Phosphorescence Studies of 1-Indanone and Acetophenone Traps
The Journal of Physical Chemistry, Vol. 82,No. 4, 1978 453
Zero Field Optically Detected Magnetic Resonance and Phosphorescence Studies of the Lowest Excited Triplet States of 1-Indanone and Acetophenone Traps at Low Temperature Shigeya Niizumat and Noboru Hirota" Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11793 and Department of Chemistry, Faculty of Science, Kyoto University, Kyoto, 606, Japan (Received August 8, 1977)
We have investigated the phosphorescence spectra, zero field splittings (zfs), and decay rates from the spin sublevels of the TI states of various 1-indanone traps at 1.2-4.2 K using the zero field ODMR technique and compared them with those of acetophenone. The properties of %7r* 1-indanone vary remarkably depending * traps were found to be significantly on the environment. The phosphorescence spectra of 3 ~ a1-indanone different from those of %a* acetophenone traps. The difference was attributed to the rigid planar structures of %a* 1-indanone. The D values and decay rate constants of the z sublevels of 3a7r*1-indanone traps were found to be much smaller than those of 3aa*benzaldehydes with similar values for the energy separations between the 3n7r*and 3aa*states. The possible causes of the differences are discussed. The 1-indanone crystal doped with 1,2-methylenedioxybenzeneshows three different types of phosphorescence. It was found that the intensities of these emissions are strongly temperature dependent. A model to explain the temperature dependence is presented and it is shown that the temperature dependence is due to intermolecular energy transfer among different traps. A brief comment on the nature of the dual phosphorescing species is given based on the results obtained. 1. Introduction Since the discovery of dual phosphorescence of 1indanone in rigid matrices by Yang and Murov, the phenomenon of dual emission as well as the properties of the lowest triplet (T1)state of 1-indanone have attracted considerable attention.'-I5 Although several explanations have been proposed concerning the causes of the dual emission, uncertainties still remain as to the exact nature of the phosphorescing species and the cause of the dual emission of 1-indanone. We thought that low temperature zero field ODMR (optically detected magnetic resonance) spectroscopy might be helpful in clarifying the detailed nature of the phosphorescing species and in answering some of the questions concerning the dual emission. Spectroscopic and magnetic properties of 1-indanone have been investigated by several workers previously.13,16,1' The TI state of 1-indanone is considered predominantly 3n7r*in character in a durene host,16but the work by Case and Kearns shows that it is a 37r7r* state in pure 1-indanone crysta1.l' Their So T1 absorption spectra, however, indicate that the radiative property of 1-indanone is considerably different from that of acetophenone. Comparison of the radiative properties of %a* 1-indanone and acetophenone may be useful in obtaining information about the relationship between the structures and dynamic properties in the 3 7 r ~ *aromatic carbonyls. Accordingly, we have investigated the properties of the TI states of various traps of 1-indanone and acetophenone crystals. We found that the properties of the T1 states of 1-indanone traps differ tremendously depending on the trap. We describe the spectroscopic, dynamic, and magnetic properties of both 3 n ~ and * 37rrp* type 1-indanone traps and discuss them in comparison with those of the acetophenone trap. We also found that the I-indanone crystal doped with 1,2-methylenedioxybenzeneexhibits a remarkable temperature dependence of the phosphorescence spectrum. We present a model to explain this observation. Finally, we briefly comment on the nature
-
Present address, Department of Chemistry, Faculty of Science, Tohoku University, Sendai, Japan. *Address correspondence to this author at Kyoto University. 0022-3654/78/2082-0453$01.00/0
of the phosphorescing species in rigid matrices based on our results. 2. Experimental Section 2.1. Samples and Sample Preparations. The follqwing systems are studied in this work (1)1-indanone in durene, (2) neat 1-indanone crystal, (3) 1-indanone crystal doped with 1,2-methylenedioxybenzene,(4) 1-indanone crystal doped with durene, (5) acetophenone in 1,4-dimethoxybenzene (DMOB) (6) acetophenone doped with anisole. 1-Indanone was purified by vacuum sublimation followed by extensive zone refining. Durene and DMOB were recrystallized twice from acetone and then zone refined extensively. Acetophenone was purified by repeated fractional distillations. 1,2-Methylenedioxybenzene was purified by vacuum sublimation. All crystals were melt grown using the standard Bridgman method. The guest concentration of the initial melt was 0.5-1 wt % in samples 1,4, 5, and 6 and 0.6 wt % in 3. The molecular structures and the axis systems used here are shown in Figure 1. 2.2. Experimental Methods and Procedures. (A) Phosphorescence Spectra. The phosphorescence spectra of all systems were taken with our ODMR apparatus at 4.2 and 1.2 K. The details of the experimental setups were given previously.ls The temperature dependence of the phosphorescence spectrum of the 1-indanone crystal doped with 1,2-methylenedioxybenzene was studied in the temperature range between 1.8 and 4.2 K. The desired temperature was obtained by carefully controlling the pumping speed of the helium gas. The temperature of the liquid helium was determined by measuring the vapor pressure of the helium. (B) Zero Field ODMR Experiments. Three types of ODMR experiments were performed in the present work. They are (1) fast passage ODMR experiments under constant light ilIumination,18-20 (2) microwave induced delayed phosphorescence (MIDP) experiment^:^-^^ and (3) microwave modulated phosphorescence ~ p e c t r o s c o p y . ~ ~ ~ ~ ~ The ODMR setup used in this work is essentially the same as that described in the previous paper.18 The zero field splittings (zfs) were determined from the microwave frequencies corresponding to the zero field
0 1978 American Chemical Society
454
S.Niizuma and N. Hirota
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
(a )
I-lndanone in durene
Ai
400
420
440
380 nrn
INDANONE Xd TRAP
(b)
QL3i,
(2)
x
i1 Acetophenone
az 1: 77soc'
x
Trap
01170 crn-'
0,1241 crn-'
ACETOPHENONE
x
I-Indanone X i Trap
TRAP
Figure 1. Molecular structures, fine structure axis system, and the zero field sublevel schemes of %7r* 1-indanone and acetophenone.
transitions. They were usually obtained by the fast passage ODMR method in the way described in the literature.18-20 The total decay rates (k,)and the relative radiative decay rates (k,T)of the individual spin sublevels were determined by using the standard MIDP method first developed by Schmidt and van der Our method of data acquisition and analysis are similar to those described in the l i t e r a t ~ r e . l ~ * ~ ~ , ~ ~ , ~ ~ When we study the phosphorescence spectra of traps, it often happens that the ordinary phosphorescence spectrum consists of the superposition of the emissions coming from different traps. In such a case we can easily separate out the emissions coming from different traps by taking the changes in the phosphorescence emissions caused by the specific microwave transition^.^^ Here, we have taken two different types of microwave modulated phosphorescence spectra for this purpose. (1) Fast passage phosphorescence ~ p e c t r a . ~The ~,~~ microwave power was swept through the resonance frequencies between i and j sublevels repeatedly and the chainges produced in phosphorescence was detected with a lock-in amplifier, while the monochromator was scanned. The intensity of the detected signal is proportional to k,' - kj'. (2) Microwave induced delayed phosphorescence (MIDP) spectra. The microwave induced delayed phosphorescence (MIDP) signals were repeatedly generated and the produced MIDP signals were detected with a lock-in amplifier, while the monochromator was scanned. Our experimental procedure is similar to that described in the l i t e r a t ~ r e The . ~ ~ spectrum ~~~ obtained also gives kf - h,' as a function of the emission wavelength.
,
!
410
400
390
380 nm
Flgure 2. Phosphorescence spectra of 1-indanone and acetophenone:
(a) 1-indanone in durene; (b) acetophenone X trap; (c) 1-lndanone XI trap. TABLE I: Vibrational Analvsis Peak v , cm-'
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
25965 25935 25800 25763 25554 25470 25407 24962 24929 24893 24814 24700 24487 24390 24364 24292
A v , cm-'
Intensitya
Assignment
(A) Acetophenone Trap S 0-0 30 W a" COCH, tortion ? 165 w a" CH, toryion ? 202 w a' S(PhCOCH,) 411 m a" v i 6 a 495 W a" 'lab 558 m a, PhCObend 1003 m a' v , 1036 W a' V i 9 a 107 2 W a' v 8 1151 W a' '16 1265 W a' "sb 1478 W a' "isb 1585 W a' "ab 1601 m a' vsa 1673 S a' CO stretch
(B) 1-Indanone X, Trapb 0- 0 2 389 a" T(CCCb) 379,382 3 454 a" r ( C = O ) 463, 467 4 574 a" r(CC& ) 570, 571 5 689 a" T ( C C ~694, ) 692 6 902 a" r ( C % ) 883, 880 7 959 a' v(C ) 9 4 7 , 946, 949 8 998 a" v ( C b ) 989 1' 1723 a' v(C=O) 1714 2' 1723 t 345 3' 1723 t 408 3. Experimental Results 4' 1 7 2 3 t 464 5' 1723 t 545 3.1. Phosphorescence and M I D P Spectra. (A) 3 n ~ * 6' 1723 t 743 I-Indanone and Acetophenone. The TI states of acetoa S, m, and w denote strong, medium, and weak, phenone in DMOB and l-indanone in durene are conAssignment was made in comparison respectively. sidered to be predominantly 3 n ~ in * character judging from with IR and Raman data by Bardet, Fleury, and ~ their very large decay rate constants of the z ~ u b l e v e l . l ~ - ~SablayrolIs (ref 35). Numbers given here are the ground Recent experiment on the Stark effect of the phosstate vibration frequencies determined from IR and phorescence of 1-indanone also supports the 3 n ~ as* Raman measurements.
.~~ signment of the T1state of 1-indanone in d ~ r e n e Their spectra are very similarly characterized by strong 0-0 bands and progressions of the C=O stretching mode just as a typical phosphorescence spectrum of the 3n7r* state aromatic carbonyl. The phosphorescence spectrum of 1-indanone in durene is shown in Figure 2a. (B) 3 ~ 7 r * Acetophenone. Acetophenone Trap. The phosphorescence spectrum of the TI state of the acetophenone trap produced by the addition of a small amount of anisole to the acetophenone crystal is shown in Figure 2. The spectrum does not change over the temperature range from 4.2 to 1.2 K. The 0-0 band of the spectrum
1
26508 26119 26054 25934 25819 25605 25549 25510 24785 24440 24377 24321 24240 24042
2
is located at 25965 cm-l which is 45 cm-I lower than the location of the triplet exciton level of the acetophenone crystal determined by Dym-Hoch~trasser~~ and CaseKearns.17 The spectrum is characterized by a strong 0-0 band and a progression of the C=O stretching mode as * carbonyls. However, in the spectrum of the 3 n ~ aromatic a number of a' and a" vibrations of moderate to weak intensities are observed. This feature is similar to those found in many 37r7r* aromatic carbonyls with nearby 3n7r* states.30 The vibrational analysis of this spectrum is given in Table I. The analysis is made in comparison with the
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
Phosphorescence Studies of 1-Indanone and Acetophenone Traps
455
TABLE 11: Spectroscopic, Magnetic, a n d Decay Properties of t h e T, States of Acetophenone a n d 1-Indanone System Acetophenone trip 1-Indanone X, t r a p 1-Indanone X, trap 1-Indanone X, t r a p 1-Indanone in durene Benzaldehydea in acetophenone
x
a
T o t a l decay rates, s-'
cm-
AE, em-'
Type
25965
43
3nn *
26221
325
3nn *
VOO,
-
%n*
26281
264
26508
37
* 3nn*
3nn
*
26120
'nn
25600
3nn
*
Zfs, c m - ' D = -0.1826 E = 0.0585 D = -0.5325 E = 0.0108 D = -0.1200 E = 0.0558 D = -0.1698 E = 0.0694 D = -0.439 E = 0.011 D = -0.3601 E = 0.0609
k, 1.4
kz 77 770
kz
4.2
30
95
17.8
0.96
1.05
47.3
1.34
1.81
2300
40
117
2.6
18
Reference 31.
vibrational frequencies of the ground state acetophenone given by Koyanagi and Goodman.34 We also checked the y sublevel phosphorescence spectrum in order to ensure the symmetry assignment. The phosphorescence spectrum of this trap is similar to that of acetophenone in the acetophenone-d8 crystal reported by Koyanagi and Goodman, indicating that the nature of the T1state of this trap is similar to that of the acetophenone crystal itself. (C) 1-lndanone Traps. Since more than one type of trap exist in 1-indanone crystals, the phosphorescence spectra of 1-indanone crystals consist of those of different traps. The phosphorescence spectra of the neat 1-indanone crystal and the 1-indanone crystal doped with durene and 1,2-methylenedioxybenzeneare all identical at 4.2 K. This spectrum is shown in Figure 2. The spectrum has a very strong 0-0 band located a t 26 221 cm-l and a progression of the C=O stretching mode just as that of the 3n7* state of 1-indanone, but each peak consists of several lines of -35-cm-l separation which are presumably due to lattice vibration. We call this a type 1 spectrum and the trap responsible for it a X1 trap. This trap gives ODMR transitions a t 15.36 and 0.648 GHz. There is also a very weak peak at 26 281 cm-'. This peak gives ODMR signals a t 4.775 and 2.425 GHz showing that this peak belongs to a different trap. The MIDP spectrum obtained by applying 4.78-GHz microwave is shown in Figure 3a. We call this a type 2 spectrum and the corresponding trap a X2 trap. Both type 1 and 2 spectra were observed in all 1-indanone crystals studied here, although the type 2 spectrum is very weak. The phosphorescence spectrum of the 1-indanone crystal doped with 1,2-methylenedioxybenzeneyields an entirely different spectrum at 1.5 K as shown in Figure 3b. This spectrum has a weak origin a t 26 508 cm-l and numerous peaks in the region between 380 and 1000 cm-l from the origin. This origin gives ODMR signals at 7.175,4.160, and 3.012 GHz. The MIDP spectrum obtained by applying 3.01-GHz microwave gives the spectrum shown in Figure 3c. We call this a type 3 spectrum and the corresponding trap a X3 trap. The MIDP spectrum confirms that the weak peak a t 26508 cm-l is actually the 0-0 band and many other peaks belong to the same emitting species. This spectrum is similar to the type 2 spectrum, but it is remarkably different from that of the 3 ~ acetophenone ~ * trap. The 0-0 band is located 37.3 cm-l lower than the location of the triplet exciton level of the 1-indanone crystal determined by Case and Kearns.17 This difference is quite reasonable for an impurity induced X trap. The vibrational analysis of this spectrum is given in Table I. The vibrational frequencies obtained from the MIDP spectrum agree very well with those of the ground state 1-indanone obtained from infrared and Raman data.35
(a) 1-lndanone X 2 Trap ( M I D P sp.)
440
420
600
, I 1
( b ) I-Indanone with 12-Methylwedioxybenzene
400
380 nrn
390
380 nrn
Figure 3. Phosphorescence and microwave induced delayed phosphorescence (MIDP) spectrum of ' m r * 1-indanone: (a) MIDP spectra of 1-indanone X, trap (4.78-GHz microwave); (b) phosphorescence spectrum of 1-indanone crystal doped with 1,2-methyIenedioxybenzene at 1.5 K; (c) MIDP spectrum of 1-indanone X3 trap (3.01-GHz microwave).
Figure 4. Temperature dependence of the phosphorescence spectrum of the 1-indanone crystal doped with 1,2-methyIenedioxybenzene in 368-380-nm region: (a) 3.36 K, (b) 3.55 K, (c) 3.70 K.
At temperatures between 4.2 and 2.0 K the phosphorescence spectrum is given as a superposition of type 1,2, and 3 spectra with varying relative intensities as shown in Figure 4. The temperature dependence of the intensities of type 1 and 3 spectra is given in Figure 5. 3.2. Zero Field Splittings (Zfs) and Decay Rates. The zfs and the decay rates from spin sublevels determined in the present ODMR study are tabulated in Table 11. We briefly comment on the results. (A) 3nr* 1-Indanone. The zfs and the decay rates of 3nn-* 1-indanone in a durene host were previously determined by Nishimura and Vincent.16 Our values are in reasonably good agreement with theirs, when we take into account the uncertainities involved in the measurements
456
The Journal of Physical Chemistry, Vol. 82,
0
I
'
2.5
3.5
3.0
4.0
0
T (K) Figure 5. Temperature dependence of the relative intensities of the and X3 (OW) traps. phosphorescence emissions from X, (00)
of very large decay rate constants. Large D and k, are characteristics of 3n7r* 1-indanone. (B) 1-Indanone and Acetophenone Traps. The zfs and the decay rates of 1-indanone traps vary tremendously depending on the trap. However, a larger D is accompanied with larger k, as in the case of 3 7 r ~ *benzaldehydes. k, >> k,, k, was also found as in other aromatic carbonyls.
4. Discussion of Spectroscopic and Magnetic Properties 4.1. Nature of the Phosphorescing Species. The properties of the 3n7r* state of 1-indanone and acetophenone are very similar. On the other hand, the properties of the 3ir7r* state of 1-indanone and acetophenone differ very significantly. The triplet state energies of the impurity induced acetophenone trap and 1-indanone X3trap are only 43 and 37 cm-I lower than the triplet exciton levels of the respective crystals. The data given in Table I as well as the phosphorescence spectra clearly show that the T1 states * This of these traps are dominantly 3 ~ in~ character. conclusion agrees with the assignments of the T1states of 1-indanone and acetophenone crystals made by Case and Kearns.17 The energy differences between the 37r7r* and 3n7r* states (AETT)in 1-indanone and acetophenone crystals are 260 and 280 cm-l, respectively. We may expect that the changes in the 3 n ~ energies * in going from pure crystals to traps are also similar to those in the %7r* states. If this is the case, AEm for these traps should not be much different from those for crystals, probably being about 300 cm-l. Despite the similarities in the molecular structures and AEm the phosphorescence spectra of 37r7r* 1-indanone and acetophenone are very different. The spectra of 1indanone X2 and X3 traps are characterized by numerous very strong a" vibrations and a weak 0-0 band. On the other hand, the spectrum of the acetophenone trap has a very strong 0-0 band and a progression of C=O stretching mode but relatively weak a" vibronic bands. This difference in the radiative property is also consistent with that found for the So-T1 excitation spectra of the ~rysta1.l~ The main part of the radiative activity of the 1-indanone X2 and X3 traps seem to be accounted for well by the Herzberg-Teller vibronic spin-orbit coupling mechanism: 3
3
r
""
the route involving vibronic interaction in the triplet manifold is much more important than that involving lnx* and ~ T A *states. Minor contributions probably arise from the direct spin-orbit mixing of the 37r7r* state with the ln7r* and lap* states. Direct configurational mixing between 37r7r* and 3n7r* states probably contributes to radiative activity because of the very small AETT. These mixings give intensities at the 0-0 band and the vibronic bands of a' vibration. The observed radiative property of 37rir* 1-indanone is consistent with the planar structure of the 37r7r* state. It is also similar to that of the T1 state of p-methyl-lindanone in methylpentane glass studied by Long and Lim.13 On the other hand, the main part of the radiative activity of the acetophenone trap cannot be explained by the Herzberg-Teller mechanism. However, this type of phosphorescence spectrum has been found in many 37r7r* type aromatic carbonyls of the benzaldehyde type with nearby 3n7r* states. It was suggested that the direct configurational mixing between 3nn* and 37r7r* states
T, (%*)
el ++
T, ("7r*)
T
z (
*
"rr*
1nr*
Since AETT is small in this system, it is considered that
so t--)
'n~*
due to the small distortion from planarity is the primary cause of the appearance of such a spectrum.30 It is considered that the distortion from planarity can take place because of the effect of the crystal field as well as vibronic interaction. This mechanism leads to the in-plane polarization of the 0-0 band despite the predominant 3 ~ character of the T1 state and explains the results of the recent Zeeman effect and polarized absorption studies of the acetophenone crystal by Tanimoto et The main difference between 1-indanone and acetophenone seems to be in the fact that the C=O group in 1-indanone is rigidly held to the planar five-membered ring. Hence the out-of-plane distortion at the C=O group in 1-indanone is expected to occur with more difficulty than in acetophenone. This structural difference is probably the main cause of the observed striking differences in the radiative properties of these two systems. The other properties such as zfs and total decay rates follow the trends found for the 37r7r* aromatic carbonyls. However, the data show that k, and D of 37r7r* 1-indanone and acetophenone are much smaller than those of benzaldehydes with similar values of AET,. Although we do not know the quantum yields of phosphorescence in these molecules, the observation seems to indicate that the radiationless decay rates in 1-indanone and acetophenone are smaller than in benzaldehyde. This is consistent with the observation by Lim, Li, and Li that the radiationless decay rates in 37rir* substituted benzaldehydes are larger than those in substituted ace top hen one^.^' The D values of the 37r7r* aromatic carbonyls with nearby %7r* states are approximately given by4Ol4' Here G is the matrix element for the spin-orbit coupling between 3n7r* and 3 7 r ~ *states, (3n7r*IH,137rir*). Hence, our observation indicates that G is strongly dependent on the system and is rather small in 1-indanone. The difference in zfs and decay rates between the X2 and X3 traps of 1-indanone are likely due to the difference in AETT. The nature of the X1 trap of 1-indanone is very different from those of the other traps. The phosphorescence spectrum appears to be similar to that of the 3n7r* aromatic carbonyl. However, we note that its k, is only one third of the k , of 3 n ~ 1-indanone * in a durene host. The value of D , on the other hand, is the largest among all the 1indanone species studied here. These observations seem
""i y
T
S. Niizuma and N. Hirota
No. 4, 1978
~
*
Phosphorescence Studies of 1-Indanone and Acetophenone Traps
The Journal of Physical Chemistry, Val. 82, No. 4, 1978 457
3
- 2
I
C
1
TRAP XI
EXCITON TRAP LEVEL
X3
C
Figure 6. Energy level scheme of the exciton and traps of the 1indanone crystal doped with 1,2-methyIenedioxybenzene to explain temperature dependence of the phosphorescence spectrum.
to indicate that the T1 state of the X1 trap is best understood as having a very mixed character of 3n7r* and 3 m * states. The 0-0 energy of the T1state of the X1 trap is 264 cm-l lower than the triplet exciton level of the 1-indanone crystal. This energy is lower than those of the Xz and X3 traps in spite of the large 3n7r* character of the X1 trap. This trap exists in all the crystals studied here regardless of the degree of purification and the presence of the other guest molecules. 4.2. Temperature Dependence of the Phosphorescence of the 1-Indanone Crystal. Since the lifetimes and the spectra of the X1, X2, and X3 traps are very different, the low temperature phosphorescence of the 1-indanone crystal doped with 1,2-methylenedioxybenzenehas the characteristics of dual phosphorescence. However, it shows a remarkable temperature dependence of the phosphorescence. We examined whether this is due to an inter- or intramolecular process. We first assume that the temperature dependence is due to the intermolecular energy transfer processes shown by the scheme in Figure 6. This scheme is somewhat similar to that used in the explanation of the trap-to-trap energy transfer in mixed crystal by Hirota and Hutchison,42 The UV excitation of the crystal produces triplet exciton T,. Because of the rapid migration of the triplet exciton, the triplet state is trapped at the X1, Xz, and X3 traps. Since the emission from X2 is much weaker than those from X1 and X3, we only consider these traps here. We denote the concentrations of the triplet states of the X1 and X3 traps by ITx1]and [Tx3],respectively. The trap depth of the XI trap is 246 cm-l, whereas that of the X3 trap is only 37 cm-l. Therefore, we only consider the depopulation of the triplet state from the X3 trap to the exciton level in the temperature range of interest. We set the following kinetic equations for the changes of concentrations with respect to time. d [ T x l l / d t = - k ~ [ T ~+~R1[Te1 l
(2)
d 0.28
0.30
0.40
0.35
Flgure 7. Plots of In I3 and In (I," - 11")vs. 1 / T X, trap.
(OA)X1 trap: (OA)
The validity of this expression a t low temperature was considered in detail by Fayer and Solving eq 1, 2,and 3 under steady state conditions and noting eq 5 we obtain
R3 R,)
(k3/ho)(1
(7)
exP(-AE/hT)
Since the intensities of the phosphorescence emissions are proportional to [T,,] and [Tx3],the intensities of the emissions due to these traps, II and 13, are given by b exp(-AE/hT) It = a [ a b exp(-AE/kT)]
+ + +
I3=
P
a
+ b + exp(-AE/hT)
(9)
Here a = R3k3/Rlk0 and b = k3/ko. a and @ are the proportionality constants which depend on the radiative decay rates of the X1 and X3 traps. Here we note ko >> k3 and a, b > a + b.
Then
(11)
+
Here, k l and k, are the decay rate constants of the triplet states of the XI and X3 traps. R1 and R3 are the rate constants for the production of TX1and T,, from the exciton T, through energy transfer. A is the rate of production of the triplet exciton T,. kd is the depopulating rate constant from the trap X3 and is expected to be given by
(12) In Figure 7 plots of In (Il"- 11)and In I3vs. 1 / T are given. The plots give good straight lines with AE = 37 cm-'. As T 0, exp(-AE/kV 0. Setting II IIo and I3 12 a t T 0, we obtain the following equations from eq 8 and 9: Il/(II - I 1 O ) = [ b ( a b)/a][l/(a b)+ exP (AE / hT) 1 (13) and
kd = ko exp(-AE/kT)
13'/13= 1+ [ l / ( a + b ) ]exp(-AE/kT)
d[Tx,I/dt=
-(h3
d[Tel/dt=A
+ kd)[Tx31 + R,[TeI
- (R1
f
R3)[TeI
hd
[Tx,l
(3)
(4)
(5)
+
-+
-
-
+
-
+
(14)
458
S. Niizuma and N. Hirota
Tbe Journal of Pbysical Chemistry, Vol. 82, No. 4, 1978
30
23
are all broad and it is difficult to make an accurate comparison with the well-resolved spectra obtained here. However, the relatively weak 0-0 band and the presence of the progression of 500-cm-l vibration are consistent with the spectrum of 37r7r* 1-indanone obtained here. The average of the lifetimes of the three spin sublevels obtained here give the high temperature lifetimes of the X2 and X3 traps as 60 and 150 ms, respectively. These values agree in order of magnitude with the reported lifetimes of the long-lived phosphorescence. It has been noted that the polarization of the long-lived phosphorescence is anomalous as a 37r7r* state.13 This, however, is not unusual, when we note the fact that the 0-0 bands of the 3 ~ 7 r *acetophenone and benzaldehyde with small AETT are in-plane polarized.3o It is possible that direct mixing between 3n7r* and 37r7r* states due to a slight distortion from planarity causes the in-plane polarization at the 0-0 band because of the very small AETTof 37r7r* 1-indanone. Lim and co-workers suggested that the long-lived phosphorescence in EPA arises from the higher lying 37r7r* state because of the very slow interconversion between the 3 n ~ and * 37rir* states which are separated by 30 cm-'.13 Here, we observed both 3nr* and 37r7r* type phosphorescence in the 1-indanone crystal doped with 1,2methylenedioxybenzene, but it was shown to originate from two different traps. In the crystalline systems studied here we did not observe any simultaneous emissions from the 3n7r* and 37r7r* states of the same molecule. N
IO
0
EXP[--dE/kT)
x IO-'
(a)
Figure 8. (a) Plot of 1 2 / 1 3vs. exp(-AElkT). vs. exp(AE/kT).
C
b)
(b) Plot of 11/(11 - 1,')
The plots of 11/(11 - I t ) vs. exp(AE/kT) and 130/13 vs. exp(-AE/kT) are shown in Figure 8. They give excellent straight lines with A E = 39 cm-l. From the slopes and and R3/R1= intersections we obtain k3/ko = 3.5 X 0.75. AE determined from the above kinetic analysis is in excellent agreement with the spectroscopic AE = 37 cm-l obtained from the energy separation between the exciton level and the T1 state of the X3 trap. This agreement shows the involvement of the exciton band of the crystal in the thermal excitation process which is responsible for the temperature dependence of the phosphorescence spectrum. Hence, in the present systems the possibility that the two emissions arise from two triplet states of the same trap is excluded. Using k3 = 4.7 X 10 s-l obtained from the decay rate constant of the X3 trap, we obtain k, = 1.5 X lo9 s-l. This Acknowledgment. This work was supported in part by value is similar in order of magnitude to ko obtained for a NSF science development grant to SUNY at Stony other systems from the experiments made at much higher Brook. temperature^.^^^^ The present experiments were made on a 1-indanone crystal in which 0.6 wt % of 1,2-methyReferences and Notes lenedioxybenzene exists in the initial melt. Although we (1) N. C. Yang and S. L. Murov, J . Cbem. fbys., 45, 4358 (1966). do not know the exact concentration of the X3 trap in the (2) G. Yamaguchi, Y. Kakinoki, and H. Tsubomura, Bull. Cbem. SOC. actual mixed crystal, it is probably on the order of a few Jpn., 40, 426 (1967). tenth of a percent. Since our analysis gives R3/R1= 0.75, (3) P. Gacoin and Y. Meyer, C. R. Acad. Sci., Paris, Ser. B, 267, 149 (1968). our sample must contain a considerable amount of the X1 (4) R. N. Griffin, Pbofocbem. Pbofobiol., 7, 159 (1968). trap. The present study does not tell how the X1 trap is (5) Y. Kanda, J. Stanislaus, and E. C. Lim, J. Am. Cbem. Soc., 91, produced. However, the observation that this trap exists 2619 (1969). in all the crystals regardless of the degree of purification (6) H. J. Pownall and J. R. Huber, J . Am. Cbem. Soc., 93,6429 (1971). (7) M. E. Long, Y. H. Li, and E. C. Lim, Mol. Pbofocbem., 3, 221 (1971). or the presence of other guest molecules seem to indicate (8) N. Y. C. Chu and D. R. Kearns, J. Am. Cbem. Soc., 94,2619 (1972). that it is rather intrinsic to the 1-indanone crystal. (9) P. J. Wagner, M. May, and H. Haug, Cbem. fbys. Lett., 13, 545 4.3. Nature of the Dual Phosphorescing Species. The (1972). present study clearly shows that the properties of the T1 (10) P. R . Callis and R. W. Wilson, Cbem. Pbys. Lett., 13, 417 (1972). (11) P. Gacoin, J. Cbem. Pbys., 57, 2418 (1972). state of 1-indanone vary tremendously depending on the (12) H. J. Pownall, R. E. Connors, and J. R. Huber, Cbem. fbys. Lett., environment. It can certainly be either the 3n7r* or '7r7r* 22, 403 (1973). state depending on the environment. We also noted that (13) M. E. Long and E. C. Lim, Cbem. Pbys. Lett., 20, 413 (1973). (14) N. Kanamaru, M. E. Long, and E. C. Lim, Cbem. fbys. Lett., 26, the 1-indanone crystal doped with 1,2-methylenedioxy1 (1974). benzene exhibits both 37r7r* and 3na* type emissions at very (15) Y. Tanimoto, N. Hirota, and S. Nagakura, Bull. Cbem. SOC.Jpn., low temperatures. Although the present result does not 48, 41 (1975). provide a direct answer to the question about the cause (16) A. Nishimura and J. Vincent, Cbem. Pbys. Lett., 13, 89 (1972). (17) W. A. Case and D. R. Kearns, J . Cbem. Pbys., 52, 2175 (1970). of the dual emission in rigid matrices, we briefly discuss (18) T. H. Cheng and N. Hirota, J . Cbem. fbys., 56, 5019 (1972). the relationship between our results and those obtained (19) C. B. Harris, J . Chem. Pbys., 54, 972 (1972). in rigid matrices. (20) D. S. Schweltzer, J. Zuchlich, and A. H. Maki, Mol. Pbys., 25, 193 (1973). It is generally agreed that the short-lived phosphorescence originates from the 3nir* state of l - i n d a n ~ n e . ~ ~ ~ ~ ~(21) , ' ~ D. ~ A. ' ~Antheunis, J. Schmidt, and J. H. van der Waals, Cbem. fbys. Left., 6, 255 (1970). This is certainly correct, although the 3n7r* state can (22) J. Schmidt, D.A. Antheunis, and J. H. van der Waals, Mol. Pbys., substantially be mixed with the 37rir* state. The Boltz22, 1 (1971). (23) J. Schmidt, Thesis, Leiden, 1972. mann averaged lifetime of the X1 trap is about 3 ms which (24) M. A. El-Sayed, D. V. Owens, and D. S. Tinti, Cbem. fbys. Lett., is close to the lifetime of the short-lived phosphorescence 6, 395 (1970). in rigid matrices. (25) A. L. Kwiram, Inf. Rev. fbys. Cbem., Ser. 7 , 1, (1975). (26) A. H. Francis and C. B. Harris, J . Cbem. Pbys., 57, 1050 (1972). There have been different suggestions as to the species and N. Hirota, J. fbys. Cbem., 80, 2966 (1976). ~ ~ ,A.~ ~Chakrabarti J~ responsible for the long-lived p h o s p h o r e s ~ e n c e . ~ ~ ~ ~ ~(27) (28) E. T. Harrigan and N. Hirota, J . Am. Cbem. SOC.,98, 3460 (1976). The suggested species include 37r7r* l - i n d a n ~ n e ' ,and ~~ (29) N. Nishi and M. Kinoshita, Cbem. fbys. Lett., 27, 342 (1974). various photoproduct^.^^^ Unfortunately the reported (30) E. T. Harrigan and N. Hirota, Mol. Pbys., 31, 681 (1976). (31) T. H. Cheng and N. Hirota, Mol. Pbys., 27, 281 (1974). spectra of the long-lived phosphorescence in rigid matrices
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
Organic Scintillators with Large Stokes’ Shifts
(32) S. J. Sheng and D. M. Hanson, Chem. Phys., 10,51 (1975). (33) S.Dym and R. M. Hochstrasser, J . Chem. Phys., 60,368 (1974). (34) (a) L. Goodman and M. Koyanagi, Mol. Photochem., 4,369 (1972);
459
(38) Y. Tanimoto, T. Azumi, and S.Nagakura, Bull. Chem. SOC.Jpn., 46, 136 (1975). (39) R. Li, Y. H. Li, and E. C. Lim, J . Chem. Phys., 53, 2443 (1970). (40) H. Hayashi and S. Nagakura, Mol. Phys., 24, 801 (1972). (41)C. R. Jones, D. R. Kearns, and R. M. Wing, J . Chem. Phys., 58, 1370 (1973). (42) (a) N. Hirota, and C. A. Hutchison, J. Chem. Phys., 42,2869 (1965); (b) N. Hirota, ibid., 43, 3354 (1965). (43) M. D. Fayer and C. 8.Harris, Phys. Rev. 6, 9,748 (1974). (44) E. T. Harrigan and N. Hirota, J . Chem. Phys., 49, 2301 (1968).
(b) M. Koyanagi, R. J. Zwarich, and L. Goodman, J. Chem. Phys., 56, 3044 (1972). (35) M. L. Bardet, G. Fleury, and M. C. Sablayrolls, J . Mol. Struct., 3,
141 (1969). (36) For example, R. M. Hochstrasser, ”Molecular Aspects of Symmetry”, W. A. Benjamin, New York, N.Y., 1966,Chapter 9. (37) E. T. Harrigan and N. Hirota, Mol. Phys., 31, 663 (1976).
Organic Scintillators with Unusually Large Stokes’ Shifts H. Gusten,” P. Schuster, and W. Seitz KernforschungszentrumKarlsruhe, Instltut fur Radiochemie, 7500 Karlsruhe, Postfach 3640, Federal Republic of Germany (Received July 11, 1977) Publication costs assisted by KernforschungszentrumKarlsruhe
A series of methyl and/or methoxy substituted 1,3-diphenyl-2-pyrazolines were synthesized for evaluation as solutes in liquid scintillation counting systems. The electronic absorption spectra, absolute fluorescence spectra, fluorescence quantum yield, fluorescence decay times, as well as the relative pulse height of 15 1,3-diphenyl-2-pyrazolineswere measured in benzene at room temperature. Unusually large Stokes’ shifts with high fluorescence quantum yields of about 0.90 are obtained when bulky methyl and/or methoxy substituents are attached in ortho, ortho‘ positions of the phenyl rings in 1 and/or 3 positions of the 2-pyrazoline ring. The sterically hindered 1,3-diphenyl-2-pyrazolines have a promising potential as scintillators for liquid scintillation counting. Due to the large Stokes’ shift the highly soluble sterically hindered 1,3-diphenyl-2-pyrazolines can be used as primary solutes without requiring a wavelength shifter.
(1)1,3-Diphenyl-2-pyrazoline, mp 151 “C,loA, 355 nm, 2.05 X lo4 (isooctane); (2) 1,3,5-triphenyl-2-pyrazoline, mp 136 “C,ll A,, 357 nm, t 2.03 X lo4 (isooctane); (3) l-phenyl-3-mesityl-2-pyrazoline, mp 96 “C, ,A, 292.5 nm, E 1.40 X lo4 (isooctane); (4) l-phenyl-3-(2’,4’,6’-trimethoxyphenyl)-Zpyrazoline, mp 150 “C, ,A, 297.5 nm, E 1.58 X lo4 (isooctane); (5) l-phenyl-3-(2’,6’-dimethoxyphenyl)-2-pyrazoline, mp 131 OC, ,A, 295 nm, E 1.34 X lo4 (isooctane); (6) 1-(2,6-dimethylphenyl)-3-(2’,5’-dimethoxyphenyl)-2-pyrazoline, mp 82 “C, ,A, 339.5 nm, t 1.73 X lo4 (isooctane); (7) 1-(2,6-dimethylphenyl)-3- (2’,6’-dimethoxyphenyl)-2-pyrazoline,mp 117 “C, A,, 285 nm, 6 0.95 X l o 4 (isooctane); (8) l-phenyl-3-(4’-methoxyphenyl)-2-pyrazoline, mp 142 “C,12A,, 358 nm, E 2.03 X lo4 (benzene); (9) l-phenyl-3-(3’-methoxyphenyl)-2pyrazoline, mp 73 “C, ,A, 364 nm, 6 1.99 X lo4 (benzene); Experimental Section (10) l-phenyl-3-(2’-methoxyphenyl)-2-pyrazoline, mp 110 Substances. The methyl and methoxy substituted 361 nm, t 1.65 X lo4 (benzene); (11) l-phenyl“C, A, 1,3-diphenyl-2-pyrazolineswere prepared by standard 3-(2’,4’-dimethoxyphenyl)-2-pyrazoline, mp 106 “C, A, procedure^.^ The condensation of the appropriately 356 nm, E 1.72 X lo4 (benzene); (12) l-phenyl-3-(2’,5’-disubstituted Mannich bases with phenylhydrazine or the methoxyphenyl)-2-pyrazoline, mp 90 “C, A,, 368 nm, t appropriately substituted phenylhydrazine gave methyl 1.73 X lo4 (benzene); (13) l-phenyl-3-(3’,4’-dimethoxyand/or methoxy substituted 1,3-diphenyl-2-pyrazolines phenyl)-2-pyrazoline, mp 124 “C, ,A, 360 nm, t 2.30 X lo4 with yields of 55 to 90%. l-Phenyl-3-mesityl-2-pyrazoline (benzene); ( 14) l-phenyl-3-(3’,5’-dimethoxyp hen yl) -2and 1-phenyl-3-(2’,4’,6’-trimethoxyphenyl)-2-pyrazoline pyrazoline, mp 126 “C, A, 363.5 nm, t 2.11 X lo4 were synthesized by condensation of mesityl vinyl ketone (benzene); (15) l-phenyl-3-(3’,4’,5’-trimethoxyphenyl)-2and 2,4,6-trimethoxyphenyl vinyl ketone with phenylpyrazoline, mp 135 “C, ,A, 363 nm, E 2.17 X lo4 (benzene). hydrazine, respectively. The compounds are purified by The photophysical data of the compounds investigated recrystallization and, if necessary, by column chromawere measured in benzene for fluorescence spectroscopy tography. All new compounds furnished correct data for (Merck Co., Darmstadt) and isooctane for UV spectroscopy elemental analysis and were further characterized by their (Merck Co., Darmstadt). Degassing was performed by the absorption and fluorescence spectra as well as by their freeze-thaw technique at 5 X lo4 Torr in special cuvettes mass spectroscopic fragmentation. with a view to fluorescence spectroscopy.13
Introduction 1,3,5-Triaryl-2-pyrazolines are essential components of a liquid or plastic s~intillator.’-~Besides in conventional liquid scintillation counting this class of highly fluorescent compounds is used as fluorescent whitening agent^.^^^ Although the synthesis of a large number of these industrial fluorescers has been described, there has been a lack of quantitative photophysical data until r e ~ e n t l y . ~ - ~ In a study on the relation between structure and fluorescence properties of 1,3-diphenyl-2-pyrazolines8 we observed unusually large Stokes’ shifts of the fluorescence in solution. We report here the photophysical data of a number of methyl and methoxy substituted 1,3-diphenyl-2-pyrazolines and their promising potential as scintillators for liquid scintillation counting.
0022-3654/78/2082-0459$0 1.00/0
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0 1978 American Chemical Societv
