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J. mys. Chem. 1983, 87,4885-4887
drew G. Dickson for his critical comments and help in discussing this work.
Acknowledgment. This work as supported by NSF Grant OCE81-13068. This paper was the result of work performed while the author was a guest of Prof. Roger Bates at the Chemistry Department of the University of Florida, Gainesville, FL. The author also thanks Dr. An-
Registry No. MgSO,, 7487-88-9; H2S04,7664-93-9; HC1, 7647-01-0.
A Spectroscopic Study of Photochromic 4,4’-Bis(3,5-diphenyIformazyl)biphenyl J. W. Lewis. and C. Sandorfy Depadement de Chimie, Universit6 de Montrhal, Montr6a1, Qusbec, Canada H3C 3V 1 (Received: January 17, 1983)
The ultraviolet-visible absorption and resonance Raman spectra of 4,4’-bis(3,5-diphenylformazyl)biphenylare presented and discussed. Solutions of the title compound in dichloromethane are highly photochromic. The ultraviolet-visible absorption spectra indicate the presence of two stable photoproducts. These spectra are interpreted in terms of a two stage, A + hu B + hu C, photoreaction. The resonance Raman spectra of the final form of the title compound indicate the presence of at least three structural isomers in unequal proportions. The term “resonance structures” is therefore inappropriate in the description of this particular compound.
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Introduction The photochromic properties of 1,3,5-triphenylformazan have been studied14 and two reaction schemes2s4have been proposed to account for the observed photoinduced spectral changes. In a recent report5 we extended the spectroscopic investigations of photochromic triphenylformazans to include resonance Raman results. By means of 15N labeling and selective substitution on the phenyl group at N-1, it was confirmed that two isomers exist, as earlier proposed6s7for both the initial and final forms of these molecules. These resonance Raman results also led to the conclusion that an excited-state proton transfer is the initial photoinduced event in the photochromism of triphenylformazans. The resonance Raman results did not show evidence for the presence of intermediate isomer^,^^^ thus it was suggested that modifications in the accepted reaction schemes might be necessary.8 The recent discove$ of the semiconductor capabilities of irradiated thin films of triphenylformazan also suggests that the photochemistry of these compounds is not as well-known as previously believed. For example, thin films of triphenylformazan, even those left unilluminated for 12 h, exhibited a photovoltage spectrum which agrees well with the optical absorption spectrum of the yellow, final form of the compound. In the present work we report ultraviolet-visible absorption and resonance Raman scattering data related to the photochromism of 4,4’-bis(3,5-diphenylformazyl)biphenyl, or bitriphenylformazan (TPF),. The photochromic transformation of this interesting double molecule proceeds step by step and, as a direct result, solutions of the comred yellowpound undergo color changes-purple upon (partially) selective irradiation. (TPF), is then a potential two-stage photochromic system.
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Experimental Section (TPF)2 was prepared by reducing 3,3’-(4,4’-biphenylene)bis[2,5-diphenyl-2H-tetrazoliumchloride], Aldrich Chemical Co., with dextrose in basic solution. *Present address: Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803. 0022-365418312087-488550 1.50lO
Following chromatography (3X) on neutral alumina and recrystallization (3x) in dichloromethane-methanol the compound wm found to be spectroscopically homogeneous. The ultraviolet-visible absorption spectra were recorded by means of a Cary 17 spectrometer. The resonance Raman spectra were recorded by means of a Spex Ramalog 6 spectrometer which was equipped with photon-counting detection and with a Coherent Radiation Innova 90-5argon-ion laser. The resonance Raman spectra of the initial and final forms of this compound were obtained by means of techniques outlined previous1y.j The radiant source was an Oriel Model 7340 equipped with a 200-W medium-pressure mercury arc. The source output was filtered by 12 cm of water to remove the infrared portion of the spectrum and by cutoff filters, either Corning 3387 or Corning 2424, to eliminate the ultraviolet portion of the spectrum and to restrict the visible output to the >450- and >580-nm regions, respectively.
Results a n d Discussion Ultraviolet-Visible Absorption Spectra. The ultraviolet-visible absorption spectra of the initial and final forms of (TPF)2 are given in Figure 1. The spectra recorded after intermediate periods of (partially) selective irradiation of the solution are also reproduced in the figure. The absorption maxima of the initial and final forms of (TPF), at 560 and 450 nm, respectively, should be compared with the values 488 and 407 nm for the corresponding forms of ~
~
~~~~~
(1) Hausser, I.; Jerchel, D.; Kuhn, R. Chem. Ber. 1949, 82, 195. (2) Kuhn, R.; Weitz, H. M. Chem. Ber. 1953, 86, 1189. (3) Langbein, H. J. Prakt. Chen. 1979, 321, 655. (4) Grummt, U.-W.; Langbein, H. J. Photochem. 1981, 15, 329. (5) Lewis, J. W.; Sandorfy, C. Can. J. Chem. 1983, 61, 809. (6) Otting, W.; Neugebauer, F. A. Chem. Ber. 1969, 102, 2520. (7) Fischer, P. B.; Kaul, B. L.; Zollinger, H. Helu. C h i n . Acta 1968, 51, 1449. (8)The two additional isomers are supposedly cis-syn and cis-anti. The N=N double bond stretching vibrations of symmetrically substituted cis-azo compounds are b a n inactive. The present azo derivatives are unsymmetrically substituted and the N=N stretching bands should be observable in the Raman spectra even though they may be of lesser intensity than those of the trans isomers. (These results do not completely rule out the presence of cis isomers since the Raman bands may be very weak and unobservable in the present experiments.) (9) Dahlberg, S. C.; Reinganum, C. B. J. Chem. Phys. 1982, 76, 2731.
0 1983 American Chemical Society
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The Journal of Physical Chemisfty, Vol. 87, No. 24, 1983
Lewis and Sandorfy
chart I ( T R I P H E N Y L F 0 R M A 2 A N)2
Ph
\
W A V E L E N G T H (nm)
Figure 1. Absorption spectra of (TPF),, 1.23 X lo4 M in CH,Ci,, l c m optical path, and the absorbance scale runs from 0.0 to 1.0. (-) Absorption spectrum of the inttiai, dark adapted solution. (- - - -) Absorption spectrum following irradiation (A > 580 nm) for 1 min. (- - - - - ) Absorption spectra following irradiation (A > 460 nm) for longer periods of time. Absorptlon spectrum of the final completely irradiated solution.
.. .-
(.--a)
TPF. Thus, in contrast to previous observations1°in which para substituents (chloro, bromo, iodo, and methoxy) on the phenyl group a t N-1 had little effect on the position of the visible absorption maxima of the two forms of TPF, substantial red shifts are observed in the present spectra. In addition, the molar absorptivities of the two forms of (TPF), are determined to be approximately twice the corresponding values of TPF., In pure, dry dichloromethane the yellow (fial) form and the mixtures of the various forms of (TPF)2are essentially stable during the period of time necessary for the recording of the absorption spectra. This stability was verified by scanning the monochromator in the opposite direction and noting the absorbance at various wavelengths. In all cases the changes in absorbance were less than &2%. The spectra shown in Figure 1are thus not significantly distorted by changes, during the process of measurement, in the relative amounts of the various forms. In subdued lighting solutions of the final form of (TPF), remain yellow for several hours, but the lifetime of this form is critically dependent on the purity of the initial compound and on the purity and nature of the solvent. In addition, even highly photochromic solutions of (TPF),.in dichloromethane are rendered nonphotochromic by brief periods of ultraviolet (220 < A < 290 nm) irradiation. The absorption spectra of the ultraviolet-irradiated solutions are not significantly different from those of unirradiated solutions; hence, the reasons for the lack of a photochromic response are unclear. Traces of acid produced in the photolytic decomposition of the solvent could be the cause since the photochromism of TPF is especially sensitive to traces of acid i m p ~ r i t i e s . ~ The two most important features in the spectra shown in Figure 1are the maximum a t 505 nm and the lack of an isosbestic point near 500 nm. Both these results indicate that a simple A B photoreaction sequence cannot explain the spectra. These findings are, however, readily explained by two successive photoreactions
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wherein the first (selective) photon results in an opening of only one of the formazan chelate rings (see Chart I). . (10)Unpublished data relative to ref 5.
\ Ph
#h
The resultant molecule would then contain two chromophores, a chelated triphenylformazan (A, = 500 nm) and an unsymmetrically substituted azo compound (Arnm = 410-450 nm). Unfortunately the present molecular system is not ideal and the absorption bands of the three forms are overlapping. A pure spectrum of form B is thus not obtainable. These spectra do, however, demonstrate that a two-state photochromic system based on the formazan ring is a possibility. Resonance Raman Spectra. The two structures of fully chelated (TPF)2shown previously in Chart I represent an enigma in structural chemistry. On the one hand, it may be argued that the two structures (two other structures are also possible) represent resonance structures and that the real structure lies somewhere in between the two extremes. It may also be argued that the two structures are spectroscopically distinguishable provided that the energy barriers separating the various forms are sufficient to isolate or freeze-in one or all of the possible structures. We now address this enigma for the specific example of (TPv2.
In previous infrared absorption,, nuclear magnetic resonance,' and resonance Raman5 studies it was suggested that the various forms of substituted triphenylformazans, both chelated and unchelated, were spectroscopically distinguishable. In these cases it may be countered that the substitution pattern on the various phenyl groups and/or 15N labeling of one or more of the formazan ring nitrogen atoms perturbs the system to the extent that one "resonance form" becomes the preferred one. The structures shown in Chart I1 are the equivalents of the final forms of TPF (a fourth form of (TPF), is possible provided that phenyl substitutents on the two formazan rings are different). Thus (TPFl2is not perturbed by substituents and its structural properties should be be greatly of those of TPF. The resonance Raman spectra of the initial and final forms of (TPF), are reproduced in Figure 2. The spectrum of the initial form is essentially the equivalent of that of TPF.5 The only difference lies in the relative intensities of the various bands. The phenyl group band near 1600 cm-' is somewhat more intense in the spectra of (TPF),, but this is entirely the expected behavior since more phenyl groups may participate than in the example of TPF. Two features of the spectrum of the f i a l form of (TPF)2 serve as markers for the distinguishability of the (four) possible structures. These are the 1600-cm-' (phenyl group) and the -1440-cm-' (azo group) regions of the spectra. Inspection of the structures in Chart I1 reveals
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The Journal of Physical Chemistty, Vol. 87, No. 24, 1983 4887
Spectroscopic Study of (TPF),
Chart I1
N %N
N
/ Ph
\
Ph
(TPFh
In view of the nature of the present results related to symmetrically substituted (TPF)2it is not entirely possible to assign a particular N=N double bond stretching band to a particular final isomer. On the basis of general chemical principles one might conclude that the initial isomer with the two azo groups nearest the biphenyl group would be the most stable. The electron deficient azo groups should prefer to locate nearer the more electron-rich environment. If our previous predictions5 are valid, the less intense, highest frequency resonance Raman band a t 1420 cm-’ in the spectrum of the final form should correspond to the N=N double bond stretching energy of that isomer with the N=N double bond furtherest from the biphenyl group. This interpretation is in keeping with our prediction of a proton transfer as the initial photoevent of the photo~hromism.~ Hence the most preferred initial isomer results, photochemically, in the least favored final isomer.
/N Ph (TRIPHENY LF OR M A 2 AN)?
knowledged to contain the N=N double bond stretching bands. The highest frequency component of this system of four bands is most likely attributable to (a) phenyl group ~ibration(s).~J’-’~ The intensities of the two phenyl group bands and of the three azo-group bands are clearly not equal. We thus conclude that the relative concentrations of the various “isomers” are not equal. If ”resonance structures” were a valid term for the description of the various forms of (TPF)2,equal intensities would be expected for the two phenyl group bands and for the three azo-group bands in the resonance Raman spectrum. This must be true since for resonance structures no single final form should be greatly preferred over another and all the possible structures should occur with nearly equal probability. It is thus likely that “isomers” exist also for the initial form of
a
Conclusions The ultraviolet-visible absorption and resonance Raman spectra of the initial and final forms of photochromic 4,4’-bis(3,5-diphenylformazyl)biphenyl have been presented and discussed. The ultraviolet-visible absorption spectra are interpreted in terms of an A + hv B + hv C photois~merization.’~The overlap of the absorption bands of isomers A and B are such that the pure spectrum of B is not attained. The resonance Raman spectra of the initial and final forms of (TPFI2indicate the presence of at least three structural isomers in unequal proportions. These latter results suggest that the term “resonance structures” is inappropriate in the description of these particular compounds. Registry No. 4,4’-Bis(3,5-diphenylformazyl)biphenyl,
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1600
1300
1000
R A M A N SHIFT.CM-I
Flguro 2. Resonance Raman spectra of (TPF)?, 1.87 X lo4 M in Ciici,, X = 5145 A: bottom curve, spectrum of the initial, red isomer: top curve, spectrum of the mixture of flnal, yellow isomers.
that three orientations of the phenyl groups are possible. The phenyl groups may be related to (a) an N-substituted aniline, (b) an azomethine group, and ( c ) an azo group. However, in (a) the phenyl group is not part of the conjugated system and is thus not expected to contribute significantly to the resonance Raman spectrum. Two distinguishable 1600-cm-’ phenyl group bands of unequal intensity are observed in the spectrum of the final form of (TPF)2.Examination of the possible structures of the final form also reveals four possible environments (two of which are equivalent) for the azo group. At least four bands are observed in the spectral region generally ac-
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23305-70-6. (11) Brandmuller, J.; Hacker, H.; SchrBtter,H. W. Chem. Ber. 1966, 99, 765. (12) Lorriaux, J. L.; Merlin, J.-C.; Dupaix, A.; Thomas, E. W. J. Raman Spectrosc. 1979,8, 81. (13) Kubler, R.; Lfittke, W.; Weckherlin, S. Z. Elektrochem. 1960,64, 650. (14) Further evidence for successive photoreactions A B C may be found in Mauser, H. ‘Formale Kinetik”; Bertelsmann Universitiitaverlag: Diisseldorf, 1974. (We are grateful to a reviewer for so kindly furnishing this reference.)
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