J. Phys. Chem. 1992, 96,9600-9602
9600
accommodate the large proton splitting of 7.7 G observed in H20EiBC'-. The tetraphenyl-iBC results also minimize the possibility that the EPR spectrum attributed to H,OEiBC'- arises from adventitious proton attack on the iBC skeleton. (24) The reduction potential of sirohydrochlorin in vivo is not known. Although H2iBCs are hard to reduce in vitro:+ the reduction potential of sirohydrochlorin could shift significantly because of hydrogen bonding and/or partial ionization of its eight carboxylic groups as well as interactions with nearby prosthetic group residues such as the Fe4S4cysteine found near the
siroheme in Escherichia coli sulfite reductase (McRee, D. E.; Richardson, D. C.; Richardson, J . S.;Siegel, L. M. J . Biol. Chem. 1986, 261, 10277.) (25) Battersby, A. R.; Frobel, K.; Hammerschmidt, F.; Jones, C. J . Chem. Soc., Chem. Commun. 1982, 455. Kodera, M.; Leeper, F. J.; Battersby, A. R. Ibid. 1992, 835. (26) Wasielewski, M. R. Chem. Rev. 1992, 92,435. Newton, M. D. Ibid. 1991, 91, 767. Gust, D.; Moore, T. A. Science 1989, 244, 31. Marcus, R. A.; Sutin, N. Biochim. Biophys. Acra 1985, 811, 265. Closs, G.L.; Miller, J . R. Science 1988, 240, 440.
Exclplex Formatlon between Anthracene and Ammonia in an Argon Matrix at 17 K
R. Fraenkel and Y. Haas* Department of Physical Chemistry and the Farkas Center for Light Induced Processes, The Hebrew University, Jerusalem, Israel (Received: August 7, 1992)
Exciplex emission from the title system was observed upon deposition of anthracene vapor and ammonia in an argon matrix. This allowed the determination of a lower limit for energy of the vibrationally relaxed exciplex state in an argon environment, 20800 200 cm-I, and a decay time of about 1.5 X 10" s.
Introduction The nature and energetics of electronically excited states are of central importance in photochemistry. In the case of bimolecular reactions, the electronic structure of the encounter complex may be considerably,different from that of either component. Exciplexes have been initially discovered in nonreacting systems',* but were later often considered as possible reaction intermedia t e ~ . ~The - ~characterization of reacting exciplexes is inherently difficult, since their existence is principally based on fluorescence spectroscopy. Therefore, if a chemical reaction proceeds at a much faster rate than the fluorescence, the latter's quantum yield may become extremely small. In fluid solutions, exciplexes are usually formed by a collision between an electronically excited molecule and a ground-state partner. It has been shown that, in supersonic jets, some van der Waals adducts can be excited to yield exciplex emission, thus enabling for the first time to obtain fluorescence excitation spectra of exciplexes.6'0 In favorable cases, one may even observe the absorption spectrum of the exciplex." The jet method was used to observe exciplexes of anthracene with ammonia:I2 Analysis of the data suggested that at least two different adduct structures are formed between anthracene and ammonia, only one of which leads to a charge-transfer type exciplex emission. This exciplex was, to our knowledge, never observed in the bulk. We have now extended this work to the study of the same system in an argon matrix. Exciplexes have often been studied in frozen soluti0ns13or in doped crystals;14it has been suggested that exciplex formation could be assisted by a suitable organization in the solid, but we are not aware of a study of exciplexes in rare gas matrices. The objective of this work was to confirm the results of the jet study and also to obtain more detailed information on the properties of the exciplex. In particular, vibrational relaxation is essentially inhibited in the case of jet-cooled adducts that are essentially collision free. In a matrix, vibrational relaxation is rapid, and one may study the properties of the system near the bottom of any electronic state, including the exciplex one. Results Matrices were prepared by premixing ammonia and argon, passing the gas over anthracene crystals, and depositing the mixture of a BaF2 window held at 26 K. The window was cooled by an Air Products cryostat (Model CS202K), and deposition was normally at a rate of 12 mmol/h, until a 0.5-1-mm-thick matrix was formed. Anthracene was held at room temperature, leading to an estimated vapor pressureI5 of Torr. Zone-refined samples led to the same results as commercially available highpurity material.
The matrix was cooled to 17 K and then irradiated by a tunable, pulsed dye laser (10-20-ns pulse width, 0.5 mJ/pulse, 0.2-1-cm-I spectral resolution). The resulting fluorescence was dispersed by a 0.75-m monochromator (Spex 1701) and detected by a photomultiplier. The signal was fed into a digital oscilloscope (Tektronics 2430A) and processed by a personal computer. When pure anthracene is deposited in a matrix, three major sites are observed by UV absorption and fluorescence spectros~ 0 p y . lThe ~ origins (0,Otransitions) of these sites are located at 26 973,27 004, and 27 159 cm-l above the ground state. This may be compared to the vapor-phase" (jet) value of 27 695 cm-I. Addition of ammonia leads to the observation of three more distinct sites, whose origins are located at 26890, 26919, and 27 111 cm-I. The absorption and fluorescence bands due to anthracene in argon and due to anthracene-ammonia sites in the matrix had a width of about 8 cm-', with the monochromator set to 3-cm-l resolution. In order to observe the exciplet emission, the monochromator slits had to be opened considerably, leading to spectral resolution of 11 A. Figure 1 shows the fluorescence emission spectrum obtained under these conditions following excitation to the origin of the lowest lying site at 26973 cm-I. It is seen that a broad and structureless emission band appears, whose width is about 3500 cm-I. The discrete bands on the left are due to locally excited (LE) states of anthracene in the matrix. Tuning the laser to energies lower than that of the main sites (as observed by fluorescence excitation of LE states), one still observes the broad-band emission, but now the discrete bands are absent (Figure 2). It was found that fluorescence due to the broad band could be detected at least down to 1000 cm-I below the energy of the lowest lying LE state. The low intensity of the signal (about 3 orders of magnitude weaker than that of the discrete bands) prohibited the detailed study of an absorption or excitation spectrum. As far as could be established, the excitation spectrum shows no vibrational structure, and the broad signal's intensity was not changed appreciably upon tuning the laser above or below the origin of the LE bands. No signal was obtained when only anthracene or ammonia was deposited. Figure 3 shows the decay profile of the broad band, as observed at 580 nm, leading to a decay time of 1.5 ps (correlation coefficient = 0.96).
Discussion The anthracene-ammonia system was previously discussed in the context of supersonic jet observations.I2 A curve-crossing model was proposed to account for the formation of the exciplex.
0022-36S4/92/2096-9600%03.00f 0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9601
Letters
AnrhracendNH, in argon matrix
-.
Y.
3-4 p time interval
R
= 0.96
. .. ..... *
I 4
450
500
550
650
Mx)
700
Wavelength, nm
1
Figure 1. Fluorescence spectrum of the anthracene-ammonia system in an argon matrix, held at 17 K upon excitation at 26973 cm-I. The top spectrum shows the integrated fluorescence intensity over the time interval 0.5-1 ps after the excitation, and the bottom one over the time interval 3-4 ps. The matrix was formed by depositing a mixture of argon/ammonia (1OOO:l ratio) on a BaF, window, after passing it over anthracene crystals held at room temperature, leading to an anthracene/ammonia ratio of 1:lOO. Resolution = 11 A, step size = 10 A.
I
1
1
2
3
4
5
Time, 1 s Figure 3. Decay behavior of the broad emission band, measured at 17 K, for the same matrix as in Figures 1 and 2. R is the correlation coefficient. 6 5
0.5-1 p time interval
Exciplex transitions
1
_____.__._ LE transitions
0
I
3-4 bs time interval I
I
3
4
R (anthracenefNH3), A
~
4M)
450
500
550
Mx)
~~
650
700
Wavelength. nm Figure 2. Same as Figure 1, with excitation at 26 538 cm-I, namely, below the energy of the locally excited anthracene-ammonia adducts.
The energy of the charge-transfer exciplex state was expressed as V ( R ) = Pm - Aan- e 2 / R - e2(aam+ a a n ) / R 4 B / R 1 * ( 1 ) where Pm is the ionization potential of ammonia, Aanis the electron affinity of anthracene, R is the separation between anthracene and ammonia, e is the charge of the electron, CY is the polarizability,
+
Figure 4. Schematic energy level diagram of the anthracene-ammonia system, showing a slice cut along the intermolecular distance. Transitions between locally excited (LE) states are allowed by symmetry and by Franck-Condon overlap considerations. The energy minimum of the exciplex state is located at a much smaller intermolecular separation than the ground, van der Waals bound state, and thus direct transitions to the origin are Franck-Condon forbidden. The exciplex emission spectrum, shown schematically, is broad, reflecting the steep repulsive part of the ground-state potential.
and the last term represents the repulsion due to Pauli forces. Equation 1 is based on a simple model that assumes complete electron transfer from ammonia to anthracene. The polarizability term was found to be essential for obtaining curve crossing with the LE state (cf. Figure 6 in ref 12). In the matrix, the energy levels are modified by interaction with the argon atoms. Nonetheless, it is instructive to use the gross feature of the model to analyze the data and obtain some insight into the nature of the system. Figure 4 shows an energy level diagram that is based on eq 1 and modified to account for the
9602 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
experimental data. It is seen that the uilibrium separation between ammonia and anthracene is 3.4 in the ground state (van der Waals interaction) and 2.8 A in the exciplex state. Application of the Franck-Condon (FC) principle would thus predict essentially prohibition of a direct transition to the minimum of the exciplex state on one hand and the Occurrence of a very large Stokes shift on the other. These predictions are indeed borne out by experiment. The very weak absorption amplitude into the CT exciplex state is accounted for by the small transition moment (see below), by the large width, and by the unfavorable FC factor. The absence of observable vibrational structure is also compatible with the proposed model: The density of vibrational states of the adduct is quite high at the energy of the vertical transition, which lies a few thousand cm-l above the origin. In addition, at the temperature of the experiment (1 7 K), phonon bands are 50 cm-I broad and will also contribute to the overall congestion. In the matrix, as mentioned, vibrational relaxation is facile, and one expects to observe emission from the minimum of the potential well. As seen from Figure 4, the FC transition would be to the repulsive part of the ground-state potential curve. Thus, one would expect it to lead to dissociation of the adduct, which indeed is the case in the gas or liquid phase. In the matrix, the cage effect will probably inhibit actual separation. The large width of the emission spectrum is a reflection of the steepness of the ground-state repulsive potential and is reproduced rather well from the model. The blue edge of the exciplex emission spectrum at 480 nm (20 800 cm-I) can be considered as the lower limit for the energy of the exciplex state. According to Figure 4, the actual location is somewhat higher-about 22 400.cm-I. The previous jet work12 was interpreted as indicating the presence of two structurally stable adducts, only one of which leads to exciplex formation. The difference was traced to a much better overlap between the lone pair orbital of the ammonia (serving as an electron donor) and the frontier orbitals of anthracene (the electron acceptor) in one of the adducts. The exciplex type excitation spectrum consisted of a broad band (several hundred cm-I) in contrast to the LE excited transition which displayed a series of narrow bands assigned to vibrational structure. In the matrix, we observed separate banded systems, which, as suggested above, indicate the existence of three distinguishable sites. All of them probably correlate to the LE state observed in the jet. The broad band found in the matrix is likely to correlate with the exciplex type one observed in the jet. We find that the excitation spectrum of the broad band extends over a very large range from below the LE state to above it. Further work is needed to characterize it better, but it is already clear that its decay time is about 2 orders of magnitude longer than that of the LE state and that the optical density is at least 3 orders of magnitude smaller. The transition dipole moment of the exciplex type emission can be estimated as follows: In the case of an electric dipole transition,
1
-
Letters the dipole moment is related to the radiative lifetime T by the relation1* lp12 =
heJ3/ 16 r 3 7
where X is the average emission wavelength and c is the permittivity of argon. In the jet, the decay time of the exciplex type transition was found to vary according to the excitation frequency, between 300 and 500 ns.I2 In the matrix, we obtain a somewhat longer decay time, 1.56 p. Assuming that this decay time is the pure radiative lifetime, we get p = 0.4 D. This result is in line with the forbidden nature of the transition and with the values reported for other CT type exciplexes.2
Conclusion In conclusion, it was shown that exciplex emission from the anthracene/ammonia system can be observed in a cryogenic matrix. The matrix isolation method provides a tool to observe directly weak optical transitions that may be difficult to observe in fluid solutions. The states revealed by these transitions are of potential importance for the elucidation of photochemical processes. Acknowledgment. The Farkas Center for Light-Induced
Processes is supported by the Minerva Gesellschaft, mbH, Munich. References and Notes ( I ) Leonhardt, H.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1963, 63, 791. (2) Beens, H.; Weller, A. In Organic Molecular Phorophysics; Birks, J. B., Ed.; Wiley: London, 1975; p 159. ( 3 ) Caldwell, R. A.; Creed, D. Acc. Chem. Res. 1980, 13, 45. (4) Lewis, F. D.; De Voe, R. J.; MacBlane, D. B. J . Org. Chem. 1982,47, 1392. (5) Mella, M.; Fasani, E.; Albini, A. J. Photochem. Photobiol. A 1991, 59, 297. (6) Saigusa, H.; Itoh, M. J . Chem. Phys. 1984, 81, 5692. (7) Castella, M.; Millie, P.; Piuzzi, F.; Caillet, J.; Langlet, P.; Claverie, P.; Tramer, A. J. Phys. Chem. 1989, 93, 3949. (8) Lahmani, F.; Zehnacker-Rentien. A,; Breheret, E. J. Phys. Chem. 1991, 95, 3647. (9) Haas, Y.; Anner, 0. In Photoinduced Electron Transfer; Fox, A. M., Chanon, M., Eds.; Elsevier: New York, 1988; Part A, p 305. (10) Saigusa, H.; Lim, E. C. J. Phys. Chem. 1991, 95, 7580. (1 1) Amirav, A.; Castella, M.; Piuzzi, F.; Tramer, A. J . Phys. Chem. 1988, 92, 5500. (12) Anner, 0.;Haas, Y. J. Phys. Chem. 1986, 90, 4298. (13) (a) McGlynn, S. P.; Boggus, T. D.; Elder, D. J . Chem. Phys. 1960, 32, 357. (b) Matsugawa. S.; Garrigues, P.; Lamotte, M.; Tamura, M. J. Phys. Chem. 1991, 95, 9676. (14) Berkovic, G . E.; Cohen, M. D.; Ludmer, Z . Chem. Phys. 1983,82, 405. (15) Macknick, A. B.; Prausnitz, J. M. J. Chem. Eng. Data 1979,24. 175. (16) Fraenkel, R.; Haas, Y.; Dick, B. To be published. (17) Lambert, W. R.; Felker, P. M.; Syage, T. A,; &wail, A. H. J. Chem. Phys. 1984.81, 2195. (18) Siegman, A. E. Lasers; University Science Books: Mill Valley, CA, 1986.
