7580
J . Phys. Chem. 1991,95, 7580-7584
Charge-Transfer Interactions in 1-Cyanonaphthaiene van der Waals Complexes with Aliphatic Amines: Dependence of Excited-State Dynamics on Donor Ionization Potential Hiroyuki Saigusa* and Edward C. Limt Department of Chemistry, The University of Akron, Akron, Ohio 44325-3601 (Received: March 1 1 , 1991)
The exciplex formation reaction that occurs within the initially excited van der Waals complexes of I-cyanonaphthalene with a variety of aliphatic amines has been studied as a function of the ionization potential (IP) of the amines and the e x w s vibrational energy of the electronically excited complexes. In all cases, the fluorescence excitation spectra are composed of only narrow-band features originating from local excitations of the acceptor chromophore. While complexes involving amines of high IP are inefficient in producing exciplexes, those with amines of low IP give rise to exciplex fluorescence from the charge-transfer state. The tributylamine complex, unlike the others, undergoes the exciplex formation upon excitation into its S,origin. These results are interpreted in terms of the crossing of the initially excited van der Waals state and the charge-transfer state.
Introduction Chemical reactions that occur between the constituents of van der Waals (vdW) complexes upon photoexcitation or photoionization have been a subject of considerable interest. The essential features of such reactivities may be explained by assuming that the initially prepared electronic or ionic states of the complex interact with the product states. However, these reactive vdW pairs often provide us with complicated reactive potential energy surfaces, leading to a rich variety of products, which are difficult to understand. Our efforts have been focused on pure and mixed clusters containing aromatic molecules, where product states are located substantially lower than the ionization or dissociation thresholds of the component molecules. Among such vdW-initiated photochemical reactions, charge-transfer (CT) complex formation that takes place between a pair of electron donor (D) and acceptor (A) molecules is a particularly well-studied reaction.' In previous p a p e r ~ , ~Saigusa J et al. reported that exciplex formation of 1cyanonaphthalene (CNN) with triethylamine (TEA) can proceed under collision-free conditions, analogous to that occurring in the static vapor phase,4 when the corresponding vdW complex is excited into its lowest excited singlet (SI) state. The term exciplex was introduced to describe the CT complex that is stable only in the excited state, to distinguish it from the initially excited vdW complex.5 The discrete nature of the excitation spectrum of exciplex fluorescence was rationalized in terms of the coupling between the initially excited locally excited state A*D and the low-lying CT state A-D+. The efficiency of exciplex formation appeared to be strongly dependent on the excess intramolecular as well as intermolecular vibrational energy of the A*D state. These results and related studies on other suggest that such exciplex formation is common for weakly bound vdW systems comprising a pair of electron donor and acceptor molecules. However, in contrast to studies of exciplexes in solutions8 and in the vapor phase,9 systematic probes of the dependence of exciplex formation on the ionization potentials (IP) and electron affinities of the constituent molecules do not yet appear to exist for jet-cooled vdW complexes. This paper describes the results of supersonic jet studies on the vdW complexes of C N N with a series of aliphatic amines, which are designed to examine the effect of donor ionization potentials on the efficiency of exciplex formation. For amines of high IP, the lowest excited state of these vdW complexes is essentially an excited state of A (viz., CNN) perturbed weakly by the vdW interaction with D, Le., A*D. With decreasing ionization potential of amine, we anticipate that a CT state corresponding to an electron-transfer configuration A-D+ 'Inaugural holder of the Goodyear Chair in Chemistry at the University of Akron.
0022-3654/9 1/2095-7580$02.50/0
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is stabilized and located below the A*D state. Thus, the exciplex formation A*D A-D+ may take place following the AD A*D excitation. The observation that the fluorescence excitation spectrum of the CNN-TEA exciplex consists of narrow-band features, of line width =1 cm-l, has an important implication on the rate of exciplex formation. Previous investigations on other donor-acceptor system^^*'*'^^'^ have yielded line widths of the excitation spectra that are between 20 and 200 cm-I. This severe line broadening has been attributed to ultrafast exciplex formation, which occurs at rates of the order 1013-1014s-I. In view of this discrepancy, it would also be important to study a possible dependence of the line width on the ionization potential of the donor molecule. The fluorescence excitation and dispersed fluorescence spectra of the complexes of C N N with NH3, dimethylamine (DMA), diethylamine (DEA), trimethylamine (TMA), tripropylamine (TPA), and tributylamine (TBA), we describe here, address both of these issues.
Experimental Section The vdW complexes of C N N with these amines were synthesized in pulsed supersonic expansions. A detailed description of the experimental setup for measuring fluorescence excitation and dispersed fluorescence spectra has been given previously.i2 The carrier gas of He at 4 atm was introduced into a stainless steel reservoir containing amine. The gas mixture of the donor and He was led to a nozzle in which the CNN sample was placed and heated to =IO0 "C. The most important procedure in this experiment was to carefully control the concentration of the donor, so as to prevent formation of higher order aggregates, which ( I ) For a recent review, see: Haas, Y.;Anner, 0. In Photoinduced Electron Transfer; Fox, M. A,, Chanon, M., Eds.; Elsevier: New York, 1988; Part A. (2) Saigusa, H.; Itoh, M.Chem. Phys. Left. 1984, 106, 391. Saigusa, H.; Itoh, M. J. Chem. Phys. 1984, 81, 5692. (3) Saigusa, H.; Itoh. M.; Baba, M.; Hanazaki, 1. J. Chem. Phys. 1987, 86. 2781. (4) Chewter, L.; OConnor, D. V.; Phillips, D. Chem. Phys. Lett. 1981,84, 39. OConnor, D. V.; Chewter, L.; Phillips, D. J. Phys. Chem. 1982,86,3400. ( 5 ) An exciplex has been defined originally as an excited-state complex of definite stoichiometry, which is dissociative in the ground state: Molecular Luminescence; Lim, E. C., Ed.; Benjamin: New York, 1969. (6) Castella, M.; Prochorow. J.; Tramer, A. J. Chem. Phys. 1984.81. 251 I . (7) Anner, 0.;Haas, Y. Chem. Phys. Lett. 1985, 119, 199. (8) Birks, J . B. Photophysics ofAromatic Molecules; Wiley: New York, 1970; pp 403-491. (9) Hirayama, S.; Abbott, G. D.; Phillips, D. Chem. Phys. Lett. 1978,56, 497; Hirayama, S.; Phillips, D. J. Phys. Chem. 1981, 85, 643. (IO) Castella, M.; Tramer. A,; Piuzzi, F. Chem. Phys. Leu. 1986. 129, 105. ( I I ) Castella, M.; MilliB, P.; Piuzzi, F.; Caillet, J.; Langlet, P.;Claverie, P.; Tramer, A. J. Phys. Chem. 1989, 93. 3949. (12) Saigusa, H.; Lim, E. C. J. Chem. Phys. 1983, 78, 91.
0 1991 American Chemical Society
1-Cyanonaphthalene Complexes with Aliphatic Amines
1
d,
1
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7581
t
CNN-TEA ,t14
i
4 00
-20
0
20
40
60
n
n
-I
I
1
I
1
1
500
600
Wavenumber/cm
1
700
-I
Wavenumber/cm Figure 1. Fluorescence excitation spectra of CNN complexes with various amines in the origin-band region of uncomplexed CNN: (a) CNNNH,; (b) CNN-trimethylamine (TMA); (c) CNN-diethylamine (DEA); (d) CNN-triethylamine (TEA); (e) CNN-tripropylamine (TPA). The frequency shifts are with respect to the origin band of uncomplexed CNN at 31 414 cm-'.
Figure 2. Fluorescence excitation spectra of CNN complexes with (a) NH3, (b) DMA, and (c) DEA in the excess energy range between Y, and v4. The vibrational frequency shown represents the energy shift from the origin band of each complex. Each spectrum was normalized to the intensity of the origin band, which is nearly half of this scale. The truncated features ocurring at 403,450,510, and 661 cm-l are assigned to vibronic bands ( v I through v4) of the CNN monomer.
interfere with the desired 1:l donor-acceptor complex. In order to obtain the vapors of NH,, DMA, and TMA, solutions of these compounds dissolved in water were used as reservoirs. The vapors of other amines were obtained from the corresponding liquid donors. Under the experimental conditions employed, the optimum temperatures of the amine reservoir were found to be -50 OC for N H 3 / H 2 0 and DMA/H20, -60 OC for DEA, 0 "C for TMA/ H20, 25-30 OC for TPA, and 9&100 OC for TBA. All chemicals were purchased from Aldrich and used without further purification. The AD complexes thus formed in the ground electronic state were excited by the second harmonics of a YAG-pumped dye laser (Quanta Ray DCR-I /Lambda Physik FL2002). The fluorescence excitation spectra were recorded by detecting either the total fluorescence (>320 nm) through a glass filter (UV-34) or the exciplex fluorescence (>420 nm) through a low-fluorescence filter (LF-418). The dispersed fluorescence spectra were obtained by scanning a 0.22-m double monochromator (SPEX 1680) at a fixed laser frequency. The fluorescence decay curves were measured by an oscilloscope (Tektronics I I402 with a 400-MHz amplifier plug-in unit).
It should be noted from Figure 1 that the excitation spectra of the complexes with DEA, TMA, and TPA exhibit weak satellite peaks, which are blue shifted, respectively, by 9.3, 9.5, and 12.8 cm-l from the corresponding origins. The intensity ratio of the satellite feature to the corresponding origin is independent of the concentration of the amine, indicating that higher order clusters are not responsible for the appearance of the satellite peaks. As demonstrated previously for the CNN-TEA complex, the satellite peak is more likely due to intermolecular vibrations between C N N and amine. The weak spectral features observed in the CNN-NH, and CNN-TMA systems are, on the other hand, assigned to the 1:2 complexes based on the dependence of their relative intensities on the amine concentration. Our previous investigation on the CNN-TEA system2 demonstrated that exciplex formation proceeds when the complex is excited into its SI vibronic levels of the C N N chromophore with excess energies of >400 cm-I. With NH3as a partner, no exciplex formation takes place up to excess energies of .=3000cm-I. The excitation spectrum of the CNN-NH, system, shown in Figure 2, consists of vibrational features that are typical of local excitations of the CNN moiety. For the sake of discussion, the intramolecular vibrations with frequencies, 402,451, 509, and 662 cm-I in the CNN-NH3 excitation spectrum are denoted uI to v4,13 in the order of increasing vibrational energy. The dispersed fluorescence spectra from these vibronic levels can be attributed either to the fragment A* generated by vibrational predissociation (for the 662-cm-l excitation) or to the background levels of the A*D state produced by vibrational relaxation (for other excitations). This clearly indicates that the CT state of the CNN-NH,
Results The fluorescence excitation spectra of the CNN complexes with different amines in the SI origin regions are shown in Figure I . For all donors, a well-resolved spectral feature appears in the vicinity of the origin band of the uncomplexed C N N molecule at 31 414 cm-I, which can be assigned to the origin band of the corresponding 1:1 complex. In going from NH3 to TPA (following the order of increasing size of the donor molecule), the origin band shifts gradually from the blue to the red side of the monomer origin. Thus, the origin of the CNN-NH3 complex is blue shifted by 36 cm-I from the monomer, while that of the CNN-TPA complex is red shifted by 7.4 cm-I. The dispersed fluorescence spectra from the origin band of each of these complexes can be unambiguously assigned to resonance fluorescence from the initially excited vdW state A*D.
( I 3) The vibrational features of the C N N monomer could be analyzed in terms of mode-mixing of the naphthalene vibrations. For vibrational assignments on substituted naphthalenes, see Jacobson, B. A,; Guest, J. A.; Novak, F. A.; Rice, S . A. J. Chem. Phys. 1987, 87, 269. (14) Klasinc. L.; Kovac, B.; Giisten, H.Pure Appl. Chem. 1983, 55, 289. (15) Watanabe, K.;Nakayama, T.; Mottle, J. J . Quant. Spectrosc. Radiat. Transfer 1962, 2. 369. (16) Nakajima. A.; Akamatu, H. Bull. Chem. SOC.Jpn. 1969,42, 3030.
7582 The Journal of Physical Chemistry, Vol. 95, No. 20, 1991
Saigusa and Lim
I I 0;
I
1402
662
CNN-TBA
1 / i , j e t . > 320 nm
- 150
- 1m
-50
1I
CNN
-'
0
Wavenumber/cm Figure 4. Fluorescence excitation spectra of the CNN-TBA system in the origin-band region of uncomplexed CNN. Fluorescence was detected at >420 nm (top) and at >320 nm (bottom). The labels mark the origin transitions of the CNN monomer and the CNN-TBA complex. A weak satellite feature blue-shifted by 8.0 cm-' from the complex origin is assigned to an intermolecular vibration of the complex, while other sharp
features, marked by asterisks, are due to the CNN dimer.
1
I
1
I
500
600
700
1
400
Wavenumber/cm
-'
Figure 3. Fluorescence excitation spectra of (a) uncomplexed CNN and CNN complexed with (b) TMA, (c) TEA, (d) TPA, and (e) TBA. Fluorescence was detected at >320 nm in (a) and at >420 nm in (b)-(e). The energy represents the displacement relative to the origin band of each complex, shown in Figure I .
system is located above the initially excited state A*D. For the complexes of DMA and of DEA, an increase in excess vibrational energy leads to a significant quenching of A* and AID fluorescence. The excitation spectrum of the CNN-DMA system in Figure 2b exhibits weak spectral features that can be assigned as uI (402 cm-I), u2 (452 cm-I), and v3 (509 cm-I) of the complex. For the CNN-DEA system, we observed only one vibrational feature at 401 cm-' above the origin (Figure 2c). Thus, the high-lying vibrational features of these complexes appear significantly weaker in intensity than the origin band, suggesting the possibility of exciplex formation as the origin of the fluorescence quenching. However, in both complexes, we failed to observe any red-shifted fluorescence due to exciplex regardless of which vibronic level is excited. Excitation of the CNN-TMA and CNN-TPA complexes at excess energies of >400 cm-' leads to exciplex formation. This can be demonstrated by the fact that only the spectral features due to the complexes are seen in the excitation spectra of the exciplex fluorescence detected at >420 nm. Thus, the electronically excited vdW complexes of these systems undergo formation of the exciplexes, which have significant radiative transition probabilities. In Figure 3, the excitation spectra of the exciplex fluorescence of the CNN-TMA and CNN-TPA systems are compared with the corresponding excitation spectrum of the CNN-TEA system. It is evident that all the spectra are composed of narrow bands, whose line widths are determined to be 0.7 cm-' by using etalon scans. The frequency shifts of the vibrational features with respect to the origin band correspond closely to those in the uncomplexed CNN. This result is consistent with the earlier conjecture* that the initially excited state of these systems is essentially an unperturbed A*D state and the exciplex is formed A-D+ process. only through the A*D Another interesting observation in Figure 3 is that the exciplex formation appears to be mode-selective with respect to excitations of intramolecular vibrations of the complex. For example, the spectrum of the CNN-TMA exciplex gives only a weak, broad spectral feature at 453 cm-' above the origin, suggesting that the AID A-D+ process is inefficient with respect to other relaxation
-
-
-
processes, such as A*D AD. Mode specificity in the excitation of a combination band involving intramolecular and intermolecular modes has been demonstrated previously for the CNN-TEA exciplex formation. The situation changes drastically when TBA is used as the donor. The fluorescence excitation spectra in the origin-band region of the CNN-TBA system are shown in Figure 4. A well-resolved transition which occurs at -1 30.4 cm-', with respect to the uncomplexed CNN, is identified as the origin band of the complex. In striking contrast to the complexes discussed earlier, the origin band of the complex is observed in the excitation spectrum of the exciplex fluorescence, detected at >420 nm. This spectral feature is completely absent when only A*D fluorescence is detected in the spectral range of 320-360 nm. This observation indicates that the exciplex formation proceeds effciently even when the vdW complex is excited to the vibrationless level of SI.The excitation spectrum of the exciplex fluorescence for the CNNTBA system is shown in Figure 3e. Vibronic features involving intramolecular excitations of the C N N chromophore are observed at 401, 444, 510, and 660 cm-' above the origin. The intensity distribution of these features, including its origin, is almost identical to that of the uncomplexed CNN, indicating that the quantum yield of the exciplex formation in the CNN-TBA system is independent of the initially excited vibronic level. The vibrational frequencies of the CNN-amine complexes are listed in Table I. The similarity of the vibrational frequencies among the complexes as well as their correspondence to those in the uncomplexed C N N demonstrates the localization of initial excitation on the acceptor chromophore. Only the frequency of the u2 band, at about e450 cm-I, varies considerably from complex to complex. As is evident from Figure 3, each intramolecular vibrational feature ( u I through u4) is accompanied by a satellite band that represents one quantum of the intermolecular vibration built on the intramolecular vibration. In Figure 5, we compare the dispersed fluorescence spectra of the complexes that display exciplex formation with that of the CNN-NH3 complex. These spectra were obtained following excitation of u, band at =O + 510 cm-I, except for the CNN-TBA complex where the origin band was excited. It should be clear from the figure that the complexes with a low-IP donor display exciplex fluorescence (as well as the fluorescence from the locally excited vdW complex), while the NH3 complex exhibits only the fluorescence from the locally excited vdW complex, A*D. In all systems that exhibit exciplex fluorescence, the maximum intensity of the exciplex emission occurs at =450 nm. The ratio of the
I-Cyanonaphthalene Complexes with Aliphatic Amines
The Journal of Physical Chemistry, Vol. 95, No. 20, 1991 7583
TABLE I: Excitation Energies and Fluorescence Lifetimes of CNN-Donor Complexes
- vo,o(C"),b
IP,'
donor none (uncomplexed CNN) NH3 dimethylamine (DMA) diethylamine (DEA) trimethylamine (TMA) triethylamine (TEA) tripropylamine (TPA) tributylamine (TBA)
~0.0
eV
cm-l
o,C
cm-'
0
8.59 10.15 8.24 8.01 7.82 7.50 7.25 7.22
35.6 32.4 17.7 29.1 13.8 -1.4 -1 30.4
9.3 9.5 12.8 8.0
VI
- VkO,
cm-
403 402 402 40 1 402 402 402 40 1
VI
- Vp.0,
cm-
v3
- vo,o1
cm-'
v4
- vo.0.
cm-I
450 45 1 452
510 509 509
66 1 662
453 453 450 444
509 509 510 510
662 662 661 660
7,d
ns
20.0 17.5
15.5 15.4
*Ionization potentials taken from refs 14-16. *Frequencyshifts relative to the origin band of the uncomplexed CNN at 31 414 cm-I. CFrequencies of intermolecular vibrational modes in the origin band regions. "Fluorescence lifetimes of the exciplexes obtained by exciting the V , transitions at about 0 + 509 cm-l.
i
sorption of the van der Waals complexes shifts to lower energy with increasing size and complexity of the donor molecule. (2) The formation of the CNN-amine exciplex from the locally excited state of the corresponding vdW complex is related to the ionization potentials of the amine. Only donor molecules (amines) with relatively low ionization potentials give rise to exciplex formation. (3) No significant broadening in the excitation spectra is observed for all the donor-acceptor systems undergoing the exciplex formation. (4) Unlike the case of the other systems, exciplex formation of the CNN-TBA system takes place efficiently upon excitation into the electronic origin of the vdW complex. The red shift of the electronic origin is related to the size, and not the ionization potential, of the donor molecule, since the electronic origin of the DEA complex is at lower energy relative to that of the TMA complex (Figure 1 and Table I). This suggests that the red shift is due to dispersion interaction between the donor and acceptor molecules, which stabilizes the excited state relative to the ground state. The observation that the excitation spectra of the exciplex fluorescence are composed of the narrow bands whose frequencies and line widths are virtually identical with those in the uncomplexed C N N indicates that the potential energy surface is very similar for the AD and A*D states, and that the A-D+ state is formed only through the excitation of the A*D state. Therefore, exciplex formation may be regarded as a unimolecular isomerization process from a specific vibronic level u of the A*D state:
a ) CNN-NHJ
-
b ) CNN-TMA
u
'7
1
C ) CNN-TEA
n
d ) CNN-TPA
11 2oooo
I
I
25000
30000
AD
Wa venumber/cm -' Figure 5. Dispersed fluorescence spectra of CNN complexes with various donors. Spectra (a)-(d) were generated by selective excitation of the u3 mode (=O + 510 cm-I), while spectrum (e) was obtained by exciting the CNN-TBA origin band shown in Figure 4. The spectral resolution was -100 cm-' in (a) and ~ 5 0 cm-' 0 in (b)-(e). Note that the severely truncated feature in each spectrum is mainly due to scattered light. The broad feature lying between 25 000 and 20000 cm-l can be assigned as A-D+ fluorescence, while the structured emission near the excitation position, A*D fluorescence.
exciplex fluorescence to the A*D fluorescence increases in going from TMA to TBA. In addition, the fluorescence spectra of the CNN-TPA and CNN-TBA exciplexes appear to be broader compared with those of the CNN-TMA and CNN-TEA exciplexes. The fluorescence decay curves of the exciplex fluorescence were measured as a function of the excess vibrational energy. The decays from all levels are nonexponential, and they can be approximated by biexponentials. In all cases, the decay is dominated by the fast component. The slow component, which appears weakly (with a preexponential factor of
