16835
J. Phys. Chem. 1995,99, 16835-16841
Total Luminescence Spectroscopy of Terrylene in Low-Temperature Shpol’skii Matrixest Krystyna Palewska,***J6zef Lipifiskit Juliusz Sworakowski,” Jerzy Sepid: Hansruedi Gygax,l Erich C. Meister,l and Urs P. WildL Institute of Physical and Theoretical Chemistry, Technical University of Wroclaw, 50-370 Wroclaw, Poland, Institute of Physical Chemistry, Polish Academy of Sciences, 01-224 Warsaw, Poland, and Laboratory of Physical Chemistry, Swiss Federal Institute of Technology, ETH Zentrum, CH-8092, Zurich, Switzerland Received: April 13, 1995; In Final Form: July 19, 1995@
Fluorescence emission and excitation spectra of tenylene in several n-alkane Shpol’skii matrixes have been studied at liquid helium temperatures. The presence of two dominant Shpol’skii sites has been detected in all matrixes; we tentatively attribute one of the sites to tenylene substitutionally entering the host lattice, and the other one to guest molecules situated in the region of defects. The experiments, supported by quantumchemical calculations, point to the existence of two stereoisomers of the solute. We found n-decane to form the best matrix for tenylene; the system seems to be suitable for single molecule spectroscopy studies.
1. Introduction Photophysical properties of terrylene (cf. Figure 1) became of special interest when it was established that terrylene solutions can be studied by the modem technique of single molecule spectroscopy (SMS). Quite severe requirements imposed on both the solute and the solvent (matrix) allowed only very few systems to be investigated by SMS. While the requirements concerning the properties of solute molecules (e.g., high photochemical stability, strong S I O So0 transition, high fluorescence quantum yield etc.) have been clearly defined (cf., e.g., refs 1-5), those concerning the matrix are less evident. The technique of SMS requires that the solute molecules be fixed in a well-defined and geometrically and energetically stable environment in order to minimize the spectral diffusion and enhance the reproducibility of spectral features. For this reason, Shpol’skii matrixes,6 allowing for incorporation of the solute molecules into the crystal lattice of the solvent, come as the evident choice. In earlier works on single molecule spectroscopy of terrylene,1-5.7-’2 three types of matrixes were employed: polycrystalline Shpol’skii matrixes (n-hexadecane), single crystals of aromatic hydrocarbons @-terphenyl and anthracene), and polymers (polyethylene, poly(vinylbutyral), poly(methy1 methacrylate), and polystyrene). Recently, results of similar measurements performed on tetra-tert-butylterrylene in poly(isobutylene) were reported.I3 The interesting behavior of terrylene in the Shpol’skii matrix n-hexadecane makes it worthwhile to carry out a more systematic study of spectroscopy of this molecule in a series of solvents known to form Shpol’skii matrixes. In this paper, we report on low-temperature fluorescence and fluorescence excitation spectra of terrylene molecules in a series of matrixes of even-numbered n-alkanes, from n-octane to n-hexadecane. We employed the technique of total luminescence spectroscopy (TLS), described in detail in refs 14-16. The luminescence intensity is measured as a function of both the excitation and emission frequency, the results being given
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* To whom correspondence should be addressed. Dedicated to Professor Zdzislaw Ruziewicz on the occasion of his 70th birthday. I Technical University of Wroclaw. 8 Polish Academy of Sciences. Swiss Federal Institute of Technology. Abstract published in Advance ACS Abstracts, October 15, 1995. +
@
1
2
Figure 1. Structural formula of the tenylene molecule, and shapes of two of its stereoisomers (“1” and “2”) determined by the quantumchemical calculations reported in this paper.
in the form of a two-dimensional (2D) intensity matrix in the (Vex,V,m) plane. TLS was earlier used for a detailed study of the spectral properties of aromatic hydrocarbon^'^-'^ and of fullerenes,Ig and is of outstanding value if the emission spectra are excitation wavelength dependent.I4-l6 For the purpose of this paper, we first summarize basic characteristics of the TLS spectra of molecules in Shpol’skii matrixes, (a more thorough discussion of TLS spectra of aromatic hydrocarbons in low-temperature n-alkane solvents can be found in ref 16). If a guest molecule in a polycrystalline matrix occupies several distinct sites in the crystal lattice, then the TLS spectra exhibit pattems that can be analyzed as a superposition of single-site vibronic patterns, each identical in structure. The peak frequencies associated with the presence of a site i fulfill the relation ?:,Y. =
+ pvlb(sl)) + Vvlb(S1)
where 6vlb(SO) and ?vlb(SI) are the vibronic frequencies of the ground and excited states, respectively. It is assumed that Vvlb(S0) and ?vlb(Sl) do not depend strongly on the specific site. Therefore, multisite TLS spectra can be described by a weighted superposition of identical single site vibronic pattems, each subspectrum i shifted relative to another one j by the site splitting
0022-3654/95/2099-16835$09.00/0 0 1995 American Chemical Society
Palewska et al.
16836 J. Phys. Chem., Vol. 99, No. 46, 1995 ABO/& in the (Vex,Vem) plane along a “diagonal” line with slope (3Gex/3Bem)= -1 (see ref 16). If the spectral features are associated with different species in a sample, such as stereoi s o m e r ~or ’ ~even products of photochemical reaction,’*the TLS spectrum is not simply formed by a superposition of identical patterns but does also contain “off-diagonal” peaks. The existence of such features on TLS spectra gives immediate reference to the presence of different species in the investigated sample.
2. Experimental Section Spectroscopic measurements were carried out on a fully computer controlled systemI4 consisting of two double-grating SPEX 1402 monqchromators. The excitation monochromator was equipped with an OSRpM XBO 2500 W xenon arc as light source. A Hamamatsu R 2949 photomultiplier in the photoncounting mode was mounted to the emission monochromator to detect the dispersed fluorescence. All excitation spectra were corrected for the spectral characteristics of the excitation subsystem by means of a rhodamine B quantum counter. The two-dimensional spectra were measured by sequentially and automatically recording emission spectra at various excitation frequencies. The spectral resolution (7 cm-I) was the same in excitation and emission. The solvents employed in the present study were purchased from Fluka in purum or puriss grade quality and were used without further purification. Terrylene was obtained from Dr. Schmidt, Institute for PAH Research, D-2070 Ahrensburg, and used as received. The concentration of terrylene ranged from 1.1 x M in n-octane to 3.0 x M in n-hexadecane matrixes. Degassed terrylene solutions were placed in an Oxford CF204 cryostat which was precooled to 25 K. The temperature of the sample decreased rapidly, and a temperature of 60 K was obtained after approximately 2 min. The polycrystalline samples thus obtained were then cooled slowly to the final temperature between 4.5 and 6 K, at which the spectra were recorded. The experiment was supplemented by quantum-chemical calculations which will be described in the next section.
3. Results (a) Quantum-Chemical Calculations. Quantum-chemical calculations have been performed to test the possibility of the existence of stable stereoisomers differing in molecular geometries. The ground-state geometry of terrylene has been calculated Initially, the structure using the AM1 method of Dewar et has been completely optimized (allowing all bond lengths and bond angles to relax). The resulting molecular structure obtained in such a way was that of the isomer referred to as “1” in Figure 1. In this modification, the central naphthalene fragment is deflected by ca. 4” with respect to the terminal ones, laying on the same plane. In the next step, a helicoid+ shape of the molecules was assumed, and then the geometry of terrylene was optimized to a local minimum yielding the structure of the second isomer, referred to as “2”. The “metastable” molecule was indeed found helicoidaltthe dihedral angle between the two terminal naphthalene planes being equal to 16.8”. The shape of this modification resembles that of 7,15-diethyltenylene, determined by X-ray diffractioi2I The optimized ground-state geometries of both isomers are shown in Figure 1. We have also performed additional calculations using the MNDO method.22 Having employed the same procedure as in the case of the Ah41 method, we arrived at qualitatively similar results: the calculations confirmed the possibility of the
existence of two stereoisomers. The equilibrium shapes of the isomers are similar to those obtained from the AM1 method, although the calculated interplanar angles were found different: the deflection -angle of the terminal naphthalenes in the isomer “1” amounted to 14”, whereas the angle between the terminal naphthalenes in the isomer “2” was found to exceed 30”. The AM1 and MNDO methods were also used to calculate the force constants and harmonic vibrational frequencies for both isomers. Both methods yielded real frequencies in all cases; this result indicates that the energies of both structures have been indeed minimized. For the isomer “l”,the main vibration found in our calculations appears at 279 cm-’ (285 cm-I), whereas for the isomer “2” the respective frequency is equal to 268 cm-‘ (256 cm-‘; the values in the brackets have been obtained from the MNDO method). Although the calculated absolute frequencies are not very accurate, the calculated frequency shifts are nevertheless in a reasonable agreement with the experiment (the experimental frequencies amount to 246 cm-I and 209 cm-I, respectively; cf. Section 3b). All other frequencies calculated for the two isomers differ by less than 3 cm-I. It is worth noticing that the results obtained for the isomer “1” coincide with those reported by TchCnio et al? for the planar molecule. It should also be stressed that the groundstate energies of both stereoisomers “1” and “2” are nearly the same: according to the calculations, the energy of isolated molecules of the “1” modification is lower than that of the form “2” by less than 1 kJ/mol, irrespective of the method used. It is therefore possible that even small modifications of the ground state energies (due to, e.g., solute-solvent interactions) may influence the relative abundance of the stereoisomers, as well as their shapes, thus contributing to the spread of the vibrational frequencies observed in various matrixes. Other parameters (energies and polarizations of the electronic transitions, charge distributions in the ground and excited states, etc.) have been evaluated employing the GRINDOL method,23 which is a modified version of an INDO-like approach including the configuration interaction (CI). 500 singly excited configurations were included in the CI procedure. The results are given in section 4 of this paper. (b) Fluorescence Measurements. Selected fluorescence and fluorescence excitation spectra of terrylene in n-alkane matrixes were recorded for all alkanes used in this study. The spectra in n-decane and n-hexadecane, shown in Figure 2, are representative of the whole group. All spectra exhibit a clearly visible doublet structure (in Shpol’skii meaning6). Within the experimental error, we observed a resonance agreement between the first emission and excitation lines. There is also a mirror symm2try between the vibronic patterns in the emission and excitation spectra over the spectral region covered by our experiment. The structure of the 0-0 region of the SO SI transition was found to be sensitive to the nature of the matrix; the bestresolved spectra are observed in n-decane. In the fluorescence spectrum (Figure 2a), the components of the dominant doublet appear at 17 609 cm-’ (OA, a weaker component) and 17 366 cm-’ (OB, a very strong line), whereas in the excitation spectra we observed the corresponding lines at 17 620 and 17 371 cm-’. Additionally, a very weak third component (OC ) appears at 17 408 cm-I in the fluorescence spectrum. The inhomogeneous broadening of the main lines mentioned above was comparable. The two dominant lines, associated with the 0-0 doublet of the SI SOtransition, are also found in the spectra of terrylene in the n-hexadecane matrix at 17749 and 17491 cm-’ in fluorescence and 17 743 and 17 497 cm-’ in excitation. It
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J. Phys. Chem., Vol. 99, No. 46, 1995 16837
Total Luminescence Spectroscopy of Terrylene
a
-em=580 nm
....em=581 nm -.-em=581.5 nm
>
t v,
5 c L w
x
-
b 17600
V
17500
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z
EXCITATION WAVENUMBERS [CM-ll
3
Figure 3. Emission wavelength dependence of the excitation spectrum of the B site in n-hexadecane matrix at 5 K.
r
19000
iaooo
17000
16000
WAVENUMBERS [CM-']
Figure 2. Fluorescence emission and excitation spectra of tenylene at 5 K in (a) n-decane ( c = 1.3 x lo-' M), and (b) n-hexadecane (c = 3.0 x lo-' M). Full lines, fluorescence spectra excited at 18 018 k 3 cm-I in both matrixes; dashed lines, excitation spectra monitored at 17 094 f 3 cm-I in n-decane and at 17241 & 3 cm-l in n-hexadecane. All lines corresponding to fundamental frequencies of the B site (cf. Table 1) are marked with arrows.
should be noted that, unlike in n-decane, the phonon wing of OA in n-hexadecane is much more pronounced and comparable in intensity with the zero-phonon line, being temperature independent between 1.8 and 6 K. The structure of the 0-0 transition multiplet was found to depend on both the emission and the excitation energy. This is shown in Figure 3, where highly resolved SI0 So0 excitation spectra are plotted as observed at selected emission wavenumbers. The TLS spectra of terrylene in all matrixes used in this study are shown in Figure 4a-f. These figures cover approximately the Vm 700 cm-I region in excitation and 600 - 1000 cm-' region in fluorescence. In all solvents, terrylene occupies one strongly dominant and well-defined site with the energy of the 0-0 transition located at 17 366 cm-I in n-decane and near 17 500 cm-I in other matrixes. This site is labeled with the letter B in the following. The inhomogeneous broadening of the line in all solvents is small (fwhm 30 cm-I). Also the widths of the cross sections of the phonon wings are small, both in emission and in excitation. The second component of the dominant doublet in Figure 2 (labeled A) appears clearly in the two-dimensional spectra, with the 0-0 transition located at ca. 17 600 cm-' in n-decane and ca. 17 750 cm-' in other solvents. As can be verified by inspection of the spectra a-e, the line shapes of the A transitions are strongly dependent on the n-alkane solvent: The best Shpol'skii effect is observed in alkanes of medium chain length (CSand Clo) whereas the line width increases in longer alkanes ( C I t~o cl6). A peculiar cockleshell shape of the contour of
-
+
the peak found in n-hexadecane, n-tetradecane, and n-dodecane matrixes is evidence for the existence of broad and intensive phonon wings, both in emission and in excitation. Apart from the dominant doublet, a much weaker component (hereafter labeled C) of the Shpol'skii multiplet has been observed in all solvents at ca. 17 400 cm-' in n-decane and ca. 17 600 cm-I in other solvents. Moreover, there exists a distribution of sites, manifesting itself as a tail (labeled D) on the diagonal V e x = Vem 246 f 7 cm-I (cf. Figure 4). The pattern corresponding to the distribution is quite intensive in n-hexadecane, clearly visible in n-tetradecane, and almost absent in n-decane &d n-dodecane. In addition to the features described above, located on the diagonal Vex = Vem 246 Z!C 7 cm-' and associated with the presence of the Shpol'skii multiplet described above, a set of less intense peaks was found on the diagonal Fex = ?em 209 & 5 cm-I (labeled X in Figure 4), whose origin will be discussed in the subsequent section of this paper. Fundamental vibrational frequencies of terrylene in the SO and SI states. determined from the analysis of the emission spectra, are given in Table 1. Both the values quoted in Table 1 and those given in Figure 4a-e point to the influence of the nature of the solvent on the position of the main vibrational peak. Apart from the real effect of the matrix (which will be discussed later), another factor could contribute to this effect. It should be pointed out that the spread of the vibrational frequencies may be influenced by a contribution of phonon wings, likely to be dependent on the nature and crystallinity of the matrix.
+
+
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4. Discussion
The most intense vibronic line appearing in the fluorescence spectra of the sites A-D in all solvents appears at (246 f 7) cm-' below the 0-0 transition, in a very good agreement with the results of Moemer et In the excitation spectra, the strongest line appears at nearly the same frequency above the 0-0 transition. The fundamental frequencies are given in Table 1 and indicated by the arrows in Figure 2a. Moreover, overtones of the strongest vibration, as well as combinations with all other vibrations are also visible in the spectra. Incidentally, the frequency of the strongest vibronic line almost coincides with the difference of energies of the two dominant sites, A and B. As was mentioned in the preceding section, the shapes of the lines associated with the B site are different f r o m those al.43539
Palewska et al.
16838 J. Phys. Chem., Vol. 99, No. 46, 1995
8
7
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v)
Bm
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EMISS ION WAVE NU MBERS [C M ]
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EMISSION WAVENUMBERS [CM-’1
Figure 4. TLS Shpol’skii spectra of terrylene in n-alkane matrixes at 5 K. The capital letters indicate sites as described in the text. The solvents M; (c) n-dodecane, c = 1.7 and concentrations (at room temperature) are (a) n-hexadecane, c = 3.0 x M; (b) n-tetradecane, c = 2.6 x x lo-’ M; (d) n-decane, c = 1.3 x lo-’ M; (e) n-octane, c = 1.1 x M. All spectra have been cut at the 1% intensity level to show also the weaker sites. (f) displays a narrow spectral region of (a) with larger magnification.
associated with the A site. In particular, the high intensity of the phonon wing in the latter one may indicate an efficient electron-phonon coupling, much stronger than that in the B site. This feature (and also the shell-like shapes of the contour lines in the TLS spectra; cf. Figure 4) seems to indicate that the arrangement of the host molecules in this site is not strictly
defined and/or the terrylene molecules enjoy some freedom of orientation. On the other hand, the shape of the TLS line attributed to the B site is due to a small width of the zerophonon line, Le., to a small inhomogeneous broadening. This indicates that the guest molecules enter the host lattice in a welldefined manner. One may therefore assume that the B site is
J. Phys. Chem., Vol. 99, No. 46, 1995 16839
Total Luminescence Spectroscopy of Tenylene
TABLE 1: Fundamental Vibrational Frequencies (in cm-I) of Tenylene Identified in the Fluorescence and Fluorescence Excitation Spectra in Some n-Alkane Matrixes, at 5 Ka excitationb fluorescenceC n-decane n-hexadecane n-decane n-hexadecane polyethylene‘ 249 vs 246 vs 249 vs 241 vs 243 435 w 438 w 436 w 439 w 439 532 m 533 m 536 m 532 m 536 573 m 572 m 582 m 581 m 584 780 830 1035 vw 1037 1035 vw 1156 vvw 1160 vvw 1244 m 1238 m 1278 m 1275 m 1275 s 1319 w 1319 w 1310 m 1309 m 1361 m 1357 m 1380 w 1380 w 1370 1404 w (or 249 + 1156) 1405 w (or 246 + 1160) 1506 m (or 241 + 1269) 1524 m (or 249 + 1275) 1522 m (or 241 1279) 1520 w (or 249 + 1278) 1512 w (or 246 + 1275) 1563 w 1560 s 1553 s 1562 1565 w 1613 w 1612 w The values given in the table were found from the analysis of the lines belonging to the “B” site. The values for polyethylene matrix were taken from ref 4. w = weak, m = medium, s = strong, v = very. Reference 4.
+
formed by tenylene substitutionally replacing the solute molecules. The same may also be true for the less-populated sites C and D; their presence in the long-chain matrixes (tetradecane and hexadecane) might be due to a misfit between the molecular dimensions of the host and guest molecules, allowing for some distribution of substitutional sites, as will be discussed later. In the A site, the guest molecules would be located in (or in the vicinity of) a defect, sufficiently well-defined yet providing a distribution of environments (the site is, to some extent, similar to “pseudo-liquid” ones, discussed in ref 24). However, it should be pointed out once again that, to within the experimental uncertainty, the vibrational pattems of all these sites are the same, Le., the molecules should be of the same (or a very similar) geometry. The analysis of the vibronic frequencies associated with the X site yields values that are different from the sites discussed above (in particular, at low frequencies; see Figure 5a). Due to a very low intensity of the lines associated with this site, the analysis was performed only for the n-tetradecane matrix, in which the interference associated with the distribution D was sufficiently weak (see Figure 4b). To get a closer insight into the nature of these lines, we recorded additional fluorescence spectra shown in Figure 5. The excitation frequency used for the spectrum presented in Figure 5a was equal to 17 427 cm-I, Le., the frequency corresponds exactly to the excitation of the X species, while the excitation frequency in Figure 5b to 17 409 cm-I, thus matching sites belonging to the distribution D (see Figure 4b). The results, presented in Figure 5, demonstrate, in a very good agreement with the l i t e r a t ~ r e , ~that . ~ .the ~ dominant feature in the spectrum of the X site is a vibration appearing at 207 cm-’ in n-tetradecane matrix and at 205 cm-’ in nhexadecane matrixes. (As will be discussed later in this paper, tenylene molecules occupying the X sites are equivalent to those referred to as “type-2” molecules in ref 9.) It was mentioned in section 2 of this paper (cf. also ref 16) that the excitation and emission spectra of rigid guest molecules entering different Shpol’skii sites exhibit, in general, identical vibronic pattems, Le., the vibronic lines appear on common diagonals in the (ije,,Ve,) plane. On the other hand, a different vibronic pattem, which manifests itself by appearance of offdiagonal features, is usually associated with the presence of different species. This is the case of the X site where the
I
EXCITATIONAT 17427 CM-’
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EXCITATIONAT 17409 CM-’
IN
IB
17500
:i 17000
8
: IL R 16500
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EMISSION WAVENUMBERS [CM-’]
Figure 5. Fluorescence spectra of tenylene in n-tetradecane, at 5 K: (a) excitation at (17427 =! 3) cm-I; (b) excitation at (17409 f 3) cm-’ (cf. Figure 4b). See text for further discussion. vibronic pattem differs from that of other sites. Having this in mind, the appearance of the X site can be rationalized assuming the presence of a different stereoisomer of the guest molecule. The presence of this species manifests itself in some matrixes examined in the present study. The energy difference of the Slo So0 electronic transition, calculated for the two stereoisomers using the GRINDOL methodz3 (see section 3a), amounts to ca. 60 cm-I, whereas the experimentally determined differences between the energies of the most stable B site and the X site, determined from the
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Palewska et al.
16840 J. Phys. Chem., Vol. 99, No. 46, 1995
analysis of TLS spectra, amount to OB - OX 60 cm-’ in n-hexadecane, OB - OX M 70 cm-’ in n-tetradecane, and OB OX -55 cm-I in n-octane (note that in the latter matrix the transition energy is lower for the B site). It seems that the matrixes influence the energy gap of the 0-0 transitions of the sites A-D to a much lesser degree than the corresponding energy of the X site: the energies Slo So0 of the former sites are practically not influenced by the change of solvent, whereas the corresponding energy of the X site changes by about 125 cm-l. It was pointed out in the preceding section of this paper that the resolution on the spectra depends on the nature of the matrix. The best-resolved spectra have been obtained in the n-decane matrix, with decreasing resolution on increasing the chain length of the n-alkane. This effect can be rationalized taking into consideration the effect of two principal factors influencing the quality of Shpol’skii matrixes: the compatibility of the host and guest molecules, and conditions of crystallization upon cooling a matrix down to the final temperature of the experiment. Let us qualitatively examine the influence of these two factors. One can safely assume that the terrylene molecules enter the Shpol’skii matrixes with their long axes parallel (or nearly parallel) to the long axes of the n-alkanes. Although the structure of terrylene has not been determined, a reasonable estimate of the length of the molecule can be obtained by an appropriate summation of experimentally determined bond lengths for q ~ a t e r r y l e n e .A ~ ~length of 1.34 nm is calculated with these data, a value that coincides with the experimentally determined dimension of the c axis of the unit cell of diethyltenylene crystal2’(1.382 nm), and not far from the length of the c axis of terrylene estimated in an early paper26 (1.47 nm). The lengths of n-alkane molecules can be estimated either by a summation of the appropriate bond lengths or by assuming them equal to the lengths of the c axes of unit cells of relevant crystals.27 The averaged lengths of the even-numbered n-alkane molecules estimated in such a way amount to 1.10, 1.35, 1.61, 1.86, and 2.09 nm for n-octane through n-hexadecane, respectively. The comparison of the molecular dimensions reveals immediately that the best fit should be expected for an n-decane matrix, as can indeed be inferred from our experiments. Octane molecules are “too short”: incorporating terrylene molecules into the n-octane matrix should result in an additional strain deforming the lattice and deteriorating the matrix quality. On the other hand, n-tetradecane and n-hexadecane molecules are “too long”; hence the guest molecules enter the host lattices with some freedom of location; consequently, one may expect a distribution of microenvironments, even in the case of substitutional sites. The crystallinity of the matrixes is crucially influenced by conditions of their formation. Although it is beyond the scope of this paper to discuss in detail all factors influencing the crystallization conditions, we shall consider here only the effect of two principal ones: the viscosity of the solvent at the freezing point and the cooling rate at that temperature. Taking approximate correlations from ref 28, one may estimate the viscosity of n-hexadecane at its freezing point (293 K) to be more than twice the corresponding value for n-octane at 216 K. On the other hand, assuming identical conditions of cooling, the rate of cooling at the freezing point always increases with increasing temperature of melting. Both these factors contribute to decreasing the crystallinity of n-alkane matrixes upon increasing the lengths of their chains. The above discussion leads to the following conclusion: the effect of the geometrical host-guest compatibility and of the
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crystallization conditions result in obtaining n-decane as an optimal n-alkane matrix for terrylene, allowing for a precise orientation of the guest and for the formation crystalline matrixes of a sufficiently good quality. Finally, we want to compare briefly the vibronic features of the spectra obtained in this work with those reported earlier. We pointed to a general agreement between the main features of our spectra and those reported in refs 4, 5, and 9. We agree with the descriptions of most vibrations. However, our interpretation differs in some details. First, we attribute the vibronic lines appearing at 780 and 830 cm-’ , associated with the stereoisomer “l”, and interpreted as fundamental frequencies in refs 4, 5, and 9, to the combinations of strong fundamental vibrations ((242 534) cm-I and (242 584) cm-I, respectively). Furthermore, the use of the technique of TLS allowed us to identify the site X which is due to a second stereoisomer of terrylene. The agreement of several vibronic frequencies of the X site with those attributed in the cited references to a molecule “type-2” (different from all other ones), allows us to draw a conclusion that the same stereoisomers were observed in both cases. However, some frequencies attributed in refs 4, 5, and 9 to the “type-2” molecule could not be identified in our spectra of the X site. We believe that at least some of these frequencies belong to the more abundant form of terrylene molecules.
+
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5. Conclusions In all n-alkane matrixes examined in this study, we found two dominant Shpol’skii sites. We tentatively attribute one of the sites to terrylene molecules substitutionally entering almost undistorted matrixes, and the other sites to guest molecules located in (or in the vicinity of) some defect of the matrix crystal lattices. The n-alkane matrix giving highest spectral resolution for terrylene is n-decane, having only the two principal sites populated. The inhomogeneous broadening associated with both sites is comparable. The photochemical stability of the terryleneh-decane guest-host pair makes the system very promising for studies using the single molecule spectroscopy technique. On the basis of the interpretation of TLS spectra, coexistence of two stereoisomers of terrylene has been postulated, supported by the results of quantum-chemical calculations.
Acknowledgment. We thank Prof. Andrzej Olszowski for helpful discussions. The help from Dr. Alois Renn (ETH Zurich) is gratefully acknowledged. This work was supported ’ by the Swiss National Science Foundation and by the Technical University of Wroclaw. References and Notes (1) Moemer, W. E. Science 1994, 265, 46. (2) Moemer, W. E.; Plakhotnik, T.; Imgartinger, T.; Croci, M.; Palm, V.; Wild, U. P. J. Phys. Chem. 1994, 98, 7382. (3) Plakhotnik, T.; Moemer, E. W.; Imgartinger, T.; Wild, U. P. Chimia 1994, 48, 3 1. (4) Myers, A. B.; TchCnio, P.; Zgierslu, M. Z.; Moemer, W. E. J. Phys. Chem. 1994, 98, 10377. ( 5 ) TchCnio, P.; Myers, A. B.; Moemer, W. E. J. Lumin. 1993, 56, 1. (6) Shpol‘skii, E. V. Vsp. Fiz. Nuuk 1960, 71, 215; 1962, 77, 321: 1963, 80, 255. (7) Omt, M.; Bemard, J.; Zumbusch, A.; Personov, R. I. Chem. Phys. Left. 1992, 196, 595; 1992, 199, 408. (8) Myers, A. B.; TchCnio, P.; Moemer, W. E. J. Lumin. 1994, 58, 161. (9) TchCnio, P.; Myers, A. B.; Moemer, W. E. Chem. Phys. Lerr. 1993, 213, 325. (10) Fleury, L.; Zumbusch, A,; Orrit, M.; Brown, R.; Bernard. J. J. Lumin. 1993, 56, 15.
Total Luminescence Spectroscopy of Terrylene (11) Kummer, S.; Basch6, Th.; Brauchle, C. Chem. Phys. Lett. 1994, 229, 309. (12) Kozankiewicz, B.; Bernard, J.; Onit, M. J. Chem. Phys. 1994,101, 9377. (13) Kettner, R.; Tittel, J.; Basche, Th.; Brauchle, C. J. Phys. Chem. 1994, 98, 6671. (14) Suter, G . W.; Kallir, A. J.; Wild, U. P. Chimia 1983, 37, 413. (15) Kallir, A. J.; Suter, G.W.; Bucher, S. E.; Meister, E.; Liiond, M; Wild, U. P. Acta Phys. Polon. 1987, A71, 755. (16) Palewska, K.; Meister, E. C.; Wild, U. P. J. Lumin. 1991, 50, 47. (17) Palewska, K.; Meister, E. C.; Wild, U. P. Chem. Phys. 1989, 138, 115. (18) Palewska, K.; Meister, E. C.; Wild, U. P. J. Photochem. Photobiol. A 1989, 50, 239. (19) Palewska, K.; Sworakowski, J.; Chojnacki, H.; Meister, E. C.; Wild, U. P. J. Phys. Chem. 1993, 97, 12167.
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