J. Phys. Chem. 1981, 85, 763-768
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ARTICLES Fluorescence Properties of Methyl Salicylate in Vapor, Liquid, and Solution Ricardo Lopez-Delgado”t Center for Interdiscipihary Studies in Chemical Physics, University of Western Ontario, London, Ontario, Canada
and Sylvain Larare CNRS- Laboratoire de Synthsse Organique, €cole Polytechnique, 9 1 128 Palalseau, France (Received: August 6, 1980; In Final Form: December 5, 1980)
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Methyl salicylate (MS) absorption, excitation, and fluorescence spectra in the 300-500-nm range have been recorded in the vapor phase (p 0.2 torr), where molecules are collision free during the lifetime of the excited state, as well as in the neat liquid and in n-hexane and water solutions. Fluorescence decay times have been measured for both MS ultraviolet (UV) and blue emissions tm a function of the sample physical state, temperature, and excitation wavelength. Also, the effect of inert gases (Nz and n-butane) on the blue fluorescence spectral and time distributions have been studied. The data analysis leads us to three main conclusions: (1)There is strong experimental evidence in favor of the ground-state equilibrium between two rotamers responsible for the MS dual fluorescence. (2) The blue fluorescence is certainly generated by excitation of the most abundant carbonyl-phenol hydrogen-bonded closed rotamer, whose population in the ground state is estimated to be -800 times larger than that of the other rotamer; the huge blue fluorescence red shift is produced by the “proton quasi-transfer” from phenol to carbonyl groups upon excitation, the molecule remaining planar; excitation and decay time data show the onset of a new very efficient (“third-channel” type) radiationless deactivation process at low vibrational energy excess (-300 cm-’) in the excited state, which is tentatively attributed to a low-frequency out-of-plane molecular motion destroying the planar symmetry and perturbing the “proton quasi-transfer” process. (3) The UV fluorescence is attributed to the other MS hydrogen-bonded closed rotamer, where now the hydrogen bridge is made between the phenol group and the ester oxygen.
Introduction Methyl salicylate (MS) complex double fluorescence was already observed as early as 1924 (vapor-phase excitation).l Much later, Weller2p3studied the molecule in aprotic solvent solutions, again finding the double emission: a very broad band whose maximum near 450 nm shows a huge red shift with respect to the absorption maximum, near 310 nm; and a much weaker band, actually as a shoulder, around 340 nm. Weller explained this behavior2p3by a rapid proton-transfer reaction occurring in the excited state according to the pathway A B C (see Figure 1). The IJV fluorescence (ca. 340 nm) was then attributed to the excited species B (Figure l),and the blue emission (ca. 450 nm) to the excited species C. Therefore the MS molecule should display in the excited-state potential-energy surface, along an appropriate normal coordinate, a double minimum. However, in 1976, Sandros4 showed that, in fact, the UV and blue emissions have different excitation spectra, which points to the preexistence of two different molecular species already in equilibrium in the ground state; it could happen that the equilibrium is too slow as compared to the excited-state lifetimes; therefore, he attributes the UV emission to tautomer D (in the ground state, see Figure 11, where there is no possibility of proton transfer upon excitation. Tautmer A already exhibits a strong hydrogen bridge in the ground state and, upon excitation, the phe-
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CNRS-Laboratoire de Photophysique MolBculaire, UniversitB de Paris Sud, 213, 91405 Orsay, France.
nolic proton is quickly and completely transferred to the carboxyl group, generating then the blue emission. The same conclusion is reached by Klopffer and Kaufmand in their vapor-phase study. Another very interesting piece of the MS fluorescence puzzle is added by Goodman and Brus6 with their work on MS excitation and emission spectra in a Ne matrix a t 4 K. Relatively well-structured spectra are observed, and the emission Franck-Condon envelope coincides with the MS blue fluorescence reported by Weller;2*sthe emission seems to be very intense, showing a wavelength-independent 12-11s lifetime. The excitation spectrum is built from the (0-0) origin in a Franck-Condon progression of an excited-state normal mode, with frequency in the 350-cm-l range; the extreme intensity increase for each successive member of the progression indicates that a large displacement has occurred along the appropriate normal coordinate; later on we will see that, for the purpose of the present work, this is an extremely important observation. From the analysis of their spectra the authors6 conclude the absence of any double minimum in the excited-state potential-energy surface.
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(1) J. K. Marsh, J. Chem. SOC.,125,418 (1924). (2)A. Weller, 2.Elektrochem., 60, 1144 (1956). (3) A. Weller, Prog. React. Kinet., 1, 188 (1961). (4)K. Sandros, Acta Chem. Scand. Ser. A , 30, 761 (1976). (5)W. Klopffer and G. Kaufmann, J.Lumin., 20, 283 (1979). (6)J. Goodman and L. E. Brus, J.Am. Chem. Soc., 100, 7472 (1978).
0022-3654/81/2085-0763$01.25/00 1981 American Chemical Society
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Also, fluorescence properties of MS in both n-hexane and water solutions have been studied in comparison with the vapor phase.
Materials and Methods MS (Eastman Kodak) has been purified by gas chromatography under control by GC and NMR analysis. n-Hexane has been treated with concentrated H2S04,in order to eliminate all unsaturated and aromatic fluorescent impurities, and then distilled several times and dried. Water was triple distilled. Concentrations of MS were adjusted so that optical densities in a 1-cm absorption path were of the order of 0.25 ( lo4 M). All experiments were performed in 1-cm square nonfluorescent fused silica cells; special care was taken with MS vapor samples (see below). Absorption spectra were recorded in a Cary 219 spectrophotometer; fluorescence and excitation spectra a t low resolution (Ah,, -8 nm) were recorded in a Perkin-Elmer MPF-4 spectrofluorimeter and compared with the ones kindly provided by the Madrid group.8 Fluorescence decay parameters have been measured by means of a commercial PRA-3000 “nanosecond spectrofluorimeter”, directly interfaced to a DEC MINK-I1 computer, allowing for fast data treatment. The system using the single photon counting method has been described in some detail ehewhere;1° it has the ability to show Flgure 1. Different MS molecular tautomers discussed in the text: (Gr) an instrumental function g(t) which is wavelength indeelectronic ground state; (Exc) electronic excited state. pendent: lo Finally, in a very recent paper, Acuiia et al.’ criticize the g(t) = Io(hl,t)R p M ( X 2 , t ) = lo(t)R p M ( t ) = constant attribution of the UV emission to an open t a u t ~ m e r ~ . ~ lo(Xl,t)= I&), the true profile of the hydrogen-filled (Figure lD), in view of the fluorescence properties which thyratron-triggered PRA-510B flash lamp at any wavethey have observed in a number of MS related comlength, is convoluted by R p M ( X 2 , t ) = R p ~ ( t ) ,a t any pounds: leading them to the proposition of a different wavelength in the 220-500-nm range. This property model consisting of the equilibrium in the ground state of greatly simplifies both data recording and data treatment; two closed (hydrogen-bonded) MS tautomers (A + E, the true fluorescence decay is recovered by iterative conFigure 1);rotamer E would then be responsible for the UV volution of the instrumental function, g(t), by one or more emission. However, the most striking feature of their reexponential decays with a weighted least-squared convoport’ is the extraoridinary shape of the MS gas-phase lution procedure until the best fit is obtained with the excitation spectrum, indicating a dramatic drop in the blue experimental data, as defined by a minimum x2 value and emission quantum yield beyond a certain excitation residuals randomly distributed at better than 95% conwavelength (see Figure 2b, in ref 7). Also, when 1atm of fidence limit. n-butane vapor is added to MS at saturated pressure (-0.2 The PRA-3000 instrument is equipped with Jobin-ettorr, a t room temperature), the excitation substantially Yvon H-10 type monochromators; however, because of the increases, and the maximum shifts by -6 nm toward the low level of emission, particularly in vapor samples, the maximum of the absorption.* It would then be possible excitation monochromator slit was 1-1.5 mm open (8-12to correlate this behavior with the opening of a new very nm excitation bandwidth) and the emission was “seen” fast radiationless pathway (of the “third-channel” through band-pass filters: (a) the UV fluorescence, by beyond a certain excess of vibrational energy in the excited means of an 18-A Kodak filter coupled with 4-mm thick state. In such a case fluorescence lifetimes should draordinary glass with transmission maximum around 340 nm matically depend on excitation wavelengths around the and -35-nm half-width; (b) the blue emission, by means possible third-channel threshold. of a Balzers K-2 filter with blue and red cutoff, a t -420 Therefore we have tried to measure in the present work and 500 nm, respectively. MS blue fluorescence lifetimes as a function of the exciSpecial care was taken with the vapor-sample excitation, tation wavelength in the vapor phase in a collision-free particularly when the UV fluorescence was studied, besituation during the excited-state lifetime, a very difficult cause of its extremely low emission level (in order, for task indeed for both the optical density at saturation example, to obtain meaningful statistics, we had to count pressures and the fluorescence quantum yield are very low. for more than 15 h) and because of the extraordinary Fluorescence lifetimes of the UV emission in the vapor ability of MS to be adsorbed to cell silica walls. Therefore have been also measured in order to understand its origin. a typical experiment was performed in the following way: (1)The cell holder was equipped in both the excitation and (7) A. U. Acuiia, F. Amat-Guerri, J. catalbn, and F. Gonzblez-Tablas, emission side with narrow (-3 mm) diaphragms, so that J. Phys. Chem., 84, 629 (1980). (8) A. U. Acuiia, J. Catalln, and F. Toribio, personal communication; the photodetector could not possibly “see” the excitation work submitted for publication in J . Phys. Chem. light through both the entrance and the opposite exit (9) M. Jacon, C. Lardeux, R. Lbpez-Delgado, and A. Tramer, Chem. windows. (2) The cell was filled with MS vapor under Phys., 24, 145 (1977). vacuum a t a temperature around 20 “C; then it was (10) E. Gudgin. R. L6uez-Delgad0, and W. R. Ware, Can. J. Chem., in press. transferred to the hotter (-30 “C) cell compartment in (11) Ramsay and Young, 2. Phys. Chem., 1, 237 (1887). order to prevent any liquid condensation on the walls. (3) (12) J. H. Callomon, J. E. Parkin. and R. Lbpez-Delgado, Chem. Phys. The statistical photon counting was run for -15 h; the Lett., 13, 125 (1972).
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Methyl Salicylate Fluorescence Lifetimes
WAVELENGTH Inm Flgure 2. MS absorption profiles at room temperature: (1) pure vapor at saturated pressure (-0.2 torr); (2) n-hexane solution, M; (3) aqueous solution. lo4 M.
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WAVELENGTH i n m Figure 4. MS vapor (collision-free) blue fluorescence spectral dlstribution as a function of the excitation wavelength (in nm): (1) 335; (2) 325; (3) 310. Room temperature.
io4
Zcn
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Z
w
t
z
n lo2 W
c!
J
a
260
280
300 320 WAVELENGTH Inm
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5 9
340
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Flgure 3. MS blue fluorescence excitation profiles at room tempera0.2 torr) N, 0.2 torr); (2) vapor ( p ture: (1) pure vapor ( p (p 1 atm); (3) va or ( p 0.2 torr) n-butane ( p 1 atm); (4) in n-hexane, 10- M liquid solution.
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excitation function g(t) was recorded and accumulated for -1000 counts every hour in an attempt to average, as much as possible, any drift in the excitation profile. (4) Finally, the cell was thoroughly evacuated, and the “blank’ experiment was performed under exactly the same conditions: after 20 h of recording the integrated number of counts in the time range scanned (256 channels at -0.07 ns per channel) was equal to or less than 200. So we can confidently state that “parasitic” effects (scattered light from the excitation source, cell-wall luminescence, and photodetector dark background counts) are negligible.
Results and Discussion An MS vapor absorption spectrum through a 10-mm quartz cell at room temperature and saturated pressure is shown in Figure 2 (line 1). The vapor pressure may be empirically calculated,’l and at 30 OC the result is -0.23 torr. The absorption spectrum in the n-hexane solution has the same profile, but it is slightly shifted (-3 nm) toward the red (Figure 2, line 2); on the other hand, in the water solution the absorption maximum is shifted by as much as 6 nm toward the blue (Figure 2, line 3). Excitation spectra compare quite well with those generated by the Madrid g r ~ u p ,although ~,~ here the resolution is 1 order of magnitude smaller, for we have to correlate them with our lifetime experiment. In Figure 3, lines 2
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IO’
IO0
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10 TIME ins
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Flgure 5. MS blue fluorescence time-dispersion best-fitted curves in 0.2 torr) as a function of the excitation wavethe vapor phase ( p length (In nm): (1) 335; (2) 325; (3) 310. (4) Instrumental function g(f). Temperature, -30 OC.
and 3, we notice that, when 1 atm of N2 and n-butane, respectively, is added to the MS pure vapor (Figure 3, line l),the excitation substantially increases, and the maxima move toward the absorption maximum. By the time we arrive at the n-hexane liquid solution, the excitation maximum is only -2 nm to the red of the solution absorption maximum (compare Figures 2 and 3). Another very important observation, first made by Acuiia and colleagues,8and confirmed here (Figure 4), is that, whatever the excitation wavelength is (remembering that in our experimental conditions, MS molecules are collision free during the excited-state lifetime; see below), the maximum of the blue emission always shows about the same red shift and a similar Franck-Condon envelope. However, collision-free MS vapor blue fluorescence lifetimes display a dramatic excitation wavelength effect, as may be seen in Figure 5 and Table I (experiments 1-4). This effect nicely correlates with the excitation spectrum profile (Figure 3, line 1)and carries the message about the opening at a certain vibration energy level in the excited state of a very efficient third-channel type9J2radiationless deactivation process.
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TABLE I: MS Fluorescence Lifetimes ( T ) ,r 2 )and Relative Integrated Intensities (Zl,I , ) as a Function of the Sample Physical State, Excitation ( A e x c ) and Emission ( h , ) Wavelengths, and Temperatures (T,“ C ) a expt Xaxcr hem, no. sample nm nm T,”C 71, ns I , , 76 7 9 , ns I,, ?6 335 450 30 -25 -75 1 vapor (vac) 70 30 450 14 2 vapor (vac) 335 55 45 325 450 30 3 vapor (vac) 30 60 40 310 450 4 vapor (vac) 5 vapor (vac) 305 340 30 40 1.5 t 0.2 60 335 450 30 100 6 vapor (N2,1 atm) 30 40 1.4 t 0.2 60 7 vapor (N2,1 atm) 305 340 325 450 20 0.55 t 0.5 100 8 n-hexane(-10-4M) 95 1.0 t 0.5 5 310 450 20 0.50 t 0.5 9 n-hexane(-10-4M) 300 450 20 0.40 t 0.07 80 1.1 t 0.5 20 10 n-hexane M) 297 340 20 0.4 * 0.1 65 1.25 t 0.1 35 11 n-hexane M) 325 450 20 0.12 f 0.05 100 12 H,O M) 0.33 t 0.1 100 13 H,O (- 10-4 M ) 300 340 20 100 325 450 20 0.52 t 0.1 14 neat liq 100 297 450 20 0.51 r 0.1 15 neatliq 0.5 f 0.1 100 300 340 20 16 neat liq Vapor-phase decay times are given in brackets for they are considered as “weighted average” lifetime values (see text). Experiments 14-16 have been done by front face excitation in a 2-mm thick cell. In experiments 5-7 and 13 the excitation bandwidth (AX,,,) was set 12 nm; in all others Ahexc 8 nm. hem = 340 means through Kodak 18-A filter, and A , , = 450 through Balzers K-2 filter.
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C lo3 Lo z
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Flgure 6. MS blue fluorescence timedispersion best-fitted curves in 0.2 torr); (2) vapor ( p 0.2 torr) N, ( p 1 atm); (3) Instrumental function g(t). temperature, -30 O C .
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the vapor phase and at bXc 335 nm: (1) pure vapor ( p
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MS vapor decay time values reproduced in Table I, as well as decay curves shown in Figure 5, are the best double-exponential fits to the experimental data; but in no case are the residuals random (see preceding section), and x2 values are always relatively high. When one tries to fit the experimental data with a triple-exponential decay, x2 values become slightly better but residuals remain always nonrandom. A reasonable explanation for our failure to properly analyze the decay data would be the following one. At our working vapor pressure (-0.2 torr), and in view of the implicated decay times (Table I), MS molecules are collision free during the excited-state lifetime. However, on one hand the ground-state Boltzmann distribution of energy levels for such a molecule certainly has to be very broad (probably several thousands of wavenumbers), and this broad spectrum of excited vibrational levels is transported by the excitation to the upper electronic state; on the other hand, and because of the low emission level, we have to use rather broad excitation bandwidths, resulting in an additional broadening of the vibrational-level spectrum transported from the ground state to the excited state by the electromagnetic excitation. Fluorescence is
then released from bunches of excited vibronic levels in spite of the collision-free situation, and then fluorescence lifetimes reported in Table I (vapor samples, A, -450 nm) have to be in fact some sort of “weighted average’’ values of multiple exponential decays with widely different extremes, since the fluorescence quantum yield changes very fast in a relatively narrow excitation energy range, as shown qualitatively by the excitation spectrum (Figure 3, line 1). Therefore, the double-exponential analysis (even tripleexponential) is a rather crude approximation, but it serves the qualitative purpose of the present phenomenological discussion. It should be easy now to understand the increase in quantum yield when N2 or n-butane gas is added to the MS (0.2 torr) vapor sample (Figure 3, lines 2 and 3); indeed, these gases are going to act as vibrational scavengers, relaxing the vibronically excited MS molecules toward the third-channel threshold, as long as the collision rate can compete with the third-channel deactivation rate, reflected by the emission lifetime. As this lifetime decreases extremely quickly with decreasing excitation wavelengths (see Table I), the N2 (or n-butane) collisional effect very quickly ceases to be important. But in the case of the n-hexane liquid solution, at room temperature, the collisional rate is so high that it should successfully compete with the third channel; in fact, this is what apparently happens since absorption and excitation profiles almost coincide (compare Figures 2 and 3). However, decay times measured in the n-hexane solution seem to be in clear contradiction with the above description, for here almost the entire excited-state population should be vibrationally relaxed below the third-channel threshold, yet the observed fluorescence lifetimes (Table I, experiments 8-10) are nearly A,, independent, in our statistical error limit, and, at least, they are 1 order of magnitude smaller than the decay times measured in the collision-freevapor samples below the threshold (compare 71in experiments 8-10 with T~ in experiments 1-3, Table 1). The clue for the understanding of this apparent contradication is given by experiment 6, Table I; indeed, by the addition of 1 atm of dry N2 to the MS vapor sample 1 (Table I), the disappearance of all long-decay components is observed; if N2 is pumped out again, the longdecay components reappear, and so on. under these experimental conditions MS molecules are no longer in a
Methyl Salicylate Fluorescence Lifetimes
collision-free situation, particularly for those vibronic excited levels whose lifetimes are longer than 1ns; the collisional rate at 1 atm is high enough to thermalize the electronically excited population of MS molecules around kT (-300 cm-’, at 30 “C).Then results obtained with the n-hexane solutions (experiments 8-10, Table I) only make sense under the hypothesis that the energy of the MS third-channel threshold is of the order of kT. Collisions are, depending on the excitation energy, simultaneously shooting up or relaxing down the vibronically excited MS molecules toward the thermally equilibrated vibrational distribution in the electronic excited state, which happens to be just beyond the third-channel threshold. The study of MS fluorescence as a function of the temperature in the vapor phase is an extremely difficult task because of the already mentioned extraordinary ability of the molecule to interact with the silica cell walls. In spite of the fact that this MS vapor property was already pointed out by Klopffer and Kaufmanq6 it deceived them, for they have reported a rather beautiful experimental artifact (see Figure 3 in ref 5). Indeed, by heating their sample, they observe a spectacular increase of the UV fluorescence intensity while the blue emission remains more or less constant. Careful experiments, first performed by the Madrid group8 and confirmed later by us here, show that the UV emission intensity is, on the contrary, rather temperature independent, while the blue emission intensity decreases very fast with increasing temperature. In fact, what Klopffer and Kaufmann are observing5 is the convolution of two opposite effects: on one hand, by heating the sample, they are desorbing more and more MS molecules out of the cell walls, increasing then the “concentration” and, consequently,the overall fluorescence intensity; on the other hand, the blue emission intensity decreases with increasing temperature, one effect compensating the other. Our measured decay times provide additional experimental evidence: in experiment 2 (Table I), when the temperature is lowered from 30 to 14 “C,the “average” (see above) observed lifetimes become much longer, and one may see the appearance of long-decay components of 9-ns lifetime. The fact that Iz is smaller in experiment 2 than in experiment 1 (Table I) seems contradictory at first sight, but it may be explained in the following way: as the 0-0 transition appears around 325 nm,6 it is obvious that, by excitation at 335 nm, most of the absorbed energy goes into “hot” transitions. If the temperature is lowered, the population of low-frequency modes, responsible for these “hot” transitions, will certainly decrease; so does the relative intensity of long-decay components, but the fluorescence total intensity substantially increases. Nevertheless, the UV fluorescence lifetime does not depend appreciably on temperature; nor does the addition of 1atm of N2 produce any dramatic effect but a small collisional quenching (see experiments 5 and 7, Table I). When the UV fluorescence is excited, a shortdecay component ( r l , in experiments 5 and 7) is always present; it certainly corresponds to the blue emission, for the 18A Kodak filter, used to select the UV emission, substantially overlaps with the blue one; this is corroborated by the dramatic effect of adding Nz on this short component (compare r1 in experiments 4-7), but not on the other one (compare r z in experiments 5 and 7). On phenomenological grounds then the temperature quenching of the blue emission could be explained in the following way. (1)In the vapor-phase collision-free sample, when the temperature is raised, the ground-state Boltzmann distribution broadens and kT increases; then for the same
The Journal of Physical Chemisfry, Vol. 85, No. 7, 1981 767
excitation wavelength, the MS molecules will end up in higher vibronic levels, further and further away from the third-channel threshold, and we already know that the rate of this new radiationless pathway increases extremely fast with excited-state vibrational energy excess, as clearly shown by the excitation spectrum profile and fluorescence lifetimes. (2) After excitation of methyl salicylate in the n-hexane liquid solution, the vibronic-level populations are thermally equilibrated during the lifetime of the excited state. This is true regardless of the excited-state arrival level or the ground-state departure one, because of the very high collision rate in the condensed medium. Therfore, for a given reason temperature, fluorescence will always be released from the same distribution of vibrational level in the excited state. If the temperature increases, so does kT, and the distribution maximum moves further and further away from the third-channel threshold, resulting in an increasing fluorescence quenching (as in the vapor case). A t very low temperature kT would certainly lie well below the threshold and the fluorescence yield should approach unity, and this is precisely what Goodman and Bruss observe in the Ne matrix at 4 K. The UV fluorescence time properties, as compared with those of the blue emission, constitute an additional and definite proof in favor of the hypothesis of two different rotamers in ground-state equilibrium. Assuming the MS natural lifetime of the order of 20 ns,6 the UV emission quantum yield r$w = 1.65/20 (from Table I),and the blue fluorescence overall quantum yield of the order of since we estimate, in our experiment the blue intensity to be 10 times stronger than the UV one, we qualitatively evaluated the ratio of rotamer populations: [rot. blue] 800[rot. UV] That is probably the reason why rotamer A (see Figure 1) is the only molecular species detected by IR spectro~copy.’~ Our lifetime results above do not allow us to go much further in the dispute about the molecular conformation of the isomer responsible for the UV fluorescence. Nevertheless, the fact that water reduces the lifetimes of both isomers by about the same factor (experiments 12 and 13, Table I) may abound in favor of the Acufia et al.’ theory of two hydrogen-bridge closed rotamers (see below). Our failure to observe the relatively long UV fluorescence decay in the MS neat liquid samples (experiment 16) might be simply due to reabsorption, in spite of the front face excitation since this emission substantially overlaps the absorption tail, for we have verified that in a 10-mm-path cell OD is > 2.0 at 360 nm, in the neat liquid sample. Let us examine now the nature of the third channel. Because of the higher number of degrees of freedom (particularly at low energy) in this kind of molecule, the Boltzmann distribution at room temperature is always very broad and, even if we were able to use a much narrower excitation bandwidth, we would always end up with a large spectrum of excited vibronic levels preventing us from properly characterizing the promoting mode of vibration responsible for the coupling (whatever it is) that opens the third channel. One could try to analyze the absorption spectrum at very high resolution around the 0 4 transition, searching for a sudden broadening of the rotational structure of a particular vibronic band, as the dramatic change in fluorescence lifetimes around the threshold allows us to foresee (this phenomenon has already been observed in the case of benzene12); this experiment is
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(13)N. Mori, Y. Asano, and Y. Tsuzuki, Bull. Chem. soc. Jpn., 42,488 (1969).
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particularly difficult since, for reasons already mentioned, vibronic transitions have to be pretty crowded and not much information is available on the frequencies of vibration of MS excited-state normal modes. Another very interesting but also very difficult way to gather more precise information on the third-channel energy threshold and symmetries would be to repeat the lifetime experiment, but this time severely reducing the vibrational temperature in a supersonic molecular beam and using very narrow excitation (tunable laser). However, with the already available experimental data we could try to do some thinking about a possible phenomenological description. In the first place, the present work shows that the blue fluorescence spectral distribution is quite independent of the vibronic level(s) from where it is released, as proved by the emission spectrum profiles as a function of the excitation wavelength in collision-free MS vapor samples (see Figure 4); nor does this spectral distribution depend much on temperature! If one assumes that the spectacular red shift in the fluorescence FranckCondon envelope is due to a “proton quasi-transfer”6between both the phenolic and carbonyl groups upon excitation, then this phenomenon is always present whenever there is blue fluorescence; consequently the event that quenches the fluorescence has to be necessarily something that prevents this “quasi-transfer” from happening. We are certainly dealing here with an n-r* electronic transition, where the C=O group is in resonance with the aromatic ring and necessarily coplanar with it; consequently the hexagonal ring made up by the hydrogen bridge formation also has to be coplanar with the aromatic one. It is not then unthinkable that a certain type of molecular motion, which will destroy this coplanarity, such as out-of-plane vibrational modes, might quench the fluorescence. The analysis of the relatively high-resolution excitation spectrum of MS in the low-temperature Ne matrix6 shows
the spectrum to be mainly built on a low-frequency (-350 cm-l) normal mode progression, presenting very peculiar Franck-Condon factors. Such a low-frequency mode can only be attributed to an oubof-plane vibration, and among the candidates one could be some sort of oscillation of the entire COOCH3 group, resulting in the loss of the molecular plane of symmetry; this vibration would then strongly couple the electronic excited state to the extremely dense manifold of high vibrational levels of the ground state (vibrationally induced internal conversion). Using this hypothesis, we may probably better understand the lifetime results in the aqueous solutions (experiments 12 and 13, Table I); indeed, H 2 0 molecules, upon collision, will successfully compete with the carbonyl group for hydrogen bonding with the phenol group (aswell as the carbonyl group being hydrogen-bonded to water molecules), resulting in the destruction of the MS intramolecular hydrogen bridge. The fact that in aqueous solutions both the UV and blue emissions are similarly quenched (but not in the aprotic solvent) would be an additional argument in favor of the MS hydrogen-bond closed isomer m ~ d e l las , ~the rotamer responsible for the UV emission (see Figure 1E).
Acknowledgment. We warmly thank the Centre National de la Recherche Scientifique (CNRS, France) for granting permission to perform this work at The University of Western Ontario. Above all, we have very much appreciated the help and material support from the Photochemistry Unit of the Chemisty Department, The University of Western Ontario. Many thanks are also due to the technical staff of Photochemical Research Associates, Inc., for their friendly collaboration. Our friends Dr. Acuiia and Professor Catalan andd colleagues put us on the track of this very interesting problem, providing us with all available information which they had on hand; in fact, their contribution to this work has been conclusive.
Electron Attachment to Halogens J. A. Ayala, W. E. Wentworth,” Department of Chemistty, University of Houston, Houston, Texas 77004
and
E. C. M. Chen
Department of Chemistry, University of Houston Clear Lake City, Houston, Texas 77058 (Received: June 72, 1980)
Thermal electron attachment to Clz,Brz,and I2 has been studied as a function of temperature. Clpand Izshow a slight dependence on temperature as shown in the Arrhenius activation energies: Clz,0.037 f 0.004 eV; Br2, 0.008 f 0.007 eV; Iz, 0.033 f 0.005 eV. Thermal electron attachment rate constants at 300 K are as follows: cm3/s; Brz, (1.34 f 0.34) X Clp, (2.8 f 0.4) X cm3/s; 12, (1.36 f 0.28) X cm3/5, Negative ion potential energy curves are calculated from the Morse potential for the neutral molecule modified by a single empirical parameter. The empirical parameter for the ground state of the negative ion %, was evaluated from alkali metal beam charge-transfer data and for the excited 211gstate from electron beam data. These negative ion potential energy curves have been used to correlate various experimental data associated with negative ion formation. Introduction The interaction of electrons with halogen molecules has been studied by many different techniques, each of which provides unique information concerning the negative ion states or the kinetics and/or the thermodynamics of the attachment processes. In Figure 1, a set of typical potential energy curves for halogens is given to provide a framework 0022-36541ai/20a5-076a$oi.2510
for the discussion of the particular experimental parameters which can be obtained from each type of experiment. The ground state of the negative ion is a 22,state1v2with a dissociation energy on the order of 1-2 eV. The first (1) W. B. Person, J. Chern. Phys., 38, 109 (1963). (2)T.L. Gilbert and A. C. Wahl, J . Chem. Phys., 55, 6247 (1971).
0 1981 American Chemical Society
