8252
J . Phys. Chem. 1992,96, 8252-8259
Transient Raman Studies on the Structure of the ChioraniCAikyibentene Triplet Charge-Transfer Complexes Tahei Tahara and Hiro-o Hamaguchi* Molecular Spectroscopy Laboratory, Kanagawa Academy of Science and Technology (KAST). KSP East 301, 3-2-1 Sakato, Takatsu, Kawasaki 213, Japan (Received: April 20, 1992; In Final Form: June 17, 1992)
Transient resonance Raman spectroscopy has been used to study the structure of chloranilalkylbenzenetriplet chargetransfer complexes in solution. Resonance excitation conditions were chosen so that only Raman bands due to chloranil vibrations were selectively enhanced. The Raman spectra of the complexes with weak donors were similar to that of TI chloranil, while those of the complexes with strong donors were similar to that of the chloranil anion radical. The observed vibrational frequencies of chloranil in the triplet complexes were plotted as a function of the ionization potential of donors, and common “S-shape (or inverted S-shape)”dependence was observed for all the vibrations. This S-shape dependence was well-reproduced by a simple theory. It was concluded that the molecular structure of the chloranil moiety in these triplet complexes is intermediate between that of the free TI state and that of the anion radical. The structure changes in accordance with the change in the electron-donating ability of donors, reflecting the change in the magnitude of the charge transfer occurring in the complexes.
1. Introduction
Electron transfer or charge transfer (CT) is one of the most important elementary p r o c a w in chemistry. The chargetransfer complexes, or electron donor-aoceptor (EDA) complexes, provide useful systems which are suitable for studying the nature of the charge transfer Occurring between molecules. Thus, a vast amount of data have been accumulated in the past 40 years concerning the physicochemical properties of the charge-transfer complexes, using spectroscopic,crystallographic, electromagnetic, and other measuring techniques.’-’ The interaction between an electron donor D and an electron acceptor A is well-explained in terms of the mixing between the no-CT cofliguration DA and the CT configuration D’A- as first shown by Mulliken in 1952.4 This CT interaction causes an attracting force between a donor and an acceptor. It is known that, in the singlet electronic manifold, the energy of the no-CT configuration is much lower than that of the CT configuration and that this large energy separation prevents these two configurations from substantial m i ~ i n g .This ~ results in complexes in the ground state with nearly no-CT character and those in the lowest excited states with nearly pure CT character. In contrast, the no-CT configuration and the CT configuration can have energies close to each other in the triplet electronic manifold, because the no-CT configuration in the triplet manifold is the locally excited (LE) configuration, DA* or D*A, and this configuration has higher energy than that of the no-CT configuration in the singlet manifold by the amount of the triplet excitation energy. This proximity of energy levels allows these two configurations to mix substantially with each other. Thus, even small changes in the electron-donating ability of the donors or in the electronwithdrawing ability of the acceptors significantly affect the magnitude of the mixing between these configurations and hence change the electronic structure of CT complexes in the triplet manifold (triplet complexes). If we fm the acceptor and appropriately change the electron-donating ability of the donor by changing the donor molecule, we have an ideal system in which triplet complexes with both LE-predominant and CT-predominant characters are obtainable. In this case, the molecular and electronic structure of the acceptor is expected to change significantly in accordance with the change of the donor molecules. The present study is concerned with this structural change in triplet complexes. The formation of CT complexes in the triplet electronic manifold was first observed for the tetracyanobenzene-all@enzene system by Nagakura and co-workers.6 By using electron spin resonance spectroscopy, they demonstrated that the magnitude of CT varies from 7% in tetracyanobenzentbenzeneto 95% in tetracyanobenzene-hexamethylbenzene in rigid glass matrices at 77 K.7 The chloranilalkylbenzenetriplet complexes form another *Author to whom correspondence should be addressed.
0022-3654/92/2096-8252$03.00/0
prototypical system which is more suitable for Raman spectroscopic studies; these complexes are nearly free from fluorescence in contrast to the strongly fluorescent tetracyanobenzene complexes. It has been made clear by now that the LE configuration is lower in energy for the complexes with mesitylene and weaker donors (LE-predominant complexes), while the CT configuration has lower energy for the complexes with durene and stronger donors (CT-predominant complexes).e19 A switchover is expected to occur between mesitylene and durene. The chloranil-alkylbenzene system has another point of interest. It is thought that chloranil in the triplet complexes abstracts hydrogen from the donor to form the chloranil ketyl radical (the chloranil semiquinone radical) The system therefore provides an opportunity for studying the mechanism of the hydrogen abstraction in terms of the CT structure of the complexes. Although considerable knowledge has been gained on the electronic structure of certain kinds of triplet complexes, mostly by using nanosecond-picosecond electronic absorption spectroscopy, information on the molecular structure of these complexes is still scant. This is because elucidating the molecular structure of short-lived species such as triplet complexes was a difficult task until quite recently. The situation has been changed by the developments in transient and timeresolved Raman spectrampy. Of particular relevance to the present study is the fact that the structure of “isoatomers” (a group of molecular species having the same atomic frame but different electronic structure including So, SI,TI, anion radical, cation radical, etc.) can be studied systematically by time-resolved Raman The present paper reports a transient and time-resolved Raman study on the structural problems in triplet CT complexes. The molecular structure of the chloranil moiety in the chloranil-alkylbenzene triplet complexes is studied in solution at room temperature. ,9J0J3314918
2. Experimental Section 21. Trradeat Reaomw Ramaa Spec&& Transient Raman spectra were measured with a nanosecond Raman system constructed in this laboratory. This system is based on two sets of Q-switched NdYAG lasers (Spactron SL801, 50 Hz, 10-ns pulse duration; SL804,SO Hz,17-ns pulse duration), a dye laser (SL400OG), a doubling-mixing unit (SL4000EMX), and home-made H2/D2 Raman shifters. The firing of these two sets of pulsed laserswere synchronized by a delay generator (Stanford Research Systems DG535). The third harmonic (355 nm, 2-5 mJ/pulse) of one of the two Nd:YAG lasers was used to photoexcite chloranil. The second harmonic (532 nm, 1-4 mJ) of the other Nd:YAG laser or a Stokes line of the D2Raman shifter (451 nm, 1 mJ)induced by the third harmonic was used to probe resoNLncc Raman scattering of the triplet complexes. The pump and probe beams were focusbd onto a thin-film-like stream of the sample solution. The scattered light at 90° to the laser beams 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8253
Structure of Triplet Charge-Transfer Complexes
TABLE I: V i b n t l d Freuuencies (in cm-') and Their Assignment of Chloradl in the Triplet Complexes, TI Chlonnll, and the Anion Radial ~~~~
~
SOE 1609
CA-BZ CA-TL (9.245)b (8.82) 1565 1571 (0.55)c 1564
TI
CA-mXY
CA-MES
CA-DU
CA-PMB
CA-HMB
(8.56) 1566
(8.39) 1566
(8.03) 1577 1560
(7.92) 1576 1558
(7.85) 1577 1559
1495
1495
1688
1418 (0.19) 1204
1413 1202
1410 1201
1405 1197
1406 1195
1007
1045 (0.45)
486 327
651 485 (0.40) 327 (0.44)
1038 804 646 481 324
1033 803 646 482 324
1030 807 647 482 325
1027 807 646 484 325
assignments
AR
1583 C=Cstr 1554 1517 1498
CO str
500 335
1017
1016
499 334
500 335
1086 1016
CC str + CCl str deform 1 + deform 2 2 X deform 2 499 ring deform 1 333 ring deform 2
'From ref 28. bTheionization potential of donors (in eV). From ref 7. CThedepolarization ratio. Abbreviations: CA = chloranil, Bz = benzene, TL = toluene, mXY = m-xylene, MES = mesitylene, DU = durene, PMB = pentamethylbenzene, HMB = hexamethylbenzene,AR = anion radical. was collected by a camera lens (Nikon) and analyzed by a triple polychromator (Jobin-Yvon T64000) equipped with an intensified photodiode array detector (Princeton Instruments IRY1024G/RB). The detector was used in the gate mode (18-ns gate width) with a gate pulser (Princeton Instruments FGlOO). The acquired data were stored and analyzed with a personal computer (NEC PC9801RL). The time resolution of the system was determined by the duration of the laser pulses (17 ns). The delay time was adjustable between 0 ns and 20 ms. Depolarization ratice wcre measured with a polarizing sheet and a polarization scrambler placed in front of the spectrometer entrance slit. t2.Tnasient Akporptioa specbp. Transient absorption spectra were measured by using a pulsed Xe lamp (Hamamatsu L2358) as the monitoring light source. The monitoring light was introduced to a 1-cm quartz sample cell through which the sample solution was circulated. The pump beam from a nanosecond Nd:YAG laser was focused by a cylindrical lens onto the sample cell. The monitoring light transmitted through the irradiated portion of the sample cell was collected by a quartz lens and analyzed by a monochromator (Jobin-Yvon HR320). Several glass filters (HOYA) were used to attenuate the monitoring light and to cut off the second-order diffraction light of the monochromator. An intensified photodiode array detector (Princeton Instruments IRY-'IOOG/B/par) and a photomultiplier (Hamamatsu R636) were attached to the two output ports of the monochromator. The former was used with the gate mode (5-11s gate width) in order to measure transient absorption spectra at a certain delay time. The latter was used with a boxcar averager (Stanford Research Systems) to measure the temporal behavior of the transient absorption at a certain wavelength. The timing of the system was adjusted by a delay generator (Stanford Research Systems DG535) and controlled by a personal computer (NEC PC9801RL). The time resolution of this system was determined by the duration of the laser pulse (10 ns). 2-3. Samples. Chloranil (Wako Chemical Co. Ltd., special grade) was purified by recrystallization from toluene several times before use. Chloroform, benzene, toluene, m-xylene, and mesitylene (Wako, special grade) were used as obtained. Durene (Wako), pentamethylbenzene (Wako), and hexamethylbenzene (Tokyo Kasei) were recrystallized several times from cyclohexane, a ethanol-water mixture, and ethanol, respectively. The sodium and potassium salts of chloranil were prepared by the method of Torrey and Hunter.21 The salts prepared by this method have been identified as 1:l salts. The chloranil anion radical was produced by dissolving the salts in polar solvents such as acetonitrile.22 3. Results and Discussion 3-1. TIState of Chloranil. Time-resolved resonance Raman spectra obtained from a chloroform solution of chloranil(1 X 10-2 mol dm-') are shown in Figure 1. The pumping and probing wavelengths are 355 and 532 nm, respectively. Several transient Raman bands are observed in the spectrum at the delay time of 20 ns, and they decay in several hundreds of nanoseconds. The
I
I
A
B
C
D 1500
1000
500
1 0
RAMAN SHIFT / cm"
Figure 1. Time-resolved resonance Raman spectra of chloranil in chloroform (1 X lo-*mol dm-3; pump laser 355 nm; probe laser 532 nm). (A) 20 ns; (B) 100 ns; (C) 200 ns; (D) probe only.
decay curves of all transient bands are identical, showing that they are due to a single transient species. This transient species is identified as the T I state of chloranil for the following reasons. (1) The TI state of chloranil exhibits transient absorption around 520 nm.8,23The probing wavelength, 532 nm, is in rwnance with this T-T absorption. The Raman bands of the T I state should be enhanced under this resonance condition. (2) The intersystem crossing from SIto TI takes place in a few tens of picoseconds with a quantum yield of almost unity.I2 The lifetime of TI chloranil is reported as 1-10 ps in oxygen-free s o l u t i ~ n s . ~ J ~ ~ ~ ~ Therefore, TI chloranil is most likely to exist in the nanosecond time region after photoexcitation. In our experiment, sample solutions were not deoxygenated, and the lifetime of the TI state was shortened by oxygen quenching. The Raman spectrum of TI chloranil obtained by subtracting the solvent and the So bands is shown in Figure 2. Seven Raman bands were observed in the region between 170 and 1800 cm-'. We assign the band at 651 cm-l to the overtone of 327 cm-I. In order to obtain information on the symmetry of the remaining six vibrations, we measured the depolarization ratios. The observed values are given in Table I. We could not obtain any reliable depolarization ratio of the weak band at 1204 cm-' because of the overlap with the strong solvent band at 1216 cm-I. Instead, we measured the depolarization ratio of the corresponding band at 1202 cm-l in benzene. This band gave the value of 0.38. Thus, these six Raman bands of T I chloranil are all polarized. In most cases of resonance Raman scattering, the Franck-Condon mechanism (Albrecht's A-term resonance)24is dominant, and the intensities of Raman bands due to totally symmetric vibrations
8254 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992
1500
Tahara and Hamaguchi
1000 500 RAMAN SHIFT icm.'
Figure 2. Transient resonance Raman spectrum of TIchloranil. The solvent bands and So bands were subtracted in the spectrum.
are enhanced. We assign these six bands to totally symmetric fundamentals, overtones, or combinations. The observed depolarization ratios are consistent with this a ~ s i g n m e n t . ~ ~ Chloranil in the So state is known to be nearly planar.26 If we assume that the planarity is retained in the T I state (DZh),T I chloranil has six totally symmetric fundamental vibrations. Vibrational assignments of Sochloranil have already been made on the basis of the polarized Raman spectra of a single crystal and normal coordinate ana lyse^.^^^^^ Referring to the vibrational assignments of the So state, we make tentative assignments of T I Raman bands in the following way. The band at 1571 cm-I is undoubtedly assigned to the C = C stretch of the ring. We assign the 1418-cm-l band to the CO stretch by analogy of T I pbenzoquinone, which gives 1496 cm-' for this m0de.2~We assign the 1045-cm-I band to a C-C stretchingvibration mixed with C-Cl stretching. This mode gives a Raman band at 1007 cm-I in the So ~ p e c t r u m . The ~ ~ *bands ~ ~ at 485 and 327 cm-I correspond to 486- and 327-cm-I bands in the So spectrum, and they are assignable to the ring deformations. The Raman band due to the totally symmetric C-Cl bending vibration is expected to appear around 200 cm-I. No Raman band was observed in this region of the T I Raman spectrum. It is not likely that the vibrational frequency of this vibration changes drastically on going from the Soto the TI state. We conclude that the corresponding Raman band was not observed under the present experimental conditions. The remaining 1204-cm-I band is assignable to an overtone or a combination. The frequencies of the observed TI Raman bands are listed in Table I together with their assignments. 3-2. Complexes with b e n e , Toluene, m-Xylene, and Mesitylene (LE-Predominant Triplet Complexes). In the complexes of chloranil with mesitylene or weaker donors such as m-xylene, toluene, and benzene, the energy of the LE configuration is lower than that of the CT configuration in the triplet electronic manifold. Therefore, the triplet complexes of these donoracceptor pairs can be described as LE-predominant triplet complexes. The nanosecond transient absorption spectra of chloranil ( I X lo-' mol dm-9 in benzene, toluene, m-xylene, and mesitylene are compared with that in chloroform (free TI state) in Figure 3. The pumping wavelength is 355 nm, and the delay time is 20 ns. These donor-acceptor pairs form complexes also in the ground state (So complexes), and hence, both free chloranil and the So complexes are expected to exist in equilibrium in solution. Both free chloranil and the So complexes exhibit absorption at 355 nm, and the pumping pulse excites both species. However, the amount of the free T I state is negligibly small in neat donors, as will be proved by the Raman results presented in the following. Therefore, the transient absorption spectra shown in Figure 3B-E can be attributed solely to the TI chloranil-alkylbenzene complexes. Transient absorption spectra of the triplet complexes of some of these donoracceptor pain have been reported previously by several group~.8+'~5~' The present spectra are in good agreement with them. The absorption band characteristic to T I chloranil (-520 nm) remains in the spectra of the triplet complexes. This means that the LE state provides the most siBnifcant contributionto the triplet complexes. The most significant spectral change on going from TIchloranil to the complexes is the appearance of new broad absorption bands in the longer wavelength region (>600 nm). These new bands have been assigned to the CT transition in the
a
1
4
J
0.0
O0.0400
500 '
600
700 *
800
900 G
WAVELENGTH inm
Figure 3. Transient absorption spectra of chloranil in chloroform (A), benzene (B), toluene (C), m-xylene (D), and mesitylene (E) (1 X lW3 mol dm-'; pump laser 355 nm; delay time 20 ns).
I
I
I
I
I
1500
1000
500
0
RAMAN SHIFT icm.'
Figure 4. Transient resonance Raman spectra of triplet complexei of chloranil with benzene (A), toluene (B), m-xylene (C), and mesitylene (D) (chloranil 1 X mol dm-' in neat donors; pump laser 355 nm; probe laser 532 nm; delay time 20 ns). The solvent bands were subtracted in each spectrum.
triplet complexes; they have been regarded as evidence for the complex formation in these donoracceptor pairs in the triplet state! The absorption maximum of this CT band is located around 850 nm for the chloranil-benzene pair, and those of the other pairs are out of our observed spectral region (>900 nm). The transient absorption spectra show that these triplet complexes are of LEpredominant character (partially charge-transfer triplet complexes). Since the transition showing a maximum at 520 nm is localized on the chloranil part of the complexes, only chloranil vibrations are expected to be selectively enhanced with the excitation at 532 nm. This resonance condition is very useful for examining the structural changes of the chloranil part of the complexes. Figure 4 shows the transient resonance Raman spectra of the complexes measured with pumping at 355 nm and probing at 532 nm. As expected, the spectral features are very similar to that of the free TIchloranil. Except for the weak Raman band around 800 an-',
The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8255
Structure of Triplet Charge-Transfer Complexes
o.4{
1
I. W
0
20ns
0.3-
z
5a
0 0.2-
2a U
0.1
-
0.0-
500
400
600
700
WAVELENGTH /nm
1440 1420 1400 1380
Figure 6. Time-resolved absorption spectra of chloranil (1 X IO-’ mol dm-’) in chloroform in the presence of hexamethylbenzene (1 X 10-1mol dm-’). Pump laser 355 nm.
1060 1040 1020 1000
RAMAN SHIFT icm”
Figure 5. Transient resonance Raman spectra of TI chloranil (A) and triplet complexes with benzene (B), toluene (C), m-xylene (D), and mesitylene (E).
o.21?====7
n., 0.0
which is assigned to the combination of two ring deformation vibrations, all the observed bands of the complexes are unequivocally related to those of the free T I chloranil; only Raman bands due to chloranil vibrations are observed in these spectra. The obtained vibrational frequencies are listed in Table I. In spite of the similarity in intensity patterns, some vibrational frequencies in the complexes are significantly deviated from those of ”free” TIchloranil. This gives definitive proof for the formation of triplet complexes under the present experimental conditions. The deviations in two typical Raman bands are more clearly seen in the expanded spectra shown in Figure 5 . The CO stretching frequencies are significantly lower in the complexes than that in free T I chloranil(l418 cm-I). They show gradual downshifts as the number of methyl groups in the donors increases (1 41 3 cm-I in benzene 1410 cm-’ in toluene 1405 cm-I in m-xylene 1400 cm-l in mesitylene). The bands assigned to the C-C stretch + C-Cl stretch exhibit very similar behavior (1045 cm-’in CHCl, 1038 cm-l in benzene 1033 cm-l in toluene 1030 cm-’ in m-xylene 1027 an-*in mesitylene). The methyl substitution is known to increase the electron-donating ability of donors. The increasing electrondonating ability of donors results in the increase of the magnitude of charge transfer occurring in the complexes. The observed gradual downshifts of the frequencies of chloranil vibrations indicate that significant structural changes are induced on the chloranil part by complex formation and that they reflect the difference in the magnitude of the charge transfer occurring in the LE-predominant triplet complexes. $3. Complexes with H e x m e t h y b e “ , Peatamethybe“, and Durew (CC-Predominant Triplet Complexes) and the Anion Rndicd. In the complexes of chloranil with durene or stronger donors such as pentamethylbenzene and hexamethylbenzene, the energy of the CT configuration is lower than that of the LE configuration in the triplet electronic manifold. Therefore, the triplet complexes of these donor-acceptor pairs can be described as CT-predominant triplet complexes. In these complexes, the magnitude of charge transfer is larger than that in LE-predominant complexes described in the previous section. Time-resolved absorption spectra obtained from a chloroform solution of chloranil and hexamethylbenzene (1 X lo-’ and 1 X 10-1 mol dm-,, respectively) are shown in Figure 6. This donor-acceptor pair generates a strong transient absorption with a maximum at 448 nm immediately after the photoexcitation. After this fmt transient decays, another transient absorption (435 nm) due to a long-lived transient is observed. This temporal change in the transient absorption is essentially the same as that observed for the chloranil-durene system reported previously by Kobashi et al.” The first transient is assigned to the triplet complex and the second transient to the chloranil semiquinone
-
- -
-
-
-
-
-0.0
D
If\ 400
500
600
700
800
900
WAVELENGTH inm
Figure 7. Transient absorption spectra of triplet complexes and absorp-
tion spectrum of the anion radical. (A) Triplet complex of chloranil and durene (chloramil 1 X lo-’ mol dm-’, durene 2 X IO-’ mol dm-’, in chloroform; pump laser 355 nm; 20 ns); (B) triplet complex of chloranil and pentamethylbenzene (chloranil 1 X IO-’ mol dm-’, pentamethylbenzene 2 X 10-1 mol dm-’, in chloroform; pump laser 355 nm; 20 ns); (C) triplet complex of chloranil and hexamethylbenzene (chloranil 1 X IO-’ mol dm-’, hexamethylbenzene 1 X 10-1 mol dm-’, in chloroform; pump laser 355 nm; 20 ns); (D) the anion radical (1.1 X IO4 mol dm-’ in acetonitrile). radical which is produced by hydrogen abstraction from the donor and/or the solvent (chloroform). The transient absorption due to free T I chloranil (precursor of the triplet complex) was not observed with the present time resolution. Transient absorption spectra measured at a delay time of 20 ns are shown in Figure 7 for the chloroform solution of chloranil (1 X lo-, mol dm-’) with durene (2 X lo-’ mol dm”), pentamethylbenzene (2 X 10-1mol dm-,), and hexamethylbenzene (1 X 10-1mol dm-9. These donor-acceptor pairs show very similar transient absorption spectra. These absorptions can be assigned to the triplet complexes, although minor contributions from the semiquinoneradical can not be excluded completely. As reported previously, the absorption spectra of the CT-predominant triplet complexes are similar to that of the chloranil anion radial (Figure 7D).I3 Triplet complexes exhibit additional very weak absorption bands in the longer wavelength region (>600nm) which are assignable to back-CT transitions. These absorption spectra clearly show that the CT configuration provides the most significant contribution to these triplet complexes. Time-resolved Raman spectra measured from a chloroform solution of chloranil(3 X lo-’ mol dm-3) with hexamethylbenzene (1 X 10-1 mol dm-’) are shown in Figure 8. The pumping
Tahara and Hamaguchi
8256 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992
B
I
I
1500
I
I
1000 500 RAMAN SHIFT cm.’
F v 8. Time-resolved resonance Raman spectra of chloranil(3 X lo-’ mol dm-’) in chloroform in the presence of hexamethylbenzene (1 X IO-’ mol dm-’) measured with 355-nm pumping and 451-nm probing. (A) 20 ns; (B) 100 ns; (C) 200 ns; (D) 1 ps; (E) probe only. Solid arrows and dashed arrows indicate Raman bands due to the triplet complex and chloranil semiquinone radical, respectively. wavelength is 355 nm, and the probing wavelength is 451 nm. This probing wavelength is in resonance. with the T-T transition of the complex. The temporal change of the Raman spectra is in good agreement with that of the transient absorption; Le., the Raman bands due to the first transient (marked by solid arrows) appear immediately after the photoexcitation and the bands due to the second transient (marked by dashed arrows) remain after the first transient decays. The first transient is undoubtedly the triplet complex, and the second one is the chloranil semiquinone radical. Since the semiquinone radical exhibits transient absorption around 435 nm, the Raman bands due to this species are also enhanced by the preresonance effect under the present excitation condition. Three Raman bands due to the semiquinone radical are thus observed at 1545, 1160, and 495 cm-I. In this paper, we focus on the triplet complexes and hence do not mention the semiquinone radical any more. Raman bands due to the triplet complex can be recognized in the probeonly spectrum (Figure 8E). Since the ground-state complex exhibits absorption in the visible region, the probing laser can produce a small amount of the triplet complex and probe it. However, the contribution from this process is small (less than 10%) in the spectrum at a delay time of 20 ns. In order to check the other effects of the probe laser on the spectrum of the triplet complex, we examined the probe laser power dependence. The high-power irradiation (1 mJ/pulse) and the lowpower irradiation (0.1 mJ) of the probe laser gave an identical transient spectrum of the triplet complex. In the case of the triplet complex between anthraquinone-2,6-disulfonateand water, high-power irradiation of the probe laser produced the radical anion, and the resultant ion was observed with sharp power dep e n d e n ~ e . ~Lack ~ * ~ ’of the probe laser power dependence in the present experiment indicates that such a process does not occur in the chloranil-hexamethylbenzene triplet complex and that we measure the Raman spectrum of the unphotolyzed triplet complex. Similar temporal changes in time-resolved Raman spectra were observed for the chloranil-pentamethylbenzeneand chloranildurene pairs, although higher concentrations of donors (2 X lo-’ mol dm-3) had to be used to obtain reliable spectra of these complexes. By subtracting the Raman bands due to the solvent, the Sostate, and the semiquinone radical from the spectra measured at 0, 10, or 20 ns, we obtain the resonance Raman spectra of the CT-predominant triplet complexes of these three pairs. They are shown in Figure 9A-C. The Raman spectrum of the free chloranil anion radical was measured with 457.9-nmexcitation. Since the chloranil anion
I
I
I
1500
io00
500
RAMAN SHIFT /cm”
Figure 9. Transient resonance Raman spectra of triplet complexes and resonance Raman spectrum of the anion radical. (A) Triplet complex of chloranil and durene (chloranil 3 X lo-’ mol dm-), durene 2 X lo-’ mol dnr3,in chloroform; pump laser 355 nm; probe laser 451 nm); (B) triplet complex of chloranil and pentamethylbenzene (chloranil3 X lo-’ mol dm-’, pentamethylbenzene 2 X lo-’ mol dm-’, in chloroform; pump laser 355 nm; probe laser 451 nm); (C) triplet complex of chloranil and hexamethylbenzene (chloranil3 X IO-) mol dm-’, hexamethylbenzene 1 X 1O-I mol dm”, in chloroform; pump l a w 355 nm; probe laser 451 nm); (D) the anion radical (1 X IO-’ mol dm-’, in acetonitrile; probe laser 457.9 nm). The Raman bands due to solvent, the So states, and the chloranil semiquinone radical were subtracted in each spectrum.
-
radical is not stable at room temperature in acetonitrile (lifetime 4 h),22we checked the degradation of the sample of the anion radical by checking the absorption spectrum before and after the Raman measurement. The degradation of the anion radical by less than 10%was detected under the present experimental conditions. The observed spectrum is given in Figure 9D. The anion radical prepared from the potassium salt and that from the sodium salt gave identical spectra; no influence of the countercation was recognized in the Raman spectra. Tripathi and Schuler reported two vibrational frequencies (1592 and 1459 cm-’) of the chloranil anion radical in 1982, although they did not show the spectrum.32 The vibrational frequenciesobtained here are quite different from theirs. They prepared the chloranil anion radical by the radiolysis of tetrachloro hydroquinone in a basic solution (pH 11). Although the reason of this discrepancy is not clear at present, it might be due to the difference in the experimental conditions, e.g., pH of the solution and the method of the preparation. The spectral features of the CT-predominant triplet complexes are very similar to that of the anion radical except that some bands of the anion radical are not observed for the complexes. This indicates that only Raman bands due to the chloranil vibrations are selectively enhanced in these resonance Raman spectra as expected from the resonance condition. Although the Raman intensity patterns of the CT-predominant triplet complexes are quite different from that of T I chloranil, prominent Raman bands of the complexes can be related to those of T I chloranil rather straightforwardly. Thus, we can make vibrational assignments for some Raman bands of the CT predominant triplet complexes. Concerning the chloranil-hexamethylbenzene pair, for example, the Raman bands at 1577,1016, 500, and 335 cm-’can be related to those at 1571 (C-C stretch), 1045 (C-C stretch + C-CI stretch), 485 (deform), and 327 m-’ (deform) of T, chloranil. The observed vibrational frequencies of the CT-predominant triplet complexes are listed in Table I. 3-4. Frequencies of Chloranil Vibrations in Complexes A TbeoreticrlTreatment We determined the frequencies of chloranil vibrations both in the LE-predominant and CT-predominant triplet complexes. We also determined those of Ti chloranil and the chloranil anion radical. In order to analyze these experimental
-
Structure of Triplet Charge-Transfer Complexes data theoretically, we derive a formula which relates the frequencies of the chloranil vibrations in the complexes with the ionization potential of donors (a good indicator of the electrondonating ability). Mulliken’s theory is used as the starting point. The wave function of a triplet complex at a fmed intermolecular geometry can be represented by a linear combination of the wave functions of the LE and CT configurations of the same geometry: 1 (&LE + Wml = (a2 b2 ~ b S ) l / ~
’
Then, eq 6 is simplified to
+ +
where the overlap integral, S = ($LEI$CT), is neglected in the second expression. The mixing coefficient 6 (=b/a) is smaller than unity for the LE-predominant complexes, while it is larger than unity for the CT-predominant complexes. The energy of the complex is given as a function of normal coordinates Q: 1 (ELE + 6’EcT 26h) (2) E(Q) (ltlW)=
+
($LEIWLE) ECTE ($cTIWCT) ELE
h E ($cTT(~$LE) Since the force constants are the second derivatives of the energy with respect to the changes in the normal coordinates Q,we obtain the following expressions of the force constants and the vibrational frequencies of normal vibrations localized on chloranil:
k = -W = Q)
aQ2
The Journal of Physical Chemistry, Vol. 96, NO.21, 1992 8257
AE
(8)
In these formulas, negative signs correspond to the lower energy state, i.e., the triplet complex in question. The energy difference between the LE and CT configurations, AE, can be given approximately by the following expression: hE = IPD - EAT - e / r 1IPD - Co (9) where IPD is the ionization potential of the donor, EAT is the electron affinity of the acceptor, and r is the distance between the donor and the acceptor. In the present case, the acceptor is the lowest excited triplet state of chloranil, and the donors are the alkylbenzenes in the ground states. If we assume that the distance r is constant for all triplet complexes, the second and the third terms of (9) can be regarded as constant and can be replaced by a parameter Co. Combining (4), (8), and (9), we obtain the final formula relating the frequency of a chloranil vibration in the triplet complexes with the ionization potential of donors. -
.
ECT- ELE
v=
I
[(UT’
-
+ [$(‘Po
- Co - [(IPD - CO)’ +
3-5. St“lCbangemChlorPnilIuducedbyChargeTranofer
where vT and vA are the frequencies of the chloranil vibrations in the LE and CT configurations, respectively. The energy and the coefficient 6 of the complex can be obtained by solving the following equation (the variation method): Et,% - E h - S E h-SE E m - E
Equation 5 can be solved exactly, and the obtained energies and the coefficients of the lower and higher states are as follows:
The CT complexes in the singlet electronic manifold are often discussad by using only low-order terms of the solution of ( S ) , since the mixing between the So state and the CT state is not large. However, when we discuss the complexes in the triplet manifold, such a treatment is not adequate because the LE configuration and the CT configuration can be highly mixed with each other. We have to use the full expression in eq 6. In order to decrease the number of parameters in (6),we use the following approximations:
in Triplet Complexes. In Figure 10, the five fundamental vibrational frequencies of chloranil in the triplet complexes are plotted as a function of the ionization potential of the donors. The corresponding frequencies of the TIstate and the anion radical are also plotted therein. Common “S-shape (or inverted S-shape)” dependence is observed for all vibrations. In order to compare the obtained experimental data with the theoretical curves, we need to fix several values of parameters in (10). First, we replace the vibrational frequencies of chloranil in the LE and CT configurations with those of free T I chloranil and the anion radical, respectively. The LE configuration corresponds to a pair of T1 chloranil and the donor So state, and the CT codiguration corresponds to that of the chloranil anion radical and the donor cation radical. The parameter C, can be rewritten as follows: Co = IPD - AE (11) This means that C, is equal to the ionization potential at AE = 0. In the triplet complexes of chloranil with alkylbenzenes, the inversion of the energy of the LE and the CT configurations occurs between mesitylene and durene. Thus, we put Coequal to the average value of the ionization potentials of mesitylene and durcne:
If we calculate the value of r from eq 9 using EAT = 4.89 eV for T, ~ h l o r a n i l we , ~ ~obtain r = 4.3 A. This value is slightly larger than the distance between molecular planes for chloranil-hexamethylbenzene pairs in crystal (3.5 A).34 The parameter can be estimated from the energy of the CT transition in the triplet
Tahara and Hamaguchi
8258 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992
1570
7
+ C-CI str
1040
6 1030 .
1380 12.0
10.0
8.0
6.0
I
5101
5
12.0
10.0
8.0
6.0
U
J,,
I
deform
4 12.0
12.0
10.0
8.0
100
80
60
6.0
Ionization potential of donors /eV Figure 10. Plots of the vibrational frequencies of chloranil against the ionization potential of donors. (A) T,chloranil; (0)LE-predominant triplet complexes; ( 0 )CT-predominant triplet complexes; (A)the anion
radical. electronic manifold. Using (8), we obtain the following expression for the energy of the CT transition of the complexes:
As d a r i b e d in section 3-2, the broad absorptions assigned to the CT transitions are observed for the LE-predominant triplet complexes. The absorption maximum for the chloranil-benzene pair is located around 850 nm (1.46 eV). Using this value, 1/31 is calculated as 0.514 eV. The wavelengths of the CT transitions for the other pairs are calculated as 1037 nm (toluene), 1142 nm (m-xylene), and 1188 nm (mesitylene). These values accord with the fact that the maxima of the CT absorption bands of these pairs are out of our observed spectral region (>go0 nm). We fix 1/31 at 0.514 eV. Using the values of parameters fixed as above, we calculated the theoretical curves for five fundamental vibrations. They are depicted with solid lines in Figure 10. For the CO stretch, we use 1380 cm-l for vA, since the corresponding frequency of the anion radical is not available. The theoretical curves reproduce the characteristic S shapes (or inverted S shapes) of the experimental data quite well. This means that the vibrational frequency shifts surely reflect the change in the magnitude of the charge transfer taking place in the complexes. The most significant deviation of the experimental data from the theoretical curve is found for the C=C stretch vibration, especially for the LE-predominant complexes. This deviation may be due to the Raman band shape. As shown in Figure 4 (especially for the chloranil-benzene and chloranil-toluene pairs), this vibration gives Raman bands having complicated features, Le., a relatively sharp band accompanied by a broad band in the higher frequency side. These complicated band features are ascribable to a Fermi resonance between the C - C stretch vibration and an unknown overtone or combination. If this is the case, the unperturbed frequencies are expected to be higher than the present value that are determined by reading the sharp peaks in the lower frequency side. The agreement between the experimental data and the theoretical curve would be better for these unperturbed frequencies. There are several more factors that need consideration. Firstly, we implicitly neglect the change of the normal coordinates Q in
the derivation of formulas 3 and 4. The structural changes taking place in chloranil in the triplet complexes cause changes in the normal coordinates of chloranil. Although these changes are expected to be small, we need to bear this fact in mind. Secondly, the vibrational frequencies of the complexed chloranil in the LE configurationare not exactly the same as those of the free TI state. The same is true for the CT configuration. This is because the wave functions of the LE and CT configurations at a certain intermolecular geometry should include the effects from all interactions other than the charge-transfer interaction, e.g., the Coulomb interaction and the van der Waals interaction. Therefore, it is probable that some errors are introduced into the theoretical curves when vT and vA are replaced with the vibrational frequencies of the free TI state and the anion radical. Thirdly, it is possible that the intermolecular geometry of the triplet complexes, which is simplified as the distance r in the present theoretical treatment, is not exactly the same for all complexes. In this case, the parameter C,, cannot be regarded as constant. For example, a larger magnitude of charge transfer occurring in the complexes should result in a larger Coulomb attraction force between the donor and the acceptor. Thus, the effective r value (the distance of the donor and the acceptor) can become smaller with increasing magnitude of the charge transfer. This effect can distort the theoretical curves in the lower ionization potential region toward the vibrational frequency of the anion radical. If C, is taken as 9.0 eV, which corresponds to r = 3.5 A, the dotted curves in Figure 10 are obtained. In this case, the frequencies for the CT-predominant complexes (filled circles) fit better with the theoretical curves. It seems that the dotted curves also accord with the fact that the vibrational frequencies in the CT-predominant complexes are not very sensitive to the change of the donors and that they are all close to those of the anion radical. The change in the intermolecular geometry with the change in the magnitude of charge transfer seems important in solution at room temperature. The magnitude of charge transfer varies significantly among the CT-predominant triplet complexes of tetracyanobenzene with durene, pentamethylbenzene, and hexamethylbenzene in a rigid glass matrix at 77 K.' In solution at room temperature, on the other hand, we did not obtain clear evidence for the difference among the CT-predominant triplet complexes of chloranil with the same donors. This fact might be ascribed to the effect of the change in the intermolecular geometry, which can occur more easily in solution at room temperature. In spite of these uncertainty factors, the general trends of the observed frequency shifts in the chloranilakylbenzene complexes are well-reproduced by the present theoretical treatment. This means that the relationship between the molecular structure and charge-transfer interaction in triplet complexes is essentially described by this simple theory. The following conclusions are drawn from the present study: (1) The molecular structure of the chloranil moiety in the triplet complexes is intermediate between those of TI chloranil and the anion radical, and (2) it changes with the change in the electron-donatingability of donors, reflecting the magnitude of charge transfer occurring between the donor and the acceptor. In this paper, we examined only one particular system of triplet complexes, Le., chloranil-alkylbenzene triplet complexes. However, these conclusions are thought to be valid for general CT complexes in the triplet electronic manifold. We finally emphasize the fact that structural information discussed in the present study was obtained only by virtue of time-resolved Raman spectroscopy. The time-resolved absorption spectra in Figure 3 give only limited information about the similarity of the electronic structures in the LE-predominant complexes. The timeresolved Raman spectra in Figure 4, on the other hand, give unique structural information that makes a clear distinction among those complexes. Acknowledgment. We are grateful to Professor H. Miyasaka of Kyoto Institute of Technology for helpful discussions. References and Notes (1) Mulliken, R.A.; Person, W. B. Molecular Complexes; Wiley: New York, 1969.
8259
J. Phys. Chem. 1992,96, 8259-8264 (2) Foster, R. Organic Charge Transfer Complexes; Academic Press: London, 1969. (3) Nagakura, S.In Excited Stares; Lim, C., Ed.; Academic Press: New York, 1975; p 321. (4) Mulliken, R. S.J. Am. Chcm. Soc. 1952, 74, 811. (5) There arc some ground-state complexes in which substantial mixing takes place between the CT and no-CT configurations. For example, it was reported that the magnitude of the charge transfer in the iodine-trimethylamine complex is as high as 40%. Sce ref 1. (6) Iwata, S.; Tanaka, J.; Nagakura, S . J. Chem. Phys. 1%7,47, 2203. (7) Hayashi, H.; Iwata, S.; Nagakura, S.J. Chem. Phys. 1969, 50,993. (8) Kawai, K.; Shirota, Y.; Tsubomura, H.; Mikawa, H.Bull. Chem. Soc. Jpn. 1972, 45, 77. (9) Kobashi, H.; Gyoda, H.; Morita, T. Bull. Chem. Soc. Jpn. 1977, 50, 1731. (10) Kobashi, H.; Nagumo, T.; Morita, T. Chem. Phys. Lerr. 1978, 57, 369. (1 1) Gschwind, R.; Haselbach, E. Hela Chim. Acta 1979, 62, 941. (12) Hilinski, E. F.; Milton, S. V.; Renzepis, P. M. J. Am. Chem. Soc. 1983,105, 5193. (13) Kobashi, H.; Funabashi, M.; Kondo, T.; Morita, T.; Okada, T.; Mataga, N. Bull. Chem. Soc. Jpn. 1984,57, 3357. (14) Kobashi, H.; Okada, T.; Mataga. N. Bull. Chem. Soc. Jpn. 1986,59, 1975. (15) Kobashi, H.; Kondo, T.; Funabashi, M. Bull. Chem. Soc. Jpn. 1986, 59, 2347. (16) Kobashi, H.; Hiratsuka, K.; Motegi, K. Bull. Chem. Soc. Jpn. 1988, 61, 298. (17) Levin, P. P.; Pulzhnikov, P. F.; Kuzmin, V. A. Chem. Phys. Lerr. 1988,152,409. (18) Jones, G. I.; Haney, W. A.; Phan, X . T. J. Am. Chem. Soc. 1988,110, 1922. (19) Kobashi, H.; Funabashi, M.; Sizuka, H.; Okada, T.; Mataga, N. Chem. Phys. Lerr. 1989, 160, 261.
(20) Hamaguchi, H. In Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: New York,1987; Vol. 16, p 227. (21) Torrey, H. A.; Hunter, W. H. J. Am. Chem. SOC.1912, 34, 702. (22) Andre, J. J.; Weil, G. Mol. Phys. 1968, 15, 97. (23) Kemp, D. R.; Porter, G. Chem. Commun. 1969, 1029. (24) Albrecht, A. C. J . Chem. Phys. 1961, 34, 1476. (25) We note the possibility that the electronic degeneracy in the triplet state may affect the polarization of Raman scattering (See: Hamaguchi, H. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E.,Eds.; Wiley: Chichester, 1985; Vol. 12, p 273). However, this effect is negligible if spin-orbit interactions are small in the resonant intermediate state of Raman scattering. (26) Chu, S.S.C.; Jeffrey, G. A.; Sakurai, T. Acta Crystallogr. 1%2, 15, 661. (27) 1291. (28) (29) (30)
Girlando, A.; Pecile, C. J. Chem. SOC.,Faraday Trans. 2 1973,69, Girlando, A.; Pecile, C. J. Mol. Spectrosc. 1979, 77. 374. Rossetti, R.; Beck,S.M.; BNS, L. E. J. Phys. Chem. 1983,87,3058. Phillips, D.; Moore, J. N.; Hester, R. E. J. Chem. Soc., Faraday
Trans. 2 1986,82, 2093. (31) Moore, J. N.; Phillips, D.; Hester, R. E. J. Phys. Chem. 1988, 92, 5619. (32) Tripathi, G. N. R.; Schuler, R. H. J. Chem. Phys. 1982, 76,2139. (33) The electron affinity of T, chloranil is estimated by using the following equation: EAT = EAs ET where EAs is the electron affinity of So
+
chloranil and ET is the energy difference between the So and T , states of chloranil. With values of EAs = 2.76 eV and ET = 2.13 eV. we obtain EAT = 4.89 eV. The values of EAs and ET were taken from the following references: Bowers, M. T. Gas Phase Ion Chemistry; Academic Press: 1979; Vol. 2. Shcheglova, N. A.; Shigoline, D. N.; Yokobson, G. G.; Tushishvili, L. S. Zh. Fiz. Khim. 1969, 43, 1984. (34) Harding, T. T.; Wallwork, S. C. Acta Crystallogr. 1955, 8, 787.
Photodlssoclatlon of Size-Selected Mg+(H,O), Ions for n = 1 and 2 Fuminori Misaim, Masaomi Sanekata, Keizo Tsukamoto, Kiyokazu Fuke,* Institute for Molecular Science, Myodaiji, Okazaki 444, Japan
and Suehiro Iwata Department of Chemistry, Faculty of Science and Technology, Keio University, Yokohama 223, Japan (Received: April 21, 1992; In Final Form: June 24, 1992)
Electronically excited states of magnesium-water cluster ions, Mg+(H20),,are studied by photodissociation after mass selection. Dissociation spectra are obtained as a function of wavelength for n = 1 (250-370 nm) and 2 (280-470 nm). The spectra show absorption peaks at 28 300, 30 500, and 38 500 cm-' for n = 1, and 25 000, 29 400, and 32 000 cm-l for n = 2. These absorption bands are assigned to the 2P-2S type transitions localized on the Mg+ ion with the aid of ab initio CI calculations. In addition to evaporation of water molecules, photoinduced chemical reaction to produce MgOH+ is found to occur efficiently. Especially, for Mg+(H20)2,the branching ratio between the former and latter processes has been found to depend sensitively on the excitation energy. On the basis of these results, the mechanism of the photodissociation of these cluster ions is discussed.
Iatroduction Thermochemical properties of monovalent metal cations solvated with water have been extensively studied for the past two decades.l-' Thcse studies were mostly devoted to determining the successive hydration energies by high-pressure mass spectrome collision-induced dissociation (CID),4qsand photodissociation and charge-transfer collision experimentsq6 Also, the intramolecular dissociation processes were observed by CID4 and photodissociation methods.6b The measured binding energies have found to be in reasonable agreement with the results of theoretical studies.' On the other hand, the electronic structures of clusters composed of one metal atom or its ion solvated with polar molecules have been the subject of much interest recently.*-1° One of the central issues of the studies is the change of the electronic structure with the stepwise solvation and how many solvent molecules are needed to have characteristic properties of bulk solution. For example, clusters of an alkali-metal-atom solvated
with water or ammonia can be considered to be a model of the bulk system that produces solvated electrons. Especially, the ionization potentials of alkali-metal atoms solvated with polar molecules have been found to show interesting dependencies on the kind and the number of solvent molecules. This observation can be discussed in line with the relationship between the stability of the ion-pair state in the clusters and of the solvated electrons in the bulk f l ~ i d s . * ~ - ~ For alkaline-earth-metalatomic ions with ligands, information about the transition from free metal ions to the liquid solution phase is expected to be obtained from the photodissociation of mass-selected singly-charged cluster ions. In the bulk solution, the doubly charged ion is the only stable ion of alkaline-earth metal atoms. It is expected that the ion-pair state, in which an electron of the metal ion delocalizes on the ligand molecules, should be stabilized with increasing number of solvent molecules. In the case of Sr+(NH3)". the features in the photodissociation spectra
0022-3654/92/2096-8259$03.00/00 1992 American Chemical Society
