3636
J . Phys. Chem. 1985,89, 3636-3641
The question arises whether an equilibrium between TIN(intra) and TIN(inter) is established in the first excited singlet state. Since the emission excitation spectrum of the blue fluorescence (Aobsd = 445 nm) of SI of TIN(inter) does not show a band at 345 nm (Figure 3, curve B, at 345 nm is the absorption maximum of TlN(intra), Figure 1 (acetonitrile)); TIN(intra) does not contribute to the fluorescence of SI of TIN(inter): or in other words, the conversion in the first excited singlet state of TIN(intra) to TIN(inter) is too slow a process to compete effectively with the other deactivation processes in accordance with the aforementioned rate constant of hydrogen bond opening as determined by the ultrasound method in the ground state.30 The equilibrium between TIN(intra) and TIN(inter) in the ground state depends on temperature (the concentration of TIN(intra) increases with temperature, see section 1.2) whereas the emission excitation spectrum of TIN(inter) (see section 1.4 and Figure 3) does not change with temperature in accordance with our assumption that in thefirst excited singlet state TIN(intra) is not converted into TIN(inter) (equilibrium not established). This is a very important result as it excludes the possibility that SIof TIN(intra) is converted into S, of TIN(inter) and then with high ISC yield to T I of TZN(inter). Otherwise TIN(intra) would lose its high photostability. Conclusion The intramolecular hydrogen bond within the stabilizer molecule must be preserved to stabilize polymers against UV degradation for the following reasons: (i) The inner filter effect of the stabilizer decreases drastically for X > 320 nm if the intramolecular hydrogen bond is changed into an intermolecular one to the polymer. (ii) The stabilizer with the intermolecular hydrogen bond TIN(inter) has a triplet yield of 0.15 at 296 K and a long triplet lifetime which may be the origin of the smaller photostability (of TIN(inter) as compared to TIN(intra)) of the stabilizer itself and of the polymer (initiation of polymer degradation processes). (iii) Intramolecular proton transfer in the excited singlet state -'~ the can only take place in the TIN(intra) m o l e ~ u l e ~whereby proton transferred species SI' ( N H species) is more rapidly deacticated ( T ~ '= 141 ps, crystalline state, 296 K)12 than SI of
TIN(inter) (7F= 0.4 ns in Me2S0, 296 K, see ref 25, p 85). Thus +' is shorter than rF by approximately a factor of 3 although T{ refers to the crystalline state and T~ to the fluid phase. The equilibrium between TIN(intra) and TIN(inter) in the ground state is mainly determined by steric factors and by the H-acceptor ability of the solvent. The intramolecular hydrogen bond of T I N might be preserved by introducing voluminous substituents in position 3' of the phenyl ring. The stabilizer molecule can be fixed by an intermolecular hydrogen bond to the polymer which, on the other hand, is not desirable since the intact intramolecular hydrogen bond is the origin of the photostability. It is therefore suggested that the stabilizer molecule should be fixed to the polymer by a second functional group thus leaving the intraring intramolecular hydrogen bond intact with its efficient deactivation mechanism. In the case of poly(m-phenyleneisophthalamide)a monomer unit was attached to the T I N molecule thus providing high compatibility of stabilizer and polymer12q16and leaving the intramolecular hydrogen bond intact as revealed by the absorption spectrum." A high photostability of the system results as was shown by Herlinger and co-workers.I6 Acknowledgment. We express our sincere thanks to Dr. T. Werner, Bayer AG., Leverkusen, for helpful suggestions. Further thanks are due to Dr. Helmut Mueller, CIBA-GEIGY, Basel, and to Prof. Dr. H. Herlinger and Dr. B. Kuester, Institute fuer Textilund Faserforschung, Wissenschaftliche Institute an der Universitaet Stuttgart, for providing us samples. The financial support of the Deutsche Forschungsgemeinschaft and of the Fonds der Chemischen Industrie is gratefully acknowledged. Registry No. I (R=H), 2440-22-4; I (R=Me), 58380-86-2; 11, 97012-33-4;triethylamine, 121-44-8;acetonitrile,75-05-8; diethyl ether, 60-29-7; tetrahydrofuran, 109-99-9; n-heptane, 142-82-5; dibutyl phthalate, 84-74-2; pyridine, 110-86-1;dimethylacetamide, 127-19-5; dimethyl sulfoxide, 67-68-5; quinuclidine, 100-76-5.
Supplementary Material Available: Fractional atomic coordinates for H atoms, anisotropic temperature factors for C, N, and 0 atoms, and calculated and observed structure factors (23 pages). Ordering information is given on any masthead page.
Sensitized Phosphorescence Excitation Spectra of Benzoic Acid Monomer and Methyl Benzoate and Their Complexes in Supersonic Jets Shin-ichi Kamei, Haruo Abe, Naohiko Mikami, and Mitsuo Ito* Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received: February 27, 1985)
Recently developed sensitized phosphorescence excitation spectroscopy has been applied to observe the electronic excited states of benzoic acid monomer and methyl benzoate in supersonic jets, states which had never been detected by any other means. We observed for the first time the '(?r,~*) states of benzoic acid monomer and methyl benzoate by the phosphorescence excitation technique. The '(?r,s*)states of the monomer and methyl benzoate have been found to be nonfluorescent but phosphorescent. The emission properties of benzoic acid and methyl benzoate change very much upon complexation, which leads to an enhancement of the fluorescence quantum yield. The emission properties of the l(?r,?r*) state have been discussed in terms of internal conversion and intersystem crossing to nearby singlet and triplet states.
(1) Poeltl, D. E.; McVey, J. K. J . Chem. Phys. 1983, 78, 4349. (2) Tomioka. Y.: Abe. H.: Mikami. N.: Ito. M. J . Phvs. Chem. 1984. 88.
2263.
0022-3654/85/2089-3636$01 .50/0
available for the excited-state monomer is the iocation of its '(a,**)state, which has been determined from the absorption spectra of benzoic acid solution at low t e m p e r a t ~ r e s . ~ In a 0 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 17, 1985 3637
Sensitized Phosphorescence Excitation Spectra 3.pH3
METHYLBENZOATE ( @ ABSORPTION ( VAPOR)
I
36000
I
I
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PHOSPHORESCENCEEX. (940)
(1272)
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36500 WAV ENUMBER (cm-9
37600
(a) 35704 I
FLUORESCENCE EX
35926 I
36000
36500 WAVENUMBER (cm-1)
37000
Figure 1. Fluorescence excitation spectrum (a) and sensitized phosphorescence excitation spectrum (b) of methyl benzoate in a supersonic jet. The numbers in parentheses are the frequency differences from the 0,O band (36 105 cm-'). (c) Electronic absorption spectrum of methyl benzoate vapor at room temperature.
previous work: we tried to observe the spectrum of the monomer in a supersonic jet by using both fluorescence excitation and multiphoton ionization techniques. Although we could observe the spectrum of the dimer, the band due to the monomer was never detected. However, definite evidence was obtained for the presence of the monomer in the jet through the detection of bands due to the monomer hydrogen bonded with water in the fluorescence excitation spectrum. We concluded from these results that the monomer itself is not fluorescent but its hydrogen-bonded complex becomes fluorescent. A similar situation has also been found for methyl benzoate2 in which the hydrogen atom of the OH of benzoic acid is substituted by a methyl group. In the fluorescence excitation spectrum of methyl benzoate in a supersonic jet, the bands due to the hydrogen-bonded species were observed but no band was found for methyl benzoate itself. Recently, we have applied sensitized phosphorescence excitation spectroscopy to a molecule seeded in a supersonic jet4 and demonstrated its potential for nonfluorescent or weakly fluorescent molecules. We applied this method to detect the benzoic acid monomer and methyl benzoate. In this paper, we report the fluorescence excitation, dispersed fluorescence, and sensitized phosphorescence excitation spectra of benzoic acid and methyl benzoate in supersonic jets. We detected for the first time the '(?r,?r*) states of methyl benzoate and the benzoic acid monomer in jets by the sensitized phosphorescence excitation technique. We also studied hydrogenbonded methyl benzoate and benzoic acid. The simultaneous measurements of the fluorescence excitation and phosphorescence excitation spectra showed that the emission properties of benzoic acid and methyl benzoate change very much upon complexation. The emission properties and their changes upon complexation will be discussed.
Experimental Section The experimental apparatus for the simultaneous measurements of fluorescence excitation spectrum and sensitized phosphorescence excitation spectrum in combination with a pulsed supersonic free (3) Ito, M. J . Mol. Specrrosc. 1960, 4, 144. Ito, M.; Tsukioka, M.; Imanishi, S. J . Am. Chem. Soc. 1960, 82, 1559. (4) Abe., H.; Kamei, S.; Mikami, N.; Ito, M. Chem. Phys. Leu. 1984, 109, 217.
jet has been described el~ewhere.~ Methyl benzoate was heated to 312 K to increase the vapor pressure to about 1 torr. Benzoic acid, was heated to 370 K. The sample vapor seeded in 3 atm of He was expanded into a vacuum chamber at torr through a 400-pm pulsed nozzle. The second harmonic of a tunable dye laser (Molectron DL-14, Coumarine 540 A) pumped by a nitrogen laser (Molectron UV-24) was used as an exciting source, and its intensity was kept constant in all spectral region studied. The laser beam crossed the jet 15-mm downstream and the molecule in the jet was excited to its l ( r , r * ) state. The fluorescence excitation spectra were obtained by detecting the total fluorescence with a photomultiplier (HTV-R562), and the photocurrent was averaged by a boxcar integrator (Brookdeal 9415/9425). On the other hand, the triplet state molecule prostate by intersystem crossing travels for duced from the '(?r,r*) 40 ps from the excitation position and collides with a liquid-nitrogen-cooled copper surface that was installed 85-mm downstream from the nozzle. The cold surface is covered by either the solid sample with the same sample vapor supplied from the previous jet or solid biacetyl which was supplied from its vapor through another nozzle. The solid serves as a phosphor. When the triplet-state molecules in the jet collide with the phosphor, energy transfer from the excited molecule to the phosphor occurs, and sensitized phosphorescence is emitted. We detected this emission by a photomultiplier (HTV-R585) and the signal was processed by a gated photon-counting system (Ortec 9302, 9315, 9325 and a home-made gate generator with a 30-ms gate width after a 50-ps delay from laser fire). With the above procedure, we obtained the sensitized phosphorescence excitation spectrum. The dispersed fluorescence spectrum was measured by using a Nalumi 0.75-m monochromator in the second order with a HTV-R928 photomultiplier. Methyl benzoate and benzoic acid, which were obtained from Wako Pure Chemical Industries, were purified by repeated vacuum distillations and vacuum sublimations, respectively.
Results and Discussion Methyl Benzoate. Figure l a shows the fluorescence excitation spectrum of methyl benzoate ('(r,n*) So) in a supersonic free jet. There exist two sharp bands at 35 704 and 35 926 cm-', and also a similar pair at 36 043 and 36 268 cm-I. It was already shown in our previous study2 that all the bands appearing in the
-
3638 The Journal of Physical Chemistry, Vol. 89, No. 17, 1985
Kamei et al.
METHYLBENZOATE H2O +
B 35926
A 35704
FLUORESCENCE EX.
c I
(a)
35897
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I
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I
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I
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C
A
_p
_.
I
36200 36000 WAVENUMBER/ cm-1 Figure 2. Fluorescence excitation spectrum (a) and sensitized phosphorescence excitation spectrum (b) of a mixture of methyl benzoate and water in a supersonic jet
35800
fluorescence excitation spectrum are due to hydrogen-bonded complexes formed between methyl benzoate and H20which was contained in the sample as an impurity. The hydrogen-bonded complex was evidenced by the splitting of each band into two bands by the addition of heavy water to the jet. It was concluded that methyl benzoate is not fluorescent but it becomes fluorescent by the formation of a hydrogen-bonded complex. Figure 1b shows the phosphorescence excitation spectrum of methyl benzoate in a supersonic free jet, which was simultaneously measured with the fluorescence excitation spectrum. It is noted that the phosphorescence excitation spectrum is quite different from the fluorescence excitation spectrum. It is also found that main bands appearing in the phosphoresoence excitation spectrum correspond closely with the broad peaks of the absorption spectrum of methyl benzoate vapor at room temperature shown in Figure IC. The close resemblance to the absorption spectrum indicates that the main bands in the phosphorescence excitation spectrum are due to methyl benzoate. From the absence of these bands in the fluorescence excitation spectrum, it is concluded that methyl benzoate does phosphoresce but not fluoresce. Hence, all the state go to the triplet state by rapid molecules in the I(*,**) intersystem crossing before they fluoresce. The strongest band a t 36 105 cm-' in the phosphorescence excitation spectrum can be assigned to the 0,O band of the '(*,**) So transition of methyl benzoate. The vibronic bands in the wavelength region shorter than the 0,O band represent the excited-state vibrational frequencies, which are shown in parentheses in the figure. On the longer wavelength side of the 0,O band, there exist two prominent bands at 35 926 and 35 914 cm-l. The former band coincides in frequency with the band observed in the fluorescence excitation spectrum. Therefore, it must be due to the hydrogen-bonded complex formed between methyl benzoate and water contained in the sample. On the other hand, the weak band at 35 914 cm-' has no counterpart in the fluorescence excitation spectrum. It is also difficult to consider it as a hot band, because its intensity does not change upon changing the stagnation pressure. From our experience in the study of the electronic spectra of rotational isomers of aromatic molecules,' the band is
-
( 5 ) Oikawa, A.; Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1984,88, 5180.
probably assigned to a rotational isomer of methyl benzoate. Methyl benzoate has two rotational isomers (I and 11). In the
CH3 0
0,
0,
,o,
6 6
CH3
I
I1
electronic ground state, isomer I will be more stable than isomer I1 when steric hindrance involving the bulky methyl group is taken into account. Therefore, the strong band at 36 105 cm-' may be assigned, to the 0,O band of isomer I, while the weaker band at 35914 cm-' can be assigned to the 0,O band of the less-stable isomer 11. The frequency difference between the two bands is 191 cm-], which is the same order of magnitude as those found in the rotational isomers of various substituted phenols reported by Oikawa et al.s If we assume that the relative intensity of the two bands reflects the concentration ratio of the two isomers, the ratio of I1 to I is less than 1/20, isomer I forming the majority. Therefore, in our further discussion, we ignore the presence of isomer 11. It is of interest to note that isomer I1 is also not fluorescent but phosphorescent, We emphasize here that the exact location of the origin of the '(*,a*) So transition of methyl benzoate was determined for the first time by sensitized phosphorescence spectroscopy. Hydrogen-Bonded Methyl Benzoate. Figure 2, a and b, shows the simultaneously measured fluorescence excitation and phosphorescence excitation spectra, respectively, for a mixture of methyl benzoate and water in a supers6nic free jet. It is seen by comparison between the phosphorescence excitation spectta shown in Figure 2b and Figure 1b that the intended addition of water induces an intensity enhancement of the A (35704 cm-l), B (35 926 cm-I), and C (35 897 cm-l) bands shown in Figure 2b relative to the intensity of the 0,O band at 36 105 cm-l. The intensity enhancement indicates that these bands are due to complexes formed with water. In the fluorescence excitation spectrum, they also appear as seen in Figure 2a, two of them, A and B, already being mentioned in a previous section. By comparison between the fluorescence excitation spectra shown in Figure 2a and Figure l a the intensity enhancement by the addition
-
The Journal of Physical Chemistry, Vol. 89, No. 17, 1985 3639
Sensitized Phosphorescence Excitation Spectra
-
difference is seen for the band at 1300 cm-I, which is assigned to C-OCH, stretching mode6 (1319, 1295, and 1301 cm-I for A, B, and C, respectively). These results also support the assignments of the A, B, and C bands as due to different hydrogen-bonded species. The 1:l complexes associated with the A and B bands are probably assigned to I11 and IV, respectively.
( a ) A(35704cml)EX 1004
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( c ) C (35897cm'l)EX. 1004
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35000 WAVENUMBER (cm-1)
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Figure 3. Dispersed fluorescence spectra of methyl benzoate-water hydrogen-bonded complexes obtained by exciting the A (35 704 cm-l) band (a), B (35926 cm-I) band, (b) and C (35897 cm-') band (c) in the fluorescence excitation spectrum (Figure 2a). Arrows indicate exciting positions. The numbers assigned to the bands are the frequency differences from the exciting position.
of water is much larger for the C band than for the A and B bands. Vibronic bands belonging to the A, B, and C bands are also found at 36 043 (A 339), 36 268 (B 342), and 36 241 cm-' (C 343), respectively. Although the A, B, and C bands appear in both the phosphorescence excitation and fluorescence excitation spectra, their relative intensities are quite different between the two spectra; the intensities of the A and B bands are comparable in the fluorescence excitation spectrum, while the A band is much weaker than the B band in the phosphorescence excitation spectrum. The intensity dependence upon the water concentration suggests that the number of water molecules contained in the hydrogenbonded complex is different between the complexes associated with the A and B bands and the complex associated with the C bands. We tentatively assign the A and B bands as due to a 1:1 complex (methyl benzoate:water) and the C band to a 1:2 complex. It is also suggested from the great difference in the relative intensity of the A band to the B band between the two spectra that the 1:l complexes associated with the A and B bands are different, that is, they are structural isomers. Therefore, in total, we have three hydrogen-bonded species: two 1:l complexes (A and B) and one 1:2 complex (C). In order to confirm this, we observed the dispersed fluorescence spectra. Parts a-c of Figure 3 show the dispersed fluorescence spectra obtained by exciting the A, B, and C bands, respectively. The excitation positions are shown by arrows. Each spectrum exhibits a typical pattern of the resonance fluorescence originating from a band origin, that is, the ground-state vibrational structure is developed from the strongest band at the excitation position. The observed ground-state vibrational structure is similar among the three spectra. However, distinct differences exist in two respects. One is the difference in the low-frequency hydrogen-bond mode frequencies: 60 and 125 cm-I for A band, 58 and 104 cm-I for B, and 79 cm-I for the C excitation. Another clear frequency
+
+
+
lv
The assignments are supported by the fact that the ground-state frequency of the 0-CH, stretching mode for the complex associated with the A band (13 19 cm-') is not greatly different from the value for methyl benzoate (13 15 cm-'),6 while the frequency for the B band (1 295 cm-') is considerably different. This may be readily explained by the assignments given above. The frequencies of the ground-state hydrogen bond modes are larger in the A complex (60 and 125 cm-I) than in the B complex (58 and 104 cm-I), indicating a stronger hydrogen bond in the former than in the latter. We have an empirical rule that the spectral red shift of a '(?r,a*)state of an aromatic molecule induced by hydrogen bonding is generally larger for stronger hydrogen bonding in the ground state.' Applying the rule to the present case, we expect that the spectral red shift is larger in the A complex than in the B complex. The observed red shift measured from the 0,O band of methyl benzoate (36 105 cm-l) is 401 and 179 cm-' for the A and B bands, respectively, in agreement with expectation. We also measured the fluorescence excitation and phosphorescence excitation spectra for mixtures of methyl benzoate and various proton-donating molecules (methanol, ethanol, acetic acid, and trifluoroacetic acid) in supersonic jets. In each mixture, two bands corresponding to the A and B bands were found. For the band corresponding to the A band, the red shift from the origin of methyl benzoate is 401 (HzO), 403 (CH,OH), 398 (C2H50H), 681 (CH,COOH), and 848 cm-I (CF,COOH). It is 179 (H20), 198 (CH,OH), 205 (C2H50H),288 (CH,COOH), and 533 cm-I (CF,COOH) for the band corresponding to the B band. The red shift increases roughly in the increasing order of the proton-donating power of the molecule, confirming again the presence of hydrogen-bonded complexes. Similar to the A and B bands, the band corresponding to the A band is weaker than the band corresponding to the B band in the phosphorescence excitation spectrum, while their intensities are comparable in the fluorescence excitation spectrum. The 1:2 complex to which the C band belongs is probably one in which two oxygen atoms of methyl benzoate are hydrogen bonded to water (V).
H.
0
A
CH3
v Benzoic Acid Monomer. Similar to methyl benzoate, the benzoic acid monomer is nonfluorescent, but it becomes fluorescent when a hydrogen-bonded complex is formed. In our previous study,2 we predicted the band origin of the '(n,?r*) state of the monomer at 36 000 cm-' from the origins of the fluorescent hydrogen-bonded complexes. We apply here the sensitized phosphorescence excitation technique to detect directly the benzoic acid monomer.
-
(6) Greem, J. H.S.;Harrison, D. J. Spectrochim. Acta 1976, 33, 5 8 3 . (7) Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1982, 86, 1768.
3640 The Journal of Physical Chemistry, Vol. 89, No. 17. 1985
Kamei et al.
O p -
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............ 35727
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35800
3 6 200 WAVENUMBER (cm-1)
36600
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Figure 4. Fluorescence excitation spectrum (a) and sensitized phosphorescence excitation spectrum (b) of benzoic acid in a supersonic jet. The star shows the band due to the benzoic acid-water hydrogen-bonded complex. The broken curve indicates the absorption spectrum of benzoic acid vapor at room temperature.
In Figure 4a is shown the fluorescence excitation spectrum of benzoic acid in a supersonic jet, which was already reported in our previous paper.2 As was concluded there, all the bands except for a weak band at 35 874 cm-' (which is shown by star in the figure) are due to the benzoic acid dimer. The strongest band at 35 727 cm-I is the origin of the ~(T,T*)state of the dimer. The band at 35 874 cm-' is due to a hydrogen-bonded complex formed between the benzoic acid monomer and water. Figure 4b shows the sensitized phosphorescence excitation spectrum simultaneously measured with the fluorescence excitation spectrum. Beside the dimer bands, several new bands appear which are completely absent from the fluorescence excitation spectrum. They are sharp bands at 35 923 and 35 943 cm-' and a broad band at 35 965 mi1. It is found that their locations nearly coincide with a longest wavelength broad peak of the absorption spectrum of benzoic acid vapor shown by a broken curve in the figure. The good correspondence indicates that they are due to the benzoic acid monomer. The two sharp bands at 35 923 and 35 943 cm-l are assigned to the origins of the two rotational isomers of the benzoic acid monomer (VI and VII). If we take into account steric repulsion
H I
oy'o VT
0,
,o,
C
H
0 m
between the hydrogen atom of OH and the ortho H atom of the benzene ring, VI would be more stable than VI1 in the ground state. Then, the stronger band at 35 923 cm-' will be assigned to VI and the weaker band at 35 943 cm-I to VI. The frequency difference of 20 cm-l is much smaller than that for the rotational isomers of substituted phenol (10O-3OO cm-l) reported by Oikawa et aL5 The small difference may be understood because the OH group is more separated than the substituted phenol from the benzene ring in which (T,T*)excitation is mainly localized. At present, we cannot make an assignment for the band a t 35 965 cm-I, although its broadness suggests some kind of complex. Thus, the benzoic acid monomer was detected for the first time by the sensitized phosphorescence excitation method. This means that
the benzoic acid monomer is phosphorescent but nonfluorescent, just as in methyl benzoate. On the longer wavelength side of the band origins, we notice a band at 35 874 cm-l which is the same band as that shown by the star in the fluorescence excitation spectrum. Therefore, this band must be due to a hydrogen-bonded complex with water. Similar to methyl benzoate, the benzoic acid monomer becomes fluorescent as well as phosphorescent when it forms a hydrogen-bonded complex. Finally, it may be worth mentioning that, in the course of the spectral measurements, we noted the following. In sensitized phosphorescence excitation spectroscopy, a solid sample deposited on a cooled copper surface supplied from a jet is usually used as a phosphor. However, benzoic acid solid is a rather poor phosphor and a spectrum with a good S / N ratio is not obtained. Therefore, we used a mixture of benzoic acid and biacetyl as a sample because the biacetyl solid is known to be a good phosphor. By using the mixture, we could obtain the phosphorescence excitation spectrum with a very good S / N ratio. The spectrum is the same as that shown in Figure 4b, except for the appearance of a strong band at 35866 cm-'. This band is completely absent from the simultaneously measured fluorescence excitation spectrum. Next, we carried out a measurement in which only benzoic acid (seeded in He) was used as the sample and the cooled copper surface was covered by biacetyl solid desposited from its vapor introduced from another nozzle. Then, we found that the band at 35 866 cm-l completely disappears. From the above results, it is apparent that the band is due to a complex formed between the benzoic acid monomer (or dimer) and biacetyl. The complex is phosphorescent and nonfluorescent. This emission property is somewhat different from that of the hydrogen-bonded complexes mentioned before, that is, the hydrogen bonded complex emits both fluorescence and phosphorescence although their relative rates vary with the kind of complex. As far as the emission property is concerned, the complex formed between benzoic acid and biacetyl is similar to that between benzoic acid monomer and methyl benzoate. Emission Properties. The most interesting aspects of the benzoic acid monomer and methyl benzoate and their complexes are their characteristic emission properties. Both benzoic acid monomer and methyl benzoate are nonfluorescent but phosphorescent. Their hydrogen-bonded complexes including the benzoic
Sensitized Phosphorescence Excitation Spectra acid dimer are fluorescent and phosphorescent. The complex of the benzoic acid monomer (or dimer) with biacetyl is nonfluorescent and phosphorescent. The emission properties mentioned above are very similar to those of the molecules in rigid glass solutions at 77 K reported by Baba and Kitamura.' They have found that the benzoic acid monomer shows phosphorescence alone, while the dimer emits not only phosphorescence but also fluorescence. They have inferred that the benzoic acid monomer has an 3(n,a*) state between the I(a,a*)state and the lowest 3(n,a*) state, and this results in efficient intersystem crossing from the '(a,**)state to the j(n,?r*) state, and hence in the nonfluorescent nature of the monomer. The appearance of fluorescence in the dimer has been explained on the assumption that the 3(n,a*) state rises above the '(?r,a*) state owing to the hydrogen bonds involved in the dimerization. Although the possible existence of the 3(n,a*) state between the '(*,a*)and 3(?r,a*) states has been denied later by Ridley and Zerner9 on the basis of their INDO/S calculation of the benzoic acid monomer, the interpretation given by Baba and Kitamura is still attractive. A problem inherent to the benzoic acid monomer and methyl benzoate is the lack of experimental information on a low-lying state '(n,?r*) state. According to the c a l c u l a t i ~ n s , the ~ ~ l'(n,a*) ~ is always predicted to lie below the '(a,n*)state. However, all the experimental efforts to find the '(n,a*) state in the energy region below the '(a,a*)state were in vain. We also searched for the '(n,a*) state of methyl benzoate in a jet by fluorescence excitation and sensitized phosphorescence excitation methods over a frequency region 4000 cm-I below the '(a,**)state, but no signal was obtained. The lack of experimental evidence for the '(n,a*) state led Baba and Kitamura' to the assumption that there is no '(n,a*) state below the I(a,a*)state and the latter is the lowest singlet excited state. However, for benzoic acid dimer, the l(n,a*) state was found to be very close to the '(a,x*) state, but the former is still below the latter.* This strongly suggests that the '(n,a*) state is the lowest singlet excited state in the monomer too. Actually, recent calculations (STO-3G + CI(MR-BWPT)) on the benzoic acid monomer by Koseki et a l l 0 predicted the lowest excited singlet state to be '(n,?r*). They also showed that 3(n,a*) is the lowest excited triplet state and there exist a 3(n,a*) state below the l(a,a*)state. With these relative energy levels, we may easily explain the phosphorescent-nonfluorescent nature of the benzoic acid monomer or methyl benzoate. Two efficient nonradiative processes can be considered; one is
--
IC
'(a,a*)
'(n,a*)
and the other
--
ISC
'(a,**)
--
ISC
3(n,a*)
3(n,a*)
Since the energy difference between the I(a,?r*)and '(n,?r*) states is believed to be more than 3000 cm-I,Io rapid internal conversion would be possible from the '(?r,?r*) state to the '(n,?r*) state, even in an isolated molecular condition. The two rapid processes leading to the triplet states explain the strong appearance of the '(a,a*) state of the benzoic acid monomer or methyl benzoate in the phosphorescence excitation spectrum and its absence in the fluorescence excitation spectrum. When the benzoic acid monomer or methyl benzoate forms a hydrogen bond between the oxygen atom of the C=O group and a proton-donating molecule (such as HI), both the energy levels of the '(n,a*) state and the 3(n,?r*) state are raised owing to the stabilization of nonbonding electrons localized on the oxygen atom. It would be very probable that the raised '(n,a*) state comes very close to or above the '(*,a*)state. In the former case, an efficient internal conversion from the '(?r,a*) to the '(n,a*) state does not occur because of the small density of states in the '(n,?r*) state under isolated molecule conditions. In the lattter case, the '(n,r*)
The Journal of Physical Chemistry, Vol. 89,No. 17, 1985 3641 state does not play a role in the emission property of '(a,a*). Therefore, in the hydrogen-bonded complexes, one of the two processes existing in the monomer or methyl benzoate '(a,a*)IC l(n,a*) ISC 3(n,a*)
--
--
is practically removed and only the direct process of ISC
'(a,**)
+
3(n,7r*)
remains. The 3 ( n , ~ * )state in the hydrogen-bonded complex is assumed to be located at an energy below the '(a,**)state where the vibrational density of states in the j(n,a*) state is large enough to induce appreciable intersystem crossing. The fluorescentphosphorescent nature of the l(a,?r*)state in the hydrogen-bonded complex may be understood with the above scheme. However, the above explanation cannot be directly applied to the hydrogen-bonded complex in which the oxygen atom of the OH group of benzoic acid or of the OCH3 group of methyl benzoate participates in hydrogen bonding (such as IV). It is known that nonbonding electrons localized on the oxygen atom of the O H or OCH3 are more stable in energy than those of the C=O group. The electronic states associated with these nonbonding electrons are assumed to be at higher energies and they are not directly involved in the emission property of the '(a,a*) state. However, we may expect appreciable effects of hydrogen bonding on the relative locations of the '(n,a*),'(n,a*), 3(n,x*), and 3(a,a*)states, although the effect would be much smaller than that of hydrogen bonding with the C=O group. The emission property of the hydrogen-bonded complex involving the O H or OCH3group is intermediate between the properties of the benzoic acid monomer or methyl benzoate and of the hydrogen-bonded complex involving the C=O group. That is, as seen from Figure 1, the quantum yield ratio @F/@P estimated from the peak heights of the band in the fluorescence excitation and phosphorescent excitation spectra is 0 for methyl benzoate and about 10 for the hydrogen-bonded complex involving the C=O group when the ratio is normalized to unity for the complex in which the OCH3 group participates in the hydrogen bonding. This suggests that the effects of the hydrogen bonding through the O H or OCH3 group on the electronic states are in the same direction as but smaller than those of the C=O hydrogen bonding. In a previous section, we mentioned a band at 35866 cm-I observed in the phosphorescence excitation spectrum for the mixture of benzoic acid and biacetyl. This band must be due to a complex formed between the benzoic acid monomer (or dimer) and biacetyl. It is probably a van der Waals complex which is strongly phosphorescent and nonfluorescent. Among the various solvent molecules used in the formation of the complex, biacetyl is unique in a sense that it has several electronic states lower in energy than the '(n,n*)state of the benzoic acid monomer (or dimer). Biacetyl is also known to be a very good phosphorescence sensitizer. It is suggested from these facts that the phosphorescent-nonfluorescent nature of the complex arises from efficient energy transfer of the '(a,a*)excitation energy which is mainly localized in the benzoic acid moiety of the complex to low-lying phosphorescent states of the biacetyl moiety. Such an energy transfer in a complex has recently been reported by Tomioka et aLil for the mixed dimer of benzoic acid and p-toluic acid. In conclusion, the '(a,a*)states of the benzoic acid monomer and methyl benzoate in jets were detected for the first time by applying sensitized phosphorescence excitation spectroscopy. It was found that both the benzoic acid monomer and methyl benzoate are nonfluorescent but phosphorescent. The hydrogen-bonded complexes formed with various proton-donating molecules were also studied and they were found to be fluorescent and phosphorescent. The emission properties of the complexes seem to be sensitively determined by the changes of the low-lying singlet and triplet states induced by hydrogen bonding. Registry No. Benzoic acid, 65-85-0; methyl benzoate, 93-58-3.
(8) Baba, H.; Kitamura, M. J. Mol. Spectrosc. 1972, 41, 302. ( 9 ) Ridley, J. E.;Zerner, M. C . J . Mol. Spectrosc. 1979, 76, 7 1 . (IO) Koseki, S.; Nakajima, T., private communication.
( 1 1 ) Tomioka, Y . ;Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1984,88,
5186.
