J . Phys. Chem. 1985,89, 3833-3841 aqueous solution are important factors for achieving good activity. Proximity of ZnS and CdS sites at the spatial location of optical absorption is also important for good activity.
Acknowledgment. The authors acknowledge financial support of this work by the Gas Research Institute (Contract No. 598260-0756). National Science Foundation support of the X-ray
3833
photoelectron spectrometer by an equipment grant, CHE 8201 179, is also acknowledged. We thank Dr. M. Schmerling for his help with the TEM and Prof. Marcin Majda and Cary Miller for supplying thin film samples of A1203. Registry No. H20, 7732-18-5; H2, 1333-74-0; CdS, 1306-23-6; ZnS, 13 14-98-3.
Solvation effects in Jet-Cooled 2-Amlnopyridlne Clusters: Excited-State Dynamics and Two-Color Threshold Photoionization Spectroscopy James Hager and Stephen C. Wallace* Department of Chemistry, University of Toronoto, Toronoto, Ontario M5S IAI, Canada (Received: February 28, 1985)
We present a detailed study of 2-aminopyridineand the 2-aminopyridineradical cation in a supersonic expansion. Dramatic changes in excited-state behavior have been observed upon the attachment of single solvent molecules. The solvent species investigated include Ar, CHI,CC14,H20, MeOH, EtOH, NH3,and 1,4-dioxane, which exhibit a wide range of excited-state interaction energits. Spectral features of the complexes were found to be quite rich in structure showing substantial vibrational excitation in the A state especially for the polar solvent molecules. We also report a novel effect of complex formation on the excited-state dynamics of these complexes as inferred from measurements of fluorescence decay times. A considerable lengthening of the lifetimes of excited-state molecular complexes with polar solvents was observed and attributed to the ability of complex formation to affect the magnitude of the excited-state singlet-triplet interactions. In addition to these studies, we have also measured the IPSof the bare molecule and these complexes using the technique of two-color photoionization spectroscopy. The IP differences have provided information regarding the intermolecularinteractionsbetween a well-characterized molecular cation and neutral solvent molecules. Together with the spectral shifts of the uncharged species, these results clearly demonstrate the approximate order of magnitude increase in the stabilization of the 2-aminopyridine ground ionic level relative to the neutral excited state.
Introduction The study of the solvation of aromatic molecules containing nonbonding electrons is particularly important in elucidating the interactions between two excited electronic states.'-3 Nitrogen heterocycles are often characterized by two closely spaced electronic transitions, nn* and aa*,leading to strong vibronic interactions manifested by complicated absorption spectra as well as easily perturbed radiative and nonradiative properties.1-2q4 We have investigated the behavior of 2-aminopyridine (2-AP) and a wide variety of its associated complexes of different interaction strengths in a supersonic jet. Condensed-phase studies5-' have demonstrated that even in hydrocarbon solvents 2-AP has an appreciable fluorescence quantum yield (-0.06 at 300 K) which is seen to increase by an order of magnitude in acidic solution. The rate constant for the radiative process was found to be independent of temperature and degree of protonation of the ring nitrogen while the nonradiative rate changes dramatically. An increase in the quantum yield of phosphorescence in nonpolar solvents relative to protic solvents a t 77 K indicates a significant contribution of intersystem crossing to the nonradiative rate.6 The change of fluorescence lifetime appears to closely follow the solvent-dependent fluorescence quantum yield. In contrast to many of the recently studied nitrogen heterocycles such as quinoline, isoquinoline, and acridine which have 1Qwest energy na* singlet states, 2-aminopyridine is characterized3 by a lowest energy a,a*transition. However, many of the observable (1) Lim, E.C. In 'Excited States"; Lim, E.C., Ed.;Academic Press: New York, 1977; Vol. 3, p 305. (2) Innes, K. K.; Byme, J. P.; Ross,I. G . J . Mol. Spectrosc. 1967,22, 125. (3) Hollas, J. M.; Kirby, G. H.; Wright, R.A. Mol. Phys. 1970, 18, 327. (4) (a) Felker, P. M.; Zcwail, A. H. Chem. Phys. Lett. 1983, 94, 448. (b) Felker, P. M.; Zewail, A. H. Chem. Phys. Le??.1983, 94, 454. (c) Felker, P. M.; Zewail, A. H. J. Chem. Phys. 1983, 78, 5266. (5) Weisstuch, A.; Testa, A. C. J . Phys. Chem. 1968, 72, 1982. (6) Kimura, K.; Takaoka, H.; Nagai, R.Bull. Chem. SOC.Jpn. 1977,50, 1343. (7) Kleinwachta, V.; Drobnik, J.; Augenstein, L. Photochem. Photobiol. 1966,5, 579.
0022-3654/85/2089-3833$01SO/O
fluorescence properties are quite similar for these two groups of probe molecules. Those with a lowest energy na* singlet state exhibit only a very weak fluorescence in hydrocarbon solvent while they show much more intense fluorescence in hydrogen-bonding solvents. This has been interpreted in terms of a greater energy gap between the na* and aa* electronic states upon the formation of hydrogen bonds leading to a smaller vibronic interaction and a decreased nonradiative rate.' Recently, Felker and Zewai14 have demonstrated the important information that can be obtained from studying the interactions between the n7r* and 7ra* states of isoquinoline via complex formation in a supersonic jet. In these investigations association with water, methanol, and acetone led to much longer fluorescence lifetimes than that of the bare molecule.4b This behavior was rationalized in terms of a change in the na*-aa* energy gap, depending on the complexing partner, decreasing the magnitude of the interaction between these two electronic states leading to a concomitant decrease in the nonradiative rate. The increase in the observed fluorescence lifetime reflects the strength of interaction of the particular solute species and its ability to form hydrogen bonds. Such direct information regarding solutesolvent interactions provides motivation for studying other molecular species and solvent partners in the unique environment of a supersonic expansion. The lowest energy electronic transition of 2-aminopyridine has only recently been unambiguously assigned to a m* electron p r ~ m o t i o n . ~Thus, amino substitution of the parent pyridine molecule in this manner gives rise to substantially different excited electronic state dynamics as indicated by the significantly higher quantum yield of fluorescence exhibited by the 2-substituted molecule. Using a supersonic expansion to prepare rotationally and vibrationally cold 2-AP, we have investigated the fluorescence excitation and oqe-color multiphoton ionization (MPI) spectra of the bare molecule, extending the assignments of Hollas et al.13 to approximately 2000 cm-' above the origin. The formation of 2-AP complexes with polar and nonpolar solvent molecules results in distinct, sometimes complex, spectral features, which have been 0 1985 American Chemical Society
3834 The Journal of Physical Chemistry, Vol. 89, No. 18, 1985 identified by using two-color photoionization mass spectrometry. Dynamic information of this electronic state has been obtained via fluorescence lifetime studies of the bare molecule and the 1:l complexes and illustrates the dramatic effects of the association of even one solvent molecule upon the observed decay time. We have also recorded the photoionization efficiency (PIE) spectra* of 2-AP and several of the 1:l complexes using two tunable dye lasers and a quadrupole mass spectrometer in order to obtain information about the interaction between the parent radical cation and a variety of solute molecules. The important points about this technique are the very soft ionization of the monomers characterized by a lack of fragmentation at low laser fluences near threshold* and the strong propensity for Au = 0 ionizing transitions producing vibrationally cold monomer ions.9 Photoionization of the weakly associated complexes on the other hand can lead to large changes in vibrational quanta of the intermolecular vibrational modes due to the large changes of binding energy upon This makes possible the observation of ground ionic state vibrational features appearing as higher energy ionization thresholds in the PIE spectra or in other cases may result in a broadened ionization onset due to the superposition of many Au # 0 ionizing transitions.I0 The energetics of these photoionization thresholds provide a direct measure of the relative magnitudes of the interaction energies of the parent cation with polar and nonpolar molecules. In what follows, we first present the results of the neutral bare molecule and complexes as probed by fluorescence excitation and one-color MPI spectroscopy as well as by measuring the fluorescence decay times. Subsequently, we discuss the results of the two-color photoionization studies and compare them with recently published results from other systems.
Experimental Section The supersonic beam apparatus has been described previously.” For the laser-induced fluorescence studies a continuous helium free jet, seeded with 2-aminopyridine vapor (vapor pressure 30-40 mtorr) and the appropriate complexing partner, was crossed with the frequency-doubled light of a Nd:YAG pumped dye laser system. Typically, a 100-pm pinhole was used as the expansion orifice with a backing pressure of 3.5-4.0 atm. The laser-free jet interaction region was maintained 5 mm (50 nozzle diameters) downstream of the pinhole. The excitation light for these fluorescence studies was produced by a Quanta Ray Nd:YAG (DCR-1A) pumped dye laser (PDL-1) using various dyes which, when frequency doubled in KD*P (WEX-l), covered the 2800-3150-A spectral region. The line width of the UV light was approximately 0.2 cm-l with pulse energies 1 5 mJ which could be reduced with neutral-density filters. Signal processing of the undispersed fluorescence was performed with a PAR 162/165 boxcar and an X-Y recorder which was ramped as a function of wavelength by a Quanta Ray CDM-1 control unit. Lifetime measurements were made with a Tektronix 79 12 AD transient digitizer and dedicated 4052 microcomputer. Each decay was averaged 400 times and base line corrected. The apparatus for the two-color photoionization studies has been described in detail in previous publications.12 In the present study the intermediate electronic level in the two-color scheme was prepared by using light from a XeCl excimer pumped dye laser (Lumonics 860-Twith an EPD-330 dye laser) frequency doubled in KD*P. This system provided pulses of 7-11s duration and 1 mJ per pulse. The Nd:YAG pumped dye laser described above was used as the ionizing laser in these studies. Fundamental laser light from this laser was frequency doubled in KD*P and the
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-
(8) Duncan, M. A.; Dietz, T. G.; Smalley, R. E. J . Chem. Phys. 1981, 75, 2118. (9) Reilly, J. P. Isr. J . Chem. 1984, 24, 266. (10) Hager, J.; Ivanco, M.; Smith, M. A,; Wallace, S. C. Chem. Phys. Lett. 1985, 113, 503. (11) (a) Hager, J.; Wallace, S . C. J . Phys. Chem. 1983, 87, 2121. (b) Hager, J.; Wallace, S. C. J. Phys. Chem. 1984, 88, 5513. (12) (a) Smith, M.A.; Hager, J. W.; Wallace, S. C. J . Chem. Phys. 1984, 80, 3097. (b) Smith, M. A.; Hager, J. W.; Wallace, S. C. J . Phys. Chem. 1984, 88, 2250.
Hager and Wallace second harmonic tracked during each scan with a Quanta Ray WEX 1 system. Temporal overlap of the lasers and power normalization was done as published earlier.I2 The ions produced in the overlap region in the supersonic expansion are accelerated into the quadrupole mass spectrometer housing with a very small accelerating voltage (C1 V/cm) where they were focused and analyzed. This very small accelerating voltage had no effect on any of the threshold positions or breadths reported here. All ionization potentials are corrected to vacuum wavenumbers. In these experiments the identity of the ionized species was initially determined with the quadrupole mass filter. In each case only the parent species was observed with no fragmentation evident. This allows a more sensitive technique to be used in obtaining the two-color photoionization efficiency spectra, namely using the quadrupole as a total ion accelerating region by turning off the ac fields associated with the mass spectrometer. This results in an ion signal increase of greater than 100 with no change in the resulting spectra. The data acquisitionI2 and sample handling” technique are the same as previously described. 2-Aminopyridine (99+%) was obtained from Aldrich and used without further purification. The samples of the various complexing partners were obtained as previously
Results and Discussion Spectroscopy of 2-AP. The A-2 system of 2-AP has only recently been unambiguously assigned to a AB* electron promotion by Hollas and c o - ~ o r k e r s .This ~ electronic IA’-]A’ transition has its origin at 33 471 cm-I. The transition moment was found to be only slightly perturbed from the first *a* transition of pyridine3 (along the in-plane axis and perpendicular to the C2-axis). Assuming that the pyridine na* transition energy is unaffected by amino substitution, this implies that the m*-na* energy gap is approximately 1300 cm-] for 2-AP. This can be considered to be a lower limit since we find no evidence for an overlapping na* transition using either one-color MPI or fluorescence excitation spectroscopy up to 2000 cm-’ above the BT* origin. Two recent publication^'^^^^ have investigated the 2-AP aa* absorption spectrum with particular emphasis on the similarity with that of aniline. In both studies it was determined that the ground-state out-of-plane angle of -32’ for the amino substituent decreases dramatically upon electronic excitation and approaches planarity. Hollas et aI.l3 have also carried out dispersed fluorescence investigations of this electronic state and have been able to assign many ground- and excited-state vibrations as well as identify appreciable hydrogen bonding between the ring nitrogen and the nearer hydrogen of the amino group. The jet-cooled fluorescence excitation spectrum of isolated 2-AP is shown in Figure 1. As is generally the case, the spectrum shows substantial simplification over that obtained at room temperature due to the substantial elimination of hot bands and sequence structure. This has allowed us to revise and extend the vibrational assignments of the A state as is demonstrated in Table I. There is excellent agreement between our results and those of Hollas et al.13afor the spectral region up to 1047 cm-I above the origin as reported by that group. The only modification to these assignments is that the weak band at 540 cm-’ has been determined to be a cold one and therefore should be associated with the 213, excitation. In addition the band at 609.5 cm-I reported by Hollas et al.13acompletely disappears under supersonic expansion conditions and thus is probably a hot band. At higher energies one can see that the vibrational modes v I (amino group inversion), vI9 (benzene-type ul), v2,, (benzene-type v I 2 ) ,and u22 (benzene-type v6& are particularly active in overtones and combination-bands in the excited state. This behavior is indeed similar to the A state of aniline. Further assignments of the higher energy fundamentals will have to wait for dispersed fluorescence investigations. (13) (a) Hollas, J. M.; Musa, H.; Ridley, T. J . Mol. Spectrosc. 1984, 104, 89. (b) Hollas, J. M.; Musa, H.; Ridley, T. J . Mol. Spectrosc. 1984,104, 107. (14) Gordon, R. D.; Clark, D.; Crawley, J.; Mitchell, R. Specrrochim. Acta, Part A 1984, 40A. 657.
The Journal of Physical Chemistry, Vol. 89, No. 18, 1985 3835
2-Aminopyridine Clusters
2030 R E L A T I V E E N E R G Y (cm-’)
Figure 1. Fluorescence excitation spectrum of the singlet X X * transition of 2-AP in a supersonic jet. This spectrum has not been power normalized. TABLE I: Band Positions of 2-Aminopyridine-Major Observed Vibronic Transitions
relative position,’ cm-’
1045 1047 1304 1344 1347 1420 1433 1437 1493 1510 1569 1731 1797 1826 1868 1883 1945 2013
assignmentb 0: 22; 211,
0 523 540 815 824 91 1 969 972 978
201, 1; 191,
1
22; 18; 978
+ 29;
20e22; 22;1; 19;22; 22%or 181,221, 201,1; 191,201,
I 20
I
I
I
I
I
1
I
I
-10 -20 IO 0 RELATIVE ENERGY (cm-’) Figure 2. Fluorescence excitation of the 2-AP S, origin transition.
h
1: 20A22; or 18h206 Iil9; 19;22;
“Uncertainties in band positions are iz1 cm-’. bAssignments are based on this dispersed fluorescence studies in ref 13a and the assumption of harmonic modes with no interactions between these modes. The good rotational cooling of our expansion is demonstrated in Figure 2 where a higher sensitivity spectrum of the 2-AP origin is displayed. In the room temperature vapor3the rotational contour is comprised of three peaks of approximately equal intensity in an interval of about 4.6 cm-l. In Figure 2 one can see that the rotation contour in this study is dominated by only one intense band with an associated fwhm of 1.8 ad.Based upon our earlier investigationsof jet-cooled anilineik this corresponds to a rotational temperature of 1 by the same two-color technique. We have therefore assigned the feature located at -500 cm-1 as the electronic origin of the 2-AP(CH,0H)I complex. This is the largest spectral shift we have yet observed for the "singly solvated" 2-AP probe molecule and thus the largest ground-state-excitedstate binding energy difference. The complexes of 2-AP with other polar molecules such as ethanol, ammonia, and 1,Cdioxane are given in Figure 7. Using mass analysis we find that in each of the spectra shown in Figure 7 there are contributions from multiply solvated 2-AP molecules
0
-100
-202
RELATIVE
-m
-400
E N E R G Y (cm-0
Figure 7. One-color MPI spectra of (a) 2-AP (CH3CH20H),(b) 2-AP (NH,), and (c) 2-AP (1,4-dioxane) near the bare molecule origin. TABLE II: Band Positions and Fluorescence Lifetimes of the Lowest Energy Transitions of 2-AminopyridineComplexes
species 2-AP 2-AP(argon), 2-AP(arg0n)~ 2-AP(methane), 2-AP(methane), 2-AP(CF.+) 1 2-AP(CC14), 2-AP(ammonia), 2-AP(dioxane), 2-AP(water) 2-AP(methanol)I 2-AP(ethanol),
relative position," cm-' 0
-32 -62
fluorescence lifetime, ns
