J. Phys. Chem. 1991, 95, 3647-3652
magnitude. The main channel remains associative detachment, A minor (10%) channel produces OH-(H20), The temperature dependence of the rate constant for reaction of the singly solvated ion is positive. A second water solvent molecule decreases the reactivity to below our detection limit. The reactivity decrease appears too large to be caused by steric factors. A possible cause for the reduced reactivity is a reduced interaction energy of H2 with @(H@) compared with that of H2 with @. NOdependence
3647
on rotational energy was found for either the rate constants or the branching ratios for n = 0. Note added in proof: Beam experiments, conducted at considerably higher energies than those of the present experiments, have been reported p r e v i o ~ s l y . ~ ~ (30) Martin, J. D.; Bailey, T. L. J . Chem. Phys. 1!J68,49, 1977. Doverspike, L. D.;Champion, R. L.;Lam, S.K.J. Chem. Phys. 1973,58, 1248.
Bimolecular Photochemlcal Intermediates: A Study of Jet-Cooled Complexes of Cyanonaphthalenes with Olefins F. Lahmani,' A. Zehnacker-Rentien, and E. Breheret Laboratoire de Photophysique Mollculaire, Bit. 21 3, UniversitP Paris-Sud, 91405 Orsay Cedex, France (Received: September 13, 1990)
The fluorescence excitation spectra, dispersed emission, and fluorescence decay times of the complexes formed in a supersonic expansion between 1- and Zcyanonaphthalene (1-CNN and 2-CNN) and 2,3-dimethyl-2-butene or 2-methyl-2-butene (DMB and MB) have been investigated in the region of the first singlet state of cyanonaphthalene. The I-CNN-DMB complexes exhibits a broad excitation spectrum and an exciplex type emission similar to that observed in nonpolar solutions. Exciplex emissions have also been identified in the case of 1-CNN-MB and 2-CNN-DMB complexes, whereas the 2-CNN-MB system behaves in the excited state as a weakly perturbed van der Waals complex. In the case of 2-CNN-DMB complex, exciplex formation efficiency is decreased by the excitation of a higher intramolecular level of 2-CNN, the fluorescence spectrum resulting mainly from the locally excited 2-CNN populated by internal vibrational redistribution. The different behavior observed in the cyanonaphthalene-olefin complexes are related to the energy gap between locally excited and charge-transfer states. The geometry of the complexes is suggested to play an important role in exciplex formation dynamics in the case of nearly resonant LE and CT states.
Introduction Exciplex formation is a general well-known mechanistic pathway involving interactions between electronically excited and ground-state molecules. Fluorescence quenching and photoreactions such as photocycloadditions involving many aromatic molecules singlet states have been postulated to proceed via so called nonemissive exciplexes.' A lot of fluorescent exciplexes have been also characterized in solution in numerous bimolecular systems by the observation of a broad long-wavelength emission band resulting from vertical transitions from the bound excited complex stabilized by charge-transfer interactions to the repulsive part of the ground state.2 Typical exciplexes involve aromatic molecules in their excited state as electron acceptors and amines ground states as electron donors. Among the very few examples known to exhibit both exciplex fluorescence and photochemical reactivity the aromatic nitriles and "rich electron" olefins represent an interesting category to investigate.3 The technique of jet spectroscopy provides a convenient means to generate and to study molecular associations with weak binding energy and has been applied recently to investigate in isolated gas-phase conditions the photophysicp of exciplexes produced by photoexcitation of ground-state van der Waals complexes. Saigusa and Itoh4J reported the first observation of exciplex fluorescence in the jet cooled l-cyanonaphthalenttriethylaminecomplex. They (1) T w o , N. J. Modern Mecunisric Phorochemisrry; Bmjamin/Cumming: Menlo Park, CA, 1978. (2) Buks, J. B. Phorophysics of Aromoric Molecules; Wiley-Interscience: London, 1970. (3) McCullough, J. J. Chem. Rev. 1987, 87, 811. (4) (a) Saigusa, H.; Itoh, M. Chem. Phys. Lcrr. 1984, 106, 391. (b) S a i g w , H.; Itoh, M. J . Chem. Phys. 1984, 81, 8692. (5) Saigusa, H.; Itoh, M.; Bata, M.; Hanazaki, I. J. Chem. Phys. 1987, 86, 2588.
0022-3654/91/2095-3647$02.50/0
demonstrated that excess internal vibrational energy was necessary to induce the exciplex formation from the van der Waals locally excited complex. Castella et al.61~ and Anner and Haass have observed exciplex fluorescence in the jetcooled mixture of perylene or anthracene with aromatic amines. In an extensive study, Castella et aI.6~~ have investigated the influence of the ionization potential (IP) of the donor and described the transition from typical van der Waals behavior with high-IP donors to characteristic exciplex formation with low-IP donors, due to the strong coupling of the locally excited complex with the charge-transfer state. Isomeric forms have been proposed to explain the mixed properties of intermediate systems such as perylene-monomethylamine. Anner et aL9 have described a similar behavior in 9-cyanoanthracene-2,5-dimethyl-2,4-hexadienecomplexes and attributed the decrease of the lifetime of the exciplex at higher vibrational excess energy to a rapid nonradiative decay that is consistent with the efficient photochemical reaction observed in solution. We report here a spectroscopic study of the complexes of 1and 2-cyanonaphthalene (1-CNN, 2-CNN) with 2J-dimethyl2-butene (DMB) and 2-methyl-2-butene (MB) formed in a supersonic expansion. Exciplex fluorescence has been observed in solution only for the 1-CNN-DMB pair whereas the other system are nonfluorescent.'O However, intermediacy of exciplexes have been postulated in the 2 + 2 photocycloaddition of these olefins to the naphthalene ring.3 Jet cooling may prevent nonradiative (6) Castella, M.; Tramer. A.; Piuui, F. Chem. Phys. Lrrr. 1986, 129, 105, 112. (7) Castella, M.; Millib, P.; Piuzzi, F.; Caillct, J.; Langlet, J.; Claverie, P.; Tramer, A. J. Phys. Chem. 1989, 93, 3949. (8) Anner, 0.;Haas, Y. Chem. Phys. Lcrr. 1985, 119, 199. (9) Anner, 0.;Haas, Y. Chem. Phys. L r r r . 1987, 137, 121. (10) Taylor, G. N. Chem. Phys. t o r r . 1971, 10, 355.
0 1991 American Chemical Society
3648 The Journal of Physical Chemistry, Vol. 95. No. 9, 1991
Lahmani et al.
(=)
0
-
190
-
100
-
300
cm"
Figure 1. Fluorescence excitation spectra of jet-cooled l-cyanonaphthalene complexes with (a) 2,3-dimethyl-2-butene and (b) 2methyl-2-butene. The l-cyanonaphthalenesamples were held at 80 'C, and 2.3-dimethyl-2-butene and 2-methyl-2-buteneat -50 and -70 'C, respectively.
decays such as internal conversion or cycloaddition, which are supposed to be activated processes, and thus allows observation of the emission from the exciplexes.
Experimental Section The supersonic free jet used in these experiments is formed by expanding through a 200-pm nozzle a flow of helium (3 atm of backing pressure) saturated with cyanonaphthalene and DMB or MB. The ethylenic compounds (liquid at room temperature) are contained in traps. Their concentration is controlled by varying the temperature of the trap. Cyanonaphthalenes (solid at room temperature) are contained in an oven and heated just before the expansion (at 80 OC for 1-CNN and 110 OC for 2-CNN). The excitation source is a KDP frequency-doubled dye laser (DCM) pumped by the first harmonic of a YAG laser (BM Industrie). The UV beam crosses the jet at right angle and is not focused to minimize saturation effects. Emitted fluorescence is collected 25 nozzle diameters downstream either directly or through a filter (Schott WG 345 or GG375) by a photomultiplier (RTC XP 2020) or through a Jobin Yvon 60-cm monochromator combined with an EM1 9568 BQ photomultiplier. The fluorescence signal is then digitized by means of a Camac-controlled multichannel analog/digital converter (Lecroy 2249 W) and processed through a homemade interface with an Olivetti PC XT microcomputer. The fluorescence decays were recorded by a 125-MHz oscilloscope (Lecroy 9400) working in the sweeping mode (200-ps resolution). The laser pulse width is about 10 ns, and the rise time of the photomultiplier is less than 3 ns. Results The fluoresxnce excitation spectra of pure 1-CNN and 2-CNN in the supersonic jet are in good agreement with previously reported data: the 0-0 transition of 1-CNN is at 31 400 cm-' and the first vibronic transitions at 404,454,510, and 610 cm-' above the origin.' The 2-CNN 0-0 transition is at 30874 cm-l, and the main vibronic transitions are found at 363, 478, and 557 cm-l
330
140
S50
SIC0
rgo SSo 390
400
nm
Figure 2. Emission spectra of jetaoled l-cyanonaphthalenecomplexes with (a) 2,3-dimethyl-2-butene(resolution 100 c d , excitation at 31 250 cm-I; the figure shows also the bare l-cyanonaphthalened level emission
(short-wavelengths spectrum) recorded under the same conditions), (b) 2-methyl-2-butene (narrow-band excitation at 3 1 304 cm-I), and (c) 2-methyl-2-butene (broad-band excitation at 31 300 cm-I). followed by a strong doublet at 709 and 722 cm-l above the origin. (1) Complexes of 1-CNN with Olefins. ( A ) 2,3-Dimethyl-2butene (DMB): Addition of 2,3-dimethyl-2-butene to the 1-CNN gives rise in the fluorescence excitation spectrum to a slightly asymetric broad absorption band peaking at 150 cm-' to the red of the 1-CNN 0-0 transition as shown in Figure la. The half-width of this absorption band is about 150 cm-I. Another similar broad band centered at 300 cm-' to the blue side of the 1-CNN 0-0 transition may correlate with three vibronic states of 1-CNN at 404,454, and 510 cm-I although no distinct maxima can be assigned to each transition. Figure 2a shows the emission spectrum obtained by exciting the broad absorption of the 1-CNN-DMB complex at -150 cm-' from the monomer origin together with the 1-CNN 00 level fluorescence. It appears as a broad structureless fluorescence red shifted with respect to the pure 1-CNN fluorescence with its maximum at 27 600 cm-' (370 nm) and a 3000-cm-' half-width. This emission spectrum is similar to that observed in hexane solution at room temperature and is thus attributed to the 1CNN-DMB exciplex. The measured decay time of 60 h 3 ns is largely lengthened with respect to the bare 1-CNN 00 level fluorescence lifetime (22 ns). This decay time does not depend on the excitation wavelength within the broad absorption band of the complex (-75, -1 50, and -225 cm-'from the 0-0 transition of the bare molecule). (B) 2-Methyl-2-butene (CH3)2C+CHCH3 (MB): The excitation spectrum of the 1-CNN-MB mixture shown in Figure l b exhibits in the red side of the monomer origin several narrow features superimposed on a broad band peaking at about -100 cm-' from the 1-CNN 0-0 transition. Dispersed fluorescence obtained by exciting either the narrow peak at -96 cm-' from the monomer origin or the continuous background absorption is shown in Figure 2b,c. In the case of the excitation of the narrow feature of the complex, a structured spectrum analogous to that of the
The Journal of Physical Chemistry. Vol. 95, No. 9, 1991 3649
Bimolecular Photochemical Intermediates
r
330
0
- AOO
cm-''
Figure 3. Fluorescence excitation spectra of jet-cooled 2-cyanonaphthalene complexes with (a) 2,3-dimethyl-2-butene and (b) 2-
methyl-2-butene. The Zcyanonaphthalene samples were held at 110 OC. bare 1-CNN 00 state is observed. This emission corresponds to the resonance fluorescence of the complex locally excited on the 1-CNN chromophore. In contrast, a continuous broad red-shifted fluorescence peaking at 354 nm results from the excitation of the broad absorption band of the 1-CNN-MB complex. The excitation spectrum in the region until 700 cm-I above the monomer 0-0 transition exhibits clearly the narrow features shifted by 96 cm-I toward the red of each vibronic band of pure 1-CNN (at 404,454, and 510 cm-I). The underlying broad continuum is much weaker and does not appear with appreciable intensity except in the region of the narrow peak correlating with the 404-cm-' vibrational level. Pumping the narrow peak of the complex which is associated with the 510-cm-I vibronic band of 1-CNN gives rise to a broad emission that may be resolved into two components: a blue emission band starting close to the energy of the 0-0 transition of the complex (322 nm) and a more redshifted one, peaking at 356 nm, similar to that obtained when pumping the broad absorption at 100 cm-' in the red of the 1-CNN origin. ( 2 ) Complexes of 2-CNN with Olefins. ( A ) 2,3-Dimethyl-2butene (DMB): The excitation spectrum of 2-CNN-DMB complex in the region of 2-CNN origin is shown in Figure 3a. It appears as a single band with a congested fine structure shifted from the 2-CNN 0-0 transition by 128 cm-'. A similar structure is observed at 127 cm-' to the red side of the strong vibronic band of pure 2-CNN at 722 cm-' above the origin. Complex absorption associated with the weaker vibronic bands at 363,478, and 557 cm-' does not show up clearly in the excitation spectrum. Dispersed emission obtained by exciting either the features at -128 cm-' to the red of the 0-0 transition or of the 722cm-' band of 2-CNN differs strongly from each other as seen on Figure 4. While fluorescence from the complex 0-0 band pumped at 30672 cm-I appears as a nonresonant broad continuum peaking at 28000 cm-' (356 nm), typical of an exciplex emission, the fluorescence from the complex excited with 722-cm-' excess internal energy is located at higher energy and exhibits two main wide peaks at
340
350
~
-~ 360
3+0
380
nm
Figure 4. Emission spectra of jet-cooled 2-cyanonaphthalenecomplexes with 2,3-dimethyl-2-butene: (a) excitation in the 0 transition region at 30 746 cm-I; (b) excitation in the 0 + 722 cm-l transition region at 31 468 cm-l. Experimental resolution is 80 cm-I.
30600 (327 nm) and 29 200 cm-'(342.5 nm). This fluorescence corresponds to 2-CNN broadened 00 level type fluorescence. This behavior is usually observed when rapid intermolecular vibrational redistribution takes place from the optically excited complex toward the dense manifold of intermolecular vibrational levels. The fluorescence decay times are also very different: in the former case the measured lifetime (89 ns) is longer than that of the 00 state of pure 2-CNN (79 ns), while in the latter case the lifetime (57 ns) is shorter than the fluorescence lifetime of the 722-cm-I level of pure 2-CNN (78 ns). Vibrational predissociation of the complex with 722-cm-' excess energy can be ruled out since it would have led to much narrower emission spectra and longer lifetime from the free 2-CNN fragment produced in the 00 level. (B) 2-Methyl-2-butene (MB): The excitation spectrum of 2-CNN-MB complex (Figure 3b) is composed of a system of closely spaced narrow bands starting at -128 cm-' from the bare 2-CNN 0-0 transition. The most intense bands are at -96, -55, and -33 an-'.The observed intensity distribution indicates a slight difference between equilibrium configurations in the ground and excited potential curves of the complex. A similar band system is repeated in the red side of the intense 722-cm-l band of pure 2-CNN. The fluorescence spectrum obtained when pumping the strongest feature at -96 cm-' from the 2-CNN 0-0 band reproduces the vibrational structure observed in the 2-CNN 00 level fluorescence. It corresponds thus to a resonant emission. Each of the 2-CNN vibrational bands appears to be broader (70 cm-')than the spectral bandwidth (30 cm-I). This is easily explained by the overlap of the intermolecular fine structure analogous to that observed in the excitation spectrum, which cannot be resolved under the experimental conditions. The fluorescencespectrum obtained when pumping the complex with 722-m-* internal energy in the 2-CNN chromophore is composed of three wide peaks at 30 500,29 100, and 27 700 cm-'showing the 14OO-cm-' progression typical of the 2-CNN Oo level emission spectrum. This spectrum is thus the result of the fast vibrational redistribution of the excess energy among the isoenergetic intermolecular levels that induces a broadened 00 level type emission. This emission is similar to that observed in the case of the 2-CNN-DMB complex excited in the 722-cm-' vibronic level of 2-CNN.
3650 The Journal of Physical Chemistry. Vol. 95, No. 9, 1991
Lahmani et al.
TABLE I: Summuv of the W o t d v d d h r t i e s of the CY^^^^ Compkxes IP, eV excitation acceDtor 0 donor (ref 11) red shift, cm-' I-CNN, 0-0 = 31 400 cm-I, E A = 0.68 eV (ref 12)
T
= 22 ns,
ccLI
I-CNN, 0-0
0-0 + 510 cm-'.
+ 122 cm-l,
H
8.72
ccLI>=L
T
= 47 ns
2-CNN, 0-0 = 30874 cm-I, EA = 0.65 eV
0-0
8.26
T
T
= 79 ns,
8.26
= 78 ns 8.72
2-CNN, 0-0
0-0 + 722 cm-l
-150 (max), diffuse
fluorescence type hv max
T,
ns
exciplex, 27000 cm-I (370 nm)
60
resonant
35
-104 (max), diffuse -96 (narrow)
exciplex, 28200 cm-I (354 nm) IVR + exciplex
54 40
-128 (congested)
exciplex, 28000 cm-l (356 nm)
89
-128 (congested)
IVR
57
-96 (max) (narrow)
resonant
64
-94 (max) (narrow)
IVR
60
-96 (narrow)
In the corresponding adiabatic representation (strong coupling case), the molecular wave function of the excited state is given by ~p = cl(p(A*D) c2dA-D')
+
EMISSION
1
I 1 LOCALLY
EXCITE0 STATE
Figure 5. Schematic representation of potential energy curves involved in the exciplex formation.
The fluorescence decay times resulting from the excitation of the most intense features at -96 and -55 cm-' from the monomer 0-0 transition are 63 ns, and the decay time measured by pumping the 722-cm-' vibration of 2-CNN in the complex is 60 ns. In both cases the fluorescence lifetime is shortened with respect to the bare 2-CNN molecule as usually observed in van der Waals complexes.
Discussion The energetic and dynamic properties of the jet-cooled complexes of cyanonaphthalenesobtained in this work are summarized in Table I. ( 1 ) Characterizationof Exciplexes and Locally Excited Stares. From Table I it can be seen that exciplex emission have been observed in jet conditions for the 1-CNN-DMB, 1-CNN-MB, and 2-CNN-DMB pairs. The identification of the exciplex formation in the excited complexes rests mainly on the fact that the emission spectra of such species are broad, lack of vibrational ' type structure, and are at lower energy than the monomer 0 fluorescence of the van der Waals complex. The mechanism generally accepted to account for the exciplex formation in jet-cooled conditions- is based on the following nonadiabatic description (Figure 5): (i) Franck-Condon excitation of the ground state of the van der Waals complex formed in the supersonic expansion to the locally excited state of the absorbing component of the complex (A*D). (ii) The crossing of the zeroorder (A*D) state with a nearby charge-transfer state (A-D+) induces the transition to the isoenergetic vibrational modes of the (A-D+) state, which deactivates radiatively by vertical transitions toward the repulsive part of the ground-state surface. (1 1) Frost, D.C.;Senthu, J. S. Indian J. Chem. 1971,9. 1105. (12) Chowdhury, S.;Kebarle. P.J . Am. Chem. Soc. 1986, 108, 5453.
Depending on the relative value of the mixing coefficients cI and c2,the exciplex state may contain more or less chargetransfer character, and since these mixing coefficients should depend on the nuclear coordinates, the correspondingpotential hypersurface may possess several local minima along the different intermolecular coordinates. Thus two factors are important to consider in the exciplex formation process. They are respectively the relative position of the (A*D) and (A-D+) states, for their equilibrium configuration, Le., the overall free energy difference between the initial and the final states, and the Franckxondon weighted electronic coupling matrix element between the two states. The different behavior observed in the four cyanonaphthalene-olefins complexes with respect to exciplex formation may be rationalized on the basis of the energy gap A€ between zero-order (A*D) and (A-D') states as described by Castella et al.' The 2-CNN-MB and the 1CNN-DMB complexes illustrate two extreme situations. (a) The lowest excited state is essentially of the (A'D) type (LE state): this case is observed here in 2-CNN-MB system. The ground and first excited states correspond respectively to the So and SIstates of the 2-CNN monomer weakly perturbed by van der Waals interactions. This perturbation results in the appearance of a system of narrow bands, due to intermolecular modes of the complex and associated with each vibronic transition of the monomer. (b) The lowest excited state is essentially of the (A-D+) type state, (CT state): the excitation spectrum still corresponds to the LE state reached in the Franck-Condon window determined by the ground-state van der Waals geometry, but strong coupling with the nearby CT state broadens the vibronic bands of the perturbed chromophore. The emission has the character of an exciplex fluorescence. This situation is illustrated here in the case of 1-CNN-DMB complex. When the (A*D) and (A-D+) states are close in energy, several intermediate behaviors can be observed, and excess energy in the (A*D) state may influence strongly the efficiency of exciplex formation. In this view, one can stress the contrasted behavior observed here in the case of 1-CNN-MB and of 2-CNN-DMB. In the case of the 1-CNN-MB complex, the superposition of a narrow-band excitation spectrum associated with resonance fluorescence with a broad-band excitation spectrum resulting in exciplex type fluorescence is observed in the region of the (HI transition. This behavior has already been described by Castella et al.6*7in the perylene-monomethylaniline and anthracene-dimethylaniline systems and by Anner et al.I3 in anthraceneanisole
Bimolecular Photochemical Intermediates
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3651
and diethyl ether complexes. The dual properties of these complexes have been interpreted by the presence of two isomeric conformations in the jet-cooled van der Waals complexes. One isomer behaves in the excited state as a van der Waals adduct, and the other gives rise to exciplex emission. There is an energy barrier that prevents conversion between the two forms in the 0" level. The different photophysical properties of the two isomeric forms reveals the importance of the intermolecular geometry to promote the electron-transfer process. A similar interpretation may apply to the 1-CNN-MB complexes where different ground-state conformers may be present in the jet because of the slightly polar character of the MB molecule. In fact, isomeric forms have been unambiguously identified in the case of the complexes of 1-CNN with polar solvent molecule^.'^ When the 1-CNN-MB van der Waals type isomer is excited with 510-cm-' vibrational energy in the (A*D) state, one obtains a double-shaped fluorescence that can be separated into two components: a "blue" emission peaking at the energy of the 0-0 transition of the locally excited and a "red" component similar to that attributed to the exciplex fluorescence. The "blue" emission corresponds to the relaxed 1-CNN 0" emission expected after rapid intramolecular vibrational redistribution (IVR) in the LE manifold. This result shows that fast IVR occurs prior to radiative decay. As both LE and CT states emission are observed, the rate for IVR into the dense manifold of LE and CT are of the same order of magnitude. In a second step, radiative decay takes place by Au = 0 transitions from both LE and CT states. The 2-CNN-DMB system presents a very peculiar behavior in both absorption and emission: (a) The excitation spectrum exhibits a single dense band composed of about 3-cm-' fine structure distributed over a 10-cm-' global envelope. This spectral shape differs strongly from that observed for the 2-CNN-MB, which shows a narrow-band structure extending over about 100 cm-', and from the 1-CNNDMB complex, which appears as a broad continuum of about 150-cm-' half-width. Comparison of 1-CNN with 2-CNN complexes with the same DMB donors shows that substitution of naphthalene with the CN electron-acceptor group in the 1 or 2 position influences strongly the exciplex formation. These results indicate that the complexes involving either 1-CNN and 2-CNN may have different equilibrium configuration that may affect the energy gap and the mixing between zero-order states. It should be noted that a similar band shape was previously seen in the excitation spectrum of benzonitrilebenzene complex.1s The single congested 0-0 feature of this complex indicates a similar geometry of the ground and excited states. The width and the fine structure of the 0-0 band of the 2-CNN-DMB may result from the coupling of the LE state with a sparse density of the almost isoenergetic charge-transfer (CT) state as previously suggested by Castella et al.7 according to the intermediate case treatment currently used in the theory of radiationless transitions. The limited number of coupled levels may be due to a small density of states in the CT state because of a small energy gap between the LE and CT states. Another explanation may be that only specific intermolecular modes from the bath are active for the coupling process. (b) While exciplex emission is observed for the complex excited at origin, excitation of the 722-cm-' vibrational level leads to a relaxed fluorescence that keeps the character of the locally excited 2-CNN. This striking result indicates that, while the electrontransfer process is faster than resonance fluorescence from the Oo level of the complex, it does not compete with the vibrational redistribution from the locally vibrationally excited states. It can be deduced consequently that in this system electron transfer and IVR are not sequential processes but rather competitive pathways. There is thus a strong coupling between (A*D) and (A-D+) zero-order states at the origin, but when internal energy is deposited in the 2-CNN moiety, IVR populates intermolecular bath
modes in the LE manifold which preferentially decay radiatively toward the ground state by numerous Au = 0 transitions (Figure 5 ) . T h e observations imply also that there is no efficient coupling between the intermolecular vibrational levels populated by IVR in the LE state and the exciplex state. This unusual behavior may be explained qualitatively by invoking the intermediate case of radiationless transitions as previously suggested by the narrow-band structure of the excitation spectrum. If, in this case,only a very few levels of the CT manifold are coupled with the initially locally excited state and assuming that the electronic part of the coupling matrix element is not modified at higher energy, the decrease of exciplex emission intensity may be due to the decrease of the vibrational overlap as expected when the energy gap between the coupling states is increased. A recent formalism developed by Deperasinska and Prochorow to describe the electron-transferp r ~ c e s s ~may ~ * be ' ~ used to explain the unusual results observed in the 2-CNN-DMB complex. The formalism is based on the potential curve crossing approach using the Landau-Zener theory. In this approach, the crossing point between the van der Waals excited state and the ionic potential energy surface is decisive to determine the rate constant for the electron-transfer process. This crossing point is defined by the shape of the potential energy surfaces. The electron transfer takes place between discrete vibronic states liv) of the initial LE state (i.e., prior to IVR) and discrete states Jfw) of the final CT state. If the charge-transfer curve crosses the van der Waals state at a shorter intermolecular distance than the equilibrium distance of the van der Waals LE curve, the electron-transfer probability is calculated to depend strongly on the initially excited vibrational level. The 2-CNN 722-cm-' frequency may be assigned to an in-plane totally symmetric vibration of the naphthalene ring by analogy with other 2-substituted naphthalene compounds.'* Thus, the coordinate corresponding to this mode and the reaction coordinate involved in the exciplex formation process may not be mixed. This could explain that the crossing point may be different for the 0-0 level and for the 722-cm-' level (or for the isoenergetic dark levels that are coupled to it by IVR). This difference between the crossing distances could explain the differences in the rate for exciplex formation from these two levels. (2) Energetics and Dynamics of the Jet-Cooled Exciplexes. Comparison with solution experiments: As already mentioned, exciplex emission of cyanonaphthalene-olefinssystems is observed in the jet experiments for the 1-CNN-DMB, 1-CNN-MB, and 2-CNN-DMB complexes. The only exciplex emission known for these pairs in nonpolar or weakly polar solutions has been evidenced in the l-CNN-DMB system. The spectral shift between the 0-0 transition of the monomer and the maximum exciplex emission is -4700 cm-' in hexane solution and 4400 cm-' in the jet-cooled complex. In solution, the exciplex fluorescence originates from the vibrationally relaxed CT state, and the spectral shift represents the sum of the stabilization energy of the exciplex with respect to the monomer excited state and of the repulsion energy in the ground state (AHH, + AHg, Figure 5). In the jet conditions, the exciplex fluorescence originates from the large number of isoenergetic heat bath states of the CT state populated by intramolecular vibrational redistribution from the initially excited LE state. The observed emission results then from the numerous Au = 0 transitions which have favorable Franck-Condon factors to the ground state and are similar to the fluorescence observed in condensed phase as described by Russell and Levy.Ig This mechanism explains thus why the exciplex energetics do not change strongly whether the exciplex is generated by selective excitation of the jet-cooled complex or from the quenching of the monomer excited state in nonpolar medium. The smaller spectral shift measured in the jet-cooled 1-CNNMB and 2-CNN-DMB complexes 3200 and 2850 cm-l respec-
(13) Anner, 0.; Haas, Y. J . Am. Chem. Soc. 1988, 110, 1416-1423. (14) Lahmani, F.; Zehnacker-Rentien, A.; Brthtret, E. Submitted for publication to J. Phys. Chem. (1 5) Lahmani, F.; Lardeux-Dedonder, C.; Zehnacker-Rentien, A. h s e r
(16) Deperasinska, I. Chem. Phys. 1988, 120,359. (17) Deperasinska, I.; Prochorow, J. Chem. Phys. Lerf. 1989, 163, 257. (18) Jacobson, B. A.; Guest, J. A.; Novak, F. A.; Rice, S. A. J . Chem. Phys. 1987, 87,269. (19) Russel, T. D.; Levy, D. H. J. Phys. Chem. 1982,86, 2718.
Chem. 1989, 10,41.
3652 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 tively indicates that exciplex emission could have been difficult to evidence in solution because of overlap with monomer emission. This smaller shift indicates also a smaller stabilization energy of these exciplexes with respect to the monomer Oo level emission. Thus in both complexes the LE and CT states are probably nearly isoenergetic and strongly interacting. In the case of 1-CNN complexes, the comparison of MB with DMB donor shows that an increase of the IP of about 3600 cm-' l 1 induces a shift of the exciplex emission of only 1200 an-'.There is not thus a simple correlation of the CT state energy with the olefin ionization potential as already mentioned by Ware et Furthermore, the assumption of two isomeric forms leading to different behaviors in the excited state implies that the relaxation toward exciplex depends on the geometric arrangement of the two partners in the ground-state van der Waals complex. The 2-CNN-olefins systems do not exhibit exciplex emission in solution, although the intermediacy of exciplexes has been postulated to explain the quenching of the monomer and the formation of cyclobutane photoadduct.21,zz This behavior has been related to the slightly higher reduction potential of 2-CNN relative to 1-CNN, which causes a smaller stabilization energy of the exciplex.z' However recent measurements of the gas-phase electronic affinity of both 1-CNN and 2-CNN give very close values of respectively 0.68 and 0.65 eV.IZ Thus the change in electronic affinity cannot be responsible for the decrease of bonding energy of the 2-CNN-olefin exciplex, but more probably the difference between 1-CNN and 2-CNN may be due to a different geometry of the exciplex as also evidenced in jet conditions. It has also been suggested that because of the smaller stabilization energy, radiationless transitions toward cycloaddition product or ground-state reactants may be more efficient from 2-CNN exciplexes than from 1-CNN ones. In the theoretical model proposed by MichIz3for 2 2 cycloadditions, the exciplex evolves along the reaction coordinate through an energy barrier to a second so-called pericyclic minimum, due to the crossing of the doubly excited state with the ground states of the reactants and products, from which deactivation or chemistry takes place. Increasing the vibrational energy of the exciplex may induce a quenching of the exciplex fluorescence by favoring the crossing over the energy barrier toward the nonradiative pathways and decrease simultaneously the intensity of higher vibronic bands in the fluorescence excitation spectrum. This is obviously not observed since the relative intensity of the 722-cm-l band of the 2-CNN-DMB
-
+
~~~
(20) Ware, W. R.; Watt, D.; Holmes, J. D. J . Am. Chem. Soc. 1974,96, 1853. (21) McCullough, J.; Miller, R. C.; Fung, D.; Wu, W. S.J . Am. Chem. Soc. 1975, 97, 5942. (22) Pac, C.; Yasuda, M.; Shimo, K.; Sakurai, H. Bull. Chem. Soc. Jpn. 1982, 55, 1605. ( 2 3 ) Michl, J. Phofochem.Photobiol. 1977, 25, 141.
Lahmani et al. complex with respect to the corresponding band of the bare molecule is of the same order as that of the 0 transition. It may be concluded that the energy barrier to the product pathway is higher than -700 cm-I. Moreover, exciplex formation seems to be prevented from higher vibronic states of the LE van der Waals complex since the dispersed fluorescence is mainly of LE 2-CNN type. The different stabilization energy and behavior of the 1-CNN-DMB and 2-CNN-DMB exciplexes are relatad more probably to a different geometry of the two complexes in their CT state. Exciplex decay: The exciplex decay rate constants have been measured in solution for 1-CNN-DMB exciplex in various solvent~.~' In n-hexane and benzene the decay depends on the temperature since the formation of the exciplex is reversible. Q,,, = 3.75 ns in n-hexane at room temperature and $qim = 0.05 at DMB limiting concentration for which 96% of the monomer emission is quenched. The radiative rate constant of the exciplex is thus kf= 1.33 X lo7s-'. The decay rate of the exciplex emission measured in the supersonic expansion is 1.66 X lo7 s-l ( T = 60 ns) very close to the value of kfin solution, indicating a very high quantum yield for the exciplex fluorescence in the jet condition.
Conclusions The photophysical properties of jet-mled complexes of 1- and 2-cyanonaphthaleneand 2,3-dimethyl-2-butene and 2-methyl-2butene have been examined in order to have informations upon the exciplexes postulated as intermediatesin the photocycloaddition of these systems. Exciplexes that are not observed in solution at room temperature have been identified from the fluorescence spectrum in the case of 1-CNN-MB and 2-CNN-DMB systems. The excitation spectrum of the jet-cooled adducts keeps the character of the cyanonaphthalene chromophore showing either narrow or broadened features in the red side of the monomer main transitions. It is thus attributed to the transitions arising from Franck-Condon excitation of the van der Waals ground state to the locally excited state. The coupling of the LE states with lower energy ionic states induces relaxation toward the exciplex state geometry. The 2-CNN-DMB complex exhibits a drastic modification of the nature of emission following the excitation of the internal vibrational level of the 2-CNN chromophore at 722 cm-I. The fluorescence which was characteristicof an exciplex state at origin becomes typical of the locally relaxed 2-CNN singlet state. This result evidences a selectivity in the exciplex formation efficiency with the intramolecular excited vibration, which should be confirmed with further experiments. (24) McCullough,J. J.; McInnis, W. K.; Lod, C. J. L.; Faggiani, R. J. Am. Chem. Soc. 1982, 104,4644.
