4298
J. Phys. Chem. 1986, 90, 4298-4302
Jet-Cooled Exciplexes. Ammonia with Perylene and with Anthracene O d d Anner and Yebuda Haas* Department of Physical Chemistry and The Fritz Haber Center for Molecular Dynamics, The Hebrew University of Jerusalem, Jerusalem 91 904, Israel (Received: December 3, 1985: I n Final Form: March 4, 1986)
Supersonic expansion of ammonia with perylene or with anthracene in helium carrier gas leads to formation of at least two types of adducts. One involves weak van der Waals interactions in both the ground and electronically excited state. The other forms, upon electronic excitation, a charge-transfer exciplex similar to that observed in the perylene (or anthracene) dimethylaniline system.
Introduction Exciplexes are considered to be important intermediates in many organic photochemical reactions.’ Their existence is confirmed in some cases by the observation of new emission bands.2 However, often the exciplexes are very short-lived, or highly reactive, and their very existence is inferred from indirect evidence.) It was actually observed that the more reactive exciplexes may be difficult to observe under normal reaction conditions, as the radiative decay rate is slow compared to nonradiative ones3 We have recently recorded the fluorescence from a jet-cooled adduct between dimethylaniline (DMA) and perylene and also between DMA and a n t h r a ~ e n e . ~On the basis of the well-known properties of these systems in liquid solutions, we assigned this emission to a jet-cooled exciplex. Since supersonic expansion allows the observation of collision-free clusters, it is reasonable to expect that exciplexes difficult to observe under usual thermal conditions may be detectable in the jet. Like aromatic amines, aliphatic amines are known to quench the fluorescence of polycyclic aromatic hydrocarbons (PAH’s). However, only in a few cases has exciplex emission been ~ b s e r v e d . ~ None was reported for ammonia, the parent compound of the amines. In this paper we describe the properties of the fluorescence emission from jet-cooled adducts between ammonia and two typical PAH’s-perylene and anthracene. Two different types are observed: one is assigned to van der Waals bound systems in both ground and electronic states, the other is considered to be due to an ammonia-hydrocarbon exciplex. This assignment is based on the shape of the spectral features and on lifetime measurements. This work appears to substantiate the observation (by Mataga and Ottolenghi, ref. 2, p 26) that “only specific ground-state aggregate pairs of suitable geometrical configuration may lead to the fluorescent complex state”.
Results The experimental setup was the same as that described prev i o ~ s l y .Briefly, ~ ammonia, highly diluted in helium, was passed over crystals of the hydrocarbon, maintained at a desired temperature, and expanded through a 0.35-mm-pulsed nozzle. The unskimmed molecular beam was crossed at right angles by a pulsed dye laser beam (7-ns pulsewidth, 1.5 cm-’ bandwidth) 30-50 nozzle diameters downstream. Fluorescence from the irradiated zone was viewed at right angles to both molecular and laser beams and detected with a monochromator-photomultiplier combination. The signal was digitized and averaged until an acceptable sig(1) Barltrop, J. A.; Coyle, J. D. Principles of Photochemistry; Wiley: New York, 1978. Turro, N. J. Modern Mechanistic Photochemistry; Benjamin/ Cumming: Menlo Park, CA, 1978. (2) Beens, H.; Weller, A. In Organic Molecular Photophysics; Birks, J. B., Ed.; Wiley: London, 1975, p 159. Gordon, M.; Ware, W. R., Eds. The Exciplex; Academic: New York, 1975. Mataga, N.; Ottolenghi, M. In Molecular Association; Forster, R., Ed.; Academic: New York, 1979; Vol. 2, p 1 . (3) Caldwell, R.A,; Creed, D. Arc. Chem. Res. 1980, 13, 45. Mattes, S. L.; Farid, S. Ace. Chem. Res. 1982, 15, 80. (4) Anner, 0.;Haas, Y. Chem. Phys. Lett. 1985, 119, 199. (5) Nakashima, N.; Mataga, N. Z. Phys. Chem. (Munich) 1972, 79, 150.
0022-3654/86/2090-4298$01.50/0
nal-to-noise ratio was achieved. Some difficulty was encountered in establishing the exact pressure of ammonia in the high-pressure side, particularly at low concentrations. Ammonia was found to adsorb strongly to the walls and pipes (all stainless) and then to desorb slowly. The pressures quoted below are nominal and may be in error by up to 50% for pressure below 1 Torr. This uncertainty does not affect the interpretation of the present results, since no quantitative measurement of signal intensities was attempted. Typical excitation spectra of the perylene-ammonia system are shown in Figure 1. In the absence of ammonia, no bands were observed to the red of the 0,O band (24065 cm-I). Upon addition of small amounts of ammonia to the helium carrier gas, two different types of new shifted bands appear in the spectrum: (1) A broad band (fwhm 80 cm-l) shifted by about 390 cm-’ with respect to the 0,O band appears, and a similar but weaker band appears at -40 cm-I. The large widths of these bands lead to an estimated error of i 5 cm-I in these shifts. Note the separation between those bands (350 cm-I), which is close to the frequency shift of the strong 353-cm-I vibrational band in perylene.6 A similar feature is observed in the perylene-DMA system as well! At low ammonia concentration the shape of the broad band is well approximated by a Lorentzian line shape, cf. Figure 2. It is evident that a Gaussian line shape is not applicable, while a Lorentzian line shape fits reasonably well within the signal-to-noise ratio of the experiment peak. Increasing the ammonia pressure leads to asymmetric broadening of this band, mainly to the red side. (2) A set of much narrower bands appears, whose number and positions are strongly pressure-dependent. At low ammonia concentration, only a few bands are observed (mainly a blue-shifted band at 30 cm-I). Increasing the pressure results in a large increase in the number of red-shifted bands. A further pressure increase causes strong congestion up to an abrupt cutoff at -220 cm-’. The two fluorescence band types differed also in their decay times. For the sharp features we measured approximately 9 f 1 ns (possibly affected by the laser pulse width), and for the broad bands a lifetime of 32 f 1 ns. Another difference was found in the relative peak intensities: the sharp features were about an order of magnitude stronger than the broad ones. Qualitatively, similar spectra were also obtained with the anthracene-ammonia system. An important difference was found upon measurement of the decay times. The narrow van der Waals bands led to emission with about the same decay time as bare anthracene (-20 ns). The broad bands displayed characteristic decay times of 300-500 ns, depending on the stagnation conditions. Figure 3 shows the excitation spectrum obtained when the emission was observed 300 ns after excitation. The two major peaks observed at low pressures and assigned to 1 : 1 adducts have maxima at -175 and -505 cm-’, respectively. Upon increasing the ammonia pressure, these peaks broaden, and additional ones appear. The added ones are assigned to adducts containing more than one ammonia molecule. The peaks assigned to 1:1 adducts have an
-
(6) Schwartz, S . A.; Topp, M. R. Chem. Phys. 1984, 86, 245. Bouzou, C.; Jouvet, C.; Leblond, J. B.; Millie, Ph.; Tramer, A. Chem. Phys. Lett. 1983, 97, 161.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 4299
Jet-Cooled Exciplexes PERYLENE-AMMONIA
1 TORR
0 -100 -200-300-400 -500 -600 -700 F r e q u e n c y Shift (cm-’ )
Figure 1. Excitation spectra of the perylene-ammonia system. Emission was observed at 460 nm 0-14 ns after excitation. The perylene crystals were maintained at 150 O C . Helium pressure was 6 atm. The intensity of the 0,O band at 24065 cm-l (zero shift in the figure) is not to scale. The other features are drawn at their correct magnitudes and are comparable in each panel and also between panels. The pressure of ammonia in the high-pressure chamber is indicated in each panel.
0
-200 -400 -600 Frequency Shift (cm-’ )
-800
Figure 3. Excitation spectra of the anthraceneammonia system observed 300 ns after the excitation pulse. Emission was observed through a cutoff filter transmitting above 470 nm. Anthracene was maintained at 120 “ C (yielding a pressure of 1 Torr). The 0,O band is at 27 695 cm-’, and its actual intensity is much larger than shown. Other details as in Figure 1. ANTHRACENE-AMMONIA
-
PERYLENE AMMONIA
K1 TORR
1
\
GAUSSIAN F I T
1 TORR
1
o
LORENTZIAN
FIT
0 -100-200-300-400-500-600-700 Frequency Shift ccm-’ ) Figure 4. Excitation spectra of the anthracene-ammonia system observed 20 ns after the excitation pulse. Other details as in Figure 3.
-100 -200 -300 -400 -500 -eo0 -700
Frequency Shift (cm”
)
Figure 2. Calculated analytical best fits for the experimentally obtained broad features. The sharp spectral features were removed in order to simplify the presentation. The Lorentzian line shape is seen to reproduce the observed line shape much better than the Gaussian one.
approximate Lorentzian line shape with a width of about 200 cm-’. Another difference between the perylene and anthracene systems involves the widths of the excitation bands assigned to van der Waals complexes (the narrow lines). For perylene we obtained about 2 cm-l, a value possibly limited by the laser line width. In the case of anthracene, the line widths were about 8 cm-’, in the presence or absence of ammonia. W e consider this result as indicating that anthracene is less efficiently rotationally cooled than perylene in our system. Some spectra displaying the narrow excitation bands are shown in Figure 4. The overall shape is similar to that observed in the perylene-ammonia system except that the bands extend to a -400-cm-’ shift from the 0,O band. Also, their intensities were about 2 orders of magnitude larger than those of the broad bands.
540
520
500 480 460 Wavelength(nm)
440
420
Figure 5. Emission spectrum of the perylene-ammonia adduct upon excitation at the broad band red-shifted by 400 cm-I with respect to the 0,O band (Aexc = 422.52 nm). Conditions are as in Figure 1 except that the spectrum was recorded 40 ns after excitation and ammonia pressure was about 0.5 Torr. The wiggles shown are due to insufficient average and do not represent fine structure. No vibrational structure was observed with the spectral resolution used (4 nm).
In both perylene-ammonia and anthracene-ammonia systems, the narrow excitation bands lead to a structured fluorescence spectrum (our resolution (4 nm) permitted only the observation of coarse vibrational structure). Excitation into the broad bands
4300
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986
Anner and Haas
TABLE I: Some Characteristics of the Fluorescence Excitation Spectra of Jet-Cooled Anthracene and Perylene in the Presence of Ammonia
shift," cm-' anthracene perylene
-400 to +30b -260 to +40b
anthracene
-1 50, -505 -40, -390'
perylene
width, cm-'
decay time, ns
fluorescence spectrum
assignment
Narrow Bands 20 f 2 structured 9 f 2 structued
-8 -2
-
van der Waals complexes van der Waals complexes
Broad Bands
200d -80d
450 f 100
32 f 2
broad and structureless broad and structureless
exciplex exciplex
'At band center. bMany lines observed at high pressure, only few at low pressure. C T wmain ~ lines observed at low pressure. At high pressure they become asymmetric and smear out. dDerived from a fit to Lorentzian line shape.
led to a strongly red-shifted emission (centered around 465 nm), which was broad and structureless (Figure 5). Its shape was very similar to that observed in the DMA-hydrocarbon system^.^ Some features of the observed fluorescence are summarized in Table I.
Discussion The narrow excitation bands observed in the perylene-ammonia system are similar to lines found in the jet-cooled spectra of perylene-alkanes.' They may thus be assigned to van der Waals complexes that are loosely bound in both the ground and the excited states. The red shift is due to somewhat stronger interaction between the electronically excited hydrocarbon and the "solvent" molecule, compared to ground-state interactions.8 The origin of the few blue-shifted bands observed with perylene is not clear at the moment. A similar observation was made by Schwartz and Topp' in perylene-alkane complexes and assigned to internal vibration of perylene, whose frequency was modified by the presence of the alkane. More work is required in order to account for these features, but since our main purpose in this study was to investigate the broad bands, we have not followed up the matter. The broad bands indicate a completely different interaction. On the basis of our previous results on the DMA-perylene (and anthracene) systems, we propose an exciplex state as the upper level in these transitions. This assignment accounts for the broad diffuse nature of the bands in both excitation and emission. It is also consistent with the longer decay times observed (much longer in the case of anthracene). As can be seen from Figures 1 and 3, the shape and intensities of the broad excitation bands vary with ammonia pressure in a nonmonotonic fashion: the signal intensity increases with ammonia pressure up to about 4 Torr and then starts to decrease. Concurrently, these excitation bands broaden considerably. Experimental difficulties (adsorption and desorption of ammonia on the walls) excluded the determination of a quantitative relationship. Yet it is clear that increasing the ammonia concentration leads to a decrease in the exciplex emission intensity. One possible reason is that big clusters decompose upon excitation, leaving behind configurations unfavorable for exciplex formation. Another is that upon excitation one or more ammonia molecules are ejected from the cluster, a process that leads to quenching of the exciplex state. In this paper we shall concentrate on the properties of 1:l adducts only. A crude model was suggested in ref 4, based on earlier work,2 wherein a curve crossing between a locally excited state and an ionic charge-transfer (CT) state led to population of the exciplex level. In this model the crossing distance was calculated by using the simplest approximation for the ionic curve: V(R) = P,(amine) - A,(hydrocarbon) - e 2 / R
+ B/RIZ
(1)
Here P I and A, are the ionization potential and electron affinity of the appropriate molecules, respectively. The third term expresses the Coulomb attraction, and the fourth, the repulsion of the electron clouds at short separations. The covalent curve was constructed from the well depth and intermolecular distance calculated by using intermolecular pairwise potential. Details of this calculation are given in this paper. The same B constant was (7) Schwartz, S. A,; Topp, M. R. J . Phys. Chem. 1984, 88, 5673. (8) McRae, E. G. J . Phys. Chem. 1957, 61, 562.
ANTHRACENE-AMMONIA
10
3
4
6
0
1
R (Angstrom)
Figure 6. Potential energy diagram for the anthracene-ammonia adduct showing the curve crossing between the covalently bound pair and the ionic curve. Curve a used the Coulomb attraction term only, while in curve b the ion-induced dipole term was added.
used for the ionic and covalent curves under the assumption that the intermolecular repulsion at short distances is similar in both states. When this expression is used for ammonia, the ionic curve is found to lie above the covalent one at all intermolecular separations, and no crossing is obtained. This is due to the much higher ionization potential of ammonia (10.16 eV) compared to DMA (7.15 eV). A similar problem was dealt with in the case of the harpoon reactions between alkali metals and halogen molecules. It was suggested that higher interactions terms be added to the potential in order to account for the experimental observation^.^ The first correction involves ion-induced dipole interaction and depends on the polarizabilities a+ and a- of the cation and anion, respectively. The correction term is e2(a+ a-)/2R4 and is important mostly when the polarizabilities are large. This indeed is the case for polycyclic aromatic hydrocarbons. The polarizability tensor was measured for solid anthracene,1° and the principal values were found to be (in A3 units) 65.7, 21.1, and 12.2, respectively. In liquid solution an average value of 25.9 A3 is reported.1° The values for perylene are not known to the same precision, but are estimated to be even larger." The polarizability of the anthracene anion is expected to be somewhat higher than that of the neutral molecule. We have used the modified potential, with a(anthracene-) = 30 A3. The results are in Figure 6 and show that this first correction suffices to lead to curve crossing, which in turn would allow exciplex formation. A more exact calculation would have to address specifically the geometry of the adduct and use the full polarizability tensor. The foregoing discussion suggests that knowledge of the exact geometry of the adducts is crucial for understanding the photophysical properties of jet-cooled exciplexes. This raises the interesting question of the feasibility of coexistence of several distinct configurations in a given system (namely, stagnation conditions,
+
(9) Gislason, E. A,; Sachs, J. G . J . Chem. Phys. 1975, 62, 2678. (IO) Dunmur, D. A. Mol. Phys. 1972, 23, 109. ( 1 1) Liptay, W.; Walz, G.; Baumann, W.; Schlosser, H.-J.; Deckers, H.; Detzer, N. Z . Nalurforsch. A: Astrophys., Phys., Phys. Chem. 1971, 26A, 2020.
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 4301
Jet-Cooled Exciplexes
TABLE II: Values of Parameters Used in the Geometry Calculations" B,E
A,b
I -550
PAH
ammonia
cm-*/(A6mol)
cm-'/mol
C,b A-l
H
H
C C H
H
9 025 34 790 314946 81 695
841608 3 631 130 10718 100 3 484 200
3.769 3.699 3.151 3.221
N N
I
-800
-650
"Compare eq 2. Values for the atomic charges qk and qI were taken from ref 20. F = 1.16 X lo5for conversion into wavenumbers when qk and ql are in electron charge units and Rkl is in A. 'The values of the A , B, and C parameters were calculated according to the usual method:
-700
A,, = (AkA,)1/2BkI = (BkBI)"2Ckr= 0.5(Ck + C,).
a
0
eo
180
270
360
(degrees) Figure 8. Potential energy diagram for the anthracene-ammonia adduct obtained by rotating the ammonia molecule about an axis parallel to the x or y axes passing through the nitrogen atom. The ring-nitrogen distance is held at z = 3.5 A. The coordinate system used has its origin at the center of the anthracene molecule and is shown in the figure. Rotation
PERYLENE-AMMONIA
b Figure 7. Most stable configuration calculated for the perylene-ammonia system. For details, see text.
nozzle shape and diameter, etc.). Assuming that the expansion process is thermodynamically controlled, stable conformers are expected to be exclusively formed. In the cold jet, coexistence of two different ones is possible only if they differ in their energy by no more than several times the average thermal energy. Under our conditions we estimate 100 cm-' to be a likely value for such a difference. In order to test the feasibility of such coexistence, we carried out potential energy calculations for several plausible geometries of the hydrocarbonammonia adduct. The calculation is based on pairwise potentials between all atoms of the two molecules.I* The potentials used were either Buckingham13 or 12,6 Lennard-J~nes,'~ giving qualitatively similar results. Further details concerning these calculations will be published e1~ewhere.l~ Results quoted in this paper are based on the use of the Buckingham potential and the addition of the electrostatic interaction in the form Kohl
= C C ( B k / exp(-CklRkl) - Ak//Rk16 r k
+fqkq//DRk/)
(2)
Rkl is the distance between atom k on the acceptor and atom I on the donor. A, B, and C are the appropriate Buckingham potential parameters. qk and qr are atomic charges, D is the "effective dielectric constant" (usually taken as 216), and f is a unit conversion factor. The parameters used are given in Table 11. The most stable configuration calculated for the two systems studied locates the NH3 molecule above the center of the aromatic hydrocarbon with its hydrogens facing the plane (Figure 7). The separation between the nitrogen atom and the aromatic plane is 3.55 A, and the energy of the anthracene-ammonia and peryleneammonia systems is -750 and -910 cm-',respectively, relative to the energy at infinite separation of the pair. Translation of (12) This calculation is similar to that carried out to obtain the structure of macromolecules. See,for instance: Levitt, M. J. Mol. Biol. 1983, 270,723. Similar calculations carried out for jet-cooled exciplexes were performed by: Ondrechen, M. J.; Berkovitch-Yellin, Z.; Jortner, J. J. Am. Chem. SOC.1981, 203, 6586 (tetracene-rare gases). Schauer, M.; Bernstein, E. R. J . Chem. Phys. 1985, 82, 3722 (small aromatic dimers). (13) Parameters for this potential were obtained from: Schauer, M.; Law, K. S.;Bernstein, E. R. J . Chem. Phys. 1985,82,736 (carbon and hydrogen interactions). Righini, R.; Neto, N.; Califano, S . Chem. Phys. 1978, 33, 345 (nitrogen atom interactions). (14) Parameters for this potential were obtained from Levit's paper." (15) Anner, 0.;Haas, Y., to be published. (16) Momany, F.A.; Carmthers, L. M.; McGuire, R. F.; Scheraga, H. A. J. Phys. Chem. 1974, 78, 1595.
Figure 9. Two metastable configurations of the perylene-ammonia adduct obtained at a rotation angle of (a) 45' and (b) 275O (see Figure 8 for definition of the coordinates). These configurationsare expected to allow better charge transfer than that of Figure 7.
the N H 3 molecule from this point along any axis leads to a steep rise in energy with no local minima. On the other hand, rotating the ammonia molecule around the axes passing through the nitrogen atom and parallel to the x or y axes leads to the potential curves shown in Figure 8. Notice two local minima obtained at about 45O and 275' angles of rotation. The conformations obtained at these angles upon rotation around the x axis are shown in Figure 9. In these conformations the lone-pair orbital is roughly parallel to the aromatic plane, while in the most stable configuration it faces away from the plane. It is likely that rotation about any axis intermediate between the x and y axes would lead qualitatively to a similar potential function. An extensive calculation checking all possibilities was not attempted and appears unwarranted. The inclusion of the electrostatic interaction affects the results in only a minor way. Energies change by at most 20 cm-', and geometries hardly change a t all. Similar calculations for the anthracene-DMA and peryleneDMA systems studied by us4show only a single stable ground-state configuration, in which the aromatic planes are parallel and the nitrogen atom is displaced by 1.7 8,from the aromatic hydrocarbon center. The interplanar distance is 3.5 A. In contrast to the ammonia adducts studied in this paper, no van der Waals bands are observed in the DMA systems, and the exciplex emission dominates the spectrum. These experimental results and the calculated potentials point to the conclusion that coexistence of two or more adduct conformations with differing energies is possible in the beam. Chemical intuition leads us to suggest that charge transfer occurs when there is sufficient overlap between the ammonia lone-pair orbital and an appropriate accepting orbital on the hydrocarbon. The most likely candidates are the highest occupied molecular orbital (HOMO), which is half-occupied after the electronic excitation, or the lowest unoccupied molecular orbital (LUMO), which is half-filled after the excitation. It is expected that the most stable configuration, shown in Figure 7, will lead to negligible overlap between the donor and acceptor orbitals, as the lone pair
4302
J. Phys. Chem. 1986, 90, 4302-4305
faces away from the hydrocarbon pair. Thus, this configuration is expected to lead to the sharp features in the fluorescence excitation and emission spectra (cf. Figures 1 and 3). The configurations shown in Figure 9, lying about 70 and 110 cm-l above the former, allow for a better overlap between the donor and acceptor orbitals and are thus expected to be responsible for the broad exciplex features. In the DMA systems the single configuration possibly leads to a good overlap between donor and acceptor, and indeed only exciplex emission is observed. More quantitative calculations based on the electronic structure of the exciplexes studied as a function of orientation are needed in order to understand these systems in depth. An approximate Lorentzian form of similar broad bands was observed also in other jet-cooled adducts capable of forming a donor-acceptor exciplex,I7 and this appears to be a common occurrence. As discussed in ref 4, this Lorentzian line shape may be accounted for by using the formalism of radiationless transitions theory,]* in which the molecule is considered to have two types of excited states: those that can be directly coupled to the ground state by a radiative process, and those for which such a transition is forbidden. Labeling the former as Is) and the latter as Il), and assuming that each Is) state may be coupled to many 11) states, the transition rate in the statistical limit is given by k = (2.rr/h)l(lIm)lZp
where Vis the coupling potential between the states and p is the density of 11) states. In the present case, the charge-transfer (exciplex) states are not readily accessible from the ground state due to unfavorable Franck-Condon factors. This is evident, inter alia, from the large spectral shift between the excitation and the emission bands. ~
~~
~
(17) We have recorded spectra with similar characteristics for the following systems: anthracene and perylene with aniline, anthracene with anisole and with diethyl ether, 9methylanthracene with aniline and with diethyl ether. These systems also exhibit, upon excitation into the broad bands, a structureless emission spectrum with decay times significantly larger than those of usual van der Waals adducts. Details will be published elsewhere. (18) Jortner, J.; Rice, S. A,; Hochstrasser, R. M. Adu. Pfiofocfiem.1969, 7, 149. Jortner, J.; Levine, R. D. Ado. Cfiem. Pfiys. 1981, 47, 1.
Additional evidence comes from the fact that the broad-band emission is much weaker than the narrow-band emission. In contrast, the local excitation transition of perylene on anthracene is highly allowed, even in the complexed molecule. Thus, according to this model Is) is a locally excited state and 11) are charge-transfer states. In the limit of a large density of states, the radiationless process leads to a broadened quasi-Lorentzian absorption line shape. A related example was recently demonstrated by Jortner et al.19 in the fluorescence excitation spectrum of porphin and azulene. The line width reported here is about 1 order of magnitude larger than that reported in ref 19. The large density of states in the charge-transfer complex is probably related to the existence of very low frequency vibrations. Such low-frequency modes arise from the conversion of six translational and rotational degrees of freedom of the free pair to vibrational ones in the complex. The fact that so many different systems4J7show similar spectral features calls for a common underlying mechanism, and the one offered by this extension of the formalism of radiationless transitions appears to account for the observations, including the approximate Lorentzian form of the excitation bands. In summary, we have shown that ammonia can form exciplexes with perylene and with anthracene in a supersonic jet. A corresponding observation was not reported in liquid solutions, in contrast with DMA and other aromatic amines. It is suggested that in the cold jet conformations leading to exciplex formation have a better chance of showing up than under usual conditions. This result should carry over to many other exciplex systems not yet observed by fluorescence spectroscopy. The experimental results indicate that isomeric clusters can coexist in the beam and that cluster configuration can have a dramatic effect on the fate of energy absorbed by the system. Registry No. Ammonia, 7664-41-7; anthracene, 120-12-7; perylene, 198-55-0. (19) Even, U.; Magen, J.; Jortner, J. Cfiem. Pfiys. Left. 1982, 88, 131. Amirav, A.; Jortner, J. J. Cfiem. Pfiys. 1984, 81,4000. (20) Hoffman, R. J. Cfiem. Pfiys.1963, 39, 1397.
Holographic Photochemical Study of 9-Nitroanthracene A. C. Testa* Department of Chemistry, S t . John's University, Jamaica, New York 11439
and U. P. Wild* Laboratorium fur Physikalische Chemie, Eidgenossische Technische Hochschule, 8092 Zurich, Switzerland (Received: October 28, 1985; In Final Form: February 28, 1986)
The holographic behavior of 9-nitroanthracene in a poly(viny1 butyral) film at 466 and 458 nm indicates that a relatively clean unimolecular one-photon process occurs at these wavelengths, leading to the formation of 9-anthrol. The UV lines of an argon ion laser resulted in a weaker and different holographic behavior, which was attributed t o the presence of secondary photochemical events. From the 458-nm laser intensity dependence on the time for the maximum efficiency in the hologram The holographic the quantum yield for the one-photon chemistry of this molecule was determined to be 4 = (8.3 & 1.7) X efficiency at the maximum in the growth curve was determined to be -0.12%. It is suggested that the wavelength dependence of the holographic behavior of a molecule may be useful in elucidating complicated photochemical processes.
Introduction The photochemistry of 9-nitroanthracene has remained an interesting research problem due to the presence of secondary photochemical and luminescence processes. Approximately ( I ) Chapman, 0. L.; Hecker, D. C.; Reasoner, J. W.; Thackaberry, S. P. J. Am. Cfiem.SOC.1966, 88, 5550.
20 years ago Chapman and co-workers' reported that anthraquinone and anthraquinone monoxime were the primary photo(2) Hamanoue, K.; Hirayama, S.;Nakayama, T.; Teranishi, H. J. Pfiys. Cfiem. 1980,84, 2074. (3) Snyder, R.; Testa, A. C. J . Pfiys. Chem. 1981, 85, 1871. (4) Hamanoue, K.; Amano, M.; Kimoto, M.; Kajiwara, Y . ;Nakayama, T.; Teranishi, H. J. Am. Cfiem.SOC.1984, 106, 5993.
0022-3654/86/2090-4302$01 .50/0 0 1986 American Chemical Society
