- Email: [email protected]
Some characteristic features of shpolskii spectra. The key and hole rule for shpolskii systems
Volume 47. number 2
SOME CHARACl-ERISTIC
CHEMICAL PHYSICS LETTERS
FEATURES
OF SHPOISKii
THE KEY AND HOLE RULE FOR SHPOtSKIl
15 April 1977
SPECTRA.
SYSTEMS
J.J. DEKKERS, G.Ph. HOORNWEG, G. VISSER, C. MACLEAN and N.H. VELTHORST Chemical Laboratory of the Free University. Amsterdam.
The Netherlands
Received 27 January 1977
The key and hole rule for Shpolskii systems has been re-ehaminedby studying fluorescence spectra of naphthatene, antluacene and naphthacene in several n-alkane polycrystals (20 K)- it is demonstrated that the key and hoCe rulehas no significance for the systems under study. From our results and literature data the Shpolskii effect appears to Be a rather general phenomenon for aromatic hydrocarbons in frozen n-alkanes, provided a correct concentration: is chasen and a proper cooling rate is applied.
a _ introduction The appearance of quasi-lines in optical spectra of many aromatic compounds dissolved in n-alkane matrices is generally called the Shpolskii effect. The literature seems to indicate that quasi-lines occur only in specific combinations of n-alkanes and aromatic molecules. Bolotnikova, for example, studied absorption and fluorescence spectra of naphthaIene, anthracene and naphthacene (tetracene) and noted the sharpest bands respectively in n-Q, n-C7 and it-C9 [ 1] . A remarkable matching in length of long axis and short axis of the guest and of the host molecules seemed to exist. She postulated a substitutional replacement of an alkane moIecule by an aromatic hydrocarbon, a phenomenon formulated as the lock and key principle by Shpoiskii [2,3] and as Shpolskii’s key and hole rule by Pfister [4,5]. For non-linear condensed aromatic compounds a similar correlation has been suggested between the dimensions of the long axis of the guest and of the host molecule [6]; examples are phenanthrene in n-C6 and chrysene in n-C8 [7]. Durocher and Leach have applied the key and hole rule to determine the probable orientation of durene molecules in an n-C, polycrystalline matrix [S, 93. Meyer, on the other hand, has questioned the validity of the key and hole rule because similar line structures are found in rare gas matrices and in inert matrices of
small molecules flol. However, geometric31 compatibility may be achieved by substituting one guest for severaL host molecules [4,11,13] _ In a subsequent paper an investigation will be reported about the concentration dependence of the fluorescence spectra of acenaphthene in several n-alkane polycrystals [ 13 1. The results can be understood by assuming that the soIutions have phase diagrams of the eutectic type [8] and that Shpolskii systems are in general thermodynamically non-equilibrium solid solutions. In the present paper fluorescence spectra of naphthalene, anthracene and naphthacene in several rr-aikane polycrystais (20 K) are discussed. It wiB be demonstrated that the key and hole rule has no significance for the Shpofskii systems investigated,
2. Experimental The experimental arrangement, the aromatic hydrocarbons and the rz-alkane solvents used have been described earlier [13,14]. The 254 nm Hg hne of a I00 W arc has been employed as the exciting wave length. All measurements were carried out at 20 K_
357
Volume 47, number 2
15 April 1977
CHEMICAL PHYSICS LElTERS
3. Results Ir?fig. 1 the 0, O-O, 1000 cm-l region of the fluorescence spectra of 10m4 M naphthalene in tz-C5 to iz-CS are reproduced. For naphthalene in n-C5 quasilines appear nearly exclusively. The fluorescence spectra of 1 Om4 M naphthalene in n-C6 to n-CS dre very similar; only broad bands are observed and no quasilines are present. In figs. 2 and 3 the 0, O-regions of the fluorescence spectra of respectively 1Om4 M anthracene in n-C5 to n-C, and (about) 5 X 10m6 M naphthacene in n-C7 to rz-Cll are given. It is noted that there are several sites (secondary origins) in the spectra [ 141; these will be discussed in a subsequent paper. Ah bands are composed of narrow zero-phonon lines and an accompanying phonon sideband. In the latter a sharp one- or multi-phonon line [ 11,15] may be present. In table 1 the half widths of the (main) 0, O-quasi-lines for naphthalene, anthracene and naphthacene (20 K) are @den.
4_ Dicussion In the fluorescence spectra of anthracene and naphthacene(figs . 2 and 3) narrow quasi-lines (line-widths about 8 cm-l) are observed. Table 1 shows that, within experimental error, there is hardly any difference in the half widths of the O,O-quasi-hnes, indicating that quasi-line spectra are not restricted to anthracene in n-C7 and naphthacene in n-C9 _ For ah combinations solid solutions exist which give rise to the occurrence of the Shpolskii effect. From investigations on the related molecules phenanthrene and chrysene in n-alkane matrices it appears that the sharpest bands are not found for phenanthrene in n-C6 and chrysene in n-C, [ 16]_ Therefore it can be concluded that for the aromatic hydrocarbons mentioned the key and hole rule is not meaningful. At first sight the exclusive appearance of broad bands in the fluorescence spectra of naphthalene in n-C6 to IZ-CS seems to corroborate the key and hole rtde. In our opinion a preferable explanation is the following. There seems to be little doubt that solutions of naphthalene in n-aIkane have phase diagrams of the eutectic type, for which the solid-solid solubihty is. very low [8, I7 1. Hence, the equihirium concentration 358
n-C,
4 n-C,
L Fig. 1. Part of the fluorescence spectra of naphthalene in n-Cs to n-C8 polycrystals (20 K).
Volume 47, number 2
CHEMICAL PHYSICS LETTERS
Fig. 2. Part of the fluorescence spectra of anthracene in n-C5
to n-C9 polycrystak
(20 K).
ES ApriI 1977
Fii. 3. Part of the fluorescence spectra of naphthacene in n-C7 to n-C, t polycrystds (20 K).
359
CHEMICAL PHYSICS LETTERS
Volume 47. number 2
Table 1 HaIf widths (20 K) of the 0. O-quasi-linesin cm-’ (experimental error * 2 cm-‘). To determine the haIf’ height of the quasitina, the phonon side-bands have been subtracted NaphthaIene
An*&acene
n-Cs n-Ca n-c,
6 a) -
7 8
n-c,3
-
t&Q
n-Cl0 mC11
Naphthacene
8 9
7 7
9
8 7 9
a) Half width of the 0.515 cm-’ vrbronic quasi-line (measured in view of low intensity of the O,Oquasi-line).
of naphthalene in n-C5 to n-C, in the sohd state is small. The influence of the cooling rate on the Shpolskii effect is well known. For example, Shpolskii et al. were able to obtain quasi-lines for naphthaleneln-C7 at high concentrations [I 81, a result that could not be reproduced by us. The difference can be explained by a faster cooling rate employed by Shpolskii et al. Our experimental set up, in which the sample is prepared, generates a fied cooling rate of the binary system, which is somewhat too high to reach equilibrium. The dependence on chain iength appears in the following way. The binary mixture consisting of naphthalene in n-C6 will approach thermodynamic equilibrium in a shorter time than in n-C,, etc. As a result under our experimental conditions nonequilibrium solid solutions will be formed for naphthalene in n-C5 but not in n-C6 (at 10d4 Mj, in which phase separation takes place. This explains not only the present experimental data, but also those for a related system, i.e. acenaphthene in n-C5 to n-C, [ 13]_ It is concluded that the considerations presented explain the experiments on naphthalene without having to invoke the key and hole rule. On the basis of the results of our work and of literature data the following conclusions seem justified. Small aromatic hydrocarbons seldom display Shpolskii spectra, when dissolved in frozen n-alkanes. To obtain Shpolskii-s effect the experimental conditions are in general less critical for large aromatics than for intermediately sized aromatic hydrocarbons. A few examples will be given to demonstrate this. 360
15 April 1977
For benzene [ 181 and some mono- and polysubstituted benzenes [9], which are aromatic compounds with a “smah size”, no Shpolskii effect has been observed in n-alkane crystals. Aromatic compounds with a “large size”, e.g. anthracene, naphthacene, coronene [5,9, 12,19],peryIene [12,15,20] and 1,12-benzperylene [12,1 S, 211 show the Shpolskii effect within a rather large range of n-alkane crystals (see e.g. figs. 2 and 3). Aromatics with an “intermediate size” (e.g. naphthaIene, acenaphthene) show Shpolskii spectra in several n-a&me matrices; the dependence on concentration and on cooling rate is obvious [ 13 1. In a recent review paper [223 Shpolskii and Bolotnikova distinguished two groups of Shpolskii systems on account of the concentration dependence of the quasi-line spectra. The first group includes systems with quasi-line spectra, which are superimposed on diffuse bands when the concentration of the dissolved aromatic molecules is enhanced. Their conclusion that such a concentration dependent behaviour is especially characteristic of systems with comparable linear size of guest and host moIecules is not confirmed by our results.
5. Conchtsion From a study of the fluorescence spectra of naphthalene, anthracene and naphthacene in several n-alkane polycrystals (20 K) it is demonstrated that the key and hole rule for Shpolskii systems in n-alkene polycrystals has no significance. The Shpolskii effect is a rather general phenomenon, displayed by aromatic hydrocarbons in frozen n-alkanes, provided a correct concentration is chosen and a proper cooling rate of the system is applied. Especially for intermediately sized aromatics (e.g. naphthalene) the permitted values of these factors are rather critical_
References [l] [2] [3] [4] [5 J [6]
T.N. Bolotnikova, Opt. Spectry. 8 (1959) 138. E.V. Shpolskii. Soviet Phys. Uspekhi 5 (1962) 522. E.V. Shpolskii, Soviet Phys. Uspekhi 6 (1963) 411. C. Pfister. Chem. Phys. 2 (1973) 171. C. Pfister, Thbse, UniversitCde Grenoble (1973). J.B. Birks. Photophysics of aromatic molecules (Wiley, New York, 1970) p. 118.
Volume 47, number 2 [7] [S] ]S] [lo] [ 1 l] 1121 1131 f14] [ 151
CHEMiCAL PHYSICS LETTERS
R.N. Nurmukhametov, Russ. Chem. Rev. 38 (1969) 180. G. Durocher and S. Leach, J. Cl&u, Phys. 66 (1969) 628. S. Leach, Pure Appl. Chem. 27 (1971) 457. 8. Meyer, Low temperature spectroscopy (Eisevier, Amsterdam, 1971) p. 60. R.M. Macoab, Thesis, University of California, Berkeley (1969). C. Pfister, J. Chim. Phys. 67 (1970) 418_ YJ. Dekkers, G.Ph. Hoornweg, C. MacLean and N.H. Velthorst, to be published. JJ. Dekkers. G.Ph. Hoornweg, C. MacLean and N.H. Velthorst, Chem. Phys. 5 (1974) 393. R.I. Personov. IS. Osadko, E.D. Godyaev and E-1. Alshits, Soviet Phys. Solid State 13 (1972) 2224.
ES April 1977
f16] JJ. Dekkers, G.PL Hoomweg, C. MacLean and N.H. Velthorst, unpublished rest&s. 1171 S. Leach and R. Lopez-De&ado, P. Chim. Phys_ 64 (1967) 1247. ]18] E-V. Shpolskii, L.A. Wimova, G-N. Nersesova and VJ. Glyadkovskii, Opt. Spectry. 24 (X968) 25. 119) K. Ohno. T. Kajiwara and H. Drokuch?, Bull_ Chem. Sac. Japan 45 (1972) 996. [20] E.V. Shpolskii and R.I. Personov, Opt. Spectry_ 8 (1960) 172. [21] D.M. Grebenshchikov, N.A. KovrJzhnykh and R-1. Personov, Opt. Spectry. 30 (1971) 32. [22] E.V. Shpolskii and T.N. Bolotnikova, Pure AppI. Chem. 37 (1974) 183.
361
SOME CHARACl-ERISTIC
CHEMICAL PHYSICS LETTERS
FEATURES
OF SHPOISKii
THE KEY AND HOLE RULE FOR SHPOtSKIl
15 April 1977
SPECTRA.
SYSTEMS
J.J. DEKKERS, G.Ph. HOORNWEG, G. VISSER, C. MACLEAN and N.H. VELTHORST Chemical Laboratory of the Free University. Amsterdam.
The Netherlands
Received 27 January 1977
The key and hole rule for Shpolskii systems has been re-ehaminedby studying fluorescence spectra of naphthatene, antluacene and naphthacene in several n-alkane polycrystals (20 K)- it is demonstrated that the key and hoCe rulehas no significance for the systems under study. From our results and literature data the Shpolskii effect appears to Be a rather general phenomenon for aromatic hydrocarbons in frozen n-alkanes, provided a correct concentration: is chasen and a proper cooling rate is applied.
a _ introduction The appearance of quasi-lines in optical spectra of many aromatic compounds dissolved in n-alkane matrices is generally called the Shpolskii effect. The literature seems to indicate that quasi-lines occur only in specific combinations of n-alkanes and aromatic molecules. Bolotnikova, for example, studied absorption and fluorescence spectra of naphthaIene, anthracene and naphthacene (tetracene) and noted the sharpest bands respectively in n-Q, n-C7 and it-C9 [ 1] . A remarkable matching in length of long axis and short axis of the guest and of the host molecules seemed to exist. She postulated a substitutional replacement of an alkane moIecule by an aromatic hydrocarbon, a phenomenon formulated as the lock and key principle by Shpoiskii [2,3] and as Shpolskii’s key and hole rule by Pfister [4,5]. For non-linear condensed aromatic compounds a similar correlation has been suggested between the dimensions of the long axis of the guest and of the host molecule [6]; examples are phenanthrene in n-C6 and chrysene in n-C8 [7]. Durocher and Leach have applied the key and hole rule to determine the probable orientation of durene molecules in an n-C, polycrystalline matrix [S, 93. Meyer, on the other hand, has questioned the validity of the key and hole rule because similar line structures are found in rare gas matrices and in inert matrices of
small molecules flol. However, geometric31 compatibility may be achieved by substituting one guest for severaL host molecules [4,11,13] _ In a subsequent paper an investigation will be reported about the concentration dependence of the fluorescence spectra of acenaphthene in several n-alkane polycrystals [ 13 1. The results can be understood by assuming that the soIutions have phase diagrams of the eutectic type [8] and that Shpolskii systems are in general thermodynamically non-equilibrium solid solutions. In the present paper fluorescence spectra of naphthalene, anthracene and naphthacene in several rr-aikane polycrystais (20 K) are discussed. It wiB be demonstrated that the key and hole rule has no significance for the Shpofskii systems investigated,
2. Experimental The experimental arrangement, the aromatic hydrocarbons and the rz-alkane solvents used have been described earlier [13,14]. The 254 nm Hg hne of a I00 W arc has been employed as the exciting wave length. All measurements were carried out at 20 K_
357
Volume 47, number 2
15 April 1977
CHEMICAL PHYSICS LElTERS
3. Results Ir?fig. 1 the 0, O-O, 1000 cm-l region of the fluorescence spectra of 10m4 M naphthalene in tz-C5 to iz-CS are reproduced. For naphthalene in n-C5 quasilines appear nearly exclusively. The fluorescence spectra of 1 Om4 M naphthalene in n-C6 to n-CS dre very similar; only broad bands are observed and no quasilines are present. In figs. 2 and 3 the 0, O-regions of the fluorescence spectra of respectively 1Om4 M anthracene in n-C5 to n-C, and (about) 5 X 10m6 M naphthacene in n-C7 to rz-Cll are given. It is noted that there are several sites (secondary origins) in the spectra [ 141; these will be discussed in a subsequent paper. Ah bands are composed of narrow zero-phonon lines and an accompanying phonon sideband. In the latter a sharp one- or multi-phonon line [ 11,15] may be present. In table 1 the half widths of the (main) 0, O-quasi-lines for naphthalene, anthracene and naphthacene (20 K) are @den.
4_ Dicussion In the fluorescence spectra of anthracene and naphthacene(figs . 2 and 3) narrow quasi-lines (line-widths about 8 cm-l) are observed. Table 1 shows that, within experimental error, there is hardly any difference in the half widths of the O,O-quasi-hnes, indicating that quasi-line spectra are not restricted to anthracene in n-C7 and naphthacene in n-C9 _ For ah combinations solid solutions exist which give rise to the occurrence of the Shpolskii effect. From investigations on the related molecules phenanthrene and chrysene in n-alkane matrices it appears that the sharpest bands are not found for phenanthrene in n-C6 and chrysene in n-C, [ 16]_ Therefore it can be concluded that for the aromatic hydrocarbons mentioned the key and hole rule is not meaningful. At first sight the exclusive appearance of broad bands in the fluorescence spectra of naphthalene in n-C6 to IZ-CS seems to corroborate the key and hole rtde. In our opinion a preferable explanation is the following. There seems to be little doubt that solutions of naphthalene in n-aIkane have phase diagrams of the eutectic type, for which the solid-solid solubihty is. very low [8, I7 1. Hence, the equihirium concentration 358
n-C,
4 n-C,
L Fig. 1. Part of the fluorescence spectra of naphthalene in n-Cs to n-C8 polycrystals (20 K).
Volume 47, number 2
CHEMICAL PHYSICS LETTERS
Fig. 2. Part of the fluorescence spectra of anthracene in n-C5
to n-C9 polycrystak
(20 K).
ES ApriI 1977
Fii. 3. Part of the fluorescence spectra of naphthacene in n-C7 to n-C, t polycrystds (20 K).
359
CHEMICAL PHYSICS LETTERS
Volume 47. number 2
Table 1 HaIf widths (20 K) of the 0. O-quasi-linesin cm-’ (experimental error * 2 cm-‘). To determine the haIf’ height of the quasitina, the phonon side-bands have been subtracted NaphthaIene
An*&acene
n-Cs n-Ca n-c,
6 a) -
7 8
n-c,3
-
t&Q
n-Cl0 mC11
Naphthacene
8 9
7 7
9
8 7 9
a) Half width of the 0.515 cm-’ vrbronic quasi-line (measured in view of low intensity of the O,Oquasi-line).
of naphthalene in n-C5 to n-C, in the sohd state is small. The influence of the cooling rate on the Shpolskii effect is well known. For example, Shpolskii et al. were able to obtain quasi-lines for naphthaleneln-C7 at high concentrations [I 81, a result that could not be reproduced by us. The difference can be explained by a faster cooling rate employed by Shpolskii et al. Our experimental set up, in which the sample is prepared, generates a fied cooling rate of the binary system, which is somewhat too high to reach equilibrium. The dependence on chain iength appears in the following way. The binary mixture consisting of naphthalene in n-C6 will approach thermodynamic equilibrium in a shorter time than in n-C,, etc. As a result under our experimental conditions nonequilibrium solid solutions will be formed for naphthalene in n-C5 but not in n-C6 (at 10d4 Mj, in which phase separation takes place. This explains not only the present experimental data, but also those for a related system, i.e. acenaphthene in n-C5 to n-C, [ 13]_ It is concluded that the considerations presented explain the experiments on naphthalene without having to invoke the key and hole rule. On the basis of the results of our work and of literature data the following conclusions seem justified. Small aromatic hydrocarbons seldom display Shpolskii spectra, when dissolved in frozen n-alkanes. To obtain Shpolskii-s effect the experimental conditions are in general less critical for large aromatics than for intermediately sized aromatic hydrocarbons. A few examples will be given to demonstrate this. 360
15 April 1977
For benzene [ 181 and some mono- and polysubstituted benzenes [9], which are aromatic compounds with a “smah size”, no Shpolskii effect has been observed in n-alkane crystals. Aromatic compounds with a “large size”, e.g. anthracene, naphthacene, coronene [5,9, 12,19],peryIene [12,15,20] and 1,12-benzperylene [12,1 S, 211 show the Shpolskii effect within a rather large range of n-alkane crystals (see e.g. figs. 2 and 3). Aromatics with an “intermediate size” (e.g. naphthaIene, acenaphthene) show Shpolskii spectra in several n-a&me matrices; the dependence on concentration and on cooling rate is obvious [ 13 1. In a recent review paper [223 Shpolskii and Bolotnikova distinguished two groups of Shpolskii systems on account of the concentration dependence of the quasi-line spectra. The first group includes systems with quasi-line spectra, which are superimposed on diffuse bands when the concentration of the dissolved aromatic molecules is enhanced. Their conclusion that such a concentration dependent behaviour is especially characteristic of systems with comparable linear size of guest and host moIecules is not confirmed by our results.
5. Conchtsion From a study of the fluorescence spectra of naphthalene, anthracene and naphthacene in several n-alkane polycrystals (20 K) it is demonstrated that the key and hole rule for Shpolskii systems in n-alkene polycrystals has no significance. The Shpolskii effect is a rather general phenomenon, displayed by aromatic hydrocarbons in frozen n-alkanes, provided a correct concentration is chosen and a proper cooling rate of the system is applied. Especially for intermediately sized aromatics (e.g. naphthalene) the permitted values of these factors are rather critical_
References [l] [2] [3] [4] [5 J [6]
T.N. Bolotnikova, Opt. Spectry. 8 (1959) 138. E.V. Shpolskii. Soviet Phys. Uspekhi 5 (1962) 522. E.V. Shpolskii, Soviet Phys. Uspekhi 6 (1963) 411. C. Pfister. Chem. Phys. 2 (1973) 171. C. Pfister, Thbse, UniversitCde Grenoble (1973). J.B. Birks. Photophysics of aromatic molecules (Wiley, New York, 1970) p. 118.
Volume 47, number 2 [7] [S] ]S] [lo] [ 1 l] 1121 1131 f14] [ 151
CHEMiCAL PHYSICS LETTERS
R.N. Nurmukhametov, Russ. Chem. Rev. 38 (1969) 180. G. Durocher and S. Leach, J. Cl&u, Phys. 66 (1969) 628. S. Leach, Pure Appl. Chem. 27 (1971) 457. 8. Meyer, Low temperature spectroscopy (Eisevier, Amsterdam, 1971) p. 60. R.M. Macoab, Thesis, University of California, Berkeley (1969). C. Pfister, J. Chim. Phys. 67 (1970) 418_ YJ. Dekkers, G.Ph. Hoornweg, C. MacLean and N.H. Velthorst, to be published. JJ. Dekkers. G.Ph. Hoornweg, C. MacLean and N.H. Velthorst, Chem. Phys. 5 (1974) 393. R.I. Personov. IS. Osadko, E.D. Godyaev and E-1. Alshits, Soviet Phys. Solid State 13 (1972) 2224.
ES April 1977
f16] JJ. Dekkers, G.PL Hoomweg, C. MacLean and N.H. Velthorst, unpublished rest&s. 1171 S. Leach and R. Lopez-De&ado, P. Chim. Phys_ 64 (1967) 1247. ]18] E-V. Shpolskii, L.A. Wimova, G-N. Nersesova and VJ. Glyadkovskii, Opt. Spectry. 24 (X968) 25. 119) K. Ohno. T. Kajiwara and H. Drokuch?, Bull_ Chem. Sac. Japan 45 (1972) 996. [20] E.V. Shpolskii and R.I. Personov, Opt. Spectry_ 8 (1960) 172. [21] D.M. Grebenshchikov, N.A. KovrJzhnykh and R-1. Personov, Opt. Spectry. 30 (1971) 32. [22] E.V. Shpolskii and T.N. Bolotnikova, Pure AppI. Chem. 37 (1974) 183.
361