J. Phys. Chem. 1994, 98, 7963-7966
7963
Temperature and Solvent Dependences of the Zero-Field Splittings and Character of the Triplet States of Methylpyrazines Tosihiro Kamei, Masahide Terazima, Seigo Yamauchi, and Noboru Hirota’ Department of Chemistry, Faculty of Science, Kyoto University, Kyoto, 606 Japan Received: February 14, 1994; In Final Form: May 6, 1994”
Temperature and solvent dependences of the zero-field splittings (zfs) of the excited triplet states of methylpyrazines are investigated. There seems to be a correlation between the temperature dependence of the zfs and that of the triplet decay. The zfs varies markedly with temperature in the systems in which the triplet decay rate constants increase rapidly at relatively low temperatures. A detailed study of the temperature dependence of the zfs of 2,6-dimethylpyrazine (DMP) indicates the presence of a 31r1r* state in the vicinity of the Tl(n?r*) state. Solvent dependence of the zfs was studied in the case of DMP and trimethylpyrazine(TMP). In nonpolar solvents the TI state of DMP is 3n1r* in character, but in T M P the TI state has dominant m* character.
Introduction Vibronic mixing between two nearby electronic states and its influenceon spectroscopicproperties have continued to be a topic of considerableinterest in recent years.’ Spectroscopicproperties of nitrogen heterocyclics and aromatic carbonyls with nearby 3 n ~ and * 3 m * states are particularly interesting in this regard. A large increase in the radiationless decay rate due to thevibronic coupling which is termed the “proximity effect”has been a subject of extensive investigations.2 Since the energy separation between the T1(3n~*)and T2( 3 m r * ) states of pyrazine can be varied systematically by methyl substitution,pyrazine and methylpyrazines are convenientsystems to study the “proximity effect”. The effect of the methyl substitution on the radiationlessdecay rate of the T l S o transition has been studied by both solid state- and supersonic jet’s8 spectroscopies. Madej et al. found a significant decrease in the phosphorescencelifetime and quantum yield caused by the methyl sub~titution.~ Sneh and co-workers recently measured the decay rate constants of various vibrational levels of the triplet pyrazine and 2-methylpyrazine(MP) in supersonicjets and examined the excess energy dependence of the T l S o radiationless They found that the decay rate constants increase dramatically with excess energy in both pyrazine and MP, but the increase starts at a much lower excess energy in MP than in pyrazine. This was explained in the frameworkof the “proximity effect” by which the frequency of the accepting mode (via) is greatly reduced in the T1 state.8 In previous works we have studied the temperaturedependence of the decay rate constants of triplet pyrazine, 2-methylpyrazine (MP), and 2,6-dimethylpyrazine (DMP) in a variety of rigid matrices.5~6It was found that the decay rate constants at 4.2 K are not so much different, but they differ tremendously at higher temperatures. In pyrazine the temperaturedependence was found to follow an Arrhenius type equation with an apparent activation energy of 1500-2000 cm-l.S On the other hand, in MP and DMP the decay rate constants were expressed by the sums of two exponential functions with much lower activation energies6 On the basis of these observations it was suggested that a new channel for radiationless decay opens up at states nearly isoenergetic with the Tz(?rs*) states. Sobolewski, Lim, and Siebrand9 suggested that these observations can be rationalized by invoking a fast radiationless decay from the T 2 ( 3 ~ ~state * ) which is strongly distorted by valence instability. It is known that the zero-field splitting (zfs) of the triplet states of some nitrogen heterocyclicsis sensitiveto the n?r*--rrr*mixing.10
* Abstract published in Aduance ACS Abstracts, July 15, 1994.
For example, the zfs of phthalazine varies remarkably with the nature of the host as well as with temperature, reflecting the extent of the vibronic mixing and thermal averaging of the n?r* and UT* states. Therefore, it is thought that we can obtain information about the nature of higher triplet states from the temperature dependence of zfs. Time-resolved EPR (TREPR) spectroscopy provides a convenient means to study zfs of shortlived triplet states. In the present work we have examined the zfs of triplet methylpyrazines in a variety of rigid matrices at different temperatures, paying attention to the correlation between the zfs and the triplet decay rate constant in the systems whose triplet decay rate constants were studied previously.6 We have found remarkable temperature dependence of the zfs in the DMP/npentane and MP/methylcyclohexane (MCH) systems which show large increases in the decay rate constants at relatively low temperatures. From the analysis of the temperature dependence of the zfs we discuss the character of the triplet states that are thermally populated at higher temperatures. We also discuss the characters of the T1 states of DMP and trimethylpyrazine (TMP) in different matrices. Experimental Section The experimental setup for the TREPR measurements is essentially the same as that reported previously,ll and only a brief description is given here. A sample placed in an EPR cavity was irradiated with a Lumonics TE861M excimer laser (KrF, A = 248 nm, 100 mJ/pulse) or a Lumonics Hyper EX400 excimer laser (XeCI, A = 308 nm, 200 mJ/pulse), and transient EPR signals were detected by using a modified microwave unit of a JEOL FE-3X spectrometer without field modulation. The transient EPR signals at 0.4-1.0 ps after the laser irradiation were fed toa boxcar integrator (PAR Model 160). Themicrowave power was about 10 mW for all the measurements. The EPR measurements from 4 to 150 K were made by flowing cold helium gas around the sample with an Oxford ESR-900 continuous flow cryostat. The temperature was measured by a AuFe-Ag thermocouplewhich was inserted directly into the sample solution. This enabled us to measure the sample temperature accurately, regardless of a rise in the temperature due to the laser irradiation. A liquid nitrogen finger tip dewar was used for the measurements at 77 K. DMP was purified by vacuum sublimation. Trimethylpyrazine and spectrograde solvents wereused without further purification. Concentrations of the sample solutions were about 10-2 M.
0022-365419412098-7963%04.50/0 0 1994 American Chemical Society
Kamei et al.
7964 The Journal of Physical Chemistry, Vol. 9%.No. 33, 1994 I
1
Figure 1. Molecular structure of 2,6-dimethylpyrazine(DMP) and the axis system used in this paper. w
( a ) 4u
I
I \
100
I
200
300
400
magnetic field / mT
Figure 3. Simulated TREPR spectra of the triplet state of DMP in n-pentane: (a) 4 K and (b) 77 K.
! I
0
100
200
300
400
magnetic field / mT Figure 2. TREPR spectra of the triplet state of DMP in n-pentane: (a) 4 K and (b) 77 K.
Results and Discussion TemperatureDependence of Zfsin DMP. We have examined the temperature dependence of the TREPR spectra of MP and DMP in several different rigid matrices. We found remarkable temperature dependence of zfs in DMP/n-pentane and MP/ MCH in which the triplet decay rate constants start to increase rapidly at relatively low temperatures.6 In other systems the temperature dependence of the zfs was found to be relatively small. Since the spectrum of MP/MCH is contaminated by broad components, we have studied only DMP/n-pentane in detail. In the following we use the axis system shown in Figure 1. The TREPR spectra of DMP/n-pentane obtained at 4 and 77 K are shown in cigure 2. The spectra are analyzed in terms of the spin Hamiltonian, 7f = g0H-S- XS:
- YS; - ZS:
(1)
X+Y+Z=O The values of zfs are first roughly estimated from the two peaks indicated by 2 and 3 in the figure using eq 2
x = -{A/3 Y = -{A/3
- [(hv + A)’- (gpHh)2]1/2)
+ [(hu + A ) ’ - ( g ~ H h ) ’ ] ’ / ’ } Z = + (2/3)A
(2a)
X>Z>O> Y.lZJ3Therelativepopulatingratetothe Ysublevel is much larger than those to the other sublevels (P,>> P,, P,), as discussed later. Then peak 1 in the spectrum measured at 4 K is assigned to the low-field peak of the Y stationary point, but peaks 2 and 3 are due to the X and Z stationary peaks superimposed on each other. The high-field Y peak located at 630 mT is very weak and is not shown here; this spectrum can be simulated well withX = 2.76 f 0.12 GHz, Y = -5.52 f 0.12 GHz, Z = 2.76 & 0.12 GHz, and Py - P,:P, - P, = 1:0 as shown in Figure 3a. As seen from Figure 2, the spectrum shows considerable temperature dependence;they and low-field X and Z peaks move toward higher fields as the temperature is raised. The observed spectrum at 77 K and its simulation are shown in Figures 2b and 3b, respectively. However, at further higher temperatures the simulated spectra with zfs determined using eq 2 begin to disagree with the observed ones. The simulated Xand Z stationary peaks begin to split into two peaks, but such split peaks were never observed. Only the Z stationary peaks were observed, though the SIN ratios were rather poor at higher temperatures. This disagreement cannot be removed by just adjusting the zfs. To explain this observation, we must consider either adjusting the relative populating rates or introducing anisotropic spin-lattice relaxation. If the population in the T, sublevel becomes comparable to that in the T, sublevel, the X stationary peaks become weak and the simulated spectrum becomes more or less similar to the observed one. Similarly, theXpeaks become weak, if we assume that the spin-lattice relaxation time between the T, and T, sublevels is much shorter than the others. We first examine the relative populating rates to the sublevels. In planar pyrazine and DMP with 3n7r* T1 states, the important process that determines the spin selectivity of the populating rate is the intersystem cross (isc) from the lowest excited singlet (S1(n?r*)) state to the higher triplet (Tz(mr*)) state below the S1 state in energy. Since it is only the Ty sublevel of the T2 state that couples with the SIstate via direct spin-orbit interaction, the major isc route is expected to be
(2b) (2c)
A = (g@)’(H; - H,2)/4hu Here Hh and H1 are the magnetic fields for the high- and lowfield resonance peaks, respectively. Small adjustments to the zfs are then madeon the basis of thecomparison between theobserved and simulated spectra. We assume that the ordering of the zerofield spin sublevels in 3n7r* MP and DMP are similar to that in pyrazine as discussed in the previous work.6 In pyrazine it is known that the T, and T, sublevels are close to each other and
Subsequent internal conversion from the higher triplet state to the TI state is expected to occur, populating the Ty sublevel predominantly. This prediction is indeed borne out by the experimental result obtained at 4 K. The dominant isc to the Ty sublevel should remain even at higher temperatures, unless the isc from the thermally excited singlet states populate the T, sublevel. Even if the population of the T, sublevel is dominant just after the isc, it may be possible to redistribute the population among the T, and Ty sublevels by the anisotropic spin-lattice relaxation between the T, andT,sublevels. Such a process would be possible,
The Journal of Physical Chemistry, Vol. 98, No. 33, 1994 7965
Temperature and Solvent Dependences of the Zfs
zfs I GH,
InF,
ov
Figure 4. Temperature dependence of the zero-field splittings (zfs) of
DMP in n-pentane.
-I/
I___.-__-
0.01
0.02
0.03 0.04
l/T
Figure 6. Temperaturedependence of the average 3 r ~character * F*( T ) of DMP in n-pentane. The straight line is drawn as a guide to the eye.
Fn 0
I 1
0.5
3nnx
"X
Figure 5. Expected changes of the zero-field splittings of pyrazine of mixed 3n~-3ar*character with the extent of mixing.
if the structures in the higher vibronic levels are distorted. Actually, an out of plane distortion in the higher vibronic level is predicted in pyrazine along the u l b mode due to the vibronic coupling between the Tl(na*) and T2(7ra*) states. Such a distortion is also likely to occur in DMP in which the Tl(na*) andTz(rr*) statesareclosetoeachother. TheT,andT,sublevels can be mixed by the distortion, and the thermal population into these higher levels may bring in a large population into the T, sublevel. It should be mentioned that such anisotropic relaxation was in fact found to be important in the triplet state of Cy0 in which rapid structural fluctuation takes place because of the dynamic Jahn-Teller distortion.14J5 Temperature dependence of the zfs of DMP/n-pentane is shown in Figure 4. It is seen that Y increases and X and 2 decrease as temperature is raised. This temperature dependence is similar to that found in phthalazine10and is rationalized in terms of the increased average 3 m * character of the triplet state. When a triplet state is of mixed 3na*and 37r3*character, the wave function of the mixed state is described by a linear combination of the zero-order wave functions of the pure 3na* and 37r7r* states, \k = CY+,,
+ p+,,
(a2 + p2 = 1)
(3)
where a and j3 are the mixing coefficients. When the contributions of the second-order effect of the spin-orbit coupling to zfs can be neglected, the sublevel energy Y of the mixed state is given by
+
Y = a2Yn @*Y,
(4)
and similar equations for X and Z. Here Y,and Y, are the values for pure 3n7r* and 3 ~ states, ~ * respectively. The change of the zero-field scheme on going from the pure 3na* state to the pure 3 m * state in pyrazine derivatives is shown schematically in Figure 5. As the 37ra* character increases, Ygoes up and Xgoes down. The observed temperature dependence of the zfs is given by the thermal average of the zfs of various states occupied a t a certain temperature. From Figure 5 and the observed change of the zfs given in Figure 4 it is concluded that the average 37ra*
character of the triplet state increases as temperature is raised. In the following we estimate the average 37r7r* character at temperature T. Suppose that the 3 m * character of the state having an excess energy E from the zeroth vibrational level of the TI state is f ( E ) and the density of the state is p ( E ) . Then the zfs, Y(T) at temperature, T, is given by
.rfimYs(E) e-E1kTp ( E ) d E + .CY,( 1 -f(E))e-ElkT p(E) dE c p ( E ) eE1-kTd E (5)
Similar equations hold for X(T) i n d Z(T). The average mr* character F,(T) is obtained from eq 5 as
We have calculated F,(T) from Y(T) using Y, = -5.52 GHz of DMP/n-pentane a t 4 K and Y, = 0.55 GHz of tetramethylpyrazine in durene.I5 The temperature dependence of F,( T ) is shown in Figure 6. The slope of the In F,( T ) vs 1 / T plot gives a nearly straight line with an activation energy of 150 cm-l in the range of T = 50-120 K. F,(T) becomes as large as 0.32 at 125 K. In the simple two-level model involving only the Tl(na*) and T2(*a*)states separated by E,, F,(T) is given by
(7) Though the experimental data cannot be fit well by this simple equation, these results seem to indicate the presence of a state of dominant mr*character in the vicinity (within 100-200 cm-1) of the Tl(na*) state. In the previous work6 the phosphorescence decay rateconstant of DMPln-pentane was found to be expressed by a sum of two exponential terms with activation energies E l = 100 cm-I and E2 = 530 cm-I. The activation energy obtained here is close to E l , and it is likely that the higher state with dominant 3 m * character has a large radiationless decay rate constant. This seems to be in agreement with the conclusion reached by Sobolewski et aL9 Effect of Solvent and Substitution. In contrast to the case of DMP/n-pentane and MP/MCH, the temperature dependent change of the zfs is much smaller in other systems. For example, in m-xylene, Y only changes from -5.86 GHz at 4.2 K to -5.66
7966 The Journal of Physical Chemistry, Vol. 98, No. 33, 1994
A
I
I
I
200
300
400
magnetic field 1 mT
Figure 7. Observed and simulated spectra of DMP in trifluoroethanol:
(upper) observed and (lower) simulated.
GHz a t 77 K compared with a change from -5.52 to -4.90 GHz in n-pentane. This difference is well explained in terms of the shift of the 37r7r* state to a much higher energy in this matrix as indicated by the relatively small temperature dependence of the decay rate constant at low temperatures and the large activation energy found for the decay rate constant (AI3 = 1570 cm-I). Thus, the temperaturedependenceof zfs found in DMP/n-pentane is not due to the specific effect of the methyl group. Generally speaking, there seems to be a good correlation between the temperature dependence of the zfs and that of the triplet decay; the zfs changes markedly only in the systems in which the triplet decays start to increase rapidly at relatively low temperatures. Thus the present result supports the proposition that the presence of a nearby 3 m * state is essential in producing the fast radiationless decay.6 The T1 states of DMP in nonpolar solvents are of dominant 3 n r * character, and its zfs does not depend much on the solvent. However, in a very polar solvent such as trifluoroethanol, the order of the triplet states is now inverted and the 3 m r * state becomes the T1state. In Figure 7 the observed spectrum is shown together with the simulated spectrum calculated with 2 = 1.26 GHz, Y = 0.75GHz,X=-2.01 GHz,andP,-P,:P,-P,= 1:0.6. The obtained zfs are similar to those of s-tetramethylpyrazine, whose T1state is determined to be 3B2,(7r7r*).17 Both the T, and T, sublevels are populated as in the case of s-tetramethylpyrazine.16~1~ The populationintotheT,andT,sublevels was explained in terms of the rotation of t h e y and z in-plane spin axes by 45" from the N-N axis caused by the vibronic coupling between the 3B2, and 3BIu states.16J7 Similar rotation of the spin axis is probably taking place in the case of DMP.
Kamei et al. We have also studied trimethylpyrazine (TMP) in a variety of rigid matrices of both polar and nonpolar solvents. The spectra due to AM = il transitions cover relatively narrow ranges of magnetic fields of 200-250 mT in all the solvents studied. The zfs of TMP in toluene estimated from the peak positions are 2 = 1.17 GHz, Y = 0.69 GHz, and X = 1.86 GHz, though the spectral shape was not simulated well. Similar spectra were obtained in other nonpolar solvents as well as in ethanol. These observationsseem to indicate that theTI stateoftrimethylpyrazine now becomes of dominant 3 m * character irrespective of the nature of the solvent.
Concluding Remark The temperature dependence of the zfs of DMP/n-pentane indicates the presence of a state of dominant 3 m * character in thevicinityof theTl(n7r*) statewhich islikely tocausean increase of the radiationless decay at relatively low temperatures. Acknowledgment. We thank Dr. M. Baba for helpful discussions. We are indebted to Dr. K. Tominaga for the computer simulation program of EPR spectra of randomly oriented triplet molecules. This work was partly supported by a Scientific Research Grant-in-Aid for the Priority Area of "Molecular Magnetism" (Area No. 228/04242 102) provided by the Ministry of Education, Science and Culture of Japan. References and Notes (1) Lim, E. C. J . Phys. Chem. 1986, 90,6770. (2) Lim, E. C. In ExcitedStates; Lim, E. C., Ed.; AcademicPress: New York, 1977; vol 3, p 305. (3) Madej, S.L.;Okajima,S.; Lim, E. C.J. Chem. Phys. 1976,65,1219. (4) Madej, S. L.; Gillespie, G. D.; Lim, E. C. Chem. Phys. 1978,32, 1. ( 5 ) Terazima, M.; Yamauchi, S.;Hirota, N. J. Phys. Chem. 1986,90, 4294. (6) Yamauchi, S.;Mibu, K.; 1987, 91, 6173.
Komada, Y.; Hirota, N. J . Phys. Chem.
(7) Sneh, 0.;Dunn-Kittenplom, D.; Cheshnovsky, 0. J. Chem. Phys. 1989, 91, 7331. (8) Sneh, 0.;Cheshnovsky, 0. J . Chem. Phys. 1991,96,8095. (9) Sobolewski,A. L.; Lim, E. C.; Siebrand,W. Int. J. Quantum Chem. 1991, 39, 309. (10) Terazima, M.; Yamauchi, S.;Hirota, N. J. Chem. Phys. 1985,83, 3234. (11) Terazima, M.; Yamauchi, S.;Hirota, N. J . Phys. Chem. 1985,89, 1220. (12) Burland, D. M.; Schmidt, J. Mol. Phys. 1991, 22, 191. (13) Nishi, N.; Kinoshita, M.; Nakajima, T.; Shimada, R.; Kanda, Y. Mol. Phys. 1977, 33, 31. (14) Terazima, M.; Hirota, N.; Shinohara, H.; Saito, Y. Chem. Phys. Lett. 1992. 195. 333. (15) Terazima, M.; Sakurada, K.; Hirota, N.; Shinohara, H.; Saito, Y. J. Phys. Chem. 1993,97, 5447. (16) Ymauchi, S.; Miyake, K.; Hirota, N. Mol. Phys. 1984, 53,479. (17) Antheunis, D. A.; Schmidt, J.; vander Waals, J. H. Mol. Phys. 1978, 36, 177.
