Matrix-Isolated Fluorescence Spectra of Aromatic Hydrocarbons
boundary and depend on its structure (we neglect any proton transfer in the interior of the micelles having low dielectric constant, since ten units lowering of it than water was observed to lower the complexation significantly under the present experimental conditions*). The ASo values given in Table I varied in the order SDS < water < T X 100 < (CTAB TX 100) < (SDS T X 100) < CTAB < PEG. The environment for the reaction was then most ordered in SDS, much less in CTAB, and the least in PEG. The free energy changes for the individual additives followed the order PEG < T X 100 < water < SDS < CTAB. Appearance of PEG and CTAB at the two extreme ends and that of SDS just before CTAB advocated the significant roles of charge and other specific effeds1i4than the water structure. The effects of micellar charge, the environmental dielectric, changed pK, water structure, and the partition of the species between the micelle and the bulk would all work in conjunction, whose dissection at present would not be meaningful. In conclusion we make a statement that inspite of accepting the error related possibility of a linear enthalpy-entropy correlation,21we observed such a nice extrathermodynamic compensation for the present proton transfer process in the presence and absence of different additives including the hydroxylic nonaqueous solvents reported earlier.4 Acknowledgment. We thank Professor M. N. Das, Head of the Department of Chemistry for use of laboratory facilities.
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References and Notes (1) Present Address: Department of Chemistry, Ciarkson College of Technology, Potsdam, N.Y. (2) S. P.Moulik, A. K. Chatterjee, and K. K. Sen Gupta, Spectrochim. Acta, Sect. A , 29, 363 (1973). (3) S. P. Moulik, A. K. Chatterjee,and K. K. Sen Gupta, Ind. J. Chem., 12, 92 (1974). (4) S. P. Moulik, S. Ray, and A. R. Das, J. Phys. Chem., 80, 157 (1976). (5) S. P. Moulik, S. Ray, and A. R. Das, Ind. J. Chem., 14A, 921 (1976). (6) S. P. Moulik, C. R. Sahu, and A. K. Mitra, Carbonhydr. Res., 25, 197 (1972). (7) S. P. Moulik, A. K. Mitra, and K. K. Gupta, Carbohydr. Res., 19, 416 (1971). (8) S. P. Moulik and A. K. Mitra, Carbohydr. Res., 23, 65 (1972). (9) J. H. Fendler and E. J. Fendler, “Catalysis in Micellar and Macromolecular Systems”, Academlc Press, New York, N.Y., 1975. (10) L. M. Casilio, E. J. Fendier, and J. H. Fendler, J. Chem. SOC.B , 1377 (1971). (11) C. A. Bunton and M. J. Minch, J. Phys. Chem., 78, 1490 (1974). (12) R. L. Reeves, J. Am. Chem. Soc., 97, 6025 (1975). (13) C. J. O’Connor, E. J. Fendler, and J. H. Fendler, J. Org. Chem., 38, 3371 (1973). (14) C. J. OConnor, E. J. Fendler, and J. H. Fendler, J. Am. Chem. SOC., 96, 370 (1974). (15) L. Kundu and B. N. Ghosh, Ind. Chem. Soc.. 46, 462 (1969). (16) R. H. Crook, D. B. Fordyce, and G. F. Trebbi, J . Phys. Cheh., 67, 1987 (1963). (17) P. Mukherjee and K. J. Mysels, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 36 (1971). (18) E. Baumgartner, R. Fernendes-Prini, and D. Turyn, J . Chem. Soc., Faraday Trans. 1 , 70, 1518 (1974). (19) J. V. Moller and U. Kragh-Hansen, Biochemistry, 14, 2312 (1975). (20) S. N. Vinogradov, R. A. Hudson, and R. M. Scott, Biochim. Biophys. Acta, 214, 6 (1970). (21) R. R. Krug, W. G. Hunter, and R. A. Grieger, Nature(London),261, 567 (1976).
Observation of Quasilinear Fluorescence Spectra (the “Shpol’skii Effect”) in Matrix-Isolated Polycyclic Aromatic Hydrocarbons P. Tokousbalides, E. L. Wehry,” and Gleb Mamantov” Department of Chemistry, University of Tennessee, Knoxville, Tennessee 379 16 (Received May 16, 1977) Publication costs assisted by the National Science Foundation and the Nectric Power Research Institute
Quasilinear “Shpol’skii” fluorescence spectra of polycyclic aromatic hydrocarbons can be observed by matrix isolation, employing an n-alkane as the matrix, if the vapor-deposited sample is annealed at elevated temperature (ca. 140 K) prior to observation of the spectrum at low temperature (16 K). Quasilinear spectra obtained by matrix isolation compare closely to those obtained by conventional frozen-solution techniques in Shpol’skii matrices. Temperature variations of vapor-deposited samples after deposition but before spectral measurements reveal the presence of matrix “memory effects”.
Introduction Broad absorption or emission bands in the electronic spectra of polycyclic aromatic hydrocarbons (PAH) observed in liquid solutions of certain n-alkanes reduce to very narrow features (“quasilines”) upon freezing at low temperature (the Shpol’skii effect1). The quasilines correspond to purely electronic transitions, i.e., to transitions in which the vibrational state of the lattice does not ~hange.~B In cases of weak coupling of the electronic and lattice vibrational transitions, the quasilines are accompanied (on the long-wavelength side of the fluorescence band and on the short-wavelength side of the absorption band) by a diffuse band, the “phonon wing”. The phonon wing arises from electronic transitions in the PAH accompanied by concomitant changes in the vibrational state of the lattice. In the case of strong coupling between the
electronic transitions of the PAH and the vibrational transitions of the lattice, only the phonon wing is observed.2 The existence of several slightly different, but well defined, types of molecular sites (impurity-host) in the alkane matrix, corresponding to different rotational isomers of the alkane molecule, is thought to be responsible for the “multiplet” structures of quasilines in many Shpol’skii spectra.l While the fluorescence spectroscopy of PAH in frozen solutions has received extensive s t ~ d y , ’ , ~very ~ ~ -few ~ spectral studies of PAH in vapor-depositedmatricedohave been described, and no reports of matrix isolation fluorescence spectra of PAH in Shpol’skii solvents have appeared. Matrices formed by deposition of vapors on a cold target usually possess a large number of poorly defined multiple sites.“ This condition results in the appearance The Journal of Physical Chemistry, Voi. 81, No. 18, 1977
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of very broad bands in the luminescence spectra of guest molecules in vapor-deposited samples. Annealing of such a sample can reduce the extent of matrix imperfections, leading, in some cases, to substantial sharpening of spectra. Depending upon the relative sizes of guest and host molecules, annealing at temperatures close to the matrix melting point can also greatly facilitate guest-guest recombination, resulting in broad spectra characteristic of guest aggregates. We now report the observation that quasilinear fluorescence spectra can be observed for PAH in n-alkane matrices if proper annealing procedures are employed; the present study pertains specifically to the fluorescence of benz[a]anthracene in such matrices.
Experimental Section Benz[a]anthracene (purum) was obtained from Fluka AG and used without further purification. Spectroquality solvents (n-hexane and n-heptane, Matheson Coleman and Bell) were purified by treatment with sulfuric acid followed by distillation over anhydrous sodium sulfate. The apparatus described p r e v i o u ~ l y for ~ ~ Jperforming ~ matrix isolation spectroscopy of PAH in “conventional” matrices (e.g., nitrogen) by Knudsen effusion-vacuum sublimation was modified by incorporation of a delivery system for n-alkane vapor. The apparatus was designed to permit degassing of the liquid n-alkane by the freeze-thaw method. Samples were deposited on a sapphire window mounted in the head of a closed-cycle refrigerator (Spectrim, CTI Cryogenics, Waltham, Mass.), with a matrix deposition rate of ca. mol The mole ratio of benz[a]anthracene to alkane ranged from 1:105to 1:1Q6.Annealing of deposits was accomplished by increasing the temperature of the deposit (which had been collected at a window temperature of 16 K) to the desired temperature (which was usually maintained for 5 min or less), then allowing the deposit to return to 16 K at the maximum cooling rate of the refrigerator (ca. 10 K/min). The brief annealing, at temperatures as much as 50 K below the matrix melting point, ensures that no significant diffusion of guest molecules occurs. For frozen-solution spectroscopy, a 6-mm sealed cylindrical quartz cell was used. The cell was placed in a copper holder which was attached to the cold tip of the cryostat, The samples were frozen to the desired temperature in an elapsed time of approximately 5 min. All fluorescence spectra were obtained with a spectrometer constructed in this 1ab0ratory.l~ Results and Discussion The fluorescence spectrum of benz[a]anthracene (BaA) in frozen solutions of n-heptane (Figure 1A) exhibits a band centered at 384 nm which consists of three wellresolved quasilines. The quasilines occur at 383.6,384.3, and 384.7 nm, in close agreement with previous results for BaA in frozen solutions of n-heptane.15 The central quasiline exhibits a half-width of 12 cm-l. A broad band appears in the frozen-solution spectrum at ca. 405 nm; the intensity of this band increases, relative to that of the quasilines, as the BaA concentration in the solution is increased. This band may accordingly originate, at least in part, from emission of microcrystalline aggregates formed during freezing. The fluorescence spectrum of BaA, matrix isolated in n-heptane at 16 K, is shown in Figure 1B; as anticipated, none of the well-defined features of the frozen-solution spectrum are observed. However, if the matrix-isolated sample, deposited at 16 K, is annealed at 140 K prior to measurement of the spectrum at 16 K, the spectrum shown The Journal of Physical Chemistry, Vol. 81, No. IS, 1977
P. Tokousbalides,
E. L. Wehry, and G. Mamantov
‘OI
I
8.
F 4.
O
L
.
:1
C
A 6.
,
400
,
,
400
z
c
“m3-J
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Figure 1. Fluorescence spectra of benz[ alanthracene in n-heptane M); B, matrlx isolated at 16 K: A, in frozen solution (C = 1.4 X (benz[ a]anthracene:heptane mole ratio = 1:200 000) before annealing: C, after annealing B at 140 K. Excitation wavelength = 278 nm in all cases.
E
^I Ul
i 400
380
X nm
400
50,
400
3Bo
Figure 2. Fluorescence spectra of benz[ alanthracene in n-hexane M); B, matrix isolated at 16 K: A, in frozen solution (C = 1.0 X (benz[a]anthracene:hexanemole ratio = 1:500 000) before anneallng: C, after annealing B at 140 K. Excttation wavelength = 278 nm In all cases.
in Figure 1C is obtained. In the spectrum of the annealed matrix-isolated sample, the structureless band at 384 nm in Figure 1B is resolved into three well-defined quasilines, the wavelength maxima and half-widths of which coincide with those observed in a frozen solution (compare Figures 1A and IC). Frozen-solution spectra of BaA in n-hexane at 16 K, although much narrower than those observed at room temperature, do not exhibit quasilinear structure (Figure 2A). Thus, crystalline n-hexane does not act as a specific Shpol’skii solvent for BaA, at least under the freezing conditions of our cryostat. As anticipated, spectra of matrix-isolated samples in n-hexane, deposited at 16 K, are very broad, as shown in Figure 2B. Once again, however, annealing of such a matrix-isolated sample provides a quasilinear spectrum of BaA (Figure 2C), with a single sharp line of 22-cm-’ half-width at 383.8 nm. Consequently,under certain conditions it appears possible to observe Shpol’skii spectra in matrix-isolated samples even in solvents in which frozen-solution spectra do not exhibit quasilinear structure. As shown in Figure 3, evolution of quasilines from the broad band at 384 nrn in a vapor-depositedBaAmheptane sample does not occur gradually with increasing annealing
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Matrix-Isolated Fluorescence Spectra of Aromatic Hydrocarbons
et
A
B
C
D
E
F
l
Figure 3. Effect of annealing temperature upon the 384-nm fluorescence band system of benz[ alanthracene in vapor-deposited n-heptane matrices: A, 16 K; B, 120 K; C, 130 K; D, 135 K; E, 140 K; F, 150 K.
temperature, but appears rather suddenly at annealing temperatures of 135 K or greater. (For n-hexane matrices, the onset of quasilines in the BaA fluorescence spectrum appears at an annealing temperature of 125 K.) The intensity ratio for the quasilines in n-heptane changes significantly as the annealing temperature is varied in the 135-150 K range but, at annealing temperatures between 150 and 170 K, the relative intensities of the three quasilines remain constant. (The melting point of n-heptane is 182.5 K.) Although there is no known phase change in crystalline hexane or heptane at such temperatures,16 it is conceivable that packing rearrangements or slight rotations of the carbon chains of the heptane molecules may conceivably occur at temperatures in the 125-150 K region. Such small matrix rearrangements appear sufficient to allow the guest (BaA) molecules to accommodate themselves in the “specific” types of lattice sites required for observation of quasilinear spectra. It is likely that the PAH molecule itself determines the nature of these “specific” sites: For annealing temperatures greater than 150 K, the distribution of quasilines in the BaA fluorescence spectrum is independent of the duration of annealing; annealing times ranging from 3 min to 3 h were employed, with no detectable effect upon the relative intensities of the quasilines. This result indicates that the number of possible types of rotational or packing rearrangements in the alkane matrix is quite limited. Moreover, this result, coupled with the fact that quasihear spectra are produced upon very brief (
