3872
J . Phys. Chem. 1990, 94, 3872-3874
Monophotonic Ionization from the Vibrationally Unrelaxed Excited Singlet State of N,N,N’,N’-Tetramethyl-p-phenylenediamine in Acetonitrile Solution Yoshinori Hirata,* Musubu Ichikawa, and Noboru Mataga* Department of Chemistry, Faculty of Engineering Science, and Research Center for Extreme Materials, Osaka University, Toyonaka, Osaka 560, Japan (Receiued: January 16, 1990)
The excitation wavelength dependence of the photoionizationyield of N,N,N’,N’-tetramethyl-p-phenylenediamine in acetonitrile has been investigated by using tunable picosecond dye laser photolysis measurements. The monophotonic ionization from the vibrationally unrelaxed state has been observed, the yield of which increased with increasing excitation energy in the S, state and decreased slightly around the onset of the S, state. This suggests that the rate of the electron photoejection depends not only on the excess energy contained but also on the excited vibrational mode.
Introduction Ionization or electron ejection from an excited molecule in condensed phase is one of the most important primary processes in photochemistry and has been investigated extensively by many researchers.’-’ In the liquid phase, ionization seems to be a rather complex process because of the stabilization of charges by the polarization of solvent. The polarization can stabilize the ion pair which should be unstable and immediately recombine in the gas phase. Such effect is prominent in highly polar solvents as we reported in the previous I n acetonitrile N,N,N’,N’-tetramethyl-p-phenylenediamine (TMPD) ionizes from its fluorescence state and the TMPD cationsolvent anion ion pair is f0rmed.I The ionization potential of TMPD is 6.65 eV in the gas phase, while the energy of the S, state is 3.5 eV. Therefore, the solvent polarization can lower the ionization energy of TMPD more than 3 eV in acetonitrile. Similar ionization of TMPD is observed also in several alcohols.* The rather long fluorescence lifetime of TMPD ( I .2 ns in acetonitrile and 4.3 ns in methanol) suggests that the ionization requires some special conformation of solvent molecules which can trap the ejected electron from the solute. On the other hand, the ionization energy in methanol glass (77 K) is 4.75 eV4 and the ionization cannot occur in the relaxed S, state. The electron can be ejected from the unrelaxed state with higher energy. Since the relaxation to the fluorescence state should be quite rapid, the ejected electron and the parent cation can be stabilized only by the electronic polarization and not by the reorientation polarization of solvent. The photoionization of TMPD in nonpolar liquids at room temperature also occurs from the unrelaxed state, which can be reached by the excitation with shorter than about 250-nm light.5 Although such an ionization process from an unrelaxed state should also occur by near-UV excitation of TMPD in polar solvents, the mechanism in this case is quite unclear. There are experimental difficulties in determining the ionization threshold and the ionization yield of polar solutions with pulsed lasers. That is, multiphoton ionization can easily occur with rather high yield, which disturbs the measurements of the ionization yield. In this respect, we have undertaken to make careful measurements of the excitation wavelength dependence of the photoionization of ( I ) Hirata, Y.; Mataga, N. J . Phys. Chem. 1983, 87, 1680. (2) Hirata, Y. Mataga, N. J . Phys. Chem. 1985, 89, 4031. (3) Hirata. Y.: Mataga, N.; Sakata, Y.; Misumi, S. J. Phys. Chem. 1983,
87. 1493. (4) Bernas, A , ; Gauthier. M.; Grand, D.; Parlant. G . Chem. Phys. Lett. 1912. 17. 439, ( 5 ) Choi, H. T.; Sethi, D. S.; Braun, C. L. J . Chem. Phys. 1982, 77, 6027. Braun, C. L.: Scott, T. W.: Albrecht, A . C. Chem. Phys. Lett. 1981,84, 248. (6) Richards, J. T.; Thomas, J. K. Trans. Faraday SOC.1970, 66, 621. (7) Labhart, H.; Heinzelman, W. In Organic Molecular Photophysics; Birks, J. B., Ed.; Wiley-Interscience: London, 1973; Vol. I , p 330 and references therein
0022-3654/90/2094-3872$02.50/0
TMPD in acetonitrile by using the transient absorption spectroscopy with a picosecond dye laser.
Experimental Section Picosecond transient absorption spectra were measured by using a dye laser photolysis system pumped by the second harmonics of a mode-locked Nd3+:YAG laser (Quantel, Picochrome). The combination of dyes rhodamine 590 (osc.)/kiton red 620 (amp.), cresyl violet 670 (o)/DCM (a), cresyl violet (o)/LDS 698 (a), and LDS 698 (o)/LDS 698 (a) can deliver a picosecond pulse at 590 nm (3 mJ), 650 nm (1.5 mJ), 682 nm (0.8 mJ), and 700 nm (1 .I mJ), respectively. The second harmonics of the dye laser output was used for excitation. The picosecond white light used to monitor the transient absorption was generated by focusing the fundamental pulse of the Nd3+:YAG laser into a quartz cell containing a H 2 0 / D 2 0 mixture. A pair of monochromators coupled to diode array detectors (Princeton Instruments, ST-1OOO) was used to measure the spectrum of the interrogating pulse. The laser was operated at 10 Hz and the signal was digitized (16 bit) and transferred to a microcomputer. Typically 30 shots of signals were accumulated in the computer and then the transient absorbance was calculated. The excitation intensity was varied by placing color glass filters in the path of the laser beam. TMPD obtained from its dihydrochloride was purified by sublimation in vacuo. Spectrograde acetonitrile and methanol dried by contacting with molecular sieves were transferred to the bulb containing TMPD and connected to the quartz sample cell without breaking the vacuum. Spectrograde cyclohexane was used without further purification. All the measurements were performed at room temperature (21 “C). Results and Discussion Figure 1 shows a UV absorption spectrum and a fluorescence spectrum of TMPD in acetonitrile; the excitation wavelengths used for the transient absorption measurements are indicated by arrows. A relative fluorescence yield is also shown in the figure. The yield decreased with increasing excitation energy in the longer wavelength region. While in the region of 3.0 X lo4-3.2 X lo4 cm-) the fluorescence yield was almost constant. Around the onset of the S, state the yield was slightly higher. The decrease of the fluorescence yield in the region of much higher excitation energy was observed even in nonpolar solvents and is attributed to photoionization. These results strongly suggest the existence of the energy dissipation channel which, in acetonitrile, can compete with the vibrational relaxation when the excitation energy is lower than 3.5 X lo4 cm-’. Photoionization seems to be a candidate for the energy dissipation channel. Such excitation energy dependence of fluorescence yield was not observed in n-hexane or in methanol. The picosecond transient absorption spectra of TMPD in acetonitrile measured at 100 ps after the 295-nm laser pulse excitation are displayed in Figure 2 as a function of excitation energy. The absorption bands peaking at 570 and 610 nm were
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 3873
Letters
i
3.0
0
1.0 [
2.0
s11 ( 1 0 - 5 ~ )
Figure 3. Plots of [TMPD+]/[Sl]against [SI] measured at 100 ps after the 325-nm (0)and 295-nm ( 0 )laser pulse excitation.
WAVE NUMBER ( 1 O3 cm-l)
Figure 1. UV absorption and fluorescence spectra of TMPD in acetonitrile. Relative fluorescence yield normalized at 2.78 X 10' cm-l is also shown in the figure. I
I
I
I
I
I
lo4 cm-I excitation, while the cation band was not so evident in the transient absorption spectra obtained by 2.82 X lo4 cm-' excitation with low-intensity laser pulse. This should mean that the rapid ionization from the unrelaxed state can occur by the shorter wavelengths excitation. In order to obtain the photoionization yield, we tried to resolve the measured transient absorption spectra into their components, SI and cation bands. The reference spectrum of Le., the S, the latter was obtained at long delay time (6 ns) in the same solution, while the transient absorption spectrum obtained in cyclohexane at 100 ps after the excitation was used for the reference of the former. In acetonitrile, we could not observe the pure S, SI absorption spectrum because of the presence of the ionization process. Since the absorption band was slightly redshifted in cyclohexane compared with that in acetonitrile, we used it as the reference after shifting the band to the blue by about 12 nm. The spectra of the components so obtained are also shown in the Figure 2. Although the sum of the components spectra is not shown in the figure, it essentially overlaps the observed data. In cyclohexane, we measured the transient absorption spectra S, up to 6 ns after the picosecond laser excitation. The S, absorption band was observed at short delay times, and with increasing delay time the spectral shape changed gradually to the superposition of the S, SI and T, TI absorption band. Since an isosbestic point was observed at 610 nm, the extinction coefficient of the S, SI band was deduced from the reported values TI of the triplet yield and the extinction coefficient of the T, band.6 The value so obtained was 19 000 M-' cm-' at 760 nm. The extinction coefficient of the cation band was reported as 10 100 M-' cm-I at 610 nm in ethanol. Although the spectral shape of the transient absorption depends to some extent on solvents, we used these values to estimate the ionization yield. If we assume that the ionization involves two distinct charge carrier generation mechanisms, monophotonic and biphotonic process, the excitation intensity, Zex, dependence of the produced ion can be described by
-
-
t-
-
I
0.1
A
-
-
-
+-
500 600 700 800 900 WAVELENGTH [ n m 1 Figure 2. Transient absorption spectra of TMPD in acetonitrile measured
400
at 100 ps after the 295-nm picosecond laser pulse excitation. The laser intensity was attenuated by using color glass filters and the relative excitation intensity is shown in the figure. The spectra were resolved successfully into the component, S, S, (dotted line) and TMPD+ (solid line), bands.
-
-
already assigned to the TMPD cation radical, while the band at SI transition of 680 and 740 nm can be assigned to the S, TMPD; a good agreement of the decay time of the band and the fluorescence lifetime of TMPD (1.2 ns) is observed. Although TMPD can ionize via the relaxed S1 state in acetonitrile as previously reported,' the delay time of 100 ps is not long enough to produce the large amount of the cation radical from that state as it is actually observed. Therefore, a rapid ionization process should exist, of which the candidates are the multiphoton ionization and the monophotonic ionization from an unrelaxed (FranckCondon) state. The ratio of the transient absorbance of the cation band to that of the S, SI absorption decreases with decreasing intensity of the excitation laser pulse. This suggests that a multiphoton (may be two photon) process is contributing to the ionization under our experimental conditions. On the other hand, even at very low excitation intensity, the cation band was observed with 3.39 X
-
[TMPD+] p Iex + AI,.
(1)
Since [SI]and [TMPD+] can be obtained simultaneously, we replaced I,,, the measurements of which were rather unreliable, by [S,]. It is then useful to transform eq 1 into [TMPD+]/[Sl] = B
+ C[Sl]
(2)
where B is proportional to the yield of monophotonic ionization, while Cis proportional to the ionization efficiency of the biphotonic process. There is an obvious restriction placed on eq 2. The concentration of the vibrationally relaxed SI state, [S,], must be linear with Zex.This may be violated when the multiphoton process is prevailing. In order to obtain the yield of the monophotonic ionization, [TMPD+Io, eq 2 calls for plotting [TMPD+]/[S,] against [SI] and extrapolating to [SI]= 0. Figure 3 demonstrates this procedure at two different excitation wavelengths. Good linear relations were obtained for both cases. For 3.08 X lo4 cm-' excitation, the slope of the plot was almost zero, meaning that the two-photon ionization is negligible. On the other hand, the bi-
3874
J . Phys. Chem. 1990,94. 3814-3816
TABLE I: Excitation Wavelength Dependence of the Monophotonic Ionization Yield from the Vibrationally Unrelaxed SI State of TMPD in Acetonitrile Solution excitn energy, excitn energy, lo4 cm-' ionizn yield lo4 cm-I ionizn yield 3.39 3.08 2.94
0.47 0.54 0.59
1
2.87 2.82
i-
0
0.50 0.21
I
@j
= [TMPD+],/([TMPD+],
+ [Si])
(3)
The values of aiare listed in Table I. The values of Oiand those of the relative fluorescence yield are plotted against excitation energy in Figure 4. The ionization yield increases with increasing excitation energy near the origin of the SIstate, while in the region of 2.9 X 104-3.1 X IO4 cm-' the yield seems constant. It should be stressed here that the yield obtained by excitation at 3.39 X lo4cm-] was less than that at 3.08 X 104 c d . This result suggests that the ionization can compete with the intramolecular vibrational redistribution (IVR) and that the rate may depend on the optically excited vibrational mode. If the IVR is faster than the ionization, the ionization yield may be determined by the excess energy and should increase monotonically with increasing excitation energy. The lower ionization yield at 3.39 X IO4 cm-I suggests that the accepting mode of the S2 SI internal conversion is different from the promoting mode of the ionization if the internal conversion occurs before ionization. If the ionization was faster than the internal conversion, the promoting mode was not excited by 3.39 x IO4 cm-I excitation. In the present study, we have demonstrated that TMPD in acetonitrile undergoes monophotonic ionization from the vibrationally unrelaxed SI state, complementing our previous findings' on its ionization from the relaxed SI state (fluorescence state). Moreover, our results indicate strongly that the rate of the electron photoejection from the vibrationally unrelaxed state depends not only on the excess vibrational energy but also on the nature of the excited vibrational mode, which seems to be very important in the elucidation of the nature of the electron ejection process from the excited solute in polar solutions in general. -+
0
-0
30 25 WAVENUMBER(103~,-1 I Figure 4. Excitation energy dependence of the monophotonic ionization yield ( 0 )at 100 ps after the excitation and the fluorescence yield (0). 35
photonic process was clearly observed at 3.39 X lo4 cm-I. The yield of the monophotonic ionization seems to be less when excited at 3.39 X lo4 cm-' compared with 3.08 X lo4 cm-I, in spite of the higher excitation energy of the former. Neglecting the nonradiative decay channel from the unrelaxed excited singlet state, except for the ionization, we can determine the monophotonic ionization yield by using the equation
Acknowledgment. The present work is supported partly by a Grant-in-Aid (No. 62065006) from the Japanese Ministry of Education, Science and Culture to N.M.
Predicted Enthalpies of Formatlon for Methyl-Substituted Disilanes Jerry A. Boatzt and Mark S. Gordon* Department of Chemistry, North Dakota State University, Fargo, North Dakota 58105 (Received: December 5, 1989)
Enthalpies of formation of the entire series of methyl-substituted disilanes, Si,(CH,)&k ( k = 1-6), are predicted by using homodesmic reactions at the MP2/6-3lG(d)//RHF/3-21G* level of theory. The calculated values are systematically higher than the values suggested by Walsh but are in excellent agreement with the kinetic data of O'Neal, Ring, and co-workers.
Introduction Accurate enthalpies of formation are an essential ingredient in understanding the thermodynamics of any given class of reactions. Furthermore, a knowledge of enthalpies of formation for a reaction or a group of related reactions is a necessary first step in the deduction of the mechanisms for these reactions. Unfortunately, the thermodynamics of many silicon compounds is not well-understood experimentally. A case in point is the series of methylated disilanes, where the enthalpies of formation are not well-known.',2 The calorimetric methods used in measuring enthalpies of formation are often plagued by incomplete combustion of the d i s i l a n e ~ . ~ Several techniques for the theoretical prediction of enthalpies of formation of molecules have recently been Among these is the methodology of Pople and co-workers,s who use large basis sets and quadratically convergent configuration +Presentaddress: Department of Chemistry, University of Utah, Salt Lake City, UT 841 12. 0022-3654/90/2094-3814$02.50/0
interaction (QCI)* to predict enthalpies of formation and other properties of small molecules to within 2-3 kcal/mol of the experimental values. For larger systems, the semiemprical methods of Dewar and Stewart et aL6 can be used to predict enthalpies of formation. (1) Walsh, R. Organomerallics 1989, 8, 1973-1978. (2) (a) Nares, K. E.; Harris, M. E.; Ring, M. A.; ONeal, H. E. Organometallics 1989,8, 1964-1967. (b) ONeal, H. E.; Ring, M. A,; Richardson, W. H.; Licciardi, G . F. Organomerallics 1989, 8, 1968-1973. (3) Walsh, R. In The Chemistry of Organosilicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1988; Chapter 5. (4) Pople, J. A.; Luke, B. T.; Frisch, M. J.; Binkley, J. S. J . Phys. Chem. 1985, 89, 2198-2203. (5) Pople, J. A.; Head-Gordon, M.; Fox, D. J.; Raghavachari, K.; Curtiss, L. A. J . Chem. Phys. 1989, 90, 5622-5629. ( 6 ) (a) Dewar, M. J. S.; Thiel, W. J. Am. Chem. Soc. 1977, 99,4899. (b) Stewart, J. J. P. MOPAC, QCPE No. 455, Department of Chemistry, Indiana University, Bloomington, IN 47405. (7) Disch, R. L.; Schulman, J. M.; Sabio, M. L. J . Am. Chem. Soc. 1985, 107, 1904. (8) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. J . Chem. Phys. 1987, 87, 5968.
0 1990 American Chemical Society
