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Far-infrared photoacoustic spectra of tetracene, pentacene, perylene and pyrene
Vibrational Spectroscopy 49 (2009) 28–31
Contents lists available at ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Far-infrared photoacoustic spectra of tetracene, pentacene, perylene and pyrene K.H. Michaelian a,*, B.E. Billinghurst a, J.M. Shaw b, V. Lastovka b a b
Natural Resources Canada, CANMET Energy Technology Centre–Devon, 1 Oil Patch Drive, Devon, Alberta T9G 1A8, Canada Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 October 2007 Received in revised form 9 April 2008 Accepted 10 April 2008 Available online 20 April 2008
Far-infrared photoacoustic (PA) spectra of four different four- and five-ring aromatic hydrocarbons (tetracene, pentacene, perylene and pyrene) were acquired in this investigation. The use of largeamplitude phase-modulation and step-scan spectrometer operation yielded research-quality PA spectra in the 100–700 cm1 region. The bands in these spectra were correlated with published infrared and Raman spectra and with calculated band positions. The PA spectra contain many previously unobserved bands and complement the information already obtained from other types of vibrational spectra for the four compounds. Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved.
Keywords: Photoacoustic infrared spectroscopy Far-infrared Aromatic hydrocarbons DFT calculations
1. Introduction Photoacoustic (PA) infrared spectroscopy is a well-established technique that has been used to characterize a wide variety of both traditional and non-traditional samples during the last three decades [1]. This method, which obviates traditional infrared sample preparation procedures, commonly employs Fourier transform infrared (FT-IR) spectrometers for study of the midinfrared (400–4000 cm1, 2.5–25 mm) region. A number of research groups also utilize PA FT-IR spectroscopy in contiguous regions. In the latter context, the present article describes a farinfrared study of four aromatic hydrocarbons recently carried out in our laboratory. PA infrared spectra of tetracene, pentacene, perylene and pyrene were obtained from about 100 to 700 cm1 in this research. These well-known aromatic compounds are involved in several industrial applications. Interest also derives from their electronic properties, potential effects on health and environment, and possible astrophysical role. Furthermore, these four- and five-ring compounds can be regarded as simple models for asphaltenes, the highly aromatic components of bitumens and heavy oils that adversely affect the extraction and upgrading processes required for the production of middle-distillate fuels. The low-wavenumber vibrational spectra of asphaltenes (and suitable model com-
* Corresponding author. Tel.: +1 780 987 8646; fax: +1 780 987 8676. E-mail address: [email protected] (K.H. Michaelian).
pounds) provide data relevant to the calculation of constantvolume heat capacities [2]. 2. Experimental Tetracene (C18H12), pentacene (C22H14), perylene (C20H12) and pyrene (C16H10) were obtained from commercial sources at purity levels of at least 98 per cent and analyzed without any further purification. Tetracene and pyrene crystals were ground manually to facilitate filling the PA sample cups. Needle-like perylene crystals and finely divided pentacene were studied as received. PA far-infrared spectra of the hydrocarbons were acquired at a resolution of 6 cm1 using a Bruker IFS 66 v/S Fourier transform infrared (FT-IR) spectrometer and an MTEC 300 PA cell. The radiation source was either a standard globar or an Hg lamp. Ambient air remained in the spectrometer during most experiments. In other tests, the instrument was purged with dry N2 to reduce absorption by water vapour. Helium was used as the PA carrier gas. A multi-layer mylar beamsplitter was installed in the interferometer and a polyethylene window was utilized to seal the PA cell. These optical components restricted the accessible region to about 100–700 cm1. Spectra of carbon black were obtained under like conditions and used to correct the sample spectra for the variation in the system response with wavelength. Phase-modulation step-scan PA spectra were acquired as an alternative to the more commonly measured rapid-scan spectra in this work. Phase modulation results in multiplication of the energy (blackbody) curve by jJ1(2pne)j where J1 is the first-order Besselfunction, n is the infrared wavenumber (cm1) and e is the
0924-2031/$ – see front matter . Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2008.04.005
K.H. Michaelian et al. / Vibrational Spectroscopy 49 (2009) 28–31
29
modulation amplitude [3]. Low frequencies (5–22 Hz) and large amplitudes (5–20l, where l is 0.6328 mm, the HeNe laser wavelength) were employed. Demodulation was effected using standard FT-IR software or, alternatively, by means of a lock-in amplifier. This experimental strategy made it possible to acquire research-quality, far-infrared PA spectra down to about 100 cm1. 3. Results 3.1. Large-amplitude phase modulation The quantity jJ1(2pne)j is displayed for e = 5, 15 and 20l in Fig. 1. It can be observed that a modulation amplitude of 15l (dotted line) produces a Bessel-function minimum near 650 cm1, close to the high-wavenumber cutoff of the beamsplitter used in this study. The plotted curves show that large amplitudes are most appropriate for far-infrared step-scan PA spectroscopy. By contrast, smaller amplitudes should be utilized to study the midinfrared region. Fig. 2 depicts a series of far-infrared PA spectra of carbon black obtained using an unpurged spectrometer and a modulation frequency of 5 Hz. Because this reference material was optically opaque and thermally thin [1] under the conditions employed in this experiment, these PA spectra can be interpreted as step-scan energy curves. The structure along the wavenumber axis arises primarily from water vapour (50–300 cm1) and beamsplitter (300–500 cm1) absorption. A gradual shift of the Bessel-function minimum to lower wavenumbers with increasing e is evident on the right-hand side of Fig. 2, the 480-cm1 value observed at e = 20l agreeing with the dashed curve in Fig. 1. PA intensity reaches its maximal value at about 230 cm1 at this amplitude. As expected, the locations of the intensity maxima and Besselfunction minima progressively shift to higher wavenumbers as e decreases.
Fig. 2. Step-scan phase-modulation PA spectra of carbon black. The transmissionlike features along the wavenumber axis are primarily due to water vapour and beamsplitter absorption. The modulation–amplitude axis is non-linear.
Far-infrared PA spectra of the catacondensed hydrocarbons (all carbons on the periphery of the ring system) tetracene and pentacene are shown in Fig. 3. Table 1 compares the positions of the bands in these spectra with representative literature data [4–7] and the results of density functional theory (DFT) calculations in this work and Ref. [8]. The low-wavenumber PA infrared bands
observed for C18H12 and C22H14 in this study generally agree well with these other sources of information, a result that affirms their authenticities. The PA spectra of tetracene (also known as naphthacene or 2,3benzanthracene) obtained in this work display several previously unobserved far-infrared bands. Features at 166, 271, 390, 459, 469, 550, 607 and 626 cm1 are in good agreement with calculated [2,8] band positions, but only those above 500 cm1 have already been reported in infrared spectra [6]. The calculations in Ref. [6] predicted a similar series of bands, except for those at 271 and 390 cm1. Analogues of the latter two features have also been identified using neutron scattering [4] and fluorescence [5], respectively. The PA band at 572 cm1, which cannot be correlated with a predicted or previously reported infrared band, occurs near a calculated Raman frequency (Table 1). This result may be attributable to activation of a gerade mode in the PA infrared
Fig. 1. Plots of jJ1(2pne)j for e = 5l (solid line), 15l (dotted line) and 20l (dashed line). Details are given in the text.
Fig. 3. Far-infrared PA spectra of catacondensed aromatics: (a) tetracene (5 Hz, 12l); (b) pentacene (22 Hz, 15l). Possible bands below 150 cm1 for tetracene require confirmation.
3.2. Spectra of tetracene and pentacene
K.H. Michaelian et al. / Vibrational Spectroscopy 49 (2009) 28–31
30
Table 1 Infrared spectra of tetracene and pentacene, 100–700 cm1 Tetracene PA infrared (this work)
Pentacene a
Literature
Predicted bands This workc
166 271 390 459 469 550 572 607 626 655 h
a b c d e f g h
272e 387f
552g 608g 628g
166 278 388 d 485 492 574 578 d 631 662
b
Ref. [8] 160 266 374 d 464 472 556 561 d 611 640
PA infrared (this work)a 105 125 198 254 356 384 454 468 511 569 621 669
Literature infrared [7]
Predicted bands b This workc d
455 470 572 622
105 121 198 266 d 369 384 481 487 498 591 655 663 d
Ref. [8] 100d 117 189 258 d 357 370d 460 467 483 572 634 642 d
Uncertainty in band positions is about 2 cm1. Predicted to be infrared-active except where noted otherwise. Uncorrected DFT calculations using 6-311 G B3LYP as described elsewhere [2]. Predicted Raman-active band. Observed using neutron scattering [4]. Observed in fluorescence fine structure [5]. Infrared data [6]. Probable combination derived from fundamentals at 271 and 390 cm1.
spectrum. As mentioned below, similar results were obtained for the other three aromatic hydrocarbons studied in this work. The PA band at 655 cm1 is probably due to a combination. The intensities of combination bands are often enhanced with regard to those arising from fundamentals in PA infrared spectra, a phenomenon that can be put down to a small degree of saturation. A total of 12 bands were observed in the far-infrared PA spectrum of pentacene. Table 1 and Fig. 3b confirm that reliable data were obtained at very low wavenumbers for this compound, with the features at 105 and 125 cm1 corresponding well with predictions. Six bands were identified below 400 cm1 for pentacene in this work; three of these features (105, 254 and 384 cm1) can be correlated with predicted Raman bands. At higher wavenumbers, the PA bands at 454, 468, 569 and 621 cm1 agree with the earlier literature [7], while those at 511 and 669 cm1 have apparently not been observed previously. Although the two common crystal polymorphs of pentacene yield distinct Raman spectra between about 40 and 100 cm1, the PA infrared
spectra obtained here are apparently unaffected by the possible existence of both phases. 3.3. Spectra of perylene and pyrene The PA spectra of the pericondensed aromatics perylene and pyrene are plotted in Fig. 4. Experimental and computed band positions are listed in Table 2 together with previously published data [9–11] and calculated [2,8] band locations. Comparison with Table 1 shows that characterization of the low-wavenumber region was more extensive for perylene and pyrene than for tetracene and pentacene prior to the current research. Although the far-infrared PA spectra of the two pericondensed compounds provide only a limited amount of new information, the existing literature allows verification of the bands observed in this work. The spectrum of perylene (Fig. 4a) contains eight bands that can be identified with certainty. These features are readily correlated with calculated band positions. The 419- and 622-cm1 PA infrared bands apparently correspond to the Raman bands [10] at 421 and 624 cm1, respectively. Pyrene yielded the most complex spectrum (Fig. 4b) in the present investigation, with 19 bands being identified below 700 cm1. Table 2 shows that the band at 121 cm1 arises from a lattice mode and that four bands at higher wavenumbers (284, 321, 435 and 654 cm1) are attributable to combinations. These four bands, in addition to a fundamental at 677 cm1, have not been reported previously. Other possible combination bands in the PA spectrum of pyrene are also under investigation. The occurrence of combination bands in the far-infrared PA spectra is significant, since it can indicate the existence of low-wavenumber fundamentals that are not directly observable in the infrared and Raman spectra. Thus, the PA spectra complement results obtained using traditional infrared techniques. 4. Discussion and conclusions
Fig. 4. Far-infrared PA spectra of pericondensed aromatics (5 Hz, 12l): (a) perylene and (b) pyrene.
The far-infrared PA spectra of tetracene, pentacene, perylene and pyrene acquired in this investigation demonstrate an important new application of step-scan phase-modulation FT-IR spectroscopy. Low modulation frequencies and large amplitudes
K.H. Michaelian et al. / Vibrational Spectroscopy 49 (2009) 28–31
31
Table 2 Infrared spectra of perylene and pyrene, 100–700 cm1 Perylene PA infrared (this work)
Pyrene a
Literature
Predicted bands This workc
185 254 419 463 542 583 622 652
a b c d e f g h i j k
178
d
g
421 461d 543d 583d 624g 650d
183 258 433f 477 565 595 647f 671f
b
PA infrared (this work)a
Literature infrared [11]
This workc
Ref. [8]
156f 219 253f
151f 209 259f
320i 349 392 406
365 409f 419f
353 396f 406f
453 484 493 525 537 572 586
470f 508 515 525f 568 595f 595f
455f 491 500 527f 549 577f 579f
698f
678f
Ref. [8] 174 250 417f 462 544 579 630f 648f
e
121 156 219 259 284 h 321 i 353 394 410 435 j 455 486 495 529 541 578 589 654 k 677
Predicted bandsb
127 158 218 258
e
Uncertainty in band positions is about 2 cm1. Predicted to be infrared-active except where noted otherwise. Uncorrected DFT calculations using 6-311 G B3LYP as described elsewhere [2]. Infrared data [9]. Probable lattice mode. Predicted Raman-active band. Raman data [10]. Possible combination derived from fundamentals at 121 and 156 cm1. Possible combination derived from 100-cm1 band and fundamental at 219 cm1. Possible combination derived from 80-cm1 band and fundamental at 353 cm1. Possible combination derived from fundamentals at 156 and 495 cm1.
were found to yield optimal results between about 100 and 700 cm1. This work confirms the feasibility of PA spectroscopy at far-infrared wavelengths and shows that the method need not be confined to the more familiar mid-infrared region. In this research, PA infrared bands were identified at wavenumbers as low as 105 cm1 (pentacene), 125 cm1 (pyrene and pentacene) and 166 cm1 (tetracene). The number of bands observed for each of the four compounds ranged from 8 to 19. One probable combination band was identified for tetracene, while another three bands detected for pyrene arise from combinations involving modes at lower wavenumbers. A total of 40 infrared-active fundamental bands were observed in the PA spectra of these aromatic hydrocarbons and correlated with predicted [2,8] band positions. About half of the bands mentioned in this paper were previously unreported in the scientific literature. A significant number of PA infrared bands were correlated with predicted or observed Raman bands in this work. This suggests that the mutual exclusion rule governing the infrared and Raman activities of the four centrosymmetric molecules does not always apply for the condensed phase. More investigation is required to determine whether this activation in the PA infrared spectra arises from a reduction in symmetry or another cause. Further improvements in far-infrared PA spectroscopy may be obtainable through the use of synchrotron radiation (SR) instead of
conventional thermal infrared sources [12]. Potential bands at low wavenumbers, such as those observed for tetracene and perylene in the current work, could be adjudicated by means of SR PA spectra. Linearization and signal recovery techniques can also improve data quality in PA infrared spectroscopy [13]. These possibilities are currently being investigated in our laboratory and will be applied to the study of aromatic hydrocarbons and asphaltenes in future research.
References [1] K.H. Michaelian, Photoacoustic Infrared Spectroscopy, Chemical Analysis Series, vol. 161, Wiley–Interscience, Hoboken, NJ, 2003. [2] N. Sallamie, J.M. Shaw, Fluid Phase Equilibr. 237 (2005) 100. [3] H. Wang, E.B. Phifer, R.A. Palmer, Fresen. J. Anal. Chem. 362 (1998) 34. [4] A.M. Pivovar, J.E. Curtis, J.B. Leao, R.J. Cherterfeild, C.D. Frisbie, Chem. Phys. 325 (2006) 138. [5] J.W. Sidman, J. Chem. Phys. 23 (1956) 122. [6] H.M. Hudgins, S.A. Sandford, J. Phys. Chem. A 102 (1998) 329. [7] L. Colangeli, V. Mennella, G.A. Baratta, E. Bussoletti, G. Strazzulla, Astrophys. J. 396 (1992) 369. [8] G. Malloci, C. Joblin, G. Mulas, Chem. Phys. 332 (2007) 353. [9] S.J. Cyvin, B.N. Cyvin, P. Klaeboe, Spectrosc. Lett. 16 (1983) 239. [10] K.K. Ong, J.O. Jensen, H.F. Hameka, J. Mol. Struct. 459 (1999) 131. [11] A. Bree, R.A. Kydd, T.N. Misra, V.V.B. Vilkos, Spectrochim. Acta A 27 (1971) 2315. [12] K.H. Michaelian, R.S. Jackson, C.C. Homes, Rev. Sci. Instrum. 72 (2001) 4331. [13] K.H. Michaelian, Rev. Sci. Instrum. 78 (2007) 051301.
Contents lists available at ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Far-infrared photoacoustic spectra of tetracene, pentacene, perylene and pyrene K.H. Michaelian a,*, B.E. Billinghurst a, J.M. Shaw b, V. Lastovka b a b
Natural Resources Canada, CANMET Energy Technology Centre–Devon, 1 Oil Patch Drive, Devon, Alberta T9G 1A8, Canada Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 October 2007 Received in revised form 9 April 2008 Accepted 10 April 2008 Available online 20 April 2008
Far-infrared photoacoustic (PA) spectra of four different four- and five-ring aromatic hydrocarbons (tetracene, pentacene, perylene and pyrene) were acquired in this investigation. The use of largeamplitude phase-modulation and step-scan spectrometer operation yielded research-quality PA spectra in the 100–700 cm1 region. The bands in these spectra were correlated with published infrared and Raman spectra and with calculated band positions. The PA spectra contain many previously unobserved bands and complement the information already obtained from other types of vibrational spectra for the four compounds. Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved.
Keywords: Photoacoustic infrared spectroscopy Far-infrared Aromatic hydrocarbons DFT calculations
1. Introduction Photoacoustic (PA) infrared spectroscopy is a well-established technique that has been used to characterize a wide variety of both traditional and non-traditional samples during the last three decades [1]. This method, which obviates traditional infrared sample preparation procedures, commonly employs Fourier transform infrared (FT-IR) spectrometers for study of the midinfrared (400–4000 cm1, 2.5–25 mm) region. A number of research groups also utilize PA FT-IR spectroscopy in contiguous regions. In the latter context, the present article describes a farinfrared study of four aromatic hydrocarbons recently carried out in our laboratory. PA infrared spectra of tetracene, pentacene, perylene and pyrene were obtained from about 100 to 700 cm1 in this research. These well-known aromatic compounds are involved in several industrial applications. Interest also derives from their electronic properties, potential effects on health and environment, and possible astrophysical role. Furthermore, these four- and five-ring compounds can be regarded as simple models for asphaltenes, the highly aromatic components of bitumens and heavy oils that adversely affect the extraction and upgrading processes required for the production of middle-distillate fuels. The low-wavenumber vibrational spectra of asphaltenes (and suitable model com-
* Corresponding author. Tel.: +1 780 987 8646; fax: +1 780 987 8676. E-mail address: [email protected] (K.H. Michaelian).
pounds) provide data relevant to the calculation of constantvolume heat capacities [2]. 2. Experimental Tetracene (C18H12), pentacene (C22H14), perylene (C20H12) and pyrene (C16H10) were obtained from commercial sources at purity levels of at least 98 per cent and analyzed without any further purification. Tetracene and pyrene crystals were ground manually to facilitate filling the PA sample cups. Needle-like perylene crystals and finely divided pentacene were studied as received. PA far-infrared spectra of the hydrocarbons were acquired at a resolution of 6 cm1 using a Bruker IFS 66 v/S Fourier transform infrared (FT-IR) spectrometer and an MTEC 300 PA cell. The radiation source was either a standard globar or an Hg lamp. Ambient air remained in the spectrometer during most experiments. In other tests, the instrument was purged with dry N2 to reduce absorption by water vapour. Helium was used as the PA carrier gas. A multi-layer mylar beamsplitter was installed in the interferometer and a polyethylene window was utilized to seal the PA cell. These optical components restricted the accessible region to about 100–700 cm1. Spectra of carbon black were obtained under like conditions and used to correct the sample spectra for the variation in the system response with wavelength. Phase-modulation step-scan PA spectra were acquired as an alternative to the more commonly measured rapid-scan spectra in this work. Phase modulation results in multiplication of the energy (blackbody) curve by jJ1(2pne)j where J1 is the first-order Besselfunction, n is the infrared wavenumber (cm1) and e is the
0924-2031/$ – see front matter . Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2008.04.005
K.H. Michaelian et al. / Vibrational Spectroscopy 49 (2009) 28–31
29
modulation amplitude [3]. Low frequencies (5–22 Hz) and large amplitudes (5–20l, where l is 0.6328 mm, the HeNe laser wavelength) were employed. Demodulation was effected using standard FT-IR software or, alternatively, by means of a lock-in amplifier. This experimental strategy made it possible to acquire research-quality, far-infrared PA spectra down to about 100 cm1. 3. Results 3.1. Large-amplitude phase modulation The quantity jJ1(2pne)j is displayed for e = 5, 15 and 20l in Fig. 1. It can be observed that a modulation amplitude of 15l (dotted line) produces a Bessel-function minimum near 650 cm1, close to the high-wavenumber cutoff of the beamsplitter used in this study. The plotted curves show that large amplitudes are most appropriate for far-infrared step-scan PA spectroscopy. By contrast, smaller amplitudes should be utilized to study the midinfrared region. Fig. 2 depicts a series of far-infrared PA spectra of carbon black obtained using an unpurged spectrometer and a modulation frequency of 5 Hz. Because this reference material was optically opaque and thermally thin [1] under the conditions employed in this experiment, these PA spectra can be interpreted as step-scan energy curves. The structure along the wavenumber axis arises primarily from water vapour (50–300 cm1) and beamsplitter (300–500 cm1) absorption. A gradual shift of the Bessel-function minimum to lower wavenumbers with increasing e is evident on the right-hand side of Fig. 2, the 480-cm1 value observed at e = 20l agreeing with the dashed curve in Fig. 1. PA intensity reaches its maximal value at about 230 cm1 at this amplitude. As expected, the locations of the intensity maxima and Besselfunction minima progressively shift to higher wavenumbers as e decreases.
Fig. 2. Step-scan phase-modulation PA spectra of carbon black. The transmissionlike features along the wavenumber axis are primarily due to water vapour and beamsplitter absorption. The modulation–amplitude axis is non-linear.
Far-infrared PA spectra of the catacondensed hydrocarbons (all carbons on the periphery of the ring system) tetracene and pentacene are shown in Fig. 3. Table 1 compares the positions of the bands in these spectra with representative literature data [4–7] and the results of density functional theory (DFT) calculations in this work and Ref. [8]. The low-wavenumber PA infrared bands
observed for C18H12 and C22H14 in this study generally agree well with these other sources of information, a result that affirms their authenticities. The PA spectra of tetracene (also known as naphthacene or 2,3benzanthracene) obtained in this work display several previously unobserved far-infrared bands. Features at 166, 271, 390, 459, 469, 550, 607 and 626 cm1 are in good agreement with calculated [2,8] band positions, but only those above 500 cm1 have already been reported in infrared spectra [6]. The calculations in Ref. [6] predicted a similar series of bands, except for those at 271 and 390 cm1. Analogues of the latter two features have also been identified using neutron scattering [4] and fluorescence [5], respectively. The PA band at 572 cm1, which cannot be correlated with a predicted or previously reported infrared band, occurs near a calculated Raman frequency (Table 1). This result may be attributable to activation of a gerade mode in the PA infrared
Fig. 1. Plots of jJ1(2pne)j for e = 5l (solid line), 15l (dotted line) and 20l (dashed line). Details are given in the text.
Fig. 3. Far-infrared PA spectra of catacondensed aromatics: (a) tetracene (5 Hz, 12l); (b) pentacene (22 Hz, 15l). Possible bands below 150 cm1 for tetracene require confirmation.
3.2. Spectra of tetracene and pentacene
K.H. Michaelian et al. / Vibrational Spectroscopy 49 (2009) 28–31
30
Table 1 Infrared spectra of tetracene and pentacene, 100–700 cm1 Tetracene PA infrared (this work)
Pentacene a
Literature
Predicted bands This workc
166 271 390 459 469 550 572 607 626 655 h
a b c d e f g h
272e 387f
552g 608g 628g
166 278 388 d 485 492 574 578 d 631 662
b
Ref. [8] 160 266 374 d 464 472 556 561 d 611 640
PA infrared (this work)a 105 125 198 254 356 384 454 468 511 569 621 669
Literature infrared [7]
Predicted bands b This workc d
455 470 572 622
105 121 198 266 d 369 384 481 487 498 591 655 663 d
Ref. [8] 100d 117 189 258 d 357 370d 460 467 483 572 634 642 d
Uncertainty in band positions is about 2 cm1. Predicted to be infrared-active except where noted otherwise. Uncorrected DFT calculations using 6-311 G B3LYP as described elsewhere [2]. Predicted Raman-active band. Observed using neutron scattering [4]. Observed in fluorescence fine structure [5]. Infrared data [6]. Probable combination derived from fundamentals at 271 and 390 cm1.
spectrum. As mentioned below, similar results were obtained for the other three aromatic hydrocarbons studied in this work. The PA band at 655 cm1 is probably due to a combination. The intensities of combination bands are often enhanced with regard to those arising from fundamentals in PA infrared spectra, a phenomenon that can be put down to a small degree of saturation. A total of 12 bands were observed in the far-infrared PA spectrum of pentacene. Table 1 and Fig. 3b confirm that reliable data were obtained at very low wavenumbers for this compound, with the features at 105 and 125 cm1 corresponding well with predictions. Six bands were identified below 400 cm1 for pentacene in this work; three of these features (105, 254 and 384 cm1) can be correlated with predicted Raman bands. At higher wavenumbers, the PA bands at 454, 468, 569 and 621 cm1 agree with the earlier literature [7], while those at 511 and 669 cm1 have apparently not been observed previously. Although the two common crystal polymorphs of pentacene yield distinct Raman spectra between about 40 and 100 cm1, the PA infrared
spectra obtained here are apparently unaffected by the possible existence of both phases. 3.3. Spectra of perylene and pyrene The PA spectra of the pericondensed aromatics perylene and pyrene are plotted in Fig. 4. Experimental and computed band positions are listed in Table 2 together with previously published data [9–11] and calculated [2,8] band locations. Comparison with Table 1 shows that characterization of the low-wavenumber region was more extensive for perylene and pyrene than for tetracene and pentacene prior to the current research. Although the far-infrared PA spectra of the two pericondensed compounds provide only a limited amount of new information, the existing literature allows verification of the bands observed in this work. The spectrum of perylene (Fig. 4a) contains eight bands that can be identified with certainty. These features are readily correlated with calculated band positions. The 419- and 622-cm1 PA infrared bands apparently correspond to the Raman bands [10] at 421 and 624 cm1, respectively. Pyrene yielded the most complex spectrum (Fig. 4b) in the present investigation, with 19 bands being identified below 700 cm1. Table 2 shows that the band at 121 cm1 arises from a lattice mode and that four bands at higher wavenumbers (284, 321, 435 and 654 cm1) are attributable to combinations. These four bands, in addition to a fundamental at 677 cm1, have not been reported previously. Other possible combination bands in the PA spectrum of pyrene are also under investigation. The occurrence of combination bands in the far-infrared PA spectra is significant, since it can indicate the existence of low-wavenumber fundamentals that are not directly observable in the infrared and Raman spectra. Thus, the PA spectra complement results obtained using traditional infrared techniques. 4. Discussion and conclusions
Fig. 4. Far-infrared PA spectra of pericondensed aromatics (5 Hz, 12l): (a) perylene and (b) pyrene.
The far-infrared PA spectra of tetracene, pentacene, perylene and pyrene acquired in this investigation demonstrate an important new application of step-scan phase-modulation FT-IR spectroscopy. Low modulation frequencies and large amplitudes
K.H. Michaelian et al. / Vibrational Spectroscopy 49 (2009) 28–31
31
Table 2 Infrared spectra of perylene and pyrene, 100–700 cm1 Perylene PA infrared (this work)
Pyrene a
Literature
Predicted bands This workc
185 254 419 463 542 583 622 652
a b c d e f g h i j k
178
d
g
421 461d 543d 583d 624g 650d
183 258 433f 477 565 595 647f 671f
b
PA infrared (this work)a
Literature infrared [11]
This workc
Ref. [8]
156f 219 253f
151f 209 259f
320i 349 392 406
365 409f 419f
353 396f 406f
453 484 493 525 537 572 586
470f 508 515 525f 568 595f 595f
455f 491 500 527f 549 577f 579f
698f
678f
Ref. [8] 174 250 417f 462 544 579 630f 648f
e
121 156 219 259 284 h 321 i 353 394 410 435 j 455 486 495 529 541 578 589 654 k 677
Predicted bandsb
127 158 218 258
e
Uncertainty in band positions is about 2 cm1. Predicted to be infrared-active except where noted otherwise. Uncorrected DFT calculations using 6-311 G B3LYP as described elsewhere [2]. Infrared data [9]. Probable lattice mode. Predicted Raman-active band. Raman data [10]. Possible combination derived from fundamentals at 121 and 156 cm1. Possible combination derived from 100-cm1 band and fundamental at 219 cm1. Possible combination derived from 80-cm1 band and fundamental at 353 cm1. Possible combination derived from fundamentals at 156 and 495 cm1.
were found to yield optimal results between about 100 and 700 cm1. This work confirms the feasibility of PA spectroscopy at far-infrared wavelengths and shows that the method need not be confined to the more familiar mid-infrared region. In this research, PA infrared bands were identified at wavenumbers as low as 105 cm1 (pentacene), 125 cm1 (pyrene and pentacene) and 166 cm1 (tetracene). The number of bands observed for each of the four compounds ranged from 8 to 19. One probable combination band was identified for tetracene, while another three bands detected for pyrene arise from combinations involving modes at lower wavenumbers. A total of 40 infrared-active fundamental bands were observed in the PA spectra of these aromatic hydrocarbons and correlated with predicted [2,8] band positions. About half of the bands mentioned in this paper were previously unreported in the scientific literature. A significant number of PA infrared bands were correlated with predicted or observed Raman bands in this work. This suggests that the mutual exclusion rule governing the infrared and Raman activities of the four centrosymmetric molecules does not always apply for the condensed phase. More investigation is required to determine whether this activation in the PA infrared spectra arises from a reduction in symmetry or another cause. Further improvements in far-infrared PA spectroscopy may be obtainable through the use of synchrotron radiation (SR) instead of
conventional thermal infrared sources [12]. Potential bands at low wavenumbers, such as those observed for tetracene and perylene in the current work, could be adjudicated by means of SR PA spectra. Linearization and signal recovery techniques can also improve data quality in PA infrared spectroscopy [13]. These possibilities are currently being investigated in our laboratory and will be applied to the study of aromatic hydrocarbons and asphaltenes in future research.
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