Fluorescence spectrum of the charge-transfer complex

2718

J. Phys. Chem. 1982, 86, 2718-2727

Fluorescence Spectrum of the Charge-Transfer Complex Tetracyanoethylene-p-Xylene Cooled In a Supersonlc Free Jet Timothy D. Russellt and Donald H. Levy’

Downloaded by UNIV OF CAMBRIDGE on September 1, 2015 | http://pubs.acs.org Publication Date: July 1, 1982 | doi: 10.1021/j100211a033

The James Franck Insfltute and The Department of Chemistty, The University of Chicago, ChlcaQo, Illlnois 60637 (Received: December 2 1, 798 1; In Final Form: March 18, 1982)

The fluorescence excitation spectrum and the dispersed fluorescence spectrum of the charge-transfer complex tetracyanoethylene-p-xylene have been observed in the gas phase in a supersonic-free-jetexpansion. In spite of extensive cooling, both the excitation and the emission spectra are broad and diffuse, and the fluorescence spectrum is insensitive to the frequency of the exciting light. Because of supersonic cooling, the spectrum of the higher-energy configuration (Y isomer) is much less intense in the free jet than in the static gas. The diffuseness in the excitation spectrum is attributed to a shift in the excited-electronic-state potential surface. The shifted excited state well requires that the Franck-Condon-allowed absorption be to a distribution of vibrational levels high in the well rather than to a single zero-point level, and the overlap of features due to the many levels causes broadening in the excitation spectrum. The broadening in the emission spectrum and the insensitivity of that spectrum to excitation frequency are attributed to intramolecular vibrational relaxation. Features assigned to larger complexes of the type A-D-A have been observed in the excitation spectrum. These features are red shifted in absorption but blue shifted in emission, and this is attributed to a horizontal shift of the excited-state potential relative to the smaller D-A complex. Features having a longer fluorescence lifetime have been observed in the excitation spectrum and have been assigned to van der Waals complexes of the charge-transfer compound with the carrier gas. It is suggested that the longer lifetime is due to vibrational predissociation which competes with intramolecular vibrational relaxation and drains energy out of the intramolecular vibrational modes before emission.

Introduction Ever since Mulliken first formulated his theory on intermolecular charge-transfer complexes,lV2much effort has been expended characterizing the ground-state properties and the observed spectra of these c o m ~ l e x e s , but ~ - ~only in the past decade has progress been made on characterizing the excited, or charge-transfer, state and its dynamic and luminescent proper tie^.^^ Many unanswered questions still remain in understanding the absorption, luminescent, and excited-state dynamical properties of these complexes. One of the difficulties in studying these phenomena has been the problem of achieving isolated molecules. Static-vapor-phase work on these complexes has generally required high temperatures and/or large partial pressures of the component species to ensure sufficient complex concentration. Although exciplex emission from the charge-transfer state has been observed in the vapor phase for several systems,l0charge-trmfer fluorescence has never been observed from a vapor-phase complex after direct excitation into the charge-transfer state. We report in this work on the application of free-jet spectroscopy to the study of the charge-transfer complex tetracyanoethylene (TCNE)-p-xylene. TCNE-p-xylene is, in the Mulliken terminology,2 a b r a r complex, where the electron is transferred from a bonding ?r orbital of the donor, p-xylene, to an antibonding 7~ orbital of the acceptor, TCNE. It is generally accepted to have the “sandwich” structure1’J2 (shown in Figure 1) predicted by Mulliken’s maximum overlap principle: and which is typical of most of these R-T* c0mp1exes.l~ Experimental work of V0igt14and ProchorowB and theoretical work of Hanna15 predict the X configuration to be the more stable of the two isomers in the ground electronic state. ‘General Electric Co., Lamp Business Division No. 1310, Nela Park, Cleveland, OH 44112. 0022-365418212086-2718$01.25/0

The static-gas-phase absorption spectrum of the complex, shown in Figure 2 (redrawn from ref 16), is, as is common for most charge-transfer complexes, broad and diffuse, with no resolvable vibrational structure. KrolP has reported the gas-phase absorption band to consist of two overlapped bands with maxima at 23 250 and 26 880 cm-l, extending from 18 000 to 34 000 cm-’ with a bandwidth (fwhm) of about 8000 cm-l. Aside from a large red shift on going from the gas phase to a condensed phase, amounting to 1000-2000 cm-’ depending on the solvent used,16-18the shape and the width of the band are fairly independent of phase.8J8Jg From temperature studies on the relative intensities of (1) Mulliken, R. S. J. Am. Chem. SOC. 1950, 72,600. (2) Mulliken, R. S.;Person, W. B. “Molecular Complexes: A Lecture and Reprint Volume”; Wiley-Interscience: New York, 1969. (3) (a) Foeter, R., Ed. ‘Molecular Complexes”; Crane, Russak: New York, 1973; Vol. I. (b) Ibid. 1974, Vol. 11. (4) Yarwood, J., Ed. ‘Spectroscopy and Structure of Molecular Complexes”; Plenum Press: New York, 1973. (5) Foster, R. ‘Organic Charge Transfer Complexes”;Academic Press: London, 1964. (6) Nagakura, S.“Excited States”; Lim, E. C., Ed.; Academic Press: New York, 1975; Vol. 2, pp 321-83. (7) Prochorow, J.; Bernard, E. J. Lumin. 1974, 8,471. (8) Prochorow, J. J. Lumin. 1974, 9, 131. (9) Deperasinska, I.; Prochorow, J.; Sobolewski, A. Chem. Phys. 1978, 32,257. (10) (a) Hirayama, S.; Abbott, G. D.; Phillips, D. Chem. Phys. Lett. 1978,56,497. (b) Prochorow, J.; Okajima, S.; Lim, E. C. Zbid. 1979,66, 590. (c) OkaJima,S.;Lim, E. C. Ibid. 1980,70,283. (d) Hirayama, S.Ibid. 1979,63,596. (e) Itoh, M.; Kotani, T.; Hanashima, Y. Zbid. 1980, 75,307. (11) Holder, D.D.;Thompson, C. C. J. Chem. SOC.,Chem. Commun. 1972,277. 1964,86, 3611. (12) Voigt, E. M. J. Am. Chem. SOC. (13) Prout, C. K.; Kamenar, B. ‘Molecular Complexes”;Foster, R., Ed.; Crane, Ruasak New York, 1973; Vol. I, Chapter 4, pp 178-204. (14) Mobley, M. J.; Riechkhoff, K. E.; Voigt, E. M. J. Phys. Chem. 1977,81, 809. (15) Lippert, J. L.; Hanna, M. W.; Trotter, P. J. J . Am. Chem. SOC. 1969,91, 4035. (16) Kroll, M. J . Am. Chem. SOC.1968,90, 1097. (17) Kroll, M.; Ginter, M. L. J . Phys. Chem. 1965, 69, 3671. (18) Voigt, E. M. J. Phys. Chem. 1966, 70, 598. (19) Rossi, M.; Buser, U.; Haselbach, E. Helu. Chim. Acta 1976, 59, 1039.

0 1982 American Chemical Society

The Journal of Physical Chemisity, Vol. 86,No. 14, 1982 2719

Fluorescence Spectrum of TCNE-p -Xylene

Y

X

Y

X


Flgure 1. Structure of the geometrical isomers of TCNE-p-xylene.

u (cm-'/1000)

2. Fluorescence excitation spectrum of TCNE-p-xylene coded in a supersonic free jet. The cenler gas was pure helium at Po = 18 atm. TCNElp-xylene ratio was approximately 1:1. Superimposed on the free-jet spectrum is the static-gas absorption spectrum at 90 OC, taken from ref 18.

the two peaks, V0igt14 and Prochorow8 have determined that the low-energy band is due to the Y configuration, while the higher-energy band comes from the X configuration. Although the X isomer is of lower energy in the ground electronic state, the donor orbital of this isomer has a higher ionization potentialm than that of the Y isomer. In the X isomer the nodal plane of the donor orbital passes through the methylated carbon atoms of the pxylene ring and perpendicular to the plane of the ring, while in the Y configuration the donor orbital has a nodal plane ale0 perpendicular to the p-xylene ring but bisecting two opposite C-C bonds of the ring. The higher ionization potential of the donor orbital of the X isomer places its charge-transfer transition at higher energy in accordance with Mulliken's theory.2 TCNE-p-xylene is one of only about 100 charge-transfer complexes that are known to fluoresce in the condensed ~ h a s e > ~ l all - ~ 'of them being of the *-a* type. Prochorow8n has observed the fluorescence band as a function (20) Baker, A. D.; May, D. P.; Turner, D. W.; J. Chem. SOC.B 1968, 22. (21) (a) Czekalla, J.; Schmillen, A.; Majer, K. J. 2.Electrochem. 1957, 61,537; 1969,63,623. (b) Czekalla, J.; Briegleb, G.; Herre, W. Ibid. 1959, 63,712. (c) Czekalla, J.; Briegleb, G.; Herre, W.; Glier, R. Zbid. 1957,61, 537. (22) h n b e r g , H.M.;Eimutis, E. C . J. Phys. Chem. 1966, 70,3494. (23) Short, G. D.; Parker, C. A. Spectrochim. Acta, Part A 1967,23, 2487. (24) Prochorow, J.; Tramer, A. J. Chem. Phys. 1966,44,4545; 1967, 47, 775. (25) Mataga, N.; Murata, Y. J. Am. Chem. SOC.1969,91, 3144. (26) (a) Iwata, S.; Tanaka, J.; Nagakura, S. J. Am. Chem. SOC.1967, 89,2813. (b) K o b a y d , T.; Iwata, S.; Nagakwa, S.Bull. Chem. SOC.Jpn. 1970,43,713. (c) Kobayanhi, T.; Yoshihara, K.; Nagakura, S. Zbid. 1971, 44,2603. (27) (a) Prochorow, J. Bull. Acad. Pol. Sci., Ser. Sci. Math., Astron. Phys. 1967, 15, 37. (b) Zbid. 1974, 22, 1283.

of the viscosity (temperature-controlled) of the medium. In the viscous liquid phase at 135 K, he reports the fluorescence to consist of a single, broad, diffuse band, peaking at 15900 cm-' independent of excitation wavelength, with a bandwidth (fwhm) of about 3500 cm-'. In rigid, glassy solution at 80.3 K, the fluorescence becomes double banded, with maxima at about 16 800 and 18OOO cm-'. These spectra were found to be dependent on excitation wavelength. Furthermore, the excitation spectra were also found to be dependent on observation wavelength, for these rigid-phase spectra, with the higher-energy excitation band becoming relatively most intense as a shorter wavelength of observation is used. From this work, Prochorow proposed a model of hindered reorientation of the two isomers of the complex during the excited-state lifetime, leading to the observation of fluorescence from both isomers in rigid phase, while in the viscous liquid phase there is a fast reorientation of the two components leading to fluorescence from only the lowest-energy excited-state isomer. In the present work, we present the total fluorescence excitation spectrum and the dispersed fluorescence spectrum for the TCNE-p-xylene complex cooled in a supersonic-free-jet expansion. The cooling attained was such that virtually only the X isomer was observed. On the basis of our observations that, even in the cold, isolated environment of the free-jet expansion the excitation spectra consist of broad, diffuse bands, while the dispersed fluorescence spectra are independent of excitation frequency, we propose a model of rapid intramolecular vibrational relaxation occurring in the charge-transfer state of the complex. In addition, we show evidence for the existence of multiple complexes, formed in the early stages of the expansion, and present the excitation and dispersed spectra for them. Fluorescence lifetimes of the complexes were also measured. Experimental Section The experiments were performed in two stages. The first set of experiments were run by using a continuously tunable CW dye laser pumped by the UV output (-3 W) of an argon ion laser. With a three-plate birefringent filter as the tuning element, excitation scans were run covering the region from 21 600 to 23 250 cm-l using the laser dye stilbene 420 with a bandwidth (fwhm) of 1 cm-' and from 19OOO to 21 400 cm-' using coumarin 480 with a bandwidth of 0.5 cm-'. Fluorescence from the jet was collected with an f/1.2, 55-mm focal length camera lens, focused onto angular slits, for spatial selection, and subsequently detected by a cooled RCA C31000A photomultiplier tube operating in the photon-counting mode. For the dispersed fluorescence experiments, light from the jet was collected with an f/1.2, 72-mm focal length planoconvex lens, focused onto the entrance slits of a f/7 Spex 1-m monochromator having a reciprocal linear dispersion of 4A/mm in first order using a 2400 g/mm grating, and subsequently detected in the same manner as described above. During a scan the distance between the jet and the collection lens was changed to maintain the focus. This was necessary because chromatic aberrations in the lens made the focusing position frequency dependent. With this setup, dispersed fluorescence experiments were accomplished with excitation frequencies ranging from 20 000 to 23 250 cm-I and with monochromator slit widths of from 1 to 5 mm. More detailed descriptions of this apparatus have appeared p r e v i o u ~ l y . ~ ~ - ~ ~ (28) Smalley, R.E.; Levy,D.H.; Wharton, L. J.Chem. Phys. 1976,64, 3266.


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The Journal of Physical Chemistry, Vol. 86, No. 14, 1982

The second set of experiments were run by using a pulsed dye laser system as the excitation source. Using nine different dye laser setups, pumped by either the second or third harmonic of an NdYAG laser, we covered the entire range from 19OOO to 30 750 cm-'. The frequency range from 19 OOO to 24 400 cm-' was covered by using the dyes stilbene 420 and the coumarins 440,460,480, and 500. With input power of 55 mJ/pulse, peak output powers ranged from 2.5 to 4.5 mJ/pulse. The low conversion efficiency was a result of defocusing the input pump beam into the dye cuvette in order to minimize superradiance. The frequency range from 24 400 to 26 800 cm-' was covered by using the dyes BBQ and DPS, and peak output power was 1 mJ/pulse. The frequency region from 26 800 to 30 750 cm-' was covered by using the second harmonic of the oxazine dyes 725 and 720. Pumping with 190 mJ/pulse, the peak output power was 0.3-1.0 mJ/pulse. For all dye laser setups, the laser bandwidth (fwhm)ranged from 0.4 to 0.6 cm-', as was found by monitoring the dye laser output with a 1-cm-' free spectral range air-spaced etalon. Details of the gated integrator detection, the electronically controlled dye laser scanning system, and the minicomputer interfacing have been previously described.30 Time-resolution capability for the fluorescence lifetime experiments was provided by a programmable digital delay generator. A minicomputer was used to send an initial, preset delay time to the delay generator, which, after receiving a trigger pulse from the Nd:YAG laser, output a pulse which was delayed in time with respect to that trigger pulse. This output pulse was then sent to a pulse generator which provided gate pulses of preset width for the integrator to perform the signal integration. The delay time was held constant for a preset number of laser pulses to provide signal averaging. Then the minicomputer incremented the delay time sent to the delay generator by a preset amount, the minimum step size being 10 ns, and the above sequence recommenced. For this system there was a jitter in the delay time of 5 ns. Total fluorescence lifetime experiments used a delay time step size of 10 ns and a gate pulse width of 10 ns. Dispersed fluorescence lifetime experiments used the same step size but with a 30-11s gate pulse width. For most experiments, 200 laser pulses per delay time were used for signal averaging. The supersonic free jet used in both sets of experiments was formed by bubbling helium through a reservoir of p-xylene, passing this over a heated sample of TCNE, and expanding the resulting mixture through a small orifice into an evacuated chamber. Many variations of the expansion conditions were attempted. The first set of experiments using the CW laser apparatus were survey scans and feasibility studies, and more varied sets of conditions were tried. The stagnation chamber pressure, Po, was varied from 1 to 100 atm, the orifice diameter ranged from 25 to 100 pm, the temperature of the TCNE was varied from 110 to 160 "C,andd that of the p-xylene from 20 to 75 "C. The second set of experiments were run more systematically by using a 50-pm nickel pinhole, a Po of 18, 86, or 100 atm, a TCNE temperature of 120 or 140 OC, and a p-xylene temperature of 20 or 0 "C. For some experiments, a few percent Ar (3-10%) was added to the helium expansion. Quartz nozzles3' were sometimes used in place of the nickel pinholes, as there was some initial concern (29) Sharfin, W.; Johnson, K. E.; Wharton, L.; Levy, D. H. J . Chem. Phys. 1979, 71, 1292. (30) Fung, K. H. Ph.D. Thesis, The University of Chicago, Chicago, IL,1980. (31) Fitch, P. S. H.; Haynam, C. A.; Levy, D. H. J. Chem. Phys. 1980, 73,1064.

Russell and Levy

that the TCNE might react32 with the nickel pinhole and/or the nickel gasket used for sealing the stainless-steel mount for the nozzle. However, no differences in the recorded spectra could be observed, nor was there any noticeable sign of reaction of the nickel pinhole and gasket, both of which could be used many times. The oven33and the temperature controller34used for heating the TCNE were found to be capable of holding the sample to within f l "C. The p-xylene used in the experiment (99+%, MCB Manufacturing Chemists; or 99%, Aldrich) was used without further purification. The TCNE (98%,Aldrich) was purified in the manner of Moser and Varchmin.= The sample was sublimed twice over active carbon at a pressure of less than 0.1 torr and a temperature of 70-75 OC. It was then sublimed twice more under the same conditions but without the carbon to remove any carbon particles that were obtained from the first two sublimations. When not in use, the TCNE was stored in a desiccator over phosphorus pentoxide and kept in a nitrogen drybox. When being used for an experiment, the TCNE was kept in a vacuum desiccator over P4010.

Results Fluorescence Excitation Experiments. In Figure 2 we show the fluorescence excitation spectrum of TCNE-pxylene covering the region from 31 OOO to 19 OOO cm-l. This spectrum and all of the excitation spectra to be shown are composites of about 15 individual scans. Each dye laser setup used to cover a particular region of the spectrum was chosen so as to provide adequate overlap with adjoining regions. As much care as possible was taken in splicing the charts together to ensure that nothing artificial was introduced. Each scan was normalized to the laser intensity with corrections being made to account for sensitivity changes in the power detection system used. Superimposed on the free-jet excitation spectrum in Figure 2 we have drawn in the static-gas-phase absorption spectrum of the complex taken from the work of Kroll.16 The most striking difference between them is the relative intensity of the two bands. The higher-energy (in the ground electronic state) Y isomer, with its absorption peak at 23 530 cm-' in the static gas phase, has essentially been cooled out in the free-jet expansion, where it appears in the spectrum as a relatively low-intensity shoulder in the region from 22 200 to 23 000 cm-'. The X isomer, with a static-gas-phase absorption peak at 26 880 cm-l, appears in the free-jet spectrum as a 3725-cm-' broad (fwhm)band with its maximum at 27950 cm-'. The shoulder which appears in the region from 25 000 to 26 000 cm-' is of uncertain origin. However, more will be said about this in the following section. When higher backing pressures are used for the expansion, several new features can be observed. In Figure 3 is the excitation spectrum of the complex obtained by using a stagnation chamber pressure, Po, equal t o 103 atm. The spectrum in Figure 2 was taken a t Po equal to 18 atm. Under the higher expansion conditions, the main band narrowed to 2875 cm-l and peaked at 28000 cm-'. Two new peaks grew in at 21 220 and 22 470 cm-l. Pressure studies of these two new features showed that both grew (32) Webster, 0.W.; Mahler, W.; Benson, R. E. J. Am. Chem. SOC. 1962,84, 3678. (33) Tellinghuisen, J.; Ragone, A.; Kim, M. S.; Auerbach, D. J.; Smalley, R. E.; Wharton, L.; Levy,D. H. J. Chem. Phys. 1979,71,1283. (34) DeKoven, B. M.Ph.D. Thesis, The University of Chicago, Chicago, IL, 1980. (35) Miller, F.A.;Sala, 0.; Devlin, P.; Overend, J.; Lippert, E.; Luder, W.; Moser, H.; Varchmin, J. Spectrochim. Acta 1964,20, 1233.


The Journal of Physical Chemistty, Vol. 86, No. 14, 7982 2721

TABLE I: Charge-Transfer Fluorescence Excitation Band Positions He

18

22

I I1 I11 I I1 I11 I I1 I11 I I1 I11 I I1 I11 I I1 I11

120

He

103

22

120

He

86

22

120

He

86

0

140

6%Ar-He

86

22

120

6% Ar-He

86

0

140

22 680 (shy 27 950 21 220 22 470 28 000 21 190 22 480 21 030 22 360 (sh y 22 575 21 140 22 680 (sh)" 26 975 21 140 22 495 (shy 27 075


" Midpoint ofshoulder.

A

1

33

'

I

31

'

"

29

l

'

I

~

25 u (cm-' I 1000

27

I

23

t

l

'

21

l

1'9

;I

Flgue 3. Fluorescence excitatlon spectrum of TCNE-p-xylene at Po = 103 atm and TCNEIp-xylene ratio = 1:l.

in gradually as Po was increased. Pressure studies of the 21 220-cm-' peak using an orifice diameter half the size of the original one (50 vs. 25 pm) indicated that the growth of the peak followed some high-order, i.e., greater than fiist-order, pressure dependence which is characteristic of complex formation in the expansion.%@ Additional experiments were run to check for complex formation and to try to determine the nature of the complex. The spectra in Figures 2 and 3 were run with the p-xylene reservoir kept at room temperature (=22 "C) and with the TCNE heated to 120 "C so that the vapor pressures of both components were approximately the same (-7 torr37938). The spectrum in Figure 4 (top) was run a t a Po of 86 atm, but with the p-xylene cooled to 0 "C (vapor pressure = 1.74 torr) and the TCNE heated to 140 "C (vapor pressure = 24 torr). These conditions, with a 13.8:l TCNE/p-xylene ratio, are more favorable for multiple complex formation, Le., TCNE,-p-xylene. The spectrum in Figure 4 (middle) was run with Po equal to 86 atm, with the vapor pressures of TCNE and p-xylene equal at -7 torr, and with about 6% argon mixed in with the helium carrier gas. These conditions are favorable for (36) Kenny,J. E.; Johneon, K. E.; Sharfin, W.; Levy, D. H. J . Chem. Phys. 1980, 72,1109. (37) Boyd, R. E. J. Chem. Phys. 1963,38, 2529. (38)Jordan, T. E. 'Vapor Pressures of Organic Compounds":Interscience: New York, 1954;

I

33

I

31

l

29

1

I

27 25 23 v icm-'/t000)

I

21

1

19

Flgure 4. Fluorescence excitation spectrum of TCNE-p-xylene: (top) heUum canler gas and TCNE/p-xylene ratlo 14:1,(middle) 94%-6% helium-argon carrier gas and TCNE/p-xylene ratio ' 1: 1, and (bottom) 94%-6% helum-argon carrier gas and TCNElp-xylene ratio = 141. Total pressure was 86 atm for all three spectra.

van der Waals complex formation as argon is known to more readilp and more strong19 complex than is helium. In Figure 4 (bottom), the spectrum was taken with a 6% argon in helium mix with Poequal to 86 atm, as was the previous spectrum, but with the p-xylene cooled to 0 "C and the TCNE heated to 140 "C. Under these conditions van der Waals and/or multiple charge-transfer complex formation should be favored. In general, the peak in the 21 OOO-cm-' region is favored relative to the band in the 22 500-cm-' region under conditions of large TCNE p-xylene ratios and/or the presence of a few percent argon, while the main band shows a red shift under these conditions. In Table I we have listed the peak positions under the various expansion conditions. The shoulder which occurs in the 25 OOO-26o00-cm-' region is not listed as its position seemed to be rather insensitive to experimental conditions. Dispersed Fluorescence Experiments. The dispersed fluorescence spectrum obtained with excitation at 22 375 (39) Johnson, K.E.; Sharfii, W.; Levy, D. H. J . Chem. Phys. 1981,74, 163. (40) Blazy, J. A.; DeKoven, B. M.; Russell, T. D.; Levy, D. H. J. Chem. Phys. 1980, 72, 2439.

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The Journal of phvsical Chemlst!y, Vol. 86 No. 14, 1982

.!!

Excitation,

jet

Fluorescence, let--

u i c m - ' / 1000)


Russell and Levy

Flgure 5. Dispersed fluorescence spectrum of TCNE-p-xylene. H a lium carrier gas at Po = 18 atm. Excitation frequency = 22375 cm-'. Superimposed on this spectnrm is the excitation spectrum taken under the same expansion conditions as Figure 2, and the dispersed flwrescence spectrum of the complex (v, = 15900 cm-') in the cdd, viscous liquid phase (ref 27b).

cm-' is shown in Figure 5. This spectrum was taken in a low-pressure (18 atm) helium expansion and is typical of spectra taken under these conditions at various excitation frequencies. Also shown in Figure 5 is the fluorescence spectrum of the complex in a cold, viscous liquid and the supersonic-jet fluorescence excitation spectrum. All spectra have been corrected for photomultiplier sensitivity and monochromator efficiency, both of which decline toward longer wavelengths. The signal-to-noiseratio was poor in some of the spectra, which produced sharp, structurelike peaks that were not repeatable and not real. Nonetheless, by comparing spectra we have determined that the fluorescence is independent of excitation frequency. From the large number of scans run covering the entire absorption band, the results yield a band that peaks at 17 240 f 170 cm-' with a fwhm equal to 3410 f 330 cm-'. It was not possible to take dispersed spectra under all of the conditions used for taking the excitation data. The temperature of the p-xylene could not be lowered as the signal-to-noise ratio was too low to obtain meaningful results. The above set of data was taken under a range of conditions, with Po ranging from 2 to 24 atm, the TCNE heated to 120-140 OC, and the p-xylene held at room temperature. All of the dispersed data taken by using the pulsed laser apparatus were with the TCNE heated to 140

"C. Using a higher stagnation chamber pressure and/or adding a few percent Ar to the carrier gas brought about changes in the dispersed spectra. From the excitation data there were four main areas of interest, the regions around 21 000, 22 500, 25 000-26 000, and 27 000-28 000 cm-I. Within our limits of sensitivity and laser power available, we took dispersed spectra from all of those regions (or near them) using expansion conditions as similar as possible to those used for the excitation data. All of the high-pressure dispersed data were taken with Po = 86 atm, TCNE heated to 140 "C,and p-xylene kept at room temperature. Fluorescence spectra obtained in either pure helium or in a 4% Ar-He mixed carrier gas with excitation at 21 200 cm-' and with Po = 86 atm were blue shifted with respect to the low-pressure spectra and had a steeper slope on the blue edge of the band. The spectrum taken with the ArHe mix appeared to have a shoulder at about 19OOO cm-' while the spectrum of the complex in a pure-helium ex-

22

20

I8

16

u (cm-l/

IOOOI

14

Figure 6. Dispersed fluorescence spectrum of TCNE-p -xylene. He at 86 atm, vE = 22375 cm-'.

pansion peaked at about 18 600 cm-I and then leveled off. Although difficult to tell from these spectra, data from other dispersed scans and lifetime measurements discussed below indicated that these fluorescence spectra resulted from two broad, overlapped bands due to emission from at least two different species. In Figure 6 is the fluorescence spectrum resulting from excitation at 22 375 cm-' in a pure-He expansion at Po = 86 atm. I t was in this region in the excitation spectrum that use of a pure-helium expansion at high pressure showed a peak that was not present (or only a shoulder) in those expansions using either a low backing pressure or an argon-helium mixed carrier gas. The spectrum in Figure 6 shows some noticeable differences from that in Figure 5 with a rather steep rise from 21000 cm-I and peaking at about 18200 cm-'. The spectrum taken with 22 375-cm-' excitation using a 4% Ar-He mixed expansion is not much different from spectra taken at low backing pressures but does exhibit a steeper slope at the blue edge of the band and possibly a small shoulder at about 19 OOO cm-', although with the low signal-to-noise ratio it is difficult to be certain that the shoulder is real. Spectra taken using a high-pressure pure-He expansion with excitation frequencies of 25 270 and 26 970 cm-' are quite similar to one another and to the low-pressure spectra. Figure 7 is the spectrum taken by using a 7.5% Ar-He carrier gas with an excitation frequency of 24810 cm-'. It shows the steep slope at the band's blue edge with a shoulder at about 19OOO cm-'. A spectrum taken by using 26 970-cm-' excitation was identical. Dispersed spectra taken by using a 4 % Ar-He expansion and exciting at 26 970 cm-' did not exhibit a shoulder around 19 OOO cm-I and were quite similar to the low-pressure dispersed data. Since our fluorescence lifetime experiments, which will be discussed more fully below, indicated the presence of a long-lived species when high backing pressures and/or a mixed Ar-He carrier gas was used, some dispersed spectra were taken with the integrator gate pulse delayed about 40 ns from the rise of the laser pulse; i.e., we only observe those species still fluorescing 40 ns after excitation. Those results are shown in Figure 8, which was taken by using the same expansion conditions as those used for the previous spectra, but with an excitation frequency of 25 270 cm-'. Several such spectra were taken, all giving a result the same as that in Figure 8, i.e., a band that rises rapidly

The Journal of Physlcal Chemistry, Vol. 86, No. 14, 7982 2723


TABLE 11: Qualitative Characterization of Dispersed Fluorescence Spectra vs. Excitation Frequency and Expansion Condition8


P o , atm

gas

Vexc,

cm-'

18

He

21 200 + 29 780

86

He

21 200

86

He

22 375

86

He

323 250

86

4% Ar-He

21200-23 250

86

4% Ar-He

86

7.5% Ar-He

2 24 240

24 240 + 27 000

Figureb

commentsC single broad diffuse band umax = 17240 i 170 cm-' sharp blue rise of band a t E+ 20000, cm-l; umax shifted t o 18600 cm-l sharp blue rise of band a t = 21000, cm-' ; vmax = 18200 cm-l single broad diffuse band vmax = 17240 cm-I sharp blue rise of band at 20000 cm-I ;shoulder a t = 19000 cm'' single broad diffuse band umax = 17240 cm-' sharp blue rise of band a t i ~20000 : cm-' ;shoulder at 19000 cm-'

5

6 (51

(5) 7,8

p-Xylene a t room temperature (22"C). TCNE at 140 "C. This column gives figure number where representative spectra are shown. Numbers in parentheses mean that spectra taken with the stated expansion conditions are n o t shown, but the given figure numbers show similar spectra. Those spectra that are characterized as having "sharp blue rise , . ." means that the slope of the band on the high-frequency side was qualitatively steeper than those spectra having a single broad band with a gradual rise on the high-frequency side. The wavenumber o n which the band started t o rise steeply is also given,

I

24

22

20 18 16 vicm-l/ 10001

a 24 24

14

Figure 7. Dispersed fluorescence spectrum of TCNE-p-xylene: 7.5% Ar in He at Po = 86 atm, vE = 24810 cm-'.

at 20000 cm-'with a rather broad flat peak that falls more quickly to the red than the spectra taken at low backing pressures. The band maximum occurs at approximately 18250 cm-', although with the signal-to-noise ratio and the broad maximum it is difficult to determine an exact value. Table I1 contains a qualitative characterization of the dispersed fluorescence spectra that have been presented. Fluorescence Lifetime Experiments. Using low-pressure expansions, total fluorescence lifetime experiments were run throughout the region from 21 150 and 29 850 cm-' and yielded a lifetime for the complex of less than 10 ns, the limit of our detection system. With higher pressure and/or He-Ar mixed expansions, the lifetime data indicated the presence of a (or some) longer-lived species. Only with the He-Ar mixed expansions in the region from 21 150 to 27 000 cm-' was there a sufficient amount of the longer-lived species present for us to determine an accurate lifetime. The data yielded an excitation wavelength independent value of 41.6 f 2.6 ns. The data are shown in Figure 9. With the high-pressure helium expansions there was an insufficient amount of the longer-lived species present to do an accurate lifetime determination. We could only establish a lower limit of 31 ns covering the region from 21 150 to 23 250 cm-'. At higher excitation frequencies, the data indicated that there was no longer-lived species present. Increasing the TCNElp-xylene ratio by raising

k

20 18 16 20 18 16 v (cm~/1000i

22 22

14 14

Flgure 8. Dispersed fluorescence spectrum of TCNE-p-xylene: 7.5% Ar in He at P o = 86 atm, vE = 25270 cm-'. Spectrum taken 40 ns after rise of laser pulse (see text).

I

!

0

,

IO

,

I

,

20 30 40 50

I

60 70 80 90 100

Time INSECI

Flgure S. Fluorescence decay curves of (A) short-lived and (e) longlived species. Nonexponential behavior at early times is due to the flnlte rise and fall times (3-4 ns) of the photomultiplier tube.

and lowering their respective reservoir temperatures generally increased the amount of the longer-lived species that was present, but under none of the experimental conditions tried for the helium expansions could any long-lived species be detected at excitation frequencies higher than 27 000 cm-l. For the He-Ar mixed expansions, increasing the Ar concentration generally led to increased amounts of the longer-lived species and some could be detected at excitation frequencies up to 29 850 cm-*.


2724

The Journal of phvsical Chemistry, Vol. 86, No. 14, 1982

Russell and Levy

kinetics of the geometrical isomerization process on the Discussion cooling of the complex in a free-jet expansion vs. cooling The major results that have been presented so far in this in a condensed phase,14the excitation spectra show that work are as follows: (A) the relative intensity of the exthese bands are broad and diffuse in either phase. citation bands of the two isomeric forms of the complex The dispersed fluorescence spectrum of the complex in in the jet vs. that of the respective absorption bands in a condensed phase (drawn in on Figure 5, taken from ref hot, static gas (see Figure 2); (B)the diffuseness of the 27b) is likewise quite similar to what we have observed. excitation bands in the cold, isolated environment of the Prochorows has found that the fluorescence is independent expansion; (C) the diffuseness and the insensitivity to of excitation wavelength in a cold, viscous condensed phase excitation wavelength of the dispersed fluorescence spectrq but shows a doubled-humped band in the rigid, solid (D) the excitation and fluorescence spectra of the long-lived phase, due to fluorescence from both isomers. species that are present in the expansion under the conIf high vibrational states of a molecule in a condensed ditions of high Po and/or large TCNE/p-xylene ratios phase are excited by light absorption, interactions of the and/or a few percent Ar mixed in with the carrier gas. molecule with its environment, acting as a heat bath, The observed relative intensity of the two bands is a generally leads to a rapid vibrational relaxation and only result of the cooling that occurs during the expansion. fluorescence from the low-lying thermalized levels of the Although excitation spectra are a product of the absorption excited electronic state are normally observed. As observed cross section and the quantum yield of the transition, by Prochorow, when the higher-energy isomer (in the exexperimental work has shown that the absorption cross sections for the two isomers are approximately e q ~ a l ’ ~ ? ~cited ~ electronic state) is able to reorient to the lower-energy form during ita excited-state lifetime, fluorescence is only and that the excitation spectrum is similar to the abobserved from the low-lying levels of the lower-energy sorption spectrums in the condensed phase. isomer. If this reorientation is hindered because of the If we ignore any small differences in the absorption cross rigid environment, fluorescence from both isomers is obsections and quantum yields of the isomers, we can deserved. termine an approximate temperature for the complex in In the cold, isolated environment of the free-jet expanthe jet using a theoretical value of the ground-state-energy sion there is no heat bath for the complex to interact with difference between the two isomers of 279.8 cm-‘, from the and relax ita excess vibrational energy into, except for the work of Lippert, Hanna, and Trotter.15 From the spectrum molecule itself. We have interpreted our observations to in Figure 2 using the relative intensities of the bands at indicate that intramolecular vibrational relaxation is oc27 950 and 22 680 cm-l yields a ratio of 22.5:l and a temcurring in the charge-transfer state of the complex. The perature of 129 K. term implies that a single state of the molecule is initially However, since the various degrees of freedom of the prepared and that this state subsequently relaxes into a complex are not at equilibrium in the jet, this temperature large density of isoenergetic state^.^^**^ In this case and does not actually give us the relative population numbers in other case^^.^' where intramolecular vibrational relaxfor each of the various vibrational modes of the complex. ation has been invoked to explain the data, no time-deWith a potential barrier to relative rotation in the ground pendent phenomena were observed. Our understanding state calculated by Lippert et al.15 of 1090 cm-’, the geoof the process under these circumstances has been demetrical isomerization process may cease early in the exscribed in more detail elsewhere.46a pansion while cooling of the vibrational and rotational Because of a large relative shift of the minima of the modes may continue to later stages of the expansion. It ground-etate and excited-state potential wells, as is genhas been generally observed that the lower the energy of erally cited for these weak b-an complexes,2”v4 excitation the mode, the better is the cooling attained. of the complex is from near the zero-point level of the Any attempt to explain the spectroscopy of the complex ground electronic state to a point high up on the interas has been presented in this paper must take into account molecular potential well of the charge-transfer state, where (1) the known properties of free-jet expansions, (2) previous the density of the levels, due to the low-frequency torsional, work done on various molecules seeded into these expanrocking, and bending modes of the complex, may be very sions, and, (3) previous work done on this complex in both high. Because of a favorable Franck-Condon factor, it is gas and condensed phases. this nth vibrational level, high up in the intermolecular Fitch et al.31have shown that a molecule as large as well, that carries all of the absorption strength. The high phthalocyanine is observed to have well-resolved vibradensity of the low-frequency modes and the coupling betional structure when cooled in an expansion. Although tween these modes and the intermolecular stretching other has shown that there are a few residual colmotion lead to a broadening of the transition and the lisions that occur between the seeded molecule and the diffuseness of the excitation spectra. This relaxation carrier gas atoms a t the point of intersection of the laser process for both the condensed phase and the isolated with the jet, much larger collisional cross sections than gas-phase molecule is shown schematically in Figure 10. those found in that work would be required for any colThe high density of states is provided by the low-frelisions to occur between the carrier gas atoms and a quency intermolecular modes, i.e., the bending, rocking, molecule with as short a fluorescence lifetime as this complex has been found to have. Therefore, the observed spectroscopy of the complex must be due to the intrinsic (433)Mukamel, S.; Smalley, R. E. J. Chem. Phys. 1980, 73, 4156. properties of the cold and isolated molecule. (44)Freed, K. F.; Nitzan, A. J. Chem. Phys. 1980, 73, 4765. What we have observed for this complex is in fact quite (45) Prochorow, J.; Tramer, A. Acta Phys. Pol. 1968, 33, 267. (46) (a) Fitch, P. S. H.; Haynam, C. A.; Levy, D. H. J. Chem. Phys. similar to what has been observed for this complex in the 1981, 74,6612. (b) Langridge-Smith, P. R. R.; Brumbaugh, D. V.; Haycold condensed phase.s Aside from differences in the nam, C. A.; Levy, D. H. J. Phys. Chem., in press. relative intensity of the two bands, presumably due to the (47) (a) Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J. Chem. Phys. (41) Holder, D.D.;Thompson, C. C. J. Chem. Soc., Chem. Commun. 1972, 277. (42) Russell, T. D.;DeKoven, B. M.; Blazy, J. A.; Levy, D. H. J.Chem. Phys. 1980, 72, 3001.

1980,72,5039. (b) Hopkins, J. B.; Powers, D. E.; Mukamel, S.; Smalley, R. E. Ibid. 1980, 72,5049. (c) Powers, D.E.; Hopkins, J. B.; Smalley, R. E. Ibid. 1980, 73,5721. (d) Hopkins, J. B.; Powers, D. E.; Smalley, R. E. Ibid. 1980, 73, 683. (e) Beck, S. M.; Hopkins, J. B.; Powers, D. E.; Smalley, R. E. Ibid. 1981, 74, 43. (f) Hopkins, J. B.; Powers, D. E.; Smalley, R. E. Ibid. 1981, 74, 745.


R; k;

FYLENE+

The Journal of Physical Chemistry, Vol. 86, No. 14, 1982 2725

R--

-B I

L

TCNE +P-XYLENE

,


R;

, ,

R; R;

.

R;

R--

Figure 10. Vibrational relaxation for (A) condensed-phase molecule and (6) isolated gas-phase molecule. After excitation, the condensebphase molecule vibrationally relaxes to its zero-point level through lntemtdecular interaction wlth the environment. The isolated molecule relaxes via a rapid intermolecular redistribution of its excess vibrational energy into a large density of isoenergetic vibrational levels.

and torsional modes, as well as other low-frequency modes of the individual molecules. If the symmetry of the complex is the same in both states, as is thought to be the case for complexes of this type,24*45 then only Au = 0 transitions will have favorable Franck-Condon factors for these lowfrequency modes? The fluorescence originates from mixed levels having low numbers of quanta in the intermolecular stretch (largely, but not exclusively, levels having zero quanta in this mode) and higher numbers of quanta in each of the low-frequency modes. Therefore, the fluorescence would be independent of excitation wavelength, as is observed, and similar to the fluorescence observed from the condensed-phase complex. The fact that the fluorescence is independent of which isomer is excited indicates that reorientation of the complex to ita lowest-energy form in the excited state must be a rapid process for the isolated molecule. It would seem probable that, since there is a barrier to relative rotation in the ground state, there would be some sort of a similar barrier to relative rotation in the excited state. Since fluorescence that could be attributed to another isomeric form was not observed, then either excitation of the complex to ita charge-transfer state is always to levels high up in the intermolecular well where there is sufficient excess energy above the zero-point level to surmount the barrier or else some low-energy pathway is provided by the excited-state potential surfaces for the relative rotation. Either case tends to support our assertion that excitation of the complex to ita chargetransfer state is to levels where there is a sufficient density of states to provide an effective heat bath which allows excess energy to spread throughout the complex. It seems likely that the band which appears in the 21 100-cm-' region is due to a multiple charge-transfer complex, e.g., TCNE2-p-xylene. Such intermolecular complexes have been observed before4*@in the condensed phase, generally under the conditions of a large excess of the donor molecule. Under those conditions the trimer was believed to be of the D-A-D type (D = donor, A = ac(48)Ho, Y. E.; Thompson, C. C. J. Chem. SOC.,Chem. Commun. 1973, 609. (49) Dodson, B.; Foster, R.; Bright, A. A. S.;Foreman, M. I.; Gorton, J. J . Chem. SOC. B 1971, 1283.

Flguro 11. Schematic potential energy surfaces of a molecular complex along the Intermolecular coordinate: (A) schematic for a 1:l complex, (6)schematic for 2: 1 complex, depicting relative horizontal shift of surface on trimer formation.

ceptor), rather than of the A-D-A type as is believed to exist here under the conditions of a large excess of the acceptor molecule. Although the position and the shape of the charge-transfer band for the condensed-phase 1:l and 1:2 complexes were apparently quite similar, a large red shift is observed for the 2:l complex charge-transfer transition in the isolated gas-phase environment of the free-jet expansion. From Table I this red shift amounts to about 1500 cm-'. Shifting of the excitation maximum on van der Waals complexations0or dimer formations1in a free-jet expansion has been observed before and has generally been taken as a measure of the difference between ground-state and excited-state well depths, i.e., some vertical displacement of the potential surfaces due to differences in the bond strength of the complexed species in the two states. Although the red shift in the excitation spectrum band maximum of the 2:l complex, as observed in this work, could be due to a lowering of the excited-state potential surface, this could not explain the blue shift that we observe of the fluorescence band of the 2:l complex vs. that of the 1:l complex. It would seem that there must be a horizontal displacement of the surfaces in order to explain the excitation spectrum red shift and the dispersed fluorescence spectrum blue shift. This is shown schematically in Figure 11. The horizontal shift would bring the potential minima of the two surfaces closer so that excitation from the zero-point level of the ground state would be to points high up in the excited-state well but, from the Franck-Condon principle, not as high up on the surface as excitation of the 1:l complex, thus producing a red-shifted excitation spectrum. After a rapid vibrational relaxation, fluorescence from low-lying levels of the charge-transfer state would be to points lower on the ground-state surface, due to the horizontal shift, than fluorescence from the 1:l complex. The only case that we are aware of where a band maxi"is cited for a termolecular complex of the b?r-ar type is from the work of Dodson et al.49 where they give a calculated A, (the bands are completely overlapped) for the 1:2 (D-A-D) complex of tetrafluoro-p-benzoquinone(50) Levy, D. H. In 'Advances in Chemical Physics"; Jortner, J., Levine, R. D., Rice, S. A., E&.; Wiley-Interscience: New York, 1981; Vol. 47 and rererences therein. (51) Brumbaugh, D. V.; Haynam, C. A.; Levy, D. H. J. Chem. Phys. 1980, 73, 5380.

The Journal of Physical Chemistry, Voi. 86, No. 14, 1982

2726

TABLE 111: TCNE-p-Xylene Charge-Transfer Band Positions in Various SolventsQ solvent C,F,, C,HM

cc1,

CHCI, CH,CI, CH,Cl, at 7 7 K MCH-3MPb at 135 K gas phase at 90 “C free jet at Po = 1 8 atm

ua,

cm-I

22900 22000 21300 21500 21700

’b

9

cm-’

25800 25100 24500 23500 24100 15300 15900

22 000 23 530 22 680 (shY

Vf!

cm-I

26 880 27 950

17 240

ref

17 17 17 11 53 54 27b 16 this work (Table I )


All solvents at room temperature unless otherwise noted. A mixture of methylcyclohexane and 3methylpentane. s h : midpoint of shoulder.

hexamethylbenzene that is blue shifted from the 1:l complex by 434 cm-I in CC14solution. Ross and la be^^^ have reported t vs. X values for the trinitrobenzene-dimethylaniline complex in CHCIBsolution. The ,e value shifts to shorter wavelengths under conditions favoring D-A-D complexes but, judging from the reported values, only seems to symmetrically broaden the band under conditions favoring A-D-A complex formation. No difference in the position of emax was reported for the TCNE-p-xylene charge-transfer band in CC14and heptane solutions as the donor concentration was increased, although the data were shown to indicate the presence of a 1:2 complex.48 We could f i e no indication from our excitation data of a band that could be attributed to a 1:2 complex. The exact position of the peak that we attribute to a 2:l complex varied, in pure-helium expansions, with Poand with the TCNE/p-xylene ratio, but this could be partly due to overlap with the 22 500-cm-l band and partly due to helium van der Waals complexation with the 2:l complex. The position of the band maximum did not change with the TCNE/p-xylene ratio in those expansions where a few percent Ar was present, but the band was broadened, particularly in the red tail, presumably because of Ar complexation with the 2:1 complex. It is possible that the potential interactions between a TCNE molecule complexing with the 1:l complex are not much different from those between a p-xylene molecule and the 1:l complex and that under some of our expansion conditions both types of complexes are formed and absorb in the same region. In table I11 we have listed the band maxima for the complex in various solvents. The solvent-dependent red shift of the charge-transfer bands is a generally occurring phenomenon for weak br-ar complexes and has been known for some years.l6vZ4These condensed-phase effects have generally been attributed to some mixture of a vertical displacement of the potential surfaces due to a stabilizing influence of the medium on the charge-transfer state and a horizontal displacement of the surfaces due to compressional effects of the medium on the loosely bound ground-state complex. Although the solvent interactions with the 1:l complex produce qualitatively the same effect as termolecular complexation in the isolated gas phase in red shifting the charge-transfer absorption maximum, the fluorescence maximum (see Table 111) of the solvated 1:l complex is red shifted from that of the gas-phase 1:l

Russell and Levy

complex as observed in this work. There are no reports of fluorescence from a termolecular species in solution. The vertical and horizontal displacements of the potential surfaces that occur in condensed phase would produce the observed red shifts in absorption and fluorewnce and may simply overwhelm and/or modify the potential interactions due to termolecular complexation leading to the similarity of the charge-transfer bands of the 1:1, 1:2, and 2:l complexes in condensed phase. We have assigned the band in the 22 500-cm-l region to be due to a helium (or multiple He) van der Waals complex with the 1:l charge-transfer complex. This is consistent with its growth with Po,its absence in the presence of large TCNElp-xylene ratios, presumably due to an exchange reaction of the TCNE with the He, 1:l charge-transfer complex leading to a 2:l complex as has been observed before in the case of CF3NO-He, c0mplexes,5~and also its absence in the presence of a few percent Ar which displaces the He atoms.% Since the peak occurs in the same region as the shoulder that we have assigned to the 1:l complex, it is consistent with the small shifts normally observed for He van der Waals complexes.50 The large effect of helium complexation on the dispersed fluorescence spectrum is unexpected. From Figure 6 there is a rather large blue shift and broadening of the band. Since the lifetime data indicated the presence of two species, one short-lived (

Fluorescence spectrum of the charge-transfer complex

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