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Infrared Stimulated Duryl Radical Fluorescence
On Infrared Stimulated Duryl Radical Fluorescence in Rigid Solutions of Durene in 3-Methylpentane at 77’K1 F. P. Schwarz and A. C. Albrecht” Department of Chemistry, Cornell University, lfhaca, New York 14850 (Received March 22, 1973) Publication costs assisted by the National lnstitutes of Health
After a dilute solution of durene in 3-methylpentane at 77°K is photosensitized in the ultraviolet, not only is the normal infrared stimulated charge recombination (ISCR) durene luminescence seen, but a weaker ISCR duryl radical fluorescence is found as well. Unlike the durene ISCR luminescence, the intensity of the duryl radical ISCR fluorescence component increases linearly with photosensitization from a nonzero initial level. This initial level, however, behaves identically with the durene ISCR luminescence with regard to changes in various parameters such as intensity of ultraviolet light and sample preparation. When perdeuterated durene is examined in a similar fashion there is no change in the durene ISCR luminescence signal but the initial duryl radical ISCR luminescence is reduced by about onethird. This isotope effect is identical with that found for two-photon @-bond cleavage and the initial ISCR duryl radical fluorescence is attributed to a charge recombination @-bondcleavage yielding excited duryl radicals. The efficiency appears to be about 1 in lo4 ISCR produced excited durene molecules. The growth of radical ISCR fluorescence with repeated cycling is attributable to the steady buildup of a new one-photon ionizable species which, when ionized, gives an ISCR luminescence in which the duryl radical fluorescence is a significant component.
I. Introduction Infrared stimulated emission is now a well-known property of rigid organic solutions at 77°K which have been photosensitized with ultraviolet light. Such photosensitized solutions normally exhibit a weak visible emission or afterglow which is dramatically enhanced by exposing the solution to near-infrared light (red to - 2 p ) . At the same time infrared stimulation is known to induce a transient increase in the electrical conductivity of the solution.2 Without prior ultraviolet photosensitization however, the sample is uiiresponsive to such long wave illumination. The ultraviolet photosensitization brings about the ejection of an electron from the absorbing solute molecule (usually a biphotonic event), and the electron is trapped in the solvent matrix in the vicinity of the partner cation. This trapped electron can be ionized with low-energy light in the near-infrared region to cause either complete charge separation (photocurrent) or charge recombination to give luminescing excited states of the original solute molecule. Ionization of the solute molecule is not always the only conspicuous biphotonic chemistry occurring in such solid solutions. When the solute is a suitable benzene derivative, a biphot-onic p-bond cleavage is often observed. Such biphotonic events appear to take place via excited solute states lying near the ionization continuum. Since infrared stimulated charge recombination (ISCR) leads to highly excited states of the solute molecule, the question is raised whether ISCR can bring about @-bond scission. Such charge recombination chemistry has been observed in solutions of 1,2,4,5-tetramethylbenzene(durene) in 3methylpentane (3-MP) a t 77°K and this forms the subject of the present report. The durene-3-MP system exhibits both biphotonic ionization and biphotonic @-bond cleavage leading to a 1,2,4-trimethplbenzyl radical (duryl radical) and a hydrogen atom. A quantitative photochemical study of this sys-
tem has just been completed3 and it is found that both photochemical channels occur with similar quantum yields when exciting durene from its lowest triplet state into triplet states near its ionization continuum.4 Incidentally, a one-photon route for p-bond cleavage via vibra. ~ ISCR in this tionally excited Si was also d i s ~ o v e r e d The system not only yields the usual durene recombination luminescence (fluorescence and phosphorescence) but in addition gives a recombination fluorescence in the green which appears to be that from an excited duryl radical. The charge recombination seems to bring about @-bond cleavage while leaving the benzyl radical product in its first excited doublet state. When such rigid solutions are photosensitized, and then subjected to ISCR, the ionization channel is reversed. The biphotonic p-bond cleavage, however, is not reversed but instead is seen to increase upon repeated recycling of the uv-photosensitization-ISCR steps. As long as the uv-photosensitization step is not severe (or the irreversible steps are not efficient), the sample can endure many such cycles without significant depletion of the parent molecule. In fact, the ISCR induced durene luminescence is found to be reproducibly constant from cycle to cycle. But this is not true of the new green recombination fluorescence attributed to ISCR induced p-bond scission. This charge recombination induced duryl radical luminescence increases linearly with repeated cycling and extrapolates to a nonzero initial value. The presentation of this study is, thus, divided into two main parts. At first the behavior of the nonzero initial ISCR duryl radical fluorescence is compared with that of the more familiar ISCR durene fluorescence under a variety of conditions. It is shown how the initial level of ISCR duryl radical fluorescence is a consequence of ISCR @-bond cleavage leaving excited duryl radicals. The efficiency for this ISCR @-bondcleavage is compared with the efficiencies3 for 6-bond cleavage The Journal of Physicai Chemistry. Voi. 77, No. 20, 1973
F. P. Schwarz and A. C. Albrecht
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by the one- and two-photon excitation of durene itself, and it is seen how still a third mechanism is indicated for this ISCR induced @-bondcleavage. In the second part, the behavior of the growth portion of the ISCR duryl radical fluorescence seen upon repeated cycles is studied as a function of the photosensitization conditions of the sample. It is argued how the increase is due to ISCR between the electron and some unknown cationic species which is produced irreversibly during the photosensitization. The species is one photon, near-uv ionizable, and is, itself, stable with respect to infrared stimulation. Its characteristics closely resemble a new species which has been found in photoconductivity studies2 of a variety of aromatic derivatives (including durene) in 3-MP at 77°K where, following an initial biphotonic sensitization, a one-photon induced photocurrent is found in the near-ultraviolet region ( a region which had been inactive prior to photosensitization in the further uv).
11. Experimental Section A. Materials Durene, perdeuterated durene, and the 3-MP were obtained and purified as described earlier.3 B. Apparatus. The apparatus has been described previously and was used with one modification.3 The green duryl radical fluorescence overlaps the spectral region of the durene phosphorescence. To separate one emission from the other, lock-in techniques were used. The ISCR radical fluorescence follows the modulation of the infrared light (100 Hz) (since recombination and fluorescence proceed on a nanosecond time scale). This modulation is achieved by a mechanical chopper placed between the sample and the infrared light source. A Par lock-in amplifier is used. ISCR radical fluorescence is thus studied by first uv photosensitizing the sample and then stimulating it with modulated near-ir light while observing the modulated emission using filters which isolate the green region of the spectrum. All samples were 1.3 X M of durene in 3-MP a t room temperature and, unless otherwise specified, were purged with dry helium before cooling to 77°K. Except where otherwise noted, the uv photosensitizations were carried out at 275 nm with a bandwidth of -15 nm. Under these conditions,3 the photon flux was typically 7 x l O 1 * photons/cm2 sec. 111. Results and Discussion A Introduction. A typical modulated ISCR green fluorescence signal, F R X ( t ) ,as detected by the lock-in amplifier with a 0.3 sec time constant is presented in Figure l a . In Figure l b the modulated ISCR durene fluorescence F x ( t ) , as detected by the same instrument (but with different detection filters) is displayed. The similarity of the time behavior of these fluorescences is apparent. (The asterisk signifies a recombination process in keeping with the notation previously used3). In this investigation the time-integrated form of the ISCR fluorescence signals is used. These are symbolized by FR* and F* and correspond to the areas under the curves in Figure l a and l b , respectively. Each of these parameters must be proportional to the total number of recombinations which lead to the given fluorescence and. as areas, are insensitive to the intensity of the infrared light used. The spectrum of the duryl radical fluorescence in 3-MP a t 77°K has been observed6 and resembles closely the The Journal ofPhysical Chemistry, Vol 77. No. 20. 1973
(0)
(1.R. on)
.!w\ n
(b)
= 5 z
time (sec)
The time course of (a) the ISCR induced green fluorescence, F R * ( t ) . and (b) the ISCR induced durene fluorescence F * ( f ) . Figure 1.
spectrum seen by others under different conditions. The green ISCR fluorescence, FR*(t), seen in this present work, and regarded as duryl radical fluorescence, is quite weak and was not fully spectrally resolved. However, great pains were taken to identify its spectral characteristics using various filter combinations. Appropriate Varian monoband-pass and Corning filters are used to provide band passes
