
Figure 2. Schematic DSC chart for aromaticcompound: (a) crystal, (b) glass.
1013 P.12 According to Adam-Gibbs theory,') the viscosity of a supercooled liquid is given by eq 1 in terms of the molar excess entropy TJ = A exp(B/TS,) (1) where S, is the entropy difference between the amorphous and crystalline phases (and is a function of temperature T') and A is a frequency factor and should be a constant ( 10-3-lV P) for the
6242 The Journal of Physical Chemistry, Vol. 97, No. 23, 1993
Naito and Miura
TABLE I: Relation between Tsand Transition-Fusion Entropies for Aromatic Compounds compound
Tm (K) 685 692 570 540 595 610 56 1 617 446 589 519 621 570 579 609 598
Alq3 DNIBPC OXD-S2 TCTA OXD-S3 OXD-SI TPTTA OXD-S4 OXD-S 1 OXD-S7 OXD-S9 OXD-6 TTF'AE OXD-S6 OXD-SO OXD-8 TDATA c-3 DIAMINE OXD-7 MTDATA D- 1 c-1 D-3 c-2
1,3,5-trinaphthylbenzene H-4 H-5 TPD H- 3 p-MTDAB PPCP DPH m-MTDAB D-4 D-5 o-terphenyl naphthyl saliciate 2-phenylethyl N-phenylcarbamate benzyl N-phenylcarbamate methyl N-phenylcarbamate phenyl saliciate ethyl N-phenylcarbamate propyl N-phenylcarbamate dimethyl phthalate diethyl phthalate benzyl alcohol tert-but ylbenzene bromobenzene n-hexylbenzene sec-butylbenzene n-pentylbenzene isopropylbenzene n- butylbenzene n-propylbenzene toluene ethylbenzene
473 456 507 476 507 427 529 43 1 472 427 477 440 43 1 483 527 435 456 465 397 329 368 360 354 325 314 326 331 274 270 258 215 243 205 198 194 177 185 174 178 178
Tg (K) 448 443 442 424 420 418 414 413 412 41 1 405 402 384 381 372 369 362 355 351 350 348 346 345 344 344 342 337 334 333 333 331 330 323 322 317 308 245 244 235 233 218 217 213 203 192 180 168 142 140 137 128 128 125 124 122 113 111
NmIN
ZWrIN
ref0
N
(J/W mol))
(J/W mol))
tw tw tw 28 tw tw 29 tw tw tw tw tw tw tw tw tw 30 tw tw tw 30 31 tw 31 tw 8 31 31 tw 31 32 tw 33 32 31 31 15 34 14 14 14 34 14 14 14 14 35 36 37 36 36 36 36 36 36 36 36
34 43 48 45 57 51 61 45 51 51 51 40 78 45 36 34 58 50 48 36 61 38 46 40 52 36 31 31 40 29 48 35 34 48 40 41 18 20 18 17 11 16 12 13 14 16 8 10 7 12 10 11 9 10 9 7 8
1.18 1.46 1.06
0 0 0
tw tw tw
0.95 1.77 0.96 1.69 0.61 1.61 1.23 1.73 0.60 1.62 0.12 1.94
0 0 0 0 b 0 0 0 0.53c 0 1.92d 0
tw tw 29 tw tw tw tw tw tw tw tw tw
1.54 2.00 1.69 1.38
0 0 0 0
tw tw tw 30
1.85
0
tw
1.62 2.50
0 0
tw 8
1.97
0
tw
2.48
0
tw
2.14
0
32
2.89
0
15
5.45 4.47 4.00 3.94 4.08 4.85 4.43 4.18 4.38
n.oeC n.0. n.0. n.0. n.0. n.0. n.0. 0
14 14 14 tw 14 14 14 14 37
6.29
n.0.
38
4.87 6.10 5.45 5.29 6.38
0 0 0 0 0
37 37 37 37 31
n.0.
ref
0 tw: thermal propertics were measured in this work. Polymorphorism was observed. The sample recrystalized from the solution melted at Tm (446 K). Crystallization was observed at 448 K to form another crystalline state, which showed a phase transition at 522 K and melted at 541 K to yield an isotropic liquid: Tc = 488 K, Tu = 522 K, 0.17 J/(K mol), T d = 541 K, 0.027 J/(K mol). Tu = 551 K. Tu = 591 K. e Not observed.
same compound family.14.15 B is a material constant.I6 Since the quantity BIT& at a given T, is almost constant, T,,can be expressed as follows:
Tg = B/CS,(T,)
(2)
where c is a constant. The material constant ( B ) is related to internal motions Of Segments and is Usually defined as in eq 3: B = E$*/k, (3) whereS* is a constantcorrespondingto the possible configurations of the liquid states, Eo is a potential barrier for the segment to be rearranged intoanother d i g u r a t i o n , and &e is the Boltzmann constant. By using S* and S,, the quantity Z* = S*/S, can be
defined, indicating the smallest number of motional segments per molecule. For polymer chains, Z* indicates the number of main-chain bonds permitting internal r ~ t a t i o n . 'In ~ the cases of nonpolymeric compounds, Z* corresponds to the number of "heavy" atoms (N)per molecule except for hydrogen atoms." Eo may be almost the Same for aromatic compound analogs, and therefore B can be expressed as B'N, where B' (=EGc/kB)is a constant. Consequently, T,, can be expressed by Tg = ( B ' / C 3 ~ / S , ( T g )
(4)
Small S,(T,)/N values are necessary for high TB. Smaller CA&,,/N values, the sum of the entropy of fusion (AS,) and
The Journal of Physical Chemistry, Vol. 97,No. 23, 1993 6243
Molecular Design for Nonpolymeric Organic Dye Glasses
TABLE II: Relation between Tgand Transition-Fusion Entropies for Aliphatic Compounds compound DL-lactic acid terf-butyl alcohol
propylene carbonate cyclohexanol cycloheptanol fer?-butylchloride n- hexanol vinyl acetate
n-pentanol n-but ylcyclohexane n-butanol methanol sec-butyl alcohol ethylcyclohexane 3-methylheptane ethanol dichloromethane n-propanol cyclohexene isobutyl bromide methylcyclohexane cycloheptane 3-methylhexane 2-methylpentane chloroform 2,3-dimethylbutane isopentane 1-butene carbon tetrachloride a
ZASt,IN
Tm (K) 299
T, (K) 204
ref
N
(JIWmol))
22
6.50
n.o
ref 14
298 218 173 280
180 160 150 140 130 125 125 120 119 119 103 100 98 98 96 93 93 92 85 85 83 80 80 79 76 65 61 60
39 40
6 5
4.60
7 7 8 5
6.28 0.86
n.o n.0. 4.43’
49 50 37
9.00 9.7 1
0 0
37 37
8.33 7.00 10.20 9.30
0 0 0 1.706
31 37 37 37
6.44 9.50 10.70 11.40 8.00 3.27
0 0 0 0 5.07c
37 37 37 51 37 38
6.57 1.00
0 5.29d
37 37
8.83 11.00
0 0 7.86‘ 0 0
37 37 51 51 37 37
248 221 180 195 198 183 175 158 162 150 161 176 146 170 156 147 265 154
120 210 145 113 88 250
41 42 37 43 5 43 36 43 44 45 36 31 36 37 36 58 36 36 58 45 46 37 47 48 37 37
1 6 6 10
5 2 5 8 8 3 3 4 6 5 7 7 7 6 4 6 5 4 5
(JIWmol))
0.97 9.20 11.00 2.60
n.0.
3.4v TU- 264 K. Tt,= 157 K. Tu = 139 K. TUI= 135 K 7.3 J/(K mol), Tt,2 = 198 K 0.3 J/(K mol), Tt,3 = 212 K 0.4 J/(K mol). e Tt,= 136
K. f T U
225 K.
T=T, A
$
primarily by the crystallization at the interface. According to the A-G theory,” the authors introduce the smallest number of segments (N)which must simultaneouslyovercomean individual barrier, E,, for diffusion at the interface (crystallization). In Figure 3, the activation energy (E,) is large at T,, because the free energy change per segment between the crystal and liquid states, AG/N, is almost zero. However, at temperatures below T,, AG/N increases abruptly, and hence E, decreases. AG can be approximated by eq 618
1
*
F E
W
Crystal
Liquid L
Reaction axis Figure3. Schematicexplanationfor crystallizationof supercooled liquids.
the entropies of phase transitions between TBand T, (CaS,) for nonpolymeric aromatic compounds, suggest a smaller configuration number difference between a liquid at T, and a crystal at T,. This indicatessmaller&( TJ/Nvalues,becausethespecific heat differences of segments (AC,/N) between amorphous and crystalline phases are similar for the same family.14 Therefore, T, can be expected to increase with a decrease in the thermodynamic parameter of phase transitions, C&”/N. Crystallization and Thermodynamic Parameters. It is known that crystal growth in a supercooled liquid involves two processes, as shown in Figure 3: one is molecular diffusion in a bulk liquid and the other is crystallization at the crystal/liquid interface.1° The diffusion process is predominant at low temperatures, while the latter prevails at high temperatures. Tc- appears near T,, for aromatic compounds, according to data in previous studies shown in Table IV, and is described by eq 5: (5) wherefisa constant (0.92 f 0.03). Therefore,MCVisdetermined
AG = CAHu,,,AT/Tm (6) where CAH,,, means the sum of the enthalpies of fusion and of phase transitions between Tc,- and Tm. Here, AT is defined as follows: AT = Tm- TC,-. According to Figure 3, E, near TW can roughly be estimated by9 (7)
where A is a constant. From eqs 5-7
where ko is a frequency factor and is a constant for aromatic compounds. Consequently, MCV can be expected to decrease with an increase in the thermodynamic parameter of phase transitions, N/( T,CAH,,,). It should be noted that amorphous parameters such as T,, MCV, and Tc,- are interrelated with the thermodynamic parameters T,, CASu,,, and EM,.,. These relations allow us to elucidate the amorphous molecular materials by experiments using thermal analyses.
Experimental Results
Glass Transition Temperature (Tg).Tables 1-111 indicate the thermal properties of aromatic, aliphatic, and polyhydroxy compounds, respectively. The data for most of the aromatic
Naito and Miura
6244 The Journal of Physical Chemistry, Vol. 97, No. 23, 1993
TABLE III: Relation between Tgpnd Trmsition-Fusion Entropies for Polyhydroxy Compounds comuound T,(K) glucose 414 resolucinol 383 glycerol 292 propane217 1,2-diol ethanediol 260 273 water 0
"IN
Tg I T;
500 -
EUirIN
~~
1 1 12 9 8 11 6 38 5
152 135
55 11
4 1
6.67 6.88 4.80
0.75' 5.70
52 51 37
.300 Y
hm
10.80 22.00
37 37
0 0
347 K.
TU
400 -
T.(KI ref N (J/(Kmol)) (J/(Kmol)) ref 290 250 186 160
*
'5
200 -
100 -
"0
100 200 300 400 500 600 700 800 TmI K
.
Figure6. Relation between glasa transition temperature (TI) and melting
Y
point (Tm)for aromatic compounds (O), for aliphatic compounds (+), and for polyhydroxy compounds (A).
I-!!
0
t 0
1
2
3
4
5
6
Z A St,,, IN / (J / K mol)
Figure 4. Relation between glass transition temperature (TI) and transition-fusion entropies (CA&,/iV) for aromatic compounds. 500
400
5 h!
300
A
2ool
soO +
A
A A
@+
100
o b
A
%$+ $+it+++
+
f
++
TCl
I
I
I
I
I
5
10
15
20
25
Tc,max Tc2 Tm
Temperature
Figure 7. Schematic relation between crystal-growth velocity and temperature.
ZA Str,,/N/ (J / K mol)
Figure 5. Relation between glass transition temperature (TI) and transition-fusion entropies (C&,/iV) for aromatic compounds (0), for aliphatic compounds (+), and for polyhydroxy compounds (A).
compounds with larger N values were measured in this study, while those for the others were taken from pertinent studies in the literature. Among them, Alq3 (see Figure 1) has the highest T,, 448 K,and TTPAE has the largest N, 78. It is seen that the class of ustar-burstn molecules such as OXD-Scompounds with a larger N shows relatively higher T, values. Figure 4 shows the plot of T, (K)and CAStr,,,,/N. An almost linear relation was found, as described by eq 9:
(9) Tg= a - b c A S , J N where a and b are constants and a corresponds to about 500 K when T, is extrapolated toZAStr,,,,!N= 0. The relation indicates that small entropies of phase transitions and/or a large molecular structure (large N ) are necessary for high Tr This is almost in accordance with the relation predicted in the previous section. In Figure 5 , plots of T, and CAS,,,,,/N for aliphatic and polyhydroxy compounds other than aromatic compounds are given. Similar relations (high T, correspondingto small EA&,/ N )wereobserved, while for polyhydroxycompounds,the T, values were higher than those for the other compounds for the same CAStrm/N values.
The relation between Tgand T, for all the compoundsin Tables 1-111 is shown in Figure 6. The well-known rule of thumb has
nearly been confirmed: Tg/Tm = 2/3-19 Crystallization. The relation between the crystal growth velocity of a supercooled liquid and its temperature has been known to give a bell-shaped curve, as shown in Figure 7.10 T,I and Tczare crystallization temperatures observed when heating a glass and when cooling a supercooled liquid, respectively. The crystallization temperatures cannot be uniquely determined since they vary with the observed time.20 On the contrary, MCVand T- are nearly independentof the observed condition. Therefore, parameters closely relating the crystallization properties are more important. The MCV and Tw values for some aromatic compoundshave been reported, as shown in Table IV. The authors found a correlation between the MCV values and the enthalpy of fusion. A closer correlation was found by taking account of T,, as shown in Figure 8, and is expressed by eq 10, as predicted in the previous section:
log(MCV) = c - d N / ( T m c A H t r m )
(10)
where c and d are constants. The compounds in Figure 8 have no phase transitions between T- and T,, and therefore, EMir,,,, equals AHmin these cases. The above relation indicates that larger N/( TmCMtrm)values will give lower MCV. Thus, the
Molecular Design for Nonpolymeric Organic Dye Glasses
The Journal of Physical Chemistry, Vol. 97,No. 23, 1993 6245
TABLE IW Relation between Maximum Crystal-Growth Velocity (MY)*and Transition-Fusion Enthalpies for Aromatic
Compounds
compound Tm (K) Thmu (K) N naphthalene 351 10 pdichlorobenzcnc 326 306 8 pdinitrobenzcne 447 406 12 pdibromobenzenc 359 332 8 dipheny1 343 327 12 1,3,5-trichlorobenzene 9 337 312 o-dichlorobenzene 256 237 8 ambenzene 340 310 14 1,2-diphenylethane 324 294 14 diphenylmethane 298 275 13 bend 368 333 16 0-crtsol 303 273 8 benzophenone 308 280 14 triphenylmethane 366 34 1 19 piperonal 310 290 11 chalcone 328 303 13 phenyl saliciate 315 295 16 o-tcrphenyl 329 314 18 1,l-diphenylethanc 255 228 14 a All MCY and To,- data were taken from refs 52 and 53. EA&.,,,
log(MCV) (m/s)
N/EAHtr,D
N / (Tmzmtr.”)
-0.2 -0.4 -0.6 -0.9 -0.9 -1 .o -1.5 -2.0 -2.0 -2.1 -2.2 -2.4 -3.0 -3.4 -4.0 -4.1 -4.3 -4.7 -4.8
0.52 0.47 0.43 0.39 0.64 0.50 0.62 0.64 0.45 0.68 0.81 0.58 0.77 0.86 0.60 0.68 0.80 0.98 0.78
0.0015 0.0014 0.0010 0.001 1 0.00 19 0.0015 0.0024 0.0019 0.0014 0.0023 0.0022 0.00 19 0.0025 0.0024 0.0019 0.0021 0.0025 0.0030 0.0030
W)
(mow)
ref 37,49 37,49 37,49 37,49 37,49 49 37,49 37,49
50 37,49 49 37,49 49 52
twb tw
tw 15 37
values were measured in this work.
TABLE V Crystallization Data for Aromatic Compounds with High T.
-1
-5
t
~
%.0
t
Figure 8. Relation between maximum crystal-growth velocity (MCY) and product of melting point ( T m ) and transition-fusion enthalpies (CA&,/N) for aromatic compounds.
amorphous phases are stabilized when the materials have large N/(TmIAHu,,) values. MCY and TC,- values have not been measured for high-T, compounds shown in Figure 1. However, the authors were able to estimate their MCV values by measuring AT, (= T,z - T,]). A large MCY is considered to give a larger AT, at the same observed time, provided that the compounds have a similar bellshaped feature. Table V shows the crystallization data for highT, aromatic compounds, and Figure 9 indicates the relation between AT, and N/(T,ZAHv,,). It can be seen again that large N/(T,ZAHvm) values are preferable for small AT,, Le., small MCV. TTPAE has a phase transition at 55 1 K, and T,, of TTPAE is presumed to be about 524 17 K according to eq 5 . This indicates that the enthalpy of the phase transition occurring between Tand T, should be included in eq 10. Among high-T, compounds, OXD-S1 showed the largest N/(TmcAHwm)value and yielded a very stable glassy phase even when its fusion was cooled very slowly. Alq3 with the highest Tg value showed a rather small N/(T,&Wtr,,) value, and therefore, its MCVvalue will be large. Actually, it was difficult to produce an Alq3 bulk glass even by cooling its fusion. TPD easily yields a glassy state, suggesting a small MCV. However, the crystal-growth velocity of Alq3 at room temperature is considered to be smaller than that of TPD, because To- of Alq3 is much higher than that of TPD according to eq 5 . This is the reason why Alq3 amorphous films, which can be prepared by
*
OXD-S1 c-2 TPTTA MTDATA c-1 OXD-S3 OXD-S9 c-3 OXD-S2 TTPAE DIAMINE OXD-7 m-MTDAB TPD OXD-S6 Alq3
1,3,5-trinaphthylbenzene OXD-S4 OXD-SI OXD-8 PPCP DNIBPC OXD-SO OXD-S7 p-MTDAB D- 1 DPH TCTA H-4 D-3 H-3 H-5 D-4 D- 5
‘Te = 551 K.
446 43 1 561 476 427 595 519 473 570 570 456 507 456 579 685 47 2
412 344 414 348 345 420 405 355 442 384 351 350 322 333 381 448 342
617 610 598 527 692 609 589 483 507 435 540 427 529 43 1 477 465 397
413 418 369 330 443 372 411 331 346 323 424 337 344 333 344 317 308
440
n.0. n.0.
470 433 n.0. 492 477 n.0. 500 429 n.0. 397 361 n.0. 416 473 438 426 380 361 467 388 425 361 399
n.0. n.0.
0 0
n.0. 552 n.0. n.0. n.0. 563 n.0. 466
0 60 0
134 0 69
479 602
0 63 129
570 603 561 502 556 571 561
132 177 181 141 89 183 136
n.0.
0.0083 0.0034 0.0033 0.0032 0.0030 0.0030 0.0030 0.0029 0.0029 0.0028’ 0.0024 0.0023 0.0022 0.0020 0.0019 0.0018 0.0018 0.0016 0.0015 0.0015 0.0014 0.0014 0.00146 0.0012
n.0.
506 n.0. 393 n.0.
n.0.
384 n.0.
Tu = 591 K.
physical vapor deposition, are more stable than TPD amorphous films at room temperature.21 Figure 10 shows the relations between the crystallization temperatures (T,1 and T,z) and CA&,/N. Plots of Tot and ZhS,,,/N show a similar relation to those of Ts and CASumm/N but do not for plots of Tc2 and ZAStr,m/N. This is due to the fact that Tgand Tcl are determined by the viscosity of the supercooled liquid.
6246 The Journal of Physical Chemistry, Vol. 97, No. 23, 1993
x
‘9
cy
0
A
d-046-0 0.002 0.004
0
41
0
00‘1 O?
Y
Naito and Miura
I
I
I
ob
I ^
0.006
I
I
I
I
L
5
10
15
20
25
0.008 Z A S,,,,
N I (T,,,CA
H,,,,) I (mol I K kJ)
Figure 9. Relation between difference in crystallization temperatures (ATc)and product of melting point (Tm)and transition-fusion enthalpies
(ZA&,/N)
for high-T, aromatic compounds.
. b”
A
A
AA4
Figure 11. Relation between bulk modulus ( K ) and transition-fusion entropies (bS,,,,,,/N) for aromatic compounds (O), for aliphatic compounds (+), and for polyhydroxy compounds (A).
TABLE VI: Relation between Bulk Modulus ( K ) . at 25 OC and Transition-Fusion Entropies ZA&,/N K x 10‘’
A
A
200 100 I
I
1
1
I
I
2
C A St,,, IN I (J I K mol) Figure 10. Relation betweencrystallizationtemperatures and transitionfor high-T, aromatic compounds: Tcl ( O ) , fusion entropies (Z&,/N)
TCZ(A). Discussion Other Relations for Amorphous Parameters. T, and Melting Point (T,,,). Glass can be formed by cooling a fusion with a velocity faster than M C V or by a nonequilibrium process such as physical vapor deposition. Even crystalline compounds such as benzene and p-xylene, which have large CAS,,,/N and high Tmvalues, will form glasses having low T,. However, these are greatly deviated from the well-known rule of “T,/Tm = */3”. In fact, groups of compounds with T,/Tm = 0.5 or 0.2 have been known.22 The value 213 had been once considered a magic number, but now it is denied.23 Compounds which yield glasses easily are considered to follow this rule. T, and Molecular Dynamics (MD). Recently, many studies have been carried out on glass transition by molecular dynamics s i m ~ l a t i o n .O~n~the ~ ~basis ~ ~ ~of a soft-sphere model, Hiwatari et al.24a have reported that glass transition takes place when the r value, defined as follows, reaches 1.5-1.6:
r = (N’/V)CT~(~,T/C)-’/~
(1 1)
where ”spheres of a diameter u occupy a volume V. e is an energy parameter, and the potential function P(r) is expressed by
P(r) = +/r)’2
(J/W mol))
compound benzene toluene m-xylene chlorobenzene bromobenzene nitrobenzene aniline n-hexane n- heptane methanol acetone n-cctane n-nonane ethyl acetate ethanol n-decane dichloromethane chloroform acetic acid 1,2-dichloroethane tetrachloroethylene water 1,2-ethanediol
A A
300
0
IN I (J I K mol)
a
(Pa) 1.03 1.08 1.15 1.33 1.49 2.00 2.13 0.60 0.71 0.80 0.8 1 0.83 0.85 0.85 0.88 0.91 1.03 1.03 1.06 1.28 1.32 2.17 2.70
5.90 5.3 1 6.44 6.06 6.29 4.60 4.40 12.20 11.00 11.10 8.10 12.00 14.00 9.20 10.60 11.80 8.70 11.00 10.10 9.30 7.00 22.00 10.80
All data were taken from the literature.5’
Y, 300
0 0 00 0 0 0 0
0 00 0
0
go%
I
0.002
I
I
0.004
I
I
0.006
I
I
0.008
I
I
0.01
N I (T, Z A Ht,,,) / (mol I K kJ) Figure 12. Relation between glass transition temperatures (Tg) and product of melting point ( Tm)and transition-fusion enthalpies (ZA&,/ N) for aromatic compounds.
(12)
where r is the distance between the spheres and e is proportional to the bulk modulus ( K ) . According to the model, T, should be proportional to K . If this is the case, we can confirm again the relation observed between T,and CASu,,,,/Nbyexamining theinterrelationbetween K and CASu,m/N. As shown in Figure 1 1 from the data of Table VI, the relation between K a t 25 OC and CAS,,/N is similar to that between T, and ~AS,,,,,,/Nshown in Figure 5. This indicates
that “hard” molecules (large K ) with a small freedom yield glasses with high T,. T, a n d MCV. Figure 12 shows plots of T, versus N / ( TmCfor aromatic compounds. No rigorous relation was found between T, and N / ( TmCAH,,,,,), Le., MCV. Low-T,, compounds such as alkylbenzenes have large N/(TmzAHu,,,,) values due to their low Tm. This is in accordance with the fact that their bulk liquids can be easily frozen into glasses.
Molecular Design for Nonpolymeric Organic Dye Glasses Crystallization Temperature. Molecular diffusion in a bulk supercooled liquid is the predominant factor for crystallization observed when a glass is heated. The viscosity of a supercooled liquid decreases as the temperature rises and reaches a certain value (vel) at which crystallization takes place during a given observed time scale (a given heating rate in DSC measurement). Therefore, the discussion developed for glass transition (vg) is applicableeven in obtaining the relation between Tcl and CAS*,,/ N, as shown in Figure 10. Amorphous Properties and Molecular Structures. Tg and Molecular Structures. As described above, small caSt,,/N values are necessary for high T,, and therefore, both large molecules (N) and relatively small CAS,,, are required. According to the Hirschfelder-Stevenson-Eyring theory,25 the entropy of phase transitions can be expressed as
where ASpisthe entropy contribution due to positionaldisordering of the liquid and equals the gas constant (R) in the case of rigid globular molecules. AS,,,t can be divided into two parts relevant to molecular motions:
S,is the entropy change of molecular rotations in the gas phase and in the crystal phase. S,, is the entropy of torsional oscillations permitted in the crystal lattice, and Sris most often given by eq 15:
S,= R [ d 2 (T/39.6)3/21060(Z~~z)1/2/u] (15) wherellis therotational moment around the iaxisoftheconsidered molecules and u is the symmetry number. Therefore, small Zxy,z and large u are necessary for small CASt,,,. It is thus suggested that highly symmetric, globular, rigid, and dense molecules yield high- Tgglasses. Actually,these structural requirementsare nearly satisfied for the cases of Alq3 and OXD-S compounds. Alq3 has a symmetric and nearly globular structure consistingof only rigid aromatic rings. Furthermore, a heavy aluminum atom exists in the center of the molecule, leading to small rotational moments (Ixys). Also, a large star-burst molecule recently reported by Shirota et aL26has a globular structure with Tg higher than 200 OC. It has a very large N value ('1 18) and probably a small cAS,,,/N value. Crystallization Properties and Molecular Structures. Large N/( T,cAH,,,) values are preferable for small MCV, Le., for easy amorphous formation and for amorphous stability. CAHt,, and T, values are affected by the crystal structures and are difficult to quantitatively estimate compared with entropy changes.2' An effective way to reduce enthalpy changes is to introduce unsymmetrical structures into the molecular skeletons. This is because molecular packing in the crystal is somewhat disordered, like toluene to benzene or m-xylene to p-xylene. However, the method often causes the entropy changes to increase, resulting in low TBand Tc,mx. It is therefore necessary to reduce CAH, (molecular cohesion) without increasing CAS,,,. For this purpose, (1) symmetric globular structures for canceling intramolecular dipoles and (2) rigid and bulky substituents like tert-butyl groups are preferable. Network Molecules. Polyhydroxy molecules like water give higher Tgvalues than aromatic and aliphatic compounds for the same CAS,,,/Nvalues. The authors found that flexible polymers also show a similar effect. This may be caused by increasing the effective "N" values due to molecular network formation via hydrogen bonds. Further studies are necessary to confirm the network effect in aromatic dye compounds. Conclusion Heat-resistive and stable nonpolymeric organic dye glasses can be formed from large, symmetric, globular, rigid, and dense
The Journal of Physical Chemistry, Vol. 97,No. 23, 1993 6247 molecules with weak intermolecular cohesion. Some close relations between their amorphous properties ( T MCV, and Tc,-) and thermodynamic parameters (Tm, fAStrm, and ZAH,,,) have been established and rationalized theoretically. The possibility of network molecules as candidates for having improved amorphous natures has also been suggested. Acknowledgment. We thankDr. S.Egusaand Mr. M. Sugiuchi (Toshiba R&D Center) for supplying DNIBPC, Alq3, C-1, C-2, and C-3. Helpful discussions with Messrs. M. Azuma, T. Nakayama, Y.Watanabe, K. Yamamoto, and T. Sasaki are also appreciated. We are also indebted to Mr. F. Umibe (consultant) for reviewing the original manuscript and suggesting revisions to its English. References and Notes (1) For example: (a) Ashwell, G. J., Ed. Molecular Electronics;Research Studies, 1992. (b) Rambidi, N. G.; Chemavskii, D. S.;Sandler, Yu, M. J. Mol. Electron. 1991, 5, 105. (c) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.;Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Lungmuir 1987,3,932. (d) Girling, I. R.; Kolinsky, P. V.; Cade, N. A,; Earls, J. D.; Peterson, I. R. Opt. Commun. 1985,55, 289. (e) Blinov, L. M.; Dubinin, N. V.; Mikhnev, L. V.; Yudin, S . G. Thin Solid Films 1984, 120, 161. (f) Roberts, G. G. Contemp. Phys. 1984,25, 109. (g) Carter, F. L., Ed. Molecular Electronic Devices; Dekker: New York, 1983. (2) For example: (a) Barraud, A.; Palacin, S.,Ed. Lungmuir-Blodgert Films 5: Proceedings of 5th International Conferenceon Langmuir-Blodgett Films; Elsevier: Amsterdam, 1991. (b) Roberts, G., Ed. Lungmuir-Blodgett Films; Plenum: New York, 1990. (c) Adams, N. K. The Physics and Chemistry of Surfaces; Dover: New York, 1968. (d) Naito, K.; Miura, A.; Azuma, M. J. Am. Chem. Soc. 1991, 113,6386. (3) For example: (a) Molecular Electronics and Bioelectronics 1992,3, 228-282 (Japanese). (b) M6bus, M.; Karl, N. Thin Solid Films 1992,215, 213. (c) Zimmermann, U.; Karl, N. Surf. Sci. 1992,268,296. (d) Haskal, E. I.; So, F. F.; Burrows, P. E.; Forrest, S.R. Appl. Phys. Lett. 1992,60,3223. (4) (a) Tang, C. W.; VanSlyke, S . A. Appl. Phys. Len. 1987,51, 913. (b) Tang, C. W.; VanSlyke, S.A.; Chen, C. H. J. Appl. Phys. 1989,65,3610. (5) For example: (a) Adachi, C.; Tsutsui, T.; Saito, D. Appl. Phys. Lett. 1989, 55, 1489. (b) Hamada, Y.; Adachi, C.; Tsutsui, T.; Saito, S . Jpn. J. Appl. Phys. 1992,31,1812. (c) Egusa,S.;Gemma,N.;Miura,A.;Mimhima, K.; Azuma, M. J. Appl. Phys. 1992,71,2042. (d) Masui, H.; Takeuchi, M. Jpn. J. Appl. Phys. 1991, 30, L864. (e) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Bums, P. L.; Holmes, A. B. Nature 1990, 347, 539. (6) For example (review): (a) Ngai, K. L.; Wright, G. B. Ed. Proceedings of the International Discussion Meeting on Relaxations in Complex Systems. J. Non-Cryst. Solids 1990,131-133. (b) Williams, G. J. Non-Crysr. Solids 1991, 131-133, 1 . (c) Schefer, G. W. J. Non-Cryst. Solids 1990, 123, 75. (d) Yonezawa, F. Solid State Phys. 1991,45, 179. (e) Bondi, A. Physical Properties of Molecular Crystals, Liquids, and Glasses; John Wiley: New York, 1968; pp 370449. (7) For example: (a) Mohanty, U. J. Chem. Phys. 1990,93,8399. (b) Chakrabarti,A.; Yashonath,S.;Rao, C.N. R. J. Phys. Chem. 1992,96,6762. (c) Sethna, J. P.; Shore, J. D.; Huang, M. Phys. Rev. B 1991,44, 4943. (d) Cohen, M. H.; Grest, G. S.Phys. Rev. B 1979,20, 1077. (e) Brawer, S. A. J . Chem. Phys. 1984,81, 954. (f) Sanchez, I. C. J. Appl. Phys. 1974,45, 4204. (g) Zhang, J.; Jonas, J. J. Phys. Chem. 1992,96,3478. (h) Stillinger, F. H. J. Chem. Phys. 1988,89, 6461. (8) Majill, J. H. J. Chem. Phys. 1967, 47, 2802. (9) Shirota, Y.; Kobata, T.; Noma, N. Chem. Lett. 1989, 1145. (10) (a) van Hook, A. Crystallization; Reinhold New York, 1961;p 138. (b) Volmer, M.; Marder, M. Z. 2.Phys. Chem. 1931, 154A, 97. (1 1) A h , K.; Takahashi, M.; Kunugi, A. Chem. Express 1990,5, 385. (12) Bondi, A. Physical Properties of Molecular Crystals, Liquids, and Glasses; John Wiley: New York, 1968; p 372. (13) Adam, G.; Gibbs, J. H. J . Chem. Phys. 1965,43, 139. (14) Privalko, V. P. J. Phys. Chem. 1980, 84, 3307. (15) Greet, R. J.; Tumbull, D J. Chem. Phys. 1967, 47, 2185. (16) Miller, A. A. Macromolecules 1978, 11, 859. (17) Wubderlich, B. J . Phys. Chem. 1960, 64, 1052. 118) Rawson. H. Inorganic Glass-formingSystems,. 33:. Academic Press: New York, 1967. (19) (a) Kauzmann, W. Chem. Rev. 1948,43,219. (b) Beaman, R. G. J. Polvm. Sci. 1952.9.470. (c) . . . , Sakka, S.:Mackenzie, J. D. 1. Non-Crysr. I
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