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Johnson for permission to quote the hyperfiltration results and for many helpful discussions. We are
also indebted to bl. D. Danford, C. G. Westmoreland, J. Csurny, and Neva Harrison for technical assistance.
Detailed Studies of a One-Electron, Two-Photon Ionization in a Rigid Organic Solution at 77"K1p2 by K. D. Cadogan and A. C. Albrecht Department of Chemistry, Cornell University, Ithaca, N e w York 1.4860 (Received August 8.4, 1967)
A quantitative spectrophotometric study is made of the photoionization of N,N,N',N'tetramethyl-p-phenylenediamine (Thf PD) in 3-methylpentane (3MP) rigid solution a t 77°K. Some extension is also made to an ether-isopentane-ethanol (EPA) rigid solution. The ionization is shown to proceed by a biphotonic mechanism with the metastable triplet state as the second photon-absorbing intermediate. It is further shown by polarization measurements that ionization of the triplet state occurs by direct excitation into the continuum or near continuum. Quantitative estimations are obtained and wavelength dependence is determined for the quantum efficiencies of both one-photon steps.
I. Introduction
The preliminary report4 has strongly implicated the lowest excited triplet state as a photoactive interFor some time now it has been known that it is mediate in the two-photon ionization process. The possible to photoeject electrons from organic molecules results of several quite distinct experimental studies in rigid solutions with light energy far below the taken together provide the evidence of triplet state gaseous ionization potential. The mechanism of this involvement. These results include the observation photoionization has been the subject of much recent that (1) the observed lifetime of the photochemical study. Photoconductivity studies3 produced early intermediate matches the phosphorescence lifetime; evidence for the involvement of two photons in the ( 2 ) the addition of 0 2 quenches both the long-lived photoionization of organic molecules in the condensed phosphorescence of TMPD, as well as the initial rate phase. The system most studied in this photoconducof formation of WB; (3) the wavelength dependence for tivity work has been N,N,N',N'-tetramethyl-pcreating the photochemical intermediate is the same as phenylenediamine (TMPD) in 3-methylpentane (3that for exciting phosphorescence; (4) ultraviolet IMP) at 77"K, which can be readily photoionized to excitation induces phosphorescence lifetime shortening, give the monopositive cation Wurster's blue (WB) and suggesting that there is an excitation-induced destruca "free" electron. Since then, several other biphotonic tion of the triplet state; and (5) polarization of the phenomena in similar systems have been observed and these will be specified below. In a preliminary r e p ~ r t , ~ second photon step, which is defined as the photoionization of the intermediate state, matches the polariwe have presented spectrophotometric evidence supzation of triplet-triplet (T-T) absorption over a conporting a biphotonic mechanism for photoionization of the TMPD-3MP system. The photoconductivity study3 naturally focuses on the mobile charge carriers (1) Supported by Public Health Research Grant A-3415 from the National Institute of Arthritis and Metabolic Diseases. (electrons), while this spectrophotometric work focuses (2).Taken in part from the Ph.D. Thesis of K. D. Cadogan, Cornell on the other photoionization product, the stable, imUniversity, Ithaca, N. Y., 1966. mobile cation (WB). The technique employed is to (3) G. E. Johnson, Thesis, Cornell University, Ithaca, N. Y . , 1965; monitor the cation photoproduct spectroscopically G. E. Johnson and A. C. Albrecht, J . Chem. Phyd., 44, 3162, 3179 (1966). while it is being formed, which is an elaboration of the (4) K. D. Cadogan and A. C. Albrecht, ibid., 43, 2550 (1965). initial spectrophotometric study made on this system ( 5 ) W. C. Meyer and A. C. Albrecht, J . Phys. Chem., 6 6 , 1168 in which the biphotonic mechanism was not discovered.6 (1962). Volume 78, Number 3 March 1068
930 siderable wavelength region. I n what follows, the details of this spectrophotometric work will be presented with a partial extension to another system. Details of the photoionization are thereby elucidated and estimates are obtained for the efficienciesand energy requirements for each of the consecutive one-photon steps.
11. Experimental Section A. Materials. Aniline (AN) (Mallinckrodt, purified) was vacuum distilled over zinc dust. N,iS,jY',N/-Tetramethyl-p-phenylenediamine(TnIPD) (from TMPD-2HC1, Eastman Kodak Co.) was sublimed from the prepared base6 then zone refined. White crystals resulted. The solvent 3-methylpentane (31IP) (Phillips petroleum, pure grade) was purified as described elsewhere.5 EPA (by volume: 5 parts diethyl ether, 5 parts isopentane, and 2 parts ethanol) (HartmanLeddon Co., Philadelphia, Pa.) was used without further purification. B. Apparatus and Procedure. Rectangular Pyrex or fused-quartz cells with 1 X 1 cm cross-sectional dimen.;ions were used to contain the samples under study. Solution concentrations varied from 0.5 x to 3 X low3N . Cooling to liquid nitrogen temperature was accomplished either by simply immersing the cell in liquid nitrogen, or by attaching copper plates to the cell and immersing the copper in liquid nitrogen inside a partially unsilvered quartz dewar. This latter method eliminated the interference of liquid nitrogen bubbles and schlieren in the optical measurements. (For further details see, ref 2.) Figure 1 is a schematic diagram of the apparatus used t o study photoionization rates. A General Electric A-H6 mercury lamp in conjunction with a Bausch and Lomb 500-mm focal length grating monochromator, having a dispersion of 33 &'mm, provided ultraviolet (primary) excitation. (Excitation within the absorption bands of the solute will be called pyimayy excitation.) The monochromator was positioned to make the exit slit horizontal. Both entrance and exit slits were usually set at l-mm width. An image 0.5 cm wide and 1 mm high was focused on the cell, which was mounted so that it could be raised or lowered independently of the dewar. Because the photoionized sections of the sample mere so thin, studies on previously unexposed sections of the rigid solution could be made at 2-mm intervals along the cell. As many as 20 individual studies could thereby be made on the same frozen sample. A second monitoring light beam was introduced by reflection off a thin quartz plate into the same optical path as the primary beam and focused at the same spot on the cell. This monitoring source consisted of a 110-W tungsten lamp stabilized by a constant-voltage transformer and focused through an Aminco grating monochromator (250-mm focal length, The Journal of Physical Chemistry
E(. D. CADOGAN AND A. C. ALBRECHT
7 5 Figure 1. Schematic, diagram of apparatus: A, monochromator; B, Hg lamp; C, tuiigsleii lamp; D, Xe-Hg lamp; E, mirror; F, thin quartz lamp; G, quartz dewar; H, sample cell; I, photomiiltiplier tube; J, Photovolt photometer (lModel 520-N); K, Varian G-14 recorder; and L, additional photomultiplier.
66-&'mm dispersion), which was also positioned to provide a horizontal slit image. Slit widths were set close to 1 mm and the focused image on the cell was typically 0.3 cm wide and 0.5 mm high. Care was taken so that the primary beam always completely overlapped the monitoring beam while passing through the cell. The rate of cation production was determined by measuring the change in optical density with the monitoring light at the 634-mp absorption peak of Wurster's blue (WB). The detector was an RCA-1P21 photomultipler tube attached to a Photovolt photometer (Series 520-11). The output of the photometer was fed into a Varian G-14 recorder with a counter voltage introduced to reduce the input signal to the recorder. In this way, only 20% of the photometer output could be displayed as a full-scale deflection on the recorder chart, and an intensity change as small as 0.2% in the monitoring light or, consequently, a 0.001 unit change in optical density, was detectable. A Corning glass filter (2424) in front of the phototube served to Beep out TnIPD lumiiiescence and a Corning filter (9863) in the primary-excitation beam removed any unwanted light from that source. A typical experimental rate study is shown in Figure 2. Ignoring the initial rise in optical density (OD) which is due to triplet-triplet (T-T) absorption, it was observed that the OD increased linearly with time up to at least a 0.04-unit change, but the rate of change would eventually approach zero after prolonged excitation (1 hr or longer). To avoid complicated analysis, rate measurements were confined to the linear region. Even so, over this period at least two separate rate
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Figure 2. A typical experimental study of photoionization rate in a 1.1 x 10-3 M rigid solution of T M P D in 3MP a t 77°K exciting a t 335 mp ( t , light on; J , light off). The optical density is measured a t 634 mp. The initial rise in optical density, as well as the decay, when the excitation is cut off, is due to the buildup and collapse of the transient (triplet) state. The essentially linear increase in optical density over this time scale represents the buildup of stable cation (WB).
measurements could be made without repositioning the sample. Since visible light can stimulate charge recombination in photoionized rigid solutions of TNPD,6 causing a decrease in WB concentration, it was checked and found that the weak red monitoring beam had no effect on the cation concentration. The photoionization rate as a function of wavelength was determined at 5-mp intervals, exposing a new section of the sample for each measurement. On any one frozen sample, a range of 50 mp could be covered while still allowing two separate measurements at most wavelengths to essay reproducibility. Occasionally, initial slopes €or two different wavelengths were measured without changing the positioning, in order to check the uniformity of the rigid solution. The average precision of initial slopes determined by these reproducibility checks was 6%. The light-intensity dependence of the photoionization rate was determined by varying the light intensity with neutral density filters over a range of almost one order of magnitude. The relative primary-light intensities a t different wavelengths were determined by using an integrating screen, which consisted of an aqueous solution of esculin (1 g/l.).' Absolute-intensity measurements were made using a ferrioxalate actinometer.8 For double-beam excitation, a method similar to that used in the photoconductivity work was employed. The additional source of excitation introduced (see Figure 1) was an Hanovia 2500-W xenon-mercury short-arc lamp intercepted by Corning filters (3850) and (5850) plus an NiSO, solution. This filter combination cuts off all light below 360 mp and passes a band peaking around 380 mp. This second source of excitation is called the secondary beam and does not directly excite the solute. When used alone, a negligible amount of photoionization occurs, but when employed in conjunction with the primary excitation, it dra-
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matically enhances the rate of ionization. Wavelengthand intensity-dependence studies of the ionization rate were repeated with this additional beam flooding the sample. As much as a 200-fold secondary-beam enhancement over primary excitation alone was observed when the primary light was sufficiently weak. I n any case, at least a tenfold enhancement was always maintained. T o accomplish this it was necessary, at times, to reduce primary-light intensities with neutral density filters in the regions of strongest A-H6 lamp output. As a check, a few two-beam measurements were also made with the secondary source focused at a right angle to the primary beam. The influence of oxygen on the photoionization was determined by studying a rigid solution with essentially three different, rather cleanly separated layers of oxygen concentration. This was accomplished simply by freezing the sample slowly from the bottom up while bubbling first helium and then oxygen through the still fluid portion. Photoionization rates as well as the steady-state phosphorescence intensity were measured in each layer. This intensity should reflect oxygen levels inversely, since oxygen quenches TR'IPD phosphorescence. Rate studies were also made with polarized excitation beams. The monitoring beam was already accidentally almost completely polarized along one laboratory axis (c), largely because of reflection off the quartz plate; however, experiments were also done with the polarization rotated 90". The intensity of the monitoring light for the unfavorable polarization was quite weak, but still suitable. Double Glan prisms were used to polarize each of the two excitation beams and a Polaroid sheet effected polarization of the monitoring beam. For these experiments, the secondary beam was focused at right angles to the primary beam (along the a laboratory axis). The above details apply to work with the TAIPD3RIP system. Some changes were necessary in order to study the photoionization of aniline (-4X) in EPA. The tungsten monitoring lamp was replaced by a compact Hanovia 150-W xenon arc lamp which was used to monitor a t 420 mp. This is, where the photoproduct, which is assumed to be the cation, has a maximum in its visible absorption band. Unfortunately the phosphorescence band of A S is in this same wavelength region. The resulting serious interference, due to phosphorescence, was eliminated by bucking with a signal from a second phototube positioned (see Figure 1) to pick up only sample phosphorescence. Photoionization proceeded much more slowly in the AN-EPA system, and the xenon lamp was somewhat less stable than the (6) W. M. McClain and A. C. Albrecht, J . Chem. Phys., 43, 465 (1965). (7) E. J. Bowen, Proc. Rou. SOC.(London), A154, 349 (1936). (8) C. G. Hatchard and C. A. Parker, ibid., A235, 518 (1956).
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