J. Phys. Chem. 1990, 94, 7936-7943
7936
Photorotamerlzatlon of Methyl Salicylate and Related Compounds in Cryogenic Matrices Edward Orton,* Meredith A. Morgan: and George C. Pimentels Laboratory of Chemical Biodynamics, University of California, Berkeley, Berkeley, California 94720 (ReceiGed: January 30, 1990)
Spectroscopic studies of methyl salicylate (MS), salicylamide (SAM), and o-hydroxyacetophenone (OHAP) isolated in 12 K matrices of, variously, SF6,Ar, Kr, or Xe are presented. Irradiation in the SIelectronic absorption bands of the normal intramolecularly hydrogen bonded conformers generates matrix-stabilized rotamers. Ground-state photorotamer conformations deduced from infrared spectra are correlated with steady-state electronic absorption, excitation, and emission spectra, as well as with emission lifetime data. Matrix-isolated SAM and OHAP photolyze to yield phosphorescent, nonintramolecularly hydrogen bonded rotamers via photochemically reversible pathways. In contrast, irradiation of MS in SF6 proceeds via a photochemically irreversible pathway to generate a rotamer with a weak intramolecular hydrogen bond between the phenol hydrogen and the methoxy oxygen of the ester moiety. The MS photorotamer exhibits both UV fluorescence and visible phosphorescence.
Introduction The spectroscopy of salicylic acid derivatives has been a focal point of interest for over 30 years. Although the anomalous emission of methyl salicylate (MS) was first observed in 1924,’ only in 1956 did WellerZ propose that the strongly Stokes-shifted blue emission be attributed to excited-state intramolecular proton transfer from the normal ground-state conformation MS-C (below). The UV emission, with its distinct lifetime and temperature-dependent intensity,3 has been attributed to fluorescence of a stable rotamer for which the structure MS-E (below) was propo~ed.~There is, however, scant spectral evidence supporting H
H
?
0 ’
MSC
U
U
MS-E
the rotamer structure; direct spectroscopic observation of MS-E has been limited to weak vibrational features arising from its equilibrium population (- 1%) present in thin-film IR spectra at room temperature: and high-resolution emission spectra of jetcooled gas-phase sample^.^ Previous attempts to trap the MS-E rotamer by irradiation in Ar, N2 and Xe matrices were unsuccessful .6 In this paper, we report the photolytic behavior of MS-C isolated at 12 K in SF6and Xe matrices. Using high-resolution infrared spectra, we identify the conformations of MS-C and MS-E. Also presented are steady-state and time-resolved emission spectra which relate the spectra of these trapped species to those observed by others at room temperature. In addition, the emission spectrum of MS-E in SF6includes previously unreported phosphorescence. We also describe the photolytic behavior in matrices of two other salicylic acid derivatives which are intramolecularly hydrogen bonded in their lowest energy ground states, salicylamide (SAMC), and o-hydroxyacetophenone (OHAP-C). Generation of a H
have been postulated to explain dual fluorescence observed in room temperature cyclohexane solutions.8 However, neither of those studies included infrared or high-resolution emission spectra. As with MS, judicious choice of photolysis wavelengths and cryogenic matrix environments permitted the generation and trapping of the corresponding photorotamers of SAM-C and OHAP-C. We present both steady-state and time-resolved emission spectra, as well as high-resolution infrared spectra for each phototautomer. We have an ongoing interest in this laboratory in excited-state reactions. In an accompanying r e p ~ r t we , ~ discuss the excited states and reaction pathways for the photorotamerization of intramolecularly hydrogen bonded o-hydroxybenzaldehyde (OHBA-C) to a non-hydrogen-bonded form (OHBA-F). Al-
OHBA-C
OHBA-F
though the normal ground states of OHBA, MS, SAM, and OHAP are the intramolecularly hydrogen bonded forms which exhibit highly Stokes shifted fluorescence (ca. 10000 cm-’) from the corresponding proton or hydrogen-transfer SI states, striking differences are apparent both in the photolytic behaviors and in the excitation and emission spectra of the rotamers. We discuss these differences in terms of the spectroscopic characteristics of the excited states. By generating and examining the photorotamers of MS-C, SAM-C, and OHAP-C in cryogenic matrices, we are able to associate specific matrix stabilized conformations with electronic absorption and emission data, and thereby clarify ambiguities that have arisen in earlier studies from the spectroscopic contributions of these rotamers. The higher resolution excitation and emission spectra presented here contribute greatly to the understanding of the excited electronic states of these compounds.
H
( 1 ) Marsh, J. K. J. Chem. Soc. 1924,125, 418. (2) Weller, A. Z. Elekfrochem. 1956,60, 1144. (3) Acuna, A. U.; Catalan, J.; Toribio, F. J . Phys. Chem. 1981,85, 241. (4) Toribio, F.; Catalan, J. Amat, Acuna, A. U. J. Phys. Chem. 1983,87, SAM-C
OHAPC
phosphorescent species by irradiation of OHAP-C in 77 K 3methylpentane glass has been reported,’ while rotamers of SAM-C
* Author to whom correspondence should be addressed.
+Present address: Rohm and Haas Co., Architectural Coatings Research, Spring House, PA 19477. *Deceased June 18. 1989
0022-3654/90/2094-7936$02.50/0
817. ( 5 ) Helmbrook, L.; Kenny, J. E.; Kohler, B. E.; Scott, G. W. J. Phys. Chem. 1983,87, 280. (6) Gebicki, J.; Krantz, A. J. Chem. Soc., Perkin Trans. 2 1984,IO, 1617. (7) Nagaoka, S.; Hirota. N.; Sumitani, M.; Yoshihara, K. J. Am. Chem. SOC.
i983; ios,4220.
(8) Schulman, S.G.; Underberg, W. J. M. Phofochem. Photobiol. 1979,
29, 937.
(9) Morgan, M.A.; Orton. E.; Pimentel, G.C. Preceding article in this issue.
0 1990 American Chemical Society
Photorotamerization of Methyl Salicylate
The Journal of Physical Chemistry, Vol. 94, No. 20, I990 7937
4Sd Figure 1. Fluorescence excitation (1 nm band-pass, A,, = 475 nm) and emission (1 nm band-pass, A,, = 320 nm) spectra of MS-C in 12 K SF,.
Experimental Section Insfrumentation. Instruments used for acquisition of cryogenic matrix IR absorption spectra, steady-state emission spectra, fluorescence lifetimes, and long (>0.1 s) phosphorescence lifetimes have been described previously.1° Filters used for selection of the excitation (ex) and emission (em) wavelengths in the nanosecond lifetime apparatus were MS, ex = 282 nm interference (29 nm band-pass, 10% maximum transmittance Baird Atomic), em = 400-4600 nm (3-74, Corning); SAM, ex = 240-420 nm (7-54, Corning), em = 460-600 nm (4-64, Corning) and, alternatively, ex = 282 nm interference, em = 400-4600 nm; OHAP, ex = 300-380 nm (7-60, Corning), em = 460-600 nm. Instrument response functions were determined by replacing the excitation filter with one transmitting the same wavelengths as the emission filter and attenuating the signal with neutral-density filters. The dye laser and the instruments used for determining short (CO.1 s) phosphorescence lifetimes and for acquisition of UV/vis absorption spectra have been described previo~sly.~ Sample Preparation. OHAP and SAM (Aldrich Chemical Co.) were purified by flash column chromatography. SAM was further purified by vacuum sublimation. M S (Aldrich) was purified by simple distillation. The purified compounds were stored under nitrogen prior to use. [CD3-0-D]-o-hydroxyacetophenone, methyl [0-Dlsalicylate, and [N,N-D2-O-D]salicylamide were prepared by exchange with CH,OD (Aldrich, 99.5% D). Argon (Pacific Oxygen Co. 99.998% Ar), krypton and xenon (Airco, 99.995% Kr, 99.9995% Xe), and SF6 (Matheson Gas Products, 99.8% SF6) were used without purification. Sample Deposition. Matrix gas mixtures were prepared by equilibrating the room temperature vapor pressure of degassed MS or OHAP with, respectively, at least 75 and 100 Torr of the matrix gas. Since the room temperature vapor pressure of M S exposed to an oil-covered Hg manometer is -0.2 Torr (MKS Baratron), and that of OHAP is somewhat lower than the 0.4 Torr of OHBA? this implies M:R's (mole ratios, matrix gas:reactant) of at least 3751 and >250:1. As discussed in the previous paper,9 this ensures that most (>94%, in the case of MS matrices) of the molecules are isolated, with only a small fraction of the nonisolated molecules in a position to participate in intermolecular hydrogen bonding. Dilution studies allowed identification of infrared absorptions due to multimers. Multimers do not play a detectable role in the matrix photochemistry of OHAP or SAM, but do in the case of M S in Kr or Ar. Comparing experiments containing traces of matrix-isolated water with others containing no experimentally detectable water, we found no evidence in the rates or courses of the reactions to indicate the spectral complications arising from intermolecular hydrogen bonding impurities. SAM was deposited by flowing the matrix gas at -0.6 mmol/h over the solid, positioned between the gas metering valve and the (IO) Morgan, M. A.; Pimentel, G.C. J . Phys. Chem. 1989, 93, 3056.
'
'360' '
'3\d ' '460' ' '440' ' '560' WAVELENGTH (NANOMETERS)
I
'510'
I
'
' 6I
Figure 2. Fluorescence excitation (0.4 nm band-pass, X, = 340 nm)and fluorescence and phosphorescence emission (0.4 nm band-pass, kX= 290 nm) spectra of MS-E in SF, following 41 h photolysis with the fluorometer source (320 nm, 16 nm band-pass). Features marked with an X are due in part to contaminating OHBA-F. TABLE I: Vibrational Features in Fluorescence Excitation and Emission of MS-E in SF, Matrix and Phosphorescence in Xe Matrix
fluorescence excitation nm A cm-''' 311.4 (0) 308.2 333 306.8 481 695 304.8 803 303.8 299.2 1309 1602 296.6
a
fluorescence emission nm A cm-la 312.0 313.8 315.4 317.4 320.2 325.2 328.2 339.0
(0)
184 345 545 821 1301 1582 2553
phosphorescence nm A cm-Ib 371.0 379.0 383.0 389.5 395.0 403.0 408.0 416.0 421.5 437.0 470.0
(0) 784 1327 1590 1638 2140 2444 2916 3229 407 1 5678
f 6 0 cm-I.
cryostat. For deposition, the solid samples were maintained at, respectively, room temperature, 42 'C, and 65 OC for emission, UV, and IR studies.
Results Methyl Salicylate. The excitation and emission spectra of methyl salicylate in 12 K SF6 matrix (Figure 1) correspond to the Stokes-shifted blue fluorescence previously observed in neon matrix." This emission has a measured lifetime of 11.0 f 1.5 ns in SF6, 10.5 f 1.5 ns in Ar, and 3.0 f 1.0 ns in Xe, in good agreement with the 12 f 2 ns lifetime measured in neon. The uncertainties in the lifetimes arise in large part from fluorescence decays which could not be fitted with a single exponential, requiring instead three lifetimes, with the middle (major) component comprising at most 90% of the intensity. Photolysis at 320 nm (8 nm band-pass, 41 h) of the SF6 matrix with the 150-W fluorometer source resulted in -50% conversion to a species characterized by the excitation and emission spectra of Figure 2. The emission between 311 and 360 nm corresponds to the UV fluorescence previously observed as the minor component in gasand solution-phase samples of methyl salicylate. This UV emission is strongly quenched in xenon matrix. We were unable to measure the lifetime of the MS-E UV fluorescence in SF,. The emission between 360 and 550 nm in Figure 2 is phosphorescence with a lifetime of 2.3 f 0.2 s in SF6 at both 369 and 392 nm, with some fluorescence from residual MS-C. Excitation spectra of the fluorescence and phosphorescence were identical and wavelength independent. Peak maxima of the fluorescence excitation and ( 1 1 ) Goodman, J.; Brus, L. E. J . Am. Chem. SOC.1978, 100, 7472.
7938 The Journal of Physical Chemistry, Vol. 94, No. 20, 1990
Orton et al.
TABLE 11: 0-0 Transition Wavelengths (nm) of MS-E, SAM-F, and OHAP-F
compd MS-E SAM-F OHAP-F
state TI SI TI T, SI
S2
argon n.d." n.d. n.d.
xenon 370.0 f 1.0 317.3 f 1.0 364.4 f 0.4
388.6 f 0.4 >360 307.2 f 0.4
n.d. n.d. n.d.
SF, 367.6 f 0.4 311.7 f 0.2
361.8 f 0.46 385.8 f 0.4 n.d.
306.8 f 0.4
a Indicates wavelength not determined. bThis value was determined from the overlap of the 0-0 bands in the fluorescence excitation and emission spectra. All others are obtained as the wavelength at halfheight of the leading peak.
16BO
:Bb3
1560
1540
1320 lZD0 IOBO WRYENUMBERS CN-3
960
E'i3
720
6 0
Figure 4. FTlR difference spectrum of SAM-C (negative features) and its photolysis product SAM-F (positive features) in 12 K Ar matrix after 1 h 45 min laser irradiation ( 5 mW, 320 nm). Insert: N-H and free 0 - H stretching region, ordinate axis compressed by factor of 1.8.
I
Figure 3. FTlR difference spectrum of MS-C (negative features) and its photolysis product MS-E (positive features) in 12 K Xe matrix after 2.5 h laser irradiation (6 mW power, 325 nm). Insert: 0 - H stretching region, ordinate axis compressed by factor of 1.4.
emission vibrational features in SF,, and the phosphorescence vibrational features in Xe, are given in Table I. The 0-0 transitions of the product are given in Table 11. The infrared difference spectrum in Figure 3 shows MS-C (negative features) and the photolysis product (positive features) in Xe after 2.5 h photolysis with 6 mW of 325-nm laser light. Frequencies of the MS-C and MS-E absorptions are given in Table I l l . IR frequencies of deuterated MS-C ( M S - C O D ) and its photolysis product (MS-E-OD) are also given in Table 111. MS-C was photolyzed to MS-E in the four matrices Ar, Kr, Xe, and SF6, with approximate photolysis rates in the ratio 1:10:15:15. However, in Ar and Kr matrices the infrared absorptions following photolysis were broad, indicating that those molecules which react are not isolated but rather share a larger lattice site with a second guest molecule. Additionally, in the time it took to reduce by 10% the carbonyl stretch absorbance at the frequency corresponding to isolated MS-C molecules in argon matrix, the carbonyl stretch band corresponding to nonisolated MS-C diminished by 90%. Photolysis of MS-C proceeds at wavelengths at least as long as 332.5 nm in xenon, and 333 nm in SF6, albeit slowly. Irradiation at all absorption wavelengths produced small amounts of C 0 2 and CO, identified by their fundamentals at, respectively, 2335.2 and 21 29.4 cm-l. Attempts to regenerate MS-C in matrices photolyzed at 320 nm to steady state (>90% MS-E) were unsuccessful in both Xe and SF, matrices at wavelengths between 280 and 310 nm. Instead, bands attributable to C O and COz, a large, broad, unidentified feature at 690 cm-l, and in SF6 a weak absorption in the H F stretching region at 3930 cm-I, were produced. Salicylamide. Figure 4 is an infrared difference spectrum of SAM-C (negative features) and its photolysis product SAM-F (positive features) in 12 K Ar after 105-min 320-nm photolysis at 5 mW with the dye laser. The uniform kinetic growth profiles of the absorptions ensure that only the SAM-C and SAM-F species contribute to the spectra. The frequencies and relative
Figure 5. Fluorescence excitation (1 nm band-pass, A,, = 414 nm) and emission ( 1 nm band-pass, A,, = 290 nm) spectra of SAM-C in 12 K SF, matrix. Features marked with an X are in part due to impurities present on window prior to deposition.
I
0.04 230
"-.-_______ y50
270 290 310 330 WAVELENGTH (NANOMETERS)
350
370
Figure 6. Upper curve: UV spectrum (0.2 nm resolution) of SAM-C in 12 K xenon matrix. Lower curve: UV spectrum after 165 min laser photolysis at 320.0 nm, 8 mW power, showing growth of structured features attributable to the photorotamer SAM-F.
intensities of the infrared absorptions of SAM-C, SAM-F, and their deuterated counterparts are compiled in Table IV. The 0-H stretch of SAM-C is not detected, presumably being broadened by the strong intramolecular hydrogen bond. In the deuterated counterpart, D,-SAM-C, the 0 - D stretch is assigned at 2220.2 cm-I, which, assuming an isotope shift by a factor of 1.35, places the 0-H stretch near 2997 cm-' in the nondeuterated form.
Photorotamerization of Methyl Salicylate TABLE 111: Infrared Absorptions and Relative Intensities of MS-C, MS-E, and Their Deuterated (0-D)Counterparts in 12 K Xe Matricesa MS-C MS-E MS-C-OD MS-E-OD Y, cm-' re1 int Y , cm-I re1 int Y, cm-l re1 int Y, cm-l re1 int 3183.1 vs 3450.5 vs 3186.7 br, we 3450.2 br, wc 1705.0 w 1754.4 w 2950.2 w 2555.1 w 1682.8 vs 1741.4 vs 2377.1 m 1748.1 sh, vw 1668.3 sh,w 1736.3 sh, m 1705.5 sh, vw 1741.1 sh, m 1620.1 m 1623.3 m 1683.3 sh, m 1739.5 vs 1615.8 m 1477.1 s 1679.7 vs 1630.2 vw 1599.9 sh, vw 1461.5 vw 1659.4 sh, vw 1623.3 b 1594.3 w 1457.0 w 1620.6 mpt, vw 1614.1 b 1590.0 sh, vw 1382.4 sh, vw 1615.8 mpt, vw 1607.3 b 1584.4 m 1379.3 w 161 1.9 mpt, w 1578.4 b 1505.8 m 1372.3 w 1585.9 w 1477.2 b 1489.9 m 1365.0 w 1574.1 w 1464.9 b 1482.2 m 1313.9 sh, vw 1489.7 w 1457.4 b 1469.2 m 1284.5 m 1481.0 m 1433.3 w 1455.4 w 1276.8 m 1469.4 w 1303.8 b 1450.4 sh, vw 1270.5 d, m 1459.6 w 1295.1 b 1441.7 vs 1268.0 d, m 1455.7 w 1277.3 w 1433.3 sh, w 1246.4 sh, w 1441.7 d, m 1271.2 sh, w 1422.4 w 1209.5 sh, m 1439.3 d, m 1268.1 b 1346.7 s 1188.6 sh, vw 1422.2 vw 1249.3 b 1336.2 m 1156.3 b 1411.6 vw 1238.0 m 1326.5 m 1152.9 m 1347.4 m 1209.3 b 1307.2 vs 1113.4 s 1337.3 m 1190.5 b 1255.8 d, s 1080.3 m 1326.7 vw 1152.5 w 1254.1 d, s 1035.5 m 1317.3 m 1131.5 vw 1229.1 w 961.2 vw 1311.3 m p t , m 1119.6 m 1215.1 d , v s 864.3 sh,vw 1309.8 m p t , m 1113.6 b 121 1.9 d, vs 797.3 w 1307.9 mpt, m 1091.2 b 1193.9 m 755.8 m 1291.0 vw 1081.0 d, vw 1182.3 m 694.8 w 1287.1 w 1079.6 d, vw 1161.3 m 580.3 m 1256.3 vs 1038.1 d, w 1156.3 m 528.4 w 1215.3 d, m 1035.7 d, w 1137.7 w 1212.4 d, m 993.8 b 1131.4 vw 1194.4 s 946.3 vw 1092.6 m 1162.3 m 797.0 b 1030.9 w 1142.0 w 755.1 b 971.4 w 1137.9 vw 697.5 b 862.4 vw 1103.5 d, m 695.1 w 850.8 w 1100.8 d, m 656.7 vw 800.7 w 1093.1 w 577.2 w 755.8 s 987.5 w 726.4 m 966.8 w 701.3 m 862.1 vw 668.5 vw 850.1 vw 525.8 vw 755.8 vs 726.6 w 701.3 m 667.3 vw 517.9 m "Abbreviations: vs = very strong, s = strong, m = medium, w = weak, vw = very weak, d = doublet, t = triplet, mpt = multiplet, sh = shoulder, br = broad. Product band superimposed on starting material band. 'Band is due to H isotope contaminant.
Both the excitation and emission spectra (shown in Figure 5), and the UV absorption spectrum (shown in Figure 6, upper curve) agree with those reported for SAM-C in durene mixed crystals at 4.2 K.I2 The fluorescence lifetime is IO f 2 ns in argon and 5.5 f 1.5 ns in xenon. The necessity of fitting the emission decay curve with both shorter (- 1 ns) and longer (- 15 ns) components prevented a more precise identification of the lifetime of the middle (major) component. Photolysis of matrix isolated SAM-C at 315 nm for 75 min with the fluorometer source results in diminution of the normal Stokes-shifted fluorescence and growth of new, structured features (see Figure 7). The lifetime of this new emission measured at 368,389, and 414 nm (4 nm band-pass) is 1.4 f 0.1 s in SF,. The emission lifetime in Xe was
