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, C2 C4), and financial support by SFB 93, project C2, and by NATO 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-bonded exciplex (RNH2-crown)* is produced by deprotonationof (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 competitive decay 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- one-way l~ 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 Conference on 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 Kg 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
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The Journal of Physical Chemistry, Vol. 90, No. 19, 1986
Shizuka and Serizawa
TABLE I: Absorption (A=) and Fluorescence (A& and Ap&,)Peaks, Fluorescence Quantum Yields (@RN+H3 and Constants of Phenanthrylammonium Ion-18-Crown-6 C!omplexes in MeOH-H20 ( 9 1 ) Mixtures at 300 K O [RNH,I/ lo-, M 2.9, 3.77 3.39 3.13 2.25
compd 1RNH, 2RNH2 3RNH2 4RNH2 9RNH2
[crown]/M 0.2 0.2 0.2 0.37 0.3
[H,S04]/M 0.02 0.025 0.02 0.025 0.02
X Zf
Amax RNHJ nm 361 366 367 371 366
nm 296 294 293 297 294
@RNH2),
and Stabilization
K l
Xmax RNHJ
nm 447
b b 436 444
*RN+HI
@RNHr
0.159 0.23, 0.259 0.0907 0.0747
0.10, b b
102h-1 2.2, lo., 13., o.2oi 1.6,
0.0804
0.0982
OData taken from ref 21. bProton dissociation in the excited state of 2-(or 3-)RN+H3-crown complex scarcely occurs TABLE 11: Fluorescence Quantum Yields (@RN+H3 and Conditionso
IRNH,
2.9,
0.2
2RNH2
3.7,
0.2
2 3 4 5 6 7.5 2.5 5 7.5 10
3RNH2
4RNH2
3.39
3.1,
0.2
0.37
2 2.5 4 5 7.5 2.5 5 7.5 10
9RNHz 4 5 6 7.5
*RNH~)
0.18, 0.18, 0.18, 0.18, 0.18, 0.19 0.255 0.247 0.247 0.242 0.28 0.27, 0.28 0.2S3 0.28, 0.1 1 0.12 0.1 23 0.12, 0.094 0.093, 0,0951 0.093 0.094 0.096
of the RNtH3-18-Crown-6 Complexes in MeOH-H,O ( 9 1 ) Mixtures under Various
0.18, 0.1S3 0.179 0.1S6 0.179 0.18, 0.239 0.233 0.232 0.22, 0.26, 0.257 0.26, 0.26, 0.26, 0.10 0.10, 0.11,
0.159 0.16 0.158 0.164 0.159 0.16, 0.23, 0.23 0.228 0.22, 0.259 0.24 0.254 0.25, 0.252 0.09, 0.10,
0.11,
0.10, 0.10~
0.087 0.0862 0.0869 0.0856 0.0872 0.087
0.0747 0.0753 0.0742 0.074 0.0744 0.074
0.14, 0.143 0.14, 0.1 47 0.14, 0.14, 0.22, 0.22 0.2 1, 0.21, 0.24, 0.24 0.24, 0.24, 0.242 0.080, 0.09 1 0.091, 0.0968 0.068, 0.0685 0.0687 0.068 0.068, 0.069,
0.13, 0.12, 0.12, 0.13, 0.132 0.12, 0.20 0.208 0.20, 0.20, 0.20, 0.20, 0.20 0.20, 0.20 0.069, 0.080, 0.08 1 0.084, 0.06 1 6 0.0613 0.061, 0.0613 0.0607 0.0625
,
0.0639 0.0692 0.070, 0.0678 0.064, 0.0647
0.0821 0.082, 0.078, 0.0804 0.0747 0.0753
0.1 o9 0.10, 0.10, 0.10~ 0.0989 0.097,
0.13, 0.12, 0.12, 0.12, 0.ll5 0.1 1,
0.155 0.14, 0.139 0.145 0.13, 0.135
o.0887 0.082, 0.0789 0.078, 0.1 o9 0.0963 0.088, 0.081, 0.0786 0.073,
0.095, 0.087, 0.085 0.0832 0.11, 0.10 0.091 0.087, 0.0795 0.0735
b
b 0.063, 0.058, 0.0572 0.057, 0.078, 0.0742 0.066, 0.0624 0.0603 0.0573
0.0722 0.0679 0.0668 0.0661 0.0902 0.080 0.0745 0.06S9 0.0676 0.0647
0.0804 0.075, 0.0739 0.072, 0.098, 0.O8S8 0.0829 0.076, 0.0752 0.0688
"Experimental errors with 5%. bProton dissociation in the excited state of 2- or 3-RN+H3-crown scracely occurs. The fluorescence quantum yields were measured by comparison with a quinine bisulfate -0.1 N H2S04solution (aF= 0.54),23*24 using a Hitachi M850 fluorimeter. Excitation wavelengths for protonated 1-, 2-, 3-, 4-, and 9-phenanthrylamines (IRN+H3, 2RN+H,, 3RN+H3,4RNfH3, and 9RN+H3) were at the corresponding absorption peaks (296, 294, 293, 297, and 294 nm, respectively). The fluorescence response functions were recorded with a nanosecond time-resolved spectrophotometer (Horiba NAES-1100, 2-11s pulse width). This single photon counting apparatus is able to measure both the exciting pulse and emission response functions simultaneously, and to compute the decay parameters by the deconvolution method.
Results and Discussion Proton-Induced Quenching. At first, proton-induced quenching of phenanthrylamines in the presence of 18-crown-6 was carried out. Under the experimental conditions, 1:1 phenanthrylammonium ion-1 8-crown-6 complexes (RN+H,-crown) were produced in the ground state. Figure 1 shows the acid concentration effects on the fluorescence quantum yields @RN+H, (for the RN+H,-crown emission) and @RNH2 [for the RNH,-crown emission produced by deprotonation of (RN+H,-crown) *] in MeOH-H,O (9:l) mixtures at 300 K: (a) for the 1RN+H3-crown system, (b) for the 4RN+H,-crown system, and (c) for the 9RN+H,-crown system. The values of @RN+H3 for the protonated amine complex with 18-crown-6 are nearly constant (0.16, 0.23, 0.25, 0.10, and 0.074 for the 1, 2, 3, 4, and 9RN+H3 systems, respectively). However, the @ R N H 2 values for the neutral amine (23) Melhuish, M. H. J. Phys. Chem. 1961, 65, 229. (24) Demas, J. N.; Crosby, G . A. J . Phys. Chem. 1971, 75, 991.
species decrease with increasing H,S04 concentration. The results indicate that proton-induced quenching is involved in the excited singlet state of the neutral amine species but not for the protonated amine complex. For 2RN+H3and 3RN+H3systems, the neutral RNH2-crown complexes are scarcely produced by deprotonation of the excited RN+H3-crown complexes because the complexes are too stable to dissociate into (RNH2-crown)* plus proton.21 It is known that proton-induced quenching occurs by electrophilic protonation at one of the carbon atoms of the aromatic ring.11.13 Electron migration from the amino group to the phenathrene ring (Le., intramolecular charge transfer) occurs effectively in the excited singlet state of phenanthrylamines.22 Measurements of the fluorescence quantum yield (@RN+H3 and @RNH2) have been performed under various conditions. The experimental data are summarized in Table 11. Dynamic Behavior of Excited RWH,-l 8-Crown-4 Complexes. The fluorescence decay functions of the RN+H,-crown complexes have been measured by means of the single photon counting method (Horiba NAES-1100). Typical results are shown in Figure 2: (a) the observed fluorescence response function IRN+H3(t) for the 9RN+H3-crown complex (excited at 294 nm; monitored at 366 nm) at various temperatures, and (b) the observed fluorescence response function Z R N H 2 ( t ) for the neutral 9RNH2-crown complex (excited at 294 nm; monitored at 444 nm) at 300 K together with the lamp function, I L ( t ) , monitored at 294 nm. The I R N + H , ( t ) and I R N H 2 ( t ) functions in Figure 2 are considerably different from those for free 9RN+H3and 9RNH2 without 18-cr0wn-6.~~For instance, the ZRN+H3(t) function in Figure 2a shows a single-exponential decay with a lifetime of 29 ns at 300 K. From the I R N H 2 ( l ) function in Figure 2b, the rise and decay rates at 300 K for the neutral amine species were
Phenanthrylammonium Ions-1 8-Crown-6 Complexes
"
0
L
O
0.02
0.04
The Journal of Physical Chemistry, Vol. 90, No. 19, 1986 4575
0.06
0.08
TIME
TIME
Cns>
010
[HZsod/M
Figure 1. Acid concentration effects upon the fluorescence quantum yields (@RN+H and *RNH2) for the 1RN+H3-crown (a), 4RN+H3-crown (b), and 9RNiH3-crown (c) systems. The concentrations of R N H 2 and 18-crown-6 were the same as those listed in Table I. For 2- and 3RN+H3-crown complexes, the excited-state proton dissociation scarcely occurs.
obtained to be 3.42 X lo7 s-l and 5.& X lo7 M-I s-l, respectively. The rise rate for the excited neutral amine species is equal to the decay rate (3.45 X lo7 s-I) of the (9RN+H3-crown)* complex in Figure 2a within experimental error, indicating that the excited 9 R N H 2 species is produced by deprotonation of the excited 9RN+H3-crown complex. The decay rate X1 for the 9RN+H3-crown complex increased with increasing temperature as shown in Figure 2a. Similar results were obtained for the 1RN+H3-crown and 4RN+H3-crown complexes. The decay parameters X1 and X2 were measured under various conditions. The experimental data are summarized in Table 111. The data show that complex formation of RN+H3with 18-crown-6 in the ground state plays an important role in the dynamic behavior of the excited protonated amines. The lifetime in the excited singlet state of phenanthrylammonium ions increases markedly in the presence of 18-crown-6, resulting from a significant decrease in the rate constant for excited-state proton transfer of the RN+H3-crown complexes as shown later. For 2RN+H3-crown and 3RN+H3-crown complexes, the proton dissociation scarcely occurs in the excited state. This is mainly due to the fact that these complexes are too stable to dissociate into the excited neutral amine species plus proton (the stabilization constants are 1.06X lo3 and 1.31 X lo3 M-' at 300 K (cf. Table I) for the 2RN+H3-crown and 3RN+H3-crown complexes, respectively21). The IRN+H3(t)function with a single-exponential decay may indicate that the rate for the back protonation process between the excited neutral amine species and protons is negligibly small compared to those of the other decay processes. This assumption is strongly supported by the experimental finding that the @ R N + H ~ values are constant even at higher acid concentrations (Figure 1 and Table 11) and that the IRN+H3(t)function comprises of only one decay component (XI) with a constant value regardless of the acid concentration at each temperature (see Table 111). The back protonation rate of the excited neutral amine species may be relatively slow because the proton dissociation of the (RN'H3crown)* complex leads to formation of the neutral (RNH2crown)* complex instead of decomposition into free RNH2* plus 18-crown-6. The neutral complex may dissociate into RNH2 plus 18-crown-6 in the ground state. The (RNH2-crown)* complex is, therefore, called a "hydrogen-bonded exciplex". The electron migration from the amino group to the phenanthryl group occurs appreciably in the excited state of RNH2, leading to a positive charge on the nitrogen atom.22 The hydrogen atoms of the amino group in the excited state may be protic compared to those in the ground state, which favors the production of the hydrogen-bonded complexes (RNH2-crown)* between RNH2* and 18-crown-6.
VI
c r
a
0 0
Figure 2. (a) Observed fluorescence response functions I R N + H ~ ( ~excited ) at 294 nm and monitored at 366 nm for the 9RN+H3-18-crown-6 complex in a MeOH-H20 (9:l) mixture at various temperatures. The concentrations of 9RNH2, 18-crown-6, and H2SO4were the same as those listed in Table 1. (b) Observed fluorescence response function IRNH2(t) excited at 294 nm and monitored at 444 nm at 300 K for the (9RNH2-crown)* complex (the sample was the same as that for (a)) and the lamp function IL(t)monitored 294 nm. For details see text.
SCHEME I
)
R
Keeping a Corey-Pauling-Kolton model25(see Scheme I) in mind as has been reported elsewhere,"-14 one can understand that the proton attack on the amino group of the excited neutral amine complex is structurally blocked by 18-crown-6 and the phenanthryl group of the complex. Reaction Mechanism for the Excited-State Proton-Transfer Reactions of m H 3 - C r o w n Complexes. The experimental results can be accounted for by the reactions in Scheme I a t sufficient concentration of 18-crown-6. In this scheme k , denotes the rate (25) (a) Izatt, R. M.; Lamb, J. D.; Rossiter, B. E., Jr.; Izatt, N. E.; Christensen, J. J. J. Chem. Soc., Chem. Commun. 1978, 386. (b) Izatt, R. M.; Lamb, J. D.; Izatt, N. E.; Rossiter, B. E., Jr.; Christensen, J. J.; Haymore, B. L. J. Am. Chem. SOC.1979, 101, 6273. (c) Izatt, R.M.; Lamb, J. D.; Swain, C . S.; Christensen, J. J.; Haymore, B. L. Ibid. 1980, 102, 3032.
4576
The Journal of Physical Chemistry, Vol. 90, No. 19, 1986
Shizuka and Serizawa
TABLE III: Fluorescence Decay Parameters XI and h2 for the RN+H3-18-Crown-6 Complexes under Various Conditions Measured by a Single-Photon Counter" [crown]/
M
compdC
0.2
0.2
0.2
0.37
0.3
[H2S041/ 10-2M 2 3 4 5 6 7.5 2 2.5 5 7.5 10 2 3 4 5 6 7.5 2.5 5 7.5 10 2 3 4 5 6 7.5
280 2.09 2.08 2.14 2.1 2.1, 2.14 1.88 1.88
1.88 1.88 1.94 1.88 1.9 1.89 1.9 1.88 1.93 3.7 3.7, 3.7, 3.68 3.0 2.92 2.92 2.9, 2.9, 2.8,
IRN+H,(~)~ X1/107 S-I at T, K 290 300 310 320 2.12 2.29 2.4, 2.7 2.14 2.3 2.42 2.6, 2.1, 2.32 2.4, 2.6, 2.1 2.2, 2.4 2.6, 2.11 2.28 2.4 2.6, 2.15 2.29 2.4, 2.64 1.95 2.17 2.02 2.08 1.9, 1.97 2.0 2.09 1.98 1.9, 2.02 2.07 1.95 1.99 2.0, 2.07 1.9, 1.99 2.01 2.08 1.98 2.04 2.1 2.15 1.98 2.02 2.07 2.16 1.98 2.02 2.07 2.16 1.9, 2.0, 2.1 2.1, 1.97 2.03 2.08 2.15 1.97 2.02 2.07 2.17 4.1 4.5 5.0 5.59 4.07 4.58 5.06 5.72 4.03 4.39 5.0 5.59 4.1 4.36 4.92 5.62 3.26 3.45 4.1 4.63 3.22 3.4, 4.03 4.3, 3.23 3.5 3.95 4.37 3.2* 3.4, 3.98 4.39 3.25 3.4, 4.0 4.37 3.2, 3.4, 3 9, 4.37
X,/107 s-I at 290 300 2.1, 2.33 2.14 2.28 2.1, 2.1, 2.Z4 2.1, 2.33 2.13 2.33
280 2.16 2.08 2.1 2.1, 2.1, 2.0,
T/K 310 2.44 2.38 2.4* 2.39 2.39 2.42
IRNH~(~) X2/108s-l at 320 280 290 300 2.66 1.07 1.16 1.25 2.68 1.12 1.2 1.34 2.68 1.1, 1.1, 1.36 2.66 l . l s 1.2, 1.4c 2.6, 1.2, 1.4 1.5, 2.7 1.24 1.44 1.63
d
d
d
d
3.75 3.72 3.68 3.6, 2.96 2.& 2.7, 2.8, 2.73 2.7
4.1 4.08 4.1 4.03 3.29 3.14 3.1 3.2, 3.06 2.99
4.46 4.52 4.4, 4.44 3.42 3.45 3.43 3.43 3.42 3.35
5.0 5.03 4.98 5.0 3.86 3.8, 3.75 3.7, 3.89 3.76
5.56 5.62 5.7, 5.62 4.5 4.38 4.36 4.2 4.1, 4.1,
2.44 2.58 2.57 2.61 0.392 0.533 0.513 0.58 0.565 0.628
2.65 2.7, 2.95 3.19 0.452 0.603 0.602 0.54, O.6& 0.769
3.02 3.36 3.55 3.64 0.521 0.61, 0.656 0.70, 0.739 0.847
T/K 310 1.42 1.49 1.58 1.6, 1.7, 1.88
320 1.67 1.79 1.8, 2.0, 2.1' 2.1,
3.44 3.6 2.77 4.02 0.585 0.719 0.769 0.828 0.801 0.902
3.9, 4.15 4.44 4.74 0.62, 0.73, 0.837 0.91, 0.99 1.02
Experimental error 5%. Fluorescence response functions fRN+H,(t) and IRNH2(f) of the RN+H3-crown complexes were measured at the excitation and the monitoring wavelengths and A?
