4441
J. Phys. Chem. 1995,99, 4441 -4446
Electron Paramagnetic Resonance of a Triplet Species Observed in 2-Nitrobiphenyl Single Crystal after Ultraviolet Light Irradiation Jiro Himchi,* Kimihiro Yoshimura, and Mikio Yagi* Department of Physical Chemistry, Faculty of Engineering, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240, Japan Received: September 30, 1994; In Final Form: January 9, 1 9 9 9
After ultraviolet light (UV) irradiation of 2-nitrobiphenyl (2-NBP) below 120 K, stable spin-triplet species can be observed in rigid glasses and in the crystalline state by the electron paramagnetic resonance (EPR) method. The zero-field splitting (ZFS) parameters obtained from the UV-irradiated single crystal of 2-NBP at 77 K are Xlhc = 0.0144, Y/hc = 0.0222, and zlhc = -0.0367 cm-l. The absolute values of these ZFS parameters are relatively larger than the corresponding ones obtained in ethanol glasses. To determine the absolute signs of the ZFS parameters and the energy of singlet-triplet separation, the temperature dependence of the EPR signals was observed between 1.93 and 4.2 K. Taking Boltzmann populations and the EPR transition probabilities into account, we found that the triplet state observed is 1.7 f 0.5 cm-' above the singlet ground state. The triplet species is inferred to be formed through the abstraction of a hydrogen atom from the 2'-position of the biphenyl ring by one of the oxygen atoms of the nitro group by taking the direction of the principal spin axes and the result of a semiempirical molecular orbital calculation into consideration.
Introduction During ultraviolet light (UV) irradiation of many aromatic molecules in rigid organic glasses at low temperatures, their lowest excited triplet (TI) states can be detected by steady state or time-resolved electron paramagnetic resonance technique (SSEPR and TREPR, respectively). However, little is known about the TI states of aromatic nitro molecules. Recently, the TREPR spectra of nitronaphthalenes in their T1 states have been observed in a mixture of diethyl ether, isopentane, and ethanol at 77 K.' The values of the zero-field splitting (ZFS) parameter D obtained for the TI states of nitronaphthalenes are about 15% smaller than that of naphthalene on the average. Similarly, the SSEPR spectrum of 4-nitrobiphenyl (4-NBP) in its TI state can be detected in ethanol (EtOH) at 77 K, and its D value is also nearly 15% smaller than that of biphenyl. For 2-nitrobiphenyl (2-NBP), however, a different type of EPR spectrum of the triplet state is obtained in EtOH glasses or in other organic rigid glasses at 77 K.* The D value so obtained is less than half of that of biphenyl. Further, the EPR signals did not change in several hours after cessation of W irradiation, at temperatures below 120 K. This is a quite unusual phenomenon which cannot be seen in most of aromatic molecules and is peculiar to the derivatives of 2-NBP. From the temperature dependence of the EPR signal intensity above 4.2 K, the triplet species was estimated to be in the ground state or in the excited state which is closely located above the singlet ground state. These facts suggest that the present triplet species is quite different in character from the TI states of 4-NBP and nitronaphthalenes. We tried the product analysis in this reaction but could not obtain any positive result. On the other hand, the signal intensities of the infrared absorption lines of the nitro group were decreased with the UV irradiation time. In consideration of these facts, this triplet species is estimated to be formed through the abstraction of a hydrogen atom from the 2'-position of biphenyl ring by one of the oxygen atoms of the nitro group as shown in Figure 1. To confirm the above @
Abstract published in Advance ACS Abstracts, March 1, 1995.
0022-3654/95/2099-444 1$09.00/0
my e
hv
Figure 1. The most probable structure of the triplet species produced from UV-irradiated 2-NBP.
estimation, EPR work on the W-irradiated 2-NBP has been carried out in some detail using a single crystal of 2-NBP as a host.
Experimental Section 2-NBP (Aldrich) was purified by recrystallization from EtOH. The pure poly(viny1 alcohol) (PVA) film was obtained by the same method as described previou~ly?.~After heat treatment for about 1 h at 80 "C and swelling in distilled water at 40 OC, the swollen film was soaked in the methanol solution of a sample until an appropriate amount had penetrated into the film by diffusion. Then the film was stretched at 70 "C using a Shibayama S-60 film stretcher. The films thus obtained have about 200% of stretch in the stretched direction s. The single crystals of 2-NBP were obtained from an EtOH solution by evaporating the solvent at room temperature. The EPR measurements above 4.2 K were carried out by using a JEOL JES-FElXG spectrometer with 100 lcHz magnetic field modulation at microwave frequencies close to 9.2 GHz. The irradiating light was provided by a Canrad-Hanovia 1 kW Xe-Hg lamp through a Toshiba W - 3 3 s glass filter and 5 cm of distilled water. The details of EPR measurements at 77 K were essentially the same as those reported in our previous paper^.^-^ The temperature was changed from 4.2 to 180 K using an Air Products LTR-3 continuous flow cryostat and a Scientific Instruments temperature controller system (Series 5500). Temperatures below 4.2 K were obtained by pumping the helium in a quartz Dewar, and the lowest temperature obtained in the present experiments was 1.93 K. In this case, 0 1995 American Chemical Society
4442 J. Phys. Chem., Vol. 99, No. 13, 1995
Higuchi et al.
TABLE 1: Parameters in Spin Hamiltonian host 2-NBP crystal EtOH glass 2.00405 2.00565 2.00220 0.01439 0.02220 -0.03669
gxx
g?? gzz
Xthclcm-I Ylhctcm-‘ Zthctcm-‘
Y
z
2.00426 2.00612 2.00208 0.01 15 0.0212 -0.0328
the EPR spectra were measured by using the same spectrometer with 80 Hz magnetic field modulation. To avoid the saturation of the EPR signals, the incident microwave power used was 0.1 pW at temperatures between 4.2 and 1.93 K.
Results Spin Hamiltonian. In the presence of a magnetic field (magnetic induction B), the EPR result can well be interpreted by the following spin Hamiltonian:
+
Hs = pBB.g.S S D S
p,B.g.S - XS: - YS,’ - ZS,‘
+
= pBBzS D[S: - S(S
+ 1)/3] + E(S:
- S)’,
Here, these symbols have their usual meaning. In the interpretation of the EPR result, we neglect the effects of all the crystalline fields or the fields of host molecules. EPR Measurements at 77 K. The triplet molecular species was detected by the EPR method for the UV-irradiated powder of 2-NBP at 77 K. The EPR spectra thus observed are scarcely changed from those in rigid glasses.2 In Table 1, the ZFS parameters and the principal g-values listed were obtained from the powdered sample, except the absolute signs of the ZFS parameters which were determined as will be described in the subsequent section. To understand its electronic structure and magnetic properties, it is necessary to determine the directions of the principal axes of the D tensor relative to the molecular axes. Although the molecular structure changes from 2-NBP by UV-irradiation, the deviation from the original structure of the biphenyl skeleton without hydrogen atoms should not be considerably large as was estimated previously.* Therefore, the above properties may roughly be obtainable without serious difficulties. For the present purpose, we first measured the EPR spectra by adopting the stretched-polymer-film t e c h n i q ~ e . ~ Stretched .~ PVA films containing 2-NBP were irradiated with a 1 kW XeHg lamp through a Toshiba UV-D33S filter at 77 K. After cessation of irradiation, EPR signals were observed at 77 K. As is clearly seen in Figure 2, the observed ordering of the principal values of the D tensor is IZI > I yl > 1x1.The intensity of the Z signals increases when the extemal magnetic field is perpendicular to the film plane (BI In),compared with the case where B is parallel to the stretched direction s (Blls). When BI Is, the intensity of mostly the X signals increases, compared with the signals observed in unstretched PVA films. These facts suggest that the longest direction of the triplet species deviates considerably from the magnetic z-axis and is relatively close to the x-axis. However, further information about the principal magnetic axes could not be obtained as long as only the stretched-PVA-film technique is used, since the present triplet species do not have a Cz axis. To obtain more precise information about them, we carried out an EPR experiment using a UV-irradiated single crystal of 2-NBP. The crystal structure of 2-NBP, determined by X-ray diffraction, is monoclinic, and the space group P21h ( z = 4, a = 1.7462, b = 0.7553, c = 0.7896 nm, p = 98.94’) with four
I
1
1
I
260
280
300
320
B/m T Figure 2. EPR spectra of the triplet species observed in stretched PVA films at 77 K: (a) Bits and (b) Bl(n. molecules per unit cells as shown in Figure 3.6 The outline of the molecular structure of 2-NBP obtained and the numbering of atoms used are shown in Figure 4. In triplet species occupying the magnetically inequivalent site of 2-NBP crystals, four pairs of absorption lines may generally be observed at an arbitrary orientation of the host crystal in the applied magnetic field due to the Ah4s = f l transition between the triplet spin sublevels. When the crystal is rotated around the a’ (a’ is perpendicular to the bc plane) or c axis, two pairs of lines can be observed. On the other hand, only a pair of lines can be detected for the rotation of the crystal around the b axis. Thus, we obtained four sets of angular dependence curves of the resonant magnetic fields around the a’ or c axis and two sets of the curves around the b axis, as shown in Figure 5 . In the present experiment, however, no hyperfine structure has been resolved. The principal axes of the D tensor were determined using the ZFS parameters and the principal g-values of the powdered sample with the aid of computer simulation as described below. At first, the spin density was assumed to be considerably high at atoms C2’ and 0 2 as in the structural formula shown in Figure 1. Taking the result of the stretched-PVA-film experiments into consideration, the magnetic axis system assumed in the first step was that the z axis was parallel to the 02-C2’ direction, the x axis was perpendicular to the z-axis in the benzene ring with carbon atoms Cl’-C6’, and the y axis was perpendicular to the x and z axes. By use of the ZFS parameters and g-values listed in Table 1, the resonance fields were calculated by rotating B around each crystal axis (a’, b, c). Thus, each of the angular dependence curves obtained for the resonance field was compared to the corresponding experimental one. Such a procedure was repeated by assuming a better set of magnetic axes (x, y , z ) , until the consistency of each calculated curve with the corresponding experimental one was obtained. The angular dependence curves simulated here are also shown in Figure 5 . Thus, we can determine the direction cosines of the principal axes of the D tensor for the triplet molecular species with one of the orientations in reference to the crystal system (a’, b, c) of the 2-NBP, as given in Table 2. It may be noted here that the z axis obtained is relatively close to the 02-C2’ direction of 2-NBP (the angle between them is about 12”) and deviates about 60” from the C4-C4’ direction of 2-NBP (the long axis of the biphenyl skeleton). As is described above, there are two kinds of molecules oriented in a 2-NBP single crystal. When the B is rotated around the a’ or c axis, the two sets of experimental angular
J. Phys. Chem., Vol. 99, No. 13, 1995 4443
EPR of Triplet Species
Figure 3. Projections of 2-NBP crystal structure (a) in the ah plane viewed down the h axis and (b) in the ac plane viewed down the c axis.
A Q
Figure 4. Outline of the structure of 2-NBP molecule in 2-NBP crystal and numberings of atoms in 2-NBP molecule.
dependence curves of resonant magnetic fields cannot be reproduced by the above procedure. However, it should be noted here that, if all the signs for the direction cosines between the b axis and each of the D-tensor axes are reversed, one can reproduce the other sets of the experimental curves which cannot be obtained by the above procedure. Singlet-Triplet Energy Separation. In the preliminary work on the UV-irradiated 2-NBP in EtOH glass,* we estimated from the intensities of EPR signals at several temperatures between 4.2 and 77 K that the observed triplet species is possibly in the ground state, although there remains some ambiguity. To eliminate such uncertainties, we have carried out a similar EPR experiment precisely using UV-irradiated 2-NBP single crystals between 1.93 and 4.2 K. In this case, the 2-NBP single crystal mounted at end of a quartz rod was UV-irradiated at 77 K and immersed into liquid helium on the condition of BIly. By reduction of the pressure above boiling helium, the temperature was lowered to 1.93 K which is the lowest temperature attainable in our apparatus. Thereafter, by warming up the sample slowly, we measured the intensity of the EPR signal at several temperatures between 1.93 and 4.2 K. Then the
intensities of the Y signals are plotted against the inverse of the temperature (lI7') as shown in Figure 6. As can be seen from this figure, the intensity of the low-field Y signal is stronger than that of the high-field one between 1.93 and 4.2 K. Taking Boltzmann distributions in the singlet state and the triplet spin sublevels and the probabilities of the A M s = f l transitions into account, the curve of the relative intensity of the EPR signal vs (1/7') can be evaluated by estimating the ZFS parameters (X. Y,and 2)and the energy separation between the singlet and the Y spin sublevel of the triplet state (Am. The ZFS parameters used here are those listed in Table 1 except their undetermined absolute signs. This procedure is similar to that of the work of naphthalene in its TI state by Homig and Hyde7 except for the inclusion of the singlet-triplet energy separation and the relative intensity used. If the D value was assumed to be negative (2 > 0), the intensity at the high-field Y signal was evaluated to be high compared to that of the lowfield one at each temperature, although any value of AE was used. This means that the D value should not be negative (2
0). The energy separation AE was estimated using the set of ZFS parameters with D > 0. At temperatures above 4.2 K, the intensity of the EPR signal decreases approximately inversely proportional to the temperature T. This suggests the fact that the AE is very small and nearly a few cm-l if the ground state is a singlet state. Therefore, the temperature dependence of the mean value of the relative intensities in the Y signals was evaluated by assuming several small values of AE . As a result, the simulated curve can be best fitted to the experimental one when the observed triplet state is 1.7 f 0.5 cm-I above the singlet ground state, as can be seen in Figure 7. On the other hand, one could not reproduce the experimental curve by assuming the triplet ground state. Thus, we can clarify the facts that the triplet molecular species observed is in the excited state only 1.7 f 0.5 cm-l (2.1 x eV) above the singlet ground state and the D value is positive (2 0 < X Y). This is a quite unusual phenomenon in the excited triplet state of aromatic molecular species although the present species is an unstable
4444 J. Phys. Chem., Vol. 99, No. 13, 1995
250
Higuchi et al.
350
300
400
250
B/mT
300
350
400
B/mT
( C w w
c u
. w
c1
r
Q
c -e c
0 43
0
c
L
0
w 4 V
r
0 00
U
d
250
300
350
400
B/mT Figure 5. Resonance fields of the AMs = f l EPR transitions for the UV-irradiated 2-NBP crystal as a function of the extemal magnetic field B in (a) bc, (b) a’c, and ( c ) a‘b planes of the 2-NBP crystal (a’ is perpendicular to the bc plane).
TABLE 2: Direction Cosines of the ZFS Tensor crystallographic axis
a’ x
principal ZFS axis
y
z a
a
0.2919 0.6688 0.6837
6
C
-0.9543 0.1559 0.2550
0.0640 -0.7269 0.6837
a’ is perpendicular to the bc plane.
photochemical intermediate at normal temperature. It is of special interest to note that the triplet state of the present molecular species becomes the lowest energy state if the extemal field of B is higher than about 2 T.
Discussion The present photochemical phenomenon observed in 2-NBP crystals or powders is essentially the same as that in EtOH glasses. A similar phenomenon can be detected in a single crystal of benzophenone containing 2-NBP as a guest obtained by evaporating from their diethyl ether solution. From these observations, we could exclude a possibility that the 2-NBP single crystal contained trapped EtOH molecules used as a solvent which would render the system almost equivalent to rigid glasses. In the UV irradiation, the intramolecular abstraction of an aromatic hydrogen atom by one of the oxygen atoms of the nitro group is not usual, although similar reactions are actually
possible to occur for a hydrogen atom of methyl group.* In these reactions, one of the oxygen atoms of the nitro group in the precursor should be located where the oxygen atom can approach closely to a hydrogen atom to be abstracted. However, such a condition is not always satisfied in most of the aromatic nitro compounds, as in the cases of 4-NBP and nitronaphthalenes which cannot produce a stable triplet species at 77 K. In the mass spectrum (EI, 70eV)9 of 2-NBP, a peak of mlz = 182 was observed with high intensity while for 2-NBP-d5 (2-(NOz)C&Cdls) a peak of mlz = 186 was similarly detected with high intensity but that of mlz = 187 was very weak. As the ion of mlz = 182 for 2-NBP corresponds to the product by the abstraction of OH, the ions of mlz = 186 and 187 for 2-NBPd5 correspond to those of OD and OH abstraction, respectively. These facts may indirectly suggest the possibility of OH bond formation by abstraction of a H2’ atom which is in the nearest location from one of the oxygen atoms of nitro group. It may be noted here that, in the TREPR spectrumlo of 2-NBP in EtOH glass at 77 K, the resonance fields observed are distinctly different from those of the SSEPR spectrum described above. Also the rate of triplet formation estimated from the B ~ ,signal , of SSEPR of 2-NBP appears to be no appreciably different from that produced by the 2-NBP-ds. These facts may suggest that at the initial stage the triplet species observed by the TREPR, possibly the excited triplet state of 2-NBP, is produced by the UV irradiation, and afterward another triplet
J. Phys. Chem., Vol. 99, No. 13, 1995 4445
EPR of Triplet Species 2.0
2.0
AE = -1.0 cm-' 1.8
0
IL
0
0 0
1.8
0 0
1.6
1.6
0
0 0 b *s
0
1.4
1.4
C
aJ
5
AA
I,
AA AE = -2.5 cm-'
1.2
AA
1.2
A
A
A
A
0
1.o
1.o
A 0.8 0.2
0.8 0.2
0.3
0.4
0.5
0.3
0.4
0.5
0.6
0.6
1/ T (K-I) Figure 6. Temperature dependence of the relative intensities,ZH (higher field) and IL (lower field), in the EPR Y signals of the UV-irradiated 2-NBP crystal. The intensities were calculated by the peak height of the fiist derivatives and normalized by the mean value of ZH and ZL at 4.2 K.
species discussed here is formed consecutively through the abstraction of a hydrogen atom from the 2'-position of biphenyl ring. As can be seen in Table 1, each of the absolute values of the ZFS parameters obtained from the W irradiated 2-NBP crystals or powders is relatively larger than the corresponding one which was obtained in EtOH glasses. Such a difference may be explained by the following reasons: the 2-NBP molecule is not so stiff, and the phenyl ring and the nitro group are partly rotatable around the C1 -C1' and C2-N bonds, respectively. Therefore, the molecular geometry and the electronic structure of the triplet species are somewhat changeable according to the environmental field. As a result, some deviations appear in the ZFS parameters from those obtained in EtOH glasses, although the molecular skeleton of the triplet species in the 2-NBP crystal may not largely be changed from that of the precursor in its crystal. On the other hand, the semiempirical restricted Hartree-Fock (RHF) calculations with AM1 approximation" were carried out for the three lowest states of the molecular species with the structural formula shown in Figure 1. The optimized structure obtained for the lowest triplet state with singly occupied orbitals of p 3 7 and (7338 is fairly close to that of the estimated one. The singlet state with the same singly occupied orbitals with optimized structure was 0.0028 eV (22 cm-') lower than the
1 / T (K-') Figure 7. Temperature dependence of the mean value of ZH and IL in the EPR Y signal of the UV-irradiated 2-NBP crystal. The circles indicate the experimental values, and the curves were calculated by assuming Z < 0 < X < Y and the singlet ground state.
above triplet state. Although such a semiquantitative agreement with the experimental result is apparently fortuitous, this singlet-triplet energy separation is evidently within error of the present calculation. This may suggest that these two states are closely located to each other in agreement with the experimental result. It may be noted here that the calculation of a singlet state with ( ~ 3 7could ) ~ not give any optimized results, possibly because of the p 3 7 MO being localized at the C2' atom largely. If the unpaired electrons of the triplet species are located at the nitro group and at the C2' atom separately as was shown by the structural formula stated above (see Figure l), the D value was estimated to be about -0.05 cm-' using a relation of D = -(3/2)g2,u~2(r-3)(I is the distance between the unpaired electrons).2 In the present experiment, however, the determined D is a positive value of +0.055 cm-' in the 2-NBP crystal, showing that its absolute value is strangely close to that of the above one. The reason for this may be qualitatively inferred from the fact that the predominant contribution in the D value is due to the one-center spin-spin interactions mainly at the C2' atom and subsequently at atoms in the nitro group. This may be explained by the coefficients of A 0 in LCAO-MO's of singly occupied orbitals of p 3 7 and v)38 obtained by the above semiempirical RHF calculation. In this case, the p 3 7 MO is mainly located at the C2' atom, while the p38 MO is relatively localized at the nitro group. However, there is some contribution
4446 J. Phys. Chem., Vol. 99, No. 13, 1995 of the C2' A 0 in 9 3 8 MO and that of AO's of the nitro group in ~ 3 MO. 7 These make possible the contributions to the positive D value by overcoming the negative value due to the above-mentioned spin-spin interaction between the separated unpaired electrons. However, the accuracy of the wavefunction obtained is not sufficiently high in evaluating the magnetic constants. Therefore, a quantitative elucidation of the ZFS parameters is actually difficult at the present stage.
Conclusion
A stable photochemical triplet intermediate produced from 2-NBP is studied by EPR at temperatures between 1.93 and 77 K. It is found that the triplet state observed is only 1.7 z t 0.5 cm-' above the singlet ground state. The inferred structural formula shown in Figure 1 is favorable for elucidating the experimental results with the aid of theoretical consideration. Acknowledgment. The authors wish to thank Professor Yuji Ohashi and Dr. Akiko Sekine of Tokyo Institute of Technology for their cooperation in the crystal structure analysis. They express their thanks to Mr. Michinori Fujisawa of our laboratory for his cooperation in the EPR experiments. Also they wish to thank Professor Yusaku Ikegami of Tohoku University for his
Higuchi et al. helpful discussion and Professors Shozo Tero-Kubota and Shinji Onodera of Tohoku University for their help in the IR measurements. The present work was partially supported by a Grant-in-Aid for Scientific Research No. 01470007 from the Japanese Ministry of Education, Science, and Culture.
References and Notes (1) Yagi, M.; Shioya, Y.; Higuchi, J. J. Photochem. Photobiol. A: Chem. 1991, 62, 65. (2) Tanigaki, K.; Yagi, M.; Higuchi, J. Chem. Phys. Lett. 1988, 153,
51.
( 3 ) Ito, T.; Higuchi, J.; Hoshi, T. Chem. Phys. Lett. 1975, 35, 141. (4) Higuchi, J.; Yagi, M.; Iwaki, T.; Bunden, M.; Tanigaki, K.; Ito, T. Bull. Chem. SOC.Jpn. 1980, 53, 890. ( 5 ) Yagi, M.; Makiguchi, K.; Ohnuki, A,; Suzuki, K.; Higuchi, J.; Nagase, S . Bull. Chem. SOC.Jpn. 1985, 58, 252. (6) Sekine, A.; Ohashi, Y.; Yoshimura, K.; Yagi, M.;Higuchi, J. Acra Crystallogr., Sect. C 1994, 50, 1101. (7) Homig, A. W.; Hyde, J. S. Mol. Phys. 1963, 6, 33. (8) Preston, P. N.; Tennant, G. Chem. Rev. 1972, 72, 627. (9) JEOL-JMS-AX500 was used. (10) A Lumonics HE-420 excimer laser (XeC1, 308 nm) was used as an exciting light source with a repetition rate of 25 Hz. (11) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. SOC.1985, 107, 3903.
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