1270
J. Phys. Chem. 1986, 90, 1270-1275
allows a rather accurate determination of the photochemical quantum yield of the investigated hole-burning process. We have presented first preliminary data of the Debye-Waller factors and Debye temperatures, which may lead into an interesting new direction, namely, relating Debye temperatures from thermal data to comparable values from optical data. In case of the optical data one has to assume that the “lattice” modes, which contribute to the specific heat, also couple to the optical transition. Our first experimental data seem to justify this assumption. More data, however, are needed to prove this point.
Acknowledgment. We thank our colleague J. Friedrich for many helpful discussions and we acknowledge the support of the Stiftung Volkswagenwerk without whose support this work would not have been possible.
Appendix: Evaluation of the Convolution Integral in the Hole Shape Function In eq 8 we had derived an expression which describes the spectral shape of a zero-phonon hole. It can be written in the following form
The sum has to be taken over all the poles in the upper complex half-plane. The residue of a functionflz), which at zo has a pole of order p , can be calculated from the following formula:I4
In order to get general expressions for the derivatives of our functions fp, we use the partial fraction analysis 1
1
The further calculation is straightforward. It yields
-
xlyp(w’) dw’ =
1
y
(w - wL
+ iy)P(w - wL)P + (‘46)
with f p ( ~ ’ ) = ([(w’
+ w - o L ) ~+ y2/4]”(~’*+ y2/4)]-l
(A21
Here the integrations over the angular coordinates have been carried out. The functions fp(w’) can be. extended over the whole complex w’ plane; they have infinities of order one at w’1,2 = 2=iy/2 and infinities of order p at = wL - w f iy/2, respectively. Therefore the integral in ( A l ) can be solved by using the residue iaw:14
Inserting this formula in (Al) directly leads to the final expression for the hole shape given in (1 la). At the burning frequency (Le., for w = wL) eq l l a is not applicable because of zero denominators. In this case we start out with eq A2 and w = wL; with the residue law we can calculate
+I ,dw’,=,, f p ( 4
= 2+)-
1 y2P+1
(‘47)
which leads to (1 lb). R e m NO. H~Pc,574-93-6;PE, 9002-88-4;PS, 9003-53-6;PMMA, 901 1-14-7.
f m
Jm
fp(w’) dw‘ = 2rriCRes Cfp,dj) J
(A31
(14) Endl, K.; Luh, W. ‘Analysis 111”; Endl, K., Ed.; Akademische Verlagsgesellschaft: Frankfurt/Main, 1974; pp 207, 203.
Electron Spfn Resonance in Phosphorescent Triplet States of 2,P’-Blpyridine and 2,2’-Biquindine Complexes with Diamagnetic Metal Ions and of 2,2‘-Binaphthaiene Jiro Higuchi,* Kazuhiro Suzuki, Hiroyuki Arai, Akira Saitoh, and Mikio Yagi Department of Chemistry, Faculty of Engineering, Yokohama National University, Tokiwadai, Hodogaya- ku, Yokohama, 240 Japan (Received: September 17, 1985)
The phosphorescent triplet states of 2,2’-bipyridine and 2,2’-biquinoline complexes with several kinds of diamagnetic group I1 (2 and 12) metal ions M2+ (M = Mg, Ca, Sr, and Zn) have been studied by ESR and phosphorescence spectroscopies. For the 2,2’-bipyridine (bpy) complexes, the ESR spectra of [M(bpy)I2+ were clearly observed. In general, the zero-field splitting parameters of these complexes are fairly close to those of (Z)-2,2’-bipyridine, which has been detected by ESR spectroscopy. For the ESR and phcsphoresknce spectra of 2,2‘-biquinoline (bq) complexes, only [Zn(bq)I2+could be detected. In connection with its conformation, the ESR and phosphorescence spectra of 2,2’-binaphthalene were studied as the parent hydrocarbon of 2,2’-biquinoline. Its ESR signals originating from either its E or Z conformer could be assigned separately by using a stretched poly(viny1 alcohol) film as a host. As a result, the zero-field splitting parameters of the undetected (Z)-2,2’-biquinoline could be discussed by comparison with those of [Zn(bq)12+and also those of (Z)-2,2’-binaphthalene. From the analysis of the phosphorescence spectra, all the lowest triplet states of the molecular species studied in the present work are estimated to have mr* character, and the electronic structure of the ligand in these metal complexes is not largely different from the corresponding metal-free ligand with Z conformation. On the basis of all the present experimentalobservations, the zero-field splitting parameters appear to be more sensitive and useful for detecting the formsltion of a metal complex with a ligand whose conformation is changeable upon coordination, in comparison with the phosphorescence spectrum.
Introduction Various kinds of metal chelates in their ground (G)state have long been studied by ESR spectroscopy with fruitful results. Because of the short lives of excited paramagnetic wamition-metal complexes, however, the phosphorescent ESR works of metal 0022-3654/86/2090-1270.$01.50/0
chelates have been restricted to the lowest excited triplet (TI) states of complexes with a diamagnetic ion. The results obtained for these molecular species may usually be explained by a relatively small perturbation of ligand by the metal ion. On the other hand, there are Some different types of molecules whose geometries are 0 1986 American Chemical Society
2,2’-Bipyridine and 2,2’-Biquinoline Complexes remarkably changed upon coordination with a metal ion, as in the cases of the typical bidentate ligands 2,2’-bipyridine and 2,2’-biquinoline,I In actuality, the stable conformation of these free ligands is E (s-trans in the G state) as is seen in most of their solutions without metal ions, while that of their metal complexes is apparently Z (s-cis in the G state). In these cases, the metal ion may possibly give a small perturbation to the ligand with Z conformation which is not always easily observable. Therefore, the study of such metal chelates is also useful for obtaining some supplemental information on their Z conformers of the metal-free ligands. Nevertheless, the phosphorescent ESR studies of their metal ion complexes have not been carried out in detail in connection with their conformations, although Rabold and Piette reported their preliminary work more than 15 years ago., In the present work, the phosphorescent triplet states (T, states) of 2,2‘-bipyridine and 2,2‘-biquinoline complexes with Znz+ or Mg2+are taken up, since Mg2+and ZnZ+are common diamagnetic metal ions and of particular interest from the viewpoint that they are sometimes coordinated with chelating biomolecules. Especially for 2,2’-bipyridine, we have succeeded in obtaining the ESR spectrum originating from the T, state of its Z conformer in a poly(viny1 alcohol) (PVA) and in mixtures of water with various alcohol^.^ As a result, we could examine the influence of diamagnetic metal ions M2+ (Mz+ = MgZ+,Ca2+, Sr2+, and Zn2+) being a small perturbation to (2)-2,2’-bipyridine in these metal chelates. In addition, the T, state of the 2,2’-biquinoline complex with Zn2+ has been studied by both ESR and phosphorescence spectroscopy. Although the 2 conformer of the metal-free biquinoline has not yet been detected, these spectra have been elucidated by comparing those of (E)- and (Z)-binaphthalenes together with that of the metal-free (E)-biquinoline. For the phosphorescent ESR studies, the magnetic fine structure of the triplet molecules (S = 1) was explained by the following spin Hamiltonian:
7f = gpBB*S+ S.D*S
= gpBB*S- XS: - Ys; - zSz2 = g/.bBB.S
+ D[S:
- (1/3)S(S
+ I)] + E[S? - S;]
Here, the principal values of the D tensor are -X,-Y, and -Z, and D and E are the zero-field splitting (ZFS) parameters, while B is an external magnetic induction field. The other symbols have their usual meaning, and the anisotropy of g was disregarded. Assuming the molecular structure of the TI state to be planar,7 the coordinate system of the principal axes was taken as follows: the x axis is relatively close to the long direction of the molecule (for the Z conformer, parallel to the central C-C bond which connects the two pyridine, quinoline, or naphthalene rings), the z axis is perpendicular to the molecular plane, and t h e y axis is perpendicular to the other two axes.
Experimental Section 2,2’-Bipyridine (bpy) (Tokyo Kasei) and 2,2’-biquinoline (bq) (Tokyo Kasei) were purified by recrystallization from ethanol. 2,2’-Binaphthalene (K & K Labs) were purified by recrystallization from benzene. [Zn(bpy),] (NO,), was prepared by heating a 50% ethanol solution of Zn(N03),.6Hz0 (Koso Chemical) and 2,2’-bipyridine ([ Zn2+]:[bipyridine] = 1:3) and purified by recrystallization from water.* [Mg(bpy)I2’ and [Zn(bpy)l2+were (1) For the ligand names, 2,Y-bipyridine and 2,2’-biquinoline are abreviated to bpy and bq, respectively. (2) Rabold, G. P.; Piette, L. H. Spectrosc. Lett. 1968, I , 211-224. (3) Ito, T.; Higuchi, J. Chem. Lett. 1974, 1519-1520. (4) Ito. T.: Hieuchi. J.: Hoshi. T. Chem. Phvs. Lett. 1975. 35. 141-145. (Sj Higuchi, J, Ito,’T.iYagi, M.;Minagawi, M.; Bunden,’M.f Hoshi, T. Chem. Phys. Lett. 1977, 46, 477-480. ( 6 ) Higuchi, J.; Yagi, M.; Iwaki, T.; Bunden, M.; Tanigaki, K.; Ito, T. Bull. Chem. SOC.Jpn. 1980, 53, 890-895. (7) Yagi, M.; Makiguchi, K.; Ohnuki, A,; Suzuki, K.; Higuchi, J.; Nagase, S . Bull. Chem. SOC.Jpn. 1985, 58, 252-257. (8) Ohno, T.; Kato, S . Bull. Chem. SOC.Jpn. 1974, 47, 2953-2957.
The Journal of Physical Chemistry, Vol. 90, No. 7, 1986 1271 U Z1
I
I
ZW
XE)
a21
023
025
0 27
0 29
BI T Figure 1. ESR spectra of the low-field Am = i l transitions for the T, states of (a) [ M P ( ~ P Y )(b) I ~ ~[Ca(bpy)l2’, , (c) [Sr(bpy)12’, (dl [Zn(bpy)I2+, and (e) (E)-2,2’-bipyridine, in ethanol-water glasses (EtOH:H,O = 9:l by volume) at 77 K.
obtained by dissolving 2,2’-bipyridine (5 X lo-, mol dm-3) and Mg(N03)2.6H20(Wako Pure Chemicals) and Zn(N03),-6H,0, respectively, in ethanol so as to satisfy the condition of [M2+]:[bipyridine] = 10:l. Quite similarly, [Ca(bpy)12+ and [Sr(bpy)12+were obtained from Ca(N03),-4Hz0 (Merck) and Sr(N03)2(Merck), respectively, except when 2,2’-bipyridine is dissolved in a mixture of ethanol-water (EtOH:H20 = 9:l by volume), since the solubilities of these complexes in ethanol are not large enough for the phosphorescent ESR detection. Therefore, only when the ESR spectra of [Mg(bpy)12+ and [Zn(bpy)l2+ were compared with those of [Ca(bpy)12+ and [Sr(bpy)12+were the measurements of these complexes carried out in a mixture of ethanol-water (EtOH:H,O = 9:l by volume). [Zn(bq)I2+was obtained by dissolving 2,2’-biquinoline (5 X mol dm-7 and Zn(C104)2.6Hz0(Mitsuwa Pure Chemicals) ( 5 X lo-* mol dm-,) in ethanol. The ESR spectra were obtained by using a JEOL JES-ME-3X spectrometer with 100-kHz modulation at microwave frequencies close to 9.2 GHz. The exciting light was provided by an Ushio USH-SOOD 5OOW mercury arc lamp or a Canrad-Hanovia 1-kW Xe-Hg lamp through a Toshiba UV-D33S glass filter and 5 cm of distilled water. The decays of ESR signals were measured by using a 1024-channel Kawasaki Electronica TM- 1610s signalaverager system. The details of ESR measurements were essentially the same as that reported previously.6 For the phosphorescence measurements, the excitations were done the same as in the ESR measurements. The emissions from a sample were passed through a Jobin-Yvon HR-1000 monochromator and detected by a Hamamatsu Photonics R453 photomultiplier tube. All the measurements were carried out at 77 K.
Results and Discussion ESR Spectra. Figure 1 shows ESR spectra of 2,2’-bipyridine (bPY), [MS(bPY)12+>[Ca(bpy)12+, [WbPY)12+,and [Zn(bpy)12+ in their TI states observed in enthanol-water glasses (EtOH:H20 = 9:l by volume) at 77 K. As has been detected in previous works,,” the observed ESR spectrum of 2,2’-bipyridine appears to originate from only its free E conformer, while the spectra of
1272 The Journal of Physical Chemistry, Vol. 90, No. 7 , 1986
Higuchi et al.
TABLE I: Zero-Field Splitting Parameters (in cm-') and Lifetimes (in s) in the Phosphorescent Triplet States at 77 K
molecule (E)-2.2'-bipyridine
counterion
solvent EtOH EtOH-HZO (9:l)' MeOH-H,O (4:l)' MeOH-H,O (4: EtOH EtOH-HZO (911)' EtOH-H,O (9:l)' EtOH-H,O (9:l)' EtOH EtOH-H,O (9:l)' EtOH EtOH EtOH EtOH EtOH EtOH EtOH
I4
Irl
0.0488 0.0242 0.0485 0.0242 0.0479 0.0242 0.04 1 1 0.0312 0.0413 0.0311 0.0415 0.03I O 0.0412 0.0313 0.0412 0.0316 0.0416 0.0305 0.0417 0.0307 0.0412 0.0296 0.0400 0.0329 0.0699 0.0037 0.0342 0.0252 0.0345 0.0228 0.0680 0.0055 0.0298 0.0312
I4
ID1
0.0732 0.0727 0.0721 0.0723 0.0724 0.0725 0.0726 0.0728 0.07 17 0.0725 0.0711 0.0730 0.0662 0.0591 0.0575 0.0625 0.0607
0.1098 0.1091 0.1081
0.1084 0. IO86 0. IO88
0.1089 0.1093 0.1076 0.1087 0.1062 0.1094 0.0992 0.0887 0.0863 0.0938 0.0910
IEl
D*(calcd)" D*(Bmin)* T~ 0.1118 0.1121 0.96 0.1111 0.1111 1 .oo 0.1 100 0.1 102 0.97 0.97 0.1087 0.1 102 0.1 116 2.0 0.1090 0.1 108 1.86 0.1092 1.61 0.1092 0.1 IO9 1.22 0.1096 0.1 108 0.1080 1.19 0.1086 0.1091 1.19 0.1090 1.19 0.1071 0.1078 4.4 0.1096 0.1096 0.1 178 0.81 0.1 I79 0.0906 0.46 0.0891 0.0878 0.32 0.0869 2.6 0.1134 0.1131 0.03 13 1.4 0.0912 0.0917 0.0007
0.0123 0.0121 0.01 I9 0.0050 0.0051 0.0052 0.0050 0.0048 0.0056 0.0055 0.0058 0.0035 0.0368 0.0045 0.0059
"D*(calcd) = [ D 2 + 3E2]"*. is obtained from the observed resonance field of the Am = 1 2 transition with Kottis and Lefebvre's correction (Kottis, P.; Lefebvre, R. J . Chem. Phys. 1963,39. 393-403). CByvolume. metal ion complexes shown in Figure 1 are superpositions of that of (E)-bipyridine on that of [M(bpy)I2+except in the case of the Zn2+ complex. For the former metal ion complexes, the line positions of each ESR spectrum were not appreciably changed when the concentration of the metal ions increased in the rigid solution. This may apparently be due to the fact that the stability constants for these [M(bpy)12' ions in their G state are really small, that is, for example log K = 0.5 for [ M g ( b ~ y ) ] ~ " . ~ Consequently, the phosphorescent ESR spectra of [ M ( b ~ y ) ~ ] ~ ' and [M(bpy)J*' are actually unobservable. On the other hand, ] ~ +be observed the ESR spectra of [Zn(bpy)12+and [ Z n ( b ~ y ) ~can separately from each other by changing the concentration ratio of [Zn2+]:[bipyridine] in the ethanol solution. This is quite the same situation as in the case of their absorption and emission spectra observed by Ohno and Katos when these complexes were prepared by a manner similar to that in the present work. Since the corresponding line positions of these two species are considerably close to each other, the ESR spectrum of [Zn(bpy)2]2' may not clearly be separable from those of the above two complex ions. In actuality, the ESR spectra of these metal ion complexes shown in Figure 1 are fairly close to that of (Z)-bipyridine rather than that of (Qbipyridine, and their peaks were assigned to be the same ordering without any contradiction. The ZFS parameters obtained from these ESR spectra are listed in Table I, together with the observed values of biphenyl as the parent hydrocarbon of bipyridine. In molecular species related to 2,2'-bipyridine, the phosphorescence lifetime (7p) obtained from the ESR Am = *I transition signal is longest for biphenyl, and those of the (E)- and (Z)-bipyridines are estimated to be fairly close to each other in the same glass. The observed 7;s of the bipyridine complexes with diamagnetic metal ions are longer than that of the metal-free (E)-bipyridine. This is quite similar to the case of 1,lOphenanthroline complexes,* although the 7p of (Z)-bipyridine has not yet been measured in an ethanol glass. The longer 7p of [Mg(bpy)I2' than those of the bipyridine complexes of Zn2+may be explained qualitatively by the contribution of spin-orbit interaction in the T, state. In examining this point more precisely, we have measured the 7;s of mono-bipyridine complexes of Mg2+, Ca2+, Sr2+, and Zn2+ from the decay of the ESR Am = =k1 transition signals in the ethanol-water glasses at 77 K. The values so obtained are given in Table I. In the mono-bipyridine complexes of group IIA (2) ions20(MgZ+,Ca2+,and Sr2+),the 7p decreases with increasing atomic weight of the metal ion, in qualitative accordance with the increase in its spin-orbit interaction. However, the 7p of [Sr(bpy)12+obtained is slightly longer than that of [Zn(bpy)]*', in disagreement with the ordering of atomic weight of the metal ion. This may possibly be due to the fact that the (9) Sone. K.; Krumholz, P.; Stammreich, H. J . Am. Chem. SOC.1955, 77, 777-780.
Y
035
Z
X
037
039
041
043
BIT Figure 2. ESR spectra of the high-field Am = 1 1 transitions for the TI states of (a) [Zn(bq)]*+ in an ethanol glass, (b) [Zn(bq)I2+in a stretched PVA film with B(ln (n is perpendicular to the film plane), (c) [2n(bq)l2' in a stretched PVA film with Blls (s is parallel to the stretched direction), and (d) (E)-2,2'-biquinolinein an ethanol glass, at 77 K.
influence of vibronic coupling in the TI state is slightly different between the group IIA (2) and IIB (12) metal ion complexes because of the difference in the coordination due to the electron configuration of these ions. In the present study of the TI state of 2,2'-biquinoline (bq) complexes with a diamagnetic ion, only an ESR spectrum of [Zn(bq)I2' could be observed. Figure 2 shows its ESR spectrum with the Am = f l transition which could not be observed by Rabold and Piette.2 In this case, however, no ESR signal of [Mg(bq)12' was detectable when any amount of Mg2+ ions was added to the ethanol solution of 2,2'-biquinoline, possibly because of its small stability constant which may be supposed from the difference between those of [Mg(bpy)l2: and [Zn(bpy)12'. As was reported previously,6 the ESR spectrum of the metal-free biquinoline observed in an ethanol glass originates from its E conformer. The ESR signals of [Zn(bq)I2' are easily distinguishable from those of the metal-free biquinoline. This may suggest the fact that the ESR spectrum of (Z)-2,2'-biquinoline is expected to be considerably different from that of (E)-2,2'biquinoline if it is observable. Each ESR peak of [Zn(bq)I2' was assigned by the aid of the technique using a stretched PVA film as a h ~ s t . ~As - ~shown in Figure 2, the Z peaks were easily
2,2’-Bipyridine and 2,2’-Biquinoline Complexes
~~~~
_
j
0150
~~~~
.
The Journal of Physical Chemistry, Vol. 90,No. 7, 1986 1273
_ i
0155
BIT Figure 3. ESR A m = f 2 transition signals for the T, states of ( E ) - and (Z)-2,2’-binaphthalenes(a) in a methanol glass, (b) in an ethanol glass, (c) in a 3-methyl-1-butanol(AmOH) glass, and (d) in an unstretched PVA film, at 77 K.
0.23
0.27
031
BIT Figure 4. ESR spectra of the low-field A m = f l transitions for the T, states of (E)- and (Z)-2,2’-binaphthalenes(a) in an ethanol glass, (b) in a stretched PVA film with Blln, and (c) in a stretched PVA film with Blls, at 7 1 K.
determined from their enhancement in the spectrum when the external magnetic induction B is perpendicular to the film plane (Blln), although the other two kinds of signals are difficult to assign directly. On the other hand, the pattern of the observed ESR spectrum of [Zn(bq)]*’ is actually similar to that of the 2,2’biquinolinium ion [H(bq)]+ which was recently reported by our laboratory,’0 since their spectra essentially originate from (Z)biquinoline. From these observations, we would assign the ESR peaks of [Zn(bq)I2+as shown in Figure 2 and estimate the se4 > Ill. The ~ ~ ’ s quence of the ZFS parameters to be IZI > 1 of (E)-2,2’-biquinoline and [Zn(bq)I2+ obtained from the decay of the Am = f 2 transition signals are 0.81 and 0.32 s, respectively. It may be noted here that the 7;s of [Zn(bq)]*’ become long (0.46 s) if its counterion is NO3- without a third-row atom, instead of C104- with a chlorine atom. This may mainly be due to the difference in the external spin-orbit interaction between the metal ion complex and its counterion. In these two cases, however, the ZFS parameters are not much different from each other, as is shown in Table I. In connection with the results of (E)-biquinoline and [Zn(bq)12+,a phosphorescent ESR study was carried out for 2,2’binaphthalene as the parent hydrocarbon of biquinoline. As shown 0145 0150 0155 in Figures 3 and 4, the spectra observed in ethanol glasses indicate superpositions of two kinds of those which originate from ( E ) E/ T and (Z)-binaphthalenes. Actually, the 7;s obtained from the Figure 5. ESR Am = f 2 transition signals for the T, states of ( E ) - and decay of the Am = 422 transition signals are different between (Z)-2,2’-binaphthalenesin ethanol glasses at 77 K (a) with A-type exthese two conformers, that is, 2.6 and 1.4 s. The intensity ratio citation (using Toshiba UV-D33S glass filter) and (b) with B-type excitation (using Toshiba UV-D33S and UV-35 glass filters). of the ESR peak with the longer 7,,to that with the shorter one is changeable when a different solvent (methanol, 3-methyl-ldivided into two groups by changing the solvent and measuring butanol, and PVA) is used (see Figure 3). Further, the ESR its decay character. spectrum as well as the intensity of these two ESR signals did The ESR Am = f l transition signals belonging to each group not change with concentration of the solute in the range of 2.6 of 2,2’-binaphthalene could be assigned to X,Y, and Z signals X to 2.6 X mol dm-3. Therefore, these ESR signals by using a stretched PVA film as a host (see Figure 4). In the should not be attributed to impurities or to aggregates. As a result, assignment of the conformers, the angles between the stretched it is possible for each line of the observed ESR spectrum to be direction of the film (s) and the external magnetic induction B were changed in a range of Oo I6’ I1 5 O in the film plane, and the origin of each group was determined such that the species with (10) Yagi, M.; Saitoh, A,; Takano, K.; Suzuki, K.; Higuchi, J. Chem. Phys. ~ (E)-binaphthalene while that with the shorter one the longer ‘ T is Lett. 1985, 118, 215-218.
1274 The Journal of Physical Chemistry, Vol. 90, No. 7, 1986
is (2)-binaphthalene. This may suggest the fact that the +p)s of molecules studied in the present work are much dependent upon the molecular geometry. By use of two types of filter combinations (A-type excitation: Toshiba UV-D33S glass filter; B-type excitation: Toshiba UV-D33S and UV-35 glass filters to cut off the short-wavelength region), the ESR spectra were measured as shown in Figure 5. The relative intensity of the ESR signal obtained from (2)-binaphthalene to that for (E)-binaphthalene increases under the lower energy excitation. This means that (Z)-binaphthalene has an absorption at a longer wavelength region compared to the case of (E)-binaphthalene. Such a situation corresponds to the fact that [Zn(bq)I2+has an absorption at a longer wavelength region than the metal-free (E)-biquinoline. From these observations, if the metal-free (2)-biquinoline is detected, T~ may be estimated to be short and the ESR spectrum to be relatively close to [Zn(bq)I2+ compared to the case of (E)-biquinoline, In the superposed X and Y signals of (Z)-binaphthalene, it may be preferable to take the ZFS parameters 4 by examining the ESR line shapes observed as IZI > IIl I1 in the stretched PVA films, although there remains another possibility of IZI > 1 4 > lyl which is the same sequence as that of [Zn(bq)I2+. Actually, the Xvalue is fairly close to the Yvalue in these two molecular species, and the spin distributions in their TI states are not significantly dissimilar from each other. ZFS Parameters. The ZFS parameters obtained from the ESR spectra observed in the present work are listed in Table I. The values which have been obtained by the previous author^^-^ are in fair agreement with the present ones except those of 2,2'-biquinoline observed at 1.9 K in different hosts by Clarke, Mitra, and Vinodgopal." In general, the D value is most influenced by the electron spin-spin interactions among the nearest-neighboring atoms which are scarcely different between the E and Z conformations of a planar molecule. However, as the secondary effect, the D value is additionally influenced by the molecular geometry, since the electron spin-spin interactions among the nonadjacent atoms which belong to the different quinolyl or naphthyl groups are largely changeable according to the conformations. On the other hand, the E value is much dependent on the molecular geometry and is largely different between the E and Z conformations. This is obvious because of the fact that, if the x axis is chosen to be parallel to the central rotatable C-C bond and t h e y axis is perpendicular to it in the molecular plane, nonzero off-diagonal elements of the electron spinspin interaction matrix appear for the E conformer while all the off-diagonal matrix elements are zero for the Z conformer.6 As can be seen in Table I, the ZFS parameters of the nitrogen heteromolecules and their metal ion complexes are relatively close to those of the corresponding parent hydrocarbons with the E and the Z conformation, respectively. This may possibly be due to a large contribution of the molecular geometry compared with the effect of substitution of the nitrogen atom. Actually, the ZFS parameters of 2,2'-bipyridine are not much different between its E and Z conformers and from those of biphenyl. In general, the D value of the metal ion complexes is expected to be fairly close to that of its metal-free ligand with the Z conformation. Actually, in methanol-water (4:l by volume) glasses the D values of (Z)-bipyridine and [Zn(bpy)l2' are very close to each other,7 and in methanol-ethanol (1 :1 by volume) glasses those of 1 , l O phenanthroline and its Zn2+complex also satisfy this relation.2J2 In the mono-bipyridine complexes studied here, the observed D values are not much different from each other and the sequence of their magnitude is Zn2+ < Mg2+ < Ca2+ < Sr2+. Also, the E values are fairly close to each other, but the sequence of their magnitudes is completely reversed to the above one. Within the group IIA (2) metal ions, the ionic radius actually increases with an increase in the atomic number.I3 With the same sequence, the metal nucleus of these complex ions tends to go away from (1 1) Clarke, R. H.; Mitra, P.; Vinodgopal, K.Chem. Phys. Lett. 1980, 76, 237-240; J . Chem. Phys. 1982, 77, 5288-5297. (12) Rabold, G. P.;F'iette, L. H. Photochem. Photobiol. 1966, 5, 733-738. (13) Huheey, J. E. 'Inorganic Chemistry, Principles of Structure and Reactivity"; Harper and Row: New York, 1978; pp 71-74.
Higuchi et al.
420
440
460
4 80
wavelength I nm
Figure 6. Phosphorescence spectra of (a) [Mg(bpy)12', (b) [Zn(bpy)12+, and (c) (E)-2,2'-bipyridine, in ethanol glasses at 77 K.
the midpoint of the central C-C bond of the ligand to the y direction and the spin delocalization to the y direction increases slightly larger compared with its changes in the other directions. As a result, the decrease in the lyl value is a little larger than the 4 and IZI values, as can be seen in Table I. changes in the 1 Consequently, the above sequences in the D and E values are reasonably explained by the magnitude of the ionic radius of the central metal atom, except for their relative values in the Mg2+ and Zn2+complexes. In this case, the ionic radii are nearly the same and the differences of the ZFS parameters between their mono-bipyridine complexes may be attributed mainly to the coordination character due to the electron configurations of the metal ions (Le., s2p6for Mg2+ and s2p6dI0for Zn2+). Even though the D value of (Z)-2,2'-bipyridine is slightly larger than that of its E conformer, the D value of the (2)-binaphthalene is fairly small compared with that of its E conformer. This is mainly due to the influence of the electron spin-spin interactions among the above-mentioned nonadjacent atoms upon the D value, since the binaphthalene is a relatively large molecule and the difference between the spin distributions originating from the geometries of these two conformers is not small. On the other hand, the observed E value of the E conformer is actually larger than that of the corresponding 2 conformer. This is apparently due to the above-mentioned off-diagonal elements of the electron spinspin interaction matrix which appeared only in E conformers. Similar relations of the ZFS parameters with those of 2,2'-binaphthalene should also be valid for 2,2'-biquinoline. Although the metal-free (Z)-biquinoline has not yet been detected, it is possible to estimate its ZFS parameters semiquantitatively from those of [Zn(bq)I2+. In actuality, the D value of [Zn(bq)I2+is fairly small compared with that of (E)rquinoline, and the difference between them is considerably larger than that between (E)- and (2)-binaphthalenes. Such a difference may possibly be due to the fact that the biquinoline consists of two polar heterocyclic groups while the binaphthalene is a nonpolar hydrocarbon. According to the aforementioned relation that the ZFS parameters of the metal ion complexes studied here are fairly close to those of its metal-free ligand with Z conformation, the D value of (Z)-biquinoline may be estimated to be fairly close to 0.0863 cm-' of [Zn(bq)I2+but smaller than 0.0938 cm-' of (E)-biquinoline
The Journal of Physical Chemistry, Vol. 90, NO. 7, 1986 1275
2,2'-Bipyridine and 2,2'-Biquinoline Complexes
n
/(b)
...
"...~.*...
i ' '\
-A
L ~
,
480
'.,
....,...' ..'...
...,
500
520
540
560
wavelength I nm
Figure 7. Phosphorescence spectra of (a) [Zn(bq)]*' and (b) (E)-2,2'biquinoline, in ethanol glasses at 77 K.
while its E value may be relatively close to 0.0059 cm-I of [Zn(bq)I2+. Phosphorescence Spectra. The phosphorescence spectra of 2,2'-bipyridine and [Zn(bpy)12+ can be observed separately as shown in Figure 6 . They are very close to those obtained in previous ~ o r k s . ~ In - ' ~these two molecular species, the (to bands of the TI G transition are observed a t the nearly same region S not much different from each other. This means and the T ~ are that the T, state of [Zn(bpy)12+has AT* character in which the local excitation within ligand is predominant. For [Mg(bpy)12+, however, the observed phosphorescence spectrum is a Superposition upon that of (E)-bipyridine as shown in Figure 6 , as in the case of the phosphorescent ESR spectrum of [Mg(bpy)12+. Since the TP(S of (E)-bipyridine and [Mg(bpy)12+ obtained from the decay of the ESR Am = f l transition signals are 0.96 and 2.0 s, respectively, the difference between them is not large enough to separate the observed phosphorescence spectra into their components by using a time-resolved technique. The phosphorescence spectra of 2,2'-biquinoline and [Zn(bq)I2+ were observed as shown in Figure 7. The former spectrum is slightly different from that observed by KlassenIs but is nearly the same as that by Bulska and Kotlicka.I6 Therefore, it is not discussed here in some detail. As is seen in Figure 7, we can reasonably assign the strong band at 522 nm to the 0-0 band of the TI G transition for [Zn(bq)I2+,although the very weak band from the metal-free biquinoline appears at 488 nm. Further, the T~ of [Zn(bq)I2 (0.32 s) is not significantly different from that of (E)-2,2'-biquinoline (0.81 s). These results indicate that the TI state of [Zn(bq)I2' has m* character with the dominant contribution of the local excitation in the ligand. Then the Forster cycle1' was applied to equilibria in the complex formationla
-
-
Zn2+
+ ligand
__ KI
[Zn(ligand)12+
by using the shift of the 0-0band of the phosphorescence spectrum in each complex formation. As a result, the difference of KI in the TI state from that of the G state is obtained to be fairly large for [Zn(bq)]2' while it is small for [Zn(bpy)l2'. This means that the TI state of [Zn(bq)I2+is more stable than the G state. The phosphorescence spectrum of 2,2'-binaphthalene has already been observed by Clar and Zander19 and by Rabold and (14) Rabold, G. P.; Piette, L. H. Specfrosc. Len. 1968, 1, 225-236. (15) Klassen, D. M. Ihorg. Chem. 1976, 15, 3166-3168. (16) Bulska, H.; Kotlicka, J. Pol. J. Chem. 1979, 53, 2103-2115. (17) FBrster, Th. Z. Elekrrochem. 1950, 54, 42-46. (18) Grabowski, 2.R. J. Lumin. 1981, 24/25, 559-562. (19) Clar, E.; Zander, M. Chem. Ber. 1956,89, 749-762. (20) In this paper the periodic group notation in parentheses is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the pblwk elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e.g., 111 3 and 13.)
-
500
550
600
wavdcngthi nm
Figure 8. Time-resolved phosphorescence spectra of 2,2'-binaphthalene in ethanol glasses at 77 K by setting the sampling times (a) 0-1 s and (b) 10-1 1 s after shutting off the exciting light.
TABLE II: Influences of the Metal Complex Formation and/or the Conformation Change upon the Molecular Constants"
AD
AE *7P AVW large large: very small very smallc 2,2'-biquinoline not large large large large 2,2'-binaphthalene small large large (very small) 2,2'-bipyridine
small
'D and E are the ZFS parameters, T~ is the phosphorescence lifetime of the Tl state, and uW is the frequency at the 0-0 band of the T1 G transition. Metal complex formation. Conformation change only.
-
Piette.14 Although this spectrum is expected to be a superposition of the component originating from (E)-binaphthalene upon that of (Z)-binaphthalene, they could not be assigned separately by these authors. In the present work, the phosphorescence spectra of 2,2'-binaphthalene were observed by setting the sampling time 0-1 and 10-1 1 s after shutting off the exciting light. The results obtained are shown in Figure 8. Although the TP)S of (E)- and (Z)-binaphthalenes obtained from the decay of the ESR Am = f l transition signals are 2.6 and 1.4 s, respectively, the observed time-resolved phosphorescence spectra are almost the same. This may show a possibility that the T1 states of these two conformers may be close to each other. On the other hand, the phosphorescence spectra observed arp scarcely changed upon the choice of excitation wavelengths, unlike the case of the ESR spectra. Further, the T~ obtained from the decay curve of phosphorescence is not shorter than 2.6 s at any wavelength monitored. As a result, there appears another possibility that (Z)-binaphthalene is a nonphosphorescent molecule and the observed phosphorescence spectrum is actually attributed to the emission of (E)-binaphthalene. In conclusion, we summarized the results on the change in the molecular constants treated here as given in Table 11. On the basis of these experimental observations, the ZFS E parameter is confirmed to be most sensitive in the metal complex formation and/or the conformation change of bidentate ligand with a rotatable c-C bond which connects two aromatic rings.
Acknowledgment. This work was supported by a Grant-In-Aid for Research No. 57470004 from the Ministry of Education, Science and Culture. Registry No. bpy, 366-18:7; bq, 119-91-5; Mg(bpy)(NO,),, 9982887-2; Cu(bpy)(NO,),, 99828-88-3; Sr(bpy)(NO,),, 99828-89-4; Zn(bpy)(NO,),, 68249- 16- 1 ; [Zn(bpy),] (NO,),, 2988 1-78-5; Zn(bq)(NO,),, 14950-33-5; Zn(bq)(ClO,),, 99828-90-7; 2,2'-binaphthalene, 61 2-78-2.
