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Tetracene-argon van der waals molecules
Chemical Physics 63 (1981) 469-474 North-Holland Publishing Company
TETRACENE-ARGON Anne M. GRIFFITHS Edward
Received
Dark
Chnical
VAN DER WAALS
MOLECULES
and Philip A. FREEDMAN Loborntories.
University
College
of
VValer,Aberystwytit,
Dyfed. SY23
INE, UK
7-7 July 1981
Expansion of tetracene vapour with excess argon carrier gas through a nozzle of a supersonic jet apparatus permits the controlled formation of (CraHta)Ar,, molecules. These have been studied by laser fluorescence techniques in the 450 nm region of the spectrum corresponding to excitation of the parent tetracene molecules to the ‘B,, electronic state. We have been able to observe up to the n = 10 complex and have identified three different binding sites on the tetracene framework. From the resolved fluorescence spectra of these species one obtains an upper limit of the van der Waals binding energy of 314 cm -t in the upper electronic states and 274 cm-t m the ground state. These resolved spectra also exhibit features identified as due to the tetracene-argon stretching vibration giving a value of 36.5&Z cm-’ for the lowest transition in the ground state of (CtsH&Ar.
1. Introduction
There has been considerable experimental advance in recent years in the study of the weak van der Waals bound molecules, arising mainly from the increased use of molecular beam apparatus. Here advantage is taken of the cooling experienced by the species as they expand from the high pressure region into the vacuum chamber. As a result of this cooling, any weakly bound species formed during the expansion process can be stabilised to provide one with a convenient source for study. This technique, pioneered by Klemperer and his colleagues [l] in the microwave and radio-frequency region, has now been extended, mainly by the use of lasers, to include studies in virtually all regions of the spectru-m. The introduction of simplified nozzle sources following the studies of Campargue [2] and of Levy et al. and their colleagues [3], has provided a simple and convenient apparatus to extend this work. The high concentrations achievable in such supersonic expansion chambers, combined with the highly sensitive detection technique of laser induced fluorescence have permitted the investigation of the properties of highly complexed species as opposed to
0301-0104/81/0000-0000/$02.75
the more “conventional” bimolecular complexes. This paper describes our work on the bonding of multiple argon atoms on the aromatic molecule tetracene ClgHr2_ The choice of the molecule tetracene for such a study was made for a number of reasons. By use of a pianar molecule one has the possibility to study the packing of the inert gas on this plane. The way in which the argon atoms occupy the possib!e sites and the order in which this is done should provide information on the bonding involved. Since we are sttidying the electronic transition of the molecule associated with the R eIectrons above and below the molecular piane, it is expected that only inert gas molecules interacting with these electrons, and hence bound to the top (or bottom), rather than the side, of the aromatic framework will produce observed spectral shifts. The choice of tetracene rather than any other aromatic hydrocarbon for this initial investigation was made since it absorbs and emits radiation in 2 convenient region of the spectrum, whilst not being too large (and hence providing too many bonding sites) to exclude meaningful analysis. Finally, since we have shown previously [4] that the van der Waals complexes containing several
@ 1981 North-Holland
bound inert gas atoms follow a simple spectral frequency shift rule as opposed to the assertions of other groups [S-7]. one retains a powerful tool to help in analysinp the recorded data.
2. Experimental Tetracene, heated to 33O’C, was mixed with excess argon carrier gas (0.5 to 5 atm) and expanded through a 0.050 mm nozzle into a vacuum chamber. The molecular beam was crossed with that of a home-built pulsed tunable dye laser. Fluorescence was detected either by a RCA 31034 photomultiplier which recorded the total emitted signal or by a Hamamatsu R36G tube which recorded the signal after dispersion throrigh a l/2 meter spectrometer. Further details of the apparatus are published elsewhere
31.
Fig. 1. Flcorescence excitation spectra of tetracene-argon van der Waals molecules. The numbering of the peaks COTresponds to the number of bound argon atoms. Trace (a1 HYGrecorded with an argon pressure 2 atm. trace (b) at 3.5 atm. and trace Ccl at 4 arm. Table 1 Obsenxd
3. Results 3. I. Tot01 (rurresolceci) fiuorescence
complex
studies
Fig. 1 shows typical spectra obtained after excitation of tetracene mixed with varying pressures of argon in the region of 446 nm corresponding to the O-O transition of the ‘B Ill_ ‘Al, electronic absorption spectrum. Spectra were also obtained after excitation of higher vibrational levels of the upper electronic state but the relative intensity of the van der Waals complex emission compared to the signal of the parent molecule was found, in all cases, to be weaker than after O-O excitation, presumably due to more efficient intramoleculer nonradiative relaxation processes for these states. In table 1 we record the position of the observed peaks, together with the measured shift from the previous peak. One should note that this measured shift does not represent the binding energy of the argon atoms, but rather the difference in this term between the two electronic states studied in the experiment. As can be seen fro-m fig. 1, peaks 3, 4, 6 and 7, where the labels correspond to the number of bound
positions
of terracenr-aryon
\-an der Waals
absorptions
Xumber of bound argon atoms
Absorption frequency (cm-‘)
Shift (cm-‘!
0
23395.1
1
22353.8
2
22315.5
3
{ 22285.7 22276.2
sh
129.8 ‘139.3
4
{ 22248.6
sh
( 37.5
41.3 38.3
27.6
22238.7 25.0 5
22210.7
6
{ 22184.4 22176.6
7
22157.3 22149.0
24.3 1 34.1 sh 27.1 35.4 sh 26.8
8
22130.5
9
22101.0
10
22074.4
29.5 26.6
471
argon atoms [4, 61, possess shoulders whilst the others appear as single features. The breadth of the absorption peak corresponding to C18H12Ar3 compared with its immediate neighbours will also be noted. The general form of this spectrum agrees with that obtained by Amirav et al. [9] allowing for differences in spectral shift [4]. From the observed separations between spectral features three different approximately constant values can be picked out. The first five
peaks (corresponding
t0 C18H12
t0 ClgHlp%)
are separated by =39 cm-‘. As had been noted previously [4] this value is not constant but decreases slightly as the number of bound argon atoms increases. The separation between peaks corresponding to C18H12Ara to CIBHr2Arr0 is again approximately constant at -27 cm-‘, with a slight decrease in value as II increases. The high frequency shoulders of peaks 3 and 4 are similarly =27 cm-’ from their main neighbour containing one fewer bound argon atom. Finally, the low frequency shoulders of peaks 6 and 7 are -34.5 cm-’ from their main neighbour containing one less bound argon atom. This suggests a total of three different binding sites on the tetracene framework with two possibilities available for the case of 3, 4, 6 and 7 bound argon atoms. In none of our spectral recordings do we find evidence for splitting or satellite features associated with peak 5. 3.2.
Resohed fluorescence studies
Since the total (unresolved) spectral studies produced quantitative information solely relating to the.difference in binding of the argon atoms to the aromatic framework, it was decided to attempt resolved emission investigations to see if information relating to a single state could be obtained. After much effort high quality resolved spectra were obtained as illustrated in fig. 2, where we show part of the spectrum obtained after excitation of the 314 cm-’ vibration of the upper ‘Br, state of uncomplexed tetracene [8]. It should be noted that all the features are assigned to ground state vibrations. On resolving the emission after excitation
Fig. 2. Resolved fluorescencespectrum obtained after excitation of the 314t 0 peak (at 440.3 nm) of uncomplexed tetracene. The figures in parentheses give the shifts of the peaks from the first (excitation) line.
of peaks corresponding to complexed tetracene in this region however, extra features were observed. As an example we show in fig. 3a the low energy region of the resolved spectrum obtained after excitation of the 314 cm-’ vibration of C18H12ArZ. A satellite is observed at ~40 cm-’ to higher energy of the 317 cm-r vibrational peak and at -36 cm-’ to lower energy_ Since the resonance excitation peak also contains an unknown intensity contribution from scattered light we will ignore it in our discussion. The high frequency satellite is easily assigned to O-O emission from Cr8H12Ar1_ Since excitation to the 314 cm-’ level of ‘Bt, provides enough energy for the complex to dissociate, the binding energy must be less than 314 cm-’ for this state, whilst using the previous unresolved data, the binding energy for the ground electronic state is then calculated to be less than 274 cm-‘. We have recorded similar spectra after excitation to the 314 cm-’ vibration of complexes up to ClsHrrArj, which is the limit of our signal to noise ratio, as well as to higher vibrational levels of the complexes. We see no evidence for multiple dissociations or for emission from intermediate vibrational levels of the upper electronic state [8] which would heip us to lower this upper limit of the binding ener,v.
fluorescence
To identify the satellite at lower energy of the 317 cm-’ vibrational peak WC recorded the (much stronger) emission after excitation at the O-O transition of the van der Waals complexes. These spectra showed similar low frequency emission as illustrated in fig. 3b. This spectrum was taken under identical conditions to that shown in fig. 3a but the differing FranckCondon factors [5], together with the much stronger resonance emission to scattered light intensity ratio. require us to concentrate our attention to around the excitation peak. As may be seen, the satellite emission is again clear!y visible (note the sensitivity change between the main peak and the sateIlite in the figure) occurring at 36.5 5 2 cm-’ to lower energy. The weak features to lower energy of the satellite represent the noise level of these spectra. Since
the intensity of the 0-O band requires the tdiatomic) bond distance to change by =kO.OS A during the electronic transition. However, if the bond decreases in length in going from the ground to the excited state one predicts the 0 + 2 band to have one third of the 0-t 1 intensity, whiIst if the bond increases the calculated intensity ratio is Iess than 3%. From the non-observation of the 0+2 transition one must therefore conclude that the van der Waals bond is increasing in length on excitation. Since this model permits only one dimensional motion we cannot, at the moment, tell how much of this bond change is to be regarded as a length increase rather than a lateral shift of the argon atom across the tetracene framework.
these satellite features
4. Discussion
only occur in the resol-
ved spectra of the van der Waals complexes we assign them to excitation of the tetracene-argon stretching vibration in The ground state. The closeness of this frequency to the =40 cm-’ separation of the peaks in the absorption spectrum could result in the corresponding features being hidden under the much stronger nonvibrationa!ly excited bands. However, since no evidence of sateIIite emission is seen after excitation of the uncomplexed tetracene origin peak, which under this premise would be hiding the stretching vibration absorption Gf CltiHIZArlr it is mGre likely that emission from complexes with this vibration excited is rapid!y quenched, these states thus not being observable in our experiment. Finally, since we have an approximate value for the relative intensity of emission to the vibrationless and the u = 1 !evel of the tetracene-argon stretch, it seemed appropriate to use a simple Morse (diatomic) model to predict bond length changes. ObviousIy the model is extremely simplistic and accurate input data are lacking. However, we have done a large number of calculations varying a11 the input parameters Gver wide ranges. The relative intensities of the bands are found to be sensitive only tG the difference in internuclear distances. For the O+ 1 vibrational band to have ~20%
Perhaps the most surprising feature of the absorption spectrilm of the van der Waals complexes is the large number of argon atoms observed to be bound to the aromatic framework. The argon-argon interatomic potentia! is well known [lo] suggesting that adj$cent atoms will not approach more than ~3.7 A to each other. Since the -‘width” of a given *‘henzene“_ring of the tetracene framework is only --3_4A across (see fig. 4) most of these argon atoms must be bound to the periphery of the framework, overlapping the hydrogen atoms. Due to the larger number of such sites, as well as the smaller effect such occupancy would have on the observed spectrum (being less affected by change in the ;r electron distribution), we assign the -27 cm-’ separation between peaks as representing the addition of an extra argon atom to such a site. Of mare interest perhaps is the geometric arrangement which results in the observed spectrum for lower number of bound argon atoms. The key to any explanation must lie in the apparently unique arrangement of five argon atoms on the molecular framework as opposed to the multiple sites possible for three, four, six and seven atoms. We suggest that for low occupancy numbers, the argon atoms will try to
Fig. 3. Resolved fluorescence spectrum obtained after excitation of the 314 cm-’ vibration of ClsH12ArZ. The figures in parentheses give the shifts of the peaks from the first (excitation) line. (b) Resolved fluorescence spectrum obtained after excitation of the 0~0 transition of CI&112Ar1_ Note the change in sensitivity of a factor of 10 between the main peak and satelIite.
sit centrally above a single “benzene” ring of the molecule and thus show maximum spectral sensitivity to 7i electron rearrangement. With the restriction that the argon atoms cannot
Fig. 4. Two possible arrangements of argon atom on the tetracene framework. In (a) room is left for two more atoms which may be accommodated cm the right hand end of the aromatic molecule.
approach each other at less than their van der Waals diameter, one finds that only two atoms may be placed on each side of the aromatic framework using such sites, either above ‘-benzene” rings 1 and 3 or above rings 1 and 4. Once these four atoms are attached, the fifth may be either on a peripheral site, as in fig. 4a, or above the centre of the tetracene framework, as in fig. 4b. These two possibilities are the natural extensions of the arrangements of the previously attached four atoms, where in both cases extra energy stabilisation comes from the argon-argon bonding between adjacent atoms. We suggest that the observed spectrum is inconsistent with the possible arrangement depicted in fig. 4b being accessible until the sixth argon atom is bonded, since this would necessitate peak 5 of the observed spectrum having a satellite. Involving the requirement that, for the lower complexes at leasr, binding of argon atoms on opposite sides of the same “benzene” ring is energetically unfavourabIe, we can explain the observed absorption spectra. Making this assumption, and assigning the ==39 cm-’ spectra1 shift to the bonding of an argon atom above the centre of a given --benzene” ring results in only one possible arrangement for four bound atoms, viz. two above rings 1 and 3 with the other two atoms below rings 2 and 4. The addition of a further atom wi!l then be to a peripheral site as shown in fig. 4a giving a spectral shift of ~27 cm-’ with this addition. At this stage binding of an argon atom to the opposite side of the tetracene framework must involve the breakdown of the bonding restriction, no matter where the addition occurs. Thus either further bonding to a peripheral site (giving a shift of 27 cm-‘) or atomic rearrangement occurs permitting the configuration illustrated in fig. 4b to occur (with a spectral shift of ==35 cm-‘). This option is seen to be open onfy for 6 or 7 bound atoms (since peripheral bonding is blocked by this arrangement), thus explaining the low frequency shoulders on these two peaks. The main peaks in these cases correspond as indicated above to a peripheral site. Finally, it will be noted that with 3 or 4 bound atoms, one can have the possibility of
peripheral, as well as ring centred, bonding explaining the high frequency shoulders of these two peaks, whilst for three bound atoms only, the configuration illustrated in fig. 4b is possible without breaking the opposite side bonding requirement. Since this results in a spectral shift of =35 cm-r rather than ~39 cm-‘, the breadth of this feature can be explained. By involving this energy restriction and using simple geometric considerations, one can thus explain the observed spectrum. For such a restriction to exist must require a co-operative bonding effect not normahy associated with pure van der Waals bonds and may indicate appreciable charge transfer occurring. Although we can find no other model which wili satisfactorily explain the details of the absorption spectrum, we do not, of course, consider the model as proven. We are therefore undertaking a series of further studies on related species to obtain further bonding information.
Acknowledgement
We wish to thank Professor helpful discussions concerning
W. J. Jones for. this work and for
the loan of apparatus. We would also like to thank the Science and Engineering Research Counci! for financia! support.
References [ll
S.E. Novick, P. Davies, S.J. Harris and W. Klemperer. J. Chem. Phys. 59 (1973) 2273; 7.A Dixon, C.H. Joyner, F.A. Baiocchi and W. Klemperrr.
[?I
J. C‘nem.
Phys.
74
(19811
6539,
6534.
6550.
R. Campargue, J. Chem. Fhys. 52 (1970) 1795: Rev. Sci. Instrum. 35 (1964) 111. 131 D-H. Levy, L. Wharton and R.E. Smalley. Chemical and biochemical applications of Iasers, Vol. 2 (Academic Press. New York, 1977) p. 1. [S] I. Raitt. A.M. Griflirhs and P.A. Freedman, Chem. Phys. Letters 80 ( I98 1 j 225. [Sj T.R. Hays. W. Henke, H.L. Selzle and E.W. Sch:ag. Chrm. Phys. Lerrers77 (1981) 19. [61 X. Amirav, U. Even and J. Jortner, Chem. Phys. Letters 67 119791 9. PI A. Amirav, U. Even and J. Jortner, J. Phys. Chem. 85 (1981) 309. I31 A.M. Griffith5 and P.A. Freedman, J. Chem. Sot. Faraday II, submitted for publication. WI A. Amirav, U. Even and J. Jorrner, Chem. Phy. 51 t1980131. El01 EA. Colbonrn and A.E. Douglas, J. Chem. Phys. 65 (1976) 1731.
TETRACENE-ARGON Anne M. GRIFFITHS Edward
Received
Dark
Chnical
VAN DER WAALS
MOLECULES
and Philip A. FREEDMAN Loborntories.
University
College
of
VValer,Aberystwytit,
Dyfed. SY23
INE, UK
7-7 July 1981
Expansion of tetracene vapour with excess argon carrier gas through a nozzle of a supersonic jet apparatus permits the controlled formation of (CraHta)Ar,, molecules. These have been studied by laser fluorescence techniques in the 450 nm region of the spectrum corresponding to excitation of the parent tetracene molecules to the ‘B,, electronic state. We have been able to observe up to the n = 10 complex and have identified three different binding sites on the tetracene framework. From the resolved fluorescence spectra of these species one obtains an upper limit of the van der Waals binding energy of 314 cm -t in the upper electronic states and 274 cm-t m the ground state. These resolved spectra also exhibit features identified as due to the tetracene-argon stretching vibration giving a value of 36.5&Z cm-’ for the lowest transition in the ground state of (CtsH&Ar.
1. Introduction
There has been considerable experimental advance in recent years in the study of the weak van der Waals bound molecules, arising mainly from the increased use of molecular beam apparatus. Here advantage is taken of the cooling experienced by the species as they expand from the high pressure region into the vacuum chamber. As a result of this cooling, any weakly bound species formed during the expansion process can be stabilised to provide one with a convenient source for study. This technique, pioneered by Klemperer and his colleagues [l] in the microwave and radio-frequency region, has now been extended, mainly by the use of lasers, to include studies in virtually all regions of the spectru-m. The introduction of simplified nozzle sources following the studies of Campargue [2] and of Levy et al. and their colleagues [3], has provided a simple and convenient apparatus to extend this work. The high concentrations achievable in such supersonic expansion chambers, combined with the highly sensitive detection technique of laser induced fluorescence have permitted the investigation of the properties of highly complexed species as opposed to
0301-0104/81/0000-0000/$02.75
the more “conventional” bimolecular complexes. This paper describes our work on the bonding of multiple argon atoms on the aromatic molecule tetracene ClgHr2_ The choice of the molecule tetracene for such a study was made for a number of reasons. By use of a pianar molecule one has the possibility to study the packing of the inert gas on this plane. The way in which the argon atoms occupy the possib!e sites and the order in which this is done should provide information on the bonding involved. Since we are sttidying the electronic transition of the molecule associated with the R eIectrons above and below the molecular piane, it is expected that only inert gas molecules interacting with these electrons, and hence bound to the top (or bottom), rather than the side, of the aromatic framework will produce observed spectral shifts. The choice of tetracene rather than any other aromatic hydrocarbon for this initial investigation was made since it absorbs and emits radiation in 2 convenient region of the spectrum, whilst not being too large (and hence providing too many bonding sites) to exclude meaningful analysis. Finally, since we have shown previously [4] that the van der Waals complexes containing several
@ 1981 North-Holland
bound inert gas atoms follow a simple spectral frequency shift rule as opposed to the assertions of other groups [S-7]. one retains a powerful tool to help in analysinp the recorded data.
2. Experimental Tetracene, heated to 33O’C, was mixed with excess argon carrier gas (0.5 to 5 atm) and expanded through a 0.050 mm nozzle into a vacuum chamber. The molecular beam was crossed with that of a home-built pulsed tunable dye laser. Fluorescence was detected either by a RCA 31034 photomultiplier which recorded the total emitted signal or by a Hamamatsu R36G tube which recorded the signal after dispersion throrigh a l/2 meter spectrometer. Further details of the apparatus are published elsewhere
31.
Fig. 1. Flcorescence excitation spectra of tetracene-argon van der Waals molecules. The numbering of the peaks COTresponds to the number of bound argon atoms. Trace (a1 HYGrecorded with an argon pressure 2 atm. trace (b) at 3.5 atm. and trace Ccl at 4 arm. Table 1 Obsenxd
3. Results 3. I. Tot01 (rurresolceci) fiuorescence
complex
studies
Fig. 1 shows typical spectra obtained after excitation of tetracene mixed with varying pressures of argon in the region of 446 nm corresponding to the O-O transition of the ‘B Ill_ ‘Al, electronic absorption spectrum. Spectra were also obtained after excitation of higher vibrational levels of the upper electronic state but the relative intensity of the van der Waals complex emission compared to the signal of the parent molecule was found, in all cases, to be weaker than after O-O excitation, presumably due to more efficient intramoleculer nonradiative relaxation processes for these states. In table 1 we record the position of the observed peaks, together with the measured shift from the previous peak. One should note that this measured shift does not represent the binding energy of the argon atoms, but rather the difference in this term between the two electronic states studied in the experiment. As can be seen fro-m fig. 1, peaks 3, 4, 6 and 7, where the labels correspond to the number of bound
positions
of terracenr-aryon
\-an der Waals
absorptions
Xumber of bound argon atoms
Absorption frequency (cm-‘)
Shift (cm-‘!
0
23395.1
1
22353.8
2
22315.5
3
{ 22285.7 22276.2
sh
129.8 ‘139.3
4
{ 22248.6
sh
( 37.5
41.3 38.3
27.6
22238.7 25.0 5
22210.7
6
{ 22184.4 22176.6
7
22157.3 22149.0
24.3 1 34.1 sh 27.1 35.4 sh 26.8
8
22130.5
9
22101.0
10
22074.4
29.5 26.6
471
argon atoms [4, 61, possess shoulders whilst the others appear as single features. The breadth of the absorption peak corresponding to C18H12Ar3 compared with its immediate neighbours will also be noted. The general form of this spectrum agrees with that obtained by Amirav et al. [9] allowing for differences in spectral shift [4]. From the observed separations between spectral features three different approximately constant values can be picked out. The first five
peaks (corresponding
t0 C18H12
t0 ClgHlp%)
are separated by =39 cm-‘. As had been noted previously [4] this value is not constant but decreases slightly as the number of bound argon atoms increases. The separation between peaks corresponding to C18H12Ara to CIBHr2Arr0 is again approximately constant at -27 cm-‘, with a slight decrease in value as II increases. The high frequency shoulders of peaks 3 and 4 are similarly =27 cm-’ from their main neighbour containing one fewer bound argon atom. Finally, the low frequency shoulders of peaks 6 and 7 are -34.5 cm-’ from their main neighbour containing one less bound argon atom. This suggests a total of three different binding sites on the tetracene framework with two possibilities available for the case of 3, 4, 6 and 7 bound argon atoms. In none of our spectral recordings do we find evidence for splitting or satellite features associated with peak 5. 3.2.
Resohed fluorescence studies
Since the total (unresolved) spectral studies produced quantitative information solely relating to the.difference in binding of the argon atoms to the aromatic framework, it was decided to attempt resolved emission investigations to see if information relating to a single state could be obtained. After much effort high quality resolved spectra were obtained as illustrated in fig. 2, where we show part of the spectrum obtained after excitation of the 314 cm-’ vibration of the upper ‘Br, state of uncomplexed tetracene [8]. It should be noted that all the features are assigned to ground state vibrations. On resolving the emission after excitation
Fig. 2. Resolved fluorescencespectrum obtained after excitation of the 314t 0 peak (at 440.3 nm) of uncomplexed tetracene. The figures in parentheses give the shifts of the peaks from the first (excitation) line.
of peaks corresponding to complexed tetracene in this region however, extra features were observed. As an example we show in fig. 3a the low energy region of the resolved spectrum obtained after excitation of the 314 cm-’ vibration of C18H12ArZ. A satellite is observed at ~40 cm-’ to higher energy of the 317 cm-r vibrational peak and at -36 cm-’ to lower energy_ Since the resonance excitation peak also contains an unknown intensity contribution from scattered light we will ignore it in our discussion. The high frequency satellite is easily assigned to O-O emission from Cr8H12Ar1_ Since excitation to the 314 cm-’ level of ‘Bt, provides enough energy for the complex to dissociate, the binding energy must be less than 314 cm-’ for this state, whilst using the previous unresolved data, the binding energy for the ground electronic state is then calculated to be less than 274 cm-‘. We have recorded similar spectra after excitation to the 314 cm-’ vibration of complexes up to ClsHrrArj, which is the limit of our signal to noise ratio, as well as to higher vibrational levels of the complexes. We see no evidence for multiple dissociations or for emission from intermediate vibrational levels of the upper electronic state [8] which would heip us to lower this upper limit of the binding ener,v.
fluorescence
To identify the satellite at lower energy of the 317 cm-’ vibrational peak WC recorded the (much stronger) emission after excitation at the O-O transition of the van der Waals complexes. These spectra showed similar low frequency emission as illustrated in fig. 3b. This spectrum was taken under identical conditions to that shown in fig. 3a but the differing FranckCondon factors [5], together with the much stronger resonance emission to scattered light intensity ratio. require us to concentrate our attention to around the excitation peak. As may be seen, the satellite emission is again clear!y visible (note the sensitivity change between the main peak and the sateIlite in the figure) occurring at 36.5 5 2 cm-’ to lower energy. The weak features to lower energy of the satellite represent the noise level of these spectra. Since
the intensity of the 0-O band requires the tdiatomic) bond distance to change by =kO.OS A during the electronic transition. However, if the bond decreases in length in going from the ground to the excited state one predicts the 0 + 2 band to have one third of the 0-t 1 intensity, whiIst if the bond increases the calculated intensity ratio is Iess than 3%. From the non-observation of the 0+2 transition one must therefore conclude that the van der Waals bond is increasing in length on excitation. Since this model permits only one dimensional motion we cannot, at the moment, tell how much of this bond change is to be regarded as a length increase rather than a lateral shift of the argon atom across the tetracene framework.
these satellite features
4. Discussion
only occur in the resol-
ved spectra of the van der Waals complexes we assign them to excitation of the tetracene-argon stretching vibration in The ground state. The closeness of this frequency to the =40 cm-’ separation of the peaks in the absorption spectrum could result in the corresponding features being hidden under the much stronger nonvibrationa!ly excited bands. However, since no evidence of sateIIite emission is seen after excitation of the uncomplexed tetracene origin peak, which under this premise would be hiding the stretching vibration absorption Gf CltiHIZArlr it is mGre likely that emission from complexes with this vibration excited is rapid!y quenched, these states thus not being observable in our experiment. Finally, since we have an approximate value for the relative intensity of emission to the vibrationless and the u = 1 !evel of the tetracene-argon stretch, it seemed appropriate to use a simple Morse (diatomic) model to predict bond length changes. ObviousIy the model is extremely simplistic and accurate input data are lacking. However, we have done a large number of calculations varying a11 the input parameters Gver wide ranges. The relative intensities of the bands are found to be sensitive only tG the difference in internuclear distances. For the O+ 1 vibrational band to have ~20%
Perhaps the most surprising feature of the absorption spectrilm of the van der Waals complexes is the large number of argon atoms observed to be bound to the aromatic framework. The argon-argon interatomic potentia! is well known [lo] suggesting that adj$cent atoms will not approach more than ~3.7 A to each other. Since the -‘width” of a given *‘henzene“_ring of the tetracene framework is only --3_4A across (see fig. 4) most of these argon atoms must be bound to the periphery of the framework, overlapping the hydrogen atoms. Due to the larger number of such sites, as well as the smaller effect such occupancy would have on the observed spectrum (being less affected by change in the ;r electron distribution), we assign the -27 cm-’ separation between peaks as representing the addition of an extra argon atom to such a site. Of mare interest perhaps is the geometric arrangement which results in the observed spectrum for lower number of bound argon atoms. The key to any explanation must lie in the apparently unique arrangement of five argon atoms on the molecular framework as opposed to the multiple sites possible for three, four, six and seven atoms. We suggest that for low occupancy numbers, the argon atoms will try to
Fig. 3. Resolved fluorescence spectrum obtained after excitation of the 314 cm-’ vibration of ClsH12ArZ. The figures in parentheses give the shifts of the peaks from the first (excitation) line. (b) Resolved fluorescence spectrum obtained after excitation of the 0~0 transition of CI&112Ar1_ Note the change in sensitivity of a factor of 10 between the main peak and satelIite.
sit centrally above a single “benzene” ring of the molecule and thus show maximum spectral sensitivity to 7i electron rearrangement. With the restriction that the argon atoms cannot
Fig. 4. Two possible arrangements of argon atom on the tetracene framework. In (a) room is left for two more atoms which may be accommodated cm the right hand end of the aromatic molecule.
approach each other at less than their van der Waals diameter, one finds that only two atoms may be placed on each side of the aromatic framework using such sites, either above ‘-benzene” rings 1 and 3 or above rings 1 and 4. Once these four atoms are attached, the fifth may be either on a peripheral site, as in fig. 4a, or above the centre of the tetracene framework, as in fig. 4b. These two possibilities are the natural extensions of the arrangements of the previously attached four atoms, where in both cases extra energy stabilisation comes from the argon-argon bonding between adjacent atoms. We suggest that the observed spectrum is inconsistent with the possible arrangement depicted in fig. 4b being accessible until the sixth argon atom is bonded, since this would necessitate peak 5 of the observed spectrum having a satellite. Involving the requirement that, for the lower complexes at leasr, binding of argon atoms on opposite sides of the same “benzene” ring is energetically unfavourabIe, we can explain the observed absorption spectra. Making this assumption, and assigning the ==39 cm-’ spectra1 shift to the bonding of an argon atom above the centre of a given --benzene” ring results in only one possible arrangement for four bound atoms, viz. two above rings 1 and 3 with the other two atoms below rings 2 and 4. The addition of a further atom wi!l then be to a peripheral site as shown in fig. 4a giving a spectral shift of ~27 cm-’ with this addition. At this stage binding of an argon atom to the opposite side of the tetracene framework must involve the breakdown of the bonding restriction, no matter where the addition occurs. Thus either further bonding to a peripheral site (giving a shift of 27 cm-‘) or atomic rearrangement occurs permitting the configuration illustrated in fig. 4b to occur (with a spectral shift of ==35 cm-‘). This option is seen to be open onfy for 6 or 7 bound atoms (since peripheral bonding is blocked by this arrangement), thus explaining the low frequency shoulders on these two peaks. The main peaks in these cases correspond as indicated above to a peripheral site. Finally, it will be noted that with 3 or 4 bound atoms, one can have the possibility of
peripheral, as well as ring centred, bonding explaining the high frequency shoulders of these two peaks, whilst for three bound atoms only, the configuration illustrated in fig. 4b is possible without breaking the opposite side bonding requirement. Since this results in a spectral shift of =35 cm-r rather than ~39 cm-‘, the breadth of this feature can be explained. By involving this energy restriction and using simple geometric considerations, one can thus explain the observed spectrum. For such a restriction to exist must require a co-operative bonding effect not normahy associated with pure van der Waals bonds and may indicate appreciable charge transfer occurring. Although we can find no other model which wili satisfactorily explain the details of the absorption spectrum, we do not, of course, consider the model as proven. We are therefore undertaking a series of further studies on related species to obtain further bonding information.
Acknowledgement
We wish to thank Professor helpful discussions concerning
W. J. Jones for. this work and for
the loan of apparatus. We would also like to thank the Science and Engineering Research Counci! for financia! support.
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