J. Phys. Chem. 1995,99,10471-10483
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AI, Fluorescence Excitation Spectrum of Liquid Benzene from 250 to 150 nm Vladimir 0. Sa&? and Sanford Lipsky* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 Received: November 8,1994; In Final Form: April 20,199P
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The BzZu At, fluorescence quantum yield of benzene has been obtained for excitations from 250 to 150 nm in the solvents n-hexane, isooctane, n-perfluorohexane, n-pentane, cyclohexane, and cis-decalin. In n-hexane and isooctane, measurements have been made over a range of benzene concentrations from 0.112 to 11.2 M (the neat liquid). In dilute solutions, the sudden losses in fluorescence as the excitation wavelength sweeps from the first into the second absorption system and then again as the excitation wavelength sweeps from the second into the third absorption system are correlated with the behavior of benzene vapor in the region of channel three. Increasing the benzene concentration causes abatements in the rate of loss of fluorescence with decreasing excitation wavelength in the regions of the second and third absorption systems and, above 2 M, causes a reversal of this rate so as to cause a recovery of the fluorescence in the region of the third absorption system. These changes are correlated with the effects of benzene concentration on the electronic absorption spectrum and with reported effects of clustering on the multiphoton photoionization efficiency.
I. Introduction
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Peculiar variations in the B2" AI, (hereafter referred to as SO) fluorescence quantum yield of liquid benzene have SI been reported for excitations into upper electronic states Sn.I Since the S I SO emission spectrum remains independent of excitation energy, these variations were interpreted to reflect changes in the efficiency with which upper states internally converted to the fluorescing state, S I . The pertinent variable in these studies was the ratio ,BAA) of the SI SO fluorescence quantum yield @.AI) for excitation at wavelength I to that at some wavelength AI on the long-wavelength side of the SO SIabsorption system. Thus, it was found that PAL), although constant for any excitation wavelength within the first absorption system (Le., equal to unity for AI = 270-230 nm), declined rapidly on entrance into the second absorption system (beginning at ca. 225 nm) and continued to drop to a value of 0.5 at 1 = 210 nm. From 210 to 200 nm, which spans the region of maximum B I , (SO absorptivity of the second absorption system,2 AI, S 2 ) , ,&I) remained relatively constant but then again plummeted on entrance into the third absorption system at A = 200 nm and continued to decline to a value of 0.3 at 1 = 190 nm. From 190 to 185 nm, an interval in which the liquid benzene AI, Elu ( S O S3) absorption exhibits a broad maximum,2 PAL) remained constant at 0.3. Below A = 185 nm, as the SO S3 absorption spectrum begins to decline in absorptivity, PAL) was observed to recover, achieving at A = 150 nm the same value as at 210 nm of ca. 0.5 (and at 140 nm a value of 0.8).3 This peculiar anticoincidence of the spectrum of PAL) vs L (hereafter referred to as the fluorescence excitation spectrum) with the absorption spectrum was also obtained for neat liquid toluene, p-xylene, and mesitylene, albeit with somewhat less pronounced declines in PdA) on entrance into the SO S2 and SO S3 absorption ~ y s t e m s . ~Indeed, .~ with increasing methyl substitution there appeared to be generally an approach of Pr (1)to unity over the entire region of variation of L from 270 to 120 nm.
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On leave from the Institute of Chemical Kinetics and Combustion, Novosibirsk 630090, Russia Abstract published in Advance ACS Abstracts, June 1, 1995. +
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A few measurements were also made on 4% by volume solutions of the aromatics in either cyclohexane or i~ooctane.',~ Again, pdA) remained unity within the SO S I absorption system, declined sharply on entrance into the SO Sz system, and then again declined sharply on entrance into the SO S3 system. Measurements in these dilute solutions were only obtained to ca. 190 nm, and accordingly, no tumaround in Pr (A) was observed. In the case of benzene, for all I from 225 to 190 nm, BAA) was significantlylower in the dilute solution than in the neat liquid. Several years later very similar results were reported by Birks et aL4 The addition of p-terphenyl to neat liquid benzene at a concentration too low (ca. 0.01 M) to provide any significant competition with the benzene for light absorption quenches the SI SO fluorescence of benzene and replaces it, via energy transfer, with an S I SO fluorescence from p-terphenyl.' Defining a new excitation spectrum variable, Pt(I),to be the ratio of the fluorescence quantum yields from p-terphenyl for excitation of the benzene at wavelength I to that for excitation at 1 1 ,it was found that, from I = 225 to 150 nm, P,(I) showed qualitatively the same wavelength variation as /?AI),except that their ratio, p,(I)lpAI), was somewhat larger than unity and increasingly so in regions where the optical absorptivity of the benzene was largest (Le., from A = 185 to 190 nm).1.3 Very similar results to these were obtained using 2,5-diphenyloxazole (PPO) instead of p - t e r p h e n ~ l . ~ The possibility of some artifactual cause for the peculiar anticoincidence of the fluorescence action spectra and the optical absorption spectra was considered in all of these early investigations. Thus, as the absorption coefficient increased and the penetration depth of the exciting light accordingly decreased, there would be increasing probability within the lifetime of the fluorescing state (ca. 30 ns for the alkylbenzenes)6 for its diffusion to the window-liquid interface. Any quenching, due to some interaction of the excited molecule with the window or with photochemical products at the interface would, perforce, generate the observed anticoincidence. However, that this could not be, at least, the entire origin of the effect was indicated by the fact that the anticoincidence was not much altered by replacing the ca. 30 ns lived SIstates of the alkylbenzenes with the ca. 1-2 ns lived S I states of p-terphenyl or P P O . ' S ~ . ~ Additionally, the effects of dilution of the alkylbenzene with
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0 1995 American Chemical Society
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10472 J. Phys. Chem., Vol. 99, No. 26, 1995
saturated hydrocarbons, which, of course, should have significantly increased the penetration depth of the exciting light at the dilutions studied (Le., 4% by volume), were found to have relatively small effects on the fluorescence excitation spectra, at least, over the spectral range A = 270- 190 r ~ m land, . ~ indeed, in the case of benzene, to even cause a decrease in pr(1). The exact extent of the correction required for the “penetration depth” artifact was obtained at one wavelength, 1. = 185 nm, by Lawson et al.7,8 It was shown that the addition of 0 2 and CCld, both of which strongly reduce the lifetime of SI benzene, increased pf(185). As the concentrations of these quenchers approached 0.01 M for 0 2 and 0.05 M for CCL, pr(18.5) approached a limiting value independent of the nature of the quencher and of further increase in its concentration. Also, it was reported that on addition of PPO, as its concentration approached 0.01 M, p,(185) also approached a limiting value which, within a few percent, was the same limiting value as had been obtained for pf(185) using 02 and CCld. These limiting values were then interpreted as being the “corrected” S3 S I conversion efficiencies (Le., corrected for surface quenching by virtue of having adequately reduced the lifetime of the emitting species so as to make such quenching negligible). For neat liquid benzene, this limiting value was 0.45 (as compared to pf(185) = 0.36 when all SI quenchers were absent). In what follows, we will refer to the corrected quantum yield simply as p( 185). Fuchs and Voltz9 suggested a possible explanation for the recovery of Pfcl) to the blue of ca. 190 nm. Their system was benzene containing a-naphthyl-2-phenyl-5-oxazole (aNP0)at 0.018 M, to which was added chloroform at concentrations from 0.05 to 0.5 M. They monitored the fluorescence of a N P 0 and reported that the chloroform strongly reduced P(1) beginning at a wavelength close to the position of the minimum in PfcA). In view of the known high-electron capture cross-section of chloroform, they suggested that at ca. 7.0 eV (180 nm) there existed a threshold for photoionization of neat liquid benzene. Accordingly, in the absence of chloroform, the recovery in p(A) at energies above the threshold was attributed to recombination of the ionized electron (or the negative ion of a N P 0 ) with the benzene positive ion to directly generate the SI state of benzene (or of aNP0). Thus would be avoided the inefficient cascade of Si to S I via the neutral states. This explanation was also adopted by Schwarz and Mautner’O to account for the effects of chloroform, methyl chloride, and ethyl chloride on PdA) for neat liquid benzene over an excitation range from 270 to 110 nm. For chloroform and methyl chloride, they observed effects very similar to those obtained by Fuchs and Voltz9 (albeit the methyl chloride had less quenching effect than chloroform) but, for ethyl chloride, reported essentially no effect even up to concentrations of 0.9 M.” Adopting the same point of view as had Fuchs and volt^,^ they suggested electron capture to be the origin of the enhanced quenching beginning, according to their data at ca. 175 nm, and thus set an ionization threshold of neat liquid benzene at 7.1 eV. The absence of an effect for ethyl chloride was considered to be consistent with its smaller capture cross-section for the thermal electrons. In a subsequent paper,” these quenching measurements were extended to other alkylbenzenes with similar results to those for benzene. In neither the work of Fuchs and Voltz9 nor of Schwarz and MautnerIo,” did these authors address the question of why the upswing in p ( l ) always occurred at significantly lower energies (Le., at ca. 6.6 eV in benzene) than their suggested ionization thresholds. Additionally, in both sets of measurements, one can see evidence of enhanced quenching by chloroform even at significantly longer wavelengths within the SO SZ absorption
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Saik and Lipsky system, and indeed, in somewhat earlier work, WuI3 had reported that for dilute solutions of the alkylbenzenes in isooctane the addition of chloroform had its first noticeable effect on p ( A ) well within the SO S2 system. Thus, were the enhanced quenching effects of the alkyl halides due to an ionization threshold, this threshold would have to be set at a much lower energy in dilute solution (e.g., for 4% benzene in isooctane at 6.2 eV). Since these dilute solutions exhibited no minimum in P ( A ) at 6.2 eV,I4 the connection of an ionization with the upswing was somewhat precarious. Additionally, recent photocurrent measurements on neat liquid benzene have reported a photocurrent threshold at an energy of 7.65 eV,I5 significantly above the 6.6 eV position of the minimum, thus mandating the development of a new explanation for the recovery of the fluorescence. The present investigation was undertaken to reopen the question of the origin of the peculiar features of the benzene liquid fluorescence excitation spectrum and to study how these features develop as the concentration of benzene increases from low to high concentrations. Benzene has had so much spectroscopic attention in recent years that it seemed appropriate to once again attempt to unravel the photophysical properties of its liquid state at high excitation energies. To assist us in this regard, we have recently reexamined the SO S? and SO S3 absorption spectrum of concentrated solutions of benzene in n-hexaneI6 and confirm previous surmises that there indeed exists a strong nonartifactual correlation with the fluorescence excitation spectrum. The origin of this mirror-like anticoincidence has not previously been addressed.
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XI. Experimental Section Measurements from 270 to 150 nm were made with a Spex F212 spectrofluorometer modified as has been previously described.” Samples were illuminated either with a 30 W Hamamatsu L879 D? lamp (fitted with a MgFz window) or with an electrodeless lamp containing 220 Torr of Xe. The Xe lamp was constructed from a Pyrex tube to which was attached via a WAgCl seal a 1 mm thick LiF window. Power for the lamp was supplied from a 2450 megacycleds Raytheon microwave generator operating at 25 W. The Xe lamp supplied a continuum from ca. 150 to 190 nm whose intensity, although lower and somewhat less stable than that from the Dz lamp, had a much slower variation with wavelength. This aspect of the Xe lamp was utilized in a search, albeit negative, for any possible artifacts in the experimental fluorescence excitation spectrum (which is a ratio of two independent scans, one of sample and one of monitor) that might have been caused by the rapid variation in intensity in the Dr lamp from 155 to 175 nm. Exciting monochromators were usually run with a band-pass of 1-2 nm, and the analyzers, at ca. 5-10 nm. The sample cells were illuminated at normal incidence, and the fluorescence was collected at an angle of 22.5” from the normal.” Sample cells were fitted with 1 mm thick LiF windows connected to the cell body either with WAgCl seals (to Pyrex) or with indium O-rings (to copper). Occasionally, a Suprasil quartz window was used in order to assess contributions from stray excitation light. Such contributions were, however, always found to be of negligible significance. The intensity of the exciting light was usually monitored with a screen of sodium salicylate that had been sprayed onto the back surface of a LiFI8 window from a saturated solution in absolute ethanol and periodically inserted into the position otherwise occupied by the cell front window. Although the windows for the screen and for the sample cells were chosen to be as similar as possible in their optical transmission, their
Fluorescence Excitation Spectrum of Liquid Benzene similarity was generally lost after some time due to disparity in their aging characteristics. Accordingly, the screens were periodically checked by placing in the sample cell a liquid monitor of either neat cis-decalin or neat perhydrophenanthrene and monitoring their fluorescence at ca. 230 nm. The cis-decalin has been previously shown to exhibit a fluorescence efficiency which is independent of excitation energy from 120 to 185 nm.I9 Perhydrophenanthrene has been found, in this investigation, to behave similarly but with somewhat higher intensity and over a larger spectral range (120-190 nm). Corrections have not been applied for reflection losses at the window-solution interfaces. For dilute solutions, these would not be particularly significant, by virtue of our use of cis-decalin or perhydrophenanthrene as monitors of the exciting light in the region where such correction would be most important. For the neat liquid, such corrections are more important but still of relatively minor significance. A calculation using the experimental absorption coefficientsI6 and refractive indices20of the neat liquid indicates that corrections for reflection loss never exceed ca. 8% and vary sufficiently slowly that none of the features that we observe in the fluorescence excitation spectrum are significantly affected either in spectral position or in magnitude. In front face fluorescence measurements, the collection efficiency of the emitted light becomes independent of excitation wavelength only for sufficiently small optical penetration depths. To satisfy this constraint for our system, it was required that the sample's optical absorption coefficient remain above ca. 10 cm-I. From examination of the absorption spectrum of benzene,2q16qz1it follows that this constraint is satisfied for all solutions that we have studied for excitation wavelengths A. 217 nm and at all A. for benzene concentrations greater than ca. 1.5 M. At lower concentrations and longer excitation wavelengths, our measured fluorescence intensity at selected values of A. will be too low by virtue of inadequate collection efficiency. This, however, is only a problem in the region of very low optical absorptivity between the SO SIand SO S 2 absorption systems from ca. 230 to 217 nm. Although no formal corrections were made for these effects, they have been qualitatively considered in interpretations of the pertinent excitation spectra. All fluorescence intensities were monitored at a single wavelength of the fluorescence spectrum either at the fluorescence maximum or (in the case of examination of the fluorescence of the saturated hydrocarbons when benzene was added to them) sufficiently to the red of the emission maximum so as to avoid any attenuation of the solvent fluorescence by the benzene. Although monitoring was done at a single wavelength, this was always preceded by a sweep of the emission spectrum to confirm its reliability. Unless otherwise specified, all measurements were made at 20 "C on nitrogenated solutions. For some preliminary measurements at lower temperatures, cooling was accomplished using precooled gaseous NZas heat exchanger. The temperature control was f 2 "C for periods sufficiently long for a reliable sweep of the excitation monochromator (ca. 20 min). Benzene (Omnisolve, glass distilled), n-hexane (Mallinckrodt, spectrophotometric grade), and isooctane (American Burdick and Jackson, high purity solvent) were usually used without purification after establishing that additional treatment22had no significant effect on the fluorescence excitation spectra. Both cis-decalin (Aldrich, 99%) and perhydrophenanthrene (Chemicals Procurement Inc.) were additionally purified by percolation through activated silica gel. All other chemicals employed in this investigation were purchased in high purity and usually
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J. Phys. Chem., Vol. 99, No. 26, I995 10473
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Wavelength, nm Figure 1. Fluorescence excitation spectra from 180 to 250 nm of benzene in n-hexane at concentrations of (a) 0.1 1 2 M, (b) 0.224 M, (c) 1 . 1 2 M, (d) 2.24 M, (e) 4.4g M, (f) 8.96 M, and (g) 11.2 M (neat liquid). The ordinate of spectrum a is shown correctly but spectra b-g are each successively shifted upward by 0.20 ordinate units.
employed without further treatment. Absorption spectra to 170 nm were obtained with a Cary 15 spectrophotometer, and those to shorter wavelengths, with a McPherson (230 S) 1 m vacuum monochromator in conjunction with a McPherson (665) doublebeam attachment.
III. Results As has been previously reported,] the emission spectrum of benzene (over both monomer and excimer was found to be invariant to excitation wavelength except for slight reabsorption effects. These were manifested only on the shortwavelength edge of the emission spectrum (and only over a rather narrow band of excitation wavelengths that are weakly absorbed within the SO S I system). The fluorescence excitation spectra were therefore monitored using analysis wavelengths sufficiently to the red of this edge to avoid the reabsorption. Figure 1 shows the fluorescence excitation spectra of benzene for a series of representative solutions in n-hexane from 0.1 12 to 11.2 M (neat liquid benzene) over an excitation spectral range from A. =' 250 to 190 nm. For convenience of presentation, each spectrum has been successively shifted upwards by 0.10 ordinate units. All spectra have been nomalized to unity at the wavelength of maximum SO SIoptical absorptivity (A. = 254-255 nm). At this wavelength, the decadic optical absorptivity, k(A), of benzene even in the 0.112 M solution (where k(254) = 250 M-' cm-1)21 is adequately large to ensure complete light absorption within the viewing geometry of our collection system. However, at shorter wavelengths within the SO SI band this is no longer the case. The average absorptivity on this edge slowly declines, ultimately reaching in the spectral region between the SO S I and SO SZ absorption systems a minimum value of k(222) = 6 M-' cm-I in dilute solutionsz1and of k(227) = 15 M-' cm-' in the neat liquid.2.16 Since saturation of our viewing geometry is not complete until the absorption coefficient is 10 cm-I, fluorescence intensities in this region will tend to be too low until benzene concentrations of ca. 1.5 M are reached. However, to the blue of 217 nm (and between 232 and 267 nm), the viewing geometry was saturated at all concentrations shown in Figure 1. Although not shown here, measurements at lower concentrations from 0.00896 to 0.112 M were found to be independent of benzene concentration between 180 and 200 nm.
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10474 J. Phys. Chem., Vol. 99, No. 26, 1995
Saik and Lipsky
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Figure 4. Fluorescence excitation spectra from 180 to 230 nm of benzene in n-hexane at concentrations of 1.12 M (open circles) and 4.48 M (filled circles). 10
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