George M. Wyman and Bizhan M. Zarnegar
1204
It has been reported that an excited orotic acid reacts with acrylonitrile to form the 'acrylonitrile radical.9 The first step is almost surely a hydrogen abstraction reaction, but the authors report that they could not observe dihydroorotic acid, the expected product from disproportionation of two H. adducts, when the H. adduct is formed a t the 5, 6 bond positions. This would suggest that in their system, the hydrogen abstraction does not add a
hydrogen atom to the' C6 or Cg positions, but rather to a carbonyl oxygen as we suggest. Acknowledgments. The authors take pleasure in acknowledging the financial support of the Medical Research Council of Canada and the National Cancer Institute of Canada. We are indebted to Drs. J. W. Hunt, T. Penner, and D. W. Whillans for helpful discussions.
Excited State Chemistry of lndigoid Dyes. 11. The Interaction of Thio- and Selenoindigo Dyes with Hydroxylic Compounds and Its Implications on the Photostability of Indigo' George M. Wyrnan" and Bizhan M. Zarnegar Department of Chemistry, University of North Carolina, Chapel Hili, North Carolina 27514
(Received November 6, 1972)
Publication costs assisted by the U.S. Army Research Office-Durham
The fluorescence and the photoisomerization of trans-thioindigo is quenched by phenol and p-nitropheno1 at the diffusion-controlled rate. The effectiveness of ethanol, cy-trifluoroethanol, and a,a'-hexafluoro2-propanol as fluorescence quenchers ranges from 2.5 to 58% of the diffusion-limited value and increases with increasing acidity of the alcohol. The fluorescence quenching rates of selenoindigo and several dyes related to thioindigo with one or more of these hydroxy compounds were also determined. The results support the idea that an intramolecular proton transfer in the excited (Si)state is responsible for the unique photostability and lack of fluorescence exhibited by indigo and its ring-substituted derivatives.
It is now generally recognized that indigo (Ia) and its ring-substituted derivatives exist only as the trans isomers2,3 and that, in contrast with other indigoid dyes,4*5 they will not undergo photochemical trans cis isomerization,2q6 nor do they exhibit any appreciable fluorescence.' When it was observed that the N,N'-dimethyl-8,9 and N,N'-diacetyl derivatives2.10 undergo photoisomerization, the photostability of the parent compound was attributed to hydrogen bonding.2 Although the existence of hydrogen bonds in indigo in its ground state is now firmly established from crystallographic data,3 this does not provide a wholly satisfactory explanation for the photostability, since the energy required for exciting the molecule to its Si state (ca. 48 kcal) is far greater than the stabilization that may be expected from the formation of two typical hydrogen bonds (ea. 10 kcal). In a recent communication an alternative explanation was proposed by one of us for this remarkable photostability,ll suggesting that a fast proton transfer occurs in the Si state (as represented by eq 1) and that this takes precedence over the other excited state processes (fluorescence and cis-trans isomerization) normally encountered in of this type. The greatly enhanced (ca. 1O6-fo1d) acidity of Ar-NH groups and the similarly increased basicity of Ar-CO functions in the SI state is well known12 proton transfer phenomena have and such been reported for other compounds that are hydrogen
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The Journalof Physical Chemistry, Vol. 77, No. 70, 1973
7
I
Ia, X = NH Ib, X = S IC, X = S, 6,6'-diethoxyId, X = S, 6,6-di-n-hexoxyIe, X = S e
$.
0-H
bonded in the ground state, e.g., derivatives of salicylic acid,l3 o-hydroxyazines,l4 and some hydro~vbenzazoles.~~ (1) Presented in part at the V I International Conference on Photochemistry, Bordeaux, Sept, 1971. (2) w. R, Brode, E, 0, pearson, and G . M, wyman,J. A ~ Chem. ~ ~ SOC.,76, 1034 (1954). ( 3 ) H. V. Eller, Bull. SOC. Chim. Fr., 106, 1444 (1955); E. A. Gribova, Kristallografiya, 1, 53 (1956). (4) G. M. Wyman and W. R. Brode, J. Amer. Chem. Soc., 73, 1487
(1951).
.
Excited State Chemistry of lndigoid Dyes
1205
The previous communication11 contained preliminary results of the interaction of excited trans-thioindigo (Ib) as the prototype for “protonless indigo”l6 with phenol and p-nitrophenol. The present paper reports an extension of this study to ethanol and to two fluorinated alcohols; in addition the excited state interactions of several dyes related to thioindigo (IC, Id, Ie, and 11) with some of these
Results (a) Fluorescence Quenching. The fluorescence quenching results, based on Stern-Volmer treatment19 of the fluorescence intensities as a function of quencher concentration, are tabulated in Table I. It should be noted that the fluorescence of these dyes is unaffected by the addition of arfisole or p-nitroanisole, indicating that the quenching is neither due to electron transfer, nor to interaction of the dye with the nitro group. A comparison of the data in the first two lines of Table I with the corresponding results reported in ref 11 shows a downward revision of the I1 quenching constants (from 9.3 to 8.2). Since both phenols hydroxy compounds is also reported. Reexamination of the quench a t the same rate, we believe that this is the cordata reported earlier disclosed that the uniquely high rate rect diffusion-controlled rate, although it is almost 20% found for the inhibition of the trans cis isomerization of lower than the value of 1.1 x 1010 obtained from the Ib by p-nitrophenol was due to an experimental artifact Debye equation.20 Refinement of these measurements was and the correct rate for this reaction was determined. probably due to the availability of the Cary 17 instrument, the high sensitivity of which permits the accurate Experimental Section measurement of the spectral absorption of the weakly ab(a) Materials. Indigo (Ia), thioindigo (Ib), and 6,6’-disorbing solutions that are necessary for fluorescence meaethoxythioindigo (IC)were research samples obtained from surements. Allied Chemical Corp., the DuPont Co., and Ciba-Geigy (b) Cis-Trans Isomerization. The Stern-Volmer-type S. A., respectively. A sample of resublimed indigo (Ia) plots of the photostationary state concentrations obtained and 3,3’-dioxo-4,4,4’,4’-tetramethyl-2,2’-bithioanylideneon the thioindigo-phenol and thioindigo-p-nitrophenol (11) were provided through the courtesy of Professor W. systems using orange cut-off filters for the irradiation reLuettke (University of Goettingen). The samples of selenosulted in straight lines with slopes of 131 and 192.5, reindigo (Ie) and 6,6’-di-n-hexoxythioindigo (Id) were made spectively.ll However, when the absorption curves were available to us by Dr. D. L. Ross (RCA Laboratories). Recomputed for each isomer in the quencher-containing soagent grade phenol was used and the p-nitrophenol was lutions by the method of Blanc and Ross,z1 it was found recrystallized from benzene before utilization. Spectrothat p-nitrophenol enhances the intensity of absorption of scopic grade benzene was used for all measurements. the cis isomer at wavelengths greater than 500 nm. Thus ( b ) Fluorescence Quenching. Solutions of the dyes in the high (greater than diffusion controlled!) Stern-Volmer spectroscopic grade benzene were preirradiated for 5 (or slope found for this system was due to this artifact and more) min in the apparatus described previouslylo to obnot to an interaction of the quencher with a second, tain the photostationary state enriched with respect to the lower-lying excited state, as had been thought. In order to trans isomer. The exciting light was filtered either eliminate this difficulty, the Ib-p-nitrophenol system was through a Corning blue filter (usually No. 5-60) or a Jenareinvestigated by a kinetic technique, where the low conGlas interference filter with a transmission peak a t 459 versions make the results independent of the absorption nm. Irradiation was carried out through a rectangular intensity ‘of the cis isomer. Moreover, the Ib-phenol syswater bath in order to eliminate heat effects. After irratem was also measured again with the use of an interferdiation the quencher solution was added (in the dark) and ence filter to provide a narrower range of wavelengths for the fluorescence measured on a Hitachi Perkin-Elmer excitation and, of course, the Cary 17 instrument. The reMPF-SA fluorescence spectrometer. ( c ) Cis-Trans Isomerization. (1) Photostationary State R. Pummerer and G. Marondel, Chem. Ber., 93, 2834 (1960). Measurements. Appropriate mixtures of Ib and phenol (in Attempted photoisomerization of indigo in a degassed benzene soiubenzene solution), contained in a 1-cm quartz absorption tion by means of a flash photolysis apparatus with a 10-psec lifetime was unsuccessful. (D. G. Whitten, personal communication.) cell, were irradiated with light from a 300-W projection A weak fluorescence has been found (near 625 nm) in some samlamp through a Jena-Glas interference filter ( Xmax 552 pies of indigo in benzene. However, a resublimed sample showed essentially no fluorescence, nm) to obtain a cis-rich photostationary state. The dye J. Weinstein and G. M. Wyman, J. Amer. Chem. Soc., 78, 4007 concentration was kept constant within each series of If l.F_I R _A _ \, . measurements. C. R. Giuiiano, L. D. Hess, and J. D. Margerum, J. Amer. Chem. SOC., 90, 587 (1968). (2) Kinetic Studies. Appropriate mixtures of Ib and p G. M. Wyman and A. F. Zenhaeusern, J. Org. Chem., 30, 2348 nitrophenol (in benzene) were irradiated in an optical (1956). G . M. Wyman, Chem. Commun., 1332 (1971). bench, using the projection lamp and interference filter Cf. A. Weiler, Discuss. FaradaySoc., 183 (1965). described above, for identical periods of time (ca. 1 min) A. Welier, Z. Elekfrochem., 60, 1144 (1956). to bring about 4-8% conversion to the cis isomer. CorrecA. Weiier and H. Wolf, Justus Liebigs Ann. Chem., 657, 84 (1962). D. L. Williams and A. Heiler, J. Phys. Chem., 74, 4473 (1970). tion for the reverse reaction was calculated from the equaIt has already been reported that the conjugate acid cation obtion developed by Lamola and Hammond.17 tained by dissolving thioindigo in concentrated Bulfuric acid behaves very much like indigo does In inert solvents, hence this selection The relative concentrations of the trans isomer were deappears weii justified (cf. W. R. Brode and G. M. Wyman, J. Amer. termined spectroscopically by means of a Cary Model 17 73, 4267 (1951)). Chem. SOC., spectrophotometer, using 565 nm as the analytical wave(17) A. A. Lamoia and G. S. Hammond, J. Chem. Phys., 43, 2129 (1965). length and the recently reported absorption curves for (18) G. M. Wyman and 6. M . Zarnegar, J. Phys. Chem., 77, 831 thioindigo in benzene.18 Correction was made for the (1973). weak absorption due to the cis isomer a t this wavelength; (19) 0. Stern and M. Voimer, Z. Phys., 20, 183 (1919). (20) Cf. J. G. Caivert and J. N. Pitts, “Photochemistry,” Wiiey, New however, this was only necessary for the photostationary York, N. Y.. 1966, p 627. state measurements (at 565 nm t t 6220, ec 160). (21) J. Blanc and D. L. Ross, J. Phys. Chem., 72,2817 (1968).
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The Journal of Physical Chemistry, Voi. 77, No. 10, 7973
George M . Wyman and Bizhan M . Zarnegar
1206
TABLE I: Stern-Volmer Rate Constants and Excited State Lifetimes -~
S l o p e : I10
Dye
Quencher
Ib Ib Ib ib ib
Phenol p-Nitrophenol
IC
II
Phenol Phenol Phenol p-Nitrophenol Phenol
II
CF3CH20H
Id
le le
-
Figure 1. Stern-Volmer type plots of the inhibition of the trans cis isomerization of thioindigo by phenol and p-nitrophenol (solvent benzene):0 , (c/t)o/(c/t)sat the photostationary state vs. phenol concentration; X , (rate)O/(rate), at 4 4 % trans cis conversion vs.p-nitrophenol concentration.
-
sults obtained are plotted by the Stern-Volmer approach in Figure 1. The slope of the single straight line is in very good agreement with the corresponding values obtained from the fluorescence-quenching measurements for these two systems (cf. Table I).
Discussion of Results The excellent agreement between the fluorescence quenching constants in Table I and the rates of the inhibition of the trans cis isomerization (Figure 1) for the Ib-phenol and Ib-p-nitrophenol systems clearly demonstrates that there is a very fast, diffusion-limited interaction between the excited (SI) trans isomer and the phenol molecule. Furthermore, the phenols interact neither with the excited cis form, nor with the twisted intermediate that is almost certainly involved in the isomerization process. On the other hand, the possibility of a triplet intermediate is still not rigorously excluded, even though it has not been possible to detect such a precursor by flash photolytic techniques.18-22 The data in Table I also indicate that for the same dye there is a correlation between the quenching rate constants and the acidity of the quencher. This is especially apparent in the series of alcohols used; in contrast with ethanol, the two fluorinated alcohols are weak acids (pK, of CF3CH20H = 12.4, (CF&CHOH = 9.3)23 and the differences observed in the quenching rates strongly suggest acid-base interactions. At first glance phenol (pK, = 10.0) appears to be anomalous in this respect; however, the relative acidities in benzene may well be somewhat different than in aqueous solution where the pK, values have been determined. While p-nitrophenol had been selected as one of the quenchers, because it is more acidic than phenol, it only helped to confirm that the rate of the Ib-phenol interaction is diffusion limited. Dye I1 was surprising in two respects: first it was not expected to exhibit fluorescence at room temperature18 and, second, it was not expected to be as strongly basic in the excited state as its aromatic analogs, due to the lack of conjugation between the carbonyl groups and benzene rings. However, there is no doubt that in the excited (SI) state it is also interacting with phenol at the diffusion-controlled rate. Thus it appears that conjugation of the carbonyl groups with the sulfur atoms within the “basic chromophore” responsible for its indigoid character (absorption in the visi-
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The Journal of Physical Chemistry, Vol. 77, No. 10, 7973
113.4a
CF3CH20H (CF3)zCHOH CzHsOH 0.9 f 0.3a 0.96 2.3b 0.7 f 0.3a
~
kq T c
k, X IO9
110 f 2 112 f 2 22 f 1 65 k 1 2.6 f 0.1 8.4 f 0.1 7.4 f 0.1 19.0 f 0.3 18.5 f 0.1 7.6 f 0.2 4.1 f 0.1
8.2 8.3 1.6 4.9 0.19 9.3d 8.2 8.2 8.1 10.8d 5.9
T, nsec
aDetermined through the kindness of Professor W. R. Ware by the single photon counting technique (in air). Determined indirectly by assuming quenching at the diffusion-controlled rate of 8.2 X IO9. Leastsquares analysis. These values are probably too high: they are based on the value of T indicated in the table, but which is subject to considerable uncertainty (as shown).
*
ble, fluorescence and cis-trans isomerization)18-24is sufficient to enhance the basicity of the carbonyl groups in the excited (SI) state. While these results do not prove that an intramolecular proton transfer occurs when indigo (or one of its ring-substituted derivatives) i s excited to its SI state, nonetheless they are completely consistent with that concept. The observed very fast reaction of each prototype dye in its excited singlet state with phenol (and, whenever attempted, with p-nitrophenol) and the apparent acidity dependence of the rate of this interaction attests to the strong basicity of excited indigoid molecules. In view of what is known of the enhanced basicity of aromatic carbonyl compounds in the excited singlet state,l2 it is reasonable to assume that in indigo dyes the basicity is largely localized in the carbonyl groups. This work has also demonstrated that, once a donor-acceptor complex is formed between the excited dye and the hydroxy compound, the adduct will neither isomerize nor fluoresce. Since in indigo the potentially acidic protons are held in the close proximity of the potentially basic carbonyl groups by means of hydrogen bonding in the ground state, the occurrence of such an intramolecular proton transfer would seem to be exceedingly likely on excitation. Recent quantum-mechanical calculations have also indicated an appreciable increase in the electron density of the carbonyl groups and a corresponding decrease on the nitrogen atoms when indigo is excited to its SI state;25 there is little doubt that such a redistribution of the electron cloud in the molecule would favor, if not require, the proposed proton transfer. By analogy with the behavior of the systems studied in the present investigation, the product of such a proton transfer would not be expected to isomerize or to fluoresce, but to decay to the trans ground state by a radiationless process.26 Thus one may conclude that, al(22) Thioindigo can be isomerized through a triplet intermediate by using an appropri4te sensitizer. (Unpublished data of the authors.) (23) R. Filler and R. M . Schure, J. Org. Chem., 32, 1217 (1967). (24) H. Herrmann and W. Luettke, Chem, Ber., 101, 1715 (1968). (25) E . Wille and W. Luettke, Angew. Chem., lnf. Ed. Engi., 10, 803 (1971). (26) One of the referees pointed out that other molecules that COntain intramolecular hydrogen bonds (e.g., o-hydro~yazobenzenes~~ and o-hydroxybenzophenoneslB) have also been found to undergo fast radiationless deactivation to the ground state. (27) D. Gegiou and E, Fischer, Chem. Phys, Left., 10,99 (1971).
Quenching of Biphenyl Fluorescence by Inorganic Ions though the photostability and lack of fluorescence exhibited by indigo may seem anomalous, when viewed from the standpoint of ground-state chemistry, this behavior is consistent with that shown by related systems in the excited state.
1207
try Department of Duke University and the Spectroscopy Department of the Max Planck Institute for Biophysical Chemistry for making their facilities availaMe for carrying out some of this work. This research was supported by the U. S. Army Research Office-Durham.
Acknowledgments. The authors are greatly indebted to Professors ~ ~ R. ~Ware, A. ~ weller, ~ and D. k G. ~ (28) Whitten for many helpful discussions and to the Chemis-
w.
w,
N. J. Turro in "Technique of Organic Chemistry," Vol. XIV, P. A. , Leermakers and A. Weissberger, Ed., Interscience, New York, N. Y., 1969, p 223.
Quenching of Biphenyl Fluorescence by Inorganic Ions A. R. Watkins Max-Planck-lnstitut fur Biophysikalische Chemie, 34 Gottingen-Nikolausberg, Germany (Received June 13, 1972) Publication costs assisted b y the Max-Planck-lnstitut fur Biophysikalische Chemie, Gottingen
Examination of the quenching of biphenyl fluorescence by a series of inorganic salts shows that the measured rate constants, in comparison to earlier measurements for anthracene, are in most cases larger. This can be correlated with the more negative value of the excited state reduction potential on going from anthracene to biphenyl. However, electron-transfer fluorescence quenching, which would be expected in view of this behavior, is shown by flash photolysis not to occur for the systems biphenyl-NH4CNS and biphenyLN(CzH5)4Br, although it may be operative to a limited extent for the system biphenyl-KI. A mechanism which accounts for these experimental observations is suggested.
The fluorescence of many organic compounds in solution is quenched by the addition of inorganic salts. The anion of the salt is generally regarded as the species responsible for the quenching process; the commonly accepted mechanism is formulated as a transfer of an electron from the anion to the electronically excited molecule, followed by a reverse electron-transfer reaction which leads to the system in its original unexcited state.172
lM* -4- A-
+
-
M-
+A
+
M hv M ARecently, evidence has become available which appears to provide support for the mechanism outlined above. An investigation of the quenching of the fluorescence of a number of dyestuffs has shown that the quenching efficiency of inorganic anions shows a trend which parallels the trend in the oxidation potentials of the anions used as quenchers.3 In addition, a semiquinone intermediate, apparently resulting from an oxidation-reduction process, has been identified in the quenching of benzhydroquinone fluorescence by the cobalticyanide anion.4 In some cases, however, doubt has been expressed as to whether electron-transfer mechanism 1 is generally applicable. The quenching action of iodide and bromide ions has been attributed to a heavy-atom enhancement of the intersystem crossing rate,E+6 and an alternative general mechanism for fluorescence quenching by inorganic anions, in which an interaction depending on the polarizabi-
lity of the inorganic anion leads to the disappearence of the fluorescing state, has been p r ~ p o s e d .On ~ the other hand, the quenching of anthracene fluorescence by inorganic anions, including iodide and bromide, was recently shown not to occur by heavy atom perturbation of the spin-orbit coupling and it was postulated that fluorescence quenching takes place via an electron transfer mechanism.* In view of the confusion surrounding this topic, it seemed advisable to investigate the mechanistic details of this type of fluorescence quenching in more detail, focussing attention on polynuclear aromatics, the chemical and spectroscopic properties of which have been well characterized. This note reports the result of'an investigation of the quenching of biphenyl fluorescence by inorganic anions.
Experimental Section Biphenyl (Schuchardt) was recrystallized several times from ethanol and zone refined. Tetrahexylammonium (1) Th. Fdrster, "Fluoreszenz Organischer Verbindungen," Vandenhoeck and Ruprecht, 1951. (2) L. E. Orgel, Quart. Rev., Chem. Soc., 422 (1954). (3) D. K.Majumdar, Z. Phys. Chem.. 217, 200 (1961). (4) J. Feitelson and N. Shaklay, J. Phys. Chem., 71, 2582 (1967). (5) A. R . Horrocks, T. Medinger, and F. Wilkinson, Photochem. Photobioi.. 6, 21 (1967). (6) H. Leonhardt and A. Weller in "Luminescence of Organic and Inorganic Materials," H. P. Kailman and G. M. SDruch. Ed., Wiley. New York, N. Y., 1962. (7) J. Glowacki, Bull. Acad. Pol. Sci., 11, 487 (1963). (8) R. Beer, K. M. C. Davis, and R. Hodgson, Chem. Commun., 840 (1970)
The Journalof Physical Chemistry, Vol. 77, No. 10, 1973
