4381
NOTES and Dr. D. H. O'Brien of Texas A & M University for their helpful discussions. This research was SUPported by the Research Council and the Chemistry Depart,ment of Texas A & & University. I It is also partially supported by Research Corporation. ACknowledgment is also made to the donors of the Petroleum Fund (PRF-1447-G), administered by the American Chemical Society, for partial support of this research.
Photoconductivity of Electron Acceptors.
11. 2,7-Dibromofluoren-Aga-malononitrile by Tapan K. Ahkherjee Energetics Branch, Air Force Cambridge Research Laboratories, Bedford, Massachu.retts 01730 (Received February 20, 1969)
I n our previous work' on the photoconductivity in nitro derivatives of fluoren-Aga-malononitrile it was noted that, in the case of the 2,7-dinitro derivative (DDF; X = NOz), the maximum photocurrent was obtained by excitation with energy less than that required to raise the photoconductor to the first singlet
state. The exact reason for this difference was not clearly understood. Replacement of the NOz-group by a heavy atom would enhance the probability of direct singlet-triplet taansition. If the S -+ T transition is involved in any way in the generation of photocarriers, then one would expect the photocurrent maximum to be shifted to a wavelength considerably longer than the first singlet-singlet absorption band. To test this possibility, 2,7-dibromo-A9"-malononitrile (DBF, X = Br) was synthesized, and its light absorption and photoconductivity characteristics were studied.z DBF, synthesized from 2,7-dibromofl~orenone~ by the previously described procedure, was purified by repeated crystallization from dimethylformamide, ethyl acetate, and chloroform4 (red crystals, mp 346-348'). Spectroscopic and photoconductivity measurements were performed as before.'
Results and Diricussion I n methylene dichloride, DBF shows a low intensity broad absorption between 550-425 mp (maximum a t 480 mp; log E , 2.49). Due to the limited solubility of DBF, the solvent dependence of the absorption spectrum could not be studied. Hence, the assignment of the lorn energy itransition (480 mp) as a -A--A* state was done by indirect means. A comparison of the absorption spectra of DBF and
Table I : Absorption Spectra in Methylene Dichloride m p (log E )
Compound
380 310 420 310 435 350 480 348
Fluorenone 2,7-Dibromofluorenone
Fluoren-Aga-malononitri~e (X = H) DBF (X = Br)
(2.42), 327 (3 .OO)
(3.28) (2.72), 312 (3.79) (2.51), 365 (3.53),295 (2.49), 360 (4.29), 317
(3.68) (3.45) (3.35) (4.25) (4.22)
fluoren-Aga-malononitrile(X = H) with those of 2,7-dibromofluorenone and fluorenone (Table I) shows that the long wavelength peaks are of similar intensities. From theoretical molecular orbital calculations5 and other experimental evidence,6 the weak absorption near 380 mp in fluorenone has been assigned to a -A--A* state, rather than to a n-a* state as previously thought. By analogy, the band a t 420 mp in 2,7-dibromofluorenone is assigned the -A--A* level. It is also known that the well characterized n--A* bands in aldehydes and ketones disappear when carbonyl oxygen (=O) atom is replaced
( )(:.
by dicyanomethylene group' =C
The positions
and the intensities of the first low-energy bands that appear in the products have the properties of -A--A* transitions.8 Table I also shows that in DBF, the low energy band is red-shifted compared to the unsubstituted fluoren-Aga-malononitrile. Similar red shift was observed in fluorenone with the electron donating substituent in the "two" position, and was considered a good test of -A--T* t r a n ~ i t i o n . ~ The absorption spectrum of the vacuum evaporated film (Figure 1) of DBF shows some vibrational structure in the long wavelength region. All peaks are somewhat red-shifted compared to those of the solution spectrum. Since the excited singlet levels in the crystals are often found to be lower than the singlet levels in solution by a few tenths of an electron volt,lo the difference between the solid and solution spectrum is not unexpected, (1) T. K. Mukherjee, J . Phys. Chem., 70,3848 (1966). (2) The monobromo derivative, as well as the 2,7-diamino and 2,7-dihydroxy derivatives were found to be nonphotoconductive. 2,7-Dibromofluorenone did not yield any photocurrent. (3) Ch. Courtot, Ann. Chim., 14,99 (1930). (4) Anal. Calcd. for ClsH~BrzNz:C, 49.77; H, 1.56; Br, 41.40; N, 7.25. Found: C, 49.70; H, 1.80; Br, 41.15; N, 7.28. The upper limit of purity was ascertained from a single spot on TLC and maximum molecular extinction coefficient. Repeated melting of a sample showed some decomposition, hence zone refining was not attempted. (5) H. Kuroda and T. L. Kunii, Theor. Chim. Acta, 9 , 51 (1967). (6) K. Yoshihara and D. R. Kearns, J . Chem. Phys., 45, 1991 (1966). (7) E. Campaigne, R. Subramaya, and D. R. Maulding, J . Org. Chem., 28, 623 (1963). There are a large number of examples scattered in the literature. (8) This test of n-a* transition may be used as a supplement to the oximation test described by Yasihara and Kearns in ref 6. (9) A. Kuboyama, Bull. Chem. SOC.Jap. 37, 1540 (1964). (10) F. Gutman and L. E. Lyons, "Organic Semiconductors," John Wiley and Sons, Inc., New York, N. Y., 1967, Table I, p 653.
Volume 78, Number 12 December 1969
KOTES
4382
1 9
i' 7;
0
I
400
I
500
,
600
700
WAVELENGTH (mp)
Figure 1. Spectral response of surface photoconductivity of 2,7-dibromofluoren-Aga-nialononitrile a t 200 V. Absorption spectrum of vacuum deposited film, -C-C-.
-,
Like most other molecular crystals, DBF is a high resistance semiconductor (pbulk = 2 X 10'6 ohm cm), and it is a weaker photoconductor compared to DDF.I' The surface photocurrent in air was about 35% lower than that in vacuum, a behavior reminescent of the 2,7-dinitro derivative. The surface photocurrent spectrum is also shown in Figure 1. Reading from the low energy side, one notices that the photocurrent maximum a t 610 mp is about 0.26 eV lower than the first absorption maximum at 560 mp. There is some evidence of vibrational structure in the photocurrent action spectrum. The height of the photocurrent peak at 610 mp increased monotonically with the applied field (60500 V). Although the solid absorption spectrum shows a weak tail towards the longer wavelength, the photocurrent drops rather sharply. This behavior, coupled with the proximity of the absorption and photocurrent peaks, is suggestive of the fact that the charge carriers in DBF may be generated within the singlet-singlet energy threshold of this compound.12 (1 1) Under the conditions described in ref 1. (12) It has not been possible to detect the triplet level in DBF, either from the absorption spectrum in heavy atom solvent or from emission spectroscopy.
Potentiometric Titration of Stereoregular Poly(acry1ic acids)
by Yoshikazu Kawaguchi and Mitsuru Nagasawa Department of Applied Chemistry, Nagoya University, Furocho, Chilcusa-lcu, Nagoga, 464,Japan (Received March 14,1969)
Theoretical treatment of the problem of potentiometric titration of polyelectrolytes, having a uniform The Journal of Physical Chemistry
distribution of ionizable groups of one kind, leads to an expression for the pH of the solution pH = pKo - log[(l - Qo/a]
+ 0.434e$/kT (1)
where \L is the electrostatic potential a t the point on the molecule from which the proton is removed, pKo the intrinsic ionization constant of the group and a! is the degree of ionization. Since the magnitude of $ is dependent on the effects of nearest charged groups, the potentiometric titration curves reflect the local conformation of the polyions and, therefore, one may expect to find a difference between the potentiometric titration curves of linear polyelectrolytes which have different local conformations in solution. Many quantitative studies were carried out with poly@-glutamic acid) to find its conformational change in neutralization and to estimate the molecular param-. eters from analysis of the potentiometric titration curves. For linear polyelectrolytes, too, it was predicted from potentiometric titration curves that isotactic poIy(methacry1ic acid), p(MAA), may have locally helical conformation, whereas syndiotactic p(IC4AA) has the planar zigzag conformation.' Therefore, it would be desirable to confirm the locally helical structure of isotactic p (MAA) using an independent experimental method. Unfortunately, no such method has been found. On the other hand, recent nmr studies show that isotactic poly(acry1ic acid), p(AA), appears to have locally helical structure in solution due to the strong electrostatic repulsion between charged groups. The purpose of the present paper is to show that a detectable difference can be observed between the potentiometric titration curves of isotactic p(AA) which was shown by nmr studies to have a helical conformation and those of syndiotactic p(AA) which presumably has a planar zigzag conformation. Concerning the difference between ionization constants of carboxyl group of isotactic and atactic p(AA), Miller, et aLI2detected no difference but Sakaguchi, et aLI3reported that syndiotactic p(AA) is a stronger acid than atactic p(AA). The present work supports the conclusion of the latter authors.
Experimental Section Samples. Isotactic poly(methy1 acrylate), p(MA), was polymerized with lithium aluminum hydride at -78" in toluene for 20 hr in vacuo. The polymerization product was poured into methanol containing 10% HC1. P(MA) thus obtained was purified by repeated precipitation from its chloroform solution with methanol, The degree of tacticity, calculated from the nmr spectrum of the methylene groups using the method of (1) M.Nagasawa, T. Murase, and K. Kondo, J . Phys. Chem., 69, 4005 (1966). (2) M. L. Miller, K. O'Donnel, and K. Skogman, J . Colloid Sci., 17, 649 (1962). (3) Y. Sakaguchi, J. Nishino, M. Okuyama, H. Shobayashi, M. Shimada, R. Nomura, andY. Ito, Kobunshi Kagalcu, 24,25 (1967).
