J. Phys. Chem. 1987, 91, 6173-6177
6173
Temperature Dependence of the Trlplet Lifetimes of Methyipyrazines. Proximity Effect Seigo Yamauchi,* KO Mibu, Yasuo Komada, and Noboru Hirota* Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan (Received: April 2, 1987; In Final Form: June 17, 1987)
Temperature dependence of the decay rate constants and sublevel properties of the triplet states of 2-methylpyrazine (MP) and 2,6-dimethylpyrazine (DMP) are investigated in order to assess the importance of the proximity effect on the triplet properties. It is shown that the proximity effect on the triplet properties is not clearly observed at 1.4 K. However, very drastic temperature dependence was observed for the decay rate constants of the systems with nearby 3n7r* and %a* states. It is found empirically that the temperature dependence is approximated by the sum of two Arrhenius type equations with very different activation energies, AEl and AE2. The decay rate constants of the states with excess energies AEl and AE2 are estimated to be in the order of lo8 and lo3 s-l, respectively. AEI decreases in the order pyrazine, MP, and DMP, following the order of the location of the T2state. It is concluded that the increase of the decay rate is brought by a strong Tl(n7r*)-T2(7r7r*) mixing due to the proximity of these states.
1. Introduction Nitrogen heterocyclic and aromatic carbonyl compounds are known to possess two nearby electronic states (3n7r* and 37r7r*).1 The effects produced by strong vibronic interaction between these nearby states on excited-state properties have been a subject of much interest in the past two decades2 This effect, termed the “proximity effect” by Lim, is now considered to manifest itself in a wide variety of phenomena related to excited-state dynamics such as substituent and hydrogen-bonding effects on luminescence properties in condensed phase3g4and rapid internal conversion in gas-phase molecule^,^^^ though unequivocal evidence for the presence of this effect is not so easy to obtain. One of the oftenquoted examples of the proximity effect is a systematic change of the triplet lifetime of 3n7r* pyrazine caused by methyl sub~titution.~ Madej et al. reported that the lifetime of 3n7r* pyrazine is remarkably shortened by methyl substitution: 18.5 ms in pyrazine, 6.8 ms in 2-methylpyrazine (MP), and 0.8 ms in 2,6-dimethylpyrazine (DMP) in a methylcyclohexane (MCH) polycrystal a t 77 Ke3 From the measurement of the quantum yield of phosphorescence they confirmed that the shortening is due to the increased radiationless decay rates. It was considered that closer proximity of the 3n7r*(T1)and 37r7r*(T2)states in M P and D M P results in the distortion of the potential surface of the T1 state via vibronic coulping, producing an increased Franck-Condon factor for the radiationless decay. They further provided spectroscopic observations which were taken as supporting evidence for the presence of the proximity effect3 On the other hand, we have recently found’ that the triplet decay rate constant of pyrazine in a rigid medium increases drastically at higher temperatures with an apparent activation energy of 1300-2700 cm-I depending on the solvent. Since this energy is considered to be close to the 3n7r*-37r7r* separation in pyrazine, it was suggested that the increased radiationless decay rates at higher vibrational levels of the T, state caused by the proximity effect are responsible for this temperature dependence. It is then expected that the decay rate constants of methylpyrazines with smaller 3n7r*-3~7r*separations are even more temperature dependent, and entire temperature dependence should be known
to correctly discuss the proximity effect in these systems. Accordingly, we have investigated the decay properties of M P and DMP in a variety of rigid matrices over a wide range of temperature. In the present work we first tried to find rigid matrices in which M P and DMP give sharp phosphorescence spectra. We then studied phosphorescence emission and excitation spectra and temperature dependence of the triplet decay rate constants in these systems. It was found that the decay rate constants are remarkably temperature dependent, even at temperatures lower than 77 K in some cases of M P and in most cases of DMP. We have also performed zero-field ODMR (optically detected magnetic resonance) experiments to determine the zero-field splittings (zfs) and sublevel decay rate constants at 1.4 K which provide relevant information about the nature of the zeroth vibrational level of the T1 state. We compare the obtained results with that of 3n7r* pyrazine and discuss how the proximity effect affects the triplet properties of methylpyrazines.
2. Experimental Section
(1) Lim, E. C. In Excited Stares; Lim, E. C . , Ed.; Academic: New York, 1977; Vol. 3, p 305.
M P was purified by distillation. DMP was purified by vacuum sublimation. Spectroscopic grade solvents (ethanol, n-hexane, cyclohexane, and methylcyclohexane) were used without further purification. Other solvents were purified by distillation. Concentrations of the solutions were about M. Temperature was controlled by an Oxford C F 204 continuous flow cryostat. When the phosphorescence spectra were obtained, samples were excited by a 900-W Xe arc lamp through a NiS04 solution filter and an UVD33S glass filter. The emission spectra were obtained with a Spex 1704 1-m monochromator equipped with an EM1 9502B photomultiplier. The excitation spectra were obtained by monitoring total phosphorescence using the xenon arc lamp with the Spex monochromator as an excitation source. In the lifetime measurements irradiation of a sample was made with an excimer laser (Lumonics TE-861M, XeC1, X = 308 nm, pulse width -14 ns). The lifetimes were measured at the 0-0 bands of the spectra by using a transient memory (Kawasaki Electronica MRSOE) and an averager (Kawasaki Electronica TM 700). ODMR and MIDP (microwave-induced delayed phosphorescence) experiments were performed at 1.4 K following the procedures already described elsewhere.8
(2) (a) Lim, E. C. In Molecular Luminescence; Lim, E. C., Ed.; Benjamin: New York, 1969; p 469. (b) Lim, E. C. J. Phys. Chem. 1986, 90, 6770 and references therein. (3) (a) Madej, S. L.; Okajima, S.; Lim, E. C. J . Chem. Phys. 1976, 65, 1219. (b) Huber, J. R.;Mahaney, M.; Morris, J. V. Chem. Phys. 1976, 16, 329. (c) Kanamaru, N.; Lim, E. C. J . Chem. Phys. 1975,62, 3252; 1976, 65,4055. (d) Wassam, W. A,; Lim, E. C. Chem. Phys. Lett. 1979,38,217. (4) Madej, S. L.; Gillispie, G. D.; Lim, E. C. Chem. Phys. 1978, 32, 1. (5) Okajima, S.; Lim, E. C. J. Chem. Phys. 1978, 69, 1929. (6) Forch, B. E.; Okajima, S.; Lim, E. C. Chem. Phys. Left. 1984, 108,
3. Results 3.1. Phosphorescence Emission and Excitation Spectra. We have examined about ten different solvent systems to obtain well-resolved phosphorescence spectra and succeeded in several systems. Representative examples are shown in Figure 1. The main vibronic bands observed are due to v5, v6a and/or V6b, vga, and vlOamodes, which make the phosphorescence spectra similar
311. (7) Terazima, M.; Yamauchi, S.; Hirota, N. J . Phys. Chem. 1986, 90, 4294.
(8) Yamauchi, S.; Ueno, T.; Hirota, N. Mol. Phys. 1982, 47, 1333.
0022-365418712091-6173$01.50/0
0 1987 American Chemical Society
6174
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
a
I'
Yamauchi et al. TABLE II: Assignment and Intensities of the Phosphorescence Spectra of Various Systems
MP/ MCH Y5
2 X YS ~ 4
1
2X
0
~
vlOa
MP/ cyclohexane
0.03a (751)b 0.05 (1498) 0.01 (929) 0.11 (1857)
0.02 0.06 0.01 0.09
(754) (1504) (933) (1863)
us
and uI0. Bands in the
DMP/ n-pentane 0.04 (756) 0.05 (1500) c
0.07 (1870)
DMP/ m-xylene 0.05 0.04 0.02 0.05
(753) (1507) (939) (1880)
l 5 a Relative intensity; Iwv/i,,+ Vibrational frequencies (in cm-I) obtained from the phosphorescence spectra at 4.2 K. CNotobserved.
a ,-. .8
b
4
I
L
375
Figure 1. Phosphorescence spectra of (a) 2-methylpyrazine in cyclohexane and (b) 2,6-dimethylpyrazine in n-pentane at 4.2 K. (See Table I for the vibrational analyses.) TABLE I: Vibrational Analyses of the Phosphorescence Spectra
wavelength/ wavenumber/ A cm-I 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Av
intensity
I 8 9 10
11 12 13 14 15 16
:1
assignt 18
*
3762.1 3781.3 3836.3 3847.1 3864.2 3891.1 3921.6 3929.7 3933.0 3941.1 3949.0 3979.6 4020.6 4037.3
26 523 26 438 26 059 25 986 25 879 25 692 25493 25440 25419 25 366 25316 25121 24865 24762
52 187 566 639 754 933 1132 1185 1206 1259 1309 1504 1760 1863
0.07 0.05 0.23 0.18 0.02 0.01 0.03 0.09 0.04 0.03 0.09 0.06 0.03 0.09
3764.8 3791.8 3808.0 3845.3 3869.9 3873.7 3915.8 3922.2 3929.4 3938.6 3943.2 3985.6 3990.2 4026.5 4031.2 4050.0
26 554 26 523 26 253 25 998 25 833 25 808 25530 25489 25442 25383 25353 25083 25054 24828 24800 24684
0 189 301 556 721 746 1024 1065 1112 1171
1201 1471
I500 1726 1754 1870
1 0.05 0.03 0.27 0.07 0.04 0.02 0.05 0.05 0.07 0.09 0.06 0.05 0.03 0.03 0.07
370
b
(a) , , 2-Methylpyrazine/Cyclohexane 1 0-0 26 625 ' 0 3754.8
phonon Ph-CH3 tor v6a
rfng
Y6b
ring
I
370
Yj
V,Oa
2 x Y6a v9, H bend V6a
+ v6b
v i 4 ring v3 H bend
2X 2x 2X
VS u6a
+ V6b
YIO~
(b) 2,6-Dimethylpyrazine/n-Pentane 1 2 3 4 5 6
+nm
0-0
Ph-CH3 tor VIS v6a
wI ~j
VI*
? 2X
vba
~9,
Y* VI
+ 85 V6a + Uga 2
X YS
Y6a
+ Y2
2 X ulOa
to that of pyrazine. We&raolved phosphorescence spectra of M p and DMP in M C H were already reported by Madej et aL3 The MP spectrum in cyclohexane shown in Figure l a consists of a sharp component and a broad one which presumably arise from different sites. The sharp component is very similar to that in M C H reported by Madej et al. The DMP spectrum obtained in n-pentane consists of only a sharp component. Vibrational analyses
I
+n m
360
Figure 2. Phosphorescence excitation spectra of (a) 2-methylpyrazine in a methylcyclohexane polycrystal and (b) 2,6-dimethylpyrazinein npentane at 4.2 K. The vibrational analysis was made by referring to ref 10.
of the spectra were made by referring to the IR and Raman data9 and are given in Table I. In Table I1 intensities of the v5 and vlOa bands obtained in several systems are compared. The phosphorescence spectra were broadened at higher temperatures, but the spectral features remained unchanged. The phosphorescence excitation spectra of M P and DMP were obtained in the systems which gave rise to well-resolved singleemission spectra. The spectra are shown in Figure 2. The assignment of the vibrations was made in reference to that for pyrazine by Nishi et a1.I0 These excitation spectra show a striking contrast to that of pyrazine in which vibronic bands up to 01 160-cm-' bands are not significantly broadened compared with the 0-0 band. On the other hand, in the M P and DMP spectra u6a bands (0-539 and 0-531 cm-') are already broadened (30 cm-I for M P and 18 cm-' for DMP) compared with those (7 cm-') of the 0-0 bands. Onset of such broadenings of the So T1 absorption spectra was taken as an indication of the presence of the T, state by Hochstrasser and Marzzacco." The spectra given in Figure 2 clearly indicate closer proximity of the T2 states in these systems. The fact that uk. bands are already broadened seems [O show that significant 3n&-37r.rr* mixing is aiready taking
-
(9) (a) Watanabe, T.; Shimada, H.; Shimada, R. Bull. Chem. SOC.Jpn. 1982,55,2564. (b) Ishibashi, Y.; Arakawa, F.; Simada, H.; Shimada, R. Zbid. 19833 56+ 1327. (10) Nishi, N.; Kinoshita, M.; Nakayama, T.; Shimada, R.; Kanda, Y. ~phys, 1 1977, , 33, 3 1 , (1 1 ) Hochstrasser, R. M.; Marzzacco, C. J . Chem. Phys. 1968, 49, 971.
~
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 6175
Triplet Lifetimes of Methylpyrazines L
x)x)o
60 40
T/K
log(& )k,l-
20
4
log(Ws-1
3
bR
-
-*-----.
2
1
1
I/T.K-1
0.05
k ( T ) = ko
0.02
/T.K-~
+ kl exp(-AEl/kT)
+ kl exp(-AE,/kT) + kz exp(-AE,/kT)
m-xylene (0). TABLE III: Relevant Decay Rate Constants and Energy Differences to Temperature Dependence of the Triplet Lifetimes ko, k , , k2/s-l AE,, AE,/cm-' MP/MCH(PC)~ ko = 66.1 k2 = 1.59 X lo3 AE2 = 150 ko = 86.1 MP/n-pentane kl = 2.0 X lo8 AEl = 1070 k2 = 1.2 x 103 AE2 = 150 MP/cyclohexane ko 86.1 k , = 5.9 X lo8 AEl = 1170 k, = 1.4 x 103 AE2 = 180
DMP/n-pentane DMP/MCH(glass) site I
site I1 DMP/octane site I site I1 DMP/m-xylene
(1)
Here k ( T ) is the decay rate constant ( 1 / ~ at ) temperature T and ko is the low-temperature limiting value of k(T). In the present systems, however, this equation cannot reproduce the observed temperature dependences well. In most systems they are better approximated by the sum of two Arrhenius type equations
(2)
as seen from the log (k( T ) - ko) vs 1 / T plots given in Figure 4. ko, k , , k2, AEl, and AE2 obtained for various systems are summarized in Table I11 together with the results for pyrazine in which the third term of eq 2 is negligibly small. AEz was found to be 150-180 cm-l for M P and 100-130 cm-I for DMP, and AE, is 1100 cm-I for M P and 600 cm-I for DMP except in m-xylene. If we compare these values with AEl = 1300-2700 cm-l for pyrazine,' we notice that MIdecreases in the order pyrazine, MP, and DMP. In the cases of most DMP systems eq 2 can reproduce the temperature dependence well about 15 K, but k still decreases slightly at lower temperatures and deviations from the predicted values become noticeable.
-
,
,
Figure 4. Plots of log ( k - ko) vs 1 / T 2methylpyrazine in n-pentane (U) and cyclohexane (0);2,6-dimethylpyrazinein n-pentane ( 0 )and
O.L,3
place at levels about 500 cm-I from the zeroth vibrational level. It is notable that in the DMP/n-pentane system only the 0531-cm-I ( v ~and ~ ) 0-1069-cm-I (vga) bands were observed in the excitation spectrum, which may indicate that other vibronic bands of lower frequencies are more broadened. 3.2. Temperature Dependence of the Triplet Decay. It was found that in all the systems examined here the triplet lifetimes (7)and the phosphorescence intensities of M P and DMP are remarkably temperature dependent. At higher temperatures the lifetimes become shorter accompanied with the weaker intensities, which indicates that the increased radiationless decay is mainly responsible for the lifetime shortenings. The decay curves were always found to be expressed by single exponentials. Some typical examples of the temperature dependence are shown in Figure 3. Though all the systems show shortenings of the lifetimes at higher temperatures, temperature dependence is rather dependent on the solvent. For example, temperature dependence of DMP/n-pentane is quite different from that of DMP/m-xylene as shown in Figure 3. As reported in the previous paper,' temperature dependence of the triplet decay of pyrazine is well-approximated by an Arrhenius type equation
-
0.01
1
0.025
Figure 3. Temperature dependence of the triplet decay rate constants in various systems: 2methylpyrazine in n-pentane (U) and cyclohexane (0); 2,6-dimethylpyrazinein n-pentane ( 0 )and m-xylene (0).
k ( T ) = ko
I
'
P/cyclohexaneb P/ benzeneb
ko = 240 k l = 2.8 x k 2 = 9.6 x ko = 63 k , = 1.2 X k2 = 8.0 X krJ = 200 k , = 5.2 x ko = 200 k , = 3.4 X ko = 50 k2 = 9.7 X ko = 48.7 k , = 2.2 X ko = kl = ko = k, =
107 103
AE, = 530
lo7 lo2
AE1 = 630
107
AE1 = 600
lo3
AE2 = 100
lo2
AE, = 130
10"
AE, = 1570
54 1.8 x 10'0 56 1.1 x 1013
AE2 = 100
AE, = 100
AE, = 1750 AE, = 2620
Polycrystalline form. *Reference7. 3.3. ODMR and MIDP Results. Two ODMR transitions were observed at zero field in most systems as in pyrazine. The zfs were determined by assuming the sublevel scheme similar to that in pyrazine, T, > T, > T, from the top with the separation between T, and T, being very small (Figure 5). The determined D values (D= -3/zY) are 8.6-9.7 GHz for M P and 7.4-8.9 GHz for DMP, which are somewhat smaller than that (9.0-10.5 GHz) of pyrazineI0J2and are dependent on the environment as shown in Table IV. The sublevel decay rate constants (k,, k,, and k,) were determined at 1.4 K by the MIDP method. The results are given in Table IV. The obtained decay rate constants are very aniso(12) Burland,
D.M.; Schmidt, J. Mol. Phys. 1971, 22,
191.
Yamauchi et al.
6176 The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
In contrast to the emission spectra, the phosphorescence excitation spectra clearly indicate the importance of vibronic mixing in the relatively low energy regions ( G O O cm-') of T I M P and DMP. This is indicative of the presence of the T2states in these regions. 4.2. ODMR Results and Proximity Effect. When the TI state is mixed with the T2state by vibronic interaction, a wave function of the mixed state is approximated by a linear combination of the zero-order wave functions of the pure 3 n ~ and * 37r7r* s t a t e ~ l ~ , ~ ~
*
I
P A
0.5
1
Figure 5. Schematic drawing for the change of zfs as a function of
3n7r*-37r7r*mixing coefficient, p.
TABLE I V Static and Dvnamic Prowrties of the Triplet Sublevels T1.41
ms
+
= CY*", p*,, (3) where a! and /3 are the mixing coefficients. The zfs ( X , Y , Z )of the mixed state is given by
T,(nn*)
kzl
kxl
kyl
s-l
SKI
s-l
T, - Tyl
GHz
TI
- Tyl
GHz
8.73 8.79 MP/n-pentane 14.5 8.76 MP/MCH(pc)c 15.7 10.7 11.8 168 8.69 8.42" MP/MCH(glass) 20.9 7.32 6.09 130 8.11 8.97 MP/EtOH 19.2 DMP/n-pentane 8.7 11.2" 323 8.10 DMP/n-hexane 20.3 3.68 3.90 140 7.92 5.04 258 7.35 7.52 DMPln-heptane 11.0 10 DMPln-octane 19.4 5.45" 144 7.4 7.5 DMP/m-xyleneb 20.5 1.63 1.41 143 8.77 8.94 "Not separated into two components. bShinmori, K.; Fukuda, K.; Nishi, N.; Kinoshita, M. Absrracrs of Papers, Symposium on Molecular Structure and Electronic State, Tokyo, Japan, 1974; p 113. Polycrystalline form.
tropic in all cases; for instance, k, = 11.8 S-I, k,, = 168 s-', and k, = 10.7 in M P / M C H and k, = 3.90 s-l, k,, = 140 s-l, and k, = 3.68 in DMP/n-hexane. These values represent the decay rate constants of the zeroth vibrational levels of the TI states. The average lifetimes at 1.4 K (71.4 = 3(k, ky k J 1 ) are also listed in Table IV. It is clearly seen that TI,^ of M P and DMP are not so different from 71.4 (-20 ms) of pyrazine. The smallest 71.4 are 14.5 and 8.7 ms for M P and DMP, respectively.
+ +
4. Discussion
The most important observation made in this work is that the triplet lifetimes of M P and DMP are extremely temperature dependent and 71.4 and T,, of some systems are very different. Lim and co-workers discussed the proximity effect using 777obtained in MCH,3 but the present results shows that these values are not appropriate to discuss the proximity effect on the zeroth vibrational ~ MP, and DMP are level of the TI state. In fact, T ~of ,pyrazine, not so different from each other, though there are some systems in which 7 1 . 4 are somewhat smaller (DMP/n-pentane, etc.; see Table IV). This result indicates that the proximity effect does not show up clearly in the lifetimes of the T1states at 1.4 K. Here we carefully examine whether or not our results are indicative of the presence of the proximity effect. 4.1. Phosphorescence Emission and Excitation Spectra. Madej et al. noted in the phosphorescence spectrum of DMP/MCH relatively high intensities of the 0-747-cm-' ( v 5 )and 0-935-cm-' (vloa) bands compared with those of their second harmonic bands, where v5 and vloa are one quanta of hydrogen out-of-plane bending modes.3 This observation was taken as evidence for the distortion of the potential surface by the proximity effect. In the present study, however, the spectrum in Figure l b and the data in Tables Ib and I1 show that the intensities of these bands are also weak in DMP/n-pentane, though q7(= 0.28 ms) of this system is smaller than T~~ (= 0.8 ms) of DMP/MCH. In fact, in all of our M P and DMP systems intensities of the v5 and viOa bands are rather weak. Thus, our phosphorescence spectra did not give any indication of the distortion of the potential surface of the TI state of DMP. This result is consistent with the fact that 71.4 of our systems are not so different from that of pyrazine.
and similar equations for Y and Z . Here X,, and X,,= are the values for pure n?r* and ira* states, respectively. In the case of 3na* pyrazine T, is the top and T,, the bottom,I2 while in 37r7r* s-tetramethylpyrazine T, is the top and T, the bottom.l5 When the contribution of the spin-orbit coupling to zfs can be neglected, zfs is expected to change with the extent of mixing as depicted in Figure 5. Such a systematic change of zfs was indeed found in the case of phthalazine whose 3n7r* character changes depending on the solvent.I6 If M P and D M P have mixed character, their zfs should be smaller than that of pyrazine. In fact, zfs of M P and DMP are generally smaller than that of pyrazine, which may seem to indicate mixed character of these systems. However, zfs can be decreased by methyl substitution when r* spin densities on the nitrogen (N) atoms are reduced by inductive and mesomeric effects of the methyl groups and the one-center integrals on N for the spinspin interaction are reduced. If the state mixing is the main cause for the decrease of the zfs and the lifetime is shortened by the proximity effect, there should be a correlation between the zfs and the triplet lifetime. As the result given in Table IV shows, there is no clear correlation between the zfs and the triplet lifetime. Therefore, it is likely that the substituent effect on the spin distribution is the main cause for the decrease of the zfs in the T I states of M P and DMP and the proximity effect is not important in the zeroth vibrational level of the T i state. We next examine the sublevel decay rate constants. In nearly planar 3n7r* pyrazine, sublevel decay is highly selective, T,, being much more active in radiative and nonradiative decays with k,, = ~ 3 0 k , . If~ a~ molecule is distorted in the direction of the out-of-plane vibration as expected in the case of double-minimum potential produced by n r * - m * interaction, the decay should become less selective with a higher decay activity of the T, sublevel. The MIDP result (Table IV) shows that the decays are still very selective in M P and DMP; for example, ky = 323 s-' and k, = 11 s-l for DMP/n-pentane. Therefore, it is concluded that there is no serious distortion of the geometry in the zeroth vibrational level of the TI state. 4.3. Temperature Dependence of the Decay Rate Constants and the Proximity Effect. Though the decay rate constants and the ODMR data obtained at 1.4 K did not provide any clear evidence of the proximity effect, temperature dependence of the decay rate constants does reveal the proximity effect clearly. The data summarized in Table I11 show that the decay rate constants of the higher vibrational levels of the TI state increase dramatically. The decay rate constant k( 7') is given by Jmk(AE) p(AE) exp(-AE/kT) dAE k(T) =
(5) J-p(AE)
exp(-AE/kT) dAE
Here k ( U ) is the decay rate constant of a state with a vibrational (13) Lim, E. C.; Stanislaus, J. J . Chem. Phys. 1970, 53, 2096. (14) Li, Y. H.; Lim, E. C. J . Chem. Phys. 1972, 56, 1004. ( 1 5 ) De Grcot, M. S.; Hesselman, I. A. M.; Reinders, F. J.; van der Waals, J. H. Mol. Phys. 1975, 29, 3 1 . (16) Terazima, M.; Yamauchi, S.; Hirota, N. J . Chem. Phys. 1985, 83, 3234.
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 6177
Triplet Lifetimes of Methylpyrazines
I P
MP
1
01
7 I
DMP
"
"
i
I
1000
'
"
" 2000
'
Figure 6. Schematic drawing for the excess energy dependence of the decay rate constants of the vibrational levels of the T, states of pyrazine and methylpyrazines.
energy of AE. p ( A E ) is the density of the states. In the present systems temperature dependence of the decay rate constant is mainly due to the change in the radiationless decay as mentioned in section 3.2. This is expected because the radiative decay rate does not change much by vibronic coupling with the T2(7r7r*) state. Then the tempeature dependence of k(T) represents that of the radiationless decay. Starting from the golden rule expression, Lin has previously obtained an expression for the temperature dependence of the radiationless decay, which is given by a sum of Arrhenius type expression"
Such an equation is derived easily from eq 5 by assuming p ( A E ) = Ci6(AE-AEi), where 6 represents the delta function. Since it is impossible to discuss the obtained temperature dependence of k(T) quantitatively using eq 5 , we discuss the qualitative feature of the temperature dependence on the basis of eq 2 and in analogy with the case of pyrazine discussed in the previous paper.' It was concluded that the observed Arrhenius type behavior of the temperature dependence in pyrazine is due to an abrupt increase of k ( M )at AE 2000 cm-' as schematically shown in Figure 6. The fact that the temperature dependences in the present systems are approximated by the sum of two Arrhenius type equations with very different activation energies, AEl and AE2, seems to suggest that k(AE) increases in a stepwise manner at these energies. The data given in Table I11 show that in M P the levels with vibrational energies of 150-200 cm-' (AE2) or higher have decay rate constants of lo3 S-I and those with 1100 cm-' (MI)have -loB s-l. MIand AE2 for DMP (with the exception of DMP/m-xylene) are 500-600 and 100-1 30 cm-I, respectively. Then the qualitative feature of the excess energy dependence of the triplet decay rate constants of methylpyrazines is given schematically as shown in Figure 6. In M P and DMP shortenings of the lifetimes take place in the states with even very low vibrational energies (100-200 cm-I).
-
(17) Lin, S.H. J . Chem. Phys. 1972, 56,2648.
-
-
AE/cm"
-
Presumably strong vibronic mixing of the 3n7r* and %7r* states already takes place at low vibrational levels because of the close proximity of the 3 n ~ and * 37r7r* states. Preliminary results of our time-resolved EPR study showed large temperature dependence of the zfs in DMP/n-pentane at relatively low temperatures, which indicates that the levels with vibrational energies of 150 cm-' have strongly mixed character of 3n7r* and 3 7 r ~ *states, supporting the above argument. The levels with excess energies of -600 cm-l in DMP and 1100 cm-' in M P have extremely large decay rate constants as those with -2000 cm-' in pyrazine. The fact that AEl decreases on going from pyrazine to DMP is consistent with the red shift of the T 2 ( m * ) state by methyl substitution as revealed in the excitation spectra. Therefore, our results clearly demonstrate the importance of the proximity effect in enhancing the decay rate constants of the higher vibrational levels of the TI state. Though the exact locations of the T2origins are not known, 600 cm-I for DMP and 1100 cm-' for M P appear to be higher than the Tl-T2 separations in these systems. A very drastic increase of the decay rate constant caused by strong vibronic mixing seems to occur when the vibrational levels of the T, state become somewhat higher than the origins of the T2state. A correlation between AEl and k,, namely, the result that larger AE,is accompanied by larger k , found in the pyrazine systems, was again observed in methylpyrazines. Stronger vibronic coupling and larger density of states in a region of a large excess energy (AE,) may result in larger k l . Finally, we comment on the case of DMP/m-xylene in which the temperature dependence of the decay rate is very different from the other DMP systems, being similar to that in pyrazine. It is well-known that the n7r*-m* separation varies considerably depending on the environment. In pyrazine AEl was found to change from 1750 cm-l in cyclohexane to 2650 cm-I in benzene. Likewise, the different behavior found for DMP/m-xylene is due to the large energy separation (AE, = 1570 cm-I) between the 3 n ~and * %7r* states in this system. This result also supports the conclusion that the temperature dependence observed in the methylpyrazine systems is due to the proximity of the 3 n ~ and * 3 ~ states ~ *and excludes the possibility that it is due to the specific effect of the methyl group.
-
5. Concluding Remarks It is shown that the proximity effect on the triplet properties of methylpyrazines is not clearly observed at 1.4 K. However, there is a very dramatic proximity effect on the decay rate constants of the higher levels of the TI state manifested in the temperature dependence of the triplet decay. Our previous suggestion that the excess energy dependence of the triplet decay rate constant of pyrazine in a rigid matrix is brought by strong 3n7r*-3~7r* interaction is further substantiated by the present results. On the basis of the present results we can also exclude the possibility that a 3Bl,(n~*)state is involved in the fast radiationless decay of the higher vibrational states,' because the 3Bl,(n7r*) state is not expected to be red-shifted by methyl substitution. We believe that the present results together with that of pyrazine provide a clear example of the proximity effect on the triplet dynamics. Registry No. MP, 109-08-0; DMP, 108-50-9; P, 290-37-9.
