J. Phys. Chem. 1992, 96, 2083-2088
2083
Size and Excess Vibrational Energy Dependence of Excimer Formation in Naphthalene Clusters Hiroyuki Saigusa,* Sheng Sun,and Edward C. Limt Department of Chemistry, The University of Akron, Akron, Ohio 44325-3601 (Received: September 30, 1991;
In Final Form: November 8, 1991)
-
Excited-state dynamics of small naphthalene clusters, following SI So excitation, has been investigated by the techniques of the resonant two-photon ionization, fluorescence excitation, and dispersed fluorescence. Highly resolved multiplet structures are obtained for the vibronic transitions of the trimer and tetramer, which provide information on the intermolecular interactions between the constituent molecules. Depending greatly upon its excess vibrational energy, each cluster exhibits broad and red-shifted bands in the dispersed fluorescence spectrum. This behavior is interpreted as evidence for the transformation of the initially excited van der Waals cluster into an excimer, which is formed by a pair of the monomers within the cluster framework. The restricted geometry of the van der Waals cluster is considered to explain the size and excess energy dependence of the excimer formation.
Introduction Spectroscopic investigation of molecular clusters is currently of great interest because it provides insight into the evolution from the characteristics of molecular scale to those of condensed phase. The localization and delocalization of elementary excitations, which have been well studied in bulk molecular crystals,' can now be studied in the van der Waals (vdW) clustersn2 Direct investigations of how the excitation evolves as a function of the size are expected to lead to a microscopic understanding of the optical properties of such geometrically restricted systems. Benzene clusters are particularly important systems in which to study intermolecular dynamics in the excited state. Well-resolved SI So excitation spectra of the benzene clusters have been reported by Hopkins et al.) and by Langridge-Smith et a1.: and subsequently by Law et ala5 Bbrnsen et al. have observed excitonic features in the spectra of the benzene dimefi and trimer,' clearly indicating the importance of excitation delocalization in these systems. The observation that the lifetimes of the benzene dimer appear to be sisnificantly shorter than those of the monomer has suggested the possibility of excimer formation occurring within the excited-state However, no direct evidence for such a large structural change has been ~ b s e r v e d . ~ Our approach to the study of the excited-state structure and dynamics of aromatic clusters involves the spectroscopic manifestation of such structural isomerization occurring within the clusters. Many aromatic molecules display strongly red-shifted structureless fluorescence bands in highly concentrated solutions, indicating the occurrence of excimer for ma ti or^.^^^ Such broad fluorescence can also be observed upon excitation of the fluorene vdW dimer generated in a supersonic expansion.'OJ' Its discrete excitation spectra reveals that the excimer formation may be considered as a conformational isomerization occurring within the initially excited vdW dimer. The large stability of the excimer state results from strong exciton resonance interactions between two monomer excited states carrying large oscillator strengths,I2 whereas the initially excited dimer is bound by weak vdW interactions. In the work reported here, we have investigated the excited-state dynamics of small naphthalene clusters (N I 5), with an emphasis on the excimer formation as a function of the cluster size and of the excess vibrational energy. Naphthalene is a favorable case for such studies, as the painvise interactions between the molecules have been determined') for the isotopically substituted crystals. In addition, Wessel and Syage have reported resolved vibronic structures in the SI So transitions of the trimer and tetramer c l ~ s t e r s . ' ~ Their J ~ analysis of the fine structure15 has predicted a herringbonetype geometry for the tetramer, whereas the trimer
-
-
Inaugural holder of the Goodycar Chair in Chemistry at the University of Akron.
analysis suggested a linear structure in which the naphthalene molecules are arrayed along the short-axis direction. The excimer of naphthalene has been previously observed only in a very concentrated solution (10.5 M).89 Its intensity maximum occurs at 4 0 0 nm.16 The stability of the excimer state has been attributed to configurational interactions of exciton resonance states originating either from the 'La state (f= 0.15) or from the IBbstate (f = 1.70), with charge resonance If such an excimer state is also stabilized in a vdW cluster of naphthalene, excimer formation may take place within the cluster. In the case of the naphthalene monomer, most of the vibronic activities in the SIspectrum derive principally from the Sl-S2 coupling. If the excimer stability indeed arises from excitonic interactions between the S2states, the SIvibronic spectra of the cluster should be strongly perturbed as a consequence of the vibronic coupling. Therefore, an analysis of the frequencies and intensities of the cluster transitions will provide detailed information on the excitonic interactions occurring in the higher electronic states. This paper presents highly resolved SIspectra of the naphthalene clusters recorded by the resonant two-photon ionization (R2PI) method and the fluorescence excitation method. Also shown are the dispersed fluorescence spectra of each cluster, which illustrate the size and excess energy dependence of the excimer formation occurring within the clusters.
(1) See, for example, Davydov, A. S. Theory of Molecular Exciton; McGraw-Hill: New York, 1970. (2) See for recent review, Whetten, R. L.; Hahn, M. Y. In Atomic and Molecular Clusters; Bernstein, E. R., Ed.; Elsevier: Amsterdam, 1990; pp 765-803. (3) Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J . Phys. Chem. 1981,85, 3739. (4) Langridge-Smith, P. R. R.; Brumbaugh, D. V.;Haynam, C. A.; Levy, D. H. J. Phys. Chem. 1981, 85, 3742. ( 5 ) Law, K. S.;Schauer, M.; Bernstein, E. R. J. Chem. Phys. 1984, 81, 4871. (6) Bornsen, K. 0.; Selzle, H. L.; Schlag, E. W. J . Chem. Phys. 1986,85, 1726. (7) Biirnsen, K. 0.; Lin, S. H.; Selzle, H. L.; Schlag, E. W. J. Chem. Phys. 1989, 90, 1299. (8) Birks, J. B. Phorophysics of Aromatic Molecules; Wiley: New York, 1970; pp 301-371. (9) Mataga, N.; Kubota, T. Molecular Interactions and Electronic Spectra; Marcel Dekker: New York, 1970; pp 411-484. (10) Saigusa, H.; Itoh, M. J . Phys. Chem. 1985, 89, 5436. (11) Saigusa, H.; Lim, E. C. J . Phys. Chem. 1991, 95, 2364. (12) Saigusa, H.; Lim, E. C. J . Phys. Chem. 1990, 94, 2631. (13) Hanson, D. M. J. Chem. Phys. 1970,52, 3409. (14) Wessel, J. E.; Syage, J. A. J. Chem. Phys. 1988, 89. 5962. (15) Wessel, J. E.; Syage, J. A. J . Phys. Chem. 1990, 94, 737. (16) Selinger, B. K. Aust. J . Chem. 1966, 19, 825. (17) Birks, J. B. Chem. Phys. Lett. 1967, I , 304.
OO22-3654192/2096-2O83%O3.OO/O 0 1992 American Chemical Society
2084 The Journal of Physical Chemistry, Vol. 96,NO. 5, 1992 Experimental Section Naphthalene clusters were generated by pulsed nozzle expansion in a supersonic beam apparatus. The desired concentration of naphthalene was obtained by flowing H e at a pressure of 4 atm through a reservoir filled with naphthalene (Aldrich >99%) at 90 OC. The gas mixture was then expanded as a 200-ps pulse through a 0.5 mm diameter nozzle (General Valve Series 9) operated at 10 Hz.The jet passed through a 2-mm nickel skimmer (Beam Dynamics Model 2) into an ionization chamber, where it intersected the laser beam 15 cm downstream from the nozzle. The chamber was maintained at 4 X 10” Torr during the nozzle operation. Tunable laser radiation was provided by the frequency-doubled output of a YAG-pumped dye laser (Quanta Ray DCR-1/ Lambda Physik FL2002). The laser beam was focused onto the cluster beam by an f = 30 cm cylindrical lens. The pulse energies were adjusted to minimize fragmentation as well as spectral broadening due to a near resonant two-photon ionization process. Typical pulse energies were 320 nm through a UV-34 filter or excimer fluorescence at >400 nm through an LF-418 filter. The dispersed fluorescence spectra were measured by a 0.64-m monochromator (Jovin Yvon THR-640 with 3600-lines/mm grating).
Saigusa et al.
hl I
(d) R 2 P I
-
-
Results The SI So transition of naphthalene has been extensively investigated by the fluorescence spectroscopy21,22and by the R2PI ~ p e c t r o s c o p y . The ~ ~ ~weak ~ ~ S,(’B3, ]A,) origin is observed at 32020 cm-’ followed by an intense transition at 435 cm-I, denoted by 8; using the notation of Stockburger et al.2s The spectrum is dominated by vibronically activated bl, transitions, such as 7; a t 91 1 cm-I and 8;s; at 1137 cm-I. In this paper, we show the R2PI and fluorescence excitation spectra of the naphthalene clusters recorded at excess energies ranging from 0 to 1200 cm-l. A. The 8; Band Region. The fluorescence excitation spectrum of the naphthalene clusters in the gf,region is shown in Figure la. Also shown for comparison are the R2PI spectra of the dimer
1~
~
0
Spec t r a 1 Sh i f t /cm - I Figure 1. Excitation spectra of the naphthalene clusters in the & region. The spectral shift is relative to the monomer 8; transition occurring at
32455 cm-‘. The spectrum in part a was obtained by detecting the total fluorescence at >320 nm,while the one-color R2PI technique was employed to record the spectra of (b) the dimer, (c) the trimer, and (d) the
tetramer. TABLE I: Spectral Shifts (in cm-’) of the Naphthalene Trimer and Tetramer, Relative to the Corresponding Monomer Transitions’
Trimer 0:
$A
7A
-43.4 -38.3
-88.3 -8 1.O
-86.8 -82.3
00, -108.7
8;
7;
-199.6 -196.3 -179.6 -173.9 -168.9 -150.5 -123.9 -1 13.6 -109.6 -107.3
-197.6 -193.8 -173.5 -167.1 -1 50.7 -147.9 -124.5 -1 14.0
%8!, -74.6 -7 1.4
Tetramer
-
(18) Schriver, K. E.; Hahn, M. Y.; Whetten, R. L. Phys. Reu. Lett. 1987, 59, 1906. (19) Wessel, J. E. Phys. Rev. Lett. 1990, 64, 2046. (20) Saigusa, H.; Lim, E. C. J . Chem. Phys. 1983, 78, 91. (21) Beck, S . M.; Powers, D. E.; Hopkins, J. B.; Smalley, R. E. J . Chem. Phys. 1980, 73, 2019. (22) Behlen, F. M.; McDonald, D. B.; Sethuraman, V.; Rice, S . A. J . Chem. Phys. 1981, 75, 5685. (23) Duncan, M. A.; Dietz, T. G.; Smalley, R. E. J . Chem. Phys. 1981, 75. 2118. (24) Cooper, D. E.; Frueholz, R. P.; KJimcak, C. M.; Wessel, J. E. J . Phys. Chem. 1982,815,4892. (25) Stockburger, M.; Gattermann, H.; Klusmann, W. J . Chem. Phys. 1975,63, 4519.
-100
-200
8h8; -193.8 -190.2 -185.7 -169.0 -142.9 -139.5
“These values are based on both the R2PI and fluorescence excita-
tion measurements.
(Figure lb), the trimer (Figure IC),and the tetramer (Figure Id). These spectra were taken with the naphthalene sample held at 90 OC. The strongest feature observed in the fluorescence excitation spectrum is the 8; band of the monomer. Extending toward lower energy, down to -250 cm-I from this peak, one finds weaker features arising from vibrationally hot monomers as well as those assignable to 8; transitions of the cold naphthalene clusters. By comparison of the fluorescence excitation spectrum with the R2PI spectra, a prominent sharp feature located a t -81 cm-I (marked by N3) with respect to the monomer & band is identified uniquely as a corresponding transition of the trimer. Further to the red of this peak, an irregular series of weaker peaks appear at shifts ranging from -100 to -250 cm-I (marked by N4), which agree very well with the multiplet structure of the tetramer spectrum. The positions of the main features coincide with those previously reported by Wessel and Syage.I5 However, the tetramer spectrum shown in Figure Id reveals additional features which
.
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2085
Excimer Formation in Naphthalene Clusters
u C 4
,
- 7500
( b ) Fluorescence
- 196
-114
I
A d e t . > 400nm
I
I
,
1
-5000
Wavenumber/cm
-2500
0
-'
Figure 2. Dispersed fluorescence spectra obtained by exciting the SA bands of (a) the monomer, (b) the trimer, and (c) the tetramer. The tetramer spectrum was obtained following the excitation of the strongest feature located at -151 cm-' in Figure Id. The spectral resolution was 10 cm-l for the spectra in parts a and b, while it was 50 cm-I for the spectrum in part c. The energy scale in each spectrum is relative to its excitation energy. For the spectra in parts a and b, no prominent features were observed in the range from -3500 to -10000 cm-l. See also Figure 7a for the trimer spectrum.
have not been resolved in their spectrum. The positions of the main features observed in the trimer and tetramer spectra are listed in Table I. It is very important to note that fluorescence excitation spectrum shown in Figure l a reveals a weak, broad background structure. As reported by Wessel and Syage,I5 and also shown in Figure 1b, the RZPI spectrum of the dimer is dominated by broad bands, with no prominent structure. The broad features appearing in the fluorescence excitation spectrum match well the dimer spectrum, indicating that the dimer emits weakly at the g; excitation. The dispersed fluorescence spectra obtained when the monomer, trimer, and tetramer are excited at the 8; bands, are compared in Figure 2. The monomer and trimer spectra were taken with a resolution of 10 cm-l, while a 50-cm-' resolution was used to record the tetramer spectrum. It is clear that there is a severe broadening in the trimer spectrum as compared to the monomer spectrum. This phenomenon is very common for weakly bound vdW clusters containing large molecule^.^ When clusters are formed, low-frequency modes are generated. These modes, which describe the intermolecular motions of the component molecules, will create a dense set of background levels. Therefore, this will lead to a relaxation of the initial excess vibrational energy into the intermolecular modes. The background states that are thus populated are responsible for the appearance of such broad features in the dispersed fluorescence spectra. The situation changes dramatically in the tetramer. The tetramer fluorescence is dominated by broad features whose intensity maximum occurs at 380-385 nm which is red-shifted by 6000-6300 cm-I from the excitation energy. This cannot be explained by such a relaxation process. One can find only weak spectral features which correspond to the trimer spectrum. On the basis of its red-shift and spectral features, we assign the tetramer fluorescence as originating from an excimer state. Having observed that the tetramer emission spectrum is dominated by the excimer fluorescence, it is tempting to measure its excitation spectra by detecting only the red-shifted fluorescence. Figure 3b shows the fluorescence excitation spectrum that is
obtained when emissions are detected at >400 nm. The RZPI spectrum of the tetramer is reproduced in Figure 3a for ease of comparison. It is clear that the two spectra are essentially identical, demonstrating that the fluorescence excitation spectrum discriminates against the transitions of other species such as the monomer and trimer. However, a close examination of the fluorescence excitation spectrum in Figure 3b reveals that the tetramer features are superimposed on an underlying background which agrees in intensity distribution to the dimer spectrum (Figure lb). This indicates that the dimer also transforms into an excimer and thus emits a t this wavelength. The 8; band of the monomer appears very weakly in the fluorescence excitation spectrum under this detection condition (>400 nm). The spectrum also exhibits a couple of sharp weaker features located in the lower energy region (-250 to -200 cm-I). These features coincide well with those of the RZPI spectrum of the pentamer (not shown). B. Tbe Origin Band Region. In Figure 4, we show the 0; bands of the naphthalene clusters. The fluorescence excitation spectrum shown in Figure 4a consists of sharp features with an underlying broad background. The sharp features located at -43 and -109 cm-' are identified uniquely as the origins of the trimer (N3) and tetramer (N4), respectively, based on comparisons with the corresponding RZPI spectra (shown in parts c and d of Figure 4). Each origin band is characterized by a prominent sharp feature. The origin bands are much weaker relative to the corresponding transitions in the &, region, roughly by an order of magnitude. It is also important to note that the broad background feature resembles the FOP1 spectrum of the dimer shown in Figure 4b. Unfortunately, because of the low fluorescence intensity, it was not possible to record dispersed fluorescence spectra following excitation of these origin bands, under the same spectral resolution as employed for the 1; bands. To obtain more information on the excited-state dynamics of the clusters in the origin band region, we measured fluorescence excitation spectra under different detection conditions. The results are displayed in Figure 5 . Compared to the excitation spectrum of the total fluorescence (Figure sa), the spectrum generated by observing emissions at 330 f 10 nm (Figure 5b) is free of broad background fluorescence. The origins of the trimer (N3) and tetramer (N4) are clearly seen in the spectrum. When the detection wavelength is set to monitor only the red-shifted fluorescence at >400 nm, no prominent features are observed, indicating that neither the trimer nor the tetramer produces excimer fluorescence following excitation into the origin band. The spectrum in Figure 5c is completely dominated by broad features which coincide with those observed in Figure 5a. Moreover, the broad spectrum is similar to the RZPI
Saigusa et al.
2086 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992
1,
(a) F l u o r e s c e n c e N4
BIB1
1
I
I
( b )F l u o r e s c e n c e
I
1 4 00
-200
0
-100
S p e c t r a 1 Sh i f t / c m
- 100
800
1000
Spectral Shift/cm-' Figure 6. Higher energy region of (a) the RZPI spectrum of the naphthalene trimer and (b) the fluorescenceexcitation spectrum obtained by detecting the emission at >400 nm. The spectral shift is relative to the origin band of the trimer at 31 977 cm-'. The peak marked by an asterisk is due to interference from the strong 8; transition of the monomer. The spectral features observed at spectral shifts of 320 nm, while the spectra of (b) the dimer, (c) the trimer, and (d) the tetramer were recorded by the RZPI method.
-200
600
the two spectra suggests that the broad features, which are associated with the dimer transitions, produce exclusively excimer fluorescence upon excitation. C. The Higher Energy Region. The large Stokes-shift of the excimer fluorescence made it possible to separate the tetramer features from those of the monomer and trimer in the 8; region. With increasing excess energy, however, the fluorescence excitation spectrum exhibits prominent peaks that can be assigned to the trimer. This behavior is presented in Figure 6. The fluorescence excitation spectrum obtained when the excimer fluorescence is monitored (Figure 6b) exhibits two prominent features appearing at higher energies. These features coincide well with the 7: and 8A8; transitions appearing in the RZPI spectrum of the trimer (Figure 6a). The frequency shifts are 871 and 1103 cm-I with respect to the trimer origin, respectively. Each transition is accompanied by a weaker feature with a splitting of 4.5 cm-I for the 7; transition and of 3.1 cm-' for the SA8; transition. To the red of each trimer transition, the fluorescence excitation spectrum displays a much weaker multiplet structure, which can be assigned to the corresponding tetramer transitions. Their spectral shifts are similar to those occurring in the 8; region. The spectral shifts are listed in Table I. The dispersed fluorescence spectra resulting from excitations of the trimer transitions are shown in F w r e 7. The excess energy dependence of the fluorescence intensity distribution is evident. Thus, the 7' spectrum (Figure 7b) consists of resolved features corresponding well to the 8' spectrum (Figure 7a), followed by broad bands with an intensity maximum at =380 nm. However, only broad features are seen when the 8i8; transition is excited, suggesting that the trimer excited at this excess energy rearranges completely into an excimer geometry.
0
-'
Spectral Shi f t/cm Figure 5. Comparison of the fluorescence excitation spectra of the naphthalene clusters in the SI origin region obtained by detecting fluorescence (a) at >320 nm, (b) at 330 f 10 nm, and (c) at >400 nm. The origins of the trimer and tetramer are marked in spectrum b by N3 and N4, respectively. The other sharp features are assigned to transitions
arising from vibrationally hot ground-state monomers. Note that the broad background is abscnt in spectrum b, while only the broad spectrum in seen in part c. The sharp feature of spectrum c is due to interference from the monomer origin. spectrum of the dimer shown in Figure 4b, except that the monomer origin leaks onto the fluorescence excitation spectrum by an interference from its strong emission. The close similarity of
Discussion The most important observation in this study is the Occurrence of excimer formation upon excitation of the naphthalene vdW clusters, which provides new information on the structures and dynamics of these clusters. We will discuss the excited-state dynamics of the three clusters separately, based both on the results presented in this paper and on the analysis of the excitonic interactions reported by Wessel and Syage." A. Tetramer. The discovery of the excimer fluorescence following the excitation into the S; transitions is most remarkable. The fluorescence spectrum is completely dominated by broad bands, while its excitation spectrum is composed only of the discrete features that are associated with the vdW tetramer
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2087
Excimer Formation in Naphthalene Clusters
transitions. The behavior indicates that large structural changes must occur following the excitation of the vdW tetramer. We will now discuss how such structural changes may occur within the excited-state tetramer, leading to the formation of a tightly bound excimer pair. Since the first discovery of the pyrene excimer in the stability of excimers has been explained in terms of exciton-resonance interactions (MI*M2 and MlM2*) and/or charge-resonance interactions (Ml+M2and MIM2+)between two electronic systems!*9 Moreover, the resulting excimer must have a closely overlapped configuration with an interplanar distance of 3.0-3.5 A,17J7 thus explaining the Occurrence of a large red-shift in the fluorescence spectrum. A herringbonetype geometry has been assumed for the tetramer based on the analysis of the excitonic interactions in the nontotally symmetric modes;I5 the long axes of naphthalene are parallel while the short axes are substantially displaced, resulting in resonance interactions of -20 cm-' between nearest neighbor pairs. This geometry is consistent with the qualitative expectation that the component molecules must be placed in the tetramer framework in such a way that structural changes can Occur easily toward the excimer configuration. This model suggests that the excimer is formed between the inner nearest neighbor pair of the herringbone framework, with the outer pair serving as solvent molecules. Once excimer formation goes to completion, the excitation energy will be localized on the strongly bound excimer pair. Thus, the excimer state will provide a deep trap for the delocalized excitonic states of the tetramer. We also believe that the binding energy of the excimer is strong compared with those between the excimer pair and the environment's, which results in evaporation of the two solvent monomers following the excimer formation. The present system provides a model for excimer formation in crystals. It has been demonstrated that excimer formation takes place in aromatic molecular crystals, with rates greatly dependent on the structure.28 The reaction coordinate is described by a simple one-dimensional motion of two neighboring molecules in
a pair toward each other. In the pyrene crystal, for example, the interplanar separation of the excimer is assumed to be 0.2 A shorter than that of the ground-state pair.29 The excimer formation time T has been determined to be -140 fs at room temp e r a t ~ r e . ~In~contrast, the rate of the excimer formation in the tetramer is estimated to be 0.1 mJ/pulse) reveal discrete spectral features corresponding to fragmentation of larger cluster ions, the broad spectra shown in Figures l b and 4b were obtained at low fluence of
