J. Phys. Chem. 1992, 96, 7904-7908
7904
(48) (a) Saxe, P.; Yamaguchi,Y.; Schaefer, H. F. 111 J . Chem. Phys. 1982, 77,5647. (b) Osamura, Y.; Yamaguchi, Y.; Saxe, P.; Vincent, M. A.; Gaw, J . F.; Schaefer, H. F. 111 Chem. Phys. 1982, 72, 131. (49) (a) Osamura, Y.; Yamaguchi, Y.; Saxe, P.; Fox, D. J.; Vincent, M. A.; Schaefer, H. F. J . Mol. Strucr. 19113, 103, 183. (b) Handy, N. C.; Schaefer, H. F. J . Chem. Phys. 1984, 81, 5031.
(50) (a) Rendell, A. P.; r#,T. J. J. Ch" Phys. 1991,946219. (b) Lee, T. J.; Rendell. A. P. J . Chem. Phys. 1991,91, 6229. (51) PSI 1.0, 1989, PSITECH, Inc., Watkinsvilk, GA. (52) TITAN is a set of electronic structure programs written by T. J. Lee, A. P. Rendell, and J. E. Rice. (53) Allen, W. D. Program INTDER, Stanford University,Stanford, CA.
Radlatlonless Decay of the S2 States of Azulene and Related Compounds: Solvent Dependence and the Energy Gap Law Brian D. Wagner, Dietrich Tittelbach-Helmrich, and Ronald P. Steer* Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N OW0 (Received: April 29, 1992)
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The UV-visible absorption spectra, S2lifetimes, and S2 SOfluorescencequantum yields of azulene, azulene-d8,1,3-di(guaiazulene) have each chloroazulene, 1,3-dibromoazulene, 4,6,8-trimethylazulene,and 1,4-dimethyl-7-isopropylazulene been measured accurately in six solvents. The S2-Sl electronic energy spacings of each solute vary by ca.500 cm-' in these solvents. The variations in the S2nonradiative relaxation rates with electronic energy spacing are interpreted within the S1 internal conversion dominates the nonradiative framework of the energy gap law of radiationless transition theory. S2 TJ is important in the halogenated derivatives. decay in azulene and azulene-d8,but intersystem crossing (likely S2 The alkyl-substitutedcompounds exhibit anomalous behavior and demonstrate that factors other than the electronic energy spacing are involved in determining the rates of their radiationless relaxation. Previous energy gap law correlations based on data from a series of structurally different compounds must be reinterpreted.
---
Introduction
Azulene is the first-discovered and best-known example of a closed-shell polyatomic molecule which exhibits "anomalous" S2 Although the number So fluorescence in condensed of compounds which are known to radiate efficiently and react chemically from upper electronic excited states is now rather large,I*l2 azulene and its derivatives continue to be the focus of considerable attention and to be quoted as the "classical examples". I3-l5 In preparation for studies of the dynamic behavior of the short-lived SI states of azulene and other compounds in condensed media by two-photon, pumpprobe methods,16 we conducted a thorough review of the literature on this subject. To our surprise, we discovered that there is a remarkable degree of disagreement on the values of the quantum yields of S2 So fluorescence and the lifetimes of the S2states of azulene, azulened8, and its closely related derivatives in various condensed media. Moreover, although azulene and other nonalternant hydrocarbons and their derivatives clearly exhibit "slow" S2nomdiative decay rates owing to their large S2-Sl (or perhaps S2-T,J electronic energy spacings, none of the previous attemptseg to measure and correlate these rates quantitatively within the framework of the energy gap law" of radiationless transition theory has been completely successful. Murata et al.495 measured the quantum yields of Sz So fluorescence, +f, of azulene and thirteen of its derivatives but did not measure the S2lifetimes directly. Instead they calculated the radiative rate constants, k, from the absorption spectra using the Strickler-Berg formalism,I8 calculated the lifetimes via T = &/k,, and then obtained the nonradiative rate constants from C k , , = (1 - 4f)/T. A linear relationship between log (Zknr) and U ( S 2 - S I ) for the 14 compounds led them to conclude that S2 SIinternal conversion constituted the major S2radiationless decay process. Later Eber et a1.6 measured both qbf and T but adopted Murata et al.'s4" value of & = 0.031 for azulene in ethanol as a secondary fluorescence standard. (Neither group applied the required "nZnrefractive index correction in the measurement of
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To whom correspondence should be addressed.
the quantum yields.I9) Eber et al. concluded that log (Ck,)was not a linear function of U ( S 2 - SI),that the brominated and chlorinated derivativesexhibited a "heavy atom" effectla and that intersystem crossing played an important role in Sis radiationless decay. However, Gillespie and Li" reevaluated the data from the previous two studies in terms of Siebrand's extended theory of radiationless transitions2* and concluded that intersystem crossing from S, to T, was not signifcant. Most recently Griesser and measured T ( S ~ of ) azulene, azulene-d8, and four derivatives in both 3-methylpentane and ethanol at 77 K. They but calculated k, from obtained Ck, by subtracting k, from i 1 the room temperature solution-phase absorption spectrum using the Stricklederg relation~hip,'~ assuming that k, was independent of both temperature and solvent. Use of the latter procedure does not introduce very large errors into C k , even though the values of k, so obtained may not be particularly accurate. The largest experimental error in Griesser and Wild's data is therefore associated with their measurements of T . However, their use of a wide variety of azulene derivatives to test the energy gap law of S2radiationless decay mechanism remains problematic because the structural changes used to vary the electronic energy spacing (Cl, Br, and alkyl substitution) will also change (i) the magnitudes of the matrix elements coupling S2to all lower states, and (ii) the numbers and/or energies of all vibrational modes in the molecule. We have accurately remeasured the S2 So fluorescence quantum yields and the S2lifetimes of azulene, azulene-d8, and four of its derivatives using current standards of experimental practice. Variations in the electronic energy gaps of each compound have been introduced by changing only the nature of the solvent. The results of these measurements and attempted correlations of the S2nonradiative decay rates with electronic energy gap are presented in this paper.
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Experigest.l Section
Azulene (AZ) and guaiazulene (GAZ, 1,Cdimethyl-7-isopropyldene), both from Aldrich, were used a received. Samples of highly purified AZ and azulene-d8were kindly supplied by Jh. B. Nickel. 1,3-Dichloroazulene (DCAZ) and 1 , 3 d i b r o m d e n e
0022-3654/92/2096-7904$03.00/00 1992 American Chemical Society
Decay of the S2 States of Azulene
The Journal of Physical Chemistry, Vol. 96, NO. 20, 1992 7905
TABLE I: Experimental Results for the Nonradiative Decay of the SIState of Azulew E(s,)/ io3 E ( s , ) / io3 AE(S2-S,)/ 10’ compd solvt cm-I cm-I cm-’ 14.39 14.26 AZ PFCH 28.65 14.37 14.01 CH 28.38 14.52 13.99 ETOH 28.51 14.66 13.95 MeCN 28.61 14.62 13.83 DCM 28.45 13.78 14.50 BNZ 28.28 14.40 14.34 28.74 AZ-ds PFCH 14.39 14.05 28.45 CH 14.56 14.02 28.58 EtOH 14.62 13.99 28.61 MeCN 14.62 13.87 28.49 DCM 14.54 13.84 28.38 BNZ 12.77 14.63 27.40 DCAZ PFCH 12.79 14.35 27.14 CH 13.10 14.30 27.40 EtOH 14.20 13.21 27.41 MeCN 13.17 14.07 27.24 DCM 13.09 14.05 BNZ 27.14 13.16 DBAZ 13.87 27.03 CH 13.35 13.68 BNZ 27.03 13.77 13.66 PFCH 27.43 GAZ 13.83 13.57 27.40 EtOH 13.90 13.53 27.43 MeCN 13.72 13.48 27.20 CH 13.81 13.34 27.15 BNZ 14.01 13.29 DCM 27.30 15.48 12.52 28.00 TMAZ PFCH 15.53 12.29 27.82 CH 15.78 12.00 EtOH 27.78 15.65 11.97 27.62 BNZ 15.90 DCM 11.84 27.74 15.90 11.82 27.72 MeCH
(DBAZ) were synthesized and purified according to the method of Anderson et al.22*234,6,8-Trimethylazulene (TMAZ) was synthesized and purified according to the method of Garst et alez4 Benzene (BNZ, BDH Omnisolv), acetonitrile (MeCN, BDH Omnisolv) and dichloromethane (DCM, BDH Omnisolv) were used as received. Cyclohexane (CH, BDH Omnisolv), perfluoro- 1,3-dimethylcyclohexane (PFCH, PCR Chemicals), and ethanol (EtOH, 95%) were fractionally distilled before use. Solutions of 104-10-5 M concentration were employed in all experiments, and pure solvent in a matched cell was used as a reference in the absorbance measurements and for background subtraction in the quantum yield and lifetime measurements. Concentrations were chosen so that solutions exhibited an absorbance of ca. 0.1 in a 1-cm cell at the excitation wavelength used in the quantum yield measurements (320 nm for AZ and AZ-d,, and 330 nm for the derivatives). All solutions were purged with dry N2 immediately prior to use. The experiments were all performed at 20 f 1 OC. Excited-state energies were determined from the wavelengths of the 0 bands in the SI So and S2 So absorption spectra, except in the case of TMAZ for which the S2energy was obtained from the fluorescence spectrum. Fluorescence quantum yields were measured by a relative method which has been described in detail previously.25 Both 9,lO-diphenylanthracene in cyclohexane ( I # J ~ = 0.9526)and quinine bisulfate in 1 N H2S04(+f = 0.5219*27) were used as standards and refractive index corrections (n2) for sample and standard solvents were emp10yed.I~ The maximum error in the quantum yield measurements is estimated to be *5%. Fluorescence decay times were measured using a SpectraPhysics synchronously pumped, cavity-dumped, frequency-doubled R6G dye laser excitation system which has been described in detail previ~usly.*~.~~ In the present experiments a cooled Hamamatsu R2809U-07 microchannel plate photomultiplier tube coupled to a Hamamatsu C4267 fast preamplifier and a modified Tennelec TC 454 constant fraction discriminator were employed in the timacorrelated, single-photon detection system. This system has an instrument response function with a fwhm of 44 ps and can be used to resolve fluorescence lifetimes as short as ca. 10 ps with
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-
and Derivatives as a
4f
T2/pS
0.052 0.046 0.04 1 0.040 0.041 0.047 0.067 0.072 0.060 0.058 0.066 0.073 0.082 0.073 0.062 0.053 0.056 0.057 0.016 0.013 0.013 0.017 0.017 0.018 0.021 0.022 0.0006 0.0005 0.0005 0.0005 0.0004 0.0003
1670 1330 1310 1350 1170 1040 2230 1710 1660 1680 1480 1360 1370 990 880 810 740 710 200 180 260 330 350 330 290 290 17 15 14 12 510 510
Function of Solvent k,/107 s-I 3.1 3.5 3.1 3.O 3.5 4.5 3.0 4.2 3.6 3.5 4.4 5.4 6.0 7.4 7.0 6.5 7.6 8.1 8.0 7.2 5.0 5.1 4.9 5.4 7.2 8.6 3.5 3.3 3.2 4.2
xk.,/ios S-I 5.68 7.17 7.32 7.1 1 8.20 9.16 4.18 5.43 5.66 5.60 6.31 6.81 6.70 9.36 10.7 11.7 12.8 13.3 49.2 54.8 37.9 29.8 28.1 29.8 33.8 33.7 590 670 710 830
moderate accuracy. The maximum error in the nanosecond lifetimes recovered in these experiments is estimated to be f 2 0 ps. An excitation wavelength of 304 nm was used throughout and emission was viewed through a Zeiss M4QIII quartz prism monochromator set at 390 nm.
Results and Discussion The azulene derivatives wed in this work are the same as those which have been used frequently in previous The six solvents were chosen to provide the largest possible measurable variations in the S2-SI and S2-So electronic energy spacings in thistarget group. Ethanol and cyclohexane were included in order to facilitate comparisons with previous work. Solvents containing heavy atoms were excluded. The absorption spectra, S2 So fluorescence quantum yields, and S2lifetimes of each solute were measured in all six solvents; the resulting values of E(S2),E @ , ) , 7(S2), and +f(S2-S0) are summarized in Table I. The two independently purified sources of AZ gave identical results. In the case of DBAZ the SI S, origin was not resolved in the polar solvents, and this compound was therefore examined only in CH and BNZ. The rate constant for the radiative decay of S2,k,, and the sum of the rate constants for all parallel first-order nonradiative decay processes of S2, Ck,,, were calculated using the standard expressions
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k, = 4f/7
(1)
The values of k, Ck, and AE(S2- S,) are also presented in Table I. Our results are compared with those obtained in previous studies in Tables I1 (for cyclohexane) and I11 (for ethanol). The values of E(S2)and E(&) all agree quite well except for the halogenated derivaties. In the latter cases our values agree well with those of Murata et aL4v5but are considerably larger than those of Eber et ale6 Our lifetimes are consistently shorter than those of Eber et a1,6 who used flash-lamp excitation methods, but are in good agreement with those of Griesser and Wild,8-9who used a
7906 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992
Wagner et al.
TABLE Ik Comparison of E x ~ m e n t dand Literature Values for Azulene rad Dcrlvrtives in Cyclohexane 91.
91 compd AZ AZ-ds DCAZ DBAZ GAZ TMAZ
exptl 14.01 14.05 14.35 13.87 13.48 12.33
exptl 0.046 0.072 0.073 0.016 0.018 0.0005
refs 4, 5 14.0 14.3 13.4
72/P
refs 4, 5 0.031 0.058 0.014
exptl 1330 1710 990 200 330 15
ref 9' 1300 1990 1010 270 395
n
k W
89.
v
e m -0 8 7 .
'In 3-methylpentane at 77 K. TABLE IIk Comparison of Experimental and Literature Values for A d e n e and Derivatives in E t b l
compd AZ AZ-ds DCAZ DBAZ GAZ TMAZ
hE(S,-S,)/ io3 cm-l exptl ref 6 13.99 14.0 14.02 14.30 13.0 13.81 12.4 13.57 13.4 12.00 12.3
9f exptl ref 6 0.041 0.031 0.060 0.062 0.058 0.011 0.044 0.017 0.014 0.0005 0.0011
72/PS exptl ref 6 ref 9 1310 1640 1280 1660 1720 880 830 890 310 300 330 2000 340 14 430
851 136
J
1
'4 0
138
14 2
14 4
14.6
148
AE(S2 -S1 )/1 O3 cm-' Figure 1. log (xknr)vs AE(S2-Sl) for the S2 decay of azulene (O), azulene-d, ( 0 ) ,and 1,3-dichloroazulene ( 0 ) in various solvents.
11 0 h
k 2
109
W
mode-locked ion laser excitation system capable of measuring lifetimes longer than ca. 150 ps with a precision of ca. f50 ps. Poor agreement exists between our values and the calculated lifetimes used by Murata et al.4*5and this highlights the error introduced by using only the Strickler-Berg formalism to obtain kr* The quantum yields of S2 So fluorescence measured in the present work are consistently 3040% larger than those obtained in previous ~tudies.4,~ In the case of AZ itself, for example, we obtain a value of #f = 0.041 f 0.002 in 95% ethanol compared with the widely quoted value of 0.031 .4 A recent determination of #f = 0.04 f 0.01 for AZ by a completely independent photoacoustic calorimetry meth0d3Otends to support the present work. The nanosecond S2 fluorescence lifetimes of AZ, AZ-d,, and DCAZ measured in the present work are reproducible to within about 20 p, whereas those for TMAZ, the shortest of the six solutes, are reproducible to within ca. 3 ps. The fluorescence quantum yields of all the target compounds except TMAZ are believed to be accurate to within 5%. The present data therefore represent a considerable improvement over those previouSly used in analyzing the kinetics of the decay of the S2states of AZ and its derivativca in condensed media. This is particularly important because we wish to examine the dependence of xknr on AE for each individual solute by changing only the nature of the solvent. Because the energy gaps vary by only ca. 500 cm-' for a given solute in the six solvents, very precise values of xk,, must be obtained if their correlations with AE are to be meaningful. Potential correlations of x k , or log (xk,) with parameters characterizing the solvent, dielectric constant, refractive index, and solvent polarity function" were examined, but no direct relationship was found. In addition, relatively poor correlations between log ( x k , ) and M ( S 2- S,,) were observed for each solute
W
0
-0m
108
10 7
-
10 6
n
k
I B 9.6
P
Y
W v
o
e
-g
n
0
9.4
921 13 1
I
I
I
13 3
135
137
AE(S2 -SI )/1 O3 cm-' F i p e 2. log (xknr) vs hE(S2-S,) for the Sz decay of 4,6,8-trimethylazulene (A) and guaiazulene (B) in various solvents. Error limits for log (xknr)are shown.
in the six solvents and for all six solutes in the same solvent. On the other hand excellent linear plots of log (xk,,J vs AE(S2 - S,) are found for each of AZ, AZ-d,, and DCAZ in the six solvents employed. These results are displayed in Figure 1. A similar plot for TMAZ (Figure 2A) is apparently also linear, but the values of Ck, are subject to considerably greater uncertainty in this case because of TMAZ's very short S2lifetime (7 < 20 ps), and the correlation is poorer. Even a casual inspection of the data
TABLE I V A ~ l y ~ iof . 9log (E&.,) vs AE Plots for Various Solut&olvent Systems correlation cocff system vs AE(S2-So) vs AE(Sz-S,) AZ in various solvents -0,832 -0.979 AZ-d8 in various solvents -0.859 -0.993 DCAZ in various solvents -0.376 -0.991 TMAZ in various solvents -0.9 10 six azulenes in CH six azulenes in 95% EtOH refs 4, 5 (various azulenes in CH) ref 9 (various azulenes in EtOH)
GAZ
-0.975 -0.980 -0.892
~10pe/10-~ cm -0.40 -0.40
0.40 0.40
-0.51
0.80"
C(Sz-Sl)/cm-' 16 14 116'
0.65* 1.02
47b 205
y(S2-SJ
-0.64 min.
+O. 10 max. -0.97 -0.87 -0.47
'y = 0.80, C = 140 cm-I if correlated with hE(Sz-Tl). See text. bRedetermined for huM= 1580 cm-l using the data of refs 4, 5.
The Journal of Physical Chemistry, Vol. 96, NO. 20, 1992 7907
Decay of the S2 States of Azulene in Table I reveals that there is no correlation between Ck,,or log (E&,)(Figure 2B) and AE(S2 - SI)for GAZ. Data for all the attempted log (Ck,,) vs AE correlations are summarized in Table IV. It is significant that in all four linear log (Ck,,) vs U ( S 2SI)plots the point for PFCH falls squarely on the line with the other five solvents. Perfluoroalkanes are very weakly interacting media and are known from previous studies of electronic relaxation processes in other systems to act solely as classical heat The fact that all six solvents, including PFCH, act similarly provides good evidence that specific solute-solvent interactions do not strongly influence the values of Ck, obtained for AZ, AZ-d8, DCAZ, and perhaps TMAZ in any of the solvents used in this study. Thus the S2-S1 electronic energy gap (or the S2-Tl gap in the case of DCAZ) appears to be the only important variable in the experiments in which xk, is determined for each of these solutes in various solvents. The x k n ,data obtained in this way are particularly amenable to analysis using the theory of radiationless transitions bemuse AE varies without changing the structure of the solute. Englman and Jortnerl' have derived the following relationship between k,, and the electronic energy gap, AE,between the two weakly coupled states:
where (4) Here C is the matrix element for vibronic coupling between the two states, and hwM and AM are the energy and reduced displacement of the accepting vibrational modes, M. Differentiation of the logarithm of (1) with respect to AE yields the equation
-=---(-) d(ln knr) d(AE)
1 2AE
r + l h ~ y
(5)
which allows y to be determined from the measured slopes of log k,, vs AE plots such as Figure 1. The -1/(2AE) term in eq 5 is often n ~ g l e c t e d ,but ~ . ~it is not necessary to do so if A(AE) is small-as is the case when only the solvent is varied in our experiments. We therefore use AE,, (ca. 14000 cm-' for AZ) in our calculations. Once y is obtained, C can be calculated directly from the experimental data for any assumed value of huM Values of C in the 10L103-cm-l range are expected for molecules conforming to the weak-coupling case.33 Griesser and Wildsu9found that perdeuterating azulene had a very small effect on Zk, ( k H / k=~ 1.57 in ethanol) and argued convincingly that this demonstrated that in-plane skeletal C-C, not high-frequency C-H(D), stretching vibrations are the important accepting modes in SI.We c o d " their measurements (our value of kH/kD = 1.30 in ethanol) and agree with their interpretation. We therefore employ Griesser and Wild's value of hw, = 1580 cm-'in the calculation of y and C, with the results shown in Table IV. For AZ, y = 0.40 and C = 16 cm-' whereas for AZ-d8, y = 0.40 and C = 14 cm-I. For DCAZ y = 0.80 and C = 116 cm-l if hwM = 1580 cm-I is also used for this molecule and it is further assumed that the S 2 S Ienergy gap is the appropriateone to employ (is., if the rate constant for S2 SIinternal conversion dominates the sum in Ck,). However, the trend in Ck,,for the series AZ,DCAZ, DBAZ in the same solvent (Table 11) suggests that intersystem crossing plays an increasingly important role in the halogenated with AE(S2-S1) may compounds. The correlation of log (Xk,,,) therefore not be the appropriate choice for DCAZ, and will TIintersystem certainly be inappropriate for DBAZ. If S2 crarphg dominates the radiationless decay of S2in the halogenated compounds, then y = 0.80 and C = 140 cm-' is obtained for DCAZ provided that its S2-Tl energy gap is a.700 cm-l larger than that for S2-S1 (as is the case for azulene itselP4). Fur-
--
--
11.2
TMAZ
n 102-
h
Y
W v
0
m
9
92-
AZ-ds 82L
12 0
Figure 3. log
I
12 5
I
1
130
135
14 0
I
14:
A E ( S ~-SI )/I O3 cm-' for the S2decay of azulene and
(Ck,,) vs AE(S,-S,)
related compounds in cyclohexane. thermore, if y = 0.80, hoM = 1580 cm-'and AE(S2-T1) = 14500 cm-'are adopted for DBAZ, then the data of Table I can be used to calculate C = 285 cm-* for S2 T I in this molecule. The trends in Ckn,and C in the series AZ, DCAZ, DBAZ and the S1 (not S2 various energy gap correlations suggest that S2 So) internal conversion dominates the radiationless decay of S2 in the parent hydrocarbon but gives way to intersystem crossing (likely S2 TI) in the dibromo compound. Both S2 SIand S2 T I may contribute significantly to Ck,,in DCAZ. The much larger values of Ck,,in TMAZ appear to be qualitatively consistent with the expectations of the energy gap law given the substantially smaller S2-SI(or S2-S0) electronic energy spacings in this compound. However a plot of log (Ck,,,) vs AE(S2-SI)for TMAZ (Figure 2A) yields values of y which range from 1.25 to -1.42 and corresponding values of C from 27 vs to 0.1 cm-l for the two extremes of the slope of log (Ck,,,) AE(S2-SI). None of these values is sigdicant and illustrates only that the data are not sufficiently accurate to draw any firm conclusions about the nonradiative relaxation processes in this compound. In the case of GAZ the S2lifetimes are shorter and the values of Ck,,are larger than in AZ itself in the same solvent. Again the fact that AE(S2-SI) (or AE(S2&)) is smaller in GAZ appears to lead to the conclusion that the nonradiative rates are determined by the magnitudes of the Franck-Condon factors for coupling with lower electronic states, in accordance with the energy gap law. However, no correlation of Xk, with any of the electronic energy spacings in GAZ is apparent from the data in Table I (see also Figure 2B). Vibrations in the alkyl substituents and the different symmetry of the carbon skeleton may both play roles in increasing the radiationless relaxation rates in GAZ relative to AZ itself. However, the role of the solvent is unclear in this case and cannot be distinguished from the effects of structural changes in the target solute. Therefore the use of compounds such as GAZ and TMAZ in correlating the S2nonradiative relaxation rates with molecular structure appears to be of doubtful value. Finally we examine the apparent correlations which exist when one plots log (Ck,,) vs AE(S2-SI)for all six solutes in a common solvent, as shown in Figure 3 for cyclohexane. The correlation appears to be good and similar to those obtained in other studies,@ particularly if one considers only the nonhalogenated compounds. An equally good correlation exists when the data from experiments in 95% ethanol are employed. However, both y and C obtained from such plots are strongly influenced by the values of Xk, for TMAZ and GAZ which exhibit the smallest AE(S2-S1)but for which the interpretation of the solvent effects is unclear.
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--
----
--
C~luSiOns Changes in the electronic energy spacings of azulene and related compounds may be introduced by varying the nature of the solvent within a group of solvents which exhibit no specific interactions with the solute. The variations in AE are sufficiently large to
J. Phys. Chem. 1992,96, 7908-7912
7908
permit the resulting changes in the S2 nonradiative rates of the solute to be measured with good accuracy. Correlations of log (Ckm)with AE for single solutes in these solvents reveal that AZ and AZ-d8 behave in a manner consistent with the expectations of the energy gap law of Englman and Jortner, provided that S2 SIinternal conversion is S i s dominant nonradiative relaxation channel. Intersystem crossing dominates in DBAZ, and DCAZ appears to exhibit intermediate behavior in which intersystem crossing and internal conversion occur with comparable rates. TMAZ and particularly GAZ do not behave in a manner which is completely compatible with the expectations of the energy gap law. The use of DBAZ, DCAZ, TMAZ and GAZ and similar heavy-atom and alkyl-substituted compounds in correlating the rates of S2 nonradiative relaxation with S2-Sl energy gap is therefore of doubtful quantitative value. Correlations involving fluorinated Czvazulenes and Hafner’s hydrocarbon^^^ may prove more useful.
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Acknowledgment. We are grateful to Dr. Bernhard Nickel, Max-Planck Institut fiir Biophysikalische Chemie, Gattingen, for supplying purified samples of azulene and azulene-d8. This research was made possible by a grant funded by the Networks of Centers of Excellence Program in association with the Natural Sciences and Engineering Research Council of Canada (NSERCC) and by an NSERCC operating grant. B.D.W. gratefully acknowledges the award of an NSERCC Postdoctoral Fellowship. Reistry NO. AZ, 215-51-4;AZ-da, 127-60-6;DCAZ, 14658-94-7; DBAZ, 14658-95-8; TMAZ, 941-81-1;GAZ, 489-84-9.
References and Notes (1)Kasha, M. Discuss. Faraday SOC.1950,9, 14. (2)Beer, M.; Longuet-Higgins, H. C. J. Chem. Phys. 1955, 23, 1390. (3) Binsch, G.; Heilbronner,E.; Jankow, R.; Schmidt, D. Chem. Phys. Left. 1967,1, 135. (4)Murata, S.; Iwanga, C.; Toda,T.;Kokubun, H. Chem. Phys. Left. 1972,13,101;erratum, 1972,15, 152.
(5) Murata, S.; Iwanga, C.; Toda, T.; Kokubun, H. Ber. Bunsenges. Phys. Chem. 1972,76,1176. (6)Eber, G.;Schneider, S.; DBrr, F. Chem. Phys. Leu. 1977,52, 59. (7)Gillespie, G.D.; Lim, E. C. Chem. Phys. Lett. 1979,63,193. (8) Griesser, H. J.; Wild, U.P. Chem. Phys. 1980,52, 117. (9)Griesser, H. J.; Wild, U. P. J . Phorochem. 1980,12,115. (10)Turro, N. J.; Ramamurthy, V.; Cherry, W.; Farneth, W. Chem. Rev. 1978,78, 126. (1 1) Plummer, 9. F.;AI-Saigh, Z. Y.;Arfan, M. Chem. Phys. Len. 1984, 104,389. (12) Ramamurthy, V.; Steer, R. P. Acc. Chem. Res. 1988,21,380. (13) Sobolewski, A. L.;Prochorow, J. J . Lumin. 1984,31/32,589. (14)Olszowski, A. Chem. Phys. Lett. 1980,73,256. (15)Woudenberg, T. M.; Kulkarni, S. K.; Kenny, J. E. J . Chem. Phys. 1988,89,2789. (16)Wagner, B. D.; Szymanski, M.; Steer, R. P. J . Chem. Phys., submitted. (17)Englman, R.; Jortner, J. Mol. Phys. 1970,18, 145. (18) Strickler, S. J.; Berg, R. A. J . Chem. Phys. 1962,37,814. (19)Meech, S.; Phillips, D. J. Phorochem. 1983,23, 193. (20)McGlynn, S.P.; Azumi, T.; Kinoshita, M.Molecular Spectroscopy of the Triplet Srare; Prentice-Hall: Englewood Cliffs, NJ, 1969. (21) Siebrand, W. J . Chem. Phys. 1967,47,2411. (22) Anderson, Jr. A. G.;Nelson, J. A.; Tazuma, J. J. J. Am. Chem. Soc. 1953,75,4980. (23)Anderson, Jr. A. G.; Steckler, B. M. J . Am. Chem. Soc. 1959,81, 4941. (24)Garst, M. E.; Hochlowski, J.; Douglass 111, J. G.;Sasse, S. J . Chem. Educ. 1983,60,510. (25)Maciejewski, A.; Steer, R. P. Chem. Phys. Left. 1983, 100, 540. (26)Morris, J. V.;Mahaney, M. A.; Humber, J. R. J . Phys. Chem. 1976, 80,969. (27)Birks, J. B.; Dyson, D. J. Proc. R . Soc. London 1963,A275, 135. (28)James, D. R.; Demmer, D. R. M.; Verrall, R. E.; Steer, R. P. Reo. Sci. Instrum. 1983,54, 1121. (29)Szymanski, M.; Maciejewski, A,; Steer, R. P. J. Phys. Chem., submitted. (30)Small, J. R.; Hutchings, J. J.; Small, E. W. Proc. SPIE 1989,1054, 26. (31)Suppan, P. J. Phorochem. Phorobiol. 1990,A50, 293. (32)For a review see: Maciejewski, A. J . Phorochem. Phorobiol. 1990, A51, 87. (33)Hochstrasser, R.; Marzzacco, C. J . Chem. Phys. 1968,49,971. (34)Nickel, B. Chem. Phys. Leu. 1979,68,17. (35)Dhingra, R. C.; Poole, J. A. J . Chem. Phys. 1968,48,4829.
Matrix Isolation Study of the Reactions of Trimethyialumlnum with Ammonla Bruce S . Ault Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221 (Received: May 8, 1992;
In Final Form: July 9, 1992) The initial and subsequent reaction products arising from the merged jet copyrolysis of (CH3)3A1with NH3 followed by trapping into argon matrices have been investigated. When the deposition line was at room temperature, the 1:l complex (CH3)3Al.NH3was isolated in high yield. Assignment to this species was verified by deuterium and I5N isotopic labeling. The infrared spectrum of this adduct was characterized by very intense, sharp absorptions. Overall, the spectra suggest a strongly bound complex, with significant distortion of the AlC3 skeleton away from planarity, and some distortion of the NH3subunit toward planarity. Pyrolysis above 165 “C of a gas-phase mixture of (CH3)3Aland ND3prior to matrix deposition led to the formation and isolation of CH3D. No other products were trapped in the matrix, probably due to decomposition of these species on the walls of the high-temperature deposition line prior to reaching the matrix surface.
Introduction Thin films of aluminum nitride, A N , have become increasingly important in recent years as a consequence of the excellent electronic, thermal, and mechanical properties of this material.I4 These films may be produced by a range of techniques, including chemical vapor deposition (CVD) and laser irradiation. A range of precursors have been employed; trimethylaluminum is one common source of aluminum while ammonia and related amines may supply nitrogen for the product. Organoaluminum compounds are known5 to form 1:l molecular adducts with ammonia and amines; thermolysis of the adducts has been shown to lead to AlN with carbon While these reactions are clearly of importance in the CVD of AlN, little is known about the mechanism leading to thin-film
formation. Interrante and co-workersss9have examined the solution-phase thermolysis reactions of (CH3)3Aland NH3 by solution calorimetry, DSC, and proton NMR. They were able to identify the trimer ((CH3)2Al.NH2)3and obtained some evidence for the intermediacy of monomeric (CH3)2AlNH2,the CH, elimination product from the adduct. They postulated a detailed set of reactions in solution leading ultimately to A1N. The matrix isolation techniquelWl2was developed for the isolation and characterization of reactive intermediate species and has recently been applied to the study of systems relevant to CVD. Adducts such as (CH3),Ga.AsH3 and (CH3)3Ga.SbH3have been studied,I3J4,as well as H2 and methane elimination products,’~” such as H 2 m H 3 and H2BSH. The recently developed merged jet deposition techniqueI5is particularly effective in modeling the
0 1992 American Chemical Society 0022-3654/92/2096-790~~03.00~0
