W. C. MEYER
2118
450 nm is a measure of the amount of photoreaction. When the lifetime of lucigenin is plotted us. AA, a straight line should result if the reaction proceeds from the singlet state, since chloride quenches the singlet state of lucigenin. Figure 4 shows the plot of hA us. the lifetime which indicates that the photoreaction proceeds through the first excited singlet state of lucigenin. It might be argued that chloride quenching of the lowest singlet and triplet states of lucigenin could account for the results of Figure 4. Although this is possible, to have the slope of Figure 4 so close to unity and the reaction proceed mainly from the triplet would require a rather fortuitous combination of excited-state
lifetimes and quenching rate constants. Long-term photolysis of lucigenin has yielded a product of yet undetermined structure. Traces of N-methylacridone are present, and it has been shown that the reaction product is not dimethylbiacridine.
Acknowledgments. The work was supported in part through funds provided for by the U. S. Atomic Energy Commission under Contract AT(30-1)-905. We wish to thank Anthony Vaudo for his assistance in the flash photolysis work. We also thank a reviewer for suggesting the explanation for the lack of oxygen quenching.
Halogen-Sensitized Photoionization of N,N,N’,N’-Tetramethyl-p-phenylenediamine in Liquid Halogenomethanes by W. C. Meyer Phusical Research Laboratory, The Dow Chemical Company, Midland, Michigan
48640
(Received October 8, 1969)
Production of Wurster’s Blue is shown to be a one-photon process independent of the solvent dielectric constant. Sensitization is confined to halogenated electron acceptors. The constancy of quantum yields with excitation energy excludes vibronic photochemistry. Yields in chlorinated solvents are distinctly less than in brominated solvents, which lends credence to the contention that the organic halide undergoes dissociative reduction to stabilize the transient ion pair of the excited donor-acceptor entity.
Introduction Photoionizations of aromatic molecules isolated in rigid solutions have been found to be biphotonic phenornena,’r2 with evidence that the triplet state is an intermediate which absorbs the second quantum to complete the ionization step.2 In fluid solution the two-photon requirement is sustained, but a partially ionized state replaces the triplet state as intermediate, and a single quantum path emerges in solvents of low polarity, thought to be due to the trace presence of oxidizers. When certain electron-accepting solutes are deliberately added to aromatic amines in hydrocarbon glasses, photoionization is promoted; in fact in some cases no ionization occurs in their a b ~ e n c e . ~(The reaction also assumes a linear dependence on light intensity.) Donor-acceptor complexes, a type of association expected in these systems, can dissociate into ion radicals in polar solventss or upon light absorption.6 The single quantum ionization path has received less attention with regard to ionization efficiency, excitation wavelength dependence, etc. Because there was a T h e Journal of Physical Chemistry
variability in the efficiency of organic halides to enhance photoionization and possible mechanisms other than dissociative reduction of the halide mere not consideredj4 a more thorough study of the role of the organic halide in the process was undertaken. How then is sensitization promoted by organic halides? Charge-transfer complexes between aromatic amines and halogenomethanes have been r e p ~ r t e d . ~ What relation, if any, does complex formation have with sensitized photoionization? Is complex formation (1) J. P. Ray and T. D. S. Hamilton, Nature, 206, 1040 (1965). (2) K. D. Cadogan and A. C. Albrecht, J . Phys. Chem., 72, 929 (1968).
(3) R. Potashnik, M. Ottolenghi, and R . Bensasson, ibid., 73, 1912 (1969). (4) M. Kondo,
M .R. Ronayne, J. P. Guarino, and W. H. Hamill, J . Amer. Chem. Soc., 86, 1297 (1964). (5) R. Foster and T. J. Thomson, Trans. Faraday SOC., 5 8 , 860 (1962). (6) C . Lagercranta and (1962).
M . Yhland, Acta Chem. Scand.,
16, 1043.
(7) K. M. C. Davis and M. F. Farmer, J . Chem. SOC.,B , 28 (1967)
HALOGEN-SENSITIZED PHOTOIONIZATION PROCESSES a necessary but perhaps not sufficient requirement for sensitization? Do halogenomethanes act as separate entities, such as energy reservoirs for photoejected electrons, or do they intimately partake in the initial step of ionization? Heavy-atom effects, which must be present to some degree, are one source of perturbation for the process. I n short, by what pathway does a single photon of 3.1 eV cause ionization in rigid solutions? Experimental evidence is amassed to answer these questions. First, photoionization of the aromatic amine N,N,N’,N’-tetramethyl-p-phenylenediamine (TMPD) is studied in liquid solution to determine the dependence of quantum yield with halogenation of the solvent and the effect of solvent dielectric constant and other variables on the process. The light intensity dependence is stringently tested. The phenomenon is extended to polymer films in the following paper. Conditions for photoionization of aromatic amines are outlined and the donor-acceptor association between the amine and halogenomethane is verified. Luminescence and ionization thresholds are compared to provide information about the nature of association and its relation to photoionization. Advantage is taken of polymer rigidity to study the nature of trapping sites from a comparison of thermaland light-induced detrapping of charged species a t room temperature and 77°K. Polymer films lend themselves nicely to luminescence studies, and the third paper treats halogen perturbation of the fluorescence and phosphorescence polarization of TRlPD in polystyrene films. When results are correlated to the oriented photoproduct (Wurster’s Blue), the overall pathway of photoionization becomes evident.
Experimental Section TMPD, obtained as the dihydrochloride salt from Eastman Kodak, was precipitated as the free amine from aqueous KOH, dried, and sublimed. Liquid halogenomethanes were all used immediately after being freshly distilled. All other materials were used as received. Room temperature absorption spectra were run on the Cary 15 spectrophotometer. Photoionization experiments were followed using an HB200 superpressure mercury light source and a uv Bausch & Lomb monochromator with a dispersion of 3.2 nm/mm for excitation. Slit widths of 2 mm gave sufficient light intensity for photoionization and allowed experiments t o be realistically conducted at 10-nm intervals. The solution was continuously stirred during the experiment to ensure a homogeneous concentration of product throughout the cell. A tungsten light source and visible monochromator set a t right angles to the uv light source provided the means for monitoring blue color formation. The monochromator was set to
2119 pass 570-nm light, a wavelength where Wurster’s Blue alone absorbs. Light transmitted through the sample was detected with a 1P-28 photocell. An interference filter in front of the photocell eliminated uv and other stray light. Light transmission with time of irradiation of uv light was recorded on a Sargent Model SR recorder. The cell was quartz with the dimensions 1.9 X 1.9 X 6 cm; an extended tubing served as a holder. The cell conveniently held 25 ml of solution. For determining the light intensity dependence, neutraldensity filters consisting of calibrated wire screens were used. The exciting light could be varied an order of magnitude. Concentrations of M T M P D in various solvents were prepared in the dark with some precaution taken to exclude oxygen; nitrogen was bubbled through the solutions for 10 min. TMPD was irradiated in its first electronic absorption band at 10-nm intervals. The rate of production of Wurster’s Blue was linear with time for 30 sec or more. To simplify calculations and eliminate effects of photoproduct light absorption in the uv region, the rate was calculated at time t = 0 from the slope of the 570-nm optical density-time curve. Fresh solutions were substituted when 10% conversion of T M P D to Wurster’s Blue had taken place. Relative lamp intensities were determined using an integrating screen of esculinU8 The absolute intensity was found for one wavelength using a ferrioxalate actinometer,S and the remainder was converted to absolute values.
Results An influence of solvent dielectric constant on photoionization of certain organic dyes has been reported by Holmes. lo A general increase in ionization efficiency with increasing solvent dielectric constant was seen, with the onset occurring a t a value greater than 4.7. Hence the possibility of change of solvent dielectric constant through addition of solutes must be probed. Solvents were chosen to cover a range of dielectric constants. The results are presented in Table I. Included is the observation of whether TRlPD fluorescence was present or absent when irradiated. Several important facts emerge from the data. The first is that the dielectric constant per se does not assume dominance until high values are reached. This parallels the results of Kainer and Uberle,l’ who found the complete transfer of an electron from TMPD to chlorani1 did not take place in solvents of low dielectric constant, but it did occur in acetonitrile; the transfer was not photoinduced, however. In the present work photoionization took place only in halogenated solvents irrespective of their dielectric constants. A correla(8) E.J. Bowen, Proc. Roy. SOC.,A154, 349 (1936). (9) C.G.Hatchard and C. A. Parker, ibid., A235, 518 (1956). (10) E.0.Holmes, Jr., J . Phys. Chem., 70, 1037 (1966). (11) H.Kainer and A. Uberle, Chem. Ber., 88, 1147 (1955). Volume 7 4 Number 10
M a y 14,1970
2120
W. C. MEYER
Table I: Solvent Effect on Wurster’s Blue Production and TMPD Fluorescence Solvent
Dielec const
Acetonitrile Cyclohexanone Pyridine CHzCla CHtBra CHaI
37.5 18.3 12.3 9.1 7.2 7.0
Methyl ether CHCL CHBrs Ethyl ether o-Xylene Toluene CCla
5.0 4.8 4.4 4.3 2.6 2.4 2.2
TMPD fluorescence
Slight Slight No No No TMPD insoluble Yes PJo No Yes Yes Yes No
96
TMPD photoionian
Slight Slight No Yes Yes
98 9 7 1
100% 55%
I /
22%
10%
Ir I O990
0
10
20
30
40
50
60
T I M E , SECONDS
No Yes Yes No No No Ppt formed
tion between Wurster’s Blue formation and the absence of TMPD fluorescence is evident. (The well-known quenching action of halogen-containing compounds toward luminescence of emitting molecules accounts for the lack of fluorescence in these solvents apart from any competitive photochemistry, and any correlation may be coincidental. Further experiments are needed to clear up this point.) Photoionization of TMPD (and other aromatic amines) in these solvents has several directions from which it might proceed. Enhanced singlet-triplet transitions induced by a heavy-atom effect could both quench fluorescence and promote ionization, the latter either through a triplet-triplet annihilation mechanism12 or through the absorption of a second photon directly.2 A nonlinear dependence on exciting light is predicted for either mechanism. Another mechanism is a donor-acceptor type of association wherein the quenching mechanism involves charge-transfer interaction similar to that witnessed for substituted naphthal e n e ~and , ~ ~ionization arises from the complete transfer of an electron when light absorption occurs within the complex. In fact an analogous path similar to the above mechanisms is possible if low-lying triplet chargetransfer energy levels are thermally populated, since then a single photon could conceivably effect ionization in the manner of intramolecular triplet-state participation. Fluorescence might be quenched because of competing photochemistry from the amine-excited singlet state, an ionization pathway advanced by Meyer and Albrecht14 but erroneously based on a one-photon absorption process. If, in addition to or in spite of the aforementioned possible roles of organic halides in sensitized photoionizations, dissociative reduction of the halide is an essential ingredient, and thus in the final sense the halide acts as a separate entity to stabilize the transient ion pair by removing excess electronic energy, then The Journal of Physical Chemistry
t
Figure 1. Variation of rate of TMPD photoionization with change in intensity of 310-nm excitation in steps from 100% intensity to 10% intensity. The ordinate is the intensity of 575-nm light transmitted by Wurster’s Blue (cell path 1.9 cm; solvent CHCI,).
sensitizer efficiency should to some extent depend on the strength of the C-X bond (X = halogen), greater efficiency occurring in compounds having weaker bonds. One crucial test of many of the potential mechanisms is the dependence of the rate of photoionization on exciting light intensity. The light intensity dependence was carefully measured for all excitation wavelengths studied in the quantum yield determinations. Figure 1 reproduces the experimental rate curves generated for one set of conditions. The linearity of the initial rise permits slopes to be easily resolved. The results for Wurster’s Blue production in various halogenomethanes are summarized in Table 11. The rate was assumed to take the form (dC/dt)po = 410” = Ic(dD/dt),=o =
- (k/I&v) (AI/At) t=o (1) Wurster’s Blue concentration, C, at time t is related to its optical density, D, through the proportionality constant k using Beer’s law. The quantum yield, 6, and incident light intensity, Io,are determined in the normal manner. The slope of decreasing transmitted visible light with time of uv irradiation, divided by the average transmitted light in the time interval, At, is thus proportional to the rate of Wurster’s Blue production (the right-hand side of eq 1). The cell path length and TMPD concentration were chosen such that all incident uv light was absorbed. The light exponent of 1 accords with results of other systems in rigid glasses.4r16 Triplet-triplet annihilation or absorption of a second photon by the triplet state or other biphotonic mechanisms are eliminated as possibilities. (12) L.P. Gary, K. de Groot, and R. C. Jarnagin, J . Chem. Phys., 49, 1577 (1968). (13) 9. Ander, H. Blume, G. Heinrich, and D. Schulte-Frohlinde, Chem. Commun., 745 (1968). (14) W. C. Meyer and A. C. Albrecht, J . Phys. Chem., 66, 1168 (1962). (15) J. P. Simons and P. E. R. Tatham, J . Chem. Soc., A , 854 (1966).
HALOGEN-SENSITIZED PHOTOIONIZATION PROCESSES
Table I1 : Light Intensity Dependence of Photoionization Rate in Different, Halogenomethanes' n
Solvent
(light exponent)
CHCla CHBr3 CHCh CHzBrz
1.12 =k 0.17 l.lOi0.04 0.91 i0.03 0.94 i 0.05
Av 1 . 0 2 & 0 . 0 7 Values for 360-nm excitation.
The extent to which fluorescence may be quenched through competition of photochemistry from the excited singlet state, and if any vibronic effects in the photochemistry are manifested,16 are investigated from the magnitude and wavelength dependence of the quantum yield. Quantum yields of Wurster's Blue production as a function of excitation wavelength and solvent are presented in Table 111. Yields for brominated methane derivatives do not extend to shorter wavelengths because of light absorption complications of the solvent. The extinction coefficient of the photoproduct in the visible region was determined for each solvent using the procedure outlined by Meyer and Albrecht. l4 Table I11 : Quantum Yields of TMPD Photoionization as a Function of Excitation Wavelength and Solvent A,
lo'+
7 -
nm
CHCls
CHzClz
280 290 300 310 320 330 340 350 360 370 380 390
5.2 4.9 5.3 5.0 4.7 3.7 4.1 3.5 4.6 4.5
5.9 5.9 3.5 3.4 3.4 3.9 4.4 4.6 3.9 2.8
...
...
-
CHaBrz
CHBra
...
...
...
8.7 6.9 7.4 7.2 8.2 7.7
...
... 7.2 7.5 6.8 8.0 7.8 6.9 6.8
The constancy of 4 with excitation energy excludes vibronic photochemistry of the excited singlet state of TRIPD. The almost random variation of 4 is taken as a measure of experimental accuracy, albeit the downward trend at longer wavelengths is probably not an experimental artifact. The low values of 4 definitely
2121 eliminate fluorescence quenching based on competitive photoionization; instead the formation of either chargetransfer complexes or enhanced singlet-triplet intersystem crossing can explain this quenching. Quantum yields in chlorinated solvents are distinctly less than in brominated solvents, a trend not quantitatively established by Hamill and coworkers4 in hydrocarbon glasses. This result supports the contention that organic halide dissociation, where a C-X bond is broken, governs the efficiency of the sensitizer. An interesting observation, which also lends credence t o halide dissociation, arose when p-chloranil was added to form a 1:1 ratio of TMPD to chloranil in CHCL. TMPD failed to ionize when irradiated. Chloranil forms a strong 1: 1 complex with TMPDlll and apparently the proximity of the strong electron acceptor shielded TMPD from the near-neighbor presence of halogenomethane solvent molecules, thus preventing stabilization of any transient electron transfer through reductive dissociation of the more unstable solvent halide. (A complex of TMPD-chlornnil in the polycrystalline state has a threshold energy of 5.0 eV for photoemission of e l e c t r ~ n san , ~energy ~ requirement not met in this work.) When added t o a photolyzed sample, chloranil increased the rate of fading of Wurster's Blue. The acceptor either must have complexed with TRilPD+ and then abstracted the ejected electron (from whatever its environment) or may first have formed an anionic species before migrating to the cation (and in a sense desolvated the electron). The overwhelming concentration of heavy-atom solvent molecules undoubtedly masked any oxygen effect on the rate of photoionization. Oxygenated and degassed samples showed no change in photoactivity. The critical and specific presence of halogenated electron acceptors which undergo dissociative reduction easily, in contradistinction to acceptors merely having differing electron affinities, is indicated but has not been thoroughly pursued. The influence of temperature and viscosity on the rate of ionization also have not been ascertained. These and other considerations, such as the extent of heavy-atom effects as a source of perturbation and the relative orientation of any donor-acceptor complexes (whether TMPD is a 7 ~ -or n-type donor), are taken up in subsequent papers where a rigid environment is employed. (16) R. S. Becker, E. Dolan, and D. E. Balke, J. Chem. Phys., 50, 239 (1969). (17) T. Hibma, J. G. Vegter, and J. K. Kommandeur, {bid., 49, 4755 (1968).
Volume 74, Number 10 May
14,1970
