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J. Phys. Chem. 1986, 90,4568-4573
When these two effects are incorporated to the master equation, the calculated results show a very good agreement with experiment in a variety of conditions, demonstrating the predictive power of the method. In regions where pressure is detrimental to the multiphoton decomposition, rotational relaxation is complete and then the simple master q u a t i o n without fractionation applies. Data in this region allow the extraction of the mean down vibrational energy removed per collision. The results also show that
the calculated reaction probabilities are independent of the shape of the laser pulse, in the sense that a smoothed Gaussian profile yields virtually the same results as when spiking is considered.
AcknowfedgTent. We thank the Consejo Nacional de Investigaciones Cientificas y Tknicas (CONICET) for partial financial support. Registry No. CDF,, 558-22-5; CHF,, 75-46-7; Ar, 7440-37-1,
Exclmer-Laser-Induced Photochemlstry of Organometallic Compounds Monitored by Dye Laser Mass Spectroscopy: Dimethyl Dltetlurlde (CH,TeTeCH,) R. Larciprete and M. Stuke* Max-Planck-lnstitut fur biophysikalische Chemie, Department Laserphysik, 0-3400 Gottingen, West Germany (Received: February 3, 1986)
Dimethyl ditelluride, CH3TeTeCH3,is a suitable organometallic gaseous precursor of tellurium. We describe a detailed investigation of the laser-induced photochemistry of CH3TeTeCH3monitored by tunable dye laser multiphoton ionization time-of-flight mass spectroscopy with nanosecond and picosecond laser pulses. In the blue spectral region, we observe the formation of the Te atoms and CH3TeCH3molecules. With KrF excimer laser (248 nm) irradiation in the UV, however, the formation of metal dimers Te2is identified, and an estimation of the internal energy distribution of the Te2photofragments is given.
Structured deposition of superconductors, metals, and semiconductors on various substrate materials and shapes can be achieved by using excimer lasers and organometallic compounds. MOCVD (metal organic chemical vapor deposition) seems to be the mast promising technique for the generation of well-defined layers of metals and semiconductors, since it combines the advantages of MBE (molecular beam epitaxy) and LPE (liquid-phase epitaxy) with a potential for mass production of electronic devices. For a review of MOCVD see ref 1 and references therein. Extending MOCVD by the use of lasers to laser-MOCVD, selective area growth can be achieved (see ref 2 therein). In the ideal case, one would like to induce photodeposition on various substrate materials and shapes only from selected adsorbate species and sites, controlled by the tunable laser wavelength and the intensity distribution impinging onto the surface. So far, however, only steps in this direction have been achieved when laser-enhanced deposition through a combination of thermal and photochemical effects3 was observed, or when UV laser photodeposition from adsorbate mixtures4 was induced. In the majority of applications up to now, thermal effects seem to be dominating. In addition to the generation of microstructures, the reduction of substrate temperatures is another promising feature of laserMOCVD when compared to the classical MOCVD technique. In special cases, the reduction of substrate temperatures may be achieved by using a less stable organometallic c o m p o ~ n d . ~But in general, the use of photon energy rather than thermal energy for the decomposition of the organometallic compounds may by far be more practical. Thus, photochemistry comes in. For a systematic and successful use of (laser) photochemistry of organometallic compounds for deposition processes, the identification and internal energy characterization of photoproducts is necessary for a detailed understanding and control of the (1) Metalorganic Vapour Phase Epitaxy 1984; Mullin, J. B., et al., Eds.; North-Holland: Amsterdam, 1984. (2) Laser Chemical Processing of Semiconductor Devices; Houle, F. A., Deutsch, T. F., Osgood, R. M., Eds.; Material Research Society: Pittsburg, PA, 1984. (3) Aoyagi, Y.; Masuda, S.;Namba, S.;Doi, A, A. Appl. Phys. Lett. 1985, 41, 95. (4) Ehrlich, D. J.; Tsao, J. Y . Appl. Phys. Lett. 1985, 46, 198. Higashi, G S.; Rothberg, L. J. Appl. Phys. Lett. 1985, 47, 1288. (5) Hoke, W. E.; Lemonias, P. J. Appl. Phys. Lett. 1985, 46, 398.
0022-3654/86/2090-4568$01.50/0
photochemical processes involved. Knowledge of the excimerlaser-induced photoproducts from a vast variety of organometallic compounds is needed for selection of the appropriate molecule, laser wavelength, and intensity for a given deposition process. The applications of lasers in (photo-) chemical analysis has brought tremendous advantages over classical techniques. Different from the purely spectroscopic techniques such as laserinduced fluorescence (LIF) and coherent antistokes raman scattering (CARS), laser mass spectroscopy gives both: the mass of the species to be detected and in addition spectroscopic information, which makes this technique unique for sensitive identification and characterization of stable and transient species. In addition, this technique has the potential for the exact evaluation of ultrafast kinetics: though this field is still in its infancy. In this respect, it is interesting to note that already in 1971 Jonah, Chandra, and Bersohn measured in an ingenious and elegant experiment’ the photodissociationof dimethylcadmium Cd (CH,), and-without using any laser-they could estimate the photodissociation kinetics to be on the femtosecond time scale. In general, however, the use of lasers will be necessary for a detailed understanding of the photoprocess: product identification, internal energy characterization of photoproduct atoms and molecules, and determination of the (ultrafast) kinetics involved. A recent review of the applications of lasers in chemical analysis is given in ref 8. Tellurium (and also selenium) is used in a vast variety of important group 11-VI (groups 12-16)17 compounds such as cadmium mercury telluride, an infrared detector material; in HgTe/CdTe superlattices; in phase-change-erasable optical data storage devices. Also some compact disk (CD) players use tellurium-containing layers. Dimethyl ditelluride, CH,TeTeCH,, is a suitable organometallic precursor of tellurium. In the following, we shall describe a (6) El-Sayed, M. A,; Gobeli, D.; Simon, J. In Ultrafast Phenomena; Auston, D. H., Eisenthal, K. B., Springer Ser. Chem. Phys. Springer: Berlin, 1984; Eds.; Vol. 38, 341. Greene, B. I.; Farrow, R. C. J. Chem. Phys. 1983, 78, 3336. Greene, B.I. In Ultrafast Phenomena; Ed. Auston, D.H., Eisenthal, K.B., Springer Ser. Chem. Phys. IV; Springer: Berlin, 1984; Eds.; Vol. 38, 308. Stuke, M. Proc. Con$ Laser Electroopt. (CLEO), June 1984, Anaheim, CA 1984, 254. (7) Jonah, C . ; Chandra, P.; Bersohn, J. Chem. Phys. 1971, 55, 1903. (8) Zare, R . N. Science 1984, 226, 298.
0 1986 American Chemical Society
Photochemistry of Organometallic Compounds
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laser
IU c a m w t e r W 10
20
30
40
50 60 7 0 80 90 100
120 140 160 180200
250
300
mAss
Figure 1. Schematic view of the experimental setup. The photolysis laser (top) produces photofragments, which are sampled by the excimer laser pumped dye laser (bottom) in a differentially pumped time-of-flight (TOF) mass spectrometer. For details see text.
detailed investigation of the excimer-laser-induced photochemistry of dimethyl ditelluride, CH3TeTeCH3,monitored by tunable dye laser multiphoton ionization time-of-flight mass spectroscopy.9JoJ6
Experimental Setup A schematic view of the experimental setup including the lasers, the UHV vacuum chamber (lo-' mbar background pressure), and the data acquisition system is shown in Figure 1. The UHV vacuum system consists of a main chamber, into which the sample is either introduced by a pulsed nozzle or through a molecular leak valve. The laser beams enter counterpropagating collinearly through Suprasil quartz windows. The photolysis excimer laser (KrF, 248 nm, 10-20 ns) is focused by a spherical lens (f = 240 mm) and its energy ( 1 0 . 5 mJ) is controlled by a set of liquid filters. The dye laser passes a variable attenuator (set of different transmission filter glasses) and a spherical focusing lens with f = 240 mm. The focal intensity is in the range of several tens of M W cm-* to several G W cm-2. Ions generated in the focus of the laser beam(s) are slightly pushed through a grid (100 V cm-') and accelerated (2000 V), before they drift through the field-free section in the differentially pumped ( l o 8 Torr) side arm serving as the T O F mass analyzer.I6 The ions are detected by a tandem channelplate detector and the resulting signal is preamplified and transferred to either a fast oscilloscope (Tektronix 7104), or a fast (200 or 400 MHz) transient recorder (Tektronix 7612D), which can store the complete mass spectrum for each laser shot. Data are transferred to a minicomputer (PDP11/23+ with two Winchester drives, DECnet connection to VAX computer), which also triggers the photolysis excimer laser and the excimer laser pumping the dye laser, controls the wavelength of the tunable dye laser, and reads and stores the reference pulse energy for each laser shot (Laser Precision Energy Meter Rj7200), which is measured after an uncoated 45-deg quartz beam splitter. Results and Discussion ( a ) Single Dye Laser Experiments (380-480 nm). The time-of-flight mass spectrum of dimethyl ditelluride, CH3TeTeCH3, obtained after dye laser ionization at 428 nm (15 ns, 4 mJ) is shown in Figure 2a. It is interesting to note that no signal appears at the parent ion mass ( m / e 290) indicated by the arrow in Figure 2a. This lack can be related to the Occurrence of a fast photodissociation of the parent molecule and/or parent molecular ion induced by further absorption of photons during the same laser pulse. When examining the main characteristics of Figure 2a, one can immediately realize that it is practically identical with the laser mass spectrum of dimethyl telluride reported in ref 9. The mass spectrum for picosecond laser excitation, however, is completely different (as in the case of CH3TeCH3): as shown in Figure 2b (9) Stuke, M.Appl. Phys. Lett. 1984,45, 1175. (10) Fantoni, R.; Stuke, M.Appl. Phys. B 1985, 38, 209.
(bl
10
20
30
4 0 50 60 70 80 90 100 120 140 168 180 208 MtSS
250
300
Figure 2. (a) Dye laser (T = 10-20 ns) induced TOF mass spectrum of dimethyl ditelluride, CH3TeTeCH3,at 428 nm and E = 4 mJ. The parent ion is not observed; see arrow at mass 290. (b) TOF mass spectrum of CH3TeTeCH3with picosecond laser ( T = 2 ps) excitation, E = 6 rJ. The parent molecular ion at mass 290 has the highest abundance in the TOF mass spectrum.
revealing the parent molecular ion as the dominating peak in the T O F mass spectrum. This circumstance allows us to postulate a photochemical dynamics that starting from dimethyl ditelluride involves as an intermediate product dimethyl telluride, obtained by elimination of one of the two tellurium atoms from the parent molecule CH3TeTeCH3. The resulting atoms can be identified by sharp atomic resonances in the dye laser wavelength dependence of the Te+ ion abundance as shown in Figure 3a (top) for CH3TeTeCH3. These atomic resonances may overlap with molecular bands of CH3TeCH3,9but also occur at wavelenghts where CH3TeCH3is not ionized readily (see arrow). Therefore, both photoelimination products-CH3TeCH3 and Te-produced during the nanosecond laser pulse can be identified. Using the observed line positions and the known Te atomic resonances, we can assign the internal energy-if necessary-as was shown in ref 10. A similar behavior is observed for the wavelength region (400-410 nm) depicted in Figure 3b, where the dye laser wavelength dependence of the CH3+and Te+ ion abundances resulting from CH,TeTeCH3 are compared. CH3+(top) and Te+ (middle trace) reveal the same broad shapes coming from CH3TeCH3, with the atomic resonances of the Te atoms being superimposed on the Te+ trace, shown on expanded scale at the bottom trace. The strong increase of the atomic resonances relative to the molecular bands in the Te+ trace may be attributed to a further photodissociation of the CH3TeCH3photofragment into additional Te atoms and methyl radicals. This would fit well to the strong increase of the single photon absorption of CH3TeCH3below 203 nm shown in Figure 4 (top). The corresponding Te+ ion abundance in the laser mass spectrum of CH3TeTeCH3 is shown as an insert in Figure 4. (b) T w o - h e r Experiments. Although so far we described only the results obtained when the CH3TeTeCH3molecule was irra-
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Larciprete and Stuke
t
Te*
-
1-m
Figure 5. TOF mass qxctrum of CH,TeTcCH, produced by excimer laser madlition at 248 nm (top) or dyr laser irradiation (scc Figure 2a) (bclou). The delay bctuecn excimer and d>c l a w is 31 = 0.5 PI. 135
L30
L25
ch Inmi
CH;
Te*
Te'
-
~ 1 o .f o
10(
Xlnm
Figure 3. (a) Dye laser wavelength dependence of the "?ef ion abundance from CH,TeTeCH, (top) and the corrcspondingdyc laser energy (bottom). Photofragments eliminated during the laser pulse are detcctcd. The broad band features are due to CH,TeCH,; the sharp spikes are due to the Te atoms eliminated during the laser pulse. The arrow marks the wavelength where the Te atoms arc detected without interference from CH,TeCH,. (b) Comparison between the dye laser wavelength dependence of CH,' (tap) and Te' (middle) in the 400-410-nm range. The Te' wavelength dependence is expanded on the bottom trace with arrow indicating the wavelength suitable far detection of CH,TeTeCH, (see text). Abs I
I
F i w 4. Single-photonabsorption spectrum of CH,TeCH, in the 20W 2lWnm range. The insert shows the Tet ion abundance obtained from CH,TeTeCH, in the dye laser mass spectrometer. diated with blue dye laser light, the main purpose o f this work i s to utilize the tunable dye laser as a probe for the excimer-laser-induced photolysis (see Figure I).
Figure 5 show the TOF mass spectrum obtained when CHITcTeCH, was exposed to 248-nm K r F excimcr laser radiation (top). For comparison, we report in the lower trace a lypcial dye-laser (blue)-induced TOT: mass spectrum. exhibiting the characteristics we discucccd previousl) (see Figure 2a). Note that the tuo lahers arc triggered at different times. the reason for this fact becoming evidcnt bclou. In Figure 5. the two mass peaks generated by the K r F excimer laser correspond to the Tcf and Te)* ions. As in the case o f dye laser irradiation (compare Figure 2a). at 248 nm also there is no evidence fur the parent ion; but unlike the lower trace. the CH,' peak also i s completely missing u i t h nanosecund U V laser excitation. The comparison bctueen the two traces shows that when the photon wavelength i s shifted from the blue region to the UV. different reaction channels become available for the system evnlulion. I n this respect the interesting meaning of Figure 5 i s the evidence that during the interaction of this molecule w i t h 5-CV photons (248 nm) oneofthe favorite photolysis pathways produces a distribution of Te, dimers which can bc detected due to an efficient MPI proccss. This major channel does not produce CH,' ions. since CH, radicals do not havc resonances for ionization at 24X nm. In order to enlarge our information, it is useful to utilize the contribution of both lasers. firing thc dye laser after the excimer laser, so that the visible photons can interact uith the ncutral fragment distribution resulting from the UV-laser-induced photochemistry. Depending on the delay time between the two laser pulses, it i s possible to sample only the neutral photoproducts (Ar > 100 ns), or ncutral and charged photoproducts together (At < 50 ns) I n the idlowing. we have used a delay time bctwccn the excimcr photolysis laser and the dye laser of Ai = 500 ns. Figure 6a illustrates the result we obtain utiliiing the experimentsl configuration in which the dye laser i s triggered 0.5 us after the excimer laser. The bottom and the ccntrsl traccc reproduce respectively the mass spectra rcmrdcd whcn only the dye (426.3 nm) or onl) the excimer (248 nm) laser beam i s sent into the reactiun chamber. The upper trace shous the resulting time-of-flight signal obtained whcn both laser pulses are working in wncert with une anuther. Two mass spectra are s u p e r i m p d : the excimer laser coming first induces naturally the same effect ac before. as i s displayed b) the t w o small peaks. corresponding to the Te' and Te,' ions, that (come at the same time and) cxhibit similar shapes in the central and in the top trace. The two other pcaks appearing in the upper trace reprexnt the dela)ed dye pulw induced MPl and coincide uith the Te' and the Te,' signals detected 0.5 us after the corresponding UV-induced peakr. The same behavior i s observed i n the case that the d)e laser is tuned to a wavelength inefficient for inni7ation when irradiating alone. This i s shoun in Figure 6b. uhcre. keeping the other experimental parameters constant. the dye laser wavelength is sent in off.rcsonancc (421.2 nm) for CH,TeCH,. as the complete lack uf the Te' signal in the bottom trace demonstrates. The main features of thc dye laser TOF mass spestroscop) for detection o f laser photolysis products are demonstrated in figure 7 for the K r F excimer laser photolysis of CII,TcTeCH,. Tuning the dye l a w in the 380-410 nm range. Figure 1reprecentc in the
Photochemistry of Organometallic Compounds le'
The Journal of Physical Chemistry. Vol. 90. No. 19. 1986 4571
CHiTeTeCHi
4
n
KIF
. 1
.
.
. . . ..
.
Ib
EXCIMER LASER ENEROV
.
!dun)
-
I
[ b)
t
Figure 6. (a) TOF mass spectra ofCH,TeTeCH, when irradiated with the excimer laser alone (KrF. middle trace), or dye laser alone (426.3 nm. bottom), or when the dye laser is delayed 0.5 11s with respect to the KIF excimer laser (top trace). (b) TOF mass spectra of CH,TeTeCH, obtained far dye laser irradiation at 424.2 nm (bottom), which is off resonance far detection of CH,TeCH,. and when the dye laser is delayed 0.5 ps with respect to the KrF excimer laser (tap trace). x)
! , ,,, 1
ib
EXCIMER LASER ENERGY larbunj
Figure 8. Dependence of the dye laser induced Te,+ signal on the excimer laser intensity ( a ) dye laser unfacused and (b) dye laser fmacused.f = 240 mm.
L100
LOO0 r-Alnm
3900
380 0
Figure 7. Dye laser wavelength dependence of the Tea+ion (see arrow in Figure 6b) i n the range 380-410 nm (below), and thecorrcspnding
dye laser energy (top). The main feature is a broad band centered around at shorter wavelength sharp spectral structures for the formation of the Te,+ ion are prominent (insert, expanded vertical scale).
405 nm. but
lower trace the wavelength dependence of the Te2+ion abundance and in the upper trace the corresponding dye laser energy. The major contribution is a broad structure a t longer wavelengths (centered around 405 nm) but toward shorter wavelengths a sham spectral structure is observed as shown in an expanded vertical scale in the insert. The occurrence of these sharp peaks, having very narrow width when compared to the longer wavelength region centered around 405 nm, may be carefully considered in a further development concerning possible isotopic effects in the Te2+spectrum." The dependence of the dye laser signal on the excimer laser intensity is shown in Figure 8a for an unfocused dye laser beam, giving a slope of n = 1. Saturation behavior can be seen in Figure ( I 1) Larciprcte. R.;Stukc. M.Appl. Phys. B. to be submitted for publiCaIlO".
Figure 9. Wavelength dependence o f d l e I d w induced fragment ions (a) Tea+and (b) Te'. The arrow marks the uauelength used for detection of CHITeTeCH,, compare Figure 3b
8 b for the case when the dye laser beam is focused. The line indicates the done n = 1 observed for the lower laser intensitv in Figure 8a. Figure 9 shows a comvarison between the Te,+ and the Te+ abundances in the same wavelength region as Figire 7. The Te+ spectrum determined by the delayed dye laser pulse presents the same qualitative features observed before, with intense atomic lines superimposed onto a broad profile. (The bottom inserts show in adapted scales the real shape of the peaks appearing cut in the spectrum.)
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The Journal of Physical Chemislry, Vol. 90, No. 19, 1986
I - tons
Te
i
Te‘
Approximate time evolution of the excimer laser pumped dye laser intensity. Due to different intensities at different times. different ions are present at different tims of the dye laser pulse. For details. see Fipre 11.
I
text.
Te
- Te
t
Figure 10. (a) Comparison of the dye laser wavelength dependence of Te+ (top) and Tea+ (bottom). The dye laser beam is facused W = 240 mm). Note that the spectral resolution achievable is different for both
ions. (b) Comparison of the dye laser wavelength dependence of Te+ (top) and Text (bottom trace). The dye laser beam is unfocused. The spectral resolution achieved is similar for the r e 2 + and Te+ ions. Comparing Figures 3b and 9 we can see that whereas the atomic lines present about the same relative amplitude in both pictures, there is a clear change in the broad band profile, manifesting the occurrence of different photodynamics when the dye laser pulse is sent directly or after the excimer pulse into the chamber. In fact, the peak present at 408.6 nm in Figure 3h (see arrow) due to the (2R + 1) ionization of the CH,TeCH, photofragment reduces in Figure 9b to a weak structure hardly recognizable (see arrow). This proves that most of the CH,TeTeCH, molecules (>95%) are destroyed by the photolysis excimer laser pulse, so that they are not available anymore for the different reaction preferably induced by the dye laser radiation. The fact that we can identify photoproducts which are produced during the dye laser pulse (10-20 ns) causes us to raise the question, are the ions forming the fragmentation pattern produced at the same time during the laser pulse? Comparing in a selected wavelength range (378.5-389.3 nm) the spectral behavior of the Te+and r e z + abundances for focused (Figure loa) and unfocused (Figure lob) dye laser radiation, it became evident that only for the low-intensity case are the spectral dependences similar. For the high intensity case however, the Te+ wavelength dependence becomes much broader compared to the Tezc spectral dependence. W e attribute this to power broadening. Therefore we conclude that-as sketched in Figure 1 I-for the high-intensity case the main contributions to the Te,+ and Te+ peaks in the TOF mass spectrum may be produced at different times (intensities) of the laser pulse. Discussion of the Possible Reaction Kinetics The structure of the dimethyl ditelluride. CH,TeTeCHl, is given in Figure 12 (middle trace), with the Te-Te bond distance and the TeTe-C bond angle estimated by extrapolating from the known values of the CH,SSCH, and CH,SeSeCH, molecules.” One possibility to readily explain our identification of the T e and CH,TeCH, photofragments occurring during the dye laser pulse is to assume a fast pbotoisomerization to an isomeric structure” like the one given in Figure 12 (bottom trace) with the observed photoproducts separated by the dashed line. Flash photolysis experiments on CH,SSCH,” at 195 nm in the gas phase (12) Landolt-Bbnstein, New Series; Springer-Verlag: Berlin, 1976: Vol. 7.
(13) Gmrlin Handbook Springer-Vcrlag:
ps If.
Berlin, 1984; Suppl. Vol. 82.
!
/CH3
TeiTe\
CH3 Figure 12. Structure of the dimethyl ditelluride, CH,TeTeCH,, moleculc
(middle trace) and a possible isomeric ~tructure(bottom). The dashed line separates the observed phatofragments,when irradiating with the dye laser alone. diluted in buffer gases and on C,H;TeTeCZH,” with a mercury lamp in the condensed phase cannot give clear insight into the reaction mechanism. The observed wavelength dependence of the Tez+ion in Figure 7 gives evidence for two different internal energy distributions in which the Te, dimer is produced by the photolysis from the CH,TeTeCH, precursor: one at higher internal energy detected by the broad spectral features centered about 405 nm, and one at lower energy characterized by the sharp spectral features below 390 nm. The photofragmentation process may be ultrafast, leaving no time for energy randomization before the separation of species. W e are continuing the work concentrating on three main aspects: (a) studying the dependence of the spectral features detected in Figure 7 on the wavelength of the photolysis laser; (b) checking if the sharp features observed in Figure 7 (insert) can be used for isotope selective excitation;” (c) using ultrafast laser pulses to directly measure the kinetics of the isomerization and fragmentation processes involved. Conclusion W e have shown that dimethyl ditelluride is a very useful photochemical precursor of tellurium. With excimer laser photolysis at 248 nm, more that 95% of the precursor molecules can Callear, A. B.; Dickson. D. R. Tram. ForodqvSoc. 1970.66. 1987. (IS) Brown, D. H.;Crorr, R.I.; Millington, D. J. Olgonomel. Chem. 1977, (14)
125, 219. (16) Technical
Gbttinger, FRG.
details: SUMOTEK
GmbH,
P.O.B. 3311, D-3400.
(17) In this paper the periodic group notation in parentheses is in accord with recent actions by IUPAC and ACS nomenclature commilees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups I and 2. Thed-transitionelcmentscomprisc groups 3 through 12. and the p-blnk elements comprise groups I3 through 18. (Note that the former Roman number designation is prscrved in the last digit of the new numbering: e.&, 111-3 and 13.)
4513
J. Phys. Chem. 1986,90,4573-4518 be dissociated within one laser pulse. With laser mass spectroscopy, the formation of metal dimers Te2 could be identified; its laser wavelength dependence was established and an estimation of the internal energy distribution of the Te2 dimers could be achieved. Therefore, laser mass spectroscopy proves to be the ideal technique for the study of the primary laser photochemical processes of organometallic compounds which are of interest for laser-MOCVD.
Acknowledgment. W e greatly acknowledge Kurt Muller's technical assistance throughout this work. Cooperation with M. A. El-Sayed about the picosecond kinetic aspect, advice and support by F. P. Schafer and J. Troe (SFB 93, C 2 C4), and financial support by S F B 93, project C2, and by N A T O Grant 250182 are gratefully acknowledged.
+
Registry No. CH,TeTeCH,, 20334-43-4; CH3TeCH3,593-80-6; Te, 13494-80-9; Te2, 10028-16-7.
Dynamic Behavior in the Excited State of Phenanthrylammonium Ions-1 8-Crown-6 Complexes: A One-way Proton-Transfer Reaction' Haruo Shizuka* and Manabu Serizawa Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan (Received: February 3, 1986)
The dynamic behavior in the excited singlet state of 1:1 phenanthrylammonium ion-18-crown-6 complexes in MeOH-water (9:l) mixtures at various temperatures has been studied by means of the single photon counting method with fluorimetry. Complex formation of phenanthrylammonium ions (RN+H3)with 18-crown-6decreases markedly the proton dissociation rate in the excited state, resulting in an increase of its lifetime or fluorescencequantum yield. 2- and 3RN+H3-crowncomplexes are especially stable and do not dissociate into the excited neutral amine species plus proton. The hydrogen-bondedexciplex (RNH2-crown)* is produced by deprotonation of (RN+H3-crown)*for 1-, 4-, and 9RN+H3-crowncomplexes. The excited-state proton-transfer reaction in the 1-, 4-, and 9RN+H3-crown systems is a one-way process since the proton association rate is negligibly small compared to those of the other competitivedecay processes. That is, there is no excited-state prototropic equilibrium in the RN+H3-crownsystems. There is a large steric effect on protonation to the amino group of the excited neutral complex. In contrast, proton-induced quenching occurs effectively in (RNH2-crown)* complexes.
Acid-base properties in the excited state of aromatic compounds are elementary processes in both chemistry and biochemistry.'-IO There has been considerable recent interest in the photochemical and photophysical properties of aromatic compounds in the presence of protons:" proton-transfer reactions in the excited state and proton-induced q u e n ~ h i n g , ' ~a- lone-way ~ proton-tranfer reaction in the excited states of hydrogen-bonded c ~ m p l e x e s , ' ~ examples for no excited-state prototropic e q ~ i l i b r i a , ' ~hydro*'~ gen-atom-transfer reactions from triplet aromatic compounds (naphthylammonium ions" and 1-naphtholIs) to the ground state (1) The preliminary accounts of the paper were presented at the XIIth International Conferenceon Photochemistry, Tokyo, August 1985. This work was supported by a Scientific Research Grant-in-Aid from the Ministry of Education of Japan (No. 58470001). (2) Fbrster, Th. Z . Elektrochem. Angew. Phys. Chem. 1950,54,42,531. (3) Weller, A. Ber. Bunsenges. Phys. Chem. 1956, 66, 1144. (4) Weller, A. Prog. React. Kinet. 1961, 1, 189. (5) Beens, H.; Grellmann, K. H.; Gurr, M.; Weller, A. Discuss. Faraday Sac. 1965, 39, 183. (6) Vander Donckt, E. Prog. React. Kinet. 1970, 5, 273. (7) Wehry, E. L.; Rogers, L. B. In Fluorescence and Phosphorescence Analyses; Hercules, D. M., Ed.; Wiley-Interscience: New York, 1966; p 125. (8) (a) Schulman, S. G.In Modern Fluorescence Spectroscopy; Wehry, E. L., Ed.; Plenum: New York, 1976; Vol. 2. (b) Schulman, S . G. In Fluorescence and Phosphorescence Spectroscopy; Pergamon: Oxford, U.K., 1917. (9) Ireland, J. F.; Wyatt, P. A. H. A d a Phys. Org. Chem. 1976, 12, 131 and a number of references therein. (10) KIBpffer, W. Adv. Photochem. 1977, 10, 311. (11) Shizuka, H. Acc. Chem. Res. 1985, 18, 141 and references cited therein. (12) Tsutsumi, K.; Shizuka, H. Chem. Phys. Lett. 1977, 52, 485; 2.Phys. Chem. (Wiesbaden) 1978, 111, 129. (13) Shizuka, H.; Tobita, S. J . Am. Chem. Sac. 1982, 104, 6919 and references cited therein. (14) Shizuka, H.; Kameta, K.; Shinozaki, T. J. Am. Chem. Sac.1985,107, 3956. (15) Shizuka, H.; Nakamura, M.; Morita, T. J . Phys. Chem. 1979, 83, 2019. (16) Shizuka, H.; Ogiwara, T.; Kimura, E. J . Phys. Chem. 19W89.4302. (17) Shizuka, H.; Fukushima, M. Chem. Phys. Lett. 1983, 101, 598. (18) Shizuka, H.; Hagiwara, H.; Fukushima, M. J . Am. Chem. Sac. 1985, 107,7816.
0022-3654/86/2090-4573$01.50/0
of aromatic ketones, proton-enhanced hydrogen-atom transfer,19 and proton-assisted photoionization of methoxynaphthalenes.20 Recently, it has been found that the complex formation of naphthylammonium ions with 18-crown-6 decreases significantly the proton-transfer rate in the excited state resulting in an increase of its lifetime, and that the back protonation rate in the excited state is negligibly ~ma1l.l~ It has also been proposed that the values of the ground-state association constants K g of the naphthylammonium ions with 18-crown-6 can be determined by the fluorimetric titration method.14 In previous work2' measurements of KBvalues of phenanthrylammonium ions (RN+H3) with 18crown-6 have been carried out, and the fluorescence titration method for determination of Kgvalues has been established. Table I shows the data for the RN+H3-crown complexes.2' This paper reports the dynamic behavior of the phenanthrylammonium ion-1 8-crown-6 complexes studied by means of the single photon counting method with fluorimetry.
Experimental Section Phenanthrylamines used were the same as those reported elsewhere.22 18-Crown-6 (Merck) were purified by repeated recrystallizations from dichloromethane. Sulfuric acid (9795, Wako) was used without further purification. Methanol (Spectrosol, Wako) and distilled water were used as a MeOH-H,O mixture (9:l in volume). The acid concentrations (0.02-0.1 M ) used were sufficient to make protonated phenanthrylamines in the ground state. The concentrations of 18-crown-6 used were 0.2-0.37 M, which were sufficient to make phenanthrylammonium ion-1 8-crown-6 complexes. All samples were thoroughly degassed by freeze-pumpthaw cycles on a high-vacuum line. (19) Shizuka, H.; Kaneko, S.; Hagiwara, H., unpublished results. (20) Shizuka, H.; Hagiwara, H.; Satoh, H.; Fukushima, M. J. Chem. Sac., Chem. Commun. 1985, 1454. (21) Shizuka, H.; Serizawa, M.; Okazaki, K.; Shioya, S. J . Phys. Chem., in ... mess. r-
(22) Tsutsumi, K.; Sekiguchi, S.; Shizuka, H. J . Chem. Soc., Faraday Trans. I 1982, 78, 1087.
0 1986 American Chemical Society
