Time-Lapse Spectrometry in Organic Photochemistry Nicolae Filipescu, Fredrick L. Minn, and James W. Pavlik Departments of Chemi.rtry, The George Washington University, Washington, D. C. 20006, and Wisconsin State University, River F a h , Wis. 54022 Time-lapse spectrometry consists of monitoring absorption spectra after short irradiations, successive determinations of absorption throughout the subsequent dark period, and periodic recording of emission and excitation during and after irradiation. These measurements permit detection of intermediates, calculation of their spectra, quantitative determination of rate constants and reaction orders. The method is invaluable in mechanistic organic photochemistry. SPECTROMETRY IS AMONG the most valuable analytical tools in chemistry. However, except for flash photolysis, it is used almost exclusively for equilibrium systems. Spectrometric determinations have been used in this laboratory to great advantage, especially in the UV and visible ranges, on systems undergoing chemical changes. These investigations included detection of unsuspected intermediates in complex sequential transformations, quantitative determination of such kinetic parameters as stoichiometry and rate constants, and examination of the behavior of transient species in their reactions with selected analytical reagents. This procedure was particularly useful for investigating photochemical transformations. Most photochemical processes involve electronically excited molecules originally having energy in excess of the thermal energies required by ground-state reactions. Rather than dissipating by collision, the excess energy tends to remain localized in relatively longlived intermediates. These species may themselves absorb light and undergo further photochemical reactions or may change to stable products in a dark process. Work on the photoreduction of aromatic ketones (1-4) stimulated the use of a technique, time-lapse spectrometry (TLS), which deserves separate discussion; its usefulness, general applicability, and limitations are discussed herein. Although TLS is not restricted to light-induced processes, its use is detailed in organic photochemistry, with some nonphotochemical examples, as well. EXPERIMENTAL
Reagents. Preparation of 2,3-exo(4,5-methylenephenanthrene-1 1 ’-spiro-1”-cyclopropane)bicyclo[2.2. llheptane (5), di(4-pyridyl) ketone dimethiodide (3), di(p-tert-buty1)benzophenone (2), and spiro(anthrone-l0,l ’-cyclopropane) (6) has been described previously. Spectrograde solvents were used throughout. Irradiations were carried out with a 9W ultraviolet hand lamp (UVS-11 or UVL-22, Ultraviolet Products). Apparatus and Procedure. Absorption spectra were recorded on Cary Model 14 or 15 Spectrophotometers; (1) N. Filipescu and F. L. Minn, J . Amer. Chem. Soc., 90, 1544 (1968). (2) N. Filipescu and F. L. Minn, J. Chem. SOC.,Secf. B, 1969, 84. ( 3 ) F. L. Minn, C. L. Trichilo, C . R. Hurt,and N. Filipescu, J . Amer. Chem. SOC.,92, 3600 (1970). (4) N. Filipescu, F. E. Geiger, C. L. Trichilo, and F. L. Minn, J. Phys. Chem., 74, 4344 (1970). (5) N. Filipescu, J. R. DeMember, and G. R. Howard, J. Chim. Phys., 1970, 84. (6) N. Filipescu and J. W. Pavlik,J. Chem. SOC.,Secf. C , 1970, 1851.
emission spectra, on an Aminco-Bowman Spectrophotofluorometer. In photochemistry, the TLS method consists essentially of three kinds of spectrometric measurements: (1) monitoring absorption spectra of solutions following successive and relatively short intervals of irradiation, (2) consecutive determinations of absorption spectra following a significant photochemical conversion and at intervals throughout the dark until no change is observed for an extended period of time, and (3) periodic recording of emission and excitation spectra, preferrably in the frozen sample but possibly in the liquid phase, in conjunction with or simultaneous to steps 1 and 2. Since many chemical and photochemical reactions are altered by the presence of atmospheric oxygen, it is convenient to degas the reagents under vacuum in reservoirs such as those shown in Figure 1, prior to mixing or irradiation. Although the cell allows examination by emission spectrometry of species detected by absorption, the two spectra could be recorded from separate cells corresponding to “halves” of that illustrated. Dilutions or concentrations of sample can be done conveniently in the sealed cells by cooling the reservoirs in liquid nitrogen. RESULTS AND DISCUSSION
TLS measurements should probably accompany most mechanistic organic photochemical investigations. Additional valuable information is obtained toward clarifying the overall reaction mechanism, information which cannot be secured by relatively long photolyses followed by product identification. TLS implies a separation of the transformations occurring during irradiation from those taking place in the subsequent dark period. Moreover, the intermediates can be mixed at any chosen time during the transformation with selected coreactants, including acids, oxidizing agents, or chemical trapping agents. Furthermore, quantitative kinetic calculations can easily be performed from TLS data. The importance of sequential emission measurements should not be minimized. First, analytical measurements based on fluorescence and phosphorescence are in many cases more sensitive and selective than those based on absorption, especially for mixtures (7-9). Second, freezing the reacting system may trap transient species physically by arresting diffusion and allow recording of their spectra. Furthermore, low temperatures enhance and clarify fluorescence and add triplet-to-ground-singlet phosphorescence, which is not normally observable in fluids, There are certain limitations, of course, to the applicability of TLS. First, the transient species must be stable enough that recording time or freezing time be minimal compared to their lifetime. Many reactions will not yield relatively longlived intermediates. Even in such cases, however, step 1 should be carried out, since it may help to establish stoichiom(7) C. A. Parker, “Photoluminescence of Solutions,” Elsevier,
Amsterdam, 1968. (8) R. S. Becker, “Theory and Interpretation of Fluorescence and Phosphorescence,” Interscience, New York, N. Y., 1969. (9) S. Udenfriend, “Fluorescence Assay in Biology and Medicine,” Academic Press, New York, N. Y., 1962.
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n
t
3.0
Figure 1. TLS sample cell C, fused quartz 1-cm2cuvette; R1and R2,degassing side reservoirs, 20 mm i.d. and 50 mm long; P, quartz probe for matrix-isolation spectrometry; F,flame-sealableconstriction tube; SIand S2, quartzto-borosilicate glass graded seals; H, holder for immersion in an
Aminco-Bodman or Hitachi-Perkin-Elmer coldfinger Dewar
Figure 3. Changes in the UV-visible spectrum of wlO-4M solution of 4 in degassed acetonitrile in the presence of zinc (1) After reaction with zinc was complete. (2)-(9) After 15,25, 36, 70, 102, 109, 137, and 160 days, respectively, in the dark. (10) After admission of air
0.3 0.2 0.1
of a certain intermediate, the dark reaction may proceed significantly or the intermediate itself may react photochemically. Several other investigations somewhat related to TLS can be found in the recent literature. Time-resolved spectrometry (11) yields concentration information from analysis of phosphorescence decay curves. One interesting study (12) follows emission spectra in time. Others (13-17) refer mainly to thermal reactions, do not exhibit the presence of intermediates, and d o not employ emission spectrometry. The examples given below have not been reported previously except for the photoreduction of di-p-tert-butylbenzophenone, which is included here because it is highly illustrative of TLS. Illustrative Examples. Figure 2 shows consecutive changes in the UV-absorption spectrum of a rigid model compound 1 undergoing a photorearrangement to 2 upon successive short
c
L 230 240
250 260 270 280 290 300 310 320 WAVELENGTH, nm
Figure 2 Changes in the UV-absorption spectrum of 1 (3.0 X 10-5Min isopropyl alcohol) on consecutive irradiation ( 1)
etry in the first or subsequent photochemical transformations. Second, TLS will not replace flash-photolysis techniques (IO), which are invaluable for identification of the reacting excited states and of the short-lived products of the primary photoprocess, such as neutral free radicals. Thus TLS essentially borders on the fraction-of-a-second range of flash photolysis. A third limitation is that the results from spectrometric concentration may not necessarily extrapolate to preparativescale reactions. For instance, during the relatively long irradiation time required to generate sizeable concentrations (10) See, for example. G. Porter. in “Techniques of Organic Chemistry, Vol. 8,” A. Weissberger, Ed., Interscience, New York, N. Y . . 1963, p 1055 and references therein. 84
1
Before irradiation. (2t(8) After 1-minute exposures
2
(min) irradiations. This conversion can be followed quantitatively also by monitoring emission spectra either at room temperature or at 77 OK. The characteristic fluorescence of phenanthrene (340-420 nm) is replaced by the intense fluorescence of tri(11) P. A. St. John and J. D. Winefordner, ANAL.CHEM., 39, 500 (1967). (12) P. F. Jones and A. R. Calloway, 160th National Meeting ACS, Chicago, Ill., Physical Abst 173, 1970. (13) K. A. Muszkat and E. Fischer, J. Chem. Soc., Sect. B , 1967, 662. (14) L. Senatore, E. Ciuffarin, and A. Fava, J . Amer. Chem. Soc., 92, 3035 (1970). (15) K. R. Huffman, C. E. Kuhn, and A. Zweig, ibid., p 599. (16) D. K. Lin and C. S. Garner, ibid.,91,6637 (1969). (17) R. D. Temple, J . Org. Chem., 35, 1275 (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
WAVELENGTH, mu Figure 40. UV absorption spectrum of 1.04 X 1OV3M solusolution of TBB in (CH3)2CHOHat different times
WAVELENGTH, mu
(1) Before irradiation, (2) after 3-min irradiation (h) to 67% conversion, (3) after 30 min of dark reaction (past to), (4) after 365 min, (5) 1.8 days, (6) 6.8 days, (7) 9.8 days, (8) 13.8 days, (9) 20.8 days, (10) 33.9 days, ( t f )projected
-1
Figure 5. Absorption spectrum of 1.01 X 10-3M solution of TBB in (CH3)2CHOH. Reaction of the enol intermediate with (CH&CHONa. (1) Before irradiation, (2) after 3-min irradiation to 52% conversion, (3) after addition of (CH&CHONa (h),(4) 8 min after to, (5) 31 min, (6) 66 min, (7) 106 min, (8) 166 min, (9) 1451 min, (10) after 116, 400 min and after bubbling 0 2 through the open cell for 1 hour (unchanged)
I
t O O ' 3d0
-----____ 13'
'-----L-
' 3 h ' 3 i O ' 3AO ' 3780
4 400
420
440
WAVELENGTH, mu
Figure 4b.
Continuation of Figure 4a
projected, (10) after 33.9 days of dark reaction, (11) 41.8 days, (12) 48.8 days, (13) 55.8 days, (14) 62.7 days, (15) 69.8 days, (16) 76.9 days, (17) 83.8 days. (18) 90.9 days, (19) 97.7 days, (20) 104.7 days, (21) 111.7 days, (22) 118.7 days, (23) 132.7 days, (24) 146.7 and 159.7 (unchanged), (25) after opening and passage of 0 2 for 3 min (tf)
benzofulvene 2 (structureless band in the 450-540 nm range with A,, 490 nm). The gradual decrease in phenanthrene phosphorescence band (450-580 nm) as the rearrangement proceeds can also be followed by recording emission from samples frozen after each irradiation. Actually, the analogous reaction of spiro-linked fluorene-norbornane was used, in connection with TLS, as the photochemical detector for intramolecular energy transfer in two rigid model compounds (18, 19).
Figure 3 illustrates the zero-order conversion of magenta 523 nm) radical cation 3, prepared from di(4-pyridyl) ketone dimethiodide 4 and zinc in degassed acetonitrile, to a
,,A(,
(18) J. M. Menter and N. Filipescu, J . Chem. Sac., Sect. B , 1970, 464. (19) N. Filipescu and J. R. Bunting, J . Chem. Sac., in press.
diamagnetic red species, presumably a zwitterion, ,,A,( 480 nm) over a period of several months in the dark. The six isosbestic points at 244, 286, 291, 291, 303, and 309 nm in Figure 2 and one at 498 nm in Figure 3 testify to the absence of side reactions and to constant stoichiometry. In addition, they indicate that molecule 2 is photostable. If the behavior is non-isosbestic, the departures may be analyzed by multicomponent absorption spectrometry to give quantitative'lnformation regarding the relationship between competitive processes and to determine the absorption spectra of intermediate species. Figures 4a and 46 display spectral changes accompanying the dark reaction of the first intermediate generated from di(p-tert-buty1)benzophenone (TBB) by an initial brief photolysis. It was demonstrated (2) that the intermediate (Curve 2) reacts in a bimolecular process with the residual starting material in competition with a significantly slower reaction in 345 nm) changes in a firstwhich the enol intermediate ,,A(, order reaction to a visible-absorbing species ,,A,( 380 nm). This is illustrated clearly in Figure 46, which gives spectral changes beyond the first stage. Despite apparent complexity, it was possible to elucidate individual steps responsible for spectral changes in the reaction of the enol intermediate with alkoxide (Figure 5) and to calculate the absorption spectra of three other intermediates,
ANALYTICAL C H E M I S T R Y , VOL. 43, NO. 1, JANUARY 1971
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200
250
300
350
400
450
500
WAVELENGTH, nm
370 400 430 460 WAVELENGTH, mu Figure 6. Calculated extinction coefficients for intermediates. The intermediate from unsubstituted benzophenone, InB, is also included for comparison 310
340
n/ I
90
50
_LLL-uLL_LLL
200
250
300
350 400 WAVELENGTHmv
450
500
5
Figure 7. Emission and excitation spectra of 10-3M TBB in isopropyl alcohol glass at 77 OK,uncorrected. Emission (1) and excitation (2) before irradiation. Emission (3) and excitation (4) after 3-min irradiation at room temperature
designated Inz, In’z, and.In8, in Figure 6. The use of lowtemperature emission spectrometry allowed the detection of still another transient fluorescent species in the photoreduction of TBB (Figure 7, Curves 3 and 4).
86
Figure 8. UV-visible absorption spectra of N ~ O - ~of M 5 in concentrated HzS04. (1) Before irradiation. (2)-(12) After consecutive 30-sec exposures to 420-nm light
A final illustration of the wide applicability of TLS to various solvents and solutes is the photolysis of a hydroxycarbonium ion in concentrated sulfuric acid. Figure 8 shows changes in the absorption spectrum of spiro(anthrone-lO,l’cyclopropane) in H2S04 upon short exposures to violet light. Again, the gradual changes and the presence of nine isosbestic points at 211, 228, 255, 270, 286, 310, 364, 405, and 439 nm indicated absence of side reactions, constancy of molar relationships among products, photostability and thermal stability of products, and general features of the photoproduct absorption. Furthermore, from knowledge of light intensity, absorbance of solution at the exciting frequency, and duration of the consecutive irradiations, one can estimate the efficiency of the photoreaction. ACKNOWLEDGMENT
We thank Mr. W. Carrion and Dr. H. Plotkin for their continued interest. RECEIVED for review July 10, 1970. Accepted October 12, 1970. Work partially supported by the Atomic Energy Commission Contract AT-(40-1)-3797. Part of this work was carried out in the Optical Systems Branch of Goddard Space Flight Center, NASA.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 1, JANUARY 1971
