Electron spin resonance of phosphorescent aromatic compounds in stretched polyvinyl alcohol films

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ELECTRONSPlNRESONANCEOFPHOSPHORESCENTAROMATICCO~OUNDS lNSTRE"iCHEDPOLYVfNYLALCOHOLFlLMS 3iro HIGUCHI, Takeshi ITO*, Mlkio YAGI, Masahiko MINAGAWA, Makoto BUNIXN Deparlmmtt of Chemistry, Yokohama National Univct wy. Minami-ku, Y&drcmta 233, Jopon and Tosbihiko HOSHI Department of Chemistry. College of S&we Sctagaya-ku, Tokyo 15 7, Japari

Receivti

22

and Etrgiuecring,

,doyarrru Gakdn

uttivcatty,

December 1976

Usmg;t strctclicd polyvinyl alcohol film as a host, elcc!ron spin reson3ncc of pttospl~orcscenttrrplct stateskrasbeen studied for scveraI aromatic compounds which are insoluble in water. The present method is demonstrated to be useFuirt for the assignment of the spectra and to be ntitabie to obtain wc~-re~I~~ spectra.

I. fntroduclion To eliminate the difficulty in findmg a host crystal, electron spin resonance (ESR) experiments have recentiy been carried out for the phosphorescent triplet state of aromatic molecules by using a stretched polyvinyl alcohol (PVA) film as a host IL] . in this CWX, the unstretched films were obtained by evaporating water from aqueous SoIut~oi~scontainirlg appropriate amounts of PVA and a sample. Efereaftet, this technique will be called the SoSution~vapora~~g method. By the use of the stretched films, the determination of the fine structure tensor was rather straightforward, because the onentations of guest molecules have a tendency to be orthorhombic in such films with more than 200% of stretch, that is, (1) the molecular plane is difficult to be oriented perpendicular to the stretching direction; (2) the long molecular axis has a tendency to align to the stretching direction;

* Resent address: Department of Chemistry, Faulty of Education, Shinshu University, Nislinagano, Nqano 380, Japan.

(3) the molecular plane is preferably oriented par& Iel to the film plane. So far as the solution-evaporating method is used, the molecules to be observed should be soluble in water. In the present work, this point has been modified by using the diffusion-penetrating method and app[icd to. several aromatic molecules which are insolubte in water. Further appIicab~ities and usefulness of the present method will be discussed.

2.Experhnental ‘The ESR measurements were carried out for naphthalene, phenanthrene, fluorene, carbazolc, biphenyl, &yanobiphenyl, 4,4’dicyanobiphcnyl, 4&orobiphenyl, 4,4’difluorobiphenyl and 2,2’-bipyridyE irt stretched PVA films. Cyanobiphenyls and fiuoroblphenyls were purchased from Eastman Kodak Co. and ~d~ch Co., respectively. Ail the other compounds were obtainer from Tokyo Kasei Co. They were puritred by recrystalIization. A pure PVA iilm 2 X HI-2 cm thick was obtained by the same method described previously [ 11. After a heat treatment for about an hour at 80°C 477

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and swelling in water for about three hours at 30°C, the film was soaked in methanol solution o f a sample until its appropriate amount was penetrated into the film by diffusion. Then the film was stretched for about five minutes at about 60°C. The stretched films obtained are about 7 X 10 -3 cm thick, and with about 250% of stretch in the stretched direction 5' and about 30% of contraction in direction c perpendicular to s in the film plane. 1he details o f the ESR experiment have been reported previously [ I ] and are not repeated here.

3. Results and discussion

15 March 1977

Hz Hx e

~

Hv

~

O°

Hit s

3o*

6 O*

lqae f'me structure parameters obtained are listed in table 1. For these molecules, the coordinate axes were taken as follows: the x axis is parallel to the longest direction of molecule and t h e y axis is perpendicular to it in the molecular plane (except for E-2,2'-bipyridyl), while the z axis is perpendicular to the molecular plane. As an example, the observed ESR spectra o f the low field Am = ±1 transition for the phosphorescent triplet state of naphthalene are shown in fig. 1. The line shape changes with a rotation around the c axis according to the general relations concerning the orientatio-ts o f guest molecules ( 1 ) - ( 3 ) . A similar trend was 3bserved for the other molecules studied here. This ma>' imply

7 5* 90*

H/In

J

,,I,,

2000

I

300o

Fig. 1. Angular dependence of ESR spectra of the low field Am = ± I transition for the phosphorescent triplet state of naphthalene on a rotation around the c axis of the stretched PVA host films at 77 K. Ois the angle between the s axis and the external magnetic field H in the sn plane of the PVA films.

Table 1 Fine structure parameters in the phosphorescent triplet slates (cm-1 ) Molecule

tXI/hc

1Yl/he

IZI/hc

IDl/hc

;EI/hc

D*/hc a)

ID**/f~cb)

naphthalene phenanthtene fluorene carbazole biphcny! 4-~anobiphenyl 4,4'-dicyanobiphenyl 4-fluorobiphenyl 4,4'-difluorobiphenyl Z-2,2'-bipyridyl E-2,2'-bipy tidy!

0.0485 0.0805 0.0392 0.0406 0.0400 0.0438 0.0429 0.0394 0.0387 0.04 11 0.0482

0.0180 0.0119 0.0323 0.0270 0.0328 0.0232 0.0183 0.0350 0.0369 0.0317 0,0237

u.0667 0.0688 0.0713 0.0676 0.0729 0.0666 0.0611 0.0741 0.0756 0.0722 0.0722

0.1001 0.1032 0.1070 0.I014 O.1093 0.0999 0,0917 0,1111 0.1134 0.1083 0.1083

0.0153 0.0462 0.0034 0,0068 0.0036 0.0103 0.0123 0.0022 0.0009 0.0047 0.0125

0.1035 0.1306 0.1071 0.1021 0.1095 0.1015 0.12941 0.1112 0.1134 0.1086 0.1104

0.1033 0.1303 0.1067 0.1019 0.1089 0.1019 0.0937 0.1117 0.1139 0.1100 0.1100

a) ' =(D z + 3E2)1/2. "' obtained from the observed reson,4ncefield of the Am = ±2 transition with Kottis-Lefebvre's correction [2].

k/

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that concerning the orientations of dissolved molecules there is no significant differeace between the film prepared by the solution-evaporating method and that by the diffusion-penetrating method. As a result, t'.le assignment of the resonance fields was straightforward, especially for molecules with long planar structure. The observed ESR spectra of naphthalene and phenanthrene are similar to those of quinoxaline and 1,10-phenanthroline in our previous work [1]. The fine structure parameters obtained are in good agreement with those of previous studies (for naphthalene see refs. [ 3 - 6 ] , for phenanthrene refs. [4,7,8] ). The ESR spectra of fluorene and earbazole comparatively resemble each other under various conditions. The ESR signals of the phosphorescent triplet state of carbazole in glass have hitherto been assigned by the magnetophotoselection technique or by analogy with those in crystals with similar structures [9 10], such as fluorene [1 I] and dibenzothiophene [7]. The present result supports the previous assignment. In the case of biphenyl with the planar lowest triplet state [13,14] ,the intensity of Z peaks is lelatively enhanced ~-hen the applied magnetic field ~ is parallel to the normal of the film plane, n (//]In), although it is considerably weak in an unstretched film. On the other hand, the intensity of X peaks is very strcng with Hlls. This may possibly imply that the guest mo!ecules have a strong tendency to align x Ils. 4-cyanobil)henyl and 4,4'.dicyanobiphenyl are the longest molecules studied here and the ESR spectra of their phosphorescent triplet states have not hitherto been observed because of the difficulty in finding a suitable host crystal. In such cases, the present method is really useful. In the stretched Films, one may expect fairly high degree of their orientations as compared with the cases of some shorter molecules. Thetobserved ESR spectra show large anisotropy and the intensity of their X peaks is actually strong. As can be seen in table 1, the IE! value increases with the increasing number of CN groups. This is due to the spin delocalizat;on by the para-substituted CN groups. In the work of fluorobiphenyls in ethanol glasses by Mispelter et al. [ 14], the 19F hyperfine splittings (hfsD in the Z peaks of the Am ±1 transition were obse;ved only when signal averaging was employed, and tile X and Y peaks of 4,,~ .dlfluoroblphenyl could be re~olved by using polarized excitation, while Taylor et al. showed E/he = 0.0000 cm-1 in EPA glass [ 15]. Adopting the

15 March | 977

present technique, we could separately observe the X and Y signals without use of polarized light (fig. 2). Further, we could resolve the 19F hfs of 4-fluorobiphenyl and 4,4'-difluorobiphenyl when//'lln without an averagirtg technique (50 ± 2 G and 50 -+ 5 G, respectively). These hfs are in good agreement with those of Mispelter et al. These facts may certainly imply that the orientation of these molecules is a fairly high degree ~s was expected from the result of the biphenyls. As contrasted with the cyanobiphenyls, the IEI value decreases with the increasing number of F atoms because of the change in spin delocalization. As a result, the pre. sent method appears to be useful not only for the ~esolution of weak and partly resolved signals in glass but also for the studies of spin distribution and hfs in phosphorescent triplet states. The ESR spectra observed for the above molecules are compatible with the general relations (1)-(3) which were deduced in previous work with the solution-evaporating method [ 1,16]. The fine structure parameters obtained previously for Z- and E..2,2'-bipyridyl in stretched PVA films agree with those obtained by the present method within the limitatio,ls of experimental error. The line shapes of spectra for the two conformers are qualitatively similar in these films but the intensity ratio is changed. The details of this point will be published in the near future. Recently, Krebs and Sackmann used liquid crystal glasses [frozen nematic mixtures of cholesteryl chloride and cholesteryl taurate (1.85 : 1.00)] in ESR stu-

HII S

,......~~ , ~ , ~

~

HI1 C

Hy

=

AF

2550

2600

2650

2700 G

•

Fig. 2. X and Y peaks of the low field .~rn = .,. i tra1~sition Ior the phosphorescent triplet stale of 4,4'-difluorobiphe:ayt in stretched PVA films at 77 K.

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PIlYSlCS UTFERS

15 March 1977

&es OFthe triplet statesof several aromatic Ilydrombans It ?,t$f . fn this caslf, the intensity of X peaks is actually strong when H&E (E is an applied electric field uxd to orient the liquid crystal) [ 181, as in the case ofHlls 6%thr present work. However, one may not exactly distinguish between the Y and Z peaks because the moleculary and .z axes are in a plane nearly perpendicuk~r to E in uniaxialfy aiignncdnernatic liquid glasses. In view of these facts, the strctchcd poiymerfiim method IS actualfy useful and favorable for some ~332sas cumpared with the use of liquid crystal &Hses.

I21 P. Rot& .md R. tcfcbvre, 5. L%m. Pbys. 39 (1963) 393. f3f CA. HotcblitenJr. and B-W. ~~n~rn, 1. Chcm. Phys 34 (1961) 908, 141 R.W. Brando~~, R.E. Gcrkin and CA. tiutchbon Jr., J. Chem. ?hys. 37 (1962) 447. IS] f. Mispeiter, 3.-Ph. G&et and J.-M. Lhoste, Mol. Phys. 21(1!.?71) 999. 161 3. deJong,J. Nqm. Kewn. 9 (1973) 185. [7J R.W. Brandan,R.E.Gerkin snd CA EtutchisonSr., J. Chem. Phys. 4 1( 1964) 37 17. [S$ K.E. Gcxkin and A.&f. Wmer, I. C-hem. Phyr 50 (1969) 3114.

The authors wish to thank Professor M. Kinoshita, The University ofTokyo, fm his kind advice and Mr. T. Yogi, Tokyo Jnstltutc of Tcchnolog~, for his help in the preliminary work whcrg he was in our laboratory.

112) h-1.Baiwir,Chm. Phyrs.Lcttcrs9 (1971) 482;J. Md. Struct. 19 (1973) 4i9. J. Mispelter,Chem. I%ys. Letters 10 (1971) 539, J. Mispeftcr, J.-Ph. Grivct rend J.-Ml Lhostc, Mol. ippfyS. 21(1971) f015. H.V. Taylor, AL. Attrcd and If-M. Hoffman, J, Am. Chem. Sw. 95 (1973) 321% TAO and 3. Hiydlhi, Chem. Letters f1974) ISIP, P_ Krcbsnnd E. ~~ckmann, Mol. Pbys. 23 119721437. P. Krebs and E. Sackmannl J. Mrrgn Reson. 22 (1976) 359.