7244
J. Phys. Chem. 1992, 96, 7244-7247
Complex Formatlon of Benzophenone Ketyl Radlcal and Trlethylamlne
Department of Chemistry, Tokyo Institute of Technology, Ohokayama, Meguro. Tokyo 152, Japan (Received: March 24, 1992; In Final Form: May 20, 1992)
Transient absorption experiments in the benzophenonttriethylamine (TEA) system have revealed that the hydrogen-bonded 1:l complex is formed by the Bssociation of benzophenone ketyl radical (BPH) and TEA in nonpolar solvents at room temperature (298 K). The complex exhibits a broad visible absorption band with an absorption coeficient of 4OOO M-'cm-' at the peak (555 nm). The equilibrium constants for complex formation are determined to be 99 f 6 M-'in cyclohexane and 37 f 4 M-l in benzene. The complex is found to be nonfluorescent, although bare BPH is known as an emissive radical.
I. Introduction Many papers have been published on the photoreduction of aromatic ketones by amines for the past 20 years.'-" Guttenplan and a h e n fmt supposed that the initialstep of the photoreduction was an electron transfer from amine to triplet ketone, forming a charge-transfer (CT) complex.2 There are pioneering works of direct detection of the CT complex using a picosecond transient absorption technique by Peters and co-workers.6 Recently, Mataga and co-workers have reported that the CT complex formed in the photolysis of the benzophenonediphenylamine system does not contribute to the formation of the ketyl radicalloand showed that the primary photochemical process in the photoreduction of benzophenone by amines cannot be explained in terms of only the electron transfer." It has become clear that the chemical behavior of photoexcited benzophenone depends on the kinds of both amines and solvents. Although a few papers have been published on the photodynamics of benzophenone ketyl radical (e.g., relaxation of the electronically excited and photodissociationIsJ6), little attention has been paid to the interaction between amines and the ketyl radicals. We have examined this interaction with laser flash photolysis and laser-induced fluorescence techniques and found that the ketyl radical (BPH) and triethylamine (TEA) form a complex. The spectral properties of the complexes and the formation dynamics are presented in this paper.
II. Experimental Section The transient absorption apparatus was reported previously." Briefly, the third harmonic of a Nd-YAG laser (Lumonics HPY-750; pulse duration of 7 ns) was used as an excitation light source. A xenon flash lamp (Ushio UXL-150DS, 150 W) was synchronously fired with the Nd-YAG laser and used as a monitoring light for the transient signals. A monochromator (Nikon P-25O)/photomultiplier (Hamamatsu R928) combination was used to obtain transient spectra with a spectral resolution of 2 nm. Signals from the photomultiplier were accumulated for a few tens of shots in a digital memory (Iwatsu DM-901) on-line with a personal computer (NEC PC-9801 VM2). A digital storage oscilloscope (Gould 7404; 400 MHz) was used to measure short-lived transients. The fluorescence and fluorescence excitation spectra were obtained with a pump and probe two-color excitation technique. A XeCl excimer laser (Lambda Physik LPXlOS; 308 nm, pulse duration 20 ns) was used as the pump source. The excimer laser beam was divided into two beams. One was used to generate the ketyl radicals. The other, which experienced an optical delay of 50 ns, was used to pump a dye laser (Lambda Physik FL 3002; 450-570 nm). The fluorescence was dispersed with the monochromator and detected with the photomultiplier. The signals were averaged by a boxcar integrator (Stanford SR250), digitized by an A/D converter and stored in the personal computer. The output of the dye laser was corrected with a Rhodamine B solution as a quantum counter and a photodiode.l8 A flow cuvette (NSG T-59FL-UV-IO) was used to avoid the influence of photoproducts. Benzophenone (Merck) was purified by recrystallization several times in ethanol. Cyclohexane (Cica Merck Uvasol) and benzene
(Cica Merck) were used without further purification. Triethylamine (Kanto, TLC grade) was distilled in a high-vacuum line. All samples were deaerated with solvent-saturated Ar. All measurements were carried out at room temperature.
III. Results Absorption Spectra of the BPH-TEA System. Triplet benzophenone (3BP*) is known to abstract a hydrogen atom from cyclohexane, yielding BPH, whose characteristic absorption spectrum consists of a sharp and intense UV band peaking at 325 nm and a structured band between 500 and 570 nm as shown by spectrum a in Figure 1. The transient absorption was found to change drastically in the presence of TEA (70 mM) as shown by spectrum b in Figure 1. The UV band of the transient is obviously different from that of BPH and becomes weaker and broader, while the visible band loses its structure with a bathochromic shift. Triplet benzophenone has UV and visible absorption bands and might be responsible for the visible broader band of spectrum b in Figure 1. Since triplet benzophenone is not observed in spectrum a in Figure 1 even in the absence of TEA and also since TEA efficiently quenches benzophenone triplet at an almost diffusion-controlled rate? it is not realistic that triplet benzophenone is alive for 1.5 ps after the laser excitation in the presence of 70 mM TEA. Although the ion pair (BP-amine+) is generally known to show a broad absorption band in the red region (-700 nm), the ion pair between benzophenone and TEA has not been detected in nonpolar s01vents.I~ The red-shifted band is not, therefore, attributed to the ion pair. The amine radical formed by hydrogen abstration from TEA could be responsible for the red-shifted band. This will be, however, ruled out based on the following experimental results. To identify the chemical specieswhich gives the red-shiftedband (spectrum b in Figure l), we examined the TEA concentration dependence of the transient absorption spectrum. Fwre 2 shows the transient absorption spectra taken at several concentrations of TEA (6.25-100 mM) under constant laser power. To avoid the influence of triplet benzophenone, the transient absorbances in Figure 2 were measured at 40-50 ns after the laser pulse when triplet benzophenone was completely quenched. The decay of the transient was neglected in this time range because the decay time was as slow as in the microsecond order. Increasing the concentration of TEA, the peak position shifted to red with two isosbestic points at 530 and 550 nm. Benzophenone in the ground state does not form a complex with TEA because no spectral change was observed with an increase in TEA concentration. The shift of the peak position observed in Figure 2 suggests that the ketyl radical and TEA yield a complex which has the red-shifted absorption and that the spectral change measured in Figure 2 reflects chemical equilibrium is established between them. The benzophenone-TEA system gave red fluorescence by UV laser irradiation. To identify the emitting species, we carried out pump and probe experiments. The probe dye laser irradiated the sample to induce the fluorescence at 50 ns after the pump UV laser when the transient absorbance became constant. The gate was set at 60-90 ns to detect the fluorescence signal only induced
0022-3654/92/2096-7244S03 .OO/O 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 18, 1992 7245
Complex Formation of Ketyl Radical and Amine
I
I-
I
1
0
/i
1
w
0
z U
9
0.5-
0
cn m a
WAVELENGTH I nm F i p 1. Transient absorption spectra obtained at 1.5 p s after the laser excitation of benzophenone in cyclohexane: (a) without TEA (open circles) and (b) with 70 mM TEA (closed circles).
0.015
I
I
I
I
b) 0.2 W
Y
s
4 K
0.1
ti OF
0
I
I
I
40
60
80
VEA] / mM
600
500
I
20
WAVELENGTH / nm Figure 2. Transient absorption spectra recorded at 40-50 ns after laser
Figure 4. (a) Reciprocal plot of the transient absorbance vs TEA concentration. (b) Plot of the transient absorbance vs TEA concentration. The solid curve is the best fitting.
excitation. The arrows in the f i r e indicate the order of increasing TEA concentrations: 6.25, 12.5, 25.0, 50.0,and 70.0 mM.
TABLE I: Kinetic Coastants for BPH + TEA + C o d e x
I
I
I
solvent cyclohexane benzene
I
KJM-' 99 f 6 37 4
*
k1/109M-' s'Ia 6.9 10
kJ108 s-I 0.7 2.7
"The diffusion-controlled rate is assumed.
where C represents the complex and n is the association number of TEA. The transient absorbance at a given wavelength is given as
OD@) = ai[BPH] + &C]
0'
I
I
I
I
600
500
I
I 700
WAVELENGTH / nm Figure 3. Fluorescence spectrum excited at 545 nm (closed circles) and fluorescence excitation spectrum monitored at 605 nm (open circles) obtained in the photolysis of the BPH-TEA system in cyclohexane. The fluorescence spectrum is not corrected for the spectral response.
by the visible dye laser. The fluorescence and fluorescence excitation spectra are plotted in Figure 3. The fluorescence and excitation spectra are in good agreement with those of the bare This fact indicates that the fluorescent species ketyl radi~al.'Z~~ is only the ketyl radical and the complex between BPH and TEA is nonfluorescent. K&tka of the BPH-TEA Complex Formation. The chemical has bem mif fed to be cstabkhed between BPH-TEA and the complex in the preceding section. We make a stoichiometrical analysis of this chemical equilibrium. The equilibrium is given as
mum
BPH
+ nTEA
C
=i
(1)
(2) where €1and c$ are molar absorption coefficients of the ketyl radical and the complex, respectively. In the spectral region where c k = 0, the reciprocal of absorbance, OD-', is given as
l/OD = (l/c&~)(K,
+ l/[TEA]"j
(3)
where k, is the equilibrium constant for reaction 1 and a is a constant being equal to [BPH] + [C]. Figure 4a shows the plot of OD-'vs [TEA]-' at 585 nm, and a good linear relationship obtained indicates n = 1 in eq 3. The 1:l complex was also formed in benzene solvent. The K, and c i values were determined from the least-squares fit of eq 2 as shown by a solid curve in Figure 4b. The X; values were obtained for five X valucs (570,575,580, 585, and 590 nm), and only the average values are listed in Table I. The absorption spectrum of the complex in the visible region in cyclohexane solvent is reproduced from the obtained K, and reported ck valuesm as shown in Figure 5. The spectrum of the complex shows a bathochromic shift of 10 nm and the decrease in the molar absorption coefficient at the peak compared with that of the ketyl radical. The vibrational structure observed in bare ketyl radical disappears in the spectrum of the complex. The kinetics of the complex formation was investigated by measuring the temporal profiles of the transient abmption. Since triplet benzophenone existed in our time window at TEA con-
7246 The Journal of Physical Chemistry, Vol. 96, No. 18, 1992 I
,I
I
_
1
.
4000
2000
0 600
500 WAVELENGTH / nm
Figure 5. Absorption spectra of bare BPH (open circles) and the BPHTEA 1:l complex (closed circles). The BPH spectrum was obtained in the photolysis of benzophenone in cyclohexane. The spectrum of the complex was calculated on the basis of the spectrum of bare BPH and the equilibrium constant K, obtained here. The e: values are calibrated with = 5100 M-l cm-l taken from ref 20.
0
100
50 TIME / ns
Figure 6. Time profiles of the transient absorbances monitored at 530 MI (open circles) and 565 nm (closed circles) with fittings shown as solid curves. The shape of the laser pulse was also plotted (triangle dot).
centrations below 50 mM, the concentration of TEA was fixed at 100 mM. The rise curves obtained at the monitor wavelengths of 530 and 565 nm are plotted in Figure 6, together with the time profde of the laser pulse. The following reaction scheme is given in our system:
-- ++ ISC
'BP lBP* 3BP* 3BP* + TEA BPH TEA' 'BP*
+ TEA
'BP
TEA
(4)
(5) (6)
BPH + TEA C K, = kl/k-l (14 where TEA' in reaction 5 represents the counter amine radical of the ketyl radical generated through the hydrogen abstraction reaction. The elactronic quenching of the triplet benzophenone (q6 ) should be included in the scheme because the quantum yield of the ketyl radical formation is ca. 0.7 even at TEA amcentrations above 50 mM.2' The transient absorbance at shorter periods after the laser pulse is given as follows:
OD = d[3BP*] + ei[BPH] + &C]
(7)
where q is the molar absorption coefficient of triplet benzophenone. The time profile of the transient absorbance, OD(r), is reproduced using the scheme described above and the time profile of the laser pulse. The formation rate constant of the from complex (k,)is roughly estimated to be 1 X 1Olo M-l the OD(t) profilea using the least-squares procedures. This value is comparable with the diffusion-controlled rate constant of cyclohexane. Although the time resolution of our syrtem is not high enough to determine the exact formation rate, we can crudely
Kajii et al. conclude the complex is formed through the diffusion-limited process. IV. Discussion Amines are known to form complexes with various substances. When the partner has a large electron affinity like Iz, the driving force for the complex formation is well explained in terms of the chargetransfer (CT) interaction.22 There arc many systems which form 1:1 hydrogen-bonded complexes between amines and hydrogen donors. Generally, the CT energy makes a major contribution to the total hydrogen-bonding energy.23In the case of hydroxy-aromatic molecules like phenol, the effluxion of the nonbonding electrons located on the oxygen atom to the aromatic ring T system becomes more important in the m* excited state than in the ground state, thus increasing the proton-donating ability (acidity) in the excited state. The similar hydrogen-bonded complex seems to be formed in the BPH-TEA system of present interest. Therefore, one can expect that the red-shifted mr* transition results from the hydrogen-bonding effect. Since BPH has an absorption spectrum similar to those of the rr* transitions of arylmethyl radicals, including the isoelectronic radical PhzCCH3, the visible and UV absorption bands of BPH can be assigned to the mr* A semiempirical calculation also supportszsthe assignment of the rr* transitions in the ahsorption spectrum of BPH. The observed red shift of the m* transition confirms the hydrogen-bonded 1:1 complex (Ph,COH-N(Et),) formation in our system. The &naphthol-TEA system was reported to form the hydrogen-bonded complex with the equilibrium constant K, = 180 M-' (at 15 "C) and the spectral shift bv, = 590 cm-l (red shift) in The values of K, = 99 M-' and bv, = 330 cm-'(red shift) obtained in the present BPH-TEA system in cyclohexane are comparable to those in the &naphthol-TEA system. As described in the Results section, the formation rate constants of the complex, k l , were estimated to be the diffusion-controlled rates: 6.9 X lo9 M-' s-' in cyclohexane and 1.0 X 1O'O M-'s-I in benzene. The rates of dissociation of the complex into BPH and TEA, k-', are, therefore, calculated to be 7.0 X lo7 s-' in cyclohexane and 2.7 X 108 s-I in benzene. It should be noted that the dissociation rate in benzene solvent is 4 times as large as that in cyclohexane. The dissociation rate of the complex is considered to be controlled with three factors. The first is the dielectric constant of the solvent: Induced dipole of the solvent molecules could stabilize the hydrogen-bonded complex, with a large dipole moment resulting from the CT interaction between BPH and TEA. However. this cannot explain the experimental results because the dissociation rate in benzene with higher dielectric constant (e = 2.28) is faster than in cyclohexane (e = 2.02). The second is the hydrogen-bonding energy. The hydrogen-bonding energies in benzene and cyclohexane, however, seem to be almost the same because the absorption spectra and the shift of peak position from that of bare ketyl radical are essentially the same in both solvents. Thus, the difference in the dissociation rates cannot originate from the hydrogen-bonding energy. The third is the cage effect of the solvent. The viscosity of cyclohexane is ca. 1.5 times as large as that of benzene. A larger cage effect is, therefore, expected in cyclohexane than in benzene, which may result in the prohibition of the dissociation in cyclohexane compared in benzene. Generally, the cage effect plays an important role in the quantum yield of the photodissociation process. The BPH-TEA complex does not fluoresce, while bare BPH has a relatively large fluorescence quantum yield. Considering the detection limit of our system, the upper limit of the excitedstate lifetime of the complex is estimated to be ca. 100 ps. The radiative lifetime of the complex will not be so different from that of the bere ketyl radical becaw the integrated absorption intensity of the complex in the visible region is almost the same as that of bare BPH, which has a fluorescence lifetime of 2-5 ns at room temperat~re.'~J~ Rapid proton transfer in the complex, yieldw both the benzophenone radical anion and HN+(Et),, or fast disfociation of the complex into BPH and TEA may be responsible for the short lifetime of the excited complex. Picosecond spec-
7247
J. Phys. Chem. 1992,96,7247-7251
(IO) Miyasaka, H.; Mataga, N . Bull. Chcm. Soc. Jpn. 1990, 63, 131.
troscopic experiments are in progress to reveal the dynamics of the complex in the excited state.
(1 1) Miyasaka, H.; Morita, K.; Kamada, K.; Mataga, N . Bull. Chem. SOC. Jpn. 1990,63,3385. Chem. Phys. Lett. 1991, 178,504. (12) Topp, M. R. Chem. Phys. Lett. 1976, 39,423. (13) Obi. K.: Yamanuchi. H. Chem. Phvs. Letr. 1978. 51.448. (14) Hiratsuka, H.; kamazaki, T.; Machwa. Y.; Hikida, T,;Mori, Y.J . Phys. Chem. 1986,90,774. (1 5 ) Nagarajan, V.; Fesscnden, R. W. Chem. Phys. Lett. 1984.11 2,207. (16) Johnston, L. J.; Lougnot, D. J.; Wintgens, V.; Scaiano, J. C. J . Am. Chem. Soc. 1988.110.518. Redmond, R. W.; Scaiano. J. C. Chem. Phvs.
Acknowledgment. This work was supported in part by the Grant-in-Aid for Scientific Research (No.03453019) from the Ministry of Education, Science and Culture. R&W NO. BPH, 16592-08-8; TEA, 121-44-8.
References md Notes
.S.G. Cohcn, J. 1..;
(\ -1I ) Cohen. H. -H. J. Am. --,-- - - - S. G.: - Chao.
Lett. 1990. 166. 20. (17) KBjii, Y.; Fujita, M.; Hiratsuh, H.; Obi, K.; Mori, Y.; Tanake I. J. Phys. Chem. 1987, 91, 2791. (18) Melhuish, W. H. J . Opt. Soc. Am. 1964, 54, 183. (19) Miyasaka, H.; Morita, K.; Mataga, N. Private communication. (20) Beckett A.: Porter, G. Trans. Faraday Soc. 1963.59. 2038. (21) Miyasaka.H.; Makga, N. Proceed& of "Dynamics and Mecha-
Chem. SOC.1968.90. , . 165. Cohen.
Phys. Chem. 1968, 72, 3782. (2) Guttenplan, J. B.; Cohen, S. G. J . Am. Chem. Soc. 1972, 94, 4040. (3) Cohen,-S. G.; Parola, A,; Parsons, G. H. Chem. Rev. 1973, 73, 141. (4) Arimitsu, S.;Masuhara, H. Chem. Phys. Lett. 1973,22,543. Arimitsu, S.;Masuhara, H.; Mataga, N.; Tsubomura, H. J. Phys. Chem. 1975, 79, 1255. (5) Inbar, S.; Linschitz, H.; Cohen, S. G.J . Am. Chem. Soc. 1980,102, 1419. (6) Peters, K. S.; Freilich, S. C.; Schaeffer, C. G.J. Am. Chem.Soc.1980, 102, 5701. Schaeffer, C. G.; Peters, K. S. Ibld. 1980, 102, 7566. (7) Bhattacharyya, K.; Das, P. K. J . Phys. Chrm. 1986, 90, 3987. (8) Harhino, M.; Shizuka. H. J. Phys. Chem. 1987,91,714. Harhino, M.; Kogure, M. Ibid. 1989, 93, 728. (9) Devadoss, C.; Fasenden, R. W. J . Phys. Chem 1990,94,4540.
nisms of Photoinduced Electron Trmfer and Related Phenomenan;Elsevier: New York, in press. (22) Nagakura, S.J . Am. Chem. Soc. 19!58,80, 520. (23) Mataga, N.; Kubota, T. Molecular Interactions and Electronic Spectra; Marcel Dekkcr: New York, 1970. (24) Bromberg, A.; Meisel, D. J. Phys. Chem. 1985, 89, 2507. (25) Minegishi, T.; Hiratsuka, H.; Tanizaki, Y.; Mori, Y. Bull. Chem. SOC. Jpn. 1984, 57, 162. (26) Mataga, N.; Kaifu, Y. Mol. Phys. 1963, 7, 137.
Cobalt, Rhodium, and Iridium Dioxide Molecules and Waish-Type Rules R. J. Van Zee, Y.M. Hamrick,+ S. Li, and W.Weltner, Jr.* Department of Chemistry and Chemical Physics Center, University of Florida, Gainesville, Florida 32611-2046 (Received: March 27, 1992; In Final Form: May 18. 1992)
COO,, Rho,, and IrO,, readily formed by the reaction of laser-vaporized metal atoms with oxygen, have been shown to be linear with 22ground states. This was established by electron-spin-resonance spectra of the molecules isolated in solid argon lo3Rh , (I = '/2): 191J931r (I = 3/2), and 170(I = hyperfine splittings have been obtained and at -2 K. 59C0(I= 'I,) indicate that the electron spin is predominantly on the metal atom and has -50% so character. Attempts to observe the metal atom-02 complexes were not successful. A Walsh-type rule applied to transition-metal dioxides is supported by the geometries found here.
I. Introduction In the course of research on the clusters of the group VI11 metals, Co, Rh, and Ir,' electron-spin-resonance (ESR) signals were detected which were established by addition of O2(and 1702) to be due to the metal dioxide molecules. There are several reasons for being interested in these dioxides. There is the question of whether a dioxide complex M+(O,-) is formed by these metals and whether its ESR spectrum can be detected, as has been the case for the alkali2-s and coinage metals.61' The IR spectrum of such a Rh complex has been reported by Hanlan and Ozin.12 Is this complex a necessary precursor to the formation of a covalently-bonded dioxide mole cule? Is the latter linear or bent and of high or low spin? The geometry and electronic properties in the ground state are also of intertst relative to the suggestion that Walsh-like rules may apply to such transition-metal (TM) triatomics. A simple molecular orbital scheme can be set up for transition-metal dioxides and dihalides,13-17 analogous to Walsh's treatment of BAB molecules1Ebut emphasizing d rather than p orbitals on A.ie2i Instead of an abrupt change from linear to bent geometry upon addition of the 17th valence electron (e.g., linear C 0 2 to bent NO,), the TM triatomics are proposed to change from bent to lincar when the 19th valeme electron is added (e.&, bent TiF2 to linear VF2).13-17Among the first-raw dioxides, the geometry change would then occur from CrO, to Mn02, and most experimental evidence supports this scheme.22-26 Some
dioxides are still controversial, such as Fe0227-m and perhaps C U O ~ . ~Here ~ * ~the * ESR evidence that COO2, Rho2,and Ir02 (with 21 valence electrons) are linear is in accord with this Walsh-type rule. Finally, these metal oxides are of interest bccause of their possible relevance to the well-known catalytic activity of these metals. It is worth noting that there is not a monotonic change in the electronic properties of these metal atoms in going down the periodic table. Rh is ammalous in having a d8s (4F) ground state, whereas Co and Ir have d7s2(4F) lowest states with the d's lying approximately 0.4 eV h i g h ~ r . ~ )Thus, . ~ ~ for Rh, a promotion energy is not needed to utilize its extended 5s electron, and its exceptional reactivity is noticeable in matrices Containing H2,02, or CH,. This difference in the atomic properties should also ultimately be reflected in the molecular orbitals of the dioxides and in their electronic and magnetic properties. This is indeed found to be the case.
II. ExperimentalSection The Heli-Tran and ESR apparatus have been described prev i o u ~ l y ?The ~ ~ metal ~ vapor was produced with a Nd:YAG laser operating at 1064 nm. Cobalt rod, 5 mm diameter X 12 mm long (99.998% purity, 100% s9c~, I = 7/2), rhodium powder pressed into a pellet ap roximately 1 mm X 10 mm diameter (99.95% purity, 100%lo Rh,I = and iridium powder p r d into a pellet approximately 1 mm X 10 mm diameter (99.95% purity, 38.5% l9IIr, I = 3/2, and 61.5% 1931r,I = 3/2) were purchased from
P
Present address: Am= Laboratory, Amos, I A 50010.
0022-3654/92/2096-7247S03.00/0
(B
1992 American Chemical Society
